Relationships between Microstructural and Mechanical Performance on Example of an Air-Hardening Steel

Abstract

Because of an excellent combination of strength and ductility, mono-phase low-alloyed steels with a bimodal grain size are an appropriate alternative to conventionally cold-rolled and annealed steels as well as to steels with a dual-phase microstructure. This study investigates how the microstructure of a low-alloyed air-hardening steel either with a homogeneous, a dual-phase, or a bimodal grain structure influences its mechanical and fatigue performances. The homogeneous ferritic grain microstructure of the steel sheets is adjusted by an intercritical annealing at 790 °C, along with subsequent air hardening to obtain a dual-phase state. Then, the ferritic-martensitic material is cold-rolled and annealed at 550–700 °C to produce different bimodal grain microstructures. The evolution of microstructure and mechanical properties are characterized. An annealing temperature of 600 °C is considered to be the optimal temperature resulting in pronounced bimodal grain size distribution. The sheet with a bimodal microstructure exhibits a higher strength and equal ductility compared to one with a homogeneous ferritic microstructure. Additionally, high-cycle fatigue tests of the material with a bimodal microstructure shows its superior fatigue behavior at a loading of above 800 000 cycles compared with both the homogeneous ferritic microstructure as well as the dual-phase microstructure.