低碳Nb-Ti二元微合金钢析出过程的演变

建立规则溶液亚点阵模型计算了不同温度(1073~1523 K)下低碳Nb-Ti二元微合金钢(Nb质量分数为0.023%,Ti质量分数为0.012%)中碳氮化物析出相的平衡摩尔分数、化学驱动力和各组元摩尔分数,对微合金钢中析出粒子演变规律进行研究,并利用透射电镜观察及能谱分析验证这种析出模式.计算结果表明,1523 K下析出粒子化学式组成为(Nb_0.15Ti_0.85)(C_0.16N_0.84),由富Ti的析出物逐渐过渡至Nb-Ti均匀析出,析出粒子演变顺序为(Nb_0.15Ti_0.85)(C_0.16N_0.84)、(Nb_xTi_1-x)(C_yN_1-y)和(Nb_0.5Ti_0.5)(C_0.56N_0.44),与实验结果符合较好.随着温度降低,Ti/Nb质量比逐渐减小,得到的TiC比NbC更难溶.对均匀形核及位错处形核的临界核心尺寸和相对形核速率进行计算,得到最大形核率即可获得最细小第二相尺寸的温度. 

Evolution of precipitates in Nb-Ti binary low-carbon microalloyed steels

A thermodynamic model of binary low-carbon microalloyed steel with 0.023% Nb and 0.012% Ti additions was established to calculate the equilibrium molar fraction and chemical driving force of carbonitride precipitation as well as the molar fraction of each element in austenite at temperatures from 1 073 to 1 523 K. The evolution of precipitation in the microalloyed steel was studied, and the precipitation pattern was verified by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The calculating results show that the chemical formula of precipitation particles is (Nb_0.15Ti_0.85) (C_0.16N_0.84) at 1523 K, Ti-rich precipitates gradually transit to Nb-Ti uniform precipitates. The evolution order of precipitation particles is (Nb_0.15Ti_0.85) (C_0.16N_0.84), (Nb_xTi_1-x) (C_yN_1-y) and (Nb_0.5 Ti_0.5) (C_0.56 N_0.44), which accord with the experimental results. The Ti/Nb mass ratio decreases gradually with decreasing temperature, and TiC is more insoluble than NbC. The critical core size and the relative nucleation rate were computed under two nucleation conditions, and the temperatures at which the size of the second phase is the smallest, i. e. the nucleation rate is maximum are 1 198 K and 1 123 K for homogeneous nucleation and dislocation nucleation, respectively.