Nb첨가 저합금강의 오스테나이트 결정립 크기에 따른 γ/α 상변태 거동

Abstract

The addition of Nb in steel has advantages to obtain the uniform and fine structure as well as high-strength with an proper control. Generally, the addition of Nb has both accelerating and delaying effects in phase transformation. Solute Nb in austenite lowers the activity of carbon and results in the delay in phase transformation. On the other hand, the soluble Nb and its precipitates accelerate phase transformation due to the formation of fine austenite grains during reheating. In this study, it was attempted to analyze the effect of Nb in γ/α transformation with the austenite grain size by constructing CCT (Continuous Cooling Transformation)diagram and AGS(Austenite Grain Size) measurement. In order to observe the effect of solute Nb on γ/α transformation, results of carbon steel were compared with those of Nb added steel by adjusting AGS nearly same. To examine the Nb effect on AGS, AGS was measured with contents of Nb(0, 0.02, 0.04, 0.06wt%) and reheating temperature(1200, 1150, 1000, 900℃). It was confirmed that soluble Nb delayed the γ/α transformation when AGS in Nb added and carbon steels were kept nearly same. However, when reheating temperature was lowered up to 850℃, the delay of γ/α transformation by Nb was not effectively observed. It was considered that smaller AGS by Nb addition lessened the delay of γ/α transformation.