The internal quality of the slab refers to the low magnification structure, composition segregation, center porosity, center segregation and cracks. After hot working of the billet, some defects can disappear, some deformed, and some remain intact, which brings different degrees of harm to the product performance. The generation of internal defects in the slab involves the effects of heat transfer, mass transfer and stress in the solidification of the slab, and the formation mechanism is extremely complex. But in general, the internal defects of the slab are controlled by the solidification process of the slab in the secondary cooling zone. Measures to improve the internal quality of the slab include:
(1) Controlling the structure of the slab: The first thing is to expand the equiaxed crystal area in the center of the slab to suppress the growth of columnar crystals. This reduces center segregation and center porosity. For this reason, the use of molten steel low superheat casting, electromagnetic stirring and other technologies are effective methods to expand the equiaxed crystal area.
(2) Reasonable secondary cooling system: In the secondary cooling zone, the surface temperature of the casting billet is evenly distributed, and the surface temperature at the straightening point is greater than 900 °C, and there is no liquid core straightening as much as possible. For this purpose, a computer is used to control the distribution of secondary cooling water, gas-water spray cooling, etc.
(3) Control the stress and deformation of the slab in the secondary cooling zone: the stress and deformation of the solidified shell in the secondary cooling zone are the root causes of cracks. To this end, multi-point bending straightening, accurate arc alignment, roll gap alignment, compression casting technology, etc. are used.
(4) Control the flow of molten steel in the liquid phase cavity to promote the floating of inclusions and improve their distribution. For example, the crystallizer adopts electromagnetic stirring technology, and the design of the submerged nozzle is improved.
