Towards engineering of mechanical properties through stabilization of austenite in ultrafine grained martensite–austenite dual phase steel processed by accumulative roll bonding

Abstract

In this study, microstructure development with respect to mechanical properties in ultrafine grained dual phase (martensite/austenite) by chemical composition of Fe-24Ni-0.1C (wt%) steel processed by 1-cycle and 6-cycle of accumulative roll bonding (ARB) process was investigated. Microstructure characterization by electron backscatter diffraction (EBSD) system showed that significant grain refinement occurred after 6 cycles ARB process and an average grain size smaller than 200nm was obtained. Moreover, phase analysis map obtained from EBSD system showed that the microstructure of starting material is mostly constituted of martensite phase (more than 70%) and transforms to retained austenite by 6 cycles of ARB process. This phenomenon has proved that retained austenite is stabilized during the ARB process through grain refinement. Uniaxial tensile tests exhibited that yield strength did not significantly improve by 1-cycle reverse transformation compared to the starting material. In contrast, further continuation of the ARB process caused gradual increase in yield and ultimate tensile strengths. The significant improvement in yield strength should be originated from significant grain regiment introduced by 6 cycles of the ARB process. Surprisingly, the uniform tensile elongation for the starting material and the 6 cycles ARB processed specimen are almost the same, though yield and ultimate tensile strengths are considerably improved by employing 6 cycles of ARB process.