Recrystallization Of Austenite Research Articles

Electropulsing of low-density duplex steel for strengthening and ductilization by microstructural refinement

The present investigation finds the effect of electropulsing treatment on the mechanisms of refining and reprecipitation of ordered L20 structure (B2) and its impact on tensile properties of low-density duplex steel of Fe-18Mn-10.5Al-1 C-6Ni composition. The selected composition without manganese is melted at 1600°C by induction heating in a vacuum but Mn is added into the melt at 1560°C, in the argon atmosphere to reduce the loss. The melt is cast into a plate form in a copper mold. The cast steel is homogenized, hot rolled, and annealed (PD1-A sample) to get elongated bands, spheroids, and platelets of B2 phase with an austenitic matrix which results in a yield strength of 1015 MPa, and tensile strength of 1285 MPa with a plastic elongation of 16.3%. Electropulsing partially dissolves the B2 phase at a temperature much lower than the equilibrium solvus by reducing barrier energy because of electron wind. As a result, the sizes of bands and spheroids are reduced, and platelets are disintegrated. Electron wind force deforms coarse precipitates, and fragments as well as spheroidized it. Electropulsing of annealed steel (PD1-AE) mobilizes dislocations, and recovers it in B2, but induces localized recrystallization of austenite. At a later time of the signal, the temperature rapidly falls and the matrix becomes supersaturated but electropulsing accelerates low-temperature precipitation of the B2 phase. Therefore, electropulsing can be adopted as a fast manufacturing process to dissolve, deform, fragment, spheroidize and reprecipitate high temperature phase at much lower temperature by dominant athermal effect of electron wind energy over thermal effect. Electropulsing also induces recovery and recrystallization at a relatively low temperature and reduces defect density. Reduction in the overall size of B2 and its distribution towards better uniformity provided an increased yield strength of 1077 MPa, tensile strength of 1355 MPa, and a plastic elongation of 21.6%. The selected duplex low-density steel follows two-stage work hardening of Ludwigson model with an additional stage of dynamic recovery. Fractography analysis confirms that both samples show mixed modes of ductile and brittle fracture, with an increase in size and area percentage of dimples.

Research on the strengthening mechanism of Nb–Ti microalloyed ultra low carbon IF steel

Interstitial free (IF) steels are characterized by their interstitial free ferritic matrix due to addition of micro-alloying elements that draw carbon and nitrogen out of matrix to form carbonitrides. The IF steels typically have excellent deep drawing ability, certain mechanical strength, and high plastic deformation performance. In this paper, the microstructure evolution and carbides precipitation behavior during the hot rolling, cold rolling and annealing processes have been studied using both experimental and thermodynamic and kinetic modeling approaches. The strengthening mechanism of Nb–Ti microalloyed IF steel has been investigated. The results of the thermodynamic simulation indicates that the addition of Nb into Ti bearing IF steel promotes recrystallization at 900 °C, leading to fine recrystallized grains. Due to the recrystallization of austenite during the final rolling process, Nb–Ti microalloyed hot rolled plates exhibit finer grain size, fewer substructures, and lower dislocation density compared to the hot rolled plates containing only Ti. Building upon the initial fine and uniform structure, Nb–Ti microalloyed steel maintains a refined structure after undergoing cold rolling and annealing. The study of the carbides precipitation behavior suggests that TiN precipitates at high temperature above 1100 °C, while a substantial amount of (Nb,Ti)C and TiC precipitate at about 900 °C in Nb–Ti and Ti bearing steels, respectively. There is no significantly difference between the amount of carbides in steels hot deformed at 900 °C and in the hot rolled plates, indicating that the carbides mainly precipitate during the hot rolling process. There are negligible carbides precipitated during the coiling process at 600 °C. In Nb–Ti containing steel, Nb and Ti in solid solution are significantly reduced due to the large amount precipitation of (Nb,Ti)C during the final rolling process. The reduction of solid solution Nb effectively promotes the recrystallization of austenite. Therefore, the grains of hot-rolled, cold-rolled, and annealed strips are refined. In addition, there are more small size carbides in Nb–Ti bearing steel than that in Ti bearing steel. Therefore, the higher strength of Nb–Ti steel is mainly from the strengthening effect of grain refinement and carbides precipitation.

ТЕОРЕТИЧНІ ТА ТЕХНОЛОГІЧНІ ЗАСАДИ ЗНЕМІЦНЮЮЧОГО ТЕРМОМЕХАНІЧНОГО ОБРОБЛЕННЯ СТАЛЕВОГО ПРОКАТУ ЗВАРЮВАЛЬНОГО ПРИЗНАЧЕННЯ

The theoretical justification of the prerequisites for the plasticization of welding wire rod from alloy steels due to the management of the processes of structure formation during cooling of the metal after hot plastic deformation is given. It is shown that the processes of metadynamic and static recrystallization of austenite, which occur after dynamic recrystallization and static polygonization, are decisive in the softening schemes of thermomechanical treatment of wire rod. The softening effect during thermomechanical treatment is achieved by adjusting the size of the dynamically recrystallized austenite grain through accelerated water cooling of the metal from the temperature of finish hot deformation to the temperature of the folding out of the wire rod into coils before the start of the air-cooling stage on the "Stelmore" line. The transformation of fine-grained austenite begins at higher temperatures compared to coarse-grained and is accompanied by the formation of a larger fraction of structurally free ferrite and a decrease in the fraction of bainite and martensite during the subsequent slow cooling on the conveyor under heat-insulating covers. The structural features of the "Stelmor" line, necessary for the effective implementation of the process of softening thermomechanical treatment of welding wire rod, are considered.

Influence of the Holding Thickness and Process in the Microstructure and Mechanical Properties of a Microalloyed 380 MPa Structural Steel

Flat low carbon microalloyed structural steels processed by controlled rolling have excellent mechanical properties, particularly regarding toughness, since austenite is rolled with full recrystallization between rolling passes (roughing stage at high temperatures), and, subsequently, with no recrystallization at all between passes (finishing stage at low temperatures), maximizing nucleation of ferrite grains during austenite transformation after hot rolling. However, rolling must be prompted in an intermediate temperature range where recrystallization of austenite between rolling passes occurs partially, as this can lead to great heterogeneity in the grain size distribution of the product, a potential condition to impairs its low temperature toughness. On the other hand, this holding period that is necessary to avoid rolling within this temperature range can last several minutes, reducing the productivity of the rolling line and, in this way, potentially harming the financial performance of the plant. This work was developed to analyze several proposed rolling conditions for the finishing stage during controlled rolling of 16 mm thick strips of a low C Nb-Ti microalloyed steel strip, 380 MPa structural grade, processed in a Steckel mill, to identify the best condition that provides maximum productivity without affecting the required microstructural, toughness and mechanical characteristics of the final product.

Revealing the precipitation kinetics of multi-stage and multi-scale Ti-bearing precipitation in a 460 MPa grade HSLA steel

The multi-stage and multi-scale Ti-bearing precipitates, containing TiN, Ti(C, N), Ti4C2S2 and TiC, were systematically examined through experimental observations and our modified precipitation kinetic analyses. The result showed that cubic TiN (several microns) mainly precipitated in molten steel. After continuous casting, Ti(C, N) and Ti4C2S2 primarily precipitated in austenite, spanning 100–1000 nm in size. Upon subjected to severe rough rolling deformation, further precipitation of Ti(C, N) and Ti4C2S2 (tens of nanometers) appeared, accompanying with austenite recrystallization. During finishing rolling, recrystallized austenite grains were subdivided into multiple microbands, leading to an accumulation of deformation stored energy (DSE). The strain-induced precipitation (SIP) of spherical TiC particles (10–20 nm) distributed along microband walls, prior austenite grain boundaries, and even within the microbands. Subsequently, sequential matrix transformations, specifically ferrite (γ → α) and pearlite (γ → α + θ), were initiated during coiling. High-density TiC interphase precipitation occurred during the γ → α transformation, while a more dispersed, low-density TiC precipitation was observed after the γ → α + θ transformation. Kinetic analyses revealed that the nose temperature for Ti(C, N) precipitation in austenite was a minimum of 200 °C higher than that of Ti4C2S2. The DSE accelerated the TiC SIP with higher nose temperature in austenite during hot rolling. Moreover, kinetic analyses also implied that the migrating γ/α interface promoted faster TiC nucleation and growth relative to dislocation nucleation, and the relative precipitation time shortened more than 20 orders of magnitude. Furthermore, both the γ/α interface migrating velocity and the interface carbon content exhibited weak influence on the kinetics of interphase precipitation. However, the pearlite formation reduced the driving force of TiC precipitation, resulting in the TiC dispersed precipitation rather than TiC interphase precipitation.

Influence of vanadium, niobium and boron on kinetics of austenite recrystallization of steels with different strength levels under hot deformation conditions

Summarized results of research of the dynamic and static austenite recrystallization kinetics of low-carbon low-alloy and alloyed steels of strength classes 420, 620, 690, 750 and 890 and medium-carbon steels of strength class 1700 containing different amounts of vanadium, niobium and boron are presented. Studies were carried out by the plasmetric method under deformation conditions close to hot rolling. It was found that vanadium has a weak effect on recrystallization, and niobium in all the studied steels significantly slows it down in the hot rolling temperature range, regardless of the total doping level; microalloying of steels with boron leads to acceleration of austenite recrystallization.

In-situ fabrication of repairing layers for large structures using follow-up hot-hammering-assisted wire arc additive manufacturing

The development of a spherical hammer head follow-up hot-hammering-assisted (FH) hybrid wire arc additive manufacturing enables the application of the hammering-assisted process to the in-situ repair of large-scale structural parts. This process allows for large deformation in the deposited weld bead with lower energy input at higher temperatures, resulting in less work hardening. A thermal, mechanical and plastic deformation coupled finite element model is established to analyze the plastic strain distribution with different shapes of hammer heads and different deformation history. Compared with the flat hammer head, the spherical hammer head is more likely to produce large plastic deformation in deposited weld bead under the same force and energy conditions, and obtain more uniform microstructure. Different hammering temperatures higher than austenite recrystallization temperature with the spherical hammer head, are selected to study the effect on grain refinement of repairing layers. The OM, XRD, SEM and EBSD are used for grain, texture evolution and strengthening mechanism analysis. The results show that by introducing the spherical hammer head FH process at 980 °C slightly higher than the austenite recrystallization temperature, the grain size decreases gradually from the bottom repairing layer to the top one, showing a genetic evolution law. The topmost repairing layer grain size is refined from 353 μm to 30 μm, the average grain size of repairing layers is refined from 361 μm to 62 μm, and the strong {100} texture of the topmost repairing layer is reduced to be approximately random. The grain refinement is attributed to the combined influence of recrystallization and co-generation crystallization. Meanwhile, the yield strength is significantly improved, and the main strengthening mechanisms are grain refinement strengthening and dislocation strengthening, and the contributions are 42.0 MPa and 101.4 MPa.

Nucleation Sites in the Static Recrystallization of a Hot-Deformed Ni-30 Pct Fe Austenite Model Alloy

In the present study, the nucleation of static recrystallization (SRX) in austenite after hot deformation is experimentally analyzed using a Ni-30 pct Fe model alloy. In agreement with the predictions by current models, nucleation rate exhibits a strong peak, early during SRX. Whereas such an early peak is explained by current models by the saturation of nucleation sites, this condition is far from reached, even after the peak declines. In addition, triple-junction and grain-boundary sites are shown to make a quantitatively similar contribution to nucleation. However, for a given boundary between deformed grains, nucleation predominantly starts at one of the triple junctions. Triple-junction nucleation initiates by strain-induced boundary migration of the nucleus (bulging) along one of the boundaries at the junction. Annealing twin boundaries contribute negligibly to nucleation through their grain-boundary sites. By contrast, their junctions with the boundaries of the parent grains do play a relevant role. The earlier nucleation at the triple junctions is attributed to the higher dislocation density observed around them, and the energy of the boundary consumed by the bulge. Both the maximum and average number of nuclei formed per boundary between deformed grains increase with increasing boundary length.

Simulation of hot deformation of cast super duplex corrosion-resistant steel

The deformation behavior of cast super duplex steel has been studied in the temperature range 1100–1250 °C and strain rates 0.1–10 s–1. Hot deformation was carried out by uniaxial compression of cylindrical samples on a Gleeble 3800 thermomechanical simulator. It is shown that the flow stresses decrease with an increase in temperature and a decrease in the strain rate in accordance with a change in the Zener‒Hollomon parameter of the temperature-speed mode of deformation. The shape of the flow curves indicates that hot deformation is accompanied by intense dy-namic softening. As a result, the flow stresses experience a drop or remain at a constant level after reaching peak values. Ferrite in all studied modes of hot deformation acquires a dynamically recrystallized structure. At the lowest deformation temperature (1100 °C) and relatively high deformation rates (1–10 s–1), the austenite softening mechanism is dynamic recovery. A decrease in the strain rate, as well as an increase in the deformation temperature, causes a partial dynamic recrystallization of austenite. Under the same deformation conditions, the level of plastic flow stresses in the steel under study is significantly higher than in standard duplex stainless steels. From the analysis of the peak stresses of the flow, the value of the effective activation energy Q = 501.31 kJ/mol of hot deformation, which is necessary for calculating the Zener‒Hollomon parameter, was determined. An expression for the peak flow stress is obtained in the form of a hyperbolic function of the Zener‒Hollomon parameter. The resulting expression describes the array of experimental data with high accuracy and can be used to estimate the required energy-power parameters of forging and rolling equipment. A comparative assessment of the hot ductility of super duplex steel is carried out by finding the amount of deformation corresponding to the appearance of the first macroscopic cracks on the sample surface. It is shown that at a strain rate of 10 s–1, which is typical for hot stamping processes, the safest temperature range of deformation of the steel under study is the temperature range of 1150–1250 °C, in which austenite undergoes partial dynamic recrystallization, and the risk of crack formation decreases.

Microstructure evolution and static recrystallization kinetics in hot-deformed austenite of coarse-grained Mo-free and Mo containing low-carbon CrNiMnB ultrahigh-strength steels

The static recrystallization characteristics and microstructure evolution in hot-deformed austenite were evaluated for a newly developed low-carbon CrNiMnB ultrahigh-strength steel with and without molybdenum addition. The time for 50% static recrystallization (t 50% ) over a wide range of strains and hot-deformation temperatures were obtained using the stress-relaxation technique on Gleeble thermomechanical simulator. Moreover, effect of deformation parameters on the size distribution and average size of prior austenite grains are investigated. A novel semi-automatic stress relaxation test reading tool with a graphical user interface was created and used successfully for the current study. The obtained results of strain´s power and the apparent activation energy are within the range stated in literature for C-Mn and microalloyed steels. Addition of molybdenum increase the power of strain and the apparent activation energy from -1.9 to -2.6 and 206 to 212 kJ/mol, respectively. The retardation effect of molybdenum addition was shown by a new regression equation devised for calculating t 50% . The developed equations show a good agreement with the experimental data and can be used in the designing of roughing during thermomechanical processing. The deformation parameters i.e., temperature, strain and holding time have a significant effect on the size distribution and average size of prior austenite grains. • The static recrystallization characteristics and microstructure evolution in hot-deformed austenite were evaluated for Mo-free and Mo-containing ultrahigh-strength steels. • A novel semi-automatic stress relaxation test reading tool with a graphical user interface is created to overcome the difficulties in determination of the onset of static recrystallization manually from stress relaxation curves. • SRX kinetic equations are derived for explaining static recrystallization in hot-deformed austenite of Mo-free and Mo-containing ultrahigh-strength steels. • Quantitative analysis of microstructural feature during SRX is performed.

Microstructure Evolution and Mechanical Properties of Ferrite-Austenite Duplex Fe-Mn-Al-(Cu)-C Steel under Different Annealing Temperatures.

The effect of Cu addition and the intercritical annealing (IA) temperature on the microstructural evolution and mechanical properties of Fe-0.4C-7Mn-4Al (wt%) was investigated via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and nanoindentation tests. The results showed that the volume fraction and the average grain size of austenite, and the fraction of high angle grain boundaries, increased with IA temperature increase in the range of 650 °C to 710 °C. The addition of Cu facilitates the formation of Cu-rich nanoparticles, raises the volume fraction of austenite, and delays the recrystallization of austenite. As IA temperature increased, the yield strength (YS), ultimate tensile strength (UTS), and Lüders bands strain (LBS) decreased in both experimental steels. The Cu addition not only increases the YS of medium Mn steel but also benefits the decrease of LBS. The best comprehensive mechanical properties were obtained at the IA temperature of 690 °C in the studied steel, with Cu addition. According to nanoindentation experiments, the Cu addition raises the hardness of ferrite and austenite from 4.7 GPa to 6.3 GPa and 7.4 GPa to 8.5 GPa, respectively, contributing to the increase of YS of medium-Mn steel.

The coupling machine learning for microstructural evolution and rolling force during hot strip rolling of steels

Microstructural evolution during hot rolling, which is complex and unperceptive but has direct effects on rolling forces, determines the final microstructure and mechanical properties of steel products. In this paper, a comprehensive set of machine learning (ML) models was developed through rolling forces to reveal the evolutions of recrystallization and grain size of austenite, in which the grain size was estimated by considering the grain shape effect after each rolling pass. By using the backpropagation neural network (BPNN) and genetic algorithm (GA) combined with industrial data, the quantitative connections of parameters in the models for static recrystallization (SRX), meta-dynamic recrystallization (MDRX), and mean flow stress (MFS) were established with steel chemical compositions and deformation conditions. Through the machine learning (ML) approach, softening kinetics and rolling forces can be more accurately predicted than traditional models. Also, the recrystallization behavior and evolution of austenite grain size during hot rolling were analyzed by using the proposed ML models, which are consistent with experimental results.

Microstructure evolution and parameters optimization of follow-up hammering-assisted hybrid wire arc additive manufacturing

Coarse columnar grains are the typical microstructure of wire arc additive manufacturing, resulting in the poor mechanical properties. A follow-up hammering-assisted (FH) hybrid wire arc additive manufacturing process is proposed to improve the microstructure of deposited weld bead. The effects of different hammering temperatures (higher than Ae3 line) and different deformation amount on the microstructure are systematically studied, and the effect on mechanical properties is investigated correspondingly by fabricating a deposited block. The Electron Back Scatter Diffraction (EBSD), Scanning Electron Microscope (SEM), and Optical Microscopy (OM) are used for characterization. The results show that the FH process can significantly refine the grain size of deposited weld bead, and meanwhile weaken the texture strength. The grain refinement mechanism for hammering higher than the austenite recrystallization temperature is austenite dynamic recrystallization. Instead, for hammering lower than the austenite recrystallization temperature, the more significant grain refinement is attributed to the high-temperature deformation increasing the ferrite phase transformation nucleation site, nucleation rate and the deformation-induced ferrite phase transformation. Hammering temperature 850 °C and hammering deformation 40% are determined to be the optimal process parameters to refine the grains. Using the optimal process parameters, the strength and hardness of the deposited layers are greatly promoted, and the anisotropy is also improved, while the ductility is reduced to some degree, but improved obviously compared to the low-temperature hammering.

Effect of deformation amount distribution in the austenite recrystallization temperature region on the microstructure and mechanical properties of the X70 pipeline steel

ABSTRACT In this study, the effect of deformation amount distribution of X70 pipeline steel on the microstructure and mechanical properties in the austenite recrystallization temperature region are investigated. The results reveal that the austenite grain size is smaller and more uniform by increasing the last-pass rough rolling deformation amount in the austenite recrystallization region. An increase in the last-pass rough-rolling deformation amount can also improve the strength and low-temperature toughness. The deformation mechanism and microstructural evolution based on the last-pass in the austenite recrystallization temperature region are clarified. The final microstructure refinement effect of granular bainite and acicular ferrite attribute to the finer and more uniform original austenite grain size. The main strengthening mechanisms are grain boundary strengthening and dislocation strengthening. The high proportion of the high-angle grain boundary is significantly greater for improving the strength and toughness, which impedes dislocations to slip across grain boundaries.

Investigation of isothermal continuous hot deformation behavior of the new as-cast alumina-forming austenitic alloy

Multi-pass isothermal continuous hot deformation experiments were designed for homogenization and recrystallization in the as-cast alumina-forming austenite (AFA) heat-resistant alloy. Flow curves and microstructure characterization were used to study the grain evolution with different deformation parameters in each pass. In particular, attention was paid to the dendrites and segregated phase. Under different hot deformation conditions, the alloys show significant work-hardening characteristics, which result from the strengthening effect of the primary segregated MC-type carbide. The increase in deformation temperature contributes significantly to dynamic recrystallization (DRX), and the main mechanism of DRX is discontinuous DRX (DDRX). The continuous DRX (CDRX) is less occurring. Static recrystallization (SRX) occurs during the isothermal holding time between deformation passes. SRX becomes more and more apparent with temperature and the holding time. The increase in the deformation passes leads to a gradual disappearance of the alloy dendrites and an increase in the recrystallization volume fraction. Meanwhile, the segregated phase gradually decomposes into particles and partially dissolves. However, the segregated phase particles eventually show band-like distribution. SRX grains around the segregated phase will exhibit similar orientations due to particle-stimulated nucleation (PSN) mechanisms. Until the grain size increases to the point that it can escape from particle restraint, it will transform to random orientation. The above study confirms that multi-pass isothermal continuous hot deformation can improve the as-cast microstructure and guide practical industrial production.

Hot Working Behavior in Multiphase Steel with Ti and V.

This study investigated the effect of hot working conditions on changes in yield stress and the softening degree in the newly developed multiphase steel with Ti and V microadditions. The research was performed on the GLEEBLE 3800 thermomechanical simulator. In order to determine the σ-ε curves, continuous compression tests were carried out. The samples were plastically deformed at temperatures from 900 °C to 1100 °C at the rate of 0.1 s−1, 1 s−1 and 10 s−1. The activation energy of the plastic deformation was 375 kJ·mol−1. The analysis of the shape and course of the curves indicated that the decrease in strain hardening was mainly the result of the continuous dynamic recrystallization process. Two-stage compression with isothermal holding of the samples was also carried out between the two stages of deformation lasting from 1 s to 50 s. The structure of primary austenite was generated using the ARPGE software. The different size of austenite grain is the result of various thermally activated processes—when increasing the strain rate from 0.1 s−1 to 10 s−1, the average grain size of the primary austenite decreases from approx. 16 µm to approx. 6 µm. The time t0.5 needed to form 50% of the austenite fraction recrystallized at 1100 °C is approx. 4 s and extends to approx. 10 s with the reduction in the plastic deformation temperature to 900 °C. The time of complete austenite recrystallization tR, which varies from approx. 50 s to approx. 90 s in the tested temperature range, lengthens even more. The obtained results make it possible to develop thermomechanical treatment technology for the production of forgings from the tested multiphase steel.

Effect of Niobium Supersaturation in Austenite on the Static Recrystallization Behavior of Carbon Structural Steels

This work describes the effect of Nb supersaturation in austenite on the suppression of static recrystallization of austenite during an isothermal holding period following hot deformation. The investigation involved three carbon structural steels with varying Nb concentration at constant C (0.20 pct) and N (0.007 pct) levels. The isothermal double-hit deformation technique led to the determination of T5 pct and T95 pct (recrystallization-stop and full recrystallization temperatures, respectively) as a function of a true strain and interpass time. The results indicate that the T5 pct increases with increasing Nb supersaturation in austenite at a rate of 40 °C per 0.006 pct Nb supersaturation for a true stain ε=0.40. At each respective T5 pct, all tested steels exhibited an Nb supersaturation ratio ≥ 7.5 in austenite. A high, localized strain-induced precipitation of Nb(CN) was observed at the austenite subgrain boundaries in the unrecrystallized microstructure. This translated into higher values for local precipitate-pinning forces (FPIN), which were significantly higher than that predicted from equilibrium thermodynamics. The critical FPIN for retardation of static recrystallization was found to be 1.6 MPa at the respective T5 pct for each steel. The present study has contributed to advancing our knowledge of the interplay between Nb solute supersaturation and volume fraction of Nb(CN) precipitation in particular for carbon structural steels. It has also highlighted an opportunity to apply niobium, even an ultra-low addition (i.e., < 100 ppm) to commodity-grade structural steels to reduce overall alloying costs.

Abnormal effect of strain rate on dynamic recrystallization of austenite in medium carbon steel alloyed by boron

According to literature data confirmed by our experiments on various steels, the strain degree that triggers the dynamic recrystallization of austenite in high-temperature deformation increases with the strain rate. However, as shown in the present work, micro-alloying of medium carbon steel by boron results in the opposite dependence in the strain rate range of 1 to 100 s−1. The corresponding features of stress-strain diagrams comply with the former austenite structures revealed by chemical etching. A specific effect of boron is discussed with allowance for its segregations at grain boundaries where the recrystallization embryos form. Such segregations hinder rearrangement of atoms involved in the grain boundary migration and hence decelerate their bulging necessary to nucleate recrystallization origins. Similar to the known “yield tooth” effect, very strong strain hardening of the steel with boron eventually enables the boundaries to free from the segregated atoms and begin the dynamic recrystallization at relatively low strain degrees. The obtained results indicate that the considered phenomenon is not due to the ultimate strain rate as such but results from its combination with the boron alloying. The revealed regularities gain in significance for evident reasons. First, they enhance ductility of high-strength steels hot deformed with very high strain rates as, for instance, in case of hammering. Second, such treatments notably refine the austenite structure and, correspondingly, improve the transformed structure and final mechanical properties after steel quenching.

Effect of Nb and Mo on Austenite Microstructural Evolution During Hot Deformation in Boron High Strength Steels

This work has focused on the study of hot working behavior of boron high strength steels microalloyed with different combinations of Nb and/or Mo. The role of Nb and Mo during the hot deformation of low carbon steels is well known: both mainly retard austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, the design of rolling schedules resulting in properly conditioned microstructures, requires microstructural evolution models that take into account the effect of the different alloying elements. In this specific case, the effect that high levels of molybdenum (0.5 pct) have in the recrystallization delay was evaluated. In that respect, hot torsion tests were performed in this work to investigate the microstructural evolution during hot deformation of four boron steels, with different Nb (0.025 pct) and Mo (0.5 pct) combinations. The retardation in recrystallization kinetics was modeled in all cases and measured kinetics agree with those predicted by equations previously developed for Nb–Mo microalloyed steels with lower Mo concentrations (< 0.3 pct). The strain-induced precipitation in the Nb and Nb–Mo bearing steels was also characterized. Finally, the fractional softening evolution during multipass rolling simulations was compared with MicroSim® model predictions, showing a good agreement with experimental results.

Effect the hot deformation on austenite structure of the carbon steel

Purpose. Determination influence temperature and degree of hot plastic deformation carbon steel on the development secondary recrystallization of the austenite. M ethodology. The carbon steels with a concentration of carbon by 0.55 and 0.65%, which correspond to the maximum and minimum values within the brand composition for all-rolled railway wheels, were used as material for the research. The number of other chemical elements met requirements of the documentation for railway wheels. The samples for research had the form of cylinders with a diameter of 20 and a height of 40 mm. Different degrees of plastic deformation were obtained during compression with a deformation rate of 10-3 s-1. Chamber-type electric heating furnaces were used to heat the samples to temperatures of the hot pressing. When heating samples, measures are taken to prevent oxidation and a local decrease concentration of the carbon atoms on their surfaces. After heat treatment of the samples and mechanical preparation of the surface, detection of the structure of austenite was carried out using of the etching based on a mixture of solutions hydrochloric and picric acids. The structure of the steels was investigated under a light microscope. The size of the austenite grain was determined by the methods of quantitative metallographic. Findings. The study structure of the carbon steel determined that under conditions of increasing the annealing temperature, the occurrence of structural heterogeneity of the austenite is caused by a change at mechanism of the development secondary recrystallization. Starting from 10% of hot pressing, in proportion to the degree of plastic deformation, development secondary recrystallization by the mechanism of movement of the large-angle boundaries leads to grinding of the austenite grain and the formation of a homogeneous structure. At degrees of hot deformation less than 10%, the density of dislocations is not enough to form a nucleus for the development secondary recrystallization by the mechanism of movement of the large-angle boundaries. As a result, recombination of the dislocations in grains of hot deformed austenite will occur much faster, which will lead to the separation of grains into fragments. Their size will be determined by the density of dislocations introduced during hot pressing. Disorientation angles between fragments have intermediate values between boundaries with large disorientation angles and subboundaries, which determines their low mobility. For such a structure, development of the secondary recrystallization in austenite will occur by the mechanism of grain boundary dissolution. As a result, there is significant heterogeneity structure of the austenite, which is determined by the difference in sizes between neighboring grains up to several times. Originality. Depending on the conditions of hot pressing of the carbon steel, the value of plastic deformation is determined, which separates development the secondary recrystallization of the austenite by qualitatively different mechanisms. At a low value of hot deformation, when the density of dislocations is not sufficient for the formation of a nucleus of the secondary recrystallization by the mechanism movement of the large-angle boundaries, the growth austenite grains occurs by the mechanism of dissolution of the grain boundaries. Practical value. Determining effect temperature and degree of hot plastic deformation on the mechanism of the development secondary recrystallization will allow to the optimize tenology crimping the blank of the all-rolled railway wheel.