水口结瘤对结晶器内钢渣界面卷渣行为影响的基础研究

The slag entrainment at the steel-slag interface during continuous casting process seriously restricts the further quality improvement of steel products. Previous studies are mainly focused on the mechanism of slag entrainment under steady conditions. For most aluminum deoxidized steel, the residual micro-grade inclusions in the molten steel would adhere to the wall of nozzle during continuous casting and subsequently grow into larger blockings, which are called cloggings. The normal flow pattern of molten steel in the nozzle and mold is changed by the existence of the cloggings. At present, the mechanism of slag entrainment is still not clear for the specific unsteady condition..The current project shall conduct the study on the mechanism of slag entrainment during the slab continuous casting with clogged submergenced entry nozzle. First of all, based on the similarity criterion of steel-slag interface, a water model is established, and the model nozzles are made according to the actual distribution and morphology of cloggings in the nozzles from the industrial practice, during which various nozzles with different casting periods are sampled. Thus, the influence of clogging degree on the behavior of slag entrainment shall be carried out, and the critical judgment criterions for various forms of slag entrainment could be determined. Then, by the commercial fluid calculation software and self-developed subroutine, the study will build a coupling model that could simultaneously calculate the growth of clogging on the inner wall of the nozzle with the attachment of inclusion particles and the slag entrainment at the steel-slag interface. The established model can study the behavior evolution of slag entrainment during the growth of clogging. The current project can improve the existing mechanism of slag entrainment especially under unsteady condition, and clearly determine the main influence factors, which could enhance the theoretical basis for the further quality improvement of clean steel.

连铸结晶器内钢渣界面卷渣问题严重制约着钢铁产品质量的进一步提高，此前对卷渣机理的研究主要集中在稳态条件下。对大多数铝脱氧钢来说，连铸过程钢液中残留的微米级夹杂物会在水口内壁附着长大，形成结瘤物。水口结瘤使得水口和结晶器内钢液正常流动模式发生改变。目前针对水口结瘤这一特定的非稳态环境，钢渣界面的卷渣机理尚不清楚。.本项目以板坯结晶器对研究对象，首先基于钢渣界面卷渣相似准则建立水力学系统，还原不同浇铸时期水口内结瘤物的形貌和分布，研究不同物性参数下水口结瘤程度对卷渣行为的影响，建立不同形式卷渣产生的临界判断准则；其次，结合流体计算商业软件和自主开发子程序，建立微米级夹杂物颗粒在水口内壁附着长大与宏观流场的耦合计算模型，研究水口结瘤过程钢渣界面卷渣类型的演变规律。通过本项目的开展，将完善现有的钢渣界面卷渣机理，明确非稳态环境下卷渣产生的机制以及主要影响因素，为进一步提升洁净钢质量奠定理论基础。

本项目以连铸板坯结晶器为研究对象，采用物理模拟和数值模拟相结合的方法，研究水口结瘤对结晶器内流场和钢渣界面卷渣的影响。通过对水口结瘤过程钢渣界面卷渣的研究，可为进一步减少卷渣缺陷提供理论指导。.首先建立了模型与原型比为1:4结晶器水模型，采用粒子测速仪（PIV）研究对比了不同水口出口形状（矩形和类椭圆形）对结晶器内流场、液面附近流速、卷渣频率等的影响，再采用大涡模拟（LES）研究非稳态下水口出口流股特征。研究发现，尽管两种水口下结晶器液面流速相当，但使用矩形水口时的液面发生卷渣的频率大于使用类椭圆形水口时的频率，原因在于此时卷渣发生受液面涡量变化率的影响。随着浇铸的进行，卷渣通常发生在涡量变化率的峰值处，即涡度变化最快的时刻，且类椭圆形水口下液面涡量变化率大于矩形水口。.然后建立水口结瘤过程的数学模型，分别采用分离涡模型（DES）计算水口内的湍流、离散相模型（DPM）跟踪夹杂物颗粒的运动、多孔介质模型模拟颗粒沉积对流场的影响。研究发现，夹杂物颗粒倾向于粘附在水口出口下部和水口底部区域；水口结瘤后，水口内的流场发生改变，随着壁面粘附物的增多，水口出口流股的平均速度逐渐降低，而射流最大速度和水口中心主流股速度逐渐增加。此外，大尺寸的夹杂物颗粒更容易在壁面沉积，且结瘤达到一定程度后，随着结瘤的进行，单位时间内沉积的颗粒数量明显增加。.为了得到水口结瘤这一特定的非稳态环境对结晶器流场和液面卷渣的影响，本项目制作结瘤物占水口内腔体积比例分别为0%、10%、30%和50%的水口。对比发现：正常水口下的流场是对称的、典型的双辊流，水口结瘤后，结晶器左右两侧流场不对称。从液面形态和油层厚度来看，结瘤水口的液面不平稳，油层向水口靠拢，油层增厚，上回流逐渐增强。当结瘤比例小于10%时，卷渣频率较小，主要发生剪切卷渣。当结瘤比例为30%和50%时，剪切卷渣发生频率明显增大，且随拉速增大、保护渣（油层）黏度的减小，卷渣频率增加的趋势更加明显。