基于热冲压与碳配分的高强钢淬火回火过程组织演变与强韧性机理研究

The quenching process of hot stamping high strength steel is not controllable, the products have fully martensite with poor ductility. Q-P steel and Q-P-T steel contents less metastable austenite, showing low product of trength and ductility,due to weak of TRIP effect. Integration of hot stamping and quchening-partitioning heat treatment processes is the new technology to ultra high strength steel needs study urgently.The project is developing hot-stamping quchening-partitioning heat treatment Q&P steel. The effect of hot stamping temperature, strain, strain rate on martensite transformation and grain refining mechanism of martensite were investigated , then cleared the refining mechanism of multiplex microstructure. The interface migration of martensite and austenite phase was researched, the mechanism of carbon migration and diffusion during quenching, partition, isothermal tempering processes were cleared. The mechanism of phase transformation and microstructure evolution during the quchening-partitioning heat treatment were also explained. Based on the experiment and theoretical calculation, the mathematical model of carbon migration and mathematical model of microstructure evolution were established. At last, the mechanisms of strengthening and toughening steel using hot stamping, quenching, partition, isothermal tempering processes to product multiphase microstructure were laborated and mechanisms of high product of trength and ductility was also cleared. The achievements of the research have important theoretical and practical significance to extension the development of ultra high strength steel and to product new generation of automotive steel with high strength and ductility.

高强度钢热冲压成形工艺淬火过程不可控，组织全部为马氏体，塑性差。Q-P钢、Q-P-T钢中亚稳残余奥氏体含量较少，TRIP效应弱，强塑积有待提高。热冲压成形、淬火、碳配分、回火热处理一体化工艺是开发高强度钢的新技术，亟待研究。项目以热冲压成形Q&P钢为研究对象，研究热冲压成形时温度、应变量、应变速率等对马氏体相变的影响，明确复相组织晶粒细化机制；研究马氏体/奥氏体相界面迁移和碳在淬火、碳配分、回火过程中的迁移扩散机制及其对试验钢多相组织的影响，探讨淬火、碳配分、回火处理过程的组织演变规律，建立碳元素扩散数学模型和组织转变数学模型，深入分析热冲压成形、相变、碳配分综合运用实现复相组织强韧化的机制，明确试验钢高强塑积的机理。项目的研究成果对于开拓超高强度钢研发思路、新一代高强塑积汽车用钢开发应用具有重要的理论和现实意义。

发展高强度钢是实现汽车轻量化、节能减排的有效途径和重要手段。为解决高强度钢在冷成形技术中变形能力差、易开裂、零件回弹量大、尺寸精度不高和热成形技术中塑韧性差等问题。本课题提出热冲压成形和Q&P热处理一体化工艺，探究热冲压成形Q&P工艺下高强钢的材料特性、马氏体相变和晶粒细化的机理。同时，研究热冲压成形Q&P钢淬火、配分处理过程中的组织转变规律。依据碳迁移扩散机制和高强钢的强塑性机理构建出碳元素扩散数学相变模型，从而进一步优化热冲压成形Q&P热处理一体化工艺。研究表明：随着奥氏体化温度的升高，高强钢的抗拉强度增加，伸长率降低，同时发现在双相区进行奥氏体Q&P热处理工艺性能优于完全奥氏体化Q&P热处理工艺。经新型热处理工艺后，组织由板条状淬火马氏体M1、块状马氏体M2、残余奥氏体和铁素体等多相组织构成。并发现残余奥氏体的稳定性在很大程度上取决于所应用的热处理工艺，且主要归因于残余奥氏体含量和形态。发现在Q&P热处理工艺下，C/Mn元素的扩散配分，有利于奥氏体的稳定性。当淬火至室温时，能使更多的残余奥氏体得以在室温下稳定存在。同时，经过研究发现，残余奥氏体大多形核于马氏体/铁素体大角度晶界处，异质的残余奥氏体结构（粒状、块状、层状），因其具有不同的稳定性，可以在不同应变阶段持续地发生TRIP效应，提升高强钢的强塑性。为开拓新型高强度钢板设计研发思路、新一代高强塑积汽车用钢的开发应用及汽车轻量化进程提供实际应用参考价值及理论意义。