Mold powder for continuous casting and method for continuous casting of high Mn steel

Abstract

To provide a mold powder for continuous casting which can stably slab with high quality even when casting a high Mn steel containing Mn within a range of 10% or more and 30% or less by mass ratio during a long time.SOLUTION: When CaO and SiO2 are main components, and components excluding C as an aggregate is 100% by mass ratio, content of MnO is 0.5% or less, content of Al2O3 is 10% or less, total content of Na2O and Li2O is 6% or more and 15% or less, content of F is 10% or more and 24% or less, and a basicity of (CaO) / (SiO2) falls within a range of 0.50 or more and 1.00 or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a mold powder for continuous casting that is supplied into a mold when continuously casting high-Mn steel containing 10% or more and 30% or less of Mn by mass, and a method for continuously casting high-Mn steel using the mold powder for continuous casting. [Background technology]

[0002] When performing continuous casting of steel, mold powder for continuous casting (hereinafter, sometimes referred to as "mold powder") is added onto the surface of molten steel in the mold. This mold powder melts on the surface of the molten steel in the mold and flows between the solidified shell and the mold. The mold powder then forms a film (powder film) between the solidified shell and the mold, some of which is cooled to form a solid (glass phase or crystalline phase), and the rest remains molten and acts as a lubricant between the solidified shell and the mold.

[0003] Here, if mold powder is entrained in the slab during continuous casting, it will cause defects such as surface flaws in the subsequent rolling process, etc. For this reason, it is necessary to suppress the entrainment of mold powder. In addition, if the flow of the mold powder between the mold and the solidified shell is hindered and the lubricating effect is impaired, the mold and the solidified shell may stick to each other, resulting in poor quality that requires maintenance of the slab surface after casting, or in the worst case, the occurrence of a breakout. For this reason, it is very important to ensure that the mold powder flows between the mold wall and the solidified shell, increase the solidification uniformity of the slab in the mold, and stably control the heat removal in the mold.

[0004] In addition, when steel containing Mn is continuously cast, Mn in the molten steel reacts with SiO2 in the mold powder, increasing the MnO concentration in the mold powder and making the mold powder more likely to vitrify. This increases the amount of heat transferred from the solidified shell to the mold, promoting uneven solidification of the shell and potentially causing surface defects such as vertical cracks. Therefore, as shown in, for example, Patent Documents 1 to 3, various molding powders have been proposed for Mn-containing steels. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-254109 [Patent Document 2] Patent No. 5637081 [Patent Document 3] JP 2020-121320 A Summary of the Invention [Problem to be solved by the invention]

[0006] Recently, from the viewpoint of improving productivity, efforts are being made to increase the casting speed and the casting time by increasing the number of continuous castings (continuous casting of many heats: continuous continuous casting, or simply continuous casting for short), and it is therefore necessary to ensure the inflow of mold powder and maintain the lubrication between the solidified shell and the mold. Here, when the mold powders of the above-mentioned Patent Documents 1 to 3 are used, the powder film formed when the powder flows between the mold and the solidified shell is designed to mainly crystallize cuspidine (Ca4Si2O7F2).

[0007] However, when the casting amount is increased, the reaction between Mn in the molten steel and SiO2 in the mold powder progresses further, and the SiO2 in the mold powder decreases significantly. This changes the basicity of the mold powder, and when the mold powder contains Al2O3, cuspidine (Ca4Si2O7F2) is not formed in the powder film, but gehlenite (Ca2Al2SiO7), which has a high melting point, is formed. This hinders the flow of the mold powder between the mold and the solidified shell and the lubrication, causing uneven heat removal and the risk of cracking of the cast piece.

[0008] The present invention has been made in consideration of the above-mentioned circumstances, and has an object to provide a mold powder for continuous casting, which makes it possible to stably obtain high-quality cast pieces even when high-Mn steel containing 10% or more and 30% or less of Mn by mass is cast for a long period of time, and a method for continuous casting of high-Mn steel. [Means for solving the problem]

[0009] In order to solve the above problems, the present inventors conducted extensive research and discovered that by crystallizing CaF2 in the powder film formed when the mold powder flows between the mold and the solidified shell, even if the amount of SiO2 in the mold powder decreases, the composition of the powder film is less affected and stable casting can be performed.

[0010] The present invention has been made based on the above findings, and the mold powder for continuous casting according to the present invention is a mold powder for continuous casting that is supplied into a mold when continuously casting high Mn steel containing 10% or more and 30% or less Mn by mass, and is characterized in that the powder contains CaO and SiO2 as main components, and when the components excluding C as an aggregate are taken as 100%, the MnO content is 0.5% or less, the Al2O3 content is 10.0% or less, the total content of Na2O and Li2O is 6.0% or more and 15.0% or less, and the F content is 10.0% or more and 24.0% or less, and when the amount of CaO is [CaO] and the content of SiO2 is [SiO2], the basicity, which is [CaO] / [SiO2], is within the range of 0.50 or more and 1.00 or less, when the amount of CaO is [CaO] and the content of SiO2 is [SiO2], by mass ratio.

[0011] Here, the content of each component is calculated by converting the concentration of the metal element in the mold powder into an oxide, assuming that all the metal elements are present as oxides, and for F, the concentration value of F in the mold powder is used as is, and the sum of the converted values ​​of these metal elements into oxides and the value of F is taken as 100%. For example, in the case of a mold powder consisting of Si, Ca, Al, O, and F, the concentrations of Si, Ca, Al, and F in the mold powder are %Si, %Ca, %Al, and %F, respectively, and W is 100%. SiO2 , W CaO , W Al2O3 , W F Let us assume the following: W SiO2 =%Si×M SiO2 / M Si W CaO =%Ca×M CaO / M Ca W Al2O3 =%Al×M Al2O3 / (M Al ×2) W F =%F The concentration of SiO2 is W SiO2 / (W SiO2 +W CaO +W Al2O3 +W F ) x 100. The concentration of CaO is W CaO / (W SiO2 +W CaO +W Al2O3 +W F ) x 100. The concentration of Al2O3 is W Al2O3 / (W SiO2 +W CaO +W Al2O3 +W F ) x 100. The concentration of F is W F / (W SiO2 +W CaO +W Al2O3 +W F ) x 100. Here, M SiO2 , M CaO , M Al2O3are the molecular weights of SiO2, CaO, and Al2O3, respectively, and M Si , M Ca , M Al are the atomic weights of Si, Ca, and Al, respectively.

[0012] Since the mold powder for continuous casting of the present invention has the composition as described above, CaF2 crystallizes in the powder film. When the mold powder is used for continuous casting of high Mn steel containing 10% or more and 30% or less of Mn, even if the amount of MnO in the mold powder increases and the amount of SiO2 decreases, the composition of the powder film is not significantly affected, and stable casting can be performed for a long period of time. In addition, when the amount of CaO is [CaO] and the amount of SiO2 is [SiO2] by mass ratio, the basicity, which is [CaO] / [SiO2], is within the range of 0.50 to 1.00, the content of MnO is 0.5% or less, and the content of Al2O3 is 10.0% or less. Therefore, even if the amount of MnO in the molding powder increases and the amount of SiO2 decreases, the basicity can be maintained within an appropriate range, and the generation of gehlenite with a high melting point during solidification can be suppressed, making it possible to stably improve flowability and lubricity.

[0013] The method for continuous casting of high-Mn steel according to the present invention is a method for continuously casting high-Mn steel containing 10% or more and 30% or less of Mn by mass, and is characterized in that the above-mentioned mold powder for continuous casting is supplied into a mold. In the continuous casting method for high Mn steel having this configuration, the above-mentioned mold powder for continuous casting is supplied into the mold, so that CaF2 crystallizes in the powder film. Even if the amount of SiO2 in the mold powder decreases after a long period of casting, the crystal phase that mainly crystallizes in the powder film does not change, and stable casting can be performed for a long period of time. Effect of the Invention

[0014] As described above, according to the present invention, it is possible to provide a mold powder for continuous casting and a method for continuous casting of high Mn steel that can stably produce high-quality cast pieces even when high Mn steel containing 10% or more and 30% or less of Mn by mass is cast for a long period of time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, a mold powder for continuous casting and a method for continuously casting a high Mn steel according to embodiments of the present invention will be described, although the present invention is not limited to the following embodiments. The mold powder for continuous casting according to this embodiment is supplied into a mold when high Mn steel containing Mn in a range of 10% to 30% by mass is continuously cast.

[0016] The mold powder for continuous casting according to the present embodiment is mainly composed of CaO and SiO2, and has, by mass ratio, an MnO content of 0.5% or less, an Al2O3 content of 10.0% or less, a total content of Na2O and Li2O of 6.0% or more and 15.0% or less, and an F content of 10.0% or more and 24.0% or less. In addition, when the amount of CaO is [CaO] and the content of SiO2 is [SiO2], the basicity, which is [CaO] / [SiO2], is within the range of 0.50 or more and 1.00 or less. In this embodiment, the total content of CaO and SiO2 is set to 50.0% or more in terms of mass ratio. In addition, unavoidable impurities such as S, FeO, etc. may be contained, and it is preferable to limit the amount of these unavoidable impurities to 1.5% or less by mass ratio.

[0017] In the continuous casting mold powder of this embodiment, the solidification point is preferably 980° C. or higher and 1200° C. or lower. In the continuous casting mold powder of this embodiment, the viscosity at 1300° C. is preferably 0.6 Pa·s or less.

[0018] The composition of the mold powder for continuous casting according to the present embodiment and the reasons for the regulations as described above will be explained below.

[0019] <〔CaO〕 / 〔SiO2〕> In the mold powder for continuous casting of the present embodiment, the basicity, which is the mass ratio 〔CaO〕 / 〔SiO2〕 of the amount of CaO 〔CaO〕 and the amount of SiO2 〔SiO2〕, is set to 0.50 or more and 1.00 or less. Here, in the present embodiment, since the basicity: 〔CaO〕 / 〔SiO2〕 is 1.00 or less and the amount of SiO2 is ensured, even when the ratio of SiO2 in the mold powder decreases due to Mn in the molten steel, the formation of high melting point crystals can be suppressed. Also, since the basicity: 〔CaO〕 / 〔SiO2〕 is 0.50 or more, it is possible to suppress the mold powder from becoming a glassy state, suppress the heat extraction from the solidified shell to the mold, and suppress the occurrence of surface cracks in the slab. Note that the lower limit of 〔CaO〕 / 〔SiO2〕 is preferably 0.55. Also, the upper limit of 〔CaO〕 / 〔SiO2〕 is preferably 0.95.

[0020] <Content of MnO> When continuously casting a high-Mn steel containing Mn in the range of 10% or more and 30% or less by mass ratio, Mn in the molten steel reacts with SiO2 in the mold powder to generate MnO. When the content of MnO in the mold powder increases, the powder film formed between the mold and the solidified shell tends to be vitrified, resulting in uneven cooling and prone to cracking on the slab surface. Therefore, in the present embodiment, MnO in the mold powder is an unavoidable impurity component, and the upper limit of its content is 0.5%.

[0021] <Content of Al2O3> If the amount of Al2O3 contained in the mold powder is large, gehlenite is likely to be formed in a state of high basicity. Also, in a state of coexisting with MgO, MgAlO4 is likely to be formed. The formation of these substances causes the slag rim to become enlarged, resulting in poor lubrication and possibly causing cracks and sticking of the slab. Therefore, in this embodiment, the content of Al2O3 in the mold powder is set to 10.0% or less by mass ratio. Note that the upper limit of the content of Al2O3 in the mold powder is preferably 8.5%. Also, there is no particular limitation on the lower limit of the content of Al2O3 in the mold powder. Furthermore, in order to suppress the formation of the above-mentioned MgAlO4, the upper limit of the content of MgO is preferably 7.0%.

[0022] <Total content of Na2O and Li2O> When the amount of Na2O and Li2O contained in the mold powder is small, the freezing point of the mold powder becomes high, resulting in poor lubrication and possibly causing cracks and sticking of the slab. On the other hand, when the amount of Na2O and Li2O contained in the mold powder is large, the powder film formed between the mold and the solidified shell is likely to be vitrified, resulting in uneven cooling and likely causing cracks on the slab surface. Therefore, in this embodiment, the total content of Na2O and Li2O in the mold powder is set to 6.0% or more and 15.0% or less by mass ratio. The effects of lowering the freezing point and promoting vitrification are equivalent at the same content, and either only Na2O or only Li2O may be contained. Note that the upper limit of the total content of Na2O and Li2O is preferably 13.0%. Also, the lower limit of the total content of Na2O and Li2O is preferably 8.0%.

[0023] <Content of F> If the mold powder contains too little F, the freezing point of the mold powder will be high, which may cause poor lubrication and lead to cracking or seizure of the cast piece. On the other hand, if the mold powder contains too much F, the immersion nozzle may be subject to significant melting damage, making it difficult to perform stable casting. In addition, there is a risk of composition changes due to the volatilization of NaF. Therefore, in this embodiment, the F content in the molding powder is set to 10.0% or more and 24.0% or less. The upper limit of the F content is preferably 22.0%. The lower limit of the F content is preferably 12.0%.

[0024] <Freezing point> In the mold powder for continuous casting of this embodiment, by setting the solidification point to 980°C or higher, it is possible to suppress the heat transfer from the solidified shell to the mold from becoming greater than necessary, and to suppress the occurrence of cracks and dents on the surface of the slab. On the other hand, by setting the solidification point to more than 1200°C, lubrication becomes insufficient, and there is a risk of cracking or seizing of the slab. Therefore, in this embodiment, it is preferable that the solidification point of the continuous casting mold powder is within the range of 980°C or higher and 1200°C or lower. The upper limit of the solidification point of the mold powder for continuous casting of this embodiment is more preferably 1180°C. On the other hand, the lower limit of the solidification point of the mold powder for continuous casting of this embodiment is more preferably 1000°C.

[0025] <Viscosity> In the mold powder for continuous casting of this embodiment, when the viscosity at 1300° C. is 0.6 Pa·s or less, the flowability and lubricity of the mold powder can be sufficiently ensured. For this reason, in this embodiment, the viscosity at 1300° C. is preferably set to 0.6 Pa·s or less. The upper limit of the viscosity of the mold powder for continuous casting of this embodiment at 1300° C. is more preferably 0.4 Pa s. On the other hand, there is no particular restriction on the lower limit of the viscosity of the mold powder for continuous casting of this embodiment at 1300° C., but when there is concern about excessive inflow or entrainment of the mold powder, the lower limit of the viscosity at 1300° C. is preferably 0.1 Pa s.

[0026] In the continuous casting method for high Mn steel according to the present embodiment, the high Mn steel containing 10% to 30% Mn by mass is targeted, and the mold powder for continuous casting according to the present embodiment is used. CaF2 crystallizes in the powder film formed between the solidified shell and the mold. It is preferable to add the mold powder so that the thickness of the molten layer of the mold powder formed on the molten steel in the mold is within the range of 5 mm to 20 mm.

[0027] The mold powder for continuous casting according to the present embodiment configured as described above contains CaO and SiO2 as main components, and the content of MnO is 0.5% or less, the content of Al2O3 is 10.0% or less, the total content of Na2O and Li2O is 6.0% or more and 15.0% or less, the content of F is 10.0% or more and 24.0% or less, and the basicity, which is [CaO] / [SiO2], is within the range of 0.50 or more and 1.00 or less, by mass ratio. Therefore, CaF2 crystallizes in the powder film, and even if the amount of MnO in the mold powder increases and the amount of SiO2 decreases when the mold powder is used for continuous casting of high Mn steel containing 10% or more and 30% or less, the crystal phase that mainly crystallizes in the powder film does not change, and casting can be performed stably for a long time.

[0028] In addition, the basicity, which is [CaO] / [SiO2], is within the range of 0.50 to 1.00, the MnO content is 0.5% or less, and the Al2O3 content is 10.0% or less. Therefore, even if the amount of MnO in the molding powder increases and the amount of SiO2 decreases, the basicity can be maintained within an appropriate range, and the generation of gehlenite, which has a high melting point, during solidification can be suppressed, making it possible to stably improve flowability and lubricity.

[0029] In the continuous casting method for high Mn steel of this embodiment, the above-mentioned mold powder for continuous casting is used when continuously casting high Mn steel containing 10% or more and 30% or less Mn by mass. Therefore, even if the amount of MnO in the mold powder increases and the amount of SiO2 decreases, the crystal phase that mainly crystallizes in the powder film does not change, and stable casting can be performed for a long period of time.

[0030] The mold powder for continuous casting and the method for continuous casting of high Mn steel according to the embodiments of the present invention have been specifically described above. However, the present invention is not limited thereto, and can be appropriately modified without departing from the technical concept of the invention. EXAMPLES

[0031] (Example) The results of experiments carried out to confirm the effects of the present invention will be described below. Using continuous casting equipment, continuous casting was carried out to produce high Mn steel having the composition shown in Table 1. The casting conditions were a slab thickness of 250 mm, a slab width of 1200 mm, and a steady casting speed of 1.2 m / min. At this time, the mold powder for continuous casting to be added to the mold was the one shown in Table 2. In this example, 3.0% by mass of C was added as an aggregate for adjusting the dissolution rate to 100% of the mold powder having the composition shown in Table 2.

[0032] Casting was carried out for a maximum of four hours, and the slabs were visually inspected for surface defects (cracks). The evaluation results are shown in Table 2. In Table 2, cases where no cracks were found are marked "absent," cases where cracks were found but could be removed by scarfing are marked "minor," and cases where cracks were found but could not be removed by scarfing are marked "present."

[0033] The viscosity of various continuous casting mold powders at 1300°C was measured using the rotating cylinder method. The continuous casting mold powder to be measured was placed in a crucible and pre-melted at 1400°C for 10 to 15 minutes, then placed in a vertical tubular furnace (heating element is SiC), the rotor of an E-type viscometer was immersed in the molten powder, and after stabilizing at 1300°C for 30 minutes, the rotor was rotated to measure the torque due to viscous resistance and determine the viscosity. It is important to calibrate the E-type viscometer with a standard viscous liquid in advance. The solidification point of the mold powder for continuous casting was determined as the temperature at which the viscosity increased significantly when the viscosity was measured at 5°C intervals during the cooling process after the powder was melted using a rotational viscometer.

[0034] Furthermore, the powder film recovered from the mold after casting was subjected to XRD measurement. The crystal with the highest peak is listed in Table 2.

[0035] [Table 1]

[0036] [Table 2]

[0037] In Comparative Example 1, the mold powder used had an Al2O3 content of 22.0% and an F content of 6.5%, both of which were outside the range of the present invention, but cracks occurred that could not be removed by scarfing. The crystals of the powder film were Ca4Si2O7F2. In Comparative Example 2, the mold powder used was out of the range of the present invention, with an Al2O3 content of 20.0%, an F content of 4.1%, and a total content of Na2O and Li2O of 3.0%, but the slag rim grew during casting, so casting was stopped. The crystals of the powder film were Ca4Si2O7F2. In Comparative Example 3, the mold powder used was out of the range of the present invention, with an Al2O3 content of 18.0%, an F content of 6.8%, a total content of Na2O and Li2O of 5.5%, and [CaO] / [SiO2] of 1.22. However, the slag rim grew during casting, so casting was stopped. The crystals of the powder film were Ca4Si2O7F2.

[0038] In Comparative Example 4, the mold powder used had an F content of 9.0% and a [CaO] / [SiO2] ratio of 0.45, both of which were outside the range of the present invention, and cracks that could not be removed by scarfing occurred. Although the crystal with the highest peak in the XRD measurement of the powder film recovered from the mold after casting was CaF2, it was mainly glassy, ​​and the heat transfer from the solidified shell to the mold became uneven, which is thought to be the cause of the cracks that could not be removed by scarfing. In Comparative Example 5, a mold powder with a total content of Na2O and Li2O of 16.0%, which is outside the range of the present invention, was used, and cracks that could not be removed by scarfing occurred. Although the crystal with the highest peak in the XRD measurement of the powder film recovered from the mold after casting was CaF2, it was mainly glassy, ​​and the heat transfer from the solidified shell to the mold became uneven, which is thought to be the cause of the cracks that could not be removed by scarfing. In Comparative Example 6, a mold powder was used that was outside the range of the present invention, with an F content of 26.3% and a [CaO] / [SiO2] ratio of 1.40. The crystal with the highest peak in the XRD measurement of the powder film recovered from the mold after casting was CaF2. However, because the F content was high, the submerged nozzle was severely damaged, and casting was discontinued.

[0039] In contrast, in Examples 1 to 6 of the present invention, in which the basicity of [CaO] / [SiO2] was in the range of 0.50 to 1.00, the MnO content was 0.5% or less, the Al2O3 content was less than 10%, the total content of Na2O and Li2O was 6.0% to 15.0%, and the F content was 10.0% to 24.0%, the occurrence of cracks was suppressed and casting could be performed stably. In particular, in Examples 1 to 3 of the present invention, the occurrence of cracks could be prevented. In addition, in all of Examples 1 to 6 of the present invention, the crystals of the powder film were CaF2.

[0040] From the above, it was confirmed that the present invention can provide a mold powder for continuous casting and a method for continuous casting of high Mn steel that can stably obtain high-quality cast pieces even when high Mn steel containing 10% or more and 30% or less by mass of Mn is cast for a long period of time.

Claims

1. A mold powder for continuous casting, which is supplied into a mold when continuously casting high Mn steel containing Mn in a range of 10% to 30% by mass, CaO and SiO ２ The main component is In terms of mass ratio, when the components excluding C as aggregate are taken as 100%, The content of MnO is 0.5% or less, A ２ O ３ The content is 10.0% or less, Na ２ O and Li ２ The total content of O is 6.0% or more and 15.0% or less, The F content is 10.0% or more and 24.0% or less, It is said that, In terms of mass ratio, the amount of CaO is [CaO], SiO ２ The content of [SiO ２ ], [CaO] / [SiO ２ %. The mold powder for continuous casting is characterized in that the basicity of the powder is in the range of 0.50 to 1.

00.

2. A method for continuously casting high Mn steel, which comprises continuously casting high Mn steel containing Mn in a range of 10% to 30% by mass, the method comprising the steps of: A method for continuous casting of high Mn steel, comprising supplying the mold powder for continuous casting according to claim 1 into a mold.

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