TW201813739A - Continuous steel casting method in which the inner wall surface of a mold copper plate of the casting mold filled with hetero-thermally conductive metal filling portion and the area ration is 10% or more and 80% or less - Google Patents

Abstract

The present invention is to prevent surface cracking due to inhomogeneous cooling of the solidified shell in the early stage of solidification and to suppress the center segregation occurring in the thickness center of the cast slab. At the inner wall surface of a mold copper plate from a position of 20 mm above the meniscus to a position at least 50 mm or more and up to 200 mm or less below the meniscus position, the mold for continuous casting is provided with a hetero-thermally conductive metal filling portion which is filled with a metal with a ratio of 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 hetero-thermally conductive metal filling portion to the area of the inner wall surface provided with the hetero-thermally conductive metal filling portion, that is, the area ration is 10% or more and 80% or less. The vibration mark pitch (OMP) and the distance (D1) derived from the vibration frequency (f) and the casting speed (Vc) satisfy the formula (1): D1 ≤ OMP=Vc X 1000/f, and the distance (D2) satisfies the formula (2): D2 ≤ 4r; wherein r in formula (2) is a radius (mm) of a circle centered on gravity of the hetero-thermal thermally conductive metal filling portion and having the same area as the area of the hetero-thermally conductive metal filling portion.

Description

Continuous casting method of steel

[0001] The present invention relates to continuous casting technology, and more particularly, to a continuous casting method for steel, by suppressing uneven solidification of the slab in the initial stage of solidification, in order to improve surface cracking and center segregation of the slab.

[0002] Generally, when a steel slab is produced by continuous casting, a molten steel injected into a mold is brought into contact with the mold to be cooled, thereby forming a thin solidified layer (hereinafter referred to as a "solidified shell"). In this manner, the molten steel is poured into the mold, and the solidified shell is pulled downward (hereinafter referred to as "normal casting") to produce a cast piece. [0003] When the cooling unevenness caused by the mold becomes uneven, the thickness of the solidified shell becomes uneven, and as a result, the surface of the solidified shell becomes uneven. 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 tiny longitudinal crack remains even after the slab is completely solidified, and becomes a longitudinal crack on the surface of the slab. When longitudinal cracking occurs on the surface of the slab, it is necessary to perform removal of the longitudinal crack (hereinafter referred to as trimming) before sending the slab to a subsequent step (for example, a rolling step). [0004] The mold vibrates in the direction of casting (hereinafter also referred to as "swing"). The vibration of the mold causes the upper end of the solidified shell to bend toward the molten steel side, and the gap between the curved solidified shell and the inner wall surface of the mold will The molten steel is caused to overflow, and a portion (hereinafter referred to as a "claw portion") protruding toward the molten steel side is formed in the solidified shell. When the surface of the solidified shell is not smooth, the gap formed by the curved solidified shell and the inner wall surface of the mold becomes larger, and the claw portion of the solidified shell becomes larger. When the claw portion protruding toward the molten steel side becomes larger, non-metallic inclusions and bubbles floating in the molten steel on the meniscus (the molten steel liquid level in the mold) will be captured by the claw portion. The captured non-metallic inclusions and air bubbles cause surface defects such as surface flaws and swelling of the steel sheet after hot rolling or steel sheet after cold rolling. [0005] The frequency of such surface defects such as longitudinal cracks, blemishes, 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 accordingly. In hot charge and direct charge in which technology is being established in recent years, the slab dressing operation has also become a factor that hinders the stabilization of the operation. Therefore, it is extremely economically advantageous to prevent uneven growth of the solidified shell thickness and occurrence of claws due to uneven cooling at the initial stage of solidification. [0006] In order to prevent uneven cooling in the initial stage of solidification, it is necessary to perform uniform and gentle cooling in the initial stage of solidification to uniformly increase the thickness of the solidified shell, thereby preventing the generation of claws. In this regard, Non-Patent Document 1 describes that in continuous casting of a 280 × 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 slab. Patent Document 1 describes that a recess having a diameter or 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. Furthermore, Patent Document 2 describes that 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 these technologies, a mold additive is put on the meniscus surface, and a mold additive layer of sufficient thickness is stably maintained for a long period of time between the mold and the solidified shell, and an air layer is formed on the uneven portion provided on the inner surface of the mold to melt it. The mold additive layer is intended to achieve gentle cooling (hereinafter referred to as "gradual cooling") by utilizing the heat insulation 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, the mold of a flat-bodied continuous casting machine that can change the width is a set of molds that use long and short sides. If the recesses provided on the inner surface of the mold are consistent with the corners of the mold at the beginning of continuous casting, There is a problem that the molten steel splashes into the recess of the corner portion at the beginning of casting. [0009] When the immersion nozzle is replaced or the tank is replaced, because the molten steel level in the mold is lower than the normal casting state, the mold additives fixed to the inner surface of the mold become easy to peel off and detach. When the casting is started again, there is a problem that molten steel and molten steel splash into the corner of the corner portion. Such a phenomenon that molten steel enters the recessed portion causes a restrictive breakout of the solidified shell. [0010] The generation mechanism of the central segregation of the slab can be imagined as follows. As solidification progresses, segregation components are concentrated in the solidified structure, that is, between the dendritic trees. The molten steel after the segregation component is concentrated flows out from the dendrite tree due to shrinkage of the slab during solidification or swell of a slab called bulging. The molten steel after the outflow of the segregated component is concentrated flows toward the final solidification part, that is, the solidification end point, and solidifies as it is to form a concentrated zone of the segregated component. This thickening zone is central segregation. It is effective to prevent the center segregation of the slab from preventing the movement of the molten steel after the concentration of the segregation components existing in the dendrite tree and the local accumulation of the molten steel after the concentration of the segregation components are effective. Several methods have been proposed. [0011] One of them is a light reduction method using a slab of a reduction roller group, and the reduction effect on the central segregation is limited to a light reduction that exceeds a certain degree of solidification shrinkage. In Patent Document 3, a method is proposed in which a slab is swelled at a position where the solid phase ratio of the slab is 0.1 or less, and the thickness of the slab in the central portion in the width direction is formed to be shorter than that of a short side portion generated in a mold. After the thickness of the cast slab is 20 to 100 mm, at least one pair of reduction rollers is used immediately before the end of solidification, and the reduction amount of each pair of reduction rollers is 20 mm or more, and the amount of inflation is performed Down. [0012] In Patent Document 4, a method is proposed in which the thickness of the slab in the center portion in the width direction is set to be equal to 10 to 50 of the thickness of the slab in the short side portion while the thickness of the unsolidified portion of the slab is 30 mm. After the inflation is performed at a thickness of%, a reduction equivalent to the amount of inflation is performed by at least one pair of reduction rollers before the solidification end point. [0013] Patent Document 5 proposes a continuous casting method for steel, in which the thickness of the slab at the beginning of bulging is 3% or more and 25% or less, and the slab position where the solid phase ratio at the center is 0.2 or more and 0.7 or less The reduction is performed at a thickness corresponding to 30% to 70% of the bulging amount. [0014] Patent Document 1: Japanese Patent Application Laid-Open No. 9-94634 Patent Document 2: Japanese Patent Application Laid-Open No. 10-193041 Patent Literature 3: Japanese Patent Application Laid-Open No. 7-210382 Patent Literature 4: Japanese Patent Application Laid-Open No. 9-206903 JP Patent Document 5: Japanese Patent Application No. 11-99285 [0015] Non-Patent Document 1: P. Perminov et al, Steel in English, (1968) No. 7.p. 560 to 562

[Problems to be Solved by the Invention] In continuous casting of steel, vertical and horizontal vibrations are applied to a mold, and the vibration is used to prevent the solidified shell from being fused to the mold. Due to the vibration of the mold, periodic irregularities called oscillation marks are formed on the surface of the slab that is deformed at the front end. When the unevenness of the vibration mark becomes large, the contact between the surface of the solidified shell and the mold becomes uneven, and the heat radiation from the mold becomes uneven, so the unevenness on 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 solidified interface of the final solidified portion becomes uneven, and even if the pressing is performed by the method described in Patent Documents 3 to 5, the effect cannot be fully 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 of steel, in which molten steel is poured into 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 provided from a position at least 20 mm upward from the position of the meniscus in the normal casting state to a position at least 50 mm or more and at most 200 mm from the position of the meniscus downward. The inner wall surface of the mold copper plate at the position is filled with a metal or metal alloy having a thermal conductivity difference ratio of 20% or more with respect to the thermal conductivity of the mold copper plate inside the plurality of grooves, and a plurality of heterothermally conductive metal fillers are provided. The ratio of the total area of all the differently thermally conductive metal-filled portions to the area of the inner wall surface provided with the plurality of differently thermally conductive metal-filled portions, that is, the area ratio is 10% to 80%. (f) The vibration mark 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), In the formula (1), Vc is the casting speed (m / min), f is the vibration frequency (cpm), OMP is the vibration mark pitch (mm), and D1 is the distance from the other filled metal with different thermal conductivity and the copper plate of the aforementioned mold. The distance (mm) from the boundary line to a boundary line between the hetero-thermally conductive metal filling portion and the mold copper plate. The other hetero-thermally conductive metal filling portion has a center of gravity with the one of the plurality of hetero-thermally conductive metal filling portions. The copper plate of the mold is disposed at the same position in the width direction and is adjacent to the aforementioned one thermally-conductive metal-filled portion in the casting direction. In formula (2), r is centered on the center of gravity of the aforementioned thermally-conductive metal-filled portion and The radius (mm) of a circle having the same area as the area of the hetero-thermally conductive metal filled portion, D2 is the distance (mm) from the center of gravity of the other heat-conductive metal filled portion to the center of gravity of the one heat-conductive metal filled portion, The other heterothermally conductive metal filling portion is located at the same position as the center of gravity of the one heterothermally conductive metal filling portion in the casting direction and is adjacent to the one differently thermally conductive metal filling portion in the width direction. [2] The continuous casting method for steel according to [1], wherein the plurality of hetero-thermally conductive metal-filled portions are provided so that the distance (D1) satisfies the following formula (3), . [3] The continuous casting method for steel according to [1] or [2], wherein the shapes of the plurality of grooves are all the same. [4] The continuous casting method for steel according to any one of [1] to [3], wherein the shape of the plurality of grooves is circular or quasi-circular without corners. [5] The continuous casting method for steel according to any one of [1] to [4], wherein the plurality of hetero-thermally conductive metal-filled portions are provided in a grid shape. [6] The continuous casting method for steel according to any one of [1] to [4], wherein the plurality of hetero-thermally conductive metal-filled portions are provided in a staggered shape. [7] The continuous casting method for steel according to any one of [1] to [6], in which the roll opening degree of the plurality of pairs of slab support rolls provided in the continuous casting machine is gradually increased toward the downstream side in the casting direction. It is increased to increase the total bulging amount of the long side surface of the slab having an unsolidified portion inside the slab thickness (thickness between the slab long side surface) and the exit of the mold in a range of more than 0 mm and less than 20 mm. For the plurality of pairs of slab support rolls, the degree of roll reduction gradually decreases toward the downstream side of the casting direction. At least when the solid phase ratio of the thickness center of the slab becomes 0.2 to 0.9, it will be equivalent to 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. The reduction force is applied to the long side surface of the slab, and the reduction force is applied by the reduction force. The long side surface of the slab is reduced by the same total reduction amount as the total expansion amount or a total reduction amount smaller than the total expansion amount. [8] The continuous casting method for steel according to any one of [1] to [7], wherein the plurality of slits along the casting direction on the outer wall surface of the copper mold plate are in the width of the copper mold plate Set the singular or plural pitch in the direction. When the plurality of slits are set with the singular pitch, set the singular pitch to Z (mm). When the plurality of slits are plural When the pitch is set, when the longest pitch among the plurality of pitches is set to Z (mm), the aforementioned Z satisfies the following formula (4), Here, the center of gravity of the hetero-thermally-conductive metal-filled portion refers to the center of gravity of the cross-sectional shape of the heat-different-metal-filled portion on the molten steel side plane of the mold copper plate. [Effects of the Invention] According to the present invention, a plurality of differently thermally conductive metal-filled portions are provided in the vicinity of the meniscus including the meniscus position along the width direction and the casting direction of the continuous casting mold. The thermal resistance of continuous casting molds can be increased or decreased periodically in the width direction and casting direction of nearby molds. Thereby, the heat flux in the vicinity of the meniscus, that is, from the solidified shell to the mold for continuous casting at the initial stage of solidification is periodically increased or decreased. By the periodic increase and decrease 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 these stresses is reduced. By reducing the deformation of the solidified shell, the non-uniform heat flux distribution due to the deformation of the solidified shell is made uniform, and the stresses generated are dispersed to reduce the respective strain variables. As a result, cracks on the surface of the solidified shell can be prevented. [0019] According to the present invention, a portion where the heat flux can be increased or decreased at least once between the pitches of the vibration marks can be made shallower in depth and the surface of the solidified shell can be made uniform. As a result, the inner surface of the solidified shell that grows with the surface is also uniformized, so that the solidified interface of the final solidified portion is smoothed, and the number of spots at which segregation is formed is reduced, thereby improving the internal quality of the flat slab.

[0021] A specific implementation method of the present invention will be described with reference to the drawings. FIG. 1 is a schematic side view of a flat-bend continuous casting machine of a vertical bending type to which the continuous casting method of steel of this embodiment can be applied. [0022] The flat-bodied continuous casting machine 1 is provided with a continuous casting mold 5 (hereinafter simply referred to as a "mold"), molten steel 11 is poured into the mold 5 and solidified to form the shell shape of the cast piece 12, and the mold 5 It vibrates in the casting direction of the slab 12. A feed trough 2 is provided at a predetermined position above the mold 5, and the feed trough 2 is used to relay supply of molten steel 11 supplied from a ladle (not shown) to the mold 5. Below the casting mold 5, a plurality of pairs of cast sheet supporting rollers composed of a support roller 6, a guide roller 7, and a pinch roller 8 are provided. The pinch roller 8 is used to support the cast slab 12 and is also a driving roller for pulling the cast slab 12 out. Nozzles (not shown) such as water nozzles or aerosol nozzles are arranged in the gap between the slab support rollers adjacent to each other in the casting direction to form a secondary cooling zone, and cooling water (hereinafter (Also called "secondary cooling water") The slab 12 is cooled while being pulled out to reduce the number of unsolidified portions 14 inside, and the solidified shell 13 is grown and 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 an immersion nozzle 4 is provided below the sliding nozzle 3. [0023] A plurality of conveying rollers 9 for conveying the cast slabs 12 are provided on the downstream side of the slab support rolls. Above the conveying rollers 9, a predetermined length is provided from the cast slabs 12 to a predetermined length. The flat slab 12a cuts the slab cutting machine 10. The roll interval of the opposing guide rollers 7 is set to gradually narrow toward the downstream of the casting direction before and after the casting direction 15 across the solidification end position 15 of the slab 12, that is, a light pressing belt 17 is provided, and the light pressing belt 17 is composed of a plurality of pairs of guide rollers to which a roller gradient is applied. [0024] In the lightly-reduced belt 17, the ingot 12 can be lightly-reduced in the entire area or a partially selected area thereof. In the present embodiment, the lightly-reduced belt 17 is provided so that at least the ingot 12 having a solid phase ratio of 0.2 to 0.9 in the thickness center portion of the slab 12 falls within the installation range of the lightly-reduced belt 17. [0025] The reduction gradient of the light reduction belt 17 is expressed by the amount of narrowing of the roll opening per 1 m in the casting direction, that is, “mm / m”; 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 slab 12 is also disposed between the slab support rollers constituting the light-reduction belt 17. Although FIG. 1 shows an example in which only the guide roller 7 is disposed on the light-reduction belt 17, the pinch roller 8 may be disposed on the light-reduction belt 17. The slab support roller arranged on the light reduction belt 17 is also called a "reduction roller." [0026] The opening degree of the guide rollers 7 disposed between the lower end of the mold 5 and the end of the liquidus liquid crater of the slab 12 toward the downstream side of the casting direction in the order of every 1 or each roll The roller opening degree is increased until the amount of expansion of the roller opening degree becomes a predetermined value. These guide rollers 7 constitute a forced inflation belt 16 for forcibly inflating the long side surface of the slab 12 having the unsolidified portion 14 inside. The slab support roller on the downstream side of the forcibly inflated belt 16 is narrowed to a certain value by the degree of roll opening or a degree corresponding to the shrinkage of the slab 12 accompanied by the decrease in temperature, and then connected to the lightly pressed belt 17. [0027] FIG. 2 shows an example of setting of a roller opening degree. As shown in FIG. 2, in the forced inflation band 16, the long side surface of the slab is forcibly inflated by static pressure of molten steel to increase the thickness of the central portion of the long side surface of the slab (region b). On the downstream side after the belt 16, the opening degree of the roll is narrowed to a certain value or a degree corresponding to the shrinkage of the cast slab 12 accompanying the temperature drop (area c). Then, the long side of the cast slab is pressed by the belt 17 under a light pressure. Down (area d). A and e in FIG. 2 indicate regions in which the roll opening degree is narrowed to a degree corresponding to the shrinkage of the cast slab 12 as the temperature decreases. A 'in FIG. 2 shows an example of a roll opening that is narrowed to a degree equivalent to the shrinkage of the cast slab 12 as the temperature decreases. However, the roll opening is not carried out by a light pressing method (known method). . [0028] In the forced inflation belt 16, the roll opening of the guide roller 7 is sequentially enlarged toward the downstream side in the casting direction, and the molten steel static pressure generated by the unsolidified portion 14 can be used to make the vicinity of the short side of the cast piece 12 The excluded long-side surface is forcibly inflated in accordance with the roller opening degree of the guide roller 7. Near the short side of the long side of the slab, because the short side of the slab after the solidification is fixed, the thickness at the start of forced inflation is maintained. Therefore, the slab 12 becomes the length of the slab caused only by forced inflation. The bulged portion of the side surface is in contact with the guide roller 7. [0029] FIG. 3 is a schematic side view of a mold long-side copper plate constituting a part of a mold provided in a flat-blade continuous casting machine. The mold 5 shown in FIG. 3 is an example of a continuous casting mold used for casting a flat embryo slab. The mold 5 is a combination of a pair of mold long-side copper plates 5a (hereinafter also referred to as "mold copper plates") and a pair of mold short-side copper plates. Fig. 3 shows a mold long side copper plate 5a therein. The short-side copper plate of the mold is the same as the long-side copper plate 5a of the mold, and the inner wall surface side is provided with a metal filler 19 having a different thermal conductivity, and the description of the short-side copper plate of the mold is omitted here. However, due to the shape of the slab 12 having a much larger width than the slab thickness, the solidified shell 13 on the long side surface side of the slab tends to cause stress concentration, and surface cracks easily occur on the long side surface side of the slab. Therefore, the mold short-side copper plate of the mold 5 for the flat slab may not be provided with the metal filler 19 having a different thermal conductivity. [0030] As shown in FIG. 3, from the Q position at least 20 mm upward from the meniscus position 18 during normal casting of the mold long-side copper plate 5a to at least 50 mm to 200 mm below the meniscus position 18 downward. The range of the inner wall surface at the R position is staggered in the range of the length W of the mold width direction perpendicular to the casting direction: the thermal conductivity of the filling relative to the long side copper plate 5a of the mold is 20% or more. A metal or metal alloy (hereinafter referred to as "differentially thermally conductive metal") having a circularly differently thermally conductive metal filling portion 19. The "meniscus" means "the molten steel surface in the mold". "Normal casting" refers to a state in which the molten steel injection into the mold 5 of the flat-blade continuous casting machine 1 starts to cruise and maintains a constant casting speed. During normal casting, the injection speed of the sliding nozzle 3 toward the molten steel 11 of the mold 5 is automatically controlled, so that the meniscus position 18 becomes constant. [0031] The different thermally conductive metal filling portions 19 are filled in circular grooves that are independently processed on the inner wall surface side of the mold copper plate, and are filled with a thermal conductivity different from that of the copper alloy constituting the mold copper plate. It is formed of a metal with a different thermal conductivity. [0032] As a 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 spraying treatment is preferably used. Although a different thermally conductive metal processed in accordance with the shape of the circular groove can be inserted into the circular groove or the like for filling, in this case, a gap or crack may occur between the different thermally conductive metal and the mold copper plate. When gaps or cracks occur between the metal with different thermal conductivity and the mold copper plate, cracks and peeling of the metal with different thermal conductivity may occur, which may reduce the life of the mold, crack the slab, or even restrict the casting leakage. Not ideal. The above-mentioned problem can be prevented by filling the heterothermally conductive metal with a plating process or a spraying process. [0033] In the present embodiment, as the copper alloy used for the mold copper plate, a copper alloy with a small amount of chromium (Cr), zirconium (Zr), or the like 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 inclusions in the molten steel from being captured 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 attenuation, a copper alloy with reduced conductivity can be used. In this case, the thermal conductivity also decreases in accordance with the decrease in the electrical conductivity. Therefore, 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 the mold copper plate generally has a lower thermal conductivity than pure copper. [0034] FIG. 4 is a conceptual diagram showing thermal resistance at three positions of a long-side copper plate of a mold having a hetero-thermally conductive metal filling portion corresponding to the position of the hetero-thermally 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 installation position of the hetero-thermal conductive metal filling portion 19. [0035] The plurality of hetero-thermally conductive metal-filled portions 19 are provided in the vicinity of the meniscus including the meniscus position 18 along the width direction and the casting direction of the continuous casting mold, as shown in FIG. The thermal resistance of the continuous casting mold in the width direction of the mold near the lunar surface and in the casting direction is formed so as to increase and decrease periodically. Thereby, the heat flux near the meniscus, that is, from the solidified shell to the mold for continuous casting at the initial stage of solidification, is formed into a distribution that increases and decreases periodically. [0036] Different from FIG. 4, in the case where a metal with a higher thermal conductivity than that of a mold copper plate is filled to form a metal filled portion 19 having a different thermal conductivity, the thermal resistance is relatively reduced at the position where the metal filled portion 19 is provided. In this case as well, the thermal resistance of the continuous casting mold in the width direction of the mold in the vicinity of the meniscus and in the casting direction is formed to have a periodic increase and decrease in the same manner as described above. In order to form a periodic distribution of the thermal resistance as described above, it is preferable that the different thermally conductive metal-filled portions 19 are independent of each other. [0037] By the periodic increase and decrease of the heat flux, the stress and thermal stress caused by the phase transition state of the solidified shell 13 (for example, the transformation of δ iron to γ iron) can be reduced, so that the stress generated by these stresses The deformation of the solidified shell 13 becomes smaller. By making the deformation of the solidified shell 13 smaller, the uneven heat flux distribution due to the deformation of the solidified shell 13 is made uniform, and the generated stresses are dispersed to reduce the respective strain variables. As a result, the occurrence of surface cracks on the surface of the solidified shell can be suppressed. [0038] Through the periodic increase and decrease of the heat flux in the initial solidification period, the thickness of the solidified shell 13 in the mold becomes uniform not only in the width direction of the slab but also in the casting direction. By uniformizing the thickness of the solidified shell 13 in the mold, the solidified interface of the solidified shell 13 of the slab 12 after being pulled out from the mold 5, even in the final solidified part of the slab, in the width direction and casting direction of the slab. It also becomes smooth. [0039] However, in order to obtain the aforementioned effect stably, the periodic increase and decrease of the heat flux caused by the provision of the hetero-thermal 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 metal-filled portion 19 with different thermal conductivity cannot be obtained; on the contrary, if the difference between the periodic increase and decrease of the heat flux is too large, it is caused by this The stress becomes larger due to surface cracking due to the stress. [0040] The difference between the increase and decrease in the heat flux caused by the provision of the hetero-thermally conductive metal filling portion 19 depends on the difference in thermal conductivity between the mold copper plate and the hetero-thermally conductive metal, and relative to the disposition of the hetero-thermally conductive metal filler 19 The area of the inner wall surface of the mold copper plate in the region of the area is the ratio of the sum of the areas of all the different thermally conductive metal filling portions 19, that is, the area ratio. [0041] When the mold copper plate used in the continuous casting method of steel of this embodiment is used, the thermal conductivity of a metal having a different thermal conductivity filled in a circular groove is set to λ. _m When using: relative to the thermal conductivity of the copper plate of the mold (λ _c ), Thermal conductivity of metal with different thermal conductivity (λ _m ) Bad ratio ((λ _c -λ _m | / λ _c ) × 100) is a metal or metal alloy of 20% or more. By using the thermal conductivity (λ _c ) Metals or metal alloys with a thermal conductivity difference ratio of 20% or more make the effect of the periodic fluctuation of the heat flux caused by the hetero-thermally conductive metal filling portion 19 fully exerted, even at high speed where cracks on the surface of the slab are likely to occur At the time of casting and medium carbon steel casting, the surface crack suppression effect of the slab can be fully exerted. The thermal conductivity of the mold copper plate and the thermal conductivity of the metal with different thermal conductivity are the thermal conductivity at room temperature (about 20 ° C). The thermal conductivity is generally lower at higher temperatures. As long as the thermal conductivity of the metal with different thermal conductivity is 20% or more at room temperature relative to the thermal conductivity of the mold copper plate, even if it is used as a continuous casting mold ( At about 200 to 350 ° C., the thermal resistance of the portion where the hetero-thermal conductive metal filling portion 19 is provided and the thermal resistance of the portion where the hetero-thermal metal filling portion 19 is not provided can still be different. [0042] The mold copper plate used in the continuous casting method of steel according to this embodiment is provided with a hetero-thermal conductive metal filling portion 19 with respect to the inner wall surface of the mold copper plate in a range where the hetero-thermal conductive metal filling portion 19 is formed. Area A (A = (Q + R) × W, unit: mm ² ), The sum of the areas B (mm) of all the different thermally conductive metal-filled portions 19 ² ), That is, the area ratio ε (ε = (B / A) × 100) is 10% to 80%. By setting the area ratio ε to 10% or more, the area occupied by the differently thermally conductive metal-filled portions 19 with different heat fluxes can be secured, and the difference in heat flux can be obtained by using the differently thermally conductive metal-filled portions 19 and the mold copper plate to obtain Slab surface crack suppression effect. On the other hand, when the area ratio ε exceeds 80%, there are too many parts of the heterothermally-conductive metal-filled portion 19 and the period of fluctuation of the heat flux becomes long, making it difficult to obtain the effect of suppressing the surface cracks of the slab. [0043] Therefore, it is more preferable to provide the hetero-thermal conductive metal filling portion 19 so that the area ratio ε becomes 30% to 60%, and it is particularly preferable to provide the different thermal conductivity so that the area ratio ε becomes 40% to 50%. Metal-filled portion 19. [0044] As long as the thermal conductivity (λ _c Thermal conductivity of the filler metal (λ _m The difference ratio may be 20% or more, and the type of the metal with different thermal conductivity is not particularly limited. For reference only, metals that can be used as filler metals include 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)), and alloys containing these metals. These pure metals and alloys have a lower thermal conductivity than copper alloys, and can be easily filled in circular grooves by plating or spraying. Pure copper with higher thermal conductivity than copper alloys can also be used as the metal to fill the circular grooves. For example, when pure copper is used as the filler metal, the thermal resistance of the portion where the metal-filled portion 19 having different thermal conductivity is provided becomes smaller than the thermal resistance of the portion of the mold copper plate. [0045] FIG. 5 shows an example of a planar shape of a groove. Although FIGS. 3 and 4 show examples in which the shape of the groove is circular as shown in FIG. 5 (a), the groove may not be circular. For example, the groove may be an oval as shown in FIG. 5 (b), a corner or a square or a rectangle formed as shown in FIG. 5 (c), and a ring as shown in FIG. 5 (d). . It can also be a triangle as shown in Figure 5 (e), a trapezoid as shown in Figure 5 (f), a pentagon as shown in Figure 5 (g), and a shape as shown in Figure 5 (h) The surface has a prominent shape (star-shaped sugar). The grooves are provided with different thermally conductive metal filling portions whose shapes correspond to the shapes of the grooves. [0046] The shape of the groove is preferably a circle as shown in FIG. 5 (a) or a shape without a “corner” as shown in (b) to (d), but it may also be as shown in FIG. 5 (e) to (h) have a "corner" shape. By making the shape of the groove not to have a "corner" shape, the boundary surface between the heterothermally conductive metal and the mold copper plate becomes a curved surface, stress is not easily concentrated on the boundary surface, and cracks are unlikely to occur on the surface of the mold copper plate. [0047] In the present embodiment, among the shapes of the grooves, a non-circular shape such as shown in FIGS. 5 (b) to (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 called a "quasi-circular groove". The radius of a quasi-circle can be evaluated by using the radius of a circle having the same area as that of the quasi-circle, that is, the equivalent circle radius r. The quasi-circular equivalent circle radius r is calculated by the following formula (5). [0048] In formula (5), S _ma Is the area of a quasi-circular groove (mm ² ). [0049] FIG. 6 is a partially enlarged view of a region in which a metal-filled portion having a different thermal conductivity is provided. As shown in FIG. 6, in the mold copper plate of this embodiment, the circular hetero-thermal-conductive metal filling part 19 is provided in a staggered shape. Here, the staggered arrangement means that the hetero-thermally conductive metal filling portions 19 are alternately provided at half-pitch positions of the hetero-thermally conductive metal filling portions 19. [0050] In FIG. 6, 19 a represents a hetero-thermally conductive metal filling portion, and 19 b represents another hetero-thermally conductive metal filling portion. The center of gravity of the differently thermally conductive metal filling portion 19a and the center of gravity of the differently thermally conductive metal filling portion 19b are provided at the same position in the width direction of the mold copper plate and at positions adjacent to each other in the casting direction. Here, the center of gravity of the hetero-thermally-conductive metal filling portion 19 refers to the center of gravity of the cross-sectional shape of the hetero-thermally-conductive metal filling portion 19 on the molten steel side plane of the mold copper plate. [0051] When the distance from the boundary line between the heterothermally conductive metal filling portion 19a and the mold copper plate in the casting direction to the boundary line between the heterothermally 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 expression (1). [0052] In formula (1), Vc is the casting speed (m / min), f is the vibration frequency (cpm), and OMP is the pitch of the vibration marks (mm). [0053] In this way, the interval between the boundary lines of the different thermally conductive metal filling portions 19 in the casting direction and the mold copper plate, that is, the interval between the different thermally conductive metal filling portions 19 in the casting direction is greater than that in the casting direction of the vibration marks. With a small pitch, the hetero-thermally conductive metal-filled portion 19 is provided on a mold copper plate. Thereby, a portion where the heat flux can be increased or decreased at least once between the pitches of the vibration marks, and the claws generated during the formation of the vibration marks are slowly cooled with a deliberately short pitch, thereby causing the cause The uneven heat flux due to the deformation of the claws is uniformized, so that the respective strain variables are reduced. As a result, it is possible to suppress the fall of the claws to reduce the depth of the vibration marks and to make the thickness of the solidified shell 13 in the casting direction uniform. By making the thickness of the solidified shell 13 in the initial stage uniform, the solidified interface of the final solidified part forming the central segregation is smoothed, thereby reducing the number of points where segregation is formed, and the internal quality can be improved. By reducing the depth of the vibration marks, it is also possible to suppress lateral cracks starting from the vibration marks. [0054] The heterothermally conductive metal filling portion 19 is provided on the inner wall surface of the long 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 heterothermally-conductive metal-filled portion 19. [0056] In this manner, the hetero-thermally conductive metal-filled portion 19 is formed such that the interval between the different-heat-conductive-metal-filled portions 19 in the casting direction is equal to or less than twice the radius of the hetero-thermally-conductive metal-filled portion 19 or the equivalent circle radius. It is set on a mold copper plate. Thereby, a difference in heat flux can be imparted everywhere in the casting direction, and the heat flux from the solidified shell to the mold for continuous casting can be periodically increased and decreased at the initial stage of solidification, and the respective strain variables can be reduced. [0057] In FIG. 6, 19 a denotes a differently thermally conductive metal filling portion, and 19 c denotes another differently thermally conductive metal filling portion. The center of gravity of the differently thermally conductive metal filling portion 19a and the center of gravity of the differently thermally conductive metal filling portion 19c are provided at the same position in the casting direction and at positions adjacent to each other in the width direction of the mold copper plate. Here, when the distance from the center of gravity of the heterothermally conductive metal filling portion 19a to the center of gravity of the heterothermally conductive metal filling portion 19c is set to D2 (mm), the heterothermally conductive metal filling portion 19 satisfies the following (2) with a distance D2 ) Type is provided 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 heterothermally-conductive metal-filled portion 19. [0059] As such, the distance from the center of gravity of the hetero-thermally-conductive metal filling portion 19a to the center of gravity of the heat-differentially-metal filling portion 19c becomes 4 times or less the radius of the heat-differentially-metal filling portion 19 so that The metal filling portion 19 is provided on a mold copper plate. As a result, the portion where the heat flux formed by the hetero-thermally conductive metal filling portion 19 increases or decreases is allowed to exist at a pitch shorter than the space period of the solidification fluctuation at the front end portion of the non-uniformly solidified solidified shell. The deformation of the solidified shell 13 at the initial stage of solidification can be reduced, and the respective strain variables are also reduced, thereby suppressing cracks on the surface of the solidified shell. [0060] FIG. 7 is a schematic view showing an outer wall surface side of a mold long-side copper plate. Fig. 8 is a schematic cross-sectional view of the DD section of Fig. 7 in a state where a back plate is provided on the outer wall surface of the long side copper plate of the mold, and the cross section of the stud bolt screwed to one of the bolt holes on the right side of the DD section is superimposed and displayed. . The outer wall surface of the long copper plate 5a of the mold is provided with a plurality of slits 30 through which the cooling water 44 passes, and bolt holes 32 screwed with the column bolts 42 for fixing the back plate 40. The slits 30 are provided at plural pitches in the width direction of the long copper plate 5a of the mold along the casting direction, avoiding the bolt holes 32. In the example shown in FIG. 7, the slits 30 are provided at a pitch of L2 at positions that avoid the bolt holes 32, and the slits 30 are provided at a pitch of L1 at other positions. Here, L2> L1. In the example shown in FIG. 7, the longest pitch of the slit 30 is L2. [0061] The back plate 40 is fixed to the outer wall surface of the mold long-side copper plate 5a by the stud bolts 42. The cooling water 44 is supplied from below the back plate 40 and is discharged from above the back plate 40 through the slit 30. In this way, the cooling water 44 is passed through the slit 30 of the long copper plate 5a of the mold, and the long copper plate 5a of the mold is cooled by the cooling water 44. [0062] Although the portion provided with the slit 30 is inferior to the metal-different-filling portion 19 having a different thermal conductivity, a periodic heat flux variation occurs in the width direction of the mold. When the space period of the slit 30 and the distance D2 in the width direction of the hetero-conductive metal-filled portion 19 are close, the so-called "beat" (hereinafter referred to as "beat") occurs in the periodic variation of the periodic heat flux between the two. "Difference"). If a beat occurs, the periodic variation of the heat flux caused by the hetero-conductive metal-filled portion 19 may be destroyed. [0063] Adjusting the pitch of L1 and the slit 30 so that the magnitude of the heat flux in the region where the slit 30 is provided at a pitch of L2 is the same as the magnitude of the heat flux in other regions, avoiding the bolt hole 32. depth. Therefore, when the longest pitch of the slit 30 is set to Z, it is preferable that the distance D2 in the width direction of the hetero-conductive metal-filled portion 19 with Z as a reference satisfies the following formula (4), and the difference is set. Thermally 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 mold long-side copper plate 5a. [0065] Thereby, the space period of the slit 30 and the distance D2 in the width direction of the hetero-thermally-conductive metal filling portion 19 can be suppressed from being close, and the periodic fluctuation of the heat flux caused by the hetero-thermally-conductive metal filling portion 19 can be suppressed. destroyed. [0066] The example shown in FIG. 7 is an example in which slits 30 are provided at a plurality of pitches on the outer wall surface of the mold long-side copper plate 5a, but it is not limited to this. The slits 30 may be provided at an odd pitch on the outer wall surface of the mold long-side copper plate 5a. When the slits 30 are provided at a single pitch, the single pitch is set to Z (mm). [0067] FIG. 9 shows another example of the arrangement of the differently thermally conductive metal-filled portions. In FIG. 9, the circular hetero-thermally conductive metal-filled portion 20 is provided on the inner wall surface of the mold copper plate in a grid pattern. Here, the arrangement of the hetero-thermally conductive metal filler 20 in a grid pattern means a group of parallel lines having a constant width in the casting direction and parallel to the mold width direction, and parallel lines having a constant width in the mold width and parallel to the casting direction. At the intersections of the clusters, a hetero-thermally conductive metal filler 20 is provided. [0068] In FIG. 9, 20a represents one hetero-thermally conductive metal filling portion, and 20b and 20c represent other hetero-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 at positions 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 located adjacent to each other in the width direction of the mold copper plate. [0069] In FIG. 9, the distance D1 is the distance along the casting direction, and is the distance from the boundary line between the heterothermally conductive metal filling portion 20a and the mold copper plate to the boundary line between the heterothermally conductive metal filling portion 20b and the mold copper plate. The distance D2 is a distance from the center of gravity of the heterothermally-conductive metal filling portion 20a to the center of gravity of the hetero-thermally conductive metal filling portion 20c. In FIG. 9, the hetero-thermally conductive metal-filled portion 20 is provided on the inner wall surface of the mold long-side copper plate 5 a so as to satisfy the formulas (1), (2), and (3). [0070] The hetero-thermally conductive metal filling portion may be provided in a grid pattern on the mold copper plate as described above, and the case where the hetero-thermally conductive metal filling portion is provided in a grid shape may also be satisfied. By satisfying the above formula (1), the claws can be suppressed. Falling down makes the depth of the vibration marks shallow, and the same effect as in the case where the hetero-thermally conductive metal-filled portions are arranged in a staggered manner is obtained. [0071] In this embodiment, an example in which the shapes of the grooves provided in the mold copper plate are all circular is shown, but it is not limited to this. As long as the area ratio is at least 10% to 80% and the formulas (1) and (2) are satisfied, the shapes of the grooves may not be all the same. [0072] Combining the mold provided with the hetero-thermally conductive metal filling portion 19 with the following method can further improve the internal quality of the cast piece. In this method, the slab is intentionally swelled to a depth of more than 0 mm and 20 mm, and the solid phase rate of the central part is 0.2 to 0.9. The slab is equivalent to a reduction speed (mm / min) and a casting speed. (m / min) Product (m · mm / min ² ) Is a reduction force of 0.30 or more and 1.00 or less, and is lightly reduced by an amount equal to or smaller than the expansion amount of the slab when it is intentionally swelled. [0073] In the present embodiment, the total amount of forced inflation of the forced inflation band 16 (hereinafter referred to as the “total inflation amount”) is set so that the thickness of the slab (thickness between the long sides of the slab) with respect to the die exit is In the range of more than 0mm and 20mm. In this embodiment, the initial solidification in the mold is controlled so that the solidified interface can be smoothed in the width direction and the casting direction of the slab even in the final solidified part of the slab 12, so the reduction force generated by light reduction It can evenly act on the solidification interface, so that the central segregation can be reduced even if the total bulging amount is more than 0mm and less than 20mm. [0074] In the lightly-reduced belt 17, the slab 12 is pressed down at least when the solid phase ratio of the thickness center portion of the slab becomes 0.2 to 0.9. In the case where the solid phase rate of the central part does not reach 0.2, the thickness of the unsolidified part of the slab is thicker at the reduction position immediately after the reduction. As the subsequent solidification progresses, central segregation will occur again. . When the reduction is performed at a period when the solid phase ratio of the central portion exceeds 0.9, the molten steel after the thickening of the segregation component is difficult to be discharged, and the effect of improving the central segregation becomes small. This is because the thickness of the solidified shell 13 of the slab at the time of reduction is thick, and the reduction force cannot sufficiently reach the center of the thickness. Further, when the solid phase ratio of the central portion exceeds 0.9 and the reduction amount is large, positive segregation occurs near the central portion of the thickness as described above. Therefore, reduction is performed at the position of the slab having a solid phase ratio of 0.2 to 0.9 in the central portion. Of course, before the solid phase ratio of the slab thickness center portion becomes 0.2 and the solid phase ratio of the slab thickness center portion exceeds 0.9, the slab 12 can be pressed down by the light pressing belt 17. [0075] The solid phase ratio at the center of the thickness of the slab can be obtained by two-dimensional heat transfer and solidification calculation. Here, the solid phase ratio is defined as: the solid phase ratio above the liquidus temperature of the steel = 0, the solid phase ratio below the solidus temperature of the steel = 1.0, and the solid phase ratio at the center of the slab thickness is 1.0 The position at is the solidification end position 15 which corresponds to the position on the most downstream side where the solid phase ratio at the center of the thickness of the slab is 1 when the slab is moved downstream. [0076] In this embodiment, the total amount of reduction of the slab 12 in the light reduction belt 17 (hereinafter referred to as the "total reduction amount") is made the same as or smaller than the total expansion amount. By making the total reduction amount equal to or smaller than the total bulge amount, it is possible to reduce the formation of a light reduction by not pressing down the solidified portion that reaches the thickness center portion of the short side of the slab 12. The load of the guide roller 7 with the belt 17 can suppress equipment failure such as breakage and breakage of the bearing of the guide roller 7. [0077] In this embodiment, the product of the reduction speed and the casting speed (mm.m / min. ² ) Is 0.30 or more and 1.00 or less. When the reduction is performed with a reduction amount smaller than 0.30, the thickness of the unsolidified portion of the slab is thick at the reduction position after the reduction, and the molten steel after the segregation component is concentrated cannot be sufficiently discharged from the dendritic tree. Therefore, center segregation will occur again after pressing. When rolling is performed at a rolling reduction of more than 1.00, almost all the molten steel in which the segregation component between the dendrite trees is concentrated is squeezed out and discharged to the upstream side in the casting direction. It is thin and is captured by the solidified shells on both sides in the thickness direction of the slab that is upstream of the casting direction from the pressing position. Therefore, positive segregation occurs near the thickness center of the slab. [0078] The effect of light reduction for preventing central segregation in the central part of the slab and positive segregation near the central part is also affected by the solidified structure of the slab. When the part is in contact with the unsolidified part In the case where the solidified structure is equiaxed, there is a thickened molten steel that causes semi-macro segregation between the equiaxed grains, and the effect due to reduction is reduced. Therefore, the solidified structure should be a columnar crystal structure rather than an equiaxed crystal. [0079] In this embodiment, under various casting conditions of continuous casting operations, the thickness of the solidified shell 13 and the solid phase ratio at the center of the thickness of the slab are determined in advance using two-dimensional heat transfer and solidification calculations, so that When the solid phase ratio at the center of the sheet thickness is 0.2 to 0.9, the amount of secondary cooling water, the limit of secondary cooling, and the casting speed can be adjusted in such a manner that the casting strip 10 is depressed by the light reduction belt 14 1 or more. Here, the "secondary cooling limit" refers to stopping the spray of cooling water toward both ends of the long side surface of the slab. The secondary cooling is performed to limit the secondary cooling to weak cooling. Generally, the solidification end position 13 extends downstream of the casting direction. [0080] As described above, by carrying out the continuous casting method of steel according to this embodiment, it is possible to prevent surface cracks of the slab caused by uneven cooling of the solidified shell at the initial stage of solidification, and at the same time, it is possible to cause vibration marks. The depth becomes shallower. The shallower vibration marks make the surface of the initial solidified shell 13 uniform, and the solidified interface at the final solidified part is also smoothed. Further deliberate inflation and light reduction can make the reduction force evenly act on the solidification. Interface, suppressing the occurrence of central segregation at the center of the thickness of the slab. As a result, stable production of high-quality slabs can be achieved. [0081] Although the above description has been made for the continuous casting of flat slabs, the continuous casting method of steel in this embodiment is not limited to the continuous casting of flat slabs. The above-mentioned explanation can also be applied to the continuous casting of the green slab. Example 1 [0082] A 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%, and S: 0.005 to 0.015 mass%, Al: 0.020 to 0.040 mass%), using a water-cooled copper mold with metal arranged on the inner wall surface under various conditions, and the total bulging volume in the forced inflation zone, and the reduction speed and The product of the casting speed was changed in various ways to perform casting, and a test was conducted to investigate surface cracks and internal quality (central segregation) of the cast slab after casting. [0083] The product of the reduction speed and the casting speed of the light reduction belt is 0.28 to 0.90 mm. m / min ² In each test, the slab was pressed down at a time when the solid phase ratio at the center of the thickness of the slab became 0.2 at least to 0.9 when the strip was lightly pressed. The total reduction of the cast piece in the case of forced inflation with the forced inflation belt is set to be the same as or smaller than the total inflation amount. The test to prevent the slab from being bulged in the forced swell belt was to lightly reduce the belt, and the solidification end position on the short side of the slab was also pressed. [0084] The mold used is a mold having an inner space dimension 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 (the molten steel level in the mold) during normal casting was set to a position 100 mm downward from the upper end of the mold. In order to grasp the effect of the continuous casting method for steel of this embodiment, a mold was prepared under the following conditions and a comparative test was performed. For each mold, a metal having a lower thermal conductivity than that of a copper plate of the mold is used as the metal having a different thermal conductivity. The shape of the heterothermally conductive metal filling portion 19 is a circular shape of 6 mm in diameter. Under these casting conditions, the pitch of the vibration marks was 13 mm. [0085] Mold 1: In a range (range length = 220 mm) from a position 80 mm downward from the upper end of the mold to a position 300 mm downward from the upper end of the mold, the ratio of the difference in thermal conductivity with respect to the thermal conductivity of copper is staggered and filled. It is a metal with a different thermal conductivity of 20%, and the metal-filled portion 19 with a different thermal conductivity is provided. The area ratio ε of the heterothermally conductive metal-filled portion 19 was 50%. The distance D1 between the different thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different thermally conductive metal filling portions 19 in the mold width direction is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold is 33.0 mm. [0086] Mold 2: In a range from a position of 190 mm downward from the upper end of the mold to a position of 750 mm downward from the upper end of the mold (range length = 670 mm), the ratio of the difference in thermal conductivity with respect to the thermal conductivity of copper is staggered and filled. It is a metal with a different thermal conductivity of 20%, and the metal-filled portion 19 with a different thermal conductivity is provided. The area ratio ε of the heterothermally conductive metal-filled portion 19 was 50%. The distance D1 between the different thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different thermally conductive metal filling portions 19 in the mold width direction is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold is 33.0 mm. [0087] Mold 3: In a range from a position 80 mm downward from the upper end of the mold to a position 300 mm downward from the upper end of the mold, the thermal conductivity relative to copper is staggered and the difference in thermal conductivity is 20%. Metal, and a hetero-thermally conductive metal filling portion 19 is provided. The area ratio ε of the heterothermally conductive metal-filled portion 19 was 50%. The distance D1 between the different thermally conductive metal filling portions 19 in the casting direction is 15 mm, and the distance D2 between the centers of gravity of the different thermally conductive metal filling portions 19 in the mold width direction is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold is 33.0 mm. [0088] Mold 4: In a range from a position 80 mm downward from the upper end of the mold to a position 300 mm downward from the upper end of the mold, the thermal conductivity relative to copper is staggered and the difference in thermal conductivity is 20%. Metal, and a hetero-thermally conductive metal filling portion 19 is provided. The area ratio ε of the heterothermally conductive metal-filled portion 19 was 50%. The distance D1 between the different thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different thermally conductive metal filling portions 19 in the mold width direction is 15 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold is 38.0 mm. [0089] Mold 5: In a range from a position of 80 mm from the upper end of the mold to a position of 300 mm from the upper end of the mold, the thermal conductivity relative to copper is staggered and the difference in thermal conductivity is 15%. Metal, and a hetero-thermally conductive metal filling portion 19 is provided. The area ratio ε of the heterothermally conductive metal-filled portion 19 was 50%. The distance D1 between the different thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different thermally conductive metal filling portions 19 in the mold width direction is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold is 33.0 mm. [0090] Mold 6: In a range from a position 80 mm downward from the upper end of the mold to a position 300 mm downward from the upper end of the mold, the heat conductivity relative to copper is staggered and the difference in thermal conductivity is 20%. Metal, and a hetero-thermally conductive metal filling portion 19 is provided. The area ratio ε of the heterothermally conductive metal-filled portion 19 was 5%. The distance D1 between the different thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different thermally conductive metal filling portions 19 in the mold width direction is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold is 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 downward from the upper end of the mold, the thermal conductivity relative to copper is staggered and the difference in thermal conductivity is 20%. Metal, and a hetero-thermally conductive metal filling portion 19 is provided. The area ratio ε of the heterothermally conductive metal-filled portion 19 was 85%. The distance D1 between the different thermally conductive metal fillers 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different thermally conductive metal fillers 19 in the mold width direction is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold is 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 downward from the upper end of the mold, the thermal conductivity of copper is filled in a grid pattern with a difference in thermal conductivity of 20%. Metal, and a hetero-thermally conductive metal filling portion 19 is provided. The area ratio ε of the heterothermally conductive metal-filled portion 19 was 50%. The distance D1 between the different thermally conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different thermally conductive metal filling portions 19 in the mold width direction is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold is 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 downward from the upper end of the mold, the thermal conductivity relative to copper is staggered and the difference in thermal conductivity is 20%. Metal, and a hetero-thermally conductive metal filling portion 19 is provided. The area ratio ε of the heterothermally conductive metal-filled portion 19 was 50%. The distance D1 between the different thermally conductive metal filling portions 19 in the casting direction is 9 mm, and the distance D2 between the centers of gravity of the different thermally conductive metal filling portions 19 in the mold width direction is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold is 33.0 mm. [0094] Mold 10: In a range from a position 80 mm downward from the upper end of the mold to a position 300 mm downward from the upper end of the mold, the thermal conductivity relative to copper is staggered and the difference in thermal conductivity is 20%. Metal, and a hetero-thermally conductive metal filling portion 19 is provided. The area ratio ε of the heterothermally conductive metal-filled portion 19 was 50%. The distance D1 between the different thermally conductive metal filling portions 19 in the casting direction is 9 mm, and the distance D2 between the centers of gravity of the different thermally conductive metal filling portions 19 in the mold width direction is 12 mm. The longest pitch of the slits 30 provided on the outer wall surface of the mold is 16.5 mm. [0095] Mold 11: A mold in which a hetero-thermally conductive metal filling portion 19 is not provided. [0096] In the continuous casting operation, as the mold additive, alkalinity ((mass% CaO) / (mass% SiO) ² )) Is 1.1, the solidification temperature is 1090 ℃, and the viscosity at 1300 ℃ is 0.15Pa. s mold additive. The solidification temperature refers to a temperature at which the viscosity of the mold additive sharply increases during the cooling of the molten mold additive. The position of the meniscus in the mold during normal casting is 100mm down from the upper end of the mold. During casting, the position of the meniscus is controlled so that the meniscus is within the setting range. The casting speed during normal casting is 1.7 ～ 2.2m / min. It is used to investigate the surface cracks and internal quality of the slab. In all tests, the casting speed during normal casting is 2.0m / min. Slab as the object. The superheat degree of molten steel in the feeding tank is 25 to 35 ° C. Regarding the temperature management of the mold, the thermocouple was buried from the back side at a depth of 5 mm from the surface (the surface on the molten steel side) at a position 50 mm below the meniscus of the mold, and the copper plate temperature was measured based on the thermocouple. 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 surface cracks was measured. The occurrence of cracks on the surface of the slab was evaluated by using the calculated value as the denominator of the casting direction length of the slab to be inspected and the casting direction length of the slab at the site where the surface crack occurred. For the evaluation of the internal quality (central segregation) of the slab, a cross-section sample of the slab was used, and the Mn concentration per 100 μm was measured by EPMA in the range of ± 10 mm from the center portion of the slab of the mirror-polished surface of the cross-section sample to evaluate the segregation degree. Specifically, the Mn concentration (C ₀ ) And the average value (C) of the Mn concentration ± 10 mm in the center (C / C ₀ ) Is defined as the degree of Mn segregation and evaluated. [0098] In addition to the above-mentioned investigations, under the conditions of each test number, the non-uniformity σ (mm) of the thickness of the solidified shell was measured. The non-uniformity of the thickness of the solidified shell is measured by putting FeS (iron sulfide) powder into the molten steel in the mold, and taking a sample from the cross section of the obtained slab to measure the thickness of the solidified shell. The thickness of the solidified shell was measured at a position of 1/4 in the width direction of the mold, from a position of the meniscus to a position of 200 mm below, and 40 points were measured at a pitch of 5 mm. σ can be calculated by the following formula (6). [0099] [0100] In the formula (6), D is an actual measured value of the thickness of the solidified shell (mm), and Di is an approximate expression using a relationship between a predetermined thickness of the solidified shell and the solidification time. Calculated value of solidified shell thickness (mm) calculated from the solidification time of the distance from the lunar surface. N represents the number of measurements, and is 40 in this example. [0101] Table 1 shows the test conditions of each of the tests Nos. 1 to 14 and the results of investigations on the surface and internal quality of the slab. [0102] [0103] Test Nos. 1, 8, 9, 10, 11, and 13, the installation conditions of the different thermally-conductive metal filling portions 19 on the mold surface were within the scope of the present invention, and the longest pitch of the slits 30 satisfied the formula (4). These test numbers are all that 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, because the product of the reduction speed and the casting speed was not in the range of 0.30 to 1.00, a slight center segregation was confirmed. As for the other numbers, the results of improved central segregation have been obtained. [0104] In Test No. 2, the range where the hetero-thermally conductive metal filling portion 19 is provided is shifted downward, and the product of the reduction 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, fine surface cracks occurred in the slab, and the effect of reducing surface cracks could not be confirmed compared with the past. The unevenness of the thickness of the solidified shell is 0.38 mm and becomes large, and the improvement effect cannot be confirmed with respect to the center segregation. [0105] In test number 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 to 1.00. In Test No. 3, although the surface cracks of the slab were improved, the unevenness of the thickness of the solidified shell became 0.37 mm and became large, and the improvement effect was not confirmed with respect to the center segregation. [0106] In test number 4, the distance D2 in the width direction of the mold 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 slab occurred, and the improvement effect of the surface crack could not be confirmed. The unevenness of the thickness of the solidified shell was a little larger than 0.31 mm, and it was confirmed that the central segregation was slight although it was slight. [0107] Test No. 5, the ratio of the difference in thermal conductivity of the different thermally conductive metal is less than 20%; Test No. 6, the area ratio of the differently thermally conductive metal filling portion 19 is less than 10%; Test No. 7, the differently thermally 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 slab occurred, and the improvement effect of the surface crack could not be confirmed. The unevenness of the thickness of the solidified shell is 0.31 to 0.33, which is a little bit large, and the center segregation is slight but confirmed. [0108] Test No. 12, although the product of the reduction speed and the casting speed is in the range of 0.30 to 1.00, the distance D1 in the casting direction is long. In Test No. 12, although the cracks on the three surfaces of the cast slab and the center segregation were improved, the unevenness of the thickness of the solidified shell was 0.37 mm and became large. [0109] In Test No. 14, the installation conditions of the hetero-thermal conductive metal filling portion 19 on the mold surface 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 cracking ratio was better than the test numbers 2 to 7, it became 1.8% and became a little larger. It was confirmed that there was a slight center segregation, and the unevenness of the thickness of the solidified shell was a little larger than 0.31 mm. [0110] In Test No. 15, although the installation conditions of the different thermally conductive metal filler 19 on the mold surface are within the scope of the present invention, the longest pitch Z of the slit 30 cannot satisfy the formula (4). In addition, 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 numbers 2 to 7, it was slightly larger at 1.5%, and it was confirmed that there was a slight center segregation, and the unevenness of the thickness of the solidified shell was slightly larger at 0.33 mm. [0111] In Test No. 16, the surface of the cast slab was cracked because the hetero-thermally conductive metal-filled portion 19 was not provided. The unevenness of the thickness of the solidified shell was 0.32 mm, which was a little large, and it was also confirmed that the center segregation was observed.

[0112]

1‧‧‧Flat embryo continuous casting machine

2‧‧‧feed trough

3‧‧‧ sliding mouth

4‧‧‧ Dip

5‧‧‧Mould for continuous casting

5a‧‧‧ long copper plate

6‧‧‧ backup roller

7‧‧‧Guide roller

8‧‧‧ pinch roller

9‧‧‧ transport roller

10‧‧‧ Slab Cutting Machine

11‧‧‧ molten steel

12‧‧‧ Cast

12a‧‧‧ Flat embryo cast sheet

13‧‧‧ frozen shell

14‧‧‧ unsolidified part

15‧‧‧ End of solidification

16‧‧‧ Forced inflation belt

17‧‧‧light pressing belt

18‧‧‧ meniscus location

19‧‧‧ Metal Filled with Different Thermal Conductivity

19a‧‧‧a metal-filled portion with a different thermal conductivity

19b‧‧‧Other hetero-thermally conductive metal filling parts

19c‧‧‧Other hetero-thermally conductive metal filling parts

20‧‧‧ Metal Filled with Different Thermal Conductivity

20a‧‧‧a metal-filled portion with different thermal conductivity

20b‧‧‧Other hetero-thermal conductive metal filling parts

20c‧‧‧Other hetero-thermal conductive metal filling parts

30‧‧‧ slit

32‧‧‧ Bolt hole

40‧‧‧ back plate

42‧‧‧Studs

44‧‧‧ cooling water

[0020] FIG. 1 is a schematic side view of a vertical-bent flat-blade continuous casting machine that can use the continuous casting method of steel of this embodiment. FIG. 2 shows an example of a roll opening profile. FIG. 3 is a schematic side view showing a mold long side copper plate constituting a part of a mold provided in a flat-blade continuous casting machine. FIG. 4 is a conceptual diagram showing the thermal resistance of three positions of a long-side copper plate of a mold having a differently thermally conductive metal filling portion corresponding to the position of the differently thermally conductive metal filling portion. Formed by metal with lower thermal conductivity than the mold copper. FIGS. 5 (a) to (h) show examples of the planar shape of the groove. FIG. 6 is a partially enlarged view of a region where a metal-filled portion having a different thermal conductivity is provided. FIG. 7 is a schematic view showing an outer wall surface side of a mold long side copper plate. Fig. 8 is a schematic cross-sectional view of the DD section of Fig. 7 in a state where a back plate is provided on the outer wall surface of the long side copper plate of the mold. . FIG. 9 shows another example of the arrangement of the metal-filled portion having different thermal conductivity.

Claims (8)

A continuous casting method of steel, in which molten steel is poured into a continuous casting mold, and the molten steel is pulled out under the condition that the continuous casting mold is vibrated in the casting direction to produce a cast piece. The mold has a plurality of grooves, and the plurality of grooves are provided in the mold from a position at least 20 mm upward from the position of the meniscus in a normal casting state to a position at least 50 mm and at most 200 mm below the position of the meniscus The inner wall surface of the copper plate is filled with a metal or metal alloy having a thermal conductivity difference ratio of 20% or more with respect to the thermal conductivity of the mold copper plate inside the plurality of grooves. The ratio of the sum of the areas of the inner wall surfaces of the plurality of differently thermally conductive metal-filled portions to the total area of all the differently thermally conductive metal-filled portions, that is, the area ratio is 10% to 80%. The vibration mark 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 formula (1), Vc is the casting speed (m / min), f is the vibration frequency (cpm), OMP is the vibration mark pitch (mm), and D1 is the difference from other The distance (mm) from the boundary line between the thermally conductive metal filling portion and the mold copper plate to the boundary line between the different thermally conductive metal filling portion and the mold copper plate. The other different thermally conductive metal filling portion is the same as the one of the plurality. The center of gravity of the differently thermally conductive metal filling portion is disposed at the same position in the width direction of the mold copper plate and is adjacent to the one of the differently thermally conductive metal filling portion in the casting direction. In the formula (2), r is the aforementioned different thermal conductivity. The radius (mm) of a circle having the same center of gravity as the center of gravity of the metal-filled portion and having the same area as the area of the metal-filled portion, and D2 is from the center of gravity of the other metal-filled portion to the metal-filled portion. Distance (mm) of the center of gravity of the part, the other hetero-thermally conductive metal filling part is located at the same position in the casting direction as the center of gravity of the one hetero-thermally conductive metal filling part and is the same as the one Heat conductive metal filling in the portions adjacent to the width direction. The continuous casting method for steel according to claim 1, wherein: the plurality of differently thermally conductive metal-filled portions are provided so that the distance (D1) satisfies the following formula (3), D1 ≦ 2r ... (3) . The continuous casting method for steel according to claim 1 or claim 2, wherein: the shapes of the plurality of grooves are all the same. The continuous casting method for steel according to any one of claim 1 to claim 3, wherein the shape of the plurality of grooves is a circle or a quasi-circular shape without corners. The continuous casting method for steel according to any one of claim 1 to claim 4, wherein: the aforementioned plurality of hetero-thermally-conductive metal filling portions are provided in a grid shape. The continuous casting method for steel according to any one of claim 1 to claim 4, wherein: the aforementioned plurality of different thermally conductive metal filling portions are provided in a staggered shape. The continuous casting method for steel according to any one of claim 1 to claim 6, wherein the roll opening degree of the plurality of pairs of 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 an unsolidified part inside is expanded to a thickness of the slab (thickness between the long side surfaces of the slab) at the exit of the mold by a total bulging amount in a range of more than 0 mm and less than 20 mm. The roll reduction of the pair of slab support rolls gradually decreases toward the downstream side of the casting direction. From the time when the solid phase ratio of the thickness center of the slab is at least 0.2 to 0.9, it is equivalent to the pressure reduction. the speed (mm / min) and the product of the casting speed (m / min) of (mm.m / min ²⁾ 0.30 1.00 less than the pressing force given to the longitudinal plane of the slab, by the pressing force The long-side surface of the slab is reduced by a total reduction amount equal to the total bulging amount or a total reduction amount less than the total bulge amount. The continuous casting method for steel according to any one of claim 1 to claim 7, wherein the plurality of slits along the casting direction on the outer wall surface of the copper mold plate are formed in the width direction of the copper mold plate by Set the singular or plural pitch. When the aforementioned plurality of slits are set at the singular pitch, set the aforementioned singular pitch to Z (mm), and when the aforementioned plural slits are set at the plural pitch. In the case where the longest pitch among the plurality of complex pitches is set to Z (mm), the aforementioned Z satisfies the following formula (4), Z ≧ 2.5 × D2 ... (4).