JP6680320B2 - Steel manufacturing method and flux for addition to container containing molten steel - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a steel manufacturing method and a flux for addition to a container containing molten steel for manufacturing steel having a small amount of oxide-based non-metallic inclusions, that is, high cleanliness steel.

  Non-metallic inclusions in steel (hereinafter referred to as "inclusions") not only cause surface defects and deterioration of properties in the final product, but, for example, alumina inclusions adhere to the inner wall of the immersion nozzle. It causes nozzle clogging and hinders operation. In recent years, the cleanliness required for steel materials has become more strict, and the cleanliness has been improved by the improvement of the secondary refining technology and the development of electromagnetic fluidized stirring in a continuous casting mold. On the other hand, even if molten steel with high cleanliness is obtained in the secondary refining stage, the molten steel may be contaminated by the tundish that plays a role of dispensing to the casting mold of continuous casting, and its prevention is highly clean. It is important in the production of steel. Among them, the addition of flux to the molten steel in the tundish not only has the effect of keeping the molten steel warm, but also has the function of blocking the atmosphere from the molten steel and absorbing the inclusions, which is one of the countermeasures against molten steel contamination in the tundish. .

As a tundish flux addition technique for producing such a high-cleanliness steel, Patent Document 1 discloses that an injection flow of molten steel from a ladle to a tundish does not retain an oxidizing force for aluminum in molten steel, and Disclosed is a continuous casting method for steel, which comprises adding a flux that retains the adsorptivity of system inclusions. Then, as a specific flux, CaO, SiO ₂ , and Al ₂ O ₃ are contained in a total amount of 50 to 95 mass%, a fluoride or an alkali oxide is contained in an amount of 5 to 30 mass%, and the ratio of CaO to SiO ₂ is further increased. A flux having a composition of 1.5 or more is described.

In Patent Document 2, in a molten steel surface heat insulating material composed of CaO, MgO and Al, or SiO ₂ , CaO, MgO and Al, the SiO ₂ content is 10 mass% or less, and the MgO content is 10 to 37 mass%. And a molten steel surface heat insulating material having a CaO content / MgO content of 1.5 or more and an Al content / MgO content of 0.2 or more, and the molten steel surface in the tundish A continuous casting method for steel is described, characterized in that it is added above.

In Patent Document 3, a low melting point heat insulating material having a melting temperature of 1500 ° C. or less is added as a lower layer side heat insulating material to a molten steel surface in a tundish for continuous casting, and a high melting point heat insulating material having a melting temperature of more than 1580 ° C. It is a method of adding as an upper layer side heat insulating material and coating the molten steel surface with a two-layer structure, wherein the composition of the lower layer side heat insulating material has a CaO / Al ₂ O ₃ mass ratio of 1.1 to 2.0, Further, the MgO content is 10% by mass or more and less than 30% by mass, the SiO ₂ content is 5% by mass or less, and the composition of the upper layer side heat insulating material has a CaCO ₃ content of 60% by mass or more. A characteristic steel continuous casting method is described.

In Non-Patent Document 1, the presence of the alkali oxide K ₂ O in the CaO—SiO ₂ —Al ₂ O ₃ -based slag forms an aluminosilicate network due to the charge compensation effect of K ⁺ for AlO ₄ . It shows that the slag viscosity is improved. Further, Non-Patent Document 2, the alkali oxide K ₂ O addition to the _{CaO-SiO 2 -Al 2 O 3} -MgO based tundish flux improves the scavenging of the inclusions, the inclusions decrease in the molten steel It shows that it contributes.

JP 2001-286999 A Japanese Patent No. 4750985 Japanese Patent No. 4441425

S. Sukenaga, N. Saito, K. Kawakami and K. Nakashima: ISIJ international, 46 (2006), p.352-358. J. Y. Yu, Y. Kang and I. Sohn: Metallurgical and Materials Transactions B, 45 (2014), p.113-122.

However, the above prior art has the following problems. That is, in Patent Document 1, since the flux that retains the alumina adsorbability is directly added to the molten steel flow, it may become a serious defect when the flux flows out to the mold due to the short-circuit flow. The ratio of CaO to SiO ₂ (basicity) is specified to be 1.5 or more, but in high cleanliness steel, the Al concentration in steel is often 0.01 mass% or more, and when the basicity is as low as 1.5, Causes SiO ₂ to be reduced by Al in the molten steel, increasing the oxygen potential in the molten steel and causing reoxidation.

  Patent Document 2 defines a continuous casting method by adding a CaO-based flux containing MgO and Al to a tundish, but if the amount of MgO added is too large, the viscosity and interfacial tension of the slag decrease, Since the tundish slag is easily caught, it may contaminate the molten steel.

  Patent Document 3 discloses a continuous casting method of steel in which heat insulating materials having different melting points are added in two layers, but as described in Patent Document 2, the MgO concentration in the flux used for the lower layer is 10 to It is as high as 30% by mass, and there is a concern that it may be easily caught.

In Non-Patent Document 1, the flux having the composition described is for investigating the influence of alkali oxides on the slag viscosity, and is not intended to absorb inclusions. Further, since the basicity is 0.67 to 1.22, even if it is used as a tundish flux, SiO ₂ is reduced by Al in molten steel by reacting with molten steel having an Al concentration of 0.01 mass% or more, which causes reoxidation. Becomes

Furthermore, in Non-Patent Document 2, it is reported that the absorption of Al ₂ O ₃ inclusions is improved by adding K ₂ O, which is an alkali oxide, to CaO—SiO ₂ —Al ₂ O ₃ —MgO based flux. As shown from the result of the lab test, the flux (CaO) / (Al ₂ O ₃ ) to which K ₂ O is added has a high value of 1.65. Therefore, the composition of this flux is not a complete liquid phase composition but a solid phase saturation composition in the molten steel temperature range in the tundish, which is disadvantageous in absorbing inclusions as compared with the liquid phase composition, and is poor in slag formation. Therefore, the molten steel surface may be exposed and reoxidized.

  Therefore, in view of the above problems, the present invention is capable of producing a highly clean steel that suppresses the oxidation of molten steel and further reduces oxide-based non-metallic inclusions in the molten steel, and a method for producing the steel and the molten steel. It is an object of the present invention to provide a flux for addition to a container accommodating.

The gist of the present invention is as follows.
[1] CaO, SiO ₂ , Al ₂ O ₃ , and R ₂ O (one or more selected from R = K, Rb, and Cs) as a main component, and the total amount is 85 mass% or more, Furthermore, (R ₂ O): 5.0 mass% or more, (MgO): 5.0 mass% or less, (T.Fe) + (MnO): 1.0 mass% or less, (CaO) / (SiO ₂ ): 4.0 or more, ( CaO) / (Al ₂ O ₃ ): 0.5 to 1.5, a method for producing a steel, characterized in that a flux satisfying a viscosity of 1.1 poise or more at 1550 ° C. is added onto the molten steel contained in a storage container.

  [2] The method for producing steel according to [1], wherein the storage container is a tundish for a continuous casting machine.

  [3] The method for producing steel according to the above [1] or [2], wherein a heat insulating material containing carbon is added onto the flux so that the upper layer has a heat insulating material and the lower layer has a double layer of flux.

  [4] In the continuous casting, an inert gas is blown into the molten steel in the tundish and the upper portion of the slag to prevent reoxidation of molten steel from before the first charge of molten steel is injected into the tundish until the casting of the final charge is completed. The method for manufacturing steel according to.

  [5] The acid-soluble Al concentration in the molten steel at the time of adding the flux is 0.001% by mass or more and 0.1% by mass or less, according to any one of [1] to [4] above. Steel manufacturing method.

[6] CaO, SiO ₂ , Al ₂ O ₃ , and R ₂ O (one or more selected from R = K, Rb, and Cs) as a main component, and the total amount thereof is 85% by mass or more, Furthermore, (R ₂ O): 5.0 mass% or more, (MgO): 5.0 mass% or less, (T.Fe) + (MnO): 1.0 mass% or less, (CaO) / (SiO ₂ ): 4.0 or more, ( CaO) / (Al ₂ O ₃ ): 0.5 to 1.5, a flux at 1550 ° C. that satisfies 1.1 poise or more, a flux for addition to a container containing molten steel.

  Use of the method for producing steel of the present invention and the flux for addition to the container containing molten steel suppresses the oxidation of molten steel and produces high-cleanliness steel in which oxide nonmetallic inclusions in molten steel are further reduced. can do.

The present inventors investigated various methods for ensuring molten steel cleanliness in a tundish, and in the tundish from CaO, SiO ₂ , Al ₂ O ₃ , and R ₂ O (R = K, Rb, and Cs It has been found that by adding a flux containing one or more selected main components) to molten steel, reoxidation is suppressed, inclusion absorbability is improved, and steel with high cleanliness can be obtained. . That is, by adding an alkali oxide R ₂ O (one or more selected from R = K, Rb, and Cs) to CaO-SiO ₂ -Al ₂ O ₃ -based slag, the alkali metal ion R causes slag in the slag. (AlO ₄ ) ^5- has a +1 charge compensation, and by polymerization with (SiO ₄ ) ^4-an aluminosilicate network is formed, and due to this polymerization promoting effect, selective Al ₂ O ₃ -based inclusions are formed. Can be expected to be adsorbed, and the inclusion trapping property is improved because the slag viscosity is improved. At this time, as the alkali oxide, it is desirable to add at least one of K ₂ O, Rb ₂ O, and Cs ₂ O. If the total is 5.0% by mass or more, two or more kinds may be used in combination. If the R ₂ O concentration is lower than 5.0% by mass, a sufficient charge compensation effect cannot be obtained, so 5.0% by mass or more must be added. The upper limit of the R ₂ O concentration is not particularly limited, but an excess of 30% by mass or more that is not used for charge compensation of Al ₂ O ₃ is generated. Therefore, it is preferably 30% by mass or less. Further, the more preferable range of the R ₂ O concentration is 10 to 30 mass%.

The total amount of CaO, SiO ₂ , Al ₂ O ₃ , and R ₂ O is 85% by mass or more and 100% by mass or less. If the total amount is less than 85% by mass, the expected effect cannot be obtained because the absolute amount is small even if the target composition ratio is satisfied.Furthermore, if the amount of impurities is large, reoxidation or inhibition of aluminosilicate network formation occurs. Is concerned. Impurities can be allowed up to 15% by mass in total, and halides and the like may be contained in addition to oxides. Regarding the oxide in the impurities, from the viewpoint of preventing reoxidation, if the oxide has the standard free energy of formation of Al ₂ O ₃ equal to or less than the standard free energy of Al ₂ O ₃ at the molten steel temperature (ZrO ₂ , BaO, SrO, etc.), The expected effect can be obtained without inhibiting the effect of the present invention. For oxides whose standard free energy of formation is higher than Al ₂ O ₃ (excluding the aforementioned SiO ₂ , Fe oxide, MnO, Na ₂ O, Li ₂ O, K ₂ O, Rb ₂ O and Cs ₂ O) Since it may be reduced by Al in molten steel and cause reoxidation, it is desirable that the total amount is 1 mass% or less. Further, when a halide such as CaF ₂ is contained, it causes a decrease in slag viscosity, so the total content is preferably 1% by mass or less. As described above, the total amount of CaO, SiO ₂ , Al ₂ O ₃ , and R ₂ O is 85 mass% or more, but the higher the proportion of these components in the flux, the more the effect of the present invention. Since it is easily expressed, it is preferably 90% by mass or more, more preferably 95% by mass or more.

  If the MgO concentration in the flux is higher than 5.0% by mass, the viscosity of the slag and the interfacial tension between the molten steel and the slag will decrease, and the slag entrainment property will increase, so it is made 5.0% by mass or less. The lower the MgO concentration, the better, and the lower limit is not particularly limited.

  If the total concentration of the lower oxides T.Fe and MnO in the flux is larger than 1.0% by mass, the reoxidation property of Al in the molten steel increases, so the content must be 1.0% by mass or less. The lower the total concentration of T.Fe and MnO, the better, and the lower limit is not particularly limited.

When (CaO) / (SiO ₂ ) is less than 4.0, in general Al-killed steel, the reaction of the following formula (1) proceeds and reoxidation of molten steel is promoted, so it is necessary to secure 4.0 or more. is there. Here, the oxides in parentheses () represent the components in the flux, and the brackets [] represent the components in the molten steel. The upper limit of (CaO) / (SiO ₂ ) is not particularly limited, but the presence of the molten silicate formed from the three-dimensional network in the slag is considered to promote the formation of the aluminosilicate network, and the SiO in the flux is ₂ is preferably 3.0% by mass or more, that is, the upper limit of (CaO) / (SiO ₂ ) is preferably 18.
3 (SiO ₂ ) +4 [Al] = 3 [Si] +2 (Al ₂ O ₃ ) ・ ・ ・ (1)

Further, when (CaO) / (Al ₂ O ₃ ) is smaller than 0.5 or larger than 1.5, the solid phase is precipitated at the temperature of the tundish molten steel, the liquid phase ratio is reduced, and the absorption of inclusions is disadvantageous. , (CaO) / (Al ₂ O ₃ ): in the range of 0.5 to 1.5 Also, in the range of (CaO) / (Al ₂ O ₃ ): 1.0 to 1.5, the melting point of the flux can be lowered to about 1400 ° C or less. Therefore, from the initial stage of addition, absorption of inclusions and prevention of atmospheric oxidation due to good slag formation can be expected. Further, since the saturation concentration of Al ₂ O ₃ and the initial flux during the Al ₂ O ₃ difference in density ΔAl ₂ O ₃ in the flux tundish molten steel temperature can be secured more than 10 wt%, even overlapping the communicating people casting, inclusions flux The absorption effect can be sustained. The more preferable range of (CaO) / (Al ₂ O ₃ ) is 1.0 to 1.5.

The flux component concentration specified in the present invention is a value obtained when the flux obtained by uniformly mixing the raw materials is analyzed by ICP emission spectroscopy (high frequency inductively coupled plasma emission spectroscopy). In addition, when carbonate is used as a raw material for CaO or R ₂ O, or when water is contained to some extent, Ig.loss, that is, loss on ignition exists in the flux, but it is removed at the molten steel temperature. Therefore, the scope of the present invention is not considered. That is, when calculating the mass concentration of each component, the mass of Ig.loss is subtracted from the flux mass of the denominator.

Further, if the slag viscosity at 1550 ° C. is lower than 1.1 poise, the ability to trap inclusions cannot be expected, and the effect of adding R ₂ O cannot be obtained sufficiently. 1550 ℃ refers to the molten steel temperature in the tundish during casting, and by setting the viscosity at that temperature to 1.1 poise or more, the flux in the tundish is entrained by the molten steel flow in the tundish and non-metallic inclusions are involved. It is possible to suppress the occurrence of the above-mentioned problem and to secure the inclusion trapping property in the flux, and it is possible to manufacture a steel having a higher cleanliness. Therefore, the flux of the present disclosure has a viscosity at 1550 ° C. of 1.1 poise or more. The upper limit of the viscosity is not particularly limited as long as the flux can sufficiently secure the liquid phase.

  The viscosity of the flux shall be measured with a coaxial double cylinder rotational viscometer. Specifically, the flux filled in the platinum-based crucible installed in the resistance heating furnace is melted and held at 1550 ° C., and the viscosity is obtained from the viscous torque obtained by immersing and rotating the platinum-based rod from above.

  The flux of the present disclosure is preferably added when the molten steel in the tundish has been poured up to the first charge of continuous casting. If the flux is added before the time when it is full, the cleanliness of the molten steel may be deteriorated by being caught in the injection flow or the like. Also, it is preferable to add the total amount of flux to the first charge as much as possible, and if added in the middle of successive ones, the flux may be entrained in the molten steel due to the impact of the addition, which may deteriorate the cleanliness in the molten steel. .

  According to the above-described flux of the present disclosure, generation of inclusions due to reoxidation in molten steel is suppressed, and high inclusion absorbability is exhibited when added to molten steel. This makes it possible to manufacture steel having extremely high cleanliness.

Further, the present inventors have found that the cleanliness of molten steel in the tundish can be improved by adding the heat insulating material containing carbon onto the tundish flux described above. By adding the heat insulating material onto the tundish flux, the temperature drop of the added flux can be prevented and the liquid phase can be secured. As a result, the effect of absorbing the inclusions of the flux can be maintained and the temperature decrease of the molten steel can be suppressed at the same time, so that the deterioration of the inclusion floating property can be prevented. Further, by containing carbon in the heat insulating material, it reacts with oxygen in the atmosphere in the tundish to generate gas, so that the oxygen concentration in the atmosphere can be reduced, and further, the molten steel and the oxygen in the atmosphere can be reduced. Since the reaction can be suppressed, the generation of Al ₂ O ₃ -based inclusions due to reoxidation can be reduced. At this time, if the carbon content is less than 30% by mass, oxygen in the atmosphere cannot be sufficiently combusted, so in order to obtain the above effect, the content is 30% by mass or more like burned rice. Is desirable.

In addition, by blowing an inert gas directly into the molten steel in the tundish and the upper part of the slab from before the molten steel is injected into the tundish to the completion of casting of the final charge, the oxygen concentration in the atmosphere can be reduced, and Al ₂ It was found that the formation of O ₃ -based inclusions can be suppressed. Tundish flux and heat insulation material should not be added because there is a concern that they may be entrained by the molten steel injection flow in the unsteady part before the tundish is full or when the pan is replaced. Therefore, at that time, an inert gas is blown into the tundish in advance to replace the atmosphere, and by preventing the inflow to the atmosphere, reoxidation of the molten steel can be prevented. Further, even during steady casting, by blowing an inert gas into the molten steel and the upper part of the slag, the inflow to the atmosphere can be reduced and the effect of suppressing reoxidation can be expected, and the combustion reaction of the carbon content in the heat insulating material can also be suppressed, The effect of suppressing oxidation can be sustained. Argon gas can be preferably used as the type of the inert gas, and nitrogen gas can be used as long as it is a steel type that does not cause a problem in picking up nitrogen.

The steel manufacturing method of the present invention is preferably applied to steel having an Al concentration of 0.001 to 0.1 mass%. When Al is less than 0.001 mass%, deoxidation products other than the Al ₂ O ₃ system are stable, and the cleaning effect of the present invention is small. When the Al concentration is higher than 0.1% by mass, SiO ₂ in the tundish slag is reduced by Al in the molten steel, and reoxidation occurs, so that Al ₂ O ₃ -based inclusions are generated. Therefore, the steel manufacturing method of the present invention is preferably applied to steel having an Al concentration of 0.001 to 0.1 mass%.

(Example 1)
Converter-RH vacuum degassing furnace-In the process of continuous casting, when casting 240 tons of 1-charge low-carbon aluminum killed molten steel with a carbon concentration of 0.04 mass% for 4 consecutive charges, the molten steel is poured to the tundish at the first charge. Then, the tundish flux having the composition and viscosity shown in Table 1 was added onto the molten steel in the tundish to produce a slab cast piece having a thickness of 230 mm and a width of 1200 mm. This slab was hot-rolled and cold-rolled to finally obtain a cold-rolled steel sheet having a thickness of 2.0 mm and a width of 1200 mm.

  At the end of the RH vacuum degassing treatment and in the tundish, molten steel samples were sampled, the total oxygen concentration in the steel was measured, and the change Δ [T.O.] Was measured. Further, the acid-soluble Al concentration [sol. Al] in the molten steel sample was measured. In the cold-rolled steel sheet, the number of inclusions per unit area was measured by the leakage magnetic flux flaw detection method. Table 1 shows changes in the total oxygen content in steel Δ [TO], acid-soluble Al concentration [sol. Al], and cold-rolled steel sheet in the tundish from the end of the RH vacuum degassing treatment in Examples of the present invention and Comparative Examples. The number density of inclusions is shown.

In the examples of the present invention, the change in the total oxygen content in the steel from RH to the tundish was about 0.0008 mass% or less, and the number density of inclusions in the steel sheet was a low level of 0.009 pieces / m ² or less. In addition, in the present invention example 1, the total concentration of CaO, SiO ₂ , Al ₂ O ₃ , and K ₂ O was as low as 86.2% by mass, but other components except MgO, T.Fe, and MnO were excluded. In addition, mainly BaO + SrO = 10 mass% and Na ₂ O = 1 mass%. On the other hand, in a comparative example in which the composition of the tundish flux does not satisfy the conditions of the present invention, the change in the total oxygen content in the steel in the tundish from RH is about 0.0011 mass% or more, and the inclusion number density in the steel sheet is also It was as high as 0.013 pieces / m ² or more. For Comparative Example 2, the total concentration of CaO, SiO ₂ , Al ₂ O ₃ and K ₂ O was as low as 80.7 mass%, but other components were excluded from MgO, T.Fe, and MnO. Mainly, BaO + SrO = 4.1 mass% and Na ₂ O + Li ₂ O = 8.9 mass%.

(Example 2)
Similar to Example 1, a slab slab was manufactured in the same manner as in Example 1 in the steps of converter-RH vacuum degassing furnace-continuous casting using an actual machine having a molten steel amount of about 240 tons per charge. Further, it was subjected to hot rolling and cold rolling to manufacture a cold rolled steel sheet having a thickness of 2.0 mm and a width of 1200 mm. At this time, in the tundish, the tundish flux having the composition and the viscosity shown in Table 2 was added, and the paddy rice having a carbon content of 50 mass% was added as a heat retaining material containing carbon onto the flux. In the same manner as in Example 1, Δ [TO], [sol. Al], and the number density of inclusions in the cold rolled steel sheet were determined. Table 2 shows the results.

In the present invention example, in the case of adding heat insulating material containing carbon, not more than about 0.0002 mass percent change in steel total oxygen amount in the tundish from RH, even inclusions number density in the steel sheet 0.003 pieces / m ² It was extremely low as below, and good cleanliness was obtained as compared with the case where no heat insulating material was used.

(Example 3)
Similar to Example 1, a slab slab was manufactured in the same manner as in Example 1 in the steps of converter-RH vacuum degassing furnace-continuous casting using an actual machine having a molten steel amount of about 240 tons per charge. Further, it was subjected to hot rolling and cold rolling to manufacture a cold rolled steel sheet having a thickness of 2.0 mm and a width of 1200 mm. At this time, in the tundish, the tundish flux having the composition and viscosity shown in Table 3 was added, and the burned rice with a carbon content of 50% by mass was added as a heat retaining material containing carbon onto the flux, and the first charge was performed. Ar gas was blown into the molten steel in the tundish and the upper part of the slag from before the molten steel was injected until the casting of the final charge was completed. In the same manner as in Example 1, Δ [TO], [sol. Al], and the number density of inclusions in the cold rolled steel sheet were determined. The results are shown in Table 3.

In the present invention example, when blowing Ar gas during casting into the tundish during casting, the total oxygen content in the steel in the tundish from RH is reduced by 0.0001 to 0.0002 mass% and reoxidation is suppressed, and in the steel sheet. The number density of inclusions was also very good at 0.001 particles / m ² .

  Use of the method for producing steel of the present invention and the flux for addition to the container containing molten steel suppresses the oxidation of molten steel and produces high-cleanliness steel in which oxide nonmetallic inclusions in molten steel are further reduced. can do.

Claims (6)

CaO, SiO ₂ , Al ₂ O ₃ , and R ₂ O (one or more selected from R = K, Rb, and Cs) as a main component, the total amount of which is 85% by mass or more, and ( R ₂ O): 5.0 mass% or more, (MgO): 5.0 mass% or less, (T.Fe) + (MnO): 1.0 mass% or less, (CaO) / (SiO ₂ ): 4.0 or more, (CaO) / (Al ₂ O ₃ ): A method for producing steel, characterized in that a flux satisfying a viscosity of 1.1 poise or higher at 0.5 to 1.5 and 1550 ° C. is added onto the molten steel contained in the storage container.   The method for manufacturing steel according to claim 1, wherein the storage container is a tundish of a continuous casting machine.   The method for producing steel according to claim 1, wherein a heat insulating material containing carbon is added onto the flux so that the upper layer has a heat insulating material and the lower layer has a two-layered structure of flux.   The steel according to claim 2, wherein in the continuous casting, an inert gas is blown into the molten steel in the tundish and the upper portion of the slag to suppress re-oxidation of molten steel from before the first charge of molten steel is injected into the tundish to the completion of casting of the final charge. Manufacturing method.   The method for producing steel according to any one of claims 1 to 4, wherein the acid-soluble Al concentration in the molten steel when the flux is added is 0.001 mass% or more and 0.1 mass% or less. CaO, SiO ₂ , Al ₂ O ₃ , and R ₂ O (one or more selected from R = K, Rb, and Cs) as a main component, the total amount of which is 85% by mass or more, and ( R ₂ O): 5.0 mass% or more, (MgO): 5.0 mass% or less, (T.Fe) + (MnO): 1.0 mass% or less, (CaO) / (SiO ₂ ): 4.0 or more, (CaO) / (Al ₂ O ₃ ): 0.5 to 1.5, a viscosity at 1550 ° C. of 1.1 poise or more, a flux for addition to a container containing molten steel.