Mold powder for continuously casting steel and method for continuously casting steel using the same

Abstract

To provide a mold powder in which: a ZrO2 source material dissolves into a molten slag within a limited time; the melting loss of a soaking nozzle is effectively suppressed; a ZrO2-based inclusion in the steel is not generated; and no prior melting process is required.SOLUTION: A mold powder comprises a Ca3ZrSi2O9 raw material as a source of a ZrO2 source.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a mold powder for continuous casting of steel and a method for continuous casting of steel using the same. [Background technology]

[0002] In continuous casting of steel, molten steel stored in a tundish is poured into a mold through an immersion nozzle to be cooled and solidified, while the solidified shell is continuously drawn below the mold by rolls to continuously produce cast pieces of various shapes such as slabs, blooms, billets, etc. Powdered or granular mold powder is scattered on the surface of the molten steel in the mold, and the tip of the immersion nozzle is inserted into the molten steel in the mold.

[0003] The mold powder melts due to the heat of the molten steel (hereinafter, the molten mold powder is referred to as "molten slag") and covers the surface of the molten steel, and the molten slag flows between the solidified shell and the mold and is consumed. The main roles of the mold powder from the time it is spread until it is consumed are (1) to keep the surface of the molten steel warm and prevent oxidation, (2) to absorb non-metallic inclusions that float to the surface of the molten steel and to purify the molten steel, (3) to maintain lubrication between the solidified shell and the mold, and (4) to control the heat flux from the solidified shell to the mold.

[0004] The main component of the mold powder is CaO-SiO2, and may contain Na2O, Li2O, K2O, Al2O3, MgO, CaF2, and other secondary components as necessary. The mold powder before use is generally a mixture of multiple raw materials, and the raw materials for the CaO-SiO2 source are, for example, wollastonite, the raw materials for the CaO source are, for example, limestone and cement, the raw materials for the SiO2 source are, for example, silica sand and diatomaceous earth, the raw materials for the Na2O source are, for example, soda ash and sodium carbonate, the raw materials for the Li2O source are, for example, lithium carbonate, the raw materials for the Al2O3 source are, for example, alumina clinker, the raw materials for the MgO source are, for example, magnesia clinker, and the raw materials for the F source are, for example, fluorite and sodium fluoride.

[0005] The main role of the SEN is to prevent oxidation by the atmosphere when the molten steel stored in the tundish is poured into the mold, to control the flow rate of the molten steel, and to control the flow of the molten steel in the mold. The tip of the SEN is inserted into the molten steel in the mold, and the surface of the molten steel is covered with molten slag, so the middle part of the SEN comes into contact with the molten slag (powder line). The material of the SEN is usually an alumina-carbon material for the main body, and a zirconia-carbon material that has relatively high corrosion resistance against molten slag is used near the powder line. The erosion rate of this zirconia-carbon material determines the service life and replacement frequency of the SEN, and therefore has a significant impact on the productivity of continuous casting. Therefore, mold powder containing ZrO2 has been proposed for the purpose of suppressing the erosion rate of the zirconia-carbon material and further improving the service life of the SEN.

[0006] For example, Patent Document 1 proposes a mold additive for continuous casting, which is characterized by containing 0.5 to 15 wt% of zirconia in a mold additive for regular casting to which a flux base material and a melting property adjusting material are selectively added. Patent Document 2 also proposes a mold additive for continuous casting, which is used when continuously casting Si-deoxidized molten steel with sol.Al of 0.002% or less with a submerged nozzle made of zirconia or a zircon-containing material or a nozzle in a slag line having these materials, and which contains 2 to 6 wt% of ZrO2 in a mold additive for regular casting to which a flux component and a melting property adjusting material are selectively added, and is used by mixing a base material that has been melt-treated in its entirety in advance with an aggregate such as carbon powder for adjusting the melting rate. Baddeleyite and zircon sand are used as raw materials for the ZrO2 source, and a pre-melted product containing ZrO2 and a pre-melted product containing ZrO2 with a small amount of NaF added to adjust the viscosity are used as the mold additive for continuous casting. Furthermore, Patent Document 3 proposes a molding powder containing CaO, SiO2, and a fluorine compound as basic components and containing 0 to 10 mass% ZrO2. Zircon sand is used as the raw material for the ZrO2 source. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 57-41862 [Patent Document 2] Japanese Patent Application Publication No. 5-57411 [Patent Document 3] JP 2001-179408 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, when baddeleyite or zircon sand is used as the raw material for the ZrO2 source of the mold powder, they dissolve slowly in the molten slag and often cannot be completely dissolved in the molten slag within a limited time. In this case, the content of ZrO2 in the molten slag is insufficient, and the effect of suppressing the melting damage of the submerged entry nozzle is reduced. In addition, since the density of undissolved baddeleyite and zircon sand is significantly higher than that of the molten slag, they may sink below the molten slag and be taken into the molten steel, becoming ZrO2-based inclusions in the steel. These problems are particularly likely to occur in continuous casting of thin slabs with a mold thickness of 100 mm or less. This is thought to be because, in continuous casting of thin slabs, the dimensions of the mold in the thickness direction of the slab are small, and the casting speed is high at 2.0 m / min or more, so that the residence time of the molten slag on the surface of the molten steel is short, and the time for dissolving baddeleyite and zircon sand is insufficient.

[0009] On the other hand, when the ZrO2-containing pre-melted product proposed in Patent Document 2 is produced on an industrial scale, a melting process and fuel are required, which increases costs. Also, baddeleyite and zircon sand may remain unmelted in the pre-melted product. Furthermore, because baddeleyite and zircon sand have a higher density than molten slag, they may sink below the molten slag during melting treatment, which may cause a problem of ZrO2 segregation, in that the ZrO2 concentration of the pre-melted product is high at the bottom and low at the top. Therefore, it is practically difficult to put pre-melted products into practical use.

[0010] The present disclosure has been made in consideration of the above-mentioned circumstances, and has an object to provide a mold powder in which a ZrO2 source raw material dissolves in molten slag within a limited time, effectively suppresses melting damage to the submerged entry nozzle, does not produce ZrO2-based inclusions in the steel, and does not require a prior melting treatment, and a method for continuous casting of steel using the same. [Means for solving the problem]

[0011] (1) One aspect of the present disclosure is The present invention relates to a mold powder containing Ca3ZrSi2O9 raw material as a ZrO2 source raw material. The Ca3ZrSi2O9 raw material dissolves quickly in molten slag and completely during continuous casting, so this mold powder effectively suppresses the melting damage of the submerged entry nozzle and does not cause ZrO2-based inclusions in the steel. Therefore, there is no need to melt the mold powder in advance.

[0012] (2) In one aspect of the present disclosure, The content of the Ca3ZrSi2O9 raw material is preferably 1 to 15 parts by mass in terms of ZrO2 per 100 parts by mass of molten slag formed from the part of the mold powder excluding the ZrO2 source raw material. When the content of the Ca3ZrSi2O9 raw material is 1 part by mass or more in terms of ZrO2 per 100 parts by mass of molten slag formed from the part of the mold powder excluding the ZrO2 source raw material, the effect of suppressing melting damage to the submerged nozzle is remarkable, and when it is 15 parts by mass or less, the role of the mold powder, such as lubricity, can be fully exhibited. In addition, the "molten slag equivalent formed from the portion of the mold powder excluding the ZrO2 source raw material" is assumed to be the molten slag obtained by melting the portion of the mold powder excluding the ZrO2 source raw material, and does not include components supplied from the ZrO2 source raw material (e.g., CaO, ZrO2, SiO2, etc. of the Ca3ZrSi2O9 raw material) or components lost during melting (e.g., carbon raw materials for adjusting the slag formation rate, CO2 contained in limestone, etc.).

[0013] (3) Another aspect of the present disclosure is The present disclosure relates to a method for continuous casting of steel, characterized by using a mold powder according to one embodiment of the present disclosure. Since the Ca3ZrSi2O9 raw material dissolves quickly in molten slag and completely during continuous casting, the mold powder effectively suppresses melting damage to the submerged entry nozzle and at the same time does not cause ZrO2-based inclusions in the steel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] A preferred embodiment of the present disclosure will be described in detail below. Note that the embodiment described below does not unduly limit the contents of the present disclosure described in the claims, and all of the configurations described in the embodiment are not necessarily essential as a means for solving the problems of the present disclosure.

[0015] <Mold powder> The mold powder of this embodiment contains Ca3ZrSi2O9 raw material as a raw material of ZrO2 source. As will be described in detail in the examples below, the Ca3ZrSi2O9 raw material dissolves in molten slag significantly faster than baddeleyite and zircon sand, and dissolves completely during continuous casting, so that this mold powder effectively suppresses the melting damage of the submerged nozzle and does not cause ZrO2-based inclusions in the steel. Therefore, there is no need to melt the mold powder in advance. This is thought to be because the diffusion of Ca ions and Si ions in the Ca3ZrSi2O9 raw material into the molten slag is very fast, which also promotes the diffusion of Zr ions. The mold powder of this embodiment preferably contains Ca3ZrSi2O9 raw material as a main raw material of the ZrO2 source, and more preferably the raw material of the ZrO2 source consists of Ca3ZrSi2O9 raw material.

[0016] The Ca3ZrSi2O9 raw material contains a mineral having a crystal phase of Ca3ZrSi2O9 as a main component, and is preferably made of the mineral. The chemical composition of the mineral is preferably a stoichiometric composition, i.e., CaO: 40.9 mass%, ZrO2: 29.9 mass%, and SiO2: 29.2 mass%, and each of the components CaO, ZrO2, and SiO2 may deviate from the stoichiometric composition by up to 5 mass%. In addition, the mineral may contain components other than CaO, ZrO2, and SiO2 (e.g., MgO, Al2O3, TiO2, Fe2O3, Cr2O3, etc.) in a total of 3 mass% or less of solid solution. The Ca3ZrSi2O9 raw material may contain impurities in a total of 8 mass% or less as an industrial raw material. The impurities may be present in the Ca3ZrSi2O9 mineral, or may be present in the mold powder as other minerals. The Ca3ZrSi2O9 raw material can be produced by sintering or electrofusion using raw materials such as a CaO source, a ZrO2 source, and a SiO2 source as starting materials.

[0017] The content of the Ca3ZrSi2O9 raw material is preferably 1 to 15 parts by mass in terms of ZrO2 per 100 parts by mass of molten slag formed from the part of the mold powder excluding the ZrO2 source raw material. When the content of the Ca3ZrSi2O9 raw material is 1 part by mass or more in terms of ZrO2 per 100 parts by mass of molten slag formed from the part of the mold powder excluding the ZrO2 source raw material, the effect of suppressing melting damage to the submerged nozzle is remarkable, and when it is 15 parts by mass or less, the role of the mold powder, such as lubricity, can be fully exhibited. In addition, the "molten slag equivalent formed from the portion of the mold powder excluding the ZrO2 source raw material" is assumed to be the molten slag obtained by melting the portion of the mold powder excluding the ZrO2 source raw material, and does not include components supplied from the ZrO2 source raw material (e.g., CaO, ZrO2, SiO2, etc. of the Ca3ZrSi2O9 raw material) or components lost during melting (e.g., carbon raw materials for adjusting the slag formation rate, CO2 contained in limestone, etc.).

[0018] The particle size of the Ca3ZrSi2O9 raw material may be approximately the same as that of commonly used mold powders (eg, 150 μm or less).

[0019] The mold powder of this embodiment can use conventional raw materials other than the Ca3ZrSi2O9 raw material. The raw material of the CaO-SiO2 source can be, for example, wollastonite, the raw material of the CaO source can be, for example, limestone, cement, the raw material of the SiO2 source can be, for example, silica sand, diatomaceous earth, the raw material of the Na2O source can be, for example, soda ash, sodium carbonate, the raw material of the Li2O source can be, for example, lithium carbonate, the raw material of the Al2O3 source can be, for example, alumina clinker, the raw material of the MgO source can be, for example, magnesia clinker, the raw material of the F source can be, for example, fluorite, sodium fluoride, etc., as needed, to adjust the slag-forming rate of the mold powder.

[0020] The mass ratio (CaO / SiO2) (basicity) of CaO to SiO2 calculated as molten slag formed from the part of the mold powder of this embodiment excluding the raw material of the ZrO2 source is not particularly limited, and is preferably 0.5 to 1.5, for example. The CaO in the mass ratio (CaO / SiO2) does not include CaO converted from CaF2. In addition, all fluorine (F) contained in the part of the mold powder excluding the raw material of the ZrO2 source is converted to CaF2, and fluorides other than CaF2 are converted to their oxides. For example, when NaF is contained, the F of NaF is converted to CaF2, and Na is converted to Na2O.

[0021] The component analysis of the mold powder and molten slag is carried out, for example, as follows. That is, a predetermined amount of sample is taken from the mold powder, which is a mixture of multiple raw materials, heated to 1300°C or higher, melted to form molten slag, held for a predetermined time, and then quenched. The quenched molten slag is subjected to component analysis by conventional analytical methods, such as chemical analysis, X-ray diffraction, and electron microscope analysis (EPMA analysis, etc.). The component analysis of the Ca3ZrSi2O9 raw material is carried out in the same manner.

[0022] The form of the molding powder of the present embodiment is not particularly limited, and examples thereof include powder, extrusion molding granules, hollow spray granules, and agitation granules.

[0023] <Continuous casting method of steel> The continuous casting method of steel according to this embodiment uses the mold powder of this embodiment. Since the Ca3ZrSi2O9 raw material dissolves quickly into the molten slag and completely dissolves during continuous casting, this mold powder can effectively suppress the erosion of the immersion nozzle and, at the same time, does not generate ZrO2-based inclusions in the steel. The mold powder of this embodiment is particularly suitable for the continuous casting of thin slabs with a mold thickness of 100 mm or less and a casting speed of 2.0 m / min or more.

Example

[0024] Hereinafter, the examples of the present disclosure will be described in detail.

[0025] <Method for measuring ZrO2 dissolution amount> For the following Examples 1 to 2, the dissolution amount of ZrO2 into the molten slag was measured as follows.

[0026] 1.5 g of a raw material of a ZrO2 source in terms of ZrO2 was uniformly spread on the bottom of a platinum crucible with an inner diameter of 20 mm, and a mold powder (corresponding to "molten slag converted from the part of the mold powder excluding the raw material of the ZrO2 source") for forming 30 g of molten slag was placed thereon. This set was placed in an electric furnace, heated to 1450°C, held for 10 minutes, and then a part of the surface of the molten slag was sampled as a sample with a platinum spoon. The cooled sample was pulverized and subjected to chemical analysis of the ZrO2 content. It can be judged that the higher the ZrO2 content, the faster the dissolution rate of the raw material of the ZrO2 source into the molten slag. In addition, the component analysis of the molten slag formed from the mold powder excluding the ZrO2 source was performed in advance.

[0027] [Example 1] The difference in the amount of ZrO2 dissolved in molten slag depending on the ZrO2 source material was evaluated using the above-mentioned method for measuring the amount of ZrO2 dissolved. That is, in the present invention product 1, Ca3ZrSi2O9 raw material was used as the ZrO2 source raw material, and in the comparative products 1 and 2, baddeleyite (ZrO2) and zircon (ZrSiO4) were used as the ZrO2 source raw materials, respectively. Both raw materials were high-purity products with a purity of 99.0 mass% or more (the total amount of impurities other than ZrO2, CaO, and SiO2 was 1.0 mass% or less) and had a particle size of 5 μm or less. The chemical compositions of the Ca3ZrSi2O9 raw material and zircon were almost the same as the stoichiometric composition, except for the impurity components (the Ca3ZrSi2O9 raw material in the present invention product 1 is referred to as high-purity product A). That is, the stoichiometric compositions of Ca3ZrSi2O9 and zircon are CaO: 40.9 mass%, ZrO2: 29.9 mass%, SiO2: 29.2 mass%, and ZrO2: 67.2 mass%, SiO2: 32.8 mass%, respectively. 5.0 g of high purity product A, 1.5 g of baddeleyite, and 2.2 g of zircon were used. This is equivalent to 1.5 g in terms of ZrO2 (5 mass parts in terms of ZrO2 for 100 mass parts in terms of molten slag formed from the part of the mold powder excluding the raw material of the ZrO2 source). The chemical compositions of the molten slag formed from the part of the mold powder excluding the raw material of the ZrO2 source were all the same. Table 1 shows the ZrO2 source material used in the present invention 1 and comparative products 1 and 2, the chemical composition of the molten slag formed from the part of the mold powder excluding the ZrO2 source material, and the ZrO2 content of the molten slag sample. [Table 1]

[0028] The ZrO2 content of the molten slag samples was 0.8% by mass for baddeleyite in Comparative Example 1 and 1.2% by mass for zircon in Comparative Example 2, while it was 4.6% by mass for the Ca3ZrSi2O9 raw material in Invention Example 1. The dissolution rate of the ZrO2 source in Invention Example 1 was significantly faster. This is thought to be because the diffusion of Ca ions in calcium zirconate into the molten slag was very fast, which also promoted the diffusion of Zr ions.

[0029] [Example 2] The difference in the amount of ZrO2 dissolved in molten slag due to the chemical composition of the Ca3ZrSi2O9 raw material was evaluated using the above-mentioned method for measuring the amount of ZrO2 dissolved. That is, the chemical composition of the Ca3ZrSi2O9 raw material, excluding impurities other than ZrO2, CaO, and SiO2, was the same as that of high-purity product A of invention 1 in invention 2, ZrO2: 25.8 mass%, CaO: 41.7 mass%, SiO2: 32.5 mass% (high-purity product B) in invention 3, and ZrO2: 34.0 mass%, CaO: 41.2 mass%, SiO2: 24.8 mass% (high-purity product C) in invention 4. The deviation from the stoichiometric composition of the Ca3ZrSi2O9 raw material of inventions 3 and 4 is within ±5 mass%. For each of the present invention products 2 to 4, 30g of molten slag was used, and 5.0g of Ca3ZrSi2O9 raw material was used. The ZrO2 content of the present invention products 2 to 4 was 1.5g, 1.3g, and 1.7g, respectively. The chemical composition of the molten slag formed from the mold powder except for the ZrO2 source raw material was the same for all of the products. Table 2 shows the ZrO2 source raw material used for the present invention products 2 to 4, the chemical composition of the molten slag formed from the mold powder except for the ZrO2 source raw material, and the ZrO2 content of the molten slag sample. [Table 2]

[0030] The ZrO2 content of the molten slag samples reached 4.4 mass% for high purity product A, 4.2 mass% for high purity product B, and 4.3 mass% for high purity product C. Inventive products 2 to 4 all had a significantly faster dissolution rate of the ZrO2 source.

[0031] [Example 3] Continuous casting of steel was performed using the mold powder of the present disclosure, and the melting rate of the submerged nozzle and the surface of the cast piece were evaluated. A mold for a thin slab with a thickness of 75 mm and a width of 1850 mm was used, the casting speed was 4.8 m / min, and the casting time was 120 minutes. As the raw material for the ZrO2 source, in the present invention product 5, a Ca3ZrSi2O9 raw material with a purity of 95 mass% or more was used, and in the comparative product 3, a zircon sand with a purity of 99 mass% or more was used. The content of the raw material for the ZrO2 source of the present invention product 5 and the comparative product 3 was 5.0 parts by mass in terms of ZrO2, relative to 100 parts by mass of molten slag formed from the part of the mold powder excluding the raw material for the ZrO2 source. The particle size of the present invention product 5 and the comparative product 3 was 50 μm or less. Table 3 shows the chemical composition of the ZrO2 source material used in the present invention product 5 and the comparative product 3, calculated as the molten slag formed from the part of the mold powder excluding the ZrO2 source material, the corrosion rate of the submerged entry nozzle, and the observation results of the slab surface. [Table 3]

[0032] When mold powder of Comparative Example 3 was used, the melting speed of the powder line of the SEN was 0.17 mm / min in the direction of the inner diameter of the nozzle, and ZrO2-based inclusions were found on the surface of the slab. On the other hand, when mold powder of Invention Example 5 was used, the melting speed of the powder line of the SEN was 0.02 mm / min in the direction of the inner diameter of the nozzle, and no ZrO2-based inclusions were found on the surface of the slab.

[0033] When the mold powder of the present disclosure is used, the melting rate of the submerged entry nozzle is significantly slowed, so that the service life of the submerged entry nozzle is extended, the frequency of replacement is reduced, and the productivity of continuous casting is improved. At the same time, since ZrO2-based inclusions are not generated on the surface of the cast piece, the quality of the cast piece is also excellent. Thus, the remarkable superiority of the mold powder of the present disclosure has been demonstrated. The mold powder of the present disclosure is particularly suitable for continuous casting of thin slabs with a mold thickness of 100 mm or less and a casting speed of 2.0 m / min or more.

[0034] Although the present embodiment has been described in detail as above, it will be easily understood by those skilled in the art that many modifications are possible without substantially departing from the novel matters and effects of the present disclosure. Therefore, all such modifications are included in the scope of the present disclosure. For example, a term described at least once in the specification together with a different term having a broader meaning or the same meaning may be replaced with that different term anywhere in the specification. In addition, the configuration and operation of the manufacturing apparatus, etc. of the present embodiment are not limited to those described in the present embodiment, and various modifications are possible.

Claims

1. ZrO ２ Ca as a source of raw material ３ ZrSi ２ O ９ A molding powder comprising raw materials.

2. The molding powder according to claim 1, The Ca ３ ZrSi ２ O ９ The content of the raw material in the mold powder is ２ ZrO ２ The molding powder is characterized in that the amount of the molded article is 1 to 15 parts by mass in terms of the total mass of the molded article.

3. A method for continuous casting of steel, comprising using the molding powder according to claim 1 or 2.

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