Cold rolled HSLA sheet and strip products

Higher strength microalloyed steel grades were the key fulfilling ultra-light weight design requirements. During the 80’s the volume fraction of such steel grades had already reached 20 % of the total weight of the body in white. In cold rolled steels the strength is increased essentially by grain refinement and by precipitation hardening. Also microalloyed steels can exhibit a higher strength in the cold formed and painted condition. This yield strength gain derives from work-hardening plus a contribution from bake hardening. It is also found that the fatigue strength of microalloyed steels increases with the static strength even in the notched state. Successful efforts have been made to reduce the scatter in properties for microalloyed higher strength steel grades. Special emphasis was placed on the development of isotropic steel grades. The continuous annealing process results in an additional benefit of a reduced scatter in mechanical properties and shows higher strength values for the same chemical composition. An early Porsche test vehicle and the Ultra Light Steel Auto Body (ULSAB) vehicle chiefly made of microalloyed higher strength steel showed favourable behaviour. The original Porsche test results were confirmed in the later ULSAB study. Microalloying will continue to be an important aspect of materials design in the future. Any further grain refinement would promote plastic instability. However ultra fine plus multi-phase structure would be an ideal approach to overcome this problem. Microalloying, especially with niobium, will retain a dominant role in the achievement of the desired structures and the resulting properties of future materials. (AU)