Manufacturing method of clean steel for thin plate

Abstract

To provide a method for producing a thin sheet clean steel reduced in nonmetallic inclusions causing a pin hole defect.SOLUTION: In a method for producing a thin sheet clean steel where, after converter refining, a molten steel is tapped out into a ladle, and the molten steel in the ladle is subjected to secondary refining treatment, the addition of a Mn source into the molten steel is performed only in a converter or during the secondary refining treatment, and, upon addition of the Mn source into the molten steel during the secondary refining treatment, the Mn source addition is performed in such a manner that an oxygen potential aO in the molten steel is controlled to 8.0 mass.ppm or less. Therefore, even in a steel sheet product after rolling having a particle diameter of 50 to 60 μm, it is spherical and is not cracked, and the number ratio of spherical inclusions in which each outside essentially consists of Al2O3 and MnO-FeO-MgO is contained at the center is reduced to produce a thin sheet clean steel.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing clean steel for thin plates. [Background technology]

[0002] Thin steel sheets can be processed into steel products by various processes. In particular, thin steel sheets for can manufacturing, such as 2-piece cans and 3-piece cans, are required to have high workability. Patent Document 1 discloses a high-strength, high-ductility steel sheet for cans, which contains components such as C: 0.04-0.1%, Mn: 0.5-1.5%, and the like, and has a specified ferrite phase structure, and a manufacturing method thereof. Patent Document 2 discloses a steel sheet for 2-piece deformed cans, which contains C: 0.0020% or less, and a manufacturing method thereof. Steel sheets for can manufacturing also include those with a plating or resin film formed on the surface.

[0003] In particular, in the case of steel sheets for can manufacturing, there is a demand for steel sheets that do not crack or have defects in the surface plating or resin film during forming such as deep drawing in the can manufacturing process. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2005-336610 A [Patent Document 2] JP 2005-320633 A Summary of the Invention [Problem to be solved by the invention]

[0005] When forming two-piece cans using thin steel sheets for can-making, pinhole defects can occur on the surface of the formed can. It was found that the cause of pinhole defects is non-metallic inclusions contained in the steel. When an inclusion investigation was conducted to investigate the actual state and causes of pinhole defects, it was found that the inclusions that cause pinhole defects have a particle size of 50 to 60 μm, are spherical and not crushed even in the finished steel sheet after rolling, and contain mainly Al2O3 on the outside and MnO-FeO-MgO in the center. Inclusions of this type were confirmed in the molten steel and in the slime extraction of the cast slab.

[0006] An object of the present invention is to provide a method for producing clean steel for thin plates which contains less nonmetallic inclusions that cause the above-mentioned pinhole defects. [Means for solving the problem]

[0007] That is, the gist of the present invention is as follows. [1] A method for producing clean steel for thin plates in which molten steel is tapped into a ladle after converter refining and the molten steel in the ladle is subjected to secondary refining, The Mn source is added to the molten steel only in the converter or during the secondary refining process. When adding a Mn source to molten steel during secondary refining, the oxygen potential a O A method for producing clean steel for sheet, comprising adding a Mn source in an amount of 8.0 mass ppm or less. [2] A method for producing clean steel for sheet steel according to [1], characterized in that when a Mn source is added to molten steel during secondary refining, the t-Fe+MnO concentration in the slag components in the ladle when the Mn source is added is 8 mass% or less. [3] A method for producing clean steel for sheet steel according to [1] or [2], characterized in that when the Mn source is added to the molten steel during the secondary refining treatment, the stirring energy added to the molten steel from the completion of tapping of the molten steel into the ladle to the addition of the Mn source is 20 to 30 kJ / ton-steel. [4] A method for producing clean steel for sheet steel according to any one of [1] to [3], characterized in that an Al source for deoxidizing molten steel is added only during secondary refining treatment. [5] A method for producing clean steel for sheet steel according to any one of [1] to [4], characterized in that the number density of MnO-Al2O3-based spherical inclusions having a diameter of 50 to 60 μm in the steel is set to 20 pieces / kg or less. [6] A method for producing clean steel for sheet metal according to any one of [1] to [5], characterized in that the sheet metal is a sheet metal for can making. Effect of the Invention

[0008] According to the present invention, it is possible to provide a method for producing clean steel for thin plates which contains few nonmetallic inclusions that may cause pinhole defects. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The thin steel plate to be manufactured in the present invention contains Mn and Al as steel components. Al also functions as a strong deoxidizing element. Mn also has a weak deoxidizing function.

[0010] In the method for producing molten steel for thin steel plates that is the subject of the present invention, the molten steel is tapped into a ladle after converter refining, and the molten steel in the ladle is subjected to secondary refining treatment. In the process of producing molten steel, Mn and Al are added to the molten steel to adjust the chemical composition of the molten steel and to deoxidize the molten steel. Usually, when tapping molten steel from a converter into a ladle, an Al source and an Mn source are added to the molten steel in the ladle during tapping to deoxidize the molten steel and roughly adjust the chemical composition, and further alloy components are added in the secondary refining stage to finely adjust the steel composition.

[0011] As the method of the secondary refining treatment, mainly used are vacuum degassing treatment such as RH, DH, REDA, V-KIP, VAD, ASEA-SKF, LF (ladle furnace), submerged lance injection treatment, and simplified ladle refining (CAS, CAB, SAB, etc.). In the present invention, the RH vacuum degassing treatment can be suitably used as the secondary refining treatment.

[0012] As described above, the present invention is aimed at reducing nonmetallic inclusions that are the cause of pinhole defects that occur on the surface of a can after forming when using a thin steel sheet for can making. The nonmetallic inclusions have a particle size of 50 to 60 μm, are spherical and not crushed even in the finished steel sheet after rolling, and contain mainly Al2O3 on the outside and mainly MnO in the center. Inclusions with such properties are hereinafter referred to as "spherical inclusions". It was speculated that the spherical inclusions with such properties were MnO-based low melting point inclusions generated when the Mn source was added to the molten steel in the ladle, which were reduced by Al added to the molten steel in the ladle to generate spherical alumina.

[0013] Metallic Mn, Mn ferroalloy, or Mn ore can be used as the Mn source added to molten steel. Metallic Al is used as the Al source added. The timing of adding the Mn source was examined as either in the converter furnace before tapping or in the RH vacuum tank during RH treatment, or both, taking the case where RH vacuum degassing is used as the secondary refining process as an example. When the Mn source is added during RH treatment, evaluations were conducted for both the case where the Al source is added before the Mn source and the case where the Al source is added after the Mn source, and the effect of the time difference between the addition of the Al source and the Mn source was also examined.

[0014] As a result of the evaluation, it was confirmed that a good cleanliness level could be obtained when the Mn source was added to the molten steel only in the converter or only during the secondary refining treatment, as compared with the case where the Mn source was added to the molten steel both in the converter and during the secondary refining treatment. Therefore, in the present invention, the Mn source was added to the molten steel only in the converter or only during the secondary refining treatment.

[0015] In addition, when adding a Mn source to molten steel during secondary refining, the oxygen potential a O It was confirmed that good cleanliness could be obtained by adding the Mn source at a concentration of 8.0 mass ppm or less. By adding the Al source before the Mn source during the secondary refining process, the oxygen potential in the molten steel at the time of adding the Mn source can be reduced. Furthermore, by ensuring the time between adding the Al source (deoxidizing Al) and adding Mn, the oxygen potential a in the molten steel at the time of adding the Mn source can be reduced.O can be reliably kept to 8.0 mass ppm or less. For the molten steel after tapping, the oxygen potential in the molten steel a O It is presumed that when a Mn source is added when the Mn content is higher than 8.0 ppm by mass, the added Mn is oxidized to form large spherical MnO oxides, and then the surfaces of the spherical MnO oxides are reduced by the Al in the molten steel and replaced with Al2O3 oxides, forming spherical inclusions with a particle size of 50 to 60 μm, with the outside of the inclusions being mainly Al2O3 and the center mainly containing MnO. Such inclusions remain spherical even in the finished steel plate after rolling and are not crushed. When a conventional method of adding an Al source and a Mn source to molten steel during tapping is adopted, it is presumed that parts with high and low oxygen potentials are unevenly mixed in the molten steel in the ladle, and Mn oxidation proceeds in the parts with high oxygen potential, resulting in the formation of the above-mentioned spherical inclusions.

[0016] When the converter finishes tapping, converter slag flows into the ladle, and a slag layer (hereinafter referred to as "ladle slag") is formed on the surface of the molten steel in the ladle after tapping. The ladle slag contains low-grade oxides such as iron oxide and MnO due to the converter slag, and these low-grade oxides are a factor in reducing the cleanliness of the molten steel. Therefore, metallic Al is sprayed on the surface of the ladle after tapping, and the sprayed metallic Al reduces the low-grade oxides in the ladle slag, thereby improving the cleanliness of the molten steel. Hereinafter, the Al added to the ladle for this purpose is referred to as "modified Al addition." In the tests, the effect of adding or not adding modifier Al was also investigated.

[0017] As a result, when the Mn source is added to molten steel during the secondary refining process, the quality evaluation result is better if the t-Fe+MnO concentration of the slag components in the ladle when the Mn source is added is 8 mass% or less. Therefore, in the present invention, when the Mn source is added to molten steel during the secondary refining process, it is more preferable to set the t-Fe+MnO concentration of the slag components in the ladle when the Mn source is added to 8 mass% or less. The above-mentioned preferable slag components can be obtained by refining so as not to blow down C more than necessary during blowing in the converter, adding modified Al to the ladle slag after tapping, etc.

[0018] The cleanliness of molten steel is evaluated by taking a molten steel sample, extracting the inclusions as slime, and evaluating the number density (pieces / kg) of spherical MnO-Al2O3 inclusions of 50-60 μm. The size of 50-60 μm is evaluated by counting the inclusions classified using a sieve, the spherical shape is evaluated by observing the extracted inclusions with a stereomicroscope, and the MnO-Al2O3 system is evaluated by measuring the chemical composition of the inclusions with an electron microscope equipped with an energy dispersive spectroscopic analyzer.

[0019] In addition, at the level where the Mn source is added during secondary refining, the oxygen potential a O The subject of evaluation is O The evaluation is performed based on the measured value obtained by an oxygen sensor using a solid electrolyte.

[0020] Furthermore, the stirring energy applied to the molten steel from the completion of tapping into the ladle to the addition of the Mn source is calculated using the following formula. <Equation for mixing energy due to bottom gas injection> ε=6.18V G T l / W M {ln(1+h0 / (1.46×10 -5 P2))}·t ε: Mixing energy (J / ton-steel) V G : Gas flow rate (Nm 3 / min) Tl : Molten steel temperature (K) W M : Molten steel weight (t) h0: Gas injection depth (m) P2: Pressure inside the vacuum chamber (Pa) t: Gas injection time (s) 《RH mixing energy》 ε=(1 / 2)·(1000·Q / 60)·V 2 / W M t V = (1000·Q / 60) / (ρ L π D 2 / 4) Q: Reflux flow rate (t / min) V: Flow velocity of molten steel in the downcomer (m / s) D: Immersion tube inner diameter (m) ρ L : Molten steel density (kg / m 3 ) t: Reflux time (s)

[0021] In the case of adding a Mn source to molten steel during secondary refining, the stirring energy applied to the molten steel from the completion of tapping of the molten steel into the ladle until the addition of the Mn source was compared. As a result, it was found that the cleanliness of the steel is improved when the stirring energy is 20 kJ / ton-steel or more. On the other hand, when the stirring energy exceeds 30 kJ / ton-steel, the amount of exogenous inclusions increases due to the inclusion of ladle slag, and the quality of the product is reduced. In other words, when adding a Mn source to molten steel during secondary refining, it is preferable that the stirring energy applied to the molten steel from the completion of tapping of the molten steel into the ladle until the addition of the Mn source is 20 to 30 kJ / ton-steel.

[0022] Although the Al source for deoxidizing molten steel can be added to the ladle during tapping before the secondary refining, better evaluation results can be obtained by adding the Al source only during the secondary refining treatment. That is, it is preferable to add the Al source for deoxidizing molten steel only during the secondary refining treatment.

[0023] By applying the above-mentioned manufacturing method, the number density of MnO-Al2O3-based spherical inclusions having a diameter of 50 to 60 μm in the steel can be reduced to 20 pieces / kg or less, thereby realizing good cleanliness.

[0024] The thin plate produced by the above-mentioned method for producing clean steel for thin plate of the present invention is preferable because when used as a thin plate for can making, it can particularly improve can making performance.

[0025] There is no limitation on the composition of the steel to which the method for producing clean steel for thin plate of the present invention is applied. By applying the present invention, good clean steel can be produced from steel of any composition. In particular, the effect of the present invention can be fully exhibited in steel within the following composition range (mass%). C: 0.01 to 0.08% Silicon: 0.001 to 0.3% Mn: 0.1-0.8% Al: 0.01 to 0.6% Nb: 0-0.1% Ti: 0 to 0.1% EXAMPLES

[0026] Tests were carried out to clarify the formation mechanism of spherical inclusions and to consider countermeasures. The molten steel used in the tests had a composition in the range of 0.02-0.05%C, 0.15-0.25%Mn, 0.04-0.09%Al, and 10-20ppm(TO).

[0027] The smelting process in which the tests were conducted involved tapping molten steel into a ladle after converter refining, and then subjecting the molten steel in the ladle to RH vacuum degassing as a secondary refining process. Metallic Mn or Mn ore was used as the Mn source added to the molten steel. Metallic Al was used as the Al source added. The Mn source was added either into the converter furnace before tapping or into the RH vacuum tank during RH treatment, or both. When the Mn source was added into the converter furnace, metallic Mn or Mn ore was used, and the Mn source added during RH treatment was metallic Mn. The Al source (metallic Al) used as deoxidizing Al was added into the RH vacuum tank during RH treatment. When the Mn source was added during RH treatment, tests were conducted for both the case where the Al source was added first and the case where the Al source was added later, and the time difference between the addition of the Al source and the Mn source was adjusted in various ways.

[0028] Metallic Al is sprayed onto the surface of the ladle after the steel is tapped from the converter, and the sprayed metallic Al reduces low-grade oxides in the ladle slag, improving the cleanliness of the molten steel. In the test, the effect of adding modified Al was also investigated.

[0029] For some charges, after arriving at the RH treatment plant, argon gas was bubbled by blowing it through a porous plug installed at the bottom of the ladle.

[0030] Regarding the evaluation of the cleanliness of the molten steel, first, a molten steel sample was taken from a continuous casting tundish, and the inclusions in the taken sample were subjected to slime extraction, and the number density (pieces / kg) of spherical MnO-Al2O3-based inclusions of 50 to 60 μm was evaluated. The size of the inclusions was evaluated by counting them through classification using a sieve, the spherical shape of the inclusions was evaluated by observing them with a stereomicroscope, and the MnO-Al2O3 system was evaluated by component analysis using an electron microscope equipped with an energy dispersive spectroscopic analyzer. Secondly, the cleanliness was evaluated by eddy current inspection of the steel sheet. Specifically, the number of voltage changes per coil when product defects occurred was measured in advance by eddy current inspection of the steel sheet after cold rolling and annealing, and the pass / fail judgment was made based on this.

[0031] In addition, at the level where the Mn source is added during secondary refining, the oxygen potential a O was evaluated. O was measured by immersing an oxygen sensor using a solid electrolyte in molten steel.

[0032] Furthermore, the stirring energy applied to the molten steel from the completion of tapping into the ladle until the addition of the Mn source was calculated using the above formula.

[0033] When a Mn source was added to molten steel during the secondary refining process, the concentration of low-grade oxides in the ladle slag when the Mn source was added was evaluated as "t-Fe+MnO" (mass%).

[0034] The results are shown in Table 1. For "time from deoxidizing Al addition to Mn addition," when the value is positive, the Al source was added first, and when the value is negative, the Mn source was added first. Of the levels in which the Mn source was added during secondary refining, only level 1 added the Mn source first, and in all other levels 0, 2 to 4, and 7 to 9, the Al source was added first.

[0035] [Table 1]

[0036] In the comparative examples, levels 0, 2 to 4, the Mn source was added both in the converter and during secondary refining, and the evaluation results were poor. From this, it can be seen that in order to reduce the spherical inclusions that are the subject of the present invention, adding the Mn source both in the converter and during secondary refining should be avoided.

[0037] In the comparative example, level 1, the Mn source was added before the Al source during the secondary refining, and the evaluation result was poor.

[0038] In contrast, in levels 5 to 9 of the present invention, the Mn source is added to the molten steel only in the converter or only during the secondary refining treatment, and when the Mn source is added to the molten steel during the secondary refining treatment, the Mn source is added after the Al source is added. In these levels, when the Mn source is added to the molten steel during the secondary refining treatment, the oxygen potential a O The Mn source was added after keeping the Mn content at 8.0 mass ppm or less, and the evaluation results were good.

[0039] In addition, the oxygen potential a in molten steel when adding the Mn source to the molten steel during the secondary refining process O In the comparative example, the Mn source was added before the deoxidizing Al was added, so the oxygen potential a O The oxygen potential at the time of Mn source addition was 12.1 ppm by mass, which was a high value, while the oxygen potential at the time of Mn source addition was 8 ppm by mass or less, which was a good level, was achieved in levels 7 to 9, where the time was 3 or 5 minutes, and the oxygen potential at the time of Mn source addition was 8 ppm by mass or less. In order to keep the oxygen potential at the time of Mn source addition to 8 ppm by mass or less, it is effective to ensure the time from the addition of deoxidizing Al to the addition of Mn.

[0040] Among the invention examples of levels 5 to 9, in levels 7 to 9 in which a Mn source is added to molten steel during secondary refining, levels 8 and 9 in which the slag component "t-Fe+MnO concentration" in the ladle when the Mn source is added is 8 mass% or less, have better quality evaluation results than level 7. That is, when a Mn source is added to molten steel during secondary refining, it is more preferable to set the t-Fe+MnO concentration in the slag component in the ladle when the Mn source is added to 8 mass% or less.

[0041] Among the inventive examples, levels 7 to 9 in which a Mn source is added to molten steel during secondary refining treatment were compared in terms of the stirring energy applied to the molten steel from the completion of tapping of the molten steel into the ladle until the addition of the Mn source. As a result, level 9, in which the stirring energy was 20 kJ / ton-steel or more, showed a good evaluation result.

[0042] In the invention examples of levels 5 to 9, the Al source for deoxidizing molten steel was added only during the secondary refining treatment, and good evaluation results were obtained.

Claims

1. A method for producing clean steel for thin plates, comprising the steps of tapping molten steel into a ladle after converter refining and subjecting the molten steel in the ladle to secondary refining, The Mn source is added to the molten steel only in the converter or only during the secondary refining process; When the Mn source is added to the molten steel during the secondary refining process, the oxygen potential a Ｏ The Mn source is added so that the content is 8.0 mass ppm or less, A method for producing clean steel for thin plates, characterized in that the number density of MnO-Al 2 O 3 -based spherical inclusions having a diameter of 50 to 60 μm in the steel is set to 20 pieces / kg or less.

2. The method for producing clean steel for sheet according to claim 1, characterized in that, when the Mn source is added to the molten steel during the secondary refining treatment, the t-Fe+MnO concentration in the slag components in the ladle when the Mn source is added is 8 mass% or less.

3. 3. The method for producing clean steel for thin plate according to claim 1 or 2, characterized in that, when the Mn source is added to the molten steel during the secondary refining treatment, stirring energy applied to the molten steel from the completion of tapping of the molten steel into the ladle to the addition of the Mn source is set to 20 to 30 kJ / ton-steel.

4. 4. The method for producing clean steel for sheet according to claim 1, wherein an Al source for deoxidizing molten steel is added only during secondary refining treatment.

5. 5. The method for producing clean steel for sheet metal according to claim 1, wherein the sheet metal is a sheet metal for can manufacturing.

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