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Continuous casting powder FAQ

[vc_row][vc_column][vc_toggle title=”What is the function of continuous casting mold flux?”]During the casting process, powder or granular slag, called mold slag, should be continuously added to the molten steel surface of the mold. The role of mold slag has the following aspects:

(1) heat insulation to prevent heat dissipation;

(2) isolate the air to prevent the oxygen in the air from entering the molten steel for secondary oxidation, which affects the quality of the steel;

(3) absorb and dissolve from the molten steel and float up Inclusions at the interface of the steel slag to purify the molten steel;

(4) There is a layer of slag film between the mold wall and the solidification shell to lubricate, reduce the drawing resistance, and prevent the bonding between the solidification shell and the copper plate;

(5) Fill the shell The air gap between the mold and the mold improves the heat transfer of the mold. A good mold slag should be able to give full play to the above five aspects, so as to improve the surface quality of the slab and ensure the smooth running of continuous casting.[/vc_toggle][vc_toggle title=”What are the requirements for the mold flux melting mode?”]In the continuous casting process, the mold slag added to the mold must have a prescribed melting mode to complete the above five functions, that is, the so-called slag layer-sintering layer-one liquid must be formed on the molten steel surface. The so-called three-layer structure of the slag layer. The slag powder with low melting point (1100-1200°C) added to the high-temperature molten steel (about 1500°C) of the mold is supplied with heat by the molten steel, forming a slag covering layer of a certain thickness (about 10~1200°C) on the molten steel surface. l5mm), the heat transfer from molten steel to the slag layer is slowed down, and the slag on the liquid slag layer is heated, and the slag and powder are sintered together to form a so-called sintered layer (temperature at 900-600 ° C), and the powder on the sintered layer is The slag receives less heat from the molten steel and has a low temperature (<500°C), so it remains in powder form and evenly covers the molten steel surface, preventing the molten steel from dissipating heat and preventing oxygen in the air from entering the molten steel. During the billet drawing process, due to the up and down vibration of the mold and the downward movement of the solidified billet shell, the liquid slag layer on the molten steel surface continuously passes through the interface between the molten steel and the copper wall and squeezes into the billet shell and the copper wall. A layer of solid slag film is formed on the surface, and a layer of liquid slag film is formed on the surface of the condensate shell. This layer of liquid slag film plays a lubricating role on the mold wall and the surface of the shell, just like adding lubricating oil when the motor shaft rotates. At the same time, the slag film fills the air gap between the shell and the copper wall, reducing the thermal resistance and improving the heat transfer of the crystallization. With the progress of billet drawing, the liquid slag on the molten steel surface is continuously consumed, and the sintered layer descends to the molten steel surface and melts into a liquid slag layer, and the powder slag layer becomes a sintered layer, and then new slag powder is added to the mold. Keep it as a three-layer structure, and in such a cycle, the mold powder is continuously consumed.[/vc_toggle][vc_toggle title=”How to realize the so-called “three-layer structure“ of the mold powder powder? “]In order to exert the five functions of the mold powder, it is necessary to make the mold powder powder form a “three-layer structure”. To form a “three-layer structure”, the key is to control the melting speed of the mold slag powder, that is, the slag powder added to the molten steel surface should not be melted into a liquid all at once, but gradually melted. For this reason, carbon particles are generally added to the mold flux as a melt rate regulator. The speed of controlling the melting rate of carbon particles depends on the type and quantity of carbon particles added. Carbon is a high-temperature resistant material, and the extremely 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 the carbon powder is not added enough, the temperature of the slag layer has not reached the sintering temperature of the slag material, and the carbon particles have been burned out, the sintered layer is developed, the melting rate is too fast, and the liquid slag layer is too thick. If too much carbon powder is added and some carbon particles still exist after the slag is completely melted, the sintered layer will shrink and the thickness of the sintered layer will be too thin. When the amount of carbon powder added is moderate, some of the carbon particles in the sintered layer are burned out, and the rest of the slag is still effectively controlled by the carbon particles, so that a sintered layer and a liquid slag layer of suitable thickness will be obtained. There are two kinds of carbon materials, graphite and carbon black. Graphite particles are coarse, with a particle size of 60-80 μm, and their separation and blocking effects are poor, but the initial oxidation temperature is high (about 560 ° C), the oxidation speed is slow, and the ability to control the melting rate in the high temperature region is strong. Carbon black has an amorphous structure with very fine particles (0.06-0.10μm), strong separation and blocking effects, low starting oxidation temperature (500 ° C), and fast oxidation speed, so carbon black is controlled in the lower temperature area of ​​the slag layer. The melting rate is strong, and the control efficiency is low 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%.[/vc_toggle][vc_toggle title=”What are the factors that affect the absorption of inclusions in molten steel by mold slag?”]The submerged nozzle injection causes the convective movement of molten steel in the mold, and the inclusions floating on the interface of the mold slag may be involved in the solidification shell due to the fluctuation of the liquid level of the mold, resulting in subcutaneous inclusions or surface inclusions of the slab. Slag, affecting the surface quality. Therefore, it is hoped that the inclusions floating up to the slag interface are quickly absorbed and dissolved by the liquid slag layer. To make the inclusions floating on the steel slag interface quickly transfer to the liquid slag, this process depends on:

(1) the contact area of ​​the steel slag interface;

(2) the viscosity of the liquid slag;

(3) the ability of the slag to dissolve the inclusions.

That is to say, the better the fluidity of the slag, the larger the contact area of ​​the steel slag, and the easier it is for inclusions to enter the slag. As long as the inclusions enter the slag, the slag can absorb and dissolve quickly, and the ability of the slag to dissolve the inclusions is mainly determined by the chemical composition of the slag, that is, the content of CaO and SiO2, (CaO%/SiO2% is called alkalinity) and the slag. The original Al2O3 content. The production test pointed out that the alkalinity increases, the ability of the slag to dissolve Al2O3 inclusions increases, when the alkalinity is greater than 1.1, the ability to dissolve Al2O3 decreases; the original Al2O3 content in the slag is greater than 10%, the slag dissolves Al2O3 rapidly decreases. Therefore, when preparing mold slag, the ratio of CaO% to SiO2% of the slag should be 0.9 to 1.0, and the original Al2O3 content should be as low as possible, generally less than 10%. How big is the dissolving ability of the liquid slag layer on the molten steel surface of the mold to Al2O3 inclusions? The study pointed out: when CaO%/SiO2% = 0.9 to 1.0, when the Al2O3 content in the slag is greater than 20%, there will be compounds with high melting points. The melting point increases, the viscosity increases, and the floating inclusions can no longer be absorbed. However, during the casting process, the mold slag is continuously consumed, and the floating inclusions are also continuously absorbed, so that the slag is enriched by Al2O3. In order to keep the slag having a good ability to absorb Al2O3 without changing the properties 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, the content of Al2O3 in the dilute slag is flushed;

(3) the Al2O3 in the slag is enriched during the casting process, and the crystallizer can be used to change the slag.[/vc_toggle][vc_toggle title=”What are the functions of the thickness of the liquid slag layer of the mold and its measurement methods?”]To achieve good use effect of mold slag, there must be a thickness of liquid slag layer that meets actual needs. If the liquid slag layer is too thick or too thin, the slab will produce surface longitudinal cracks. If the slab pulling speed is 1.2~1.5m/min, the thickness of the liquid slag layer is less than 5mm, the longitudinal cracks of the slab increase significantly (from 50mm/m to 200mm/m), the thickness of the liquid slag layer is 6~15mm, and the longitudinal cracks almost disappear. , the liquid slag layer is larger than 20mm, and the longitudinal cracks have increased. When the thickness of the liquid slag layer is less than a certain value, the slag ring formed along the periphery of the mold will block the channel between the meniscus liquid slag flowing into the billet shell and the copper wall, so that the liquid slag cannot smoothly flow into the billet shell surface and form a uniform slag film, longitudinal cracks may occur on the surface of the corresponding slab. Then, what is the thickness of the liquid slag required for the passage of the liquid slag to pass through the meniscus without being blocked? According to theoretical calculations, it is pointed out that the pulling speed is less than lm/min, the thickness of the liquid slag layer is 5-7 mm, and the pulling speed is greater than lm/min. min, the thickness of the liquid slag layer is 7-15mm. This is consistent with the thickness of the critical liquid slag layer measured in production practice. The method of measuring the thickness of the liquid slag layer in production: tie a steel wire and a copper wire (or aluminum wire) together and insert it into the mold slag layer. Since the temperature of the liquid slag is higher than the melting point of copper, the copper wire melts. , the length of the molten copper wire is measured to be the thickness of the liquid slag layer. Since the molten steel temperature is different at each point of the slab mold section (such as the submerged nozzle area and the mold edge), the thickness of the liquid slag layer is also different, so the thickness of the liquid slag layer at different positions can be measured.[/vc_toggle][vc_toggle title=”How does mold slag play a lubricating role?”]During the casting process, the mold vibrates up and down, the billet moves downward, and friction occurs between the surface of the solidified shell and the copper wall. In severe cases, the shell will be cracked. Therefore, lubrication must be carried out between the billet shell and the copper wall, which can only be achieved by mold flux. To ensure good lubrication, there must be a liquid slag film with suitable properties and uniform thickness between the solidified shell and the copper wall. The liquid slag layer on the molten steel surface of the mold is the source of continuous supply of the liquid slag film. To this end, it is necessary to ensure that the channel between the liquid slag near the meniscus of the mold flowing into the blank shell and the copper wall is unobstructed, and is 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 mold, a primary shell is formed, and mold slag powder is added to the liquid surface, and the slag powder melts to form a layer of liquid slag, which is close to the liquid around the copper wall. The slag is cooled to form a slag ring, and as the mold moves downward, the slag is gradually squeezed into the space between the 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 near the copper wall remains as a solid slag skin, while the surface temperature of the condensed shell is high, and the slag on the side of the blank shell is a liquid slag film with fluidity. In this way, the liquid slag film is used to lubricate the mold copper wall and the billet shell, which is consumed as the billet is pulled out, and the solid slag skin attached to the copper wall is basically not consumed with the vibration of the mold. While the slag film is continuously consumed, the liquid slag on the molten 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 the viscosity of the slag, the pulling speed, and the vibration of the mold. Knowing that the viscosity of the slag is constant, the pulling speed increases, and the thickness of the slag film increases; while the pulling speed is constant and the viscosity increases, the thickness of the slag film decreases. Generally, the thickness of slag film is 50-200μm, and the consumption of slag is 0.4-0.6kg/t. Therefore, in order to make the lubrication of the slag film to the solidified shell in the best state, the thickness of the slag film, the consumption of the slag, and the viscosity of the slag must be properly matched. When the vibration of the mold is constant, the viscosity (η) and the pulling speed (V) should be properly matched. It is not advisable to combine low viscosity and low pulling speed, or high viscosity and high pulling speed. The product of the two is η·V As an index to evaluate the lubrication condition, if the value of η·V is too small or too large, it means that the thickness and consumption of the slag film are not appropriate, and the lubrication condition is poor.

[/vc_toggle][vc_toggle title=”What are the main raw materials used in the preparation of mold slag?”]As the raw materials for the preparation of mold slag, there are: natural minerals, industrial wastes 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. The auxiliary materials for the flux include: caustic soda, fluorite, barite, cryolite, borax, lithium carbonate, etc. Melting rate regulators include natural graphite, carbon black, lamp black, etc.[/vc_toggle][vc_toggle title=”What is the effect of mold flux on the quality of continuous casting slabs?”]Molding flux is added to the molten steel surface of the mold, and the quality of the mold flux mainly affects the surface quality of the slab:

(1) Longitudinal cracks on the surface of the slab: The longitudinal cracks originate from the thickness of the primary shell in the meniscus area of ​​the mold. inhomogeneity. The liquid slag on the molten steel surface cannot flow into and distribute evenly around the casting billet, resulting in uneven thickness of the solidified shell. Stress concentration is likely to occur in the thin shell of the billet. When the stress exceeds the high temperature strength of the solidified shell, cracks occur. The study pointed out that the liquid slag layer on the molten steel surface of the mold is maintained at 5-15 mm, which can significantly reduce the longitudinal cracks on the surface of the slab. Longitudinal cracking is also related to slag viscosity (η), melting speed (tf) and pulling speed (V). It was pointed out that the larger the η/tf ratio, the smaller the longitudinal crack index. If the slag temperature is 1300℃, η/tf=1, the longitudinal crack index is 6, η/tf=2, and the longitudinal crack index is 0. Some people think that: the continuous casting slab η · V is controlled at 2 ~ 3.5. The η·V of the billet is controlled at 5, which can make the slag film uniform, the heat transfer is stable, the lubrication is good, and the cracks can be significantly reduced.

(2) Slag inclusion: Slag inclusion in the slab 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 slag inclusions have different depths on the surface. Slag inclusions seriously impair the surface quality of the product and must therefore be removed prior to hot working. The mold shell is involved in 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 condensed shell is thin, forming a high temperature “hot spot”, which is one of the reasons for the breakout of the mold shell. The slag inclusions on the surface of the slab are mainly composed of anorthite and anorthite, both of which contain more than 20% A12O3, and their melting points are 1550°C and 1590°C respectively, which are easy to agglomerate the slag. When the liquid level of the crystallizer fluctuates too much, the immersion nozzle is inserted too shallowly, and the liquid level will be involved in the slag. “Steelmaking and continuous casting flame cutting high-efficiency and energy-saving technology” was listed as a key scientific and technological achievement promotion project by the Ministry of Science and Technology.

This technical product has been successfully applied in several steelmaking enterprises, which has reversed the high consumption of gas, large cutting kerf, rough cutting section, high oxygen pressure, high dust in the workshop, high noise, heavy environmental pollution, and more damage to cutting tools in the past. The labor intensity of workers is high, which shows great energy saving and consumption reduction power and excellent environmental protection effect.

The 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 is less slag, and the yield is high; it can realize automation, cutting and continuous casting speed match.

2. Saving steel: The cutting seam of cutting billets and slabs can be maintained at about 3mm, and the cutting damage can be reduced by more than 0.5 kg per ton of steel.

3. Energy saving: the gas pressure of the energy-saving continuous casting cutting nozzle is 1/2 to 1/3 of other cutting nozzles, and the oxygen pressure is 1/2 of other cutting nozzles, which can save more than 50% of gas and 40%-50% of oxygen. %, automatic fire off and ignition can be realized during cutting.[/vc_toggle][vc_toggle title=”What are the types of continuous casting mold flux?”]According to the composition of the designed mold powder, the mold powder is made by selecting suitable raw materials through crushing, ball milling, mixing and other production processes. There are four types.

(1) Powdered mold powder: It is a mechanical mixture of various powdery materials. In the process of long-distance transportation, due to the long-term vibration, the materials of different specific gravity are segregated, and the uniform state of the slag is destroyed, which affects the stability of the use effect. At the same time, when adding slag powder to the crystallizer, the dust is flying, which pollutes the environment.

(2) Granular mold powder: In order to overcome the disadvantage of polluting the environment, an appropriate amount of binder is added to the powdery slag to make a granular mold powder similar to millet grains. The production process is complicated and the cost has increased.

(3) Pre-melting mold slag: Mix the slag-forming materials and put them into a pre-melting furnace to melt them into one. After cooling, they are crushed and ground, and an appropriate melting rate regulator is added to obtain a pre-melting powder mold slag. The premelted mold powder can also be further processed into granular mold powder. The production process of premelting mold flux is complicated and the cost is high. But the advantage is to improve the uniformity of mold slag formation.

(4) Heat-generating mold slag: add a heat-generating agent (such as aluminum powder) to the slag powder to make it oxidize to release heat and quickly form a liquid slag layer. However, the slag-forming speed of this kind of slag is not easy to control, and the cost is high, so it is less used.[/vc_toggle][vc_toggle title=”What are the main physical and chemical properties of continuous casting mold flux?”]After the mold slag is prepared, the physical and chemical properties of the slag should be measured. The main physical and chemical indicators are as follows:

(1) Chemical composition: the chemical composition of each brand of mold slag should be analyzed, and the content of each oxide should be within the specified range. Inside, this is the bare minimum indicator.

(2) Melting temperature, the slag powder is made into a sample of Φ3×5mm, and the sample is heated on a special instrument to the temperature at which the cylinder turns into a hemisphere, and the temperature reaching the hemispherical 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 influence on the lubricating effect of slag absorbing inclusions and shell. Usually, the viscosity of the slag at 1300°C is measured with a torsional viscometer or a rotational viscometer to compare the fluidity of different slags.

(4) Melting speed: The melting speed is a measure of the speed of the slag melting process, which is related to whether a stable three-layer structure can be formed on the molten steel surface of the mold and the required thickness of the slag layer. The melting rate 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 represented by a certain weight of mold slag powder, heated to a specified temperature, and the amount of liquid slag formed per unit area and time.

(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 ​​powder powder in a certain volume that flows down from the specified height to the spreading area on the plate.

(6) Moisture: Mold powder is easy to absorb moisture. If the amount of adsorbed moisture exceeds the specified requirements (such as 0.5%), the slag powder will agglomerate, which will endanger the use effect.[/vc_toggle][vc_toggle title=”How to control the moisture of mold slag?”]The mold residue moisture is divided into two categories: adsorbed water and crystal water. Moisture can make mold powder agglomerate and deteriorate its quality. Moisture should be limited to less than 0.5%. Certain substances in the base material, such as soda, solid, and body water glass, have a strong ability to absorb water. When water is absorbed, the powder slag is rolled into a ball, which brings trouble to the continuous casting operation. The water absorption of mold slag is mainly determined by the type and particle size of raw materials. The finer the particle size, the greater the water absorption rate. At 200 mesh, the cement water absorption rate is 0.41%, the solid water glass is 3.24%, the fluorite is 0.45%, the soda is 15.9%, and the graphite is trace. Moisture control method: the baking temperature of raw materials is not 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 packaged in time. For steel grades with higher quality requirements, it is best to bake the mold flux raw materials to above 800 °C to remove crystal water, or to use pre-melted mold flux.[/vc_toggle][vc_toggle title=”What are the design principles of mold flux components?”]To realize the five functions of mold powder, the key is to formulate mold powder with appropriate components. The mold slag slag commonly used in continuous casting is based on the slag system composed of CaO-SiO2-Al2O3 ternary compounds. And contains an appropriate amount of Na2O, CaF2, K20 and other compounds. This kind of slag material is weakly acidic or neutral liquid slag after melting, and has good wettability to molten steel, and the slag viscosity changes smoothly with temperature. Continuous casting mold slag is basically composed of three materials: (1) basic slag. Contains CaO, SiO2, Al203 basic slag. According to the ternary phase diagram of CaO-SiO2-Al203, the composition range of these three compounds is: Ca0 10-38%, Si0240-60%, Al203 less than 10%. The melting point is higher than 1300℃. (2) Flux. Such as Na2O, 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, and the amount added depends on the melting point of the slag. (3) Regulator. The carbon particles are melt rate modifiers. The added amount is 5-7%. According to the requirements of steel grades, the appropriate content of each compound in the mold flux is determined through experiments.[/vc_toggle][/vc_column][/vc_row][vc_row][vc_column][vc_btn title=”About LMM GROUP” link=”|target:_blank”][vc_column_text css=”.vc_custom_1649296177864{margin-top: 50px !important;}”]friendship Link:

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