基于CaO-SiO2-Al2O3三元渣系超低碳钢连铸保护渣的基础研究

For the CaO-SiO2 silicate based mold fluxes of ultra low carbon steel, at present, there are still slab quality issues that are difficult solved, namely how to avoid slag entrapment of slab, slag patch in hook,and which Na2O in slag results in the white spot defect. However, it is found that CaO-SiO2-Al2O3 ternary slag system based mold fluxes have the advantage of both higher surface tension and chemical stability. Therefore, an idea of CaO-SiO2-Al2O3 ternary slag system based mold fluxes instead of lime-silica based mold fluxes is put forward. The fundamentals of CaO-SiO2-Al2O3 ternary slag system based mold fluxes will be investigated, including 1) melting and viscosity property,2) interfacial properties ( surface tension,interfacial tension and spreading ratio),3) solidification and crystallization behavior of slag film, and 4) chemical stability during melt slags water leaching process. And then, basic data about influence of Al2O3 and Li2O, other oxide and addition amount on physical properties and ability of controlling F and Na transfer will be obtained. On the basis of the above studies，it is expected to seek for the chemistry composition ranges of CaO-SiO2-Al2O3 ternary slag system based mold fluxes with satisfying among the "four high" performances (high viscosity, high surface tension,high interfacial tension and high chemical stability), metallurgy functions (especially lubrication) and environmental requirements (less secondary cooling water pollution and equipment corrosion) for ultra low carbon slab casting. All of the work mentioned in this project will lay a theoretical foundation for CaO-SiO2-Al2O3 ternary slag system based mold fluxes instead of lime-silica based mold fluxes in order to solve the quality problems of high quality steel and environmental protection in the casting production.

现用CaO-SiO2硅酸盐为基超低碳钢保护渣还存在难以解决的铸坯质量问题，即如何避免保护渣卷入、凝固钩夹渣以及由夹渣中Na2O引起的白斑缺陷。然而，发现CaO-SiO2-Al2O3三元渣系保护渣具有较高表面张力和化学稳定性。为此，课题提出采用CaO-SiO2-Al2O3三元渣系保护渣代替现用硅酸盐渣系保护渣的思路。通过开展CaO-SiO2-Al2O3三元渣系保护渣熔化流变特征、界面性质（表面张力、界面张力及铺展率）、渣膜凝固结晶行为以及水浸过程中化学稳定性的基础研究，得到适宜Al2O3和Li2O含量、其它氧化物和熔剂对其物理性能影响的基础数据，寻求出适合超低碳钢既具有高粘度、高表面张力、高界面张力和高化学稳定性的“四高”性能，又满足连铸保护渣润滑功能和环保要求的CaO-SiO2-Al2O3三元渣系连铸保护渣成分范围。为解决高品质钢连铸生产中的质量问题和保护渣在使用过程中环保问题打下基础。

传统的硅酸盐为基超低碳钢保护渣还存在难以解决的铸坯质量问题，即如何避免保护渣卷入、凝固钩夹渣以及由夹渣中Na2O引起的白斑缺陷。然而，发现CaO-SiO2-Al2O3三元渣系保护渣具有较高表面张力和化学稳定性。为此，本课题提出采用CaO-SiO2-Al2O3三元渣系保护渣代替现用硅酸盐渣系保护渣的思路。开展了CaO-SiO2-Al2O3三元渣系保护渣熔化流变特征、界面性质（表面张力、界面张力及铺展率）、渣膜凝固结晶行为以及水浸过程中化学稳定性的基础研究，得到了适宜Al2O3和Li2O含量、其他氧化物和熔剂对其物理性能影响的基础数据。主要结论如下：1）随着Al2O3/SiO2比值增大，熔渣粘度、转折温度以及对温度的敏感性均呈现出增大的趋势，并且F调控熔渣粘度的能力比Na2O强。2）基于CaO-SiO2-Al2O3渣系熔渣结构特点和网络结构的形成机制，提出了修正公式NBO*/T。在修正公式中，Al2O3 被分为三部分：[AlO4]四面体、[AlO3F]四面体以及网络外体（VAl和VIAl）；CaF2 则分为两部分：[AlO3F]四面体（BF和NBF）和稀释剂（FF）。 3）随着Al2O3含量增加，熔渣剪切稀化行为（非牛顿特性）先增强后减弱；当熔渣中Al2O3含量为9.5%（摩尔比）时，熔渣体现出最强的剪切稀化行为；熔渣中Q3(1Al)和Q2(1Al)结构对剪切力的变化最敏感，是调控熔渣剪切稀化行为的关键。4）CaO-SiO2-Al2O3三元渣系的熔渣结晶行为一方面受熔渣聚合度的影响，同时熔剂类型（Li2O、MgO）也不可忽视；尤其是Li2O的加入可以明显降低熔渣的结晶温度，增加1200C以上的孕育时间。5）随着Al2O3/SiO2比、Li2O和MgO含量增加，熔渣表面张力增大、铺展系数减小，钢渣润湿性能降低。6）熔渣在水浸过程中的化学稳定性强烈依赖于熔渣结构。由于CaO-SiO2-Al2O3三元渣系的熔渣结构中有大量的[AlO4]和[AlO3F]四面体生成，抑制了熔渣中阳离子（Na+）和阴离子（F-）两类离子交换的交换量。此外，当用静电势较大的阳离子替换静电势较小的阳离子（如Li+替换Na+，Mg2+替换Ca2+）时，也可减少离子交换量，这是由于静电势较小的阳离子更容易溶出所致。