低碳钢铸坯皮下凝固钩形成及其对夹杂物行为影响研究

The solidification hooks which usually exist in the subsurface of oscillation marks in low carbon steel slab are somehow deteriorate the slab quality. In one hand, the discontinuities of micro structure and composition in the hook region might induce cracks; on the other hand, bubbles and inclusions are easy to be captured by the hooks and then become product defects. Thus, it is necessary to study the hook formation and its effect on the behavior of inclusion capture, which has important significance on improving the cleanliness and product quality of low carbon steel.. Method of combining plant experimental research and mathematical modeling research is used to study the contents which mainly focus on the following two aspects: (1) plant experiments on the formation mechanism and influencing factors of subsurface hooks in low carbon steel slabs; (2) theoretical research on the mathematical model of hook formation and inclusion motion and capture.. Through the research, it is aimed to determine the formation mechanism and infuencing factors of subsurface hooks in low carbon steel slab, to establish the coupling model of initial solidification and heat transfer of molten steel in the mold, as well as the inclusion motion and capture model, and accordingly to propose the effect rule of hooks on the capture behavior of inclusions in slab subsurface. The results above could be important guidances on reducing large inclusions in slab subsurface and improving surface quality of high quality steel.

常出现于低碳钢铸坯振痕处皮下的凝固钩会一定程度恶化铸坯质量。一方面凝固钩处显微组织和成分的不连续易导致其成为裂纹源；另一方面凝固钩易捕捉气泡和夹杂物成为产品缺陷。因此有必要掌握凝固钩形成及其对夹杂物捕捉等行为的影响规律，这对提高低碳钢洁净度和产品质量有着重要意义。. 项目采用工厂实验研究与数学模型研究相结合的方法重点研究两方面内容：（1）基于企业生产低碳钢连铸坯皮下凝固钩生成机理及影响因素的实验研究；（2）连铸坯皮下凝固钩生成的数学模型以及夹杂物运动和捕捉模型研究。. 通过研究确定低碳钢连铸坯皮下凝固钩的形成机理及影响因素；建立结晶器内钢液初始凝固传热耦合模型以及夹杂物运动和捕捉模型；并在此基础上找到凝固钩对铸坯亚表层夹杂物行为的影响规律。上述结果对铸坯表层大型夹杂物的去除与控制和高品质钢表面质量的提高具有重要指导意义。

在低碳钢连铸过程中，由于结晶器振动的采用，在浇铸的铸坯表层总会出现周期性的振痕，而凝固钩结构也会伴随振痕的出现而形成。凝固钩是在结晶器内的弯月面凝固形成的，该结构处存在微观偏析，同时该结构向结晶器内部生长延伸，会捕获结晶器内上浮的气泡和夹杂物等，从而恶化铸坯的表面质量。因此本项目对低碳钢（特别是超低碳钢）铸坯皮下凝固钩的形成及其对夹杂物等的捕获行为进行了研究。.研究内容主要包括：(1) 对比凝固钩主线与Bikerman方程得到的弯月面形貌，证实了凝固钩的形成机理；(2) 钢液成分特别是碳含量对凝固钩的影响；(3) 连铸工艺参数对凝固钩的影响，本部分主要考察了拉速、浇铸钢水的过热度、结晶器冷却水流量、水口浸入深度、电磁制动和铸坯宽度等参数；(4) 凝固钩对夹杂物的捕获规律的实验研究，并研究了夹杂物在凝固钩处的捕获位置及夹杂物的尺寸分布；(5) 对结晶器内的多相流动及钢液凝固以及夹杂物运动进行了数值计算。.结果表明：通过对比侵蚀得到的凝固钩主线与通过Bikerman方程计算的弯月面形状曲线，证实了凝固钩是弯月面凝固后钢液溢流形成的；对比不同碳含量和连铸参数的凝固钩结果发现，减小碳含量、提高拉速、提高浇铸钢水的过热度、减小结晶器冷却水流量、减小水口浸入深度、采用电磁制动均能减小凝固钩结构；研究发现，凝固钩会捕获夹杂物和气泡，且夹杂物的尺寸越大，被凝固钩捕获的现象越明显。在结晶器内被捕获的夹杂物和气泡留在铸坯表面，会在铸坯轧制过程中形成Sliver和Pencil pipe缺陷，因此现场常采用表面清理的方式消除此危害。通过控制凝固钩结构，可以适当减小甚至消除对铸坯表面的修磨。因此该研究是很有实际意义的。