基于第一性原理的相间析出强化型高强度高局部成形性能多相钢的组织性能控制

Application of 1GPa grade ultra-high strength hot rolled automotive steel is an effective way to reduce the weight of car body, which is of significance for energy saving and emission reduction. In this project, the key problems consisting of the interphase precipitation behavior, multi-phase microstructure control and the combination of strength, ductility and local formability in the 1GPa ultra high strength hot-rolled automotive steel are systematically investigated. The multi-phase microstructure consisting of the ferrite with combined nanometer-sized precipitates, bainite and retained austenite is obtained. Ferrite is strengthened in virtue of interphase precipitation strengthening by novelly designed (Mo,V,Ti)C and stable retained austenite is obtained through addition of Al. Therefore, the combination of high strength, high ductility and high local formability can be realized. On the basis of the first principles, the effect of element doping on the stability of (Mo,V,Ti)C is to be elucidated and the precipitation thermodynamics and kinetics model of (Mo,V,Ti)C/α-Fe is to be established. The mechanisms of interphase precipitation and multi-phase transformation behaviors will be investigated by means of thermo-mechanical tests, hot-rolling tests and cooling path control. Moreover, the effects of grain size, precipitate morphology and phase proportion on the combination of strength, ductility and local formability will be revealed. Finally, the 1GPa hot rolled automotive steel with high strength, high ductility and high local formability would be produced in laboratory.

开发和应用1GPa级超高强度热轧汽车用钢是降低车身自重实现汽车轻量化的有效措施，对汽车工业节能减排具有重要意义，本项目围绕相间析出、多相组织控制和强塑性与局部成形性能匹配等问题开展研究。利用(Mo,V,Ti)C相间析出相强化铁素体，通过以Al代Si提高残余奥氏体稳定性，获得相间析出强化型铁素体+贝氏体+残余奥氏体的多相组织，实现强塑性和局部成形性能的良好匹配。基于第一性原理，揭示元素掺杂对(Mo,V,Ti)C稳定性的影响机制，结合准谐德拜模型，建立高温条件下析出热动力学模型。通过热模拟、热轧和不同冷却路径等实验，揭示(Mo,V,Ti)C相间析出机制，系统研究多相钢组织性能演变的物理冶金学规律，确定相间析出和多相组织耦合控制的最佳工艺窗口。揭示晶粒尺寸、析出粒子尺寸与分布、相比例等对强塑性和局部成形性能的影响规律，制备出1GPa级高强度、高塑性和高局部成形性热轧汽车用钢的原型钢。

1GPa级超高强度汽车用钢是降低车身自重实现汽车轻量化、节能减排的关键材料，但强塑性与成形性能往往难以匹配。本项目以围绕相间析出、多相组织控制和强塑性与局部成形性能匹配等问题开展研究。取得如下重要结果：基于第一性原理，研究了V、Cr、Zr、Nb、Mo、W元素和空位掺杂对三元B1型(Ti1-xMx)C碳化物和四元B1型(TixV0.75-xM0.25)C和(TixNb0.75-xM0.25)C碳化物晶格常数和结构稳定的影响，求得不同析出物的固体热容，热膨胀系数，为高温条件下析出热动力学模型提供基础参数。探究了线性插值法在多元复合碳化物的适用范围，阐明了多元复合碳化物稳定性的电子特性，研究了TiC析出及(Ti4-x, Mx)C4/α-Fe界面稳定性的影响规律，为最优的成分设计方案提供理论指导。利用复合析出相固溶析出模型和经典形核长大理论，分别构建了(Ti,Mo,V)C复合沉淀析出和(Ti,Mo)C与ε-Cu共沉淀析出热动力学模型，绘制形核率-温度(NrT)曲线和析出-时间-温度(PTT)曲线，揭示了等温析出的最佳工艺窗口，沥清了共沉淀析出钢(Ti,Mo)C中Ti/Mo比、(Ti,Mo)C与ε-Cu析出次序等关键机理。利用热力学软件建立还有Al、Ti、V及Mo等不同元素的平衡相图，分别研究了连续冷却条件下相变行为、相间析出行为，绘制了实验钢动态连续冷却相变曲线，对多元复合析出进行了形貌、成分和高分辨晶格等表征，摸清了(Mo,V,Ti)C的析出演变规律，构建了(MoxTiyVz)C粗化动力学模型，结合析出PTT曲线，找出了(Mo,V,Ti)C相间析出与铁素体相变的动力学匹配工艺窗口，揭示铁素体基体上(Mo,V,Ti)C 相间析出与铁素体、贝氏体和残余奥氏体多相组织耦合控制机理。对Ti-Mo-V、Ti-Cr典型实验钢进行轧制及冷却实验，并进行力学性能检测。提出两步短时配分法，获得了贝氏体双峰组织的多相钢，大幅提高了实验钢的均匀延伸率和扩孔性能，成功实现了1GPa高强钢高强度、高塑性和高局部成形的良好匹配。该项目的实施，丰富了汽车用钢组织性能调控的基本理论，对超高强钢的应用和推广具有促进作用。共发表和已录用论文16篇，其中SCI论文14篇，EI 2篇，授权发明专利1项，培养硕士研究生7名，已毕业6名，培养博士研究生2名，出站博士后1名。