JPH06170506A - Molding powder for continuous casting - Google Patents

Abstract

PURPOSE:To provide the molding power for continuous casting which lessens the migration of carbon from the powder to molten steel of even the extra-low- carbon steel and is adequately controllable in melting even if the amt. of the aggregate carbon in the powder is lowered. CONSTITUTION:The carbonate concn. in powder raw material is specified to >=4mass% and <15mass% in terms of CO2 concn. and the (CaO+CaF2X0.718)/ SiO2 mass concn. ratio thereof to >=0.8 to <1.2. The powder having <0.5 (CaO+ CaF2X0.718)/SiO2. mass concn. ratio is specified to <15mass% of the constituting raw material powder and the concn. of the added carbon to <1.0mass%.

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, and more particularly to a mold powder for continuous casting which reduces the amount of carbon transferred to a slab.

[0002]

2. Description of the Related Art In continuous casting of steel, powder or granules are formed on the surface of the molten steel in the mold for the purpose of preventing oxidation of the molten steel in the mold and maintaining good lubricity between the mold and the slab. A so-called mold powder (hereinafter sometimes simply referred to as powder) is added. The added powder melts little by little, and the melt of this powder (powder slag) flows into between the mold and the slab, and sufficient lubrication between the mold and the slab is performed to ensure stable casting. it can.

[0003] for the conventional continuous casting powder is, CaO -SiO ₂ -
Using Al ₂ O ₃ as a base material, add Na, Li, etc. in the form of carbonate or fluoride to generate alkali metal oxides such as Na ₂ O, Li ₂ O, or add fluorite. Therefore, the freezing point and viscosity are adjusted.

Carbon powder is generally added to powder for the purpose of controlling the melting rate. Carbon powder prevents contact between base material particles and molten steel, delays the progress of melting, and absorbs radiant heat from the surface of molten steel, which eventually leads to the occurrence of flaws on the surface of the slab. Although it has an excellent effect of preventing and making a sound slab, it has a problem that carbon migrates into molten steel during casting. If the carbon content of the slab increases, in the case of an ultra-low carbon steel aiming for a C concentration of 30 ppm or less, even a very small amount of carbon migration will be out of specification, so actual operation Must be avoided from above.

[0005]

As described above, conventionally, when carbon powder is added to powder, it is difficult to avoid transition to molten steel during casting. However, if carbon is not added or the content is reduced to 1% by mass or less, the effect of delaying the progress of the original melting of carbon is not exerted, and the powder added to the mold is completely melted and the temperature of the molten steel surface can be kept warm. However, there are problems that the surface defects are increased and that the molten steel surface is solidified and casting cannot be performed.

Thus, the object of the present invention is to reduce the transfer of carbon from the powder to the steel during casting, even when continuously casting, for example, ultra low carbon steel having a carbon concentration of 30 ppm or less, It is to develop a mold powder for continuous casting which can reduce the amount of aggregate carbon in the powder and can appropriately control melting.

[0007]

[Means for Solving the Problems] As described above, when the carbon concentration in the powder is reduced, the amount transferred to the molten steel is reduced accordingly. Currently, the carbon concentration of industrially used powder is usually 2.0 to 4.0% by mass.
The carbon concentration is 1.0 ~ for ultra-low carbon steel.
About 1.5 mass% of powder is used on a trial basis.

The inventors of the present invention have studied under such circumstances.
Continuing development, adding a large amount of carbonate to the powder raw material,
Limiting the raw material concentration of high SiO ₂ concentration, Si as powder composition
By specifying the O ₂ concentration, the aggregate carbon concentration is 1.
The inventors have completed the present invention knowing that the melting characteristics can be normally secured even if the content is reduced to less than 0 mass%.

The present invention is a powder for continuous casting of steel, wherein the carbonate concentration in the powder raw material is 4 mass% or more and less than 15 mass% in terms of CO ₂ concentration, The (CaO + CaF ₂ × 0.718) / SiO ₂ mass concentration ratio of the entire powder is 0.8 or more.
It is less than 1.2 and (CaO + CaF ₂ × 0.71
8) For continuous casting to reduce the amount of carbon transfer to the slab, characterized by less than 15% by mass of components with a mass ratio of less than 0.5 / SiO ₂ and less than 1.0% by mass of added carbon. It is a mold powder. Incidentally, the conventional powder for ultra-low carbon steel had the following composition.

CO ₂ equivalent carbonate concentration: 2.0 mass% (CaO + CaF ₂ × 0.718) / SiO ₂ : 0.7 to 1.0 (CaO + CaF ₂ × 0.718) / SiO ₂ ratio of components with a concentration ratio of less than 0.5
: 20% by mass Carbon amount: 1 to 1.5% by mass

[0011]

Next, the reason for limiting the composition of the mold powder in the present invention as described above will be explained. When carbonate is added to powder, it reacts with carbon and acidic oxides in powder when added on molten steel to generate CO ₂ or CO gas. Since these gases pass through between the base material and rise, they have a function of preventing sintering of the base materials and, as a result, preventing surface defects of the slab.

If the concentration of such a carbonate is less than 4% by mass in terms of CO ₂ concentration, the effect of preventing sintering will not be obtained, while if it is 15% by mass or more, it will be adversely affected by the generated gas. In addition, there is a problem that the melting becomes unstable, the surface defects of the slab become noticeable, the generation of dust becomes severe, and handling problems occur.

The (CaO + CaF ₂ × 0.718) / SiO ₂ mass concentration ratio is
It is a parameter showing the basic characteristics of the chemical composition of powder, but if the average value of the entire composition is less than 0.8, the sintering of the base material is promoted and the heat removal amount between the mold and the slab increases. It is not realistic that the slab will crack if it is too much. Also,
Even if the mass concentration ratio is 1.2 or more, the sintering of the base material is promoted, so that it becomes difficult to control the melting and the surface flaw of the slab is unavoidable.

Further, as the raw material powder, for example, silica stone powder,
If a large amount of SiO ₂ is contained, that is, if the above mass concentration ratio is less than 0.5 and a large amount of 15 mass% or more is used, the sintering of the base material in the low temperature region is promoted, which makes melting control difficult, The occurrence of surface defects is unavoidable.

With the above component composition, even if the amount of aggregate carbon is less than 1.0% by mass, normal melting control can be performed, and the transfer of carbon from powder to molten steel can be reduced. Becomes The powder may be in the form of fine powder or granules and is not particularly limited.

Here, a typical composition example of the mold powder for continuous casting according to the present invention is shown. Blended material mass% wollastonite 25-60% fluorite 2-10% quartzite 0-10% carbon 0.1 to 1.0% Composition Examples _{CaO 25 ~ 35% CaF 2 2} ~ 10% SiO 2 25 ~ 35% Al 2 O 3 0 to 10% Next, the function and effect of the present invention will be described more specifically with reference to Examples.

[0017]

[Example] Various test powder No. 1 having the composition shown in Table 1
No. 15 to No. 15 were actually used for the casting test. The basic compounding materials of powder used in this example are as described above. CO
Increase or decrease of ₂ terms carbonate concentration is carried out by adjusting the carbonate, modify the total composition _{(CaO + CaF 2 × 0.718)} / SiO 2 mass ratio of concentration by adjusting the wollastonite or Portland cement amount, a higher SiO ₂ The concentration of raw materials, such as silica stone powder, was less than 15%.

The radius of curvature was used in the continuous casting of this example.
It is a single point straightening continuous casting machine with a length of 10 m and a slab size of 1800 mm.
It was × 150 mm. The molten steel composition is shown in Table 2, which was an extremely low carbon steel. The casting speed was 1.5 m / min.

First, when the powders Nos. 1 and 2 were used, the casting was successful for the first 40 minutes, but after that,
The molten layer thickness of the powder suddenly increased, and finally melted, so that the temperature of the molten steel could not be kept and the surface of the molten steel solidified, so that casting was stopped. The defects on the surface of the slab were 5 times or more than usual.

Next, No. 3, 4, 7, 8, which is an example of the present invention,
10, 11, 13 and 14 powders were used. These had a small amount of transferred carbon, and had no flaws in the slab. Furthermore, the thickness of the melted layer of the powder was 12 mm to 14 mm, which was sufficient.

As for the No. 12 powder, like the No. 1 powder, the melting control could not be performed during the casting, and the casting was stopped. The No. 15 powder was operating satisfactorily, but the amount of carbon transferred to the slab was large, as described later, and the prescribed components could not be satisfied.

Regarding No. 6 and 9 powders, like the No. 1 powder, melting control could not be performed during casting and the casting was stopped. For No. 5 powder, the concentration of carbonate is high,
Dust generation was severe and caused operational problems. each,
The amount of carbon transferred to the slab and the surface flaw occurrence rate in the test are summarized in Figures 1 and 2.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

EFFECTS OF THE INVENTION According to the present invention, the amount of carbon transferred from the powder to the slab during continuous casting of an ultra-low carbon steel of C: 30 ppm or less can be reduced, and other quality and operation restrictions are not caused.

[Brief description of drawings]

FIG. 1 is a graph of carbon migration showing the results of an example of the present invention.

FIG. 2 is a graph of surface flaw occurrence rate showing the results of the examples of the present invention.

Claims (1)

[Claims] 1. A mold powder for continuous casting of steel, wherein the carbonate concentration in the powder raw material is 4% by mass or more and less than 15% by mass in terms of CO ₂ concentration, and CaO + CaF ₂ × 0.718) / SiO ₂ mass concentration ratio of 0.8 or more 1.2
Is less than (CaO + CaF ₂ × 0.718) / S of the constituent raw material powders
A mold powder for continuous casting that reduces the amount of carbon transferred to a slab, characterized in that the component having an iO ₂ mass concentration ratio of less than 0.5 is less than 15 mass% and the added carbon concentration is less than 1.0 mass%.