Continuous casting method for steel

Abstract

The present invention provides a continuous casting method for steel that can improve the durability of immersion nozzles. [Solution] A continuous casting method for steel using an immersion nozzle and mold powder, wherein the powder line portion of the immersion nozzle contains a zirconia-carbon refractory material containing zirconia and graphite, and a nitride in an external amount of 0.5% to 5.0% relative to the total mass of the zirconia-carbon refractory material, and the mold powder slag formed on the surface of the molten steel contains 1% to 15% titanium oxide.

Description

Technical Field

[0001] The present invention relates to a method for continuous casting of steel using a dipping nozzle and mold powder.

Background Art

[0002] In the continuous casting of steel, a dipping nozzle is used to introduce molten steel from a tundish into a mold. The dipping nozzle plays an important role in preventing oxidation of the molten steel by the atmosphere, rectifying and controlling the flow direction of the molten steel. Its stable use is extremely important for ensuring the productivity of continuous casting and the quality of steel products. Materials such as alumina-graphite and alumina-silica-graphite are arranged in the dipping nozzle, especially in the part that contacts the molten steel.

[0003] In the continuous casting of steel, mold powder is supplied onto the molten steel for the purposes of suppressing reoxidation of the molten steel, absorbing inclusions in the molten steel, ensuring lubricity by flowing between the molten steel and the mold, and controlling heat extraction from the molten steel to the mold. It melts by heat on the surface of the molten steel to form mold powder slag (hereinafter abbreviated as slag). Mold powder typically has CaO and SiO2 as main components and may contain components such as various metal oxides and fluorine. The erosiveness of slag against the components of general refractories is very high.

[0004] In order to reduce the erosion of the dipping nozzle by slag, materials such as zirconia-carbon (ZrO2·C), which have relatively excellent corrosion resistance against slag, are arranged in the part where the slag contacts on the outer peripheral side of the dipping nozzle (hereinafter referred to as the powder line part). Nevertheless, in the dipping nozzle, the erosion of the powder line part is the largest, and the corrosion resistance of this part restricts the durability of the dipping nozzle.

[0005] Furthermore, when casting steel types that contain titanium as a component (hereinafter referred to as titanium-added steel), the reaction shown in Equation 1 below may occur between the titanium element in the molten steel and the SiO2 component in the slag, which may increase the amount of titanium oxide (TiO2) in the slag. Ti + SiO2 → TiO2 + Si (Equation 1)

[0006] Slag containing titanium oxide is more prone to eroding the material in the powder line than slag that does not contain titanium oxide. The reaction in Equation 1 does not occur stably during continuous casting, and the composition of the slag can fluctuate during continuous casting. As mentioned above, the corrosion resistance of the powder line governs the lifespan of the immersion nozzle, so fluctuations in the slag composition affect the corrosion resistance of the powder line, which in turn affects the lifespan of the immersion nozzle. As a result, the lifespan of the immersion nozzle can vary greatly depending on the type of steel being produced, hindering the stable operation of continuous casting.

[0007] Common methods for improving the corrosion resistance of the powder line include using refractories with a high zirconia content and a low graphite content, and adjusting the composition of the mold powder to produce a highly viscous slag that suppresses the erosive properties of the slag. Furthermore, Patent Document 1 discloses a method of adding raw materials such as silica to assist in the sintering of zirconia in the graphite-burned portion caused by the carburization of graphite in molten steel. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 57-007366 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, excessively reducing the graphite content may result in insufficient thermal shock resistance, thus limiting the improvement in corrosion resistance achieved by increasing the zirconia content. Furthermore, adjusting the composition of the mold powder may impair the functions originally expected of the mold powder, making significant changes to the mold powder's composition impractical in some cases. Moreover, even with the invention described in Patent Document 1, further improvement in the corrosion resistance of the powder line portion may be desired.

[0010] The object of the present invention is to provide a method for improving the corrosion resistance of the powder line section in continuous casting of steel. In particular, it is to achieve stable operation without significantly changing the corrosion resistance of the powder line section, even when using molten steel containing titanium. [Means for solving the problem]

[0011] The present invention relates to a continuous casting method for steel using an immersion nozzle and mold powder, wherein the powder line portion of the immersion nozzle contains a zirconia-carbon refractory material containing zirconia and graphite, and a nitride in an external amount of 0.5% to 5.0% by mass relative to the total mass of the zirconia-carbon refractory material, and the slag formed on the surface of the molten steel contains 1% to 15% by mass of titanium oxide.

[0012] This method improves the corrosion resistance of the powder line and stabilizes the operation of the continuous steel casting process.

[0013] According to a preferred embodiment of the method of the present invention, at least a portion of the titanium elements in the titanium oxide are derived from the mold powder.

[0014] According to this embodiment, even when the molten steel does not contain titanium, it is easy to obtain the effect of improving the corrosion resistance of the powder line portion.

[0015] According to a preferred embodiment of the method according to the present invention, the mold powder is a composition in which at least one selected from a cuspidine phase and a glass phase is a main component when solidified.

[0016] According to this aspect, a phase generally used in continuous casting of steel is obtained, and it is easy to control the heat extraction characteristics of the slag.

[0017] According to a preferred embodiment of the method according to the present invention, a layer containing titanium nitride is formed on the surface of the powder line part of the immersion nozzle that contacts the slag.

[0018] According to this aspect, since titanium nitride has a high melting point, it substantially functions as a protective layer for the powder line part of the immersion nozzle, and it is easy to improve the corrosion resistance of the powder line part.

Brief Description of the Drawings

[0019] [Figure 1] It is a schematic diagram of an immersion nozzle and its surroundings in continuous casting of steel. [Figure 2] It is a scanning electron microscope (SEM) image of the molten loss part of the powder line part of Example 1. [Figure 3] It is a scanning electron microscope (SEM) image of the molten loss part of the powder line part of Comparative Example 1.

Embodiments for Carrying Out the Invention

[0020] 〔Outline of the continuous casting method〕 In continuous casting of steel, molten steel whose components and temperature are adjusted is supplied to a ladle and then supplied to a tundish. The molten steel supplied to the tundish is supplied to a continuous casting mold through an immersion nozzle for continuous casting. The molten steel is gradually withdrawn from the bottom and rolled into steel while being cooled and solidified in the continuous casting mold.

[0021] FIG. 1 shows a continuous casting immersion nozzle 1 and a part of the mold. The immersion nozzle 1 having a cylindrical shape is attached to the lower part of a tundish (not shown). Molten steel 6 is supplied from the tundish to the mold through the immersion nozzle 1.

[0022] The immersion nozzle 1 has a main body part 2 and a powder line part 3. The main body part 2 has an inlet 4 communicating with the tundish and an outlet 5 arranged below the liquid surface of the molten steel 6 in the mold.

[0023] In continuous casting, mold powder is scattered on the molten steel 6, and the mold powder is melted by the heat of the molten steel 6 to become slag 7. The slag 7 serves to prevent oxidation and temperature drop of the molten steel 6 by preventing the surface of the molten steel 6 from contacting air, adsorbing impurities and the like floating on the surface of the molten steel 6, etc., but on the other hand, it may exert an erosion effect on the immersion nozzle 1.

[0024] In order to suppress the wear due to the slag 7 and improve the durability of the immersion nozzle 1, a powder line part 3 is provided in the circumferential direction of the part of the immersion nozzle 1 that contacts the slag 7. The powder line part 3 is made of a material having relatively high corrosion resistance as will be described later and is provided so as to surround the main body part 2.

[0025] Note that the immersion nozzle 1 of the present invention is not limited to the structure of the immersion part and the inner tube of the main body part 2, such as the inner tube diameter, outer diameter, inner tube structure, discharge hole shape, etc. Further, the powder line part 3 of the present invention is not limited to the shape, dimensions, etc.

[0026] The main component of molten steel 6 is iron, but it may also contain carbon, silicon, manganese, phosphorus, sulfur, and other elements. When casting special steel materials, other metallic elements are intentionally added to the molten steel 6. For example, when casting high-hardness tool steel, chromium and molybdenum may be added, and when casting heat-resistant alloys, chromium, molybdenum, nickel, cobalt, tungsten, titanium, niobium, and other elements may be added. If the molten steel 6 contains titanium, as mentioned above, titanium oxide may form in the slag 7, and slag 7 containing titanium oxide is highly corrosive. Therefore, when manufacturing steel materials containing titanium, the powder line section 3 of the immersion nozzle 1 is particularly susceptible to corrosion.

[0027] [Configuration of the immersion nozzle] The immersion nozzle 1 includes a main body 2 and a powder line section 3, and the main body 2 has an inlet 4 and a discharge port 5 (Figure 1). However, the structure of the immersion nozzle 1 shown in Figure 1 is just one example, and the structure of the immersion nozzle is not limited in the present invention.

[0028] The material of the main body 2 can be any refractory material commonly used for the main body of an immersion nozzle, and may be, for example, an alumina-carbon refractory material. Constructing the main body 2 from a carbon-containing refractory material is preferable because it tends to increase the thermal shock resistance of the immersion nozzle 1. It is preferable that the material of the main body 2 be determined so as to achieve the necessary thermal shock resistance across the entire operating temperature range of the immersion nozzle 1.

[0029] The material of the powder line section 3 is a zirconia-carbon refractory, comprising a zirconia-carbon refractory material containing zirconia and graphite, and a nitride added to the zirconia-carbon refractory material in an external amount of 0.5% to 5.0% by mass. The material of the powder line section 3 is a zirconia-carbon refractory material with relatively excellent corrosion resistance, to which nitride is added, which plays a role in providing corrosion resistance to the powder line section 3 in environments where titanium is present. In environments where titanium is absent, the corrosion resistance is mainly provided by the zirconia-carbon refractory material, similar to conventional immersion nozzles.

[0030] The nitride is not particularly limited as long as it is a refractory raw material containing nitrogen, and may include, for example, aluminum nitride, zirconium nitride, silicon nitride, boron nitride, silicon iron nitride, sialon, and various oxynitrides. The nitride functions as a source of nitrogen in the reactions that occur during casting (described later). The nitride may be a single compound or a mixture of several types of nitrides.

[0031] The particle size of the nitride added during the manufacture of refractories is preferably 1 μm or larger, more preferably 5 μm or larger, and even more preferably 10 μm or larger. A particle size of 1 μm or larger is preferable because it is easy to knead and mold during manufacture, and the nitrogen release rate during use is not too fast, making it suitable as a nitrogen source. The particle size of the nitride is preferably 500 μm or smaller, more preferably 250 μm or smaller, and even more preferably 100 μm or smaller. A particle size of 500 μm or smaller is preferable because the nitrogen release rate during use is not too slow, making it suitable as a nitrogen source. Note that particle size is a value measured according to JIS Z8825:2022 or JIS Z8815:2022.

[0032] The nitride content added to the powder line section 3 is 0.5% by mass or more, preferably 1% by mass or more, and more preferably 1.5% by mass or more, relative to the total mass of the zirconia-carbon refractory material. When the nitride content is 0.5% by mass or more, it is easier to sufficiently form a titanium nitride layer (described later) on the surface of the powder line section 3 during casting.

[0033] The nitride content is 5.0% by mass or less, preferably 4% by mass or less, and more preferably 3% by mass or less, relative to the total mass of the zirconia-carbon refractory material. By setting the nitride content to 5.0% by mass or less, a sufficient amount of zirconia-carbon refractory material, which is responsible for corrosion resistance in environments where titanium is absent, is incorporated, making it easier to exhibit corrosion resistance. In particular, under operating conditions in which multiple types of steel are cast in a single continuous casting operation, it is required to achieve both corrosion resistance in environments where titanium is present and corrosion resistance in environments where titanium is absent, and this balance can be achieved by setting the nitride content to 5.0% by mass or less.

[0034] The zirconia contained in zirconia-carbon refractory materials may be stabilized zirconia with solid solutions of components such as calcium oxide, magnesium oxide, or yttrium oxide (Y2O3), unstabilized zirconia without solid-solution components, or mixtures thereof.

[0035] In the zirconia-carbon refractory material of the powder line portion 3, the zirconia content is preferably 74% by mass or more, more preferably 78% by mass or more, and particularly preferably 82% by mass or more. A zirconia content of 74% by mass or more makes it easier to improve corrosion resistance to slag 7. The zirconia content is preferably 95% by mass or less, more preferably 93% by mass or less, and particularly preferably 90% by mass or less. A zirconia content of 95% by mass or less makes it easier to exhibit thermal shock resistance because the content of other components (e.g., graphite) does not become excessively small. The powder line portion 3 may contain, for example, 74 to 95% by mass of zirconia.

[0036] In the zirconia-carbon refractory material of the powder line portion 3, the graphite content is preferably 5% by mass or more, more preferably 7% by mass or more, and particularly preferably 10% by mass or more. A graphite content of 5% by mass or more makes it easier to improve thermal shock resistance. The graphite content is preferably 25% by mass or less, more preferably 20% by mass or less, and particularly preferably 18% by mass or less. When the graphite content is 25% by mass or less, the content of other components (e.g., zirconia) does not become excessively small, making it easier to exhibit corrosion resistance.

[0037] The zirconia-carbon refractory material of the powder line section 3 may contain components other than the aforementioned zirconia and graphite (hereinafter referred to as "other components"). These other components may include additives added as needed, such as silica, silicon, silicon carbide, zirconium boride, zirconium carbide, magnesia, and spinel, as well as components derived from the firing residue of the binder added to the refractory material when forming the immersion nozzle 1. In order to fully exhibit both the corrosion resistance provided by zirconia and the thermal shock resistance provided by graphite, the content of other components (additives, binder firing residue, etc.) in the zirconia-carbon refractory material is, for example, 15% or less, preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 5% by mass or less, and most preferably 3% by mass or less. If the zirconia-carbon refractory material of the powder line section 3 contains multiple types of other components, it is preferable that the total content of all other components is within the above preferred range.

[0038] Furthermore, the powder line portion 3 may contain other components in addition to the zirconia-carbon refractory material and nitride.

[0039] The immersion nozzle 1 can be manufactured by a method known as a method for manufacturing refractory materials. For example, the materials corresponding to the main body portion 2 and the powder line portion 3 are filled and placed in predetermined positions within a mold, and then hydrostatic molding is performed. The hydrostatically molded body is then fired at a predetermined temperature (for example, 800°C or higher) to manufacture the immersion nozzle 1 according to the present invention.

[0040] [Composition of mold powder] Mold powder is added to molds in the form of powders or granules obtained by kneading raw materials such as oxides, fluorides, carbonates, and carbon. For example, mold powder is an inorganic composition mainly composed of CaO and SiO2. In this example, the CaO / SiO2 mass ratio is called the "basicity" of the mold powder.

[0041] The chemical composition of mold powder for continuous casting of steel is, for example, SiO2: 20-50% by mass, CaO: 20-50% by mass, Al2O3: 0.5-15% by mass, MgO: 0.5-10% by mass, Na2O: 1-15% by mass, F: 2-15% by mass, and C: 1-10% by mass. In addition, various components such as Li2O, MnO, B2O3, BaO, and SrO may be added to the mold powder as needed.

[0042] During continuous casting, titanium oxide may be added to the mold powder to adjust its properties. Since titanium oxide in the slag originates from both the molten steel and the powder, the amount of titanium oxide added to the mold powder is adjusted considering the titanium content of the molten steel. For example, when casting special steel with added titanium (usually containing 1% by mass or less of titanium), it may not be necessary to add titanium oxide to the mold powder. On the other hand, if titanium is not added to the molten steel, the titanium oxide in the slag originates solely from the powder. In this case, the titanium oxide content of the slag can be considered to be the same as the titanium oxide content of the mold powder. The titanium oxide content in the mold powder is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 4% by mass or more. When the titanium oxide content in the mold powder is 1% by mass or more, the titanium nitride layer described later is easily formed. The titanium oxide content in the mold powder is preferably 15% by mass or less, more preferably 13% by mass or less, and even more preferably 10% by mass or less. When the titanium oxide content of the mold powder is 15% by mass or less, it is easier to prevent the formation of undesirable crystalline phases in the slag, and the mold powder is more likely to perform its function.

[0043] Mold powder can be characterized by the phases formed when it is melted and then solidified. The phases that can be formed at this time are broadly classified into crystalline phases and glass phases, and examples of crystalline phases include casspidyne (3CaO·2SiO2·CaF2), ghelenite (2CaO·Al2O3·SiO2), and akermaite (2CaO·MgO·2SiO2). The phases formed when mold powder solidifies can be controlled by adjusting the composition of the mold powder. Preferably, the mold powder used in the present invention is a composition in which at least one of the casspidyne phase and the glass phase is selected as the main component when solidified. Using such mold powder makes it easier to control the heat removal from molten steel to the mold. Furthermore, when the solidified slag is composed of the casspidyne phase and the glass phase, it is particularly preferable from the viewpoint of stable heat removal characteristics. Furthermore, a composition whose main component is at least one selected from the caspedyne phase and the glass phase means that, when the composition is analyzed by X-ray crystal structure analysis, the peak originating from the caspedyne phase is detected more strongly than the peaks originating from the other crystal phases, or no peaks from any of the crystal phases, including the caspedyne phase, are detected.

[0044] [Phenomena that occur during continuous casting] During continuous casting, slag 7 forms on top of the molten steel 6, and the slag 7 contains titanium oxide. The titanium oxide in the slag 7 may originate solely from the mold powder, solely from the molten steel 6, or from both the mold powder and the molten steel 6. The titanium oxide originating from the mold powder is titanium oxide that was blended into the mold powder. The titanium oxide originating from the molten steel 6 is titanium that was added to the molten steel 6 for the production of titanium-containing steel and incorporated into the slag 7 as an oxide.

[0045] The titanium oxide content in the slag 7 is 1% by mass or more, preferably 2% by mass or more, and more preferably 4% by mass or more. A titanium oxide content of 1% by mass or more makes it easier to improve the corrosion resistance of the powder line section 3. The titanium oxide content is 15% by mass or less, preferably 13% by mass or less, and more preferably 10% by mass or less. A titanium oxide content of 15% by mass or less makes it easier to exhibit the general functions required of the slag 7. Note that the titanium oxide content is the sum of the titanium oxide content derived from the mold powder and the titanium oxide content derived from the molten steel 6.

[0046] The powder line portion 3 of the immersion nozzle 1 contains nitride. When titanium oxide in the slag 7 comes into contact with the nitride contained in the powder line portion 3 of the immersion nozzle, a reaction like the one shown in Equation 2 below occurs, and titanium nitride is formed at the contact surface between the powder line portion 3 and the slag 7. In Equation 2, R is any element that combines with nitrogen to form nitride. mTiO2+R n N m →mTiN+R n O (Formula 2)

[0047] Since the reaction in Equation 2 occurs at the contact surface between the powder line section 3 and the slag 7, the titanium oxide contained in the slag 7 is consumed only in the portion in contact with the powder line section 3. Therefore, the titanium oxide concentration is locally reduced only in the portion of the slag 7 in contact with the powder line section 3, and there is no significant fluctuation in the titanium oxide concentration in the remaining portion of the slag 7. This reduces the erosion effect of the powder line section 3 by the titanium oxide-containing slag 7 and prevents the reaction in Equation 2 from affecting the macroscopic properties of the slag 7.

[0048] Furthermore, titanium nitride is formed by the reaction in Equation 2. Since the reaction in Equation 2 occurs at the contact surface between the powder line portion 3 and the slag 7, the titanium nitride is formed on the surface of the powder line portion 3, and typically a layer mainly composed of titanium nitride is formed. The melting point of titanium nitride is 2930°C, which is much higher than the temperature of molten steel (approximately 1560°C). Therefore, the titanium nitride-rich portion formed on the surface of the powder line portion 3 becomes a protective layer that protects the powder line portion 3 from erosion by the slag 7. The formation of the protective layer has been experimentally confirmed as shown in the examples described later.

[0049] [Examples] The present invention will be further described below with reference to examples. However, the following examples are not limiting to the present invention.

[0050] (1) Erosion test and observation of the damaged area Example 1 A zirconia-carbon refractory material containing 84% by mass of zirconia and 13% by mass of graphite, with a 3% remainder (additives and binder firing residue), was mixed thoroughly with 2% by mass of silicon nitride added externally. The mixture was then placed in the powder line section of an immersion nozzle, molded, and fired. The refractory material constituting the main body consisted of alumina, silica, and graphite.

[0051] Continuous casting of steel was performed using the immersion nozzle obtained in Example 1.

[0052] As the mold powder, a known mold powder mainly composed of CaO and SiO2 was used, to which titanium oxide was added, resulting in a mixture containing 10% by mass of TiO2. The basicity of the mold powder (the mass ratio of CaO / SiO2, abbreviated as C / S in the table below) was 1.2.

[0053] After continuous casting of steel, the slag was crushed, and the mass percentages of titanium oxide, CaO, and SiO2 were analyzed using X-ray fluorescence analysis in accordance with JIS R2216:2005. The titanium oxide content in the slag of Example 1 was 10% by mass, and the C / S ratio calculated from CaO and SiO2 was 1.2.

[0054] Furthermore, after continuous casting of steel, the corrosion resistance of the powder line section was evaluated based on the amount of damage on one side. The method for measuring the amount of damage on one side is as follows: Before use, the outer diameter of the powder line section of the immersion nozzle is measured with a caliper and designated as D1. After use, the part of the powder line section of the immersion nozzle with the maximum amount of melting (maximum melting area) is identified, and the outer diameter of the powder line section at the maximum melting area is measured with a caliper and designated as D2. The value obtained by (D1-D2) / 2 is the amount of damage on one side.

[0055] Comparative Example 1 An immersion nozzle was manufactured in the same manner as in Example 1, except that nitride was not added to the powder line section, and continuous casting of steel was performed under the same conditions as in Example 1. Subsequently, the amount of damage on one side was measured in the same manner as in Example 1.

[0056] Comparing the amount of unilateral erosion in the powder line section of Example 1 and Comparative Example 1, the amount of unilateral erosion in the powder line section of Example 1, which had nitride added, was less than 80% of that of Comparative Example 1, which did not have nitride added. This confirms that Example 1 achieved higher corrosion resistance than Comparative Example 1.

[0057] The cross-sectional state of the eroded portions of Example 1 and Comparative Example 1 was observed using a scanning electron microscope (SEM). Figure 2 shows the backscattered electron composition image (COMPO image), titanium distribution image, and nitrogen distribution image of Example 1, and Figure 3 shows the backscattered electron composition image, titanium distribution image, and nitrogen distribution image of Comparative Example 1.

[0058] The images in Figures 2 and 3 were acquired with the cross-section 31 of the powder line area positioned on the right side of the field of view and the slag (hereinafter referred to as "adhesion 71") adhering to the surface of the powder line area positioned on the left side of the field of view. The granular areas with relatively high brightness in the COMPO image are zirconia that constitute the powder line area.

[0059] Comparing the titanium distribution images in Figure 2 and Figure 3, it was confirmed that in the deposit 71 of Figure 2 (Example 1), there is a layer A in which titanium is concentrated in an area close to the powder line. On the other hand, in the deposit 71 of Figure 3 (Comparative Example 1), titanium is scattered throughout the area of ​​the deposit 71.

[0060] Comparing the nitrogen distribution images in Figure 2 and Figure 3, it was confirmed that nitrogen was concentrated in layer A of deposit 71 in Figure 2 (Example 1), where titanium was concentrated. On the other hand, no areas of nitrogen concentration were observed in Figure 3 (Comparative Example 1).

[0061] As described above, the scanning electron microscope (SEM) measurements confirmed the presence of layer A on the surface of the powder line, where both titanium and nitrogen are concentrated. Given that the elements present are titanium and nitrogen, layer A is considered to contain a large amount of titanium nitride. In other words, a titanium nitride layer was formed on the surface of the powder line. This titanium nitride layer functions as a protective layer for the powder line.

[0062] (2) Changes to the composition of the powder line and slag Continuous casting of steel was performed under various conditions by changing the composition of the powder line and slag. After continuous casting, the slag composition was analyzed, and the corrosion resistance of the powder line was evaluated. The evaluation of corrosion resistance focused on the presence or absence of a titanium nitride layer on the surface of the powder line and the corrosion resistance of the powder line itself.

[0063] The presence or absence of a titanium nitride layer formation on the surface of the powder line portion in contact with the slag (hereinafter referred to as the observation surface) was evaluated by observing the immersion nozzle after continuous casting using a scanning electron microscope (SEM). The observed state of the powder line portion was classified into the following three levels. +: A titanium nitride layer was observed across the entire observation surface. ±: Formation of a titanium nitride layer was observed in a portion of the observation surface. -: No titanium nitride layer was observed on the observation surface.

[0064] The corrosion resistance of the powder line section was evaluated by quantitatively comparing the degree of erosion in the powder line section. The degree of erosion in each powder line section was quantified as the aforementioned one-sided damage amount. The one-sided damage amount of Comparative Example 2, which was tested using a combination of an immersion nozzle without nitrides and slag without titanium oxides in the powder line section, was used as a baseline, and compared with the one-sided damage amount of the powder line section of each immersion nozzle. Each example was classified into the following four levels according to the degree of erosion. For examples at level B or higher, it was determined that the corrosion resistance was clearly improved compared to the baseline Comparative Example 2. A: The amount of damage on one side is less than 80% of the amount of damage on one side in Comparative Example 2. B: The amount of damage on one side is 80% or more but less than 95% of the amount of damage on one side in Comparative Example 2. C: The amount of damage on one side is 95% or more but less than 100% of the amount of damage on one side in Comparative Example 2. Z: The amount of damage on one side is 100% or more of the amount of damage on one side in Comparative Example 2.

[0065] Under the same manufacturing conditions as in Example 1, immersion nozzles equipped with powder line sections having the compositions shown in Table 1 were manufactured. If the combined content of zirconia and graphite was less than 100%, the remainder consisted of additives and binder calcination residues. The manufactured immersion nozzles were then used in continuous casting of steel in slag having the compositions shown in Table 1. The method for determining the C / S and TiO2 content of the slag was the same as in Example 1.

[0066] The evaluation results for each immersion nozzle are shown in Table 1. Examples 2-4 and Comparative Example 5 shown in Table 1 differ only in the amount of nitride added to the powder line.

[0067] [Table 1]

[0068] As shown in Table 1, in Examples 2-4, where the material of the powder line portion contained nitride and the slag contained TiO2, higher corrosion resistance was observed than in Comparative Examples 2-4, which lacked one or both of these characteristics.

[0069] For examples with different graphite content in the powder line section, the same tests as in Table 1 were conducted. Immersion nozzles with powder line sections having the compositions shown in Table 2 were manufactured under the same manufacturing conditions as in Example 1. Then, continuous casting of steel was performed using the manufactured immersion nozzles in slag having the compositions shown in Table 2. The method for determining the C / S and TiO2 content of the slag was the same as in Example 1.

[0070] The evaluation results for each immersion nozzle are shown in Table 2. Examples 5 and 6 shown in Table 2 differ from Example 3 shown in Table 1 only in the graphite content of the powder line. Similar to the examples shown in Table 1, Examples 5 and 6, in which the material of the powder line contains nitride and the slag contains TiO2, showed higher corrosion resistance than Comparative Examples 6 to 11, which lacked one or both of these features.

[0071] [Table 2]

[0072] For examples with different slag basicities, the same tests as in Table 1 were performed. Under the same manufacturing conditions as in Example 1, immersion nozzles equipped with powder line sections having the compositions shown in Table 3 were manufactured. Then, continuous casting of steel was performed using the manufactured immersion nozzles in slag having the compositions shown in Table 3. The method for determining the C / S and TiO2 content of the slag was the same as in Example 1.

[0073] The evaluation results for each immersion nozzle are shown in Table 3. Examples 7 and 8 shown in Table 3 differ from Example 3 shown in Table 1 only in the basicity C / S of the slag. Similar to the examples shown in Table 1, Examples 7 and 8, in which the material of the powder line section contains nitride and the slag contains TiO2, showed higher corrosion resistance than Comparative Examples 12-17, which lacked one or both of these characteristics.

[0074] [Table 3]

[0075] For examples with different slag TiO2 content, the same tests as in Table 1 were conducted. Under the same manufacturing conditions as in Example 1, immersion nozzles equipped with powder line sections with the compositions shown in Table 4 were manufactured. Then, continuous casting of steel was performed using the manufactured immersion nozzles in slag having the compositions shown in Table 4. The method for determining the C / S and TiO2 content of the slag was the same as in Example 1.

[0076] Table 4 shows the evaluation results for each immersion nozzle. Examples 9 to 12 shown in Table 4 differ from Comparative Examples 2, 18, and 19 only in the TiO2 content of the slag. In Examples 9 to 12, where the immersion nozzles were used in slag containing 1% to 15% by mass of TiO2, higher corrosion resistance was observed than in Comparative Examples 2, 18, and 19, which had different TiO2 content. In addition, the slag in Comparative Example 19 had insufficient function to control heat removal from molten steel to the mold, making continuous casting difficult.

[0077] [Table 4] [Industrial applicability]

[0078] The method according to the present invention is suitably used in the iron and steel industry. [Explanation of symbols]

[0079] 1: Immersion nozzle 2: Nozzle body 3: Powder line section 4:Inlet 5:Discharge hole 6: Molten steel 7: Molded Powder Slag 31: Cross-section of the powder line 71: Mold powder slag adhering to the surface of the powder line. A: A layer where titanium is concentrated.

Claims

1. A continuous casting method for steel using an immersion nozzle and mold powder, The powder line portion of the immersion nozzle is Zirconia-carbon refractory materials containing zirconia and graphite, The above zirconia-carbon refractory material contains 0.5% to 5.0% by mass of nitride on an external basis, Includes, The mold powder slag formed on the surface of the molten steel contains 1% to 15% by mass of titanium oxide. A continuous casting method for steel.

2. The continuous casting method for steel according to claim 1, wherein at least a portion of the titanium element in the titanium oxide is derived from the mold powder.

3. The method for continuous casting of steel according to claim 1, wherein the mold powder is a composition in which at least one selected from the caspidine phase and the glass phase is the main component when solidified.

4. A continuous casting method for steel according to any one of claims 1 to 3, wherein a layer containing titanium nitride is formed on the surface of the powder line portion of the immersion nozzle that is in contact with the mold powder slag.

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