Influence of subsurface microstructure on the bendability of ultrahigh-strength strip steel

The bendability of thermomechanically rolled and direct-quenched martensitic - bainitic ultrahigh-strength steels can be very anisotropic. Unlike in most steels, it can be good when the bend axis is parallel to the rolling direction but poor when the bend axis is perpendicular to the rolling direction as a result of shear band formation. This paper shows how bendability can be significantly improved and made more isotropic by modifying the subsurface microstructure to include a relatively soft polygonal ferrite and granular bainite layer. It is shown that the thickness of this layer should be at least 300 μm to prevent shear band formation and thereby improve bendability during transverse bending. The results are interpreted in terms of the opposing effects of geometric softening and the strain hardening of the different subsurface microstructures. The role of subsurface shear texture in explaining the anisotropic nature of geometric softening and shear band formation is discussed.