Effect of microstructure and mechanical properties on tensile loading-unloading characteristics of cold-rolled DP780 steels

Abstract Tensile loading-unloading characteristic of three cold-rolled DP780 steels was studied. Similar hardening behaviour was showed by these steels even with different chemical composition and microstructures. Tensile loading-unloading tests with prestrain between 0.5% and 8% were carried out on Instron tensile testing machine. Sample microstructures were analysed by EBSD and SEM. Hardness of ferrite and martensite were measured by nanohardness instrument. Total unloading recovery strain, which is related to unloading chord modulus (E chord ), can be divided into elastic strain and microplastic strain. The results showed that there was no correlation between unloading microplastic strain (ε mp ) and phase fraction or phase size within the scope of this study. However, with the increase of flow strength increment, ε mp increases while E chord reduction becomes larger. Furthermore, ε mp depends on the phase yield strength ratio between martensite phase and ferrite phase (σ m /σ f ), emp increases as the σ m /σ f increase for a given flow strength increment. The results showed that the springback of DP780 steels can be improved by reducing the yield strength difference between martenstie and ferrite.