超低碳钢连铸坯钩状坯壳的演变与夹杂物的捕集

超低碳钢常用于生产汽车面板等表面质量要求较高的产品.连铸坯皮下的钩状坯壳很容易捕集夹杂物导致冷轧钢板表面出现翘皮、亮/暗线等缺陷,对产品质量具有严重危害.采用数值模拟分析了钩状坯壳的形成和演变过程.将计算的初生坯壳形状制作成物理模型,模拟了夹杂物在凝固前沿被捕集的过程,并对凝固钩区域不同位置的夹杂物的受力特征进行了分析.结果表明,凝固钩在弯月面中形成以后,不会直接湮没进坯壳内,而是要经历熔化、变粗、生长、湮没等逐步演变的过程.数值模型预测拉速1.3 m·min^-1条件下最终存留在坯壳中的凝固钩深度约为2.5 mm,这与实际观察到的钩状坯壳的尺寸基本一致.模拟得到的钩状坯壳形貌与铸坯表层和漏钢坯壳的金相特征较为接近.夹杂物最容易在凝固钩下表面被捕集,不容易在凝固钩上表面被捕集,特别是对尺寸相对较大的夹杂物.但是溢流发生时,靠近弯月面处的夹杂物可能随着钢流进入到初生凝固钩上部而被快速冷却的钢液包裹.两道凝固钩之间的坯壳由于其凝固前沿处于垂直分布,小于100 μm夹杂物可能被捕集而大尺寸夹杂物不易被捕集.

Hook evolution and inclusion entrapment of ultralow-carbon steel slabs

Ultralow-carbon (ULC) steel slabs are usually used for manufacturing high surface quality products such as automobile panel. Severe hooks in the subsurfaces of ultralow-carbon steel slabs usually degrade the surface quality of slab because of inclusions entrapment, which results in unacceptable sliver and blister defects on the surface of the final cold-rolled strip products. The hook formation and evolution process during the initial solidification of a continuous casting slab were studied through numerical modelling. A physical model based on the numerical simulation results was constructed to simulate the process of inclusion entrapment near the hook region, and the forces of inclusions in different positions of the solidified hook region were analyzed. The results demonstrate that following formation, the hook is not immediately buried in the shell; it sustained several stages such as melting, coarsening, growing, and burying. It is predicted that the final hook depth, as buried in the shell, is 2.5 mm when the casting speed is 1.3 m·min-1, which is basically the same as the actual size of the hooked shell observed by a metallographic experiment. The calculated shape of the shell inner face with hooks is similar to morphologies of the slab surface region and breakout shell. The results of physical simulation and force analysis show that inclusions are most likely to be caught by the lower face of the solidified hooks, but they are more difficult to be entrapped by the upper face of the hook, especially for large-size inclusions. However, when overflow occurs, the inclusions near the me-niscus may be wrapped by the rapidly cooled molten steel above the primary hook. In the vertical shell between the two adjacent hooks, small-size inclusions (less than 100 μm) may be wrapped by the solidified front, but large-size inclusions are difficult to be wrapped.