Introduction to stainless steel smelting process technology

Stainless steel has many excellent properties, beautiful appearance, long service life, and can be 100% recycled, so it is widely used.

The advancement of stainless steel smelting process technology, especially since the 1960s, the use of off-furnace refining and continuous ingot technology has further promoted the rapid development of stainless steel, improved the quality of stainless steel, increased the yield rate, and reduced production costs.

China began to produce stainless steel in the early 1950s. At that time, it was smelted in a 3t electric arc furnace without external refining equipment and continuous casting. The annual output was only a few hundred tons to several thousand tons. The quality, variety and cost could not meet the requirements. Since the 1980s, Taiyuan Iron and Steel Corporation and the Iron and Steel Research Institute have taken the lead in developing the refining technology outside the AOD furnace in China. Subsequently, each special steel plant successively installed 18-40tAOD and 15-60tVOD refining devices, and the refining ratio was also significantly improved. Prior to 1985, the ingots of stainless steel in China had always been outdated. After the TISCO 1280 vertical continuous casting machine was put into operation in 1985, the slab and billet continuous casting machines of the Shanghai No. 3 Steel Plant, Chongqing Special Steel, and the Shanghai No. 5 Steel Plant were successively put into production. In 1989, the horizontal continuous casting machine for stainless steel tube billets of the Chengdu Seamless Steel Tube Plant was put into operation, ending the history of China's stainless steel ingot completely using die casting. However, due to various reasons, the annual output of stainless steel in China has been hovering around 300,000 tons, and the slow progress of stainless steel smelting technology has severely restricted the development of stainless steel in China. Since 2000, the state has adjusted the structure of the iron and steel industry and adopted a series of policies and measures to encourage the development of the stainless steel industry, so that China's stainless steel production has achieved leapfrog development, from 350,000 tons in 1999 to 1.778 million tons in 2003. With an average annual growth rate of about 50%, it has become the world's fourth largest producer of stainless steelmaking.

Development of oven equipment and technology

Oven is the main equipment for refining stainless steel. At present, there are about 155 to 175 tAOD furnaces in the world, of which 1/2 is in the stainless steel plant and the rest is in the foundry. AOD refined stainless steel output accounts for more than 80% of the world's stainless steel.

Since the first 18t AOD furnace in China was put into production in September 1983, there are currently more than 20 AOD furnaces of 1-40t, including 8 AOD furnaces above 18t (including TISCO 3x40t, Dalian 1x40t, Pudong 1x30t, Shanghai Steel No. 1 Plant 1x18t , Great Wall 118t and Yida lx18t, etc.). In 2004, the commissioning of the 60tAOD furnace at Shanghai No. 5 Steel Plant and the 120tAOD furnace at Shanghai No. 1 Steel Plant significantly improved the equipment level of China's AOD furnace.

Yantai Steel is the earliest enterprise in China to produce stainless steel with AOD furnace. At present, AOD furnace has accumulated more than 2 million tons of stainless steel and has accumulated a lot of experience. In order to further expand the production capacity, TISCO implemented two technical transformations on the 18tAOD furnace. After the first transformation, the AOD furnace capacity was expanded from 18t to 40t, and the production capacity was increased from 160,000t to 400,000t. The second transformation was implemented in 2004. The furnace capacity was further expanded to 45t. The addition of a top-blown oxygen lance shortened the smelting time. The introduction of the voestalpine expert automation control system improved the smelting control accuracy. The ammonia consumption was reduced and the dust removal was increased. The dust removal ability of the fan improves the environmental quality. After two transformations, TISCO AOD furnace equipment has reached the international advanced level. In addition, TISCO also plans to implement a yen loan environmental protection project. Six 18t chemical steel electric furnaces will be transformed into a 90t ultra-high-power chemical steel electric furnace, which not only eliminates the slow production bottleneck of chemical steel, but also further increases production capacity. By 500,000 tons, the environmental quality has been greatly improved, and real clean production has been achieved.

In recent years, the operating technology of China's AOD production has made significant achievements. The main progress is as follows:

(1) Improvement of furnace lining life

The lining life of the furnace is the main technical and economic indicator of AOD production. After years of technical research, especially in improving the operating process (for example, reducing the silicon content in the electric furnace tapping, improving the AOD slagging system, the alkalinity in the decarbonization period is A lot of work has been done in increasing the 0.5 to 1.0, the alkalinity in the reduction period from 0.8-1 to 2.0-3.5, and improving the furnace building process by using high-quality refractory materials. The life of AOD linings has generally improved. Taigang AOD The average life of the furnace lining reaches 110 times, the highest is 130 times, and the average life of the refractory produced by TISCO is 92 times.

) Improvement of desulfurization process

Most of China's AOD furnaces adopt single slag blowing process. In order to reduce the S content in steel, the rapid desulfurization process is adopted, and the alkalinity in the slag during refining is controlled to about 2.5. Sica is added to the ladle to desulfurize by strong stirring during tapping, so that an appropriate amount of calcium remains in the steel. After improving the AOD process, the desulfurization rate is above 70%. At present, the sulfur content in Taigang stainless steel is stable below 0.005%, with an average of 0.0034%.

NStainless steel smelting

的 There are two main ways for nitrogen alloying in N-containing stainless steel: one is to add alloys such as manganese nitride and chromium nitride, and the other is to directly alloy with nitrogen, which has lower production costs. The AOD furnace can be directly alloyed with nitrogen, so smelting stainless steel has great advantages. TISCO applied the theory of nitrogen dissolution and removal in stainless steel in the 18t and 40t AOD furnaces, and established a nitrogen alloying process model. In the smelting process, the nitrogen content in the finished product can be controlled more accurately without the need to analyze the nitrogen content in the steel. Control precision ± 0.0135%. At present, TISCO has applied this model to mass-produce OCr19Ni9N, OCr19Ni9NbN, 1Cr17Mn6Ni5N, 00Cr18Ni5Mo3Si2N, and 00Cr22Ni5Mo3N and other nitrogen-containing stainless steels in batches using the direct nitrogen alloying method. In 2003, the production of 35,000 tons of various types of stainless steel containing N, achieved significant economic benefits.

 Usage of dust removal

The amount of dust during furnace smelting is 0.7% to 1.0% of steel output. Generally, AOD dust contains Cr2O315%, NiO4%, CaO26%, Fe27%, MgO15% and other substances. The particle size of the dust is ≤20μm, and the Cr2O3 and NiO in the dust. It is a precious metal oxide. If not recycled, it will not only cause waste of resources, but also pollute the environment. Therefore, how to recover Cr and Ni in the AOD dust is an important issue for each stainless steel plant.

The recovery process adopted by TISCO after research is to mix and shape the dust with the sic amount required for the reduction of oxides. After drying at 200 ° C, it is sent to an intermediate frequency induction furnace for melting reduction and cast into a high-C nickel-chromium alloy (Cr13% -Ni6%). ), And then sent it back to the electric furnace for smelting. The AOD furnace dust recovered by this method has achieved good economic benefits.

progress Progress of smelting stainless steel technology in hot metal

China is a developing country. The accumulation of scrap steel, especially stainless steel scrap, is very small. Therefore, the development of smelting stainless steel with all or part of the molten iron is the goal of Chinese stainless steel plants. There are 3 plants in the world using molten iron to smelt stainless steel, including Japan's Nippon Steel Hachiman Plant, JFE Group Kawasaki Chiba Plant and Brazil Acesita Plant. China Taigang established a three-step production process with molten iron as the main raw material in 2003, that is, molten iron pretreatment desiliconization and dephosphorization -30tUHP electric furnace pre-melting alloy-75tK-OBM-S double blowing converter decarburization-80tVOD refining 80tLF furnace refining —Vertical curved square slab continuous casting machine. Its key equipment for hot metal pretreatment was imported from Japan's JFE Group Kawasaki, 75tK-B0M-S converter was imported from voestalpine, and 80tVOD was imported from Danieli. The production line is designed to produce 55t of stainless steel.

 The converter has the following characteristics for smelting stainless steel:

 The raw materials are flexible. EAF can be used to premelt alloys (15-30t) + partially dephosphorized molten iron (30-50t), or all dephosphorized molten iron (65t) can be used to smelt stainless steel.

The large capacity of the converter can ensure smooth operation without splashing;

 The top gun with gun position control can obtain high decarbonization speed and CO post-blowing operation to ensure the thermal energy requirements in the smelting operation;

The cooling gas flow and pressure at the bottom tuyere can be individually controlled to ensure optimal cooling and prolong the service life of the tuyere;

Using VAI-CON-TEMP continuous temperature measurement device;

Adopt pneumatic slag stopper and IRIS slag measuring system to reduce the excessive slag amount during tapping to ensure the VOD operation in the next process.

Adopt advanced LevelII automatic control system and software model to ensure full-automatic control in converter smelting stainless steel and improve stainless steel quality.

After one year of practice, TISCO has basically mastered the K-OBM-S converter stainless steel process and achieved the following results:

Liaoyuan has a designed output of 350,000 t / a. Since it was put into production in April 2003, it produced 265,500 tons of stainless steel that year. In 2004, a K-OBNI-S converter shell replacement car was installed, which shortened the furnace shell replacement time. After the transformation, the designed output of stainless steel is 550,000 t / a.

The life of the furnace lining has been continuously improved, from about 200 times at the beginning to nearly 700 times at present, and the life of the furnace bottom has also been continuously improved, from 50 times at the beginning to nearly 300 times at present (each furnace lining is replaced 2-3 times) bottom).

The consumption of tritium has been greatly reduced. The original design for smelting 304 stainless steel was 8m3 / t. Now the whole process of the bottom tuyere uses N2 generation of Ar, which has achieved obvious economic benefits.

(2) The process of melting the alloy in the electric furnace was canceled, and the 100% dephosphorized molten iron was implemented. The ferrochrome was added to the converter to smelt 400 series stainless steel in the two-step production process, which greatly reduced the production cost.

Since stainless steel is smelted with de-P molten iron, the P content in steel is lower than stainless steel smelted with scrap steel as the main raw material. As, Su, Cu and other harmful elements in steel are also greatly reduced.

The first steel plant of Baosteel Group's Shanggang No. 1 Plant has an annual output of 720,000 tons, and a stainless steel production line with hot metal and chrome hot metal as the main raw materials has been put into operation. Its process route is pretreatment of molten iron-100tUHP electric furnace premelted chrome hot metal and alloy-120tVOD refining-120tVOD refining-continuous casting. The converter of this production line uses AOD-L. The commissioning of this production line has further improved China's technical level of smelting stainless steel with molten iron.

Progress of continuous casting technology

China's stainless steel continuous casting technology started late. Until 1985, TISCO put into operation the first vertical slab stainless steel continuous casting machine in China. Because this continuous casting machine uses a single furnace continuous casting offline cutting method, the ladle capacity is only 18t, and the maximum annual output is only 58,000 tons, which cannot achieve full continuous casting production. In 1999, TISCO decided to implement technical transformation of this continuous casting machine. The purpose of transformation is to increase the capacity of ladle and tundish, adopt multi-furnace continuous casting by using online cutting method, change the secondary cooling method, and improve the quality of slab. In 2004, it was decided to implement the second technical transformation of this continuous casting machine. The purpose of this transformation is to increase the thickness of the slab to 200mm, improve the quality of the slab, and reduce the grinding loss of the slab. Its main content is to change the crystallizer vibration mode to DYNAFLEX hydraulic vibration, the second cooling system implements dynamic cooling (DYNACS), installs the VAI-Q quality control expert system, and adds automatic granular slag adding device and casting printer.

Transform After transformation, TISCO has made the following progress in continuous casting process:

Stainless steel continuous casting ratio is significantly improved. The continuous casting ratio of the vertical continuous caster increased from 51.4% in 1999 to 97.5% in 2003.

The number of continuous casting furnaces increased significantly. For 304 steel, the number of continuous casting furnaces is more than 6 furnaces, and the yield of continuous casting slabs has increased by 2.37 percentages, with an average of 96.97%.