Mold powder for continuous casting of steel and method for continuous casting of 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 Art

[0002] In the continuous casting of steel, molten steel stored in a tundish is poured into a mold through a submerged nozzle, cooled and solidified, and a solidified shell (solidification shell) is continuously drawn downward from the mold by rolls to continuously produce slabs, blooms, billets, and other various-shaped cast slabs. A powdery or granular mold powder is scattered on the surface of the molten steel in the mold, and the tip of the submerged nozzle is inserted into the molten steel in the mold.

[0003] The mold powder is melted by the heat of the molten steel (hereinafter, the molten mold powder is referred to as "molten slag") to cover the surface of the molten steel, and the molten slag flows into and is consumed between the solidification shell and the mold. The main roles of the mold powder from scattering to consumption are: (1) heat preservation and oxidation prevention of the molten steel surface; (2) absorption of non-metallic inclusions floating from the molten steel and purification of the molten steel; (3) maintenance of lubrication between the solidification shell and the mold; (4) control of the heat flux from the solidification shell to the mold, etc.

[0004] The main component of the mold powder is a CaO-SiO2 system, and it contains sub-components such as Na2O, Li2O, K2O, Al2O3, MgO, CaF2, etc. as required. The mold powder before use is generally a mixture of multiple raw materials. As the raw material for the CaO-SiO2 source, for example, wollastonite is used; as the raw material for the CaO source, for example, limestone, cement is used; as the raw material for the SiO2 source, for example, silica sand, diatomaceous earth is used; as the raw material for the Na2O source, for example, soda ash, sodium carbonate is used; as the raw material for the Li2O source, for example, lithium carbonate is used; as the raw material for the Al2O3 source, for example, alumina clinker is used; as the raw material for the MgO source, for example, magnesia clinker is used; as the raw material for the F source, for example, fluorite, sodium fluoride is used.

[0005] The main roles of the immersion nozzle are to prevent oxidation by the atmosphere when pouring the molten steel stored in the tundish into the mold, control the flow rate of the molten steel, control the flow of the molten steel in the mold, etc. The tip of the immersion nozzle is inserted into the molten steel in the mold, and since the surface of the molten steel is covered with molten slag, the middle part of the immersion nozzle comes into contact with the molten slag (powder line). As the material of the immersion nozzle, generally, alumina-carbon material is used for the main body, and zirconia-carbon material with relatively high corrosion resistance to molten slag is used near the powder line. Since the erosion rate of this zirconia-carbon material determines the service life and replacement frequency of the immersion nozzle, it has a great influence on the productivity of continuous casting. Therefore, in order to suppress the erosion rate of the zirconia-carbon material and further improve the service life of the immersion nozzle, mold powder containing ZrO₂ has been proposed.

[0006] For example, Patent Document 1 proposes a continuous casting mold additive characterized in that 0.5 to 15 wt% of zirconia is contained in an additive for continuous casting obtained by selectively adding a flux base material and a melting property adjusting material. Further, Patent Document 2 discloses a mold additive used when continuously casting Si-killed molten steel with sol.Al of 0.002% or less using an immersion nozzle or a nozzle having these materials in a slag line made of zirconia or a zircon-containing material. The mold additive for continuous casting is obtained by adding 2 to 6 wt% of a ZrO₂ component to a mold additive for continuous casting obtained by selectively adding a flux component and a melting property adjusting material, and mixing a base material previously melted as a whole with an aggregate such as carbon powder for adjusting the melting rate. As the raw material of the ZrO₂ source, baddeleyite and zircon sand are used, and as the continuous casting mold additive, a pre-melted product containing ZrO₂ and a pre-melted product containing ZrO₂ with a little NaF added to adjust the viscosity are used. Furthermore, Patent Document 3 proposes a mold powder having CaO, SiO₂, and a fluorine compound as basic components and containing 0 to 10 mass% of ZrO₂. As the raw material of the ZrO₂ source, zircon sand is used.

Prior Art Documents

Patent Documents

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 57-41862 [Patent Document 2] Japanese Patent Application Laid-Open No. 5-57411 [Patent Document 3] Japanese Patent Application Laid-Open No. 2001-179408 [Summary of the Invention] [Problems to be Solved by the Invention]

[0008] However, when using baddeleyite or zircon sand as the raw material for the ZrO2 source of the mold powder, their dissolution rate into the molten slag is slow, and in many cases, they 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 erosion of the immersion nozzle decreases. In addition, since the undissolved baddeleyite and zircon sand have a significantly higher density than the molten slag, they settle below the molten slag and may be incorporated into the molten steel, becoming ZrO2-based inclusions in the steel. These problems are particularly likely to occur in the continuous casting of thin slabs with a mold thickness of 100 mm or less. This is because in the 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. Therefore, the residence time of the molten slag on the molten steel surface is short, and the dissolution time of baddeleyite and zircon sand is insufficient.

[0009] On the other hand, when the pre-melted product containing ZrO2 proposed in Patent Document 2 is manufactured on an industrial scale, processes and fuel for melting are required, resulting in an increase in cost. In addition, baddeleyite and zircon sand may remain undissolved in the pre-melted product. Furthermore, since baddeleyite and zircon sand have a higher density than the molten slag, they settle below the molten slag during the melting process, and a problem of ZrO2 segregation occurs, where the ZrO2 concentration is higher at the bottom and lower at the top of the pre-melted product. Therefore, the practical use of the pre-melted product is substantially difficult.

[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 continuous casting method of steel characterized by using a mold powder according to one aspect of the present disclosure. Since the Ca3ZrSi2O9 raw material dissolves quickly into the molten slag and completely dissolves during continuous casting, this mold powder effectively suppresses the erosion of the immersion nozzle and at the same time does not generate ZrO2-based inclusions in the steel.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, preferred embodiments of the present disclosure will be described in detail. It should be noted that the embodiments described below do not unduly limit the content of the present disclosure described in the claims, and not all of the configurations described in these embodiments are essential as the solution means of the present disclosure.

[0015] <Mold Powder> The mold powder of this embodiment contains a Ca3ZrSi2O9 raw material as a raw material for a ZrO2 source. As will be described in detail in the examples below, the Ca3ZrSi2O9 raw material dissolves significantly faster into the molten slag than baddeleyite and zircon sand and completely dissolves during continuous casting. Therefore, this mold powder effectively suppresses the erosion of the immersion nozzle and at the same time does not generate ZrO2-based inclusions in the steel. Thus, prior melting treatment of the mold powder is also unnecessary. This is presumably because the diffusion of Ca ions and Si ions in the Ca3ZrSi2O9 raw material into the molten slag is very fast, which promotes the diffusion of Zr ions. The mold powder of this embodiment preferably contains a Ca3ZrSi2O9 raw material as the main raw material for the ZrO2 source, and more preferably the raw material for the ZrO2 source consists of the Ca3ZrSi2O9 raw material.

[0016] The Ca3ZrSi2O9 raw material contains, as the main component, a mineral with a crystal phase of Ca3ZrSi2O9, and preferably consists of such a mineral. The chemical composition of the mineral is preferably the stoichiometric composition, i.e., CaO: 40.9% by mass, ZrO2: 29.9% by mass, SiO2: 29.2% by mass, and each component of CaO, ZrO2, and SiO2 may deviate from the stoichiometric composition by up to 5% by mass. Further, the mineral may contain up to 3% by mass in total of components other than CaO, ZrO2, and SiO2 (for example, MgO, Al2O3, TiO2, Fe2O3, Cr2O3, etc.) in solid solution. The Ca3ZrSi2O9 raw material may contain impurities of 8% by mass or less in total 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 in terms of molten slag formed from the part of the mold powder excluding the raw material of the ZrO2 source. When the content of the Ca3ZrSi2O9 raw material is 1 part by mass or more in terms of ZrO2 per 100 parts by mass in terms of molten slag formed from the part of the mold powder excluding the raw material of the ZrO2 source, the effect of suppressing the melting loss of the immersion nozzle is remarkable, and when it is 15 parts by mass or less, the role of the mold powder such as lubricity can be sufficiently exerted. Note that "in terms of molten slag formed from the part of the mold powder excluding the raw material of the ZrO2 source" assumes the molten slag obtained by melting the part of the mold powder excluding the raw material of the ZrO2 source, and does not include components supplied from the raw material of the ZrO2 source (for example, CaO, ZrO2, SiO2, etc. of the Ca3ZrSi2O9 raw material) and components that disappear during melting (for example, carbon raw material for adjusting the slagging rate, CO2 contained in limestone, etc.).

[0018] The particle size of the Ca3ZrSi2O9 raw material may be on the same order as that of a commonly used mold powder (for example, 150 μm or less).

[0019] For the mold powder of the present embodiment, conventional raw materials other than the Ca3ZrSi2O9 raw material can be used. As the raw material for the CaO-SiO2 source, for example, wollastonite can be used. As the raw material for the CaO source, for example, limestone, cement can be used. As the raw material for the SiO2 source, for example, silica sand, diatomaceous earth can be used. As the raw material for the Na2O source, for example, soda ash, sodium carbonate can be used. As the raw material for the Li2O source, for example, lithium carbonate can be used. As the raw material for the Al2O3 source, for example, alumina clinker can be used. As the raw material for the MgO source, for example, magnesia clinker can be used. As the raw material for the F source, for example, fluorite, sodium fluoride, etc. can be used. For adjusting the slagging rate of the mold powder, a carbon raw material, for example, carbon black, coke powder, graphite, etc. may be added as necessary.

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

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

[0022] The form of the mold powder of the present embodiment is not particularly limited, and examples include powder, extruded granules, hollow spray granules, agitation granulation, etc.

[0023] <Continuous casting method of steel> The continuous casting method of steel in this embodiment uses the mold powder of this embodiment. Since the Ca3ZrSi2O9 raw material dissolves quickly in 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 in 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 sprayed on the bottom of a platinum crucible with an inner diameter of 20 mm, and a mold powder (corresponding to "molten slag formed 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 in 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 the molten slag depending on the raw material of the ZrO2 source was evaluated by the above method for measuring the ZrO2 dissolution amount. That is, in Invention Product 1, Ca3ZrSi2O9 raw material was used as the raw material of the ZrO2 source, and in Comparative Products 1 and 2, baddeleyite (ZrO2) and zircon (ZrSiO4) were used as the raw materials of the ZrO2 source, respectively. All of these raw materials were high-purity products with a purity of 99.0% by mass or more (the total amount of impurity components other than ZrO2, CaO, and SiO2 was 1.0% by mass or less), and those with a particle size of 5 μm or less were used. The chemical compositions of the Ca3ZrSi2O9 raw material and zircon were almost the same as the stoichiometric compositions except for the impurity components (the Ca3ZrSi2O9 raw material of Invention Product 1 is referred to as high-purity product A). That is, the stoichiometric compositions of Ca3ZrSi2O9 and zircon are 40.9% by mass of CaO, 29.9% by mass of ZrO2, 29.2% by mass of SiO2, and 67.2% by mass of ZrO2, 32.8% by mass of SiO2, respectively. 5.0 g of high-purity product A, 1.5 g of baddeleyite, and 2.2 g of zircon were used. This corresponds to 1.5 g in terms of ZrO2 (5 parts by mass in terms of ZrO2 with respect to 100 parts by mass of the molten slag formed from the part of the mold powder excluding the raw material of the ZrO2 source). The chemical compositions in terms of the molten slag formed from the part of the mold powder excluding the raw material of the ZrO2 source were made the same. Table 1 shows the raw materials of the ZrO2 source used in Invention Product 1 and Comparative Products 1 and 2, the chemical compositions in terms of the molten slag formed from the part of the mold powder excluding the raw material of the ZrO2 source, and the ZrO2 content of the molten slag sample. [Table 1]

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

[0029] [Example 2] The difference in the amount of ZrO₂ dissolved in the molten slag depending on the chemical composition of the Ca₃ZrSi₂O₉ raw material was evaluated by the above method for measuring the ZrO₂ dissolution amount. That is, the chemical composition of the Ca₃ZrSi₂O₉ raw material excluding impurity components other than ZrO₂, CaO, and SiO₂ was made the same as that of the high-purity product A of the present invention product 1 in the present invention product 2, and in the present invention product 3, ZrO₂: 25.8% by mass, CaO: 41.7% by mass, SiO₂: 32.5% by mass (high-purity product B), and in the present invention product 4, ZrO₂: 34.0% by mass, CaO: 41.2% by mass, SiO₂: 24.8% by mass (high-purity product C). The deviation of the Ca₃ZrSi₂O₉ raw materials of the present invention products 3 and 4 from the stoichiometric composition is within ±5% by mass. For all of the present invention products 2 to 4, 30 g of molten slag formed from the mold powder excluding the Ca₃ZrSi₂O₉ raw material and 5.0 g of the Ca₃ZrSi₂O₉ raw material were used. The ZrO₂-equivalent contents of the present invention products 2 to 4 are 1.5 g, 1.3 g, and 1.7 g, respectively. The chemical compositions in terms of molten slag formed from the part of the mold powder excluding the raw material of the ZrO₂ source were made the same. Table 2 shows the chemical compositions 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 raw materials of the ZrO₂ source used in the present invention products 2 to 4, and the ZrO₂ contents of the molten slag samples.

Table 2

[0030] The ZrO₂ content of the molten slag sample reached 4.4% by mass in the high-purity product A, 4.2% by mass in the high-purity product B, and 4.3% by mass in the high-purity product C, and in all of the present invention products 2 to 4, the dissolution rate of the ZrO₂ source was significantly faster.

[0031] [Example 3] Continuous casting of steel was carried out using the mold powder of the present disclosure, and the erosion rate of the immersion nozzle and the surface of the slab were evaluated. A mold for thin slabs 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 of the ZrO2 source, in the inventive product 5, a Ca3ZrSi2O9 raw material with a purity of 95% by mass or more was used, and in the comparative product 3, zircon sand with a purity of 99% by mass or more was used. The content of the raw material of the ZrO2 source in the inventive product 5 and the comparative product 3 was set to 5.0 parts by mass in terms of ZrO2 with respect to 100 parts by mass of the molten slag formed from the part of the mold powder excluding the raw material of the ZrO2 source. The particle size of the inventive product 5 and the comparative product 3 was 50 μm or less. Table 3 shows the chemical composition in terms of the molten slag formed from the raw material of the ZrO2 source used in the inventive product 5 and the comparative product 3 and the part of the mold powder excluding the raw material of the ZrO2 source, the erosion rate of the immersion nozzle, and the observation results of the slab surface.

Table 3

[0032] When the mold powder of the comparative product 3 was used, the erosion rate of the powder line of the immersion nozzle was 0.17 mm / min in the inner diameter direction of the nozzle, and ZrO2-based inclusions were observed on the surface of the slab. On the other hand, when the mold powder of the inventive product 5 was used, the erosion rate of the powder line of the immersion nozzle was 0.02 mm / min in the inner diameter direction of the nozzle, and no ZrO2-based inclusions were observed on the surface of the slab.

[0033] When the mold powder of the present disclosure is used, since the erosion rate of the immersion nozzle is significantly slow, the service life of the immersion nozzle is prolonged, the replacement frequency is reduced, and the productivity of continuous casting is improved. At the same time, since no ZrO2-based inclusions are generated on the surface of the slab, the slab quality is also excellent. Thus, the remarkable superiority of the mold powder of the present disclosure was demonstrated. The mold powder of the present disclosure 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.

[0034] Although the present embodiment has been described in detail as above, those skilled in the art will easily understand that many modifications can be made without substantially departing from the novel matters and effects of the present disclosure. Therefore, all such modified examples are included in the scope of the present disclosure. For example, in the specification, a term that is described at least once together with a different term that is broader or synonymous can be replaced with that different term anywhere in the specification. Also, the configuration and operation of the manufacturing apparatus and the like of the present embodiment are not limited to those described in the present embodiment, and various modifications are possible.

Claims

1. ZrO ２ uses Ca as the main raw material of the source ３ ZrSi ２ O ９ contains raw materials In the mold powder which may or may not contain baddeleyite, zircon or zircon sand as a secondary raw material of the ZrO₂ source, The mold powder is characterized in that the content of the Ca₃ZrSi₂O₉ raw material is 1 to 15 parts by mass in terms of ZrO₂ with respect to 100 parts by mass of the molten slag formed from the part of the mold powder excluding the raw material of the ZrO₂ source.

2. In the mold powder according to Claim 1, The mold powder is characterized in that the raw material of the ZrO₂ source consists of the Ca₃ZrSi₂O₉ raw material.

3. A continuous casting method for steel, characterized by using the mold powder according to Claim 1 or 2.

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