粗粒鋼におけるオーステナイト結晶粒の成長性に関する二,三の知見

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Following the previous report, (Kawai and V. Masuko, Tetsu-to-Hagane, vol. 41, 1955 p. 435) similarly from the view point of grain-growth characteristics of austenite, the author observed experimentally some practical problems concerning the grain size control in coarse-grained steels for high-pressure and high-temperature piping service.Results obtained are summarized as follows:(1) A finished product has uniform coarse grains of No. 3-No. 4 in McQuaid-Ehn Grain Size Number which are by No. 1-No. 3 less than a ladle sample. And the difference of grainsize between a finished product and a ladle sample increases by increasing forging ratio.(2) The grain growth characteristics of austenite is of the so-called "gradual coarsening" type having no abrupt coarsening stage, and the coarsening temperature is lower in a fini-shed product than in a ladle sample.(3) In the melting of these steels, as the reducing proceeds, austenite grains become finer and the coarsening temperature rises. And by the addition of Al, austenite grains become still more finer and the coarsening temperature rises higher, but as the time passes on, the coarsening temperature falls. Moreover, the grain-refining effect of Cr additions is also observed.(4) Austenite grain size and the coarsening temperature have a close connection with the acid-soluble Al in steel as in the case of fine-grained steels in the previons report.As to Al2O3 in steel, however, the clear relation is not observed.(5) The reason for the difference of grain size between a finished product and a ladle sample is considered to be mainly the diminution of acid-soluble Al in steel, but, especially in the case of low-content level of it, the difference of forging ratio and the distribution state of carbides are also considerably concerned.(6) The relative relation of grain growth characteristics of austenite among fine-grained steel, coarse-grained steel and duplex-grained steel was clarified.