WO2018016101A1

WO2018016101A1 - Continuous casting mold and method for continuous casting of steel

Info

Publication number: WO2018016101A1
Application number: PCT/JP2017/001146
Authority: WO
Inventors: 直道岩田; 則親荒牧; 鍋島　誠司; 三木　祐司; 孝平古米
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2015-07-22
Filing date: 2017-01-16
Publication date: 2018-01-25
Also published as: EP3488946A1; KR102245013B1; KR20190017978A; TWI630962B; CN109475930B; CN109475930A; EP3795274B1; EP3488946A4; TW201803664A; EP3795274A1

Abstract

A continuous casting mold having low-thermal-conductivity metal filled parts formed by filling a low-thermal-conductivity metal into a plurality of recessed grooves provided in a range from an arbitrary position above a meniscus to an arbitrary position below the meniscus in an inner wall surface of a copper alloy mold copper plate for configuring a water-cooled continuous casting mold, the thermal conductivity λm (W/(m × K)) of the low-thermal-conductivity metal being 80% or less of the thermal conductivity λc (W/m × K)) of the mold copper plate, and the thermal resistance ratio R defined by equation (1) being 5% or greater. (1): R = [(T-H)/(1000 × λc) + H/(1000 × λm) – T/(1000 × λc)]/[T/(1000 × λc)] × 100. In the formula, R is the thermal resistance ratio (%) of the low-thermal-conductivity metal filled parts and the mold copper plate, T is the distance (mm) from a bottom surface of a slit in the mold copper plate as a flow path for mold cooling water to the surface of the mold copper plate, and H is the filling thickness (mm) of the low-thermal-conductivity metal.

Description

Continuous casting mold and steel continuous casting method

The present invention relates to a continuous casting mold capable of continuously casting molten steel while suppressing slab surface cracking due to non-uniform cooling of the solidified shell in the mold, and a continuous casting method of steel using this mold. About.

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 uneven, the thickness of the solidified shell becomes uneven 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. When the non-uniformity of the solidified shell thickness is large, a vertical crack is generated in the mold, and a breakout in which the molten steel flows out from the vertical crack may occur. Since the cracks present in the slab become surface defects in the subsequent rolling process, it is necessary to care for the surface of the slab and remove the surface cracks at the stage of the cast slab after casting.

不 Uniform solidification in the mold is likely to occur particularly in steels having a carbon content of 0.08 to 0.17% by mass (referred to as medium carbon steels). 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) of steel types that are prone to surface cracks.

For example, Patent Document 1 proposes that mold powder having a composition that is easily crystallized is used to increase 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.

In Patent Document 2, a vertical groove and a horizontal groove are provided on the inner wall surface of the mold, and mold powder is allowed to flow into the vertical groove and the horizontal groove, thereby slowing down the cooling of the mold and at the same time uniforming in the mold width direction. Techniques for suppressing vertical cracks in slabs have been proposed. However, when the inner wall surface of the mold is worn due to contact with the slab, and the groove provided on the inner wall surface of the mold becomes shallow, the amount of mold powder flowing in decreases and the slow cooling effect is reduced. There is a problem of not persisting. In addition, when molten steel is poured into an empty mold space at the start of casting, the injected molten steel penetrates into the groove provided on the inner wall surface of the mold and solidifies, and the mold copper plate and the solidified shell adhere to each other, thereby solidifying the shell. There is also a problem that there is a risk that a constraining breakout may occur due to the inability to withdraw.

In Patent Document 3, a longitudinal groove or a lattice groove parallel to the casting direction, in which the groove width and the groove depth are set according to the viscosity of the mold powder, is provided at the center in the width direction of the inner wall surface of the mold. Without filling with mold powder, a void is formed inside the groove, and air is allowed to flow into the void, thereby slowing down the cooling of the mold and at the same time uniforming in the mold width direction, Techniques for suppressing cracking have been proposed. However, in this case as well, the groove is exposed on the inner wall surface of the mold, and similarly to Patent Document 2, there is a problem that the slow cooling effect is not sustained due to wear of the inner wall surface of the mold. There is also a problem that at the start of casting, molten steel enters the inside of the groove provided on the inner wall surface of the mold and solidifies, so that the solidified shell cannot be pulled out, and a restrictive breakout may occur.

Patent Document 4 proposes a mold in which a lattice-shaped groove is provided on the inner wall surface of the mold, and a mold in which the lattice-shaped groove is filled with a different metal (Ni, Cr) or ceramics (BN, AlN, ZrO ₂ ). Has been. This technology periodically creates a difference in the amount of heat removal between the groove and the part other than the groove, and disperses the stress of the solidified shell from δ iron to γ iron and heat shrinkage in the low heat removal region. It is a technology that suppresses vertical cracks in slabs. However, the grooves are in a lattice shape, and in the lattice groove shape, the boundary between the groove on the inner wall surface of the mold and the mold copper plate (made of copper or copper alloy) is a straight line, and the boundary surface is cracked due to the difference in thermal expansion. However, there is a problem that it is easy to propagate and the mold life is reduced.

Patent Document 5 discloses a mold in which a vertical groove parallel to the casting direction is provided on the inner wall surface of the mold, and a mold in which the vertical groove is filled with a different metal (Ni, Cr) or ceramics (BN, AlN, ZrO ₂ ). There has been proposed a continuous casting method that uses a slab drawing speed and a mold vibration cycle within a predetermined range. According to Patent Document 5, by optimizing the mold vibration period according to the slab drawing speed, the oscillation mark formed on the slab works as if a lateral groove was provided. It is said that the same effect of reducing surface cracks is observed. However, as in Patent Document 4, the boundary between the groove on the inner wall surface of the mold and the mold copper plate (made of copper or copper alloy) is a straight line, and the boundary surface is easily cracked and propagates due to the difference in thermal expansion. There is a problem that the mold life is reduced.

In Patent Document 6, a concave groove having a diameter of 2 to 10 mm is provided near the molten steel surface in the mold (hereinafter also referred to as “meniscus”) on the inner wall surface of the mold, and a dissimilar metal (Ni, stainless steel) is provided inside the concave groove. ) Or ceramics (BN, AlN, ZrO _2, etc.) are embedded, and a mold is proposed in which the embedded interval is 5 to 20 mm. Similarly to Patent Documents 4 and 5, this technique is also a technique for reducing the uneven solidification by providing a periodic heat transfer distribution and suppressing vertical cracks in the slab. However, in Patent Document 6, since a drill hole is opened on the surface of the mold copper plate, and the dissimilar metal or ceramic formed into the shape of the drill hole is embedded therein, the contact between the back surface of the embedded dissimilar metal or ceramic and the mold copper plate The state is not constant, and there is a high possibility that a gap is formed at the contact portion. In the case where a gap is formed, the amount of heat removed from each groove portion is greatly changed by the gap, and there arises a problem that cooling of the solidified shell cannot be properly controlled. There is also a problem that the embedded dissimilar metal or ceramic is easily peeled off from the mold copper plate.

JP 2005-297001 A JP-A-9-276994 Japanese Patent Laid-Open No. 10-193041 JP-A-1-289542 Japanese Patent Laid-Open No. 2-6037 JP-A-1-170550

The present invention has been made in view of the above circumstances, and the object of the present invention is to generate a constraining breakout at the start of casting and to reduce the mold life due to cracks on the surface of the mold copper plate, and at the initial stage of solidification. Slab surface cracks due to non-uniform cooling of solidified shells and slab surface cracks due to non-uniform solidified shell thickness due to transformation from δ iron to γ iron in peritectic reaction It is to provide a continuous casting mold that can be suppressed over a long time, and to provide a continuous casting method of steel using this continuous casting mold.

The gist of the present invention for solving the above problems is as follows.
[1] A water-cooled continuous casting mold,
A plurality of concave grooves provided in a range from any position above the meniscus on the inner wall surface of the copper alloy mold copper plate constituting the mold to any position below the meniscus is filled with a low heat conductive metal. Having a low thermal conductivity metal filling formed;
The thermal conductivity of the mold copper plate _{λ c (W / (m ×} K)) the relative thermal conductivity of the low thermal conductive metal _{λ m (W / (m ×} K)) is not more than 80%,
A continuous casting mold having a thermal resistance ratio R defined by the following formula (1) of 5% or more.
R = {(TH) / (1000 × λ _c ) + H / (1000 × λ _m ) −T / (1000 × λ _c )} / {T / (1000 × λ _c )} × 100 (100) 1)
Here, R is a thermal resistance ratio (%) between the low thermal conductive metal filling portion and the mold copper plate, and T is a distance from the bottom surface of the mold copper plate slit to the mold copper plate surface, which becomes a flow path of the mold cooling water. (Mm) and H are filling thicknesses (mm) of the low thermal conductive metal.
[2] The concave groove is longer than the meniscus by a length L ₀ (mm) or more calculated from the following formula (2) from an arbitrary position above the meniscus by the slab drawing speed Vc (m / min). The continuous casting mold according to the above [1], provided in the range of the inner wall surface of the mold copper plate up to an arbitrary position below.
L ₀ = 2 × Vc × 1000/60 (2)
[3] The above-mentioned [1] or [2], wherein the continuous casting mold has a periodic thermal resistance distribution or a heat flux distribution in a range of an inner wall surface of the mold copper plate provided with the low thermal conductive metal filling portion. ] The casting mold for continuous casting described in the above.
[4] The above-mentioned [1] to [3], wherein the opening shape of the recess in the inner wall surface of the mold copper plate is a circle or a pseudo circle, and the diameter of the circle or the equivalent circle diameter of the pseudo circle is 2 to 20 mm. The continuous casting mold according to any one of the above.
[5] The continuation according to [4], wherein an interval between the low thermal conductive metal filling portions satisfies the relationship of the following expression (3) with respect to the diameter or the equivalent circle diameter of the low thermal conductive metal filling portions. Casting mold.
P ≧ 0.25 × d (3)
Here, P is the interval (mm) between the low heat conductive metal filling parts, and d is the diameter (mm) or the equivalent circle diameter (mm) of the low heat conductive metal filling parts.
[6] The ratio of the total area B (mm ² ) of all the low thermal conductive metal filling portions to the area A (mm ² ) of the inner wall surface of the mold copper plate within the range where the low thermal conductive metal filling portions are formed. Total area length S (S = (B / A) × 100) is 10% or more, and the total boundary length between all the low thermal conductive metal filling portions and the mold copper plate with respect to the area A (mm ² ) The continuous casting mold according to any one of [1] to [5] above, wherein a ratio ε (ε = C / A) of C (mm) satisfies the relationship of the following formula (4):
0.07 ≦ ε ≦ 0.50 (4)
[7] The continuous casting mold according to [6], wherein the low thermal conductive metal filling portions are formed independently.
[8] The continuous casting mold according to any one of [1] to [7], wherein the low heat conductive metal is filled in the concave groove by plating or spraying.
[9] On the inner wall surface of the mold copper plate, a plating layer of nickel having a thickness of 2.0 mm or less or an alloy containing nickel is formed, and the low thermal conductive metal filling portion is covered with the plating layer. The casting mold for continuous casting according to any one of [1] to [8] above.
[10] A continuous casting method of steel using the continuous casting mold according to any one of [1] to [9],
Medium carbon steel having a carbon content of 0.08 to 0.17% by mass is poured into the mold, and the slab slab having a slab thickness of 200 mm or more is cast at a slab drawing speed of 1.5 m / min or more. A continuous casting method of steel in which the medium carbon steel is drawn from the steel and continuously cast.

In the present invention, the thermal resistance ratio R between the low thermal conductive metal filling portion and the mold copper plate is 5% or more, and the low thermal conductivity metal is filled with the thermal conductivity of 80% or less with respect to the thermal conductivity of the mold copper plate. A plurality of low heat conductive metal filling portions formed in this manner are installed in the width direction and casting direction of the continuous casting mold in the vicinity of the meniscus including the meniscus position. As a result, the thermal resistance of the continuous casting mold in the mold width direction and the casting direction in the vicinity of the meniscus periodically increases and decreases, and the heat flux from the solidified shell to the continuous casting mold in the vicinity of the meniscus, that is, in the initial stage of solidification, is periodic. Increase or decrease. By periodically increasing or decreasing the heat flux, stress and thermal stress due to transformation from δ iron to γ iron are reduced, and 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 due to the deformation of the solidified shell is made uniform, and the generated stress is dispersed to reduce the amount of individual strains. Is suppressed.

FIG. 1 is a schematic side view of a mold long-side copper plate constituting a part of a water-cooled continuous casting mold according to the present embodiment as viewed 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 shown in FIG. 1. FIG. 3 is a diagram conceptually showing the thermal resistance at three positions of the long copper plate having a low heat conductive metal filling portion corresponding to the positions of the low heat conductive metal filling portions. FIG. 4 is a schematic view showing an example in which a plating layer for protecting the mold surface is provided on the inner wall surface of the long-side copper plate of the mold. FIG. 5 is a diagram showing the results of investigating the influence of the thermal conductivity of the low thermal conductive metal filled in the low thermal conductive metal filling portion on the slab surface crack. FIG. 6 is a diagram showing the results of investigating the influence of the thermal resistance ratio R between the low thermal conductive metal filling portion and the mold copper plate on the slab surface crack. FIG. 7 is a diagram showing the results of investigating the influence of the area ratio S of the low thermal conductive metal filling portion and the boundary length ratio ε on the slab surface crack. FIG. 8 is a diagram showing the results of investigating the influence of the diameter d of the low thermal conductive metal filling portion on the slab surface crack. 9 is a schematic side view showing the arrangement of the low thermal conductive metal filling portion in Test No. 40-44. 10 is a schematic view showing the arrangement of the low thermal conductive metal filling portion in the test No. 45. 11 is a schematic view showing the arrangement of the low thermal conductive metal filling portion in Test No. 46.

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 according to the present embodiment, wherein a mold long-side copper plate 1 having a low thermal conductive metal filling portion 3 formed on the inner wall surface side. It is the schematic side view seen from the inner wall surface side. FIG. 2 is a cross-sectional view taken along the line X-X ′ of the long copper plate 1 shown in FIG.

The continuous casting mold shown in FIG. 1 is an example of a continuous casting mold for casting a slab slab. A water-cooled copper alloy continuous casting mold for a slab slab is configured by combining a pair of copper alloy long mold copper plates and a pair of copper alloy short mold copper plates. FIG. 1 shows the long-side copper plate 1 of the mold. Similarly to the mold long-side copper plate 1, the mold short-side copper plate is also provided with the low thermal conductive metal filling portion 3 on the inner wall surface side, and the description of the mold 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 low heat conductive metal filling part 3 does not have 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 long copper plate 1 up to a position below L, a plurality of low heat conductive metal filling portions 3 having a diameter d are installed with the interval between the low heat conductive metal filling portions as P. Yes. 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. Therefore, even if the meniscus position is a position 50 mm below the upper end of the mold long side copper plate 1 or a position 200 mm below the upper end, the length Q and the length L are described below. What is necessary is just to arrange | position the low heat conductive metal filling part 3 so that these conditions may be satisfied.

As shown in FIG. 2, the low thermal conductive metal filling portion 3 is plated or formed inside a circular groove 2 having a diameter d, which is independently processed on the inner wall surface side of the long-side copper plate 1. By the thermal spraying treatment, a metal having a thermal conductivity λ _m of 80% or less with respect to the thermal conductivity λ _c of the copper alloy constituting the long-side copper plate 1 is filled (hereinafter referred to as “low thermal conductivity metal”). Is formed. Here, the groove 2 having a circular opening on the inner wall surface of the mold copper plate is referred to as a “circular groove”. Further, reference numeral 4 in FIG. 2 is a slit serving as a flow path for mold cooling water installed on the back side of the long mold copper plate 1, and reference numeral 5 is a back plate that is in close contact with the back of the long mold copper sheet 1. It is.

FIG. 3 is a diagram conceptually showing the thermal resistance at three positions of the long copper plate 1 having the low thermal conductive metal filling portion 3 corresponding to the position of the low thermal conductive metal filling portion 3. As shown in FIG. 3, the thermal resistance is relatively high at the installation position of the low thermal conductive metal filling portion 3.

By installing the plurality of low heat conductive metal filling portions 3 in the width direction and casting direction of the continuous casting mold near the meniscus including the meniscus position, the heat of the continuous casting mold in the mold width direction and casting direction near the meniscus A distribution in which the resistance increases or decreases periodically is formed. 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 addition, the comparison between the thermal conductivity λ _c of the copper alloy and the thermal conductivity λ _m of the low thermal conductive metal is defined by comparing the thermal conductivities at room temperature (about 20 ° C.). The thermal conductivity of the copper alloy and the low thermal conductive metal generally decreases as the temperature increases, but the thermal conductivity λ _m of the low thermal conductive metal at room temperature is 80% of the thermal conductivity λ _c of the copper alloy at normal temperature. In the following cases, even at the operating temperature (about 200 to 350 ° C.) as a casting mold for continuous casting, the thermal resistance of the portion where the low thermal conductive metal filling portion 3 is installed and the low thermal conductive metal filling portion 3 are installed. It is possible to make a difference in the thermal resistance of the part that is not present.

In this embodiment, in order to form a distribution in which the heat flux from the solidified shell to the continuous casting mold is periodically increased or decreased, in other words, the thermal resistance of the portion where the low thermal conductive metal filling portion 3 is installed, and the low heat In order to make a clear difference in the thermal resistance of the part where the conductive metal filling part 3 is not installed, the thermal resistance between the low thermal conductive metal filling part 3 and the mold copper plate defined by the following equation (1) The low heat conductive metal filling portion 3 is installed according to the shape of the casting copper plate so that the ratio R is 5% or more. Here, the thermal resistance ratio R between the low thermal conductive metal filling portion 3 and the mold copper plate is, as shown in the equation (1), from the bottom surface 4a of the slit 4 of the mold copper plate serving as the mold cooling water flow path to the mold copper plate surface. The distance T, the filling thickness H of the low thermal conductive metal in the low thermal conductive metal filling portion 3, the thermal conductivity λ _c of the mold copper plate, and the thermal conductivity λ _m of the low thermal conductive metal.

R = {(TH) / (1000 × λ _c ) + H / (1000 × λ _m ) −T / (1000 × λ _c )} / {T / (1000 × λ _c )} × 100 (100) 1)
However, in the formula (1), R is the thermal resistance ratio (%) between the low thermal conductive metal filling part and the mold copper plate, and T is the mold copper plate surface from the bottom surface of the mold copper plate slit which becomes the flow path of the mold cooling water. Distance (mm), H is the filling thickness (mm) of the low thermal conductivity metal, λ _c is the thermal conductivity of the mold copper plate (W / (m × K)), and λ _m is the thermal conductivity of the low thermal conductivity metal (W / (m × K)).

If the thermal resistance ratio R is larger than 100%, solidification in the low heat conductive metal filling portion 3 is significantly delayed, and thus uneven solidification is promoted, and surface cracks and breakout of the slab may occur. Therefore, the thermal resistance ratio R is preferably 100% or less.

Considering the influence on the initial solidification, the installation position of the low thermal conductive metal filling portion 3 is a meniscus having a length L ₀ or more calculated from the following equation (2) according to the slab drawing speed Vc during steady casting. It is preferable to set it to a lower position. That is shown in FIG. 1, the length L from the meniscus position, it is preferable that the length L ₀ or more.

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

The length L ₀ is related to the time during which the slab after the start of solidification passes through the range where the low heat conductive metal filling portion 3 is installed. It is preferable that the slab stays in the range where the low heat conductive metal filling portion 3 is installed for at least 2 seconds. In order for the slab to be present in the range where the low thermal conductive metal filling portion 3 is installed for at least 2 seconds after the start of solidification, the length L ₀ needs to satisfy the formula (2).

By securing the time for the cast slab after the start of solidification to stay in the range where the low thermal conductive metal filling part 3 is installed for 2 seconds or more, the effect of the periodic fluctuation of the heat flux by the low thermal conductive metal filling part 3 is obtained. The effect of suppressing the surface cracking of the slab can be enhanced at the time of high speed casting which is sufficiently obtained and the surface crack is likely to occur in the solidified shell, or during the casting of medium carbon steel. In order to stably obtain the effect of periodic fluctuation of the heat flux by the low heat conductive metal filling part 3, it is necessary to secure a time for the slab to pass through the range where the low heat conductive metal filling part 3 is installed for 4 seconds or more. More preferable. On the other hand, although it is not necessary to set an upper limit for the length L, the length L is selected from the viewpoint of suppressing the groove processing cost and the plating processing cost or the thermal spraying processing cost on the surface of the mold copper plate for installing the low thermal conductive metal filling portion 3. It is preferably within 5 times ₀ .

On the other hand, the position of the upper end portion of the low thermal conductive metal filling portion 3 may be any position as long as it is above the meniscus position. Therefore, the length Q shown in FIG. Any value is acceptable. However, since the meniscus fluctuates in the vertical direction during casting, the upper end portion of the low heat conductive metal filling portion 3 is set to the upper position of the meniscus so that the upper end portion of the low heat conductive metal filling portion 3 is always positioned above the meniscus. Is preferably about 10 mm above, and more preferably about 20 mm to 50 mm above the set meniscus at the upper end of the low heat conductive metal filling portion 3.

1 and FIG. 2 show an example in which the opening shape in the inner wall surface of the long-side copper plate 1 of the low thermal conductive metal filling portion 3 is circular, but the opening shape is not limited to a circle. For example, any shape may be used as long as it does not have a so-called “corner” such as an ellipse and has a shape close to a circle. Hereinafter, a shape close to a circle is referred to as a “pseudo circle”. When the opening shape of the low thermal conductive metal filling portion 3 is a pseudo circle, the concave groove 2 processed on the inner wall surface of the long copper plate 1 for forming the low thermal conductivity metal filling portion 3 is referred to as a “pseudo circular groove”. Called. The pseudo circle is, for example, an ellipse or a shape having no corners such as a rectangle whose corners are circles or ellipses, and may be a petal pattern. The size of the pseudo circle is evaluated by a circle-equivalent diameter obtained from the opening area on the inner wall surface of the pseudo circular long-side copper plate 1.

When Patent Document 4 and Patent Document 5 are provided with a longitudinal groove or a lattice groove and filled with a low heat conductive metal, the low heat conductivity is reduced at the boundary surface between the low heat conductive metal and copper and the orthogonal portion of the lattice portion. The stress due to the thermal strain difference between the conductive metal and copper concentrates, causing a problem of cracking on the mold copper plate surface. In contrast, in the continuous casting mold according to the present embodiment, the shape of the low thermal conductive metal filling portion 3 is circular or pseudo-circular. As a result, the boundary surface between the low thermal conductive metal and copper becomes a curved surface, so that the stress is less likely to concentrate on the boundary surface and the advantage that cracks are unlikely to occur on the surface of the mold copper plate is manifested.

The diameter d and equivalent circle diameter d of the low thermal conductive metal filling part 3 are preferably 2 to 20 mm. By setting the diameter d and the equivalent circle diameter d of the low heat conductive metal filling portion 3 to 2 mm or more, the heat flux in the low heat conductive 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 2 mm or more, it becomes easy to fill the inside of the circular or pseudo-circular concave groove 2 with a low heat conductive metal by a plating process or a thermal spraying process. On the other hand, by setting the diameter d and the equivalent circle diameter d of the low heat conductive metal filling portion 3 to 20 mm or less, a decrease in heat flux in the low heat conductive metal filling portion 3 is suppressed, that is, solidification in the low heat conductive metal filling portion 3. The delay is suppressed, stress concentration on the solidified shell at that position is prevented, and the occurrence of surface cracks in the solidified shell can be suppressed. That is, when the diameter d and the equivalent circle diameter d exceed 20 mm, surface cracks in the solidified shell tend to increase. Therefore, the diameter d and equivalent circle diameter d of the low thermal conductive metal filling portion 3 should be 20 mm or less. preferable. When the shape of the low thermal conductive metal filling portion 3 is a pseudo circle, the equivalent circle diameter d of the pseudo circle is calculated by the following equation (5).

Equivalent circle diameter = (4 × S / π) ^1/2 (5)
However, in Formula (5), S is an opening area (mm < ² >) in the inner wall face of the casting copper plate of the low heat conductive metal filling part 3. FIG.

The thermal conductivity λ _m of the low thermal conductive metal used by filling the circular concave groove and the pseudo circular concave groove needs to be 80% or less with respect to the thermal conductivity λ _c of the copper alloy constituting the mold copper plate. By using a low thermal conductivity metal of 80% or less with respect to the thermal conductivity of the copper alloy, the effect of periodic fluctuations in the heat flux due to the low thermal conductivity metal filling portion 3 is sufficient, and surface cracks occur in the slab. Even during easy high-speed casting and medium carbon steel casting, the effect of suppressing the surface cracking of the slab is sufficiently obtained.

The low thermal conductive metal used in the continuous casting mold according to this embodiment can be easily filled by plating or thermal spraying, so nickel (Ni, thermal conductivity: 90.5 W / (m × K) ), Nickel-based alloy, chromium (Cr, thermal conductivity; 67 W / (m × K)), cobalt (Co, thermal conductivity: 70 W / (m × K)), and the like are preferable. In addition, the numerical value of the thermal conductivity described in this specification is the thermal conductivity at normal temperature (about 20 ° C.).

Further, as a copper alloy used as a mold copper plate, a copper alloy to which chromium or zirconium (Zr) is added in a small amount, which is generally used as a casting mold for continuous casting, may be used. 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. In such a continuous casting mold, the difference in thermal conductivity between the mold copper plate and the low thermal conductive metal is small, but by setting the thermal resistance ratio R shown in the above formula (1) to 5% or more, The effect of reducing surface cracks is exhibited.

The filling thickness H of the low thermal conductive metal filling portion 3 is preferably 0.5 mm or more. By setting the filling thickness H to 0.5 mm or more, the heat flux in the low heat conductive metal filling portion 3 is sufficiently lowered, and the effect of suppressing the surface cracking of the slab can be obtained.

Further, the filling thickness H of the low thermal conductive metal filling portion 3 is preferably set to a diameter d or less and an equivalent circle diameter d or less of the low thermal conduction metal filling portion 3. Since the filling thickness H is equal to or smaller than the diameter d and equivalent circle diameter d of the low thermal conductive metal filling portion 3, the filling of the low thermal conductive metal into the concave groove 2 by the plating process or the thermal spraying process is facilitated, and No gaps or cracks occur between the filled low thermal conductivity metal and the mold copper plate. If gaps or cracks occur between the low thermal conductivity metal and the mold copper plate, the filled low thermal conductivity metal will crack or peel off, resulting in reduced mold life, cracking of the slab, or even a restrictive breakout. It becomes.

It is preferable that the space | interval P between low heat conductive metal filling parts is 0.25 times or more of the diameter d of the low heat conductive metal filling part 3, and the equivalent circle diameter d. That is, it is preferable that the interval P between the low thermal conductive metal filling portions satisfies the relationship of the following expression (3) with respect to the diameter d or equivalent circle diameter d of the low thermal conductive metal filling portion 3.

P ≧ 0.25 × d (3)
However, in Formula (3), P is a space | interval (mm) between low heat conductive metal filling parts, and d is a diameter (mm) or a circle equivalent diameter (mm) of a low heat conductive metal filling part.

Here, the interval P between the low thermal conductive metal filling portions is the shortest distance between the ends of the adjacent low thermal conductive metal filling portions 3 as shown in FIG. By setting the interval P between the low thermal conductive metal filling portions to “0.25 × d” or more, the interval between the low thermal conductive metal filling portions is sufficiently large, and the heat flux and the copper alloy portion ( The difference from the heat flux of the portion where the low heat conductive metal filling portion 3 is not formed becomes large, and the effect of suppressing the surface cracking of the slab can be obtained. The upper limit value of the interval P between the low thermal conductive metal filling portions may not be determined. However, when the interval P is increased, the area ratio of the low thermal conductive metal filling portion 3 is decreased. It is preferable to do.

The arrangement of the low heat conductive metal filling portions 3 is preferably a staggered arrangement as shown in FIG. 1, but is not limited to the staggered arrangement, and any arrangement is acceptable as long as the arrangement satisfies the interval P between the low heat conductive metal filling portions. Good.

Area ratio S which is the ratio of the total area B (mm ² ) of the areas of all the low thermal conductive metal filling portions 3 to the area A (mm ² ) of the inner wall surface of the mold copper plate within the range where the low thermal conductive metal filling portions 3 are formed. (S = (B / A) × 100) is preferably 10% or more. By securing the area ratio S of 10% or more, the area occupied by the low heat conduction metal filling portion 3 having a small heat flux is secured, and a heat flux difference is obtained between the low heat conduction metal filling portion 3 and the copper alloy portion. The effect of suppressing surface cracking can be stably obtained. The upper limit of the area ratio S occupied by the low thermal conductive metal filling portion 3 may not be set, but as described above, the interval P between the low thermal conductive metal filling portions is set to “0.25 × d” or more. Therefore, the condition “P = 0.25 × d” may be considered as the maximum area ratio S.

Further, the sum C (mm) of the boundary lengths of all the low heat conductive metal filling portions 3 and the mold copper plate with respect to the area A (mm ² ) of the inner wall surface of the mold copper plate within the range where the low heat conductive metal filling portion 3 is formed. It is preferable that the ratio ε (ε = C / A) satisfies the following formula (4).

0.07 ≦ ε ≦ 0.60 (4)
As a result of investigating the influence of the ratio ε on the slab surface crack, when the ratio ε was outside the range of the formula (4), the effect of reducing the surface crack was small. The ratio ε varies depending on the diameter d or equivalent circle diameter d of the low heat conductive metal filling portion 3 and the number of low heat conductive metal filling portions 3.

When the ratio ε is less than 0.07, the number of the low thermal conductive metal filling portions 3 is small, and the stress caused by volume shrinkage or heat shrinkage during the δ / γ transformation is difficult to be uniformly dispersed throughout the shell. The effect of suppressing cracking on one surface is reduced. On the other hand, when the ratio ε is larger than 0.60, the number of the low heat conductive metal filling portions 3 is too large. As a result, the periodic increase / decrease in the heat flux does not reach the target level, and the effect of suppressing the slab surface cracking is achieved. Is reduced. Further, when the ratio ε was larger than 0.60, slab bulging was also observed directly under the mold.

The low heat conductive metal filling portion 3 is basically installed on both the long side mold copper plate and the short side mold copper plate of the casting mold for continuous casting. When the long side length of the slab is remarkably large, surface cracks tend to occur on the long side of the slab, and even if the low thermal conductive metal filling portion 3 is installed only on the long side mold copper plate, the slab surface cracks An inhibitory effect can be obtained.

Further, as shown in FIG. 4, a plating layer 6 is formed on the inner wall surface of the mold copper plate on which the low thermal conductive metal filling portion 3 is formed for the purpose of preventing wear due to the solidified shell and cracking of the mold surface due to thermal history. It is preferable to provide it. The plating layer 6 is obtained by plating a commonly used nickel or nickel-containing alloy such as a nickel-cobalt alloy (Ni-Co alloy) or a nickel-chromium alloy (Ni-Cr alloy). It is done. The thickness h of the plating layer 6 is preferably 2.0 mm or less. By setting the thickness h of the plating layer 6 to 2.0 mm or less, the influence of the plating layer 6 on the heat flux can be reduced, and the effect of the periodic fluctuation of the heat flux by the low heat conductive metal filling portion 3 is sufficient. Can get to. However, when the thickness h of the plating layer 6 is greater than 0.5 times the filling thickness H of the low thermal conductive metal filling portion 3, the formation of a periodic heat flux distribution difference by the low thermal conductive metal filling portion 3 is suppressed. Therefore, the thickness h of the plating layer 6 is preferably 0.5 times or less the filling thickness H of the low thermal conductive metal filling portion 3. As long as this condition is satisfied, the plating layer 6 may have the same thickness from the upper end to the lower end of the mold or may have a different thickness from the upper end to the lower end. FIG. 4 is a schematic view showing an example in which a plating layer for protecting the mold surface is provided on the inner wall surface of the long-side copper plate of the mold.

The continuous casting mold configured in this way is particularly continuous casting of slab slabs (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.

As described above, in the continuous casting mold according to the present embodiment, the plurality of low thermal conductive metal filling portions 3 having the thermal resistance ratio R defined by the equation (1) of 5% or more include the meniscus position. It is installed in the width direction and casting direction of the continuous casting mold near the meniscus. As a result, the thermal resistance of the continuous casting mold in the mold width direction and in the casting direction near the meniscus of the continuous casting mold periodically increases and decreases, and the heat flux from the solidified shell to the continuous casting mold in the initial stage of solidification periodically Increase or decrease. Due to the periodic increase / decrease of the heat flux, stress and thermal stress due to δ / γ transformation are reduced, and 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 due to the deformation of the solidified shell is made uniform, and the generated stress is dispersed to reduce the amount of individual strains. Is suppressed.

In addition, in FIG. 1, although the example which installed the low heat conductive metal filling part 3 of the same shape in the casting direction or the mold width direction was shown, the shape of the low heat conductive metal filling part 3 may not be the same. If the diameter d or equivalent circle diameter d of the low heat conductive metal filling portion 3 is in the range of 2 to 20 mm, the low heat conductive metal filling portions 3 having different diameters may be installed in the casting direction or the mold width direction. However, if the diameter d or equivalent circle diameter d of the low heat conductive metal filling portion 3 varies greatly depending on the location, solidification is delayed in a region where the area ratio of the low heat conductive metal filling portion 3 is locally high, and the slab is placed at that position. Since there is a risk of surface cracking, it is preferable to use a single diameter or equivalent circle diameter.

2 shows an example in which the low thermal conductive metal filling portion 3 having the same filling thickness H is installed in the casting direction, the filling thickness of the low thermal conductive metal filling portion 3 installed in the mold width direction or the slab width direction is shown. The H may not be the same, and the filling thickness H may be different in each low heat conductive metal filling portion 3. However, it is preferable that the filling thickness H of any low thermal conductive metal filling portion 3 is 0.5 mm or more.

Furthermore, although the example which installed the low heat conductive metal filling part 3 by the same space | interval in the casting direction or the mold width direction was shown in FIG. 1, the space | interval which installs the low heat conductive metal filling part 3 does not need to be the same. However, also in this case, it is preferable that the interval P between the low thermal conductive metal filling portions satisfies the relationship of the expression (3).

In addition, the above description has been made with respect to a continuous casting mold for a slab slab. However, the continuous casting mold according to the present embodiment is not limited to a continuous casting mold for a slab slab, but for a bloom slab. The present invention can also be applied to continuous casting molds for billets and billets.

C; 0.05 to 0.25% by mass, Si; 0.10 to 0.35% by mass, Mn; 0.70 to 1.30% by mass, P; 0.010 to 0.030% by mass, S; Molten steel containing 0.002 to 0.006% by mass, Al; 0.02 to 0.05% by mass, an inner wall surface of a copper alloy long mold copper plate and an inner wall surface of a copper alloy short mold copper plate In addition, using a water-cooled copper alloy continuous casting mold in which a low heat conductive metal filling portion is installed under various conditions, continuous slab slabs with a slab long side width of 1500 to 2450 mm and a slab short side thickness of 220 mm are used. The test which investigated the surface crack of the cast slab after casting was performed.

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 processed on the inner surface of the mold copper plate in a range from a position 60 mm below the upper end of the mold to a position below the set meniscus position and a length L (mm) below, and then the circular concave groove is formed by electroplating. Filled with low thermal conductivity 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. Thereafter, a Ni—Co alloy was plated on the entire inner wall surface of the mold copper plate, and a plating layer having a thickness of 0.2 mm at the upper end of the mold and a thickness of 2.0 mm at the lower end of the mold was applied.

As the mold copper plate, two types of copper alloys having different thermal conductivities with thermal conductivities of 298.5 W / (m × K) and 120.0 W / (m × K) are used, and a low thermal conductive metal for filling (Hereinafter also referred to as “filling metal”) include pure nickel (thermal conductivity: 90.5 W / (m × K)), pure cobalt (thermal conductivity: 70 W / (m × K)), pure chromium ( Thermal conductivity: 67 W / (m × K)) and pure copper (thermal conductivity: 398 W / (m × K)) were 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 with a length of 2 mm or more detected by penetrant flaw detection was measured, and the value (pieces / m) obtained by dividing the sum by the casting direction length (m) of the slab where surface cracks were investigated It was defined as a crack index, and the occurrence of surface cracks was evaluated using this surface crack index.

Table 1 shows the mold construction conditions and slab surface inspection results of tests No. 1 to 26, and Table 2 shows the mold construction conditions and slab surface inspection results of tests No. 27 to 48. In addition, in the remarks column of Table 1 and Table 2, the test using the water-cooled copper alloy continuous casting mold within the scope of the present invention is the present invention example, and the scope of the present invention has a low heat conductive metal filling part. A test using a mold for continuous casting made of water-cooled copper alloy which is not satisfied is shown as a comparative example, and a test using a mold for continuous casting made of water-cooled copper alloy which does not have a low heat conductive metal filling part is shown as a conventional example. .

Tests Nos. 1 to 8 are tests in which the influence of the thermal conductivity λ _m of the filled metal on the slab surface cracking with respect to the thermal conductivity λ _c of the mold copper plate was investigated. As shown in the test results of tests No. 1 to No. 8 in FIG. 5, the surface cracking of the slab is suppressed when the thermal conductivity λ _m of the filled metal is 80% or less of the thermal conductivity λ _c of the mold copper plate. It was confirmed that

Test Nos. 9 to 19 are tests in which the influence of the thermal resistance ratio R between the low heat conductive metal filling portion and the mold copper plate on the slab surface crack was examined. As shown in the test results of Test Nos. 9 to 19 in FIG. 6, it was confirmed that the slab surface crack was suppressed when the thermal resistance ratio R was 5% or more. However, it was found that when the thermal resistance ratio R exceeds 100%, the effect of reducing surface cracks is reduced. As shown in Test No. 9, even when the thermal conductivity λ _m of the filling metal is in the range of 80% or less of the thermal conductivity λ _c of the mold copper plate, the thermal resistance ratio R is not 5% or more. It was confirmed that the effect of suppressing cracking of the slab surface could not be obtained.

Test Nos. 20 to 26 are the sum B (mm ² ) of the areas of all the low thermal conductive metal filling portions with respect to the area A (mm ² ) of the inner wall surface of the mold copper plate within the range where the low thermal conductive metal filling portions are formed. The ratio of the area ratio S, which is the ratio, to the slab surface cracks, and all the low thermal conductive metal filling portions for the area A (mm ² ) of the inner wall surface of the mold copper plate within the range where the low thermal conductive metal filling portions are formed This is a test for investigating the influence of the ratio C of the total boundary length C (mm) with the mold copper plate on the slab surface crack. As shown in the test results of Test Nos. 20 to 26 in FIG. 7, when the area ratio S is 10% or more and the ratio ε is in the range of 0.07 to 0.60, slab surface cracks are observed. Suppressed. When the area ratio S was 10% or more, or when the ratio ε was outside the range of 0.07 to 0.60, slight surface cracks occurred in the slab.

Test Nos. 27 to 32 are tests in which the influence of the diameter d of the low thermal conductive metal filling portion on the slab surface crack was investigated. As shown in the test results of Test Nos. 27 to 32 in FIG. 8, it was confirmed that cracking of the slab surface was suppressed when the diameter d of the low thermal conductive metal filling portion was 2 to 20 mm.

Test Nos. 33 to 36 are tests in which the influence of the interval P between the low thermal conductive metal filling portions on the slab surface crack was investigated. When the condition of “P ≧ 0.25 × d” was satisfied, slab surface cracking was suppressed. When the interval P deviated from the condition of “P ≧ 0.25 × d”, a slight surface crack occurred in the slab.

Test Nos. 37 to 39 are tests in which the effect on the surface crack of the slab having a length L in the range where the low thermal conductive metal filling portion was arranged was investigated. The length L is larger range for the length L ₀ that is calculated by the slab drawing speed Vc, it was confirmed that the cast slab surface cracks is suppressed.

Test Nos. 40 to 46 are a series of water-cooled copper alloys in which a plurality of low thermal conductive metal filling portions are connected to the inner wall surfaces of a copper alloy long mold copper plate and a copper alloy short mold copper plate. This is a test using a casting mold, that is, a continuous casting mold made of a water-cooled copper alloy in which each low heat conductive metal filling portion is not independent.

Test Nos. 40 to 44, of which, as shown in FIG. 9, three low heat conductive metal filling parts in which three low heat conductive metal filling parts having a diameter of 3 mm are combined. This is a test arranged by changing the interval P between them. Also in the tests No. 40 to 44, the thermal conductivity λ _m of the filling metal is 80% or less of the thermal conductivity λ _c of the mold copper plate, the thermal resistance ratio R is 5% or more, and the slab drawing speed V _{When the} length L is larger than the length L ₀ calculated by _c and the diameter d, the interval P, the area ratio S, and the ratio ε satisfy the preferable conditions, the slab surface crack is suppressed. It was confirmed that When the area ratio S or the ratio ε deviated from suitable conditions, a slight surface crack occurred in the slab.

Test No. 45 is a test using a water-cooled continuous casting mold in which low heat conductive metal filling portions are connected in the width direction of the mold as shown in FIG. As shown in Fig. 5, the test is performed using a water-cooled continuous casting mold in which all the low heat conductive metal filling parts are connected in the width direction and the casting direction of the mold. FIGS. 10- (A) and 11- (A) are schematic side views of the long side copper plate having a low heat conductive metal filling portion formed on the inner wall surface side, as viewed from the inner wall surface side. B) is a YY ′ cross-sectional view of the mold long-side copper plate shown in FIG. 10- (A), and FIG. 11- (B) is a YY view of the mold long-side copper plate shown in FIG. 11- (A). 'Cross section.

In Test No. 45, a low heat conductive metal filling portion having a diameter d: 8 mm, a filling thickness H: 4 mm, and a spacing P: 4 mm is provided in the width direction of the long copper plate and the short copper plate. In this case, a low thermal conductive metal filling portion having a diameter d of 4 mm and a filling thickness H of 1 mm is provided. Since the filling thickness H of the 8 mm diameter low heat conductive metal filling portion is larger, the stress caused by volume shrinkage or heat shrinkage during the δ / γ transformation is dispersed in the solidified shell portion in that region, and It is thought that surface cracking has been reduced.

On the other hand, in test No. 46, all of the low thermal conductive metal filling parts are connected, and solidification is always delayed at the same position of the solidified shell during continuous casting. Therefore, stress and thermal stress due to δ / γ transformation are present at that location. It is thought that the surface cracking occurred and minor surface cracks occurred.

Test Nos. 47 and 48 are tests using a conventional continuous casting mold in which a low heat conductive metal filling portion is not installed. In Test Nos. 47 and 48, many slab surface cracks occurred.

DESCRIPTION OF SYMBOLS 1 Mold long side copper plate 2 Concave groove 3 Low heat conductive metal filling part 4 Slit 5 Back plate 6 Sheet metal layer

Claims

A water-cooled continuous casting mold,
A plurality of concave grooves provided in a range from any position above the meniscus on the inner wall surface of the copper alloy mold copper plate constituting the mold to any position below the meniscus is filled with a low heat conductive metal. Having a low thermal conductivity metal filling formed;
The thermal conductivity of the mold copper plate λ c (W / (m × K)) the relative thermal conductivity of the low thermal conductive metal λ m (W / (m × K)) is not more than 80%,
A continuous casting mold having a thermal resistance ratio R defined by the following formula (1) of 5% or more.
R = {(TH) / (1000 × λ c ) + H / (1000 × λ m ) −T / (1000 × λ c )} / {T / (1000 × λ c )} × 100 (100) 1)
Here, R is a thermal resistance ratio (%) between the low thermal conductive metal filling portion and the mold copper plate,
T is the distance (mm) from the bottom of the slit of the mold copper plate to the mold copper plate surface, which becomes the flow path of the mold cooling water,
H is the filling thickness (mm) of the low thermal conductive metal.
The concave groove is an arbitrary length below the meniscus by a length L 0 (mm) or more calculated from the following formula (2) from an arbitrary position above the meniscus by the slab drawing speed Vc (m / min). The continuous casting mold according to claim 1, wherein the casting mold is provided in a range of an inner wall surface of the mold copper plate up to the position.
L 0 = 2 × Vc × 1000/60 (2)
The continuous casting mold according to claim 1 or 2, wherein the continuous casting mold has a periodic thermal resistance distribution or heat flux distribution in a range of an inner wall surface of the mold copper plate provided with the low thermal conductive metal filling portion. Casting mold.
The opening shape in the inner wall surface of the mold copper plate of the recess is circular or pseudo-circular,
The continuous casting mold according to any one of claims 1 to 3, wherein the circular diameter or the equivalent circular diameter of the pseudo-circle is 2 to 20 mm.
5. The continuous casting mold according to claim 4, wherein an interval between the low heat conductive metal filling portions satisfies a relationship of the following expression (3) with respect to the diameter or the equivalent circle diameter of the low heat conductive metal filling portions.
P ≧ 0.25 × d (3)
Here, P is the interval (mm) between the low thermal conductive metal filling parts,
d is the diameter (mm) or equivalent circle diameter (mm) of the low thermal conductive metal filling portion.
Area ratio S which is the ratio of the total area B (mm 2 ) of the areas of all the low thermal conductive metal filling portions to the area A (mm 2 ) of the inner wall surface of the mold copper plate within the range where the low thermal conductive metal filling portions are formed. (S = (B / A) × 100) is 10% or more,
In addition, the ratio ε (ε = C / A) of the sum C (mm) of the boundary lengths of all the low heat conductive metal filling portions and the mold copper plate with respect to the area A (mm 2 ) is expressed by the following formula (4). The continuous casting mold according to any one of claims 1 to 5, which satisfies the relationship.
0.07 ≦ ε ≦ 0.50 (4)
The continuous casting mold according to claim 6, wherein each of the low thermal conductive metal filling portions is formed independently.
The continuous casting mold according to any one of claims 1 to 7, wherein the low thermal conductive metal is filled in the concave groove by a plating process or a thermal spraying process.
On the inner wall surface of the mold copper plate, a plating layer of nickel having a thickness of 2.0 mm or less or an alloy containing nickel is formed,
The continuous casting mold according to any one of claims 1 to 8, wherein the low thermal conductive metal filling portion is covered with the plating layer.
A steel continuous casting method using the continuous casting mold according to any one of claims 1 to 9,
While injecting medium carbon steel having a carbon content of 0.08 to 0.17 mass% into the mold,
A steel continuous casting method in which the medium carbon steel is drawn from the mold at a casting speed of 1.5 m / min or more and continuously cast as a slab casting having a thickness of 200 mm or more.