Continuous steel casting mold and continuous steel casting method

Abstract

A continuous steel casting mold enables continuous casting while suppressing mold plate surface abnormalities even at casting speeds ≥2.5 m / min. The mold includes a copper alloy mold plate with a front surface defining the mold's inner wall and a back surface defining cooling water channels, plus a backup plate covering these channels. The front surface region including the meniscus contains recesses filled with dissimilar material having different thermal conductivity than the mold plate. The total sectional area of cooling water channels in the range including these dissimilar material-filled portions is smaller than the total sectional area below, enhancing cooling efficiency and preventing surface defects.

Description

[0001] This application is a National Stage of International Application No. PCT / JP2024 / 007978 filed Mar. 4, 2024, which is based upon and claims benefit of priority from Japanese Patent Application No. 2023-077840 filed May 10, 2023, the entire contents of the prior applications being incorporated herein by reference.TECHNICAL FIELD

[0002] This application pertains to a continuous steel casting mold that stably allows for continuous casting even at a high casting speed of 2.5 m / min or more, and to a related continuous steel casting method.BACKGROUND

[0003] There have been stricter demands for high productivity and high quality in the continuous casting of steel. Thus, techniques are desired that enable high-speed casting without giving rise to surface defects on the slab in the mold and in the secondary cooling zone. If, in particular, surface cracks occur on the surface of the slab during continuous casting, the slab needs to be cooled and repaired by removing the surface defects in order to avoid the entry of defects to downstream processes. This hinders the hot direct processing of the slab and causes a major problem in terms of productivity.

[0004] Typical surface cracks include longitudinal surface cracks, transverse surface cracks, and corner cracks. Transverse surface cracks and corner cracks are known to occur when strain is applied in the embrittlement temperature range in the secondary cooling zone of continuous casting. Transverse surface cracks and corner cracks are generally prevented by, for example, designing the secondary cooling specifications so that the temperature at the bend correction point will be outside the embrittlement temperature range or by adding anti-embrittlement elements.

[0005] On the other hand, it is known that longitudinal surface cracks occur easily in medium carbon steel in the hypo-peritectic region where uneven solidification is likely to occur, and that the cracks tend to originate in the mold. In view of these facts, a mild cooling mold powder that can improve uneven solidification is used to reduce the stress concentration in the solidifying shell at the initial stage of solidification.

[0006] When a mild cooling mold powder has been added, less heat is removed from the mold. Thus, in high-speed casting, the solidifying shell that has reached the lower end of the mold has not grown thick sufficiently and has an increased risk of breakout. For this reason, types of steels that require the use of mild cooling powder have a problem in that the casting speed is relatively low compared to other types of steels. Another problem is that the use of mild cooling powder increases the cost.

[0007] To address the above problems, techniques have been proposed, and have come into use, that prevent the occurrence of longitudinal surface cracks by filling the mold plates with a low-thermal conductivity material. Patent Literature 1 discloses a continuous casting mold in which dissimilar material-filled portions are formed by filling the inner wall surface of the mold with a dissimilar material having a thermal conductivity different from that of a copper alloy constituting the mold. According to Patent Literature 1, the thermal resistance R between the mold surface at a position where the dissimilar material-filled portion is disposed and a cooling water channel provided on the back surface of the mold is controlled to a predetermined range. This control allegedly makes it possible to reduce the occurrence of surface cracks in the slab and to retard the shortening of mold life due to the cracking on the mold surface.

[0008] Patent Literature 2 discloses a continuous casting mold in which dissimilar material-filled portions are formed by filling a mold plate with a dissimilar material having a different thermal conductivity from a copper alloy constituting the mold plate. In this continuous casting mold, the mold plate has cooling water channels on its back surface and water flow disrupting portions are disposed on a region of the cooling water channels corresponding to the dissimilar material-filled portions, so as to disrupt the water flow. This configuration allegedly can reduce the occurrence of surface cracks in the slab and can extend the mold life.CITATION LISTPatent Literature

[0009] PTL 1: Japanese Unexamined Patent Application Publication No. 2017-39165

[0010] PTL 2: International Publication No. 2020 / 095932SUMMARYTechnical Problem

[0011] The present inventors carried out casting tests using filled molds in which the mold plates disclosed in Patent Literature 1 or 2 was filled with Ni on the molten steel side, and confirmed that the filled molds were highly effective in reducing the occurrence of longitudinal surface cracks in medium carbon steels. However, the present inventors have found that when the filled molds are used on a high-speed continuous casting machine, the temperature of the mold plate surface is steadily above 300° C. when the casting speed is as high as 2.5 m / min or more. When the temperature of the mold plate surface is steadily above 300° C., abnormalities, such as coating separation and deformation, occur on the mold plates to make it impossible to use the mold continuously.

[0012] The present application has been made in view of the problems discussed above. It is therefore an object of the present application to provide a continuous steel casting mold that enables continuous casting while suppressing the occurrence of surface cracks on the slab and the occurrence of abnormalities on the mold plate surface even under 2.5 m / min or higher casting speed conditions. Another object of the present application is to provide a continuous steel casting method that involves the continuous steel casting mold.Solution to Problem

[0013] The present application solves the problems described above through the following.[1] A continuous steel casting mold including a mold plate including a copper alloy, the mold plate having a front surface defining an inner wall surface of the mold, and a back surface defining cooling water channels; and a backup plate attached to the mold plate so as to cover the cooling water channels, wherein a region of the front surface of the mold plate including at least a meniscus has a plurality of recesses each defining a dissimilar material-filled portion in which the recess is filled with a dissimilar material having a thermal conductivity different from a thermal conductivity of the mold plate, and a total sectional area of the cooling water channels on the back surface of the mold plate in a range including the region having the dissimilar material-filled portions is smaller than a total sectional area of the cooling water channels below the range.[2] The continuous steel casting mold according to [1], wherein the dissimilar material-filled portions are disposed so that a heat flux passing from the inner wall surface of the mold to each cooling water channel changes periodically in the region of the front surface of the mold plate.[3] The continuous steel casting mold according to [1] or [2], wherein the dissimilar material-filled portions and the cooling water channels are disposed so as to satisfy at least one of expressions (1) to (3) below:d<P≤S(1)e≤L≤1⁢0⁢0⁢0×Vc / f(2)A⁢1≤0.8×A⁢2(3)wherein d is a width (mm) of each dissimilar material-filled portion in a mold width direction, P is a spacing (mm) in the mold width direction between adjacent dissimilar material-filled portions, S is a spacing (mm) in the mold width direction between adjacent cooling water channels among the cooling water channels disposed on the back surface of the mold plate, e is a width (mm) of each dissimilar material-filled portion in a slab withdrawal direction, L is a spacing (mm) in the slab withdrawal direction between adjacent dissimilar material-filled portions, Vc is a slab withdrawal speed (m / min) in a continuous steel casting process, f is an oscillation frequency (1 / min) of the continuous casting mold in the continuous steel casting process, A1 is the total sectional area (mm2) of the cooling water channels in the range including the region, and A2 is the total sectional area (mm2) of the cooling water channels below the above range.[4] The continuous steel casting mold according to any one of [1] to [3], wherein a coating layer is formed on the front surface of the mold plate so as to cover the dissimilar material-filled portions.[5] The continuous steel casting mold according to any one of [1] to [4], wherein the mold plate is a mold plate among a pair of longer mold plates and a pair of shorter mold plates, and the continuous steel casting mold has a common pump that supplies cooling water to the longer mold plates and the shorter mold plates, and a boost pump that supplies cooling water to at least the shorter mold plates.[6] The continuous steel casting mold according to any one of [1] to [5], wherein the mold plate has a funnel shape, and the dissimilar material-filled portions are arranged also in a 50 mm region extending across and along a boundary of the funnel shape in a range from 0 mm to 200 mm below the meniscus.[7] A continuous steel casting method using the continuous steel casting mold described in any one of [1] to [6], wherein continuous casting at a casting speed of 2.5 m / min or more brings a temperature of the front surface of the mold plate at the meniscus up to 350° C. or below.[8] A continuous steel casting method using the continuous steel casting mold described in any one of [1] to [6], the method including estimating a temperature of the front surface of the mold plate from a temperature measured with a thermocouple embedded in a region of the mold plate from 20 mm to 100 mm below the meniscus, and controlling a flow rate of cooling water and the total sectional area of the cooling water channels on the back surface of the mold plate in the region having the dissimilar material-filled portions so that the estimated temperature of the front surface of the mold plate is 350° C. or below.[9] A continuous steel casting method using the continuous steel casting mold described in any one of [1] to [6], wherein a mold powder having a basicity of less than 1.2 is used.

[10] A continuous steel casting method using the continuous steel casting mold described in any one of [1] to [6], wherein cooling water is passed in a direction from an upper end of the mold plate toward a lower end of the mold plate.

[11] A continuous steel casting method using the continuous steel casting mold described in any one of [1] to [6], wherein a slab that is produced has a thickness of 150 mm or less.Advantageous EffectsIn the continuous steel casting mold according to the present application, the total sectional area of the cooling water channels in a range that includes a region having the dissimilar material-filled portions is made smaller than the total sectional area of the cooling water channels below that range. In the cooling water channels with a smaller total sectional area, the heat transfer coefficient between the water flow and the cooling water channels is increased to allow a larger amount of heat to be convectively transferred. Thus, the mold plate can be cooled by effectively removing heat through the region of the mold plate having the dissimilar material-filled portions. This effective cooling of the dissimilar material-filled portions and the mold plate can reduce the thermal stress that occurs at the boundary between the mold plate and the dissimilar material-filled portions. As a result, surface cracking of a slab of a type of steel that involves peritectic reaction can be suppressed, and continuous casting can be performed while suppressing coating separation or copper plate deformation even under 2.5 m / min or higher casting speed conditions. Furthermore, the protection from coating separation and copper plate deformation can extend the life of the mold having the dissimilar material-filled portions.BRIEF DESCRIPTION OF DRAWINGS

[0016] FIG. 1 is a perspective view illustrating an exemplary continuous steel casting mold according to an embodiment.

[0017] FIG. 2 is a schematic view illustrating an example of the front surface of a mold plate constituting a long side of the mold according to the present embodiment.

[0018] FIG. 3 is a set of views illustrating the structure of the mold plate 22 with respect to the region C enclosed by a square in FIG. 2.

[0019] FIG. 4 is a set of a sectional view of a conventional mold plate cut in the casting direction, and a front view of the conventional mold plate.

[0020] FIG. 5 is a set of a sectional view taken along line U-U and a sectional view taken along line V-V in FIG. 4.

[0021] FIG. 6 is a set of a sectional view of the mold plate of the mold according to the present embodiment cut in the casting direction, and a front view of the mold plate.

[0022] FIG. 7 is a set of a sectional view taken along line U-U and a sectional view taken along line V-V in FIG. 6.

[0023] FIG. 8 is a graph illustrating relationships between the distance from the mold upper end and the mold plate surface temperature.

[0024] FIG. 9 is a set of a sectional view of the mold plate cut in the casting direction, and a front view of the mold plate.

[0025] FIG. 10 is a schematic view illustrating an exemplary installation of pumps for supplying cooling water to the mold.

[0026] FIG. 11 is a graph illustrating relationships between the total pumped water volume and the pump source pressure.

[0027] FIG. 12 is a schematic view illustrating an exemplary installation of pumps for supplying cooling water to the mold.

[0028] FIG. 13 is a schematic view illustrating a mold used in Inventive Example 36.

[0029] FIG. 14 is a schematic view illustrating a mold used in Comparative Example 30.DESCRIPTION OF EMBODIMENTS

[0030] The present application will be described below through an embodiment of the present application. FIG. 1 is a perspective view illustrating an exemplary continuous steel casting mold 10 according to the present embodiment. The continuous steel casting mold 10 (hereinafter, sometimes written simply as the “mold 10”) used for continuous casting of a slab has a pair of opposing long sides of mold 12 and a pair of opposing short sides of mold 14 that are sandwiched between the long sides of mold 12. A tundish (not shown) containing a molten steel 16 is arranged above the mold 10, and a submerged nozzle 18 is connected to the bottom of the tundish. The pair of long sides of mold 12 and the pair of short sides of mold 14 form a rectangular internal space in the mold 10, and the submerged nozzle 18 is inserted into this internal space. The long sides of mold 12 and of the short sides of mold 14 are composed of mold plates made of a copper alloy on their sides that come into contact with the molten steel 16, and backup plates are arranged on the back of these mold plates.

[0031] In the copper alloy mold plates that constitute the long sides of mold 12 and the short sides of mold 14, cooling water channels are disposed on the back surface opposite from the surface that is brought into contact with the molten steel 16. The mold 10 is cooled by passing cooling water through the cooling water channels. In the operation of continuous steel casting, the molten steel 16 is poured into the internal space of the mold 10 through the submerged nozzle 18, and the molten steel 16 is cooled and solidified with the mold 10 to form a solidifying shell on the surface of the mold 10 in contact therewith. The slab including the solidifying shell as the outer shell and the unsolidified molten steel 16 inside the solidifying shell is continuously withdrawn in the slab withdrawal direction A, that is, a vertically downward direction, thus giving a steel slab. In the mold 10, the mold plates are in contact with the molten steel 16 and the high-temperature slab. Due to this contact, the surface temperature of the mold plates (the temperature on the side in contact with the molten steel) is increased and is highest near the position of a meniscus M in the mold (the level of the molten steel surface in the mold). In FIG. 1, the position of the meniscus M is indicated by a dash-dot line.

[0032] Although it depends on the type of steel, it is preferable that heat be removed from the solidifying shell uniformly in the slab withdrawal direction A and the mold width direction B at the position of the meniscus M in the mold. This uniform removal of heat from the solidifying shell in the slab withdrawal direction A and the mold width direction B can promote uniform thickening of the solidifying shell. Here, the slab withdrawal direction A and the mold width direction B are orthogonal to each other. The mold plates are preferably made of a copper alloy that has high resistance to thermal stress deformation and has high thermal conductivity allowing for high cooling effects of the cooling water.

[0033] A plurality of slab support rolls (not shown) are arranged below the mold 10, and water spray nozzles or air mist spray nozzles are disposed between adjacent slab support rolls. While the cooling water is sprayed onto the slab surface through the water spray nozzles or the air mist spray nozzles to cool the slab, the slab supported by the slab support rolls is withdrawn. After the completion of solidification to the center of the slab, the slab is cut into a predetermined length. Slabs with a predetermined length to be subjected to hot rolling in the next step are thus produced.

[0034] In the mold 10 according to the present embodiment, the total sectional area of the cooling water channels that cool the region of the mold plates where the dissimilar material-filled portions are disposed is made smaller than the total sectional area of the cooling water channels below the region. This configuration increases the linear velocity of the cooling water in the region having the dissimilar material-filled portions, and the heat transfer coefficient in the region is increased to allow a larger amount of heat to be convectively transferred. Thus, heat can be effectively removed from the dissimilar material-filled portions and the mold plates. As a result, the dissimilar material-filled portions and the vicinities thereof in the mold plates are effectively cooled and the thermal stress that occurs at the boundaries between the mold plates and the dissimilar material-filled portions is reduced. In this manner, the mold 10 is extended in life and the casting speed can be increased.

[0035] FIG. 2 is a schematic view illustrating an example of the front surface of a mold plate 22 constituting the long sides of mold 12 of the mold 10 according to the present embodiment. Each of the long sides of mold 12 and the short sides of mold 14 constituting the mold 10 includes a mold plate 22 that has a front surface defining an inner wall surface of the mold, and a back surface defining cooling water channels; and a backup plate attached to the mold plate 22 with bolts and nuts.

[0036] On the front surface of the mold plate 22, a region including the meniscus M has recesses, and dissimilar material-filled portions 20 independent from one another are formed by filling of the recesses with a dissimilar material having a thermal conductivity different from that of the mold plate 22. The dissimilar material-filled portions 20 are arranged in the slab withdrawal direction A and the mold width direction B in the vicinity of the meniscus M including at least the meniscus M. The dissimilar material-filled portions 20 may be formed by shaping the dissimilar material into a shape corresponding to the recesses and fitting the shapes into the recesses, or may be formed by filling the recesses with the dissimilar material by, for example, plating or thermal spraying. By filling the recesses with the dissimilar material by plating, thermal spraying, or the like, the occurrence of gaps between the recesses and the dissimilar material can be prevented.

[0037] It is preferable that the dissimilar material-filled portions 20 be regularly arranged on the front surface of the mold plate 22 so that the heat flux passing from the mold inner wall surface to each cooling water channel will be increased and decreased periodically at the mold inner wall surface.

[0038] As a result of the dissimilar material-filled portions 20 being arranged on the front surface of the mold plate 22 including the vicinity of the meniscus M, the region of the mold plate 22 including the vicinity of the meniscus M shows regular and periodic changes in thermal resistance in the slab withdrawal direction A and the mold width direction B. Consequently, the heat flux passing from the solidifying shell to the mold plate 22 in the vicinity of the meniscus M, that is, in the early stage of solidification increases and decreases regularly and periodically. The regular and periodic changes in heat flux reduce thermal stress and the stress generated by 8 iron to y iron transformation, and consequently the deformation of the solidifying shell caused by these stresses is reduced. Small deformation of the solidifying shell improves the non-uniformity in heat flux distribution caused by the deformation of the solidifying shell, and the stress that is generated is dispersed to produce small individual strains. As a result, the occurrence of longitudinal cracks on the surface of the solidifying shell is suppressed.

[0039] The recesses are not necessarily circular recesses that have a perfect circular shape on the front surface of the mold plate 22 and may be quasi-circular recesses having a quasi-circular shape. The quasi-circular shape is a shape having no sharp corners, such as, for example, an elliptical shape or a square or rectangular shape with circular or elliptical corners. Alternatively, the recesses may have a petal-like shape.

[0040] In order to ensure that the heat flux at the mold inner wall surface will change periodically, it is preferable that the distances between any adjacent dissimilar material-filled portions 20 be the same. The thermal conductivity of the dissimilar material is preferably 80% or less or 125% or more of the thermal conductivity of the mold plate 22. The thermal conductivity of the dissimilar material varies with the atmosphere temperature. Thus, the thermal conductivity of the dissimilar material and that of the mold plate assume that the temperature is room temperature (ambient temperature) at the time of mold production. When the thermal conductivity of the dissimilar material at room temperature differs by about 20% from the thermal conductivity of the mold plate 22, the heat flux passing through the mold inner wall surface is allowed to change regularly and periodically so that thermal stress and the stress generated by 8 iron to y iron transformation can be reduced. As long as the above stresses, such as the stress generated by the transformation, can be reduced and the occurrence of surface cracking in the slab can be prevented, the thermal conductivity of the dissimilar material may be outside the above range and the distances between any dissimilar material-filled portions 20 may be different from one another.

[0041] Examples of the dissimilar materials having a thermal conductivity that is 80% or less of the thermal conductivity of the mold plate 22 include Ni (thermal conductivity: about 90 W / (m×K)) and Ni alloys (thermal conductivity: about 40 to 90 W / (m×K)) that can be easily plated or thermally sprayed. For the mold plate 22, a copper alloy (thermal conductivity: about 100 to 385 W / (m×K)) may be used and, for example, a highly thermally conductive copper alloy (thermal conductivity: about 318 W / (m×K)) or a low-thermal conductivity copper alloy (thermal conductivity: about 119 to 239 W / (m×K)) for electromagnetic stirring may be used. However, the dissimilar material and the mold plate 22 may be metals other than Ni alloys and copper alloys.

[0042] The mold plate 22 may be pure copper (thermal conductivity: about 398 W / (m×K)) or the copper alloy described above. When, in particular, the molten steel in the mold is electromagnetically stirred, the attenuation of the intensity of the magnetic field from the coil to the molten steel is preferably avoided by using a copper alloy that contains several percent by mass of a component other than the copper component and thereby exhibits a lower electric conductivity. The thermal conductivity of a copper alloy is lower than pure copper. Based on this fact, the thermal conductivities of the dissimilar material and of the mold plate 22 are preferably controlled by appropriate selection of the dissimilar material and / or the material of the long sides of mold 12 depending on the use application of the mold 10.

[0043] Dissimilar material-filled portions 20 similar to those on the long sides of mold 12 may be formed also on the front surface of the short sides of mold 14. Due to the shape of a slab, however, stress concentration in a slab tends to occur on the sides of the solidifying shell next to the long sides of mold 12, and surface cracks are likely to occur on the surface facing the long sides. Thus, the mold 10 according to the present embodiment is necessarily provided with the dissimilar material-filled portions 20 on the long sides of mold 12, but the short sides of mold 14 do not necessarily have the dissimilar material-filled portions 20.

[0044] In consideration of the impact on initial solidification, it is preferable that the dissimilar material-filled portions 20 be provided in a region that extends from a position remote from the meniscus M in steady casting operation upward by a distance Q to a position remote from the meniscus M downward by a distance R. The distance Q is an appropriate value greater than zero. The distance R can be calculated from expression (4) below:R=2×Vc×1000 / 60(4)

[0045] In the above expression (4), R is the distance (mm), and Vc is the slab withdrawal speed (m / min) in the continuous steel casting process.

[0046] The distance R is related to the time in which the solidifying shell after the start of solidification passes through the region having the dissimilar material-filled portions 20. It is preferable that the solidifying shell stay in the region having the dissimilar material-filled portions 20 for at least 2 seconds after the start of solidification. In order to ensure that the solidifying shell will stay in the region having the dissimilar material-filled portions 20 for at least 2 seconds after the start of solidification, the dissimilar material-filled portions 20 are to be disposed to a location that is below the meniscus M by at least the distance R determined from expression (4).

[0047] By allowing the slab to stay in the region having the dissimilar material-filled portions 20 for at least 2 seconds after the start of solidification, the dissimilar material-filled portions 20 can produce sufficient effects by giving rise to periodic changes in heat flux passing from the mold inner wall surface to each cooling water channel. Specifically, a residence time of at least 2 seconds for the solidifying shell to stay in the region having the dissimilar material-filled portions 20 ensures effective suppression of surface cracking in the slab even at a high casting speed or in the casting of medium carbon steel where surface cracking is likely to occur. In order to obtain the effects of the periodic changes in heat flux stably by the dissimilar material-filled portions 20, it is more preferable to ensure 4 or more seconds for the solidifying shell to pass through the region having the dissimilar material-filled portions 20. In the case of a continuous thin-slab casting machine, the slab withdrawal speed is high and the distance R is extended. Thus, the dissimilar material-filled portions 20 need to be provided in a larger region in the slab withdrawal direction A. This increases the mold processing cost. Even in this case, at least 1 second is to be ensured for the solidifying shell to pass through the dissimilar material-filled portions 20. In this manner, the effect of the periodic changes in heat flux corresponding to the amount of time can be obtained.

[0048] The position of the upper end of the region having the dissimilar material-filled portions 20 is not particularly limited as long as the upper end is above the meniscus M. That is, the distance Q may be any value greater than zero. In view of the fact that the meniscus M fluctuates vertically during casting, however, it is preferable that the dissimilar material-filled portions 20 be arranged up to 20 mm above the meniscus M so that the upper end of the region having the dissimilar material-filled portions 20 will be always above the meniscus M. The dissimilar material-filled portions 20 are more preferably arranged up to 40 mm above the meniscus M. The position of the meniscus M is generally 60 to 150 mm below the upper end of the mold plates 22, and the region in which the dissimilar material-filled portions 20 are formed may be determined appropriately in accordance with this position.

[0049] In continuous casting of steel, hot molten steel is poured into the internal space of the mold and the temperature of the mold plates 22 is increased. Thus, cooling water channels are formed in the mold plates 22 constituting the long sides of mold 12 and the short sides of mold 14, and cooling water is passed through the cooling water channels to cool the mold plates 22. However, because the thermal expansion coefficient of the dissimilar material-filled portions 20 differs from the thermal expansion coefficient of the long sides of mold 12, thermal stress is concentrated at their boundaries and may cause cracking on the front surface of the mold plates 22.

[0050] To address the above phenomenon, the mold 10 according to the present embodiment is configured such that the total sectional area of the cooling water channels in a range of the mold plate 22 that includes the region having the dissimilar material-filled portions 20 is smaller than the total sectional area of the cooling water channels below that range. By reducing the sectional area of the water channels in the region having the dissimilar material-filled portions 20, the linear velocity of the cooling water in the water channels is increased and consequently the heat transfer coefficient between the cooling water and the wall surface of the copper plate slits is increased. As a result, heat is removed in an accelerated manner from the region of the mold plate 22 having the dissimilar material-filled portions 20, and the region can be cooled effectively.

[0051] Next, the dissimilar material-filled portions 20 and the cooling water channels will be described. FIG. 3 is a set of views illustrating the structure of the mold plate 22 with respect to the region C enclosed by a square in FIG. 2. In FIG. 3, (a) is a plan view illustrating the front surface of the mold plate 22, (b) is a plan view illustrating the back surface of the mold plate 22, (c) is a vertical sectional view of the region, and (d) is a horizontal sectional view of the region. As illustrated in FIGS. 3(b) and (d), cooling water channels 26 are disposed on the back surface of the mold plate 22. As illustrated in FIGS. 3(c) and (d), a backup plate 30 is attached to the back surface of the mold plate 22 so as to cover the cooling water channels 26.

[0052] FIG. 4 is a set of a sectional view (a) of a conventional mold plate 40 cut in the casting direction, and a front view (b) of the conventional mold plate 40. FIG. 5 is a set of a sectional view (a) taken along line U-U and a sectional view (b) taken along line V-V in FIG. 4. FIG. 6 is a set of a sectional view (a) of the mold plate 22 of the mold 10 according to the present embodiment cut in the casting direction, and a front view (b) of the mold plate 22. FIG. 7 is a set of a sectional view (a) taken along line U-U and a sectional view (b) taken along line V-V in FIG. 6.

[0053] As illustrated in FIGS. 6 and 7, the mold 10 according to the present embodiment differs from the conventional mold plate 40 in that a spacer 28 is installed between the cooling water channels 26 in the region having the dissimilar material-filled portions 20, and the backup plate 30. As a result of installing the spacer 28, the total sectional area of the cooling water channels 26 in that region becomes smaller than the total sectional area below the region. The spacer 28 is to be installed in a range that includes the region having the dissimilar material-filled portions 20.

[0054] When, in particular, casting is performed at a high casting speed of 2.5 m / min or more, it is preferable to further reduce the total sectional area of the cooling water channels 26 and thereby increase the linear velocity of the cooling water. If the solidifying shell does not grow sufficiently in the mold, the solidifying shell cannot withstand the static pressure of the molten steel at the lower end of the mold and a breakout occurs in the worst case. The above countermeasure prevents this steel leakage problem. When a low-thermal conductivity material is used for the dissimilar material-filled portions 20, the amount of heat removed from the mold is smaller than the amount of heat removed through the conventional mold plates 40. This gives rise to a concern that the solidifying shell may not be formed sufficiently. The thickness of the solidifying shell can be calculated from the following expression (5):z_shell=K×t⁢s1 / 2=K×(z_m / Vc)1 / 2(5)

[0055] In the above expression (5), z_shell is the thickness (mm) of the solidifying shell, K is the solidification constant (20 to 27 mm / min0.5), ts is the solidification time (min), z_m (m) is the distance from the meniscus M in the casting direction, and Vc is the casting speed (m / min).

[0056] As understood from expression (5), the impact on the growth of the solidifying shell is increased with decreasing distance from the meniscus M. Thus, the solidifying shell is allowed to grow to a larger thickness by increasing the solidification constant K of the dissimilar material-filled portions 20 that are close to the meniscus M.

[0057] Copper alloy is generally used for the mold plates 22. When the temperature exceeds 350° C., a copper alloy is softened and is deformed easily due to a significant decrease in yield stress. The outermost surface of the mold plate 22 that will come next to the molten steel is generally plated with a Ni—Co or Ni—Cr coating layer 32. However, the hardness of Ni alloys is significantly lowered when the temperature exceeds 350° C.

[0058] As described above, heat removal is insufficient when casting is performed at a high speed or when a low-thermal conductivity material is used for the dissimilar material-filled portions 20, and, if the surface temperature of the mold plate 22 exceeds 350° C., separation or cracks tend to occur on the copper plate surface or the coating layer surface. In contrast, the mold 10 according to the present embodiment involves the spacer 28 to increase the linear velocity of the cooling water in the region having the dissimilar material-filled portions 20. This configuration increases the heat transfer coefficient between the cooling water and the cooling water channels 26 to allow a larger amount of heat to be convectively transferred. Thus, the mold plate 22 and the dissimilar material-filled portions 20 can be effectively cooled by effectively removing heat from the region of the mold plate 22 having the dissimilar material-filled portions 20. As a result, the slab can be prevented from surface cracking, and separation or cracking on the front surface of the mold plate 22 and the coating layer 32 can be suppressed.

[0059] The linear velocity of the cooling water in the cooling water channels 26 is preferably 7.0 m / sec or more. At this linear velocity, the Reynolds number Re of the water in the water channels is about 40000 and the water flow is turbulent (Re>2300). To increase the local heat removal near the meniscus M, the linear velocity of the cooling water may be controlled to 10.0 m / sec or more. In principle, cooling water at a linear velocity of 10.0 m / sec or more can effectively lower the surface temperature of the mold plate 22 by about 30° C.

[0060] Increasing the linear velocity of the cooling water requires increasing the water supply pressure. When the water supply pressure is raised, a larger pressure loss occurs in each water channel. This results in a greater variation in the linear velocity of the cooling water among many slits. Thus, if the linear velocity is excessively increased, cooling produces a large difference between the results of cooling at hardly cooled regions near the stud bolts and at easily cooled regions, giving rise to a concern that longitudinal cracks will be facilitated.

[0061] If the linear velocity of the cooling water is so increased that a large difference in linear velocity is produced among the slits, the solidifying shell near the meniscus M may rise to have fine cracks that will grow to longitudinal cracks. If the linear velocity of the cooling water remains fast even in the lower part, the risen portion of the solidifying shell is subjected to continuous tensile stress in the width direction and the cracks may be widen to longitudinal cracks or grow to deep depressions. When, in contrast, the sectional area is increased for the cooling water channels 26 located below the region from the upper end of the mold to about 200 mm below the meniscus M, the linear velocity of the cooling water is slowed down. This advantageously reduces the tensile stress and prevents cracks from being widened to longitudinal cracks or growing to deep depressions. Thus, high-speed casting and the cooling of the surface temperature of the mold plates 22 benefit most effectively from the configuration in which the cooling water flows at a high linear velocity in a range that includes the region extending from the vicinity of the meniscus M to the end of the dissimilar material-filled portions 20, and the linear velocity below that range is slowed down.

[0062] The present inventors have confirmed that longitudinal cracks tend to occur markedly in a slab when a general copper alloy mold having no dissimilar material-filled portions 20 across the meniscus M is cooled at a linear velocity of 12 m / sec or more. The present inventors have also confirmed that the occurrence of longitudinal cracks in the slab tends to be suppressed when this mold is modified so that the sectional area of cooling water channels 26 in the vicinity of the meniscus M is made smaller than that below the vicinity. Furthermore, it is preferable that the mold satisfy the relationship A1≤0.8×A2 where A1 is the total channel area of the cooling water channels 26 in a range of the back surface of the mold plate that includes the region having the meniscus M and the dissimilar material-filled portions 20, and A2 is the total sectional area of the cooling water channels 26 below that range. The present inventors have confirmed that by adopting this configuration, a slab can be cast while suppressing the occurrence of severe longitudinal cracks with a depth of 2 mm or more not only when the mold 10 according to the present embodiment is used but also when using a conventional mold.

[0063] The present inventors have also confirmed that when the cooling water channels 26 satisfy the above expression regarding the total area, the mold 10 according to the present embodiment that has the dissimilar material-filled portions 20 across the meniscus M does not cause longitudinal cracks even when a slab is produced at a high casting speed of 2.5 m / min or more.

[0064] In the mold 10 according to the present embodiment, it is preferable that the dissimilar material-filled portions 20 and the cooling water channels 26 be formed in the mold plate 22 so as to satisfy at least one of the conditions in the following expressions (1) to (3):d<P≤S(1)e≤L≤1⁢0⁢0⁢0×Vc / f(2)A⁢1≤0.8×A⁢2(3)

[0065] In expressions (1) to (3), d is the width (mm) of each dissimilar material-filled portion 20 in the mold width direction B; P is the spacing (mm) in the mold width direction B between adjacent dissimilar material-filled portions 20; S is the spacing (mm) in the mold width direction B between adjacent cooling water channels 26 among the cooling water channels 26 disposed on the back surface of the mold plate 22; e is the width (mm) of each dissimilar material-filled portion 20 in the slab withdrawal direction A; L is the spacing (mm) in the slab withdrawal direction A between adjacent dissimilar material-filled portions 20; Vc is the slab withdrawal speed (m / min) in the continuous steel casting process; f is the oscillation frequency (1 / min) of the mold 10 in the continuous steel casting process; A1 is the total sectional area (mm2) of the cooling water channels 26 in a range of the back surface of the mold plate 22 that includes the region having the meniscus M and the dissimilar material-filled portions 20; and A2 is the total sectional area (mm2) of the cooling water channels 26 below the above range. The term “spacing” indicates the center-to-center distance between two adjacent portions in the slab withdrawal direction A or in the width direction B of the mold plate 22 (see FIG. 3).

[0066] Next, the total sectional area A1 and the total sectional area A2 will be described. The boundary between the total sectional area A1 and the total sectional area A2 resides between a position including the dissimilar material-filled portions 20 and a position including no dissimilar material-filled portions 20. That is, the range of the total sectional area A1 may include, as part thereof, a region having no dissimilar material-filled portions 20 as long as the range includes the region having the dissimilar material-filled portions 20. It is, however, preferable that the range of the total sectional area A1 be the region having the dissimilar material-filled portions 20. The total sectional area A1 and the total sectional area A2 are each the sum of the sectional areas of the respective slits. The linear velocity of the cooling water in the cooling water channels 26 may be calculated by dividing the volume flow rate by the total sectional area.

[0067] In the mold 10 according to the present embodiment, the dissimilar material-filled portions 20 are provided on the front surface of the copper plate near the meniscus. Because of this, the region having the dissimilar material-filled portions 20 is cooled about 10% weakly compared to the region where there are no dissimilar material-filled portions 20. Thus, the mold 10 according to the present embodiment is configured so that the total sectional area A1 is 20% smaller than the total sectional area A2. This configuration increases the linear velocity of the cooling water in the region having the dissimilar material-filled portions 20 by about 20%, which enhances the heat transfer from the slits. Thus, the region having the dissimilar material-filled portions 20 can be constantly cooled on the same level as the conventional cooling of a region having no dissimilar material-filled portions 20.

[0068] The mold plate 22 is cooled with the cooling water flowing through the cooling water channels 26 on the back surface of the mold plate 22. Thus, the manner in which heat is removed from the mold plate 22 through the cooling water channels 26 is radial. This gives rise to uneven cooling of the front surface of the mold plate 22 between portions close to the cooling water channels 26 and portions remote from the cooling water channels 26. In order to ensure that thermal stress and the stress generated by & iron to y iron transformation will be reduced more effectively by the periodic changes in thermal resistance offered by the dissimilar material-filled portions 20, it is preferable to cause the heat flux to vary at smaller intervals than the spacings S between the cooling water channels 26. This is preferably achieved by satisfying the above-described expression (1). That is, the spacing P between the dissimilar material-filled portions 20 in the mold width direction B is preferably made equal to or less than the spacing S between the cooling water channels 26, and the width d of the dissimilar material-filled portions 20 is preferably smaller than the spacing P.

[0069] The width d of the dissimilar material-filled portions 20 is preferably 2 mm or more and 20 mm or less. When the dissimilar material-filled portions 20 are quasi-circular, the equivalent circular diameter determined from expression (6) below may be used as the width d.Equivalent⁢ circular⁢ diameter=(4×S / ∏)1 / 2(6)

[0070] In expression (6), S is the area (mm2) of the dissimilar material-filled portion 20.

[0071] When the width d or the equivalent circular diameter is 2 mm or more, the circular or quasi-circular recesses can be easily filled with the dissimilar material by plating or thermal spraying. When, on the other hand, the width d or the equivalent circular diameter is 20 mm or less, the reduction in heat flux by the dissimilar material-filled portions 20 will not cause a delay in solidification and the solidifying shell at the corresponding positions is prevented from stress concentration and will not suffer surface cracking.

[0072] When the molten steel 16 is poured into the mold 10, the mold 10 is oscillated while adding a mold powder onto the surface of the molten steel 16 in order to prevent sticking of the molten steel 16 to the mold 10. It is known that such oscillations leave periodic oscillation marks on the surface of the slab in the slab withdrawal direction A, and the slab thickness tends to vary periodically in the slab withdrawal direction A.

[0073] Transverse cracks in the slab can be suppressed by ensuring that the width of the dissimilar material-filled portions 20, the spacing between adjacent dissimilar material-filled portions 20, the slab withdrawal speed, and the oscillation frequency f of the mold satisfy the above-described expression (2). Specifically, transverse cracks in the slab can be suppressed by ensuring that the width of the dissimilar material-filled portions 20 in the slab withdrawal direction A is smaller than the length of one cycle (pitch) of the change in slab thickness in the slab withdrawal direction A caused by oscillation marks.

[0074] As the mold 10 oscillates, each cycle of oscillation forms one concave or depression in the slab. This depression is an oscillation mark. At a casting speed Vc and a mold oscillation frequency f, oscillation marks are formed at pitches of 1000×Vc / f. In view of the fact that the oscillation of the mold 10 generates oscillation marks at pitches of 1000×Vc / f, avoiding strain or stress concentration at these locations is significantly important to suppress the occurrence of transverse cracks. Thus, the mold 10 according to the present embodiment is preferably such that the dissimilar material-filled portions are provided at pitches of 1000×Vc / f or less. This makes it possible to avoid artificial thermal stress acting on valleys of the oscillation marks. As a result, fine cracks are prevented from occurring on valleys of the oscillation marks during the initial stage of solidification, and there will be no or little expansion or propagation of cracks in the downstream secondary cooling zone.

[0075] In the mold 10 according to the present embodiment, it is preferable that the dissimilar material-filled portions 20 be formed in the mold plate 22 so as to satisfy the condition in the following expression (7):0.5≤t≤d(7)

[0076] In expression (7), t is the filling thickness (mm) of the dissimilar material in the dissimilar material-filled portions 20, and d is the width (mm) of the dissimilar material-filled portions 20 in the mold width direction B.

[0077] If the filling thickness of the dissimilar material-filled portions 20 (see FIG. 3(d)) is less than 0.5 mm, the dissimilar material-filled portions 20 may fail to produce sufficient amounts of changes in heat flux. If, on the other hand, the filling thickness is excessively large, it becomes difficult for the dissimilar material to fill the recesses. Thus, the filling thickness is preferably equal to or less than the width d (mm) of the dissimilar material-filled portions in the mold width direction. The filling thickness t is preferably 10 mm or less. If the filling thickness is more than 10 mm, the dissimilar material encounters difficulty in filling the recesses. Furthermore, if the dissimilar material has a lower thermal conductivity than the mold plate 22 and the filling thickness is more than 10 mm, the amount of heat removed from the mold plate 22 is reduced. This causes an increase in the surface temperature of the mold plate 22 and, if the surface temperature exceeds 350° C., abnormalities will occur in the copper plate or the coating layer. Thus, it is preferable that the filling thickness be 10 mm or less.

[0078] Preferably, as illustrated in FIG. 6, a coating layer 32 is formed on the front surface of the mold plate 22 so as to cover the dissimilar material-filled portions 20. This provides protection from wear by the solidifying shell or cracking on the mold surface due to thermal history. The coating layer 32 may be formed by plating or thermal spraying of a commonly used material, such as nickel or a nickel-containing alloy, for example, nickel-cobalt alloy (Ni—Co alloy) or nickel-chromium alloy (Ni—Cr alloy).

[0079] The cooling intensity (the heat transfer coefficient) of the mold 10 is determined by the linear velocity of the cooling water flowing through the cooling water channels 26. Thus, high-speed casting preferably involves increasing the linear velocity of the cooling water to 7.0 m / sec or more. The average linear velocity of the cooling water is determined from the following expression (8). In order to increase the linear velocity of the cooling water to 7.0 m / sec or more, it is important to appropriately design the volume flow rate of the cooling water and the cooling water channels 26.Average⁢ linear⁢ velocity⁢ of⁢ cooling⁢ water⁢ (m / sec)=volume⁢ flow⁢ rate⁢ of⁢ cooling⁢ water⁢ (m3 / sec) / total⁢ sectional⁢ area⁢ (m2)⁢ of⁢ cooling⁢ water⁢ channels(8)

[0080] When molten steel is solidified through the mold 10 in continuous casting of steel, the amount of heat removed from the mold 10 can be estimated from the difference between the temperatures of the cooling water entering and leaving the mold and the temperature of thermocouples embedded in the copper mold plates. The amount of heat removed from the mold 10 reaches the maximum in the range from 20 mm to 80 mm below the meniscus M in the casting direction and tends to decrease gradually thereafter.

[0081] FIG. 8 is a graph illustrating relationships between the distance from the mold upper end and the surface temperature of the mold plate 40. The surface temperature of the copper mold plate is shown to change in a manner corresponding to the amount of heat removal described above. As illustrated in FIG. 8, the surface temperature of the mold plate 40 reaches the maximum at the peak top temperature point immediately below the meniscus M. If the surface temperature of the mold plate 40 exceeds 350° C., the copper plate reaches its yield stress and the hardening point of the surface coating, which gives rise to a concern that the coating layer may separate from the mold. As illustrated in FIG. 8, this curve shifts to the higher temperature side as the casting speed is increased. Thus, if it is expected that the surface temperature of the mold plate will exceed 350° C., the linear velocity of the cooling water needs to be increased by increasing the volume flow rate of the cooling water or reducing the total sectional area of the cooling water channels 26.

[0082] If the material embedded in the recesses is of low thermal conductivity, the temperature increase at the filled portions will be greater. The surface temperature of the mold plate varies depending on the filling depth of the low-thermal conductivity material, the type of material of the mold plate, and the thickness of the mold plate. It is therefore necessary to give attention to the surface temperature of the mold plate at the time of high-speed casting.

[0083] As illustrated in FIG. 8, the surface temperature of the mold plate 40 reaches the maximum near the meniscus M and then shows a temperature distribution that is relatively low and gentle. Thus, the risk of separation or damages on the copper plates or the coating layers tends to reside mainly in the region extending up to about 200 mm below the meniscus M. It is therefore preferable to increase the amount of heat removed in this region so as to keep the surface temperature of the mold plate at 350° C. or below.

[0084] In the mold 10 according to the present embodiment, the total sectional area of the cooling water channels 26 in a range of the back surface of the mold plate that includes the region having the meniscus M and the dissimilar material-filled portions 20 is made smaller than the total sectional area of the cooling water channels 26 below that range. This configuration enhances the heat removal from the mold plate 22 and enables keeping the surface temperature of the mold plate 22 at 350° C. or below even when continuous casting is performed at a casting speed of 2.5 m / min or more.

[0085] It is preferable to embed thermocouples in a range of the mold plate 22 that extends from a position 20 mm below the meniscus M to a position 100 mm below the meniscus M, and use the temperatures measured with the thermocouples. Specifically, the surface temperature of the mold plate 22 can be calculated from the temperature measured with the thermocouple, and the required volume flow rate of the cooling water and the required total sectional area of the cooling water channels 26 can be determined so that the surface temperature of the mold plate 22 will be 350° C. or below. Furthermore, the results thus obtained also determine, for example, the upper limit of feasible casting speed.

[0086] Next, the flow direction of the cooling water will be described. FIG. 9 is a set of a sectional view (a) of the mold plate 22 cut in the casting direction, and a front view (b) of the mold plate 22. The flow direction of the cooling water will be explained with reference to FIG. 9. In the continuous steel casting method according to the present embodiment, as illustrated in FIG. 9, the cooling water flows in a direction from the mold upper end near the meniscus M toward the mold lower end. This allows low-temperature cooling water to be supplied to the cooling water channels 26 near the meniscus M and thereby can increase the cooling capacity for the mold 10 near the meniscus M.

[0087] It was conventionally considered undesirable to supply low-temperature cooling water to the upper end of a mold near the meniscus M because it would lead to uneven cooling and the presence of uneven cooling near the meniscus M would cause longitudinal crack defects in the slab. In contrast, uniform cooling effects can be expected in the continuous steel casting method of the present embodiment by virtue of the dissimilar material-filled portions 20 embedded near the meniscus M. Thus, in the continuous steel casting method of the present embodiment, the cooling water is preferably passed in a direction from the mold upper end near the meniscus M toward the mold lower end. In this manner, the mold upper end most likely to have a high surface temperature is cooled with an increased cooling capacity and the surface temperature of the mold plate 22 is easily kept at 350° C. or below. Even in an operation at a casting speed of 2.5 m / min or more where the surface temperature of the mold plate 22 is easily brought to above 350° C., it is preferable to introduce low-temperature cooling water from the mold upper end. This allows the cooling water to flow in a direction from the mold upper end near the meniscus M toward the mold lower end, and the mold upper end most likely to have a high surface temperature can be cooled with an enhanced cooling capacity.

[0088] In the continuous steel casting method according to the present embodiment, it is preferable to supply cooling water to the cooling water channels 26 with high accuracy. In the mold 10, cooling water is generally supplied from a common pump to the cooling water channels 26 of the long sides of mold 12 and the short sides of mold 14. Due to the flow rate-pressure relationship determined by the piping diameter, however, the above configuration may fail to attain enhanced accuracy of cooling water supply.

[0089] Specifically, an attempt to increase the volume flow rate of the cooling water in any of the long sides of mold 12 and the short sides of mold 14 may fail because sufficient pressure is not available for the remaining sides.

[0090] FIG. 10 is a schematic view illustrating an exemplary installation of pumps for supplying cooling water to the mold 10. A total of four independent pumps, two for the front and rear long sides of mold 12 and two for the left and right short sides of mold 14, may be installed as illustrated in FIG. 10. However, enormous costs are incurred and the control is complicated. Furthermore, multiple backup pumps will be necessary in case of breakdown. Thus, such installation is not desirable as equipment for continuous steel casting that is operated without interruption almost 365 days a year.

[0091] To ensure continuous operation, the system is preferably one that can supply cooling water to all of the four mold sides, namely, the long sides of mold 12 and the short sides of mold 14, even in small volumes without suspension even in the event of a pump failure. It is therefore preferable that the long sides of mold 12 and the short sides of mold 14 share a main pump.

[0092] FIG. 11 is a graph illustrating relationships between the total pumped water volume and the pump source pressure. In general, a pump characteristically has a relationship between the total pumped water volume and the pump source pressure as shown by the solid line in FIG. 11. The dashed lines are system diagrams of the pump. As illustrated in FIG. 11, increasing the total volume of cooling water supplied requires that the valve opening degree be closer to full and the pump source pressure be lowered. A mold used in continuous casting of steel is composed of a pair of long sides of mold 12 and a pair of short sides of mold 14, and the piping diameter for the short sides of mold 14 receiving less water is generally smaller than the piping diameter for the long sides of mold 12 that are supplied with a larger volume of water. Thus, an attempt to supply a volume of water corresponding to the required linear velocity to the mold 10 for distribution to the long sides of mold and the short sides of mold will succeed in supplying water to the long sides of mold but may fail to achieve the desired flow rate in the short sides of mold due to significant influence of pressure loss.

[0093] FIG. 12 is a schematic view illustrating an exemplary installation of pumps for supplying cooling water to the mold. In order to solve the above problem, it is preferable that, as illustrated in FIG. 12, common pumps P that handle the overall water supply and boost pumps BP that further increase the volume flow rate of water supplied to the short sides be installed in series. Specifically, increasing the volume flow rate of cooling water supplied to the short sides of mold 14 connected to a narrow-diameter piping requires that the water pressure in the piping be increased over the pressure loss. The boost pumps BP installed as water pressure-increasing pumps separate from the common pumps P can add pressure in the piping to the short sides of mold 14.

[0094] The pumps are preferably controlled in such a manner that the boost pumps BP are operated when a specific casting speed is reached. When the casting speed is high, the volume flow rate of cooling water supplied to the long sides of mold 12 is increased and also the water pressure is raised with the boost pumps BP to adjust the supply of cooling water to the short sides of mold 14 to the target volume. By adopting this approach, the number of backup pumps for use in the event of a pump failure can be minimized, and further the minimum of cooling water required can be supplied to the mold 10 even in the event of a pump failure, thereby preventing operational abnormalities.

[0095] The volume flow rate of cooling water may be effectively controlled by changing the volume flow rate of cooling water correspondingly to the casting speed. Because the change in mold cooling capacity is not as steep as that of secondary cooling water, the control may be performed in a simplified manner in which the volume flow rate of water is changed in stages in accordance with the casting speed.

[0096] Ideally, it is preferable to construct a system that automatically controls the pumps and the total sectional area of the cooling water channels 26 based on the temperatures measured with thermocouples embedded in the mold plate 22 so that the surface temperature of the mold plate 22 will be 350° C. or below. This control may be made in such a manner that the water volumes from the mold pumps are preset, and the pump settings and the total sectional area of the cooling water channels 26 are controlled so that the surface temperature of the mold plate determined from the thermocouple temperature in the mold plate 22 will be 350° C. or below. To realize more advanced control, the spacer 28 that is installed on the backup plate to increase the linear velocity at locations corresponding to the positions of the dissimilar material-filled portions may be made movable with, for example, a servo motor, a pneumatic cylinder, or a hydraulic cylinder. In this manner, the total sectional area A1 of the cooling water channels 26 in the upper part of the mold including the meniscus M and the dissimilar material-filled portions 20 can be altered without dismantling the mold 10.

[0097] Next, the mold powder used in the continuous steel casting method according to the present embodiment will be described. In the continuous casting of medium carbon steel, typically hypo-peritectic steel, a mild cooling mold powder is used in order to suppress the occurrence of longitudinal cracks in the slab. In medium carbon steel, the growth of the solidifying shell tends to be nonuniform due to factors, such as transformation stress stemming from δ→γ transformation, and stress concentration occurring in the circumferential direction gives rise to defects, such as longitudinal cracks or depressions, at that location. In order to suppress this phenomenon, a crystallization powder is used for medium carbon steel. This type of mold powder actively forms crystalline phases and achieves uniform mild cooling in the mold. In order to stably generate crystalline phases from high temperatures, it is necessary to use a high-basicity mold powder having a basicity CaO / SiO2 of 1.2 or more.

[0098] Low-basicity mold powders are mainly vitreous and can be used to cool the mold more strongly. On the other hand, high-basicity mold powders are highly effective in preventing longitudinal cracks but entail a decrease in cooling capacity of the mold. Thus, it is difficult to achieve high-speed casting and longitudinal crack prevention at the same time with a high-basicity mold powder. Furthermore, high-basicity mold powders are expensive. From the point of view of production costs, it is advantageous if a mold powder with a basicity Cao / SiO2 of less than 1.2 can be used where possible.

[0099] In the continuous steel casting method according to the present embodiment, the mold plate 22 is provided with the dissimilar material-filled portions 20 so as to apply regular temperature changes to the solidifying shell. This suppresses an increase in local heat flux at a specific location, thereby suppressing the occurrence of longitudinal cracks and depressions in the slab. In view of this, the continuous steel casting method according to the present embodiment preferably involves a mold powder having a basicity Cao / SiO2 of less than 1.2. The use of such a mold powder is compatible with the strengthening of mold cooling and thus allows the casting speed to be increased, and further can suppress the occurrence of longitudinal cracks in the slab.

[0100] The continuous steel casting method according to the present embodiment is preferably applied to continuous casting of a thin slab at a high casting speed of 3.0 m / min or more and 8.0 m / min or less that produces a slab with a thickness of 150 mm or less. In continuous casting of a thin slab, the cast slab is basically fed directly to the rolling process while being heated in a tunnel furnace, and thus cannot be subjected to conditioning processes, such as slab repairing. If slab defects, such as longitudinal cracks, have occurred, the coil quality is deteriorated and the yield is lowered. These facts have hindered active implementation of high-speed continuous thin slab casting in the production of hard-to-cast types of steels, such as hypo-peritectic medium carbon steels and high-alloy special steels.

[0101] In contrast, the continuous steel casting method according to the present embodiment is capable of producing hypo-peritectic medium carbon steel by continuous thin slab casting, thus markedly enhancing productivity. Furthermore, in continuous thin slab casting, the mold thickness is very small and the outer diameter of the submerged nozzle that supplies molten steel may not be accommodated into the mold. This problem is avoided by using a mold that has a special funnel shape (a hollow cone shape) between the widthwise central portions of the long sides. In the casting of medium carbon steel, not only uneven solidification occurs at the meniscus M but also cooling tends to be nonuniform at the boundary of this funnel shape, thus giving rise to the occurrence of longitudinal cracks in the slab. Because this boundary is located below the meniscus M, it was impossible to suppress longitudinal cracks completely through improvements in mold powder alone. This problem is preferably addressed by arranging the dissimilar material-filled portions 20 also in a 50 mm region extending across and along the boundary of the funnel shape in a range from 0 mm to 200 mm below the meniscus M. In this manner, uneven cooling is suppressed even at the boundary of the funnel shape, and the thin slab can be prevented from longitudinal cracks or breakout.EXAMPLESExample 1

[0102] Next, Examples will be described in which the mold according to the present embodiment was evaluated by operation on an actual continuous casting machine. In EXAMPLE 1, the following three types of molds were used for evaluation.

[0103] Comparative Example 1: A general continuous casting mold in which there are no dissimilar material-filled portions on the inner wall surfaces of the mold and the sectional area of cooling water channels is constant in the casting direction.

[0104] Comparative Example 2: A continuous casting mold in which dissimilar material-filled portions are disposed on the inner wall surfaces of the mold and the sectional area of cooling water channels is constant in the casting direction.

[0105] Inventive Example 1: A continuous casting mold in which dissimilar material-filled portions are disposed on the inner wall surfaces of the mold, and a spacer is installed on the backup plate side so that the total sectional area A1 of cooling water channels corresponding to the locations where the dissimilar material-filled portions are embedded satisfies the relationship A1=0.7×A2 in relation to the sectional area A2 below the locations.

[0106] All of the above molds were composed of long sides of mold with a length of 2.1 m and short sides of mold with a length of 0.22 m, and had a rectangular internal space. The mold plates constituting the long sides of mold and the short sides of mold were made of a copper alloy having a thermal conductivity of about 380 (W / (m×K)) at room temperature.

[0107] The steel types of interest here for continuous casting were all types of steels that were typical choices for casting, ranging from very low carbon steel to medium carbon steel. The chemical composition included C: 0.0008 to 0.25 mass %, Si: 0.002 to 1.2 mass %, Mn: 0.10 to 2.0 mass %, P: 0.005 to 0.030 mass %, S: 0.001 to 0.02 mass %, and Al: 0.001 to 0.06 mass, the balance being Fe and incidental impurities. The mass of molten steel per charge was 300 tons. While pouring the molten steel into the provided mold, the mold was cooled while being oscillated in the slab withdrawal direction to form a solidifying shell. The solidifying shell was withdrawn to give a cast slab. The slab withdrawal speed Vc was 0.3 to 2.6 (m / min).

[0108] In EXAMPLE 1, a mold powder was added onto the molten steel in the oscillating mold to prevent the molten steel from sticking to the mold. The mold powders used here had a basicity ((mass % CaO) / (mass % SiO2)) of 0.6 to 1.8. Among the mold powders, those having a high basicity of 1.5 to 1.8 were used for medium carbon steel.

[0109] In EXAMPLE 1, the goal was to perform continuous casting of 3000 charges without replacing the mold, and surface cracks on the long sides of mold were checked after every 100 charges had been cast. The surface of the long sides of mold was visually inspected for separation or cracks in the coating layer or the copper plate, and the continuous casting operation was stopped immediately when any abnormalities were found. Surface cracks on the slab were checked after every continuous casting. After medium carbon steel highly susceptible to cracking was cast, the slab was subjected to penetrant testing (color checking) and the surface of the slab was visually inspected for longitudinal cracks along the slab withdrawal direction.

[0110] The molds of Comparative Example 2 and Inventive Example 1 had a plurality of circular recesses in the mold plates constituting the long sides of mold. A nickel alloy (thermal conductivity at room temperature: 80 (W / (m×K))) as a dissimilar material was applied by plating to fill the recesses, thereby forming dissimilar material-filled portions. For all the molds, a nickel alloy coating layer was formed on the surface of the mold inner walls as illustrated in FIGS. 4 and 6.

[0111] In Comparative Example 2 and Inventive Example 1, the filling depth t of the dissimilar material was 1 mm, and the dissimilar material-filled portions 20 were arranged so as to satisfy expressions (1) and (2).

[0112] In Comparative Example 1, Comparative Example 2, and Inventive Example 1, cooling water was supplied to the mold while setting the volume flow rate of the cooling water so that the linear velocity in the cooling water channels would be 7.0 m / sec. In Inventive Example 1, a spacer 28 was installed to increase the linear velocity in the cooling water channels to 10.0 m / sec.

[0113] The operational results of Comparative Example 1, Comparative Example 2, and Inventive Example 1 were as follows.

[0114] Comparative Example 1: Cracks occurred in the coating layers of the coated mold plates at the completion of casting of 2600 charges, and the casting operation was canceled. The incidence of longitudinal cracks in the medium carbon steel slab was 8.0%.

[0115] Comparative Example 2: Separation and cracks occurred in the coating layers of the coated mold plates at the completion of casting of 2000 charges, and the casting operation was canceled. The incidence of longitudinal cracks in the medium carbon steel slab was 0.5%.

[0116] Inventive Example 1: There was no separation or crack in the coating layers of the coated mold plates after the completion of casting of 3000 charges. The incidence of longitudinal cracks in the medium carbon steel slab was 0.2%.

[0117] As demonstrated above, Inventive Example 1 resulted in no surface cracks in the mold plates constituting the long sides of mold even after the completion of casting of 3000 charges, and also achieved effective reduction in the incidence of longitudinal cracks in the medium carbon steel slab.

[0118] In Inventive Example 1, furthermore, the casting speed was increased to 3.0 m / min and the casting was continued until 3500 charges were reached. Even at this maximum casting speed of 3.0 m / min that would increase the thermal load, the mold plates constituting the long sides of mold were free from surface cracks and no tendency was observed for longitudinal cracks to increase in the medium carbon steel slab.

[0119] As demonstrated above, Inventive Example 1 successfully achieved continuous casting of the target number of charges, 3000 charges, without replacing the mold, and showed that the mold had a longer service life than the molds of Comparative Examples 1 and 2. Furthermore, the mold surface suffered no damages even when subjected to operation at a casting speed of 2.5 m / min or more. These results are most likely due to the fact that the mold was cooled more efficiently as a result of reducing the total sectional area of the cooling water channels in the region having the dissimilar material-filled portions 20 and the consequent increase in linear velocity.

[0120] The slabs cast in Comparative Example 2 and Inventive Example 1 were inspected for surface cracks, but no surface cracks that would require repairing were found in the slabs. The present inventors believe that the molds in Comparative Example 2 and Inventive Example 1 successfully suppressed the occurrence of surface cracks in the slab as a result of the fact that the dissimilar material-filled portions effectively suppressed the uneven thickening of the solidifying shell due to the δ iron to γ iron transformation during the casting of the medium carbon steel and thereby suppressed the occurrence of surface cracks. The absence of surface cracks that would require repairing showed that these molds would allow for direct rolling.Example 2

[0121] Next, EXAMPLE 2 (Inventive Examples 2 to 35, and Comparative Examples 3 to 29) will be described in which continuous steel casting was performed in the same manner as in EXAMPLE 1. In EXAMPLE 2, the dissimilar material-filled portions and the like were provided in the short sides of mold to facilitate changing the dissimilar material-filled portions, the cooling water channels, and the casting conditions among the tests by inserting and removing the respective molds, and continuous steel casting was performed while changing the conditions of the short sides of mold.

[0122] In EXAMPLE 2, the preset number of casting charges per Inventive Example or Comparative Example was 5. The steel types of interest here for continuous casting were limited to medium carbon steels susceptible to the occurrence of longitudinal cracks on the slab surface. The steels had a chemical composition including C: 0.08 to 0.17 mass %, Si: 0.10 to 0.30 mass %, Mn: 0.50 to 1.20 mass %, P: 0.010 to 0.030 mass %, S: 0.005 to 0.015 mass %, and Al: 0.020 to 0.040 mass %, the balance being Fe and incidental impurities. In Inventive Examples 2 to 35 and Comparative Examples 3 to 29, conditions were changed, such as, referring to FIG. 3, the width d (mm) of the dissimilar material-filled portions 20 in the mold width direction, the spacing P (mm) between the dissimilar material-filled portions 20 in the mold width direction, the width e (mm) of the dissimilar material-filled portions 20 in the slab withdrawal direction A, the total sectional area A1 of the cooling water channels in the upper part, and the total sectional area A2 of the cooling water channels in the lower part. Furthermore, the continuous casting in Inventive Examples 2 to 35 and Comparative Examples 3 to 29 involved changes in oscillation frequency (1 / min), slab withdrawal speed Vc (m / min), and mold powder basicity.

[0123] Each operation consisted of continuous casting of 5 charges. In the mold used, thermocouples were embedded in a region extending from the meniscus M to 50 mm below the meniscus in the casting direction, and the temperature was measured with the thermocouples. The temperature was measured at one-second intervals, and the temperature data was recorded. The distance from the thermocouple temperature measurement point to the surface of the mold plate 22 next to the molten steel was 15 mm. Based on a heat transfer model, the surface temperature of the mold plate 22 was calculated from the thermocouple temperature.

[0124] Table 1-1 below describes the conditions in Inventive Examples 2 to 35, such as the width d, the spacing P, and the total sectional areas A1 and A2 of the cooling water channels, and the calculated results of the copper mold plate surface temperature. The molds in Comparative Examples 3 to 29 were general continuous casting molds having no dissimilar material-filled portions, or molds that were provided with dissimilar material-filled portions but had a constant sectional area of cooling slit channels at and below the positions of the dissimilar material-filled portions.TABLE 1Total sectionalTotal sectionalWidthOscillationCasting1000 ×Fillingarea of waterarea of waterWidth dSpacingeSpacingfrequencyspeedVc / fdepthSpacing Schannel A1channels A2Items(mm)P (mm)(mm)L (mm)f (1 / cpm)Vc (m / min)(mm)t (mm)(mm)(mm2)(mm2)Inv. Ex. 213251451.6110.5208333.311458.3Inv. Ex. 325251451.6111.0208333.311458.3Inv. Ex. 456581451.6115.0208333.311458.3Inv. Ex. 556591451.6115.0208333.311458.3Inv. Ex. 6107691451.6115.0208333.311458.3Inv. Ex. 71010691451.6115.0208333.311458.3Inv. Ex. 82010881451.61110.0208333.311458.3Inv. Ex. 91820891451.61110.0208333.311458.3Inv. Ex. 105105101822.0113.0208333.311458.3Inv. Ex. 115105101822.0115.0208333.311458.3Inv. Ex. 125105102092.3115.0208333.311458.3Inv. Ex. 13325251451.6111.0208333.311458.3Inv. Ex. 145225121451.6115.0208333.311458.3Inv. Ex. 15565151451.61112.0208333.311458.3Inv. Ex. 1610126121451.6110.3208333.311458.3Inv. Ex. 171010651451.61117.0208333.311458.3Inv. Ex. 1813251451.6110.3208333.311458.3Inv. Ex. 1956591451.6115.0208333.311458.3Inv. Ex. 205105151451.6113.0808333.311458.3Inv. Ex. 215105101451.6115.0808333.311458.3Inv. Ex. 2213252272.5110.5208333.311458.3Inv. Ex. 2325252272.5111.0208333.311458.3Inv. Ex. 2456582272.5115.0208333.311458.3Inv. Ex. 2556592272.5115.0208333.311458.3Inv. Ex. 26107692272.5115.0208333.311458.3Inv. Ex. 271010692272.5115.0208333.311458.3Inv. Ex. 285105102272.5115.0208333.311458.3Inv. Ex. 295105102272.5115.0208333.311458.3Inv. Ex. 30325252733.0111.0208333.311458.3Inv. Ex. 3110126122272.5110.3208333.311458.3Inv. Ex. 3213252272.5110.3208333.311458.3Inv. Ex. 3356592272.5115.0208333.311458.3Inv. Ex. 345105152272.5113.0808333.311458.3Inv. Ex. 355105102272.5115.0808333.311458.3Comp. Ex. 313252272.5110.52012222.212222.2Comp. Ex. 425252272.5111.02012222.212222.2Comp. Ex. 556582272.5115.02012222.212222.2Comp. Ex. 656592272.5115.02012222.212222.2Comp. Ex. 7107692272.5115.02012222.212222.2Comp. Ex. 81010692272.5115.02012222.212222.2Comp. Ex. 92010882272.51110.02012222.212222.2Comp. Ex. 101820892272.51110.02012222.212222.2Comp. Ex. 115105102552.8113.02012222.212222.2Comp. Ex. 125105102272.5115.02012222.212222.2Comp. Ex. 13325252733.0111.02012222.212222.2Comp. Ex. 145225122452.7115.02012222.212222.2Comp. Ex. 15565152272.51112.02012222.212222.2Comp. Ex. 1610126122272.5110.32012222.212222.2Comp. Ex. 171010652272.51117.0207638.97638.9Comp. Ex. 1813252272.5110.3207638.97638.9Comp. Ex. 1956592272.5115.0207638.97638.9Comp. Ex. 205105152272.5113.0807638.97638.9Comp. Ex. 215105102272.5115.0807638.97638.9Comp. Ex. 22————1451.611—2012222.212222.2Comp. Ex. 23————1641.811—2012222.212222.2Comp. Ex. 24————2272.511—207638.97638.9Comp. Ex. 25————2552.811—207638.97638.9Comp. Ex. 26————1451.611—2012222.212222.2Comp. Ex. 27————1641.811—2012222.212222.2Comp. Ex. 28————2272.511—207638.97638.9Comp. Ex. 29————2552.811—207638.97638.9Separation orcracks onMaximumMoldIncidence ofcoating layertemperature ofpowderExpressionExpressionExpressionlongitudinaland coppercopper platebasicityItems(1)(2)(3)cracks (%)platesurface (° C.)CaO / SiO2Inv. Ex. 2∘∘∘0∘1571.55Inv. Ex. 3∘∘∘0∘1731.55Inv. Ex. 4∘∘∘0∘2161.55Inv. Ex. 5∘∘∘0∘2161.55Inv. Ex. 6x∘∘0∘2161.55Inv. Ex. 7x∘∘0∘2161.55Inv. Ex. 8x∘∘0∘2391.55Inv. Ex. 9∘∘∘0∘2391.55Inv. Ex. 10∘∘∘0∘2521.55Inv. Ex. 11∘∘∘0∘2711.55Inv. Ex. 12∘∘∘0∘3111.55Inv. Ex. 13x∘∘0∘1731.55Inv. Ex. 14xx∘14∘2161.55Inv. Ex. 15∘x∘8∘2451.55Inv. Ex. 16∘x∘0∘1461.55Inv. Ex. 17xx∘0∘2571.55Inv. Ex. 18∘∘∘12∘1461.55Inv. Ex. 19∘∘∘0∘2161.55Inv. Ex. 20∘x∘0∘2021.55Inv. Ex. 21∘∘∘0∘2161.55Inv. Ex. 22∘∘∘0∘2451.55Inv. Ex. 23∘∘∘0∘2701.55Inv. Ex. 24∘∘∘0∘3381.55Inv. Ex. 25∘∘∘0∘3381.55Inv. Ex. 26x∘∘0∘3381.55Inv. Ex. 27x∘∘0∘3381.55Inv. Ex. 28∘∘∘0∘3381.55Inv. Ex. 29∘∘∘0∘3381.10Inv. Ex. 30x∘∘0∘3241.55Inv. Ex. 31∘x∘0∘2281.55Inv. Ex. 32∘∘∘12∘2281.10Inv. Ex. 33∘∘∘0∘3381.55Inv. Ex. 34∘x∘0∘3151.55Inv. Ex. 35∘∘∘0∘3381.10Comp. Ex. 3∘∘x0∘2451.55Comp. Ex. 4∘∘x0∘2701.55Comp. Ex. 5∘∘x0Δ3381.55Comp. Ex. 6∘∘x0Δ3381.55Comp. Ex. 7x∘x0Δ3381.55Comp. Ex. 8x∘x0Δ3381.55Comp. Ex. 9x∘x0x3731.55Comp. Ex. 10∘∘x0x3731.55Comp. Ex. 11∘∘x0x3531.55Comp. Ex. 12∘∘x0Δ3381.55Comp. Ex. 13x∘x0Δ3241.55Comp. Ex. 14xxx14x3651.55Comp. Ex. 15∘xx8x3821.55Comp. Ex. 16∘xx0∘2281.55Comp. Ex. 17xxx32x4011.55Comp. Ex. 18∘∘x20∘2281.55Comp. Ex. 19∘∘x24Δ3381.55Comp. Ex. 20∘xx24Δ3151.55Comp. Ex. 21∘∘x32Δ3381.55Comp. Ex. 22———20∘1381.55Comp. Ex. 23———24∘1551.55Comp. Ex. 24———30∘2161.55Comp. Ex. 25———50∘2411.55Comp. Ex. 26———40∘1381.10Comp. Ex. 27———46∘1551.10Comp. Ex. 28———66∘2161.10Comp. Ex. 29———88∘2411.10

[0125] In Tables 1-1 and 1-2, “o” in the columns “Expression (1)”, “Expression (2)”, and “Expression (3)” means that the respective expressions are satisfied, and “x” in the columns means that the respective expressions are not satisfied. The values described in the column “Maximum temperature of copper plate surface” in Tables 1-1 and 1-2 were calculated in such a manner that the average surface temperature of the mold plate was calculated from the thermocouple temperatures based on the heat transfer model, and the average temperatures thus obtained during the steady operation time of the continuous casting of 5 charges were further averaged by being divided by the number of data samples. A smaller value of this “Maximum temperature of copper plate surface” means that the mold plate surface at the position of the meniscus M is cooler, and the maximum temperature of the copper plate surface tended to be higher with increasing casting speed. It can be said that the mold plate is cooled stably when the maximum temperature of the copper plate surface remains at 350° C. or below. The tests have confirmed that coating separation or cracks do not occur on the copper plate surface when the maximum temperature of the copper plate surface is 350° C. or below.

[0126] In EXAMPLE 2, surface cracks on the slab were checked after each operation of continuous casting. Continuous casting of one charge can produce 10 pieces of slab. Since 5 charges were continuously cast in each of Inventive Examples and Comparative Examples, 50 pieces of slab were cast in each of Inventive Examples and Comparative Examples. Penetrant testing was performed on all of these slabs, and the surface of the penetrant-tested slabs was visually inspected for surface cracks on the slabs. Since the detailed specifications of the molds here were changed only with respect to the short sides of mold, the data evaluated in EXAMPLE 2 was limited to surface cracks on the short sides. When longitudinal cracks were found on the slab surface, the slab counted. The percentage of the total number of longitudinally cracked slabs relative to the total number of slabs (=50) was described in the column “Incidence of longitudinal cracks”. This crack incidence includes even slabs that had visible but very fine cracks. Thus, even those slabs having an incidence of longitudinal cracks of more than 0 are substantially not problematic as long as the crack incidence is 15% or less.

[0127] In Inventive Examples 2 to 35, in which continuous casting was performed using a mold provided with dissimilar material-filled portions 20, the incidence of longitudinal cracks was 15% or less in all the slabs obtained in one continuous casting operation. These results have confirmed that longitudinal cracks on the slab surface can be prevented by the use of the mold according to the present embodiment.

[0128] Furthermore, Inventive Examples 2 to 35 satisfied expression (3) and maintained the copper plate surface temperature at the meniscus position at 350° C. or below. Consequently, the occurrence of cracks and abnormalities on the copper plate surface was suppressed, and no cracks or abnormalities occurred on the copper plate surface even under high-speed casting conditions at a casting speed of 2.5 m / min or more.

[0129] Longitudinal cracks occurred markedly when the type of mold described in Comparative Example 1 of EXAMPLE 1 was used (Comparative Examples 22 to 29). Furthermore, longitudinal cracks were suppressed effectively when the basicity of the mold powder was 1.55, but the incidence of longitudinal cracks was above 40% when the mold powder basicity was 1.10.

[0130] When the type of mold described in Comparative Example 2 of EXAMPLE 1 was used (Comparative Examples 3 to 21) and when expression (3) was not satisfied and the maximum temperature of the copper plate surface exceeded 300° C., coating separation or cracks occurred on the copper plate surface after 100 or more charges. When the mold did not satisfy expression (3) and the maximum temperature of the copper plate surface exceeded 350° C., coating separation or cracks occurred on the copper plate surface after 5 charges. Furthermore, although not described in the table, the maximum mold surface temperature was lowered to about 300° C. when the linear velocity through the cooling slits was increased from 7.0 m / sec to 12.0 m / sec. In this case, however, stable operation was difficult and depression-shaped longitudinal cracks occurred near the corners of the shorter sides at an incidence of 50%.

[0131] From the above results, it was confirmed that the continuous casting of medium carbon steel using the mold according to the present embodiment can benefit from suppressed occurrence of surface cracks in the steel slab and effective reduction of the mold plate temperature in the vicinity of the meniscus where the dissimilar material-filled portions are provided. Furthermore, the effective reduction of the mold plate temperature leads to suppressed occurrence of cracks or abnormalities on the copper plate surface. Thus, it was confirmed that the mold design according to the present embodiment can achieve both an extended life of the mold provided with the dissimilar material-filled portions, and the suppression of surface cracks in the slab.Example 3

[0132] In EXAMPLE 3, the effect of the mold according to the present embodiment in high-speed casting was evaluated by continuously casting steel on a thin slab continuous casting machine fitted with a mold that had a funnel-shaped (hollow cone-shaped) curved surface 42 in the widthwise central portions of the copper mold plates (Inventive Example 36 and Comparative Example 30). The continuous casting in EXAMPLE 3 produced a slab with a constant size in which the slab width was 1250 mm and the slab thickness at the mold lower end was 75 mm.

[0133] The steel types of interest here for continuous casting were medium carbon steels that had a chemical composition including C: 0.08 to 0.17 mass %, Si: 0.10 to 0.30 mass %, Mn: 0.50 to 1.20 mass %, P: 0.010 to 0.030 mass %, S: 0.005 to 0.015 mass %, and Al: 0.020 to 0.040 mass %, the balance being Fe and incidental impurities. The casting speed was 4.0 to 5.0 m / min, the mold oscillation frequency was f=400 to 500 (1 / min), and the mold oscillation stroke was 6 mm. The mold powder basicity was Cao / SiO2=1.25.

[0134] FIG. 13 is a schematic view illustrating the mold used in Inventive Example 36. FIG. 14 is a schematic view illustrating the mold used in Comparative Example 30. As illustrated in FIG. 13, the mold used in Inventive Example 36 was such that dissimilar material-filled portions 44 in which recesses 5 mm in diameter x 1.5 mm in depth were filled with a nickel alloy were arranged across and along the meniscus M and also across and along a boundary of the funnel-shaped curved surface 42 in a range from 0 mm to 200 mm below the meniscus M. Furthermore, the cooling slits were designed by installing a spacer 28 on the backup plate side so that the total sectional area of the cooling water channels from their upper end to 200 mm below the meniscus taken as 0 mm would be 0.75 times the total sectional area from the bottom of the above range to the lower end. On the other hand, the mold used in Comparative Example 30 had no dissimilar material-filled portions 44 and no spacer 28.

[0135] Five charges were cast in the test of Inventive Example 36 and Comparative Example 30. In a thin slab continuous casting machine, the cast slab is generally heated continuously in a tunnel furnace and is then rolled. In this test, however, the thin slabs were cooled to check the status of surface cracks after casting.

[0136] In Inventive Example 36, five charges were continuously cast at a casting speed of 4.0 m / min or more and 5.0 m / min or less. The continuous casting resulted in no longitudinal cracks on the surface of the cast slab and was shown to achieve satisfactory quality. In contrast, Comparative Example 30 encountered a breakout in one of the five charges and failed to achieve complete casting, and longitudinal cracks were present in 66% of all the cast slab sheets. Most of the longitudinal cracks had occurred at positions corresponding to the boundary of the funnel shape.

[0137] In Inventive Example 36, the surface temperature of the copper plate estimated from the thermocouple temperatures in the copper plate near the meniscus during casting was stable at around 320° C. even at a casting speed of 5.0 m / min, and no abnormalities were found on the copper plate surface after casting. In Comparative Example 30, in contrast, the surface temperature of the copper plate estimated from the thermocouple temperatures in the copper plate near the meniscus M during casting tended to exceed 350° C. at a casting speed of around 4.0 m / min, and thus casting at a higher speed was abandoned.

[0138] These results have confirmed that the use of the mold of Inventive Example 36 is highly effective for continuous casting of thin slab that requires a high casting speed of 2.5 m / min or more. Besides the vertical slit cooling system described above, the canal cooling system is also frequently used for molds of continuous thin-slab casting machines, in which round holes are machined in the casting direction and the mold is cooled by passing water through the holes. Similar effects can be obtained also in the canal system by changing the upper hole diameter and the lower hole diameter. Thus, either of these cooling systems may be adopted.

[0139] As described above, the mold according to the present embodiment allows steel to be continuous cast while suppressing the occurrence of longitudinal cracking defects originating within the mold during continuous casting, and also suppressing coating separation or copper plate deformation even under 2.5 m / min or higher casting speed conditions, thereby realizing a longer mold life. Furthermore, the mold provides the option of using a mold powder having high cooling capacity and thus enables casting to be performed under higher speed conditions, thus making it possible to further enhance the productivity of slabs.

[0140] Repairing in the conditioning processes associated with longitudinal cracking defects can be omitted. This allows the slab to be hot direct rolled. Thus, the present application can save the cost of heating the slab in a heating furnace and can also contribute to energy savings.

Claims

1. A continuous steel casting mold comprising:a mold plate comprising a copper alloy, the mold plate having a front surface defining an inner wall surface of the mold, and a back surface defining cooling water channels; anda backup plate attached to the mold plate so as to cover the cooling water channels, whereina region of the front surface of the mold plate including at least a meniscus has a plurality of recesses each defining a dissimilar material-filled portion in which the recess is filled with a dissimilar material having a thermal conductivity different from a thermal conductivity of the mold plate, anda total sectional area of the cooling water channels on the back surface of the mold plate in a range including the region having the dissimilar material-filled portions is smaller than a total sectional area of the cooling water channels below the range.

2. The continuous steel casting mold according to claim 1, wherein the dissimilar material-filled portions are disposed so that a heat flux passing from the inner wall surface of the mold to each cooling water channel changes periodically in the region of the front surface of the mold plate.

3. The continuous steel casting mold according to claim 1, wherein the dissimilar material-filled portions and the cooling water channels are disposed so as to satisfy at least one of expressions (1) to (3) below:d<P≤S(1)e≤L≤1⁢0⁢0⁢0×Vc / f(2)A⁢1≤0.8×A⁢2(3)wherein d is a width (mm) of each dissimilar material-filled portion in a mold width direction, P is a spacing (mm) in the mold width direction between adjacent dissimilar material-filled portions, S is a spacing (mm) in the mold width direction between adjacent cooling water channels among the cooling water channels disposed on the back surface of the mold plate, e is a width (mm) of each dissimilar material-filled portion in a slab withdrawal direction, L is a spacing (mm) in the slab withdrawal direction between adjacent dissimilar material-filled portions, Vc is a slab withdrawal speed (m / min) in a continuous steel casting process, f is an oscillation frequency (1 / min) of the continuous casting mold in the continuous steel casting process, A1 is the total sectional area (mm2) of the cooling water channels in the range including the region, and A2 is the total sectional area (mm2) of the cooling water channels below the above range.

4. The continuous steel casting mold according to claim 1, wherein a coating layer is formed on the front surface of the mold plate so as to cover the dissimilar material-filled portions.

5. The continuous steel casting mold according to claim 1, wherein the mold plate is a mold plate among a pair of longer mold plates and a pair of shorter mold plates, andthe continuous steel casting mold has a common pump that supplies cooling water to the longer mold plates and the shorter mold plates, and a boost pump that supplies cooling water to at least the shorter mold plates.

6. The continuous steel casting mold according to claim 1, wherein the mold plate has a funnel shape, andthe dissimilar material-filled portions are arranged also in a 50 mm region extending across and along a boundary of the funnel shape in a range from 0 mm to 200 mm below the meniscus.

7. A continuous steel casting method using the continuous steel casting mold described in claim 1, whereincontinuous casting at a casting speed of 2.5 m / min or more brings a temperature of the front surface of the mold plate at the meniscus up to 350° C. or below.

8. A continuous steel casting method using the continuous steel casting mold described in claim 1, the method comprising:estimating a temperature of the front surface of the mold plate from a temperature measured with a thermocouple embedded in a region of the mold plate from 20 mm to 100 mm below the meniscus, and controlling a flow rate of cooling water and the total sectional area of the cooling water channels on the back surface of the mold plate in the region having the dissimilar material-filled portions so that the estimated temperature of the front surface of the mold plate is 350° C. or below.

9. A continuous steel casting method using the continuous steel casting mold described in claim 1, whereina mold powder having a basicity of less than 1.2 is used.

10. A continuous steel casting method using the continuous steel casting mold described in claim 1, whereincooling water is passed in a direction from an upper end of the mold plate toward a lower end of the mold plate.

11. A continuous steel casting method using the continuous steel casting mold described in claim 1, whereina slab that is produced has a thickness of 150 mm or less.

12. The continuous steel casting mold according to claim 2, wherein the dissimilar material-filled portions and the cooling water channels are disposed so as to satisfy at least one of expressions (1) to (3) below:d<P≤S(1)e≤L≤1⁢0⁢0⁢0×Vc / f(2)A⁢1≤0.8×A⁢2(3)wherein d is a width (mm) of each dissimilar material-filled portion in a mold width direction, P is a spacing (mm) in the mold width direction between adjacent dissimilar material-filled portions, S is a spacing (mm) in the mold width direction between adjacent cooling water channels among the cooling water channels disposed on the back surface of the mold plate, e is a width (mm) of each dissimilar material-filled portion in a slab withdrawal direction, L is a spacing (mm) in the slab withdrawal direction between adjacent dissimilar material-filled portions, Vc is a slab withdrawal speed (m / min) in a continuous steel casting process, f is an oscillation frequency (1 / min) of the continuous casting mold in the continuous steel casting process, A1 is the total sectional area (mm2) of the cooling water channels in the range including the region, and A2 is the total sectional area (mm2) of the cooling water channels below the above range.

13. The continuous steel casting mold according to claim 2, wherein a coating layer is formed on the front surface of the mold plate so as to cover the dissimilar material-filled portions.

14. The continuous steel casting mold according to claim 3, wherein a coating layer is formed on the front surface of the mold plate so as to cover the dissimilar material-filled portions.

15. The continuous steel casting mold according to claim 12, wherein a coating layer is formed on the front surface of the mold plate so as to cover the dissimilar material-filled portions.

16. The continuous steel casting mold according to claim 2, wherein the mold plate is a mold plate among a pair of longer mold plates and a pair of shorter mold plates, andthe continuous steel casting mold has a common pump that supplies cooling water to the longer mold plates and the shorter mold plates, and a boost pump that supplies cooling water to at least the shorter mold plates.

17. The continuous steel casting mold according to claim 3, wherein the mold plate is a mold plate among a pair of longer mold plates and a pair of shorter mold plates, andthe continuous steel casting mold has a common pump that supplies cooling water to the longer mold plates and the shorter mold plates, and a boost pump that supplies cooling water to at least the shorter mold plates.

18. The continuous steel casting mold according to claim 4, wherein the mold plate is a mold plate among a pair of longer mold plates and a pair of shorter mold plates, andthe continuous steel casting mold has a common pump that supplies cooling water to the longer mold plates and the shorter mold plates, and a boost pump that supplies cooling water to at least the shorter mold plates.

19. The continuous steel casting mold according to claim 12, wherein the mold plate is a mold plate among a pair of longer mold plates and a pair of shorter mold plates, andthe continuous steel casting mold has a common pump that supplies cooling water to the longer mold plates and the shorter mold plates, and a boost pump that supplies cooling water to at least the shorter mold plates.

20. The continuous steel casting mold according to claim 13, wherein the mold plate is a mold plate among a pair of longer mold plates and a pair of shorter mold plates, andthe continuous steel casting mold has a common pump that supplies cooling water to the longer mold plates and the shorter mold plates, and a boost pump that supplies cooling water to at least the shorter mold plates.

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