Correlation between microstructure and fatigue properties of hot-rolled thick-plate complex-phase steel

Complex-phase (CP) steels, with a multiphase microstructure, offer an excellent combination of high strength, ductility, and formability, making them an attractive alternative to conventional high-strength low-alloy (HSLA) steels in the automotive industry. However, the microstructure and fatigue property relation in CP steels is complex. This limits the full exploitation of CP steels in applications, such as heavy-vehicles, where excellent fatigue performance of thick-plates after punching holes is the critical parameter. In this work, we initiate the study of the relation between microstructure and fatigue properties of a commercial CP steel (800CP) and compare it with a conventional HSLA (500MC) steel. Fatigue property, tensile property and fatigue crack growth rate (FCGR) testing is conducted and the performance of the two steels is rationalized using detailed microstructure characterization, before and after fatigue testing. FCGR testing shows that, despite a higher yield strength of 800CP, both steels have a similar propagation rate due to a more tortuous crack propagation path and a higher quantity of secondary crack formation in the 800CP microstructure. The high cycle fatigue (HCF) testing shows that the fatigue limit in 800CP is 25% higher. This increase in fatigue limit is attributed to the improved resistance to fatigue crack initiation in 800CP due to its larger fraction of bainite.