Mold powder for continuous casting and method for producing steel

Abstract

Provided is a mold powder for continuous casting, with which it is possible to produce a high-quality cast slab and to prevent operational troubles when a high-Al steel is continuously cast. This mold powder for continuous casting has a component composition consisting of at least one oxide that is selected from the group consisting of CaO, BaO, MgO, La2O3, and ZrO2, aggregate carbon, Al2O3, a fluorine compound, Li2O, and the balance made up of unavoidable impurities, wherein: the Al2O3 content is 25 to 35% by mass; the total content of Ca, Ba, Mg, La, and Zr in terms of oxides is 40 to 60% by mass; the F content is 3 to 10% by mass; the Li content in terms of Li2O is 6 to 12% by mass; the total content of the unavoidable impurities is 0 to 2.0% by mass; and Y represented by -12.7 × (%C) + 51.8 is 5 to 35 inclusive.

Description

Mold powder for continuous casting and method for manufacturing steel

[0001] The present invention relates to a mold powder for continuous casting and a method for producing steel.

[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. ２When continuous casting is performed using powder containing Al ２ 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] JP 2017-170494 A JP 5-185195 A

[0009] However, the powder described in the above document has the following problems.

[0010] For example, the powder described in Patent Document 1 is SiO ２ The thermite reaction occurs when the powder is used. ２ O ３ Not only do adverse effects arise from the increase in concentration, but also from heat generation. First, the solidification of molten steel is delayed at the meniscus, causing the solidified shell to break (bleed) along the oscillation marks. This impairs the surface quality of the cast slab. Furthermore, the occurrence of flames can cause equipment failure and shutdowns.

[0011] In addition, the powder described in Patent Document 2 contains SiO ２ does not contain Na ２ O and TiO ２ The thermite reaction occurs between these components and the Al in the molten steel. ２ O ３ As the concentration increases, operational problems and deterioration of the quality of the slab are observed.

[0012] The gist of the present invention for solving the above problems is as follows.

[0013] 1. Aggregate carbon, Al ２ O ３ , CaO, BaO, MgO, La ２ O ３ , and ZrO ２at least one oxide selected from the group consisting of a fluorine compound, Li ２ O, and the balance being unavoidable impurities, ２ O ３ Content: 25 to 35 mass%, Total content of Ca, Ba, Mg, La, and Zr in terms of oxides: 40 to 60 mass%, F content: 3 to 10 mass%, Li content: ２ A mold powder for continuous casting, wherein the content in terms of O is 6 to 12 mass%, the total content of the unavoidable impurities is 0 to 2.0 mass%, and when the content of the aggregate carbon expressed in mass% is (%C), Y represented by the following formula (2) is 5 or more and 35 or less: Y = -12.7 x (%C) + 51.8 (2)

[0014] 2. The mold powder for continuous casting according to item 1 above, wherein the component composition further contains a carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr.

[0015] 3. The mold powder for continuous casting according to the above item 1 or 2, wherein the maximum particle size of the mold powder is 250 μm or less.

[0016] 4. The mold powder for continuous casting according to any one of 1 to 3 above, wherein 50 mass % or more of the mold powder is made of a premelt raw material.

[0017] 5. A method for producing steel, comprising continuously casting steel containing 0.5 mass % or more of Al using the mold powder for continuous casting according to any one of 1 to 4 above.

[0018] 6. The method for producing steel according to 5 above, wherein the powder consumption per ton of molten steel is 0.4 kg or more.

[0019] According to the present invention, it is possible to provide 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, and also to provide a method for producing steel using the powder.

[0020] Fig. 1 is a graph showing the relationship between the molten layer thickness and the number of temperature reversals between the upper and lower thermocouples. Fig. 2 is a graph showing the relationship between the molten layer thickness and the number density of powder defects.

[0021] The present invention will be specifically described below. In this specification, "%" as a unit of content refers to "% by mass" unless otherwise specified.

[0022] 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.

[0023] Al in the powder during casting ２ O ３ The factors that cause the concentration to increase 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 It is necessary to set the powder preparation and steel manufacturing conditions taking into consideration that the amount will increase to some extent during casting.

[0024] As a result of investigations from the above viewpoints, the inventors focused on the solidification start temperature after the composition change of the powder during casting. They then concluded that it would be preferable to lower the solidification start temperature after the composition change (hereinafter, the solidification start temperature after the composition change may be simply referred to as the solidification start temperature). In order to lower the solidification start temperature, the composition of the powder and the manufacturing conditions of the steel are important.

[0025] The component composition of the powder according to one embodiment of the present invention will be described below.

[0026] 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%. The total C content, Al ２ O ３ Content, Ca, Ba, Mg, La, and Zr contents in terms of oxides, F content, Li content ２ The above normalization is applied to the content in terms of O, the content of aggregate carbon, and the content of unavoidable impurities.

[0027] 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 (%T.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 = %T.C' + %Al ２ O ３ '+%BaO'+%CaO'+%Li ２ O'+%MgO'+%F'...(3-1) %T. C=%T. C'÷A×100...(3-2) %Al ２ O ３ = % Al ２ O ３ '÷A×100...(3-3) %BaO=%BaO'÷A×100...(3-4) %CaO=%CaO'÷A×100...(3-5) %Li ２O = % Li ２ %T.C and %F are the total C content and F content after normalization, 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 carbon content of the aggregate after normalization is %C, and the carbon content of the aggregate in 100% by mass of the molding powder is %C', then the following formula is established: %C = %C' ÷ A × 100 (3-9)

[0028] 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 ', the following formula is used instead of the formula (3-1): A = % T.C' + % Al ２ O ３ '+%BaO'+%CaO'+%Li ２ O'+%MgO'+%F'+%SiO ２ ' ... (3-10) 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……(3-11)

[0029] Next, the reasons for limiting each component will be explained.

[0030] [Aggregate Carbon] The 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 content of aggregate carbon will be described later.

[0031] If the total C content is low, the effect as aggregate cannot be obtained. From this viewpoint, the total C content is preferably 1.3% or more. On the other hand, if the total C content exceeds 5.0%, when a thermite reaction occurs, C burns and a flame is generated, which may cause equipment failure and operation shutdown. Therefore, from the viewpoint of preventing operation troubles, the total C content is preferably 5.0% or less. Here, the total C content can be calculated as the sum of the aggregate carbon content and the C content derived from carbonates described below. Furthermore, the C derived from carbonates is considered to be the sum of C derived from carbonates remaining in the premelt raw material described below and C derived from carbonates blended other than the premelt raw material. The total C content can be measured by a combustion method.

[0032] [Al ２ O ３ ] 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 25%, such an effect cannot be obtained. ２ O ３ The Al content is set to 25% or more, preferably 28% or more. ２ O ３ The content may be 25.0% or more, or 28.0% or more. ２ O ３ If the content is higher than 35%, 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 35% or less, preferably 33% or less. ２ O ３ The Al content may be 35.0% or less, or 33.0% or less. ２ O ３ The Al content was determined by X-ray fluorescence analysis. ２ O ３Convert to and find the answer.

[0033] [CaO, BaO, MgO, La ２ O ３ , and ZrO ２ As a result of the investigations by the present inventors, it has been found that at least one oxide selected from the group consisting of CaO, BaO, MgO, La ２ O ３ , and ZrO ２ It has been found that the solidification start temperature of the powder can be lowered by including at least one oxide selected from the group consisting of Al. This is thought to be due to the following reasons. First, all of the oxides have a standard free energy of formation at 1300°C of Al. ２ O ３ The average temperature of the powder in the molten state poured into the mold is 1300°C. Here, the standard free energy of formation at 1300°C is ２ O ３ Oxides lower than this hardly react with Al in molten steel, so the Al in the powder during casting ２ O ３ Furthermore, the oxide is Al ２ O ３ This forms a complex oxide with Al, lowering the solidification start temperature of the powder. ２ O ３ In order to suppress the increase in the concentration and lower the powder solidification start temperature, the standard free energy of formation at 1300 °C must be less than Al. ２ O ３ For the above reasons, in one embodiment of the present invention, the powder should contain less than CaO, BaO, MgO, La. ２ O ３ , and ZrO ２ The material contains at least one oxide selected from the group consisting of:

[0034] [Total content of Ca, Ba, Mg, La, and Zr in terms of oxides: 40 to 60 mass %] When the total content of Ca, Ba, Mg, La, and Zr in terms of oxides is less than 40%, Al ２ O ３If the total content exceeds 60%, the contents of other components will be reduced, and the effects of containing other components will not be obtained. Therefore, the total content is set to 40% or more, preferably 45% or more. The total content may be 40.0% or more, or 45.0% or more. On the other hand, if the total content exceeds 60%, the contents of other components will be reduced instead, and the effects of containing other components will not be obtained. Therefore, the total content is set to 60% or less, preferably 55% or less. The total content may be 60.0% or less, or 55.0% or less. The total content is determined by determining the contents of Ca, Mg, La, and Zr elements by X-ray fluorescence analysis, and the content of Ba element by ICP atomic emission spectroscopy, and then subtracting the contents of CaO, MgO, and La, respectively. ２ O ３ , ZrO ２ , and are converted to BaO and then summed up.

[0035] Among the above oxides, CaO, BaO and MgO are excellent in terms of cost and have a low Al content in the powder. ２ O ３ Even if the amount of Ca increases, a liquid phase can be formed more efficiently at a low temperature. From this viewpoint, it is preferable that the oxide is at least one component selected from the group consisting of CaO, BaO, and MgO. In this case, the total content of Ca, Ba, and Mg calculated as oxides is preferably 40% or more, more preferably 45% or more. The total content may be 40.0% or more, or may be 45.0% or more. On the other hand, the total content is preferably 60% or less, more preferably 55% or less. The total content may be 60.0% or less, or may be 55.0% or less.

[0036] The upper and lower limits of the respective contents of Ca and Ba are not particularly limited. However, the Ca content in terms of CaO may be 20.0% or more. The Ca content in terms of CaO may be 45.0% or less. The Ba content in terms of BaO may be 10.0% or more. The Ba content in terms of BaO may be 35.0% or less.

[0037] The upper and lower limits of the contents of Mg, La, and Zr are not particularly limited. However, these elements have the effect of lowering the melting point. Therefore, when Mg is contained, the content of Mg converted to MgO is preferably 1.0% or more. When La is contained, the La content of La is preferably 1.0% or more. ２ O ３ The content in terms of Zr converted to ZrO is preferably 0.5% or more. ２ The content of Mg converted to MgO is preferably 0.5% or more. On the other hand, if it is contained in excess, the melting point will increase. Therefore, the content of Mg converted to MgO is preferably 5.0% or less. ２ O ３ The content in terms of ZrO is preferably 3.0% or less. ２ The content in terms of converted value is preferably 3.0% or less.

[0038] [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 CaF ２ , LiF and BaF ２ However, from the viewpoint of cost, CaF ２ It is preferable to use

[0039] [F content: 3 to 10 mass%] If the F content of the powder is less than 3%, the above-mentioned effects of the fluorine compound cannot be obtained. Therefore, the F content of the powder is set to 3% or more. The F content may be 3.0% or more. On the other hand, if the F content of the powder exceeds 10%, 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 of the powder is set to 10% or less. The F content may be 10.0% or less. The F content can be measured using absorptiometry.

[0040] [Li ２ O] Li ２ O has a standard free energy of formation of Al at 1300°C. ２ O ３Although its carbon content is higher than that of Al, it is an oxide that is relatively difficult to reduce with Al, and is added because it has the effect of lowering the solidification start temperature.

[0041] [Li's Li ２ O content: 6 to 12 mass%] Li ２ If the content in terms of O is less than 6%, the effect of lowering the solidification start temperature is not exhibited. ２ The content of Li in terms of O is set to 6% or more, preferably 8% or more. The content may be 6.0% or more, or 8.0% or more. ２ If the content in terms of O exceeds 12%, Li will be 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 The effect of increasing the solidification temperature due to the increase in Li is greater than the effect of increasing the Li content. ２ The content calculated as O is 12% or less, preferably 11% or less. The content may be 12.0% or less, or 11.0% or less. The content is determined by ICP emission spectroscopy to determine the content of Li element. ２ Convert to O and calculate.

[0042] [Carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr] The powder may contain a carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr. This is for the following reason: In the production of powder, the component composition of an intermediate raw material such as a premelt raw material may deviate from the target. In such cases, the content of the element can be adjusted by mixing the carbonate with the intermediate raw material. Here, even when elements such as Li, Ca, Ba, Mg, La, and Zr are contained as carbonates, the same effect as when oxides of the elements are contained can be obtained.

[0043] When elements such as Ca, Ba, Mg, La, and Zr are contained as carbonates, the total content of the elements converted into oxides is calculated by adding the content of the oxides of the elements and the content of the carbonates of the elements converted into oxides. ２ The content in terms of O is Li ２ O content and Li ２ CO ３ The amount is calculated by adding up the amount of the oxides obtained by converting the amount of the oxides into the amount of the oxides.

[0044] Here, the carbonate may be a carbonate of at least one element selected from the group consisting of Li, Ca, Ba, and Mg, or may be a carbonate of at least one element selected from the group consisting of Li and Ba.

[0045] The total content of the carbonates is the total content of Ca, Ba, Mg, La, and Zr in terms of oxides, and the total content of Li in terms of Li. ２ There are no limitations as long as the content in terms of O is satisfied. However, by lowering the total content of the carbonates, it is possible to increase the content of other components and also to prevent the melting property during casting from being changed by carbon dioxide gas. Furthermore, it is possible to further suppress the generation of bare spots. Therefore, the total content of the carbonates relative to the powder is preferably 10% or less, more preferably 5% or less. On the other hand, there is no particular limitation on the lower limit of the total content of the carbonates, and it may be 0%, and the carbonates may not be contained. From the same viewpoint, the Li content of the Li carbonate in the powder ２ The content calculated as O is preferably 10% or less. The lower limit of the content is not particularly limited and may be 0%. From the same viewpoint, the content of each of the elements Ca, Ba, Mg, La, and Zr in the powder calculated as an oxide of the carbonate of the element is preferably 10% or less. The lower limit of the content of each element is not particularly limited and may be 0%.

[0046] From the same viewpoint, Li ２ Li carbonate content in terms of O ２The ratio of the content calculated as O is preferably 50% or less. The lower limit of the ratio is not particularly limited, and may be 0%, and Li carbonate may not be included. From the same viewpoint, for each of the elements Ca, Ba, Mg, La, and Zr, the ratio of the content of the carbonate of the element calculated as an oxide to the content of the element calculated as an oxide is preferably 50% or less. The lower limit of the ratio is not particularly limited, and may be 0%, and carbonate of each element may not be included.

[0047] 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.

[0048] [Total content of the unavoidable impurities: 0 to 2.0 mass %] The unavoidable impurities include Al ２ O ３ Higher oxide (Li ２ Examples of the oxides include SiO ２ , Na ２ O, TiO ２ , Fe ２ O ３ These oxides may be unavoidably mixed into the powder raw materials, and although it is difficult to completely remove them, it is necessary to intentionally avoid adding them and to keep their content as low as possible in order to prevent a thermite reaction with Al in the molten steel. Therefore, the total content of the above unavoidable impurities is set to 2.0% or less.

[0049] [Y: 5 or more and 35 or less] In the present invention, it is necessary to determine the component composition of the powder so that Y represented by the above formula (2) is 5 or more and 35 or less.

[0050] First, the thickness of the molten powder layer during casting will be described. If the thickness of the molten powder layer is less than 5 mm, the flow between the mold and the solidified shell becomes unstable, resulting in a localized powder supply shortage and an increased risk of the molten steel coming into direct contact with the mold, a phenomenon known as seizure.

[0051] On the other hand, if the thickness of the molten layer exceeds 35 mm, the following problems arise.

[0052] First, sensor failure makes it difficult to control the molten steel level. Continuous casting machines are typically equipped with a molten steel level detection sensor installed above the mold to control the molten steel level. If the molten steel layer becomes too thick, the temperature of the sensor itself will rise, increasing the risk of sensor failure. Continuous casting machines typically control the amount of molten steel poured into the mold by detecting the molten steel level, but if the sensor fails, the automatic control of the molten steel level will stop, bringing operations to a halt.

[0053] Furthermore, quality problems can occur when the powder gets caught in the molten steel. Normally, the molten powder floats on top of the molten steel, with the unmelted powder sitting on top of that, but if the molten layer becomes too thick, the solid phase that precipitates from the molten powder will sink within the molten powder, increasing the risk of it getting caught in the molten steel.

[0054] As a result of investigations by the present inventors, these phenomena are observed only when the thickness of the molten layer exceeds 35 mm, and therefore the thickness of the molten layer must be controlled to 35 mm or less.

[0055] Here, the thickness of the powder molten layer can be controlled by the powder melting rate. That is, if the melting rate is slow, the powder molten layer thickness will be thin, and conversely, if the melting rate is fast, the powder molten layer thickness will be thick. Furthermore, as mentioned above, the powder melting rate can be adjusted by the amount of aggregate added to the powder. The inventors investigated the relationship between the molten layer thickness Y (unit: mm) and the aggregate carbon content %C (unit: mass %) for powders within the composition range of the present invention, and found that the above formula (2) holds true within an error range of 10%.

[0056] In general, the thickness and melting rate of a powder melt layer can be adjusted by changing the components other than the aggregate and the melting point of the powder. However, considering the other component compositions, it is believed that the thickness and melting rate of the powder melt layer in the powder of the present invention can be adjusted only by the aggregate carbon content. That is, in the present invention, by appropriately using a powder having a predetermined component composition, it is possible to control the solidification start temperature of the powder between the mold and the solidified shell. Here, the solidification start temperature is determined by the composition of the powder after component change. On the other hand, the thickness of the powder melt layer in the molten steel is determined by the composition before component change. Therefore, from the perspective of controlling both the composition before and after component change, it is believed that the thickness and melting rate of the powder melt layer can be adjusted only by the aggregate carbon content.

[0057] Therefore, when the aggregate carbon content expressed in mass % is (%C), Y expressed by the above formula (2) is set to 5 or more and 35 or less. This makes it possible to produce high-quality cast slabs and prevent operational problems. It is preferable that Y is 10 or more. It is also preferable that Y is 30 or less. Here, Y is a value rounded off to the nearest whole number. It is also preferable that the aggregate carbon content is 1.3% or more, and more preferably 1.5% or more. It is preferable that the aggregate carbon content is 3.7% or less, and more preferably 3.5% or less.

[0058] As described above, the powder according to one embodiment of the present invention comprises aggregate carbon, Al ２ O ３ , CaO, BaO, MgO, La ２ O ３ , and ZrO ２ at least one oxide selected from the group consisting of a fluorine compound, Li ２ O, and the balance being unavoidable impurities. ２ O ３ Content: 25 to 35 mass%, Total content of Ca, Ba, Mg, La, and Zr in terms of oxides: 40 to 60 mass%, F content: 3 to 10 mass%, Li content: ２The content in terms of O is 6 to 12 mass%, the total content of the unavoidable impurities is 0 to 2.0 mass%, and when the content of the aggregate carbon expressed in mass% is (%C), it is important that Y expressed by the above formula (2) is 5 or more and 35 or less.

[0059] The composition may further contain a carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr.

[0060] 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 and Al. ２ O ３ and CaO, BaO, MgO, La ２ O ３ , and ZrO ２ At least one oxide selected from the group consisting of: ２ The raw material may be composed of Li, Ca, Ba, Mg, La, and Zr. The raw material may further contain a carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr. 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.

[0061] [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 ３Carbonates such as ammonium nitrate, ammonium nitrate, and ammonium nitrate may be blended into the powder. However, blending with 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 mass% or more of the premelt raw material, and more preferably 50 mass% or more of the premelt raw material. The upper limit of the premelt raw material content is not particularly limited and may be 100 mass%. However, considering the blending of aggregate carbon, it is preferable that the powder comprises 99.5 mass% or less of the premelt raw material. It is also more preferable that the powder comprises 90 mass% or less of the premelt raw material.

[0062] 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 composition. For example, Al ２ O ３ When there is a shortage of Al ２ O ３ If CaO is insufficient, CaF ２ Alternatively, CaO may be added. If the F content is insufficient, CaF ２ If BaO is insufficient, BaCO ３ Li ２ When O is insufficient, Li ２ CO ３ can be mixed.

[0063] 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%.

[0064] 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 gas. Furthermore, this may 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 Li, Ca, Ba, Mg, La, and Zr, the proportion of the carbonate other than the premelt raw material relative to the total amount of powder is preferably 20% or less, more preferably 10% or less. The lower limit of this proportion is not particularly limited and may be 0%.

[0065] [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.

[0066] [Method for Producing Steel] A method for producing steel according to one embodiment of the present invention will now be described. The method for producing steel according to one embodiment of the present invention is a method for continuously casting steel containing 0.5 mass % or more of Al using the powder described above.

[0067] In the continuous casting of high-Al steel, the use of the above-described powder minimizes the occurrence of thermite reactions, enabling high-Al steel of stable quality to be produced with high productivity. Therefore, the steel subjected to continuous casting by the steel production method according to one embodiment of the present invention contains 0.5 mass % or more of Al, preferably 1.0 mass % or more. In other words, the steel production method according to one embodiment of the present invention is a method for continuously casting steel containing 0.5 mass % or more of Al, preferably a method for continuously casting steel containing 1.0 mass % or more of Al.

[0068] [Solidification Start Temperature] In continuous casting, in order to produce high-quality cast pieces while preventing operational problems, it is advantageous to lower the solidification start temperature of the powder during casting. Lowering the solidification start temperature results in higher quality cast pieces and further reduces the risk of operational problems.

[0069] The average temperature of the molten powder during casting is 1300° C. Therefore, it is preferable that the solidification start temperature of the powder 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.

[0070] [Viscosity] According to the present invention, in order to produce a high-quality cast slab, 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.

[0071] 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.

[0072] 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.

[0073] [Thickness of molten layer] As described above, in the continuous casting of high-Al steel, it is preferable that the thickness of the molten layer is 5 mm or more and 35 mm or less. Note that the thickness of the molten layer can be measured, for example, by inserting a copper wire into the powder, removing it, and then observing the state of the deposits.

[0074] [Powder Consumption] In a steel manufacturing method according to one embodiment of the present invention, 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 is low, 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.

[0075] 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 ) ... (4-1) 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 ) kｉ : 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) ... (4-2) 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.

[0076] 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.

[0077] The inventors have found that when continuously casting steel containing 0.5% by mass 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 made 1300°C or lower and the viscosity can be made 10 poise or lower. This allows stable operation and the production of cast slabs with no problems in surface quality.

[0078] From this perspective, in a method for producing steel according to an embodiment of the present invention, it is preferable to set the powder consumption amount per ton of molten steel to 0.4 kg or more.

[0079] 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.

[0080] 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.

[0081] First, mold powders for continuous casting were prepared having the 12 kinds of component compositions shown in Table 1. ２ O ３ ), quicklime (CaO), fluorite (CaF ２ ), barium carbonate (BaCO ３ ), magnesium carbonate (MgCO ３ ), lithium carbonate (Li ２ CO ３ ) was blended as needed, and then melted to prepare a premelt raw material. Next, alumina, quicklime, and fluorite were blended as needed with the premelt raw material, and aggregate carbon was further blended to obtain the composition shown in Table 1. Carbon black was used as the aggregate carbon. Powder C further contained SiO ２ and Na ２ The Al, Ca, Mg, and Si contents were measured by X-ray fluorescence analysis. The total C content was measured by a combustion method. The Ba, Li, and Ti contents were measured by ICP atomic emission spectrometry. The Na content was measured by atomic absorption spectrometry. The F content was measured by absorptiometry. The Al in Table 1 ２ O ３ , Li ２ O, CaO, BaO, MgO, SiO ２ , Na ２ O, TiO ２The values ​​in the table indicate the content of each element converted to oxide. Furthermore, since carbonates are converted to oxides by the premelt and the raw materials added after the premelt are all oxides or fluorides, almost no carbon derived from carbonates is contained in any of the powders. Therefore, the total C in Table 1 corresponds to the amount of carbon added as aggregate. The proportion of premelt raw materials in all of the powders shown in Table 1 is 50% or more, and the maximum particle size is 250 μm or less.

[0082]

[0083] Next, molten steel with a carbon concentration of 0.0040% and an aluminum concentration of 5% was produced, and continuous casting of a slab with a mold cross-sectional size of 200 mm x 1000 mm was performed at a slab withdrawal speed of 0.7 m / min. Powder was supplied as shown in Table 2 while adjusting the mold vibration conditions to achieve the powder consumption per ton of molten steel shown in Table 2. Two thermocouples (an upper thermocouple and a lower thermocouple) were inserted into the mold at positions 90 mm and 200 mm below the molten steel surface. When casting was performed without problems, the temperature of the upper thermocouple was higher than that of the lower thermocouple. However, when the molten steel directly contacted the mold, the temperature of the upper and lower thermocouples reversed. To prevent a breakout, the slab withdrawal speed was reduced during casting, and the casting speed was returned to normal after the thermocouple temperatures stabilized.

[0084] 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 (5): Trimming yield (%) = (weight of slab after trimming) / (weight of slab before trimming) × 100 (5) 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.

[0085] The results are shown in Table 2. In Comparative Example No. 4, breakout occurred approximately 10 minutes after the start of casting. Therefore, it was not possible to collect molten powder during casting, and the solidification start temperature and viscosity could not be measured. Furthermore, it was not possible to evaluate the number of bleeds and the maintenance yield. In Comparative Example No. 8, corrosion progressed throughout the continuous casting equipment, making continued use difficult. In Comparative Example No. 9, due to an insufficient thickness of the molten powder layer, molten steel adhered to the mold immediately after the start of casting, and breakouts occurred from the adhered areas. Therefore, it was not possible to collect molten powder during casting, and it was not possible to measure the solidification start temperature and viscosity. Furthermore, it was not possible to evaluate the number of bleeds and the maintenance yield. In Comparative Example No. 10, the sensor for detecting the molten steel level broke down approximately 5 minutes after the start of casting, and casting was halted. Therefore, it was not possible to collect molten powder during casting, and it was not possible to measure the solidification start temperature and viscosity. Furthermore, it was not possible to evaluate the number of bleeds and the maintenance yield. In the comparative example No. 13, no particular operational problems were observed during casting, and the number of bleeds in the cast pieces and the trimming yield were the same as in the inventive example, but powder defects, a type of surface defect, occurred frequently in the steel material after rolling. This was because the thick molten layer caused frequent powder entrapment.

[0086]

[0087] As shown in Table 2, all of the inventive examples were able to achieve high quality cast slabs and stable operation, while all of the comparative examples either had inferior quality or experienced more operational problems.

[0088] The relationship between the thickness of the molten layer and the number of times the temperature of the upper and lower thermocouples reversed during continuous casting is shown in Figure 1. When the thickness of the molten layer was 5 mm or more, the above phenomenon never occurred, but when it was 4 mm, it occurred frequently, and casting was stopped when the number of times reached 8.

[0089] In addition, powder defects were investigated after rolling the slabs obtained by the above continuous casting. Figure 2 shows the relationship between the molten layer thickness in the above continuous casting and the number density of powder defects. When the molten layer thickness was 35 mm or less, almost no powder defects were observed in the steel material after rolling, but when the molten layer thickness was 36 mm, powder defects due to powder entrapment increased sharply. When the molten layer thickness was 40 mm, casting was stopped midway through casting due to a malfunction of the sensor for detecting the molten metal level. However, when the slabs that had been cast partway were rolled to produce steel, many powder defects were observed.

Claims

1. Aggregate carbon, Al ２ O ３ , CaO, BaO, MgO, La ２ O ３ , and ZrO ２ At least one oxide selected from the group consisting of a fluorine compound, Li ２ O, and the balance being unavoidable impurities, ２ O ３ Content: 25 to 35 mass%, Total content of Ca, Ba, Mg, La, and Zr calculated as oxides: 40 to 60 mass%, F content: 3 to 10 mass%, Li content: ２ A mold powder for continuous casting, comprising: a carbon content calculated as O: 6 to 12 mass%, a total content of the unavoidable impurities: 0 to 2.0 mass%, and, when the aggregate carbon content expressed in mass% is (%C), Y represented by the following formula is 5 or more and 35 or less. Y=-12.7×(%C)+51.8 2. The mold powder for continuous casting according to claim 1, wherein the composition further contains a carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr.

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 a premelt raw material.

5. A method for producing steel, comprising continuously casting steel containing 0.5 mass % or more of Al using the mold powder for continuous casting according to any one of claims 1 to 4.

6. The method for producing steel according to claim 5, wherein the powder consumption per ton of molten steel is 0.4 kg or more.

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