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

Tensile and dilatation tests were conducted in order to study the effects of Mn, Si and Nb on the transfermation behavior in the 0.14C-(2.1 similar to2.4)Mn-(0.5 similar to1.0)Si-(0.00 similar to0.02)Nb TRIP steels. Ali the specimens for dilation tests were annealed under the condition to form the same austenite fraction, followed by cooling to the austempering temperature. The volume fraction of bainite transformed during austempering, DeltaV(B), and the martensite start temperature, M-s, were measured. DeltaV(B) was dependent on the austempering temperature and a minimum in DeltaV(B) was obtained at 450 degreesC for all the examined steels. The M-s temperature was linearly decreased as DeltaV(B) was increased. The decreasing rate of M-s to DeltaV(B) was notdependent on the steel composition, but on the austempering temperature, exhibiting the maximum value when the austempering temperature was 450 degreesC. it was also observed that volume fraction of retained austenite was linearly increased with the decrease of M-s for the steels austempered at various conditions. The highest elongation was obtained when the rate of M-s to DeltaV(B) was the maximum. M-s was increased when the amount of Mn was increased from 2.1 % to 2.4 %. II 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-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 DeltaV(B) and the M-s.