Microstructural evolution during the progressive transformation-induced plasticity effect in a Fe–0.1C–5Mn medium manganese steel

Abstract

The microstructural evolution of a cold-rolled and intercritical annealed medium-Mn steel (Fe–0.10C–5Mn) was investigated during uniaxial tensile testing. In-situ observations under scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis were conducted to characterize the progressive transformation-induced plasticity process and associated fracture initiation mechanisms. These findings were discussed with the local strain measurements via digital image correlation. The results indicated that Lüders band formation in the steel was limited to 1.5% strain, which was mainly due to the early-stage martensitic phase transformation of a very small amount of the less stable large-sized retained austenite (RA), which led to localized stress concentrations and strain hardening and further retardation of yielding. The small-sized RA exhibited high stability and progressively transformed into martensite and contributed to a stably extended Portevin–Le Chatelier effect. The volume fraction of RA gradually decreased from 26.8% to 8.2% prior to fracture. In the late deformation stage, fracture initiation primarily occurred at the austenite/martensite and ferrite/martensite interfaces and the ferrite phase.

1. Introduction

Given the demand for automotive steel with high strength and excellent impact energy absorption capacity, medium manganese steel (MMnS) has become attractive candidates for automotive sheet-forming applications [1–3]. MMnS achieves an ultrafine and multiphase microstructure through cold rolling (CR) and intercritical annealing (IA) processes in the ferrite–austenite two-phase region [4–7]. A certain amount of austenite shows stability at room temperature (RT), but it can transform into martensite during deformation and remain preserved in the microstructure. The volume fraction and stability of the retained austenite (RA) play vital roles in controlling the strength, ductility, and work-hardening capability of MMnS [8–11]. Hu et al. [12] proposed a multi-alloyed design principle together with a direct intercritical rolling process to obtain RA with high content and optimal stability and provided persistent transformation-induced plasticity (TRIP) effect on work hardening and damage resistance. A detailed description of the specific behavior of RA during the deformation process is currently unavailable. Therefore, in terms of size and morphology, the effect of the progressive TRIP process, particularly the influence of the RA, on strain hardening and fracture initiation behavior throughout the entire deformation process must be understood.

The formation of Lüders band and Portevin–Le Chatelier (PLC) bands during the tensile deformation of MMnS shows an association with localized deformation in banded structures [13–18]. Lüders strain, which typically exceeds 5% strain [19–20], negatively affects the application of CR MMnS. Wang et al. [21] observed that the optimization of CR reduction in IA MMnS is an effective means of reducing Lüders strain. Li et al. [22] limited Lüders strain through prestraining, which resulted in the increased average stability of the RA; as a result, the occurrence of the TRIP effect was delayed, and the initiation strain of Lüders bands was suppressed. Although these methods effectively reduced Lüders strain through suppression of the TRIP effect during yielding, they can also reduce plasticity. Furthermore, the formation of PLC bands leads to uneven deformation and early necking, although it increases the strength of steel [14]. Revealing the microstructural evolution under the effect of progressive TRIP will help in elucidating the mechanisms of fracture initiation.