Improvement of Anti-aging Property at Low Temperature by Cr Addition in Bake Hardenable Ultra Low Nitrogen Steels

Abstract

The effect of Cr, Mn, Mo, V, Nb, and Al on bake hardenability (BH) at 170°C and anti-aging properties, i.e. the inhibition of the appearance of yield point elongation (YPE) after low temperature aging, in ultra low N steels was investigated. It was found that Cr addition of 0.5–1.0 mass% apparently inhibit the appearance of YPE after aging for 3.6 ks at 100°C without much decreasing the amount of 2%BH at 170°C. The reason for the inhibition by Cr addition was discussed on the diffusion trap model, by using the interactions among Cr atom, N atom, and vacancy evaluated by first-principles calculation. The estimation indicates that (1) Cr would not inhibit the diffusion of N atoms as a repulsive interaction was expected for Cr–N complexes, (2) a small amount of vacancies introduced by skinpass rolling, which attracts strongly with N atoms, seems to play a main role on the inhibition of the diffusion of N atoms, (3) Cr could retard the annihilation of vacancies which trap N atoms during aging, since Cr atom attractively interacts with vacancy–N complexes. Moreover, the effect of annealing temperature and coiling temperature on bake hardenability was investigated, and the effect was found to be understood by the change of solute N, owing to the precipitation of CrN which has a precipitation nose at 650–700°C.