Definition of corner pit
The corner pit defect of continuous casting billet is one of the common surface defects in the continuous casting process, which is mainly manifested as local depressions or small pits at the corners of continuous casting billets (such as slabs, billets, etc.). This defect may have an adverse effect on subsequent rolling or processing, and even lead to more serious quality problems such as cracks and folding.
The varieties that the editor has experienced have also had many such defects. In summary, such defects appear more on medium-carbon steel, slightly less on high-carbon steel, and basically not on ultra-low carbon steel. From the position of the corner pit defect, it is basically distributed on the two chamfers of the outer arc of the billet, and the inner arc surface of the billet appears in some furnaces. Moreover, for a certain flow, the position of the defect in the same package is relatively fixed, basically distributed on one side, and occasionally appear on both sides of the billet at the same time. From the defect morphology, most defects are continuously distributed, and a very small number appear occasionally. The depth of the pit varies from 2 to 20 mm, and the bottom of the pit is uneven.
Causes of pits at the corners of ingots
Protective slag factors
Protective slag plays a vital role in the continuous casting process, and its physical and chemical properties directly affect the surface quality of ingots. When the protective slag has a high melting point and high viscosity, it will quickly denature after absorbing inclusions in the molten steel, and the viscosity will further increase, and the melting point will be greatly increased, resulting in excessive formation of slag rings. In actual production, the probability of corner pits increases significantly in the later stage of pouring of some medium-carbon steels due to changes in the performance of the protective slag. Mismatches between the melting rate and melting temperature of the protective slag can also cause problems. If the melting rate is too slow, a liquid slag layer of suitable thickness cannot be formed on the surface of the molten steel, affecting the adsorption and lubrication of inclusions; if the melting rate is too fast, it is difficult to maintain a stable original slag layer, which can easily lead to an increase in slag inclusions or slag rings. Carbonaceous materials are the key factors affecting the melting rate of protective slag, and changes in their content will directly change the melting rate.
Casting process factors
In the early stage of pouring, the temperature of the molten steel in the crystallizer is low, the protective slag is not easy to melt, and it is easy to slag. When the contractor adds protective slag for the first time, if the amount of slag is too large, the formation of slag rings will be aggravated. In the early stage of the first furnace pouring, due to the low temperature of the molten steel, the protective slag is not melted well, and the probability of corner pits is relatively high. The continuous adjustment of the pulling speed during pouring will cause the liquid level of the crystallizer to fluctuate, increasing the probability of slag roll. When the liquid level fluctuates, the liquid slag layer of the protective slag fluctuates accordingly, and it is easy to adhere to the copper wall of the crystallizer to form a slag ring. The large fluctuation of the molten steel liquid level in the crystallizer will cause the slag pits at the corners where the slag rolls form.
Molten steel quality factors
There are many inclusions in the molten steel, especially high melting point inclusions, such as MgO, CaO, TiOâ‚‚, etc., which will cause the protective slag to denature rapidly, resulting in increased viscosity of the protective slag, increased melting point, and more slag rings. These inclusions float up in the molten steel and enter the protective slag, changing the performance of the protective slag, and thus affecting the quality of the ingot.
Equipment factors
If the submerged nozzle is inserted too deep into the molten steel, the temperature of the molten steel surface in the crystallizer will be reduced, which cannot meet the temperature required for the normal melting of the protective slag, resulting in excessive viscosity of the protective slag and the formation of slag rings. If the nozzle is not inserted in place or not centered, it will cause the slag to turn over on the liquid surface of the crystallizer, uneven thickness of the liquid slag layer, and defects such as slag inclusion in the ingot, increasing the probability of corner pits.
Prevention and control measures for corner pits in ingots
Optimizing the performance of protective slag: Selecting the appropriate crystallizer protective slag is the key. By adjusting the chemical composition of the protective slag, such as appropriately increasing the basicity, reducing the amount of flux added, and optimizing the carbonaceous material content, the melting performance, viscosity and melting rate of the protective slag can be improved, and the generation of corner pits can be reduced. A steel plant increased the basicity of the protective slag of medium carbon steel from 0.67 to 0.78, the hemispherical temperature from 1030℃ to 1095℃, and the melting rate from 38s to 42s, and the corner pits of the ingot were significantly reduced.
Standardize the pouring operation Change the way and amount of protective slag addition. In the early stage of pouring, the first addition of protective slag is controlled at about 4 spoons to ensure a thickness of about 40mm. Then manually add it evenly from all sides. After the drawing speed is stable, change to automatic slag addition to ensure that the thickness of the protective slag is uniform and stable. Standardize the drawing speed adjustment. When the first furnace is poured, the ingot is increased to the normal drawing speed as soon as possible according to the temperature after it leaves the lower mouth of the crystallizer 1m, and the drawing speed adjustment interval is less than 30s, and the ingots in the drawing speed adjustment section are concentrated on the head billet for scrapping.
Improve the quality of molten steel. Strengthen the refining treatment of molten steel and adopt advanced refining processes, such as refining outside the furnace and vacuum degassing, to effectively remove inclusions in molten steel, reduce the content of inclusions, and reduce the probability of the protective slag being denatured due to inclusions, thereby reducing the possibility of corner pits.
Optimize equipment parameters. Accurately control the insertion depth and centering of the submerged nozzle. According to the cross-sectional size of the billet, steel type and casting process requirements, select the appropriate nozzle insertion depth and ensure that the nozzle is well centered to avoid slag ring formation and billet defects caused by nozzle problems.
