Micro-Segregation induced strain inhomogeneity in >900 MPa UTS martensitic hot rolled advanced high strength steel

Abstract

High strength martensitic strip steels used in the yellow goods industry require good ductility and high strength. This paper examines how local compositional inhomogeneity of alloying elements (Mn, Mo, and Cr), caused by interdendritic microsegregation during casting and subsequent deformation, affect local straining behaviour in a >900 MPa autotempered martensite hot rolled strip steel. The alloy rich and alloy depleted bands have been characterised using micro-XRF and SEM-EDX through thickness for a 10 mm thick strip. Tensile samples were selectively machined from areas with a range of microsegregation severity in the longitudinal and transverse orientations. Digital Image Correlation (DIC) during tensile testing was used to examine local straining behaviour and shear band formation. Differential straining of up to 2.5x between solute enriched (lower strain) and solute depleted (high strain) bands was found in the strongly microsegregated transverse samples due to the lower hardness of the solute depleted bands (reduced solid solution strengthening). The high local strain in solute depleted bands in the strongly segregated sample led to premature failure (at lower global applied strain) compared to the less segregated samples. No significant strain partitioning was observed for the longitudinal oriented samples regardless of segregation level.