摘要 The relationship of the P and C grain boundary segregation and its effect on bake hardening behavior were investigated in ultra-low carbon bake hardening (ULC-BH) steel with and without P addition annealed at 810 °C for various time using electron probe micro-analyzer, electron backscattered diffraction, and three-dimensional atomic probe techniques. Results revealed that P addition and annealing duration considerably affected the bake hardening behavior of experimental steel. The BH value of ULC-BH steel without P addition is lower than that with P addition within a short annealing time, and the difference in the BH value gradually decreases as the annealing duration is prolonged. P segregation is dominant in terms of a high P bulk content in steels with P addition at the expense of C segregation during annealing. By contrast, opposite effects are observed in low carbon bake hardening steel. The high residual solute C content in steel with P addition is due to P segregation at the grain boundary. Site competition is mainly responsible for the lower BH value in ULC-BH steel without P addition than that with P addition. As the annealing time is further extended, C segregation begins at grain boundary despite the delayed P segregation, leading to a gradual decrease in the solute concentration in the matrix of steels with P addition. C and P segregations reach the equilibrium as the annealing time increases to 60 min at 810 °C in the two steel samples. Theoretical calculations reveal that the residual solute C concentration in the matrix decreases to zero, and this finding is consistent with the change trend of the bake hardening value. Hence, the C segregation at grain boundary adversely influences the bake hardening property of ULC-BH steel.
Abstract：The relationship of the P and C grain boundary segregation and its effect on bake hardening behavior were investigated in ultra-low carbon bake hardening (ULC-BH) steel with and without P addition annealed at 810 °C for various time using electron probe micro-analyzer, electron backscattered diffraction, and three-dimensional atomic probe techniques. Results revealed that P addition and annealing duration considerably affected the bake hardening behavior of experimental steel. The BH value of ULC-BH steel without P addition is lower than that with P addition within a short annealing time, and the difference in the BH value gradually decreases as the annealing duration is prolonged. P segregation is dominant in terms of a high P bulk content in steels with P addition at the expense of C segregation during annealing. By contrast, opposite effects are observed in low carbon bake hardening steel. The high residual solute C content in steel with P addition is due to P segregation at the grain boundary. Site competition is mainly responsible for the lower BH value in ULC-BH steel without P addition than that with P addition. As the annealing time is further extended, C segregation begins at grain boundary despite the delayed P segregation, leading to a gradual decrease in the solute concentration in the matrix of steels with P addition. C and P segregations reach the equilibrium as the annealing time increases to 60 min at 810 °C in the two steel samples. Theoretical calculations reveal that the residual solute C concentration in the matrix decreases to zero, and this finding is consistent with the change trend of the bake hardening value. Hence, the C segregation at grain boundary adversely influences the bake hardening property of ULC-BH steel.
XI-LIANG -ZhHANG,HOU Hua-Feng,LIU Shou, et al. Effect of annealing time and phosphorus addition on bake hardening behavior of ultra-low carbon bake hardening steel[J]. Journal of Iron and Steel Research International, 2018, 25(12): 1287-1295.
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2018, Vol. 25 
