JPH0741384B2 - 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 of steel, which is excellent in productivity and can contribute to improvement of operation stability and stability of steel quality. Regarding

[0002]

2. Description of the Related Art In recent years, the progress of continuous casting of steel has been remarkable, and high-temperature slabs have become unmaintained (HCR, HDR), high-speed casting, diversified mold cross-sectional shapes, and multi-casting. Improving and stabilizing the operation rate of the continuous casting machine by reducing defects and making the slab defect-free is the biggest issue in the CC process. Under these circumstances, the mold powder plays a large role.

The quality of the mold powder is designed so as to meet various steel components and various casting conditions. However, there are variations in quality especially when the quality design is inappropriate or the quality design is appropriate. In some cases, high-speed casting may cause the continuous casting machine to stop due to problems such as breakout,
Further, it often causes a slab defect. With the improvement of the quality design level of mold powder, the stability of quality becomes important depending on the type of steel.

In continuous casting of steel, mold powder is added in the mold. The mold powder added on the molten steel surface injected into the mold through the immersion nozzle forms a layered structure of molten slag layer and unmelted raw powder layer from the molten steel surface by receiving heat from the molten steel and plays various roles. It is consumed by flowing between the solidified shell of steel and the mold. Lubrication between the mold and the solidified shell, dissolution of inclusions floating in molten steel, absorption, prevention of molten steel oxidation, heat retention, and function as a heat medium between the solidified shell and the mold are important roles. That is, when a certain amount of the molten slag layer is flowed while ensuring a certain thickness, the roles of ~ can be fulfilled and a sound cast piece can be obtained.

Particularly, the thickness of the molten slag layer is important, and if it is too thin, the unmelted raw powder layer comes close to the molten steel surface, and a slight fluctuation in the molten metal surface causes a state of easy contact. Defects tend to be a problem. If it is too thick, there is a problem such as generation of slag bear. Further, if the powder slag is too thin or too thick, the powder slag may be unevenly inflowed or insufficiently inflowed, which may cause cracking or breakout.
Therefore, it is extremely important to set the slagging speed that determines the thickness of the molten slag layer, and it is necessary to stabilize the slagging speed.

Typical mold powders include Portland cement, yellow phosphorus slag, wollastonite, blast furnace slag, dicalcium silicate (2CaO.Si).
O ₂ ), synthetic calcium silicate, etc. as the main raw material base material, and if necessary, silica raw material for adjusting powder characteristics such as basicity and bulk specific gravity, and further fluorite, cryolite, borax, fluorinated A mixed type in which a fluoride raw material such as magnesium, a carbonate such as sodium carbonate and barium carbonate, a flux raw material as a melting property adjusting material such as viscosity, a carbonaceous raw material as a melt slag speed adjusting material, and the like, There are a pre-melt type in which all of the components except the carbonaceous raw material are previously melt-water granulated, and a semi-pre-melt type in which a part of them are previously melt-water granulated. In terms of shape, there are a powder type in which powder raw materials are mixed and a granule type in which granulation is performed by various methods.

The molten slag formation rate, which determines the thickness of the molten powder slag layer, is basically made of carbonaceous raw materials, though it may be made of some metals or other raw materials, and the particle size and use of the carbonaceous raw materials are used. The reality is that the amount is adjusted.

[0008]

As described above, the control of the molten slag formation rate that determines the thickness of the molten powder slag layer is performed by the carbonaceous raw material, and is also adjusted by the particle size and the amount of use of the carbonaceous raw material. It is the actual situation, but quite a fine powder such as carbon black is also used,
Since various raw materials used, including glassy raw materials, have various surface conditions (unevenness, static electricity, etc.), it is easy to form aggregates,
Even if the blending amount is accurate, it is not easy to make the degree of dispersion of the carbonaceous raw material constant during mixing. Therefore, the degree of dispersion between the production lots, and hence the molten slag formation speed, is likely to differ, and if the difference is large, lockout re-manufacturing is required, and the productivity is unavoidably reduced. Moreover, it was difficult to control the degree of dispersion within a narrow range. Therefore, even if the quality design itself of the mold powder is appropriate, this subtle difference in the degree of dispersion leads to variations in the molten slag formation rate in the actual machine, which in turn results in variations in the thickness of the molten powder slag layer, and ultimately in the slab quality. Deterioration, and in some cases operating problems such as breakout. Although these problems were not readily apparent in conventional operations with slow casting speeds, they became apparent due to severe operating conditions and a marked improvement in quality design technology for mold powder.

However, it is difficult to stabilize the quality of the mold powder such as the stabilization of the degree of dispersion, and the current situation is that sufficient measures have not been taken.

[0010]

As a result of various studies to solve the above problems, the present inventors have clarified the mechanism of dispersion and found that all of the above-mentioned drawbacks can be overcome.

That is, the mold powder for continuous casting of steel according to the present invention is a mixture of various raw materials, a burning type in which raw materials excluding carbon are pre-fired, or a molten type of powder or granules in which pre-melting is performed. Specific surface area of mold powder for continuous casting is 9
It is characterized by containing 0.2 to 5% by weight of carbon black having a pH of 5 BET-m ² / g or more and a pH of less than 8.

[0012]

[Function] As a result of various studies and investigations by the present inventors,
The following findings were obtained. Since the carbonaceous raw material exists around various raw materials, the melting and slagging rate can prevent contact between the raw materials and sintering during heat reception,
After the raw materials are melted, they enter the molten slag layer as the surrounding carbonaceous raw material is oxidized. Therefore, the rate of molten slag can be controlled by adjusting the type, particle size, and addition amount of the carbonaceous raw material.

A carbonaceous raw material having a small particle size and a large specific surface area is effective for finely controlling the rate of molten slag formation. However, it is more difficult to stabilize the dispersion degree as the carbonaceous raw material has a smaller particle size and a larger specific surface area. For example, conventional carbon black, which has been often used conventionally, is more effective in adjusting the melting and slagging rate than other carbonaceous raw materials because it has “small particle size and large specific surface area”. However, it is extremely difficult to keep the degree of dispersion constant because of the characteristic of easy aggregation.

As a result of conducting studies on various carbon blacks for improving the above-mentioned disadvantages of the carbon blacks,
Specific surface area 95 with acid functional groups
It has been found that carbon black having a BET-m ² / g or more and a pH of less than 8 is effective. That is, dispersibility is improved by the action of the acidic functional group regardless of the type of other raw materials,
Moreover, it was found that the degree of dispersion was stable.

A carbon black having a specific surface area of 95 BET-m ² / g or more and a pH of less than 8 which has been subjected to an oxidation treatment and is provided with an acidic functional group has excellent dispersibility, so that a small amount of carbon black is equivalent to an ordinary carbon black. The slagging rate can be obtained.

As a result of investigating the influence of the carbon raw material on the carburizing rate in the molten steel, carbon black has the slowest carburizing rate, the larger the specific surface area, and the smaller the particle size, the slower. It was also found that the carburizing rate becomes slower as the dispersion of carbon black in the mold powder is sufficient and the amount of aggregates is smaller.

The mold powder for continuous casting of steel according to the present invention is based on the above findings. That is, as a carbonaceous raw material which is a melting rate adjusting agent, a specific surface area of 95 BET-
By containing 0.2 to 5% by weight of carbon black having a pH of m ² / g or more and a pH of less than 8, the stability of the degree of dispersion is enhanced, and thereby the stability of the melt slagging rate is enhanced. It is possible to eliminate variations in the slag layer thickness and ultimately prevent operational problems such as deterioration of slab quality and breakout. Further, in manufacturing, the difference in the degree of dispersion between lots can be eliminated, the frequency of lot-out can be drastically reduced, and the productivity can be remarkably improved.

Table 1 shows that only the carbon black, which is a part of the carbonaceous raw material of the mold powder of the present invention of the premelt type, was changed and was produced 10 times under the same production conditions. The result of having compared the variation of speed is shown. As a carbonaceous raw material that affects the rate of molten slag, coke is 2% by weight and carbon black is 1.5 to 2.
It uses 5% by weight.

[0019]

[Table 1]

As is clear from Table 1, in order to obtain the same level of melting and smelting rate as in the case of ordinary carbon black A and B and C, it is necessary to add more carbon black. The variation between manufacturing lots becomes large. When this variation is used in an actual machine, it leads to variation in the molten slag formation rate, variation in the thickness of the molten powder slag layer, and eventually deterioration of the slab quality, causing operational problems such as breakout. Strict product inspection was necessary, and remanufacturing was required by lot out. With carbon black A, as a practical matter, it was impossible to control the molten slag formation rate within a narrow range.

Further, since carbon black having a specific surface area of 95 BET-m ² / g or more and a pH of less than 8 is excellent in dispersibility, it is possible to adjust the melting and slagging rate with a small amount, so that the amount of carbon in the mold powder can be reduced. Therefore, it is particularly important for ultra-low carbon steel and stainless steel in which carburization due to carbon in powder is a problem. That is, since the amount of carbon is small, the probability that molten steel comes into contact with the carbon in the powder is reduced, so that carburization can be reduced. Also,
If the specific surface area is large and the particle size is small, it is difficult to wet with the molten powder slag or molten steel, and if the carbon black has a low pH, it hardly exists in the molten powder because it hardly exists in the aggregated form, and even if it remains Since the carburizing speed in molten steel is slow, it is extremely effective in reducing carburizing.

Carbon black having a specific surface area of less than 95 BET-m ² / g or a pH of 8 or more which is not subjected to oxidation treatment is not preferable because the dispersibility is poor and the above effects cannot be obtained.

The amount of carbon black having a specific surface area of 95 BET-m ² / g or more and a pH of less than 8 is 0.2-5.
Weight percent is preferred. If it is less than 0.2% by weight, it is difficult to control the melt slag formation rate, which is not preferable. On the other hand, if it exceeds 5% by weight, the rate of melt slag formation becomes too slow because of good dispersibility, which is not preferable.

The mold powder for continuous casting of steel according to the present invention has a specific surface area of 95 BE as a carbonaceous raw material.
Other carbon black, graphite, anthracite, charcoal or the like can be used in combination if carbon black having a T-m ² / g or more and a pH of less than 8 is used in the range of 0.2 to 5% by weight. That is, in steel types that do not have the problem of carburization and in which the heat retention of the mold powder is important, the heat of oxidation combustion of the carbonaceous raw material is effective in improving the heat retention, and these carbonaceous raw materials can be used. The specific surface area is 9 for adjusting and stabilizing the melting and slag formation rate.
This is because carbon black having a pH of 5 BET-m ² / g or more and a pH of less than 8 can be used. However, it is desirable that the total amount of carbonaceous raw material used is less than 10% by weight.

The function of the mold powder for continuous casting of steel according to the present invention will be described in more detail for each application. In extremely low carbon steel, low carbon steel, and stainless steel, a large amount of deoxidized product Al ₂ O ₃ floats in the mold,
Since it is absorbed in the powder slag, it has the characteristic that the composition variation of the powder slag becomes larger than that of other steel types.
Since the increase of Al ₂ O ₃ concentration in this powder slag leads to the increase of viscosity, it is easy to cause poor lubricity due to the decrease of the flow-in amount, and even if the Al ₂ O ₃ concentration increases, the fluctuation of the melting characteristics (viscosity Small rise is desirable.

In order to obtain a composition having a small characteristic variation, M
Although a method of using about 1 to 20% by weight of gO has been disclosed, it has not always been practical. That is, M
Since the basic component increases due to the use of the gO component, it becomes easy to sinter in the process of being added to the mold and melted, and it becomes easy to impair the heat retention and the stable inflow due to the formation of slag bear. There was a drawback.

However, the specific surface area 95 according to the present invention is
By using carbon black having a BET-m ² / g or more and a pH of less than 8, the carbon black is sufficiently dispersed and other raw materials can be prevented from coming into contact with each other. It has become possible to reduce the amount of slag bear that is produced. Further, the use of lithium carbonate causes a reaction between the lithium carbonate and the carbon raw material to accelerate the oxidation of the carbon raw material, so that it is possible to suppress carburization of the molten steel or the solidified shell.

When cracking defects become a problem in medium carbon steel and high carbon steel, it is effective to raise the crystallization temperature (solidification temperature) of the powder slag, and the composition is high CaO / SiO. _{Two-component} powders are often used. As a means for increasing the CaO / SiO ₂ , there is a case where some or all of the raw materials except the carbonaceous raw material are melted in advance, or fluorite is used, but it is easy to sinter like the above case. However, the limited use of carbon black can solve these problems.

In the mold powder for continuous casting of steel of the present invention, other raw materials except the above carbon black such as base material, siliceous raw material, flux raw material, carbon raw material, etc. are not particularly limited, and are commonly used. Various types of can be used.

[0030]

EXAMPLES The mold powder for continuous casting of the present invention will be further described below with reference to examples. Examples Table 1 below shows the formulations of the products of the present invention and comparative products, and the results of use in actual machines.

[0031]

[Table 2]

[0032]

As described above, the mold powder for continuous casting of steel according to the present invention has a specific surface area of 95 BET-m ² / g or more,
In addition, since carbon black having a pH of less than 8 is blended, it is possible to eliminate variations in the degree of dispersion of the mold powder, and there is no problem such as carburization.

Claims (1)

[Claims] 1. A mold powder for continuous casting of steel comprising powder or granules of a mixed type in which various raw materials are mixed, a calcined type in which raw materials excluding carbon are preliminarily burned, or a molten type in which preliminarily melted is used, and a specific surface area of 9
A mold powder for continuous casting of steel, comprising 0.2 to 5% by weight of carbon black having a pH of 5 BET-m ² / g or more and a pH of less than 8.