2015 年 101 巻 1 号 p. 51-58
The effect of cold-working and Mo addition on creep behavior in 15Mn-17Cr-0.09C-0.39N-(0.01, 0.5, 1.0, 1.5, 2.0) Mo steels has been investigated. The steels with solution treatment at 1373 K and ones with 40% cold-rolling are subjected to a creep test at 873 K under 200-400 MPa, respectively. Time to rupture (tr) of both steels increases with increasing the Mo concentration. Since tr in the cold-rolled steels is shorter then that in solution-treated ones in a same creep condition, cold-working is suggested to be harmful for creep strength. Creep rate in a primary creep region of cold-rolled steels with more than 1.0 Mo is smaller than that for solution-treated ones because many dislocations are induced by the cold-working. This nature is advantageous as sealing materials for high temperature. Mo addition causes decreasing of minimum creep rate in the solution-treated steels. A linear relationship exists between logarithm of minimum creep rate and Mo content. In the cold-rolled steels with 0.01 Mo and 0.5 Mo, recrystallization is partially occurred during a prescribed heat treatment before the creep tests. Therefore, the creep rate is promptly accelerated. In the cold-rolled steels with more than 1.0 Mo, the suppression of recrystallization by Mo yields the small minimum creep rate and the extension of tr. Fine precipitates of Laves phase are generated in grain interior in tertiary creep region of cold-rolled steels with 1.5 and 2.0 Mo. It also contributes to the extension of tr due to the retarding effect of creep rate acceleration.
