JP2001347351A - Mold powder and continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mold powder and a continuous casting method with which a cast slab having good cleanliness on the surface layer part thereof without generating the longitudinal crack can be obtained. SOLUTION: In the mold powder containing CaO, Si02, Al2O3 and fluorine compound as the basis components, the ratio in mass% of CaO/SiO2 is 1.1-1.5, and 5-15 mass% Al2O3, <=5 mass% of the total of content converted into the oxide of alkaline metals, <=1 mass% ZrO2, 0-8 mass% TiO2 and 4-12 mass% F, are contained, and the viscosity at 1300 deg.C is 2-6 Poise and the solidified point is >=1100 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold powder added to the surface of molten steel in a mold, and further relates to a continuous casting method using the mold powder.

[0002]

2. Description of the Related Art In continuous casting of steel, mold powder is added to the surface of molten steel in a mold for casting. Usually, a mixture of powders of a plurality of types of oxides, fluorine compounds, carbon, and the like is used for the mold powder.

In a mold powder added to the surface of molten steel in a mold, a molten layer is formed at a contact portion with the surface of the molten steel due to heat received from the molten steel. This molten layer flows into the gap between the inner wall of the mold and the solidified shell, and forms a film. This film is composed of two phases, a solid phase portion cooled by a mold and a liquid phase portion.

[0004] The mold powder plays the following roles in the state of a molten layer and a film. Heat retention and prevention of oxidation of molten steel Absorption of bubbles or inclusions in molten steel Ensuring lubricity between inner wall of mold and solidified shell Adjusting cooling rate of solidified shell The effect of mold powder on the surface quality of slab by continuous casting is large, Adjustment of the cooling rate of the solidified shell is important for depletion or vertical cracks generated on one surface, among the above roles. By uniformizing the cooling rate of the solidified shell in the mold, the thickness of the solidified shell becomes uniform in the width direction, the stress caused by solidification shrinkage is relaxed, and the occurrence of depletion or vertical cracks generated on the surface can be suppressed .

When crystals are precipitated in the mold powder film during the cooling process, the heat transfer resistance of the film increases, so that the solidification shrinkage of the slab is suppressed, the adhesion to the mold wall is increased, and the inside of the mold is reduced. Conventionally, a method for promoting crystal precipitation in a film has been proposed because uneven cooling of a solidified shell can be prevented.

For example, JP-A-5-15955 discloses that CaO contained in mold powder and CaF ₂
As the sum of the Ca component estimated to be present as CaO converted to CaO, the T.C. Ca
% Of O to SiO ₂ CaO / SiO ₂
Is increased to about 0.9 to 1.3, crystals are precipitated in the film, the cooling rate of the solidified shell in the mold is made uniform, and the cooling is performed slowly.

However, the basicity of the mold powder (C
aO / SiO ₂ or T.A. In the method of increasing CaO / SiO ₂ ) to precipitate a large amount of crystals in the film, the volume of the film where the crystals are deposited shrinks, and voids are easily formed between the mold and the solidified shell. When a gap is formed between the mold and the solidified shell, heat transfer resistance is dramatically increased, and a local solidification delay may occur in the solidified shell. Longitudinal cracks and the like are likely to occur in the portion of the solidified shell where solidification delay has occurred.

Therefore, Japanese Patent Application Laid-Open No. Hei 7-214263 discloses that the basic content of mold powder is set to 0.6 to 0.9, and the total content of ZrO ₂ , TiO ₂ and CeO ₂ which are accelerators for crystal precipitation. To 1% by mass or less, and the freezing point is 115
A method has been proposed in which the temperature is kept at 0 ° C. or higher so that the amount of crystals precipitated in the film of the mold powder does not become excessive and is appropriately maintained. Thereby, uniformization of the cooling rate of the solidified shell in the mold is promoted, and it can be expected that generation of depletion or vertical cracks generated on the surface is suppressed.

However, according to this method, oxides such as Al ₂ O ₃ and MnO may increase in molten steel in a mold,
These oxides are trapped in the solidified shell in the mold, and the cleanliness of the surface layer of the slab tends to deteriorate. Surface flaws and the like are likely to occur in products that are hot rolled using these slabs as raw materials.

[0010]

SUMMARY OF THE INVENTION According to the present invention, it is possible to prevent the occurrence of depletion or vertical cracks on the surface of a slab and to obtain a slab of good quality with good cleanliness of the surface layer of the slab. It is an object of the present invention to provide a mold powder that can be used and a continuous casting method using the mold powder.

[0011]

The gist of the present invention resides in a mold powder shown in the following (1) and (2) and a continuous casting method shown in the following (3). (1) CaO, and SiO _2, Al ₂ O ₃ and fluorine compound a mold powder having a basic composition, mass% ratio CaO / SiO ₂ with respect to SiO ₂ of CaO is 1.1
To 1.5, 5 to 15% by mass of Al ₂ O ₃ , 5% by mass or less in total of alkali metal conversion contents, Zr
O ₂ 1 wt% or less, the TiO ₂ 0 to 8 wt%, the F 4
And a viscosity at 1300 ° C of 2 to 6%.
Poise, mold powder for continuous casting having a solidification point of 1100 ° C. or higher. (2) The mold powder for continuous casting according to the above (1), wherein the contents of Al ₂ O ₃ , TiO ₂ and F satisfy the following formula (A). 2 ≦ (Al ₂ O ₃ ) × (TiO ₂ ) ≦ 10 + 15 × (F) (A) where (Al ₂ O ₃ ): Al ₂ O in the mold powder
( ₃ ) Content (% by mass) (TiO ₂ ): Content of TiO _{2 in} mold powder (% by mass) (F): Content of F in mold powder (% by mass) (3) The above (1) or (2) Wherein the chemical composition Cp of molten steel defined by the following formula (B) and Al ₂ O in the mold powder are provided.
₃ the relationship between the content of a continuous casting method for casting under conditions satisfying the following formula (C) or (D) type. Cp = C + 0.02 × Mn + 0.04 × Ni-0.01 × Si + 0.02 × Cr + 0.07 × S (B) Here, C, Mn, Ni, Si, Cr and S: in molten steel Content (% by mass) | Cp−0.12 | ≦ 0.05; 5 + 2000 × | Cp−0.12 | ² ≦ (Al ₂ O ₃ ) ≦ 10 + 2000 × | Cp−0.12 | ^2.・ (C) When | Cp−0.12 |>0.05; 10 ≦ (Al ₂ O ₃ ) ≦ 15 (D) where | Cp−0.12 |: 0 from the value of Cp .12 Absolute value of the value obtained by subtracting 12 (Al ₂ O ₃ ): Al ₂ O ₃ content in the mold powder (% by mass) Based on the following findings (a), the present inventors have made the following (b) The above-mentioned problem has been solved by taking the measures (a) and (c). (A) When the basicity of the mold powder is as low as 0.6 to 0.9, the increase in oxides such as Al ₂ O ₃ and MnO in the molten steel in the mold is caused by the SiO _{2 in} the molten layer. This is because the activity is increased, and components in the molten steel such as Al and Mn are easily oxidized at the interface between the molten steel in the mold and the molten layer of the mold powder.

Therefore, in order to make the cooling rate of the solidified shell in the mold uniform, it is important to increase the basicity of the mold powder. However, the method of increasing the basicity of the mold powder and causing a large amount of crystals to precipitate in the film has a problem that voids are formed between the mold and the solidified shell, and local solidification delay is likely to occur in the solidified shell.

The present inventors have proposed CaO, SiO_Two And
In a mold powder containing a nitrogen compound as a basic component,
CaO SiO_Two Ratio of mass% of CaO to CaO /
SiO_Two Is 1.1 to 1.5, Al_Two O_Three 5
By blending ％ 15% by mass,
Uniform cooling of solidified shell in the mold without excessive precipitation
It has been found that it is possible to maintain the state.
Al, an amphoteric oxide _Two O_Three When combined with
Al in the molten layer of the mold powder_Two O_ThreeIs acidic
And the precipitation of crystals in the film is moderately suppressed.
It is done. (B) Therefore, the mold powder of the present invention
Then, based on the knowledge of the above (a), CaO, SiO_Two ,
Al_Two O_Three And a fluorine compound as a basic component,
A mold powder having the composition shown in (1) is used.

Thus, even if the basicity is as high as 1.1 to 1.5, the precipitation of crystals in the film is appropriately suppressed, and the cooling of the solidified shell in the mold can be made uniform. Further, since the basicity is high, it is possible to prevent an increase in oxides such as Al ₂ O ₃ and MnO in the molten steel in the mold. (C) In the method of the present invention, the relationship between the chemical composition Cp of molten steel defined by the above formula (B) and the Al ₂ O ₃ content in the mold powder is determined by the above formula (C) or ( D) Cast under conditions that satisfy the formula.

| Cp-0.12 | is an index indicating whether or not the chemical composition of steel is a composition that easily causes a subperitectic reaction. It is well known that vertical cracks are likely to occur on the surface of a steel slab that causes a subperitectic reaction. Therefore, | Cp-
In the case of 0.12 | ≦ 0.05, that is, in a steel which is liable to cause a subperitectic reaction, the Al ₂ O ₃ content in the mold powder is set to a low content so as to satisfy the above formula (C). And

By lowering the Al ₂ O ₃ content to moderately increase the precipitation of crystals in the film, suppressing the shrinkage of the solidified shell due to the subperitectic reaction, and increasing the adhesion to the mold wall, It suppresses uneven cooling of the solidified shell inside and prevents the occurrence of vertical cracks on the slab surface.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The mold powder of the present invention and a continuous casting method using the mold powder will be specifically described below.

The powder of the present invention comprises CaO, SiO ₂ , A
The basic composition is l ₂ O ₃ and a fluorine compound. The content is about 25 to 50% for CaO and ₂ for SiO2.
About 0 to 40%, Al ₂ O ₃ 5 to 15%, F 4 to 1
2%. Further, the total content of alkali metals in terms of oxide is 5% by mass or less, ZrO ₂ is 1% by mass or less, and TiO ₂ can be contained in a range of 0 to 8% by mass as necessary. .

The raw materials used for producing the powder of the present invention may be those generally used. CaO
Raw lime, limestone, cement as raw materials, silica sand and light algae soil as SiO ₂ raw materials, alumina as Al ₂ O ₃ raw materials, fluorite as F raw materials, oxides of alkali metals, for example, Na ₂ O raw materials Examples of soda ash, sodium carbonate, sodium fluoride, and TiO ₂ raw materials include rutile sand, and ZrO ₂ raw materials include zircon sand. Further, it is preferable that the raw material has a particle size of 100 μm or less. These raw materials include Fe ₂ O ₃ , F
Although oxides such as e ₃ O ₄ are contained and are inevitably contained in powder, even if these impurities exist,
No problem. CaO, SiO of mold powder
_Although the contents of ₂ have already been described, these contents are only for reference, and in the present invention, CaO / SiO
The value of ₂ is set to 1.1 to 1.5. When the basicity is less than 1.1, the effect of suppressing molten steel oxidation is small. Further, the basicity is 1.
If it exceeds 5, crystals are excessively precipitated in the film, and the solidification point becomes excessively high. Therefore, there is a problem that lubricity in the mold is deteriorated and cooling of the solidified shell tends to be uneven.

The Al ₂ O ₃ content in the mold powder is 5
To 15% by mass. If it is less than 5% by mass, crystals are excessively precipitated in the film, and the cooling of the solidified shell in the mold tends to be uneven. On the other hand, when the content exceeds 15% by mass, the viscosity of the molten layer of the mold powder is excessively increased, the solidification point is increased, and the molten layer is less likely to flow into the gap between the mold and the solidified shell.

The total content of alkali metal oxides in the mold powder in terms of oxide is 5% by mass or less. The oxide of the alkali metal is contained in the mold powder by a usual raw material blending. However, if the content exceeds 5% by mass,
Unmelted mold powder easily sinters in the mold, which deteriorates the heat retention of molten steel in the mold. Further, it becomes difficult for the molten layer to uniformly flow into the gap between the mold and the solidified shell. Therefore, the total content of alkali metal in terms of oxide is set to 5% by mass or less.

The content of ZrO _{2 in} the mold powder is 1
% By mass or less. ZrO ₂ is contained in the mold powder when a TiO ₂ raw material is blended. However,
Even in a state where a small amount of about several percent is present in the mold powder, the precipitation of crystals is promoted and the freezing point is significantly increased. Therefore, the content of ZrO ₂ is set to 1% by mass or less.

The F content in the mold powder is 4 to 12
% By mass. F is contained for the purpose of adjusting the freezing point and moderate crystal precipitation. If it is less than 4% by mass, the effect is small. If the content exceeds 12% by mass, the viscosity of the molten layer is excessively reduced, so that the molten layer is not only easily entangled in the molten steel, but also excessively flows into the gap between the mold and the solidified shell, and rather, longitudinal cracks are easily generated on the slab surface.

In the mold powder, if necessary,
The O ₂ is added. At that time, the content of TiO ₂ is set to 0 to 8% by mass. TiO ₂ has been conventionally used as an accelerator for crystal precipitation in a film, but in the present invention, the addition of a suitable amount of TiO ₂ has a remarkable effect on lowering the freezing point. It was found that the viscosity was hardly reduced. Therefore, without decreasing the viscosity, for the purpose of lowering the freezing point, the TiO ₂ may be contained 0-8 wt%.

The solidification point of the mold powder is 1100 ° C. or higher, and the viscosity at 1300 ° C. is 2 to 6 poise. The upper limit of the desired freezing point is 1300 ° C.
By setting the solidification point in the above range, the thickness of the solid phase portion, which is a crystal precipitation site in the film, can be ensured. By setting the viscosity in the above range, the molten mold powder can be prevented from being caught in the molten steel in the mold.

Al ₂ O ₃ , TiO _{2 in} mold powder
It is desirable that the contents of F and F fall within a range that satisfies the expression (A). The present inventors have found that the desired content of TiO ₂ is affected by the content of Al ₂ O ₃ and F. That is, TiO ₂ and Al ₂ O
_By setting the product of the content of ₃ within a certain range, it becomes easy to appropriately adjust the solidification point of the mold powder or the viscosity of the molten layer. Furthermore, it has been found that the appropriate range of the product of the TiO ₂ and Al ₂ O ₃ contents is affected by the F content. TiO ₂ and Al ₂ O ₃
If the product of the content ratios is less than 2, the decrease in the freezing point is small,
If the value exceeds the value on the right side of the above-described formula (A), the freezing point tends to increase.

In the method of the present invention, the chemical composition Cp of the molten steel defined by the above-mentioned formula (B) and the Al ₂ in the mold powder are used.
Casting is performed under the condition that the relationship with the O ₃ content satisfies the above-mentioned formula (C) or (D). | Cp-0.12
In the case of | ≦ 0.05, that is, in a steel which is liable to cause a subperitectic reaction, the Al ₂ O ₃ content in the mold powder is
The content is set low so as to satisfy the above-mentioned formula (C).
By lowering the Al ₂ O ₃ content to moderately increase the precipitation of crystals in the film, suppressing solidification shell shrinkage associated with the subperitectic reaction and increasing the adhesion to the mold wall, solidification in the mold The uneven cooling of the shell is suppressed, and the occurrence of vertical cracks on the surface of the slab is prevented.

When | Cp-0.12 |> 0.05, the occurrence of vertical cracks on the surface of the slab can be prevented without reducing the Al ₂ O ₃ content. Rather, Al ₂ O ₃
By slightly increasing the content to 10 to 15% by mass,
It is desirable to increase the viscosity of the molten layer of the mold powder to an appropriate level, to suppress excessive crystal precipitation in the film, and to uniformly cool the solidified shell in the mold.

[0029]

EXAMPLE Using a vertical bending type continuous casting machine, a thickness of 230 mm was used.
mm and a width of 1220 mm were cast at a speed of 1.1 m / min or 1.4 m / min. Table 1 shows the chemical composition of the steel used.
Shown in The steel is a low carbon steel and a hypoperitectic medium carbon steel, and Table 1 shows the chemical composition Cp defined by the method of the present invention. Cp of low carbon steel is 0.043% by mass, Cp of medium carbon steel is 0.117% by mass, and | Cp−
0.12 | is 0.077 for low carbon steel and 0.0 for medium carbon steel.
003, and | Cp−0.12 | ^{2 of} medium carbon steel.
Is 0.000009. Therefore, when casting low-carbon steel, Al ₂ O ₃ in the mold powder is desirable.
The content is 10 to 15% by mass according to the above formula (D). Desirable Al in the mold powder when casting medium carbon steel is used.
_The content of ₂ O ₃ is 5 to 10% by mass from the formula (C) described above.

[0030]

[Table 1] Table 2 shows the composition of the mold powder used in the test. Mold powder a has basicity and viscosity, mold powder b has basicity, mold powder c has Al ₂ O ₃ content, mold powder d has Al ₂ O ₃ content and viscosity, and mold powder e has TiO ₂ content. Mold powders that do not satisfy the conditions specified in the invention. Table 3 shows the test conditions and test results.

[0031]

[Table 2]

[Table 3] Among the indices shown in Table 3, "Noro Pin" is the number of occurrences of the lazy flaws and pinhole flaws observed when the slab surface was scarfed. Index A: None, Index B:
1 to 5, index C: 6 to 10, index D: 11 to 20
, Index E: 21 to 50, index F: 51 or more, and the indexes A and B are the number of occurrences without any problem. The “vertical crack” is the ratio of molecules in which a vertical crack has occurred in the cast slab. Index A: less than 3%, Index B: 3% or more and less than 6%, Index C: 6% or more and less than 10%,
Indices D: 10% or more, and indices A and B are occurrence situations without any particular problems. Furthermore, "copper plate temperature fluctuation"
Is the temperature variation of the thermocouple buried in the mold copper plate during casting, small index: less than ± 3 ° C, medium index: less than ± 10 ° C, large index: ± 10 ° C or more. ,
There is no problem in particular.

In the test in which low carbon steel was cast at a speed of 1.4 m / min, the test No. In comparison with Test No. 6, Test No. 1 to No. In the case of No. 5, it can be seen that all of the flaws and pinhole flaws are reduced and the surface quality of the slab is improved. Test No. of the comparative example. 7 and No. 7 In No. 10, the solidification point was as high as 1300 ° C. or more, and it could not be used for casting. Test No. of the comparative example. 8 and No. 9
In this case, since the crystal precipitation was excessive or unstable, the temperature of the mold copper plate greatly fluctuated, and in some cases, vertical cracks occurred. In addition, since the solidification point or the viscosity was high, the lubricity was poor, and many defective portions such as restraint marks were observed on the slab surface.

In the test in which medium carbon steel was cast at a speed of 1.1 m / min, the test No. Compared with Test No. 16 of the present invention, 11-No. In No. 15, it can be seen that all of the flaws, pinhole flaws, longitudinal cracks, and variations in the temperature of the mold copper plate are reduced, and the surface quality of the slab is improved. Test No. of the comparative example. 17 and No. In the case of No. 20, the solidification point was as high as 1300 ° C. or higher, and it was impossible to use for casting. Test No. of the comparative example. 18 and No.
In 19, since the crystal precipitation is excessive or unstable,
The temperature of the mold copper plate greatly fluctuated. In 19, a vertical crack also occurred.

[0034]

According to the mold powder of the present invention and the continuous casting method of the present invention using the mold powder, no depletion or vertical cracking occurs on the slab surface, and the cleanness of the surface layer of the slab is good. A simple slab can be obtained.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Masayuki Kawamoto 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. F-term (reference) 4E004 MB14 NC04

Claims (3)

[Claims] 1. A mold powder comprising CaO, SiO ₂ , Al ₂ O ₃ and a fluorine compound as basic components,
The ratio CaO / SiO ₂ mass% with respect to SiO ₂ of aO is 1.1 to 1.5, the Al ₂ O ₃ 5 to 15 wt%,
5 wt% of an alkali metal in a total of oxide conversion content below the ZrO ₂ 1 wt% or less, the TiO ₂ 0 to 8 wt%, and contains 4-12 wt% of F,. 2 to a viscosity at 1300 ° C. A mold powder for continuous casting, characterized in that the powder has a solidification point of 1100 ° C. or more. 2. The method according to claim 1, wherein the contents of Al ₂ O ₃ , TiO ₂ and F satisfy the following formula (A).
3. A mold powder for continuous casting according to item 1. 2 ≦ (Al ₂ O ₃ ) × (TiO ₂ ) ≦ 10 + 15 × (F) (A) where (Al ₂ O ₃ ): Al ₂ O in the mold powder
( ₃ ) Content (% by mass) (TiO ₂ ): Content of TiO _{2 in} mold powder (% by mass) (F): Content of F in mold powder (% by mass) 3. A continuous casting method using the mold powder according to claim 1 or 2, wherein a chemical composition Cp of molten steel defined by the following formula (B) and A in the mold powder are provided.
A continuous casting method characterized in that casting is performed under the condition that the relationship with the l ₂ O ₃ content satisfies the following formula (C) or (D). Cp = C + 0.02 × Mn + 0.04 × Ni-0.01 × Si + 0.02 × Cr + 0.07 × S (B) Here, C, Mn, Ni, Si, Cr and S: in molten steel 5 + 2000 × | Cp-0.12 | ² ≦ (Al ₂ O ₃ ) ≦ 10 + 2000 × | Cp−0.12 | ² ... When the content (mass%) | Cp−0.12 | ≦ 0.05;・ (C) When | Cp−0.12 |>0.05; 10 ≦ (Al ₂ O ₃ ) ≦ 15 (D) where | Cp−0.12 |: 0 from the value of Cp Absolute value of the value obtained by subtracting 12 (Al ₂ O ₃ ): Al ₂ O ₃ content in the mold powder (% by mass)