含Nb低碳钢相变温度A_(r3)预测模型

PREDICTION OF A_(r3)FOR Nb-CONTAINING LOW CARBON STEELS

根据热膨胀实验测定的4种不同Nb含量低碳钢在不同连续冷却条件下先共析铁素体相变温度(A_(r3)),建立了一个新的铁素体形核和长大模型,用来预测含Nb低碳钢的A_(r3),模型中考虑了Nb的溶质拖曳作用.结果表明,先共析铁素体相变形核长大的临界碳浓度随钢中Nb含量的增加而呈线性增大,但对温度和冷却速率的变化不敏感;在连续冷却过程中钢中Nb含量的增加会降低先共析铁素体形核温度,但当Nb含量大于0.023%之后形核温度并不会继续降低;固溶Nb会抑制铁素体相变的发生,NbC的析出会促进铁素体形核;本模型适合于Nb含量低于0.04%的低碳钢,使用该模型预测的A_(r3)与实测值吻合.

Proeutectoid ferrite transformation start temperatures （Ar3） under variant continuous cooling rates were measured by dilatometer experiments. A new model, based on ferrite nucleation growth model and considering the solute drag-like effect of Nb, was used to predict Ar3 of Nb-containing low carbon steels. It is shown that a limiting carbon concentration in the vicinity of the growing ferrite nuclei is invariable with the variations of temperature and cooling rate, while increases linearly with the Nb content increasing. During the continuous cooling process, the increases of Nb content results in decreasing of the nuclei temperature of proeutectoid ferrite when Nb content is not above 0.023%. Nb solutionized in austenite inhabits the proeutectoid ferrite transformation, and NbC accelerates ferrite nucleation. This model is suitable to predict Ar3 of low carbon steels with Nb content lower than 0.04%, and the calculated Ar3 is in good agreement with the measured value.