Microstructure and mechanical property of complex phase steel consisting of martensite and isothermal product formed below Ms temperature

Abstract

The bainite structure containing 1.5 wt.% high silicon has been expected as an alternative structure that can improve the brittleness of martensite by implementing a cementite free matrix and retaining a nano-scale austenite layer. However, since the incomplete phase transformation phenomena and the carbon solid solution effect conflict, the bainite structure has been evaluated to have a limitation in that it can be applied only to complex steels composed of martensite and bainite based on a medium carbon alloy system of 0.5 wt.% or less. In addition, the temperature at which bainite structure can be formed has been delimited to be between the temperature range where reconstructive phase transformation can proceed and the start temperature of martensite phase transformation. Therefore, methods for realizing the complex phase of bainite and martensite have been limited to the accellerated cooling method, controlling the austempering time method, and the 2-step quenching and partitioning method. However, recently, studies on isothermal displacive phase transformation that persist in the region below the Ms temperature have been reported. The definitive explanation for the identity of this isothermal product is deficient, and whether this structure can be classified as lower bainite is still controversial. Obviously, if this isothermal product is utilized, a homogeneous matrix can be realized in characteristics such as hardness deviation relative to the existing complex phase steel consisting of austempered bainite and athermal martensite. In this study, changes in mechanical properties of complex steel consisting isothermal products formed below Ms were analyzed. And analysis of the microstructural evolution and crystallographic features of this structure progressed to elucidate the causal relationship on the increase in mechanical properties of complex steel.