Types of secondary refinement

Dephosphorization of molten steel

At present, the single-use converter process can reach a phosphorus content of 40-100 ppm, which depends on the silicon and phosphorus content in the molten iron. According to the amount of slag to determine the silicon content of molten iron, P2O5 must be formed during dephosphorization. At present, it is more popular to conduct converter less slag blowing after the molten iron is dephosphorized, especially in Japan. According to the discussion. Dephosphorization is carried out by the converter double method, and the molten steel contains up to 40 ppm of phosphorus. However, in this case. It must be noted that hot metal dephosphorization must first be desiliconized. As a result, the converter is smelted with ultra-low silicon content, so the converter has the advantage of less slag operation. On the other hand, this process does not allow high scrap loading ratio. Two-stage converter process (duplex method), the slag of the first converter is stripped off, and after the second converter is tapped, the slag is returned to the converter for the next conversion, so that the final phosphorus content of the converter can reach 30ppm, where all the phosphorus is The content refers to the content at the end of converter blowing. If there is a small amount of slag during tapping, the reduction of P2O5 in the slag can return the molten steel to phosphorus. In addition, adding a small amount of phosphorus-containing alloying elements and ferromanganese can also cause a small increase in phosphorus content. The phosphorus content of the final product is about 10 ppm higher than the phosphorus content at the end of the converter.

In fact, adding a ladle furnace and using different methods for secondary refining of molten steel can reduce the phosphorus content to a lower level. This process reduces the tapping temperature of the converter by about 50°C. The reduced molten steel temperature must be balanced by heating the molten steel in the next secondary refining process. The phosphorus balance value of the second converter of the double converter has been compared. The iron content in the converter slag is about 18%, the P2O5 content is about 0.4%, and the tapping temperature is 1700℃. We can get steel with 20ppm phosphorus

Desulfurization 

During the BOF furnace steelmaking production, desulfurization is divided into three stages: molten iron desulfurization, in-converter desulfurization, and molten steel desulfurization outside the furnace. By spraying calcium carbide, magnesium or a mixture of calcium oxide and magnesium, the sulfur content in the molten iron can be reduced to 20 ppm. The result is mainly determined by the amount of desulfurization agent sprayed. The desulfurization in the converter or electric furnace is small, and the molten steel desulfurization is left in the end.

In order to achieve efficient desulfurization, the following measures must be taken: sufficient aluminum must be added, and the ladle slag must be saturated with calcium oxide. In order to ensure the kinetic conditions for efficient desulfurization, the molten steel in the ladle must be violently boiling. It has been stipulated that the so-called "calcium oxide saturation" is used to express the characteristics of the top slag of the ladle. Calcium oxide saturation is used to identify the composition of normal ladle top slag related to calcium oxide saturation. When the calcium oxide saturation is equal to 1, it means that the top slag of the ladle is saturated calcium oxide. When the calcium oxide saturation is less than 1, it means that the top slag of the ladle is an unsaturated calcium oxide uniform liquid slag. When the calcium oxide saturation is greater than 1, Mai shows that the top slag of the ladle is a supersaturated uneven slag of calcium oxide. The relationship between the change in calcium oxide saturation and the desulfurization rate during molten steel desulfurization has been tested. Under the condition that the top slag of the ladle is saturated with calcium oxide, the desulfurization rate can reach 95%. Under the condition that the top slag of the ladle is unsaturated calcium oxide, the efficiency of desulfurization decreases. This is caused by the decrease in the activity of CaO in the top slag of the ladle. Under the condition that the top slag of the ladle is supersaturated calcium oxide, the desulfurization efficiency also decreases.

During the intense boiling of molten steel in the ladle furnace, in addition to desulfurization, other reactions occur. On the one hand, the SiO2 in the top slag of the ladle reacts with [Al] to form Al2O3 and increase the Si content. At the same time, AI and O2 in the air also produce secondary oxidation. Considering these different reactions, the content of S and AL is a function of time. In the case of simple desulfurization, there will be a small amount of Al burning loss. Due to the reduction of the amount of slag, the consumption of aluminum will increase simultaneously during the boiling process of the molten steel. In addition, when the molten steel is reoxidized, more will occur again. A1 is burned out. Due to the decrease in the amount of slag, the amount of Mn in the top slag of the ladle decreases, and the Mn content in the furnace increases slightly. but. The decrease of the SiO2 content in the top slag of the ladle will cause the Si content to increase significantly. The increase in Si content is not conducive to desulfurization, especially for the production of low-silicon steel, especially for thin plates and strips. It can reduce the violent degree of molten steel boiling, which will inevitably seriously affect the efficiency of desulfurization. Under the best operating conditions, the desulfurization efficiency can indeed reach 92%. The weakening of the stirring intensity will cause the desulfurization efficiency to drop to 75%. When the stirring intensity in the furnace is not enough and the air flow rate is low, and the boiling cannot be reached, the desulfurization efficiency can only reach 35%.

Under the optimal desulfurization conditions, the lowest value of sulfur content after hot metal desulfurization can only reach 10 or 20 ppm. For high sulfur content, the desulfurization of molten steel alone can fully meet the requirements. The efficiency of molten iron desulfurization is generally required to reach 75%. The sulfur content after hot metal desulfurization must be less than 30 ppm. In the case of relatively low desulfurization efficiency during the secondary refining, such as 35%, the molten iron must also be enhanced desulfurization. In this case, the sulfur content must be reduced to about 30 ppm after hot metal desulfurization, so that the final sulfur content can reach the minimum value of 50 ppm. Generally speaking, the sulfur content must be reduced to 150ppm after hot metal desulfurization, so that the lowest value of the final sulfur content can reach 100ppm.