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 630ºC and 650ºC 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 630ºC to 650ºC, 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 630ºC. The impact energy obtained at -60ºC was greater than 80 J. When the annealing temperature was increased to 650ºC, 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 improved weldability [5,7]. Currently used low alloy heavy plate steels for high-strength structural applications are essentially quenched and tempered steels with low-to medium-carbon content [8].Generally expensive alloying elements, such as Ni, Cr, Cu, and Mo, are added to obtain required high strength in the mid-thickness of the plate to improve the microstructure, mechanical properties and homogeneity. Furthermore, the traditional high-strength structural steels adopt multi-stage heat treatment process, which consumes significant energy and is against the concept of green economy.Mn is an important alloying element in the design of advanced high-strength steels, because it increases the stability of austenite and also impacts the kinetics of transformation [9-10]. In recent years, a number of studies have been carried out on low-carbon medium-manganese steels to explore new possibilities in the development of advanced high-strength steels [11][12][13][14][15]. It is known that BCC bainite/martensite transforms to FCC austenite during intercritical annealing in the two-phase region, referred as austenite revert...

Austenite reversion in low-carbon martensitic stainless steels – a CALPHAD-assisted review

“…Reverted austenite may also transform to martensite by strain-induced transformation and in this way enhances the plastic regime [45,71,72,81]. This mechanism known as transformation induced plasticity, has not been treated in depth in low-carbon martensitic stainless steels, but advanced in-situ experiments and modelling of strain-induced martensite formation have been performed in other variations of steels [82][83][84][85][86][87][88].…”

Enhanced Impact Toughness of Heat Affected Zone in Gas Shield Arc Weld Joint of Low‐C Medium‐Mn High Strength Steel by Post‐Weld Heat Treatment

“…The microstructure of the BM is presented in Figure . Based on the previous research, the OM (Figure a) of BM indicated tempered martensite and reversed austenite. In the EBSD image quality map with grain boundary misorientation distribution, the red lines represent low misorientation boundaries of 2–15° and blue lines correspond to high misorientation boundaries of 15° and greater (Figure b).…”

“…At the same time, C and Mn atoms diffused from martensite to reversed austenite. This was mainly attributed to the fact that FCC austenite can accommodate more C and Mn atoms than BCC martensite . Thereby, the stability of reversed austenite was enhanced with increased C and Mn‐content .…”

“…At present, the medium‐Mn TRIP steels with 3–10% Mn content has received significant attention as advanced high strength steels (AHSS) because of good combination of strength and ductility . Majority of the studies on medium‐Mn steels focused on microstructure and mechanical properties.…”

“…Jang et al studied the effects of annealing time on 6Mn‐TRIP steel. Liu et al and Lee et al discussed the work hardening behavior of medium‐Mn TRIP steel. However, the weldability of medium‐Mn steels has not been studied.…”