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Advanced high strength steel (AHSS) strip used in the yellow goods sector needs a balance of high strength with good toughness and bendability. Cracking during bending can result in failure depending on radius to thickness ratio, in view of the relative low ductility and complex microstructure of these steels. This paper examines how local compositional inhomogeneity can affect bending performance in tempered martensite AHSS.
Localised inhomogeneity in the microstructure can generate regions of strain concentration during deformation, affecting the bending performance of the material. Inhomogeneity at the material scale includes inclusion distributions, mixed microstructures, compositional segregation and texture. Inclusions and other impurities in the steel have been reported to be dominating factors in formability. However, as advanced steels become cleaner, other factors become more influential in limiting bendability. When considering bending performance, the sub-surface region is of particular significance as it experiences the greatest strain and stress during bending. These are commonly the nucleation points for failure, primarily through the formation of rippling, micro-shearband, tensile triaxiality and cleavage fracture.
A range of multiscale characterisation methods including micro-XRF, SEM-EDS and nanoindentation have been combined with SEM in-situ mechanical tests to examine the scale and role of local inhomogeneity. These techniques have been used to assess compositional inhomogeneity (Inter dendritic segregation of Mn formed during solidification subsequently hot rolled to give Mn enriched and depleted bands) on the local hardness (instrumented nano-indentation mapping) and therefore strength. Variation in the severity of segregation banding has been observed in strip steels of different thickness, and with respect to position through thickness. The influence of the different banding severity on strain partitioning and shear band formation during deformation has been identified using DIC. The significance of the correlation between these factors is discussed in the context of processability.