JPH11267803A - Mold powder for continuous casting of steel and continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide powder capable of preventing occurrence of bubbles defects on a cast piece surface and obtaining the cast piece with good surface quality, and a continuous casting method using it. SOLUTION: This mold powder for continuous casting of steel consists of CaO, Al2 O3 and fluorine compound, as basic components, with the ratio in weight% of T. CaO to Al2 O3 (T. CaO/Al2 O3 ) expressed by formula T.CaO (weight%)=CaO (weight%)+CaF2 (weight%)×(56/78) (CaF2 (weight%))=F (weight%)×(78/38) being 0.5-2.5, and contains 15 weight% or less SiO2 , 5-30 weight% F as fluorine compound, 0-10 weight% Na2 O, and 0-10 weight% MgO.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold powder for continuous casting of steel (hereinafter abbreviated as "powder"), and more particularly to a method for preventing the occurrence of cellular defects on the surface of a slab and improving the surface quality. The present invention relates to a powder capable of obtaining a slab and a continuous casting method using the same.

[0002]

2. Description of the Related Art In continuous casting of steel, powder generally used conventionally is calcium oxide (hereinafter referred to as CaO).
), Silicon dioxide (hereinafter abbreviated as SiO ₂ ), calcium fluoride (hereinafter abbreviated as CaF ₂ ) as main chemical compositions, and sodium oxide (hereinafter abbreviated as Na _2).
O) and the like.

Among them, SiO ₂ is 20 to 40% by weight.
(Hereinafter simply abbreviated as%). Ca
% Of O to SiO ₂ CaO / SiO ₂ had been adjusted to a range of about 0.8 to 1.3. here,
T. CaO is represented by the following formula (1).

[0004] T. CaO (% by weight) = CaO (% by weight) + CaF ₂ (% by weight) × (56/78) (1) where CaF ₂ (% by weight) = F (% by weight) × (78 /
38).

[0005] By using these conventional powders containing about 20 to 40% of SiO ₂ , for example, Mn is reduced to 0.5%.
In the case of continuously casting steel (hereinafter, referred to as high manganese steel) containing more than a certain degree, there is a problem that porosity defects occur on the surface of the slab. The cellular defects of the slab become the surface defects of the product in the subsequent rolling.

[0006] This bubble-like defect is caused especially when the powder is melted and flows into the gap between the inner wall of the mold and the solidified shell.
When some unmelted powder is entrained, it is likely to occur.

This defect is caused by a slab having a relatively small cross-sectional area, for example, a slab having a circular cross section (hereinafter referred to as a round slab).
Tends to occur when the molten steel is cast with electromagnetic stirring.

[0008]

SUMMARY OF THE INVENTION The present invention is capable of preventing the occurrence of cellular defects on the surface of a slab and obtaining a slab having a good surface quality.
It is another object of the present invention to provide a powder which can be used both when casting a round slab having a small cross-sectional area and when casting a slab having a small cross-sectional area at a high speed, and a continuous casting method using the same.

[0009]

The gist of the present invention lies in a powder for continuous casting shown in the following (1) and (2) and a continuous casting method shown in (3).

(1) A mold powder for continuous casting of steel containing CaO, Al ₂ O ₃ and a fluorine compound as basic components. CaO and Al ₂ O
₃ % “T.CaO / Al ₂ O ₃ ” is 0.5 to
2.5, a SiO ₂ 15% or less, the F as fluorine compound 5-30%, 0-10% of Na ₂ O, and MgO 0
A mold powder for continuous casting of steel, characterized in that it contains 10 to 10%. T. CaO (%) = CaO (%) + CaF ₂ (%) × (56/78) (1) where CaF ₂ (%) = F (%) × (78/38).

(2) The viscosity at 1300 ° C. is 2.5 to
The mold powder for continuous casting of steel according to the above (1), which is 10 poise.

(3) A continuous casting method for steel having a Mn content of 0.5% or more, characterized by using the mold powder according to (1) or (2).

When a high-manganese steel is continuously cast using a conventional powder containing about 20 to 40% of SiO ₂ , there is a problem that a bubble defect is generated as described above. The present inventors have obtained the following findings as a result of investigating the cause of the occurrence of the defect.

First, from the viewpoint of the operation of continuous casting, the occurrence of this defect is examined. In particular, when the powder is in a molten state and flows into the gap between the inner wall of the mold and the solidified shell, a part of the unmelted powder is unreacted. It has been found that this defect is likely to occur when powder is entrained.

[0015] Further, it may be particularly likely to occur depending on casting conditions. For example, when a round cast slab is cast while electromagnetically stirring molten steel, this cellular defect is likely to occur.

This is because the molten steel is rotated in the mold by electromagnetic stirring, so that the molten steel surface becomes mortar-shaped, so that the powder added on the molten steel surface and the molten powder gather at the center of the molten surface. As a result, especially near the inner wall of the mold, the thickness of the molten powder tends to be thin, so that the unmelted powder as well as the molten powder easily gets caught in the gap between the inner wall of the mold and the solidified shell.

Regarding the occurrence of this defect, the present inventors have found that the MnO content in the powder film, in which the molten powder has been solidified in the gap between the inner wall of the mold and the solidified shell, also decreases, especially as the SiO ₂ content in the powder decreases. Also, it was found that when the MnO content in the powder film was high, the number of cellular defects was large.

As described above, the carbon monoxide gas (hereinafter referred to as CO gas) generated in the vicinity of the meniscus of the molten steel in the mold is trapped by the solidified shell and becomes a cellular defect on the surface of the slab. I understood.

That is, bubble defects caused by CO gas are as follows:
It is caused by a series of reactions shown below. M in molten steel
n (hereinafter referred to as [Mn]) is an oxidation-reduction reaction represented by the following formula (2) with SiO _{2 in} unmelted powder or molten powder caught in the gap between the inner wall of the mold and the solidified shell. To form manganese oxide (hereinafter referred to as MnO). SiO _{2 in} these powders is reduced, and Si moves into the molten steel (hereinafter, Si in the molten steel is referred to as [Si]). 2 [Mn] + (SiO ₂ ) = 2 (MnO) + [Si] (2) Next, the generated Mn0 is combined with carbon in powder and molten steel by oxidation reduction represented by the following formula (3). Cause a reaction. (MnO) + C = [Mn] + CO (3)
The CO gas generated by the equation (3) forms bubbles. These bubbles are trapped in the solidified shell, and cellular defects occur in the slab.

Therefore, it has been found that means for suppressing the generation of CO gas bubbles should be taken in order to reduce the bubble defects. For this purpose, for example, Si in powder
It was conceived that the above reaction was suppressed by reducing the O ₂ content, thereby suppressing the bubble defect.

By the way, among conventional powders, a powder having a low content of SiO ₂ as a main component or containing no SiO ₂ has been proposed. That is, JP 57
No. 184,563 discloses that when casting aluminum-killed steel, the oxidation of Al in molten steel is suppressed,
to l ₂ O ₃ based nonmetallic inclusions absorbed by intended to prevent immersion nozzle clogging, composed mainly of Al ₂ O ₃ -CaO based, reduced the SiO ₂ content of below 7.0% powder Has been proposed.

Japanese Patent Application Laid-Open No. Sho 60-133965 discloses an Al ₂ O 3 for preventing clogging of an immersion nozzle.
By using O ₃ , CaO, and MgO as main components, SiO ₂
Powders containing no are proposed.

However, Japanese Patent Application Laid-Open No. 57-184563 discloses
The powder proposed in Japanese Patent Application Laid-Open No. 60-133965 or Japanese Patent Application Laid-Open No. Sho 60-133956 has a relatively high melting temperature of 1200 ° C. or higher, which contains or does not contain SiO ₂ .

Therefore, when continuously casting round cast pieces, or when casting is performed at a high speed of, for example, 2.0 m / min or more to improve productivity due to small cross-sectional area of cast pieces, these casts may be used. Powder is too slow to melt and cannot be used. When the slab cross-sectional area is small or in the case of high-speed casting, since the amount of molten steel passing through the mold per unit time is relatively small, the heat supply from the molten steel to the powder decreases, and the melting of the powder decreases. Because it is slow.

The powder of the present invention is characterized in that the content of SiO ₂ , which was used as a main component in conventional powders, is greatly reduced, and a CaO—Al ₂ O ₃ —fluorine compound system is used as a basic component. And Note that CaF ₂ is mainly used as the fluorine compound.

By the way, in the conventional powder containing CaO—SiO ₂ —CaF ₂ as a main component, when the content of SiO ₂ is reduced, the melting temperature of the powder increases.
It was very difficult to reduce the content of _{2 from} the conventional range of 20 to 40%. In the present invention, A
By adding l ₂ O ₃ and increasing the CaF ₂ content, it was possible to reduce SiO ₂ while suppressing the rise in melting temperature.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic compounds constituting the powder of the present invention are CaO, Al ₂ O ₃ and CaF ₂ as a fluorine compound. Hereinafter, the basic concept of the chemical composition of the powder of the present invention will be described.

CaO is about 20 to 50%, Al ₂ O ₃
Is contained in an amount of about 20 to 50%, but these compounding ratios are only a guide. In the present invention, T. CaO
/ Al ₂ O ₃ ratio as O.D. 5 to 2.5.
By adding Al ₂ O ₃ , it is possible to reduce the SiO ₂ while suppressing the rise in the melting temperature. However, when the ratio is less than 0.5 or more than 2.5, any powder is used. The melting temperature of the powder exceeds 1300 ° C., and the melting of the powder becomes slow, which hinders the operation of continuous casting. Therefore, this ratio was set to 0.5 to 2.5.

The content of SiO ₂ needs to be 15% or less. FIG. 1 shows the results of examining the amount of increase in the MnO content in the molten powder during casting by changing the SiO ₂ content in the powder containing the CaO—Al ₂ O ₃ —CaF ₂ system of the present invention as a basic component. Shown in Here, MnO% in the molten powder is:
The powder film discharged immediately below the mold during casting was collected, its content was measured by chemical analysis, and the increase from MnO% in the powder before use was calculated and used as an index.

As can be seen from FIG. 1, SiO in the powder
MnO% increase amount in the molten powder with _2% decrease is reduced, the trend SiO _2% becomes particularly noticeable in a range of 15% or less. In order to prevent the occurrence of bubble defects, the amount of MnO in the powder must be reduced as much as possible. Therefore, the content of SiO ₂ in the powder needs to be 15% or less. Desirable is 5% or less.
Although SiO ₂ may not be positively added, it may be inevitably included as an impurity of another raw material.

Next, in a pure ternary composition of the CaO--Al ₂ O ₃ --SiO ₂ system, even at the lowest melting temperature, 13
It is about 30 ° C., and the melting temperature is too high for use as a powder. For use as powder, 1000
It is necessary to lower the melting temperature to about 1300 ° C.

Further, when casting a slab having a small cross-sectional area, or when performing a high-speed casting at a casting speed of, for example, 2.0 m / min or more in order to improve productivity, etc. The temperature needs to be adjusted to 1050-1250 ° C. That is, from the viewpoint of high-speed casting, the upper limit of the powder melting temperature is desirably 1250 ° C. On the other hand, even if the melting temperature is too low, the thickness of the molten powder is too thin, and the unmelted powder is likely to be entrained. The lower limit is desirably 1050 ° C.

Incidentally, this CaO-Al ₂ O ₃ -Si
Addition of F as a fluorine compound in an O ₂ ternary composition has an effect of lowering the melting temperature of powder. T. CaO / Al ₂
If the content of F is 5% in the range where the ratio of O ₃ is 0.5 to 2.5 and the content of SiO ₂ is 15% or less, the melting temperature becomes 125%.
The melting temperature can be lowered to about 0 ° C., and when further 30% is blended, the melting temperature decreases to about 1050 ° C. Therefore, it is desirable to mix F by 5 to 30%. When 5 to 30% of F is converted to CaF ₂ , about 10 to 6
0%. The compounding form may be a commonly used one such as fluorite or sodium fluoride.

Na ₂ O and MgO are compounds to be added as required. Na ₂ O is an oxide having a low melting point, and conversely, MgO is an oxide having a high melting point. by the way,
The CaO-Al ₂ O ₃ -CaF ₂ system of the present invention in a powder having a basic component, F5~30% as described above, S
The melting temperature of a powder containing 15% or less of iO ₂ is about 1050 to 1250 ° C., but depending on the composition,
The temperature may slightly deviate from this range.

That is, in some cases, it is necessary to raise or lower the melting temperature of the powder. In that case, Na ₂ O or MgO may be added according to the situation. This makes it easy to freely adjust the melting temperature. The effect begins to be obtained when the amounts to be blended are each about 2%. 1
It is preferable to adjust the melting temperature to be a target in a range up to 0%. If it exceeds 10%, the melting temperature will be
The above-mentioned upper and lower appropriate temperatures are respectively deviated. Therefore, in the case of compounding, 2 to 10
% Is desirable.

The viscosity of the powder is adjusted by the content of CaF _{2 and the} like. If the content of CaF ₂ is large, the viscosity decreases. This viscosity is preferably higher when, for example, a round slab is cast while electromagnetically stirring molten steel. By increasing the viscosity, the flow of the molten powder is suppressed, and the thickness of the molten slag near the inner wall of the mold can be secured, which is effective for preventing unmelted powder from being entrained. The lower limit of viscosity is 1300 ° C
2.5 poise is desirable. The upper limit is 10po
If it exceeds this value, the lubricating property deteriorates and the operation is hindered. Therefore, the viscosity at 1300 ° C. is 2.5 to 10
Poise is preferred.

Next, the continuous casting method of the present invention will be described. The powder of the present invention particularly has a Mn content of 0.5%.
It is suitable for continuous casting of the above high manganese steel.

As described above, particularly in the continuous casting of high manganese steel, the cause of bubble defects is that CO gas generated in the vicinity of the meniscus of molten steel in the mold is trapped in the solidified shell, and the bubble property of the slab surface is reduced. It is a defect.
The mechanism of CO gas generation is [Mn] in molten steel.
When the reaction of the SiO ₂ in the powder unmelted caught into the gap between the mold inner wall and the solidified shell or the molten powder, continue and MnO of the reaction product at this time, the carbon in the and in the molten steel powder In the reaction produces CO gas.

Therefore, this phenomenon is likely to occur when the Mn content of steel is high, and is remarkable at a content of 0.5% or more. As mentioned above, considering the reaction mechanism,
The most preferable is to reduce SiO ₂ in the powder. In the present invention, the reduction of SiO _{2 in} the powder is intended. Therefore, when the powder of the present invention is used, the Mn content is 0.5% or more. Can be cast without the occurrence of cellular defects.

[0040]

EXAMPLE In a curved continuous caster, 11 kinds of powders of the present invention shown in Table 2 and 4 kinds of powders as comparative examples were used for high manganese steels having a chemical composition shown in Table 1, and
By performing continuous casting at a casting speed of 2.0 m / min, a round slab having a cross-sectional shape of 300 mm in diameter was produced. The surface of the obtained cast slab was examined, and the performance of each powder was evaluated from the total number of generated porosity defects.

[0041]

[Table 1]

[0042]

[Table 2]

In Table 2, nine types of Examples 1 to 9 of the present invention are shown.
The powder is CaO, Al of the basic composition. _TwoO_ThreeAnd CaF
_TwoAnd powders of the invention, wherein Examples 1 to 4 of the present invention are contained.
Is T. CaO / Al_TwoO_Three0.7 to 2.1
Inventive Examples 5 to 7 have a F content of 8.0 to 8.0.
26.0%, and Examples 8 and 9 of the present invention
_TwoIs defined by the present invention up to 8 to 12%.
The effect was changed within a certain range. N
a_TwoO and MgO are not blended, and the weight inevitably contained
% Is shown. The two types of Invention Examples 10 and 11 are basically
Na in composition_TwoMix one or both of O and MgO,
The effect was adjusted by adjusting the melting temperature.

The two types of powders of Comparative Examples 12 and 13 are powders of a general composition conventionally used and have a SiO ₂ content of about 30 to 35%. The two types of Comparative Examples 14 and 15 have a SiO ₂ content of 15% or less and within the range specified in the present invention. A powder having CaO / Al ₂ O ₃ of 0.3 and 3.1, which exceeds the lower and upper limits of the range specified in the present invention.

Table 3 shows evaluation indices of the slab surface quality obtained by casting using the powders of the inventive examples and comparative examples. Table 4 shows the relationship between the occurrence frequency of cellular defects and the evaluation index of the slab surface quality.

[0046]

[Table 3]

[0047]

[Table 4]

When the powders of Examples 1 to 4 of the present invention were used,
T. Since the ratio of CaO / Al ₂ O ₃ was within the range specified in the present invention, the slab was extremely good with few cellular defects. However, T. The ratio of CaO / Al ₂ O ₃ is 2.1
In the case of Inventive Example 4 which was slightly higher, the cast slab evaluation was B, and Inventive Examples 1 to 3 were better.

The melting temperature of Example 4 of the present invention was 1250
Since the temperature was slightly higher than 1264 ° C., the cast slab was evaluated as B. On the other hand, the evaluation of the cast slab using the powder of Inventive Example 11 in which Na ₂ O and Mg 0 were blended and the melting temperature was adjusted was the best.

In the results of the powders of Examples 5 to 7 of the present invention in which the influence of the F content was examined in the range of 8.0 to 26.0%, the powder viscosity decreased as the F content increased. F to 2
The viscosity of Inventive Example 7 containing 6.0% was 2.5 poise, which was slightly lower. However, in each case, the slab evaluation was the best, and it was confirmed that a slab free of cellular defects could be obtained if the F content was within the range specified in the present invention.

In the powders of Examples 8 and 9 of the present invention, SiO
₂ The effect of the content was investigated. With the increase of SiO ₂
Although the melting temperature is lower, the SiO ₂
The evaluation of the cast slab using Inventive Example 9 of 12.0%, which is close to the upper limit of the range, is lowered to B. This is due to the occurrence of some bubble defects.

The powder of Inventive Example 10 is obtained by mixing Na ₂ O with the chemical composition of Inventive Example 2. The melting temperature of this powder was lowered as expected, and the evaluation of the slab using this powder was also good.

The two types of powders in Comparative Examples 12 and 13 were Si
Because O ₂ content greater and 30% to 35%, bubble defects often occur in the slab, the evaluation of the cast slab surface was poor and D to E.
The two powders of Comparative Examples 14 and 15 had a melting temperature of 130.
The temperature was so high as to be 0 ° C. or higher and could not be used for the test of continuous casting.

[0054]

The powder of the present invention is capable of continuously casting a high manganese steel having a Mn content of 0.5% or more, in which porosity defects easily occur in a slab, into a slab having a relatively small cross-sectional area at a high speed. It is suitable for. By using the powder of the present invention, it is possible to produce a slab with good surface quality while maintaining good lubricity.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the SiO ₂ content in powder and the amount of increase in the MnO content in molten powder during casting.

Claims (3)

[Claims] 1. A mold powder for continuous casting of steel containing CaO, Al ₂ O ₃ and a fluorine compound as basic components. The ratio “T. CaO / Al ₂ O ₃ ” of the weight% of CaO and Al ₂ O ₃ is 0.5 to
2.5, a SiO ₂ 15 wt% or less, 5 to 30 wt% of F as a fluorine compound, a Na ₂ O 0 wt%, the steel is characterized in that it contains MgO 0 wt% Mold powder for continuous casting. T. CaO (% by weight) = CaO (% by weight) + CaF ₂ (% by weight) × (56/78) (1) where CaF ₂ (% by weight) = F (% by weight) × (78 /
38). 2. The mold powder for continuous casting of steel according to claim 1, wherein the viscosity of the molten powder at 1300 ° C. is 2.5 to 10 poise. 3. A method for continuously casting steel having a Mn content of 0.5% by weight or more, comprising using the mold powder according to claim 1 or 2.