Continuous casting mold flux knowledge

 1. What is the role of continuous casting mold powder?

During the pouring process, powdery or granular slag is continuously added to the molten steel surface of the mold, which is called mold flux. The role of mold powder has the following aspects:

(1) Thermal insulation to prevent heat dissipation;

(2) Separate the air to prevent oxygen in the air from entering the molten steel to cause secondary oxidation, which will affect the quality of the steel; (3) Absorb and dissolve the inclusions that float up from the molten steel to the slag interface to purify the molten steel;

(4) There is a layer of slag film between the mold wall and the solidified shell for lubrication, reducing the drawing resistance and preventing the adhesion of the solidified shell and the copper plate; (5) Filling the air gap between the blank shell and the mold to improve Heat transfer in the mold.

A good mold flux should be able to fully play the role of the above five aspects to achieve the purpose of improving the surface quality of the cast slab and ensuring the smooth flow of continuous casting.

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

The mold slag added to the mold during the continuous casting process must fulfill the functions of the above five aspects. The mold slag powder must have a prescribed melting mode, that is, the so-called powder slag layer-sintered layer one liquid is required to form on the molten steel surface. The so-called three-layer structure of the slag layer.

The slag powder with low melting point (1100～1200℃) added to the high temperature molten steel (about 1500℃) surface of the mold, relying on the molten steel to provide heat, forms a certain thickness of liquid slag covering layer (about 10～10) on the molten steel surface. l5mm), the heat transfer of molten steel to the slag layer slows down, the slag on the liquid slag layer is heated, and the slag powder sinters together to form a so-called sintered layer (temperature at 900～600℃). The slag accepts less heat transferred from the molten steel and has a low temperature (<500°C), so it remains powdery and evenly covers the surface of the molten steel, preventing the molten steel from dissipating heat 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 molten steel surface continuously squeezes into the space between the shell and the copper wall through the interface between the molten steel 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 acts as a lubrication on the mold wall and the surface of the blank shell, just like adding lubricating oil when the motor shaft rotates. At the same time, the slag film fills the air gap between the blank shell and the copper wall, reduces the thermal resistance, and improves the heat transfer of the crystallization.

As the drawing progresses, 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 powder slag layer becomes a sintered layer, and then new slag powder is added to the mold. Keep it in a three-layer structure, so the cycle will continue to consume the powder of protective slag.

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

To play the 5 functions of the mold powder, it is necessary to form a "three-layer structure" with the powder added to the mold slag. The key to forming a "three-layer structure" is to control the melting speed of the powder, that is to say, the slag powder added to the steel surface should not melt into liquid all at once, but gradually melt. For this reason, carbon particles are generally added to the mold flux as a regulator of the melting rate.

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 to hinder the contact and fusion between the slag materials and slow down the melting speed. If the addition of carbon powder is insufficient and the temperature of the slag layer has not reached the sintering temperature of the slag material, the carbon particles have been burned out, 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 will still exist after the slag is completely melted, and the sintered layer will shrink and the thickness of the sintered layer will be too thin. When the amount of carbon powder is moderate, some of the carbon particles in the sintered layer are burned out, and the remaining part of the slag is still effectively controlled by the carbon particles, so that a sintered layer and a 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 60-80μm, and their separation and retarding effect is poor, but the initial oxidation temperature is higher (about 560℃), the oxidation speed is slow, and the ability to control the melting rate in the high temperature zone is strong. Carbon black has an amorphous structure, with very fine particles (0.06～0.10μm), strong separation and retardation, low initial oxidation temperature (500℃), and fast oxidation speed. Therefore, carbon black is in the lower temperature zone of the slag layer. The melting rate is strong, and the control efficiency is low in the high temperature zone. Even if the dosage is increased, the improvement effect is limited.

Generally, the amount of carbon powder added is 4 to 7%.

4. What are the factors that affect the absorption of inclusions in molten steel by the mold flux?

The immersion nozzle injection causes the convective movement of the molten steel in the mold, and the inclusions floating on the slag interface of the mold may be involved in the solidified shell due to the fluctuation of the mold liquid level, resulting in inclusions under the skin of the cast slab or surface inclusions Slag affects the surface quality. Therefore, it is hoped that the inclusions floating up to the steel slag interface will be quickly absorbed and dissolved by the liquid slag layer.

To make the inclusions floating up to the steel slag interface quickly transfer to 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 slag to dissolve inclusions.

In other words, the better the fluidity of the slag and the larger the contact area of ​​the steel slag, the easier it is for inclusions to enter the slag. As long as the inclusions enter the slag, the slag can quickly absorb and dissolve, 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 original Al2O3 content.

The production test pointed out that the alkalinity increases, the slag dissolves Al2O3 inclusions. When the alkalinity is greater than 1.1, the Al2O3 dissolving ability decreases; the original Al2O3 content in the slag is greater than 10%, and the slag dissolves Al2O3 rapidly. Therefore, when preparing mold flux, the ratio of CaO% to SiO2% of the slag should be 0.9-1.0, 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 mold steel to dissolve Al2O3 inclusions? Studies have pointed out: when CaO%/SiO2%=0.9～1.0, the content of Al2O3 in the slag is greater than 20%, and high melting point compounds will be precipitated, causing the slag As the melting point increases, the viscosity increases, and the floating inclusions can no longer be absorbed.

However, during the pouring process, the mold powder is continuously consumed and the floating inclusions are 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 slag performance, the following measures can be taken:

(1) When preparing slag powder, select appropriate raw materials and reduce the content of Al2O3 in the original slag as much as possible; (2) Increase the consumption of slag powder and the content of Al2O3 in the diluted slag;

(3) Al2O3 is enriched in the slag during the pouring process, and the crystallizer can be used to replace the slag.

5. What are the effects of the thickness of the liquid slag layer of the mold and its measurement methods?

To achieve a good use effect, the mold powder must have a thickness of the liquid slag layer that meets the actual needs. If the liquid slag layer is too thick or too thin, the slab will have 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 obviously (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 greater than 20mm, and the longitudinal cracks have increased.

The thickness of the liquid slag layer is less than a certain value, and the slag ring formed along the periphery of the mold will block the channel between the meniscus liquid slag flowing into the blank shell and the copper wall, so that the liquid slag cannot flow smoothly into the surface of the blank shell and form a uniform The slag film may cause longitudinal cracks on the surface of the corresponding cast slab. So what is the thickness of the slag required for the slag to pass through the channel downstream of the meniscus without being blocked? According to theoretical calculations, the pulling speed is less than lm/min, the thickness of the slag layer is 5-7mm, 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 critical liquid slag layer thickness 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 slag layer of the mold. Because the temperature of the liquid slag is higher than the melting point of copper, the copper wire melts , Measure the length of the melted copper wire is the thickness of the 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), 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 mold powder play a lubricating effect?

During the pouring process, the mold vibrates up and down, and the casting billet moves downwards, creating friction between the surface of the solidified shell and the copper wall, causing the billet shell to bond with the copper wall, which increases the drawing resistance, and the lighter causes the billet shell to crack , The heavy ones will cause the shell to crack. Therefore, lubrication must be carried out between the blank shell and the copper wall, and this effect can only be realized by the mold flux.

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 liquid surface of the mold steel is the source of continuous supply of the liquid slag film. For this reason, it is necessary to ensure that the passage between the liquid slag near the meniscus of the crystallizer flows into the blank shell and the copper wall is unblocked, 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, the primary green shell is formed. When the powder of protective slag is added to the liquid surface, the slag powder is melted to form a layer of liquid slag. The slag is cooled to form a slag ring. As the crystallizer moves downward, the slag is gradually squeezed between the shell and the copper wall to completely fill the slag. The temperature of the copper wall is low, 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 green shell is a liquid slag film with fluidity. In this way, the liquid slag film is smoothed between the copper wall and the shell of the mold, which is consumed as the cast slab is pulled out, while the solid slag attached to the copper wall is basically not consumed with the vibration of the mold. While the slag film is continuously consumed, the molten steel surface liquid slag is continuously replenished downward through the meniscus channel, forming a stable liquid slag film.

The thickness of the slag film is related to the slag viscosity, pulling speed, mold vibration and other factors. Knowing that the slag viscosity is constant, the drawing speed increases, and the slag film thickness increases; while the drawing speed is constant and the viscosity increases, the slag film thickness decreases. Generally, the thickness of the slag film is 50-200μm, and the slag consumption is 0.4-0.6kg/t. Therefore, 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 should be properly matched. When the mold vibration is constant, the viscosity (η) and the drawing speed (V) should be properly matched. Low viscosity and low drawing speed, or high viscosity and high drawing speed are not advisable. The product of the two is η·V As an indicator to evaluate the lubrication status, 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 status is poor.

7. What are the design principles of the mold flux composition?

To realize the five functions of mold powder, the key is to formulate mold powder of suitable composition.

The mold flux slag widely used in continuous casting is based on the slag system composed of CaO-SiO2-Al203 ternary compound. And contains appropriate amount of Na2O, CaF2, K20 and other compounds. This kind of slag is weakly acidic or neutral liquid slag after melting, which has good wettability to molten steel, and the viscosity of the slag changes smoothly with temperature. Continuous casting mold powder is basically composed of three materials:

(1) Basic slag material. Basic slag containing CaO, SiO2, Al203. According to the CaO-SiO2-Al203 ternary phase diagram, the composition range of these three compounds is: Ca0 10～38%, Si0240～60%, Al203 less than 10%. The melting point is higher than 1300°C.

(2) Flux. Such as Na2O, CaF2 can reduce the melting point and viscosity of the slag. According to the resource situation, LiO2, K20, BaO, NaF, B2O3, etc. can also be used as fluxes, and the amount of addition depends on the melting point of the slag.

(3) Regulators. Carbon particles are melting rate regulators. The addition amount is 5 to 7%.

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

8. What are the main raw materials used in the preparation of mold powder?

The raw materials used for the preparation of protective slag include: 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. As auxiliary flux materials, there are: caustic soda, fluorite, barite, cryolite, borax, lithium carbonate, etc. Melting rate regulators include natural graphite, carbon black, lamp black and so on.

9.What is the impact of mold flux on the quality of continuous casting slab?

The mold flux is added to the molten steel surface of the mold. The quality of the mold flux mainly affects the surface quality of the casting slab:

(1) Longitudinal cracks on the surface of cast slab: Longitudinal cracks originate from the unevenness of the thickness of the primary slab in the meniscus area of ​​the mold. The liquid slag on the surface of the molten steel cannot even flow and distribute around the cast slab, resulting in uneven thickness of the solidified shell. Stress concentration is likely to occur in the thinner part of the blank shell. When the stress exceeds the high temperature strength of the solidified shell, cracks will occur.

The study pointed out that the liquid slag layer on the liquid surface of the mold steel is maintained at 5 to 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). Someone pointed out: the larger the η/tf ratio, the smaller the longitudinal fissure index. For example, 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: control the continuous casting slab η·V at 2～

3.5. When the η·V of the billet 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: The slag inclusion of the casting billet can be divided into surface slag inclusion and subcutaneous slag inclusion. The size of slag inclusion varies. From a few millimeters to ten millimeters, the depth of the slag inclusion on the surface is also different. Slag inclusion seriously harms the surface quality of the product, so it must be removed before thermal processing.

The mold shell is involved in the slag, which is an important source of slag inclusion. For example, slag spots are formed on the surface of the blank 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 composition of the slag inclusions on the surface of the cast slab is mainly anorthite and calcium feldspar. A12O3 in these two compounds is more than 20%, and their melting points are 1550°C and 1590°C, which are easy to agglomerate the slag. The liquid level in the crystallizer fluctuates too much, the immersion nozzle is inserted too shallow, and the slag will be drawn in by the liquid level turning.

"Steel-making and continuous casting flame cutting high-efficiency and energy-saving technology" is 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 steel-making enterprises, reversing the high consumption of gas, large cutting gaps, and cutting sections in the past. Roughness, high oxygen pressure, excessive dust in the workshop, high noise, heavy environmental pollution, multiple damage to the cutting tool, and high labor intensity of workers, showing great energy saving power and excellent environmental protection effect. 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 is less slag, and the yield is high; it can be automated, and the cutting and continuous casting speeds are matched.

2. Saving steel: cutting square billet and slab slits can be kept at about 3mm, and cut 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. , It can automatically cut off and ignite during cutting.

10.What are the types of continuous casting mold powder?

According to the designed mold powder composition, the mold powder is made by selecting suitable raw materials through crushing, ball milling, mixing and other manufacturing processes. There are four types.

(1) Powder mold powder: It is a mechanical mixture of various powder materials. In the process of long-distance transportation, due to long-term vibration, materials of different specific gravity will segregate, and the uniform state of slag will be destroyed, which affects the stability of the use effect. At the same time, when adding slag powder to the crystallizer, the dust flew up and polluted 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 powdery slag to make a granular protective slag similar to millet grains. The production process is complicated and the cost has increased.

(3) Pre-melted mold powder: After mixing the slag-making materials, they are put into a pre-melting furnace and melted into one body. After cooling, they are crushed and ground, and appropriate melting rate regulators are added to obtain pre-melted powdered mold powder. The pre-melted mold powder can be further processed into granular mold powder. The manufacturing process of pre-melted mold slag is complicated and the cost is relatively high. But the advantage is to improve the uniformity of mold powder formation.

(4) Heat-generating mold powder: Add 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 formation speed of this kind of slag is not easy to control, and the cost is relatively high, so it is less applied.

11.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 should be measured. The main physical and chemical indicators are as follows:

(1) Chemical composition: The chemical composition of the mold powder of each brand should be analyzed, and the content of each oxide should be within the specified range, which is the minimum index.

(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 becomes hemispherical. The temperature at which the hemispherical point is reached is defined as the melting temperature.

(3) Viscosity: It represents the fluidity of slag powder melted into liquid. The fluidity of the slag has an important influence on the slag absorption of inclusions and the lubrication effect of the blank shell. The viscosity of the slag at 1300°C is usually measured with a torsion 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 liquid surface of the mold steel and the required thickness of the slag layer.

The melting rate can be expressed by the time it takes 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 a certain weight of protective slag powder, heated to a specified temperature, and the amount of liquid slag formed per unit area and time.

(5) Spreadability: It represents the covering ability and covering uniformity of the powder slag added to the molten steel surface. It can be measured by the area of ​​mold powder in a certain volume, which flows down from the specified height and spreads on the plate.

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

12.How to control the moisture of the mold powder?

Moisture of mold flux is divided into two types: adsorption water and crystal water. Moisture can agglomerate mold powder and deteriorate its quality. The moisture content should be limited to less than 0.5%.

Certain substances in the base material, such as soda, solid, and water glass, have a strong ability to absorb water. When the water is absorbed, the powder slag is wrapped into a mass, which brings trouble to the continuous casting operation.

The water absorption of the mold powder is mainly determined by the type and particle size of the 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 traced.

Method of controlling moisture: the baking temperature of raw materials is not lower than 110℃. Properly extend the baking time. The raw materials after baking should be mixed and mixed in time, and the prepared slag powder should be sealed and sealed in time.

For steel grades with higher quality requirements, it is best to bake the mold powder raw materials to a temperature above 800°C to remove crystal water, or use pre-melted mold powder.