The molten steel changes from liquid to solid. As the temperature drops, the shrinkage can be divided into:
(1) Liquid shrinkage: shrinkage from the pouring temperature to the liquidus temperature. For low carbon steel, it is generally 1%;
(2) Solidification shrinkage: the volume shrinkage of the liquid completely turning into a solid. For steel, it is generally 3 to 4%. Volume shrinkage can leave shrinkage cavities in the ingot.
(3) Solid state shrinkage: shrinkage from solidus temperature cooling to room temperature. Generally 7 to 8%. The solid state shrinkage is the linear shrinkage of the entire steel ingot, which is related to the phase transformation during the cooling process of the steel. It has an important influence on the cracking of steel ingots. The density of liquid steel is 7.0g/cm3, and the density of solid steel is 7.8g/cm3, then the liquid becomes solid and the shrinkage is: ((7.8-7.0)/7.0)×100%=11.4%, of which the liquid shrinkage is about 1% , Solidification shrinkage of 3 to 6%, solid shrinkage of 7 to 8%. The volume shrinkage of 3 to 4% during solidification will leave shrinkage holes in the steel ingot, and a protective cap is used to concentrate the shrinkage holes on the head of the steel ingot. During continuous casting, molten steel is continuously replenished to the liquid phase, so there is no concentrated shrinkage cavity in the continuous casting billet. The linear shrinkage of the slab with liquid core when it continues to solidify has an important influence on the quality of the slab and the safety of production. Therefore, the mold should maintain a certain reverse taper, and the roll gap of the backup roll in the secondary cooling zone should conform to the law of the billet line shrinkage from top to bottom. That is to say, the slab with liquid core runs in the inner and outer arc envelope space formed by many pairs of rollers. The opening between the rolls should decrease with the linear shrinkage of the slab during cooling. For example, a vertical bending slab continuous casting machine with a vertical section has 99 pairs of rolls arranged in the secondary cooling area. It is difficult to make the opening of the rolls decrease continuously from top to bottom, which is difficult in terms of mechanical structure. Therefore, the opening degree of the roller pitch is set to shrink in a stepped shape. For example, the thickness of the slab is 250mm, the thickness of the narrow surface of the upper mouth of the mold is 258mm, and the thickness of the lower mouth is 257mm.