超细晶中锰钢作为国家973项目最新研发的第三代汽车钢，经温热成形后，具备与目前的热成形高强钢强度相当而塑性翻番的显著优势，更利于汽车安全性品质的提升。因中锰钢成分、超细晶微观结构等的新特征属性，导致其温热成形过程中的系列基础科学问题亟需解决，以保证复杂结构精确成形的工艺开发。本项目将研究变温、变加载条件下中锰钢马氏体多尺度微观结构中各力学性能的组织控制单元，以构建理想的超细晶微观结构形成路径；揭示多场耦合作用下的马氏体相变行为特征,并完善相变动力学和相变塑性模型，为冲压仿真分析奠定理论基础；开展不同应变路径下板材的热成形极限试验，绘制全温度域的三维成形极限图，阐明板材高温成形能力和断裂机制。本项目提出的中锰钢在热-力-相变耦合作用下多尺度微观结构形成、理论模型构建和高温成形性评价等均是全新课题，研究成果将丰富马氏体相变理论和弹塑性成形理论，高强度、高塑性中锰钢的成功应用将具有重要现实意义。

Ultrafine grained medium-Mn steel is the newly developed third generation automobile steels from national “973”project. Compared with the current hot-stamped high strength steels, the medium-Mn steel treated by warm/hot stamping process possesses the similar strength and higher plasticity, which is more beneficial to promote the safety of automobile. In order to develop the precise warm/hot forming technology of automobile structures for the mudium-Mn steel with special components and ultrafine grained microstructure characteristics, it is necessary to resolve series of basic scientific problems during warm/hot stamping process. In this project, firstly, under various temperature and loading conditions, the microstructural units in multi-scale martensite influencing the strength and plasticity will be analyzed to construct the forming path of ideal mircrostruture with the optimal properties. Secondly, the martensitic transformation behavior of medium-Mn steel under multi-field coupling effect will be revealed, and the kinetics model and plasticity model of phase transformation will be improved, which can lay the theoretical foundation for the warm/hot stamping simulation analysis. Furthermore, for clarifying the high temperature formability and fracture mechanism of medium-Mn steel, a 3D thermal forming limit diagram for sheets within full temperature range will be also obtained based on the thermal forming limit tests. This project proposes three new subjects for the medium-Mn steel under the coupling effect of thermal-machanical-phase transformation, such as the formation of multi-scale microstructure, the construction of theoretical models and the evaluation of formability. The research results will enrich the theory of martensitic transformation and elastic-plastic forming. The successful application of warm/hot stamped medium-Mn steels with high strength and plasticity will have important realistic significance.