High-purity steel melting method

Abstract

To provide a high cleanliness steel melting method capable of more efficiently removing inclusions present in a molten steel upon degassing treatment by an RH type vacuum degassing device.SOLUTION: In a high cleanliness steel melting method in which, in an RH type vacuum degassing device, an immersion tube of a vacuum tank is immersed in a molten steel to decompress the inside of the vacuum tank, a reflux gas is made to flow from the immersion tube to reflux the molten steel and to execute degassing treatment, next, metallic Al is added during the degassing treatment, and oxygen is blown from a top-blown lance provided in the vacuum tank after the addition of the metallic Al. An oxygen feed rate upon the oxygen blow is controlled within a range of 7.5 to 10 Nm3 / hr / t and an oxygen feed amount is controlled within a range of 0.7 to 1.2 Nm3.t, and a reflux time of the molten steel after the blow end of the oxygen is controlled within a prescribed range.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] In particular, the present invention relates to a method for producing high-cleanliness steel by adding Al during degassing in the secondary refining step and blowing oxygen from a top lance to remove inclusions. [Background technology]

[0002] Coarse inclusions in steel significantly degrade the properties of steel materials, regardless of the steel type. Therefore, when producing many types of steel, coarse inclusions are removed from molten steel in secondary refining processes. Patent Document 1 discloses a method for refining molten steel at elevated temperatures, which combines CaO powder top blowing with oxygen supply to convert inclusions formed at the hot point into CaO-Al2O3, facilitating their floating and removal into slag. Patent Document 2 also discloses a method for adding Al to molten steel at a specific timing during oxygen supply, rather than before, thereby maintaining a low Al concentration in the molten steel during oxygen supply and lowering the melting point of the formed inclusions, thereby facilitating their floating and removal. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3267177 [Patent Document 2] Patent No. 5131827 [Non-patent literature]

[0004] [Non-Patent Document 1] Kuwahara et al.: Iron and Steel, Vol. 73 (1987) p. S176 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, there has been a demand for producing high-purity steel from which coarse inclusions have been further removed. In the method described in Patent Document 1, the conditions for oxygen supply and the conditions after oxygen supply are unclear, the inclusion removal effect by oxygen supply is insufficient, and the use of CaO powder is costly. In addition, the method described in Patent Document 2, which aims to produce low-Al steel, oxidizes a small amount of Al, and therefore is insufficient in the inclusion removal effect.

[0006] In view of the above-mentioned problems, an object of the present invention is to provide a method for producing high-cleanliness steel that can more efficiently remove inclusions present in molten steel when performing degassing treatment in an RH-type vacuum degassing apparatus. [Means for solving the problem]

[0007] The inventors conducted extensive research into a method for degassing molten steel using an RH-type vacuum degasser. During the degassing process, metallic Al was added to the molten steel, and oxygen was blown onto the molten steel from a top-blowing lance in a vacuum vessel to generate Al2O3, which then aggregated and coalesced inclusions and facilitated their flotation and removal. While the circulating flow of molten steel in an RH vacuum degasser causes inclusions to aggregate and coalesce, the larger the inclusion diameter, the greater the buoyancy acting on the inclusions, making them more likely to float to the surface of the molten steel and facilitate their removal from the molten steel. However, the TO concentration in Al-killed steel before degassing is several tens of ppm, which reduces the frequency of aggregation and coalescence, and inclusions of a certain size or larger tend to remain in the steel. In contrast, top-blowing oxygen oxidizes oxygen on the order of several hundred ppm to generate a large amount of Al2O3, significantly increasing the frequency of inclusion aggregation and coalescence. This allows inclusions that cannot be completely removed by conventional processes to be efficiently floated and removed.

[0008] The present inventors have therefore discovered that there are optimal conditions for generating a large amount of Al2O3, allowing it to agglomerate and coalesce with inclusions present in steel, thereby efficiently removing the inclusions, and have arrived at the present invention.

[0009] The present invention is as follows. (1) In the RH type vacuum degassing equipment, the immersion tube of the vacuum chamber is immersed in molten steel to Vacuum it A method for producing high cleanliness steel, comprising the steps of reducing pressure, flowing a reflux gas from the immersion tube to reflux the molten steel, thereby carrying out a degassing treatment, adding metallic Al during the degassing treatment, and after adding metallic Al, blowing oxygen from a top lance provided in the vacuum vessel, The oxygen blowing speed is 7.5 to 10 Nm 3 / hr / t, oxygen supply rate 0.7~1.2Nm 3 / t, and the molten steel reflux time t after the end of the oxygen blowing is after A method for producing high-cleanliness steel, characterized in that: 1.8 W / Q≦t after ≦4.0 W / Q (1) Q=11.4 G 1 / 3 D 4 / 3 ·{ln(P1 / P0)} 1 / 3 ···(2) where W is the mass of molten steel (t), Q is the molten steel circulation rate (t / min), G is the circulation gas flow rate (Nl / min), D is the inner diameter of the immersion tube (m), P1 is the pressure at the gas injection position (Pa), and P0 is the pressure inside the vacuum vessel (Pa). (2) The molten steel contains, in mass%, C:0.05~0.20%、 Si: 0.1 to 0.5%, and Mn: 0.2 to 0.6% The method for producing a high-cleanliness steel according to (1) above, comprising: [Effects of the Invention]

[0010] According to the present invention, inclusions present in molten steel can be removed more efficiently when degassing is performed in an RH-type vacuum degassing apparatus. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 10 is a diagram for explaining a preferred range in the relationship between the oxygen supply amount and the oxygen supply rate in OB treatment. [Figure 2] FIG. 10 is a diagram for explaining a suitable range of the reflux time after OB treatment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described in detail. First, a specific procedure for generating a large amount of Al2O3 and removing inclusions will be described.

[0013] In this embodiment, degassing is performed using an RH-type vacuum degassing apparatus. Specifically, the immersion tube of the vacuum vessel is immersed in the molten steel in a ladle, the vacuum vessel is then depressurized, and the molten steel is drawn into the vacuum vessel. A reflux gas is then circulated through the immersion tube to perform the degassing process. Then, at any timing during the degassing process, metallic Al is added to the molten steel in an amount corresponding to the oxygen supply rate. Oxygen is then blown from a top lance in the vacuum vessel while the molten steel is still circulating, performing oxygen blowing (OB) processing. The blowing of oxygen oxidizes the Al in the molten steel, generating Al2O3, which floats up and separates as coarse inclusions. At this time, the reflux of the molten steel continues within the range described below even after the OB processing is completed.

[0014] Note that, assuming typical molten steel compositions, the inclusions contained in the molten steel before the addition of metallic Al are generally inclusions close to a single phase of Al2O3, but depending on the composition specifications and process, they may have a low melting point composition containing CaO, SiO2, TiO2, etc. In this embodiment, these inclusions are agglomerated with Al2O3 produced by the OB treatment and float to be removed as an agglomerated and coalesced inclusion.

[0015] Next, the conditions for oxygen blowing during OB treatment and the post-reflux time after OB treatment will be described in detail. Figure 1 is a diagram showing the relationship between the amount of oxygen sent and the oxygen sending rate. In Figure 1, as a result of the experiment, the circle marks indicate conditions under which cleanliness was improved (more inclusions were removed from the steel) compared to when OB treatment was not performed, and the cross marks indicate conditions under which cleanliness was worsened (more inclusions remained in the steel) compared to when OB treatment was not performed.

[0016] From the above results, the oxygen flow rate from the top lance in OB treatment is 7.5 to 10 Nm 3 / hr / t. The oxygen supply rate is 7.5Nm 3 If the oxygen jet blow rate is less than 10 Nm / hr / t, the oxygen jet will blow softly, reducing the efficiency of oxygen deposition and weakening the stirring at the hot point where the oxygen jet collides with the molten steel surface. This will cause oxidation of not only Al but also Si and Mn contained in the molten steel, reducing the effectiveness of removing inclusions. On the other hand, if the oxygen blow rate is 10 Nm / hr / t, the oxygen jet will blow softly, reducing the stirring at the hot point where the oxygen jet collides with the molten steel surface. This will cause oxidation of not only Al but also Si and Mn contained in the molten steel, reducing the effectiveness of removing inclusions. 3 If the flow rate is greater than 1 / hr / t, the TO concentration in the steel becomes too high, and the rate of formation of the inclusions becomes excessive compared to the rate at which the inclusions float up. As a result, the rate at which the inclusions float up and separate during the reflux cannot keep up, resulting in a deterioration in cleanliness.

[0017] In addition, from the results shown in Figure 1, the oxygen supply rate in OB treatment was 0.7 to 1.2 Nm 3 / t. Oxygen supply rate is 0.7Nm 3 If the oxygen supply rate is less than 1.2 Nm / t, the amount of Al2O3 produced by the OB treatment is too small, and the effect of promoting the floating of inclusions by aggregation and coalescence cannot be obtained. 3 If the amount of metallic Al added is greater than 1 / t, the amount of metallic Al added during degassing treatment will also increase because it is determined by the oxygen supply rate, and as a result, the floating and separation of inclusions during the reflux process will not be able to keep up, resulting in a deterioration in cleanliness.

[0018] Next, the post-return time after OB treatment will be explained. The Al2O3 produced by OB treatment must be removed by floating separation in the post-return flow after OB treatment. Here, since the return conditions vary depending on the equipment specifications, the conditions must be standardized using the molten steel recirculation flow rate calculated using the formula by Kuwahara et al. described in Non-Patent Document 1. Figure 2 is a diagram for explaining the preferred range of the return time after OB treatment. In Figure 2, the horizontal axis represents the post-return time (min) after OB treatment, and the vertical axis represents the ratio of the number of inclusions in the molten steel to that in the case where OB treatment is not performed. In other words, a value on the vertical axis of less than 1 indicates that the inclusions were effectively removed. Experimental results confirmed that there is a preferred range for the post-return time after OB treatment.

[0019] From the above results, the post-circulation time t after (min) satisfies the following conditions (1) and (2). 1.8 W / Q≦t after ≦4.0 W / Q (1) Q=11.4 G 1 / 3 D 4 / 3 ·{ln(P1 / P0)} 1 / 3 ···(2) In equation (1), W represents the mass of molten steel (t), Q represents the molten steel circulation rate (t / min), G represents the circulation gas flow rate (Nl / min), D represents the inner diameter of the immersion tube (m), P1 represents the pressure at the gas injection position (Pa), and P0 represents the pressure inside the vacuum vessel (Pa).

[0020] Post-circulation time t after OB treatment after If the reflux time t (min) is shorter than 1.8W / Q, the Al2O3 generated during the OB treatment cannot be completely removed, and inclusions remain in the molten steel. after If the time (min) is longer than 4.0 W / Q, Al continues to burn and re-oxidation causes the Al2O3 content to increase, deteriorating cleanliness. Here, re-oxidation refers to a reaction in which the slag in the upper part of the ladle, which contains lower oxides such as FeO and MnO originating from the slag flowing out of the converter, reacts with the Al in the molten steel, and the reduction of the lower oxides causes Al2O3 inclusions to be continuously supplied to the molten steel.

[0021] The number of inclusions in molten steel can be evaluated by measuring the number of inclusions over 10 μm in size per unit area using an electron microscope. Regarding whether or not an improvement in cleanliness has been achieved, specifically, samples are taken from the same steel type to be tested after degassing treatment with and without top oxygen blowing, and the number of inclusions over 10 μm in size is counted. If the number is lower than the number of inclusions without top oxygen blowing, it can be determined that an improvement in cleanliness has been achieved.

[0022] As described above, by setting the oxygen supply rate and oxygen supply amount during OB treatment within the above ranges and the reflux time after OB treatment within the above ranges, the Al2O3 produced during OB treatment aggregates and floats up to separate inclusions that do not easily float to the surface of molten steel, thereby further improving the cleanliness of molten steel. Furthermore, because the inclusions in molten steel float up and are removed as aggregates regardless of their composition, stable purification is possible for a wide range of steel types. [Example]

[0023] Next, an example 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 this example. 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.

[0024] A 320-ton molten steel sample was tapped into a ladle and alloys were added to adjust the composition to 0.05-0.20 mass% C, 0.1-0.5 mass% Si, 0.2-0.6 mass% Mn, 0.01-0.03 mass% P, 0.001-0.005 mass% S, and 0.01-0.05 mass% Al. The RH-type vacuum degasser submerged pipe (inner diameter D = 0.6 m) was then immersed in the molten steel from the ladle, and the vacuum chamber was evacuated to draw a vacuum, suctioning the molten steel into the vacuum chamber. Argon gas was then introduced into the molten steel at a reflux gas flow rate G (Nl / min) shown in Table 1 below. The degassing process was initiated while circulating the molten steel. The pressure inside the vacuum chamber, P0, was 6.67 kPa, and the pressure at the gas injection point, P1, was 100 kPa.

[0025] Next, at any timing during the degassing treatment, metallic Al was added to the molten steel in an amount corresponding to the oxygen supply rate shown in Table 1. After all the necessary metallic Al was added, oxygen was blown from the top lance installed in the vacuum vessel, and OB treatment was performed at the oxygen supply rate and oxygen supply rate shown in Table 1. After the OB treatment was completed, the post-reflux time t afterThe molten steel was continued to flow for 10 seconds, and the degassing treatment was completed. At this point, a sample of the molten steel after the degassing treatment was taken and the number of inclusions was examined using an electron microscope. At the same time, an experiment was also conducted using the same steel type as the test target without top oxygen blowing, and the number of inclusions in the molten steel sample after the degassing treatment was also examined. If the number of inclusions was lower than that without top oxygen blowing (the ratio of the number of inclusions without top oxygen blowing was less than 1), it was determined that the invention was effective.

[0026] [Table 1]

[0027] The underlined parts in the table indicate values ​​outside the scope of the present invention. For Ch. Nos. 1 to 3, the oxygen flow rate during OB treatment was 7.5 to 10 Nm 3 / hr / t range, and oxygen supply rate is 0.7~1.2Nm 3 / t, and the post-recirculation time satisfied the conditions of equations (1) and (2), confirming the improvement in cleanliness.

[0028] On the other hand, in Channel No. 4, the oxygen flow rate was too low, resulting in a soft blow of oxygen, which resulted in oxidation of Si and Mn in addition to Al. This reduced the effectiveness of inclusion removal. In Channel No. 5, the oxygen flow rate was too high, resulting in the inclusion formation rate exceeding the rate at which the inclusions rose, resulting in low cleanliness.

[0029] In Channel No. 6, the amount of oxygen supplied was too small, so the effect of promoting the floating of inclusions through aggregation and coalescence was not sufficient.In Channel No. 7, the amount of oxygen supplied was too large, so a large amount of Al2O3 was generated, and the inclusions could not be completely removed during the reflux, resulting in low cleanliness.

[0030] In Ch. No. 8, the post-reflux time after OB treatment was too short, which meant that the Al2O3 formed during OB treatment could not be sufficiently removed, resulting in poor cleanliness. In Ch. No. 9, the post-reflux time after oxygen supply was too long, which resulted in excessive reoxidation of Al in the molten steel, resulting in poor cleanliness.

Claims

1. A method for producing high-cleanliness steel in an RH type vacuum degassing apparatus, comprising the steps of: immersing an immersion tube of a vacuum vessel in molten steel, drawing a vacuum inside the vacuum vessel to reduce pressure; flowing a reflux gas through the immersion tube to reflux the molten steel, thereby carrying out a degassing treatment; adding metallic Al during the degassing treatment; and blowing oxygen from a top lance provided in the vacuum vessel after the metallic Al has been added, The oxygen blowing speed is 7.5 to 10 Nm 3 / hr / t, oxygen supply rate 0.7 to 1.2 Nm 3 / t, and the reflux time t of the molten steel after the end of the oxygen blowing is after a method for producing a high-cleanliness steel, characterized in that: 1.8・W / Q≦t after ≦4.0・W / Q ・・・(1) Q=11.4・G 1 / 3 ・D 4 / 3 ・{ln(P 1 / P 0 )} 1 / 3 ・・・(2) where W is the mass of molten steel (t), Q is the amount of molten steel circulated (t / min), G is the amount of circulating gas (Nl / min), D is the inner diameter of the immersion pipe (m), and P 1 is the pressure at the gas injection position (Pa), P 0 represents the pressure inside the vacuum chamber (Pa).

2. The molten steel comprises, in mass %, C: 0.05-0.20%, Si: 0.1 to 0.5%, and Mn: 0.2-0.6% 2. The method for producing a high-cleanliness steel according to claim 1, further comprising the steps of:

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