高Crフェライト系耐熱鋼のオーステナイト化と再結晶挙動に及ぼすBの影響

概要

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By using 10CrMoVNbWCoBN steel which developed for steam turbine rotor material, the effect of the boron addition on γ grain behavior at approximately the quenching temperature during the heating austenitizing temperature was investigated. During the heating austenitizing temperature, at first γ phase is formed after α/γ transformation, and then granular grains are formed by the recrystallization process through the high dislocation density as driving force. The temperature of completing the recrystallization is 1025℃ in boron free steel, and is 1075℃ in 90 ppm boron containing steel. It shifts to the high temperature side by containing boron. From the microstructural observation, it was confirmed that the solid solution temperature of M23C6 and (Nb, V) C shifts to 50℃ high temperature side by containing the boron, therefore, the recrystallization temperature also shifts to the 50℃ high temperature side. In order to increase the creep strength of the boron containing steel, it was indicated that the quenching from higher temperature was important for dissolving the precipitates. If the similar effect is obtained in the martensite, it is estimated that the addition of boron contributes to the creep strengthening by increasing high-temperature long term stability of the carbonitride of Nb and/or V.
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