Influence of C Content and Annealing Temperature on Micro-structure and Mechanical Properties of 400°C Transformed Steel Containing Retained Austenite

Abstract

In order to evaluate the influence of C content and annealing temperature on the mechanical properties of steels containing retained austenite, cold-rolled sheets containing 0.12 to 0.4 C, 1.2 Si, and 1.5 Mn have been intercritically annealed and isothermally transformed at 400°C. Annealing near AC1 temperature followed by the 400°C isothermal transformation for 100 to 300 sec results in the best combination of strength and ductility. The ultimate tensile strength ranges from 590 MPa in the 0.12 C steel to 980 MPa in the 0.4 C steel. The total elongation varies 39 to 33%, and is ranked well above that of conventional ferrite-martensite dual-phase steels at the comparable strength. Amounts of retained austenite in these specimens are 7 to 20% and linearly related to the C contents of the steel. Mechanical stability of the retained austenite is fairly improved compared to that found in conventional dual-phase steels, and enhances the ductility at high strength. Better combinations of strength and ductility are maintained even in the lower C steels due to the contribution of an increased amount of highly ductile ferrite. © 1991, The Iron and Steel Institute of Japan. All rights reserved.