Complex phase (CP) steels are widely used in automotive components such as frame rails, rocker panels, and tunnel stiffeners owing to their high strength and good local formability. The subtle hardness difference between microstructures allows CP steels to exhibit excellent hole expansion performance, with the high-hardness martensite-austenite (MA) constituents being the critical structure. The distribution of MA constituents is crucial to the mechanical properties of the product. This study aims to improve the hole expansion property by constructing a continuous distribution of MA constituents along the rolling direction at the thickness center. Microstructures and hole expansion behavior were investigated using CLSM, SEM, EBSD, and hole expansion tests. Results indicate that after thermodynamic treatment, the MA constituents were aggregated at the thickness center in a continuous distribution along the rolling direction with a long axis of approximately 1.25 μm, and an average distance of less than 1.0 μm. Microhardness quantification of the plastic damage on the punching edge suggests that the advanced steel exhibits the highest hardening at the thickness center with a 41% hardness increase after punching, which is higher than the 31% hardening in the maximum hardening burr zone of the base steel. The advanced steel, despite suffering severe punching damage, exhibited a hole expansion ratio of approximately 43%, higher than the 34% of the base steel. Quasi in situ interrupted hole expansion tests indicate that at the thickness center of the advanced steel, the circumferential cracks formed through a multiple void interaction mechanism which promotes the stress release. In the matrix, pit-like damage is caused by a void coalescence mechanism. Both mechanisms lead to the mechanical instability and eventual failure of the steel. The damaging position of the hole edge had a decisive impact on the fracture mode.

摘 要 复相钢兼具高强度和良好的局部可成形性，被广泛应用于汽车车架导轨、摇臂板和隧道加强件等典型汽车零部件。复相钢中各类微观结构之间较小的硬度差异使其具有优异的扩孔性能，其中高硬度马氏体-奥氏体(MA)组元是影响复相钢扩孔性能的关键组织，其分布对扩孔性能的影响至关重要。本工作提出了构造厚度中心沿轧向连续分布MA组元以提升复相钢扩孔率的方法，利用CLSM、SEM、EBSD手段和扩孔实验，研究了构造MA组元特征分布前后复相钢的微观结构和扩孔行为特性。结果表明，基准钢的MA组元均匀分布，长轴为0.98 μm，平均中心间距为1.2 μm。构造特征组织后的实验用钢MA组元聚集在厚度中心，长轴约1.25 μm，沿轧向连续分布，平均间距小于1.0 μm。微观硬度量化冲裁边的塑性损伤结果表明，冲孔损伤后实验钢板厚度中心处硬化最高，较损伤前硬化41%，高于基准钢最大硬化区(毛刺区，31%)。冲孔损伤更高的实验用钢的扩孔率约43%，高于基准钢(约34%)。利用准原位中断扩孔实验分析了扩孔行为与显微组织特征的关系。实验用钢通过多孔隙相互作用机制在厚度中心处形成环状裂纹促使应力释放，同时在基体中通过单一孔隙机制形成坑状损伤，导致材料局部失稳并最终失效。受损质点处于孔缘位置对断裂方式具有一定程度的影响。关键词 复相钢，马氏体-奥氏体(MA)组元，扩孔率，断裂