Electric furnace steelmaking process and features

Electric arc steel making features

Electric furnace steelmaking mainly uses arc heat. In the arc action zone, the temperature is as high as 4000 °C. The smelting process is generally divided into a melting period, an oxidation period and a reduction period. In the furnace, not only an oxidizing atmosphere but also a reducing atmosphere can be created, so the efficiency of dephosphorization and desulfurization is very high.

The iron-containing raw materials used are mainly scrap steel, accounting for more than 70%. In addition, molten iron, pig iron, direct reduced iron, hot briquetting, etc. can be added. The smelting time is longer, generally at least twice that of the converter smelting time. The melting temperature of the electric furnace is high and easy to control and adjust. The temperature of the arc zone of the electric arc furnace is as high as 3000-6000 °C, and the furnace temperature is as high as 2000 °C, which is much higher than the temperature required for smelting general steel grades. It can be used for smelting alloy steel and Stainless steel.

The atmosphere in the furnace is easy to control and adjust. At different stages of smelting, the furnace can not only create an oxidizing atmosphere, but also a reducing atmosphere. The former is beneficial to decarburization and dephosphorization, while the latter is beneficial to deoxidation, desulphurization, addition of easily oxidizable alloys, recovery of metal elements and Control molten steel composition. Electric furnace equipment is simpler, less investment, faster to build, less land occupation, and easy to control pollution.

However, because the required energy is provided by the high temperature generated by the electrode during short circuit, the power consumption is relatively large. The electric energy consumed by smelting 1 ton of steel is about 350-600kwh; the hydrogen and nitrogen content in the finished steel is relatively high. Because under the action of the arc, a large amount of water vapor in the air in the furnace dissociates, and the generated hydrogen and nitrogen, such as entering the molten steel, will affect the quality of the steel. The arc is a "point" heat source, and the temperature distribution in the furnace is uneven. When the molten pool is calm, the temperature of each part of the molten steel is quite different. The existence of carbonaceous electrodes will increase the carbonization of molten steel, which brings difficulties to the smelting of low carbon steel.

Scrap steel is an energy-carrying resource, and using scrap steel to make steel can save a lot of energy. In large iron and steel complexes, from mining, beneficiation, sintering, coking, iron making to steelmaking and rolling, energy consumption and pollution emissions are mainly concentrated before the steelmaking process. Studies have shown that direct steelmaking with scrap steel can save 60% of energy and 40% of water compared to steelmaking with ore iron.

Scrap iron and steel is an environmentally friendly resource. Compared with direct steelmaking with scrap steel and iron making with ore, it can reduce waste gas by 86%, waste water by 76% and waste residue by 97%, which is conducive to cleaner production and waste reduction.

Steel making electric arc furnace steps

1. Feeding

The operation of adding raw materials such as molten iron or scrap steel to the electric furnace is the first step in the steelmaking operation of the electric furnace.

2. Slag making

The operation of adjusting slag composition, basicity and viscosity and its reactivity in steel and iron production. For example, the oxygen blowing operation is to generate slag with sufficient fluidity and basicity, which can transfer enough oxygen to the metal liquid surface, so as to reduce the sulfur and phosphorus below the upper limit of the planned steel grade, and make the splashing during oxygen blowing. and the amount of spillage is minimized.

3. Slag

The slag discharge or slag removal operation adopted in the smelting process according to different smelting conditions and purposes during electric arc furnace steelmaking. For example, when the single-slag method is used for smelting, the oxidized slag must be removed at the end of the oxidation; when the double-slag method is used to make the reduced slag, the original oxidized slag must be completely released to prevent phosphorus return.

4. Molten pool stirring

Energy is supplied to the molten metal pool to move the molten metal and slag to improve the kinetics of metallurgical reactions. The molten pool stirring can be achieved by means of gas, mechanical, electromagnetic induction and other methods.

5. Dephosphorization

A chemical reaction that reduces the phosphorus content in molten steel. Phosphorus is one of the harmful impurities in steel. Steel containing more phosphorus is easy to be brittle when used at room temperature or lower, which is called "cold brittleness". The higher the carbon content in the steel, the more severe the brittleness caused by phosphorus. Generally, the phosphorus content in ordinary steel is not more than 0.045%, and the high-quality steel requires less phosphorus.

6. Bottom blowing electric furnace bottom blowing

N2, Ar, CO2, CO, CH4, O2 and other gases are blown into the molten pool in the furnace according to the process requirements through the nozzle placed at the bottom of the furnace to accelerate the melting and promote the metallurgical reaction process. The bottom blowing process can shorten the smelting time, reduce power consumption, improve dephosphorization and desulfurization operations, increase the residual manganese in the steel, and improve the yield of metals and alloys. And can make molten steel composition and temperature more uniform, thereby improving steel quality, reducing costs and increasing productivity.

7. Melting period

The melting period of steelmaking is mainly for open hearth and electric furnace steelmaking. The melting period of electric arc furnace steelmaking from the start of electrification to the time when the furnace steel flowers and materials are completely melted, and the time from the completion of molten iron mixing to the end of all furnace materials in open hearth steelmaking is called the melting period. The task of the melting period is to melt and heat up the charge as soon as possible, and to make the slag in the melting period.

8. Oxidation period and decarbonization period

The oxidation period of ordinary power electric arc furnace steelmaking usually refers to the process stage from the melting of the charge, sampling and analysis to the completion of the slag removal. It is also believed that it started from blowing oxygen or adding ore to decarbonize. The main tasks of the oxidation period are to oxidize carbon and phosphorus in molten steel; remove gas and inclusions; and heat molten steel evenly. Decarbonization is an important operating process in the oxidation period. In order to ensure the purity of the steel, the decarburization amount is required to be greater than 0.2%. With the development of off-furnace refining technology, most of the oxidative refining in electric arc furnaces is moved to ladle or refining furnace.

9. Refining period

During the steelmaking process, some elements and compounds that are harmful to the quality of steel are selected into the gas phase or discharged and floated into the slag by chemical reaction through slag making and other methods, so that they are excluded from the molten steel during the process operation period. Continuous casting machine out of billet Continuous casting machine out of billet.

10. Restoration period

In the ordinary power electric arc furnace steelmaking operation, the period from the completion of slag removal at the end of oxidation to the tapping is usually called the reduction period. Its main task is to make reduced slag for diffusion, deoxidation, desulfurization, chemical composition control and temperature adjustment. The reduction period has been eliminated for high-power and ultra-power EAF steelmaking operations.

11. Refining outside the furnace

The steelmaking process in which the primary molten steel in the steelmaking furnace (converter, electric furnace, etc.) is moved to another container for refining, also called secondary metallurgy. The steelmaking process is therefore divided into two steps: primary smelting and refining. Primary smelting: The charge is melted, dephosphorized, decarburized and mainly alloyed in an oxidizing atmosphere furnace. Refining: Degassing, deoxidizing, desulfurizing, removing inclusions and fine-tuning components, etc. The benefits of splitting steelmaking into two steps are: improved steel quality, reduced smelting time in the steelmaking workshop, simplified process and lower production costs. There are many types of out-of-furnace refining, which can be roughly divided into two categories: out-of-furnace refining under atmospheric pressure and out-of-furnace refining under vacuum. According to the different treatment methods, it can be divided into ladle processing type out-of-furnace refining and ladle refining type out-of-furnace refining.

12. Stirring of molten steel

Stirring of molten steel during refining outside the furnace. It homogenizes the composition and temperature of molten steel and can promote metallurgical reactions. Most metallurgical reaction processes are phase interface reactions, and the diffusion rate of reactants and products is the limiting link of these reactions. When molten steel is in a static state, its metallurgical reaction speed is very slow. For example, it takes 30 to 60 minutes to desulfurize molten molten steel in an electric furnace; while in furnace refining, it takes only 3 to 5 minutes for desulfurization by stirring molten steel. When the molten steel is in a static state, the inclusions are removed by floating, and the removal speed is slow; when the molten steel is stirred, the removal speed of the inclusions increases exponentially, and is related to the stirring intensity, type, and the characteristics and concentration of the inclusions.

13. Ladle feeding wire

Feed deoxidation, desulfurization and fine-tuning powders, such as Ca-Si powder, or direct feeding of aluminum wire, carbon wire, etc., into the ladle through the wire feeder to carry out deep desulfurization, calcium treatment and fine-tuning in the steel. Methods for ingredients such as carbon and aluminum. It also has the function of cleaning molten steel and improving the morphology of non-metallic inclusions.