Steelmaking: adding scrap iron and cast iron ingots → high temperature (blowing oxygen with electricity, etc. ) → molten steel+steel slag → steel ingot (or continuous rolling)+steel slag enters the electric arc furnace or converter.
According to the requirements of steelmaking varieties, the process of removing the carbon content in pig iron to a specified range and reducing or increasing the content of other elements to a specified range. Simply put, it is the process of reducing carbon, desulfurization and phosphorus and adjusting the content of silicon and manganese in pig iron. This process is basically an oxidation process, using different sources of oxygen (such as oxygen in the air, pure oxygen, oxygen in iron ore) to iron oxide carbon, silicon, manganese and other elements in water. Chemical reactions mainly include:
2FeO+Si 2Fe+SiO2
FeO+Mn Fe+MnO
The carbon monoxide produced by the reaction is easily discharged from molten iron into the furnace gas and removed. The generated silicon dioxide, manganese oxide and ferrous oxide interact to form slag floating on the steel surface. Sulfur and phosphorus in pig iron are generally harmful to steel and must be removed as much as possible when steelmaking. Adding lime (CaO) into the steelmaking furnace can remove sulfur and phosphorus;
2P+5FeO+3CaO 5Fe+Ca2(PO4)2 (turned into slag)
After reducing carbon and other elements to the specified range, molten steel still contains a lot of oxygen, which is a harmful impurity, making the plasticity of steel worse and prone to cracks during rolling. Therefore, deoxidizers (such as ferromanganese, ferrosilicon and aluminum, etc. ) must be added in the final stage of steelmaking to remove excess oxygen in molten steel;
Manganese+ferrous manganese oxide+iron
Silicon +2FeO silicon dioxide +2Fe
Aluminum+ferric oxide aluminum oxide+ferric oxide
At the same time, the composition and temperature of molten steel are adjusted, and when it meets the requirements, it can be tapped and cast into ingots.
There are three main methods of steelmaking: converter, electric furnace and open hearth furnace. The main feature of open hearth steelmaking is that it can use more scrap steel (20 ~ 50% scrap steel can be used), and the raw materials have strong adaptability, but the smelting time is long. At present, open hearth steelmaking is mainly used in China. Oxygen top-blown converter (see figure) is widely used in converter steelmaking, with fast production speed (the blowing time of 1 300 tons converter is less than 20 minutes, and the auxiliary time is less than 1 hour, while it takes 7 hours for 300 tons of open hearth smelting 1 furnace steel), with many varieties and good quality. It can be used to smelt both ordinary steel and alloy steel. Electric furnace steelmaking is smelting with electric energy as heat source. It can be used to refine stainless and acid-resistant steel needed by chemical industry, advanced silicon steel and pure iron needed by electronic industry, ball steel and heat-resistant steel needed by aviation industry, bearing steel needed by machinery industry, high-speed cutting tool steel, precision alloy needed by instrument industry, etc.
Iron ore, coke, limestone, fluorite and other raw materials are put into the blast furnace in proportion, and hot air is blown in to heat it to above 1000 degrees. In this way, elemental iron is reduced by the reduction of carbon. This is the process of ironmaking. Then molten iron is injected into the steelmaking converter, and then a certain amount of scrap steel is added to blow oxygen into the converter, which is the most decisive work in steelmaking. Oxygen can iron oxide excess carbon in water and convert it into gas. Then, part of ferromanganese or ferrosilicon is put into the oxidized molten steel to be oxidized with the residual oxygen in the molten steel, and then the molten steel is cast into steel ingots or parts, or directly enters a steel rolling plant to be rolled into usable profiles. This is the whole process of steelmaking.
Simply put, the purity of iron is refined to 98%. To be refined like this, you must keep beating.
Quenching makes impurity C fully contact with oxygen to generate CO2. Therefore, in ancient iron and steel smelting, there was constant beating, constant barbecue and constant quenching in water.
Modern steelmaking steps are as follows
slag making
Slagging: the operation of adjusting slag composition, alkalinity, viscosity and reaction ability in iron and steel production. The purpose is to pass through the steel blast furnace
The slag-metal reaction produces a metal with the required composition and temperature. For example, the operation of oxygen top-blown converter slagging and oxygen blowing is to generate slag with sufficient fluidity and alkalinity, which can transfer enough oxygen to the metal surface, so that sulfur and phosphorus can be reduced below the upper limit of planned steel grade, and the amount of splashing and slag overflow during oxygen blowing can be reduced as much as possible.
taphole
Slag discharge: Slag discharge or skimming operation during smelting according to different smelting conditions and purposes. If smelting by single slag method, the oxide slag must be scraped off at the end of oxidation; When making reducing slag by double slag method, all original oxide slag must be discharged to prevent phosphorus from flowing back.
Molten pool stirring
Molten pool stirring: providing energy to the metal molten pool to make the molten metal and slag move, thus improving the kinetic conditions of metallurgical reaction. The stirring of molten pool can be realized by gas, machinery, electromagnetic induction and other methods.
Electric furnace bottom blowing
Electric furnace bottom blowing: according to the process requirements, N2, Ar, CO2, CO, CH4, O2 and other gases are blown into the furnace molten pool through the nozzle placed at the bottom of the furnace to accelerate melting and promote the metallurgical reaction process. Bottom blowing process can shorten smelting time, reduce energy consumption, improve dephosphorization and desulfurization operations, increase the amount of residual manganese in steel and improve the yield of metals and alloys. It can make the composition and temperature of molten steel more uniform, thus improving the quality of steel, reducing the cost and improving the productivity.
melting stage
Melting period: The melting period of steelmaking is mainly open hearth and electric furnace steelmaking. Electric arc furnace steelmaking is busy with my steelmaking from electrification to furnace steel flower.
The melting period is called the melting period until all materials are melted, and the open hearth steelmaking is called the melting period from the beginning of mixing molten iron to the complete melting of the burden. The task of the melting period is to melt and raise the temperature of the burden as soon as possible, and to make slag during the melting period.
Oxidation stage and decarbonization stage
Oxidation stage and decarbonization stage: the oxidation stage of common EAF steelmaking usually refers to the process stage from charge dissolution, sampling analysis to removal of oxide slag. Some people think that it started with blowing oxygen or adding ore to decarburize. The main task of oxidation stage is to oxidize carbon and phosphorus in molten steel; Removing gas and impurities; The molten steel is heated uniformly to raise the temperature. Decarburization is an important operation process in the oxidation stage. In order to ensure the purity of steel, decarbonization is required to be greater than 0.2%. With the development of external refining technology, oxidation refining of EAF is mostly carried out in ladle or refining furnace.
refining period
Refining period: the process operation period in which some elements and compounds harmful to steel quality are selected into the gas phase or discharged or floated into slag through chemical reaction, so as to be excluded from molten steel. Continuous casting machine discharging
Reduction period
Reduction period: In common EAF steelmaking operation, the period from the end of oxidation to tapping is usually called reduction period. Its main task is to produce reduced slag for diffusion, deoxidation, desulfurization, chemical composition control and temperature regulation. At present, the reduction period has been cancelled for steelmaking operations of high-power and ultra-high-power electric arc furnaces.
External refining
Refining outside the furnace: the steelmaking process of moving the molten steel preliminarily refined in the steelmaking furnace (converter, electric furnace, etc.). ) to another container for refining, also known as secondary metallurgy. Therefore, the steelmaking process is divided into two steps: primary smelting and refining. Primary smelting: melting, dephosphorization, decarbonization and main alloying of furnace charge in oxidizing atmosphere. Refining: degassing, deoxidizing, desulfurizing, removing inclusions and fine-tuning the composition of molten steel in a container with vacuum, inert gas or reducing atmosphere. The advantages of two-step steelmaking method are: it can improve the quality of steel, and the steelmaking workshop
Shorten smelting time, simplify technological process and reduce production cost. There are many kinds of refining outside the furnace, which can be roughly divided into two kinds: atmospheric refining and vacuum refining. According to different treatment methods, it can be divided into ladle treatment type external refining and ladle refining type external refining.
Molten steel stirring
Molten steel stirring: the stirring of molten steel during refining outside the furnace. It homogenizes the composition and temperature of molten steel and promotes metallurgical reaction. Most metallurgical reactions are phase interface reactions, and the diffusion speed of reactants and products is the limiting link of these reactions. In the static state, the metallurgical reaction speed of molten steel is very slow, for example, it takes 30 ~ 60 minutes for desulfurization of static molten steel in electric furnace; When refining in the furnace, the molten steel can be desulfurized by stirring for only 3-5 minutes. In the static state of molten steel, inclusions * float up and are removed, and the removal speed is slow; When stirring molten steel, the removal rate of inclusions increases exponentially, which is related to the intensity and type of stirring, the characteristics and concentration of inclusions.
Ladle feeding
Ladle wire feeding: the method of feeding the powder with deoxidizing, desulfurizing and fine-tuning components wrapped with iron sheets into the ladle through a wire feeder, or directly feeding aluminum wires and carbon wires for Ca-Si tapping for deep desulfurization, calcium treatment and fine-tuning of carbon and aluminum components in steel. It also has the functions of purifying molten steel and improving the shape of nonmetallic inclusions.
Ladle treatment
Ladle treatment: short for ladle treatment external refining. It is characterized by short refining time (about 10 ~ 30 minutes) and converter steelmaking.
The refining task is single, there is no heating device to compensate the temperature drop of molten steel, the process operation is simple, and the equipment investment is less. It has devices for degassing and desulfurizing molten steel, controlling composition and changing the shape of inclusions. Such as vacuum cycle degassing method (RH, DH), ladle vacuum argon blowing method (Gazid), ladle powder spraying treatment method (IJ, TN, SL) and so on.
Barrel refining
Ladle refining: short for ladle refining outside the furnace. Its characteristic is that the refining time is longer than that of ladle treatment (about 60 ~ 180 minutes). It has many refining functions and heating devices to compensate for the temperature drop of molten steel. Suitable for refining all kinds of high alloy steel and special performance steel (such as ultra-pure steel). Vacuum oxygen blowing decarburization (VOD), vacuum arc heating degassing (VAD), ladle refining (ASEA-SKF), closed argon blowing fine tuning (CAS), etc. , all belong to this category; Similarly, there is argon-oxygen decarbonization (AOD).
Inert gas treatment
Inert gas treatment: inert gas Ar is blown into molten steel, which does not participate in metallurgical reaction, but each small bubble rising in molten steel is equivalent to a "small vacuum chamber" (the partial pressure of H2, N2 and CO in the bubble is close to zero), which has the function of "gas washing". The principle of stainless steel production by out-of-furnace refining method is to apply the equilibrium relationship between carbon, chromium and temperature under different CO partial pressures. Refining decarbonization with inert gas and oxygen can reduce the partial pressure of CO in carbon-oxygen reaction. At lower temperature, the carbon content decreases and chromium is not oxidized.
Prealloying
Pre-alloying: the operation process of adding one or more alloying elements to molten steel to meet the requirements of finished steel composition specification is called alloying. In most cases, deoxidation and alloying are carried out at the same time, and part of the deoxidizer added to steel is consumed by deoxidation of steel and converted into deoxidation products to be discharged; The other part is absorbed by 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 with the deoxidizer is absorbed by molten steel, which is called prealloying.
Composition control
Composition control: the operation to ensure that all components of finished steel meet the standard requirements. Composition control runs through every link from batching to tapping, but the key point is to control the composition of alloying elements during alloying. For high-quality steel, it is often required to accurately control the composition in a narrow range; Generally, under the premise of not affecting the performance of steel, it is controlled according to the middle and lower limits.
Increase silicon
Increasing silicon: At the end of blowing, the silicon content in molten steel is extremely low. In order to meet the requirements of various steel grades for silicon content, a certain amount of silicon must be added in the form of alloy. Besides being used as the consumption part of deoxidizer, it also increases the silicon in molten steel. The amount of silicon added should be accurately calculated and should not exceed the allowable range of steel blowing.
Terminal control
Terminal control: control to make the chemical composition and temperature of metal meet the planned tapping requirements at the end of steelmaking and blowing in oxygen converter. There are two ways to control the end point: increasing carbon and pulling carbon.
Tap, tap
Tapping: tapping operation when the temperature and composition of molten steel meet the specific requirements of steelmaking varieties. Pay attention to prevent slag from flowing into ladle when tapping. Additives used to adjust the temperature, composition and deoxidation of molten steel are added to ladle or tapping flow when tapping, which is also called deoxidation alloying.