Process Description of Refining Stainless Steel by AOD Furnace

Process Description of Refining Stainless Steel by AOD Furnace

There are two ways of smelting stainless steel: one is the two-step method developed in the early 1970s, namely, EAF+AOD or VOD method, and the other is the three-step method developed in the 1980s, namely, EAF+converter (MRP, K-OBM or LD-OB)+VOD method. Both methods have their own characteristics. The two-step method is mainly adopted by EAF+AOD in professional stainless steel manufacturers, and the smelting of stainless steel is basically not limited (except C+N ≤ 250ppm). It is easy to match with continuous casting, and the investment and production cost are low.

Basic Operation Process of AOD Furnace

The appearance of AOD furnace is similar to that of converter. The furnace lining is made of various specifications and varieties of basic refractory materials (such as magnesium calcium brick, chrome magnesium brick, etc.). Two or more double-layer consumable spray guns are installed at the bottom of AOD side. The inner tube of the spray gun is copper tube, and the mixture of oxygen and argon (nitrogen) with different proportions is blown for decarburization. The outer pipe is a stainless steel pipe. Argon or nitrogen is blown into the gap between the inner pipe and the outer pipe, and the compressed air is used as the cooling gas. AOD process is essentially a two-step steel making process. First, the furnace charge (including scrap steel and iron alloy) is melted in the electric arc furnace or induction furnace. Nickel, chromium and manganese are generally added according to the lower and middle specifications. According to the minimum cost requirements of furnace charge, carbon generally varies from 0.5% to 3.0%. Most steel mills are 1.5% to 2.0%. After the furnace charge is melted, it can be reduced. When the temperature of molten steel in the electric furnace is about 1550 ℃, it is sent to the AOD furnace for decarburization after slag removal. The carbon content of molten steel entering the furnace is generally 1.5%~2.0%, and the requirement of 0.03% is reached after three stages of decarburization. The ratio of oxygen to argon (nitrogen) in the first phase is 3:1 decarburization to 0.25%, the second phase is 1:1 decarburization to 0.10%, and the third phase is 3:1~4:1 decarburization to the terminal carbon. The end point temperature is between 1710 ℃ and 1730 ℃ for reduction refining.

AOD Furnace Decarbonization Process

For AOD furnace decarbonization process, pure oxygen blowing or high oxygen-nitrogen ratio can be adopted for decarburization in high carbon area. The converting strength of AOD furnace is 0.8~1.8Nm3/(t · min). The key to the decarbonization process of AOD furnace is the interval of C<0.7%~0.08%. In order to improve the decarburization speed and prevent a large amount of oxidation of chromium. The oxygen to argon ratio must be reduced continuously with decreasing carbon content to reduce Pco. The conventional AOD furnace decarbonization process uses several stages of oxygen to argon ratio based on the carbon content in the steel. The decarburization rate of AOD furnace is between VOD furnace and combined blowing converter.

Reduction Process of AOD Furnace

At the end of decarburization, AOD furnace generally has 2% Cr oxidation, VOD furnace has about 1% Cr oxidation, and the oxygen content in the steel. Approximately 700 ppm and 200 ppm, respectively, in order to reduce Cr2O3 in the slag and deoxidize the molten steel, Fe-Si is generally added to the steel for reduction. The addition amount of Fe-Si in AOD furnace is generally 8-12kg/t. After silicon deoxidation, pure argon blowing shall be carried out for about 6 minutes to facilitate the floating of impurities. Oxygen content in steel can be reduced to 50~80 ppm and 30~50 ppm by AOD furnace using Fe-Si and aluminum deoxidation.

In order to reduce Cr2O3 in slag, silicon content in steel and alkalinity of slag shall be controlled. In order to achieve 98.5% recovery of chromium in AOD furnace The Si content in steel shall be controlled at about 0.5%, and the alkalinity of steel slag shall be controlled at about 2.0. In order to control the alkalinity of steel slag, lime shall be added by stages. 15kg/t and 9kg/t MgO shall be added before steel mixing. 22kg/t shall be added during the decarbonization period, and 22kg/t shall be added after the end point. In order to fully reduce slag, CaF2 with 12% lime amount shall be added at the end point of decarbonization.

Desulfurization Process of AOD Furnace

Sulfur in stainless steel has obvious influence on thermoplastic property, rust resistance and corrosion resistance, and the sulfur content in steel after AOD furnace refining generally has reached ≤ 0.030%. The double slag process can achieve the sulfur content ≤ 10ppm. AOD furnace can perform strong stirring operation, so desulfurization is easier than VOD furnace and lower content can be achieved. Desulfurization of AOD furnace has a great relationship with slag alkalinity. In addition, in order to strengthen desulfurization, aluminum powder is added to the steel slag during refining period to make the composition of steel slag close to 60% CaO, 32% AbO3 and 8% SiO2, which is more effective for desulfurization and faster for desulfurization. As mentioned above, stainless steel has three different decarburization mechanisms under different carbon contents. In order to make the AOD furnace decarburization process closer to these mechanisms, Japan Kobelco has developed a new AOD furnace decarburization refining process, as shown in Table 3-2.

Top Bowing AOD Furnace Decarbonization Process

At present, about half of AOD furnaces adopt the combined blowing converter technology, and the top blowing process is adopted when the carbon content is ≥ 0.4%. The main effect is that the temperature rise rate of molten steel is increased from 127 ℃/C% to 195 ℃/C%, and the decarburization rate is increased from 0.055%/min to 0.085%/min. CRE can reach 90% Therefore, 10 min of smelting is shortened and 2~3kg of Fe-Si consumption is reduced. The test results of 70t furnace show that the process of mixed gas of top blowing oxygen+bottom blowing oxygen and argon is the best, and the temperature rise rate and decarbonization rate reach a high level, as shown in Table 3-3.

AOD-L top blowing can be "soft blowing" and "hard blowing". Soft blowing with sonic or subsonic oxygen gun is to burn most of the oxygen and CO to generate CO2, so that 75% - 90% of the generated heat is transmitted to the molten steel tank to increase the temperature of molten steel at a fast speed, accelerate decarburization and reduce the oxidation of chromium. Soft blowing is carried out with the method of supplying oxygen through the tuyere embedded in the refractory material at the furnace mouth, with the minimum investment, and 100% of the top blowing oxygen is used for secondary combustion. Soft blow operation instead of hard blow operation Use subsonic LD gun, basically all top blowing oxygen reacts with molten pool to improve decarburization speed. Hard blowing operation can also blow oxygen and nitrogen mixture to further reduce the blowing time. In addition, it can prevent air (nitrogen) infiltration and improve CRE during the late decarburization or smelting of ultra-low carbon stainless steel.

Improve the Service Life of AOD Furnace Lining

AOD furnace lining is a major problem in the process. In AOD operation, the erosion degree of furnace lining is 30% in decarbonization period, 60% in reduction desulfurization period and 10% in analysis waiting for adjustment component period In recent years, the service life of furnace lining has been continuously improved through AOD furnace process measures. For example, the service life of 90t AOD furnace lining of POSCO Iron&Steel Co., Ltd. in Korea is 240 times averagely and 270 times at most. The specific practices are as follows:

 ① Improve the molten steel charging conditions, i.e. control the Si content in the molten steel of the electric furnace at the lower limit, reduce the slag amount of the electric furnace, rake slag before mixing into the AOD furnace, and control the tapping temperature of the electric furnace at 1550 ℃; 

②  AOD furnace temperature shall be controlled, especially the furnace temperature at the end of decarbonization shall be kept below 1720 ℃ to prevent melting loss of refractory; 

③  The smelting time shall be shortened as much as possible. The decarbonization efficiency shall be increased by improving the converting model to shorten the decarbonization time. In addition, the desulfurization time shall be shortened to reduce the reaction time of refractory and liquid slag; 

④  Reduce slag as much as possible, use high-quality lime (CaO ≥ 92%), reduce lime addition, improve converting model and reduce metal oxidation; 

⑤  Dolomite is added for slag making, and MgO is added at the early stage of converting, and the content in slag shall be controlled within 6%~10%.