基于中断淬火-碳配分-临界区退火的中锰钢热冲压成形组织演变及增强增韧机理研究

Hot stamping of high-performance medium Mn steel sheets is rapidly increasing in automotive industry, due to the needs for light weight, higher passive safety, energy conservation and emission reduction. The conventional hot stamping process mainly focuses on improvement of the component strength by obtaining homogeneous martensite, which cannot match the higher strength and elongation requirements of the third generation high strength steel. This project is considering to combine the plasticity forming and heat treatment. A new process named hot stamping - interrupted quenching - carbon partitioning - intercritical annealing is proposed to obtain hot stamped medium Mn parts with high strength and ductility (Strength Ductility Balance > 35GPa %). The effect of dynamic load during hot stamping on phase transformation and grain refinement is investigated. The element diffusion during C/Mn partitioning can greatly affect the strength and ductility of the hot stamped parts. The mathematical model of C/Mn partitioning is built and the full flow – multi scale – multi physics numerical simulation is realized. The process design rules is established according the experimental and simulative results. The effect of phase transformation on TRIP effect, strength and ductility is studied to improve the mechanical properties of hot stamped parts. Carrying out of this project can help to greatly improve the overall performance of the hot stamped medium Mn parts. The research can promote the combination of plasticity forming and heat treatment. The development of this project has important scientific significance and engineering value.

在轻量化、节能减排和提高安全性能等需求的驱动下，中锰钢及其热冲压成形技术的应用倍受汽车行业的关注和期待，但传统热冲压成形中锰钢制件的强塑性尚不能满足第三代高强钢碰撞安全性能需求。因此，本项目拟将热处理工艺和板料塑性成形相结合，创新性提出热冲压成形-中断淬火-碳配分-临界区退火新工艺，从而获得具有高强韧性（强塑积>35GPa%）的中锰钢热成形零件。主要研究热冲压成形过程中动态载荷对于组织转变及晶粒细化影响机制，探索碳锰配分过程中元素扩散及其对于强韧性影响规律，建立碳锰配分数学模型并实现工艺流程的多物理场多尺度数值模拟，制定基于实验和数值模拟的工艺设计准则，揭示组织转变对TRIP效应及强韧性影响机理，获取提高制件强韧性的方法。本项目的开展能有效提高中锰钢热冲压制件的综合性能，促进塑性成形与热处理工艺的有机结合，具备重要的科学意义和工程价值。

在轻量化、节能减排和提高安全性能等需求的驱动下，中锰钢及其热冲压成形技术的应用倍受汽车行业的关注和期待，但传统热冲压成形中锰钢制件的强塑性尚不能满足第三代高强钢碰撞安全性能需求。在此背景下，本项目将热冲压与淬火配分（QP）工艺和临界区退火工艺（IA）相结合，创新性提出了热冲压中断淬火-碳配分-临界区退火工艺（QA）。研究了不同QP/IA/QA工艺参数下零件组织变化和力学性能变化规律，通过SEM、TEM、EBSD和XRD等检测方法，分析热处理过程中的元素配分、组织形貌、位错密度和残余奥氏体含量等。所选用中锰钢材料直接热冲压后零件的强塑积为15.2GPa%，通过QP工艺得到零件的强塑积最高达到26.2GPa%，通过IA工艺得到零件的强塑积最高达到30.8GPa%，而通过本项目所提出QA工艺得到零件的强塑积最高达到43.6GPa%。QA工艺中通过QP获得的先驱奥氏体能促进后续IA过程中逆奥氏体的形成，最终获得包含回火马氏体、残余奥氏体和新鲜马氏体的多相细化组织，且残余奥氏体含量更高，稳定性更好。建立了QA工艺过程中的碳锰配分模型，结合ThermoCalc热力学软件对于传统相变驱动力模型参数进行修正，提高了数值模拟精度。对Cr合金化热成形钢进行了在线QP工艺的有益尝试，试制得到的汽车B柱零件和纵梁零件，其力学性能显著优于传统22MnB5热成形零件，为本项目的应用转化奠定了基础。