Microstructure and mechanical properties of 0.2C-3.9Al-1.12Mn-0.3Mo δ-TRIP steel as a function of isothermal bainitic transformation temperature

Abstract

The present investigation employed a two-stage heat treatment on a δ-transformation-induced plasticity (TRIP) steel comprising 0.2C–4Al–1.2Mn–0.3Mo (wt%). The mechanical and microstructural characteristics that result from varied isothermal bainitic transformation (IBT) temperatures following inter-critical annealing at 820℃ for 10 min are thoroughly analyzed. The microstructure of the steels consisted of δ-ferrite, α-ferrite, bainitic ferrite, retained austenite (RA), and martensite, resulting in an optimum combination of the ultimate tensile strength (UTS) and total elongation (TE). The results of the investigation showed that IBT temperature had an effect on the stability of RA and the martensitic transition. Due to the increased mechanical stability of RA, the study revealed that the TRIP phenomenon was more prominent at lower IBT temperatures. Both tensile and yield strengths, as well as elongation, decreased as a consequence of the increase in IBT temperature. Maximum values of UTS, TE, and the product of these two properties (PSE) are attained (860 MPa, 41%, and 35260 MPa∙%, respectively) under optimal processing conditions (at 350℃ IBT temperature in 10 min).