Bake-hardenable Al-killed Steel Sheet by Box-annealing Process

Abstract

The bake-hardenable cold-rolled steel sheets for the outer panels of auto-bodies exhibiting a sufficient formability and an excellent resistance to denting have been developed in box-annealing process. Examinations in the effects of chemical composition and box-annealing condition on the bakehardenability and on Snoek peak height of Al-killed steels revealed that lowering total carbon content to less than 0.02% for tight coil annealing and raising annealing temperature to α/γ intercritical range for open coil annealing, besides, adding silicon or phosphorus and reducing manganese content in steel can increase the bake-hardenability to 40 MPa, retaining 10ppm carbon in solution. The mill produced bake-hardenable Al-killed steel sheets showed practically non-aging quality and high r-value. Discussions were made on the compatibility of retarded aging property and bake-hardenability of the steels by using Hundy's equation. A retarded aging and bake-hardenable rimmed steel was also produced by controlling nitrogen to 8 ppm in the open coil annealing furnace.