Cold rolled steel sheet is widely used in daily production and life, hole defect is one of the most serious defects in cold rolled steel sheet, because the defect must be removed before it can be used, greatly reducing the rate of production of cold rolled steel sheet, and make the downstream user production efficiency of cold rolling mill greatly reduced, need to be analyzed and controlled.In actual production, hole defects are mainly caused by inclusions.
The holes caused by inclusions are characterized by tear and dimple fracture, or crescent shape.There are three types of inclusions:
1. Inclusion of Ti elements
Ti element in molten steel is easy to form titanium nitride and titanium carbide with N and C in molten steel. The inclusion has regular geometric shape, high melting point and poor plasticity, and combines with silicoaluminate inclusion to form compound oxide.Due to the poor plasticity and high hardness of Ti compounds inclusion, during the cold rolling process, due to the thinning of rolling, cracks in some areas and the interface of the metal matrix, finally forming holes.
2. Compound inclusion
Silicate inclusions, silicon aluminate belongs to half plastic inclusions, its plasticity is good, but its easy to brittle inclusions, calcium aluminate inclusions is not easy to deformation, the compound mixed with the steel substrate interface cracks, due to the rolling direction vertical Yu Gangban crack, as the steel plate rolling, not synchronous deformation, the final hole defects in the crack.
3. Al2O3 inclusion
Since Al has the strongest deoxidizing ability, its deoxidizing products have a high melting point and are easy to form fine solid particles and float to the surface, so it is often used as deoxidizing agent.In addition, the nodules at 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 refractories during the open pouring to form Al2O3.
Under the effect of thermal stress, the large deformation of Al2O3 brittle inclusion break into inclusions with sharp edges, and chains are distributed in the matrix, it is easy to substrate scratch, and stress concentration around the inclusion lead to the field, until at the interface crack or gap formation, as rolling, eventually produce hole defects.
Measures taken to avoid inclusion causing hole defects are as follows:
The first is to ensure the fluidity of molten steel in the ladle to avoid the opening of the ladle or reduce the opening time.Secondly, control the superheat of molten steel in the ladle to ensure that inclusions in the molten steel grow up and float up.Thirdly, the change of drawing speed is reduced in the production process to prevent the nozzle plug of tundish and avoid the slag rolling and other inclusions caused by the fluctuation of liquid level of crystallizer.Finally, in the process of pouring avoid the occurrence of water hole, slag change, punching rod and other abnormal operation behavior, reduce the rate of hole defect of cold rolled plate.