There are currently three methods for smelting stainless steel in the world, namely one-step method, two-step method, and three-step method.
One-step method: the electric furnace smelts stainless steel in one step. Because the one-step method requires harsh raw materials (returning stainless steel scrap, low-carbon ferrochromium and metallic chromium), the production of raw materials and energy media consumes high, high cost, long smelting cycle, low productivity, low product variety, poor quality, and furnace lining life. Short, high consumption of refractory materials, so this method is rarely used to produce stainless steel.
Two-step method In 1965 and 1968, VOD and AOD refining equipment were successively produced, which played a decisive role in the transformation of stainless steel production technology. The former is decarburization by vacuum blowing oxygen, and the latter is decarburization by diluting argon and nitrogen gas. Cooperate any of these two refining facilities with an electric furnace to form a two-step stainless steel production process.
The use of electric furnace and VOD two-step steelmaking process is more suitable for small-scale and multi-variety compatible stainless steel production.
The two-step steelmaking process using electric furnace and AOD to produce stainless steel has the following advantages:
1. The AOD production process has low requirements for raw materials, and the electric furnace tapping contains C up to about 2%. Therefore, cheap high-carbon FeCr and 20% stainless steel scrap can be used as raw materials to reduce operating costs.
2. The AOD method can support 0.08% of the carbon in the molten steel in one step. If the smelting time is extended and the amount of Ar is increased, the molten steel can be further reduced to below 0.03%, except for ultra-low carbon. Except for ultra-low nitrogen stainless steel, 95% of varieties can be produced.
3. The production cycle of stainless steel is shorter than VOD and has better flexibility.
4. The total investment in production system equipment is more expensive than VOD, but less than the three-step method.
5. AOD furnace produces steel in one step, with fewer personnel and less equipment, so the overall cost is lower.
6. AOD can use the initial molten steel containing less than 1.5% C, so low-priced high-carbon FeCr, FeNi40 and 35% carbon steel scrap can be used for ingredients, and the raw material cost is low.
The disadvantages are:
1. The lining service life is short;
2. Reduced ferrosilicon consumes a lot;
3. It is currently unable to produce ultra-low C, ultra-low nitrogen, and stainless steel, and the gas content in the steel is high;
4. Large consumption of argon gas.
At present, 88% of stainless steel in the world is produced by the two-step method, 76% of which are produced by AOD furnace. Therefore, it is more suitable for large-scale professional stainless steel factories.
Three-step method: electric furnace + combined blowing converter + VOD three-step smelting of stainless steel. Its characteristic is that the electric furnace, as the melting equipment, is only responsible for supplying semi-finished molten steel containing Cr and Ni to the converter. The main task of the combined blowing converter is to blow oxygen to quickly decarburize to achieve the maximum recovery of Cr. VOD vacuum oxygen blowing is responsible for further decarburization, degassing and fine adjustment of composition. The three-step method is more suitable for areas with relatively short supply of argon gas, and uses molten iron with a higher carbon content as raw materials, and is adopted by professional plants that produce a large proportion of low-C and low-N stainless steel.