Mold powder

Abstract

To provide a mold powder that can achieve both a surface tension sufficient to prevent slag inclusion even during high-speed continuous casting, and slow cooling of the cast slab due to crystallization within the slag film. [Solution] The mold powder mainly consists of CaO and SiO2, with a mass ratio of CaO to SiO2 (CaO / SiO2) of 1.5 to 2.3, and contains 2.0 to 11.0 mass% SrO, 0.0 to 3.0 mass% MgO, 2.0 to 9.9 mass% Na2O, 0.0 to 4.0 mass% K2O, 0.0 to 4.0 mass% Li2O, and 7.0 to 15.0 mass% F.

Description

Technical Field

[0001] The present disclosure relates to a mold powder suitable for continuous casting of steel.

Background Art

[0002] In the continuous casting of steel, molten steel stored in a tundish is poured into a mold through a submerged nozzle, cooled and solidified, and a solidified shell (solidification shell) is continuously drawn downward from the mold by rolls, whereby slabs, blooms, billets and other various shaped castings are continuously produced. A powdery or granular mold powder (sometimes referred to as a mold flux) is scattered on the surface of the molten steel in the mold. The mold powder is melted by the heat of the molten steel to form a molten slag layer (hereinafter, the melted mold powder is referred to as powder slag). The powder slag flows into the space between the mold and the solidification shell, is cooled by the mold and solidifies into a film shape (referred to as a slag film), and is discharged from the lower end of the mold. The main roles of the mold powder from scattering to discharge are shown below. (1) Heat preservation of the molten steel surface (2) Prevention of oxidation of the molten steel surface (3) Absorption of non-metallic inclusions floating from the molten steel and purification of the molten steel (4) Ensuring lubrication between the solidification shell and the mold (5) Controlling the heat flux from the solidification shell to the mold

[0003] In the continuous casting of medium carbon steel with a carbon content of 0.08 to 0.25% by mass, slow cooling of the casting is necessary to suppress surface cracking (casting cracking) of the casting. For slow cooling of the casting, it is important to quickly crystallize crystals in the slag film, and for this purpose, it is effective to increase the basicity (mass ratio (CaO / SiO2)) of the mold powder.

[0004] On the other hand, rapid crystallization within the slag film reduces the liquid phase in the powder slag, leading to decreased lubricity. To compensate for the loss of lubricity due to crystallization within the slag film, it is effective to reduce the viscosity of the powder slag and increase its flow rate between the mold and the solidified shell.

[0005] Patent Document 1 discloses a mold powder for continuous casting characterized by containing CaO, SrO, and SiO2 as the main components, having a basicity A expressed as a molar ratio of (CaO + SrO) / SiO2 of 1.39 to 4.0, a SrO concentration of 15 to 40% by mass, containing 1 to 8% by mass of B2O3, and having a viscosity of 0.05 to 0.5 Pa·s at 1300°C. This mold powder for continuous casting does not cause powder entrapment (slag entrapment) even in high-speed continuous casting and can maintain the lubricity of the powder film (slag film).

[0006] Non-patent document 1 discloses a highly basic mold powder with a basicity (CaO / SiO2) of 1.5 or higher. This highly basic mold powder increases the crystal growth rate in the slag film, achieving slow cooling near the meniscus, and by adjusting the crystallization temperature, it can ensure an appropriate solidification shell thickness and suppress cast slab cracking. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2017-119286 [Non-patent literature]

[0008] [Non-Patent Document 1] Junya Ito et al., Suppression of Cast Slab Cracking by High-Basicity Mold Powder that Achieves Both Slow Cooling of Initial Solidification Shell and Proper Heat Removal within the Mold, Shinagawa Technical Report, No. 61, 2018, pp. 35-50. [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The mold powder for continuous casting disclosed in Patent Document 1 maintains a surface tension sufficient to prevent slag inclusion by SrO even when high-speed continuous casting is performed. However, crystal formation in the slag film is suppressed to improve lubricity, which presents challenges in slowing down the cooling of the cast slab.

[0010] The highly basicized mold powder disclosed in Non-Patent Literature 1 has a low viscosity of 0.06 to 0.09 Pa·s, which presents challenges in slag inclusion, particularly in high-speed continuous casting (e.g., 1.6 m / min) where molten metal level fluctuations are large.

[0011] This disclosure is made in view of the above circumstances and aims to provide a mold powder that can achieve both a surface tension sufficient to prevent slag inclusion even when high-speed continuous casting is performed, and slow cooling of the cast slab due to crystallization in the slag film. [Means for solving the problem]

[0012] One aspect of this disclosure is, It mainly contains CaO and SiO2, with a mass ratio of CaO to SiO2 (CaO / SiO2) of 1.5 to 2.3. SrO 2.0~11.0% by mass, MgO 0.0~3.0% by mass, Na2O 2.0~9.9% by mass, K2O 0.0~4.0% by mass, Li2O 0.0~4.0 mass%, and, This invention relates to a mold powder characterized by containing F 7.0 to 15.0% by mass.

[0013] When mold powder according to one embodiment of this disclosure is used in continuous casting of steel, it is possible to achieve both a surface tension sufficient to prevent slag inclusion even during high-speed continuous casting, and slow cooling of the cast slab due to crystallization in the slag film, thereby suppressing slab cracking. Since the main crystal species crystallized in the slag film can be cuspidine (3CaO·2SiO2·CaF2), it is believed that the heat flux from the solidified shell to the mold is suppressed.

[0014] In one aspect of this disclosure, Preferably, the surface tension at 1300°C is 300 mN / m or higher, and the crystallization temperature is 1100 to 1200°C.

[0015] By achieving both suppression of slag inclusion and slow cooling through caspidine crystallization, high-quality medium-carbon steel can be obtained even when performing high-speed continuous casting. [Modes for carrying out the invention]

[0016] Preferred embodiments of this disclosure will be described in detail below. It should be noted that these embodiments are not intended to unduly limit the scope of the claims of this disclosure, and not all configurations described in these embodiments are necessarily essential as solutions of this disclosure.

[0017] The mold powder of this embodiment mainly contains CaO and SiO2, with a mass ratio of CaO to SiO2 (CaO / SiO2) of 1.5 to 2.3, and also contains 2.0 to 11.0 mass% SrO, 0.0 to 3.0 mass% MgO, 2.0 to 9.9 mass% Na2O, 0.0 to 4.0 mass% K2O, 0.0 to 4.0 mass% Li2O, and 7.0 to 15.0 mass% F.

[0018] In this specification, the content (i.e., chemical composition) of each component of the mold powder is the one converted to oxides in the molten state at 1300°C, and F is the sum obtained by separating F from fluorides. When the raw material contains carbonate, carbon is decomposed into carbon dioxide and released into the atmosphere during melting by the heat of the molten steel, so it is not considered in the chemical composition. For example, when the mold powder contains CaF2, the Ca in CaF2 is converted to CaO, and is added to the CaO converted from CaO and other Ca-containing compounds contained in the mold powder, and F is separated and added to the F of other fluorides.

[0019] <Mass ratio (CaO / SiO2)> The mold powder of this embodiment contains CaO and SiO2 as main components, and the mass ratio (CaO / SiO2) (basicity) of CaO and SiO2 is 1.5 to 2. It is preferably 1.6 to 2.1. As a result, crystal precipitation in the slag film becomes active, the viscosity decreases, and the lubricity can be enhanced. Further, by including F in the mold powder, the main crystal species that crystallize in the slag film can be made to be cuspidine, which is effective for slow cooling.

[0020] <sro> The mold powder of this embodiment contains 2.0 to 11.0% by mass of SrO, preferably 2.0 to 8.0% by mass of SrO. SrO increases the surface tension of the powder slag. On the other hand, SrO has a smaller effect on viscosity and crystallization temperature than Na2O, Li2O, and MgO. Therefore, it is possible to ensure the surface tension while maintaining each property of the powder slag and suppress slag entrainment. If the SrO content is less than 2.0% by mass, the surface tension of the powder slag decreases and slag entrainment is likely to occur. If it exceeds 11.0% by mass, the crystallization of cuspidine is hindered, crystal seeds other than cuspidine and CaF2 crystallize in the slag film, and slow cooling is hindered.

[0021] <mgo> The mold powder of this embodiment contains 0.0 to 3.0% by mass of MgO, preferably 0.0 to 1.0% by mass. MgO enhances the surface tension and lubricity of the powder slag, but it hinders the crystallization of casspidyne. Therefore, a smaller amount of MgO is preferable for mold powders that aim for slow cooling through the crystallization of casspidyne.

[0022] <Na2O、K2O> The mold powder of this embodiment contains 2.0 to 9.9% by mass of Na2O. If the Na2O content is less than 2.0% by mass, excessive crystals may crystallize in the slag film, reducing lubricity and potentially causing seizing. If it exceeds 9.9% by mass, the surface tension, crystallization temperature, and viscosity of the powder slag will decrease.

[0023] The mold powder of this embodiment may contain up to 4.0% by mass of K2O. Adding up to 4.0% by mass of K2O results in, for example, Na6Al4Si4O 17 This can suppress the formation of associated minerals such as [mention specific minerals].

[0024] <li2o> The mold powder of this embodiment contains 0.0 to 4.0% by mass of Li2O. If the Li2O content exceeds 4.0% by mass, the crystallization temperature, viscosity, and surface tension decrease, and crystal species other than caspedyne may crystallize in the slag film, which is undesirable.

[0025] <f> The mold powder of this embodiment contains 7.0 to 15.0% by mass of F, preferably 7.5 to 13.0% by mass of F, and more preferably 10.0 to 13.0% by mass of F. When the content of F is less than 7.0% by mass, the crystallization of cuspidine is insufficient and crystal seeds other than cuspidine crystallize, and the crystallization temperature and uniform heat extraction property decrease, preventing slow cooling. When the content of F exceeds 15.0% by mass, the surface tension of the powder slag decreases, and the quality of the slab may deteriorate due to slag entrainment.

[0026] <Other components> The mold powder of this embodiment may contain Fe2O3, BaO, P, S, etc. as inevitable impurities. Since BaO hinders crystallization and reduces slow cooling, less than 0.1% by mass is preferred.

[0027] <Raw materials> Known raw materials can be used for the mold powder of this embodiment. As raw materials for CaO, for example, Portland cement, limestone, quicklime, synthetic calcium silicate, wollastonite, phosphorus slag, blast furnace slag, etc. can be used. As raw materials for SiO2, for example, silica sand, silica powder, diatomaceous earth, etc. can be used. As raw materials for Na2O, K2O, Li2O, SrO and F, for example, sodium carbonate, potassium carbonate, lithium carbonate, strontium carbonate, calcium fluoride (fluorite), lithium fluoride, magnesium fluoride, strontium fluoride, etc. can be used.

[0028] <F source> There is no particular limitation on the F source of the mold powder of this embodiment, but CaF2 is preferred. When CaF2 is used as the F source, cuspidine becomes the dominant crystal seed that crystallizes in the slag film, which is effective for slow cooling.

[0029] <Viscosity> The viscosity of the mold powder in this embodiment at 1300°C is preferably 0.03 to 0.09 Pa·s. When the viscosity at 1300°C is 0.09 Pa·s or less, the lubricity between the mold and the solidified shell is improved, and restrictive breakout can be suppressed during casting.

[0030] <Surface tension> The higher the surface tension of the powder slag, the more effective it is in suppressing slag inclusion. At 1300°C, a surface tension of 300 mN / m or higher is preferable, and 320 mN / m or higher is more preferable.

[0031] <Crystallization temperature> In this embodiment, the crystallization temperature of the mold powder is preferably 1100 to 1200°C, and more preferably 1120 to 1180°C. This allows for a good balance between slow cooling and lubricity. If the crystallization temperature is too high, the lubricity decreases. On the other hand, if the crystallization temperature is too low, the slow cooling decreases. In this specification, "crystallization temperature" refers to the temperature at which exothermic reaction begins when 120g of mold powder is heated to 1300°C to melt, and the molten powder slag is cooled at 4°C / min.

[0032] <Crystal seeds> The preferred primary crystal species to crystallize in the slag film is casspidyne, and CaF2 may crystallize as an associated crystal species. When crystallized species other than casspidyne and CaF2 crystallize in the slag film, uniform heat removal and slow cooling may decrease. While a higher amount of casspidyne crystallizes improves slow cooling, it can reduce lubricity and cause slag inclusion. Therefore, the preferred ratio of crystals to glassy material in the slag film is 40% or more and less than 90% crystals, and 10% or more and less than 60% glassy material. [Examples]

[0033] The embodiments of this disclosure will be described in detail below.

[0034] [Experimental Method] Tables 1-3 show the chemical composition of the mold powder used in the experiment. The content (mass %) of each component in the chemical composition is calculated by converting the molten state at 1300°C to oxides, and F is calculated by separating F from the fluoride and summing them up. If the raw material contains carbonates, carbon is released into the atmosphere as carbon dioxide during melting and is therefore not considered in the chemical composition. [Table 1] [Table 2] [Table 3]

[0035] Experimental Examples 1 to 15 in Table 1 are examples of the present disclosure, all of which contain CaO and SiO2 as the main components. In Experimental Examples 1 to 7, the mass ratio (CaO / SiO2) was fixed at 2.10, while in Experimental Examples 1 to 3, the MgO content was increased, in Experimental Examples 4 to 7, the SrO content was increased, and in Experimental Example 5, the Li2O content was decreased. In Experimental Examples 8 to 15, the mass ratio (CaO / SiO2) was fixed at 1.60, while in Experimental Examples 8 to 13, the SrO content was increased, in Experimental Examples 8 and 14, the Na2O content was increased, in Experimental Example 10, the MgO content was increased, in Experimental Example 11, the Na2O content was decreased, in Experimental Example 12, the Li2O content was increased, and in Experimental Example 15, the K2O content was increased.

[0036] Experimental Examples 16-22 in Table 2 are comparative examples of the present disclosure, all of which contain CaO and SiO2 as the main components. Experimental Examples 16-18 had a fixed mass ratio (CaO / SiO2) of 2.10, while Experimental Examples 19-22 had a fixed mass ratio (CaO / SiO2) of 1.60. Experimental Example 16 had an increased MgO content, Experimental Example 17 had an increased Li2O content, Experimental Examples 18 and 19 had an increased SrO content, Experimental Example 20 had a decreased Na2O content, Experimental Example 21 had a decreased SrO content to 0.0 mass%, and Experimental Example 22 had an increased Na2O content.

[0037] Experimental Examples 23 and 25 in Table 3 are embodiments of the present disclosure, and Experimental Examples 24 and 26 are comparative examples of the present disclosure. All contain CaO and SiO2 as the main components, and the mass ratio (CaO / SiO2) was fixed at 1.60. In Experimental Examples 23 and 24, the fluorine content was increased, and in Experimental Examples 25 and 26, the fluorine content was decreased.

[0038] For the mold powders of Experimental Examples 1 to 26, viscosity, surface tension, crystallization temperature, and crystallized crystal species were measured using the method described below.

[0039] <Viscosity> The viscosity of the mold powder was measured using the platinum ball pulling method. Specifically, approximately 120 g of mold powder was placed in a platinum crucible, heated to 1300°C in an electric furnace and melted. A 10 mm diameter platinum ball was suspended in the molten powder slag and pulled up at a speed of 8.5 mm / s. The load was measured, and the viscosity (η) (unit: Pa·s) was calculated using Stokes' equation.

[0040] <Surface tension> The surface tension of the mold powder was measured using the platinum ball pulling method, similar to the viscosity measurement method. Specifically, approximately 120 g of mold powder was placed in a platinum crucible, heated to 1300°C in an electric furnace, and melted. A 10 mm diameter platinum ball was suspended in the molten powder slag and pulled up at a speed of 8.5 mm / s. The surface tension (in mNm) was determined from the maximum load exhibited at the moment the platinum ball separated from the powder slag liquid surface and the droplet was cut.

[0041] <Crystallization temperature> The crystallization temperature of the mold powder was measured by differential thermal analysis. Specifically, approximately 120g of mold powder was placed in a platinum crucible and heated to 1300°C in an electric furnace. A thermocouple was inserted into the molten powder slag, and the temperature of the powder slag was measured while the temperature was reduced at 4°C / min. The temperature at which exothermic reaction associated with crystallization began was defined as the crystallization temperature.

[0042] <Crystallized crystal seeds> Crystallized species in the slag film were identified by X-ray diffraction. Specifically, approximately 120g of mold powder was melted at 1300°C, poured into a copper water-cooled trough, and the resulting slag film was crushed. X-ray diffraction was then measured to identify the crystallized species. Furthermore, the amount of crystallized material was visually inspected on the fracture surface of the slag film obtained by pouring it into the copper water-cooled trough, and a five-level index was established. That is, since the glassy and crystalline areas can be visually distinguished on the fracture surface, the index was set as follows: "++++" for 90% or more of the crystalline area (crystallized amount), "++++" for 65% to less than 90%, "+++" for 40% to less than 65%, "++" for 20% to less than 40%, and "++" for less than 20%. A higher crystallized amount indicates a higher slow cooling effect.

[0043] [Measurement results] Table 1 shows the measurement results for Experimental Examples 1 to 15, which are embodiments of this disclosure; Table 2 shows the measurement results for Experimental Examples 16 to 22, which are comparative examples of this disclosure; and Table 3 shows the measurement results for Experimental Examples 23 to 26.

[0044] As for the main crystalline species, casspidyne and CaF2 were successfully crystallized in Experimental Examples 1-7 and 23, while an appropriate amount of casspidyne was successfully crystallized in Experimental Examples 8-15 and 25. Furthermore, Experimental Examples 1-15, 23, and 25 all exhibited high surface tensions of 300 mN / m or more, enabling the suppression of slag inclusion and resulting in a balance between lubricity and slow cooling. Therefore, the mass ratio of CaO to SiO2 (CaO / SiO2) is preferably 1.5-2.3, more preferably 1.6-2.1, and the F content is preferably 7.0-15.0 mass%, more preferably 7.5-13.0 mass%, and even more preferably 10.0-13.0 mass%. The following describes each experimental example in detail.

[0045] In Experimental Examples 1-3 and 10, the crystallization temperature and crystallization amount decreased with increasing MgO content, but this was within a favorable range. In Experimental Example 16, with 3.6 mass% MgO, the crystallization temperature and crystallization amount decreased further. This hindered the crystallization of caspidine, resulting in a poor balance between slow cooling and lubricity. Therefore, an MgO content of 0.0-3.0 mass% is preferred, and 0.0-1.0 mass% is considered more preferred.

[0046] In experimental examples 4-7, the crystallization temperature and crystallization amount decreased with increasing SrO content, but this was within a favorable range. On the other hand, in experimental examples 18 and 19 with 11.6 mass% SrO content, (Ca,Sr)2SiO4 crystallized, reducing the slow cooling effect. In experimental example 21, which did not contain SrO, the surface tension was less than 300 mN / m, resulting in a poor balance between slow cooling effect and lubricity. Therefore, a SrO content of 2.0-11.0 mass% is preferred, and 2.0-8.0 mass% is considered more preferred.

[0047] In Experimental Example 5, the crystallization temperature increased with decreasing Li2O. In Experimental Example 12, viscosity and crystallization temperature decreased slightly with increasing Li2O, but within a favorable range. On the other hand, in Experimental Example 17, with 4.7 mass% Li2O, the surface tension fell to less than 300 mN / m, and Ca2SiO4 crystallized, resulting in poor performance in achieving both slow cooling and lubricity. Therefore, a Li2O content of 0.0 to 4.0 mass% is considered preferable.

[0048] In Experimental Example 11, the crystallization temperature and crystallization amount increased with decreasing Na2O content. On the other hand, in Experimental Example 22, with 12.0 mass% Na2O, the surface tension and crystallization temperature decreased even with the inclusion of SrO, resulting in a poor balance between slow cooling and lubricity. Furthermore, in Experimental Example 20, with 1.9 mass% Na2O, the crystallization temperature and crystallization amount increased excessively, raising concerns about seizure due to decreased lubricity. Therefore, a Na2O content of 2.0 to 9.9 mass% is considered preferable.

[0049] In Experimental Example 15, the viscosity, surface tension, and crystallization temperature decreased slightly with the addition of K2O, but these were within a favorable range. Therefore, a K2O content of 0.0 to 4.0 mass% is considered preferable.

[0050] Experimental Example 23, with an F content of 15.0 mass%, showed viscosity, surface tension, and crystallization temperature within favorable ranges, whereas Experimental Example 24, with an F content of 15.2 mass%, had a surface tension of less than 300 mN / m, resulting in a poor balance between slow cooling and lubricity. On the other hand, Experimental Example 25, with an F content of 7.0 mass%, showed viscosity, surface tension, and crystallization temperature within favorable ranges, while Experimental Example 26, with an F content of 6.6 mass%, had a decreased crystallization temperature, resulting in a poor balance between slow cooling and lubricity. Considering the measurement results of Experimental Examples 1 to 15, it is considered that an F content of 7.0 to 15.0 mass% is preferable, 7.5 to 13.0 mass% is more preferable, and 10.0 to 13.0 mass% is even more preferable.

[0051] Although this embodiment has been described in detail above, it will be readily apparent to those skilled in the art that many modifications are possible without substantially departing from the novelty and effects of this disclosure. Therefore, all such modifications are included within the scope of this disclosure. For example, any term that appears in the specification at least once alongside a broader or synonymous term may be replaced with that different term anywhere in the specification.< / f> < / mgo> < / sro>

Claims

1. CaO and SiO ２ It mainly contains CaO and SiO ２ The mass ratio (CaO / SiO ２ ) is 1.5 to 2.3, SrO 2.0 to 11.0% by mass, MgO 0.0-3.0% by mass, Na ２ O 2.0-9.9% by mass K ２ O 0.0~4.0% by mass Li ２ O 0.0 to 4.0% by mass, and, A mold powder characterized by containing F 7.0 to 15.0% by mass.

2. In the mold powder according to claim 1, A mold powder characterized by having a surface tension of 300 mN / m or more at 1300°C and a crystallization temperature of 1100 to 1200°C.

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