Effects of Carbon, Phosphorus, and Sulfur Content on the Tensile Properties and Spot Weldability of High-Strength Cold-Rolled Sheet

Low-carbon aluminum-killed rephosphorized steels containing 0.06 to 0.11 percent phosphorus have been developed to produce highly formable cold-rolled sheets with minimum yield strengths of 40 ksi, suitable for automobile body-panel formations. In the present study the effects of carbon (0.03 to 0.09%), phosphorus (0.06 to 0.12%), and sulfur (0.01 and 0.025%) on the spot-welding characteristics of rephosphorized cold-rolled-steel sheet with yield strengths ranging from 37 to 50 ksi were investigated. Within the compositional ranges explored, the transition from good weldability (full button pullout in peel and cross-tension tests) to poor weldability (partial button pullout) was primarily related to higher levels of carbon and sulfur. Increasing the phosphorus content from 0.06 to 0.12 percent also decreased spot-weld integrity.