Microscopic Response of TRIP Steels to Prestrain During Plastic Deformation

In order to uncover the mechanism of elastic modulus degradation during plastic deformation, uniaxial tensile test of transformation-induced plasticity (TRIP) steels under different prestrain levels was carried out. The real elastic modulus unloaded at each prestrain was calculated by linearly fitting. The microstructure evolution with plastic strain and the fracture morphology were monitored by using a scanning electron microscope (SEM). Dislocation density and its distribution were detected under a transmission electron microscope (TEM). Microscopic mechanism of the elastic modulus degradation of TRIP steels was discussed in detail. Experimental results indicated that the investigated TRIP600 steel was of severe elastic modulus degradation during plastic deformation. The new-born martensite distributed among the retained austenite, resulting in the combination of good ductility and high strength for TRIP steels. It was the change of dislocation movement that induced the variation of atomic binding force and finally led to the variation of elastic modulus.     

Deformation-induced austenite grain rotation and transformation in TRIP-assisted steel

Uniaxial straining experiments were performed on a rolled and annealed Si-alloyed TRIP (transformation-induced plasticity) steel sheet in order to assess the role of its microstructure on the mechanical stability of austenite grains with respect to martensitic transformation. The transformation behavior of individual metastable austenite grains was studied both at the surface and inside the bulk of the material using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD) by deforming the samples to different strain levels up to about 20%. A comparison of the XRD and EBSD results revealed that the retained austenite grains at the surface have a stronger tendency to transform than the austenite grains in the bulk of the material. The deformation-induced changes of individual austenite grains before and after straining were monitored with EBSD. Three different types of austenite grains can be distinguished that have different transformation behaviors: austenite grains at the grain boundaries between ferrite grains, twinned austenite grains, and embedded austenite grains that are completely surrounded by a single ferrite grain. It was found that twinned austenite grains and the austenite grains present at the grain boundaries between larger ferrite grains typically transform first, i.e. are less stable, in contrast to austenite grains that are completely embedded in a larger ferrite grain. In the latter case, straining leads to rotations of the harder austenite grain within the softer ferrite matrix before the austenite transforms into martensite. The analysis suggests that austenite grain rotation behavior is also a significant factor contributing to enhancement of the ductility.     

The Effect of Size and Shape of Austenite Grains on the Mechanical Properties of a Low‐Alloyed TRIP Steel

The effects of size and shape of austenite grains on the extraordinary hardening of steels with transformation induced plasticity (TRIP) have been studied. The deformation and transformation of austenite was followed by interrupted ex situ bending tests using electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). A finite element model (FEM) was used to relate the EBSD based results obtained in the bending experiments to the hardening behavior obtained from tensile experiments. The results are interpreted using a simple rule of mixture for stress partitioning and a short fiber reinforced composite model. It is found that both, the martensite transformation rate and the flow stress difference between austenite and martensite significantly influence the hardening rate. At the initial stage of deformation mainly larger grains deform, however, they do not reach the same strain level as the smaller grains because they transform into martensite at an early stage of deformation. A composite model was used to investigate the effect of grain shape on load partitioning. The results of the composite model show that higher stresses develop in more elongated grains. These grains tend to transform earlier as it is confirmed by the EBSD observations.     

Effect of Testing Temperature on Retained Austenite Stability of Cold Rolled CMnSi Steels Treated by Quenching and Partitioning Process

The effect of testing temperature on retained austenite (RA) stability of industrially cold rolled CMnSi sheet steel treated by quenching and partitioning (Q&P) process has been investigated by observing the deformation and transformation behavior of RA at different testing temperatures. Uniaxial tensile properties at different temperatures were determined and a correlation between RA stability and mechanical properties were also established. Ultimate tensile strength increases monotonously when temperature decreases, while total elongation reaches an optimum value between 0 and 20°C, where RA exhibits the greatest TRIP effect. Work hardening rate was calculated to decrease through three different stages in an oscillation manner, leading to significant enhancement in both strength and ductility. The kinetic of deformation‐induced martensite transformation is also studied and the stability of RA can be evaluated by comparing the kinetic parameter β.     

Dynamic shear response and evolution mechanisms of adiabatic shear band in an ultrafine-grained austenite–ferrite duplex steel

The dynamic properties of an intercritically annealed 0.2C5Mn steel with ultrafine-grained austenite–ferrite duplex structure were studied under dynamic shear loading. The formation and evolution mechanisms of adiabatic shear band in this steel were then investigated using interrupted experiments at five different shear displacements and the subsequent microstructure observations. The dynamic shear plastic deformation of the 0.2C5Mn steel was observed to have three stages: the strong linear hardening stage followed by the plateau stage, and then the strain softening stage associated with the evolution of adiabatic shear band. High impact shear toughness was found in this 0.2C5Mn steel, which is due to the following two aspects: the strong linear strain hardening by martensite transformation at the first stage, and the suppressing for the formation of shear band by the continuous deformation in different phases through the proper stress and strain partitioning at the plateau stage. The evolution of adiabatic shear band was found to be a two-stage process, namely an initiation stage followed by a thickening stage. The shear band consists of two regions at the thickening stage: a core region and two transition layers. When the adjoining matrix is localized into the transition layers, the grains are refined along with increasing fraction of austenite phase by inverse transformation. However, when the transition layers are transformed into the core region, the fraction of austenite phase is decreased and almost disappeared due to martensite transformation again. These interesting observations in the core region and the transition layers should be attributed to the competitions of the microstructure evolutions associated with the non-uniformly distributed shear deformation and the inhomogeneous adiabatic temperature rise in the different region of shear band. The 0.2C5Mn TRIP steel reported here can be considered as an excellent candidate for energy absorbers in the automotive industry.     

Effect of Silicon Content on Thermodynamics of Austenite Decomposition in C-Si-Mn TRIP Steels

Some numerical models such as central atoms model （CAM） and superelement model were used to simulate the thermodynamics of austenite decomposition in the Fe-C-Mn-Si TRIP （transformation induced plasticity） steels. Thermodynamic calculations were carried out under a para-equilibrium （PE） condition. The results show that certain silicon content can accelerate the polygonal ferritic transformation and increase the volume fraction and stability of retained austenite by retarding the precipitation of carbides during the bainitic transformation.     

Austempering of Hot Rolled SiMn TRIP Steels

 The austempering after hot rolling in hot rolled Si Mn TRIP (transformation induced plasticity) steels was investigated. The mechanism of TRIP was discussed through examination of the microstructure and the mechanical properties of this kind of steel. The results showed that the strain induced transformation to martensite of retained austenite occurs in hot rolled Si Mn TRIP steels. The sample exhibited a good combination of ultimate tensile strength and total elongation when it was held at the bainite transformation temperature after hot deformation. The stability of retained austenite increases with an increase in isothermal holding time, and a further increase in the holding duration resulted in the decrease of stability. The mechanical properties were optimal when holding for 25 min, and tensile strength and total elongation reached the maximum values (774 MPa and 33%, respectively).     

不同屈强比的冷轧DP780钢工艺调控技术分析

采用不同的工艺调控技术,实现了一种成分体系可生产具有不同屈强比的经济型冷轧DP780钢,并通过分析力学性能测试结果、TEM和SEM组织形貌特征,得到热轧初始组织、冷轧压下量、连续退火工艺对屈强比的影响。结果表明,当热轧初始组织为F＋P(铁素体＋珠光体)时,随着平均晶粒尺寸细化至约7.5 μm,屈服强度增加了50 MPa,屈强比由0.48增至0.56;当热轧初始组织变为F＋B(铁素体＋贝氏体）、以贝氏体为主时,屈服强度达到532 MPa,屈强比增至0.65,同时有利于保证DP780钢的扩孔性和塑性,扩孔率达到86%,特别适用于有扩孔翻边要求的汽车结构件和加强件。此外,适当增加冷轧压下量和降低退火保温温度,均有利于增强基体的强化效应,从而提高屈强比。     

TRIP钢中残余奥氏体及其稳定性的研究

采用扫描电镜、透射电镜、X射线衍射仪等对贝氏体等温转变后TRIP钢中的残余奥氏体及其稳定性进行了研究.结果表明,TRIP钢在贝氏体转变区400～440 ℃保温120～300 s,随着等温温度的升高和保温时间的延长,钢中残余奥氏体的含量不断增多,残余奥氏体碳含量呈降低趋势.TRIP钢中的残余奥氏体主要以薄膜状、粗大块状和细小粒状的形态存在.粗大块状的残余奥氏体稳定性最差,薄膜状次之,细小粒状最稳定.残余奥氏体的含量不足,或残余奥氏体的含量偏高造成碳含量的不足,都会导致TRIP钢综合成形性能的降低.此外,贝氏体等温处理时间过长,渗碳体的出现大大降低了残余奥氏体中的碳含量,从而降低了残余奥氏体的稳定性.