What are the characteristics of the solidification process of continuous casting slabs?

Compared with die casting, the characteristics of continuous casting solidification process are:

(1) The solidification of the continuous casting slab is a heat transfer process. The molten steel is poured into the mold for heat transfer, solidification and operation, forming a continuous casting slab with a long liquid phase cavity (the slab is more than 20 meters long). In order to accelerate the solidification, three cooling zones are arranged in the continuous casting machine:

Primary cooling zone: The molten steel forms a sufficiently thick and uniform shell in the mold to ensure that the mold does not leak.​​

Secondary cooling zone: water spray cooling to accelerate the transfer of internal heat to completely solidify the slab.​​

Tertiary cooling zone: to homogenize the billet temperature.​​

(2) The solidification of the continuous casting slab is the process of converting the liquid into a solid along the liquid phase in the solidification temperature range. The continuous casting billet can be regarded as a steel ingot with a long liquid phase, which moves along an arc track in the continuous casting machine at a fixed speed. The billet solidifies in motion. In essence, it is a process of latent heat release and transfer along the liquid-solid-liquid interface. However, the crystal strength at the solidification interface is very small (only 1 to 3 N/mm2), and the strain from deformation to fracture is 0.2 to 0.4%. Therefore, when the external force on the slab (such as bulging force, straightening force, thermal stress, etc.) exceeds the above critical value, cracks will occur at the solid-liquid interface and expand along the columnar crystal until the solidified shell can resist the external force. This is the cause of internal cracks in the slab.​​

(3) The solidification of the continuous casting slab is a staged solidification process. Solidification growth goes through three stages:

– The molten steel forms a primary shell in the mold.​​

– The slab with liquid core grows stably in the secondary cooling zone.​​

– Accelerated growth of the liquid phase near the end of solidification.​​

During the solidification process, the flow and mixing caused by the mold injection in the liquid phase have an important influence on the solidification of the slab. The research points out that the upper part of the liquid phase is a forced convection area, and the height of the convection area depends on the injection mode, the type of immersion nozzle and the section of the slab. The liquid flow in the lower part of the liquid phase is mainly the natural convection caused by the shrinkage of the billet shell and the sinking of the crystal, or the flow caused by the bulging of the billet. The flow has an important influence on the structure of the slab, the floating of inclusions and the segregation of solute elements.​​

(4) The cooling of the solidified shell in the continuous casting machine can be regarded as undergoing deformation heat treatment. On the one hand, the solidified shell is subjected to the action of force, and on the other hand, it is cooled by water spray. As the temperature decreases, the phase transition occurs, and the structure also changes. It is possible that sulfide and nitride points may precipitate at the grain boundary, increasing the high temperature brittleness. The source of surface cracks in the blank. Therefore, it is necessary to deeply understand the law of mutual connection and mutual restriction of the above four aspects, in order to formulate correct countermeasures in equipment and technology, so that the continuous casting machine can achieve the purpose of high production efficiency and good quality of cast slabs.​​

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