Method for continuous casting of steel

Abstract

In the continuous casting method of steel, both the melting loss of the sliding nozzle plate of the ladle and tundish and the adhesion of inclusions to the outlet hole of the sliding nozzle plate and the resulting nozzle blockage are taken into account. The continuous casting method of steel of the present invention is a continuous casting method for continuously casting molten steel containing calcium without the addition of aluminum and deoxidized by silicon. In this method, the molten steel is contained in a ladle equipped with a sliding nozzle composed of a first sliding nozzle plate, which is formed of alumina-zirconia-carbon refractory. Molten steel is poured from the ladle into a tundish equipped with a sliding nozzle composed of a second sliding nozzle plate, which is formed of magnesia-spinel refractory. Molten steel is poured from the tundish into a mold while an inactive gas is blown into the molten steel flowing from the outlet hole of the second sliding nozzle plate, and the molten steel is continuously cast.

Description

Technical Field

[0001] This invention relates to a continuous casting method for steel made by continuously casting molten steel without aluminum (Al), deoxidized with silicon (Si), and with added calcium (Ca). Background Technology

[0002] In the continuous casting process of molten steel, molten steel is poured from the ladle into the tundish while simultaneously pouring molten steel from the tundish into the mold while maintaining a nearly constant amount of molten steel in the tundish. Sliding nozzles are used in both the ladle and the tundish as means of controlling the amount of molten steel poured from the ladle into the tundish and from the tundish into the mold. In the ladle, the sliding nozzle is located at the molten steel outlet and is configured with two or three sliding nozzle plates arranged between the upper and lower nozzles. Similarly, in the tundish, the sliding nozzle is located at the molten steel outlet at the bottom of the tundish and is configured with two or three sliding nozzle plates arranged between the upper nozzle and the immersion nozzle.

[0003] The sliding nozzle plate has outflow holes penetrating the upper and lower surfaces of each sliding nozzle plate, through which molten steel flows down. For a two-plate sliding nozzle with two sliding nozzle plates, the upper sliding nozzle plate is usually fixed, and the lower sliding nozzle plate is moved (slid) in close contact with the fixed upper sliding nozzle plate to adjust the opening area of ​​the overlapping outflow holes, thereby controlling the injection volume of molten steel. For a three-plate sliding nozzle with three sliding nozzle plates, the upper and lower sliding nozzle plates are usually fixed, and the middle sliding nozzle plate is moved in close contact with the fixed upper and lower sliding nozzle plates to adjust the opening area of ​​the overlapping outflow holes, thereby controlling the injection volume of molten steel.

[0004] As refractory materials for sliding nozzle plates in ladles and tundishes, alumina (Al2O3)-carbon (C) refractories and magnesia (MgO)-carbon refractories, which have good thermal shock resistance and low raw material costs, are widely used. Regarding the material of the sliding nozzle plates for ladles and tundishes, as will be discussed later, the selection depends on the composition (steel grade) of the molten steel to be continuously cast. Generally, sliding nozzle plates with the same main component are used for both ladles and tundishes. That is, if the sliding nozzle plate for the ladle is alumina-carbon, then the sliding nozzle plate for the tundish is also alumina-carbon; if the sliding nozzle plate for the ladle is magnesia-carbon, then the sliding nozzle plate for the tundish is also magnesia-carbon.

[0005] However, for certain steel grades, refining (calcium addition treatment) is sometimes carried out by adding Ca-Si alloys to molten steel to achieve a specified amount of calcium content, for purposes such as controlling the morphology of non-metallic inclusions (hereinafter also referred to as "inclusions"). Additionally, aluminum-based deoxidation (aluminum killing treatment) is the primary method for deoxidizing molten steel; however, for certain steel grades such as silicon steel, deoxidation (killing treatment) is sometimes performed without adding aluminum, using only deoxidizing elements other than aluminum, such as silicon.

[0006] When Ca-Si alloy is added to aluminum-killed steel, the inclusions in the resulting molten steel are mostly composed of the CaO-SiO2-Al2O3 system. On the other hand, when Ca-Si alloy is added to steel that has undergone silicon killing treatment without the addition of aluminum, the inclusions in the resulting molten steel are mostly composed of the CaO-SiO2 system.

[0007] In continuous casting, as molten steel flows through the outlet orifice of the sliding nozzle plate in the ladle or tundish, some inclusions in the molten steel adhere to the inner surface of the outlet orifice. If the sliding nozzle plate is made of an alumina-carbonaceous refractories or similar material with alumina as its main component, and if calcium addition treatment generates CaO-SiO2-Al2O3 inclusions or CaO-SiO2 inclusions that adhere to the inner surface of the outlet orifice, these inclusions and Al2O3 in the sliding nozzle plate will form low-melting-point compounds, leading to melting and damage to the sliding nozzle plate. This melting and damage reduces the durability of the sliding nozzle plate, sometimes resulting in a decrease in the number of continuous-continuous casting operations (charges) and operational failures such as steel leakage from the sliding nozzle plate.

[0008] To address this problem, it is known that sliding nozzle plates with high resistance to melting and primarily composed of magnesia are used in the continuous casting of calcium-added steel. The main challenge for sliding nozzle plates with magnesia as the main component is their resistance to cracking, and much effort has been made to solve this problem.

[0009] For example, Patent Document 1 proposes a sliding nozzle plate refractory in which, in a magnesia (MgO)-spinel (MgAl2O4)-carbon (C) sliding nozzle plate, when the content of magnesia, spinel and carbon is set to 100% by mass, magnesia is 27-88% by mass, spinel is 10-62% by mass, and carbon is 2-8% by mass. In the particle size of 0.3 mm to 4 mm, the content is 22-73% by mass relative to the aforementioned 100% by mass, of which magnesia raw material is 0-63% by mass and spinel raw material is 0-65% by mass. In the particle size of less than 0.3 mm, the content is 27-78% by mass relative to the aforementioned 100% by mass, of which magnesia is 25-50% by mass, spinel is 0-20% by mass and carbon raw material is 2-8% by mass, and carbon black as carbon raw material is 1% by mass or more.

[0010] In addition, Patent Document 2 proposes a sliding nozzle plate for a ladle, which is a sliding nozzle plate formed of brick. The brick is obtained by adding a binder to a compound containing refractory aggregate, metallic aluminum and carbon, mixing and molding, and then firing at a temperature below 1000°C. The refractory aggregate is formed of 35 to 75% by mass of magnesia and 20 to 60% by mass of alumina relative to the amount of the aforementioned compound.

[0011] Existing technical documents

[0012] Patent documents

[0013] Patent Document 1: Japanese Patent Application Publication No. 2017-149596

[0014] Patent Document 2: Japanese Patent Application Publication No. 2004-141899 Summary of the Invention

[0015] The problem that the invention aims to solve

[0016] However, the aforementioned prior art has the following problems.

[0017] That is, if a magnesia-based sliding nozzle plate, used for calcium-added steel to improve melt resistance, is used as described above, inclusions may sometimes adhere to the inner surface of the outlet orifice of the sliding nozzle plate in the ladle and tundish. These inclusions may grow during continuous casting and block the outlet orifice. This can lead to problems such as the inability to complete continuous casting of the primary filler portion of molten steel in the ladle, the need to halt continuous casting, or the inability to perform continuous casting involving other fillers.

[0018] The present invention was made in view of the above circumstances, and its object is to provide a continuous casting method for steel in which steel is continuously cast from molten steel without aluminum, deoxidized with silicon, and with added calcium to cast steel sheets, which can simultaneously prevent inclusions from adhering to the outlet orifice of the sliding nozzle plate of the ladle and the resulting nozzle blockage, and suppress the melting loss of the sliding nozzle plate of the tundish and prevent inclusions from adhering to the outlet orifice of the sliding nozzle plate.

[0019] Methods for solving problems

[0020] To address the aforementioned issues, the essence of this invention is as follows.

[0021] [1] A continuous casting method for steel, wherein a casting sheet is produced by continuously casting molten steel containing calcium without the addition of aluminum and deoxidized by silicon, and with the addition of calcium, wherein the molten steel is contained in a ladle provided with a sliding nozzle consisting of a first sliding nozzle plate formed of alumina-zirconia-carbon refractory, the molten steel is injected from the ladle into an intermediate ladle provided with a sliding nozzle consisting of a second sliding nozzle plate formed of magnesia-spinel refractory, and the molten steel is injected from the intermediate ladle into a mold while blowing an inactive gas into the molten steel flowing down from the outlet hole of the second sliding nozzle plate, thereby continuously casting the molten steel.

[0022] [2] The continuous casting method for steel as described in [1] above is characterized in that the composition of the first sliding nozzle plate is 70-81% by mass of Al2O3, less than 10% by mass of SiO2, 5-18% by mass of ZrO2, and 3-10% by mass of fixed carbon.

[0023] [3] The continuous casting method for steel as described in [1] or [2] above is characterized in that the composition of the aforementioned second sliding nozzle plate is 89-97% by mass of MgO and 4-7% by mass of Al2O3.

[0024] [4] The continuous casting method of steel as described in any one of [1] to [3] above, characterized in that the composition of the molten steel contains 1.5% by mass or more of silicon, 0.003% by mass or less of acid-soluble aluminum (including zero), and 0.001% by mass or more of total calcium.

[0025] [5] The continuous casting method for steel as described in [4] above is characterized in that the composition of the molten steel contains less than 0.0050% by mass of carbon, 1.5 to 5.0% by mass of silicon, less than 3.0% by mass of manganese, less than 0.2% by mass of phosphorus, less than 0.0050% by mass of sulfur, less than 0.003% by mass of acid-soluble aluminum (including zero), and 0.001 to 0.008% by mass of total calcium.

[0026] Invention Effects

[0027] According to the present invention, in the continuous casting of calcium-added molten steel without aluminum addition and after silicon deoxidation, a sliding nozzle plate made of alumina-based material is used for the sliding nozzle of the ladle, where inclusion adhesion and blockage are prone to occur. On the other hand, for the sliding nozzle of the tundish, where the inclusion density in the molten steel is lower than that in the ladle, and where improved durability is directly related to increased filler count in continuous casting, and where more precise control of the amount of molten steel injected into the mold is required, a sliding nozzle plate made of magnesia-based material is used, and molten steel is injected into the mold while blowing an inert gas into the molten steel flowing from the outlet orifice of the sliding nozzle plate. Thus, it is possible to simultaneously prevent inclusion adhesion to the outlet orifice of the sliding nozzle plate in the ladle and prevent nozzle blockage, and to suppress melting of the sliding nozzle plate in the tundish and prevent inclusion adhesion to the outlet orifice of the sliding nozzle plate, thereby preventing continuous casting interruptions, enabling continuous casting operations, and improving the filler count in continuous casting. Detailed Implementation

[0028] The embodiments of the present invention will be described in detail below.

[0029] The continuous casting method of molten steel involved in this invention continuously casts molten steel containing calcium but without the addition of aluminum (Al), after silicon (Si) deoxidation and the addition of calcium (Ca) to produce castings. In this method, molten steel is contained in a ladle equipped with a sliding nozzle consisting of a first sliding nozzle plate formed of an alumina (Al2O3)-zirconia (ZrO2)-carbon (C) refractory. Molten steel is poured from the ladle into an intermediate ladle equipped with a sliding nozzle consisting of a second sliding nozzle plate formed of a magnesia (MgO)-spinel (MgAl2O4) refractory. Molten steel is poured from the intermediate ladle into a mold while blowing an inactive gas into the molten steel flowing from the outlet of the second sliding nozzle plate.

[0030] First, the reasons for selecting the material of the sliding nozzle plate (hereinafter also referred to as "plate") of the ladle in this invention will be explained.

[0031] Typically, calcium in molten steel reacts with Al2O3 in the molten steel, slag, and refractory materials to form CaO-Al2O3 oxides. These CaO-Al2O3 oxides can form a composition with a melting point lower than that of the molten steel. Therefore, if a sliding nozzle plate made primarily of alumina is used for continuous casting of calcium-added steel, the calcium in the molten steel reacts with Al2O3 in the plate at the contact point between the molten steel and the plate (e.g., at the outlet), forming low-melting-point CaO-Al2O3 oxides. Furthermore, CaO-Al2O3 inclusions generated during calcium treatment and suspended in the molten steel adhere to the plate and react with Al2O3 in the plate, forming further low-melting-point CaO-Al2O3 oxides. The formation of these low-melting-point CaO-Al2O3 oxides leads to plate melting and loss.

[0032] On the other hand, compared with CaO-Al2O3 oxides, the calcium in molten steel and the CaO-MgO oxides and CaO-Al2O3-MgO oxides formed by the reaction of CaO-Al2O3 generated from calcium treatment with MgO in the plate have higher melting points, which are in most cases higher than the temperature of the molten steel. Therefore, even when using plates made primarily of magnesia to continuously cast calcium-added steel, plate loss is less likely to occur.

[0033] However, in continuous casting, molten steel passes through the slab while in contact with the outlet holes, resulting in localized cooling of the molten steel in contact with the outlet holes. At this point, inclusions suspended in the molten steel sometimes adhere to and solidify within the outlet holes. Furthermore, once inclusion adhesion begins, subsequent inclusions suspended in the molten steel that come into contact with this area will also aggregate and solidify, and the adhesion and growth of inclusions continues until the nozzle eventually closes, preventing further molten steel injection.

[0034] Therefore, in this invention, for the first sliding nozzle plate which serves as the sliding nozzle plate of the ladle, a plate with magnesia as the main component is not used, but an alumina-zirconia-carbon refractory is used to prevent inclusions from adhering to and solidifying into the outlet hole of the plate.

[0035] By using an alumina-zirconia-carbonaceous refractory on the first sliding nozzle plate, calcium in the molten steel and CaO-SiO2 inclusions generated by calcium treatment and suspended in the molten steel (in this invention, silicon-killed inclusions without added aluminum) react with Al2O3 in the plate, thereby causing moderate melting of the plate. By causing moderate melting of the sliding nozzle plate, inclusions in the molten steel, even if they adhere to the outlet holes of the plate, will separate and not solidify on the plate.

[0036] It should be noted that there is a risk of excessive melting of the steel plate due to the reaction between calcium and CaO-SiO2 inclusions in the molten steel and Al2O3 in the plate. However, for calcium-added steels that are deoxidized with silicon without adding aluminum, the activity of calcium in the molten steel is lower than that in typical aluminum-deoxidized calcium-added steels, and the reaction between calcium in the molten steel and Al2O3 in the plate becomes more stable. Therefore, excessive melting of the plate does not occur. Moreover, unlike the sliding nozzle plate of the tundish described later, even in continuous casting, the sliding nozzle plate of the ladle only needs to be durable enough to withstand the pouring of at least one filler of molten steel from the ladle into the tundish, and therefore will not hinder operation.

[0037] Next, the reasons for selecting the material of the sliding nozzle plate of the intermediate tundish in this invention will be described.

[0038] In this invention, a magnesia-spinel refractory is used for the second sliding nozzle plate, which serves as the sliding nozzle plate in the tundish. As mentioned earlier, when using plates made primarily of alumina, calcium and CaO-SiO2 inclusions in the molten steel react with Al2O3 in the plate, causing melting loss. Since the sliding nozzle plate in the tundish requires more precise control of the amount of molten steel poured into the mold, even minor melting loss must be avoided. Therefore, plates made primarily of alumina are unsuitable for the tundish.

[0039] On the other hand, there is a risk that inclusions may adhere to the plate outlet holes due to the use of magnesia-spinel refractory for the second sliding nozzle plate. However, some of the inclusions in the molten steel float and separate within the ladle and tundish, reducing the amount of inclusions in the molten steel. Furthermore, by blowing inactive gas into the molten steel flowing down from the plate outlet holes in the tundish and cleaning the inner surface of the plate outlet holes, the adhesion of inclusions to the plate outlet holes is inhibited. Therefore, even if a plate with magnesia-based material is used for the tundish plate, the outlet holes will not become blocked, making it possible to increase the number of continuous casting cycles. Regarding the method of blowing inactive gas into the molten steel flowing down from the plate outlet holes in the tundish, inactive gas can be blown in from the upper nozzle located above the sliding nozzle, or it can be blown in from the inner surface of the outlet holes of the sliding nozzle plate.

[0040] The "alumina-zirconia-carbonaceous refractory" in this invention only needs to contain at least "alumina (Al2O3)", "zirconia (ZrO2)", and "fixed carbon" as components. Furthermore, the first sliding nozzle plate used as the sliding nozzle of the ladle preferably has the following composition: Al2O3 70-81% by mass, SiO2 10% or less by mass, ZrO2 5-18% by mass, and fixed carbon 3-10% by mass.

[0041] Here, if the Al2O3 content is less than 70% by mass, adequate melting loss cannot be obtained, so it is not preferred. On the other hand, if the Al2O3 content is greater than 81% by mass, the melting loss is too large, so it is not preferred.

[0042] Adding ZrO2 improves the plate's resistance to calcium corrosion in molten steel. However, excessive ZrO2 content can easily cause cracking; therefore, the preferred ZrO2 content is 5–18% by mass.

[0043] SiO2 is an unavoidable component in the ZrO2 raw material when ZrO2 is added. As long as the SiO2 content is below 10% by mass, it will not affect the properties. Therefore, the SiO2 content is preferably set to below 10% by mass.

[0044] Graphite is a raw material used to make fixed carbon. Other materials that can be used include carbon black and pitch. Adding graphite improves the refractory's resistance to rupture. However, if too much fixed carbon is added, oxygen in the molten steel reacts with the carbon in the fixed carbon and escapes as CO gas, forming pores in the plate. Therefore, the content of fixed carbon is preferably set at 3–10% by mass.

[0045] The second sliding nozzle plate used as the sliding nozzle of the intermediate liner preferably has a composition of 89-97% by mass of MgO and 4-7% by mass of Al2O3.

[0046] Here, if the MgO content is below 89% by mass, the corrosion resistance decreases, so it is not preferred. If the MgO content is above 97% by mass, the crack resistance decreases, so it is also not preferred. It should be noted that, as a raw material for MgO, sintered magnesia or fused magnesia with a purity of 95% by mass or higher is preferred. If the purity is below 95% by mass, impurities easily form low-melting-point substances, reducing corrosion resistance, so it is also not preferred.

[0047] The Al2O3 contained in the second sliding nozzle plate is a component included by using spinel as the raw material of the plate. Spinel is a compound formed by the equimolar combination of MgO and Al2O3, and has solid solubility on both the MgO and Al2O3 sides. That is, the "magnesia-spinel refractory" of the present invention only needs to contain at least "magnesia (MgO)" and "spinel (MgAl2O4)" as components, and may also contain fixed carbon. As the spinel raw material used in the present invention, a raw material with an Al2O3 and MgO content of 95% by mass or more and an MgO content of 10 to 50% by mass is used. In addition, either sintered spinel or fused spinel can be used as the spinel raw material.

[0048] By using spinel as a raw material, the crack resistance of refractory materials is improved. If the amount of spinel is small and the Al2O3 content is less than 4% by mass, the improvement in crack resistance brought by spinel cannot be fully obtained, so it is not preferred. On the other hand, if the amount of spinel is large and the Al2O3 content exceeds 7% by mass, the corrosion resistance decreases, so it is also not preferred.

[0049] Graphite is a raw material for fixed carbon. Other materials that can be used include carbon black and pitch. Fixed carbon exists in a mixture with magnesia and spinel phases. The addition of graphite and the like improves the refractory's resistance to blasting. However, if too much fixed carbon is added, oxygen (O) in the molten steel reacts with carbon (C) in the fixed carbon and escapes as CO gas, forming holes in the sliding nozzle plate. Therefore, the content of fixed carbon is more preferably set at 1 to 5% by mass.

[0050] The inventors of this application obtained the above research results based on the laboratory experiments described below.

[0051] The following experiment was conducted: Test pieces were cut from the sliding nozzle plates of Level 1 (mainly composed of magnesia: magnesia-spinel refractories) and Levels 2 and 3 (mainly composed of alumina: alumina-zirconia-carbonaceous refractories) shown in Table 1, and immersed in molten steel containing calcium and deoxidized with silicon. The immersion time was 30 minutes, and the rotation speed of the test pieces was 300 rpm.

[0052] [Table 1]

[0053]

[0054] After the experiment, SEM observation and EPMA analysis were performed on the steel-molten steel interface of each test piece to investigate the adhesion thickness of CaO-SiO2 inclusions. The results are shown in Table 2.

[0055] [Table 2]

[0056]

[0057] As shown in Table 2, CaO-SiO2 inclusions were observed to adhere in Level 1 (mainly composed of magnesia). On the other hand, no CaO-SiO2 inclusions were observed to adhere in Levels 2 (mainly composed of alumina) and 3 (mainly composed of alumina).

[0058] It was confirmed that CaO-SiO2 inclusions in the molten steel initially adhered to the surface of the test piece. However, in test pieces made primarily of alumina, the CaO-SiO2 inclusions reacted with the Al2O3 components in the test piece, forming liquid-phase reaction products at the molten steel temperature. Therefore, the adhesion of the CaO-SiO2 inclusions ceased. In other words, it can be seen that by using an alumina-based material for the sliding nozzle plate of the ladle, the clogging of the plate outlet orifice, a problem that has been identified in continuous casting operations, can be suppressed.

[0059] The continuous casting method for molten steel involved in this invention is suitable for the continuous casting of molten steel with a composition of silicon (Si) of 1.5% or more by mass, acid-soluble aluminum (sol.Al) of 0.003% or less by mass (including zero), and total calcium (T.Ca) of 0.001% or more by mass. Here, total calcium refers to the sum of calcium dissolved in molten steel and calcium contained in inclusions in molten steel.

[0060] Furthermore, the continuous casting method for molten steel according to the present invention is more suitable for the continuous casting of molten steel with a composition of less than 0.0050% by mass of carbon, 1.5 to 5.0% by mass of silicon, less than 3.0% by mass of manganese, less than 0.2% by mass of phosphorus, less than 0.0050% by mass of sulfur, less than 0.003% by mass of acid-soluble aluminum (including zero), and less than 0.001 to 0.008% by mass of total calcium. Moreover, this molten steel is preferably molten steel that serves as a raw material for non-directional electromagnetic steel sheets.

[0061] The reasons for specifying the chemical composition of non-directional electromagnetic steel sheets in the above manner are as follows.

[0062] Carbon (C); less than 0.0050% by mass

[0063] Carbon is an element that causes magnetic aging and increases iron loss, especially when it exceeds 0.0050% by mass, where the increase in iron loss becomes significant. Therefore, the content is limited to below 0.0050% by mass, preferably below 0.0030% by mass. It should be noted that regarding the lower limit, the lower the content, the better, so there is no specific specification.

[0064] Silicon (Si); 1.5–5.0% by mass

[0065] Silicon is an effective element for increasing the resistivity of steel and reducing iron loss. In particular, in this invention, silicon is contained at least 1.5% by mass in order to reduce aluminum, which has a similar effect to silicon. However, if silicon exceeds 5.0% by mass, not only does the magnetic flux density decrease, but steel also becomes embrittled, and cracks occur during cold rolling, significantly reducing manufacturability. Therefore, the upper limit is set at 5.0% by mass.

[0066] Manganese (Mn); less than 3.0% by mass

[0067] Similar to silicon, manganese is also an effective element for increasing the resistivity of steel and reducing iron loss. On the other hand, if the manganese content exceeds 3.0% by mass, the magnetic flux density decreases, so the upper limit is set at 3.0% by mass. More preferably, it contains 0.05% by mass or more of manganese.

[0068] Acid-soluble aluminum (sol.Al); less than 0.003% by mass (inclusive)

[0069] Similar to silicon, aluminum is also an effective element for increasing the resistivity of steel and reducing iron loss. However, in this invention, for the purpose of improving texture and increasing magnetic flux density, the amount of aluminum is reduced and limited to less than 0.003% by mass, based on acid-soluble aluminum. Regarding the lower limit, the lower the amount, the better; therefore, no specific limit is specified.

[0070] Phosphorus (P); less than 0.2% by mass

[0071] Phosphorus is a useful element that significantly increases the hardness of steel even in trace amounts, and its content can be adjusted appropriately according to the required hardness. However, excessive phosphorus content can lead to a decrease in cold rollability; therefore, the upper limit for phosphorus content is set at 0.2% by mass.

[0072] Sulfur (S); less than 0.0050% by mass

[0073] Sulfur forms sulfides and inclusions, reducing manufacturability (hot-rollability) and the magnetic properties of the finished steel sheet; therefore, a lower concentration is preferred. Thus, in this invention, the upper limit can be 0.0050% by mass, but when magnetic properties are of primary concern, it is preferably set to 0.0025% by mass or less. It should be noted that lower sulfur concentrations are preferred, therefore no specific lower limit is specified.

[0074] Total calcium (T.Ca); 0.001–0.008% by mass

[0075] Calcium forms coarse sulfides (CaS) and inhibits the precipitation of fine sulfides such as MnS, thus improving grain growth and reducing iron loss. Therefore, the total calcium content is set to 0.001% by mass or higher. On the other hand, if the total calcium content exceeds 0.008% by mass, the amount of calcium sulfides and oxides increases, hindering grain growth and consequently reducing iron loss characteristics. Therefore, the upper limit of total calcium is set to 0.0080% by mass.

[0076] As explained above, according to the present invention, it is possible to simultaneously prevent inclusions from adhering to the outlet hole of the sliding nozzle plate of the ladle and prevent nozzle blockage, and to suppress melting loss of the sliding nozzle plate of the tundish and prevent inclusions from adhering to the outlet hole of the sliding nozzle plate. This enables the prevention of continuous casting interruption, the implementation of continuous casting, and the improvement of continuous casting filler number.

[0077] Example

[0078] For the sliding nozzles of the ladle and the tundish, plates of material 1 (mainly magnesia) and material 2 or 3 (mainly alumina) shown in Table 1 are used for continuous casting of calcium-added molten steel (250 tons per fill) without aluminum addition and after silicon deoxidation. The composition of the calcium-added molten steel includes less than 0.0050% by mass of carbon, 3.5 to 5.0% by mass of silicon, less than 3.0% by mass of manganese, less than 0.02% by mass of phosphorus, less than 0.0010% by mass of sulfur, less than 0.003% by mass of acid-soluble aluminum, and 0.001 to 0.003% by mass of total calcium.

[0079] To address the following situations, tests were conducted comparing the durability (filling count) of the sliding nozzles in the tundish (continuous casting) and the clogging failure rate of the sliding nozzles in the ladle: For the ladle sliding nozzles using plates made primarily of magnesia, and for the tundish sliding nozzles using plates made primarily of alumina (existing method 1); for both the ladle and tundish sliding nozzles using plates made primarily of magnesia (existing method 2); and for the ladle sliding nozzles using plates made primarily of alumina, and for the tundish sliding nozzles using plates made primarily of magnesia (the present invention). The test results are shown in Table 3. It should be noted that no difference was observed in the test results for plates made of material level 2 and material level 3; Table 3 shows the results when using plates made of material level 2.

[0080] [Table 3]

[0081]

[0082] As shown in Table 3, in the existing method 1, the sliding nozzle plate of the tundish, which is mainly composed of alumina, is melted and damaged due to calcium in the molten steel, and the continuous casting process involves two fillings. In addition, in the sliding nozzle of the ladle, blockage failures caused by the adhesion of CaO-SiO2 inclusions occurred at a rate of about 4% of the fillings.

[0083] In existing method 2, by using a highly corrosion-resistant magnesia-based material for the sliding nozzle plate of the tundish, the number of fillers in continuous casting is increased to 5. On the other hand, in the sliding nozzle of the ladle, similar to existing method 1, blockage failures caused by the adhesion of CaO-SiO2 inclusions occur at a rate of about 4% of the filler number.

[0084] In this invention, the sliding nozzle plate of the ladle is made of a material mainly composed of alumina, which is a reaction product of CaO-SiO2 inclusions in the molten steel to generate a liquid phase. This eliminates the occurrence of inclusion blockage faults in the sliding nozzle of the ladle.

[0085] In addition, in the sliding nozzle plate of the tundish, the adhesion of inclusions is suppressed by the floating and separation effect of inclusions in the ladle and tundish, and the cleaning effect brought about by blowing inactive gas into the molten steel flowing down from the plate outlet hole of the tundish. By using a plate made of magnesia as the main component, the number of fillings in continuous casting can be increased to 5.

Claims

1. A continuous casting method for steel, wherein, A continuous casting method is used to manufacture steel sheets by continuously casting molten steel that contains calcium but without added aluminum, has undergone silicon deoxidation, and contains calcium. The continuous casting method is characterized by... The molten steel is contained in a ladle equipped with a sliding nozzle consisting of a first sliding nozzle plate, the first sliding nozzle plate being formed of alumina-zirconia-carbonaceous refractory. The molten steel is poured from the ladle into an tundish equipped with a sliding nozzle consisting of a second sliding nozzle plate made of magnesia-spinel refractory. While blowing inactive gas into the molten steel flowing down from the outlet of the second sliding nozzle plate, the molten steel is injected from the tundish into the mold, thereby continuously casting the molten steel. The composition of the first sliding nozzle plate includes 70-81% Al2O3, less than 10% SiO2, 5-18% ZrO2, and 3-10% fixed carbon. The composition of the second sliding nozzle plate is 89-97% MgO and 4-7% Al2O3.

2. The continuous casting method for steel as described in claim 1, characterized in that, The molten steel contains silicon of 1.5% by mass or more, acid-soluble aluminum of 0% by mass or more and less than 0.003% by mass, and total calcium of 0.001% by mass or more.

3. The continuous casting method for steel as described in claim 2, characterized in that, The molten steel contains less than 0.0050% by mass of carbon, 1.5 to 5.0% by mass of silicon, less than 3.0% by mass of manganese, less than 0.2% by mass of phosphorus, less than 0.0050% by mass of sulfur, more than 0% by mass of acid-soluble aluminum and less than 0.003% by mass of total calcium, and 0.001 to 0.008% by mass of total calcium.

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