基于动态碳配分行为的高强塑积热轧复相组织调控机理

Currently the widely used production on Q&P steel and related microstructure controlled mechanism is based on cold rolling-heat treatment process including isothermal partitioning. The traditional Q&P process concept was combined with hot rolling process, the objective of which was to obtain advanced high-strength steels by a short hot rolling process to replace cold rolling one. In this study, the relaxation was introduced in the process of DQ&P including direct quenching and nonisothermal partitioning. The concept to obtain advanced high-strength steels with the high product of strength and plasticity by multiphase (ferrite + martensite + retained austenite) controlled process was proposed. That is to say, the higher plastic forming properties can be obtained by the compatible deformation between ferrite and martensite combined with TRIP effect of retained austenite. The project was studied basing on three points and their coupling interaction, which was included of transformation control of supercooling deformed austenite, ferrite phase transition during relaxation and carbon partition in the nonisothermal condition. The key scientific problems of nonisothermal partitioning behavior, coupling interaction of partition and transformation, microscopic features of retained austenite and controlled mechanism were systematically studied. The microstructure controlled process to obtain ferrite + martensite + retained austenite based on nonisothermal partitioning was established. And finally, the hot-rolled multiphase original steel with the high product of strength and plasticity can be obtained on the basis of the integrated control technology.

目前研究Q&P钢及相关组织控制原理广泛采用的生产工艺为基于等温碳配分的“冷轧—热处理”工艺路径，将传统Q&P原理引入热连轧生产工艺中，目的在于通过“以热代冷”的减量化工艺流程制备先进高强度钢。本研究将空冷弛豫处理引入直接淬火动态配分DQ&P工艺中，提出以铁素体+马氏体+残余奥氏体复相组织调控来获得高强塑积先进高强度钢的理念，即利用铁素体与马氏体的软硬相协调变形机制并结合残余奥氏体TRIP效应获得更高的塑性成形性能。课题以轧制压缩变形获得形变奥氏体、引入弛豫处理生成铁素体软相、卷取冷却过程中动态碳配分及三者之间耦合作用为核心，系统研究动态碳配分行为及其与复相组织演变的耦合交互作用、动态碳配分条件下的残余奥氏体微观特征与控制原理等关键科学问题，明确基于动态碳配分原理的以马氏体+铁素体+残余奥氏体复相组织为目标的组织控制工艺路径，并基于关键工艺窗口的一体化控制制备出先进高强塑积热轧复相钢原型钢。

目前研究Q&P钢及相关组织控制原理广泛采用的生产工艺为基于等温碳配分的“冷轧—热处理”工艺路径，将传统Q&P原理引入热连轧生产工艺中，目的在于通过“以热代冷”的减量化工艺流程制备先进高强度钢。本研究将空冷弛豫处理引入直接淬火动态配分DQ&P工艺中，提出以铁素体+马氏体+残余奥氏体复相组织调控来获得高强塑积先进高强度钢的理念，主要围绕铁素体相变一次碳配分和缓冷过程二次动态碳配分机理、残余奥氏体（RA）稳定性控制及基于动态碳配分的组织演变及相变诱发塑性一体化控制理论研究等三个方面展开研究。. 研究结果表明：（1）铁素体的引入，可促进边界局部富碳，从而使得室温保留较多亚微米级别的块状RA。同时，揭示了卷取冷却过程中相变和碳配分的耦合机理，获得了适于动态碳配分的热/动力学工艺窗口：在0.04℃/s冷却时，淬火工艺窗口为240℃-Ms之间；当淬火温度为320℃时，冷速小于1℃/s时，可完成马氏体板条间的碳配分，从而获得复合型RA；（2）复相组织调控可最大程度的稳定RA，使得0.2C-1.6Si-1.6Mn(wt%) 成分保留大于10%的RA。同时，明确了实验钢碳含量对马氏体板条间薄膜状RA的影响，特别是当碳含量低至0.078%时，马氏体板条间不能获得纳米级RA，而相变组织为孪晶马氏体。在变形过程中，复合型RA可提供持续的TRIP效应，获得大于20%的延伸率；（3）采用复相组织调控思路，针对0.2%C的实验钢，明确了铁素体引入的最佳弛豫温度区间为760-800℃。同时，明确了在较高淬火温度下，一般大于280℃，可以获得强塑性良好的性能，抗拉强度大于1000MPa， 延伸率大于20%，强塑积大于20GPa.%。. 该研究肯定了DQ&P工艺的可行性及优势，并且获得了工艺窗口较宽的生产工艺以及性能优良的实验原型钢，为热轧DQ&P钢的生产提供了基础理论支撑和工艺指导。同时，也为新一代先进高强钢的开发提供了新思路。