JPH0825009A - Powder for continuous casting of steel - Google Patents

Abstract

PURPOSE:To drastically decrease the surface flaws of a slab even with the powder to be added into casting molds at the time of continuous casting of steel. CONSTITUTION:This powder for continuous casting of steel has the following components (the components are wt.%). The powder contains (a) 60 to 100% first base material which is powder consisting essentially of calcium silicate as base material and contains 0.001 to 10% carbon powder of a grain size <=5mum in its particles, (b) 0 to 40% property control agent for adjusting the properties of the calcium silicate and (d) 0.5 to 7% carbon as a melting speed control agent.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a powder for continuous casting of steel (hereinafter simply referred to as "powder") which is used by adding it to a mold in continuous casting of steel.

[0002]

2. Description of the Related Art Conventionally, powders used for continuous casting of steel have been required to have the following characteristics. The powder added to the molten steel surface in the mold forms a molten layer in contact with the molten steel surface (hereinafter referred to as the molten layer) and an unmelted layer above it, and prevents molten steel oxidation by air by covering the molten steel surface. And have a heat retention effect. The melted powder enters the space between the mold and the solidified shell and is consumed as a lubricant, so it is always supplied in an appropriate amount.At this time, the melt rate that provides an appropriate melt layer thickness while maintaining a balance with the consumption rate is maintained. To have.

The molten layer has the ability to absorb non-metallic inclusions floating above the molten steel, and the physical properties (viscosity, melting temperature, etc.) of the non-metallic inclusions are not significantly changed even if the non-metallic inclusions are absorbed. The melted powder must flow between the mold and the solidified shell to form a uniform melted powder film, which has a lubricating effect. The molten layer should have appropriate viscosity and surface tension and should not be caught in molten steel.

Among the above, in particular, the entrainment of the molten layer into the molten steel (hereinafter referred to as entrainment of powder) has a casting speed of 1.5 m / mi.
Frequently occurs in high-speed casting that is larger than n.
Even in low-speed casting, there may be a problem in steel grades that have strict quality requirements such as tin plate materials and steel plate materials for automobiles. To prevent this powder from being entrapped, powders with high viscosity and high surface tension, such as Japanese Patent Publication No. 40103, have been developed. However, in recent years, further high-speed casting and further improvement in quality have been demanded, so that further higher viscosity and higher surface tension of powder have been aimed.

Usually, powders for continuous casting are SiO ₂ , CaO,
_{Al 2 O 3, Fe 2 O} 3, MgO, MnO, BaO, B 2 as a base material of an oxide of O _3,, Na ₂ O Other physical properties modifiers, K ₂ O, Li ₂ O
Metal oxides such as NaF, KF, LiF, CaF ₂ , MgF ₂ , AlF ₃ ,
Fluorides such as Na ₃ AlF ₆ and carbonates and nitrates of these metals are added.

As a base material of the powder, a pre-melt base material of calcium silicate dissolved in an electric furnace or cupola,
Further, existing base materials include blast furnace slag, glass powder, Portland cement, fly ash, and natural materials such as basalt, wollastonite, and shirasu. Components added to these base materials, as auxiliary materials for adjusting the physical properties, sodium fluoride, cryolite, sodium carbonate, lithium carbonate, fluorite, and for adjusting the components of the main component,
Glass powder, diatomaceous earth, etc. are added as the SiO ₂ source, and calcium carbonate, fluorite, etc. are added as the CaO source.

Carbon powder, carbonaceous materials such as natural and artificial graphite, coke powder, and coal powder, or nitrides such as boron nitride are blended with the powder as a melting rate adjusting agent. Conventionally, carbonaceous powder has an average particle size of about 30
Ultra-fine carbon black having a size of nm, coke powder of about 40 μm, and wood powder of about 100 μm are used. The powder is generally in powder form or in the form of granules by adding a binder.

[0008] Conventionally, as a method of changing the viscosity of powder, it has been general to increase or decrease the content of F and Na added as auxiliary materials. That is, the viscosity of the powder is particularly inversely proportional to the contents thereof, and the high-viscosity powder generally reduces the F 2 and Na contents.

[0009]

However, in order to prevent the entrainment of the powder by changing the viscosity of the conventional powder,
When the F and Na contents are changed and the composition is changed to have high viscosity,
For example, the melting point and the like changed, and it was impossible to prepare a powder suitable for the purpose. The present invention achieves high viscosity without changing the composition, which is a basic characteristic of powder,
It is intended to provide a powder capable of obtaining a high-quality product without powder entrapment.

[0010]

[Means for Solving the Problems] The inventors of the present invention have conducted various experiments and studied the powder flow-in conditions in actual operation, and as a result, have produced a powder having a high viscosity while maintaining a softening temperature similar to that in the past. I was able to find out that The present invention is based on this finding.

(1) The invention of claim 1 is a powder for continuous casting of steel having the following components (the components are wt%). (A) A powder containing calcium silicate as a main component, and 0.001 of carbon powder having a particle size of 5 μm or less inside the particles.
The content of the first base material containing 10% to 10% is 60 to 100%, and (b) the physical property adjusting agent for adjusting the physical properties of calcium silicate is 0 to 100%.
40% and (c) 0.5 to 7 carbon as a melting rate adjusting agent
% Is included.

(2) The invention according to claim 2 is the steel according to claim 1, wherein the powder containing calcium silicate as a main component of the first base material has the following component composition (component is wt%). Powder for continuous casting. _{CaO: 20~50%, SiO 2:} 20~45%, Al
₂ O ₃ : 0.5-10%, MgO: 0.5-20%, Na
₂ O: 1 to 20%, F: 2 to 20%.

(3) The invention of claim 3 is characterized in that the physical property adjusting agent is one or more of oxides, fluorides or carbonates of alkali metals or alkaline earth metals. Powder for continuous casting of the listed steels.

[0014]

As described above, in the continuous casting method using powder, entrainment of powder becomes a serious problem. As a result of various studies to prevent the entrainment of the powder while ensuring the lubricity and heat retention of the powder, it was decided to increase the viscosity of the powder by dispersing carbon of an appropriate particle size in the molten powder. It was successful. The operation will be described below.

The molten layer produced by melting the powder for continuous casting on the molten steel surface is a silicate melt (molten silicate), and is chemically an ionic melt composed of various ions and complex ions. is there. In the molten layer, alkali metal oxides, alkaline earth metal oxides, etc. with high ionic bond simply become metal cations and oxygen ions, but those with high covalent bond and large bonding force like SiO _2. Does not simply become ions, but exists as complex ions such as SiO ₄ ⁴⁻ and Si ₂ O ₇ ⁶⁻ . Fluoride exists as metal cation and F ⁻ ion.

Although the carbonaceous substances added to the powder have various forms of existence, covalent bonds between carbon atoms occupy almost 100%, so that even when the carbonaceous substances are present in the molten layer, they do not become ions. Therefore, once it is taken into the molten layer, it floats and separates due to the density difference with the molten powder, or remains in the molten layer unless it is removed as CO gas by reacting with oxygen diffused from the outside in the molten layer. Will be done.

However, in actual operation, the powder on the molten steel forms two layers of the melt layer and the powder layer, which are completely covered by the melt layer and the heat-insulating powder layer, and the conventional powder has the structure shown in FIG. As shown in a) and (b), the carbonaceous material does not exist in the base material and is simply mixed with the base material, so it is easily oxidatively decarburized in the powder layer part, and It contains almost no carbon.

As shown in FIG. 1 (c), the powder of the present invention contains, as a main component, a first base material in which carbon is dispersed in a base material containing calcium silicate as a main component. The method for producing the first base material is as follows.

Raw materials such as lime, silica stone, bauxite, and magnesia containing CaO, SiO ₂ , Al ₂ O ₃ , and MgO are melted in, for example, an electric furnace, and a premelt material containing calcium silicate as a main component is melted and crushed. After that, dry 10
Fine powder of less than μm. To this fine powder, as a physical property adjusting agent, fluorite, sodium carbonate, etc. are added, and further water, alcohol, etc. are made into a slurry, and fine carbon such as carbon black having an average particle size of about 30 nm is added, It is dried by a method such as a spray dryer to give granules.

The component composition (wt%) of the granules is as follows. _{CaO: 20~50%, SiO 2:} 20~45%, Al
₂ O ₃ : 0.5-10%, MgO: 0.5-20%, Na
₂ O: 1-20%, F: 2-20% Carbon: 0.001-10%

The above-mentioned granules made into fine powder having a particle size of 10 μm or less are used as a first base material, and 60 to 100% of this is used, 0.5 to 7% of carbon such as carbon black is used, and alkali metal or alkaline earth metal is used as a physical property adjusting material. The powder for continuous casting according to the present invention is produced by blending 0 to 40% of at least one of the oxides, fluorides, and carbonates.

The reason for such blending is firstly to impart the necessary characteristics to the powder as described above, and secondly, when the powder is added to the molten steel in the mold as described later, it is about 13
By forming a molten layer at 00 ° C and using 0.001 to 6% of carbon having a particle size of 5 µm or less in the molten layer on the molten steel as a powder for continuous casting of steel, surface flaws of the slab are prevented. This is because it is possible to greatly reduce it.

Therefore, even if 60% of the first base material is blended, the carbon content of the first base material is set to 10 as described above so that at least 6% of carbon remains in the molten layer on the molten steel.
%. When such powder is added on molten steel,
In the powder layer of the two layers of the molten layer and the powder layer, decarburization occurs only near the surface of the base material, and carbon inside the base material remains. The melting rate of the powder is adjusted by the carbon near the surface of the base material or the carbon added by simply mixing. After that, the powder in the powder portion melts.

Depending on the casting conditions, the molten layer is 5
It remains on the molten steel for about 30 minutes, during which the carbon in the molten layer gradually decreases due to oxidation. However, the carbon remaining in the molten layer does not float because it is fine and is surrounded by an oxide base material, so its oxidation rate is low, and even when it flows between the mold and the solidified shell of molten steel, Most of the carbon inside the material remains as it is. Since this molten layer containing carbon is a solid-liquid mixed fluid, its viscosity is 0.1 to 5 poise higher than that of the molten layer containing no carbon, depending on its content.

Since the powder of the present invention has a portion in which carbon is forcibly dispersed in the powder base material, the basic performance (surface tension, crystallization temperature) other than viscosity is the same as that of the base material. Further, since the addition of carbon used for controlling the melting rate is carried out as usual, there is no adverse effect on the melting rate due to the dispersion of carbon in the base material. In this way, by forcibly dispersing the carbon in the base material, it becomes possible to maintain the state in which the carbon is dispersed even during melting, and the viscosity of the powder is controlled by controlling the viscosity of the powder without changing other characteristics. Surface defects can be greatly reduced.

The viscosity mentioned here is at 1500 ° C. Generally, for viscosity, the measured value at 1300 ° C has been conventionally used, but this is because the average temperature of the molten powder film directly above the meniscus is 1300 ° C, and the measured value is relatively easy and stable. However, the temperature of the molten layer on the molten steel surface in the mold is 1500 ° C, and it is necessary to measure the powder entrainment from the molten layer on the molten steel surface in the mold at 1500 ° C. Is more desirable.

Next, the size of the carbon particles in the molten powder,
The reason for limiting the content and its action will be described. In the present invention, it is desirable that carbon particles are uniformly dispersed in the molten layer. However, if carbon of 5 μm or more exists in the molten layer, the buoyancy on the carbon particles cannot be ignored,
A phenomenon such as floating separation from the molten layer occurs. Therefore, it is desirable that the carbon particle diameter in the molten layer is 5 μm or less.

The carbon content in the molten layer has a great effect on its viscosity. If the amount of carbon is 0.001% (10 ppm) or less, the viscosity is almost the same as that of no carbon added, and the effect of reducing the viscosity due to carbon cannot be expected. Also, the amount of carbon is 6%
When it exceeds the above range, when the powder enters between the mold and the solidification shell and comes into contact with the solidification shell, carbon is carburized in the solidification shell, which lowers the melting point of the solidification shell, which may cause breakout. Therefore, it is desirable that the carbon content in the molten layer be 0.001 to 6%.

As described above, 5 μm is contained in the molten layer on the molten steel.
By using the following powder for continuous casting of steel containing 0.001 to 6% of carbon, the viscosity of the molten powder can be arbitrarily increased without changing other physical properties. As a result, it becomes possible to significantly reduce the surface flaws of the slab.

[0030]

[Example] Silica stone, lime, bauxite, magnesia, and fluorite were melted in an electric furnace so that the basicity (CaO / SiO ₂ , weight ratio) was 1.15, thoroughly degassed, and then granulated. Then
The calcium silicate base material produced by drying is finely pulverized to 10 μm or less, carbon black having an average particle size of 30 nm is added thereto, wet-mixed, and the average outer diameter is obtained by a spray dryer.
It was made into 100 μm granules. The amount of carbon black added is 0.
It is in the range of 001 to 10%.

The granules were pulverized again to 10 μm or less to prepare a calcium silicate base material in which carbon was uniformly dispersed in the base material powder. The first base material in which this carbon is uniformly dispersed is 60 to 100%
In addition, as a physical property modifier, diatomaceous earth, calcium carbonate, an auxiliary raw material of sodium silicate silicate, and as a melting rate modifier, carbon black and artificial graphite powder 30 nm to 100 μm
The above carbonaceous powders were added and mixed to prepare continuous casting powders having the formulations shown in Tables 1 to 3.

The produced powders are roughly classified into three types: ultra low carbon steel slab powders, low carbon steel slab powders, and medium carbon steel billet powders. Only the carbon content of the modifier is changed, and the main component composition is the same.

On the other hand, for comparison, silica stone, lime, bauxite, magnesia and fluorite were melted in an electric furnace and thoroughly degassed so that the basicity (CaO / SiO 2, weight ratio) was 1.15. , Water crushed, dried calcium silicate base material, diatomaceous earth, calcium carbonate,
Three types of powders were prepared by adding and mixing carbon black powder of artificial silica powder of 30 nm to 100 μm as an auxiliary raw material of fluorinated sodium carbonate and a melting rate adjusting agent. These comparative powders are very low carbon steel slab powders, low carbon steel slab powders, medium carbons according to the present invention, except that the base material does not contain carbon black and the amount of carbon as a melting rate modifier is different. It has the same composition as the powder for steel billets.

Using the powder of the present invention and the comparative powder, a slab continuous casting machine (mold size 220 × 2000 m)
m) cast ultra low carbon steel and low carbon steel,
Medium carbon steel was cast in a billet continuous casting machine (220 x 220 mm), and the melting characteristics of the powder during casting were visually inspected, and the molten layer of powder was sampled from the mold during casting, and the carbon in the molten layer was sampled. Was analyzed and the viscosity of the molten layer (1500 ° C) was measured by the platinum pulling method, and the cast slab was investigated.

The measurement results and survey results are shown in Tables 1, 2 and 3. The amount of carbon in the table is the amount of the target components when the powder to be finally produced is 100% by weight, and the carbonaceous components that inevitably enter from carbonates are excluded. The melting property index of powder, carburization of cast slab, carburization index and cast cleanliness index were defined as follows.

Melting property index of powder: The melting property evaluation of powder is displayed in five stages, the normal melting property is set to 3, the smaller the number, the worse the melting property, and the larger the number, the better. Carburization of cast slabs , carburization index ; carburization degree is displayed in 5 levels, and when there is no carburization, it is set to 5, and the smaller the number, the worse the carburization degree. Slab cleanliness index ; cleanliness is displayed in 5 levels, the current cleanliness is set to 3, the smaller the number, the worse the cleanliness, and the larger the number, the better the cleanliness.

Although the powders shown in Comparative Examples 1 and 2 in Table 1 have different total carbon contents, they are a simple mixture of carbon contents, and therefore carbon is not contained in the molten layer, and the viscosity of the molten layer is comparative example. It was 0.5 poise for both 1 and 2. On the other hand, in Examples 1, 2, and 3 in which the carbon content was dispersed in the base material, although the main component had the same composition as the comparative example,
The viscosity of the molten layer was high, and the cleanliness of the slab was good.

Further, the higher the carbon content added to the base material in advance, the higher the carbon content in the molten layer. The higher the carbon content in the molten layer, the higher the viscosity and the better the cleanliness of the slab. Become. Although the effect of carburization was not observed in Examples 1, 2, and 3, it was confirmed that carburization was performed in Comparative Example 2 in which the total carbon content was 4.5%. The melting characteristics of the powder slightly affect the amount of carbon added to the base material, but most depended on the amount of carbon added to control the melting.

Further, as shown in Tables 2 and 3, similar effects were observed in other examples. Figure 2 shows the carbon content of the powder in the molten layer of ultra-low carbon steel powder and 1500 ° C.
The relationship with the viscosity in Table 1 was shown, but as described above, it was clear that the higher the carbon content of the powder, the higher the viscosity. As described above, all of the powders shown in the examples can be cast under the same conditions as normal powders, have no effect of carburization, and have extremely good slab cleanliness, and surface scratches and non-oxide-based inclusions. It was possible to obtain a slab with extremely few objects.

[Table 1]

[Table 2]

[Table 3]

EFFECTS OF THE INVENTION The powder according to the present invention provides a powder which can achieve a high viscosity without changing the composition which is a basic characteristic and can obtain a high quality slab without powder entrapment. To do.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a mixed state of carbonaceous powder in a powder base material of the present invention in comparison with a mixed state of conventional carbonaceous powder in a powder base material.

FIG. 2 is a diagram showing the relationship between the carbon content and the viscosity in the molten layer of the ultra low carbon steel powder according to the present invention at 1500 ° C.

Claims (3)

[Claims] 1. A powder for continuous casting of steel having the following components (the components are wt%). (A) A powder containing calcium silicate as a main component, and 0.001 of carbon powder having a particle size of 5 μm or less inside the particles.
The content of the first base material containing 10% to 10% is 60 to 100%, and (b) the physical property adjusting agent for adjusting the physical properties of calcium silicate is 0 to
40% and (c) 0.5 to 7 carbon as a melting rate adjusting agent
% Is included. 2. The powder for continuous casting of steel according to claim 1, wherein the powder containing calcium silicate as a main component of the first base material has the following component composition (the components are wt%). CaO: 20 to 50%, SiO ₂ : 20 to 45%, Al ₂ O ₃ : 0.5 to 10%, MgO: 0.5 to 20%, Na ₂ O: 1 to 20%, F: 2 to 20%. 3. The powder for continuous casting of steel according to claim 1, wherein the physical property modifier is at least one selected from oxides, fluorides and carbonates of alkali metals or alkaline earth metals. .