Typical surface defects and control of wide plates

Wide and thick plate varieties are widely used in petroleum pipelines, clean energy wind power and other engineering projects. However, with the increasing demand for steel plate thickness, the rolling compression is relatively low, and the surface defects of the steel plate increase. The main types of surface defects are:

1. Star cracks at the edge of the steel plate

Mainly exist within 80mm of the edge of the steel plate, expanding inward with the increase of the width-to-thickness ratio. Generally, when the width-to-thickness ratio is greater than 1.35, it is in the shape of a mountain peak or chicken claws. This is because the micro-alloyed steel, with the addition of B and alloying elements such as Nb, V, Ti, and Ni in the composition of wide and thick plate varieties, is sensitive to cracks and easily produces corner cracks, which appear as edge star cracks after rolling extension.

Measures:

(1) By realizing dynamic water distribution and dynamic gas distribution, eliminate the influence of fluctuations in steel water superheat, cooling water, and compressed air temperature, ensure uniform and stable casting cooling, and improve the accuracy of casting cooling control.

(2) Strictly control w(N) in molten steel, requiring w(N) ≤ 10 × 10 ^-6 to avoid the formation of NbN, AlN, TiN, etc., during billet cooling and precipitation at grain boundaries, causing surface and corner cracks.

2. Transverse cracks on the upper surface of the steel plate

Defects can occur on both the upper and lower surfaces of the steel plate, showing a mountain-peak shape. Normal edge cutting cannot remove them, and some minor ones can be polished. This is mainly due to the excessive cooling intensity in the overlapping area of adjacent nozzles of secondary cooling water on the surface of the casting, and the cracks extend along the trough of the vibration mark, causing transverse cracks on the surface of the steel plate.

Measures:

(1) Adjust the water volume according to changes in air temperature and medium temperature. On the basis of ensuring internal quality, reduce the water volume in the straightening fan-shaped section area and keep the straightening area weakly cold.

(2) Argon protection casting in the tundish to reduce the nitrogen content in the molten steel and avoid the formation of NbN, TiN, BN, etc., compounds in alloy steel. The precipitation of nitrides at grain boundaries induces cracks.

3. Black lines at the edge of the steel plate

This defect is characterized by the longitudinal existence of black lines at the edge of the steel plate, and the larger the width-to-thickness ratio, the closer the defect is to the inside. Due to the rapid temperature drop of the metal at the corners of the slab and the low temperature of the cooling water, the edges are always in a low-temperature, high-stress state, producing microcracks that cannot be welded after oxidation at high temperatures, resulting in black lines.

Measures:

(1) Chamfer the corners of the billet or use a chamfered crystallizer to make the corners of the billet close to an arc shape, avoiding the folding of sharp corners onto the steel plate to produce this defect.

(2) Increase the billet lifting time to reduce the contact time between the lower surface of the billet and the refractory material, so that the atmosphere in the heating furnace can fully heat the lower surface of the billet and reduce the temperature difference between the upper and lower surfaces after the billet is taken out of the furnace.

(3) Install protective baffles on the cooling water of the rolling mill roll pass to avoid cooling the lower surface of the steel plate and reduce the temperature difference between the upper and lower surfaces. (From Rolling Mill Expert Network)