Mold powder for continuous casting

Abstract

To provide inexpensive mold powder for continuous casting that enables the production of high-quality cast pieces and the prevention of operational problems when continuously casting high-Al steel. [Solution] A mold powder for continuous casting having a component composition consisting of aggregate carbon, Al2O3, BaO, CaO, Li2O, MgO, a fluorine compound, and the balance being unavoidable impurities, with a total C content of 0.5 to 5.0 mass%, an Al2O3 content of 17 to 25 mass%, a Ba content calculated as BaO of 22 to 30 mass%, a Ca content calculated as CaO of 31 to 41 mass%, a Li content calculated as Li2O of 2.0 to 5.0 mass%, a Mg content calculated as MgO of 0.8 to 2.0 mass%, an F content of 12 to 20 mass%, and a total content of the unavoidable impurities of 2.0 mass% or less.

Description

[Technical Field]

[0001] The present invention relates to a molding powder for continuous casting. [Background technology]

[0002] When continuously casting molten steel using a continuous casting machine, the molten steel is first poured from a ladle through a tundish into a mold, where an initial solidified shell is formed. The molten steel is then cooled in the subsequent secondary cooling zone, where solidification progresses to the interior. During casting, mold powder for continuous casting (hereinafter sometimes referred to as powder) is constantly added to the mold, and the molten powder penetrates between the mold and the solidified shell. During continuous casting, the powder serves to improve lubrication between the solidified shell and the mold and to keep the molten steel warm.

[0003] Generally, powders are primarily composed of CaO and SiO2, with Al2O3, Na2O, fluorine compounds, C, and other ingredients added depending on the desired properties. SiO2 is added to lower the melting point and promote vitrification. However, if the Al concentration in the molten steel is high, a thermite reaction occurs during continuous casting, and oxides in the powder, particularly SiO2, are reduced by the Al in the molten steel. The thermite reaction between SiO2 and Al is expressed by the following equation (1): 4[Al]+3(SiO2)→2(Al2O3)+3[Si]……(1) Here, [Al] and [Si] refer to the components in the molten steel, and (SiO2) and (Al2O3) refer to the components in the powder.

[0004] The thermite reaction increases the Al2O3 concentration of the powder. It is known that an increase in the Al2O3 concentration in the powder can cause various problems. For example, when continuous casting is performed using powder containing CaO and SiO2, an increase in the Al2O3 content leads to the formation of 2CaO·Al2O3·SiO2 (gehlenite), a high-melting point alloy. Gehlenite significantly reduces the lubricity of the powder, increasing the risk of an operational problem called breakout, in which the initial solidification shell breaks during casting and molten steel leaks. Furthermore, as the Al2O3 content increases, the melting point increases, resulting in the formation of sintered powder chunks called bare chunks on the mold. When the bare chunks become large, they push into the initial solidification shell, sometimes resulting in depressions on the slab surface.

[0005] Against this background, various powders have been proposed for continuous casting of high-Al steels, with the aim of producing high-quality cast slabs and preventing operational problems.

[0006] For example, Patent Document 1 proposes a mold powder for continuous casting of steel, characterized by an F content of 16 to 25 mass%, a CaO to SiO2 mass ratio (CaO / SiO2) of 1.0 to 1.8, an Al2O3 content of 5 mass% or less (including zero), and an MgO content of 1.5 mass% or less (including zero). It is said that this powder can prevent restrictive breakouts and slag bear enlargement that are caused by large compositional fluctuations. Here, "compositional fluctuation" refers to a decrease in SiO2 and an increase in Al2O3 in the molten slag.

[0007] Patent Document 2 proposes a mold powder for continuous casting that contains 10-35 wt% CaO, 10-35 wt% Al2O3, 3-15 wt% TiO2, 3-20 wt% Li2O, 5-40 wt% BaO, 15 wt% or less F, and 20 wt% or less Na2O, and further contains one or more of the following aggregates: 0.5-4.0 wt% BN and 0.5-4.0 wt% C, with the remainder consisting of unavoidable impurities. This powder is said to be able to prevent deterioration of slab surface quality and breakouts due to poor lubrication (alteration) of the powder when continuously casting steel containing slag-reducing metal elements.

[0008] Furthermore, Patent Document 3 proposes a mold powder for continuous casting having a composition containing 0.5 to 4.0 mass% C, 10 to 40 mass% CaO, 10 to 40 mass% Al2O3, 3 to 20 mass% Li2O, 5 to 40 mass% BaO, 15 mass% or less of an F compound calculated as F, and the remainder consisting of unavoidable impurities. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-170494 [Patent Document 2] Japanese Patent Application Publication No. 5-185195 [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-33849 Summary of the Invention [Problem to be solved by the invention]

[0010] However, the powder described in the above document has the following problems.

[0011] For example, the powder described in Patent Document 1 contains SiO2, and its use causes a thermite reaction. The thermite reaction increases the Al2O3 concentration in the powder, causing not only the aforementioned problems but also problems due to heat generation. First, the solidification of molten steel is delayed at the meniscus, causing the solidified shell to rupture (bleed) along the oscillation marks, impairing the surface quality of the slab. Furthermore, the generation of flames causes equipment failure and shutdowns.

[0012] Furthermore, the powder described in Patent Document 2 does not contain SiO2, but does contain Na2O and TiO2, and a thermite reaction occurs between these components and Al in the molten steel. As a result, operational problems and deterioration in the quality of the cast slabs were observed along with the increase in heat generation and Al2O3 concentration.

[0013] The powder described in Patent Document 3 not only does not contain SiO2, but also, as a rule, does not contain components that are easily reduced to Al, such as TiO2 and Na2O. Therefore, the thermite reaction rate was lower than that of the powders described in Patent Documents 1 and 2. However, even when using the powder described in Patent Document 3, the thermite reaction could not be sufficiently suppressed. As a result, the composition and physical properties of the mold powder changed significantly during casting, making it impossible to maintain the powder's liquid phase, resulting in operational problems and deterioration of the quality of the cast pieces. Furthermore, due to the recent sharp rise in the price of lithium, the price of mold powders containing Li2O has also risen sharply. Furthermore, increasing the Li2O content to maintain the liquid phase after the composition change resulted in extremely high production costs. [Means for solving the problem]

[0014] The gist of the present invention for solving the above problems is as follows.

[0015] 1. The composition of the aggregate is composed of carbon, Al2O3, BaO, CaO, Li2O, MgO, fluorine compounds, and the balance being unavoidable impurities; Total C content: 0.5~5.0% by mass, Al2O3 content: 17~25% by mass, Ba content in terms of BaO: 22 to 30 mass% Ca content in terms of CaO: 31 to 41 mass% Li content in terms of Li2O: 2.0 to 5.0 mass% Mg content in terms of MgO: 0.8 to 2.0 mass% F content: 12~20% by mass, A mold powder for continuous casting, wherein the total content of the unavoidable impurities is 2.0 mass % or less.

[0016] 2. The mold powder for continuous casting according to 1 above, wherein the component composition further contains a carbonate of at least one element selected from the group consisting of Ba, Ca, Li and Mg.

[0017] 3. The mold powder for continuous casting according to 1 or 2 above, wherein the maximum particle size of the mold powder is 250 μm or less.

[0018] 4. The mold powder for continuous casting according to any one of 1 to 3 above, wherein 30 mass % or more of the mold powder is made of a premelt raw material. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide, at low cost, a mold powder for continuous casting that enables the production of high-quality cast pieces and the prevention of operational problems when continuously casting high-Al steel. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be specifically described below.

[0021] As mentioned above, an increase in the Al2O3 concentration in the powder leads to the formation of gehlenite, which has a high melting point, and the coarsening of bare grains, which impairs operational stability and reduces the surface quality of the cast slab. Therefore, in order to obtain stable productivity and quality in the continuous casting of high-Al steel, it is desirable to keep the increase in the Al2O3 concentration in the powder during casting as small as possible.

[0022] The Al2O3 concentration in the powder increases during casting due to two factors: oxidation of Al in the molten steel by oxides in the powder, and oxidation of Al in the molten steel by the atmosphere. Because atmospheric oxidation is difficult to prevent, it is necessary to minimize the increase in the Al2O3 concentration by preventing oxidation of Al in the molten steel by oxides in the powder. Furthermore, to produce steel without causing operational or quality problems, the powder preparation and steelmaking conditions must be set with consideration given to the fact that the Al2O3 concentration in the powder will increase to some extent during casting.

[0023] As a result of investigations from the above viewpoints, the inventors focused on the solidification start temperature after the powder changes composition during casting, and came to the conclusion that it is preferable to lower the solidification start temperature after the change in composition (hereinafter, the solidification start temperature after the change in composition may be simply referred to as the solidification start temperature), and that to achieve this, the composition of the powder should be controlled.

[0024] Furthermore, the inventors have concluded that in order to stabilize operations and improve the surface quality of cast slabs, the content of oxides with a standard free energy of formation higher than Al2O3 at 1300°C should be minimized. This is due to the following reasons: First, 1300°C corresponds to the average temperature of the molten powder during casting. Oxides such as SiO2, Na2O, TiO2, and Li2O, which have a standard free energy of formation higher than Al2O3 at that temperature, will cause a thermite reaction with Al in the molten steel. This will cause the various adverse effects described above. Therefore, it is necessary to produce powder that contains almost no oxides with a standard free energy of formation higher than Al2O3 at 1300°C.

[0025] In addition, in light of the rising price of Li, it is necessary to control the content of each component under conditions that reduce the LiO content, specifically, under conditions that the Li content is 5.0 mass% or less in terms of LiO.

[0026] The component composition of the powder according to one embodiment of the present invention will be described below. The unit of content "%" refers to "% by mass" unless otherwise specified.

[0027] First, the content of each component will be described. In this specification, the content of each component is a value normalized so that the sum of the content of the metal element converted into an oxide and the content of elements excluding the metal element and O (oxygen) is 100%. The above normalization is applied to the total C content, Al2O3 content, Ba content converted into BaO, Ca content converted into CaO, Li content converted into Li2O, Mg content converted into MgO, F content, and the content of unavoidable impurities.

[0028] For example, in the case of a mold powder whose component elements are C, Al, Ba, Ca, Li, Mg, F, and O, the content is determined as follows: First, the content of each component other than O in 100% by mass of the mold powder (%C', %Al', %Ba', %Ca', %Li', %Mg', %F') is measured by the method described below. Next, the contents of Al, Ba, Ca, Li, and Mg converted into oxides (%Al2O3', %BaO', %CaO', %Li2O', %MgO') are calculated using %Al', %Ba', %Ca', %Li', and %Mg'. Next, a coefficient (100 (mass%) ÷ A (mass%)) is calculated from the total A (mass%) of the oxide-equivalent contents of Al, Ba, Ca, Li, and Mg, the C content, and the F content, and the content of each component is normalized by multiplying the content of each component by the coefficient. Specifically, the content of each component is normalized using the following formula. A=%C´+%Al2O3´+%BaO´+%CaO´+%Li2O´+%MgO´+%F´……(2-1) %C=%C´÷A×100……(2-2) %Al2O3=%Al2O3´÷A×100……(2-3) %BaO=%BaO´÷A×100……(2-4) %CaO=%CaO´÷A×100……(2-5) %Li2O=%Li2O´÷A×100……(2-6) %MgO=%MgO´÷A×100……(2-7) %F=%F´÷A×100……(2-8) Here, %C and %F are the normalized C content and F content, respectively. Also, %Al2O3, %BaO, %CaO, %Li2O, and %MgO are the normalized Al2O3 content, Ba content converted into BaO, Ca content converted into CaO, Li content converted into Li2O, and Mg content converted into MgO, respectively.

[0029] The same applies when the molding powder further contains unavoidable impurities. For example, when the unavoidable impurities are composed of SiO2 and the SiO2 content in 100 mass% of the molding powder is %SiO2', the following formula is used instead of the above formula (2-1). A=%C´+%Al2O3´+%BaO´+%CaO´+%Li2O´+%MgO´+%F´+%SiO2´……(2-9) Furthermore, the content of unavoidable impurities %Im is equal to the normalized SiO2 content (%SiO2) and can be calculated using the following formula: %Im=%SiO2=%SiO2´÷A×100……(2-10)

[0030] Next, the reasons for limiting each component will be explained.

[0031] [Aggregate carbon] The aggregate carbon plays a role in adjusting the melting speed of the powder. Examples of aggregate carbon include, but are not limited to, carbon black and coke powder. The content of aggregate carbon is preferably 0.5 to 3.5%.

[0032] [Total C content: 0.5~5.0%] If the total C content is less than 0.5%, the aggregate will not be effective. From this perspective, the total C content is set to 0.5% or more. On the other hand, if the total C content exceeds 5.0%, the C will burn and generate a flame when a thermite reaction occurs, which can cause equipment failure and operational shutdowns. Therefore, from the perspective of preventing operational problems, the total C content is set to 5.0% or less. Here, the total C content is considered to be the sum of the aggregate carbon content and the C content derived from carbonates, which will be described later. Furthermore, the C derived from carbonates is considered to be the sum of the C derived from carbonates remaining in the premelt raw material, which will be described later, and the C derived from carbonates blended other than the premelt raw material. The total C content is measured using a combustion method.

[0033] [Al2O3: 17-25%] Al2O3 is added to increase the activity of Al2O3 in the molten powder and suppress the oxidation reaction of Al in the molten steel. If the Al2O3 content is less than 17%, this effect cannot be obtained. Therefore, the Al2O3 content is set to 17% or more, preferably 19% or more. The Al2O3 content may be 17.0% or more, or even 19.0% or more. On the other hand, if the Al2O3 content is higher than 25%, the powder solidification start temperature becomes very high, taking into account that the Al2O3 content will further increase during casting. Therefore, the Al2O3 content is set to 25% or less, preferably 23% or less. The Al2O3 content may be 25.0% or less, or even 23.0% or less. The above content is calculated by determining the Al element content by X-ray fluorescence analysis and converting it to Al2O3.

[0034] [BaO] BaO has a lower standard free energy of formation at 1300°C than Al2O3 and hardly reacts with Al in molten steel, which prevents the increase in Al2O3 concentration in the powder during casting. BaO also forms a complex oxide with Al2O3, lowering the solidification start temperature of the powder.

[0035] [Ba content in BaO equivalent: 22-30%] If the Ba content in terms of BaO is less than 22%, the above-mentioned effect of lowering the solidification start temperature of the powder cannot be obtained. Therefore, the Ba content in terms of BaO is set to 22% or more, preferably 24% or more. The content may be 22.0% or more, or may be 24.0% or more. On the other hand, if the Ba content in terms of BaO is higher than 30%, the solidification start temperature will increase. Therefore, the Ba content in terms of BaO is set to 30% or less, preferably 28% or less. The content may be 30.0% or less, or may be 28.0% or less. The content is calculated by determining the content of Ba element by ICP atomic emission spectroscopy and converting it into BaO.

[0036] [CaO] The standard free energy of formation of CaO at 1300°C is lower than that of Al2O3, and it hardly reacts with Al in molten steel, so the increase in the Al2O3 concentration in the powder during casting is suppressed. CaO also forms a complex oxide with Al2O3, lowering the solidification start temperature of the powder.

[0037] [Ca content in CaO equivalent: 31-41%] If the Ca content in terms of CaO is less than 31%, the above-mentioned effects of suppressing the increase in Al2O3 concentration and lowering the solidification start temperature of the powder cannot be obtained. Therefore, the Ca content in terms of CaO is set to 31% or more, preferably 33% or more. The content may be 31.0% or more, or may be 33.0% or more. On the other hand, if the Ca content in terms of CaO is higher than 41%, the solidification start temperature will increase. Therefore, the Ca content in terms of CaO is set to 41% or less, preferably 40% or less. The content may be 41.0% or less, or may be 40.0% or less. The content is calculated by determining the Ca element content by X-ray fluorescence analysis and converting it into CaO.

[0038] [LiO] Since Li2O has a higher standard free energy of formation at 1300°C than Al2O3, it reacts with Al in molten steel to some extent, resulting in a certain increase in the Al2O3 concentration in the powder during casting. However, because Li2O has the effect of lowering the solidification start temperature and viscosity, the powder of the present invention contains Li2O.

[0039] [Li content in terms of Li2O: 2.0-5.0%] If the Li content in terms of Li2O is less than 2.0%, the effect of lowering the solidification start temperature is not achieved. Therefore, the Li content in terms of Li2O is set to 2.0% or more. On the other hand, if the Li content in terms of Li2O exceeds 5.0%, Li2O is easily reduced by Al in the molten steel, resulting in an increase in Al2O3. As a result, the effect of increasing the solidification start temperature due to the increase in Al2O3 exceeds the effect of lowering the solidification start temperature due to the addition of Li2O. Furthermore, reducing the Li content can reduce production costs. Therefore, from the viewpoints of the solidification start temperature and production costs, the Li content in terms of Li2O is set to 5.0% or less, preferably 4.0% or less. From the viewpoint of further reducing production costs, it is more preferably set to less than 3%, and even more preferably 2.9% or less. Note that the above content is calculated by determining the Li element content by ICP atomic emission spectroscopy and converting it to Li2O.

[0040] [MgO] MgO has a lower standard free energy of formation at 1300°C than Al2O3 and hardly reacts with Al in molten steel, which prevents the increase in Al2O3 concentration in the powder during casting. It also lowers the powder's solidification start temperature.

[0041] [Mg content in MgO equivalent: 0.8-2.0%] If the Mg content in terms of MgO is less than 0.8%, the aforementioned effect of lowering the solidification start temperature of the powder cannot be obtained. Therefore, the Mg content in terms of MgO is set to 0.8% or more, preferably 1.0% or more. On the other hand, if the Mg content in terms of MgO exceeds 2.0%, the solidification start temperature will increase. Therefore, the Mg content in terms of MgO is set to 2.0% or less, preferably 1.5% or less. The above content is calculated by determining the Mg element content by X-ray fluorescence analysis and converting it into MgO.

[0042] [Fluorine compounds] The fluorine compound is blended to lower the powder solidification initiation temperature and viscosity (described later). The fluorine compound is not particularly limited, and at least one selected from the group consisting of fluorides, such as CaF, LiF, and BaF, can be used. However, from the viewpoint of cost, it is preferable to use CaF.

[0043] [F content: 12~20%] If the F content is less than 12%, the above-mentioned effects of the fluorine compound cannot be obtained. Therefore, the F content is set to 12% or more, preferably 13% or more, more preferably more than 15%, and even more preferably 16% or more. The F content may be 12.0% or more, 13.0% or more, more than 15.0%, or 16.0% or more. On the other hand, if the F content exceeds 20.0%, not only will the effect saturate, but the risk of oxidation and deterioration of the continuous casting machine will dramatically increase. Therefore, the F content is set to 20% or less. The F content may be 20.0% or less. The F content is measured using absorptiometry.

[0044] [Carbonate of at least one element selected from the group consisting of Ba, Ca, Li, and Mg] The powder may contain a carbonate of at least one element selected from the group consisting of Ba, Ca, Li, and Mg. Even when elements such as Ba, Ca, Li, and Mg are contained as carbonates, the same effect as when oxides of the elements are contained can be obtained. In powder production, the component composition of intermediate raw materials such as premelt raw materials may deviate from the target. In such cases, the content of the elements can be adjusted by mixing the carbonate with the intermediate raw materials.

[0045] When the powder contains carbonate, the content of each element converted to its oxide is calculated as the sum of the content of the oxide of the element and the content of the carbonate of the element converted to its oxide.

[0046] Here, the carbonate may be a carbonate of at least one element selected from the group consisting of Li and Ba, or may be a carbonate of Ba.

[0047] The total content of the carbonates is not limited as long as the total content of each of the elements Ba, Ca, Li, and Mg is sufficient. However, by reducing the total content of the carbonates, the content of other components can be increased and the melting property during casting can be prevented from being affected by carbon dioxide gas. Furthermore, the formation of bare particles can be further suppressed. Therefore, the total content of the carbonates is preferably 20% or less, and more preferably 10% or less, of 100% by mass of the powder. On the other hand, the lower limit of the total content of the carbonates is not particularly limited and may be 0%, meaning that the carbonates may not be present. The total content of the carbonates can be calculated from the total C content and the amount of aggregate carbon added.

[0048] There are no particular limitations on the upper and lower limits of the content of each of BaCO3, CaCO3, Li2CO3 and MgCO3 contained in the carbonates.

[0049] However, from the same viewpoint, the content of BaCO3 is preferably 20% or less of 100% by mass of the powder. Also, the ratio of the content of BaCO3 converted into BaO to the content of Ba converted into BaO is preferably 50% or less. The lower limit of the content of BaCO3 is not limited, and it may be 0%, or BaCO3 may not be contained.

[0050] From the same viewpoint, the content of CaCO3 is preferably 20% or less of 100% by mass of the powder. Also, the ratio of the content of CaCO3 converted into CaO to the content of Ca converted into CaO is preferably 50% or less. The lower limit of the CaCO3 content is not limited, and it may be 0%, or CaCO3 may not be included.

[0051] From the same viewpoint, the content of Li2CO3 is preferably 5% or less of 100% by mass of the powder. Also, the ratio of the content of Li2CO3 converted into Li2O to the content of Li converted into Li2O is preferably 50% or less. The lower limit of the content of Li2CO3 is not limited, and it may be 0%, or Li2CO3 may not be contained.

[0052] From the same viewpoint, the content of MgCO3 is preferably 2% or less of 100% by mass of the powder. Also, the ratio of the content of MgCO3 converted into MgO to the content of Mg converted into MgO is preferably 50% or less. There is no lower limit for the content of MgCO3, and it may be 0%, or MgCO3 may not be contained.

[0053] The component composition of the powder according to one embodiment of the present invention has been described above. The powder according to one embodiment of the present invention contains the above components, with the remainder consisting of unavoidable impurities.

[0054] The unavoidable impurities may also include oxides whose standard free energy of formation at 1300°C is higher than that of Al2O3. Examples of such oxides include SiO2, Na2O, and TiO2. Because these oxides may be unavoidably mixed into the powder raw materials, it is difficult to completely remove them. However, their content must be as low as possible to prevent a thermite reaction with Al in the molten steel. From this perspective, the total content of the unavoidable impurities is set to 2.0% or less. In other words, the total content of aggregate carbon, Al2O3, BaO, CaO, Li2O, MgO, and fluorine compounds is set to 98.0% or more. The lower limit of the total content of the unavoidable impurities is not limited and may be 0%, meaning that the unavoidable impurities may not be present. The total content of the unavoidable impurities can be determined by measuring the contents of elements corresponding to components that may be contained as unavoidable impurities, converting metal elements into oxides, and calculating the total value.

[0055] As described above, the powder according to one embodiment of the present invention has a component composition consisting of aggregate carbon, Al2O3, BaO, CaO, Li2O, MgO, a fluorine compound, and the balance being unavoidable impurities. Total C content: 0.5~5.0% by mass, Al2O3 content: 17.0~25.0% by mass, Ba content in terms of BaO: 22.0 to 30.0 mass% Ca content in terms of CaO: 31.0 to 41.0 mass% Li content in terms of Li2O: 2.0 to 5.0 mass% Mg content in terms of MgO: 0.8 to 2.0 mass% F content: 12.0~20.0% by mass, Total content of the unavoidable impurities: 2.0% by mass or less It is important that

[0056] The component composition further includes It may contain carbonate of at least one element selected from the group consisting of Ba, Ca, Li and Mg.

[0057] A powder according to one embodiment of the present invention can be produced, for example, by mixing raw materials to achieve the aforementioned component composition. The raw materials may comprise aggregate carbon, Al2O3, BaO, CaO, Li2O, MgO, and a fluorine compound. The raw materials may further contain a carbonate of at least one element selected from the group consisting of Ba, Ca, Li, and Mg. Some or all of the raw materials may be premelt raw materials. The particle size of the powder may be adjusted by techniques such as pulverization or classification.

[0058] [Premelt raw materials] Premelt raw material refers to a material obtained by melting and pulverizing raw materials in advance. As mentioned above, carbonates such as Li2CO3 and BaCO3 may be blended as raw materials for powder. However, blending carbonates can generate gas when the powder melts during casting, which can adversely affect the quality of the cast slab, such as pinhole defects and inclusion entrapment defects, or can adversely affect operational aspects, such as melting of the submerged entry nozzle. Premelt raw material is used to reduce these effects and form a stable molten layer. Therefore, in a powder according to one embodiment of the present invention, the powder preferably comprises 30% or more of premelt raw material, and more preferably 50% or more of premelt raw material. The upper limit of the premelt raw material content is not particularly limited and may be 100%. However, considering the addition of aggregate carbon, it is preferable that the powder comprises 99.5% or less of premelt raw material. It is also more preferable that the powder comprises 90% or less of premelt raw material.

[0059] A powder according to one embodiment of the present invention can be produced by blending additional materials with the premelt raw material in order to adjust the powder's component composition, etc. For example, if Al2O3 is insufficient, Al2O3 can be blended. If CaO is insufficient, CaF2 or CaO can be blended. If the F content is insufficient, CaF2 can be blended. If BaO is insufficient, BaCO3 can be blended. The blending amount of BaCO3 is preferably 20% or less of 100% by mass of the powder. If Li2O is insufficient, Li2CO3 can be blended. The blending amount of Li2CO3 is preferably 5% or less of 100% by mass of the powder. If MgO is insufficient, MgCO3 can be blended.

[0060] Here, the aggregate carbon cannot normally be used as a premelt raw material, that is, the ratio of the premelt raw material to the total amount of aggregate carbon may be 0%.

[0061] If carbonate is added after premelt to adjust the composition, the total carbon content increases, reducing the content of other components, and the molten state changes due to the influence of carbon dioxide. This can also lead to the generation of dust and flames. In other words, reducing the amount of carbonate added can further improve operability and quality. Therefore, with regard to carbonate of at least one element selected from the group consisting of Ba, Ca, Li, and Mg, the proportion of the carbonate other than the premelt raw material relative to the total powder mass is preferably 20% or less, more preferably 10% or less. The lower limit of this proportion is not particularly limited and may be 0%.

[0062] [Particle size] The use of finely ground powder facilitates the powder melting during casting, resulting in the formation of a stable molten layer. Therefore, the powder according to one embodiment of the present invention preferably has a maximum particle size of 250 μm or less. Here, a maximum particle size of 250 μm or less refers to a particle size that passes entirely through a 60-mesh (250 μm opening) standard sieve (SIK THE IIDA TESTING SIEVE) specified in JIS Z 8801. In other words, the powder preferably has a particle size that passes entirely through a 60-mesh sieve.

[0063] [Steel manufacturing method] A method for producing steel using the above powder will be described below. The above powder can be suitably used when continuously casting steel containing 0.5% or more of Al.

[0064] In the continuous casting of high-Al steel, the use of the above powder minimizes the occurrence of the thermite reaction, enabling the production of high-Al steel of stable quality with high productivity. Therefore, in the steel production method using the above powder, the steel to be subjected to continuous casting contains 0.5% or more Al, preferably 1.0% or more Al.

[0065] [Solidification start temperature] In continuous casting, lowering the powder solidification temperature during casting is effective in preventing operational problems while producing high-quality slabs. When the solidification temperature rises due to an increase in the Al2O3 concentration in the powder caused by the thermite reaction, the lubrication between the mold and slab is impaired, increasing the risk of operational problems. The increased friction between the mold and slab can also cause cracks in the slab, resulting in a loss of quality.

[0066] Generally, the average temperature of molten powder during casting is about 1300°C. Therefore, it is preferable that the powder solidification start temperature is 1300°C or lower. In other words, it is preferable to control the powder so that it becomes a completely liquid phase at 1300°C during casting.

[0067] [viscosity] Furthermore, to produce high-quality cast slabs, it is desirable to reduce the viscosity of the powder at 1300°C during casting (hereinafter, the viscosity of the powder at 1300°C during casting will be simply referred to as viscosity). This is because lowering the viscosity at 1300°C, which is the average temperature in the molten state, ensures uniform flow when the powder is added, reducing the unevenness of the cast slab surface. From this perspective, it is desirable to control the viscosity to 10 Poise or less. Note that 1 Poise = 0.1 Pa·s.

[0068] The solidification start temperature and viscosity are measured as follows: First, the molten powder is sampled during casting. Since the powder composition is changing in the early stages of casting and the measured values ​​fluctuate, the sample is sampled just before the end of the casting period when the powder composition has reached a steady state.

[0069] Next, the solidification initiation temperature and viscosity are measured. The solidification initiation temperature can be measured using differential thermal analysis. Specifically, a sample is cooled from a molten state at a constant cooling rate, and the differential heat peak is measured using differential thermal analysis. The starting point on the high-temperature side of the peak (peak initiation temperature) is defined as the solidification initiation temperature. The viscosity can also be measured using a rotational viscometer.

[0070] [Powder consumption] In the steel manufacturing method, the powder is added to the meniscus when molten steel, which has been melted to achieve the above-mentioned Al content, is poured from a ladle into a mold via a tundish. After being supplied to the meniscus, the powder melts, penetrates the gap between the mold and the solidified shell, and is withdrawn from the mold along with the slab. During this continuous casting process, the molten steel and the powder come into contact at the meniscus, and the Al in the molten steel reacts with the powder only during this contact. Therefore, if the powder consumption rate is low, the reaction time with the molten steel per unit amount of powder increases, which increases the Al2O3 concentration in the powder and therefore the solidification start temperature. Furthermore, if the powder consumption rate is low, the Al2O3 content, which leads to high viscosity, increases, resulting in a high viscosity at 1300°C.

[0071] Here, the reaction between the powder and molten steel does not reach equilibrium instantly, but changes gradually, and the components in the powder saturate at a certain value lower than the equilibrium value. Here, the balance equation for component i in the powder is expressed as follows using the powder consumption amount: W·(dX i / dt)=Q P ·(X i,0 -X i )+k i ·A·(X i,E -X i )……(3-1) W: Weight of fused powder layer (kg) X i : Concentration of component i in the powder X i,0 : initial concentration of component i in the powder X i,E : Equilibrium concentration of component i between molten steel and powder t: time (s) Q P : Powder consumption (kg / s) A: Reaction area between molten steel and powder (m 2 ) k i : Reaction rate constant of component i (kg / m 2 / s) Here, the concentration of component i no longer changes (dX i / dt=0), the concentration of component i, X i can be expressed as follows: X i =(Q P X i,0 +k i A X i,E ) / (Q P +k i A)……(3-2) From the above formula, powder consumption Q P As X increases, i In other words, if component i is Al2O3, increasing the powder consumption dilutes the amount of Al2O3 that increases during casting, lowering the Al2O3 concentration at saturation, and ultimately achieving a low solidification start temperature and low viscosity.

[0072] Therefore, in order to further reduce the solidification initiation temperature and viscosity and achieve more stable operation, it is advisable to increase the powder consumption.

[0073] The inventors have found that when continuously casting steel containing 0.5% or more Al, if powder is added at a powder consumption rate of less than 0.4 kg per ton of molten steel, the solidification start temperature exceeds 1300°C. On the other hand, by adding powder according to one embodiment of the present invention at a powder consumption rate of 0.4 kg or more per ton of molten steel, the solidification start temperature can be kept at 1300°C or lower and the viscosity can be kept at 10 Poise or lower. This allows for stable operation and the production of cast slabs with no problems in surface quality.

[0074] From this viewpoint, in the steel manufacturing method, it is preferable that the powder consumption amount per ton of molten steel is 0.4 kg or more.

[0075] The method for controlling the powder consumption is not particularly limited. However, since the mold generally vibrates in the casting direction, it is preferable to control the amplitude or frequency of the mold vibration. The powder consumption may also be controlled by adjusting the physical properties of the powder. Specifically, the powder consumption can be controlled by adjusting the viscosity and crystallization temperature. The vibration conditions of the mold, such as the amplitude or frequency, may also be changed depending on the physical properties of the powder. [Example]

[0076] The present invention will now be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and appropriate modifications can be made within the scope of the present invention, and all such modifications are within the technical scope of the present invention.

[0077] First, a mold powder for continuous casting was prepared having the chemical composition shown in Table 1. In preparation, Al2O3, CaO, MgCO3, CaF2, BaCO3, and Li2CO3 were first blended and then melted to prepare a premelt raw material. Next, Al2O3, CaO, CaF2, BaCO3, Li2CO3, and MgCO3 were additionally blended into the premelt raw material as needed, and aggregate carbon was further blended to obtain the chemical composition shown in Table 1. Carbon black was used as the aggregate carbon.

[0078] The total C, Al, Ca, Ba, Li, Mg, and F contents were measured using the methods described above. In Table 1, Al2O3, BaO, CaO, Li2O, and MgO represent the Al2O3 content, Ba content calculated as BaO, Ca content calculated as CaO, Li content calculated as Li2O, and Mg content calculated as MgO, respectively. The total content of unavoidable impurities was calculated as the sum of the oxide-equivalent contents of Si, Na, Ti, Fe, and Mn, plus the S and P contents. Specifically, the contents of Si, Fe, Mn, S, and P were determined by X-ray fluorescence analysis, the Na content by atomic absorption analysis, and the Ti content by ICP atomic emission spectroscopy. The total metal elements were converted into oxides (SiO2, Na2O, TiO2, Fe2O3, and MnO) and calculated. In Table 1, SiO2, Na2O, and TiO2 respectively represent the Si content converted into SiO2, the Na content converted into Na2O, and the Ti content converted into TiO2.

[0079] The proportion of premelt raw material in each of the powders shown in Table 1 was 30 mass % or more of the powder. The maximum particle size of each of the powders shown in Table 1 was 250 μm or less.

[0080] [Table 1]

[0081] Next, molten steel with a C concentration of 0.0040% and an Al concentration of 5% was produced, and continuous casting of slabs with a mold cross-sectional size of 200 mm × 1000 mm was carried out at a casting withdrawal speed of 0.7 m / min. During this process, the powder shown in Table 2 was supplied while adjusting the mold vibration conditions so as to obtain the powder consumption per ton of molten steel shown in Table 2.

[0082] Next, the solidification start temperature and viscosity were measured using the methods described above. Furthermore, as an operational evaluation, the occurrence of smoke and flames during continuous casting was investigated. Additionally, the resulting cast pieces were observed to determine the number of bleeds (pieces / m 2 ) was investigated. Furthermore, the trimming yield was investigated as an evaluation of the quality of the slab. The trimming yield is a value calculated by the following formula (4). Trimming yield (%) = (weight of slab after trimming) / (weight of slab before trimming) × 100 (4) When defects such as depressions occur on the slab surface, the slab surface must be scraped off with a scarf or grinder to remove them, resulting in a decrease in yield. In other words, the higher the slab surface yield, the higher the quality of the slab.

[0083] The results are shown in Table 2. Inventive Examples Nos. 1 to 3, high-quality cast slabs and stable operation were achieved. On the other hand, the comparative examples all had inferior quality or more operational problems. For example, in Comparative Example No. 7, corrosion progressed throughout the continuous casting equipment, making continued use difficult. Furthermore, Comparative Example No. 10 experienced operational problems, such as white smoke and flames emitting from the mold during casting.

[0084] [Table 2]

Claims

1. Aggregate carbon, Al ２ O ３ , BaO, CaO, Li ２ The composition of the composition is O, MgO, a fluorine compound, and the balance is unavoidable impurities, Total C content: 0.5 to 5.0% by mass, Al ２ O ３ Content: 17-25% by mass, Ba content in terms of BaO: 22 to 30 mass% Ca content in terms of CaO: 31 to 41 mass% Li's Li ２ Content in terms of O: 2.0 to 5.0 mass% Mg content in terms of MgO: 0.8 to 2.0 mass% F content: 12 to 20% by mass, A mold powder for continuous casting, wherein the total content of the unavoidable impurities is 2.0% by mass or less.

2. 2. The molding powder for continuous casting according to claim 1, wherein the component composition further contains a carbonate of at least one element selected from the group consisting of Ba, Ca, Li, and Mg.

3. 2. The mold powder for continuous casting according to claim 1, wherein the maximum particle size of said mold powder is 250 μm or less.

4. 3. The mold powder for continuous casting according to claim 2, wherein the maximum particle size of said mold powder is 250 μm or less.

5. 5. The mold powder for continuous casting according to claim 1, wherein 30 mass % or more of the mold powder is made of a premelt raw material.

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