Influence of the Al and Mn content on the structure-property relationship in density reduced TRIP-assisted sheet steels

In the present study four different density reduced TRIP steel concepts with varying Mn and Al contents were investigated with regard to their microstructure, mechanical properties and retained austenite (RA) stability. For the hot rolled strips, scanning electron microscopy (SEM) revealed a microstructure consisting of ferrite and two types of carbides. Using both X-ray diffraction (XRD) and SEM with backscattered electron (BSE)-detection, the carbides were identified as cementite for the steel grades containing 2.8 and 3.3 wt% Al and kappa-carbides for the compositions with 5.2 wt% Al. The microstructure of the laboratory continuously annealed cold rolled sheets consisted of numerous inclusions of bainite and RA, embedded in a ferritic matrix. As an aftermath of the increased Al content, resulting in an increased ferrite content, the bainitic transformation was significantly reduced, which led to a destabilization of RA and in turn to the formation of martensite upon final cooling to room temperature (RT). With increasing Al- and Mn-contents the tensile strength rose from 720 to 1050 MPa, whereas the total elongation decreased from 39% to 17%. Both highest mechanical and chemical RA stability were found for the steel grades containing 2.8 and 3.3 wt% Al, resulting in the superior combination of strength and ductility, namely R(m)XA(80) of almost 30,000 MPa% by a density reduction up to almost 5%.