Effect of Mo on Softening Behavior During Annealing in Cold-Rolled High Mn Austenitic Stainless Steels

The effect of molybdenum (Mo) on softening behavior of cold-rolled high manganese austenitic stainless steels was investigated. The high temperature hardness of cold-rolled Mo-free steel was drastically decreased by prolonged annealing at 873 K. On the other hand, there is not remarkable degradation of high temperature hardness in the steels containing more than 1 mass% Mo. Fully recrystallized structure is observed in Mo-free steel after annealing at 873 K for 1440 ks. Since deformation structures were not disappeared in the steels containing more than 1 mass% Mo even after annealing, the addition of Mo retarded the recrystallization in the work-hardening steels. After annealing, fine particles of M23C6 type carbide precipitated at the grain boundaries in the steel containing Mo. The amount of Mo in M23C6 carbides was increased with increasing Mo content. On the other hand, the size of these carbides slightly decreased with increasing Mo content. These fine carbides strongly prevented the grain boundary movement (migration), therefore recrystallizaiton in the work-hardening steels was retarded. Moreover, the growth of M23C6 carbides at the grain boundary was consistent with Ostwald ripening equation substituted Mo for Cr. This result suggested that the growth rate of M23C6 was controlled by the diffusion of Mo.