JP2000158105A - Mold powder for continuous casting of steel and continuous casting method - Google Patents

Abstract

[PROBLEMS] To be suitable for high-speed casting as typified by a thin slab continuous casting method, and for continuous casting of steel such as medium carbon steel in which vertical cracks are likely to occur on a slab surface. Provided is a mold powder and a method of continuously casting steel using the mold powder. A CaO, and SiO ₂ and fluorine compounds as basic components, in weight percentages, the ratio of the percent (CaO) h represented by the following formula (A), and% of SiO ₂ (CaO) h
/ SiO ₂ is at 0.9-1.9, comprises CaF ₂ represented by the following formula (B) 15 to 60%, and a Na ₂ O 0 to 1
Mold powder for continuous casting of steel containing 5% and 1-20% of MgO. Here, (CaO) h = T. CaO-F
× (56/38) (A), CaF ₂ = F × (7
8/38) (B), T.P. CaO: Total C in powder
a content of Ca content (amount%), F: Total F in powder
Content (%)

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for casting steel at a high speed or for casting steel such as medium carbon steel.
The present invention relates to a mold powder for continuous casting capable of obtaining a cast piece with good surface quality and performing a stable casting operation. Further, the present invention relates to a continuous casting method using the mold powder.

[0002]

2. Description of the Related Art Automotive exterior steel sheets and the like that require good surface quality and cold workability are usually manufactured from continuous cast slabs having a thickness of about 250 mm. The reason is,
The purpose is to use cast pieces with good surface quality and internal quality and to obtain high productivity. The casting speed at this time is generally about 2 to 3 m / min. This is because when casting is performed at a casting speed higher than this, vertical cracks may occur on the slab surface, or nonmetallic inclusions may easily remain inside the slab, which may cause defects in the product.

[0003] On the other hand, from several years ago, an electric furnace minimill process in which a thin slab continuous casting facility and a simple hot rolling facility are arranged on one production line has been adopted. In this thin slab continuous casting method, at least 5
The aim is to operate at a casting speed of about m / min.

[0004] Medium carbon steel is susceptible to longitudinal cracks on the surface of the slab due to peritectic reaction during solidification from molten steel. However, this medium carbon steel is produced by thin slab continuous casting as described above. Casting at a high casting speed promotes the occurrence of vertical cracks in the slab. In the case of low-alloy steels as well, when an alloy component that increases crack sensitivity is included, vertical cracks are likely to occur in the slab.

It is well known that the occurrence of vertical cracks on the slab surface and the mold powder are closely related.
In the continuous casting method, molten steel is supplied into a mold from an immersion nozzle, and mold powder is poured into the surface of the molten steel in the mold during casting. Usually, a mixture of powders of a plurality of types of oxides and carbon is used for the mold powder. Mold powder put into the mold,
The molten steel is melted by the heat of the molten steel, and molten slag is formed on the surface of the molten steel. This molten slag flows into the gap between the inner wall of the mold and the solidified shell to form a slag film. This slag film is cooled and solidified by contact with the mold. The solidified portion comprises a vitreous portion and a crystalline portion.

[0006] The mold powder exhibiting such behavior has the following effects.

1) Insulation of molten steel surface in the mold and prevention of oxidation of the molten steel 2) Absorption of bubbles and oxides existing in the molten steel and floating on the molten steel surface 3) Lubricity between the inner wall of the mold and the solidified shell 4) Adjustment of cooling rate of solidified shell by adjusting heat transfer resistance of molten slag.

[0008] In these roles, when casting at high speed, it is important to ensure the lubricity by the molten slag shown in 3). Also, to prevent vertical cracks on the slab surface,
It is important to adjust the cooling rate of the solidified shell in the mold of 4), that is, the surface of the slab at the initial stage of solidification.

In general, when the casting speed is increased in the continuous casting method, the amount of molten slag flowing into the gap between the inner wall of the mold and the solidified shell decreases. When the inflow of the molten slag decreases and the slag film thickness decreases, the solidified shell is restrained by the inner wall of the mold due to poor lubrication, and in extreme cases, an operation accident such as breakout occurs. Then, in order to secure the inflow amount of the molten slag, the solidification point at the time of melting the mold powder is lowered or the viscosity is lowered. However, if the solidification point and viscosity of the mold powder are excessively reduced, the thickness of the slag film tends to be uneven. Therefore, the cooling rate of the solidified shell in the mold becomes uneven, and vertical cracks are likely to occur on the surface of the slab. In this way, a mold powder having both properties of lubrication, which is effective for increasing the casting speed, and uniformity of the cooling rate of the solidified shell, which is effective for preventing vertical cracks on the slab surface, is produced. That is an important technical issue.

The following method has been proposed for preventing longitudinal cracks on the surface of a slab when casting at high speed.
That is, in Japanese Patent Application Laid-Open No. 3-193248, ZrO ₂ , TiO ₂ , Sc ₂ O ₃ , Y ₂ O ₃
A method has been proposed in which an oxide of a Group IIIA or Group IV element such as the above is added as a crystal precipitation accelerator. In this publication, the viscosity of molten slag is 1 pois at 1300 ° C.
It is said that by lowering the value to e or less, a higher casting speed can be achieved. The mold powder precipitates crystals in the process of cooling from the molten state, and the crystals slowly cool the surface of the slab in the mold. Slow cooling of the slab surface
The cooling rate on the slab surface is made uniform.

[0011] Also, in Japanese Patent Application Laid-Open No. Hei 5-15955,
Reducing the viscosity of the molten slag; CaO SiO
Wt% of the specific T. for ₂ It has been proposed to increase CaO / SiO ₂ . Here, T. CaO is the sum of CaO contained in the mold powder and CaO converted to CaO estimated to be present as CaF ₂ and is defined by the following formula (C).

T. CaO (%) = CaO (%) + CaF ₂ (%) × (56/78) (C) Increasing the CaO / SiO ₂ of about 1.2 to 1.3, crystals during the cooling process of the molten slag is precipitated, the crystals are to slow cool the slab surface within the mold.

However, the above-mentioned Japanese Patent Laid-Open Publication No.
When the mold powder proposed in Japanese Patent Application Laid-Open No. 5-15955 and medium carbon steel containing peritectic steel is cast at a casting speed of 2 to 3 m / min or more, the surface of the slab by the mold powder In some cases, the slow cooling effect of the steel is insufficient, and vertical cracks may occur on the slab surface.

In addition to the above, a mold powder having a large CaO / SiO ₂ value has been proposed in Japanese Patent Application Laid-Open No. 5-269560. This mold powder is CaO / Si
O ₂ is set to 1.1 to 1.8, and CaO / F is set to 9 to 4
0 is satisfied. The purpose is to prevent vertical cracking during high-speed casting and to prevent erosion of the immersion nozzle. Japanese Patent Application Laid-Open No. 54-35129 proposes a mold powder having a relatively high CaO / SiO ₂ content. That is,
It has been proposed that the CaO / SiO ₂ be 0.6 to 1.4 and that fluorite be blended up to 10% by weight. However, when using these proposed mold powders, when casting at a casting speed of 2 to 3 m / min or more, vertical cracks may occur on the slab surface.

As described above, when casting at a casting speed of about 5 m / min or more, or when casting medium carbon steel or the like containing peritectic steel, the occurrence of longitudinal cracks on the slab surface is prevented. In order to prevent the occurrence of operation accidents such as breakouts, it is necessary to further improve the performance of the mold powder.

[0016]

The present invention is suitable for high-speed casting as typified by a thin slab continuous casting method, and tends to cause vertical cracks on the surface of a slab such as medium carbon steel. An object of the present invention is to provide a mold powder suitable for continuous casting of steel and suitable for a stable casting operation, and a continuous casting method using the mold powder.

[0017]

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

(1) (CaO) represented by the following formula (A) containing CaO, SiO ₂ and a fluorine compound as basic components:
h (% by weight) and the SiO ₂ content (% by weight) (C
aO) h / SiO ₂ is 0.9 to 1.9, further contains 15 to 60% by weight of CaF ₂ represented by the following formula (B), 0 to 15% by weight of Na ₂ O, and MgO From 1 to 20
Mold powder for continuous casting of steel containing wt%.

Here, (CaO) h = T. CaO-F × (56/38) (A) CaF ₂ = F × (78/38) (B) CaO: CaO equivalent amount of total Ca content in powder (% by weight) F: Total F content in powder (% by weight) (2) A continuous casting method of steel using the mold powder described in (1) above.

In the mold powder of the present invention (hereinafter simply referred to as powder), since (CaO) h / SiO ₂ , that is, the basicity is increased, a large amount of crystals are precipitated in the process of solidification from a molten state. However, simply increasing the basicity of the powder will increase the freezing point of the powder. In this case, it is difficult to cast at high speed. on the other hand,
Simply increasing the content of CaF ₂ or Na ₂ O, etc., and lowering the freezing point and viscosity will result in the formation of a sintered layer in the molten slag or the erosion of the immersion nozzle, resulting in a stable casting operation. It will be difficult. Then, the present inventors have solved the powdery high basicity and the solidification point and the characteristic which are difficult to achieve for the powder such as a decrease in viscosity by means of the following (a) and (b).

(A) Acceleration of Crystal Precipitation from Molten Slag In order to prevent vertical cracking of a slab, it is important to equalize the cooling rate of the solidified shell in the mold, that is, the surface of the slab at the early stage of solidification. When the cooling rate of the slab surface in the early stage of solidification is not uniform, the thickness of the solidified shell becomes uneven in the width direction of the slab. Therefore, stress generated in the solidified shell due to solidification shrinkage is not uniformly relieved in the width direction of the slab, and a vertical crack is generated on the slab surface.

In order to cool the solidified shell in the mold at a uniform cooling rate through a slag film (hereinafter simply referred to as a film) between the mold and the solidified shell, it is important to increase the heat transfer resistance of the film. It is. If the heat transfer resistance of the film is small, the solidified shell is greatly affected by the variation in the cooling effect of the mold. In that case, the variation in the cooling rate of the solidified shell at each position of the mold becomes large, and the thickness of the solidified shell becomes
It becomes uneven in the width direction of the mold. Conversely, by increasing the heat transfer resistance of the film and slowing the cooling rate of the solidified shell,
The thickness of the solidified shell is made uniform in the width direction of the mold. Therefore, the occurrence of vertical cracks on the surface of the slab can be prevented.

In the powder of the present invention, the conventional index T.I. Ca
By adopting a new index of (CaO) h / SiO ₂ represented by the above formula (A) instead of O / SiO ₂ , a ternary system having CaO—SiO ₂ —CaF ₂ as a basic component is adopted. A composition range in which crystals are easily precipitated is selected. Therefore, in the process of solidifying the molten slag, more crystals are precipitated, and the heat transfer resistance of the molten slag is large.

Among the ternary powder compositions containing CaO—SiO ₂ —CaF ₂ as a basic component, (CaO) h / S
Because iO ₂ is large and 0.9-1.9, in the process of the molten slag solidifies, many crystals precipitated.

FIG. 1 shows (CaO) h-SiO ₂ —CaF ₂ 3 for explaining the range of the chemical composition of the powder of the present invention.
FIG. 3 is a composition diagram of an original system. In the powder of the present invention, since most of the fluorine compound to be blended is CaF ₂ , the value of (CaO) h calculated from the above formula (A) and the content of the blended CaO are almost the same value. It is. Therefore, the powder of the present invention will be described with reference to the above ternary composition diagram for convenience.

As shown by the shaded area in FIG.
O) The chemical composition of the powder of the present invention having h / SiO ₂ of 0.9 to 1.9 and a CaF ₂ content of 15 to 60% by weight (hereinafter simply referred to as%) is (CaO) h / Boundary line 1 with SiO ₂ of 0.9 (CaO.SiO ₂ and CaF ₂ 100
%), (CaO) h / SiO ₂ is 1.9.
Border ₂ (2CaO · SiO 2 and CaF ₂ 100% and a straight line connecting the), CaF ₂ content of 15% linear, Ca
This is a range surrounded by a straight line having an F ₂ content of 60%.

For comparison, as in the case of the powder of the present invention, the range of the chemical composition of the conventional powder used for slowly cooling the solidified shell is indicated by the symbol (a). Further, the range of the chemical composition of the conventional powder having a high basicity and preventing the immersion nozzle from being damaged is indicated by the symbol (b). (CaO) h / S, such as the powder of the present invention
iO ₂ is 0.9 to 1.9 and CaF ₂ content is 15 to 6
In the case of 0%, during the cooling process from the molten state, 3CaO.2SiO ₂ .CaF ₂ or 3
Many crystals represented by the molecular formula of CaO · 2SiO ₂ precipitate.

(B) Reducing the viscosity of high basicity powder and optimizing the solidification point The thickness of the film during continuous casting of steel is determined by the amount of molten slag flowing into the gap between the inner wall of the mold and the solidified shell in the mold. . If the inflow is excessive, the difference in the film thickness between the position where the inflow is small and the position where the inflow is large increases, and the cooling rate of the solidified shell becomes uneven. On the other hand, when the inflow amount is too small, the thickness of the film is reduced as a whole. At this time, a slight difference in the thickness of the film tends to make the cooling rate of the solidified shell uneven. As described above, even if the amount of inflow of the molten slag is large or small, vertical cracks are easily generated on the slab surface. When the inflow of molten slag is extremely small, a breakout operation accident may occur.

In particular, when casting at a high speed, the amount of molten slag flowing into the gap between the inner wall of the mold and the solidified shell in the mold tends to decrease. Therefore, it is important to secure and uniform the film thickness. It is.

In general, the thickness of the film can be controlled by adjusting the freezing point of the powder and the viscosity at the time of melting to an appropriate range.

In the powder of the present invention, the solidification point of the powder is lowered by adding an appropriate amount of MgO.
Compared with CaF ₂ and Na ₂ O, it is possible to prevent the immersion nozzle from being melted and to prevent the formation of a powder sintered layer on the molten slag.

If the immersion nozzle has a large melt damage, it breaks at that portion. If casting is continued using such an immersion nozzle whose tip has been lost, the quality of the inside and surface of the slab deteriorates. In extreme cases, it causes a breakout. In addition, when the sintered layer of the powder is formed, the lubricating performance of the molten slag deteriorates, and in extreme cases, breakout is caused.

In the powder of the present invention, MgO is contained in an amount of 1 to 20%.
By containing CaF _{2 in an amount} of 15 to 60% and Na ₂ O in an amount of 0 to 15%, an appropriate freezing point and viscosity are obtained.

[0034]

BEST MODE FOR CARRYING OUT THE INVENTION The powder of the present invention and a continuous casting method using the powder will be specifically described below. % Means% by weight.

(1) Chemical Composition of Powder The basic components of the powder of the present invention are three components of CaO, SiO ₂ and a fluorine compound. The approximate content of each is 20-45% for CaO, 10-35% for SiO ₂ ,
The content of the fluorine compound is 15 to 60% in terms of CaF ₂ . In addition to these, the MgO 1 to 20%, further 0 to 15% of Na ₂ O, are contained respectively. Hereinafter, the appropriate range of the chemical composition of the powder of the present invention will be described.

((CaO) h / SiO ₂ ) The ratio of (CaO) h to SiO ₂ defined in the present invention (CaO) h / SiO ₂
(CaO) h used for calculation of the value of ₂ is (A)
And (B). That is, T. in powder. From the analytical values of Ca, all of them are C
It is not CaO obtained assuming that it is aO. First, as shown in the above equation (B), it is assumed from the analytical value of F that the total amount of F exists as CaF ₂ ,
₂ Determine the content. Next, assuming that Ca excluding Ca existing in the form of CaF ₂ is CaO, (CaO)
Find h. The value of SiO _{2 is} a value obtained based on the analysis value of Si in the powder.

In the powder of the present invention, the (CaO) h /
The value of SiO ₂ is set to 0.9 to 1.9. (CaO) h /
When SiO ₂ is within this range, a sufficient amount of crystals precipitate when the molten slag is cooled and solidified. (Ca
O) If h / SiO ₂ is less than 0.9, precipitation of a sufficient amount of crystals does not occur. Also, (CaO) h / SiO
_{When 2} exceeds 1.9, the solidification point of the powder is too high, so that the powder is not easily melted at the temperature of molten steel when casting steel. In such a case, it is difficult to optimize the thickness of the molten slag on the surface of the molten steel in the mold and the thickness of the film between the inner wall of the mold and the solidified shell of the slab, which hinders the operation of continuous casting. Therefore, (CaO) h / SiO
₂ is set to 0.9 to 1.9.

As will be described later, (CaO) h / Si
When the value of O ₂ is 1.9 and its vicinity, and 0.9 and its vicinity, the precipitation of the crystal is rather difficult, so the preferable range is 1.1 to 1.7.

(CaF ₂ content rate)
The F ₂ content is a value determined by the above-described equation (B). Therefore, the analysis value of F in all fluorine compounds such as sodium fluoride as well as CaF _2
2 Value converted to content. However, in the case of the powder of the present invention, most of the compounded fluorine compound is CaF _2.
It is.

In the powder of the present invention, the proper CaF
The content of ₂ is 15 to 60%. When the content of CaF ₂ is within this range, a sufficient amount of crystals precipitate when the molten slag is cooled and solidified. CaF ₂ is 15%
If it is less than 3%, it is difficult to lower the freezing point, and CaF ₂
Is more than 60%, the immersion nozzle is significantly melted.

(CaO) h / SiO ₂ is 0.9 to 1.9
When the CaF ₂ content is 15 to 60%, as described above, many crystals called caspidin are precipitated during the cooling process from the molten state.

(Na ₂ O content) Na ₂ O is a component to be added as required. Even with the basic composition of the present invention,
The freezing point may be 1300 ° C. or higher. At this time, the incorporation of Na ₂ O is effective in lowering the freezing point. In order to obtain this effect, it is desirable to add 2% or more. On the other hand, if Na ₂ O exceeds 15%, the effect of lowering the solidification point is reduced even if it is added more, and a powder sintered layer is more likely to be formed on the molten slag. Therefore, Na ₂ O is 0 to 15%
And

(MgO Content) In the powder of the present invention, the content of MgO is set to 1 to 20%. When the MgO content is within this range, the freezing point and the viscosity of the powder decrease. If the content of MgO is less than 1%, the effect of lowering the freezing point and viscosity of the powder is insufficient.
The freezing point of the powder rises instead.

(2) When casting at a powder viscosity, a melting temperature and a melting speed of 5 m / min or more, it is difficult to secure the thickness of the film as described above. In order to ensure the thickness of the film, it is necessary to set the solidification point of the powder and the viscosity of the molten slag within appropriate ranges.

In the case of the powder having the chemical composition specified in the present invention, the freezing point of most of the powder is 1100 to 1300 ° C.
About. The range of the solidification point of the powder is a temperature suitable for the above-described high-speed casting. When the freezing point is out of this range, it is desirable to adjust the temperature with MgO, Na ₂ O, or the like, as described above.

The powder melt viscosity suitable for casting at a high speed such as 5 m / min or more is 1.5 at 1300 ° C.
Poise or less is desirable. If the viscosity exceeds 1.5 poise, the amount of molten slag flowing into the gap between the inner wall of the mold and the solidified shell in the mold is insufficient, and vertical cracks occur on the slab surface and continuous casting becomes difficult. There are cases.

(3) Powder Raw Materials The raw materials used for producing the powder of the present invention may be generally used powder raw materials. Quicklime as CaO raw material, limestone, cement, as SiO ₂ raw material, silica sand, Keimodo, as the CaF ₂ raw materials, fluorite, Na
_{As the} 2O raw material, soda ash and sodium carbonate may be used, and as the MgO raw material, MgO clinker, magnesium carbonate and the like may be used.

It is desirable that the raw material has a particle size of 100 μm or less. These raw materials include Al ₂ O ₃ , Fe
Oxides such as ₂ O ₃ and Fe ₃ O ₄ are contained and are inevitably contained in powder. However, even if these impurities are present, they are not particularly hindered.

(4) Continuous casting method using powder of the present invention By using these powders of the present invention, steel, especially steel having a C content of 0.05 to 0.20%, can be applied to the surface of a slab. Continuous casting can be performed stably at high speed without generating vertical cracks.

Particularly, when the C content is 0.08 to 0.20%
Steel undergoes a phase transformation called peritectic reaction during the solidification process from molten steel. Due to this phase transformation, vertical cracks easily occur on the surface of the slab. In addition, when casting at a high speed, the effect of uneven cooling on the solidified shell in the mold is added, and vertical cracks on the slab surface are more likely to occur.

When the C content is 0.05 to 0.20%,
Low alloy steel containing alloying elements such as n, Cr, Ni, Ti, Mo, Nb, V, etc., which enhances the susceptibility of cast slabs to cracking, is characterized by the fact that these alloying elements and C act synergistically to produce longitudinal cracks in slabs. Is easy to occur.

Even with a low carbon steel having a C content of about 0.05%, vertical cracks are likely to occur on the slab surface, especially when casting at a high speed of about 5 m / min or more.

Therefore, when the above-described steel is cast, the powder of the present invention can be used for continuous casting at a casting speed of 5 m / min or more without vertical cracks on the slab surface. Is possible.

The amount of powder used depends on the type of the continuous casting machine, the type of steel, the casting speed, etc., but is not particularly limited. However, at a casting speed of 5 m / min, approximately 0.2 kg / steel-
about t.

The powder of the present invention has a C content of 0.05%.
It goes without saying that it is also suitable for continuous casting of less than steel. Further, using the powder of the present invention, 2.
Even when casting at a low casting speed of about 0 m / min, the thickness of the film is sufficiently ensured, and no vertical cracks occur on the slab surface.

[0056]

[Example] Using a curved continuous casting machine, a thickness of 120 m
m, a slab having a width of 1000 mm was continuously cast.

For medium-carbon steel, low-carbon steel and low-carbon high-Mn steel having the chemical compositions shown in Table 1, low-carbon steel has a casting speed of 6%.
m / min, and the other steels were cast at a casting speed of 5 m / min.

[0058]

[Table 1]

The surface of the obtained slab was examined for longitudinal cracks.
The degree of occurrence of vertical cracks is indicated by evaluations A to E, and the evaluation criteria are as follows. The total value of the length of occurrence of vertical cracks per 1 m of the slab length was less than 5 mm for evaluation A, less than 5 to 10 mm for evaluation B, and 10 to 100 for evaluation C.
mm, the evaluation D is 100 to less than 500 mm, and the evaluation E is 500 mm or more. If the slabs of evaluations C to E are rolled as they are, there is a problem in the surface quality of the product, but the slabs of evaluations A and B do not pose any practical problems if they are rolled as they are.

During the casting, it was observed whether or not a sintered layer was formed in the molten powder in the mold, and the state of the formation was evaluated by the following index. Evaluation ○ indicates that almost no sintered layer was generated during the entire casting period, and the casting operation could be carried out stably.Evaluation △ indicates that the sintered layer was slightly generated during the latter half of the casting period. In particular, when there was no problem in the casting operation, the evaluation x indicates that a large number of sintered layers were generated from the beginning of casting, and the casting operation had to be stopped halfway.

Further, the state of erosion of the immersion nozzle was observed during casting, and the state of erosion was evaluated by the following index. Evaluation ○
No erosion occurs in the immersion nozzle during the entire period during casting,
When the casting operation was stable, the evaluation △ indicates that the immersion nozzle was slightly damaged during casting.Especially, when there was no hindrance to the casting operation, the evaluation 、 indicates that the immersion nozzle was damaged during casting. This shows a case where the casting operation had to be stopped halfway due to an increase in the diameter of the immersion nozzle or breakage of the tip of the immersion nozzle.

Table 2 shows test conditions and test results.

[0063]

[Table 2]

In the following examples, the casting test using the powder of the present invention is also a test of the continuous casting method of the present invention.

Test No. 1 to 9, the steel N shown in Table 1
o. One medium carbon steel was cast. Test No. 1
To 6 are tests using the powder of the present invention,
o. Nos. 7 to 9 are tests using powders of comparative examples in which (CaO) h / SiO ₂ was out of the lower limit specified in the present invention, or in which Na ₂ O or MgO was out of the upper limit specified in the present invention. .

Test No. Test Nos. 1 to 6 1 to
Powders used for No. 3 were CaO, SiO ₂ and CaF ₂
This is a powder in which MgO is blended into the basic component system. Test N
o. The powders used in Nos. 4 to 6 were powders in which Na ₂ O and MgO were blended in the basic component system. These tests N
o. The viscosity at 1300 ° C. of the powder used in each of Examples 1 to 6 was 0.3.
27-0.83 poise, freezing point 1124-11
88 ° C.

The evaluation of the vertical cracks of the slabs cast using the powders of the examples of the present invention was conducted according to Test No. With the exception of 1, 4 and 5, the evaluation A was good. Test No. Nos. 1, 4 and No. The powder used in No. 5 had a low (CaO) h / SiO ₂ of 0.9, and had a relatively small amount of crystal precipitation, and a slightly small cooling effect of the solidified shell. Were evaluated B. However, as described above, there is no practical problem if the rolling is performed as it is. Also,
In some tests, a sintered layer was formed on the molten slag and immersion nozzles were damaged by slag.

Test No. 130 of powder used in 7-9
The viscosity at 0 ° C is 0.20 to 0.71 poise and the freezing point is 1110 to 1325 ° C.

Test No. 1 was cast using low basicity powder. 7 and the basicity are as high as 1.4, but Na ₂
Test No. O using powder whose O deviated from the upper limit specified in the present invention. In each of the slabs of No. 8, remarkable vertical cracks of the vertical crack evaluation D occurred. Further, the test No. In No. 8, significant generation of a sintered layer and melting of the immersion nozzle were observed. Test No.
In No. 9, since the MgO content was too high and the solidification point was higher than 1300 ° C., the casting operation was difficult and the casting was stopped.

Test No. In Nos. 10 to 13, the steel Nos. No. 2 low carbon steel was cast. Test No. Test Nos. 10 and 11 are tests using the powder of the present invention. Tests 12 and 13 are tests using powders of comparative examples in which (CaO) h / SiO ₂ is out of the lower limit specified in the present invention.

Test No. The powders used in Nos. 10 and 11 were based on the basic components CaO, SiO ₂ and CaF ₂ .
It is a powder containing gO or MgO and Na ₂ O.
These powders have (CaO) h / SiO ₂ of 1.4 to
1.5, CaF ₂ content of 45 to 50%, viscosity at 1300 ° C. of 0.40 to 0.42 poise, freezing point of 11
96-1219 ° C.

The evaluation of the vertical cracks in the cast pieces cast using the powders of the examples of the present invention was all “A”. In addition, the formation of a sintered layer and the erosion of the immersion nozzle hardly occurred, and a smooth casting operation was performed.

Test No. The powders used in Nos. 12 and 13 are those containing CaO, SiO ₂ and CaF ₂ as basic components, and containing Na ₂ O and MgO. However, (CaO) h / SiO ₂ is a powder which is 0.8 and deviates from the lower limit specified in the present invention.

The evaluation of the vertical cracks of the slabs cast using the powders having a low basicity of these comparative examples was B. The reason that only a small number of vertical cracks occurred was attributed to the fact that low-carbon steel originally had few vertical cracks on the slab surface. However, the vertical crack evaluation B of the cast slab cast using this powder was worse than the vertical crack evaluation A of the cast slab cast using the powder of the present invention. In addition, a sintered layer was generated in some tests, and erosion of the immersion nozzle occurred,
All were minor.

Test No. In Nos. 14 to 17, the steel No. shown in Table 1 was used. No. 3 low carbon high Mn steel was cast. Test No. 14
And 15 are tests using the powder of the present invention example,
Test No. 16 and 17 are (CaO) h / SiO ₂
Is a test using a powder of a comparative example outside the lower limit specified in the present invention.

Test No. The powders used for Nos. 14 and 15 were based on the basic components CaO, SiO ₂ and CaF ₂ .
It is a powder containing gO or MgO and Na ₂ O.
These powders have (CaO) h / SiO ₂ of 1.4,
CaF ₂ content is 48 to 54%, viscosity at 1300 ° C. is 0.38 to 0.41 poise, and freezing point is 1168 to 1
209 ° C.

The evaluation of the vertical cracks of the cast pieces cast using the powders of the present invention was all good in evaluation A.
Slight erosion of the immersion nozzle was observed, but no sintered layer was formed.

Test No. The powders used in Nos. 16 and 17 were based on the basic components of CaO, SiO ₂ and CaF _2.
It contains a ₂ O and MgO. However,
(CaO) h / SiO ₂ is 0.7, which is outside the lower limit specified in the present invention.

The evaluation of the vertical cracks of the slabs cast using the powders of these comparative examples was poor in evaluation B and evaluation C.
Along with the formation of a sintered layer, erosion of the immersion nozzle was partially caused.

[0080]

By using the mold powder of the present invention, it is possible to perform casting at a high casting speed typified by a continuous thin slab casting method, and to cast slab surfaces such as medium carbon steel. It is possible to cast steel that is likely to cause vertical cracks without generating vertical cracks, and it is possible to perform a stable casting operation.

[Brief description of the drawings]

FIG. 1 is a (CaO) h—SiO ₂ —CaF ₂ ternary composition diagram for explaining the range of the chemical composition of the powder of the present invention.

Claims (2)

[Claims] 1. A ratio (CaO) of (CaO) h (% by weight) represented by the following formula (A) to SiO ₂ content (% by weight), containing CaO, SiO ₂ and a fluorine compound as basic components.
h / SiO ₂ is 0.9 to 1.9, and further contains 15 to 60% by weight of CaF ₂ represented by the following formula (B);
And Na ₂ O 0 to 15 wt%, for continuous casting of steel, characterized in that it contains MgO 1 to 20 wt% mold powder. Here, (CaO) h = T. CaO-F × (56/38) (A) CaF ₂ = F × (78/38) (B) CaO: CaO converted amount of total Ca content in powder (% by weight) F: Total F content in powder (% by weight) 2. A method for continuously casting steel, comprising using the mold powder according to claim 1.