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(A) Inverse pole figure map of 9XC grade steel after isothermal heat treatment. (B) Phase distribution map. Ferrite is marked in green, austenite is marked in red. (C) Phase distribution map (ferrite -white, austenite -gray), with the indicated K-S misorientation angles (43 • ±1) in red and N-W misorientation angles (46 • ±1) in blue. (D) Distribution of the misorientation angles. K-S, Kurdjumov-Sachs; N-W, Nishiyama-Wassermann.
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Both qualitative and quantitative analyses play a key role in the microstructural characterization of nanobainitic steels focused on their mechanical properties. This research demonstrates various methods of microstructure analysis using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron backscatter diffraction...
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Context 1
... determine the morphology of the presented structural components, quantitative EBSD analysis was performed based on the misorientation angles. It was identified that bainitic sheaves were characterized by different crystallographic orientations ( Figure 9A). Besides, the presence of both filmlike and blocky austenite was confirmed ( Figure 9B). ...
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... was identified that bainitic sheaves were characterized by different crystallographic orientations ( Figure 9A). Besides, the presence of both filmlike and blocky austenite was confirmed ( Figure 9B). However, it should be emphasized that the EBSD technique does not detect the strongly refined austenite films that were identified by TEM. ...
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... analysis of the misorientation angles enables quantitative evaluation of both the crystallographic orientations and the related structural constituents. It can be noticed that in the tested area, the high-angle boundaries (HABs) in the range of 42 • -47 • were distinctly dominant ( Figure 9D). In the case of nanobainitic steels, the misorientation angle of 46 • corresponds to the misorientation angle between bainitic ferrite and retained austenite, which is related to the ideal angle of the Nishiyama-Wassermann (N-W) OR -45.99 • [26]. ...
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... well-known K-S and N-W ORs define the typical relations between ferrite and austenite; therefore, in the case of bainite steels, they indicate the presence of bainitic ferrite and retained austenite. This was also confirmed on the phase distribution maps with the K-S and N-W ideal angles ( Figure 9C). It was also identified that the dominant orientation was the N-W OR (approximately 25%), rather than the K-S OR (approximately 16%). ...
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... The most intensive abrasion occurs in the process of band extrusion, where the production mass is mixed, homogenized, vented, and formed [7]. This process is currently realized in horizontal band plungers, which are made up of a two-shaft mixer and a pug mill [8,9]. The production mass processed by these machines is at a humidity level of 20%-25% and is compressed in the pug mill head under 2-10 MPa of pressure. ...
... Nanoscale microstructure, containing a mixture of very thin bainitic ferrite plates separated by carbonenriched austenite, are the main characteristics of the so-called nanobainitic steel family [7]. Królicka et al. [8] demonstrated that the microstructure of nanobainitic steels significantly influences the properties of the manufactured products. Currently, steel manufacturers and users of steel products have recognized the advantages resulting from obtaining this type of microstructure [9]. ...
Nanobainitic steels exhibit an exceptional combination of high strength, good plasticity, impact toughness, and wear resistance. They are suitable for the production of large mass components through the open-die forging process. Subsequently, the forgings are air-cooled. An obstacle of this method is the extended time required for the large forgings to undergo a bainitic transformation, making the industrial implementation of this process economically unjustifiable. Nevertheless, nanobainitic steels also allow for the open-die forging of small batches of structural elements with high property requirements. A technological limitation lies in the necessity of performing a series of operations, leading to a prolonged processing time dependent on the shape of the product and the degree of deformation. Therefore, inter-operational reheating is often necessary, incurring costs and time consumption. This is particularly relevant to forgings with a mass ranging from a few to several dozen kilograms, which, due to their low thermal capacity, rapidly dissipate heat to the surroundings and tools. Designing an economical process with a limited number of reheating cycles requires advanced knowledge of material behavior under thermo-mechanical deformation parameters, including boundary conditions where a significant decrease in plasticity occurs and the risk of crack initiation. To obtain this information, a comprehensive analysis of the influence of thermo-mechanical parameters applied during the deformation of nanobainitic steel at relatively low temperatures on the flow characteristics and crack formation was conducted. To achieve this goal, the Digital Image Correlation method, the finite element method modeling considering damage criteria, and the macrostructural evaluation of deformed specimens were employed.
... To this day, nanobain steels are the subject of intensive research aimed at improving their microstructure. For example, Królicka et al. [7] demonstrated that both qualitative and quantitative analyzes play a key role in the microstructural characterization of nanobainitic steels, with particular emphasis on their mechanical properties. Bainite is an aggregate of phases, consisting of plates of ferrite doped with minority phases such as carbides or residual [8], compared to many high-performance steels [9]. ...
The processing maps developed by dynamic material modelling (DMM) method are now widely used in the design of hot forming processes. However, this applies to those processes that are relatively fast or are carried out under isothermal conditions, when it is possible to maintain the deformation parameters within the processing window. In the case of multi-stage free forging, the temperature successively decreases during subsequent operations and is increased during inter-process reheating. Under such conditions, processing maps in direct form are not applicable. The proposed solution is to implement the data obtained by the DMM method into calculations carried out by the finite element method (FEM). This approach leads to obtain the distributions of DMM parameters in the volume of the feedstock at successive forging stages. Selected results of a combined DMM/FEM analysis of a multi-stage hot forging process of 80MnSi8-6 steel are presented. The starting data for the analysis were the flow curves of this material, determined from compression tests. The processing maps were developed and processing windows were determined. The results of the DMM analysis were verified by microstructure observations. Data from the DMM analysis were implemented into QForm software using LUA scripts. An integrated FEM/DMM numerical analysis of the process of a multi-step hot free forging of an example product was performed. The geometry of the tools and a sequence of operations were developed. The distributions of the DMM parameters and the hot deformation activation energy in the forging volume after successive forging sequences were analyzed.
... In time, together with the technical progress and the general need of building new railway lines according to the technical interoperability standards [2], a necessity occurred to standardize the steel grades used to make the railways as well as to determine standards for their production [3]. It should be mentioned that there is also research into bainitic steel for application in rails [4]. The European Standard for railway rails is given in two norms [5,6]. ...
The aim of this research is a systematic investigation of heat treatments of 60E1 profile rails made of steel R350HT, which would ensure the properties required by the standard EN 16273. Additionally, it presents a concept of cooling rails on a semi-industrial station, which will make it possible to obtain the desired properties. The dilatometric tests have demonstrated that the optimal cooling rate is within the scope of 3 °C/s to 6 °C/s, when both the EN 16273 standard’s hardness distribution and microstructure requirements are fulfilled. The tests on the designed and built station showed that the optimal pressure with respect to the microstructure and properties of the rail equals 6.5 bar. For these parameters, measurements of the interlamellar distance were also performed—the cooling rate obtained at the surface was 3.68 °C/s, with an interlamellar distance of about 80 nm, whereas inside the rail the rate was 2.63 °C/s and the distance 110 nm. The achieved results confirm that the designed station can be used for controlled cooling of rail steels from R350HT steel.
... This enforces a search for solutions that will enable the fulfilment of these objectives. One of the main directions in that area is the application of modern materials [1,2], advanced high-strength steels (AHSS) belonging to the most important ones [3,4], which are used in the automotive industry for elements of the bodywork responsible for its strength and energy consumption [5]. Their application makes it possible to reduce the mass of vehicle enforced by the external environmental requirements as well as the global competition [6]. ...
The study discusses the subject of a temperature change of TWIP steels during their deformation as a result of a conversion of the plastic deformation work into heat, based on a literature review and the authors' own research. The methods of measuring or modelling the changes of these temperatures are presented. It also points out that the heat generated during plastic deformation of TWIP steels has higher values than in the case of conventional steels, due to their higher yield stresses and limit strains. The heat has a very important effect on the microstructure of the deformed material and thus also on its properties. Its high increase can lead to e.g. a change of the deformation mechanism from twinning to dislocation glide, which is also connected with worse workability and thus also the energy consumption of the bodywork elements. On the basis of the selected literature works, the study determines the possible microstructural changes of TWIP steels related to the generated heat and demonstrates that materials with similar chemical compositions can behave differently at high deformation rates.
... Overall, the performed clean-up procedure did not exceed 5% of the measurement pixels. Parent grain diameters were measured based on EBSD and misorientation angles in the range from 20 to 40 [51]. The analysis concerned the average equivalent diameter of grains, where approx. ...
In response to the growing interest of the industry in advanced high-strength steels subjected to extreme operating conditions, two novel grades BainTS and BainNiAlCu bainitic steels have been developed. Since the in-use properties of bainitic steel are directly related to the thermal stability of the bainitic structure, two different alloy design concepts to improve tempering resistance considering various strengthening mechanisms were proposed. BainTS steel follows the standard assumptions of nanocrystalline bainitic steel design, where the retained austenite is stabilized with silicon. A potential secondary hardening was also considered using Cr, Mo, and V alloying additives. On the other hand, BainNiAlCu steel was designed taking into account the precipitation strengthening with intermetallic compounds (nickel aluminide and copper particles). It was revealed that both steels after heat treatment consist of bainitic ferrite laths and a significant content of retained austenite (above 25%) with film and blocky morphologies. After the tempering process (350 °C–650 °C), there was significant structural evolution following the gradual decomposition of the metastable bainitic structure. It was proved that BainTS steel is characterized by the higher thermal stability of both bainitic ferrite and retained austenite compared to BainNiAlCu. Despite this, the hardening effect after tempering BainNiAlCu steel was higher (by about 140 HV compared to heat treatment, approx. 30% increase). The presented results suggest that bainitic steels strengthened with intermetallic phases are extremely promising in the context of future industrialization in applications where operating conditions are exposed to elevated temperatures.
... To sum up, depending on the chemical composition of the steel and the parameters of hot galvanizing (surface preparation, time, temperature, etc.), metal products are covered with zinc coatings of certain thickness and properties in accordance with the applicable EN standards [24][25][26][27]. ...
The paper shows a significant influence of the multi-stage wire drawing technology on deformability and phase transformations in the zinc coating. SEM tests proved that the coating after hot-dip galvanizing consists of a number of thin layers, ranging from 1 to 5 µm, and differing in thickness, chemical composition and properties. When pulled through the drawing die the zinc coating heats up (as a result of friction between the material and the tool) and its dynamic plastic deformation. It resulted in the fracture and partial crushing of the hard-intermetallic phases. It has been proven that as the wire passes through successive drawing dies, the coating is thinned and diffusion as well as phase remodelling of individual structural components occurs; in the place of phase ζ, the intermetallic phase δ1 develops, increasing its share in the diffusion layer. The crystals of intermetallic phases located on the border of the diffusion and outer layers break up and remain dispersed in the zinc. An analysis of the microhardness of the coating has proven that the level of the increase in the microhardness of the zinc coating is contingent on percentage of iron in particular layers of coating.
... where L T is the mean intercept and 2/π is the stereological correction [25][26][27]. The same procedure has been used to calculate the thickness of RA for both specimens. ...
Prior austenite grain size (PAGS) can significantly impact the bainitic lath thickness formed during austempering of high carbon steels. This study examines the effect of two different PAGS on the microstructure development and in turn the tensile properties and fracture toughness of nano-structured bainitic steels. Two PAGS have been obtained by austenitizing at 900 °C and 1000 °C respectively but the bainite fraction has been maintained to be around the same range by austempering at 250 °C. Less amount of blocky austenite is formed when the PAGS is larger. It is also observed that with an increase in the PAGS, the thickness of bainitic lath and retained austenite (RA) decreases and the tensile strength, tensile ductility and the plane strain fracture toughness is enhanced. The fraction of RA transformed to martensite is less for larger PAGS during a tensile test while showing greater ductility owing to the greater stability of finer RA films.
... Where L T is the mean intercept length and 2/π is the stereological correction applied [36][37][38]. The average bainitic lath thickness has been reported in table 2. ...
Prior austenite grain size (PAGS) significantly affects the microstructure and in turn the mechanical properties of nano-structured bainite. In this study, information about crystallographic variants/blocks as well as packets has been extracted using electron backscattered diffraction (EBSD) technique for two blocks of nano-baintic steels with different PAGS obtained by austenitization at two different temperatures of 900°C and 1000°C respectively. However, the bainitic fraction has been maintained to be similar by choosing the same isothermal temperature of 250°C for transformation to bainite. The child-parent orientation relationship (OR) was determined to be close to Kurdjumov-Sachs (K-S) using EBSD for larger PAGS specimen while smaller PAGS specimen displayed closeness to K-S OR predominantly and Nishiyama-Wassermann (N-W) OR in some regions. It was observed that a higher PAGS leads to finer bainitic lath thickness, packet size and block width size. Stricter variant selection was found in smaller PAGS sample even though variant pairing was observed between variants of higher misorientation. This hints towards small number of nucleation sites for a grain and space constraint, rather than the tendency for pairing as the reason for selection.
