Effects of auto-tempering behaviour of martensite on mechanical properties of ultra high strength steel sheets

Abstract

Influences of alloying elements on mechanical properties were investigated for ultra high strength steel sheets, focusing particularly on martensitic transformation and its auto-tempering behaviour; auto-temper is a phenomenon in which the first-formed martensite near martensitic transformation start temperature (MS) is tempered during remaining process of quenching or cooling. Fe–0.15–0.30C–1.5Si–2.3Mn (mass%) steels and Cr and/or B added Fe–0.12–0.21C–1.5Si–2.3Mn steels were used in this study. The steel sheets were heated to 1093 K and held for 180 s, then cooled at 10 K/s to 773 K and held for 90 s followed by cooling at 10 K/s to room temperature. The structures of the steels consist mainly of martensite and ferrite. When increasing the tensile strength from 1050 MPa of the Fe–0.15C–1.5Si–2.3Mn steel to over 1180 MPa by additional alloying, change in stretch flangeability has a significant correlation with the hardness of martensite dependant upon the MS. Increasing carbon lowers MS, and causes serious deterioration of hole expansion ratio with increasing tensile strength. In contrast, the Cr and/or B added steels of which the MS is relatively high exhibit good stretch flangeability. This would be because the high MS leads to auto-tempering of the martensite during cooling after martensitic transformation, which results in decreasing the difference of hardness between ferrite and martensite and improving the formability of martensite. Total elongation has little appreciable difference in a range between 1200 and 1400 MPa of tensile strength. On the other hand, when increasing tensile strength over 1500 MPa, the Cr and B added steel, which is well auto-tempered, exhibits higher ductility than the carbon increased steel which contains less-tempered hard martensite. Wide hardness variation of martensite in the well auto-tempered steels may contribute to the improvement of ductility by increasing work-hardenability owing to inhomogeneous deformation of auto-tempered martensite during straining.