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Abstract
The grain refinement and strength improvement resulted from hot rolling temperature in Ti microalloyed low carbon martensitic steel was investigated in this study. Two different started hot rolling temperatures (950 °C and 1100 °C) but with the same reheating quenching process were applied to the steel. The microstructures and second precipitated particles were examined by scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-rays diffraction and phase analysis method. It was found that hot rolling could induce the precipitation of TiC. Moreover, the amount of the TiC in steel rolled at 950 °C is higher and the size of the precipitates is much finer than that in steel rolled at 1100 °C. Both the large deformation without recrystallization and the precipitates in steel rolled at 950 °C are more effective on the grain refinement after the reheating process, in which the effective grain size (EGS) can be refined to 1.4 μm. In addition, the steel rolled at 950 °C exhibits a much higher strength than that rolled at 1100 °C due to the additional dislocation strengthening and more precipitation strengthening
... High-strength low-carbon steel has been widely used in multifarious structural plates and engineering components for centuries. The strengthening phase has evolved from general ferrite and pearlite to a low-carbon martensite [1][2][3], especially for harsh conditions. The strong solid solution strengthening of carbon atoms is adopted, indicating pronounced strength increasing [4]. ...
... The microstructure was examined by using scanning electron microscopy (SEM, JEOL-7800F), transmission electron microscopy (TEM, JEOL-2100), electron back-scattered diffraction (EBSD, GAIA3-GMU Model) and X-ray diffraction (XRD, Cu Kα, 2 deg./min). The samples for SEM measurement were etched by a solution consisting of 4 vol% HNO 3 . The EBSD and XRD measurement samples were polished using an ethanol electrolyte with about 6 vol% perchloric acid at -20 • C. The thin samples for TEM measurement were mechanically polished to a final thickness of approximately 50 μm. ...
... Hot rolling temperature has a significant effect on the grain refinement and precipitation behaviour, and therefore affects the mechanical properties of MART steels. Han et al. [209] compared two TMP routes on a Ti-microalloyed low-carbon MART steel with different start rolling temperatures (950°C and 1100°C) but with the same reheating temperature (1200°C) and holding time (1 h), as schematically illustrated in Fig. 28. They found that deformation without recrystallisation (950°C) was beneficial for the refinement of both martensitic microstructure and precipitates. ...
... Schematic diagram of TMP on a Ti-microalloyed low-carbon MART steel[209]. ...
Advanced high strength steels (AHSSs) are regarded as the most promising materials for vehicles in the 21st century. AHSSs are complex and sophisticated materials, with microstructures being controlled by precise thermomechanical processing (TMP) technologies. TMP is an established and strategic method for improving the mechanical properties of AHSSs through control of microstructures and is among the most important industrial technologies for producing high quality AHSSs with the necessary mechanical properties. This article aims to provide a comprehensive review of recent progress in TMP of AHSSs, with focus on the processing-microstructure-property relationships of the processed AHSSs. We first present an introduction to the background of the TMP of AHSSs. Then, the recent progress and the latest achievements in TMP of the first, second and third generations of AHSSs and Nano Hiten steels are reviewed in detail, and the mechanisms of the TMP-induced microstructural evolution and mechanical properties variation are addressed and discussed. The present review concludes with a summary on the TMP of AHSSs currently under development, and also offers an outlook of the future opportunities which will inspire more in-depth research and eventually advance practical applications of this innovative field.
... The energy spectra of Figure 10a show that the precipitated phase in the austenite at a reheating temperature of 1000 • C is a carbide complex precipitate of titanium-niobium with undissolved Cu, and Figure 10b,c show that the precipitated phase in the austenite of the test steels at reheating temperatures of 1150 • C and 1200 • C is a carbide complex precipitate of titanium-niobium. The finely dispersed carbonitrides played a vital role in refining grains, enhancing precipitation strength, and controlling recrystallization [19][20][21][22][23]. A smaller size of the second-phase particles precipitated during the heating process indicated a greater number of these particles, which effectively pinned the austenite grain boundary to obtain relatively fine original austenite. ...
This study investigated the effects of reheating temperature on the microstructure and mechanical properties of Cu-containing 440 MPa grade non-tempered ship plate steel. The mechanical properties test, thermodynamic simulation, optical microscopy, scanning electron microscopy, transmission electron microscopy, and other tests were performed. The results revealed that with increasing reheating temperature, the ferrite grain size of Cu-containing 440 MPa non-tempered ship plate steel increased. Also, with increasing reheating temperature, the size of copper particles and niobium–titanium composite precipitates in the original austenite decreased. Consequently, this led to a weakening of the pinning effect on the original austenite and an increase in the size of the transformed ferrite grains. Moreover, with increasing reheating temperature, the number of Cu precipitates in the test steel after air cooling and rolling increased, while the size of the precipitates decreased, thereby weakening the solid solution strengthening effect of Cu, and precipitation was enhanced. Additionally, as the reheating temperature increased, the tensile strength and yield strength of the air-cooled test steel after rolling increased, while the impact toughness decreased.
... The deformation-induced dislocation microstructures could act as nucleation sites for precipitation, which can further improve the mechanical properties [18]. The excellent mechanical properties, the yield strength of 485 MPa, the ultimate tensile strength of 540 MPa, and the ductility of 11%, were achieved for an Al-Cu alloy through CR and subsequence aging [19]. ...
The mechanical properties and microstructure evolution of an Al-Cu-Li alloy sheet processed via hot rolling (HR) (at 400 °C and 500 °C) or cryorolling (CR) (at −100 °C and −190 °C) and subsequence aging at 160 °C for 10 h were investigated. Before aging, the highest ultimate tensile strength of 502 MPa was achieved when the sheets were cryorolled at −190 °C, while the better ultimate tensile strength of 476 MPa and the best elongation rate of 11.1% was achieved simultaneously when the sheets were cryorolled at −100 °C. The refined grains and numerous uniform deformation-induced dislocations microstructures were responsible for the improved strength and enhanced ductility of the cryorolled sheets compared to that of the alloy processed by hot rolling with a low dislocation density zone (LDDZ) and high dislocation density zone (HDDZ). After aging at 160 °C for 10 h, the ultimate tensile strength further improved resulted from the greater precipitation strengthening, and the increased precipitates provided greater resistance to dislocations movement resulting in the increased ductility although the dislocation density decreased. The uniform dislocation microstructures in the cryorolled sheets provide numerous nucleation sites for the precipitates, leading to higher strength after aging.
... It is also observed that precipitate count of 10− 20 μm 2 decreased from AR to SA to ST condition. Presence of precipitates influence the mechanical properties of the steels as they hinder the dislocation movement [43,44]. From Fig. 6b and 6c, it can also be seen that, TB30 steel has a substantial higher number of precipitates count in comparison to TB1 steel. ...
Effect of boron on microstructure and its influence on elevated temperature properties of titanium stabilized AISI 321 steel was studied. A comparative study on mechanical properties of AISI 321 steels with and without boron addition is done. Both steels were worked to get appropriate reduction using forging-rolling process followed by solution annealing (SA) and stabilizing treatment (ST) at the appropriate temperature. Optical micrographs revealed uniform distribution of precipitates in steel. The precipitate count for Ti-stabilized AISI 321 steel with 30 ppm boron (TB30) was higher than Ti-stabilized AISI 321 steel (TB1) in which TiN and TiCN precipitates were observed. In addition to this, for TB30 steel containing boron precipitates of Fe2B and Cr2B were also observed. Precipitates refined the austenite matrix and provided yield strength stability to the AISI 321 steels. Boron addition in TB30 steel improved YS and UTS of AISI 321 steel with a marginal loss in ductility. SA and ST, TB30 steel exhibited best mechanical properties at room and elevated temperature (350 °C). Further, quantification of intergranular corrosion was done by sensitization of samples at 675⁰C for one hour and evaluated as per ASTM A262 specification practice A and practice E. Addition of nearly 30 PPM boron is highly beneficial in titanium stabilized AISI 321 steel that led to increased resistance to sensitization (both according to practice A and practice E) and improved the microstructural stability at elevated temperature resulting a reduction in intergranular corrosion and improved the elevated temperature mechanical properties. Thus, SA and ST, TB30 steels showed superior results as compared to TB1 steel. Study shows that TB30 steel is more suitable material to replace the cooling water inlet and outlet tubes of VVER type nuclear reactor.
... During rolling, the grain size of rolled steels is dependent on the austenite grain size [34]. Because TiN and TiC particles are stable even at high temperature, they remain in the particle state in the base steel during the hot forming process and effectively suppress grain growth through grain boundary pinning [33,[35][36][37]. Ti-bearing steels therefore have rather small prior austenite grain sizes, which result in the formation of relatively small pearlite grains in the cooling process after rolling. ...
We determined the effects of alloying elements and heat treatment temperature on the graphitization behavior of a medium-carbon high-silicon steel (Fe-0.55C-2.3Si, wt%). Trace amounts (<150 ppm) of B and Ti were individually added to base steel and graphitization heat treatment was applied to the three compositions (base steel, B-bearing steel, and Ti-bearing steel) at 700 and 750 °C. The grain size of the B-bearing steel was similar to that of the base steel, whereas that of the Ti-bearing steel was ∼37% lesser, which can be attributed to grain boundary pinning effects induced by TiN particles. For all three steels, cementite in pearlite decomposed during heat treatment and graphite formed mostly along the grain boundaries where carbon diffusion readily occurred. The graphitization rate follows the order of Ti-bearing steel > B-bearing steel > base steel, which is attributed to the numerous grain boundaries in the Ti-bearing steel and presence of BN particles in the B-bearing steel because the grain boundaries and BN particles act as major graphite nucleation sites. A higher abundance of nucleation sites for graphitization in the initial microstructure leads to higher graphite density but reduced average graphite size. For all three steels, higher heat treatment temperature promoted both cementite decomposition and graphite formation by accelerated carbon diffusion, which reduced the graphitization completion time, decreased the graphite number density, and increased the graphite size. Fine and uniformly distributed graphite forms within a short time (1 h) in the Ti-bearing steel treated at 750 °C, which significantly decreased its strength and improved its ductility.
... Therefore, controlling the precipitates and hindering the grain growth is very significant for improving product properties. Furthermore, precipitates and grain size are greatly affected by processing parameters during hot deformation [11,12]. Therefore, investigation of deformation temperatures, as well as the strain rate, is of vital importance. ...
The 0.1C-18Cr-1Al-1Si ferritic heat-resistant stainless steel has attracted considerable attention to high-temperature applications due to its favorable combination of creep and oxidation resistance. In this paper, the microstructural evolution and precipitation behavior of the 0.1C-18Cr-1Al-1Si ferritic heat-resistant stainless steel is studied from the compression deformation data in the temperature range of 850 °C–1050 °C and the strain rate range of 0.01–1 s ⁻¹ . Experimental results demonstrate that higher temperatures and lower strain rates enhance the dynamic recrystallization (DRX) process with remarkable effectiveness. The main precipitates are proved as the AlN phases and the (Cr,Fe) 23 C 6 carbides during hot deformation. With an increase in the deformation temperature, the size of (Cr,Fe) 23 C 6 and AlN gradually increases, and volume fraction gradually decreases. When the strain rate decreases, the average size and volume fraction of (Cr,Fe) 23 C 6 and AlN gradually increase. At the lower temperatures, the occurrence of dynamic recrystallization (DRX) is strongly influenced by (Cr,Fe) 23 C 6 formed on the grain boundaries, mainly because it causes a pinning effect, which hinders the movement of dislocations and delays the occurrence of the DRX.
... It shows that moderately increasing the deformation temperature has an obvious softening effect on the material, which can release the work hardening caused by large plastic deformation and deformation resistance. In actual production, the finishing rolling temperature of the steel is generally 900 °C [22,23], because lower temperature leads to higher deformation resistance, which necessitates a higher load requirement. Therefore, factors such as grain size and deformation resistance should be taken into account in the selection of the deformation temperature. ...
In this paper, we have studied the influence of deformation on the microstructure and mechanical properties of 20Mn2SiCrNi bainitic high strength steel processed through a hot rolling route. Simulation of different temperatures and degrees of deformation was carried out via Gleeble-1500. The study suggested that grain size is refined when the deformation is carried out at lower temperature (> Ac3). When the degree of deformation was increased from 20% to 60%, grain size and microstructure were both refined and the size of retained austenite was reduced. The tensile strength increased from 1345 MPa to 1432 MPa. The impact toughness increased from 115 J/cm² to 210 J/cm² at room temperature, from 63 J/cm² to 142 J/cm² at −40 °C. Furthermore, it was observed that the microstructure after air cooling was composed of granular bainite (GB), lath bainite (LB) and martensite/austenite (MA) island for different deformation conditions. The study reveals that the impact toughness of 20Mn2SiCrNi bainitic high strength steel can be increased by increasing the degree of deformation.
... However, improvisation of its properties is still going on, which can be achieved by altering the chemical composition or by tailoring the thermo-mechanical process. One important means of improving the mechanical properties is by grain refinement [19] which can be achieved during the thermo-mechanical processing of the material. The evolution of microstructure, at high temperature, is a complex process involving recovery, recrystallization and growth [20]. ...
Microstructure and texture development in an austenitic stainless steel at high temperatures and low strain rate were studied through laboratory scale compression and rolling experiments. The uniaxial compression tests were performed under constant temperature with sample temperatures of 900 °C, 1000 °C and 1100 °C whereas, the rolling experiments were performed at the same initial sample temperatures but in a transient condition. The average strain rate was 1 s⁻¹ and 1.23 s⁻¹, during the compression and rolling experiments, respectively. After deformation the microstructure and texture developed in the samples were examined through the electron back scattered diffraction technique. For compression test, deformation at 900 °C produces mostly deformed grains whereas, at 1000 °C and 1100 °C only recrystallized grains were observed. In contrast, rolling produces partially recrystallized microstructures at all the three temperatures. Finite element (FE) modeling was used to calculate the variation of the state variables like temperature, strain and strain rate and the differences in microstructure for the two processes were analyzed in the light of FE simulation results. In texture, tube orientation like α fiber was developed by the compression at 900 °C whereas, at 1000 °C and 1100 °C random texture with cube as the significant texture component, was observed. Similarly in rolling, α, β and τ fibers with different intensities were developed at the center and random texture was observed near the surface of the samples.
... The transformation temperatures and enthalpies after hot rolling and quenching are presented in Table 2. It is worth noting that, the transformation temperatures and the enthalpies decreases after hot rolling as compared to before hot rolling, attributes to increase in density of the precipitates [35] with change in matrix elemental composition. However, the hysteresis doesn't affect much before and after hot rolling. ...
In the present study, the influence of rare earth element gadolinium (Gd) on Cu-Al-Be polycrystalline shape memory alloy has been investigated. Cu88.13 Al11.42 Be0.45 ternary alloy with addition of Gd from 0.05 to 0.15 wt% has been used for investigation. The tests have been carried out for microstructure, morphology,ductility, phases, crystal structure, phase transformation temperatures and shape recovery ratio. Refinement of the grain size resulted as gadolinium increased from 0 to 0.08 wt%, the grain size decreases from 463.45 to 81.80 µm with reduction of 82.34%. The tensile strength has increased from 398.93 to 581.42 MPa with improvement in the ductility from 10.05% to 23.72% at 0.08 wt% gadolinium. The phase transformation temperatures increases as gadolinium increases and reduction in shape recovery ratio from 97% to 65%.
... The results, expressed as hole expansion ratio (HER %) are listed in Table 5.5. In general, the rolling in the austenite recrystallization region will bring about fine and recrystallized austenite grains [122] , while rolling in the non-recrystallized region will introduce pancaked grains, more grain boundary area per unit volume, a high density of dislocations and substructures in the austenite [123] . During the continuous cooling to the coiling temperatures, the deformed austenite grains will transform in to one or more different forms of ferrite grains, and the retained austenite region will be further enriched with carbon which is rejected from the ferrite phase and become stabilized [61] . ...
... Although the results of the present work on the deformation and fracture of the specific steel 17Mn1Si cannot be directly extended to all pipe steels used in oil and gas pipeline industry, the common property for all these steels is high fracture toughness (or the ability to resist ductile crack initiation and growth). The fracture toughness is determined by the amount of the work of plastic deformation in the fracture zone, which depends on the type of the microstructure formed during thermomechanical processing of the initial hot-rolled sheet [24][25][26]. Unlike other steels, the pipeline steels have a well-defined ratio of strength, ductility and fracture toughness, which is specified by the thermomechanical processing scheme [27][28][29]. The simulation results of this study were verified for 17Mn1Si pipe steel because of its widespread use. ...
The paper deals with a theoretical and experimental study of the relationship between the microstructural parameters, mechanical properties, and impact deformation and fracture of steels using the example of 17Mn1Si pipe steel. A model for the behavior of a polycrystalline grain conglomerate under impact loading at different temperatures was proposed within a cellular automata framework. It was shown that the intensity of dissipation processes explicitly depends on temperature and these processes play an important role in stress relaxation at the boundaries of structural elements. The Experimental study reveals the relationship between pendulum impact test temperature and the deformation/fracture energy of the steel. The impact toughness was shown to decrease almost linearly with the decreasing test temperature, which agrees with the fractographic analysis data confirming the increase in the fraction of brittle fracture in this case. It was shown with the aid of the proposed model and numerical simulations that the use of the excitable cellular automata method and an explicit account of test temperature through the possibility of energy release at internal interfaces help to explain the experimentally observed features of impact failure at different temperatures.
... The microstructure evolution can be explained as follows. The rolling in the austenite recrystallization region brings about a continuous recrystallization of austenite grains during the rolling process and this can remarkably refine the austenite grain size [28]. Subsequently, high densities of substructure and dislocation (deformation bands) are generated in the austenite when the rolling is conducted in the non-recrystallized austenite region [29]. ...
The relationship betweenmicrostructure and tensile properties of anNb–Timicroalloyed X90 pipeline steel was studied as a function of finish rolling temperature using a Gleeble 3500 simulator, an optical and scanning electron microscope, electron back scattered diffraction (EBSD), a transmission electron microscope (TEM) and X-ray diffraction. The results indicate that the microstructure is primarily composed of non-equiaxed ferrite with martensite/austenite (M/A) constituent dispersed at grain boundaries for the specimens with different finish rolling temperatures. With a decrease in the finish rolling temperature, the yield strength increases, following a significant increase in the grain refinement strengthening contribution and dislocation strengthening contribution, although the precipitation strengthening contribution decreases. The increasing yield ratio (YR) shows that the strain hardening capacity declines as a result of the microstructure evolution when decreasing the finish rolling temperature.
... The elemental mass fraction of the MC precipitates after the SPR test was determined by PCPA [25,26] using 80 9 25 9 10 mm 3 samples cut from the rolled plates. The precipitates were first obtained by microporous membrane filtration after the electrolysis at -10°C (current density: 0.04-0.06 ...
The present work aims to reveal the effect of Mo on the precipitation behavior of MC-type (M=Ti and Mo)
carbides in the austenite matrix of titanium micro-alloyed steel. The precipitation start-time–temperature curve was determined by a double-pass compression test on a Gleeble simulator, and the elemental mass fraction of MC-type carbides was measured by a physical–chemical phase analysis after a single-pass rolling test. The results shows that 0.2 wt% Mo accelerates the precipitation kinetics of MC-type carbides. During the initial stage of precipitation, Mo tends to distribute in the outer region of precipitates by replacing Ti despite of the high solubility of MoC in austenite. The replacement of Ti by Mo in TiC lattice leads to two opposite effects: First, it restrains MC precipitation due to the higherGibbs free energy of (Ti, Mo)C relative to TiC; Second, it promotes MC precipitation by decreasing the interfacial chemical energy of MC/austenite system. The second effect is more pronounced during the initial stage of precipitation when MC precipitates are relatively small and hence MC precipitation is accelerated by Mo addition. Compared to TiC, (Ti, Mo)C with stronger coarsening resistance suppresses austenite recovery and recrystallization more effectively, which favors maintaining the deformation microstructures at high temperatures.
... Several studies have been performed on the effect of austenitizing temperature on the structure and mechanical properties of steels containing Nb or Ti [11], [12], [13], [14], [15], especially on grain refinement [16], [17]. However, early efforts mainly focused on the effect of microalloying with Nb or Ti, grain growth behavior of coarse-grained austenite or strips manufactured through the compact strip production (CSP) process, which has a low deformation ratio. ...
The effect of austenitizing temperature on the microstructure and mechanical properties of Nb–Ti microalloyed steel was
investigated. Steels were subjected to different austenitizing treatments (temperatures ranging from 850 °C to 1250 °C for
5-120 min) and rolled after being austenitized at different temperatures (i.e. 1020 °C, 1070 °C and 1150 °C). The results
showed that austenite grain coarsening temperature was around 1000 °C. The mean grain size of the rolled steels initially
increased and then decreased, but the ferrite content decreased with increasing austenitizing temperature. The precipitates
in the prior austenite and rolled steel were both complex Nb–Ti carbonitrides. As the austenitizing temperature increased
from 1020 °C to 1150 °C, most precipitates were dissolved and re-precipitated as dispersive particles with mean size
decreasing from 30 nm to 10 nm. Meanwhile, the 80 nm to 100 nm rectangular Ti-rich carbonitrides were not dissolved
and varied during the subsequent cooling process.The Yield strength and ultimate tensile strength increased but the
elongation and reduction in area (in percent) decreased with the elevated austenitizing temperature.
Low- and high-Cu high-strength steels are designed and cast through strip and ingot casting. The results show that the Cu suppresses the formation of polygonal ferrite (PF) and intragranular acicular ferrite (IAF), but promotes the formation of granular bainite (GB) and bainite ferrite (BF). The microstructure is refined by the high cooling rate as indicated by the smaller width and length of acicular or lath ferrites for the strip casts. For the high-Cu strip cast, most Cu is retained in solid solution above equilibrium concentration due to the rapid secondary cooling rate. And the rest of Cu is found in the form of Cu-bearing precipitates, i.e., duplex sulfide-oxide particle, shell-like sulfide, and Cu-rich precipitate. However, only duplex sulfide-oxide particle can be found in the low-Cu strip cast. Most of Mn is captured between the dendrites because of the rapid solidification, leading to an evident Mn microsegregation in the interdendritic region of the strip casts. It is found that the number density of nano-scale precipitates in both two strip casts is much lower than in ingot casts, which can be attributed to the incomplete precipitation caused by the rapid secondary cooling rate during strip casting. For either the low- or high-Cu high-strength steels, a higher tensile strength of the strip casts than the ingot casts results from solution strengthening, dislocation strengthening, and fine-grain strengthening. It is noted that the increasing of Cu content from 0.01 to 0.52 wt pct reduces the strength and total elongation of the ingot or strip casts, because the predominant microstructure of IAF turns into BF.Graphical Abstract
First, strip cast samples of high strength microalloyed steel with sub-rapid solidification characteristics were prepared by simulated strip casting technique. Next, the isothermal growth of austenite grain during the reheating treatment of strip casts was observed in situ through confocal laser scanning microscope (CLSM). The results indicated that the time exponent of grains growth suddenly rise when the isothermal temperature higher than 1000°C. And the activation energy for austenite grain growth were calculated to be 538.0 kJ/mol in the high temperature region (above 1000°C) and 693.2 kJ/mol in the low temperature region (below 1000°C), respectively. Then, the kinetics model of austenite isothermal growth was established, which can predict the austenite grain size during isothermal hold very well. Besides, high density of second phase particles with small size was found during the isothermal hold at the low temperature region, leading to the refinement of austenite grain. After isothermal hold at different temperature for 1800 s, the bainite transformation in microalloyed steel strip was also observed in situ during the continuous cooling process. And growth rates of bainite plates with different nucleation positions and different prior austenite grain size (PAGS) were calculated. It was indicated that the growth rate of the bainite plate is not only related to the nucleation position but also to the PAGS.
Introducing a second phase into steel to improve the wear resistance has become an effective approach for the design of wear‐resistant materials in recent years. However, Efficient control of the precipitation behavior, morphology and size of the second phase requires further research. In this work, a micron‐submicron dual‐scale TiC precipitated bainitic steel is prepared by treating titanium‐alloyed bainitic steel with a solid solution and forging process (SSF). By comparing as‐cast sample (NSSFS) and SSF treated sample (SSFS) of titanium alloyed bainitic steel (HTBS) in terms of microstructure and TiC morphology, the regulation mechanism of the SSF process on the microstructure and the distribution of TiC is verified. The solid solution process places the matrix in the supersaturated state of Ti in the low‐temperature deformation zone. And the subsequent forging treatment eliminates the grain boundary segregation of eutectic TiC and induces the precipitation of submicron TiC. The wear behaviors and mechanical properties of traditional forged ball mill liner steel (BMS) and SSFS samples are compared, and the wear performance strengthening mechanism of dual‐scale TiC is studied. The strength and wear resistance of SSFS are significantly higher than those of BMS, while its elongation and impact toughness do not decrease significantly. This article is protected by copyright. All rights reserved.
Ultra-fast cooling has been increasingly applied in producing high-strength steels. In this study, the microstructure and mechanical properties of a martensitic steel quenched at an ultra-fast cooling rate were studied and compared with those of the steel quenched at a conventional water-cooling rate. The results showed that the tensile strength and yield strength of the ultra-fast cooled specimens were approximately 150 and 208 MPa higher than those of the conventional water-cooled specimens, respectively. The increases could be attributed mainly to dislocation strengthening according to the theoretically calculated results of dislocation strengthening on the basis of the measured values of dislocation density, which was increased from 1.26 10–16 to 1.60 × 10–16 m–2 when the cooling rate increased to ultra-fast region. Thus, the results of this study mean that the strength of martensitic steels could be improved through ultra-fast cooling.
Rebar is the most important material in large-scale engineering structures, and its fine structure determines the strength and seismic resistance of rebar. The objective of this study is to investigate the effect of Nb content on the microstructure and mechanical properties of high-strength anti-seismic rebar under the condition of thermal deformation. The Gleeble-3800 thermal simulator was used to simulate the continuous rolling process, which analyzed that effect of precipitation behavior of Nb on the microstructure and mechanical properties of rebar. Scanning electron microscope (SEM), transmission electron microscope (TEM) and universal tensile testing machine were used to characterize the microstructure, precipitation and mechanical properties of high-strength anti-seismic rebar. Under the condition of process I or process II, when the Nb content changed from 0.012 wt% to 0.020 wt%, the microstructure of the experimental steel was mainly composed of ferrite and pearlite, resulting in relatively low the tensile strength and good the plasticity, and the tensile fracture belonged to ductile fracture. Under the condition of process I or process II, when the Nb content changed from 0.035 wt% to 0.060 wt%, the microstructure of the experimental steel was mainly composed of ferrite, pearlite and bainite, resulting in relatively high the tensile strength, which reached 995.30±5 MPa and low the plasticity, and the tensile fracture belonged to shear fracture. Whether under the condition of process I or process II, when the Nb content changed from 0.012 wt% to 0.060 wt%, TEM confirmed that the main precipitates of the experimental steels were mainly (Ti, Nb) C, (Ti, Nb, V) C, (Nb, Ti, V) C, respectively. These carbides were mainly distributed on the ferrite grains or grain boundaries, and the shapes were elliptical, square, strip, respectively. The average particle size of these carbides was less than 50 nm, and the crystal structure of these carbides belonged to the cubic structure.
Ferrium® M54® is a recently developed high-performance steel with medium C and high Ni–Co contents. A processing route involving ausforming (rolling) and tempering was investigated. An ultra-fine martensitic block structure with high dislocation density was obtained, and the fully processed specimen showed an outstanding strength-elongation balance with a 1726 MPa yield strength, 2222 MPa ultimate tensile strength, and a 15.5% total elongation. Comparing the ausformed specimen and the direct-quenched (non-ausformed) baseline specimen, higher degrees of block refinement and dislocation number density in the former both contributed to the high strength of the as-ausformed condition. After tempering, the dislocation density of the ausformed specimen decreased to levels close to that of the direct-quenched specimen, but microstructural refinement persisted. Meanwhile, M2C precipitation hardening compensated for the loss of strength from dislocation recovery. Also, the acceleration of precipitation due to ausforming enabled a lower tempering temperature or shorter soaking time. Ausforming has been demonstrated as an effective way to highly refine the martensitic microstructure and further enhance the performance of secondary hardenable steel products.
In this work, the effect of Ti content on the microstructure and mechanical properties of low-density Fe-30Mn-10Al-1.57C-2.3Cr-0.3Si-χTi (χ = 0, 0.3, 0.6, and 0.9 wt%) alloys was systematically investigated. The results reveal that hot deformation, followed by solid-solution treatment at 1050 °C (3 h) and thermal aging at 350 °C (6 h), ensures outstanding mechanical properties. The low-density alloys exhibit completely austenite microstructures with carbides. The original carbide phase, the so-called κ-carbide, is observed in the low-density Ti-free specimen. However, increase in the Ti content leads to the reduction of this phase and the emergence of a new type of carbide, i.e., TiC. Moreover, the grain size of austenite phase is refined with Ti addition, resulting in a superior yield strength. In addition, the low-density alloy with a Ti concentration of 0.6 wt% demonstrates optimal mechanical properties with the yield strength of 1031.75 MPa, the ultimate tensile strength of 1158.55 MPa, and the total elongation of 23.96%. It is worth noting that the novel low-density alloy exhibits a density of 6.65 g/cm³, which is ~14.74% lower than that of traditional steel (7.8 g/cm³) and can be ascribed to the incorporation of lightweight Al element.
The low-carbon bainitic–martensitic steel added with microalloying elements was designed, and samples with different cooling rates were produced through ingot casting and simulated strip casting. The ingot cast presented a multiphase microstructure constituted of allotriomorphic ferrite, polygonal ferrite, acicular ferrite and pearlite. The air-cooled strip consisted of polygonal ferrite, acicular ferrite and bainite ferrite, and the gas-cooled strip was composed of acicular ferrite, bainite ferrite and martensite. The results indicated that the duplex sulfide-oxide particles were firstly formed during solidification and refined by the high solidification rate during strip casting; the embedded structure of copper sulfide in duplex particles suggested that the copper sulfide precipitated from the liquid oxide. Independent copper sulfides with an average diameter of ~ 80 nm were formed in the ingot cast and a large number of nanoscale carbides or nitrides less than 25 nm was also observed. However, independent copper sulfides and nanoscale precipitates were hardly observed in the gas-cooled strip cast. For the air-cooled strip cast, only a small number of the nanoscale precipitates were observed. Vickers hardness, yield strength (YS) and ultimate tensile strength (UTS) increased with the fraction of bainite ferrite and martensite, in which the gas-cooled strip showed the highest YS and UTS, and the ingot cast presented the highest total elongation (TE). Nevertheless, the gas-cooled strip showed the best toughness due to the highest product of UTS and TE.
The microstructural evolution, mechanical and electrochemical properties of the rotary friction welding joint of 1MnCrMoNi maraging steel were investigated after post-weld heat treatment (PWHT). The joint could be divided into two zones: weld zone and thermo-mechanically affected zone. After PWHT, the parallel alignment of the laths turned ‘blurred’; sub-grain and cell structure appeared; M23C6 and ε -Cu precipitated, respectively; the joint was softened. But the microhardness was still higher than that of base materials and distributed unevenly. The closer it was to the weld centre, the higher the microhardness was. No deformation occurred in the joints during tensile test, indicating the excellent bonding strength. PWHT process slightly reduced the impact toughness and corrosion resistance of the joint.
The microstructural evolution and mechanical properties in a low carbon Nb‐Ti‐Mo bearing steel subjected to hot‐rolling followed by four different processing schedules (air‐cooling, direct‐quenching, low temperature tempering, and high temperature tempering) were studied to elucidate the significance of structural refinement and nanoscale precipitates for obtaining the optimized combination of strength and toughness. The microstructure of air‐cooled steel composed of coarse polygonal ferrite and granular bainite, and minimum yield strength of 473 MPa and lowest impact energy at ‐40 °C of 40 J were obtained. The microstructure of steel direct‐quenched and tempered at 520 °C consisted of sub‐micron tempered martensitic and bainitic laths with high relative fraction of high misorientation boundaries and high density of 3‐5 nm (Nb, Ti, Mo)C precipitates, and maximum yield strength of 726 MPa and highest impact energy at ‐40 °C of 213 J were obtained. The formation of nanoscale precipitates coupled with fine tempered martensitic and bainitic blocks contributed to 253 MPa improvement of yield strength and more than 5 times enhancement of impact energy. The small blocks of tempered martensite and bainite were capable of deviating and finally arresting the crack propagation. Whereas, the coarse polygonal ferrite and bainite provided inadequate resistance for crack propagation.
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An ultrahigh-strength steel with sufficient uniform elongation (UE) and high strain-hardening ability was fabricated by a combination method of heavy warm rolling (HWR) on the metastable austenite and subsequent quenching (Q). The WR-Qed steel shows an ultra-fine martensitic structure with an average effective grain size (EGS) of 0.57 μm, yield strength (YS) of 1911 MPa and ultimate tensile strength (UTS) of 2411 MPa. The tempering (T) effect on microstructure and properties of the steel was investigated. Compared with the heavily hot-rolled and tempered steel, the WR-QTed steels exhibited substantial enhancement in strength, ductility and strain-hardening ability. After tempering at 200 ℃ for 1 h, the WRed steel demonstrated an excellent strength-ductility combination with a YS of 1905 MPa, UTS of 2163 MPa and total elongation of 11.7%. Grain refinement and dislocation strengthening play major roles in enhancing strength. It is believed that the heavy warm deformation can effectively refine the metastable austenite grains and consequently result in the ultrafine-structured steel by martensitic transformation during the subsequent quenching. The excellent strength-ductility combinations of the WR-Qed steel are also closely associated with effect of ultrafine lamellar structures of martensite, nano-twins and high density low-angle grain boundaries.
An austenitic Hadfield steel sheet shows a relatively low yield strength of 0.4–0.5 GPa and serrated flows in spite of excellent tensile strength and ductility along with highly-sustained strain hardening. In order to overcome the shortcomings, a multi-layer steel (MLS) sheet was fabricated by a roll-bonding with an ultra-high-strength martensitic hot-press-forming (HPF) steel sheet. Near the Hadfield/HPF interface, the carburized and decarburized layers were formed by the carbon diffusion from the Hadfield (1.2% of C) to HPF (0.23% of C) layers, and could generate a kind of very thin multi-layers of 35 μm in thickness. All tensile properties of the Hadfield/HPF MLS sheet (yield strength; 946 MPa, tensile strength; 1291 MPa, elongation; 44.5%) were superior to those of the Hadfield sheet. Interestingly, the persistent elongation up to 44.5%, which is higher than that of the Hadfield steel, in the present MLS sheet is a quite unique and interesting characteristic. The simultaneous enhancement of strength and ductility of the MLS sheet was explained by the contributions of 1) populated twin formation, 2) generation of geometrically necessary dislocations (GNDs), and 3) increase of back stress inside thin interfacial layers.
The undissolved phases and carbide precipitation in Ti and Ti–Zr microalloyed low-carbon steels were investigated by scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectrometry. At 1225 °C, the replacement of Ti by Zr formed Zr2CS and (Zr, Ti)N (the Ti/Zr atomic ratio is 0.11) and reduced the consumption of Ti. At 925 °C, it was identified that TiC phases were precipitated at first and Zr was incorporated into the TiC lattice in the subsequent precipitation process, which promoted the precipitation of titanium carbide. The calculation of the interaction coefficient between Ti, C, N and Zr showed that Zr reduced the activity of Ti and C and increased the activity of N in the iron matrix. Therefore, with the addition of Zr, the solubility of Ti was increased, and the consumption of Ti was reduced at high temperature in Ti microalloyed low-carbon steel. The thermodynamic calculation of carbide precipitation transformation showed that the replacement of Ti by Zr increased the nucleation driving force and the nucleation rate of titanium carbide, while the critical core size and the critical nuclear energy were reduced. As the holding time was extended, the Zr/Ti atomic ratio increased and the size of the precipitates also increased. When the Zr/Ti atomic ratio reached a certain level, the size of the precipitates did not increase with further increase in atomic ratio. When the Zr/Ti atomic ratio in (Ti, Zr)C was 0.05–0.17, (Ti, Zr)C was the most stable carbide and the easiest to nucleate at 925 °C. There was more of the (Ti, Zr)C phase than TiC at 925 °C after 50 and 100 s, and the time to complete the coarsening behavior of (Ti, Zr)C was shorter than that of TiC.
The precipitation behavior of nanometer-sized carbides in ferrite in Nb–V-bearing low-carbon steel was studied by electron microscopy and nanoindentation hardness measurements. The results indicated that interphase precipitation and random precipitation could occur simultaneously for the specimen isothermally treated at 700 °C for 60 min, while in other specimens, only random precipitation was observed. This phenomenon might be explained by mass balance criterion during the diffusional phase transformation. Nanohardness result indicated that the average hardness of the specimens isothermally held at 600 °C for 20 min was 3.87 GPa. For the specimen isothermally holding at 650 °C for 20 min, the average hardness was 4.10 GPa and the distribution of the nanohardness was in a narrower range compared with that of the specimen isothermal holding at 600 °C for 20 min. These implied that the carbides in the specimens isothermal treated at 650 °C were more uniformly dispersed, and the number density of the carbides was greater than that treated at 600 °C. Using Ashby–Orowan model, the contribution of precipitation strengthening to yield strength was estimated to be ~110 MPa for the specimen isothermally treated at the temperature of 650 °C for 20 min.
The behavior of austenite grain growth and the solution of Ti were researched by heated in an electric furnace at the temperature of 850 °C to 1250 °C for 30 min. The precipitates of Ti during the heating process were examined by transmission electron microscopy (TEM). The results indicate that the tested steel has two grain coarsening temperatures of austenite grains with the increase of heating temperature, 1050 °C and 1250 °C respectively, which corresponds to the solution temperature of two precipitates of Ti, but lower than the value of the solution temperature. It is found that the activation energy of grain growth reduced from 26,561 J/mol to 239.8 KJ/mol with the dissolving of TiC precipitates by analyzing two stages of austenite grain growth process.
Effects of galvanizing simulation parameters on microstructures and mechanical properties of Ti-microalloyed cold rolled hot-dip galvanizing DP980 steel were investigated in this study by optical microscopy (OM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and tensile test. Moreover, the precipitation behavior of Ti in the experimental steel was also studied. The results show that, as the heating temperature increases, the tensile strength of experimental galvanizing DP980 steel decreases while the yield ratio and elongation of the steel are enhanced. The microstructures of experimental steels exhibit typical dual phase steel character and the volume fractions of MA islands are almost 30%. In addition, lots of nano-sized TiC precipitates can be found in the ferrite grains.
The behavior of austenite grain growth and the solution of Ti were researched by heated in an electric furnace at the temperature of 850°C to 1250°C for 30 min. The precipitates of Ti during the heating process were examined by transmission electron microscopy (TEM). The results indicate that the tested steel has two grain coarsening temperatures of austenite grains with the increase of heating temperature, 1050°C and 1250°C respectively, which corresponds to the solution temperature of two precipitates of Ti, but lower than the value of the solution temperature. It is found that the activation energy of grain growth reduced from 26,561 J/mol to 239.8 KJ/mol with the dissolving of TiC precipitates by analyzing two stages of austenite grain growth process. © 2016 by The Minerals, Metals & Materials Society. All rights reserved.
Effects of galvanizing simulation parameters on microstructures and mechanical properties of Ti- microalloyed cold rolled hot-dip galvanizing DP980 steel were investigated in this study by optical microscopy (OM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and tensile test. Moreover, the precipitation behavior of Ti in the experimental steel was also studied. The results show that, as the heating temperature increases, the tensile strength of experimental galvanizing DP980 steel decreases while the yield ratio and elongation of the steel are enhanced. The microstructures of experimental steels exhibit typical dual phase steel character and the volume fractions of MA islands are almost 30%. In addition, lots of nano-sized TiC precipitates can be found inthe ferrite grains. © 2016 by The Minerals, Metals & Materials Society. All rights reserved.
Increasing demands with regard to performance of special steels necessitate the further development of process technology as well as steel metallurgy. The present paper exemplifies recent developments based on alloy modifications with microalloying elements enhancing the performance of special steel types. The focus is thereby on molybdenum alloyed carburizing grades that are typically applied e.g. in the powertrain of vehicles, machinery and power generation equipment. A dedicated addition of niobium enables better mechanical properties and higher carburizing temperatures. Simultaneously the carburizing time can be shortened and distortion by heat treatment is reduced, as the microstructure remains more homogeneous. Further development potential offered by microalloying in carburizing steels is seen in an improved toughness due to the finer sized grain structure allowing to produce higher strength and more economical steel. These measures when applied in new steel concepts enable significant cost savings along the entire manufacturing chain especially in the production of larger transmissions. Finally, additional cost savings can be expected by a reduction and optimization ofprocesses andprocedures at theend-user. © 2016 by The Minerals, Metals & Materials Society. All rights reserved.
The goal of this study is to empirically investigate the major hot rolling process parameters affecting the yield strength, ultimate tensile strength and strain-hardening exponent of Nb-microalloyed steel sheets. The parameters considered were the roughing, finishing and coiling temperatures. Three levels for each parameter were used to develop a model relating the process parameters to mechanical properties. By applying the response surface methodology, analysis of variance was done to determine the mathematical models related to each response. The results indicated that decreasing the coiling and finishing temperatures significantly influenced the mechanical properties. Also, the models predicted that the maximum yield strength, ultimate tensile strength and strain-hardening exponent are simultaneously obtained under the following conditions: roughing temperature = 1097 °C, finishing temperature = 837 °C and coiling temperature = 580 °C. The predicted values were close to the experimental values, indicating the suitability of the models.
The influence of rolling reduction on the microstructural variations in Ti-containing tool steel was investigated. The microstructures, including the prior-austenite grain structure and primary/secondary carbides, were characterized using optical and electron microscopy; the carbides were quantitatively analyzed by small-angle neutron scattering data collected from macro-samples. In parallel with refining the primary carbides, increases in the degree of rolling reduction significantly changes the precipitation kinetics of the secondary MC and M23C6 (M = metallic element) carbides, and thus their mechanical properties, especially the hardness to toughness balance throughout the aging. The improvement of this balance is attributed to the refinement of the prior-austenite grain structure, as well as the retardation of M23C6 precipitation. The reaction kinetics of the precipitation reactions were analyzed by the reduced heat of reaction observed in a calorimetry experiment.
This paper presents the various ways of forging large forged pieces such as shafts, plates, and discs. A finite element method was used to simulate open die forging processes such as forging by flat dies with a slant, forging by staged dies, and upsetting of profiled workpiece. The parameters of the stress strain state of the billet in these processes were determined, along with the rational parameters for forging plates by flat dies with slant such as tool size and mechanical modes. The optimal die step dimension and mechanical regime for forging shafts by staged tools was determined. Also, the rational geometric parameters of the workpiece for forging discs by upsetting were determined. For these forging schemes, a simulation was done of the microstructure evolution in forgings obtained by traditional technology and new proposed technology. The paper shows the advantages of new forging schemes which provide a uniform distribution of grain size in the obtained billets.
Rolling temperature was found to play a decisive role in the precipitation hardening of Ti–Mo microalloyed hot-rolled high strength steel, despite identical chemical composition and phase transformation condition. The different microstructural features of prior austenite grain structure and dislocation structures inside austenite and/or ferrite grain, and the change of austenite/ferrite phase transformation characteristic, depending on rolling temperature, significantly influenced the precipitation characteristics of interphase-precipitated nanometer-sized carbides and consequently led to a totally different precipitation hardening effect.
In this study, a 23 factorial design analysis was conducted to screen the significant factors influencing the mechanical properties and formability of Nb-microalloyed steel sheets in hot-rolling process and for optimizing these factors to predict maximum yield strength (YS), ultimate tensile strength (UTS), elongation (El), and dome height (DH), simultaneously. For this purpose, two levels for the major hot-rolling process parameters of roughing temperature (RT), finishing temperature (FT), and coiling temperature (CT) were chosen; eight experiments for each response were conducted. From the analysis of variance, the most important parameters affecting the YS, UTS, El, and DH were determined, and satisfactory prediction regression models were derived. It was observed that optimal condition of factors for the best combination of response variables was obtained at 1150 °C of RT, 836 °C of FT, and 604 °C of CT. Findings of this research show that the obtained predicted values were close to the experimental values indicating suitability of the models.
The UOE process is widely recognized as an effective approach to
manufacture large-diameter submerged-arc welding pipes. During the UOE
process, the plate is crimped along its edge, pressed into U and O
shape, welded closed and then expanded to obtain a circular pipe. In
this study, the mechanical properties tests are carried out on the API
X80 pipeline steel to establish the constitutive model. A
two-dimensional finite element model of the UOE forming process is
established under plane strain assumption. The deformed geometry of
different forming steps and the distribution of equivalent plastic
strain are obtained. The model is validated in practical manufacturing
by comparing the forming quality, which shows a significant coherence in
their geometric configurations. In particular, the effect of process
parameters on the forming quality is analyzed. Those parameters are the
yield strength of the plate, the location of C-die, the initial location
of U-rollers, the displacement of U-rollers, the location of V-anvil,
the final spacing between O-punch and O-die. The main forming quality
includes the opening after O-forming, the ovality after pre-welding and
expansion. The relation curves of parameters versus forming quality are
obtained and illustrated.
We describe here the precipitation behavior and mechanical properties of 560MPa Ti–Nb and 770MPa Ti–Nb–Mo–V steels. The precipitation characteristics were analyzed in terms of chemistry and size distribution of precipitates, with particular focus on the crystallography of precipitates through an analysis of electron diffraction patterns. In addition to pure carbides (NbC, TiC, Mo2C, and VC), Nb containing titanium-rich carbides were also observed. These precipitates were of a size range of 4–20nm. The mechanism of formation of these Ti-rich niobium containing carbides is postulated to involve epitaxial nucleation of NbC on previously precipitated TiC. Interface precipitation of NbC was an interesting observation in compact strip processing which is characterized by an orientation relationship of [001]NbC//[001]α-Fe, implying that the precipitation of NbC occurred during austenite–ferrite transformation.
Yield strength enhancement for martensitic steel fabricated by vacuum induction melting is investigated. It is found that
the addition of Ti can improve the yield strength property of the martensitic steel, which can be attributed to increase in
precipitation hardening from formation of TiC precipitates in the martensitic matrix. Moreover, the yield strength can be
further enhanced by tempering and reheat quenching process, which can be ascribed to the formation of a superfine sized (~8 μm)
grains and large amount of freshly nano-sized (1–10 nm) precipitates in the final martensitic structure for martensitic steel
containing Ti. The experimental and theoretical results on the contribution of TiC precipitates to hardening of the martensitic
steel are in excellent agreement, showing that the precipitation hardening of 188 MPa caused by TiC precipitates is the main
reason why the yield strength for martensitic steel is enhanced via titanium addition.
The correlation of microstructure and Charpy V-notch (CVN) impact properties of a high-toughness API X70 pipeline steel was
investigated in this study. Six kinds of steel were fabricated by varying the hot-rolling conditions, and their microstructures,
effective grain sizes, and CVN impact properties were analyzed. The CVN impact test results indicated that the steels rolled
in the single-phase region had higher upper-shelf energies (USEs) and lower energy-transition temperatures (ETTs) than the
steels rolled in the two-phase region because their microstructures were composed of acicular ferrite (AF) and fine polygonal
ferrite (PF). The decreased ETT in the steels rolled in the single-phase region could be explained by the decrease in the
overall effective grain size due to the presence of AF having a smaller effective grain size. On the other hand, the absorbed
energy of the steels rolled in the two-phase region was considerably lower because a large amount of dislocations were generated
inside PFs during rolling. It was further decreased when coarse martensite or cementite was formed during the cooling process.
DMR-249A is a low carbon microalloyed high-strength low-alloy (HSLA) steel. While DMR-249A plates of thickness less than 18 mm meet the specified room temperature yield strength (390 MPa) and Charpy impact toughness (78J at −60 °C) in the as-rolled condition, thicker plates require water quenching and tempering. Elimination of the quenching and tempering treatment can result in significant cost and energy savings besides offering increased productivity. Therefore, in the present work, modifications to the base DMR-249A steel composition have been investigated with the objective of producing thicker gage plates (24 mm) capable of meeting the specified properties in the normalized condition. Plates from three modified compositions i.e., containing 0.015 wt.% titanium and 0.06, 0.09 and 0.12 wt.% vanadium respectively and one composition with 0.10 wt.% vanadium, and without any titanium were investigated over a range of normalizing temperatures (875–1000 °C). In all cases, only the steel without titanium met the specified properties in the normalized condition. Microstructural investigations using scanning and transmission electron microscopy, as well as support evidence from calculations performed using ThermoCalc software, suggest that this is due to the presence of nanoscale vanadium rich carbonitride particles distributed throughout the matrix. These particles were absent in the titanium-containing steel at a similar vanadium level.
It is well known that Nb and Ti are used to prevent sensitization in stainless steel. However, the amounts of Nb and Ti of standard are not enough to prevent completely this phenomenon. Therefore in this paper, several austenitic stainless steel heats were alloyed with different contents of Nb and Ti then cast in sand mould using an open air-induction melting furnace. The Nb content which used in this paper (more than standard values) is ranging from 0.004 to 0.410 wt% and Ti content is ranging from 0.007 to 1.10 wt%. The cast ingots were homogenized at 1200 °C and water quenched. The materials were austenitized at 1200 °C and then hot forged up to 85% in cross-sectional area. Solution treatment was carried out at temperatures above 1150 °C to ensure dissolution of carbides and rapid quenching to a temperature lower than the precipitation range of 875–425 °C was done. The cold rolling, welding and sensitization tests, EDS-SEM investigations, XRD pattern and corrosion resistance were carried out to study the effect of alloying elements (Nb and Ti) on sensitization behavior and mechanical properties.
The possibility of a progressive austenite grain refinement due to dynamic recrystallization during low temperature finish rolling has been studied on a microalloyed Mn-steel. By using hot working simulation tests the range of deformation conditions was determined in which an austenite mean grain size of 1–4 μm can be achieved. The subsequent formation of a fine ferrite structure leads to excellent mechanical properties superior to those of heavily deformed but not recrystallized austenite. An einem mikrolegierten Mn-Stahl wurde die Möglichkeit einer wirkungsvollen Austenitkornfeinung durch dynamische Rekristallisation während eines Endwalzens bei niedrigen Temperaturen untersucht. Mit der Technik der Warmumformsimulation wurde ein Bereich der Umformbedingungen ermittelt, in dem ein extrem feines Austenitkorn von 1–4 μm erreichbar ist. Ein daraus resultierendes feines Ferritgefüge führt zu exzellenten mechanischen Eigenschaften, die nach der Umwandlung aus einem stark verformten, aber nicht rekristallisierten Austenit nicht erzielt wurden.
High strength low alloy (HSLA) steels have demonstrated superior mechanical properties through controlled rolling (CR). In the present investigation, the effects of processing parameters, such as finish rolling temperature, rolling reduction, inter-pass time and cooling rate, on the final microstructure and mechanical properties of a grade X52 type HSLA steel has been studied by tensile and charpy impact tests and optical microscopy and scanning electron microscopy. To yield better mechanical properties of X52 microalloyed steel grade, the optimization of the rolling process in the laboratory experiment and rolling mill has been carried out. It has been found that the tensile and impact properties of X52 steel are significantly improved by controlled rolling in the (alpha+gamma) two-phase region. The cause of such improvement in mechanical properties of the control-rolled steel is correlated with changes in microstructure, i.e. ferrite grain refinement, a large number of subgrains and high dislocation density. Furthermore, the low carbon contents exhibited by these steels cause evident improvement in their toughness and weldability. Finally, a controlled rolling process in the (alpha+gamma) two-phase region of X52 steel plates has been suggested.
The grain refinement and mechanical properties improvement resulted from Ti addition and reheating quenching were demonstrated in this study. The direct quenched medium manganese steel with low carbon content (0.05C) was treated by reheating quenching process. The yield strength and Charpy impact energy were measured. The microstructures and the second precipitated particles were examined by optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM), X-rays diffraction and phase analysis method. It was found that reheating quenching at 900–1000 °C resulted in significant grain refinement, especially the refinement of effective grain size (EGS), which was attributed to the large amount nano-sized precipitation of TiC. In addition, high elastic modulus was also obtained from the large amount TiC precipitated from the matrix. It is concluded that reheating quenching process is a useful method to refine the grain size and improve the combined mechanical properties of the martensitic steel through Ti addition.
Three series of titanium-containing steels with different nitrogen contents were used to evaluate the effect of a small titanium addition on properties. With the addition of titanium, an obvious ferrite grain refinement was observed in the as-rolled condition. This refinement is attributed to the TiN grain-boundary-pinning effect. The TiN grain-boundary-pinning effect is most effective when the titanium content is around that in the titanium-to-nitrogen stoichiometric ratio, i.e. 3.42. Also the grain refinement effect from TiC might be obtained when the titanium content is higher than that in the titanium-to-nitrogen stoichiometric ratio. In contrast with the observed grain refinement, the yield strength of steels with a titanium content around that in the titanium-to-nitrogen stoichiometric ratio suffered a considerable loss when compared with plain carbon steels. This loss is due to the depletion of free nitrogen for solid solution hardening. However, the toughness was improved through grain refinement and depletion of free nitrogen. When the titanium content was over the stoichiometric ratio, a precipitation-hardening effect resulting from TiC was observed which resulted in the increase in strength and deterioration of toughness.
The microstructure and nanometric precipitates present in advanced structured steel have been studied by high resolution transmission electron microscopy equipped with energy dispersion X-ray microanalysis, in order to relate the nanometric precipitates and grain size with the improvement of the yield strength value of the API steel. The microstructure and nanometric precipitates of the advanced steel were obtained by a combination of thermo-mechanical controlled hot rolling and accelerated cooling procedures. The API steel composition consisted of hot rolled Nb–Ti microalloyed with: 0.07C, 1.40Mn, 0.24Si, 0.020Al, 0.009P, 0.001S, 0.05Mo, 0.5Cr, 0.05Nb, 0.25Ni, 0.10Cu, 0.012Ti, 0.05N in wt%. As a result, this hot rolled steel tested at a strain rate of 5×10−3s−1 showed an improved yield strength from 798MPa to 878MPa due to the micrometric grain size of 2.2μm and to the nanometric precipitates with a size of around 5nm in the microstructure of the steel studied.
The precipitation behavior of titanium treated by different processes and its effect on microstructure and mechanical properties has been investigated in low carbon medium manganese steel. It is found that the formed precipitates during both tempering and reheating-quenching processes are TiCs. The size of them mainly ranges from 1 to 18nm and 1 to 36nm, respectively. And tempering treatment especially promotes a peak of TiC in diameter from 1 to 5nm, but it also boosts parts of precipitates nucleating along the grain boundaries. The precipitation of TiC is useful for the refinement of austenite grains and this effect depends on the reheating temperature. The mean grain size can be refined to 7.8μm when the reheating temperature is 900°C. Furthermore, it is found that the mechanical properties are determined by the reheating-quenching process. However, the tempering has nearly no influence on it when the sample is treated by the same reheating temperature. It is concluded that dislocation strengthening mechanism induced by TiC precipitation is the dominant factor for the changes of both strength and toughness.
By means of torsion tests using small specimens, the influence of austenite grain size on strain induced precipitation kinetics has been determined in a vanadium microalloyed steel. Determination of recrystallisation–precipitation–time–temperature (RPTT) diagrams for two austenite grain sizes allows values of the aforementioned magnitudes to be determined. An ample discussion is made of the quantitative influence found and its relation with nucleation and growth mechanisms of precipitates. The results are compared with the quantitative influence exerted by the other variables, reaching the conclusion that the austenite grain size has a notable influence on strain induced precipitation kinetics which should not be underestimated. Finally, the influence of austenite grain size is included in a strain induced precipitation model constructed by the authors of this work and which also takes into account the other aforementioned variables.
In this work the pinning forces exerted by TiN particles in the austenitic phase in four Ti-microalloyed steels have been determined and compared with the driving forces for recrystallisation determined in each rolling pass. The thermomechanical simulation has been carried out in the laboratory by means of torsion tests in a sequence of 20 passes with the final passes in a mixed austenitic/ferritic phase. The driving forces were found to be approximately two orders of magnitude higher than the pinning forces, which explains why the austenite in these steels barely experiences hardening during rolling and why the accumulated stress prior to the austenite→ferrite transformation is insufficient to refine the ferritic grain.
Microalloy steels are generally Nb, V, or Nb, V, Ti composite microalloyed. Because of the high price of niobium and vanadium, the development of titanium microalloyed steels is a very interesting subject. In this study, steels with different Ti contents were refined and forged. Tensile tests were conducted and microstructures of samples were analyzed. Fine precipitates were observed using a transmission electron microscope. The results show that nanoscale Ti precipitates is the main factor enhancing strength of steels. The strength of steels increases with the Ti content. The optimum content range of titanium is between 0.04 and 0.10wt.% while below 0.04wt.% and higher than 0.10wt.%, Ti has little effect on the strength of steels.
Transmission electron microscopy was used to study the precipitation in four different dual-phase steels microalloyed with titanium. TiN, Ti4C2S2 and TiC were characterized. Detailed statistical analysis of hundreds of images of the various Ti compounds fit rather well with thermodynamical calculations and they show TiC particles (approximately 10nm) to be the most dominant factor in strengthening.
Distribution and densities of dislocations, determined by electron transmission microscopy, flow stress and stored energy measurements (by microcalorimetry) on cold-worked polycrystalline silver are correlated with each other, The dislocations are arranged in dense networks forming the boundaries of an otherwise relatively dislocation-free cell structure. The flow stress is explained quantitatively in terms of the forest intersection mechanism at the boundaries. The stored energy after recovery is of the same order as the total self-energy of the dislocation arrangement, so that the long-range stresses must be largely relaxed. The considerable energy release during the recovery stage produces no observable change in dislocation distribution. This recovery stage is thought to be due to the relief of long-range stresses or to the removal of point defects. While the values of flow stress and stored energy can be accounted for in terms of the observed non-uniform distribution of dislocations (cell st...
The strain-induced precipitation kinetics of TiC in a 0.05% C–0.10% Ti HSLA steel was investigated by two-stage interrupted compression method. The precipitation–time–temperature (PTT) diagram for TiC precipitation was obtained by analyzing the softening kinetics curves of deformed austenite, which was confirmed to be of validity by employing transmission electron microscopy (TEM). Experimental results showed that the PTT diagram for TiC precipitation exhibited a typical “C” shaped and the nucleation of strain-induced TiC precipitation was a very rapid process in the temperature range 900–925°C. The relatively severe deformation applied on the steel was considered to be the main factor resulting in the fast kinetics of TiC precipitation. The TiC precipitates were heterogeneously distributed in either a chain-like or a cell like manner, implying that the precipitates nucleated on dislocations or on dislocation sub-structures, which were produced by deformation. The growth of TiC precipitates approximately followed a parabolic law. In addition, the coarsening of strain-induced TiC precipitates had already started before the completion of precipitation.
Based on analysis of austenite deformation behaviour during thermomechanical processing of Nb–Ti microalloyed steel, the rolling schedules were designed to produce (i) recrystallized austenite, (ii) unrecrystallized austenite, and (iii) ferrite-pearlite. The effects of austenite conditioning on the final ferrite-pearlite microstructure and mechanical properties of steel were investigated. To rationalise the variation in final ferrite grain size with different thermomechanical processing schedules, it is necessary to consider the ferrite grain growth in addition, to the density of ferrite nucleation sites. Mechanical properties were the means to evaluate the variation in austenite solutioning and deformation conditions introduced into individual applied rolling schedules. The benefit of tensile tests, especially yield strength and ductility values, in determining the optimum deformation schedule and coiling condition for given steel is demonstrated.
A thermomechanical control process consisting of slab reheating, controlled rolling, and accelerated cooling has been adopted at the plate mill, Bhilai Steel Plant, India for achieving high strength and toughness in C–Mn and microalloyed steels while keeping the mechanical properties and the flatness constant. A mathematical model has been developed to predict the temperature distribution in the plate during accelerated cooling, taking into account the heat generation of the phase transformation. Effects of chemistry and mill parameters on ferrite grain refinement are explained in terms of nucleation and growth rate.
Ultrafine grained steels with grain sizes below about 1 m offer the prospect of high strength and high toughness with traditional steel com-positions. These materials are currently the subject of extensive research efforts worldwide. Ultrafine grained steels can be produced either by advanced thermomechanical processes or by severe plastic deformation strategies. Both approaches are suited to produce submicron grain structures with attractive mechanical properties. This overview describes the various techniques to fabricate ultrafine grained bcc steels, the corresponding microstructures, and the resulting spectrum of mechanical properties.
Stress relaxation measurements were carried out on a plain carbon and four solution-treated Ti steels over the temperature
range 850 to 1050 °C. The results show that the stress relaxation of plain carbon austenite after a 5 pct prestrain (i.e.,
in the absence of precipitation) can be described by the relation σ = σ0 -α ln(l + βt). By contrast, in the solution-treated Ti steels, relaxation is arrested at the start of precipitation and is
resumed when precipitation is completed. As a result, this mechanical method is particularly suitable for following carbonitride
precipitation in microalloyed austenite at hot working temperatures. A new model regarding the effect of precipitation on
dislocation motion is proposed, on the basis of which, the phenomenology of the stress relaxation technique is clarified.
Precipitation-time-temperature (PTT) diagrams were determined for the Ti bearing steels containing 0.05, 0.11, 0.18, and 0.25
pct Ti. The PTT curves obtained are C-shaped for all the steels. The upper parts of these curves are shifted to significantly
longer times as the Ti and C concentrations are reduced. By contrast, the positions of the lower arms of the curves are relatively
independent of the compositions of the steels tested.
Optimum thermomechanically controlled process parameters have been established for the production of Ti-V-N microalloyed high-strength
low-alloy (HSLA) steels. On the basis of laboratory simulation and full-scale processing, it has been shown that nitrogen
is an essential alloying element addition and full appreciation of its effects leads to the ability to utilize high nitrogen
steel in connection with hot rolling in a high-temperature regime to produce HSLA products with very favorable combinations
of yield strength and toughness. The effects of reheating temperature, rolling reduction, cooling rate, and finish-cooling
temperature (FCT) on the ferrite grain size and mechanical properties have been examined. It has been shown that the potential
for precipitation strengthening is dependent on vanadium, nitrogen, and cooling parameters. Accelerated cooling (ACC) prevents
precipitation of vanadium nitrides in austenite and enhances both grain refinement and precipitation strengthening. By adjusting
nitrogen content and processing parameters, a yield strength of 500 MPa and impact transition temperature (ITT) below -60
‡C can be obtained in the as-hot-rolled condition in Ti-V-N steels, using high finish-temperature hot rolling and accelerated
cooling.
Acicular ferrite formation, promoted by the intragranular nucleation of ferrite plates, is well known to be beneficial for
achieving a good combination of mechanical properties. However, the set of microstructures that can be obtained during the
subsequent development of the transformation from the primary plates generated at particles can be quite complex and depends
on a certain number of variables: steel composition, temperature range, prior austenite grain size, and particle density.
In the present work, acicular ferrite microstructures have been produced by isothermal treatments in three different steels
with different active particle types and densities. The morphology of the obtained intragranular microstructures has been
found to depend on the steel composition, the prior austenite grain size, and the density of particles able to promote intragranular
nucleation. Electron backscattered diffraction (EBSD) techniques have been used to define the microstructural unit controlling toughness in these types of microstructures.
Hot rolled Nb–Mo steel of yield strength 600MPa and Nb–Ti steel of yield strength 525MPa with polygonal and acicular ferrite
microstructure have been developed. Using physicochemical phase analysis, XRD, TEM and EDS, the distribution, morphology,
composition, crystal structure and particle size of precipitates were observed and identified in these steels. The results
revealed that the steels containing both Nb and Mo exhibited fine and uniformly distributed MC-type carbides, while the carbides
were coarse and sparsely distributed in the steels containing Nb and Ti. The physicochemical phase analysis showed MC-type
carbides contain both Nb and Mo, and the ratio of Mo/Nb was 0.41. Meanwhile, the mass% of the fine particles (<10nm in size)
of Nb–Mo steel was 58.4%, and higher than that of Nb–Ti steel with 30.0%. Therefore, the results of strengthening mechanisms
analysis showed the higher strength of Nb–Mo steel than that of Nb–Ti steel is attributed to its relatively more prominent
precipitation strengthening effect. The yield strength increments from precipitation hardening of Nb–Mo steel attained 182.7MPa
and higher than that of Nb–Ti steel.
The purpose of this study is to clarify the correlation between microstructural factors and mechanical properties of ultrafine
steels processed by thermomechanical controlled treatments. Three steels deformed at high strain rates in a pilot plant rolling
mill showed very fine ferritic microstructure, whose grains became more equiaxed and finer with increasing fraction of alloying
elements, and had good tensile and fracture properties, although they contained only about 0.01 pct carbon. Especially in
the Ni-added steel, tensile properties were greatly improved because of the high dislocation density and the fineness of the
ferritic substructure, readily satisfying the requirements for commercial-grade high-strength, high-toughness steels. The
formation of ultrafine equiaxed grains in the steels might be explained by a possible strain-induced dynamic transformation
mechanism associated with the austenite → ferrite transformation caused by heavy deformation in the austenite range.
Analytical transmission electron microscopy was employed to characterize the precipitation at each step of the fabrication
process and thermomechanical treatment of an industrial dual-phase steel microalloyed with titanium. Theoretical thermodynamic
calculations as well as experimental analysis showed that more than half of the titanium carbosulfide (Ti4C2S2) precipitates would dissolve during reheating at 1240 °C. Despite this dissolution at 1240 °C, the remaining titanium carbonitrides
and carbosulfides were effective in pinning austenitic grain boundaries, keeping the austenitic grain size at around 40 µm (at 1240 °C). It is also shown that, during hot rolling, there exist three regions of titanium carbide precipitation. The
first is defined by an increase of titanium carbide precipitation due to deformation. The second region is marked by the insignificant
change in precipitation. The third region is indicated by another increase in precipitation due to the austenite-to-ferrite
transformation. The experimental and theoretical results on the contribution of TiC precipitation to hardening of ferrite
(Orowan mechanism) were in excellent agreement, showing that TiC precipitates have the most important effect on increasing
the yield strength, overshadowing the austenitic grain-boundary pinning contributions by Ti(C,N) and Ti4C2S2 precipitates.
A new hot rolled titanium-microalloyed steel with yield strength of 700 MPa has been developed by CSP (compact strip production) process based on commercial weather resistant steel. EBSD results showed that the average size of its grains with high angle boundaries (>15°) was 3.3 μm. High-density dislocations and large number of nanometer particles were observed in the steel product by TEM. X-ray analysis on the electrolytically extracted phase from the steel indicated that fraction of MX phase was 0.0793 wt%, in which the particles smaller than 10 nm accounted for 33.7%. The contribution of precipitation hardening resulting from nanometer particles was calculated as approximate 158 MPa. The commercial weather resistant steel, reference steel for comparison with 450 MPa yield strength, was also prepared and investigated. It can be concluded that grain refinement is still a major strengthening mechanism in this high strength steel, but precipitation hardening of nanometer TiC precipitates is the dominant factor to increasing the yield strength in new developed steel compared with the reference steel.
This paper analyzes the recrystallization kinetics in Ti-microalloyed steels processed using “beam blank” casting technology. The faster solidification rates associated with this technology brings a finer precipitation of TiN particles which are very effective in controlling austenite grain growth during hot working. Furthermore, these small precipitates have been shown to delay static and dynamic recrystallization. The finer the precipitates the higher the delay in recrystallization. Nevertheless, beyond particle size and distribution, the level of delay is very dependent on microstructure (above all austenite grain size) and deformation conditions (strain and temperature). This paper studies the effects of this recrystallization delay on the microstructure evolution during hot rolling. Special attention was paid to the study of the occurrence of partial recrystallization during the final stages of rolling, which could lead to the presence of mixed microstructures before transformation. The possibility of achieving an additional austenite grain size refinement prior to transformation was evaluated.
The effects of Ti, Ti–Mo, and Ti–Nb microalloy additions on the precipitation strengthening in three experimental high-strength low-alloy steels have been investigated. The objective of this work was to study the carbide precipitation under the conditions of continuous cooling and interrupted cooling. It was found that titanium molybdenum complex carbide, (Ti, Mo)C, can strongly maintain nanometer-scaled sizes and has the largest contribution to the hardness as compared to titanium carbide, TiC, and titanium niobium complex carbide, (Ti, Nb)C. The result emphasizes that (Ti, Mo)C particles possess an excellent behavior of thermal stability.
The correlation among thermo-mechanical controlled processing (TMCP) parameters, microstructures and mechanical properties of an acicular ferrite (AF) pipeline steel was investigated in this study. The steel was hot rolled by four different kinds of TMCP to obtain different AF microstructures, and the corresponding mechanical properties were analyzed. Electron backscatter diffraction (EBSD) analysis was conducted to determine the effective grain size (EGS) in the steel. It was found that the EGS in the steel reduced obviously with decrease of the finish rolling temperature (FRT), but little changed with the cooling rate (CR) and the simulated coiling temperature (SCT). Additionally, the fraction of low angle grain boundaries (LAGBs) increased with increasing CR in the experimental range. It was shown that yield strength of the steel was enhanced by the increased CR and SCT, and reduced FRT, which were corresponding with the increases of LAGB fraction and precipitated carbonitrides as well as the decrease of EGS, respectively. Charpy impact results showed that the low temperature toughness of the steel with FRT about 40 °C above Ar3 tended to be the best, which was in good accordance with the highest fraction of high angle grain boundaries (HAGBs), but seemed not to be related with the EGS.
Purpose: The aim of the paper is to present the development and the technical importance of the HSLA-typemicroalloyed constructional steels (High Strength Low Alloy) in selected industry branches.Design/methodology/approach: A mechanism of the interaction of Nb, Ti, V and B microadditions introducedinto the steel on mechanical properties of selected metallurgical products with the fine-grained structure,forming under the properly selected hot-working conditions with the of (M-Nb,Ti,V; X-N,C) interstitial phasesMX-type is discussed.Findings: The requirements concerning the metallurgical process, continuous casting of steel and the necessityof adjusting the hot-working conditions to the precipitation kinetics of the dispersive MX phases particles inaustenite, in the controlled rolling or thermo-mechanical treatment processes are indicated.Research limitations/implications: The continuation of investigations concerning the thermo-mechanicalrolling of automotive sheets with the multiphase structure of microalloyed steels is planned.Practical implications: The indicated data, coming also from the own research, are of practical use in relationto manufacturing the metallurgical products and machine elements of high strength and crack resistance, alsoat low temperatures.Originality/value: The results contribute to the development of rolling and forging technologies of themicroalloyed steel (HSLA) elements produced using the energy-saving thermo-mechanical treatment methods.
Anthracyclines are very potent drugs in the therapy of malignancies in childhood. The major dose limiting adverse effect of these drugs is the risk of dilated cardiomyopathy. We performed a retrospective study on 168 patients who were treated with anthracyclines for a malignant disease with or without chest radiation at the department of Pediatric Hematology and Oncology at the University of Duesseldorf between 2000 and 2004. During and after chemotherapy the patients were screened by echocardiography and ECG examinations prior to each administration of anthracyclines. Only four patients presented with adverse cardiac events, one of whom developed acute cardiac failure. This patient was additionally treated with chest radiation. Three of the four patients showed intermittent arrhythmias, mainly supraventricular tachycardia. One of them presented with atrial ectopic tachycardia and left ventricular dysfunction. We conclude that the frequency of cardiac sequelae after chemotherapy with anthracyclines is low under present guidelines. Detection of early cardiac sequelae may be more difficult than in the past. Only one patient with cardiac sequelae in our study group was diagnosed by regular performed examinations for cardiac sequelae of chemotherapy. We therefore need to modify our screening methods to increase the effectiveness of detection of cardiac dysfunction prior to clinical manifestation.