Mold powder and steel continuous casting method

Abstract

To provide mold powders that can achieve both of slow cooling of a solidified shell and promotion of lubrication, and a steel continuous casting method.SOLUTION: A mold powder for continuously casting steel comprises Fe content of 30 mass% or more, Na2O content of 1 mass% or more and 10 mass% or less, SiO2 content of 1 mass% or more and 15 mass% or less, and T.CaO concentration of 40 mass% or more, and has a coagulation point of 1100 to 1300°C. T.CaO, Na2O, and SiO2 contents mean a mass concentration when considering all of Ca, Na and Si in the mold powder to be oxides. Further, the mold powder contains Al2O3 of 2 to 5 mass%. The present invention further discloses a steel continuous casting method that uses the mold powder.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a mold powder for continuous casting and a method for continuous casting of steel. [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 composed of CaO, SiO 2 , Al 2 O 3 A mixture of powders of oxides such as fluorine compounds, carbon, etc. is used. Mold powder added to the surface of 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 (hereinafter abbreviated as film). The main functions of this mold powder are (1) to prevent reoxidation of the molten steel and to keep it warm, (2) to capture inclusions that rise from the molten steel, (3) to lubricate the mold and solidified shell, and (4) to control the heat removal from the solidified shell. The present invention relates to (3) and (4).

[0003] The film formed between the mold and the solidified shell is composed 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 the solidified shell.

[0004] In the continuous casting of steel, defects may occur on the surface and inside of the slab, which adversely affect the quality of the steel. Therefore, it is necessary to reduce the occurrence of defects on the surface and inside of the slab. In particular, medium carbon steel with a C concentration of 0.08 to 0.25 mass% is prone to have a cast defect called vertical cracks, due to the uneven growth of the solidified shell. In order to prevent vertical cracks, it is effective to cool the solidified shell slowly in the very early stage when the solidified shell is formed in the upper part of the mold, and to make the solidified shell grow uniformly. As a method for this, a casting method using mold powder has been developed, as shown in the following patent document.

[0005] In Patent Document 1, in order to promote the crystallization of crystals, the basicity and F content of the molding powder are adjusted and the solidification point is designed to be high, at 1250° C. or higher. The molding powder with a high solidification point promotes the crystallization of the crystalline phase of the film that flows between the mold and the solidified shell.

[0006] In Patent Document 2, SiO 2 It contains CaO as the main component, and K 2 O, Na 2 O, Li 2 The molding powder is characterized in that it contains a predetermined amount of K, O, 3.0 to 15.0% of F, has a crystallization temperature (same as the solidification point) of 1080 to 1280°C, and the primary crystal species is cuspidine. 2 O greatly reduces the surface tension without significantly changing the crystallization temperature, which allows slow cooling to be maintained at a high level, and the greatly reduced surface tension makes it easier for the molten film to flow between the solidified shell and the mold.

[0007] In Patent Document 3, CaO, SiO 2 and fluorine compounds as the basic components, CaF 2 The content is 60% by weight or less (29.2% or less in terms of F), and Na 2 A mold powder for continuous casting containing O and C is disclosed. CaF 2 By adjusting the content, the powder's freezing point and viscosity are lowered, and crystallization mainly consisting of cuspidine is promoted. 2 It is said that when the content exceeds 60%, the precipitation of cuspidine decreases. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2011-147979 A [Patent Document 2] Patent Publication No. 2021-074782 [Patent Document 3] Japanese Patent Application Publication No. 11-320058 [Non-patent literature]

[0009] [Non-Patent Document 1] PP Fedorova, et. al., Russian Journal of inorganic chemistry 61 (2016) 1472. Summary of the Invention [Problem to be solved by the invention]

[0010] In the case of mold powders in which crystallization of the crystalline phase is promoted as described in Patent Documents 1 to 3, the film in which the crystalline phase is crystallized has a higher heat transfer resistance than the liquid phase film, so it works to reduce the heat extraction of the solidified shell. On the other hand, it is difficult to achieve both slow cooling and lubrication by mold powder. That is, when the crystallization of the mold powder is promoted by a means such as increasing the solidification point, it is effective in slow cooling in the initial solidification part in the upper part of the mold, but the liquid phase part of the powder film in the lower part of the mold is too small, resulting in insufficient lubrication in the mold. Conversely, if the solidification point is designed to be low, lubrication in the mold works, but there is a problem that the slow cooling effect is weak.

[0011] The present invention aims to provide a mold powder and a method for continuous casting of steel that can achieve both slow cooling of the solidified shell by crystallization of the crystalline phase of the mold powder in the initial solidification stage in the mold, and promote lubrication between the mold and the solidified shell by maintaining the liquid phase of the powder film in the lower part of the mold. That is, the object of the present invention is to provide a mold powder and a method for continuous casting of steel that can achieve both slow cooling of the solidified shell and promotion of lubrication. [Means for solving the problem]

[0012] That is, the gist of the present invention is as follows. [1] A mold powder for continuous casting of steel, containing 30% or more by mass of F and 10% or less of Na. 2O content is 1% by mass or more and 10% by mass or less, SiO 2 A molding powder having a CaO content of 1% by mass or more and 15% by mass or less, a T.CaO concentration of 40% by mass or more, and a solidification point of 1100 to 1300°C. The relative mass ratio of the metal elements and F in the mold powder is evaluated, and the relative mass ratio of the metal oxides is calculated based on the relative mass ratio of the metal elements, assuming that all the metal elements are oxides. 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%. T. CaO, Na 2 O, SiO 2 The content means the content (mass %) of each of the oxides of Ca, Na, and Si calculated above. [2] Furthermore, Al 2 O 3 The molding powder according to [1], characterized in that it contains 2 to 5 mass % of [3] A method for continuous casting of steel, comprising using the mold powder according to [1] or [2]. Effect of the Invention

[0013] By applying the mold powder for continuous casting and the method for continuous casting of steel according to the present invention, it is possible to reduce longitudinal cracks and breakouts on the surface of continuously cast slabs. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] As described above, the present invention aims to achieve both slow cooling of the solidified shell and lubrication inside the mold by using mold powder in continuous casting of steel. The method involves rapidly crystallizing the crystalline phase of the film in the initial solidification portion, slow cooling of the solidified shell at the meniscus, suppressing uneven growth of the solidified shell that is the starting point of vertical cracks, while ensuring the liquid phase of the film toward the lower part of the mold, thereby obtaining lubrication inside the mold and inflow of the powder.

[0015] CaF 2 From the binary phase diagram of CaF and NaF, 2It is presumed that, when the temperature is lowered from the molten state, the solid phase is crystallized, and then when the temperature is further lowered, the liquid phase is formed again. From this fact, it is presumed that the main crystal of the molding powder is CaF 2 When the powder film is cooled, the crystalline phase (CaF 2 ) would crystallize stably, and as the temperature of the powder film further decreased at the bottom of the mold, the liquid phase would reappear.

[0016] So, CaF 2 A mold powder was prepared that contains 40% by mass or more of T.CaO and 30% by mass or more of F as a composition that is likely to crystallize, and has a solidification point of 1100 to 1300°C. When the temperature of this mold powder was measured while decreasing the temperature at 5°C / min, heat generation was confirmed twice at 1000°C or higher. It is considered that the mold powder melts again after crystallization occurs, and then crystallization occurs. Next, when this mold powder was actually used in continuous casting of medium carbon steel, which is prone to vertical cracks, it was confirmed that the surface vertical cracks of the continuously cast slab were reduced, and the load for pulling the slab out of the mold (hereinafter sometimes referred to as "load in the mold") was reduced, and breakouts were reduced, leading to the completion of the present invention. The details will be described below.

[0017] <Mold powder composition> In the present invention, the component composition of the molding powder is determined as follows. The relative mass ratio of the metal elements and F in the molding powder is evaluated, and the relative mass ratio of the metal oxides is calculated based on the relative mass ratio of the metal elements, assuming that all the metal elements are oxides, and the contents (mass%) of the metal oxides and F are calculated so that the total content of all the metal oxides and F is 100 mass%. For example, T.CaO, Na 2 O, SiO 2 , Al 2 O 3 The content means the content (mass%) of each of the oxides of Ca, Na, Si and Al calculated above.

[0018] Methods for evaluating the components of mold powder include ICP-MS, SEM-EDS, X-ray fluorescence analysis, and wet chemical analysis. ICP-MS is particularly suitable for use in estimating the F concentration or the oxide concentrations of Ca, Na, Si, Al, etc. in the mold powder.

[0019] [T.CaO] The Ca component of the mold powder is CaO or CaF 2 It is often added as CaO. T.CaO means the mass concentration when all Ca in the mold powder is regarded as CaO. T.CaO is calculated by evaluating the relative mass ratio of the total Ca content in the mold powder, multiplying this value by 56 / 40 to calculate the relative mass ratio of Ca metal oxide, and then calculating as described above. The T.CaO calculated as described above should be 40 mass% or more. If it is less than 40 mass%, it is considered that CaF 2 This is because CaO does not crystallize stably and the desired crystals cannot be obtained. There is no particular upper limit for the T.CaO content, but it is preferably 65 mass% or less. If it is more than 65 mass%, SiO 2 ,Na 2 O,Al 2 O 3 This is because the amount of added is reduced, and depending on the composition of the ingredients, the solidification point may exceed 1300°C.

[0020] [SiO 2 ] SiO calculated as above 2 is 1 mass% or more and 15 mass% or less. SiO 2 At concentrations higher than 15% by mass, the desired crystalline phase, CaF 2 The primary crystals are out of the range of CaF 2 On the other hand, SiO 2 If the concentration is less than 1% by mass, the viscosity becomes too low.

[0021] [F] The content of F calculated as above is 30 mass% or more. This is because the F component has the effect of adjusting the viscosity and solidification point of the molding powder and also has the effect of reducing the CaF 2When the F content is less than 30 mass%, the primary crystal is CaF 2 The primary crystals are CaF 2 If F does not crystallize, the liquid phase ratio of the powder film cannot be ensured in the lower part of the mold. When a thermocouple is embedded in the copper plate of the mold to evaluate the temperature performance of the copper plate of the mold during continuous casting, the thermocouple waveform during casting is defective, especially in the lower part of the mold, making it impossible to perform stable casting. The higher the F content, the lower the viscosity and solidification point of the mold powder can be.

[0022] [Alkali metal oxides] As an alkali metal oxide added to the mold powder, K 2 O, Na 2 O, Li 2 O. Alkali metal oxides are added for the purpose of adjusting and lowering the solidification point and viscosity of the mold powder. All of these components have a high affinity with the F in the mold powder, and exist as KF, NaF, and LiF in the molten state. As described in Non-Patent Document 1, at a certain concentration, NaF crystallizes out and dissolves as the temperature decreases. 2 Therefore, Na 2 O is an essential component for achieving the effects of the present invention, and Na calculated as above 2 The O concentration is 1 mass% or more, preferably 3 mass% or more. 2 When the O concentration exceeds 10 mass%, CaF 2 The remelting effect of the crystal phase of Na is not manifested. 2 The upper limit of the O concentration is 10 mass%.

[0023] Other, K 2 O,Li 2 The O component is Na to optimize the freezing point and viscosity. 2 It may be mixed separately from O. 2 O,Li 2 The O content is evaluated in the same manner as the other components above.

[0024] [Al2 O 3 ] Al 2 O 3 The addition of CaF to the mold powder 2 It is desirable to have as little content as possible, since it reduces the amount of crystallization of SiO and may cause other crystals to crystallize, resulting in deterioration of melting characteristics and poor heat removal. 2 Contains high CaF 2 The mold powder with this concentration undergoes liquid-phase separation in the molten state. 2 O 3 SiO 2 This reduces the activity of Al, which can prevent the occurrence of two-liquid phase separation. 2 O 3 The content of Al in the mold powder is preferably 2 to 5 mass %. 2 O 3 Regarding the content, one of the metal elements in the aforementioned composition calculation method is Al, and Al metal oxide is Al. 2 O 3 It is calculated as:

[0025] [C] Furthermore, in addition to the above components, it is preferable to add C to the molding powder of the present invention, and the content of the other elements is preferably 1 to 10 mass%. C has the effect of adjusting the melting rate of the molding powder, and the higher the C content, the slower the melting rate becomes. If the C content is less than 1 mass%, the melting rate becomes too high, and if it exceeds 10 mass%, the melting rate becomes too low, and the powder flowability into the mold and solidified shell deteriorates.

[0026] <Characteristics of mold powder> [Freezing point] The solidification point of the mold powder should be more than 1100℃ and less than 1300℃. If it exceeds 1300℃, the film crystal phase will be formed excessively, leading to insufficient lubrication in the mold. Mold powder with a solidification temperature lowered to 1100℃ or less is CaF 2In order to adjust the freezing point of the molding powder to more than 1100°C and not more than 1300°C, the F content and the alkali metal oxide content of the molding powder are adjusted within the range of the present invention using the above-mentioned contents as indicators.

[0027] [viscosity] If a mold powder with high viscosity is used, the lubrication between the solidified shell and the mold does not work sufficiently. Therefore, the viscosity of the mold powder is preferably 1 poise or less at 1300 ° C., which is effective as a lubricant in the mold. In addition, when a mold powder with a solidification point of about 1150 ° C. or more is used, the thickness of the crystal phase of the film increases and the lubrication in the mold decreases, so it is preferable that the viscosity of the film liquid phase is small, and it is preferably 0.2 poise or less, about 0.1 poise at 1300 ° C. The viscosity of the mold powder can be adjusted by adjusting the F content and alkali metal oxide content of the mold powder within the range of the present invention using the above-mentioned contents as indicators.

[0028] 《Raw materials》 The raw materials used in the mold powder of the present invention may be any commonly used raw materials. The raw materials for CaO include quicklime, limestone, cement, etc., and SiO 2 The raw materials are silica sand, diatomaceous earth, etc. 2 The raw materials for O include lithium carbonate, 2 Examples of O raw materials include sodium carbonate and soda ash, examples of F raw materials include fluorite, sodium fluoride, and cryolite, and examples of C raw materials include carbon black and coke powder.

[0029] The shape of the raw material for the mold powder is not limited. For example, any shape, such as powder or granules, can be used. These raw materials include Fe 2 O 3 It contains oxides such as magnesium oxide and magnesium oxide. Even if these impurities are present, they are in very small amounts and do not pose any particular problems.

[0030] <<Method for continuous casting of steel>> In the continuous casting method of steel of the present invention using the above-mentioned mold powder of the present invention, it is possible to reduce surface longitudinal cracks and breakouts in the continuously cast slab, particularly in the continuous casting of medium carbon steel, which is prone to longitudinal cracks. EXAMPLES

[0031] The hypoperitectic steel with the composition in Table 1 was used to cast a slab with a width of 1250 mm and a thickness of 250 mm using a vertical bending type continuous casting machine under the conditions of a tundish molten steel superheat of 25°C and a casting speed of 1.6 m / min. The composition of the mold powder is shown in Table 2.

[0032] [Table 1]

[0033] [Table 2]

[0034] <Viscosity and freezing point> The viscosity of the mold powder was measured using a vibrating element viscometer. The viscosity value at 1300°C was taken as the viscosity (poise) of the mold powder. The temperature was lowered from 1400°C at 2°C / min, and the temperature at which the viscosity suddenly increased with the decrease in temperature was taken as the solidification point (°C) of the mold powder.

[0035] <Main crystal> The main crystal of the mold powder was identified as the crystal with the highest peak in the X-ray diffraction analysis of the sample obtained by cooling and solidifying the molten mold powder at 2℃ / min.

[0036] <Remelting of mold powder> The temperature of 200g of mold powder was measured while cooling at 5℃ / min. If heat generation was confirmed twice at 1000℃ or higher, it was determined that the mold powder had crystallized and then melted again, and then crystallized. Remelting of the mold powder was observed at a temperature 50 to 100℃ lower than the solidification point. If remelting of the mold powder occurred in this way, it was marked as "Good", and if remelting did not occur during cooling, it was marked as "Poor", and these were recorded in the "Remelting" column in Table 2.

[0037] <Load inside the mold (determination of lubrication inside the mold)> Operational stability was evaluated by the load in the mold (kgf). The load in the mold was calculated from the torque of the oscillation cylinder during casting. Specifically, when the torque during casting matched the torque value when a weight was placed on the mold during cold operation, it was considered that a load in the mold equivalent to the weight of the weight had occurred. The lower this value, the better the lubrication in the mold and the lower the risk of the slab being confined to the mold. If this value exceeds 400kgf, the lubrication between the mold and the solidified shell is insufficient, making the slab more likely to stick to the mold.

[0038] <Vertical crack evaluation> The length of the longitudinal cracks that occurred per 1 m of the slab was defined as the longitudinal crack occurrence rate (mm / m). When the longitudinal crack occurrence rate was 0.1 mm / m or more, it was evaluated as "x", when it was more than 0 mm / m and less than 0.1 mm, it was evaluated as "○", and when no longitudinal cracks occurred at all, it was evaluated as "◎".

[0039] The results are shown in Table 2. Values ​​outside the range of the present invention are underlined.

[0040] Focusing on the load in the mold, in all of Comparative Examples 1-7, the main crystals were cuspidine, so the film did not remelt. As a result, the load in the mold was large during continuous casting. In Comparative Examples 1 and 3-7, the load in the mold was large, and the lubrication in the mold was insufficient. Among Comparative Examples 1 to 7, Comparative Example 2 was the best in terms of the load in the mold. This is presumably because the solidification point was low. Among the comparative examples, Comparative Examples 3 and 4, which had high solidification points, had particularly high loads in the mold, and furthermore, constraint marks occurred on the slab, resulting in the most insufficient lubrication in the mold. This is presumably due to the combined effect of the high solidification point and the lack of remelting of crystals. If constraint occurs between the mold surface and the solidified shell in the mold, the occurrence of constraint is detected by a breakout prediction device, and the constraint is repaired by rapidly slowing down the casting speed. In such cases, constraint marks are observed when observing the surface of the slab after casting.

[0041] In Comparative Examples 1 and 3 to 7, the solidification point of the mold powder was within the range of the present invention, so the solidified shell was cooled slowly at the meniscus, and the vertical crack incidence was evaluated as good, with ○ or ◎. On the other hand, in Comparative Example 2, the solidification point of the mold powder was lower than the range of the present invention, so the solidified shell was not cooled slowly at the meniscus, and the vertical crack incidence was also evaluated as poor, with ×.

[0042] On the other hand, in Example 1-11, the solidification point was in the range of 1100 to 1300°C, and the solidified shell of the meniscus portion could be cooled slowly, and the vertical crack occurrence rate was good in all cases. 2 Therefore, remelting of the film occurred, and the load within the mold was lower than in Comparative Examples 1 and 3 to 7, and was approximately the same as or lower than Comparative Example 2 having a low solidification point.

[0043] The occurrence of breakouts in continuous casting was evaluated based on the occurrence of restraint marks and the occurrence of an alarm by the breakout prediction device. As a result, in the examples of the present invention, no restraint marks were generated and no alarm was issued by the breakout prevention device, whereas in the comparative examples 1 and 3-7, an alarm was issued by the breakout prevention device and the occurrence of restraint marks was confirmed.

[0044] By using the molding powder of the present invention, it is possible to reduce longitudinal cracks and improve lubrication inside the mold at the same time.

Claims

1. A mold powder for continuous casting of steel, the powder having an F content of 30 mass% or more and an Na content of 100 mass% or more. ２ O content is 1% by mass or more and 10% by mass or less, SiO ２ A molding powder characterized in that the content is 1 mass% or more and 15 mass% or less, the T. CaO concentration is 40 mass% or more, and the solidification point is 1100 to 1300 ° C. The relative mass ratio of the metal elements and F in the mold powder is evaluated, and the metal elements are all considered to be oxides. The relative mass ratio of the metal oxides is calculated based on the relative mass ratio 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%. T. CaO, Na ２ O, SiO ２ The content means the content (mass%) of each of the oxides of Ca, Na, and Si calculated as above.

2. Furthermore, Al ２ O ３ The molding powder according to claim 1, characterized in that it contains 2 to 5 mass % of

3. A method for continuous casting of steel, comprising using the molding powder according to claim 1 or 2.

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