Metallurgical aspects in cold rolled high strength steel sheets

Abstract

Cold rolled high strength steel sheets with yield strength from 300 to 500 N/mm² have been developed by using conventional equipment for producing commercial cold rolled steel sheet, that is, cold rolling, box annealing, and temper rolling. Effective alloying elements for strengthening are carbon, silicon, manganese, phosphorus, niobium, etc. The sheets up to 400 N/mm² yield strength grade are easily produced by selecting appropriate chemical compositions. The sheets with higher yield strength grade than 450 N/mm² are obtained by introducing the new idea that the steel with more than 2 pct manganese is annealed between A₁ and A₃ transformation temperatures, and moderately temper rolled. Increase of tensile strength does not affect deep drawability while it deteriorates stretch-forming and stretch-flanging properties. As for electric resistance spot welding, shear tension strength increases in proportion to tensile strength, but cross tension strength hardly increases or tends to decrease. These sheets have been applied to door beams and bumper reinforcements.