Description：The molten iron contains impurities such as C, S, and P, which affect the strength and brittleness of the iron. It is necessary to re-smelt the molten iron to remove the above-mentioned impurities, and add Si, Mn, etc. to adjust its composition. The process of remelting molten iron to adjust its composition is called steelmaking.
Keyword: Steelmaking process
The operation of adjusting the composition, alkalinity and viscosity of slag and its reactivity in steel and iron production. The purpose is to refine the metal with the required composition and temperature through the slag-metal reaction. For example, the slagging and oxygen blowing operation of the oxygen top-blown converter is to generate slag with sufficient fluidity and basicity, so as to reduce the sulfur and phosphorus below the upper limit of the planned steel grade, and to prevent splashing and slag overflow during oxygen blowing. amount to a minimum.
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 oxidized slag must be removed at the end of oxidation; when the reduced slag is made by the double slag method, the original oxidized slag must be completely released to prevent phosphorus return.
Melt 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 molten pool stirring can be realized by means of gas, mechanical, electromagnetic induction and other methods.
Electric bottom blowing:
Gases such as N2, Ar, CO2, CO, CH4, O2 are blown into the molten pool in the furnace through the nozzles placed on the bottom of the furnace according to the process requirements to achieve the purpose of accelerating melting and 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.
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 charge, and the open hearth steelmaking is called the melting period from the time the molten iron is mixed to the end of the 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 steel, the decarburization amount is required to be greater than about 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.
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.
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 of chemical composition and temperature adjustment. The reduction period has been eliminated for current high-power and ultra-power electric arc furnace steelmaking operations.
Refining outside the furnace:
The steelmaking process in which the molten steel that has been initially smelted 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: it can improve the quality of steel, 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 molten steel is in a static state, its metallurgical reaction speed is very slow. For example, it takes 30 to 60 minutes to desulfurize static molten steel in an electric furnace; while it only takes 3 to 5 minutes to desulfurize by stirring molten steel in furnace refining. When the molten steel is in a static state, the inclusions will float up and be removed at a slow rate; when the molten steel is stirred, the removal rate of the inclusions increases exponentially, and is related to the stirring intensity, type, and the characteristics and concentration of the inclusions.
Ladle feeding wire:
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.
Ladle treatment: the abbreviation of ladle treatment type out-of-furnace refining. It is characterized by short refining time (about 10-30 minutes), single refining task, no heating device to compensate for the temperature drop of molten steel, simple process operation, and low equipment investment. It has devices such as molten steel degassing, desulfurization, composition control and changing the shape of inclusions. Such as vacuum circulation degassing method (RH, DH), ladle vacuum argon blowing method (Gazid), ladle spraying treatment method (IJ, TN, SL), etc. all belong to this category.
The abbreviation of ladle refining type furnace refining. It is characterized by longer refining time (about 60-180 minutes) than ladle treatment, has multiple refining functions, and has a heating device to compensate for the temperature drop of molten steel. It is suitable for various high-alloy steels and special-performance steels (such as ultra-pure steel species) refining. Vacuum oxygen blowing decarburization method (VOD), vacuum arc heating degassing method (VAD), ladle refining method (ASEA-SKF), closed argon blowing composition fine-tuning method (CAS), etc., all belong to this category; similar There is also argon oxygen decarburization (AOD).
Inert gas treatment:
Inert gas is blown into the molten steel. This gas itself does not participate in the metallurgical reaction, but each small bubble rising from the molten steel is equivalent to a “small vacuum chamber” (the partial pressure of H2, N2, and CO in the bubble is close to Zero), with “air washing” effect. The principle of producing stainless steel by refining outside the furnace is to apply the balance relationship between carbon chromium and temperature under different partial pressures of CO. Refining and decarburizing with inert gas and oxygen can reduce the partial pressure of CO in the carbon-oxygen reaction, and at a lower temperature, the carbon content is reduced and the chromium is not oxidized.
The operation process of adding one or several alloying elements to the molten steel to make it meet the composition specifications of the finished steel is called alloying. In most cases, deoxidation and alloying are carried out at the same time. Part of the deoxidizer added to the steel is consumed in the deoxidation of the steel and converted into deoxidized products to be discharged; the other part is absorbed by the molten steel and plays an alloying role. Before the deoxidation operation is completely completed, the alloying effect of the alloy added at the same time as the deoxidizer is absorbed by the molten steel is called pre-alloying.
It is an operation to ensure that the composition of the finished steel meets the requirements of the standard. Composition control runs through all links from batching to tapping, but the focus is on the control of alloying element composition during alloying. For high-quality steel, it is often required to precisely control the composition within a narrow range; generally, it is controlled according to the middle and lower limits without affecting the performance of the steel.
At the end of blowing, the silicon content in molten steel is extremely low. In order to meet the silicon content requirements of each steel grade, a certain amount of silicon must be added in the form of alloy material. In addition to being used as a deoxidizer to consume part, it also increases the silicon in molten steel. The amount of silicon increase should be calculated accurately, and should not exceed the allowable range of blowing steel grades.
End point control: the control to make the chemical composition and temperature of the metal meet the tapping requirements of the planned steel grade at the end of oxygen converter steelmaking blowing (end of oxygen blowing). There are two methods of end point control: carbon increasing method and carbon pulling method.
The operation of releasing the molten steel when the temperature and composition of the molten steel meet the specified requirements of the type of steel being made. When tapping, care should be taken to prevent slag from flowing into the ladle. Additives for adjusting the temperature, composition and deoxidation of molten steel are added to the ladle or the tapping stream during the tapping process.