JP3249429B2 - Mold powder for continuous casting of steel - Google Patents

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 which is added to molten steel in a mold during continuous casting of steel (hereinafter, simply referred to as mold powder or powder).
It is about. The powder of the present invention relates to a mold powder suitable for continuous casting of medium carbon steel, in particular, in which a surface crack of a slab is likely to occur.

[0002]

2. Description of the Related Art In continuous casting of steel, mold powder has an effect of preventing oxidation of molten steel on the upper surface of molten steel, keeping the temperature of the molten steel surface and absorbing inclusions floating on the molten metal surface.
In addition, the powder melted on the molten steel surface flows between the mold and the slab to form a slag film layer, acts as a lubricant between the mold and the slab, and further controls the heat removal between the mold and the slab. ing.

[0003] Chemical analysis of a conventional mold powder has the following composition. _{SiO 2: 20~40wt%, CaO:} 20~40wt%, Al 2 O 3: 1~10wt%, Na: 1~30wt%, F: 1~20wt%, Li: 0~20wt%, MgO: 2~ 10 wt%, ZrO2: 0 to ₂₀ wt%, B: 0 to 10 wt%

In the case of producing the above-mentioned mold powder, a so-called pre-melted calcium silicate obtained by melting a raw material such as calcined lime or silica stone in an electric furnace in addition to cement and calcium silicate and converting the raw material into slag is used as a base material. Is used, a low melting point compound such as an alkali metal carbonate or a fluorine compound is compounded as a physical property adjusting agent, and a carbon material powder is further compounded as a melting rate adjusting agent.

[0005] By the way, when the carbon concentration of steel is 0.08-0.
In continuous casting of 16 wt% medium carbon steel (hereinafter referred to as medium carbon steel), the solidification shrinkage when the steel changes from a liquid phase to a solid phase is large, cracks are easily generated on the slab surface, and high speed Casting has been difficult.

[0006] Therefore, Materials and Processes, Vol. 4 and No. 4 (1991, p. 1247) describe a method for reducing the heat flux between the mold and the slab surface as a countermeasure against cracking of medium carbon steel. Wt% of CaO converted from the weight percentage of Ca
And wt% of SiO ₂ converted from the weight percentage of total Si
Basicity (hereinafter simply referred to as basicity (CaOwt
% / SiO ₂ wt%). ) Is set to 1.3 or more, the contact thermal resistance is increased by volume shrinkage due to crystallization during cooling, and the powder film is made opaque by adding ZrO ₂ at 3.0 wt% or more, thereby reducing radiant heat transfer. And that a good cast slab can be produced (Prior Art 1).

Further, Shinagawa Technical Report No. 32 (1989, 1
On page 47), in order to prevent longitudinal cracking of the slab, which is a problem when casting the medium carbon steel, the heat removal rate in the mold and the characteristics of the powder were investigated, and the crystallization of the molten powder during the solidification process was performed. It is reported that by raising the temperature, it has been found that the heat removal in the mold can be reduced and the occurrence of vertical cracks can be reduced. Furthermore, it is described that the higher the basicity powder, the higher the crystallization temperature tends to be (prior art 2).

[0008] In the above, crystallization is a phenomenon in which a molten powder flows between a mold and a slab surface, and a part of a molten material precipitates as crystals when solidifying. It also occurs when the premelt flux, which is the base material of the casting powder, solidifies.

In recent years, as the speed of continuous casting has been increased, the pre-melt type calcium silicate flux described above has become the mainstream as a base material of mold powder in order to achieve quick and uniform melting of powder.

[0010] This pre-melt calcium silicate flux is usually blended in powder in an amount of 50% by weight or more.
Basicity of mold powder itself (CaOwt% / SiO
It is common to use a premelt calcium silicate having a basicity of about ₂ wt%). For example, as a powder raw material for medium carbon steel, a high basicity premelt flux having a basicity of 1.3 or more is used.

As a mold powder developed under such a concept, there is Japanese Patent Publication No. Hei 2-27063.
The claims of this patent claim that “the content of CaO and SiO ₂ is 70 wt% or more, and the content of CaO wt% / SiO ₂
A synthetic calcium silicate having a wt% (basicity) of 1.2 to 2.3, wherein F is 1 to 10 wt% of inevitable impurities.
%, Al ₂ O ₃ is 8 wt% or less, and Fe ₂ O ₃ is 1 w
A mold additive for continuous casting, comprising at least 50 wt% of synthetic calcium silicate having a content of t% or less as a base material. ".

However, in this publication, CaO / SiO ₂ is 1.2 to 2.3, preferably 1.2 to 1.9, because synthetic calcium silicate, which is a raw material of mold powder, is used as an amorphous raw material. Is desirable. But,
Regarding the relationship between the mold powder prepared from the synthetic calcium silicate as a raw material and the surface cracks of the slab, it does not consider or mention how the component composition should be.

[0013]

As described above, the prior art focuses on reducing the heat flow velocity in the mold, that is, slowly cooling the inside of the mold, and is effective in suppressing vertical cracks in the slab. Have been. However, in the above prior art, the macroscopically captured inside of the mold is only slow cooling, and the surface cracks of the slab still occur due to uneven shrinkage due to a local cooling rate difference of the slab. However, since the surface of the slab needs to be ground due to quality problems, it has not been possible to cope with a reduction in yield, an increase in labor costs, and an improvement in productivity due to a recent increase in casting speed.

[0014]

Means for Solving the Problems In view of the above circumstances, the present inventors have conducted various studies and found that it is important to uniformly cool a slab in a mold. As a result of further studies, the standard deviation of the slag film thickness is important for uniform cooling. To control the standard deviation of the film thickness, the CaO wt% of the mold powder / SiO ₂
The inventors have found that the wt% and the content of the premelt flux in the mold powder are important factors, and have accomplished the present invention.

In the first invention, at least the basicity (Ca
O wt% / SiO ₂ wt%) is in the range of 1.1 to 2.4, and fluorine is contained in _{2 to 12} wt%, and Al ₂ O ₃
, A mold powder for continuous casting, wherein the content of the premelt flux of the powder satisfies the following formula (1). . (Wt% of premelt flux) ≧ 25 × (CaOwt% / SiO ₂ wt%) +35 (1)

According to a second aspect of the present invention, the basicity (CaO wt% / SiO ₂ wt%) of the powder for continuous casting is from 1.3 to 1.3.
The present invention provides the continuous casting mold powder according to the first invention, wherein the content is 1.6 and the content of the premelt flux is 85% by weight or less.

A third invention provides the mold powder for continuous casting according to the first or second invention, further comprising the following compound or simple substance. (A) cryolite, NaF, fluorite, soda ash, Li content of any one or more ₂ O is 3~25wt%, 1~8wt% content of (b) carbon material.

According to a fourth aspect of the present invention, at least CaO: 35
６０60 wt%, SiO ₂ : 25-40 wt%, Al ₂ O
₃ : 1 to 7 wt%, MgO <1.5 wt%, Na ₂ O:
_{1~4wt%, Li 2 O: 1~5wt} %, F: 2~12
Provided is a continuous casting mold powder containing 0.1 wt%, wherein the content of the premelt flux of the powder satisfies the following formula (1). (Wt% of premelt flux) ≧ 25 × (CaOwt% / SiO ₂ wt%) +35 (1)

According to a fifth aspect of the present invention, there is provided a mold powder for continuous casting, which further comprises the following compound or simple substance. (A) cryolite, NaF, fluorite, soda ash, Li content of any one or more ₂ O is 3~25wt%, 1~8wt% content of (b) carbon material.

The sixth invention provides a powder for continuous casting according to the first to fifth inventions, wherein the premelt flux is amorphous.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, two types of mold powders for continuous casting are provided. The first mold powder has at least a basicity (CaOwt% / Si
O ₂ wt%) is in the range of 1.1 to 2.4, and _{2 to 12} wt% of fluorine and 1 to 7 wt% of Al ₂ O ₃
It contains. Note that the basicity is more preferably in the range of 1.3 to 1.6.

The second mold powder contains at least the following components. _{CaO: 35~60wt%, SiO 2:} 25~40wt
%, Al ₂ O ₃ : 1 to 7 wt%, MgO <1.5 wt%
%, Na ₂ O: 1 to 10 wt%, Li ₂ O: 1 to 5 wt%
%, F: 2 to 8 wt%.

The second mold powder contains Na ₂ O: 1 to 10 wt% and Li _{2 based on} the first mold powder.
O: contains 1 to 5 wt%, has a lower melting point, and is suitable for higher speed continuous casting. However, any of these mold powders can be made amorphous by quenching.

The component compositions of the first and second mold powders are basic component compositions, and are suitable for adjusting melting characteristics such as cryolite, NaF, fluorite, soda ash, and Li ₂ O. And a material for adjusting the melting rate, and a carbon material, for example, carbon black, scaly graphite and the like.

Further, the feature of the mold powder of the present invention is that the powder is melted in advance and cooled (hereinafter referred to as pre-melt flux) in the range of the following formula (1). (Wt% of premelt flux) ≧ 25 × (CaOwt% / SiO ₂ wt%) + 35 (1) Here, the wt% of CaO is the wt% of CaO calculated from the weight percentage of the total Ca. , SiO ₂ wt% is all S
It is wt% of SiO ₂ converted from the weight percentage of i.

The above equation (1) was obtained as follows. That is, various mold powders in which the composition of the pre-melt flux was changed were prepared, casting experiments were performed, the composition ratio of the pre-melt flux and the thickness of the slag film adhering to the slab were measured, and the standard deviation was measured. It was measured.

The pre-melt flux is obtained by mixing a raw material obtained by blending silica, coal, fluorite, glass powder, alkali metal carbonate, alkaline earth carbonate and the like in a composition within the above-mentioned range in a three-phase arc furnace. The mixture was melted, quenched by a jet water stream, turned into a scale, dried, and then pulverized to 100 mesh or less to prepare.

The premelt flux contains cryolite, N
The content of at least one of aF, fluorite, soda ash, and Li ₂ O is 3 to 25 wt%, and the content of at least one of carbon black and scale graphite as a carbon material is 1%. To 8 wt% to prepare mold powders (hereinafter, referred to as invention examples) 1 to 19. For comparison, mold powders (hereinafter referred to as comparative examples) 1 to 1 having a premelt flux content outside the scope of the present invention were used.
12 was created.

FIG. 4 shows Invention Examples 1 to 19, and FIG.
No. 12, chemical component value (wt%), viscosity (Pa · s), premelt ratio (wt%), standard deviation (mm) of film thickness of slag film when using these mold powders, cast slab And the required pre-melt content (wt%) are shown respectively.

Using the powders shown in FIGS. 4 and 5, a casting experiment to be described later was conducted, and the effect of the standard deviation of the film thickness of the slag film on the index of occurrence of vertical cracks in the obtained slab is shown in FIG. It is apparent from FIG. 1 that the occurrence of vertical cracks in the slab can be suppressed to a low level in a region where the standard deviation of the film thickness of the slag film is 0.4 or less. In other words, the smaller the variation in the thickness of the slag film, the more the slag film having a uniform thickness is generated, which means that the occurrence of vertical cracks on the slab surface is reduced.

FIG. 2 shows the effect of the premelt ratio on the standard deviation of the thickness of the slag film in FIGS. From FIG. 2, it is clear that a premelt content equal to or greater than the premelt content corresponding to a region where the film thickness standard deviation of the slag film is 0.4 or less is desirable from the viewpoint of suppressing the occurrence of vertical cracks in the slab.

FIG. 3 shows the relationship between the premelt content and the basicity of the mold powder in which the standard deviation of the thickness of the slag film in FIG. 2 is 0.4 or less. From FIG. 3, the region of the premelt content that suppresses the occurrence of vertical cracks is:
From the relationship with the basicity, it was determined as in the following equation. (Wt% of premelt flux) ≧ 25 × (CaOwt% / SiO ₂ wt%) +35 (1)

Therefore, it has been clarified that the use of a mold powder having a premelt content satisfying the above formula (1) can suppress the occurrence of vertical cracks on the surface of a continuously cast slab. As shown in FIG. 6, in the range of the above formula (1), the range in which the basicity of the powder is 1.3 to 1.6 and the premelt flux is 85 wt% or less is a slab. It is suitable because there are few vertical cracks on the surface.

With respect to the above pre-melt flux, at least one of cryolite, NaF, fluorite, soda ash, and Li ₂ O is contained in an amount of 3 to 25 as a melting property modifier.
Addition of 1% to 8% by weight as a content of at least one of carbon black and scaly graphite as a carbon material as a melting rate adjuster can provide a desirable mold powder.

After blending as described above, the mold powder of the present invention is obtained by uniformly mixing with a V-type blender or the like. In addition, after mixing in this manner, water can be kneaded and obtained as columnar granules by an extrusion granulator, or the mixture can be slurried and then spherical powder can be obtained by spray granulation.

[0036]

EXAMPLE In the examples, mold powders as shown in FIGS. 4 and 5 were prepared by using the above-mentioned two types of mold powders for continuous casting. Further, in the second mold powder raw material, MgO is used as a continuous cast powder component for medium carbon steel in order to maintain proper viscosity.
It was less than 5 wt%.

The invention example shown in FIG. 4 will be described in comparison with a comparative example shown in FIG. In the invention example of FIG. 4, invention examples 13 to 19 are the first powder, and invention examples 1 to 12
Is a second powder, and was prepared so as to have the respective chemical component values.

In FIG. 5, Comparative Examples 1, 2, 9,
10, 11 and 12 are the first powder compositions, but the premelt flux content does not satisfy the formula (1), and Comparative Examples 3 to 8 are the second powder compositions but the premelt flux content Does not satisfy the expression (1).

Casting experiments were conducted using the mold powders of these invention examples and comparative examples. The casting conditions were as follows: carbon concentration in steel = 0.09 to 0.16 wt%, casting speed =
1.4 to 1.8 m / min, slab thickness = 220 mm,
The slab width was 2000 mm. The number of vertical cracks having a length of 2 mm or more in the cross section of the slab obtained under the casting conditions described above was counted and indexed over the entire width of the slab and over a length of 1 m. This is the vertical crack occurrence index in FIGS.

It is clear from FIGS. 4 and 5 that the invention cracking index is much smaller in the invention example than in the comparative example. Also, from FIG. 4, in the same invention example, invention examples 1 to 12 which are examples using the second mold powder are better than invention examples 13 to 19 which are examples using the first mold powder. It is clear that the vertical crack occurrence index is smaller.

As described above, the desired content of the premelt flux of the present invention was expressed by a mathematical expression using the basicity of the mold powder as a parameter, and it was clarified that the occurrence of vertical cracks in the slab could be suppressed.

[0042]

As described above, according to the present invention, the desired content of the premelt flux, which is the main raw material of the mold powder for high-speed casting of medium carbon steel, is converted into a mathematical expression using the basicity of the mold powder as a parameter. This makes it possible to suppress the occurrence of vertical cracks in the slab. As a result, it has become possible to continuously cast high-quality steel having no cracking defects at a high speed even in a medium carbon steel having a high crack sensitivity.

[Brief description of the drawings]

FIG. 1 is a view showing the influence of the standard deviation of the thickness of a slag film on the index of occurrence of vertical cracks in a slab.

FIG. 2 is a diagram showing the influence of the premelt flux content on the standard deviation of the film thickness of the slag film in FIGS.

3 is a diagram showing the relationship between the premelt flux content and the basicity of a mold powder for making the standard deviation of the film thickness of the slag film in FIG. 2 0.4 or less.

FIG. 4 shows the chemical component values, viscosity, and premelt flux content of the mold powders of Invention Examples 1 to 19, the standard deviation of the film thickness of the slag film when these mold powders are used, and the occurrence of longitudinal cracks in slabs. It is a figure which shows an index and the minimum premelt flux content.

FIG. 5 shows the chemical component values, viscosity, and premelt flux content of the mold powders of Comparative Examples 1 to 12, the standard deviation of the film thickness of the slag film when these mold powders are used, and the occurrence of longitudinal cracks in slabs. It is a figure which shows an index and the minimum premelt flux content.

FIG. 6 is a graph showing the effect of the premelt flux content on the vertical crack occurrence index in a slab.

Continued on the front page (72) Inventor Hiroyuki Kondo 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Akira Miyamoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Keiji Watanabe 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Hidenori Sakai 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Kobunko Mining Co., Ltd. (72) Invention Hiroshi Izumi 2-3-2 Marunouchi, Chiyoda-ku, Tokyo (72) Inside the Steel Pipe Mining Co., Ltd. 13 (56) References JP-A-7-323354 (JP, A) JP-A-5-38560 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/108 B22D 11 / 07 C21C 7/076

Claims (2)

(57) [Claims] 1. At least CaO: 35 to 60 wt.
%, SiO2: 25~40wt%, Al 2 O 3: 1~7w
t%, MgO <1.5wt%, Na 2 O: 1~10wt
%, Li ₂ O: 1 to 5 wt%, and F: _{2 to} 12 wt%, characterized in that the content of the premelt flux of the powder satisfies the following formula (1). Mold powder for continuous casting. (Wt% of premelt flux) ≧ 25 × (CaOwt% / SiO ₂ wt%) + 35 (1) 2. The mold powder for continuous casting according to claim 1, further comprising the following compound or a simple substance. (A) cryolite, NaF, fluorite, soda ash, Li content of any one or more ₂ O is 3~25wt%, 1~8wt% content of (b) carbon material.