JP2848231B2 - Mold powder for continuous casting - 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 to be added to the surface of molten steel supplied into a mold through a dipping nozzle during continuous casting.

[0002]

BACKGROUND ART mold powder is a powder or granular form, typically CaO, SiO ₂ is the main component, other Al
_{It is} composed of metal compounds such as ₂ O ₃ and MgO, carbonates such as Na ₂ CO ₃ , fluorides, and carbon. The role of preventing the oxidation of the surface of the molten steel in the mold, keeping the surface of the molten steel in the mold warm, lubricating between the mold and the slab, and melting the floating inclusions are well known in the art.

However, recently, attempts have been made to control the heat flux in the mold by changing the overall heat transfer coefficient of the powder film layer interposed between the mold and the slab, in addition to the basic role described above. It is becoming. This is because, during continuous casting of a so-called hypoperitectic steel having a carbon content of about 0.1% by weight (generally 0.08 to 0.17% by weight), a δ → γ transformation including a peritectic reaction during solidification occurs. This is because rapid solidification shrinkage occurs and the slab is separated from the mold to cause a local solidification delay, which causes defects such as vertical cracks. Therefore, in order to alleviate the solidification shrinkage due to the δ → γ transformation, the heat flux in the mold is reduced to cool slowly.

Conventionally, as means for reducing the overall heat transfer coefficient of the powder film layer, both the component ratio of CaO and SiO ₂ [CaO / SiO ₂ ] (hereinafter referred to as “basicity”) and the solidification temperature have been increased. , ZrO ₂ or [CAMP-ISIJ Vol. 6 (1993) -28
3], one containing ZrO ₂ or CeO ₂ [Japanese Unexamined Patent Publication No.
198961].

[0005]

However, when the basicity exceeds 0.9, as shown by a circle in FIG.
The crystallization ratio when the mold powder flowing between the mold and the slab solidifies is increased. Since crystallization involves volume shrinkage, when the crystallization rate increases, the volume shrinks during solidification, and voids occur between the mold and the slab. Since the heat flux at this void generation site is significantly reduced, local solidification delay and accompanying vertical cracking may occur, and the temperature indicated by the thermocouple for predicting breakout may fluctuate, causing operation hindrance. Become.

Even when the basicity is 0.9 or less, when ZrO ₂ , TiO ₂ , or CeO ₂ as a crystallization agent for glass is added, the crystallization ratio increases, and A problem arises (see FIG. 1).

[0007] These problems are caused by the high solidification temperature for slow cooling in the mold, which simultaneously increases the crystallization rate excessively. Therefore, crystallization when the mold powder solidifies is suppressed. It can only be solved by

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and solves problems in operation and quality caused by excessive crystallization of a powder film. It is an object of the present invention to provide a mold powder for continuous casting capable of stably producing high-quality steel by reducing the fluctuation range of the mold.

[0009]

To achieve the above object SUMMARY OF THE INVENTION The continuous casting mold powder of the present invention are, within the scope of basicity 0.6-0.9, and ZrO _2, TiO
_The total concentration of ₂ , CeO ₂ is 1% by weight or less, and the solidification temperature is 1150 ° C. or more.

In the mold powder for continuous casting of the present invention, the basicity is set in the range of 0.6 to 0.9.
If it is less than 0.6, the crystallization ratio of the powder film becomes completely zero (see FIG. 1), and the slow cooling effect in the mold is lost. On the other hand, if it exceeds 0.9, the crystallization ratio of the mold powder flowing between the mold and the slab at the time of solidification becomes high (see FIG. 1).

Further, in the mold powder for continuous casting according to the present invention, the concentration of ZrO ₂ , TiO ₂ , and CeO ₂ is set to 1% by weight or less according to the experimental results of the present inventors. ZrO ₂ , TiO ₂ , CeO ₂
When the sum of the concentrations exceeds 1% by weight, the crystallization rate during solidification of the mold powder flowing between the mold and the slab becomes high, and longitudinal cracking breakout occurs [Table 3]. f, h, i].

Further, in the mold powder for continuous casting of the present invention, the solidification temperature is set to 1150 ° C. or more, because when the solidification temperature is lower than 1150 ° C., the incidence of vertical cracks increases rapidly. (See FIG. 2).

The mold powder for continuous casting of the present invention has a basicity of ZrO ₂ or TiO ₂ which is a crystallization agent for glass.
Any other chemical component system may be used as long as the sum of the concentrations of ₂ , CeO ₂ and the solidification temperature satisfy the above-mentioned values.

[0014]

Since the solidification temperature of the mold powder for continuous casting according to the present invention is as high as 1150 ° C. or more, the generation rate of longitudinal cracks is reduced even in the case of subperitectic steel casting due to the effect of slow cooling in the mold (see FIG. 3). . Further, the basicity is 0.6 to 0.9.
And the sum of the concentrations of ZrO ₂ , TiO ₂ , and CeO ₂ is set to 1% by weight or less, so that the crystallization ratio of the powder film is low and the local fluctuation of the heat flux in the mold is reduced [FIG. 5].

FIG. 5A shows a mold powder (comparative example) having a basicity of 1.25 and a solidification temperature of 1180 ° C.
In (b), a mold powder having a basicity of 0.85 and a solidification temperature of 1200 ° C. was used (example of the present invention). Each of the molten steels having a carbon concentration [C] of 0.14% by weight was 2.0 m / m2.
It is a figure which shows the result at the time of continuous casting at the speed of minute.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the mold powder for continuous casting according to the present invention will be described. FIG. 6 shows the chemical composition and the chemical composition shown in Table 1 below on the surface of molten steel in the mold when continuously casting molten steel having a carbon concentration [C] of 0.18% by weight at a casting speed of 2.0 m / min. It shows the result of measuring the mold copper plate temperature index for each distance from the meniscus when adding mold powder at the solidification temperature.

[0017]

[Table 1] (Unit: wt%)

As shown in FIG. 6, mold powder A having a low solidity and low solidification temperature (conventional example) is compared with mold powder B having a high solidification temperature (comparative example) and mold powder C.
In each of (Examples of the present invention), the average value of the mold copper plate temperature at a point 120 mm below the meniscus is equally reduced, and the slow cooling effect near the meniscus is apparent.

However, when the mold powder B is added for continuous casting, the temperature of the mold copper plate varies greatly, and the temperature of the mold copper plate decreases more as it goes downward from the meniscus. This is because the crystallization ratio (volume shrinkage) of the powder film is large due to the high basicity of the mold powder B, and voids are generated between the mold and the slab, and the voids are increased toward the lower part of the mold. It is the result of having

Due to the heat removal characteristics in the mold, when the mold powder A is added and continuous casting is performed, a vertical crack and a vertical crack breakout occur, and when the mold powder B is added and continuous casting is performed. No vertical cracking occurred, but a vertical cracking breakout occurred. On the other hand, when the mold powder C of the present invention was added and continuous casting was performed, neither the vertical crack nor the vertical crack breakout occurred at all.

In another embodiment of the present invention, a mold powder having a chemical composition and a solidification temperature shown in Table 2 was added to the surface of the molten steel in the mold so that the carbon concentration [C] in the molten steel was 0.18% by weight. Table 2 also shows the results of an examination of the occurrence of vertical cracks and breakage of vertical cracks when the molten steel was continuously cast at a casting speed of 3.0 m / min.
As is clear, when the mold powder of the present invention was added, neither vertical crack nor vertical crack breakout occurred at all.

[0022]

[Table 2] (Unit: wt%)

In addition, a mold powder having a chemical composition and a solidification temperature shown in Table 3 outside the range of the present invention was added to the surface of the molten steel in the mold to reduce the carbon concentration [C] in the molten steel to 0.1.
Table 3 also shows the results of an examination of the occurrence of vertical cracks and breakage of vertical cracks when 8% by weight of molten steel was continuously cast at a casting speed of 3.0 m / min. As described above, when the comparative mold powder was added, a vertical crack or a vertical crack breakout occurred in each case.

[0024]

[Table 3] (Unit: wt%)

[0025]

As described above, according to the continuous casting mold powder of the present invention, the mold powder flowing between the mold and the slab does not have to increase the crystallization rate when solidifying, and thus the mold powder in the mold can be loosened. Cooling and local fluctuation of heat flux can be realized, and high-quality steel can be continuously cast stably.

[Brief description of the drawings]

FIG. 1 is a view showing a change in the crystallization ratio of a powder film according to a change in basicity of a mold powder.

FIG. 2 is a graph showing a change in the rate of occurrence of vertical cracks due to a change in solidification temperature of mold powder.

FIG. 3 is a drawing showing a relationship between a casting speed and a heat removal amount in an example of the present invention and a comparative example.

FIG. 4 is a view showing a change in a heat flux local fluctuation index in a mold with a change in basicity of a mold powder.

FIG. 5 is a diagram showing a variation in a heat removal amount according to an example of the present invention and a comparative example.

FIG. 6 is a drawing showing the relationship between the distance from the meniscus and the mold copper plate temperature index in the present invention example, the comparative example, and the conventional example.

Claims (1)

(57) [Claims] A composition ratio of CaO and SiO ₂ [CaO
/ SiO ₂ (weight ratio)] in the range of 0.6 to 0.9,
And the sum of the concentrations of ZrO ₂ , TiO ₂ , and CeO ₂ is 1% by weight.
A mold powder for continuous casting, wherein the solidification temperature is 1150 ° C. or higher.