Austenite Microstructure Evolution Research Articles

Physical and Numerical Simulations for Predicting Distribution of Microstructural Features during Thermomechanical Processing of Steels.

The design of modern construction materials with heterogeneous microstructures requires a numerical model that can predict the distribution of microstructural features instead of average values. The accuracy and reliability of such models depend on the proper identification of the coefficients for a particular material. This work was motivated by the need for advanced experimental data to identify stochastic material models. Extensive experiments were performed to supply data to identify a model of austenite microstructure evolution in steels during hot deformation and during the interpass times between deformations. Two sets of tests were performed. The first set involved hot compressions with a nominal strain of 1. The second set involved hot compressions with lower nominal strains, followed by holding at the deformation temperature for different times. Histograms of austenite grain size after each test were measured and used in the identification procedure. The stochastic model, which was developed elsewhere, was identified. Inverse analysis with the objective function based on the distance between the measured and calculated histograms was applied. Validation of the model was performed for the experiments, which were not used in the identification. The distance between the measured and calculated histograms was determined for each test using the Bhattacharyya metric and very low values were obtained. As a case study, the model with the optimal coefficients was applied to the simulation of the selected industrial hot-forming process.

Modeling of the Dynamic Recrystallization of Ti Microalloyed High‐Strength Steel

The dynamic recrystallization (DRX) and austenite microstructure evolution of Ti microalloyed high‐strength steel are investigated and modeled by thermal simulation experiments. The corresponding mathematical models are established based on DRX‐type flow stress curves and the austenite microstructure. A highly accurate austenite grain growth model under different heating conditions is established, which can predict the initial austenite grain sizes before deformation. Considering the effect of initial austenite grain sizes and deformation conditions, the key parameters (peak stress, steady stress, peak strain, and constants C and C1) of the DRX‐type flow stress model are determined, and the predicted DRX‐type flow stresses are in good agreement with the experimental values. The relationships between critical strain and peak strain(εc = 0.77εp), critical stress, and peak stress(σc = 0.82σp) are determined for experimental steel. The relationships between the initial austenite grain size and the key parameters (k, M, a, h) in the DRX kinetic model and the DRX grain size model are determined. The calculated and experimental DRX volume fractions and grain sizes are in good agreement. The results show that these mathematical models can accurately describe the DRX behavior of experimental steel.

Microstructure evolution during continuous cooling process in Ti microalloyed steel

The phase transformation law and microstructure evolution of austenite in Ti microalloyed steel under different continuous cooling rates (0.1-30°C/s) after a two-stage deformation were studied based on the Gleeble-3800 Thermo-Mechanical Simulator. The continuous cooling transformation (CCT) curve was obtained by thermal expansion method combining with microstructures. The results showed that the Ac1 and Ac3 of the steel were 835°C and 902°C, respectively. With the increase of cooling rate, the austenite phase transition temperature decreased, from 778.5-712.2°C at 0.1°C/s to 633.7-488.7°C/s at 30°C/s. Accordingly, the austenite transformation structure changed from polygonal ferrite and pearlite to granular bainite. In addition, with the increase of cooling rate, the ferrite transformation was inhibited, and the transformation region of granular bainite was increased.

A comparative study on the evolutions of thermal and mechanical parameters and microstructure of 30Si2MnCrMoVE steel during hot ACDR and upsetting deformation

Axial closed-die rolling (ACDR), as a new type of local loading and labor-saving forming method, has a broad application prospect in the production of large-scale rotary parts, compared with traditional upsetting forming. However, the different evolutions of thermal and mechanical parameters and microstructure between the two forming modes are still ambiguous. In this study, the thermal and mechanical parameters and microstructure evolutions of 30Si2MnCrMoVE UHSS after ACDR and upsetting deformation were studied by means of numerical simulation, metallographic observation, and scanning electron microscope (SEM). The temperature and effective strain evolution of the upper surface, central axis, and middle height (1/2h) of the workpiece were analyzed by numerical simulation. Combined with the evolution of temperature and equivalent strain, the mechanisms of original austenite microstructure evolution of the two deformation modes were analyzed. The results show that there is only one deformation dead zone at the lower end of the workpiece after ACDR deformation which is different from upsetting. The deformation temperature is the primary reason for the difference of microstructure evolution at 1/2h between ACDR and upsetting deformation. The microhardness at 1/2h of the sample after ACDR deformation and direct quenching is lower than that of upsetting.

Through-Scale Analysis of Strain Path Effects in Microalloyed Austenite Subjected to Reverse Rolling

Thermomechanical hot rolling processes are often realized using reverse rolling stands, where the rolled stock is fed forward and backward through the rolling gap. During those processes, material undergoes several strain reversals that significantly alter microstructure evolution of austenite with respect to continuously rolled counterparts. In Nb-microalloyed steels, where precipitation hardening is usually expected, the effects of strain reversal are especially complex. When rolling direction is reversed, both static recrystallization (SRX) kinetics and strain-induced precipitation (SIP) processes are slowed down due to decreasing dislocation density. It affects the competition between driving force for SRX and pinning pressure for SIP and, in turn, changes the non-recrystallization temperature (Tnr), compared to the case where strain path is linear. In the present paper, detailed through-scale analysis of strain path effects in microalloyed austenite will be presented. Physical simulation and detailed microstructural analysis will be employed to study global and local effects in microalloyed austenite after complex deformation histories. Conclusions regarding the influence of strain path changes on the interactions between SRX and SIP will be drawn.

Dynamic Restoration Processes in a 23Cr-6Ni-3Mo Duplex Stainless Steel: Effect of Austenite Morphology and Interface Characteristics

The austenite and ferrite microstructure evolution and restoration mechanisms were studied during hot uniaxial compression of a 23Cr-6Ni-3Mo duplex stainless steel with two markedly different austenite morphologies (i.e., equiaxed and Widmanstatten). The deformation was performed at a temperature of 1273 K (1000 °C) at a strain rate of 0.1 s−1. The strain was preferentially partitioned in ferrite for both the microstructures studied. Both austenite morphologies displayed frequent splitting into complex-shaped deformation bands, containing dislocation cells and local stacking faults. Equiaxed austenite was favorable to the local development of microbands (MBs), while its Widmanstatten counterpart appeared to be completely resistant to their formation. This was attributed to the complexity of deformation inside the irregularly shaped Widmanstatten plates precluding the formation of self-screening MB arrays. The MB boundaries were typically aligned along highly stressed slip planes. The presence of discontinuous dynamic recrystallization (DDRX) within both the austenite morphologies was very limited. A slightly higher fraction of DDRX was detected in Widmanstatten austenite, compared to equiaxed austenite, which was ascribed to its higher contribution to the overall deformation and lower fraction of low-mobility coherent twin boundaries. Furthermore, it was demonstrated that continuous dynamic recrystallization (CDRX) was the main restoration mechanism within ferrite for both the microstructure types studied. The CDRX development within ferrite was accelerated in the microstructure with equiaxed austenite. This was related to the comparatively lower fraction of coherent interphases in this microstructure, which would hinder the slip transmission across the interphase and make the strain concentrate within ferrite.

The austenite microstructure evolution in a duplex stainless steel subjected to hot deformation

The austenite microstructure evolution and softening processes have been studied in a 23Cr–6Ni–3Mo duplex stainless steel, comprising equal fractions of austenite and ferrite, deformed in uniaxial compression at 1000 °C using strain rates of 0.1 and 10 s−1. The texture and microstructure evolution within austenite was similar in character for both the strain rate used. The observed large-scale subdivision of austenite grains/islands into complex-shaped deformation bands, typically separated by relatively wide transition regions, has been attributed to the complex strain fields within this phase. Organised, self-screening microband arrays were locally present within austenite and displayed a crystallographic character for a wide range of austenite orientations. The microband boundaries were aligned with the traces of {1 1 1} slip planes containing slip systems having high, although not necessarily the highest possible, Schmid factors. The slightly lower mean intercept length and higher mean misorientation obtained for the sub-boundaries at the higher strain rate can be ascribed to the expected more restricted dynamic recovery processes compared to the low strain rate case. Dynamic recrystallisation within austenite was extremely limited and mainly occurred via the strain-induced migration of the distorted original twin boundaries, followed by the formation of multiple twinning chains.

Development and Industrial Applying on Rolling Mill 5000 of the Model of Austenite Grain Size Evolution in Nb-Microalloyed Steels

In this research evolution of austenite grain size in Nb-microalloyed steels X65÷X120 grades during slab reheating and roughing rolling was studied. A mathematical model has been development to obtain the target temperature and soaking time in furnace, which ensure a uniform austenite structure and maximum possible dissolution of the carbonitride particles prior to roughing rolling. The Hot Rolling Recrystallization Model (HRRM) has also development to predict the austenite microstructure evolution during roughing rolling. The model is based on empirical equations and organized following a tree-structure. A validation of the model has been carried out in the laboratory by multipass compression tests. The models jointly have been used to create new strategies of processing technology of rolled plates on rolling mill 5000 for the South Stream pipeline. The industrial application has confirmed a great benefit of the models in point of cold resistance of rolled plates.

Austenite Static Recrystallization Kinetics in Microalloyed B Steels

Boron is added to steels to increase hardenability, substituting of more expensive elements. Moreover, B acts as a recrystallization delaying element when it is in solid solution. However, B can interact with N and/or C to form nitrides and carbides at high temperatures, limiting its effect on both phase transformation and recrystallization. On the other hand, other elements like Nb and Ti are added due to the retarding effect that they exert on the austenite softening processes, which results in pancaked austenite grains and refined room microstructures. In B steels, Nb and Ti are also used to prevent B precipitation. However, the complex interaction between these elements and its effect on the austenite microstructure evolution during hot working has not been investigated in detail. The present work is focused on the effect the B exerts on recrystallization when added to microalloyed steels. Although B on its own leads to retarded static recrystallization kinetics, when Nb is added a large delay in the static recrystallization times is observed in the 1273 K to 1373 K (1000 °C to 1100 °C) temperature range. The effect is larger than that predicted by a model developed for Nb-microalloyed steels, which is attributed to a synergistic effect of both elements. However, this effect is not so prominent for Nb-Ti-B steels. The complex effect of the composition on recrystallization kinetics is explained as a competition between the solute drag and precipitation pinning phenomena. The effect of the microalloying elements is quantified, and a new model for the predictions of recrystallization kinetics that accounts for the B and Nb+B synergetic effects is proposed.

The microstructure evolution and softening processes during high-temperature deformation of a 21Cr–10Ni–3Mo duplex stainless steel

The austenite and ferrite microstructure evolution and softening mechanisms have been investigated in a 21Cr–10Ni–3Mo duplex stainless steel, containing about 60% austenite, deformed in torsion at 1200 °C using a strain rate of 0.7 s−1. The above experimental conditions led to the formation of a small volume fraction of new austenite grains through discontinuous dynamic recrystallization (DDRX), which could not account for the observed large softening on the flow curve. DDRX grains mainly formed through the strain-induced migration of the pre-existing austenite grain boundaries, known to dominate in single-phase austenite, complemented by subgrain growth in the interface regions with ferrite. A significant portion of austenite dynamic softening has been attributed to the large-scale subgrain coalescence, the extent of which increased with strain, which seems to have contributed substantially to the observed flow stress decrease. The above process thus appears to represent an alternative mode of austenite dynamic softening to the classical DDRX in the duplex austenite/ferrite microstructure, characterised by limited availability of the pre-existing austenite/austenite high-angle boundaries, deformed at a high temperature. The softening mechanism within ferrite has been classified as “continuous DRX”, characterised by a gradual increase in misorientations between neighbouring subgrains with strain and resulting in the progressive conversion of subgrains into “crystallites” bounded partly by low-angle and partly by large-angle boundaries.

On the use of model alloys as a way to understand the effects of complex deformation conditions on austenite microstructure evolution during hot metal forming processes

During industrial hot metal forming processes, deformed metal undergoes complex strain history that in most cases is non-linear. This very often leads to improper prediction of microstructure evolution and properties of final metal forming products by the computer models that are used currently as in most cases, they are not strain-path sensitive. In the present work, effect of strain path changes on the austenite microstructure evolution has been studied using a special group of model alloys that were designed for this purpose. These alloys are characterised by similar stacking fault energy and flow behaviour to low carbon steels at elevated temperatures, and most of all, by the fact, that austenite in these alloys is stable all the way down to room temperature, what in turn, allows to study its behaviour directly. In the current works, the results of the study of the effects of complex strain path changes on the kinetics of static recrystallisation, strain-induced precipitation process and development of grain boundaries of deformed austenite are shown. Presented results will be very useful as an input data for the computer simulation of metal forming processes towards their improvement in order to take into account full strain path history during deformation processes at elevated temperatures.

Investigation of Austenite Evolution in Low-Carbon Steel by Combining Thermo-Mechanical Simulation and EBSD Data

Abstract The thermo-mechanical simulator Gleeble 3800 was used in combination with the electron backscattered diffraction (EBSD) technique to analyze the austenite microstructure evolution during hot deformation in a low-carbon, microalloyed steel. The parent austenite grain structure was automatically reconstructed from EBSD datasets by applying the commercially available ARPGE software [Cayron, C., Artaud, B., and Briottet, L., “Reconstruction of Parent Grains from EBSD Data,” Mater. Charact., Vol. 57, Nos. 4–5, 2006, pp. 386–401]. The work aims at two aspects: the ability to quantify the austenite evolution depending on different process parameters, and the validation of the EBSD reconstruction software ARPGE to evaluate the accuracy of reconstruction and demonstrating the application in low-carbon steels. For these steels, the conventional metallographic methods like Bechet-Beaujard [Bechet, S. and Beaujard, L., “Nouveau réactif pour la mise en évidence micrographique du grain austénitique des aciers trempés ou trempés-revenus,” Rev. Met., 1955] are limited and not reproducible in determining the former austenite. The presented approach provides an attractive alternative. In this context, hot compression tests were performed on cylindrical samples using a Gleeble 3800 simulator to create defined austenite conditions. The samples were rapidly quenched after processing to preserve the austenite microstructure. The obtained bainitic/martensitic microstructure is an essential precondition for the reconstruction procedure. For the validation of the results, correlative measurements using classical metallographic techniques and ARPGE data were performed to check the reliability of the reconstruction. The results obtained in this study provide a direct correlation of the microstructure simulation by Gleeble and the austenite microstructure evolution during processing of low-carbon steels. Especially, austenite grain-growth phenomena and grain refinement could be successfully observed and quantified. Furthermore, the measurements show that even on large EBSD maps (1000 μm × 1000 μm), the shape of the austenite grains are calculated with good accuracy.

Interaction Between Recovery, Recrystallization, and NbC Strain-Induced Precipitation in High-Mn Steels

The interaction between recovery, recrystallization, and strain-induced precipitation in two high-Mn steels, one of them microalloyed with Nb (0.1 pct) was investigated using mechanical testing and advanced microscopy techniques. Double-hit torsion tests were carried out in the 1373 K to 1173 K (1100 °C to 900 °C) temperature range in order to characterize the fractional softening behavior. Quenched specimens were analyzed using electron backscatter diffraction and transmission electron microscopy to determine the recrystallized fraction, the precipitation state, and the austenite microstructure evolution. At the highest temperature, 1373 K (1100 °C), similar softening kinetics were found in both steels. However, at temperatures lower than 1273 K (1000 °C) for the Nb steel, strain-induced precipitation was observed to take place resulting in significant softening retardation. For the base steel at all the temperatures investigated, and for the Nb steel in the absence of strain-induced precipitation, the mechanical softening corresponded well with the recrystallized fraction. However, when strain-induced precipitation took place, a major deviation was observed denoting a significant contribution of recovery to the fractional softening. Within the deformed grains, a substructure consisting of “subgrain bands” or microbands was developed. The precipitates were found mainly on the elongated subgrain boundaries, or at dislocations within the subgrains. This configuration was maintained after the migration of the recrystallization front.

Microstructure evolution during continuous cooling in niobium microalloyed high carbon steels

Effects of microalloyed niobium (Nb) on the austenite decomposition behaviors and microstructure evolution during continuous cooling in the near eutectoid steels were investigated. Compared to the Nb free steel, the Nb microalloyed steel was refined with regard to polygonal ferrite grain, pearlite block and colony sizes. This was because its austenite grain size was smaller. The volume fraction of polygonal ferrite transformed was more in the Nb microalloyed steels, which indicated the eutectoid carbon content exceeded that of pure carbon steel. The spheroidization of pearlite during continuous cooling was enhanced by Nb microalloying, mainly due to a higher critical transformation temperature and the finer pearlite structure with smaller colony size and narrower interlamellar spacing. Hot deformation right above the equilibrium eutectoid temperature accelerated the spheroidization kinetics of pearlite, especially in the Nb microalloyed steel.

Austenite grain growth and microstructure control in simulated heat affected zones of microalloyed HSLA steel

The roles of microalloying niobium, titanium and vanadium for controlling austenite grain growth, microstructure evolution and hardness were investigated at different simulated heat affected zones (HAZ) for high strength low alloy (HSLA) S690QL steel. High resolution FEG-SEM has been used to characterize fine bainitic ferrite, martensite and nanosized second phases at simulated coarse and fine grain HAZs. It was found that for Ti bearing steel (Ti/N ratio is 2) austenite grain had the slowest growth rate due to the presence of most stable TiN. The fine cuboidal particles promoted intragranular acicular ferrite (IGF) formation. Nb bearing steel exhibited relatively weaker grain growth retardation compared with titanium bearing steels and a mixed microstructure of bainite and martensite was present for all simulated HAZs. IGF existed at coarse grain HAZ of Ti+V bearing steel but it was totally replaced by bainite at fine grain HAZs. Hardness result was closely related to the morphology of bainitic ferrite, intragranular ferrite and second phases within ferrite. The microstructure and hardness results of different simulated HAZs were in good agreement with welded experimental results.

Heat Treatments Analysis of Steel Using Coupled Phase Field and Finite Element Methods

Steels are known for their remarkable mechanical properties being extensively used in industry. Furthermore, phase transformations in metals and alloys, particularly in steels, are widely studied due to their importance. The understanding of the microstructure evolution in this type of materials is vital to reproduce the thermomechanical behaviour and to create new materials. To analyse the thermomechanical behaviour of steel during phase transition of steels, a phase field model was coupled with a finite element model in order to simulate the heat treatment and microstructure evolution of austenite to pearlite/ferrite. The thermoelastoplastic constitutive equations for each phase were implemented through a user routine in commercial FE software. This procedure presents a more quantitative understanding of the phase transformation in steels and a deeper comprehension of the mechanical behaviour of these materials when subject to heat treatments.

Physical Modelling of the Interaction between Softening and Nb (C,N) Strain-Induced Precipitation in High-Mn Steels

The applicability of a physical model to describe the austenite microstructure evolution after hot deformation in High-Mn steels was investigated. Double-hit torsion tests were carried out to determine the softening behaviour of two High-Mn steels, one of them microalloyed with 0.11 wt% Nb. The values of the unknown parameters included in the model were determined by fitting experimental results. The model describes adequately the softening evolution of the steels. At high temperatures recovery and recrystallization contribute to mechanical softening, the latter having the main contribution. In contrast, when strain-induced precipitation occurs recovery has a larger effect.

Modeling of Microstructure and Mechanical Properties of Hot Rolled Steels

An integral mathematical physically based model is developed for prediction of the microstructure and mechanical properties of steels processed in accordance with a given hot deformation and accelerated cooling regimes. The model predicts austenite microstructure evolution under hot deformation, as well as its transformation during subsequent cooling with account of formation of ferrite, pearlite, bainite and martensite. Structure-property relationships are developed using an extensive experimental database chemical composition - microstructure - mechanical properties obtained for 10 steel grades. Austenite transformation depending on grain size, cooling rate and preliminary plastic deformation was investigated with the help of Gleeble 3800 system to obtain a set of practically important morphologically different microstructures for each steel grade. A quantitative analysis of the microstructures was performed using optical and scanning electron microscopy (EBSD-method). Investigation of the mechanical properties of steels with wide spectrum of obtained microstructures was carried out on the double-samples processed using Gleeble 3800. The predicted microstructure parameters for investigated steels obtained using the developed model, as well as their mechanical properties, are in good agreement with the experimental data.

Impact Analysis of Microstructure Evolution in Hot Rolling of H-Beams

To improve the product properties of H-beams, it is essential to understand the effects of hot rolling parameters on the microstructure evolution of the beams. For this purpose, a thermo mechanical model was built with the finite element Package ABAQUS. By re-meshing the model, multipass large-deformation hot rolling process was simulated under the boundary conditions predefined in accordance with the practical production. Based on the hot rolling simulation, an impact analysis of strain rate, initial rough rolling temperature, and time interval between passes on the microstructure evolution of H-beam austenite was conducted. The analytical results are meaningful for optimizing hot rolling parameters and improving H-beam properties.

Dynamic Recrystallization Behavior and Microstructure Evolution of Bridge Weathering Steel in Austenite Region

AbstractThe austenite dynamic recrystallization (DRX) behavior and microstructure evolution of a bridge weathering steel was systematically investigated at a deformation temperature range of 800–1100°C and strain rate of 0.1–10 s−1 by using hot compression test and optical microscopy. The stress exponent and hot deformation energy were obtained by regression method to determine thermal deformation constitutive equation. The curve of stress versus strain is used, combined with high order polynomial fitting, to accurately determine the critical value of DRX. The relationships between critical strain, critical stress, and Z parameter of the bridge weathering steel were obtained by regression method. Moreover, the influence factors of DRX kinetics of the bridge weathering steel were studied in the light of the experimental results. It is shown that the strain rate has a more significant effect on the rate of DRX than that of the deformation temperature, and there is almost 0.85 orders of magnitude increment in the rate of DRX as the strain rate increases an order of magnitude. The dynamically recrystallized grain size can be decreased with decreasing the deformation temperature and increasing the strain rate during the austenite deformation.