Abstract: Based on double sub-lattice model, a compound thermodynamic model of (Nb_xTi_1-x) (C_yN_1-y) is established ,and the mol fraction, atom site ratio and precipitation sequence of each ingredient in precipitated phase of Nb-Ti microalloying ultra-low carbon steel (/% ：0.02C, 0.12Si, 1.70Mn, 0.012P, 0.004S, 0.101Nb, 0.009Ti, 0.010Als) at 1023-1623 K have been calculated. The effect of precipitates on hot ductility of 230 mm casting slab of steel is studied and the established thermodynamic model is verified with test by using Gleeble thermal simulator, transmission electron microscope and energy dispersive spectrometer. Results show that at 1523 K the solute mol fraction of Nb and Ti in steel are respectively 5.4 x 10^-4 and 3.87 x 10^-5 , with temperature decreasing to 1023 K the solute content of Nb and Ti tends to zero. With decreasing temperature the Ti and N atom site ratio gradually decreases while the Nb and C atom site ratio gradually increases, the sequence of evolution of precipitates is Nb_0.315Ti_0.685C_0.02N_0.98 , (Nb_xTi_1-x) (C_yN_1-y) , Nb_0.85Ti_0.15C_0.71 N_0.29, the calculated values are coordinate with experimental results. With precipitates size less than 60nm and number of particles more than 5 per/μm² , the hot ductility o£ casting slab decreases obviously； at 1241 K the tensile strength of steel is 63.8 MPa to easily form cracks； meanwhile it is found at fracture of Gleeble specimen that Al, Si, Mn, Nb and Ti concentrate at grain boundaries, and the caves caused by carbo-nitride lead to formation of cracks at action of stress. Therefore during casting process the straightening temperature of casting slab should be ≥ 1241 K.

Key words: Nb-Ti Microalloying Ultra-Low Carbon Steel, Precipitates, Thermodynamic Model, Casting Slab, Hot Ductility

摘要： 建立了基于双亚点阵模型的 (Nb_xTi_1-x) (C_yN_1-y)复合热力学模型,并计算了1023~1623 K时Nb-Ti微合金化超低碳钢(/%:0.02C，0.12Si，1.70Mn，0.012P，0.004S，0.101Nb，0.009Ti，0.010Als)的析出相中各组分的摩尔分数、占位比以及析出顺序。通过Gleeble热模拟机、透射电镜和能谱分析仪研究了析出物对该钢230 mm铸坯热塑性的影响和验证所建立的热力学模型。结果表明,1523 K时,钢中Nb、Ti的固溶摩尔分数分别为5.4×10^-4和3.87×10^-5,降至1023 K时,Nb、Ti固溶含量趋于0。随温度降低析出物中Ti、N占位比逐渐下降,而Nb、C占位比逐步上升,析出物的演变顺序为Nb_0.315Ti_0.685C_0.02N_0.98 , (Nb_xTi_1-x) (C_yN_1-y) , Nb_0.85Ti_0.15C_0.71 N_0.29, 计算值与实验结果基本吻合。析出物尺寸小于60 nm,数量高于5个/μm²时,铸坯热塑性明显降低;1241 K钢的抗拉强度临界应力为63.8 MPa,裂纹易形成;同时,Gleeble试样断口处发现Al、Si、Mn、Nb、Ti在晶界处富集,碳氮化物引起空洞,应力作用下形成裂纹。因此连铸过程的铸坯矫直温度应≥1241 K。 

关键词: Nb-Ti微合金化超低碳钢, 析出物, 热力学模型, 铸坯, 热塑性