Bake hardening and ageing properties of hot-dip galvanized ULC steel grades

An optimum combination of good forming properties and dent resistance can be achieved in ULC - Bake Hardening steel grades. In order to provide the necessary amount of solute carbon essential for the BH-effect, it is common practice to use ULC steel containing small amounts of titanium and/or niobium. From thermodynamic computations it could be derived that in case of a titanium-alloyed ULC-BH-steel the sulphide formation is mainly controlled by manganese instead of titanium. This has a major effect on the precipitation sequence. For different Ti- and Nb alloyed steel types appropriate formulas for the calculation of the amount of solute carbon were presented. Yield strength and bake hardening properties have been compared for a commercially produced sample of titanium and niobium alloyed ULC-BH-steel. It is shown that the optimal adjustment of the solute carbon can better be realized in the case of a niobium alloyed steel. (AU)