Product trends

Application of silicon nitride in metallurgical industry

With the progress of science and technology, metallurgical enterprises are developing towards large-scale, continuous, automation, no (less) pollution and low consumption. Therefore, metallurgical enterprises must adopt new technology, new equipment and new materials. Among many materials, Sialon ceramic materials composed of silicon nitride and silicon nitride are continuously used by metallurgical enterprises all over the world, and are increasingly widely used in metallurgical industry [1 ~ 12].

1. Application in aluminum, copper, zinc and other industries

Silicon nitride ceramic is not corroded by liquid aluminum and its excellent thermal performance makes it have great potential in aluminum industry [6]. In aluminum casting continuous rolling production line, aluminum smelting and melting operation, silicon nitride ceramic can be used as thermowell, lining of aluminum smelting furnace, ladle lining of aluminum liquid, and mold of aluminum crucible, all of which can be made of silicon nitride. High quality silicon carbide combined with silicon nitride and Sialon brick has been used in aluminum reduction cell to replace the traditional carbon material, and remarkable benefits have been achieved. It has been gradually popularized in Europe, America and Japan. For example, it has been used in 110KVA aluminum reduction cell for three years, and the situation is still good. Its advantages are: 1) increasing service life, because of its strong anti-corrosion ability of cryolite, and good oxidation resistance; 2) increasing production, because of its high mechanical strength, it can reduce the lining thickness, increase the working volume in the tank, and correspondingly increase the electrode energy; 3) reduce the unit power consumption, because of the high conductivity, the corresponding electric loss is small [6]. Such products have also been used in steel-making industry, such as lining of blast furnace, burner, chute and slag copper separator, and as refractory parts of continuous casting copper.

At present, silicon nitride ceramic thermowells have been widely used in liquid aluminum temperature measurement in China. The performance of this kind of thermowells is better than that of stainless steel and corundum ceramic thermowells. Stainless steel is easy to be corroded by molten aluminum, and will be damaged after 20 hours of continuous use. Corundum can not withstand thermal shock, while silicon nitride ceramics can be stable for a long time in liquid aluminum, and can not crack after 1200 times of intermittent temperature measurement. Silicon nitride ceramic thermowell can also be used to measure the temperature of liquid zinc, and the effect is very good.

2. Application in steel making and rolling industry

The new technology of ladle lining, such as high corrosion resistance material and refractory material, has been put forward. In recent 10 years, silicon nitride combined with silicon carbide has been used in ladle in developed countries such as Europe and America, and obvious economic benefits have been achieved. In China, it has been used in the past two years. The experiment of a steel plant in Shanghai shows that silicon nitride combined with silicon carbide brick can achieve the following effects [S]; 1. The original ladle wall is built with high alumina brick. Under the repeated erosion of molten steel, the ladle wall is corroded seriously and the service effect is not ideal. The average service life is only 37 heats. After using silicon nitride and silicon carbide brick, the ladle service times can reach 156 heats, which is 4 times of the original ladle life. ② the refractory consumption and masonry cost are reduced. The original ladle masonry needs 608kg refractory material, the masonry cost is 498.8 yuan, a total of 81.4 tons of steel, refractory consumption per ton of steel is 7.47kg, the masonry cost per ton of steel is 6.13 yuan. After using silicon nitride combined with silicon carbide brick, 830kg of refractory material is needed, the masonry cost is 1696.6 yuan, 342.2 tons of steel is made, 2.41kg of refractory material is consumed per ton of steel, and the masonry cost per ton of steel is 4.98 yuan. Compared with the original ladle, the refractory consumption per ton steel is reduced by 5.06kg, and the masonry cost per ton steel is reduced by 1.19 yuan; ③ it is beneficial to improve the quality of steel. Because of using silicon nitride and silicon carbide brick in the test ladle, the corrosion of refractory is very little and the inclusions in steel are reduced.

Silicon nitride, as a refractory material, is the most important application in the steel-making industry as a horizontal continuous casting separation ring [9]. In horizontal continuous casting, the separating ring divides the molten steel flow into molten steel zone and solidification zone, which plays a role in separating the liquid-solid interface of steel, and plays a great role in maintaining the stable solidification starting point and casting billet quality. Separation ring is the key component of horizontal continuous casting technology, which has strict requirements for refractory materials: accurate size, easy machining, good thermal conductivity, resistance to high thermal stress and thermal shock, long time corrosion and corrosion resistance of liquid steel, and long service life.

Various kinds of refractories have been used for horizontal continuous casting separation rings. When alumina, zirconia and other oxide materials are used, they are unfavorable to the drawing operation of steel billet, and the effect of using alumina graphite separation ring is not good. Table 1 lists the performance comparison of refractory materials for separation ring. From the perspective of application effect, the separation ring made of silicon nitride refractory is the best. This kind of refractory has high mechanical strength, good heat shock resistance, and will not be wetted by liquid steel. It meets the technical requirements of horizontal continuous casting for heat shock resistance, corrosion resistance and non blocking. It is a promising refractory for continuous casting.

For the submerged nozzle used in continuous casting, it is not dependent on air blowing, but using material to solve the problem of nozzle blockage has been attached great importance [10 ~ 11]. The reason is that air blowing from the side wall slit of the submerged nozzle can easily cause pinholes in the steel and reduce the quality of the steel. At present, the submerged nozzle test has been carried out on the composite materials of AI2O3, ZrO2, Si3N4, ai3n4, BN, B, C, Sialon and so on, and the adhesion between Sialon and graphite is the least. Using this material as the lining of submerged nozzle without argon blowing, 5 ~ 9 heats can be operated continuously.

Table 1 performance comparison of refractories for horizontal continuous casting separator ring

Material heat resistance, heat resistance, dimensional accuracy and resistance to invasion

And easy to machine wear resistance

Seismic stress property Erodibility

Alumina cccaa

Zirconia cccaa

Fused quartz aaabc

Zirconia molybdenum cermet AACA

Reaction bonded silicon nitride bbaaa

Reaction bonded silicon nitride and boron chloride aaaaa

Hot pressed silicon nitride aacaa

There are many industrial furnaces, in order to prevent some parts from overheating damage, water cooling measures are often used. But with it comes a lot of heat loss. For example, in a typical continuous heating furnace water-cooled tube slide rail system in a rolling mill, the heated ingot is supported on the slide rail supported by the water-cooled pipe supports in the longitudinal, transverse and vertical directions, and glides and moves on it. The temperature of the high temperature heating zone in the furnace is about 1300 ℃. These water-cooled pipes often take about 25% of the heat at such high furnace temperature. This not only greatly increases the heat consumption of the heating furnace, but also is often forced to shut down and overhaul due to the burning or bending deformation of the water-cooling pipe, which reduces the operation rate of the heating furnace. Therefore, the ceramic slide rail is one of the technical development directions of heating furnace. At present, the silicon nitride materials have been tested, and it is expected to be practical in the near future. In addition, the new technology worth mentioning in the rolling system is the vitrification of cold roll. In recent years, the United States has carried out the use of Sialon ceramic refrigeration, the wear resistance is 20 times higher than that of steel rolls, and the cold-rolled low-carbon strip is 25% thinner than that of the same material rolled by steel roll [1,9].

3. Application in ironmaking industry

With the large-scale blast furnace, the main composition of refractory materials for blast furnace has changed from oxide based to oxide, non oxide and graphite composite. This transformation and development will not only continue, but also require the development and use of a new generation of products with the development of ironmaking industry technology. In this transformation and development process of blast furnace refractories, silicon nitride and Sialon bonded silicon carbide refractories are very attractive [3 ~ 5].

Table 2 number of industrial blast furnaces in the world / refractory utilization rate of bonded products

Hearth diameter (mm) bonded products

Percent of district total furnaces

5 ~ 77 ~ 99 ~ 1010 ~ 1212 ~ 15 ratio (%)

USA, Canada 4 / 431 / 1613 / 97 / 72 / 157 / 3765

Europe 4 / 416 / 1226 / 1518 / 46 / 470 / 4970

Japan, far east 0 / 02 / 12 / 214 / 723 / 1641 / 2663

Latin America 5 / 26 / 33 / 15 / 32 / 121 / 1048

Australia 2 / 25 / 32 / 22 / 11 / 112 / 975

Number of furnaces / number of furnaces for masonry bonded products 15 / 1260 / 3556 / 2946 / 3234 / 23201 / 131

The utilization rate (%) was 755255706861

Table 3 properties of silicon carbide bonded silicon nitride refractory for blast furnace

Project product 1 product 2 product 3

Bulk density (g / cm3) 2.632.652.70

The porosity (%) of tail gas is 161614

Compressive strength (mnm-2) 138138213

Flexural strength at room temperature (mnm-2) 383847

The bending strength (mnm-2) at 135 ℃ is 424248

The average thermal expansion rate of 20 ~ 1400 ℃ is 4.74.7-5.1

Thermal conductivity [w (MAK)] 25 ℃ 353541



Alkali resistance

Weight change (%) - 6 ~ - 20-4 + 0.7

The residual flexural strength (mnm-2) was 19 ~ 283047

Change of flexural strength (%) - 26 ~ - 50-200

Resistance to steam oxidation

600 hours volume expansion rate (%) 1255.7

Weight change (%) - 45-25-4

The 100 hour co resistance test has no effect

In recent years, silicon nitride and Sialon bonded silicon carbide products for blast furnace have developed rapidly. It has been used in 61% of foreign blast furnaces, as shown in Table 2, especially in large blast furnaces with hearth diameter of 12-15m. According to statistics, in the past 10 years, more than 3500 tons of such high-quality products have been built in 127 blast furnaces, and the usage of most furnaces is 270-800 tons, The application parts are from tuyere, bosh and waist to the lower middle part of furnace body. This is because the high-quality combined products have the advantages of corrosion resistance, wear resistance and thermal shock resistance. The main damage factor of refractories in the middle section of blast furnace is alkali erosion. At present, silicon nitride bonded products are mainly used, but the recent test results show that the newly developed Sialon bonded products have the best alkali resistance, so 10 blast furnaces have been put into trial use (the total consumption is 840 tons). Silicon nitride bonded silicon carbide brick has been used in 105m3 blast furnace in China for 3 years, and it is still in use with good effect. Table 3 shows the performance comparison of new BF refractories. Ai2o3-sic-c brick added with β - Si3N4 was used in the mixer car in Guchuan iron and steel plant of Kobe iron and Steel Company of Japan. The experimental results show that the slag corrosion resistance and oxidation resistance are good.

4. Application in metal heat treatment industry

The working conditions of metal heat treatment are various, and silicon nitride ceramics are not necessarily suitable for every occasion. For example, it is not suitable to use in the salt dissolving tank. Molten salts often corrode silicon nitride ceramic products quickly. But in some cases, silicon nitride ceramics have a good effect. For example: gear is quenched to the spindle of equipment, which must be subject to strong thermal shock. The silicon nitride gear is heated in the silicon nitride furnace at room temperature for more than 500 seconds, and then the gear shaft is heated for more than 500 times a week. Silicon nitride ceramics are suitable as fixture of workpiece and hook of heating body in vacuum heat treatment because of its high temperature resistance and dimensional stability under high temperature.