Developing 1000 MPa grade LCLA steels through continuous cooling: Effects of Cr and cooling rate on bainitic and martensitic transformations

Abstract

In this study, the effects of Cr addition and cooling rate on bainitic and martensitic transformations of 1000 MPa grade low carbon low alloy (LCLA) steels during continuous cooling were investigated, and the formation mechanism of C-rich martensite (CRM) was discussed. The appearance of CRM was closely related to the high concentration of C enrichment because of the incomplete formation of bainitic ferrite (BF). Most of CRM formed in the low temperature zone was distributed near the prior austenite grain boundaries (PAGB), with the maximum lattice imperfections, and the twinned substructure of high C martensite could be seen in it. With the same composition, the volume fraction and size of CRM were decreased as the cooling rate was increased. With the same cooling rate, the addition of Cr increased the volume fraction of CRM and reduced its size. Moreover, in the medium temperature zone, the addition of Cr and the increase in cooling rate lowered the transformation start temperature (TST), made the transformation after the formation of BF gradually transition to single martensitic transformation.