This article explains the understanding of electric furnace steelmaking, and describes the characteristics and steps of electric furnace steelmaking in detail.
Key words: electric furnace steelmaking; characteristics; steps
Modern steelmaking methods mainly include converter steelmaking method, open hearth furnace steelmaking method and electric furnace steelmaking method. The open hearth furnace steelmaking method has basically been eliminated. The fundamental difference between the electric furnace steelmaking method and the converter steelmaking method is that the electric furnace steelmaking method uses electric energy as the heat source, while the electric arc furnace steelmaking method is the most widely 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 electroslag furnaces produce less steel.
Electric arc furnace steelmaking is based on the electric arc generated by the discharge between the electrode and the charge, so that the electric energy is converted into heat energy in the arc light, and the metal and slag are heated and melted by the direct action of radiation and electric arc, and a kind of steel and alloy of various components are smelted. A method of steelmaking.
Characteristics of Electric Furnace Steelmaking
Electric furnace steelmaking mainly uses arc heat, and in the arc action area, the temperature is as high as 4000 °C. 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 formed, 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%, and molten iron, pig iron, direct reduced iron, hot briquette, etc. can be added. The smelting time is longer, generally at least twice as long as the converter smelting time. The melting temperature of the electric furnace is high and easy to control and adjust. The temperature in the arc zone of the electric arc furnace is as high as 3000-6000 ℃, and the furnace temperature is as high as 2000 ℃, which is much higher than the temperature required for smelting general steel types. It can be used to smelt 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 phosphorus removal, and the latter is beneficial to deoxidation, desulfurization, addition of easily oxidizable alloys, recovery of metal elements and Control the composition of molten steel. Electric furnace equipment is relatively simple, less investment, faster to build a factory, less land occupation, and easy to control pollution.
However, since the required energy is provided by the high temperature generated by the electrodes when they are short-circuited, the power consumption is relatively large, and the power consumption for 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, the water vapor in the air in the furnace dissociates in large quantities, and the generated hydrogen and nitrogen, if they enter 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 molten steel in each part is quite different. The existence of carbonaceous electrodes will increase the carbon content of molten steel, which will bring difficulties to smelting low-carbon steel.
Scrap iron and steel is an energy-carrying resource, and using scrap iron and steel to make steel can save a lot of energy. In large-scale iron and steel complexes, from mining, mineral processing, sintering, coking, ironmaking to steelmaking, steel 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 with ironmaking with ore and then steelmaking.
Scrap iron and steel is an environmentally friendly resource. Compared with using ore to iron-smelt steel, using scrap steel directly to make steel can reduce waste gas by 86%, waste water by 76% and waste residue by 97%, which is conducive to clean production and waste reduction.
Steps of electric furnace steelmaking
Feeding
The operation of adding raw materials such as molten iron or steel scrap to the electric furnace is the first step in the electric furnace steelmaking operation.
Slagging
The operation of adjusting the slag composition, alkalinity 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 sulfur and phosphorus below the upper limit of the planned steel grade, and make the splashing during oxygen blowing And the amount of spilled slag is minimized.
Slag
According to different smelting conditions and purposes, the slag discharge or slag removal operation is adopted in the smelting process during electric arc furnace steelmaking. For example, when the single slag method is used for smelting, the oxidation slag must be removed at the end of oxidation; It must be released completely to prevent back phosphorus and so on.
Pool stirring
Supply energy to the molten metal pool to make the molten metal and slag move, so as to improve the kinetic conditions of the metallurgical reaction. The stirring of molten pool can be realized by means of gas, mechanical, electromagnetic induction and other methods.
Dephosphorization
A chemical reaction to reduce 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 brittle”. The higher the carbon content in the steel, the more serious the brittleness caused by phosphorus. Generally, the phosphorous content in ordinary steel shall not exceed 0.045%, and the high-quality steel requires less phosphorus.
Electric Bottom Blowing Electric Bottom Blowing
Gases such as N2, Ar, CO2, CO, CH4, O2 are blown into the melting pool in the furnace through the nozzles placed on the bottom of the furnace according to the process requirements to achieve accelerated 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 yield of metals and alloys. And it can 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 electric arc furnace steelmaking is called the melting period from the start of electrification to the melting of the furnace steel flower and accessories, and the open hearth furnace steelmaking is called the melting period from the completion of mixing the molten iron to the completion of the furnace charge. 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.
Oxidation period and decarburization period
The oxidation period of ordinary power electric arc furnace steelmaking usually refers to the process stage of furnace material dissolution, sampling and analysis to the removal of oxide slag. It is also believed that it started from oxygen blowing or ore decarburization. The main task of the oxidation period is to oxidize carbon and phosphorus in the molten steel; remove gas and inclusions; make the molten steel evenly heated. Decarburization is an important operating process in the oxidation stage. In order to ensure the purity of the steel, the amount of decarburization is required to be greater than 0.2%. With the development of refining technology outside the furnace, most of the oxidation refining of the electric arc furnace is moved to the ladle or refining furnace.
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 or floated into the slag through chemical reactions through slagging and other methods, so that they are excluded from the molten steel. Continuous casting machine billet continuous casting machine billet.
Recovery period
In ordinary power electric arc furnace steelmaking operations, the period from the completion of slagging at the end of oxidation to tapping is usually called the reduction period. Its main task is to create reducing slag for diffusion, deoxidation, desulfurization, control chemical composition and adjust temperature. The reduction period has been eliminated for high power and ultra power EAF steelmaking operations.
Refining
The steelmaking process in which the molten steel that has been refined in a steelmaking furnace (converter, electric furnace, etc.) is moved to another container for refining is also called secondary metallurgy. The steelmaking process is therefore divided into two steps: initial smelting and refining.
Primary smelting: The charge is melted, dephosphorized, decarburized and main alloyed in a furnace with an oxidizing atmosphere.
Refining: Degassing, deoxidizing, desulfurizing, removing inclusions and fine-tuning the composition of the initially refined molten steel in a container with vacuum, inert gas or reducing atmosphere.
The advantages of dividing steelmaking into two steps are: the quality of steel can be improved, the steelmaking workshop can shorten the smelting time, simplify the process and reduce production costs. There are many types of out-of-furnace refining, which can be roughly divided into two types: out-of-furnace refining under atmospheric pressure and out-of-furnace refining under vacuum. According to different processing methods, it can be divided into ladle processing type external refining and ladle refining type external refining.
Stirring of molten steel
Stirring of molten steel during refining outside the furnace. It homogenizes the composition and temperature of molten steel 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 the molten steel is in a static state, its metallurgical reaction speed is very slow. For example, it takes 30-60 minutes to desulfurize the static molten steel in an electric furnace; while it only takes 3-5 minutes to desulfurize by stirring the molten steel in the furnace refining. When the molten steel is in a static state, the inclusions are floated up and removed, 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, characteristics and concentration of the inclusions.
Ladle wire feeding
Feed deoxidation, desulfurization and fine-tuning powder wrapped in iron sheet into the ladle through the wire feeder, such as Ca-Si powder, or directly feed aluminum wire, carbon wire, etc. to carry out deep desulfurization, calcium treatment and fine-tuning of molten steel. method for components such as carbon and aluminum. It also has the functions of cleaning molten steel and improving the shape of non-metallic inclusions.