Abstract
A medium Mn steel has been designed to achieve an excellent combination of strength and ductility based on the TRIP (Transformation Induced Plasticity) concept for automotive applications. Following six passes of hot rolling at 850 °C, the Fe-7.9Mn-0.14Si-0.05Al-0.07C (wt.%) steel was warm-rolled at 630 °C for seven passes and subsequently air cooled to room temperature. The sample was subsequently intercritically annealed at various temperatures for 30 min to promote the reverse transformation of martensite into austenite. The obtained results show that the highest volume fraction of austenite is 39% for the sample annealed at 600 °C. This specimen exhibits a yield stress of 910 MPa and a high ultimate tensile stress of 1600 MPa, with an elongation-to-failure of 0.29 at a strain rate of 1 × 10−3/s. The enhanced work-hardening ability of the investigated steel is closely related to martensitic transformation and the interaction of dislocations. Especially, the alternate arrangement of acicular ferrite (soft phase) and ultrafine austenite lamellae (50–200 nm, strong and ductile phase) is the key factor contributing to the excellent combination of strength and ductility. On the other hand, the as-warm-rolled sample also exhibits the excellent combination of strength and ductility, with elongation-to-failure much higher than those annealed at temperatures above 630 °C.
Highlights
With the rapid increase in the number of motor vehicles worldwide and the associated environmental impact, automobile lightweighting has become an urgent global initiative
We demonstrate that excellent combination of strength and ductility of a medium Mn TRIP steel can be achieved by controlling the volume fraction and the morphology of austenite through hot rolling followed with warm rolling, replacing the traditional three-stage processes including hot rolling, cold rolling, and annealing
This is similar to the measured austenite volume fraction reported by Lee and De Cooman [5] for a 6Mn steel, and the reason may lie in the variations of C and Mn concentration in austenite, affecting austenite’s thermal stability
Summary
With the rapid increase in the number of motor vehicles worldwide and the associated environmental impact, automobile lightweighting has become an urgent global initiative. The excellent performances of this class of steels are closely related to both the multiphase character of their microstructure and the TRIP effect, i.e., the martensitic transformation of the austenite induced by deformation [1]. Good ductility results from ductile and soft ferrite and retained austenite (RA), while high strength originates from the martensite induced by martensite transformation from RA [1,2,3]. In order to obtain a microstructure with a specific amount and stability of RA, quenching and partitioning (Q&P) treatments [3,8] have proven effective in producing high-strength steels with a mixed microstructure of tempered martensite and retained austenite [3,8]. The typical Q&P process consists of a first quench (from either full or partial austenization) to a temperature between the martensite start and martensite finish temperature, and a subsequent isothermal treatment at the same or higher temperature to promote the partitioning of carbon from the supersaturated martensite to the retained austenite
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