Converter steelmaking

1, classification of converter steelmaking method converter is a modern steelmaking method with molten iron as the main raw material. The steelmaking furnace consists of a frustum-shaped furnace cover, a cylindrical furnace body and a spherical bottom. The furnace body is provided with a trunnion that can rotate around it to meet the operation of charging, tapping and slag pouring, hence the name. Acid air bottom-blown converter-Bessemer furnace (UK 1856)? Air converter {Basic air bottom-blown converter-Thomas furnace (Germany 1878) Basic air side-blown converter (China 1952) converter {Oxygen top-blown converter -LD (Austria 1952)? Oxygen converter {oxygen bottom blowing converter -OBM (Germany 1967) top-bottom combined blowing converter (France 1975) 2. The background and abbreviation of oxygen top-blown converter steelmaking (1) is firstly an oxidation refining process. The original bainite furnace and Thomas furnace used blast to utilize oxygen. After World War II, industrial oxygen generators appeared in the United States, which made it possible to make steel with pure oxygen. But the original bottom blowing method is easy to burn the bottom of the furnace and the spray gun. The United Carbide Company of the United States conducted an oxygen top-blown converter experiment in the laboratory on 1947, and successfully named it BOF. Austria sent relevant experts to visit and study as soon as they heard it. After coming back, they conducted a semi-industrial test on a 2-ton converter at 1949, and achieved success. 1952 and 1953 30-ton oxygen top-blown converters were built and put into operation in Linz and Donowitz respectively, so they are often referred to as LD for short. 1967 12 Germany and Canada jointly invented the oxygen bottom-blown converter, which used a double-layer sleeve nozzle and introduced gaseous hydrocarbons for cooling. 1975, France developed a top-bottom combined blowing converter, which integrated the advantages of LD and OBM, and put forward it at the 77-year world annual meeting. (2) Characteristics of oxygen top-blown converter 1) Advantages Oxygen top-blown converter has shown great advantages since it came out, and all countries in the world have developed simultaneously and become the most important steelmaking method at present. Its advantages are: (1) fast melting speed and high productivity (only 20 minutes for one heat of steel); (2) High thermal efficiency, no additional heat source is needed during smelting, and 10% ~ 30% of scrap steel can be utilized; (3) There are many kinds of steels with good quality (both high carbon steel and low carbon steel can be smelted, and the contents of S, P, H, N, O and inclusions are low); (4) It is convenient for comprehensive utilization and computer control of the production process. 2) Disadvantages Of course, LD still has some problems, such as high blowing loss (10%), limited steel grade (it is difficult to smelt high alloy steel containing a lot of refractory elements and oxidizable elements) and so on. The development trend of oxygen converter is the first to popularize oxygen top-blown converter in Japan, leading the development trend of LD: (1) large capacity (relatively small investment); (2) Increase refining outside the furnace, increase varieties and improve quality (theoretically, any steel can be refined); (3) Introducing bottom blowing technology and implementing compound blowing (reducing splashing and blowing loss); (5) The computer control of smelting process is realized. 1 introduction of raw materials for converter steelmaking raw materials used in converter steelmaking can be divided into two categories: metallic materials and nonmetallic materials. The quality of raw materials is not only related to the difficulty of blowing operation, but also affects the output, quality and production cost of steel. 1. 1 metal materials The main metal materials for converter steelmaking are molten iron, scrap steel and ferroalloy. 1. 1. 1 the role of molten iron 1: The main raw materials for converter steelmaking generally account for more than 70% of the burden. Hot metal is required to meet certain requirements in order to simplify and stabilize the operation and obtain good technical and economic indicators. 1) The temperature is ≥ 1250℃ and the temperature of molten iron is stable, indicating physical heat. Higher molten iron temperature can not only ensure the smooth blowing of converter, but also increase the consumption of scrap steel and reduce the production cost. Therefore, it is hoped that the temperature of hot metal should be as high as possible, and it should generally be kept above 1250℃ ~ 1300℃ during charging. In addition, it is also hoped that the temperature of molten iron is relatively stable, which is beneficial to smelting operation and production scheduling. 2) The composition is appropriate and the fluctuation is small? Converter steelmaking has strong adaptability and can convert molten iron with various components into steel. However, in order to facilitate converter operation and reduce production costs, the composition of molten iron should be appropriate and stable. (1) Phosphorus content in hot metal ≤0.4%: Phosphorus will cause "cold brittleness" in steel and is one of the harmful elements in steel. The dephosphorization effect of converter single slag smelting is 85% ~ 95%, and the phosphorus content of plain carbon steel is usually less than 0.04%. Therefore, the national standard stipulates that the phosphorus content of hot metal is less than 0.4%. It should be pointed out that dephosphorization cannot be carried out in blast furnace. If the phosphorus content of hot metal exceeds 0.4%, or low phosphorus steel is blown, it is necessary to adopt double slag smelting or pre-dephosphorization treatment for hot metal. (2) Sulfur content in hot metal ≤0.07%: Sulfur will make steel "hot brittle" and is also a harmful element in steel. The desulfurization effect of converter is not ideal, and the desulfurization rate of single slag smelting is only 30% ~ 35%, while the sulfur content of molten steel is usually required to be below 0.05%, so the national standard stipulates that the sulfur content of molten iron is ≤0.07%. If the sulfur content of hot metal exceeds 0.07% or low sulfur steel is blown, double slag smelting or pre-desulfurization treatment of hot metal is needed. (3) Silicon content in molten iron: Silicon in molten iron is one of the main heating elements in converter steelmaking. When the silicon content increases by 0. 1%, the scrap rate can increase by 1.3% ~ 1.5%. For large and medium-sized converters, the silicon content of hot metal should be 0.5% ~ 0.8%. The heat loss of small converter is large, and the silicon content of hot metal can be higher. If the silicon content is less than 0.5%, the chemical heat of molten iron will be insufficient, which will lead to the decrease of scrap rate and even the normal blowing of small capacity converter; On the other hand, if the silicon content of hot metal is higher than 0.8%, it will not only increase the consumption of slagging materials, but also increase the amount of slag in the furnace, which will easily lead to splashing and increase metal loss. In addition, when the silicon content of hot metal is high, the basicity of slag in the early stage is low, and the erosion of furnace lining is intensified; At the same time, the high content of silica in the initial slag will relatively reduce the contents of FeO and MnO in the slag, and it is easy to form a layer of 2CaO with melting point of 2 130℃ on the surface of lime block. The SiO2 _ 2 shell prevents lime from melting and slows down the slagging speed, which is not conducive to early dephosphorization. It should be pointed out that the silicon content of hot metal in some steel mills exceeds 65438 0.2%, and some even reach 65438 0.5%, so pre-desilication should be carried out to reduce the burden of converter. (4) Manganese content in molten iron: Manganese in molten iron is a kind of beneficial elements, which is mainly reflected in that manganese oxide generated after manganese oxidation can promote lime dissolution, which is beneficial to prolonging furnace life and reducing steel sticking of oxygen lance. The manganese content of hot metal used in most steel plants in China is not high, mostly 0.2% ~ 0.4%. The main way to increase the manganese content in hot metal is to add manganese ore to blast furnace raw materials, but this will increase the coke ratio in ironmaking production and reduce the productivity of blast furnace. The rationality of increasing manganese in hot metal needs detailed technical and economic comparison, and there is no hard and fast requirement for manganese content in hot metal at present. (5) Carbon content of molten iron: Carbon is also the main heating element in converter steelmaking, and the requirement of smelting can be met with a carbon content of ≥3.5%, while molten iron usually contains about 4% carbon, which is generally not required. See table (6) 1- 1 for the composition control of molten iron in some domestic converter steelmaking plants. 3) Blast furnace slag with slag content ≤0.5% contains a lot of S and SiO2 _ 2, so it is hoped that the molten iron charged into the converter will have as little slag as possible to reduce the desulfurization task and slag content, and the slag content is usually required to be less than 0.5%. Definition of hot metal pretreatment: the process of desulfurization, dephosphorization or desilication of hot metal before charging into converter is called hot metal pretreatment. Objective: To reduce the burden of blast furnace and converter and improve productivity. 1) The desulfurization conditions of molten iron outside the furnace are better than that of molten steel (high content of carbon, silicon, phosphorus and other elements in molten iron, high activity coefficient of sulfur and low oxygen content in molten iron), and the desulfurization efficiency is 4-6 times higher than that of molten steel, which is more economical than the converter double slag method, so the pre-desulfurization technology of molten iron has been widely adopted at home and abroad. Basic idea: add desulfurizer to molten iron to combine it into slag. (1) Desulfurization agent and its characteristics: At present, there are mainly the following four kinds of hot metal pre-desulfurization agents. ① desulfurization reaction of calcium carbide powder (CaC2): CaC2(S)+[S]=CaS(S)+2[C] Features: strong desulfurization ability, but a small amount of CO and C2H2 escaped during desulfurization, which brought out calcium carbide powder and polluted the environment, so a dust removal device must be installed; The price is more expensive. ② desulfurization reaction of lime powder (CaO): 2cao (s)+[s]+1/2 [si] = cas (s)+1/22cao? SiO _ 2 (S) has the characteristics of low price and low desulfurization cost, but when it is used alone, its desulfurization ability is poor, and C2S will appear on the surface of lime, which hinders the desulfurization reaction from continuing and reduces the desulfurization speed and efficiency. Therefore, a proper amount of aluminum powder or baking soda powder is often added to avoid the generation of C2S: Cao (S)+[S]+2/3 [Al] = CAS (S)+655. ③ Desulfurization reaction of soda powder (Na2CO3): Na2CO3 (L)+[S]+[Si] = Na2S (L)+SiO2 (S)+{CO} Features: The desulfurization ability is very strong, and the generated gas has stirring effect, so the desulfurization speed is fast, but it is expensive and seriously polluted, so it is rarely used now, and sometimes it is formed by mixing with other powders. ④ Desulfurization reaction of metallic magnesium: The boiling point of metallic magnesium is only 1 107℃, and it is a gas at the temperature of molten iron, so the desulfurization reaction is {Mg}+[S]=MgS(S) Features: When metallic magnesium is directly added to molten iron, explosive gasification reaction will occur, which will not only greatly reduce the utilization rate of magnesium, but also cause molten iron to splash, causing accidents. Under the same hot metal conditions, the capacity order of each desulfurizer is Na2CO3, CaC2, Mg and CaO, as shown in Table 9, 1-3. Some of the above desulfurizers can be used alone, but most of them are used in combination, such as calcium carbide powder+lime powder, magnesium metal+calcium carbide powder, lime powder+soda powder, magnesium metal+lime powder, etc. And their desulfurization ability is very different. (2) Method and effect of desulfurization: The basic process of pre-desulfurization of hot metal is to add desulfurizer to hot metal and mix to produce desulfurization reaction. At present, the most widely used methods are mechanical stirring and injection. (1) mechanical stirring and mixing method: add the desulfurizer into the ladle, insert a stirrer made of refractory material into the molten iron and stir it to fully mix it with the desulfurizer. Features: The desulfurization effect is related to the agitator speed and the type of desulfurizer, as shown in Figures (10), 1-3 and 1-4. There are many forms of this method, and the representative one is the Japanese KR method (mainly calcium carbide powder) introduced by WISCO No.2 Smelter in 1979. After digestion and transformation, the desulfurizer mainly made of lime powder is used. (2) Jet mixing method: using air or inert gas as carrier, spraying powdered desulfurizer into molten iron with a spray gun, so that molten iron and desulfurizer are fully mixed. In 1980s, Baosteel introduced DTS method to inject calcium carbide powder from Japan. See figure 1-6 for the application effect of various desulfurizers in spray method. In actual production, each factory should choose the most suitable desulfurizer and desulfurization method according to the required desulfurization degree, hot metal heat loss and iron loss, desulfurization equipment cost and environmental pollution. 2) The basic idea of pre-desilication of hot metal: adding oxidation desilication agent to hot metal to make it oxidized into SiO2 _ 2 and enter the slag. (1) desilication agent: The commonly used desilication agents are mainly iron oxide scale and sintered mineral powder, and a small amount of lime and fluorite are added to reduce the viscosity of slag. The proportion used by various manufacturers is not exactly the same: Fukuyama, Japan? Mill scale 70- 100%, lime 0-20%, fluorite 0-10%; Japan Water Island? 75% sintered mineral powder and 25% lime. The dosage of desilication agent is about 15 ~ 30k g/t(2) desilication method: There are two commonly used methods for desilication outside the furnace: input method and top spraying method. The input method is to put desiliconizing agent into the iron ditch when the blast furnace is tapping, and make full mixing and reaction by using the impact and stirring effect of molten iron flowing into the ladle. This is the earliest desilication method, and its efficiency is low, generally around 50%. Top-spraying method is to inject desiliconizing agent from a certain height above the liquid level of molten iron (iron ditch or molten iron pouring into molten iron) by using air with pressure of 0.2 ~ 0.3 MPa for mixing reaction. Because the hot metal and desilication agent are mixed twice in this way, the desilication efficiency is as high as 70 ~ 80%, and the silicon content of hot metal can reach below 0. 1 ~ 0. 15%. 3) The dephosphorization efficiency of molten iron pre-dephosphorization converter is high, especially for double slag smelting, but it will increase the consumption of slag-making materials, prolong the smelting time and increase the production cost. In recent years, great progress has been made in the research of dephosphorization of hot metal outside the furnace and it has been used in industrial production. Basic idea: add dephosphorizing agent to molten iron to oxidize phosphorus in fixed slag. (1) dephosphorizing agent: At present, soda ash system and lime system are widely used. Soda dephosphorizing agent: 2[P]+5[O]+3Na2CO3(S)=(Na2O? P2O5)+3{CO} lime dephosphorizing agent: 2[P]+5[O]+4CaO(S)=(4 CaO? P2O5), which is often equipped with a certain amount of iron oxide or sintered mineral powder and fluorite powder flux. (2) Desilication: Because the affinity between phosphorus and oxygen is less than that between silicon and oxygen, and molten iron always contains some silicon, desiliconization is needed before dephosphorization. When dephosphorizing with soda system, [Si] is required to be less than 0. 1%, and when treating with lime system, [Si] is required to be less than 0. 15%. (3) Method and effect of dephosphorization of hot metal outside the furnace: At present, the main method of dephosphorization of hot metal is injection method, which uses gas as carrier to inject dephosphorization agent into the ladle to make it fully mixed and dephosphorize quickly. Nippon Steel injected 45kg/t argon for 20min minutes, and the dephosphorization rate reached about 90%. 3) simultaneous desulfurization and dephosphorization of hot metal As can be seen from the above, soda ash and lime are both desulfurization and dephosphorization agents, so simultaneous desulfurization and dephosphorization of hot metal is not only low in cost, but also high in productivity. At present, there are two simultaneous stripping processes that have been applied in industry. (1)SARP process: Sumitomo's alkaline refining process, in which molten iron is desiliconized first, slag is removed when [Si] is less than 0. 1%, and then baking soda powder is injected at19 kg/t. The desulfurization rate and dephosphorization rate can reach 96% and 95%. This method is characterized by high desulfurization efficiency, but high treatment cost, serious erosion of refractory materials and gas (CO) pollution. (2)ORP method: desilication is also carried out first. When [Si] is less than 0. 15%, slag is removed, and then 52kg/t lime-based powder is injected. The desulfurization rate and dephosphorization rate can reach 80% and 88% respectively. This method is characterized by low treatment cost, but large amount of slag and high iron loss (TFE = 20 ~ 30%). 1. 1.2 scrap steel 1 function: scrap steel is another metal charge for converter steelmaking, and its function is to cool the molten pool. In oxygen top-blown converter steelmaking, the physical and chemical heat of the main raw material molten iron is enough to heat the bath temperature from 1250℃ ~ 1300℃ to the steelmaking temperature 1600℃, and there is residual heat. Scrap steel is used to consume the excess heat to adjust the temperature of the molten pool. 2. Requirements (1) are clean and free of rust, impurities and non-ferrous metals; (2) The maximum length shall not exceed half of the diameter of the furnace mouth, and the maximum cross-sectional area shall not exceed one-fifth of the area of the furnace mouth. 3 Processing and preheating of scrap steel 1) Processing of scrap steel Most of the scrap steel used in converter steelmaking is purchased scrap steel. It has a wide range of sources, different sizes and shapes, and many of them are mixed. According to the characteristics of outsourced scrap steel, corresponding processing treatments such as cutting, packaging, incineration, sorting and washing are carried out to meet the basic requirements of converter steelmaking for scrap steel. 2) The purpose of scrap preheating is to increase the scrap ratio (see table 17, 1-8) and reduce the production cost. Method and effect: Using the residual heat of ladle and fuel combustion for heating. (Shougang) put scrap steel into a ladle, roast it in a gas roasting furnace for 30-40 min, and then pour it into a converter together with molten iron. The scrap ratio increased by 10%. 1. 1.3 ferroalloy function: deoxidizer, alloying agent. Type: mainly Fe-Si, Fe-Mn, Mn-Si, Al, and possibly Fe-Cr, Fe-W, Fe-Mo, Ni and other alloys. Requirements: accurate composition, appropriate block size (5 ~ 40 mm), and baking before use. Thinking about the problem 1 Briefly describe the development trend of oxygen converter. What are the requirements for molten iron in converter steelmaking? 3 What are the commonly used desulfurizers? How about their desulfurization ability? What are the pre-desulfurization methods for hot metal outside the furnace? What are the factors that affect the desulfurization effect of mechanical stirring? 5. Briefly describe the simultaneous desulfurization and dephosphorization process of SARP. 6 What requirements should be met by lime for steelmaking? 2.2 bottom blowing gas jet 2.2. 1 behavior of bottom blowing gas; metallurgists such as Sen Yimei blow water or mercury from the bottom with nitrogen, and take photos of its outflow with a high-speed camera. It is found that there are two behaviors when the gas flows out of the submerged nozzle: (1) When the bubbling speed is low, the gas bulges at the outlet of the nozzle to form bubbles and grows up gradually. When the bubbles grow to a certain extent (buoyancy is greater than), (2) when the jet velocity is higher, the gas forms a continuous airflow on the nozzle and shoots into the liquid. This phenomenon is called submerged jet. It is found that when the apparent Mach number Ma/ calculated from the flow rate increases to above 1, the gas flowing out of the nozzle changes from bubbling to spraying, that is, the speed of the apparent Mach number Ma/ equal to 1 is the critical flow rate, as shown in Figure 2- 12. The apparent Mach number Ma/ is calculated by Formula 2-9: Ma/= nu/a = q/aa? υ-gas outlet velocity, m/s; A—— the speed of sound at room temperature, m/s; A—— nozzle cross-sectional area, m2; Q—— gas flow, m3/s