Denaturation of secondary refining inclusions

Denaturation of secondary refining inclusions

The inclusions are denatured by adding calcium-containing materials, such as CaSi. CaSi can be blown into the furnace with a spray gun, or added to the furnace in the form of a core wire. The denaturation of inclusions can be studied with the CaO-Al2O3-CaS ternary phase diagram. The inclusions are the residual deoxidation products Al2O3 at first, and the inclusions are transformed from Al2O3 to CaO after calcium is added. If the sulfur content is high, in addition to the transformation of the deoxidized product Al2O3 into liquid calcium aluminate, CaS inclusions are also generated. For this reason, it is very important to avoid the precipitation of CaS caused by the change of sulfur content and transform the deoxidized product into liquid calcium aluminate. Because before the alumina is converted into liquid calcium aluminate, CaS is mainly generated, especially when the sulfur content is increased, it is impossible to convert the deoxidized product into calcium aluminate. Obviously, the transformation process of deoxidation products from Al2O3 to liquid calcium aluminate varies with the addition of calcium. However, the solid calcium sulfide produced by adding a certain amount of calcium depends on the sulfur content. The amount of liquid calcium aluminate produced before the precipitation of solid calcium sulfide is the amount of liquid inclusions. During the secondary refining, the goal must be to accurately hit the "liquid inclusions". Since calcium is oxidized to form calcium sulfide, it is impossible to denature the oxide with such a high sulfur content.

Denitrogenation

Using the converter bottom blowing gas for stirring, such as nitrogen or argon, the nitrogen content at the end of the converter blowing can reach 20ppm, and the nitrogen content in the electric furnace is higher. When the molten steel is tapped from the converter or electric arc furnace into the ladle, the amount of nitrogen absorbed depends on the exact time of the addition of a deoxidizing element. Only rimmed steel tapping does not absorb nitrogen, and then deoxidation is carried out in the ladle.

During vacuum treatment, in principle, it is feasible to remove nitrogen from molten steel. Since nitrogen, sulfur and oxygen are also surface active elements, ultra-low sulfur and oxygen content are the prerequisites for a large amount of nitrogen removal during vacuum processing. Before the vacuum treatment, the nitrogen content is 50ppm, after the vacuum treatment, the nitrogen content can reach 30ppm. Under the same other conditions, the initial nitrogen content is relatively high (90ppm), and the nitrogen content can reach 40ppm after vacuum treatment. This means that during the vacuum treatment, especially when the initial nitrogen content is increased and the sulfur and oxygen content is very low, the denitrification effect is obvious. When the initial nitrogen content is low, the denitrification effect during the vacuum treatment is not obvious. The influence of sulfur content is obvious, that is, ultra-low sulfur content is a basic prerequisite to ensure effective denitrification in vacuum treatment. During the vacuum treatment, if the sulfur content increases, the effect of denitrification is poor.

Decarburization

At present, vacuum decarburization is generally used to achieve ultra-low carbon content. The smelting of non-deoxidized steel generally adopts RH device or ladle furnace for degassing. Therefore, the dissolved oxygen reacts with carbon to form CO. Carbon dissolves into the molten steel from the lining of the vacuum treated ladle to cause carbon increase, or carbon dissolves from the mold flux into the molten steel during molten steel pouring. On a ladle degassing furnace, in order to obtain low carbon content, improvements were made in vacuum decarburization. The optimization of the analysis system has made this wish possible, and the carbon content in the final sample has been reduced from the original average value of 38 ppm to about 36 ppm. The use of low-carbon continuous casting mold powder can further reduce the carbon content to an average of about 31ppm. The carbon content can also be significantly reduced by choosing different ladle furnace linings. In the first test, a dolomite ladle was used for lining and magnesia sand was used for the slag line. The carbon content of dolomite and magnesia is 5%-8%. When bauxite is used for the lining of the ladle and magnesia is used for the slag line, the average carbon content can be reduced to 20 ppm. These results confirm the influence of ladle lining materials on carbon content. Using non-carbon ladle furnace lining material, after vacuum treatment and pouring, ultra-low carbon content can be obtained.