Continuous casting mold powder knowledge essence post

　　1. what is the function of continuous casting mold powder?

　　During the pouring process, powdery or granular slag materials shall be continuously added to the water surface of the mold steel, which is called protective slag. The functions of protective slag are as follows:

　　(1) Thermal insulation to prevent heat dissipation;

　　(2) Separate the air to prevent the oxygen in the air from entering the molten steel to cause secondary oxidation and affect the quality of the steel;

　　(3) Absorbing and dissolving inclusions floating from molten steel to slag interface to purify molten steel;

　　(4) There is a layer of slag film between the mold wall and the solidified shell for lubrication, reducing the blank drawing resistance and preventing the bonding between the solidified shell and the copper plate;

　　(5) Filling the air gap between the green shell and the mold to improve the heat transfer of the mold.

　　A good mold flux should give full play to the above five aspects, so as to improve the surface quality of slab and ensure the smooth running of continuous casting.

　　2. what are the requirements for mold flux melting mode?

　　In order to complete the functions of the above five aspects, the mold flux added to the mold during continuous casting must have a specified melting mode, that is, the so-called three-layer structure of the so-called powder slag layer sintering layer liquid slag layer is required to be formed on the steel surface.

　　The slag powder with low melting point (1100 ~ 1200 ℃) added to the high-temperature molten steel (about 1500 ℃) surface of the crystallizer forms a certain thickness of liquid slag covering layer (about 10 ~ l5mm) on the steel liquid surface by providing heat from the molten steel. The heat transfer from the molten steel to the powder slag layer slows down. The powder slag on the liquid slag layer is heated, and the slag powders are sintered with each other to form a so-called sintering layer (at 900 ~ 600 ℃). The powder slag on the sintering layer receives less heat from the molten steel, The temperature is low (<500 ℃), so it is kept in powder form and evenly covered on the steel surface, preventing the heat dissipation of molten steel and preventing the oxygen in the air from entering the molten steel.

　　During the drawing process, due to the up and down vibration of the mold and the downward movement of the solidified shell, the liquid slag layer on the steel level is continuously squeezed between the shell and the copper wall through the interface between the molten steel and the copper wall, forming a solid slag film on the surface of the copper wall and a liquid slag film on the surface of the condensate shell. This liquid slag film acts as a lubricant on the surface of the mold wall and 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 billet shell and the copper wall, reduces the thermal resistance and improves the heat transfer of crystallization.

　　With the progress of billet drawing, the liquid slag on the steel liquid level is continuously consumed, while the sintering layer drops to the steel liquid level and melts into a liquid slag layer. The powder slag layer becomes a sintering layer, and then new slag powder is added to the crystallizer to maintain a three-layer structure. In this way, the protective slag powder is continuously consumed.

　　3. how to realize the so-called "three-layer structure" of mold powder?

　　In order to give full play to the five functions of mold powder, it is necessary to form a "three-layer structure" of slag powder added to the crystallizer. The key to forming a "three-layer structure" is to control the melting rate of the protective slag powder, that is, the slag powder added to the steel level should not melt into liquid at once, but gradually melt. For this reason, carbon particles are usually added into the mold flux as a regulator of melting rate.

　　The speed of controlling melting rate by carbon particles depends on the type and quantity of carbon particles. 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 the carbon powder is insufficient, the temperature of slag layer has not reached the starting sintering temperature of slag, and the carbon particles have been burned out, the sintering layer is developed, the melting speed 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 sintering layer will shrink and the thickness of the sintering layer will be too thin. When the amount of carbon powder is moderate, some carbon particles are burned out in the sintering layer, and the rest of the slag is still effectively controlled by carbon particles. In this way, the sintering layer and liquid slag layer with appropriate thickness can be obtained.

　　Carbon materials include graphite and carbon black. Graphite particles are coarse, with particle size of 60 ~ 80 μ m. Its separation and retardation are poor, but the initial oxidation temperature is high (about 560 ℃), the oxidation speed is slow, and the ability to control the melting rate in the high temperature zone is strong. Carbon black is amorphous structure with fine particles (0.06 ~ 0.10 μ m) , with strong separation and retardation, low initial oxidation temperature (500 ℃) and fast oxidation speed, carbon black has strong ability to control melting rate in the zone with low slag layer temperature and low control efficiency in the zone with high temperature. Even if the dosage is increased, the improvement effect is limited.

　　Generally, the amount of toner added is 4 ~ 7%.

　　4. what factors affect the absorption of inclusions in molten steel by mold flux?

　　The submerged nozzle injection causes the convection movement of molten steel in the mold, and the inclusions floating to the steel slag interface of the mold may be drawn into the solidification shell due to the fluctuation of the mold liquid level, resulting in subcutaneous inclusions or slag inclusions on the surface of the slab, affecting the surface quality. Therefore, it is hoped that the inclusions floating to the steel slag interface will be absorbed and dissolved by the liquid slag layer soon.

　　The inclusion floating to the steel slag interface shall be quickly transferred to the liquid slag. This process depends on:

　　(1) Contact area of steel slag interface;

　　(2) Viscosity of liquid slag;

　　(3) The ability of slag to dissolve inclusions.

　　That is to say, the better the fluidity of slag, the larger the contact area of steel slag, and the easier the inclusion will enter the slag. Once the inclusions enter the slag, the slag can quickly absorb and dissolve. The ability of slag to dissolve inclusions mainly depends on the chemical composition of slag, that is, the content of Cao and SiO2 (cao%/ sio2% is called alkalinity) and the original Al2O3 content in the slag.

　　The production test shows that with the increase of alkalinity, the slag's ability to dissolve Al2O3 inclusions increases. When the alkalinity is greater than 1.1, the slag's ability to dissolve Al2O3 decreases; If the original Al2O3 content in the slag is greater than 10%, the dissolved Al2O3 in the slag decreases rapidly. Therefore, when preparing protective slag, the ratio of cao% to sio2% of slag should be 0.9 ~ 1.0, and the original Al2O3 content should be as low as possible, generally less than 10%.

　　How big is the ability of liquid slag layer on the water surface of crystallizer steel to dissolve Al2O3 inclusions? It is pointed out that when cao%/ sio2%=0.9 ~ 1.0, the Al2O3 content in the slag is greater than 20%, there will be compounds with high melting point precipitated, which will increase the melting point and viscosity of the slag, so that the floating inclusions can no longer be absorbed.

　　However, during the pouring process, the mold flux is continuously consumed and the floating inclusions are continuously absorbed, so that the slag is enriched by Al2O3. In order to keep the slag with good Al2O3 absorption capacity without changing the slag performance, 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) Properly increase the consumption of slag powder and dilute the Al2O3 content in the slag;

　　(3) During pouring, with the enrichment of Al2O3 in the slag, the crystallizer can be used for slag replacement.

　　That is to say, the better the fluidity of slag, the larger the contact area of steel slag, and the easier the inclusion will enter the slag. Once the inclusions enter the slag, the slag can quickly absorb and dissolve. The ability of slag to dissolve inclusions mainly depends on the chemical composition of slag, that is, the content of Cao and SiO2 (cao%/ sio2% is called alkalinity) and the original Al2O3 content in the slag.

　　The production test shows that with the increase of alkalinity, the slag's ability to dissolve Al2O3 inclusions increases. When the alkalinity is greater than 1.1, the slag's ability to dissolve Al2O3 decreases; If the original Al2O3 content in the slag is greater than 10%, the dissolved Al2O3 in the slag decreases rapidly. Therefore, when preparing protective slag, the ratio of cao% to sio2% of slag should be 0.9 ~ 1.0, and the original Al2O3 content should be as low as possible, generally less than 10%.

　　How big is the ability of liquid slag layer on the water surface of crystallizer steel to dissolve Al2O3 inclusions? It is pointed out that when cao%/ sio2%=0.9 ~ 1.0, when the Al2O3 content in the slag is greater than 20%, compounds with high melting point will precipitate, which will increase the melting point and viscosity of the slag, so that the floating inclusions can no longer be absorbed.

　　However, during the pouring process, the mold flux is continuously consumed and the floating inclusions are continuously absorbed, so that the slag is enriched by Al2O3. In order to keep the slag with good Al2O3 absorption capacity without changing the slag performance, 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) Properly increase the consumption of slag powder and dilute the Al2O3 content in the slag;

　　(3) During pouring, with the enrichment of Al2O3 in the slag, the crystallizer can be used for slag replacement.

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

　　In order to achieve good application effect, the protective slag must have the thickness of liquid slag layer that meets the actual needs. If the liquid slag layer is too thick or too thin, the slab will have longitudinal surface cracks. For example, if the slab casting speed is 1.2 ~ 1.5m/min, the thickness of liquid slag layer is less than 5mm, the longitudinal cracks of slab increase significantly (from 50mm/m to 200mm/m), the thickness of liquid slag layer is 6 ~ 15mm, the longitudinal cracks almost disappear, and the liquid slag layer is more than 20mm, and the longitudinal cracks increase again.

　　If the thickness of the liquid slag layer is less than a certain value, the slag circle formed along the periphery of the crystallizer will block the channel between the meniscus liquid slag and the billet shell and the copper wall, so that the liquid slag cannot smoothly flow into the surface of the billet shell and form a uniform slag film, which may produce longitudinal cracks on the corresponding slab surface. What is the thickness of liquid slag required for the passage of liquid slag downstream through the meniscus not to be blocked? According to the theoretical calculation, the casting speed is less than lm/min, the thickness of liquid slag layer is 5 ~ 7mm, the casting speed is greater than lm/min, and the thickness of 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 liquid slag layer in production: bind a steel wire and a copper wire (or aluminum wire) together and insert them into the mold slag layer. Since the temperature of liquid slag is higher than the melting point of copper, the copper wire melts. The length of molten copper wire measured is the thickness of liquid slag layer. Since the molten steel temperature at each point of the slab mold section is different (such as the submerged nozzle area and the mold edge), and the liquid slag layer thickness is also different, the liquid slag layer thickness at different positions can be measured.

　　How does the protective slag lubricate?

　　During the pouring process, the crystallizer vibrates up and down, and the casting slab moves down, resulting in friction between the solidified shell surface and the copper wall, which makes the shell and the copper wall bond and increases the blank drawing resistance. The light one will cause cracks in the shell, and the heavy one will cause cracks in the shell. Therefore, lubrication must be carried out between the green shell and the copper wall, which can only be realized by the protective slag.

　　To ensure good lubrication, there must be a layer of liquid slag film with appropriate properties and uniform thickness between the solidified shell and the copper wall. The liquid slag layer on the mold steel level is the source of continuous supply of liquid slag film. To this end, it is necessary to ensure that the channel for liquid slag near the meniscus of the crystallizer to flow into the blank shell and the copper wall is unblocked and not blocked by the slag circle around the copper wall.

　　So how is the lubricating slag film formed? When the mold is filled with molten steel, the primary shell is formed. Add protective slag powder to the liquid surface, the slag powder melts and forms a layer of liquid slag layer. The liquid slag near the copper wall cools and forms a slag circle. As the mold moves downward, the slag is gradually squeezed between the shell and the copper wall to completely fill the slag. The copper wall temperature is low, and the slag shell near the copper wall remains a solid slag skin, while the surface temperature of the solidified shell is high. The slag near the green shell is a liquid slag film with fluidity. In this way, the copper wall and shell of the mold are lubricated by liquid slag film, which is consumed as the casting slab is pulled out, while the solid slag skin attached to the copper wall is basically not consumed as the mold vibrates. While the slag film is continuously consumed, the liquid slag on the steel surface is continuously supplemented downward through the meniscus channel to form a stable liquid slag film.

　　The thickness of slag film is related to slag viscosity, casting speed, mold vibration and other factors. When the viscosity of slag is constant, the casting speed increases and the thickness of slag film increases; When the casting speed is constant and the viscosity increases, the slag film thickness decreases. General slag film thickness 50 ~ 200 μ m. Slag consumption is 0.4 ~ 0.6kg/t., Therefore, in order to make the lubrication of slag film to solidified shell in the best state, the thickness of slag film, slag consumption and slag viscosity should be properly coordinated. When the crystallizer vibration is constant, the viscosity( η) And casting speed (V) shall be properly matched. Low viscosity and low casting speed, or high viscosity and high casting speed are not recommended. The product of the two shall be used η· V is used as an index to evaluate the lubrication condition, η· If the 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 flux?

　　In order to realize the five functions of mold flux, the key is to prepare mold flux with appropriate composition.

　　The protective slag commonly used in continuous casting is based on the slag system composed of cao-sio2-al203 ternary compound. It also contains an appropriate amount of Na2O, CaF2, K20 and other compounds. After melting, the slag is weakly acidic or neutral liquid slag, which has good wettability to molten steel, and the viscosity of slag changes gently with temperature. Continuous casting mold powder basically consists of three materials:

　　(1) Foundation slag. Containing CaO, SiO2 and Al203 basic slag. According to the ternary phase diagram of cao-sio2-al203, the composition range of the three compounds is: Ca0 10 ~ 38%, si0240 ~ 60%, Al203 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. According to the resources, LiO2, K20, Bao, NaF, B2O3, etc. can also be used as flux, and the amount added depends on the slag melting point.

　　(3) Regulator. Carbon particles are used as melting rate regulators. The addition amount is 5 ~ 7%.

　　According to the requirements of steel grade, the appropriate compound content of protective slag is determined through experiments.

　　What are the main raw materials used to prepare protective slag?

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

　　What is the effect of mold flux on the quality of continuous casting slab?

　　The protective slag is added to the steel surface of the crystallizer. The quality of the protective slag mainly affects the surface quality of the slab:

　　(1) Longitudinal cracks on the slab surface: the longitudinal cracks originate from the non-uniformity of the thickness of the primary shell in the meniscus area of the mold. The liquid slag on the steel surface can not flow evenly into and distribute around the slab, resulting in uneven thickness of the solidified shell, which is easy to produce stress concentration at the thin part of the slab shell, and cracks occur when the stress exceeds the high-temperature strength of the solidified shell.

　　It is pointed out that the longitudinal cracks on the slab surface can be significantly reduced if the liquid slag layer on the mold steel surface is kept at 5 ~ 15mm. Longitudinal crack and slag viscosity( η)、 Melting speed (TF) is related to drawing speed (V). It was noted that: η/ The larger the TF ratio, the smaller the longitudinal crack index. If the slag temperature is 1300 ℃, η/ Tf=1, longitudinal crack index is 6, η/ Tf=2, longitudinal crack index is 0. Some people think that for continuous casting slab η· V is controlled between 2 and 3.5. Square billet η· When V is controlled at 5, the slag film can be uniform, the heat transfer is stable, the lubrication is good, and the cracks can be significantly reduced.

　　(2) Slag inclusion: slab slag inclusion can be divided into surface slag inclusion and subcutaneous slag inclusion. Slag inclusions vary in size. From a few millimeters to tens of millimeters, the depth of slag inclusion on the surface is also different. The slag inclusion seriously endangers the product surface quality, so it must be removed before hot processing.

　　Slag entrainment in mould shell 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 condensing shell is thin, forming a high-temperature "hot spot", which is one of the reasons for steel leakage of the billet shell of the crystallizer.

　　The slag inclusions on the slab surface mainly consist of anorthite and anorthosite. A12O3 in these two compounds is more than 20%, and their melting points are 1550 ℃ and 1590 ℃ respectively. It is easy to agglomerate the slag. If the liquid level of the crystallizer fluctuates too much and the immersion nozzle is inserted too shallow, the liquid level will be turned and the slag will be involved.

　　"High efficiency and energy-saving technology for steel-making and continuous casting flame cutting" has been listed as a key scientific and technological achievement promotion project by the Ministry of science and technology of the people's Republic of China. This technical product has been successfully applied in several steel-making enterprises, reversing the past conditions of billet cutting gas consumption, large cutting gap, rough cutting section, high oxygen pressure, workshop dust, large noise, heavy environmental pollution, cutting tool damage, and high labor intensity, showing great power of energy saving and consumption reduction and excellent environmental protection effect. The technology has the following characteristics:

　　1. Advanced technology: concentrated flame during cutting, fast cutting speed; The cutting section is smooth, the upper edge does not collapse, the lower edge has less slag, and the yield is high; It can realize automation and match cutting and casting speed.

　　2. Steel saving: cutting square billets and slab slits can be kept at about 3mm, and cutting loss per ton of steel can be reduced by more than 0.5kg.

　　3. Energy saving: the fuel gas pressure of the energy-saving continuous casting cutting nozzle is 1/2 to 1/3 of that of other cutting nozzles, and the oxygen pressure is 1/2 of that of other cutting nozzles, which can save more than 50% of the fuel gas and 40-50% of the oxygen. Automatic fire cut-off and ignition can be realized during cutting.

　　What are the types of mold flux for continuous casting?

　　According to the composition of the designed protective slag, the protective slag is made by selecting appropriate raw materials through crushing, ball milling, mixing and other production processes. There are four types.

　　(1) Powdery protective slag: it is a mechanical mixture of various powdery materials. In the long-distance dynamic transportation process, due to long-time vibration, the materials with different specific gravity are segregated, and the uniform state of slag is damaged, affecting the stability of the use effect. At the same time, when slag powder is added to the crystallizer, the dust flies and pollutes the environment.

　　(2) Granular protective slag: in order to overcome the shortcomings of polluting the environment, an appropriate amount of binder is added to the powdered slag to make a granular protective slag like millet. The manufacturing process is complex and the cost has increased.

　　(3) Premelted protective slag: put all slagging materials into the premelting furnace to melt them into a whole. After cooling, break and grind them into fine pieces, and add appropriate melting rate regulator to obtain premelted powdered protective slag. The premelted protective slag can be further processed into granular protective slag. The manufacturing process of premelted protective slag is complex and the cost is high. But the advantage is to improve the uniformity of slag formation.

　　(4) Heating type protective slag: add heating agent (such as aluminum powder) into the slag powder to oxidize it to give off heat and quickly form a liquid slag layer. However, the slag forming speed is difficult to control and the cost is high, so the application is less.

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

　　After the protective slag is prepared, the physical and chemical properties of the slag shall be measured. The main physical and chemical indexes are as follows:

　　(1) Chemical composition: the chemical composition of each grade of protective slag shall be analyzed, and the content of each oxide shall be within the specified range, which is the minimum index.

　　(2) Melting temperature, making slag powder Φ three × For a 5mm sample, heat the sample to the temperature at which the cylinder becomes hemispherical on a special instrument, and define the temperature at which the hemispherical point is reached as the melting temperature.

　　(3) Viscosity: it indicates the fluidity of slag powder melting into liquid. The fluidity of slag has an important influence on the inclusion absorption by slag and the lubrication effect of green shell. Generally, the viscosity of 1300 ℃ slag is measured by torsion pendulum viscometer or rotary viscometer to compare the fluidity of different slag.

　　(4) Melting rate: the melting rate is a measure of the speed of slag melting process, which is related to whether a stable three-layer structure can be formed on the steel level of the crystallizer and the required liquid slag layer thickness.

　　The melting rate can be expressed by the time required for the standard specimen to completely melt into liquid at the specified temperature (such as 1300 ℃ or 1400 ℃). It can also be expressed by the amount of liquid slag formed per unit area and time when a certain weight of protective slag powder is heated to the specified temperature.

　　(5) Spreadability: it indicates the covering ability and uniformity of powder slag added to the steel liquid level. It can be measured by the area that the protective slag powder in a certain volume flows down to the plate from the specified height.

　　(6) Moisture: the protective slag powder is easy to absorb moisture. If the amount of adsorbed water exceeds the specified requirements (e.g. 0.5%), the slag powder will agglomerate and endanger the use effect.

　　How to control the moisture content of protective slag?

　　The water content of protective slag can be divided into adsorption water and crystallization water. Moisture can cause agglomeration and quality deterioration of protective slag powder. Moisture shall be limited to less than 0.5%.

　　Some substances in the base material, such as soda, solid and water glass, have strong water adsorption capacity. When water is adsorbed, the powder slag is wrapped into a ball, which brings trouble to the continuous casting operation.

　　The water absorption of protective slag 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%, that of solid sodium silicate is 3.24%, that of fluorite is 0.45%, that of soda is 15.9%, and that of graphite is trace.

　　Moisture control method: the baking temperature of raw materials shall not be lower than 110 ℃. Extend the baking time properly. The baked raw materials shall be batched and mixed in time, and the prepared slag powder shall be sealed in time.

　　For the steel with high quality requirements, it is better to bake the raw material of protective slag to above 800 ℃ to remove the crystal water, or use the premelted protective slag.