The role of refining slag in steelmaking

The process of moving molten steel from a converter, open hearth or electric furnace to another vessel for refining. Also called "secondary steelmaking". The steelmaking process is therefore divided into two steps: primary refining and refining. Primary smelting: the charge is melted, dephosphorized, decarbonized and main alloyed in an oxidizing atmosphere furnace. Refining: degassing, deoxidizing, desulfurizing, removing inclusions and fine-tuning composition of the molten steel in vacuum, inert gas or reductive atmosphere containers. In this way, steel-making can be carried out in two steps, which can improve the quality of steel, shorten the smelting time, simplify the process and reduce the production cost. In 1933, R. Perrin (France) used specially prepared synthetic slag with high alkalinity to conduct "slag scrubbing desulphurization" on molten steel in the process of steel extraction, which was the germination of out-of-furnace refining technology. In 1950, in Federal Republic of Germany, liquid steel was vacuumed to remove hydrogen from steel to prevent "white spots". Since the late 1960s, the technology of refining outside the furnace has been continuously developed. At present, dozens of methods have been applied in industrial production, and a new branch of steelmaking process has been gradually formed. China began to study the vacuum treatment of molten steel in 1957. The equipment of liquid steel degassing and vacuum ingot casting was established. In the 1970s, the equipment of outside furnace refining, such as argon oxygen furnace, ladle refining furnace and ladle dusting device, was established. Principle refining mainly through the following action: vacuum degassing liquid steel gas solubility obeying the square root law, the content of hydrogen in the steel. Liquid steel vacuum processing, reduce the partial pressure of hydrogen in the refining container p 啹, can achieve the goal of liquid steel dehydrogenation. The equilibrium constant of hydrogen dissolution reaction kH is a function of temperature, at 1600℃, kH = 0.0027. The dissolution equilibrium constant of hydrogen in molten steel is low and the diffusion rate is fast, so the dehydrogenation rate of molten steel is fast, which can make the hydrogen content in steel close to the equilibrium value. Similarly, denitrification can also be carried out, but the dissolution equilibrium constant of nitrogen in molten steel is high, Kn = 0.040, and the diffusion rate is slow. Therefore, in the vacuum treatment of molten steel, the nitrogen removal rate is only 10 ~ 25% (see Degasing of steel, vacuum metallurgy). Two deoxidization methods are usually used for vacuum deoxidization outside furnace refining. Carbon deoxidization under vacuum and precipitation deoxidization by adding alloying elements such as silicon, manganese and aluminum. Under vacuum, the carbon-oxygen reaction is: [C] + [O] ─→ CO ↑, then [C] % · [O] % = PPCO/K = MPPCO. The equilibrium constant K is a function of temperature, and at 1600℃ and PPCO = 1 atmosphere, the value is 0.0020 ~ 0.0025. Therefore, the deoxidization ability of carbon under vacuum is very strong, which can exceed the deoxidized elements silicon, manganese and aluminum. The reaction product CO is gaseous rather than inclusions and is easily removed under vacuum (see Deoxidation of steel). The inert gas treatment blows inert gas into the molten steel. The gas itself does not participate in the metallurgical reaction, but each small bubble rising from the molten steel acts as a "small vacuum chamber" (the partial pressure of H2, N2, and Co in the bubble is close to zero) with a "gas wash" effect. The principle of off-furnace refining for stainless steel production is the balance between carbon chromium and temperature under different CO partial pressures (Fig. 1). Refining and decarburization with oxygen added with inert gas (changing the ratio of argon/oxygen during the process) can reduce the partial pressure of CO in the carbon-oxygen reaction, and reduce the carbon content without oxidizing chromium at lower temperatures.