Mold copper plate, mold for continuous casting, and continuous steel casting method

Abstract

Provided is a mold copper plate that can suppress vertical cracks on the surface of a slab by suppressing uneven solidification of a solidified shell and extend the life of a mold for continuous casting.　The mold copper plate used in a mold for continuous casting has: a base material layer containing copper or a copper alloy; a layer provided on the base material layer and including a recessed groove or a round groove; and a coating layer provided on the layer including the recessed groove or the round groove. The coating layer contains at least one among nickel, cobalt, a nickel base alloy, a cobalt base alloy, a stainless alloy, a metal nitride, a metal carbide, and a metal oxide. The layer including the recessed groove or the round groove is composed of a different material having a thermal conductivity that is at most 80% or at least 120% of the thermal conductivity of the base material layer.

Description

Molded copper plate, continuous casting mold, and steel continuous casting method

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

[0002] In continuous casting of steel, if the cooling of molten steel in the mold becomes uneven, the thickness of the solidified shell will become uneven in the casting direction of the slab and the width direction of the mold. When the thickness of the solidified shell becomes uneven, stress due to shrinkage and deformation of the solidified shell acts, and in the early stage of solidification, this stress concentrates in the thin-walled portion of the solidified shell, causing surface cracks in the solidified shell.

[0003] These cracks then expand due to external forces such as thermal stress and bending and straightening stresses from the rolls of the continuous casting machine, leading to larger surface cracks. In addition, in medium-carbon steels such as hypoperitectic steels, the solidified shell is deformed by δ / γ transformation in the early stages of solidification, further increasing the risk of surface cracks. Large nonuniformities in the solidified shell thickness can lead to longitudinal cracks in the mold, which can cause breakouts, where molten steel leaks through these cracks. Because cracks present in slabs become surface defects in the subsequent rolling process, it is necessary to treat the slab surface after casting to remove surface cracks.

[0004] Patent Document 1 discloses a continuous casting mold having a dissimilar material filled portion in which a recess in the inner wall surface of the mold is filled with a metal or nonmetal having a thermal conductivity different from that of the mold copper plate. According to Patent Document 1, providing a dissimilar material filled portion in the continuous casting mold makes the thermal conductivity of the mold copper plate non-uniform, thereby suppressing non-uniform solidification in the early stages of solidification and preventing vertical cracks in the solidified shell and on the surface of the produced slab.

[0005] Japanese Patent Application Laid-Open No. 2018-192530

[0006] The continuous casting mold disclosed in Patent Document 1 has the problem that repeated thermal contraction due to molten steel poured into the mold causes protrusion of the foreign substance-filled portion and cracks to occur in the foreign substance-filled portion, thereby shortening the mold's lifespan. Furthermore, when high-speed casting is performed using a thin slab continuous casting machine, the mold temperature becomes high due to high-speed casting, making it more likely that cracks will occur not only in the foreign substance-filled portion but also on the mold surface, thereby shortening the mold's lifespan. In particular, funnel-shaped molds used in thin slab continuous casting machines have the problem that cracks are more likely to occur in the mold due to their shape, shortening the mold's lifespan.

[0007] The present invention has been made in view of the above-mentioned problems, and has an object to provide a mold copper plate, a continuous casting mold, and a method for continuous casting of steel using the mold, which can suppress uneven solidification of the solidified shell to suppress vertical cracks on the surface of the slab and extend the life of the continuous casting mold.

[0008] The means for solving the above problems are as follows. [1] A molded copper plate for use in a continuous casting mold, comprising: a base layer containing copper or a copper alloy; a layer containing grooves or rounded grooves provided on the base layer; and a coating layer provided on the layer containing grooves or rounded grooves, wherein the coating layer is composed of at least one of nickel, cobalt, a nickel-based alloy, a cobalt-based alloy, a stainless steel alloy, a metal nitride, a metal carbide, and a metal oxide, and the layer containing grooves or rounded grooves is composed of a heterogeneous material whose thermal conductivity ratio to the thermal conductivity of the base layer is 80% or less or 120% or more. [2] The molded copper plate according to [1], which has an interface layer at the interface between the coating layer and the layer containing grooves or rounded grooves. [3] A continuous casting mold including the mold copper plate according to [1] or [2], wherein the thickness of the coating layer in the lamination direction is 2.0 mm or less, and the layer including the concave grooves or round grooves is provided in a region from the meniscus to 200 mm below. [4] The continuous casting mold according to [3], wherein the tensile strength of the coating layer at 300°C is 250 MPa or more. [5] The continuous casting mold according to [3] or [4], which includes the mold copper plate in a funnel shape. [6] A method for continuously casting steel using the continuous casting mold according to any one of [3] to [5], wherein a CaO / SiO ２ and continuously casting the mixture at a casting speed of 1.0 m / min to 6.0 m / min.

[0009] By using a continuous casting mold including the mold copper plate according to the present invention, it is possible to suppress uneven solidification of the solidified shell and to suppress the occurrence of cracks in the continuous casting mold during continuous casting, thereby suppressing vertical cracks on the surface of a slab cast using the continuous casting mold including the mold copper plate and extending the life of the continuous casting mold.

[0010] FIG. 1 is a cross-sectional schematic diagram showing an example of a continuous casting machine having a continuous casting mold made of a mold copper plate according to this embodiment. FIG. 2 is a perspective view showing an example of a mold. FIG. 3 is a cross-sectional view of the mold copper plate. FIG. 4 is a schematic diagram of the mold copper plate as seen from the internal space side. FIG. 5 is a schematic diagram of a sample whose tensile strength was measured. FIG. 6 is a graph showing the number of vertical cracks on the surface of the slabs of Conventional Examples 1 and 2, Inventive Examples 1 to 27, and Comparative Examples 1 to 12. FIG. 7 is a graph showing the number of charges, which indicates the lifespan of the molds of Conventional Examples 1 and 2, Inventive Examples 1 to 27, and Comparative Examples 1 to 12.

[0011] The present invention will be described below through embodiments of the present invention. The following embodiments are preferred examples of the present invention, and the present invention is not limited to these embodiments.

[0012] 1 is a cross-sectional schematic diagram showing an example of a continuous casting machine 10 having a continuous casting mold 12 (hereinafter referred to as "mold 12") made of a molded copper plate according to this embodiment. First, a method for continuously casting steel using the mold 12 made of a molded copper plate according to this embodiment will be described with reference to FIG. 1.

[0013] The continuous casting machine 10 has a mold 12, a tundish 14 installed above the mold 12, and a plurality of strand support rolls 16 arranged in a row below the mold 12. Although not shown, a ladle containing 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 solidifies as heat is removed from the inner surface of the mold 12, forming a solidified shell 24. This results in a strand 28 having the solidified shell 24 as its outer shell and an unsolidified layer 26 made of the molten steel 18 inside.

[0014] The upper surface of the molten steel 18 in the mold 12 is covered with molten mold flux 22. The mold flux 22 flows between the mold 12 and the molten steel 18 and serves as a lubricant. The mold flux 22 flows between the mold 12 and the molten steel 18, thereby suppressing seizure between the mold 12 and the solidified shell 24.

[0015] In the gaps between adjacent strand support rolls 16 in the casting direction, multiple secondary cooling zones 30, each equipped with spray nozzles (not shown), are installed along the casting direction from directly below the mold 12. The strand 28 pulled out of the mold 12 is cooled by cooling water sprayed from the spray nozzles in the secondary cooling zones 30. While the strand 28 is transported by the strand support rolls 16 and passes through the multiple secondary cooling zones 30, the solidified shell 24 is appropriately cooled, solidification of the unsolidified layer 26 progresses, and solidification of the strand 28 is completed.

[0016] A plurality of transport rolls 17 are installed downstream in the casting direction for continuing to transport the slab 28. A slab cutter 32 for cutting the slab 28 is disposed above the transport rolls 17. After solidification is complete, the slab 28 is cut into slabs 34 of a predetermined length by the slab cutter 32. In this manner, slabs 34 are cast by the continuous steel casting method using the mold 12.

[0017] 2 is a perspective view showing an example of the mold 12. The mold 12 is composed of a pair of molded copper plates 40 arranged opposite each other and a pair of molded copper plates 42 sandwiched between the molded copper plates 40 and arranged opposite each other.

[0018] The pair of molded copper plates 40 and the pair of molded copper plates 42 are formed in the shape of rectangular plates. The pair of molded copper plates 40 are arranged on the front and rear sides in Fig. 2. The pair of molded copper plates 42 are arranged on the right and left sides in Fig. 2.

[0019] When viewed from above, the mold 12 has an inner wall surface formed into a rectangular shape by a pair of molded copper plates 40 and a pair of molded copper plates 42. The mold 12 is configured into a hollow cylindrical shape by the pair of molded copper plates 40 and the pair of molded copper plates 42.

[0020] Recesses that serve as cooling water channels (not shown) are formed on the back surfaces of the mold copper plates 40 and 42. The back surface of the mold copper plate 42 is the surface opposite to the surface of the mold 12 that faces the internal space into which the molten steel 18 is poured. The mold copper plates 40 and 42 are cooled by passing cooling water through the cooling water channels, whereby the molten steel 18 is cooled and a solidified shell 24 is formed.

[0021] The meniscus refers to the surface position of the molten steel 18 in the mold 12. In Fig. 2, the symbol M indicates the position of the meniscus. In this embodiment, continuous casting is performed by controlling the position of the meniscus to be 100 mm from the upper end of the mold 12. In Fig. 2, arrow A indicates the casting direction, and arrow B indicates the width direction of the mold 12. In the mold 12, a pair of mold copper plates 40 and a pair of mold copper plates 42 are arranged so that the casting direction A and the width direction B are perpendicular to each other.

[0022] The mold copper plate 40 and the mold copper plate 42 that make up the mold 12 have the same configuration and differ only in size. Therefore, in the following explanation, the configuration of the mold copper plate 40 will be explained, and an explanation of the mold copper plate 42 will be omitted.

[0023] Figure 3 is a cross-sectional view of the molded copper plate 40. Figure 3(a) is a cross-sectional view of the molded copper plate 40, and Figure 3(b) is an enlarged view of part C in Figure 3(a). As shown in Figure 3, the molded copper plate 40 has a layer 52 including grooves 54 formed on a base layer 50, and a coating layer 56 formed on the layer 52 including the grooves 54. The coating layer 56 is provided on the surface facing the internal space of the mold 12 into which molten steel 18 is poured.

[0024] The substrate layer 50 is made of a material containing copper or a copper alloy. When a copper alloy is used as the substrate layer 50, for example, a copper alloy to which a small amount of chromium, zirconium, or the like is added may be used. The layer 52 including the grooves 54 is provided between the substrate layer 50 and the coating layer 56. The grooves 54 are formed so as to be recessed from the surface of the substrate layer 50 facing the coating layer 56.

[0025] FIG. 4 is a schematic diagram showing the surface of the molded copper plate 40 facing the internal space. In FIG. 4 , the area surrounded by a dotted circle indicates the internal structure of the molded copper plate 40. The layer 52 containing the grooves 54 is preferably formed in a region R extending 200 mm below the meniscus of the molded copper plate 40. That is, the layer 52 containing the grooves 54 may be formed in a region extending 200 mm below the meniscus, or may be formed in a region further below the meniscus. The layer 52 containing the grooves 54 may also be formed in a region Q above the meniscus. As described above, continuous casting is performed so that the meniscus is located 100 mm from the top of the mold 12. Therefore, in the molded copper plate 40 according to this embodiment, the layer 52 containing the grooves 54 is formed in a region R extending 200 mm below the meniscus (300 mm from the top of the mold 12) and a region Q extending 30 mm above the meniscus (70 mm from the top of the mold 12).

[0026] In the layer 52 including the grooves 54, the grooves 54 are composed of a plurality of vertical grooves extending in the casting direction A and arranged in the width direction B so as to be parallel to one another, and a plurality of horizontal grooves extending in the width direction B and arranged in the casting direction A so as to be parallel to one another. The plurality of vertical grooves and horizontal grooves form a lattice-like groove 54. However, the grooves 54 are not limited to a lattice-like shape, and a plurality of cylindrical grooves may be provided in the region, for example, in a staggered pattern.

[0027] 3 , the layer 52 including the grooves 54 is made of a different material whose thermal conductivity has a ratio of 80% or less or 120% or more to the thermal conductivity of the base layer 50. The layer 52 including the grooves 54 includes a filling portion 57 in which the different material is filled in the grooves 54, and a layer portion 58 of the different material formed between the coating layer 56 and the base layer 50.

[0028] The depth of the groove 54 from the surface of the base layer 50 is preferably 0.5 mm to 5.0 mm, and more preferably 1.0 mm to 3.0 mm. The width of the groove 54 is preferably 2.0 mm to 20.0 mm, and more preferably 4.0 mm to 10.0 mm. The thickness of the layer portion 58 in the stacking direction is preferably 0.0 mm to 0.5 mm, and more preferably 0.0 mm to 0.2 mm.

[0029] The dissimilar materials are not limited to metals and may be non-metals. The ratio of the thermal conductivity of the dissimilar materials to the base layer 50 at room temperature (approximately 20°C) should be 80% or less or 120% or more. The thermal conductivity of metals generally decreases as the temperature increases. However, if the ratio of the thermal conductivity of the dissimilar materials to the thermal conductivity of the base layer 50 at room temperature is 80% or less or 120% or more, a sufficient difference in thermal conductivity can be generated even at the operating temperature of the mold 12.

[0030] The dissimilar material may be a metal with a lower thermal conductivity than copper and copper alloys, such as Ni (thermal conductivity: approximately 90 W / (m×K)) or Ni alloy (thermal conductivity: approximately 40 to 90 W / (m×K)). Cu, Co, NiCo, nickel-based alloys (Hastelloy, Inconel), stainless steel alloys, etc. may also be used. The dissimilar material in the mold copper plates 40, 42 included in one continuous casting mold is either a low thermal conductive material whose thermal conductivity is 80% or less relative to the base layer 50, or a high thermal conductive material whose thermal conductivity is 120% or more relative to the base layer 50.

[0031] Because the layer 52 including the grooves 54 is made of different materials, the thermal resistance of the mold 12 in the casting direction A and width direction B of the mold 12 near the meniscus increases and decreases regularly and periodically. This creates a distribution in which the heat flux from the solidified shell to the mold 12 increases and decreases regularly and periodically during the early solidification stage of the molten steel near the meniscus. This periodic increase and decrease in heat flux reduces the stress and thermal stress generated in the solidified shell due to the transformation of δ iron to γ ​​iron, thereby reducing deformation of the solidified shell caused by these stresses. The reduced deformation of the solidified shell homogenizes the non-uniform heat flux distribution caused by the deformation of the solidified shell, thereby suppressing non-uniform solidification. These effects suppress vertical cracks on the surface of the solidified shell and the occurrence of vertical cracks on the surface of the cast slab 34.

[0032] The coating layer 56 may be provided at least on the layer 52 including the grooves 54, but may also be provided on the surface of the base layer 50 where the layer 52 including the grooves 54 is not formed. The coating layer 56 is preferably provided so that its thickness in the stacking direction is 2.0 mm or less. By making the thickness of the coating layer 56 2.0 mm or less, the effect of the coating layer 56 on heat flux is reduced, thereby enabling the effect of periodic fluctuations in heat flux caused by different substances to be fully obtained. The coating layer 56 is preferably provided so that its thickness in the stacking direction is 10 μm or more. A thickness of the coating layer 56 in the stacking direction of less than 10 μm is not preferable because it results in portions where the coating layer 56 is not formed during the formation of the coating layer 56.

[0033] The coating layer 56 is composed of at least one of nickel, cobalt, a nickel-based alloy, a cobalt-based alloy, a stainless steel alloy, a metal nitride, a metal carbide, and a metal oxide. The coating layer 56 may be composed of Hastelloy, Inconel, or stainless steel. These alloys have high heat resistance and corrosion resistance, and therefore, by including these alloys, the heat resistance and corrosion resistance of the coating layer 56 are improved.

[0034] The coating layer 56 may be configured to include at least one of a metal nitride, a metal carbide, and a metal oxide. Examples of the metal nitride include TiN, TiSiN, WN, WSiN, TaN, and TaSiN. Examples of the metal carbide include TiC, TiSiC, WC, WSiC, TaC, and TaSiC. Examples of the metal oxide include TiO and TiSiO.

[0035] The coating layer 56 containing at least one of a metal nitride, a metal carbide, and a metal oxide can improve the heat resistance of the coating layer 56 and also improve the wettability between the mold flux 22 and the mold copper plate 40. If the wettability with the mold flux 22 is improved, the mold flux 22 can be uniformly flowed into the mold 12.

[0036] If the mold flux 22 can be uniformly introduced into the mold 12, fluctuations in the amount of heat removal are suppressed, further suppressing uneven solidification of the solidified shell 24. The introduced mold flux 22 causes the cooling rate at the initial stage of solidification to be slower than the cooling rate at the final stage of solidification, so the molten steel 18 is cooled gradually, further suppressing vertical cracks on the surface of the slab 34. More preferably, the coating layer 56 is made of an alloy containing at least one of nickel, cobalt, a nickel-based alloy, a cobalt-based alloy, and a stainless steel alloy, and further containing at least one of a metal nitride, a metal carbide, and a metal oxide.

[0037] The mold 12 constructed with the mold copper plate 40 according to the present embodiment is preferably applied to a thin slab continuous casting machine having a mold thickness of 40 mm to 100 mm, and performing continuous casting at a casting speed of 3.0 m / min to 6.0 m / min. For application to such a thin slab continuous casting machine, the mold copper plate 40 according to the present embodiment is preferably funnel-shaped. A funnel-shaped mold copper plate is a funnel-shaped mold copper plate in which the thickness of only the central portion in the width direction, where the submerged entry nozzle 20 is inserted, is expanded due to the narrow width of the mold used to cast the thin slab. For example, a funnel-shaped mold copper plate with a mold thickness of 40 mm is provided with a funnel-shaped portion in which the central portion in the width direction of the mold copper plate is widened into a funnel shape so that a submerged entry nozzle 20 with an outer diameter of 80 to 100 mm can be used.

[0038] Thin slab continuous casters cast at high speeds, which increases the mold temperature near the meniscus, promoting uneven solidification and making vertical cracks more likely to occur on the surface of the cast slab 34. In a funnel-shaped mold copper plate, thermal stress is likely to occur in the central portion in the bulged width direction, and heat removal from the molten steel 18 is also uneven at that location. For this reason, a mold including a funnel-shaped mold copper plate further promotes uneven solidification of the solidified shell, making vertical cracks more likely to occur on the surface of the cast slab 34.

[0039] In contrast, the molded copper plate 40 according to the present embodiment includes a layer 52 including grooves 54 made of a dissimilar material whose thermal conductivity ratio relative to that of the base layer 50 is 80% or less or 120% or more. This suppresses uneven solidification of the solidified shell 24 and prevents vertical cracks on the surface of the cast slab 34. Furthermore, the molded copper plate 40 according to the present embodiment includes a coating layer 56 containing at least one of nickel, cobalt, a nickel-based alloy, a cobalt-based alloy, a stainless steel alloy, a metal nitride, a metal carbide, and a metal oxide. This suppresses cracks in the coating layer 56 even when the surface temperature near the meniscus increases during high-speed casting. Because the coating layer 56 is provided on the layer 52 including grooves 54, suppressing cracks in the coating layer 56 also suppresses cracks in the dissimilar material below it.

[0040] In a thin slab continuous casting machine that continuously casts at a high casting speed of 3.0 m / min or more and 6.0 m / min or less, the surface temperature of the mold copper plate that constitutes the mold 12 during casting may reach approximately 300° C. If the surface temperature of the mold copper plate that constitutes the mold reaches 300° C., the strength of the coating layer may decrease, and even if the mold copper plate has a coated surface, the lifespan of the mold copper plate may be significantly reduced.

[0041] To prevent a significant decrease in lifespan even when the surface temperature reaches 300° C. or higher, the mold copper plate 40 preferably has a coating layer 56 with a tensile strength of 250 MPa or higher at 300° C. By having a coating layer 56 with a tensile strength of 250 MPa or higher at 300° C., even when the mold copper plate 40 is used in a thin slab continuous casting machine in which the surface temperature of the mold reaches 300° C. or higher, the occurrence of cracks in the coating layer 56 is suppressed, and a decrease in mold lifespan can be suppressed.

[0042] 5 is a schematic diagram of a sample whose tensile strength was measured. The tensile strength of the coating layer 56 at 300°C can be measured by the method described in JIS G 0567:2020 (High-temperature tensile test method for steel materials and heat-resistant alloys). It has been confirmed that Hastelloy, Inconel, stainless steel, metal nitrides (TiN, TiSiN, WN, WSiN, TaN, TaSiN), and metal carbides (TiC, TiSiC, WC, WSiC, TaC, TaSiC) are resistant to deterioration in tensile strength even at temperatures above 300°C, and have a tensile strength of 250 MPa or more at 300°C.

[0043] When the coating layer 56 is provided on the layer 52 having the grooves 54, it is preferable to form an interface layer at the interface between the layer 52 having the grooves 54 and the coating layer 56. By providing an interface layer at the interface between the layer 52 having the grooves 54 and the coating layer 56, it is possible to prevent gaps from being generated at the interface or uneven deformation from occurring even when the surface temperature of the molded copper plate 40 reaches 300°C or higher.

[0044] The interface layer is formed by providing a coating layer 56 on the layer 52 having the grooves 54 using a laser cladding method. On the other hand, if the coating layer 56 is provided using electroplating or hot-dip plating, no interface layer is formed.

[0045] The cross section of a sample in which a TiC coating layer was applied to the surface of a copper alloy substrate was examined using a laser cladding method. As a result, it was confirmed that an interfacial layer was formed at the interface between the substrate and the coating layer, in which the copper alloy constituting the substrate and the TiC constituting the coating layer were mixed. On the other hand, in the sample in which the coating layer was applied by hot-dip plating, no interfacial layer was formed, and the copper alloy constituting the substrate and the TiC constituting the coating layer were separated without mixing at the interface. From these results, it was confirmed that an interfacial layer is preferably formed at the interface between the layer 52 having the grooves 54 and the coating layer 56, and that this can suppress peeling and deformation at the interface.

[0046] The basicity (CaO / SiO ２It is preferable that the mass ratio of the slab to the slab (mass ratio of the slab to the slab) is 0.8 or more and 2.2 or less, and the casting speed is 1.0 m / min or more and 6.0 m / min or less. Under these continuous casting conditions, by performing continuous casting using a mold including the mold copper plate according to this embodiment, it is possible to continuously cast a slab while suppressing vertical cracks on the surface, and also to extend the life of the continuous casting mold.

[0047] As described above, the molded copper plate 40 according to this embodiment includes the layer 52 including the grooves 54 provided on the base layer 50, and the coating layer 56 provided on the layer 52 including the grooves 54. The layer 52 including the grooves 54 is made of a different material whose thermal conductivity ratio relative to the thermal conductivity of the base layer 50 is 80% or less or 120% or more. This suppresses uneven solidification of the solidified shell and vertical cracks on the surface of the cast slab 34.

[0048] The coating layer 56 is composed of at least one of nickel, cobalt, nickel-based alloy, cobalt-based alloy, stainless steel alloy, metal nitride, metal carbide, and metal oxide. This improves the heat resistance of the coating layer, improves the heat resistance of the molded copper plate, and suppresses the occurrence of cracks in the coating layer 56 even if the surface temperature of the molded copper plate 40 becomes high. Furthermore, because the coating layer 56 is provided on the layer 52 containing the grooves 54 made of a different material, the occurrence of cracks in the different material can also be suppressed. As a result, the life of the mold 12 made of the molded copper plates 40, 42 can be extended.

[0049] In the present embodiment, the mold 12 is described as being composed of a pair of molded copper plates 40 and a pair of molded copper plates 42, but the present invention is not limited to this. As described above, the molded copper plate according to the present embodiment can suppress vertical cracks on the surface of the slab 34 to be cast and can also suppress the occurrence of cracks, so it is sufficient that at least one of the molded copper plates constituting the mold 12 is the molded copper plate 40, 42 according to the present embodiment. This makes it possible to suppress vertical cracks on the surface of the slab 34 to be cast and extend the mold life more effectively than a mold that does not include any of the molded copper plates 40, 42 according to the present embodiment.

[0050] Next, an example will be described in which medium-carbon steel that had been oxygen-blown in a converter and then RH-degassed was continuously cast into a slab. The chemical compositions of the medium-carbon steel used in the example were: C: 0.08-0.17% by mass, Si: 0.10-0.30% by mass, Mn: 0.50-1.20% by mass, P: 0.010-0.030% by mass, S: 0.005-0.015% by mass, and Al: 0.020-0.040% by mass. 300 tons of this medium-carbon steel was introduced into a ladle, and continuous casting was carried out using the continuous casting machine shown in FIG. 1.

[0051] In Examples 1 to 27, a mold was used that was composed of a molded copper plate on which a layer containing grooves and a coating layer were formed. In Examples 1 to 27, the layer containing grooves was formed using a different material whose thermal conductivity at room temperature was 80% or less or 120% or more relative to the substrate layer. In Examples 1 to 18, 25, and 27, the coating layer was formed using a laser cladding method, thereby forming an interface layer at the interface between the layer containing grooves and the coating layer. On the other hand, in Examples 19 to 24 and 26, the coating layer was formed using electroplating, thereby not forming an interface layer at the interface between the layer containing grooves and the coating layer.

[0052] In the conventional example, a mold was used that was made of a molded copper plate on which a Ni or NiCo coating layer was formed by electroplating. In comparative example 1, a mold was used that was made of a molded copper plate with only a base layer. In comparative examples 2 to 12, a mold was used that was made of a molded copper plate on which a coating layer was provided by laser cladding. In comparative examples 9 to 12, a mold was used that was made of a molded copper plate on which a layer containing grooves was formed using a different material whose thermal conductivity at room temperature was greater than 80% or less than 120% of the base layer. The continuous casting conditions and evaluation results of the cast slabs for the conventional example, inventive examples, and comparative examples are shown in Table 1 below. In the "Molded steel sheet coating layer" column in Table 1, "%" means mass %, and "TiC10%-Ni" in inventive example 6 means a Ni alloy containing 10 mass % TiC.

[0053]

[0054] The tensile strength of the coating layer at 300°C is a value measured using the method described in JIS G 0567:2020 (High-temperature tensile test method for steel materials and heat-resistant alloys). The mold temperature during casting is the maximum surface temperature near the meniscus measured using a thermocouple installed in the mold copper plate. The number of vertical cracks on the slab surface is a value measured to determine the number of vertical cracks that occurred on the surface of the slab after casting. The number of charges is the cumulative number of charges from the time the mold was used until cracks appeared on the mold surface.

[0055] 6 is a graph showing the number of vertical cracks on the slab surface in Conventional Examples 1 and 2, Inventive Examples 1 to 27, and Comparative Examples 1 to 12. As shown in FIG. 6, the number of vertical cracks on the slab surface in Inventive Examples 1 to 27 was significantly smaller than that in Conventional Examples 1 and 2 and Comparative Examples 1 to 12. These results confirm that by forming a layer with grooves using a different material whose thermal conductivity ratio with respect to the base layer is 80% or less and 120% or more, vertical cracks on the surface of the solidified shell can be suppressed, and vertical cracks on the surface of the cast slab can also be suppressed.

[0056] 7 is a graph showing the number of charges, which indicates the mold lifespan of Conventional Examples 1 and 2, Inventive Examples 1 to 27, and Comparative Examples 1 to 12. As shown in FIG. 7, it was confirmed that the number of charges of Inventive Examples 1 to 27 was equal to or greater than that of Conventional Examples 1 and 2 and Comparative Example 1. From these results, it was confirmed that by using molds made of the molded copper plates of Inventive Examples 1 to 27, it is possible to suppress the occurrence of vertical cracks on the slab surface without reducing the mold lifespan compared to conventional methods.

[0057] Inventive Examples 19 to 24, and 26 are inventive examples that use molded copper plates in which no interface layer is formed at the interface between the coating layer and the layer with concave grooves. As shown in Figure 7, the number of charges in Inventive Examples 1 to 18, 25, and 27, which used molded copper plates in which an interface layer is formed at the interface between the coating layer and the layer with concave grooves, was higher than the number of charges in Inventive Examples 19 to 24 and 26, which used molded copper plates in which no interface layer was formed. These results confirmed that the life of the mold can be extended by using a molded copper plate in which an interface layer is formed at the interface between the coating layer and the layer with concave grooves.

[0058] Inventive Examples 1 and 2 are inventive examples using molded copper plates having a coating layer that does not satisfy the tensile strength requirement of 250 MPa or more at 300°C. Inventive Examples 3 to 18, 25, and 27 are inventive examples using molded copper plates having a coating layer that satisfies the tensile strength requirement of 250 MPa or more at 300°C. As shown in Figure 7, the number of charges in Inventive Examples 3 to 18, 25, and 27 was greater than the number of charges in Inventive Examples 1 and 2. These results confirmed that the life of the mold can be further extended by using a molded copper plate having a coating layer that satisfies the tensile strength requirement of 250 MPa or more at 300°C.

[0059] REFERENCE SIGNS LIST 10 Continuous casting machine 12 Continuous casting mold 14 Tundish 16 Strand support roll 17 Conveyor roll 18 Molten steel 20 Submerged nozzle 22 Mold flux 24 Solidified shell 26 Unsolidified layer 28 Strand 30 Secondary cooling zone 32 Strand cutting machine 34 Slab 40 Mold copper plate 42 Mold copper plate 50 Base layer 52 Layer containing grooves 54 Groove 56 Coating layer 57 Filling portion 58 Layer portion

Claims

1. A molded copper plate used in a continuous casting mold, comprising: a base layer containing copper or a copper alloy; a layer containing grooves or round grooves provided on the base layer; and a coating layer provided on the layer containing grooves or round grooves, wherein the coating layer is composed of at least one of nickel, cobalt, a nickel-based alloy, a cobalt-based alloy, a stainless steel alloy, a metal nitride, a metal carbide, and a metal oxide, and the layer containing grooves or round grooves is composed of a heterogeneous material whose thermal conductivity ratio to the thermal conductivity of the base layer is 80% or less or 120% or more.

2. The molded copper plate according to claim 1, which has an interface layer at the interface between the coating layer and the layer containing the concave grooves or rounded grooves.

3. A continuous casting mold including the molded copper plate according to claim 1 or 2, wherein the thickness of the coating layer in the lamination direction is 2.0 mm or less, and the layer including the concave grooves or round grooves is provided in a region up to 200 mm below the meniscus.

4. The continuous casting mold according to claim 3, wherein the coating layer has a tensile strength of 250 MPa or more at 300°C.

5. A continuous casting mold according to claim 3 or 4, comprising the mold copper plate in a funnel shape.

6. A method for continuously casting steel using the continuous casting mold according to any one of claims 3 to 5, comprising the steps of: applying CaO / SiO2 to the surface of molten steel in the continuous casting mold; ２ and continuously casting the mixture at a casting speed of 1.0 m / min to 6.0 m / min.