连铸保护渣固渣膜凝固结构演变行为及传热特性研究

Lubricating slabs and controlling heat transfer are significant functions of mold fluxes, which achieved by liquid and solid slag films between initial slab surfaces and mold wall, respectively. However, the mechanism of heat flux control near meniscus is still unclear, and the understanding of structure evolutions of solid slag films under mold conditions also are rarely reported. Those disadvantages restrict the development of continuous casting technology. Thus, based on the contradiction between heat transfer control and lubrication functions of mold fluxes in the casting of crack sensitive steels, high-basicity and high-fluorine mold fluxes, fluorine-free mold fluxes, and CaO-Al2O3 based mold fluxes are chosen as the bases of this project. The effect of water capacity of molten mold fluxes on the surface roughness and closed porosity of solidified slag films will be investigated. The method of controlling the roughness and closed porosity of solid slag films via water capacity regulating will also be discussed. Besides, the structure features of solid films, its influence factors, possible controlling methods will be investigated by an improved water-cooling copper probe with a small size. The relationship between those structure features and comprehensive heat transfer characteristics of solid films will also be calculated. By means of this project, methods to control the structure characteristics of solid slag films can be obtained, and a system using structure characteristics of solid slag films to estimate its comprehensive heat transfer features also can be established. The result of this project can contribute the basic theory of controlling and optimizing the films structure and property of mold fluxes, which also can expand the varieties of mold fluxes and has a high academic significance and practical application value.

润滑铸坯和控制传热是连铸保护渣重要的冶金功能，由结晶器壁与初生坯壳间的液、固渣膜实现。然而，目前结晶器弯月面附近固渣膜控制传热机理仍存争议，对结晶器条件下固渣膜凝固结构演变规律和影响因素仍缺少认识，严重阻碍了连铸技术的发展。基于此，本项目针对裂纹敏感钢连铸过程中，保护渣渣膜控制传热与润滑铸坯功能的矛盾，选取典型高碱度高氟、无氟及CaO-Al2O3基保护渣，重点研究保护渣水容量特性对渣膜表面粗糙度、闭孔率等特征演变的影响及调控机制；使用小尺寸水冷探头，全面研究固渣膜结构特征的影响因素、可能的调控手段，以及固渣膜结构特征参数与其综合传热特性之间的定量关系。通过本项目的研究，可以阐明保护渣固渣膜凝固结构特征的调控方法和途径，建立使用固渣膜结构参数定量评价其综合传热特性的方法和体系。研究结果能为固渣膜结构性能调控及优化奠定理论基础，能扩大保护渣应用品种，具有较高的学术意义和实际应用价值。

保护渣凝固渣膜的结构及演变行为决定了其控制传热的能力和对润滑的影响，尤其是渣膜凝固初期，其结构对弯月面附近横向热流的控制有重要作用。由于结晶器内条件复杂，影响因素众多，目前无法直观地研究结晶器内固渣膜的凝固生长过程。如何获取与结晶器冷却条件类似的渣膜样，是研究渣膜的重要基础。基于此，项目在大宽厚比小尺寸水冷探头的基础上，实现了对固渣膜凝固过程中冷却速率的连续调整，为复杂及连续可变冷却条件下固渣膜的获取提供了条件。为明确保护渣烧结特性，以便调节和控制弯月面渣圈、渣条行为特性，获得厚度和结构均匀的固、液渣膜，项目通过保护渣加热过程中的宏观应力-应变特性，研究开发了保护渣烧结特性测试方法和微区液相生成量评价途径。针对典型保护渣凝固渣膜内，玻璃体和晶体真密度有差异的特点，开发使用了真密度法，评价渣膜内晶体的析出量和析出速率，并解释了超高碱度保护渣现场应用性能优越的原因。此外，项目重点研究了CaO-Al2O3渣系条件下，固渣膜结构演变行为及其对渣膜传热等冶金性能的影响。结果提示，在增加氧化铝的条件下，即使初始只含2 wt%的氧化镁，铝镁尖晶石也会大量析出，急剧促进渣膜晶体比例和厚度的上升，且先期析出的微小尖晶石可作为铝酸锂、铝酸钡的析出核心，显著促进晶体的大量析出。凝固渣膜的传热特性直接由其结构决定，项目通过建立固渣膜依附水冷铜壁凝固生长的传热模型，分析了渣膜厚度、传热系数、渣-铜界面热阻、液渣凝固温度等参数对热流的影响。基于此，可在获取有代表性的凝固渣膜基础上，分析不同结构参数对传热的影响。项目研究结果为保护渣品种设计、优化提供了参考，深化了保护渣基础理论。