薄板坯亚包晶钢渣膜矿相结构对铸坯纵裂纹影响机理研究

Continuous casting and rolling for thin slab has the advantages of high production efficiency, low cost and strong adaptability, but longitudinal crack appears easily in the slab surface because of high casting speed and low-volume, and longitudinal crack phenomenon of hypo-peritectic is especially obvious due to peritectic reaction. The mineralogical structure of flux films for the thin slab of hypo-peritectic steel will be quantitatively analyzed by using theoretical methords of process mineralogy. Based on the study, the adsorption capacity of mold powder will be justified, and the different characteristics between normal flux film and the fault flux film will be found out, and the texture characteristics of flux films for the thin slab of hypo-peritectic steel will be justified. According to the quality of slab and the technological conditions, the quantitative relationship between the kinds of crystallite and thermoresistance will be analyzed, and influence of the texture of flux films on the quality of slab and processing parameter will be studied. On this basis, the different constituents of mold powder and flux film will be confected in the laboratory, and then the quantitative relationship between the texture of flux film and viscosity, heat conduction will be researched. The formation mechanism of the longitudinal cracks of hypo-peritectic steel will be investigated, according to the crystal structure and phase equilibrium theory of the minerals in the flux film, and the rational constituent of mould powder for the thin slab of hypo-peritectic steel will be found. The study will provide a theoretical basis to exploit new mold powder, to rich the continuous casting technology and to reduce incidence of the longitudinal cracks of slab.

薄板坯连铸连轧生产效率高、成本低、适应性强，但由于其拉速高、结晶器容量小，铸坯表面极易出现纵裂纹，亚包晶钢因包晶反应，纵裂纹现象尤为明显。本课题运用工艺矿物学研究方法，定量分析现场薄板坯亚包晶钢渣膜的矿物组成、分层特征、结晶率，揭示现场用保护渣吸附夹杂物的能力，找出正常渣膜及事故渣膜矿相结构的差异，厘定薄板坯亚包晶钢渣膜矿相结构特点。结合现场工艺条件及铸坯质量，厘清渣膜显微结构中玻璃相脱玻化微晶种类及含量与热阻之间的定量关系，查明渣膜矿相结构与工艺条件的关系及其对铸坯表面纵裂纹的影响规律。在此基础上，实验室自制不同成分的保护渣及渣膜，厘清渣膜的矿相结构与粘度、热流密度之间的定量关系，结合渣膜中每种矿物的晶体结构特点及相平衡理论，分析铸坯纵裂纹形成的机理，确定薄板坯亚包晶钢合理的保护渣成分，为合理开发保护渣、丰富连铸技术、降低铸坯纵裂纹的发生率提供理论依据。

薄板坯连铸连轧生产效率高、成本低、适应性强，但由于其拉速高、结晶器容量小，铸坯表面极易出现纵裂纹；而亚包晶钢因包晶反应，纵裂纹现象尤为明显。结晶器与铸坯之间渣膜的形成、结构及其性状，直接影响着铸坯的润滑与传热，对防止亚包晶钢薄板坯表面纵裂纹起着决定性的作用。为此，在对现场薄板坯渣膜矿相结构定量分析的基础上，利用FactSage模拟现场保护渣渣膜的析晶过程，查明了渣膜矿相结构形成机理及其对铸坯表面纵裂的影响规律；选用多种矿物原料自制不同成分的保护渣及渣膜，在建立保护渣矿物原料与其物化性能、渣膜矿相之间关系的基础上，确定了薄板坯亚包晶钢合理的保护渣成分。. 研究结果表明，薄板坯亚包晶钢现场渣膜的厚度和结晶率与其它钢种存在明显差别，结晶矿物的形态、粒度及百分含量也略有不同，但矿相的成层分布规律具有一致性；渣膜形成过程中枪晶石在1450℃～1500 ℃时首先析出，黄长石和硅灰石则在1100℃～1250 ℃范围内析晶，其结果是现场渣膜中枪晶石在靠近结晶器侧成层分布，与黄长石和硅灰石之间存在明显界线。薄板坯亚包晶钢事故渣膜中结晶率为60%～65%，枪晶石和黄长石含量都是20%～25%，较正常渣膜偏低，这在一定程度上降低了渣膜的热阻能力，使其无法正常对传热速度及均匀性进行控制，可能是导致铸坯表面出现纵裂的直接原因。. 通过对矿物原料配渣体系中物化性能和渣膜矿相特征的系统研究，以及对薄板坯亚包晶钢纵裂的影响机制分析，厘清了渣膜矿相、工艺条件、保护渣性能与铸坯表面纵裂之间的关系，最终确定适用于薄板坯亚包晶钢连铸用保护渣的理想基渣为：29.6%水泥熟料、17.4%石英砂、17%硅灰石、12%萤石、14%纯碱；10%硼砂。这些研究成果对薄板坯亚包晶钢改善铸坯纵裂有较好的适用性，又为实际连铸生产或炼钢厂选择合理配渣原料、开发优质高效保护渣提供了理论依据。