An experimental apparatus for simulating copper mold is used to quantify the heat flux through the slag film and to obtain a solid slag for further determining its crystallization behavior.The result indicates that both the chemical composition of the mold powder and the cooling rate have an important influence on the heat flux through the slag film. With increasing the binary basicity, the heat flux of slag film decreases at first, reaches the minimum at the basicity of 1.4, and then increases, indicating that the maximum binary basicity is about 1.4 for selecting'mild cooling'mold powder. The heat transfer through the slag film can be specified in terms of the crystalline ratio and the thickness of the slag film. Recrystallization of the solid slag occurs and must be considered as an important factor that may influence the heat transfer through the solid slag layer.
The isothermal and non-isothermal experiments were performed to construct the continuous cooling trans- formation (CCT) and temperature time transformation(TTT)diagrams of four industrial mold fluxes through visual observations in an experimental apparatus based on the single hot thermocouple technique(SHTT).The results of the CCT diagrams indicate that①the crystallization temperature of mold fluxes lowers as the cooling rate increases,②the mold fluxes have larger critical cooling rate,higher crystallization temperature, and less onset time of crystal- lization when the basicity increases or the viscosity decreases,③the influences of the melting points of the mold flu- xes on their crystallization tendency are not significant.Isothermal tests show that the onset time of crystallization decreases at first, and then increases, and finally represents a'C'shape with increasing isothermal temperature. The TTT diagrams of four industrial mold fluxes were divided into two separate'C'shape regions. The crystal phase of C20A selected was analyzed by X-ray diffraction, which is cuspidine (Ca_4 Si_2O_7 F_2) over 1100℃and calci- um silicon oxide fluoride (Ca_2SiO_2F_2) below 1100℃.When compared with the TTT diagram, the CCT diagram can provide a more realistic estimate of the critical cooling rate of the mold fluxes. Thus, both the CCT and TTT di- agrams can unambiguously describe the crystallization phenomena of the mold fluxes.
采用热丝法构建CaO-SiO_2-CaF_2-Na_2O四元渣系的连续冷却转变(Continuous Cooling Transformation,CCT)和等温转变(Temperature Time Transformation,TTT)曲线.实验结果表明,碱度越高,保护渣临界冷却速度越大,碱度1.5的渣样临界冷却速度达20℃/s.等温实验X射线衍射结果显示,低碱度渣样在高温区析出硅灰石(CaO·SiO_2),高渣样析出硅酸二钙[Ca_2(SiO_4)],后者导热系数低,控制传热效果更好.随碱度增加,保护渣TTT曲线鼻尖点孕育时间缩短,动力学分析可用推导方程描述渣的等温结晶过程.对裂纹敏感性钢种,CaO-SiO_2-CaF_2-Na_2O四元渣系保护渣碱度可达1.5.
