Mold and continuous casting method for steel

Abstract

To provide a casting mold that can improve wettability with melting mold flux.SOLUTION: In a casting mold 12 used for continuous casting of steel, a molten steel contact surface of the casting mold 12 is coated with a coating 22 containing one or more kinds of a metal carbide, a metal nitride, and a metal carbonitride.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a mold used for continuous casting of steel and a continuous casting method for steel using the same. [Background Art]

[0002] In continuous casting, increasing the casting speed is essential for improving productivity. However, increasing the casting speed causes problems such as seizure of the solidified shell due to insufficient consumption of mold flux and uneven inflow of mold flux. When uneven inflow of mold flux occurs, an air gap layer with low thermal conductivity is formed between the mold flux and the mold, resulting in a partial reduction in the amount of heat removed from the molten steel. This creates regions with high heat removal and regions with low heat removal from the molten steel, giving rise to the problem of uneven solidification where the thickness of the solidified shell becomes non-uniform. When the problem of uneven solidification occurs, surface defects such as longitudinal cracks occur in slabs produced by continuous casting. Furthermore, in medium-carbon steel grades such as hypoperitectic steel, in addition to the above-mentioned problems during high-speed casting, the volume change due to δ / γ transformation during solidification becomes large, so the problem of uneven solidification tends to become even more pronounced.

[0003] Patent Document 1 discloses a continuous casting mold in which the molten steel contact surface of the mold is coated with an amorphous alloy made of an iron alloy or a nickel alloy. According to Patent Document 1, applying the coating can impart high wear resistance to the mold, thereby enabling smooth continuous casting of steel and extending the service life of the mold. [Prior Art Literature] [Patent Literature]

[0004] [Patent Document 1] Japanese Patent Laid-Open No. 2001-105103 [Summary of the Invention] [Problem to be Solved by the Invention]

[0005] In the mold for continuous casting disclosed in Patent Document 1, the surface facing the casting space is coated with a molten layer of amorphous alloy, but this coating has low wettability with molten mold flux. As a result, under high-speed casting conditions, the molten mold flux cannot flow between the molten steel and the mold, causing stalemate between the mold and the solidified shell. Furthermore, even under normal casting conditions, the low wettability between the coating and the molten mold flux causes the molten mold flux to flow unevenly, forming an air gap layer and resulting in the problem of uneven solidification of the molten steel. In addition, the formation of the air gap layer reduces the amount of heat removed from the mold, making it difficult to improve the casting speed and thus the mold's production capacity.

[0006] The present invention has been made in view of the above problems, and its object is to provide a mold that can improve wettability with molten mold flux and a method for continuous casting of steel using the mold. [Means for solving the problem]

[0007] The means to solve the above problems are as follows: [1] A mold used for continuous casting of steel, wherein the molten steel contact surface of the mold is covered with a coating containing one or more of metal carbides, metal nitrides, and metal carbonitrides. [2] The coating comprises 20% by mass or more of a metal or alloy consisting of two or more of Ni, Cr, Co, and Fe, as described in [1]. [3] The mold according to [1] or [2], wherein the thickness of the coating is 50 μm or more and 10 mm or less. A method for continuous casting steel, comprising continuously casting steel using the mold described in [4] [1] or [2]. A method for continuous casting steel, comprising continuously casting steel using the molds described in [5][3]. [Effects of the Invention]

[0008] The mold according to the present invention has higher wettability with molten mold flux at high temperatures than conventional molds, and the contact angle with the molten mold flux is smaller, so uneven flow of molten mold flux can be suppressed. As a result, the formation of an air gap layer between the mold flux and the mold is suppressed, and uneven solidification of molten steel can be suppressed. In addition, since the molten mold flux, which acts as a lubricant in the mold, flows more easily between the molten steel and the mold, sticking between the solidified shell and the mold is suppressed, enabling stable operation of high-speed continuous casting of steel. Furthermore, by suppressing the formation of an air gap layer, the amount of heat removed from the mold is increased, so the casting speed can be improved and the production capacity of the mold can be increased. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a schematic cross-sectional view showing an example of a continuous casting apparatus having a mold according to this embodiment. [Figure 2] Figure 2 is a schematic cross-sectional view of the mold. [Figure 3] Figure 3 is a graph showing the relationship between the TiC content of each metal and the contact angle with the molten mold flux. [Modes for carrying out the invention]

[0010] The present invention will be described below through embodiments of the present invention. The following embodiments are preferred examples of the present invention and are not limiting in any way.

[0011] Figure 1 is a schematic cross-sectional view showing an example of a continuous casting apparatus 10 having a mold 12 according to this embodiment. The continuous casting apparatus 10 includes a mold 12, a tundish 14 installed above the mold 12, and a plurality of slab support rolls 16 arranged in a row below the mold 12. Although not shown in the figure, a ladle for holding molten steel 18 is installed above the tundish 14, and the molten steel 18 is poured into the tundish 14 from the bottom of the ladle. An immersion nozzle 20 is installed at the bottom of the tundish 14, and the molten steel 18 is poured into the mold 12 through the immersion nozzle 20. The molten steel 18 is cooled and solidified from the inner surface of the mold 12, forming a solidified shell 24. As a result, a slab 28 is formed with the solidified shell 24 as its outer shell and an unsolidified layer 26 made of molten steel 18 inside.

[0012] Multiple secondary cooling zones 30, each equipped with a spray nozzle (not shown), are installed in the gaps between adjacent slab support rolls 16 in the casting direction, extending from directly below the mold 12 along the casting direction. The slab 28 is cooled as it is pulled out by the cooling water sprayed from the spray nozzles of the secondary cooling zones 30. As the slab 28 is conveyed by the slab support rolls 16 and passes through the multiple secondary cooling zones 30, the solidified shell 24 is properly cooled, the solidification of the unsolidified layer 26 progresses, and the solidification of the slab 28 is completed.

[0013] Downstream in the casting direction, multiple conveyor rolls 17 are installed to continue transporting the cast slab 28. Above the conveyor rolls 17, a cast slab cutting machine 32 is positioned to cut the cast slab 28. After solidification is complete, the cast slab 28 is cut into cast slabs 28a of a predetermined length by the cast slab cutting machine 32.

[0014] Figure 2 is a schematic cross-sectional view showing a part of the mold 12. The mold 12 is composed of four mold copper plates 13 that are assembled to form a roughly rectangular tubular shape according to the shape of the slab to be cast. The inner surfaces of the four mold copper plates 13 (the sides that come into contact with the molten steel 18) are covered with a coating 22.

[0015] Furthermore, the upper surface of the molten steel 18 inside the mold 12 is covered with molten mold flux 19 (hereinafter referred to as molten mold flux 19). The molten mold flux 19 flows between the mold 12 and the molten steel 18 and acts as a lubricant. The flow of the molten mold flux, which acts as a lubricant, between the mold 12 and the molten steel 18 suppresses sticking between the mold 12 and the solidified shell 24. While covered with molten mold flux 19, the molten steel 18 is injected into the mold 12 through the immersion nozzle 20. The molten steel 18 injected into the mold 12 is cooled by the mold 12. As a result, the molten steel 18 at the interface between the mold 12 and the molten steel 18 solidifies, and the solidified shell 24 is formed.

[0016] In the cooling of molten steel 18 using such a mold 12, if the molten mold flux 19 flows unevenly between the mold 12 and the molten steel 18, an air gap layer with low thermal conductivity is formed in the areas where the molten mold flux 19 did not flow, reducing the amount of heat removed from the molten steel 18. When there are areas where the amount of heat removed is partially reduced in this way, the solidification of the molten steel 18 in contact with those areas slows down, resulting in the problem of uneven solidification. Furthermore, as mentioned above, since the molten mold flux 19 acts as a lubricant, there is a risk of seizing between the mold 12 and the solidified shell 24 in areas where the air gap layer is not formed.

[0017] In contrast, in the mold 12 according to this embodiment, the inner surface of the mold copper plate 13 that comes into contact with the molten steel 18 is coated with a coating 22 containing one or more of metal carbides, metal nitrides, and metal carbonitrides. This improves the wettability between the inner surface of the mold copper plate 13 and the molten mold flux 19 at high temperatures, and suppresses the uneven inflow of the molten mold flux 19. As a result, it is possible to suppress the occurrence of air gaps due to the uneven inflow of the molten mold flux 19 in the initial stages of solidification of the molten steel 18, and the resulting uneven growth and longitudinal cracking of the slab due to uneven thermal resistance between the mold and the molten steel. Furthermore, the risk of smearing between the mold 12 and the solidified shell 24 can also be reduced.

[0018] The coating 22 is provided for the purpose of suppressing uneven inflow of molten mold flux during the initial solidification stage of molten steel 18. For this reason, the entire inner surface of the mold copper plate 13 does not need to be covered with the coating 22, and it is sufficient that at least the molten steel contact surface that comes into contact with the molten steel 18 below the meniscus is covered with the coating 22.

[0019] As the metal carbide, metal nitride and metal carbonitride contained in the coating 22, for example, one or more selected from the group consisting of TiC, SiC, ZrC, TiN, CrN, TiAlN and TiCN can be used. Among these, it is preferable to use TiC, which is a metal carbide. Further, the content of the metal carbide, metal nitride and metal carbonitride contained in the coating 22 is preferably 0.1% by mass or more and 80.0% by mass or less. When the content of the metal carbide, metal nitride and metal carbonitride contained in the coating 22 exceeds 80.0% by mass, cracks may occur in the coating layer of the coating 22, which is not preferable. In addition, when the content of the metal carbide, metal nitride and metal carbonitride contained in the coating 22 is less than 0.1% by mass, the effect of improving wettability with the molten mold flux is reduced, which is not preferable.

[0020] Further, it is preferable that the coating 22 contains 20% by mass or more of a metal composed of one or an alloy composed of two or more selected from the group consisting of Ni, Cr, Co and Fe. When the content of the metal composed of one or the alloy composed of two or more selected from the group consisting of Ni, Cr, Co and Fe contained in the coating 22 is less than 20% by mass, formation of the coating 22 becomes difficult, which is not preferable. Since the coating 22 contains at least 0.1% by mass of metal carbide, metal nitride and metal carbonitride, the upper limit of the content of the metal composed of one or the alloy composed of two or more selected from the group consisting of Ni, Cr, Co and Fe contained in the coating 22 is 99.9% by mass.

[0021] Further, the coating 22 may contain unavoidable impurities in an amount of 5% by mass or less. If the content is 5% by mass or less, the wettability with the molten mold flux 19 is not affected even when the impurities are contained.

[0022] The thickness of the coating 22 is preferably not less than 50 μm and not more than 10 mm. Reducing the thickness of the coating 22 to less than 50 μm is not preferred, because there is a risk that a portion of the coating 22 will disappear due to wear. In addition, increasing the thickness of the coating 22 to more than 10 mm is not preferred, because the amount of heat extracted from the mold 12 decreases, which may increase the number of cracks in the cast slab 28 during casting. It is more preferable that the thickness of the coating 22 is not less than 0.1 mm and not more than 0.3 mm. By setting the thickness of the coating 22 to be not less than 0.1 mm and not more than 0.3 mm, both the disappearance of the coating 22 due to wear and the reduction in the amount of heat extracted from the mold 12 can be suppressed.

[0023] The method for coating the mold copper plate 13 with the coating 22 is not particularly limited. For example, laser cladding welding may be used to coat the contact surface of the mold copper plate 13 that comes into contact with molten steel 18 with the coating 22.

[0024] Next, experimental results confirming that coating the mold copper plate 13 with the coating 22 improves wettability with molten mold flux 19 at high temperatures will be described. In Inventive Example 1, a powder material obtained by mixing TiC powder at 20 mass% and Inconel powder at 80 mass% was laser-clad welded onto a 10 mm × 10 mm metal piece to produce a test piece covered with a coating having a thickness of 0.2 mm.

[0025] In Inventive Example 2, a powder material obtained by mixing TiC powder at 50 mass% and Ni powder at 50 mass% was laser-clad welded onto a metal piece of the same size to produce a test piece covered with a coating having a thickness of 0.2 mm. In Inventive Example 3, a powder material obtained by mixing TiC powder at 50 mass% and Co powder at 50 mass% was laser-clad welded onto a metal piece of the same size to produce a test piece covered with a coating having a thickness of 0.2 mm. In Inventive Example 4, a powder material obtained by mixing TiC powder at 50 mass% and Fe powder at 50 mass% was laser-clad welded onto a metal piece of the same size to produce a test piece covered with a coating having a thickness of 0.2 mm.

[0026] In Invention Example 5, a test specimen was prepared by thermal spraying a powder material, which was a mixture of 0.1% by mass of TiC powder and 99.9% by mass of Ni powder, onto a metal piece of the same size, resulting in a coating with a thickness of 0.2 mm. In Invention Example 6, a test specimen was prepared by thermal spraying TiC powder onto a metal piece of the same size, resulting in a coating with a thickness of 0.2 mm. In Invention Example 7, a test specimen was prepared by thermal spraying TiN powder onto a metal piece of the same size, resulting in a coating with a thickness of 0.2 mm. On the other hand, in Conventional Example 1, a test specimen was prepared by thermal spraying a Ni-Co (Ni:Co=50:50) plated coating with a thickness of 0.2 mm onto a metal piece of the same size.

[0027] A 2 mm square mold flux was placed on the test specimens of Invention Examples 1-6 and Conventional Example 1, and the temperature was raised from room temperature in a point-focused infrared imaging furnace. The mold flux used in the experiment mainly consisted of SiO2: 30% by mass, Al2O3: 5% by mass, and CaO: 30% by mass, and also contained MgO, Na2O, and LiO2. The basicity of the mold flux (CaO / SiO2) was 1. While videotaping the furnace, the temperature of the mold flux was raised to 1000°C, melted into droplets, and then the contact angle between the droplet-shaped molten mold flux and the test specimen was measured. The results of the contact angle measurement are shown in Table 1 below.

[0028] [Table 1]

[0029] As shown in Table 1, the contact angle between the conventional Ni-Co plating coating and the molten mold flux was 59°, whereas the contact angles between the coatings in Experimental Examples 1-7, which contain metal carbides or metal nitrides, and the molten mold flux were all smaller than in the conventional example. From these results, it was confirmed that coating the contact surface of the mold copper plate 13 with the molten steel 18 with a coating containing metal carbides or metal nitrides can improve the wettability with the molten mold flux at high temperatures compared to conventional methods. This improvement in wettability with the molten mold flux is presumed to be due to the improved reactivity between the molten mold flux and the coating material caused by the metal carbides or metal nitrides distributed on the mold surface, which in turn reduces the surface tension between the molten mold flux and the mold. Since a similar effect can be expected with metal carbonitrides, it is thought that coating the contact surface of the mold copper plate 13 with the molten steel 18 with a coating containing metal carbonitrides can improve the wettability with the molten flux compared to conventional methods. Furthermore, since the reactivity between the molten mold flux and the coating material is improved regardless of the composition of the mold flux, the above effects can be obtained similarly regardless of the composition of the mold flux.

[0030] Next, the influence of the metal carbide and metal content in coating 22 will be explained. Test specimens were prepared by coating them with a 0.2 mm thick coating using a powder material prepared by mixing TiC powder with Ni powder, Fe powder, Co powder, or Inconel powder in predetermined ratios. Mold flux was placed on these test specimens, and the temperature was raised from room temperature in a focusing infrared imaging furnace to melt the mold flux into droplets. The contact angle between the droplet-shaped molten mold flux and the test specimen was then measured. The results of the contact angle measurement are shown in Table 2 below. The mold flux used mainly consists of SiO2: 30% by mass, Al2O3: 5% by mass, and CaO: 30% by mass, and also contains MgO, Na2O, and LiO2. The basicity of the mold flux (CaO / SiO2) is 1.

[0031] [Table 2]

[0032] Figure 3 is a graph showing the relationship between the TiC content of each metal and the contact angle with the molten mold flux, and is a graph of Table 2. In Figure 3, the horizontal axis is the TiC content (mass%), and the vertical axis is the contact angle with the mold flux (°). As shown in Figure 3, among Ni, Fe, Co, and Inconel, coatings using Ni or Inconel had a smaller contact angle with the molten mold flux at high temperatures than coatings using Fe or Co. From this result, it can be seen that among Ni, Fe, Co, and Inconel, it is preferable to use Ni or Inconel as the metal to be mixed with TiC.

[0033] Furthermore, when using Ni or Inconel, the contact angle with the molten mold flux at high temperatures was reduced by setting the TiC content to 20% to 60% by mass. From these results, it can be seen that when using Ni or Inconel as the metal mixed with TiC, it is preferable to set the TiC content to 20% to 60% by mass.

[0034] Furthermore, when using Ni or Inconel, the contact angle with the molten mold flux at high temperatures was further reduced by setting the TiC content to 20% to 40% by mass. From these results, it can be seen that when using Ni or Inconel as the metal mixed with TiC, it is preferable to set the TiC content to 20% to 40% by mass.

[0035] As described above, the mold 12 according to this embodiment has high wettability with the molten mold flux and a small contact angle with the molten mold flux 19. As a result, the molten mold flux 19 can flow uniformly between the molten steel 18 and the mold 12, thereby suppressing uneven flow. Consequently, the formation of an air gap layer between the mold flux and the mold is suppressed, and uneven solidification of the molten steel can be suppressed. Furthermore, since sticking between the solidified shell and the mold is suppressed and the amount of heat removed from the mold increases, stable operation of high-speed continuous casting can be achieved by performing continuous casting of steel using this mold.

[0036] In this embodiment, an example is shown in which all inner surfaces of the four mold copper plates 13 constituting the mold 12 are covered with coating 22, but this is not the only example. It is sufficient if at least one of the four mold copper plates constituting the mold has its inner surface covered with coating 22. By using a mold made of such mold copper plates, uneven solidification of the molten steel can be suppressed more effectively than when using a mold in which all four mold copper plates have their inner surfaces not covered with coating 22, and smearing between the solidified shell and the mold can be suppressed. [Examples]

[0037] Test castings of medium carbon steel (chemical composition: C: 0.08~0.17 mass%, Si: 0.10~0.30 mass%, Mn: 0.50~1.20 mass%, P: 0.010~0.030 mass%, S: 0.005~0.015 mass%, Al: 0.020~0.040 mass%) were carried out using the same mold flux as used in the above-described experiment, under conditions of a casting speed of 2.3 m / min. The inner surface of the mold for continuous casting was coated with TiC 20 mass% - Inconel 80 mass% (Inventive Example 11), TiC 50 mass% - Ni 50 mass% (Inventive Example 12), TiC 50 mass% - Co 50 mass% (Inventive Example 13), TiC 50 mass% - Fe 50 mass% (Inventive Example 14), or Ni-Co plating (Conventional Example 11). The thicknesses of each coating are 0.1mm, 0.2mm, 0.5mm, 1.0mm, 25.0mm, 10.0mm, 15.0mm, and 20.0mm.

[0038] The number of longitudinal cracks in slabs cast using molds under each condition was measured, and the number of longitudinal cracks was measured along the slab area (1 m²). 2 The longitudinal crack density was calculated by dividing by ). The longitudinal crack density of slabs cast using a continuous casting mold coated with a thickness of 0.2 mm Ni-Co was used as the baseline, and Table 3 shows the values ​​obtained by dividing the longitudinal crack density of slabs cast using each mold by the baseline longitudinal crack density. The number of longitudinal cracks in the slabs was measured by penetrant testing.

[0039] [Table 3]

[0040] In Table 3, a value less than 1.00 indicates a reduction in longitudinal cracking of the slab compared to the conventional Ni-Co plated product (0.2 mm thick), resulting in improved slab quality. On the other hand, a value greater than 1.00 indicates an increase in longitudinal cracking of the slab compared to the conventional Ni-Co plated product (0.2 mm thick), resulting in a deterioration of slab quality.

[0041] As shown in Table 3, all slabs cast using the molds of Invention Examples 11 to 14 had a wettability of less than 1.00. The coatings of Invention Examples 11 to 14 have the same composition as the coatings of Invention Examples 1 to 4 shown in Table 1, and these coatings were found to improve the wettability between the mold and the molten mold flux. These results confirm that improving the wettability between the mold and the molten mold flux reduces longitudinal cracking of the slab and improves slab quality. Furthermore, all slabs cast using the molds of Invention Examples 11 to 14 with a coating thickness of 10 mm or less had a wettability of 0.50 or less. These results confirm that reducing the coating thickness to 10 mm or less can significantly reduce longitudinal cracking of the slab and greatly improve slab quality. [Explanation of symbols]

[0042] 10 Continuous casting equipment 12 molds 13. Copper mold plate 14 Tan Dish 16 Cast slab support rolls 17 Conveyor Rolls 18 Molten steel 19. Molten Molding Flux 20 Immersion nozzles 22 Coating 24 Solidified Shell 26 Unsolidified layer 28 cast slabs 30 Secondary Cooling Zone 32 Slab cutting machine

Claims

1. A mold used in continuous casting of steel, A mold in which the molten steel contact surface of the mold is coated with a mixture of Inconel and TiC.

2. The mold according to claim 1, wherein the TiC content in the coating is 10% by mass or more and 80% by mass or less.

3. The mold according to claim 1 or claim 2, wherein the thickness of the coating is 50 μm or more and 10 mm or less.

4. A method for continuous casting steel, comprising continuously casting steel using a mold according to claim 1 or claim 2.

5. A method for continuous casting steel, comprising continuously casting steel using the mold described in claim 3.

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