연속 주편의 3차원 응고와 벌징에 관한 수치해석적 연구

Abstract

In the continuous casting process, the internal cracks should be prevented to achieve the improvement of productivity by high speed casting. The internal cracks can occur when tensile strains at the solidification front exceed a certain threshold level. The bulging of the strand is presumed to play a major role in occurrence of tensile strains that cause internal cracks. In particular, when the casting speed is high, strains caused by the bulging become large because of the decrease of the solidified shell thickness and the increase of the surface temperature of the strand.

In this paper, solidification analysis of strand-casting under air-mist spray condition, which uses air-mist mixture, is carried out by the two-dimensional finite difference method. In the solidification analysis, the boundary conditions of width face of slab are defined by considering zones directly cooled by sprays and indirectly by rolls and radiation, and the boundary conditions of narrow face of slab are only defined by considering zones by radiation. In order to deal with complicated deformation of the slab, the three-dimensional elastic-plastic and creep models are used for analyzing bulging in the strand casting. The bulging deflections and strains are obtained by using the well-known finite element code ANSYS and are compared with those from the experiments under the same condition. For efficient bulging analysis, the output data of the finite difference method are automatically transformed to input data for ANSYS. The effects of important parameters in casting process of the slabs, such as casting speed, roll pitch and slab shape, are analyzed by implementation of the presented method.

When comparing with different casting speeds, the bulging strain obtained in the high speed casting appears to be larger than that at the low speed casting. Also, creep has great influence on the casting directional strain, which increases up to three times of the resulted value by the elasto-plastic model. Therefore, the effect of creep cannot be neglected in the computation of the bulging. When the ratio of width to thickness is smaller than 3.0, the values of casting directional strain and bulging deflection drastically decrease with decreasing of the slab width.

It is shown that the secondary cooling condition, roll pitch of the continuous caster and slab width are important process parameters to restrain the bulging and prevent the crack in high speed casting.