1.What is the role of continuous casting slag?
During the pouring process, powdered or granular slag is continuously added to the surface of the molten steel in the crystallizer, which is called protective slag. The functions of protective slag are as follows:
(1) Insulation and heat preservation to prevent heat dissipation;
(2) Isolation of air to prevent oxygen in the air from entering the molten steel and causing secondary oxidation, which affects the quality of the steel;
(3) Absorption and dissolution of inclusions that float from the molten steel to the slag interface to purify the molten steel;
(4) There is a layer of slag film between the crystallizer wall and the solidified shell to act as a lubricant, reduce the resistance of the billet, and prevent the solidified shell from sticking to the copper plate;
(5) Fill the air gap between the billet shell and the crystallizer to improve the heat transfer of the crystallizer.
A good protective slag should be able to fully play the above five roles to achieve the purpose of improving the surface quality of the billet and ensuring the smooth continuous casting.
2.What are the requirements for the casting mold slag melting mode?
In the continuous casting process, the protective slag added to the crystallizer must have a specified melting mode to complete the above five functions, that is, it is required to form a so-called three-layer structure of powder slag layer, sintering layer and liquid slag layer on the surface of molten steel.
The slag powder with a low melting point (1100-1200℃) added to the surface of the high-temperature molten steel (about 1500℃) in the crystallizer forms a liquid slag covering layer (about 10-15mm) of a certain thickness on the surface of the molten steel by providing heat with 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 powder is sintered together to form the so-called sintering layer (temperature is 900-600℃). On the sintering layer, the powder slag receives less heat transferred from the molten steel and the temperature is low (<500℃), so it remains in powder form and evenly covers the surface of the molten steel, 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 crystallizer and the downward movement of the solidified billet shell, the liquid slag layer on the surface of the molten steel is constantly squeezed into the space between the billet 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 solidified shell. This layer of liquid slag film lubricates the surface of the crystallizer wall and 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 the heat transfer of crystallization.
As the billet drawing progresses, the liquid slag on the surface of the molten steel is continuously consumed, and the sintering layer drops to the surface of the molten steel and melts into a liquid slag layer, and the powder slag layer becomes a sintering layer. New slag powder is added to the crystallizer to keep it in a three-layer structure. In this cycle, the protective slag powder is continuously consumed.
3.How to achieve the so-called “three-layer structure” of mold protection slag powder?
In order to play the five functions of the protective slag, the slag powder added to the crystallizer must form a “three-layer structure”. The key to forming a “three-layer structure” is to control the melting speed of the protective slag powder, that is, the slag powder added to the steel liquid surface should not melt into liquid all at once, but melt gradually. For this reason, carbon particles are generally added to the protective slag as a melting rate regulator. The speed of carbon particles controlling the melting rate depends on the type and amount of carbon particles added. Carbon is a high-temperature resistant material. The extremely fine carbon powder is adsorbed around the slag particles, separating the slag particles from each other, hindering the contact and fusion between the slag materials, and slowing down the melting rate. If insufficient carbon powder is added, the slag layer temperature has not reached the slag sintering temperature, and the carbon particles have been burned out, then the sintering layer is developed, the melting rate is too fast, and the liquid slag layer is too thick. If too much carbon powder is added, some carbon particles still exist after the slag is fully melted, which will shrink the sintering layer and make the sintering layer too thin. When a moderate amount of carbon powder is added, part of the carbon particles in the sintered layer are burned out, while the remaining slag is still effectively controlled by the carbon particles, so that a sintered layer and liquid slag layer of appropriate thickness can be obtained.
There are two types of carbon materials: graphite and carbon black. Graphite particles are coarse, with a particle size of 60-80μm. Its separation and retardation effects are poor, but the initial oxidation temperature is high (about 560℃), the oxidation rate is slow, and the ability to control the melting rate in the high temperature area is strong. Carbon black has an amorphous structure, very fine particles (0.06-0.10μm), strong separation and retardation effects, a low initial oxidation temperature (500℃), and a fast oxidation rate. Therefore, carbon black has a strong ability to control the melting rate in the low temperature area of the slag layer, and a low control efficiency in the high temperature area. Even if the amount added is increased, the improvement effect is limited. The amount of carbon powder generally added is 4-7%.
4.What are the factors that affect the absorption of inclusions in molten steel by protective slag?
The submerged nozzle injection causes molten steel convection in the crystallizer. The inclusions that float to the interface of the crystallizer slag may be drawn into the solidified shell due to the fluctuation of the crystallizer liquid level, causing subcutaneous inclusions or surface slag inclusions in the ingot, affecting the surface quality. Therefore, it is hoped that the inclusions that float to the slag interface will be quickly absorbed and dissolved by the liquid slag layer.
In order to quickly transfer the inclusions that float to the slag interface to the liquid slag, this process is determined by:
(1) The contact area of the slag interface;
(2) The viscosity of the liquid slag;
(3) The ability of the slag to dissolve inclusions.
In other words, the better the slag fluidity, the larger the contact area of the slag, and the easier it is for inclusions to enter the slag. As soon as the inclusions enter the slag, the slag can quickly absorb and dissolve them. 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 basicity) and the original Al2O3 content in the slag.
Production tests indicate that the ability of slag to dissolve Al2O3 inclusions increases with increasing basicity. When basicity is greater than 1.1, the ability to dissolve Al2O3 decreases. When the original Al2O3 content in the slag is greater than 10%, the ability of slag to dissolve Al2O3 decreases rapidly. Therefore, when preparing protective slag, the ratio of CaO% to SiO2% in the slag should be between 0.9 and 1.0, and the original Al2O3 content should be as low as possible, generally less than 10%.
How much is the ability of the liquid slag layer on the surface of the molten steel in the crystallizer to dissolve Al2O3 inclusions? Research indicates that when CaO%/SiO2%=0.9~1.0, the Al2O3 content in the slag is greater than 20%, and high-melting-point compounds precipitate, which increases the melting point of the slag and increases the viscosity, and it can no longer absorb the floating inclusions.
However, during the casting process, the crystallizer protective slag is continuously consumed and continuously absorbs the floating inclusions, causing the slag to be enriched with Al2O3. In order to maintain the slag’s good ability to absorb Al2O3 without changing the slag’s properties, the following measures can be taken:
(1) When preparing slag powder, select appropriate raw materials and try to reduce the Al2O3 content in the original slag as much as possible;
(2) Appropriately increase the slag powder consumption to dilute the Al2O3 content in the slag;
(3) During the casting process, as Al2O3 is enriched in the slag, the crystallizer slag replacement operation can be used.
5.What are the functions and measurement methods of the crystallizer slag layer thickness?
In order to achieve good use effect of protective slag, the thickness of liquid slag layer must meet actual needs. Too thick or too thin liquid slag layer will cause surface longitudinal cracks on the slab. If the slab pulling speed is 1.2-1.5m/min, the thickness of liquid slag layer is less than 5mm, the longitudinal cracks of the slab will 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 greater than 20mm, the longitudinal cracks will increase again.
When 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 channel between the meniscus liquid slag flowing into the billet shell and the copper wall, so that the liquid slag cannot flow smoothly into the billet shell surface to form a uniform slag film, which may cause longitudinal cracks on the corresponding billet surface. So what is the thickness of the liquid slag required for the channel of the liquid slag flowing down through the meniscus to not be blocked? According to theoretical calculations, when the pulling speed is less than 1m/min, the thickness of the liquid slag layer is 5-7mm, and when the pulling speed is greater than 1m/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.
The method for measuring the thickness of the liquid slag layer in production is: tie a steel wire and a copper wire (or aluminum wire) together and insert them into the slag layer of the crystallizer. Since the temperature of the liquid slag is higher than the melting point of copper, the copper wire melts, and the length of the copper wire melted is the thickness of the liquid slag layer. Since the temperature of the molten steel at each point of the slab crystallizer section is different (such as the submerged nozzle area and the edge of the crystallizer), the thickness of the liquid slag layer is also different, so the thickness of the liquid slag layer at different positions can be measured.
6.How does the protective slag play a lubricating role?
During the pouring process, the crystallizer vibrates up and down, and the billet moves downward, which generates friction between the solidified shell surface and the copper wall, causing the billet shell to adhere to the copper wall, increasing the resistance to billet drawing, and causing cracks in the billet shell in the lightest case, and tearing the billet shell in the worst case. Therefore, lubrication must be performed between the billet shell and the copper wall, and this effect can only be achieved by protective slag.
To ensure good lubrication, there must be a layer of liquid slag film with suitable properties and uniform thickness between the solidified shell and the copper wall. The liquid slag layer on the steel liquid surface of the crystallizer is the source of continuous supply of liquid slag film. To this end, it is necessary to ensure that the channel between the billet shell and the copper wall for the liquid slag near the meniscus of the crystallizer to flow is unobstructed and not blocked by the slag ring around the copper wall.
So how is the lubricating slag film formed? When the crystallizer is filled with molten steel, the primary billet shell is formed. When protective slag powder is added to the liquid surface, the slag powder melts to form a layer of liquid slag. 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 copper wall temperature is low, and the slag shell near the copper wall remains as a solid slag skin, while the surface temperature of the solidified shell is high, and the slag near the shell is a liquid slag film with fluidity. In this way, the copper wall of the crystallizer and the shell are lubricated by the liquid slag film, 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 when the slag viscosity is constant, the pulling speed increases and the slag film thickness increases; when the pulling speed is constant and the viscosity increases, the slag film thickness decreases. Generally, the slag film thickness is 50-200μm, and the slag consumption is 0.4-0.6kg/t. Therefore, in order to make the slag film lubricate the solidified shell in the best state, the slag film thickness, slag consumption, and slag viscosity should be properly matched. When the crystallizer vibration is constant, the viscosity (η) and pulling speed (V) should be properly matched. Low viscosity and low pulling speed, or high viscosity and high pulling speed are not advisable. The product of the two, η·V, is used as an indicator to evaluate the lubrication condition. If the η·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.
7.What are the design principles for mold slag composition?
To achieve the five functions of protective slag, the key is to prepare protective slag with appropriate composition.
The protective slag commonly used in continuous casting is based on the slag system composed of CaO-SiO2-Al2O3 ternary compounds. It also contains appropriate amounts of Na2O, CaF2, K2O and other compounds. This slag is weakly acidic or neutral liquid slag after melting, has good wettability to molten steel, and the slag viscosity changes slowly with temperature. Continuous casting protective slag is basically composed of three materials:
(1) Basic slag. Contains CaO, SiO2, and Al2O3 basic slag. According to the CaO-SiO2-Al2O3 ternary phase diagram, the composition range of these three compounds is: CaO 10~38%, SiO2 40~60%, and Al2O3 less than 10%. The melting point is above 1300℃.
(2) Flux. Such as Na2O and CaF2 can reduce the melting point and viscosity of the slag. According to the resource conditions, LiO2, K20, BaO, NaF, B2O3, etc. can also be used as flux, and the amount added depends on the melting point of the slag.
(3) Regulator. Carbon particles are melting rate regulators. The addition amount is 5-7%.
According to the requirements of the steel grade, the appropriate content of each compound in the protective slag is determined through experiments.
8.What are the main raw materials used to prepare protective slag?
The raw materials 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 dust, blast furnace slag, electric furnace white slag, etc. The auxiliary materials used as flux include caustic soda, fluorite, barite, cryolite, borax, lithium carbonate, etc. The melting rate regulator includes natural graphite, carbon black, lamp black, etc.
9.What is the impact of mold slag on the quality of continuous casting?
Mold powder is added to the surface of the steel surface of the mold. The quality of the mold powder mainly affects the surface quality of the cast slab:
(1) Longitudinal cracks on the surface of the cast slab: Longitudinal cracks originate from the uneven thickness of the primary green shell in the meniscus area of the mold. 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 slag inclusions on the surface of the ingot are mainly composed of calcium feldspar and calcium feldspar. The A12O3 in these two compounds is greater than 20%. Their melting points are 1550℃ and 1590℃ respectively, which easily cause the slag to agglomerate. If the liquid level in the crystallizer fluctuates too much and the immersion nozzle is inserted too shallowly, the liquid level will roll in the slag.
“High-efficiency and energy-saving technology for flame cutting of steelmaking continuous casting” 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 enterprises, reversing the past situation of high gas consumption, large cutting slits, rough cutting sections, high oxygen pressure, high dust in the workshop, high noise, heavy environmental pollution, many damages to cutting tools, and high labor intensity of workers, showing great energy-saving power and excellent environmental protection effect. This technology has the following characteristics:
- Advanced technology: The flame is concentrated during cutting, and the cutting speed is high. Fast; the cutting section is smooth, the upper edge does not collapse, the lower edge has less slag, and the yield rate is high; it can be automated, and the cutting and continuous casting speeds are matched.
- Save steel: The cutting seam of cutting square billets and slabs can be kept at about 3mm, and the cutting loss can be reduced by more than 0.5 kg per ton of steel.
- Save energy: The 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 gas and 40-50% of oxygen. Automatic fire-off and ignition can be achieved during cutting.
10.What types of continuous casting protection slag are there?
According to the designed protective slag composition, suitable raw materials are selected and processed through crushing, ball milling, mixing and other production processes to produce protective slag. There are four types.
(1) Powdered protective slag: It is a mechanical mixture of various powdered materials. During long-distance transportation, due to long-term vibration, materials with different specific gravities are segregated, the uniform state of the slag is destroyed, and the stability of the use effect is affected. At the same time, when adding slag powder to the crystallizer, dust flies and pollutes the environment.
(2) Granular protective slag: In order to overcome the disadvantage of polluting the environment, an appropriate amount of binder is added to the powdered slag to make granular protective slag similar to millet grains. The production process is complicated and the cost is increased.
(3) Pre-melted protective slag: After the slag-making materials are mixed, they are put into the pre-melting furnace to melt into one, and then crushed and ground after cooling, and an appropriate melting rate regulator is added to obtain pre-melted powdered protective slag. Pre-melted protective slag can also be further processed into granular protective slag. The production process of pre-melted protective slag is complicated and the cost is relatively high. But the advantage is to improve the uniformity of protective slag formation.
(4) Exothermic protective slag: Add an exothermic agent (such as aluminum powder) to the slag powder to make it oxidize and release heat, quickly forming a liquid slag layer. However, the slag formation speed of this type of slag is difficult to control and the cost is high, so it is rarely used.
11.What are the main physical and chemical properties of continuous casting mold slag?
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. This is the minimum indicator.
(2) Melting temperature: The slag powder is made into a Φ3×5mm sample, and the sample is heated on a special instrument to the temperature at which the cylinder becomes a hemisphere. The temperature reaching the hemisphere point is defined as the melting temperature.
(3) Viscosity: It indicates the flowability of the slag powder when it melts into a liquid. The slag fluidity has an important influence on the slag’s absorption of inclusions and the lubrication of the billet shell. The viscosity of the slag at 1300℃ is usually measured using a torsion viscometer or a rotational viscometer to compare the fluidity of different slags.
(4) Melting rate: The melting rate 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 steel liquid surface of the crystallizer and the required liquid slag layer thickness.
The melting rate can be expressed by the time required for a standard sample to completely melt into liquid at a 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 a specified temperature.
(5) Spreadability: It indicates the covering ability and uniformity of the powder slag added to the surface of the molten steel. It can be measured by the area of protective slag powder in a certain volume flowing down from a specified height to a flat plate.
(6) Moisture: Protective slag powder easily absorbs moisture. If the amount of moisture absorbed exceeds the specified requirement (such as 0.5%), the slag powder will clump, endangering the use effect.
12.How to control the moisture content of casting mold slag?
The moisture content of protective slag can be divided into two categories: adsorbed water and crystallized water. Moisture can cause the protective slag powder to clump and deteriorate in quality. The moisture content should be limited to less than 0.5%.
Some substances in the base material, such as soda, solid water glass, etc., have a strong ability to absorb water. After absorbing water, the powder slag is rolled into a ball, which brings trouble to the continuous casting operation.
The water absorption of protective slag is mainly determined by the type and particle size of the raw materials. The finer the particle size, the greater the water absorption rate. At 200 mesh, the water absorption rate of cement is 0.41%, solid water glass is 3.24%, fluorite is 0.45%, soda is 15.9%, and graphite is trace.
Methods for controlling moisture: The baking temperature of the raw materials is not less than 110℃. Prolong the baking time appropriately. The raw materials after baking should be mixed and mixed in time, and the prepared slag powder should be sealed and encapsulated in time.
For steel grades with higher quality requirements, the protective slag raw materials are best baked to above 800℃ to remove crystallized water, or pre-melted protective slag should be used.