JPH09308951A - Mold powder for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of breakout and to improve the quality of a cast slab by using a mold powder having a specific composition and restraining the occurrence of surface skin at the time of continuously casting a Cr-containing steel, B-containing steel or Cr-B-containing steel. SOLUTION: The composition of the mold powder is by wt.% of 50-90% basic raw material, <=25% silica raw material, 2-15% fluoride, 3-5% aluminum oxide raw material, 2-3.8% carbon powder and 0.7-1.3 ratio of CaO/SiO2 and substantially 0-2% Na2 O. Simultaneously, the solidification temp. of the powder having this composition is in the range of 1040-1180 deg.C and the viscosity η (Poise) at 1300 deg.C is made to satisfy the condition of the equation. Then, as means for obtaining this composition and viscosity, it is desirable to contain <=3% LiO2 without blending Na2 CO2 , NaF, Na3 AlFe in the raw material constituting the powder and use the powder of 0.45-0.65kg/ton of steel.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a mold powder for continuous casting of steel, more specifically, C
The present invention relates to a mold powder suitable for continuous casting of r-containing steel, B-containing steel or Cr and B-containing steel.

[0002]

Molds for continuous casting of steel include portland cement, synthetic calcium silicate, wollastonite,
It is common to use yellow phosphorus slag as a main raw material, add a siliceous raw material if necessary, and further add soda ash, fluorite, a fluorine compound, an alkali metal compound and a carbonaceous raw material as a melting rate adjusting agent. .

[0003] Mold powder is added to the surface of molten steel in a mold and is consumed while performing various functions.
The main role is to lubricate the mold and solidified shell,
Its main roles are to dissolve and absorb inclusions floating from the molten steel, to keep the molten steel warm, and to control the heat removal rate.

It is important to adjust the softening point, viscosity, etc. of the mold powder, and it is important to select the chemical composition. For the above, powder properties such as melting rate, bulk specific gravity, and spreadability, which are mainly adjusted by carbonaceous raw materials, are important. With regard to, it is necessary to adjust the solidification temperature and the like, and it is important to select the chemical composition.

The progress of continuous casting technology in Japan is remarkable and is continuing to develop. Also, H
Improvements in CR and HDR ratios, high-speed casting, etc. are being actively promoted. Therefore, the demand for mold powder that greatly affects the casting quality and operation stability is becoming more stringent, and the mold powder required is now in a wide variety.

The mold powders used conventionally are
Portland cement, rinse slag,
Synthetic slag, wollastonite, etc. are used, and Na ₂ CO ₃ , Li ₂ CO ₃ , MgCO ₃ , and Ca are used as melting regulators.
Carbonates such as CO ₃ , SrCO ₃ , MnCO ₃ , and BaCO ₃ , and further NaF, Na ₃ AlF ₆ , fluorite, MgF ₂ ,
It is general to use LiF, borax, spodumene, etc., and to add a carbonaceous raw material as a melting rate adjusting agent.

On the other hand, a type using a synthetic calcium silicate as a base material (semi-premelt type) or a mold powder excluding carbon powder is dissolved in advance and crushed to an appropriate particle size,
Complete melting type mold powder to which carbon powder is added after that
(Premelt type) is also used.

[0008]

There are various types of special steel such as Cr-containing steel, B-containing steel or Cr- and B-containing steel, and C is 0.1 to 0.4% by weight as a typical component of steel. Si is 0.1 to 0.4% by weight, Mn is 0.6 to 1.5% by weight, Al is 0.02 to 0.06% by weight, Cr is about 0.03% by weight or more, and B is More than 0.001% by weight is added.

Currently, the above Cr-containing steel, B-containing steel or C
The mold powder used in the continuous casting of r- and B-containing steel contains a considerable amount of Na ₂ O as a melt adjusting component.
(For example, about 6 to 12% by weight), and such mold powder has the following problems.

The mold powder receives heat from the molten steel in the mold, and is slagged and melted. It successively flows into the gap between the mold and the solidified shell to form a slag film and is consumed. When using the conventional mold powder as described above, throwing the mold powder into the mold, slag of the mold powder, called deckle when melted,
A product with a skin-tension is formed on the surface of the molten steel. If this skin called so-called deckle is generated during casting, not only breakout will occur, but the operation will be impaired, rolling defects such as stepping will occur, and the yield of steel will be reduced. Because the slag comes into contact,
Defects in inclusion properties frequently occur, which deteriorates the quality of steel and becomes a major problem.

This phenomenon is caused by Cr-containing steel, B-containing steel or C
Particularly remarkable for r- and B-containing steels, the deckle formation is
It is considered to be peculiar to these steel types. Regarding the generation of this deckle, the heat retaining property of the mold powder is a problem, and therefore, the following means are taken as the mold powder. One of them is to increase the carbonaceous raw material used as a melting rate adjusting agent for mold powder to improve heat retention. However, by taking this measure, the melting rate becomes slow, and the thickness of the mold powder molten layer inevitably decreases, causing breakout. Further, when the fluctuation of the molten metal surface during casting becomes large, the mold powder may come into contact with the molten steel, promoting the formation of inclusions, and conversely deteriorating the steel quality.

There are some cases where sodium carbonate, lithium carbonate, or a carbonate such as magnesium carbonate is used in the mold powder, and it is said that deckle is formed by an endothermic reaction when the carbonate decomposes. ing. Therefore, reduction of the amount of the carbonate raw material or no use of the carbonate raw material has been studied, but the present situation is that the solution has not been completely solved.

When casting the above steel types, the frequency of breakout increases as the number of continuous castings increases, which is a serious problem. However, a mold powder that can completely solve these various problems is obtained. Not not.

Therefore, an object of the present invention is to provide a Cr-containing steel, B
It is an object of the present invention to provide a mold powder that does not generate deckle during continuous casting of a steel containing steel or a steel containing Cr and B.

[0015]

[Means for Solving the Problems] As a result of various studies to solve the above problems, the present inventors clarified the mechanism of deckle formation in the mold, and were able to overcome all the above-mentioned drawbacks. Found.

That is, according to the present invention, in the mold powder for continuous casting of steel, the base materials 50 to 90 are used as constituent raw materials.
% By weight, 25% by weight or less of silica raw material, 2 to 15% by weight of fluorspar, 0.5 to 5% by weight of aluminum oxide raw material and 2 to 3.8% by weight of carbon powder.
aO / SiO ₂ weight ratio is within the range of 0.7 to 1.3,
The present invention relates to a mold powder for continuous casting, which has a Na ₂ O content of 2% by weight or less.

Further, according to the present invention, in the mold powder for continuous casting of steel, the base materials 50 to 90 are used as constituent raw materials.
% By weight, 25% by weight or less of silica raw material, 2 to 15% by weight of fluorspar, 0.5 to 5% by weight of aluminum oxide raw material and 2 to 3.8% by weight of carbon powder.
aO / SiO ₂ weight ratio is within the range of 0.7 to 1.3,
The present invention relates to a mold powder for continuous casting, which is characterized by the substantial absence of Na ₂ O.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of repeated studies and studies, the present inventors have obtained the following findings. It has been found that the deckle formed during continuous casting of Cr-containing steel, B-containing steel or Cr- and B-containing steel is formed by the reaction of molten steel and powder slag. As the substances involved in this reaction, Cr and / or B in molten steel and SiO ₂ and Na ₂ O in mold powder are considered. When this reaction occurs, it is considered that deckle is generated by an endothermic reaction among the following reactions.

In the case of Cr-containing steel, B-containing steel, Cr and B-containing steel, the following reactions occur. In addition, () in a formula shows the component in slag, and the underline shows the component in molten steel.

Embedded image

Embedded image

Embedded image

In the reactions (1) to (4), the exothermic and endothermic reactions are thermodynamically calculated as follows.

In the above reaction, ΔH ° is positive (> 0) and, that is, an endothermic reaction. Therefore,
In order to prevent the reactions (1) and ( ₂₎ from occurring, it is effective and important to reduce Na ₂ O in the mold powder.

Next, the mold powder of the present invention will be described in detail. In order to prevent the deckle generated during continuous casting, it is desirable to suppress the reaction between the above-mentioned mold powder and molten steel. Conventional mold powders generally use Na ₂ O as a viscosity modifier, but it is effective for the mold powder to reduce Na ₂ O as much as possible. Therefore, the content of Na ₂ O as the chemical composition of the mold powder is 2% by weight or less,
Preferably, by substantially making it absent, the Na ₂ O activity in the powder slag can be made sufficiently small, whereby the reaction is suppressed.

In the present specification, "substantially absent" of Na ₂ O means that the mold powder is not mixed with Na ₂ CO ₃ , NaF, Na ₃ AlF ₆ or the like as a raw material. It means that only Na ₂ O contained as an impurity in the raw material constituting the powder is contained.

Further, the reaction may be generated by the Cr ₂ O ₃ in the powder slug, since the Cr ₂ O ₃ is not completely dissolved in the powder slag, present suspended in the form of an oxide, with powder slug It flows between the mold and the solidified shell. However, Cr ₂ O ₃ is not completely dissolved and exists as particles, so that heat removal is made uneven in the slag film, and the slag film is cut from undissolved Cr ₂ O ₃ to restrain It causes a breakout.

However, as described above, by reducing the Na ₂ O content in the mold powder, the reaction is suppressed and the formation of Cr ₂ O ₃ in the powder slag can be suppressed, and the heat removal Not only does it homogenize, but also breakout does not occur, which can contribute to stable casting.

Further, if the Na ₂ O content of the mold powder is reduced, the viscosity of the powder slag will inevitably increase, so it is necessary to include a component in place of this.

As an alternative to Na ₂ O, F and Mg
O is contained in the mold powder. However, if a large amount of F is contained, the melting loss of the immersion nozzle becomes large, and further, the problem of corrosion of the continuous casting machine occurs, so it is preferable to contain F in an amount of 6% by weight or less. On the other hand, when MgO is contained in a large amount, it causes non-uniform melting during softening and melting, the components of the powder slag layer tend to become non-uniform, and further promotes deterioration of the quality of the slab. It is preferable to contain it in an amount of not more than%.

In order to contain F and MgO in the mold powder in the above range and further reduce the viscosity, L
Utilize i ₂ O. However, when a large amount of Li ₂ O is contained in the mold powder, the cost becomes very high. Therefore, it is preferably 5% by weight or less, more preferably 3% by weight.
It is the following. Further, when a large amount of Li ₂ O is contained, the solidification temperature is significantly lowered, the heat removal is increased, and surface cracking may occur depending on the carbon content of the steel. Also, L
i ₂ O not only improves the fluidity of the slag, but also lowers the activation energy, which reduces variations in the amount of mold powder consumed due to changes in the casting temperature, so i ₂ O should be used within the above range. preferable.

Next, the physical properties of the mold powder of the present invention will be described. As described above, the solidification temperature is apt to cause surface cracks depending on the carbon content of steel, for example, 0.10 to 10.
In the range of 0.25% by weight, at least the solidification temperature is 10
It is necessary to have a temperature of 85 ° C or higher, and at least 1040 ° C for a carbon amount exceeding 0.25% by weight. However, if the solidification temperature is too high, the solidification shell becomes more uniform, but the thickness of the solidification shell is thin and the hot strength of the solidification shell is significantly reduced. Therefore, it is necessary to secure a certain degree of thickness, and a solidification temperature exceeding 1180 ° C. is not preferable for that.

Further, in order to set the solidification temperature to 1040 ° C. or higher, Ca in the chemical composition of the mold powder is set.
The O / SiO ₂ weight ratio is preferably 0.7 or more, and in order to set the solidification temperature to 1180 ° C or less, CaO / S
The iO ₂ weight ratio is preferably 1.3 or less.

The viscosity and casting speed of the mold powder of the present invention should be in the range given by the following equation:

[Equation 2] (In the formula, η represents the viscosity expressed in poise, and Vc represents the casting speed expressed in m / min.) By satisfying this relational expression,
Slag uniformly flows in, heat removal is extremely stable, and steel with few surface defects can be cast. As shown in FIG. 1, when the value is below the appropriate range within the range of the above formula, the powder slag flows in unevenly, the heat removal becomes uneven, surface cracks occur, and breakability of cracking also occurs. It occurs frequently. On the other hand, if the suitable range is exceeded, the lubrication of the solidified shell and the mold will be significantly impaired, and in some cases, the mold and the solidified shell will seize, leading to a breakout. Furthermore, the frictional force between the mold and the shell is significantly increased due to insufficient consumption,
On the contrary, another surface defect such as lateral cracking occurs.

Further, it is extremely important to set the viscosity of the mold powder to be suitable for the above-mentioned casting conditions,
Thereby, the consumption amount of the mold powder can be set to the optimum range. The optimum range of consumption is Cr-containing steel,
From the characteristics of B-containing steel or Cr and B-containing steel, 0.45
~ 0.65 kg / ton (for 1 ton of steel, the amount of mold powder used is 0.45 to 0.65 kg).

Next, the raw materials used for the mold powder of the present invention will be described. As a raw material constituting the mold powder of the present invention, 50 to 90% by weight of a base material is used. As the base material, Portland cement, phosphorus slag, synthetic slag, wollastonite, synthetic calcium silicate or the like can be used. Here, if the amount of the base material used is less than 50% by weight, the flux component inevitably increases and the viscosity becomes too low, which is not preferable.
On the other hand, if it exceeds 90% by weight, the amount of other components used becomes small, which is not preferable.

Further, the silica raw material is used in the mold powder of the present invention in an amount of 25% by weight or less. When the amount of the silica raw material used exceeds 25% by weight, the bulk specific gravity becomes extremely small, and dust is likely to be generated, which is not preferable.

Further, a fluorite raw material is used for the mold powder of the present invention. Generally, sodium fluoride, cryolite, or fluorspar is generally used as the F source of the mold powder, but for the reasons described above, the amount of Na ₂ O in the mold powder is 2% by weight or less, preferably Is substantially absent, so it is necessary to use sodium fluoride and cryolite within the range of 2 wt% or less of Na ₂ O in the chemical composition of the mold powder. Therefore, as the F source, fluorite is used within the range of 2 to 15% by weight. Here, if the amount of fluorite used is less than 2% by weight, it is not preferable because sufficient F cannot be provided to the mold powder, and if it exceeds 15% by weight, the softening point of the mold powder becomes too high. Not preferable.

Aluminum oxide is used as a raw material for forming the mold powder. The reason is that by adding aluminum oxide, the slag melting of the mold powder becomes significantly smoother, the components of the molten slag layer can be made uniform, and it can contribute to the uniform inflow of the slag, without cracking. This is because a sound slab can be obtained. In this case, the addition amount is preferably 0.5% by weight or more. However, if it exceeds 5% by weight, crystals called gerenite are precipitated in the slag film in addition to the crystals of caspitaine, resulting in non-uniform heat removal and the thermoelectric conversion of the mold shell plate to detect breakout. A pair is set, but this thermocouple temperature greatly fluctuates, in some cases it causes the malfunction of the breakout alarm, the coagulation temperature becomes too high, and it is not preferable because it promotes the above result. .

In the mold powder of the present invention, the combined use of fluorite having the characteristic of a high melting point mineral and aluminum oxide suppresses the formation of a sintered layer when the mold powder is melted and melted. If the sintered layer is not generated, not only the molten slag layer is homogenized, but also the generation of slag bear is completely suppressed, and as a result, a uniform inflow of slag can be achieved and good continuous casting can be performed. it can.

Further, it is preferable to use 2 to 5% by weight of the carbon raw material as the melting rate adjusting agent. If it is less than 2% by weight, the thickness of the molten slag layer becomes thick during actual casting, and if it exceeds 3.8% by weight, the thickness of the molten slag layer becomes too thin, which is not preferable.

The mold powder according to the present invention can be obtained by blending the respective raw materials and then uniformly mixing them with a mixer. In addition, a liquid and, if necessary, an organic or inorganic binder are added, and the mixture is granulated by a method such as extrusion granulation, stirring granulation, tumbling granulation, fluidized granulation, spray granulation, etc., and used in a granule form You can also

[0040]

The mold powder of the present invention will be further described below with reference to examples. Example The raw materials were blended in the blending ratios shown in Table 1 below to obtain mold powders of the present invention product and comparative product. In addition,
Inventive product 6 and comparative product 2 are granular products obtained by adding water and an organic binder to the powder raw material mixture and stirring and granulating. Others are powder products obtained by mixing the powder raw material mixture with a V-type mixer. The chemical composition of the obtained mold powder is also shown in Table 1. Also, synthetic calcium silicate 1 is Ca
The O / SiO ₂ weight ratio is 1.1 and the synthetic calcium silicate 2 has a CaO / SiO ₂ weight ratio of 1.3.

The results of using the obtained mold powder in continuous casting are also shown in Table 1. The product of the present invention 1
In the present invention, the steel containing 0.1% by weight B, the steel containing 1.8% by weight Cr, the product 2 of the present invention, the steel containing 0.03% by weight B, the steel containing 0.5% by weight Cr, In the product 3, 0.004% by weight B-containing steel and 13% by weight Cr-containing steel are used. In the product 4 of the present invention, 0.003% by weight B and 0.003% by weight B are contained.
Steel containing 2% by weight of Cr is 0.0 in the product 5 of the present invention.
Steels containing 1 wt% B and 0.7 wt% Cr, and in the product 6 of the present invention, steel containing 0.4 wt% B and steel containing 20 wt% Cr were used, respectively. Comparative products 1 and 2 use the same steel as the product 4 of the present invention, comparative product 3 uses the same steel as the product 5 of the present invention, and comparative product 4 uses the same steel as the product 1 of the present invention. Using.

[0042]

[Table 1]

In the evaluation of the use results in Table 1, ⊚ indicates extremely good, ∘ indicates good, Δ indicates bad, and x indicates remarkably bad.

[0044]

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide a mold powder that does not generate deckle when continuously casting B-containing steel, Cr-containing steel, and B- and Cr-containing steel.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the viscosity of mold powder and the casting speed.

Claims (4)

[Claims] 1. A mold powder for continuous casting of steel, comprising 50 to 90% by weight of a base material, 25% by weight or less of a silica raw material, 2 to 15% by weight of fluorite and 0.5 to 5% of an aluminum oxide raw material as constituent raw materials. %, And 2 to 3.8% by weight of carbon powder, and has a chemical composition of CaO / SiO ₂
A mold powder for continuous casting, characterized in that the weight ratio is in the range of 0.7 to 1.3 and the Na ₂ O content is 2% by weight or less. 2. A mold powder for continuous casting of steel, comprising 50 to 90% by weight of a base material as constituent raw materials, 25% by weight or less of silica raw material, 2 to 15% by weight of fluorite, 0.5 to 5% by weight of aluminum oxide raw material. % And carbon powder 2-3.8% by weight
A mold powder for continuous casting, characterized in that the CaO / SiO ₂ weight ratio in the chemical composition is within the range of 0.7 to 1.3, and Na ₂ O is substantially absent. 3. Mold powder for continuous casting of steel according to claim 1 or 2, wherein the solidification temperature is 1040 to 1180 ° C., and the viscosity η (poise) at 1300 ° C. satisfies the following formula: (In the formula, Vc represents a casting speed expressed in m / min) 4. The steel is Cr-containing steel, B-containing steel or Cr.
Mold powder for continuous casting of steel according to any one of claims 1 to 3, which is a B-containing steel.