Continuous casting method for extra-low carbon steel

Abstract

To provide a continuous casting method for extra-low carbon steel, capable of stably casting a slab of extra-low carbon steel in which occurrence of surface defects of a rolled material caused by non-metallic oxides can be suppressed.SOLUTION: This invention relates to a continuous casting method for ultra-low carbon steel for continuously casting a slab of ultra-low carbon steel, including: injecting molten steel in a tundish 4 into a mold 30 through a molten steel supply path 20 disposed at a bottom of the tundish 4, and blowing Ar gas into an interior of the molten steel supply path 20 when injecting the molten steel, wherein the blown Ar gas has a purity of 99.999 vol.% or more and an oxygen concentration of 0.003 vol.ppm or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a continuous casting method for ultra-low carbon steel, which continuously casts slabs of ultra-low carbon steel. [Background technology]

[0002] In the continuous steel casting process, molten steel, whose composition and temperature have been adjusted in the refining process, is stored in a refractory container called a ladle and transported to a continuous casting machine where the continuous casting process is carried out. The transported molten steel is transferred to an intermediate container called a tundish, where coarse inclusions are floated and separated. After that, the molten steel is poured from the tundish into a mold of the continuous casting machine, and slabs are continuously cast. Molten steel is poured from the tundish into the mold through a molten steel supply passage (a passage formed by the inner walls of a sliding plate, a submerged nozzle, etc.) disposed at the bottom of the tundish.

[0003] Patent Documents 1 and 2 propose a technique in which the Ar gas blown into the inside of a molten steel supply passage is highly purified to efficiently suppress adhesion of high-melting-point nonmetallic inclusions to the inner surface of the molten steel supply passage, thereby preventing blockage of the molten steel supply passage and suppressing the occurrence of bubble defects. Patent Document 1 proposes a method of continuous casting while flowing Ar gas with a purity of 99.95% or more, an oxygen concentration of 2 ppm or less, and a dew point of -65°C or less into the flow path of an immersion nozzle. Patent Document 2 presents a method for continuous casting of ultra-low carbon steel slabs, characterized in that high-purity Ar gas with a purity of 99.999 vol% or more, an oxygen concentration of 0.1 volppm or less, and a dew point of -70°C or less is flowed through the flow path of an immersion nozzle. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5768773 [Patent Document 2] Patent No. 6065688 Summary of the Invention [Problem to be solved by the invention]

[0005] In continuous casting of steel, a problem may occur in that the molten steel supply passage is clogged with non-metallic oxides such as alumina. To prevent this, an inert gas (e.g., Ar gas) is blown into the molten steel supply passage. However, Ar gas injected into the molten steel supply channel leaks into the mold, and this Ar gas does not float up and is removed from the mold, but is instead entrapped in the solidified shell, which can cause bubble defects in the cast slab. On the other hand, if the flow rate of Ar gas supplied into the molten steel supply channel is reduced in order to suppress bubble defects, there is a risk that clogging of the molten steel supply channel cannot be sufficiently suppressed.

[0006] In particular, in ultra-low carbon steel with a carbon content of 0.01% by mass or less, surface defects can occur in rolled products obtained by rolling slabs obtained by continuous casting. These surface defects can be caused by non-metallic oxides such as alumina. To prevent the formation of oxides caused by the reaction of oxygen in the air with Al and Si in molten steel, it is necessary to block oxygen from entering the molten steel before it solidifies and becomes a cast slab. Blocking oxygen from the submerged entry nozzle (SEN) and the sliding plate that controls the flow rate just before the molten steel is poured into the mold is particularly important, as most of the inclusions generated by reoxidation flow into the mold.

[0007] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a method for continuous casting of ultra-low carbon steel that can stably cast a slab that can suppress the occurrence of surface defects in the rolled material caused by non-metallic oxides. Here, in this specification, the term "surface defect" refers to a linear defect that occurs on the surface of a rolled material with a width of 2 mm or less and extending over several tens of mm in the rolling direction. The "linear defect" referred to here is caused by a partial protrusion of the steel material or an inclusion being exposed on the surface of the steel material, and may occur continuously or intermittently over several tens of mm in the rolling direction. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the present inventors conducted extensive research and found that the cause of defects in ultra-low carbon steel during rolling is mainly large cluster-shaped inclusions formed by aggregation of minute oxides such as alumina. Here, we believe that trace amounts of oxygen present in Ar gas blown into the inside of the molten steel supply channel (sliding plate, submerged entry nozzle, etc.) first react with FeO, the main component of molten steel, to form liquid-phase oxides, and then react with deoxidizing elements such as Al to form solid-phase oxides such as alumina. In such cases, the liquid-phase oxide FeO acts as a binder for the solid-phase oxides generated by reaction with oxygen in the molten steel or Ar gas, promoting the formation of cluster-shaped inclusions, which then become large cluster-shaped inclusions that cause defects during rolling.

[0009] Furthermore, if the oxygen concentration in Ar gas is high anywhere on the inner surface of the molten steel supply passage (sliding plate, submerged entry nozzle, etc.) closest to the mold, FeO is generated, promoting the formation of cluster-shaped inclusions, which are then mixed into the mold, significantly affecting the cleanliness of the cast slab. However, at 1550°C when the molten steel is in a molten state, the equilibrium oxygen concentration at which FeO is generated in the molten steel is 0.003 vol ppm (oxygen partial pressure 3.0 × 10 -9 At temperatures below this level, FeO theoretically does not form and large cluster-shaped inclusions are less likely to occur.

[0010] The present invention has been made based on the above findings, and the method for continuous casting of ultra-low carbon steel, which is the invention according to claim 1 of the present application, is a method for continuous casting of ultra-low carbon steel for continuously casting a slab of ultra-low carbon steel, which includes injecting Ar gas into a molten steel supply passage disposed at the bottom of a tundish when pouring molten steel from the tundish into a mold through the molten steel supply passage, and is characterized in that the purity of the injected Ar gas is 99.999 vol% or more and the oxygen concentration is 0.003 volppm or less.

[0011] According to the method for continuous casting of ultra-low carbon steel of the invention set forth in claim 1 of the present application, the purity of the Ar gas blown into the molten steel supply passage is 99.999 vol% or more and the oxygen concentration is 0.003 volppm or less, so that FeO is not generated in the molten steel inside the molten steel supply passage and the generation of large cluster-shaped inclusions is suppressed, thereby suppressing the generation of surface defects in the rolled material.

[0012] The method for continuous casting of ultra-low carbon steel according to claim 2 of the present invention is characterized in that in the method for continuous casting of ultra-low carbon steel according to claim 1 of the present invention, the dew point of the Ar gas injected is −83°C or lower. According to the method for continuous casting of ultra-low carbon steel of the invention according to claim 2 of the present application, the dew point of the Ar gas blown into one or both of the inner surfaces of the molten steel supply passages is set to −83° C. or lower, so that the generation of FeO in the molten steel can be more reliably suppressed and the generation of large cluster-shaped inclusions can be further suppressed. [Effects of the Invention]

[0013] As described above, the present invention provides a method for continuous casting of ultra-low carbon steel that can stably cast a slab that can suppress the occurrence of surface defects in the rolled material caused by non-metallic oxides. [Brief explanation of the drawings]

[0014] [Figure 1]1 is an explanatory diagram showing an example of a continuous casting apparatus for carrying out a continuous casting method for ultra-low carbon steel according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic explanatory view of the periphery of a mold of the continuous casting machine shown in FIG. [Figure 3] FIG. 1 is a diagram illustrating a mechanism for generating cluster-shaped inclusions. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following embodiments, the metal to be cast will be described as steel. However, the present invention is not limited to the following embodiments.

[0016] In the method for continuous casting of ultra-low carbon steel according to an embodiment of the present invention, a slab made of ultra-low carbon steel having a carbon content of 0.001% by mass or less is continuously cast. First, an example of a continuous casting apparatus for carrying out a method for continuous casting of ultra-low carbon steel according to an embodiment of the present invention will be described with reference to FIGS.

[0017] In the continuous casting apparatus 1 shown in FIG. 1, molten steel is transferred from a converter using a ladle 2, and then transferred to a tundish 4 via a long nozzle 3. After large inclusions are floated and separated in the tundish 4, the molten steel is supplied into a mold 30 via a molten steel supply passage 20 disposed at the bottom of the tundish 4, and a cast piece is continuously cast. The cast strip produced from the mold 30 is pulled downward by a support roll group 40 consisting of a plurality of support rolls 41 and is curved by a bending roll 42.

[0018] As shown in FIG. 2, the continuous casting apparatus 1 includes a mold 30, an immersion nozzle 21 that discharges molten steel into the mold 30, and an electromagnetic stirring device 50 that stirs the molten steel in the upper part of the mold 30. The mold 30 in this embodiment is cylindrical and has a casting space with a rectangular cross section, and is used to cast a slab with a cross section that matches the cross section of this casting space. Furthermore, the above-mentioned submerged nozzle 21 is positioned at the center of the mold cross section in the vertical direction, and is arranged so that the openings of the discharge holes 22 (22A, 22B) face the end of the mold 30 (the inner wall farthest from the discharge holes 22 (22A, 22B)).

[0019] Furthermore, as shown in FIG. 2, a solidified shell S grows on the surface of the mold plate 31 of the mold 30. Furthermore, continuous casting powder 9 is supplied onto the molten steel stored in the mold space in order to maintain the temperature of the molten steel and ensure lubrication between the solidified shell S and the mold plate 31. The electromagnetic stirring device 50 generates a moving magnetic field in the molten steel in the mold 30 to force the molten steel to flow.

[0020] As shown in Fig. 2, the tundish 4 and the submerged entry nozzle 21 are connected via an upper nozzle 6 and a sliding plate 7, and the upper nozzle 6, the sliding plate 7, and the submerged entry nozzle 21 form a molten steel supply passage 20. The molten steel in the tundish 4 is supplied into the mold 30 through the interior of the upper nozzle 6, the interior of the sliding plate 7, and the interior of the submerged entry nozzle 21. The sliding plate is a component made of two or three stacked perforated plate-like refractory plates, and controls the flow rate of molten steel from the tundish to the mold.

[0021] Furthermore, in the continuous casting apparatus 1, an Ar gas introducing means 10 for blowing Ar gas into the interior of the molten steel supply passage 20 is disposed. 2, the Ar gas introducing means 10 is provided with a first introduction part 11 for introducing Ar gas into the upper nozzle 6 and a second introduction part 12 for introducing Ar gas into the submerged nozzle 21, and is configured so that Ar gas is blown into the molten steel supply path 20. There are no limitations on the means for introducing Ar gas into the molten steel supply path 20 as long as the effects of this embodiment are achieved.

[0022] In the method for continuous casting of ultra-low carbon steel according to this embodiment, the purity of the Ar gas blown into the molten steel supply passage 20 is set to 99.999 vol% or more, and the oxygen concentration is set to 0.003 volppm or less.

[0023] Here, there is a risk that oxides will adhere to the inner surface of the molten steel supply passage 20, which supplies molten steel from the tundish 4 to the mold 30, and cause clogging. In particular, the submerged entry nozzle 21, which is located downstream, is prone to clogging with oxides. Large cluster-shaped inclusions formed by aggregation of minute oxides such as alumina cause clogging of the molten steel supply passage 20. The size of the inclusions is, for example, about 100 μm, and may be larger than 100 μm.

[0024] When oxygen is present in the Ar gas being blown in, Fe, the main component of molten steel, reacts with the oxygen to produce FeO (liquid phase oxide), and deoxidizing elements such as Al react with oxygen to form solid phase oxides such as alumina. As shown in Figure 3, the liquid phase oxide, FeO, acts as a binder for the solid phase oxide, resulting in the formation of cluster-shaped inclusions.

[0025] Therefore, in the method for continuous casting of ultra-low carbon steel according to the present embodiment, the purity of the Ar gas blown into the inside of the molten steel supply passage 20 is set to 99.999 vol% or more, and the oxygen concentration is set to 0.003 volppm or less, thereby suppressing the generation of FeO (liquid phase oxide) that acts as a binder and suppressing the formation of cluster-shaped inclusions.

[0026] Here, the purity of the Ar gas injected into the inside of the molten steel supply passage 20 is preferably 99.999 vol % or more. The oxygen concentration of the Ar gas blown into the inside of the molten steel supply passage 20 is preferably 0.003 volppm or less, and more preferably 0.001 volppm or less.

[0027] It is preferable to set the dew point of the Ar gas blown into the inside of the molten steel supply passage 20 to −83° C. or lower in order to suppress the generation of FeO (liquid phase oxide) which acts as a binder. Here, it is more preferable that the dew point of the Ar gas blown into the inside of the molten steel supply passage 20 is −85° C. or lower.

[0028] According to the method for continuous casting of ultra-low carbon steel of this embodiment, the purity of the Ar gas blown into the inside of the molten steel supply passage 20 is 99.999 vol% or more and the oxygen concentration is 0.003 volppm or less, so that FeO is not generated in the molten steel inside the molten steel supply passage 20 and the generation of large cluster-shaped inclusions is suppressed. This makes it possible to suppress the generation of surface defects in the rolled material.

[0029] In the method for continuous casting of ultra-low carbon steel according to the present embodiment, when the dew point of the Ar gas blown into the molten steel supply passage 20 is set to −83° C. or lower, even if oxygen is generated due to the decomposition of water at high temperatures, FeO in the molten steel can be more reliably suppressed, and the generation of large cluster-shaped inclusions can be further suppressed.

[0030] The method for continuous casting of ultra-low carbon steel according to an embodiment of the present invention has been specifically described above, but the present invention is not limited to this and can be modified as appropriate within the scope of the technical concept of the invention. For example, in this embodiment, the continuous casting apparatus shown in Figs. 1 and 2 is used, but the present invention is not limited to this, and other continuous casting apparatuses may be used. [Example]

[0031] The results of experiments carried out to confirm the effects of the present invention will be described below.

[0032] A slab made of ultra-low carbon steel containing 0.005 mass% C, 0.3 mass% Si, 0.5 mass% Mn, 0.01 mass% P, 0.01 mass% S, and 0.3 mass% Al was continuously cast.

[0033] 260 tons of molten steel was refined in a converter-RH vacuum degassing device, and the molten ultra-low carbon steel containing 0.005 mass% C, 0.3 mass% Si, 0.5 mass% Mn, 0.01 mass% P, 0.01 mass% S, and 0.3 mass% Al was placed in a ladle. Molten steel was poured from a discharge hole at the bottom of the ladle via a long nozzle into a tundish located below the ladle.

[0034] Then, molten steel was fed from the tundish into the mold through a feed channel, and a 250 mm thick slab was continuously cast. At this time, Ar gas was blown into the SEN through the porous refractory material on the inner surface of the SEN, and the oxygen concentration of the Ar gas was changed depending on the conditions.More than 400 tons of molten steel was supplied to the mold per SEN.

[0035] The obtained slab was rolled to a thickness of 1 mm, and the coil surface was observed. Linear defects with inclusions were considered to be inclusion defects. 2 The number of defects per 1000m was measured under condition No. 1 in Table 1. 2 The normalized value was divided by the number of defects per unit area and evaluated as the inclusion defect number index. The evaluation results are shown in Tables 1 and 2.

[0036] [Table 1]

[0037] [Table 2]

[0038] In conditions No. 1 to No. 8, the oxygen concentration of the Ar gas was 1 volppm and the dew point was −63° C., and the inclusion defect number index was 0.90 to 1.00. In conditions No. 9 to No. 16, the oxygen concentration of the Ar gas was 0.01 volppm and the dew point was −78° C., and the inclusion defect number index was 0.87 to 0.93.

[0039] In conditions No. 17 to No. 33, the oxygen concentration of the Ar gas was 0.003 volppm and the dew point was −85 to −57° C., and the inclusion defect number index was 0.25 to 0.60. In addition, in conditions No. 34 to No. 50, the oxygen concentration of the Ar gas was 0.001 volppm and the dew point was −85 to −57° C., and the inclusion defect number index was 0.18 to 0.55. In this way, by setting the oxygen concentration of the Ar gas blown into the molten steel supply passage to 0.003 volppm or less, it is possible to significantly reduce inclusion defects in the rolled material.

[0040] From the above results, it has been confirmed that the present invention can provide a continuous casting method for ultra-low carbon steel that can stably cast a slab that can suppress the occurrence of surface defects in the rolled material caused by non-metallic oxides.

Claims

1. A method for continuously casting ultra-low carbon steel for continuously casting a slab of ultra-low carbon steel, comprising the steps of: When molten steel in the tundish is poured into a mold through a molten steel supply passage disposed at the bottom of the tundish, Ar gas is blown into the molten steel supply passage, A method for continuously casting ultra-low carbon steel, characterized in that the purity of the Ar gas to be blown in is 99.999 vol % or more and the oxygen concentration is 0.003 vol ppm or less.

2. 2. The method for continuous casting of ultra-low carbon steel according to claim 1, wherein the Ar gas to be blown has a dew point of −83° C. or lower.

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