Continuous casting mold powder

Abstract

Provided is a low-cost continuous casting mold powder with which it is possible to produce a high-quality cast slab and to prevent operational trouble in the continuous casting of Al-rich steel. This continuous casting mold powder has a component composition comprising aggregate carbon, Al2O3, BaO, CaO, Li2O, MgO, and a fluorine compound, with the balance consisting of unavoidable impurities, wherein: the total C content is 0.5-5.0 mass%; the Al2O3 content is 17-25 mass%; the Ba content in terms of BaO is 22-30 mass%; the Ca content in terms of CaO is 31-41 mass%; the Li content in terms of Li2O is 2.0-5.0 mass%; the Mg content in terms of MgO is 0.8-2.0 mass%; the F content is 12-20 mass%; and the total content of the unavoidable impurities is 2.0 mass% or less.

Description

Mold powder for continuous casting

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

[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 consist of CaO and SiO ２ The main component is Al, depending on the required properties. ２ O ３ , Na ２ Components such as O, fluorine compounds, and C are blended. ２ However, if the Al concentration in the molten steel is high, a thermite reaction occurs during continuous casting, and the oxides in the powder, especially SiO ２ etc. are reduced by Al in the molten steel. Here, SiO ２ The thermite reaction between Al and SiO is expressed by the following formula (1): 4[Al] + 3(SiO ２ ) → 2 (Al ２ O ３ ) + 3 [Si] (1) where [Al] and [Si] are components in molten steel, (SiO ２ ) and (Al ２ O ３ ) refers to the ingredients in the powder.

[0004] The thermite reaction causes the powder Al ２ O ３ The concentration of Al in the powder increases. ２ O ３ It is known that increasing the concentration of CaO and SiO can cause various problems. ２ When continuous casting is performed using powder containing Al２ O ３ By increasing the amount, the high melting point 2CaO.Al ２ O ３ SiO ２ Gehlenite (Gehlenite) is generated. Gehlenite significantly deteriorates the lubricity of the powder, increasing the risk of an operational problem called breakout, in which the initial solidified shell breaks during casting and molten steel leaks out. ２ O ３ As the content increases, the melting point rises, and lumps of sintered powder called bare particles are formed on the mold. When the bare particles become large, they push into the initial solidification shell, which can cause depressions on the surface of the slab.

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

[0006] For example, in Patent Document 1, the amount of F is 16 to 25 mass %, CaO and SiO ２ The mass ratio (CaO / SiO ２ ) is 1.0 to 1.8, Al ２ O ３ A mold powder for continuous casting of steel has been proposed, characterized in that the amount of SiO in the molten slag is 5% by mass or less (including zero) and the amount of MgO is 1.5% by mass or less (including zero). This powder is said to be able to prevent restrictive breakouts and the enlargement of slag bears caused by large compositional fluctuations. Here, the compositional fluctuations refer to the amount of SiO in the molten slag. ２ decreases, and Al ２ O ３ This means that the number of people who have a disability increases.

[0007] In addition, Patent Document 2 discloses a composition containing CaO: 10 to 35 wt %, Al ２ O ３ :10~35wt%, TiO ２ :3~15wt%, Li ２ O: 3 to 20 wt%, BaO: 5 to 40 wt%, F: 15 wt% or less, Na ２A mold powder for continuous casting has been proposed, which contains 20 wt% or less O, and further contains one or more of 0.5 to 4.0 wt% BN and 0.5 to 4.0 wt% C as aggregates, with the remainder consisting of unavoidable impurities. This powder is said to be able to prevent deterioration of the surface quality of cast slabs and breakouts caused by poor lubrication (alteration) of the powder when continuously casting steel containing slag-reducing metal elements.

[0008] Furthermore, Patent Document 3 discloses a composition containing 0.5 to 4.0 mass % of C, 10 to 40 mass % of CaO, and Al. ２ O ３ 10 to 40 mass % of Li ２ A mold powder for continuous casting has been proposed, which has a composition containing 3 to 20 mass % of O, 5 to 40 mass % of BaO, 15 mass % or less of an F compound calculated as F, and the remainder consisting of unavoidable impurities.

[0009] JP 2017-170494 A JP 5-185195 A JP 2003-33849 A

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

[0011] For example, the powder described in Patent Document 1 is SiO ２ The thermite reaction occurs when the powder is used. ２ O ３ In addition to the adverse effects of the increased concentration mentioned above, heat generation also caused adverse effects. First, the solidification of molten steel was delayed at the meniscus, causing the solidified shell to break (bleed) along the oscillation marks, which impaired the surface quality of the cast slab. Furthermore, the occurrence of flames caused equipment failure and shutdowns.

[0012] In addition, the powder described in Patent Document 2 contains SiO ２ does not contain Na ２ O and TiO ２ The thermite reaction was occurring between these components and the Al in the molten steel. ２ O３ As the concentration increased, operational problems and deterioration in the quality of the slab were observed.

[0013] The powder described in Patent Document 3 is SiO ２ Not only does it not contain TiO ２ , Na ２ Since it does not contain, as a rule, components such as O that are easily reduced to Al, the amount of thermite reaction that occurs is smaller than that of the powders described in Patent Documents 1 and 2. However, even when the powder described in Patent Document 3 is used, the progress of thermite reaction could not be sufficiently suppressed. As a result, the components and physical properties of the mold powder change significantly during casting, and the powder cannot be maintained in a liquid phase state, resulting in operational troubles and deterioration in the quality of the cast pieces. In addition, the price of Li has recently risen sharply, so the use of Li ２ The price of mold powder containing O is also rising. ２ If the O content is increased, the production cost becomes extremely high.

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

[0015] 1. Aggregate carbon, Al ２ O ３ , BaO, CaO, Li ２ The composition of the alloy is O, MgO, a fluorine compound, and the balance is unavoidable impurities, and the total C content is 0.5 to 5.0 mass %. ２ O ３ Content: 17 to 25 mass%, Ba content in terms of BaO: 22 to 30 mass%, Ca content in terms of CaO: 31 to 41 mass%, Li content ２ A mold powder for continuous casting, comprising: an O content of 2.0 to 5.0 mass%, an Mg content 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.

[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.

[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.

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

[0021] As mentioned above, the Al powder ２ O ３ If the Al concentration is increased, the formation of gehlenite with a high melting point and the coarsening of bare grains occur, which impairs the stability of the operation 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 necessary to reduce the Al concentration of the powder during casting. ２ O ３ A small increase in concentration is desirable.

[0022] Al in the powder during casting ２ O ３ The reasons for the increase in concentration include oxidation of Al in the molten steel by oxides in the powder and oxidation of Al in the molten steel by the atmosphere. ２ O ３ In order to minimize the increase in Al concentration, it is necessary to prevent oxidation of Al in molten steel by oxides in the powder as much as possible. ２ O ３ The powder preparation and steel manufacturing conditions must be set taking into consideration that the concentration increases 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 undergoes a chemical change during casting, and concluded that it is preferable to lower the solidification start temperature after the chemical change (hereinafter, the solidification start temperature after the chemical change may be simply referred to as the solidification start temperature), and that to achieve this, the chemical composition of the powder should be controlled.

[0024] Furthermore, the present inventors have found that in order to stabilize the operation and improve the surface quality of the cast slab, it is necessary to make the standard free energy of formation at 1300°C equal to Al. ２ O ３ The conclusion was reached that the content of oxides higher than 1300°C should be minimized as much as possible. This is for the following reasons: First, 1300°C corresponds to the average temperature of the molten powder during casting. And, the standard free energy of formation at this temperature is Al ２ O ３ Higher SiO ２ , Na ２ O, TiO ２ and Li ２ Oxides such as O cause a thermite reaction with Al in the molten steel, which causes the various adverse effects mentioned above. Therefore, the standard free energy of formation at 1300°C is ２ O ３ It is necessary to produce powders that contain very few oxides higher than 100%.

[0025] In addition, in light of the rising price of Li, ２ The conditions under which the content of O was reduced, specifically, Li was replaced with Li ２ It is necessary to control the content of each component so that it is 5.0 mass % or less in terms of O.

[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 normalized value such 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%. ２ O ３ Content, Ba content in BaO conversion, Ca content in CaO conversion, Li content ２ The above-mentioned normalization is applied to the content calculated as O, the content of Mg calculated as MgO, the 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 contents are determined as follows. First, the contents of each component other than O in 100 mass% of the mold powder (%C', %Al', %Ba', %Ca', %Li', %Mg', %F') are measured by the method described below. Next, the contents of Al, Ba, Ca, Li, and Mg converted into oxides (%Al ２ O ３ ', %BaO', %CaO', %Li ２ Next, a coefficient (100 (mass%) ÷ A (mass%)) is calculated from the total A (mass%) of the contents of Al, Ba, Ca, Li, and Mg converted into oxides, 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 by the following formula: A = %C' + %Al ２ O ３ '+%BaO'+%CaO'+%Li ２ O'+%MgO'+%F'...(2-1) %C=%C'÷A×100...(2-2) %Al ２ O ３ = % Al ２ O ３ '÷A×100...(2-3) %BaO=%BaO'÷A×100...(2-4) %CaO=%CaO'÷A×100...(2-5) %Li ２ O = % Li ２O'÷ A × 100 (2-6) %MgO = %MgO'÷ A × 100 (2-7) %F = %F'÷ A × 100 (2-8) where %C and %F are the normalized C content and F content, respectively. ２ O ３ , %BaO, %CaO, %Li ２ O and %MgO are normalized values ​​of Al ２ O ３ Content, Ba content in BaO conversion, Ca content in CaO conversion, Li content ２ The content is calculated as O, and the content is calculated as MgO.

[0029] The same applies when the molding powder further contains unavoidable impurities. For example, the unavoidable impurities may be SiO ２ and SiO in 100% by mass of the mold powder. ２ Content is %SiO ２ In the case of A, the following formula is used instead of the formula (2-1): A = % C' + % Al ２ O ３ '+%BaO'+%CaO'+%Li ２ O'+%MgO'+%F'+%SiO ２ ' ... (2-9) Furthermore, the content % Im of unavoidable impurities is normalized to SiO ２ Content (%SiO ２ ) and can be calculated using the following formula: %Im = %SiO ２ = % SiO ２ ´÷A×100……(2-10)

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

[0031] [Aggregate Carbon] Aggregate carbon plays a role in adjusting the melting rate of the powder. Examples of aggregate carbon include, but are not limited to, carbon black and coke powder. The lower limit of the aggregate carbon content is not particularly limited, but is preferably 0.5% or more. The upper limit of the aggregate carbon content is also not particularly limited, but is preferably 3.5% or less.

[0032] [Total C Content: 0.5 to 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] [Al ２ O ３ :17~25%] Al ２ O ３ is the Al in the molten powder ２ O ３ It is added to increase the activity of Al and suppress the oxidation reaction of Al in molten steel. ２ O ３ If the content is less than 17%, such an effect cannot be obtained. ２ O ３ The Al content is set to 17% or more, preferably 19% or more. ２ O ３ The Al content may be 17.0% or more, or 19.0% or more. ２ O ３ If the content is higher than 25%, additional Al is added during casting. ２ O ３ Considering the increase in Al content, the solidification temperature of the powder becomes very high. ２ O ３ The Al content is set to 25% or less, preferably 23% or less. ２ O ３ The content may be 25.0% or less, or 23.0% or less. The content is determined by X-ray fluorescence analysis of the Al element. ２ O ３ This is calculated by converting it into

[0034] [BaO] BaO has a standard free energy of formation of Al at 1300 ° C. ２ O ３ It is lower than that and hardly reacts with Al in molten steel, so the Al in the powder during casting ２ O ３ The increase in the concentration is suppressed. ２ O ３ It forms a complex oxide with the powder, lowering the solidification start temperature of the powder.

[0035] [Ba Content in BaO Equivalent: 22 to 30%] If the Ba content in BaO equivalent is less than 22%, the aforementioned effect of lowering the solidification start temperature of the powder cannot be obtained. Therefore, the Ba content in BaO equivalent 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 BaO equivalent is higher than 30%, the solidification start temperature will increase. Therefore, the Ba content in BaO equivalent 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 Ba element content by ICP atomic emission spectroscopy and converting it to BaO.

[0036] [CaO] CaO has a standard free energy of formation of Al at 1300 ° C. ２ O ３ It is lower than that and hardly reacts with Al in molten steel, so the Al in the powder during casting ２ O ３ The increase in the concentration of CaO is suppressed. ２ O ３ It forms a complex oxide with the powder, lowering the solidification start temperature of the powder.

[0037] [Ca content in terms of CaO: 31 to 41%] When the Ca content in terms of CaO is less than 31%, Al ２ O ３If the CaO content is too high, the above-mentioned effects of suppressing the increase in the Ca 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 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 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] [Li ２ O] Li ２ O has a standard free energy of formation of Al at 1300°C. ２ O ３ Since the Al content is higher than that of the molten steel, it reacts with the Al in the molten steel to some extent, and the Al content of the powder during casting ２ O ３ However, the powder of the present invention has the effect of lowering the solidification initiation temperature and viscosity, so that the Li ２ It is assumed to include O.

[0039] [Li's Li ２ O content: 2.0 to 5.0%] Li ２ If the content in terms of O is less than 2.0%, the effect of lowering the solidification start temperature is not exhibited. ２ The content of Li in terms of O is 2.0% or more. ２ If the content in terms of O exceeds 5.0%, Li is dissolved by Al in the molten steel. ２ O becomes more easily reduced and Al ２ O ３ increases, so Li ２ The effect of adding O is greater than the effect of lowering the solidification start temperature. ２ O ３ In addition, by reducing the Li content, the manufacturing cost can be reduced. Therefore, from the viewpoint of the solidification start temperature and manufacturing cost, the Li content of Li ２The content calculated as O is 5.0% or less, preferably 4.0% or less, and from the viewpoint of further reducing the manufacturing cost, more preferably less than 3%, further preferably 2.9% or less. The content is determined by measuring the content of Li element by ICP emission spectroscopy, and ２ Convert to O and calculate.

[0040] [MgO] MgO has a standard free energy of formation of Al at 1300°C. ２ O ３ It is lower than the Al content in the molten steel and hardly reacts with the Al in the powder during casting. ２ O ３ This suppresses the increase in concentration and also lowers the powder solidification start temperature.

[0041] [Mg Content in MgO Equivalent: 0.8 to 2.0%] If the Mg content in MgO equivalent 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 MgO equivalent is set to 0.8% or more, preferably 1.0% or more. On the other hand, if the Mg content in MgO equivalent exceeds 2.0%, the solidification start temperature will increase. Therefore, the Mg content in MgO equivalent 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 Compound] The fluorine compound is blended to lower the solidification start temperature of the powder and the viscosity described later. The fluorine compound is not particularly limited, and may be a fluoride, for example, CaF ２ , LiF and BaF ２ At least one selected from the group consisting of: ２ It is preferable to use

[0043] [F Content: 12 to 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%, not only does the effect saturate, but the risk of oxidation and deterioration of the continuous casting machine increases dramatically. 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. Note that, in the production of powder, 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 satisfied. 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 spots 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] BaCO contained in the above carbonate ３ , CaCO ３ , Li ２ CO ３ and MgCO ３ The upper and lower limits of the content of each of the above are not particularly limited.

[0049] However, from the same point of view, BaCO ３ The content of Ba is preferably 20% or less of 100% by mass of the powder. ３ The content ratio of BaCO in terms of BaO is preferably 50% or less. ３ The lower limit of the content of BaCO is not limited and may be 0%. ３ may not be included.

[0050] From a similar perspective, CaCO ３ The content of Ca is preferably 20% or less of 100% by mass of the powder. ３ The content ratio of CaCO in terms of CaO is preferably 50% or less. ３ The lower limit of the content of is not limited and may be 0%, ３ may not be included.

[0051] From a similar perspective, Li ２ CO ３The content of Li is preferably 5% or less of 100% by mass of the powder. ２ Li content in terms of O ２ CO ３ Li ２ The content ratio calculated as O is preferably 50% or less. ２ CO ３ The lower limit of the content is not limited and may be 0%, ２ CO ３ may not be included.

[0052] From a similar perspective, MgCO ３ The content of MgCO relative to the content of Mg in terms of MgO is preferably 2% or less of 100% by mass of the powder. ３ The content ratio of MgCO in terms of MgO is preferably 50% or less. ３ The lower limit of the content of MgCO is not limited and may be 0%. ３ may not be included.

[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 include those having a standard free energy of formation of oxides at 1300°C of Al. ２ O ３ The oxide may also contain oxides having a higher content than SiO. ２ , Na ２ O, TiO ２ These oxides may be unavoidably mixed into the powder raw materials, and therefore it may be difficult to completely remove them. However, it is necessary to keep the content as low as possible to prevent thermite reaction with Al in the molten steel. From this viewpoint, the total content of the unavoidable impurities is set to 2.0% or less. In other words, the total content of aggregate carbon, Al ２ O ３ , BaO, CaO, Li ２The total content of O, MgO, and fluorine compounds is 98.0% or more and 100% or less. The lower limit of the total content of the unavoidable impurities is not limited and may be 0%, and the unavoidable impurities may not be contained. 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] The content of each component that may be contained as an unavoidable impurity is not particularly limited as long as the total content satisfies the above-mentioned range. However, SiO ２ When it contains, SiO of Si ２ The content of Na as an unavoidable impurity is preferably 0.5% or less. ２ If O is included, Na ２ The content in terms of O is preferably 0.5% or less. ２ When TiO is contained, ２ The content in terms of carbon is preferably 0.5% or less.

[0056] As described above, the powder according to one embodiment of the present invention contains aggregate carbon, Al ２ O ３ , BaO, CaO, Li ２ The composition of the alloy is O, MgO, a fluorine compound, and the balance being unavoidable impurities, wherein the total C content is 0.5 to 5.0 mass %, Al ２ O ３ Content: 17 to 25 mass%, Ba content in terms of BaO: 22 to 30 mass%, Ca content in terms of CaO: 31 to 41 mass%, Li content ２ It is important that the content in terms of O is 2.0 to 5.0 mass%, the content of Mg in terms of MgO is 0.8 to 2.0 mass%, the content of F is 12.0 to 20.0 mass%, and the total content of the unavoidable impurities is 2.0 mass% or less.

[0057] The composition may further contain a carbonate of at least one element selected from the group consisting of Ba, Ca, Li, and Mg.

[0058] The powder according to one embodiment of the present invention can be produced by, for example, mixing raw materials to have the above-mentioned component composition. Here, the raw materials include aggregate carbon, Al ２ O ３ , BaO, CaO, Li ２ The raw material may be composed of O, MgO, and a fluorine compound. The raw material may further contain a carbonate of at least one element selected from the group consisting of Ba, Ca, Li, and Mg. A part or all of the raw material may be a premelt raw material. The particle size of the powder may be adjusted by a method such as pulverization or classification.

[0059] [Premelt raw material] The premelt raw material refers to a material that is melted and crushed in advance. As described above, Li ２ CO ３ and BaCO ３ Carbonates such as ammonium nitrate, ammonium nitrate, and ammonium nitrate may be blended into the powder. However, blending of 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. The 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 the premelt raw material, and more preferably 50% or more of the premelt raw material. The upper limit of the premelt raw material content is not particularly limited and may be 100%. However, considering the blending of aggregate carbon, it is preferable that the powder comprises 99.5% or less of the premelt raw material. It is also more preferable that the powder comprises 90% or less of the premelt raw material.

[0060] The powder according to one embodiment of the present invention can be prepared by blending additional materials with the premelt raw material in order to adjust the powder's component composition. For example, Al ２ O ３ When there is a shortage of Al ２ O ３ If CaO is insufficient, CaF ２Alternatively, CaO may be added. If the F content is insufficient, CaF ２ If BaO is insufficient, BaCO ３ BaCO ３ The blending amount of Li is preferably 20% or less of 100% by mass of the powder. ２ When O is insufficient, Li ２ CO ３ Li ２ CO ３ The amount of MgO blended is preferably 5% or less of 100% by mass of the powder. ３ can be mixed.

[0061] Here, the aggregate carbon cannot normally be used as a premelt raw material, i.e., the ratio of the premelt raw material to the total amount of aggregate carbon may be 0%.

[0062] If carbonate is added after premelt to adjust the composition, the total C content increases, which reduces the content of other components and changes the melting state due to the influence of carbon dioxide. Furthermore, this can 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 the 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 amount is preferably 20% or less, more preferably 10% or less. The lower limit of this proportion is not particularly limited and may be 0%.

[0063] [Particle Size] Using a finely pulverized powder facilitates the powder melting during casting, forming 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.

[0064] [Steel Manufacturing Method] A method for manufacturing steel using the above powder will now be described. The above powder can be suitably used when continuously casting steel containing 0.5% or more of Al.

[0065] In the continuous casting of high-Al steel, the use of the above powder minimizes the occurrence of the thermite reaction, enabling high-Al steel of stable quality to be produced with high productivity. Therefore, in the steel manufacturing 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.

[0066] [Solidification Start Temperature] In order to produce high-quality cast pieces and prevent operational problems during continuous casting, it is advisable to lower the solidification start temperature of the powder during casting. ２ O ３ If the solidification start temperature rises due to an increase in concentration, the lubrication between the mold and the slab is impaired, not only increasing the risk of operational problems, but also increasing the friction between the mold and the slab, which can cause cracks in the slab and impair quality.

[0067] 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.

[0068] [Viscosity] In order to produce high-quality cast slabs, it is preferable to reduce the viscosity of the powder at 1300°C during casting (hereinafter, the viscosity of the powder at 1300°C during casting may be simply referred to as viscosity). This is because reducing 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 preferable to control the viscosity to 10 Poise or less. 1 Poise = 0.1 Pa s.

[0069] 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.

[0070] 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.

[0071] [Powder Consumption Amount] In the above-mentioned steel manufacturing method, powder is added to the meniscus when molten steel, which has been melted to have the above-mentioned Al content, is poured from a ladle into a mold via a tundish. After being supplied to the meniscus, the powder becomes molten, penetrates into the gap between the mold and the solidified shell, and is withdrawn to the rear of the mold together with the slab. During this continuous casting process, the molten steel and the powder come into contact with each other at the meniscus, and the Al in the molten steel reacts with the powder only while this contact is occurring. Therefore, if the powder consumption amount is small, the reaction time with the molten steel per unit amount of powder becomes long, and the Al in the powder decreases. ２ O ３ The concentration increases, which increases the solidification start temperature. Furthermore, when the powder consumption is low, the Al content, which causes high viscosity, ２ O ３ As a result, the viscosity at 1300°C increases.

[0072] 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: Powder melt layer weight (kg) X ｉ : Concentration of component i in powder X ｉ，０ : initial concentration of component i in 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 By increasing ｉ In other words, when component i is changed to Al, ２ O ３ Therefore, by increasing the powder consumption, the amount of Al ２ O ３ The increase in Al is diluted, and the saturated Al ２ O ３ The concentration is reduced, and thus a lower solidification onset temperature and lower viscosity are achieved.

[0073] 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.

[0074] The inventors have found that when continuous casting steel containing 0.5% or more of 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 stable operation and the production of cast slabs with no problems in surface quality.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] First, a mold powder for continuous casting having the component composition shown in Table 1 was prepared. ２ O ３ , CaO, MgCO ３ , CaF ２ , BaCO ３ , Li ２ CO ３ After blending, the mixture was melted to prepare a premelt raw material. Then, Al was added to the premelt raw material as needed. ２ O ３ , CaO, CaF ２ , BaCO ３ , Li２ CO ３ , MgCO ３ These were mixed together, and aggregate carbon was further mixed to obtain the composition shown in Table 1. Carbon black was used as the aggregate carbon.

[0079] The total contents of C, Al, Ca, Ba, Li, Mg, and F were measured by the methods described above. ２ O ３ , BaO, CaO, Li ２ O and MgO are Al ２ O ３ Content, Ba content in BaO conversion, Ca content in CaO conversion, Li content ２ The total content of unavoidable impurities was calculated as the sum of the oxide-converted contents of Si, Na, Ti, Fe, and Mn, and 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 was determined by atomic absorption analysis, and the Ti content was determined by ICP atomic emission spectroscopy. Metal elements were also calculated as oxides (SiO ２ , Na ２ O, TiO ２ , Fe ２ O ３ , MnO) and the total was calculated. ２ , Na ２ O, TiO ２ are Si and SiO ２ Conversion of Na to Na ２ O content, TiO ２ Represents the converted content.

[0080] The proportion of the 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.

[0081]

[0082] Next, molten steel having a C concentration of 0.0040% and an Al concentration of 5% was produced, and continuous casting of a slab having a cross-sectional size of 200 mm × 1000 mm in a mold was carried out under the condition of a slab withdrawal rate of 0.7 m / min. During this process, powder shown in Table 2 was supplied while adjusting the vibration conditions of the mold so as to obtain the powder consumption per ton of molten steel shown in Table 2.

[0083] Next, the solidification start temperature and viscosity were measured by the above-mentioned methods. Furthermore, as an operational evaluation, the occurrence of smoke and flames during continuous casting was investigated. Additionally, the obtained cast pieces were observed to measure 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, trimming work is carried out to remove them by scraping the slab surface with a scarf or grinder, which reduces the yield. In other words, the higher the trimming yield, the higher the quality of the slab.

[0084] 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. In Comparative Example No. 10, operational problems were observed, such as white smoke and flames emitting from the mold during casting.

[0085]

Claims

1. Aggregate carbon, Al ２ O ３ , BaO, CaO, Li ２ The composition of the alloy is O, MgO, a fluorine compound, and the balance is unavoidable impurities, and the total C content is 0.5 to 5.0 mass %. ２ O ３ Content: 17 to 25 mass%, Ba content in terms of BaO: 22 to 30 mass%, Ca content in terms of CaO: 31 to 41 mass%, Li content ２ A mold powder for continuous casting, comprising: an O content of 2.0 to 5.0 mass%, an Mg content 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.

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

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

4. The mold powder for continuous casting according to any one of claims 1 to 3, wherein 30 mass % or more of the mold powder is made of premelt raw material.

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