Effects of alloying elements on mechanical properties and phase transformation of TRIP cold-rolled steel sheets

摘要

Tensile and dilatation tests were conducted in order to study the effects of Mn, Si and Nb on the transformation behavior in the 0.14C -(2.1～2.4)Mn -(0.5～1.0)Si -(0.00～0.02)Nb TRIP steels. The volume fraction of bainite transformed during austempering, ΔV{sub}B, and the martensite start temperature, M{sub}S, were measured. ΔV{sub}B was dependent on the austempering temperature and a minimum in ΔV{sub}B was obtained at 450°C for all the examined steels. M{sub}S temperature was linearly decreased as ΔV{sub}B was increased. The ratio of M{sub}S decrease to ΔV{sub}B was not dependent on the composition of steels, but on the austempering temperature, exhibiting a maximum when the austempering temperature was 450°C. It was also observed that elongation was the greatest when the ratio was maximum. M{sub}S was increased when the amount of Mn was increased from 2.1% to 2.4%. It was because Mn retarded the ferrite transformation rate so that the soluble C diffused from the ferrite to the austenite was decreased. The addition of Si decreased M{sub}S because it activated the diffusion of C to austenite during the cooling to the austempering temperature. The addition of Nb was observed to give little influence on ΔV{sub}B and the M{sub}S.