Cold-rolled steel plates are widely used in daily production and life. Hole defects are one of the most harmful defects in cold-rolled thin plates. Since the defective parts must be removed before they can be used, the yield rate of cold-rolled steel plates is greatly reduced, and cold-rolled The production efficiency of the downstream users of the plant is greatly reduced, which needs to be analyzed and controlled. In actual production, hole defects are mainly caused by inclusions.
The holes caused by inclusions are characterized by tear-shaped and crypt-shaped fractures, or crescent-shaped, and the size of the front and back of the steel plate is not significantly different. The following three types of inclusion defects:
1. Inclusion with Ti element
The Ti element in molten steel easily forms titanium nitride and titanium carbide with N and C in molten steel. The inclusions have a regular geometry, high melting point, and poor plasticity. Combined with aluminosilicate inclusions to form composite oxidation Thing. Due to the poor plasticity of Ti inclusions and high hardness, in the cold rolling process, due to rolling thinning, the interface with the metal matrix is ​​cracked in some areas, and eventually hole defects are formed.
2. Compound inclusion
Silicate and aluminosilicate inclusions are semi-plastic inclusions, which have good plasticity, but they are easy to wrap brittle inclusions, calcium aluminate inclusions are not easy to deform, these composite inclusions produce cracks at the interface with the steel matrix, Since the crack is perpendicular to the rolling direction of the steel plate, as the steel plate is rolled, it cannot be deformed synchronously, and eventually a hole defect occurs in the crack.
3, Al2O3 inclusion
Because Al has the strongest deoxidizing ability, its deoxidized product has a high melting point, is easy to form fine solid particles, and is easy to float, so it is often used as a deoxidizer. In addition, the nodules in the water outlet are involved in the molten steel, and the Al element of the molten steel reacts with oxygen in the air, slag, and oxides in the refractory during open casting to generate Al2O3.
Under the stress of hot working, large pieces of Al2O3 brittle inclusions are deformed and broken into inclusions with sharp edges and corners, and are distributed in a chain shape in the matrix. It is easy to scratch the matrix and generate a stress concentration field around the inclusions , Until cracks or voids are formed at the interface, and as the rolling progresses, hole defects eventually occur.
Avoiding defects caused by inclusions has several aspects:
The first is to ensure the fluidity of the molten steel in the ladle to avoid open pouring of the ladle or reduce the open pouring time; secondly, to control the superheat of the molten steel in the ladle to ensure that the inclusions in the molten steel are grown and floated; again, in the production process Reduce the change of the drawing speed, prevent the tundish water port from clogging, avoid the slag coiling and other inclusions caused by the liquid level fluctuation of the crystallizer; Finally, avoid the abnormal operation behaviors such as pour water port, slag replacement, punching rod during the pouring process, and reduce the cold rolling Plate hole defect rate.