相间析出强化双相结构的组织性能控制机理研究

A unique dual-phase structure composed of interphase precipitation strengthened "ferrite + martensite" is proposed in this research. The remarkable strengthening effect in ferrite, which is produced by the massive, orderly arranged interphase precipitated carbides, results in the improved strain uniformity between constituent phases during deformation due to the decreased strength difference between ferrite and martensite and the enhanced martensite plasticity. This provides a new idea to solve the crucial problem of fracture liability during forming process (bending and hole-expansion) of high-strength dual-phase steel. Therefore, experiments and model calculation will be employed to investigate the relation between the morphology of interphase precipitated carbides and nucleation rate of carbides on the phase interface, velocity of phase interface, and to build the theoretical model for interphase precipitation kinetics. Based on the modified law of mixtures, the stress-strain relation during tensile testing will be constructed. The mechanism by which the ferrite/martensite hardness ratio and the phase fraction affect the martensite plasticity will be studied. The internal link between formability and microstructure will be investigated. Eventually, the model for comprehensive control of microstructure, mechanical properties and formability of interphase precipitation strengthened dual-phase structure will be established. The expected achievements in this research can provide crucial guidance for the precise control of interphase precipitation. Meanwhile, from theoretical perspective, the understanding of the relation between microstructure and deformation behavior, fracture behavior will be enhanced. Finally, the results of this research can be used to develop hot-rolled dual-phase steel with high strength and excellent formability, and finally broaden its application.

本课题提出一种独特的相间析出强化"铁素体+马氏体"双相结构，大量规则排列的相间析出碳化物使铁素体获得显著强化，一方面可降低铁素体与马氏体的强度差，另一方面通过应力传递，可促进马氏体的塑性行为，从而改善形变过程中两相应变的均匀性。这为解决高强双相钢在成形(弯曲和扩孔)中易于开裂的关键问题提供了一个新的思路。为此，拟采用实验与模型计算，研究相间析出碳化物形貌与相界上碳化物形核率、相界面迁移速度的关系，建立相间析出动力学理论模型；建立基于修正混合法则的拉伸应力-应变模型，研究铁素体/马氏体硬度比值、相比例对马氏体塑性行为的作用机理；确定成形性能与微观组织的内在联系，最终建立相间析出强化双相结构的组织、力学与成形性能的综合控制模型。本课题研究成果可为相间析出的精确控制提供关键性理论指导，可加深对微观组织与形变行为、断裂行为关系的理论认识，可用于指导高成形性高强热轧双相钢的开发，拓宽双相钢应用范围。

本课题从相变与析出的热动力学理论研究出发，针对常规的“碳化钛钒相间析出”与特殊的“片层珠光体”两种相间析出形式，深入开展了相间析出的热动力学理论、控制机制及其在高强、高成形性汽车用钢中的应用研究，主要完成的研究工作包括：①在厘清相变热动力学理论框架的前提下，耦合“界面迁移”与“溶质拖曳”建立了微合金钢连续冷却相变动力学模型，分析了等温相变条件下相界面热力学平衡条件随时间的转变、及其对相变动力学的影响；②与台湾大学合作，创新性地采用脱碳实验方法开展微合金碳化物相间析出的热动力学行为研究，完成了从实验装置设计、合金体系选择到相界面取样分析方法的细致工作，在聚焦离子束与透射电镜支持下最终获得了脱碳试样相界面区域的析出形貌；③基于碳化钛钒在铁素体中的析出动力学计算，开展了低碳钒钛微合金钢的合金设计以实现等温铁素体相变与碳化钛钒析出鼻温区间的重合，在等温相变模拟实验与热轧实验中均获得了大量致密排列的碳化钛钒相间析出；④在“超快冷+空冷+超快冷”冷却模式下，降低铁素体相变温度区间，获得了碳化钛钒析出强化的双相结构，获得析出强化的铁素体/贝氏体与马氏体强度差异减小，通过应力传递促进了应变在铁素体/贝氏体、马氏体间更加均匀的分配，在抗拉强度为≥780MPa时扩孔率可达93-140%，该技术正在推广至高强、高成形性双相钢的开发；⑤创新性地通过相界面热力学平衡条件的调控，破坏片层珠光体中铁素体与渗碳体相变的协同性，实现奥氏体/铁素体、奥氏体/渗碳体相界面迁移速率的差异化，获得了Fe-C-Mn-Cr合金热轧条件下的离散化珠光体组织(铁素体+弥散渗碳体)，该技术正在推广至汽车用精冲钢的开发，可显著缩短现有的“冷轧+球化退火”冗长工艺流程。