Mold powder for continuous casting and continuous casting method

Abstract

To achieve both stable operation and good quality of cast slab in continuous casting of steel, especially in casting of high-Al steel.SOLUTION: There is provided a mold powder for continuous casting of steel, wherein the concentration of T.CaO is 25 mass% or more, the concentration of F is more than 25 mass%, the concentration of SiO2 is 1 mass% or more and 20 mass% or less, and the concentration of an alkali metal oxide is 10 mass% or more and 25 mass% or less. There is also provided a continuous casting method characterized by using the mold powder for continuous casting to cast steel containing 0.1 mass% or more of Al.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a mold powder for continuous casting used for casting high-Al steel in continuous casting, and a continuous casting method. [Background technology]

[0002] In continuous casting of steel, mold powder is added to the surface of molten steel in the mold before casting. Mold powder is mainly a mixture of powders of oxides such as CaO, SiO2, and Al2O3, fluoride, and carbon. The mold powder added to the surface of the molten steel from above the mold melts due to the heat it receives 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 abbreviated as film).

[0003] The main functions of this mold powder are (1) preventing reoxidation of molten steel and maintaining its temperature, (2) capturing inclusions that float from the molten steel, (3) lubricating the mold and solidified shell, and (4) controlling heat removal from the solidified shell. The present invention relates to (3) and (4). The film that forms between the mold and solidified shell 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 and acts to reduce the cooling rate of the solidified shell. On the other hand, the liquid phase acts as a lubricant and prevents seizure between the mold and solidified shell.

[0004] In continuous casting of steel, defects can occur on the surface and inside of the cast slab, which have a negative effect on the quality of the steel, so it is necessary to reduce the occurrence of defects.

[0005] In high-Al steel with an Al concentration of 0.1% or more, an interface reaction occurs between the molten layer of mold powder and the molten steel as shown in the following formula (1), reducing the SiO2 in the mold powder while increasing the Al2O3 concentration. 4[Al]+3(SiO2)→2(Al2O3)+3[Si] (1) Here, [ ] means the components in the steel, and ( ) means the components in the mold powder.

[0006] Therefore, when casting steel with a high Al content, the change in the composition of the mold powder due to the reaction in equation (1) above cannot be ignored.The change in the composition of the mold powder can significantly change the viscosity and solidification point of the mold powder from the physical properties at the time of design, causing problems such as deterioration of the lubrication in the mold and the surface quality of the cast slab.

[0007] To address these problems, two main countermeasures have been considered, and several methods have been disclosed. The first method is to suppress the oxidation reaction of Al and reduce the changes in the composition and physical properties of the mold powder. The second method is to select an initial composition and physical properties that are tolerant to the oxidation reaction of Al and less susceptible to it.

[0008] An example of a disclosed technique related to the first method is Patent Document 1. In Patent Document 1, the Al2O3 concentration in the mold powder is set to 20 to 40 mass% and the SiO2 concentration to 7.0 mass% or less, thereby suppressing the interfacial reaction of formula (1) above and reducing changes in the physical properties of the mold powder during casting. While this composition is designed to reduce the likelihood of interfacial reactions, the softening temperature of the mold powder in Patent Document 1 is as high as 1200 to 1300°C, compared to the softening temperature of ordinary mold powders, which is 1050 to 1250°C. This results in insufficient lubrication within the mold, and places significant limitations on its practical use. Here, the softening temperature refers to the temperature at which the sample surface begins to melt when the mold powder is heated from room temperature.

[0009] Disclosed techniques relating to the second method are, for example, Patent Documents 2 and 3.

[0010] Within the composition range of typical mold powders, cuspidine (3CaO·2SiO2·CaF2) forms as a crystalline phase. During the continuous casting of high-Al steel, an increase in the Al2O3 concentration in the molten layer of mold powder changes the composition to one that is more likely to produce gehlenite (2CaO·Al2O3·SiO2), a high-melting-point crystal. When high-melting-point crystals form, the viscosity and solidification point of the mold powder change significantly from the designed physical properties, degrading the lubrication within the mold and the surface quality of the cast slab.

[0011] In contrast, the invention described in Patent Document 2 aims to produce Melite, a complete solid solution of Akermanite (2CaO·MgO·2SiO2) and Gehlenite, in the mold, and adjusts the initial composition to one that will produce Akermanite. Even if the Al2O3 concentration in the molten layer of the mold powder increases in the mold and Gehlenite precipitates, the invention aims to form a complete solid solution with Akermanite, minimizing changes in the solidification point and viscosity.

[0012] On the other hand, in Patent Document 3, the basicity is 1.0 to 1.8, the F content is 16 to 25 mass%, and the initial composition is designed to produce CaF2. As will be described later, a higher basicity is effective in suppressing the interfacial reaction, but in Patent Document 3, when the basicity is high, the increase in the crystallization temperature (freezing point) after the composition change could not be ignored. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 57-184563 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-223599 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-170494 [Non-patent literature]

[0014] [Non-Patent Document 1] Masafumi Hanao et al., Iron and Steel, 93 (2007), pp. 362-366 Summary of the Invention [Problem to be solved by the invention]

[0015] The above-mentioned prior art techniques have not resolved problems such as unstable heat removal in the mold and poor mold lubrication caused by reactions between the mold powder and molten steel, which can lead to a decline in cast quality or an impediment to stable operation. The present invention has been conceived with the aim of solving the above problems, and aims to provide a mold powder for continuous casting and a continuous casting method that achieve both stable operation and the maintenance of good cast quality when continuously casting steel containing 0.1 mass% or more of Al. [Means for solving the problem]

[0016] That is, the gist of the present invention is as follows. [1] A mold powder for continuous casting of steel, characterized in that the concentration of T.CaO is 25% by mass or more, the concentration of F is more than 25% by mass, the concentration of SiO2 is 1% by mass or more and 20% by mass or less, and the concentration of alkali metal oxides is 10% by mass or more and 25% by mass or less. The relative mass ratios of the metal elements and F in the molding powder are evaluated, and assuming that all metal elements are oxides, the relative mass ratios of the metal oxides are calculated based on the relative mass ratios of the metal elements, and the contents (mass%) of the metal oxides and F are calculated so that the total content of all metal oxides and F is 100 mass%. The contents of T. CaO, SiO2, and alkali metal oxides refer to the contents (mass%) of each oxide of Ca, Si, and alkali metal calculated above. [2] A continuous casting method, characterized by using the mold powder for continuous casting according to [1] above to cast steel having an Al content of 0.1 mass % or more. [Effects of the Invention]

[0017] According to the present invention, it is possible to stably continuously cast steel containing 0.1 mass % or more of Al, and to obtain cast pieces with good surface quality. DETAILED DESCRIPTION OF THE INVENTION

[0018] An object of the present invention is to achieve stable operation and good cast quality in continuous casting of steel, particularly in casting of high Al steel.

[0019] [T.CaO, T.CaO / SiO2] The Ca component of molding powder is often added as CaO or CaF2. In this case, the amount of CaO when all the Ca components are considered to be CaO is described as "T.CaO".

[0020] Increasing the T.CaO / SiO2 ratio is an effective way to reduce the reaction between mold powder and molten steel. However, with conventional inventive technology, if the T.CaO / SiO2 ratio is increased, the solidification point after compositional fluctuations in the molten layer in the mold becomes excessively high, and the desired crystals do not precipitate stably, making it unsuitable for continuous casting. For example, in Patent Document 3, the basicity (T.CaO / SiO2) is designed to be 1.0 to 1.8 (paragraph

[0018] of the same document).

[0021] As a result of intensive research, the inventors have discovered that by designing a composition in which CaF2 is the main crystal and increasing the amount of F blended, it is possible to design a wide range of solidification points, from 900 to 1200°C, while increasing T.CaO / SiO2, and that it is possible to suppress fluctuations in solidification point and viscosity caused by the formation of Gehlenite due to an increase in the Al2O3 concentration, which inhibits stable casting. Specifically, the mold powder satisfies the following requirements:

[0022] [T.CaO] The molding powder of the present invention contains 25% by mass or more of T.CaO. If the content is less than 25% by mass, CaF2 does not precipitate stably, and the desired crystals cannot be obtained. It is more preferable that the content of T.CaO is 30% by mass or more.

[0023] [F] The F content is more than 25% by mass. This is because the F component has the effect of adjusting the solidification point of the molding powder and is also a component that constitutes CaF2. If the F content is 25% by mass or less, CaF2 will not crystallize sufficiently, and other crystals may crystallize out as the main crystals. It is more preferable that the F content is 30% by mass or more.

[0024] [SiO2] The SiO2 content is 20 mass% or less. If the SiO2 concentration exceeds 20 mass%, the increase in Al2O3 concentration due to the interface reaction of the above formula (1) cannot be ignored, and the influence of the composition fluctuation of the mold powder becomes large. In addition, the addition of SiO2 is essential from the viewpoint of optimizing the solidification point, so the content is set to 1 mass% or more.

[0025] [Alkali metal oxides] Alkali metal oxides that can be added to molding powder include K2O, Na2O, and Li2O. Alkali metal oxides are added to lower the solidification point and viscosity of the molding powder, and it is desirable to add 10 to 25 mass% of alkali metal oxides to optimize the solidification point. Furthermore, from the viewpoint of cost, Na2O is usually used, but Li2O and / or K2O may also be added in part.

[0026] [Main crystal] The main crystals refer to the crystals derived from the first ferromagnetic line when the mold powder sample is melted at 1400°C, cooled at 2°C / min to 800°C, and then naturally cooled by X-ray diffraction.

[0027] In the present invention, by using a molding powder with the above-mentioned component composition, CaF2 can be made the main crystal. By using CaF2 as the main crystal, as described above, by increasing the amount of F blended, it is possible to design the freezing point in a wide range of 900 to 1200°C while increasing T.CaO / SiO2, and it is also possible to suppress fluctuations in freezing point and viscosity caused by the formation of gehlenite due to an increase in the Al2O3 concentration, which inhibits stable casting.

[0028] [T.CaO / SiO2 (basicity)] As a result of the above, by using a mold powder with the above-mentioned component composition, the present invention makes it possible to design a wide range of solidification points, from 900 to 1200°C, while adopting a high basicity of T.CaO / SiO2 (basicity) of 1.8 or more. Furthermore, as a result of adopting a high basicity of T.CaO / SiO2 (basicity) of 1.8 or more, it is possible to suppress the formation of gehlenite and to suppress fluctuations in solidification point and viscosity caused by the formation of gehlenite. By adopting such a high basicity, the interfacial tension of the mold powder with molten steel increases, thereby reducing the entrainment of the mold powder into molten steel. It is more preferable that the T.CaO / SiO2 (basicity) be 2.1 or more.

[0029] [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 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.

[0030] [Freezing point] The freezing point of the molding powder is not particularly specified, but it is preferably above 900°C and below 1250°C. If it exceeds 1250°C, excessive film crystalline phase will form, resulting in insufficient lubrication inside the mold. Mold powders with a freezing temperature lowered below 900°C will not crystallize enough CaF2. In the present invention, even when a high basicity of T.CaO / SiO2 (basicity) of 1.8 or higher is used, by using a mold powder with the above component composition, it is possible to design the freezing point within the above preferred range.

[0031] [viscosity] There are no particular restrictions on the viscosity of the mold powder, but it is desirable that it be 1 poise or less at 1300°C. If the viscosity exceeds 1 poise, the flow of mold powder between the mold and the solidified shell will be insufficient, making it more likely to cause defects in the cast slab.

[0032] If the viscosity of mold powder is reduced, the mold powder becomes more likely to be entrained in the molten steel in the mold, and the entrained mold powder droplets become non-metallic inclusions near the surface of the slab, resulting in a problem of degrading its cleanliness. However, Non-Patent Document 1 discloses that even if the viscosity at 1300°C is reduced to 1 poise or less, entrainment can be prevented if the mold powder has a high basicity. The present invention achieves this effect by setting the T.CaO / SiO2 (basicity) to 1.8 or more.

[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, sodium fluoride, and cryolite, 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. [Example]

[0035] <Ingredients> The content of component elements in the molding powder was evaluated by chemical analysis. The relative mass ratio of the metal elements and F in the molding powder was evaluated, and assuming that all metal elements were oxides, the relative mass ratio of the metal oxides was calculated based on the relative mass ratio of the metal elements. The contents (mass%) of metal oxides and F were calculated so that the total content of all metal oxides and F was 100 mass%. The total CaO, SiO2, and alkali metal oxide contents refer to the contents (mass%) of each oxide of Ca, Si, and alkali metals calculated above.

[0036] <Viscosity and freezing point> The viscosity of the mold powder was measured using a vibrating reed viscometer. The temperature was lowered from 1400°C at a rate of 2°C / min, and the viscosity value at 1300°C was taken as the viscosity of the mold powder. The temperature at which the viscosity suddenly increased with the decrease in temperature was taken as the solidification point of the mold powder. The results are shown in Table 1.

[0037] <Main crystal species> The main crystal species of the mold powder was determined by melting the sample at 1400°C, then cooling it at 2°C / min until it reached 800°C, and then allowing it to cool naturally. The obtained sample was analyzed by X-ray diffraction, and the crystals derived from the first strong ray were identified as the main crystals. Furthermore, if the detected intensity was 1 / 3 or more of the main crystal, it was also identified as the second main crystal due to competitive crystal precipitation. These results are shown in Table 1. In Table 1, values ​​that deviate from the present invention are underlined.

[0038] [Table 1]

[0039] <Reaction with molten steel> A melting test in a crucible was conducted for Examples 1 to 5 and Comparative Examples 1 to 4, simulating the reaction between the molten mold powder layer and molten steel in the above-mentioned mold. A 10 kg steel sample with an Al concentration of 0.5% by mass was placed in a magnesia crucible and melted at 1500°C. 100 g of the mold powder sample was added to the molten steel and held for 20 minutes, after which it was air-cooled and solidified. The solidified mold powder sample was collected to obtain a simulated sample. The obtained sample was analyzed by XRD to check for the presence or absence of high-melting-point crystals with melting points of 1500°C or higher. The threshold for the presence of high-melting-point crystals was set at 1 / 10 of the peak intensity of the main crystals. The presence of high-melting-point crystals was determined when a peak higher than this was detected. The results are shown in Table 2. The melting points of the various crystals are Cuspidine: 1410°C, CaF2: 1420°C, and Gehlenite: 1590°C, with Gehlenite being the high-melting crystal. If there is no precipitation of high-melting crystals, it can be determined that fluctuations in the freezing point and viscosity have been suppressed.

[0040] [Table 2]

[0041] In all of Examples 1 to 5, the main crystal species was CaF2, and no precipitation of high-melting-point crystals was observed in the powder samples after the experiment. Furthermore, in Examples 1 to 5, the change in freezing point before and after the experiment was 30°C or less. This is because the powder of the present invention has an effect of suppressing the interfacial reaction with molten steel by increasing the T.CaO concentration and decreasing the SiO2 concentration. On the other hand, in Comparative Examples 1 to 3 and 5, precipitation of Gehlenite, a high-melting-point crystal, was observed after the experiment, and the change in freezing point was 40°C or more.

[0042] In Comparative Example 4, the concentration of alkali metal oxides including Na2O exceeded the range of the present invention, and therefore NaF was confirmed as a second main crystal in addition to CaF2 as the main crystal, and the solidification point was low at 870°C.

[0043] In Comparative Example 5, the concentrations of F and alkali metal oxides are outside the ranges of the present invention, but the main crystal is CaF. This powder exhibited a high freezing point of 1260°C. Simply increasing the basicity from the powder of Patent Document 3 raises the concern that the freezing point will be higher, resulting in insufficient lubrication within the mold.

[0044] In Comparative Example 3, the concentration of alkali metal oxides including Na2O was within the range of the present invention, as compared to Comparative Example 5. Only the F content was outside the range of the present invention, and the main crystal was cuspidine. Gehlenite was confirmed in the sample after reaction with molten steel. In order to prevent the precipitation of gehlenite, it is necessary to increase the F content so that the main crystal was CaF2. Even if the main crystal was CaF2, as in Comparative Examples 1 and 2, if the SiO2 content was higher than the range of the present invention or the T.CaO was lower than the range of the present invention, the concentration of Al2O3 was high in the sample after reaction with molten steel, and gehlenite was confirmed.

[0045] Therefore, by increasing the F concentration so that the main crystals are CaF2, and further increasing the T.CaO concentration and decreasing the SiO2 concentration, the reaction with molten steel can be kept low and the precipitation of high-melting-point crystals can be prevented.

[0046] <Continuous casting test> Continuous casting tests were conducted using the mold powders of Examples 2 and 3 and Comparative Examples 1 and 3. Using a continuous casting machine with a mold size of 1500 mm wide x 250 mm thick, 250 t of molten steel was cast at a casting speed of 1.2 m / min for steel with Al = 0.1 to 0.5%. In the test using the mold powders of Examples 2 and 3, no particular problems occurred during casting, and the cast slabs also showed no problems. On the other hand, in the casting using the mold powders of Comparative Examples 1 and 3, the waveform of the thermocouple attached to the copper plate of the mold became distorted, triggering the breakout detection. In other words, there were signs of a restrictive breakout occurring due to insufficient lubrication of the film. This is thought to be due to the mold powder of the comparative example being unable to suppress the precipitation of high-melting-point crystals, resulting in insufficient lubrication of the formed film. Furthermore, in Comparative Example 3, longitudinal cracks were observed on the surface of the slab.

[0047] As described above, the molding powder of the present invention is particularly effective when continuously casting steels that have characteristics that cause large variations in the composition of the molding powder, such as high-Al content steels (0.1% Al).The molding powder of the present invention can also be used with other steels without any problems.

Claims

1. A mold powder for continuous casting of steel, comprising: T. CaO concentration of 25% by mass or more; F concentration of more than 25% by mass; SiO ２ and a concentration of alkali metal oxides is 10% by mass or more and 25% by mass or less. The relative mass ratios of the metal elements and F in the molding powder are evaluated, and the metal elements are all considered to be oxides. The relative mass ratios of the metal oxides are calculated based on the relative mass ratios of the metal elements, and the contents (mass%) of the metal oxides and F are calculated so that the total content of all metal oxides and F is 100 mass%. ２ The content of alkali metal oxides means the content (mass %) of each of the oxides of Ca, Si, and alkali metals calculated above.

2. A continuous casting method, comprising the step of casting a steel having an Al content of 0.1 mass % or more using the mold powder for continuous casting according to claim 1.

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