TWI655979B - Steel continuous casting method - Google Patents

Abstract

In order to prevent surface cracking due to uneven cooling of the solidified shell at the initial stage of solidification, center segregation occurring at the center portion of the thickness of the cast piece is suppressed. In the inner wall surface of the mold copper plate at a position where the meniscus position of the continuous casting mold is 20 mm from the meniscus position to at least 50 mm or more and 200 mm or less from the meniscus position, the ratio of the difference in thermal conductivity to the mold copper plate is 20% or more of the metal is provided with a different heat conductive metal filling portion, and the ratio of the total area of all the different thermally conductive metal filling portions, that is, the area ratio is 10%, with respect to the area of the inner wall surface on which the heterothermally conductive metal filling portion is provided Above 80% or less, the vibration pitch pitch (OMP) and the distance (D1) derived from the vibration frequency (f) and the casting speed (Vc) satisfy the formula (1), and the distance (D2) satisfies the formula (2). r of the formula (2) is a radius (mm) of a circle having an area equal to the area of the heterothermally conductive metal-filled portion centering on the center of gravity of the different thermally conductive metal-filled portion.

Description

Steel continuous casting method

[0001] The present invention relates to a continuous casting technique, and more particularly to a continuous casting method for steel, which improves surface cracking and center segregation of a cast piece by suppressing uneven solidification of a cast piece at an initial stage of solidification.

[0002] Generally, in the case of producing a steel slab by continuous casting, a molten steel injected into a mold is brought into contact with a mold to be cooled, thereby forming a thin solidified layer (hereinafter referred to as a "solidified shell"). In this way, the molten steel is poured into the mold while the solidified shell is pulled downward (hereinafter referred to as "normal casting") to produce a cast piece. [0003] When the cooling by the mold is not uniform, the thickness of the solidified shell becomes uneven, and as a result, the surface of the solidified shell becomes unsmooth. In particular, in the initial stage of solidification, if the thickness of the solidified shell grows unevenly, stress concentration occurs on the surface of the solidified shell, and minute longitudinal cracks occur. This slight longitudinal crack remains even after the cast piece is completely solidified, and becomes a longitudinal crack on the surface of the cast piece. When longitudinal cracking occurs on the surface of the cast piece, it is necessary to perform longitudinal crack removal (hereinafter referred to as trimming) before the cast piece is sent to a subsequent step (for example, a rolling step or the like). [0004] The mold vibrates in the casting direction (hereinafter also referred to as "swing"), and the vibration of the mold causes the upper end portion of the solidified shell to be bent toward the molten steel side, and the gap between the curved solidified shell and the inner wall surface of the mold The molten steel is overflowed, and a portion where the solidified shell protrudes toward the molten steel side (hereinafter referred to as "claw portion") is formed. When the surface of the solidified shell is not smooth, the void formed by the curved solidified shell and the inner wall surface of the mold becomes large, and the claw portion of the solidified shell becomes large. When the claw portion protruding toward the molten steel side becomes large, non-metallic inclusions and bubbles floating in the molten steel are caught by the claw portion on the meniscus (melting molten steel surface in the mold). The captured non-metallic inclusions and air bubbles are the cause of surface defects such as surface flaws and swelling of the steel sheet after hot rolling or the steel sheet after cold rolling. [0005] The frequency of occurrence of surface defects such as longitudinal cracks, flaws, and swelling tends to increase as the casting speed increases. The casting speed of today's general slab continuous casting machine is about 1.5 to 2 times higher than that of 10 years ago, and the dressing operation is also increased. In the hot charge and direct charge that the technology is being established in recent years, the dressing operation of the cast piece has also become a major cause of stabilization of the work. Therefore, it is economically advantageous if it is possible to prevent the uneven growth of the thickness of the solidified shell due to the uneven cooling in the initial stage of solidification and the occurrence of the claw portion. [0006] In order to prevent uneven cooling in the initial stage of solidification, it is necessary to uniformly and moderately cool in the initial stage of solidification to uniformly grow the thickness of the solidified shell, thereby preventing the formation of the claw portion. In this regard, in the continuous casting of a 280 x 280 mm billet, it is effective to provide unevenness on the inner surface of the mold in order to improve the surface properties of the cast piece. Patent Document 1 discloses that a concave portion having a diameter or a width of 3 to 80 mm and a depth of 0.1 to 1.0 mm is provided on the inner surface of the mold. Further, in Patent Document 2, a groove having a width of 0.2 to 2 mm and a depth of 6 mm or less is provided on the inner surface of the mold. [0007] In the technique, the mold additive is put on the meniscus, and a mold additive layer having a sufficient thickness is stably maintained for a long time in the gap between the mold and the solidified shell, and an air layer is formed and melted in the uneven portion provided on the inner surface of the mold. The mold additive layer is intended to achieve gentle cooling (hereinafter referred to as slow cooling) by the heat insulating properties of the air layer and the molten mold additive layer. [0008] However, when these techniques are actually applied to continuous casting, various problems occur. For example, a mold of a flat continuous casting machine of a variable width can be changed, because a long-side and short-side group mold is used, and if the concave portion provided on the inner surface of the mold coincides with the corner portion of the mold at the start of continuous casting, There is a problem that the molten steel is splashed into the concave portion of the corner portion at the start of the casting. [0009] When replacing the dip nozzle or when replacing the tundish, since the molten steel surface in the mold is lower than the normal casting state, the mold additive fixed to the inner surface of the mold is easily peeled off and detached. When the casting is started again, there is a problem that the molten steel and the molten steel are splashed into the concave portion of the corner portion. The phenomenon in which the molten steel enters the concave portion causes a constrained breakout of the solidified shell to occur. [0010] The mechanism for generating the center segregation of the cast piece can be imagined as follows. As the solidification progresses, the segregation component is concentrated between the solidified structure, that is, the dendritic tree. The molten steel in which the segregation component is concentrated flows out from the dendritic trees due to shrinkage of the cast piece at the time of solidification or swelling of a cast piece called bulging. The molten steel which has been concentrated by the eluted segregation component flows toward the final solidified portion, that is, the solidification end point, and solidifies to form a concentrated band which is a segregation component. This concentrated zone is the central segregation. As a countermeasure against the center segregation of the slab, it is effective to prevent the movement of the molten steel which is present after the segregation of the segregation components between the dendritic trees, and to prevent local accumulation of the molten steel after the segregation of the segregation components, and to use these principles. Several methods have been proposed. [0011] One of them is a soft reduction method using a cast piece of a roll group, and the effect of improving the center segregation is limited by a light press which is more than a certain amount of solidification shrinkage. Patent Document 3 proposes a method in which the slab is inflated at a position where the solid phase ratio of the center portion of the slab is 0.1 or less, and the thickness of the slab at the center portion in the width direction is formed to be shorter than the short side portion formed in the mold. After the thickness of the slab is 20 to 100 mm thick, the amount of bulging is equivalent to 20 mm or more by the pressing rolls of at least one pair immediately before the end of solidification, under the condition that the pressing amount of each pair of pressing rolls is 20 mm or more. Under pressure. [0012] Patent Document 4 proposes a method in which the thickness of the slab at the center portion in the width direction is 10 to 50 the thickness of the slab corresponding to the short side portion while the thickness of the unsolidified portion of the slab is 30 mm. After the thickness of % is bulged, the pressing corresponding to the bulging amount is performed by at least one pair of pressing rolls before the solidification end point. [0013] Patent Document 5 proposes a continuous casting method for steel in which the solid phase ratio of the center portion is 0.2 or more and 0.7 or less after bulging at 3% or more and 25% or less of the thickness of the slab at the start of bulging. The pressing is performed at a thickness corresponding to 30% or more and 70% or less of the bulging amount. [Patent Document 1] Japanese Patent Application Laid-Open No. Hei 9- 196 134. Patent Document 3: Japanese Patent Application No. Hei 7-210382. Japanese Patent Publication No. Hei 11-99285 [0015] Non-Patent Document 1: P. Perminov et al, Steel in English, (1968) No. 7. p. 560-562

[Problems to be Solved by the Invention] [0016] In continuous casting of steel, vibration in the vertical direction is imparted to a mold, and the vibration is used to prevent the solidified shell from being fused to the mold. By the vibration of the mold, periodic irregularities called oscillation marks are formed on the surface of the slab which is deformed at the tip end portion. When the unevenness of the vibration marks becomes large, the contact between the surface of the solidified shell and the mold becomes uneven, and the heat discharged from the mold also becomes uneven, so that the unevenness of the inner surface of the solidified shell also becomes large. When the unevenness of the inner surface of the initial solidified shell becomes large, the solidification interface of the final solidified portion becomes unsmooth, and even if it is pressed by the method described in Patent Documents 3 to 5, the effect cannot be sufficiently obtained. [Technical means for solving the problem] [0017] The gist of the present invention for solving the above problems is as follows. [1] A continuous casting method for steel, in which a molten steel is injected in a continuous casting mold, and the molten steel is pulled out while the continuous casting mold is vibrated in a casting direction to produce a cast piece. The continuous casting mold has a plurality of grooves, and the plurality of grooves are disposed at least 20 mm from a position of a meniscus in a normal casting state to at least 50 mm or more and 200 mm or less from a position of the meniscus. The inner wall surface of the mold copper plate at the position is filled with a plurality of different thermally conductive metal fillings in the inside of the plurality of grooves, with a metal or metal alloy having a thermal conductivity difference of 20% or more with respect to the thermal conductivity of the mold copper plate. The ratio of the sum of the areas of all the different thermally conductive metal filling portions, that is, the area ratio, is 10% or more and 80% or less with respect to the area of the inner wall surface on which the plurality of different thermally conductive metal filling portions are provided, and the vibration frequency is (f) The vibration pitch pitch (OMP) and the distance (D1) derived from the casting speed (Vc) satisfy the following formula (1), and the distance (D2) satisfies the following formula (2). In the formula (1), Vc is the casting speed (m/min), f is the vibration frequency (cpm), OMP is the vibration pitch pitch (mm), and D1 is from the other isothermally-conductive metal filling portion and the aforementioned mold copper plate. a distance (mm) from a boundary line to a boundary line between the different thermally conductive metal filling portion and the mold copper plate, and the other different thermally conductive metal filling portion is a center of gravity of the one of the plurality of different thermally conductive metal filling portions The mold copper plate is disposed at the same position in the width direction and adjacent to the one different heat conductive metal filling portion in the casting direction. In the formula (2), r is centered on the center of gravity of the different heat conductive metal filling portion and a radius (mm) of a circle having an area of the same area of the different heat conductive metal filling portion, and D2 is a distance (mm) from a center of gravity of the other heat conductive metal filling portion to a center of gravity of the one different heat conductive metal filling portion. The other different thermally conductive metal filling portion is disposed at the same position in the casting direction as the center of gravity of the one different heat conductive metal filling portion, and is adjacent to the one different heat conductive metal filling portion in the width direction. [2] The continuous casting method of steel according to the above [1], wherein the plurality of different thermally conductive metal filling portions are provided so that the distance (D1) satisfies the following formula (3). . [3] The continuous casting method of steel according to [1] or [2], wherein the plurality of grooves have the same shape. [4] The continuous casting method of steel according to any one of [1] to [3] wherein the plurality of grooves have a circular shape or a quasi-circular shape without an angle. [5] The continuous casting method of steel according to any one of [1] to [4] wherein the plurality of different thermally conductive metal filling portions are provided in a lattice shape. [6] The continuous casting method of steel according to any one of [1] to [4] wherein the plurality of different thermally conductive metal filling portions are provided in a staggered shape. [7] The continuous casting method of steel according to any one of [1] to [6] wherein the roll opening degree of the plurality of pairs of the slab support rolls provided in the continuous casting machine is gradually toward the downstream side in the casting direction. Increasing, the length of the slab of the slab having the unsolidified portion inside is increased with respect to the thickness of the slab at the exit of the mold (thickness between the long sides of the slab) by a total bulging amount exceeding a range of 0 mm and 20 mm or less, and then The light reduction belt whose roller opening degree of the plurality of pairs of the slab support rolls is gradually decreased toward the downstream side in the casting direction is at least the time point when the solid phase ratio at the center portion of the thickness of the slab is 0.2 to 0.9. The product of the reduction speed (mm/min) and the casting speed (m/min) (mm.m/min ² ) is 0.30 or more and 1.00 or less, and the pressing force is applied to the long side surface of the cast piece by the aforementioned pressing force. The long side surface of the cast piece is pressed at a total reduction amount equal to the total bulge amount described above or a total reduction amount smaller than the total bulge amount. [8] The continuous casting method of steel according to any one of [1] to [7] wherein, in the outer wall surface of the mold copper plate, a plurality of slits along the casting direction are in the width of the mold copper plate. The direction is set in a singular or plural pitch. When the plurality of slits are set at a singular pitch, the singular pitch is set to Z (mm), and when the plurality of slits are plural In the case of the pitch setting, when the longest pitch among the aforementioned complex pitches is Z (mm), the aforementioned Z satisfies the following formula (4). Here, the center of gravity of the different heat conductive metal filling portion refers to the center of gravity of the cross-sectional shape of the different heat conductive metal filling portion on the molten steel side surface of the mold copper plate. [Effect of the Invention] According to the present invention, a plurality of different thermally conductive metal-filled portions are provided along the width direction of the continuous casting mold and the casting direction in the vicinity of the meniscus including the meniscus position, and thus the meniscus is formed. The thermal resistance of the continuous casting mold can be periodically increased or decreased in the vicinity of the width direction of the mold and the casting direction. Thereby, the heat flux from the solidified shell to the continuous casting mold is periodically increased or decreased in the vicinity of the meniscus, that is, in the initial stage of solidification. By the increase and decrease of the periodicity of the heat flux, the stress and thermal stress caused by the transformation from δ iron to γ iron are reduced, and the deformation of the solidified shell due to the stress is reduced. By reducing the deformation of the solidified shell, the uneven heat flux distribution due to the deformation of the solidified shell is made uniform, and the generated stress is dispersed to reduce the respective strains. As a result, cracking of the surface of the solidified shell can be prevented. [0019] According to the present invention, the portion where the heat flux is increased or decreased can be present at least once between the pitches of the vibration marks, so that the depth of the vibration marks can be made shallow, and the surface of the solidified shell can be made uniform. Thereby, the inner surface of the solidified shell which grows together with the surface is also uniformized, and the solidification interface of the final solidified portion is smoothed, and the spot which forms segregation is reduced, and the internal quality of the flat embryo cast sheet can be improved.

[0021] A specific implementation method of the present invention will be described with reference to the drawings. Fig. 1 is a schematic side view showing a vertical bending type flat embryo continuous casting machine to which the continuous casting method of steel of the present embodiment can be applied. [0022] The continuous casting mold 1 is provided with a continuous casting mold 5 (hereinafter simply referred to as "molding mold"), and the molten steel 11 is injected into the mold 5 to be solidified to form the outer shape of the cast sheet 12, and the mold 5 is to be molded. The vibration is made in the casting direction of the cast piece 12. The feed tank 2 is provided at a predetermined position above the mold 5, and the feed tank 2 is for relaying the molten steel 11 supplied from the tub (not shown) to the mold 5. Below the mold 5, a plurality of pairs of slab support rolls composed of a backup roll 6, a guide roll 7, and a pinch roll 8 are provided. Among them, the pinch roller 8 is used to support the cast piece 12, and is also a drive roller for pulling the cast piece 12 out. A nozzle (not shown) such as a water nozzle or a gas mist nozzle is disposed in a gap between the slab support rolls adjacent to each other in the casting direction to constitute a secondary cooling zone, and the cooling water is discharged from the nozzle of the secondary cooling zone (hereinafter The slab 12 is cooled while being pulled out, and the internal unsolidified portion 14 is reduced, and the solidified shell 13 is grown to be cast. A sliding nozzle 3 for adjusting the flow rate of the molten steel 11 is provided at the bottom of the feeding tank 2, and a dipping nozzle 4 is provided below the sliding nozzle 3. [0023] A plurality of conveying rollers 9 for conveying the cast slab 12 are disposed on the downstream side of the slab supporting roller, and a predetermined length is set from the cast slab 12 above the conveying roller 9. The slab cut piece 10 is cut by the flat slab 12a. Before and after the casting direction of the solidification end position 15 of the cast piece 12, the roll interval of the opposed guide rolls 7 is set to be gradually narrowed toward the downstream of the casting direction, that is, the light reduction belt 17 is provided, and the light reduction belt is provided. 17 is composed of a group of guide rollers to which a plurality of pairs of roller gradients are applied. [0024] The strip 12 can be lightly depressed under lightly pressing the strip 17 over its entire area or selected portion. In the present embodiment, the soft reduction belt 17 is provided so that at least the slab 12 having a solid phase ratio of the center portion of the thickness of the slab 12 is 0.2 to 0.9 enters the installation range of the soft reduction belt 17. [0025] The pressing gradient of the soft pressing belt 17 is expressed by the amount of narrowing of the opening degree per 1 m in the casting direction, that is, "mm/m"; and the pressing of the casting sheet 12 of the belt 17 under light pressing The speed (mm/min) is obtained from the product of the reduction gradient (mm/m) and the casting speed (m/min). A nozzle for cooling the cast piece 12 is also disposed between the slab support rolls constituting the soft reduction belt 17. Although FIG. 1 shows an example in which only the guide roller 7 is disposed on the light press belt 17, the pinch roller 8 may be disposed on the belt 17 in the light press. The slab support roll disposed on the light reduction belt 17 is also referred to as a "press roll". [0026] The opening degree of the guide roller 7 disposed between the lower end of the mold 5 and the liquidus liquid crater end position of the cast piece 12, toward the downstream side of the casting direction, per 1 roller or each plural roller sequentially The roller opening degree is increased until the amount of expansion of the roller opening degree becomes a predetermined value. The forced bulging belt 16 is constituted by these guide rollers 7, and the forced bulging belt 16 is for forcibly bulging the long side surface of the slab 12 having the unsolidified portion 14 therein. The cast piece supporting roll on the downstream side of the forced bulging belt 16 is connected to the soft reduction belt 17 after the roll opening degree is narrowed to a certain value or to the extent that the slab 12 has a shrinkage amount accompanying the temperature drop. 2 is a view showing an example of setting of a roll opening degree. As shown in Fig. 2, the forced bulging belt 16 is set to forcibly swell the long side surface of the cast piece by the static pressure of the molten steel to increase the thickness of the central portion of the long side surface of the cast piece (region b), and to force the bulging. The downstream side of the belt 16 is such that the opening degree of the roller is narrowed to a certain value or a degree corresponding to the contraction amount of the slab 12 which is accompanied by the temperature drop (region c), and then the belt is pressed at the long side of the slab Next (area d). A and e in Fig. 2 indicate a region in which the opening degree of the roll is narrowed to a level corresponding to the amount of shrinkage of the cast piece 12 accompanying the temperature drop. A' in Fig. 2 shows an example in which the opening degree of the roll is narrowed to a level corresponding to the amount of shrinkage accompanying the temperature drop of the cast piece 12, but the roll opening degree of the casting method (conventional method) which is not subjected to the light press is not performed. . [0028] In the forced bulging belt 16, the roller opening degree of the guide roller 7 is sequentially expanded toward the downstream side in the casting direction, and the molten steel static pressure generated by the unsolidified portion 14 is allowed to be near the short side of the cast piece 12. The excluded long side faces are forcibly inflated in accordance with the roll opening degree of the guide rolls 7. In the vicinity of the short side of the long side of the cast piece, since the short side of the cast piece after the solidification is completed is fixed and maintained, the thickness at the start of forced bulging is maintained, and therefore, the cast piece 12 becomes the length of the cast piece only by forced bulging. The swollen portion of the side surface is in contact with the guide roller 7. 3 is a schematic side view showing a mold long-side copper plate constituting a part of a mold of a continuous casting machine for flat embryos. The mold 5 shown in Fig. 3 is an example of a mold for continuous casting for casting a flat blank cast piece. The mold 5 is composed of a pair of mold long side copper plates 5a (hereinafter also referred to as "cast copper plates") and a pair of mold short side copper plates. Fig. 3 shows a mold long side copper plate 5a therein. The mold short-side copper plate is also the same as the long-side copper plate 5a of the mold, and a heat-conductive metal-filled portion 19 is provided on the inner wall surface side thereof, and the description of the mold short-side copper plate is omitted here. However, in the cast piece 12, since the width of the flat embryo is much larger than the thickness of the flat embryo, the solidified shell 13 on the long side of the cast piece tends to have stress concentration, and surface cracking easily occurs on the long side of the cast piece. Therefore, the mold short-side copper plate of the mold 5 for the flat embryo casting sheet may not be provided with the different heat conductive metal filling portion 19. [0030] As shown in FIG. 3, the Q position at least 20 mm from the meniscus position 18 at the time of normal casting of the long-side copper plate 5a of the mold is at least 50 mm or more and 200 mm or less downward from the meniscus position 18. The range of the inner wall surface of the R position is arranged in a staggered manner in the range of the length W in the width direction of the mold perpendicular to the casting direction: the ratio of the thermal conductivity difference of the thermal conductivity of the copper plate 5a to the long side of the mold is 20% or more. A circular or different thermally conductive metal filling portion 19 made of a metal or a metal alloy (hereinafter referred to as "different heat conductive metal"). "Mental surface" means "the molten steel surface in the mold". The "normal casting" is a state in which the molten steel of the casting mold 5 of the flat embryo continuous casting machine 1 is started to maintain a constant casting speed. At the time of normal casting, the injection speed of the sliding nozzle 3 toward the molten steel 11 of the mold 5 is automatically controlled, and is controlled so that the meniscus position 18 becomes constant. [0031] The thermally conductive metal-filled portion 19 is filled in a circular groove which is independently processed on the inner wall surface side of the mold copper plate, and is filled with a thermal conductivity different from that of the copper alloy constituting the mold copper plate. It is formed by a different heat conductive metal. [0032] As means for filling the inside of the circular groove with a thermally conductive metal having a thermal conductivity different from that of the copper alloy constituting the mold copper plate, a plating treatment or a spray treatment is preferably employed. Although a different heat conductive metal processed by a shape of a circular groove may be filled in a circular groove or the like to be filled, in this case, a gap or crack may occur between the different heat conductive metal and the mold copper plate. When a gap or crack occurs between the different heat conductive metal and the mold copper plate, cracking and peeling of the heat conductive metal may occur, which may cause a decrease in the life of the mold, cracking of the cast piece, or even constraint casting. Not ideal. The above problem can be prevented by filling the isothermally conductive metal by a plating treatment or a spray treatment. In the present embodiment, as the copper alloy used for the mold copper plate, a copper alloy to which chromium (Cr), zirconium (Zr) or the like is added, which is generally used as a mold for continuous casting, can be used. In recent years, in order to homogenize the solidification in the mold or prevent the inclusions in the molten steel from being caught in the solidified shell, an electromagnetic stirring device for stirring the molten steel in the mold is generally provided, in order to suppress the magnetic field strength of the electromagnetic coil to the molten steel. For the attenuation, a copper alloy with reduced conductivity can be used. In this case, the thermal conductivity is also lowered in accordance with the decrease in the electrical conductivity, so that the thermal conductivity of the mold copper plate is about 1/2 of that of pure copper (thermal conductivity: about 400 W/(m × K)). The copper alloy used as a mold copper plate generally has a lower thermal conductivity than pure copper. [0034] FIG. 4 is a conceptual view showing a thermal resistance at a position of three places of a long-side copper plate of a mold having a different heat conductive metal filling portion, corresponding to a position of a different thermally conductive metal filling portion, and a different heat conductive metal filling portion. It is formed by filling a metal whose thermal conductivity is lower than that of a mold copper plate. As shown in FIG. 4, the thermal resistance is relatively increased at the position where the heterothermally-conductive metal filling portion 19 is provided. [0035] By placing a plurality of different thermally conductive metal filling portions 19 in the width direction and the casting direction of the continuous casting mold near the meniscus including the meniscus position 18, as shown in FIG. The thermal resistance of the continuous casting mold in the width direction of the mold near the lunar surface and the casting direction is formed as a periodic increase or decrease. Thereby, the heat flux from the solidified shell to the continuous casting mold in the vicinity of the meniscus, that is, in the initial stage of solidification, is formed into a periodic increase and decrease distribution. [0036] The case where the metal having a higher thermal conductivity than the mold copper plate is filled to form the different heat conductive metal filling portion 19 is different from that of FIG. 4, and the thermal resistance is relatively lowered at the position where the different thermally conductive metal filling portion 19 is provided. Also in this case, as in the above, the thermal resistance of the continuous casting mold in the width direction of the mold and the casting direction in the vicinity of the meniscus is formed as a periodic increase or decrease. In order to form a periodic distribution of the thermal resistance as described above, the different thermally conductive metal filling portions 19 are preferably independent of each other. [0037] By increasing or decreasing the periodicity of the heat flux, the stress and thermal stress caused by the phase transition state of the solidified shell 13 (for example, the metamorphosis of the δ iron to the γ iron) can be reduced, and the stress is generated due to the stress. The deformation of the solidified shell 13 becomes small. By making the deformation of the solidified shell 13 small, the uneven heat flux distribution resulting from the deformation of the solidified shell 13 is made uniform, and the generated stress is dispersed to reduce the respective strain amount. As a result, the occurrence of surface cracks on the surface of the solidified shell can be suppressed. [0038] The thickness of the solidified shell 13 in the mold is made uniform not only in the width direction of the cast piece but also in the casting direction by the periodic increase or decrease of the heat flux at the initial stage of solidification. By homogenizing the thickness of the solidified shell 13 in the mold, the solidification interface of the solidified shell 13 of the cast piece 12 pulled out from the mold 5, even in the final solidified portion of the cast piece, in the width direction and the casting direction of the cast piece It also becomes smooth. [0039] However, in order to stably obtain the aforementioned effects, the periodic increase and decrease of the heat flux caused by the provision of the different heat conductive metal filling portion 19 must be appropriate. That is, if the difference between the periodic increase and decrease of the heat flux is too small, the effect of providing the heterothermally-conductive metal filling portion 19 cannot be obtained; on the contrary, if the difference between the periodic increase and decrease of the heat flux is too large, it occurs due to this. The stress becomes large, and surface cracking occurs due to the stress. [0040] The difference in the increase or decrease in the heat flux caused by the provision of the different heat conductive metal filling portion 19 depends on the difference in thermal conductivity between the mold copper plate and the different heat conductive metal, and the metal heat filling portion 19 disposed with respect to the heat conductive material. The area of the inner wall surface of the mold copper plate in the area, the ratio of the total area of all the different thermally conductive metal filling portions 19, that is, the area ratio. [0041] The mold copper plate used in the continuous casting method for steel of the present embodiment has a thermal conductivity of λ of a different heat conductive metal filled in a circular groove. _m When used, the thermal conductivity (λ) relative to the mold copper plate _c ), thermal conductivity of different thermally conductive metals (λ _m ) the ratio of the difference ((|λ _c -λ _m |/λ _c ) × 100) is a metal or metal alloy of 20% or more. By using the thermal conductivity (λ) relative to the copper alloy constituting the mold copper plate _c A metal or a metal alloy having a ratio of a difference in thermal conductivity of 20% or more causes the effect of periodically changing the heat flux caused by the heat conductive metal filling portion 19 to be sufficient, even in the case where cracking of the surface of the cast piece is likely to occur. At the time of casting and casting of medium carbon steel, the surface crack suppressing effect of the cast piece can be sufficiently exerted. The thermal conductivity of the mold copper plate and the thermal conductivity of the different thermal conductivity metal are the thermal conductivity at normal temperature (about 20 ° C). The thermal conductivity is generally lower as the temperature is higher, and the ratio of the difference in thermal conductivity of the different thermally conductive metal is 20% or more as long as the thermal conductivity of the molten metal relative to the mold plate is normal temperature, even in the use temperature as a continuous casting mold ( At about 200 to 350 ° C, the thermal resistance of the portion where the different heat conductive metal filling portion 19 is provided and the thermal resistance of the portion where the heat conductive metal filling portion 19 is not provided may be different. In the mold copper plate used in the continuous casting method for steel according to the present embodiment, the heat conductive metal filling portion 19 is provided so as to be opposed to the inner wall surface of the mold copper plate in the range in which the different heat conductive metal filling portion 19 is formed. Area A (A = (Q + R) × W, unit: mm ² ), the sum of the areas of all the different thermally conductive metal filling portions 19 (mm) ² The ratio of the area ratio ε (ε = (B / A) × 100) is 10% or more and 80% or less. By setting the area ratio ε to 10% or more, it is possible to secure an area occupied by the heat-conductive metal filling portion 19 having a different heat flux, and obtain a heat flux difference by using the different heat conductive metal filling portion 19 and the mold copper plate. The surface crack suppression effect of the cast piece. On the other hand, when the area ratio ε exceeds 80%, the portion of the heat conductive metal-filled portion 19 is excessively large, and the fluctuation period of the heat flux is long, and it is difficult to obtain the surface crack suppressing effect of the cast piece. Therefore, it is more preferable to provide the heterothermally conductive metal filling portion 19 such that the area ratio ε is 30% or more and 60% or less, and it is particularly preferable to provide the isothermal thermal conductivity so that the area ratio ε is 40% or more and 50% or less. Metal filling portion 19. [0044] as long as the thermal conductivity relative to the mold copper plate (λ _c The thermal conductivity of the filler metal (λ) _m The ratio of the difference is 20% or more, and the type of the different heat conductive metal is not particularly limited. For reference only, it can be used as a metal for filling metals, including pure nickel (Ni, thermal conductivity: 90W/(m×K)), pure chromium (Cr, thermal conductivity: 67W/(m×K)), pure cobalt. (Co, thermal conductivity: 70 W/(m×K)), an alloy containing the metals, and the like. These pure metals and alloys have lower thermal conductivity than copper alloys and can be easily filled in circular grooves by plating treatment and spray processing. Pure copper having a higher thermal conductivity than the copper alloy can also be used as the metal filled in the circular groove. For example, when pure copper is used as the filler metal, the thermal resistance of the portion where the different heat conductive metal filling portion 19 is provided becomes smaller than the thermal resistance of the portion of the mold copper plate. [0045] FIG. 5 shows an example of the planar shape of the groove. 3 and 4 show an example in which the groove shape is a circle as shown in Fig. 5(a), but the groove may not be circular. For example, the groove may also be an ellipse as shown in FIG. 5(b), and the corner portion may be formed into a rounded square or rectangle as shown in FIG. 5(c), and the ring shape is as shown in FIG. 5(d). . It may be a triangle as shown in Fig. 5(e), a trapezoid as shown in Fig. 5(f), a pentagon as shown in Fig. 5(g), and as shown in Fig. 5(h). The surface has a prominent shape (star shape). A heat conductive metal filling portion whose shape corresponds to the shape of the groove is provided in the grooves. [0046] The shape of the groove is preferably a circular shape as shown in FIG. 5(a) or a shape having no "angle" as shown in (b) to (d), but may be as shown in the drawing. A shape having a "corner" as shown in 5(e) to (h). By setting the shape of the groove to a shape having no "angle", the boundary surface between the different heat conductive metal and the mold copper plate becomes a curved surface, stress on the boundary surface is not easily concentrated, and cracking is less likely to occur on the surface of the mold copper plate. In the present embodiment, among the shapes of the grooves, for example, a non-circular shape as shown in FIGS. 5(b) to 5(h) is referred to as a quasi-circular shape. When the shape of the groove is quasi-circular, the groove processed on the inner wall surface of the mold copper plate is referred to as a "quasi-circular groove". The radius of the quasi-circular shape can be evaluated by the radius of a circle having the same area as the area of the quasi-circular shape, that is, the equivalent circle radius r. The equivalent circle radius r of the quasi-circular shape is calculated by the following formula (5). [0048] In (5), S _Ma The area of the quasi-circular groove (mm ² ). [0049] FIG. 6 is a partially enlarged view of a region in which a different thermally conductive metal filling portion is provided. As shown in Fig. 6, in the mold copper plate of the present embodiment, the circular different thermally conductive metal filling portions 19 are provided in a staggered manner. Here, the staggered arrangement means that the heterothermally-conductive metal filling portion 19 is alternately provided at a half-pitch position of the different heat conductive metal filling portion 19. [0050] In FIG. 6, 19a denotes a different thermally conductive metal filling portion, and 19b denotes another heterothermally conductive metal filling portion. The center of gravity of the different thermally conductive metal filling portion 19a and the center of gravity of the different thermally conductive metal filling portion 19b are provided at the same position in the width direction of the mold copper plate, and are disposed adjacent to each other in the casting direction. Here, the center of gravity of the different heat conductive metal filling portion 19 means the center of gravity of the cross-sectional shape of the different heat conductive metal filling portion 19 on the molten steel side surface of the mold copper plate. [0051] When the distance from the boundary line between the different heat conductive metal filling portion 19a and the mold copper plate in the casting direction to the boundary line between the different thermally conductive metal filling portion 19b and the mold copper plate is D1 (mm), the heterothermally conductive metal The filling portion 19 is provided on the inner wall surface of the mold copper plate so that the distance D1 satisfies the following formula (1). [0052] In the formula (1), Vc is a casting speed (m/min), f is a vibration frequency (cpm), and OMP is a vibration pitch pitch (mm). [0053] In this manner, the interval between the boundary line of the heat conductive metal filling portion 19 and the mold copper plate in the casting direction, that is, the interval between the different heat conductive metal filling portions 19 in the casting direction is larger than the casting direction of the vibration marks. In a manner in which the pitch is small, the different heat conductive metal filling portion 19 is provided on the mold copper plate. Thereby, the portion where the heat flux is increased or decreased can be present at least once between the pitches of the vibration marks, and the claws generated at the time of forming the vibration marks are slowly cooled at an intentionally short pitch, thereby causing the cause. The uneven heat flux in the claw deformation is uniformized, and the respective strain variables become small. As a result, it is possible to suppress the falling of the claws and to make the depth of the vibration marks shallow, and to make the thickness of the solidified shell 13 in the casting direction uniform. By making the thickness of the initial solidified shell 13 uniform, the solidification interface of the final solidified portion forming the center segregation is smoothed, whereby the point of segregation is also reduced, and the internal quality can be improved. By making the depth of the vibration mark shallow, it is also possible to suppress the lateral crack starting from the vibration mark. The heat-conductive metal-filled portion 19 is provided on the inner wall surface of the long-side copper plate 5a of the mold so that the distance D1 satisfies the following formula (3). [0055] In the formula (3), r represents a radius (mm) or an equivalent circle radius (mm) of the different thermally conductive metal filling portion 19. [0056] In the same manner, the space of the heat conductive metal filling portion 19 in the casting direction is equal to or less than twice the radius of the heat conductive metal filling portion 19 or the equivalent circular radius, and the heat conductive metal filling portion 19 is formed. Set on the mold copper plate. Thereby, the heat flux difference can be imparted everywhere in the casting direction, and the heat flux from the solidified shell to the continuous casting mold can be periodically increased and decreased at the initial stage of solidification, and the respective strains can be made small. In FIG. 6, 19a denotes a different heat conductive metal filling portion, and 19c denotes another heat conductive metal filling portion. The center of gravity of the different heat conductive metal filling portion 19a and the center of gravity of the different heat conductive metal filling portion 19c are provided at the same position in the casting direction, and are disposed adjacent to each other in the width direction of the mold copper plate. When the distance from the center of gravity of the different heat conductive metal filling portion 19a to the center of gravity of the different heat conductive metal filling portion 19c is set to D2 (mm), the different heat conductive metal filling portion 19 satisfies the following at a distance D2 (2). The method is set on the inner wall surface of the long copper plate 5a of the mold. [0058] In the formula (2), r represents a radius (mm) or an equivalent circle radius (mm) of the different thermally conductive metal filling portion 19. In the same manner, the distance from the center of gravity of the different thermally conductive metal filling portion 19a to the center of gravity of the different thermally conductive metal filling portion 19c is equal to or less than four times the radius of the thermally conductive metal filling portion 19, and the thermal conductivity is different. The metal filling portion 19 is provided on a mold copper plate. Thereby, the portion where the heat flux formed by the different heat conductive metal filling portion 19 is increased or decreased is present at a shorter pitch than the spatial cycle of the solidification fluctuation of the tip end portion of the solidified shell which is not uniformly solidified. The deformation of the solidified shell 13 at the initial stage of solidification can be made small, and the respective strains can be reduced, and the crack on the surface of the solidified shell can be suppressed. [0060] FIG. 7 is a schematic view showing the outer wall surface side of a mold long side copper plate. Figure 8 is a cross-sectional view showing the DD section of Figure 7 in a state in which the backing plate is provided on the outer wall surface of the long-side copper plate of the mold, and further, the one of the bolt holes on the right side of the DD section is screwed with the cross-section of the stud. . On the outer wall surface of the mold long side copper plate 5a, a plurality of slits 30 through which the cooling water 44 passes, and bolt holes 32 which are screwed to the stud bolts 42 for fixing the back plate 40 are provided. The slits 30 are provided at a plurality of pitches in the width direction of the long-side copper plate 5a of the mold along the casting direction, avoiding the bolt holes 32. In the example shown in FIG. 7, the slit 30 is provided at a position of L2 at a position avoiding the bolt hole 32, and the slit 30 is provided at a position other than this at a pitch of L1. Here, L2>L1, in the example shown in Fig. 7, the longest pitch of the slit 30 is L2. [0061] The backing plate 40 is fixed to the outer wall surface of the long-side copper plate 5a of the mold by the stud bolts 42. The cooling water 44 is supplied from the lower side of the backing plate 40, and is discharged from the upper side of the backing plate 40 through the slit 30. In this manner, the cooling water 44 is passed through the slit 30 of the long-side copper plate 5a of the mold, and the long-side copper plate 5a of the mold is cooled by the cooling water 44. The portion in which the slit 30 is provided is not comparable to the thermally conductive metal filling portion 19, but causes a variation in the periodic heat flux in the width direction of the mold. When the spatial period of the slit 30 and the distance D2 in the width direction of the different thermally conductive metal filling portion 19 are close to each other, a periodic variation of the periodic heat flux between the two occurs, which is called "beat" (hereinafter referred to as "beat"). "Poet"). If a beat is generated, the periodic variation of the heat flux caused by the different thermally conductive metal filling portion 19 may be broken. [0063] The pitch of the slit and the slit 30 are adjusted so that the size of the heat flux in the region where the slit 30 is provided at the pitch of L2 avoids the bolt hole 32 and the magnitude of the heat flux in the other region are the same. depth. Therefore, when the longest pitch of the slits 30 is set to Z, it is preferable that the distance D2 in the width direction of the different heat conductive metal filling portion 19 based on Z satisfies the following formula (4), and the difference is set. Thermal conductive metal filling portion 19. [0064] In the formula (4), Z represents the longest pitch (mm) of the slit 30 in the width direction of the long-side copper plate 5a of the mold. Thereby, it is possible to suppress the spatial period of the slit 30 from being close to the distance D2 in the width direction of the different thermally conductive metal filling portion 19, and it is possible to suppress the periodic variation of the heat flux caused by the heterothermally conductive metal filling portion 19. destroyed. In the example shown in FIG. 7, the slit 30 is provided at a plurality of pitches on the outer wall surface of the long copper plate 5a of the mold, but the invention is not limited thereto. The slits 30 may also be provided at a single pitch on the outer wall surface of the long side copper plate 5a of the mold. When the slits 30 are arranged at a single pitch, the singular pitch is set to Z (mm). [0067] FIG. 9 shows another example of the arrangement of the different thermally conductive metal filling portions. In FIG. 9, the circular different thermally conductive metal filling portion 20 is provided in a lattice shape on the inner wall surface of the mold copper plate. Here, the provision of the different heat conductive metal filling portion 20 in a lattice shape means that the parallel line group having a constant width in the casting direction and parallel to the width direction of the mold and a parallel line having a constant width in the width direction of the mold and parallel to the casting direction. At the position of the intersection of the groups, a different thermally conductive metal filling portion 20 is provided. In FIG. 9, 20a denotes a different thermally conductive metal filling portion, and 20b, 20c denote other different thermally conductive metal filling portions. The center of gravity of the different thermally conductive metal filling portion 20a and the center of gravity of the different thermally conductive metal filling portion 20b are provided at the same position in the width direction of the mold copper plate, and are disposed adjacent to each other in the casting direction. The center of gravity of the different thermally conductive metal filling portion 20a and the center of gravity of the different thermally conductive metal filling portion 20c are provided at the same position in the casting direction, and are disposed adjacent to each other in the width direction of the mold copper plate. In FIG. 9, the distance D1 is the distance along the casting direction, which is the distance from the boundary line of the different thermally conductive metal filling portion 20a and the mold copper plate to the boundary line between the different thermally conductive metal filling portion 20b and the mold copper plate. The distance D2 is the distance from the center of gravity of the different thermally conductive metal filling portion 20a to the center of gravity of the different thermally conductive metal filling portion 20c. In Fig. 9, the heterothermally-conductive metal filling portion 20 is provided on the inner wall surface of the long-side copper plate 5a of the mold so as to satisfy the above formulas (1), (2), and (3). [0070] In the case where the isothermally conductive metal-filled portion is provided in a lattice shape on the mold copper plate, the isothermally conductive metal-filled portion may be provided in a lattice shape, and the claw (1) can be suppressed. When the depth is reduced, the depth of the vibration marks is made shallow, and the same effect as the case where the different heat conductive metal filling portions are provided in a staggered manner is obtained. In the present embodiment, the shape of the groove provided in the mold copper plate is similarly circular, but the shape is not limited thereto. As long as at least the above area ratio is 10% or more and 80% or less and the formulas (1) and (2) are satisfied, the shapes of the grooves may not all be the same. [0072] The combination of the mold provided with the different heat conductive metal filling portion 19 and the following method can further improve the internal quality of the cast piece. In this method, the slab is intentionally swelled to a degree exceeding 0 mm and 20 mm or less, and the slab having a solid phase ratio of the center portion of 0.2 or more and 0.9 or less is used for the pressing speed (mm/min) and the casting speed. Product of (m/min) (m.mm/min ² The pressing force of 0.30 or more and 1.00 or less is lightly pressed in the same amount or less than the amount of expansion of the cast piece when it is intentionally inflated. In the present embodiment, the total amount of forced bulging of the forced bulging belt 16 (hereinafter referred to as "total bulging amount") is set so that the thickness of the slab (the thickness between the long sides of the cast piece) with respect to the exit of the mold is More than 0mm, 20mm or less. In the present embodiment, the initial solidification in the mold is controlled, and even in the final solidified portion of the cast piece 12, the solidification interface can be made smooth in the width direction and the casting direction of the cast piece, so that the pressing force generated by the light pressing is suppressed. It can act equally on the solidification interface, so that the central segregation can be alleviated even if the total bulging amount is more than 0 mm and 20 mm or less. When the belt 17 is lightly pressed, the cast piece 12 is pressed at least at a point point when the solid phase ratio at the center portion of the thickness of the cast piece becomes 0.2 to 0.9. When the solid phase ratio of the center portion is less than 0.2, the pressing is performed because the thickness of the unsolidified portion of the cast piece is thicker at the depressed position immediately after the pressing, and the center segregation occurs again as the solidification progresses thereafter. . When the solid phase ratio of the center portion is reduced by more than 0.9, the molten steel after the concentration of the segregation component is hardly discharged, and the effect of improving center segregation is small. This is because the thickness of the solidified shell 13 of the cast piece at the time of pressing is thick, and the pressing force cannot sufficiently reach the center portion of the thickness. Further, when the solid phase ratio of the center portion exceeds 0.9 and the amount of reduction is large, positive segregation occurs in the vicinity of the center portion of the thickness as described above. Therefore, the position of the slab having a solid phase ratio of 0.2 or more and 0.9 or less in the center portion is pressed. Of course, the slab 12 can be pressed down by the soft reduction belt 17 before the solid phase ratio at the center portion of the slab thickness is 0.2 and the solid phase ratio at the center portion of the slab thickness exceeds 0.9. [0075] The solid phase ratio at the center portion of the slab thickness can be obtained by two-dimensional heat transfer solidification calculation. Here, the solid phase ratio is defined as a solid phase ratio of 0 or more above the liquidus temperature of the steel, a solid phase ratio of 1.0 or less below the solidus temperature of the steel, and a solid phase ratio of 1.0 at the center of the thickness of the cast piece. The solidification end position 15 corresponds to the position on the most downstream side of the solid portion ratio of the center portion of the slab thickness in the state where the slab is moved to the downstream side. In the present embodiment, the total amount of reduction (hereinafter referred to as "total reduction") of the slab 12 of the lightly depressed belt 17 is equal to or smaller than the total bulging amount. By making the total reduction amount the same as the total bulging amount or smaller than the total bulging amount, the portion where the solidification of the center portion of the thickness of the short side of the cast piece 12 is not completed can be reduced, and the light reduction can be reduced. The load of the guide roller 7 of the belt 17 can suppress equipment failure such as breakage or breakage of the guide roller 7. [0077] In the present embodiment, the product of the reduction speed and the casting speed when the belt 17 is lightly pressed is lightly pressed (mm.m/min). ² The pressing force of 0.30 or more and 1.00 or less is added to the long side surface of the cast piece. When the pressure is reduced by a reduction amount smaller than 0.30, the thickness of the unsolidified portion of the cast piece is thicker at the depressed position after the pressing, and the molten steel after the segregation component is concentrated cannot be sufficiently discharged from the dendritic tree. Therefore, central segregation will occur again after pressing. When the reduction is carried out at a reduction of more than 1.00, the molten steel which is concentrated in the dendritic tree is almost completely extracted and discharged to the upstream side in the casting direction, but the thickness of the unsolidified portion is higher. It is thin and is caught by the solidified shell on both sides in the thickness direction of the cast piece on the upstream side in the casting direction than the pressed position, so that positive segregation occurs in the vicinity of the center portion of the thickness of the cast piece. [0078] The effect of preventing the center segregation of the center portion of the cast piece and the light segregation occurring in the vicinity of the center portion is also affected by the solidification structure of the cast piece, and the portion in contact with the unsolidified portion In the case where the solidified structure is equiaxed, there is a concentrated molten steel which is a cause of semi-macro segregation between the equiaxed crystals, and the effect by the reduction is reduced. Therefore, the solidified structure should be a columnar crystal structure rather than an equiaxed crystal. [0079] In the present embodiment, the solid phase ratio of the solidified shell 13 and the solid phase ratio of the central portion of the slab thickness are determined by two-dimensional heat transfer solidification calculation or the like under various casting conditions of the continuous casting operation, at least in casting. When the solid phase ratio of the center portion of the sheet thickness is 0.2, the time when the solid phase ratio is 0.2 to 0.9, the cast piece 10 can be pressed down by the light press belt 14, and the amount of secondary cooling water, the limit of secondary cooling, and the casting speed can be adjusted. One or two or more. Here, the "limitation of secondary cooling" means that the cooling water is stopped toward the ejection of both end portions of the long side surface of the cast piece. By performing the limiting of the secondary cooling, the secondary cooling is weakly cooled. Generally, the solidification end position 13 extends toward the downstream side in the casting direction. [0080] As described above, by carrying out the continuous casting method of steel according to the present embodiment, it is possible to prevent surface cracking of the cast piece due to uneven cooling of the solidified shell at the initial stage of solidification, and also to cause vibration marks. The depth is shallower. Since the surface of the initial solidified shell 13 becomes uniform by making the vibration marks shallower, the solidification interface at the final solidified portion is also smoothed, and further intentional bulging and light pressing are performed, and the pressing force can be uniformly applied to the solidification. The interface suppresses center segregation occurring at the center portion of the thickness of the cast piece. Thereby, stable production of high quality cast pieces can be achieved. The above description is directed to the continuous casting of the flat slabs, but the continuous casting method of the steel of the present embodiment is not limited to the continuous casting of the flat slabs, and the blooms are small in the middle. The above description can also be applied to the continuous casting of the slab. Example 1 [0082] Medium carbon steel (chemical composition: 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%, Al: 0.020 to 0.040 mass%), a water-cooled copper mold in which metal is placed under various conditions on the inner wall surface, and the total bulging amount in the forced bulging belt and the pressing speed of the belt under light pressing The product of the casting speed was cast in various changes, and a test for the surface crack and internal quality (central segregation) of the cast piece after casting was investigated. [0083] The product of the pressing speed and the casting speed of the belt is 0.28 to 0.90 mm. m/min ² In each test, the slab was pressed at a time point when the solid phase ratio at the center portion of the thickness of the slab became 0.2 at a time of 0.9. The total reduction amount of the case where the cast piece is forcedly inflated in the forced bulging belt is set to be equal to or smaller than the total bulging amount. The test for not allowing the slab to be inflated in the forced bulging belt is performed by gently pressing the belt, and the solidification end position on the short side of the continuous casting sheet is also pressed. The mold used was a mold having an inner space size of a long side length of 2.1 m and a short side length of 0.26 m. The length from the upper end to the lower end (=mold length) of the water-cooled copper mold used was 950 mm, and the position of the meniscus (melting molten steel surface in the mold) at the time of normal casting was set to be 100 mm downward from the upper end of the mold. In order to grasp the effects of the continuous casting method of steel of the present embodiment, a mold of the following conditions was produced and a comparative test was performed. For each mold, as the different heat conductive metal, a metal whose thermal conductivity is lower than that of the mold copper plate is used. The shape of the different thermally conductive metal filling portion 19 is a circular shape of φ6 mm. Under this casting condition, the pitch of the vibration marks was 13 mm. [0085] Mold 1: a ratio of a thermal conductivity difference with respect to the thermal conductivity of copper in a range from a position 80 mm below the upper end of the mold to a position 300 mm below the upper end of the mold (range length = 220 mm) The heat conductive metal filling portion 19 is provided as a heat conductive metal of 20%. The area ratio ε of the different thermally conductive metal filling portion 19 is 50%. The distance D1 between the thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the thermally conductive metal filling portions 19 in the width direction of the mold is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold was 33.0 mm. [0086] Mold 2: a ratio of a difference in thermal conductivity from a position of 190 mm from the upper end of the mold to a position 750 mm from the upper end of the mold (range length = 670 mm) in a staggered manner with respect to the thermal conductivity of copper The heat conductive metal filling portion 19 is provided as a heat conductive metal of 20%. The area ratio ε of the different thermally conductive metal filling portion 19 is 50%. The distance D1 between the thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the thermally conductive metal filling portions 19 in the width direction of the mold is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold was 33.0 mm. [0087] Mold 3: in a range from a position 80 mm below the upper end of the mold to a position 300 mm below the upper end of the mold, the ratio of the thermal conductivity difference with respect to the thermal conductivity of the copper is 20% in a staggered manner. The metal is provided with a different heat conductive metal filling portion 19. The area ratio ε of the different thermally conductive metal filling portion 19 is 50%. The distance D1 between the thermally conductive metal filling portions 19 in the casting direction is 15 mm, and the distance D2 between the centers of gravity of the thermally conductive metal filling portions 19 in the width direction of the mold is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold was 33.0 mm. [0088] Mold 4: in a range from a position 80 mm below the upper end of the mold to a position 300 mm below the upper end of the mold, the ratio of the difference in thermal conductivity to the thermal conductivity of copper is 20% in a staggered manner. The metal is provided with a different heat conductive metal filling portion 19. The area ratio ε of the different thermally conductive metal filling portion 19 is 50%. The distance D1 between the thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the thermally conductive metal filling portions 19 in the width direction of the mold is 15 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold was 38.0 mm. [0089] Mold 5: in a range from a position 80 mm downward from the upper end of the mold to a position 300 mm below the upper end of the mold, the ratio of the thermal conductivity difference with respect to the thermal conductivity of copper is staggered to 15%. The metal is provided with a different heat conductive metal filling portion 19. The area ratio ε of the different thermally conductive metal filling portion 19 is 50%. The distance D1 between the thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the thermally conductive metal filling portions 19 in the width direction of the mold is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold was 33.0 mm. [0090] Mold 6: in a range from a position 80 mm below the upper end of the mold to a position 300 mm below the upper end of the mold, the ratio of the thermal conductivity difference with respect to the thermal conductivity of the copper is 20% in a staggered manner. The metal is provided with a different heat conductive metal filling portion 19. The area ratio ε of the different thermally conductive metal filling portion 19 is 5%. The distance D1 between the thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the thermally conductive metal filling portions 19 in the width direction of the mold is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold was 33.0 mm. [0091] Mold 7: in a range from a position 80 mm downward from the upper end of the mold to a position 300 mm below the upper end of the mold, the ratio of the difference in thermal conductivity to the thermal conductivity of copper is 20% in a staggered manner. The metal is provided with a different heat conductive metal filling portion 19. The area ratio ε of the different thermally conductive metal filling portion 19 is 85%. The distance D1 between the thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the thermally conductive metal filling portions 19 in the width direction of the mold is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold was 33.0 mm. [0092] Mold 8: In a range from a position 80 mm downward from the upper end of the mold to a position 300 mm below the upper end of the mold, the ratio of the thermal conductivity difference with respect to the thermal conductivity of the copper is 20%. The metal is provided with a different heat conductive metal filling portion 19. The area ratio ε of the different thermally conductive metal filling portion 19 is 50%. The distance D1 between the thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the thermally conductive metal filling portions 19 in the width direction of the mold is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold was 33.0 mm. [0093] Mold 9: in a range from a position 80 mm downward from the upper end of the mold to a position 300 mm below the upper end of the mold, the ratio of the thermal conductivity difference with respect to the thermal conductivity of the copper is 20% in a staggered manner. The metal is provided with a different heat conductive metal filling portion 19. The area ratio ε of the different thermally conductive metal filling portion 19 is 50%. The distance D1 between the thermally conductive metal filling portions 19 in the casting direction is 9 mm, and the distance D2 between the centers of gravity of the thermally conductive metal filling portions 19 in the width direction of the mold is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold was 33.0 mm. [0094] The mold 10: in a range from a position 80 mm below the upper end of the mold to a position 300 mm below the upper end of the mold, the ratio of the thermal conductivity difference with respect to the thermal conductivity of the copper is 20% in a staggered manner. The metal is provided with a different heat conductive metal filling portion 19. The area ratio ε of the different thermally conductive metal filling portion 19 is 50%. The distance D1 between the thermally conductive metal filling portions 19 in the casting direction is 9 mm, and the distance D2 between the centers of gravity of the thermally conductive metal filling portions 19 in the width direction of the mold is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold was 16.5 mm. [0095] Mold 11: A mold in which the different heat conductive metal filling portion 19 is not provided. [0096] In the continuous casting operation, as a mold additive, alkalinity ((% by mass CaO) / (% by mass SiO) is used. ² )) is 1.1, the solidification temperature is 1090 ° C, and the viscosity at 1300 ° C is 0.15 Pa. s mold additive. The solidification temperature refers to a temperature at which the viscosity of the mold additive sharply increases during cooling of the molten mold additive. The position of the meniscus in the mold during normal casting is 100 mm from the upper end of the mold. During the casting, the position of the meniscus is controlled in such a manner that the meniscus is within the set range. The casting speed during normal casting was 1.7 to 2.2 m/min, and the cast piece for investigating the surface crack and internal quality of the cast piece was used in all tests with a casting speed of 2.0 m/min at the time of normal casting. Casting the object as an object. The molten steel in the feed tank has a superheat of 25 to 35 °C. The temperature management of the mold is at a position 50 mm below the meniscus of the mold, and the thermocouple is buried from the back surface at a depth of 5 mm from the surface (surface on the molten steel side), and the temperature of the copper plate obtained from the thermocouple is measured. To estimate the surface temperature of the mold. [0097] After the continuous casting was completed, the surface of the long side of the cast piece was pickled to remove the scale, and the number of occurrence of surface cracks was measured. The occurrence of cracks on the surface of the cast piece is determined by using the calculated numerical value as the denominator in the casting direction of the cast piece to be inspected, and the length of the cast piece in the portion where the surface crack occurs is taken as a numerator. The evaluation of the internal quality (central segregation) of the cast piece is carried out by taking a cross-sectional sample of the cast piece and measuring the Mn concentration per 100 μm by EPMA in the range of ±10 mm in the center portion of the cast surface of the cross-sectional sample. degree. Specifically, the Mn concentration at the end where segregation should not occur (C ₀ And the ratio (C) of the Mn concentration of ±10 mm in the center portion (C/C) ₀ ) was defined as the degree of Mn segregation and was evaluated. [0098] In addition to the above discussion, the measurement of the unevenness σ (mm) of the thickness of the solidified shell was carried out under the conditions of each test number. The unevenness of the thickness of the solidified shell was measured by introducing FeS (iron sulfide) powder into a molten steel in a mold, and sampling the profile of the obtained cast piece to measure the thickness of the solidified shell. The thickness of the solidified shell was measured at a position of 1/4 of the width direction of the mold, and from the position of the meniscus to a position of 200 mm downward, 40 points were measured at a pitch of 5 mm. σ can be calculated according to the following formula (6). [0099] In the formula (6), D is a measured value (mm) of the thickness of the solidified shell, and Di is an approximate expression using a relationship between a predetermined solidified shell thickness and a solidification time, and is used at a position corresponding to the thickness of the solidified shell. The calculated value (mm) of the solidified shell thickness calculated from the solidification time of the lunar surface distance. N represents the number of measurements, which is 40 in this embodiment. Table 1 shows the results of investigations on the test conditions of each test of Test Nos. 1 to 14 and the quality of the surface and interior of the cast piece. [0102] Test Nos. 1, 8, 9, 10, 11, and 13, the setting conditions of the different heat conductive metal filling portion 19 on the surface of the mold are within the scope of the present invention, and the longest pitch of the slit 30 satisfies the formula (4). These test numbers are all, and the surface crack ratio can be greatly improved. The unevenness of the thickness of the solidified shell is 0.30 or less, and the thickness of the solidified shell becomes uniform. However, regarding test No. 1, since the product of the reduction speed and the casting speed was not in the range of 0.30 or more and 1.00 or less, it was confirmed that there was slight center segregation. Regarding other numbers, there is a result that central segregation is improved. In Test No. 2, the range in which the different heat conductive metal filling portion 19 is provided is shifted downward, and the product of the pressing speed and the casting speed is not in the range of 0.30 or more and 1.00 or less. Therefore, in Test No. 2, the surface of the cast piece was slightly cracked, and the effect of reducing the surface crack was not confirmed in the past. The unevenness of the thickness of the solidified shell was 0.38 mm, which was large, and the improvement effect was not confirmed about the center segregation. In Test No. 3, the distance D1 in the casting direction was long, and the product of the reduction speed and the casting speed was not in the range of 0.30 or more and 1.00 or less. In Test No. 3, although the crack on the surface of the cast piece was improved, the unevenness of the thickness of the solidified shell was 0.37 mm, which was large, and the improvement effect was not confirmed about the center segregation. In Test No. 4, the distance D2 in the mold width direction was long, and the product of the reduction speed and the casting speed was not in the range of 0.30 or more and 1.00 or less. In Test No. 4, it was confirmed that the surface crack of the cast piece occurred, and the effect of improving the surface crack could not be confirmed. The unevenness of the thickness of the solidified shell was 0.31 mm and somewhat increased. Regarding the center segregation, it was confirmed to be slight. [0107] Test No. 5, the ratio of the difference in thermal conductivity of the different thermally conductive metal was less than 20%; Test No. 6, the area ratio of the different thermally conductive metal filling portion 19 was less than 10%; Test No. 7, isothermally conductive metal filling The area ratio of the part 19 is higher than 80%. Therefore, in Test Nos. 5 to 7, it was confirmed that the surface crack of the cast piece occurred, and the effect of improving the surface crack could not be confirmed. The unevenness of the thickness of the solidified shell was slightly increased from 0.31 to 0.33, and the center segregation was confirmed to be slight. In Test No. 12, the product of the reduction speed and the casting speed was in the range of 0.30 or more and 1.00 or less, but the distance D1 in the casting direction was long. In Test No. 12, although the surface crack and center segregation of the cast piece were improved, the thickness unevenness of the solidified shell was 0.37 mm and became large. Test No. 14, the setting conditions of the different heat conductive metal filling portion 19 on the surface of the mold are within the scope of the present invention, and the longest pitch Z of the slit 30 satisfies the formula (4). However, the distance D1 in the casting direction is long, and although the formula (1) is satisfied, the formula (3) cannot be satisfied. Therefore, although the surface crack ratio was better than the test Nos. 2 to 7, it was 1.8% and was somewhat enlarged, and slight center segregation was confirmed, and the unevenness of the thickness of the solidified shell was 0.31 mm and was somewhat increased. In Test No. 15, the setting conditions of the different heat conductive metal filling portion 19 on the surface of the mold were within the scope of the present invention, but the longest pitch Z of the slit 30 could not satisfy the formula (4). Further, the distance D1 in the casting direction is long, and although the formula (1) is satisfied, the formula (3) cannot be satisfied. Therefore, although the surface crack ratio was better than the test number 2 to 7, it was 1.5% and was somewhat enlarged, and it was confirmed that there was slight center segregation, and the unevenness of the thickness of the solidified shell was 0.33 mm and was somewhat large. In Test No. 16, since the heterothermally-conductive metal filling portion 19 was not provided, the surface crack of the cast piece was confirmed. The unevenness of the thickness of the solidified shell was 0.32 mm and was somewhat enlarged, and center segregation was also confirmed.

[0112]

1‧‧‧ flat embryo continuous casting machine

2‧‧‧ Feeding trough

3‧‧‧Sliding mouth

4‧‧‧impregnation mouth

5‧‧‧Continuous casting mold

5a‧‧‧Molded long edge copper plate

6‧‧‧Support roller

7‧‧‧guide roller

8‧‧‧Pinch roller

9‧‧‧Transport roller

10‧‧‧ Casting machine

11‧‧‧Fused steel

12‧‧‧ cast

12a‧‧‧ flat embryo casting

13‧‧‧ solidified shell

14‧‧‧Unsolidified part

15‧‧‧End of solidification

16‧‧‧forced bulging belt

17‧‧‧Light reduction belt

18‧‧‧Moonmoon position

19‧‧‧Different thermal conductivity metal filling

19a‧‧‧A different thermally conductive metal filling section

19b‧‧‧Other heterothermal metal filling parts

19c‧‧‧Other heterothermally conductive metal filling parts

20‧‧‧Differential thermal conductivity metal filling

20a‧‧‧A different thermally conductive metal filling section

20b‧‧‧Other heterothermal metal filling parts

20c‧‧‧Other heterothermally conductive metal filling parts

30‧‧‧slit

32‧‧‧Bolt holes

40‧‧‧ Backboard

42‧‧‧ column bolt

44‧‧‧ cooling water

1 is a schematic side view showing a vertical bending type flat embryo continuous casting machine to which the continuous casting method of steel of the present embodiment can be applied. Fig. 2 is a view showing an example of a profile of the roll opening degree. Fig. 3 is a schematic side view showing a long-side copper plate of a mold constituting a part of a mold set in a continuous casting machine for flat embryos. 4 is a conceptual diagram showing the thermal resistance at three positions of a long-side copper plate of a mold having a different heat conductive metal filling portion, corresponding to the position of the different thermally conductive metal filling portion, and the different thermally conductive metal filling portion is filled with the same. The thermal conductivity is lower than that of the mold copper plate. 5(a) to (h) show an example of the planar shape of the groove. Fig. 6 is a partially enlarged view of a region in which a different thermally conductive metal filling portion is provided. Fig. 7 is a schematic view showing the outer wall surface side of the long side copper plate of the mold. Figure 8 is a cross-sectional view showing the DD section of Figure 7 in a state in which the backing plate is provided on the outer wall surface of the long-side copper plate of the mold, and further, the one of the bolt holes on the right side of the DD section is screwed with the cross-section of the stud. . Fig. 9 shows another example of the arrangement of the different thermally conductive metal filling portions.

Claims (9)

A continuous casting method for steel, in which a molten steel is injected in a mold for continuous casting, and the molten steel is pulled out while the continuous casting mold is vibrated in a casting direction to produce a cast piece, which is used for continuous casting. The casting mold has a plurality of grooves, and the plurality of grooves are disposed at a position at least 20 mm from the position of the meniscus in the normal casting state to a position at least 50 mm or more and 200 mm or less from the position of the meniscus. The inner wall surface of the copper plate is filled with a plurality of different thermally conductive metal filling portions in a plurality of grooves or with a metal or metal alloy having a thermal conductivity difference of 20% or more with respect to the thermal conductivity of the mold copper plate. The area of the inner wall surface of the plurality of different thermally conductive metal filling portions is set, and the ratio of the total area of all the different thermally conductive metal filling portions, that is, the area ratio is 10% or more and 80% or less, and the vibration frequency (f) The vibration pitch pitch (OMP) and distance (D1) derived from the casting speed (Vc) satisfy the following formula (1), and the distance (D2) satisfies the following formula (2), D1≦OMP=Vc×1000/f. ..(1) D2≦4r...(2) in (1) , Vc is a casting speed (m / min), f is a vibration frequency (cpm), OMP as chatter marks pitch (mm), D1 is the other side of the thermally conductive metal filler isobutyl and the copper plate from the mold a distance (mm) from a boundary line between a different thermally conductive metal filling portion and the mold copper plate, and the other different thermally conductive metal filling portion is a center of gravity of the one of the plurality of different thermally conductive metal filling portions The copper plate is disposed at the same position in the width direction and adjacent to the one different heat conductive metal filling portion in the casting direction. In the formula (2), r is centered on the center of gravity of the different thermally conductive metal filling portion and is The radius (mm) of the circle of the same area of the different heat conductive metal filling portion, and D2 is the distance (mm) from the center of gravity of the other heat conductive metal filling portion to the center of gravity of the one different heat conductive metal filling portion, and the other The different thermally conductive metal filling portion is disposed at the same position in the casting direction as the center of gravity of the one different heat conductive metal filling portion, and is adjacent to the one different heat conductive metal filling portion in the width direction. The continuous casting method for steel according to claim 1, wherein the plurality of different thermally conductive metal filling portions are provided such that the distance (D1) satisfies the following formula (3), and D1≦2r... 3). The continuous casting method of steel according to claim 1, wherein the plurality of grooves have the same shape. A continuous casting method for steel according to claim 1, wherein The plurality of grooves have a circular shape or a quasi-circular shape without an angle. The continuous casting method for steel according to claim 1, wherein the plurality of different thermally conductive metal filling portions are provided in a lattice shape. The continuous casting method of steel according to claim 1, wherein the plurality of different thermally conductive metal filling portions are provided in a staggered shape. The continuous casting method of steel according to any one of claims 1 to 6, wherein the roll opening degree of the plurality of pairs of the slab support rolls provided in the continuous casting machine is gradually increased toward the downstream side in the casting direction, and The long side surface of the slab having the unsolidified portion inside is enlarged with respect to the thickness of the slab at the exit of the mold (thickness between the long sides of the slab) in a range of more than 0 mm and 20 mm or less, and then, in the above plural The light reduction belt whose taper opening degree of the slab support roll is gradually decreased toward the downstream side in the casting direction, and the time when the solid phase ratio of the center portion of the thickness of the slab is at least 0.2 to 0.9, which is equivalent to the pressure The product of the lower speed (mm/min) and the casting speed (m/min) (mm.m/min ² ) is 0.30 or more and 1.00 or less, and the pressing force is applied to the long side surface of the cast piece by the aforementioned pressing force. The total reduction amount of the same amount as the total bulge amount described above or the total reduction amount smaller than the total bulge amount described above is pressed against the long side surface of the cast piece. The continuous casting method of steel according to any one of the preceding claims, wherein, in the outer wall surface of the mold copper plate, a plurality of slits along the casting direction are in the width direction of the mold copper plate The singular or plural pitch setting, when the plurality of slits are set at a singular pitch, the pitch of the singular number is set to Z (mm), and when the plurality of slits are set at a plurality of pitches In the case where the longest pitch among the plurality of pitches is Z (mm), the Z satisfies the following formula (4), Z ≧ 2.5 × D2 (4). The continuous casting method for steel according to claim 7, wherein, in the outer wall surface of the mold copper plate, a plurality of slits along the casting direction are disposed at a singular or plural pitch in the width direction of the mold copper plate. When the plurality of slits are set at a singular pitch, the pitch of the singular number is set to Z (mm), and when the plurality of slits are set at a complex pitch, the plurality of sections are When the longest pitch among the distances is Z (mm), the above Z satisfies the following formula (4), Z ≧ 2.5 × D2 (4).