DUCTILITY AND STRAIN-INDUCED TRANSFORMATION IN A HIGH-STRENGTH TRANSFORMATION-INDUCED PLASTICITY-AIDED DUAL-PHASE STEEL

The influence of forming temperature and strain rate on the ductility and strain-induced transformation behavior of retained austenite in a ferritic 0.4C-1.5Si-1.5Mn (wt pct) dual-phase steel containing fine retained austenite islands of about 15 vol pct has been investigated. Excellent combinations of total elongations (TELs), about 48 pct, and tensile strength (TS), about 1000 MPa, were obtained at temperatures between 100-degrees-C and 200-degrees-C and at a strain rate of 2.8 X 10(-4)/S. Under these optimum forming conditions, the flow curves were characterized by intensive serrations and increased strain-hardening rate over a large strain range. The retained austenite islands were mechanically the most stable at temperatures between 100-degrees-C and 200-degrees-C, and the retained austenite stability appeared to be mainly controlled by strain-induced martensite and bainite transformations (SIMT and SIBT, respectively), with deformation twinning occurring in the retained austenite. The enhanced TEL and forming temperature dependence of TEL were primarily connected with both the strain-induced transformation behavior and retained austenite stability.