抑制中锰钢塑性失稳现象的新型变形工艺的开发与研究

Medium Mn steel always exhibit plastic instability during tensile deformation, leading to the appearance of yielding plateau and the stress serration on the stress-strain curves. This is one of the reasons why the medium Mn steel has not been industrialized yet. The existing research on the plastic instability of medium Mn steel mainly focuses on the optimization of microstructure and deformation temperature, which tends to reduce the strength or plasticity while eliminating plastic instability. Our previous research found that the simultaneous loading of electric pulse during the tensile deformation of medium Mn steel can reduce the Lüders strain and eliminate the PLC effect without significant affecting their mechanical properties. Therefore, we are going to study the effect of electric pulse loading during tensile deformation on the plastic instability and tensile properties of medium Mn steel. Firstly, both conventional tensile tests and electrical pulse tensile tests are adopted to study the effect of strain rate and electrical pulse on mechanical properties of medium Mn steel, and the influencing mechanism will be clarified by microstructural examination; then the influence of the electric pulse on the microstructure evolution during the nucleation and propagation of local deformation bands will be studied by using the interrupted tensile test, the high energy X-ray diffraction and digital image correlation (DIC) methods; finally, the optimization criteria of the electric pulse deformation process of medium Mn steel will be proposed, which is expected to eliminate the plastic instability under the premise of ensuring the mechanical properties.

中锰钢在拉伸变形过程中往往会发生局部变形不稳定现象，在应力-应变曲线上表现为屈服平台和应力锯齿，这是性能优异的中锰钢尚未得到广泛应用的原因之一。现有关于中锰钢塑性失稳现象的研究主要集中于显微组织和变形温度的优化，然而在消除塑性不稳定现象的同时往往会降低强度或者塑性。申请人前期研究发现，中锰钢拉伸变形时同步加载电脉冲能够同时降低吕德斯应变和消除PLC效应，但是对力学性能几乎没有影响。因此，本项目拟开展电脉冲同步加载对塑性不稳定现象和拉伸性能影响的研究。首先，采用常规拉伸和电脉冲拉伸实验研究应变速率和电脉冲对力学性能的影响规律，并且通过显微组织表征阐明机理；在此基础上，采用中断拉伸实验，原位高能X射线衍射和数字图像相关技术（DIC）等研究电脉冲对局部变形带的形核和传播，以及相应组织演变的影响规律；最后，提出中锰钢电脉冲变形工艺的优化准则，旨在保持力学性能的前提下，消除塑性不稳定现象。

中锰钢兼具超高强度，优良韧塑性和低合金成本被认为是最有潜力的新一代先进高强钢。然而，高强韧中锰钢在变形中往往发生局部塑性变形带的形核和传播，在应力-应变曲线上表现为屈服下降，屈服平台和应力锯齿，使其在冲压变形过程容易发生条带状褶皱和延迟断裂，从而限制了此类钢的大规模工业化制备和应用。.本项目首先探究了应变速率和电脉冲同步加载处理对中锰钢塑性失稳和拉伸性能的影响机制。研究发现，随应变速率增加，吕德斯前沿的奥氏体转变量和吕德斯应变均不变。然而，随应变速率提升，PLC带的类型由Type A转变为Type C。这是因为高应变速率下变形时，高应变下会产生自绝热，抑制奥氏体转变为马氏体，促进标距范围内其他区域的PLC带形核。电脉冲同步加载能同时抑制吕德斯带和PLC带的形核和传播，但导致加工硬化能力显著下降。基于此，项目组转化研究思路在变形前对研究中锰钢进行电脉冲处理，不仅有效抑制了不连续屈服和PLC效应，而且显著提升了加工硬化能力。至此，我们开发出一种能同时抑制中锰钢塑性失稳和提升加工硬化能力的新技术。其机理为，电脉冲的电效应能够降低原子间键合力，导致奥氏体可以在更高的温度下达到向马氏体转变的临界应变。另外，电脉冲处理产生的焦耳热还会促进C原子由马氏体配分至奥氏体，提升奥氏体的稳定性而提升高应变下的加工硬化能力。.项目组在同一中锰钢中引入等轴+片层两种形貌的双相组织探究了组织形貌对屈服行为的影响。研究发现，位错易于在共格片层相界面发射产生屈服前的微观应变，引起临近等轴奥氏体应力集中而在更低应力下发生相变和吕德斯带的形核。吕德斯带传播过程，未变形区域的微观变形导致奥氏体转变，诱发其他吕德斯带形核而缩短吕德斯应变。项目组采用空冷和水冷的方式实验研究了冷却速率对屈服行为的影响机理。研究发现，冷却速率越慢相界面会形成更显著的C/Mn偏聚，导致界面发射位错困难，发生不连续屈服。