Bubbles in the head of continuous casting billets are common quality defects in steel production, which seriously affect the quality of the billets and the performance of subsequent rolled products.
Main causes of bubbles in the head of continuous casting billet
Poor deoxidation and gas precipitation
Insufficient deoxidation of molten steel is one of the important reasons for the formation of bubbles in the head. When the content of deoxidizing elements such as carbon (C) and silicon (Si) in the molten steel is insufficient, the oxygen (O) content in the steel will be high. During the solidification of molten steel, as the temperature decreases, the carbon-oxygen reaction balance is destroyed, and carbon monoxide (CO) gas will be generated. When the partial pressure of CO gas exceeds the sum of the static pressure of the molten steel and the atmospheric pressure, bubbles will form. For example, when the carbon content of TL08 steel is less than 0.04%, the bubble defect of the casting billet is the most serious, and the unqualified rate is as high as 40%. At the same time, when the content of gases such as hydrogen (H) and nitrogen (N) in the molten steel is too high, as the temperature decreases, the solubility of the gas in the steel decreases, and it will also precipitate from the molten steel and form bubbles. When the refractory material of the tundish is poorly baked, the water in it will decompose into hydrogen and oxygen, which will increase the hydrogen content after entering the molten steel, and precipitate to form bubbles during the solidification process.
Tundish-related factors
The state of the refractory and covering agent of the tundish has a significant impact on the bubbles in the head billet. If the refractory material of the tundish is not baked sufficiently, the moisture in it will decompose into hydrogen and oxygen at the beginning of pouring, and increase the gas content after entering the molten steel. For example, when the baking temperature of the tundish is lower than 400℃, the proportion of 1.5-level bubble defects in the billet reaches 30%, while when the baking temperature is higher than 800℃, the bubble defects are significantly reduced. In addition, if the tundish covering agent contains more moisture or has poor performance, it may be involved in the molten steel during pouring, which not only introduces gas, but also may cause slag inclusion, thereby inducing bubbles.
Secondary oxidation and gas inclusion during pouring
During the pouring stage, the molten steel is exposed to the air and is prone to secondary oxidation, which leads to an increase in the oxygen content in the steel, aggravates the carbon-oxygen reaction, and produces more CO bubbles. At the same time, when argon protection casting is used, if the argon flow rate is not properly controlled, too much argon will be involved in the molten steel to form bubbles. For example, in the crystallizer, argon bubbles may be captured by the solidified billet shell to form subcutaneous bubbles. In addition, if the moisture content of the mold protection slag exceeds the standard or is used improperly, it may be drawn into the molten steel during pouring, introducing gas and inclusions, and promoting bubble formation.
Improper control of process parameters
The superheat of molten steel has an important influence on bubble formation. When the superheat exceeds 30°C, the solidification rate of the molten steel accelerates, the width of the two-phase zone decreases, the bubble radius decreases, and it is not easy to float up and discharge. The probability of residual bubbles in the steel is 3-4 times higher than that at normal superheat. The fluctuation of the pulling speed and the instability of the crystallizer liquid level will cause the flow state of the molten steel to change, increase the risk of gas and protection slag entrainment, and thus promote the formation of bubbles in the head billet.
Control measures for bubbles in the head of continuous casting billet
Optimize the steelmaking deoxidation process
Strictly control the content of deoxidizing elements such as carbon and silicon in the molten steel to ensure that it meets the lower limit requirements. For example, the carbon content of Q235 steel is controlled to be greater than 0.20%, the silicon content is greater than 0.17%, and the carbon content of TL08 steel is controlled to be greater than 0.06%, and the silicon content is greater than 0.10% to ensure that the molten steel is fully deoxidized and the oxygen content is reduced. Strengthen the deoxidation operation, adopt appropriate deoxidizers and deoxidation processes, such as reasonably adding strong deoxidizers such as aluminum (Al) during the steelmaking process to ensure that the molten steel is fully deoxidized. At the same time, strengthen the real-time monitoring of the composition of the molten steel, and adjust the amount of deoxidizer in time to avoid insufficient or excessive deoxidation.
Improve the management of the tundish
Improve the baking quality of the refractory materials of the tundish and ensure that the baking temperature reaches above 800℃. Adjust the baking time according to the quality of the coal gas. When the CO content in the coal gas is lower than 35%, extend the baking time to more than 1.5 hours to fully discharge the moisture in the refractory materials. Select high-quality, low-moisture tundish covering agents, and strictly check their physical and chemical properties before use. Optimize the tundish structure design, such as setting reasonable weirs, improving the flow state of molten steel, and promoting the floating and discharge of inclusions and bubbles.
Strengthen the control of the pouring process
Adopt full-process protective casting technology to reduce the contact between molten steel and air and reduce the risk of secondary oxidation. Reasonably control the argon flow rate to avoid excessive argon from being involved in the molten steel while ensuring that the nozzle is not blocked. For example, by installing an argon flowmeter, accurately adjust the argon flow rate to reduce bubble defects. Keep the crystallizer liquid level stable, control the liquid level fluctuation within ±3mm, and reduce the involvement of protective slag. Select a crystallizer protective slag with good performance, ensure that its moisture content is less than 0.5%, and use it correctly when pouring, and avoid adding it too early or too late.
Optimize process parameters
Strictly control the superheat of molten steel within the range of 20-30℃ to provide good dynamic conditions for the floating of bubbles. Stabilize the pulling speed, avoid frequent fluctuations, and ensure that the molten steel flows smoothly in the crystallizer. Reasonably design the matching relationship between pouring temperature and drawing speed, adjust process parameters according to steel type and billet section, and reduce bubble defects caused by improper process parameters.
Strengthen raw material and equipment management
Ensure that raw materials such as alloy materials, covering agents, and protective slag are dry, and take moisture-proof measures during storage to prevent moisture from entering the molten steel. Regularly check the baking conditions of equipment such as tundishes and ladles to ensure that the equipment is dry. Strengthen the maintenance of the crystallizer water cooling system to prevent water leakage from penetrating into the molten steel and increasing the hydrogen content in the steel.
Practice has shown that measures such as controlling the degree of molten steel deoxidation, optimizing the tundish baking process, strictly managing the pouring process, and reasonably setting process parameters can effectively reduce the bubble defects of the head of the continuous casting billet and improve the quality and production efficiency of the billet. The formation of bubbles in the head of the continuous casting billet is the result of the combined action of multiple factors, and requires systematic control in all links from steelmaking to continuous casting.
