Mold for continuous casting of steel and method for continuous casting of steel

Abstract

The present invention provides a continuous casting mold and a continuous casting method, with which it is possible to carry out continuous casting, while suppressing the occurrence of abnormalities on the surface of a mold plate even under a high-speed casting condition of 2.5 m / minute or more.　The present invention specifically provides a mold for continuous casting of steel, the mold comprising: a mold plate which is formed of a copper alloy, the front surface of which forms an inner wall surface of the mold, and the back surface of which is provided with a cooling water path; and a backup plate which is attached to the mold plate so as to cover the cooling water path. The mold is provided with a plurality of dissimilar material filled parts that are each formed by filling a recessed part, which is formed in a region including at least a meniscus on the surface of the mold plate, with a dissimilar material that has a thermal conductivity different from the thermal conductivity of the mold plate. The total cross-sectional area of the cooling water path on the back surface of the mold plate in a range that includes the region where the plurality of dissimilar material filled parts are formed is smaller than the total cross-sectional area of the cooling water path below the range.

Description

Mold for continuous casting of steel and method for continuous casting of steel

[0001] The present invention relates to a mold for continuous casting of steel and a method for continuous casting of steel, which are capable of stably realizing continuous casting even at high speeds of 2.5 m / min or more.

[0002] In the continuous casting of steel, the demand for high productivity and high quality is becoming more stringent, and there is a need for technology that can achieve high-speed casting without generating surface defects in the mold and secondary cooling zone. In particular, if surface cracks occur on the surface of the cast slab during continuous casting, it is necessary to remove the surface defects by cold slab conditioning to prevent the defects from being transported to downstream processes. This hinders the direct delivery process of the cast slab, posing a major problem in terms of productivity.

[0003] Typical surface cracks include vertical surface cracks, transverse surface cracks, and surface key cracks. It is known that transverse surface cracks and surface key cracks occur when strain is applied in the embrittlement temperature range during the secondary cooling zone of continuous casting. Common methods for preventing transverse surface cracks and surface key cracks include designing secondary cooling specifications that avoid the embrittlement temperature range at the bending correction point, and adding embrittlement suppression elements.

[0004] On the other hand, it is known that surface vertical cracks are likely to occur in medium carbon steel in the hypoperitectic region, where uneven solidification is likely to occur, and that the cracks are likely to start inside the mold. For this reason, slow-cooling mold powder, which can suppress uneven solidification, is used to alleviate stress concentration on the solidified shell during initial solidification.

[0005] When using slow-cooling mold powder, the mold heat removal is weakened, so during high-speed casting, the solidified shell does not grow thick enough at the bottom of the mold, making breakout more likely. As a result, steel types that require the use of slow-cooling powder have the problem of casting speeds that are relatively slow compared to other steel types. Another problem with slow-cooling powder is its high cost.

[0006] In response to this, a technology has been proposed and adopted that prevents vertical surface cracks by filling the mold plate with a material with low thermal conductivity. Patent Document 1 discloses a continuous casting mold in which a dissimilar-substance-filled portion is formed on the inner wall surface of the mold, where the dissimilar-substance-filled portion is filled with a dissimilar substance having a thermal conductivity different from that of the copper alloy constituting the mold. Patent Document 1 claims that by keeping the thermal resistance R between the mold surface at the position where the dissimilar-substance-filled portion is formed and a cooling water channel provided on the back surface of the mold within a predetermined range, surface cracks in the cast slab can be reduced and a reduction in mold life due to mold surface cracks can be suppressed.

[0007] Patent Document 2 also discloses a continuous casting mold in which a dissimilar-substance-filled portion is formed in the mold plate, where the dissimilar-substance-filled portion is filled with a dissimilar substance having a thermal conductivity different from that of the copper alloy constituting the mold plate. In this continuous casting mold, a water flow disturbing portion is formed in the cooling water channel on the back surface of the mold plate in the region where the dissimilar-substance-filled portion is formed, thereby reducing surface cracks in the cast slab and extending the mold life.

[0008] JP 2017-39165 A International Publication No. 2020 / 095932

[0009] The present inventors conducted casting tests using a packed mold in which Ni was filled into the molten steel-side surface of the mold plate as disclosed in Patent Documents 1 and 2, and confirmed that this packed mold was highly effective in reducing surface vertical cracks in medium-carbon steel. However, when this packed mold was used in a high-speed continuous caster, it was found that the temperature of the mold plate surface steadily exceeded 300°C at casting speeds of 2.5 m / min or higher. When the temperature of the mold plate surface steadily exceeded 300°C, abnormalities such as plating peeling and deformation of the mold plate occurred, making it impossible to continue using the mold.

[0010] The present invention has been made in view of the above problems, and an object of the present invention is to provide a continuous casting mold for steel that can perform continuous casting while suppressing the occurrence of surface cracks of the slab and abnormalities on the surface of the mold plate even under high-speed casting conditions of 2.5 m / min or more. Another object of the present invention is to provide a continuous casting method for steel that uses the continuous casting mold for steel.

[0011] The means for solving the above problems are as follows. [1] A mold for continuous casting of steel, comprising a mold plate made of a copper alloy, the surface of which forms the mold inner wall surface and the back surface of which has cooling water channels, and a backup plate attached to the mold plate so as to cover the cooling water channels, wherein a plurality of dissimilar-substance-filled sections filled with a dissimilar substance having a thermal conductivity different from that of the mold plate are formed in recesses formed in a region of the surface of the mold plate that includes at least the meniscus, and the total cross-sectional area of ​​the cooling water channels on the back surface of the mold plate in a range including the region where the plurality of dissimilar-substance-filled sections are formed is smaller than the total cross-sectional area of ​​the cooling water channels below the region. [2] The mold for continuous casting of steel according to [1], wherein the plurality of dissimilar-substance-filled sections are formed so that the heat flux from the mold inner wall surface toward the cooling water channels changes periodically in the region on the surface of the mold plate. [3] The mold for continuous casting of steel according to [1] or [2], wherein the dissimilar substance filling portion and the cooling water channel are formed so as to satisfy at least one of the following formulas (1) to (3): d<P≦S (1) e≦L≦1000×Vc / f (2) A1≦0.8×A2 (3) In the above formulas (1) to (3), d is the width (mm) of the foreign substance filled portion in the mold width direction, P is the spacing (mm) in the mold width direction between adjacent ones of the foreign substance filled portions, S is the spacing (mm) in the mold width direction between adjacent ones of the cooling water channels formed in the back surface of the mold plate, e is the width (mm) of the foreign substance filled portion in the slab withdrawal direction, L is the spacing (mm) in the slab withdrawal direction between adjacent ones of the foreign substance filled portions, Vc is the slab withdrawal speed (m / min) in the continuous casting process of steel, f is the vibration frequency (1 / min) of the continuous casting mold in the continuous casting process of steel, and A1 is the total cross-sectional area (mm) of the cooling water channels in the range including the above region. ２ ), and A2 is the total cross-sectional area (mm ２). [4] The mold for continuous casting of steel according to any one of [1] to [3], wherein a plating layer is formed on the surface of the mold plate so as to cover the dissimilar substance-filled portion. [5] The mold for continuous casting of steel according to any one of [1] to [4], wherein the mold plates are a pair of long-side mold plates and a pair of short-side mold plates, and a booster pump is provided that supplies cooling water to at least the short-side mold plates, in addition to a common pump that supplies cooling water to the long-side mold plates and the short-side mold plates. [6] The mold plate has a funnel shape, and the dissimilar substance-filled portion is also formed within a 50 mm range of the boundary of the funnel shape within a range of 0 to 200 mm below the meniscus. [7] A method for continuous casting steel using the mold for continuous casting steel according to any one of [1] to [6], wherein the maximum temperature of the surface of the mold plate at the meniscus is 350°C or less when continuous casting is performed at a casting speed of 2.5 m / min or more. [8] A method for continuous casting steel using the mold for continuous casting steel according to any one of [1] to [6], wherein the surface temperature of the mold plate is estimated from the temperature measured by a thermocouple embedded in the mold plate within a range of 20 to 100 mm below the meniscus, and the amount of cooling water supplied and the total cross-sectional area of ​​the cooling water channels on the back surface of the mold plate in the region where the dissimilar substance-filled portion is formed are adjusted so that the estimated surface temperature of the mold plate is 350°C or less. [9] A method for continuous casting steel using the mold for continuous casting steel according to any one of [1] to [6], wherein mold powder having a basicity of less than 1.2 is used.

[10] A method for continuous casting steel using the mold for continuous casting steel according to any one of [1] to [6], wherein the direction of flow of cooling water is from the upper end of the mold plate to the lower end of the mold plate.

[11] A method for continuous casting steel using the mold for continuous casting steel according to any one of [1] to [6], wherein the thickness of the cast strand is 150 mm or less.

[0012] In the mold for continuous casting of steel according to the present invention, the total cross-sectional area of ​​the cooling water channels in the range including the region where the dissimilar-substance-filled portion is formed is smaller than the total cross-sectional area of ​​the cooling water channels below that range. In cooling water channels with a reduced total cross-sectional area, the heat transfer coefficient between the water flow and the cooling water channels increases, increasing the amount of convective heat transfer, thereby effectively removing heat from the mold plate in the region where the dissimilar-substance-filled portion is formed. Effectively cooling the dissimilar-substance-filled portion and the mold plate reduces thermal stress at the interface between the mold plate and the dissimilar-substance-filled portion. As a result, surface cracking of slabs in steel types involving peritectic reactions can be suppressed, and continuous casting can be performed while suppressing plating peeling and copper plate deformation, even under high-speed casting conditions of 2.5 m / min or more. Furthermore, suppressing plating peeling and copper plate deformation can extend the life of molds with dissimilar-substance-filled portions.

[0013] FIG. 1 is a perspective view showing an example of a mold for continuous casting of steel according to this embodiment. FIG. 2 is a schematic diagram showing an example of the surface of a mold plate constituting the long side of the mold in the mold according to this embodiment. FIG. 3 is a diagram showing the structure of the mold plate 22 in the rectangular area C in FIG. 2. FIG. 4 is a cross-sectional view in the casting direction and a front view of a conventional mold plate. FIG. 5 is a U-U cross-sectional view and a V-V cross-sectional view of FIG. 4. FIG. 6 is a cross-sectional view in the casting direction and a front view of a mold plate of the mold according to this embodiment. FIG. 7 is a U-U cross-sectional view and a V-V cross-sectional view of FIG. 6. FIG. 8 is a graph showing the relationship between the distance from the top of the mold and the surface temperature of the mold plate. FIG. 9 is a cross-sectional view in the casting direction and a front view of the mold plate. FIG. 10 is a schematic diagram showing an example of the installation of a pump that supplies cooling water to the mold. FIG. 11 is a graph showing the relationship between the total pump water volume and the pump source pressure. FIG. 12 is a schematic diagram showing an example of the installation of a pump that supplies cooling water to the mold. Fig. 13 is a schematic diagram showing the mold used in Inventive Example 36. Fig. 14 is a schematic diagram showing the mold used in Comparative Example 30.

[0014] The present invention will be described below through embodiments of the present invention. FIG. 1 is a perspective view showing an example of a continuous casting mold 10 for steel according to this embodiment. The continuous casting mold 10 for steel (hereinafter, sometimes simply referred to as "mold 10") used for continuously casting a cast strip has a pair of opposing long mold sides 12 and a pair of opposing short mold sides 14 sandwiched between the long mold sides 12. A tundish (not shown) containing molten steel 16 is disposed above the mold 10, and an immersion nozzle 18 is installed at the bottom of the tundish. The pair of long mold sides 12 and the pair of short mold sides 14 form a rectangular internal space in the mold 10, into which the immersion nozzle 18 is inserted. The sides of the long mold sides 12 and the short mold sides 14 that come into contact with the molten steel 16 are formed by mold plates made of a copper alloy, and a backup plate is disposed behind the mold plates.

[0015] The copper alloy mold plates constituting the mold long sides 12 and short sides 14 have cooling water channels formed on the backside of the surfaces that contact molten steel 16. Cooling water is passed through these channels to cool the mold 10. In continuous steel casting, molten steel 16 is injected into the interior space of the mold 10 through a submerged nozzle 18. The molten steel 16 is cooled and solidified by the mold 10, forming a solidified shell on the contact surface with the mold 10. The cast slab, with this solidified shell as its outer shell and unsolidified molten steel 16 inside, is continuously withdrawn vertically downward in a slab withdrawal direction A to cast the steel slab. In the mold 10, contact with the molten steel 16 and the high-temperature slab causes the surface temperature of the mold plate (the temperature on the side in contact with the molten steel) to rise, reaching a maximum temperature near the meniscus M (the surface of the molten steel in the mold) within the mold. In FIG. 1 , the position of the meniscus M is indicated by a dashed line.

[0016] Although it depends on the type of steel, it is preferable to uniformly remove heat from the solidified shell at the position of the meniscus M in the mold in both the slab withdrawal direction A and the mold width direction B. By uniformly removing heat from the solidified shell in both the slab withdrawal direction A and the mold width direction B in this way, it is possible to promote uniform growth in the thickness of the solidified shell. Here, the slab withdrawal direction A and the mold width direction B are perpendicular to each other. For the mold plate, it is preferable to use a copper alloy that has high deformation resistance against thermal stress and high thermal conductivity that can enhance the cooling effect of cooling water.

[0017] A plurality of strand support rolls (not shown) are arranged below the mold 10, and water spray nozzles or air mist spray nozzles are arranged between adjacent strand support rolls. The strand is cooled by spraying cooling water onto the surface of the strand from the water spray nozzles or air mist spray nozzles, while being supported by the strand support rolls and pulled out. After solidification is complete up to the center of the strand, the strand is cut to a predetermined length. In this way, a strand of a predetermined length is produced, which is to be subjected to the next process of hot rolling.

[0018] In the mold 10 according to this embodiment, the total cross-sectional area of ​​the cooling water channels that cool the mold plate in the region where the dissimilar-substance-filled section is provided is smaller than the total cross-sectional area of ​​the cooling water channels below it. This increases the linear velocity of the cooling water in the region where the dissimilar-substance-filled section is provided, increasing the heat transfer coefficient in that region and the amount of convective heat transfer, thereby enabling effective heat removal from the dissimilar-substance-filled section and the mold plate. As a result, the dissimilar-substance-filled section and the surrounding mold plate are effectively cooled, suppressing thermal stress at the boundary between the mold plate and the dissimilar-substance-filled section, thereby extending the life of the mold 10 and enabling faster casting speeds.

[0019] 2 is a schematic diagram showing an example of the surface of a mold plate 22 that constitutes the mold long side 12 of the mold 10 according to this embodiment. Each of the mold long side 12 and mold short side 14 that constitute the mold 10 has a mold plate 22 whose surface forms the mold inner wall surface and whose back surface has a cooling water channel, and a backup plate that is attached to this mold plate 22 with bolts and nuts.

[0020] On the surface of the mold plate 22, recesses formed in an area including the meniscus M are filled with a dissimilar material having a thermal conductivity different from that of the mold plate 22, forming a plurality of mutually independent dissimilar material-filled portions 20. The dissimilar material-filled portions 20 are formed in the slab withdrawal direction A and the mold width direction B at least near the meniscus M, including the meniscus M. The dissimilar material-filled portions 20 may be formed by processing the dissimilar material into a shape that fits into the recess and fitting it into the recess, or by filling the dissimilar material into the recess by plating means, thermal spraying means, or the like. Filling the recess with the dissimilar material by plating means, thermal spraying means, or the like can prevent the formation of voids between the recess and the dissimilar material.

[0021] It is preferable that each of the multiple dissimilar substance filling sections 20 be regularly arranged on the surface of the mold plate 22 so that the heat flux on the mold inner wall surface heading from the mold inner wall surface toward the cooling water channel increases and decreases periodically.

[0022] By arranging multiple dissimilar-substance-filled sections 20 on the surface of the mold plate 22, including the vicinity of the meniscus M, the thermal resistance of the mold plate 22 in the slab withdrawal direction A and the mold width direction B in the region including the vicinity of the meniscus M increases and decreases regularly and periodically. This causes the heat flux from the solidified shell to the mold plate 22 in the vicinity of the meniscus M, i.e., in the early stages of solidification, to increase and decrease regularly and periodically. The regular and periodic increase and decrease in heat flux reduces the stresses and thermal stresses generated by the transformation of δ iron to γ ​​iron, thereby reducing the 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, and the generated stress is dispersed, reducing the amount of individual strain. As a result, the occurrence of vertical cracks on the solidified shell surface is suppressed.

[0023] The recesses do not have to be perfectly circular recesses on the surface of the mold plate 22, but may be pseudo-circular recesses. A pseudo-circular recess is a shape without corners, such as an ellipse, or a square or rectangle with circular or elliptical corners. Furthermore, the recesses may be shaped like flower petals.

[0024] To ensure a periodic change in heat flux on the mold inner wall surface, the spacing between adjacent dissimilar-material-filled sections 20 is preferably uniform. The thermal conductivity of the dissimilar material is preferably 80% or less or 125% or more relative to the thermal conductivity of the mold plate 22. The thermal conductivity of the dissimilar material changes with changes in ambient temperature. Therefore, the thermal conductivities of the dissimilar material and mold plate are based on room temperature (normal temperature) during mold manufacture. If the thermal conductivity of the dissimilar material differs from the thermal conductivity of the mold plate 22 by about 20% at room temperature, the regular and periodic increase and decrease in heat flux on the mold inner wall surface can reduce stress and thermal stress caused by the transformation of δ iron to γ ​​iron. However, as long as stress caused by the transformation is reduced and surface cracking of the cast slab is prevented, the thermal conductivity of the dissimilar material does not necessarily have to be within the above range, and the spacing between the dissimilar-material-filled sections 20 does not necessarily have to be uniform.

[0025] Examples of dissimilar materials with thermal conductivity of 80% or less of the thermal conductivity of the mold plate 22 include Ni (thermal conductivity: approximately 90 W / (m×K)) and Ni alloys (thermal conductivity: approximately 40 to 90 W / (m×K)), which are easily plated or sprayed. Copper alloys (thermal conductivity: approximately 100 to 385 W / (m×K)) can be used for the mold plate 22. For example, high-thermal-conductivity copper alloys (thermal conductivity: approximately 318 W / (m×K)) and low-thermal-conductivity copper alloys for electromagnetic stirring (thermal conductivity: approximately 119 to 239 W / (m×K)) can be used. However, metals other than Ni alloys and copper alloys may also be used for the dissimilar materials and mold plate 22.

[0026] The mold plate 22 can be made of pure copper (thermal conductivity: approximately 398 W / (m×K)) or the aforementioned copper alloy. In particular, when electromagnetically stirring the molten steel in the mold, it is preferable to use a copper alloy with low electrical conductivity, which contains a few mass percent of components other than copper, to prevent attenuation of the magnetic field strength from the coil into the molten steel. The thermal conductivity of copper alloys is lower than that of pure copper. In other words, it is preferable to adjust the thermal conductivity of the dissimilar material and / or the mold long side 12 by appropriately selecting the material for the dissimilar material and / or the mold long side 12 depending on the application of the mold 10.

[0027] A foreign-substance-filled portion 20 similar to that on the mold long sides 12 may also be formed on the surface of the mold short sides 14. However, in a cast slab, stress is likely to concentrate in the solidified shell on the mold long sides 12 side due to its shape, and surface cracks are likely to occur on the long side surface. Therefore, while it is necessary to provide a foreign-substance-filled portion 20 on the mold long sides 12 of the mold 10 according to this embodiment, it is not necessarily necessary to provide a foreign-substance-filled portion 20 on the mold short sides 14.

[0028] Considering the influence on initial solidification, it is preferable to provide the different material filling section 20 in a region from an upper position that is a distance Q away from the position of the meniscus M during steady casting to a lower position that is a distance R away from the meniscus M. The distance Q is an arbitrary value greater than zero. The distance R can be calculated using the following equation (4).

[0029] R=2×Vc×1000 / 60 (4) In the above formula (4), R is the distance (mm), and Vc is the slab withdrawal speed (m / min) in the steel continuous casting process.

[0030] The distance R relates to the time it takes for the solidified shell, after solidification has begun, to pass through the region where the dissimilar-substance-filled section 20 is formed. The solidified shell preferably stays in the region where the dissimilar-substance-filled section 20 is installed for at least two seconds after solidification has begun. In order for the solidified shell to remain in the region where the dissimilar-substance-filled section 20 is installed for at least two seconds after solidification has begun, the dissimilar-substance-filled section 20 may be installed at least the distance R calculated by equation (4) below the meniscus M.

[0031] If the slab after solidification begins resides in the region where the dissimilar-substance filling section 20 is installed for two seconds or more, the effect of the periodic change in heat flux from the mold inner wall surface toward the cooling channel by the dissimilar-substance filling section 20 can be fully achieved. That is, by ensuring that the solidified shell resides in the region where the dissimilar-substance filling section 20 is installed for two seconds or more, the effect of suppressing surface cracking of the slab can be achieved even during high-speed casting or when casting medium-carbon steel, in which surface cracking is likely to occur. To stably achieve the effect of the periodic change in heat flux by the dissimilar-substance filling section 20, it is more preferable to ensure that the time for the solidified shell to pass through the region where the dissimilar-substance filling section 20 is installed is four seconds or more. On the other hand, in the case of a thin slab continuous caster, the slab withdrawal speed is fast, so the distance R is long, and the region in the slab withdrawal direction A where the dissimilar-substance filling section 20 should be installed is large, resulting in increased mold processing costs. In such cases, ensuring that the time for passing through the dissimilar-substance filling section 20 is one second or more can achieve the effect of the periodic change in heat flux commensurate with that time.

[0032] The upper end of the region where the dissimilar substance filling portion 20 is formed need only be above the meniscus M and is not particularly limited. Therefore, the distance Q may be any value greater than zero. However, the meniscus M moves up and down during casting. For this reason, the dissimilar substance filling portion 20 is preferably formed up to 20 mm above the meniscus M so that the upper end of the region where the dissimilar substance filling portion 20 is always above the meniscus M. It is more preferable that the dissimilar substance filling portion 20 is formed up to 40 mm above the meniscus M. The position of the meniscus M is generally 60 to 150 mm below the upper end of the mold plate 22, and the region where the dissimilar substance filling portion 20 is formed can be determined accordingly.

[0033] In continuous casting of steel, high-temperature molten steel is poured into the internal space of the mold, causing the temperature of the mold plate 22 to rise. For this reason, cooling water channels are formed in the mold plate 22, which constitutes the mold long sides 12 and the mold short sides 14, and cooling water is passed through these channels to cool the mold plate 22. However, because the thermal expansion coefficient of the dissimilar material filled portion 20 differs from that of the mold long sides 12, thermal stress concentrated at the boundary between them may cause cracks to form on the surface of the mold plate 22.

[0034] Therefore, in the mold 10 according to this embodiment, the total cross-sectional area of ​​the cooling water channels in the range including the region of the mold plate 22 where the dissimilar material-filled portion 20 is formed is made smaller than the total cross-sectional area of ​​the cooling water channels below that range. By reducing the cross-sectional area of ​​the channel in the region where the dissimilar material-filled portion 20 is formed, the linear flow velocity of the cooling water in the channel increases, thereby increasing the heat transfer coefficient between the cooling water and the copper plate slit wall surface. As a result, heat removal from the mold plate 22 in the region where the dissimilar material-filled portion 20 is formed is promoted, and this region can be cooled effectively.

[0035] Next, the foreign material filling section 20 and the cooling water channels will be described. Figure 3 is a diagram showing the structure of the mold plate 22 in the rectangular area C in Figure 2. In Figure 3, (a) is a plan view showing the front surface of the mold plate 22, and (b) is a plan view showing the back surface of the mold plate 22. (c) is a vertical cross-sectional view of the relevant area, and (d) is a horizontal cross-sectional view of the relevant area. As shown in Figures 3(b) and (d), cooling water channels 26 are formed on the back surface of the mold plate 22. As shown in Figures 3(c) and (d), a backup plate 30 is attached to the back surface of the mold plate 22 so as to cover the cooling water channels 26.

[0036] Figure 4 is a cross-sectional view (a) of a conventional mold plate 40 taken along the casting direction and a front view (b). Figure 5 is a U-U cross-sectional view (a) and a V-V cross-sectional view (b) of Figure 4. Figure 6 is a cross-sectional view (a) of a mold plate 22 of the mold 10 according to this embodiment taken along the casting direction and a front view (b). Figure 7 is a U-U cross-sectional view (a) and a V-V cross-sectional view (b) of Figure 6.

[0037] 4 to 7, in the mold 10 according to this embodiment, unlike the conventional mold plate 40, a spacer 28 is installed between the cooling water channel 26 in the region where the dissimilar substance-filled portion 20 is formed and the backup plate 30. By installing this spacer 28, the total cross-sectional area of ​​the cooling water channel 26 in that region becomes smaller than the total cross-sectional area below it. The spacer 28 may be installed in a range that includes the region where the dissimilar substance-filled portion 20 is formed.

[0038] In particular, when performing high-speed casting at a casting speed of 2.5 m / min or more, it is preferable to reduce the total cross-sectional area of ​​the cooling water channels 26 and increase the linear flow velocity of the cooling water. This is to prevent the occurrence of steel leakage problems, such as breakout, if the solidified shell does not grow sufficiently in the mold, as it will not be able to withstand the static pressure of the molten steel at the mold bottom. If a low thermal conductivity material is used for the dissimilar substance filling section 20, the amount of heat removed from the mold will be less than with a conventional mold plate 40, and there is a concern that the formation of the solidified shell will be insufficient. The solidified shell thickness can be calculated using the following equation (5).

[0039] z_shell=K×ts １／２ =K×(z_m / Vc) １／２ ...(5) In the above formula (5), z_shell is the solidified shell thickness (mm). K is the solidification constant (20 to 27 mm / min) ０．５ ) where ts is the solidification time (min). z_m (m) is the distance from the meniscus M in the casting direction. Vc is the casting speed (m / min).

[0040] As can be seen from the above formula (5), the influence on the growth of the solidified shell increases as the distance from the meniscus M decreases. Therefore, by increasing the solidification constant K of the dissimilar-substance-filled portion 20 that is closer to the meniscus M, the solidified shell can be grown thicker.

[0041] Copper alloys are generally used for the mold plate 22, but copper alloys become easily deformed when the temperature exceeds 350°C due to a significant drop in yield stress and softening. The outermost surface of the mold plate 22 facing the molten steel is generally plated with a Ni-Co or Ni-Cr plating layer 32, but the hardness of Ni alloys drops significantly when the temperature exceeds 350°C.

[0042] Therefore, when high-speed casting is performed or a low-thermal-conductivity material is used for the dissimilar-substance-filled portion 20, heat removal is insufficient, and when the surface temperature of the mold plate 22 exceeds 350°C, peeling and cracks are likely to occur on the copper plate surface and the plating surface. In contrast, the mold 10 according to this embodiment is equipped with a spacer 28 to increase the linear flow velocity of the cooling water in the region where the dissimilar-substance-filled portion 20 is formed. This increases the heat transfer coefficient between the cooling water and the cooling water channel 26, increasing the amount of convective heat transfer. This allows for effective heat removal from the mold plate 22 in the region where the dissimilar-substance-filled portion 20 is formed, thereby effectively cooling the mold plate 22 and the dissimilar-substance-filled portion 20. As a result, surface cracking of the cast slab can be suppressed, and peeling and cracking on the surface of the mold plate 22 and the plating layer 32 can be suppressed.

[0043] The linear flow velocity of the cooling water in the cooling water channel 26 is preferably 7.0 m / sec or higher. In this linear flow velocity region, the Reynolds number Re of the water in the channel is approximately 40,000, resulting in turbulent flow (Re>2300). To increase local heat removal near the meniscus M, the linear flow velocity of the cooling water should be set to 10.0 m / sec or higher. In principle, setting the linear flow velocity of the cooling water to 10.0 m / sec or higher can reduce the surface temperature of the mold plate 22 by approximately 30°C.

[0044] To increase the linear flow velocity of the cooling water, it is necessary to increase the water supply pressure. Increasing the water supply pressure increases the pressure loss in each water channel. This increases the variation in the linear flow velocity of the cooling water for each of the many slits. Therefore, if the linear flow velocity is increased too much, the cooling difference between the uncooled areas near the stud bolt and the cooled areas will increase, and there is a concern that vertical cracks may actually worsen.

[0045] If the linear flow velocity of the cooling water is too high, the difference in linear flow velocity between the slits becomes large, which may cause the solidified shell to lift up near the meniscus M, resulting in the formation of fine cracks that serve as the initiation points for vertical cracks. If the linear flow velocity of the cooling water remains high even in the lower part, tensile stress in the width direction of the lifted portion of the solidified shell continues to occur, which may lead to the opening of vertical cracks or the growth of deep depressions. In contrast, if the cross-sectional area of ​​the cooling water channel 26 is increased from about 200 mm below the meniscus M from the upper end of the mold, the linear flow velocity of the cooling water is slowed, which has the advantage of alleviating tensile stress and preventing the opening of vertical cracks or the growth of depression depth. Therefore, increasing the linear flow velocity of the cooling water in the range from the vicinity of the meniscus M to the region where the foreign-substance-filled portion 20 is formed and decreasing the linear flow velocity below that range is the most effective method for high-speed casting and reducing the surface temperature of the mold plate 22.

[0046] The inventors have confirmed that in a typical copper alloy mold in which the dissimilar-substance-filled portion 20 is not embedded across the meniscus M, longitudinal cracks tend to become apparent when the linear flow velocity is 12 m / sec or higher. It has also been confirmed that, for this mold, reducing the cross-sectional area of ​​the cooling water channels 26 near the meniscus M relative to the area below it tends to suppress the occurrence of longitudinal cracks in the slab. Furthermore, it is preferable that the total channel area A1 of the cooling water channels 26 on the back surface of the mold plate in the range including the area where the meniscus M and the dissimilar-substance-filled portion 20 are formed and the total cross-sectional area A2 of the cooling water channels 26 below that range satisfy the relationship A1 ≦ 0.8 × A2. This confirmed that the occurrence of severe longitudinal cracks of 2 mm or more in the slab is suppressed not only when the mold 10 according to this embodiment is used, but also when a conventional mold is used.

[0047] It was also confirmed that by satisfying the relational expression for the total area of ​​the cooling water channels 26 described above, the mold 10 according to this embodiment, in which the dissimilar substance filling section 20 is embedded across the meniscus M, does not cause longitudinal cracks in the cast piece even when high-speed casting of 2.5 m / min or more is performed.

[0048] In the mold 10 according to this embodiment, it is preferable to form the foreign substance filling section 20 and the cooling water channel 26 in the mold plate 22 so as to satisfy at least one of the conditions of the following equations (1) to (3).

[0049] d<P≦S (1) e≦L≦1000×Vc / f (2) A1≦0.8×A2 (3) In equations (1) to (3), d is the width (mm) of the foreign-substance-filled section 20 in the mold width direction B. P is the spacing (mm) in the mold width direction B between adjacent ones of the foreign-substance-filled section 20. S is the spacing (mm) in the mold width direction B between adjacent ones of the cooling water channels 26 formed on the back surface of the mold plate 22. e is the width (mm) of the foreign-substance-filled section 20 in the slab withdrawal direction A. L is the spacing (mm) in the slab withdrawal direction A between adjacent ones of the foreign-substance-filled section 20. Vc is the slab withdrawal speed (m / min) in the continuous steel casting process. f is the vibration frequency (1 / min) of the mold 10 in the continuous steel casting process. A1 is the total cross-sectional area (mm ２ A2 is the total cross-sectional area (mm ２ The "spacing distance" refers to the center-to-center distance between two adjacent portions of each portion in the slab withdrawal direction A or the width direction B of the mold plate 22 (see FIG. 3).

[0050] Next, the total cross-sectional area A1 and the total cross-sectional area A2 will be described. The boundary between the total cross-sectional area A1 and the total cross-sectional area A2 is divided into a position that includes the foreign substance filling section 20 and a position that does not include the foreign substance filling section 20. In other words, the range of the total cross-sectional area A1 includes the region where the foreign substance filling section 20 is formed, and may also include a portion of the region where the foreign substance filling section 20 is not formed. However, it is preferable that the range of the total cross-sectional area A1 is the region where the foreign substance filling section 20 is formed. Both the total cross-sectional area A1 and the total cross-sectional area A2 are the sum of the cross-sectional areas of the respective slits. The linear flow velocity of the cooling water in the cooling water channel 26 can be calculated by dividing the water volume by the total cross-sectional area.

[0051] In the mold 10 according to this embodiment, the dissimilar-substance-filled portion 20 is provided on the copper plate surface near the meniscus, so that the cooling of the region where the dissimilar-substance-filled portion 20 is provided is about 10% weaker than the region where the dissimilar-substance-filled portion 20 is not provided. Therefore, in the mold 10 according to this embodiment, the total cross-sectional area A1 is made 20% smaller than the total cross-sectional area A2. This increases the linear velocity of the cooling water in the region where the dissimilar-substance-filled portion 20 is provided by about 20%, enhancing heat transfer from the slits, so that the cooling of the region where the dissimilar-substance-filled portion 20 is provided can be maintained at the same level as in the conventional region where the dissimilar-substance-filled portion 20 is not provided.

[0052] Because the mold plate 22 is cooled by cooling water flowing through the cooling water channels 26 on the back surface of the mold plate 22, heat is removed from the mold plate 22 radially through the cooling water channels 26. Therefore, uneven cooling occurs on the surface of the mold plate 22 between areas close to and far from the cooling water channels 26. To maximize the effect of reducing the stress and thermal stress generated by the transformation of δ iron to γ ​​iron due to the periodic increase and decrease in thermal resistance caused by the foreign material filling section 20, it is preferable to create a heat flux difference at a distance smaller than the spacing distance S of the cooling water channels 26. Therefore, it is preferable to satisfy the above formula (1). That is, it is preferable that the spacing distance P of the foreign material filling section 20 in the mold width direction B be equal to or less than the spacing distance S of the cooling water channels 26, and it is preferable that the width d of the foreign material filling section 20 be less than the spacing distance P.

[0053] The width d of the foreign substance filling portion 20 is preferably 2 mm or more and 20 mm or less. When the foreign substance filling portion 20 has a pseudo-circular shape, the width d may be the circle equivalent diameter calculated by the following formula (6).

[0054] Equivalent circle diameter = (4 x S / π) １／２ ...(6) In equation (6), S is the area of ​​the foreign substance filling portion 20 (mm ２ )

[0055] By setting the width d or the equivalent circle diameter to 2 mm or more, it becomes easy to fill the circular or pseudo-circular recesses with a dissimilar material by plating or thermal spraying. On the other hand, by setting the width d and the equivalent circle diameter to 20 mm or less, delay in solidification due to a decrease in heat flux at the dissimilar material filling portion 20 is suppressed, stress concentration on the solidified shell at that position is prevented, and the occurrence of surface cracks in the solidified shell is easily suppressed.

[0056] When molten steel 16 is poured into the mold 10, in order to prevent the molten steel 16 from sticking to the mold 10, the mold 10 is vibrated while mold powder is poured onto the surface of the molten steel 16. It is known that this vibration causes periodic oscillation marks to form on the surface of the slab in the slab withdrawal direction A, and the thickness of the slab tends to change periodically in the slab withdrawal direction A.

[0057] Transverse cracks in the slab can be suppressed by making the width of the dissimilar-substance-filled portion 20, the spacing between adjacent dissimilar-substance-filled portions 20, the slab withdrawal speed, and the vibration frequency f of the mold satisfy the above formula (2). In other words, transverse cracks in the slab can be suppressed by making the width of the dissimilar-substance-filled portion 20 in the slab withdrawal direction A smaller than the length (pitch) of one cycle in the slab withdrawal direction A of the thickness increase or decrease of the slab caused by oscillation marks.

[0058] As the mold 10 oscillates, concave depressions are formed in the slab, one per oscillation cycle. These depressions are called oscillation marks. When the casting speed Vc and the mold frequency f are used, the oscillation marks occur at a pitch of 1000 × Vc / f. Because the oscillation of the mold 10 causes the oscillation marks to occur at a pitch of 1000 × Vc / f, it is extremely important to avoid applying concentrated strain or stress to these areas in order to suppress transverse cracks. Therefore, in the mold 10 according to this embodiment, it is preferable to provide dissimilar material-filled portions at a pitch of 1000 × Vc / f or less. This prevents artificial thermal stress from acting on the oscillation mark valleys, thereby suppressing the generation of fine cracks in the oscillation mark valleys during the initial solidification stage and suppressing the subsequent expansion and propagation of cracks during the secondary cooling zone.

[0059] In the mold 10 according to this embodiment, it is preferable that the foreign substance filling section 20 is formed in the mold plate 22 so as to satisfy the condition of the following formula (7).

[0060] 0.5≦t≦d (7) In the above formula (7), t is the filling thickness (mm) of the foreign substance in the foreign substance filling section 20, and d is the width (mm) of the foreign substance filling section 20 in the width direction B of the mold.

[0061] If the filling thickness of the foreign substance filling section 20 (see Figure 3(d)) is less than 0.5 mm, the fluctuation of heat flux in the foreign substance filling section 20 may be insufficient. On the other hand, if the filling thickness is too thick, it becomes difficult to fill the recesses with the foreign substance. Therefore, it is preferable that the filling thickness be less than the width d (mm) of the foreign substance filling section in the mold width direction. The filling thickness t is preferably 10 mm or less. If the filling thickness is greater than 10 mm, it becomes difficult to fill the recesses with the foreign substance. Furthermore, if the thermal conductivity of the foreign substance is lower than that of the mold plate 22 and the filling thickness is greater than 10 mm, the amount of heat removed from the mold plate 22 will be reduced. This will increase the surface temperature of the mold plate 22, and if the surface temperature exceeds 350°C, this will lead to abnormalities in the copper plate and plating. Therefore, it is preferable that the filling thickness be 10 mm or less.

[0062] 6, a plating layer 32 is preferably formed on the surface of the mold plate 22 so as to cover the dissimilar substance-filled portion 20. This makes it possible to suppress cracking of the mold surface due to wear caused by the solidified shell and thermal history. The plating layer 32 can be formed by plating or thermal spraying commonly used nickel or alloys containing nickel, such as nickel-cobalt alloys (Ni—Co alloys) and nickel-chromium alloys (Ni—Cr alloys).

[0063] Since the cooling strength (heat transfer coefficient) of the mold 10 is determined by the linear flow velocity of the cooling water flowing through the cooling water passages 26, it is preferable to increase the linear flow velocity of the cooling water to 7.0 m / sec or more during high-speed casting. The average linear flow velocity of the cooling water is calculated by the following formula (8), so in order to increase the linear flow velocity of the cooling water to 7.0 m / sec or more, it is important to appropriately design the amount of cooling water and the cooling water passages 26.

[0064] Average linear flow velocity of cooling water (m / sec) = Amount of cooling water (m ３ / sec) / total cross-sectional area of ​​cooling water channel (m ２ ) ... (8)

[0065] When molten steel is solidified through a mold 10 in continuous casting of steel, the amount of heat removed from the mold 10 can be estimated from the temperature difference between the cooling water entering and leaving the mold and the temperature of a thermocouple embedded in a copper plate of the mold. The amount of heat removed from the mold 10 tends to reach a maximum value in the range of 20 mm to 80 mm below the meniscus M in the casting direction, and then gradually decrease.

[0066] Figure 8 is a graph showing the relationship between the distance from the top of the mold and the surface temperature of the mold plate 40. In response to the heat dissipation amount, the surface temperature of the mold copper plate reaches its maximum temperature at the temperature maximum point directly below the meniscus M, as shown in Figure 8. If the surface temperature of the mold plate 40 exceeds 350°C, the yield stress of the copper plate and the hardening point of the surface plating are reached, raising concerns about the occurrence of mold plating peeling. As shown in Figure 8, this curve shifts toward higher temperatures as the casting speed increases. Therefore, if the surface temperature of the mold plate is likely to exceed 350°C, it is necessary to increase the amount of cooling water or reduce the total cross-sectional area of ​​the cooling water channels 26 to increase the linear flow velocity of the cooling water.

[0067] If the recesses are filled with a low thermal conductivity material, the temperature rise at the filled area will be even greater. The surface temperature of the mold plate varies depending on the filling depth of the low thermal conductivity material, the material of the mold plate, and the thickness of the mold plate, so attention must be paid to the surface temperature of the mold plate during high-speed casting.

[0068] As shown in Figure 8, the surface temperature of the mold plate 40 reaches a maximum near the meniscus M, and below that, the temperature distribution is relatively low and gentle. For this reason, the areas where the copper plate or plating is likely to peel or be damaged tend to be concentrated within about 200 mm below the meniscus M, and it is preferable to increase the amount of heat dissipation in these areas to keep the surface temperature of the mold plate below 350°C.

[0069] In the mold 10 according to this embodiment, the total cross-sectional area of ​​the cooling water channels 26 on the back surface of the mold plate in the range including the region where the meniscus M and the foreign material filled portion 20 are formed is smaller than the total cross-sectional area of ​​the cooling water channels 26 below this range. This improves heat removal from the mold plate 22, and the surface temperature of the mold plate 22 can be kept at 350°C or less even during continuous casting at a casting speed of 2.5 m / min or more.

[0070] It is preferable to embed a thermocouple in the mold plate 22 within a range of 20 to 100 mm below the meniscus M and use the temperature measured by the thermocouple. That is, the surface temperature of the mold plate 22 can be calculated using the temperature measured by the thermocouple, and the amount of cooling water required and the total cross-sectional area of ​​the cooling water channels 26 can be determined so that the surface temperature of the mold plate 22 is 350°C or less. From this result, the upper limit of the possible casting speed can also be determined.

[0071] Next, the flow direction of cooling water will be described. Figure 9 shows a cross-sectional view (a) of the mold plate 22 in the casting direction and a front view (b) of the mold plate 22. The flow direction of cooling water will be described using Figure 9. In the continuous steel casting method according to this embodiment, as shown in Figure 9, the flow direction of cooling water is from the upper end of the mold near the meniscus M to the lower end of the mold. This allows low-temperature cooling water to be supplied to the cooling water channels 26 near the meniscus M, thereby improving the cooling capacity of the mold 10 near the meniscus M.

[0072] In the past, supplying low-temperature cooling water to the upper end of the mold near the meniscus M was considered undesirable because it led to uneven cooling, and uneven cooling near the meniscus M would result in longitudinal crack defects in the cast slab. In contrast, in the continuous steel casting method according to the present embodiment, the foreign-substance-filled section 20 is embedded near the meniscus M, which is expected to provide a uniform cooling effect. Therefore, in the continuous steel casting method according to the present embodiment, it is preferable to direct the cooling water from the upper end of the mold near the meniscus M toward the lower end of the mold. This improves the cooling capacity at the upper end of the mold, where the surface temperature is likely to be high, and makes it easier to keep the surface temperature of the mold plate 22 below 350°C. Furthermore, even in operations with a casting speed of 2.5 m / min or higher, where the surface temperature of the mold plate 22 is likely to exceed 350°C, it is preferable to introduce low-temperature cooling water from the upper end of the mold. This directs the cooling water from the upper end of the mold near the meniscus M toward the lower end of the mold, thereby improving the cooling capacity at the upper end of the mold, where the surface temperature is likely to be high.

[0073] In the continuous steel casting method according to this embodiment, it is preferable to supply cooling water to the cooling water channels 26 with high precision. In the mold 10, cooling water is generally supplied from a common pump to the cooling water channels 26 of the mold long sides 12 and the mold short sides 14. However, with this configuration, even if an attempt is made to improve the precision of the cooling water supply, due to the flow rate-pressure relationship determined by the piping diameter, when an attempt is made to increase the amount of cooling water on either the mold long sides 12 or the mold short sides 14, the amount of cooling water may not be increased because pressure is not generated on the other side.

[0074] Figure 10 is a schematic diagram showing an example of the installation of pumps for supplying cooling water to a mold 10. As shown in Figure 10, a total of four pumps could be installed independently, with two pumps at the front and rear of the mold long sides 12 and two pumps at the left and right of the mold short sides 14. However, this would not only be extremely costly, but also complicate control. Since this would require having multiple backup pumps in case of failure, this is not desirable for operating facilities for continuous steel casting, which operates without interruption for almost 365 days a year.

[0075] For continuous operation, it is preferable that even in the event of a pump failure, a small amount of cooling water continues to flow to all four sides of the mold, including the long sides 12 and the short sides 14. Therefore, it is preferable that the main pump be shared by both the long sides 12 and the short sides 14 of the mold.

[0076] FIG. 11 is a graph showing the relationship between the total pump water volume and the pump source pressure. Generally, the relationship between the total pump water volume and the pump source pressure is shown by the solid line in FIG. 11 as a pump characteristic. The dashed line is a pump system diagram. As shown in FIG. 11, increasing the total amount of cooling water supplied requires opening the valve closer to full open and reducing the pump source pressure. A mold used in continuous steel casting consists of a pair of mold long sides 12 and a pair of mold short sides 14. The pipe diameter of the mold short sides 14, which have a lower water volume, is typically smaller than the pipe diameter of the mold long sides 12, which have a higher water volume. Therefore, even if an amount of water corresponding to the required linear flow velocity is supplied to the mold 10 along the long and short sides, the pressure loss on the short sides may be so great that the target flow rate cannot be achieved, even if the water can be supplied to the long sides.

[0077] Figure 12 is a schematic diagram showing an example of the installation of a pump for supplying cooling water to the mold. To solve the above-mentioned problems, as shown in Figure 12, it is preferable to install a common pump P, which is responsible for the overall water supply, and a boost pump BP, which further increases the amount of water on the short side, in series piping. That is, to increase the amount of cooling water supplied to the short side 14 of the mold, which has a smaller piping diameter, it is necessary to increase the water pressure in the piping enough to overcome pressure loss. By installing a boost pump BP separate from the common pump P as a pump for increasing this water pressure, the pump can compensate for the pressure in the piping on the short side 14 of the mold.

[0078] A preferred pump control method is to operate the boost pump BP when a specific casting speed is reached. When the casting speed is high, the amount of cooling water at the mold narrow side 14 can be adjusted to a target amount by increasing the water pressure with the boost pump BP in conjunction with an increase in the amount of cooling water at the mold long side 12. By adopting this method, not only can the number of backup pumps used in the event of a pump failure be minimized, but a minimum amount of cooling water can also be supplied to the mold 10 even in the event of a pump failure, thereby avoiding operational abnormalities.

[0079] One effective method for controlling the amount of cooling water is to vary the amount of cooling water in response to the casting speed. However, because the change in mold cooling capacity is not as steep as that of the secondary cooling water, it is easier to control the amount of water by setting the amount of water at several steps in relation to the casting speed.

[0080] Ideally, a system would be constructed that automatically controls the total cross-sectional area of ​​the pump and cooling water channels 26 based on the temperature of a thermocouple embedded in the mold plate 22 so that the surface temperature of the mold plate 22 is kept below 350°C. The control method involves determining the water flow rate of the mold pump, and then adjusting the pump setting and the total cross-sectional area of ​​the cooling water channels 26 so that the surface temperature of the mold plate, determined from the thermocouple temperature of the mold plate 22, is kept below 350°C. For more advanced control, a spacer 28 installed on the backup plate may be movable using a servo motor, pneumatic cylinder, hydraulic cylinder, or the like to increase the linear flow velocity at a location corresponding to the foreign material filling portion. This allows the total cross-sectional area A1 of the cooling water channels 26 above the mold, including the meniscus M and the foreign material filling portion 20, to be changed without dismantling the mold 10.

[0081] Next, the mold powder used in the continuous casting method for steel according to this embodiment will be described. In the continuous casting of medium carbon steel, which is a typical example of hypoperitectic steel, a slow-cooling mold powder is used to suppress the occurrence of longitudinal cracks in the cast slab. With medium carbon steel, the growth of the solidified shell tends to be uneven due to the transformation stress associated with the δ → γ transformation, and when stress concentration occurs in the circumferential direction, defects such as longitudinal cracks and depression occur at that location. To suppress this phenomenon, a crystallizing powder is used for medium carbon steel, which actively generates a crystalline phase in the mold powder and achieves uniform slow cooling within the mold. In order to stably generate a crystalline phase from a high temperature, a basicity of CaO / SiO ２ It is necessary to use a molding powder with a high basicity of 1.2 or more.

[0082] Low-basicity mold powders are mainly composed of glass, and can be used to achieve strong cooling of the mold. On the other hand, high-basicity mold powders are highly effective in preventing vertical cracks, but their mold cooling capacity is reduced. For this reason, it is difficult to achieve both high-speed casting and prevention of vertical cracks with high-basicity mold powders. Furthermore, high-basicity mold powders are expensive, and from the viewpoint of production costs, the basicity CaO / SiO ２ It is preferable that the mold powder can be substituted with a mold powder having a refractive index of less than 1.2.

[0083] In the continuous casting method for steel according to this embodiment, the mold plate 22 is provided with a dissimilar substance filled section 20, and a regular temperature change is applied to the solidified shell, thereby suppressing an increase in local heat flux to a specific location, thereby suppressing the occurrence of longitudinal cracks and depressions in the slab. ２ It is preferable to use a molding powder having a ρ of less than 1.2. By using such a molding powder, it is possible to achieve strong cooling of the mold, which makes it possible to accommodate higher casting speeds and also to suppress longitudinal cracks in the slab.

[0084] The continuous steel casting method according to this embodiment is preferably applied to the continuous casting of thin slabs with a casting speed of 3.0 m / min to 8.0 m / min, with a thickness of 150 mm or less. In the continuous casting of thin slabs, the cast slab is heated in a tunnel furnace and directly connected to the rolling process, which does not allow for refinement processes such as slab conditioning. Therefore, the presence of slab defects such as longitudinal cracks leads to deterioration of coil quality and reduced yield. Therefore, continuous thin slab casting at high speeds has not been actively implemented in the production of hypoperitectic medium-carbon steels and special steels with high alloy content, which are difficult to cast.

[0085] In contrast, by applying the continuous steel casting method according to this embodiment, hypoperitectic medium-carbon steel can be produced by continuous thin-slab casting, significantly improving productivity. Furthermore, in continuous thin-slab casting, the mold thickness becomes so thin that the outer diameter of the submerged entry nozzle (SEN) supplying molten steel cannot fit inside the mold. To prevent this, a mold with a special funnel shape at the center of the long side width is used. When casting medium-carbon steel, not only does uneven solidification occur at the meniscus M, but uneven cooling also easily occurs at the boundary of this funnel shape, resulting in vertical cracks in the cast slab. Because this location is below the meniscus M, vertical cracks cannot be completely suppressed by improving the mold powder alone. In response, it is preferable to form a dissimilar-substance filling portion 20 within a 50-mm range between the funnel-shaped boundary and the 0-200 mm range below the meniscus M. This suppresses uneven cooling at the boundary of the funnel shape, thereby preventing vertical cracks and breakouts in the thin slab.

[0086] Example 1 Next, an example will be described in which a mold according to this embodiment was evaluated using an actual continuous casting operation. In Example 1, the following three types of molds were used for evaluation. Comparative Example 1: A general continuous casting mold in which a foreign substance-filled portion is not formed on the mold inner wall surface and the cross-sectional area of ​​the cooling water channel is constant in the casting direction. Comparative Example 2: A continuous casting mold in which a foreign substance-filled portion is formed on the mold inner wall surface, but the cross-sectional area of ​​the cooling water channel is constant in the casting direction. Inventive Example 1: A continuous casting mold in which a foreign substance-filled portion is formed on the mold inner wall surface. A mold in which a spacer is installed on the backup plate side so that the total cross-sectional area A1 of the cooling water channel corresponding to the location where the foreign substance-filled portion is embedded satisfies the relationship A1 = 0.7 × A2 relative to the cross-sectional area A2 below it.

[0087] Each of the above molds had a rectangular inner space with a long side length of 2.1 m and a short side length of 0.22 m, and the mold plates constituting the long and short sides of the mold were made of a copper alloy with a thermal conductivity of approximately 380 (W / (m×K)) at room temperature.

[0088] The target steel types for continuous casting were all steel types normally cast, from ultra-low carbon steel to medium carbon steel. The chemical compositions were: C; 0.0008-0.25% by mass, Si; 0.002-1.2% by mass, Mn; 0.10-2.0% by mass, P; 0.005-0.030% by mass, S; 0.001-0.02% by mass, Al; 0.001-0.06% by mass, with the balance being Fe and unavoidable impurities. The mass of molten steel per charge was 300 tons. While pouring molten steel into a prepared mold, the mold was cooled while vibrating it in the slab withdrawal direction to form a solidified shell, which was then withdrawn to cast a slab. The slab withdrawal speed Vc was 0.3-2.6 (m / min).

[0089] In Example 1, mold powder was poured onto the molten steel in the vibrating mold to prevent the molten steel from sticking to the mold. The mold powder had a basicity ((mass % CaO) / (mass % SiO ２ For medium carbon steel, mold powders with a basicity of 1.5 to 1.8 were used.

[0090] In Example 1, the goal was to perform 3,000 charges of continuous casting without changing the mold, and surface cracks on the long side of the mold were checked after every 100 charges of casting. The surface of the long side of the mold was visually inspected for peeling or cracks in the plating or copper plate, and if any abnormalities were found, the continuous casting operation was stopped at that point. Surface cracks were checked for slabs after every continuous casting. Surface cracks were checked for slabs by visually inspecting the surface of the slabs that had been subjected to a penetrant test (color check) after casting of medium carbon steel, which is highly sensitive to cracking, and vertical cracks along the slab drawing direction were confirmed.

[0091] In the molds of Comparative Example 2 and Inventive Example 1, multiple circular recesses were formed in the mold plate constituting the long sides of the mold, and the interiors of these recesses were filled with a nickel alloy (thermal conductivity at room temperature: 80 (W / (m×K))) as a dissimilar material using plating means to form dissimilar material-filled portions. For all molds, a nickel alloy plating layer was provided on the surface of the mold inner wall as shown in Figures 4 and 6.

[0092] In Comparative Example 2 and Inventive Example 1, the filling depth t of the foreign substance was set to 1 mm, and the foreign substance filling section 20 was provided so as to satisfy the formulas (1) and (2).

[0093] In Comparative Example 1, Comparative Example 2, and Inventive Example 1, the cooling water was supplied to the mold at a rate set so that the linear flow velocity in the cooling water channel was 7.0 m / sec. In Inventive Example 1, spacers 28 were installed to increase the linear flow velocity in the cooling water channel to 10.0 m / sec.

[0094] The operational results of Comparative Example 1, Comparative Example 2, and Inventive Example 1 were as follows. Comparative Example 1: At the end of casting 2,600 charges, cracks occurred in the plating layer of the mold plating plate, and casting was halted. The incidence of longitudinal cracks in the medium-carbon steel slab was 8.0%. Comparative Example 2: At the end of casting 2,000 charges, peeling and cracks occurred in the plating layer of the mold plating plate, and casting was halted. The incidence of longitudinal cracks in the medium-carbon steel slab was 0.5%. Inventive Example 1: At the end of casting 3,000 charges, no peeling or cracks occurred in the plating layer of the mold plating plate. The incidence of longitudinal cracks in the medium-carbon steel slab was 0.2%. Thus, in Inventive Example 1, no surface cracks occurred in the mold plates that make up the long sides of the mold, even at the end of casting 3,000 charges, and a reduction in longitudinal cracks in the medium-carbon steel slab was confirmed.

[0095] In Example 1, the casting speed was further increased to 3.0 m / min, and casting was continued until 3,500 charges were reached. As a result, even when casting was carried out at the maximum casting speed of 3.0 m / min, which increases the thermal load, no surface cracks occurred on the mold plates that form the long sides of the mold, and no tendency for longitudinal cracks to increase in the medium carbon steel slabs was observed.

[0096] Thus, in Example 1, continuous casting of 3,000 charges was possible without replacing the mold, confirming that the mold's service life was improved compared to Comparative Examples 1 and 2. Furthermore, even when operated at a casting speed of 2.5 m / min or more, there was no damage to the mold surface. This result is thought to be due to the fact that the mold was cooled more efficiently by reducing the total cross-sectional area of ​​the cooling water channels in the region where the dissimilar substance-filled portion 20 was formed and increasing the linear flow velocity.

[0097] The slabs cast in Comparative Example 2 and Inventive Example 1 were examined for surface cracks, but no surface cracks requiring maintenance were found in either case. It is believed that the molds of Comparative Example 2 and Inventive Example 1 could effectively suppress the occurrence of surface cracks caused by the uneven solidified shell thickness resulting from the transformation of δ iron to γ ​​iron, which occurs in medium carbon steel casting, due to the presence of foreign substance-filled portions, and thus could suppress the occurrence of surface cracks in the slab. Thus, the absence of surface cracks requiring maintenance demonstrated that direct rolling is possible by using these molds.

[0098] Example 2 Next, Example 2 will be described (Invention Examples 2 to 35, Comparative Examples 3 to 29), in which continuous steel casting was performed in the same manner as in Example 1. In Example 2, in order to change the foreign substance filling section, cooling water passages, and casting conditions, the foreign substance filling section and the like were provided in the narrow side mold, which allows the conditions to be easily changed by inserting and removing the mold for each test, and continuous steel casting was performed by changing the conditions of the narrow side mold.

[0099] In Example 2, the number of casting charges in one example and one comparative example was 5. Furthermore, because the occurrence of vertical cracks on the surface of a cast slab is likely to be a problem, the target steel type for continuous casting was limited to medium carbon steel with chemical compositions of 0.08 to 0.17 mass% C, 0.10 to 0.30 mass% Si, 0.50 to 1.20 mass% Mn, 0.010 to 0.030 mass% P, 0.005 to 0.015 mass% S, 0.020 to 0.040 mass% Al, with the balance being Fe and unavoidable impurities. In each of Inventive Examples 2 to 35 and Comparative Examples 3 to 29, the width d (mm) of the disparate-substance-filled portion 20 in the mold width direction, the spacing distance P (mm) of the disparate-substance-filled portion 20 in the mold width direction, the width e (mm) of the disparate-substance-filled portion 20 in the slab withdrawal direction A, the total upper cross-sectional area A1 of the cooling water channels, the total lower cross-sectional area A2 of the cooling water channels, etc. were changed. Furthermore, in each of Inventive Examples 2 to 35 and Comparative Examples 3 to 29, continuous casting was carried out while changing the vibration frequency (1 / min), the slab withdrawal speed Vc (m / min), and the mold powder basicity.

[0100] In each run, a continuous casting run of five charges was carried out once. Thermocouples were embedded in the mold used in a region extending from the meniscus M to 50 mm below in the casting direction, and the temperature was measured using the thermocouples. The temperature was measured at one-second intervals, and the temperature data was recorded. The distance from the thermocouple temperature measurement point to the molten steel-side surface of the mold plate 22 was 15 mm. Based on a heat transfer model, the surface temperature of the mold plate 22 was calculated from the temperature of the thermocouples.

[0101] The width d, spacing distance P, total cross-sectional areas A1 and A2 of the cooling water channels, and the calculated surface temperatures of the mold copper plate for Examples 2 to 35 are shown in Table 1. Comparative Examples 3 to 29 are general continuous casting molds that do not have portions filled with foreign substances, or molds that have portions filled with foreign substances but the cross-sectional area of ​​the cooling slit water channel corresponding to those portions is the same as that below it.

[0102]

[0103] In Table 1, a "〇" in the "Equation (1)," "Equation (2)," and "Equation (3)" columns indicates that the respective equation is satisfied, and an "×" in the corresponding column indicates that the respective equation is not satisfied. The "Maximum Copper Plate Surface Temperature" column in Table 1 lists the value calculated by calculating the average mold plate surface temperature based on a heat transfer model from the thermocouple temperatures and then averaging this average temperature over the number of data samples within the steady-state operation time of five charges of continuous casting. The lower this "maximum copper plate surface temperature," the more cooled the mold plate surface at the meniscus M. The faster the casting speed, the higher the maximum copper plate surface temperature. If this maximum copper plate surface temperature is 350°C or less, it can be said that the mold plate is being cooled stably. It has been confirmed that if the maximum copper plate surface temperature is 350°C or less, plating peeling or cracking does not occur on the copper plate surface.

[0104] In Example 2, surface cracks were also investigated for each slab after every continuous casting run. Ten slabs can be produced in a single continuous casting run. Five charges were continuously cast in each of the inventive and comparative examples, resulting in 50 slabs. Penetrant testing was performed on all of these slabs, and the slab surfaces that had been penetrant tested were visually inspected to confirm any surface cracks. Because the detailed specifications of the mold were changed only on the short sides, the data evaluated in Example 2 was limited to surface cracks at the short sides. When longitudinal cracks were observed on the surface of the slab, they were counted, and the percentage of the total number of slabs with longitudinal cracks relative to the total number of slabs (= 50) was entered in the "Longitudinal Crack Occurrence Rate" column. Since this crack occurrence rate includes even very fine cracks visually observed, even if the longitudinal crack occurrence rate is not zero, a crack occurrence rate of 15% or less is not substantially problematic.

[0105] In Examples 2 to 35, in which continuous casting was performed using a mold provided with the dissimilar material filling section 20, the incidence of longitudinal cracks in all of the strands obtained in a single continuous casting run was 15% or less. This result confirmed that the use of the mold according to this embodiment can prevent longitudinal cracks on the surface of the strands.

[0106] Furthermore, in Examples 2 to 35, the formula (3) was satisfied, and the copper plate surface temperature at the meniscus was 350°C or less. This suppressed the occurrence of cracks and abnormalities on the copper plate surface, and no cracks or abnormalities occurred on the copper plate surface even under high-speed casting conditions of a casting speed of 2.5 m / min or more.

[0107] When using the molds (Comparative Examples 22 to 29) of the type shown in Comparative Example 1 of Example 1, the occurrence of vertical cracks was significant. Furthermore, when the basicity of the mold powder was 1.55, the effect of suppressing vertical cracks was confirmed, but when the basicity of the mold powder was 1.10, it was confirmed that the rate of vertical cracks occurred exceeded 40%.

[0108] When using a mold of the type shown in Comparative Example 2 of Example 1 (Comparative Examples 3 to 21), which did not satisfy formula (3) and in which the maximum copper plate surface temperature exceeded 300°C, plating peeling and cracks occurred on the copper plate surface after 100 charges or more. When using a mold which did not satisfy formula (3) and in which the maximum copper plate surface temperature exceeded 350°C, plating peeling and cracks occurred on the copper plate surface after 5 charges. Furthermore, although not shown in the table, when the linear flow velocity of the cooling slit was increased from 7.0 m / sec to 12.0 m / sec, the maximum mold surface temperature could be reduced to approximately 300°C. However, depression-shaped vertical cracks were observed near the short side corners at a rate of 50%, and it was determined that stable operation would be difficult to achieve.

[0109] From the above results, it was confirmed that continuous casting of steel using the mold according to this embodiment can suppress the occurrence of surface cracks in a slab of medium carbon steel and can effectively lower the temperature of the mold plate near the meniscus where the foreign-substance-filled portion is formed. Furthermore, since the occurrence of cracks and abnormalities on the surface of the copper plate can be suppressed by effectively lowering the temperature of the mold plate, it was confirmed that the mold according to this embodiment can achieve both a longer mold life with the foreign-substance-filled portion formed and suppression of surface cracks in the slab.

[0110] Example 3 To confirm the effect of the mold according to this embodiment in high-speed casting, continuous casting of steel was performed using a thin slab caster in Example 3. In Example 3, continuous casting of a fixed-size slab with a slab width of 1,250 mm and a slab thickness at the mold bottom end of 75 mm was performed using a mold having a funnel-shaped curved surface 42 in the center in the width direction of the mold copper plate (Invention Example 36 and Comparative Example 30).

[0111] The target steel type for continuous casting was medium carbon steel with the following chemical components: 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%, with the balance being Fe and unavoidable impurities. The casting speed was 4.0-5.0 m / min, the mold vibration frequency f = 400-500 (1 / min), and the mold vibration stroke was 6 mm. The basicity of the mold powder was CaO / SiO ２ = 1.25.

[0112] FIG. 13 is a schematic diagram showing the mold used in Example 36. FIG. 14 is a schematic diagram showing the mold used in Comparative Example 30. As shown in FIG. 13 , Example 36 used a mold in which a dissimilar substance-filled portion 44 containing a 5 mm diameter x 1.5 mm depth nickel alloy was embedded in the meniscus M and in the boundary portion of the funnel-shaped curved surface 42 within a range of 0 to 200 mm below the meniscus M. Furthermore, for the cooling slit, a spacer 28 was installed on the backup plate side so that the total cross-sectional area of ​​the cooling water channel within a range of 0 to 200 mm below the meniscus from the upper end was 0.75 times the total cross-sectional area below that to the lower end. On the other hand, Comparative Example 30 used a mold in which a dissimilar substance-filled portion 44 was not embedded and a spacer 28 was not installed.

[0113] A casting test of five charges each was carried out for Inventive Example 36 and Comparative Example 30. In a thin slab continuous caster, the slabs after casting are generally heated continuously in a tunnel furnace and then rolled, but in this test, the thin slabs after casting were turned into cold pieces to check the condition of surface cracks.

[0114] As a result of casting, in Example 36, continuous casting was performed for all five charges at a casting speed of 4.0 m / min or more and 5.0 m / min or less. No longitudinal cracks were observed on the surface of the cast slab, confirming that the quality was satisfactory even when subjected to continuous rolling. In contrast, in Comparative Example 30, breakout occurred in one of the five charges, and complete casting was not achieved. After casting, longitudinal cracks occurred in 66% of the slabs. Most of the longitudinal cracks occurred at positions corresponding to the boundaries of the funnel shape.

[0115] In Example 36, the copper plate surface temperature estimated from the copper plate thermocouple temperature near the meniscus during casting was stable at around 320°C even at a casting speed of 5.0 m / min, and no abnormalities were observed on the copper plate surface after casting. In contrast, in Comparative Example 30, the copper plate surface temperature estimated from the copper plate thermocouple temperature near the meniscus M during casting tended to exceed 350°C from a casting speed of around 4.0 m / min, so further increases in the casting speed were abandoned.

[0116] These results confirmed that the use of the mold of Example 36 is extremely effective for thin slab continuous casting, which requires high-speed casting of 2.5 m / min or more. In mold cooling for thin slab continuous casters, in addition to the vertical slit cooling method described above, the canal cooling method, in which round holes are drilled in the casting direction and water is passed through them for cooling, is also widely used. Even with the canal cooling method, similar effects can be obtained by changing the upper and lower hole diameters, so either cooling method is acceptable.

[0117] In this way, by performing continuous casting of steel using the mold according to this embodiment, it is possible to suppress the occurrence of vertical crack defects originating within the mold during continuous casting, and to suppress plating peeling and copper plate deformation even under high-speed casting conditions of 2.5 m / min or more, thereby realizing a longer mold life. Since it is also possible to use mold powder with high cooling capacity, it is possible to accommodate even higher-speed casting conditions, thereby further improving the productivity of cast pieces.

[0118] Since the treatment process associated with vertical crack defects can be omitted in the finishing process, a direct rolling process becomes possible, slab heating costs in the heating furnace can be reduced, and energy savings can also be achieved.

[0119] REFERENCE SIGNS LIST 10 Continuous casting mold 12 Mold long side 14 Mold short side 16 Molten steel 18 Submerged entry nozzle 20 Portion filled with different material 22 Mold plate 26 Cooling water channel 28 Spacer 30 Backup plate 32 Plating layer 40 Conventional mold plate 42 Curved surface 44 Portion filled with different material

Claims

1. A mold plate made of a copper alloy, the surface of which forms the inner wall surface of the mold and the back surface of which is formed with a cooling water channel; A backup plate attached to the mold plate so as to cover the cooling water passage, A plurality of dissimilar material filled portions are formed in recesses formed in a region including at least a meniscus on the surface of the mold plate, the dissimilar material having a thermal conductivity different from that of the mold plate being filled therein; A mold for continuous casting of steel, wherein a total cross-sectional area of ​​the cooling water channels on the back surface of the mold plate in a range including the region in which the multiple dissimilar substance filled portions are formed is smaller than a total cross-sectional area of ​​the cooling water channels below the range.

2. 2. The mold for continuous casting of steel according to claim 1, wherein the plurality of dissimilar substance filled portions are formed so that a heat flux from the mold inner wall surface toward the cooling water channel changes periodically in the region on the surface of the mold plate.

3. 3. The mold for continuous casting of steel according to claim 1, wherein the disparate substance filling portion and the cooling water passage are formed so as to satisfy at least one of the following formulas (1) to (3): d<P≦S...(1) e≦L≦1000×Vc / f...(2) A1≦0.8×A2 (3) In the above formulas (1) to (3), d is the width (mm) of the dissimilar substance filled portion in the mold width direction, P is the spacing (mm) between adjacent dissimilar substance filled portions in the mold width direction, S is the spacing (mm) between adjacent cooling water channels formed in the back surface of the mold plate in the mold width direction, e is the width (mm) of the dissimilar substance filled portion in the slab withdrawal direction, L is the spacing (mm) between adjacent dissimilar substance filled portions in the slab withdrawal direction, Vc is the slab withdrawal speed (m / min) in the continuous casting process of steel, f is the vibration frequency (1 / min) of the continuous casting mold in the continuous casting process of steel, and A1 is the total cross-sectional area (mm) of the cooling water channels in the range including the above region. ２ ), and A2 is the total cross-sectional area (mm ２ ).

4. 3. The mold for continuous casting of steel according to claim 1, wherein a plating layer is formed on the surface of said mold plate so as to cover said portion filled with said different kind of substance.

5. The mold plates include a pair of long side mold plates and a pair of short side mold plates; 3. The mold for continuous casting of steel according to claim 1, further comprising a booster pump for supplying cooling water to at least the mold plate on the short side, in addition to a common pump for supplying cooling water to the mold plate on the long side and the mold plate on the short side.

6. The mold plate has a funnel shape, 3. The mold for continuous casting of steel according to claim 1, wherein the dissimilar substance filled portion is also formed within a range of 50 mm of the boundary portion of the funnel shape within a range of 0 to 200 mm below the meniscus.

7. A method for continuous casting of steel using the mold for continuous casting of steel according to claim 1 or 2, comprising the steps of:

13. A method for continuous casting of steel, wherein the maximum temperature of the surface of the mold plate at the meniscus is 350° C. or less when continuous casting is performed at a casting speed of 2.5 m / min or more.

8. A method for continuous casting of steel using the mold for continuous casting of steel according to claim 1 or 2, comprising the steps of: A method for continuous casting of steel, comprising the steps of: estimating a surface temperature of the mold plate from a temperature measured by a thermocouple embedded in the mold plate within a range of 20 to 100 mm below the meniscus; and adjusting the amount of cooling water supplied and the total cross-sectional area of ​​the cooling water channels on the back surface of the mold plate in the region where the disparate substance-filled portion is formed so that the estimated surface temperature of the mold plate is 350°C or lower.

9. A method for continuous casting of steel using the mold for continuous casting of steel according to claim 1 or 2, comprising the steps of: A process for continuous casting of steel using a mold powder having a basicity of less than 1.

2.

10. A method for continuous casting of steel using the mold for continuous casting of steel according to claim 1 or 2, comprising the steps of: A method for continuously casting steel, wherein the direction of flow of cooling water is from an upper end of the mold plate to a lower end of the mold plate.

11. A method for continuous casting of steel using the mold for continuous casting of steel according to claim 1 or 2, comprising the steps of: A method for continuous casting of steel, wherein the thickness of the cast slab is 150 mm or less.