Physical Metallurgy of Automotive High-Strength Steels

Summary

This paper briefly reviews the various available types of high-strength steels — conventional, recovery-annealed, and dual-phase steels. A judgmental analysis of a large number of stamped automotive components shows that dual-phase steels, with their superior strength at a given elongation (formability), offer the best potential for weight savings. The rest of the paper concerns the physical metallurgy and mechanical response of dual-phase steels. It is shown that strength is a function of the percent martensite in the structure, although the strength is less than predicted by a simple rule of mixtures. The enhanced ductility of dual-phase steels is shown to be a consequence of the high s train-hardening rate observed in these steels, the reason for which is not well understood but probably reflects the properties of the ferrite matrix. The strain-rate sensitivity of dual-phase steels is found to be higher than that of conventional high-strength steels of comparable tensile strengths. Fatigue studies of dual-phase steels show them to be comparable to conventional high-strength steels in the notched condition; energy absorption during the axial collapse of tubes is shown to be solely a function of tensile strength. Also included are data on the temperature dependence of tensile strength in both the notched and unnotched conditions; high-strength dual-phase steels exhibit notch sensitivity even at room temperature. A study of formability (springback) shows that high-strength dual-phase steel is slightly worse than SAE 950X but much better than SAE 980X steel.