Technical progress of refractory materials for steelmaking converters

The converter is an important equipment for steelmaking, which does not require an external heat source and mainly uses hot pig iron, scrap steel, and ferroalloys as raw materials. According to the properties of refractory materials in the furnace lining, it is divided into acidic converter and alkaline converter. According to the location of gas blowing into the furnace, it is divided into bottom blown converter, top blown converter, side blown converter, and top bottom composite blowing converter.

The history of converter steelmaking is long. As early as 1856, the British inventor Bessemer invented the bottom blown acidic converter, marking the beginning of modern steelmaking. In 1879, Thomas alkaline converter appeared again, which could process high phosphorus pig iron. In 1864, Frenchman Martin used the principle of heat storage to establish the open hearth steelmaking method, which once became the world's main steelmaking equipment. In the 1950s, about 85% of the world's crude steel was smelted by the open hearth. In 1952, Austria introduced a pure oxygen top blown converter, which solved the problem of nitrogen and other harmful impurities in steel, while reducing the heat loss with exhaust gas. It can blow pig iron at lower temperatures, save coke consumption in blast furnaces, and use more scrap steel. The steelmaking speed is particularly fast (about 10 minutes for one furnace of steel, compared to 7 hours for open furnaces). In 1968, an oxygen bottom blown converter was introduced, In 1970, the bottom blowing method of a double-layer casing type bottom blowing oxygen gun protected by hydrocarbons was invented. Various types of bottom blowing methods (such as OBM, Q-TOP, LSW, etc.) have shown many advantages over top blowing in actual production. Austrians studied the top and bottom combined blowing steelmaking process in 1973 and put it into production in the early 1980s. It is superior to top and bottom blowing, and some countries have basically phased out pure top blown converters.

In China, the air side blown converter steelmaking method test at Tangshan Steel Plant was successful in 1951, and it was put into operation in 1952. In 1954, small-scale oxygen top blown converters were successfully tested. In 1962, Shougang changed the air side blown converter to a 3T oxygen top blown converter, and in 1964, two 30T top blown oxygen converters were put into operation. With the renovation of 3.5-5T top blown oxygen converters by steel plants in Tangshan, Shanghai, Hangzhou, and other places, some large steel enterprises, such as Ansteel Wuhan Iron and Steel Group changed the open hearth furnace to a 250-300 ton oxygen top blown converter, which was put into operation in 1985. Baosteel, which has advanced technology, also used a 300 ton large oxygen top blown converter for steelmaking. Since then, China has basically phased out open hearth steelmaking. In recent years, there have been advantages such as top and bottom blowing combined converter and sliding plate slag blocking technology, which shorten smelting time, make steel composition and temperature more uniform, reduce inclusions in steel, and improve steel cleanliness. Some steel mills in China also commonly use top and bottom combined blowing technology. According to statistics, converter steel accounts for over 70% of the world's total crude steel production, while China accounts for 90%.

The converter is basically made of refractory materials, and the cost of refractory materials used in steelmaking accounts for about 9% to 13% of the cost of the steelmaking process. Due to the different compositions of molten iron and refractory raw material resources in different countries, the selection of refractory materials for furnace linings also has different focuses. Developed industrial countries such as Europe, America, and Russia have developed from dolomite and magnesium products to the current magnesium carbon bricks. Japan is the country that first used magnesium carbon bricks. In China, the lining of the converter has been made of tar dolomite, tar magnesium, magnesium dolomite, and magnesium dolomite carbon bricks, and now magnesium carbon bricks are widely used. Due to the corrosion resistance, thermal shock resistance, wear resistance, good thermal conductivity, anti stripping, and stable performance at high temperatures, magnesia carbon bricks are widely used as converter linings in various countries. The performance and quality of refractory materials directly affect the steelmaking method and service life of converters, and are of great significance in improving product quality and economic benefits. The issue of refractory materials used in converters has attracted people's attention, and new technologies for refractory materials have emerged, such as spray repair technology, slag splashing protection technology, sliding nozzle slag blocking technology, etc.

Structure of converter lining and damage mechanism of refractory materials

The lining structure can be divided into parts such as furnace bottom, melt pool, furnace wall, furnace cap, slag line, ear shaft, furnace mouth, tapping hole, bottom blowing gas supply brick, etc. The lining of the converter consists of an insulation layer, a permanent layer, and a working layer. The insulation layer is generally built with asbestos board or polycrystalline refractory fiber, and now some use magnesium sand filling layer instead. The furnace cap insulation layer is made of resin magnesium sand; Different parts of the permanent layer are used, and low grade magnesia carbon bricks, tar dolomite bricks, or sintered magnesia bricks are often used for masonry; The working layer is entirely constructed with magnesia carbon bricks. There are three grades of magnesia carbon bricks in our country. The performance requirements of magnesia carbon bricks used in different parts of the converter are different. Shinagawa Refractory Company in Japan has developed a series of magnesia carbon brick products that meet the requirements of different parts, as shown in Table 1. In the production process of the converter, due to the direct mechanical impact and wear of the working layer caused by adding scrap steel, direct contact with high-temperature molten steel slag, infiltration and erosion of high-temperature slag and molten steel, as well as erosion by molten steel, slag and furnace gas, the working conditions are very harsh, and the working conditions of each part are different, and the degree of damage is also different. During use, it is generally necessary to repeatedly remove the working layer, and if the permanent layer is relatively intact, it should not be replaced.

Technological progress in the application of refractory materials in converter 2

Furnace age is an important technical indicator for converter steelmaking. Improving furnace life not only reduces production costs, but also improves production efficiency. The refractory material of the converter lining is the main factor affecting the furnace life. In recent years, through the development and research of scientific and technological personnel, the service life of the lining has been significantly improved, and the unit consumption of refractory material has been reduced to 2-0.38kg/t steel. The specific measures are as follows.

Table 1 Performance of Series Magnesium Carbon Bricks

2.1 Comprehensive balanced furnace construction

Based on the different erosion conditions in different parts of the furnace lining, 3-6 different grades of magnesia carbon bricks are selected and built on slag lines, ear shafts, furnace walls, and molten pools respectively, in order to achieve the best economic benefits. The most severely eroded area is the brick with the best masonry quality. The inner lining is constructed using the dry method of magnesia carbon bricks, or using clay to build some magnesia dolomite bricks containing carbon. These bricks have varying degrees of expansion at high temperatures and do not need to be built too tightly. The working conditions of each part are roughly as follows:

(1) The temperature change at the furnace mouth is drastic, the slag and high-temperature furnace gas are severely eroded, and the furnace mouth is impacted during feeding and cleaning of residual steel residue. Therefore, it is required that magnesium carbon bricks have high thermal shock resistance and slag resistance, and are not easy to stick slag;

(2) The furnace cap is most severely corroded by slag, and is also affected by sudden temperature changes and exhaust gas erosion. Magnesia carbon bricks with good thermal shock stability and slag resistance should be used;

(3) During the blowing process, the lining on the loading side of the furnace is not only subjected to erosion and chemical erosion by molten steel and slag, but also to impact and erosion by the loading of scrap steel and the addition of molten iron. It is required to build magnesia carbon bricks with high slag resistance, high strength, and high thermal shock resistance;

(4) The lining on the tapping side is mainly affected by the thermal shock and erosion of the molten steel during tapping, and the damage speed is lower than that on the loading side. If the same magnesium carbon bricks are used on both sides, it can be slightly thinner;

(5) During the blowing process, the slag line is in long-term contact with the slag, especially on the slag discharge side, which is strongly eroded by the slag. The furnace lining is severely damaged, and magnesium carbon bricks with good slag resistance should be used;

(6) There is no protective slag covering layer on the surface of the lining in the ear shaft area, and the carbon in the magnesia carbon brick is easily oxidized and difficult to repair. Therefore, it is necessary to build high-grade magnesia carbon bricks with good slag resistance and strong oxidation resistance;

(7) Although the molten pool and furnace bottom are eroded by molten steel, they are less damaged compared to other parts and can be used to build magnesium carbon bricks with lower carbon content. If it is a top and bottom combined blowing converter, the central part of the furnace bottom is prone to damage, and magnesium carbon bricks of the same quality as the charging side can be built.

2.2 Improving the quality of magnesia carbon bricks

As mentioned earlier, magnesia carbon bricks have excellent properties such as thermal shock resistance and corrosion resistance, and are widely used in converter linings. However, during use, it was found that due to the easy oxidation of magnesia carbon bricks, their resistance to thermal shock and corrosion decreased. Researchers have explored a new type of antioxidant self-healing composite antioxidant: using metal Al, Si powder, or Al Si composite powder as antioxidants, high corrosion-resistant phases such as SiC and AIN are generated in situ during heat treatment or high-temperature service; Another approach is to add various pre synthesized nanocarbons, such as carbon black and nano graphite oxide composite powders, and select suitable transition elements (Fe, Co, Ni) inorganic or organic compounds as catalysts. Phenolic resin is cracked to produce gases such as CO, C2H2, CH4, etc. Under the catalysis of transition metals, low dimensional graphitized carbon such as carbon nanotubes and nano carbon fibers are formed. Through these technologies, magnesia carbon bricks maintain good corrosion resistance and thermal shock resistance.

2.3 Development of amorphous refractory materials and repair of furnace lining

In order to improve the service life of the converter lining, local damaged parts should be repaired at any time. The application of new amorphous refractory materials and repair techniques has significantly improved the service life of the converter lining, generally exceeding 8000 heats, and some exceeding 20000 heats. At present, large-scale hot repair is commonly used to maintain the feeding side, furnace bottom, and tapping side. Spray repair materials are used for the rounded corners and ear shafts of the melt pool, and grouting materials are used to fill the gaps and maintain the steel outlet area during the replacement of the converter steel outlet. The main types of surface repair materials include MgO-SiO2 (also known as water-based large fabrics), MgO-C, MgO-CaO, etc; Spray repair materials mainly include MgO, MgO-CaO, MgO-Cr2O3, etc. These amorphous refractory materials are divided into anhydrous repair materials (mainly asphalt, coal tar, asphalt powder, resin, etc.) and water-based repair materials (MgO-SiO2-H2O binding and phosphate binding) according to the binder. The types and performance characteristics of repair materials are shown in Table 2.

Table 2 Types and Performance Characteristics of High Temperature Repair Materials

Using organic compounds such as coal tar, asphalt, and resin as binders can cause environmental pollution, long sintering time, and poor corrosion resistance. Qin Yan et al. developed a carbon free and environmentally friendly large area repair material using high-purity magnesium sand powder (MgO97.02%) and medium grade magnesium sand (MgO94.80%) particles as the main raw materials, and SiO2 ultrafine powder (SiO2=97.02%) as the binder. This water-based large-area repair material adopts a wet direct current casting method, which has good high-temperature spreading ability. During the high-temperature sintering process, no harmful gases are generated, forming ceramic bonding. The sintering time is shortened by more than 50%, and the structure is dense, anti-oxidation, and erosion resistant. After multiple uses, there is no smoke or dust on site, and the service life is extended by 2-3 times. The specific ingredients of the large surface repair material are shown in Table 3.

For easily damaged parts such as ear shafts and debris lines, spray repair methods are used. At present, magnesium based spray repair materials are widely used in China, with a relatively low service life. Someone has developed magnesium carbon spray repair materials, mainly consisting of 3-0mm magnesium sand (MgO95.2%), 3-0mm carbon (C=94.2%), asphalt A (fixed C=46.3%, softening point 140-160 ℃), asphalt B (fixed C=43.5%, softening point 100-120 ℃), and additives. After practical use, it has been found that its rebound rate is low, adhesion is good, sintering strength is high, and its service life is 30% longer than that of magnesium based spray repair materials, reducing the number of converter repairs.

Table 3 Particle Ratio of Large Surface Repair Material (w%)

There is also the iron block slag furnace repair technology, which utilizes the rapid heat exchange between iron blocks and high-temperature liquid slag, and efficient cold solid bonding to repair local weak areas, saving the cost of furnace repair materials. The slag retention operation saves the consumption of slag making materials, reduces and slows down the erosion of the furnace lining, improves the metal recovery rate, and extends the service life of the furnace lining. Fujian San'an Iron and Steel Company controls parameters such as slag alkalinity, MgO content, and FeO content. After tapping, an appropriate amount of pig iron blocks are used to accelerate slag cooling, allowing the mixed pig iron blocks to condense and bond in the impact area of converter scrap and molten iron, achieving the purpose of furnace repair. This method can use production gaps for furnace repair, which only takes 10 minutes at a time, saving more than 1 hour compared to conventional furnace repair, and is very environmentally friendly.

In order to solve the problem of slag sticking at the furnace mouth and furnace cap, a type of anti sticking slag spraying material is adopted, which mainly uses recycled magnesia carbon bricks as the main raw material. Through semi dry spraying construction, the spraying layer thickness is 35-50mm, and the adhesion rate is above 80%, achieving good usage effect.

Shinagawa Refractory Company in Japan has developed a MgO-C spray repair material that can rapidly harden (see Table 4), significantly reducing the hardening time.

Table 4 Performance of MgO-C spray repair material with rapid hardening

2.4 Furnace protection

Good operation can protect the furnace lining. In the 1990s, the United States successfully developed the technology of splashing slag to protect the converter. After steelmaking is completed, the liquid slag in the furnace is not poured out, and some lightly burned magnesium balls or dolomite materials are added to the furnace to increase the melting point and viscosity of the slag. Nitrogen gas is released into the furnace through an oxygen gun, causing the liquid slag to splash onto the furnace lining because the slag contains MgO, which is beneficial to the furnace lining. After promoting slag splashing protection in China, the furnace life has increased by 3-4 times, generally above 20000 heats. The consumption of refractory materials per ton of steel has decreased by 0.2-1.0kg, the consumption of supplementary materials has decreased by 0.5-1.0kg/t, and the utilization coefficient of the converter has increased by 2% to 3%. The cost of splashing slag to protect the furnace is lower than that of repairing it, but the usage effect is not as good as repairing it, and it is also unstable.

2.5 Improving the lifespan of gas supply components in top and bottom combined blowing converters

By blowing and stirring the molten pool at the bottom, the steel yield and quality can be improved. Gas supply components are divided into two categories: nozzle type and brick type. Among them, brick type is the mainstream application, which mainly includes dispersed type, circumferential seam type, and through hole type. The through-hole type has the advantages of low gas supply resistance, large air flow regulation range, good airtightness, less air leakage, and the metal pipe has a reinforcing effect on bricks, making them less prone to peeling and cracking, making it the mainstream of development. Luonai Institute uses high-purity magnesium sand and flake graphite as the main raw materials, adds Al, Si powder and B4C antioxidant, uses thermosetting asphalt modified resin as the binder, adds an appropriate amount of asphalt powder as the ingredient, mixes the mud evenly, and uses isostatic pressing to prepare MgO-C gas supply components. The use result is that the erosion rate is 0.28mm per furnace, and the maximum service life is 2113 furnaces. In order to reduce the carburization rate of stainless steel pipes and improve their service life, a coating larger than 1mm is applied on the surface to form a dense protective isolation layer that is thermally stable and resistant to carbon reduction on the stainless steel surface.

Shinagawa Refractory Company in Japan has developed breathable bricks with high fracture toughness and excellent thermal shock resistance, as shown in Table 5. When used on a 220t converter, the loss rate is reduced by about 40% compared to traditional breathable bricks. The converter has been running for 4000 cycles without replacing the breathable bricks.

Table 5 Performance of converter gas supply components

The steelmaking plant of Xuangang Group adopts metal tube type magnesium carbon breathable bricks, with 30 stainless steel pipes embedded in each brick, which are stamped and formed using a 300t brick press. Breathable bricks are used to blow N2 and Ar gas into the lower part of the 80t combined blowing converter, achieving a converter life of 5000 heats. The supply of gas in the lower part is synchronized with the furnace life, striving to synchronize with 10000 heats.

2.6 Using skateboard slag blocking technology to improve the quality of skateboards

Skateboard slag blocking technology is a rapidly developing new technology in recent years. The skateboard slag blocking device is a device that transplants a sliding water gate mechanism system similar to a ladle to the tapping port of the converter, and opens or closes the tapping port through mechanical or hydraulic control to achieve the purpose of slag blocking. The sliding water outlet slag blocking technology has outstanding advantages in improving the yield of molten steel, reducing inclusions in steel, improving the cleanliness of molten steel, reducing slag sticking in the ladle, and extending the service life of the ladle.

The inner water outlet of the steel outlet is connected to the end of the steel outlet brick, the lower is connected to the upper slide plate of the steel outlet, and the outer water outlet is connected to the lower slide plate of the steel outlet. The inner/outer water outlet and skateboard should have good resistance to slag erosion, high temperature oxidation, thermal shock, etc. The skateboard should also have wear resistance. At present, there are mainly non fired magnesium carbon, non fired aluminum zirconia carbon, and zirconia (embedded inner core) products for external water outlets. Their physical and chemical indicators and service life are shown in Table 6. Most non fired water outlets are treated with asphalt impregnation, with a service life of 30-90 heats.

Table 6 Materials, Physical and Chemical Indicators, and Service Life of External Water Outlets

At present, the material, physical and chemical indicators, and service life of domestic slag blocking skateboards are shown in Table 7. The service life is relatively low. Studies have shown that introducing expanded graphite and Si powder can promote the formation of SiC whiskers in bricks, improve the toughness of skateboards, resist crack propagation, improve the thermal shock resistance of skateboards, and extend their service life.

Table 7 Physical and chemical indicators and service life of skateboards made of different materials

The Luoyang Institute summarized the causes of damage to the slag blocking skateboard and found that the upper skateboard was mainly damaged by erosion and hole expansion when in contact with molten steel, while the lower skateboard was mainly damaged by thermal shock crack propagation when in contact with external air. Therefore, by embedding zirconium rings on the upper slide plate and zirconium plates on the lower slide plate, the corrosion resistance and thermal shock resistance of the zirconium ring zirconium plate have been improved, and the service life of the slide plate has been stabilized at around 20 heats. Develop three series of products to meet the needs of different working conditions (see Table 8).

Table 8 Physical and chemical indicators and properties of zirconium rings and zirconium plates for skateboards

The development trend and optimization technology direction of refractory materials for converter 3

The development of converter steelmaking technology has promoted the technological progress of refractory materials, but the service life and usage methods of refractory materials in some parts are still unsatisfactory. The future development trend is: (1) to develop high-performance, wear-resistant, and thermal shock resistant low-carbon magnesia carbon bricks; (2) Develop fast sintering, environmentally friendly hot repair materials without pollution, long-lasting spray repair materials, and good repair methods; (3) Developing flame spray repair, although the large surface water-based semi dry spray repair material has no smoke and dust, it has moisture and is prone to generating steam, leaving hidden dangers for the bonding and use of the working face. However, flame spray repair and the working face are immediately melted and sintered into one, with a short time and long service life; (4) Develop cooling technology for the furnace body to further improve the service life of the furnace lining, splash slag to protect the furnace, increase the furnace life, but reduce the thickness of the furnace lining, which will cause deformation of the furnace shell. Cooling technology can suppress the deformation of the furnace shell, and increase the service life of the furnace shell to 10-15 years; (5) Develop long-life composite blowing gas supply components, such as composite gas supply components, optimize the layout of the bottom tuyere structure, and adapt to advanced technologies such as top and bottom blowing, low oxygen steelmaking, low part powder spraying, low part oxygen supply, and low part CO2 blowing; (6) Improve the performance and structure of slag blocking skateboards, such as composite skateboards, to extend their service life and reduce the number of replacements; (7) Develop lightweight and energy-saving refractory materials with good high-temperature performance; (8) Research and develop auxiliary equipment such as long-life water-cooled smoke hoods and flues.