EP2035169A1

EP2035169A1 - Mold flux and continuous casting method using the same

Info

Publication number: EP2035169A1
Application number: EP07747064A
Authority: EP
Inventors: Jung Wook Cho; Hyun Seok Jeong; Goo Hwa Kim; Joong Kil Park; Sang Ho Lee; Ki Hyeon Moon
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2006-06-22
Filing date: 2007-06-22
Publication date: 2009-03-18
Anticipated expiration: 2027-06-22
Also published as: EP2035169B1; JP2009541060A; EP2035169A4; JP5037612B2; WO2007148939A1

Abstract

A mold flux including 20 to 50 wt% of CaO, 20 to 50 wt% of SiO2, 20 wt% or less of Al2O3, 20 wt% or less of Na2O, 10 wt% or less of Li2O, 20 wt% or less of B2O3, and 10 wt% or less of MgO, and a continuous casting method using the mold flux. The quality of products can be improved, since the occurrence of surface cracks of cast pieces is suppressed. Further, since the occurrence of break-out is prevented, it is possible to improve the process stability.

Description

[DESCRIPTION] [Invention Title]

MOLD FLUX AND CONTINUOUS CASTING METHOD USING THE SAME [TECHNICAL FIELD] The present invention relates to a mold flux and a continuous casting method using the mold flux, and more particularly, to a continuous casting method using a molten mold flux. [BACKGROUND ART]

A pre-treating process of a molten iron, a converter refining process, a secondary refining process and a continuous casting process are sequentially performed in a general steelmaking method.

As shown in Fig. 1, in order to manufacture a cast piece (which is a general term for a slab, a billet, a bloom, a beam blank, and the like) by a continuous casting process, molten steel is supplied from a ladle, and then passes through a tundish 1 for storing the molten steel, a submerged nozzle 2, and a mold 3. The molten steel is then cooled in a mold 3 by cooling effect thereof, and forms a solidified shell 5. The solidified shell 5, which is formed by cooling the molten steel, is solidified by secondary cooling water sprayed through spray nozzles while being guided by guide rollers provided below the mold, whereby a complete solid cast piece is manufactured.

When molten steel is supplied to the mold during the continuous cast ing process, a mold flux used as an additional substance is also supplied to the mold. In general, mold flux is supplied to the mold in a solid form such as powder or granule, and then melted by the heat of the molten steel supplied to the mold. The mold flux controls the heat transfer between the molten steel and the mold, and improves lubrication performance.

The function of the mold flux in a continuous casting mold 10 will be described in more detail with reference to FIG.2. The mold flux, which is supplied to a mold 10 in the form of powder or granule, is melted on the surface of molten steel 12, and sequentially forms a liquid layer 21, a sintered layer 23, and a powder layer 25 (that is, a molten slag layer 21, a semi-molten layer 23, and an unmelted layer 25) in this order from the surface of molten steel. Since the molten slag layer 21 is substantially transparent, the molten slag layer easily transmits radiation waves that are radiated from the molten steel 12 and have a wavelength in the range of 500 to 4000 nm. In contrast, since the semi-molten layer 23 and the unmelted layer 25 are optically opaque, the semi-molten layer and the unmelted layer shield the radiation waves so as to prevent the temperature on the surface of the molten steel from being rapidly lowered. However, the conventional mold flux in the form of powder or granule is melted by the heat of the molten steel 12. Then, the molten slag layer 21 flows between the mold 10 and a sol idified shel 111. The molten slag layer is solidified on the inner wall of the mold 10, so that a solid slag film 27 is formed on the inner wall of the mold and a liquid slag film 21 is formed adjacent to the molten steel 12. Accordingly, the mold flux controls heat transfer between the molten steel 12 and the mold 10, and improves lubrication performance.

In this case, the mold flux, which is attached to the mold 10 at a position where the molten slag flows between the solid slag film 27 and the solidified shell 11, protrudes toward the inside of the mold 10. The mold flux protruding toward the inside of the mold is called a slag bear 29. The slag bear 29 prevents molten slag from flowing between the mold flux film 27 and the solidified shell 11.

The mold flux consumption per unit area of the cast piece is limited due to the slag bear 29. In general, as the casting speed increases, the mold flux consumption decreases. For this reason, the lubrication performance deteriorates between the cast piece and the mold, the occurrence of break-out increases. In addition, since the thickness of liquid mold flux becomes irregular due to the slag bear 29, the shape of the solidified shell becomes irregular in the mold 10 and surface cracks are caused, which gets worse as the casting speed increases. In recent years, there has been proposed a method of injecting a mold flux onto the surface of molten steel after the mold flux is melted outside the mold. Since the molten mold flux is substantially transparent in the wavelength range of 500 to 4000 run as described above, the molten mold flux easily transmits radiation waves emitted from the molten steel, thereby increasing radiation heat transfer . Accordingly, it is not possible to keep temperature of the surface of the molten steel. For this reason, when a predetermined time elapses in the casting process, the surface of the molten steel is solidified. Therefore, the continuous casting process cannot be smoothly performed.

Meanwhile, early slow cooling by facilitating crystallization has been introduced in the related art. However, more mold flux is mixed into the molten steel and the mean amount of transferred heat decreases. As a result, break-out is caused. [Disclosure]

[Technical Problem]

The present invention provides a mold flux capable of suppressing the occurrence of surface cracks, preventing the occurrence of break-out, and preventing the surface solidification of molten steel by early slow cooling that is achieved by controlling the composition and thermal conductivity of a mold flux. Further, the present invention provides a continuous casting method using the mold flux.

In addition, the present invention provides a mold flux capable of improving a heat insulation effect on the surface of the molten steel during casting in order to prevent surface solidification and improving the quality of the cast pieces and the process stability by adjusting an absorption coefficient of the mold flux during a molten mold flux process. Further, the present invention provides a continuous casting method using the mold flux.

[Technical Problem] According to an aspect of the present invention, a mold flux contains 20 to 50 wt% of CaO, 20 to 50 wt% of SiO₂, 20 wt% or less of Al₂O₃, 20 wt% or less of Na₂O, 10 wt% or less of Li₂O, 20 wt% or less of B₂O₃, and 10 wt% or less of MgO.

In this case, a thermal conductivity of the mold flux may be 1.2 W/m.k or more. When the mold flux is melted, the mold flux may have an absorption coefficient of 1000/m or more in a wavelength range of 500 to 4000 nm.

The mold flux may contain 3 parts by weight or less with respect to the 100 parts by weight of the mold flux, and may have basicity in the range of 0.5 to 1.5. In addition, a continuous casting method includes melting a mold flux, which contains 20 to 50 wt% of CaO, 20 to 50 wt% of SiO₂, 20 wt% or less of Al₂O₃, 20 wt% or less of Na₂O, 10 wt% or less of Li₂O, 20 wt% or less of B₂O₃, and 10 wt% or less of MgO, outside a mold; and supplying the molten mold flux to the mold throughout the entire continuous casting process while a flow rate of the molten mold flux is controlled.

In this case, a thermal conductivity of the mold flux may be 1.2 W/m.k or more.

The mold flux may have an absorption coefficient of 1000/m or more in a wavelength range of 500 to 4000 nm outside the mold.

[Advantageous Effects]

As described above, according to the present invention, it is possible to remove a slag bear by using a molten mold flux process, to prevent the surface solidification of molten steel by early slow cooling, and to suppress the occurrence of surface cracks. In particular, the mean amount of transferred heat is increased by controlling a thermal conductivity depending on the composition of a mold flux. Accordingly, it is possible to prevent break-out from occurring, to effectively control heat transfer between moIten steel and a mold, and improve lubrication performance. In addition, since a heat insulation effect on the surface of molten steel is improved by adjusting an absorption coefficient of the mold flux with respect to molten steel radiation heat , it is possible to stably perform a process over a long period of time, to improve process stability and productivity, and improve quality of cast pieces. [Description of Drawings]

FIG. 1 is a schematic view illustrating a general continuous casting process .

FIG. 2 is a schematic view illustrating the shape of a mold flux existing in a continuous casting mold. FIG. 3 is a schematic view of a continuous casting machine using a molten mold flux according to an embodiment of the present invention.

FIG. 4 is a graph showing a relationship between an absorption coefficient and a wavelength of a mold flux in the related art.

FIG. 5 is a graph showing a relationship between an absorption coefficient of a molten mold flux and a radiation heat flow rate on the surface of molten steel .

FIG. 6 is a photograph of the surface of molten steel during the continuous casting process using a mold flux according to a fourth comparative example. FIG. 7 is a photograph of the surface of molten steel during the continuous casting using a mold flux according to a second example. [Best Mode]

An embodiment of the present invention will be described in detail below with reference to accompanying drawings. However, the present invention is not limited to the following embodiment. Further, the present invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art. Like reference numerals refer to like elements in drawings. FIG. 3 is a view showing the schematic structure of a continuous casting machine using a molten mold flux.

Referring to FIG.3, a continuous casting machine includes a mold 10, a submerged nozzle 30 for supplying molten steel to the mold 10, a mold cover 100 for covering an upper portion of the mold 10, a mold flux melting unit 200 for melting a mold flux that is to be supplied to the mold, and a mold flux feeding unit 300 for feeding a molten mold flux 20, which is melted by the mold flux melting unit 200, to the mold 10.

Since the mold 10 and the submerged nozzle 30 are typical configuration used in a conventional continuous casting machine, the descriptions thereof will be omitted herein.

Since the mold cover 100 is provided on the upper surface of the mold 10 so as to cover the entire surface of molten steel , the mold cover prevents radiation waves from being radiated from the surface of molten steel 12 to the outside. For this purpose, an inner surface of the mold cover 100, that is, a surface of the mold cover facing the molten steel is made of a material having high reflexibi Ii ty, such as an aluminum mirror or a gold-coated mirror . Accordingly, the mold cover reflects the radiation waves radiated from the surface of molten steel 12 so that the radiation waves are absorbed into the molten mold flux 20 or the surface of molten steel 12. As a result, it is possible to minimize the drop in temperature of the surface of molten steel 12 and to prevent the molten mold flux 20 from being solidified again on the wall of the mold 10.

The mold flux melting unit 200 includes a mold flux supplier 205, a crucible 210 for receiving a mold flux raw material supplied from the mold flux supplier 205 in a form of provisionally molten liquid, granule, a mold flux heater 220 such as a heating coil that is provided around the crucible 210 to melt the mold flux, an outlet 230 through which the molten mold flux appropriately melted in the crucible 210 is discharged, and a stopper 240 for opening or closing the outlet 230 to control the amount of the molten mold flux to be discharged. The stopper 240 is moved up and down above the outlet 230 to adjust a distance between an edge of the outlet 230 and a lower end of the stopper 240. Accordingly, the stopper controls the amount of the molten mold flux to be discharged. In this case, the up and down movement of the stopper 240 is accurately controlled by a hydraulic or pneumatic cylinder (not shown). The feeding unit 300 includes an injection pipe 310 and an injection pipe heater 320 such as a heating coil. One end of the injection pipe 310 is connected to the mold flux melting unit 200. Further, an inject ion nozzle 312, which penetrates the mold cover 100 and supplies the molten mold flux 20 to the mold, is provided at the other end of the injection pipe. The injection pipe heater 320 is provided around the injection pipe 310 between the mold flux melting unit 200 and the mold cover 100 to heat the injection pipe 310. In this case, the outer portions of the injection pipe 310 and the injection pipe heater 320 may be insulated with a heat insulator in order to maintain the molten mold flux 20 at a constant temperature. The mold flux melting unit 200 and the feeding unit 300 may be completely or partially made of platinum (Pt) or a platinum alloy such as platinum-rhodium (Pt-Rh). Since the mold flux should quickly melt the nonmetallic inclusions floating on the surface of molten steel in the mold during the casting, the mold flux should have a low viscosity and quickly melts oxides such as AI₂O3. Accordingly, a refractory furnace used in the conventional glass industry may be quickly corroded due to the molten mold flux 20. In particular, when the outlet 230 through which the molten mold flux 20 is discharged from the mold flux melting unit 200, and the injection pipe 310 including the lower end of the stopper 240 and the injection nozzle 312 of the mold flux feeding unit 300 become corroded, it is not possible to accurately control the flow rate of the molten mold flux. As a result, it is not possible to perform a stable continuous casting process. For this reason, the injection pipe 310 and portions which is connected to or in contact with the injection pipe, that is, the outlet 230 through which the molten mold flux is discharged, the stopper 240, and the injection pipe 310 may be made of platinum or a platinum al loy in order to prevent the corrosion by the mold flux.

Meanwhile, the flow rate of the molten mold flux is changed depending on the amount of molten steel that is supplied to the mold per unit time. The stopper 240 is moved up and down to control a space between the lower end of the stopper 240 and the edge of the outlet 230. Accordingly, it is possible to accurately adjust the flow rate of the molten mold flux 20.

The flow rate of the molten mold flux has been controlled using the stopper in the above-mentioned embodiment. However, the present invention is not limited to the above-mentioned embodiment, and the flow rate of the molten mold flux may be control led by a ladle ti It ing method, a siphon method using pressure difference, or various members such as a sliding gate.

In order to effectively achieve early slow cooling and to control the amount of transferred heat when a continuous casting process for injecting a molten mold flux is performed using the above-mentioned machine, the mold flux according to the embodiment of the present invention contains 20 to 50 wt% of CaO, 20 to 50 wt% of SiO₂, 20 wt% or less of Al₂O₃, 20 wt% or less of Na₂O, 10 wt% or less of Li₂O, 20 wt% or less of B₂O₃, and 10 wt% or less of MgO. Further, the mold flux may have a thermal conductivity of 1.2 W/m.k or more. In this case, CaO and S1O2 are ingredients for forming basicity of the mold flux. If the content of CaO is larger than 50 wt% or the content of S1O2 is smaller than 20 wt%, the viscosity of the slag significantly decreases. This results in an excessive amount of slag supplied to the molten steel, which is not desirable. In contrast, if the content of CaO is smaller than 20 wt% or the content of Siθ2 is larger than 50 wt%, the viscosity of slag significantly increases and thereby the slag supply to the molten steel becomes difficult. Therefore, lubrication performance of the mold deteriorates, and increases a possibility of break-out.

AI2O3 is an ingredient for adjusting the viscosity of the mold flux. If the content of AI2O3 is larger than 20 wt%, the viscosity of the mold flux excessively increases and the performance for absorbing nonmetallic inclusions in the molten steel deteriorates.

Na2θ is an ingredient for adjusting the melting point of the mold flux, similar to AI₂O3. If the content of Na₂θ is larger than 20 wt%, the melting point of the mold flux is lowered. This results in a significant decrease of viscosity and surface tension of the mold flux, and the effect, by which the mold flux is suppressed to be mixed into the molten steel, significantly deteriorates. For this reason, in order to increase the viscosity, the content of Na2θ may be decreased and the content of AI₂O3 may be increased. Further, in order to decrease the viscosity, the content of Na₂θ may be increased and the content of AI2O3 may be decreased.

L12O and B2O3 are ingredients for controlling the melting point and the viscosity or the thermal conductivity of the mold flux. As the contents of L19O and B2O3 are increased, the melting point and the viscosity are decreased and the thermal conductivity is increased. However, if the contents of LiaO and B2O3 are excessively large, the melting point and the viscosity of the mold flux also become excessively decreased and the mold flux may be easily mixed into the molten steel. Accordingly, the content of Li₂O may be 10 wt% or less and the content of B2O3 may be 20 wt% or less.

MgO has an effect simi lar to CaO, but the effect of MgO per unit weight is smaller than that of CaO per unit weight. When the content of MgO is excessively large, high temperature precipitations are formed and thereby the viscosity of the mold flux is increased or crystallization is enhanced.

The content of MgO may be 10 wt% or less.

Further, according to the embodiment of the present invention, it is possible to adjust the amount of transferred heat required for securing the quality of cast pieces and to perform a stable continuous casting process during the continuous cast ing using the mold flux, by controlling the thermal conductivity of the mold flux. Accordingly, if the thermal conductivity of the mold flux according to the embodiment of the present invention is controlled to be 1.2 W/m.k or more as described above, it is possible to increase the mean amount of transferred heat and to prevent break-out from occurring during the continuous casting.

As the composition of the above-mentioned mold flux is optimized, the thermal conductivity of a vitreous mold flux is increased. Accordingly, it is possible to control the composition of the mold flux so that the mold flux has a thermal conductivity of 1.2 W/m.k or more. In particular, as the basicity (CaOZSiO₂) is decreased, the chain structure of SiO₂ is grown, phonons easily move, and the thermal conductivity increases.

According to the embodiment of the present invention, a molten mold flux process using the above-mentioned mold flux is applied to a continuous casting process. That is, after a mold flux is completely melted outside a mold by the above-mentioned continuous casting machine, the molten mold flux is periodically and continuously injected into the mold while a flow rate of the molten mold flux is controlled.

According to the embodiment of the present invention, since the molten mold flux process is appl ied to the continuous casting process, it is possible to remove a slag bear. Further, since mold flux consumption is increased, the thickness of a mold flux film is increased and heat transfer is suppressed. Further, since early slow cooling is achieved, it is possible to prevent surface solidification of the molten steel and to decrease defects such as surface cracks. Furthermore, since the mold flux having an improved thermal conductivity is used as described above, the mean amount of transferred heat is increased. That is, it is possible to prevent the occurrence of break-out by increasing the thickness of a solidified shell.

The present invention will be described in more detail below with reference to examples. [EXAMPLES]

The following Table 1 shows ingredients of mold fluxes according to first to third comparative examples and ingredients of a mold flux according to an example of the present invention. [Table 1]

Each of the first to third comparative examples had a composition corresponding to a thermal conductivity of 1.2 W/m.k or less. In contrast, the first example had a composition corresponding to a thermal conductivity of 1.2 W/m.k or more.

The following Table 2 shows process conditions and process results of the first to third comparative examples and the first example. The first to third comparative examples and the first example were obtained from slab casting. Slab casting was performed using the mold fluxes having the compositions shown in Table 1 and a mold that has a width of 1012 mm and a thickness of 140 mm. The type of steel was an extra-low carbon steel having a carbon concentration of 60 ppm.

The first example and the first comparative example were obtained as follows: after a mold flux was completely melted outside a mold, the molten mold flux was injected into the mold by using a flow rate control device in the form of a stopper . When the molten mold flux was injected into the mold, the temperature of the molten mold flux was 1300⁰C. When molten steel was filled in the mold before the beginning of casting, the thickness of the molten pool was increased up to a target thickness. As the casting started, the mold was covered with a mold cover. Next, as the casting was performed, molten mold flux was continuously supplied to the mold so as to supplement the consumed molten mold flux. The mold cover is made of aluminum, and the surfaces of the mold cover have a mean reflexibi lity of 85% against infrared rays in the range of 500 to 4000 nm corresponding to radiation waves of molten steel.

The second and third comparative examples were obtained as follows-^' like a general process using powder mold flux, when molten steel was filled in the mold before the beginning of casting, a powder mold flux was injected into molten steel and then the casting began to be performed. During the casting, a powder mold flux was supplied as needed to the mold so as to supplement consumed the powder mold flux. [Table 2]

Referring to Table 2, according to the first example, it is possible to remove a slag bear by applying the molten mold flux process to the continuous casting process. For this reason, the mold flux consumption in the first example is significantly increased as compared to the second and third comparative examples, which were obtained by applying a powder mold flux process to the continuous casting process. Therefore, it is possible to reduce friction between the mold and solidified shells. Further, the first example has a smaller ratio of the maximum amount of transferred heat to the mean amount of transferred heat as compared to the second and third comparative examples. Therefore, it can be understood that early slow cooling is achieved. This is because the maximum amount of transferred heat directly under the surface of the molten steel is decreased by injecting the molten mold flux as compared to when the powder mold flux is injected. That is, a ratio of the maximum amount of transferred heat to the mean amount of transferred heat is 2.0 to 2.5 in a general process using the powder mold flux. However, if a process using molten mold flux according to the example of the present invention is performed, the ratio is significantly decreased by 1.2 to 1.5.

Further, it is shown that the mean amount of transferred heat of the first example is increased as compared to the first comparative example which used the same molten mold flux process by controlling the thermal conductivity of the first example to be 1.2 W/m.k or more. That is, early slow cooling is achieved by injecting the molten mold flux, and the ratio of the maximum amount of transferred heat to the mean amount of transferred heat is substant ial Iy the same . However , the mean amount of transferred heat of the example, whose thermal conductivity 1.30 W/m.k, is larger than that of the first comparative example whose thermal conductivity of 1.05 W/m.k. Therefore, according to the example, the thickness of a solidified shell is increased and it is possible to prevent break-out from occurring.

According to the present invention, a molten mold flux which is controlled to have a thermal conductivity of 1.2 W/m.k or more depending on the composition of the mold flux is used. Therefore, it is possible to prevent the surface solidification of the molten steel and to decrease surface cracks. As a result, it is possible to improve the quality of the products. Further, since the occurrence of break-out is prevented, it is possible to improve the process stability.

Meanwhile, the molten mold flux is substantially transparent in the wavelength range of 500 to 4000 nm corresponding to radiation waves of molten steel . Accordingly, when the continuous casting using the molten mold flux is performed, the radiation heat radiated from the molten steel is easily transmitted through the molten mold flux, so that the surface temperature of the molten steel is not kept and is solidified. As a result, the surface temperature of the molten steel is kept using the above-mentioned mold cover or the like.

In addition, according to the present invention, a heat insulation effect on the surface of the molten steel is improved by adjusting the absorption coefficient of the mold flux. In order to effectively control the heat insulation on the surface of the molten steel during the continuous casting process where a molten mold flux is injected, the mold flux according to the example of the present invention may have a mean absorption coefficient of 1000/m or more in the wavelength range of 500 to 4000 nm corresponding to radiation waves of molten steel.

FIG. 4 is a graph showing a relationship between an absorption coefficient and a wavelength of a mold flux in the related art. It is shown in FIG.4 that the mold flux in the related art has an absorpt ion coefficient of 100/m to 800/m, that is, 1000/m or less for the wavelength range of 500 to 4000 nm.

FIG.5 is a graph showing a relationship between a reflexibi lity of the mold cover and a radiation heat flow rate on the surface of the molten steel whi Ie an absorpt ion coefficient of the mold flux in the wavelength range of 500 to 4000 nm changes with the molten mold flux having a thickness of 20 mm. In this case, as the radiation heat flow rate becomes large, heat loss is increased, which means that a possibi lity of solidifying the surface of the molten steel is increased.

It can be understood from FIG. 5 that the radiation heat flow rate on the surface of molten steel is decreased as the absorption coefficient of the mold flux is increased. That is, it is possible to improve a heat insulation effect on the surface of the molten steel by reducing heat loss from the molten steel . (A) indicates heat loss on the surface of molten steel during the powder mold flux process, that is, a calculated amount of heat required to increase the temperature of a powder mold flux and to melt the powder mold flux when a powder mold flux instead of a molten mold flux at room temperature is supplied to the molten steel. (B) indicates the calculated amount of heat corresponding to a condition where the surface of the molten steel is not solidified when a molten mold flux is used. It is shown that the surface of the molten steel is not solidified regardless of the reflexibility of the mold cover when the mold flux has an absorption coefficient of 1000/m or more. Accordingly, the absorption coefficient of the mold flux in the wavelength range of 500 to 4000 nm may be 1000/m or more.

As shown in FIG.5, when the mold flux has an absorption coefficient of 1000/m or more, it is possible to prevent the surface solidification of the molten steel regardless of the reflexibi lity of the mold cover provided on the mold. However, when the mold cover is not provided on the mold, heat transfer is increased due to convection. For this reason, the mold cover may be provided on the mold. Further, according to the present invention, if the absorption coefficient of the mold flux is adjusted to be 1000/m or more, it is possible to minimize heat loss from the molten steel. Accordingly, even though the reflexibi lity of the mold cover deteriorates due to the oxidation of the surface of the mold cover or the volatilization of the mold flux, it is possible to secure a heat insulation effect on the surface of the molten steel . As a result , it is possible to perform a stable process. As described above, in order to allow the mold flux to have an absorption coefficient of 1000/m or more, the proper amount of transition metal oxides such as Fe2θ3, T1O2, NiO, and O2O3 may be added to the mold flux or the basicity (CaO/SiO?) of the mold flux may be lowered as much as possible.

Since a transit ion metal element may react to Al of molten steel , total amount of the transition metal oxide to be added may be 3 parts by weight or less with respect to the 100 parts by weight of the mold flux. Further, when the basicity of the mold flux is excessively low, the viscosity is excessively increased, and lubrication performance in the mold deteriorates. Therefore, the basicity may be maintained in the range of 0.5 to 1.5.

According to the present invent ion, since the molten mold flux process is applied as described above, it is possible to increase the mold flux consumption by reducing the thickness of a slag bear. Therefore, it is possible to effectively control heat transfer between the molten steel and the mold, and to improve lubrication performance. Further, it is possible to secure a heat insulation effect on the surface of the molten steel by using a mold flux where an absorption coefficient of molten steel radiation heat is adjusted as described above. As a result, it is possible to improve the quality of cast pieces, and to improve productivity and process stability.

The following Table 3 shows ingredients and absorption coefficients of mold fluxes according to a fourth comparative example and a second example. [Table 3]

As shown in Table 3, the fourth comparative example has a mean absorption coefficient of 470/m in the wavelength range of 0.5 to 4 μm, and the second example has a mean absorption coefficient of 1250/m in the wavelength range of 0.5 to 4 μm. Slab casting was performed in a continuous casting machine by using a mold that has a width of 1012 mm and a thickness of 140 mm and the mold fluxes according to the fourth comparative example and the second example. The type of steel was an extra-low carbon steel having a carbon concentration of 60 ppm. After a mold flux was completely melted outside a mold, the molten mold flux was injected into the mold by using a flow rate control device in the form of a stopper. When the molten mold flux was injected into the mold, the temperature of the molten mold flux was 1300⁰C. When molten steel was filled in the mold before the beginning of casting, the thickness of the molten pool increased up to a target thickness. Then, the casting began to be performed and the mold was covered with a mold cover. After that , as the cast ing was performed, molten mold flux was cont inuously suppl ied to the mold so as to supplement the consumed molten mold flux. The mold cover is made of aluminum, and the surfaces of the mold cover is finished with #1600 sandpaper so as to have a mean reflexibi lity of 40% for infrared rays in the range of 500 to 4000 nm that is a range corresponding to radiation waves of molten steel .

FIGS.6 and 7 are photographs of the surface of molten steel during the continuous casting using mold fluxes according to a fourth comparative example and a second embodiment. Referring to FIG.6 showing a photograph of the surface of molten steel corresponding to a fourth comparative example, it can be seen that a surface solidified layer is formed in the case of the fourth comparative example, which causes the quality of cast pieces and process stability to deteriorate. In contrast, referring to FIG.7 showing a photograph of the surface of molten steel corresponding to a second embodiment, it can be seen that a surface solidified layer is not formed in the case of the second embodiment. This is because loss of radiation heat from the molten steel is prevented by adjusting an absorption coefficient of the mold flux in the wavelength range of 500 to 4000 nm. That is, according to the second example, the heat loss radiated from the molten steel is prevented due to the relatively large absorption coefficient of the mold flux. Therefore, it is possible to achieve early slow cooling and to prevent the surface of the molten steel from being solidified.

According to the present invention, the mold flux, which has an absorption coefficient of 1000/m or more in the wavelength range of 500 to 4000 nm, is used during the continuous casting process using a molten mold flux. Accordingly, it is possible to achieve early slow cooling and to prevent the surface of the molten steel from being solidified. In particular, it is possible to prevent deckel that is an abnormal solidification phenomenon of slag pool of the mold flux in the early casting. Further, since a heat insulation effect on the surface of the molten steel is secured, it is possible to stably perform a molten mold flux process over a long period of time. In addition, since an "F" value indicating the degree of fluctuation of the surface of the mold, is reduced, it is possible to prevent slag from being mixed into the molten steel and to improve the quality of the cast pieces.

Although the invention has been described with reference to the accompanying drawings and the preferred embodiments, the invention is not limited thereto, but is defined by the appended claims. Therefore, it should be noted that various changes and modifications can be made by those skilled in the art without depart ing from the technical spirit of the appended claims .

Claims

[CLAIMS] [Claims 1]

A mold flux for continuous casting, the mold flux comprising 20 to 50 wt% of CaO, 20 to 50 wt% of SiO₂, 20 wt% or less of Al₂O₃, 20 wt% or less of Na₂O, 10 wt% or less of Li₂O, 20 wt% or less of B₂O₃, and 10 wt% or less of MgO. [Claims 2]

The mold flux of claim 1, wherein a thermal conductivity of the mold flux is 1.

2 W/m.k or more.

[Claims 3]

The mold flux of claim 1, wherein the mold flux has an absorption coefficient of 1000/m or more in a wavelength range of 500 to 4000 nm in a molten state.

[Claims 4] The mold flux of claim 1, wherein the mold flux contains 3 parts by weight or less with respect to the 100 parts by weight of the mold flux, and has basicity in the range of 0.5 to 1.

5. [Claims 5]

A continuous casting method comprising^'- melting a mold flux including 20 to 50 wt% of CaO, 20 to 50 wt% of SiO₂, 20 wt% or less of Al₂O₃, 20 wt% or less of Na₂O, 10 wt% or less of Li₂O, 20 wt% or less of B₂O₃, and 10 wt% or less of MgO outside a mold; and supplying the molten mold flux to the mold throughout the entire continuous casting process while a flow rate of the molten mold flux is controlled.

[Claims 6]

The continuous casting method of claim 5, wherein a thermal conductivity of the mold flux is 1.2 W/m.k or more.

[Claims 7] The continuous casting method of claim 5, wherein the mold flux has an absorption coefficient of 1000/m or more in a wavelength range of 500 to 4000 nm outside the mold.