JP2019171435A - Method of continuous casting - Google Patents

Abstract

To provide a method of continuous casting adapted to prevent surface cracks of a billet resulting from uneven cooling of a solidified shell within a continuous casting mold.SOLUTION: A method of continuous casting in the present invention is for performing continuous casting using a mold that has dissimilar-metal filled portions 3 having a diameter of from 1.0 mm to 10.0 mm filled with a metal having a heat conductivity of 80% or lower or 120% or higher relative to a heat conductivity of a copper mold in a vicinity of a meniscus on an inner wall surface of the mold, and the dissimilar-metal filled portions adjacent to each other in a width direction of the mold are in contact with or parts thereof are overlapped with each other and that a center-to-center distance W of the dissimilar-metal filled portions adjacent to each other in the width direction of the mold satisfies a relationship of the following Expression (3) with respect to a diameter d of the dissimilar-metal filled portion and/or a center-to-center distance L of the dissimilar-metal filled portions adjacent to each other in a casting direction of the mold satisfies a relationship of the following Expression (4) with respect to the diameter d of the dissimilar-metal filled portions. 0.70<W/d≤1.00...(3) 0.70<L/d≤1.00...(4)SELECTED DRAWING: Figure 1

Description

  The present invention relates to a continuous casting method for steel in which molten steel is continuously cast while suppressing slab surface cracks resulting from uneven cooling of a solidified shell in a mold.

  In continuous casting of steel, molten steel injected into a mold is cooled by a water-cooled continuous casting mold, and the molten steel is solidified at a contact surface with the mold to generate a solidified shell (also referred to as “solidified layer”). The slab having the solidified shell as an outer shell and the inside as an unsolidified layer is continuously drawn below the mold while being cooled by a water spray or an air / water spray installed on the downstream side of the mold. The slab is solidified to the center of thickness by cooling with water spray or air-water spray, and then cut by a gas cutter or the like to produce a slab of a predetermined length.

  If the cooling in the mold becomes non-uniform, the thickness of the solidified shell becomes non-uniform in the casting direction of the slab and in the mold width direction. A stress caused by the shrinkage or deformation of the solidified shell acts on the solidified shell, and in the initial stage of solidification, this stress is concentrated on the thin portion of the solidified shell, and the stress causes cracks on the surface of the solidified shell. This crack expands due to subsequent external stresses such as thermal stress, bending stress due to the roll of a continuous casting machine, and straightening stress, resulting in a large surface crack. Since cracks present in the slab become surface defects in the subsequent hot rolling process, it is necessary to clean the surface of the slab and remove the surface cracks at the stage of the cast slab after casting. .

  Inhomogeneous solidification in the mold is likely to occur particularly in steel (referred to as medium carbon steel) having a carbon content of 0.08 to 0.17% by mass. In a steel having a carbon content of 0.08 to 0.17% by mass, a peritectic reaction occurs during solidification. It is believed that the inhomogeneous solidification in the mold is caused by transformation stress due to volume shrinkage during transformation from δ iron (ferrite) to γ iron (austenite) by this peritectic reaction. That is, the solidified shell is deformed by the strain caused by the transformation stress, and the solidified shell is separated from the inner wall surface of the mold by this deformation. The portion separated from the inner wall surface of the mold is cooled by the mold, and the thickness of the solidified shell at the portion away from the inner wall surface of the mold (the portion away from the inner wall surface of the mold is referred to as “depression”) is reduced. It is considered that the stress is concentrated on this portion and the surface cracks are generated by reducing the thickness of the solidified shell.

  In particular, when the slab drawing speed is increased, not only the average heat flux from the solidified shell to the mold increases (the solidified shell is rapidly cooled), but also the heat flux distribution is irregular and non-uniform. Therefore, the occurrence of slab surface cracks tends to increase. Specifically, in a slab continuous casting machine having a slab thickness of 200 mm or more, surface cracks are likely to occur when the slab drawing speed is 1.5 m / min or more.

  Therefore, conventionally, various means have been proposed in order to suppress surface cracks (particularly longitudinal cracks) in the steel types that are liable to generate surface cracks.

  For example, Patent Document 1 proposes using mold powder having a composition that is easily crystallized, and increasing the thermal resistance of the mold powder layer to slowly cool the solidified shell. This is a technique of suppressing surface cracking by reducing the stress acting on the solidified shell by slow cooling. However, only the slow cooling effect due to the mold powder has not sufficiently improved the non-uniform solidification, especially in the medium carbon steel in which transformation from δ iron to γ iron occurs due to a slight temperature drop accompanying solidification, The fact is that the occurrence of surface cracks cannot be sufficiently suppressed.

  Therefore, a metal having a thermal conductivity lower than that of the mold copper plate or higher than that of the mold copper plate is filled in the vicinity of the meniscus (also referred to as “molten steel surface in the mold”) of the inner wall surface of the continuous casting mold. In other words, the solidified shell near the meniscus is forcibly given a regular and periodic heat flux distribution to the solidified shell due to transformation from δ iron to γ iron or thermal stress. There has been proposed a method for reducing the load on the slab and preventing the surface crack of the slab.

  For example, Patent Document 2 discloses that the thermal conductivity of copper is within the range of an inner wall surface from an arbitrary position above the meniscus on the inner wall surface of the water-cooled copper mold to a position 20 mm or more below the meniscus. % Of a plurality of different kinds of metals having a diameter of 2 to 20 mm or equivalent circular diameters of 2 to 20 mm, which are formed by filling a metal having a thermal conductivity of less than or equal to% into the circular concave groove or pseudo circular concave groove provided on the inner wall surface. Continuously having a metal filling part independently, and a filling thickness of the metal in the different metal filling part being 0.5 mm or more and a diameter d of the different metal filling part or an equivalent circle diameter d or less. Casting molds have been proposed.

  Further, Patent Document 3 discloses that the thermal conductivity of the mold copper plate is within the range of the inner wall surface from a position above the meniscus on the inner wall surface of the water-cooled copper mold to a position below the meniscus by a predetermined distance or more. Using a continuous casting mold having a dissimilar metal filling portion filled with a metal having a thermal conductivity of 80% or less or 125% or more, the distance L between the centers of dissimilar metal filling portions adjacent to the casting direction is A method for controlling the vibration conditions of the mold and the slab drawing speed so as to satisfy “A / 2 ≦ L ≦ 1000 × Vc / f” with respect to the amplitude A, the frequency f, and the slab drawing speed Vc is proposed. Has been. Patent Document 3 states that slab surface cracks caused by oscillation marks can be reduced.

JP 2005-297001 A International Publication No. 2014/002409 Japanese Patent Laid-Open No. 2006-168610

  According to Patent Document 2 and Patent Document 3, the surface cracks of the medium carbon steel were reduced, but when the slab drawing speed was 2.0 m / min or more, the surface cracks were scattered and still improved. There is room.

  The present invention has been made in view of the above circumstances, and its object is to form a dissimilar metal filling portion having a lower or higher thermal conductivity than the copper mold on the inner wall surface of the continuous casting mold, thereby The slab is made by forcibly giving a regular and periodic heat flux distribution to the solidified shell in the vicinity of the meniscus, reducing the stress due to transformation from δ iron to γ iron and the stress on the solidified shell. In order to prevent surface cracking of steel, it is caused by surface cracking due to non-uniform cooling of the solidified shell at the initial stage of solidification and transformation from δ iron to γ iron in a medium carbon steel with a peritectic reaction. It is an object of the present invention to provide a continuous casting method of steel capable of preventing surface cracking due to uneven thickness of a solidified shell.

The gist of the present invention for solving the above problems is as follows.
[1] While injecting molten steel into the continuous casting mold and vibrating the continuous casting mold in the casting direction, the solidified shell formed by cooling the molten steel is pulled out from the continuous casting mold, A continuous casting method of steel,
The continuous casting mold is lower than the meniscus by a length R ₀ (mm) or more determined by the following equation (1) based on the slab drawing speed Vc (m / min) from an arbitrary position above the meniscus. Up to the position of the inner wall surface of the water-cooled copper mold, a metal having a thermal conductivity of 80% or less or 120% or more with respect to the thermal conductivity of the copper mold has a diameter of 1.0 mm on the inner wall surface. A plurality of dissimilar metal filling portions formed by being filled in circular grooves processed as in the range of 10.0 mm to 10.0 mm,
The filling thickness H (mm) of the dissimilar metal filling portion satisfies the relationship of the diameter d (mm) of the circular groove and the following equation (2):
The circular grooves adjacent to each other in the width direction of the continuous casting mold are in contact with each other, or a part thereof overlaps between the centers of the different metal filling portions adjacent to each other in the width direction of the continuous casting mold. The distance W (mm) satisfies the relationship of the following formula (3) with the diameter d (mm) of the circular concave groove and / or the dissimilar metal filling portion adjacent to the casting direction of the continuous casting mold. The center-to-center distance L (mm) is a continuous casting method of steel that satisfies the relationship of the diameter d (mm) of the circular groove and the following equation (4).
R ₀ = 2 × Vc × 1000/60 (1)
0.5 ≦ H ≦ d (2)
0.70 <W / d ≦ 1.00 (3)
0.70 <L / d ≦ 1.00 (4)
[2] The center-to-center distance L (mm) of the dissimilar metal filling portion adjacent to the casting direction of the continuous casting mold is the mold vibration amplitude A (mm) of the continuous casting mold and the frequency f of the mold vibration. (Times / min) and the slab drawing speed Vc (m / min) and the continuous casting method for steel according to the above [1], which satisfies the relationship of the following expression (5).
A / 2 ≦ L ≦ 1000 × Vc / f (5)

  According to the present invention, at least one of the distribution period of the heat flux in the mold width direction or the distribution period of the heat flux in the casting direction is appropriate for preventing uneven solidification, and Surface cracking due to non-uniform cooling and surface cracking due to non-uniform solidification shell thickness due to transformation from δ iron to γ iron in peritectic reaction is prevented more stably than in the prior art. Is realized. Furthermore, when the relationship of the formula (5) is satisfied, the surface crack of the slab caused by the oscillation mark can be reduced.

It is the schematic side view which looked at the mold long side copper plate which comprises a part of mold for water cooling type continuous casting used by this invention from the inner wall surface side. It is X-X 'sectional drawing of the casting_mold | template long side copper plate 1 shown in FIG. It is a figure which shows notionally the thermal resistance in the position of three places of the casting_mold | template long side copper plate which has a different metal filling part corresponding to the position of a different metal filling part. The figure which shows the example of arrangement | positioning of the dissimilar metal filling part which adjoins the width direction and casting direction of the casting_mold | template for continuous casting in this invention, and the example of long side of a mold is W / d = 0.80 and L / d = 0.80 It is the schematic side view seen from the inner wall surface side of a copper plate. The figure which shows the example of arrangement | positioning of the dissimilar metal filling part which adjoins the width direction and casting direction of the casting_mold | template for continuous casting in this invention, and the long side of a mold is an example of W / d = 0.90 and L / d = 0.90 It is the schematic side view seen from the inner wall surface side of a copper plate. The figure which shows the example of arrangement | positioning of the dissimilar metal filling part which adjoins the width direction and casting direction of the casting mold for continuous casting in this invention, W / d = 0.70 and L / d = 1.00 It is the schematic side view seen from the inner wall surface side of a copper plate. It is a figure which shows the result of having investigated L / d to 1.00, changing W / d in the range from 0.10 to 2.00, and having investigated the influence on the surface crack number density of slab slab of W / d. is there. It is a figure which shows the result of having investigated W / d as 1.00, changing L / d in the range from 0.10 to 2.00, and having investigated the influence on the surface crack number density of the slab slab of L / d. is there.

  Hereinafter, the present invention will be specifically described through embodiments of the invention. FIG. 1 shows a mold long side copper plate 1 constituting a part of a water-cooled continuous casting mold used in the steel continuous casting method according to the present invention, and a dissimilar metal filling portion on the inner wall surface side of the water-cooled copper mold. It is the schematic side view which looked at the mold long side copper plate 1 in which 3 was formed from the inner wall surface side. FIG. 2 is a cross-sectional view taken along the line X-X ′ of the mold long-side copper plate 1 shown in FIG. 1.

  The continuous casting mold shown in FIG. 1 is an example of a continuous casting mold for casting a slab slab. A water-cooled continuous casting mold for a slab slab is configured by combining a pair of copper or copper alloy mold long side copper plates and a pair of copper or copper alloy mold short side copper plates. FIG. 1 shows the long-side copper plate 1 of the mold. Similarly to the long-side copper plate 1, the short-side copper plate is also formed with the dissimilar metal filling portion 3 on the inner wall surface side, and the description of the short-side copper plate is omitted here. However, in slab slabs, stress concentration is likely to occur in the solidified shell on the long side of the slab due to the shape of the slab width being extremely large relative to the slab thickness, and surface cracks occur on the long side of the slab It's easy to do. Therefore, the dissimilar metal filling part 3 does not need to be installed on the short side copper plate of the continuous casting mold for the slab slab.

  As shown in FIG. 1, the length of the long side copper plate 1 is longer than the meniscus from a position above the length Q (length Q is an arbitrary value greater than zero) away from the position of the meniscus during steady casting. In the range of the inner wall surface of the mold long side copper plate 1 up to a position lower by R, a plurality of dissimilar metal filling portions 3 are installed. Here, “meniscus” is “molten steel surface in mold” and its position is not clear during non-casting, but in the normal continuous casting operation of steel, the meniscus position is 50 mm to 200 mm from the upper end of the mold copper plate. An arbitrary position below the degree is set. Accordingly, even if the meniscus position is a position 50 mm below the upper end of the long copper plate 1 and 200 mm below the upper end, the length Q and the length R are described below. What is necessary is just to arrange | position the dissimilar metal filling part 3 so that these conditions may be satisfied.

As shown in FIG. 2, the dissimilar metal filling portion 3 is formed on the inner wall surface side of the long-side copper plate 1 and has a circular groove 2 with a diameter d (hereinafter, “circular groove 2”). And a metal having a thermal conductivity λ _m of 80% or less with respect to the thermal conductivity λ _c of the long copper plate 1 (hereinafter referred to as “low thermal conductivity metal”). Or a metal having a thermal conductivity λ _m of 120% or more with respect to the thermal conductivity λ _c of the long copper plate 1 (hereinafter referred to as “high thermal conductive metal”). Is. The diameter of the dissimilar metal filling portion 3 filled in the circular groove 2 having a diameter d is also d. Here, reference numeral 4 in FIG. 2 is a slit, which is a flow path for the mold cooling water, installed on the back side of the long mold copper plate 1, and reference numeral 5 is a back that is in close contact with the back of the long mold copper sheet 1. It is a plate.

  In FIG. 3, the thermal resistance at three positions of the mold long side copper plate 1 having the dissimilar metal filling portion 3 using the low heat conductive metal as the filling metal in the circular groove 2 corresponds to the position of the dissimilar metal filling portion 3. Conceptually. As shown in FIG. 3, the thermal resistance is relatively high at the installation position of the dissimilar metal filling portion 3. FIG. 3 is an example in which a low heat conductive metal is used as the filling metal in the circular groove 2, and when a high heat conductive metal is used as the filling metal in the circular groove 2, contrary to FIG. The thermal resistance becomes relatively low at the installation position of the metal filling portion 3.

  By disposing a plurality of different metal filling portions 3 in the width direction and casting direction of the continuous casting mold near the meniscus including the meniscus position, the thermal resistance of the continuous casting mold in the mold width direction and casting direction near the meniscus A distribution is formed that periodically increases and decreases. This forms a distribution in which the heat flux from the solidified shell in the vicinity of the meniscus, that is, in the initial stage of solidification, to the continuous casting mold periodically increases and decreases.

  Due to the periodic increase and decrease of the heat flux, the stress and thermal stress generated in the solidified shell due to the transformation from δ iron to γ iron (hereinafter referred to as “δ / γ transformation”) are reduced, and the solidified shell generated by these stresses The deformation of becomes smaller. By reducing the deformation of the solidified shell, the non-uniform heat flux distribution resulting from the deformation of the solidified shell is made uniform, and the generated stress is dispersed to reduce the amount of individual strain. As a result, the occurrence of surface cracks on the surface of the solidified shell is suppressed.

In the present invention, the comparison between the thermal conductivity λ _c of the long-side copper plate 1 and the thermal conductivity λ _m of the low thermal conductivity metal or the high thermal conductivity metal is made by comparing the thermal conductivities at room temperature (about 20 ° C.). Define in. The thermal conductivity of copper, a copper alloy, a low thermal conductivity metal, and a high thermal conductivity metal generally becomes smaller as the temperature becomes higher. However, the thermal conductivity metal at room temperature with respect to the thermal conductivity λ _c of the mold long-side copper plate 1 at room temperature. 80% thermal conductivity lambda _m is less than, or as long as the thermal conductivity lambda _m the high thermal conductive metal at room temperature to the thermal conductivity lambda _c of the mold long sides copper plate 1 at room temperature 120% or more, Even at the operating temperature (about 200 to 350 ° C.) as a casting mold for continuous casting, the thermal resistance of the part where the dissimilar metal filling part 3 is installed and the thermal resistance of the part where the dissimilar metal filling part 3 is not installed , Can make a sufficient difference. As a filling metal for the circular concave groove 2, only one of the low heat conductive metal and the high heat conductive metal is used in one continuous casting mold.

Considering the influence on the initial solidification, the dissimilar metal filling portion 3 is installed at a length R ₀ or more determined by the following equation (1) from the meniscus according to the slab drawing speed Vc at the time of steady casting. It is necessary to reach the lower position. That is, the length R from the meniscus position shown in FIG. 1 needs to be not less than the length R ₀ calculated from the equation (1).

R ₀ = 2 × Vc × 1000/60 (1)
Here, in the formula (1), R ₀ is the length (mm), and Vc is the slab drawing speed (m / min).

The length R ₀ is related to the time during which the slab after the start of solidification passes through the range where the dissimilar metal filling portion 3 is installed. For at least 2 seconds, the slab needs to stay within the range where the dissimilar metal filling portion 3 is installed. In order for the slab to exist in the range where the dissimilar metal filling portion 3 is installed for at least 2 seconds after the start of solidification, the length R ₀ needs to satisfy the expression (2).

  By securing the time for the cast slab after the start of solidification to stay in the range where the dissimilar metal filling part 3 is installed for 2 seconds or more, the effect of the periodic fluctuation of the heat flux by the dissimilar metal filling part 3 is sufficient. As a result, the effect of suppressing the surface cracking of the slab can be enhanced during high-speed casting in which surface cracks are likely to occur in the solidified shell or during casting of medium carbon steel.

In order to stably obtain the effect of the periodic fluctuation of the heat flux by the dissimilar metal filling part 3, it is more necessary to secure the time for the slab to pass through the range where the dissimilar metal filling part 3 is installed for 4 seconds or more. preferable. On the other hand, although it is not necessary to set an upper limit for the length R, the length R _{0 is used} from the viewpoint of suppressing the concave groove processing cost and the plating processing cost or the thermal spraying processing cost on the mold copper plate surface for installing the dissimilar metal filling portion 3. Is preferably within 5 times.

  On the other hand, the position of the upper end portion of the dissimilar metal filling portion 3 may be anywhere as long as it is above the meniscus position. Therefore, the length Q shown in FIG. 1 is an arbitrary value exceeding zero. If it is. However, since the meniscus fluctuates in the vertical direction during casting, the upper end portion of the dissimilar metal filling portion 3 is always positioned above the meniscus, so that the upper end portion of the dissimilar metal filling portion 3 is 10 mm from the meniscus set. Preferably, the upper position of the dissimilar metal filling portion 3 is more preferably about 20 mm to 50 mm above the set meniscus.

  The diameter d of the circular concave groove 2, in other words, the diameter d of the dissimilar metal filling portion 3 is in the range of 1.0 mm to 10.0 mm. By setting the diameter d of the circular groove 2 to 1.0 mm or more, the heat flux in the dissimilar metal filling portion 3 is sufficiently lowered, and the effect of suppressing the surface cracking of the slab can be enhanced. Moreover, by setting it as 1.0 mm or more, it becomes easy to fill the inside of the circular circular ditch | groove 2 with a low heat conductive metal or a high heat conductive metal by a plating process or a thermal spraying process. On the other hand, by setting the diameter d of the circular groove 2 to 10.0 mm or less, a decrease in heat flux in the dissimilar metal filling part 3 is suppressed, that is, a solidification delay in the dissimilar metal filling part 3 is suppressed, Stress concentration on the solidified shell at the position is prevented, and the occurrence of surface cracks in the solidified shell can be suppressed. That is, when the diameter d exceeds 10.0 mm, surface cracks in the solidified shell tend to increase, and therefore the diameter d of the circular groove 2 needs to be 10.0 mm or less.

  The filling thickness H of the dissimilar metal filling portion 3 is 0.5 mm or more, and it is necessary to satisfy the relationship of the diameter d (mm) of the circular groove 2 and the following equation (2). Here, in the formula (2), H is the filling thickness (mm), and d is the diameter (mm) of the circular groove 2.

0.5 ≦ H ≦ d (2)
By setting the filling thickness H of the dissimilar metal filling portion 3 to 0.5 mm or more, the heat flux in the dissimilar metal filling portion 3 is sufficiently lowered, and the effect of suppressing the surface cracking of the slab can be obtained. Further, by making the filling thickness H equal to or smaller than the diameter d of the circular groove 2, it becomes easy to fill the circular groove 2 with the low heat conductive metal and the high heat conductive metal by the plating process or the thermal spraying process. In addition, no gaps or cracks occur between the filled low heat conductive metal and high heat conductive metal and the mold copper plate. When gaps or cracks occur between the low thermal conductivity metal and the high thermal conductivity metal and the mold copper plate, cracks and peeling of the filled low thermal conductivity metal and the high thermal conductivity metal occur, reducing the mold life, cracking the slab, Furthermore, it causes a restrictive breakout.

  In the present invention, the center-to-center distance W (mm) of the dissimilar metal filling portion 3 adjacent to the width direction of the continuous casting mold and the center-to-center distance L (of the dissimilar metal filling portion 3 adjacent to the casting direction of the continuous casting mold. mm) is required to satisfy the following formula (3) or the following formula (4) with respect to the diameter d of the circular groove 2.

0.70 <W / d ≦ 1.00 (3)
0.70 <L / d ≦ 1.00 (4)
That is, at least one of the dissimilar metal fillers adjacent to each other in the width direction of the continuous casting mold or the dissimilar metal fillers adjacent to each other in the casting direction of the continuous casting mold are in contact with each other, or Some of them need to overlap. FIG. 1 and FIG. 2 show a state in which both dissimilar metal filling portions adjacent in the width direction of the continuous casting mold and dissimilar metal filling portions adjacent in the casting direction of the continuous casting mold are in contact (W / d = 1). .00 and L / d = 1.00).

  FIG. 4 to FIG. 6 show examples of disposition of the dissimilar metal filling portions 3 adjacent to each other in the width direction and the casting direction of the continuous casting mold in the present invention. 4-6 is the schematic side view which looked at a part of casting_mold | template long side copper plate 1 from the inner wall surface side.

  FIG. 4 shows that the diameter d of the circular groove 2 is 5.0 mm, the distance W (mm) between the centers of the dissimilar metal fillers 3 adjacent in the mold width direction, and the center of the dissimilar metal fillers 3 adjacent in the casting direction. In this example, both distances L (mm) are 4.0 mm (W / d = 0.80, L / d = 0.80).

  FIG. 5 shows that the diameter d of the circular concave groove 2 is 5.0 mm, the distance W (mm) between the centers of the dissimilar metal filling portions 3 adjacent to each other in the mold width direction, and the distance between the centers of the dissimilar metal filling portions 3 adjacent to each other in the casting direction. In this example, both distances L (mm) are 4.5 mm (W / d = 0.90, L / d = 0.90).

  FIG. 6 shows that the diameter d of the circular groove 2 is 5.0 mm, the distance W (mm) between the centers of the dissimilar metal filling portions 3 adjacent to each other in the mold width direction is 3.5 mm, and the dissimilar metal fillings adjacent to each other in the casting direction. This is an example in which the center-to-center distance L (mm) of the portion 3 is 5.0 mm (W / d = 0.70, L / d = 1.00).

  The inventors of the present invention determined that the ratio (W / d) between the center-to-center distance W (mm) of the dissimilar metal filling portion 3 adjacent in the mold width direction and the diameter d (mm) of the circular groove 2 is the surface of the slab slab. The effect on cracking was investigated. Specifically, the diameter d of the circular groove 2 is constant at 5.0 mm, and the distance L (mm) between the centers of the dissimilar metal filling portions 3 adjacent to each other in the casting direction and the diameter d (mm) of the circular groove 2. ) And the ratio (W / d) was varied in the range from 0.10 to 2.00, and the surface crack number density of the slab slab was investigated.

  The survey results are shown in FIG. When W / d was 0.70 or more and 1.00 or less, the surface crack number density of the slab cast was lowered. This is because when W / d is greater than 0.70 and less than or equal to 1.00, the distribution period of the heat flux in the mold width direction is appropriate for preventing uneven solidification, and the effect of suppressing the surface cracking of the slab is enhanced. it is conceivable that.

  On the other hand, when W / d is 0.70 or less, the heat flux period in the mold width direction becomes too small, and thus the effect of suppressing non-uniform solidification is considered to be small. On the other hand, if W / d exceeds 1.00, the heat flux period in the mold width direction becomes too large, the solidification delay increases in the slow cooling region, and the effect of suppressing non-uniform solidification becomes small.

  Similarly to the above, the ratio (L / d) between the center distance L (mm) of the dissimilar metal filling portion 3 adjacent to the casting direction of the mold and the diameter d (mm) of the circular groove 2 is slab cast. The effect of surface cracking on the surface was investigated. Specifically, the diameter d of the circular groove 2 is constant at 5.0 mm, and the distance W (mm) between the centers of the dissimilar metal filling portions 3 adjacent in the mold width direction and the diameter d ( mm) and the ratio (W / d) was kept constant at 1.00, the ratio (L / d) was varied in the range from 0.10 to 2.00, and the surface crack number density of the slab slab was investigated. .

  The survey results are shown in FIG. When L / d was 0.70 or more and 1.00 or less, the surface crack number density of the slab slab was lowered. When L / d is 0.70 or more and 1.00 or less, the distribution cycle of the heat flux in the casting direction is appropriate for preventing uneven solidification, and the effect of suppressing the surface cracking of the slab is enhanced. Conceivable.

  On the other hand, when L / d is 0.70 or less, the heat flux period in the casting direction becomes too small, so that the effect of suppressing non-uniform solidification is considered to be small. On the other hand, if L / d exceeds 1.00, the heat flux period in the casting direction becomes too large, the solidification delay increases in the slow cooling region, and the effect of suppressing non-uniform solidification becomes small.

In the present invention, the thermal conductivity λ _m of the low thermal conductive metal used by filling the circular concave groove 2 needs to be 80% or less with respect to the thermal conductivity λ _c of the mold long side copper plate 1. In addition, the thermal conductivity λ _m of the high thermal conductivity metal used by filling the circular concave groove 2 needs to be 120% or more with respect to the thermal conductivity λ _c of the mold long side copper plate 1. By using a low thermal conductivity metal having a thermal conductivity λ _m of 80% or less or a high thermal conductivity metal of 120% or more with respect to the thermal conductivity λ _c of the long-side copper plate 1 of the mold, The effect of periodic fluctuation 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 in which surface cracks are likely to occur in the slab and casting of medium carbon steel.

  As the low heat conductive metal to be used, nickel (Ni, thermal conductivity: 90 W / (mxK)), nickel alloy, chromium (Cr, thermal conductivity) can be easily filled by plating or thermal spraying. (Rate: 67 W / (m × K)), cobalt (Co, thermal conductivity: 70 W / (m × K)) and the like are suitable. In addition, the numerical value of the thermal conductivity described in this specification is the thermal conductivity at normal temperature (about 20 ° C.).

  Moreover, as a copper alloy used as a mold copper plate, a copper alloy that is generally used as a casting mold for continuous casting and to which a small amount of chromium, zirconium (Zr), or the like is added may be used. Of course, pure copper may be used as the mold copper plate.

  In recent years, an electromagnetic stirrer that stirs molten steel in the mold has been installed in the continuous casting mold in order to homogenize the solidification in the mold or prevent the inclusion of inclusions in the molten steel in the solidified shell. Is common. In this case, in order to suppress the attenuation of the magnetic field strength from the electromagnetic coil to the molten steel, a copper alloy with reduced conductivity is used. The copper alloy has a reduced thermal conductivity in accordance with a decrease in its conductivity. Therefore, in recent years, a copper alloy mold copper plate having a thermal conductivity of about 1/2 that of pure copper is also used. Incidentally, the thermal conductivity of pure copper is about 385 W / (m × K).

  As described above, in a continuous casting mold using a copper alloy having a low thermal conductivity as a mold copper plate, as a high thermal conductive metal to be used by filling the circular concave groove 2, the thermal conductivity having a small content of pure copper or an alloy component is used. High copper alloys can be used.

  Further, in the present invention, in order to prevent slab surface cracks due to the oscillation mark, the mold vibration amplitude A (mm) and frequency f (times / min) of the continuous casting mold are used for continuous casting. It is preferable to set according to the distance L (mm) between the centers of the dissimilar metal filling portions 3 adjacent to the casting direction of the mold and the slab drawing speed Vc (m / min).

  Here, when the vibration waveform of the mold is a sine waveform, for example, the displacement y due to the vibration of the mold is “y = Asin (2πft), where A: amplitude (mm), f: frequency (times / min. ), T; time (min) ". That is, the amplitude A of the mold vibration is ½ of the distance (stroke) between the top dead center and the bottom dead center of the vibration.

  The oscillation mark of the slab is formed for each cycle of the mold vibration. Therefore, the interval SD (mm) between the oscillation marks formed on the slab is calculated by the following equation (6).

Interval SD = 1000 × Vc / f (6)
If the solidified shell is in contact with both the dissimilar metal filling portion 3 and the mold copper plate while the solidified shell advances the distance SD at the lower side in the casting direction, the solidified shell in the interval SD is subjected to the heat flux fluctuation almost evenly. Therefore, uneven solidification at the site is improved. Thereby, the difference in cooling between the top and valley of the oscillation mark is reduced, and the occurrence of surface cracks due to the oscillation mark in the slab is suppressed.

  That is, as shown in the following formula (7), if the distance L (mm) between the centers of the dissimilar metal filling portions 3 adjacent to each other in the casting direction is equal to or less than the interval SD (mm), during one cycle of mold vibration. All the solidified shells in the interval SD are surely in contact with both the dissimilar metal filling portion 3 and the mold copper plate, and the occurrence of surface cracks due to the oscillation mark in the slab is suppressed.

L ≦ interval SD = 1000 × Vc / f (7)
On the other hand, when the center-to-center distance L (mm) of the dissimilar metal filling portion 3 adjacent to the casting direction becomes smaller than half of the amplitude A (mm), the solidified shell always has a low heat conduction metal portion during one period of oscillation. 3 or the mold copper plate, and as a result, the heat removal at the protrusion between the oscillation marks is not subject to periodic heat flux fluctuations, and the solidified shell is subject to periodic heat flux fluctuations. It will be cooled in an unacceptable state. That is, the effect of reducing the difference between the top and valley of the oscillation mark is less likely to occur. Therefore, the center-to-center distance L (mm) of the dissimilar metal filling portion 3 adjacent to the casting direction needs to be at least half of the amplitude A (mm), and the following equation (5) is derived.

A / 2 ≦ L ≦ 1000 × Vc / f (5)
That is, in the present invention, in order to prevent slab surface cracking due to the oscillation mark, the distance L (mm) between the centers of the dissimilar metal filling portions is the mold vibration amplitude A (mm) of the continuous casting mold. It is preferable to satisfy the relationship of the above formula (5) with the frequency f (times / min) of the mold vibration and the slab drawing speed Vc (m / min).

  In FIG. 1, the dissimilar metal filling part 3 is arranged in such a manner that the centers of the dissimilar metal filling parts 3 are aligned in the casting direction, but the center position of the dissimilar metal filling part 3 may be shifted in the casting direction. The dissimilar metal filling portion 3 is basically installed on both the long-side mold copper plate and the short-side mold copper plate of the continuous casting mold, but with respect to the short-side length of the slab like a slab slab. When the slab long side length is remarkably large, surface cracks tend to occur on the long side of the slab, and even if the dissimilar metal filling portion 3 is installed only on the long side mold copper plate, An inhibitory effect can be obtained.

  While injecting molten steel into the continuous casting mold constructed as described above, the solidified shell formed by cooling the molten steel is pulled out from the continuous casting mold while vibrating the continuous casting mold in the casting direction. The slab is manufactured. The continuous casting mold having such a structure continuously casts a slab slab (thickness: 200 mm or more) of medium carbon steel having a high surface cracking sensitivity and a carbon content of 0.08 to 0.17% by mass. It is preferable to use it. Conventionally, when continuously casting a slab slab of medium carbon steel, it is common to reduce the slab drawing speed in order to suppress surface cracking of the slab. Because it is possible to suppress slab surface cracks by using, it is possible to continuously cast slabs with no surface cracks or very few surface cracks even at slab drawing speeds of 1.5 m / min or more. Is done. During continuous casting, it is preferable to control the vibration conditions of the mold and the slab drawing speed so as to satisfy the expression (5).

  As described above, according to the present invention, at least one of the periodic heat flux distribution period in the mold width direction and the periodic heat flux distribution period in the casting direction is used to prevent uneven solidification. Appropriate surface cracks due to non-uniform cooling of the solidified shell at the initial stage of solidification, and surface cracks due to non-uniform solidified shell thickness due to transformation from δ iron to γ iron in a medium carbon steel with peritectic reaction It is possible to prevent more stably than in the prior art.

  In addition, although the said description was performed regarding the continuous casting of a slab slab, this invention is not limited to the continuous casting of a slab slab, and it is applied along with the above also in the continuous casting of a bloom slab or a billet slab. can do.

  Medium carbon steel (chemical component, 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) 0.030 mass%, S; 0.005 to 0.015 mass%, Al; 0.020 to 0.040 mass%) on the inner wall surface of a copper alloy long mold copper plate under various conditions It cast using the water-cooled copper mold in which the filling part was installed, and the test which investigates the surface crack of the cast piece after casting was done (invention example 1-7, comparative examples 1-5). The water-cooled copper mold used is a mold for casting a slab slab having a slab long side width of 1500 to 2450 mm and a slab thickness of 260 mm. For comparison, a test using a water-cooled copper alloy continuous casting mold that does not have a low heat conductive metal filling part was also performed (conventional example).

  The length from the upper end to the lower end of the water-cooled copper alloy continuous casting mold used was 950 mm, and the position of the meniscus (molten steel surface in the mold) during steady casting was set at a position 100 mm below the upper end of the mold. A circular concave groove is cut on the inner wall surface of the mold copper plate in a range from a position 60 mm below the upper end of the mold to a position 200 mm below the upper end of the mold (length R = 100 mm). Was filled with a low thermal conductive metal. After the electroplating treatment, surface grinding is performed to remove the low heat conductive metal adhering to the portion other than the circular groove, and the process of electroplating is repeated a plurality of times to make the low heat conductive metal into the circular groove. Completely filled to form a low thermal conductive metal filling. In this case, the low thermal conductive metal filling portion and the surrounding copper alloy portion (portions where the low thermal conductive metal filling portion is not formed) were formed on a smooth surface having no step.

  As the mold copper plate, a copper alloy having a thermal conductivity of 298.5 W / (m × K) is used, and as the low thermal conductive metal for filling (hereinafter also referred to as “filled metal”), pure nickel (thermal conductivity; 90.5 W / (m × K)) was used.

In the continuous casting operation, the basicity ((mass% CaO) / (mass% SiO ₂ )) is 1.0 to 1.5 and the viscosity at 1300 ° C. is 0.05 to 0.20 Pa · s as the mold powder. The mold powder was used. After continuous casting was completed, the state of cracking on the surface of the slab was investigated by dye penetration testing. The number of surface cracks of a length of more than 2mm detected by penetrant inspection measures, and the sum, surface cracks and a value obtained by dividing (number / m ²⁾ by the surface area of the slab of the examination of surface cracks (m ²⁾ It was defined as the number density, and the occurrence of surface cracks was evaluated using this surface crack number density.

  Table 1 shows the slab drawing speed Vc, mold vibration conditions, low heat conductive metal filling portion installation conditions, and slab surface inspection results during steady casting in each test.

  In Invention Examples 1 to 7, the surface crack number density of the slab was low, and it was confirmed that the surface crack of the slab was suppressed.

  On the other hand, in Comparative Example 1, W / d and L / d are too large, and the periodic heat flux distribution period in the mold width direction and the casting direction due to the installation of the low heat conductive metal filling portion is unevenly solidified. It did not act effectively for prevention, and the surface crack number density of the slab was 1.7 times or more that of the example of the present invention.

  In Comparative Example 2, since the diameter d of the circular groove is too small, the heat flux in the dissimilar metal filling portion is not sufficiently lowered, W / d is too large, and L / d is too small. In addition, the periodic heat flux distribution period in the mold width direction and the casting direction due to the installation of the low heat conductive metal filling portion does not effectively work to prevent non-uniform solidification, and the surface crack number density of the slab is in the example of the present invention. It was 1.9 times or more.

  In Comparative Example 3, since the diameter d of the circular groove is too large, solidification delay occurs in the dissimilar metal filling part, and W / d and L / d are too large, thereby installing a low heat conductive metal filling part. Thus, the periodic heat flux distribution period in the mold width direction and the casting direction did not work effectively to prevent non-uniform solidification, and the surface crack number density of the slab was 2.1 times or more that of the present invention example. .

  In Comparative Example 4, W / d is too small and L / d is too large. Accordingly, the periodic heat flux distribution period in the mold width direction and the casting direction due to the installation of the low heat conductive metal filling portion is obtained. It did not work effectively to prevent uneven solidification, and the number density of surface cracks in the slab was 1.2 times or more that of the present invention.

  In Comparative Example 5, the diameter d of the circular concave groove is too large, the solidification delay occurs in the dissimilar metal filling portion, the W / d is too small, and the L / d is too large. The periodic heat flux distribution period in the mold width direction and the casting direction due to the installation of the conductive metal filling portion does not work effectively to prevent non-uniform solidification, and the surface crack number density of the slab is 1. It was more than 3 times.

  In the conventional example, the surface crack number density of the slab was about 5 times or more that of the example of the present invention, and many slab surface cracks occurred.

1 Mold long side copper plate 2 Circular groove 3 Dissimilar metal filling part 4 Slit (mold cooling water flow path)
5 Back plate

Claims (2)

While injecting molten steel into a continuous casting mold, vibrating the continuous casting mold in the casting direction, drawing the solidified shell formed by cooling the molten steel from the continuous casting mold to produce a slab A continuous casting method of steel,
The continuous casting mold is lower than the meniscus by a length R ₀ (mm) or more determined by the following equation (1) based on the slab drawing speed Vc (m / min) from an arbitrary position above the meniscus. Up to the position of the inner wall surface of the water-cooled copper mold, a metal having a thermal conductivity of 80% or less or 120% or more with respect to the thermal conductivity of the copper mold has a diameter of 1.0 mm on the inner wall surface. A plurality of dissimilar metal filling portions formed by being filled in circular grooves processed as in the range of 10.0 mm to 10.0 mm,
The filling thickness H (mm) of the dissimilar metal filling portion satisfies the relationship of the diameter d (mm) of the circular groove and the following equation (2):
The circular grooves adjacent to each other in the width direction of the continuous casting mold are in contact with each other, or a part thereof overlaps between the centers of the different metal filling portions adjacent to each other in the width direction of the continuous casting mold. The distance W (mm) satisfies the relationship of the following formula (3) with the diameter d (mm) of the circular concave groove and / or the dissimilar metal filling portion adjacent to the casting direction of the continuous casting mold. The center-to-center distance L (mm) is a continuous casting method of steel that satisfies the relationship of the diameter d (mm) of the circular groove and the following equation (4).
R ₀ = 2 × Vc × 1000/60 (1)
0.5 ≦ H ≦ d (2)
0.70 <W / d ≦ 1.00 (3)
0.70 <L / d ≦ 1.00 (4) The center-to-center distance L (mm) of the dissimilar metal filling portion adjacent to the casting direction of the continuous casting mold is the mold vibration amplitude A (mm) of the continuous casting mold and the frequency f (times / times) of the mold vibration. min) and the slab drawing speed Vc (m / min) and the following formula (5):
A / 2 ≦ L ≦ 1000 × Vc / f (5)