Effect of Cold Reduction on Microstructure and Mechanical Property of 5% Mn Steel

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-E1 relationship. Medium Mn steel can retain a lot of austemte by reheating to an (alpha+gamma) intercntical 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 intercntical annealing was discussed by using 0.2C-2Si-5Mn steel with softened baimte structure at 575 degrees C and the following results were obtained. (1) Hot rolling and intercntical annealed steel sheet showed a lath type structure of ferrite and retained austemte. On the other hand, cold rolling and intercntical annealed steel sheet showed a mixture of equiaxed ultrafine ferrite and retained austemte. The volume fraction of retained austemte increased as the intercntical 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%, TSxUE1; 33,592 MPa% were obtained for the steel intercntically annealed at 675 degrees C for 30 mm 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.