EP3795274A1

EP3795274A1 - Continuous casting mold and method for continuous casting of steel

Info

Publication number: EP3795274A1
Application number: EP20206258.4A
Authority: EP
Inventors: Naomichi IWATA; Norichika ARAMAKI; Seiji Nabeshima; Yuji Miki; Kohei FURUMAI
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2015-07-22
Filing date: 2017-01-16
Publication date: 2021-03-24
Anticipated expiration: 2037-01-16
Also published as: KR102245013B1; KR20190017978A; TWI630962B; CN109475930A; EP3488946A4; TW201803664A; WO2018016101A1; EP3488946A1; EP3795274B1; CN109475930B

Abstract

A surface crack due to the inhomogeneous cooling of a solidified shell in the early solidification stage and a surface crack due to a variation in the thickness of a solidified shell caused by transformation from δ iron to γ iron in medium-carbon steel are inhibited.A continuous casting mold includes portions filled with a metal of low thermal conductivity formed by filling a plurality of concave grooves with the metal of low thermal conductivity in a region, on an inner wall surface of a mold copper plate which is made of a copper alloy and constitutes a water-cooled continuous casting mold, from a position located above a meniscus to a position located below the meniscus, in which a ratio of a thermal conductivity λ<sub>m</sub> (W/(m × K)) of the metal of low thermal conductivity to a thermal conductivity λ<sub>c</sub> (W/(m × K)) of the mold copper plate is 80% or less, and in which a thermal resistance ratio R defined by expression (1) below is 5% or more. R=T+H/1000×λc+H/1000×λm−T/1000×λc/T/1000×λc×100Here, R denotes a thermal resistance ratio (%) of the portions filled with the metal of low thermal conductivity to the mold copper plate, T denotes a distance (mm) from a bottom surface of a slit of the mold copper plate, which is used as a flow channel of mold-cooling water, to a surface of the mold copper plate, and H denotes a filling thickness (mm) of the metal of low thermal conductivity.

Description

Technical Field

The present invention relates to a continuous casting mold with which continuous casting can be performed on molten steel while inhibiting a surface crack from occurring in a cast piece caused by inhomogeneous cooling of a solidified shell in the mold, and to a method for continuous casting of steel by this mold.

Background Art

In a continuous casting process for steel, since molten steel which is injected into a mold is cooled using a water-cooled continuous casting mold, a solidified shell (also referred to as a "solidified layer") is formed as a result of the surface portion of the molten steel which is in contact with the mold being solidified. A cast piece having the solidified shell as an outer shell and a non-solidified layer inside the shell is continuously drawn in a downward direction through the mold while the cast piece is cooled by water sprays or air-water sprays which are installed on the downstream side of the mold. The central portion in the thickness direction of the cast piece is solidified as a result of being cooled by the water sprays or the air-water sprays, and then the cast piece is cut into cast pieces having a predetermined length by a gas cutting machine, for example.
In the case where inhomogeneous cooling occurs in the mold, there is a variation in the thickness of the solidified shell in the casting direction of the cast piece and width direction of the mold. Since the solidified shell is subjected to stress caused by the shrinkage and deformation of the solidified shell, and, since this stress is concentrated in a thin portion of the solidified shell in the early solidification stage, a crack occurs on the surface of the solidified shell due to this stress. Subsequently, such a crack grows into a large surface crack due to an external force caused by, for example, bending stress and leveling stress which are applied by the rolls of the continuous casting machine and thermal stress. In the case where a variation in the thickness of the solidified shell is large, a longitudinal crack occurs in the mold, which may result in a breakout in which molten steel flows out through the longitudinal crack. Since the crack existing in the cast piece becomes a surface defect in a subsequent rolling process, it is necessary to remove the surface crack at the cast piece stage after casting by repairing the surface of the cast piece.
Inhomogeneous solidification in the mold tends to occur, in particular, in the case of steel (referred to as "medium-carbon steel") having a carbon content of 0.08 mass% to 0.17 mass%. In the case of steel having a carbon content of 0.08 mass% to 0.17 mass%, a peritectic reaction occurs at the time of solidification. It is considered that inhomogeneous solidification in the mold is caused by transformation stress due to a decrease in volume which occurs when transformation from δ iron (ferrite phase) to γ iron (austenite phase) occurs due to this peritectic reaction. That is, the solidified shell is deformed due to strain caused by this transformation stress, and the solidified shell is detached from the inner wall surface of the mold due to this deformation. Since the portion which has been detached from the inner wall surface of the mold becomes less likely to be cooled through the mold, there is a decrease in the thickness of the solidified shell of this portion which has been detached from the inner wall surface of the mold (this portion which is detached from the inner wall surface of the mold is referred to as a "depression"). It is considered that, since there is a decrease in the thickness of the solidified shell, a surface crack occurs due to the stress described above being concentrated in this portion.
In particular, in the case where there is an increase in a cast piece drawing speed, since there is an increase in average thermal flux from the solidified shell to the mold (the solidified shell is rapidly cooled), and since the distribution of thermal flux becomes irregular and inhomogeneous, there is a tendency for the number of surface cracks occurring in the cast piece to increase. Specifically, in the case of a machine for continuously casting a slab having a cast-piece thickness of 200 mm or more, a surface crack tends to occur when the cast piece drawing speed is 1.5 m/min or more.
Therefore, to date, various methods have been proposed to inhibit a surface crack (in particular, a longitudinal crack) from occurring in a steel grade in which a surface crack tends to occur.
For example, Patent Literature 1 proposes a technique in which mold powder having a chemical composition which tends to cause crystallization is used to increase the thermal resistance of a mold powder layer, so that a solidified shell is slowly cooled. This is a technique for inhibiting a surface crack from occurring by decreasing stress applied to the solidified shell as a result of slow cooling. However, with only the effect of slow cooling utilizing mold powder, since there is an insufficient improvement in inhomogeneous solidification, it is not possible to sufficiently inhibit a surface crack from occurring, in particular, in the case of medium-carbon steel in which transformation from δ iron to γ iron occurs due to a small decrease in temperature when solidification occurs.
Patent Literature 2 proposes a technique in which a longitudinal crack is inhibited from occurring in a cast piece by making mold powder flow into vertical grooves and horizontal grooves formed on the inner wall surface of a mold to slowly decrease the cooling rate of the mold and to homogenize the cooling rate in the width direction of the mold. However, there is a problem in that, in the case where there is a decrease in the depth of the grooves formed on the inner wall surface of the mold due to the abrasion of the inner wall surface of the mold caused by the contact with the cast piece, there is a decrease in the effect of slow cooling due to a decrease in the amount of mold powder flowing into the grooves, that is, there is a problem in that the effect of slow cooling does not last. In addition, there is also a risk of, when molten steel is injected into an empty space of a mold at the start of casting, injected molten steel flowing into the grooves formed on the inner wall surface of the mold and solidifying, so that it is not possible to draw a solidified shell due to a mold copper plate and the solidified shell adhering to each other, which results in a constrained breakout occurring.
Patent Literature 3 proposes a technique in which a longitudinal crack is inhibited from occurring in a cast piece by forming vertical grooves parallel to the casting direction or grid grooves in the central portion in the width direction of the inner wall surface of a mold with the width and depth of the grooves being controlled in accordance with the viscosity of mold powder, so that gaps for letting air flow thereinto are formed in the grooves without filling the grooves with the mold powder to slowly decrease the cooling rate of the mold and to homogenize the cooling rate in the width direction of the mold. However, also in this case, since the grooves are exposed on the inner wall surface of the mold, there is a problem in that the effect of slow cooling does not last due to the abrasion of the inner wall surface of the mold as in the case of Patent Literature 2. In addition, there is also a risk of, at the start of casting, molten steel flowing into the grooves formed on the inner wall surface of the mold and solidifying, so that it is not possible to draw a solidified shell, which results in a constrained breakout occurring.
Patent Literature 4 proposes a mold in which grid grooves are formed on an inner wall surface thereof and a mold in which the grid grooves are filled with a foreign metal (such as Ni or Cr) or a ceramic (BN, AlN, or ZrO₂). This technique is a technique in which a longitudinal crack is inhibited from occurring in a cast piece by providing a periodic variation in the amount of heat removed through the grooves and the portions other than the grooves to disperse stress caused by transformation from δ iron to γ iron or thermal shrinkage of a solidified shell in the regions where the removed amount of heat is small. However, there is a problem in that, in the case of the grid grooves, since the boundary line between the groove formed on the inner wall surface of the mold and a mold copper plate (made of copper or a copper alloy) has a straight-line shape, a crack tends to occur and propagate on the boundary surface due to a difference in thermal expansion, which results in a decrease in the life of the mold.
Patent Literature 5 proposes a continuous casting method in which a mold in which vertical grooves parallel to the casting direction are formed on an inner wall surface thereof, or a mold in which the vertical grooves are filled with a foreign metal (such as Ni or Cr) or a ceramic (BN, AlN, or ZrO₂) is used with a cast piece drawing speed and a mold oscillation period being specified to be within predetermined ranges. According to Patent Literature 5, by optimizing the mold oscillation period in accordance with the cast piece drawing speed, since oscillation marks formed on the cast piece have a function of horizontal grooves, the effect of inhibiting a surface crack from occurring is realized as in the case of Patent Literature 4 by forming only vertical grooves. However, as in the case of Patent Literature 4, there is a problem in that, since the boundary line between the groove formed on the inner wall surface of the mold and a mold copper plate (made of copper or a copper alloy) has a straight-line shape, a crack tends to occur and propagate on the boundary surface due to a difference in thermal expansion, which results in a decrease in the life of the mold.
Patent Literature 6 proposes a mold in which concave grooves having a diameter of 2 mm to 10 mm are formed in a part of an inner wall surface of the mold in the vicinity of the upper surface of molten steel in the mold (hereinafter, also referred to as a "meniscus") and filled with a foreign metal (such as Ni or stainless steel) or a ceramic (such as BN, AlN, or ZrO₂) with the distance between the grooves being 5 mm to 20 mm. This technique is also a technique in which, as in the cases of Patent Literature 4 and Patent Literature 5, by providing a periodic distribution of thermal conduction to inhibit inhomogeneous solidification, a vertical crack is inhibited from occurring in a cast piece. However, in the case of Patent Literature 6, since the groove openings are formed by a drill on the surface of a mold copper plate, and since the grooves are filled with the foreign metal or the ceramic formed into the shape of the grooves, the conditions of contact between the back surfaces of filling pieces of the metal or the ceramic and the mold copper plate are not constant, which increase a risk of gaps being formed in the contact parts. In the case where such gaps are formed, there is a large variation in the amount of heat removed among the concave grooves depending on such gaps, which results in a problem in that it is not possible to appropriately control the cooling of a solidified shell. In addition, there is also a problem in that the filling pieces of the foreign metal or the ceramic tend to be separated from the mold copper plate.

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2005-297001
PTL 2: Japanese Unexamined Patent Application Publication No. 9-276994
PTL 3: Japanese Unexamined Patent Application Publication No. 10-193041
PTL 4: Japanese Unexamined Patent Application Publication No. 1-289542
PTL 5: Japanese Unexamined Patent Application Publication No. 2-6037
PTL 6: Japanese Unexamined Patent Application Publication No. 1-170550

Summary of Invention

Technical Problem

The present invention has been completed in view of the situation described above, and an object of the present invention is to provide a continuous casting mold with which it is possible to inhibit, for a long period of time, a surface crack from occurring in a cast piece due to the inhomogeneous cooling of a solidified shell in the early solidification stage and a surface crack from occurring in a cast piece due to a variation in the thickness of a solidified shell caused by transformation from δ iron to γ iron accompanied by a peritectic reaction in medium-carbon steel without a constrained breakout occurring at the start of casting or a decrease in the life of the mold due to a surface crack occurring in a mold copper plate, and to provide a continuous casting method for steel utilizing this continuous casting mold.

Solution to Problem

The subject matter of the present invention for solving the problems described above is as follows.

[1] A continuous casting mold, the mold being a water-cooled continuous casting mold, including:
- portions filled with a metal of low thermal conductivity formed by filling a plurality of concave grooves with the metal of low thermal conductivity in a region, on an inner wall surface of a mold copper plate which is made of a copper alloy and constitutes the mold, from a position located above a meniscus to a position located below the meniscus, in which,
- a ratio of a thermal conductivity λ_m (W/(m × K)) of the metal of low thermal conductivity to a thermal conductivity λ_c (W/(m × K)) of the mold copper plate is 80% or less, and
- a thermal resistance ratio R defined by expression (1) below is 5% or more. $R = \{(T - H) / (1000 \times λ_{c}) + H / (1000 \times λ_{m}) - T / (1000 \times λ_{c})\} / \{T / (1000 \times λ_{c})\} \times 100$
  
  Here, R denotes a thermal resistance ratio (%) of the portions filled with the metal of low thermal conductivity to the mold copper plate, T denotes a distance (mm) from a bottom surface of a slit of the mold copper plate, which is used as a flow channel of mold-cooling water, to a surface of the mold copper plate, and H denotes a filling thickness (mm) of the metal of low thermal conductivity.
[2] The continuous casting mold according to item [1] above, in which the concave grooves are formed in a region, on the inner wall surface of the mold copper plate, from a position located above the meniscus to a position located below the meniscus and at a distance equal to or more than a length L₀ (mm) from the meniscus, where L₀ is calculated by expression (2) below from a cast piece drawing speed Vc (m/min). $L_{0} = 2 \times V c \times 1000 / 60$
[3] The continuous casting mold according to item [1] or [2] above, having a periodic thermal resistance distribution or a periodic thermal flux distribution in the region, on the inner wall surface of the mold copper plate, where the portions filled with the metal of low thermal conductivity are formed.
[4] The continuous casting mold according to any one of items [1] to [3] above, in which, openings of the concave grooves on the inner wall surface of the mold copper plate have a circular shape or a quasi-circular shape, and in which the circular shape has a diameter of 2 mm to 20 mm and the quasi-circular shape has a circle-equivalent diameter of 2 mm to 20 mm.
[5] The continuous casting mold according to item [4] above, in which a distance between the portions filled with the metal of low thermal conductivity satisfies expression (3) below with respect to the diameter or circle-equivalent diameter of the portions filled with the metal of low thermal conductivity. $P \geq 0.25 \times d$

Here, P denotes a distance (mm) between the portions filled with the metal of low thermal conductivity and d denotes a diameter (mm) or circle-equivalent diameter (mm) of the portions filled with the metal of low thermal conductivity.
[6] The continuous casting mold according to any one of items [1] to [5] above, in which, a ratio of a total area B (mm²) of all the portions filled with the metal of low thermal conductivity to an area A (mm²) of the region, on the inner wall surface of the mold copper plate, where the portions filled with the metal of low thermal conductivity are formed, that is, an area ratio S (S = (B/A) × 100), is 10% or more, and in which a ratio of a total length C (mm) of boundaries between all the portions filled with the metal of low thermal conductivity and the mold copper plate to the area A (mm²) , that is, a ratio ε (ε = C/A), satisfies expression (4) below. $0.07 \leq ε \leq 0.50$
[7] The continuous casting mold according to item [6] above, in which the portions filled with the metal of low thermal conductivity are separately formed.
[8] The continuous casting mold according to any one of items [1] to [7] above, in which the concave grooves are filled with the metal of low thermal conductivity by a plating treatment or a thermal spraying treatment.
[9] The continuous casting mold according to any one of items [1] to [8] above, in which a coating layer having a thickness of 2.0 mm or less and containing nickel or an alloy containing nickel is formed on the inner wall surface of the mold copper plate, and in which the portions filled with the metal of low thermal conductivity are covered with the coating layer.
[10] A method for continuous casting of steel, using the continuous casting mold according to any one of items [1] to [9] above, the method including the steps of:
injecting medium-carbon steel having a carbon content of 0.08 mass% to 0.17 mass% into the mold, and drawing the medium-carbon steel from the mold in a form of a cast slab having a thickness of 200 mm or more at a cast piece drawing speed of 1.5 m/min or more.

Advantageous Effects of Invention

In the present invention, plural portions filled with a metal of low thermal conductivity, which are formed by filling the portions with a metal of low thermal conductivity having a thermal resistance ratio R, that is, the thermal resistance ratio of the portions filled with a metal of low thermal conductivity to the mold copper plate, of 5% or more and thermal conductivity which is 80% or less of the thermal conductivity of the mold copper plate, are disposed in the width direction and casting direction of a continuous casting mold in a region including a meniscus position in the vicinity of the meniscus. With this, since the thermal resistance of the continuous casting mold periodically increases and decreases in the width direction and casting direction of the continuously casting mold in the vicinity of the meniscus, the thermal flux from a solidified shell to the continuous casting mold periodically increases and decreases in the vicinity of the meniscus, that is, in the early solidification stage. As a result of such periodic increase and decrease in thermal flux, since there is a decrease in stress due to transformation from δ iron to γ iron and in thermal stress, there is a decrease in the amount of deformation of the solidified shell caused by these stresses. As a result of a decrease in the amount of deformation of the solidified shell, since an inhomogeneous distribution of thermal flux caused by the deformation of the solidified shell is homogenized, and since generated stress is dispersed, there is a decrease in the amounts of various strains, which results in a surface crack being inhibited from occurring in the solidified shell.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a schematic side view of a mold copper plate on the long side of a mold constituting a part of the water-cooled continuous casting mold according to the present embodiments viewed from the inner wall surface side.
[Fig. 2] Fig. 2 is an X-X' cross-sectional view of the mold copper plate on the long side of a mold in Fig. 1.
[Fig. 3] Fig. 3 is a conceptual diagram illustrating the thermal resistance distributions in accordance with the positions where portions filled with a metal of low thermal conductivity are formed at three positions on a mold copper plate having portions filled with a metal of low thermal conductivity on the long side of a mold.
[Fig. 4] Fig. 4 is a schematic diagram illustrating an example in which a coating layer is formed on the inner wall surface of a mold copper plate on the long side of a mold to protect the surface of the mold.
[Fig. 5] Fig. 5 is a diagram illustrating the results of investigations regarding the influence of the thermal conductivity of a metal of low thermal conductivity used for portions filled with a metal of low thermal conductivity on a surface crack occurring in a cast piece.
[Fig. 6] Fig. 6 is a diagram illustrating the results of investigations regarding the influence of the thermal resistance ratio R of portions filled with a metal of low thermal conductivity to a mold copper plate on a surface crack occurring in a cast piece.
[Fig. 7] Fig. 7 is a diagram illustrating the results of investigations regarding the influences of the area ratio S and boundary length ratio ε of portions filled with a metal of low thermal conductivity on a surface crack occurring in a cast piece.
[Fig. 8] Fig. 8 is a diagram illustrating the results of investigations regarding the influence of the diameter d of portions filled with a metal of low thermal conductivity on a surface crack occurring in a cast piece.
[Fig. 9] Fig. 9 is a schematic side view illustrating the arrangements of portions filled with a metal of low thermal conductivity in test Nos. 40 to 44.
[Fig. 10] Fig. 10 is a schematic diagram illustrating the arrangement of portions filled with a metal of low thermal conductivity in test No. 45.
[Fig. 11] Fig. 11 is a schematic diagram illustrating the arrangement of portions filled with a metal of low thermal conductivity in test No. 46.

Description of Embodiments

Hereafter, the present invention will be specifically described through the embodiments of the invention. Fig. 1 is a schematic side view of a mold copper plate 1 on the long side of a mold constituting a part of the water-cooled continuous casting mold according to the present embodiments, that is, the mold copper plate 1 on the long side of the mold having portions 3 filled with a metal of low thermal conductivity formed on an inner wall surface side thereof, viewed from the inner wall surface side. In addition, Fig. 2 is an X-X' cross-sectional view of the mold copper plate 1 on the long side of a mold in Fig. 1.
The continuous casting mold in Fig. 1 is an example of a continuous casting mold used for casting a cast slab. A water-cooled continuous casting mold made of a copper alloy for a cast slab consists of a combination of a pair of mold copper plates made of a copper alloy on the long side of the mold and a pair of mold copper plates made of a copper alloy on the short side of the mold. Of such mold copper plates, a mold copper plate 1 on the long side of the mold is illustrated in Fig. 1. Under the assumption that the mold copper plate on the short side of the mold has portions 3 filled with a metal of low thermal conductivity formed on an inner wall surface side thereof as in the case of the mold copper plate 1 on the long side of the mold, the description of the mold copper plate on the short side of the mold is omitted here. However, in the case of a cast slab, since stress concentration tends to occur in a solidified shell on the long side of the cast piece due to its shape having a width notably exceeding its thickness, a surface crack tends to occur on the long side of the cast piece. Therefore, it is not necessary to form portions 3 filled with a metal of low thermal conductivity on the mold copper plate on the short side of the mold of a continuous casting mold used for a cast slab.
As illustrated in Fig. 1, plural portions 3 having a diameter d and filled with a metal of low thermal conductivity are formed with the distance P between the portions filled with a metal of low thermal conductivity in a region, on the inner wall surface of the mold copper plate 1 on the long side of the mold, from a position located above the position of a meniscus when stationary casting is performed on the mold copper plate 1 on the long side of the mold and at a distance equal to a length Q (length Q is assigned a value larger than 0) from the meniscus to a position located below the meniscus and at a distance equal to a length L from the meniscus. Here, the term "meniscus" refers to "the upper surface of molten steel in a mold", and, although its position is not clear when casting is not performed, the meniscus position is set to be about 50 mm to 200 mm lower than the upper edge of the mold copper plate in an ordinary continuous casting operation for steel. Therefore, even in the case where the meniscus position is 50 mm or 200 mm lower than the upper edge of the mold copper plate 1 on the long side of the mold, the portions 3 filled with a metal of low thermal conductivity may be disposed so that a length Q and a length L satisfy the conditions according to the present invention as described below.
As illustrated in Fig. 2, the portions 3 filled with a metal of low thermal conductivity are formed on the inner wall surface of the mold copper plate 1 on the long side of the mold by filling separately formed circular concave grooves 2 having a diameter of d with a metal (hereinafter, referred to as a "metal of low thermal conductivity") having thermal conductivity λ_m, which is 80% or less of the thermal conductivity λ_c of the copper alloy used to construct the mold copper plate 1 on the long side of the mold through a plating treatment or a thermal spraying treatment. Here, the concave grooves 2 having a circular opening shape on the inner wall surface of the mold copper plate are referred to as "circular concave grooves". In addition, reference 4 in Fig. 2 denotes a slit which is used as a flow channel of mold-cooling water and which is disposed on the back-surface side of the mold copper plate 1 on the long side of the mold, and reference 5 denotes a back plate which is in close contact with the back surface of the mold copper plate 1 on the long side of the mold.
Fig. 3 is a conceptual diagram illustrating the thermal resistance distributions in accordance with the positions where portions 3 filled with a metal of low thermal conductivity are formed at three positions on a mold copper plate 1 having portions 3 filled with a metal of low thermal conductivity on the long side of a mold. As illustrated in Fig. 3, there is a relative increase in thermal resistance at the positions where portions 3 filled with a metal of low thermal conductivity are formed.
By disposing plural portions 3 filled with a metal of low thermal conductivity in the width direction and casting direction of a continuous casting mold in a region including a meniscus in the vicinity of the meniscus, a distribution of the thermal resistance of the continuous casting mold, in which the thermal resistance periodically increases and decreases in the width direction and casting direction of the continuously casting mold in the vicinity of the meniscus, is provided. With this, a distribution of the thermal flux from a solidified shell to the continuous casting mold, in which the thermal flux periodically increases and decreases in the vicinity of the meniscus, that is, in the early solidification stage, is provided.
As a result of such periodic increase and decrease in thermal flux, since there is a decrease in stress generated in the solidified shell due to transformation from δ iron to γ iron (hereinafter, referred to as "δ/γ transformation") and in thermal stress, there is a decrease in the amount of deformation of the solidified shell caused by these stresses. As a result of a decrease in the amount of deformation of the solidified shell, since an inhomogeneous distribution of thermal flux caused by the deformation of the solidified shell is homogenized, and since generated stress is dispersed, there is a decrease in the amounts of various strains, which results in a surface crack being inhibited from occurring in the solidified shell.
Here, the ratio between the thermal conductivity λ_c of the copper alloy and the thermal conductivity λ_m of the metal of low thermal conductivity is defined as the ratio at room temperature (about 20°C). Although the thermal conductivities of the copper alloy and the metal of low thermal conductivity generally decrease with an increase in temperature, in the case where the ratio of the thermal conductivity λ_m of the metal of low thermal conductivity to the thermal conductivity λ_c of the copper alloy is 80% or less at room temperature, it is possible to provide a difference in thermal resistance between the positions where the portions 3 filled with a metal of low thermal conductivity are formed and the positions where no such portion 3 filled with a metal of low thermal conductivity is formed even at a temperature (about 200°C to 350°C) at which the continuous casting mold is used.
In the present embodiments, to provide a distribution of the thermal flux from a solidified shell to the continuous casting mold, in which the thermal flux periodically increases and decreases, that is, to provide a clear difference in thermal resistance between the positions where the portions 3 filled with a metal of low thermal conductivity are formed and the positions where no such portion 3 filled with a metal of low thermal conductivity is formed, the portions 3 filled with the metal of low thermal conductivity are formed in accordance with the shape of the mold copper plate so that the thermal resistance ratio R of the portions filled with a metal of low thermal conductivity to the mold copper plate, which is defined by expression (1) below, is 5% or more. Here, the thermal resistance ratio R is, as expressed in expression (1), defined by the distance T from the bottom surface 4a of a slit 4 of the mold copper plate, which is used as a flow channel of mold-cooling water, to the surface of the mold copper plate, the filling thickness H of the metal of low thermal conductivity in the portions 3 filled with a metal of low thermal conductivity, the thermal conductivity λ_c of the mold copper plate, and the thermal conductivity λ_m of the metal of low thermal conductivity. $R = \{(T - H) / (1000 \times λ_{c}) + H / (1000 \times λ_{m}) - T / (1000 \times λ_{c})\} / \{T / (1000 \times λ_{c})\} \times 100$
Here, in expression (1), R denotes the thermal resistance ratio (%) of the portions filled with a metal of low thermal conductivity to the mold copper plate, T denotes a distance (mm) from the bottom surface of a slit of the mold copper plate, which is used as a flow channel of mold-cooling water, to the surface of the mold copper plate, H denotes a filling thickness (mm) of the metal of low thermal conductivity, λ_c denotes the thermal conductivity (W/(m × K)) of the mold copper plate, and λ_m denotes the thermal conductivity (W/(m × K)) of the metal of low thermal conductivity.
Here, in the case where the thermal resistance ratio R is more than 100%, since there is a significant delay in solidification at the portions 3 filled with a metal of low thermal conductivity, inhomogeneous solidification is promoted, which increases a risk of a surface crack and a breakout occurring in a cast piece. Therefore, it is preferable that the thermal resistance ratio R be 100% or less.
In consideration of an influence on early-stage solidification, it is preferable that the lower edge of a region where the portions 3 filled with a metal of low thermal conductivity be located below the meniscus and at a distance equal to or more than a length L₀ from the meniscus, where L₀ is calculated by expression (2) below from a cast piece drawing speed Vc when stationary casting is performed. That is, it is preferable that a length L from the meniscus in Fig. 1 be equal to or more than the length L₀. $L_{0} = 2 \times Vc \times 1000 / 60$
Here, in expression (2), L₀ denotes a length (mm), Vc denotes a cast piece drawing speed (m/min).
The length L₀ relates to the time taken for a cast piece which has started being solidified to pass through the region where the portions 3 filled with a metal of low thermal conductivity are formed, and, to inhibit a surface crack from occurring in the cast piece, it is preferable that the cast piece stay in the region where the portions 3 filled with a metal of low thermal conductivity are formed at least 2 seconds after solidification has started. To ensure that a cast piece to stay in the region where the portions 3 filled with a metal of low thermal conductivity are formed at least 2 seconds after solidification has started, it is necessary that the length L₀ satisfy expression (2).
By ensuring that a cast piece which has started being solidified stays in the region where the portions 3 filled with a metal of low thermal conductivity are formed at least 2 seconds, since the effect of a periodic variation in thermal flux caused by the portions 3 filled with a metal of low thermal conductivity is sufficiently realized, it is possible to increase the effect of inhibiting a surface crack from occurring in a cast piece in the case where a surface crack tends to occur in a solidified shell, that is, when high-speed casting is performed or when medium-carbon steel is cast. To stably realize the effect of a periodic variation in thermal flux caused by the portions 3 filled with a metal of low thermal conductivity, it is more preferable to ensure that the time taken for a cast piece to pass through the region where the portions 3 filled with a metal of low thermal conductivity are formed is 4 seconds or more. On the other hand, although it is not necessary to put a limitation on the upper limit of the length L, it is preferable that the length L be 5 times or less of the length L₀ from the viewpoint of decreasing costs for forming concave grooves and performing a plating treatment or a thermal spraying treatment on the surface of the mold copper plate to form the portions 3 filled with a metal of low thermal conductivity.
On the other hand, since the upper edge of the region where the portions 3 filled with a metal of low thermal conductivity are formed may be located at any position as long as the position is above the meniscus, a length Q in Fig. 1 may take a value larger than 0. However, since the meniscus moves in an up and down direction when casting is performed, to ensure that the upper edge of the region where the portions 3 filled with a metal of low thermal conductivity are formed is always located at a position above the meniscus, it is preferable that the upper edge of the portions 3 filled with a metal of low thermal conductivity be located about 10 mm, or more preferably about 20 mm to 50 mm, higher than a set meniscus.
Although, in the case of the example illustrated in Fig. 1 and Fig. 2, the opening shape of the portions 3 filled with a metal of low thermal conductivity on the inner wall surface of a mold copper plate 1 on the long side of a mold is circular, it is not necessary that the opening shape be circular. Any kind of shape may be used as long as the shape is one similar to a circle such as an ellipse which does not have a so-called "angulated corner". Hereinafter, a shape similar to a circle will be referred to as a "quasi-circle". In the case where the opening shape of portions 3 filled with a metal of low thermal conductivity is a quasi-circle, grooves 2 formed on the inner wall surface of the mold copper plate 1 on the long side of the mold to form the portions 3 filled with a metal of low thermal conductivity are referred to as "quasi-circle grooves". Examples of a quasi-circle include an ellipse and a rectangle having corners having a shape of a circular arc or an elliptic arc which have no angulated corner, and, further, a shape such as a petal-shaped pattern may be used. The size of a quasi-circle is measured in terms of circle-equivalent diameter, which is calculated from the area of the opening having a quasi-circular shape on the inner wall surface of the mold copper plate 1 on the long side of the mold.
In the case of Patent Literature 4 and Patent Literature 5 where vertical grooves or grid grooves are formed and the grooves are filled with a metal of low thermal conductivity, there is a problem in that, since stress caused by a difference in thermal strain between the metal of low thermal conductivity and copper is concentrated at the boundary surface between the metal of low thermal conductivity and copper and at the intersections of the grid portions, surface cracks occur in the mold copper plate. In contrast, in the case of the continuous casting mold according to the present embodiments, the shape of the portions 3 filled with a metal of low thermal conductivity is set to be a circle or a quasi-circle. With this, since the boundary surface between the metal of low thermal conductivity and copper is a curved surface, stress is less likely to be concentrated at the boundary surface, which results in an advantage in that a surface crack is less likely to occur in a mold copper plate.
It is preferable that the portions 3 filled with a metal of low thermal conductivity have a diameter d or a circle-equivalent diameter d of 2 mm to 20 mm. By controlling the diameter d or the circle-equivalent diameter d of the portions 3 filled with a metal of low thermal conductivity to be 2 mm or more, since there is a sufficient decrease in thermal flux in the portions 3 filled with a metal of low thermal conductivity, it is possible to increase the effect of inhibiting a surface crack from occurring in a cast piece. In addition, by controlling the diameter d or the circle-equivalent diameter d to be 2 mm or more, it is easy to fill circular concave grooves 2 or quasi-circular concave grooves 2 with the metal of low thermal conductivity by performing a plating treatment or a thermal spraying treatment. On the other hand, by controlling the diameter d or circle-equivalent diameter d of the portions 3 filled with a metal of low thermal conductivity to be 20 mm or less, since a decrease in thermal flux in the portions 3 filled with a metal of low thermal conductivity is inhibited, that is, since solidification delay in the portions 3 filled with a metal of low thermal conductivity is inhibited, stress concentration in a solidified shell at positions corresponding to the portions 3 is prevented, which results in a surface crack being inhibited from occurring in the solidified shell. That is, since there is a tendency for the number of surface cracks occurring in a solidified shell to increase in the case where the diameter d or the circle-equivalent diameter d is more than 20 mm, it is preferable that the portions 3 filled with a metal of low thermal conductivity have a diameter d or a circle-equivalent diameter d of 20 mm or less. Here, in the case where the portions 3 filled with a metal of low thermal conductivity have a quasi-circular shape, the circle-equivalent diameter d of the quasi-circle is calculated by expression (5) below. $circle - equivalent diameter = {(4 \times S / π)}^{1 / 2}$
Here, in expression (5), S denotes the area (mm²) of the opening of the portions 3 filled with a metal of low thermal conductivity on the inner wall surface of the mold copper plate.
It is necessary that the ratio of the thermal conductivity λ_m of a metal of low thermal conductivity with which circular concave grooves or quasi-circular concave grooves are filled to the thermal conductivity λ_c of a copper alloy to be used to construct a mold copper plate be 80% or less. By the metal of low thermal conductivity whose thermal conductivity is 80% or less of the thermal conductivity of the copper alloy, since the effect of a periodic variation in thermal flux caused by the portions 3 filled with the metal of low thermal conductivity is sufficiently realized, it is possible to sufficiently realize the effect of inhibiting a surface crack from occurring in a cast piece even in the case where a surface crack tends to occur, that is, when high-speed casting is performed or when medium-carbon steel is cast.
Examples of the metal of low thermal conductivity used for the continuous casting mold according to the present embodiments include nickel (Ni, having a thermal conductivity of 90.5 W/(m × K)), a nickel-based alloy, chromium (Cr, having a thermal conductivity of 67 W/(m × K)), and cobalt (Co, having a thermal conductivity of 70 W/(m × K)), which are easy to use as filling materials in a plating treatment or a thermal spraying treatment. Here, the thermal conductivity described in the present description is that determined at room temperature (about 20°C) .
In addition, as a copper alloy used for a mold copper plate, a copper alloy to which, for example, minute amounts of chromium and zirconium (Zr) generally used for a continuous casting mold are added may be used. Nowadays, to homogenize solidification in a mold or to prevent inclusions in molten steel from being trapped in a solidified shell, a continuous casting mold is generally provided with an electromagnetic stirring device, with which molten steel in a mold is stirred. In this case, to inhibit the attenuation of the strength of a magnetic field applied from an electromagnetic coil to molten steel, a copper alloy whose electrical conductivity is decreased is used. In the case of such a copper alloy, since thermal conductivity decreases with a decrease in electrical conductivity, there is a case where a copper alloy whose thermal conductivity is about 1/2 of that of pure copper is used as a mold copper plate nowadays. In the case of such a continuous casting mold, although there is a decrease in the difference in thermal conductivity between a mold copper plate and a metal of low thermal conductivity, the effect of inhibiting a surface crack from occurring in a cast piece is realized by controlling a thermal resistance R defined by expression (1) above to be 5% or more.
It is preferable that the filling thickness H of the portions 3 filled with a metal of low thermal conductivity be 0.5 mm or more. By controlling the filling thickness H to be 0.5 mm or more, since there is a sufficient decrease in thermal flux in the portions 3 filled with a metal of low thermal conductivity, it is possible to realize the effect of inhibiting a surface crack from occurring in a cast piece.
In addition, it is preferable that the filling thickness H of the portions 3 filled with a metal of low thermal conductivity be equal to or less than the diameter d or circle-equivalent diameter d of the portions 3 filled with a metal of low thermal conductivity. Since the filling thickness H is controlled to be equal to or less than the diameter d or circle-equivalent diameter d of the portions 3 filled with a metal of low thermal conductivity, it is easy to fill the concave grooves 2 with the metal of low thermal conductivity by performing a plating treatment or a thermal spraying treatment, and a gap or a crack does not occur between the filling pieces of metal of low thermal conductivity and the mold copper plate. In the case where a gap or a crack occurs between the filling pieces of metal of low thermal conductivity and the mold copper plate, a crack or separation occurs in the metal of low thermal conductivity, which results in a decrease in mold life and results not only in a crack but also in a constrained breakout occurring in a cast piece.
It is preferable that a distance P between the portions filled with a metal of low thermal conductivity be 0.25 times or more of the diameter d or circle-equivalent diameter d of the portions 3 filled with a metal of low thermal conductivity. That is, it is preferable that a distance P between the portions filled with a metal of low thermal conductivity satisfy the expression (3) below with respect to the diameter d or circle-equivalent diameter d of the portions 3 filled with a metal of low thermal conductivity. $P \geq 0.25 \times d$
Here, in expression (3), P denotes the distance (mm) between the portions filled with a metal of low thermal conductivity and d denotes the diameter (mm) or circle-equivalent diameter (mm) of the portions filled with a metal of low thermal conductivity.
Here, the expression a "distance P between the portions filled with a metal of low thermal conductivity" refers to the shortest distance between the edges of the adjacent portions 3 filled with a metal of low thermal conductivity as illustrated in Fig. 1. By controlling the distance P between the portions filled with a metal of low thermal conductivity to be equal to or more than "0.25 × d", since the distance is sufficiently large and the difference in thermal flux between the portions 3 filled with a metal of low thermal conductivity and a copper alloy portion (in which no such portion 3 filled with a metal of low thermal conductivity is formed) is sufficiently large, it is possible to realize the effect of inhibiting a surface crack from occurring in a cast piece. Although there is no particular limitation on the upper limit of the distance P between the portions filled with a metal of low thermal conductivity, since there is a decrease in the area ratio of the portions 3 filled with a metal of low thermal conductivity in the case where the distance P is large, it is preferable that the distance P be equal to or less than "2.0 × d".
Although it is preferable that the portions 3 filled with a metal of low thermal conductivity are arranged in a zigzag pattern as illustrated in Fig. 1, the arrangement pattern of the portions 3 filled with a metal of low thermal conductivity is not limited to a zigzag pattern, and any arrangement may be used as long as the distance P between the portions filled with a metal of low thermal conductivity satisfies expression (3) above.
It is preferable that the ratio of a total area B (mm²) of all the portions 3 filled with the metal of low thermal conductivity to an area A (mm²) of the region, on the inner wall surface of the mold copper plate, where the portions 3 filled with the metal of low thermal conductivity are formed, that is, an area ratio S (S = (B/A) × 100), be 10% or more. By controlling the area ratio S to be 10% or more, since it is possible to achieve sufficient area occupied by the portions 3 filled with a metal of low thermal conductivity and having low thermal flux, it is possible to achieve sufficient difference in thermal flux between the portions 3 filled with a metal of low thermal conductivity and the copper alloy portion, which results in the effect of inhibiting a surface crack from occurring in a cast piece being stably realized. Here, although it is not necessary to put a particular limitation on the upper limit of the area ratio S of the area occupied by the portions 3 filled with a metal of low thermal conductivity, since it is preferable that the distance P between the portions filled with a metal of low thermal conductivity be equal to or more than "0.25 × d" as described above, the expression "P = 0.25 × d" may be used to determine the maximum area ratio S.
In addition, it is preferable that the ratio of a total length C (mm) of boundaries between all the portions 3 filled with the metal of low thermal conductivity and the mold copper plate to the area A (mm²) of the region, on the inner wall surface of the mold copper plate, where the portions 3 filled with the metal of low thermal conductivity are formed, that is, a ratio ε (ε = C/A), satisfy expression (4) below. $0.07 \leq ε \leq 0.60$
From the results of investigations regarding the influence of the ratio ε on a surface crack occurring in a cast piece, it is clarified that there was only a slight effect of inhibiting a surface crack from occurring in the case where the ratio ε did not satisfy expression (4). The ratio ε varies depending on the diameter d or circle-equivalent diameter d of portions 3 filled with a metal of low thermal conductivity and the number of portions 3 filled with a metal of low thermal conductivity.
When the ratio ε is less than 0.07, since the number of portions 3 filled with a metal of low thermal conductivity is small, stress caused by a decrease in volume when δ/γ transformation occurs or thermal shrinkage is less likely to be dispersed across a shell, which results in a decrease in the effect of inhibiting a surface crack from occurring in a cast piece. On the other hand, when the ratio ε is more than 0.60, since the number of portions 3 filled with a metal of low thermal conductivity is excessively large, the amount of periodic increase and decrease in thermal flux does not reach a target level, which results in a decrease in the effect of inhibiting a surface crack from occurring in a cast piece. In addition, when the ratio ε is more than 0.60, bulging occurred in a cast piece immediately below a mold.
Although the portions 3 filled with a metal of low thermal conductivity are basically formed on the mold copper plates on both of the long side and short side of the continuous casting mold, in the case of a cast slab where the ratio of the long side length of the cast piece to the short side length of the cast piece is significantly large, since a surface crack tends to occur on the long side of the cast piece, it is possible to realize the effect of inhibiting a surface crack from occurring in a cast piece even in the case where the portions 3 filled with a metal of low thermal conductivity are formed only on the mold copper plates on the long side of a mold.
In addition, as illustrated in Fig. 4, it is preferable that a coating layer 6 be formed on the inner wall surface of a mold copper plate on which the portions 3 filled with a metal of low thermal conductivity is formed to prevent abrasion caused by a solidified shell and a surface crack occurring in the mold surface due to a thermal history. It is possible to form the coating layer 6 by performing a plating treatment utilizing commonly used nickel or a nickel-containing alloy such as a nickel-cobalt alloy (Ni-Co alloy) or a nickel-chromium alloy (Ni-Cr alloy). It is preferable that the thickness h of the coating layer 6 be 2.0 mm or less. By controlling the thickness h of the coating layer 6 to be 2.0 mm or less, since there is a decrease in the influence of the coating layer 6 on thermal flux, it is possible to sufficiently realize the effect of a periodic variation in thermal flux caused by the portions 3 filled with a metal of low thermal conductivity. However, in the case where the thickness h of the coating layer 6 is larger than 0.5 times of the filling thickness H of the portions 3 filled with a metal of low thermal conductivity, a periodic variation in thermal flux caused by the portions 3 filled with a metal of low thermal conductivity is inhibited from being provided. Therefore, it is preferable that the thickness h of the coating layer 6 be 0.5 times or less of the filling thickness H of the portions 3 filled with a metal of low thermal conductivity. As long as this condition is satisfied, the thickness of the coating layer 6 may be constant or variable from the upper edge of the mold to the lower edge of the mold. Fig. 4 is a schematic diagram illustrating an example in which a coating layer is formed on the inner wall surface of a mold copper plate on the long side of a mold to protect the surface of the mold.
It is preferable that the continuous casting mold, which is configured as described above, be used when a cast slab (having a thickness of 200 mm or more) of medium-carbon steel having a carbon content of 0.08 mass% to 0.17 mass%, which has high surface crack sensitivity, is manufactured by performing continuous casting. In the case where a cast slab of medium-carbon steel is manufactured by performing continuous casting, a cast piece drawing speed has been generally decreased to inhibit a surface crack from occurring in the cast piece. However, by using the continuous casting mold, which is configured as described above, since it is possible to inhibit a surface crack from occurring in a cast piece, it is possible to manufacture a cast piece with no surface crack or a significantly small number of surface cracks even by performing continuous casting at a cast piece drawing speed of 1.5 m/min or more.
As described above, in the continuous casting mold according to the present invention, plural portions 3 filled with a metal of low thermal conductivity and having a thermal resistance ratio R, which is defined by expression (1), of 5% or more are disposed in the width direction and casting direction of the continuous casting mold in a region including a meniscus position in the vicinity of the meniscus. With this, since the thermal resistance of the continuous casting mold in the vicinity of the meniscus of the continuous casting mold periodically increases and decreases in the width and casting direction of the mold, the thermal flux from a solidified shell to the continuous casting mold in the early solidification stage periodically increases and decreases. As a result of such periodic increase and decrease in thermal flux, since there is a decrease in stress due to δ/γ transformation and in thermal stress, there is a decrease in the amount of deformation of the solidified shell caused by these stresses. As a result of a decrease in the amount of deformation of the solidified shell, since an inhomogeneous distribution of thermal flux caused by the deformation of the solidified shell is homogenized, and since generated stress is dispersed, there is a decrease in the amounts of various strains, which results in a surface crack being inhibited from occurring in the solidified shell.
Here, although an example in which identically shaped portions 3 filled with a metal of low thermal conductivity are disposed in the casting direction or the mold width direction is illustrated in Fig. 1, it is not necessary that identically shaped portions 3 filled with a metal of low thermal conductivity be disposed. As long as the diameter d or circle-equivalent diameter d of the portions 3 filled with a metal of low thermal conductivity is within a range of 2 mm to 20 mm, portions 3 filled with a metal of low thermal conductivity and having various diameters may be disposed in the casting direction or the mold width direction. However, in the case where the diameter d or circle-equivalent diameter d of the portions 3 filled with a metal of low thermal conductivity widely varies from place to place, since solidification delay occurs in a region where the area ratio of the portions 3 filled with a metal of low thermal conductivity is locally high, there is a risk of a surface crack occurring in the region of the cast piece. Therefore, it is preferable that the diameter or the circle-equivalent diameter of the portions be identical.
In addition, although an example in which portions 3 filled with a metal of low thermal conductivity and having an identical filling thickness H are disposed in the casting direction of a mold is illustrated in Fig. 2, it is not necessary that the portions 3 filled with a metal of low thermal conductivity and disposed in the width direction of the mold or in the width direction of the cast piece have an identical filling thickness H, that is, the filling thicknesses H of portions 3 filled with a metal of low thermal conductivity may vary one portion to another. However, it is preferable that all the portions 3 filled with a metal of low thermal conductivity have a filling thickness H of 0.5 mm or more.
Moreover, although an example in which portions 3 filled with a metal of low thermal conductivity are disposed at regular intervals in the casting direction or width direction of a mold is illustrated in Fig. 1, it is not necessary the portions 3 filled with a metal of low thermal conductivity be disposed at regular intervals. However, also in this case, it is preferable that the distance P between the portions filled with a metal of low thermal conductivity satisfy expression (3).
In addition, although a continuous casting mold for a cast slab is described above, the continuous casting mold according to the present embodiments is not limited to a continuous casting mold for a cast slab, that is, it is possible to use the continuous casting mold described above for casting a bloom or a billet.

EXAMPLES

A test was conducted in which molten steel having a chemical composition containing C: 0.05 mass% to 0.25 mass%, Si: 0.10 mass% to 0.35 mass%, Mn: 0.70 mass% to 1.30 mass%, P: 0.010 mass% to 0.030 mass%, S: 0.002 mass% to 0.006 mass%, and Al: 0.02 mass% to 0.05 mass% was made into cast slabs having a long side length of 1500 mm to 2450 mm and a short side length of 220 mm by performing continuous casting utilizing water-cooled copper-alloy continuous casting molds in which portions filled with a metal of low thermal conductivity were formed under various conditions on the inner wall surfaces of copper-alloy mold copper plates on the long sides of the mold and the inner wall surfaces of copper-alloy mold copper plates on the short sides of the mold, and surface cracks occurring in the cast pieces after casting were investigated.
The length from the upper edge to the lower edge of the water-cooled copper-alloy continuous casting mold used for the test was 950 mm, and the position of the meniscus (the upper surface of the molten steel in the mold) when stationary casting is performed was set to be located at a position 100 mm lower than the upper edge of the mold. Circular concave grooves were formed in a region, on the inner wall surface of the mold copper plate, from a position 60 mm lower than the upper edge of the mold to a position located below the set meniscus and at a distance equal to a length L (mm) from the meniscus, and the circular concave grooves were filled with a metal of low thermal conductivity by performing an electroplating treatment. By repeating a process plural times in which, after having performed an electroplating treatment, surface grinding was performed to remove the metal of low thermal conductivity which adhered to portions other than the circular concave grooves, and an electroplating treatment was then performed again, the circular concave grooves were completely filled with the metal of low thermal conductivity to form portions filled with the metal of low thermal conductivity. In this case, a smooth surface was formed so that there was no difference in level between the portions filled with the metal of low thermal conductivity and the surrounding copper-alloy portions (portions in which no such portion filled with the metal of low thermal conductivity was formed). Subsequently, by performing a Ni-Co-alloy plating treatment across the whole inner wall surface of the mold copper plate, a coating layer having a thickness of 0.2 mm at the upper edge of the mold and 2.0 mm at the lower edge of the mold was formed.
As a mold copper plate, two kinds of copper alloys having different thermal conductivities, that is, 298.5 W/(m × K) and 120.0 W/(m × K), were used. As a metal of low thermal conductivity for filling (hereafter, also referred to as a "filling metal"), pure nickel (having a thermal conductivity of 90.5 W/(m × K)), pure cobalt (having a thermal conductivity of 70 W/(m × K)), pure chromium (having a thermal conductivity of 67 W/(m × K)), and pure copper (having a thermal conductivity of 398 W/(m × K)) were used.
In a continuous casting operation, mold powder having a basicity ((mass%CaO)/(mass%SiO₂)) of 1.0 to 1.5 and a viscosity at a temperature of 1300°C of 0.05 Pa·s to 0.20 Pa·s was used. After the continuous casting operation had been finished, a dye penetrant inspection was performed to investigate a state in which surface cracks occurred in a cast piece. By counting the number of the surface cracks having a length of 2 mm or more detected in the dye penetrant inspection, and by defining a value (number/m) calculated by dividing the total number of such surface cracks by the length (m) in the casting direction of the cast piece, for which the number of the surface cracks was investigated, as a surface crack index, the state in which a surface crack occurred was evaluated by the surface crack index.

The construction conditions of the molds and the investigation results of the surface of the cast pieces for test Nos. 1 through 26 are given in Table 1, and the construction conditions of the molds and the investigation results of the surface of the cast pieces for test Nos. 27 through 48 are given in Table 2. Here, in the column "Note" in Table 1 and Table 2, the term "Example" refers to a case where a water-cooled copper-alloy continuous casting mold within the scope of the present invention was used, the term "Comparative Example" refers to a case where a water-cooled copper-alloy continuous casting mold outside the scope of the present invention despite having portions filled with a metal of low thermal conductivity was used, and the term "Conventional Example" refers to a case where a water-cooled copper-alloy continuous casting mold having no portion filled with a metal of low thermal conductivity was used.

[Table 1]

Test No.	Thermal Conductivity λ_c of Mold Copper Plate (W/(m·K))	Thermal Conductivity λ_m of Filling Metal (W/(m·K))	λ_m/λ_c × 100 (%)	Thickness T of Mold Copper Plate (mm)	Diameter d (mm)	Thickness H (mm)	Distance P (mm)	Thermal Resistance Ratio R (%)	Area Ratio S of Filling Metal (%)	Boundary Length Ratio ε (-)	Cast Piece Drawing Speed Vc (m/min)	Length L₀ (mm)	Length L (mm)	Cast Piece Surface Crack Index (number/m)	Note
1	298.5	90.5	30.3	20.0	6	2	4	23.0	32.7	0.218	2.0	66.7	150	0.0	Example
2	298.5	70.0	23.5	20.0	6	2	4	32.6	32.7	0.218	2.0	66.7	150	0.0	Example
3	298.5	67.0	22.4	20.0	6	2	4	34.6	32.7	0.218	2.0	66.7	150	0.0	Example
4	298.5	398.0	133.3	20.0	6	2	4	-2.5	32.7	0.218	2.0	66.7	150	0.7	Comparative Example
5	120.0	90.5	75.4	10.0	8	4	3	13.0	48.0	0.240	2.0	66.7	150	0.0	Example
6	120.0	70.0	58.3	10.0	8	4	3	28.6	18.0	0.240	2.0	66.7	150	0.0	Example
7	120.0	67.0	55.8	10.0	8	4	3	31.6	48.0	0.240	2.0	66.7	150	0.0	Example
8	120.0	398.0	331.7	10.0	8	4	3	-27.9	48.0	0.240	2.0	66.7	150	0.8	Comparative Example
9	120.0	90.5	75.4	10.0	10	1	5	3.3	40.4	0.161	2.5	83.3	200	1.0	Comparative Example
10	120.0	90.5	75.4	10.0	10	2	5	6.5	40.4	0.161	2.5	83.3	200	0.0	Example
11	120.0	90.5	75.4	10.0	10	3	5	9.8	40.4	0.161	2.5	83.3	200	0.0	Example
12	120.0	90.5	75.4	10.0	10	5	5	16.3	40.4	0.161	2.5	83.3	200	0.0	Example
13	120.0	90.5	75.4	10.0	10	7	5	22.8	40.4	0.161	2.5	83.3	200	0.0	Example
14	120.0	90.5	75.4	10.0	10	7	5	22.8	40.4	0.161	2.5	83.3	200	0.0	Example
15	298.5	90.5	30.3	20.0	10	2	5	23.0	40.4	0.161	2.5	83.3	200	0.0	Example
16	298.5	90.5	30.3	20.0	10	5	5	57.5	40.4	0.161	2.5	83.3	200	0.0	Example
17	298.5	90.5	30.3	20.0	10	7	5	80.4	40.4	0.161	2.5	83.3	200	0.0	Example
18	298.5	90.5	30.3	20.0	10	10	5	114.9	40.4	0.161	2.5	83.3	200	0.1	Example
19	298.5	90.5	30.3	20.0	10	15	5	172.4	40.4	0.161	2.5	83.3	200	0.3	Example
20	298.5	90.5	30.3	20.0	2	1	1	11.5	40.4	0.807	2.0	66.7	100	0.1	Example
21	298.5	90.5	30.3	21.0	2	1	1.5	10.9	29.6	0.593	2.0	66.7	100	0.0	Example
22	298.5	90.5	30.3	20.0	2	1	2	11.5	22.7	0.454	2.0	66.7	100	0.0	Example
23	298.5	90.5	30.3	20.0	2	1	4	11.5	10.1	0.202	2.0	66.7	100	0.0	Example
24	298.5	90.5	30.3	20.0	2	1	7	11.5	4.5	0.090	2.0	66.7	100	0.3	Example
25	298.5	90.5	30.3	20.0	20	5	10	57.5	40.4	0.081	2.0	66.7	100	0.0	Example
26	298.5	90.5	30.3	20.0	20	5	20	57.5	22.7	0.045	2.0	66.7	100	0.2	Example

[Table 2]

Test No.	Thermal Conductivity λ_c of Mold Copper Plate (W/(m·K))	Thermal Conductivity λ_m of Filling Metal (W/(m·K))	λ_m/λ_c ×100 (%)	Thickness T of Mold Copper Plate (mm)	Diameter d (mm)	Thickness H (mm)	Distance P (mm)	Thermal Resistance Ratio R (%)	Area Ratio S of Filling Metal (%)	Boundary Length Ratio ε (-)	Cast Piece Drawing Speed Vc (m/min)	Length L₀ (mm)	Length L (mm)	Cast Piece Surface Crack Index (number/m)	Note
27	298.5	90.5	30.3	20.0	1.5	1	1	11.5	32.7	0.872	2.0	66.7	100	0.5	Example
28	298.5	90.5	30.3	20.0	2	1	1	11.5	40.4	0.807	2.0	66.7	100	0.0	Example
29	298.5	90.5	30.3	20.0	5	2	2	23.0	46.3	0.371	2.0	66.7	100	0.0	Example
30	298.5	90.5	30.3	20.0	10	4	4	46.0	46.3	0.185	2.0	66.7	100	0.0	Example
31	298.5	90.5	30.3	20.0	20	5	10	57.5	40.4	0.081	2.0	66.7	100	0.0	Example
32	298.5	90.5	30.3	20.0	25	5	10	57.5	46.3	0.074	2.0	66.7	100	0.6	Example
33	298.5	90.5	30.3	20.0	6	3	1	34.5	66.7	0.445	1.5	50.0	100	0.1	Example
34	298.5	90.5	30.3	20.0	6	3	2	34.5	51.1	0.340	1.5	50.0	100	0.0	Example
35	298.5	90.5	30.3	20.0	6	3	3	34.5	40.4	0.269	1.5	50.0	100	0.0	Example
36	298.5	90.5	30.3	20.0	6	3	5	34.5	27.0	0.180	1.5	50.0	100	0.0	Example
37	120.0	70	58.3	10.0	10	2	4	14.3	46.3	0.185	2.5	83.3	50	0.5	Example
38	120.0	70	58.3	10.0	10	2	4	14.3	46.3	0.185	2.5	83.3	100	0.0	Example
39	120.0	70	58.3	10.0	10	2	4	14.3	46.3	0.185	2.5	83.3	150	0.0	Example
40	298.5	90.5	30.3	20.0	3	2	1	23.0	49.7	0.660	2.0	66.7	150	0.1	Example
41	298.5	90.5	30.3	20.0	3	2	3	23.0	32.4	0.410	2.0	66.7	150	0.0	Example
42	298.5	90.5	30.3	20.0	3	2	6	23.0	21.9	0.290	2.0	66.7	150	0.0	Example
43	298.5	90.5	30.3	20.0	3	2	14	23.0	11.0	0.150	2.0	66.7	150	0.0	Example
44	298.5	90.5	30.3	20.0	3	2	24	23.0	5.1	0.068	2.0	66.7	150	0.3	Example
45	298.5	90.5	30.3	20.0	8&4	4 & 1	0	46.0 & 23.0	50.4	0.302	2.0	66.7	150	0.2	Example
46	298.5	90.5	30.3	20.0	8	4	0	46.0	78.5	0.393	2.0	66.7	150	0.4	Example
47	298.5	-	-	20.0	-	-	-	-	-	-	2.0	66.7	-	1.8	Conventional Example
48	120.0	-	-	10.0	-	-	-	-	-	-	2.0	66.7	-	1.1	Conventional Example

In test Nos. 1 through 8, the influence of the ratio of the thermal conductivity λ_m of the filling metal to the thermal conductivity λ_c of the mold copper plate on a surface crack occurring in a cast piece was investigated. From the results of test Nos. 1 through 8 illustrated in Fig. 5, it is clarified that it was possible to inhibit a surface crack from occurring in a cast piece in the case where the ratio of the thermal conductivity λ_m of the filling metal to the thermal conductivity λ_c of the mold copper plate was 80% or less.
In test Nos. 9 through 19, the influence of the thermal resistance ratio R of the portions filled with the metal of low thermal conductivity to the mold copper plate on a surface crack occurring in a cast piece was investigated. From the results of test Nos. 9 through 19 illustrated in Fig. 6, it is clarified that it was possible to inhibit a surface crack from occurring in a cast piece in the case where the thermal resistance ratio R was 5% or more. However, it is clarified that there was a decrease in the effect of inhibiting a surface crack in the case where the thermal resistance ratio R was more than 100%. Here, from the result of test No. 9, it is clarified that it was not possible to realize the effect of inhibiting a surface crack from occurring in a cast piece unless the thermal resistance ratio R was less than 5% even in the case where the ratio of the thermal conductivity λ_m of the filling metal to the thermal conductivity λ_c of the mold copper plate was 80% or less.
In test Nos. 20 through 26, the influence of the ratio of a total area B (mm²) of all the portions filled with the metal of low thermal conductivity to an area A (mm²) of the region, on the inner wall surface of the mold copper plate, where the portions filled with the metal of low thermal conductivity are formed, that is, an area ratio S on a surface crack occurring in a cast piece, and the influence of the ratio of a total length C (mm) of boundaries between all the portions filled with the metal of low thermal conductivity and the mold copper plate to the area A (mm²), that is, a ratio ε on a surface crack occurring in a cast piece were investigated. From the results of test Nos. 20 through 26 illustrated in Fig. 7, it is clarified that a surface crack was inhibited from occurring in the cast piece in the case where the area ratio S was 10% or more and the ratio ε was 0.07 to 0.60. In the case where the area ratio S was out of the range of 10% or more or in the case where the ratio ε was out of the range of from 0.07 to 0.60, a slight surface crack occurred in the cast piece.
In test Nos. 27 through 32, the influence of the diameter d of portions filled with a metal of low thermal conductivity on a surface crack occurring in a cast piece was investigated. From the results of test Nos. 27 through 32 illustrated in Fig. 8, it is clarified that it was possible to inhibit a surface crack from occurring in a cast piece in the case where the diameter d of portions filled with a metal of low thermal conductivity was 2 mm to 20 mm.
In test Nos. 33 through 36, the influence of the distance P between portions filled with a metal of low thermal conductivity on a surface crack occurring in a cast piece was investigated. It is clarified that a surface crack was inhibited from occurring in the cast piece in the case where the expression "P ≥ 0.25 × d" was satisfied. In the case where the distance P did not satisfy the expression "P ≥ 0.25 × d", a slight surface crack occurred in the cast piece.
In test Nos. 37 through 39, the influence of a length L indicating a region where portions filled with a metal of low thermal conductivity are formed on a surface crack occurring in a cast piece was investigated. It is clarified that a surface crack was inhibited from occurring in the cast piece in the case where the length L was equal to or more than the length L₀, which was calculated from a cast piece drawing speed Vc.
In test Nos. 40 through 46, water-cooled copper-alloy continuous casting molds, in which plural portions filled with a metal of low thermal conductivity were formed on the inner wall surfaces of copper-alloy mold copper plates on the long sides of the molds and inner wall surfaces of copper-alloy plates on the short sides of the molds so that the portions were combined with each other, that is, water-cooled copper-alloy continuous casting molds, in which portions filled with a metal of low thermal conductivity were not separated, were used.
Among such tests, in test Nos. 40 through 44, as illustrated in Fig. 9, the combined portions filled with a metal of low thermal conductivity which were formed by combining three portions filled with a metal of low thermal conductivity having a diameter of 3 mm were disposed with various distances P between the combined portions filled with a metal of low thermal conductivity. Also, in test Nos. 40 through 44, it is clarified that a surface crack was inhibited from occurring in a cast piece in the case where the ratio of the thermal conductivity λ_m of the filling metal to the thermal conductivity λ_c of the mold copper plate was 80% or less, the thermal resistance ratio R was 5% or more, the length L was equal to or more than the length L₀, which was calculated from a cast piece drawing speed Vc, and the diameter d, the distance P, the area ratio S, and the ratio ε satisfy the preferable conditions. In the case where the one of area ratio S, and the ratio ε did not satisfy the preferable condition, a slight surface crack occurred in the cast piece.
In test No. 45, as illustrated in Fig. 10, a water-cooled copper-alloy continuous casting mold in which the portions filled with a metal of low thermal conductivity were disposed with the portions being combined in the width direction of the mold was used. In test No. 46, as illustrated in Fig. 11, a water-cooled copper-alloy continuous casting mold in which the portions filled with a metal of low thermal conductivity were disposed with all the portions being combined in the width direction and casting direction of the mold was used. Here, Fig. 10-(A) and Fig. 11-(A) are schematic side views of a mold copper plate on the long side of a mold on which portions filled with a metal of low thermal conductivity are formed as viewed from the inner wall surface side. Fig. 10-(B) is the Y-Y' cross-sectional view of the mold copper plate on the long side of a mold illustrated in Fig. 10-(A), and Fig. 11-(B) is the Y-Y' cross-sectional view of the mold copper plate on the long side of a mold illustrated in Fig. 11-(A).
In test No. 45, portions filled with a metal of low thermal conductivity having a diameter d of 8 mm, a filling thickness H of 4 mm, and a distance P of 4 mm, between which portions filled with a metal of low thermal conductivity having a diameter d of 4 mm and a filling thickness H of 1 mm were interposed, were disposed in the width direction of each of the mold copper plate on the long side of the mold and the mold copper plate on the short side of the mold. It is considered that, since the filling thickness H of the portions filled with a metal of low thermal conductivity having a diameter of 8 mm was larger than that of the others, stress caused by a decrease in volume when δ/γ transformation occurs or thermal shrinkage was dispersed in the region of the solidified shell corresponding to such portions, which resulted in a decrease in the number of surface cracks occurred in the cast piece.
On the other hand, in test No. 46, it is considered that, since all the portions filled with a metal of low thermal conductivity are combined, solidification delay always occurred at the same position in the solidified shell when continuous casting was performed, which results in stress caused by δ/γ transformation or thermal stress being concentrated at such a position and then a slight surface crack being occurred.
In test Nos. 47 and 48, conventional continuous casting molds in which no portion filled with a metal of low thermal conductivity was formed were used. In test Nos. 47 and 48, many surface cracks occurred in the cast pieces.

Reference Signs List

1 mold copper plate on the long side of a mold
2 concave groove
3 portion filled with a metal of low thermal conductivity
4 slit
5 back plate
6 coating layer

Claims

A continuous casting mold, the mold being a water-cooled continuous casting mold, comprising:
portions (3) filled with a metal of low thermal conductivity formed by filling a plurality of concave grooves (2) with the metal of low thermal conductivity in a region, on an inner wall surface of a mold copper plate (1) which is made of a copper alloy and constitutes the mold, from a position located above a meniscus to a position located below the meniscus, wherein,

a ratio of a thermal conductivity λ_m (W/(m × K)) of the metal of low thermal conductivity to a thermal conductivity λ_c (W/(m × K)) of the mold copper plate (1) is 80% or less, and

a thermal resistance ratio R defined by expression (1) below is 5% or more: $R = \{(T - H) / (1000 \times λ_{c}) + H / (1000 \times λ_{m}) - T / (1000 \times λ_{c})\} / \{T / (1000 \times λ_{c})\} \times 100$
where

R denotes a thermal resistance ratio (%) of the portions (3) filled with the metal of low thermal conductivity to the mold copper plate (1),

T denotes a distance (mm) from a bottom surface of a slit (4) of the mold copper plate (1), which is used as a flow channel of mold-cooling water, to a surface of the mold copper plate (1), and

H denotes a filling thickness (mm) of the metal of low thermal conductivity, wherein

the portions (3) filled with the metal of low thermal conductivity include portions (3) formed by combining plural portions (3) filled with the metal of low thermal conductivity with each other.
The continuous casting mold according to Claim 1, wherein the concave grooves (2) are formed in the region as specified in claim 1 and at a distance equal to or more than a length L₀ (mm) from the meniscus, where L₀ is calculated by expression (2) below from a cast piece drawing speed Vc (m/min): $L_{0} = 2 \times Vc \times 1000 / 60$
The continuous casting mold according to Claim 1 or 2, having a periodic thermal resistance distribution or a periodic thermal flux distribution in the region, on the inner wall surface of the mold copper plate (1), where the portions (3) filled with the metal of low thermal conductivity are formed.
The continuous casting mold according to any one of Claims 1 to 3, wherein,
openings of the concave grooves (2) on the inner wall surface of the mold copper plate (1) have a circular shape or a quasi-circular shape, and
the circular shape has a diameter of 2 mm to 20 mm and the quasi-circular shape has a circle-equivalent diameter of 2 mm to 20 mm.
The continuous casting mold according to Claim 4, wherein a distance between the portions (3) filled with the metal of low thermal conductivity satisfies expression (3) below with respect to the diameter or circle-equivalent diameter of the portions (3) filled with the metal of low thermal conductivity: $P \geq 0.25 \times d$
where
P denotes a distance (mm) between the portions (3) filled with the metal of low thermal conductivity, and

d denotes a diameter (mm) or circle-equivalent diameter (mm) of the portions (3) filled with the metal of low thermal conductivity.
The continuous casting mold according to any one of Claims 1 to 5, wherein,
a ratio of a total area B (mm²) of all the portions (3) filled with the metal of low thermal conductivity to an area A (mm²) of the region, on the inner wall surface of the mold copper plate (1), where the portions (3) filled with the metal of low thermal conductivity are formed, that is, an area ratio S (S = (B/A) × 100) is 10% or more, and
a ratio of a total length C (mm) of boundaries between all the portions (3) filled with the metal of low thermal conductivity and the mold copper plate (1) to the area A (mm²), that is, a ratio ε (ε = C/A), satisfies expression (4) below: $0.07 \leq ε \leq 0.60$
The continuous casting mold according to Claim 6, wherein the portions (3) filled with the metal of low thermal conductivity are separately formed in the casting direction.
The continuous casting mold according to any one of Claims 1 to 7, wherein the concave grooves (2) are filled with the metal of low thermal conductivity by a plating treatment or a thermal spraying treatment.
The continuous casting mold according to any one of Claims 1 to 8, wherein,
a coating layer (6) having a thickness of 2.0 mm or less and containing nickel or an alloy containing nickel is formed on the inner wall surface of the mold copper plate (1), and
the portions (3) filled with the metal of low thermal conductivity are covered with the coating layer.
A method for continuous casting of steel, using the continuous casting mold according to any one of Claims 1 to 9, the method comprising the steps of:
injecting medium-carbon steel having a carbon content of 0.08 mass% to 0.17 mass% into the mold, and

drawing the medium-carbon steel from the mold in a form of a cast slab having a thickness of 200 mm or more at a cast piece drawing speed of 1.5 m/min or more.