Abstract
The bake-hardenable cold-rolled steel sheets for the outer panels of auto-bodies exhibiting an excellent resistance to the denting have been developed in the box-annealing process, by examining the effects of chemical compositions and box-annealing conditions on the bake-hardenability and on Snoek peak height of Al-killed steels. It has been found that lowering the total carbon content to less than 0.02% or raising the annealing temperature to air intercritical range bears a bake-hardenable steel sheet in the box-annealing process. Additions of silicon and phosphorus and reduction of manganese are also found to increase the bake-hardenability. The bake-hardening is due to the strain aging caused by the solute carbon of approximately 10ppm. Furthermore, it is shown that the bake-hardenable steel sheets can be successfully manufactured in steel mills and exhibits non-aging quality as well as the high r-value.
