The Third Generation Auto Sheet Steel: Theory and Practice

Abstract

Strength upgrading generally increases the contradictions between strength and ductility of steels, especially lacking of enough ductility during stamping. Nowadays, many efforts have been undertaken to improve ductility of auto sheet steels. Based on the a novel microstructure modification idea featured by Multi-phase, Meta-stable and Multi-scale (M3) proposed by the authors, over 30 % of ultrafine grained austenite were designed to improve ductility, balanced with remained martensite to maintain higher strength. It was resumed that the proposed M3 microstructure would result in a improvement of product of ultimate tensile strength and total elongation to over 30 GPa %. New low carbon steels alloyed with medium manganese (3–9 % Mn) developed in laboratory presented really an remarkable improvement in ductility to 40–30 % as ultimate tensile strength ranging from 800 to 1100 MPa. An industrial scale melt and stamping were taken over in the TISCO and FAW respectively. It was shown by the result that hot-rolled 6 mm thick steel sheets with ultimate tensile strength of 800 MPa presented better stamping behavior than the conventional 380 MPa grade steel sheets when they were stamped into cross beams, which means the new steel developed could be used as a potential candidate material for lightweight and cold forming components. Cold-rolled sheets annealed at intermediate temperature also showed an remarkable improvement in ductility even up to 40 % of total elongation at strength level of 800 MPa due to the same reverse austenite transformation phenomenon.