Evidence and effects of heterogeneities in complex phase and dual phase steels

Heterogeneous materials are extensively used in daily life. As a typical example of heterogeneous materials, advanced high-strength steels (AHSS) normally consist of various constituents with distinct properties to achieve enhanced combination of strength and ductility. However, a comprehensive description of heterogeneities in AHSS is currently absent. In this study, the correlation among the compositional, microstructural and property heterogeneities is explored by employing four commercial grade dual-phase (DP) and complex-phase (CP) steels. A correlative characterization approach combining various electron microscopic techniques and nanoindentation is developed and utilized to characterize these heterogeneities. Based on the results, Mn is alternately enriched and depleted in bands parallel to the rolling direction, while the local C content is closely associated with the spatial phase distribution. The comparison between DP800 and CP800, which were fabricated from the same cast but differently heat treated in the intercritical annealing regime indicates Mn segregation impacts the phase transformation and C distribution throughout the whole manufacturing process. In DP800, the heterogeneity of C distribution was more pronounced and martensite inherited from the Mn banding due to the lower intercritical annealing temperature, which led to the smaller fraction of austenite and intensive C partitioning. On the other hand, CP800, heat treated at higher intercritical annealing temperature, exhibited a more homogenous C distribution and resulted in a more homogenous distribution of martensite. The Mn segregation bands and their impact on martensite morphology and location is reflected by the local hardness distribution. In addition to their varying partitioning in present phases, C and Mn distribute heterogeneously in some individual grains/area, which cause variation in local properties. The hardness distributions of investigated materials were characterized through nanoindentation. The results indicate the conventional hardness difference between phases is not applicable to correlate the heterogeneous microstructure with the stretch-flangeability of the material. Therefore, a new approach combining the Shannon entropy of hardness distribution and spread of local hardness with respect to microstructure is developed, which successfully links the microstructure and hardness heterogeneity with material’s local formability. The present work deepens and expands the current understanding and knowledge of the heterogeneous microstructure and hopefully the derived conclusions provide guideline for future material design by fabricating the heterogeneities of microstructure to achieve desired properties