Ni-Cr-Mo-V 鋼の過熱脆化と旧オーステナイト粒界に析出した MnS 量の関係

概要

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The effect of overheating on the amount of MnS (V_S) and the number of MnS (n) precipitated on unit area of ductile intergranular facets and hence on the change in upper shelf energy (υE_s) was investigated in this study. The υE_s decreased proportionally with an increase in V_S in steels overheated by isothermal holding. A change in υE_s related to both V_S and n in steels overheated by continous cooling. The increase in υE_s due to slow cooling at a cooling rate less than 2℃/min was caused by an decrease in n ; despite an adequate amount of V_S. The V_S was 0.1 to 0.5 times as small as the amount of MnS precipitated on unit area of grain boundaries (V_S') which was calculated from the Mn/S solubility product and grain boundary area per unit volume. The V_S/V_S' tended to decrease when S content increased. This suggests that the increase in S content enhances the intragranular precipitation of MnS. It appears that an decrease in the onset temperature of overheating with the decrease in S content can be explained by the increase in V_S' due to coarsening austenite grain and the prevention of intragranular precipitation of MnS. The critical value of V_S' for preventing overheating was 5×10^<-5>mm^3/mm^2, which was obtained from the relationship between υE_s and V_S'. Combination of Mn and S contents to get the V_S' of less than the critical value was determined.
社団法人日本鉄鋼協会の論文
1990-03-01