不同变形温度下稀土Ce改性2507超级双相不锈钢的微观组织演变

采用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)、X射线衍射(XRD)等试验手段对不同温度(室温、高温)下变形后的稀土Ce改性2507超级双相不锈钢的微观组织演变进行了表征。结果表明:在不同温度的变形过程中,双相不锈钢中铁素体相比例明显增加,但其增加机理却不相同,热变形过程中奥氏体相向高温铁素体发生转变导致铁素体相含量增加,而冷变形过程中奥氏体相则发生形变诱导马氏体转变导致铁素体相含量增加。热变形过程中奥氏体发生动态再结晶,铁素体晶粒发生动态回复导致晶粒细化;而冷变形过程中奥氏体发生形变诱导马氏体转变和形变孪晶,铁素体晶粒则发生碎化而导致晶粒细化。     

介观尺度上热变形奥氏体储存能演化的计算机模拟

为了定量描述热变形奥氏体在介观尺度上微观变形的非均匀性,采用晶体塑性有限元方法(crystal plasticity finite element method,CPFEM)模拟了C—Mn钢在不同变形条件下的变形行为,得到了在介观尺度上奥氏体的微观应力应变和变形储存能分布．模拟所得到的应力一应变曲线与文献测定的应力一应变曲线基本一致．通过对真应变为0．5,变形速率为50s^-1的热变形奥氏体的研究发现,即使在外部的均匀变形条件下,无论是在晶粒内部还是晶粒间,材料内部变形都非常不均匀．这种变形不均匀性主要是由晶粒的初始取向不同,近邻晶粒的取向差,以及变形时滑移系的运动特性不同所引起的．本文定量描述了介观尺度上奥氏体变形储存能不均匀分布,为结合介观尺度组织模拟,实现组织演变的多尺度耦合计算提供了参考．     

热变形参数对0.05C-0.128Nb钢组织演变规律的影响

采用Thermecmastor-Z热模拟试验机,对0.05C- 0.128Nb微合金钢及X65(0.045%Nb)钢进行了热变形模拟试验,研究了其微观组织演变规律,得出高铌钢变形温度与变形抗力之间的关系.结果表明,铌含量的增加显著提高了再结晶温度,延迟了奥氏体再结晶.在低于1000 ℃变形时,低碳高铌钢形成大量扁平的"薄饼"状奥氏体组织,显著抑制了奥氏体再结晶,促进了变形诱导铁素体相变的发生.变形温度与变形抗力的对数值之间呈线性关系.     

预变形和双级时效对Fe-30Mn-11Al-1.2C奥氏体低密度钢显微组织和力学性能的影响

采用EBSD、TEM和万能试验机等研究了冷轧预变形和双级时效对Fe-30Mn-11Al-1.2C (质量分数,%)奥氏体低密度钢微观组织演变和力学性能的影响。结果表明,双级时效可以显著地提高材料的屈服强度,从固溶时的580 MPa到1120 MPa,但同时使得均匀延伸率急剧降低至几乎为0;而经过轧制预变形+双级时效处理后的样品,材料的屈服强度进一步提高,达到1220 MPa,同时材料的均匀延伸率大幅提高至18.2%,钢的综合力学性能得到明显提升。微观组织分析表明,双级时效后材料屈服强度的提升归因于κ′碳化物的有序化强化;预变形可以在奥氏体基体中引入有效的异质形核点,诱导晶内析出;该析出相(析出强化)结合预变形引入位错(形变强化)进一步提高材料的屈服强度,同时提高了材料的应变硬化能力,这是材料高塑性的根本原因。该工艺为奥氏体低密度钢的性能改善提供了新思路。     

一种节镍型奥氏体耐热钢热变形行为的研究

通过热压缩实验研究了21Cr~(-1)1Ni-N-RE节镍型奥氏体耐热钢的热力学行为和微观组织演变过程。通过对实验数据的回归分析,得到实验钢热变形激活能为451 k J/mol,应力指数为5.12。建立了热变形方程,确定了最大变形抗力和动态再结晶临界应变预测模型。通过对微观组织演变过程的分析,得到了实验钢获得均匀细小的完全动态再结晶组织的热变形条件为1150℃和10 s~(-1)。     

变形温度对形变强化相变完成时临界应变量的影响

利用“形变强化相变”机制研究了低碳钢过冷奥氏体在740℃和780℃,10s-1变形时的变形温度对相变完成时临界应变量εc的影响。结果表明,变形温度对εc和组织演变的影响很大。在740℃和780℃变形时,εc分别为0.96和1.39,变形温度降低明显促进了相变。变形温度对εc的影响在组织演变上主要表现为铁素体形核地点的不同。740℃变形时,铁素体由奥氏体晶界形核过渡到以形变带形核为主,形核速率极高;780℃变形时,铁素体由奥氏体晶界形核过渡到在铁素体/奥氏体相界面前沿高畸变区快速形核。     

节约型双相不锈钢2101高温变形过程中微观组织演化

采用电子背散射衍射技术(EBSD)和TEM研究了节约型双相不锈钢2101在温度为1000℃和应变速率为5 s~(-1)的高温变形过程中的微观组织演化.结果表明,铁素体和奥氏体都发生以小角度晶界不断向大角度晶界转变为特征的连续动态再结晶(CDRX).固溶退火后双相不锈钢奥氏体内出现大量退火孪晶.随变形量增加,奥氏体中具有∑3位向关系的晶界逐渐消失.高温变形过程中双相微观组织演化机制的耦合作用共同决定了流变曲线特征.     

热变形奥氏体动态再结晶的组织表征

研究了Fe-32%Ni合金奥氏体在1000℃、形变速率2×10-3 s-1试验条件下高温变形时的微观组织特征及其演变过程。采用光学显微镜(OM)、电子背散射衍射(EBSD)和透射电镜(TEM)对组织结构进行分析。结果表明:热变形奥氏体发生了不连续动态再结晶,根据形变奥氏体晶粒的位错分布特征,热变形动态再结晶晶粒分为3类:一是位错密度很低的细小晶粒;二是具有位错密度梯度的晶粒;三是高密度位错大体均匀分布的晶粒。     

热塑性变形过程中钛合金微观组织演变的可视化仿真

热塑性加工过程中材料的微观组织一般经历晶粒长大、静及动态再结晶等微观组织的演变,而微观组织状态直接影响着被加工零件的使用性能.因此,了解材料在热变形过程中组织的演变规律具有很重要的工程应用价值.在大量试验的基础上,利用有限元软件Marc模拟TC4钛合金的热锻过程.通过有限元软件,对热成形工艺参数如等效应力、等效应变、等效应变速率和变形温度的分布进行求解,再利用人工神经网络建立微观组织演变和热成形工艺参数的本构关系.然后,基于Voronoi图用软件Visual C++6.0来实现微观组织演变的可视化模拟.初步实现了TC4钛合金热变形过程微观组织的长大、再结晶等组织演变的二维可视化模拟.研究结果表明通过计算机模拟仿真,可以有效地、直观地预测热成形过程的材料微观组织变化规律,从而为热成形加工参数的优化提供参考.但要更精确地模拟和仿真热塑性变形中材料的微观组织的演变,还需更多地掌握在热塑性变形过程中的演变机制.     

热变形参数对40CrNiMo钢组织的影响

利用Gleeble1500热/力学模拟实验机,对40CrNiMo钢进行双道次热模拟单向压缩试验。分析了40CrNiMo钢在变形温度为950、760℃,变形速率为0.5～30s-1,变形量为0.05～0.4,热变形后的奥氏体组织特征。结果表明,在950℃时,奥氏体组织能发生动态再结晶,最终奥氏体晶粒形状取决于变形速率和变形量;40CrNiMo钢在760℃时奥氏体组织发生动态回复,最终奥氏体晶粒呈扁平的"薄饼"状,奥氏体晶粒的变形程度取决于变形量的大小。     

工艺参数对过共析钢过冷奥氏体动态转变的影响

利用Gleeble 1500热模拟试验机进行单轴热压缩实验,研究了含Al过共析钢和不含Al过共析钢过冷奥氏体形变过程中,形变温度和应变速率对组织演变的影响。结果表明:过共析钢过冷奥氏体形变过程中的动态转变经历动态相变和相变所得珠光体的动态球化、超细化。过冷奥氏体动态相变抑制晶界网状渗碳体的形成。动态转变所经历的两个过程均为热激活过程,受形变温度和应变速率的影响。降低形变温度,使过冷度增加;降低应变速率,使完成应变的时间延长,两者均减小了过冷奥氏体完成动态相变所需的应变量,使相变所得的珠光体经历的变形程度增加,有利于珠光体实现球化、超细化。合金元素Al的添加阻碍铁、碳原子的扩散,推迟过冷奥氏体动态相变的发生,细化所得的球化、超细化复相组织。     

A Multi-phase Field Model for Static Recrystallization of Hot Deformed Austenite in a C–Mn Steel

A multi-phase-field model has been developed to simulate the microstructure evolution and kinetics of the austenite static recrystallization(SRX) in a C–Mn steel. In this model, the bulk free energy that coupling the deformation stored energy with a special interpolation function is incorporated. Both the deformed grain topology and the deformation stored energy have been included in order to investigate the influence of pre-deformation on the subsequent austenite SRX at different hot deformation levels. Diverse scenarios of microstructure evolution show different deformation-dependent recrystallized grain sizes. The transformation kinetics is then discussed by analyzing the overall SRX fraction and the average interface velocity on the recrystallization front.     

DEFORMATION ENHANCED FERRITE TRANSFORMATION IN PLAIN LOW CARBON STEEL

1. IntroductionGrain refinement is an effective way of increasing strength and ductility of metallicmaterials simultaneously. In recent ten years the approach to the grain refinement in steelsexperienced a period from thermal mechanical controlling processing (TMCP) in the yearsof 60--70's to the strain induced transformation in 80's. Its basic concept is to the controlof recrystallization, transformation and grain growth in different periods of hot working byutilizing microalloying elemellt…     

Stress-assisted martensitic transformations in steels: A 3-D phase-field study

A 3-D elastoplastic phase-field model is developed for modeling, using the finite-element method, the stress-assisted martensitic transformation by considering plastic deformation as well as the anisotropic elastic properties of steels. Phase-field simulations in 3-D are performed by considering different loading conditions on a single crystal of austenite in order to observe the microstructure evolution. The thermodynamic parameters corresponding to an Fe–0.3% C steel as well as the physical parameters corresponding to commercial steels, acquired from experimental results, are used as input data for the simulations. The simulation results clearly show the well-known Magee effect and the Greenwood–Johnson effect. The results also show that even though the applied stresses are below the yield limit of the material, plastic deformation initiates due to the martensitic transformation, i.e. the well-known transformation-induced plasticity (TRIP) phenomenon. It is concluded that the loading conditions, TRIP as well as autocatalysis play a major role in the stress-assisted martensitic microstructure evolution.     

STUDY OF DYNAMIC RECRYSTALLIZATION OF LOW CARBON STEEL IN THIN SLAB CONTINUOUS ROLLING PROCESS

Combined with the technological characteristics of thin slab continuous rolling process （TSCR）, dynamic recrystallization of an extremely coarse austenite of low carbon steel is studied by Thermecmaster-Z hot simulator. By the analysis of true stress-strain curves and the observation of microstructures at different deformation stages, the critical stress and critical strain are determined under different deformation conditions. The effect of Z parameter on dynamic recrystallization of coarse austenite is studied. The microstructure evolution in real production is also discussed.     

低碳-硅-锰系TRIP钢的组织演变规律研究

采用彩色金相、SEM、TEM和X射线衍射技术研究了低碳-硅-锰TRJP钢在单向拉伸状态下的组织演变规律.结果表明,TRIP钢变形前的组织为F、B和残余奥氏体,经拉伸变形后部分残余奥氏体在应变作用下转变为孪晶结构的马氏体,提高了钢的强度;TRIP钢的断裂为韧性断裂,位于F晶界处的残余奥氏体发生相变从而松弛了应力,延缓了断裂的产生,使TRIP钢板获得高塑性.     

Investigation and Modeling of Austenite Grain Evolution for a Typical High-strength Low-alloy Steel during Soaking and Deformation Process

The final mechanical properties of components greatly depend on their grain size. It is necessary to study the grain evolution during different thermomechanical processes. In the study, the real-time austenite grain evolution of a high-strength low-alloy (HSLA) steel during the soaking process is investigated by in situ experiments. The effects of different deformation parameters on the dynamic recrystallization (DRX) kinetic behaviors are investigated by hot compression experiments. Based on the observations and statistics of the microstructures at different thermomechanical processes, a unified grain size model is established to evaluate the effects of soaking parameters and deformation parameters on the austenite grain evolution. Also, the DRX kinetic model and critical strain model are established, which can describe the effects of the soaking process on the DRX kinetics process well. The established grain size model and DRX kinetic model are compiled into the numerical simulation software using Fortran language. The austenite grain evolution of the material under different deformation conditions is simulated, which is consistent with the experimental results. It indicates that the established model is reliable, and can be used to simulate and predict the grain size during different thermomechanical processes accurately.     

Dynamic Phase Transformation and Spheroidization of Cementite of Hypereutectoid Steel Containing Aluminum during Deformation

利用Gleeble 1500热模拟试验机进行单轴热压缩实验, 研究了合金元素Al对过共析钢缓冷相变和 过冷奥氏体动态相变组织的影响. 结果表明: 在缓冷相变时, Al的加入抑制网状渗碳体形成, 细化珠光体 片层间距; 在过冷奥氏体形变过程中, 动态转变经历动态相变和相变所得珠光体中渗碳体球化及铁素体动 态再结晶等过程. 在动态相变过程中, 没有形成晶界网状渗碳体, 而直接产生珠光体. Al的加入使动态相变过程中奥氏体的稳定性提高、珠光体转变推迟, 进一步细化了珠光体片层间距. 在相变所得珠光体中渗碳体球化及铁素体动态再结晶的过程中, Al阻碍渗碳体粗化, 使渗碳体颗粒和铁素体晶粒尺寸细化.     

Thermally induced surface roughness in austenitic–ferritic duplex stainless steels

Austenitic–ferritic duplex steels hot forged to rods demonstrate a complex deformational behaviour even in the absence of mechanical loads: purely thermal cycling in the temperature interval from 20 °C to 900 °C can cause either accumulation of inelastic strains (thermal ratchetting) or plastic shakedown, depending on microstructure morphology and thermomechanical properties of the constituting phases. The structural macroscopic response of entire specimens to cyclic thermal loading is investigated by dilatometry experiments. The influence of traction-free external surfaces on the microscopic deformation of ferrite and austenite is examined by measuring the surface roughness evolution by three-dimensional profile scans. Besides an increase of the roughness parameters due to thermal cycling, the measured surface profiles also reveal a strong anisotropy linked to microstructural orientation. Spatial surface roughness parameters based on the autocorrelation function allow a quantitative correlation between roughness and microstructure. Measurements are compared with micromechanical finite-element predictions.     

Prediction of post-dynamic austenite-to-ferrite transformation and reverse transformation in a low-carbon steel by cellular automaton modeling

The post-dynamic transformation that takes place during the subsequent isothermal holding for the case when dynamic strain-induced transformation (DSIT) from austenite to ferrite occurs during hot deformation is investigated by cellular automaton modeling. The simulation provides a better understanding of carbon diffusion in retained austenite and the resulting microstructure evolution during the post-dynamic transformation. The predictions reveal that continuing transformation from retained austenite to ferrite and the reverse transformation can occur simultaneously in the same microstructure during post-deformation isothermal holding owing to the locally acting chemical equilibrium conditions. Competition between forward and reverse transformation exists during the early stage of post-dynamic heat treatment. It is also revealed that increasing the final strain of DSIT might promote the reverse transformation, whereas the continuous austenite-to-ferrite transformation yields a diminishing effect. The influence of the DSIT final strain on the grain size of ferrite and the characteristics of the resultant microstructure is also discussed.