Mold powder and method for continuous casting of steel using the same

Abstract

To provide mold powder in which the raw materials of a ZrO2 source are melted into molten slag within a limited time, the erosion of an immersion nozzle is effectively suppressed, further, ZrO2 inclusions in steel are not generated, and preliminary melting treatment is made unnecessary.SOLUTION: The mold powder contains a zirconia-mullite material as a main material of the ZrO2 source.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In continuous steel casting, molten steel stored in a tundish is poured into a mold through an immersion nozzle, where it is cooled and solidified, while the solidified shell is continuously drawn below the mold by rolls, thereby continuously producing cast pieces of various shapes such as slabs, blooms, and billets. 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. The molten slag flows between the solidified shell and the mold, and is discharged and consumed alongside the solidified shell. The main roles of mold powder from the time it is spread until it is consumed are as follows: (1) Heat retention of the molten steel surface (2) Prevention of oxidation on the surface of molten steel (3) Absorption of non-metallic inclusions that float up from the molten steel and purification of the molten steel (4) Ensuring lubrication between the solidified shell and the mold (5) Control of heat flux from the solidified shell to the mold

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

[0005] The primary functions of the SEN are to prevent atmospheric oxidation when pouring molten steel from the tundish into the mold, to control the flow rate of molten steel, and to control the flow of molten steel within 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. The intermediate portion of the SEN comes into contact with the molten slag (the powder line). SENs are typically made of alumina-carbon materials for the main body, while zirconia-carbon materials, which have relatively high corrosion resistance to molten slag, are used near the powder line. The rate of corrosion of this zirconia-carbon material determines the service life and replacement frequency of the SEN, significantly affecting the productivity of continuous casting. Therefore, mold powders containing ZrO2 have been proposed to suppress the rate of corrosion of zirconia-carbon materials and thereby extend the service life of the SEN.

[0006] For example, Patent Document 1 proposes a mold additive for continuous casting (mold powder) characterized by containing 0.5 to 15 wt% of zirconia in a steady-state casting additive containing a flux base material and a melt property adjuster as selected. Patent Document 2 also proposes a mold additive for continuous casting used when continuously casting Si-deoxidized molten steel containing 0.002% or less of sol. Al using a submerged entry nozzle or slag line (powder line) made of a zirconia or zircon-containing material. The mold additive for continuous casting is used when a steady-state casting additive containing a flux component and a melt property adjuster as selected is added, and contains 2 to 6 wt% of ZrO2, and is mixed with a base material that has been melt-treated in its entirety in advance and an aggregate such as carbon powder for adjusting the melting rate. Baddeleyite (ZrO2) and zircon sand (ZrSiO4) are used as raw materials for the ZrO2 source, and a ZrO2-containing pre-melt product and a ZrO2-containing pre-melt product with a small amount of NaF added to adjust the viscosity are used as mold additives for continuous casting. Patent Document 3 proposes a mold powder containing CaO, SiO2, and a fluorine compound as basic components and containing 0 to 10 mass% ZrO2. Zircon sand is used as a raw material for the ZrO2 source. Patent Document 4 proposes a mold powder characterized by containing calcium zirconate (CaZrO3) as a 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] Japanese Patent Application Laid-Open No. 2001-179408 [Patent Document 4] Japanese Patent Application Publication No. 2023-184091 Summary of the Invention [Problem to be solved by the invention]

[0008] However, when baddeleyite or zircon sand is used as the ZrO2 source material for mold powder, its dissolution rate in molten slag is slow and it often fails to dissolve completely in the molten slag within the limited time. In this case, the ZrO2 content in the molten slag is insufficient, reducing the effectiveness of preventing SEN corrosion. Furthermore, because 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 incorporated into the molten steel surface, forming ZrO2-based inclusions in the steel. These problems are particularly likely to occur in thin slab continuous casting with a mold thickness of 100 mm or less. This is thought to be due to the small mold dimensions in the slab thickness direction and the high casting speed of 2.0 m / min or more, which results in a short residence time of the molten slag above the molten steel surface, resulting in insufficient time for the baddeleyite and zircon sand to dissolve.

[0009] On the other hand, the ZrO2-containing pre-melted product proposed in Patent Document 2 requires a melting process and fuel for industrial-scale production, which increases costs. Furthermore, 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 settle to the bottom of the molten slag during the melting process, resulting in a problem of ZrO2 segregation, with the ZrO2 concentration being higher at the bottom and lower at the top of the pre-melted product. Therefore, the practical use of pre-melted products is substantially difficult.

[0010] Although it has been shown that calcium zirconate proposed in Patent Document 4 has a faster dissolution rate in molten slag than baddeleyite and zircon sand, this is not sufficient.

[0011] The present disclosure has been made in consideration of the above-mentioned circumstances, and its object is to provide a mold powder in which a ZrO source raw material dissolves in molten slag within a limited time, effectively suppressing melting damage to the submerged entry nozzle while at the same time not producing ZrO-based inclusions in the steel, and not requiring a prior melting treatment, and a method for continuous casting steel using the same. [Means for solving the problem]

[0012] (1) One aspect of the present disclosure is This mold powder is characterized by containing a zirconia-mullite raw material as the main ZrO2 source. The zirconia-mullite raw material dissolves quickly in molten slag and completely during continuous casting. This mold powder effectively suppresses corrosion of the submerged entry nozzle and does not produce ZrO2-based inclusions in the steel. Therefore, prior melting treatment of the mold powder is not required.

[0013] (2) In one aspect of the present disclosure, The zirconia-mullite raw material preferably contains, as a chemical composition, 30 to 50 mass% ZrO2, 40 to 55 mass% Al2O3, and 10 to 25 mass% SiO2, allowing a sufficient amount of ZrO2 to be rapidly supplied to the molten slag.

[0014] (3) In one aspect of the present disclosure, The content of the zirconia-mullite raw material is preferably 1 to 15 parts by mass, calculated as ZrO2, relative to 100 parts by mass of molten slag formed from the portion of the molding powder excluding the ZrO2 source raw material. When the content of the zirconia-mullite raw material is 1 part by mass or more, calculated as ZrO2, relative to 100 parts by mass of molten slag formed from the portion of the molding powder excluding the ZrO2 source raw material, the effect of suppressing melting damage to the submerged entry nozzle is significant, and when the content is 15 parts by mass or less, the role of the molding powder, such as lubricity, can be fully exhibited. It should be noted that "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 (for example, ZrO2, Al2O3, SiO2, etc. of the zirconia-mullite raw material) or components that disappear during melting (for example, carbon raw materials for adjusting the slag formation rate, CO2 contained in limestone, etc.).

[0015] (4) Another aspect of the present disclosure is The present disclosure relates to a continuous casting method for steel, characterized by using a mold powder according to one embodiment of the present disclosure. Since the zirconia-mullite raw material dissolves quickly in molten slag and completely dissolves during continuous casting, this mold powder effectively suppresses corrosion of the submerged entry nozzle and prevents the formation of ZrO-based inclusions in the steel. DETAILED DESCRIPTION OF THE INVENTION

[0016] Preferred embodiments of the present disclosure will be described in detail below. Note 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 the embodiments are necessarily essential as means for solving the problems of the present disclosure.

[0017] <Mold powder> The molding powder of this embodiment contains a zirconia-mullite raw material as the main raw material of the ZrO2 source, and more preferably, the raw material of the ZrO2 source is a zirconia-mullite raw material. As will be shown in detail in the examples described below, the zirconia-mullite raw material dissolves in molten slag significantly faster than baddeleyite, zircon sand, and calcium zirconate, and dissolves completely during continuous casting. Therefore, this molding powder effectively suppresses corrosion of the submerged entry nozzle and does not produce ZrO2-based inclusions in the steel. Therefore, prior melting treatment of the molding powder is not required. This is thought to be due to the high surface activity of the zirconia phase in the zirconia-mullite raw material, which diffuses very quickly into the molten slag.

[0018] The zirconia-mullite raw material contains zirconia and mullite phases as the main crystalline phases. The mullite phase may have a stoichiometric composition of 3Al2O3·2SiO2, or a non-stoichiometric composition rich in Al2O3 or SiO2. The zirconia-mullite raw material preferably contains 30-50 mass% ZrO2, 40-55 mass% Al2O3, and 10-25 mass% SiO2. This allows for the rapid supply of a sufficient amount of ZrO2 to the molten slag. The zirconia-mullite raw material may also contain 5 mass% or less of impurities other than ZrO2, Al2O3, and SiO2. The zirconia-mullite raw material can be produced, for example, by electromelting at approximately 2500°C or higher using raw materials such as a ZrO2 source, an Al2O3 source, and a SiO2 source.

[0019] The content of the zirconia-mullite raw material is preferably 1 to 15 parts by mass, calculated as ZrO2, relative to 100 parts by mass of molten slag formed from the portion of the mold powder excluding the ZrO2 source raw material. When the content of the zirconia-mullite raw material is 1 part by mass or more, calculated as ZrO2, relative to 100 parts by mass of molten slag formed from the portion of the mold powder excluding the ZrO2 source raw material, the effect of suppressing melting damage to the submerged entry nozzle is significant, and when the content is 15 parts by mass or less, the role of the mold powder, such as lubricity, can be fully exhibited. It should be noted that "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 (for example, ZrO2, Al2O3, SiO2, etc. of the zirconia-mullite raw material) or components that disappear during melting (for example, carbon raw materials for adjusting the slag formation rate, CO2 contained in limestone, etc.).

[0020] The particle size of the zirconia-mullite raw material may be approximately the same as the particle size of commonly used mold powder (for example, 150 μm or less).

[0021] The molding powder of this embodiment can use conventional raw materials other than the zirconia-mullite raw material. Examples of the CaO-SiO2 source raw material include wollastonite, CaO source raw materials include limestone and cement, SiO2 source raw materials include silica sand and diatomaceous earth, Na2O source raw materials include soda ash and sodium carbonate, Li2O source raw materials include lithium carbonate, Al2O3 source raw materials include alumina clinker, MgO source raw materials include magnesia clinker, and F source raw materials include fluorite and sodium fluoride, etc. Carbon raw materials such as carbon black, coke powder, and graphite may be added as needed to adjust the slag formation rate of the molding powder.

[0022] The mass ratio (CaO / SiO2) (basicity) of CaO to SiO2 calculated as molten slag formed from the portion of the molding powder of this embodiment excluding the ZrO2 source raw material is not particularly limited, and is preferably 0.5 to 1.5, for example. Note that the CaO in the mass ratio (CaO / SiO2) does not include CaO converted from CaF2. In addition, all fluorine (F) contained in the portion of the molding 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.

[0023] The component analysis of mold powder and molten slag is carried out, for example, as follows: A predetermined amount of sample is taken from 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 rapidly cooled. The rapidly cooled molten slag is subjected to component analysis using conventional analytical methods, such as chemical analysis, X-ray diffraction, and electron microscope analysis (EPMA analysis, etc.).

[0024] The form of the molding powder of this embodiment is not particularly limited, and examples thereof include powder, extrusion-molded granules, hollow spray granules, and agitation granules.

[0025] <Continuous steel casting method> The continuous casting method of steel in this embodiment uses the mold powder of this embodiment. Since the zirconium light raw material dissolves quickly in 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. 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

[0026] Hereinafter, the examples of the present disclosure will be described in detail.

[0027] <Measurement method of ZrO2 dissolution amount> For the following Examples 1 to 2, the dissolution amount of ZrO2 in the molten slag was measured as follows.

[0028] On the bottom of a platinum crucible with an inner diameter of 20 mm, 1.5 g of a raw material of a ZrO2 source was uniformly sprayed, and a mold powder (corresponding to "molten slag formed from the part of the mold powder excluding the raw material of the ZrO2 source") forming 30 g of molten slag was placed thereon. This set was placed in an electric furnace, heated to 1400 °C, held for 10 minutes, and then a part of the surface of the molten slag was sampled with a platinum spoon as a sample. The cooled sample was pulverized, and the ZrO2 content was measured by chemical analysis. 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.

[0029] [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. Specifically, 3.7 g of zirconia-mullite material was used as the ZrO2 source material for Product 1 of the present invention, while 1.5 g of baddeleyite (ZrO2), 2.2 g of zircon (ZrSiO4), and 2.2 g of calcium zirconate (CaZrO3) were used for Comparative Products 1-3, respectively. The mass of the ZrO2 source material for Product 1 of the present invention and Comparative Products 1-3 was 1.5 g in terms of ZrO2 (5 parts by mass of ZrO2 per 100 parts by mass of molten slag formed from the mold powder excluding the ZrO2 source material). All ZrO2 source materials had a particle size of 5 μm or less and a purity of 99.5% by mass or more (a total of 0.5% by mass or less of impurities). The main crystalline phase of the zirconia-mullite raw material is composed of a zirconia phase and a stoichiometric mullite phase, with a chemical composition of 40.5 mass% ZrO2, 42.7 mass% Al2O3, and 16.8 mass% SiO2 (hereinafter, the zirconia-mullite raw material used in Invention Product 1 is referred to as High Purity Product A). Meanwhile, the chemical compositions of zircon and calcium zirconate are 67.2 mass% ZrO2, 32.8 mass% SiO2, and 68.7 mass% ZrO2, 31.3 mass% CaO, respectively.

[0030] The chemical composition of the molding powders of invention product 1 and comparative products 1 to 3, calculated as molten slag formed from the portion excluding the raw material of the ZrO source, was the same, that is, CaO: 33.6 mass%, SiO2: 25.4 mass%, Na2O: 5.2 mass%, Li2O: 2.9 mass%, Al2O3: 4.7 mass%, MgO: 0.9 mass%, and CaF2: 27.3 mass%.

[0031] The amount of dissolved ZrO2 was measured for Invention Product 1 and Comparative Products 1 to 3. Table 1 shows the ZrO2 source materials used in Invention Product 1 and Comparative Products 1 to 3, the chemical compositions of the main components of the ZrO2 source materials, the chemical compositions of the molten slag formed from the portion of the molding powder excluding the ZrO2 source materials, and the ZrO2 content of the molten slag sample. [Table 1]

[0032] The ZrO2 content of the molten slag samples was 0.9 mass% for baddeleyite in Comparative Product 1, 1.1 mass% for zircon in Comparative Product 2, and 2.3 mass% for calcium zirconate in Comparative Product 3, while it was 4.7 mass% for the zirconia-mullite raw material in Invention Product 1. The dissolution rate of the ZrO2 source in Invention Product 1 was significantly faster. This is thought to be due to the high surface activity of the zirconia phase in the zirconia-mullite raw material, which diffuses very quickly into the molten slag.

[0033] [Example 2] The difference in the amount of ZrO dissolved in the molten slag due to the chemical composition of the zirconia-mullite raw material was evaluated using the above-mentioned method for measuring the amount of ZrO dissolved. Specifically, for Inventions 2 to 4, 5.0 g of zirconia-mullite raw material was used as the ZrO source raw material, and the molten slag formed from the portion excluding the zirconia-mullite raw material was 30 g. The chemical composition of the zirconia-mullite raw material for Invention 2 was the same as that of High-Purity Product A of Invention 1 (40.5 mass% ZrO, 42.7 mass% AlO, 16.8 mass% SiO). Invention 3 had 44.2 mass% ZrO, 40.1 mass% AlO, 15.7 mass% SiO (High-Purity Product B). Invention 4 had 36.1 mass% ZrO, 45.9 mass% AlO, 18.0 mass% SiO (High-Purity Product C). The masses of the zirconia-mullite raw materials of Inventions 2 to 4 were equivalent to 2.0 g, 2.2 g, and 1.8 g, respectively, in terms of ZrO2. All of the zirconia-mullite raw materials used were high-purity products with a particle size of 5 μm or less and a purity of 99.5 mass% or more (total impurities of 0.5 mass% or less).

[0034] The chemical composition of the molding powders of inventions 2 to 4, calculated as molten slag formed from the portion excluding the raw material of the ZrO2 source, was the same, that is, CaO: 21.4 mass%, SiO2: 38.9 mass%, Na2O: 11.6 mass%, Li2O: 1.3 mass%, Al2O3: 5.2 mass%, MgO: 2.4 mass%, and CaF2: 19.2 mass%.

[0035] The amount of dissolved ZrO2 was measured for Invention Products 2 to 4. Table 2 shows the zirconia-mullite raw material (ZrO2 source raw material) used for Invention Products 2 to 4, the chemical composition of the zirconia-mullite raw material, the chemical composition of the molten slag formed from the portion of the mold powder excluding the ZrO2 source raw material, and the ZrO2 content of the molten slag sample. [Table 2]

[0036] The ZrO2 content of the molten slag samples reached 4.8 mass% for high-purity product A, 4.7 mass% for high-purity product B, and 4.4 mass% for high-purity product C, and the dissolution rate of the ZrO2 source was significantly faster for all of invention products 2 to 4.

[0037] [Example 3] Continuous casting of steel was performed using the mold powder of the present disclosure, and the erosion rate of the submerged entry nozzle and the cast slab surface were evaluated. A mold for thin slabs, 75 mm thick and 1850 mm wide, was used, with a casting speed of 4.8 m / min and a casting time of 120 minutes. As the ZrO2 source material, Inventive Product 5 used a zirconia-mullite raw material (high-purity product A) with a purity of 99% by mass or more, while Comparative Product 4 used zircon sand with a purity of 99% by mass or more. The content of the ZrO2 source material for Inventive Product 5 and Comparative Product 4 was 5.0 parts by mass, calculated as ZrO2, per 100 parts by mass of molten slag formed from the mold powder excluding the ZrO2 source material. The particle size of Inventive Product 5 and Comparative Product 4 was 50 μm or less.

[0038] Table 3 shows the ZrO2 source materials used in Invention Product 5 and Comparative Product 4, the chemical compositions of the main components of the ZrO2 source materials, the chemical composition of the molten slag formed from the mold powder excluding the ZrO2 source material, the corrosion rate of the submerged entry nozzle, and the observation results of ZrO2 inclusions on the slab surface. [Table 3]

[0039] When mold powder of Comparative Example 4 was used, the rate of wear of the powder line of the submerged entry nozzle in the radial direction of the nozzle was 0.17 mm / min, and ZrO2-based inclusions were observed on the surface of the slab. On the other hand, when mold powder of Invention Example 5 was used, the rate of wear of the powder line of the submerged entry nozzle in the radial direction of the nozzle was 0.02 mm / min, and no ZrO2-based inclusions were observed on the surface of the slab.

[0040] When the mold powder of the present disclosure is used, the rate of wear of the submerged entry nozzle is significantly slowed, thereby extending the service life of the submerged entry nozzle, reducing the frequency of replacement, and improving the productivity of continuous casting. At the same time, ZrO2-based inclusions are not formed on the surface of the cast slab, resulting in excellent cast slab quality. In this way, the significant advantages of the mold powder of the present disclosure have 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.

[0041] Although the present embodiment has been described in detail above, it will be readily apparent to those skilled in the art that many modifications are possible without substantially departing from the novel features and advantages of the present disclosure. Therefore, all such modifications are included within 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 or equivalent meaning may be replaced with that different term anywhere in the specification. Furthermore, 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 A molding powder characterized by containing a zirconia-mullite raw material as a main raw material of the source.

2. The molding powder according to claim 1, The zirconia-mullite raw material has a chemical composition of ZrO ２ :30 to 50% by mass, Al ２ O ３ :40 to 55% by mass, SiO ２ : A molding powder characterized by containing 10 to 25 mass %.

3. The molding powder according to claim 1, The content of the zirconia-mullite raw material is ２ ZrO per 100 parts by mass of molten slag formed from the portion excluding the source raw material ２ The molding powder is characterized in that the amount is 1 to 15 parts by mass in terms of the total mass of the molding powder.

4. A method for continuous casting of steel, characterized by using the mold powder according to any one of claims 1 to 3.

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