高频电磁场对15CrMo连铸坯表面质量和等轴晶率的影响机理

采用小线圈法对结晶器内的磁场分布进行了测量,并对结晶器内磁场,电磁力和钢液流速分布进行了数值模拟.在实验室连铸机上进行了合金结构钢15CrMo的连铸实验,对连铸坯表面形貌进行了观察与分析.提出了高频电磁场对连铸坯质量影响的机理:施加高频电磁场后,保护渣通道拓宽,铸坯与结晶器壁间的渣道动压减小,有效地抑制了铸坯表面振痕的产生;受电磁场Joule热以及保护渣热阻增加的影响,已结晶同相的温度梯度减小,柱状晶生长受到抑制.此外,实验测量和数值模拟结果表明,由于磁场在拉坯方向分布不均匀,在弯月面区域形成上,下2个,,向相反的涡流,钢液环流造成固/液界面前沿液柑的温度梯度减小,有利于形成成分过冷而获得发达的等轴晶组织.

EFFECT MECHANISM OF HIGH FREQUENCY ELECTRO-MAGNETIC FIELD ON THE SURFACE QUALITY AND EQUIAXED CRYSTAL RATIO OF 15CrMo BILLET

An induced coil surrounding a segmented mold used in soft contact electromagnetic casting (soft-contact EMC) was used to produce a high frequency magnetic field for reducing ferrostatic pressure between the mold and melt. The distribution of magnetic field in the mold was examined using a magnetic probe of the induction coil type. Then mathematical model was developed to study the distributions of magnetic field, electromagnetic force and flowing velocity of molten steel in the mold. Finally, continuous casting experiments were conducted with alloy constructional steel 15CrMo in the laboratory caster. The surface morphologies and macrostructure were examined and analyzed. Based on the comprehension of the distributions of magnetic field, electromagnetic force and flowing velocity of molten steel in the mold through measurements and numerical simulation, the effects of electromag-netic field were systematically investigated. The results indicate that when the electromagnetic field was applied in the initially solidified area, the mold flux consumption was increased dramatically. As a result, the surface quality of continuously cast billets is greatly improved, for example, oscillation marks disappeared due to the decrease of flux pressure. Moreover, the growth of columnar grains is suppressed for two main reasons. The first one is that the mold near meniscus is heated by Joule heat generated by the high frequency electromagnetic field. The other one is that the thermal resistance between mold and the solidified shell is increased as the increase of mold flux thickness. Inhomogeneous distributions of magnetic field in the mold along the casting direction were confirmed both by measurement and numerical simulation. And the Lorentz force on the molten steel along the casting direction is uneven likewise. Under the drive of Lorentz force, two counter-rotational vortices are formed below the meniscus. Moreover, the temperature gradient in front of the solid/liquid interface is decreased as a result of the circulation of liquid steel. Therefore, composition supercooling is easily obtained in the liquid core, which is helpful to the growth of equiaxed crystals.