Interplay between reversed austenite and plastic deformation in a directly quenched and intercritically annealed 0.04C-5Mn low-Al steel

Abstract

We have elucidated here the interplay between reversed austenite and plastic deformation in a directly quenched and intercritically annealed medium-manganese low-Al steel in terms of microstructural evolution and mechanical properties. The ultra-low carbon 5Mn steel was subjected to controlled rolling and direct quenching, followed by intercritical annealing at 630 °C and 650 °C for 30 min, respectively. The directly quenched steel consisted of thin lath-martensite of ∼0.2–0.9 μm thickness and ∼1.7–2.0% retained austenite with ∼50 nm thickness. Intercritical annealing led to a microstructure consisting of alternate sub-micron laminated structure of tempered martensite and reversed austenite. With increase in annealing temperature from 630 °C to 650 °C, the volume fraction of austenite was increased from 12.6% to 19.1% at 1/4 thickness (t/4), and from 8.4% to 12.7% at 1/2 thickness (t/2). The yield strength, tensile strength, and elongation of 728 MPa, 826 MPa, and 25.5%, were obtained at t/4 of the plate, and 714 MPa, 814 MPa, and 24.2% at t/2, on annealing at 630 °C. The impact energy obtained at −60 °C was greater than 80 J. When the annealing temperature was increased to 650 °C, strength was marginally decreased, but toughness was significantly increased to more than 110 J. Extremely small variation in microstructure and mechanical properties at t/4 and t/2 plate thickness were observed. Energy dispersive x-ray spectroscopy (EDS) confirmed ∼7.2–10.1 wt% Mn enrichment in reversed austenite during annealing, which is of significance in stabilization of reversed austenite. Work hardening behavior and tensile experiments were conducted to study the effect of reversed austenite during plastic deformation. With increased annealing temperature, the stability of reversed austenite was decreased because of less enrichment of C and Mn in reversed austenite and grain growth. At 0.05 tensile strain, reversed austenite transformed to twin-type martensite, implying occurrence of transformation induced plasticity (TRIP) effect.