High Strength Cold-Rolled Sheet Steels for Automobiles

The strengthening mechanisms of an extra-low carbon cold- rolled sheet steel containing both P and Ti have been studied by changing annealing temperature. Yield and tensile strength decreased and r value increased as annealing temperature increased. A maximum and a minimum in total elongation were observed at the annealing temperature of 810°C and 870°C, respectively. The tensile properties were interpreted in terms of different combinations of dislocation strengthening, solid solution strengthening by P, precipitation strengthening due to (Fe, Ti) P, and softening due to ferrite grain growth depending on the annealing temperature. The strengthening contribution due to the (Fe, Ti) P precipitate calculated with a theoretical model was about 80 MPa in the steel continuously annealed in the temperature range of 750 - 810°C for 30 seconds. It is suggested that in the extra-low carbon cold-rolled steel containing both P and Ti, effective Ti content can be more realistically expressed by Ti**=total Ti-(48/14)N-(48/32)S-Ti in (Fe, Ti) P than the previous equation, Ti*=total Ti-(48/14)N- (48/32)S.