JP2002273553A - Flux for continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flux for continuously casting steel having suitable melting characteristic with which development of sintered block or slag bear developed in a mold when the steel is continuously cast, is prevented and the stable casting is secured and also, the development of surface defect on a cast slab is prevented. SOLUTION: In a hollow granular body flux used for continuous casting, the greater part of low melting point material is contained in a basis material and a little low melting point material is added later and accumulated in the inner part of the hollow granular body.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended to prevent the formation of sintered ingots or slag bears in a mold that occurs during continuous casting of steel, to ensure stable casting, and to prevent the occurrence of surface defects in slabs. The present invention relates to a flux for continuous casting of steel having appropriate melting characteristics in order to prevent the occurrence of cracks.

[0002]

2. Description of the Related Art Flux added to a mold during continuous casting of steel plays various roles and has the following functions. That is, the upper surface of the molten steel is completely covered with the flux layer melted by the heat of the molten steel and the unmelted flux layer thereon,
Prevent oxidation of molten steel by the atmosphere and have a heat retaining effect. The molten flux dissolves and absorbs the non-metal oxides that float from the molten steel, thereby preventing the non-metal oxides from becoming surface stalks, surface subcutaneous inclusions, and internal inclusions of the slab. The flux film that has flowed between the mold and the slab performs a lubricating action between the mold and the slab, uniformizes the heat removal from the slab to the mold through the film, and slowly cools the solidified shell of the slab. To obtain a slab that is uniformly developed and free from surface defects. And so on.

[0003] These effects of the flux have the effect of eliminating the surface defects of the slab and forming a beautiful casting surface. In particular, the aim is to ensure the stability of the casting operation in the continuous casting operation and to improve the slab casting yield. It is indispensable for it.

[0004] The flux is usually in the form of powder or granules, and its components are generally composed mainly of CaO and SiO ₂ , as well as Al ₂ O ₃ , alkaline earth metals and alkali metal compounds (oxides, carbonates). , Fluoride, etc.) to adjust the melting temperature, viscosity, etc., and further add carbon to adjust the melting rate to form a flux composition. An organic or inorganic binder or the like is used to keep a certain shape.

[0005] In recent years, there has been a demand for an improvement in productivity, and measures to increase the continuous casting speed have been taken to meet the demand, and an increase in the amount of casting per unit time has been attempted. In high-speed casting of steel, it is important to prevent sintering of the flux during the melting process.

Conventionally, measures have been taken to increase the amount of aggregate (carbon) added or to improve the addition method (adoption of an outer surface coat), and to add an organic substance to avoid sintering as much as possible. However, especially when high-speed casting with a small cross-sectional size and / or high basicity and high solidification temperature flux is used, it is not sufficient to cope with it. This caused abnormal development of the slag bear, and also hindered the casting operation due to the occurrence of flame.

However, when high-speed casting is performed,
The amount of flux used must increase in proportion to the casting speed.
Because the melting rate is slow, the required amount of molten flux layer on the surface of molten steel cannot be secured, and the appropriate amount of molten flux is not supplied between the slab and the mold, so the flux film is thin, and the flow becomes uneven, Therefore, since the degree of heat removal from the slab was extremely large, the occurrence of the above phenomenon was increased, and the surface quality of the slab was affected by various surface cracks, which caused surface flaws at the time of rolling. Had deteriorated.

As described above, regarding the surface quality of the slab,
There were many difficulties in obtaining a satisfactory product. In addition, the inflow of the molten flux was insufficient, resulting in poor lubrication, sometimes leading to breakout. In order to overcome such a phenomenon, as described above, reduce the amount of carbon in the aggregate that adjusts the melting rate of the flux,
It is sufficient to increase the flux melting speed and secure the amount of molten flux on the surface of the molten metal to maintain the thickness of the flux film commensurate with the casting speed.However, if this aggregate is reduced, sintering occurs in the flux melting process As a result, the melting speed of the flux is reduced, which contributes to the reduction in the thickness of the flux film.

Conventionally, the carbonaceous raw material in the flux not only plays a role as a slag-melting rate adjusting material, but also serves as a sintering inhibitor between the various raw materials in the unmelted flux layer as described above. Recognizes that carbon as an aggregate is an indispensable composition in the surface protection material, as it contributes to maintaining the low thermal conductivity layer and plays a large role in the heat retention of molten steel due to the exothermic reaction during oxidation. Has been generalized.

[0010] From such a viewpoint, there are many inventions using carbon as a melting rate regulator.
No. 11346 describes the technology "Continuous steel consisting mainly of metal oxides and containing at least one of alkali metal and alkaline earth metal fluorides, alkali metal oxides and carbonates. It is characterized by blending 0.5 to 5 wt% of carbonaceous powder of 100 mesh or less and 0.1 to 4 wt% of organic fiber material of 100 mesh or less as a melting rate modifier to the base material of the casting mold additive. "A mold additive for continuous casting of steel".

[0011]

Conventionally, in low-speed casting, a flux having a low solidification temperature has been used in casting large-sized slabs as a means for preventing the slag of sintering of the flux. Increases, addition of organic substances, etc. were applied, but no major problems occurred. However, when casting small size slabs at high speed and / or using a flux having a high solidification temperature, the occurrence of operational troubles due to abnormal slag bear development and an increase in the casting speed increase the flow rate of molten steel from the immersion nozzle. Fluctuations in the internal molten metal level have increased, and the generation of sintered lumps of unmelted flux and the generation of flames due to the decomposition of organic substances in the flux have caused a defect that the state of the molten metal cannot be confirmed.

Under such circumstances, reducing the amount of carbon in the surface protection material leads to the occurrence of surface and internal defects of the cast slab in the continuous casting operation. There has been a problem that the melting rate of the surface protection material must be increased while maintaining the amount of carbon in the protection material in a normal range.

However, the above-mentioned Japanese Patent Publication No. 2-11346.
In the invention of No. 2, fusion of particles in the flux occurs at the time of flux melting, and as a result, slag bear occurs, and it is not possible to avoid causing the various defects described above. The present invention has been made to solve such a conventional problem, and an object thereof is to provide an appropriate means therefor.

[0014]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and employs the following means. (1) In the hollow granule flux used for continuous casting, most of the low-melting substance is contained in the base material, the low-melting substance added later is reduced in a small amount, and the low-melting substance added later is placed inside the hollow granule. An integrated flux for continuous casting. (2) The low-melting substance added after the above is added in an amount of 15
The flux for continuous casting according to (1), wherein the flux is not more than mass% and the average particle diameter is larger than the average particle diameter of the base material. (3) As a post-addition substance, 0.5 to 5 mass% of an acid-treated graphite having a particle size of 150% or less and a particle size of 70% or more, which is smaller than the average particle size of the substrate, is added as a post-addition substance to the base material. The flux for continuous casting according to any one of the above.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Generally, a billet or the like having a small cast slab size is a flat slab (slab) having a large size.
As compared with, the ratio of the surface area to the cross-sectional area is large, so the amount of heat removed from the slab is large, and for the flux used for casting small-sized slabs, extremely strict conditions are required compared to the casting of slabs. And more severe conditions have been imposed by the spread of high speed casting.

This manifests as a result of the generation of sintered lumps in the mold and the abnormal development of the slag bear as described above, and makes it difficult to continue a stable continuous casting operation. Thus, the present invention has attempted to develop a flux having an excellent effect even when applied to continuous casting of a small-sized slab having many problems as described above.

The present inventors have conducted various experimental studies on preventing the sintering of the flux even if the amount of carbon in the aggregate is reduced. As a result, the inventors have found that the amount of the low-melting substance in the flux is not changed. By accumulating more low-melting substances inside the hollow granules than in the surface layer, it is possible to prevent fusion between the flux particles,
By using finely divided acid-treated graphite, the acid-treated graphite accumulates on the surface of the flux particles, and a more appropriate melting rate can be obtained without observing the generation of slag bears. Can keep
It has been concluded that the inflow of the molten flux between the mold and the slab is constantly supplied at a constant rate, and the occurrence of surface defects and breakout of the slab can be prevented.

That is, a low-melting substance is added to the flux raw material at the time of manufacturing the hollow-granular flux in such a manner that the low-melting substance is made as coarse as possible from the average particle diameter of the base material, and, if necessary, the acid-treated graphite is added in the form of fine particles. In the process of manufacturing the hollow granular flux by the drying method, the flux suspension water is sprayed and ejected in the form of particles in the flux manufacturing / drying step, and is exposed to a hot air atmosphere. As a result, when the water in the flux particles evaporates and the water moves from the inside of the particle to the surface layer, the energy causes the fine particle part to move to the surface layer of the hollow granular flux particles, but the reverse is true. Furthermore, attention is paid to the fact that the coarse particle portion remains and accumulates in the inner layer.

When the flux particles thus produced are added to the mold surface in the mold, the softening and melting proceeds from the inside containing a large amount of low-melting-point substances, thereby preventing sintering of the particles while maintaining a hollow shape. can do. For this reason, the growth of the slag bear generated due to the fusion of the flux particles does not occur, and the original role as the flux can be sufficiently fulfilled.

There are various low-melting substances among the substances constituting the flux. For example, NaF, Na ₂ CO ₃ ,
Na ₂ AlF ₆ , Li ₂ CO ₃ , LiF, Na ₂ B ₄ O
₇ etc. correspond to it. However, Na ₂ CO ₃ is excluded from the scope of the present invention because it has a high solubility in water and does not maintain the shape of solid particles during slurry production.

In the present invention, the low-melting substance to be added later is preferably added in an amount of 15 mass% or less because if the low-melting substance exceeds 15 mass%, the low-melting substance easily fuses to the base material in the melting process. As a result, the sinterability increases,
This is because a slag bear is easily generated. Furthermore, the average particle diameter is larger than the average particle diameter of the substrate,
The larger the difference, the better. In practice, the average particle size depends on the average particle size of the substrate, but the preferable average particle size is about 25 to 150 μm. The reason is that when the average particle diameter is less than 25 μm, the particles do not accumulate inside the hollow granules, accumulate near the surface layer and the particles are easily fused together, and when the average particle diameter exceeds 150 μm, coarse particles contain a low melting point substance. This is because operating troubles such as nozzle clogging are likely to occur during the granulation of hollow granules.

Since the present invention adopts such a constitution, the low melting point material in the base material is accumulated more in the hollow granules than in the surface layer portion, so that the particles can be properly melted without causing a fusion phenomenon between the particles. Speed can be maintained. Therefore, even if the flux is repeatedly subjected to heating and cooling due to fluctuations in the molten metal level, fusion of the particles is unlikely to occur, so that a large sintered body is not formed.

FIG. 1 shows an outline of the distribution of the low melting point substance in the flux of the present invention having such a configuration, together with a conventional example. FIG. 1 shows the general tendency, and it can be seen that the substance having a lower melting point than the conventional flux is located on the inner side in the hollow granules.

The addition of the acid-treated graphite makes it possible to further suppress the fusion between the particles. In this case, while controlling the particle size of the molten base material to 250 μm or less,
Acid-treated graphite of 50% or less, 70% or more of 0.5 to 5 m
When ass% is added, acid-treated graphite accumulates on the surface layer of the hollow granules. The flux particles produced in this manner, when added to the surface of the molten metal in the mold, the acid-treated graphite of the fine particles present in the surface layer of the flux particles, due to the properties of the acid-treated graphite, rapidly due to the heat of the molten steel. The acid-treated graphite which has expanded and swelled from the particle surface layer forms a state in which it is partially adhered to most of the particle surface, thereby preventing the fusion of the flux between the particles.

In the present invention, the mixing of organic fibers having an effect similar to the object of the present invention at 3 mass% or less may be used. Since the flux of the present invention is sufficiently effective when applied to continuous casting of small-sized slabs with severe use conditions, it is, of course, applicable to continuous casting of large-sized slabs without any problem. it can.

[0026]

The effects of the present invention will be described below in detail with reference to examples. The chemical composition of the flux used in the examples of the present invention is shown in Table 1, and the physical properties of the flux are shown in Table 2. Table 3 shows the operating conditions under which the slab was cast using the flux and the quality of the slab obtained thereby. For comparison, the chemical composition of the flux, the casting operation conditions, and the quality of the slab are also shown in Tables 1 to 3 as comparative examples.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

As is clear from Table 3, the casts Nos. 1 to 7 cast using the flux of the present invention did not generate slag bears during casting, and did not generate any flames. There was almost no occurrence of flaws and the like, and there was no problem in any of the slab quality.

On the other hand, in Examples Nos. 8 to 12 using fluxes that deviate from the scope of the present invention, some slag bears are generated during casting, and good results are also obtained with respect to cracks and the like in slabs. No defective products were found. In Examples 8 and 10, the amount of the post-added low-melting substance is within the scope of the present invention. However, in Example 8, the low-melting substance accumulates in the hollow surface layer because the particle diameter of the post-added low-melting substance is small. In this case, the particles are easily fused to each other.

In the case of Example No. 10, since the form of the flux was a real spherical granule, the difference between the particle sizes was not effectively distributed, and the formation of a sintered mass and a slag bear during casting was observed. In Run Nos. 9, 11, and 12, stable castability and good cast slab quality could not be obtained due to the large amount of the low-melting substance added later. In particular, in Example No. 13, the addition amount and particle size of the post-added low-melting substance are within the scope of the present invention. However, since the particle size of the acid-treated graphite to be added later is large, nozzle clogging occurs during the production of the hollow and stable. It could not be manufactured and did not reach the actual casting test.

[0033]

According to the present invention, the following effects can be expected. In particular, the sinterability (fusion of product particles in the melting process) of a flux having a high basicity and a high solidification temperature can be completely prevented, and as a result, as shown in the examples, the flux is small compared to the conventional flux. There is no sinter and no slag bear at the time of cross section and high speed casting, and there is no operation trouble. At the time of small cross section and high speed casting, there is no problem of flame generation, the appropriate thickness of the molten flux layer is ensured, the castability is stabilized, and multiple casting is possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing an outline of a distribution state of a low melting point substance in a hollow granular flux particle.

Claims (3)

[Claims] 1. In a hollow granule flux used for continuous casting, most of the low-melting-point substance is contained in a base material, the low-melting-point substance added later is reduced in a small amount, and the low-melting-point substance added later is added to the hollow granule. Flux for continuous casting characterized by being integrated inside. 2. The flux for continuous casting according to claim 1, wherein the amount of the low-melting substance added later is 15 mass% or less, and the average particle diameter is larger than the average particle diameter of the base material. . 3. The post-added substance, wherein 0.5 to 5 mass% of acid-treated graphite having a particle diameter of 150 μm or less and 70% or more, which is smaller than the average particle diameter of the substrate, is added as a post-addition substance. Item 3. The flux for continuous casting according to any one of Items 1 or 2.