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With the development of modern science and technology and the improvement of industrial and agricultural requirements for steel quality, steel mills generally adopt out-of-furnace refining process, which has become an indispensable and important link in modern steelmaking process. Because this technology can improve the production capacity of steelmaking equipment, improve the quality of steel, reduce energy consumption, and reduce the consumption of refractories, energy and ferroalloys, external refining technology has become the development direction of steel metallurgy in the world today. It is necessary to discuss the existing problems and development direction of refining technology outside the furnace.

Development course and present situation of 1 external refining technology at home and abroad

With the continuous progress of steelmaking technology, refining outside the furnace has played an important role in modern steel production. The traditional production process (blast furnace → steelmaking furnace (electric furnace or converter) → ingot casting) has been gradually replaced by the new process (blast furnace → hot metal pretreatment → steelmaking furnace → external refining → continuous casting). It has become the main technological process of large iron and steel enterprises at home and abroad, especially in the field of special steel, refining and continuous casting technology is becoming more and more mature. The refining process plays an important role in the whole process. On the one hand, it can improve the purity of steel and remove harmful inclusions, microalloyed and denatured inclusions; On the other hand, refining is also a buffer link, which is beneficial to balanced continuous casting production.

In the 1970s, in order to reduce the cost of steelmaking and improve the purity and quality of steel, Japan took the lead in applying out-of-furnace refining technology to the production of special steel, and then steel enterprises in Western Europe joined in the promotion and use of this technology. According to data reports, as early as 1985, the refining rate in Japan reached 65.9%, and it rose to 73.4% in 1989, and the refining rate of special steel reached 94%. The newly-built electric furnace short-flow steel plant 100% adopts external refining technology. With the rapid development of continuous casting technology in 1980s, the original steelmaking furnace can no longer meet the technical requirements of continuous casting, which further promotes the development of refining technology outside the furnace. By 1990, there were more than 1000 sets of external refining equipment in major industrial countries in the world.

As early as the late 1950s and mid-1960s, some primary smelting technologies were adopted in steelmaking in China, such as desulfurization of bearing steel with high basicity synthetic slag during tapping and static degassing of ladle, but there was no refining equipment. From the mid-1960s to the 1970s, some special steel enterprises (Daye, WISCO, etc. ) introduced a batch of vacuum refining equipment. In 1980s, China's self-developed refining equipment (such as LF furnace, powder spraying and stirring equipment) was gradually put into use, and Heilongjiang Metallurgical Research Institute and other units jointly developed wire feeder, core-covering machine and alloy core wire, which improved the auxiliary process of external refining technology. Now this technology is very mature, and the "trinity" short process technology with out-of-furnace refining technology as the core has been widely used in domestic iron and steel enterprises and achieved good results. Primary smelting (electric furnace or converter) → refining → continuous casting has become a typical short process of modernization.

2 characteristics and functions of refining technology outside the furnace

Extrafurnace refining refers to the process of smelting in ladle by combining vacuum treatment, argon blowing and stirring, heating and temperature control, wire feeding and powder spraying, micro-alloying and other technologies in different forms. Before tapping, the oxide slag is removed as much as possible, and the reduced slag is regenerated in the ladle to maintain the reducing atmosphere in the ladle. The purpose of refining outside the furnace is to reduce the content of elements such as C, P, S, O, H and N in steel, so as to avoid segregation, white spots and large particle inclusions and reduce the tensile strength, toughness, fatigue strength and crack resistance of steel. These tasks are only performed on the refining furnace, and their features and functions are as follows:

1) can change metallurgical reaction conditions. The reaction products of deoxidation, decarbonization and degassing in steelmaking are gases, which can be refined under vacuum conditions, which is beneficial to the forward reaction. Usually the working pressure is ≥50Pa, which is suitable for degassing molten steel.

2) The mass transfer speed of the molten pool can be accelerated. The speed of mass transfer in liquid phase determines the speed of metallurgical reaction. In the refining process, various stirring forms (gas stirring, electromagnetic stirring, mechanical stirring) are used to make the melt flow in the system, accelerate the heat and mass transfer process in the melt, and achieve the purpose of uniform mixing.

3) The reaction area of slag steel can be increased. All kinds of refining equipment have stirring devices, which can emulsify steel slag during the stirring process, and alloy and steel slag will melt, melt and polymerize in the floating process with bubbles. Usually, the reaction area of steel slag for 1 ton molten steel is 0.8 ~1.3mm2. When the slag content is 6% of the original, emulsified steel slag will form slag drops with a radius of 0.3mm, and the reaction interface will increase by 65,438+. Micro-alloying and denaturation treatment use this principle to improve the refining effect.

4) It can play a buffering role between electric furnace (converter) and continuous casting, and the refining furnace is flexible, which makes the operation time and temperature control more coordinated and forms a smoother production process with continuous casting.

3 Application of out-of-furnace refining technology in production At present, the out-of-furnace refining methods are widely recognized and used: LF method, RH method and VOD method.

3. 1 LF method (ladle refining furnace method)

It was invented by Japan Datong Iron and Steel Company at 197 1. Heating with electric arc, stirring with argon blowing at the bottom of ladle.

3. Advantages of1.1process

1) arc heating has high thermal efficiency, wide temperature rise range and temperature control accuracy up to 5℃;

2) it has the functions of stirring and alloying, and argon stirring is easy to control the alloy composition in a narrow range, thus improving the stability of the product;

3) Less equipment investment and low refining cost, which is suitable for producing ultra-low sulfur steel and ultra-low oxygen steel.

3. Key points of production process of1.2lf method

1) heating temperature control LF adopts arc heating, which has high thermal efficiency. The average temperature rise of molten steel at 65438 0℃ consumes 0.5 ~ 0.8 kW·h, and the heating speed of LF depends on the specific power supply (kVA/t), which in turn depends on the melting loss index of ladle refractories. Due to the adoption of submerged arc foam slag technology, the thermal radiation loss of arc is reduced, the thermal efficiency is increased by 10% ~ 15%, and the end temperature accuracy is ≤ 5℃.

2) White slag refining process is adopted. Slag quantity should be controlled at ≤ 5 kg/t, generally Al2O3-Cao-SiO _ 2 slag should be used, and slag alkalinity R≥3 to avoid slag reoxidation. Avoid exposure of molten steel during argon blowing and stirring.

3) Fine adjustment of alloy and control of narrow composition range. According to the test report, the metal recovery rate can be improved by using alloy core wire technology. The average recovery rate of titanium in gear steel is 87.9%, the recovery rate of boron is 64.3%, the recovery rate of carbon wire in ladle is 90%, and the recovery rate of rare earth wire in ZG 30 CrMnmore is 68%. High recovery rate can realize narrow composition control.

3. Application of1.3lf method in production practice

In June 2000, the newly-built continuous casting workshop of Angang No.1 Steelmaking Plant was officially put into operation. The refining equipment consists of two LF ladle refining furnaces, with an annual processing capacity of 2 million tons of molten steel. VD vacuum treatment device with an annual output of 800,000 tons of molten steel. The highest heating rate of LF furnace is 4℃, and the average treatment time of LF furnace is ≤28 minutes. Treatment effect: average [h] ≤ 0.0002%; Minimum value [H]≤0.000 1%.

The typical production process of domestic heavy rail manufacturers (Pangang, Baotou Steel, Angang and WISCO) is LD→LF→VD→WF→CC. After the ladle is hoisted on the ladle car of LF processing line, the argon blowing quick connector at the bottom of the ladle is manually connected, and the input of materials (including feeding line) is determined according to the required molten steel composition and temperature. Therefore, it is very important to improve the carbon content of heavy rail steel. The carbon content of molten steel is controlled at 0.2% ~ 0.3% (wt) when tapping in converter, and raised to 0.60% ~ 0.65% (wt) after tapping. When treating in LF furnace, the upper limit of standard composition is increased by 0. 10% ~ 0. 15% (wt), and the steel composition can be reached after VD treatment.

3.2 RH method (vacuum cycle degassing method) This method was invented by West Germany in 1958. Its basic principle is to use bubbles to continuously lift molten steel into a vacuum chamber for degassing and decarbonization, and then return to the ladle.

3.2. Advantages of1RH method

1) has high reaction speed. Vacuum degassing cycle is short, general degassing operation can be completed within 10 minute, alloying and temperature homogenization can be completed within five minutes, and it can be used with converter.

2) the reaction efficiency is high. The molten steel reacts directly in the vacuum chamber, and the ultra-pure steel can reach [H] ≤ 1.0× 10-6, [N] ≤ 25× 10-6 and [C] ≤10.

3) Oxygen blowing decarburization and secondary combustion heat compensation can be carried out to reduce the temperature drop during refining.

RH process parameters

1)RH cycle. Circulation refers to the amount of steel water passing through the riser or downcomer per unit time, and the unit is t/min. The calculation formula given by relevant data is: q = 0.002× du 1.5 g0.33, where: Q—— circulating flow, t/min; Du—— diameter of riser, cm; G—— argon flow rate in the riser, liter/minute.

2) cyclical factors. He refers to the ratio of molten steel passing through the vacuum chamber to the treatment capacity during RH treatment, and its formula is: μ = W T/V, where μ-cycle coefficient is times; W-circulation, t/min; T- cycle time, minv-ladle capacity, t

3) Relationship between oxygen supply intensity and carbon content. Blowing oxygen into RH can improve the speed of decarbonization, that is, RH-OB method. When [c]/[o] >: is 0.66, the mass transfer speed of oxygen in the ladle determines the decarbonization speed, and its calculation formula is:

Qo2 = 27.3× q [c], where: Qo2- oxygen intensity, Nm3/min；; Q—— molten steel circulation, t/min; [c]- carbon content, nm3/t

3.2.3 Application of RH method in production practice

The bearing steel production line EF-LF-RH-CC is jointly produced by LF and RH in Yang Shan Steel Works, and the total oxygen content in steel reaches 5.8× 10-6. LF-RH method is to heat molten steel with LF furnace first, and then carry out reduction refining with LF stirring and slag refining functions, that is, desulfurization and pre-deoxidation of molten steel, and then send molten steel to RH for dehydrogenation and secondary deoxidation. After this treatment, the cleanliness of molten steel is greatly improved and the temperature of molten steel reaches the temperature required for continuous casting.

Baosteel's external refining equipment includes RH-OB, ladle powder injection device and CAS refining device. The smelting effect of RH-OB is ideal, with dehydrogenation rate of 50% ~ 70% and denitrification rate of 20% ~ 40%. After general RH-OB treatment, [h]≤2.5× 10-6, [c]. The total oxygen content in steel is ≤25× 10-6, and the finished product rate and control accuracy of the alloy can be improved by treating the alloy in RH. The control accuracy of [C], [Si] and [Mn] can reach 0.0 1%, and the accuracy of aluminum can reach 1.5× 10.

3.3 VOD method (vacuum tank ladle deoxidation method)

3.3. Characteristics of1VOD The VOD method was first developed and applied in West Germany in 1965. It puts the ladle into a vacuum tank, blows oxygen into the ladle from the oxygen lance at the top, and blows argon upward from the bottom of the ladle for stirring. This method is suitable for producing ultra-low carbon stainless steel, achieving the purpose of maintaining chromium and removing carbon, and can be used in conjunction with converter. His advantage is that he has realized the necessary thermodynamic and kinetic conditions for smelting low-carbon stainless steel-high temperature, vacuum and stirring.

3.3.2 Application of VOD method in production practice

In the early 1990s, Shanghai Dalong Casting and Forging Plant imported 15tVODC 1 sets of key equipment and technical software from leybold Company in Germany. Ultra-low carbon stainless steel, medium-low alloy steel and carbon steel were refined by VODC external refining process, and good metallurgical results were obtained. The nonmetallic inclusions in steel are reduced, the hydrogen content is less than 3× 10-6, the oxygen content is less than 6.5× 10-6, and the recovery rate of chromium in stainless steel is 98% ~ 99%. Refined steel is very high quality. VODC refining process is mature and easy to control, which can meet the needs of multi-steel and small-tonnage refining production in small and medium-sized steel mills and steel casting plants, and will play a great positive role in developing refining production in steel casting industry, with broad development prospects 10.

Fushun Special Steel Co., Ltd. has a 30tVOD furnace. Using electric arc furnace +VOD technology to refine stainless steel can make [H] ≤ 2.58× 10-6, T [O] ≤ 4 1.9× 10-6, the chromium recovery rate reaches 99.5%, and the desulfurization rate is 64.2%.

The problems to be solved and the development direction of the development of out-of-furnace refining technology have played an important role in improving the purity of steel, accurately controlling the composition content and refining the microstructure, greatly reducing the smelting cost and improving the quality and properties of steel. However, there are also some problems in the development process, which need to be solved to make this technology more perfect.

1) to realize the intelligent control of refining process outside the furnace. According to various technical parameters of molten steel, the best refining process scheme is made by using information technology, and each refining process is controlled by computer. The refining station is equipped with rapid analysis equipment to realize data networking and reduce the waiting time of hot stop.

2) The treatment equipment outside the furnace will be "multifunctional". In Shuigang refining equipment, slag washing refining, vacuum metallurgy, stirring process and heating temperature control functions are all combined to realize refining to meet the social needs of ultra-clean steel production.

3) Develop high-quality basic refractory with high purity, high density and high strength to meet the needs of different refining furnaces and pay attention to the stability of product quality. The use conditions of refractories should be adapted to the slag as much as possible to minimize the erosion rate. According to the actual situation of refining equipment, different levels of supporting materials should be formed, and insulation and repair technologies should be developed to improve the service life of furnace lining.

4) Reduce pollution emission in the process of oil refining. The refining process will produce a lot of waste gas, including SO2, Pb, metal oxides, suspended particles and so on. Vacuum degassing cooling water contains suspended solids, lead, zinc, etc. These pollutants must be treated internally to reduce the pollution level and reach the discharge standard before being discharged, so as to strengthen environmental awareness.

5 concluding remarks

Refining technology outside the furnace is an advanced technology to improve product quality and reduce production cost, and it is an indispensable link in modern steelmaking process. It has metallurgical functions such as precise control of chemical composition and temperature, removal of inclusions, top slag reduction and removal of S and Ca, shape control of inclusions, removal of impurities such as H, O, C and S, and vacuum degassing. Only by strengthening the functions can we give full play to the advantages of refining outside the furnace and produce high-quality pure steel.