| Effect of Ti content on the bake hardening behavior was investigated in cold-rolled and continuously-annealed ultra-low carbon steels containing 0.075% P. TiN and TiS precipitates were observed in steel with Ti content lower than (48/14)N+(48/32)S while in the steel with Ti content higher than (48/14)N+(48/32)S, in addition to TiN and TiS, (Fe, Ti)P and Ti(C, N) were observed. A bake hardening of about 3kgf/㎟ was obtained in a steel with carbon content of about 25 weight ppm and Ti content controlled in N(48/14) ≤ Ti ≤ N(48/14)+S(48/32). Substantial amount of solute carbon might remain without affecting bake hardening. In the 0.0029-0.021% range, sulfur had little effect on the bake hardening. On the other hand, the solute carbon levels that resulted in yield point elongation of about 4.0 percent or less in as-annealed steel appeared to be too small to contribute to bake hardening and aging index. |
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