低碳包晶钢板坯连铸过程枝晶生长和相变行为研究

The peritectic reaction, occurring during the continuous casting of low carbon peritectic steels, could cause a significant solidification shrinkage, which can result in interdendritic hot cracking and deterioration of strand quality. In present study, experiments and multi-scale numerical simulations are all carried out to investigate the dendritic growth and peritectic phase transformation in order to fundamentally reveal the interdendritic hot cracking mechanism. The main studies are listed as follows: Firstly, in situ experimental observations are performed to investigate the kinetics of peritectic reaction and transformation of low carbon peritectic steels using a high temperature laser scanning confocal microscopy (HTLSCM). Secondly, thermodynamic calculations are carried out to investigate the effects of multi-component interactions on the phase equilibrium and solute diffusion coefficient during the solidification of low carbon peritectic steels using the commercial software FactSage. Last but not the least, based on the kinetics and thermodynamics study, a multi-phase/multi-component dendritic growth model will be proposed, and coupled with macro heat transfer and solidification model to investigate the nucleation, dendritic growth, peritectic reaction and transformation, interdendritic solute diffusion, and the effects of the cooling rate and chemical compositions on the phase interface velocity (L/δ，δ/γ and L/γ), solute microsegregtion, solidification shrinkage and characteristic parameters of dendrites (primary dendrite arm spacing λ1, secondary dendrite arm spacing λ2) during the slab continuous casting of low carbon peritectic steels. Based on the above mentioned studies, the dendritic growth patterns and phase transformation mechanism during the slab continuous casting of low carbon peritectic steels are finally revealed, and according to the RDG hot cracking criterion, a quantitative relationship between solidification structure and hot cracking susceptibility is determined to provide theoretical guidance for preventing the interdendritic hot cracking during the slab continuous casting of low carbon peritectic steels.

在低碳包晶钢连铸过程中，包晶相变加剧了枝晶间凝固收缩，极易引发枝晶间热裂，影响铸坯质量和成材率。为此，本研究拟采用实验研究和宏微观多尺度数值模拟相结合的手段，研究低碳包晶钢板坯连铸过程的枝晶生长和相变行为，揭示枝晶间热裂机理。首先，采用高温激光扫描共聚焦显微镜原位观察实验和FactSage热力学计算软件分别研究低碳包晶钢凝固过程相变动力学和热力学。然后，在此基础上，开发多相/多组元枝晶生长数学模型，并耦合宏观凝固传热模型，研究低碳包晶钢板坯连铸过程的形核、枝晶生长、包晶相变和溶质扩散现象，考察冷却速率和钢种成分对低碳包晶钢凝固相界面演变速率、晶间溶质偏析、凝固收缩和凝固组织的影响规律。最后，揭示枝晶生长规律和相变机理，并根据RDG热裂准则建立凝固组织与热裂敏感性之间的定量关系，为控制低碳包晶钢连铸坯热裂提供理论指导。

包晶钢连铸生产过程，包晶相变容易造成铸坯初凝坯壳的不均匀生长，诱发铸坯表面凹陷、裂纹等缺陷，恶化铸坯表面质量。因此，研究包晶相变，将有助于探究低碳包晶钢凝固过程中的包晶反应和包晶转变机理，揭示包晶相变过程中的限制性环节，为包晶钢连铸坯提供理论指导。本研究首先采用FactSage商业软件开展了低碳包晶钢凝固热力学研究。结果表明，溶质元素的加入导致包晶三相区的形成，包晶点处C含量随着S和Mn含量的增加而降低，而增大P将会使得包晶点处的碳含量增加，合金元素Cr含量对包晶点处C含量的影响不大；包晶相变温度随着S和P元素的增加而降低，而Mn和Cr含量对包晶转变线的温度几乎没有影响。其次，采用高温共聚焦金相显微镜对低碳包晶钢凝固相变过程进行了原位观察。结果表明，在冷却速率为0.1 K/s条件下，初生铁素体的析出温度为1777.8 K，并呈胞状结构。温度降到1676.6 K时将发生包晶反应，反应速率为2.94 mm/s，包晶转变时δ/γ与γ/L界面移动速率分别为50.3 μm/s与19.28 μm/s。然后，建立了包晶钢多相场数学模型，开展了包晶钢凝固相变数值模拟。研究表明，初始凝固时界面生长速率受到界面前沿溶质富集的影响而逐渐减慢；包晶反应时三相点平均移动速率为2.451 mm/s，而包晶转变时δ/γ及γ/L界面的移动速率分别为41.13 μm/s和8.43 μm/s。随着冷却速率和界面厚度的增加，平均包晶反应速率逐渐增加；相反，增大界面能将明显降低包晶反应三相点的平均移动速率。最后，根据数值模拟和实验研究结果表明，包晶反应过程中奥氏体包裹铁素体的同时，也将向液相及铁素体相生长。本研究在此基础上提出了新的包晶相变机制，即包晶转变的起始点与包晶反应相同，并揭示了低碳包晶钢凝固过程中包晶反应和包晶转变时的限制性环节为C扩散。