Continuous casting method for molds and steel

Abstract

To provide a casting mold capable of suppressing the occurrence of the embossing and crack of a different-material filling part.SOLUTION: A casting mold according to the invention is composed of a plurality of mold copper plates and is used for continuous casting of steel. Therein: a molten steel contact surface of the casting mold is coated with a coating made of a metal consisting of one of Ni, Cr, Co and Fe, or two or more alloys thereof, or a mixture of a metal or an alloy and a non-metal; and there is disposed, under the coating, a heterogeneous material-filled part filled with a metal or a non-metal having a thermal conductivity different from that of the mold copper plate.SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

[0002] In continuous casting of steel, when the cooling in the mold becomes non-uniform, the thickness of the solidification shell becomes non-uniform in the casting direction and the mold width direction of the slab. When the thickness of the solidification shell becomes non-uniform, stress caused by shrinkage and deformation of the solidification shell acts, and in the initial stage of solidification, this stress concentrates on the thin part of the solidification shell, and cracks occur on the surface of the solidification shell. These cracks are enlarged by external forces such as subsequent thermal stress, bending stress and correction stress by the rolls of the continuous casting machine, resulting in large surface cracks. In addition, in medium carbon steel grades such as hypoeutectoid steel, non-uniformity of the solidification shell due to δ / γ transformation occurs in the initial stage of solidification, increasing the concern about surface cracks. When the non-uniformity degree of the solidification shell thickness is large, vertical cracks occur in the mold, and in some cases, breakouts occur where molten steel flows out from these vertical cracks. Since the cracks present in the slab become surface defects in the rolling process of the next step, it is necessary to clean the surface of the slab to remove surface cracks at the stage of the slab after casting.

[0003] Patent Document 1 discloses a continuous casting mold having a heterogeneous material filling portion in which a metal or non-metal having a thermal conductivity different from that of the mold copper plate is filled in a recess on the inner wall surface of the mold. According to Patent Document 1, by providing a heterogeneous material filling portion in the continuous casting mold, the thermal conductivity of the mold copper plate becomes non-uniform, thereby reducing non-uniform solidification in the initial stage of solidification and suppressing vertical cracks in the solidification shell.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the mold provided in Patent Document 1, a dissimilar material filling section is provided on the side facing the casting space. However, repeated thermal contraction due to molten steel injected into the mold can cause the dissimilar material filling section to bulge or cracks to occur in the dissimilar material filling section. The present invention has been made in view of the above problems, and its objective is to provide a mold that can suppress the bulging and cracking of the dissimilar material filling section in a mold provided with a dissimilar material filling section, and a continuous casting method for steel using said mold. [Means for solving the problem]

[0006] The means to solve the above problems are as follows: [1] A mold for continuous casting of steel, comprising a plurality of mold copper plates, wherein the molten steel contact surface of the mold is covered with a coating made of one of Ni, Cr, Co, and Fe, or two or more alloys, or a mixture of the metal or alloy and a nonmetal, and a dissimilar material filling section is provided below the coating, in which a metal or nonmetal with a different thermal conductivity than the mold copper plates is filled. [2] The mold according to [1], 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 [3] [1] or [2]. [Effects of the Invention]

[0007] According to the present invention, the change in heat flux due to the dissimilar material filling section can suppress uneven solidification of the solidified shell while also suppressing the lifting and cracking of the dissimilar material filling section. [Brief explanation of the drawing]

[0008] [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 showing a portion of the mold 12. [Figure 3] Figure 3 is an enlarged view of section A in Figure 2. [Figure 4] Figure 4 is a graph showing the tensile strength of metals and alloys at 300°C. [Modes for carrying out the invention]

[0009] 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.

[0010] 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.

[0011] 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.

[0012] Downstream in the casting direction, multiple conveyor rolls 17 are installed to continue transporting the cast slabs 28. Above the conveyor rolls 17, a cast slab cutting machine 32 is positioned to cut the cast slabs 28. After solidification is complete, the cast slabs 28 are cut into cast slabs 28a of a predetermined length by the cast slab cutting machine 32. In this way, steel cast slabs 28a are continuously cast using the continuous casting equipment 10.

[0013] 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 cast slab. The inner surfaces of the four mold copper plates 13 (the sides in contact with the molten steel 18) are covered with a coating 22. In addition, beneath the coating 22, on the surface of the mold copper plates, there is a dissimilar material filling section 23 in which a metal or nonmetal with a different thermal conductivity than the mold copper plates is filled. The dissimilar material filling section 23 is formed, for example, by filling a circular recess independently machined on the inner surface of the mold copper plate 13 with a metal or nonmetal. The filling thickness of the dissimilar material filling section 23 is preferably 50 μm or more. The means of filling may be by applying a plating treatment or a thermal spray treatment, or by fitting metals and nonmetals that match the shape of the circular recess into the circular recess.

[0014] 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.

[0015] Figure 3 is an enlarged view of section A in Figure 2. As shown in Figure 3, beneath the coating 22, there is a dissimilar material filling section 23, which is filled with a metal or nonmetal having a different thermal conductivity than the mold copper plate. By providing this dissimilar material filling section 23, the thermal resistance of the mold can be periodically increased or decreased, reducing the stress caused by the phase transformation of the solidified shell. As a result, the deformation of the solidified shell is reduced, non-uniform solidification of the solidified shell is suppressed, and the stress is dispersed, reducing the amount of strain in each individual shell. Consequently, the occurrence of surface cracks on the surface of the solidified shell can be suppressed.

[0016] In a mold 12 provided with a dissimilar material filling section 23, if a coating 22 is not provided, repeated thermal contraction due to molten steel 18 poured into the mold 12 will cause the dissimilar material filling section 23 to bulge or crack. On the other hand, by providing a coating 22, the occurrence of bulging and cracking of the dissimilar material filling section 23 is suppressed even at high temperatures.

[0017] The coating 22 is provided for the purpose of suppressing the lifting and cracking of the dissimilar material filling portion 23. For this reason, it is not necessary for the entire inner surface of the mold copper plate 13 to be covered with the coating 22; it is sufficient for at least the area up to where the dissimilar material filling portion 23 is provided to be covered with the coating 22.

[0018] The coating 22 is a metal or alloy consisting of one of Ni, Cr, Co, and Fe, or two or more such metals or alloys with nonmetals, and among these, Ni, Co, Ni-Co alloy (Ni:Co=50:50), Inconel, and SUS are preferred. The coating 22 may contain impurities that are inevitably mixed in, in amounts of 5% by mass or less. If the amount is 5% by mass or less, the presence of such impurities will not affect the strength of the coating 22.

[0019] The thickness of the coating 22 is preferably 50 μm or more and 10 mm or less. By setting the thickness of the coating 22 to be 50 μm or more and 10 mm or less, disappearance of the coating 22 due to wear can be suppressed, reduction of the heat extraction amount from the mold 12 can be suppressed, and a solidified shell thickness that enables continuous casting can be ensured. On the other hand, if the thickness of the coating 22 is less than 50 μm, a part of the coating 22 may disappear due to wear, which is not preferable. Also, if the thickness of the coating 22 is greater than 10 mm, the heat extraction amount from the mold 12 decreases, the solidified shell thickness becomes thin, and casting of the slab becomes difficult, which is not preferable. Incidentally, the thickness of the coating 22 is more preferably 0.1 mm or more and 0.3 mm or less. By setting the thickness of the coating 22 to be 0.1 mm or more and 0.3 mm or less, disappearance of the coating 22 due to wear and reduction of the heat extraction amount from the mold 12 can be further suppressed. Further, the thickness of the dissimilar material filling portion 23 is preferably 10 mm or less in total with the thickness of the coating 22. Thereby, reduction of the heat extraction amount from the mold 12 is suppressed, and a solidified shell thickness that enables continuous casting can be ensured.

[0020] The method of coating the mold copper plate 13 with the coating 22 is not particularly limited. For example, the contact surface of the mold copper plate 13 that contacts the molten steel 18 may be coated with the coating 22 using laser cladding welding.

Example

[0021] Next, an example will be described in which the density of mold defects after casting was compared using molds according to this embodiment (Examples 1 to 8) and molds that have a dissimilar material filling section but do not have a coating (Comparative Example). In the Examples of Invention, Ni was filled as the dissimilar material on the molten steel contact surface of the mold copper plate, and the upper layer was coated with Co plating (Example 1), Ni-Co (Ni:Co=50:50) plating (Example 2), Inconel 625 plating (Example 3), SUS316 plating (Example 4), TiC10 mass%-Inconel (TiC:Inconel=10:90) plating (Example 5), Ni plating (Example 6), Cr plating (Example 7), or Fe plating (Example 8) to a thickness of 0.1 mm, 0.2 mm, 0.3 mm, and 0.5 mm, respectively. In Comparative Example 1, Ni was used as a dissimilar material to fill the contact surface of the mold copper plate, and a conventional mold without coating 22 was used.

[0022] Continuous casting of 1500 charges was performed using the molds of Invention Examples 1-3 described above and a conventional mold (comparative example), and the number of bulges and cracks that occurred in the Ni-filled portion of the mold after continuous casting was confirmed. The number of bulges and cracks that occurred was divided by the area of ​​the region where the dissimilar material-filled portion 23 was provided, and this value was defined as the mold defect density. Each mold was evaluated using an index with the mold defect density of the conventional mold (comparative example) set to 1.00. The evaluation results are shown in Table 1 below. An index of less than 1.00 means that the bulges and cracks in the dissimilar material-filled portion were reduced compared to the conventional mold (comparative example), and the mold life was improved. On the other hand, an index greater than 1.00 means that the bulges and cracks in the dissimilar material-filled portion were increased compared to the conventional mold (comparative example), and the mold life was reduced.

[0023] [Table 1]

[0024] As shown in Table 1, the index for Invention Examples 1 to 8 was less than 1.00 for all coating thicknesses from 0.1 to 0.5 mm. From these results, it was confirmed that the molds of Invention Examples 1 to 8 can suppress the lifting and cracking of the dissimilar material filling area more effectively than conventional molds without coating 22 when the coating thickness is 0.5 mm or less.

[0025] Next, we will explain the results of high-temperature tensile tests conducted on representative examples of metals and alloys used in the inventive examples, measuring their tensile strength at 300°C. The high-temperature tensile tests were performed according to the method described in JIS G 0567:2020. The metals and alloys subjected to high-temperature tensile tests were Ni-Co (Inventive Example 2), Inconel 625 (Inventive Example 3), SUS316 (Inventive Example 4), TiC10 mass%-Inconel (Inventive Example 5), and Ni (Inventive Example 6).

[0026] Figure 4 is a graph showing the tensile strength of metals and alloys at 300°C. As shown in Figure 4, the tensile strength at 300°C increased in the order of Ni-Co, Ni, SUS316, Inconel 625, and TiC10 mass%-Inconel. Thus, at around 300°C, the mold surface temperature during continuous casting of steel, the tensile strength of Inconel 625, SUS316, and TiC10 mass%-Inconel was higher than that of Ni and Ni-Co.

[0027] By using metals or alloys with high tensile strength, crack formation in the coating formed by these metals or alloys is suppressed. Therefore, it is preferable to use Inconel 625, SUS316, or TiC10 mass%-Inconel, which have a tensile strength of 400 MPa or more at 300°C, for the mold surface coating. It is even more preferable to use Inconel 625 or TiC10 mass%-Inconel, which have a tensile strength of 500 MPa or more at 300°C, for the mold surface coating. This suppresses crack formation in the coating on the mold surface more effectively than when using Ni or Ni-Co, thereby improving the durability of the mold. [Explanation of symbols]

[0028] 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 23 Different substance filling section 24 Solidified Shell 26 Unsolidified layer 28 cast slabs 30 Secondary Cooling Zone 32 Slab cutting machine

Claims

1. A mold composed of multiple mold copper plates, used for continuous casting of steel, A mold in which the molten steel contact surface of the mold is covered with a coating of SUS316, Inconel 625, or TiC 10 mass%-Inconel, and a dissimilar material filling section is provided below the coating, in which a metal or nonmetal with a different thermal conductivity than the mold copper plate is filled.

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

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

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