Heterogeneity Based Microstructure Control in Advanced High Strength Steels

Uniformity has served as a primary scheme for the microstructure control of ferrous alloys, which has been successful, pervading all aspects of steel products. Nevertheless, ever increasing demands for the safety and environment issues requires unprecedented properties that is difficult to be obtained by the standard practice. As a new pathway for the microstructure control, we consider an active utilization of chemical heterogeneity. In general, the heterogeneity in microstructure is supposed to be suppressed to avoid any possible degradation in the structural properties. However, a sophisticated application of heterogeneity enables a precise control of microstructure in various scales that provides more enhanced properties compared to the conventional ones. As a case study, we demonstrate the influence of chemical heterogeneity on microstructure and tensile properties of ultra-high strength steels mostly covered with martensite. Chemical heterogeneity was produced by intercritical annealing prior to austenitization. It generated micro-scale and nano-scale Mn enriched domains, which originated from the austenite and cementite evolving during intercritical annealing. Chemical heterogeneity in both Mn-enriched domains were almost maintained after austenitization, eventually contributing to simultaneous improvement in strength and ductility. Micro-scale Mn enriched domain generated strain partitioning between Mn-depleted and Mn-enriched domains during tensile deformation due to dissimilar solid solution strengthening. The strain partitioning resulted in higher back stress, which contributed to the improvement of strength. Meanwhile, nano-scale Mn enriched domain evolved into exceptional stable nano-sized austenite, which enhanced the ductility by exhibiting a persistent TRIP effect.