温热成形超细晶中锰钢的马氏体相变行为和成形性分析

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.

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。

为了实现汽车轻量化并提高被动安全性，更多的汽车安全件和结构件都使用了高强钢热成形技术，热成形零件通常使用经典22MnB5硼钢制造，工业加热温度一般为950℃，但其存在生产成本高、氧化脱碳（针对无涂层板料）、工业窗口窄以及零件塑性较低等问题。温成形是一种基于中锰钢显著降低加热温度的热成形工艺，坯料加热至完全奥氏体化后，在模具中成形的同时完成淬火，获得完全马氏体组织及超高强度力学性能，为了区别于传统热成形，定义为温成形工艺。项目研究要点1) 掌握多场耦合作用下中锰钢马氏体相变行为和多尺度微观结构的形成规律；2) 得到能够体现超细晶中锰钢多场耦合关系的数值仿真方法；3) 阐述中锰钢的高温成形极限能力；4) 构建制备具有较好成形性、优异力学性能的超细晶中锰钢件全新的、科学的温成形方法和理论模型。资助期间，上述研究要点均取得一定成果，不但丰富了马氏体相变理论，也完善了中锰钢高温塑性成形理论，并基于已积累的成果和经验，成功获得了工业条件下中锰钢工艺窗口，完成了大变形条件下多个典型复杂汽车零件的应用型基础研究，为后续的应用推广奠定了理论基础和必要数据。2016年北京车展中，北汽集团将温成形中锰钢B柱作为未来轻量化的主要方向之一进行展出，得到各大汽车厂和钢厂的关注。第一或通讯，项目负责人完成了高水平文章8篇(学科交叉特色，中科院TOP期刊或领域知名期刊)；与合作单位主要参加者完成中文综述性论文1篇；另外，在线1篇；第一发明人，授权国家发明专利12项；获奖方面，排名第一，获得辽宁省自然科学学术成果奖2项、大连市优秀科技成果奖2项，同时，合作者王存宇老师2019年获得第九届中国金属学会“冶金青年科技奖”；国内外学术会议，参加包括国外学术会议TMS(美国)、以及国内顶级学术会议十余次，并作邀请报告和口头报告等。