Use of interphase boundaries quantification as a means to analyze the microstructural distribution in heat treated complex-phase advanced high strength steels and their influence in void nucleation

Complex-phase (CP) steels belong to the first generation of advanced high-strength steels (AHSS) and are widely employed as replacements for dual-phase (DP) steels due to their superior work flangeability, higher yield strength, and enhanced resistance to hole expansion. These improved properties are attributed mainly to the presence of bainite, which mitigates the hardness contrast between martensite and ferrite, thereby reducing void nucleation and promoting a more homogeneous load distribution during deformation. Understanding void nucleation mechanisms is therefore essential for improving the mechanical performance of CP steels, a point that became evident during recent efforts to upgrade a CP1100 steel to a CP1200 grade, which yielded better-than-expected mechanical properties and raised the question of which mechanism was responsible for the observed improvement in elongation. However, direct observation of void nucleation is challenging in refined microstructures with grain sizes of 1–2 μm, even with advanced techniques such as X-ray microtomography. To address this limitation, a novel approach is proposed that analyzes the distribution of interphase boundaries. Electron backscatter diffraction (EBSD) data were segmented and quantitatively evaluated using image processing to assess the microstructural distribution. The relative amounts of bainite/ferrite, bainite/martensite, and ferrite/martensite interphase boundaries were used to interpret the mechanical property improvements of the newly developed CP1200 steel in comparison with its predecessor, CP1100. The results reveal a distinct improvement in the distribution of interphase boundaries, demonstrating that interphase boundary quantification provides a viable and informative alternative to direct void-nucleation analysis in refined CP steels.