Continuous casting mold, continuous casting machine, and continuous casting method

Abstract

A continuous casting mold for a continuous casting process of steel using a mold flux satisfies one or more of [A] a cross-sectional area of a cooling-water channel is smaller in an upper mold part than in a lower mold part, [B] a distance between an end of the cooling-water channel and a mold surface is shorter in the upper mold part than in the lower mold part, and [C] a thermal conductivity of a copper mold plate is higher in the upper mold part than in the lower mold part. This makes it possible to make the cooling performance in the upper mold part, in the vicinity of a molten steel surface, higher than that in the lower part of the mold.

Description

TECHNICAL FIELD

[0001] The present invention relates to a continuous casting mold, a continuous casting machine, and a continuous casting method, in particular, relates to a continuous casting mold for a continuous casting process of steel using a mold flux, a continuous casting machine provided with the continuous casting mold, and a continuous casting method using them.BACKGROUND ART

[0002] In the continuous casting process of steel, a method has been widely used in which molten steel is fed from an intermediate vessel, which is a tundish, into a continuous casting mold using a submerged entry nozzle, and a mold flux is supplied to a molten steel surface inside the mold. A water-cooling copper mold plate is placed on a surface of the continuous casting mold that comes into contact with the molten steel. As illustrated in FIG. 2, molten steel 10 solidifies at a portion in contact with the continuous casting mold 1, forming a solidified shell 13. A mold flux 11 supplied to the molten steel surface in the mold melts upon contact with the high-temperature molten steel 10 and flows into a gap between the continuous casting mold 1 and the solidified shell 13 as a mold flux film 14. The mold flux 11 has a function of keeping the temperature of the molten steel surface. Also, the inflowing mold flux film 14 provides lubrication between the continuous casting mold 1 and the solidified shell 13, and serves to control the heat flux from the solidified shell 13 to the continuous casting mold 1, thereby controlling the cooling of the cast steel surface. Hereinbelow, the continuous casting mold will be also simply referred to as “mold”.

[0003] The method of controlling the cooling of the cast steel surface by controlling the heat flux in the mold often uses mild cooling of the cast steel from the viewpoint of preventing cracks in the cast steel surface. The following two methods are known to slowly cool the cast steel: (1) a method of reducing heat conduction by decreasing the thermal conductivity of a copper mold plate and increasing the temperature of the mold surface in contact with the cast steel to reduce a temperature difference with the cast steel surface; (2) a method of reducing heat flux by promoting crystallization of the flux film formed from molten flux flowing into a gap between the mold and the cast steel and obtaining a shielding effect against radiation heat transfer and an inhibitory effect against heat conduction provided by fine voids.

[0004] There are many examples of known techniques that fall under (1), such as Patent Literatures 1 to 7. Patent Literature 1 discloses a method of disposing a sheet with low thermal conductivity in an upper part of a mold. Patent Literature 2 discloses a method of separating a cooling slit for an upper part of a mold from a surface on a molten steel side. Patent Literature 3 discloses a method of separating cooling water systems for upper and lower parts of a mold to reduce cooling in the upper part. Patent Literature 4 discloses a method of providing holes in an upper part of a mold. Patent Literature 5 discloses a method of providing multiple cooling water systems in a horizontal direction of a mold to reduce cooling in the upper part. Patent Literature 6 discloses a method of providing a sprayed layer with low thermal conductivity on a surface of an upper part of a mold. Patent Literature 7 discloses a method of embedding a heating element in an upper part of a mold. These techniques are expected to be effective to a certain extent when only the heat conduction in the mold is considered. However, the problem is that an increase in the mold surface temperature inhibits the crystallization of the mold flux film, and therefore may be counterproductive to method (2). Furthermore, an increase in the mold surface temperature may cause problems such as sticking of the cast steel to the mold surface, cracks in the mold surface, or deformation of the copper mold plate, making it difficult to put these techniques into practical use.

[0005] Examples of known techniques that fall under (2) include as follows. Patent Literatures 8 and 9 disclose a method for reducing the heat flux in a mold by causing cuspidine to crystallize and precipitate in a mold flux film. Patent Literature 10 discloses a method of causing a small amount of perovskite having a high melting point as a single substance to crystallize and precipitate, and using it as a nucleus to promote precipitation of melilite that is a main crystal. These methods are effective in mild cooling of the cast steel surface and preventing cracks, and are widely used. On the other hand, the control of crystallization of the mold flux film relies on composition designs of the mold flux, and there has been no idea of combining the composition design of the mold flux with the control of the mold surface temperature to increase the freedom of crystallization control and maximize its effect.

[0006] The methods described in Patent Literatures 8, 9, and 10, which are effective in mild cooling of the cast steel and preventing cracks, are widely used. Although the crystallization of the flux film is effective in inhibiting cracking, excessive crystallization not only causes poor lubrication in the mold, but also increases the risk of bulging due to reduced heat removal from the cast steel, breakout (BO), or false detection of BO, which can be factors that hinder good operation. In other words, in order to inhibit cracking of the cast steel, it is desirable to use a mold flux that does not simply cause the crystalline phase of the flux film to crystallize and precipitate, but that causes instant crystallization and precipitation in the upper part of the mold while satisfying an appropriate crystallization ratio through the lower part of the mold. To meet this demand, mold fluxes with adjusted crystallization speeds and solidification temperatures have been developed (Patent Literatures 11, 12, and 13).

[0007] In all of the inventions described in Patent Literatures 1 to 7, the cast steel is slowly cooled in the upper part of the mold, in the vicinity of the meniscus, in order to prevent surface cracks of the cast steel, and the cooling performance is lower in the upper part of the mold than in the lower part of the mold. In contrast, when considering high-speed continuous casting, there is a technical idea of strengthening heat removal in the upper part of the mold in order to strengthen the cooling of the cast steel in the upper part of the mold. Since it is also necessary for the high-speed continuous casting to prevent overcooling in the lower part of the mold, there are known inventions for high-speed continuous casting that strengthen the cooling in the upper part of the mold while preventing the overcooling in the lower part of the mold (Patent Literatures 14 and 15). Patent Literature 14 discloses a mold in which a cooling surface of the mold is divided into two parts, upper and lower parts, with the upper part having a spray cooling structure and the lower part having a forced water flow cooling structure in which cooling water is passed parallel to the cooling surface. Patent Literature 15 discloses a mold for continuous casting having a divided structure in which upper and lower molds having the divided structure include a water-cooling copper plate. The upper mold is of a jet heat transfer type with multiple horizontal slit-shaped water channels, and the lower mold is of a turbulent heat transfer type with multiple vertical slit-shaped water channels.CITATION LISTPatent Literature(s)

[0008] Patent Literature 1: JPS61-195742 A

[0009] Patent Literature 2: JPS61-195746 A

[0010] Patent Literature 3: JPH01-143742 A

[0011] Patent Literature 4: JPH02-197352 A

[0012] Patent Literature 5: JPH02-200353 A

[0013] Patent Literature 6: JPH08-267182 A

[0014] Patent Literature 7: JP 2000-202583 A

[0015] Patent Literature 8: JPH11-320058 A

[0016] Patent Literature 9: JP 2000-158105 A

[0017] Patent Literature 10: JP 2010-214387 A

[0018] Patent Literature 11: JP 2006-247744 A

[0019] Patent Literature 12: JP 2020-146719 A

[0020] Patent Literature 13: JP 2021-074782 A

[0021] Patent Literature 14: JPH07-314096 A

[0022] Patent Literature 15: JPH10-128513 ANon-Patent Literature(s)

[0023] Non-Patent Literature 1: The Japan Society of Mechanical Engineers, “JSME, Journal of the Heat Transfer Society of Japan”

[0024] Non-Patent Literature 2: Hanao et, al. “Mold Flux for High Speed Continuous Casting of Hypoperitectic Steel Slabs” Tetsu-to-Hagane, Vol. 88 (2002) No. 1 pp. 23-28SUMMARY OF THE INVENTIONProblem(s) to be Solved by the Invention

[0025] As described above, in order to prevent the cast steel surface from cracking, it is important to control the heat flux in the mold to control the cooling of the cast steel surface. The control can be performed through the multiple means described in (1) and (2) above, and each of them is expected to have a certain effect. However, these two mechanisms have been considered as individual inventions without any discussion of their interaction.

[0026] The present invention has been made to achieve closer-to-ideal heat flux control in the mold than ever before, by taking into consideration the interaction between the cooling function of the mold itself and the heat flux control in the mold via the mold flux film, and by implementing both in an appropriate combination. An object of the invention is to provide a continuous casting mold for a continuous casting process of steel using a mold flux, a continuous casting machine provided with the continuous casting mold, and a continuous casting method performed using them.Means for Solving the Problems

[0027] That is, the gist of the invention is as follows.

[0028] [1] A continuous casting mold for a continuous casting process of steel using a mold flux, a side of the continuous casting mold that comes into contact with molten steel configured including a water-cooling copper mold plate, a cooling-water channel being disposed in the copper mold plate so that cooling water flows in a vertical direction of the mold, the cooling-water channel being connected to an upper water supply and drainage channel at an upper end of the mold and to a lower water supply and drainage channel at a lower end of the mold, and one or more of [A], [B], and [C] below being satisfied,

[0029] [A] a cross-sectional area of the cooling-water channel is smaller in an upper part of the mold than in a lower part of the mold,

[0030] [B] a distance between an end of the cooling-water channel and a mold surface is shorter in the upper part of the mold than in the lower part of the mold, where the end of the cooling-water channel refers to an end of the cooling-water channel on a mold surface side, and

[0031] [C] a thermal conductivity of the copper mold plate is higher in the upper part of the mold than in the lower part of the mold.

[0032] [2] A continuous casting machine including the continuous casting mold according to [1], in which when a heat flux Q calculated by formula (1) is given according to a set casting speed Vc of the continuous casting machine, regarding a mold surface temperature Ts calculated by formula (2), a mold surface temperature TsU in the upper part of the mold is lower by 20 degrees C. or more than a mold surface temperature TsL in the lower part of the mold,[Formula⁢ 1]Q=1.16×106·VC0.7(1)[Formula⁢ 2]Ts=(Xλm+4·A0.023Re0.8·Pr0.4·λw·L)⁢Q+Tw(2)Re=Vw / (η / ρ)(3)Pr=η⁢CP / λw(4)d=4⁢A / L(5)where, Q: heat flux [W / m2], Vc: set casting speed [m / min], Ts: mold surface temperature [degrees C.], Tw: cooling water temperature [degrees C.], X: distance between end of cooling-water channel and mold surface [m], λm: thermal conductivity of copper mold plate [W / (m·K)], λw: thermal conductivity of cooling water [W / (m·K)], A: cross-sectional area of cooling-water channel [m2], L: perimeter of cooling-water channel [m], Re: Reynolds number of cooling water in cooling-water channel [-], Pr: Prandtl number of cooling water in cooling-water channel [-], d: equivalent diameter [m], Vw: flow velocity of cooling water in cooling-water channel [m / s], η: viscosity of water [Pa·s], ρ: density of water [kg / m3], CP: specific heat of water [J / (kg·K)]).[3] A continuous casting method, including performing continuous casting of steel using a continuous casting machine provided with the continuous casting mold according to [1] or the continuous casting machine according to [2], while supplying a mold flux into the continuous casting mold.

[0035] [4] The continuous casting method according to [3], in which a mold flux having a crystallization ratio of 20% or more in terms of area ratio when melted, cooled at a cooling rate of 10 degrees C. / min and solidified, is used as the mold flux.

[0036] [5] The continuous casting method according to [4], in which a mold flux in which a main crystal of the solidified mold flux is cuspidine and a viscosity at 1,300 degrees C. is less than 1.5 poise, is used.

[0037] [6] The continuous casting method according to [3], in which a mold flux in which: a CaO′ / SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; Li2O is contained, among alkali metal oxides, at 1 mass % or more and 15 mass % or less; and a solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less, is used as the mold flux,

[0038] CaO′ above being determined by formula (6) and formula (7),CaO’⁢(mass⁢ %)=T·CaO-CaF2’×0.718(6)CaF2’⁢(mass⁢ %)=(F-Li2⁢O×1.27-Na2⁢O×0.613-K2⁢O×0.403)×2.05(7)where:in a case where a right side of the formula (7) is negative, a left side of the formula (7) is set to 0%,

[0041] F: content ratio (mass %) of F in mold flux,

[0042] a total content of components in the mold flux excluding C is taken as 100 mass %, and a content of each of the components is determined, and

[0043] T·CaO, Li2O, Na2O, and K2O refer to contents of oxides (mass %) calculated assuming that Ca, Li, Na, and K in the mold flux are all oxides.

[0044] [7] The continuous casting method according to [3], in which a mold flux in which: a CaO′ / SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; among alkali metal oxides, Li2O is contained at 1 mass % or more and 10 mass % or less and Na2O is contained at 1 mass % or more and 10 mass % or less; and a solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less, is used as the mold flux,

[0045] CaO′ above being determined by formula (6) and formula (7),CaO’⁢(mass⁢ %)=T·CaO-CaF2’×0.718(6)CaF2’⁢(mass⁢ %)=(F-Li2⁢O×1.27-Na2⁢O×0.613-K2⁢O×0.403)×2.05(7)where:in a case where a right side of the formula (7) is negative, a left side of the formula (7) is set to 0%,

[0048] F: content ratio (mass %) of F in mold flux,

[0049] a total content of components in the mold flux excluding C is taken as 100 mass %, and a content of each of the components is determined, and

[0050] T·CaO, Li2O, Na2O, and K2O refer to contents of oxides (mass %) calculated assuming that Ca, Li, Na, and K in the mold flux are all oxides.BRIEF DESCRIPTION OF DRAWINGS

[0051] FIG. 1 is a plan cross-sectional view taken along line B-B of FIG. 6, illustrating a partial cross-section of a continuous casting mold.

[0052] FIG. 2 schematically illustrates a cross section inside the mold during continuous casting.

[0053] FIG. 3 is a sectional side view illustrating a partial cross-section of the continuous casting mold in an exemplary embodiment in which a distance XU between an end of a cooling-water channel and a mold surface in an upper part of the mold is shorter than a distance XL between the end of the cooling-water channel and the mold surface in a lower part of the mold.

[0054] FIG. 4 is a sectional side view illustrating a partial cross-section of the continuous casting mold in an exemplary embodiment in which a cross-sectional area of the cooling-water channel is smaller in the upper part of the mold than in the lower part of the mold.

[0055] FIG. 5 is a sectional side view illustrating a partial cross-section of the continuous casting mold in an exemplary embodiment in which a thermal conductivity of a copper mold plate is larger in the upper part of the mold than in the lower part of the mold.

[0056] FIG. 6 is a sectional side view taken along line A-A of FIG. 1, illustrating a partial cross-section of the continuous casting mold.

[0057] FIG. 7 is a schematic plan view of the entire continuous casting mold as seen from above.

[0058] FIG. 8 is a schematic side view of a continuous casting machine.DESCRIPTION OF EMBODIMENT(S)

[0059] The invention relates to a technique for a heat flux control in a mold for continuous casting of steel, which utilizes a mold flux to maximize an effect of inhibiting heat conduction by shielding radiation heat transfer and by forming fine voids with crystals precipitating in a flux film formed from a molten flux flowing into a gap between the mold and a cast steel, thereby mild cooling a surface of the cast steel.

[0060] A continuous casting mold 1 used for continuous casting of steel, in particular, a continuous casting mold for a continuous casting machine for casting slabs, is typically configured by combining two opposing wide faces 31 and two opposing narrow faces 32, as illustrated in FIG. 7. As illustrated in FIG. 1, a copper mold plate 2 formed from copper with excellent thermal conductivity is provided for the wide and narrow faces of the continuous casting mold 1 that come into contact with molten steel. Water-cooling is performed from a rear side of the copper mold plate 2 to keep the mold surface temperature in a steady state at approximately 300 degrees C. or less. Accordingly, it is possible to prevent sticking of the cast steel to a mold surface, softening or deformation of the copper material. In molds with a relatively large cross section for a continuous slab caster, a continuous bloom caster, and the like, a long and narrow vertical slit is typically formed on the rear side of the copper mold plate 2, and this slit and a back frame 4 on the rear side of the copper mold plate 2 form a cooling-water channel in the mold 3. As illustrated in the sectional side view of FIG. 6, the cooling-water channel 3 is connected to an upper water supply and drainage channel 8 at an upper end of the mold and a lower water supply and drainage channel 9 at a lower end of the mold, and is arranged to circulate the cooling water in the vertical direction of the mold. The cooling water supplied from the lower water supply and drainage channel 9 flows in the cooling-water channel 3 and is discharged from the upper water supply and drainage channel 8. As illustrated in the plan cross-sectional view of FIG. 1, many cooling-water channels 3 are arranged in a width direction 25 to increase the surface area of the cooling-water channels 3 in contact with the cooling water, thereby obtaining sufficient cooling performance.

[0061] As described in the Background Art section, there have been many ideas for making the cooling performance of the mold different in its vertical direction from the viewpoint of preventing cracks in the surface of the cast steel. All of these ideas aim to slow down the cooling of an initial solidified shell in the upper part of the mold by making the cooling performance in the upper part of the mold lower than that in the lower part of the mold. To achieve that purpose, the above-described two methods have been addressed: (1) a method of reducing heat conduction by decreasing the thermal conductivity of the copper mold plate and increasing the temperature of the mold surface to reduce a temperature difference between the mold surface and the cast steel surface; and (2) a method of reducing heat flux from the cast steel to the mold by promoting crystallization of the mold flux film flowing into a gap between the mold and the cast steel and obtaining the shielding effect against radiation heat transfer and the inhibitory effect against heat conduction provided by fine voids.

[0062] However, according the actual measurement of the growth rate of the initial solidified shell and the heat flux in the mold during continuous casting, it has been found out that if a mold flux that easily crystallizes is used and the cooling performance in the upper part of the mold is reduced to increase the mold surface temperature, the initial solidified shell may be strongly cooled, which is the opposite of what the invention intended. This is presumably because the crystallization of the mold flux film in the gap between the mold and the cast steel is inhibited. On the other hand, it has been found that if a mold flux that easily crystallizes is used and the cooling performance in the upper part of the mold is increased to decrease the mold surface temperature, the initial solidified shell is cooled slowly. This is presumably because the crystallization of the mold flux film in the gap between the mold and the cast steel is promoted. The inventors have found this phenomenon, which may be said to be cooling paradox, and reached a method for achieving a closer-to-ideal heat flux control in the mold by utilizing this phenomenon while solving the problems of the conventional techniques.

[0063] Features of the invention will be described below along with the above-described configurations of the invention.

[0064] First, the upper part and the lower part of the continuous casting mold will be described with reference to FIGS. 1 and 2. As illustrated in FIG. 2, an upper mold part 21 refers to an upper part of the continuous casting mold 1, which at least ranges from an upper end 23 of the mold to a position 0.2 m below the upper end 23. A lower mold part 22 refers to a lower part of the continuous casting mold 1, which is an area at least below a position 0.6 m below the upper end 23 of the mold. A lower end of the upper mold part 21 is preferably located at a position higher than a position 0.3 m below the upper end 23 of the mold.

[0065] A cross-sectional area of the cooling-water channel 3 in the upper mold part 21 being smaller than that of the cooling-water channel 3 in the lower mold part 22 means, but is not limited to, that the cross-sectional area of the cooling-water channel 3 in the upper mold part 21, i.e., the upper part of the continuous casting mold 1 at least ranging from the upper end 23 of the mold to a position 0.2 m below the upper end 23 of the mold, is smaller than that of the cooling-water channel 3 in the lower mold part 22, i.e., the lower part of the continuous casting mold 1 that is an area at least below the position 0.6 m below the upper end 23 of the mold. Therefore, for example, a cross-sectional area of the cooling-water channel 3 in a range from the upper end 23 of the mold to the position 0.3 m below the upper end 23 of the mold may be smaller than that of the cooling-water channel 3 in an area at least below the position 0.5 m below the upper end 23 of the mold.

[0066] The cross-sectional area of the cooling-water channel 3 in the upper mold part 21 refers to an area of a cross section formed when the cooling-water channel 3 in the upper mold part 21 is cut in a horizontal direction (see FIG. 1).First Exemplary Embodiment

[0067] A first exemplary embodiment of the invention will be described with reference to FIGS. 1 to 6.

[0068] The first exemplary embodiment of the invention relates to a continuous casting mold 1 for a continuous casting process of steel using a mold flux, in which a side of the continuous casting mold 1 that comes into contact with molten steel is configured including the water-cooling copper mold plate 2, the cooling-water channel 3 arranged in the copper mold plate 2 allows cooling water to flow in a vertical direction of the mold, and the cooling-water channel 3 is connected to the upper water supply and drainage channel 8 at the upper end of the mold and the lower water supply and drainage channel 9 at the lower end of the mold, and one or more of the following [A], [B], and [C] are satisfied between the upper part of the mold and the lower part of the mold.

[0069] [A] A cross-sectional area A of the cooling-water channel 3 in the upper mold part 21 is smaller than the cross-sectional area A of the cooling-water channel 3 in the lower mold part 22. In an example illustrated in FIG. 4, for a depth D of the cooling-water channel, a depth DU in the upper part of the mold is smaller than a depth DL in the lower part of the mold, so that a cross-sectional area AU in the upper part of the mold is smaller than a cross-sectional area AL in the lower part of the mold.

[0070] [B] A distance XU between a cooling-water channel end 7 and a mold surface 6 in the upper mold part 21 is shorter than a distance XL between the cooling-water channel end 7 and the mold surface 6 in the lower mold part 22 (see FIG. 3). The cooling-water channel end 7 refers to an end of the cooling-water channel 3 on a mold surface side.

[0071] [C] A thermal conductivity λmU of the copper mold plate 2 in the upper mold part 21 is larger than a thermal conductivity λmL of the copper mold plate 2 in the lower mold part 22 (see FIG. 5).

[0072] By satisfying one or more of the above [A], [B], and [C], it is possible to make the cooling performance in the vicinity of a molten steel surface in the upper part of the mold higher than that in the lower part of the mold. The cooling-water channel is connected to the upper water supply and drainage channel at the upper end of the mold and the lower water supply and drainage channel at the lower end of the mold, and the flow rate of cooling water in the cooling-water channel is constant from the upper part of the mold to the lower part of the mold. Therefore, it is possible to make the flow velocity of the cooling-water in the upper mold part 21 faster than that in the lower mold part 22 by making the cross-sectional area AU of the cooling-water channel 3 in the upper mold part 21 smaller than the cross-sectional area AL of the cooling-water channel 3 in the lower mold part 22.

[0073] The continuous casting mold according to the first exemplary embodiment is characterized in that the cooling performance in the vicinity of the molten steel surface in the upper part of the mold is made higher than the cooling performance in the lower part of the mold in order to maximize the effect of mild cooling caused by crystallization of a mold flux film, that is, the effect of reducing heat flux from a cast steel to the mold.

[0074] If the mild cooling effect accompanying the crystallization of the mold flux film is required over the entire vertical direction of the mold, including the upper part and the lower part of the mold defined above, as in the conventional method, it is sufficient to increase the overall cooling performance from the upper part to the lower part of the mold. The reason why the cooling performance in the upper part of the mold is made higher than the cooling performance in the lower part of the mold in the exemplary embodiment of the invention, is as follows.

[0075] Since the mold is used to cool and solidify molten steel to form a cast steel, the mold basically needs to provide strong cooling. Therefore, even if mild cooling is necessary to prevent cracks in a surface of the cast steel, infinite mild cooling leads to the denial of the original mold function, and thus mild cooling should be kept to the minimum necessary. The mild cooling required to prevent surface cracking of the cast steel is required only in the vicinity of the molten steel surface in the upper part of the mold, specifically, only above a position 50 mm to 200 mm below the molten steel surface. For a part of the mold lower than the above, excessive mild cooling should be avoided.

[0076] From the cooling paradox phenomenon via the mold flux film described above, there is a possibility that the crystallization of the mold flux film can be promoted by increasing the cooling performance in the vicinity of the molten steel surface in the upper part of the mold, and the amount of heat removed from a solidified shell can be reduced. On the other hand, for the lower part of the mold, there is a possibility that the amount of heat removed from the solidified shell can be increased by inhibiting the excessive crystallization of the mold flux film by reducing the cooling performance. If such a phenomenon is achievable, it would result in the moderate mild cooling in the vicinity of the molten steel surface in the upper part of the mold and the maintaining of sufficient heat flux in the lower part of the mold. In view of the above, the continuous casting of steel was performed using the continuous casting mold according to the first exemplary embodiment and a mold flux that easily crystallizes. This revealed that the cooling of the cast steel in the upper part of the mold was reduced and the cooling of the cast steel in the lower part of the mold was increased compared to a case where a conventional continuous casting mold was used. Details will be described in Examples below. According to a continuous casting method using a continuous casting machine provided with the continuous casting mold according to the first exemplary embodiment, the surface temperature of the copper plate in the vicinity of the molten steel surface in the upper part of the mold is kept low. This results in the secondary effect of inhibiting sticking of the cast steel to the mold surface, cracks in the mold surface, and deformation of the copper mold plate.

[0077] In the exemplary embodiment, it is possible to make the cooling performance in the vicinity of the molten steel surface in the upper mold part 21 higher than the cooling performance in the lower mold part 22 by satisfying one or more of the above [A], [B], and [C]. The cooling-water channel 3 is connected to the upper water supply and drainage channel 8 at the upper end of the mold and the lower water supply and drainage channel 9 at the lower end of the mold, and the flow rate of cooling water in the cooling-water channel 3 is constant from the upper part to the lower part of the mold. Thus, it is possible to make the flow velocity of the cooling water different between the upper part of the mold and the lower part of the mold (see FIG. 4) by making the cross-sectional area A of the cooling-water channel 3 different between the upper part of the mold and the lower part of the mold. These measures can be implemented at low cost and are effective enough for the exemplary embodiment.

[0078] Since the exemplary embodiment utilizes the cooling paradox phenomenon via the mold flux film, the exemplary embodiment is applicable only to the continuous casting using the mold flux.Second Exemplary Embodiment

[0079] FIG. 8 illustrates a general overall view of a continuous casting machine 41 for casting steel, in particular, the continuous casting machine 41 for casting slabs. Molten steel is injected into the continuous casting mold 1 from a tundish 42 through a submerged entry nozzle 5, and a cast steel 45 in which solidification has progressed is drawn from a lower part of the continuous casting mold 1 and pulled out while being supported by support rolls 43. The continuous casting machine 41 illustrated in FIG. 8 is of a vertical bending type. A casting distance from a surface of the molten steel in the continuous casting mold 1 to the most downstream support roll 43 is called a machine length 44.

[0080] The second exemplary embodiment relates to a continuous casting machine provided with the continuous casting mold 1 described in the first exemplary embodiment, characterized in that when a heat flux Q determined by the formula (1) is given according to a set casting speed Vc of the continuous casting machine, a mold surface temperature Ts calculated by the formula (2) in the upper mold part 21 is lower by 20 degrees C. or more than that in the lower mold part 22.

[0081] The second exemplary embodiment relates to a continuous casting machine provided with the continuous casting mold of the first exemplary embodiment, and specifies more specifically the contents described in the first exemplary embodiment. In the exemplary embodiment, the crystallization of the mold flux is affected by the mold surface temperature. The behavior of heat conduction and heat transfer from the mold surface to the cooling water can be calculated as follows.

[0082] As illustrated in FIG. 1, the continuous casting mold 1 typically has the slit-shaped cooling-water channels 3 with width W, depth D, and cross-sectional area A on the rear side of the copper mold plate 2. Cooling water flows through the cooling-water channels 3 at a velocity of 5 m / s to 10 m / s. As illustrated in FIG. 6, the cooling-water channel 3 arranged in the copper mold plate 2 allows the cooling water to flow in the vertical direction of the mold, and the cooling-water channel 3 is connected to the upper water supply and drainage channel 8 at the upper end of the mold and the lower water supply and drainage channel 9 at the lower end of the mold. Generally, the cooling water is flowed from bottom to top. The interval between the cooling-water channels in the width direction 25 of the mold is not specified here, but the cooling-water channels are ordinarily arranged densely enough that unevenness in the mold surface temperature is acceptable. Specifically, the interval between the cooling-water channels is within a range not exceeding three times the width W of the cooling-water channel. Although the above range may be exceeded in some places due to structural constraints, the effect on the overall cooling performance is small.

[0083] The heat transfer from the mold surface 6 to the end of the cooling-water channel 3 on the mold surface side is controlled by the thermal conductivity λm of the copper mold plate material. Here, the end of the cooling-water channel 3 on the mold surface side is referred to as the cooling-water channel end 7. The distance between the cooling-water channel end 7 and the mold surface 6 is denoted by X (see FIG. 1). There may be a thin plating layer (typically, a thickness of approximately tens to one hundred micrometers) on the mold surface, but its effect is negligible. Then, a heat transfer coefficient hm between the mold surface 6 and the cooling-water channel end 7, which is used in the following formula (8), is given by the following formula (9).Ts=(1 / hm)⁢Q+Tf(8)hm=λm / X(9)where, Ts: mold surface temperature, Q: heat flux, Tr: mold surface temperature in contact with cooling-water channel, λm: thermal conductivity of copper mold plate, and X: distance from cooling-water channel end 7 to mold surface 6 (see FIG. 1).

[0085] The heat transfer between the cooling water in the cooling-water channel 3 and the copper mold plate 2 is represented by the following formula (10) using a heat transfer coefficient hw between the mold and the cooling water, the heat flux Q, and a cooling water temperature Tw. The heat transfer coefficient hw between the mold and the cooling water is represented by the following formula (11) using a Nusselt number Nu.Tf=(1 / hw)⁢Q+Tw(10)hw=Nu×λw / d(11)

[0086] For determining the Nusselt number, there are many empirical formulas related to turbulent heat transfer in pipes. In an experiment for the cooling behavior of the continuous casting mold, it was found that an accurate calculation was possible by obtaining the heat transfer coefficient hw between the mold and cooling water substituted into the formula (11) through a Dittus-Boelter empirical formula (Nu=0.023Re0.8Pr0.4 (12)) (see, for example, Non-Patent Literature 1).

[0087] Here, a Reynolds number Re is defined by the formula (3), and a Prandtl number Pr is defined by the formula (4). In the formula (10), λw is a thermal conductivity of water. An equivalent diameter d in the formulae (3) and (11) is defined as an equivalent diameter d defined by the formula (5). The Nusselt number Nu is a dimensionless number indicating a magnitude of convective heat transfer relative to conductive heat transfer, the Reynolds number is a dimensionless number indicating intensity of turbulence, and the Prandtl number Pr is a dimensionless number indicating a ratio between a thickness of a velocity boundary layer and a thickness of a temperature boundary layer.

[0088] In the Reynolds number defined by the formula (3), Vw: flow velocity of cooling water in cooling-water channel, d: equivalent diameter, v: kinetic viscosity of water, and v=η / p (η: viscosity of water, p: density of water). In the Prandtl number defined by the formula (4), η: viscosity of water, λw: thermal conductivity of water, and CP: specific heat of water. The flow velocity Vw of the cooling water in the cooling-water channel is determined by dividing the total flow rate of the cooling water of the entire mold by the product of the cross-sectional area A of the cooling-water channel and the total number of the cooling-water channels. The total flow rate of the cooling water of the entire mold can be evaluated as a measurement value of a flow meter installed on a mold inlet or outlet side of cooling water piping, and is typically controlled to a constant value throughout the casting period. The equivalent diameter d defined by the formula (5) is calculated using the cross-sectional area A of the cooling-water channel and the perimeter of the cooling-water channel L=2 (W+D), and can be obtained regardless of the cross-sectional shape of the cooling-water channel. At this time, the heat transfer coefficient hw between the mold cooling water and the copper mold plate is obtained by substituting the formula (5) into the formula (11), as follows: hw=Nu×λw / (4A / L) (13).

[0089] The heat transfer from the mold surface 6 to the cooling water in the cooling-water channel 3 is represented by the following formula (14). A heat transfer coefficient hm-w can be calculated, using the heat transfer coefficient hm between the mold surface 6 and the cooling-water channel end 7 and the heat transfer coefficient hw between the mold and the cooling water, by substituting the formula (10) into the formula (8) and eliminating Tr, as represented by the following formula (15).Ts=(1 / hm-w)⁢Q+Tw(14)1 / hm-w=1 / hm+1 / hw(15)

[0090] Using these relationships, substituting the formula (15) into the formula (14), substituting the formula (9) into hm in the formula (14), and substituting the formulae (11) and (13) into hw in the formula (14), the mold surface temperature Ts for the amount of heat transferred from the cast steel to the cooling water, i.e., heat flux Q, can be determined as represented by the formula (2).

[0091] For the heat flux Q in the formula (2), the formula (1), which is an empirical formula for the casting speed, is used. Q in the formula (1) represents an average heat flux from a meniscus portion of the mold to the lower end of the mold. The heat flux in the mold during casting differs between the upper part of the mold and the lower part of the mold, and the heat flux is larger in the upper part of the mold where the surface temperature of cast steel is higher. Here, the difference in cooling performance between the upper part of the mold and the lower part of the mold is evaluated, so for the heat flux Q in the formula (2), the value of the formula (1), which depends only on the casting speed and does not depend on the mold part, is used.

[0092] The set casting speed Vc in the formula (1) is a representative casting speed of the continuous casting machine used, and refers to a casting speed between 0.7 and 0.8 times the maximum casting speed calculated from the cast steel thickness and the machine length 44 of the continuous casting machine 41 (see FIG. 8).

[0093] Here, for the mold surface temperature Ts calculated by the formula (2), the mold surface temperature in the upper mold part 21 is denoted by TsU, and the mold surface temperature in the lower mold part 22 is denoted by TsL.

[0094] The second exemplary embodiment relates to the continuous casting machine provided with the continuous casting mold characterized in that, with regard to the mold surface temperature Ts calculated by the formula (2), the mold surface temperature TsU in the upper mold part 21 is lower by 20 degrees C. or more than the mold surface temperature TsL in the lower mold part 22. That is, when ATs is defined by the following formula (16), each parameter used in the formula (2) (Q: heat flux [W / m2], Vc: set casting speed [m / min], Ts: mold surface temperature [degrees C.], Tw: cooling water temperature [degrees C.], X: distance between cooling-water channel end and mold surface [m], λm: thermal conductivity of copper mold plate [W / (m·K)], λw: thermal conductivity of cooling water [W / (m·K)], A: cross-sectional area of cooling-water channel [m2], L: perimeter of cooling-water channel [m], Re: Reynolds number of cooling water in cooling-water channel [-] (formula (3)), Pr: Prandtl number of cooling water in cooling-water channel [-] (formula (4)), d: equivalent diameter [m] (formula (5)), Vw: flow velocity of cooling water in cooling-water channel [m / s], η: viscosity of water [Pa·s], ρ: density of water [kg / m3], CP: specific heat of water [J / (kg·K)]) is adjusted so that ΔTs is 20 degrees C. or more.Δ⁢TS=TsL-TsU(16)

[0095] When the conditions under which ATs in the above formula (16) is 20 degrees C. or more are satisfied, if a mold flux that easily crystallizes is used for continuous casting of steel, the crystallization of the mold flux film is promoted in the upper mold part 21, and the cast steel surface can be cooled slowly. At the same time, excessive crystallization of the mold flux film in the lower mold part 22 can be inhibited, and the growth of the solidified shell can be promoted. Furthermore, it is possible to prevent sticking of the cast steel to the mold surface, prevent cracks in the mold surface, and inhibit deformation of the copper mold plate.

[0096] The right side of the formula (2) includes the cooling water temperature Tw. In a typical continuous casting mold, the cooling water supplied from the lower water supply and drainage channel 9 flows from bottom to top in the cooling-water channel 3 and is discharged from the upper water supply and drainage channel 8. Since the cooling water temperature increases as the cooling water passes through the cooling-water channel 3, a cooling water temperature TwU in the upper mold part 21 is higher than a cooling water temperature TwL in the lower mold part 22. If the flow direction of the cooling water is reversed, the cooling water temperature TwL in the lower mold part 22 becomes higher than the cooling water temperature TwU in the upper mold part 21. Therefore, although the relationship between the cooling water temperature in the upper mold part 21 and the cooling water temperature in the lower mold part 22 is not a direct controlling factor in the exemplary embodiment, making the flow direction of the cooling water in the cooling-water channel 3 from top to bottom (see FIG. 5) contributes to increasing ΔTs calculated by the formula (16). The cooling water temperature can be evaluated using measurement values of water temperature meters (thermocouples) installed on the mold inlet and outlet sides of the cooling water piping.Third Exemplary Embodiment

[0097] A third exemplary embodiment relates to a continuous casting method characterized by performing continuous casting of steel using a continuous casting machine provided with the continuous casting mold described in the first exemplary embodiment or the continuous casting machine described in the second exemplary embodiment, while supplying a mold flux into the continuous casting mold.Mold Fluxes in Fourth and Fifth Exemplary Embodiments

[0098] A fourth exemplary embodiment relates to a continuous casting method characterized in that, in a case where a mold flux sample is melted, cooled at a cooling rate of 10 degrees C. / min, and solidified, and the crystallization ratio of the sample is expressed as an area ratio, a mold flux having a crystallization ratio of 20% or more is used as the mold flux in the continuous casting method according to the third exemplary embodiment. The crystallization ratio (area ratio) of the mold flux is determined as a ratio (%) of an area of a region where crystals are observed in the electron microscope observation field to the total field area for the sample obtained by melting the mold flux sample, cooling the sample at a cooling rate of 10 degrees C. / min, and solidifying the sample.

[0099] As illustrated in FIG. 2, the molten steel 10 is supplied into the continuous casting mold 1 from the submerged entry nozzle 5. The mold flux 11 supplied to the surface of the molten steel 10 in the continuous casting mold 1 melts on the surface of the molten steel 10 to form a molten flux layer 12, which then flows between the continuous casting mold 1 and the solidified shell 13 to become a mold flux film 14. The mold flux film 14 is present in a gap between the cast steel and the mold, and has a total thickness of about 0.1 mm or more and 1 mm or less. The mold flux film 14 has a two-layer structure, with a solid film 15 at the mold side and a liquid film 16 at the cast steel side. Crystals may precipitate in the solid film 15, or the solid film 15 may be solidified into a glass-like state.

[0100] The exemplary embodiment utilizes the cooling paradox phenomenon found by the inventors, which is caused by the fact that crystallization of the mold flux film provides a shielding effect against radiation heat transfer and an inhibitory effect against heat conduction. Therefore, the effect is exhibited by using the mold flux 11 in which crystals precipitate in the mold flux film 14. Thus, a mold flux having a crystallization ratio of 20% or more in terms of area ratio when melted, cooled at a cooling rate of 10 degrees C. / min and solidified, is used as the mold flux 11. When solidified under the same conditions, the mold flux more preferably has a crystallization ratio of 50% or more in terms of area ratio. For the solidified sample, the type of crystal is determined for each crystal grain using SEM-EDS, and the crystal area ratio % for the total crystal grains is taken as the crystallization ratio.

[0101] When the basicity (CaO / SiO2 mass ratio) of the mold flux is higher than a predetermined value of about 1.0, the crystallization ratio falls within the preferred range of the exemplary embodiment described above.

[0102] On the other hand, if a mold flux with a high solidification temperature is used, in the lower part of the mold where the surface temperature of the cast steel is low, the liquid phase may not remain in the mold flux film, and the entire mold flux film may become a single layer of crystallized solid film. In such a case, the fluidity and void-filling properties of the entire mold flux film may be lost, and the cooling of the cast steel may become unstable. It is thus preferable to use a mold flux whose solidification temperature is lower than the surface temperature of the cast steel at the lower end of the mold. This prevents the entire mold flux film from becoming a single layer of crystallized solid film. The surface temperature of the cast steel at the lower end of the mold depends on the casting speed. When the casting speed in a steady state during continuous casting is denoted by Vc′, it is preferable that the solidification temperature of the mold flux is 1,250 degrees C. or less and the following formula (17) is satisfied, and it is more preferable that the solidification temperature of the mold flux is 1,200 degrees C. or less and the following formula (18) is satisfied.Upper⁢ Limit⁢ of⁢ Solidification⁢ Temperature⁢ (degrees⁢ C)=
1170+Vc’⁢(m / min)×50(17)Upper⁢ Limit⁢ of⁢ Solidification⁢ Temperature⁢ (degrees⁢ C)=
1150+Vc’⁢(m / min)×30(18)

[0103] Here, the solidification temperature means the same temperature as a crystallization temperature, which is a temperature at which the molten flux crystallizes, i.e., solidifies, when cooled from the molten state at 10 degrees C. / min. The solidification temperature can be determined by a rapid increase in viscosity in the case of viscosity measurement, or an exothermic peak associated with crystallization in the case of thermal analysis. The term solidification temperature is often used for the viscosity measurement. The term crystallization temperature is often used for the thermal analysis measurement.

[0104] The solidification temperature of the mold flux can be adjusted to be within the preferred range by increasing the content of alkali metal elements, fluorine, and the like in the mold flux, in addition to adjusting the crystallization ratio of the mold flux to be within the preferred range of the exemplary embodiment.Fifth Exemplary Embodiment

[0105] A fifth exemplary embodiment is characterized in that a mold flux, in which a main crystal of the molten and solidified mold flux is cuspidine and the viscosity at 1,300 degrees C. is less than 1.5 poise, is applied to the continuous casting method of the fourth exemplary embodiment.

[0106] In the exemplary embodiment, a mold flux that easily crystallizes is used as the mold flux 11, and the mold surface temperature in the upper part of the mold is lowered to promote crystallization of the mold flux film 14. After the mold flux 11 is melted on the molten surface in the mold to form the molten flux layer 12, the mold flux film 14 flowing into a gap between the solidified shell 13 and the mold surface 6 is rapidly cooled. In this case, if the crystallization speed of the mold flux film 14 is slow, even the easily crystallizable mold flux 11 solidifies into a glass-like state and crystallization is delayed. The object of the invention thus cannot be sufficiently achieved. Among the crystals generated by solidification of the molten mold flux 11, cuspidine (3CaO·2SiO2·CaF2) is known to have a fast crystallization speed. Thus, the mold flux 11 in which the main crystal is cuspidine is preferably used. Here, the mold flux 11 is melted, cooled at a cooling rate of 10 degrees C. / min, and solidified into a sample, which is then subjected to X-ray diffraction evaluation, and a crystal with the highest peak height is determined to be the main crystal. Also, in the fifth exemplary embodiment, it is desirable to reduce the viscosity of the molten mold flux, specifically, to make the viscosity at 1,300 degrees C. less than 1.5 poise, in order to increase the diffusion rate. More preferably, the viscosity at 1,300 degrees C. is less than 1.0 poise. Satisfying the requirement of the fifth exemplary embodiment reliably provides the effects of the invention. Note that “poise” is a unit of viscosity in the CGS system, and is equivalent to Pa·S in the SI system. Since 1 poise is equivalent to 0.1 Pa·S, less than 1.5 poise means less than 0.15 Pa·S, and less than 1.0 poise means less than 0.1 Pa·S.

[0107] When the basicity (CaO / SiO2 mass ratio) of the mold flux 11 is higher than a predetermined value of about 1.0 and the F content in the mold flux is 5 mass % or more, cuspidine can serve as the main crystal while keeping the crystallization ratio within the preferred range of the fifth exemplary embodiment.

[0108] It is also possible for the invention to cast steel that does not require mild cooling by promoting the growth of the solidified shell in the upper part of the mold using the continuous casting mold described in the first exemplary embodiment or the continuous casting machine described in the second exemplary embodiment, and using the mold flux in which neither crystallization nor precipitation occurs or a small amount of crystals precipitate when the mold flux is melted, cooled at a cooling rate of 10 degrees C. / min, and solidified. Alternatively, as an application of the invention, it is possible to control the degree of crystallization of the flux film with the same mold flux by changing the cooling conditions of the mold, and thus a mold flux of one brand can be adapted to a wide range of steel grades and casting conditions. As described above, the invention increases variations of the heat flux in the mold by combining the mold cooling performance and the mold flux, enabling operations suitable for a wide range of steel grades and casting conditions.

[0109] By implementing the first exemplary embodiment through the fifth exemplary embodiment, it is possible to properly control the crystallization of the mold flux film, bring the heat flux in the mold closer to an ideal state, and ensure good solidification of the cast steel.Mold Fluxes in Sixth and Seventh Exemplary Embodiments

[0110] The mold flux designed with a high solidification temperature has a high crystallization ratio, which promotes the crystallization of the film in the upper part of the mold, but causes excessive crystallization of the film in the lower part of the mold. On the other hand, if the solidification temperature is designed to be low, a liquid phase is present in the mold flux film formed between the mold surface and the solidified shell. This is effective for lubrication inside the mold and for promoting the growth of the solidified shell, but it is not possible to provide sufficient mild cooling performance in the upper part of the mold. That is, from the viewpoints of the solidification temperature and the crystallization ratio, both of the mold fluxes described above are suitable for steel casting. However, conventionally, the control of the crystallization has relied only on the composition design of the mold flux, and there has been no idea of increasing the freedom of crystallization control in combination with the control of the mold surface temperature and maximizing the effect.

[0111] The sixth and seventh exemplary embodiments aim to provide a continuously casting method for steel, and achieve more ideal in-mold heat removal than ever before by considering the interaction between the cooling performance of the mold and the in-mold heat removal control via the mold flux film and implementing them in an appropriate combination.

[0112] The sixth and seventh exemplary embodiments use the water-cooling copper mold of the first or second exemplary embodiment, and take into account the interaction with the mold flux to achieve ideal in-mold heat removal. In the mold flux, the concentrations of the components CaO, SiO2, and F are adjusted to a composition that facilitates the crystallization of cuspidine, and the crystallization ratio is increased by adding Li2O and, in the seventh exemplary embodiment, Na2O.

[0113] The component composition of the mold flux used in the continuous casting method of each of the sixth exemplary embodiment and the seventh exemplary embodiment will be described. When the mold flux contains C, the total of components other than C in the mold flux is taken as 100 mass %, and the content of each component is calculated. The C content is thus treated as not included in the total mass. “%” for the content of the mold flux refers to mass %.[CaO′ / SiO2]

[0114] The mold flux contains T·CaO, SiO2, and F as the main components. Here, T·CaO is treated as a component in which the total of Ca content in the mold flux is considered as CaO. F in a powder state is added, as CaF2, to the mold flux. In the molten state of the mold flux, F has a stronger affinity for alkali metals than Ca. Thus, in the molten state of the mold flux, it is known that F reacts with alkali metals and exists as alkali metal fluorides and the remaining F content reacts with the Ca content and exists as CaF2. Then, the CaF2 concentration in the flux having the molten state is determined by CaF2′ in the above formula (7). When the right side of the formula (7) is negative, it is considered that all the F in the flux has reacted with alkali metal oxides, the remaining F is zero, and the CaF2′ concentration is also zero; therefore, the left side of formula (7) is set to 0%. The CaF2′ concentration is determined using the formula (7) as described above, and then CaO′ is calculated using the formula (6). The CaO′ defined by the formula (6) indicates a component amount in which F and the unreacted Ca content are regarded to exist as an oxide (CaO).

[0115] CaO′ / SiO2 is an indicator of components that cause cuspidine to crystallize out. Preferably, CaO′SiO2 is 0.9 or more and 2.0 or less at a mass concentration ratio. When CaO′SiO2 is less than 0.9 at the mass concentration ratio, the precipitation amount of crystals is small, and a film having sufficient mild cooling performance cannot be obtained. When CaO′SiO2 exceeds 2.0 at the mass concentration ratio, crystal phases other than cuspidine crystallize, and a film having sufficient mild cooling performance cannot be obtained. More preferably, CaO′ / SiO2 is 1.1 or more and 1.8 or less at the mass concentration ratio.[F]

[0116] The F content is 5 mass % or more. F has the effect of adjusting the solidification temperature of the mold flux and also has the effect on the crystallization of cuspidine. If the F content is less than 5 mass %, the effects are small. On the other hand, when a large amount of F is added, the viscosity of the mold flux decreases significantly, and the mold flux is entrapped in the molten steel. Thus, the F content is preferably 24 mass % or less, and more preferably 10 mass % or more.Alkali Metal Oxide

[0117] Examples of the alkali metal oxide added to the mold flux include K2O, Na2O, and Li2O. The alkali metal oxide is added to obtain the effect of increasing the crystallization speed of the mold flux and to reduce the solidification temperature and viscosity. Na2O is often used in view of the costs. Hereinafter, the contents of the components Li2O, Na2O, and K2O in the mold flux refer to the contents of oxides (mass %) calculated assuming that Li, Na, and K in the mold flux are all oxides.

[0118] It is known that Li2O has the greatest effect on improving the crystallization speed by adding alkali metal oxides, followed by Na2O and K2O (Li2O>Na2O>K2O). Li2O has the greatest effect on increasing the crystallization speed of the mold flux. In the sixth exemplary embodiment in which Na2O is not contained, the content of Li2O is 1 mass % or more and 15 mass % or less, and is preferably 3 mass % or more. In addition, the content of Li2O is preferably 10 mass % or less. In the seventh exemplary embodiment in which Na2O is contained at 1% or more as described below, the content of Li2O is 1 mass % or more and 10 mass % or less, and is preferably 3 mass % or more and 8 mass % or less. When the Li2O content is less than 1 mass %, the crystallization speed is low. Furthermore, the solidification temperature of the mold flux becomes high, and it is not possible to sufficiently prevent the occurrence of constraint BO, solidification-retarded BO, or false detection of BO. On the other hand, when Li2O is contained in excess, the amount of cuspidine crystallized decreases, making it impossible to obtain sufficient mild cooling performance.

[0119] It is known that the addition of K2O increases the viscosity and decreases the crystallization speed compared to conventional mold fluxes. This makes it difficult for a mold powder film to flow between the mold surface and the solidified shell and the film is difficult to crystallize. Thus, K2O is not preferably contained, and the K2O content is preferably less than 0.5 mass %.

[0120] Increasing the content of Li2O can ensure both the improvement in the crystallization speed and the decrease in the solidification temperature, but the raw material price of Li2O is high. Thus, in the seventh exemplary embodiment, 1 mass % or more of Na2O is added in addition to Li2O as an alkali metal oxide for decreasing the solidification temperature. On the other hand, the addition of a large amount of Na2O reduces the amount of cuspidine crystallized, so that sufficient mild cooling performance cannot be obtained. The upper limit of the Na2O content is thus set to 10 mass %. Accordingly, the Na2O content is 1 mass % or more and 10 mass % or less, and desirably 2 mass % or more and 8 mass % or less.[Al2O3 and MgO]

[0121] The total content of Al2O3 and MgO is preferably 5 mass % or less. In the design of the mold flux, Al2O3 and MgO are unavoidable impurities. Both Al2O3 and MgO reduce the amount of cuspidine crystallized, and components crystallize. As a result, the melting characteristics deteriorate and heat removal becomes difficult, so it is desirable to keep the contents of Al2O3 and MgO as small as possible. The contents of Al2O3 and MgO in the mold flux refer to the contents of oxides (mass %) calculated assuming that Al and Mg in the mold flux are all oxides.[C]

[0122] Furthermore, it is desirable to add C to the mold flux of the exemplary embodiment in addition to the above components. C is desirably contained at 1 mass % or more and 10 mass % or less as a content rate not included in the total mass of the mold flux. C has the effect of adjusting the melting speed of the mold flux. As the C content increases, the melting speed of the mold flux decreases. If the C content is less than 1 mass %, the melting speed is too high, and if the C content exceeds 10 mass %, the melting speed is too low. In either case, the flux inflow between the mold surface and the solidified shell is impaired.[Solidification Temperature]

[0123] The solidification temperature of the mold flux is specified to be 900 degrees C. or more and 1,300 degrees C. or less. If the solidification temperature exceeds 1,300 degrees C., the crystalline phase of the film is excessively formed in the lower part of the mold, and the growth of the solidified shell is insufficient. The solidification temperature is more preferably 1,250 degrees C. or less. On the other hand, when the solidification temperature of the mold flux is less than 900 degrees C., the crystallization of cuspidine is insufficient and mild cooling performance cannot be obtained satisfactorily. The solidification temperature is thus set to 900 degrees C. or more. The solidification temperature more preferably exceeds 1,100 degrees C. It is possible to adjust the solidification temperature of the mold flux to within the range from 900 degrees C. to 1300 degrees C. by adjusting the CaOVSiO2 mass ratio, F content, and alkali metal content of the mold flux within the range of the invention based on the above description.Viscosity

[0124] The viscosity of the mold flux is desirably 1 poise or less at 1,300 degrees C. If the viscosity of the mold flux exceeds 1 poise, the inflow of mold flux between the mold and solidified shell is insufficient, which easily causes the constraint BO and cast steel defects.Raw Material

[0125] Commonly used raw materials are usable as the raw materials for the mold flux in the exemplary embodiment without any problem. Examples of CaO raw materials include quicklime, limestone, and cement. Examples of SiO2 raw materials include silica sand and diatomaceous earth. Examples of Li2O raw materials include lithium carbonate. Examples of Na2O raw materials include sodium carbonate and soda ash. Examples of F raw materials include fluorite and sodium fluoride. Examples of C raw materials include carbon black and coke powder.

[0126] The shape of the raw materials for the mold flux is not limited, and may be any shape, such as powder or granules. The raw materials for the mold flux contain oxides such as Fe2O3, Al2O3, and MgO. Even if these impurities are mixed in, the amount is small and does not cause any problems.

[0127] In the continuous casting method of the sixth exemplary embodiment, the mold flux that easily crystallizes is used in which the CaO′SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; Li2O is contained, among the alkali metal oxides, at 1 mass % or more and 15 mass % or less; and the solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less.

[0128] In the seventh exemplary embodiment, the mold flux is used in which: the CaO′ / SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; among the alkali metal oxides, Li2O is contained at 1 mass % or more and 10 mass % or less and Na2O is contained at 1 mass % or more and 10 mass % or less; and the solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less.

[0129] In the sixth and seventh exemplary embodiments, the continuous casting mold of the first or second exemplary embodiment is used in which the mold surface temperature in the upper mold part 21 is lowered to promote crystallization of the mold flux film 14. The mold flux film 14 that flows into the gap between the solidified shell 13 and the mold surface 6 is rapidly cooled. This allows a crystalline phase to rapidly crystallize out in the mold flux film 14 in the upper mold part 21, and slows down the cooling of the solidified shell 13 in the vicinity of the meniscus, thereby inhibiting heterogeneous growth of the solidified shell 13, which may otherwise become a starting point of vertical cracks. In the lower mold part 22, the growth of the crystalline phase of the mold flux film 14 is inhibited, and the solidified shell 13 is strongly cooled to promote the growth of the solidified shell 13. As a result, even when medium-carbon steel is continuously cast at a high casting speed with a carbon concentration of 0.06 mass % or more and 0.20 mass % or less, it is possible to adequately prevent vertical cracks in the cast steel and also satisfactorily prevent the occurrence of solidification-retarded BO or frequent false detection of BO. The same applies to the seventh exemplary embodiment.

[0130] In the sixth and seventh exemplary embodiments, when the mold flux is melted and cooled at a cooling rate of 150 degrees C. / min to be solidified, the crystallization ratio is preferably 70% or more in terms of area ratio.

[0131] As described with reference to FIG. 2 in the fourth and fifth exemplary embodiments, the sixth and seventh exemplary embodiments also utilize the cooling paradox phenomenon found by the inventors, which is caused by the fact that crystallization of the mold flux film provides a shielding effect against radiation heat transfer and an inhibitory effect against heat conduction. Therefore, the effect is exhibited by using the mold flux in which crystals precipitate therein. Thus, a mold flux having a crystallization ratio of 70% or more in terms of area ratio when melted, cooled at a cooling rate of 150 degrees C. / min and solidified, is preferably used in the sixth and seventh exemplary embodiments. When solidified under the same conditions, the mold flux more preferably has a crystallization ratio of 80% or more in terms of area ratio. For the solidified sample, the type of crystal is determined for each crystal grain using SEM-EDS, and the crystal area ratio % for the total crystal grains is taken as the crystallization ratio.

[0132] By setting the CaO′SiO2 mass ratio to 0.9 or more and Li2O to 1 mass % or more in the mold flux, the crystallization ratio falls within the preferred range of the exemplary embodiment(s).EXAMPLES

[0133] Example 1 given below includes Examples and Comparatives to specifically explain the continuous casting molds in the exemplary embodiments.Example 1

[0134] As illustrated in FIG. 1, the continuous casting mold 1 including the cooling-water channels 3 extending vertically on the rear side of the copper mold plate 2 was used. The mold surface temperature Ts defined by the formula (2) when the heat flux Q defined by the formula (1) was given was calculated by changing the shape and cooling conditions of the cooling-water channels 3. Here, the conditions were made different for the upper part of the mold and the lower part of the mold, and the mold surface temperature TsU in the upper part of the mold and the mold surface temperature TsL in the lower part of the mold were calculated. Tables 1 and 2 show the mold conditions and casting conditions used for the calculation. The molds used in Example A to Comparative F in Tables 1 and 2 are referred to as “mold A” to “mold F,” respectively. ΔTS defined by the formula (16) is described as the “difference between upper and lower mold surface temperatures” in Tables 1 and 2.TABLE 1ExamplesABCDistance from upper end of(m)Upper moldLower moldUpper moldLower moldUpper moldLower moldmold to upper / lowerpartpartpartpartpartpartpart of mold0.05 to 0.200.50 to 0.900.04 to 0.250.60 to 0.900.05 to 0.200.20 to 1.10Distance from upper end of(m)0.100.120.10mold to molten surfaceMold length(m)0.900.901.10Set casting speedVc(m / min)1.6 2.1 2.5 Heat flux in moldQ(W / m2)1.612 × 1061.950 × 1062.203 × 106(average valueaccording to Vc)Distance betweenX(m)0.0110.0180.0150.0150.0120.015cooling-waterchannel end andmold surfaceThermalλm(W / (m · K))340340340340340270conductivityof coppermold plateFlow velocity ofVw(m / s)8.008.0010.005.009.819.81cooling waterTemperature ofTw(° C.)373237303139cooling waterDensity of waterρkg / m3993.3995.0993.3995.6995.3992.6Thermalλw(W / (m · K))0.62430.61720.62430.61420.61570.6271conductivityof coolingwaterSpecific heatCpJ / (kg · K)417841784178417841784178of waterViscosityη(Pa · s)0.00068940.00076250.00068940.00079590.00077890.0006637of waterWidth ofW(m)0.0050.0050.0050.0050.0050.005cooling-waterchannelDepth ofD(m)0.0220.0220.0140.0280.0220.022cooling-waterchannelCross-sectionalA(m2)0.000110.000110.000070.000140.000110.00011area ofcooling-waterchannelPerimeter ofL(m)0.0540.0540.0380.0660.0540.054cooling-waterchannelEquivalent diameterd(m)0.0081480.0081480.0073680.0084850.0081480.008148Reynolds numberRe939288506010617553072102146119548Prandtl numberPr4.6135.1624.6135.4145.2864.422Calculation valueTs(° C.)141.3172.0174.8215.0172.8220.9of moldsurface temperatureDifference betweenΔTs(° C.)31   40   48   upper and lowermold surfacetemperaturesTABLE 2ComparativesDEFDistance from upper end of(m)Upper moldLower moldUpper moldLower moldUpper moldLower moldmold to upper / lowerpartpartpartpartpartpartpart of mold0.05 to 0.200.50 to 0.900.05 to 0.200.50 to 0.900.05 to 0.200.20 to 1.10Distance from upper end of(m)0.100.100.10mold to molten surfaceMold length(m)0.900.901.10Set casting speedVc(m / min)1.6 1.6 2.5 Heat flux in moldQ(W / m2)1.612 × 1061.612 × 1062.203 × 106(average valueaccording to Vc)Distance betweenX(m)0.0180.0180.0110.0110.0150.012cooling-waterchannel end andmold surfaceThermalλm(W / (m · K))340340340340290340conductivityof coppermold plateFlow velocity ofVw(m / s)8.008.008.008.0012.8012.80cooling waterTemperature ofTw(° C.)393237303428cooling waterDensity of waterρkg / m3992.6995.0993.3995.6994.4996.2Thermalλw(W / (m · K))0.62710.61720.62430.61420.62010.6110conductivityof coolingwaterSpecific heatCpJ / (kg · K)417841784178417841784179of waterViscosityη(Pa · s)0.00066370.00076250.00068940.00079590.00073160.0008319of waterWidth ofW(m)0.0050.0050.0050.0050.0050.005cooling-waterchannelDepth ofD(m)0.0220.0220.0220.0220.0220.022cooling-waterchannelCross-sectionalA(m2)0.000110.000110.000110.000110.000110.00011area ofcooling-waterchannelPerimeter ofL(m)0.0540.0540.0540.0540.0540.054cooling-waterchannelEquivalent diameterd(m)0.0081480.0081480.0081480.0081480.0081480.008148Reynolds numberRe97491850609392881546141755124899Prandtl numberPr4.4225.1624.6135.4144.9295.689Calculation valueTs(° C.)175.6172.0141.3137.9198.2159.1of moldsurface temperatureDifference betweenΔTs(° C.)−4   −3   −39   upper and lowermold surfacetemperaturesThe mold A, in which the length of the copper mold plate 2 in the casting direction 26 was 0.90 m, was used in a continuous casting machine designed so that the distance from the upper end 23 of the mold to the molten surface was 0.10 m. As illustrated in FIG. 3, the distance X between the cooling-water channel end 7 and the mold surface 6 was set to XU=0.011 m in the upper mold part 21 (the range starting from a position 0.05 m below the upper end 23 of the mold and ending at a position 0.20 m below the upper end 23 of the mold), and was set to XL=0.018 m in the lower mold part 22 (the range starting from a position 0.50 m below the upper end 23 of the mold and ending at a position 0.90 m below the upper end 23 of the mold). The distance XU in the upper mold part 21 was shorter than the distance XL in the lower mold part 22 in Example A, satisfying the requirement [B] of the first exemplary embodiment. In this configuration, regarding the mold surface temperature Ts calculated by the formula (2) when the heat flux Q in the mold defined by the formula (1) was given according to the set casting speed VC=1.6 m / min, the mold surface temperature TsU in the upper mold part 21 was 141.3 degrees C., and the mold surface temperature TsL in the lower mold part 22 was 172.0 degrees C., with a difference of ΔTs=31 degrees C. That is, Example A satisfied the requirement of the second exemplary embodiment that ΔTs of the formula (16) was 20 degrees C. or more. In Example A, the mold cooling water was flowed from the bottom to the top of the mold. The average value of the cooling water temperature TwU in the upper mold part 21 was 37 degrees C., and the average value of the cooling water temperature TwL in the lower mold part 22 was 32 degrees C., so the upper part of the mold had a higher cooling water temperature. In addition, in a range from a position 0.20 m away from the upper end 23 of the mold as the upper end to a position 0.50 m away from the upper end 23 of the mold as the lower end, the distance X between the cooling-water channel end 7 and the mold surface 6 was designed to change gradually from 0.011 m to 0.018 m.

[0136] The mold B, in which the length of the copper mold plate 2 in the casting direction 26 was 0.90 m, was used in a continuous casting machine designed so that the distance from the upper end 23 of the mold to the molten surface was 0.12 m. As illustrated in FIG. 4, a depth D of the cooling-water channel was set to a depth DU=0.014 m in the upper mold part 21 (the range starting from a position 0.04 m below the upper end 23 of the mold and ending at a position 0.25 m below the upper end 23 of the mold), and was set to a depth DL=0.028 m in the lower mold part 22 (the range starting from a position 0.60 m below the upper end 23 of the mold and ending at a position 0.90 m below the upper end 23 of the mold), with the depth of the upper part of the mold being half the depth of the lower part of the mold. The width W of the cooling-water channel was constant at 0.005 m as shown in Table 1. Thus, in the cross-sectional area A of the cooling-water channel 3, the cross-sectional area AU in the upper mold part 21 was smaller than the cross-sectional area AL in the lower mold part 22. The cross-sectional area AU was half the cross-sectional area AL. That is, Example B satisfied the requirement [A] of the first exemplary embodiment. Regarding the flow velocity of cooling water Vw, the flow velocity of the cooling water in the upper mold part 21 was designed to be twice as fast as that in the lower mold part 22. In this configuration, regarding the mold surface temperature Ts calculated by the formula (2) when the heat flux Q in the mold defined by the formula (1) was given according to the set casting speed VC=2.1 m / min, the mold surface temperature TsU in the upper mold part 21 was 174.8 degrees C., and the mold surface temperature TsL in the lower mold part 22 was 215.0 degrees C., with a difference of ΔTs=40 degrees C. That is, Example B satisfied the requirement of the second exemplary embodiment that ΔTs of the formula (16) was 20 degrees C. or more. In Example B, the mold cooling water was flowed from the bottom to the top of the mold. The average value of the cooling water temperature TwU in the upper mold part 21 was 37 degrees C., and the average value of the cooling water temperature TwL in the lower mold part 22 was 30 degrees C., so the upper mold part 21 had a higher cooling water temperature. In addition, in a range from a position 0.25 m away from the upper end 23 of the mold as the upper end to a position 0.60 m away from the upper end 23 of the mold as the lower end, the depth D of the cooling-water channel 3 was designed to change gradually from 0.014 m to 0.028 m.

[0137] The mold C, in which the length of the copper mold plate 2 in the casting direction 26 was 1.10 m, was used in a continuous casting machine designed so that the distance from the upper end of the copper mold plate 2 to the molten surface was 0.10 m. As illustrated in FIG. 5, it was designed such that the thermal conductivity λm of the copper mold plate 2 was made different between the upper mold part 21 (the range starting from a position 0.05 m below the upper end 23 of the mold and ending at a position 0.20 m below the upper end 23 of the mold) and the lower mold part 22 (the range starting from a position 0.20 m below the upper end 23 of the mold and ending at a position 1.10 m below the upper end 23 of the mold) on either side of a boundary position 17 in FIG. 5 and the thermal conductivity λmU in the upper mold part 21 was designed to be greater than the thermal conductivity λmL in the lower mold part 22. That is, Example C satisfied the requirement [C] of the first exemplary embodiment. In this configuration, regarding the mold surface temperature Ts calculated by the formula (2) when the heat flux Q in the mold defined by the formula (1) was given according to the set casting speed VC=2.5 m / min, the mold surface temperature TsU in the upper mold part 21 was 172.8 degrees C., and the mold surface temperature TsL in the lower mold part 22 was 220.9 degrees C., with a difference of ΔTs=48 degrees C. That is, Example C satisfied the requirement of the second exemplary embodiment that ΔTs of the formula (16) was 20 degrees C. or more. In Example C, it was designed such that the mold cooling water was flowed from the top to the bottom of the mold (see FIG. 5), and the average value of the cooling water temperature TwU in the upper mold part 21 was 31 degrees C., and the average value of the cooling water temperature TwL in the lower mold part 22 was 39 degrees C. The condition that the cooling water temperature was lower in the upper part than in the lower part of the mold also effectively serves to lower the mold surface temperature in the upper mold part 21 below the mold surface temperature in the lower mold part 22.

[0138] The mold D is a comparative using a common mold. As illustrated in FIG. 6, the cross-sectional area A of the cooling-water channel 3, the distance X between the cooling-water channel end 7 and the mold surface 6, and the thermal conductivity λm of the copper mold plate 2, those of which are related to the cooling of the mold, were the same from the upper part to the lower part of the mold. The same applies to the mold E. In Comparatives D and E, the flow direction of the cooling water was a normal design in which the mold cooling water was flowed from the bottom to the top of the mold. The average value of the cooling water temperature TwU in the upper part of the mold was thus higher than the average value of the cooling water temperature TwL in the lower part of the mold. In this configuration, regarding the calculated value of the mold surface temperature when the heat flux Q in the mold was given according to the set casting speed VC=1.6 m / min, the value for the upper part of the mold was slightly larger than that for the lower part of the mold, with ΔTs being a negative value.

[0139] The mold F, in which the length of the copper mold plate 2 in the casting direction 26 was 1.10 m, was used in a continuous casting machine designed so that the distance from the upper end of the copper mold plate 2 to the molten surface was 0.10 m. Example F is a comparative in which the thermal conductivity λm of the copper mold plate 2 was designed so that the thermal conductivity λmU in the upper mold part 21 (the range starting from a position 0.05 m below the upper end 23 of the mold and ending at a position 0.20 m below the upper end 23 of the mold) was smaller than the thermal conductivity λmL in the lower mold part 22 (the range starting from a position 0.20 m below the upper end 23 of the mold and ending at a position 1.10 m below the upper end 23 of the mold). In this configuration, regarding the mold surface temperature Ts calculated by the formula (2) when the heat flux Q in the mold defined by the formula (1) was given according to the set casting speed VC=2.5 m / min, the mold surface temperature TsU in the upper mold part 21 was higher than the mold surface temperature TsL in the lower mold part 22, with ΔTs=−39 degrees C., which was a negative value. In Comparative F, the mold cooling water was flowed from the bottom to the top of the mold as in a normal design, so that the average value of the cooling water temperature TwU in the upper mold part 21 was higher than the average value of the cooling water temperature TwU in the upper mold part 21. This also contributed to making the mold surface temperature TsU in the upper mold part 21 higher than the mold surface temperature TsL in the lower mold part 22. The mold F, in which the mold surface temperature in the upper mold part 21 tends to be excessively high, is disadvantageous in terms of sticking of the cast steel to the mold surface, cracks in the mold surface, and deformation of the copper mold plate.Example 2

[0140] In Example 2, the continuous casting molds of Example 1 and the mold fluxes of the fourth and fifth exemplary embodiments were used, and continuous casting was actually performed using a continuous casting machine.

[0141] Molten steel having the composition shown in Table 3 was cast using a continuous casting machine provided with the continuous casting mold shown in each of Example A (using mold A), Comparative D (using mold D), and Comparative E (using mold E) in Table 1 for the casting conditions related to the mold. The cross-sectional dimensions of the mold were 1,250 mm in width and 250 mm in thickness. The degree of superheat of the molten steel was 25 degrees C. immediately before pouring into the mold, and the casting speed was 1.6 m / min.

[0142] As the mold fluxes, mold fluxes a to c shown in Table 4 were used. Examples and Comparatives using the mold fluxes a to c are A-a, D-a, and E-a in Table 5, and A-b, A-c, and D-c in Table 6. For example, A-a means that the mold A and the mold flux a were used.

[0143] The chemical composition of each mold flux shown in Table 4 does not include carbon or carbon oxides that are lost through combustion or thermal decomposition during melting, and shows values that represent the composition after melting. In Table 4, it is assumed that all Ca obtained by analysis is CaO and all Na obtained by analysis is Na2O.

[0144] The solidification temperature in Table 4 was determined by reading, from the mold flux temperature measurement results, a temperature at which the heat generation due to crystallization was maximum when the mold flux, which was once molten in a graphite crucible in a furnace, was solidified while decreasing the furnace atmosphere temperature at a cooling rate of 10 degrees C. / min. Here, the temperature at which the heat generation due to crystallization was maximum means either a temperature at which the temperature decrease rate is the smallest, or a temperature at which the temperature increase rate due to heat generation is the largest.

[0145] Each mold flux was solidified under the above conditions. For the solidified sample, the type of crystal was determined for each crystal grain using SEM-EDS, and the crystal area ratio % for the total crystal grains was taken as the crystallization ratio. In addition, the solidified sample was subjected to X-ray diffraction evaluation, and a crystal with the highest peak height was determined to be the main crystal, as shown in Table 4.

[0146] The mold flux a in Table 4 satisfies the requirements of the fourth and fifth exemplary embodiments, and mold flux b satisfies the requirement of the fourth exemplary embodiment. In the mold flux b, the main crystal and the viscosity at 1,300 degrees C. fall outside the requirement of the fifth exemplary embodiment.TABLE 3Steel composition (mass %)CSiMnPSSol. AlN0.110.020.650.0130.0060.0350.003TABLE 4CrystallizationChemical composition (mass %)ViscosityratioSolidificationMainCaOSiO2Al2O3MgOTiO2Na2OLi2OF(1300° C.)(%)temperaturecrystala39.032.01.92.3—10.0—9.80.8 poise821,160° C.Cuspidineb38.231.89.98.46.01.01.03.13.4 poise651,170° C.Melilitec41.031.410.20.87.00.7—3.06.1 poise81,170° C.(Unclear)(Unclear)(Glass-like)TABLE 5ExampleComparativeA-aD-aE-aDistance from upper end of mold to(m)Upper moldLower moldUpper moldLower moldUpper moldLower moldupper / lower part of moldpartpartpartpartpartpart0.05 to 0.200.50 to 0.900.05 to 0.200.50 to 0.900.05 to 0.200.50 to 0.90Distance from upper end of(m)0.100.100.10mold to molten surfaceHeat flux evaluation range(m)Upper moldLower moldUpper moldLower moldUpper moldLower mold(Distance from upper end of mold)partpartpartpartpartpart0.10 to 0.200.50 to 0.900.10 to 0.200.50 to 0.900.10 to 0.200.50 to 0.90Mold flux brandaaaHeat flux index in Mold8635100378627Solidification heterogeneity index31   100   67   TABLE 6ExampleComparativesA-bA-cD-cDistance from upper end of mold to(m)Upper moldLower moldUpper moldLower moldUpper moldLower moldupper / lower part of moldpartpartpartpartpartpart0.05 to 0.200.50 to 1.100.05 to 0.200.50 to 0.900.05 to 0.200.50 to 0.90Distance from upper end of(m)0.100.100.10mold to molten surfaceHeat flux evaluation range(m)Upper moldLower moldUpper moldLower moldUpper moldLower mold(Distance from upper end of mold)partpartpartpartpartpart0.10 to 0.200.50 to 0.900.10 to 0.200.50 to 0.900.10 to 0.200.50 to 0.90Mold flux brandbccHeat flux index in mold93351284411844Solidification heterogeneity index48   155   131   As an actual heat flux value during casting in each of the upper mold part 21 and the lower mold part 22, a value obtained by estimating an average heat flux in each heat flux evaluation range shown in Tables 5 and 6 from temperature measurements of thermocouples installed at multiple points in the height direction and two points in the depth direction in the mold. The heat flux value in the upper part of the mold of Comparative D-a was set to 100, and the heat flux values in the upper mold part 21 and the lower mold part 22 in each of Examples and Comparatives were indexed and shown in “Heat flux index in mold” of Tables 5 and 6. When the values of heat flux measured by the thermocouples fluctuated over time, the values at local maximum points of the fluctuation curve were linked to obtain the average value. When the measured value of the heat flux fluctuates over time, the cause of the fluctuation is that the distance between the mold and the cast steel increases due to abnormal shrinkage of the solidified shell 13, or that a gap occurs between the solid phase of the mold flux film 14 and the mold. The above fluctuation causes decrease the heat flux, so in order to evaluate the inherent heat flux of the mold-mold flux film system, it is desirable to use the values at the local maximum points of the fluctuation curve. The region for evaluating the heat flux was an effective region below the height of the molten surface in the mold. Specifically, a range with a start point at 0.10 m away from the upper end 23 of the mold (a height of the molten surface) and an end point at 0.20 m away from the upper end 23 of the mold was defined as the upper mold part 21. A range with a start point at 0.50 m away from the upper end of the mold and an end point at the lower end of the mold was defined as the lower mold part 22.The size of irregularities on the surface of the cast steel was measured with a laser distance meter, and the standard deviation of the measured distance was taken as solidification heterogeneity. The solidification heterogeneity in Comparative D-a was set to 100, and the respective indexed values are shown in “Solidification heterogeneity index” in Tables 5 and 6.First, a description will be made on Comparative D-a in Table 5, which is a combination of a normal mold and the mold flux. D-a is a comparative in which hypo-peritectic steel with the composition shown in Table 3 was cast using the mold D in which the cooling performance for the mold was constant from the upper part to the lower part of the mold, and the mold flux a in Table 4 in which cuspidine crystallized and precipitated in the mold flux film as the main crystal. In Comparative D-a, the heat flux in the mold showed a normal heat flux distribution, being large in the upper mold part 21 where the surface temperature of the solidified shell was high, and being small in the lower mold part 22 where the surface temperature of the solidified shell was low.

[0150] In Comparative D-a, the surface of the obtained cast steel had irregularities due to the degree of solidification shrinkage specific to hypo-peritectic steel. The heterogeneity of solidification observed in Comparative D-a is probably due to insufficient crystallization of the mold flux film and insufficient reduction in the heat flux in the upper part of the mold.

[0151] In Example A-a in Table 5 using the mold A satisfying the requirements of the first and second exemplary embodiments and the mold flux a satisfying the requirements of the fourth and fifth exemplary embodiments, the cooling performance in the upper part of the mold was strengthened to promote crystallization of the mold flux film. As a result, the heat flux index in the upper part of the mold was decreased to 86, and the solidification heterogeneity index was improved to 31. Although the cooling performance in the lower part of the mold of Example A-a was the same as that of Comparative D-a, the heat flux index in the lower part of the mold of Example A-a was slightly lower than that of Comparative D-a due to the effect of the mold flux film of which crystallization was promoted in the upper part of the mold.

[0152] In Example A-b in Table 6 using the mold A and the mold flux b satisfying the requirement of the fourth exemplary embodiment, the same effect as in Example A-a was obtained, but the mold flux b had a higher viscosity and a smaller crystallization ratio than the mold flux a, so the heat flux index in the upper mold part 21 of Example A-b was slightly higher than of Example A-a. In addition, although the solidification heterogeneity of Example A-b was improved compared to Comparative D-a, it was not as good as Example A-a. The heat flux index in the lower mold part 22 of Example A-b was approximately the same as that of Example A-a, because crystallization proceeded while the mold flux film moved to the lower mold part 22 even if the viscosity of the mold flux was high and the crystallization ratio was small, and the solidification temperatures of the mold flux a and the mold flux b were approximately the same.

[0153] Comparative E-a in Table 5 used the mold E. The upper mold part 21 of the mold E had the same cooling structure as the mold A. The mold E was a mold with strong cooling all over the surface, in which the cooling structure of upper mold part 21 of mold A was extended to the lower mold part 22. In Comparative E-a, the heat flux index in the upper mold part 21 was 86, which was the same as that of Example A-a. In contrast, in the lower mold part 22, the crystallization of the mold flux film progressed excessively, and the heat flux index of the mold decreased to 27. Further, the heat flux greatly varied in the lower mold part 22. This is probably because the excessive crystallization of the mold flux film occasionally caused voids between the mold flux film and the mold. The solidification heterogeneity index of Comparative E-a was 67, and the improvement effect thereof was insufficient compared to Example A-a. This is probably because the growth of the solidified shell was inhibited by the decrease in heat flux due to the excessive crystallization of the mold flux film in the lower mold part 22 and the uniform growth of the solidified shell was inhibited by the variation in the heat flux that may be caused by the excessive crystallization of the mold flux film.

[0154] In Comparatives A-c and D-c in Table 6, it was used the mold flux c of which crystallization ratio was less than 20%, the mold flux c not satisfying the requirement of the fourth exemplary embodiment. Since the crystallization of the mold flux film was insufficient, the cooling properties of the mold alone directly appeared as the properties of cooling the cast steel in Comparatives A-c and D-c. That is, the heat flux in the upper mold part 21 using the mold A in which the cooling performance in the upper mold part 21 was high (Comparative A-c) was larger than that using the mold D in which the cooling performance in the upper mold part 21 was lower than that of the mold A (Comparative D-c). Thus, the solidification heterogeneity index of Comparative A-c was also larger than that of Comparative D-c. In addition, since the mold flux c not satisfying the requirement of the fourth exemplary embodiment was used, the overall heat flux from the upper part to the lower part of the mold of Comparative D-c was larger than that of Comparative D-a, which also promoted heterogeneous solidification.

[0155] The mold flux satisfying the requirement of the fifth exemplary embodiment is not limited to that described in the mold flux a in Table 4, and for example, a mold flux as described in Non-Patent Literature 2 may be used. Or, a similar effect can be achieved by using the mold flux (e.g., the mold flux b in Table 4) that does not satisfy the requirement of the fifth exemplary embodiment but satisfies the requirement of the fourth exemplary embodiment, as described in Patent Literature 10. It should be noted that a mold flux with a high crystallization ratio and a high crystallization speed is preferably used, that is, a mold flux with a small drop in solidification temperature when the cooling rate increases is preferably used.Example 3

[0156] In Example 3, the mold fluxes in the sixth and seventh exemplary embodiments were used, and continuous casting was actually performed using a continuous casting machine.

[0157] Molten steel having the composition shown in Table 3 was cast using the molds A, B, C, D, and E shown in Tables 1 and 2. The cross-sectional dimensions of each mold were 1,250 mm in width and 250 mm in thickness. The degree of superheat of the molten steel was 25 degrees C. immediately before pouring into the mold, and the casting speed was the set casting speed shown in Tables 1 and 2, and was in a range from 1.6 m / min to 2.5 m / min.

[0158] In Example 3, the mold fluxes shown in Tables 7 and 9 were used.

[0159] Each chemical composition shown in Tables 7 and 9 does not include carbon that is lost through combustion or thermal decomposition during melting, and shows values that represent the composition after melting. In Table 3, it is assumed that all Ca obtained by analysis is CaO and all Na obtained by analysis is Na2O. “C′ / S” in Table 3 refers to the mass ratio of CaO′ / SiO2.

[0160] The solidification temperature in Tables 7 and 9 was determined by reading, from the mold flux temperature measurement results, a temperature at which the heat generation due to crystallization was maximum when the mold flux, which was once molten in a graphite crucible in a furnace, was solidified while decreasing the furnace atmosphere temperature at a cooling rate of 10 degrees C. / min, i.e., a temperature at which the temperature decrease rate was the smallest, or a temperature at which the temperature increase rate due to heat generation was the largest.

[0161] The viscosity was measured by an oscillating-plate type viscometer at 1,300 degrees C.

[0162] Each mold flux was heated to 1,350 degrees C. in a graphite crucible to melt, and then solidified at 150 degrees C. / min. For the solidified sample, the type of crystal was determined for each crystal grain using SEM-EDS, and the crystal area ratio % for the total crystal grains was taken as the crystallization ratio. A high crystallization ratio means that the mold flux crystallizes even at a high cooling rate.Sixth Exemplary Embodiment

[0163] The mold flux d to the mold flux g in Table 7, which are products according to the invention, satisfied the component composition and solidification temperature defined in the sixth exemplary embodiment. For the mold flux h, the Li2O content was lower than the lower limit of the sixth exemplary embodiment, and the contents of other alkali metal oxides were also small, so that the solidification temperature was higher than the upper limit of the sixth exemplary embodiment. For the mold flux i, the Li2O content was lower than the lower limit of the sixth exemplary embodiment, and although cuspidine crystallized, the solidification temperature was low and the crystallization ratio was small. For the mold flux j, the Li2O content and C′ / S (CaO′ / SiO2 mass ratio) were lower than the respective lower limits of the sixth exemplary embodiment, resulting in a significantly low crystallization ratio.TABLE 7Solidi-ficationCrystal-temper-lizationLi2ONa2OK2OFT•CaOSiO2Al2O3MgOCaF2′CaO′atureViscosityratioMainmass %C′ / S° C.poise%crystalExamplesd3.00.00.012.651.728.63.50.618.038.81.412400.7292Cuspidinee5.00.00.014.749.427.43.00.517.137.11.411700.4584Cuspidinef8.30.20.411.148.628.72.10.90.848.01.710550.2481Cuspidineg1.90.00.07.051.037.82.10.29.444.21.212490.9187CuspidineComparativesh0.00.00.310.451.233.54.00.621.136.11.113051.1089Cuspidinei0.010.50.010.041.134.02.02.47.335.91.111600.7267Cuspidinej0.016.00.011.023.743.33.12.92.421.90.59802.704—TABLE 8Heat fluxHeat fluxindex inindex inupperSolidificationlowerMoldmoldheterogeneitymoldMoldfluxpartindexpartExamples1Ad8011352Ae8314383Af8517424Ag8113345Bd8427416Be8630457Bf8930478Cf883048Compar-9Ah722625atives10Ai86313311Aj1571154412Dd85313313Df94464014Dh85412415Di1001003716Dj1401104317Ed80272118Eh721021719Ei862923Table 8 shows the evaluation results of the sixth exemplary embodiment.

[0165] The heat flux in each of the upper mold part 21 and the lower mold part 22 was evaluated in the same manner as in Example 2. The heat flux value in the upper mold part 21 of Comparative 15 (D-i) was set to 100, and the heat flux values in the upper mold part 21 and the lower mold part 22 in each of Examples and Comparatives were indexed and shown in “Heat flux index in upper mold part” and “Heat flux index in lower mold part” of Table 8.

[0166] Similar to Example 2 described above, the size of irregularities on a wide face surface of the cast steel was measured with a laser distance meter, and the standard deviation of the measured distance was taken as solidification heterogeneity. The solidification heterogeneity in Comparative 15 (D-i) was set to 100, and the indexed values are shown in “Solidification heterogeneity index” in Table 8.

[0167] First, a description will be made on Comparative 15 (D-i), which is a combination of the normal mold D and the ordinary mold flux i that crystallizes but has a rather low crystallization ratio. D-i is a comparative in which hypo-peritectic steel with the composition shown in Table 3 was cast using the mold D in which the cooling performance for the mold was constant from the upper part to the lower part of the mold, and the mold flux i in Table 7 in which cuspidine crystallized and precipitated in the flux film as the main crystal.

[0168] In Comparative 15 (D-i), the heat flux in the mold showed a normal heat flux distribution, being large in the upper mold part 21 where the solidified shell surface temperature was high, and being small in the lower mold part 22 where the solidified shell surface temperature was low.

[0169] In Comparative 15 (D-i), the surface of the obtained cast steel had irregularities due to the degree of solidification shrinkage specific to hypo-peritectic steel. The heterogeneity of solidification observed in Comparative 15 (D-i) is probably due to insufficient crystallization of the mold flux film and insufficient reduction in the heat flux in the upper part of the mold.

[0170] In contrast, in Comparative 10 (A-i) using the mold A, the cooling performance in the upper mold part 21 was strengthened to promote crystallization of the mold flux film. As a result, the heat flux index in the upper mold part 21 was decreased to 86, and the solidification heterogeneity index was improved to 31. Although the cooling performance in the lower mold part 22 of Comparative 10 (A-i) was the same as that of Comparative 15 (D-i), the heat flux index in the lower mold part 22 of Comparative 10 (A-i) was slightly lower than that of Comparative 15 (D-i) due to the effect of promoting the crystallization of the mold flux film in the upper mold part 21.

[0171] In Comparative 12 (D-d) in which the solidification temperature and crystallization ratio of the mold flux were increased over Comparative 15 (D-i), the crystallization of the mold flux film was promoted in the upper mold part 21, the heat flux index in the upper mold part 21 decreased, and the solidification heterogeneity index improved to 31. Since the solidification temperature of the mold flux of Comparative 12 (D-d) was higher than that of Comparative 15 (D-i), crystals in the mold flux film grew through the lower part of the mold, and the heat flux index in the lower mold part 22 decreased slightly in Comparative 12 (D-d).

[0172] Furthermore, in Example 1 (A-d) in which the cooling performance in the upper mold part 21 was strengthened over Comparative 12 (D-d), the crystallization of the mold flux film was promoted, the heat flux index in the upper mold part 21 decreased to 80, and the solidification heterogeneity index improved to 11.

[0173] The mold flux f had an equivalent or lower solidification temperature compared to the mold flux i, but had an increased crystallization ratio compared thereto. In Comparative 13 (D-f) using the mold flux f, the crystallization of the mold flux film in the upper mold part 21 was promoted over Comparative 15 (D-i). The heat flux index in the upper mold part 21 decreased and the solidification heterogeneity index improved in Comparative 13 (D-f).

[0174] In Comparative 9 (A-h) and Comparative 14 (D-h) using the mold flux h with the highest solidification temperature, the heat flux index in the upper mold part 21 decreased and the solidification heterogeneity index improved. However, the heat flux also decreased in the lower mold part 22, and the growth of the solidified shell was inhibited throughout the entire mold length. In addition, since the mold flux h had a high solidification temperature, the film excessively crystallized through the lower part of the mold, and the heat flux index in the lower mold part 22 of Comparative 9 (A-h) and Comparative 14 (D-h) was smaller than that of Comparative 15 (D-i), making it difficult to fully obtain the effect of the invention mold A. Excessively high solidification temperature of the mold flux should be avoided because it leads to insufficient heat removal from the solidified shell and destabilizes the operation.

[0175] The mold E that had increased cooling performance throughout the entire length of the mold, was used for the tests. When the mold flux d (Comparative 17 (E-d)) and the mold flux i (Comparative 19 (E-i)) with solidification temperatures of 1,240 degrees C. or less were used, the crystallization of the film in the upper mold part 21 was promoted, and the solidification heterogeneity index decreased. In contrast, in Comparative 18 (E-h) using the mold flux h with a high solidification temperature, poor inflow of the mold flux occurred and the solidification heterogeneity index was large. In all the conditions, the film excessively crystallized through the lower mold part 22, and the heat flux index was small. High cooling performance along the entire mold length as in the mold E should be avoided because it leads to insufficient heat removal from the solidified shell and destabilizes the operation.

[0176] In Examples 5 to 8 using the molds B and C and the mold fluxes d to F, the heat flux in the lower mold part 22 was high and the growth of the solidified shell in the lower mold part 22 was promoted. The effect was the greatest in Example 8 (C-f). In addition, the solidification heterogeneity index of Examples 5 to 8 was 30 or less, which was better results than Comparatives 12 and 13 using the mold D. The reason why the solidification heterogeneity of Examples 5 to 8 was slightly inferior to that of Examples 1 to 4 is that the casting speed of Examples 5 to 8 was higher than that of Examples 1 to 4.

[0177] In the test using the mold flux j that does not crystallize, crystallization of the mold flux is not promoted even if the cooling performance in the upper mold part 21 is strengthened. Therefore, the heat flux index in the upper mold part 21 was high, 157 in Comparative 11 (A-j) and 140 in Comparative 16 (D-j).

[0178] The above results of the sixth exemplary embodiment revealed that improving the cooling performance in the upper mold part 21 or improving the crystallization ratio of the mold flux promoted the crystallization of the mold flux film 14 in the upper mold part 21 and contributed to reducing the solidification heterogeneity index. When both the improvement in cooling performance and the improvement in crystallization ratio were achieved, as in Example 1 (A-d), Example 2 (A-e), Example 3 (A-f), and Example 4 (A-g), the effect of reducing the solidification heterogeneity index was the greatest among Examples and Comparatives. In particular, the heat flux index in the lower part of the mold of Example 2 (A-e) and Example 3 (A-f), which used the mold fluxes with a high crystallization ratio and a low solidification temperature, was larger than that of Example 1 (A-d). The heat flux index in the lower part of the mold tends to be larger as the solidification becomes more uniform. Thus, when the solidification heterogeneity is small relative to a value estimated from the crystallization ratio of the mold flux film, the heat flux index in the lower part of the mold becomes slightly large.Seventh Exemplary Embodiment

[0179] The mold flux k to the mold flux o in Table 9, which are products according to the invention, satisfied the component composition and solidification temperature defined in the seventh exemplary embodiment.

[0180] For the mold flux p, each of the Li2O content and the Na2O content was lower than the lower limit of the seventh exemplary embodiment, and the solidification temperature was higher than the upper limit of the seventh exemplary embodiment. For the mold flux q, the Li2O content was lower than the lower limit of the seventh exemplary embodiment, the Na2O content was higher than the upper limit of the seventh exemplary embodiment, and although cuspidine crystallized, the crystallization ratio was 67%, which was lower than the preferred range (70% or more). For the mold flux r, the Li2O content was lower than the lower limit of the seventh exemplary embodiment, the Na2O content was higher than the upper limit of the seventh exemplary embodiment, C′ / S (CaO′ / SiO2 mass ratio) was lower than the lower limit of the seventh exemplary embodiment, and the crystallization ratio was significantly low. For the mold flux s, the Li2O content fell within the component range of the seventh exemplary embodiment, but the Na2O content was lower than the lower limit of the seventh exemplary embodiment and the solidification temperature was higher than the upper limit of the seventh exemplary embodiment.TABLE 9Solidi-ficationCrystal-temper-lizationLi2ONa2OK2OFT•CaOSiO2Al2O3MgOCaF2′CaO′atureViscosityratioMainmass %C′ / S° C.poise%crystalExamplesk2.02.10.012.551.229.12.80.317.838.41.312300.4391Cuspidinel3.04.00.014.548.526.92.70.416.836.41.411700.4883Cuspidinem4.98.20.316.743.024.52.20.210.835.21.410900.2481Cuspidinen1.12.30.09.752.032.91.90.214.141.91.312500.6186Cuspidineo6.32.40.017.246.225.91.40.615.834.81.311200.1582CuspidineComparativesp0.00.00.310.451.233.54.00.621.136.11.113051.1089Cuspidineq0.010.5 0.010.041.134.02.02.47.335.91.111600.7267Cuspidiner0.016.0 0.011.023.743.33.12.92.421.90.5 9802.704—s1.50.00.010.453.430.63.50.617.440.91.313200.9290CuspidineThe underlines indicate the values outside the scope of the invention.TABLE 10Heat fluxHeat fluxindex inindex inupperSolidificationlowerMoldmoldheterogeneitymoldMoldfluxpartindexpartExamples21Ak81113622Al83153823Am84154124An82133425Ao84154326Bk85274127Bl87314528Bm88304629Cm873048Compar-31Ap722625atives32Aq86313333Ar1571154434As70252135Dk86333336Dm92434237Dp85412438Dq1001003739Dr1401104340Ds80351941Ek81272242Ep721021743Eq862923The heat flux in each of the upper mold part 21 and the lower mold part 22 was evaluated in the same manner as in Example 2. The heat flux value in the upper mold part 21 of Comparative 38 (D-q) was set to 100, and the heat flux values for the upper mold part 21 and the lower mold part 22 in each of Examples and Comparatives were indexed and shown in “Heat flux index in upper mold part” and “Heat flux index in lower mold part” of Table 10.

[0182] Similar to Example 2 described above, the size of irregularities on a wide face surface of the cast steel was measured with a laser distance meter, and the standard deviation of the measured distance was taken as solidification heterogeneity. The solidification heterogeneity in Comparative 38 (D-q) was set to 100, and the indexed values are shown in “Solidification heterogeneity index” in Table 10.

[0183] First, a description will be made on Comparative 38 (D-q), which is a combination of the normal mold D and the ordinary mold flux q that crystallizes but has a rather low crystallization ratio. Comparative 38 (D-q) is a comparative in which hypo-peritectic steel with the composition shown in Table 3 was cast using the mold D in which the cooling performance for the mold was constant from the upper part to the lower part of the mold, and the mold flux q in Table 9 in which cuspidine crystallized and precipitated in the mold flux film as the main crystal.

[0184] In Comparative 38 (D-q), the heat flux in the mold showed a normal heat flux distribution, being large in the upper mold part 21 where the surface temperature of the solidified shell 13 was high, and being small in the lower mold part 22 where the surface temperature of the solidified shell 13 was low.

[0185] In Comparative 38 (D-q), the surface of the obtained cast steel had irregularities due to the degree of solidification shrinkage specific to hypo-peritectic steel. The heterogeneity of solidification observed in Comparative 38 (D-q) is probably due to insufficient crystallization of the mold flux film and insufficient reduction in the heat flux in the upper mold part 21.

[0186] In contrast, in Comparative 32 (A-q) using the mold A, the cooling performance in the upper mold part 21 was strengthened to promote crystallization of the mold flux film. As a result, the heat flux index in the upper mold part 21 was decreased to 86, and the solidification heterogeneity index was improved to 31. Although the cooling performance in the lower mold part 22 of Comparative 32 (A-q) was the same as that of Comparative 38 (D-q), the heat flux index in the lower mold part 22 of Comparative 32 (A-q) was slightly lower than that of Comparative 38 (D-q) due to the effect of the mold flux film of which crystallization was promoted in the upper mold part 21.

[0187] In Comparative 35 (D-k) in which the solidification temperature and crystallization ratio of the mold flux were increased over Comparative 38 (D-q), the crystallization of the mold flux film was promoted in the upper mold part 21, the heat flux index in the upper mold part 21 decreased, and the solidification heterogeneity index improved to 33. Since the solidification temperature of the mold flux of Comparative 35 (D-k) was higher than that of Comparative 38 (D-q), crystals in the mold flux film grew through the lower part of the mold, and the heat flux index in the lower mold part 22 decreased slightly in Comparative 35 (D-k).

[0188] In Example 21 (A-k) in which the cooling performance in the upper mold part 21 was strengthened over Comparative 35 (D-k), the crystallization of the mold flux film was promoted, the heat flux index in the upper mold part 21 decreased to 81, and the solidification heterogeneity index improved to 11.

[0189] In Comparative 36 (D-m) using the mold flux that had an equivalent or lower solidification temperature compared to Comparative 38 (D-q) but had an increased crystallization ratio compared thereto, the crystallization of the mold flux film was promoted in the upper mold part 21, the heat flux index in the upper mold part 21 decreased, and the solidification heterogeneity index improved, but those were insufficient.

[0190] In Comparative 31 (A-p), Comparative 37 (D-p), Comparative 34 (A-s), and Comparative 40 (D-s) using the mold flux p and the mold flux s with high solidification temperatures, the heat flux index in the upper mold part 21 decreased and the solidification heterogeneity index improved. However, the heat flux also decreased in the lower mold part 22, and the growth of the solidified shell was inhibited throughout the entire mold length. In addition, since these mold fluxes had high solidification temperatures, the film excessively crystallized through the lower part of the mold, and the heat flux index in the lower mold part 22 of Comparative 31 (A-p), Comparative 37 (D-p), Comparative 34 (A-s), and Comparative 40 (D-s) was smaller than that of Comparative 38 (D-q), making it difficult to fully obtain the effect of the invention mold A. Excessively high solidification temperature of the mold flux should be avoided because it leads to insufficient heat removal from the solidified shell and destabilizes the operation.

[0191] The mold E that had increased cooling performance throughout the entire length of the mold, was used for the tests. When the mold flux k (Comparative 41 (E-k)) and the mold flux q (Comparative 43 (E-q)) with solidification temperatures of 1,240 degrees C. or less were used, the crystallization of the film in the upper mold part 21 was promoted, and the solidification heterogeneity index decreased. In contrast, in Comparative 42 (E-p) using the mold flux p with a high solidification temperature, poor inflow of the mold flux occurred and the solidification heterogeneity index was large. In all conditions, the film excessively crystallized through the lower mold part 22, and the heat flux index was small. High cooling performance along the entire mold length as in the mold E should be avoided because it leads to insufficient heat removal from the solidified shell and destabilizes the operation.

[0192] In Examples 26 to 29 using the molds B and C and the mold fluxes k to m, the heat flux in the lower mold part 22 was high and the growth of the solidified shell in the lower mold part 22 was promoted. The effect was the greatest in Example 29 (C-m). In addition, the solidification heterogeneity index of Examples 26 to 29 was 31 or less, which was better results than Comparatives 35 and 36 using the mold D. The reason why the solidification heterogeneity of Examples 26 to 29 was slightly inferior to that of Examples 21 to 25 is that the high casting speed of Examples 26 to 29 was higher than that of Examples 21 to 25.

[0193] In the test using the mold flux r that does not crystallize, crystallization of the mold flux is not promoted even if the cooling performance in the upper mold part 21 is strengthened. Therefore, the heat flux index in the upper mold part 21 was high, 157 in Comparative 33 (A-r) and 140 in Comparative 39 (D-r).

[0194] The above results of the sixth exemplary embodiment revealed that improving the cooling performance in the upper mold part 21 or improving the crystallization ratio of the mold flux promoted the crystallization of the mold flux film in the upper mold part 21 and contributed to reducing the solidification heterogeneity index. When both the improvement in cooling performance and the improvement in the crystallization ratio were achieved, as in Example 21 (A-k), Example 22 (A-l), Example 23 (A-m), Example 24 (A-n), and Example 25 (A-o), the effect of reducing the solidification heterogeneity index was the greatest among Examples and Comparatives. In particular, the heat flux index in the lower mold part 22 of Example 22 (A-l), Example 23 (A-m), and Example (A-o), which used the mold flux with a high crystallization ratio and a low solidification temperature, was larger than that of Example 21 (A-k). The heat flux index in the lower part of the mold tends to be larger as the solidification becomes more uniform. Thus, when the solidification heterogeneity is small relative to a value estimated from the crystallization ratio of the mold flux film, the heat flux index in the lower part of the mold becomes slightly large.

[0195] As described above, by implementing the invention in combination with Examples 2 and 3, the heat flux index in the upper mold part 21 was lower and the heat flux index in the lower mold part 22 was higher than in Comparatives, and it is found out that a close-to-ideal heat flux control in the mold can be achieved. The close-to-ideal heat flux control in the mold refers to a control by which the heat flux index in the upper part of the mold becomes low and the heat flux index in the lower part of the mold becomes high. In the invention, the close-to-ideal heat flux control in the mold is achieved by using the continuous casting mold in which the cooling performance in the upper part of the mold is higher than that in the lower part of the mold, and the mold flux that is easy to crystallize.EXPLANATION OF CODES1 continuous casting mold, 2 copper mold plate, 3 cooling-water channel, 4 back frame, 5 submerged entry nozzle, 6 mold surface, 7 cooling-water channel end, 8 upper water supply and drainage channel, 9 lower water supply and drainage channel, 10 molten steel, 11 mold flux, 12 molten flux layer, 13 solidified shell, 14 mold flux film, 15 solid film, 16 liquid film, 17 boundary position, 21 upper mold part, 22 lower mold part, 23 upper end of mold, 24 lower end of mold, 25 width direction, 26 casting direction, 31 wide face, 32 narrow face, 41 continuous casting machine, 42 tundish, 43 support roll, 44 machine length, 45 cast steel

Examples

first exemplary embodiment

[0067]A first exemplary embodiment of the invention will be described with reference to FIGS. 1 to 6.

[0068]The first exemplary embodiment of the invention relates to a continuous casting mold 1 for a continuous casting process of steel using a mold flux, in which a side of the continuous casting mold 1 that comes into contact with molten steel is configured including the water-cooling copper mold plate 2, the cooling-water channel 3 arranged in the copper mold plate 2 allows cooling water to flow in a vertical direction of the mold, and the cooling-water channel 3 is connected to the upper water supply and drainage channel 8 at the upper end of the mold and the lower water supply and drainage channel 9 at the lower end of the mold, and one or more of the following [A], [B], and [C] are satisfied between the upper part of the mold and the lower part of the mold.[0069][A] A cross-sectional area A of the cooling-water channel 3 in the upper mold part 21 is smaller than the cross-sectio...

second exemplary embodiment

[0079]FIG. 8 illustrates a general overall view of a continuous casting machine 41 for casting steel, in particular, the continuous casting machine 41 for casting slabs. Molten steel is injected into the continuous casting mold 1 from a tundish 42 through a submerged entry nozzle 5, and a cast steel 45 in which solidification has progressed is drawn from a lower part of the continuous casting mold 1 and pulled out while being supported by support rolls 43. The continuous casting machine 41 illustrated in FIG. 8 is of a vertical bending type. A casting distance from a surface of the molten steel in the continuous casting mold 1 to the most downstream support roll 43 is called a machine length 44.

[0080]The second exemplary embodiment relates to a continuous casting machine provided with the continuous casting mold 1 described in the first exemplary embodiment, characterized in that when a heat flux Q determined by the formula (1) is given according to a set casting speed Vc of the con...

third exemplary embodiment

[0097]A third exemplary embodiment relates to a continuous casting method characterized by performing continuous casting of steel using a continuous casting machine provided with the continuous casting mold described in the first exemplary embodiment or the continuous casting machine described in the second exemplary embodiment, while supplying a mold flux into the continuous casting mold.

Mold Fluxes in Fourth and Fifth Exemplary Embodiments

[0098]A fourth exemplary embodiment relates to a continuous casting method characterized in that, in a case where a mold flux sample is melted, cooled at a cooling rate of 10 degrees C. / min, and solidified, and the crystallization ratio of the sample is expressed as an area ratio, a mold flux having a crystallization ratio of 20% or more is used as the mold flux in the continuous casting method according to the third exemplary embodiment. The crystallization ratio (area ratio) of the mold flux is determined as a ratio (%) of an area of a region w...

TECHNICAL FIELD

[0001] The present invention relates to a continuous casting mold, a continuous casting machine, and a continuous casting method, in particular, relates to a continuous casting mold for a continuous casting process of steel using a mold flux, a continuous casting machine provided with the continuous casting mold, and a continuous casting method using them.BACKGROUND ART

[0002] In the continuous casting process of steel, a method has been widely used in which molten steel is fed from an intermediate vessel, which is a tundish, into a continuous casting mold using a submerged entry nozzle, and a mold flux is supplied to a molten steel surface inside the mold. A water-cooling copper mold plate is placed on a surface of the continuous casting mold that comes into contact with the molten steel. As illustrated in FIG. 2, molten steel 10 solidifies at a portion in contact with the continuous casting mold 1, forming a solidified shell 13. A mold flux 11 supplied to the molten steel surface in the mold melts upon contact with the high-temperature molten steel 10 and flows into a gap between the continuous casting mold 1 and the solidified shell 13 as a mold flux film 14. The mold flux 11 has a function of keeping the temperature of the molten steel surface. Also, the inflowing mold flux film 14 provides lubrication between the continuous casting mold 1 and the solidified shell 13, and serves to control the heat flux from the solidified shell 13 to the continuous casting mold 1, thereby controlling the cooling of the cast steel surface. Hereinbelow, the continuous casting mold will be also simply referred to as “mold”.

[0003] The method of controlling the cooling of the cast steel surface by controlling the heat flux in the mold often uses mild cooling of the cast steel from the viewpoint of preventing cracks in the cast steel surface. The following two methods are known to slowly cool the cast steel: (1) a method of reducing heat conduction by decreasing the thermal conductivity of a copper mold plate and increasing the temperature of the mold surface in contact with the cast steel to reduce a temperature difference with the cast steel surface; (2) a method of reducing heat flux by promoting crystallization of the flux film formed from molten flux flowing into a gap between the mold and the cast steel and obtaining a shielding effect against radiation heat transfer and an inhibitory effect against heat conduction provided by fine voids.

[0004] There are many examples of known techniques that fall under (1), such as Patent Literatures 1 to 7. Patent Literature 1 discloses a method of disposing a sheet with low thermal conductivity in an upper part of a mold. Patent Literature 2 discloses a method of separating a cooling slit for an upper part of a mold from a surface on a molten steel side. Patent Literature 3 discloses a method of separating cooling water systems for upper and lower parts of a mold to reduce cooling in the upper part. Patent Literature 4 discloses a method of providing holes in an upper part of a mold. Patent Literature 5 discloses a method of providing multiple cooling water systems in a horizontal direction of a mold to reduce cooling in the upper part. Patent Literature 6 discloses a method of providing a sprayed layer with low thermal conductivity on a surface of an upper part of a mold. Patent Literature 7 discloses a method of embedding a heating element in an upper part of a mold. These techniques are expected to be effective to a certain extent when only the heat conduction in the mold is considered. However, the problem is that an increase in the mold surface temperature inhibits the crystallization of the mold flux film, and therefore may be counterproductive to method (2). Furthermore, an increase in the mold surface temperature may cause problems such as sticking of the cast steel to the mold surface, cracks in the mold surface, or deformation of the copper mold plate, making it difficult to put these techniques into practical use.

[0005] Examples of known techniques that fall under (2) include as follows. Patent Literatures 8 and 9 disclose a method for reducing the heat flux in a mold by causing cuspidine to crystallize and precipitate in a mold flux film. Patent Literature 10 discloses a method of causing a small amount of perovskite having a high melting point as a single substance to crystallize and precipitate, and using it as a nucleus to promote precipitation of melilite that is a main crystal. These methods are effective in mild cooling of the cast steel surface and preventing cracks, and are widely used. On the other hand, the control of crystallization of the mold flux film relies on composition designs of the mold flux, and there has been no idea of combining the composition design of the mold flux with the control of the mold surface temperature to increase the freedom of crystallization control and maximize its effect.

[0006] The methods described in Patent Literatures 8, 9, and 10, which are effective in mild cooling of the cast steel and preventing cracks, are widely used. Although the crystallization of the flux film is effective in inhibiting cracking, excessive crystallization not only causes poor lubrication in the mold, but also increases the risk of bulging due to reduced heat removal from the cast steel, breakout (BO), or false detection of BO, which can be factors that hinder good operation. In other words, in order to inhibit cracking of the cast steel, it is desirable to use a mold flux that does not simply cause the crystalline phase of the flux film to crystallize and precipitate, but that causes instant crystallization and precipitation in the upper part of the mold while satisfying an appropriate crystallization ratio through the lower part of the mold. To meet this demand, mold fluxes with adjusted crystallization speeds and solidification temperatures have been developed (Patent Literatures 11, 12, and 13).

[0007] In all of the inventions described in Patent Literatures 1 to 7, the cast steel is slowly cooled in the upper part of the mold, in the vicinity of the meniscus, in order to prevent surface cracks of the cast steel, and the cooling performance is lower in the upper part of the mold than in the lower part of the mold. In contrast, when considering high-speed continuous casting, there is a technical idea of strengthening heat removal in the upper part of the mold in order to strengthen the cooling of the cast steel in the upper part of the mold. Since it is also necessary for the high-speed continuous casting to prevent overcooling in the lower part of the mold, there are known inventions for high-speed continuous casting that strengthen the cooling in the upper part of the mold while preventing the overcooling in the lower part of the mold (Patent Literatures 14 and 15). Patent Literature 14 discloses a mold in which a cooling surface of the mold is divided into two parts, upper and lower parts, with the upper part having a spray cooling structure and the lower part having a forced water flow cooling structure in which cooling water is passed parallel to the cooling surface. Patent Literature 15 discloses a mold for continuous casting having a divided structure in which upper and lower molds having the divided structure include a water-cooling copper plate. The upper mold is of a jet heat transfer type with multiple horizontal slit-shaped water channels, and the lower mold is of a turbulent heat transfer type with multiple vertical slit-shaped water channels.CITATION LISTPatent Literature(s)

[0008] Patent Literature 1: JPS61-195742 A

[0009] Patent Literature 2: JPS61-195746 A

[0010] Patent Literature 3: JPH01-143742 A

[0011] Patent Literature 4: JPH02-197352 A

[0012] Patent Literature 5: JPH02-200353 A

[0013] Patent Literature 6: JPH08-267182 A

[0014] Patent Literature 7: JP 2000-202583 A

[0015] Patent Literature 8: JPH11-320058 A

[0016] Patent Literature 9: JP 2000-158105 A

[0017] Patent Literature 10: JP 2010-214387 A

[0018] Patent Literature 11: JP 2006-247744 A

[0019] Patent Literature 12: JP 2020-146719 A

[0020] Patent Literature 13: JP 2021-074782 A

[0021] Patent Literature 14: JPH07-314096 A

[0022] Patent Literature 15: JPH10-128513 ANon-Patent Literature(s)

[0023] Non-Patent Literature 1: The Japan Society of Mechanical Engineers, “JSME, Journal of the Heat Transfer Society of Japan”

[0024] Non-Patent Literature 2: Hanao et, al. “Mold Flux for High Speed Continuous Casting of Hypoperitectic Steel Slabs” Tetsu-to-Hagane, Vol. 88 (2002) No. 1 pp. 23-28SUMMARY OF THE INVENTIONProblem(s) to be Solved by the Invention

[0025] As described above, in order to prevent the cast steel surface from cracking, it is important to control the heat flux in the mold to control the cooling of the cast steel surface. The control can be performed through the multiple means described in (1) and (2) above, and each of them is expected to have a certain effect. However, these two mechanisms have been considered as individual inventions without any discussion of their interaction.

[0026] The present invention has been made to achieve closer-to-ideal heat flux control in the mold than ever before, by taking into consideration the interaction between the cooling function of the mold itself and the heat flux control in the mold via the mold flux film, and by implementing both in an appropriate combination. An object of the invention is to provide a continuous casting mold for a continuous casting process of steel using a mold flux, a continuous casting machine provided with the continuous casting mold, and a continuous casting method performed using them.Means for Solving the Problems

[0027] That is, the gist of the invention is as follows.

[0028] [1] A continuous casting mold for a continuous casting process of steel using a mold flux, a side of the continuous casting mold that comes into contact with molten steel configured including a water-cooling copper mold plate, a cooling-water channel being disposed in the copper mold plate so that cooling water flows in a vertical direction of the mold, the cooling-water channel being connected to an upper water supply and drainage channel at an upper end of the mold and to a lower water supply and drainage channel at a lower end of the mold, and one or more of [A], [B], and [C] below being satisfied,

[0029] [A] a cross-sectional area of the cooling-water channel is smaller in an upper part of the mold than in a lower part of the mold,

[0030] [B] a distance between an end of the cooling-water channel and a mold surface is shorter in the upper part of the mold than in the lower part of the mold, where the end of the cooling-water channel refers to an end of the cooling-water channel on a mold surface side, and

[0031] [C] a thermal conductivity of the copper mold plate is higher in the upper part of the mold than in the lower part of the mold.

[0032] [2] A continuous casting machine including the continuous casting mold according to [1], in which when a heat flux Q calculated by formula (1) is given according to a set casting speed Vc of the continuous casting machine, regarding a mold surface temperature Ts calculated by formula (2), a mold surface temperature TsU in the upper part of the mold is lower by 20 degrees C. or more than a mold surface temperature TsL in the lower part of the mold,[Formula⁢ 1]Q=1.16×106·VC0.7(1)[Formula⁢ 2]Ts=(Xλm+4·A0.023Re0.8·Pr0.4·λw·L)⁢Q+Tw(2)Re=Vw / (η / ρ)(3)Pr=η⁢CP / λw(4)d=4⁢A / L(5)where, Q: heat flux [W / m2], Vc: set casting speed [m / min], Ts: mold surface temperature [degrees C.], Tw: cooling water temperature [degrees C.], X: distance between end of cooling-water channel and mold surface [m], λm: thermal conductivity of copper mold plate [W / (m·K)], λw: thermal conductivity of cooling water [W / (m·K)], A: cross-sectional area of cooling-water channel [m2], L: perimeter of cooling-water channel [m], Re: Reynolds number of cooling water in cooling-water channel [-], Pr: Prandtl number of cooling water in cooling-water channel [-], d: equivalent diameter [m], Vw: flow velocity of cooling water in cooling-water channel [m / s], η: viscosity of water [Pa·s], ρ: density of water [kg / m3], CP: specific heat of water [J / (kg·K)]).[3] A continuous casting method, including performing continuous casting of steel using a continuous casting machine provided with the continuous casting mold according to [1] or the continuous casting machine according to [2], while supplying a mold flux into the continuous casting mold.

[0035] [4] The continuous casting method according to [3], in which a mold flux having a crystallization ratio of 20% or more in terms of area ratio when melted, cooled at a cooling rate of 10 degrees C. / min and solidified, is used as the mold flux.

[0036] [5] The continuous casting method according to [4], in which a mold flux in which a main crystal of the solidified mold flux is cuspidine and a viscosity at 1,300 degrees C. is less than 1.5 poise, is used.

[0037] [6] The continuous casting method according to [3], in which a mold flux in which: a CaO′ / SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; Li2O is contained, among alkali metal oxides, at 1 mass % or more and 15 mass % or less; and a solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less, is used as the mold flux,

[0038] CaO′ above being determined by formula (6) and formula (7),CaO’⁢(mass⁢ %)=T·CaO-CaF2’×0.718(6)CaF2’⁢(mass⁢ %)=(F-Li2⁢O×1.27-Na2⁢O×0.613-K2⁢O×0.403)×2.05(7)where:in a case where a right side of the formula (7) is negative, a left side of the formula (7) is set to 0%,

[0041] F: content ratio (mass %) of F in mold flux,

[0042] a total content of components in the mold flux excluding C is taken as 100 mass %, and a content of each of the components is determined, and

[0043] T·CaO, Li2O, Na2O, and K2O refer to contents of oxides (mass %) calculated assuming that Ca, Li, Na, and K in the mold flux are all oxides.

[0044] [7] The continuous casting method according to [3], in which a mold flux in which: a CaO′ / SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; among alkali metal oxides, Li2O is contained at 1 mass % or more and 10 mass % or less and Na2O is contained at 1 mass % or more and 10 mass % or less; and a solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less, is used as the mold flux,

[0045] CaO′ above being determined by formula (6) and formula (7),CaO’⁢(mass⁢ %)=T·CaO-CaF2’×0.718(6)CaF2’⁢(mass⁢ %)=(F-Li2⁢O×1.27-Na2⁢O×0.613-K2⁢O×0.403)×2.05(7)where:in a case where a right side of the formula (7) is negative, a left side of the formula (7) is set to 0%,

[0048] F: content ratio (mass %) of F in mold flux,

[0049] a total content of components in the mold flux excluding C is taken as 100 mass %, and a content of each of the components is determined, and

[0050] T·CaO, Li2O, Na2O, and K2O refer to contents of oxides (mass %) calculated assuming that Ca, Li, Na, and K in the mold flux are all oxides.BRIEF DESCRIPTION OF DRAWINGS

[0051] FIG. 1 is a plan cross-sectional view taken along line B-B of FIG. 6, illustrating a partial cross-section of a continuous casting mold.

[0052] FIG. 2 schematically illustrates a cross section inside the mold during continuous casting.

[0053] FIG. 3 is a sectional side view illustrating a partial cross-section of the continuous casting mold in an exemplary embodiment in which a distance XU between an end of a cooling-water channel and a mold surface in an upper part of the mold is shorter than a distance XL between the end of the cooling-water channel and the mold surface in a lower part of the mold.

[0054] FIG. 4 is a sectional side view illustrating a partial cross-section of the continuous casting mold in an exemplary embodiment in which a cross-sectional area of the cooling-water channel is smaller in the upper part of the mold than in the lower part of the mold.

[0055] FIG. 5 is a sectional side view illustrating a partial cross-section of the continuous casting mold in an exemplary embodiment in which a thermal conductivity of a copper mold plate is larger in the upper part of the mold than in the lower part of the mold.

[0056] FIG. 6 is a sectional side view taken along line A-A of FIG. 1, illustrating a partial cross-section of the continuous casting mold.

[0057] FIG. 7 is a schematic plan view of the entire continuous casting mold as seen from above.

[0058] FIG. 8 is a schematic side view of a continuous casting machine.DESCRIPTION OF EMBODIMENT(S)

[0059] The invention relates to a technique for a heat flux control in a mold for continuous casting of steel, which utilizes a mold flux to maximize an effect of inhibiting heat conduction by shielding radiation heat transfer and by forming fine voids with crystals precipitating in a flux film formed from a molten flux flowing into a gap between the mold and a cast steel, thereby mild cooling a surface of the cast steel.

[0060] A continuous casting mold 1 used for continuous casting of steel, in particular, a continuous casting mold for a continuous casting machine for casting slabs, is typically configured by combining two opposing wide faces 31 and two opposing narrow faces 32, as illustrated in FIG. 7. As illustrated in FIG. 1, a copper mold plate 2 formed from copper with excellent thermal conductivity is provided for the wide and narrow faces of the continuous casting mold 1 that come into contact with molten steel. Water-cooling is performed from a rear side of the copper mold plate 2 to keep the mold surface temperature in a steady state at approximately 300 degrees C. or less. Accordingly, it is possible to prevent sticking of the cast steel to a mold surface, softening or deformation of the copper material. In molds with a relatively large cross section for a continuous slab caster, a continuous bloom caster, and the like, a long and narrow vertical slit is typically formed on the rear side of the copper mold plate 2, and this slit and a back frame 4 on the rear side of the copper mold plate 2 form a cooling-water channel in the mold 3. As illustrated in the sectional side view of FIG. 6, the cooling-water channel 3 is connected to an upper water supply and drainage channel 8 at an upper end of the mold and a lower water supply and drainage channel 9 at a lower end of the mold, and is arranged to circulate the cooling water in the vertical direction of the mold. The cooling water supplied from the lower water supply and drainage channel 9 flows in the cooling-water channel 3 and is discharged from the upper water supply and drainage channel 8. As illustrated in the plan cross-sectional view of FIG. 1, many cooling-water channels 3 are arranged in a width direction 25 to increase the surface area of the cooling-water channels 3 in contact with the cooling water, thereby obtaining sufficient cooling performance.

[0061] As described in the Background Art section, there have been many ideas for making the cooling performance of the mold different in its vertical direction from the viewpoint of preventing cracks in the surface of the cast steel. All of these ideas aim to slow down the cooling of an initial solidified shell in the upper part of the mold by making the cooling performance in the upper part of the mold lower than that in the lower part of the mold. To achieve that purpose, the above-described two methods have been addressed: (1) a method of reducing heat conduction by decreasing the thermal conductivity of the copper mold plate and increasing the temperature of the mold surface to reduce a temperature difference between the mold surface and the cast steel surface; and (2) a method of reducing heat flux from the cast steel to the mold by promoting crystallization of the mold flux film flowing into a gap between the mold and the cast steel and obtaining the shielding effect against radiation heat transfer and the inhibitory effect against heat conduction provided by fine voids.

[0062] However, according the actual measurement of the growth rate of the initial solidified shell and the heat flux in the mold during continuous casting, it has been found out that if a mold flux that easily crystallizes is used and the cooling performance in the upper part of the mold is reduced to increase the mold surface temperature, the initial solidified shell may be strongly cooled, which is the opposite of what the invention intended. This is presumably because the crystallization of the mold flux film in the gap between the mold and the cast steel is inhibited. On the other hand, it has been found that if a mold flux that easily crystallizes is used and the cooling performance in the upper part of the mold is increased to decrease the mold surface temperature, the initial solidified shell is cooled slowly. This is presumably because the crystallization of the mold flux film in the gap between the mold and the cast steel is promoted. The inventors have found this phenomenon, which may be said to be cooling paradox, and reached a method for achieving a closer-to-ideal heat flux control in the mold by utilizing this phenomenon while solving the problems of the conventional techniques.

[0063] Features of the invention will be described below along with the above-described configurations of the invention.

[0064] First, the upper part and the lower part of the continuous casting mold will be described with reference to FIGS. 1 and 2. As illustrated in FIG. 2, an upper mold part 21 refers to an upper part of the continuous casting mold 1, which at least ranges from an upper end 23 of the mold to a position 0.2 m below the upper end 23. A lower mold part 22 refers to a lower part of the continuous casting mold 1, which is an area at least below a position 0.6 m below the upper end 23 of the mold. A lower end of the upper mold part 21 is preferably located at a position higher than a position 0.3 m below the upper end 23 of the mold.

[0065] A cross-sectional area of the cooling-water channel 3 in the upper mold part 21 being smaller than that of the cooling-water channel 3 in the lower mold part 22 means, but is not limited to, that the cross-sectional area of the cooling-water channel 3 in the upper mold part 21, i.e., the upper part of the continuous casting mold 1 at least ranging from the upper end 23 of the mold to a position 0.2 m below the upper end 23 of the mold, is smaller than that of the cooling-water channel 3 in the lower mold part 22, i.e., the lower part of the continuous casting mold 1 that is an area at least below the position 0.6 m below the upper end 23 of the mold. Therefore, for example, a cross-sectional area of the cooling-water channel 3 in a range from the upper end 23 of the mold to the position 0.3 m below the upper end 23 of the mold may be smaller than that of the cooling-water channel 3 in an area at least below the position 0.5 m below the upper end 23 of the mold.

[0066] The cross-sectional area of the cooling-water channel 3 in the upper mold part 21 refers to an area of a cross section formed when the cooling-water channel 3 in the upper mold part 21 is cut in a horizontal direction (see FIG. 1).First Exemplary Embodiment

[0067] A first exemplary embodiment of the invention will be described with reference to FIGS. 1 to 6.

[0068] The first exemplary embodiment of the invention relates to a continuous casting mold 1 for a continuous casting process of steel using a mold flux, in which a side of the continuous casting mold 1 that comes into contact with molten steel is configured including the water-cooling copper mold plate 2, the cooling-water channel 3 arranged in the copper mold plate 2 allows cooling water to flow in a vertical direction of the mold, and the cooling-water channel 3 is connected to the upper water supply and drainage channel 8 at the upper end of the mold and the lower water supply and drainage channel 9 at the lower end of the mold, and one or more of the following [A], [B], and [C] are satisfied between the upper part of the mold and the lower part of the mold.

[0069] [A] A cross-sectional area A of the cooling-water channel 3 in the upper mold part 21 is smaller than the cross-sectional area A of the cooling-water channel 3 in the lower mold part 22. In an example illustrated in FIG. 4, for a depth D of the cooling-water channel, a depth DU in the upper part of the mold is smaller than a depth DL in the lower part of the mold, so that a cross-sectional area AU in the upper part of the mold is smaller than a cross-sectional area AL in the lower part of the mold.

[0070] [B] A distance XU between a cooling-water channel end 7 and a mold surface 6 in the upper mold part 21 is shorter than a distance XL between the cooling-water channel end 7 and the mold surface 6 in the lower mold part 22 (see FIG. 3). The cooling-water channel end 7 refers to an end of the cooling-water channel 3 on a mold surface side.

[0071] [C] A thermal conductivity λmU of the copper mold plate 2 in the upper mold part 21 is larger than a thermal conductivity λmL of the copper mold plate 2 in the lower mold part 22 (see FIG. 5).

[0072] By satisfying one or more of the above [A], [B], and [C], it is possible to make the cooling performance in the vicinity of a molten steel surface in the upper part of the mold higher than that in the lower part of the mold. The cooling-water channel is connected to the upper water supply and drainage channel at the upper end of the mold and the lower water supply and drainage channel at the lower end of the mold, and the flow rate of cooling water in the cooling-water channel is constant from the upper part of the mold to the lower part of the mold. Therefore, it is possible to make the flow velocity of the cooling-water in the upper mold part 21 faster than that in the lower mold part 22 by making the cross-sectional area AU of the cooling-water channel 3 in the upper mold part 21 smaller than the cross-sectional area AL of the cooling-water channel 3 in the lower mold part 22.

[0073] The continuous casting mold according to the first exemplary embodiment is characterized in that the cooling performance in the vicinity of the molten steel surface in the upper part of the mold is made higher than the cooling performance in the lower part of the mold in order to maximize the effect of mild cooling caused by crystallization of a mold flux film, that is, the effect of reducing heat flux from a cast steel to the mold.

[0074] If the mild cooling effect accompanying the crystallization of the mold flux film is required over the entire vertical direction of the mold, including the upper part and the lower part of the mold defined above, as in the conventional method, it is sufficient to increase the overall cooling performance from the upper part to the lower part of the mold. The reason why the cooling performance in the upper part of the mold is made higher than the cooling performance in the lower part of the mold in the exemplary embodiment of the invention, is as follows.

[0075] Since the mold is used to cool and solidify molten steel to form a cast steel, the mold basically needs to provide strong cooling. Therefore, even if mild cooling is necessary to prevent cracks in a surface of the cast steel, infinite mild cooling leads to the denial of the original mold function, and thus mild cooling should be kept to the minimum necessary. The mild cooling required to prevent surface cracking of the cast steel is required only in the vicinity of the molten steel surface in the upper part of the mold, specifically, only above a position 50 mm to 200 mm below the molten steel surface. For a part of the mold lower than the above, excessive mild cooling should be avoided.

[0076] From the cooling paradox phenomenon via the mold flux film described above, there is a possibility that the crystallization of the mold flux film can be promoted by increasing the cooling performance in the vicinity of the molten steel surface in the upper part of the mold, and the amount of heat removed from a solidified shell can be reduced. On the other hand, for the lower part of the mold, there is a possibility that the amount of heat removed from the solidified shell can be increased by inhibiting the excessive crystallization of the mold flux film by reducing the cooling performance. If such a phenomenon is achievable, it would result in the moderate mild cooling in the vicinity of the molten steel surface in the upper part of the mold and the maintaining of sufficient heat flux in the lower part of the mold. In view of the above, the continuous casting of steel was performed using the continuous casting mold according to the first exemplary embodiment and a mold flux that easily crystallizes. This revealed that the cooling of the cast steel in the upper part of the mold was reduced and the cooling of the cast steel in the lower part of the mold was increased compared to a case where a conventional continuous casting mold was used. Details will be described in Examples below. According to a continuous casting method using a continuous casting machine provided with the continuous casting mold according to the first exemplary embodiment, the surface temperature of the copper plate in the vicinity of the molten steel surface in the upper part of the mold is kept low. This results in the secondary effect of inhibiting sticking of the cast steel to the mold surface, cracks in the mold surface, and deformation of the copper mold plate.

[0077] In the exemplary embodiment, it is possible to make the cooling performance in the vicinity of the molten steel surface in the upper mold part 21 higher than the cooling performance in the lower mold part 22 by satisfying one or more of the above [A], [B], and [C]. The cooling-water channel 3 is connected to the upper water supply and drainage channel 8 at the upper end of the mold and the lower water supply and drainage channel 9 at the lower end of the mold, and the flow rate of cooling water in the cooling-water channel 3 is constant from the upper part to the lower part of the mold. Thus, it is possible to make the flow velocity of the cooling water different between the upper part of the mold and the lower part of the mold (see FIG. 4) by making the cross-sectional area A of the cooling-water channel 3 different between the upper part of the mold and the lower part of the mold. These measures can be implemented at low cost and are effective enough for the exemplary embodiment.

[0078] Since the exemplary embodiment utilizes the cooling paradox phenomenon via the mold flux film, the exemplary embodiment is applicable only to the continuous casting using the mold flux.Second Exemplary Embodiment

[0079] FIG. 8 illustrates a general overall view of a continuous casting machine 41 for casting steel, in particular, the continuous casting machine 41 for casting slabs. Molten steel is injected into the continuous casting mold 1 from a tundish 42 through a submerged entry nozzle 5, and a cast steel 45 in which solidification has progressed is drawn from a lower part of the continuous casting mold 1 and pulled out while being supported by support rolls 43. The continuous casting machine 41 illustrated in FIG. 8 is of a vertical bending type. A casting distance from a surface of the molten steel in the continuous casting mold 1 to the most downstream support roll 43 is called a machine length 44.

[0080] The second exemplary embodiment relates to a continuous casting machine provided with the continuous casting mold 1 described in the first exemplary embodiment, characterized in that when a heat flux Q determined by the formula (1) is given according to a set casting speed Vc of the continuous casting machine, a mold surface temperature Ts calculated by the formula (2) in the upper mold part 21 is lower by 20 degrees C. or more than that in the lower mold part 22.

[0081] The second exemplary embodiment relates to a continuous casting machine provided with the continuous casting mold of the first exemplary embodiment, and specifies more specifically the contents described in the first exemplary embodiment. In the exemplary embodiment, the crystallization of the mold flux is affected by the mold surface temperature. The behavior of heat conduction and heat transfer from the mold surface to the cooling water can be calculated as follows.

[0082] As illustrated in FIG. 1, the continuous casting mold 1 typically has the slit-shaped cooling-water channels 3 with width W, depth D, and cross-sectional area A on the rear side of the copper mold plate 2. Cooling water flows through the cooling-water channels 3 at a velocity of 5 m / s to 10 m / s. As illustrated in FIG. 6, the cooling-water channel 3 arranged in the copper mold plate 2 allows the cooling water to flow in the vertical direction of the mold, and the cooling-water channel 3 is connected to the upper water supply and drainage channel 8 at the upper end of the mold and the lower water supply and drainage channel 9 at the lower end of the mold. Generally, the cooling water is flowed from bottom to top. The interval between the cooling-water channels in the width direction 25 of the mold is not specified here, but the cooling-water channels are ordinarily arranged densely enough that unevenness in the mold surface temperature is acceptable. Specifically, the interval between the cooling-water channels is within a range not exceeding three times the width W of the cooling-water channel. Although the above range may be exceeded in some places due to structural constraints, the effect on the overall cooling performance is small.

[0083] The heat transfer from the mold surface 6 to the end of the cooling-water channel 3 on the mold surface side is controlled by the thermal conductivity λm of the copper mold plate material. Here, the end of the cooling-water channel 3 on the mold surface side is referred to as the cooling-water channel end 7. The distance between the cooling-water channel end 7 and the mold surface 6 is denoted by X (see FIG. 1). There may be a thin plating layer (typically, a thickness of approximately tens to one hundred micrometers) on the mold surface, but its effect is negligible. Then, a heat transfer coefficient hm between the mold surface 6 and the cooling-water channel end 7, which is used in the following formula (8), is given by the following formula (9).Ts=(1 / hm)⁢Q+Tf(8)hm=λm / X(9)where, Ts: mold surface temperature, Q: heat flux, Tr: mold surface temperature in contact with cooling-water channel, λm: thermal conductivity of copper mold plate, and X: distance from cooling-water channel end 7 to mold surface 6 (see FIG. 1).

[0085] The heat transfer between the cooling water in the cooling-water channel 3 and the copper mold plate 2 is represented by the following formula (10) using a heat transfer coefficient hw between the mold and the cooling water, the heat flux Q, and a cooling water temperature Tw. The heat transfer coefficient hw between the mold and the cooling water is represented by the following formula (11) using a Nusselt number Nu.Tf=(1 / hw)⁢Q+Tw(10)hw=Nu×λw / d(11)

[0086] For determining the Nusselt number, there are many empirical formulas related to turbulent heat transfer in pipes. In an experiment for the cooling behavior of the continuous casting mold, it was found that an accurate calculation was possible by obtaining the heat transfer coefficient hw between the mold and cooling water substituted into the formula (11) through a Dittus-Boelter empirical formula (Nu=0.023Re0.8Pr0.4 (12)) (see, for example, Non-Patent Literature 1).

[0087] Here, a Reynolds number Re is defined by the formula (3), and a Prandtl number Pr is defined by the formula (4). In the formula (10), λw is a thermal conductivity of water. An equivalent diameter d in the formulae (3) and (11) is defined as an equivalent diameter d defined by the formula (5). The Nusselt number Nu is a dimensionless number indicating a magnitude of convective heat transfer relative to conductive heat transfer, the Reynolds number is a dimensionless number indicating intensity of turbulence, and the Prandtl number Pr is a dimensionless number indicating a ratio between a thickness of a velocity boundary layer and a thickness of a temperature boundary layer.

[0088] In the Reynolds number defined by the formula (3), Vw: flow velocity of cooling water in cooling-water channel, d: equivalent diameter, v: kinetic viscosity of water, and v=η / p (η: viscosity of water, p: density of water). In the Prandtl number defined by the formula (4), η: viscosity of water, λw: thermal conductivity of water, and CP: specific heat of water. The flow velocity Vw of the cooling water in the cooling-water channel is determined by dividing the total flow rate of the cooling water of the entire mold by the product of the cross-sectional area A of the cooling-water channel and the total number of the cooling-water channels. The total flow rate of the cooling water of the entire mold can be evaluated as a measurement value of a flow meter installed on a mold inlet or outlet side of cooling water piping, and is typically controlled to a constant value throughout the casting period. The equivalent diameter d defined by the formula (5) is calculated using the cross-sectional area A of the cooling-water channel and the perimeter of the cooling-water channel L=2 (W+D), and can be obtained regardless of the cross-sectional shape of the cooling-water channel. At this time, the heat transfer coefficient hw between the mold cooling water and the copper mold plate is obtained by substituting the formula (5) into the formula (11), as follows: hw=Nu×λw / (4A / L) (13).

[0089] The heat transfer from the mold surface 6 to the cooling water in the cooling-water channel 3 is represented by the following formula (14). A heat transfer coefficient hm-w can be calculated, using the heat transfer coefficient hm between the mold surface 6 and the cooling-water channel end 7 and the heat transfer coefficient hw between the mold and the cooling water, by substituting the formula (10) into the formula (8) and eliminating Tr, as represented by the following formula (15).Ts=(1 / hm-w)⁢Q+Tw(14)1 / hm-w=1 / hm+1 / hw(15)

[0090] Using these relationships, substituting the formula (15) into the formula (14), substituting the formula (9) into hm in the formula (14), and substituting the formulae (11) and (13) into hw in the formula (14), the mold surface temperature Ts for the amount of heat transferred from the cast steel to the cooling water, i.e., heat flux Q, can be determined as represented by the formula (2).

[0091] For the heat flux Q in the formula (2), the formula (1), which is an empirical formula for the casting speed, is used. Q in the formula (1) represents an average heat flux from a meniscus portion of the mold to the lower end of the mold. The heat flux in the mold during casting differs between the upper part of the mold and the lower part of the mold, and the heat flux is larger in the upper part of the mold where the surface temperature of cast steel is higher. Here, the difference in cooling performance between the upper part of the mold and the lower part of the mold is evaluated, so for the heat flux Q in the formula (2), the value of the formula (1), which depends only on the casting speed and does not depend on the mold part, is used.

[0092] The set casting speed Vc in the formula (1) is a representative casting speed of the continuous casting machine used, and refers to a casting speed between 0.7 and 0.8 times the maximum casting speed calculated from the cast steel thickness and the machine length 44 of the continuous casting machine 41 (see FIG. 8).

[0093] Here, for the mold surface temperature Ts calculated by the formula (2), the mold surface temperature in the upper mold part 21 is denoted by TsU, and the mold surface temperature in the lower mold part 22 is denoted by TsL.

[0094] The second exemplary embodiment relates to the continuous casting machine provided with the continuous casting mold characterized in that, with regard to the mold surface temperature Ts calculated by the formula (2), the mold surface temperature TsU in the upper mold part 21 is lower by 20 degrees C. or more than the mold surface temperature TsL in the lower mold part 22. That is, when ATs is defined by the following formula (16), each parameter used in the formula (2) (Q: heat flux [W / m2], Vc: set casting speed [m / min], Ts: mold surface temperature [degrees C.], Tw: cooling water temperature [degrees C.], X: distance between cooling-water channel end and mold surface [m], λm: thermal conductivity of copper mold plate [W / (m·K)], λw: thermal conductivity of cooling water [W / (m·K)], A: cross-sectional area of cooling-water channel [m2], L: perimeter of cooling-water channel [m], Re: Reynolds number of cooling water in cooling-water channel [-] (formula (3)), Pr: Prandtl number of cooling water in cooling-water channel [-] (formula (4)), d: equivalent diameter [m] (formula (5)), Vw: flow velocity of cooling water in cooling-water channel [m / s], η: viscosity of water [Pa·s], ρ: density of water [kg / m3], CP: specific heat of water [J / (kg·K)]) is adjusted so that ΔTs is 20 degrees C. or more.Δ⁢TS=TsL-TsU(16)

[0095] When the conditions under which ATs in the above formula (16) is 20 degrees C. or more are satisfied, if a mold flux that easily crystallizes is used for continuous casting of steel, the crystallization of the mold flux film is promoted in the upper mold part 21, and the cast steel surface can be cooled slowly. At the same time, excessive crystallization of the mold flux film in the lower mold part 22 can be inhibited, and the growth of the solidified shell can be promoted. Furthermore, it is possible to prevent sticking of the cast steel to the mold surface, prevent cracks in the mold surface, and inhibit deformation of the copper mold plate.

[0096] The right side of the formula (2) includes the cooling water temperature Tw. In a typical continuous casting mold, the cooling water supplied from the lower water supply and drainage channel 9 flows from bottom to top in the cooling-water channel 3 and is discharged from the upper water supply and drainage channel 8. Since the cooling water temperature increases as the cooling water passes through the cooling-water channel 3, a cooling water temperature TwU in the upper mold part 21 is higher than a cooling water temperature TwL in the lower mold part 22. If the flow direction of the cooling water is reversed, the cooling water temperature TwL in the lower mold part 22 becomes higher than the cooling water temperature TwU in the upper mold part 21. Therefore, although the relationship between the cooling water temperature in the upper mold part 21 and the cooling water temperature in the lower mold part 22 is not a direct controlling factor in the exemplary embodiment, making the flow direction of the cooling water in the cooling-water channel 3 from top to bottom (see FIG. 5) contributes to increasing ΔTs calculated by the formula (16). The cooling water temperature can be evaluated using measurement values of water temperature meters (thermocouples) installed on the mold inlet and outlet sides of the cooling water piping.Third Exemplary Embodiment

[0097] A third exemplary embodiment relates to a continuous casting method characterized by performing continuous casting of steel using a continuous casting machine provided with the continuous casting mold described in the first exemplary embodiment or the continuous casting machine described in the second exemplary embodiment, while supplying a mold flux into the continuous casting mold.Mold Fluxes in Fourth and Fifth Exemplary Embodiments

[0098] A fourth exemplary embodiment relates to a continuous casting method characterized in that, in a case where a mold flux sample is melted, cooled at a cooling rate of 10 degrees C. / min, and solidified, and the crystallization ratio of the sample is expressed as an area ratio, a mold flux having a crystallization ratio of 20% or more is used as the mold flux in the continuous casting method according to the third exemplary embodiment. The crystallization ratio (area ratio) of the mold flux is determined as a ratio (%) of an area of a region where crystals are observed in the electron microscope observation field to the total field area for the sample obtained by melting the mold flux sample, cooling the sample at a cooling rate of 10 degrees C. / min, and solidifying the sample.

[0099] As illustrated in FIG. 2, the molten steel 10 is supplied into the continuous casting mold 1 from the submerged entry nozzle 5. The mold flux 11 supplied to the surface of the molten steel 10 in the continuous casting mold 1 melts on the surface of the molten steel 10 to form a molten flux layer 12, which then flows between the continuous casting mold 1 and the solidified shell 13 to become a mold flux film 14. The mold flux film 14 is present in a gap between the cast steel and the mold, and has a total thickness of about 0.1 mm or more and 1 mm or less. The mold flux film 14 has a two-layer structure, with a solid film 15 at the mold side and a liquid film 16 at the cast steel side. Crystals may precipitate in the solid film 15, or the solid film 15 may be solidified into a glass-like state.

[0100] The exemplary embodiment utilizes the cooling paradox phenomenon found by the inventors, which is caused by the fact that crystallization of the mold flux film provides a shielding effect against radiation heat transfer and an inhibitory effect against heat conduction. Therefore, the effect is exhibited by using the mold flux 11 in which crystals precipitate in the mold flux film 14. Thus, a mold flux having a crystallization ratio of 20% or more in terms of area ratio when melted, cooled at a cooling rate of 10 degrees C. / min and solidified, is used as the mold flux 11. When solidified under the same conditions, the mold flux more preferably has a crystallization ratio of 50% or more in terms of area ratio. For the solidified sample, the type of crystal is determined for each crystal grain using SEM-EDS, and the crystal area ratio % for the total crystal grains is taken as the crystallization ratio.

[0101] When the basicity (CaO / SiO2 mass ratio) of the mold flux is higher than a predetermined value of about 1.0, the crystallization ratio falls within the preferred range of the exemplary embodiment described above.

[0102] On the other hand, if a mold flux with a high solidification temperature is used, in the lower part of the mold where the surface temperature of the cast steel is low, the liquid phase may not remain in the mold flux film, and the entire mold flux film may become a single layer of crystallized solid film. In such a case, the fluidity and void-filling properties of the entire mold flux film may be lost, and the cooling of the cast steel may become unstable. It is thus preferable to use a mold flux whose solidification temperature is lower than the surface temperature of the cast steel at the lower end of the mold. This prevents the entire mold flux film from becoming a single layer of crystallized solid film. The surface temperature of the cast steel at the lower end of the mold depends on the casting speed. When the casting speed in a steady state during continuous casting is denoted by Vc′, it is preferable that the solidification temperature of the mold flux is 1,250 degrees C. or less and the following formula (17) is satisfied, and it is more preferable that the solidification temperature of the mold flux is 1,200 degrees C. or less and the following formula (18) is satisfied.Upper⁢ Limit⁢ of⁢ Solidification⁢ Temperature⁢ (degrees⁢ C)=
1170+Vc’⁢(m / min)×50(17)Upper⁢ Limit⁢ of⁢ Solidification⁢ Temperature⁢ (degrees⁢ C)=
1150+Vc’⁢(m / min)×30(18)

[0103] Here, the solidification temperature means the same temperature as a crystallization temperature, which is a temperature at which the molten flux crystallizes, i.e., solidifies, when cooled from the molten state at 10 degrees C. / min. The solidification temperature can be determined by a rapid increase in viscosity in the case of viscosity measurement, or an exothermic peak associated with crystallization in the case of thermal analysis. The term solidification temperature is often used for the viscosity measurement. The term crystallization temperature is often used for the thermal analysis measurement.

[0104] The solidification temperature of the mold flux can be adjusted to be within the preferred range by increasing the content of alkali metal elements, fluorine, and the like in the mold flux, in addition to adjusting the crystallization ratio of the mold flux to be within the preferred range of the exemplary embodiment.Fifth Exemplary Embodiment

[0105] A fifth exemplary embodiment is characterized in that a mold flux, in which a main crystal of the molten and solidified mold flux is cuspidine and the viscosity at 1,300 degrees C. is less than 1.5 poise, is applied to the continuous casting method of the fourth exemplary embodiment.

[0106] In the exemplary embodiment, a mold flux that easily crystallizes is used as the mold flux 11, and the mold surface temperature in the upper part of the mold is lowered to promote crystallization of the mold flux film 14. After the mold flux 11 is melted on the molten surface in the mold to form the molten flux layer 12, the mold flux film 14 flowing into a gap between the solidified shell 13 and the mold surface 6 is rapidly cooled. In this case, if the crystallization speed of the mold flux film 14 is slow, even the easily crystallizable mold flux 11 solidifies into a glass-like state and crystallization is delayed. The object of the invention thus cannot be sufficiently achieved. Among the crystals generated by solidification of the molten mold flux 11, cuspidine (3CaO·2SiO2·CaF2) is known to have a fast crystallization speed. Thus, the mold flux 11 in which the main crystal is cuspidine is preferably used. Here, the mold flux 11 is melted, cooled at a cooling rate of 10 degrees C. / min, and solidified into a sample, which is then subjected to X-ray diffraction evaluation, and a crystal with the highest peak height is determined to be the main crystal. Also, in the fifth exemplary embodiment, it is desirable to reduce the viscosity of the molten mold flux, specifically, to make the viscosity at 1,300 degrees C. less than 1.5 poise, in order to increase the diffusion rate. More preferably, the viscosity at 1,300 degrees C. is less than 1.0 poise. Satisfying the requirement of the fifth exemplary embodiment reliably provides the effects of the invention. Note that “poise” is a unit of viscosity in the CGS system, and is equivalent to Pa·S in the SI system. Since 1 poise is equivalent to 0.1 Pa·S, less than 1.5 poise means less than 0.15 Pa·S, and less than 1.0 poise means less than 0.1 Pa·S.

[0107] When the basicity (CaO / SiO2 mass ratio) of the mold flux 11 is higher than a predetermined value of about 1.0 and the F content in the mold flux is 5 mass % or more, cuspidine can serve as the main crystal while keeping the crystallization ratio within the preferred range of the fifth exemplary embodiment.

[0108] It is also possible for the invention to cast steel that does not require mild cooling by promoting the growth of the solidified shell in the upper part of the mold using the continuous casting mold described in the first exemplary embodiment or the continuous casting machine described in the second exemplary embodiment, and using the mold flux in which neither crystallization nor precipitation occurs or a small amount of crystals precipitate when the mold flux is melted, cooled at a cooling rate of 10 degrees C. / min, and solidified. Alternatively, as an application of the invention, it is possible to control the degree of crystallization of the flux film with the same mold flux by changing the cooling conditions of the mold, and thus a mold flux of one brand can be adapted to a wide range of steel grades and casting conditions. As described above, the invention increases variations of the heat flux in the mold by combining the mold cooling performance and the mold flux, enabling operations suitable for a wide range of steel grades and casting conditions.

[0109] By implementing the first exemplary embodiment through the fifth exemplary embodiment, it is possible to properly control the crystallization of the mold flux film, bring the heat flux in the mold closer to an ideal state, and ensure good solidification of the cast steel.Mold Fluxes in Sixth and Seventh Exemplary Embodiments

[0110] The mold flux designed with a high solidification temperature has a high crystallization ratio, which promotes the crystallization of the film in the upper part of the mold, but causes excessive crystallization of the film in the lower part of the mold. On the other hand, if the solidification temperature is designed to be low, a liquid phase is present in the mold flux film formed between the mold surface and the solidified shell. This is effective for lubrication inside the mold and for promoting the growth of the solidified shell, but it is not possible to provide sufficient mild cooling performance in the upper part of the mold. That is, from the viewpoints of the solidification temperature and the crystallization ratio, both of the mold fluxes described above are suitable for steel casting. However, conventionally, the control of the crystallization has relied only on the composition design of the mold flux, and there has been no idea of increasing the freedom of crystallization control in combination with the control of the mold surface temperature and maximizing the effect.

[0111] The sixth and seventh exemplary embodiments aim to provide a continuously casting method for steel, and achieve more ideal in-mold heat removal than ever before by considering the interaction between the cooling performance of the mold and the in-mold heat removal control via the mold flux film and implementing them in an appropriate combination.

[0112] The sixth and seventh exemplary embodiments use the water-cooling copper mold of the first or second exemplary embodiment, and take into account the interaction with the mold flux to achieve ideal in-mold heat removal. In the mold flux, the concentrations of the components CaO, SiO2, and F are adjusted to a composition that facilitates the crystallization of cuspidine, and the crystallization ratio is increased by adding Li2O and, in the seventh exemplary embodiment, Na2O.

[0113] The component composition of the mold flux used in the continuous casting method of each of the sixth exemplary embodiment and the seventh exemplary embodiment will be described. When the mold flux contains C, the total of components other than C in the mold flux is taken as 100 mass %, and the content of each component is calculated. The C content is thus treated as not included in the total mass. “%” for the content of the mold flux refers to mass %.[CaO′ / SiO2]

[0114] The mold flux contains T·CaO, SiO2, and F as the main components. Here, T·CaO is treated as a component in which the total of Ca content in the mold flux is considered as CaO. F in a powder state is added, as CaF2, to the mold flux. In the molten state of the mold flux, F has a stronger affinity for alkali metals than Ca. Thus, in the molten state of the mold flux, it is known that F reacts with alkali metals and exists as alkali metal fluorides and the remaining F content reacts with the Ca content and exists as CaF2. Then, the CaF2 concentration in the flux having the molten state is determined by CaF2′ in the above formula (7). When the right side of the formula (7) is negative, it is considered that all the F in the flux has reacted with alkali metal oxides, the remaining F is zero, and the CaF2′ concentration is also zero; therefore, the left side of formula (7) is set to 0%. The CaF2′ concentration is determined using the formula (7) as described above, and then CaO′ is calculated using the formula (6). The CaO′ defined by the formula (6) indicates a component amount in which F and the unreacted Ca content are regarded to exist as an oxide (CaO).

[0115] CaO′ / SiO2 is an indicator of components that cause cuspidine to crystallize out. Preferably, CaO′SiO2 is 0.9 or more and 2.0 or less at a mass concentration ratio. When CaO′SiO2 is less than 0.9 at the mass concentration ratio, the precipitation amount of crystals is small, and a film having sufficient mild cooling performance cannot be obtained. When CaO′SiO2 exceeds 2.0 at the mass concentration ratio, crystal phases other than cuspidine crystallize, and a film having sufficient mild cooling performance cannot be obtained. More preferably, CaO′ / SiO2 is 1.1 or more and 1.8 or less at the mass concentration ratio.[F]

[0116] The F content is 5 mass % or more. F has the effect of adjusting the solidification temperature of the mold flux and also has the effect on the crystallization of cuspidine. If the F content is less than 5 mass %, the effects are small. On the other hand, when a large amount of F is added, the viscosity of the mold flux decreases significantly, and the mold flux is entrapped in the molten steel. Thus, the F content is preferably 24 mass % or less, and more preferably 10 mass % or more.Alkali Metal Oxide

[0117] Examples of the alkali metal oxide added to the mold flux include K2O, Na2O, and Li2O. The alkali metal oxide is added to obtain the effect of increasing the crystallization speed of the mold flux and to reduce the solidification temperature and viscosity. Na2O is often used in view of the costs. Hereinafter, the contents of the components Li2O, Na2O, and K2O in the mold flux refer to the contents of oxides (mass %) calculated assuming that Li, Na, and K in the mold flux are all oxides.

[0118] It is known that Li2O has the greatest effect on improving the crystallization speed by adding alkali metal oxides, followed by Na2O and K2O (Li2O>Na2O>K2O). Li2O has the greatest effect on increasing the crystallization speed of the mold flux. In the sixth exemplary embodiment in which Na2O is not contained, the content of Li2O is 1 mass % or more and 15 mass % or less, and is preferably 3 mass % or more. In addition, the content of Li2O is preferably 10 mass % or less. In the seventh exemplary embodiment in which Na2O is contained at 1% or more as described below, the content of Li2O is 1 mass % or more and 10 mass % or less, and is preferably 3 mass % or more and 8 mass % or less. When the Li2O content is less than 1 mass %, the crystallization speed is low. Furthermore, the solidification temperature of the mold flux becomes high, and it is not possible to sufficiently prevent the occurrence of constraint BO, solidification-retarded BO, or false detection of BO. On the other hand, when Li2O is contained in excess, the amount of cuspidine crystallized decreases, making it impossible to obtain sufficient mild cooling performance.

[0119] It is known that the addition of K2O increases the viscosity and decreases the crystallization speed compared to conventional mold fluxes. This makes it difficult for a mold powder film to flow between the mold surface and the solidified shell and the film is difficult to crystallize. Thus, K2O is not preferably contained, and the K2O content is preferably less than 0.5 mass %.

[0120] Increasing the content of Li2O can ensure both the improvement in the crystallization speed and the decrease in the solidification temperature, but the raw material price of Li2O is high. Thus, in the seventh exemplary embodiment, 1 mass % or more of Na2O is added in addition to Li2O as an alkali metal oxide for decreasing the solidification temperature. On the other hand, the addition of a large amount of Na2O reduces the amount of cuspidine crystallized, so that sufficient mild cooling performance cannot be obtained. The upper limit of the Na2O content is thus set to 10 mass %. Accordingly, the Na2O content is 1 mass % or more and 10 mass % or less, and desirably 2 mass % or more and 8 mass % or less.[Al2O3 and MgO]

[0121] The total content of Al2O3 and MgO is preferably 5 mass % or less. In the design of the mold flux, Al2O3 and MgO are unavoidable impurities. Both Al2O3 and MgO reduce the amount of cuspidine crystallized, and components crystallize. As a result, the melting characteristics deteriorate and heat removal becomes difficult, so it is desirable to keep the contents of Al2O3 and MgO as small as possible. The contents of Al2O3 and MgO in the mold flux refer to the contents of oxides (mass %) calculated assuming that Al and Mg in the mold flux are all oxides.[C]

[0122] Furthermore, it is desirable to add C to the mold flux of the exemplary embodiment in addition to the above components. C is desirably contained at 1 mass % or more and 10 mass % or less as a content rate not included in the total mass of the mold flux. C has the effect of adjusting the melting speed of the mold flux. As the C content increases, the melting speed of the mold flux decreases. If the C content is less than 1 mass %, the melting speed is too high, and if the C content exceeds 10 mass %, the melting speed is too low. In either case, the flux inflow between the mold surface and the solidified shell is impaired.[Solidification Temperature]

[0123] The solidification temperature of the mold flux is specified to be 900 degrees C. or more and 1,300 degrees C. or less. If the solidification temperature exceeds 1,300 degrees C., the crystalline phase of the film is excessively formed in the lower part of the mold, and the growth of the solidified shell is insufficient. The solidification temperature is more preferably 1,250 degrees C. or less. On the other hand, when the solidification temperature of the mold flux is less than 900 degrees C., the crystallization of cuspidine is insufficient and mild cooling performance cannot be obtained satisfactorily. The solidification temperature is thus set to 900 degrees C. or more. The solidification temperature more preferably exceeds 1,100 degrees C. It is possible to adjust the solidification temperature of the mold flux to within the range from 900 degrees C. to 1300 degrees C. by adjusting the CaOVSiO2 mass ratio, F content, and alkali metal content of the mold flux within the range of the invention based on the above description.Viscosity

[0124] The viscosity of the mold flux is desirably 1 poise or less at 1,300 degrees C. If the viscosity of the mold flux exceeds 1 poise, the inflow of mold flux between the mold and solidified shell is insufficient, which easily causes the constraint BO and cast steel defects.Raw Material

[0125] Commonly used raw materials are usable as the raw materials for the mold flux in the exemplary embodiment without any problem. Examples of CaO raw materials include quicklime, limestone, and cement. Examples of SiO2 raw materials include silica sand and diatomaceous earth. Examples of Li2O raw materials include lithium carbonate. Examples of Na2O raw materials include sodium carbonate and soda ash. Examples of F raw materials include fluorite and sodium fluoride. Examples of C raw materials include carbon black and coke powder.

[0126] The shape of the raw materials for the mold flux is not limited, and may be any shape, such as powder or granules. The raw materials for the mold flux contain oxides such as Fe2O3, Al2O3, and MgO. Even if these impurities are mixed in, the amount is small and does not cause any problems.

[0127] In the continuous casting method of the sixth exemplary embodiment, the mold flux that easily crystallizes is used in which the CaO′SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; Li2O is contained, among the alkali metal oxides, at 1 mass % or more and 15 mass % or less; and the solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less.

[0128] In the seventh exemplary embodiment, the mold flux is used in which: the CaO′ / SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; among the alkali metal oxides, Li2O is contained at 1 mass % or more and 10 mass % or less and Na2O is contained at 1 mass % or more and 10 mass % or less; and the solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less.

[0129] In the sixth and seventh exemplary embodiments, the continuous casting mold of the first or second exemplary embodiment is used in which the mold surface temperature in the upper mold part 21 is lowered to promote crystallization of the mold flux film 14. The mold flux film 14 that flows into the gap between the solidified shell 13 and the mold surface 6 is rapidly cooled. This allows a crystalline phase to rapidly crystallize out in the mold flux film 14 in the upper mold part 21, and slows down the cooling of the solidified shell 13 in the vicinity of the meniscus, thereby inhibiting heterogeneous growth of the solidified shell 13, which may otherwise become a starting point of vertical cracks. In the lower mold part 22, the growth of the crystalline phase of the mold flux film 14 is inhibited, and the solidified shell 13 is strongly cooled to promote the growth of the solidified shell 13. As a result, even when medium-carbon steel is continuously cast at a high casting speed with a carbon concentration of 0.06 mass % or more and 0.20 mass % or less, it is possible to adequately prevent vertical cracks in the cast steel and also satisfactorily prevent the occurrence of solidification-retarded BO or frequent false detection of BO. The same applies to the seventh exemplary embodiment.

[0130] In the sixth and seventh exemplary embodiments, when the mold flux is melted and cooled at a cooling rate of 150 degrees C. / min to be solidified, the crystallization ratio is preferably 70% or more in terms of area ratio.

[0131] As described with reference to FIG. 2 in the fourth and fifth exemplary embodiments, the sixth and seventh exemplary embodiments also utilize the cooling paradox phenomenon found by the inventors, which is caused by the fact that crystallization of the mold flux film provides a shielding effect against radiation heat transfer and an inhibitory effect against heat conduction. Therefore, the effect is exhibited by using the mold flux in which crystals precipitate therein. Thus, a mold flux having a crystallization ratio of 70% or more in terms of area ratio when melted, cooled at a cooling rate of 150 degrees C. / min and solidified, is preferably used in the sixth and seventh exemplary embodiments. When solidified under the same conditions, the mold flux more preferably has a crystallization ratio of 80% or more in terms of area ratio. For the solidified sample, the type of crystal is determined for each crystal grain using SEM-EDS, and the crystal area ratio % for the total crystal grains is taken as the crystallization ratio.

[0132] By setting the CaO′SiO2 mass ratio to 0.9 or more and Li2O to 1 mass % or more in the mold flux, the crystallization ratio falls within the preferred range of the exemplary embodiment(s).EXAMPLES

[0133] Example 1 given below includes Examples and Comparatives to specifically explain the continuous casting molds in the exemplary embodiments.Example 1

[0134] As illustrated in FIG. 1, the continuous casting mold 1 including the cooling-water channels 3 extending vertically on the rear side of the copper mold plate 2 was used. The mold surface temperature Ts defined by the formula (2) when the heat flux Q defined by the formula (1) was given was calculated by changing the shape and cooling conditions of the cooling-water channels 3. Here, the conditions were made different for the upper part of the mold and the lower part of the mold, and the mold surface temperature TsU in the upper part of the mold and the mold surface temperature TsL in the lower part of the mold were calculated. Tables 1 and 2 show the mold conditions and casting conditions used for the calculation. The molds used in Example A to Comparative F in Tables 1 and 2 are referred to as “mold A” to “mold F,” respectively. ΔTS defined by the formula (16) is described as the “difference between upper and lower mold surface temperatures” in Tables 1 and 2.TABLE 1ExamplesABCDistance from upper end of(m)Upper moldLower moldUpper moldLower moldUpper moldLower moldmold to upper / lowerpartpartpartpartpartpartpart of mold0.05 to 0.200.50 to 0.900.04 to 0.250.60 to 0.900.05 to 0.200.20 to 1.10Distance from upper end of(m)0.100.120.10mold to molten surfaceMold length(m)0.900.901.10Set casting speedVc(m / min)1.6 2.1 2.5 Heat flux in moldQ(W / m2)1.612 × 1061.950 × 1062.203 × 106(average valueaccording to Vc)Distance betweenX(m)0.0110.0180.0150.0150.0120.015cooling-waterchannel end andmold surfaceThermalλm(W / (m · K))340340340340340270conductivityof coppermold plateFlow velocity ofVw(m / s)8.008.0010.005.009.819.81cooling waterTemperature ofTw(° C.)373237303139cooling waterDensity of waterρkg / m3993.3995.0993.3995.6995.3992.6Thermalλw(W / (m · K))0.62430.61720.62430.61420.61570.6271conductivityof coolingwaterSpecific heatCpJ / (kg · K)417841784178417841784178of waterViscosityη(Pa · s)0.00068940.00076250.00068940.00079590.00077890.0006637of waterWidth ofW(m)0.0050.0050.0050.0050.0050.005cooling-waterchannelDepth ofD(m)0.0220.0220.0140.0280.0220.022cooling-waterchannelCross-sectionalA(m2)0.000110.000110.000070.000140.000110.00011area ofcooling-waterchannelPerimeter ofL(m)0.0540.0540.0380.0660.0540.054cooling-waterchannelEquivalent diameterd(m)0.0081480.0081480.0073680.0084850.0081480.008148Reynolds numberRe939288506010617553072102146119548Prandtl numberPr4.6135.1624.6135.4145.2864.422Calculation valueTs(° C.)141.3172.0174.8215.0172.8220.9of moldsurface temperatureDifference betweenΔTs(° C.)31   40   48   upper and lowermold surfacetemperaturesTABLE 2ComparativesDEFDistance from upper end of(m)Upper moldLower moldUpper moldLower moldUpper moldLower moldmold to upper / lowerpartpartpartpartpartpartpart of mold0.05 to 0.200.50 to 0.900.05 to 0.200.50 to 0.900.05 to 0.200.20 to 1.10Distance from upper end of(m)0.100.100.10mold to molten surfaceMold length(m)0.900.901.10Set casting speedVc(m / min)1.6 1.6 2.5 Heat flux in moldQ(W / m2)1.612 × 1061.612 × 1062.203 × 106(average valueaccording to Vc)Distance betweenX(m)0.0180.0180.0110.0110.0150.012cooling-waterchannel end andmold surfaceThermalλm(W / (m · K))340340340340290340conductivityof coppermold plateFlow velocity ofVw(m / s)8.008.008.008.0012.8012.80cooling waterTemperature ofTw(° C.)393237303428cooling waterDensity of waterρkg / m3992.6995.0993.3995.6994.4996.2Thermalλw(W / (m · K))0.62710.61720.62430.61420.62010.6110conductivityof coolingwaterSpecific heatCpJ / (kg · K)417841784178417841784179of waterViscosityη(Pa · s)0.00066370.00076250.00068940.00079590.00073160.0008319of waterWidth ofW(m)0.0050.0050.0050.0050.0050.005cooling-waterchannelDepth ofD(m)0.0220.0220.0220.0220.0220.022cooling-waterchannelCross-sectionalA(m2)0.000110.000110.000110.000110.000110.00011area ofcooling-waterchannelPerimeter ofL(m)0.0540.0540.0540.0540.0540.054cooling-waterchannelEquivalent diameterd(m)0.0081480.0081480.0081480.0081480.0081480.008148Reynolds numberRe97491850609392881546141755124899Prandtl numberPr4.4225.1624.6135.4144.9295.689Calculation valueTs(° C.)175.6172.0141.3137.9198.2159.1of moldsurface temperatureDifference betweenΔTs(° C.)−4   −3   −39   upper and lowermold surfacetemperaturesThe mold A, in which the length of the copper mold plate 2 in the casting direction 26 was 0.90 m, was used in a continuous casting machine designed so that the distance from the upper end 23 of the mold to the molten surface was 0.10 m. As illustrated in FIG. 3, the distance X between the cooling-water channel end 7 and the mold surface 6 was set to XU=0.011 m in the upper mold part 21 (the range starting from a position 0.05 m below the upper end 23 of the mold and ending at a position 0.20 m below the upper end 23 of the mold), and was set to XL=0.018 m in the lower mold part 22 (the range starting from a position 0.50 m below the upper end 23 of the mold and ending at a position 0.90 m below the upper end 23 of the mold). The distance XU in the upper mold part 21 was shorter than the distance XL in the lower mold part 22 in Example A, satisfying the requirement [B] of the first exemplary embodiment. In this configuration, regarding the mold surface temperature Ts calculated by the formula (2) when the heat flux Q in the mold defined by the formula (1) was given according to the set casting speed VC=1.6 m / min, the mold surface temperature TsU in the upper mold part 21 was 141.3 degrees C., and the mold surface temperature TsL in the lower mold part 22 was 172.0 degrees C., with a difference of ΔTs=31 degrees C. That is, Example A satisfied the requirement of the second exemplary embodiment that ΔTs of the formula (16) was 20 degrees C. or more. In Example A, the mold cooling water was flowed from the bottom to the top of the mold. The average value of the cooling water temperature TwU in the upper mold part 21 was 37 degrees C., and the average value of the cooling water temperature TwL in the lower mold part 22 was 32 degrees C., so the upper part of the mold had a higher cooling water temperature. In addition, in a range from a position 0.20 m away from the upper end 23 of the mold as the upper end to a position 0.50 m away from the upper end 23 of the mold as the lower end, the distance X between the cooling-water channel end 7 and the mold surface 6 was designed to change gradually from 0.011 m to 0.018 m.

[0136] The mold B, in which the length of the copper mold plate 2 in the casting direction 26 was 0.90 m, was used in a continuous casting machine designed so that the distance from the upper end 23 of the mold to the molten surface was 0.12 m. As illustrated in FIG. 4, a depth D of the cooling-water channel was set to a depth DU=0.014 m in the upper mold part 21 (the range starting from a position 0.04 m below the upper end 23 of the mold and ending at a position 0.25 m below the upper end 23 of the mold), and was set to a depth DL=0.028 m in the lower mold part 22 (the range starting from a position 0.60 m below the upper end 23 of the mold and ending at a position 0.90 m below the upper end 23 of the mold), with the depth of the upper part of the mold being half the depth of the lower part of the mold. The width W of the cooling-water channel was constant at 0.005 m as shown in Table 1. Thus, in the cross-sectional area A of the cooling-water channel 3, the cross-sectional area AU in the upper mold part 21 was smaller than the cross-sectional area AL in the lower mold part 22. The cross-sectional area AU was half the cross-sectional area AL. That is, Example B satisfied the requirement [A] of the first exemplary embodiment. Regarding the flow velocity of cooling water Vw, the flow velocity of the cooling water in the upper mold part 21 was designed to be twice as fast as that in the lower mold part 22. In this configuration, regarding the mold surface temperature Ts calculated by the formula (2) when the heat flux Q in the mold defined by the formula (1) was given according to the set casting speed VC=2.1 m / min, the mold surface temperature TsU in the upper mold part 21 was 174.8 degrees C., and the mold surface temperature TsL in the lower mold part 22 was 215.0 degrees C., with a difference of ΔTs=40 degrees C. That is, Example B satisfied the requirement of the second exemplary embodiment that ΔTs of the formula (16) was 20 degrees C. or more. In Example B, the mold cooling water was flowed from the bottom to the top of the mold. The average value of the cooling water temperature TwU in the upper mold part 21 was 37 degrees C., and the average value of the cooling water temperature TwL in the lower mold part 22 was 30 degrees C., so the upper mold part 21 had a higher cooling water temperature. In addition, in a range from a position 0.25 m away from the upper end 23 of the mold as the upper end to a position 0.60 m away from the upper end 23 of the mold as the lower end, the depth D of the cooling-water channel 3 was designed to change gradually from 0.014 m to 0.028 m.

[0137] The mold C, in which the length of the copper mold plate 2 in the casting direction 26 was 1.10 m, was used in a continuous casting machine designed so that the distance from the upper end of the copper mold plate 2 to the molten surface was 0.10 m. As illustrated in FIG. 5, it was designed such that the thermal conductivity λm of the copper mold plate 2 was made different between the upper mold part 21 (the range starting from a position 0.05 m below the upper end 23 of the mold and ending at a position 0.20 m below the upper end 23 of the mold) and the lower mold part 22 (the range starting from a position 0.20 m below the upper end 23 of the mold and ending at a position 1.10 m below the upper end 23 of the mold) on either side of a boundary position 17 in FIG. 5 and the thermal conductivity λmU in the upper mold part 21 was designed to be greater than the thermal conductivity λmL in the lower mold part 22. That is, Example C satisfied the requirement [C] of the first exemplary embodiment. In this configuration, regarding the mold surface temperature Ts calculated by the formula (2) when the heat flux Q in the mold defined by the formula (1) was given according to the set casting speed VC=2.5 m / min, the mold surface temperature TsU in the upper mold part 21 was 172.8 degrees C., and the mold surface temperature TsL in the lower mold part 22 was 220.9 degrees C., with a difference of ΔTs=48 degrees C. That is, Example C satisfied the requirement of the second exemplary embodiment that ΔTs of the formula (16) was 20 degrees C. or more. In Example C, it was designed such that the mold cooling water was flowed from the top to the bottom of the mold (see FIG. 5), and the average value of the cooling water temperature TwU in the upper mold part 21 was 31 degrees C., and the average value of the cooling water temperature TwL in the lower mold part 22 was 39 degrees C. The condition that the cooling water temperature was lower in the upper part than in the lower part of the mold also effectively serves to lower the mold surface temperature in the upper mold part 21 below the mold surface temperature in the lower mold part 22.

[0138] The mold D is a comparative using a common mold. As illustrated in FIG. 6, the cross-sectional area A of the cooling-water channel 3, the distance X between the cooling-water channel end 7 and the mold surface 6, and the thermal conductivity λm of the copper mold plate 2, those of which are related to the cooling of the mold, were the same from the upper part to the lower part of the mold. The same applies to the mold E. In Comparatives D and E, the flow direction of the cooling water was a normal design in which the mold cooling water was flowed from the bottom to the top of the mold. The average value of the cooling water temperature TwU in the upper part of the mold was thus higher than the average value of the cooling water temperature TwL in the lower part of the mold. In this configuration, regarding the calculated value of the mold surface temperature when the heat flux Q in the mold was given according to the set casting speed VC=1.6 m / min, the value for the upper part of the mold was slightly larger than that for the lower part of the mold, with ΔTs being a negative value.

[0139] The mold F, in which the length of the copper mold plate 2 in the casting direction 26 was 1.10 m, was used in a continuous casting machine designed so that the distance from the upper end of the copper mold plate 2 to the molten surface was 0.10 m. Example F is a comparative in which the thermal conductivity λm of the copper mold plate 2 was designed so that the thermal conductivity λmU in the upper mold part 21 (the range starting from a position 0.05 m below the upper end 23 of the mold and ending at a position 0.20 m below the upper end 23 of the mold) was smaller than the thermal conductivity λmL in the lower mold part 22 (the range starting from a position 0.20 m below the upper end 23 of the mold and ending at a position 1.10 m below the upper end 23 of the mold). In this configuration, regarding the mold surface temperature Ts calculated by the formula (2) when the heat flux Q in the mold defined by the formula (1) was given according to the set casting speed VC=2.5 m / min, the mold surface temperature TsU in the upper mold part 21 was higher than the mold surface temperature TsL in the lower mold part 22, with ΔTs=−39 degrees C., which was a negative value. In Comparative F, the mold cooling water was flowed from the bottom to the top of the mold as in a normal design, so that the average value of the cooling water temperature TwU in the upper mold part 21 was higher than the average value of the cooling water temperature TwU in the upper mold part 21. This also contributed to making the mold surface temperature TsU in the upper mold part 21 higher than the mold surface temperature TsL in the lower mold part 22. The mold F, in which the mold surface temperature in the upper mold part 21 tends to be excessively high, is disadvantageous in terms of sticking of the cast steel to the mold surface, cracks in the mold surface, and deformation of the copper mold plate.Example 2

[0140] In Example 2, the continuous casting molds of Example 1 and the mold fluxes of the fourth and fifth exemplary embodiments were used, and continuous casting was actually performed using a continuous casting machine.

[0141] Molten steel having the composition shown in Table 3 was cast using a continuous casting machine provided with the continuous casting mold shown in each of Example A (using mold A), Comparative D (using mold D), and Comparative E (using mold E) in Table 1 for the casting conditions related to the mold. The cross-sectional dimensions of the mold were 1,250 mm in width and 250 mm in thickness. The degree of superheat of the molten steel was 25 degrees C. immediately before pouring into the mold, and the casting speed was 1.6 m / min.

[0142] As the mold fluxes, mold fluxes a to c shown in Table 4 were used. Examples and Comparatives using the mold fluxes a to c are A-a, D-a, and E-a in Table 5, and A-b, A-c, and D-c in Table 6. For example, A-a means that the mold A and the mold flux a were used.

[0143] The chemical composition of each mold flux shown in Table 4 does not include carbon or carbon oxides that are lost through combustion or thermal decomposition during melting, and shows values that represent the composition after melting. In Table 4, it is assumed that all Ca obtained by analysis is CaO and all Na obtained by analysis is Na2O.

[0144] The solidification temperature in Table 4 was determined by reading, from the mold flux temperature measurement results, a temperature at which the heat generation due to crystallization was maximum when the mold flux, which was once molten in a graphite crucible in a furnace, was solidified while decreasing the furnace atmosphere temperature at a cooling rate of 10 degrees C. / min. Here, the temperature at which the heat generation due to crystallization was maximum means either a temperature at which the temperature decrease rate is the smallest, or a temperature at which the temperature increase rate due to heat generation is the largest.

[0145] Each mold flux was solidified under the above conditions. For the solidified sample, the type of crystal was determined for each crystal grain using SEM-EDS, and the crystal area ratio % for the total crystal grains was taken as the crystallization ratio. In addition, the solidified sample was subjected to X-ray diffraction evaluation, and a crystal with the highest peak height was determined to be the main crystal, as shown in Table 4.

[0146] The mold flux a in Table 4 satisfies the requirements of the fourth and fifth exemplary embodiments, and mold flux b satisfies the requirement of the fourth exemplary embodiment. In the mold flux b, the main crystal and the viscosity at 1,300 degrees C. fall outside the requirement of the fifth exemplary embodiment.TABLE 3Steel composition (mass %)CSiMnPSSol. AlN0.110.020.650.0130.0060.0350.003TABLE 4CrystallizationChemical composition (mass %)ViscosityratioSolidificationMainCaOSiO2Al2O3MgOTiO2Na2OLi2OF(1300° C.)(%)temperaturecrystala39.032.01.92.3—10.0—9.80.8 poise821,160° C.Cuspidineb38.231.89.98.46.01.01.03.13.4 poise651,170° C.Melilitec41.031.410.20.87.00.7—3.06.1 poise81,170° C.(Unclear)(Unclear)(Glass-like)TABLE 5ExampleComparativeA-aD-aE-aDistance from upper end of mold to(m)Upper moldLower moldUpper moldLower moldUpper moldLower moldupper / lower part of moldpartpartpartpartpartpart0.05 to 0.200.50 to 0.900.05 to 0.200.50 to 0.900.05 to 0.200.50 to 0.90Distance from upper end of(m)0.100.100.10mold to molten surfaceHeat flux evaluation range(m)Upper moldLower moldUpper moldLower moldUpper moldLower mold(Distance from upper end of mold)partpartpartpartpartpart0.10 to 0.200.50 to 0.900.10 to 0.200.50 to 0.900.10 to 0.200.50 to 0.90Mold flux brandaaaHeat flux index in Mold8635100378627Solidification heterogeneity index31   100   67   TABLE 6ExampleComparativesA-bA-cD-cDistance from upper end of mold to(m)Upper moldLower moldUpper moldLower moldUpper moldLower moldupper / lower part of moldpartpartpartpartpartpart0.05 to 0.200.50 to 1.100.05 to 0.200.50 to 0.900.05 to 0.200.50 to 0.90Distance from upper end of(m)0.100.100.10mold to molten surfaceHeat flux evaluation range(m)Upper moldLower moldUpper moldLower moldUpper moldLower mold(Distance from upper end of mold)partpartpartpartpartpart0.10 to 0.200.50 to 0.900.10 to 0.200.50 to 0.900.10 to 0.200.50 to 0.90Mold flux brandbccHeat flux index in mold93351284411844Solidification heterogeneity index48   155   131   As an actual heat flux value during casting in each of the upper mold part 21 and the lower mold part 22, a value obtained by estimating an average heat flux in each heat flux evaluation range shown in Tables 5 and 6 from temperature measurements of thermocouples installed at multiple points in the height direction and two points in the depth direction in the mold. The heat flux value in the upper part of the mold of Comparative D-a was set to 100, and the heat flux values in the upper mold part 21 and the lower mold part 22 in each of Examples and Comparatives were indexed and shown in “Heat flux index in mold” of Tables 5 and 6. When the values of heat flux measured by the thermocouples fluctuated over time, the values at local maximum points of the fluctuation curve were linked to obtain the average value. When the measured value of the heat flux fluctuates over time, the cause of the fluctuation is that the distance between the mold and the cast steel increases due to abnormal shrinkage of the solidified shell 13, or that a gap occurs between the solid phase of the mold flux film 14 and the mold. The above fluctuation causes decrease the heat flux, so in order to evaluate the inherent heat flux of the mold-mold flux film system, it is desirable to use the values at the local maximum points of the fluctuation curve. The region for evaluating the heat flux was an effective region below the height of the molten surface in the mold. Specifically, a range with a start point at 0.10 m away from the upper end 23 of the mold (a height of the molten surface) and an end point at 0.20 m away from the upper end 23 of the mold was defined as the upper mold part 21. A range with a start point at 0.50 m away from the upper end of the mold and an end point at the lower end of the mold was defined as the lower mold part 22.The size of irregularities on the surface of the cast steel was measured with a laser distance meter, and the standard deviation of the measured distance was taken as solidification heterogeneity. The solidification heterogeneity in Comparative D-a was set to 100, and the respective indexed values are shown in “Solidification heterogeneity index” in Tables 5 and 6.First, a description will be made on Comparative D-a in Table 5, which is a combination of a normal mold and the mold flux. D-a is a comparative in which hypo-peritectic steel with the composition shown in Table 3 was cast using the mold D in which the cooling performance for the mold was constant from the upper part to the lower part of the mold, and the mold flux a in Table 4 in which cuspidine crystallized and precipitated in the mold flux film as the main crystal. In Comparative D-a, the heat flux in the mold showed a normal heat flux distribution, being large in the upper mold part 21 where the surface temperature of the solidified shell was high, and being small in the lower mold part 22 where the surface temperature of the solidified shell was low.

[0150] In Comparative D-a, the surface of the obtained cast steel had irregularities due to the degree of solidification shrinkage specific to hypo-peritectic steel. The heterogeneity of solidification observed in Comparative D-a is probably due to insufficient crystallization of the mold flux film and insufficient reduction in the heat flux in the upper part of the mold.

[0151] In Example A-a in Table 5 using the mold A satisfying the requirements of the first and second exemplary embodiments and the mold flux a satisfying the requirements of the fourth and fifth exemplary embodiments, the cooling performance in the upper part of the mold was strengthened to promote crystallization of the mold flux film. As a result, the heat flux index in the upper part of the mold was decreased to 86, and the solidification heterogeneity index was improved to 31. Although the cooling performance in the lower part of the mold of Example A-a was the same as that of Comparative D-a, the heat flux index in the lower part of the mold of Example A-a was slightly lower than that of Comparative D-a due to the effect of the mold flux film of which crystallization was promoted in the upper part of the mold.

[0152] In Example A-b in Table 6 using the mold A and the mold flux b satisfying the requirement of the fourth exemplary embodiment, the same effect as in Example A-a was obtained, but the mold flux b had a higher viscosity and a smaller crystallization ratio than the mold flux a, so the heat flux index in the upper mold part 21 of Example A-b was slightly higher than of Example A-a. In addition, although the solidification heterogeneity of Example A-b was improved compared to Comparative D-a, it was not as good as Example A-a. The heat flux index in the lower mold part 22 of Example A-b was approximately the same as that of Example A-a, because crystallization proceeded while the mold flux film moved to the lower mold part 22 even if the viscosity of the mold flux was high and the crystallization ratio was small, and the solidification temperatures of the mold flux a and the mold flux b were approximately the same.

[0153] Comparative E-a in Table 5 used the mold E. The upper mold part 21 of the mold E had the same cooling structure as the mold A. The mold E was a mold with strong cooling all over the surface, in which the cooling structure of upper mold part 21 of mold A was extended to the lower mold part 22. In Comparative E-a, the heat flux index in the upper mold part 21 was 86, which was the same as that of Example A-a. In contrast, in the lower mold part 22, the crystallization of the mold flux film progressed excessively, and the heat flux index of the mold decreased to 27. Further, the heat flux greatly varied in the lower mold part 22. This is probably because the excessive crystallization of the mold flux film occasionally caused voids between the mold flux film and the mold. The solidification heterogeneity index of Comparative E-a was 67, and the improvement effect thereof was insufficient compared to Example A-a. This is probably because the growth of the solidified shell was inhibited by the decrease in heat flux due to the excessive crystallization of the mold flux film in the lower mold part 22 and the uniform growth of the solidified shell was inhibited by the variation in the heat flux that may be caused by the excessive crystallization of the mold flux film.

[0154] In Comparatives A-c and D-c in Table 6, it was used the mold flux c of which crystallization ratio was less than 20%, the mold flux c not satisfying the requirement of the fourth exemplary embodiment. Since the crystallization of the mold flux film was insufficient, the cooling properties of the mold alone directly appeared as the properties of cooling the cast steel in Comparatives A-c and D-c. That is, the heat flux in the upper mold part 21 using the mold A in which the cooling performance in the upper mold part 21 was high (Comparative A-c) was larger than that using the mold D in which the cooling performance in the upper mold part 21 was lower than that of the mold A (Comparative D-c). Thus, the solidification heterogeneity index of Comparative A-c was also larger than that of Comparative D-c. In addition, since the mold flux c not satisfying the requirement of the fourth exemplary embodiment was used, the overall heat flux from the upper part to the lower part of the mold of Comparative D-c was larger than that of Comparative D-a, which also promoted heterogeneous solidification.

[0155] The mold flux satisfying the requirement of the fifth exemplary embodiment is not limited to that described in the mold flux a in Table 4, and for example, a mold flux as described in Non-Patent Literature 2 may be used. Or, a similar effect can be achieved by using the mold flux (e.g., the mold flux b in Table 4) that does not satisfy the requirement of the fifth exemplary embodiment but satisfies the requirement of the fourth exemplary embodiment, as described in Patent Literature 10. It should be noted that a mold flux with a high crystallization ratio and a high crystallization speed is preferably used, that is, a mold flux with a small drop in solidification temperature when the cooling rate increases is preferably used.Example 3

[0156] In Example 3, the mold fluxes in the sixth and seventh exemplary embodiments were used, and continuous casting was actually performed using a continuous casting machine.

[0157] Molten steel having the composition shown in Table 3 was cast using the molds A, B, C, D, and E shown in Tables 1 and 2. The cross-sectional dimensions of each mold were 1,250 mm in width and 250 mm in thickness. The degree of superheat of the molten steel was 25 degrees C. immediately before pouring into the mold, and the casting speed was the set casting speed shown in Tables 1 and 2, and was in a range from 1.6 m / min to 2.5 m / min.

[0158] In Example 3, the mold fluxes shown in Tables 7 and 9 were used.

[0159] Each chemical composition shown in Tables 7 and 9 does not include carbon that is lost through combustion or thermal decomposition during melting, and shows values that represent the composition after melting. In Table 3, it is assumed that all Ca obtained by analysis is CaO and all Na obtained by analysis is Na2O. “C′ / S” in Table 3 refers to the mass ratio of CaO′ / SiO2.

[0160] The solidification temperature in Tables 7 and 9 was determined by reading, from the mold flux temperature measurement results, a temperature at which the heat generation due to crystallization was maximum when the mold flux, which was once molten in a graphite crucible in a furnace, was solidified while decreasing the furnace atmosphere temperature at a cooling rate of 10 degrees C. / min, i.e., a temperature at which the temperature decrease rate was the smallest, or a temperature at which the temperature increase rate due to heat generation was the largest.

[0161] The viscosity was measured by an oscillating-plate type viscometer at 1,300 degrees C.

[0162] Each mold flux was heated to 1,350 degrees C. in a graphite crucible to melt, and then solidified at 150 degrees C. / min. For the solidified sample, the type of crystal was determined for each crystal grain using SEM-EDS, and the crystal area ratio % for the total crystal grains was taken as the crystallization ratio. A high crystallization ratio means that the mold flux crystallizes even at a high cooling rate.Sixth Exemplary Embodiment

[0163] The mold flux d to the mold flux g in Table 7, which are products according to the invention, satisfied the component composition and solidification temperature defined in the sixth exemplary embodiment. For the mold flux h, the Li2O content was lower than the lower limit of the sixth exemplary embodiment, and the contents of other alkali metal oxides were also small, so that the solidification temperature was higher than the upper limit of the sixth exemplary embodiment. For the mold flux i, the Li2O content was lower than the lower limit of the sixth exemplary embodiment, and although cuspidine crystallized, the solidification temperature was low and the crystallization ratio was small. For the mold flux j, the Li2O content and C′ / S (CaO′ / SiO2 mass ratio) were lower than the respective lower limits of the sixth exemplary embodiment, resulting in a significantly low crystallization ratio.TABLE 7Solidi-ficationCrystal-temper-lizationLi2ONa2OK2OFT•CaOSiO2Al2O3MgOCaF2′CaO′atureViscosityratioMainmass %C′ / S° C.poise%crystalExamplesd3.00.00.012.651.728.63.50.618.038.81.412400.7292Cuspidinee5.00.00.014.749.427.43.00.517.137.11.411700.4584Cuspidinef8.30.20.411.148.628.72.10.90.848.01.710550.2481Cuspidineg1.90.00.07.051.037.82.10.29.444.21.212490.9187CuspidineComparativesh0.00.00.310.451.233.54.00.621.136.11.113051.1089Cuspidinei0.010.50.010.041.134.02.02.47.335.91.111600.7267Cuspidinej0.016.00.011.023.743.33.12.92.421.90.59802.704—TABLE 8Heat fluxHeat fluxindex inindex inupperSolidificationlowerMoldmoldheterogeneitymoldMoldfluxpartindexpartExamples1Ad8011352Ae8314383Af8517424Ag8113345Bd8427416Be8630457Bf8930478Cf883048Compar-9Ah722625atives10Ai86313311Aj1571154412Dd85313313Df94464014Dh85412415Di1001003716Dj1401104317Ed80272118Eh721021719Ei862923Table 8 shows the evaluation results of the sixth exemplary embodiment.

[0165] The heat flux in each of the upper mold part 21 and the lower mold part 22 was evaluated in the same manner as in Example 2. The heat flux value in the upper mold part 21 of Comparative 15 (D-i) was set to 100, and the heat flux values in the upper mold part 21 and the lower mold part 22 in each of Examples and Comparatives were indexed and shown in “Heat flux index in upper mold part” and “Heat flux index in lower mold part” of Table 8.

[0166] Similar to Example 2 described above, the size of irregularities on a wide face surface of the cast steel was measured with a laser distance meter, and the standard deviation of the measured distance was taken as solidification heterogeneity. The solidification heterogeneity in Comparative 15 (D-i) was set to 100, and the indexed values are shown in “Solidification heterogeneity index” in Table 8.

[0167] First, a description will be made on Comparative 15 (D-i), which is a combination of the normal mold D and the ordinary mold flux i that crystallizes but has a rather low crystallization ratio. D-i is a comparative in which hypo-peritectic steel with the composition shown in Table 3 was cast using the mold D in which the cooling performance for the mold was constant from the upper part to the lower part of the mold, and the mold flux i in Table 7 in which cuspidine crystallized and precipitated in the flux film as the main crystal.

[0168] In Comparative 15 (D-i), the heat flux in the mold showed a normal heat flux distribution, being large in the upper mold part 21 where the solidified shell surface temperature was high, and being small in the lower mold part 22 where the solidified shell surface temperature was low.

[0169] In Comparative 15 (D-i), the surface of the obtained cast steel had irregularities due to the degree of solidification shrinkage specific to hypo-peritectic steel. The heterogeneity of solidification observed in Comparative 15 (D-i) is probably due to insufficient crystallization of the mold flux film and insufficient reduction in the heat flux in the upper part of the mold.

[0170] In contrast, in Comparative 10 (A-i) using the mold A, the cooling performance in the upper mold part 21 was strengthened to promote crystallization of the mold flux film. As a result, the heat flux index in the upper mold part 21 was decreased to 86, and the solidification heterogeneity index was improved to 31. Although the cooling performance in the lower mold part 22 of Comparative 10 (A-i) was the same as that of Comparative 15 (D-i), the heat flux index in the lower mold part 22 of Comparative 10 (A-i) was slightly lower than that of Comparative 15 (D-i) due to the effect of promoting the crystallization of the mold flux film in the upper mold part 21.

[0171] In Comparative 12 (D-d) in which the solidification temperature and crystallization ratio of the mold flux were increased over Comparative 15 (D-i), the crystallization of the mold flux film was promoted in the upper mold part 21, the heat flux index in the upper mold part 21 decreased, and the solidification heterogeneity index improved to 31. Since the solidification temperature of the mold flux of Comparative 12 (D-d) was higher than that of Comparative 15 (D-i), crystals in the mold flux film grew through the lower part of the mold, and the heat flux index in the lower mold part 22 decreased slightly in Comparative 12 (D-d).

[0172] Furthermore, in Example 1 (A-d) in which the cooling performance in the upper mold part 21 was strengthened over Comparative 12 (D-d), the crystallization of the mold flux film was promoted, the heat flux index in the upper mold part 21 decreased to 80, and the solidification heterogeneity index improved to 11.

[0173] The mold flux f had an equivalent or lower solidification temperature compared to the mold flux i, but had an increased crystallization ratio compared thereto. In Comparative 13 (D-f) using the mold flux f, the crystallization of the mold flux film in the upper mold part 21 was promoted over Comparative 15 (D-i). The heat flux index in the upper mold part 21 decreased and the solidification heterogeneity index improved in Comparative 13 (D-f).

[0174] In Comparative 9 (A-h) and Comparative 14 (D-h) using the mold flux h with the highest solidification temperature, the heat flux index in the upper mold part 21 decreased and the solidification heterogeneity index improved. However, the heat flux also decreased in the lower mold part 22, and the growth of the solidified shell was inhibited throughout the entire mold length. In addition, since the mold flux h had a high solidification temperature, the film excessively crystallized through the lower part of the mold, and the heat flux index in the lower mold part 22 of Comparative 9 (A-h) and Comparative 14 (D-h) was smaller than that of Comparative 15 (D-i), making it difficult to fully obtain the effect of the invention mold A. Excessively high solidification temperature of the mold flux should be avoided because it leads to insufficient heat removal from the solidified shell and destabilizes the operation.

[0175] The mold E that had increased cooling performance throughout the entire length of the mold, was used for the tests. When the mold flux d (Comparative 17 (E-d)) and the mold flux i (Comparative 19 (E-i)) with solidification temperatures of 1,240 degrees C. or less were used, the crystallization of the film in the upper mold part 21 was promoted, and the solidification heterogeneity index decreased. In contrast, in Comparative 18 (E-h) using the mold flux h with a high solidification temperature, poor inflow of the mold flux occurred and the solidification heterogeneity index was large. In all the conditions, the film excessively crystallized through the lower mold part 22, and the heat flux index was small. High cooling performance along the entire mold length as in the mold E should be avoided because it leads to insufficient heat removal from the solidified shell and destabilizes the operation.

[0176] In Examples 5 to 8 using the molds B and C and the mold fluxes d to F, the heat flux in the lower mold part 22 was high and the growth of the solidified shell in the lower mold part 22 was promoted. The effect was the greatest in Example 8 (C-f). In addition, the solidification heterogeneity index of Examples 5 to 8 was 30 or less, which was better results than Comparatives 12 and 13 using the mold D. The reason why the solidification heterogeneity of Examples 5 to 8 was slightly inferior to that of Examples 1 to 4 is that the casting speed of Examples 5 to 8 was higher than that of Examples 1 to 4.

[0177] In the test using the mold flux j that does not crystallize, crystallization of the mold flux is not promoted even if the cooling performance in the upper mold part 21 is strengthened. Therefore, the heat flux index in the upper mold part 21 was high, 157 in Comparative 11 (A-j) and 140 in Comparative 16 (D-j).

[0178] The above results of the sixth exemplary embodiment revealed that improving the cooling performance in the upper mold part 21 or improving the crystallization ratio of the mold flux promoted the crystallization of the mold flux film 14 in the upper mold part 21 and contributed to reducing the solidification heterogeneity index. When both the improvement in cooling performance and the improvement in crystallization ratio were achieved, as in Example 1 (A-d), Example 2 (A-e), Example 3 (A-f), and Example 4 (A-g), the effect of reducing the solidification heterogeneity index was the greatest among Examples and Comparatives. In particular, the heat flux index in the lower part of the mold of Example 2 (A-e) and Example 3 (A-f), which used the mold fluxes with a high crystallization ratio and a low solidification temperature, was larger than that of Example 1 (A-d). The heat flux index in the lower part of the mold tends to be larger as the solidification becomes more uniform. Thus, when the solidification heterogeneity is small relative to a value estimated from the crystallization ratio of the mold flux film, the heat flux index in the lower part of the mold becomes slightly large.Seventh Exemplary Embodiment

[0179] The mold flux k to the mold flux o in Table 9, which are products according to the invention, satisfied the component composition and solidification temperature defined in the seventh exemplary embodiment.

[0180] For the mold flux p, each of the Li2O content and the Na2O content was lower than the lower limit of the seventh exemplary embodiment, and the solidification temperature was higher than the upper limit of the seventh exemplary embodiment. For the mold flux q, the Li2O content was lower than the lower limit of the seventh exemplary embodiment, the Na2O content was higher than the upper limit of the seventh exemplary embodiment, and although cuspidine crystallized, the crystallization ratio was 67%, which was lower than the preferred range (70% or more). For the mold flux r, the Li2O content was lower than the lower limit of the seventh exemplary embodiment, the Na2O content was higher than the upper limit of the seventh exemplary embodiment, C′ / S (CaO′ / SiO2 mass ratio) was lower than the lower limit of the seventh exemplary embodiment, and the crystallization ratio was significantly low. For the mold flux s, the Li2O content fell within the component range of the seventh exemplary embodiment, but the Na2O content was lower than the lower limit of the seventh exemplary embodiment and the solidification temperature was higher than the upper limit of the seventh exemplary embodiment.TABLE 9Solidi-ficationCrystal-temper-lizationLi2ONa2OK2OFT•CaOSiO2Al2O3MgOCaF2′CaO′atureViscosityratioMainmass %C′ / S° C.poise%crystalExamplesk2.02.10.012.551.229.12.80.317.838.41.312300.4391Cuspidinel3.04.00.014.548.526.92.70.416.836.41.411700.4883Cuspidinem4.98.20.316.743.024.52.20.210.835.21.410900.2481Cuspidinen1.12.30.09.752.032.91.90.214.141.91.312500.6186Cuspidineo6.32.40.017.246.225.91.40.615.834.81.311200.1582CuspidineComparativesp0.00.00.310.451.233.54.00.621.136.11.113051.1089Cuspidineq0.010.5 0.010.041.134.02.02.47.335.91.111600.7267Cuspidiner0.016.0 0.011.023.743.33.12.92.421.90.5 9802.704—s1.50.00.010.453.430.63.50.617.440.91.313200.9290CuspidineThe underlines indicate the values outside the scope of the invention.TABLE 10Heat fluxHeat fluxindex inindex inupperSolidificationlowerMoldmoldheterogeneitymoldMoldfluxpartindexpartExamples21Ak81113622Al83153823Am84154124An82133425Ao84154326Bk85274127Bl87314528Bm88304629Cm873048Compar-31Ap722625atives32Aq86313333Ar1571154434As70252135Dk86333336Dm92434237Dp85412438Dq1001003739Dr1401104340Ds80351941Ek81272242Ep721021743Eq862923The heat flux in each of the upper mold part 21 and the lower mold part 22 was evaluated in the same manner as in Example 2. The heat flux value in the upper mold part 21 of Comparative 38 (D-q) was set to 100, and the heat flux values for the upper mold part 21 and the lower mold part 22 in each of Examples and Comparatives were indexed and shown in “Heat flux index in upper mold part” and “Heat flux index in lower mold part” of Table 10.

[0182] Similar to Example 2 described above, the size of irregularities on a wide face surface of the cast steel was measured with a laser distance meter, and the standard deviation of the measured distance was taken as solidification heterogeneity. The solidification heterogeneity in Comparative 38 (D-q) was set to 100, and the indexed values are shown in “Solidification heterogeneity index” in Table 10.

[0183] First, a description will be made on Comparative 38 (D-q), which is a combination of the normal mold D and the ordinary mold flux q that crystallizes but has a rather low crystallization ratio. Comparative 38 (D-q) is a comparative in which hypo-peritectic steel with the composition shown in Table 3 was cast using the mold D in which the cooling performance for the mold was constant from the upper part to the lower part of the mold, and the mold flux q in Table 9 in which cuspidine crystallized and precipitated in the mold flux film as the main crystal.

[0184] In Comparative 38 (D-q), the heat flux in the mold showed a normal heat flux distribution, being large in the upper mold part 21 where the surface temperature of the solidified shell 13 was high, and being small in the lower mold part 22 where the surface temperature of the solidified shell 13 was low.

[0185] In Comparative 38 (D-q), the surface of the obtained cast steel had irregularities due to the degree of solidification shrinkage specific to hypo-peritectic steel. The heterogeneity of solidification observed in Comparative 38 (D-q) is probably due to insufficient crystallization of the mold flux film and insufficient reduction in the heat flux in the upper mold part 21.

[0186] In contrast, in Comparative 32 (A-q) using the mold A, the cooling performance in the upper mold part 21 was strengthened to promote crystallization of the mold flux film. As a result, the heat flux index in the upper mold part 21 was decreased to 86, and the solidification heterogeneity index was improved to 31. Although the cooling performance in the lower mold part 22 of Comparative 32 (A-q) was the same as that of Comparative 38 (D-q), the heat flux index in the lower mold part 22 of Comparative 32 (A-q) was slightly lower than that of Comparative 38 (D-q) due to the effect of the mold flux film of which crystallization was promoted in the upper mold part 21.

[0187] In Comparative 35 (D-k) in which the solidification temperature and crystallization ratio of the mold flux were increased over Comparative 38 (D-q), the crystallization of the mold flux film was promoted in the upper mold part 21, the heat flux index in the upper mold part 21 decreased, and the solidification heterogeneity index improved to 33. Since the solidification temperature of the mold flux of Comparative 35 (D-k) was higher than that of Comparative 38 (D-q), crystals in the mold flux film grew through the lower part of the mold, and the heat flux index in the lower mold part 22 decreased slightly in Comparative 35 (D-k).

[0188] In Example 21 (A-k) in which the cooling performance in the upper mold part 21 was strengthened over Comparative 35 (D-k), the crystallization of the mold flux film was promoted, the heat flux index in the upper mold part 21 decreased to 81, and the solidification heterogeneity index improved to 11.

[0189] In Comparative 36 (D-m) using the mold flux that had an equivalent or lower solidification temperature compared to Comparative 38 (D-q) but had an increased crystallization ratio compared thereto, the crystallization of the mold flux film was promoted in the upper mold part 21, the heat flux index in the upper mold part 21 decreased, and the solidification heterogeneity index improved, but those were insufficient.

[0190] In Comparative 31 (A-p), Comparative 37 (D-p), Comparative 34 (A-s), and Comparative 40 (D-s) using the mold flux p and the mold flux s with high solidification temperatures, the heat flux index in the upper mold part 21 decreased and the solidification heterogeneity index improved. However, the heat flux also decreased in the lower mold part 22, and the growth of the solidified shell was inhibited throughout the entire mold length. In addition, since these mold fluxes had high solidification temperatures, the film excessively crystallized through the lower part of the mold, and the heat flux index in the lower mold part 22 of Comparative 31 (A-p), Comparative 37 (D-p), Comparative 34 (A-s), and Comparative 40 (D-s) was smaller than that of Comparative 38 (D-q), making it difficult to fully obtain the effect of the invention mold A. Excessively high solidification temperature of the mold flux should be avoided because it leads to insufficient heat removal from the solidified shell and destabilizes the operation.

[0191] The mold E that had increased cooling performance throughout the entire length of the mold, was used for the tests. When the mold flux k (Comparative 41 (E-k)) and the mold flux q (Comparative 43 (E-q)) with solidification temperatures of 1,240 degrees C. or less were used, the crystallization of the film in the upper mold part 21 was promoted, and the solidification heterogeneity index decreased. In contrast, in Comparative 42 (E-p) using the mold flux p with a high solidification temperature, poor inflow of the mold flux occurred and the solidification heterogeneity index was large. In all conditions, the film excessively crystallized through the lower mold part 22, and the heat flux index was small. High cooling performance along the entire mold length as in the mold E should be avoided because it leads to insufficient heat removal from the solidified shell and destabilizes the operation.

[0192] In Examples 26 to 29 using the molds B and C and the mold fluxes k to m, the heat flux in the lower mold part 22 was high and the growth of the solidified shell in the lower mold part 22 was promoted. The effect was the greatest in Example 29 (C-m). In addition, the solidification heterogeneity index of Examples 26 to 29 was 31 or less, which was better results than Comparatives 35 and 36 using the mold D. The reason why the solidification heterogeneity of Examples 26 to 29 was slightly inferior to that of Examples 21 to 25 is that the high casting speed of Examples 26 to 29 was higher than that of Examples 21 to 25.

[0193] In the test using the mold flux r that does not crystallize, crystallization of the mold flux is not promoted even if the cooling performance in the upper mold part 21 is strengthened. Therefore, the heat flux index in the upper mold part 21 was high, 157 in Comparative 33 (A-r) and 140 in Comparative 39 (D-r).

[0194] The above results of the sixth exemplary embodiment revealed that improving the cooling performance in the upper mold part 21 or improving the crystallization ratio of the mold flux promoted the crystallization of the mold flux film in the upper mold part 21 and contributed to reducing the solidification heterogeneity index. When both the improvement in cooling performance and the improvement in the crystallization ratio were achieved, as in Example 21 (A-k), Example 22 (A-l), Example 23 (A-m), Example 24 (A-n), and Example 25 (A-o), the effect of reducing the solidification heterogeneity index was the greatest among Examples and Comparatives. In particular, the heat flux index in the lower mold part 22 of Example 22 (A-l), Example 23 (A-m), and Example (A-o), which used the mold flux with a high crystallization ratio and a low solidification temperature, was larger than that of Example 21 (A-k). The heat flux index in the lower part of the mold tends to be larger as the solidification becomes more uniform. Thus, when the solidification heterogeneity is small relative to a value estimated from the crystallization ratio of the mold flux film, the heat flux index in the lower part of the mold becomes slightly large.

[0195] As described above, by implementing the invention in combination with Examples 2 and 3, the heat flux index in the upper mold part 21 was lower and the heat flux index in the lower mold part 22 was higher than in Comparatives, and it is found out that a close-to-ideal heat flux control in the mold can be achieved. The close-to-ideal heat flux control in the mold refers to a control by which the heat flux index in the upper part of the mold becomes low and the heat flux index in the lower part of the mold becomes high. In the invention, the close-to-ideal heat flux control in the mold is achieved by using the continuous casting mold in which the cooling performance in the upper part of the mold is higher than that in the lower part of the mold, and the mold flux that is easy to crystallize.EXPLANATION OF CODES1 continuous casting mold, 2 copper mold plate, 3 cooling-water channel, 4 back frame, 5 submerged entry nozzle, 6 mold surface, 7 cooling-water channel end, 8 upper water supply and drainage channel, 9 lower water supply and drainage channel, 10 molten steel, 11 mold flux, 12 molten flux layer, 13 solidified shell, 14 mold flux film, 15 solid film, 16 liquid film, 17 boundary position, 21 upper mold part, 22 lower mold part, 23 upper end of mold, 24 lower end of mold, 25 width direction, 26 casting direction, 31 wide face, 32 narrow face, 41 continuous casting machine, 42 tundish, 43 support roll, 44 machine length, 45 cast steel

Examples

first exemplary embodiment

[0067]A first exemplary embodiment of the invention will be described with reference to FIGS. 1 to 6.

[0068]The first exemplary embodiment of the invention relates to a continuous casting mold 1 for a continuous casting process of steel using a mold flux, in which a side of the continuous casting mold 1 that comes into contact with molten steel is configured including the water-cooling copper mold plate 2, the cooling-water channel 3 arranged in the copper mold plate 2 allows cooling water to flow in a vertical direction of the mold, and the cooling-water channel 3 is connected to the upper water supply and drainage channel 8 at the upper end of the mold and the lower water supply and drainage channel 9 at the lower end of the mold, and one or more of the following [A], [B], and [C] are satisfied between the upper part of the mold and the lower part of the mold.[0069][A] A cross-sectional area A of the cooling-water channel 3 in the upper mold part 21 is smaller than the cross-sectio...

second exemplary embodiment

[0079]FIG. 8 illustrates a general overall view of a continuous casting machine 41 for casting steel, in particular, the continuous casting machine 41 for casting slabs. Molten steel is injected into the continuous casting mold 1 from a tundish 42 through a submerged entry nozzle 5, and a cast steel 45 in which solidification has progressed is drawn from a lower part of the continuous casting mold 1 and pulled out while being supported by support rolls 43. The continuous casting machine 41 illustrated in FIG. 8 is of a vertical bending type. A casting distance from a surface of the molten steel in the continuous casting mold 1 to the most downstream support roll 43 is called a machine length 44.

[0080]The second exemplary embodiment relates to a continuous casting machine provided with the continuous casting mold 1 described in the first exemplary embodiment, characterized in that when a heat flux Q determined by the formula (1) is given according to a set casting speed Vc of the con...

third exemplary embodiment

[0097]A third exemplary embodiment relates to a continuous casting method characterized by performing continuous casting of steel using a continuous casting machine provided with the continuous casting mold described in the first exemplary embodiment or the continuous casting machine described in the second exemplary embodiment, while supplying a mold flux into the continuous casting mold.

Mold Fluxes in Fourth and Fifth Exemplary Embodiments

[0098]A fourth exemplary embodiment relates to a continuous casting method characterized in that, in a case where a mold flux sample is melted, cooled at a cooling rate of 10 degrees C. / min, and solidified, and the crystallization ratio of the sample is expressed as an area ratio, a mold flux having a crystallization ratio of 20% or more is used as the mold flux in the continuous casting method according to the third exemplary embodiment. The crystallization ratio (area ratio) of the mold flux is determined as a ratio (%) of an area of a region w...

Claims

1. A continuous casting mold for a continuous casting process of steel using a mold flux,a side of the continuous casting mold that comes into contact with molten steel configured including a water-cooling copper mold plate,a cooling-water channel being disposed in the copper mold plate so that cooling water flows in a vertical direction of the mold,the cooling-water channel being connected to an upper water supply and drainage channel at an upper end of the mold and to a lower water supply and drainage channel at a lower end of the mold, andone or more of [A], [B], and [C] below being satisfied,[A] a cross-sectional area of the cooling-water channel is smaller in an upper part of the mold than in a lower part of the mold,[B] a distance between an end of the cooling-water channel and a mold surface is shorter in the upper part of the mold than in the lower part of the mold, where the end of the cooling-water channel refers to an end of the cooling-water channel on a mold surface side, and[C] a thermal conductivity of the copper mold plate is higher in the upper part of the mold than in the lower part of the mold.

2. A continuous casting machine comprising the continuous casting mold according to claim 1, whereinwhen a heat flux Q calculated by formula (1) is given according to a set casting speed Vc of the continuous casting machine, regarding a mold surface temperature Ts calculated by formula (2), a mold surface temperature TsU in the upper part of the mold is lower by 20 degrees C. or more than a mold surface temperature TsL in the lower part of the mold,Q=1.16×106·VC0.7(1)Ts=(Xλm+4·A0.023Re0.8·Pr0.4·λw·L)⁢Q+Tw(2)Re=Vw / (η / ρ)(3)Pr=η⁢CP / λw(4)d=4⁢A / L(5)where, Q: heat flux [W / m2], Vc: set casting speed [m / min], Ts: mold surface temperature [degrees C.], Tw: cooling water temperature [degrees C.], X: distance between end of cooling-water channel and mold surface [m], λm: thermal conductivity of copper mold plate [W / (m·K)], λw: thermal conductivity of cooling water [W / (m·K)], A: cross-sectional area of cooling-water channel [m2], L: perimeter of cooling-water channel [m], Re: Reynolds number of cooling water in cooling-water channel [-], Pr: Prandtl number of cooling water in cooling-water channel [-], d: equivalent diameter [m], Vw: flow velocity of cooling water in cooling-water channel [m / s], η: viscosity of water [Pa·s], ρ: density of water [kg / m3], CP: specific heat of water [J / (kg·K)]).

3. A continuous casting method, comprising performing continuous casting of steel using a continuous casting machine provided with the continuous casting mold according to claim 1, while supplying a mold flux into the continuous casting mold.

4. The continuous casting method according to claim 3, wherein a mold flux having a crystallization ratio of 20% or more in terms of area ratio when melted, cooled at a cooling rate of 10 degrees C. / min and solidified, is used as the mold flux.

5. The continuous casting method according to claim 4, wherein a mold flux in which a main crystal of the solidified mold flux is cuspidine and a viscosity at 1,300 degrees C. is less than 1.5 poise, is used.

6. The continuous casting method according to claim 3, wherein a mold flux in which: a CaO′ / SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; Li2O is contained, among alkali metal oxides, at 1 mass % or more and 15 mass % or less; and a solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less, is used as the mold flux,CaO′ above being determined by formula (6) and formula (7),CaO’⁢(mass⁢ %)=T·CaO-CaF2’×0.718(6)CaF2’⁢(mass⁢ %)=(F-Li2⁢O×1.27-Na2⁢O×0.613-K2⁢O×0.403)×2.05(7)where:in a case where a right side of the formula (7) is negative, a left side of the formula (7) is set to 0%,F: content ratio (mass %) of F in mold flux,a total content of components in the mold flux excluding C is taken as 100 mass %, and a content of each of the components is determined, andT·CaO, Li2O, Na2O, and K2O refer to contents of oxides (mass %) calculated assuming that Ca, Li, Na, and K in the mold flux are all oxides.

7. The continuous casting method according to claim 3, wherein a mold flux in which: a CaO′ / SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; among alkali metal oxides, Li2O is contained at 1 mass % or more and 10 mass % or less and Na2O is contained at 1 mass % or more and 10 mass % or less; and a solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less, is used as the mold flux,CaO′ above being determined by formula (6) and formula (7),CaO’⁢(mass⁢ %)=T·CaO-CaF2’×0.718(6)CaF2’⁢(mass⁢ %)=(F-Li2⁢O×1.27-Na2⁢O×0.613-K2⁢O×0.403)×2.05(7)where:in a case where a right side of the formula (7) is negative, a left side of the formula (7) is set to 0%,F: content ratio (mass %) of F in mold flux,a total content of components in the mold flux excluding C is taken as 100 mass %, and a content of each of the components is determined, andT·CaO, Li2O, Na2O, and K2O refer to contents of oxides (mass %) calculated assuming that Ca, Li, Na, and K in the mold flux are all oxides.

8. A continuous casting method, comprising performing continuous casting of steel using the continuous casting machine according to claim 2, while supplying a mold flux into the continuous casting mold.

9. The continuous casting method according to claim 8, wherein a mold flux having a crystallization ratio of 20% or more in terms of area ratio when melted, cooled at a cooling rate of 10 degrees C. / min and solidified, is used as the mold flux.

10. The continuous casting method according to claim 9, wherein a mold flux in which a main crystal of the solidified mold flux is cuspidine and a viscosity at 1,300 degrees C. is less than 1.5 poise, is used.

11. The continuous casting method according to claim 8, wherein a mold flux in which: a CaO′ / SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; Li2O is contained, among alkali metal oxides, at 1 mass % or more and 15 mass % or less; and a solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less, is used as the mold flux,CaO′ above being determined by formula (6) and formula (7),CaO’⁢(mass⁢ %)=T·CaO-CaF2’×0.718(6)CaF2’⁢(mass⁢ %)=(F-Li2⁢O×1.27-Na2⁢O×0.613-K2⁢O×0.403)×2.05(7)where:in a case where a right side of the formula (7) is negative, a left side of the formula (7) is set to 0%,F: content ratio (mass %) of F in mold flux,a total content of components in the mold flux excluding C is taken as 100 mass %, and a content of each of the components is determined, andT·CaO, Li2O, Na2O, and K2O refer to contents of oxides (mass %) calculated assuming that Ca, Li, Na, and K in the mold flux are all oxides.

12. The continuous casting method according to claim 8, wherein a mold flux in which: a CaO′ / SiO2 mass ratio is 0.9 or more and 2.0 or less; F is contained at 5 mass % or more; among alkali metal oxides, Li2O is contained at 1 mass % or more and 10 mass % or less and Na2O is contained at 1 mass % or more and 10 mass % or less; and a solidification temperature is 900 degrees C. or more and 1,300 degrees C. or less, is used as the mold flux,CaO′ above being determined by formula (6) and formula (7),CaO’⁢(mass⁢ %)=T·CaO-CaF2’×0.718(6)CaF2’⁢(mass⁢ %)=(F-Li2⁢O×1.27-Na2⁢O×0.613-K2⁢O×0.403)×2.05(7)where:in a case where a right side of the formula (7) is negative, a left side of the formula (7) is set to 0%,F: content ratio (mass %) of F in mold flux,a total content of components in the mold flux excluding C is taken as 100 mass %, and a content of each of the components is determined, andT·CaO, Li2O, Na2O, and K2O refer to contents of oxides (mass %) calculated assuming that Ca, Li, Na, and K in the mold flux are all oxides.

Images