Microstructural evolution and mechanical behaviors of the third-generation automobile QP980 steel under continuous tension and loads

As one of the representatives of the third-generation automobile steel, QP980 is a high-quality material for the improvement of automobile lightweight and crash safety. In this paper, tension and compression tests are conducted for the QP980 steel based on a self-designed testing platform. Microstructural evolution and mechanical behavior under monotonic and continuous tension/compression loads are investigated. The results show that the martensitic transformation in QP980 steel is more easily excited in the monotonic tension state than in the monotonic compression state. In the tension-compression-tension (T-C-T) and compression-tension-compression (C-T-C) modes, the Bauschinger effect of QP980 depends on the pre-strain amount and the pre-strain mode. The Bauschinger effect is stronger at a higher pre-strain. It is also stronger in T-C-T than in C-T-C because the transformation of retained austenite after T-C-T is greater than that after C-T-C. By analyzing the hardening and softening behaviors in the forward, reverse and reforward loading branches of T-C-T and C-T-C, the hardening ability during reverse and reforward loading branches is higher than that during forward loading. Permanent softening occurs between reverse loading and forward loading of T-C-T instead of C-T-C. The research result of this paper is favorable for understanding the relation between mechanical behavior and microstructural evolution of QP980 steel under the condition of complex loads, and promote its large-scale application.