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Analysis of types of converter steelmaking and damage mechanism of refractory linings used

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This article describes the types of steelmaking converters and the analysis of the lining damage mechanism of refractory materials.

Keywords: steelmaking converter; refractory materials; damage mechanism

Converter steelmaking is one of the most widely used steelmaking methods in the world. It has become the mainstream steelmaking method because of its high efficiency, short smelting cycle, low steel production cost, and suitability for the smelting of a variety of steel types. According to statistics, converter steelmaking accounts for more than 70% of the world’s total crude steel production. In my country, converter steelmaking accounts for more than 90% of crude steel production. As the country continues to increase its efforts to eliminate backward production capacity, my country’s converters will further develop in the direction of large-scale production.

In recent years, converter steelmaking technology has made great progress, and top-bottom repeated blowing technology and sliding plate slag-blocking technology have appeared successively. In particular, the emergence of top-bottom repeated blowing technology has significantly shortened the smelting time, made the composition and temperature of the molten steel more uniform, and at the same time reduced the content of S, P, and N elements in the molten steel, and improved the metal yield. However, as the re-blowing ratio of the converter increases, the age of the converter decreases significantly. In order to increase the age of the converter, there are methods of regularly repairing the front and rear surfaces with self-flowing repair materials, and gunning repairs on the trunnions and other parts. If this maintenance method is used and timely gunning is used, the service life of the furnace lining can be reached more than 8,000 times. The method of sputtering slag to protect the furnace is often used, that is, adding some light-burned magnesia balls or dolomite materials into the furnace to increase the melting point and viscosity of the slag. The slag is sprayed onto the furnace lining by high-pressure nitrogen. The service life of the furnace lining can be as high as more than 20,000 furnaces. However, this method consumes a large amount of nitrogen and has a high heat loss. Each furnace needs to be splashed with slag once, which affects the steelmaking efficiency. In addition, the air supply components (permeable bricks) at the bottom of the converter have a short lifespan, which affects the re-blowing effect. Sliding nozzle slag blocking technology has become a new mainstream development due to its advantages of reducing phosphorization of molten steel, increasing alloy yield, reducing inclusions in steel, and improving the cleanliness of molten steel. However, the sliding nozzle system, especially the slag blocking slide, is easily damaged and its service life needs to be improved. Under the new smelting technology conditions, there are many problems with refractory materials used in steelmaking converters, which have attracted great attention from relevant researchers. A lot of research and exploration work has been carried out to address the new demands for converter refractories, and many new refractory technologies have emerged.

Main types and smelting processes of steelmaking converters

Converters can be divided into acid converters and alkaline converters according to the properties of the furnace lining refractory materials. According to the part where the gas is blown into the furnace, the converter is divided into bottom blowing, top blowing, side blowing and top and bottom combined blowing converters; according to the type of gas, it is divided into air converter and oxygen converter. Basic oxygen top-blown and top-bottom multiple-blown converters have become the most commonly used steelmaking equipment due to their fast production speed, large output, high single-furnace output, low cost, and low investment. Converters are mainly used to produce carbon steel, alloy steel, etc. The schematic diagram of the converter steelmaking process is shown in Figure 1.

Figure 1 Schematic diagram of converter steelmaking process

Converter steelmaking uses molten iron, scrap steel, and ferroalloy as the main raw materials, adds a small amount of quicklime, and blows in air or oxygen to oxidize impurities such as silicon, manganese, phosphorus, sulfur, and carbon. A large amount of heat is released during the oxidation process (containing 1% (w) silicon can increase the temperature of pig iron by 200°C), which can reach a sufficiently high temperature in the furnace (without the need for external energy). The steelmaking process is completed in the converter by relying on the physical heat of the molten iron itself and the heat generated by the chemical reaction between the components of the molten iron.

Refractory materials used in steelmaking converters and their damage mechanisms

The refractory materials used in the lining of steelmaking converters are mainly MgO-C bricks, and a small amount of high-purity magnesia bricks and magnesia dolomite fired bricks are also used. The amorphous refractory materials used include MgO-SiO2, MgO-C, MgO-CaO, high-purity MgO, etc.

Table 1 Types of refractory materials used in converters

material typeChinaJapan
Shaped productsMagnesia carbon brickMagnesia carbon brick
Unshaped repair materialMgO-SiO₂ quality, MgO-C quality, MgO-CaO quality, high-purity MgO qualityMgO-SiO₂ quality, MgO-C quality, MgO-CaO quality, high-purity MgO quality
breathable elementDispersed MgO-C breathable bricks, ring-seam MgO-C breathable bricks, straight hole type breathable bricksDispersed MgO-C breathable bricks, annular seam MgO-C breathable bricks, porous plug type air supply element (MHP)
Slag blocking slide/nozzleMagnesium carbon slide gate plate, aluminum carbon slide gate plate, aluminum zirconium carbon slide gate plate, aluminum zirconium carbon inlaid zirconium slide gate plate; unburned aluminum zirconium carbon nozzle, unburned aluminum carbon nozzle, unburned magnesium carbon outer nozzleMagnesium carbon slide gate plate, aluminum carbon slide gate plate, aluminum zirconium carbon slide gate plate, aluminum zirconium carbon inlaid slide gate plate; unburned aluminum zirconium carbon nozzle, unburned aluminum carbon nozzle, unburned magnesium carbon outer nozzle

During the smelting process of converter steel, the furnace lining is eroded by a series of strong mechanical, physical and chemical effects. The converter re-blowing process is to install breathable bricks at the bottom of the converter, and blow oxygen, carbon dioxide, argon or nitrogen into the furnace through the breathable bricks, which strengthens the stirring of the molten pool and improves the smelting reaction. This shortens the steelmaking time, improves the quality of molten steel and reduces steelmaking costs. However, repeated blowing also accelerates the erosion of the furnace lining refractory materials, and various parts of the converter are eroded by different conditions.

(1) Erosion or mechanical impact. Operations such as adding scrap steel and adding molten iron directly face the large surface of the converter lining, causing strong impact, wear and erosion on the large surface of the furnace lining, which is the main factor in the erosion of the refractory material of the furnace lining. During the smelting process, the gas flow in the furnace scouring the furnace wall, furnace cap and other refractory materials, the molten steel and slag melting and scouring the furnace lining, and the high temperature reaction during the smelting process causing melting damage to the furnace lining and other physical erosion.

(2) Oxidation and chemical attack. Oxidation is a major cause of erosion of magnesia-carbon bricks in converter linings. During this process, the carbon component in the magnesia-carbon brick is oxidized by oxygen-containing components (such as high-temperature oxidizing gas, iron oxide, oxygen, magnesium oxide), causing the material structure to become loose and brittle.

FeO+C(s)=Fe+CO(g) (1)

O2(g)+2C(s)=2CO(g) (2)

MgO(s)+C(s)=Mg(g)+CO(g) (3)

The iron oxide in the slag reacts with the graphite or tar/resin in the hot side of the brick lining, or oxygen attacks the graphite or binding agent on the cold side of the brick lining. In both cases, the strength of the bricks is reduced and they are corroded due to erosion by gas and molten metal fluids.

The chemical reaction between iron oxide (FeO) or acidic components in the slag, such as SiO2, CaO and MgO is as follows:

FeO+MgO=FeO·MgO, (4)

SiO2+2MgO=2MgO·SiO2, (5)

CaO+SiO2+MgO=CaO·MgO·SiO2. (6)

All of the above reactions can turn the furnace lining into molten slag and cause damage to the refractory material.

(3) Thermal shock peeling. The working environment of the air supply component is high pressure and large flow (pressure greater than 1MPa, flow rate 0.15~0.2m3·min-1·t-1), and its damage mechanism is spalling and erosion wear caused by thermal stress concentration .

(4) Abrasion, melting loss and spalling. During the converter tapping process, the sliding nozzle and sliding plate must withstand the erosion of high-temperature molten steel and steel slag; the erosion and penetration of strong alkaline slag; and the intermittent high-temperature (~1600°C) strong thermal shock impact. In addition, during the slag blocking operation, the sliding plate has to withstand the abrasion of steel slag.

Therefore, the main damage methods are the erosion and erosion of high-temperature molten steel and steel slag, high-temperature oxidation, roughening and abrasion of the sliding surface, and thermal shock damage. The future technical direction of steelmaking converter refractory optimization technology is:

(1) Develop high-performance wear-resistant, thermal shock-resistant low-carbon magnesia-carbon bricks;

(2) Develop fast-sintering, pollution-free hot patching materials and long-lasting gunning patching materials;

(3) Develop long-life re-blowing gas supply components, optimize the structure and layout of the furnace bottom tuyere, and adapt to the requirements of advanced steelmaking technologies such as top-bottom re-blowing, low-oxygen tapping, bottom-blowing powder spraying, bottom oxygen supply, and bottom blowing CO2;

(4) Improve the performance and structure of the slag-blocking slide plate, extend its service life, and reduce the number of daily replacements.

New technology of refractory materials for steelmaking converters

Development and application of high-performance lining bricks

Magnesia-carbon bricks are widely used as lining bricks for converters due to their excellent resistance to slag erosion, thermal shock resistance, spalling resistance and wear resistance, as well as good stability at high temperatures. Since magnesia carbon bricks have problems with thermal shock resistance and erosion resistance caused by easy oxidation, researchers have conducted extensive exploration and research. Its key technical hot spots are reflected in: 1) The application of new anti-oxidation and self-healing composite antioxidants: using metal Al, Si powder, or Al-Si composite powder as antioxidants for magnesia carbon bricks, which are in situ after heat treatment or high temperature service The reaction generates SiC, AlN and other highly corrosion-resistant phases, which significantly improves the performance of low-carbon magnesium carbon materials. 2) Preparation and application of low-dimensional graphitized carbon: the addition and application of various pre-synthesized nanocarbons such as nanocarbon black, nanographite-oxide composite powder, etc.; in-situ synthesis of low-dimensional graphitized carbon and selection of appropriate transitions Inorganic or organic compounds of elements (Fe, Co, Ni) are used as catalysts, and phenolic resin is cracked to produce gases such as CO, C2H2, and CH4, which are catalyzed by transition metals to form low-dimensional graphitized carbon such as carbon nanotubes and nanocarbon fibers. Through the development and application of these new technologies, MgO-C bricks can maintain good corrosion resistance and thermal shock resistance.

The MgO-C bricks used in different parts of the converter have different performance requirements. In order to meet the needs, Japan’s Shinagawa Refractory Company has developed a series of MgO-C bricks that meet the performance requirements of different parts. Their basic performance and typical characteristics are shown in Table 2. The microstructure analysis of used MgO-C bricks shows that the damage of MgO-C bricks is mainly caused by slag erosion damage. By examining the damage factors of MgO-C bricks, Japanese researchers found that limiting the diffusion rate of Mg(g) in MgO-C bricks at high temperatures can reduce the brick damage rate. The results of the slag corrosion resistance test show that as the apparent porosity of MgO-C bricks (heat treated at 1500°C) increases, the corrosion index of the bricks increases linearly (see Figure 2). Based on this, dense structure MgO-C bricks (B, see Table 2) were prepared by adjusting the amount of antioxidant added, raw material particle gradation and production process.

Table 2 Performance of MgO-C bricks

project HSMRCBABCDE
Chemical composition(w)/% MgO7779797878766784
C1513141717163010
Physical propertiesApparent porosity/%3.03.81.0     
  Volume density/(gcm³)2.992.972.922.953.062.982.713.08
Room temperature flexural strength/MPa   3631493853
Typical characteristicsWear-resistantImpact resistance Impact resistanceUniversalResistant to erosionWear-resistantImpact resistanceanti-oxidation

Figure 2 The change trend of the erosion index of MgO-C bricks with the apparent porosity after firing at 1500℃

MgO-C bricks in the converter charging area are often subject to mechanical impact from scrap steel raw materials, which are prone to cracks and expansion, leading to damage to the lining. Different types of MgO-C bricks have very different creep resistance, as shown in Figure 3, which results in differences in the thermal shock and mechanical impact resistance of MgO-C bricks. Researchers from Japan’s Shinagawa Refractory Company found that MgO-C bricks (HS) with higher high-temperature flexural strength cannot alleviate the occurrence of damage. MgO-C bricks with higher fracture energy (fracture toughness) can effectively inhibit the expansion of cracks. Therefore, two new types of MgO-C bricks were developed, namely matrix-reinforced MgO-C bricks (MR) and carbon-bonded reinforced MgO-C bricks (CB). The properties are shown in Table 2. The two new types of MgO-C bricks have high fracture energies (MR0.40kJ and CB0.49kJ, HS is only 0.26kJ). Crack expansion is suppressed after mechanical impact, and both have better corrosion resistance than HS bricks. Among them, the carbon-bonded reinforced MgO-C brick has better corrosion resistance.

Figure 3 Load-displacement curves of several MgO-C bricks after firing at 1200℃

The service life of magnesia carbon bricks at the converter taphole is often affected by carbon oxidation, thermal shock spalling and flow steel wear. Therefore, the development of low carbon magnesia carbon bricks with good wear resistance and thermal shock resistance is an inevitable trend of development. Researchers at Taigang’s Second Steelmaking Plant used fused magnesia with m(CaO):m(SiO2)≥2 and high-purity flake graphite (C mass fraction ≥98%) as the main raw materials, using Al, Mg-Al , Si, B4C, and CaB6 are used as antioxidants, and thermosetting phenolic resin is used as a binding agent to prepare high-quality low-carbon MgO-C bricks. Its properties are shown in Table 3. Using newly developed low-carbon magnesia-carbon bricks in steelmaking converters, the service life of a furnace is stable at 500 to 700 times, which is significantly higher than the 300-400 times of imported low-carbon magnesia-carbon bricks.

Table 3 Properties of low carbon MgO-C bricks

project w/%Normal temperature compressive strength/MPaBulk density/(g/cm³) Apparent porosity/%
  CMgO
Newly developed bricks5.9889.7064.7 3.104.6
Imported bricks  BP5.0788.9940.2 3.055.5
  AT6.1889.0743.5 3.085.2
AQ5.5788.5241.6 3.10 5.5

Development and application of unshaped refractory materials

Due to the increase in the amount of scrap steel used in steelmaking and the application of new top-to-bottom blowing technology, the working conditions of the converter have become more severe and the damage of refractory materials has accelerated. In order to improve the service life of the converter, higher requirements are placed on the refractory materials used in various parts of the converter. The development of new amorphous refractory materials and the application of repair technology have been greatly developed. It can greatly increase the overall service life of the furnace lining without affecting normal production, making the service life of the furnace lining reach more than 8,000 times, or even more than 20,000 times. At present, large-surface hot patching materials are commonly used to maintain the charging side, furnace bottom and tapping side of the converter. Gunpowder materials are used to maintain the molten pool, fillets and trunnions of the converter, and grouting materials are used for caulking and maintenance of the taphole area of the converter when replacing the taphole. Among them, large fabrics mainly include MgO-SiO2 material (also called water-based large fabric), MgO-C material, MgO-CaO material, etc., and gunning materials mainly include MgO material, MgO-CaO material, MgO-Cr2O3 material, etc. These amorphous refractory materials are divided into water-free repair materials (mainly asphalt, coal tar and asphalt powder, phenolic resin combined) and water-based repair materials (MgO-SiO2-H2O combined and phosphate combined) according to different binders. Repair materials with different combination systems have their own advantages and disadvantages in performance, as shown in Table 4. In view of the shortcomings and application requirements of existing patching materials, researchers have conducted a lot of research and developed large fabrics or spray patching materials with better performance.

Table 4 Types and performance characteristics of high-temperature repair materials

Classification Phenolic resin system Asphalt systemWater system
liquidPowder
fluidityexcellent   excellentexcellentexcellent
Adhesiongenerally   excellentexcellentgenerally
hardening timegood   goodgoodexcellent
Durabilitygenerally   goodgoodAverage/poor
othercostlyNot easy to storeOverall more popular

The large-area converter repair materials currently used for on-site repair mainly use coal tar, asphalt (about 8%~15% (w)), resin and other organic substances as binding agents, which inevitably have some problems. For example: if the sintering time is too long, the material will have many pores and poor density after the organic matter is burned out, resulting in a material that is not resistant to slag erosion, has low strength, has a short service life, and seriously pollutes the on-site workshop environment. Qin Yan et al. selected high-purity magnesia powder (w(MgO)=97.02%) and mid-grade magnesia particles (w(MgO)=94.80%) as the main raw materials. Using silicon oxide ultrafine powder (w(SiO2)=96.0%) as a binder, a new long-lasting, carbon-free and environmentally friendly converter large-area repair material was developed.

The environmentally friendly water-based converter large-surface repair material does not produce harmful gases during the sintering process, and is safe and environmentally friendly. The product adopts wet self-flow casting method and has good high-temperature spreading performance. After high-temperature sintering, it forms a ceramic bond. It has a dense structure, anti-oxidation and erosion resistance, and the volume density can reach 2.83g·cm-3. Applied by many converter steelmaking users, the on-site use is smoke-free, the sintering time is shortened by more than 50% compared with conventional carbon-based large fabrics, and the service life is extended by 2 to 3 times.

Table 5 Converter large surface patching material particle proportion w/%

projectMid-grade magnesiaHigh purity magnesiaotherDispersant E8Water Amount Flow Value/mm
5~3 mm3~1 mm1~0 mm≤0.074mm
3#23123035              0.55270

Due to the constant impact of mechanical force and erosion of slag, the refractory materials at the bottom, trunnion and two major surfaces of the oxygen top-blown converter are easily damaged, and the trunnion and slag line areas need to be repaired frequently by gunning. At present, the most commonly used converter gunning material in China is magnesium gunning material. In order to improve its shortcomings such as intolerance to erosion, erosion resistance, and low service life, Yao Yashuang et al. developed a new type of magnesium carbon gunning material for converters. The raw materials used for gunning materials are mainly 3~0mm magnesia (w(MgO)=95.2%), 3~0mm carbon (w(C)=94.2%), asphalt A (fixed C46.2%, softening point 140~160 ℃), asphalt B (fixed C43.5%, softening point 100~120℃), additives, etc. Mix carbon and asphalt B in different proportions as the carbon source of the gunning material. The fixed carbon mass fraction is about 5% to 7%. It can be seen from the test that due to the large particle size of asphalt B, its heating and carbonization speed is slower, the degree of carbonization is higher, and it is more beneficial to the adhesion of gunning materials. Among them, the gunning material with a carbon to asphalt mass ratio of 7:2 has the best performance, greater strength after burning, and better erosion resistance. The use results show that the new converter magnesium carbonaceous gunning material has low rebound rate, good adhesion and high sintering strength; the service life of the original magnesia gunning material is 7 to 8 furnaces. The service life of the new magnesium carbon gunning material is 10 to 13 furnaces, which is increased by more than 30%, greatly shortening the number of converter repairs.

In order to solve the problem of slag sticking to the slag sticking surface of the converter furnace mouth and furnace cap, Zhu Shanhe et al. developed a converter anti-slag gunning material using recycled magnesia carbon bricks as the main raw material. The application results show that the developed anti-slag gunning material has good construction performance. Through semi-dry gunning construction, a complete and uniform slag isolation spray layer can be formed on the surface of the converter mouth. The thickness of the spray layer can reach 35~50mm. The spraying adhesion rate reaches more than 80%, and only one gunning operation per shift can meet the needs of sticky slag isolation; the thick gunning layer itself and the bonding strength with the gunning surface are low, which reduces the peeling off and cleaning of sticky slag. Difficulty, reducing the number and time of cleaning sticky slag, effectively improving the cleaning efficiency of sticky slag at the converter mouth, shortening non-production operation time; without changing the production process, the converter production capacity can be improved.

Japan’s Shinagawa Refractory Company has developed MgO-C converter hot repair material that can be rapidly hardened. Its performance is shown in Table 6. The developed carbon-bonded MgO-C gunning material has good adhesion even when the brick surface is above 1300°C after slagging. It can significantly improve the thermal repair efficiency of the converter. It can be used for ultra-high temperature construction and the hardening time is greatly improved. shortened, which helps reduce patching time.

Table 6 Characteristics of Shinagawa Company’s rapid hardening MgO-C gunning materials

serial number FRX-M90-D104FRX-M90-D110FRX-M90-D58
Chemical composition(w)/%  MgO777577
 CaO0.80.70.8
C7.67.27.6
Physical properties after 1000 ℃Apparent porosity/%25.525.325.7
Bulk density/(g/cm³)2.462.472.42
Adhesion strength/MPa2.62.32.5
Heat flow index/%10098100
Hardening time index/%8870100

Top and bottom repeated blowing technology

The converter combined blowing process stirs the molten pool with bottom-blown gas to bring the steel slag reaction close to equilibrium, avoids overoxidation of molten steel and improves metal yield and molten steel quality. The gas supply components at the bottom of the converter are divided into two categories: nozzle type and brick type. Among them, brick-type air supply components have become the mainstream development direction due to their stable performance. There are three main types of brick-type air supply components: diffuse type, ring-slit type and straight hole type. Dispersed breathable bricks have the disadvantages of high gas bypass resistance and low lifespan. Ring-seam type breathable bricks are relatively dense and have a longer lifespan than dispersion type bricks. They are widely used and well received, but their stability is far less than that of straight-hole type breathable bricks. Therefore, straight-hole ventilating bricks are the new mainstream in the development and application of air supply components in the future. Optimizing its position at the bottom of the molten pool is crucial for bottom blowing to achieve good metallurgical results. The most widely used straight-hole air supply component (Multiple Hole Plug, MHP for short) was first successfully developed by the Japanese Steel Pipe Company. Its advantages are small air supply resistance, large gas flow adjustment range, good air tightness, and not easy to leak. The reinforcement of refractory bricks by metal pipes makes the bricks less likely to peel and crack.

In order to optimize the performance of the gas supply components, through the adjustment of the proportion of refractory raw materials used to manufacture the gas supply components and the special treatment of the internal stainless steel tubes, their service life has been greatly improved and their matching with the age of the converter has been increased. Zhang Yueming uses high-purity fused magnesite (w(MgO)≥97%) and natural flake graphite (w(C)≥98%) as the main raw materials. Add Al, Si powder and B4C (total amount <6%) as antioxidants, select thermosetting and asphalt modified resin as the binding agent, and add an appropriate amount of asphalt powder. High-performance MgO-C gas supply components are prepared using isostatic pressing method. The prepared MgO-C gas supply element has significantly improved oxidation resistance, fracture toughness and thermal shock performance, good air tightness, an erosion rate of 0.28mm/heat, and a maximum service life of 2113 heats. In order to reduce the carburization speed of stainless steel pipes and improve their service life, the surface coating method of α-Al2O3/ALCH series slurry is adopted, and the ratio of α-Al2O3/ALCH is controlled to be greater than 3/7, and the coating thickness is greater than 1mm. , forming a dense protective isolation layer with stable thermal performance and resistance to carbon reduction on the surface of the stainless steel pipe.

Due to the large temperature difference between the inside and outside of the refractory bricks used in the gas supply components of the converter, the temperature gradient difference in the refractory bricks increases; in addition, the temperature drops sharply after tapping, and the breathable bricks are subject to a great deal of thermal shock. Due to the existence of these thermal stresses, cracks occur and expand inside the breathable bricks, causing intermittent peeling of the refractory bricks. Researchers from Shinagawa Refractory Company found that the cracks in the breathable bricks are mainly cracks parallel to the hot surface, and the damage caused by thermal spalling is far more serious than the damage caused by factors such as liquid steel erosion and wear. On the basis of fully investigating the damage mechanism of breathable bricks, high-performance breathable bricks with high fracture toughness and excellent thermal shock resistance were developed. The specific properties are shown in Table 7. First, by improving the toughness of the material, the occurrence and expansion of cracks are suppressed, significantly reducing spalling damage. Secondly, by increasing the size of the breathable bricks, the distance for cracks to extend to the spalling surface is extended, which also effectively reduces the occurrence of spalling. It was used in a 220t converter. Compared with traditional breathable bricks, the loss rate was reduced by about 40%, and the spalling damage was improved. The converter ran for more than 4,000 times without replacing the breathable bricks.

Table 7 Performance of refractory bricks for converter gas supply components

project MHP breathable bricks M HP bricksTraditional breathable bricks
Chemical composition(w)/%MgO8075.570
C152027
Apparent porosity/%2.64.61.0
Volume density/(gcm³)2.872.802.72
Normal temperature compressive strength/MPa353332

Slide gate plate slag blocking technology

Converter sliding plate slag blocking technology is an emerging technology that has developed rapidly in recent years. This technology mainly consists of three parts: a sliding plate slag blocking system, an infrared slag detecting system and a hydraulic drive system. It is combined with infrared slag detecting technology and PLC control technology. , realizing automatic slag judgment and slag blocking, is currently the best and latest production technology and equipment with the best slag blocking effect for converter tapping. Sliding nozzle slag blocking technology has outstanding advantages in reducing rephosphorization of molten steel, increasing alloy yield, reducing inclusions in steel, improving cleanliness of molten steel, reducing ladle slag adhesion, and extending ladle service life.

The inner nozzle of the converter taphole is connected to the end of the taphole brick, and the lower part is connected to the upper slide plate of the taphole, while the outer nozzle is connected to the lower slide plate of the taphole. Therefore, during the tapping process of the converter, the inner/outer nozzles and sliding plates must not only withstand the erosion of high-temperature molten steel and steel slag, but also the erosion and penetration of strong alkaline slag. During discontinuous tapping, it also has to withstand strong thermal shock shock at high temperatures (~1600°C). In addition, during frequent slag blocking operations, the cast holes and slideways of the sliding plate must withstand the erosion and melting loss of high-temperature molten steel and steel slag. Therefore, it is required that the material selection of the inner/outer nozzles and slide gate plates should pay attention to good slag erosion resistance, high-temperature oxidation resistance and excellent thermal shock resistance. The skateboard materials should also have excellent wear resistance.

Currently on the market, the main materials for the outer nozzle of the converter tap are unburned magnesium carbon, unburned aluminum zirconium carbon and zirconia (inlaid inner core). Among them, magnesium carbon nozzles dominate the market due to cost advantages, and aluminum-zirconium carbon nozzles have slightly better performance than magnesium carbon nozzles. The inlaid zirconia core nozzle is in the research and development stage, and its service life can reach more than 120 heats, and can even be synchronized with the life of the tap hole. Most non-burning nozzles are impregnated with asphalt to seal the pores, improve density and corrosion resistance, and their service life ranges from 30 to 90 heats.

Table 8 Physical and chemical indicators and service life of external water outlets made of different materials

Material Chemical composition(w)/%Bulk density/(gcm³)Apparent porosity/%Service life/time
Al₂O₃MgOCZrO₂
Magnesium Carbon ≥80≥9 ≥2.90≤1030~80
Aluminum zirconium carbon ≥75 ≥6  ≥3≥3.00≤1050~90
Zirconia (inner core)   ≥90>4.5≤1590-120

The slag blocking slide is one of the most critical components in the application of sliding nozzle slag blocking technology. At present, the material of domestic slag-stop slide gate plates is mostly aluminum-zirconium carbon. As a material for flow-control slide gate plates, it has high strength, good thermal shock resistance, and excellent erosion and erosion resistance. However, for slag-stopping technology, the service life of the slide plate is relatively low and can only be stabilized at about 10 to 14 furnaces. Therefore, researchers have improved the performance of the slag-blocking slide in a variety of ways. Studies have shown that the introduction of expanded graphite and Si powder can promote the formation of SiC whiskers in aluminum-carbon refractory materials to a certain extent. The toughness of the slide gate plate is improved, the ability to resist crack expansion is enhanced, and the thermal shock resistance and service life of the slide gate plate are improved.

In order to meet the high service life requirements of 18 to 20 heats or even more than 25 heats proposed by steel companies, the material of the slide plate has been transformed from conventional dead-burned aluminum zirconium carbon to a composite structure in which the body is made of aluminum zirconium carbon and the inlaid layer is made of zirconium material. . At present, there are mainly three categories: ① The upper sliding plate inlaid with zirconium rings and the lower sliding plate inlaid with zirconium plates. ②The upper sliding plate is inlaid with zirconium plates and the lower sliding plate is inlaid with zirconium plates. ③The upper slide plate is inlaid with zirconium rings and the lower slide plate is inlaid with zirconium plates in the anti-slip area of the slide. The service life of the inlaid slide plate on the 120-300t converter (front and rear slag retaining) can be stable at least 15-18 heats. If other slag blocking methods are used in the early stage, and only the sliding plate is used to block slag in the later stage, the service life of the sliding plate can reach 20 to 25 heats.

Table 9 Physical and chemical indicators and service life of slide gate plates made of different materials

Material Chemical composition(w)/%Physical propertiesService life/heats
  Al₂O₃MgOCZrO₂Bulk density/(gcm³) Apparent porosity/%
Magnesium Carbon  ≥4>75≥2.5 >2.85  ≤106~8
Aluminum carbon  ≥85 ≥6 ≥2,95  ≤86~10
spinel carbonaceous  ≥75≥6≥6 ≥2.95  ≤86~10
aluminum zirconium carbon  ≥70 ≥6≥6≥3.10  ≤810~15
  Aluminum zirconium carbon inlaid zirconium ring/plate      23-25

The disadvantage of the sliding nozzle slag blocking technology is that the service life of the refractory components is relatively low. To this end, Interstop Company cooperated with RHI Company to improve the traditional steel pouring system and developed a new type CG120 sliding nozzle for the taphole of steelmaking converter. The CG120 sliding nozzle system adopts a new structure, which can replace the inner slide plate without removing the entire sliding nozzle device from the steel shell. This system improves the use of refractory materials and significantly shortens the converter shutdown time. The service life of the upper connecting element of the CG new sliding nozzle is 24.8 furnaces, and the service life of the refractory material element is 24.8 times.

Sinosteel Luonai Institute summarized the reasons for the damage of the converter slag retaining slide and found that the upper slag retaining slide was in contact with the molten steel, which was mainly damaged by erosion and hole expansion, and the lower slide was in contact with the outside air, which was mainly damaged by the expansion of thermal shock cracks. Therefore, by inlaying a zirconium ring on the upper sliding plate and a zirconium plate on the lower sliding plate, and through measures such as phase composition control and microstructure regulation, the thermal shock resistance of the zirconium ring and zirconium plate is improved, and the life of the slag retaining slide is stabilized. At about 20 heats, the cost-effectiveness of sliding plate slagging is significantly improved. The three series of products developed can meet the needs of different working conditions. The specific performance is shown in Table 10.

Table 10 Physical and chemical indicators and properties of zirconium rings and zirconium plates on converter sliding plates

performanceGH/GB-AGH/GB-BGH/GB-C
w(ZrO₂+HfO₂)/%≥95.0≥95.0>95.0
Apparent porosity/%≤8.0≤12.0≤18.0
Volume density/(gcm³)≥5.2≥5.0>4.8
   Compressive strength/MPa≥180≥150≥120
   Thermal shock resistance (1100℃, water cooling)/timeNo cracking after 2 timesNo cracking after 3 timesNo cracking after 3 times

By continuously improving and optimizing the appearance structure design and inlaid structure design of the slide gate plate, the locking force of the expansion and deformation of the slide gate plate under high temperature is increased, preventing the occurrence and expansion of abnormal cracks, and preventing the formation of abnormal leakage steel channels. Increase the accuracy of the infrared slag detection system and the stability of the operating speed of the hydraulic drive unit cylinder, focus on solving the deficiencies found in the application, and greatly improve the safety and reliability of the slide gate plate slag blocking technology.

Conclusion

The advancement of steelmaking technology promotes the development of new technologies for refractory materials in steelmaking converters. The refractory materials involved in the new technology not only have the characteristics of high-temperature structural stability, resistance to spalling, wear resistance, and slag resistance, but also reflect the development concepts of energy saving, longevity, low carbon and environmental protection. The future development trends of refractory materials for steelmaking converters are: 1) Develop and promote high-performance magnesium carbon refractory materials. 2) Promote and apply new environmentally friendly amorphous refractory materials and high-temperature repair technology. 3) Develop new composite structural refractory materials, such as composite gas supply components, composite slide gate plates, etc. 4) Research and develop refractory materials that are lightweight, energy-saving and have good high-temperature properties.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

Our Product have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time

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