Characteristics and Steps of Electric Furnace Steelmaking

Characteristics and Steps of Electric Furnace Steelmaking

What are the methods of electric furnace steelmaking?

Modern steelmaking methods primarily include the converter steelmaking process, the open-hearth steelmaking process, and the electric furnace steelmaking process. The open-hearth steelmaking process has largely been phased out. The fundamental difference between the two processes is that the electric furnace steelmaking process uses electricity as its heat source, while the electric arc furnace (EAF) is the most widely used electric furnace steelmaking method. When we refer to electric furnace steelmaking, we primarily refer to electric arc furnace steelmaking, as other types of electric furnaces, such as induction furnaces and electric slag furnaces, produce smaller quantities of steel. Electric arc furnace steelmaking relies on an arc generated by discharges between electrodes and the charge, converting electrical energy into heat in the arc. Radiation and the direct action of the arc heat and melt the metal and slag, producing steel and alloys of various compositions.

Characteristics of Electric Furnace Steelmaking:

Electric furnace steelmaking primarily utilizes arc heat, with temperatures reaching as high as 4000°C in the arc zone. The smelting process is generally divided into melting, oxidation, and reduction phases. The furnace creates both an oxidizing and a reducing atmosphere, resulting in highly efficient dephosphorization and desulfurization.

The primary iron-containing raw material used is scrap steel, accounting for over 70%. Molten iron, pig iron, direct-reduced iron, and hot-pressed briquettes may also be added. The smelting time is relatively long, generally at least twice that of a converter. Electric furnaces offer high and easily controlled melting temperatures. The arc zone temperature of an electric arc furnace can reach as high as 3000-6000°C, and the furnace temperature can reach over 2000°C, far exceeding the temperatures required for smelting common steel grades. This allows for the smelting of alloy steels and stainless steels that cannot be smelted in converters.

The furnace atmosphere is easily controlled and adjusted. At different stages of the smelting process, the furnace can create both an oxidizing and a reducing atmosphere. The former facilitates decarburization and dephosphorization, while the latter facilitates deoxidation, desulfurization, the addition of easily oxidizable alloys, the recovery of metal elements, and the control of molten steel composition. Electric furnaces are simpler to operate, require less investment, are quick to build, occupy less space, and are easier to control pollution.

However, because the energy required is generated by the high temperatures generated by the short-circuited electrodes, it consumes a significant amount of electricity, consuming approximately 350-600 kWh to produce one ton of steel. The finished steel also contains relatively high levels of hydrogen and nitrogen. The arc dissociates a significant amount of water vapor in the furnace air, generating hydrogen and nitrogen that, if incorporated into the molten steel, can affect its quality. The arc is a point heat source, resulting in uneven temperature distribution within the furnace. When the molten pool is calm, the temperature of the molten steel varies significantly from one part to another. The presence of carbonaceous electrodes increases carbonization in the molten steel, making it difficult to produce low-carbon steel.

Scrap steel is an energy-carrying resource, and using it to produce steel can save significant energy. In large steel mills, energy consumption and pollution emissions are primarily concentrated in the pre-steelmaking process, from mining and beneficiation to sintering, coking, ironmaking, steelmaking, and rolling. Research shows that using scrap steel directly for steelmaking can save 60% energy and 40% water compared to using ore for ironmaking and then refining.

Scrap steel is an environmentally friendly resource. Using scrap steel directly for steelmaking can reduce waste gas by 86%, waste water by 76%, and waste slag by 97%, compared to using ore for ironmaking and then refining, contributing to cleaner production and reduced waste emissions.

Steps in Electric Furnace Steelmaking:

1. Charging

Adding raw materials such as molten iron or scrap steel to the electric furnace is the first step in electric furnace steelmaking.

2. Slag Formation

The process of adjusting the composition, alkalinity, viscosity, and reactivity of slag during steel and iron production. For example, oxygen blowing is designed to produce a slag with sufficient fluidity and alkalinity to transfer sufficient oxygen to the molten metal surface, thereby reducing sulfur and phosphorus levels to below the upper limits of the intended steel grade and minimizing splashing and slag overflow during oxygen blowing.

3. Slag Tapping

The slag removal or skimming process during electric arc furnace steelmaking, depending on the smelting conditions and objectives. When using the single-slag method, the slag must be removed at the end of oxidation. When using the double-slag method to produce reducing slag, the original slag must be completely removed to prevent rephosphorization.

4. Molten Pool Agitation

Energy is supplied to the molten metal pool to create movement in the molten metal and slag, thereby improving the kinetic conditions of the metallurgical reaction. Molten pool agitation can be achieved using gas, mechanical, or electromagnetic induction methods.

5. Dephosphorization

A chemical reaction that reduces the phosphorus content in molten steel. Phosphorus is a harmful impurity in steel. Steel with a high phosphorus content is prone to brittle cracking when used at room temperature or lower, a condition known as "cold brittleness." The higher the carbon content in the steel, the more severe the brittleness caused by phosphorus. The phosphorus content in ordinary steel is generally specified to not exceed 0.045%, and high-quality steel requires even lower phosphorus contents.

6. Electric Furnace Bottom Blowing

Gases such as N2, Ar, CO2, CO, CH4, and O2 are blown into the molten pool in the furnace through nozzles located at the bottom of the furnace, according to process requirements, to accelerate melting. The purpose of promoting metallurgical reactions is to accelerate the process. The bottom blowing process can shorten smelting time, reduce power consumption, improve dephosphorization and desulfurization operations, increase residual manganese in steel, and improve metal and alloy yields. It also achieves more uniform composition and temperature of the molten steel, thereby improving steel quality, reducing costs, and increasing productivity.

7. Melting Period

The melting period of steelmaking primarily applies to open-hearth and electric furnace steelmaking. In electric arc furnace steelmaking, the melting period lasts from the moment the power is applied to the furnace until all the steel and accompanying materials are completely melted; in open-hearth steelmaking, the melting period lasts from the time the molten iron is added to the time the entire charge is completely melted. The melting period's mission is to quickly melt and heat the charge and to create the slag required for the melting period.

8. Oxidation and Decarburization Periods

In conventional electric arc furnace steelmaking, the oxidation period typically refers to the process phase from charge dissolution, sampling and analysis, to slagging. Some believe it begins with oxygen blowing or ore addition for decarburization. The main tasks of the oxidation period are to oxidize carbon and phosphorus in the molten steel, remove gases and inclusions, and ensure uniform heating of the molten steel. Decarburization is a key process during the oxidation period. To ensure steel purity, the required decarburization is greater than approximately 0.2%. With the development of off-furnace refining technology, oxidation refining in electric arc furnaces has mostly been moved to ladles or refining furnaces.

9. Refining Period

The steelmaking process involves removing elements and compounds that are detrimental to steel quality from the molten steel by chemical reactions, through slagging and other methods, into the gas phase or by discharging or floating them into the slag.

10. Reduction Period

In conventional electric arc furnace steelmaking, the period from the end of the oxidation period, when slagging is completed, to steel tapping is generally referred to as the reduction period. Its primary tasks are to produce reducing slag for diffusion, deoxidize and desulfurize, control chemical composition, and adjust temperature. High-power and ultra-power electric arc furnace steelmaking operations have eliminated the reduction period.

11. Secondary Refining

The steelmaking process of transferring molten steel from a steelmaking furnace (such as a converter or electric furnace) to another vessel for refining is also called secondary metallurgy. The steelmaking process is therefore divided into two steps: primary refining and refining.

Primary refining: The charge is melted in a furnace under an oxidizing atmosphere, undergoing dephosphorization, decarburization, and main alloying.

Refining: The primary refining molten steel is removed from the vessel under vacuum, inert gas, or a reducing atmosphere for degassing, deoxidation, and desulfurization, removal of inclusions, and composition adjustment.

The benefits of splitting steelmaking into two steps include improving steel quality, shortening smelting time in the steelmaking plant, simplifying the process, and reducing production costs. There are many types of secondary refining, which can be broadly divided into two categories: normal pressure refining and vacuum refining. Depending on the treatment method, it can be further divided into ladle treatment refining and ladle refining refining.

12. Molten Steel Agitation

The agitation of the molten steel during secondary refining. It homogenizes the composition and temperature of the molten steel and promotes metallurgical reactions. Most metallurgical reactions occur at interfacial interfaces, where the diffusion rate of reactants and products is the limiting factor. When molten steel is static, its metallurgical reactions are very slow. For example, desulfurization of static molten steel in an electric furnace takes 30 to 60 minutes, while desulfurization using stirring in furnace refining only takes 3 to 5 minutes. When the molten steel is static, inclusions float up and are removed more slowly. When the molten steel is stirred, the rate of inclusion removal increases exponentially, depending on the stirring intensity, type, and nature and concentration of the inclusions.

13. Ladle Wire Feeding

A wire feeder is used to feed deoxidation, desulfurization, and composition adjustment powders, such as Ca-Si powder, directly into the ladle. This method involves deep desulfurization, calcium treatment, and fine-tuning the carbon and aluminum content of the steel. It also cleans the molten steel and improves the morphology of non-metallic inclusions.