The influence of chemical heterogeneity on microstructure and mechanical properties of advanced high strength steel (AHSS)

Abstract

Heterogeneous structured materials (HSMs) have been paid a lot of attention for future strategy to break through strength-ductility tradeoff of metallic materials. HSMs were referred as materials having domain with different strength; microstructure mixture of hard domain and soft domain. HSMs shows superior tensile properties beyond simple rule of mixture of hard domain and soft domain due to additional back stress hardening caused by interaction between hard domain and soft domain. Dissimilar strength between domains in most of previous studies was realized by using dissimilar grain size strengthening. However, strength difference can be also realized by dissimilar solid solution strengthening in ferrous material induced from chemical heterogeneity. The present study tries to apply chemical heterogeneity to make HSMs in advanced high strength steel (AHSS). The chemical heterogeneity can be made by prior annealing treatment, e.g. austenite formation and cementite precipitation. Mechanical properties and microstructure were investigated on chemically heterogeneous martensitic steels. They showed improved tensile properties and the mechanism responsible for the improvement was analyzed in terms of microstructure and deformation behavior during straining. Different from previous HSMs using dissimilar grain size, chemical heterogeneity highly affects to phase transformation behavior during heat treatment. Unprecedented phase transformation behavior was observed and how to utilize it to improve mechanical properties of advanced high strength steel (AHSS) was suggested in this study.