5th International High Manganese Steel Conference - HMNS 2022

9%Ni steels, which have excellent strength and toughness in the low temperature range, are used as the cryogenic steel mainly used for LNG storage tanks, Mn is similar to Ni as an alloying element in steels, such as improvement of hardenability and stabilization of γ, so that Mn is expected as an alternative element to Ni. However, because the addition of Mn to steel causes remarkable intergranular fracture, the microstructure control to suppress the intergranular fracture is important for applying the medium Mn steels to cryogenic materials. In previous study, we found that toughness of 10%Mn-0.1%C steel could be improved by thermomecanical control process (TMCP).　However, the effect of intercritical annealing on the toughness of medium-Mn steel subjected to TMCP has not been investigated yet. In this study, we attempted to further strengthen the TMCPed 10%Mn-0.1%C steel by performing intercritical annealing.
10%Mn-0.1%C steel was austenitized at 1473 K, and then hot-rolled with the reduction of 75% at 973 K, followed by water-quenching. The quenched sample was annealed at 873 K for 1 h. In addition, a sample intercritical annealed without TMCP was also prepared as a reference specimen.
The toughness of intercritical annealed sample without TMCP was hardly improved, whose fracture mode was found to be intergranular fracture with orange peel like surface. On the other hand, the intercritical annealed sample with TMCP showed remarkable improvement with occurrence of fracture separation and the main fracture mode was intragranular fracture. It is considered that this is because the softening of the matrix by intercritical annealing contributed only in the case of intragranular fracture. In the presentation, we will also discuss the contribution of retained γ stabilized by intercritical annealing to the toughness.