No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Occurrence of dynamic ferrite transformation in low-carbon steel above Ae3
Abstract
Dynamic ferrite transformation behavior was investigated over a wide temperature range using a 6Ni–0.1C steel. Softening in flow stress due to dynamic transformation was observed at temperatures above Ae3, the ortho-equilibrium austenite-to-ferrite transformation temperature. Microstructural observation revealed that ferrite grains formed at temperatures above Ae3 showed deformation microstructures, and those grains were reversely transformed to austenite during subsequent holding at the same temperature. Therefore, we concluded that dynamic ferrite transformation could certainly occur even above Ae3.
... and stress ( σ ) obtained by the curve of stress-strain may reflect the evolution of microstructure. The curve of work hardening rate-stress would have an inflection point when dynamic recrystallization occurred [33]. ...
... Transformation"(DIFT) and "Dynamic Strain Induced Transformation"(DSIT) [32,33]. ...
... Some researchers have reported that the presence of double minima in with true stress (σ) curves, (where θ is work hardening rate) represents dynamic transformation [32]. It has been reported that the first minima are due to dynamic transformation and the second minima are due to dynamic recrystallization [32,33]. ...
The microstructure evolution has a significant effect on the mechanical properties of P92 steel. Grain growth behavior is quite complex in these steel because it undergoes phase transformation. The grain growth behavior is also affected by the presence of precipitate and the secondary phase. So it is essential to establish grain growth kinetics in these steel. Different type of heat treatment has been done to understand growth behavior. Other etching techniques are used to accurately identify prior austenite grain boundary (PAGB) size. Also, a correlation of grain size with mechanical properties is established for P92 steel.
The need for a favorable mix of properties has prompted the tailoring of steel microstructures. Noteworthy examples are the stabilization of softer ferrite and austenite in a hard and brittle martensitic matrix for potential benefits of better workability during manufacturing and improved toughness in products. The austenite can dynamically transform to ferrite; this ‘dynamic transformation (DT)’, is the first time reported for the first time in a 9% Cr steel designed for high-temperature power plant applications. The present study reveals how thermo-mechanical processing can be used to induce DT in a 9%Cr steel to alter microstructure and properties.
... The formation of DT ferrite above the Ae 3 has also been reported by Park et al. [96] in their study of a Fe-6 wt%Ni-0.1 wt% C steel. Here they deformed the samples in compression at a strain rate of 10 À1 s À1 to strain levels of e = 0.33 and 0.96. ...
... Such results have been published in Refs. [23,24,29,90,96]. The formation of DT ferrite in a 0.06 wt%C-0.01 ...
... 34 and 35, respectively. Some similar observations regarding the formation of DT ferrite in a Fe-6 wt%Ni-0.1 wt%C steel have been reported by Park et al. [96] using EBSD techniques. An example of their microstructure is reproduced here in Fig. 36. ...
... Therefore, the term DSIT will be used in the present work for this type of phase transformation. Recently, several research groups reported that DSIT could even occur at temperatures above the Ae3 [13][14][15]. This could be attributed to that the plastic deformation effectively raises the Ae3 temperature for deformed austenite by reducing the energy barrier for nucleation and increasing the driving force for transformation. ...
... This could be attributed to that the plastic deformation effectively raises the Ae3 temperature for deformed austenite by reducing the energy barrier for nucleation and increasing the driving force for transformation. Nevertheless, the dynamic nature of such transformation was confirmed by the observation of dynamic flow softening and deformation microstructure in ferrite grains [14]. DSIT phenomena were reported as early as the 1980s by various researchers [16][17][18][19]. ...
... Therefore, the term DSIT will be used in the present work for this type of phase transformation. Recently, several research groups reported that DSIT could even occur at temperatures above the Ae3 [13][14][15]. ...
Dynamic strain-induced transformation (DSIT) in a low-carbon microalloyed steel was studied by hot cyclic torsion to understand the interactions between DSIT and strain path reversals, and the subsequent microstructure evolution when subjected to continuous cooling. The critical strain for DSIT (εc,DSIT) can be determined by analysing the dynamic softening of the flow stress–strain curves. When deformed to the same total accumulative strain of 2.0, deformation with a small strain amplitude in each pass and multiple strain path reversals led to the suppression of DSIT compared to the extensive DSIT ferrite produced by deformation with a large strain amplitude and a single strain reversal. The results reveal that the amplitude of monotonic strain, not the total accumulative strain, in relation to εc,DSIT determines the occurrence of DSIT. The suppression or promotion of DIST can be attributed mainly to the increment of the austenite grain boundary area associated with deformation, especially the development of serration and bulging, and, to a lesser extent, to the generation of high-angle boundaries by austenite grain subdivision. The evolution of these planar defects, which are believed to be the primary ferrite nucleation sites, is strongly influenced by strain path changes, and lead to significantly different DSIT behaviours. DSIT ferrite also showed very limited coarsening after continuous cooling as the ongoing deformation produces further nucleation sites in the austenite matrix and causes orientation variation of the DSIT ferrite inherited from austenite parent grains. Based on these observations, it is believed that the transformation mechanisms for DSIT are essentially the same as the reconstructive mechanism during static phase transformations.
... The conditions under which the transformation occurs indicate a dynamic ferritic transformation taking place during the plastic deformation below the A C3 temperature. This phenomenon is in line with previous studies [32][33][34]. Small differences in the course of the curves may be related to a band segregation of the alloying elements. Small, local differences in the chemical composition may affect the dynamics and the extent of the transformation, which is an argument supporting the thesis of dynamic transformation. ...
... In order to determine the amount of dynamic ferrite, a sample was deformed at 900 • C and subsequently quenched at a cooling rate of 60 • C/s. The obtained microstructure is showed in Figure 7. Due to the high deformation temperature, the fraction of dynamic ferrite is 1.8 ± 0.3%, and its morphology is in line with the literature [23,34]. The start of transformation during isothermal holding is also significantly accelerated. ...
The kinetics of ferritic transformation and the corresponding microstructural evolution in 0.17C-3.1Mn-1.6Al-0.04Nb-0.22Mo-0.22Si medium-Mn steel during isothermal annealing was investigated in dilatometric studies. The material was subjected to thermal and thermo-mechanical treatments aimed at obtaining, by the austenite → ferrite transformation, a sufficient fraction of ferrite to stabilize the retained austenite by C and eventual Mn partitioning. The samples were isothermally held for 5 h in a temperature range from 600 to 750 °C to simulate simplified temperature conditions of an industrial coiling process following hot rolling. Some of the samples were plastically deformed at a temperature of 900 °C before isothermal holding in order to study the effect of hot deformation on the kinetics of phase transformations. After the dilatometric investigations the material was subjected to light and scanning electron microscopy to reveal relationships between the holding temperature, deformation and microstructure evolution. Hardness tests were performed to assess the mechanical behavior. A significant effect of manganese in slowing down diffusional transformations during the cooling of steel was found. The influence of austenite deformation on the kinetics of austenite to ferrite transformation was noted. The plastically deformed samples showed an accelerated start of ferritic transformation and the extension of its range. During dilatometric tests, low-range dynamic ferritic transformation was recorded, which was also confirmed by the microscopic tests.
... It took two decades to attract numerous researchers to extensively study this unusual metallurgical behavior and identify the effects of DT in the industrial hot rolling operations. [19][20][21][22] Several thermodynamic models have been proposed to explain and predict the occurrence of dynamic transformation. The driving force to dynamic transformation could be from the stored energy of dislocations, 23 the mechanical activation by applied stress, 24 or the energy generated from dynamic phase softening. ...
The use of thermomechanical control processing during hot rolling has played a significant role in improving the mechanical properties of steels in the past several decades. Accurate control of various metallurgical phenomena such as recrystallization, phase transformation and strain-induced precipitation has been the main target of steel industries around the world to push the properties of steels to their limit. Therefore, numerous physical and numerical simulations of hot rolling were developed to predict the microstructure and properties of steels during thermomechanical processing, which allows for quick optimization of manufacturing parameters. One of the most common techniques to physically simulate the actual hot rolling process in a laboratory scale is through hot torsion testing. In the present work, various grades of steels subjected to torsion simulation of hot strip and plate rolling were analyzed to determine the effect of deformation on the initiation of various metallurgical phenomena. The results show that high-temperature deformation can induce unusual metallurgical phenomena, such as dynamic phase transformation, which affects the final microstructure and mechanical properties of steels. These new findings can be employed to accurately control the volume fraction of phases in steels during cooling on the runout table after hot rolling.
... It took two decades to attract numerous researchers to extensively study this unusual metallurgical behavior and identify the effects of DT in the industrial hot rolling operations. [19][20][21][22] Several thermodynamic models have been proposed to explain and predict the occurrence of dynamic transformation. The driving force to dynamic transformation could be from the stored energy of dislocations, 23 the mechanical activation by applied stress, 24 or the energy generated from dynamic phase softening. ...
... The striking contrast between the two experiments indicates that the hot deformation of austenite significantly accelerated the kinetics of ferrite transformation. Because the specimen was waterquenched immediately after the hot-compression, it is considered that the present ferrite transformation is the dynamic transformation in which austenite to ferrite transformation occurs during plastic deformation [26][27][28]. Figure 4(b) shows an EBSD phase map of the identical area in Figure 4(a), in which red colour represents BCC phases and green colour represents austenite with FCC structure. Only a very small amount of retained austenite (f γ = 2.3%) was detected in this specimen compressed at 650°C. ...
Medium-Mn steels are energetically investigated as a candidate of the third generation advanced high strength steels (AHSSs). However, their phase transformation and microstructaure evolution during various heat treatments and thermomechanical processing are still unclear. The present study first confirmed the kinetics of static phase transformation behaviour in a 3Mn-0.1C medium-Mn steel. Hot compression tests were also carried out to investigate the influence of high-temperature deformation of austenite on subsequent microstructure evolution. It was found that static ferrite transformation was quite slow in this steel, but ferrite transformation was greatly accelerated by the hot deformation in austenite and ferrite two-phase regions. Characteristic dual-phase microstructures composed of martensite and fine-grained ferrite were obtained, which exhibited superior mechanical properties.
This paper is part of a Thematic Issue on Medium Manganese Steels.
... Similarly, at a constant strain rate of 0.25 s -1 , the ferrite volume fraction was respectively 77% and 72% at 1200 °C and 1150 °C. The above findings are consistent with those reported by N. Park et al. [8,40] who observed that with the decrease in the strain rate, the ferrite volume fraction increased in a 6Ni-0.1C steel. ...
In this work, the effect of strain rate on the dynamic transformation (DT) of austenite to ferrite during high
temperature forging of an as-cast medium-carbon low-alloy steel was investigated. Hot deformation experiments
were carried out in the 1150°C to 1200°C temperature range (400–450°C above Ae3) and 0.25 s−1 to 2 s−1
strain rate range using a Gleeble 3800® thermomechanical simulator. The critical strains for DT and dynamic
recrystallization (DRX) were determined to be in the 0.08 to 0.18 strain range. A microstructural analysis was
conducted using optical and electron microscopy. The kernel average misorientation (KAM) technique was applied
to electron back-scattered diffraction (EBSD) images to quantify the internal misorientation of grains for
the characterization of DT ferrite. Furthermore, it was found that an increase in strain rate decreased the amount
of dynamically formed ferrite under the same applied strain and testing temperature. The obtained results were
correlated with the influence of deformation parameters on carbon diffusivity and its impact on the growth of
dynamically formed ferrite. It was found that an increase in the strain rate decreased the diffusion distance of
carbon, which was the responsible mechanism for decreased ferrite formation at higher strain rates.
... Reactions of DFT can be divided into dynamic transformation (DT) and dynamic strain-induced transformation (DSIT) depending on the deformation temperature. DT is usually conducted by applying a large strain to induce ferrite transformation at a temperature above Ae 3 [7,[10][11][12][13][14][15][16][17][18][19][20]. Additional driving force for ferrite transformation above Ae 3 can be obtained during the deformation. ...
In this work, the coopetitive relationships among dynamic ferrite transformation, reverse ferrite transformation, and austenite recrystallization were investigated by using dynamic dilatometry, optical metallography, and electron backscattering diffraction in an aluminum-containing low-carbon steel subjected to hot compressions in the two-phase region. The microscopic mechanism of concurrent dynamic softening was studied based on the analysis of transformation crystallography. Moreover, this analysis method was used to discover the occurrence of reverse-transformation-induced recrystallization. In this microscopic mechanism, new austenite grains formed by reverse transformation can act as seeds for recrystallization. These paths for microstructural control in steels are not common, but they can be enabled by critical thermo-mechanical treatments combined with proper alloy design.
... Very recently, several studies from different research groups also reported that dynamic transformation (DT) was observed when the deformation temperatures were above the Ae3 [15][16][17][18]. This was probably due to the fact that plastic deformation of austenite could increase the driving force for ferrite transformation and reduce the energy barriers for ferrite nucleation, therefore, effectively raising the Ae3 temperature of deformed austenite. ...
In the present work, the effect of strain path reversals on dynamic transformation (DT) above Ae3 temperature was studied using an API grade X-70 microalloyed steel deformed by torsion with single and multiple strain path reversals. The results revealed the important role played by strain path reversals on influencing the evolution of austenite grain boundaries through inhomogeneous deformation, therefore, affecting DT behaviours. In addition to flow stress–strain analysis and microstructure investigation, finite element method combined with 3D digital materials representation approach was used to gain insights into the effects of deformation with strain path reversals on the development of microstructural features in the prior-austenite grains.
... Their results confirmed the increasing presence of the (110) -Fe diffraction pattern during straining. Their experiments elicited numerous studies of DT around the world [4][5][6]. ...
The conventional thermal-mechanical control processing technology governed by a heavy reduction at a relatively lower temperature has been applied successfully in the refinement of the final microstructure of hot-rolled steels. However, it might not be the best method for the manufacture of large-size structural steels due to their limited reduction ratio and relatively high temperature during hot rolling. Boundary induced transformation (BIT), is proposed in the present paper as an alternative approach for the grain refinement of large-size structural steels. The austenite grain size during reheating and rough rolling is controlled by Ti microalloying with the aim of reducing the critical strain for the onset of dynamic recrystallization (DRX) in the subsequent rolling. The precipitation of NbC particles and their inhibition of static recrystallization during the interpass interval is responsible for the accumulation of strain to ensure the occurrence of DRX in finish rolling. Consequently, the large amount of austenite grain boundaries resulted from the formation of fine DRXed grains will provide potent nucleation sites for the phase transformation from austenite to ferrite and then lead to an effective grain refinement. The experimental results demonstrate that the ferrite grain size can be significantly refined via this approach, illustrating that BIT mechanism is very applicable to refine grain size effectively for the large-size structural steels with limited reduction ratio.
Development of thermo-mechanical processing (TMP) or thermo-mechanically controlled processing (TMCP) of steels was overviewed, putting a special emphasis on the progress of physical metallurgy about microstructural evolution and processing technology. TMP/TMCP are strong tools for controlling microstructures and mechanical properties of steels, and they have continuously developed in last 60 years. Since steel industries use huge equipment, application of TMP/TMCP is strongly restricted by installed facility. However, like the progress of direct quenching after the success of controlled rolling, innovation of processes makes industrialization of advanced TMP/TMCP possible in long spans, leading to advanced steels having superior properties. Advanced TMP/TMCP has also clarified new metallurgical phenomena, and has deepened fundamentals of physical metallurgy about microstructural evolution.
Double-hit hot compression tests were carried on medium-carbon low-alloy steels using Gleeble 3800® thermomechanical simulator. The experiments were performed at strain rates of 0.25 and 0.5 s−1 and temperatures of 1150 and 1200 °C with interpass times of 5, 15, and 25 s. The onset of critical stresses for dynamic transformation (DT) for both first and second hit were detected using the double-differentiation method. It was found that the critical stress for DT increased with a decrease in temperature and an increase in strain rate. The presence of dynamically transformed ferrite was observed and quantified using electron-backscatter diffraction, kernal average misorientation, and grain boundary maps. Then, a thermodynamic analysis was carried out using JmatPro software. A method of determining the change in Gibbs energy during DT phenomenon is proposed for double hit deformation.
Nowadays, a new concept of process utilizing dynamic ferrite transformation, which can achieve ultrafine-grained structure with a mean grain size of approximately 1 μm, has been proposed. This paper reports transformation mode of dynamic ferrite transformation and formation mechanism of ultrafine-grained structure revealed by our novel technique of in-situ neutron diffraction analysis during thermomechanical processing. Dynamic ferrite transformation occurs in a diffusional manner, whose partitioning behavior changes from para- to ortho-equilibrium with the progress of transformation. Moreover, we propose that dynamic recrystallization of dynamically-transformed ferrite is the main mechanism for the formation of ultrafine-grained structure.
There have been previous attempts to observe the occurrence of dynamic ferritic transformation at temperatures even above Ae3 in a low-carbon steel, and not only in steels, but recently also in titanium alloys. In this study, a new approach is proposed that involves treating true stress-true strain curves in uniaxial compression tests at various temperatures, and different strain rates in 0.1C-6Ni steel, which is a model alloy used to decelerate the kinetics of ferrite transformation from austenite. The initial flow stress up to peak stress was used to analyze the change in dynamic softening phenomena, such as dynamic recovery, dynamic recrystallization, and dynamic transformation. It is worth mentioning that for predicting the occurrence of dynamic transformation, flow stress before reaching peak stress is much more sensitive to the change in the dynamic softening rate due to dynamic transformation, compared to peak stress. It was found that the occurrence of dynamic ferritic transformation could be successfully obtained even at temperatures above Ae3 once the deformation condition was satisfied. This deformation condition is a function of both the strain rate and the deformation temperature, which can be described as the Zener - Hollomon parameter. In addition, the driving force of dynamic ferritic transformation might be much less than that of the dynamic recrystallization of austenite at a given deformation condition. By applying this technique, it is possible to predict the occurrence of dynamic transformation more sensitively compared with the previous analysis method using peak stress during deformation.
Developing appropriate thermomechanical processing routes for decreasing macrohardness of gear steels without heat treatment has a numerous futuristic applications in rolling industry. The effect of different deformation processes applied to Cr–Mn–Ti gear steel on the evolution of microstructure and hardness are studied by using a thermal simulator. It is found that imposing second‐pass deformation near Ae3 temperature inhibits occurrence of dynamic recrystallization (DRX), and increases stored deformation energy in austenite, which significantly promotes ferrite and pearlite transformation. As a result, the maximum volume fraction of ferrite and pearlite approaches ≈90%, and the corresponding macrohardness decreases to ≈215 HV. The difference of ferrite transformation between different deformation processes is analyzed using a thermodynamic equation by taking into account the stored deformation energy. The present calculations indicate that interval between Ae3 and deformation temperature can only be avoided at lower deformation temperature of second‐pass. This restricts static recovery or recrystallization, and quantities of dislocations that act as nucleation sites are retained before ferrite transformation. The different thermomechnical routes are applied to investigate the microstructure and hardness in a Cr–Mn–Ti gear steel. The microstructural observation suggests that ferrite and pearlite transformation are promoted by adding one‐pass deformation near Ae3, which leads to the decrease of macrohardness. A mathematical model is developed to predict ferrite transformation in different processes.
Dynamic transformation (DT) of deformed austenite to ferrite at temperatures above Ae3 occurs during a multi-step hot torsion test of a Nb-bearing medium manganese steel. In torsion tested specimens, equiaxed grains are dispersed within a martensite matrix, and the average grain size is less than 1 μm. Electron back-scattering diffraction results confirm that most of the equiaxed grains are recrystallized ferrite, and there is a small fraction of retained austenite. © 2018 The Minerals, Metals & Materials Society and ASM International
The dynamic transformation (DT) of austenite to ferrite and the retransformation of the dynamically transformed ferrite back into the more stable austenite were studied at temperatures above the Ae3. For this purpose, compression tests were carried out on a C-Mn and a Nb microalloyed steel over the temperature range 870–960 °C. The samples were deformed to a strain of 0.75 at a strain rate of 1 s−1 and held isothermally for times of 1–1000 s to permit retransformation. The samples were water quenched after testing and the volume fractions of retransformed ferrite were measured to allow for the construction of TTRT (time–temperature–reverse transformation) curves. The present type of diagram can be combined with strain–temperature–transformation (STT) curves to model the occurrence of dynamic transformation and retransformation during thermomechanical processing.
Plate rolling simulations were carried out on an X70 Nb and a low C steel by means of torsion testing. A seven-pass rolling schedule was employed where the last pass was always applied above the respective Ae3 temperature of the steel. Interpass intervals of 10 and 30s were employed, which corresponded to cooling rates of 1.5 and 0.5 C/s. The mean flow stresses (MFS`s) applicable to each schedule increased less rapidly than expected from the decreases in temperature due to the dynamic transformation (DT) that took place during straining. The amounts of ferrite that retransformed into austenite during holding were determined by optical metallography. These increased with length of the interpass intervals and were reduced in the X70 steel due to the presence of Nb. The holding times after rolling required to increase the amount of austenite available for microstructure control on subsequent cooling were also determined for the two steels.
Dynamic recrystallization (DRX) is an important grain refinement mechanism to fabricate steels with high strength and high ductility (toughness). The conventional DRX mechanism has reached the limitation of refining grains to several microns even though employing high-strain deformation. Here we show a DRX phenomenon occurring in the dynamically transformed (DT) ferrite, by which the required strain for the operation of DRX and the formation of ultrafine grains is significantly reduced. The DRX of DT ferrite shows an unconventional temperature dependence, which suggests an optimal condition for grain refinement. We further show that new strategies for ultra grain refinement can be evoked by combining DT and DRX mechanisms, based on which fully ultrafine microstructures having a mean grain size down to 0.35 microns can be obtained without high-strain deformation and exhibit superior mechanical properties. This study will open the door to achieving optimal grain refinement to nanoscale in a variety of steels requiring no high-strain deformation in practical industrial application.
Compression tests were carried out on a 0.06wt%C-0.3wt%Mn-0.01wt%Si steel at temperatures high in the austenite phase field. Eight deformation temperatures were selected in the range from 1000 to 1350 °C at 50 °C intervals. The quenched samples were examined using optical microscopy and EBSD techniques. It was observed that dynamic transformation took place and that the volume fraction of transformed ferrite first decreased with temperature (up to 1050 °C) and then increased as the delta ferrite temperature domain was approached. The EBSD results revealed the presence of Widmanstätten ferrite plates under all testing conditions, right up to 1350 °C.
To research the effect of large precipitates (size > 0.2 μm) on strain-induced dynamic transformation, the variation of V contents in large precipitates has been investigated quantitatively in two V–Ti micro-alloyed steels. The results showed that high N content promoted V precipitation on the surface of Ti large precipitates rapidly. Subsequently, large precipitates containing V induced the formation of intragranular ferrite, which accelerated the dynamic transformation process remarkably, promoted the occurrence of continuous dynamic recrystallization of ferrite and improved the refinement effect.
Deformation-induced ferrite transformation (DIFT) is one of the most effective ways of refining ferrite grains in steel. In this study, we employed a multi-phase-field (MPF) model to simulate both variations in macroscopic flow stress and microstructural evolution during DIFT. Using the MPF model, two-dimensional simulations of DIFT in a Fe–C alloy were performed to investigate the effects of strain rate, austenite grain size, and dynamic recrystallization (DRX) of the ferrite phase on flow stress curve and ferrite grain size. The results demonstrated that increasing the rate of ferrite nucleation by increasing the strain rate and reducing the austenite grain size is essential to obtaining fine-grained ferrite. The results of the simulations also indicated that it is important to reduce the interfacial mobility and increase the nucleation rate of the ferrite grains subjected to DRX in order to obtain ultrafine-grained ferrite by DIFT when it is accompanied by DRX of the ferrite phase. Thus, the MPF model is an effective tool for elucidating the correlation between the variation in the flow stress and the evolution of the ferrite grains during DIFT.
After ausforming appeared as the first thermomechanical processing of steels in the first half of the 1960 s, various thermomechanical processings have been developed for the improvement of mechanical properties over the last fifty years. Their application was mainly to martensitic steels in the 1960 s such as ausforming and TRIP, and moved to ferrite (+ pearlite) structures by the development of controlled rolling and accelerated cooling of HSLA steels in the 1970 similar to 4980 s. However, recently, interest has returned to martensite (and also bainite) because of the demand for higher strength, and the ausforming and TRIP have been revived and successfully applied to commercial practice. Very recently, severe plastic deformation (SPD) is the focus of attention as a new method of producing a very fine-grained structure with grain size of less than 1 mu m. By the application of SPD, dynamic phenomena such as dynamic recrystallization and dynamic ferrite transformation occur in the process. We need more systematic studies on such phenomena for the development of new type of thermomechanical processing in steels.
Recent observations regarding the dynamic transformation of deformed austenite at temperatures above the Ae 3 are reviewed for four different steels of increasing carbon contents. The structures observed are Widmanstätten in nature and appear to have formed displacively. The effect of deformation on the Gibbs energy of austenite in these steels is estimated by assuming that the austenite continues to work harden after initiation of the transformation and that its flow stress and dislocation density can be derived from the experimental flow curve. By further taking into account the inhomogeneity of the dislocation density, Gibbs energy contributions are derived that are sufficient to promote transformation as much as 100 1C above the Ae 3 . The present calculations indicate that the dislocation densities in the regions that experience transformation are 2–15 times higher than the average values. It is also suggested that, at the lower carbon levels, plate growth is followed by C diffusion, while it is accompanied by C diffusion at the higher carbon contents.
When austenite is deformed above the equilibrium transformation temperature Ae(3), it is dynamically transformed into Widmanstatten ferrite by a displacive mechanism. On removal of the load it is slowly retransformed into austenite by diffusional processes. The forward transformation has recently been explained in terms of a thermodynamic model in which the lower free energy of austenite is raised above that of normally unstable ferrite as a result of the additional stored energy associated with the dislocations introduced by straining. This model is here shown to be unable to account for the initiation of transformation at critical strains of about 0.1, at which only low densities of dislocations are present. Of particular importance is the observation that dynamic transformation can be initiated at temperatures 100 C and more above the Ae3 and that the critical strain actually decreases with increasing temperature and increasing chemical free energy barrier. This discrepancy is removed by allowing for mechanical (stress-based) activation of the transformation. The latter provides the energy required to accommodate the shear of the parent austenite into Widmanstatten plates, as well as the volume change or dilatation accompanying ferrite formation. The work of dilatation and the shear accommodation work, omitted from the previous analysis, are introduced here as barriers to the transformation that are overcome by the applied stress. This modified approach is able to account for the very rapid forward (mechanically activated) transformation compared with the much slower reverse transformation that takes place in the absence of stress.
The capability of strain-induced ferrite transformation in refining the microstructure of a promising transformation-induced plasticity (TRIP) steel has been studied in the present investigation. This was performed employing the hot compression testing technique to deform the experimental TRIP steel at temperatures below the austenite-to-ferrite transformation temperature (i.e., Ac3). The strain-induced ferrite transformation (DIFT) has been identified as the most effective mechanism contributing to the grain refinement at temperatures (860 and 880 degrees C) just below the corresponding Ac3. The influence of this phenomenon on the mechanical properties of the material during and after deformation has been fully addressed. The results show that the DIET would lead to a work softening behavior during deformation. However, the grain refinement through DIET has significantly improved the mechanical properties of the deformed specimens characterized by the shear punch testing method.
Phase transformation from austenite to ferrite is an important process to control the microstructures of steels. To obtain finer ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static ferrite transformation have been carried out for austenite phase. This article reviews the dynamic transformation (DT), in which ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress-true strain curve analysis. This analysis predicted the formation of ferrite grains even above the Ae3 temperature (ortho-equilibrium transformation temperature between austenite and ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae3 was experimentally confirmed. Moreover, the change in ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of ferrite.
Ultrafine cellular microstructures around alumina particles in a low-carbon steel were observed, which survived even after cyclic austenitization. This indicates that their formation is closely related to internal stress because of a structural heterogeneity during phase transformation rather than to externally applied forces or deformation. Thermo-elasto-plastic finite element analysis confirmed the evolution of a large hydrostatic pressure around an alumina particle due to thermal mismatch during cooling. Therefore, the fine cellular microstructure might be generated as a result of the hydrostatic pressure, which retards the phase transformation around the particle during cooling. In addition, we observed microstructural similarity with the same steel processed under an ultra-high pressure, which was the evidence for the role of the delay in the transformation caused by the hydrostatic pressure.
The effect of austenite grain size on kinetics of dynamic ferrite transformation above Ae(3) in a 6Ni-0.1C steel was studied. As the austenite grain size decreased, the onset of dynamic transformation was accelerated. The increase in the fraction of dynamically transformed ferrite was in good agreement with the change in flow stress, i.e. dynamic softening. The kinetics of dynamic transformation could be evaluated by an Avrami-type formula. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
In order to study about dynamic transformation phenomenon, Fe-6Ni-0.1C alloy was hot-deformed in uniaxial compression using thermo-mechanical simulator at various temperatures ranging from 600 to 1000 °C at various strain rates from 0.001 to 1 s-1 after austenitization. As the value of Zener-Hollomon (Z) parameter increased, softening of the stress from the empirically expected value, which was extrapolated from stresses deformed at low Z value, was observed through systematical analysis of peak stresses. It suggested that this softening phenomenon was attributed to the dynamic transformation, since ferrite is softer than austenite at elevated temperature. The microstructural observation also supported that ferritic transformation occurred during compressive deformation. Even above Ae3 temperature the softening of the peak stress of austenite was still observed, which implied that dynamic ferritic transformation might occur above Ae3 temperature.
Physical meaning of dynamic transformation was re-considered with a particular attention to the effect of post-transformation deformation on crystallography and kinetics of transformed ferrite. An advanced in-situ neutron diffraction experiment was performed to examine the microstructural evolution besides an advanced EBSD measurement. In particular, the deformation behavior in austenite/ferrite two-phase region was investigated as a function of deformation temperature and ferrite volume fraction. Based on these findings, the specific features of dynamic transformation were extensively discussed.
A Monte Carlo (MC) technique has been used to model deformation-induced ferrite transformation (DIFT) in an Fe-C binary system
on a mesoscale. The effects of strain rate, strain, and recrystallization of the matrix on DIFT are investigated. Increasing
the strain rate slightly retards the onset of DIFT. The volume fraction of ferrite increases gradually as the strain increases
before the volume fraction of ferrite reaches its saturation value. After the volume fraction of ferrite becomes saturated,
it oscillates around its saturation value. The recrystallization of austenite slightly retards the onset of the DIFT. Although
the recrystallization of austenite reduces the equilibrium volume fraction of ferrite significantly, it cannot completely
suppress DIFT. The stress concentration has been shown to induce the nucleation of ferrite near the grain boundaries and phase
boundaries. The significance of the reverse transformation has been investigated. We found that there is a temporal oscillation
of the volume fraction of ferrite and the stored energy after they arrive at their saturation values. We conclude that this
oscillation and the effect of the strain rate on DIFT are both brought about by the reverse transformation from induced ferrite
to undeformed austenite. The diffusion behavior of carbon atoms in the systems is different for different strain rates. The
simulation shows that the dynamic recovery of austenite cannot occur in the system during deformation under the present conditions.
The results of the simulation show that, other than the oscillation of the equilibrium volume fraction of ferrite and the
unusual diffusion behavior of carbon atoms, the simulation agrees well with the corresponding experimental results. The temporal
oscillation of the volume fraction of ferrite and stored energy and the unusual diffusion behavior are two new phenomena that
have not been reported by other researchers.
Strain-induced dynamic transformation (SIDT) is of great advantage to obtaining ultrafine-grained ferrite in low carbon steels partly by early impingement of ferrite nuclei which are very rapidly and concurrently formed due to the deformation and partly by random crystallographic orientation distribution of ferrite grains. The SIDT fraction increases with the increase of strain. There is, however, a critical strain under which no SIDT occurs. In order to apply this refining mechanism to actual production lines, it is necessary to reduce the critical stain and enhance the kinetics of SIDT. It has been known that refining prior austenite grain is the most effective for this purpose. The influences of deformation temperature and strain rate on SIDT were also examined. Because SIDT is a kind of softening mechanism of strained austenite, it competes with dynamic recrystallization (DRX) at below Ae3 temperature. Comparing the critical strains of SIDT and DRX, SIDT is predominant softening mechanism, which enables us to utilize it for grain refinement. This provides an important clue to overcome the limitation of conventional thermomechanical control process (TMCP) in grain refinement area.
The attainment of ultrafine ferrite grain structures in low carbon, low alloy steels is of interest because of the improvement in yield strength and Charpy impact transition temperature predicted by extrapolation of known data to very fine grain sizes. This paper presents a summary of research aimed at producing ultrafine ferrite in a niobium microalloyed, low carbon steel by three processing routes. Transformational grain refinement (TGR), in which extrafine austenite is hot rolled and cooled rapidly, has been shown to be capable of producing grain sizes of < 1 m in a surface layer, and 1.5 mum in the centre of 3 mm thick plate. Dynamic recrystallisation of ferrite during multipass warm rolling was shown to be neither complete nor uniform within the cross-section of the plate. Nevertheless, a partly recrystallised, partly recovered grain structure with an average grain size of 1.5 mum was obtained in the centre of 3 mm thick plate. Cold rolling and recrystallisation of ferrite that had been previously refined by TGR to an intermediate grain size was shown to produce an ultrafine grain microstructure (<1 m grain size) throughout the section of 1 mm thick strip. The hardness of ultrafine ferrite was shown to obey a linear relationship with the inverse square root of grain size, but with a lower slope than expected from the Fetch relationship for yield strength.
The design of thermomechanical processing schedules to control microstructures requires the knowledge of the austenite-to-ferrite transformation start temperature (Ar3). In this industrial process, during deformation, the temperature usually decreases continuously. Thus, two new methods to determine the Ar3, based on continuous cooling compression (CCC) and continuous cooling torsion (CCT), have been developed. While the former is applicable for low strains only, the latter can be used for low and high strain processes.
The aim of this investigation was to determine the effect of deformation in the single phase austenite and two phase austenite plus ferrite region on the transformation and dynamic transformation behaviour of austenite-to-ferrite. CCC tests were carried out on a low carbon steel and the influence of strain was examined.
As expected, deformation in the single phase austenite region increased the kinetics of the austenite-to-ferrite transformation, raising the Ar3. The faster kinetics leads to a finer polygonal ferrite grain size after transformation. Straining in the two phase region causes strain concentration on the softer ferrite and, consequently, recrystallization of this phase. Deforming close to the Ar3 maximizes the strain effect on dynamically transformed ferrite.
The effect of austenite grain size on kinetics of dynamic ferrite transformation above Ae(3) in a 6Ni-0.1C steel was studied. As the austenite grain size decreased, the onset of dynamic transformation was accelerated. The increase in the fraction of dynamically transformed ferrite was in good agreement with the change in flow stress, i.e. dynamic softening. The kinetics of dynamic transformation could be evaluated by an Avrami-type formula. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
In order to clarify the occurrence of dynamic ferrite transformation in a 6Ni–0.1C steel, the stress–strain behavior in uniaxial compression was analyzed for a wide range of temperatures and strain rates. Significant softening of flow stress for austenite was observed at lower temperatures at a constant strain rate, which seemed to correspond with the occurrence of dynamic transformation to ferrite. Analysis of the maximum stress in the stress–strain curves indicated that dynamic ferrite transformation occurred above a certain value of the Zener–Hollomon parameter (Z). The critical deformation condition (ZC) for the occurrence of dynamic transformation was determined. Increasing the amount of softening resulted in an increase in the fraction of ferrite, and the maximum flow stress came close to the flow stress of ferrite. Microstructural observations revealed that the specimens exhibiting softening consisted of ferrite grains with typical characteristics of deformation microstructure, such as a change in crystal orientation within the ferrite grain, inhomogeneity in ferrite morphology and dislocation substructures inside the grains. All these characteristics confirmed the occurrence of ferrite transformation during deformation, i.e. dynamic ferrite transformation.
A 0.036% Nb microalloyed steel was deformed in torsion over the temperature range 816-8960C in a 2%H2-Ar gas atmosphere. Strains of 0.5-5.0 were applied at strain rates of 0.04 and 0.4 s-1. The experimental parameters were varied in order to study the effects of strain, strain rate and temperature on the formation of ferrite by dynamic transformation (DT) at temperatures above the Ae3. The critical strain for ferrite formation by DT was 0.5 and the volume fraction formed increased with strain and slightly with strain rate. It was also observed that the applied strain has a far greater influence on the transformation than the time. Average ferrite grain sizes of 2 to 3.5 μm were produced, the size increasing with the transformation temperature and decreasing strain rate. By comparison with the behavior of plain C steels, it is evident that the addition of niobium slows the reverse transformation to a considerable degree. Two stages were detected in the reverse transformation: i) in stage I, observed during the initial 200 s of isothermal holding, the deformation-induced ferrite was fairly stable; ii) in stage II, observed after 200 s of holding, the reverse transformation began to take place, going to completion in about 400 s. The results of these experiments support the view that it is the stored energy of the 'inhomogeneously' distributed dislocation (i.e. those in shear bands and sub-boundaries) that provides the driving force for such ";non-equilibrium" transformation.
The post-dynamic transformation (post-DT), which could occur during isothermal holding after hot deformation, was investigated by using both dilatometry method and optical microstructural observation in a plain low carbon steel. The results indicates that the kinetics of post-DT at deformation temperature between Ae(3) and Ar-3 can be well described by the Avrami equation: X=1 -exp(-kt(n)), but the n value is lower than that of the corresponding static transformation due to the early impingement of formed ferrite grains. Furthermore, the ferrite-to-austenite re-transformation was measured by dilatometry during the isothermal holding after hot deformation above Ae(3) temperature, which suggests that dynamic transformation can indeed occur even above Ae(3).
Fe–6mass%Ni–(0.0008∼0.29)mass%C alloys were hot-deformed in torsion at 600–720°C (above the cooling transformation start temperatures Ar3 ) after austenitization. An in-situ X-ray diffraction study re-vealedthat γ→α transformation occurred during deformation in a wide range of condition, even above A3p(paraequilibrium γ→α transformation temperature). Corresponding to this transformation, apparent decrease in deformation stress from that expected for austenite was observed. Microstructural study of the specimens quenched after the deformation showed that a large amount of fine-grained ferrite was formed due to the deformation. The analysis of deformation stress and chemical driving-force of the transformation indicated that the transformation occurred in order to reduce the total energy of deformed material since the deformation of energy of α was revealed to be considerably smaller than that of γ and the amount of deformation energy saved by the transformation was shown to be much greater than the chemical energy consumed by the transformation at the tested temperatures.
Massive ferrite was observed to form during deformation of plain low carbon steel within (γ+α) 2-phase field at temperatures below T0, whereas polygonal ferrite was formed at higher temperatures by conventional strain-induced transformation. The grain refinement occurred through continuous recrystallization of massive ferrite during deformation.
The constitutive equation relating peak stress , strain rate and temperature T for hot working: , was re-examined to develop a new algorithm for determining the material constants . By graphical or numerical techniques, are determined in a manner which re-checks the suitability of the equation and of the value of α which must be initially estimated or taken from the literature. The values for data on 301, 304 and 317 steels were derived by the two techniques and tested by calculating the stresses for each experimental condition. Comparison is made with previously derived constants, agreement to within about 5% being shown. The wide utility of the sinh equation is discussed.
The present study using IF steel confirmed that dynamic recrystallization can occur also in ferrite where it has been generally considered that recovery is an only restoration process during hot deformation. Although the occurrence of DRX has been clarified by microstructural observations and crystallographic determinations, stress-strain curves do not show obvious drop of stress which has been typically reported in the case of DRX of austenite. This result indicates that it is quite difficult to distinguish whether DRX occurs in ferrite only by stress-strain behavior. The noticeable feature of DRX of ferrite is inhomogeneity of recrystallization, i.e., some of the initial grains are hard to recrystallize. This is presumably due to orientation dependence of recrystallization, which is the essential feature of ferrite.
The austenite decomposition has been investigated in a hypo-eutectoid plain carbon steel under continuous cooling conditions using a dilatometer and a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary systems Fe-C-Mn. The austenite to ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates. The formation of ferrite nuclei takes place with equilibrium composition on austenite grain boundaries. The nuclei are assumed to have a pill box shape in accordance with minimal interfacial energy. For higher cooling rates, early growth has to be taken into account to describe the transformation start. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium; i.e. only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute drag effect on the boundary movement. Thus, growth kinetics is controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements.
Hot-deformation behavior of super-cooled austenite within the austenite–ferrite (γ–α) phase field was studied in low carbon steels (0.15 wt.%) at various strain rates. Flow softening was observed during the hot-deformation at temperatures lower than critical temperature. The critical temperature was observed to be significantly higher than T0 temperature. This was due to large shift of γ–α phase-equilibrium to a higher temperature during hot-deformation. A thermodynamic model to account the effect of non-uniform deformation-induced stress on the γ–α phase-equilibrium during hot-deformation was proposed and its prediction was in good agreement with the experimentally measured critical temperature.
The critical strain criterion εp = εx for the transition from cyclic to single peak recrystallization is demonstrated to be invalid for the high temperature deformation of f.c.c. metals in tension and compression. The role of the strain and strain rate gradients present in solid torsion bars in raising the apparent torsion peak strain εp above the εp values obtained from homogeneous tension or compression testing is clarified. A similar, and larger, effect is shown to cause discrepancies in the torsion values of the recrystallization strain εx. An alternative criterion for the transition is described, based on grain size considerations. The latter indicate that cyclic flow curves are associated with grain coarsening and that single peak flow curves are associated with grain refinement. The critical condition is D0 = 2Ds, where D0 and Ds are the initial and stable grain sizes respectively. The transition in flow curve shape under strain rate change conditions is also analyzed. It appears that after an increase in strain rate, the flow curve displays a single peak, whereas, after a strain rate decrease, multiple peaks are observed. The critical condition at which the shape of the stress-strain curve changes from the multiple to the single peak type is Ds1 = Ds2, where Ds1 and Ds2 are the stable dynamically recrystallized grain sizes before and after the change in strain rate, respectively. The results indicate that single peak behaviour is caused by the “necklace” or “cascade” recrystallization of coarse-grained materials, which produces a large spread in the nucleation strain εc, and accordingly a highly unsynchronized form of local recrystallization. The growth process (and consequently the grain size) in this case appears to be deformation limited. By contrast, recrystallization is nearly completely synchronized in fine-grained materials, because the high density of grain nuclei leads to a small spread in the nucleation strain. The grain size under these conditions is determined by impingement, and is thus nucleation not growth controlled. Finally, it is concluded that the interpretation given to the transition in flow curve shape by the relative grain size model, expressed in terms of the spread Δεc in nucleation strain εc, is in broad agreement with the one derived by earlier workers on the basis of computer simulations, and in the absence of grain size considerations.RésuméNous démontrons que le critère de la déformation critique εp = εx pour la transition entre les recristallisations cyclique et à pic unique n'est pas valable pour la déformation à haute température des métaux c.f.c. en traction et en compression. Nous clarifions le rôle des gradients de déformation et de vitesse de déformation présents dans barres de torsion solides, dans l'augmentation de la déformation apparente au pic de torsion εp au-dessus des valeurs εp obtenues pour des essais de traction ou de compression homogènes. Nous montrons qu'un effet analogue et plus grand provoque des différences dans les valeurs de la déformation de recristallisation en torsion εx. Nous présentons un autre critère pour la transition, critère qui repose sur des considérations concernant la taille des grains. Il montre que des courbes d'écoulement cycliques sont associées à un grossissement des grains et que des courbes d'écoulement à pic unique sont associées à un affinage des grains. La condition critique est D0 = 2Ds, oùD0 et Ds sont respectivement les tailles de grains initiale et stable. Nous avons également analysé la transition de la forme des courbes d'écoulement en fonction de variations de la vitesse de déformation. Lorsqu'on augmente la vitesse de déformation, la courbe d'écoulement présente un pic unique, alors qu'on observe des pics multiples après une diminution de la vitesse de déformation. La condition critique pour laquelle la forme des courbes contrainte-déformation passe du type à pics multiples au type à pic unique est: Ds1 = Ds2, où Ds1 et Ds2 sont les tailles des grains stables après recristallisation dynamique, respectivement avant et après le changement de la vitesse de déformation. Ces résultats montrent que le comportement à pic unique est provoqué par la recristallisation en “collier” ou en “cascade” des matériaux à gros grains, qui conduit à une grande dispersion dans la déformation de germination εc et donc à une forme très désynchronisée de recristallisation locale. Le processus de croissance (et par suite la taille des grains) est limité, dans ce cas, par la déformation. Au contraire, la recristallisation est pratiquement entièrement synchronisée dans les matériaux à grains fins, car la forte densité de germes de grains conduit à une faible dispersion dans les valeurs de la déformation de germination. La taille des grains est déterminée dans ces conditions par leur rencontre; elle est ainsi contrôlée par la germination et non par la croissance. Pour conclure, nous remarquons que l'interprétation que nous avançons pour la transition dans la forme des courbes d'écoulement à partir d'un modèle de taille relative des grains, exprimée par la dispersion Δεc de la déformation de germination εc, est en bon accord général avec celle qu'ont obtenue antérieurement d'autres auteurs à partir de simulations sur ordinateur et en l'absence de toute considération concernant la taille des grains.ZusammenfassungEs wird gezeigt, daß das Kriterium für die kritische Dehnung εp = εx für den Übergang der Rekristallisation mit zyklischem zum einfachen Maximum ungültig ist für den Fall der Hochtemperaturverformung von kfz. Metallen im Zug- und im Druckversuch. Außerdem wird die Rolle geklärt, die die Gradienten der Verformung und der Verformungsrate in massiven. Torsionsstäben für die Erhöhung der maximalen Torsionsdehnung εp über die εp-Werte für homogene Zug- oder Druckverformung hinaus haben. Ein ähnlicher, aber größerer Effekt führt zu Diskrepanzen in der Rekristallisationsdehnung εx in Torsion. Ein alternatives Kriterium, welches von der Korngröße ausgeht, wird für den Übergang beschrieben. Die Korngrößen zeigen, daß zyklische Fließkurven mit Kornvergröberung, Fließkurven mit einem Maximum mit Kornverfeinerung einhergehen. Die kritische Bedingung ist D0 = 2Ds (D0: anfängliche, Ds: stabile Korngröße). Der Übergang in der Form der Fließkurve nach einer änderung der Dehnungsrate wird ebenfalls untersucht. Es scheint, als ob nach einem Anstieg der Dehnungsrate die Fließkurve ein einzelnes Maximum aufweist, wohingegen nach Erniedrigung der Dehnungsrate mehrfache Maxima beobachtet werden. Die kritische Bedingung, bei der sich die Form der Verfestigungs-kurve vom Typ mit mehrfachem zum Typ mit einfachem Maximum ändert, ist Ds1 = Ds2 (Ds: stabile dynamisch rekristallisierte Korngröße vor (Ds1) und nach (Ds2 der Änderung in der Dehnungsrate). Die Ergebnisse legen nhe, daß das Verhalten mit einzelnem Maximum durch eine “Kaskaden-” oder “Ketten-” artige Rekristallisation grobkörnigen Materials verursacht wird. Ein solches Material weist eine weite Spanne in der Keimbildungsdehnung εc auf und hat dementsprechend ein lokal sehr wenig synchron ablaufendes Rekristallisationsverhalten. Der Wachstumsprozeß und folglich die Korngöße scheinen in diesem Falle verformungsbegrenzt zu sein. Dagegen ist die Rekristallisation in feinkörnigem Material nahezu vollständig synchronisiert, da die hohe Dichte an Keimen zu einer kleinen Spanne in der Nukleationsdehnung führt. Die Korngröße ist unter diesen Umständen bestimmt durch Aufeinandertreffen und daher keimungs- und nicht wachstumskontrolliert. Schließlich wird gezeigt, daB die Interpretation der änderungen in der Fließkurvenform mit dem Modell der relativen Korngrößen, ausgedrückt als als die Spanne Δεc in der Nukleationsdehnung εc in breiter übereinstimmung steht mit dem Modell, welches frühere Autoren auf der Basis von Rechnersimulationen ohne Rücksicht auf die Korngrößen abgeleitet hatten.
Two-dimensional cellular automaton modeling has been performed to investigate the mechanism of ferrite refinement during the dynamic strain-induced transformation (DSIT) from austenite (γ) to ferrite (α) in a low-carbon steel. The simulated results indicate that the refinement of ferrite grains derived from the DSIT could be interpreted in context of “un-saturated” nucleation and limited growth.
The strength characteristics of microphases in ultra-fine-grained steels were analyzed using nanoindentation and AFM. It was found that there were fine ferrite grains (m) formed by a strain-induced dynamic transformation in ultra-fine-grained steels. They had equiaxed and polygonal grain shape, and higher hardness and elastic modulus than coarse ferrite transformed statically. Strengthening factors of strain-induced dynamic transformation ferrite were analyzed in terms of cementite particles and dislocation density.
The post-dynamic transformation that takes place during the subsequent isothermal holding for the case when dynamic strain-induced transformation (DSIT) from austenite to ferrite occurs during hot deformation is investigated by cellular automaton modeling. The simulation provides a better understanding of carbon diffusion in retained austenite and the resulting microstructure evolution dur-ing the post-dynamic transformation. The predictions reveal that continuing transformation from retained austenite to ferrite and the reverse transformation can occur simultaneously in the same microstructure during post-deformation isothermal holding owing to the locally acting chemical equilibrium conditions. Competition between forward and reverse transformation exists during the early stage of post-dynamic heat treatment. It is also revealed that increasing the final strain of DSIT might promote the reverse transformation, whereas the continuous austenite-to-ferrite transformation yields a diminishing effect. The influence of the DSIT final strain on the grain size of ferrite and the characteristics of the resultant microstructure is also discussed.
In the processing of steel, the design of any kind of heat treatment and/or thermomechanical processing schedule, to obtain
a given microstructure, is greatly facilitated by the knowledge of the austenite-to-ferrite transformation characteristics.
In the past, isothermal and continuous cooling tests were used in the laboratory to create time-temperature-transformation
and continuous cooling transformation diagrams, respectively, which then served as the source of transformation data. The
problem with such information is that it is only truly applicable to one particular microstructure, usually one resulting
from a simple reheating cycle in the austenite region. Most industrial steel processing operations additionally involve several
stages of high-temperature deformation leading to changes in the microstructure emerging from the final pass. To account for
this situation, a novel laboratory method for the determination of the transformation characteristics, based on continuous
cooling deformation testing, was developed. A major attraction of this test technique is that the specific microstructure,
for which the transformation characteristics are required, can be generated by hot deformation and then immediately evaluated
by continuous cooling deformation. In this article, the basic continuous cooling deformation test technique and general methods
of data analysis are illustrated, using results from several different grades of steel.
The high temperature deformation of vacuum-melted iron and zone-refined iron has been studied in the temperature range 500°
to 800°C over a wide range of strain rates in torsion. Changes in stress-strain behavior and metallographic observations show
a transition in the dynamic restoration process from recovery at high stresses to recrystallization at low stresses. The results
are discussed in terms of a model for dynamic recrystallization. It is shown that the dependence of the critical strain for
the onset of recrystallization on experimental conditions is the most important factor in determining the deformation characteristics.
The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions
using a dilatometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations
based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferrite transformation start temperature
can be predicted from a nucleation model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at
austenite grain corners. The nuclei are assumed to have an equilibrium composition and a pillbox shape in accordance with
minimal interfacial energy. For higher cooling rates or larger austenite grain sizes, early growth has to be taken into account
to describe the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries.
In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium;i.e., only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However,
Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike
effect on the boundary movement. Thus, growth kinetics are controlled by carbon diffusion in austenite modified by interfacial
segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements.
The constitutive equations for the flow behavior of a commercial 0.34 pct C-1.5 pct Mn-0.7 pct Si-0.083 pct V-0.018 pct Ti
microalloyed steel were determined. For this purpose, uniaxial hot compression tests were carried out over a wide range of
strain rates (10−4 to 10 s−1) and temperatures (1123 to 1423 K). In combination with models developed in the literature, the experimental results permit
the flow stress of the present steel to be predicted within ± 5 pct. It is shown that the classical constitutive equations
must be modified to take the grain size into account, particularly when the latter is below 30 µm.
Ultrafine ferrite grain sizes were produced in a 0.11C-1.6Mn-0.2Si steel by torsion testing isothermally at 675 °C after air
cooling from 1250 °C. The ferrite was observed to form intragranularly beyond a von Mises equivalent tensile strain of approximately
0.7 to 0.8 and the number fraction of intragranular ferrite grains continued to increase as the strain level increased. Ferrite
nucleated to form parallel and closely spaced linear arrays or “rafts” of many discrete ultrafine ferrite grains. It is shown
that ferrite nucleates during deformation on defects developed within the austenite parallel to the macroscopic shear direction
(i.e., dynamic strain-induced transformation). A model austenitic Ni-30Fe alloy was used to study the substructure developed in
the austenite under similar test conditions as that used to induce intragranular ferrite in the steel. It is shown that the
most prevalent features developed during testing are microbands. It is proposed that high-energy jogged regions surrounding
intersecting microbands provide potential sites for ferrite nucleation at lower strains, while at higher strains, the walls
of the microbands may also act as nucleation sites.
A C–Mn–V steel was used to study ultrafine ferrite formation (1–3 μm) through dynamic strain-induced transformation (DSIT) using hot torsion experiments. A systematic study determined the critical strain for the start of DSIT (εC,DSIT), although this may not lead to a fully ultrafine microstructure. Therefore, the strain to produce an ultrafine ferrite (UFF) as final microstructure (εC,UFF) during deformation was also determined. In addition, the effect of thermomechanical parameters such as deformation temperature, prior austenite grain size, strain rate and cooling rate on εC,DSIT and εC,UFF has been evaluated. DSIT ferrite nucleated on prior austenite grain boundaries at an early stage of straining followed by intragranular nucleation at higher strains. The prior austenite grain size affected the distribution of DSIT ferrite nucleation sites at an early stage of transformation and the subsequent coarsening behaviour of the grain boundary and intragranular ferrite grains during post-deformation cooling. Also, εC,DSIT and εC,UFF increased with an increase in the prior austenite grain size and deformation temperature. The post-deformation cooling had a strong effect not only on εC,UFF but also the UFF microstructure (i.e. final ferrite grain size and second phase characteristics).
Effects of the Nb addition on the strain induced ferrite transformation just above Ar3 temperature were investigated. Hot compression tests were performed with varying the true strain up to 1.6 (80% reduction) using Gleeble 1500. After the hot deformation, samples were immediately water-quenched to examine ferrite formation characteristics. The grain boundary misorientation angles were measured by electron backscatter diffraction in order to observe evolution of the ferrite grains. For reheating temperatures such as 900 and 1000 °C, where Nb was mostly precipitated as NbC, strain induced ferrite grains of 1–2 μm were formed homogeneously within the austenite grain in Nb steel. In the cases of higher reheating temperatures 1100 and 1250 °C, where most of Nb was dissolved, the strain induced ferrite transformation was remarkably reduced and the ferrite morphology was changed to elongated grains. It was considered that the ferrite transformation during deformation was retarded by both the solute drag effect of Nb and the consumption of strain energy for the dynamic precipitation of NbC.
A recent OIM study of the substructure in hot compressed Al has observed an increase in the fraction of boundaries both of 15–20° and above 20° as strain rises from 0.9 to 1.5. This was interpreted as evidence of continuous dynamic recrystallization being the mechanism for the steady state deformation. However, when the original grain boundaries and transition boundaries between deformation bands are discounted, the fraction of 15–20° boundaries is reduced to less than 20% and would be much lower if subboundaries less than 0.5° visible in TEM were taken into account. The present authors argue that dynamic recovery maintains the subgrains of constant size, low misorientation and equiaxed to produce a steady state and can permit a limited number of discrete segments with higher misorientation notably as temperature falls. Moreover, continuous dynamic recrystallization is not appropriate terminology because it is far from reaching the completion observed in other instances of continuous recrystallization.
The onset of dynamic recrystallization is treated in terms of a model based on the principles of irreversible thermodynamics. In the analysis, the initiation of dynamic recrystallization is governed by both energetic and kinetic critical conditions. The former requires that the energy stored during a given deformation schedule attain its maximum, while the latter requires the dissipative processes associated with deformation to decelerate to a critical level. The kinetic critical condition leads to a minimum value of (θ is the conventional strain hardening rate) when the critical state is attained and to the appearance of an inflection point in the θ-σ curve. In a range where the strain rate sensitivity is constant, the onset of dynamic recrystallization corresponds to a constant value of Γ = ∂ ln . The initiation of dynamic recrystallization has much in common with the beginning of flow localization. These conclusions of the model are verified using experimental data obtained during the high temperature compression of nickel and of a type 305 austenitic stainless steel.
Deformation induced ferrite transformation (DIFT) is a kind of solid state transformation induced through deformation, which can be applied to be as the effective method to produce fine or ultrafine ferrite grains. This paper reviews the research progress in the theory and application of DIFT from five aspects: evidence and study methods, thermodynamics and kinetics, transformation mechanisms, factors influencing DIFT, application of DIFT in production of fine grained C–Mn steel and ultrafine-grained microalloyed steel.
- N Park
- A Shibata
- D Terada
- N Tsuji
N. Park, A. Shibata, D. Terada, N. Tsuji, Acta Mater. 61
(2013) 163.
- S C Hong
- S H Lim
- H S Hong
- K J Lee
- D H Shin
- K S Lee
S.C. Hong, S.H. Lim, H.S. Hong, K.J. Lee, D.H. Shin,
K.S. Lee, Mater. Sci. Eng. A 355 (2003) 241.
- M Tong
- D Li
- Y Li
- J Ni
- Y Zhang
M. Tong, D. Li, Y. Li, J. Ni, Y. Zhang, Metall. Mater.
Trans. A 35 (2004) 1565.
- X Sun
- H Luo
- H Dong
- Q Liu
- Y Weng
X. Sun, H. Luo, H. Dong, Q. Liu, Y. Weng, ISIJ Int. 48
(2008) 994.
- C Zheng
- D Raabe
- D Li
C. Zheng, D. Raabe, D. Li, Acta Mater. 60 (2012) 4768.
- R Pandi
- S Yue
R. Pandi, S. Yue, ISIJ Int. 34 (1994) 270.
- N Park
- A Shibata
- N Tsuji
N. Park, A. Shibata, N. Tsuji, Adv. Mater. Res. 409
(2012) 707.
- A Cingara
- H Mcqueen
- J Mater
A. Cingara, H. Mcqueen, J. Mater. Process. Technol. 36
(1992) 17.
- H Dong
- X Sun
H. Dong, X. Sun, Curr. Opin. Solid State Mater. Sci. 9
(2005) 269.
- R Wang
- T Lei
- Scripta Metall
R. Wang, T. Lei, Scripta Metall. Mater. 28 (1993) 725.
- J.-K Choi
- D.-H Seo
- J.-S Lee
- K.-K Um
- W.-Y Choo
J.-K. Choi, D.-H. Seo, J.-S. Lee, K.-K. Um, W.-Y. Choo,
ISIJ Int. 43 (2003) 746.
- H Yada
- C.-M Li
- H Yamagata
H. Yada, C.-M. Li, H. Yamagata, ISIJ Int. 40 (2000) 200.
- H Beladi
- G Kelly
- A Shokouhi
- P Hodgson
H. Beladi, G. Kelly, A. Shokouhi, P. Hodgson, Mater.
Sci. Eng. A 367 (2004) 152.
- Y Adachi
- P G Xu
- Y Tomota
Y. Adachi, P.G. Xu, Y. Tomota, ISIJ Int. 48 (2008) 1056.
- A Z Hanzaki
- R Pandi
- P D Hodgson
- S Yue
A.Z. Hanzaki, R. Pandi, P.D. Hodgson, S. Yue, Metall.
Trans. A 24 (1993) 2657.
- R Priestner
- A K Ibraheem
R. Priestner, A.K. Ibraheem, Mater. Sci. Technol. 16
(2000) 1267.
- J H Chung
- J K Park
- T H Kim
- K H Kim
- S Y Ok
J.H. Chung, J.K. Park, T.H. Kim, K.H. Kim, S.Y. Ok,
Mater. Sci. Eng. A 527 (2010) 5072.
- A Cingara
- H Mcqueen
A. Cingara, H. Mcqueen, J. Mater. Process. Technol. 36
(1992) 17.
- H Mcqueen
- W Blum
H. Mcqueen, W. Blum, Mater. Sci. Eng. A 290 (2000) 95.