JP2024124883A - Continuous casting method for steel - Google Patents

Abstract

To provide a continuous steel casting method capable of sufficiently preventing longitudinal cracking of a cast piece and a solidification delay BO and using a continuous casting mold flux and a continuous casting mold.SOLUTION: This continuous steel casting method uses, as a continuous casting mold, a continuous casting mold increased in cooling performance near a molten steel hot water surface which is a casting mold upper part by varying, between the casting mold upper part and a casting mold lower part, one or more factors of: a shape (with and depth) of a casting mold cooling path (cooling slit), a distance of the casting mold cooling water path end from a casting mold surface, heat conductivity of a cast mold copper plate, a cast mold cooling water flow velocity, and a casting mold cooling water temperature and applies, as a mold flux, a mold flux in which a CaO'/SiO2 mass ratio is 0.9-2.0, F is contained by 5 mass% or more, among alkali metal oxides, Li2O is contained by 1-10 mass% and Na2O is contained by 1-10 mass%, and a solidification point is 900-1300°C.SELECTED DRAWING: Figure 3

Description

The present invention relates to a method for continuous casting of steel, and in particular, to a method for continuous casting of steel using mold flux.

In the continuous casting process, molten steel from a tundish is fed into a water-cooled copper mold, and mold flux is added to the surface of the molten steel in the mold to lubricate the mold, keep the molten steel warm, remove impurities, and control heat removal in the mold (heat flux in the mold). During the solidification process of the molten steel in the mold, uneven solidification occurs, which can cause surface cracks in the slab, and slow cooling is promoted to suppress this. Specific methods include (1) lowering the thermal conductivity or heat transfer coefficient of the mold to increase the mold surface temperature and reduce the temperature difference with the slab surface, and (2) promoting the crystallization of the flux film formed by the mold flux that flows between the mold and the slab, reducing radiation heat transfer, and shielding conductive heat transfer by forming an air gap due to crystallization.

There are many examples of known techniques that fall under (1), such as Patent Document 1 (a method of placing a sheet with low thermal conductivity on the upper part of the mold), Patent Document 2 (a method of retracting the cooling slits on the upper part of the mold from the surface on the molten steel side), Patent Document 3 (a method of separating the cooling water systems on the top and bottom of the mold to ease the cooling of the upper part), Patent Document 4 (a method of placing holes on the upper part of the mold), Patent Document 5 (a method of providing multiple cooling water systems in the horizontal direction of the mold to ease the cooling of the upper part), Patent Document 6 (a method of providing a sprayed layer with low thermal conductivity on the upper surface of the mold), and Patent Document 7 (a method of embedding a heating element in the upper part of the mold). Although these techniques can be expected to have a certain effect when only conductive heat transfer is considered, there is a high hurdle to practical use because an increase in the mold surface temperature induces problems that reduce the mold life, such as the sticking of the cast piece to the mold surface, cracks on the mold surface, and deformation of the copper plate. In addition, an increase in the mold surface temperature inhibits the crystallization of the mold flux, which may be counterproductive to the above method (2).

Publications of Patent Documents 8, 9, and 10 include a method of crystallizing cuspidine or melilite in a mold flux film. These methods are effective in slowing the cooling of the slab and preventing cracking, and are widely used in practice. Although crystallization of the flux film is effective in suppressing cracking, excessive crystallization not only causes poor lubrication in the mold, but also increases the risk of bulging and breakout (BO) detection due to reduced heat removal from the slab, and is a factor that hinders good operation. In other words, to suppress cracking of the slab, it is desirable to use a mold flux that does not simply crystallize the crystalline phase of the flux film, but that instantly crystallizes in the upper part of the mold while satisfying an appropriate crystallization rate throughout the lower part of the mold. To this end, mold fluxes with adjusted crystallization rates and solidification points have been developed (Patent Documents 11, 12, and 13).

Japanese Unexamined Patent Publication No. 61-195742 Japanese Unexamined Patent Publication No. 195746/1983 Japanese Patent Application Publication No. 01-143742 Japanese Patent Application Publication No. 02-197352 Japanese Patent Application Publication No. 02-200353 Japanese Patent Application Publication No. 08-267182 JP 2000-202583 A Japanese Patent Application Publication No. 11-320058 JP 2000-158105 A JP 2010-214387 A JP 2006-247744 A JP 2020-146719 A JP 2021-74782 A

Edited by the Japan Society of Mechanical Engineers, "JSME Text Series Heat Transfer Engineering"

Mold fluxes designed with a high freezing point have a high crystallization rate, which promotes film crystallization in the upper part of the mold, but causes excessive film crystallization in the lower part of the mold. On the other hand, a low freezing point is effective for lubrication inside the mold and promoting the growth of the solidified shell because a liquid phase of the mold flux exists between the mold and the cast piece, but does not provide sufficient slow cooling ability in the upper part of the mold. In other words, while both are appropriate for steel casting from the standpoint of both freezing point and crystallization rate, control of the crystallization relies solely on the composition design of the mold flux, and there has been no thought of combining it with control of the mold surface temperature to increase the freedom of crystallization control and maximize its effects.

As mentioned above, controlling the heat flux in the mold is essential to prevent cracks on the surface of the slab. There are two ways to achieve this, using the mold and mold flux, as mentioned above in (1) and (2). However, in the past, the interaction between these two mechanisms was not discussed, and they were only considered as individual inventions.

The present invention aims to provide a method for continuous casting of steel that achieves more ideal heat removal from within the mold than conventional methods by taking into account the interaction between the cooling capacity of the mold and heat removal control within the mold via the flux film, and by implementing an appropriate combination of the two.

Slow cooling is effective in preventing cracking of the slab, but strictly speaking slow cooling is desirable in the upper part of the mold. Towards the lower part of the mold, it is desirable to promote heat removal from within the mold in order to promote the growth of the solidified shell.

The present invention considers the interaction between the water-cooled mold and the mold flux to obtain an ideal heat extraction from the mold. The mold is a continuous casting mold characterized by having a higher cooling capacity near the molten steel surface than that of the lower part of the mold. The concentrations of the components CaO, _SiO2 , and F of the mold flux are adjusted to a composition that makes it easy for cuspidine to crystallize, and _Li2O and _Na2O are added to increase the crystallization rate.

Ｒｅ＝Ｖ_ｗｄ／（η／ρ） （５）
Ｐｒ＝ηＣ_Ｐ／λ_ｗ （６）
ｄ＝４Ａ／Ｌ （７）
ここで、Ｑ：熱流束［Ｗ／ｍ^２］、Ｖｃ：設定鋳造速度［ｍ／ｍｉｎ］、Ｔ_ｓ：鋳型表面温度［℃］、Ｔ_ｗ：冷却水温度［℃］、Ｘ：鋳型冷却水路端の鋳型表面からの距離［ｍ］、λ_ｍ：鋳型銅板熱伝導率［Ｗ／（ｍ・Ｋ）］、λ_ｗ：冷却水熱伝導率［Ｗ／（ｍ・Ｋ）］、Ａ：鋳型冷却水路（冷却スリット）断面積［ｍ^２］、Ｌ：鋳型冷却水路（冷却スリット）周長［ｍ］、
Ｒｅ：鋳型冷却水路（冷却スリット）内冷却水レイノルズ数［－］、
Ｐｒ：鋳型冷却水路（冷却スリット）内冷却水プラントル数［－］、
ｄ：相当直径［ｍ］、
Ｖ_ｗ：鋳型冷却水路（冷却スリット）内の冷却水流速［ｍ／ｓ］、η：水の粘度［Ｐａ・ｓ］、ρ：水の密度［ｋｇ／ｍ^３］、Ｃ_Ｐ：水の比熱［Ｊ／（ｋｇ・Ｋ）］ That is, the gist of the present invention is as follows.
[1] A continuous casting machine is used that uses a continuous casting mold in which the cooling capacity in the vicinity of the molten steel surface, which is the upper part of the mold, is increased compared to the cooling capacity in the lower part of the mold by making different one or more factors between the upper part and the lower part of the mold, such as the shape (width and depth) of the mold cooling water channel (cooling slit), the distance of the end of the mold cooling water channel from the mold surface, the thermal conductivity of the copper plate of the mold, the mold cooling water flow rate, and the mold cooling water temperature;
A method for continuous casting of steel, comprising applying in combination as a mold flux a CaO'/ _SiO2 mass ratio of 0.9 to 2.0, containing 5 mass% or more of F, containing 1 mass% or more and 10 mass% or less of _Li2O and 1 mass% or more and 10 mass% or less of _Na2O among alkali metal oxides, and having a solidification point of 900°C or more and 1300°C or less.
Here, CaO' is determined from formulas (1) and (2).
CaO' (mass%) = T. CaO- _CaF2 '×0.718...(1)
CaF ₂ ′ (mass%) = (F-Li ₂ O x 1.27-Na ₂ O x 0.613-K ₂ O x 0.403) x 2.05...(2)
(However, if the right-hand side of equation (2) is negative, the left-hand side of equation (2) is set to 0%.)
F: F content in mold flux (mass%),
The content of each component is determined by taking the total content of the components excluding C as 100 mass% in the evaluation results of the content of components in the mold flux. T. CaO, _Li2O , _Na2O , and _K2O refer to the oxide content (mass%) calculated assuming that Ca, Li, Na, and K in the mold flux are all oxides.
[2] The method for continuously casting steel according to [1], characterized in that a continuous casting machine using a continuous casting mold in which, when a heat flux Q calculated by equation (3) according to a set casting speed Vc of the continuous casting machine is applied, the mold surface temperature _Ts calculated by equation (4) is 20°C or more lower in an upper part of the mold than in a lower part of the mold.

Re=V _w d/(η/ρ) (5)
Pr=ηC _P /λ _w (6)
d = 4A / L (7)
where Q is heat flux [W/ ^m2 ], Vc is set casting speed [m/min], _Ts is mold surface temperature [°C], _Tw is cooling water temperature [°C], X is distance from mold surface to mold cooling water channel end [m], _λm is mold copper plate thermal conductivity [W/(m·K)], _λw is cooling water thermal conductivity [W/(m·K)], A is mold cooling water channel (cooling slit) cross-sectional area [ ^m2 ], L is mold cooling water channel (cooling slit) circumference [m],
Re: Reynolds number of cooling water in the mold cooling channel (cooling slit) [-],
Pr: number of cooling water Prandtl in the mold cooling water channel (cooling slit) [-],
d: equivalent diameter [m],
V _w : cooling water flow velocity in the mold cooling channel (cooling slit) [m/s], η: viscosity of water [Pa·s], ρ: density of water [kg/m ³ ], C _p : specific heat of water [J/(kg·K)]

The present invention relates to a composition having a CaO'/ _SiO2 mass ratio of 0.9 to 2.0, containing 5 mass% or more of F, and containing 1 mass% or more and 10 mass% or less of _{Li2O and Na2O} in the alkali metal oxides. A mold flux for continuous casting is used, which contains 1 mass % or more and 10 mass % or less of O and has a freezing point of 900° C. or more and 1,300° C. or less. In addition, a water-cooled copper mold for continuous casting is used, and one or more of the following factors are made different between the upper and lower parts of the mold: the shape (width and depth) of the mold cooling water passage (cooling slit), the distance of the end of the mold cooling water passage from the mold surface, the thermal conductivity of the mold copper plate, the mold cooling water flow rate, and the mold cooling water temperature. This makes it possible to increase the cooling capacity in the vicinity of the molten steel surface in the upper part of the mold compared to the cooling capacity in the lower part of the mold, thereby quickly crystallizing the crystalline phase of the film in the upper part of the mold, slowly cooling the solidified shell in the meniscus, and suppressing uneven growth of the solidified shell that is the starting point of vertical cracks, and to suppress the growth of the crystalline phase of the film in the lower part of the mold and strongly cooling the solidified shell, thereby promoting the growth of the solidified shell. As a result, even when medium carbon steel with a carbon concentration of 0.06 to 0.20 mass% is continuously cast at a high casting speed, the occurrence of longitudinal cracks in the slab can be sufficiently prevented, and the occurrence of solidification-retarded BO can be sufficiently prevented.

FIG. 2 is a plan sectional view showing a partial cross section of a continuous casting mold. FIG. 2 is a schematic diagram showing a cross section inside a mold during continuous casting. FIG. 2 is a side cross-sectional view showing a partial cross section of a continuous casting mold. FIG. 2 is a side cross-sectional view showing a partial cross section of a continuous casting mold. FIG. 2 is a side cross-sectional view showing a partial cross section of a continuous casting mold. FIG. 2 is a side cross-sectional view showing a partial cross section of a continuous casting mold.

This invention relates to a mold heat flux control technology for continuous casting of steel, which uses mold flux to maximize the effect of crystals that precipitate in the flux film formed by molten flux flowing into the gap between the mold and the slab, blocking radiative heat transfer and forming minute voids that inhibit conductive heat transfer, thereby slowly cooling the slab surface.

《Mold Flux》
The composition of the mold flux for continuous casting of the present invention will be described. When the mold flux for continuous casting contains C, the total of the components other than C in the mold flux is taken as 100% by mass, and the content of each component is calculated. Therefore, the C content is treated as an extrapolation. The % for the content of the mold flux means % by mass.

[CaO'/ _SiO2 ]
The mold flux contains T.CaO, _SiO2 , and F as the main components. T.CaO is treated as a component in which the Ca component in the mold flux is considered to be CaO in total. F is CaF in powder form. _However , in the molten state, F has a stronger affinity to alkali metals than Ca. Therefore, in the molten state, F reacts with the alkali metal components and exists as alkali metal fluorides, and the remaining F It is known that the Ca component reacts with the Ca component and exists as _CaF2 . Therefore, the _CaF2 concentration in the flux in a molten state is determined by the above formula (2). When is negative, all the F in the flux has reacted with the alkali metal oxide, the remaining F is zero, and the _CaF2 concentration is also considered to be zero. Therefore, the left side of equation (2) is set to 0%. In this way, the _CaF2 concentration is determined by the formula (2), and then CaO' is calculated by the formula (1). CaO' defined by formula (1) indicates the amount of Ca component that has not reacted with F and is considered to be present as an oxide (CaO).

CaO'/ _SiO2 is a component index that crystallizes cuspidine. It is preferable that the mass concentration ratio of CaO'/ _SiO2 is 0.9 or more and 2.0 or less. When the mass concentration ratio CaO'/ _SiO2 is less than 0.9, the amount of crystals crystallized is small, and a film having sufficient slow cooling ability cannot be obtained. On the other hand, when the mass concentration ratio CaO'/ _SiO2 exceeds 2.0, a crystal phase other than cuspidine crystallizes, and therefore a film having sufficient slow cooling ability cannot be obtained. It is more preferable that the mass concentration ratio of CaO'/ _SiO2 is 1.1 or more and 1.8 or less.

[F]
The F content is 5% by mass or more. This is because the F component has the effect of adjusting the solidification point of the mold flux and also has an effect on the crystallization of cuspidine, and if the F content is less than 5% by mass, this effect is reduced. On the other hand, if a large amount of F is added, the viscosity of the mold flux is significantly reduced, and the mold flux is entrained in the molten steel. Therefore, the F content is preferably 24% by mass or less. It is more preferable that the F content is 10% by mass or more.

[Alkali Metal Oxides]
Alkali metal oxides added to mold flux include _K2O , _Na2O , and _Li2O . Alkali metal oxides are added to improve the crystallization rate of mold flux and to adjust the solidification point and viscosity. _Na2O is often used from the viewpoint of cost. Hereinafter, _Li2O , _Na2O , and _K2O as the component contents in mold flux mean the oxide contents (mass%) calculated assuming that Li, Na, and K in the mold flux are all oxides.

It is known that the effect of adding an alkali metal oxide on improving the crystallization rate is greatest in the order Li ₂ O > Na ₂ O > K ₂ O. Li ₂ O has the greatest effect of increasing the crystallization rate of the mold flux. The content of Li ₂ O is 1 mass% or more and 10 mass% or less, and preferably 3 mass% or more and 8 mass% or less. If the content of Li ₂ O is less than 1 mass%, the freezing point of the mold flux is high and it is not possible to prevent delayed solidification BO. On the other hand, if Li ₂ O is contained in excess, the amount of cuspidine crystallization decreases and sufficient slow cooling ability cannot be obtained.

It is known that the addition of K ₂ O increases the viscosity and decreases the crystallization rate compared to conventional mold fluxes. Since the flow between the mold and the solidified shell is reduced and the crystallization rate is reduced, it is preferable to avoid the inclusion of K ₂ O, and it is preferable for the K ₂ O content to be less than 0.5 mass%.

Increasing the content of _Li2O can ensure both an improvement in the crystallization rate and a decrease in the freezing point, but since the raw material price of _Li2O is high, in the present invention, 1 mass% or more of _Na2O is added in addition to _Li2O as an alkali metal oxide for decreasing the freezing point. On the other hand, adding a large amount of _Na2O reduces the amount of crystallization of cuspidine and makes it impossible to obtain sufficient slow cooling ability, so the upper limit of the content is set to 10 mass%. Therefore, the content of _Na2O is 1 mass% or more and 10 mass% or less, and preferably 2 mass% or more and 8 mass% or less.

[ _{Al2O3 and} MgO]
The total content of _Al2O3 and MgO is preferably 5 mass% or less. In the design of mold flux, _Al2O3 and MgO are present as unavoidable impurities. Both reduce the amount of _{cuspidine crystallized} and other crystals crystallize, which deteriorates melting characteristics _and causes poor heat removal, so it is desirable to have as small a content as possible. The content of _Al2O3 and MgO in mold flux means the oxide content (mass%) calculated assuming that all Al and Mg in mold flux are oxides.

[C]
Furthermore, in addition to the above components, it is preferable to add C to the mold flux of the present invention, and the content of C in the mold flux is preferably 1 to 10 mass %. C has the effect of adjusting the melting rate of the mold flux, and the higher the C content, the slower the melting rate becomes. If the C content is less than 1 mass %, the melting rate becomes too high, and if it exceeds 10 mass %, the melting rate becomes too low, and the flux flowability between the mold and the solidified shell deteriorates.

[Freezing point] (freezing temperature (℃)) (equal to crystallization temperature)
The solidification point of the mold flux is specified to be 900°C or more and 1300°C or less. If the solidification point exceeds 1300°C, the crystal phase of the film is excessively formed at the bottom of the mold, and the growth of the solidified shell is insufficient. The solidification point is more preferably 1250°C or less. On the other hand, mold flux with a solidification point lowered to less than 900°C has insufficient crystallization of cuspidine and cannot obtain sufficient slow cooling ability, so the solidification point is set to 900°C or more. It is more preferable to set the solidification point to more than 1100°C. In order to adjust the solidification point of the mold flux to be within the range of 900°C or more and 1300°C or less, it can be realized by adjusting the CaO'/ _SiO2 mass ratio, F content, and alkali metal content of the mold flux within the range of the present invention using the above-mentioned contents as indicators.

[viscosity]
The viscosity of the mold flux is preferably 1 poise or less at 1300° C. If it exceeds 1 poise, the flow of the mold flux between the mold and the solidified shell becomes insufficient, which tends to cause defects in the cast piece.

[Raw materials]
The raw materials used for the mold flux of the present invention may be any commonly used raw materials. CaO raw materials include quicklime, limestone, cement, etc., _SiO2 raw materials include silica sand, diatomaceous earth, _Li2O raw materials include lithium carbonate, _Na2O raw materials include sodium carbonate and soda ash, F raw materials include fluorite and sodium fluoride, and C raw materials include carbon black and coke powder.

The shape of the raw material of the mold flux is not limited. For example, all shapes such as _powder and granules can be used. These raw materials 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.

<<Continuous casting mold>>
As shown in FIG. 1, a continuous casting mold 1 used for continuous casting of steel has a mold copper plate 2 made of copper, which has excellent thermal conductivity, arranged as the material on the side in contact with molten steel, and is water-cooled from the back side of the mold copper plate 2 to keep the mold surface temperature at about 300° C. or less in a steady state, thereby preventing the casting piece from sticking to the mold surface and the softening or deformation of the copper material. Molds with a relatively large cross section, such as slab continuous casters and bloom continuous casters, usually have elongated vertical grooves formed on the back side of the mold copper plate 2, and these grooves and a back frame 4 on the back side of the mold copper plate 2 form a cooling water channel. This cooling water channel is called a cooling slit 3. As shown in the side cross-sectional view of FIG. 6, cooling water supplied from a lower water supply and drainage channel 9 flows through the cooling slit 3 and is discharged from an upper water supply and drainage channel 8. As shown in the plan cross-sectional view of FIG. 1, a large number of cooling slits 3 are arranged in the width direction to increase the surface area in contact with the cooling water, thereby obtaining sufficient cooling capacity. As described in the Background Art, from the viewpoint of preventing the surface cracking of the slab, there have been many ideas of making the cooling capacity different between the top and bottom of the mold, and all of them aim to make the cooling capacity of the upper part of the mold lower than that of the lower part of the mold to cool the initial solidified shell of the upper part of the mold slowly. For this purpose, as described above, measures have been taken in two directions: (1) a method of lowering the thermal conductivity of the copper plate of the mold to raise the temperature of the mold surface (working surface of the slab side) to reduce the temperature difference with the slab surface and to ease the conductive heat transfer, and (2) a method of easing the heat flux by promoting the crystallization of the flux film formed by the molten flux flowing into the gap between the mold and the slab, obtaining a radiation heat transfer shielding effect and a conductive heat transfer inhibition effect by forming minute voids.

However, when the growth rate of the initial solidified shell and the heat flux in the mold during continuous casting were measured, it was found that if a mold flux that easily crystallizes is used and the cooling capacity of the upper part of the mold is reduced to raise the mold surface temperature, the initial solidified shell may be strongly cooled, contrary to the intended purpose. It is presumed that this inhibits the crystallization of the mold flux film in the gap between the mold and the slab. Conversely, it was found that if the cooling capacity of the upper part of the mold is increased and the mold surface temperature is reduced, the initial solidified shell may be cooled slowly. It is presumed that this promotes the crystallization of the mold flux film in the gap between the mold and the slab. The inventors discovered this phenomenon, which could be called a cooling paradox, and devised a method of utilizing this phenomenon to achieve more ideal heat flux control in the mold while solving the problems of the conventional technology.

The features of the present invention will be described below in accordance with the above-mentioned configuration of the present invention.
First, the upper and lower parts of the mold for continuous casting will be described. The upper part of the mold means the range from the upper end of the mold copper plate to 200 mm or 300 mm. The lower part of the mold means the range from the upper part of the mold copper plate to the lower end of the mold copper plate, and at least the range from the upper end of the mold copper plate to 600 mm below.

The first invention of the present invention is a continuous casting method using the mold flux of the present invention and a water-cooled copper mold for continuous casting, in which the cooling capacity of the upper part of the mold near the molten steel surface is increased compared to the cooling capacity of the lower part of the mold by making the mold cooling water channel (slit) shape (width and depth), the distance from the mold surface to the front of the mold cooling water channel, the thermal conductivity of the mold copper plate, the mold cooling water flow rate, and the mold cooling water temperature different between the upper part and the lower part of the mold.

In the continuous casting mold used in the first invention, the cooling capacity near the molten steel surface in the upper part of the mold is made higher than the cooling capacity in the lower part of the mold in order to maximize the effect of slow cooling caused by the crystallization of the mold flux film, i.e., reducing the heat flux from the cast piece to the mold.

Conventionally, if one wanted to enjoy the slow cooling effect associated with the crystallization of the mold flux film throughout the entire vertical direction of the mold, including the upper and lower parts of the mold as defined above, it was sufficient to increase the overall cooling capacity from the upper to lower parts of the mold. In contrast, in the present invention, the cooling capacity of the upper part of the mold is increased more than that of the lower part of the mold for the following reasons.

Since the mold is where molten steel is cooled and solidified to form the slab, strong cooling is inherently required. Therefore, even if slow cooling is necessary to prevent surface cracking of the slab, infinite slow cooling would negate the original function of the mold, and in that sense slow cooling should be kept to a minimum. The slow cooling required to prevent surface cracking of the slab is only required near the molten steel surface at the top of the mold, specifically above 50 mm to 200 mm below the molten steel surface. Rather, excessive slow cooling should be avoided below that point.

From the paradox phenomenon of cooling through the mold flux film, it was found that by increasing the cooling capacity near the molten steel surface in the upper part of the mold, the crystallization of the mold flux film can be promoted, and the amount of heat removed from the solidified shell can be reduced. On the other hand, by reducing the cooling capacity in the lower part of the mold, excessive crystallization of the mold flux film can be suppressed, and the amount of heat removed from the solidified shell can be increased. If such a phenomenon can be realized, it will lead to moderate cooling near the molten steel surface in the upper part of the mold and the maintenance of sufficient heat flux in the lower part of the mold. Therefore, when continuous casting of steel was performed using the continuous casting mold of the present invention and the mold flux of the present invention, it was found that the cooling of the slab in the upper part of the mold could be mitigated and the cooling of the slab in the lower part of the mold could be increased compared to the case where a conventional continuous casting mold and mold flux were used. Details will be described in detail in the examples described later. According to the continuous casting method according to the first aspect of the present invention, the surface temperature of the copper plate near the molten steel surface in the upper part of the mold is kept low, which has the secondary effect of suppressing the sticking of the slab to the mold surface, cracks on the mold surface, and deformation of the copper plate of the mold.

In the present invention, the cooling capacity near the molten steel surface in the upper part of the mold is increased more than that in the lower part of the mold by making one or more of the following factors different between the upper and lower parts of the mold: mold cooling water channel (cooling slit) shape (width and depth), distance from the mold surface to the front of the mold cooling water channel, thermal conductivity of the copper plate of the mold, mold cooling water flow rate, and mold cooling water temperature. The reason for this is that these means can be implemented at low cost and are sufficiently effective for the present invention.

Because the present invention utilizes the paradox phenomenon of cooling through the mold flux film, the present invention is applicable only to continuous casting using mold flux.

A second aspect of the present invention is a continuous casting method, characterized in that in a continuous casting machine equipped with the continuous casting mold described in the first aspect of the invention, when a heat flux Q calculated by the formula (3) according to the set casting speed Vc of the continuous casting machine is applied, the mold surface temperature _Ts calculated by the formula (4) is 20°C or more lower in the upper part of the mold than in the lower part of the mold.

In the second invention, the contents of the invention described in the first invention are more specifically defined for the continuous casting mold used in the first invention. In this invention, it is the mold surface temperature that affects the crystallization of the mold flux. The behavior of heat conduction and heat transfer from the mold surface to the cooling water can be calculated as follows.

As shown in Figure 1, a continuous casting mold 1 typically has cooling slits 3 of width W and depth D on the back side of the mold copper plate 2, and cooling water flows through the cooling slits 3 at a speed of 5 to 10 m/s. As shown in Figure 6, the cooling slits 3 generally extend in the vertical direction, and the cooling water flows from bottom to top. The slit spacing in the mold width direction is not specified here, but it is common sense to arrange them closely enough that unevenness in the mold surface temperature can be tolerated. Specifically, it is within a range not exceeding three times the slit width W. Due to structural constraints, the above range may be exceeded in some areas, but the impact on the overall cooling capacity is small.

The heat transfer from the mold surface to the surface in contact with the cooling water channel is governed by the thermal conductivity of the mold copper plate material. Here, a thin plating layer (usually several tens of μm to 100 μm) may exist on the mold surface, but its effect can be ignored. In this case, the heat transfer coefficient _hm from the mold surface to the surface in contact with the cooling water channel, as defined by the following formula (8), is given by the following formula (9).
T _s = (1/ _hm )Q+T _f (8)
h _m =λ _m /X (9)
Here, _Ts is the mold surface temperature, Q is the heat flux, _Tf is the mold surface temperature in contact with the cooling water flow path, _λm is the thermal conductivity of the mold copper plate, and X is the distance from the mold cooling water path end 7 (the end of the cooling slit 3) to the mold surface 6 (see Figure 1).

The heat transfer coefficient between the mold cooling water and the mold copper plate (the mold-cooling water heat transfer coefficient h _w defined by the following formula (10)) is expressed by the following formula, Nu: Nusselt number:
T _f = (1/h _w )Q+T _w (10)
h _w =Nu×λ _w /d (11)
There are many experimental formulas for determining the Nusselt number related to turbulent heat transfer in a pipe. When an experiment was conducted on the cooling behavior of a continuous casting mold, the Dittus-Boelter experimental formula Nu = 0.023Re ^0.8 Pr ^0.4 (12) was selected as the most suitable formula for determining the Nusselt number.
(For example, see Non-Patent Document 1) and substitute it into the above formula (11) to obtain the heat transfer coefficient _hw between the mold and the cooling water, and it was found that this can be calculated with high accuracy. Here, Re: Reynolds number is defined by the above formula (5), and Pr: Prandtl number is defined by the above formula (6). _λw in formula (11) is the thermal conductivity of water. Regarding d: equivalent diameter in formulas (5) and (11), the equivalent diameter d was determined by the above formula (7). Note that the Nusselt number is a dimensionless number indicating the magnitude of convective heat transfer relative to conductive heat transfer, the Reynolds number is the intensity of turbulence, and the Prandtl number is a dimensionless number indicating the ratio of the velocity boundary layer thickness to the temperature boundary layer thickness.

In the Reynolds number defined by formula (5), _Vw is the cooling water flow velocity in the slit, d is the equivalent diameter, v is the kinetic viscosity of water, and further v=η/ρ, η is the viscosity of water, and ρ is the density of water. In the Prandtl number defined by formula (6), η is the viscosity of water, _λw is the thermal conductivity of water, and _Cp is the specific heat of water. The equivalent diameter d defined by formula (7) can be calculated regardless of the cross-sectional shape of the cooling water channel, such as A is the cross-sectional area of the cooling channel (slit), and L is the perimeter of the cooling channel (slit) = 2 (W + D). At this time, the heat transfer coefficient _hw between the mold cooling water and the mold copper plate can be calculated by substituting formula (7) into formula (11),
h _w = Nu×λ _w / (4A/L) (13)
It becomes.

The heat transfer from the mold surface to the mold cooling water is expressed by the following formula (14): The heat transfer coefficient hm _-w can be calculated by substituting formula (10) into formula (8) and eliminating Ts using the heat transfer coefficient _hm from the mold surface to the surface in contact with the cooling water channel and the heat transfer _{coefficient hw} between the mold and the cooling water, as shown in the following formula (15):
T _s = (1/h _m−w )Q+T _w (14)
1/h _m−w =1/h _m +1/h _w (15)

Using these relationships, substituting equation (15) into equation (14), equation (9) into _hm in equation (14), and equation (13) into _hw in equation (14), the mold surface temperature _Ts can be obtained as shown in equation (4) for the amount of heat transferred from the slab to the cooling water, i.e., the heat flux Q.

For the heat flux Q in equation (4), equation (3), an empirical formula for the casting speed, is used. Q in equation (3) means the average heat flux from the meniscus part of the mold to the bottom of the mold. The heat flux in the mold during casting differs between the upper and lower parts of the mold (the heat flux is greater in the upper part of the mold where the surface temperature of the cast slab is higher), but since we are evaluating the difference in cooling capacity between the upper and lower parts of the mold here, we use the value of equation (3) for Q, which depends only on the casting speed and is not dependent on the part of the mold.

The set casting speed Vc in formula (3) is evaluated using a value within the design casting speed range of the continuous casting machine. Here, a value within the design casting speed range is a typical casting speed of the continuous casting machine being used, and means a casting speed that is 0.7 to 0.8 times the maximum casting speed calculated from the thickness of the cast slab and the length of the continuous casting machine.

The second invention is a continuous casting machine equipped with a mold for continuous casting, characterized in that the mold surface temperature _Ts calculated by formula (4) is 20°C or more lower in the upper part of the mold than in the lower part of the mold. When this condition is satisfied, when continuous casting of steel is performed using the mold flux of the present invention, crystallization of the mold flux film is promoted in the upper part of the mold, and the surface of the slab can be cooled slowly. At the same time, excessive crystallization of the mold flux film in the lower part of the mold is suppressed, and the growth of the solidified shell can be promoted. In addition, the effects of preventing the slab from sticking to the mold surface, preventing cracks on the mold surface, and suppressing deformation of the copper plate of the mold can be obtained.

In the continuous casting method of the present invention, as described above, a mold flux for continuous casting is used, which has a CaO'/ _SiO2 mass ratio of 0.9 to 2.0, contains 5 mass% or more of F, contains 1 mass% to 10 mass% of _Li2O and 1 mass% to 10 mass% of _Na2O among alkali metal oxides, has a solidification point of 900°C to 1300°C, and is easy to crystallize. In addition, since the mold for continuous casting is intended to promote crystallization of the flux film by lowering the mold surface temperature in the upper part of the mold, the flux film that melts on the molten metal surface in the mold and flows into the gap between the slab and the mold is rapidly cooled. This allows the crystalline phase of the film to crystallize quickly in the upper part of the mold, and the solidified shell in the meniscus to be slowly cooled, thereby suppressing uneven growth of the solidified shell that becomes the starting point of vertical cracks. On the other hand, in the lower part of the mold, the growth of the crystalline phase of the film can be suppressed and the solidified shell can be strongly cooled to promote the growth of the solidified shell. As a result, even when medium carbon steel with a carbon concentration of 0.06 to 0.20 mass% is continuously cast at a high casting speed, the occurrence of longitudinal cracks in the slab can be sufficiently prevented, and the occurrence of solidification-retarded BO can be sufficiently prevented.

In the present invention, it is preferable to use a mold flux that has a crystallization rate of 70% or more in terms of area ratio when melted and cooled at a cooling rate of 150°C/min and solidified.

As shown in FIG. 2, molten steel 10 is supplied from an immersion nozzle 5 into a continuous casting mold 1. 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 flux molten layer 12, which flows between the continuous casting mold 1 and the solidified shell 13 to become a mold flux film 14. The mold flux film 14 exists in the gap between the slab and the mold, has a total thickness of about 0.1 to 1 mm, and has a two-layer structure with a solidified solid phase film 15 on the mold side and a liquid phase film 16 on the slab side. In the solid phase film 15, crystals may precipitate or it may solidify into a glass-like state.

The present invention utilizes the cooling paradox phenomenon discovered by the present inventors, which occurs because the crystallization of the mold flux film has the effect of shielding radiation heat transfer and suppressing conductive heat transfer. Therefore, the effect is achieved by using a mold flux film in which crystals are precipitated. Therefore, it is preferable to use a mold flux that has a crystallization rate of 70% or more in area ratio when melted and cooled at a cooling rate of 150°C/min and solidified. It is more preferable that the crystallization rate is 80% or more in area ratio when solidified under the same conditions. The type of crystal was determined for each crystal grain of the solidified sample by SEM-EDS, and the total crystal area rate % was taken as the crystallization rate.
By setting the CaO'/ _SiO2 mass ratio to 0.9 or more and the _Li2O content to 1 mass % or more in the mold flux, the crystallization rate can be set within the preferred range of the present invention.

As shown in Figure 1, a continuous casting mold 1 having cooling slits 3 extending in the vertical direction on the back side of a mold copper plate 2 was used, and the shape of the cooling slits 3 and the cooling conditions were changed in various ways to calculate the mold surface temperature _Ts defined by formula (4) when the heat flux Q defined by formula (3) was given. Here, the conditions were made different between the upper and lower parts of the mold. The mold conditions and casting conditions used in the calculation are shown in Table 1.

As shown in FIG. 3 and Table 1, mold A is a mold designed to use a mold with a length of 0.90 m in the casting direction of the mold copper plate 2, and to have a distance of 0.10 m from the upper end of the mold copper plate 2 to the molten metal surface. The distance X from the front of the cooling water channel (end 7 of the mold cooling water channel) to the mold surface 6 is different: X _U =0.011 m at the upper part of the mold (between 0.05 m and 0.20 m from the upper end of the mold copper plate), and X _L =0.018 m at the lower part of the mold (between 0.50 m and 0.90 m from the upper end of the mold copper plate). As a result, when the casting speed is 1.6 m/min and the heat flux Q in the mold defined by formula (3) is given, the mold surface temperature T _s calculated by formula (4) is 141.3 ° C. at the upper part of the mold and 172.0 ° C. at the lower part of the mold, which is a difference of 31 ° C., and satisfies the requirements of the present invention. In mold A, the mold cooling water flows from the bottom to the top of the mold, so the mold cooling water temperature is higher at the top of the mold, with an average value of 37°C at the top of the mold and 32°C at the bottom of the mold. In addition, the design is such that when the distance from the top end of the mold copper plate is in the range of 0.20m to 0.50m, the distance X from the front of the cooling water channel to the mold surface changes gradually from 0.011m to 0.018m.

Mold B, as shown in Fig. 4 and Table 1, is a mold designed to use a mold copper plate 2 with a length of 0.90 m in the casting direction, and to have a distance of 0.10 m from the upper end of the mold copper plate 2 to the molten metal surface. The depth D of the slits, which are cooling water channels, was set to D _U = 0.014 m in the upper part of the mold (between 0.04 m and 0.25 m from the upper end of the mold copper plate) and D _L = 0.028 m in the lower part of the mold (between 0.60 m and 0.90 m from the upper end of the mold copper plate), and the upper part of the mold was half that of the lower part, thereby halving the flow path cross-sectional area and doubling the flow velocity _Vw of the cooling water. As a result, when the casting speed is 2.1 m/min and the heat flux Q in the mold defined by formula (3) is given, the mold surface temperature _Ts calculated by formula (4) is 174.8°C at the upper part of the mold and 215.0°C at the lower part of the mold, which is a difference of 40°C, and this mold satisfies the requirements of the present invention. In mold B, the mold cooling water flows from the bottom to the top of the mold, so the mold cooling water temperature is higher at the upper part of the mold, with an average value of 37°C at the upper part and 30°C at the lower part. In addition, the depth D of the slit, which is the cooling water channel, is designed to change gradually from 0.014m to 0.028m when the distance from the upper end of the mold copper plate is in the range of 0.25m to 0.60m.

As shown in FIG. 5 and Table 1, mold C uses a mold with a length of 1.10 m in the casting direction of the mold copper plate 2, and uses a continuous casting machine designed to set the distance from the upper end of the mold copper plate 2 to the molten metal surface height to 0.10 m. The thermal conductivity λ _m of the mold copper plate is made different between the upper part of the mold (between 0.05 m and 0.20 m from the upper end of the mold copper plate) and the lower part of the mold (between 0.20 m and 1.10 m from the upper end of the mold copper plate) on either side of the boundary position 17 in FIG. 5, and the thermal conductivity λ m of the mold copper plate is designed to be larger in the upper part of the mold than in the lower part of the mold. As a result, when the casting speed is 2.5 m/min and the heat flux Q in the mold defined by the formula (3) is given, the mold surface temperature T _s calculated by the formula (4) is 172.8 ° C. in the upper part of the mold and 220.9 ° C. in the lower part of the mold, which is a difference of 48 ° C. and satisfies the requirements of the present invention. Mold C is designed so that mold cooling water flows from the top to the bottom of the mold (see Figure 5). This results in a condition where the mold cooling water temperature is lower in the upper part of the mold (average value of 31°C in the upper part of the mold and average value of 39°C in the lower part of the mold), which is also effective in lowering the surface temperature of the upper part of the mold compared to the surface temperature of the lower part of the mold.

As shown in Figure 6 and Table 1, Molds D and E are comparative examples of normal molds in which the cooling structure of the mold (mold cooling water channel shape, distance from the mold surface to the front of the mold cooling water channel, thermal conductivity of the mold copper plate, and mold cooling water flow rate) is the same from the top to the bottom of the mold. Molds D and E are normal designs in which the mold cooling water flows from the bottom to the top of the mold, and as a result, the average mold cooling water temperature is higher in the upper part of the mold than in the lower part of the mold. As a result, when a heat flux Q in the mold equivalent to a casting speed of 1.6 m/min is applied, the calculated mold surface temperature is slightly higher in the upper part of the mold than in the lower part of the mold. In addition, in Mold E, the cooling capacity of the entire mold is increased by reducing the distance X between the front of the cooling water channel and the mold surface in both the upper and lower parts of the mold.

Next, the results of actual continuous casting using a continuous casting machine are shown.
Molten steel with the composition shown in Table 2 was cast using molds A, B, C, D, and E. The mold cross-sectional dimensions were 1250 mm in width and 250 mm in thickness, the molten steel was superheated to 25° C. just before pouring into the mold, and the casting speed was 1.6 to 2.5 m/min. Examples of the present invention and comparative examples are shown in Table 4.

The mold flux used had the specifications shown in Table 3.
The chemical compositions shown in Table 3 are compositions excluding carbon lost by combustion or thermal decomposition during melting, and are values representative of the compositions after melting. The Ca content obtained by analysis is shown in Table 3 as being all CaO, and the Na content obtained by analysis is shown as being all Na ₂ O. "C'/S" in Table 3 means the CaO'/SiO ₂ mass ratio.
The solidification points in Table 3 were determined by reading the mold flux temperature measurement results and determining the temperature at which the heat generation due to crystallization becomes maximum (the temperature at which the slope of the temperature decrease becomes the smallest, or the slope of the temperature increase due to heat generation becomes the largest) when the mold flux, which was once molten in a graphite crucible in a furnace, is solidified while the furnace atmosphere temperature is lowered at a cooling rate of 10° C./min.
The viscosity was measured at 1300° C. using a vibrating element viscometer.
The mold flux melted at 1350°C in a graphite crucible was solidified at 150°C/min, and the type of crystals was determined for each crystal grain in the solidified sample by SEM/EDS, and the total crystal area ratio % was taken as the crystallization ratio. A high crystallization ratio means that the mold flux crystallizes even at a high cooling rate.

Mold fluxes a to e in Table 3 are products according to the present invention and have the component composition and solidification point specified by the present invention.
Mold flux f has a Li ₂ O content and a Na ₂ O content outside the lower limit of the present invention, and a solidification point outside the upper limit of the present invention. Mold flux g has a Li ₂ O content outside the lower limit of the present invention, a Na ₂ O content outside the upper limit of the present invention, and cuspidine crystallizes, but the crystallization rate is 67%, which is lower than the preferred range (70% or more). Mold flux h has a Li ₂ O content outside the lower limit of the present invention, a Na ₂ O content outside the upper limit of the present invention, and a C'/S (CaO'/SiO ₂ mass ratio) outside the lower limit of the present invention, resulting in a significantly low crystallization rate. Mold flux i has a Li ₂ O content within the component range of the present invention, but a Na ₂ O content outside the lower limit of the present invention, and a solidification point outside the upper limit of the present invention.

The heat flux was estimated from the average heat flux of the corresponding mold region based on the temperature measurements of thermocouples installed at multiple points in the height direction and two points in the depth direction in the upper and lower parts of the mold. The heat flux value in the upper part of the mold for Comparative Example 28 (D-g) was indexed to 100, and is shown in "Heat Flux Index for Upper Mold" and "Heat Flux Index for Lower Mold" in Table 4. When the heat flux measured using a thermocouple fluctuates with time, the values of the maximum points of the fluctuation curve were linked to obtain the average value. When the heat flux measured value fluctuates with time, the fluctuation factors are the distance between the mold and the slab due to abnormal shrinkage of the solidified shell, and the occurrence of a gap between the solid phase of the mold flux film and the mold. These fluctuation factors act in the direction of reducing the heat flux, so it is best to evaluate the original heat flux of the mold-mold flux film system at the maximum value of the fluctuation curve. The area for evaluating the heat flux was the effective area below the molten metal surface in the mold. Specifically, in the upper part of the mold, the area was from 0.10 m above the top of the mold copper plate, which is the molten metal surface height, to 0.20 m above the top of the mold copper plate. In the lower part of the mold, the area was from 0.20 m above the top of the mold copper plate to the bottom of the mold.

The size of the unevenness on the long side of the slab was measured with a laser distance meter, and the standard deviation of the measured distances was taken as the degree of solidification unevenness. The degree of solidification unevenness in Comparative Example 28 (D-g) was set at 100, and the index is shown in the "Solidification unevenness index" in Table 4.

Here, we will first explain Comparative Example 28 (Dg), which is a combination of a normal mold D and a conventional mold flux g that crystallizes but has a slightly low crystallization rate. Comparative Example 28 (Dg) is a comparative example in which hypoperitectic steel with the composition shown in Table 2 was cast using mold D, whose cooling capacity due to the mold structure is constant from the top to the bottom, and mold flux shown in g in Table 3, which crystallizes and precipitates in the flux film with cuspidine as the main crystal.
In Comparative Example 28 (Dg), the heat flux in the mold was large in the upper part of the mold where the surface temperature of the solidified shell was high, and was small in the lower part of the mold where the surface temperature of the solidified shell was low, showing a normal heat flux distribution.
In Comparative Example 28 (Dg), unevenness was observed on the surface of the obtained slab due to the magnitude of solidification shrinkage specific to hypoperitectic steel. It was believed that the non-uniform solidification observed in Comparative Example 28 (Dg) was due to insufficient crystallization of the mold flux film, which resulted in an insufficient decrease in the heat flux in the upper part of the mold.
In contrast, in Comparative Example 22 (A-g) using Mold A, the cooling capacity of the upper part of the mold was strengthened to promote crystallization of the mold flux film, and as a result, the mold heat flux index of the upper part of the mold decreased to 86, and the solidification non-uniformity index improved to 31. Although the cooling capacity of the lower part of the mold was the same as that of Comparative Example 28 (D-g), the mold heat flux index of the lower part of the mold was slightly decreased compared to Comparative Example 28 (D-g) due to the influence of the mold flux film that promoted crystallization in the upper part of the mold.

Next, in Comparative Example 25 (D-a), in which the freezing point and crystallization rate of the mold flux were increased compared to Comparative Example 28 (D-g), crystallization of the mold flux film was promoted in the upper part of the mold, the mold heat flux index in the upper part of the mold decreased, and the solidification non-uniformity index was improved to 33. Because the freezing point of the mold flux was higher than that of Comparative Example 28 (D-g), the mold flux film grew toward the lower part of the mold, and the heat flux index in the lower part of the mold decreased slightly.

Furthermore, in Example 1 (A-a) of the present invention, in which the cooling capacity of the upper part of the mold was strengthened compared to Comparative Example 25 (D-a), crystallization of the mold flux film was promoted, the mold heat flux index of the upper part of the mold was reduced to 81, and the solidification non-uniformity index was improved to 11.

Comparative Example 26 (D-c), which used a mold flux with an increased crystallization rate, had a similar or lower solidification point to Comparative Example 28 (D-g), but the crystallization of the mold flux film in the upper part of the mold was promoted, and a decrease in the mold heat flux index in the upper part of the mold and an improvement in the solidification non-uniformity index were observed, but these were insufficient.

In Comparative Example 21 (A-f), Comparative Example 27 (D-f), Comparative Example 24 (A-i), and Comparative Example 30 (D-i), which used mold fluxes f and i with high freezing points, the heat flux in the upper part of the mold decreased and the solidification non-uniformity index improved. However, the heat flux also decreased in the lower part of the mold, and the growth of the solidified shell was suppressed over the entire length of the mold. In addition, because these mold fluxes have high freezing points, the film crystallized excessively even in the lower part of the mold, and the heat flux index was smaller than in Comparative Example 28 (D-g), and the effect of the invention mold A was not fully obtained. Excessively high freezing points of mold fluxes should be avoided because they lead to insufficient heat removal from the solidified shell and cause unstable operation.

In a test using mold E, in which the cooling capacity was increased along the entire length of the mold, the solidification non-uniformity index decreased with mold fluxes a and g (Comparative Example 31 (E-a) and Comparative Example 33 (E-g) respectively) that had a freezing point of 1240°C or less, due to the promotion of crystallization of the film in the upper part of the mold, but in Comparative Example 32 (E-f), in which mold flux f, which has a high freezing point, was used, poor mold flux flow occurred and the solidification non-uniformity index increased. In either case, the film crystallized excessively toward the lower part of the mold, and the heat flux index decreased. Increasing the cooling capacity along the entire length of the mold should be avoided as it leads to insufficient heat removal from the solidified shell and leads to unstable operation.

Inventive examples 6 to 9, which used molds B and C and mold fluxes a to c, had a high heat flux in the lower part of the mold, which promoted the growth of the solidified shell in the lower part of the mold, with invention example 9 (C-c) showing the greatest effect. In addition, the solidification non-uniformity index was suppressed to 31 or less, which was a better result than comparative examples 25 and 26, which used mold D.

In addition, in tests using mold flux h, which does not crystallize, crystallization of the mold flux was not promoted even when the cooling capacity of the upper part of the mold was strengthened, and the heat flux index of the upper part of the mold was high, at 157 (Comparative Example 23 (A-h)) and 140 (Comparative Example 29 (D-h)).

From the above results, improving the cooling capacity of the upper part of the mold or improving the crystallization rate of the mold flux promotes the crystallization of the mold flux film in the upper part of the mold, and contributes to reducing the solidification non-uniformity index. When both are achieved, as in invention example 1 (A-a), invention example 2 (A-b), invention example 3 (A-c), invention example 4 (A-d), and invention example 5 (A-e), the effect of reducing the solidification non-uniformity index is greatest. In particular, the heat flux index of the lower part of the mold in invention example 2 (A-b), invention example 3 (A-c), and invention example 5 (A-e), which used mold flux with a high crystallization rate and low solidification point, was higher than that in invention example 1 (A-a).

REFERENCE SIGNS LIST 1 continuous casting mold 2 mold copper plate 3 cooling slit 4 back frame 5 submerged nozzle 6 mold surface 7 mold 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

Claims (2)

A continuous casting machine is used that uses a continuous casting mold in which the cooling capacity in the vicinity of the molten steel surface, which is the upper part of the mold, is increased more than the cooling capacity in the lower part of the mold by making different one or more of the following factors between the upper and lower parts of the mold: the shape (width and depth) of the mold cooling water passage (cooling slit), the distance of the end of the mold cooling water passage from the mold surface, the thermal conductivity of the copper plate of the mold, the mold cooling water flow rate, and the mold cooling water temperature;
A method for continuous casting of steel, comprising applying in combination as a mold flux a CaO'/ _SiO2 mass ratio of 0.9 to 2.0, containing 5 mass% or more of F, containing 1 mass% or more and 10 mass% or less of _Li2O and 1 mass% or more and 10 mass% or less of _Na2O among alkali metal oxides, and having a solidification point of 900°C or more and 1300°C or less.
Here, CaO' is determined from formulas (1) and (2).
CaO' (mass%) = T. CaO- _CaF2 '×0.718...(1)
CaF ₂ ′ (mass%) = (F-Li ₂ O x 1.27-Na ₂ O x 0.613-K ₂ O x 0.403) x 2.05...(2)
(However, if the right-hand side of equation (2) is negative, the left-hand side of equation (2) is set to 0%.)
F: F content in mold flux (mass%),
The content of each component is determined by taking the total content of the components excluding C as 100 mass% in the evaluation results of the content of components in the mold flux. T. CaO, _Li2O , _Na2O , and _K2O refer to the oxide content (mass%) calculated assuming that Ca, Li, Na, and K in the mold flux are all oxides. 2. The method for continuously casting steel according to claim 1, characterized in that a continuous casting machine using a continuous casting mold in which, when a heat flux Q calculated by equation (3) in accordance with a set casting speed Vc of the continuous casting machine is applied, the mold surface temperature _Ts calculated by equation (4) is 20° C. or more lower at an upper part of the mold than at a lower part of the mold.

Re=V _w d/(η/ρ) (5)
Pr=ηC _P /λ _w (6)
d = 4A / L (7)
where Q is heat flux [W/ ^m2 ], Vc is set casting speed [m/min], _Ts is mold surface temperature [°C], _Tw is cooling water temperature [°C], X is distance from mold surface to mold cooling water channel end [m], _λm is mold copper plate thermal conductivity [W/(m·K)], _λw is cooling water thermal conductivity [W/(m·K)], A is mold cooling water channel (cooling slit) cross-sectional area [ ^m2 ], L is mold cooling water channel (cooling slit) circumference [m],
Re: Reynolds number of cooling water in the mold cooling channel (cooling slit) [-],
Pr: cooling water Prandtl number in the mold cooling water channel (cooling slit) [-],
d: equivalent diameter [m],
V _w : cooling water flow velocity in the mold cooling channel (cooling slit) [m/s], η: viscosity of water [Pa·s], ρ: density of water [kg/m ³ ], C _p : specific heat of water [J/(kg·K)]

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