Analysis of the surface-roughening phenomenon in P-added bake-hardened steel sheets

Abstract

The surface-roughening problem with phosphorous-added bake-hardened steel sheets was investigated. Surface undulation in the sheets occurs after 2% to 3% deformation during press forming action and disappears with further deformation over 10%. The roughened sheets exhibit heterogeneity of the elemental P (phosphorous) distribution, and the segregation of P increases the hardiness of ferrite grains locally. This local strengthening of ferrite grains causes a concentration of strains in the other ferrite regions that do not contain the elemental P-segregation. The strain concentration is because of the surface undulation in the sheets under small amount of straining. The evolution of the deformation texture and the corresponding Lankford values are shown using a series of viscoplastic self-consistent (VSPC) simulations, which indicated that a strong {554}[225] texture develops, and subsequently, the Lankford value increases in the non-segregated ferrite region. The higher Lankford value makes it more difficult to deform the sheet in the through-thickness direction. The different texture developments of the segregated and segregation-free regions are the main causes of the disappearance of the surface undulation with a high amount of plastic straining greater than 10%. Finally, we conclude that the segregation of P inherited from hot band affects the plastic deformation and developed textures of the thin steel sheets.