2018 Volume 104 Issue 5 Pages 274-283
Recently, medium Mn steel has been focused on as one of the promising candidates for third generation AHSS, due to it having an excellent TS-El relationship. Medium Mn steel can retain a lot of austenite by reheating to an (α+γ) intercritical temperature. Research on this material was performed on hot rolled steel sheet and cold rolled steel sheet, using the martensite as the starting microstructure.
The effect of cold reduction on the microstructure and mechanical property after intercritical annealing was discussed by using 0.2C-2Si-5Mn steel with softened bainite structure at 575°C and the following results were obtained.
(1) Hot rolling and intercritical annealed steel sheet showed a lath type structure of ferrite and retained austenite. On the other hand, cold rolling and intercritical annealed steel sheet showed a mixture of equiaxed ultrafine ferrite and retained austenite. The volume fraction of retained austenite increased as the intercritical annealing time increased. The increasing behavior was promoted by cold reduction. The maximum volume fraction of retained austenite was about 40%, and was obtained for the longest annealing time, 300 min, in all steels.
(2) Excellent mechanical properties, for example, TS; 1217 MPa, UEl; 27.6%, TS×UEl; 33,592 MPa% were obtained for the steel intercritically annealed at 675°C for 30 min after 50% cold reduction. Hot rolled steel showed continuous yielding, while cold rolled steel exhibited about 7% yield point elongation.
These results were almost accorded with the previously reported results whose initial microstructure was martensite.
