Effects of retained austenite on hydrogen embrittlement in TRIP-aided bainitic ferrite steel sheet

Abstract

In order to suppress global warming, it is necessary to reduce the weight of autobodies, whereas improvement of the collision safety of automobiles is required which results in a weight increase. Research and development of high-strength steel sheets for autobodies are being carried out to achieve both two objectives. Among various high-strength steel sheets, low alloy steels utilizing transformation-induced plasticity of retained austenite (TRIP-aided steels) have attracted attention as body frame and automotive parts materials because of their high strength and ductility. However, there is concern about hydrogen embrittlement in low alloy TRIP-aided steels with a tensile strength exceeding 1 000 MPa, as in conventional high-strength steels. In this study, in order to reveal the role of retained austenite for hydrogen embrittlement on TRIP-aided bainitic ferrite steel (TBF steel), (i) several TBF steel sheets with different volume fraction and carbon concentration of retained austenite were prepared by austempering the original sheet for different durations, and (ii) slow strain rate technique tensile tests were carried out on hydrogen-charged and uncharged test pieces of the sheets together with microstructure observations and X-ray diffractions. It was revealed that hydrogen embrittlement of TBF steel sheet became suppressed with increasing austempering time, which was attributable to the increase of surface area of lath and/or filmy metastable retained austenite acting as a trapping site for hydrogen.