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

Abstract

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. All 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, ΔVB, and the martensite start temperature, Ms, were measured. ΔVB was dependent on the austempering temperature and a minimum in ΔVB was obtained at 450°C for all the examined steels. The Ms temperature was linearly decreased as ΔVB was increased. The decreasing rate of Ms to ΔVB was not dependent on the steel composition, but on the austempering temperature, exhibiting the maximum value when the austempering temperature was 450°C. It was also observed that volume fraction of retained austenite was linearly increased with the decrease of Ms for the steels austempered at various conditions. The highest elongation was obtained when the rate of Ms to ΔVB was the maximum. Ms 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 Ms 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 ΔVB and the Ms.