Influence of prior austenite grain size on the critical strain for completion of DEFT through hot compression test

A low carbon steel was used to determine the critical strain εc for completion of deformation enhanced ferrite transformation （DEFT） through a series of hot compression tests. In addition, the influence of prior austenite grain size （PAGS） on the critical strain was systematically investigated. Experimental results showed that the critical strain is affected by PAGS. When γ→α transformation completes, the smaller the PAGS is, the smaller the critical strain is. The ferrite grains obtained through DEFT can be refined to about 3 μm when the DEFT is completed.

A NEURAL NETWORK-BASED MODEL FOR PREDICTION OF HOT-ROLLED AUSTENITE GRAIN SIZE AND FLOW STRESS IN MICROALLOY STEEL

For the great significance of the prediction of control parameters selected for hot-rolling and the evaluation of hot-rolling quality for the analysis of prod uction problems and production management, the selection of hot-rolling control parameters was studied for microalloy steel by following the neural network principle. An experimental scheme was first worked out for acquisition of sample data, in which a gleeble-1500 thermal simolator was used to obtain rolling temperature, strain, stain rate, and stress-strain curves. And consequently the aust enite grain sizes was obtained through microscopic observation. The experimental data was then processed through regression. By using the training network of BP algorithm, the mapping relationship between the hotrooling control parameters (rolling temperature, stain, and strain rate) and the microstructural paramete rs (austenite grain in size and flow stress) of microalloy steel was function appro ached for the establishment of a neural network-based model of the austeuite grain size and flow stress of microalloy steel. From the results of estimation made with the neural network based model, the hot-rolling control parameters can be effectively predicted.

Modification for prediction model of austenite grain size at surface of microalloyed steel slabs based on in situ observation

The initial solidification process of microalloyed steels was simulated using a confocal scanning laser microscope,and the growth behavior of austenite grain was observed in situ.The method for measuring the initial austenite grain size was studied,and the M_(0)^(*)(the parameter to describe the grain boundary migration)values at different cooling rates were then calculated using the initial austenite grain size and the final grain size.Next,a newly modified model for predicting the austenite grain size was established by introducing the relationship between M_(0)^(*)and the cooling rate,and the value calculated from the modified model closely corresponds to the measured value,with average relative error being less than 5%.Further,the relationship between T^(γ)(the starting temperature for austenite grain growth)and equivalent carbon content C_(P)(C_(P)>0.18%)was obtained by in situ observation,and it was introduced into the modified model,which expanded the application scope of the model.Taking the continuous casting slab produced by a steel plant as the experimental object,the modified austenite grain size prediction model was used to predict the austenite grain size at different depths of oscillation mark on the surface of slab,and the predicted value was in good agreement with the actual measured value.

Effect of austenite grain size and accelerated cooling start temperature on the transformation behaviors of multi-phase steel

The transformation behaviors and microstructures of a low carbon multi-phase steel were investigated by the simulation of deformation-relaxation-accelerated cooling processing,using a Gleeble 3500 thermal-mechanical simulator.A pre-treatment of solid solution at 1200°C was implemented to minimize the influence on transformation from solid solution/precipitation qualities of 0.08%Nb in this steel.On this basis,the effect of austenite grain size and accelerated cooling start temperature were studied individually.The results indicated that the transformation of ferrite in multi-phase steel could be significantly promoted by the refinement of austenite grains and the increase of relaxation time,while the hard phase,such as lath bainite or martensite,could still be obtained with the following accelerated cooling.In contrast,more uniform lower temperature transformed microstructure could form from coarse grain austenite.The potential benefit of austenite grain size on adjusting the proportion of phases in multiphase steel was also discussed.

Analytical Investigation of Prior Austenite Grain Size Dependence of Low Temperature Toughness in Steel Weld Metal

Prior austenite grain size dependence of the low temperature impact toughness has been addressed in the bainitic weld metals by in situ observations.Usually,decreasing the grain size is the only approach by which both the strength and the toughness of a steel are increased.However,low carbon bainitic steel with small grain size shows a weakening of the low temperature impact toughness in this study.By direct tracking of the morphological evolution during phase transformation,it is found that large austenite grain size dominates the nucleation of intragranular acicular ferrite,whereas small austenite grain size leads to grain boundary nucleation of bainite.This kinetics information will contribute to meet the increasing low temperature toughness requirement of weld metals for the storage tanks and offshore structures.

Austenite Grain Size Evolution in Railway Wheel During Multi-Stage Forging Processes

The knowledge of microstructure evolution of railway wheel during hot forming process is the prerequisite of improving mechanical properties of the final product.In order to investigate the austenite grain size evolution of railway wheel during multi-stage forging process,mathematical models of recrystallization and austenite grain growth were derived firstly by hot compression tests for railway wheel steel CL50D,which then were integrated with a thermal-mechanical finite element model by the developed subroutines.The information about kinetics of recrystallization and grain size distribution during the forging process was obtained by simulation.The predicted results were validated by experiments in an industrial scale,and the average error between the predicted grain sizes and the measured ones is about 5%.The result shows that,under the current railway wheel forging process,the grain size distribution after final forging is inhomogeneous extremely.There is a narrow coarse grain zone between the external part and center of the hub caused by static recrystallization after final forging.With cooling of 60 s after final forging,the grain size is about 85 μm for the areas near the web surface and 175 μm for center areas of the hub and rim.

Effect of deformation parameters on the austenite dynamic recrystallization behavior of a eutectoid pearlite rail steel

Understandings of the effect of hot deformation parameters close to the practical production line on grain refinement are crucial for enhancing both the strength and toughness of future rail steels.In this work,the austenite dynamic recrystallization(DRX)behaviors of a eutectoid pearlite rail steel were studied using a thermo-mechanical simulator with hot deformation parameters frequently employed in rail production lines.The single-pass hot deformation results reveal that the prior austenite grain sizes(PAGSs)for samples with different deformation reductions decrease initially with an increase in deformation temperature.However,once the deformation temperature is beyond a certain threshold,the PAGSs start to increase.It can be attributed to the rise in DRX volume fraction and the increase of DRX grain with deformation temperature,respectively.Three-pass hot deformation results show that the accumulated strain generated in the first and second deformation passes can increase the extent of DRX.In the case of complete DRX,PAGS is predominantly determined by the deformation temperature of the final pass.It suggests a strategic approach during industrial production where part of the deformation reduction in low temperature range can be shifted to the medium temperature range to release rolling mill loads.

Grain Size Prediction after Continuous Cooling Transformation from Deformed Austenite to Ferrite

On the basis of transformation thermodynamics and kinetics theories,an algorithm for predicting ferrite grain size after continuous cooling transformation from deformed austenite to ferrite is suggested.The calculated results of computer simulation with the algorithm are in so good agreement with the measured ones in controlled rolling and controlled cooling experiments that the theoretical algorithm is feasible.

Research on Heredity of Coarse Ferrite Grains

The changes in austenite grain size of the specimens with coarse ferrite grains under different heat treatment process were investigated.The focus was on studying the effect of annealing on refining coarse ferrite grains,as well as the influence of the ferrite grain size on the main technical indicators of gas carburizing.The results show that coarse ferrite grains may not necessarily cause the coarse austenite grains,but may result in mixed austenite grains.After annealing treatment,the coarse ferrite grains can be significantly refined and homogenized.Moreover,the coarse ferrite grains have no significant effects on hardnessand intergranular oxidationof gas carburizing.

Effects of initial grain size and strain on grain boundary engineering of high-nitrogen CrMn austenitic stainless steel

18 Mn18 Cr0.5 N steel with an initial grain size of 28–177 μm was processed by 2.5%–20% cold rolling and annealing at 1000°C for 24 h,and the grain boundary character distribution was examined via electron backscatter diffraction.Low strain(2.5%) favored the formation of low-Σ boundaries.At this strain,the fraction of low-Σ boundaries was insensitive to the initial grain size.However,specimens with fine initial grains showed decreasing grain size after grain boundary engineering processing.The fraction of low-Σ boundaries and the(Σ9 + Σ27)/Σ3 value decreased with increasing strain; furthermore,the specimens with fine initial grain size were sensitive to the strain.Finally,the effects of the initial grain size and strain on the grain boundary engineering were discussed in detail.

Grain size effect on cyclic deformation behavior and springback prediction of Ni-based superalloy foil

In order to clarify the influence of grain size on cyclic deformation response of superalloy sheets and springback behavior,cyclic loading-unloading and shearing tests were performed on the superalloy foils with 0.2 mm in thickness and diverse grain sizes.The results show that,the decline ratio of elastic modulus is weakened with increasing grain size,and the Bauschinger effect becomes evident with decreasing grain size.Meanwhile,U-bending test results determine that the springback is diminished with increasing grain size.The Chaboche,Anisotropic Nonlinear Kinematic(ANK)and Yoshida-Uemori(Y-U)models were utilized to fit the shear stress-strain curves of specimens.It is found that Y-U model is sufficient of predicting the springback.However,the prediction accuracy is degraded with increasing grain size.

Austenite grain growth behavior of Q1030 high strength welded steel

The austenite grain growth behavior of Q1030 steel was studied under different heating conditions. The austenite grain size increases with the heating temperature and holding time increasing. Austenite grains grow in an exponential manner with rising heating tem- perature and in a parabolic manner with prolonging holding time. A mathematical model for describing the austenite grain growth behavior of Q 1030 steel was obtained on the basis of experimental results using regression analysis. When the heating temperatures lie between 1000 and 1100℃ at a certain holding time, abnormal grain growth appears, which causes mixed grains in Q1030 steel.

基于正交实验法的BS960E高强钢的淬火工艺优化

基于正交实验法研究了淬火过程中加热速率、保温温度、保温时间和冷却速率等参数对BS960E贝氏体高强钢显微组织和力学性能的影响。结果表明,淬火后BS960E钢的组织均为板条马氏体;通过正交实验法设计的淬火工艺获得最小平均晶粒尺寸的最优参数组合为:加热速率50℃/s、保温温度920℃及保温时间2 min;获得最大维氏硬度的最优参数组合为:加热速率50℃/s、保温温度1010℃、保温时间2 min及冷却速度100℃/s。通过增设对照实验组验证了正交实验法的正确性,正交实验设计的试样最小平均晶粒尺寸为6.36μm,马氏体板条群、马氏体板条块和马氏体板条分别为5.2μm、1.24μm和336.3 nm。正交实验设计的试样最大硬度为424.3 HV,马氏体板条群、马氏体板条块和马氏体板条分别为8.5μm、1.65μm和333.5 nm。相比于前者,后者在冷却速率相同(100℃/s)的情况下,由更高的温度(1010℃)降低至室温,产生了更大的过冷度,马氏体相变驱动力增加导致位错密度增加,大角度晶界占比由70.5%提高至83.3%,因此硬度更高。

35MnBH钢链轨节表面开裂原因

圆钢35MnBH经锻打、热处理加工成链轨节后表面出现开裂现象,采用宏观观察、化学成分分析、金相检验、淬透性测试等方法对表面开裂原因进行分析。结果表明:链轨节表面裂纹为淬火裂纹,加热工艺不当是产生淬火裂纹的主要原因。

42CrMoA奥氏体晶粒长大行为研究

利用箱式电阻炉以及金相显微镜,试验研究不同Al含量以及热处理工艺对42CrMoA晶粒度长大行为的影响,确定不同Al含量、奥氏体化温度及对应保温时间下该钢种晶粒度长大行为的规律。结果表明:42CrMoA晶粒度长大行为与铝氮比和温度有直接关系,晶粒度临界长大温度随着铝氮比的降低而降低。当铝含量低于0.021时,随着Al/N值的降低,晶粒度临界长大温度与异常长大温度均降低,当铝含量高于0.021时,晶粒度异常长大临界开始升高。且当其他化学成分基本一致时,不同Al含量与奥氏体温度对应的晶粒度尺寸存在一定线性关系。

真空气体淬火热处理对粉末冶金高速钢S390硬度的影响

对粉末冶金高速钢S390进行真空气体淬火热处理,初步探讨了在一定冷却压力和冷却速率条件下真空气体淬火热处理方式的可行性以及其对S390钢硬度的影响。结果表明:粉末冶金高速钢S390的有效厚度为20 mm,在冷却压力为0.8 MPa,冷却速率为3℃/s条件下,可以对S390钢进行真空淬火,该钢不会发生开裂现象;在相同条件下,四次回火后S390钢的硬度比盐浴热处理后该钢的硬度小1.2 HRC,其红硬性较盐浴热处理后小3.1~3.7 HRC。

冷轧连退带钢力学性能不合格原因

采用金相检验、力学性能测试等方法,对某冷轧连退带钢屈服强度、抗拉强度、断后伸长率等力学性能指标不合格的原因进行分析。结果表明:当退火速率较慢时,明火段板带温度升高,当板带温度比珠光体开始奥氏体化温度高30℃时,组织中珠光体趋于球化转变,材料的强度明显降低;当退火速率较快时,明火段板带温度下降,当板带温度比珠光体开始奥氏体化温度低10℃时,晶粒再结晶或者晶粒长大不完全,造成材料强度偏高、断后伸长率偏小。

疲劳试验对GIS波纹管补偿器材料显微组织的影响

文中对疲劳试验前后的GIS波纹管补偿器进行硬度测试、显微组织观察以及化学成分分析等试验,为改进GIS波纹管补偿器制造工艺提供参考。试验结果表明:未经过疲劳试验条件下,试样直边段处的硬度最低,波谷处的硬度最高;而相同位置条件下,经过疲劳试验的试样硬度均大于未经过疲劳试验的试样。试样显微组织均为奥氏体+马氏体,其中疲劳试验后的试样波谷(断口)位置晶粒度最大;较大的晶粒度导致样品波谷处强度增加。化学成分分析结果表明:疲劳试验前后试样化学成分发生偏析,元素的偏析会导致奥氏体不锈钢产生马氏体相变。

热处理工艺及氮含量对20CrMoH奥氏体粗化行为的影响

利用马弗炉热处理和光学金相显微镜对20CrMoH钢原始奥氏体金相组织进行了观察和晶粒尺寸测量,研究了热处理工艺和不同N含量对20CrMoH钢原始奥氏体组织生长规律的影响。研究表明:在热轧组织均匀和细小的情况下,热处理工艺和氮含量均对钢中奥氏体晶粒的影响显著。当N含量为78 ppm时,890℃热处理时晶粒长大不明显;随着加热温度升高,晶粒异常长大现象逐渐显著;当加热温度达到950℃时,晶粒粗化明显,并呈现出不均匀长大现象,出现混晶。当N含量为110 ppm时,890℃和950℃热处理时晶粒长大程度不明显;但是当加热温度达到甚至超过1000℃时,原始奥氏体晶粒出现快速、不均匀长大现象,加热2 h以内即出现混晶。整体而言,热处理温度对20CrMoH材料的原始奥氏体晶粒尺寸和均匀性有显著影响。相比较而言,110 ppm氮含量较78 ppm氮含量材料抵抗晶粒粗化的能力大幅度提升。氮含量的增加能够较大幅度提高20CrMoH齿轮钢的热处理粗化临界温度,拥有更宽的工艺窗口,更有助于产品后续热处理工艺的制定与执行。

检测方法对20CrNiMoH钢奥氏体晶粒度的影响

奥氏体晶粒度是衡量钢材质量的一个重要指标,研究了奥氏体晶粒度显示方法对20CrNiMoH渗碳齿轮钢奥氏体晶粒度检测结果的影响。结果表明,氧化法和直接淬硬法检测的奥氏体晶粒度差别较小,渗碳法检测的奥氏体晶粒比氧化法和直接淬硬法低0.2~0.3级,模拟渗碳法检测奥氏体晶粒度存在混晶现象。对造成模拟渗碳法出现混晶的原因进行了分析,并给出了改进措施,确保使用模拟渗碳法检测奥氏体晶粒度仍没有混晶现象出现。