Microstructure–mechanical property relationship in hot-rolled high-Al-low-Si dual-phase steel

Abstract

ABSTRACTIn this study, the effect of finish rolling temperature and coiling temperature on the microstructure and mechanical properties of high-Al-low-Si dual-phase (DP) steels is explored. Two different finish rolling temperatures (850 and 790°C) and three different coiling temperatures (200, 250 and 300°C) were studied. The results indicated that all the different processing conditions led to ferrite-martensite DP microstructure. With the decrease in finish rolling temperature, the volume fraction of ferrite was increased and martensite content was decreased. When the coiling temperature was increased to 300°C, autotempered martensite was obtained, which led to the softening of martensite and decrease in tensile strength and strain hardening ability, but higher post-necking elongation. Moreover, the nanoscale Nb-based carbides played a crucial role in refining the microstructure of hot-rolled high-Al-low-Si DP steel. EBSD (Electron Backscattered Diffraction) analysis revealed that the ferrite grains were fine, and decrease in finish rolling temperature and coiling temperature led to an increase in low-angle boundaries. When the finish rolling temperature was decreased to 790°C and coiling temperature was decreased to 200°C, the steel had excellent mechanical properties with tensile strength of 885 MPa, uniform and total elongation of 16.0 and 25.94%, respectively, and the product of tensile strength and total elongation was 20 264 MPa%. The improvement of strength and plasticity can be attributed to the fraction of ferrite and martensite, precipitation of NbC, fine microstructure.