Variation of solute distributions during deformation and bake hardening process and their effect on bake hardening phenomenon in ultra-low carbon bake hardening steels

摘要

Three-dimensional atom probe was used to investigate solute carbon and other elements distributions during bake hardening process after pre-deformation and also to analyze their effect on bake hardening phenomenon of the steels. Two different kinds of bake hardening steels were prepared and annealed by water quenching. The as-received samples were pre-deformed at different levels (from 0 to 10%), and baked at 170 °C for 20 min. Distributions and concentrations of solute elements in the steels were characterized with three-dimensional atom probe. Bake hardening values of the steels were examined by tensile experiments. Three dimensional atom probe detection results indicate that C distribution changes little with the increase of pre-deformation in BH-Mn steel. In BH-P steel, however, with the increase of pre-deformation, more C clusters form in the matrix and C concentration decreases. Distribution patterns and the maximum separation distance method results prove that the C cluster is just C segregation or C together with P segregation rather than vanadium carbides precipitate. Moreover, bake hardening experiment results indicate that BH values are similar in the two BH steels and the BH values change only a little as the pre-deformation increases from 2 to 10%. The 4% pre-deformation induces the highest BH values in the two BH steels, and is considered to be the critical pre-deformation in making the balance of Cottrell atmosphere in the two BH steels.