Effect of annealing temperature on microstructure and mechanical properties of hot-rolled medium-Mn steel

Abstract

This paper emphasizes on the effect of annealing temperature. Hot-rolled medium manganese steel (0.17C-7.57Mnwt%) was used for quench and partition (Q&P) process. Different temperature parameters were set for the study. Various aspects of steel microstructural evolution, alloying element partitioning, and stability of retained austenite (RA) have been investigated. They were held at an annealing temperature of 671/680/700/720/760 °C for 15 min, respectively, and then quickly transferred to a 100 °C water bath for 90 s to complete the fractionation. When the annealing temperature is increased, the RA content shows an increasing and then decreasing trend, with the highest RA content at 680 °C, and a decreasing trend in RA content starting at 680 °C. The focus is on the correlation between the stability of RA and the overall mechanical properties of medium-Mn steel. This result found different RA contents in the specimens at different annealing temperatures. The transformation-induced plasticity (TRIP) effect continued to occur for different durations. These differences affect the mechanical properties of medium-Mn steel. When the annealing temperature is 680 °C, there is a phenomenon of transformation of large-angle grain boundaries to small-angle grain boundaries during stretching. The transformation and generation of angular grain boundaries have an important influence on the mechanical properties of steel. When the annealing temperature was 680 °C, the geometric dislocation density increased from 4.2824 × 10¹⁶ m⁻² to 5.7384 × 10¹⁶ m⁻² during stretching. The results show that the microstructure and phase composition morphology of the specimens are different at different annealing temperatures, and the morphology is lath-like, block-like, and granular at 680 °C, while the bit granular at 760 °C. The RA content is also different, with higher RA content at 680 °C, and lower RA content at 760 °C. The stability of RA has an effect on the overall mechanical properties of the specimens. Proper RA stabilization results in specimens with a continuous TRIP effect. The continuous TRIP effect causes the specimen to exhibit excellent tensile properties. Specifically, the tensile strength was 1070 MPa, the elongation was 33.84%, and the product of tensile strength and elongation was 36.2 GPa·%.