厚坯连铸喂钢带技术与冷带重熔/热坯凝固的协同相变理论

One of the effective technologies, which can eliminate the defects of centerline segregation and loosen in slab and thick slab or bloom, is to add steel strip toward the continuous casting mold. Since we lacks the profound understanding on the interaction between heat strand and cool strip about technology of adding steel strip toward the mold, it can be widely applied in industrial application. The present project focuses on the physical characteristics of adding steel strip toward the continuous casting mold, we propose the scientific hypothesis of collaborative phase change for the strand/strip system. Furthermore, we develop the theories of heat transfer based on the collaborative phase change, and reveal the heat flux of molten steel inward strip and outward shell, the grain growth and macro/micro- segregation of solute element in strand. We develop the mathematical model for the phase change heat transfer and solidifying processes in strand/strip system based on the generalized enthalpy method. We put forward the technological process of liquidus remelting of strip steel to calculate the adding steel strip. The superheat can be rapidly reduced and large zero superheat zone in the front of solidification is formed by the adding steel strip technology, so that the center structure of slab or bloom can be compactly produced, center-segregation can be significantly reduced and ratio of equiaxed dendrite increase. The theoretical basis of industrial application of adding steel strip technology toward the continuous casting mold is built; the strand quality of slab/bloom can be improved in our country.

向连铸结晶器内喂入钢带是解决中厚、超厚板坯及较厚矩形坯中心偏析及中心疏松等内部缺陷的有效技术之一。由于对喂入钢带导致连铸体系内冷带重熔与热坯凝固的相互作用缺乏深入认识，迄今尚无法大范围工业化应用。本项目提出热坯/冷带协同相变的假说，发展坯/带协同相变传热理论，揭示连铸体系内钢液、坯壳、钢带复合传热规律，揭示连铸坯凝固结晶规律和溶质元素再分配规律。基于广义热焓法建立喂钢带工艺的连铸坯/钢带体系中相变传热及凝固结晶过程的数学模型。在掌握坯壳内过热钢液中喂入钢带的相变传热规律基础上，提出液相线共熔法建立钢带喂入量与钢带喂入参数和铸坯状态之间的关系，使得喂入钢带不但在浇注区能快速降低钢液过热度，而且在凝固前沿的液芯中形成较大范围的零或接近零过热度钢液区，凝固后将会密实铸坯中心结构，大幅度减轻铸坯中心偏析，同时增加等轴晶率。为连铸结晶器喂钢带技术的工业化应用奠定理论基础。

连铸坯中心偏析、中心疏松等内部质量问题严重影响产品的力学性能，向结晶器内喂入钢带是提高铸坯内部质量的方法之一。钢带喂入会引起原凝固体系中复杂多物理场的变化，由于对钢带-铸坯相变体系内热质传递过程缺乏深入认识，喂钢带工艺制度仍不完善，致其应用受到了极大的限制。本项目采用实验和数值模拟相结合的方法揭示了喂钢带工艺改善凝固组织的作用机理，探索了连铸喂钢带工艺涉及的复杂的流动、相变、传热及溶质传输等基础科学问题。考察了结晶器冷态模型内晶粒微观结构及宏观凝固组织演化，并建立了柱状晶-等轴晶混合凝固-熔化模型，探究了中心加入内部冷源时凝固组织的发展变化。发现内部冷源通过加速形核、增强对流和促进柱状晶向等轴晶转变的作用机制提高凝固组织质量且内冷源过冷形核速率具有先增大到极值而后减小的过程特征。认为若要在喂钢带期间进一步促进钢液形核，可考虑在引入过冷度的同时向熔体内加入大量细小弥散的微粒，即利用钢带夹带形核剂提高喂钢带工艺的效率。建立了凝固熔化共存体系内流动-传热耦合数学模型，结合凝固组织实验区别考虑了熔化糊状区渗透率和凝固糊状区渗透率的影响。发现钢带在喂入过热钢液后没有直接熔化而是经历了钢鞘形成、钢鞘熔化以及钢带熔化三个阶段。分析了钢带尺寸、熔体过热度以及喂带速度对熔化速率的影响，并提出了将总熔化时间与上述参数关联的数学关系式。在上一模型的基础上，将钢液中溶质传输考虑在内，基于预测的溶质中心偏析度结果，设计了增材稀释钢带中合金元素含量。通过本项目研究明确了喂带参数的计算方法，为完善喂带技术奠定理论基础。