Yielding Behavior of Low Carbon Martensitic Steel Sheet Containing Retained Austenite

Abstract

Quenching and Partitioning (Q&P) steel sheets, which utilize the transformation induced plasticity (TRIP) effect of retained austenite to improve the elongation of high strength steel sheets, are expected to become an important material for next-generation automotive structural parts. Although it has been reported that the yield strength (YS) of the Q&P steels consisting of tempered martensitic microstructure with retained austenite (hereafter ”Q&P steels” in this study) is affected by retained austenite, the mechanism has not yet been discussed in detail. The purpose of this study is to clarify the effect of the carbon content in retained austenite on the yielding behavior of the Q&P steels. The chemical composition of the model steel used here was 0.18%C-1.5%Si-3.0%Mn (mass%). The steels were annealed at 1143 K, then cooled to 473 K, followed by holding at the temperatures between 523 K and 673 K for 600 s. The increased carbon content in retained austenite increased the YS of the Q&P steels. It was found that the yielding of the Q&P steels was caused by the stress-induced transformation of retained austenite when the critical stress for the stress-induced transformation was lower than the elastic limit of tempered martensitic matrix. This result revealed that the increased carbon content in retained austenite was able to achieve the higher elastic limit of martensitic steels containing retained austenite.