Casting mold for continuous casting of steel and continuous casting method of steel

Abstract

Provided are a continuous casting mold and a continuous casting method with which it is possible to perform continuous casting while suppressing the occurrence of abnormalities on the surface of a casting plate even under high-speed casting conditions of 2.5 m / min or more. A mold for continuous casting of steel is provided with: a mold plate made of a copper alloy, the surface of which forms a mold inner wall surface and the rear surface of which forms a cooling water path; and a support plate attached to the casting plate so as to cover the cooling water path, in which a plurality of dissimilar substance filling parts filled with a dissimilar substance having a thermal conductivity different from the thermal conductivity of the casting plate are formed in a recess formed in a region including at least a meniscus on the surface of the casting plate, and the support plate is attached to the casting plate so as to cover the cooling water path. The total cross-sectional area of the cooling water path on the back surface of the casting plate in a range including a region in which the plurality of dissimilar substance filling parts are formed is smaller than the total cross-sectional area of the cooling water path below the range.

Description

Technical Field

[0001] This invention relates to a mold for continuous casting of steel and a method for continuous casting of steel that can stably achieve continuous casting even at high speeds of 2.5 m / min or higher. Background Technology

[0002] In continuous steel casting, the demands for high productivity and high quality have become more stringent, requiring high-speed casting technology that eliminates surface defects within the mold and secondary cooling zone. In particular, if surface cracks occur on the surface of the casting during continuous casting, cold lamination is necessary to remove these defects and prevent them from flowing into subsequent processes. Therefore, direct feeding of the castings is hindered, becoming a significant problem in terms of productivity.

[0003] Representative surface cracks include longitudinal surface cracks, transverse surface cracks, and key-shaped surface cracks. Transverse and key-shaped surface cracks are known to occur in the secondary cooling zone of continuous casting due to strain applied in the embrittlement temperature region. Prevention methods for transverse and key-shaped surface cracks typically include designing secondary cooling specifications that avoid the embrittlement temperature range at the bending correction point and adding embrittlement-inhibiting elements.

[0004] On the other hand, it is known that surface longitudinal cracks are prone to occur in medium carbon steel in the subperitectic region where uneven solidification is likely to form, and that crack initiation points are easily generated within the mold. Therefore, in order to mitigate stress concentration towards the solidified shell during the initial solidification, a slow-cooling protective slag that can suppress uneven solidification is used.

[0005] If a slow-cooling protective slag is used, the mold deheating is weakened. Therefore, during high-speed casting, the solidified shell thickness at the lower end of the mold is insufficient, easily leading to steel leakage. Consequently, steel grades requiring slow-cooling powder have a relatively lower casting speed compared to other steel grades. The increased cost of slow-cooling powder is also a problem.

[0006] To address this, a technique has been proposed and adopted to prevent surface longitudinal cracks by filling the casting mold with a material of low thermal conductivity. Patent Document 1 discloses a continuous casting mold in which a foreign material filling portion is formed on the inner wall surface of the mold. The foreign material filling portion is filled with a foreign material having a thermal conductivity different from that of the copper alloy constituting the mold. According to Patent Document 1, by setting the thermal resistance R between the mold surface where the foreign material filling portion is formed and the cooling water channel provided on the back of the mold within a specified range, surface cracks in the casting can be reduced, and the reduction in mold life caused by surface cracks can be suppressed.

[0007] Patent Document 2 also discloses a continuous casting mold in which a foreign material filling portion is formed on the casting template. This foreign material filling portion is filled with a foreign material whose thermal conductivity differs from that of the copper alloy constituting the casting template. In this continuous casting mold, by forming a water flow turbulence section in the cooling water channel on the back side of the casting template in the region where the foreign material filling portion is formed, surface cracks in the casting can be reduced, and the mold life can also be extended.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2017-39165

[0011] Patent Document 2: International Publication No. 2020 / 095932 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] The inventors conducted casting tests using the infill mold disclosed in Patent Documents 1 and 2, which fills the molten steel side surface of the casting template with Ni. They confirmed that this infill mold significantly reduces surface longitudinal cracks in medium carbon steel. However, it was found that when this infill mold is used in a high-speed continuous casting machine, if the casting speed is 2.5 m / min or higher, the surface temperature of the casting template consistently exceeds 300°C. If the surface temperature of the casting template consistently exceeds 300°C, abnormalities such as gold plating peeling and deformation occur, making the mold unusable.

[0014] This invention was made in view of the following problem, and its object is to provide a continuous casting mold for steel that can perform continuous casting while suppressing surface cracks in the casting sheet and abnormalities on the surface of the casting mold, even under high-speed casting conditions of 2.5 m / min or higher. Another object of this invention is to provide a continuous casting method for steel using the continuous casting mold for steel.

[0015] Methods for solving problems

[0016] The methods used to solve the above problems are as follows.

[0017] [1] A continuous casting mold for steel, comprising: a casting template made of copper alloy, the surface of which forms an inner wall surface of the mold and a cooling water channel is formed on the back side; and a support plate installed on the casting template to cover the cooling water channel, wherein a plurality of dissimilar material filling portions are formed in recesses, the recesses being formed in a region on the surface of the casting template that includes at least a meniscus, the dissimilar material filling portions being filled with dissimilar materials having a thermal conductivity different from that of the casting template, and the total cross-sectional area of ​​the cooling water channel on the back side of the casting template within the region containing the plurality of dissimilar material filling portions being smaller than the total cross-sectional area of ​​the cooling water channel below the range.

[0018] [2] According to the continuous casting mold for steel as described in [1], the plurality of foreign material filling portions are formed in the region on the surface of the casting mold in such a way that the heat flux from the inner wall of the mold toward the cooling water channel changes periodically.

[0019] [3] A continuous casting mold for steel according to [1] or [2], wherein the foreign material filling part and the cooling water channel are formed in a manner that satisfies at least one of the following formulas (1) to (3):

[0020] d <P≤S···(1)

[0021] e≤L≤1000×Vc / f···(2)

[0022] A1≤0.8×A2···(3)

[0023] In equations (1) to (3) above, d is the width (mm) of the dissimilar material filling portion in the mold width direction, P is the distance (mm) between adjacent dissimilar material filling portions in the mold width direction, S is the distance (mm) between adjacent cooling water channels in the multiple cooling water channels formed on the back of the mold template, e is the width (mm) of the dissimilar material filling portion in the casting sheet drawing direction, L is the distance (mm) between adjacent dissimilar material filling portions in the casting sheet drawing direction, Vc is the casting sheet drawing 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 (mm²) of the cooling water channels covering the area. 2 A2 is the total cross-sectional area (mm²) of the cooling water passages located below the aforementioned range. 2 ).

[0024] [4] A continuous casting mold for steel according to any one of [1] to [3], wherein the plating layer is formed on the surface of the casting mold in such a way as to cover the foreign material filling portion.

[0025] [5] A continuous casting mold for steel according to any one of [1] to [4], wherein the casting mold is a pair of long-side casting molds and a pair of short-side casting molds, and in addition to a common pump for supplying cooling water to the long-side casting molds and the short-side casting molds, it also has a booster pump for supplying cooling water to at least the short-side casting molds.

[0026] [6] A continuous casting mold for steel according to any one of [1] to [5], wherein the casting mold has a funnel shape, and the foreign material filling portion is also formed in a range of 50 mm of the boundary portion of the funnel shape in a range of 0 to 200 mm below the meniscus.

[0027] [7] A continuous casting method for steel, using a continuous casting mold for steel as described in any one of [1] to [6], wherein, when continuous casting is performed at a casting speed of 2.5 m / min or higher, the maximum temperature of the surface of the casting mold at the meniscus is 350°C or less.

[0028] [8] A continuous casting method for steel, using a continuous casting mold for steel as described in any one of [1] to [6], wherein the surface temperature of the casting mold is estimated based on the temperature measured by a thermocouple embedded in the casting mold within a range of 20 to 100 mm below the meniscus, and the supply of cooling water and the total cross-sectional area of ​​the cooling water passage on the back side of the casting mold in the area where the foreign material filling portion is formed are adjusted so that the estimated surface temperature of the casting mold is below 350°C.

[0029] [9] A continuous casting method for steel, using a mold for continuous casting of steel as described in any one of [1] to [6], wherein a protective slag with a basicity of less than 1.2 is used.

[0030]

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

[0031]

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

[0032] Invention Effects

[0033] In the continuous casting mold for steel according to the present invention, the total cross-sectional area of ​​the cooling water channel including the region where the foreign material filling portion is formed is smaller than the total cross-sectional area of ​​the cooling water channel below this region. In the cooling water channel with a reduced total cross-sectional area, the heat transfer coefficient between the water flow and the cooling water channel increases, and convective heat transfer increases, thus enabling effective heat dissipation and cooling from the casting mold in the region where the foreign material filling portion is formed. By effectively cooling the foreign material filling portion and the casting mold, thermal stress generated at the boundary between the casting mold and the foreign material filling portion can be suppressed. As a result, surface cracks in the casting sheets in steel grades accompanied by peritectic reactions can be suppressed, and continuous casting can be performed while suppressing gold plating peeling and copper plate deformation, even under high-speed casting conditions of 2.5 m / min or higher. Furthermore, by suppressing gold plating peeling and copper plate deformation, the lifespan of the mold with the foreign material filling portion can be extended. Attached Figure Description

[0034] Figure 1 This is a perspective view showing an example of a mold for continuous casting of steel according to this embodiment.

[0035] Figure 2 This is a schematic diagram showing an example of the surface of the casting template that constitutes the long side of the casting mold in this embodiment.

[0036] Figure 3 It means Figure 2 A diagram showing the construction of the casting template 22, which is a region C surrounded by quadrilaterals.

[0037] Figure 4 These are the casting direction sectional view and front view of the traditional casting template.

[0038] Figure 5 yes Figure 4 UU sectional view and VV sectional view.

[0039] Figure 6 These are the casting direction sectional view and front view of the casting template of the mold in this embodiment.

[0040] Figure 7 yes Figure 6 UU sectional view and VV sectional view.

[0041] Figure 8 It is a graph showing the relationship between the distance from the top of the mold and the surface temperature of the mold plate.

[0042] Figure 9 These are the casting direction sectional view and front view of the casting template.

[0043] Figure 10 This is a schematic diagram illustrating an example of a pump that supplies cooling water to a casting mold.

[0044] Figure 11 It is a graph showing the relationship between the total water volume of the pump and the pump source pressure.

[0045] Figure 12 This is a schematic diagram illustrating an example of a pump that supplies cooling water to a casting mold.

[0046] Figure 13 This is a schematic diagram showing the mold used in Invention Example 36.

[0047] Figure 14 This is a schematic diagram showing the mold used in Comparative Example 30. Detailed Implementation

[0048] The present invention will now be described through embodiments thereof. Figure 1 This 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") has a pair of opposing long sides 12 and a pair of opposing short sides 14 held by the long sides 12. An tundish (not shown) for containing molten steel 16 is disposed above the mold 10, and an immersion nozzle 18 is provided at the bottom of the tundish. A rectangular internal space is formed in the mold 10 through the pair of long sides 12 and the pair of short sides 14, and the immersion nozzle 18 is inserted into this internal space. The side of the long sides 12 and short sides 14 that contacts the molten steel 16 is made of a copper alloy casting template, and a support plate is disposed on the back of this template.

[0049] Cooling water channels are formed on the back side of the copper alloy casting template that forms the long side 12 and short side 14 of the mold, on the side in contact with 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 internal space of the mold 10 through the immersion 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. A casting sheet with this solidified shell as its outer shell and the unsolidified molten steel 16 inside is continuously drawn along a casting sheet drawing direction A that is vertically downward, thus casting a steel casting sheet. In the mold 10, through contact with molten steel 16 and the high-temperature casting sheet, the surface temperature of the casting template (the temperature on the side in contact with the molten steel) rises, reaching its highest temperature near the meniscus M (the surface of the molten steel inside the mold). Figure 1 In the diagram, the position of the meniscus M is indicated by a single-dotted line.

[0050] While this also depends on the steel grade, it is preferable to ensure uniform heat dissipation from the solidified shell along both the casting drawing direction A and the mold width direction B at the meniscus M within the mold. This uniform heat dissipation from the solidified shell along both the casting drawing direction A and the mold width direction B promotes uniform growth of the solidified shell thickness. Here, the casting drawing direction A and the mold width direction B are orthogonal. As the casting template, a copper alloy with high thermal conductivity that exhibits high resistance to thermal stress deformation and improves the cooling effect of the cooling water is preferred.

[0051] Multiple casting support rollers (not shown) are arranged below the mold 10, and water spray nozzles or air spray nozzles are arranged between adjacent casting support rollers. Cooling water is blown onto the surface of the casting from the water spray nozzles or air spray nozzles to cool the casting. The casting is drawn while being supported by the casting support rollers. After solidification to the center of the casting, the casting is cut into a specified length. In this way, a casting of a specified length is manufactured for the next hot rolling process.

[0052] In the mold 10 of this embodiment, the total cross-sectional area of ​​the cooling water channels cooling the mold plate where the foreign material filling portion is located is smaller than the total cross-sectional area of ​​the cooling water channels below it. As a result, the linear velocity of the cooling water in the region where the foreign material filling portion is located increases, the heat transfer coefficient in that region increases, and convective heat transfer increases, enabling effective heat dissipation from the foreign material filling portion and the mold plate. Consequently, the foreign material filling portion and the surrounding mold plate can be effectively cooled, suppressing thermal stress generated at the boundary between the mold plate and the foreign material filling portion, extending the lifespan of the mold 10, and achieving higher casting speeds.

[0053] Figure 2 This is a schematic diagram showing an example of the surface of the casting template 22 constituting the long side 12 of the mold in this embodiment. The long side 12 and short side 14 of the mold constituting the mold 10 each have: a casting template 22, the surface of which forms the inner wall surface of the mold and a cooling water channel is formed on the back side; and a support plate, which is installed on the casting template 22 by bolts and nuts.

[0054] On the surface of the casting template 22, recesses in the region containing the meniscus M are filled with a dissimilar material whose thermal conductivity differs from that of the casting template 22, forming multiple independent dissimilar material filling portions 20. Multiple dissimilar material filling portions 20 are formed in the casting sheet drawing direction A and the mold width direction B near the meniscus M, at least including the meniscus M. The dissimilar material filling portions 20 can be formed by machining the dissimilar material into a shape that fits into the recess and embedding it into the recess, or by filling the recess with the dissimilar material using methods such as gold plating or spray plating. By filling the recess with the dissimilar material using methods such as gold plating or spray plating, gaps between the recess and the dissimilar material can be prevented.

[0055] For the multiple foreign material filling parts 20, it is preferable to arrange the multiple foreign material filling parts 20 regularly on the surface of the casting template 22 in such a way that the heat flux from the inner wall surface of the mold to the inner wall surface of the cooling water channel increases or decreases periodically.

[0056] By arranging multiple dissimilar material filling portions 20 on the surface of the casting template 22 near the meniscus M, the thermal resistance of the casting template 22 in the casting sheet drawing direction A and the mold width direction B in the region near the meniscus M increases and decreases regularly and periodically. As a result, the heat flux from the solidified shell to the casting template 22 near the meniscus M, i.e., in the early stage of solidification, increases and decreases regularly and periodically. This regular and periodic increase and decrease in heat flux reduces the stress and thermal stress generated by the phase transformation from δ-iron to γ-iron, and decreases the deformation of the solidified shell caused by these stresses. With the decrease in solidified shell deformation, the uneven heat flux distribution caused by the deformation of the solidified shell is homogenized, and the generated stress is dispersed, resulting in smaller individual strains. Consequently, the formation of longitudinal cracks on the surface of the solidified shell is suppressed.

[0057] The recess may not be a perfectly circular recess on the surface of the casting template 22, but may be an approximate circular recess that resembles a circle. An approximate circle may be an ellipse, a square or rectangle with rounded or elliptical corners, or other shapes without corners. Furthermore, the recess may also be shaped like a petal pattern.

[0058] To ensure that the change in heat flux on the inner wall of the mold is periodic, it is preferable that the spacing between adjacent dissimilar material filling portions 20 is the same. The thermal conductivity of the dissimilar material relative to the thermal conductivity of the mold template 22 is preferably 80% or less or 125% or more. The thermal conductivity of the dissimilar material varies with the atmosphere temperature. Therefore, the thermal conductivity of the dissimilar material and the mold template is based on the room temperature (room temperature) at which the mold is manufactured. At room temperature, if the thermal conductivity of the dissimilar material differs from that of the mold template 22 by about 20%, the stress and thermal stress generated by the phase transformation from δ-iron to γ-iron can be reduced through the regular and periodic increase and decrease of heat flux on the inner wall of the mold. However, as long as the stress generated by the aforementioned phase transformation is reduced to prevent surface cracking of the casting, the thermal conductivity of the dissimilar material does not need to be within the above range, and the spacing between the dissimilar material filling portions 20 does not need to be the same.

[0059] Examples of dissimilar materials whose thermal conductivity relative to the template 22 is 80% or less include Ni (thermal conductivity: approximately 90 W / (m×K)) and Ni alloys (thermal conductivity: approximately 40~90 W / (m×K)), which are easily plated or sprayed. The template 22 can be made of copper alloys (thermal conductivity: approximately 100~385 W / (m×K)), 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 119~239 W / (m×K)). However, dissimilar materials and the template 22 can also be made of metals other than Ni alloys and copper alloys.

[0060] As the casting template 22, pure copper (thermal conductivity: approximately 398 W / (m×K)) or the aforementioned copper alloy can be used. In particular, when electromagnetic stirring of molten steel is performed within the mold, in order to prevent the magnetic field strength from the coil to the molten steel from weakening, it is preferable to use a copper alloy with lower conductivity due to the addition of components other than copper by a few percent by mass. The thermal conductivity of the copper alloy is lower than that of pure copper. That is, it is preferable to appropriately select the dissimilar material and / or the material of the long side 12 of the mold according to the application of the mold 10, and adjust the thermal conductivity of the dissimilar material and the casting template 22.

[0061] The same dissimilar material filling portion 20 as the long side 12 of the mold can be formed on the surface of the short side 14 of the mold. However, in the casting, due to its shape, stress concentration is easily generated in the solidified shell on one side of the long side 12 of the mold, and surface cracks are easily generated on the long side surface. Therefore, it is necessary to provide the dissimilar material filling portion 20 on the long side 12 of the mold 10 in this embodiment, but it is not necessary to provide the dissimilar material filling portion 20 on the short side 14 of the mold.

[0062] Considering the impact on initial solidification, the foreign material filling section 20 is preferably located in the region from a position above the meniscus M during stable casting, at a distance Q, to a position below the meniscus M, at a distance R. The distance Q is any value greater than zero. The distance R can be calculated according to the following equation (4).

[0063] R = 2 × Vc × 1000 / 60 ···(4)

[0064] In equation (4) above, R is the distance (mm) and Vc is the drawing speed of the casting sheet in the continuous casting process of steel (m / min).

[0065] The distance R is related to the time it takes for the solidified shell to pass through the area where the foreign substance filling part 20 is formed after the start of solidification. Preferably, the solidified shell remains in the area where the foreign substance filling part 20 is provided for at least 2 seconds after the start of solidification. In order to ensure that the solidified shell exists in the area where the foreign substance filling part 20 is provided for at least 2 seconds after the start of solidification, it is only necessary to set the foreign substance filling part 20 at a distance R below the meniscus M as calculated by equation (4).

[0066] If the time the cast sheet remains in the area where the foreign material filling section 20 is provided after solidification begins is ensured to be 2 seconds or more, the effect of the periodic change in heat flux from the inner wall of the mold to the cooling water channel caused by the foreign material filling section 20 can be fully obtained. That is, by ensuring that the solidified shell remains in the area of ​​the foreign material filling section 20 for 2 seconds or more, the effect of suppressing surface cracks in the cast sheet can be obtained even in high-speed casting and casting of medium carbon steel, where surface cracks are prone to occur. In order to stably obtain the effect of periodic change in heat flux caused by the foreign material filling section 20, it is more preferable to ensure that the time for the solidified shell to pass through the area where the foreign material filling section 20 is provided is 4 seconds or more. On the other hand, in the case of a thin billet continuous casting machine, the casting sheet drawing speed is fast, so the distance R becomes longer, and the area in the casting sheet drawing direction A where the foreign material filling section 20 should be provided becomes larger, which increases the processing cost of the mold. In this case, if the time for passing through the foreign material filling section 20 is ensured to be more than 1 second, the effect of periodic change in heat flux corresponding to that time can be obtained.

[0067] The upper end of the region forming the foreign material filling portion 20 only needs to be above the meniscus M, and there is no particular limitation. Therefore, the distance Q can be any value greater than zero. However, the meniscus M varies in the vertical direction during casting. Therefore, the foreign material filling portion 20 is preferably formed such that the upper end of the region of the foreign material filling portion 20 is always above the meniscus M, up to 20 mm above the meniscus M. More preferably, the foreign material filling portion 20 is formed up to 40 mm above the meniscus M. The position of the meniscus M is generally set to be 60 to 150 mm below the upper end of the casting template 22, and the region where the foreign material filling portion 20 is formed is determined accordingly.

[0068] In continuous steel casting, molten steel at high temperature is poured into the internal space of the mold, causing the temperature of the casting template 22 to rise. Therefore, cooling water channels are formed in the casting template 22, which forms the long side 12 and short side 14 of the mold, through which cooling water is channeled to cool the casting template 22. However, the thermal expansion rate of the dissimilar material filling portion 20 differs from that of the long side 12 of the mold, potentially causing cracks on the surface of the casting template 22 due to thermal stress concentrated at their boundaries.

[0069] Therefore, in the mold 10 of this embodiment, the total cross-sectional area of ​​the cooling water channels in the casting mold 22, including the region where the foreign material filling portion 20 is formed, is smaller than the total cross-sectional area of ​​the cooling water channels below that region. By reducing the cross-sectional area of ​​the water channels in the region where the foreign material filling portion 20 is formed, the linear flow velocity of the cooling water in the water channels increases, thereby increasing the heat transfer coefficient between the cooling water and the copper plate slot wall. As a result, heat dissipation of the casting mold 22 in the region where the foreign material filling portion 20 is formed is promoted, and this region can be effectively cooled.

[0070] Next, the foreign material filling section 20 and the cooling water circuit will be explained. Figure 3 It means Figure 2 A diagram illustrating the construction of the casting template 22, which is a region C enclosed by a quadrilateral. Figure 3 In the diagram, (a) is a plan view showing the surface of the casting template 22, (b) is a plan view showing the back of the casting template 22, (c) is a vertical sectional view of this part, and (d) is a horizontal sectional view of this part. Figure 3 As shown in (b) and (d), cooling water channels 26 are formed on the back side of the casting mold 22. Figure 3 As shown in (c) and (d), a support plate 30 is installed on the back of the casting mold 22 in a manner that covers the cooling water passage 26.

[0071] Figure 4 These are the casting direction sectional view (a) and front view (b) of the conventional casting template 40. Figure 5 yes Figure 4 UU sectional view (a) and VV sectional view (b). Figure 6 These are a cross-sectional view (a) and a front view (b) of the casting template 22 of the casting mold 10 in this embodiment, taken in the casting direction. Figure 7 yes Figure 6 UU sectional view (a) and VV sectional view (b).

[0072] like Figures 4-7 As shown, in the mold 10 of this embodiment, unlike the conventional mold 40, a spacer 28 is provided between the cooling water channel 26 and the support plate 30 in the region where the foreign material filling portion 20 is formed. By providing this spacer 28, the total cross-sectional area of ​​this region of the cooling water channel 26 is smaller than the total cross-sectional area below it. The spacer 28 only needs to be provided in the area including the region where the foreign material filling portion 20 is formed.

[0073] Especially in high-speed casting with a casting speed of 2.5 m / min or higher, it is preferable to further reduce the total cross-sectional area of ​​the cooling water channel 26 to increase the linear flow rate of the cooling water. This is to prevent the solidified shell from failing to withstand the static pressure of the molten steel at the bottom of the mold if the solidified shell growth inside the mold is insufficient, which could lead to a steel leakage accident in the worst case. When a low thermal conductivity material is used in the foreign material filling part 20, the heat dissipation from the mold is less than that of the conventional casting template 40, so the formation of the solidified shell may be insufficient. The thickness of the solidified shell can be calculated using the following formula (5).

[0074] z_shell=K×ts 1 / 2 =K×(z_m / Vc) 1 / 2 ···(5)

[0075] In equation (5) above, z_shell is the solidified shell thickness (mm). K is the solidification constant (20~27mm / min). 0.5 ). 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).

[0076] As can be seen from equation (5) above, the closer the location is to the meniscus M, the greater the impact on the growth of the solidified shell. Therefore, by increasing the solidification constant K of the foreign material filling part 20 that is closer to the meniscus M, the solidified shell can be made to grow thicker.

[0077] The casting mold 22 is generally made of copper alloy, but if the temperature exceeds 350°C, the copper alloy will soften due to a significant decrease in yield stress and will be easily deformed. Usually, a Ni-Co or Ni-Cr based plating layer 32 is applied to the outermost surface of the molten steel side of the casting mold 22 for use, but if the temperature exceeds 350°C, the hardness of the Ni alloy will decrease significantly.

[0078] Therefore, in high-speed casting, when a low thermal conductivity material is used in the foreign material filling section 20, insufficient heat dissipation occurs. If the surface temperature of the casting mold 22 exceeds 350°C, peeling and cracking are likely to occur on the copper plate surface and the plating surface. In contrast, in the casting mold 10 of this embodiment, a spacer 28 is provided to accelerate the linear flow rate of the cooling water in the area where the foreign material filling section 20 is formed. As a result, the heat transfer coefficient between the cooling water and the cooling water path 26 increases, and the convective heat transfer increases, enabling effective heat dissipation from the casting mold 22 in the area where the foreign material filling section 20 is formed, and effectively cooling the casting mold 22 and the foreign material filling section 20. As a result, surface cracks in the casting sheet can be suppressed, and peeling and cracking of the plating layer 32 on the surface of the casting mold 22 can be suppressed.

[0079] The linear velocity of the cooling water in the cooling water channel 26 is preferably 7.0 m / sec or higher. In the region of this linear velocity, the Reynolds number Re of the water in the channel is around 40,000, resulting in turbulent flow (Re > 2300). To increase local heat dissipation near the meniscus M, the linear velocity of the cooling water should be 10.0 m / sec or higher. By maintaining a linear velocity of cooling water of 10.0 m / sec or higher, it is theoretically possible to reduce the surface temperature of the casting mold 22 by approximately 30°C.

[0080] To increase the linear velocity of the cooling water, the supply pressure needs to be increased. However, increasing the supply pressure increases pressure loss in each water path. This leads to a greater deviation in the linear velocity of the cooling water in each of the multiple slits. Therefore, if the linear velocity is excessively increased, the cooling difference between the uncooled and cooled areas near the stud bolts will increase, potentially worsening the longitudinal crack.

[0081] If the linear velocity of the cooling water is too high, the difference in linear velocity between the slits will increase, and a solidified shell may sometimes rise near the meniscus M, creating micro-cracks that can become the initiation point of longitudinal cracks. If the linear velocity of the cooling water remains constant and is relatively high below, tensile stress in the width direction of the rising solidified shell may persist, leading to the opening of longitudinal cracks and the growth of deeper depressions. Conversely, by increasing the cross-sectional area of ​​the cooling water passage 26 approximately 200 mm below the meniscus M from the top of the mold, the linear velocity of the cooling water decreases, thus mitigating tensile stress and preventing the opening of longitudinal cracks and the growth of deeper depressions. Therefore, increasing the linear velocity of the cooling water and decreasing the linear velocity below it in the area including the region where the foreign material filling portion 20 is formed from the vicinity of the meniscus M is the most effective method for high-speed casting or when it is desirable to reduce the surface temperature of the mold plate 22.

[0082] The inventors have confirmed that in a typical copper alloy mold where the foreign material filling portion 20 is not embedded across the meniscus M, longitudinal cracks in the casting tend to become more pronounced when the linear flow rate is set to 12 m / sec or higher. They have also confirmed that when the cross-sectional area of ​​the cooling water passages 26 near the meniscus M is smaller than that below it relative to the mold, there is a tendency to suppress the generation of longitudinal cracks in the casting. Furthermore, the total area A1 of the cooling water passages 26 on the back side of the mold template, including the area of ​​the meniscus M and the region where the foreign material filling portion 20 is formed, and the total cross-sectional area A2 of the cooling water passages 26 below that area preferably satisfy the relationship A1 ≤ 0.8 × A2. Therefore, it has been confirmed that not only when using the mold 10 of this embodiment, but also when using conventional molds, the generation of longitudinal cracks in castings with a depth of 2 mm or more can be suppressed.

[0083] It was also confirmed that, by satisfying the above-mentioned relationship of the total area of ​​the cooling water channel 26, in the mold 10 of this embodiment, in which the foreign material filling part 20 is embedded in the part separated from the curved surface M, no longitudinal cracks of the casting sheet will be generated even when high-speed casting of 2.5 m / min or more is carried out.

[0084] In the mold 10 of this embodiment, it is preferable to form the foreign material filling part 20 and the cooling water channel 26 on the mold template 22 in a manner that satisfies at least one of the following formulas (1) to (3).

[0085] d <P≤S···(1)

[0086] e≤L≤1000×Vc / f···(2)

[0087] A1≤0.8×A2···(3)

[0088] Here, in equations (1) to (3), d is the width (mm) of the dissimilar material filling portion 20 in the mold width direction B. P is the distance (mm) between adjacent dissimilar material filling portions 20 in the mold width direction B. S is the distance (mm) between adjacent cooling water channels 26 in the mold width direction B among the plurality of cooling water channels 26 formed on the back of the casting template 22. e is the width (mm) of the dissimilar material filling portion 20 in the casting sheet drawing direction A. L is the distance (mm) between adjacent dissimilar material filling portions 20 in the casting sheet drawing direction A. Vc is the casting sheet drawing speed (m / min) in the continuous casting process of steel. f is the vibration frequency (1 / min) of the casting mold 10 in the continuous casting process of steel. A1 is the total cross-sectional area (mm²) of the cooling water channels 26 on the back of the casting template 22 within the area including the meniscus M and the region where the dissimilar material filling portion 20 is formed. 2 A2 is the total cross-sectional area (mm²) of the cooling water passage 26, which is located below the range mentioned above. 2 "Interval distance" refers to the center-to-center distance between two adjacent parts in the casting sheet drawing direction A or the width direction B of the casting template 22 (refer to...). Figure 3 ).

[0089] Next, the total cross-sectional areas A1 and A2 will be explained. The boundaries of the total cross-sectional areas A1 and A2 are defined by the location containing or not containing the foreign material filling portion 20. That is, the range of the total cross-sectional area A1 may include the area where the foreign material filling portion 20 is formed, or it may include a portion of the area where the foreign material filling portion 20 is not formed. However, the range of the total cross-sectional area A1 is preferably the area where the foreign material filling portion 20 is formed. Both the total cross-sectional areas A1 and A2 are the sum of the cross-sectional areas of each slit. The linear velocity of the cooling water in the cooling water passage 26 can be calculated by dividing the water volume by the total cross-sectional area.

[0090] In the mold 10 of this embodiment, since a foreign material filling portion 20 is provided on the copper plate surface near the meniscus, the cooling of the area where the foreign material filling portion 20 is provided is about 10% weaker than that of the area where the foreign material filling portion 20 is not provided. Therefore, in the mold 10 of this embodiment, the total cross-sectional area A1 is 20% smaller than the total cross-sectional area A2. As a result, the linear velocity of the cooling water in the area where the foreign material filling portion 20 is provided is increased by about 20%, and the heat transfer from the slit is improved, so the cooling of the area where the foreign material filling portion 20 is provided can be maintained at the same level as in the conventional case where the foreign material filling portion 20 is not provided.

[0091] Since the casting mold 22 is cooled by cooling water flowing in the cooling water channel 26 on the back side of the casting mold 22, the casting mold 22 dissipates heat radially from the cooling water channel 26. Therefore, uneven cooling occurs on the surface of the casting mold 22, in the areas near and far from the cooling water channel 26. In order to further utilize the effect of reducing the stress and thermal stress caused by the phase transformation from δ iron to γ ​​iron by the periodic increase and decrease of thermal resistance caused by the foreign material filling portion 20, it is preferable to generate a heat flux difference with an interval smaller than the interval distance S of the cooling water channel 26. Therefore, it is preferable to satisfy the above equation (1). That is, it is preferable to set the interval distance P of the foreign material filling portion 20 in the mold width direction B to be less than or equal to the interval distance S of the cooling water channel 26, and the width d of the foreign material filling portion 20 is preferably less than the interval distance P.

[0092] The width d of the foreign material filling part 20 is preferably 2 mm or more and 20 mm or less. When the foreign material filling part 20 is approximately circular, the width d can also be the equivalent circle diameter calculated according to the following formula (6).

[0093] Equivalent circle diameter = (4 × S / π) 1 / 2 …(6)

[0094] Here, in equation (6), S is the area (mm²) of the foreign material filling part 20. 2 ).

[0095] By making the width d or the equivalent circle diameter 2 mm or more, it is easy to fill the circular or near-circular recesses with dissimilar materials using gold plating or spraying methods. On the other hand, by setting the width d and the equivalent circle diameter to 20 mm or less, the solidification delay caused by the decrease in heat flux at the dissimilar material filling part 20 is suppressed, stress concentration towards the solidified shell at this location is prevented, and the generation of surface cracks in the solidified shell is easily suppressed.

[0096] When molten steel 16 is poured into mold 10, in order to prevent the molten steel 16 from sintering into mold 10, mold 10 is vibrated while a protective slag is added to the surface of the molten steel 16. It is known that due to this vibration, periodic oscillation marks are formed on the surface of the casting along the casting drawing direction A, and there is a tendency for the thickness of the casting to change periodically in the casting drawing direction A.

[0097] By ensuring that the width of the foreign material filling portion 20, the spacing between adjacent foreign material filling portions 20, the casting speed, and the vibration frequency f of the mold satisfy the above equation (2), transverse cracks in the casting can be suppressed. That is, by making the width of the foreign material filling portion 20 in the casting drawing direction A smaller than the length (spacing) of one cycle of the casting thickness increase or decrease caused by the oscillation mark in the casting drawing direction A, transverse cracks in the casting can be suppressed.

[0098] As the mold 10 oscillates, a concave depression forms on the casting sheet during one cycle of oscillation. This depression serves as an oscillation mark. When using a casting speed Vc and a mold vibration frequency f, oscillation marks are generated at intervals of 1000 × Vc / f. Thus, because the oscillation marks are generated at intervals of 1000 × Vc / f through the oscillation of the mold 10, it is crucial to avoid applying concentrated strain and stress to this area, which is essential for suppressing transverse cracks. Therefore, in the mold 10 of this embodiment, it is preferable to provide dissimilar material filling portions at intervals of 1000 × Vc / f or less. This avoids artificial thermal stress acting on the oscillation mark valleys, resulting in the suppression of fine cracks forming in the oscillation mark valleys during the early stages of solidification, and inhibiting the propagation and expansion of cracks in the subsequent secondary cooling zone.

[0099] In the mold 10 of this embodiment, it is preferable that the foreign material filling part 20 is formed on the mold template 22 in such a way that it satisfies the conditions of the following formula (7).

[0100] 0.5≤t≤d···(7)

[0101] In the above formula (7), t is the filling thickness (mm) of the foreign material in the foreign material filling part 20, and d is the width (mm) of the foreign material filling part 20 in the mold width direction B.

[0102] The filling thickness of the foreign material filling section 20 (refer to) Figure 3 When the thickness (d) is less than 0.5 mm, the variation in heat flux in the foreign material filling portion 20 sometimes becomes insufficient. On the other hand, if the filling thickness is too thick, it is difficult to fill the recess with foreign material. Therefore, the filling thickness is preferably less than or equal to the width d (mm) of the foreign material filling portion in the mold width direction. The filling thickness t is preferably less than or equal to 10 mm. If the filling thickness is greater than 10 mm, it is difficult to fill the recess with foreign material. Furthermore, if the thermal conductivity of the foreign material is lower than that of the mold template 22, the heat dissipation from the mold template 22 will be less if the filling thickness is greater than 10 mm. As a result, the surface temperature of the mold template 22 rises, and if the surface temperature exceeds 350°C, it will cause abnormalities in the copper plate and gold plating. Therefore, the filling thickness is preferably less than or equal to 10 mm.

[0103] like Figure 6 As shown, a plating layer (gold plating layer) 32 is preferably formed on the surface of the casting mold 22 in a manner that covers the foreign material filling portion 20. This suppresses wear caused by the solidified shell and cracks on the mold surface caused by the thermal process. The plating layer 32 can be formed by gold plating or spraying commonly used nickel or nickel-containing alloys, such as nickel-cobalt alloys (Ni-Co alloys) and nickel-chromium alloys (Ni-Cr alloys).

[0104] The cooling intensity (heat transfer rate) of the mold 10 is determined by the linear velocity of the cooling water flowing in the cooling water path 26. Therefore, it is preferable to increase the linear velocity of the cooling water to 7.0 m / sec or higher during high-speed casting. The average linear velocity of the cooling water is calculated by the following equation (8). Therefore, in order to increase the linear velocity of the cooling water to 7.0 m / sec or higher, the appropriate design of the cooling water volume and the cooling water path 26 becomes important.

[0105] Average linear velocity of cooling water (m / sec) = Cooling water flow rate (m³ / s) 3 / sec) / Total cross-sectional area of ​​cooling water channels (m²) 2 (8)

[0106] In continuous casting of steel, when molten steel is solidified through mold 10, the heat dissipation from mold 10 can be estimated based on the temperature difference between the cooling water entering and exiting the mold and the temperature of the thermocouple embedded in the copper plate of the mold. The heat dissipation from mold 10 reaches its maximum value in the casting direction within a range of 20 mm to 80 mm below the meniscus M, and then shows a tendency to decrease gradually.

[0107] Figure 8 This is a graph showing the relationship between the distance from the top of the mold and the surface temperature of the mold plate 40. Corresponding to the aforementioned heat dissipation, the surface temperature of the mold copper plate is... Figure 8The temperature maximum point directly below the meniscus M is the surface temperature of the casting mold 40. If the surface temperature of the casting mold 40 exceeds 350°C, it reaches the yield stress of the copper plate and the solidification point of the gold plating, thus potentially causing the gold plating to peel off. Figure 8 As shown, the curve shifts towards the high-temperature side as the casting speed increases. Therefore, when the surface temperature of the casting mold needs to exceed 350°C, it is necessary to increase the cooling water volume or reduce the total cross-sectional area of ​​the cooling water channel 26 to accelerate the linear flow rate of the cooling water.

[0108] When the recess is filled with a material with low thermal conductivity, the temperature rise in the filled area increases further. The surface temperature of the casting mold varies depending on the depth of the low thermal conductivity material filling, the material of the casting mold, and the thickness of the casting mold; therefore, the surface temperature of the casting mold during high-speed casting needs to be carefully monitored.

[0109] like Figure 8 As shown, the surface temperature of the casting template 40 reaches its maximum near the meniscus M, and then becomes a relatively low and gradual temperature distribution below it. Therefore, areas where the copper plate and gold plating are easily peeled off or damaged tend to concentrate about 200 mm below the meniscus M. It is preferable to increase the heat dissipation in this area to keep the surface temperature of the casting template below 350°C.

[0110] In the mold 10 of this embodiment, the total cross-sectional area of ​​the cooling water channels 26 on the back side of the mold plate, including the area containing the meniscus M and the region where the foreign substance filling portion 20 is formed, is smaller than the total cross-sectional area of ​​the cooling water channels 26 below this area. As a result, heat dissipation from the mold plate 22 is enhanced, and even when continuous casting is performed at a casting speed of 2.5 m / min or higher, the surface temperature of the mold plate 22 can be kept below 350°C.

[0111] Preferably, a thermocouple is embedded in the casting mold 22 within a range of 20mm to 100mm below the meniscus M, and the temperature measured by the thermocouple is used. That is, the surface temperature of the casting mold 22 can be calculated using the temperature measured by the thermocouple, and therefore the required cooling water volume and the total cross-sectional area of ​​the cooling water passage 26 can be determined if the surface temperature of the casting mold 22 is below 350°C. Moreover, based on this result, the upper limit of the casting speed that can be cast can also be determined.

[0112] Next, the direction of cooling water flow will be explained. Figure 9 These are the casting direction sectional view (a) and front view (b) of the casting template 22. (Using...) Figure 9 Instructions are provided regarding the direction of cooling water flow. In the continuous casting method for steel according to this embodiment, as... Figure 9As shown, the cooling water is directed from the upper end of the mold near the meniscus M towards the lower end of the mold. This supplies cool water at a lower temperature to the cooling water path 26 near the meniscus M, thereby improving the cooling capacity of the mold 10 near the meniscus M.

[0113] Conventionally, supplying low-temperature cooling water to the upper part of the mold near the meniscus M results in uneven cooling. Uneven cooling near the meniscus M can lead to longitudinal cracks in the casting, which is therefore undesirable. In contrast, in the continuous casting method of this embodiment, the dissimilar material filling portion 20 is embedded near the meniscus M, thereby achieving a uniform cooling effect. Therefore, in the continuous casting method of this embodiment, it is preferable that the cooling water flows from the upper part of the mold near the meniscus M towards the lower part of the mold. This improves the cooling capacity of the upper part of the mold, where the surface temperature is prone to rise, and makes it easier to keep the surface temperature of the casting template 22 below 350°C. Furthermore, in casting operations with a casting speed of 2.5 m / min or higher where the surface temperature of the casting template 22 is prone to exceed 350°C, it is also preferable to introduce low-temperature cooling water from the upper part of the mold. Therefore, the cooling water flows from the upper part of the mold near the meniscus M towards the lower part of the mold, which can improve the cooling capacity of the upper part of the mold where the surface temperature is prone to rise.

[0114] In the continuous steel casting method of this embodiment, it is preferable to supply cooling water to the cooling water passage 26 with high precision. In the mold 10, cooling water is generally supplied to the cooling water passage 26 of the long side 12 and the short side 14 of the mold from a common pump. However, in this structure, even if it is desired to improve the precision of the cooling water supply, due to the flow-pressure relationship determined by the pipe diameter, if it is desired to increase the cooling water volume of either the long side 12 or the short side 14 of the mold, it may sometimes be impossible to increase the cooling water volume because the pressure on the remaining surface does not rise.

[0115] Figure 10 This is a schematic diagram illustrating an example of a pump supplying cooling water to the mold 10. (Example) Figure 10 As shown, four pumps can also be set up independently: two at the front and rear of the long side 12 of the mold and two on the left and right sides of the short side 14 of the mold, but this is not only very expensive, but also makes control more complicated. Because it has multiple backup pumps in case of failure, it is not a preferred operating equipment for continuous steel casting that operates almost 365 days a year.

[0116] To ensure continuous operation, it is preferable to maintain a continuous water supply to all four sides of the mold (long side 12, short side 14) even in the event of pump failure, provided that even a small amount of cooling water is supplied. Therefore, it is preferable to use a large pump shared across both the long side 12 and the short side 14 of the mold.

[0117] Figure 11This is a graph showing the relationship between the total pump flow rate and the pump source pressure. Typically, as a characteristic of the pump, there exists a relationship between the total pump flow rate and the pump source pressure. Figure 11 The solid lines represent the relationships. The dashed lines represent the pump system diagram. (See diagram for example.) Figure 11 As shown, to increase the total supply of cooling water, the valve opening needs to be close to full open, reducing the pump's source pressure. The mold used for continuous casting of steel consists of a pair of long sides 12 and a pair of short sides 14. Typically, the piping diameter of the short side 14 (where the water flow is less) is smaller than the piping diameter of the long side 12 (where the water flow is more). Therefore, even if it is possible to supply water to the mold 10 with the required linear flow rate relative to the long and short sides, the supply can be directed towards the long side, but sometimes the pressure loss on the short side is too significant to achieve the target flow rate.

[0118] Figure 12 This is a schematic diagram illustrating an example of a pump supplying cooling water to a casting mold. To address the aforementioned issue, such as... Figure 12 As shown, to further increase the water volume of the common pump P, which supplies water to the entire system, and the short side, it is preferable to install a booster pump BP via series piping. That is, to increase the cooling water supply to the short side 14 of the mold where the piping diameter is smaller, the water pressure within the piping needs to be increased to a level not less than the pressure loss. By installing a booster pump BP, which is different from the common pump P, to increase this water pressure, the pressure within the piping of the short side 14 of the mold can be supplemented.

[0119] As a pump control method, the preferred configuration is that the booster pump BP operates at a specific casting speed. When the casting speed is high, as the cooling water volume on the long side 12 of the mold increases, the booster pump BP increases the water pressure, thereby adjusting the cooling water volume on the short side 14 of the mold to the target cooling water volume. By employing this method, not only can the use of a backup pump in case of pump failure be minimized, but also a minimum amount of cooling water can be supplied to the mold 10 even in the event of pump failure, thus preventing operational abnormalities.

[0120] As a method for controlling the amount of cooling water, changing the amount of cooling water in accordance with the casting speed is also effective. However, since the change in the cooling capacity of the mold is not as drastic as that of the secondary cooling water, it is simple to control the amount of water at multiple points in a stepwise manner for the casting speed.

[0121] Ideally, a system is preferably constructed that automatically controls the total cross-sectional area of ​​the pump and cooling water passage 26 based on the temperature of the thermocouple embedded in the casting mold 22, so that the surface temperature of the casting mold 22 is kept below 350°C. Regarding the control method, the setpoint of the water volume of the mold pump is determined, and the pump setpoint and the total cross-sectional area of ​​the cooling water passage 26 are adjusted in such a way that the surface temperature of the casting mold, determined based on the thermocouple temperature of the casting mold 22, is kept below 350°C. Furthermore, as for height control, in order to increase the linear flow velocity at the location corresponding to the foreign material filling section, a servo motor, pneumatic cylinder, hydraulic cylinder, etc., can be used to move the spacer 28 provided on the support plate. Thus, without disassembling the casting mold 10, the total cross-sectional area A1 of the cooling water passage 26 on the upper side of the mold, including the meniscus M and the foreign material filling section 20, can be changed.

[0122] Next, the protective slag used in the continuous casting method of steel according to this embodiment will be explained. In the continuous casting of medium carbon steel, represented by semi-peritectic steel, a slow-cooling protective slag is used to suppress the formation of longitudinal cracks in the casting. Due to phase transformation stresses accompanying the δ→γ phase transformation, the solidified shell growth of medium carbon steel is prone to become uneven. If stress concentration occurs in the circumferential direction, defects such as longitudinal cracks and depressions will occur in that location. To suppress this phenomenon, crystallizing powder is used in medium carbon steel to actively generate crystalline phases in the protective slag and achieve uniform and slow cooling within the mold. To stably generate crystalline phases from high temperatures, a high-basicity protective slag with a basicity of CaO / SiO2 of 1.2 or higher is required.

[0123] Low-basicity mold fluxes, primarily glassy, ​​provide strong cooling for the mold. In contrast, while high-basicity mold fluxes are highly effective at preventing longitudinal cracks, they reduce the mold's cooling capacity. Therefore, high-basicity mold fluxes cannot simultaneously achieve high-speed casting and longitudinal crack prevention. Moreover, high-basicity mold fluxes are expensive; from a manufacturing cost perspective, it is preferable to use mold fluxes with a basicity of CaO / SiO2 below 1.2.

[0124] In the continuous casting method for steel according to this embodiment, a foreign material filling section 20 is provided in the casting mold 22 to impart a regular temperature change to the solidified shell, thereby suppressing the increase of local heat flux to specific areas and thus suppressing the formation of longitudinal cracks and depressions in the casting. Therefore, in the continuous casting method for steel according to this embodiment, a protective slag with a basicity of CaO / SiO2 less than 1.2 is preferably used. By using this protective slag, strong cooling of the mold can be achieved, high casting speeds can be accommodated, and longitudinal cracks in the casting can be suppressed.

[0125] In the continuous casting method for steel according to this embodiment, it is preferably applicable to the continuous casting of thin billets with a casting thickness of 150 mm or less, performed at a high-speed casting speed of 3.0 m / min or higher and 8.0 m / min or lower. Continuous casting of thin billets essentially involves directly connecting the cast billet to the rolling process while it is being heated in a subsequent tunnel furnace, without any finishing processes such as billet trimming. Therefore, if defects such as longitudinal cracks exist in the billet, the quality of the coil deteriorates, and the yield decreases. Consequently, continuous casting of thin billets using high-speed casting is not actively implemented in the manufacture of subperiterite medium-carbon steel and special steels with high alloy content, which are difficult-to-cast steel grades.

[0126] In contrast, by applying the continuous casting method for steel according to this embodiment, it is possible to cast subperiterite medium carbon steel through continuous casting of thin billets, thus improving productivity at each stage. Furthermore, in the continuous casting of thin billets, to prevent the outer diameter of the immersion nozzle supplying molten steel from being unable to be contained within the mold due to the mold thickness becoming too thin, a mold with a special funnel shape (funnel shape) in the center of its long side width is used. In the case of casting medium carbon steel, not only is uneven solidification at the meniscus M prone to occur, but uneven cooling also easily occurs at the boundary of this funnel shape, resulting in longitudinal cracks in the casting. This area is located below the meniscus M, so longitudinal cracks cannot be completely suppressed simply by improving the mold flux. Therefore, it is preferable to also form a foreign material filling section 20 within a 50mm range of the boundary of the funnel shape, within a range of 0mm to 200mm below the meniscus M. This suppresses uneven cooling at the boundary of the funnel shape, thus suppressing the generation of longitudinal cracks and steel leakage in the thin billet.

[0127] Example

[0128] [Example 1]

[0129] Next, an example of evaluating the mold of this embodiment through continuous casting on a real machine will be described. In Example 1, the following three types of molds were used for evaluation.

[0130] Comparative Example 1: A typical continuous casting mold in which no foreign material filling part is formed on the inner wall of the mold and the cross-sectional area of ​​the cooling water channel is constant in the casting direction.

[0131] Comparative Example 2: A continuous casting mold in which the foreign material filling part is formed on the inner wall of the mold, but the cross-sectional area of ​​the cooling water channel is constant in the casting direction.

[0132] Invention Example 1: A continuous casting mold in which a foreign material filling portion is formed on the inner wall surface of the mold. The mold is configured such that the total cross-sectional area A1 of the cooling water passage corresponding to the portion where the foreign material filling portion is embedded satisfies the relationship A1 = 0.7 × A2 with respect to the cross-sectional area A2 below it.

[0133] The above-mentioned molds are all rectangular inner surface spaces with a long side length of 2.1m and a short side length of 0.22m. The mold templates that make up the long side and the short side are made of copper alloy with a thermal conductivity of about 380 (W / (m×K)) at room temperature.

[0134] The steel grades used for continuous casting include all steel grades used in conventional casting, ranging from very low carbon steel to medium carbon steel. The chemical composition is: C: 0.0008–0.25 wt%, Si: 0.002–1.2 wt%, Mn: 0.10–2.0 wt%, P: 0.005–0.030 wt%, S: 0.01–0.02 wt%, Al: 0.001–0.06 wt%, with the balance being Fe and unavoidable impurities. One charge contains 300 tons of molten steel. Molten steel is poured into a prepared mold while the mold is vibrated in the direction of sheet drawing and cooled to form a solidified shell. This solidified shell is then drawn to cast the sheet. The sheet drawing speed Vc is 0.3–2.6 m / min.

[0135] In Example 1, a protective slag was added to the molten steel inside the vibrating mold to prevent sintering of the molten steel on the mold. As the protective slag, a protective powder with a basicity ((mass% CaO) / (mass% SiO2)) of 0.6 to 1.8 was used. For medium carbon steel, a high-basicity protective slag with a basicity of 1.5 to 1.8 was used.

[0136] In Example 1, the goal was to perform continuous casting with 3000 loads without changing the mold. After every 100 loads of casting, surface cracks on the long edge of the mold were checked. The surface of the long edge of the mold was visually inspected for gold plating, copper plate peeling, and cracks. If any abnormalities were confirmed, the continuous casting operation was stopped at that point. For each of all continuous castings, surface cracks on the cast sheet were checked. For surface cracks on the cast sheets, the surface of the cast sheets after penetrant testing (color inspection) was visually inspected for cracks in medium carbon steel, which is highly susceptible to cracking, to confirm longitudinal cracks along the casting direction.

[0137] In the molds of Comparative Example 2 and Invention Example 1, multiple circular recesses are formed on the mold template constituting the long side of the mold. These recesses are then filled with a nickel alloy (thermal conductivity at room temperature: 80 W / (m×K)) as a dissimilar material using a gold-plating method, forming a dissimilar material filling portion. For all molds, a [missing information - likely a surface treatment] is provided on the inner wall surface of the mold. Figure 4 , Figure 6 The nickel alloy plating shown.

[0138] In Comparative Example 2 and Invention Example 1, the filling depth t of the foreign material is set to 1 mm, and the foreign material filling part 20 is set in a manner that satisfies equations (1) and (2).

[0139] Regarding the cooling water supply rate in Comparative Example 1, Comparative Example 2, and Invention Example 1, the cooling water volume is set such that the linear flow velocity in the cooling water path is 7.0 m / sec, and cooling water is supplied to the mold. In Invention Example 1, a spacer 28 is provided to increase the linear flow velocity in the cooling water path to 10.0 m / sec.

[0140] The operational results of Comparative Example 1, Comparative Example 2, and Invention Example 1 are as follows.

[0141] Comparative Example 1: At the end of the casting process at 2600°C, cracks appeared in the plating layer of the gold-plated mold plate, leading to the termination of casting. The longitudinal crack incidence rate of the medium carbon steel casting was 8.0%.

[0142] Comparative Example 2: At the end of the casting process at 2000 charge, the plating on the gold-plated mold plate peeled off and cracked, causing the casting to stop. The longitudinal crack rate of the medium carbon steel casting was 0.5%.

[0143] Example 1: At the end of the casting process with a charge of 3000, no peeling or cracking occurred in the plating layer of the gold-plated mold plate. The longitudinal crack rate of the medium carbon steel casting was 0.2%.

[0144] Thus, in Example 1 of the invention, even at the end of the casting process with a charge of 3000, no surface cracks were generated on the casting template forming the long side of the mold, and the reduction effect on longitudinal cracks in the medium carbon steel casting was also confirmed.

[0145] In Invention Example 1, the casting speed was further increased to 3.0 m / min, and casting continued until 3500 tons of material were loaded. As a result, even when casting was carried out at the highest casting speed of 3.0 m / min with increased thermal load, no surface cracks were observed in the casting template forming the long side of the mold, and no tendency to increase longitudinal cracks was observed in the medium carbon steel castings.

[0146] Thus, it was confirmed that in Invention Example 1, continuous casting with 3000 loading cycles could be performed without changing the mold, thus improving the mold's service life compared to Comparative Examples 1 and 2. Furthermore, even when used in casting speeds of 2.5 m / min or higher, there was absolutely no damage to the mold surface. This result is believed to be due to the increased linear flow rate achieved by reducing the total cross-sectional area of ​​the cooling water channels in the area where the foreign material filling portion 20 is formed, resulting in more efficient mold cooling.

[0147] An investigation was conducted into whether surface cracks occurred in the cast sheets cast in Comparative Example 2 and Invention Example 1, but no surface cracks requiring repair were identified. It is believed that the molds of Comparative Example 2 and Invention Example 1 can effectively suppress the generation of surface cracks caused by uneven solidified shell thickness due to the phase transformation from δ-iron to γ-iron during medium carbon steel casting by utilizing the foreign material filling portion, thus suppressing the generation of surface cracks in the cast sheets. Therefore, since no surface cracks requiring repair were identified, it can be confirmed that direct-feed rolling can be performed using these molds.

[0148] [Example 2]

[0149] Next, Example 2 (Examples 2-35 of the Invention and Examples 3-29 of the Comparative Invention) will be described, in which the continuous casting of steel is performed using the same method as in Example 1 above. In Example 2, in order to change the foreign material filling part, cooling water channel, and casting conditions, the mold is inserted and removed during each test, and a foreign material filling part is set in the short side mold where the conditions can be easily changed, thereby changing the conditions of the short side mold to perform the continuous casting of steel.

[0150] In Example 2, the casting charge in one of the Invention Examples and Comparative Examples was set to 5 charges. Furthermore, considering the potential problem of longitudinal cracks on the surface of the casting, only medium carbon steel with the following chemical composition was used: C: 0.08~0.17 wt%, Si: 0.10~0.30 wt%, Mn: 0.50~1.20 wt%, P: 0.010~0.030 wt%, S: 0.005~0.015 wt%, Al: 0.020~0.040 wt%, with the balance being Fe and unavoidable impurities. In Invention Examples 2~35 and Comparative Examples 3~29, the following modifications were made respectively. Figure 3 The width d (mm) of the foreign material filling section 20 in the mold width direction, the spacing P (mm) of the foreign material filling section 20 in the mold width direction, the width e (mm) of the foreign material filling section 20 in the casting sheet drawing direction A, the upper total cross-sectional area A1 of the cooling water channel, and the lower total cross-sectional area A2 of the cooling water channel are shown. Furthermore, in Invention Examples 2 to 35 and Comparative Examples 3 to 29, continuous casting was implemented by changing the vibration frequency (1 / min), the casting sheet drawing speed Vc (m / min), and the basicity of the protective slag.

[0151] In each operation, a continuous casting operation with 5 charges is performed. A thermocouple is embedded in the mold, extending 50 mm below the meniscus (M) in the casting direction, and its temperature is measured. Temperature measurements are taken at 1-second intervals, and the data are recorded. The distance from the thermocouple's measuring point to the molten steel side surface of the casting mold 22 is 15 mm. Based on the heat transfer model, the surface temperature of the casting mold 22 is calculated from the thermocouple's temperature.

[0152] The calculation results of the width d, spacing distance P, total cross-sectional area A1, A2, etc. of the cooling water channel in Invention Examples 2 to 35, and the calculated surface temperature of the copper plate of the mold are shown in Table 1 below. Comparative Examples 3 to 29 are general continuous casting molds without foreign material filling parts or molds with foreign material filling parts but with the cross-sectional area of ​​the cooling slit water channel corresponding to that part and the area below it remaining unchanged.

[0153] [Table 1]

[0154]

[0155] In Table 1, “〇” in columns “(1)”, “(2)”, and “(3)” indicates that each equation is satisfied, and “×” in the column indicates that each equation is not satisfied. In the column “Maximum Copper Plate Surface Temperature” of Table 1, the following values ​​are recorded: the average surface temperature of the casting template is calculated based on the temperature of the thermocouple using a heat transfer model; then, this average temperature is further averaged using data samples from a stable continuous casting operation with 5 loads. The lower the “Maximum Copper Plate Surface Temperature”, the more the surface of the casting template at the meniscus M is cooled, and there is a tendency for a higher casting speed to result in a higher maximum copper plate surface temperature. If the maximum copper plate surface temperature is below 350°C, it can be said that the casting template can be cooled stably. It was confirmed that if the maximum copper plate surface temperature is below 350°C, it will not cause gold plating peeling or cracking on the copper plate surface.

[0156] In Example 2, surface cracks in the castings were investigated for each of all continuous casting operations. Ten castings can be produced in a single continuous casting operation. In each of the inventive and comparative examples, five charges were continuously cast, resulting in 50 castings in each example. Penetrant testing was performed on all the castings, and the surfaces of the castings subjected to penetrant testing were visually inspected to confirm surface cracks. Since the detailed specifications of the mold were only changed on the short side, the data evaluated in Example 2 is limited to surface cracks at the short side. When longitudinal cracks were confirmed on the surface of the castings, the castings were counted, and the percentage of the total number of castings with confirmed longitudinal cracks relative to the total number of castings (=50) was recorded in the "Longitudinal Crack Occurrence Rate" column. Even when very small cracks were visually confirmed, the castings were counted for this crack incidence rate; therefore, even if the longitudinal crack incidence rate is not zero, as long as the crack incidence rate is below 15%, there is no substantial problem.

[0157] In Invention Examples 2 to 35, where continuous casting operations were performed using a mold equipped with a foreign material filling section 20, the longitudinal crack incidence rate of all castings obtained in a single continuous casting operation was 15% or less. Based on this result, it was confirmed that by using the mold of this embodiment, surface longitudinal cracks in the castings can be prevented.

[0158] Furthermore, in Invention Examples 2 to 35, the above formula (3) is satisfied, and the surface temperature of the copper plate is below 350°C as measured by the thermometer at the meniscus. As a result, the generation of cracks or abnormalities on the surface of the copper plate can be suppressed, and even under high-speed casting conditions with a casting speed of 2.5 m / min or higher, cracks or abnormalities will not occur on the surface of the copper plate.

[0159] When using the type of mold shown in Comparative Example 1 of Example 1 (Comparative Examples 22 to 29), longitudinal cracks were significantly generated. Furthermore, when the basicity of the protective slag was 1.55, a longitudinal crack suppression effect was confirmed, but when the basicity of the protective slag was 1.10, a longitudinal crack generation rate of over 40% was confirmed.

[0160] When using a mold of the type shown in Comparative Example 2 of Example 1 (Comparative Examples 3 to 21), the mold that did not satisfy formula (3) and whose maximum copper plate surface temperature exceeded 300°C produced gold plating peeling and cracks on the copper plate surface when loading 100 or more. The mold that did not satisfy formula (3) and whose maximum copper plate surface temperature exceeded 350°C produced gold plating peeling and cracks on the copper plate surface when loading 5. In addition, although not shown in the table, when the linear flow rate of the cooling slit was increased from 7.0 m / sec to 12.0 m / sec, the maximum surface temperature of the mold could be reduced to about 300°C. However, longitudinal cracks with a concave shape were confirmed near the short corner with an incidence rate of 50%, indicating that stable operation was difficult to achieve.

[0161] Based on the above results, it has been confirmed that by using the mold of this embodiment to continuously cast steel, the generation of surface cracks in medium carbon steel castings can be suppressed, and the temperature of the mold plate near the meniscus where the foreign material filling portion is formed can be effectively reduced. Furthermore, by effectively reducing the temperature of the mold plate, the generation of cracks and abnormalities on the copper plate surface can also be suppressed. Therefore, in the mold of this embodiment, it has been confirmed that both the long lifespan of the mold with the foreign material filling portion and the suppression of surface cracks in the castings can be achieved simultaneously.

[0162] [Example 3]

[0163] To confirm the effectiveness of the mold in this embodiment of high-speed casting, Example 3, which describes the continuous casting of steel using a thin-slab continuous casting machine, will be described. In Example 3, a mold with a funnel-shaped curved surface 42 at the center of the mold copper plate in the width direction was used to continuously cast a blank with a constant size of 1250 mm in width and a blank thickness of 75 mm at the lower end of the mold (Invention Example 36 and Comparative Example 30).

[0164] The steel grade used for continuous casting is medium carbon steel with the following chemical composition: C: 0.08~0.17 wt%, Si: 0.10~0.30 wt%, Mn: 0.50~1.20 wt%, P: 0.010~0.030 wt%, S: 0.005~0.015 wt%, Al: 0.020~0.040 wt%, with the balance being Fe and unavoidable impurities. The casting speed is 4.0~5.0 m / min, the mold vibration frequency is f = 400~500 (1 / min), and the mold vibration stroke is 6 mm. The basicity of the protective slag is CaO / SiO2 = 1.25.

[0165] Figure 13 This is a schematic diagram showing the mold used in Invention Example 36. Figure 14 This is a schematic diagram showing the mold used in Comparative Example 30. For example... Figure 13As shown, in Invention Example 36, a mold was used in which a dissimilar material filling portion 44, with a nickel alloy of 5 mm φ × 1.5 mm depth embedded, was embedded in the meniscus M and the boundary portion of the funnel-shaped curved surface 42 within a range of 0 to 200 mm below the meniscus M. Furthermore, regarding the cooling slit, the spacer 28 was provided on the support plate side such that the total cross-sectional area of ​​the cooling water passage within a range of 0 to 200 mm below the meniscus from the upper end was 0.75 times the total cross-sectional area to the lower end. On the other hand, in Comparative Example 30, a mold without the dissimilar material filling portion 44 and without the spacer 28 was used.

[0166] Casting tests were conducted on each of Invention Example 36 and Comparative Example 30 with 5 charges. In thin billet continuous casting machines, the cast billet is usually continuously heated in a tunnel furnace and carried out to the rolling process. However, in this test, in order to confirm the condition of surface cracks in the cast thin billet, cold sheets were made for confirmation.

[0167] As a result of the casting process, in Invention Example 36, continuous casting was performed at a casting speed of 4.0 m / min or higher and 5.0 m / min or lower for all five charges. It was confirmed that no longitudinal cracks occurred on the surface of the cast sheets, and the quality remained completely problem-free even with continuous rolling. In contrast, in Comparative Example 30, a run-in occurred in charge 1 of the five charges, making complete casting impossible. Longitudinal cracks appeared in 66% of the cast sheets after casting. Most of the longitudinal cracks occurred at locations corresponding to the boundaries of the funnel shape.

[0168] The copper plate surface temperature, estimated from the thermocouple temperature near the meniscus in the casting of Example 36, remained stable at around 320°C even at a casting speed of 5.0 m / min, and no abnormalities were observed on the surface of the cast copper plate. In contrast, the copper plate surface temperature, estimated from the thermocouple temperature near the meniscus M in the casting of Comparative Example 30, tended to exceed 350°C from around a casting speed of 4.0 m / min, thus further increases in casting speed were abandoned.

[0169] These results confirm that the mold of Invention Example 36 is extremely effective for continuous casting of thin billets requiring high-speed casting of 2.5 m / min or higher. In mold cooling for thin billet continuous casting machines, in addition to the longitudinal slit cooling method described above, channel cooling, which involves machining circular holes in the casting direction and circulating water through them, is also commonly used. In the channel method, the same effect can be achieved by changing the upper and lower hole diameters; therefore, the cooling method can be arbitrary.

[0170] Thus, by using the mold of this embodiment to perform continuous casting of steel, the generation of longitudinal crack defects originating from within the mold during continuous casting can be suppressed, and even at high-speed casting conditions of 2.5 m / min or higher, gold plating peeling and copper plate deformation can be suppressed, thereby extending the life of the mold. Since a protective slag with high cooling capacity can also be used, it is possible to cope with even higher-speed casting conditions, thereby further improving the productivity of the cast sheets.

[0171] It can eliminate the finishing process in the finishing process that is accompanied by longitudinal crack defects, thus enabling direct feeding rolling process, reducing the cost of billet heating in the heating furnace, and also contributing to energy saving.

[0172] Explanation of reference numerals in the attached figures

[0173] 10 Continuous casting molds

[0174] 12 mold long side

[0175] 14 casting mold short side

[0176] 16 molten steel

[0177] 18 Dipping Nozzles

[0178] 20 foreign material filling sections

[0179] 22 casting template

[0180] 26 Cooling Water Circuit

[0181] 28 spacers

[0182] 30 support plate

[0183] 32 coating layers

[0184] 40 Previous casting molds

[0185] 42-curved surface

[0186] 44. Foreign material filling section.

Claims

1. A mold for continuous casting of steel, comprising: a mold plate made of copper alloy, a surface of the mold plate forming a mold inner wall surface, and a cooling water passage being formed on a back surface of the mold plate; and a support plate installed on the mold plate in a manner of covering the cooling water passage, a plurality of heterogeneous substance filling portions are formed in a recess formed in a region of the surface of the mold plate including a meniscus, the heterogeneous substance filling portions being filled with a heterogeneous substance having a thermal conductivity different from that of the mold plate, a total cross-sectional area of the cooling water passage of the back surface of the mold plate in a range including the region in which the plurality of heterogeneous substance filling portions are formed is smaller than a total cross-sectional area of the cooling water passage below the range. wherein 2. The mold for continuous casting of steel according to claim 1, wherein the plurality of heterogeneous substance filling portions are formed in the region of the surface of the mold plate in a manner that a heat flux from the mold inner wall surface toward the cooling water passage periodically changes.

3. The mold for continuous casting of steel according to claim 1 or 2, wherein the heterogeneous substance filling portions and the cooling water passage are formed in a manner satisfying at least one of the following (1) to (3): d < P ≤ S (1) e ≤ L ≤ 1000 x Vc / f (2) A1 ≤ 0.8 x A2 (3) 4. The mold for continuous casting of steel according to any one of claims 1 to 3, wherein a plating layer is formed on the surface of the mold plate in a manner of covering the heterogeneous substance filling portions.

5. The mold for continuous casting of steel according to any one of claims 1 to 4, wherein the mold plate is a pair of long-side mold plates and a pair of short-side mold plates, In the above (1) to (3), d is the width (mm) of the heterogeneous substance filling portion in the mold width direction, P is the interval distance (mm) of adjacent heterogeneous substance filling portions in the mold width direction, S is the interval distance (mm) of adjacent cooling water channels in the mold width direction, e is the width (mm) of the heterogeneous substance filling portion in the strip drawing direction, L is the interval distance (mm) of adjacent heterogeneous substance filling portions in the strip drawing direction, Vc is the strip drawing 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, A1 is the total cross-sectional area (mm 2 ) of the cooling water channel including the range, and A2 is the total cross-sectional area (mm 2 ) of the cooling water channel below the range. 2 2 in addition to a common pump that supplies cooling water to the long-side mold plates and the short-side mold plates, a booster pump that supplies cooling water to at least the short-side mold plates is provided.

6. The mold for continuous casting of steel according to any one of claims 1 to 5, wherein the mold plate has a funnel shape, the heterogeneous substance filling portions are also formed in a range of 50 mm of a boundary portion of the funnel shape in 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 any one of claims 1 to 6, wherein a maximum temperature of the surface of the mold plate at the meniscus in a case where continuous casting is performed at a casting speed of 2.5 m / minute or more is 350°C or less.

8. A method for continuous casting of steel using the mold for continuous casting of steel according to any one of claims 1 to 6, wherein a surface temperature of the mold plate is estimated from a temperature measured by a thermocouple embedded in the mold plate in a range of 20 to 100 mm below the meniscus, and a total cross-sectional area of the cooling water passage of the back surface of the mold plate in a region in which the heterogeneous substance filling portions are formed is adjusted in a manner that the estimated surface temperature of the mold plate becomes 350°C or less, and a supply amount of cooling water.

9. A method for continuous casting of steel using the mold for continuous casting of steel according to any one of claims 1 to 6, wherein ​ ​ ​ ​ A protective slag having a basicity of less than 1.2 is used.

10. A continuous casting method of steel using the continuous casting mold for steel according to any one of claims 1 to 6, wherein The cooling water is supplied in a direction from the upper end of the mold plate toward the lower end of the mold plate.

11. A continuous casting method of steel using the continuous casting mold for steel according to any one of claims 1 to 6, wherein The cast slab is cast to a thickness of 150 mm or less.

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