1. Definition of corner pits:
The pit defect at the corner of the 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 the continuous casting billet (such as slabs, square 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 folds.
The varieties that the author has experienced have also had this type of defect many times. In summary, this type of defect appears more on medium carbon steel, slightly less on high carbon steel, and almost none 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 some furnaces appear on the inner arc surface of the billet. Moreover, for a certain flow, the position of the defect in the same batch 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.
2. Cause Analysis of the Pits at the Corners of Cast Billets
1. Protective slag factors
Shielding slag plays a vital role in the continuous casting process, and its physical and chemical properties directly affect the surface quality of the ingot.
When the shielding slag has a high melting point and high viscosity, it will quickly denature after absorbing the inclusions in the molten steel, and the viscosity will further increase, and the melting point will be greatly increased, resulting in excessive slag ring formation.
In actual production, the probability of corner pits increases significantly in the later stage of pouring of some medium-carbon steel due to changes in the performance of the shielding slag. The mismatch between the melting rate and melting temperature of the shielding slag will 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 is easy to cause slag inclusions or slag rings.
As a key factor affecting the melting rate of shielding slag, changes in its content will directly change the melting rate.
2. 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, it will aggravate the formation of slag rings. 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 liquid level of the molten steel in the crystallizer will cause the slag pits at the corners of the slag roll to form.
3. Molten steel quality factors
There are many inclusions in the molten steel, especially high melting point inclusions such as MgO, CaO, TiO₂, which will cause the mold slag to rapidly degenerate, resulting in increased viscosity of the mold slag, increased melting point, and more slag rings.
These inclusions float up into the mold slag in the molten steel, changing the properties of the mold slag and affecting the quality of the ingot.
4. Equipment factors
If the immersion 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 a slag ring.
If the nozzle is not inserted in place or not centered, it will cause 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 casting, increasing the probability of corner pits.
3. Preventive measures for pits at the corners of ingots
1. Optimizing the performance of protective slag: Choosing the right mold 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 medium carbon steel protective slag 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.
2. Standardize pouring operation. Change the method and amount of adding protective slag. In the early stage of pouring, control the amount of protective slag added for the first time to about 4 spoons to ensure a thickness of about 40mm. Then add it evenly from all sides manually. After the drawing speed is stable, change to automatic slag addition to ensure uniform and stable thickness of protective slag.
Standardize the drawing speed adjustment. When the first furnace is poured, the casting is increased to the normal drawing speed as soon as possible according to the temperature after the casting billet exits the lower mouth of the crystallizer 1m, and the drawing speed adjustment interval is less than 30s, and the casting billet in the drawing speed adjustment section is concentrated on the head billet for scrapping.
3. 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 the molten steel, reduce the inclusion content, and reduce the probability of the protective slag being denatured due to inclusions, thereby reducing the possibility of corner pits.
4. Optimize equipment parameters. Accurately control the insertion depth and centering of the submerged nozzle. Select the appropriate nozzle insertion depth according to the cross-sectional size of the ingot, steel type and casting process requirements, and ensure that the nozzle is well centered to avoid slag ring formation and ingot defects caused by nozzle problems.
