Mold powder for continuous casting

Abstract

In the present invention, a mold powder for continuous casting that is effective in preventing operational troubles and quality deterioration caused by longitudinal cracks of slabs in continuous casting of hypo-peritectic steel is provided at low cost. The mold powder for continuous casting satisfies a basicity expressed by the mass ratio of the content of Ca in terms of CaO to the content of Si in terms of SiO2, i.e. (%CaO) / (%SiO2), of 1.30 to 1.60, an Al content in terms of Al2O3 of 0 mass% to 3.0 mass%, an Mg content in terms of MgO of 1.0 mass% to 3.0 mass%, an F content of 8.0 mass% to 12.0 mass%, an Na content in terms of Na2O of more than 5.0 mass% and 9.0 mass% or less, and an Mn content in terms of MnO of 2.0 mass% to 5.0 mass%.

Description

Mold powder for continuous casting

[0001] The present invention relates to a mold powder for continuous casting, and more particularly to a mold powder for continuous casting that can be suitably used for the continuous casting of hypoperitectic steel.

[0002] When continuously casting molten steel using a continuous casting machine, the molten steel is first poured from a ladle through a tundish into a mold, where an initial solidified shell forms within the mold. The molten steel then cools in the secondary cooling zone, allowing solidification to progress throughout the mold. During casting, a composite oxide called mold powder (hereinafter sometimes referred to as "powder") is constantly added to the mold to improve lubrication between the solidified shell and the mold, maintain the temperature of the molten steel, and prevent oxidation of the molten steel bath surface. The molten mold powder penetrates between the mold and the solidified shell, where it is cooled by the mold, forming a crystalline phase that controls the heat flux from the molten steel to the mold.

[0003] In hypoperitectic steel, solidification shrinkage and transformation shrinkage occur simultaneously, resulting in a larger volume change during solidification than other steels. This makes it more susceptible to uneven solidification during the initial solidification process in the mold. This uneven solidification not only causes defects such as dents in the slab, but also longitudinal cracks on the surface of the slab. Vertical cracks in the slab not only degrade product quality, but can also cause molten steel to leak into the machine, a condition known as breakout, making them a defect that must be prevented.

[0004] To prevent vertical cracks in the slab, it is extremely important to properly control the heat flux during the initial solidification stage using mold powder. Specifically, it is necessary to utilize the crystalline phase in the powder to suppress fluctuations in the heat flux.

[0005] However, the cooling rate is fast because the initially solidified part of the mold is cooled by water cooling, etc. Generally, complex oxides vitrify without crystallizing when the cooling rate is fast, but in the case of mold powder used in the continuous casting of hypoperitectic steel, a high proportion of the oxides must crystallize even under conditions of a fast cooling rate.

[0006] In light of the above, Patent Documents 1 to 3 propose mold powders for preventing longitudinal cracks from occurring on the surface of a cast piece during continuous casting of hypoperitectic steel.

[0007] JP 2011-147979 A JP 8-267204 A JP 2018-153813 A

[0008] For example, Patent Document 1 proposes a powder that promotes crystallization by controlling the solidification temperature to 1250°C or higher, which is significantly higher than usual, to reduce vertical cracks during continuous casting of hypoperitectic steel. However, raising the solidification temperature increases the risk of molten steel sticking to the mold due to insufficient melting. Furthermore, the powder becomes more susceptible to sintering, increasing the risk of the powder forming lumps known as bare particles and adhering to the mold wall, thereby deteriorating operability and quality.

[0009] In Patent Document 2, MgO and Li ２ By controlling the sum of the content of Li and O to 2 to 6 mass %, a powder has been proposed that can achieve a crystallization rate of 50% or more, which is necessary for slow cooling during initial solidification, even when cooled at a high cooling rate assumed to be in a mold. However, the price of Li has recently risen, and ２ When O is added to the mold powder, the powder unit price increases by several to ten times. Therefore, from the viewpoint of reducing the powder cost, ２ It is not preferable to add O in excess of the impurity amount. ２ Even when MgO was added instead of O, it was sometimes impossible to obtain the crystalline phase required for slow cooling. Furthermore, when the above powder was used, the heat flux in the mold became unstable, leading to operational problems.

[0010] In Patent Document 3, the above-mentioned Li ２ Due to the rising price of O, Li ２ B as a replacement for O ２ O ３ However, it is difficult to obtain a sufficient crystalline phase with this powder when the cooling rate is high. In addition, even when using this powder, operational problems may occur due to unstable heat flux in the mold.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide, at an inexpensive cost, a molding powder for continuous casting that is effective in preventing operational troubles and deterioration in quality caused by longitudinal cracks in the slab during the continuous casting of hypo-peritectic steel.

[0012] The gist of the present invention for solving the above problems is as follows.

[0013] Si SiO ２ The basicity (% CaO) / (% SiO ２ ): 1.30 to 1.60, Al of Al ２ O ３ Mg content in MgO equivalent: 0 to 3.0 mass%, Mg content in MgO equivalent: 1.0 to 3.0 mass%, F content: 8.0 to 12.0 mass%, Na content ２ A mold powder for continuous casting, comprising: a content, calculated as O, of more than 5.0 mass% and not more than 9.0 mass%; and a Mn content, calculated as MnO, of 2.0 to 5.0 mass%.

[0014] According to the present invention, it is possible to provide, at an inexpensive cost, a molding powder for continuous casting that is effective in preventing operational troubles and deterioration in quality caused by longitudinal cracks in the slab during continuous casting of hypoperitectic steel.

[0015] The component composition of a powder according to one embodiment of the present invention will be described below. Note that "%" as a unit of content refers to "% by mass" unless otherwise specified. In the following description, %C: total carbon (TC) content, %CaO: content of Ca converted to CaO, %SiO ２ : Si SiO ２ Content in terms of Al, % ２ O ３ : Al of Al ２ O ３ %MgO: Mg content in MgO equivalent; %F: F content; %Na ２ O: Na of Na ２ %MnO: Mn content converted into MnO.

[0016] A powder according to one embodiment of the present invention has the following composition: (% CaO) / (% SiO ２ ): 1.30-1.60 ・%Al ２ O ３ :0~3.0% ・%MgO: 1.0~3.0% ・%F: 8.0~12.0% ・%Na ２ O: more than 5.0% and not more than 9.0% %MnO: 2.0 to 5.0% By controlling the component composition of the powder, it is possible to control the physical properties (crystallization temperature and viscosity in the molten state) and crystallization behavior (crystallization rate and crystalline phase ratio upon rapid solidification). Furthermore, by controlling the physical properties and crystallization behavior, it is possible to prevent operational problems and quality deterioration caused by longitudinal cracks in the slab during continuous casting of hypoperitectic steel.

[0017] First, the calculation method for the content of each component will be explained. In this specification, the content of each component is a normalized value such that the sum of the content of the metal element converted to oxide and the content of elements excluding the metal element and O (oxygen) is 100%. % C, % CaO, % SiO ２ , % Al ２ O ３ , %MgO, %F, %Na ２ The contents of O, % MnO and unavoidable impurities are values ​​after applying the normalization described above.

[0018] For example, in the case of a mold powder whose component elements are C, Ca, Si, Al, Mg, F, Na, Mn, and O, the contents are determined as follows. First, the contents of each component other than O in 100 mass% of the mold powder (%C', %Ca', %Si', %Al', %Mg', %F', %Na', %Mn') are measured by the method described below. Next, the contents of Ca, Si, Al, Mg, Na, and Mn converted into oxides (%CaO', %SiO ２ ´,%Al ２ O ３ ', %MgO', %Na ２Next, a coefficient (100 (mass%) ÷ A (mass%)) is calculated from the total A (mass%) of the oxide-equivalent contents of Ca, Si, Al, Mg, Na, and Mn, the C content, and the F content, and the content of each component is normalized by multiplying the content of each component by the coefficient. Specifically, the content of each component is normalized by the following formula: A = %C' + %CaO' + %SiO ２ ´+%Al ２ O ３ '+%MgO'+%F'+%Na ２ O'+%MnO'...(1-1) %C=%C'÷A×100...(1-2) %CaO=%CaO'÷A×100...(1-3) %SiO ２ = % SiO ２ '÷A×100...(1-4)%Al ２ O ３ = % Al ２ O ３ '÷A×100...(1-5) %MgO=%MgO'÷A×100...(1-6) %F=%F'÷A×100...(1-7) %Na ２ O = % Na ２ O'÷A×100...(1-8) %MnO=%MnO'÷A×100...(1-9)

[0019] The same applies when the molding powder further contains unavoidable impurities. For example, when the unavoidable impurities are composed of Li, the Li content of 100% by mass of the molding powder is ２ The content in terms of O is % Li ２ In the case of O', the following formula is used instead of the formula (1-1): A = % C' + % CaO' + % SiO ２ ´+%Al ２ O ３ '+%MgO'+%F'+%Na ２ O′+%MnO′+%Li ２ O' (1-10) Furthermore, the content % Im of inevitable impurities is the Li of Li after normalization ２ O content (% Li ２ O) and is calculated by the following formula: % Im = % Li ２ O = % Li ２ O' ÷ A × 100 ... (1 - 11)

[0020] Next, the reasons for limiting each component will be explained.

[0021] [Basicity] The basicity is the mass ratio (% CaO) / (% SiO ２ ) If the basicity is low, the amount of cuspidine crystallized decreases, so in order to obtain a sufficient crystallization rate, the basicity is set to 1.30 or more, preferably 1.40 or more. On the other hand, the crystallization temperature increases with increasing basicity. If the basicity is too high, the crystallization temperature will be 1250°C or higher, so the basicity is set to 1.60 or less, preferably 1.50 or less.

[0022] [CaO, SiO ２ ] As long as the above-mentioned mass ratios are satisfied, % CaO and % SiO ２ is not limited. For example, %CaO can be 35% or more. %CaO can be 50% or less. %SiO ２ can be 25% or more. ２ can be 35% or less.

[0023] [Al ２ O ３ ] % Al ２ O ３ When the %Al content increases, crystallization is inhibited, the crystallization rate decreases, and the viscosity and crystallization temperature also increase. ２ O ３ The content of Al is set to 3.0% or less, preferably 2.5% or less. ２ O ３ The lower the content, the better, so it does not need to be included, and there is no lower limit, so it can be 0. However, Al is contained in ores that are used as raw materials for other components. ２ O ３ Therefore, if a raw material with high purity is used, the cost will increase significantly. ２ O ３ can be set to, for example, 1.0% or more.

[0024] [MgO] By increasing the %MgO, it is possible to inexpensively reduce the viscosity and the crystallization temperature. If the %MgO is less than 1.0%, the above effects cannot be obtained. Therefore, the %MgO is set to 1.0% or more, preferably 1.5% or more. On the other hand, if it exceeds 3.0%, the crystallization temperature increases. Therefore, the %MgO is set to 3.0% or less, preferably 2.5% or less.

[0025] [F] F is one of the components that make up Cuspidyne, and is added to adjust the viscosity and crystallization temperature. If the %F is less than 8.0%, the amount of Cuspidyne produced will be insufficient, so the %F is set to 8.0% or more, preferably 9.0% or more. Conversely, if the %F exceeds 12.0%, CaF ２ Therefore, %F is set to 12.0% or less, preferably 11.0% or less.

[0026] [Na ２ O] %Na ２ The increase in O can promote crystallization. If the O content is 5.0% or less, the effect is insufficient. ２ O is set to more than 5.0%, preferably 6.0% or more. ２ When O exceeds 9.0%, NaF and Na ６ Al ４ Si ４ O １７ These crystals are CaF ２ Although the adverse effect is relatively small because it crystallizes at a lower temperature than Na, it causes fluctuations in the heat flux in the mold. ２ O is set to 9.0% or less, preferably 8.0% or less.

[0027] [MnO] By increasing the % MnO, it is possible to inexpensively reduce the viscosity and the crystallization temperature. If the % MnO is less than 2.0%, the above effects cannot be obtained. Therefore, the % MnO is set to 2.0% or more, preferably 3.0% or more. On the other hand, if it exceeds 5.0%, the MnO content will be too high. ２ SiO ４The crystals such as MnO crystallize out, which causes the aforementioned fluctuation in the heat flux in the mold. Therefore, the %MnO is set to 5.0% or less, preferably 4.5% or less, and more preferably 4.0% or less.

[0028] [T.C] Although there are no limitations on %C, adding carbon as an aggregate as necessary can adjust the melting speed of the molding powder. Therefore, it is preferable that %C is 1.0% or more. There is also no upper limit on %C, but it is preferable that it is 8.0% or less.

[0029] The component composition of the powder has been described above. It is preferable that the molding powder contains the above-mentioned component elements, with the remainder being O and unavoidable impurities. For example, Li ２ Addition of O can provide the same effect as that of MgO and MnO. However, in order to reduce the manufacturing cost, ２ O(Li's Li ２ It is preferable to set the content (as converted into O) to 0% or to set the amount of impurities (for example, 0.5% or less).

[0030] Of the above-mentioned component compositions, Si, Ca, Al, Mg, and Mn can be measured by X-ray fluorescence analysis. F can be measured by absorptiometry. Na can be measured by atomic absorption spectrometry. C can be measured by a combustion method. Li can be measured by ICP emission spectrometry.

[0031] The powder can be produced by mixing raw materials according to the above-mentioned composition. The raw materials include, for example, aggregate, CaO, SiO ２ , Al ２ O ３ , MgO, fluorine compounds, Na ２ In other words, the powder may consist of, for example, aggregates, CaO, SiO ２ , Al ２ O ３ , MgO, fluorine compounds, Na ２ The fluorine compound may be a mixture of CaF ２and NaF. Because NaF is a deleterious substance, it is preferable that its content be 6% or less of the raw materials. The raw materials may further contain Ca, Mg, and Na as carbonates. The raw materials are not particularly limited, and for example, a part or all of the raw materials may be premelt raw materials. For example, the raw materials may contain oxides obtained by premelting carbonates (premelt). The particle size of the powder may be adjusted by techniques such as pulverization or classification.

[0032] The aggregate can be added as needed to adjust the melting rate of the molding powder. Carbon can be used as the aggregate, and carbon black, graphite, etc. can be used as the carbon. There is no lower limit to the amount of aggregate added, but it is preferably 1.0% or more. There is no upper limit to the amount of aggregate added, but it is preferably 8.0% or less.

[0033] The shape of the molding powder of the present invention is not particularly limited. For example, it may be a powder having a particle size that passes through a 100 mesh (150 μm opening) sieve. It may also be granules, for example, granules molded by extrusion granulation. The screen used in the extrusion granulation may be, for example, 0.5 to 2 mm.

[0034] Next, the physical properties and crystallization behavior of the molding powder will be explained. As a result of investigating the physical properties and crystallization behavior of various molding powders, the present inventors have found that it is preferable to satisfy the following conditions (1) to (4): (1) The crystallization ratio when solidified at a cooling rate of at least 300°C / min or more from a molten state of 1300°C is 80% or more. (2) In the crystalline phase crystallized under the conditions for measuring the crystallization ratio, cuspidine is 80% or more by weight, and CaF ２ (3) The crystallization temperature is 1150°C or higher and lower than 1250°C. (4) The viscosity at 1300°C is 0.3 to 0.8 poise.

[0035] [Crystallization Ratio] The mold powder preferably has a crystallization ratio of 80% or more when solidified from a molten state of 1300°C at a cooling rate of at least 300°C / min. Here, 1300°C corresponds to the average temperature of the molten mold powder in contact with molten steel, and 300°C / min corresponds to the minimum cooling rate when a water-cooled copper plate is used as the mold. The cooling rate in the mold varies depending on the flow rate of the cooling water used for water cooling and the thermal conductivity of the mold itself, but is typically 300°C / min or more. The cooling rate may be 1000°C / min, which corresponds to the cooling rate at the mold surface of a continuous casting machine. A crystallization ratio of 80% or more can effectively prevent quality deterioration and operational problems caused by uneven solidification during the continuous casting of hypoperitectic steel. The upper limit of the crystallization ratio is not limited and may be 100%. The crystallization ratio can be obtained by pouring 300 to 700 g of molten powder into a water-cooled mold and observing the cross section after solidification.

[0036] [Crystalline Phase] The molding powder is solidified from a molten state at 1300°C at a cooling rate of at least 300°C / min. In the crystallized phase, cuspidine is 80% or more by weight, and CaF ２ is preferably 5% or less.

[0037] If multiple phases competitively precipitate during the crystallization of the molding powder, the crystallization behavior becomes unstable due to a decrease in the crystal melting point, which causes fluctuations in the heat flux within the mold and results in non-uniform solidification. Therefore, it is desirable that the crystalline phase be close to a single phase, and specifically, it is preferable that the crystalline phase contain cuspidine in a weight ratio of 80% or more. On the other hand, the upper limit of the weight ratio is not limited and may be 100%.

[0038] In addition, there are various crystalline phases that can crystallize in general powders, among which CaF ２ crystallizes at high temperatures and forms a complex crystal with cuspidine, so CaF ２ is preferably 5% or less by weight. ２If the weight ratio exceeds 5%, the fluctuation in the heat flux becomes significant. On the other hand, the lower limit of the weight ratio is not limited and may be 0%.

[0039] The cooling rate may be 1000° C. / min. The weight ratio of the crystalline phase can be determined by analyzing the crystallized portion of the sample in which the crystallization ratio was observed by XRD and quantifying it by the Rietveld method or the like.

[0040] [Crystallization Temperature] The crystallization temperature of the molding powder is preferably 1150°C or higher and lower than 1250°C. A high crystallization temperature does not necessarily result in a high crystallization rate, but an excessively low crystallization temperature will not result in a sufficient crystallization rate. Therefore, the crystallization temperature is preferably 1150°C or higher. On the other hand, if it is 1250°C or higher, there is an increased risk of problems such as the aforementioned burning and bare spots occurring. Therefore, the crystallization temperature is preferably lower than 1250°C, and more preferably lower than 1220°C. The crystallization temperature can be measured using a differential thermal analyzer.

[0041] [Viscosity] The viscosity of the molding powder at 1300°C is preferably 0.3 to 0.8 Poise. If the viscosity is too high, mass transfer within the powder is suppressed, hindering crystallization when the cooling rate is fast. Therefore, the viscosity is preferably 0.8 Poise or less. On the other hand, if the viscosity is too low, the molten powder may become entrained in the molten steel. If this powder remains in the product, not only will it not be able to meet the properties originally required of the product steel, but surface defects known as slivers may occur. For these reasons, the viscosity is preferably 0.3 Poise or more. The viscosity can be measured using a rotational viscometer or the like. 1 Poise = 0.1 Pa·s.

[0042] A method for producing steel using the above powder will be described below. The above powder can be suitably used when continuously casting hypoperitectic steel.

[0043] By appropriately controlling the heat flux at the initial stage of solidification using the powder, it is possible to prevent uneven solidification during continuous casting of hypo-peritectic steel, which typically has a carbon concentration of 0.07 to 0.17%.

[0044] The present invention will now be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and appropriate modifications can be made within the scope of the present invention, and all such modifications are within the technical scope of the present invention.

[0045] First, a mold powder for continuous casting was prepared having the components shown in Table 1. In the preparation, cement raw material, silica sand, CaF ２ , NaF, Na ２ CO ３ , MgCO ３ , and MnO were blended and then melted to prepare a premelt raw material. Next, cement raw material, silica sand, CaF were added to the premelt raw material as needed. ２ , NaF, Na ２ CO ３ , MgCO ３ , MnO, and further aggregate were mixed to obtain the composition shown in Table 1. The cement raw materials were CaO, Al ２ O ３ and SiO ２ The main component was carbon black.

[0046] The contents of T.C, Ca, Si, Al, Mg, F, Na and Mn were measured by the above-mentioned method. In Table 1, the contents of metal elements are expressed in terms of oxides. The content of unavoidable impurities was calculated as the sum of the contents of Fe and Ti in terms of oxides and the contents of S and P. Specifically, the contents of Fe, Ti, S and P were determined by fluorescent X-ray analysis, and the contents of metal elements were calculated as oxides (Fe ２ O ３ , TiO ２ ) and calculated the total.

[0047]

[0048] Table 2 shows the viscosity, crystallization temperature, crystallization ratio, and crystalline phase ratio at 1300°C for each molding powder, which were measured by the above-mentioned methods. The crystallization ratio and crystalline phase ratio are values ​​measured under the condition of a cooling rate of 1000°C / min.

[0049]

[0050] Next, continuous casting of slabs was carried out on hypoperitectic medium carbon steel with C=0.11% using the mold powders shown in Tables 1 and 2. The conditions for the casting were a cross-sectional size of the mold of 260 mm × 1500 mm and a slab withdrawal speed of 1.5 m / min.

[0051] The trimming yield was investigated to evaluate the quality of the slab. The trimming yield is a value calculated by the following formula (2): Trimming yield (%) = (weight of slab after trimming) / (weight of slab before trimming) × 100 (2) When vertical cracks occur on the surface of a slab, trimming work is carried out to remove them by scraping the surface of the slab with a scarf or grinder, which reduces the yield. In other words, the higher the trimming yield, the higher the quality of the slab.

[0052] Furthermore, the thermocouple embedded in the mold was monitored for any abnormalities in its behavior. An abnormality in the thermocouple behavior here means that the temperature fluctuation per unit time exceeded a predetermined value. If an abnormality was detected, the withdrawal speed was reduced, and after confirming that the thermocouple behavior had stabilized, the withdrawal speed was restored to the above speed.

[0053] The results are shown in Table 3. All of the inventive products were able to achieve high-quality cast slabs and stable operation. On the other hand, all of the comparative products were either inferior in quality or had more operational problems. Note that comparative product 1 suffered a restrictive breakout about 10 minutes after the start of casting, making it impossible to evaluate its quality.

[0054]

Claims

1. SiO of Si ２ The basicity (% CaO) / (% SiO ２ ): 1.30 to 1.60, Al of Al ２ O ３ Mg content in MgO equivalent: 0 to 3.0 mass%, Mg content in MgO equivalent: 1.0 to 3.0 mass%, F content: 8.0 to 12.0 mass%, Na content ２ A mold powder for continuous casting, comprising: a content, calculated as O, of more than 5.0 mass% and not more than 9.0 mass%; and a Mn content, calculated as MnO, of 2.0 to 5.0 mass%.

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