应变速率对新型Q-P-T汽车钢TRIP效应和动态力学行为的影响及其机制：相场模拟与实验验证

The high ductility of novel quenching-partitioning-tempering steels is attributed to considerable retained austenite and its transformation induced plasticity (TRIP) effect, which is affected by high strain rate. Our elementary investigation indicates that the dynamic mechanical behaviors of low-carbon Q-P-T automobile steels are obviously different from their quasi-static ones. To reveal theoretically the nature of TRIP effect evolution and the origin of difference between dynamic and quasi-static mechanical behaviors of Q-P-T automobile steels, which is the base of Q-P-T automobile steels applied in high strain rate deformation, the phase field simulation combining with the experimental verification will be used in this application program related to the important foundation in the advanced high strengthen automobile steels, and the following investigations will be performed: Phase-field dynamic model connected with stress relaxation effect and strain rate will be proposed; Based on the simulation of microstructural evolution and redistribution of inner stress under the conditions of dynamic uniaxial compression/tension tests or quasi-static ones, respectively, quantitative relationship between strength/plasticity/product of strength and elongation and strain rate will be built under different deformation styles and the mechanism of TRIP effect evolution related to high strain rate will be revealed; Digital image correlation （DIC）technology combined with experimental obsevation and characterization will be used to determine the distribution of micro-strain and the preferential nucleation positions and propagation path of cracks according to the microstructures in mechanical tests before or after, in which the strain distribution determined by DIC technology will be compared with that simulated by phase field method. This program will help supplying theoretical direction in the industrial application and development of Q-P-T automobile steels at high strain rate.

新型淬火-分配-回火(Q-P-T)汽车钢的高塑性归因于其具有大量的残留奥氏体及其相变诱发塑性(TRIP)效应，应变速率对其有重要影响。我们初步研究发现，低碳Q-P-T汽车钢的动态拉伸和压缩力学行为明显不同于准静态。为了从理论上揭示应变速率关联的TRIP效应演化机制和Q-P-T汽车钢动态/准静态力学行为差异的起因，本项目针对新一代高强汽车钢中这一重要基础问题，拟开展以下研究：1)建立包含应力松弛效应和应变速率的相场动力学模型；2)相场模拟研究应变速率对TRIP效应的影响规律及其机制；3)相场模拟不同应变速率/应变方式下微观组织演变及应力再分布，结合实验得到基于应变速率的动态力学的定量变化规律；4)基于微观组织形貌观察与表征，运用数字图像关联(DIC)新技术获得应变分布，揭示与应变速率对应的裂纹优先萌生位置和扩展的路径。本项目研究将为新型Q-P-T汽车钢在高应变速率下的工业应用提供理论指导。

动态冲击实验及相场模拟相互结合研究表明，冲击载荷下变体的相互协调及变体重排对TRIP效应有贡献，随应变速率增加强度会增加，但相变及TRIP效应会受到抑制。将相场方法与有限元结合起来可实现温度场-组织场-应力场的耦合模拟，这对于研究大尺度试样并与实验结果直接比较具有很强的优越性。实验结果显示动态载荷下裂纹与切变带会协同发生，高速形变下裂纹为扩张型裂纹，能穿透晶粒内的各种相；相场有限元模拟表明裂纹可以在新相与母相界面形核并扩展至另外一片新相微裂纹自身会阻止新相穿过，在裂纹端部由于应力集中又会导致新相在其周围优先形核并长大，这些变化会导致内应力出现再分布；数值模拟显示，结构相变会导致表面浮凸的形成，并能得到宏观塑性变形，此浮凸的宏观塑性变形会对微裂纹的自愈合具有积极的作用，同时能阻止微裂纹的扩展。基于相场方法，结合晶体学和界面形态学可以对复杂组织的连续结构转变进行模拟和分析，这些工作对于钢铁材料按照CCT曲线进行工艺设计特别是其中复杂的结构相变演化具有积极的参考价值。