1. Analysis of the causes of shrinkage
The influence of overheating on shrinkage
The high degree of superheating of molten steel will make the columnar crystals of the cast slab coarser, and aggravate the generation of intergranular cracks. At the same time, the probability of “bridging” molten steel in the final solidification stage will be further increased.
The influence of secondary cooling on shrinkage cavity
The intensity of water supply in each section of the secondary cooling zone determines the cooling effect and the solidification behavior of the cast slab. The method of weak cooling helps to reduce the formation of shrinkage cavities.
The change of drawing speed and secondary cooling system will affect the temperature gradient of molten steel during solidification. When the temperature gradient is large, the grain growth rate is faster; when the temperature gradient is small, the grain growth rate is slower. During the solidification of the cast slab, the instability of the heat transfer of the cast slab causes the growth rate of the columnar crystals to be different. The preferentially grown columnar crystals meet at the center of the slab to form a “lattice bridge”, and the molten steel in the liquid cavity is separated by the solidified columnar crystals. And wrapped, the molten steel wrapped by the columnar product cannot be supplemented by the upper molten steel during solidification and shrinkage and forms loose or shrinkage holes, accompanied by center segregation.
2. Improvement measures
Reduce the temperature of the molten steel in the tundish and control the superheat of the molten steel so that the molten steel can be poured at a temperature close to the liquidus temperature to obtain a fine as-cast structure.
Stable drawing speed, prevent the occurrence of high drawing speed operation, control the amount of secondary cooling water, and improve the water quality of the secondary cooling section.
Electromagnetic stirring breaks up the columnar crystals that have grown and increased the width of the equiaxed crystals.