KR100252483B1 - Mold powder for continuous casting of molten steel - Google Patents

Abstract

The present invention relates to a continuous mold mold powder added to molten steel in a mold during continuous casting of steel, the basicity (CaOwt% / SiO ₂ wt%) is 1.1 to 2. A continuous casting mold containing ₄ to 8 wt% of fluorine and 1 to 7 wt% of Al ₂ O ₃ , wherein the content of the premelt melt of the powder satisfies the following formula. Powder, (wt% of premelt flux) ≧ 25 × (CaOwt% / SiO ₂ wt%) + 35, wherein the basicity is from 1-3. And more preferably it is 6, the addition can also be added cryolite, NaF, fluorite, soda ash, 3~25wt% of any one 1 or more kinds of Li ₂ O, the carbon material 1~8wt% with respect to the flux loop rimelteu It features.

Description

Mold powder for continuous casting of steel

The present invention relates to a continuous mold mold powder (hereinafter simply referred to as mold powder or powder) added to molten steel in a mold during continuous casting of steel. The powder of the present invention relates to a mold powder that is most suitable for continuous casting of medium carbon steel, which is particularly susceptible to surface cracking of cast steel.

In the continuous casting of steel, the mold powder prevents the oxidation of molten steel on the upper surface of the molten steel and absorbs the inclusions that have risen to the warmth and hot water surface of the molten steel surface. In addition, the powder melted from the molten steel surface flows between the mold and the slab to form a slag film layer, acting as a lubricant between the mold and the slab, and controlling the heat generation between the mold and the slab.

When chemical analysis of a conventional mold powder is carried out, the composition is as follows.

SiO ₂ : 20-40wt%, CaO: 20-40wt%, Al ₂ O ₃ : 1-10wt%,

Na: 1-30 wt%, F: 1-20 wt%, Li: 0-20 wt%,

MgO: 2-10 wt%, ZrO ₂ : 0-20 wt%, B: 0-10 wt%.

In the case of manufacturing the mold powder, so-called pre-melted calcium silicate obtained by melting and rehydrating raw materials such as slaked lime and silica in addition to cement and calcium silicate as a substrate is used. Low melting compounds such as metal carbonates and fluorine compounds are blended, and carbon ash powder is blended as a melting rate regulator.

However, the carbon concentration of the steel is 0.08 to 0. In continuous casting of 16wt% medium carbon steel (hereinafter referred to as medium carbon steel), the solidification shrinkage rate when steel is changed from liquid phase to solid phase is large, and crack defects easily occur on the surface of cast steel, making high speed casting difficult.

Thus, Japan's “Materials and Processes”, Vol. 4, No. 4 (p. 1247, 1991), is a measure of crack defects in the medium-carbon steel, converted from the weight percentage of the total Ca to reduce the heat flux between the mold and the surface of the cast steel. The basicity (hereinafter, simply referred to as basicity (CaOwt% / SiO ₂ wt%)) of the ratio of the wt% of CaO and the wt% of SiO ₂ converted from the weight percentage of the total Si is set to 1. 3 or more and cooled. In order to increase the contact heat resistance by volume shrinkage accompanying crystallization in city, and to add ZrO ₂ more than 3.0 wt%, to make the powder film opaque and to reduce radiant heat transfer, to manufacture good cast steel Discloses that it is possible (prior art 1).

In addition, Shinagawa Gibo No. 32 (1989, p. 147) investigated the exothermic rate and the characteristics of the powder in the mold to prevent longitudinal cracking of the slab, which is a problem when casting the medium carbon steel. It has been reported that by increasing the crystallization rate during the solidification process of a powder, it is possible to alleviate the heat generation in the mold and reduce the occurrence of longitudinal cracking. It is also described that the higher the base weight of the powder, the higher the crystallization temperature tends to be (Prior Art 2).

In the above, crystallization is a phenomenon in which a part of the melt precipitates as crystal when the molten powder flows between the mold and the surface of the cast steel and solidifies. It also occurs when the premelt flux, which is the base of the casting powder, solidifies.

In recent years, in order to achieve rapid and uniform melting of powders with the speed of continuous casting, the above-mentioned premelt type calcium silicate flux is mainly used as the base material of the mold powder.

This premelt calcium silicate flux is usually blended with powder at least 50% wt, and it is common to use a premelt calcium silicate having a basicity equal to the basicity (CaOwt% / SiO ₂ wt%) of the mold powder itself. For example, as a powder raw material for medium carbon steels, a high basic degree or higher basic salt having a primary salt of at least 1.3 is used.

As a mold powder developed under such a concept, there is Japanese Patent Publication No. 90-27063. This patent claims that the content of CaO and SiO ₂ is 70 wt% or more, and CaO wt% / SiO ₂ wt% (base) is 1.2 to 2. It is a synthetic calcium silicate of 3, containing at least 50 wt% or more of synthetic calcium silicate having 1-10 wt% of F, 8 wt% or less of Al ₂ O ₃ , and 1 wt% or less of Fe ₂ O ₃ as an indispensable impurity. Mold additives for continuous casting.

However, in this publication, in order to make synthetic calcium silicate, which is a raw material of a mold powder, as an amorphous raw material, CaO / SiO ₂ is 1.2. 3, Preferably it is 1-2. 9 is said to be preferable. However, no consideration is made regarding how the composition of the composition is related to the relationship between the mold powder prepared from the synthetic calcium silicate as a raw material and the surface crack of the cast steel.

As described above, the prior art has been effective in suppressing the longitudinal cracking of the cast, focusing on reducing the heat flux in the mold, that is, completely cooling the mold. However, in the prior art, it is only a slow cooling that accepts the mold macroscopically, and surface cracks of the cast steel still occur due to the nonuniformity of shrinkage due to the difference in the local cooling speed of the cast steel, and the surface grinding of the cast steel due to quality problems With this necessity, it was not possible to cope with the reduction of the product ratio with respect to the raw material, the increase of the labor cost, and furthermore, the improvement of productivity due to the recent increase in the casting speed.

The inventors have conducted various studies. It has been found that it is important to cool the cast evenly within the mold. As a result of research, standard deviation of slag film thickness is important for uniform cooling, and CaOwt% / SiO ₂ wt% of mold powder and the content of premelt flux in mold powder are important for controlling surface deviation of the film thickness. The present invention has been achieved by gaining insight into factors.

The present invention has at least a basicity (CaOwt% / SiO ₂ wt%) of 1.1 to 2. In the range of 4, and a mold powder for continuous casting containing fluorine respectively 2~12wt% and Al ₂ O ₃ 1~7wt%, satisfying the following the content of the program rimelteu flux powder of the formula (1) Provided is a mold powder for continuous casting, characterized in that.

(Wt% of premelt flux) ≥ 25 × (CaOwt% / SiO ₂ wt%) + 35 (1)

In the mold casting for continuous casting of the present invention, the basicity (CaOwt% / SiO ₂ wt%) of the powder is 1.3 to 1. It is in the range of 6, and it is preferable that content of the said premelt melter is 85 wt% or less.

In the mold casting for continuous casting of the present invention, it is preferable to further contain the following.

(a) 3-25 wt% of one or more of any one of cryolite, NaF, fluorite, soda ash and Li ₂ O;

(b) Content of 1-8 wt% of carbon material.

In the mold casting for continuous casting of the present invention, it is preferable that the premelt flux is amorphous.

BRIEF DESCRIPTION OF THE DRAWINGS The figure which shows the influence of the standard deviation of the film thickness of a slag film on the longitudinal crack generation index in a slab.

2 is a diagram showing the effect of the premelt flux content on the standard deviation of the film thickness of the slag film layered by basicity.

FIG. 3 is a graph showing the relationship between the premelt melt content and the basicity of a mold powder in which the standard deviation of the film thickness of the slag film in FIG. 2 is 0.4 or less; FIG.

Fig. 4 is a diagram showing the effect of the premelt melt content on the longitudinal crack generation index in cast steel.

In the present invention, a mold powder for continuous casting comprising:

At least basicity (CaOwt% / SiO ₂ wt%) is 1.1 to 2. It is in the range of 4, and contains ₂ to 12 wt% of fluorine and 1 to 7 wt% of Al ₂ O ₃ .

Moreover, basicity is 1.3-3. It is more preferable to exist in the range of 6.

In addition, the composition of the mold powder is a basic composition of the composition, and the adjustment of the melting characteristics such as suitable cryolite, NaF, fluorite, soda ash, Li ₂ O and the like adjusting the melting rate, carbon materials, such as carbon black, scale graphite Etc. may be included.

Moreover, the characteristic of the mold powder of this invention includes the part which melt | dissolved the said powder beforehand and cooled (hereinafter called premelt flux) in the range of following formula (1).

(Wt% of premelt flux) ≥ 25 × (CaOwt% / SiO ₂ wt%) + 35 (1)

Here wt% of CaO is the wt% of CaO converted from the weight percentage of the total Ca, wt% of SiO ₂ is the wt% of SiO ₂ converted from the weight percentage of the total Si.

Formula (1) above was obtained as follows. In other words, various mold powders with different formulations of premelt flux were prepared and casting experiments were conducted. The mixing ratio of the premelt flux and the thickness of the slag film attached to the cast steel were measured, and the standard deviation was measured.

The premelt melter melts a raw material blended with silica, coal, fluorspar, glass powder, alkali metal carbonate, alkaline earth carbonate and the like to form a composition within the above range in a three-phase arc, and the melt is cooled and watered by jet water. It was prepared by pulverizing to 100 mesh or less after saponification and drying.

The premelt flux has a content of at least one of cryolite, NaF, fluorite, soda ash, and Li ₂ O in a content of 3 to 25 wt%. The mold powder (hereinafter referred to as the invention example) 1 to 19 were prepared by adding so that it was -8 wt%. For comparison, mold powders (hereinafter referred to as comparative examples) 1 to 12 were prepared with a premelt melt content outside the scope of the present invention.

In Table 1, Examples 1 to 19 and Table 2 of Comparative Example 1 to 12 showed that the mold powder chemical composition value (wt%), viscosity (Pa · s), premelt ratio (wt%), and these mold powders were used. The standard deviation (mm) of the film thickness of the slag film, the vertical crack generation index in the cast steel, and the required premelt content (wt%) are shown.

The effect of the standard deviation of the film thickness of the slag film in the longitudinal crack generation index in the obtained cast was performed using the powders shown in Tables 1 and 2, and the results are shown in FIG. It is clear from Fig. 1 that the vertical deviation of the slag film can be suppressed low in the region where the standard deviation of the film thickness of the slag film is 0.4 or less.

In other words, the smaller the film thickness variation of the slag film, the more the film thickness of the slag film is produced, which means that less vertical cracking occurs on the surface of the slab.

The influence of the premelt ratio on the film thickness standard deviation of the slag films in Tables 1 and 2 is shown in FIG. It is clear from Fig. 2 that the premelt content or more corresponding to the region where the film thickness standard deviation of the slag film corresponds to 0.4 or less is preferable in suppressing the occurrence of longitudinal cracking in the cast steel.

3 shows the relationship between the premelt content and basicity of the mold powder whose standard deviation of the film 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 longitudinal cracking was determined as follows from the relationship with the basicity.

(Wt% of premelt flux) ≥ 25 × (CaOwt% / SiO ₂ wt%) + 35 (1)

Therefore, it became clear that the use of a mold powder having a premelt content satisfying the above formula (1) enabled the suppression of the occurrence of longitudinal cracking on the surface of the continuously cast slab. Moreover, as shown in FIG. 4, the basicity of powder in the range of said Formula (1) is 1.3-3. 6 and the range of 85 wt% or less of the premelt flux is most suitable because there is little longitudinal crack on the surface of the cast steel.

To the premelt flux, ₃ to 25 wt% of one or more of cryolite, NaF, fluorspar, soda ash, and Li ₂ O is added as a content as a modifier of melting characteristics, and carbon black, which is a carbon material, as a modifier of melting rate, scales When 1-8 wt% of any one of shape graphite is added as content, a preferable mold powder is obtained.

After blending in this way, the mold powder of the present invention is obtained by uniformly mixing with a V-type brander or the like. In this way, the mixture may be obtained by mixing with a hydrophilic stirring granule to obtain columnar granules, or by splicing the mixture to obtain a ball-shaped powder.

EXAMPLE

In Example, the mold powder as shown in Table 1 and Table 2 was produced using the above-mentioned mold powder for continuous casting. The invention example shown in Table 1 is demonstrated, comparing with the comparative example shown in Table 2. FIG.

Casting experiments were carried out using mold powders of these invention examples and comparative examples. Casting condition is carbon concentration in steel = 0. 09-0. 16 wt%, casting speed = 1. 4 to 1. 8 m / min, slab thickness = 220 mm and slab width = 2,000 mm. The number of longitudinal cracks of 2 mm or more in the slag cross section obtained under the casting conditions was counted over the entire slab width and the length of 1 m and indexed. This is the vertical crack generation index in Table 1 and Table 2.

From Table 1 and Table 2, it is clear that the longitudinal crack generation index of the invention example is less than that of the comparative example. In Table 1, CaO / SiO ₂ was 1.3 to 1. It is evident that the longitudinal crack generation index is less when the mold powder having a premelt ratio of less than 85% and satisfying the formula (1) in the range of 6 is used.

As described above, it is evident that the preferred content of the premelt flux of the present invention is formulated using the basicity of the mold powder as a parameter, thereby suppressing the occurrence of longitudinal cracking of the cast steel.

As described above, according to the present invention, it is possible to formulate the preferred content of the premelt flux, which is the main raw material of the mold powder for medium-carbon steel high speed casting, using the basicity of the mold powder as a parameter, thereby suppressing the occurrence of longitudinal cracking of the cast steel. Was done. As a result, even in medium carbon steels with high cracking susceptibility, it is possible to continuously cast steel having excellent quality without cracking defects at high speed.

Claims (4)

At least basicity (CaOwt% / SiO ₂ wt%) is 1.1 to 2. In the range of 4, and a mold powder for continuous casting of the fluorine-containing 2~12wt% and Al ₂ O ₃ 1~7wt%, is satisfied to the content of the program rimelteu flux powder of the formula (1) Mold powder for continuous casting of steel. (Wt% of premelt flux) ≥ 25 × (CaOwt% / SiO ₂ wt%) + 35 (1) The basicity (CaOwt% / SiO ₂ wt%) of the mold casting for continuous casting is 1. 3 to 1. It is the range of 6 and content of the said pre melt melt is 85 wt% or less, The continuous casting mold powder characterized by the above-mentioned. The method of claim 1, (a) 3-25 wt% of one or more of any one of cryolite, NaF, fluorite, soda ash and Li ₂ O; (b) A mold powder for continuous casting, wherein the carbon material contains 1 to 8 wt%. 2. The mold powder for continuous casting according to claim 1, wherein the premelt flux is amorphous.