Wide and heavy plate steels are widely used in petroleum pipelines, clean new energy wind power and other engineering projects. However, as the demand for steel plate thickness increases, rolling compression is relatively low, and the surface defects of steel plates increase. The main types of surface defects are:
1. Star cracks on the edge of the steel plate
Mainly exist within 80mm of the edge of the steel plate, which expands inward with the increase of the aspect ratio. Generally, the aspect ratio is greater than 1.35, which is mountain-shaped and chicken-foot-shaped. This is due to the addition of B, Nb, and V to the steel composition of the wide and thick plate. , Ti, Ni and other alloy components, micro-alloyed steel is sensitive to cracks, it is easy to produce corner cracks, and edge star cracks appear after rolling and extension.
(1) By realizing functions such as dynamic water distribution and dynamic air distribution, the influence of molten steel superheat, cooling water and compressed air temperature fluctuations is eliminated, so as to ensure uniform and stable cooling of the slab during the casting process, and improve the cooling control accuracy of the slab.
(2) Strictly control the w(N) in the molten steel, and require w(N) in the steel to be ≤10×10-6 to avoid the formation of NbN, AlN, TiN, etc. during the cooling of the slab and the precipitation at the grain boundary to produce surface and corners Department cracks.
2. Transverse cracks on the upper surface of the steel plate
Defects may occur on both the upper and lower surfaces of the steel plate. They are in the shape of mountain peaks. Normal cutting edges cannot be removed. They can be ground to a slight degree. The main reason is that the cooling intensity of the adjacent nozzles of the secondary cooling water on the surface of the cast slab is too large, and the cracks vibrate along the edge. The extension of the trace wave trough causes transverse cracks on the surface of the steel plate.
(1) Adjust the water volume according to the changes in air temperature and medium temperature. On the basis of ensuring the internal quality, reduce the water volume in the straightening sector and keep the straightening area weakly cold.
(2) Tundish argon blowing protection casting, reduce the nitrogen content in molten steel, avoid the combination of N in alloy steel to form NbN, TiN, BN and other compounds, and nitrides will induce cracks when the grain boundaries are precipitated.
3. Black line at the edge of the steel plate
The defect morphology is the presence of the longitudinal length of the edge of the steel plate, and the larger the stretch ratio, the deeper the defect is. Due to the rapid metal temperature drop at the edges and corners of the slab and the low temperature of the medium cooling water, the edges are always in a low temperature and high stress state, causing micro cracks and cannot be welded after oxidation at high temperatures, resulting in black lines.
(1) Clean the corners of the cast slab by chamfering or use a chamfering mold to make the corners of the cast slab close to the arc shape, and avoid this defect when the sharp corners are folded on the steel plate.
(2) Increase the lifting time of the slab to reduce the contact time between the lower surface of the slab and the refractory material, so that the atmosphere in the heating furnace can fully heat the lower surface of the slab and reduce the temperature difference between the upper and lower surfaces of the slab after being discharged from the furnace.
(3) Install protective baffles on the cooling water of the rolling mill roller table to avoid cooling the lower surface of the steel plate and reduce the difference in temperature drop between the upper and lower surfaces. (Transfer from Steel Rolling Expert Network)