JP2007277647A - Method for refining extra-low sulfur high cleanliness steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently and stably refining an extra-low sulfur high cleanliness steel with which the desulfurization and the cleanliness are simultaneously promoted by suiting the slag component composition, and the heat-raising treatment and the stirring treatment to molten steel. <P>SOLUTION: The method for refining the extra-low sulfur high cleanliness steel, is to treat the molten steel with the following processes 1-4. Process 1: the process, in which CaO base flux is added into the molten steel in a ladle under the atmosphere, Process 2: the process, in which after the process 1, stirring gas is blown into the molten steel in the ladle and stirred and also, oxidizing gas is supplied into the molten steel and the generated oxide is mixed with the CaO-base flux, Process 3: the process, in which the supply of the oxidizing gas is stopped and desulfurization and inclusion-removal are performed by blowing stirring gas into the molten steel in the ladle, Process 4: the process, in which after the process 3, when the molten steel is treated by using a RH vacuum-degassing apparatus, the molten steel temperature is raised by supplying the oxidizing gas into the RH vacuum vessel. In the above refining method, in the process 1 or the process 2, Al can be added and the process 4 can be omitted. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for melting ultra-low sulfur steel having excellent cleanliness. Specifically, desulfurization and inclusion removal are performed by adding CaO-based flux, gas stirring of molten steel, and supply of oxidizing gas. The present invention relates to a method of melting an ultra-low sulfur high-clean steel having an S content of 10 ppm or less and a low total oxygen content.

In recent years, strict and high performance and characteristics are required for steel materials, and the steel has a high cleanliness and the sulfur (S) content in the steel is, for example, 10 ppm or less (hereinafter referred to as “high-sulfur steel”). There is a growing need for use of “clean ultra-low sulfur steel”. In the case of melting extremely low-sulfur steel, it is common to add a desulfurizing agent such as CaO, Na ₂ CO ₃ , or metal Mg in the pre-treatment step for melting and desulfurize it. However, the hot metal desulfurized in the hot metal pretreatment process is contaminated with sulfur by intrusion of a sulfur source from scrap and solvent in the next converter blowing process and by sulfurization from residual slag in the hot metal pretreatment process. It is extremely difficult to produce extremely low-sulfur steel only with hot metal pretreatment. Therefore, when extremely low sulfur steel is melted, the molten steel desulfurization process is performed in the secondary refining process after the converter steel, depending on the level of S content required for the product.

  When performing molten steel desulfurization treatment in the secondary refining process and producing high cleanliness ultra low sulfur steel, it is extremely important to compensate for the temperature drop of the molten steel during the treatment, to ensure the desulfurization ability and to ensure the cleanliness of the molten steel. It is.

There are electric arc heating method, electromagnetic induction heating method and the like for compensation of the temperature drop, but since electric energy is used, the heat-up energy is expensive, which is economically disadvantageous. On the other hand, a method of adding oxygen by adding Al (hereinafter also referred to as “Al heating method”) may be used because it is less expensive than a heating method using electrical energy, There is a disadvantage that the production of Al ₂ O ₃ or FeO causes an increase in inclusions. As a countermeasure, for example, a heating method as disclosed in Patent Document 1 in which a conical dip tube or the like is immersed in a ladle molten steel and Al is heated inside the dip tube is implemented. However, in this method, since the stirring strength by the stirring gas blown through the porous nozzle is weak, there is a problem that it takes a long time to equalize the temperature of the molten steel in the ladle. In addition, when the Al temperature raising method is used in the desulfurization treatment, the Al ₂ O ₃ concentration in the slag is increased and the sulfide capacity (desulfurization ability) of the slag is lowered. It was a technique used after allowing Thus, since extremely low sulfidation by using slag is difficult, in the example of the document, mixing of the generated Al ₂ O ₃ and slag is eliminated as much as possible, and for the desulfurization treatment, A method of blowing desulfurization flux is shown.

A method for ensuring desulfurization ability has also been proposed. Patent Document 2 discloses a calcium aluminate-based synthetic desulfurization agent that regulates each component composition and melting point range of CaO, MgO, Al ₂ O ₃ , and SiO ₂ and ensures fluidity. However, since a synthetic desulfurizing agent needs to be adjusted to a predetermined component composition in advance, a process for premelting or premixing the raw material is necessary, which increases the desulfurization cost and impairs the economy. Further, although the desulfurization agent disclosed in the same document is an invention based on the knowledge that the presence of an appropriate amount of Al ₂ O ₃ in the presence of CaO is advantageous for promoting desulfurization, It was necessary to add a large amount, and further, there was a problem that the desulfurization treatment time was long.

Patent Document 3 discloses iron oxide as T.I. A desulfurizing agent containing 0.5 to 6% of Fe and having optimized values of CaO / (CaO + Al ₂ O ₃ + M _X O _y ) and Al ₂ O ₃ / (CaO + Al ₂ O ₃ + M _X O _y ) It is disclosed. Moreover, this desulfurizing agent is said to exhibit a remarkable melting acceleration effect and improve the desulfurization rate by containing iron oxide. However, since this desulfurizing agent contains iron oxide, heat loss due to addition to molten steel is large, and as shown in the Examples of the same document, the S content in molten steel is reduced only to about 20 to 30 ppm. Therefore, it cannot meet the recent quality requirements for which the S content in steel is required to be 10 ppm or less.

Further, Patent Document 4 discloses a method for producing ultra-low sulfur and ultra-low oxygen steel by depressurizing and vacuum desulfurizing and refining the inside of a cylindrical dip tube immersed in ladle molten steel. A method is disclosed in which a flux composed of quicklime, alumina, and meteorite is added to a pan, and then the molten steel is gas stirred to adjust the slag component composition to an appropriate range. This method uses a ladle slag having a high absorption capacity for Al ₂ O ₃ and an excellent desulfurization capacity to melt high clean steel only by gas stirring. In this method, the only means of actively melting the flux to utilize slag is the use of meteorite. In addition, the melting acceleration method using meteorite not only requires a long time for mixing slag, but also restricts operations due to fluorine emission regulations derived from recent environmental problems. Therefore, it is necessary to develop a smelting method that can produce ultra-low sulfur ultra-low oxygen steel without using meteorite.

In Patent Document 5, molten steel decarburized and refined in a converter is received in a ladle, CaO and Al ₂ O ₃ are charged according to the S content in the steel, and the ladle is vacuumed. A method is disclosed in which an inert gas is blown from the bottom to stir slag and metal to melt ultra-low sulfur steel. Here, there is further disclosed a method in which metal Al is introduced into molten steel in place of charged Al ₂ O ₃ and is burned by supplying oxygen to be used for heating the molten steel. However, this method requires strong stirring of slag under vacuum refining. Moreover, Al is introduced to maintain the temperature of the molten steel, and no suggestion or consideration is given to the slagging promoting action of CaO based on the optimization of the oxidation reaction by Al. Accordingly, there is no specific disclosure about a method for raising the temperature by sending acid in a vacuum.

Patent Document 6 discloses a method for refining the molten steel by supplying an oxidizing gas to the molten steel while stirring the molten steel containing Al in the ladle and the slag on the molten steel with an inert gas. Thus, an extremely low sulfidation and cleaning method for molten steel is disclosed in which the ratio between the stirring power and the pure oxygen supply rate in the oxidizing gas is within an appropriate range. However, this method is intended to prevent the temperature drop of the molten steel or to raise the temperature, and uses the slag on the molten steel to quickly absorb the Al ₂ O ₃ generated by supplying the oxidizing gas into the slag. Just do it. Moreover, although improvement of the desulfurization capacity by the presence of slag has been pointed out, there is no idea that such slag is actively generated and used. Therefore, as shown in the Examples, the amount of slag that can be stirred is as small as 9 to 10 kg / t.

  As described above, the conventional technology focuses on any one of molten steel heating, slag control, and stirring, and promotes desulfurization by controlling the slag to a component composition suitable for desulfurization. It is hard to say that it is a refining method that can clean up the water. Therefore, there are still issues to be solved in order to establish a steel melting method with extremely low sulfur content and high cleanliness based on excellent economic efficiency, including reduction of energy costs. .

JP-A-63-69909 (Claims and page 2, upper right column, line 11 to lower left column, line 3) JP 2002-60832 A (claims and paragraphs [0004] to [0006]) JP 2004-204307 A (claims and paragraphs [0012] to [0014]) JP-A-8-109411 (Claims and paragraphs [0047] to [0055]) JP 2002-339014 A (claims, paragraphs [0007] to [0012] and [0019]) JP 9-87730 A (claims, paragraphs [0014], [0015] and [0070])

As described above, the following problems remain in the conventional methods for melting ultra-low sulfur steel and high clean steel. That is, (a) when the Al temperature raising method is used to compensate for the temperature decrease in the secondary refining process, the formation of Al ₂ O ₃ or FeO causes an increase in inclusions in the molten steel, and the Al _{2 in} the slag. In the combined use of the melt acceleration method using the flux containing the meteorite and the gas stirring method, the slag mixing takes time, as the O ₃ content rate increases and the slag desulfurization ability decreases. In addition, the operation is restricted by the regulation of fluorine emission, (c) In the method of stirring the slag with an inert gas while oxidizing the Al in the molten steel by blowing the oxidizing gas, the amount of slag that can be stirred is too small, Etc.

  The present invention has been made in view of the above-mentioned problems, and its problem is to optimize the desulfurization efficiency by adding CaO-based flux, gas stirring of molten steel and the above-mentioned flux, and supply of oxidizing gas. It is intended to provide a method for melting ultra-low sulfur high-clean steel having an S content in steel of 10 ppm or less and high cleanability, in particular, while ensuring and removing inclusions effectively.

  The present invention provides a very low sulfur molten steel having high cleanliness by simultaneously promoting desulfurization and cleaning by controlling the heat treatment of the molten steel and controlling the composition of the slag component and optimizing the stirring treatment of the molten steel and slag. This is a refining method that enables efficient and stable production. That is, the gist of the present invention resides in the method for melting ultra-low sulfur high clean steel shown in the following (1) to (9).

(1) A method for melting ultra-low sulfur high-purity steel (hereinafter, also referred to as “first invention”), characterized in that molten steel is processed by the steps shown in the following steps 1 to 4.
Step 1: Adding CaO-based flux to the molten steel in the ladle under atmospheric pressure Step 2: After the step 1, the molten steel and the CaO flux are blown into the molten steel in the ladle under atmospheric pressure after step 1. Stirring, supplying an oxidizing gas to the molten steel, and mixing the oxide produced by the reaction between the oxidizing gas and the molten steel with a CaO-based flux Step 3: Stop the supply of the oxidizing gas, and at atmospheric pressure Step 4: Performing desulfurization and inclusion removal by blowing a stirring gas into the molten steel in the ladle in Step 4: When the molten steel in the ladle is treated with an RH vacuum degassing apparatus after Step 3, the RH vacuum is used. A process for increasing the temperature of molten steel by supplying an oxidizing gas into the tank (2) A method for melting ultra-low sulfur high-clean steel in which the molten steel is processed in the following order by the processes shown in the following processes 1 to 3. Then, after adding Al to the molten steel in the following step 1 or step 2, whether or not an oxidizing gas of 0.4 Nm ³ or more per 1 ton (t) of molten steel is blown into the molten steel in the amount of pure oxygen in step 2 Alternatively, a method for melting ultra-low sulfur high-clean steel characterized by spraying (hereinafter also referred to as “second invention”).
Step 1: Adding CaO-based flux to the molten steel in the ladle under atmospheric pressure Step 2: After the step 1, the molten steel and the CaO flux are blown into the molten steel in the ladle under atmospheric pressure after step 1. Stirring, supplying an oxidizing gas to the molten steel, and mixing the oxide produced by the reaction between the oxidizing gas and the molten steel with a CaO-based flux Step 3: Stop the supply of the oxidizing gas, and at atmospheric pressure The step of desulfurization and inclusion removal by blowing a stirring gas into the molten steel in the ladle of (3) In the step 3, the time for blowing the stirring gas after stopping the supply of oxidizing gas is 4 minutes or more The method for melting ultra-low sulfur high-clean steel according to (2) (hereinafter also referred to as “third invention”).

(4) The ratio of the mass content of CaO and Al ₂ O ₃ in the slag after the completion of the treatment in Step 3 is 0.9 to 2.5, and the total mass content of FeO and MnO in the slag The method for melting ultra-low sulfur high-clean steel according to any one of the above (1) to (3) (hereinafter, also referred to as “fourth invention”), wherein the content is 8% or less.

  (5) In the step 4, after the supply of the oxidizing gas is stopped, the process of continuously removing the inclusions in the molten steel by continuously circulating the molten steel in the RH vacuum degassing apparatus is performed. The method for melting ultra-low sulfur high-clean steel according to (1) or (4) (hereinafter also referred to as “fifth invention”).

  (6) Melting of ultra-low sulfur high clean steel according to any one of (1) to (5), wherein the oxidizing gas is oxygen gas or a mixed gas of oxygen gas and inert gas Method (hereinafter also referred to as “sixth invention”).

  (7) The supply of the oxidizing gas is performed by spraying on the surface of the molten steel using an upper blowing lance having a water cooling structure, and is extremely low according to any one of (1) to (6) above A method for melting high-sulfur steel (hereinafter also referred to as “seventh invention”).

  (8) In one or more of the steps 1 to 3, the molten steel is processed in the molten steel in the ladle without immersing a dip tube that isolates the surface of the molten steel inside and outside the tube. The method for melting ultra-low sulfur high-clean steel according to any one of (1) to (7) above (hereinafter also referred to as “eighth invention”).

  (9) When the molten steel blown in the converter is put out into the ladle, the inflow of the converter slag into the ladle is suppressed. (1) to (8) above A method for melting ultra-low sulfur high-clean steel according to any one (hereinafter, also referred to as “ninth invention”).

  In the present invention, “ultra-low sulfur high clean steel” means a steel having a low total oxygen content in which the S content is reduced to an extremely low level. For example, in a steel product in a state where no Ca addition treatment is performed. Steel having an S content of 10 ppm or less and a total oxygen content (hereinafter also referred to as “T. [O]”) of 30 ppm or less, which is the total amount of dissolved oxygen in the steel and oxygen in the inclusions. Say. More preferably, the S content in the steel product is 6 ppm or less. [O] means steel with 15 ppm or less.

The “CaO-based flux” means a flux having a CaO content of 45% by mass or more. For example, a flux containing simple components of quick lime and components such as Al ₂ O ₃ and MgO mainly composed of quick lime. Applicable.

  “Oxidizing gas” means a gas having an ability to oxidize alloy elements such as Al, Si, Mn, Fe, etc. in the melting temperature region of steel, simple gas such as oxygen gas, carbon dioxide gas, etc. The mixed gas of simple gas and the mixed gas of the said gas, an inert gas, or nitrogen correspond. The “inert gas” means an element belonging to Group 18 of the periodic table, such as argon, helium, neon, and the like.

  In the following description, “%” representing the component content means “mass%”.

  In order to solve the above-mentioned problems, the present inventors have studied a melting method of ultra-low sulfur high-clean steel that can simultaneously achieve desulfurization acceleration and high cleaning, and the following findings (a) to (c) And the present invention described above was completed.

(A) Basic knowledge of the first invention and the function of each process In order to melt the ultra-low sulfur high-purity steel satisfying both the ultra-low sulfur and high cleanliness as described above, the following (a) -1 It is effective to refine the molten steel by the processes of Step 1 to Step 4 shown in (a) -4.

When Al and oxygen are supplied to the molten steel, the molten steel temperature rises and Al ₂ O ₃ is generated. This Al ₂ O ₃ floats on the surface of the molten steel as the molten steel temperature rises, and is absorbed by the slag after floating. At this time, Al ₂ O ₃ and slag are integrated at a high temperature, and the component composition of the slag changes due to the absorption of Al ₂ O ₃ by the slag. Furthermore, since Al ₂ O ₃ is gradually generated with the supply of oxygen and gradually rises, the compositional composition of the slag changes slowly, as in the case of adding Al ₂ O ₃ or synthetic flux. No rapid slag composition change occurs. In addition, Al ₂ O ₃ floats uniformly over the entire surface of the molten steel, so it is dispersed throughout the slag and does not become a local addition as in the case of batch addition. As well as the mixing time.

Therefore, by utilizing the Al ₂ O ₃ component generated by supplying Al and oxygen to the molten steel for the control of the slag component composition, mixing of the Al ₂ O ₃ component at a high temperature and a gradual composition change, and the Al ₂ O ₃ component The slag component composition can be controlled by uniformly dispersing the slag. As a result, strong stirring and use of meteorite can be avoided.

As described above, the production of Al ₂ O ₃ by supplying Al and oxygen to the molten steel and the absorption of the Al ₂ O ₃ component into the slag are possible. In order to advance desulfurization under such conditions, it is necessary to add a CaO component in advance.

(A) -1 Step 1
In process 1, in order to advance desulfurization, a CaO type flux is added to molten steel under atmospheric pressure. Here, the reason why CaO is added under atmospheric pressure is that oxidation refining is performed in the next step, so that it is not necessary to perform addition of CaO under reduced pressure that increases the refining cost in step 1. Al is basically supplied to the molten steel before the addition of the CaO-based flux, but may be simultaneous with the addition of the CaO-based flux.

(A) -2 Step 2
Next, in step 2, the molten steel and the added flux are stirred by blowing an inert gas into the ladle molten steel under atmospheric pressure, and an oxidizing gas is supplied to the molten steel. The oxide produced | generated by reaction is mixed with CaO type | system | group flux. This treatment is for reacting Al in molten steel with oxygen and controlling the slag component composition by utilizing the generated Al ₂ O ₃ component and promoting melting of the slag. Here, the inert gas is blown in order to facilitate the absorption of the oxidizing gas into the molten steel. If only the oxidizing gas is supplied without blowing the inert gas, the oxidation reaction proceeds only in the collision region between the oxidizing gas and the molten steel surface, and the uniform dispersion of Al ₂ O ₃ is inhibited.

In step 2, the slag component composition is controlled and melted, and the desulfurization reaction proceeds. However, this oxidizing gas supply time does not proceed until the desulfurization reaction is saturated, and desulfurization surplus power remains in the slag. Here, “desulfurization surplus power” means desulfurization ability governed by the component composition of slag, as will be described later. Further, although not so much as to change the component composition of the slag, several tens of ppm of Al ₂ O ₃ remains as inclusions in the molten steel.

(A) -3 Step 3
Therefore, after Step 2, in Step 3, the supply of the oxidizing gas is stopped, and the stirring gas is blown into the molten steel under atmospheric pressure to perform desulfurization and inclusion removal processing. By this treatment, further desulfurization by slag having a desulfurization surplus capacity and unnecessary residual inclusions are removed. The “desulfurization capacity” as used herein means sulfide capacity that is governed by the component composition of slag, that is, “desulfurization capacity”. This sulfide capacity decreases when lower oxides such as FeO and MnO are present in the slag. Therefore, in order to maximize the desulfurization power, it is necessary to control the slag component composition and reduce the concentration of the lower oxide.

  In the step 2, the lower oxide is inevitably generated by supplying the oxidizing gas. For this reason, desulfurization can be further promoted by blowing an inert gas in step 3 after step 2 and reducing the concentration of these lower oxides.

(A) -4 Step 4
Finally, step 4 is performed. In steps 1 to 3, the molten steel in the ladle is treated under atmospheric pressure. After this treatment, the ladle is also referred to as an RH vacuum degassing apparatus (hereinafter referred to as “RH apparatus”). The treatment is also referred to as “RH treatment”). In the RH treatment, an oxidizing gas is supplied to the molten steel to raise the molten steel temperature, and then the molten steel is circulated in the RH apparatus. By passing through this step, the desulfurization rate and cleanliness can be further increased. This is due to the following reasons.

That is, the temperature can be raised also in the step 2, but the main purpose is to promote desulfurization by controlling the slag component composition. For this reason, even if the molten steel temperature is too low, the amount of temperature rise of the molten steel by supplying oxygen may be limited. For example, when the molten steel temperature before processing is lower than the planned value, it is necessary to increase the molten steel temperature by increasing the supply amount of oxidizing gas. However, when the supply amount of the oxidizing gas is increased, the amount of Al ₂ O ₃ produced increases, so the amount of CaO input must be increased. As a result, the amount of slag increases.

  Therefore, in the present invention, the following method is adopted. That is, the oxidizing gas supply amount in step 2 is set to an oxygen supply amount suitable for controlling the component composition of slag mainly for desulfurization. In this case, the molten steel temperature may be slightly lower. This lack of temperature needs to be compensated at any stage. As described above, when the temperature is raised using an oxidizing gas, the concentration of FeO and MnO in the slag increases, and there is a risk that slag to molten steel may occur. Therefore, in the RH treatment, attention was paid to the fact that the reaction between the slag and the molten steel hardly proceeds.

Since the reaction rate between the slag and molten steel in the RH treatment is slow, even if the FeO, MnO concentration or Al ₂ O ₃ concentration in the slag increases during the RH treatment, resulfurization hardly occurs. Therefore, when the molten steel temperature is insufficient in step 2, as the fourth step, an oxidizing gas may be supplied in the RH treatment to raise the molten steel temperature. By this method, the desulfurization effect in steps 1 to 3 can be enhanced, and the molten steel temperature can be compensated without impairing the desulfurization effect.

  By performing the processes in steps 1 to 4 described above in the above order, it is possible to simultaneously perform the temperature rise of the molten steel and the component composition control of the slag. As a result, strong stirring of the molten steel and slag and synthetic desulfurization are possible. The desulfurization can be efficiently achieved by suppressing the use of the agent and the meteorite.

(B) Confirmation of processing effect by Step 1 to Step 3 Based on the method of the present invention, a preliminary test 1 for confirming the effect of the processing by Step 1 to Step 3 was performed.

FIG. 1 is a diagram schematically showing the processing status of steps 1 to 3 in the method of the present invention. After adding 8 kg of CaO per 1 ton (t) of molten steel (hereinafter also referred to as “8 kg / t”) to 250 t of molten steel 2 accommodated in the ladle 1, 0.012 Nm per 1 t of molten steel under atmospheric pressure While blowing Ar gas at ³ / min (hereinafter also referred to as “0.012 Nm ³ / min · t”) through an immersion lance 4 for blowing an inert gas that can be raised and lowered, oxygen gas is supplied at 0.150 Nm ³ / min · t. A total of 1.5 Nm ³ per 1 ton of molten steel (hereinafter also referred to as “1.5 Nm ³ / t”) was blown over 10 minutes through an oxidizing gas upper blowing lance 5 that can be raised and lowered at the blowing speed. After finishing the top blowing of oxygen, Ar gas was continuously blown into the molten steel and stirred for 7 minutes. In addition, the number 3 in a figure shows a slag and the number 6 shows the cover of a ladle.

As a comparative test in Preliminary Test 1, a test in which only 8Og / t of CaO was added and stirring was performed by blowing Ar gas for 17 minutes without supplying oxygen, and a mixing ratio of CaO and Al ₂ O ₃ was a mass ratio. A test was performed in which 9 kg / t of a mixed flux of 60:40 was added and stirring was performed by blowing Ar gas for 17 minutes without supplying oxygen. In all the tests, Al was added to the molten steel at the time of steel removal from the converter, and the Al content in the molten steel before stirring by blowing Ar gas was adjusted to 0.25%. The test results are shown in Table 1.

From the results shown in the table, in the test by the method of the present invention in Preliminary Test 1, the S content after treatment is lower than that of the other two comparative tests, and T. is an index of the amount of inclusions. It can be seen that [O] is also low. This is because it is easier to control the slag component composition and accelerate the desulfurization reaction when using Al ₂ O ₃ produced by the reaction between Al and supplied oxygen in accordance with the method of the present invention. T. The reason why the value of [O] is low is that melting of the slag is promoted and the ability to absorb inclusions by the slag is improved. In other words, these results show that the mixing of the produced Al ₂ O ₃ and slag was promoted, and as a result, T.W. This means that the value of [O] has decreased, and therefore, the composition composition of slag is controlled using Al ₂ O ₃ generated by supplying oxidizing gas to the molten steel as in the method of the present invention. It shows that the method is effective for simultaneous desulfurization and cleaning.

  Furthermore, the knowledge which became the basis of the 2nd invention-the 9th invention which raises the effect of the present invention is explained below.

(C) Basic knowledge of the second invention to the ninth invention (c) -1 Amount of supply of oxidizing gas In the second invention, after adding Al to the molten steel, supplying oxidizing gas in step 2 In the reaction of oxygen with oxygen, a preferable supply amount of oxidizing gas is specified.

The present inventors investigated the effect on the desulfurization rate by changing the supply amount of the oxidizing gas variously in order to grasp the preferable range of the supply amount of the oxidizing gas. As a result, it was possible to obtain a desulfurization rate of 90% or more by setting the supply amount of the oxidizing gas to 0.4 Nm ³ / t or more in terms of pure oxygen. On the other hand, when the supply amount of the oxidizing gas was less than 0.4 Nm ³ / t in terms of pure oxygen, the desulfurization rate was 5 to 10% lower than the above value. This is because when the supply amount of the oxidizing gas decreases, the amount of Al ₂ O ₃ produced decreases and the slag melts incompletely. Therefore, it has been found that the desulfurization rate can be further increased by setting the supply amount of the oxidizing gas to 0.4 Nm ³ / t or more in terms of pure oxygen. Needless to say, the Al addition amount and the CaO addition amount must be adjusted according to the supply amount of the oxidizing gas.

(C) -2 Stirring gas blowing time The third invention prescribes a preferable blowing time of the stirring gas after the supply of the oxidizing gas is stopped.

In the present invention, since the slag has a desulfurization surplus even after the production of Al ₂ O ₃ , the inert gas is blown in and stirring is continued to promote desulfurization. A preferable stirring time in this desulfurization acceleration treatment was examined by the following method. The effect on the desulfurization rate was investigated by changing the blowing time with the Ar gas blowing flow rate in the range of 0.008 to 0.020 Nm ³ / min · t. As a result, when the blowing time was 4 minutes or more, the desulfurization rate was high and showed a substantially constant value, whereas when the blowing time was less than 4 minutes, the desulfurization rate was 3 to 10 higher than the above value. % And the fluctuation amount increased. This is due to the fact that FeO and MnO in the slag could not be sufficiently reduced, so that desulfurization by the slag was inhibited. In order to ensure sufficient slag desulfurization power, it is necessary to reduce FeO and MnO. From the above results, it became clear that the desulfurization rate can be stabilized at a higher level by blowing inactive gas for stirring for 4 minutes or more after the supply of the oxidizing gas is stopped, which is preferable.

(C) -3 Slag Component Composition The fourth invention prescribes a preferred component composition of slag after the completion of the treatment in Step 3.

In the present invention, desulfurization is promoted by utilizing Al ₂ O ₃ produced by supplying oxidizing gas to molten steel for controlling the slag component composition. Therefore, it is considered that there is a preferable component composition in the slag that advances desulfurization, and the relationship between the slag component composition after treatment and the desulfurization rate was investigated. As a result, the ratio of the mass content of CaO and Al ₂ O _{3 in} the slag after treatment is 0.9 to 2.5, and the total mass content of FeO and MnO in the slag is 8 It was found that a desulfurization rate of 80% or more can be obtained when the content is less than or equal to%. On the other hand, when the above range was not satisfied, the desulfurization rate was 13 to 25% lower.

Moreover, when the ratio of the mass content of CaO and Al ₂ O _{3 in} the slag after treatment is 0.9 to 2.5, the T.O. [O] was 14 to 20 ppm. [O] was as high as 18 to 25 ppm.

From these results, it was found that, in the present invention, by setting the component composition of the treated slag within the above range, a higher desulfurization rate can be obtained and the cleanliness of the molten steel can be further increased. Control of this slag component composition can be adjusted with the addition amount of CaO-type flux mentioned above. In the present invention, the above object can be achieved without using the meteorite, but it does not exclude the presence of the CaF ₂ component mixed in the added flux. In addition, when the meteorite is used in combination, it is of course more advantageous in terms of improving the desulfurization efficiency and reducing the S content after the treatment.

(C) -4 RH treatment after desulfurization treatment The fifth invention is a method of performing inclusion removal treatment by continuously circulating the molten steel in the RH apparatus after the supply of oxidizing gas is stopped in the step 4.

Although the effect of cleaning the molten steel can be obtained by the treatment in the step 3, when high cleanliness higher than that obtained in the step 3 is required, after the supply of the oxidizing gas is stopped, the molten steel is circulated in the RH apparatus. By continuing the process, the cleaning can be further improved. In addition to some of the inclusions remaining after the treatment in Step 3, when the temperature of the molten steel is adjusted by performing the heat treatment in Step 4 while maintaining the desulfurization efficiency at a high level, the Al ₂ O is obtained by the heat treatment. ₃ Inclusions may be generated and remain in the molten steel. In such a case, in order to remove these inclusions, the cleanliness of the molten steel can be further increased by performing a reflux treatment for a certain time after supplying the oxidizing gas.

(C) -5 Kind of oxidizing gas The sixth invention is a method of melting steel using oxygen gas or a mixed gas of oxygen gas and inert gas as the oxidizing gas.

  In the method of the present invention, it is preferable to use a single oxidizing gas such as oxygen from the viewpoint of shortening the processing time, but from the viewpoint of improving the controllability of the reaction, it is possible to mix an inert gas with the oxidizing gas. it can. In the sixth invention, the “oxidizing gas” means a gas having an ability to oxidize elements such as Al, Si, Mn, Fe, etc. in the melting temperature region of steel as described above. A simple gas such as a gas, a mixed gas of these simple gases, and a mixed gas of the above gas and an inert gas or nitrogen gas are applicable. Further, “inert gas” means an element belonging to Group 18 of the periodic table, such as argon, helium, neon, and the like.

  By mixing the inert gas, the oxygen supply amount per unit time can be controlled, and the oxygen partial pressure in the supplied oxidizing gas can be reduced. Controllability of component composition is improved.

(C) -6 Supplying method of oxidizing gas The seventh invention is a melting method for supplying an oxidizing gas by blowing an oxidizing gas onto the surface of the molten steel using an upper blowing lance having a water cooling structure. .
In the method of the present invention, the desulfurization rate can be stabilized by spraying the oxidizing gas onto the surface of the molten steel through the top blowing lance. The reason is as follows. That is, for supplying the oxidizing gas, there is another method in which the oxidizing gas is blown into the molten steel. In this case, since the oxidizing gas receives the static pressure of the molten steel, the partial pressure of the oxidizing gas is reduced. To rise. When the partial pressure of the oxidizing gas increases excessively, the oxidizing gas directly reacts with elements other than Al in the molten steel, such as Fe and Mn. As a result, it becomes difficult to control the amount of Al ₂ O ₃ produced, and it is difficult to maintain the total content of the FeO content in the slag and the MnO content below the preferred upper limit defined in the fourth invention. It is. Further, when the oxidizing gas is blown up as in the seventh invention, a high temperature region is generated in the vicinity of the slag, which is more advantageous for melting and hatching of the slag.

  In addition, the lance for blowing the oxidizing gas into the molten steel needs to be devised to prevent the melting loss, but in the method of spraying the molten steel surface through the upper blowing lance without immersing the lance, It is also advantageous in that lance melting hardly becomes a problem.

  For the above reasons, the desulfurization reaction can be made to proceed more stably by spraying an oxidizing gas on the surface of the molten steel. In addition, since the top blowing lance receives strong radiant heat from molten steel or slag, a water-cooled structure is preferable from the viewpoint of durability.

(C) -7 Ensuring uniform flow of slag as a whole The eighth invention is a method of treating molten steel in a molten steel in a ladle without immersing a dip tube that isolates the surface of the molten steel inside and outside the tube. It is.

In the melting method of the present invention, it is important that Al ₂ O ₃ produced in the molten steel floats on the upper surface of the molten steel and is uniformly distributed and supplied to the entire slag existing on the upper surface of the molten steel. Therefore, the slag needs to flow uniformly over at least the entire molten steel surface.

Depending on the refining method, unlike the refining apparatus using the ladle shown in FIG. 1, the dip tube is immersed in molten steel accommodated in the smelting vessel, and an oxidizing gas is supplied into the dip tube. Methods for refining are also known. However, in the refining method in which the dip tube is immersed in the molten steel, the uniform dispersion of Al ₂ O ₃ may be hindered by the presence of the dip tube. Considering such points, in order to maximize the refining effect in the melting method of the present invention, as shown in FIG. 1, the dip tube is immersed in the molten steel accommodated in the ladle. It is preferable to refine the molten steel without causing it.

(C) -8 Inflow suppression of converter slag into the ladle The ninth invention suppresses the inflow of converter slag into the ladle when the molten steel that has been blown in the converter is put into the ladle. This is a method for melting molten steel.

  In the melting method of the present invention, the control of the slag component composition is an important factor. In order to improve the control accuracy of the slag component composition including the first invention, it is important to minimize the amount of slag mixed from the converter. Therefore, in the converter refining, which is the previous step of the melting method of the present invention, the effect of the present invention can be obtained by suppressing the slag generated in the converter from flowing into the ladle used in the present invention. It can be further increased.

  According to the melting method of the present invention, high desulfurization efficiency can be ensured and inclusions can be effectively removed by optimizing the addition of CaO-based flux, gas stirring of molten steel and flux, and the supply of oxidizing gas. Therefore, it is possible to stably smelt highly clean steel with the S content reduced to an extremely low level. Therefore, the method of the present invention is extremely low, for example, the S content in steel is 10 ppm or less and the total oxygen content is 30 ppm or less, without performing Ca addition treatment and the like, with excellent economic efficiency. High purity steel can be melted.

  As described above, the method of the present invention is a method for melting ultra-low sulfur high-purity steel in which molten steel is processed by the following steps 1 to 4. That is, Step 1: Add CaO flux to the molten steel in the ladle under atmospheric pressure, Step 2: Stir the molten steel and CaO flux by blowing a stirring gas into the molten steel in the ladle under atmospheric pressure after Step 1. At the same time, an oxidizing gas is supplied to the molten steel, and the oxide produced by the reaction between the oxidizing gas and the molten steel is mixed with the CaO-based flux. Step 3: Stop the supply of the oxidizing gas and take it under atmospheric pressure. When desulfurization and inclusion removal are performed by blowing a stirring gas into the molten steel in the pan, and when the molten steel in the ladle is processed using the RH vacuum degassing apparatus after the step 4: step 3, it is placed in the RH vacuum tank. This is a method for producing ultra-low sulfur high-clean steel, which is processed by supplying an oxidizing gas to raise the molten steel temperature.

  Below, it demonstrates still in detail about the suitable aspect for enforcing the melting method of the ultra-low sulfur high clean steel which concerns on this invention.

(1) Step 1
(1) -1 Addition time, addition method and addition amount of CaO-based flux In this step, the CaO used for the molten steel desulfurization treatment is formed on the upper part of the molten steel which has been produced after the converter blowing and has been accommodated in the ladle. Add some or all of the system flux. Since the Al addition amount and the oxidizing gas supply amount are determined according to the target temperature, the target Al content rate, and the target S content rate, an appropriate amount of CaO-based flux is added. A predetermined amount of the CaO-based flux may be added all at once, or may be added in divided portions.

In the case of batch addition, the treatment is simple, and in the case of divided addition, the melting and hatchability of slag are likely to be improved. However, it is necessary to finish adding all of the CaO-based flux before the supply of the oxidizing gas in Step 2 is completed, adding the additions in Step 1 and Step 2. This is because, in the present invention, the nature utilizes Al ₂ O ₃ which generated, even with the addition of CaO-type flux after the supply of the oxidizing gas, a sufficient reaction with generation Al ₂ O ₃ and flux This is because it does not proceed and the melting and hatching promotion may be insufficient. Further, since the CaO-based flux has a high melting point, it is preferable to further promote the melting and hatching of the CaO-based flux by utilizing the high temperature region formed by supplying the oxidizing gas in the subsequent step 2. .

  Although the CaO-based flux may be added for the purpose of increasing the melting point of the slag in the ladle after the completion of the supply of the oxidizing gas, it is an improved technique of the present invention, and the present invention It does not exclude the addition of flux.

The CaO-based flux means a flux having a CaO content of 45% or more. For example, a simple lime and a flux containing Al ₂ O ₃ or MgO as a main component of quick lime can be used. Moreover, you may use the premelt synthetic | combination fossilizer with good hatchability like a calcium aluminate. In order to achieve desulfurization and cleaning for melting ultra-low sulfur high clean steel, it is necessary to control the slag component composition on the molten steel within an appropriate range in Step 3 and thereafter. It is preferable to add a CaO-based flux of 6 kg / t or more, more preferably 8 kg / t or more before the supply of the oxidizing gas is completed.

  As a method of adding CaO-based flux, a method of injecting powder into molten steel through a lance, a method of spraying powder onto the surface of the molten steel, a method of placing on the molten steel in a ladle, Any method such as a method of adding into the ladle at the time of steel output from the converter can be used. However, in the method of the present invention that is processed under atmospheric pressure, the method of adding the entire amount of CaO-based flux into the pan at the time of steel output is simple and suitable without using dedicated equipment such as blowing or spraying. is there.

  The composition of the molten steel in the ladle before addition of the CaO-based flux is as follows: C: 0.03 to 0.2%, Si: 0.001 to 1.0%, Mn: 0.05 to 2.5%, P : 0.003 to 0.05%, S: 11 to 60 ppm, Al: 0.005 to 2.0%, and the temperature is preferably about 1580 to 1700 ° C. However, the adjustment of these molten steel components may be performed after the addition of CaO and before the supply of the oxidizing gas.

(1) -2 Al addition method, addition amount, etc. The addition of Al provides a heat source and an Al ₂ O ₃ source for molten steel heat-up in a later step. Al reduces oxygen in the molten steel and iron oxide in the slag, and finally becomes Al ₂ O _{3 in} the slag, which lowers the melting point of the slag and effectively acts on desulfurization and cleaning of the molten steel. .

In order to achieve desulfurization and cleaning for melting ultra-low sulfur high-clean steel, it is necessary to control the slag component composition on the molten steel within an appropriate range in Step 3 and later, and total of Step 1 and Step 2 Then, it is preferable to add 1.5 kg / t or more of Al in terms of metallic Al until the supply of the oxidizing gas is completed, and more preferably 2 kg / t or more. If the amount of Al added is less than 1.5 kg / t, the amount of Al ₂ O ₃ produced is too small, and the effect of utilizing Al for slag control becomes small, and adjustment of the amount of CaO added is also necessary. is there. In addition, since the effect of sufficiently reducing the lower oxide in the slag is also reduced, the effect varies slightly.

  As for the method of adding Al, as with the method of adding CaO-based flux, a method of blowing powder into the molten steel through a lance, a method of spraying powder onto the surface of the molten steel, and placing it on the molten steel in the ladle In addition, any method such as a method of adding to the ladle at the time of steel removal from the converter can be used. Further, as the Al source, pure metal Al or Al alloy may be used, or a residue obtained during Al refining may be used.

In addition, when taking out the molten steel which blew the converter into the ladle, it is preferable to suppress the inflow of the converter slag into the ladle. The converter slag contains P ₂ O ₅ , which not only causes an increase in the P content in the molten steel in the subsequent desulfurization process, but also changes the amount of slag flowing into the ladle. This is because it becomes difficult to control the composition. For this reason, reducing the amount of converter slag generated, or introducing a blade-shaped dart directly above the outlet hole at the time of outgoing from the converter, suppressing vortex formation above the outlet hole, Slag outflow from the converter is reduced by means such as detecting the outflow of slag from the electric, optical or mechanical method and stopping the steel flow at the timing of slag outflow, It is preferable to suppress the inflow of slag into the ladle.

  Not only step 1 but also steps 2 and 3 described later are performed under atmospheric pressure. The reason is that, in the present invention, in addition to the necessity of performing a strong stirring operation under reduced pressure, in order to perform the processes of Steps 1 to 3 under reduced pressure, the equipment cost and running cost increase.

(2) Step 2
In step 2, the molten steel and the CaO-based flux are stirred by blowing the stirring gas into the molten steel in the ladle under atmospheric pressure to which the CaO-based flux is added in step 1, and an oxidizing gas is supplied to the molten steel to oxidize. An oxide such as Al ₂ O ₃ produced by the reaction between the reactive gas and the molten steel is mixed with the CaO flux.

  As described above, a part or all of the CaO-based flux may be added in step 2, or a part or all of Al may be added in step 2. However, the addition amount of CaO and Al directly targeted in the present invention includes the oxidizability in step 2 from the start of steel output, including those charged in the ladle before the start of steel output from the converter. This means that the gas supply is completed.

(2) -1 Supplying method of oxidizing gas The oxidizing gas is supplied to the molten steel in the step 2 by heating the molten steel or lowering the temperature by utilizing the oxidation exothermic reaction caused by the reaction between the molten steel component and the oxidizing gas. This is because the composition of the slag is controlled by generating Al ₂ O ₃ while suppressing the slag. As this oxidizing gas, it is possible to use the kind of gas having the ability to oxidize elements in molten steel.

  As a method for supplying the oxidizing gas, a method of blowing an oxidizing gas into the molten steel or a method of blowing an oxidizing gas from a lance or nozzle disposed above the molten steel can be used. From the viewpoint of melting the slag by utilizing the region and improving hatchability, a method of spraying on the surface of the molten steel using an upper blowing lance is preferable. Thereby, the CaO type flux can be directly heated using the high temperature region formed by the reaction between the oxidizing gas and the molten steel in the ladle, and the hatching of the CaO type flux can be promoted.

  When the oxidizing gas is blown to the molten steel from the lance or nozzle disposed above the molten steel, it is necessary to secure the blowing strength of the oxidizing gas to some extent in order to effectively transmit the generated heat to the slag. In order to secure this spray strength, it is necessary to lower the lance height and approach the molten steel. As a result, the lance life is reduced by the radiant heat received from the molten steel, and the lance replacement work is increased, so that it is difficult to maintain high productivity. Therefore, when the oxidizing gas is sprayed onto the molten steel through the lance or nozzle, it is preferable that the lance or nozzle has a water cooling structure.

  The height of the lance or nozzle from the hot water surface (vertical distance between the hot water surface and the lower end of the lance) is preferably in the range of about 0.5 to 3 m. If the lance or nozzle height is less than 0.5 m, spitting of the molten steel becomes severe and the lance or nozzle life may be reduced. On the other hand, if it exceeds 3 m, the oxidizing gas jet reaches the molten steel surface. This is because the oxygen efficiency of refining may be significantly reduced.

(2) -2 supply amount of the oxidizing gas in the supply amount such as step 2 of the oxidizing gas is preferably to 0.4 Nm ³ / t or more in pure oxygen equivalent amount, and 1.2 Nm ³ / t or more More preferably. This oxygen supply amount is a preferable oxygen supply amount for obtaining a heat source for maintaining the temperature and increasing the temperature of molten steel by oxidizing Al, and also for promoting the slag formation of the CaO source added in step 1 A preferable supply amount. By using the above oxygen supply amount, a suitable amount of Al ₂ O ₃ for slag formation is generated, the controllability of the slag component composition is further improved, and the desulfurization and cleaning action of molten steel is further improved. To do.

The supply rate of the oxidizing gas is preferably in the range of 0.075 to 0.24 Nm ³ / min · t in terms of pure oxygen. If the supply rate of the oxidizing gas is less than 0.075 Nm ³ / min · t, the processing time may be long and productivity may be reduced. On the other hand, if it exceeds 0.24 Nm ³ / min · t, the CaO flux can be heated sufficiently, but the supply time of the oxidizing gas is shortened and Al ₂ O _{3 is} generated per unit time. If the amount increases too much, there is a risk that sufficient time for melting the slag and making the slag component composition uniform cannot be secured. In addition, the life of the lance and ladle refractories may be reduced. Note that it is more preferable that the supply rate of the oxidizing gas is 0.1 Nm ³ / min · t or more from the viewpoint of securing productivity.

In Step 2, by supplying the oxidizing gas performed as described above, Al ₂ O ₃ is generated and the molten steel temperature is raised. And the melting and hatching of slag are promoted using the high temperature region existing at the fire point. Further, by blowing a stirring gas from a lance immersed in the molten steel, Al ₂ O ₃ generated by the reaction between the oxidizing gas and the molten steel is mixed with the CaO-based flux to control the component composition of the slag.

The oxide produced by the reaction between the oxidizing gas and the molten steel is mainly Al ₂ O _3, but at the same time, a small amount of FeO, MnO and SiO ₂ are also produced. These are all oxides that lower the melting point of CaO. These oxides further promote the hatching of the CaO-based flux because they exhibit the action of lowering the melting point of the slag when mixed with CaO. Here, among these oxides, FeO and MnO have the effect of increasing the oxygen potential of the slag, and thus adversely affect the desulfurization of the molten steel thermodynamically. It reacts with Al in the molten steel by gas stirring in 3 and disappears.

(2) -3 Stirring gas blowing method and blowing amount As the stirring method in Step 2, the stirring gas is introduced into the molten steel through a lance immersed in the molten steel, and the stirring gas is introduced from the porous plug installed at the bottom of the ladle. However, it is preferable to introduce a stirring gas into the molten steel through a lance immersed in the molten steel. This is because, if such a method for introducing agitating gas from the porous plug installed in preparative pan bottom portion, for difficult to introduce a sufficient flow rate of the gas, the mixing of the slag and the Al ₂ O ₃ not This is because, as a result, it may be difficult to melt extremely low-sulfur steel.

The flow rate of the stirring gas is preferably in the range of 0.0035 to 0.02 Nm ³ / min · t. When the blowing flow rate is less than 0.0035 Nm ³ / min · t, the stirring force becomes insufficient, the stirring of the slag and Al ₂ O ₃ becomes insufficient, and the oxygen potential of the slag increases, resulting in a post process. This is because the reduction of the oxygen potential of the slag in Step 3 is insufficient, which may be disadvantageous for desulfurization. On the other hand, if the blowing flow rate exceeds 0.02 Nm ³ / min · t, the occurrence of splash is extremely increased, and the productivity may be reduced. In order to reduce the oxygen potential of the slag as much as possible and avoid a reduction in productivity, it is more preferable to set the blowing flow rate to 0.015 Nm ³ / min · t or less.

(3) Process 3
In step 3, the supply of the oxidizing gas using the top blowing lance is stopped, and the molten steel and slag are stirred by blowing the stirring gas through the lance immersed in the molten steel in the ladle at atmospheric pressure. Continue to desulfurize and remove inclusions.

(3) -1 Stirring gas blowing method and blowing amount The stirring gas blowing time after the supply of oxidizing gas is stopped is preferably 4 minutes or longer, and more preferably 20 minutes or shorter. The amount of stirring gas blown is preferably in the range of 0.0035 to 0.02 Nm ³ / min · t. The reason why it is preferable to continue stirring under the above conditions for melting ultra-low sulfur high-clean steel will be described below.

  In step 2, in order not to increase the oxygen potential of the slag when supplying the oxidizing gas, the oxidizing gas is decreased while the oxidizing gas is supplied or a large amount of stirring gas is blown into the molten steel at atmospheric pressure. Can be considered.

  However, if the supply rate of the oxidizing gas is extremely reduced, the temperature rise rate of the molten steel is lowered, and the productivity is lowered. Also, if an extremely large amount of stirring gas is blown into molten steel under atmospheric pressure, the scattering of molten iron increases, resulting in increased costs due to a decrease in iron yield and decreased productivity due to adhesion of scattered metal to peripheral equipment. Invite them.

In the method of the present invention, the stirring of the molten steel and the slag in the ladle is performed in the oxidizing gas supply period (step 2) in order to prevent the oxygen potential of the slag from increasing due to the supply of the oxidizing gas without causing the above problem. ) And the subsequent period (step 3) when no oxidizing gas is supplied. That is, even after the supply of the oxidizing gas by the top blowing lance or the like is stopped, the stirring gas is continuously blown into the molten steel through the lance immersed in the molten steel in the ladle. By passing through this process, the density | concentration of the lower oxide in slag can be reduced and the desulfurization capability of slag can be exhibited to the maximum. Note that, under normal gas supply conditions, the ratio (t / t ₀ ) of the stirring gas blowing time t in step 3 to the oxidizing gas supply time t ₀ in step 2 is preferably 0.5 or more. .

  In step 3, the oxide inclusions generated by supplying the oxidizing gas in step 2 are simultaneously separated with desulfurization. The gas stirring time by stirring gas blowing is preferably 4 minutes or longer. If the gas stirring time is less than 4 minutes, it is difficult to sufficiently reduce the oxygen potential of the slag that has been raised by the supply of the oxidizing gas in Step 2 in Step 3, and the desulfurization rate is increased. This is because it becomes difficult to secure a reaction time for sufficiently reducing [O]. The longer the gas agitation time, the lower the sulfidation effect and the cleaning effect. However, on the other hand, the productivity is lowered and the molten steel temperature is also lowered.

  The blowing of the stirring gas performed in step 3 is also preferably performed by a method of introducing the stirring gas through a lance immersed in the molten steel. The reason is that, for example, when the stirring gas is introduced from a porous plug installed at the bottom of the ladle, it is difficult to introduce a sufficient flow rate of gas into the molten steel. Therefore, in step 3, FeO in the slag and This is because the MnO component cannot be sufficiently reduced and it may be difficult to melt the extremely low sulfur steel.

  The method of the present invention is characterized in that gas stirring is performed under atmospheric pressure. This is because it is difficult to stir slag and metal vigorously with a small amount of gas blowing, such as gas stirring under reduced pressure, and it is difficult to perform gas stirring under a stable gas flow rate condition. .

As described above, the stirring gas blowing flow rate is preferably 0.0035 to 0.02 Nm ³ / min · t. When the blowing flow rate is less than 0.0035 Nm ³ / min · t, the stirring force is insufficient, and the oxygen potential of the slag in step 3 is insufficiently reduced, and further desulfurization may not be promoted. Moreover, if the blowing flow rate exceeds 0.02 Nm ³ / min · t, the occurrence of splash becomes extremely large, which may cause a decrease in productivity. In order to reduce the oxygen potential of the slag as much as possible and to avoid a reduction in productivity, it is more preferable that the blowing flow rate be 0.015 Nm ³ / min · t or less.

(3) -2 Slag component composition after completion of step 3 As defined in the fourth invention, the slag component composition after the completion of the treatment in step 3 is the ratio of the mass content of CaO and Al ₂ O ₃ (hereinafter referred to as the slag component composition). , “CaO / Al ₂ O ₃ ”) is 0.9 to 2.5, and the total mass content of FeO and MnO in the slag (hereinafter also referred to as “FeO + MnO”) is 8% or less. It is preferable. More preferably, the slag component composition is in the range of CaO: 45 to 60%, Al ₂ O ₃ : 33 to 46%, CaO / Al ₂ O ₃ ≧ 1.3, and (FeO + MnO) ≦ 4%. Particularly preferred ranges are CaO: 50-60%, Al ₂ O ₃ : 33-40%, CaO / Al ₂ O ₃ ≧ 1.5, and (FeO + MnO) ≦ 1%.

  In order to clarify the effect of the slag component composition, the following preliminary test 2 and preliminary test 3 were performed. C: 0.05 to 0.07%, Al: 0.08 to 0.25%, P: 0.003 to 0.015%, S: 0.0012 to 0.0035%, Mn: 0.25 A molten steel 250t having a component composition of 1.75% and Si: 0.01 to 0.55% was desulfurized using the apparatus shown in FIG.

In Preliminary Test 2, after adding CaO and Al to the molten steel in Step 1 in accordance with the method of the present invention, oxygen gas was blown with an upper blowing lance in Step 2, followed by stirring by blowing Ar gas for 9 minutes. It was. The oxygen gas supply amount was 0.5 to 1.5 Nm ³ / t, and the CaO addition amount was adjusted according to the oxygen gas supply amount. No meteorite was used.

Preliminary test 3 is a comparative test in which CaO and Al ₂ O ₃ are added and meteorite is added so that the content of meteorite in the slag is 10 to 15%, without supplying oxidizing gas. Only Ar gas was blown in, and only a stirring operation was performed for 13 minutes. In both preliminary test 2 and preliminary test 3, the total amount of slag is 18-22 kg / t. The desulfurization rate after the treatment was measured and arranged as a relationship with CaO / Al ₂ O _{3 in} slag and (FeO + MnO) content in slag.

FIG. 2 is a graph showing the relationship between the desulfurization rate in preliminary test 2 and the CaO / Al ₂ O ₃ content in slag and (FeO + MnO) content in slag, and FIG. 3 shows the desulfurization rate and slag in preliminary test 3 medium CaO / Al ₂ O _3, and in the slag is a diagram showing the relationship between (FeO + MnO) content.

From the result of the preliminary test 2 shown in FIG. That is, according to the method of the present invention, the higher the value of CaO / Al ₂ O _{3 in} the slag and the lower the (FeO + MnO) content, the better the desulfurization rate. Further, as defined in the fourth invention, when the value of CaO / Al ₂ O ₃ is 0.9 to 2.5 and the (FeO + MnO) content is 8% or less, the desulfurization rate is 80% or more. Obtained and preferred. Further, when the value of CaO / Al ₂ O ₃ is 1.3 or more and the (FeO + MnO) content is 3% or less, a desulfurization rate of 90% or more is obtained, and in particular, CaO / Al _When the value of ₂ O ₃ is 1.5 or more and the (FeO + MnO) content is 1% or less, a desulfurization rate of 95% or more is obtained, which is extremely preferable.

On the other hand, according to the preliminary test 3 shown in FIG. That is, even in the comparative test conditions, the tendency that the desulfurization rate increases as the CaO / Al ₂ O ₃ value in the slag increases and the (FeO + MnO) content decreases is the same as in the preliminary test 2. However, in Preliminary Test 3, even if the value of CaO / Al ₂ O ₃ and the content of (FeO + MnO) are the same, the desulfurization rate may reach 90% or more, and may be less than 80%. To do. That is, in the preliminary test 3, compared with the preliminary test 1, the desulfurization rate in the same slag composition is low and the fluctuation of the desulfurization rate is large.

From the above test results, as in the method of the present invention, after adding the CaO-based flux, an oxidizing gas is supplied to generate Al ₂ O _3, and a method for controlling the component composition of slag using this is as follows: It has been shown that the controllability of the slag component composition is superior to the control of the slag composition by other methods, and as a result, a high desulfurization rate can be stably achieved.

(3) -3 Steel component composition, inclusion control, etc. after completion of step 3 By finishing the treatment in step 3, the S content in the molten steel is 10 ppm or less and T.I. [O] 30 ppm or less ultra-low sulfur high clean steel, for example, C: 0.03 to 0.2%, Si: 0.001 to 0.65%, Mn: 0.05 to 2.5%, P: 0.005 to 0.05%, S: 10 ppm or less, sol. Al: 0.005 to 2.0%, T.I. [O]: An ultra-low sulfur high-clean steel having a steel component composition of 30 ppm or less is produced. The temperature at the end of step 3 is about 1590 to 1665 ° C.

  Table 2 shows typical steel component composition ranges at the end of step 3.

  In the same table, the component composition of the product is also shown. It can be seen that the reduction of [O] is completed by the end of step 3. T. T. [O] is lower in the product than at the end of step 3, which is due to the effect of step 4 following step 3.

  Moreover, as above-mentioned, in process 1-process 3, it is preferable to process without immersing dip pipes, such as a snorkel, in the molten steel in a ladle from a viewpoint of ensuring the amount of slag which acts effectively on desulfurization. When a dip tube of a degassing device is immersed, the slag is divided inside and outside the dip tube, and although the hatching of the slag existing in the region where the oxidizing gas is supplied is promoted, the slag present in other regions This is because the hatching of the slag is delayed, the slag existing outside the dip tube is not sufficiently stirred, and the amount of slag that effectively acts on desulfurization may be reduced.

  In addition, it is preferable that the amount of slag after completion | finish of the process 3 is about 13-32 kg / t. If the amount of slag is less than 13 kg / t, the amount of slag is small and it is difficult to obtain a stable desulfurization rate. On the other hand, if the amount of slag exceeds 32 kg / t, the time required for controlling the slag component composition becomes longer, and as a result, the processing time may be extended.

  In particular, when hydrogen-induced cracking resistance is required, or when it is necessary to prevent nozzle clogging in the continuous casting process, a Ca-containing substance such as CaSi, CaAl, FeCa, FeNiCa, or the like after completion of step 3 The inclusions are preferably added to make the inclusions spherical. In this case, the addition amount of CaSi is preferably in the range of about 0.2 to 1.2 kg / t. In addition, it is preferable that the CaO content rate in a spherical inclusion is 45 to 75%. This is because when the CaO content is less than 45%, the spheroidizing action becomes unstable. On the other hand, when the CaO content exceeds 75%, the stretchability of inclusions increases, which can be a starting point for hydrogen-induced cracking. This is because the nature increases.

By passing through the process of the process 1-the process 3 demonstrated above, desulfurization to the ultra-low-sulfur area | region and the cleaning of steel are achieved by use of CaO type | system | group flux, and while S content is 10 ppm or less, T.I. An ultra-low sulfur high-clean steel with [O] of 30 ppm or less can be melted at low cost.
Moreover, it is preferable not to use the meteorite because desulfurization up to the extremely low sulfur range and the cleaning action of the steel can be ensured without adding the meteorite (CaF ₂ ) to the molten steel in the ladle. In recent years, meteorites have been difficult to obtain due to depletion of resources, and use tends to be restricted due to consideration of environmental problems. Therefore, the method of the present invention that does not require the use of meteorites is environmentally friendly. It is also suitable as a method for melting steel.

  In the melting method of the present invention in which the refining reaction proceeds by supplying an oxidizing gas to the molten steel, a splash is formed along with the oxidation reaction of the molten steel, smoke generation and dust generation. Therefore, a cover is provided above the ladle. While preventing these dissipation, it is preferable to process by dust collection equipment. Furthermore, by controlling the pressure in the cover to a positive pressure, it is possible to prevent air entrainment and prevent reoxidation of molten steel and nitrogen intrusion. Further, a non-consumable upper blow lance is generally used for supplying the oxidizing gas, and a water-cooled lance is preferably used in order to increase the cooling efficiency.

(4) Step 4
Step 4 is a step that is performed in order to suppress the resulfurization and perform temperature compensation while maintaining an extremely low S content, and to further improve the cleanliness. For this purpose, it is necessary to use an RH apparatus. In the RH treatment, two dip tubes provided at the bottom of the vacuum chamber are immersed in the molten steel in the ladle, and the molten steel in the ladle is circulated through these dip tubes, so the slag is weakly agitated and the slag is less entrained Since inclusions can be separated, it is possible to achieve further higher cleaning. In addition, since the reaction rate between the slag and molten steel is low, resulfurization can be suppressed even if a heat treatment is performed using an RH apparatus.

(4) -1 Molten Steel Heat Treatment Method in RH Treatment A molten steel heat treatment method using an RH apparatus will be described. While circulating the molten steel between the vacuum tank and the ladle using the RH apparatus, the oxidizing gas is blown into the molten steel in the vacuum tank or the oxidizing gas is supplied to the molten steel in the vacuum tank through an upper blowing lance provided in the vacuum tank. Spray. Oxygen in the oxidizing gas reacts with Al in the molten steel to produce Al ₂ O ₃ , and at the same time, heat of reaction is generated, and the molten steel temperature rises due to the reaction heat. In addition, Al ₂ O ₃ inclusions, FeO, and MnO are generated by the reaction between Al and oxygen. The produced Al ₂ O ₃ , FeO and MnO migrate into the slag on the surface of the molten steel in the ladle, increase the (FeO + MnO) content in the slag, and lower the desulfurization ability of the slag.

  At this time, if the reaction rate between the slag and the molten steel is high, a sulfuration phenomenon occurs in which S in the slag moves into the molten steel. However, in the RH treatment, since the reaction rate between the slag and the molten steel is slow, the slag is suppressed. be able to. Therefore, by transferring a part of the heat treatment from the desulfurization process to the RH process, it becomes possible to increase the temperature while suppressing the resulfurization and maintaining the S content of the molten steel at an extremely low level.

  In addition, when further cleaning is required than at the end of the step 3, the inclusion can be further removed and the cleanliness can be further improved by continuing the reflux after the supply of the oxidizing gas is stopped. The RH reflux treatment time after stopping the oxidizing gas supply in step 4 is preferably 8 minutes or more, more preferably 10 minutes or more, and further preferably 15 minutes or more. The RH reflux treatment time may be appropriately determined according to the required inclusion amount level or hydrogen content level. Moreover, what is necessary is just to determine the supply amount of oxidizing gas suitably according to the target molten steel temperature after temperature rising.

The supply rate of the oxidizing gas in step 4 is preferably 0.08 to 0.20 Nm ³ / min · t in terms of pure oxygen. When the supply rate of the oxidizing gas is less than 0.08 Nm ³ / min · t, the treatment time becomes longer, and when it exceeds 0.20 Nm ³ / min · t, the production amount of FeO and MnO increases too much. It is not preferable.

  As the oxidizing gas, a simple gas such as oxygen gas or carbon dioxide, a mixed gas of these simple gases, and a mixed gas of the above gas and an inert gas or nitrogen gas can be used. From the viewpoint, oxygen gas is preferably used.

  The method of supplying the oxidizing gas can be a method such as blowing into the molten steel or spraying onto the surface of the molten steel in the vacuum chamber through the top blowing lance, but considering the good operability, it is by spraying. Is preferred. In this case, the top blowing lance nozzle may have any shape such as a straight type, a rapid expansion type, or a Laval type. The lance height (vertical distance between the lance lower end and the molten steel surface in the vacuum chamber) is preferably 1.5 to 5.0 m. When the lance height is less than 1.5 m, the lance is easily worn by spitting of the molten steel. When the lance height is higher than 5.0 m, the oxidizing gas jet hardly reaches the surface of the molten steel, and the heating efficiency decreases. .

  The atmospheric pressure in the vacuum chamber during the supply of the oxidizing gas is preferably 8000 to 1100 Pa. When recirculation is continued after the supply of the oxidizing gas is stopped, the pressure is preferably 8000 Pa or less, and more preferably 700 Pa or less. If the atmospheric pressure in the vacuum chamber is higher than 8000 Pa, the recirculation speed is slow, and it takes a long time to remove inclusions, which is not preferable. Moreover, in addition to being able to remove inclusions efficiently at 700 Pa or less, the H content and N content in the molten steel can be simultaneously reduced.

  In addition, an alloy element or the like may be added to the molten steel during or after the supply of the oxidizing gas to adjust components such as Si, Mn, Cr, Ni, and Ti in the molten steel.

(4) -2 Inclusion amount reduction and desulfurization suppression effect in step 4 Oxidizing gas supply rate (pure oxygen conversion amount) X (Nm ³ / min · t) in step 4 is 0.08 to 0.20 Nm ³ / min. The number of inclusions N ₀ having a size of 50 μm or more in the sample taken from the molten steel that has been changed in the range of t and having finished Step 3 and a size of 50 μm or more in the molten steel sample after the RH treatment in Step 4 The number of inclusions N was measured with an optical microscope, and the inclusion number index calculated by N / N ₀ was investigated. The number of inclusions was measured in accordance with the method defined in JIS G 0555 by measuring the number of inclusions having a size of 50 μm or more existing in a sample cross section of 700 mm ² .

Further, the difference between the S content [S] i (%) in the molten steel after step 3 and the S content [S] e (%) after step 4, ΔS = [S] i− [S] e was investigated. The oxidizing gas supply amount was 0.2 to 0.8 Nm ³ / t in terms of oxygen gas, and the reflux time after stopping the oxidizing gas supply was 8 minutes.

FIG. 4 is a diagram showing the relationship between the oxidizing gas supply rate (pure oxygen equivalent amount) (X) and the inclusion number index (N / N ₀ ) in Step 4, and FIG. It is a figure which shows the relationship between gas supply rate (pure oxygen conversion amount) (X), and the difference ((DELTA) S) of S content rate after completion | finish of the process 3 and the process 4. From the results shown in FIG. 4 and FIG. 5, it is possible to raise the temperature of the molten steel by supplying the oxidizing gas while suppressing the resulfurization in the RH treatment regardless of the supplying speed of the oxidizing gas in the step 4. In addition, it can be seen that large inclusions of 50 μm or more can be efficiently reduced by the RH reflux treatment after stopping the supply of the oxidizing gas.

  In order to confirm the effect of the melting method of the ultra-low sulfur high clean steel according to the present invention, the following steel melting test was conducted and the results were evaluated.

(1) Melting test method The hot metal that had been subjected to hot metal desulfurization and hot metal dephosphorization treatment as needed was charged into a 250 ton (t) scale bottom-bottom converter, and the C content in the molten iron was 0. The crude decarburized blow smelting is performed until 0.03 to 0.2%, the end temperature is set to 1630 to 1690 ° C., the crude decarburized molten steel is put into a ladle, and various deoxidizers and alloys are added at the time of steel output. The molten steel components in the ladle are as follows: C: 0.03-0.2%, Si: 0.001-1.0%, Mn: 0.05-2.5%, P: 0.003-0.05 %, S: 27-28 ppm, sol. Al: 0.005 to 2.0%, T.I. [O]: 50 to 100 ppm.

  The amount of the converter slag that flows out when the molten steel is discharged is left as it is without adjustment, or is adjusted by suppressing the inflow into the ladle using the dart described above. Moreover, at the time of steel output, it is used for deoxidation and deoxidizes the molten steel by adding Al required for the reaction with the oxidizing gas blown up in step 2, and also deoxidation of slag by stirring the steel output flow went. The processes of Step 1 to Step 4 of the method of the present invention were performed as follows.

  As step 1, 8 kg / t of quicklime in terms of CaO was collectively added to the molten steel in the ladle at the time of steel output under atmospheric pressure. Moreover, 400 kg of metal Al was added all at once to this steel.

As step 2, the immersion lance is immersed in the molten steel in the ladle, Ar gas is blown at a supply rate of 0.012 Nm ³ / min · t, and oxygen gas is 0.14 Nm ³ / min from the top blowing lance having a water cooling structure. -It sprayed on the surface of molten steel with the supply speed of t. At this time, the vertical distance between the lower end of the lance and the molten steel surface was 1.8 m, and the oxygen supply time was 6 minutes. Further, the dip tube was not immersed in the molten steel, a cover was installed above the ladle, and the generated gas, splash, dust, etc. were guided to the dust collector for treatment.

In Step 3, after the supply of oxygen gas was stopped, stirring was performed by blowing Ar gas at the Ar gas supply rate for 10 minutes. As for the slag component composition after completion of Step 3, CaO / Al ₂ O ₃ is 0.9 to 2.4, and (FeO + MnO) content is 0.7 to 6.1%.

In Step 4, oxygen gas was blown at 1.6 Nm ³ / t from an upper blowing lance installed in the vacuum chamber immediately after the start of the RH treatment. The lance nozzle was a straight type, the vertical distance between the lower end of the lance and the surface of the molten steel in the vacuum chamber was 2.5 m, and the oxygen gas supply rate was 0.14 Nm ³ / min · t. The RH apparatus has a dip tube diameter of 0.66 m, a reflux Ar gas flow rate of 2.0 Nm ³ / min, and an ultimate vacuum of 140 Pa. After the supply of oxygen gas was stopped, a 10-minute reflux treatment was performed to complete the treatment. The amount of slag in the melting test is about 18 kg / t.

  In Table 3 and Table 4, it becomes an index of the test conditions about the test numbers 1-14 of this invention example, and the test numbers 15-27 of a comparative example, and a desulfurization rate, S content rate in molten steel, and the number of inclusions in steel. T.A. Test results such as [O] are shown.

  Test numbers 1 to 6 are tests performed by changing the slag component composition under the above conditions, and test numbers 7 and 8 are tests in which the stirring time after stopping the supply of oxygen gas in step 3 was 3 minutes. It is. Test Nos. 9 and 10 are tests in which the molten steel was not recirculated after the supply of oxygen gas was stopped in Step 4, and Test Nos. 11 to 14 were processed in Step 4 by adding Al in Step 1. This is a test that was not performed.

  Next, as Comparative Example A, a test that did not perform the process of Step 3 was performed, and as Comparative Example B, a test that did not perform the process of Step 2 was performed by the following method.

That is, in Comparative Example A, 8 kg / t quicklime in a CaO equivalent amount was added all at once as Step 1, and 400 kg of metal Al was added all at once. Next, as step 2, the immersion lance is immersed in the molten steel in the ladle, Ar gas is blown at a supply rate of 0.012 Nm ³ / min · t, and oxygen gas is supplied from the top blowing lance having a water cooling structure to 0.14 Nm ^3. / Min · t was sprayed on the surface of the molten steel. At this time, the vertical distance between the lower end of the lance and the molten steel surface was 1.8 m, and the oxygen supply time was 6 minutes. Then, the process of the process 3 was abbreviate | omitted and oxygen gas was sprayed from the top blowing lance installed in the vacuum chamber immediately after the start of RH process as the process 4 at 1.0Nm < ³ > / t. The operating conditions of RH were the same as in the example of the present invention.

Further, in Comparative Example B, 8 kg / t quicklime in a CaO equivalent amount is added all at once in Step 1, and 400 kg of metal Al is added all at once, and then oxygen gas is supplied as Step 2. In Step 3, the immersion lance was immersed in the molten steel in the ladle, Ar gas was blown at a supply rate of 0.012 Nm ³ / min · t, and stirring was performed for 16 minutes. Thereafter, the ladle was transferred to the RH apparatus, and in Step 4, oxygen gas was sprayed at 2.7 Nm ³ / t from an upper blowing lance installed in the vacuum chamber immediately after the start of the RH treatment. The lance nozzle was a straight type, the vertical distance between the lower end of the lance and the surface of the molten steel in the vacuum chamber was 2.5 m, and the oxygen gas supply rate was 0.14 Nm ³ / min · t.

Since no heat-up process was performed in the previous step of the RH treatment, the temperature increase amount in the RH treatment increased, and the oxygen supply amount in the RH treatment increased compared to the case of the present invention example. The RH apparatus has a dip tube diameter of 0.66 m, a reflux Ar gas flow rate of 2.0 Nm ³ / min, and an ultimate vacuum of 140 Pa. After the supply of oxygen gas was stopped, a 10-minute reflux treatment was performed to complete the treatment.

  As described above, Table 3 and Table 4 show the test conditions and test results of the comparative examples. In test numbers 18 to 22, only CaO and Al were added, and in test numbers 23 to 27, 1.5 t of meteorite was added as a flux.

(2) Evaluation of melting test results Test numbers 1 to 14, which are tests for the present invention examples, have a significantly improved desulfurization rate compared to test numbers 15 to 27 which are tests for comparative examples. The S content after 3 and after step 4 is greatly reduced, and is an index of the number of inclusions. [O] is also reduced. In Test Nos. 11 to 14 in which Step 4 is omitted, since the oxidizing gas is used only in Step 2, the amount of Al ₂ O ₃ in Step 2 and Step 3 increases, and desulfurization and cleanliness are slightly inferior. .

  For example, even when tests having substantially the same level of slag component composition, such as test number 7 of the present invention and test number 22 of the comparative example, are compared, there is a difference in the desulfurization rate. From this result, it can be seen that it is difficult to melt a highly clean ultra-low sulfur steel simply by adjusting the slag component composition within a certain range.

In the test numbers 18 to 27 of the comparative example, the oxygen supply amount in the RH treatment was increased, so that the (FeO + MnO) content rate was increased and a slight amount of sulfurization was observed. Further, since the oxygen supply amount is large, the T.V. Compared to [O], the T.O. [O] increased. Therefore, in order to stably melt ultra-low sulfur high clean steel, it is effective to utilize Al ₂ O ₃ produced by the reaction of Al and oxygen as in the method of the present invention. In order to increase the temperature, it is understood that the temperature increase is shared by both the desulfurization treatment process and the RH treatment process, and the desulfurization and inclusion removal process by the stirring gas blowing in the process 3 are important.

Further, the present invention example is examined in detail as follows. In Test Nos. 1 to 4, the higher the value of CaO / Al ₂ O _{3 and} the lower the (FeO + MnO) content, the higher the desulfurization rate. [O] is also generally low, and the remarkable effects of the present invention are obtained.

  In addition, when test numbers 4 and 6 in which the stirring time after stopping the supply of oxygen gas in Step 3 was sufficiently secured were compared with test numbers 7 and 8 in which the stirring time was shortened to less than 4 minutes and 3 minutes, In 7 and 8, the desulfurization rate is slightly low, and the S content after step 3 is slightly high. This has shown that the desulfurization effect of this invention becomes larger by making the blowing time of the inert gas in process 3 into 4 minutes or more.

  Further, when test numbers 3 and 4 in which the molten steel was circulated after the supply of oxygen gas was stopped in step 4 were compared with test numbers 9 and 10 in which the circulatory flow was not performed, test numbers 3 and 4 showed a higher T.V. [O] is low. Therefore, it can be seen that the cleanliness of the molten steel can be increased by circulating the molten steel after stopping the supply of oxygen gas in Step 4.

  As explained above, by carrying out the method of the first invention of the present invention, an ultra-low sulfur steel having excellent cleanliness can be melted, and the conditions specified in the second to ninth inventions By satisfying the above, the effect of the present invention can be further enhanced.

  According to the melting method of the present invention, high desulfurization efficiency can be ensured and inclusions can be effectively removed by optimizing the addition of CaO-based flux, gas stirring of molten steel and flux, and the supply of oxidizing gas. Therefore, it is possible to stably smelt highly clean steel with the S content reduced to an extremely low level. Therefore, in the method of the present invention, for example, the S content in steel is 10 ppm or less and T.P. As a refining method capable of melting ultra-low sulfur high clean steel with [O] of 30 ppm or less, it can be widely applied in the steelmaking technical field.

It is a figure which shows typically the processing condition of the process 1-the process 3 in the method of this invention. It is a diagram illustrating the relationship between the degree of desulfurization and the slag CaO / Al ₂ O ₃ and the slag in the preliminary test 2 (FeO + MnO) content. It is a diagram illustrating the relationship between the degree of desulfurization and the slag CaO / Al ₂ O ₃ and the slag in the preliminary test 3 (FeO + MnO) content. Is a diagram showing the relationship between the oxidizing gas supply rate in the step 4 (X) and inclusion number index (N / N _0). It is a figure which shows the relationship between the oxidizing gas supply rate (X) in process 4, and the difference ((DELTA) S) of S content rate after completion | finish of process 3 and after completion | finish of process 4.

Explanation of symbols

1: ladle, 2: molten steel, 3: slag, 4: immersion lance for blowing inert gas,
5: Oxidizing gas blowing lance, 6: Cover

Claims (9)

A method for producing ultra-low sulfur high-clean steel, characterized by treating molten steel by the steps shown in the following steps 1 to 4.
Step 1: Adding CaO-based flux to the molten steel in the ladle under atmospheric pressure Step 2: After the step 1, the molten steel and the CaO flux are blown into the molten steel in the ladle under atmospheric pressure after step 1. Stirring, supplying an oxidizing gas to the molten steel, and mixing the oxide produced by the reaction between the oxidizing gas and the molten steel with a CaO-based flux Step 3: Stop the supply of the oxidizing gas, and at atmospheric pressure Step 4: Performing desulfurization and inclusion removal by blowing a stirring gas into the molten steel in the ladle in Step 4: When the molten steel in the ladle is treated with an RH vacuum degassing apparatus after Step 3, the RH vacuum is used. A process to raise the molten steel temperature by supplying oxidizing gas into the tank A method for producing ultra-low sulfur high-purity steel in which molten steel is processed by the steps shown in the following Steps 1 to 3, wherein Al is added to the molten steel in Step 1 or Step 2 below, and then pure oxygen is added in Step 2 A method for producing ultra-low sulfur high-clean steel, characterized in that an oxidizing gas of 0.4 Nm ³ or more per ton (t) of molten steel is blown or blown into the molten steel in a converted amount.
Step 1: Adding CaO-based flux to the molten steel in the ladle under atmospheric pressure Step 2: After the step 1, the molten steel and the CaO flux are blown into the molten steel in the ladle under atmospheric pressure after step 1. Stirring, supplying an oxidizing gas to the molten steel, and mixing the oxide produced by the reaction between the oxidizing gas and the molten steel with a CaO-based flux Step 3: Stop the supply of the oxidizing gas, and at atmospheric pressure A step of desulfurization and inclusion removal by blowing a stirring gas into the molten steel in the ladle   3. The method for melting ultra-low sulfur high-purity steel according to claim 2, wherein in step 3, the stirring gas is blown for 4 minutes or longer after the supply of the oxidizing gas is stopped. The ratio of the mass content of CaO in the slag and Al ₂ O ₃ in the slag after the completion of the process 3 is 0.9 to 2.5, and the total mass content of FeO and MnO in the slag is 8%. The method for melting ultra-low sulfur high-clean steel according to any one of claims 1 to 3, wherein:   In the step 4, after the supply of the oxidizing gas is stopped, the molten steel is continuously circulated in the RH vacuum degassing apparatus to perform a process of removing inclusions in the molten steel. The method for melting ultra-low sulfur high-clean steel according to 1 or 4.   The method for producing ultra-low sulfur high-clean steel according to any one of claims 1 to 5, wherein the oxidizing gas is oxygen gas or a mixed gas of oxygen gas and inert gas.   The supply of the oxidizing gas is performed by spraying on the surface of the molten steel using an upper blowing lance having a water cooling structure. Manufacturing method.   In one or more of the steps 1 to 3, the molten steel is treated in the molten steel in the ladle without immersing a dip tube that isolates the molten steel surface inside and outside the tube. A method for melting ultra-low sulfur high-clean steel according to any one of claims 1 to 7. The pole according to any one of claims 1 to 8, wherein when the molten steel blown in a converter is discharged into the ladle, the inflow of the converter slag into the ladle is suppressed. A method for melting low-sulfur high-clean steel.

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