Continuous casting mold powder knowledge

What is the function of continuous casting mold slag?

During the pouring process, powdery or granular slag, called mold slag, is continuously added to the surface of the molten steel surface of the mold. The functions of mold powder include the following aspects: (1) Insulation and insulation to prevent heat dissipation. (2) Separate the air to prevent oxygen in the air from entering the molten steel and causing secondary oxidation, which affects the quality of the steel. (3) Absorb and dissolve inclusions floating from the molten steel to the interface of the steel slag to purify the molten steel. (4) There is a layer of slag film between the crystallizer wall and the solidified shell to lubricate, reduce the resistance to drawing, and prevent the solidified shell from adhering to the copper plate. (5) Fill the air gap between the billet shell and the crystallizer to improve heat transfer in the crystallizer. A good mold powder should be able to fully play the above five aspects to achieve the purpose of improving the surface quality of the slab and ensuring smooth continuous casting.

What are the requirements for the melting mode of mold slag?

In order for the mold slag added to the crystallizer during the continuous casting process to fulfill the above five functions, the mold slag powder must have a prescribed melting mode. That is to say, it is required to form a so-called three-layer structure of a so-called slag layer, a sintered layer and a liquid slag layer on the surface of the steel surface. Slag powder with low melting point (1100~1200℃) is added to the surface of high-temperature molten steel (around 1500℃) of the crystallizer. Relying on the heat provided by the molten steel, a liquid slag covering layer of a certain thickness (about 10~15mm) is formed on the liquid steel surface. The heat transfer from the molten steel to the slag layer slows down, and the slag on the liquid slag layer is heated. The slag powder is sintered together to form a so-called sintered layer (temperature between 900 and 600°C). On the sintered layer, the slag receives less heat transferred from the molten steel and the temperature is low (<500°C), so it remains in powder form. Evenly covering the surface of the molten steel prevents heat dissipation of the molten steel and prevents oxygen in the air from entering the molten steel. During the billet drawing process, due to the up and down vibration of the crystallizer and the downward movement of the solidified billet shell, the liquid slag layer on the molten steel surface continuously squeezes into the space between the billet shell and the copper wall through the interface between the molten steel and the copper wall. A solid slag film is formed on the surface of the copper wall, and a liquid slag film is formed on the surface of the condensation shell. This layer of liquid slag film lubricates the crystallizer wall and the surface of the billet shell, just like adding lubricating oil when the motor shaft rotates. At the same time, the slag film fills the air gap between the billet shell and the copper wall, reducing thermal resistance and improving crystallization heat transfer. As billet drawing proceeds, the liquid slag on the molten steel surface is continuously consumed, and the sintered layer drops to the molten steel surface and melts into a liquid slag layer. The slag layer becomes a sintered layer, and then new slag powder is added to the crystallizer. Keep it in a three-layer structure, and in this cycle, the mold slag powder is continuously consumed.

How to achieve the so-called “three-layer structure” of mold powder?

To exert the five functions of mold slag, the slag powder added to the mold must form a “three-layer structure”. The key to forming a “three-layer structure” is to control the melting speed of the mold slag powder. In other words, the slag powder added to the molten steel surface should not melt into liquid all at once, but gradually melt. For this reason, carbon particles are generally added to the mold powder as a melting rate regulator. The speed at which carbon particles control the melting rate depends on the type and quantity of carbon particles added. Carbon is a high-temperature resistant material. Very fine carbon powder is adsorbed around the slag particles, which separates the slag particles from each other, hinders the contact and fusion between the slag materials, and slows down the melting speed. If insufficient carbon powder is added, the carbon particles will be burned out before the slag layer temperature reaches the starting sintering temperature of the slag material, and the sintering layer will be developed, the melting speed will be too fast, and the liquid slag layer will be too thick. If too much carbon powder is added and some carbon particles remain after the slag is completely melted, the sintered layer will shrink and the thickness of the sintered layer will be too thin. When a moderate amount of carbon powder is added, part of the carbon particles in the sintered layer will be burned out, and the remaining slag material will still be effectively controlled by the carbon particles, so that a sintered layer and liquid slag layer of appropriate thickness will be obtained. There are two types of carbon materials: graphite and carbon black. Graphite particles are coarse, with a particle size of 60 to 80 μm. Its separation and blocking effects are poor, but the starting oxidation temperature is relatively high (about 560°C), the oxidation rate is slow, and the ability to control the melting rate in high temperature areas is strong. Carbon black has an amorphous structure, very fine particles (0.06~0.10μm), strong separation and blocking effects, low starting oxidation temperature (500°C), and fast oxidation speed. Therefore, carbon black has a strong ability to control the melting rate in the lower temperature area of the slag layer, but the control efficiency is lower in the high temperature area. Even if the dosage is increased, the improvement effect is limited. Generally, the amount of toner added is 4 to 7%.

What are the factors that affect the absorption of inclusions in molten steel by protective slag?

The injection flow from the immersed nozzle causes convective movement of molten steel in the mold, and the inclusions float to the steel-slag interface in the mold. Due to the fluctuation of the crystallizer liquid level, it may be involved in the solidification shell, causing subcutaneous inclusions or surface slag inclusions in the cast slab, affecting the surface quality. Therefore, it is hoped that the inclusions floating to the steel slag interface will be quickly absorbed and dissolved by the liquid slag layer. To quickly transfer the inclusions that float to the steel slag interface into the liquid slag, this process is determined by: (1) the contact area of the steel slag interface; (2) the viscosity of the liquid slag; (3) the ability of the slag to dissolve inclusions. In other words, the better the fluidity of the slag, the larger the contact area with steel slag, and the easier it is for inclusions to enter the slag. As soon as inclusions enter the slag, they can be quickly absorbed and dissolved, and the ability of the slag to dissolve inclusions mainly depends on the chemical composition of the slag. That is, the CaO and SiO2 content, (CaO%/SiO2% is called alkalinity) and the original Al2O3 content in the slag. Production tests have pointed out that as the alkalinity increases, the slag’s ability to dissolve Al2O3 inclusions increases. When the alkalinity is greater than 1.1, the ability to dissolve Al2O3 decreases; when the original Al2O3 content in the slag is greater than 10%, the slag’s ability to dissolve Al2O3 decreases rapidly. Therefore, when preparing mold slag, the ratio of CaO% to SiO2% should be between 0.9 and 1.0, and the original Al2O3 content should be as low as possible, generally less than 10%.

What is the ability of the liquid slag layer on the surface of the mold steel surface to dissolve Al2O3 inclusions? Studies have pointed out that when CaO%/SiO2%=0.9~1.0, the Al2O3 content in the slag is greater than 20%, and high melting point compounds will precipitate, which will increase the melting point and viscosity of the slag, and it will no longer be able to absorb floating inclusions. . However, during the pouring process, the mold mold slag is continuously consumed and floating inclusions are continuously absorbed, causing the slag to be enriched by Al2O3. In order to maintain the good ability of the slag to absorb Al2O3 without changing the performance of the slag, the following measures can be taken:. (1) When preparing slag powder, select appropriate raw materials and reduce the Al2O3 content in the original slag as much as possible. (2) Appropriately increase the consumption of slag powder and flush out the Al2O3 content in the dilute slag. (3) During the pouring process, Al2O3 is enriched in the slag, and the crystallizer slag replacement operation can be used.

What are the functions of the thickness of the crystallizer liquid slag layer and its measurement methods?

To achieve good use effects, the mold slag must have a liquid slag layer thickness that meets actual needs. If the liquid slag layer is too thick or too thin, it will cause vertical cracks on the slab surface. If the slab pulling speed is 1.2~1.5m/min and the thickness of the liquid slag layer is less than 5mm, the longitudinal cracks in the slab will increase significantly (from 50mm/m to 200mm/m). The thickness of the liquid slag layer is 6 to 15 mm, and the longitudinal cracks almost disappear. The liquid slag layer is greater than 20 mm, and the longitudinal cracks increase. If the thickness of the liquid slag layer is less than a certain value, the slag ring formed along the periphery of the crystallizer will block the passage between the meniscus liquid slag flowing into the billet shell and the copper wall. As a result, the liquid slag cannot smoothly flow into the surface of the billet shell to form a uniform slag film, which may cause longitudinal cracks on the corresponding surface of the cast billet. So what is the thickness of the liquid slag required for the liquid slag to flow down the meniscus without being blocked? According to theoretical calculations, it is pointed out that when the pulling speed is less than lm/min, the thickness of the liquid slag layer is 5~7mm; when the pulling speed is greater than lm/min, the thickness of the liquid slag layer is 7~15mm. This is consistent with the critical liquid slag layer thickness measured in production practice. Method for measuring the thickness of the liquid slag layer in production: tie a steel wire and a copper wire (or aluminum wire) together and insert them into the mold slag layer. Since the temperature of the liquid slag is higher than the melting point of copper, the copper wire melts , measuring the length of the melted copper wire is the thickness of the liquid slag layer. Since the molten steel temperature is different at each point on the slab mold section (such as the immersed nozzle area and the edge of the mold), the thickness of the liquid slag layer is also different, so the thickness of the liquid slag layer at different locations can be measured.

How does mold powder lubricate?

During the pouring process, the crystallizer vibrates up and down, and the cast slab moves downward, which generates friction between the surface of the solidified shell and the copper wall, causing the shell to bond with the copper wall and increasing the resistance to casting. The lighter case will cause cracks in the shell, and the severe case will cause the shell to crack. Therefore, lubrication must be carried out between the billet shell and the copper wall, and this function can only be achieved by using protective slag. To ensure good lubrication, there must be a liquid slag film of suitable properties and uniform thickness between the solidified shell and the copper wall. The liquid slag layer on the mold steel liquid surface is a source of continuous supply of liquid slag film. To this end, it is necessary to ensure that the channel between the liquid slag near the meniscus of the crystallizer and the flow of liquid slag into the billet shell and the copper wall is unblocked and not blocked by the slag ring around the copper wall. So how is the lubricating slag film formed? When the molten steel is poured into the crystallizer, a green shell is formed, and protective slag powder is added to the liquid surface. The slag powder melts to form a liquid slag layer, and the liquid slag near the copper wall cools to form a slag ring. As the crystallizer moves downward, the slag is gradually squeezed into the space between the billet shell and the copper wall so that it is completely filled with slag. The temperature of the copper wall is low, and the slag shell on the side close to the copper wall remains a solid slag skin, while the surface temperature of the condensation shell is high, and the slag on the side close to the shell is a liquid slag film, which is fluid. In this way, the liquid slag film between the copper wall of the mold and the billet shell is lubricated, which is consumed as the billet is pulled out, while the solid slag skin attached to the copper wall is basically not consumed as the crystallizer vibrates. While the slag film is continuously consumed, the liquid slag on the steel surface is continuously replenished downward through the meniscus channel, forming a stable liquid slag film. The thickness of the slag film is related to factors such as slag viscosity, pulling speed, and crystallizer vibration. It is known that the viscosity of the slag is constant, the pulling speed increases, and the thickness of the slag film increases; and the pulling speed is constant, the viscosity increases, the thickness of the slag film decreases. Generally, the slag film thickness is 50~200μm, and the slag consumption is 0.4~0.6kg/t. , Therefore, to ensure that the lubrication of the solidified shell by the slag film is in the best state, the thickness of the slag film, the consumption of slag, and the viscosity of the slag must be properly coordinated. When the vibration of the crystallizer is constant, the viscosity (η) and the pulling speed (V) should be matched appropriately. The combination of low viscosity and low pulling speed, or high viscosity and high pulling speed is not advisable. The product of the two, eta·V, is used as an index to evaluate the lubrication condition. If the eta·V value is too small or too large, it means that the slag film thickness and consumption are inappropriate, and the lubrication condition is poor.

What are the design principles for the composition of mold powder?

To realize the five functions of mold powder, the key is to prepare mold powder with appropriate ingredients. The protective slag material currently commonly used in continuous casting is based on a slag system composed of CaO-SiO2-Al203 ternary compounds. And contains appropriate amounts of Na2O, CaF2, K20 and other compounds. This kind of slag becomes weakly acidic or neutral liquid slag after melting. It has good wettability to molten steel and the viscosity of the slag changes gently with temperature. Continuous casting mold slag is basically composed of three materials: (1) Basic slag. Contains basic slag material of CaO, SiO2 and Al203. According to the CaO-SiO2-Al203 ternary phase diagram, the composition range of these three compounds is: Ca0 10~38%, Si0240~60%, and Al203 is less than 10%. The melting point is higher than 1300℃. (2) Flux. For example, Na2O and CaF2 can reduce the melting point and viscosity of slag. Depending on the resources, LiO2, K20, BaO, NaF, B2O3, etc. can also be used as fluxes. The amount added depends on the melting point of the slag. (3) Regulator. Carbon particles are melting rate regulators. The added amount is 5~7%. According to the requirements of the steel type, the appropriate content of each compound in the mold flux is determined through experiments.

What are the main raw materials used to prepare mold powder?

The raw materials for preparing protective powder include: natural minerals, industrial waste and industrial products. The raw materials that have been used as basic slag materials include: cement, cement clinker, wollastonite, feldspar, quartz, power plant flue ash, blast furnace slag, electric furnace white slag, etc. As flux auxiliary materials include: caustic soda, fluorite, barite, cryolite, borax, lithium carbonate, etc. Melting speed regulators include natural graphite, carbon black, lamp black, etc.

What impact does mold powder have on the quality of continuous casting billets?

The protective slag is added to the surface of the steel surface of the mold. The quality of the protective slag mainly affects the surface quality of the cast slab: (1) Longitudinal cracks on the surface of the cast slab: Longitudinal cracks originate from the thickness of the primary green shell in the meniscus area of the mold. Unevenness. The liquid slag on the surface of the steel surface cannot flow evenly and distribute around the cast slab, resulting in uneven thickness of the solidified shell. Stress concentration is likely to occur in the thinner parts of the shell. When the stress exceeds the high temperature strength of the solidified shell, cracks will occur. Studies have pointed out that maintaining the liquid slag layer on the mold steel liquid surface at 5 to 15 mm can significantly reduce longitudinal cracks on the slab surface. Longitudinal cracking is also related to slag viscosity (eta), melting speed (tf) and pulling speed (V). Someone pointed out that the larger the eta/tf ratio, the smaller the longitudinal fissure index. For example, if the slag temperature is 1300°C, eta/tf=1, the longitudinal cracking index is 6, and eta/tf=2, the longitudinal cracking index is 0. Some people believe that the continuous casting slab η·V should be controlled at 2 to 3.5. Controlling the billet η·V at 5 can make the slag film uniform, stabilize heat transfer, provide good lubrication, and significantly reduce cracks. (2) Slag inclusion: Slag inclusion in the cast billet can be divided into surface slag inclusion and subcutaneous slag inclusion. Slag inclusions vary in size. From a few millimeters to more than ten millimeters, the depth of slag inclusions on the surface is also different. Slag inclusions seriously harm the surface quality of the product, so they must be removed before thermal processing. The mold shell is entangled with slag, which is an important source of slag inclusion. For example, slag spots are formed on the surface of the billet shell, where the thermal conductivity is poor and the solidification shell is thin, forming a high-temperature “hot spot”, which is one of the causes of steel leakage in the mold billet shell. The main components of slag inclusions on the surface of the cast slab are anorthite and anorthite. The A12O3 in these two compounds is greater than 20%. Their melting points are 1550°C and 1590°C respectively, which can easily cause slag to agglomerate. If the liquid level in the crystallizer fluctuates too much and the immersion nozzle is inserted too shallowly, the stirring of the liquid level will drag in the slag. “Steelmaking and Continuous Casting Flame Cutting High Efficiency and Energy Saving Technology” has been listed as a key scientific and technological achievement promotion project by the Ministry of Science and Technology. This technology product has been successfully applied in several steelmaking companies, which has reversed the high gas consumption, large cutting slits, and cutting cross-sections of steel billets in the past. Roughness, high oxygen pressure, lots of dust in the factory, loud noise, heavy environmental pollution, many damaged cutting tools, high labor intensity for workers, etc. show huge energy-saving and consumption-reducing power and excellent environmental protection effects. This technology has the following characteristics : 1. Advanced technology: The flame is concentrated during cutting, and the cutting speed is fast; the cutting section is smooth, the upper edge does not collapse, the lower edge has less slag, and the yield rate is high; automation can be realized, and the cutting and continuous casting speeds match. 2. Save steel: The cutting seams of billets and slabs can be kept at about 3mm, and each ton of steel can reduce cutting losses by more than 0.5 kilograms. 3. Energy saving: The gas pressure of the energy-saving continuous casting cutting nozzle is 1/2 to 1/2 of that of other cutting nozzles. 1/3, the oxygen pressure is 1/2 of other cutting nozzles, which can save more than 50% of gas and 40-50% of oxygen. Automatic flameout and ignition can be realized during cutting.

What are the types of continuous casting mold slag?

According to the designed composition of mold powder, suitable raw materials are selected and processed through crushing, ball milling, mixing and other production processes to make mold powder. There are four types. (1) Powdered protective powder: It is a mechanical mixture of various powdery materials. During the long-distance transport process, due to long-term vibration, materials with different specific gravity segregate and the uniform state of the slag material is destroyed, affecting the stability of the use effect. At the same time, when slag powder is added to the crystallizer, the dust flies up and pollutes the environment. (2) Granular protective slag: In order to overcome the shortcomings of environmental pollution, an appropriate amount of binder is added to the powdery slag to make granular protective slag similar to millet grains. The production process is complicated and the cost increases. (3) Pre-melted mold slag: Mix the various slag-making materials and put them into a pre-melting furnace to melt them into one body. After cooling, crush and grind them into fine pieces, and add appropriate melting speed regulators to obtain pre-melted powdery mold slag. Premelted mold powder can also be further processed into granular mold powder. The production process of pre-melted mold slag is complicated and the cost is high. But the advantage is to improve the uniformity of mold slag formation. (4) Heating type mold slag: Add exothermic agent (such as aluminum powder) to the slag powder to oxidize it and release heat, and quickly form a liquid slag layer. However, this kind of slag formation speed is difficult to control and the cost is high, so it is rarely used.

What are the main physical and chemical properties of continuous casting mold slag?

After the protective slag is prepared, the physical and chemical properties of the slag must be measured. The main physical and chemical indicators are as follows: (1) Chemical composition: The chemical composition of each brand of mold powder should be analyzed. The content of each oxide should be within the specified range. This is the minimum indicator. (2) Melting temperature: Make the slag powder into a Φ3×5mm sample, and heat the sample on a special instrument to the temperature at which the cylinder becomes a hemisphere. The temperature reaching the hemispheric point is defined as the melting temperature. (3) Viscosity: It indicates the flow properties of slag powder melted into liquid. The fluidity of slag has an important impact on the slag’s absorption of inclusions and the lubrication effect of the billet shell. Usually, a torsion viscometer or rotational viscometer is used to measure the viscosity of slag at 1300°C to compare the fluidity of different slags. (4) Melting speed: The melting speed is a measure of the speed of the slag melting process, and is related to whether a stable three-layer structure can be formed on the liquid surface of the mold steel and the required thickness of the liquid slag layer. The melting speed can be expressed by the time required for a standard sample to completely melt into a liquid at a specified temperature (such as 1300°C or 1400°C). It can also be expressed by the amount of liquid slag formed per unit area and time when a certain weight of mold slag powder is heated to a specified temperature. (5) Spreadability: It indicates the covering ability and uniformity of the slag added to the molten steel surface. It can be measured by the area of mold powder within a certain volume that flows down from a specified height onto a flat plate. (6) Moisture: Protective slag powder easily absorbs moisture. If the adsorbed moisture content exceeds the specified requirements (such as 0.5%), the slag powder will agglomerate, endangering the use effect.

How to control the moisture content of mold powder?

The moisture content of mold powder is divided into two categories: adsorbed water and crystal water. Moisture can cause slag powder to clump and deteriorate its quality. Moisture should be limited to less than 0.5%. Certain substances in the base material, such as soda, solids, and sodium silicate, have a strong ability to absorb water. When water is absorbed, the powder residue becomes rolled into balls, causing trouble to the continuous casting operation. The water absorption of mold powder mainly depends on the type and particle size of raw materials. The finer the particle size, the greater the water absorption. At 200 mesh, the water absorption of cement is 0.41%, solid water glass is 3.24%, fluorite is 0.45%, soda is 15.9%, and graphite is trace. Method to control moisture: The baking temperature of raw materials should not be lower than 110℃. Properly extend the baking time. The raw materials after baking should be batched and mixed in time, and the prepared slag powder should be sealed and encapsulated in time. For steel types with higher quality requirements, the raw materials for mold flux are best baked to above 800°C to remove crystal water, or pre-melted mold flux is used.

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