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

Abstract

To provide a mold powder for continuous casting that, even if casting steel containing Si of less than 2.0% and Al of 0.2% or more by mass ratio over a long time, the mold powder is not hindered in its flowing-in and lubrication properties but suppressed from being entrained, so as to stably obtain a high quality slab.SOLUTION: A mold powder for continuous casting, to be supplied into a mold upon continuously casting steel containing Si of less than 2.0% and Al of 0.2% or more by mass ratio, comprises CaO and SiO2 as main components with basicity, a mass ratio [CaO] / [SiO2] of a content of CaO [CaO] to a content of SiO2 [SiO2], given smaller than 0.70, in which provided that components excepting C as aggregate are taken 100% by mass ratio, then the content of Na2O is less than 10%, the content of MgO is less than 5%, the content of Li2O is 1% or more, the content of Al2O3 is less than 4% and the content of BaO is less than 7%, and a solidification point is lower than 1000°C.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 steel containing, by mass, less than 2.0% Si and 0.2% or more Al, and a method for continuously casting steel using this mold powder for continuous casting. [Background technology]

[0002] When continuous casting steel, mold powder for continuous casting (hereinafter sometimes referred to as "mold powder") is added onto the surface of molten steel in a mold. This mold powder melts on the surface of the molten steel in the mold and flows into the gap between the mold wall and the solidified shell. Some of the mold powder cools between the mold wall and the solidified shell to form a powder film, while the rest remains molten and acts as a lubricant between the mold wall and the solidified shell.

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

[0004] Furthermore, when continuously casting steel containing Al, the Al in the molten steel reduces SiO2, the main component of the mold powder, causing the Al2O3 ratio in the mold powder to increase while the SiO2 ratio decreases, resulting in changes in the viscosity and other properties of the mold powder during casting, making stable casting impossible.In addition, when the Al2O3 ratio in the mold powder increases while the SiO2 ratio decreases, when the molten mold powder flows between the mold and the solidified shell and forms a powder film, gehlenite (Ca2Al2SiO7), which has a high melting point, is formed in the crystalline layer of the powder film, hindering the mold powder's flow and lubrication between the mold and the solidified shell, making heat removal unstable and potentially causing cracks in the cast slab. Therefore, various molding powders have been proposed for Al-containing steels, as shown in Patent Documents 1 to 3, for example. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-039745 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-186813 [Patent Document 3] Japanese Patent Application Publication No. 2017-018978 Summary of the Invention [Problem to be solved by the invention]

[0006] Recently, from the viewpoint of improving productivity, efforts have been made to increase the casting speed and the casting time, and it has become necessary to ensure the inflow of mold powder and maintain the lubrication between the solidified shell and the mold. The mold powder disclosed in Patent Document 1 contains a relatively large amount of MgO, and when casting is performed for a long period of time, solidified mold powder may adhere to the mold, causing the slag rim to become coarse. Furthermore, the molding powder disclosed in Patent Document 2 has a solidification point of 1150°C or higher, which means that the molding powder is likely to solidify and crystallize, potentially reducing the flowability of the molding powder. Furthermore, the molding powder disclosed in Patent Document 3 contains a relatively large amount of Na2O, which tends to form nepheline (NaAlSiO4), possibly reducing the flowability of the molding powder.

[0007] The present invention has been made in view of the above-mentioned circumstances, and has an object to provide a mold powder for continuous casting, and a method for continuous casting of steel, which do not impede the inflow and lubricity of mold powder, and which can suppress mold powder entrainment, even when steel containing, by mass, less than 2.0% Si and 0.2% or more Al, is cast for a long period of time, thereby making it possible to stably obtain high-quality cast pieces. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention provides a mold powder for continuous casting that is supplied into a mold when continuously casting steel containing, by mass ratio, less than 2.0% Si and 0.2% or more Al, and In terms of mass ratio, when the components excluding C as aggregate are taken as 100%, CaO and SiO2 The total content is 50% or more, The basicity, which is the mass ratio of the CaO content [CaO] to the SiO2 content [SiO2], [CaO] / [SiO2], is less than 0.70, the Na2O content is less than 10%, the MgO content is less than 5%, the Li2O content is 1% or more, the Al2O3 content is less than 4%, and the BaO content is less than 7%. F content is between 11% and 16% It is characterized by having a freezing point of less than 1000°C.

[0009] 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 of the metal elements exist 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 respectively taken as %Si, %Ca, %Al, and %F, and W SiO2 , W CaO , W Al2O3 , W F Let be 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 SiO2 content is W SiO2 / (W SiO2 +W CaO +W Al2O3 +W F ) x 100. The content of CaO is W CaO / (W SiO2 +W CaO +W Al2O3 +W F ) x 100. The content of Al2O3 is W Al2O3 / (W SiO2 +W CaO +W Al2O3 +W F ) x 100. The F content is W F / (W SiO2 +W CaO +W Al2O3 +W F ) x 100. where 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.

[0010] In the mold powder for continuous casting of the present invention, the basicity, which is the mass ratio [CaO] / [SiO2] of the CaO content [CaO] to the SiO2 content [SiO2], is less than 0.70, and the Al2O3 content is less than 4% when the components excluding C as an aggregate are taken as 100% by mass. Therefore, even if the ratio of Al2O3 in the mold powder increases and the ratio of SiO2 decreases during long-term casting, a large increase in basicity can be suppressed, and the formation of gehlenite can be suppressed.

[0011] Furthermore, since the content of MgO, which is a high melting point oxide, is limited to less than 5%, the generation of slag rim during casting can be suppressed. Furthermore, since the Na2O content is limited to less than 10%, the formation of nepheline (NaAlSiO4) can also be suppressed. In addition, since the content of Li2O is set to 1% or more, the viscosity of the molding powder can be kept low. Furthermore, since the BaO content is limited to less than 7%, it is possible to suppress the entrainment of mold powder during casting. Furthermore, since the solidification point is set to be less than 1000°C, the flowability of the mold powder can be sufficiently ensured, and casting can be carried out stably for a long period of time.

[0012] Here, in the mold powder for continuous casting of the present invention, the content of BaO is preferably less than 4% by mass. In this case, since the BaO content is limited to less than 4% by mass, it is possible to further suppress the entrainment of mold powder during casting, and it becomes possible to continuously cast even higher quality slabs.

[0013] A method for continuous casting steel according to the present invention is a method for continuously casting steel containing, by mass, less than 2.0% Si and 0.2% or more Al, and is characterized by supplying the above-mentioned mold powder for continuous casting into a mold. In the steel continuous casting method having this configuration, the above-mentioned mold powder for continuous casting is supplied into the mold. Therefore, even if the amount of SiO2 in the mold powder decreases and the amount of Al2O3 increases after long-term casting, the generation of gehlenite and other crystals can be suppressed, the flowability and lubricity of the mold powder are not hindered, and stable casting can be performed for a long period of time. [Effects 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 steel that do not impede the inflow and lubricity of mold powder, and can suppress mold powder entrainment, even when steel containing, by mass, less than 2.0% Si and 0.2% or more Al is cast for a long period of time, thereby making it possible to stably obtain high-quality cast pieces. [Brief explanation of the drawings]

[0015] [Figure 1] This is a ternary phase diagram of CaO-SiO2-Al2O3. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, a mold powder for continuous casting and a method for continuous casting of steel according to embodiments of the present invention will be described, although the present invention is not limited to the following embodiments.

[0017] The mold powder for continuous casting of this embodiment is mainly composed of CaO and SiO2, and when the components excluding C as an aggregate are taken as 100% by mass, the content of Na2O is less than 10%, the content of MgO is less than 5%, the content of Li2O is 1% or more, the content of Al2O3 is less than 4%, and the content of BaO is less than 7%. Furthermore, the mold powder for continuous casting of this embodiment contains F to adjust the viscosity. In this embodiment, the total content of CaO and SiO2 is set to 50% or more by mass, assuming that the total components excluding C as an aggregate are 100%. In addition, unavoidable impurities such as S and FeO may be contained, and it is preferable to limit the amount of these unavoidable impurities to 1.5% or less by mass.

[0018] In addition, in the mold powder for continuous casting according to this embodiment, the basicity, which is the mass ratio [CaO] / [SiO2] of the CaO content [CaO] to the SiO2 content [SiO2], is less than 0.70. The mold powder for continuous casting according to this embodiment has a solidification point of less than 1000°C. The continuous casting molding powder of this embodiment preferably has a viscosity at 1300°C within the range of 0.15 Pa·s or more and less than 0.45 Pa·s.

[0019] The reasons for specifying the composition, properties, etc. of the molding powder for continuous casting according to this embodiment as described above will be explained below.

[0020] <[CaO] / [SiO2]> In the continuous casting mold powder of this embodiment, the basicity, which is the mass ratio [CaO] / [SiO2] of the CaO content [CaO] to the SiO2 content [SiO2], is set to less than 0.70. Here, in the ternary phase diagram shown in FIG. 1, by setting [CaO] / [SiO2] to less than 0.70, even when the amount of SiO2 decreases and the amount of Al2O3 increases, it is possible to suppress the formation of high-melting-point gehlenite (C2AS in FIG. 1). In addition, the basicity: [CaO] / [SiO2] is more preferably less than 0.60.

[0021] <Content of Na2O> In the mold powder for continuous casting of the present embodiment, if the content of Na2O is high, nepheline (NaAlSiO4) is likely to be formed, and there is a risk that the inflowability of the mold powder will decrease. Therefore, in the present embodiment, the content of Na2O in the mold powder is less than 10% by mass ratio. In addition, the content of Na2O is preferably less than 10% by mass ratio, and more preferably less than 9% by mass ratio. There is no particular limitation on the lower limit of the content of Na2O, but it is preferably 3% or more.

[0022] <Content of MgO> In the mold powder for continuous casting of the present embodiment, when the content of MgO, which is a high-melting-point oxide, increases, slag lumps are likely to occur, and there is a risk that the inflow of the mold powder will be inhibited. Therefore, in the present embodiment, the content of MgO in the mold powder is less than 5% by mass ratio. In addition, the content of MgO is preferably less than 5% by mass ratio, and more preferably less than 4% by mass ratio. There is no particular limitation on the lower limit of the content of MgO, but it is preferably 0.5% or more.

[0023] <Content of Li2O> In the mold powder for continuous casting of the present embodiment, if the content of Li2O is low, the viscosity of the mold powder may increase and the inflowability may decrease. In particular, in the present embodiment, since the content of Na2O is limited to less than 10% as described above, it is necessary to lower the viscosity with Li2O. Therefore, in this embodiment, the content of Li2O in the mold powder is 1% or more in terms of mass ratio. The content of Li2O is preferably 1% or more and more preferably 2% or more in terms of mass ratio. There is no particular limitation on the upper limit of the content of Li2O, but it is preferably less than 10%.

[0024] <Content of Al2O3> When casting Al-containing steel, the content of Al2O3 in the mold powder will increase. Therefore, in this embodiment, by setting the content of Al2O3 in the mold powder supplied during casting to less than 4% in terms of mass ratio, it is possible to suppress the excessive increase in the content of Al2O3 in the mold powder during casting. The content of Al2O3 is preferably less than 4% and more preferably less than 3% in terms of mass ratio.

[0025] <Content of F> In the mold powder for continuous casting of this embodiment, by adding F, it becomes possible to adjust the viscosity. Here, in this embodiment, there is no particular limitation on the content of F, but it is preferably within the range of 10% or more and 16% or less in terms of mass ratio, and more preferably within the range of 11% or more and 15% or less.

[0026] <Content of BaO> In the mold powder for continuous casting of this embodiment, if the content of BaO with a high density is large, the density of the molten mold powder will also increase, and there is a risk that the mold powder will be entrained at the interface between the molten steel surface and the molten layer of the mold powder. Therefore, in this embodiment, the content of BaO in the mold powder is set to less than 7% in terms of mass ratio. The content of BaO is preferably less than 7% and more preferably less than 4% in terms of mass ratio.

[0027] <Solidification point> In the mold powder for continuous casting of this embodiment, by setting the solidification point to less than 1000°C, solidification of the mold powder during casting can be suppressed, the generation of slag rim and crystallization of the mold powder can be suppressed, and the flowability of the mold powder can be ensured. In this embodiment, the solidification point is preferably less than 1000°C, and more preferably less than 970°C.

[0028] <Viscosity> In the mold powder for continuous casting of this embodiment, if the viscosity at 1300°C is 0.15 Pa s or more, it is possible to suppress entrainment of the mold powder at the interface between the surface of the molten steel and the molten layer of the mold powder. On the other hand, if the viscosity at 1300°C is less than 0.45 Pa s, it is possible to ensure sufficient flowability and lubricity of the mold powder. Therefore, in the continuous casting mold powder of this embodiment, the viscosity at 1300°C is preferably in the range of 0.15 Pa·s or more and less than 0.45 Pa·s, and more preferably in the range of 0.20 Pa·s or more and less than 0.35 Pa·s.

[0029] In the continuous casting method for steel according to the present embodiment, steel containing less than 2.0% by mass of Si and 0.2% by mass or more of Al is targeted, and the above-described mold powder for continuous casting according to the present embodiment is supplied into a mold. At this time, it is preferable to add the mold powder for continuous casting of this embodiment so that the thickness of the molten layer of mold powder formed on the molten steel in the mold is in the range of 5 mm to 20 mm.

[0030] According to the mold powder for continuous casting and the method for continuous casting of steel of this embodiment configured as described above, the basicity, which is the mass ratio [CaO] / [SiO2] of the CaO content [CaO] to the SiO2 content [SiO2], is less than 0.70, and the Al2O3 content is less than 4% when the components excluding C as an aggregate are taken as 100% by mass. Therefore, even if the amount of SiO2 in the mold powder decreases and the amount of Al2O3 increases after long-term casting, as shown in FIG. 1, it is possible to suppress the generation of gehlenite, which has a high melting point, and flowability and lubricity are ensured, allowing stable casting.

[0031] Furthermore, since the content of MgO, which is a high melting point oxide, is limited to less than 5%, the generation of slag rim during casting can be suppressed. Furthermore, since the Na2O content is limited to less than 10%, the formation of nepheline (NaAlSiO4) can also be suppressed. In addition, since the content of Li2O is set to 1% or more, the viscosity of the molding powder can be kept low. Furthermore, since the solidification point is set to be less than 1000°C, the flowability of the mold powder can be sufficiently ensured, and casting can be carried out stably for a long period of time.

[0032] The mold powder for continuous casting and the method for continuous casting of steel according to the embodiments of the present invention have been specifically described above, but the present invention is not limited thereto and can be modified as appropriate within the scope of the technical concept of the invention. [Example]

[0033] 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 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 1.0 m / min. At this time, the mold powder for continuous casting added to the mold was the mold powder shown in Table 2. In this example, 3% by mass of aggregate carbon for adjusting the dissolution rate was added to 100% of the mold powder having the composition shown in Table 2.

[0034] Casting was carried out for a maximum of two hours, and the slabs were visually inspected for surface defects. The evaluation results are shown in Table 2. In Table 2, the cases where no surface defects were found were marked "none," the cases where surface defects were found but could be removed by scarfing were marked "minor," and the cases where surface defects were found but could not be removed by scarfing were marked "present."

[0035] Furthermore, among the cast slabs obtained by casting, those whose evaluation results showed that there were no or very few surface defects were hot-rolled, pickled, cold-rolled, and annealed in the usual manner to produce thin sheets for automobiles, which were then pressed to inspect for the occurrence of press cracks, and the mold powder inclusion potential was evaluated based on the rate of occurrence.

[0036] The solidification points and viscosities at 1300°C of various continuous casting mold powders were measured as follows. The viscosity of continuous casting mold powder 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 10-15 minutes, then placed in a vertical tubular furnace (with a SiC heating element). The rotor of an E-type viscometer was immersed in the molten powder and allowed to stabilize at 1300°C for 30 minutes. The rotor was then rotated, and the torque due to viscous resistance was measured to determine the viscosity. It is important to calibrate the E-type viscometer with a standard viscosity liquid beforehand. The solidification point of the mold powder for continuous casting was determined by measuring the viscosity at 5°C intervals during the cooling process after melting the powder with a rotational viscometer, and determining the temperature at which the viscosity rose significantly.

[0037] [Table 1]

[0038] [Table 2]

[0039] In Comparative Example 1, the basicity was 1.35 and the solidification point was 1030°C, and surface defects that could not be removed by scarfing were observed. In Comparative Example 2, the basicity was 0.82, the Na2O content was 10.5%, the MgO content was 5.5%, and the solidification point was 1070°C, and surface defects that could not be removed by scarfing were confirmed.

[0040] In Comparative Example 3, the Na2O content was 15.0%, the MgO content was 8.0%, the Li2O content was 0.0%, and the solidification point was 1030°C, and surface defects that could not be removed by scarfing were confirmed. In Comparative Example 4, the basicity was 0.90, the Na2O content was 7.0%, the MgO content was 8.0%, and the solidification point was 1030°C, and casting was discontinued due to coarsening of the slag rim.

[0041] In Comparative Example 5, the basicity was 1.20, the NaO content was 16.5%, the MgO content was 8.0%, the LiO content was 0.9%, and the solidification point was 1190°C. Casting was discontinued due to coarsening of the slag rim. In Comparative Example 6, the BaO content was 10.0%, and mold powder was entrained in the cast slab, resulting in a press crack incidence rate of 0.4%.

[0042] In contrast, in Example 1-9 of the present invention, in which the basicity was less than 0.70, the solidification point was less than 1000°C, and the contents of each component were within the ranges of the present invention, the occurrence of surface defects was suppressed, and stable casting was possible. In addition, the occurrence rate of press cracks was also suppressed. In particular, in Examples 1-7 and 9 of the present invention, in which the BaO content was less than 4%, the occurrence rate of press cracks was reduced to 0%. This is presumably because further limiting the BaO content further suppressed the inclusion of mold powder.

[0043] From the above, it was confirmed that according to the present invention, even when steel containing, by mass, less than 2.0% Si and 0.2% or more Al is cast for a long period of time, the inflowability and lubricity of the mold powder are not hindered, and the entrainment of the mold powder can be suppressed, and it is possible to provide a mold powder for continuous casting and a method for continuous casting of steel that are capable of stably obtaining high-quality cast pieces.

Claims

1. A mold powder for continuous casting, which is supplied into a mold when continuously casting steel containing, by mass ratio, less than 2.0% Si and 0.2% or more Al, In terms of mass ratio, when the components excluding C as aggregate are taken as 100%, CaO and SiO ２ The total content of CaO [CaO] and SiO ２ Content [SiO ２ ] mass ratio [CaO] / [SiO ２ The basicity is less than 0.

70. Na ２ O content is less than 10%; MgO content less than 5%; Li ２ O content is 1% or more, Al ２ O ３ The content is less than 4% BaO content less than 7%; F content is between 11% and 16% It is said that, A mold powder for continuous casting, characterized in that the solidification point is less than 1000°C.

2. 2. The mold powder for continuous casting according to claim 1, wherein the content of BaO is less than 4% by mass.

3. A method for continuously casting steel, comprising the steps of: continuously casting steel containing, by mass, less than 2.0% Si and 0.2% or more Al, 3. A method for continuous casting of steel, comprising supplying the mold powder for continuous casting according to claim 1 or 2 into a mold.

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