JP6947737B2 - Continuous steel casting method - Google Patents

Description

The present invention relates to a continuous casting technique, and more particularly to a steel continuous casting method suitable for improving surface cracking and central segregation of slabs by suppressing non-uniform solidification of slabs in the initial stage of solidification.

Generally, when steel slabs are produced by continuous casting, the molten steel injected into the mold is first cooled in contact with the mold to form a thin solidified layer (hereinafter referred to as "solidified shell"). In this way, the solidified shell is pulled downward while injecting molten steel into the mold (hereinafter referred to as "steady casting") to manufacture slabs.

Non-uniform cooling by the mold results in non-uniform thickness of the solidified shell, resulting in a non-smooth surface of the solidified shell. In particular, if the thickness of the solidified shell grows unevenly in the initial stage of solidification, stress concentration occurs on the surface of the solidified shell and minute vertical cracks occur. These minute vertical cracks remain even after the slab is completely solidified, and become vertical cracks on the surface of the slab. When vertical cracks occur on the surface of a slab, it is necessary to remove the vertical cracks (hereinafter referred to as “maintenance”) prior to feeding the slab to a subsequent process (for example, a rolling process).

The mold vibrates in the casting direction (hereinafter, also referred to as "oscillation"), and the upper end of the solidified shell is bent toward the molten steel by the vibration of the mold, creating a gap between the bent solidified shell and the inner wall surface of the mold. When the molten steel overflows, a portion (hereinafter referred to as “claw”) overhanging the molten steel side is formed in the solidified shell. If the surface of the solidified shell is not smooth, the voids formed between the bent solidified shell and the inner wall surface of the mold become large, and the claws of the solidified shell become large. When the claws overhanging the molten steel side become large, non-metal inclusions and air bubbles floating in the molten steel are trapped in the meniscus (the surface of the molten steel in the mold). The trapped non-metal inclusions and air bubbles cause surface defects such as surface flaws and swelling in the steel sheet after hot rolling or the steel sheet after cold rolling.

The frequency of occurrence of such surface defects such as vertical cracks, scratches, and swelling tends to increase as the casting speed increases. Today, the casting speed of a general slab continuous casting machine has increased by about 1.5 to 2 times compared to 10 years ago, and the maintenance work has increased accordingly. In recent years, even in direct heating (so-called hot charge) and direct rolling (so-called direct charge), which are becoming technically established, the maintenance work of slabs is a factor that hinders the stabilization of operations. Therefore, it would be extremely economically advantageous to prevent uneven growth of solidification shell thickness and generation of claws due to non-uniform cooling in the initial stage of solidification.

In order to prevent non-uniform cooling in the initial stage of solidification, it is necessary to prevent the formation of claws by performing uniform and gradual cooling in the initial stage of solidification and uniformly growing the thickness of the solidified shell. In this regard, Non-Patent Document 1 describes that it is effective to impart irregularities to the inner surface of the mold in order to improve the surface texture of the slab in the continuous casting of billets of 280 × 280 mm. .. 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. Further, 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.

In all of these techniques, mold powder is poured into the meniscus portion to stably maintain a mold powder layer of sufficient thickness in the gap between the mold and the solidified shell for a long period of time, and the uneven portion provided on the inner surface of the mold. An air layer or a molten powder layer is formed in the air layer, and the heat insulating property of the air layer or the molten powder layer is utilized to realize gentle cooling (hereinafter referred to as slow cooling).

However, when these techniques are actually used for continuous casting, various problems arise. For example, the mold of a slab continuous casting machine whose width can be changed is a set mold of a long side and a short side. There is a problem that the splash of molten steel is inserted into the recesses at the corners when starting.

When the dipping nozzle is replaced or the tundish is replaced, the molten steel level in the mold is lower than in the steady casting state, so the mold powder adhering to the inner surface of the mold is easily peeled off and separated, and casting is performed again. There is a problem that the molten steel or the splash of molten steel is inserted into the recesses in the corners at the start. Such a phenomenon that the molten steel is inserted into the recess causes a restrictive breakout of the solidified shell.

The mechanism for generating central segregation of slabs is considered as follows. As coagulation progresses, segregation components are concentrated between the dendrite trees, which are coagulated structures. The molten steel with a concentrated segregation component flows out from the dendrite trees due to shrinkage of the slab during solidification or bulging of the slab called bulging. The outflowed molten steel in which the segregation component is concentrated flows toward the solidification completion point, which is the final solidification part, and solidifies as it is to form a concentration zone of the segregation component. This concentrated zone is the central segregation. As measures to prevent central segregation of slabs, it is effective to prevent the movement of molten steel with concentrated segregation components existing between dendrite trees and to prevent local accumulation of molten steel with concentrated segregation components. There are several methods proposed that utilize these principles.

One of them is a light reduction method for slabs using a reduction roll group, but there is a limit to the effect of improving central segregation under a light reduction method that slightly exceeds the amount of solidification shrinkage. In Patent Document 3, the slab is bulged at a position where the solid phase ratio at the center of the slab is 0.1 or less, and the thickness of the slab at the center in the width direction is the thickness of the slab at the short side generated in the mold. A method has been proposed in which the bulging equivalent amount is reduced by at least one reduction roll pair immediately before the solidification completion point after making the thickness 20 to 100 mm thicker, under the condition that the reduction amount per reduction roll pair is 20 mm or more. ..

In Patent Document 4, the thickness of the slab in the central portion in the width direction is bulging by a thickness equivalent to 10 to 50% of the thickness of the slab in the short side until the thickness of the unsolidified portion of the slab reaches 30 mm. A method has been proposed in which the bulging equivalent amount is reduced by at least one reduction roll pair until the solidification completion point.

In Patent Document 5, after bulging 3% or more and 25% or less of the thickness of the slab at the start of bulging, the position of the slab having a solid phase ratio of 0.2 or more and 0.7 or less in the central portion is determined by the amount of bulging. A continuous casting method of steel has been proposed in which a thickness corresponding to 30% or more and 70% or less is reduced.

Japanese Unexamined Patent Publication No. 9-94634 Japanese Unexamined Patent Publication No. 10-193041 Japanese Unexamined Patent Publication No. 7-21382 Japanese Unexamined Patent Publication No. 9-20603 Japanese Unexamined Patent Publication No. 11-99285

P. Perminov et al, Steel in English, (1968) No. 7. p. 560-562

In continuous steel casting, vertical vibration is applied to the mold to prevent the solidified shell from seizing on the mold due to the vibration. Periodic irregularities called oscillation marks are formed on the surface of the slab whose tip is deformed by the vibration of the mold. When the unevenness of the oscillation mark becomes large, the contact between the surface of the solidified shell and the mold becomes non-uniform, and the amount of heat removed from the mold also becomes non-uniform, 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 solidification shell becomes large, the solidification interface in the final solidification portion becomes not smooth, and the effect may not be sufficiently obtained even if the pressure is reduced by the methods described in Patent Documents 3 to 5. There are challenges.

The gist of the present invention for solving the above problems is as follows.
[1] A method for continuous casting of steel for producing slabs by drawing out the molten steel while injecting molten steel into a mold for continuous casting and vibrating the mold for continuous casting in the casting direction. The mold has a plurality of recessed grooves provided on the inner wall surface of the mold copper plate from a position at least 20 mm above the meniscus position in the constant casting state to a position at least 50 mm or more and at most 200 mm or less below the meniscus position. However, the inside of the plurality of concave grooves is filled with a plurality of different heat conductive metal filling portions filled with a metal or a metal alloy having a ratio of the difference in thermal conductivity to the thermal conductivity of the mold copper plate of 20% or more. The area ratio, which is the ratio of the total area of all the different heat conductive metal filling parts to the area of the inner wall surface provided with the plurality of different heat conductive metal filling parts, is 10% or more and 80% or less. , The oscillation mark pitch (OMP) and the distance (D1) derived from the oscillation frequency (f) and the casting speed (Vc) satisfy the following equation (1), and the distance (D2) is the following equation (2). Satisfying, continuous casting method of steel.
D1 ≤ OMP = Vc x 1000 / f ... (1)
D2 ≦ 4r ・ ・ ・ (2)
However, in the equation (1), Vc is the casting speed (m / min), f is the oscillation frequency (cpm), OMP is the oscillation mark pitch (mm), and D1 is. The center of gravity of one of the plurality of different heat conductive metal filling portions and another different heat conductive metal filling portion provided at the same position in the width direction of the mold copper plate, wherein the different heat conductive metal filling portion is provided. It is the distance (mm) from the boundary line between the different heat conductive metal filling part and the mold copper plate adjacent to each other in the casting direction to the boundary line between the different heat conductive metal filling part and the mold copper plate. In the equation (2), r is the radius (mm) of a circle having the same area as the area of the different heat conductive metal filling portion centered on the center of gravity of the different heat conductive metal filling portion, and D2 is the above 1 Another different heat conductive metal filling part provided at the same position in the casting direction as the center of gravity of the different heat conductive metal filling part of the above 1 and adjacent to the different heat conductive metal filling part in the width direction. It is a distance (mm) from the center of gravity of the heat conductive metal filling portion to the center of gravity of the different heat conductive metal filling portion of 1.
[2] The method for continuous casting of steel according to [1], wherein the plurality of different heat conductive metal filling portions are provided so that the distance (D1) satisfies the following equation (3).
D1 ≤ 2r ... (3)
[3] The method for continuously casting steel according to [1] or [2], wherein the shapes of the plurality of recessed grooves are all the same.
[4] The method for continuously casting steel according to any one of [1] to [3], wherein the shape of the plurality of concave grooves is a circle or a pseudo-circle without corners.
[5] The method for continuously casting steel according to any one of [1] to [4], wherein the plurality of different heat conductive metal filling portions are provided in a lattice pattern.
[6] The method for continuously casting steel according to any one of [1] to [4], wherein the plurality of different heat conductive metal filling portions are provided in a staggered pattern.
[7] By gradually increasing the roll opening degree of a plurality of pairs of slab support rolls provided in the continuous casting machine toward the downstream side in the casting direction, the long side surface of the slab having an unsolidified portion inside. Is expanded with a total bulging amount in the range of more than 0 mm and not more than 20 mm with respect to the slab thickness (thickness between the long side faces of the slab) at the mold outlet, and then the roll opening of the plurality of pairs of slab support rolls is cast. A light reduction zone that gradually decreases toward the downstream side in the direction, and the reduction rate (mm / min) from the time when the solid phase ratio at the center of the thickness of the slab becomes at least 0.2 to 0.9. ) And the casting speed (m / min) (mm · m / min ² ) is 0.30 or more and 1.00 or less. The steel according to any one of [1] to [6], which reduces the long side surface of the slab with a total reduction amount equal to the total bulging amount or a total reduction amount smaller than the total bulging amount. Continuous casting method.
[8] On the outer wall surface of the mold copper plate, a plurality of slits along the casting direction are provided at a single or a plurality of pitches in the width direction of the mold copper plate, and the plurality of slits are provided at a single pitch. In this case, the singular pitch is Z (mm), and when the plurality of slits are provided at a plurality of pitches, the longest pitch among the plurality of pitches is Z (mm). The method for continuously casting steel according to any one of [1] to [7], wherein the method satisfies the following formula (4).
Z ≧ 2.5 × D2 ・ ・ ・ (4)
Here, the center of gravity of the different heat conductive metal filling portion means the center of gravity of the cross-sectional shape of the different heat conductive metal filling portion on the molten steel side plane of the mold copper plate.

According to the present invention, since a plurality of different heat conductive metal filling portions are installed in the width direction and the casting direction of the continuous casting mold near the meniscus including the meniscus position, they are continuous in the mold width direction and the casting direction near the meniscus. The thermal resistance of the casting mold increases and decreases periodically. As a result, the heat flux near the meniscus, that is, from the solidified shell at the initial stage of solidification to the continuous casting mold, increases and decreases periodically. Due to the periodic increase and decrease of the heat flux, the stress and thermal stress due to the transformation from δ iron to γ iron are reduced, and the deformation of the solidified shell caused by these stresses is reduced. By reducing the deformation of the solidified shell, the non-uniform heat flux distribution caused by the deformation of the solidified shell is made uniform, and the generated stress is dispersed to reduce the individual strain amount. As a result, cracking on the surface of the solidified shell can be prevented.

According to the present invention, since a portion where the heat flux increases or decreases can be present at least once during one pitch of the oscillation mark, the depth of the oscillation mark is made shallow and the surface of the solidified shell is made uniform. Can be made to. As a result, the inner surface of the solidified shell that grows together with the surface is also made uniform, the solidified interface at the final solidified portion becomes smooth, the spots that form segregation are reduced, and the internal quality of the slab slab can be improved.

FIG. 1 is a side view of a vertical bending type slab continuous casting machine to which the steel continuous casting method according to the present embodiment can be applied. FIG. 2 is a diagram showing an example of a roll opening profile. FIG. 3 is a schematic side view of a mold long-sided copper plate forming a part of a mold installed in a slab continuous casting machine. FIG. 4 shows the thermal resistance of the different heat conductive metal filling portion at three positions of the mold long side copper plate having the different heat conductive metal filling portion formed by filling the metal having a lower thermal conductivity than the mold copper plate. It is a figure which shows conceptually corresponding to a position. FIG. 5 is a diagram showing an example of the planar shape of the concave groove. FIG. 6 is a partially enlarged view of a region provided with a different heat conductive metal filling portion. FIG. 7 is a schematic view showing the outer wall surface side of the long-sided copper plate of the mold. FIG. 8 is a cross section of DD in FIG. 7 in a state where a backup plate is provided on the outer wall surface of the long-sided copper plate of the mold, and a stud bolt is screwed into one of the bolt holes on the right side of the cross section of DD. It is a cross-sectional schematic diagram which showed the cross section by superimposing. FIG. 9 is a diagram showing another example of the arrangement of the different heat conductive metal filling portion.

A specific method for carrying out the present invention will be described with reference to the drawings. FIG. 1 is a side view of a vertical bending type slab continuous casting machine to which the steel continuous casting method according to the present embodiment can be applied.

Molten steel 11 is injected into the slab continuous casting machine 1 to solidify it to form the outer shell shape of the slab 12, and the continuous casting mold 5 vibrates in the casting direction of the slab 12 (hereinafter, simply referred to as "mold"). ) Is installed. A tundish 2 for relay-supplying the molten steel 11 supplied from the ladle (not shown) to the mold 5 is installed at a predetermined position above the mold 5. Below the mold 5, a plurality of pairs of slab support rolls including a support roll 6, a guide roll 7, and a pinch roll 8 are installed. Of these, the pinch roll 8 is also a drive roll for pulling out the slab 12 at the same time as supporting the slab 12. A secondary cooling zone in which a spray nozzle (not shown) such as a water spray nozzle or an air mist spray nozzle is arranged is formed in the gap between the slab support rolls adjacent to each other in the casting direction. The slab 12 is cooled while being pulled out by the cooling water sprayed from (hereinafter, also referred to as “secondary cooling water”), the unsolidified portion 14 inside is reduced, and the solidified shell 13 is grown so that the casting is performed. conduct. A sliding nozzle 3 for adjusting the flow rate of the molten steel 11 is installed on the bottom of the tundish 2, and a dipping nozzle 4 is installed on the lower surface of the sliding nozzle 3.

A plurality of transport rolls 9 for transporting the cast slab 12 are installed on the downstream side of the slab support roll, and above the transport roll 9, a predetermined number from the cast slab 12 is installed. A slab cutting machine 10 for cutting a slab slab 12a having a length is arranged. Before and after the solidification completion position 15 of the slab 12, the roll spacing of the opposing guide rolls 7 is set to be gradually narrowed toward the downstream in the casting direction, that is, a roll gradient is applied. A light reduction band 17 composed of a plurality of pairs of guide rolls is installed.

In the light reduction zone 17, light reduction can be performed on the slab 12 in the entire area or a partially selected region thereof. In the present embodiment, the light reduction zone is provided so that the slab 12 having a solid phase ratio at the center of the thickness of the slab 12 is at least 0.2 to 0.9 within the installation range of the light reduction zone 17. 17 is installed.

The reduction gradient in the light reduction zone 17 is indicated by the roll opening narrowing amount per 1 m in the casting direction, that is, "mm / m", and the reduction speed (mm / min) of the slab 12 in the light reduction zone 17 is determined. It is obtained by the product of the reduction gradient (mm / m) and the casting speed (m / min). A spray nozzle for cooling the slab 12 is also arranged between the slab support rolls constituting the light reduction band 17. Although FIG. 1 shows an example in which only the guide roll 7 is arranged in the light reduction band 17, the pinch roll 8 may be arranged in the light reduction band 17. The slab support roll arranged in the light reduction band 17 is also referred to as a “reduction roll”.

The opening degree of the guide roll 7 arranged between the lower end of the mold 5 and the liquidus line crater end position of the slab 12 is until the amount of expansion of the roll opening degree reaches a predetermined value toward the downstream side in the casting direction. The roll opening is gradually widened every one roll or every few rolls. These guide rolls 7 form a forced bulging zone 16 for forcibly bulging the long side surface of the slab 12 having the unsolidified portion 14 inside. The slab support roll on the downstream side of the forced bulging zone 16 is narrowed so that the roll opening degree is constant or the amount of shrinkage due to the temperature drop of the slab 12 is commensurate with the roll opening, and then is connected to the light reduction zone 17.

FIG. 2 is a diagram showing an example of a roll opening profile. As shown in FIG. 2, the forced bulging zone 16 forcibly bulges the long side surface of the slab by static pressure of molten steel to increase the thickness of the central portion of the long side surface of the slab (region b), and passes through the forced bulging zone 16. On the too downstream side, the roll opening is narrowed to a constant value or to the extent corresponding to the shrinkage amount due to the temperature drop of the slab 12 (region c), and then the long side surface of the slab is reduced by the light reduction band 17 (region c). The profile is d). Reference numerals a and e in FIG. 2 are regions in which the roll opening degree is narrowed to a extent commensurate with the amount of shrinkage of the slab 12 due to the temperature drop. Reference numeral a'in FIG. 2 is an example of the roll opening degree in a casting method (conventional method) in which the roll opening degree is narrowed to match the amount of shrinkage of the slab 12 due to the temperature drop and light reduction is not performed.

In the forced bulging zone 16, the roll opening of the guide roll 7 is gradually widened toward the downstream side in the casting direction, so that the long side surface of the slab 12 except for the vicinity of the short side is subjected to the static pressure of molten steel by the unsolidified portion 14. It is forcibly bulged according to the roll opening degree of the guide roll 7. Since the vicinity of the short side of the long side surface of the slab is held by the short side surface of the slab that has been solidified, the thickness at the time when forced bulging is started is maintained, and therefore the slab 12 is forced. Only the bulging portion of the long side surface of the slab comes into contact with the guide roll 7 due to the bulging.

FIG. 3 is a schematic side view of a mold long-sided copper plate forming a part of a mold installed in a slab continuous casting machine. The mold 5 shown in FIG. 3 is an example of a continuous casting mold for casting slab slabs. The mold 5 is formed by combining a pair of mold long-sided copper plates 5a (hereinafter, also referred to as “mold copper plates”) and a pair of mold short-sided copper plates. FIG. 3 shows the mold long-sided copper plate 5a. Similar to the mold long side copper plate 5a, the mold short side copper plate is also provided with the different heat conductive metal filling portion 19 on the inner wall surface side thereof, and the description of the mold short side copper plate will be omitted here. However, in the slab 12, due to the shape that the slab width is extremely large with respect to the slab thickness, stress concentration is likely to occur in the solidification shell 13 on the long side surface side of the slab, and surface cracks occur on the long side surface side of the slab. Likely to happen. Therefore, it is not necessary to provide the different heat conductive metal filling portion 19 on the mold short side copper plate of the mold 5 for the slab slab.

As shown in FIG. 3, from the Q position above the meniscus position 18 at least 20 mm away from the meniscus position 18 during steady casting of the long-sided copper plate 5a of the mold to the lower R position at least 50 mm or more and at most 200 mm or less away from the meniscus position 18. In the range of the inner wall surface of, a metal or a metal alloy (hereinafter referred to as "different heat conductive metal") in which the ratio of the difference in thermal conductivity to the heat conductivity of the long-sided copper plate 5a of the mold is 20% or more is 20% or more. The filled circular different heat conductive metal filling portion 19 is provided in a staggered manner in a range of length W in the mold width direction perpendicular to the casting direction. "Meniscus" means "hot metal surface in the mold". “Steady casting” refers to a state in which a cruising state is maintained at a constant casting speed after the injection of molten steel into the mold 5 of the slab continuous casting machine 1 is started. During steady casting, the sliding nozzle 3 automatically controls the injection speed of the molten steel 11 into the mold 5 so that the meniscus position 18 becomes constant.

The different heat conductive metal filling portion 19 has a different heat conductivity different from that of the copper alloy constituting the mold copper plate inside the circular concave groove independently processed on the inner wall surface side of the mold copper plate. It is formed by filling with a conductive metal.

It is preferable to apply plating treatment or thermal spraying treatment as a means for filling the inside of the circular concave groove with a different heat conductive metal having a different thermal conductivity from that of the copper alloy constituting the mold copper plate. The different heat conductive metal processed according to the shape of the circular concave groove may be filled by fitting it into the circular concave groove, but in that case, a gap or crack may occur between the different heat conductive metal and the mold copper plate. May occur. If a gap or crack occurs between the different heat conductive metal and the mold copper plate, the different heat conductive metal cracks or peels off, which causes a decrease in mold life, cracks in the slab, and a restrictive breakout. Therefore, it is not preferable. By filling the different heat conductive metal by plating or thermal spraying, such a problem can be prevented.

In the present embodiment, as the copper alloy used as the mold copper plate, a copper alloy to which a small amount of chromium (Cr), zirconium (Zr) or the like, which is generally used as a mold for continuous casting, may be added. In recent years, in order to homogenize the solidification in the mold or prevent inclusions in the molten steel from being trapped in the solidified shell, an electromagnetic stirrer for stirring the molten steel in the mold is generally installed. A copper alloy with reduced conductivity may be used to suppress the decay of the magnetic field strength from the coil to the molten steel. In this case, since the thermal conductivity also decreases as the conductivity decreases, the thermal conductivity of the mold copper plate becomes about 1/2 of that of pure copper (thermal conductivity; about 400 W / (m × K)). Copper alloys used as mold copper plates generally have lower thermal conductivity than pure copper.

FIG. 4 shows the thermal resistance of the different heat conductive metal filling portion at three positions of the mold long side copper plate having the different heat conductive metal filling portion formed by filling the metal having a lower thermal conductivity than the mold copper plate. It is a figure which shows conceptually corresponding to a position. As shown in FIG. 4, the thermal resistance is relatively high at the installation position of the different heat conductive metal filling portion 19.

By providing a plurality of different heat conductive metal filling portions 19 in the width direction and the casting direction of the continuous casting mold in the vicinity of the meniscus including the meniscus position 18, as shown in FIG. 4, the mold width direction and the casting in the vicinity of the meniscus are formed. A distribution is formed in which the thermal resistance of the continuous casting mold in the direction increases and decreases periodically. As a result, a distribution is formed in which the heat flux from the solidified shell to the continuous casting mold periodically increases or decreases near the meniscus, that is, at the initial stage of solidification.

When the different thermal conductive metal filling portion 19 is formed by filling a metal having a higher thermal conductivity than the mold copper plate, the thermal resistance is relative at the installation position of the different thermal conductive metal filling portion 19, unlike FIG. However, in this case as well, a distribution is formed in which the thermal resistance of the continuous casting mold in the mold width direction and the casting direction near the meniscus periodically increases or decreases. In order to form the periodic distribution of thermal resistance as described above, it is preferable that the different heat conductive metal filling portions 19 are independent of each other.

Due to the periodic increase and decrease of the heat flux, the stress and thermal stress due to the phase transformation of the solidified shell 13 (for example, the transformation from δ iron to γ iron) are reduced, and the deformation of the solidified shell 13 caused by these stresses is reduced. .. By reducing the deformation of the solidified shell 13, the non-uniform heat flux distribution caused by the deformation of the solidified shell 13 is made uniform, and the generated stress is dispersed to reduce the individual strain amount. As a result, the occurrence of surface cracks on the surface of the solidified shell is suppressed.

Due to the periodic increase and decrease of the heat flux at the initial stage of solidification, the thickness of the solidified shell 13 in the mold is made uniform not only in the width direction of the slab but also in the casting direction. By making the thickness of the solidified shell 13 uniform in the mold, the solidified interface of the solidified shell 13 of the slab 12 after being pulled out from the mold 5 is the width direction and the casting direction of the slab even in the final solidified portion of the slab. Becomes smooth.

However, in order to obtain these effects in a stable manner, the periodic increase / decrease in heat flux due to the installation of the different heat conductive metal filling portion 19 must be appropriate. That is, if the difference in the periodic increase / decrease in the heat flux is too small, the effect of installing the different heat conductive metal filling portion 19 cannot be obtained, and conversely, if the difference in the periodic increase / decrease in the heat flux is too large, this. The stress generated due to this increases, and this stress causes surface cracking.

The difference in increase / decrease in heat flux due to the installation of the different heat conductive metal filling portion 19 is the difference in thermal conductivity between the mold copper plate and the different heat conductive metal and the mold copper plate in the region where the different heat conductive metal filling portion 19 is arranged. It depends on the area ratio, which is the ratio of the total area of all the different heat conductive metal filling portions 19 to the area of the inner wall surface of the above.

In the mold copper plate used in the continuous steel casting method according to the present embodiment, when the thermal conductivity _{of the different heat conductive metal filled in the circular groove is λ m} , the difference with respect to _{the thermal conductivity (λ c) of the mold copper plate.} A metal or metal alloy in which the ratio of the difference in thermal conductivity (λ _{m ) of the heat conductive metal ((| λ c} − λ _m | / λ _c ) × 100) is 20% or more is used. _{By using a metal or metal alloy in which the ratio of the difference to the thermal conductivity (λ c} ) of the copper alloy constituting the mold copper plate is 20% or more, the periodic fluctuation of the heat flux due to the different heat conductive metal filling portion 19 The effect of the above is sufficient, and the effect of suppressing the surface cracking of the slab can be sufficiently obtained even at the time of high-speed casting or the casting of medium carbon steel in which the surface cracking of the slab is likely to occur. The thermal conductivity of the mold copper plate and the thermal conductivity of the different thermal conductive metal are the thermal conductivity at room temperature (about 20 ° C.). Generally, the thermal conductivity decreases as the temperature rises, but if the ratio of the difference in thermal conductivity of the different thermal conductive metal to the thermal conductivity of the mold copper plate at room temperature is 20% or more, it can be used as a continuous casting mold. Even at the operating temperature (about 200 to 350 ° C.), the thermal resistance of the part where the different heat conductive metal filling part 19 is installed and the heat resistance of the part where the different heat conductive metal filling part 19 is not installed. It can make a difference.

In the mold copper plate used in the continuous steel casting method according to the present embodiment, the area A (A = (Q + R) × W, unit; mm) of the inner wall surface of the mold copper plate within the range in which the different heat conductive metal filling portion 19 is formed. ^The area ratio ε (ε = (B / A) × 100), which is the ratio of ^{the total area B (mm 2} ) of all the different heat conductive metal filling portions 19 to 2), is 10% or more and 80% or less. Is provided with a different heat conductive metal filling portion 19. By setting this area ratio ε to 10% or more, the area occupied by the different heat conductive metal filling portions 19 having different heat fluxes is secured, and the heat flux difference is obtained between the different heat conductive metal filling portions 19 and the mold copper plate. The effect of suppressing surface cracking of the slab can be obtained. On the other hand, when the area ratio ε exceeds 80%, the number of parts of the different heat conductive metal filling portion 19 becomes too large, and the cycle of heat flux fluctuation becomes long, so that it becomes difficult to obtain the effect of suppressing surface cracking of the slab. ..

Therefore, it is more preferable to provide the different heat conductive metal filling portion 19 so that the area ratio ε is 30% or more and 60% or less, and the different heat conductive metal filling portion 19 is provided so that the area ratio ε is 40% or more and 50% or less. It is more preferable to provide the portion 19.

The type of the different heat conductive metal does not need to be specified as long as the ratio of the difference in _{the thermal conductivity (λ m} ) of the packed metal to the thermal conductivity _{(λ c} ) of the mold copper plate is 20% or more. For reference, the metals that can be used as the filling metal are pure nickel (Ni, thermal conductivity; 90 W / (m × K)), pure chromium (Cr, thermal conductivity; 67 W / (m × K)), Pure cobalt (Co, thermal conductivity; 70 W / (m × K)), alloys containing these metals, and the like are suitable. These pure metals and alloys have a lower thermal conductivity than copper alloys, and can be easily filled in the circular concave groove by plating treatment or thermal spraying treatment. Pure copper, which has a higher thermal conductivity than the copper alloy, may be used as the metal to be used for filling the circular concave groove. For example, when pure copper is used as the filling metal, the thermal resistance of the portion where the different heat conductive metal filling portion 19 is installed is smaller than that of the portion of the mold copper plate.

FIG. 5 is a diagram showing an example of the planar shape of the concave groove. Although FIGS. 3 and 4 show an example in which the shape of the concave groove is circular as shown in FIG. 5 (a), the concave groove does not have to be circular. For example, the concave groove may be an ellipse as shown in FIG. 5 (b), or may be a square or a rectangle whose corners are circles as shown in FIG. 5 (c), and may be a square or a rectangle as shown in FIG. 5 (d). ) May be in the shape of a donut. It may be a triangle as shown in FIG. 5 (e), a trapezoid as shown in FIG. 5 (f), or a pentagon as shown in FIG. 5 (g). It may be in the form of konpeito as shown in 5 (h). In these concave grooves, a different heat conductive metal filling portion having a shape corresponding to the shape of the concave groove is installed.

The shape of the concave groove is preferably a circular shape as shown in FIG. 5 (a) or a shape having no "corner" as shown in FIGS. It may have a shape having "corners" as shown in (h). By making the shape of the concave groove into a shape that does not have "corners", the boundary surface between the different heat conductive metal and the mold copper plate becomes a curved surface, stress is less likely to concentrate on the boundary surface, and the mold copper plate surface cracks. It is less likely to occur.

In the present embodiment, among the shapes of these concave grooves, for example, the non-circular shape shown in FIGS. 5 (b) to 5 (h) is defined as a pseudo-circular shape. When the shape of the concave groove is a pseudo-circular shape, the concave groove processed on the inner wall surface of the mold copper plate is called a "pseudo-circular concave groove". The radius in the pseudo-circle is evaluated by the circle-equivalent radius r, which is the radius of the circle having the same area as the area of the pseudo-circle. The quasi-circular equivalent radius r is calculated by the following equation (5).

Circle equivalent radius r = (S _ma / π) ^1/2 ... (5)
However, in the equation (5), _Sma is the area (mm ² ) of the pseudo-circular concave groove.

FIG. 6 is a partially enlarged view of a region provided with a different heat conductive metal filling portion. As shown in FIG. 6, in the mold copper plate of the present embodiment, circular different heat conductive metal filling portions 19 are provided in a staggered manner. Here, the staggered arrangement means that the different heat conductive metal filling portions 19 are alternately provided at half-pitch positions of the different heat conductive metal filling portions 19.

In FIG. 6, 19a is a different heat conductive metal filling part of 1, and 19b is another different heat conductive metal filling part. The different heat conductive metal filling portion 19a and the different heat conductive metal filling portion 19b are provided at the same positions in the width direction of the mold copper plate and adjacent to each other in the casting direction. Here, the center of gravity of the different heat conductive metal filling portion 19 is the center of gravity of the cross-sectional shape of the different heat conductive metal filling portion 19 on the molten steel side plane of the mold copper plate.

Assuming that the distance from the boundary line of the different heat conductive metal filling portion 19a with the mold copper plate in the casting direction to the boundary line of the different heat conductive metal filling portion 19b with the mold copper plate is D1 (mm), the different heat conductive metal filling portion 19 Is provided on the inner wall surface of the mold copper plate so that the distance D1 satisfies the following equation (1).

D1 ≤ OMP = Vc x 1000 / f ... (1)
However, in the formula (1), Vc is the casting speed (m / min), f is the oscillation frequency (cpm), and OMP is the oscillation mark pitch (mm).

In this way, the distance between the boundary lines of the different heat conductive metal filling portions 19 in the casting direction from the mold copper plate, that is, the distance between the different heat conductive metal filling portions 19 in the casting direction is larger than the pitch in the casting direction of the oscillation mark. Different heat conductive metal filling portions 19 are provided on the mold copper plate so as to have a small interval. As a result, a portion where the heat flux increases or decreases can be present at least once during one pitch of the oscillation mark, so that the claws generated when the oscillation mark is formed are intentionally slowly cooled at a short pitch. By doing so, the non-uniform heat flux caused by the deformation of the nail is made uniform, and the amount of individual strain is reduced. As a result, the collapse of the nail can be suppressed, the depth of the oscillation mark can be made shallow, and the thickness of the solidified shell 13 in the casting direction can be made uniform. By making the thickness of the initial solidification shell 13 uniform, the solidification interface at the final solidification portion forming the central segregation is smoothed, whereby the spots forming the segregation are also reduced, so that the internal quality is improved. By making the depth of the oscillation mark shallow, it is possible to suppress lateral cracking starting from the oscillation mark.

The different heat conductive metal filling 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 equation (3).

D1 ≤ 2r ... (3)
However, in the equation (3), r is the radius (mm) or the radius equivalent to a circle (mm) of the different heat conductive metal filling portion 19.

In this way, the different heat conductive metal filling portion 19 is placed on the mold copper plate so that the distance between the different heat conductive metal filling portions 19 in the casting direction is not more than twice the radius of the different heat conductive metal filling portion 19 or the equivalent radius of the circle. prepare. As a result, the heat flux difference can be evenly applied in the casting direction, the heat flux from the solidification shell to the continuous casting mold at the initial stage of solidification can be periodically increased or decreased, and the individual strain amount can be reduced.

In FIG. 6, 19a is a different heat conductive metal filling part of 1, and 19c is another different heat conductive metal filling part. The different heat conductive metal filling portion 19a and the different heat conductive metal filling portion 19c are provided at the same positions with respect to the casting direction and at positions adjacent to each other in the width direction of the mold copper plate. Here, assuming that 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 D2 (mm), the distance D2 of the different heat conductive metal filling portion 19 is as follows (2). It is provided on the inner wall surface of the long-sided copper plate 5a of the mold so as to satisfy the formula.

D2 ≤ 4r ... (2)
However, in the equation (2), r is the radius (mm) or the radius equivalent to a circle (mm) of the different heat conductive metal filling portion 19.

In this way, the different heat conductive metal filling portion 19a is filled with the different heat conductive metal so that 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 four times or less the radius of the different heat conductive metal filling portion 19. The portion 19 is provided on the mold copper plate. As a result, the portion where the heat flux increases or decreases formed by the different heat conductive metal filling portion 19 can be allowed to exist at a pitch shorter than the spatial period of the solidification fluctuation of the solidification shell tip portion that uniformly solidifies, and is present at the initial stage of solidification. The deformation of the solidified shell 13 is reduced, the amount of individual strain is also reduced, and cracking on the surface of the solidified shell can be suppressed.

FIG. 7 is a schematic view showing the outer wall surface side of the long-sided copper plate of the mold. FIG. 8 is a cross section of DD in FIG. 7 in a state where a backup plate is provided on the outer wall surface of the long-sided copper plate of the mold, and a stud bolt is screwed into one of the bolt holes on the right side of the cross section of DD. It is a cross-sectional schematic diagram which showed the cross section by superimposing. The outer wall surface of the long-sided copper plate 5a of the mold is provided with a plurality of slits 30 through which the cooling water 44 passes and a plurality of bolt holes 32 screwed with the stud bolts 42 for fixing the backup plate 40. 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 slits 30 are provided at a pitch of L2 at positions avoiding the bolt holes 32, and the slits 30 are provided at a pitch of L1 at other positions. Here, L2> L1, and in the example shown in FIG. 7, the longest pitch of the slit 30 is L2.

The backup 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 backup plate 40, passes through the slit 30, and is discharged from above the backup plate 40. In this way, the cooling water 44 passes through the slit 30 of the mold long side copper plate 5a, so that the mold long side copper plate 5a is cooled by the cooling water 44.

The portion provided with the slit 30 causes a periodic fluctuation of the heat flux in the mold width direction, although it is not as large as that of the different heat conductive metal filling portion 19. When the space period in which the slit 30 is provided and the distance D2 in the width direction of the different heat conductive metal filling portion 19 become close to each other, the so-called “beat” (hereinafter, “beat”” is caused by the periodic fluctuation of the heat flux of both. ) Occurs. When a beat occurs, there is a concern that the periodic fluctuation of the heat flux due to the different heat conductive metal filling portion 19 may be disrupted.

The slit 30 has a pitch of L1 and a depth of the slit 30 so that the size of the heat flux in the region provided at the pitch of L2 avoiding the bolt hole 32 and the size of the heat flux in the other region are the same. Is adjusted. Therefore, assuming that the longest pitch of the slit 30 is Z, the distance D2 in the width direction of the different heat conductive metal filling portion 19 with reference to Z satisfies the following equation (4). It is preferable to provide.

Z ≧ 2.5 × D2 ・ ・ ・ (4)
However, in the formula (4), Z is the longest pitch (mm) of the slit 30 in the width direction of the mold long side copper plate 5a.

As a result, it is suppressed that the space period in which the slit 30 is provided and the distance D2 in the width direction of the different heat conductive metal filling portion 19 are close to each other, and the periodic fluctuation of the heat flux by the different heat conductive metal filling portion 19 is caused. It can prevent it from collapsing.

In the example shown in FIG. 7, the outer wall surface of the long-sided copper plate 5a of the mold is provided with slits 30 at a plurality of pitches, but the present invention is not limited to this. The slits 30 may be provided on the outer wall surface of the long-sided copper plate 5a of the mold at a single pitch. When the slits 30 are provided with a singular pitch, the singular pitch is Z (mm).

FIG. 9 is a diagram showing another example of the arrangement of the different heat conductive metal filling portion. In FIG. 9, circular different heat conductive metal filling portions 20 are provided on the inner wall surface of the mold copper plate in a grid pattern. Here, when the different heat conductive metal filling portions 20 are provided in a grid pattern, a group of parallel lines having a constant width in the casting direction and parallel to the mold width direction and a group of parallel lines having a constant width in the mold width direction and the casting direction This means that the different heat conductive metal filling portion 20 is provided at a position at the intersection of parallel lines parallel to the above.

In FIG. 9, 20a is a different heat conductive metal filling part of 1, and 20b and 20c are other different heat conductive metal filling parts. The different heat conductive metal filling portion 20a and the different heat conductive metal filling portion 20b are provided at the same positions with respect to the width direction of the mold copper plate, and are provided at positions adjacent to each other in the casting direction. The different heat conductive metal filling portion 20a and the different heat conductive metal filling portion 20c are provided at the same positions with respect to the casting direction and at positions adjacent to each other in the width direction of the mold copper plate.

In FIG. 9, the distance D1 is a distance along the casting direction, from the boundary line between the different heat conductive metal filling portion 20a and the mold copper plate to the boundary line between the different heat conductive metal filling portion 20b and the mold copper plate. It is a distance, and the distance D2 is the distance from the center of gravity of the different heat conductive metal filling portion 20a to the center of gravity of the different heat conductive metal filling portion 20c. In FIG. 9, the different heat conductive metal filling portion 20 is provided on the inner wall surface of the mold long side copper plate 5a so as to satisfy the above equations (1), (2) and (3).

As described above, the different heat conductive metal filling portion may be provided on the mold copper plate in a grid pattern, and even when the different heat conductive metal filling portion is provided in a grid pattern, the claws can be satisfied by satisfying the above equation (1). The collapse of the oscillating mark can be suppressed and the depth of the oscillation mark can be made shallow, and the same effect as when the different heat conductive metal filling portion is provided in a staggered pattern can be obtained.

In the present embodiment, an example is shown in which the shapes of the concave grooves provided in the mold copper plate are all the same circular shape, but the present invention is not limited to this. If at least the above-mentioned area ratio is 10% or more and 80% or less and the equations (1) and (2) are satisfied, the shapes of the concave grooves do not have to be the same.

A mold provided with a different heat conductive metal filling portion 19 and a slab in which the slab is intentionally bulged over 0 mm and 20 mm or less and the solid phase ratio at the center is 0.2 or more and 0.9 or less are reduced. ^{When the product (m · mm / min 2} ) of (mm / min) and the casting speed (m / min) is intentionally bulging with a rolling force corresponding to 0.30 or more and 1.00 or less, the slab The internal quality of the slab can be further improved by combining with a method of lightly reducing an amount equal to or less than the amount of swelling.

In the present embodiment, the total amount of forced bulging of the forced bulging zone 16 (hereinafter referred to as “total bulging amount”) is more than 0 mm and 20 mm with respect to the slab thickness (thickness between the long side faces of the slab) at the mold outlet. The range is as follows. In the present embodiment, the initial solidification in the mold is controlled, and the solidification interface can be smoothed in the width direction and the casting direction of the slab even in the final solidification portion of the slab 12, so that the reduction force under light reduction is equal to the solidification interface. As a result, central segregation can be reduced even when the total bulging amount is more than 0 mm and 20 mm or less.

In the light reduction zone 17, the slab 12 is reduced at least from the time when the solid phase ratio at the center of the thickness of the slab becomes 0.2 to 0.9. When the solid phase ratio of the central portion is less than 0.2, the unsolidified portion of the slab is thick at the reduced position immediately after the reduction, so that central segregation occurs again as the solidification progresses thereafter. .. When the pressure is reduced when the solid phase ratio of the central portion exceeds 0.9, the molten steel having a concentrated segregation component is less likely to be discharged, and the effect of improving the central segregation is reduced. This is because the solidification shell 13 of the slab at the time of reduction is thick, and the reduction force does not sufficiently reach the center of the thickness. Further, when the solid phase ratio of the central portion exceeds 0.9 and the amount of reduction is large, positive segregation occurs in the vicinity of the central portion of the thickness as described above. Therefore, the position of the slab having a central solid phase ratio of 0.2 or more and 0.9 or less is reduced. As a matter of course, the slab is formed in the light reduction zone 17 before the solid phase ratio at the center of the slab thickness reaches 0.2 and after the solid phase ratio at the center of the slab thickness exceeds 0.9. 12 may be reduced.

The solid phase ratio at the center 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 = 0 above the liquidus temperature of the steel and a solid phase ratio = 1.0 below the solid phase temperature of the steel, and is centered on the thickness of the slab. The position where the solid phase ratio of the portion is 1.0 is the solidification completion position 15, and at the solidification completion position 15, the solid phase ratio at the center of the slab thickness becomes 1 while the slab moves to the downstream side. Corresponds to the downstream position.

In the present embodiment, the total amount of reduction of the slab 12 in the light reduction zone 17 (hereinafter referred to as “total reduction amount”) is equal to or smaller than the total bulging amount. By making the total reduction amount equal to or smaller than the total bulging amount, the portion of the slab 12 that has been solidified to the center of the thickness on the short side is not reduced and constitutes the light reduction zone 17. The load on the guide roll 7 is reduced, and equipment troubles such as bearing breakage and breakage of the guide roll 7 can be suppressed.

In the present embodiment, the length of the slab is the reduction force corresponding to ^{the product (mm · m / min 2} ) of the reduction speed and the casting speed when lightly reducing in the light reduction band 17 is 0.30 or more and 1.00 or less. It is given to the side surface. Under reduction with a reduction amount smaller than 0.30, the thickness of the unsolidified part of the slab at the reduction position after reduction is thick, and the molten steel with a concentrated segregation component is not sufficiently discharged from the dendrite trees. Central segregation occurs. When the reduction amount exceeds 1.00, almost all of the molten steel with concentrated segregation components existing between the dendrite trees is squeezed out and discharged to the upstream side in the casting direction, but the unsolidified portion. Because the thickness of the slab is thin, it is trapped in the solidified shells on both sides of the slab on the thickness direction of the slab slightly upstream of the casting position, which causes positive segregation in the vicinity of the center of the thickness of the slab.

The effect of light pressure on the prevention of central segregation of the central part of the slab and positive segregation near the central part is also affected by the solidified structure of the slab, and the solidified structure of the part in contact with the unsolidified part is equiaxed. In some cases, concentrated molten steel that causes semi-macro segregation exists between equiaxed crystals, and the effect of reduction is small. Therefore, it is desirable that the solidified structure has a columnar crystal structure instead of an equiaxed crystal structure.

In the present embodiment, under various casting conditions of the continuous casting operation, the thickness of the solidified shell 13 and the solid phase ratio at the center of the slab thickness are obtained in advance by using two-dimensional heat transfer solidification calculation or the like, and at least the slab thickness center. Of the amount of secondary cooling water, the width of the secondary cooling, and the casting speed so that the slab 10 can be reduced by the light reduction zone 14 from the time when the solid phase ratio of the part becomes 0.2 to 0.9. Adjust any one or more of the above. Here, "cutting the width of the secondary cooling" means stopping the injection of the cooling water to both ends of the long side surface of the slab. By performing the width cutting of the secondary cooling, the secondary cooling is weakened, and the solidification completion position 13 is generally extended to the downstream side in the casting direction.

As described above, by carrying out the steel continuous casting method according to the present embodiment, it is possible to prevent surface cracking of the slab due to non-uniform cooling of the solidified shell at the initial stage of solidification, and at the same time, it is possible to prevent the surface cracking of the slab. The depth can also be shallow. By making the oscillation mark shallow and making the surface of the initial solidification shell 13 uniform, the solidification interface at the final solidification part is also smoothed, and by intentional bulging and light reduction, the reduction force is reduced to the solidification interface. Can be evenly acted on, and the central segregation generated in the central portion of the thickness of the slab can be suppressed. As a result, it is possible to stably produce high-quality slabs.

Although the above description has been made for continuous casting of slab slabs, the method for continuous casting of steel according to the present embodiment is not limited to continuous casting of slab slabs, and is used for continuous casting of bloom slabs and billet slabs. Can also be applied according to the above.

Medium carbon steel (chemical composition, C: 0.08 to 0.17% by mass, Si: 0.10 to 0.30% by mass, Mn: 0.50 to 1.20% by mass, P: 0.010 to 0 .030% by mass, S: 0.005 to 0.015% by mass, Al: 0.020 to 0.040% by mass) using a water-cooled copper mold in which a metal is arranged on the inner wall surface under various conditions, and A test was conducted to investigate the surface cracking and internal quality (center segregation) of the slab after casting by changing the total bulging amount in the forced bulging zone and the product of the rolling speed and the casting speed in the light rolling zone. ..

The product of the reduction speed and the casting speed in the light reduction zone is 0.28 to 0.90 mm ・ m / min. ²In each test, in the light reduction zone, the slab was reduced from the time when the solid phase ratio at the center of the thickness of the slab became at least 0.2 to 0.9. When the slab was forcibly bulged in the forced bulging zone, the total reduction amount was equal to or smaller than the total bulging amount. In the test in which the slab was not bulged in the forced bulging zone, the solidification completion position on the short side of the slab was also reduced in the light reduction zone.

The mold used is a mold having an inner space size of 2.1 m in long side length and 0.26 m in short side length. The length (= mold length) from the upper end to the lower end of the water-cooled copper mold used was 950 mm, and the position of the meniscus (hot metal surface in the mold) during steady casting was set to a position 100 mm below the upper end of the mold. In order to understand the effect of the continuous steel casting method according to this embodiment, a mold under the following conditions was prepared and a comparative test was conducted. In each of the molds, a metal having a thermal conductivity lower than that of the mold copper plate was used as the different heat conductive metal. The shape of the different heat conductive metal filling portion 19 is a circular shape having a diameter of 6 mm. Under the casting conditions, the oscillation mark pitch was 13 mm.

Mold 1: In the range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold (range length = 220 mm), the ratio of the difference in thermal conductivity to the thermal conductivity of copper is 20%. The different thermal conductive metal was filled in a staggered manner, and the different thermal conductive metal filling portion 19 was installed. The area ratio ε of the different heat conductive metal filling portion 19 is 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different heat 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.

Mold 2: In the range from the position 190 mm below the upper end of the mold to the position 750 mm below the upper end of the mold (range length = 670 mm), the ratio of the difference in thermal conductivity to the thermal conductivity of copper is 20%. The different thermal conductive metal was filled in a staggered manner, and the different thermal conductive metal filling portion 19 was installed. The area ratio ε of the different heat conductive metal filling portion 19 is 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different heat 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.

Mold 3: Staggered different heat conductive metals with the ratio of the difference in thermal conductivity to the thermal conductivity of copper being 20% in the range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold. 19 was installed. The area ratio ε of the different heat conductive metal filling portion 19 is 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction is 15 mm, and the distance D2 between the centers of gravity of the different heat 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.

Mold 4: Staggered different heat conductive metals with the ratio of the difference in thermal conductivity to the thermal conductivity of copper being 20% in the range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold. 19 was installed. The area ratio ε of the different heat conductive metal filling portion 19 is 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different heat 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.

Mold 5: Staggered different heat conductive metals with the ratio of the difference in thermal conductivity to the thermal conductivity of copper is 15% in the range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold. 19 was installed. The area ratio ε of the different heat conductive metal filling portion 19 is 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different heat 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.

Mold 6: Staggered different heat conductive metals 19 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, in which the ratio of the difference in thermal conductivity to the thermal conductivity of copper is 20%. The different heat conductive metal filling portion 19 was installed by filling in a shape. The area ratio ε of the different heat conductive metal filling portion 19 is 5%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different heat 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.

Mold 7: Staggered different heat conductive metals with the ratio of the difference in thermal conductivity to the thermal conductivity of copper being 20% in the range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold. 19 was installed. The area ratio ε of the different heat conductive metal filling portion 19 is 85%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different heat 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.

Mold 8: In the range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold, different heat conductive metals having the ratio of the difference in thermal conductivity to the thermal conductivity of copper of 20% are arranged in a grid pattern. 19 was installed. The area ratio ε of the different heat conductive metal filling portion 19 is 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction is 6 mm, and the distance D2 between the centers of gravity of the different heat 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.

Mold 9: Staggered different heat conductive metals with the ratio of the difference in thermal conductivity to the thermal conductivity of copper being 20% in the range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold. 19 was installed. The area ratio ε of the different heat conductive metal filling portion 19 is 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction is 9 mm, and the distance D2 between the centers of gravity of the different heat 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.

Mold 10: Staggered different heat conductive metals with the ratio of the difference in thermal conductivity to the thermal conductivity of copper being 20% in the range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold. 19 was installed. The area ratio ε of the different heat conductive metal filling portion 19 is 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction is 9 mm, and the distance D2 between the centers of gravity of the different heat 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.

Mold 11: A mold not provided with a different heat conductive metal filling portion 19.

In the continuous casting operation, as a mold powder, a mold having a basicity ((mass% CaO) / (mass% SiO ₂ )) of 1.1 and a solidification temperature of 1090 ° C. and a viscosity of 0.15 Pa · s at 1300 ° C. I used powder. The solidification temperature is a temperature at which the viscosity of the mold powder rapidly increases during the cooling of the molten mold powder. The position of the meniscus in the mold during steady casting was 100 mm below the upper end of the mold, and the position of the meniscus was controlled so that the meniscus was within the installation range during casting. The casting speed during steady casting is 1.7 to 2.2 m / min, and the casting speed for steady casting is 2.0 m / min in all tests for slabs for investigating surface cracks and internal quality of slabs. The target was min slabs. The degree of superheat of molten steel in the tundish is 25 to 35 ° C. To control the temperature of the mold, a thermocouple is embedded 50 mm below the meniscus of the mold at a depth of 5 mm from the surface (the surface on the molten steel side) from the back, and the surface temperature of the mold is estimated from the measured value of the copper plate temperature by the thermocouple. bottom.

After the continuous casting was completed, the surface of the long side of the slab was pickled to remove the scale, and the number of surface cracks was measured. The occurrence of slab surface cracks was evaluated using a value calculated using the casting direction length of the slab to be inspected as the denominator and the casting direction length of the slab at the site where the surface cracks occurred as the numerator. Regarding the evaluation of the slab internal quality (center segregation), a cross-sectional sample of the slab is taken, and the Mn concentration is measured every 100 μm by EPMA within a range of ± 10 mm at the center of the slab on the mirror-polished surface of the cross-sectional sample. Then, the segregation degree was evaluated. _{Specifically, the ratio (C / C 0 ) of the Mn concentration (C 0} ) at the end where segregation will not occur and the average value (C) of the Mn concentration at the central portion ± 10 mm is defined as the Mn segregation degree. Defined and evaluated.

Apart from these studies, the non-uniformity σ (mm) of the solidified shell thickness was measured under the conditions at each test level. To measure the non-uniformity of the solidified shell thickness, FeS (iron sulfide) powder was added to the molten steel in the mold, and sulfaprint was taken from the cross section of the obtained slab to measure the solidified shell thickness. The solidification shell thickness was measured at 40 points at a pitch of 5 mm from the meniscus position to a position 200 mm below the position 1/4 in the width direction of the mold. The calculation of σ was performed according to the following equation (6).

However, in the formula (6), D is the measured value (mm) of the solidification shell thickness, and Di is the solidification shell thickness measured using an approximate formula that defines the relationship between the solidification shell thickness and the solidification time. It is a calculated value (mm) of the solidification shell thickness calculated by using the solidification time corresponding to the distance from the meniscus of the position. N is the number of measurements, which is 40 in this embodiment.

Table 1 shows the test conditions of each test of test levels 1 to 14 and the survey results of the surface and internal quality of the slab.

In the test levels 1, 8, 9, 10, 11, and 13, the installation conditions of the different heat conductive metal filling portion 19 on the mold surface are within the scope of the present invention, and the longest pitch of the slit 30 satisfies the equation (4). do. At all of these test levels, the surface crack ratio was significantly improved. The non-uniformity of the solidified shell thickness was also 0.30 or less, and the solidified shell thickness could be made uniform. However, with respect to test level 1, since the product of the rolling speed and the casting speed was not within the range of 0.30 or more and 1.00 or less, a slight central segregation was confirmed. At other levels, central segregation was also improved.

At the test level 2, the range in which the different heat conductive metal filling portion 19 is installed is shifted downward, and the product of the reduction speed and the casting speed is not within the range of 0.30 or more and 1.00 or less. Therefore, at the test level 2, fine surface cracks were generated in the slab, and the effect of reducing the surface cracks could not be confirmed as compared with the conventional case. The non-uniformity of the solidification shell thickness also increased to 0.38 mm, and the improvement effect on the central segregation could not be confirmed.

At the test level 3, the distance D1 in the casting direction is long, and the product of the reduction speed and the casting speed is not within the range of 0.30 or more and 1.00 or less. At test level 3, the surface cracking of the slab was improved, but the non-uniformity of the solidification shell thickness was as large as 0.37 mm, and the improvement effect could not be confirmed for the central segregation.

At the test level 4, the distance D2 in the mold width direction is long, and the product of the reduction speed and the casting speed is not within the range of 0.30 or more and 1.00 or less. At test level 4, surface cracks in the slab were confirmed, and the effect of improving the surface cracks was not confirmed. The non-uniformity of the solidified shell thickness was also slightly large at 0.31 mm, and the central segregation was also slightly confirmed.

In the test level 5, the ratio of the difference in thermal conductivity of the different heat conductive metals is lower than 20%, in the test level 6, the area ratio of the different heat conductive metal filling portion 19 is lower than 10%, and in the test level 7, the test level 7 is The area ratio of the different heat conductive metal filling portion 19 is higher than 80%. Therefore, at these test levels 5 to 7, surface cracks in the slab were confirmed, and the effect of improving the surface cracks was not confirmed. The non-uniformity of the solidified shell thickness was also slightly large, 0.31 to 0.33, and the central segregation was also slightly confirmed.

In the test level 12, the product of the rolling speed and the casting speed is in the range of 0.30 or more and 1.00 or less, but the distance D1 in the casting direction is long. At test level 12, surface cracking and central segregation of the slab were improved, but the non-uniformity of the solidification shell thickness was as large as 0.37 mm.

In the test level 14, the installation conditions of the different heat conductive metal filling portion 19 on the mold surface are within the range of the present invention, and the longest pitch Z of the slit 30 satisfies the equation (4). However, although the distance D1 in the casting direction is long and the equation (1) is satisfied, the equation (3) is not satisfied. Therefore, although the surface cracking ratio was improved from the test levels 2 to 7, it was slightly larger at 1.8%, slight central segregation was confirmed, and the non-uniformity of the solidification shell thickness was also slightly larger at 0.31 mm. ..

In the test level 15, the installation condition of the different heat conductive metal filling portion 19 on the mold surface is within the range of the present invention, but the longest pitch Z of the slit 30 does not satisfy the equation (4). Further, although the distance D1 in the casting direction is long and the equation (1) is satisfied, the equation (3) is not satisfied. Therefore, although the surface cracking ratio was improved from the test levels 2 to 7, it was slightly larger at 1.5%, slight central segregation was confirmed, and the non-uniformity of the solidification shell thickness was also slightly larger at 0.33 mm. ..

At the test level 16, since the different heat conductive metal filling portion 19 was not provided, surface cracking of the slab was confirmed. The non-uniformity of the solidified shell thickness was also slightly large at 0.32 mm, and central segregation was also confirmed.

1 Slab continuous casting machine 2 Tandish 3 Sliding nozzle 4 Immersion nozzle 5 Continuous casting mold 5a Mold long side copper plate 6 Support roll 7 Guide roll 8 Pinch roll 9 Conveyance roll 10 Shard cutting machine 11 Molten steel 12 Slab 12a Slab slab 13 Solidified shell 14 Unsolidified part 15 Solidified completed position 16 Forced bulging band 17 Light reduction band 18 Meniscus position 19 Different heat conductive metal filling part 19a 1 Different heat conductive metal filling part 19b Other different heat conductive metal filling part 19c Other Different heat conductive metal filling part 20 Different heat conductive metal filling part 20a 1 Different heat conductive metal filling part 20b Other different heat conductive metal filling part 20c Other different heat conductive metal filling part 30 Slit 32 Bolt hole 40 Backup plate 42 Stud Bolt 44 cooling water

Claims (8)

A method for continuously casting steel to produce slabs by injecting molten steel into a mold for continuous casting and pulling out the molten steel while vibrating the mold for continuous casting in the casting direction.
The mold for continuous casting has a plurality of recesses provided on the inner wall surface of the mold copper plate from a position at least 20 mm above the meniscus position in the steady casting state to a position at least 50 mm or more and at most 200 mm or less below the meniscus position. Has a groove,
Inside the plurality of recesses, a plurality of different heat conductive metal filling portions filled with a metal or a metal alloy having a ratio of the thermal conductivity difference to the thermal conductivity of the mold copper plate of 20% or more are provided. The area ratio, which is the ratio of the total area of all the different heat conductive metal filling parts to the area of the inner wall surface provided with the plurality of different heat conductive metal filling parts, is 10% or more and 80% or less.
The oscillation mark pitch (OMP) and the distance (D1) derived from the oscillation frequency (f) and the casting speed (Vc) satisfy the following equation (1), and the distance (D2) satisfies the following equation (2). Satisfied continuous casting method of steel.
D1 ≤ OMP = Vc x 1000 / f ... (1)
D2 ≦ 4r ・ ・ ・ (2)
However, in equation (1),
Vc is the casting speed (m / min).
f is the oscillation frequency (cpm).
OMP is the oscillation mark pitch (mm).
D1 is the center of gravity of one of the plurality of different heat conductive metal filling portions and another different heat conductive metal filling portion provided at the same position in the width direction of the mold copper plate, and is the different heat conductive metal filling portion of 1. Distance (mm) from the boundary line between the mold copper plate and another different heat conductive metal filling part adjacent to the metal filling part in the casting direction to the boundary line between the different heat conductive metal filling part 1 and the mold copper plate. And
In equation (2)
r is the radius (mm) of a circle having the same area as the area of the different heat conductive metal filling portion centered on the center of gravity of the different heat conductive metal filling portion.
D2 is another different heat conductive metal filling part provided at the same position in the casting direction as the center of gravity of the different heat conductive metal filling part of the above 1, and is formed in the different heat conductive metal filling part of the above 1 in the width direction. It is the distance (mm) from the center of gravity of the other adjacent different heat conductive metal filling portions to the center of gravity of the different heat conductive metal filling portion of 1. The method for continuous casting of steel according to claim 1, wherein the plurality of different heat conductive metal filling portions are provided so that the distance (D1) satisfies the following equation (3).
D1 ≤ 2r ... (3) The method for continuously casting steel according to claim 1 or 2, wherein the shapes of the plurality of concave grooves are all the same. The method for continuously casting steel according to any one of claims 1 to 3, wherein the shape of the plurality of concave grooves is a circle or a pseudo-circle without corners. The method for continuously casting steel according to any one of claims 1 to 4, wherein the plurality of different heat conductive metal filling portions are provided in a lattice pattern. The method for continuously casting steel according to any one of claims 1 to 4, wherein the plurality of different heat conductive metal filling portions are provided in a staggered pattern. By gradually increasing the roll opening of a plurality of pairs of slab support rolls provided in the continuous casting machine toward the downstream side in the casting direction, the long side surface of the slab having an unsolidified portion inside is formed at the mold outlet. The total bulging amount in the range of more than 0 mm and 20 mm or less with respect to the slab thickness (thickness between the long side faces of the slab) in
After that, in a light reduction zone in which the roll opening degree of the plurality of pairs of slab support rolls is gradually reduced toward the downstream side in the casting direction, the solid phase ratio at the center of the thickness of the slab is at least 0.2. ^{The rolling force corresponding to the product (mm · m / min 2} ) of the rolling speed (mm / min) and the casting speed (m / min 2) of 0.30 or more and 1.00 or less from the time point to the time point of 0.9. Is applied to the long side surface of the slab, and the long side surface of the slab is reduced by the reducing force with a total reduction amount equal to the total bulging amount or a total reduction amount smaller than the total bulging amount. The method for continuously casting steel according to any one of items 1 to 6. A plurality of slits along the casting direction are provided on the outer wall surface of the mold copper plate at a single pitch or a plurality of pitches in the width direction of the mold copper plate.
When the plurality of slits are provided at a single pitch, the single pitch is Z (mm), and when the plurality of slits are provided at a plurality of pitches, the most of the plurality of pitches. When the long pitch is Z (mm)
The method for continuously casting steel according to any one of claims 1 to 7, wherein Z satisfies the following equation (4).
Z ≧ 2.5 × D2 ・ ・ ・ (4)

Images