METHOD FOR CONTINUOUS CASTING OF Al DEOXIDIZED STEEL

Abstract

To provide a method for continuous casting of Al-deoxidized steel that allows for stable casting while preventing nozzle clogging.SOLUTION: A method for continuous casting of molten steel, containing, by mass%, Al: 0.010% or more and 0.100% or less and T.O: 0.0060% or less, includes: carrying out continuous casting using a submerged nozzle having an interior body containing, by mass%, CaO in an amount of 40% or more and 50% or less, as a submerged nozzle for injecting molten steel from a tundish into a mold.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for continuous casting of Al-deoxidized steel to prevent nozzle clogging. [Background technology]

[0002] Molten steel refined in a converter or vacuum treatment vessel contains a large amount of dissolved oxygen, which is generally deoxidized by Al, a strong deoxidizing element with a strong affinity for oxygen. However, deoxidization with Al produces alumina-based inclusions in the molten steel, which aggregate and coalesce to form coarse alumina clusters. These alumina clusters adhere to the inner wall of the submerged entry nozzle used to inject the steel from the tundish into the mold, causing the problem of nozzle blockage.

[0003] In order to solve such a problem of nozzle clogging, Patent Document 1 discloses a continuous casting nozzle in which a coating layer of a predetermined thickness is formed on the inner bore wall of the nozzle body and / or the discharge port, using a calcareous material containing 50 to 100% by weight of CaO. In this continuous casting nozzle, the CaO in the calcareous material coating layer reacts with Al2O3, which is a nonmetallic inclusion in the molten steel, to generate an Al2O3-CaO-based compound with a low melting point and low specific gravity, thereby removing the nonmetallic inclusions. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Unexamined Patent Publication No. 63-132755 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the continuous casting nozzle described in Patent Document 1 is used as an immersion nozzle, the following problem may occur. That is, if the humidity in the air is high when the immersion nozzle is manufactured, or if the humidity in the atmosphere is high when the immersion nozzle is stored, a slaking reaction occurs in which moisture in the outside air reacts with CaO in the calcareous material. If a slaking reaction occurs, calcium hydroxide is produced and the strength of the coating layer is weakened. This causes the coating layer to fall off during casting, resulting in breakage of the immersion nozzle and weakening the effect of preventing nozzle clogging.

[0006] In view of the above problems, an object of the present invention is to provide a method for continuous casting of Al-deoxidized steel that is capable of stably casting while preventing nozzle clogging. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention has the following configuration. (1) This is a continuous casting method for molten steel containing, by mass%, Al: 0.010% or more and 0.100% or less and TO: 0.0060% or less, characterized in that continuous casting is performed using an immersion nozzle equipped with an inner body containing, by mass%, CaO of 40% or more and less than 50% as an immersion nozzle for injecting a mold from a tundish. (2) The method for continuous casting of Al deoxidized steel according to (1) above, characterized in that the molten steel further contains, by mass%, C: 0.1000% or less, Si: 1.000% or less, Mn: 3.00% or less, P: 0.150% or less, S: 0.0500% or less, and N: 0.0040% or less, with the balance being Fe and impurities. (3) The method for continuous casting of Al-deoxidized steel according to (2) above, characterized in that the molten steel further contains, by mass%, one or more selected from the group consisting of Ti: 0.050% or less, Nb: 0.100% or less, and B: 0.0500% or less. (4) The method for continuous casting of Al deoxidized steel according to any one of (1) to (3) above, wherein the inner body of the submerged nozzle further contains, by mass%, one or more selected from the group consisting of at least one of ZrO2 and MgO: a total of 40% to 60%, SiO2: 1% to 15%, and C: 5% to 20%. Effect of the Invention

[0008] According to the present invention, it is possible to stably perform continuous casting of Al deoxidized steel while preventing nozzle clogging. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an embodiment of the present invention will be described in detail. In the following description, "%" refers to "% by mass" unless otherwise specified.

[0010] First, the inventors conducted a detailed study on the mechanism of preventing nozzle clogging by using a material containing CaO for the inner body of the SEN used for injecting from the tundish to the mold and reacting the CaO contained in the inner body with Al2O3 in the molten steel. It was found that if the inner body contains an excessive amount of CaO, the slaking reaction between the moisture in the outside air and the CaO in the calcareous material proceeds excessively while continuous casting is not in progress, reducing the effect of CaO in preventing nozzle clogging, whereas if the inner body of the SEN contains a certain amount of CaO, the reaction between the CaO in the inner body and Al2O3 in the molten steel can be sufficiently ensured.

[0011] Therefore, the inventors conducted continuous casting tests using immersion nozzles having inner bodies of various compositions, and investigated the extinguishing state of the inner body before the test and the nozzle clogging state during continuous casting. As a result, when the CaO concentration in the inner body is too low, the supply of CaO from the immersion nozzle is small, the reaction with Al2O3 in the molten steel does not progress, and nozzle clogging occurs, and when the CaO concentration is high, the extinguishing reaction progresses more and the inner body is frequently extinguished. From the results of this experiment, it was found that if the CaO concentration in the inner body is 40% or more and less than 50%, the reaction with Al2O3 in the molten steel progresses while the extinguishing reaction due to moisture in the outside air can be suppressed. In other words, by setting the CaO concentration in the inner body within the above range, it is possible to perform stable continuous casting while preventing nozzle clogging.

[0012] As described above, in the continuous casting method according to the present embodiment, the submerged nozzle used when pouring from the tundish into the mold is provided with an inner body containing 40% or more and less than 50% CaO. If the CaO content in the inner body is less than 40%, the reaction between the CaO in the inner body and the Al2O3 in the molten steel becomes insufficient, making it impossible to prevent nozzle clogging. On the other hand, if the CaO content in the inner body is 50% or more, the digestion reaction with moisture in the outside air progresses, reducing the effect of preventing nozzle clogging due to CaO, making it impossible to perform stable continuous casting.

[0013] The inner body of the submerged nozzle may contain at least one of the following components as other components. ZrO2 and MgO have the effect of maintaining the strength of the inner body, and when these components are contained in the inner body of the submerged nozzle, continuous casting can be performed more stably. However, if the total content of ZrO2 and / or MgO is less than 40%, the effect of maintaining the strength of the inner body may be insufficient. If the total content of ZrO2 and / or MgO exceeds 60%, the CaO concentration cannot be set within the above range. Furthermore, the amount of simple ZrO2 and MgO increases on the inner surface of the submerged nozzle, the fluidity of the Al2O3-CaO-based compound decreases, and even if a liquid phase exists, it may be difficult to wash it away by the molten steel flow. For the above reasons, it is preferable that the inner body of the submerged nozzle contains at least one of ZrO2 and MgO in a total content of 40% to 60%.

[0014] The inner body of the submerged nozzle may contain SiO2. SiO2 has the effect of lowering the melting point of the reaction product of the CaO component and Al2O3 in the molten steel, and when SiO2 is contained in the inner body of the submerged nozzle, nozzle clogging can be more effectively prevented. However, if the SiO2 content is less than 1%, this effect is not achieved. If the SiO2 content exceeds 15%, SiO2 may react with sol. Al in the molten steel or with C in the inner body, which may cause contamination of the molten steel. For these reasons, it is preferable that the inner body of the submerged nozzle contains SiO2 at 1% to 15%.

[0015] The inner body of the submerged nozzle may contain C. C has the effect of increasing the thermal shock resistance of the inner body, and when C is contained in the inner body of the submerged nozzle, continuous casting can be performed more stably. However, if C is less than 5%, the thermal shock resistance is insufficient and the effect cannot be obtained. If C exceeds 20%, the melting damage resistance decreases and it may not be able to withstand long-term casting. For these reasons, it is preferable that the inner body of the submerged nozzle contains C at 5% to 20%.

[0016] Next, the components of the molten steel to be continuously cast will be described. The molten steel of interest in this embodiment is Al-deoxidized steel, and the casting of Al-deoxidized steel is carried out in the following procedure. First, metallic Al or an Al-containing alloy is added to the molten steel to react with the dissolved oxygen in the molten steel and deoxidize it. This produces nonmetallic inclusions mainly composed of Al2O3 in the molten steel. Then, the deoxidized molten steel is poured from the ladle into a tundish through a long nozzle, and further poured from the tundish into a mold through a submerged nozzle equipped with the above-mentioned inner body, thereby performing continuous casting and producing a cast slab.

[0017] As described above, in Al-deoxidized steel, metallic Al or an alloy containing Al is added, and the Al content in the molten steel to be continuously cast is set to 0.010% to 0.100%. In this embodiment, since Al-deoxidized steel is used as a premise, the lower limit of Al in the molten steel is set to 0.010%. On the other hand, if the Al content in the molten steel exceeds 0.100%, the amount of Al2O3 produced in the molten steel increases, and clogging of the nozzle cannot be prevented even in a submerged nozzle equipped with an inner body having the above-mentioned composition.

[0018] Furthermore, the TO content in the molten steel is set to 0.0060% or less. If the TO content in the molten steel exceeds 0.0060%, the proportion of sol. Al in the molten steel decreases, and the amount of Al2O3 produced increases, so that even if the submerged entry nozzle is equipped with an inner body having the above-mentioned composition, it becomes impossible to prevent nozzle clogging. Although there is no particular lower limit for TO in the molten steel, TO in the molten steel is substantially set to more than 0% because TO adsorbs O from the atmosphere.

[0019] It is also preferable that the other components of the molten steel are in the following ranges: In addition to Al and TO, the molten steel preferably contains C: 0.1000% or less, Si: 1.000% or less, Mn: 3.00% or less, P: 0.150% or less, S: 0.0500% or less, N: 0.0040% or less, with the balance being Fe and impurities. The details of each component are as follows:

[0020] C is preferably 0.1000% or less in order to be applied to a steel plate with high workability. Si has the effect of strengthening the steel, and an appropriate amount is added according to the required strength, but if it exceeds 1.000%, deep drawability decreases, so it is preferably 1.000% or less. Mn also has the effect of strengthening the steel, and an appropriate amount is added according to the required strength, but if it exceeds 3.00%, deep drawability decreases, so it is preferably 3.00% or less. The lower limits of these components are not particularly limited, but are to be appropriately adjusted depending on the target steel type.

[0021] P also has the effect of strengthening steel, and is added in an appropriate amount according to the required strength, but if it exceeds 0.150%, deep drawability decreases, so it is preferable to keep it at 0.150% or less. If it exceeds 0.0500%, S will cause defects during rolling, so it is preferable to keep it at 0.0500% or less. If it exceeds 0.0040%, N will decrease workability, so it is preferable to keep it at 0.0040% or less.

[0022] Furthermore, if necessary, one or more elements selected from the group consisting of Ti: 0.050% or less, Nb: 0.100% or less, and B: 0.0500% or less may be contained.

[0023] Ti is a deoxidizing element and is also effective in refining the grains of steel sheets. However, if it exceeds 0.050%, it generates oxides due to the deoxidizing effect, and forms a complex oxide with Al2O3, which causes surface defects, so it is preferable to keep it at 0.050% or less. Nb is an element that is effective in refining the grains of steel sheets and is effective in improving the deep drawability of thin steel sheets. However, if it exceeds 0.100%, it may cause a problem that the deformation resistance of steel increases significantly, so it is preferable to keep it at 0.100% or less. B has the effect of preventing secondary processing embrittlement when deep drawing or the like is performed. However, if it exceeds 0.0500%, it may cause a problem that the deformation resistance of steel increases significantly, so it is preferable to keep it at 0.0500% or less. EXAMPLES

[0024] Next, an embodiment of the present invention will be described, but the conditions are merely examples of conditions for confirming the feasibility and effects of the present invention, and the present invention is not limited to the description of the embodiment. The present invention can be implemented in various ways to achieve the object of the present invention without departing from the gist of the present invention.

[0025] First, a ladle containing 300 tons of molten steel after the converter was placed in an RH vacuum degassing apparatus and decarburized, and further deoxidized by adding metallic Al to the molten steel. Then, metallic Ti, Fe-Nb alloy, or the like was added to the deoxidized molten steel as necessary to adjust the composition. The composition of the molten steel used in the experiment was as shown in Table 1 below. This process was performed five times for each example, and a total of 1500 tons of molten steel was cast using a two-strand slab continuous casting apparatus to produce a slab with a thickness of 240 mm. During continuous casting, a submerged nozzle having an inner body with the composition shown in Table 1 below was used.

[0026] As for the evaluation method, the status of the slaking reaction caused by outside air in the submerged entry nozzle was checked by visually observing the inner body before use to see if slaking had occurred, and if the area ratio of the slaking part was 20% or less, it was considered that there was no slaking, and it was evaluated that stable continuous casting could be performed. In addition, as for the clogging status of the submerged entry nozzle, if the opening degree of the sliding gate during continuous casting exceeded a threshold value, it was considered that nozzle clogging had occurred, and the occurrence of nozzle clogging was confirmed. The status of nozzle clogging and the status of slaking occurring in the inner body are shown in Table 1.

[0027] [Table 1]

[0028] The underlined parts in Table 1 indicate that the conditions of the present invention were not met. In all of the present invention examples 1 to 12, the change in the opening degree of the sliding gate during continuous casting was small, and almost no nozzle clogging was observed. Furthermore, when the submerged entry nozzle was observed before casting, almost no digestion was observed on the surface of the inner body of the submerged entry nozzle.

[0029] On the other hand, in Comparative Examples 13 and 14, continuous casting was performed using an immersion nozzle in which the CaO concentration of the inner body was less than 40%. In both cases, the change in the opening degree of the sliding gate during continuous casting was very large. In addition, when the immersion nozzle was observed after the completion of continuous casting, adhesion of Al2O3 to the immersion nozzle was observed, and nozzle clogging had occurred.

[0030] In both Comparative Examples 15 and 16, continuous casting was performed using an immersion nozzle whose inner body had a CaO concentration of 50% or more. In both cases, the change in the opening degree of the sliding gate during continuous casting was small, and nozzle clogging was hardly observed. However, when the immersion nozzle was observed before casting, significant digestion was observed on the surface of the inner body.

[0031] Comparative Example 17 is an example in which continuous casting was performed with molten steel having a TO content of more than 0.0060%. Since the TO content in the molten steel was too high and a large amount of Al2O3 was generated in the molten steel, the change in the sliding gate opening during continuous casting was very large, and significant nozzle clogging occurred.

[0032] Comparative Example 18 is an example in which continuous casting was performed with molten steel containing more than 0.100% Al. The Al concentration in the molten steel was too high, resulting in a large amount of Al2O3 being generated in the molten steel. As a result, the change in the sliding gate opening during continuous casting was very large, and significant nozzle clogging occurred.

Claims

1. A continuous casting method for molten steel containing, by mass%, Al: 0.010% or more and 0.100% or less and T.O: 0.0060% or less, characterized in that continuous casting is performed using an immersion nozzle equipped with an inner body containing, by mass%, CaO of 40% or more and less than 50% as an immersion nozzle for injecting a mold from a tundish.

2. 2. The method for continuous casting of Al-deoxidized steel according to claim 1, characterized in that the molten steel further contains, by mass%, C: 0.1000% or less, Si: 1.000% or less, Mn: 3.00% or less, P: 0.150% or less, S: 0.0500% or less, and N: 0.0040% or less, with the balance being Fe and impurities.

3. The method for continuous casting of Al-deoxidized steel according to claim 2, characterized in that the molten steel further contains, by mass%, one or more elements selected from the group consisting of Ti: 0.050% or less, Nb: 0.100% or less, and B: 0.0500% or less.

4. The inner body of the submerged nozzle further comprises, in mass%, ZrO 2 and at least one of MgO: 40% or more and 60% or less in total, SiO 2 4. The method for continuous casting of Al deoxidized steel according to claim 1, further comprising the step of: adding at least one selected from the group consisting of: Mn: 1% or more and 15% or less; and C: 5% or more and 20% or less.

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