Modern steelmaking methods mainly include converter steelmaking, open-hearth steelmaking and electric furnace steelmaking.
Open-hearth steelmaking has been basically eliminated. The most fundamental difference between electric furnace steelmaking and converter steelmaking is that electric furnace steelmaking uses electricity as a heat source, while electric arc furnace steelmaking is the most commonly used electric furnace steelmaking method.
What we usually call electric furnace steelmaking mainly refers to electric arc furnace steelmaking, because other types of electric furnaces such as induction furnaces and electric slag furnaces produce less steel.
Electric arc furnace steelmaking is a steelmaking method that relies on the electric arc generated by the discharge between the electrode and the charge to convert the electric energy into heat energy in the arc light, and heats and melts the metal and slag with the help of radiation and the direct action of the arc to smelt steel and alloys of various compositions.
Characteristics of electric furnace steelmaking:
Electric furnace steelmaking mainly uses arc heat. In the arc action zone, 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 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 pressed blocks, etc. can be added.
The smelting time is relatively long, 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℃, and the furnace temperature is as high as 2000℃ or more, which is much higher than the temperature required for smelting general steel grades. It can be used to smelt alloy steel and stainless steel that cannot be smelted in converters.
The atmosphere in the furnace is easy to control and adjust. In different stages of smelting, the furnace can not only create an oxidizing atmosphere, but also a reducing atmosphere. The former is conducive to decarburization and dephosphorization, and the latter is conducive to deoxidation, desulfurization, the addition of easily oxidized alloys, recovery of metal elements and control of molten steel composition. Electric furnace equipment is simpler, with less investment, fast plant construction, less land occupation, and easy pollution control.
However, since the required energy is provided by the high temperature generated by the electrode during short circuit, the power consumption is large. The power consumed for smelting 1 ton of steel is about 350-600kwh; the hydrogen and nitrogen content in the finished steel is high. Because under the action of the electric arc, the water vapor in the air in the furnace dissociates in large quantities, and the generated hydrogen and nitrogen, if entering the molten steel, will affect the quality of the steel. The electric arc is a “point” heat source, the temperature distribution in the furnace is uneven, and when the molten pool is calm, the temperature of each part of the molten steel varies greatly. The presence of carbonaceous electrodes will increase carbon in the molten steel, which brings difficulties to the smelting of low-carbon steel.
Scrap steel is an energy-carrying resource. Using scrap steel to make steel can save a lot of energy. In large steel joint ventures, from mining, ore dressing, sintering, coking, ironmaking to steelmaking and steel rolling, energy consumption and pollution emissions are mainly concentrated before the steelmaking process. Studies have shown that using scrap steel directly to make steel can save 60% of energy and 40% of water compared to using ore to make iron and then make steel.
Scrap steel is an environmentally friendly resource. Using scrap steel directly to make steel can reduce waste gas by 86%, waste water by 76% and waste slag by 97% compared to using ore to make iron and then make steel, which is conducive to clean production and waste reduction.
Steps of electric furnace steelmaking:
- Charging: The operation of adding raw materials such as molten iron or scrap steel to the electric furnace is the first step in the electric furnace steelmaking operation.
- Slag making is the operation of adjusting the composition, alkalinity, viscosity and reactivity of slag in steel and iron production. For example, oxygen blowing is to generate slag with sufficient fluidity and alkalinity, which can transfer enough oxygen to the metal liquid surface, so as to reduce sulfur and phosphorus to below the upper limit of the planned steel grade, and reduce the amount of splashing and slag overflow during oxygen blowing.
- Slag discharge is the slag discharge or slag removal operation taken during the smelting process according to different smelting conditions and purposes in electric arc furnace steelmaking. For example, when using the single slag method for smelting, the oxidized slag must be removed at the end of oxidation; when using the double slag method to make reducing slag, the original oxidized slag. Must be completely discharged to prevent phosphorus reversion, etc.
- Molten pool stirring supplies energy to the metal molten pool to make the molten metal and slag move to improve the kinetic conditions of the metallurgical reaction. Molten pool stirring can be achieved by means of gas, mechanical, electromagnetic induction and other methods.
- Dephosphorization is a chemical reaction that reduces the phosphorus content in molten steel. Phosphorus is one of the harmful impurities in steel. Steel with a high phosphorus content is prone to brittle cracking when used at room temperature or lower temperatures, 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 specified to be no more than 0.045%, and high-quality steel requires even less phosphorus.
- Bottom blowing of electric furnace Bottom blowing of electric furnace
Gases such as N2, Ar, CO2, CO, CH4, O2, etc. 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 melting. The purpose of promoting the metallurgical reaction process. The bottom blowing process can shorten the smelting time, reduce power consumption, improve dephosphorization and desulfurization operations, increase the amount of residual manganese in steel, and increase the metal and alloy recovery rate. It can also make the composition and temperature of molten steel more uniform, thereby improving steel quality, reducing costs and increasing productivity. - 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 power-on to the melting of the furnace steel flower and the melting of open-hearth steelmaking from the addition of molten iron to the melting of all the furnace materials is called the melting period. The task of the melting period is to melt and heat the furnace materials as soon as possible, and to make the slag for the melting period. - Oxidation period and decarburization period The oxidation period of ordinary power electric arc furnace steelmaking usually refers to the process stage from melting and clearing the charge, sampling and analysis to the removal of oxidized slag. Some people also believe that it starts from oxygen blowing or adding ore for decarburization. The main task of the oxidation period is to oxidize carbon and phosphorus in the molten steel; remove gas and inclusions; and heat the molten steel evenly. Decarburization is an important operation process in the oxidation period. In order to ensure the purity of steel, the decarburization amount is required to be greater than about 0.2%. With the development of refining technology outside the furnace, the oxidation refining of the electric arc furnace is mostly moved to the ladle or refining furnace.
- Refining period The process of steelmaking process uses slag making and other methods to select some elements and compounds that are harmful to the quality of steel into the gas phase or discharge and float into the slag through chemical reactions, so that they are removed from the molten steel. Continuous casting machine billet discharge.
- Reduction period In the operation of ordinary power electric arc furnace steelmaking, the period from the end of oxidation to steel discharge is usually called the reduction period. Its main tasks are to make reducing slag for diffusion, deoxidation, desulfurization, control chemical composition and adjust temperature. The reduction period has been cancelled in high-power and ultra-power electric arc furnace steelmaking operations.
- Off-furnace refining is the process of moving the steel liquid that has been initially refined in the steelmaking furnace (converter, electric furnace, etc.) to another container for refining, also known as secondary metallurgy. The steelmaking process is therefore divided into two steps: primary refining and refining.
Primary refining: The charge is melted, dephosphorized, decarburized and alloyed in a furnace in an oxidizing atmosphere.
Refining: The primary refining liquid steel is degassed, deoxidized, desulfurized, inclusions are removed and the composition is fine-tuned in a container in a vacuum, inert gas or reducing atmosphere.
The benefits of dividing steelmaking into two steps are: it can improve the quality of steel, shorten the smelting time in the steelmaking workshop, simplify the process and reduce production costs. There are many types of off-furnace refining, which can be roughly divided into two categories: off-furnace refining under normal pressure and off-furnace refining under vacuum. According to different processing methods, it can be divided into ladle treatment type off-furnace refining and ladle refining type off-furnace refining. - Stirring of molten steel Stirring of molten steel during refining outside the furnace. It makes the composition and temperature of molten steel uniform and promotes 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 rate is very slow. For example, it takes 30 to 60 minutes to desulfurize molten steel in a static electric furnace; while it only takes 3 to 5 minutes to desulfurize molten steel by stirring it in furnace refining. When molten steel is in a static state, inclusions float up and are removed, and the removal rate is slow; when molten steel is stirred, the removal rate of inclusions increases exponentially and is related to the stirring intensity, type, characteristics and concentration of inclusions.
- Ladle wire feeding is a method of feeding deoxidation, desulfurization and fine-tuning powders wrapped in iron sheets into the ladle through a wire feeder, such as Ca-Si powder, or directly feeding aluminum wire, carbon wire, etc. to deep desulfurize molten steel, treat calcium, and fine-tune the carbon and aluminum components in steel. It also has the function of cleaning molten steel and improving the morphology of non-metallic inclusions.
