Tensile deformation of third generation advanced high strength sheet steels under high strain rates

Abstract

New third generation advanced high strength steels (AHSS) are being developed for structural components of vehicles to reduce vehicle weight and improve crash performance. The goal of this work was to gain an understanding of the role of retained austenite in the high strain rate tensile deformation behavior of third generation AHSS and to understand the effect of adiabatic heating on the austenite to martensite transformation. Four steels were selected for the study: an experimental medium Mn transformation induced plasticity (TRIP) steel (TRIP7Mn), an experimental quenched and partitioned (Q&P) steel (QP3Mn), and two commercially produced steels, DP980 and QP980. The effect of strain rate on the deformation behavior of QP3Mn and TRIP7Mn, was examined through tensile tests in the range of 0.0001 – 200 /s and retained austenite X-ray diffraction measurements from interrupted tensile tests at various strain rates. The effects of adiabatic heating on flow stress and retained austenite stability were further investigated with quasi-static tensile tests at 22 – 85 °C for QP3Mn and 22 – 115 °C for TRIP7Mn. The combined effects of temperature and strain rate on flow stress and the austenite to martensite transformation were also examined with tensile tests at -10, 22, and 70 °C at 0.0005, 0.1, and 10 /s. Room temperature tensile tests in the range of 0.0001 – 100 /s were performed on the DP980 and QP980 for as-received and pre-strained conditions. The QP3Mn has positive YS strain rate sensitivity (meaning the overall trend in flow stress with increasing strain rate) possibly related to the contribution of austenite to yielding, and it has low UTS strain rate sensitivity due in part to the mostly martensitic microstructure at UTS. The TRIP7Mn upper and lower YS have positive strain rate sensitivity, but the TRIP7Mn has significant negative UTS strain rate sensitivity up to 1 /s with slight positive strain rate sensitivity above 10 /s. Elongation follows a similar trend as UTS. The UTS and elongation trends are due to a combination of factors: the disappearance of serrated plastic flow with increasing strain rate, a softening effect from adiabatic heating, and stabilization of austenite due to adiabatic heating. The austenite in the QP980 may lead to a slightly higher YS strain rate sensitivity compared to the DP980, but it does not affect trends in UTS or ductility with respect to strain rate. The DP980 and QP980 have slightly negative UTS strain rate sensitivity below 1 /s due to a softening effect from adiabatic heating. The austenite to martensite transformation is not affected by strain rate for the QP980 and QP3Mn; whereas, the TRIP7Mn has a lower transformation rate above 10% strain at high strain rates (0.1 – 100 /s) due to adiabatic heating stabilizing the austenite.