Under the inspection and analysis, from the skin defect causes are the following:
(1) oxide inclusions. Slab of large inclusions mainly from powder entrapment. In analyzing the composition of slab surface cracks found near the crack contains a lot of O, Ca, Al and Si and other elements, brittle inclusions composed of high melting point such elements can not easily be adsorbed on the protection of molten slag, it is easy into the liquid steel billet and thus remain in the interior. In the casting process, the large inclusions (such as Al2O3 inclusions) will first be peeled and shell matrix form nascent microcracks, this crack propagation along the strand matrix shell spherical inclusions, and then in the slab surface cracks. Such cracks can lead to lamellar tearing, serious damage to the welding properties of the material, thick steel plate to reduce ductility.
(2) the impact of Cu. Cu case of low melting point elements, easy to segregation at the austenite grain boundaries, and precipitates between the iron oxide layer and the substrate layer, resulting in the formation of low-melting slab surface enrichment-rich phase. Meanwhile, Cu segregation reduces the austenite grain boundary energy to reduce the grain boundary strength.
(3) N elements effect. Steel [N] content and plays a significant layer defects correspondence. The parameters in the mold and cooling the same water flux performance is substantially the same conditions, with the steel [N] content is reduced, significantly reduced from the skin defect. Tests showed that the steel of the [N] is the mass fraction of less than 60 × 10-6 from the skin when the defect rate of 5.8%, [N] is equal to the mass fraction of from greater than 60 × 10-6 at a substantial increase in the defect rate of the skin, can be as high as 76.2 %. For aluminum-containing steel, since Al forms AlN precipitation can reduce the low temperature ductility of the steel, at the same time, AlN tend along the austenite grain boundaries under stress in the vicinity of precipitates will form micro-cracks, leading to crystallization boundary embrittlement. This can be found from the detection of high aluminum slab cracks containing proven.
(4) mold cooling water, pouring temperature and casting speed impact. Appropriate to reduce the amount of water can reduce mold slab surface microcracks, improve the slab quality, reducing hot-rolled sheet rolled skin defects. Reduce the amount of water mold, in essence, with the use of high crystallization rate fluxes similar effect is to reduce the heat transfer rate, slow to realize the slab in the mold of cold, promote uniform growth of shell to improve slab quality. The ability to reduce the pouring temperature and casting speed will help to improve the slab quality, which is a slab mold shell thickness related to high pouring temperature and casting speed will increase the heat flux density, shell thickness thinning, solidified shell to withstand external forces lower, the larger the probability of cracks.