JP5590167B2 - Powder for continuous casting - Google Patents

Description

  The present invention relates to a powder for continuous casting used when continuously casting a low carbon or extremely low carbon aluminum killed steel used for, for example, a tin steel plate or a thin steel plate for automobiles.

Low carbon or extremely low carbon aluminum killed steel is a steel type that is frequently used for high-grade thin steel sheets such as tinplate and automotive steel sheets.
In automotive thin steel sheets, quality has become stricter in recent years, and it is particularly necessary to prevent inclusion inclusions in the slab during continuous casting. Inclusions include alumina clusters clustered with alumina, the deoxidation product, and powder containing oxide as a main component used to maintain the lubricity of the mold and slab during continuous casting. It is roughly divided into

The role of powder for continuous casting is
(1) Guarantee of lubrication between mold and solidified shell,
(2) Cooling control between mold and solidified shell,
(3) Absorption of inclusions (mainly alumina) that have floated in the mold,
(4) There is oxidation prevention and heat retention on the surface of the molten steel.
When the powder added to the molten steel surface by the molten steel flow in the mold is wound into the molten steel and remains in the slab, it becomes a surface flaw and an internal defect (starting point in press cracking). Therefore, as a characteristic required for the powder, a characteristic that it is difficult to be caught other than the characteristic that satisfies the role of the above four points is also required.
In general, medium carbon steel of C = 0.09 to 0.17% by mass tends to cause slab surface cracks due to inhomogeneous solidification or shrinkage due to phase transformation, especially cooling control between mold and solidified shell. (Slow cooling) is aimed at.
For this reason, high basicity (high CaO / SiO ₂ ), low-viscosity powder is used for the purpose of uniform inflow of powder and promotion of crystallization in solid powder (mainly caspidine).

On the other hand, low carbon steel is less susceptible to slab surface cracking than medium carbon steel, and powder with higher viscosity is used from the viewpoint of preventing powder entrainment.
Among the low carbon steels (including extremely low carbon steels), the inclusion strict materials (such as tinplate and automotive thin steel plates) where defects in the inclusion properties are a problem require a powder design especially from the viewpoint of preventing powder entrainment. There is a trend toward higher viscosity than low carbon steels other than inclusion strict materials (see, for example, Patent Documents 1 and 2).

JP 2000-280051 A JP 2003-290888 A

However, even in the case of low carbon steel or extremely low carbon steel, it is necessary to allow powder to flow between the mold and the solidified shell for lubrication between the mold and the solidified shell. It is necessary to reduce the viscosity of the powder.
Thus, it is necessary to have the contradictory characteristics of increasing viscosity to prevent entrainment of powder and decreasing viscosity to ensure lubrication. The current casting of strict inclusion materials is focused on powder entrainment, so highly viscous powders are used. It must be manufactured by slow low-speed casting, which is a factor that reduces the productivity of continuous casting.

  An object of the present invention is to provide a powder for continuous casting that enables high-speed casting while preventing powder entrainment in an inclusion-strict material by making both powder entrainment prevention and lubricity compatible.

  In order to solve the above problems, the present inventors have studied basic powder physical properties, and found a powder for continuous casting capable of ensuring lubricity while suppressing powder entrainment as described below. .

The first invention is an oxide-based powder that is added to the surface of molten steel in a mold for continuous casting of steel, mainly composed of CaO and SiO ₂ , a solidification temperature of 1200 ° C. or less, and an activation energy E ( J / mol). ) And a viscosity η (Pa · s) at 1300 ° C. satisfy the following formula (1).
-0.5108-0.00000519 × E > ln (η) > -2.303 + 0.00000862 × E (1)
However, E> 85 kJ / mol

In the second invention, in the oxide-based powder added to the surface of the molten steel in the mold in continuous casting of steel, CaO, SiO ₂ , Na _{2 with} respect to the total amount of oxide and fluoride excluding C and volatile components The continuous casting according to the first invention, wherein the total amount of O and CaF ₂ is 90 mass% or more and Al ₂ O ₃ , MgO and Li ₂ O are each 3 mass% or less and satisfy the following formula (3): Powder.
15-1.85 CaO / SiO ₂ ≦ Na ₂ O + CaF ₂ ≦ 25-6 CaO / SiO ₂
... (3)
However, 0.5 ≦ CaO / SiO ₂ ≦ 1, 0.7 ≦ Na ₂ O / CaF ₂ ≦ 1.2, and CaO (mass%), SiO ₂ (mass%), Na ₂ O (mass%) , CaF ₂ (mass%).

  If the powder for continuous casting of the present invention is used, it is possible to improve the casting speed while suppressing the entrainment of powder in the inclusion strict material.

The graph showing the relationship between the viscosity of 1300 ° C. and the powder entrainment index. The graph showing the relationship between the viscosity of 1500 ° C. and the powder entrainment index. The graph showing the relationship of the viscosity of 1200 degreeC and 1300 degreeC, and a powder consumption index. The graph showing the relationship between 1300 degreeC viscosity and activation energy. Graph showing the relationship between the required characteristic area in CaO / SiO ₂ and Li ₂ O. Graph showing the relationship between the required characteristic area in CaO / SiO ₂ and _{Na 2} O + CaF 2. Graph showing the relationship between the required characteristic area in Na ₂ O and CaF _2. The graph showing the relationship between a casting speed, BO detection incidence, and an internal defect flaw detection result.

The inventors of the present invention have studied the basic physical properties of the powder, and found a powder for continuous casting that can ensure the lubricity while suppressing the entrainment of the powder by controlling the temperature dependence of the viscosity.
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to this.

Conventionally, the viscosity, melting temperature, and solidification temperature (temperature at which the viscosity rapidly increases) measured at 1300 ° C. have been used as the basic characteristics of powder used when continuously casting molten steel.
Although the viscosity has been evaluated with 1300 ° C. as the representative temperature, the temperature dependence of the viscosity actually changes depending on the powder composition, and even if the viscosity at 1300 ° C. is the same, the viscosity may be different at other temperatures. The inventors paid attention to the temperature distribution in the thickness direction of the molten pool of powder added on the molten steel surface. That is, while the temperature at which the powder is wound on the molten steel is about 1500 ° C., which is almost the same as the molten steel temperature, the amount of powder flowing in between the mold and the solidified shell and the uniform inflow property are more The molten powder on the low temperature side was considered to flow in.
Therefore, the viscosity of 1500 ° C. near the interface with the molten steel is important from the viewpoint of preventing powder entrainment. Further, from the viewpoint of powder inflow, it was considered that viscosity at a temperature lower than 1300 ° C., which has been used as a conventional evaluation index, is important.

As described above, the molten pool of powder has a distribution from around 1500 ° C. on the molten steel side to the melting temperature at the top of the molten pool. Since the viscosity increases as the temperature decreases, it is estimated that the viscosity at a temperature as close as possible to the melting temperature affects the powder inflow property.
Since the melting temperature of ordinary powder is 1200 ° C. or less, in evaluating the inflow property, it was considered to design the powder by paying attention to the viscosity of 1200 ° C. Until now, the viscosity of 1300 ° C. has been used as an index of the viscosity of the powder, but attention has been paid to the activation energy when expressed in the Arrhenius type as the temperature dependence of the viscosity.

For the purpose of verifying the above-mentioned concept, first, a powder used in the past (viscosity at 1300 ° C. is 0.1 to 0.4 Pa · s) by a laboratory test is used. The relationship between the embedding property and the powder inflow property was evaluated. As the molten steel, low-carbon Al-K steel (0.04% C-0.01% Si-0.3% Mn-0.04% Al) was used. As a powder entrainment characteristic, powder was melted on the molten steel using a 20 kg vacuum melting furnace, and the molten steel was sucked from a position 1 cm below the powder / molten steel interface to evaluate the mass of the entrained powder. The powder in the sucked molten steel was separated and extracted by the slime method, and evaluated as the powder entrainment mass per unit mass of the sucked molten steel.
The entrained powder is
(1) Due to the floating characteristics of inclusions in an actual continuous casting machine, those with a thickness of 300 μm or more should float.
(2) The minimum size of defects in the tinplate material is about 50 μm.
Thus, inclusions having a particle size of 53 to 297 μm were evaluated using a 53 μm sieve and a 297 μm sieve defined by the JIS method as sieves used in the slime method.

As an evaluation index, the amount of entrainment of a powder having a viscosity of 0.2 Pa · s at 1300 ° C. used in ordinary low carbon steel was indexed as 1.
FIG. 1 shows the relationship between the viscosity at 1300 ° C. and the powder entrainment index, and FIG. 2 shows the relationship between the viscosity estimated at 1500 ° C. and the powder entrainment index.
Here, the viscosity at 1500 ° C. is not a direct measurement result, but is a value obtained by extrapolating from the relationship between temperature and viscosity by measuring the viscosity at a plurality of temperatures of 1350 ° C. or lower.

  Note that the rotating cylinder method was used for the measurement of viscosity. The powder excluding C was uniformly melted at 1400 ° C. in a carbon crucible and slowly cooled to 1350 ° C., and then the rotor of an E-type viscometer was immersed in the molten powder and held for 30 minutes to measure the viscosity. Thereafter, the operation of decreasing the temperature by 10 ° C. and holding it for 30 minutes and then measuring the viscosity is continued up to the solidification temperature. The activation energy is obtained from the relationship between the viscosity and the temperature thus obtained.

The reason for using the extrapolated value is that Na ₂ O or CaF ₂ that is usually contained in the powder may evaporate at high temperatures and change its viscosity.
In the measurement of powder viscosity, the effect of evaporation is large because measurement is performed while the molten powder is kept at a high temperature uniformly, but during actual casting, there is a temperature gradient in the molten pool, and evaporation is suppressed.
As shown in FIG. 1, even if the viscosity at 1300 ° C. is the same, the powder entrainment index varies, and as shown in FIG. 2, the viscosity at 1500 ° C. has a better correlation with the powder entrainment index. I understood. In other words, the powder entrainment property is almost determined by the viscosity at 1500 ° C., and has a powder entrainment preventing property equivalent to the powder having a viscosity at 1300 ° C. of 0.4 Pa · s used for the material for tinplate. It can be seen that the viscosity at 1500 ° C. is 0.1 Pa · s.

  Next, the powder inflow characteristics were evaluated using a small test continuous casting machine. The small test continuous casting machine had a mold size of 150 mm × 500 mm, and evaluated the amount of powder consumed during casting. The amount of powder consumed was the amount of powder used per unit mass of molten steel, by subtracting the amount of powder that had floated after casting from the total amount of powder charged. The casting speed was fixed at 1 m / min, and 8 ton molten steel was cast. The casting time is about 14 minutes.

  The consumption of powder having a viscosity of 1300 ° C. and a viscosity of 0.2 Pa · s used in ordinary low carbon steel was indexed as 1. The same low-carbon Al-K steel as described above was used as the molten steel for the powder consumption index. FIG. 3 shows the relationship between the powder consumption index and the viscosity at 1300 ° C and 1200 ° C. It can be seen that the powder consumption index varies at a viscosity of 1300 ° C., but when the viscosity at 1200 ° C. exceeds 0.6 Pa · s, the powder consumption index rapidly decreases as the viscosity increases. That is, it is considered that the powder inflow characteristic is almost determined by the powder viscosity at 1200 ° C. Since a small test continuous caster cannot perform a high-speed casting test, the test is performed and verified in an actual continuous casting machine as described later.

  Based on the above results, the viscosity at 1500 ° C. is greater than 0.1 Pa · s and the viscosity at 1200 ° C. is 0.6 Pa in order to maintain the powder inflow characteristics between the mold and the solidified shell while preventing the entrainment of powder. -It must be smaller than s.

When the temperature dependence of viscosity is approximated by an Arrhenius type, the viscosity η (Pa · s) is expressed by the following equation (4).
Indicated by
η (T) = η ₀ exp (E / R / T) (4)
R: Gas constant, T: Absolute temperature (K), η ₀ : Viscosity frequency factor Here, the viscosity at 1500 ° C. is greater than 0.1 Pa · s, and the viscosity at 1200 ° C. is 0.6.
When the simultaneous inequality is solved assuming that it is smaller than Pa · s, the inequality shown in the following equation (5) is obtained.
-0.5108-0.00000519 × E > ln (η) > -2.303 + 0.00000862 × E (5)
η: Viscosity at 1300 ° C. (Pa · s), E ( J / mol )

Next, the powder composition satisfying the formula (5) obtained above was examined.
FIG. 4 shows the relationship between the required characteristic region obtained from the inequality of equation (5) and the conventional powder. As shown in FIG. 4, it can be seen that there is no conventional powder that falls within the necessary characteristic region, and a drastic study of the powder composition is necessary. It is necessary to reduce the activation energy while having the same viscosity (1300 ° C.) as the conventional powder. The activation energy is a factor that defines a change in viscosity with respect to a change in temperature. The higher the activation energy, the greater the temperature dependency. In other words, the viscosity decreases rapidly as the temperature increases.

Considering the mechanism of manifestation of the viscosity of the molten powder, the SiO ₂ network structure of the molten powder is important, and the temperature dependence of the viscosity is presumed to be because the network structure of SiO ₂ is divided as the temperature rises. Therefore, in order to reduce the temperature dependence of the viscosity, it is important that the SiO ₂ network structure is not changed as much as possible by increasing the temperature. As shown in FIG. 4, when the viscosity at 1300 ° C. and the activation energy of various powders are observed, the activation energy tends to decrease as the viscosity decreases. Powders with low viscosity at 1300 ° C. generally have a low SiO ₂ content (ie, high basicity CaO / SiO ₂ ), and the activity that is an indicator of temperature dependence of viscosity because the SiO ₂ network is not developed. It is thought that the chemical energy is also decreasing.

In order to minimize the change in the network structure of SiO ₂ due to a temperature rise, the present inventors appropriately blended a component called a network modifier that divides the network, and the network unit of SiO ₂ on the low temperature side. We thought that it was effective to reduce the viscosity change with the temperature rise by dividing it into small pieces in advance. However, the decrease in viscosity when cutting small network units of SiO ₂ is significant, it was decided to reduce the maintenance of viscosity by dispersing a large number of network units SiO ₂ increases the content of SiO _2.
Representative examples of the network structure of SiO ₂ include basic oxides such as CaO, MgO, Li ₂ O, and Na ₂ O and fluorides such as CaF ₂ .

As a result of analyzing the activation energy representing the temperature dependency of the composition and viscosity of the conventional powder, Li ₂ O has the largest effect of reducing the activation energy (the effect of reducing the temperature dependency), Next, the order was CaF ₂ , CaO, Na ₂ O. If a large amount of Na ₂ O or CaF ₂ is added when Li ₂ O is added, the effect of lowering the viscosity is greater than the effect of reducing the activation energy, making it difficult to obtain appropriate characteristics. In addition, Al ₂ O ₃ usually contained in the powder has an effect of increasing the viscosity like SiO _2, but there is a tendency to increase the temperature dependence, and it is better to make Al ₂ O ₃ as small as possible. .

First, the powder composition based on Li ₂ O addition was examined. As a result, Li 2 _O is greater effect of lowering the viscosity with decreasing activation energy, it was found that it is necessary to the high viscosity composition in the Li 2 _O before addition of the powder composition.
As described above, in order to increase the viscosity, it is necessary to increase the content of SiO ₂ (that is, to decrease the basicity represented by CaO / SiO ₂ ). As described above, when Li ₂ O is added, when a large amount of Na ₂ O or CaF ₂ other than CaO, SiO ₂ , or Li ₂ O is contained, the viscosity is too low to obtain appropriate powder characteristics. . The remaining components other than CaO, SiO ₂ and Li ₂ O are each 3 mass% or less for adjusting the melting temperature and the solidification temperature, and the total mass of the remaining components is less than 10 mass%, and CaO / SiO ₂ and Li The temperature dependency of viscosity was measured while changing ₂ O, and a region satisfying the required characteristics was determined.
The reason why the components other than CaO, SiO ₂ and Li ₂ O are each 3 mass% or less and the total mass of the remaining components is less than 10 mass% is that it is a sufficient range for adjusting the melting temperature and the solidification temperature, and This is to prevent an extreme decrease in viscosity.

In FIG. 5, there is an appropriate range in relation to CaO / SiO ₂ and Li ₂ O, as indicated by ◯ when the condition of the expression (5) is satisfied, and by × when not satisfying the expression (5). The range is as shown in (7).
6CaO / SiO ₂ + Li ₂ O ≦ 11 (6)
1.85CaO / SiO ₂ + Li ₂ O ≧ 6 (7)
However, 0.5 ≦ CaO / SiO ₂ ≦ 1.2, and CaO (mass%), SiO ₂ (mass%), and Li ₂ O (mass%).

The viscosity on the low temperature side becomes high when it deviates above the appropriate region shown in FIG. 5, and the viscosity on the high temperature side becomes too low when it deviates below. On the other hand, when CaO / SiO ₂ is less than 0.5, the melting point of the powder becomes high and the melting rate becomes slow, so that it cannot withstand actual casting. CaO /
When SiO ₂ is larger than 1.2, even if Li ₂ O is added, it does not enter an appropriate region. In the composition entering the appropriate region, never activation energy falls below 85 (k J / mol). While maintaining proper powder viscosity, the activation energy is 85
In order to make it (kJ / mol ) or less, CaO / SiO ₂ should be less than 0.5, and Na ₂ O or CaF ₂ must be further added to lower the melting temperature. However, a further decrease in CaO / SiO ₂ lowers the absorption efficiency of inclusions (particularly alumina) in the molten steel and may cause a large amount of alumina to be generated in the molten steel due to the reduction reaction of SiO _{2 in} the powder. . For the above reasons, powders having an activation energy of 85 (kJ / mol ) or less are not suitable for actual casting.

Since Li ₂ O is rare, the production cost becomes very high when used in large amounts in powder. Therefore, when actually used for casting, it is desirable to reduce the addition of Li ₂ O as much as possible, and studies have been made on reducing Li ₂ O.
Based on the knowledge obtained when examining the proper composition of Li ₂ O for CaO, SiO ₂ , Na ₂ O, and CaF ₂ under the condition of Li ₂ O <3%, CaO / SiO ₂ and Na ₂ O, were studied proper range of the added amount of CaF _2. As in the case of obtaining the conditions for adding Li ₂ O, the remaining components other than CaO, SiO ₂ , Na ₂ O, and CaF ₂ are each 3 mass% or less, and the total mass of the remaining components is 10 mass% or less. did. It was first organized in relation CaO / SiO ₂ and Na ₂ O and the total amount of CaF _2. As the handling of the F content, F min is treated as CaF _2, Ca content added as CaF ₂ is not included in the CaO content in the case of calculating basicity (CaO / SiO _2).

In FIG. 6, there is a composition that satisfies the required characteristics in the range represented by the following formula (8) and formula (9), as indicated by ◯ when the condition of the formula (5) is satisfied and by x when the condition is not satisfied. As can be seen, even if the expressions (8) and (9) are satisfied, the required characteristics are not always satisfied.
6CaO / SiO ₂ + Na ₂ O + CaF ₂ ≦ 25 (8)
1.85CaO / SiO ₂ + Na ₂ O + CaF ₂ ≧ 15 (9)
However, 0.5 ≦ CaO / SiO ₂ ≦ 1, and CaO (mass%), SiO ₂ (mass%), Na ₂ O (mass%), and CaF ₂ (mass%).

Therefore, when arranging by mass% of Na ₂ O and CaF ₂ within the range of formula (8) and formula (9), in FIG. As shown, it was found that necessary characteristics were obtained in the range of 0.7 ≦ Na ₂ O / CaF ₂ ≦ 1.2. The viscosity on the low temperature side increases when it falls outside the appropriate region shown in FIG. 6, and the viscosity on the high temperature side becomes too low when it falls below the same as in the case of studying addition of Li ₂ O. In addition, the meaning of the upper and lower limits of the appropriate range of CaO / SiO ₂ is the same as that studied when Li ₂ O is added. Compared to Li ₂ O addition, the effect of lowering the activation energy of Na ₂ O and CaF ₂ is small, so a large amount needs to be added. Further, the reason why there is an appropriate addition ratio between Na ₂ O and CaF ₂ is that the CaF ₂ excess side tends to generate SiO ₂ crystals and is difficult to melt. On the contrary, on the Na ₂ O excess side, the activation energy lowering effect is not sufficient and the required characteristics cannot be obtained.
Normally used powder contains C for melt control and heat retention, but is excluded from the mass ratio of the powder in the examples of the present invention. Further, carbonate may be used as a powder raw material, but in this case as well, it is an oxide in a molten state and excludes volatile components.
The powder composition is determined as follows. First, after the powder is completely melted and the carbon content of C and carbonate is completely removed, it is rapidly cooled and vitrified. The vitrified powder is subjected to the X-ray fluorescence method, and the concentration of each element is determined by removing the oxygen content. When the F component exists, it is assumed that it exists as CaF ₂ and the remaining Ca component is calculated as CaO. Assuming that other metal elements and elements such as B are present as oxides, the total of oxides and fluorides is finally calculated to be 100 mass%.

  Next, a cast test was conducted using a prototype powder with an actual continuous caster to verify the effect. Continuous casting is performed using powder having the characteristics shown in Table 1 and molten steel having the components shown in Table 2, and the BO (breakout) detection occurrence rate is used as an index of powder inflow characteristics and the inside of the cold rolled sheet as an index of powder entrainment characteristics Evaluation was performed using defect inspection results (magnetic particle inspection).

The BO detection occurrence rate is the number of times of BO detection reported from the temperature distribution of the thermocouple installed in the mold as the number of times per unit casting length. When there is a position where the powder inflow between the mold and the solidified shell is not uniform or does not flow at all, the solidified shell is seized on the mold and the slab is broken (so-called breakout).
The occurrence rate of BO detection due to non-uniformity of powder inflow is most suitable as an index of powder inflow property, and is one of factors determining the casting speed in actual casting. In a good casting state, the BO detection rate is 0.0001 (1 / m) or less. The internal defect inspection result is a result of defect inspection using the leakage magnetic flux on the cold-rolled steel sheet after hot rolling and cold rolling of the slab. The internal defect flaw detection result is 0.03 (1 / m ² ) or less in a favorable range.

The casting conditions were such that the mold size was 1800 mm × 280 mm, the casting speed was changed, and the casting limit speed due to powder inflow and the casting limit speed due to powder entrainment in each powder were evaluated.
FIG. 8 shows the relationship between the BO detection occurrence rate and the internal defect inspection index with respect to the casting speed. The internal defect inspection index is indexed with 0.03 (1 / m ² ) being 1. Powder A (viscosity at 1300 ° C. is 0.2 Pa · s) is very good in terms of castability with almost no BO detection up to a casting speed of 1.7 m / min, but the casting speed is 1.3 m / min. If it exceeds, it can be seen that the internal defect inspection index greatly exceeds 1.

With powder B (viscosity at 1300 ° C. of 0.4 Pa · s), although the deterioration of the internal defect flaw detection results is not seen in the region where the casting speed is 1.5 m / min or less, the BO detection is performed at a casting speed of 1.3 m / min. The BO detection is increased at a rate not lower than min and is not able to withstand stable casting at 1.7 m / min.
On the other hand, the prototype powder (powder C) showed almost no increase in BO detection at 1.5 m / min or more, and good internal defect flaw detection results were obtained. From this result, it was confirmed that both the powder inflow property and the prevention of entrainment of powder can be achieved by appropriately controlling the temperature dependency of the viscosity described above.

Hereinafter, the results of experiments on the powder for continuous casting according to the present invention will be described in detail.
Continuous casting was performed under the conditions shown in Table 3, and the occurrence rate of BO detection and internal defect inspection results were compared. The form of continuous casting is described below.
First, the molten steel decarburized in the converter is received in a ladle and decarburized using an RH (vacuum degasser). After decarburization, Al was added for deoxidation, and after stirring for a predetermined time, alloys for component adjustment were added. The molten steel whose component adjustment was completed was supplied from a ladle to a tundish, which is an intermediate container, through a refractory nozzle. The casting conditions are a casting width of 1800 mm, a casting thickness of 280 mm, and a casting speed of 1.7 m / min. Table 5 shows the components of the molten steel. Table 5 shows the range of components at each level.
The BO detection occurrence rate shown in Table 4 is the number of BO detections reported from the temperature distribution of the thermocouple installed in the mold as the number per unit casting length. In a good casting state, the BO detection rate is 0.0001 or less. The internal defect inspection results shown in Table 4 are the results of defect inspection of the cold-rolled steel sheet after hot rolling and cold rolling of the slab using leakage magnetic flux. The internal defect flaw detection result is 0.03 (1 / m ² ) or less in a favorable range.
In Table 4, the case where both the BO detection occurrence rate and the internal defect flaw detection result are good is evaluated as “good”, and the case where it is not good is indicated as “poor”.

In Experimental Example 3 (NO. 3 in Tables 3 to 5), the amounts of Na ₂ O and CaF ₂ are appropriate for CaO / SiO ₂ , and thus the viscosity at 1200 ° C. and 1500 ° C. is within a predetermined range. The BO detection occurrence rate and the internal defect flaw detection result are both low and stable.

On the other hand, in Experimental Example 4 (NO. 4 in Tables 3 to 5), the amount of Li ₂ O or the amount of Na ₂ O and CaF _{2 with} respect to CaO / SiO ₂ is not appropriate, so the viscosity at 1200 ° C. is 0.720 Pa · S is high and the BO detection rate is high.
In Experimental Example 5 (No. 5 in Tables 3 to 5), the Li ₂ O amount with respect to CaO / SiO ₂ is not appropriate, so the viscosity at 1500 ° C. is as low as 0.080 Pa · s, and the internal defect flaw detection result. Is high. Similarly, in Experimental Example 6 (NO. 6 in Tables 3 to 5), the viscosity at 1500 ° C. is as low as 0.073 Pa · s, and the internal defect flaw detection result is high.

  As described above, by performing continuous casting using the powder for continuous casting according to the present invention, it is possible to perform high-speed casting with less powder entrainment while suppressing the generation of BO that causes a decrease in productivity.

Claims (2)

In the oxide-based powder added to the molten steel surface in the mold for continuous casting of steel, CaO and SiO ₂ are the main components, the solidification temperature is 1200 ° C or less, and the activation energy
Continuous casting powder characterized in that E ( J / mol ) and viscosity η (Pa · s) at 1300 ° C. satisfy the following formula (1).
-0.5108-0.00000519 × E > ln (η) > -2.303 + 0.00000862 × E (1)
However, E> 85 kJ / mol In the oxide-based powder added to the molten steel surface in the mold for continuous casting of steel, CaO, SiO ₂ with respect to the total amount of oxide and fluoride excluding C and volatile components,
The total amount of Na ₂ O and CaF ₂ is 90 mass% or more, and Al ₂ O ₃ , MgO and Li ₂ O
The powder for continuous casting according to claim 1, wherein each is 3 mass% or less and satisfies the following formula (3).
15-1.85 CaO / SiO ₂ ≦ Na ₂ O + CaF ₂ ≦ 25-6 CaO / SiO ₂
... (3)
However, 0.5 ≦ CaO / SiO ₂ ≦ 1, 0.7 ≦ Na ₂ O / CaF ₂ ≦ 1.2,
_{CaO (mass%), SiO 2} (mass%), Na 2 O (mass%), CaF 2 (
mass%).