Microstructure and Mechanical Properties of Cold Rolled 10 wt% Medium Mn Steel

Abstract

To obtain a high tensile strength with ductility medium Mn steel room temperature Q&P process was adopted. In the conventional Q&P process, high quenching temperature is one of the obstacles in the mass production of medium Mn Q&P steel. This research attempted to decrease the quenching temperature to room temperature (25 °C) by adjusting alloy elements. Room temperature Q&P process aim for not only high strength with continuous yielding of Q&P steel from TRIP effect but also no need for sophisticated control of quenching temperature. The manganese concentration was higher than that of the conventional Q&P steel, so 25 to 30 % austenite was maintained after room temperature Q&P process. Carbon partitioning was observed at relative low partitioning temperatures (200 to 300 °C). The stabilized austenite by carbon partitioning retarded DIMT which resulted in high tensile strength (~1500 MPa) and ductility (~23.5 %). Silicon, which is effective in inhibiting carbide formation, has been added to confirm its effectiveness. The carbon partitioning on silicon-added alloy occurred very actively due to the less -carbide formation. Silicon addition decreased mechanical stability and SFE of austenite. Nevertheless, the large amount of carbon partitioning in silicon-added alloy increased SFE and austenite stability led to a proper rate of transformation with strain. As a results, tensile properties were comparable to those produced by conventional quenching and partitioning. The relationship among the phase morphology, austenite stability and discontinuous yielding in microstructure consisting ferrite and austenite was also studied. The shape of the grain was changed by adding an austenitization heat treatment. The 10 wt% Mn steel was austenitized before intercritical annealing. Austenite and ferrite fractions and austenite stability remained equivalent to those of the non-austenitized counterpart. After intercritical annealing without austenitization, recrystallized equiaxed ferrite + austenite was formed. However, martensite that was generated on quenching after austenitization did not recrystallize during intercritical annealing, and austenite formed along lath boundaries of martensite to yield a ferrite + austenite lamellar structure. The transition from interface strengthening to strain hardening in the lamellar structure was not as evident as in equiaxed structure, owing to the higher dislocation density and lower interface strengthening in the lamellar structure than in the equiaxed structure. Consequently, discontinuous yielding is effectively prohibited in the lamellar structure of the 10 wt% Mn steel.