Effect of Rapid Heat Treatment on Bake Hardening Behavior of a Low Carbon Steel

A low carbon steel was firstly annealed with an ultra- rapid heating rate of 200℃/s at 660℃ and 750℃ respectively, then the steel was pre-strained to 2% and finally suffered a baking hardening treatment at 180℃ for 20 min in order to reveal the synergistic effect of the above two heat treatments of the performance of the low carbon steel in terms of heating temperature, soaking time and cooling rate etc. Results indicated that the bake hardening was enhanced with the increase in annealing temperature for the dissolution of the carbides. The BH value of the low carbon steel annealed at 660℃ decreased significantly with increasing soaking time. But the BH value showed a complex variation with soaking time for the steel annealed at 750℃ because the carbides dissolved incompletely during the ultra-rapid heating process: with an increase in soaking time in the range of 0 to 10 s, the BH value made an apparent increment, and then decreased obviously when the soaking time exceeded 10 s. The bake hardening of the steel is enhanced significantly with increasing cooling rate due to the increment of the solute carbon.