Enhancement of Balance in Strength, Ductility, and Stretch Flangeability by Two-Step Austempering in a 1000 MPa Grade Cold Rolled Bainitic Steel

Abstract

The microstructural evolution and properties for varied austempering routes are investigated in a cold-rolled bainitic steel. Special attention is given to the effect of retained austenite (RA) in terms of its fraction, carbon concentration, and morphology resulting from different austempering routes on mechanical properties and stretch flangeability. Bimodal sized bainitic laths are provided, and the carbon concentration of RA maintains the highest value through the two-step austempering. Total elongation (TEL) is remarkably enhanced for the two-step austempering, deviating from the exponential relationship between tensile strength (TS) and TEL as maintained by the one-step austempering. Considering the two plateaus of the strain-hardening exponent, it is considered that the hierarchical stability of RA is provided by the two-step austempering, leading to the postponed necking point so as to improve the uniform elongation. Two-step austempering could provide more complete bainitic transformation as well as more stable film-like RA, supplying a promising way to improve the combination of strength, ductility, and stretch flangeability.