Mold powder for continuous casting and continuous casting method

Abstract

To provide mold powder for continuous casting and a continuous casting method using the mold powder that can sufficiently prevent longitudinal crack of a cast piece and BO generation due to solidification delay.SOLUTION: Mold powder for continuous casting has a CaO' / SiO2 mass ratio of 1.1 to 1.8, 10 mass% or more of F, less than 0.5 mass% of K2O, 1 mass% or more and 20 mass% or less of Li2O, and the freezing point of more than 1100°C and 1250°C or less. CaO'=T.CaO-CaF2×0.718 ...(1) CaF2=(F-Li2O×1.27-Na2O×0.613-K2O×0.403)×2.05 ...(2) F: The F content in mold powder (mass %), T.CaO, Li2O, Na2O, and K2O mean the oxide content (mass%) calculated assuming that Ca, Li, Na, and K in mold powder are all oxides.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a molding powder for continuous casting and a continuous casting method. [Background technology]

[0002] In continuous steel casting, mold powder is added to the surface of molten steel in the mold. Mold powder is typically a mixture of powders of oxides such as CaO, SiO2, and Al2O3, fluorine compounds, and carbon. The mold powder, added to the surface of the molten steel from above the mold, melts due to the heat from the molten steel, forming a molten layer of mold powder on the surface of the molten steel. This molten layer flows between the mold and the solidified shell, forming a mold powder film (hereafter referred to as film). The main functions of this mold powder are: (1) preventing reoxidation of the molten steel and maintaining its temperature; (2) capturing inclusions that rise from the molten steel; (3) lubricating the mold and solidified shell; and (4) controlling heat removal from the solidified shell. This film consists of two phases: a crystalline phase formed by cooling the mold, and a liquid phase. The crystalline phase of the film has a high heat transfer resistance, which reduces the cooling rate of the solidified shell.

[0003] During continuous casting of steel, defects can occur on the surface and inside of the slab. These have a negative impact on steel quality, so reducing the occurrence of defects is essential. In particular, medium-carbon steel with a C concentration of 0.06 to 0.25% by mass tends to produce a cast defect called longitudinal cracks due to uneven growth of the solidified shell. To prevent longitudinal cracks, slow cooling of the solidified shell in the very early stages of its formation above the mold is effective in allowing it to grow uniformly. By slowing cooling using the powder film in the mold, the cooling rate of the slab can be reduced, preventing longitudinal cracks from appearing on the slab surface during continuous casting.

[0004] When the crystallization of the mold powder film in the mold is promoted, the radiation heat transfer in the film layer is suppressed, the heat transfer resistance increases, the cooling of the solidified shell through the film is slowed, and the solidified shell is formed and grows uniformly, which has the effect of preventing longitudinal cracks on the slab surface. Therefore, the occurrence of longitudinal cracks can be prevented when continuously casting varieties that are highly susceptible to longitudinal cracks, such as those described above (see Non-Patent Document 1).

[0005] Cuspidine (3CaO·2SiO2·CaF2) is a typical crystal that typically precipitates from mold powder films. In order to promote the precipitation of cuspidine, it is effective to increase the basicity of the mold powder and bring it closer to the pure composition of cuspidine.

[0006] Patent Document 1 discloses a mold powder for continuous casting of steel, which is based on CaO, SiO2, and a fluorine compound, has a CaO' / SiO2 ratio of 1.1 to 2.8, satisfies a predetermined CaF2 content, and further contains 0 to 25 wt% Na2O and 0 to 10 wt% C. The CaF2 content is calculated assuming that all F in the powder is CaF2, and CaO' is calculated by subtracting the CaF2 content from T.CaO. This promotes the precipitation of crystals such as cuspidine, while lowering the powder's freezing point and viscosity. The freezing point is said to be approximately 1100 to 1300°C.

[0007] Patent Document 2 cites Patent Documents 3 and 4 as examples, and states that the highly basic mold powders proposed in these prior documents have been added with Na2O, Li2O, F, etc. to lower the solidification point in order to ensure lubrication between the mold inner wall and the solidified shell, and that the solidification point is adjusted to a range of 1000-1250°C. Patent Document 2 discloses a mold powder for continuous casting that contains CaO, SiO2, and a fluorine compound as basic components, contains 0-10 mass% ZrO2, has a CaO' / SiO2 ratio of 0.9-1.9, and has CaF2 and alkali metal fluoride contents that satisfy specified conditions. The CaF2 content is calculated assuming that F in the powder preferentially forms alkali metal fluorides with Li, Na, and K in the powder, and the remaining F forms CaF2, and W CaO CaO' is calculated by subtracting CaF2 from (T.CaO).

[0008] In these patent documents, the basicity and F content of the mold powder are adjusted to promote the crystallization of cuspidine, which promotes the crystallization of the crystalline phase of the film that flows between the mold and the solidified shell. A film with a crystallized phase has a higher heat transfer resistance than a film with a glass phase, so this serves to reduce heat removal from the solidified shell.

[0009] Recently, there has been a trend toward high-speed casting in order to improve productivity. However, when casting is performed at high speed, the time that the solidified shell is in contact with the mold is shortened, which makes it difficult to ensure a sufficient thickness of the solidified shell at the bottom of the mold. This raises concerns about an operational problem in which the solidified shell breaks at the bottom of the mold, causing molten steel to leak (delayed solidification breakout (hereinafter referred to as "delayed solidification BO"). Therefore, in high-speed casting, it is necessary to intensively cool the solidified shell to promote its growth and obtain a solidified shell with sufficient thickness.

[0010] The use of conventional mold powders that promote crystallization is effective in suppressing vertical cracks, but the cooling of the solidified shell through the film is slowed, slowing the solidification process in the mold and increasing the risk of delayed solidification BO. The surface temperature of the solidified shell drops to near the freezing point of the mold powder toward the bottom of the mold. In particular, when mold powders with high freezing points are used, the crystallization of the film continues even in the bottom of the mold, further reducing heat removal from the solidified shell, which has the adverse effect of suppressing the growth of the solidified shell. Therefore, although the use of such mold powders is effective in preventing vertical cracks, there is a concern that it may increase the incidence of delayed solidification BO during high-speed casting.

[0011] Patent Document 5 discloses a continuous casting method for steel, in which mold powder containing a CaO / SiO2 mass ratio of 1.5 to 2.5, less than 2% Na2O by mass, and at least 1% Li2O by mass is used, and continuous casting is performed at a casting speed of 1.6 m / min or greater. By using a mold powder with a composition that is easy to crystallize and increasing the crystallization rate, dense, uniform crystals are rapidly formed in the meniscus, achieving ultra-gentle cooling. Furthermore, by using a mold powder composition with a low crystallization temperature that can suppress crystal growth, strong cooling is achieved below the meniscus. The CaO / SiO2 mass ratio (basicity) is set to 1.5 to 2.5 to achieve gentle cooling in the meniscus of the mold. The Na2O content is further reduced to less than 2% by mass to control the crystallization rate and achieve ultra-gentle cooling in the meniscus. Furthermore, the Li2O content is adjusted to 1% by mass or greater to lower the crystallization temperature to less than 1100°C, achieving strong cooling below the meniscus. In Example No. 1 in Table 1 (ultra-slow cooling in the meniscus and strong cooling in the lower part), the F concentration in the mold powder is 9.0%, the crystallization speed is high, ultra-slow cooling is performed in the meniscus, and the crystallization temperature is a low 1055°C, with strong cooling in the lower part. The crystallization temperature is the same temperature as the solidification point.

[0012] Patent Documents 6 and 7 disclose molding powders containing SiO2 and CaO as main components, with a mass ratio (CaO / SiO2) of 1.1 or more and 2.5 or less, a K2O content of 1.0 to 10.0 mass%, a total content of Na2O and Li2O of 1.0 to 18.0 mass%, F, MgO, Al2O3, and total carbon contents of 3.0 to 15.0 mass%, 0.5 to 3.0 mass%, 0.5 to 10.0 mass%, and 1.0 to 20.0 mass%, respectively, a viscosity at 1300°C of 0.03 to 0.7 Pa s, a crystallization temperature of 1080 to 1280°C, and cuspidine as the primary crystal species. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Patent No. 3463567 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-179408 [Patent Document 3] Japanese Patent Application Publication No. 8-141713 [Patent Document 4] Japanese Patent Application Publication No. 10-216907 [Patent Document 5] Japanese Patent Application Laid-Open No. 2006-247744 [Patent Document 6] Japanese Patent Application Publication No. 2020-146719 [Patent Document 7] Patent Publication No. 2021-74782 [Non-patent literature]

[0014] [Non-Patent Document 1] 5th Edition Iron and Steel Handbook, Vol. 1, Iron and Steelmaking, pp. 417-419 Summary of the Invention [Problem to be solved by the invention]

[0015] The present invention solves the problem of high speed casting of hypoperitectic steel in continuous casting. The present invention aims to provide a casting method that can slowly cool the solidified shell in the upper part of the mold and rapidly cool the solidified shell in the lower part of the mold, with the aim of realizing high-speed casting of steel that is prone to longitudinal cracks. The method involves rapidly crystallizing the crystalline phase of a film in the upper part of the mold and slowly cooling the solidified shell at the meniscus to suppress uneven growth of the solidified shell, which is the starting point for longitudinal cracks, and suppressing the growth of the crystalline phase of the film in the lower part of the mold and rapidly cooling the solidified shell to promote growth.

[0016] Specifically, it is effective to design the molding powder to have a composition that facilitates crystallization, thereby increasing the crystallization rate, while lowering the freezing point. A high crystallization rate promotes the crystallization of the film's crystalline phase in the upper part of the mold, while a low freezing point inhibits the growth of the film's crystalline phase in the lower part of the mold.

[0017] The invention described in the aforementioned Patent Document 5 is based on the same technical idea as the present invention. However, it has become clear that the invention described in Patent Document 5 is insufficient in preventing the occurrence of longitudinal cracks in a cast slab, and is unable to fully eliminate the concern about delayed solidification BO.

[0018] The present invention aims to provide a mold powder for continuous casting, and a continuous casting method using the same, which can sufficiently prevent the occurrence of longitudinal cracks in a cast slab and the occurrence of solidification-delayed BO by quickly crystallizing a film crystalline phase in the upper part of the mold, slowly cooling the solidified shell at the meniscus to suppress uneven growth of the solidified shell, which is the starting point of vertical cracks, and by suppressing the growth of the film crystalline phase in the lower part of the mold and strongly cooling the solidified shell to promote growth of the solidified shell. [Means for solving the problem]

[0019] [1] A mold powder for continuous casting, characterized in that the CaO' / SiO2 mass ratio is 1.1 to 1.8, it contains 10 mass% or more of F, and among the alkali metal oxides, it contains less than 0.5 mass% of K2O and 1 mass% or more and 20 mass% or less of Li2O, and it has a solidification point of more than 1100°C and 1250°C or less. where: CaO' (mass%)=T.CaO-CaF2×0.718 …(1) CaF2 (mass%) = (F-Li2O×1.27-Na2O×0.613-K2O×0.403)×2.05 …(2) (However, if the right side of equation (2) is negative, the left side of equation (2) is set to 0%.) F: F content in mold powder (mass%) Of the evaluation results of the component contents of the molding powder, the total content of the components excluding C is set to 100 mass%, and the content of each component is determined. T. CaO, Li2O, Na2O, and K2O refer to the oxide contents (mass%) calculated assuming that Ca, Li, Na, and K in the molding powder are all oxides. [2] The mold powder for continuous casting according to [1], further comprising 1 mass % or more of Na2O. [3] The mold powder for continuous casting according to [1] or [2], further characterized in that the total content of Al2O3 and MgO is 5 mass% or less. [4] A continuous casting method for casting medium carbon steel having a carbon concentration of 0.06 to 0.20 mass % using the mold powder for continuous casting according to any one of [1] to [3]. [Effects of the Invention]

[0020] The present invention uses a mold powder for continuous casting that has a CaO' / SiO2 mass ratio of 1.1 to 1.8, contains 10 mass% or more of F, contains less than 0.5 mass% of KO among alkali metal oxides, and contains 1 mass% to 20 mass% of Li2O, and has a solidification point greater than 1100°C and less than 1250°C. This allows rapid crystallization of a film crystalline phase in the upper mold, slow cooling of the solidified shell at the meniscus to suppress uneven growth of the solidified shell, which is the starting point for vertical cracks, and suppresses growth of the film crystalline phase in the lower mold, and strong cooling of the solidified shell to promote growth of the solidified shell. This sufficiently prevents the occurrence of vertical cracks in the slab and the occurrence of delayed solidification BO, even when continuous casting medium-carbon steel with a carbon concentration of 0.06 to 0.20 mass% at a high casting speed. DETAILED DESCRIPTION OF THE INVENTION

[0021] The component composition of the mold powder for continuous casting of the present invention will be explained. When the mold powder for continuous casting contains C, the total of the components other than C in the mold powder is taken as 100% by mass, and the content of each component is calculated. Therefore, the C content is treated as an external number (extrapolated). The % in the mold powder content means % by mass.

[0022] [CaO' / SiO2] The molding powder contains T.CaO, SiO2, and F as its main components. In this case, T.CaO is treated as a component where the total amount of Ca in the molding powder is considered to be CaO. F is added as CaF2 in the powder state, but in the molten state, F has a stronger affinity for alkali metals than Ca. Therefore, in the molten state, F reacts with alkali metal components and exists as alkali metal fluorides, while the remaining F reacts with Ca components and exists as CaF2 (see, for example, Patent Document 2). Therefore, the CaF2 concentration in the powder in the molten state is determined by the above-mentioned formula (2). If the right-hand side of formula (2) is negative, all of the F in the powder has reacted with alkali metal oxides, the remaining F is zero, and the CaF2 concentration is also considered to be zero. Therefore, the left-hand side of formula (2) is set to 0%. After determining the CaF2 concentration using formula (2) in this way, CaO' is calculated using formula (1). CaO' defined by formula (1) indicates the amount of Ca that has not reacted with F and is considered to exist as an oxide (CaO).

[0023] CaO' / SiO2 is a component index for crystallizing cuspidine. The mass concentration ratio of CaO' / SiO2 is preferably 1.1 or more and 1.8 or less. If the mass concentration ratio CaO' / SiO2 is less than 1.1, the amount of crystallization is small, and a film with sufficient slow cooling ability cannot be obtained. On the other hand, if the mass concentration ratio CaO' / SiO2 exceeds 1.8, crystalline phases other than cuspidine crystallize, and a film with sufficient slow cooling ability cannot be obtained.

[0024] [F] The F content is 10% by mass or more. This is because the F component has the effect of adjusting the solidification point of the mold powder and also has an effect on the crystallization of cuspidine, and if the F content is less than 10% by mass, this effect is reduced. On the other hand, if a large amount of F is added, the viscosity of the mold powder decreases significantly, causing the mold powder to be entrained in the molten steel. Therefore, the F content is preferably 24% by mass or less.

[0025] [Alkali metal oxides] Alkali metal oxides that are added to mold powder include KO, NaO, and LiO. Alkali metal oxides are added to improve the crystallization rate of the mold powder and to adjust the solidification point and viscosity. NaO is often used from a cost perspective. Hereinafter, LiO, NaO, and KO as the component contents in the mold powder refer to the oxide contents (mass%) calculated assuming that all of the Li, Na, and K in the mold powder are oxides.

[0026] It is known that the effect of adding alkali metal oxides on improving the crystallization rate is greatest in the order Li2O > Na2O > K2O. Li2O has the greatest effect on increasing the crystallization rate of mold powder. The Li2O content is 1% by mass or more and 15% by mass or less, and preferably 3% by mass or more and 10% by mass or less. If the Li2O content is less than 1% by mass, the freezing point of the mold powder is high and delayed solidification BO cannot be prevented. On the other hand, if Li2O is added in excess, the amount of cuspidine crystallization decreases and sufficient slow cooling ability cannot be obtained.

[0027] It is known that the addition of K2O increases the viscosity and slows the crystallization rate compared to conventional molding powders. Because this reduces the flow between the mold and the solidified shell and reduces the crystallization rate, the inclusion of K2O should be avoided, and the present invention specifies that the K2O content be less than 0.5 mass%.

[0028] Furthermore, the molding powder of the present invention may contain 1% by mass or more of Na2O. Na2O is a commonly used component, and although its inclusion does not improve the crystallization rate beyond that of conventional components, it does have the effect of lowering the solidification point. However, if it is included in excess, the amount of cuspidine crystallization decreases, making it impossible to obtain sufficient slow cooling ability, so the Na2O content is preferably 10% by mass or less.

[0029] [Al2O3 and MgO] The total content of Al2O3 and MgO is preferably 5% by mass or less. Al2O3 and MgO are unavoidable impurities in the design of the molding powder. Both reduce the amount of cuspidine crystallized and cause other crystals to crystallize, resulting in poor melting characteristics and poor heat removal, so it is desirable to keep their contents as low as possible. The contents of Al2O3 and MgO in the molding powder refer to the oxide contents (% by mass) calculated assuming that all of the Al and Mg in the molding powder are oxides.

[0030] [C] Furthermore, in addition to the above components, it is desirable to add C to the molding powder of the present invention, and the content of other elements in the molding powder is desirably 1 to 10 mass%. C has the effect of adjusting the melting rate of the molding powder, and as the C content increases, the melting rate decreases. If the C content is less than 1 mass%, the melting rate will be too high, and if it exceeds 10 mass%, the melting rate will be too low, resulting in poor powder flowability into the mold and solidified shell.

[0031] [Freezing point] (freezing temperature (℃)) (equal to the crystallization temperature) The freezing point of the mold powder is specified to be greater than 1100°C and less than 1250°C. If the freezing point exceeds 1250°C, excessive film crystalline phases are formed in the lower part of the mold, resulting in insufficient growth of the solidified shell. On the other hand, mold powders with a freezing point lowered to 1100°C or less will not crystallize cuspidine sufficiently and will not be able to obtain sufficient slow cooling ability, so it is more preferable to set the freezing point to greater than 1100°C. Adjusting the freezing point of the mold powder to be greater than 1100°C and less than 1250°C can be achieved by adjusting the CaO' / SiO2 mass ratio, F content, and alkali metal content of the mold powder within the range of the present invention using the above-described contents as indicators.

[0032] [viscosity] The viscosity of the mold powder should be 1 poise or less at 1300°C. If it exceeds 1 poise, the flow of mold powder between the mold and the solidified shell will be insufficient, making it more likely to produce defects in the cast slab.

[0033] [Raw materials] The raw materials used in the molding powder of the present invention can be any commonly used raw materials. CaO raw materials include quicklime, limestone, and cement, SiO raw materials include silica sand and diatomaceous earth, LiO raw materials include lithium carbonate, NaO 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.

[0034] Furthermore, there are no limitations on the shape of the raw materials for the molding powder. 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.

[0035] (Continuous casting method) In the continuous casting method using the mold powder for continuous casting of the present invention, the effects of the present invention can be particularly demonstrated when casting medium-carbon steel, which is a hypoperitectic steel with a carbon concentration of 0.06 to 0.20 mass %, and when casting the medium-carbon steel, it becomes possible to set the casting speed to 1.8 m / min or more. [Example]

[0036] Examples of the molding powder of the present invention will be described below. Mold powders with the compositions shown in Table 1 were prepared. The content of component elements in the mold powders was evaluated by chemical analysis. The total content of components excluding C in the mold powder component content evaluation results was set as 100 mass%, and the content of each component was determined. T. CaO, Li2O, Na2O, and K2O refer to the oxide content (mass%) calculated assuming that Ca, Li, Na, and K in the mold powder are all oxides.

[0037] (Mold powder evaluation method) The freezing point, viscosity, and crystallization rate index of the present invention and conventional products were measured. The viscosity was measured using a vibrating reed viscometer while cooling mold flux molten at 1400°C in a graphite crucible at 2°C / min, and the viscosity was measured at 1300°C. The temperature was continued to be lowered from 1300°C, and the temperature at which the viscosity rose sharply was taken as the freezing point. The freezing point is the same as the crystallization temperature. The crystallization rate index was measured by optically observing the behavior of crystallization within a 4mm x 4mm field of view when the temperature was lowered from 1350°C at a rate of 150°C / min using a laser microscope. The crystallization rate index was calculated as the rate at which the crystal phase spread from the liquid phase within a 4mm x 4mm field of view. The results are shown in Table 1.

[0038] In Table 1, values ​​outside the scope of the present invention are underlined. Nos. 1 to 8 in Table 1 are products according to the present invention, and Nos. 9 to 13 in Table 1 are comparative products. Comparative Examples Nos. 9, 10, and 13 have freezing points higher than the range of the present invention. No. 11 has a freezing point lower than the range of the present invention, and a CaO' / SiO2 ratio higher than the range of the present invention. No. 12 contains K2O, which is outside the range of the present invention, and is the largest amount contained among the alkali metal oxides. Comparative Example No. 13 in the table is a high-freezing-point molding powder that contains Na2O but no Li2O, as described in Patent Document 4. No. 13 has a Li2O content outside the range of the present invention, no Li2O is contained at all, and has a high freezing point.

[0039] (Continuous casting experiment) A continuous casting test was carried out using the mold powder in Table 1. Hypoperitectic steel with a C content of 0.11 mass% was cast at a speed of 1.8 m / min in a mold 200 mm thick and 1600 mm wide, and the effect was evaluated.

[0040] (Casting result evaluation method) <Evaluation of cast surface properties> The four surfaces of the slab were visually inspected, and the longitudinal cracks were evaluated based on their total length. The results are shown in Table 1. A rating of ◯ was given when the total length of the longitudinal cracks per 1 m of casting length was 0.05 m or less, or when no longitudinal cracks were observed at all. A rating of × was given when longitudinal cracks longer than 0.05 m in total length per 1 m of casting length were observed.

[0041] <Evaluation of heat transfer behavior to the mold> The slow cooling ability was also evaluated, and the maximum local heat flux (MW / m 2 ) was investigated. Thermocouples were embedded near the surface of the mold copper plate at three locations in the mold height direction, ranging from the meniscus to 100 mm below the meniscus. The heat flux was evaluated based on the difference between the thermocouple temperature readings and the mold cooling water temperature, and the maximum value of the evaluation results is listed in Table 1 as the "maximum local heat flux." If the maximum local heat flux is smaller than that of Comparative Example No. 12, whose solidification point falls within the range of the present invention, and is smaller or slightly larger than the maximum local heat flux of Comparative Example No. 13, whose solidification point falls outside the upper limit of the range of the present invention, then it can be evaluated that the solidified shell is cooled slowly, and that slow cooling by the mold powder has been achieved.

[0042] To evaluate the promotion of solidified shell growth, the total heat removal amount up to the bottom of the mold was investigated. The thickness of the solidified shell at the bottom of the mold can be evaluated by the total heat removal amount taken from the slab from the meniscus to the bottom of the mold. If the total heat removal amount is large, it indicates that the solidified shell is strongly cooled at the bottom of the mold, and the growth of the solidified shell within the mold is promoted. Specifically, the evaluation was carried out as follows. Thermocouples were embedded near the surface of the mold copper plate at a total of eight locations in the height direction of the mold, ranging from the meniscus to 600 mm below the meniscus. The heat flux at each position in the height direction was evaluated based on the difference between the temperature measurement results of the thermocouples and the mold cooling water temperature. The total heat removal amount (MJ / m) was calculated assuming that heat is removed by the heat flux at each position in the height direction evaluated above while the slab, moving downward at the casting speed of the slab, moves from the meniscus to the bottom of the mold. 2 ) was calculated.

[0043] <Evaluation using breakout prediction equipment> Thermocouples are embedded in continuous casting molds for the purpose of breakout prediction. Their primary purpose is to predict the occurrence of constraint-type breakouts, enabling early detection of the downward movement of the pseudo-meniscus due to constraint between the mold and solidified shell at the meniscus. Analyzing the thermocouple temperature history also allows for the evaluation of abnormal behavior targeted by this invention. When uneven solidification (the cause of vertical cracks) occurs, the temperature measurement results of the thermocouple installed near the meniscus fluctuate. This indicates that uneven solidification is detected, and the BO prediction signal is marked as "false positive (△)." Furthermore, when excessive powder crystallization occurs at the bottom of the mold, causing the crystal layer to peel off and bring the mold and solidified shell into contact, the temperature measurement results of the thermocouple installed at the bottom of the mold fluctuate. This indicates that insufficient solidified shell formation at the bottom of the mold is detected, and the BO prediction signal is marked as "false negative (×)." When neither of these false positives is detected, the BO prediction signal is marked as "good."

[0044] As described above, "Issue 1: Slow cooling of the solidified shell in the upper part of the mold" can be evaluated based on the maximum local heat flux, the presence or absence of vertical cracks, and the presence or absence of false BO detection (false detection △). Also, "Issue 2: Strong cooling of the solidified shell in the lower part of the mold" can be evaluated based on the total heat dissipation amount and the presence or absence of false BO detection (false detection ×). For the overall evaluation, if issues 1 and 2 are resolved, it will be expressed as "Good", and if either one is not resolved, it will be expressed as "Poor".

[0045] [Table 1]

[0046] The results are shown at the bottom of Table 1. Products No. 1 to No. 8 of the present invention in Table 1 are the results of continuous casting using the mold powder specified in the present invention, and although hypoperitectic steel with a C content of 0.11 mass % was cast at a speed of 1.8 m / min, good results were obtained in all indicators.

[0047] Comparative Examples 9 and 10 had freezing points higher than the range of the present invention, resulting in a low total heat transfer rate and a false positive result of BO. Comparative Example 11 had a freezing point lower than the range of the present invention and a CaO' / SiO2 ratio higher than the range of the present invention. As a result, the maximum local heat flux was high, resulting in a false positive result of BO and longitudinal cracks in the slab. Comparative Example 12 had a K2O content outside the upper limit of the present invention and contained the highest amount of K2O among the alkali metal oxides. As a result, the maximum local heat flux was high, resulting in a false positive result of BO and longitudinal cracks in the slab. Comparative Example 13 in the table had a Li2O content outside the range of the present invention and contained no Li2O at all. The freezing point was high, resulting in a low total heat transfer rate and a false positive result of BO.

[0048] According to the information from the breakout prediction device, as mentioned above, for Nos. 9, 10, and 13, excessive crystallization of the film caused peeling at the bottom of the mold, which resulted in thermocouple temperature hunting and a false positive result of BO (×). On the other hand, for Nos. 11 and 12, thermocouple hunting due to uneven solidification resulted in a false positive result of BO (△).

[0049] From the above results, it was found that adding LiO to Comparative Example No. 13 improved the crystallization rate and optimized the solidification point, thereby resulting in slower cooling of the solidified shell just below the meniscus and promoting the growth of the solidified shell in the lower part of the mold.

Claims

1. CaO' / SiO ２ The mass ratio is 1.1 to 1.8, and the content of F is 11.2 mass % or more. Among the alkali metal oxides, K ２ O is less than 0.5 mass %, Na ２ O is 1 mass% or more and 3.7 mass% or less, and Li ２ A mold powder for continuous casting, characterized by containing 2.3 mass % or more and 10 mass % or less of O, having a solidification point exceeding 1100°C and 1250°C or less, and having a viscosity at 1300°C of 1 poise or less. where: CaO' (mass%) = T. CaO - CaF ２ ×0.718 …(1) CaF ２ (mass%) = (F - Li) ２ O×1.27-Na ２ O×0.613-K ２ (0 × 0.403) × 2.05 … (2) (However, if the right side of equation (2) is negative, the left side of equation (2) is set to 0%.) F: F content in mold powder (mass%), In the evaluation results of the component contents of the molding powder, the total content of the components excluding C is set to 100 mass %, and the content of each component is determined. ２ O, Na ２ O.K. ２ O means the oxide content (mass %) calculated assuming that all of Ca, Li, Na, and K in the molding powder are oxides.

2. Furthermore, Al ２ O ３ 2. The mold powder for continuous casting according to claim 1, wherein the total content of the Cr, Ni, and MgO is 5% by mass or less.

3. A continuous casting method for casting medium carbon steel having a carbon concentration of 0.06 to 0.20 mass % using the mold powder for continuous casting according to claim 1 or 2.

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