Molded copper plate, mold for continuous casting, and casting method for slabs

Abstract

Provided is a mold copper plate which makes it possible to form a solidified slab shell in an adequate state and prolong the life of a coating applied to the mold copper plate.　The mold copper plate for continuous-casting mold comprises: a base layer comprising copper or a copper alloy; and a coating layer disposed on the base layer. The coating layer comprises an alloy including at least one constituent element selected from among nickel, cobalt, nickel-based alloys, cobalt-based alloys, and stainless-steel alloys and at least one other constituent element selected from among metal nitrides, metal carbides, and metal oxides. According to the present invention, it is possible to form a solidified slab shell in an adequate state and prolong the life of the coating applied to the mold copper plate.

Description

Technical Field

[0001] The present invention relates to a mold copper plate used for a continuous casting mold, a continuous casting mold, and a method for casting a slab.

Background Art

[0002] For example, when manufacturing a steel slab (hereinafter also referred to as a slab) of a medium carbon steel type such as a hypo-peritectic steel using a mold, a mold flux is used to maintain the lubricity between the mold and the solidified shell.

[0003] The mold flux sprayed onto the surface of the molten steel in the mold melts by the heat of the molten steel, enters between the mold and the solidified shell, and then becomes a film shape. When spraying the mold flux on the surface of the mold, there may be variations in the inflow amount depending on the part of the mold. When there are variations in the inflow amount of the mold flux, there are variations in the degree of solidification of the molten steel. As a result, longitudinal cracks in the slab are likely to occur at the initial stage when the molten steel solidifies. Also, when there are variations in the degree of solidification of the molten steel, solidification at a specific part becomes slow, so there is a risk of breakout.

[0004] Also, efforts have been made to suppress variations in the degree of solidification of the mold flux. Specifically, raising the crystallization temperature of the mold flux, promoting the precipitation of cuspidine when the mold flux solidifies, and reducing the types of crystals to be precipitated have been carried out.

[0005] However, with such a method of adjusting the physical properties and composition of the precipitated crystals, it becomes difficult to ensure the lubricity between the mold and the solidified shell, which is one of the purposes of the mold flux. For this reason, efforts have been made to lower the viscosity of the mold flux.

[0006] If the viscosity of the mold flux decreases, there is a risk of the mold flux becoming trapped in the molten steel, a phenomenon known as entrapment. Furthermore, the solidification of the mold flux requires consideration of a complex system including the crystallization temperature, degree of vitrification (degree of crystallinity), and type of crystal, making it extremely complicated.

[0007] To solve these problems, a method has been proposed to reduce variations in the degree of solidification of molten steel by improving the wettability between the molten mold flux and the mold. Specifically, this involves applying a TiN coating to the surface of the mold (see Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 9-314288 [Patent Document 2] International Publication No. 2003 / 064077 [Overview of the project] [Problems that the invention aims to solve]

[0009] Incidentally, molded copper plates are used for the inner walls of the mold. Furthermore, TiN, which is used for the coating, has a lower thermal conductivity than copper. As a result, the thickness of the solidified shell formed in the mold becomes thinner, which may cause breakout. In addition, although ceramic materials such as TiN have excellent wear resistance, their thermal and mechanical impact resistance and elongation properties are lower than those of metals, which may shorten the lifespan of the crack-resistant coating.

[0010] This invention has been made in view of the above problems, and aims to provide a molded copper plate, a mold for continuous casting, and a method for casting a slab that can form a solidified shell of the slab in an appropriate state and extend the lifespan of the coating applied to the molded copper plate. [Means for solving the problem]

[0011] To solve the above problems, the present invention has the following features.

[0012] [1] A copper mold plate for continuous casting, It comprises a base layer containing copper or a copper alloy, and a coating layer provided on the base layer, The aforementioned coating layer comprises at least one of nickel, cobalt, nickel-based alloys, cobalt-based alloys, and stainless steel alloys as one component element, and an alloy comprising at least one of metal nitrides, metal carbides, and metal oxides as another component element, wherein the mold copper plate is for continuous casting. [2] A mold for continuous casting, comprising a base layer containing copper or a copper alloy, and a molded copper plate having a coating layer provided on the base layer, the coating layer having an alloy containing at least one of nickel, cobalt, nickel-based alloys, cobalt-based alloys, and stainless steel alloys as one component element, and at least one of metal nitrides, metal carbides, and metal oxides as other component elements. [3] The continuous casting mold according to [2], wherein the coating layer has a thickness of 2.0 mm or less in the lamination direction and is provided in a region that includes at least 200 mm below the meniscus. [4] The molded copper plate has a recessed groove or a rounded groove provided between the base material layer and the coating layer. The continuous casting mold according to [2] or [3], wherein the grooves or circular grooves are filled with a different material having a different thermal conductivity from the molded copper plate. [5] A method for casting slabs by adding mold flux to molten steel and using continuous casting, A method for casting slabs with a thickness of 80 mm or more and 600 mm or less, using a continuous casting mold as described in any of [2] to [4]. [Effects of the Invention]

[0013] According to the present invention, it becomes possible to form a solidified shell of a slab in an appropriate state and to extend the life of the coating applied to the mold copper plate.

Brief Description of the Drawings

[0014] [Figure 1] It is a perspective view of a mold for continuous casting according to an embodiment. [Figure 2] It is a cross-sectional view of the mold copper plate of FIG. 1. [Figure 3] It is a plan view of the long side of the mold of FIG. 1. [Figure 4] It is a graph showing the variation in the thickness of the solidified shell in an example. [Figure 5] It is a graph showing the number of longitudinal cracks on the surface of the slab in an example. [Figure 6] It is a graph showing the number of life charges based on the wear resistance evaluation of the coating film in an example. [Figure 7] It is a graph showing the number of life charges based on the crack resistance evaluation of the coating film in an example.

Mode for Carrying Out the Invention

[0015] As an embodiment of the present invention, an example in which a mold copper plate is used for a continuous casting mold (hereinafter also referred to as a mold) will be described.

[0016] As shown in FIG. 1, the mold 100 is formed in a hollow cylindrical shape. The mold 100 has a pair of mold copper plates 11 arranged opposite to each other and a pair of mold copper plates 12 sandwiched between the mold copper plates 11 and arranged opposite to each other.

[0017] The pair of mold copper plates 11 and the pair of mold copper plates 12 are formed in a rectangular plate shape. The pair of mold copper plates 11 are arranged on the front and back surfaces in FIG. 1. The pair of mold copper plates 12 are arranged on the right and left side surfaces in FIG. 1.

[0018] In a top view, the mold 100 has an inner wall surface formed in a rectangular shape by a pair of molded copper plates 11 and a pair of molded copper plates 12. The mold 100 has an internal space surrounded by the pair of molded copper plates 11 and a pair of molded copper plates 12.

[0019] A tundish (not shown) for containing the molten steel 20 is positioned above the mold 100. An immersion nozzle 30 extending toward the mold 100 is installed at the bottom of the tundish. The immersion nozzle 30 is inserted into the internal space of the mold 100.

[0020] The molded copper plates 11 and 12 have recesses formed on their back surfaces that serve as cooling water channels (not shown). The mold 100 is cooled by passing cooling water through these cooling water channels.

[0021] The surface of the molten steel 20 in the mold 100 is called the meniscus M. In Figure 1, the meniscus M of the mold 100 is located at the position indicated by the dashed line. Also, A in the figure indicates the casting direction. B in the figure indicates the width direction of the mold 100. That is, the mold 100 is arranged so that the casting direction A and the width direction B are perpendicular to each other.

[0022] Figure 2 shows a cross-section of the molded copper plate 11(12). As shown in Figure 2, the molded copper plate 11(12) has a coating layer 18 formed on a base layer 17. More specifically, the coating layer 18 is located on the side facing the internal space.

[0023] The base layer 17 contains copper or a copper alloy. Examples of materials for the base layer 17 include copper alloys containing chromium, zirconium, etc.

[0024] The coating layer 18 is preferably formed in the region near the meniscus M of the mold 100 shown in Figure 1. More specifically, the coating layer 18 is preferably provided including the region R described later. The coating layer 18 is preferably 2.0 mm or less in thickness in the stacking direction.

[0025] The coating layer 18 has an alloy containing at least one of nickel, cobalt, nickel-based alloys, cobalt-based alloys, and stainless steel alloys as one component element, and at least one of metal nitrides, metal carbides, and metal oxides as another component element.

[0026] Preferably, one of the constituent elements is a metal that has both high heat resistance and high corrosion resistance.

[0027] Examples of nickel-based alloys include Hastelloy and Inconel.

[0028] By including one component in the alloy contained in the coating layer 18, it becomes possible to bring the thermal conductivity of the coating layer 18 closer to that of copper.

[0029] Examples of metal nitrides with other elemental components include TiN and TiSiN.

[0030] Examples of metal carbides with other elemental components include TiC and TiSiC.

[0031] Examples of metal carbides with other elemental components include TiO and TiSiO.

[0032] The alloy contained in the coating layer 18, by including other elemental components, improves the wettability between the mold flux and the surface of the mold copper plate 11 (12). Therefore, variations in the amount of mold flux flowing into the mold 100 can be reduced in the width direction B and the casting direction A, that is, the mold flux can be uniformly flowed into the mold 100. Furthermore, because the coating layer 18 has such a composition, the thermal conductivity is reduced in the initial stages of cooling the molten steel compared to the final stages, allowing the molten steel 20 to cool more slowly. As a result, it is possible to improve the occurrence of longitudinal cracks in the slab.

[0033] The coating layer 18 is not particularly limited, but can be formed on the substrate layer 17 by, for example, PVD (physical vapor deposition).

[0034] Furthermore, the mold flux is not particularly limited and any known type can be used. Examples of mold fluxes include those containing SiO2, A1203, CaO, MgO, F, Na2O, etc.

[0035] The wettability between the molten mold flux and the coating layer 18 is preferably lower than the wettability between the mold flux and Ni or NiCo.

[0036] A groove 15 is provided between the base layer 17 and the coating layer 18. In this embodiment, the groove 15 is formed recessed from the surface of the base layer 17 facing the coating layer 18. The groove 15 may also be formed recessed from the surface of the coating layer 18 facing the base layer 17. In this embodiment, the manner in which the groove 15 is formed is described, but the groove 15 is provided as appropriate depending on the embodiment, and does not necessarily have to be formed in the molded copper plate 11.

[0037] Figure 3 shows the internal space side of the molded copper plate 11. In Figure 3, the internal structure of the area enclosed by the dotted circle is schematically shown. As shown in Figure 3, it is preferable that the molded copper plate 11 has grooves 15 provided in a region that includes at least 200 mm below the meniscus M.

[0038] In this embodiment, the grooves 15 are provided in the region R from the meniscus M downward to 200 mm, and in the region Q from the meniscus M upward to 30 mm, taking into consideration fluctuations in the molten metal level.

[0039] In this embodiment, the groove 15 has a plurality of longitudinal grooves that extend along the casting direction A and are arranged along the width direction B so as to be parallel to each other. The groove 15 also has a plurality of transverse grooves that extend along the width direction B and are arranged along the casting direction A so as to be parallel to each other. Therefore, the groove 15 is formed in a grid pattern by the plurality of longitudinal grooves and the plurality of transverse grooves. Note that the groove 15 may be replaced with a round groove.

[0040] The groove 15 is filled with a dissimilar material having a different thermal conductivity than the molded copper plate. Examples of dissimilar materials include metals with lower thermal conductivity than copper, such as Ni (thermal conductivity: approximately 90 W / (m·K)) and Ni alloys (thermal conductivity: approximately 40-90 W / (m·K)). Other dissimilar materials that can be used include Cu, Co, NiCo, nickel-based alloys (Hastelloy, Inconel), stainless steel alloys, etc. The dissimilar material only needs to have a different thermal conductivity than the molded copper plate, and is not limited to metals; it may also be nonmetals.

[0041] By filling the grooves 15 with different materials, the thermal resistance of the mold 100 in the casting direction A and width direction B near the meniscus M can be increased or decreased regularly and periodically. Furthermore, the lifespan of the mold 100 can be improved while reducing variations in the amount of mold flux flowing into the mold 100. In this way, by increasing or decreasing the thermal resistance of the mold 100 regularly and periodically, uneven solidification can be reduced and surface cracking of the slab can be suppressed.

[0042] By arranging different materials in this manner, the heat flux from the solidified shell to the mold 100 in the vicinity of the meniscus M, that is, in the initial stage of solidification of the molten steel, increases and decreases regularly and periodically. This regular and periodic increase and decrease in heat flux reduces stress due to δ / γ transformation and thermal stress, thereby reducing the degree of deformation of the solidified shell caused by these stresses.

[0043] When the degree of deformation of the solidified shell is reduced, the variation in the heat flux distribution is reduced, and the generated stress is dispersed, resulting in smaller individual strains. As a result, cracking on the surface of the solidified shell can be suppressed.

[0044] The process of casting a slab using the mold 100 described above will now be explained. First, molten steel 20 is poured into the mold 100 through the immersion nozzle 30, and mold flux is added. The molten steel 20 that has cooled in the mold 100 is cooled and solidifies from the parts that are in contact with the inner wall surface of the mold 100, forming a so-called solidified shell.

[0045] The solidified shell is cooled by a water spray or gas-water spray installed downstream of the mold and continuously withdrawn in the casting direction A, which is vertically downwards, along with the unsolidified layer inside. When the solidified shell is cooled by the water spray or gas-water spray, it solidifies all the way to the center. The solidified shell is then cut by a gas cutting machine or the like to produce slabs of a predetermined length.

[0046] The temperature of the molten steel 20 is highest near the meniscus M. Depending on the type of steel, it is desirable to remove heat from the solidified shell, especially at the position of the meniscus M, from the width direction B perpendicular to the casting direction A from the inner wall surface of the mold 100. By removing heat from the solidified shell in this way, variations in the thickness of the solidified shell can be reduced.

[0047] The slab manufactured in this manner is preferably 80 mm or more and 600 mm or less in thickness, more preferably 80 mm or more and 300 mm or less, and more preferably 80 mm or more and 250 mm or less.

[0048] A slab thickness of 80 mm or more reduces the likelihood of uneven solidification of molten steel even during high-speed casting. Furthermore, a slab thickness of 600 mm or less suppresses uneven solidification of molten steel within the mold even at low casting speeds.

[0049] In this case, if the alloy contained in the coating layer is composed only of metal nitrides and metal carbides, as in the conventional technology described above, the thermal and mechanical impact resistance and elongation properties will be reduced. As a result, cracks and other damage are more likely to occur in the coating layer, and the lifespan of the coating layer will be reduced.

[0050] In contrast, the molded copper sheet 11(12) of this embodiment has an alloy in its coating layer 18 that contains one elemental component and other elemental components. Therefore, the molded copper sheet 11(12) has good wettability with the mold flux. As a result, variations in the solidification of the mold flux can be reduced, and a solidified shell with small variations in thickness can be produced. Furthermore, because the coating layer 18 of the molded copper sheet 11(12) contains one elemental component, it has good thermal and mechanical impact resistance and elongation properties. Therefore, it is possible to improve the lifespan of the coating layer 18.

[0051] In particular, because the coating layer 18 is provided including the region R from the meniscus M down to 200 mm, slow cooling can be performed only during the initial stages of solidification of the molten steel. As a result, it becomes possible to generate a solidified shell under more appropriate conditions. [Examples]

[0052] The embodiments of the present invention will be described below in comparison with conventional examples and comparative examples. (Test Example 1: Coating Comparison Test) Molten steel that had undergone oxygen blowing and RH vacuum degassing treatment in a converter was used. 300 tons of molten steel were introduced into a ladle, and slabs were manufactured using a continuous casting mold.

[0053] The mold copper plate was coated in the manner shown in Table 1 to create Conventional Example 1, Invention Examples 1-20, and Comparative Examples 1-4. The coatings for Conventional Example 1, Invention Examples 1-20, and Comparative Examples 1-4 were applied to a 200 mm area below the meniscus M. Conventional Example 1 was coated with Ni plating. In the "Dissimilar Materials" column of Table 1, "None" indicates that no grooves were formed in the mold copper plate. Examples where "Present" is used indicate that Ni was used as the dissimilar material.

[0054] [Table 1]

[0055] For each of Conventional Example 1, Invention Examples 1-20, and Comparative Examples 1-4, the variation in solidified shell thickness in the width direction, the number of longitudinal cracks on the slab surface, and the coating life were predicted. The variation in solidified shell thickness in the width direction, the number of longitudinal cracks on the slab surface, and the coating life were predicted based on the temperature of thermocouples installed in the mold copper plate of the casting mold.

[0056] Figure 4 shows the variation in the thickness of the solidified shell in the width direction for Conventional Example 1, Inventive Examples 1-20, and Comparative Examples 1-4. As shown in Figure 4, it was found that the variation in the thickness of the solidified shell in the width direction for Inventive Examples 1-20 was less than that for Conventional Example 1 and Comparative Examples 1-4.

[0057] Figure 5 shows the number of longitudinal cracks on the surface of each slab for Conventional Example 1, Inventive Examples 1-20, and Comparative Examples 1-4. As shown in Figure 5, it was found that the number of longitudinal cracks on the surface of the slabs for Inventive Examples 1-20 was fewer than that of Conventional Example 1 and Comparative Examples 1-4.

[0058] Figure 6 shows the number of charge cycles for the lifespan of each coating for Conventional Example 1, Invention Examples 1-20, and Comparative Examples 1-4. Figure 6 also shows the wear resistance evaluation of the coatings. As shown in Figure 6, the wear resistance of the coatings in Invention Examples 1-20 was found to be longer than that of Comparative Examples 2-4. The wear resistance of the coatings in Invention Examples 2, 3, 5, 6, 9, 10, and 20 was found to be longer than that of Conventional Example 1. The wear resistance of the coatings in Invention Examples 2, 3, 6, 9, and 20 was found to be longer than that of Comparative Example 1.

[0059] Figure 7 shows the number of charge cycles required for the lifespan of each coating in Conventional Example 1, Invention Examples 1-20, and Comparative Examples 1-4. In Figure 7, the crack resistance of the coatings is evaluated. The crack resistance was evaluated by measuring the tensile strength at 300°C using the method described in JIS G 0567:2020.

[0060] Furthermore, the surface temperature of the mold during continuous steel casting reaches approximately 300°C. To evaluate crack resistance in such a high-temperature environment, tensile strength tests were conducted at 300°C. Table 1 shows the tensile strength of the metal and alloy coating layers at 300°C.

[0061] As shown in Figure 7 and Table 1, the tensile strength of Invention Examples 1 to 20 is higher than that of Conventional Example 1 and Comparative Examples 1 to 4, indicating that the coatings have high crack resistance. In particular, Invention Examples 7 to 15 and Invention Examples 18 to 20, where one component is an alloy component consisting of Hastelloy, Inconel, and stainless steel, exhibit superior crack resistance compared to the other Invention Examples.

[0062] The lifespan of the coating was evaluated by combining the wear resistance evaluation in Figure 6 and the crack resistance evaluation in Figure 7. The results are also shown in the "Overall Evaluation Mold Life ch Number" column of Table 1. As shown in this column, Invention Examples 7-15 and 18-20, in which one component is an alloy component consisting of Hastelloy, Inconel, and stainless steel, are superior to the other Invention Examples.

[0063] As described above, Invention Examples 1 to 20 can reduce the variation in the thickness of the solidified shell in the width direction of the mold compared to Conventional Example 1 and Comparative Examples 1 to 4. Furthermore, Invention Examples 1 to 20 can significantly reduce the number of surface cracks in the slab compared to Conventional Example 1 and Comparative Examples 1 to 4. Moreover, Invention Examples 1 to 20 can extend the life of the coating compared to Conventional Example 1 and Comparative Examples 1 to 4.

[0064] (Test Example 2: Comparison Test of Coating Area and Thickness) The mold copper plates were coated in the manner shown in Table 2 to create Invention Examples 21-28 and Comparative Examples 5-13.

[0065] Specifically, in Invention Examples 21 to 28, one component element of the coating layer alloy was Ni, and the other component element was TiN, and either the region where the coating layer was provided (distance below the meniscus M) or the thickness of the coating layer was changed.

[0066] Specifically, Comparative Examples 5-1 2Comparative Example 13 was created by using TiN as the coating layer and changing either the area where the coating layer is provided (distance below the meniscus M) or the thickness of the coating layer.

[0067] [Table 2]

[0068] As shown in Table 2, Invention Examples 21-28 yielded better results than Comparative Examples 5-13 in terms of variation in solidified shell thickness, number of slab cracks, and coating life. [Explanation of Symbols]

[0069] 100 molds 11,12 Molded copper plate 15 grooves 17 Base material layer 18 Coating layer 20 Molten steel M Meniscus Q,R region

Claims

[Claim 1] A method for casting slabs by adding mold flux to molten steel and using continuous casting, A mold for continuous casting is used, which has a base layer containing copper or a copper alloy, and a molded copper plate having a coating layer provided on the base layer and having an alloy that contains at least one of Hastelloy, Inconel, and stainless steel alloy as one component element, and at least one of titanium nitride, titanium carbide, and titanium oxide as other component elements. The mold flux is added in a molten state, such that its wettability with the coating layer is lower than the wettability between the mold flux and Ni or NiCo. A method for casting slabs with a thickness of 80 mm or more and 600 mm or less.

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