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R D K Misra - One of the best experts on this subject based on the ideXlab platform.
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effect of hot deformation parameters on flow stress and microstructure in a low carbon Microalloyed Steel
2019Co-Authors: Liangyun Lan, Wei Zhou, R D K MisraAbstract:Abstract Hot deformation behavior of low carbon Microalloyed Steel was investigated by single-pass compression tests using a thermomechanical simulator. After the specimens were subjected to different hot deformation processes, their microstructures were obtained by constant cooling rate and then examined using optical microscopy and electron backscattering diffraction (EBSD) technique. The results showed that the absence of stress peak in the flow stress curves does not necessarily indicate the absence of dynamic recrystallization (DRX) because in some cases the partial DRX phenomenon does occur when the stress peaks are absent in the flow curves. The metallurgical events that occur during hot deformation directly influence the nature of phase transformation products. High dislocation density in the deformed austenite can promote the intragranular nucleation of acicular ferrite, while the DRX austenite grains prefer to form lath bainite/martensite. The EBSD results confirmed that the microstructures that were transformed from prior pancaked austenite grains always contain higher fraction of middle misorientation angle boundaries and larger average local strain as compared to those with prior DRX grain shape.
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grain refinement in surface layers through deformation induced ferrite transformation in Microalloyed Steel plate
2017Co-Authors: X J Shen, R D K Misra, Shuai Tang, Jun Chen, G D WangAbstract:Abstract A novel thermo-mechanical process was adopted to obtain deformation-induced ultrafine-grained ferrite in the surface layers of a Microalloyed Steel plate. The new process referred as rolling coupling water-cooling control process applies water-cooling in the midst of rolling. Deformation-induced ferrite transformation led to ultrafine ferrite grains (1–3 μm) in the surface layer of the plate, with ultrafine-grained layer accounting to ~ 13.5% of the total thickness of the plate. In the mid-thickness, relatively coarse ferrite grains (~ 7.8 μm) and pearlite microstructure was obtained. The ductile-brittle transition temperature corresponding to the ultrafine-grained structure was − 118 °C and excellent toughness was obtained in the surface layer such that the average impact toughness of the complete plate was high. The mechanism of formation of ultrafine-grained structure at the surface is elucidated.
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on the determining role of acicular ferrite in v n Microalloyed Steel in increasing strength toughness combination
2016Co-Authors: M Zang, S J Yin, Y G Wang, X H Gao, R D K MisraAbstract:Abstract Controlled rolling followed by accelerating cooling was adopted during industrial processing of a low-C V-N Microalloyed Steel. Furthermore, the effect of finish cooling temperature on the microstructure and mechanical properties was studied. The microstructure consisted of polygonal ferrite and coarse pearlite at finish cooling temperature of 660 °C, while fine pearlite was obtained instead of coarse pearlite at finish cooling temperature of 630 °C. With further decrease in finish cooling temperature to 570 °C, the microstructure comprised of fine polygonal ferrite, acicular ferrite, and a small amount of fine pearlite, such that high yield strength of 625 MPa and excellent impact toughness of 149 J at − 60 °C was obtained. Acicular ferrite characterized by fine non-parallel ferrite platelets and high degree of misorientation played a determining role in enhancing strength and toughness.
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microstructure and mechanical properties of tmcp heavy plate Microalloyed Steel
2014Co-Authors: Hui Xie, Xiuhua Gao, R D K MisraAbstract:Abstract A Microalloyed Steel heavy plate was subjected to two-stage controlled rolling and two-stage continuous cooling to explore the microstructure and mechanical properties in the plate. The objective was to obtain superior mechanical properties in the plate by exploiting the advantage of Microalloyed precipitates in stimulating intragranular ferrite nucleation. Yield strength, tensile strength, and percentage of elongation of 550 MPa, 655 MPa, and 26.5% were obtained at quarter-thickness and 515 MPa, 645 MPa, and 29.5% at mid-thickness. The impact energy determined at −40 °C was ~93 J and 71 J for quarter-thickness and mid-thickness regions, respectively. The microstructure consisted of polygonal ferrite, acicular ferrite, and pearlite. Microalloyed precipitates provided effective nucleation sites for intragranular ferrite and ensured near-homogenous microstructure and mechanical properties in the heavy Steel plate.
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toward interplay between substructure evolution dislocation configuration and yield strength in a Microalloyed Steel
2014Co-Authors: P K C Venkatsurya, R D K Misra, M D Mulholland, Murali Manohar, J E HartmannAbstract:Abstract We focus our attention here on the directional dependence of yield strength in high strength Microalloyed Steel using transmission electron microscopy and x-ray diffraction. The primary objective is to study the interplay between substructural evolution, notably cell size, dense dislocation walls (DDWs), dislocation tangle zones (DTZs), lamellar boundaries, crystallographic texture, and yield strength. The study elucidates for the first time the strong impact of thermo-mechanical deformation-induced dislocation and lamellar structures, which are likely to modify the slip pattern, leading to directional dependence of yield strength. Majority of the dislocations tend to pile along the {110} slip planes as dense dislocation walls. At low strains, grains are first divided into cell blocks that are nearly dislocation-free. At higher strains and with progress in thermo-mechanical processing dislocation tangled zones and lamellar boundaries develop. It is hypothesized that the differences in dislocation configurations, dislocations cells and cell blocks, and lamellar boundaries synergistically contribute to directional dependence of the yield strength in the high strength ferrous alloy. The presumption is envisaged on the basis of observations that the microstructural constituents were similar in the entire plane of the hot rolled strip and the crystallographic texture was weak.
Sebastián F. Medina - One of the best experts on this subject based on the ideXlab platform.
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evolution of microstructure and precipitation state during thermomechanical processing of a x80 Microalloyed Steel
2011Co-Authors: Manuel Gomez, P. Valles, Sebastián F. MedinaAbstract:Abstract A series of anisothermal hot torsion tests were carried out to simulate hot rolling on a high-strength low-carbon CMnNbMoTi Microalloyed Steel corresponding to an industrial X80 grade for pipeline construction. Mean Flow Stress was graphically represented against the inverse of temperature to characterize the evolution of austenite microstructure during rolling, which was also studied by optical microscopy and SEM on samples quenched from several temperatures. On the other hand, particles precipitated at different temperatures during rolling were analyzed by means of TEM using the carbon extraction replica technique and their size distribution and mean size were determined, as well as their morphology, nature and chemical composition. The effect of rolling temperature and austenite strengthening obtained at the end of thermomechanical processing on final microstructure and precipitation state was studied. Austenite strengthening was characterized by means of the parameter known as accumulated stress (Δ σ ). It was found that ferrite grains are finer and more equiaxed when the austenite is more severely deformed during finishing (higher values of Δ σ ) but lower values of Δ σ generate a higher density of acicular structures after cooling, which should improve the balance of mechanical properties. The increase in strength associated to acicular ferrite compared to polygonal ferrite is revealed by the higher values of Vickers microhardness measured on samples corresponding to low Δ σ . On the other hand, (Ti, Nb)-rich carbonitrides can be found from reheating and their size keeps a constant value near 20–30 nm during thermomechanical processing. A second population of much finer (Nb, Mo)-rich carbonitrides whose size is close to 5 nm forms from lower temperatures, near 1000 °C. The accomplishment of two different levels of Δ σ at the end of hot rolling schedule does not seem to introduce major differences in precipitation state before final cooling.
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assessment of austenite static recrystallization and grain size evolution during multipass hot rolling of a niobium Microalloyed Steel
2009Co-Authors: Manuel Gomez, Lucía Rancel, Sebastián F. MedinaAbstract:Double-deformation isothermal tests and multipass continuous-cooling hot torsion tests were used to study the evolution of austenite microstructures during isothermal and non-isothermal hot deformation of an Nb Microalloyed Steel. These tests, coupled with microstructural characterization, have verified that the no-recrystallization temperature (T nr ) corresponds roughly to the temperature where recrystallization starts to be incomplete during rolling. An accurate method to estimate the recrystallized fraction during hot rolling based on stress-strain data, and which does not require metallographic studies, is proposed. The results of this method have been successfully compared to metallographic measurements, the values of non-isothermal fractional softening and the accumulated stress measured in the plots of mean flow stress (MFS) versus the inverse of temperature. A remarkable austenite grain refinement occurs in the first hot rolling passes after reheating. The correlation of isothermal and continuous cooling tests is better understood if the effect of grain size on recrystallization and precipitation is taken into account.
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Evolution of austenite static recrystallization and grain size during hot rolling of a V-Microalloyed Steel
2009Co-Authors: Manuel Gomez, Lucía Rancel, Bernardo J. Fernández, Sebastián F. MedinaAbstract:Abstract Laboratory double-deformation isothermal tests and multipass continuous cooling hot torsion tests were used to study the static recrystallization of austenite under isothermal and anisothermal conditions as well as to simulate the hot rolling of a 0.13% V-Microalloyed Steel. Characterization of the evolution of austenite microstructure was carried out. It has been verified that no-recrystallization temperature ( T nr ) approximately corresponds to the temperature where recrystallization starts to be incomplete during rolling. However, incomplete recrystallization is visually evident at temperatures 25–50 °C below T nr , where grain elongation and increase in aspect ratio with temperature drop start to be significant. An accurate method to estimate the recrystallized fraction during hot rolling from stress–strain data and with no need of metallographic studies has been designed. The results of this method have been compared to metallographic measurements, the values of anisothermal fractional softening and the accumulated stress measured in the MFS plots at T T nr . A pronounced austenite grain refinement has been detected in the first hot rolling passes after reheating, as grain size decreases from 155 μm to 27 μm in six passes. If the effect of grain size on recrystallization and precipitation is taken into account, the correlation of isothermal and continuous cooling tests as well as the relationship between SRCT and T nr or RLT temperatures can be better understood.
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grain refinement by intragranular nucleation of ferrite in a high nitrogen content vanadium Microalloyed Steel
2008Co-Authors: Sebastián F. Medina, Manuel Gomez, Lucía RancelAbstract:The intragranular nucleation of ferrite was studied in a hot strained V-Microalloyed Steel. By means of torsion tests, the recrystallized fraction was determined for a strain of 0.35 and temperature of 950 °C. Tests at the same strain and holding times corresponding to the start and end of precipitation with subsequent cooling of the specimen showed that the intragranular nucleation of ferrite on VN precipitates leads to a significant decrease in the grain size, close to 50%.
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Influence of Accumulated Stress in Austenite on Transformed Ferrite Grain Size by Hot Rolling for a V-Microalloyed Steel
2008Co-Authors: Sebastián F. Medina, Manuel Gomez, Elisabeth Rodríguez, Lucía RancelAbstract:Torsion test rolling simulations have been performed in different conditions (pass strain, interpass time) for a V-Microalloyed Steel (C=0.165; V=0.170 wt%). The accumulated stress (Δσ) in austenite at temperatures below the no-recrystallisation temperature (Tnr) has been measured. The accumulated stress is directly related to the dislocation density. The ferrite grain size (Dα) obtained after hot rolling simulations for different conditions and a cooling rate of 3.5 K/s has been measured. Dα is found to be dependent on Δσ and on the austenite grain size prior to the austenite–ferrite transformation during cooling. On the other hand, a higher strain accelerates recrystallisation between passes, lowers the Tnr value, and consequently leads to a smaller accumulated stress. It is seen that a minimum value of 15 MPa must be reached in order for Dα refinement to begin.
Manuel Gomez - One of the best experts on this subject based on the ideXlab platform.
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evolution of microstructure and precipitation state during thermomechanical processing of a x80 Microalloyed Steel
2011Co-Authors: Manuel Gomez, P. Valles, Sebastián F. MedinaAbstract:Abstract A series of anisothermal hot torsion tests were carried out to simulate hot rolling on a high-strength low-carbon CMnNbMoTi Microalloyed Steel corresponding to an industrial X80 grade for pipeline construction. Mean Flow Stress was graphically represented against the inverse of temperature to characterize the evolution of austenite microstructure during rolling, which was also studied by optical microscopy and SEM on samples quenched from several temperatures. On the other hand, particles precipitated at different temperatures during rolling were analyzed by means of TEM using the carbon extraction replica technique and their size distribution and mean size were determined, as well as their morphology, nature and chemical composition. The effect of rolling temperature and austenite strengthening obtained at the end of thermomechanical processing on final microstructure and precipitation state was studied. Austenite strengthening was characterized by means of the parameter known as accumulated stress (Δ σ ). It was found that ferrite grains are finer and more equiaxed when the austenite is more severely deformed during finishing (higher values of Δ σ ) but lower values of Δ σ generate a higher density of acicular structures after cooling, which should improve the balance of mechanical properties. The increase in strength associated to acicular ferrite compared to polygonal ferrite is revealed by the higher values of Vickers microhardness measured on samples corresponding to low Δ σ . On the other hand, (Ti, Nb)-rich carbonitrides can be found from reheating and their size keeps a constant value near 20–30 nm during thermomechanical processing. A second population of much finer (Nb, Mo)-rich carbonitrides whose size is close to 5 nm forms from lower temperatures, near 1000 °C. The accomplishment of two different levels of Δ σ at the end of hot rolling schedule does not seem to introduce major differences in precipitation state before final cooling.
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assessment of austenite static recrystallization and grain size evolution during multipass hot rolling of a niobium Microalloyed Steel
2009Co-Authors: Manuel Gomez, Lucía Rancel, Sebastián F. MedinaAbstract:Double-deformation isothermal tests and multipass continuous-cooling hot torsion tests were used to study the evolution of austenite microstructures during isothermal and non-isothermal hot deformation of an Nb Microalloyed Steel. These tests, coupled with microstructural characterization, have verified that the no-recrystallization temperature (T nr ) corresponds roughly to the temperature where recrystallization starts to be incomplete during rolling. An accurate method to estimate the recrystallized fraction during hot rolling based on stress-strain data, and which does not require metallographic studies, is proposed. The results of this method have been successfully compared to metallographic measurements, the values of non-isothermal fractional softening and the accumulated stress measured in the plots of mean flow stress (MFS) versus the inverse of temperature. A remarkable austenite grain refinement occurs in the first hot rolling passes after reheating. The correlation of isothermal and continuous cooling tests is better understood if the effect of grain size on recrystallization and precipitation is taken into account.
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Evolution of austenite static recrystallization and grain size during hot rolling of a V-Microalloyed Steel
2009Co-Authors: Manuel Gomez, Lucía Rancel, Bernardo J. Fernández, Sebastián F. MedinaAbstract:Abstract Laboratory double-deformation isothermal tests and multipass continuous cooling hot torsion tests were used to study the static recrystallization of austenite under isothermal and anisothermal conditions as well as to simulate the hot rolling of a 0.13% V-Microalloyed Steel. Characterization of the evolution of austenite microstructure was carried out. It has been verified that no-recrystallization temperature ( T nr ) approximately corresponds to the temperature where recrystallization starts to be incomplete during rolling. However, incomplete recrystallization is visually evident at temperatures 25–50 °C below T nr , where grain elongation and increase in aspect ratio with temperature drop start to be significant. An accurate method to estimate the recrystallized fraction during hot rolling from stress–strain data and with no need of metallographic studies has been designed. The results of this method have been compared to metallographic measurements, the values of anisothermal fractional softening and the accumulated stress measured in the MFS plots at T T nr . A pronounced austenite grain refinement has been detected in the first hot rolling passes after reheating, as grain size decreases from 155 μm to 27 μm in six passes. If the effect of grain size on recrystallization and precipitation is taken into account, the correlation of isothermal and continuous cooling tests as well as the relationship between SRCT and T nr or RLT temperatures can be better understood.
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grain refinement by intragranular nucleation of ferrite in a high nitrogen content vanadium Microalloyed Steel
2008Co-Authors: Sebastián F. Medina, Manuel Gomez, Lucía RancelAbstract:The intragranular nucleation of ferrite was studied in a hot strained V-Microalloyed Steel. By means of torsion tests, the recrystallized fraction was determined for a strain of 0.35 and temperature of 950 °C. Tests at the same strain and holding times corresponding to the start and end of precipitation with subsequent cooling of the specimen showed that the intragranular nucleation of ferrite on VN precipitates leads to a significant decrease in the grain size, close to 50%.
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Influence of Accumulated Stress in Austenite on Transformed Ferrite Grain Size by Hot Rolling for a V-Microalloyed Steel
2008Co-Authors: Sebastián F. Medina, Manuel Gomez, Elisabeth Rodríguez, Lucía RancelAbstract:Torsion test rolling simulations have been performed in different conditions (pass strain, interpass time) for a V-Microalloyed Steel (C=0.165; V=0.170 wt%). The accumulated stress (Δσ) in austenite at temperatures below the no-recrystallisation temperature (Tnr) has been measured. The accumulated stress is directly related to the dislocation density. The ferrite grain size (Dα) obtained after hot rolling simulations for different conditions and a cooling rate of 3.5 K/s has been measured. Dα is found to be dependent on Δσ and on the austenite grain size prior to the austenite–ferrite transformation during cooling. On the other hand, a higher strain accelerates recrystallisation between passes, lowers the Tnr value, and consequently leads to a smaller accumulated stress. It is seen that a minimum value of 15 MPa must be reached in order for Dα refinement to begin.
Zu Qing Sun - One of the best experts on this subject based on the ideXlab platform.
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prediction of grain growth of coarse grained austenite in nb v ti Microalloyed Steel
2011Co-Authors: Qing Yun Sha, Zu Qing SunAbstract:Grain growth of coarse-grained austenite in Nb–V–Ti Microalloyed Steel during equalisation was predicted by extending the previous investigation. The prediction worked with initial austenite grain size distribution instead of average grain size. An improved model taking into account the holding time was used in the prediction. The result showed that only part of initial austenite grains grow at each equalisation temperatures, but the grain size of growing grains is expanded to a wider range with increasing equalisation temperature, which indicates that grain size distribution should be considered when grain growth of coarse-grained austenite is evaluated. The predicted austenite grain size distribution is close fit to the measured one and further work is expected to improve the quality of model prediction.
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comparison of grain growth between fine grained and coarse grained austenite in a nb v ti Microalloyed Steel
2010Co-Authors: Qing Yun Sha, Li Feng Qiao, Guo Jian Huang, Zu Qing SunAbstract:Comparison of grain growth of fine-grained and coarse-grained austenite in a Nb-V-Ti Microalloyed Steel during reheating or equalization at the same temperature was investigated using cold-charging and hot-charging specimens respectively in this study. The results show that the different grain growth behavior appears in fine-grained and coarse-grained austenite. The uniform grain growth and lower growth rate at reheating temperature studied was found in fine-grained austenite, while partial grain growth and higher growth rate was present in coarse-grained austenite. During reheating or equalization, the slow growth rate in fine-grained austenite may be contributed to stronger pinning force of fine precipitates while higher grain growth rate in coarse-grained austenite were believed to the result of lager size difference among part of grains. Grain coarsening occurs in fine-grained austenite as result of precipitate unpinning at extending holding time, but coarse-grained austenite remained wide size distribution at the same condition and this should not be considered as grain coarsening. coarse-grained austenite remained wide size distribution at the
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grain growth behavior of coarse grained austenite in a nb v ti Microalloyed Steel
2009Co-Authors: Qing Yun Sha, Zu Qing SunAbstract:Abstract Grain growth of coarse-grained austenite during equalization in a simulated thin slab casting and direct rolling process (TSDR) has been studied on a Nb–V–Ti Microalloyed Steel. Coarse-grained austenite produced by reheating at 1350 °C for 90 min were reheated to four equalization temperatures (1100, 1150, 1200 and 1250 °C) for 30 min, then the austenite grain sizes and precipitates were examined. The experimental results show that coarse-grained austenite keep its wide range of grain sizes at elevated equalization temperatures. Ti-rich carbonitrides retain its strong pinning effect on grain boundary of coarse-grained austenite at temperatures below 1250 °C. The occurrence of abnormal grain growth in coarse-grained austenite may be difficult when particle coarsening occurs as it is hard for the smaller grains which has the larger driving forces to grow into neighboring larger grain matrix.
J. I. Chaves - One of the best experts on this subject based on the ideXlab platform.
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Static Recrystallization of Austenite in a Medium-Carbon Vanadium Microalloyed Steel and Inhibition by Strain-Induced Precipitates
2020Co-Authors: M Gomez, S F Medina, J. I. ChavesAbstract:Keywords: vanadium Microalloyed Steel, static recrystallization of austenite, strain induced precipitation, driving and pinning forces. Abstract. The austenite static recrystallization kinetics at several temperatures and the recrystallization-precipitation-time-temperature (RPTT) diagrams of a medium-carbon vanadium Microalloyed Steel have been determined for a strain ε = 0.35. Unlike many other studies carried out previously on V Microalloyed Steels, the recrystallized fraction against time curves showed the formation of a double plateau that indicates two stages of inhibition of recrystallization due to the formation of different types of strain induced precipitates. This work makes use of transmission electron microscopy to study the nature and size distribution of these precipitates capable of inhibiting recrystallization. The values of driving and pinning forces for static recrystallization are calculated and an analysis of the relationship between the net balance of these forces, the precipitation state and the progress or inhibition of the recrystallization is accomplished. A value of driving force that decreases as recrystallized fraction grows during isothermal holding time is estimated and helps to interpret the behavior of austenite after deformation
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influence of austenite grain size on recrystallisation precipitation interaction in a v Microalloyed Steel
2007Co-Authors: Alberto Quispe, Sebastián F. Medina, Manuel Gomez, J. I. ChavesAbstract:Abstract By means of torsion tests using small specimens, the influence of austenite grain size on strain induced precipitation kinetics has been determined in a vanadium Microalloyed Steel. Determination of recrystallisation–precipitation–time–temperature (RPTT) diagrams for two austenite grain sizes allows values of the aforementioned magnitudes to be determined. An ample discussion is made of the quantitative influence found and its relation with nucleation and growth mechanisms of precipitates. The results are compared with the quantitative influence exerted by the other variables, reaching the conclusion that the austenite grain size has a notable influence on strain induced precipitation kinetics which should not be underestimated. Finally, the influence of austenite grain size is included in a strain induced precipitation model constructed by the authors of this work and which also takes into account the other aforementioned variables.