Compositional heterogeneity in multiphase steels: Characterization and influence on local properties

Local variations in chemical composition, i.e., compositional heterogeneities, play an important role in the thermodynamic stability and spatial distribution of phases in multiphase steels. The influence of heterogeneous elemental distribution on the resulting local microstructure and mechanical properties has been investigated in the present work. Two steels, one with dual-phase (DP) and one with complex-phase (CP) microstructure, were fabricated from the same cast but differently heat treated in the intercritical annealing regime to yield variations in local microstructure formation. The heterogenous distribution of C and Mn that exists on different length scales are quantified by a characterization approach correlating local chemistry, microstructure, and hardness by using electron probe microanalysis (EPMA), electron backscattered diffraction (EBSD), and nanoindentation. Mn was alternately enriched and depleted in bands parallel to the rolling direction caused by segregation during continuous casting. The local C content was closely associated with the spatial phase distribution resulting from intercritical annealing. In the DP steel, the heterogeneity of C distribution was more pronounced and martensite inherited from the Mn banding due to the lower intercritical annealing temperature. On the other hand, the CP steel, heat treated at higher intercritical annealing temperature, exhibited a more homogenous C distribution resulting in a homogeneous distribution of martensite. In addition to their varying partitioning into the present phases, C and Mn in large martensite islands tended to concentrate in the vicinity of phase interfaces and deplete in the center. Furthermore, chemical boundaries with the smooth transition in local properties were observed in the CP steel. The Mn segregation bands and their impact on martensite morphology and location is reflected by the local hardness distribution. The correlation between compositional heterogeneity, microstructure formation, and microscopic as well as macroscopic mechanical properties is discussed in detail.