How is steel made in an electric arc furnace

Electric furnace steelmaking mainly uses arc heat. In the arc action area, the temperature is as high as 4000 ℃. The smelting process is generally divided into melting period, oxidation period and reduction period. In the furnace, not only oxidation atmosphere but also reduction atmosphere can be caused. Therefore, the efficiency of dephosphorization and desulfurization is very high.

Steel making process with electricity as energy

This kind of steel-making furnace, i.e. electric furnace, includes electric arc furnace, induction furnace, electroslag furnace, electron beam furnace, consumable electric arc furnace, etc. Generally speaking, electric furnace steel is produced by alkaline electric arc furnace.

Electric furnace steel is mostly used to produce high-quality carbon structural steel, tool steel and alloy steel. This kind of steel has excellent quality and uniform properties. At the same carbon content, the strength and plasticity of electric furnace steel are better than that of open hearth steel. Electric furnace steel uses scrap steel of similar steel grades as the main raw material, and sponge iron can also be used to replace some scrap steel. Adjust the chemical composition and alloy element content by adding ferroalloy.

The electric furnace steelmaking with scrap as raw material has less capital investment than the blast furnace converter method. At the same time, due to the development of direct reduction, metallized pellets are provided for the electric furnace to replace most of the scrap, which greatly promotes the electric furnace steelmaking. There are about 1400 large electric furnaces in the world. Electric furnaces are developing towards large-scale, ultra-high power and computer automatic control. The maximum electric furnace capacity is 400 tons.

Almost all electric furnaces with more than 150 tons abroad are used to smelt ordinary steel, and 60 ~ 80% of the output of electric furnace steel in many countries are low carbon steel. Due to the shortage of electric power and scrap steel in China, it is mainly used to smelt high-quality steel and alloy steel.

Ultra high power electric arc furnace means that the energy input into the electric furnace per unit time is 2 ~ 3 times greater than that of ordinary electric arc furnace. The main advantages are: the melting time is greatly shortened and the labor productivity is improved; Improved thermal efficiency and further reduced power consumption; The use of high current short arc, heat concentration, arc stability, little impact on the power grid, etc. Supporting equipment and related technologies include: large capacity transformer is adopted, which can change voltage under load; Use water-cooled furnace wall and water-cooled furnace cover in a large area on the furnace body; Use oil oxygen spray gun to help melt the dead corner cold zone; Use computer control, etc.

Eccentric bottom tapping of electric arc furnace

In 1979, Germany first changed the traditional 50t ultra-high power electric arc furnace to the central furnace bottom tapping, and then changed it to a more perfect eccentric furnace bottom tapping. The biggest feature of eccentric furnace bottom tapping is to move the tapping port to the outside of the furnace shell, which is convenient for maintenance and overhaul. Eccentric furnace bottom tapping is matched with ultra-high power, which has developed and popularized rapidly in developed countries, especially for large electric arc furnace without slag operation. Its advantages are mainly as follows:

(1) More than 98% slag can be retained in the molten pool;

(2) The consumption of refractory materials can be reduced by about 25%;

(3) During tapping, the temperature of molten steel generally decreases by only about 25 ℃;

(4) The tapping time is short, and the tapping time of 60t electric arc furnace is only about 80s;

(5) The power consumption per ton of steel can be reduced by about 20kw • h;

(6) The consumption of steel electrode per ton can be reduced by about 0.5kg;

(7) The service life of refractory lining at tapping hole can reach about 250 times.

One of the biggest disadvantages of eccentric bottom tapping is that the electric arc furnace is required to be an overhead structure. In China, the structure of electric arc furnace is mostly pit type and less elevated type. In addition, the furnace capacity is small, the workshop equipment is aging and the transformation is difficult. There will be a long-term process to develop and popularize eccentric bottom tapping.

DC electric arc furnace

In 1982, the world's first DC electric arc furnace for actual production was made in Germany. Its central graphite electrode is used as the cathode to access the circuit, and the bottom electrode is the anode, which is composed of two horizontal metals. The metal plate is equipped with air guide cooling sheets, many contact pins are attached to the metal plate, and magnesia is built between the contact pins. The current is introduced into the contact pin through the horizontal metal plate at the bottom of the furnace, and then into the molten pool. The operation of DC electric arc furnace is not different from that of AC electric arc furnace, but to ensure good contact between furnace charge and bottom electrode, a part of molten steel shall be reserved during tapping. If the steel grade is to be changed, the molten steel in the furnace must be drained.

For DC electric arc furnace, new technologies such as eccentric bottom tapping, water-cooled furnace wall, water-cooled furnace cover, oxygen burner and scrap preheating are more suitable and effective.

The main advantages of DC electric arc furnace are: low electrode consumption, only about half of that of three-phase AC electric arc furnace; Low production cost. However, because it is still in the process of continuous improvement, there are also many problems: poor contact between the bottom electrode and the charge; Steel rod bottom electrode produces oxidation and boiling; The maintenance of steel pin (needle) bottom electrode is difficult; Graphite magnesia brick is easy to carburize; Uneven temperature in the furnace; Poor heat dissipation of bottom electrode, etc. The disadvantage is that when the electricity price is high, the local cost is high.