Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/0075—Treating in a ladle furnace, e.g. up-/reheating of molten steel within the ladle
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
  • C21C7/06—Deoxidising, e.g. killing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
  • C21C7/064—Dephosphorising; Desulfurising
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
  • C21C7/076—Use of slags or fluxes as treating agents
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/10—Handling in a vacuum
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• the present invention relates to a method for producing an ultra-low sulfur low nitrogen steel having high cleanliness, and more particularly, a sulfur content in a product is 10 ppm or less, a total oxygen content is 30 ppm or less, and a nitrogen content
• the present invention relates to a method for melting ultra-low sulfur, low nitrogen, high cleanliness steel with a content of 50 ppm or less.
• the hot metal desulfurized in the treatment process is contaminated with sulfur by intrusion of sulfur (S) from the scrap medium solvent in the next converter blowing process and by sulfurization from residual slag during hot metal pretreatment. It is extremely difficult to produce extremely low-sulfur steel with only pretreatment. Therefore, when ultra-low-sulfurized steel is melted, molten steel desulfurization is performed in the secondary refining process after the converter steel, depending on the level of S content required for the product.
• the A1 heating method was a technique that was used after allowing the progress of desulfurization to be inhibited.
• the examples in this document exclude mixing of the generated Al 2 O and slag as much as possible, and desulfurization treatment is separately performed.
• Japanese Patent Application Laid-Open No. Hei 8-109411 discloses a method for producing an ultra-low sulfur ultra-low oxygen steel by reducing the pressure in a cylindrical dip tube immersed in ladle molten steel and performing vacuum desulfurization.
• a method is disclosed in which quick lime, alumina, and a flux of fluorite strength are added to the ladle when steel is discharged from the furnace, and then the molten steel is gas-stirred to adjust the slag component composition to an appropriate range.
• This method uses ladle slag with high absorption capacity of Al O and excellent desulfurization capacity.
• JP 2002-339014 molten steel decarburized and refined in a converter is received in a ladle, CaO and Al 2 O are added according to the S content in the steel, and vacuum is applied. At the bottom of the ladle
• a method is disclosed in which an inert gas is blown in to stir slag and metal to produce ultra-low sulfur steel.
• the metal A1 is introduced into the molten steel instead of the input Al O,
• a method of burning by supplying oxygen and using it for heating the molten steel is also disclosed.
• this method requires strong stirring of slag under vacuum purification.
• A1 is added to maintain the temperature of the molten steel, and the suggestion and consideration should be given to the CaO slag-promoting action based on the optimization of the oxidation reaction by A1. Therefore, no specific disclosure has been made regarding the method of raising the temperature by sending acid in a vacuum. There is no description about the behavior of N content in molten steel.
• 9 87730 discloses that the molten steel containing A1 in the ladle and the slag on the molten steel are stirred with an inert gas while supplying an acidic gas to the molten steel.
• a method for heating and purifying molten steel in which the ratio between the stirring power and the pure oxygen supply rate in the acidic gas is adjusted to an appropriate range, and the method for ultra-low sulfidation and cleaning of molten steel is disclosed.
• the purpose of this method is to prevent the temperature drop of the molten steel or to raise the temperature. In order to quickly absorb Al O generated by the supply of oxidizing gas, the slag on the molten steel is absorbed.
• the amount of slag that can be stirred is as small as 9 to: LOkg Zt. There is no mention of the suppression of nitrogen absorption by the generation of force berth slag.
• Japanese Patent Laid-Open No. 2000-63933 discloses that a single dip tube is immersed in molten steel, and the inside of the dip tube is depressurized while blowing an inert gas from the bottom of the ladle, thereby decarburizing and degassing refinement.
• a flux is added to the dip tube and desulfurizing after decarburization and degassing is disclosed.
• the method disclosed in the same document is a scouring method using a vacuum scouring apparatus exclusively, and a method of desulfurization using slag under atmospheric pressure and preventing nitrogen absorption as in the present invention. Is not described at all.
• the melting promotion method using the flux to be used is restricted in operation due to fluorine emission regulations. Also
• the present invention has been made to solve the above-mentioned problems, and the problem is that the steel has a very low sulfur content and a high cleanliness steel with a low nitrogen concentration, in particular, the sulfur content in the product is 10 ppm or less. It is an object of the present invention to provide a method for stably melting extremely low sulfur low nitrogen high cleanliness steel having a total oxygen content of 30 ppm or less and a nitrogen content of 50 ppm or less.
• the present invention promotes desulfurization and nitrogen reduction at the same time by controlling the heat treatment and slag component composition of molten steel and optimizing the stirring treatment of molten steel and slag, and has high cleanliness.
• This is a refined method that can produce ultra-low sulfur low nitrogen steel efficiently and stably.
• the gist of the present invention resides in the method for producing ultra-low sulfur low nitrogen high cleanliness steel shown in the following (1) to (9).
• first invention Method of melting ultra-low sulfur, low nitrogen, high cleanliness steel characterized by treating molten steel in the order shown in the following steps 1 to 3 (hereinafter also referred to as “first invention”) .
• Process 1 Process of adding CaO flux to molten steel in the ladle under atmospheric pressure
• Process 2 Covering the upper opening of the ladle under atmospheric pressure ⁇ ⁇ ⁇ Lance insertion hole for heating the molten steel and for stirring A lance insertion hole provided in the lid by installing a lid provided with at least one of the lance insertion hole and the alloy addition hole and blowing a stirring gas into the molten steel in the ladle;
• Step 3 Stop supplying the oxidizing gas and stir gas into the molten steel in the ladle under atmospheric pressure. For desulfurization and inclusion removal by blowing
• Process 1 Process of adding CaO flux to molten steel in the ladle under atmospheric pressure
• Process 2 Covering the upper opening of the ladle under atmospheric pressure ⁇ ⁇ ⁇ Lance insertion hole for heating the molten steel and for stirring
• a lid having at least one of a lance insertion hole and an alloy addition hole is installed, the opening of the lid is purged with an inert gas or a carbon dioxide gas, and the molten steel in the ladle is The molten steel and the CaO-based flux are stirred while the molten steel and the CaO-based flux are agitated while suppressing the intrusion of air into the lid from the lance insertion hole and the Z or alloy addition hole provided in the lid by blowing a stirring gas.
• a process in which an acidic gas is supplied and an acidic product generated by a reaction between the acidic gas and molten steel is mixed with a CaO-based flux to form a force slag.
• Step 3 Stopping the supply of the oxidizing gas and performing desulfurization and inclusion removal by blowing a stirring gas into the molten steel in the ladle under atmospheric pressure
• Process 4 When the molten steel in the ladle is processed using an RH vacuum degasser, the inclusions in the molten steel are reduced and denitrified.
• a CaO-based flux of 6 kg or more and 16 kg or less per 1 ton (t) of molten steel is calculated in terms of CaO until the supply of the acidic gas in the step 2 is completed.
• Add to the ladle and in the step 1 or step 2 add 1.5 kg or more of A1 per molten metal It in terms of metal A1 to the ladle.
• Method for melting nitrogen high cleanliness steel hereinafter also referred to as “fourth invention”).
• step 3 the time for blowing the stirring gas after the supply of the oxidizing gas is stopped is set to 4 minutes or more, (1) to (6), The method for melting ultra low sulfur low nitrogen high cleanliness steel according to item 1.
• the ratio of the ratios is 0.9 to 2.5, and the total mass content of FeO and MnO in the slag is 8% or less.
• ultra low sulfur low nitrogen high cleanliness steel means that the S content is reduced to an extremely low level.
• the S content in steel products is 10 ppm or less
• the total oxygen content (hereinafter referred to as “T. [0]”) is the total amount of dissolved oxygen in steel and oxygen in inclusions.
• T. [0] is the total amount of dissolved oxygen in steel and oxygen in inclusions.
• T. [O] is 15 ppm or less
• the N content power is Oppm or less.
• a special ladle refiner such as LRF
• LRF low-density metal-oxide-semiconductor
• special aids such as meteorites or premelt synthetic ironmaking agents
• Ca treatment to promote molten steel desulfurization
• CaO-based flux means a flux having a CaO content of 45% by mass or more.
• quick lime is simple and contains components such as Al 2 O 3 and MgO mainly composed of quick lime.
• Oxidizing gas means a gas having the ability to oxidize alloy elements such as Al, Si, Mn, Fe, etc. in the melting temperature region of steel. These include simple gases such as gases, mixed gases of these simple gases, and mixed gases of the above gases with inert gases or nitrogen. “Inert gas” means an element belonging to Group 18 of the periodic table, such as argon, helium, and neon.
• pressing the opening of the lid with an inert gas or a carbon dioxide gas refers to an inert gas or diacid in the space between the lid installed on the ladle and the molten steel surface. This means that an inert gas or a carbon dioxide gas is blown into the opening of the lance insertion hole and the Z or alloy addition hole provided in the lid.
• % representing the component content means “mass%”.
• This Al O floats on the surface of the molten steel as the molten steel temperature rises and is absorbed by the slag after rising.
• Step 1 the CaO-based flack is applied to the molten steel at atmospheric pressure in order to proceed with desulfurization later. Add water.
• the reason for adding CaO under atmospheric pressure is that the acid is refined in the next step. It is not necessary to do this.
• the CaO flux addition time may be either during converter steelmaking or after completion of steelmaking and before the start of secondary refining. Since it is possible to shorten the time required for adding the flattening in the process, it is preferable to add it to the converter steel.
• the flux covers the upper surface of the molten steel in the pan, so that the molten steel in the pan is also shielded from atmospheric force, and there is an effect of suppressing nitrogen absorption of the molten steel.
• A1 is added before the addition of CaO-based flux or CaO-based in order to improve desulfurization efficiency. Basically, it should be supplied to the molten steel at the same time that the flux is added. However, if the molten steel is deoxidized with the addition of A1, the subsequent molten steel is likely to absorb nitrogen in the atmosphere. Therefore, in the case of simultaneously promoting low nitrogenization in addition to low sulfur, A1 is CaO. Basically, at least a part or all of the flux is added to the molten steel.
• step 1 at least a part of the CaO-based flux used for desulfurization treatment of molten steel during or after the converter blowing, or on the upper part of the molten steel that has been removed and stored in the ladle
• the force based on the addition of may also be added in step 2.
• target temperature target A1 content of the molten steel, and target S content
• the amount of A1 added and the amount of acid gas supplied are determined, so an appropriate amount of CaO-based flux is added.
• the melting point of the CaO-based flux is high, it is preferable to further promote the melting and melting of the CaO-based flux by utilizing the high temperature region formed by supplying the oxidizing gas in the subsequent step 2. Because.
• the CaO-based flux means a flux having a CaO content of 45% or more.
• the raw stone ash is simple, and the flux containing Al O or MgO mainly composed of quick lime is used.
• A1 reduces iron oxide in the oxygen slag in the molten steel and eventually becomes Al O in the slag, which lowers the melting point of the slag and effectively works for desulfurization and cleaning of the molten steel.
• the slag component composition on the molten steel should be within an appropriate range after step 3. It is necessary to control, and it is preferable to add A1 of 1.5 kgZt or more in terms of metal A1 before the supply of acidifying gas is completed by adding up step 1 and step 2, more preferably 2 kgZt Add the above. If the amount of A1 added is less than 1.5 kgZt, the amount of Al 2 O produced is too small and slag control
• A1 is an expensive metal, and the amount of A1 added and the amount of acidic gas supplied are determined according to the target temperature and target A1 content of the molten steel, and the target S content. In Step 2, the amount of A1 added until the supply of the acidic gas is completed is actually 7. OkgZt or less.
• step 2 in order to strengthen the prevention of contact between molten steel in the ladle and the atmosphere, a lid (cover) covering the upper opening of the ladle is installed in the ladle.
• a lid (cover) covering the upper opening of the ladle is installed in the ladle.
• an inert gas for stirring the molten steel such as Ar gas is blown into the molten steel, and at the same time, the space between the molten steel surface and the lid is filled with the inert gas.
• the lid is provided with at least one of a lance insertion hole for heating the molten steel, a lance insertion hole for stirring, and an alloy addition hole for a subsequent refinement treatment operation.
• the lance for heating the molten steel generally has a metal water-cooled structure, and the tip of the lance is 0. It is inserted so that it is 5 to 3m high. Then, an oxidizing gas is blown toward the upper surface of the molten steel through the central tube of the lance.
• a refractory lance When a lance for stirring is used, a refractory lance is inserted into the pan through a lance insertion hole provided in the lid, and an inert gas is passed through the center tube of the lance through the lance tip. It is immersed in molten steel while being ejected.
• Each lance has an outer diameter of 100 to 300 mm, and the distance between the inner wall of the lance insertion hole provided in the lid and the outer surface of the lance is 20 to 50 mm.
• an alloy iron-added calorific opening for adding alloy iron such as ferromanganese or aluminum may be provided on the lid.
• the opening of the lid is purged with an inert gas or a carbon dioxide-dioxide carbon gas
• an inert gas or a carbon dioxide-dioxide carbon gas Prior to stirring the molten steel, purging with an inert gas or a carbon dioxide gas for about 2 minutes can further enhance the effect of suppressing the increase in nitrogen content.
• the inert gas is blown in order to facilitate the absorption of the acidic 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 area between the acidic gas and the molten steel surface, and the uniform dispersion of Al 2 O is inhibited. It is.
• an acidic gas is supplied in step 2 to react A1 and oxygen in the molten steel.
• the present inventors investigated the effect on the desulfurization rate by variously changing the supply amount of the oxidizing gas in order to ascertain the preference and range of the acidic gas supply amount. As a result, it was possible to obtain a desulfurization rate of 80% or more by setting the supply amount of the oxidizing gas to 0.4 Nm 3 Zt or more in terms of pure oxygen. On the other hand, when the supply amount of the oxidizing gas was less than 0.4 NmVt in terms of pure oxygen, the desulfurization rate was 5 to 20% lower than the above value. This is because when the supply amount of oxidizing gas decreases, the generated Al O
• the desulfurization rate can be further increased by setting the supply amount of the oxidizing gas to 0.4 Nm 3 Zt or more in terms of pure oxygen.
• the amount of additive A1 and the amount of additive CaO according to the supply amount of the acidic gas.
• the desulfurization rate can be stabilized by spraying an acidic gas on the surface of the molten steel through the top blowing lance.
• the reason is as follows.
• there is another method for supplying the oxidizing gas in which an oxidizing gas is blown into the molten steel.
• the oxidizing gas is subjected to the static pressure of the molten steel. Pressure increases.
• the partial pressure of the oxidizing gas rises excessively, the acidic gas reacts directly with Fe, Mn, and other elements other than A1 in the molten steel. As a result, it is difficult to control Al O production.
• the lance for blowing the oxygen-containing gas into the molten steel requires a device for preventing the melting damage, but the surface of the molten steel is not inserted into the lance via the upper blowing lance. In the method of spraying, the lance melting is hardly an issue, which is also advantageous.
• the desulfurization reaction can be made to proceed more stably by spraying an acidic gas on the surface of the molten steel. Since the top blow lance receives strong radiant heat from molten steel or slag, it is preferable to have a durable surface-force water-cooled structure.
• a force using a synthetic steel material having a good flexibility such as calcium aluminate is not particularly required in the present invention.
• the reason is as follows. That is, in the present invention, the supply point of the oxidizing gas locally becomes high due to the oxidation reaction heat of A1 generated by supplying an acidic gas to the molten steel and reacting with A1 in the molten steel. It is. At the same time, the strong stirring by the inert gas blown from the immersed lance promotes the mixing and reaction of the oxide produced by the reaction between the acidic gas and the molten steel and the CaO flux, and the desulfurization ability.
• the high-grade slag is quickly made.
• the slag having a high desulfurization ability covers the surface of the molten steel, it is possible to block contact between the atmosphere and the molten steel surface even during the stirring of the molten steel.
• the temperature rise of the molten steel is simultaneously achieved by the acidity of A1
• the viewpoint of the steelmaking and heat compensation at the time of desulfurization is more advantageous than using calcium aluminate or the like.
• the acidic gas in the step 2 is preferably oxygen gas or a mixture of oxygen gas and inert gas. Moreover, the method of performing the said process without immersing dip tubes, such as a snorkel, is still more preferable from a viewpoint of enlarging reaction interface area.
• step 2 the slag component composition is controlled and melted, and the desulfurization reaction proceeds.
• the desulfurization reaction does not proceed sufficiently during the supply time of the acidic gas, and desulfurization capacity remains in the slag.
• “desulfurization capacity” means desulfurization ability governed by the composition of slag, as will be described later. Also, although not so much as to change the composition of the slag, several tens of ppm of Al O is included as inclusions in the molten steel.
• the slag still has a desulfurization surplus, and the molten steel contains inclusions of AlO.
• step 3 Since 2 3 remains slightly, after step 2, in step 3, the supply of oxidizing gas is stopped, and the stirring gas is blown into the molten steel at atmospheric pressure, so that desulfurization and inclusions are removed. Perform removal processing. By this treatment, further desulfurization with slag having surplus desulfurization capacity and unnecessary residual inclusions will be removed.
• “desulfurization capacity” means sulfide capacity controlled by the composition of slag, that is, “desulfurization capacity”. The sulfur eyed capacity is reduced when lower acid compounds such as FeO and MnO are present in the slag. Therefore, in order to maximize the desulfurization power, it is necessary to control the composition of slag components and reduce the concentration of lower oxides.
• step 2 the lower oxide is inevitably generated by the supply of the acidic 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.
• the process of step 4 may be performed after the process of step 3.
• Step 1 to Step 3 the molten steel in the ladle is treated under atmospheric pressure.
• the ladle is transferred to an RH vacuum degassing treatment device (hereinafter also referred to as “RH device”, and treatment by the RH device is also referred to as “RH treatment”), and denitrification is performed in the RH treatment.
• RH device an RH vacuum degassing treatment device
• RH treatment treatment by the RH device
• denitrification is performed in the RH treatment.
• an acidic gas may be supplied to the molten steel to raise the molten steel temperature, and then the molten steel may be circulated in the RH apparatus. Through this process, the desulfurization rate and cleanliness can be further increased, and the nitrogen content can be reduced.
• an acidic gas is supplied using an RH device to increase the temperature of the molten steel.
• the sol. Al content before RH treatment was basically higher than the upper limit of the component specification for A1.
• the temperature rise of the molten steel in this process is limited to the amount necessary to adjust sol.Al to the component standard, and the other heat compensation is the above-mentioned desulfurization from the viewpoint of promoting slag formation and removing inclusions. Performed in the previous molten steel heating It is preferable. It is effective to melt the ultra-low sulfur low nitrogen steel by continuously decirculating nitrogen after the acid transfer.
• step 1 to step 3 and preferably step 4 described above By performing the processes from step 1 to step 3 and preferably step 4 described above in the above order, the temperature rise of the molten steel and the slag component composition are suppressed while suppressing the absorption of nitrogen from the atmosphere. It is possible to control at the same time, resulting in efficient desulfurization and inclusion reduction by avoiding or suppressing the strong stirring of molten steel and slag and the use of synthetic desulfurization agents and meteorites. Can be planned.
• FIG. 1 is a diagram schematically showing the processing status and processing apparatus of steps 1 to 3 in the method of the present invention. While the 250 tons (t) of molten steel blown in the converter is also discharged to the ladle 1, the converter force is directed toward the outlet of the molten steel surface in the ladle 1 After adding lOkgZt of quick lime with a CaO component content of 92%, metal Al was added to complete the steel output.
• quick lime and metal A1 were added at the same time during steel production, and the above-mentioned quick lime was added by 2.5 kgZt until 20% had elapsed after the start of steel production, and then 50% Add metal A1 by the time it passes, and then add 7% of the quicklime until 70% has passed.
• Molten steel composition at the end of converter blowing is mass%, C: 0.03-0.15%, Si: not more than 0.01%
• Mn 0.05 to 0.4%
• P 0.05% or less
• S 27 to 28 ppm
• N 13 to 14 ppm.
• top-blown oxygen gas through the lance 5 blown on liftable oxidizing gas at 0. 150Nm 3 ZminZt spraying speed was blown 1. 2 Nm 3 Zt in total. After finishing the top blowing of oxygen, Ar gas was continuously blown into molten steel 2 and stirred for 10 minutes. The number 3 in the figure indicates slag.
• the upper surface area (A) of the molten steel in the ladle was 12 m 2
• the purge Ar gas flow rate (V) was 0 to 21 Nm ”Zmin.
• the pre-melt mixed flux is added to the molten steel 2 in the 15 kg Zt ladle 1 and the ladle 1 is covered with the lid 6, but the opening of the lid is purged with an inert gas or carbon dioxide gas.
• This test is omitted, and stirring is performed by blowing Ar gas for 17 minutes without supplying oxygen.
• oxygen gas was not used in step 2 as in the comparative test (b2). Therefore, the A1 content in the molten steel at the time before stirring by blowing Ar gas in step 2 was set to 0.06. Adjusted to%.
• Test conditions are shown in Table 1, and test results are shown in Table 2, respectively.
• T. [o] The low value of T. [o] is also the force that promotes melting of slag and improves the ability to absorb inclusions by slag. It should be noted that the mixing ratio of CaO and Al 2 O
• the method has been shown to be effective for simultaneous treatment of desulfurization and cleaning.
• the N content after step 3 is reduced to a method (test number 19). Compared to comparative tests bl and b2), it was confirmed to be 8 to 16 ppm lower. In the method of the sixth invention (Test Nos. 1 to 6) in which sardine coconut and quicklime were added in portions, good results were obtained with the lowest N content.
• the stirring gas after the supply of oxidizing gas is stopped. It prescribes.
• the slag has a desulfurization surplus even after the production of Al 2 O.
• Blowing gas and continuing stirring to promote desulfurization A preferable stirring time in this desulfurization acceleration treatment was examined by the following method.
• the Ar gas injection flow rate was set to the range of 0.008 to 0.020 Nm 3 ZminZt, and the effect on the desulfurization rate was adjusted by changing the injection time.
• the eighth invention prescribes a preferable component composition of the slag after the completion of the treatment by the step 3.
• Al O produced by the supply of an acidic gas to molten steel is controlled by the slag component composition.
• T. [O] in the molten steel after treatment was 14-24 ppm. If this range was not satisfied, T. [O] was 18-28 ppm, which was a high value.
• 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. I was able to do what I could.
• the control of the slag component composition can be adjusted by adding the CaO-based flux described above. In the present invention, the above object can be achieved without using meteorite, but the presence of CaF component mixed in the added flux can be eliminated.
• meteorite is further advantageous in terms of improving the desulfurization efficiency and reducing the S content after treatment.
• an opening of the lid of the ladle is provided with an inert gas or a carbon dioxide gas having an appropriate flow rate.
• FIG. 2 is a graph showing the relationship between the purge gas flow rate in steps 2 and 3 and the N content in steel after step 3 is completed.
• the horizontal axis represents the purge gas flow rate: V (Nm 3 Zmin) divided by the upper surface area of the molten steel in the ladle: A (m 2 ) (VZA).
• a force that is effective even if a small amount of Ar gas is blown into the space above the molten steel Specifically, when VZ A is 0.05 or higher, the effect of using purge gas clearly appears, and the N content in steel is 40ppm. The following low nitrogen steels can be obtained. When VZA is further increased to 0.16 or more, a low nitrogen steel having a N content of 37 ppm or less can be obtained. If it is less than 0.16, the purge gas flow rate is relatively small, so air easily enters from the opening and As the amount of nitrogen absorbed in steel increases, the N content in steel increases.
• the purge gas flow rate V (Nm 3 Zmin) and the upper surface area of the molten steel in the ladle: A (m 2 ) so that the purge gas flow rate satisfies the relationship expressed by the following equation (1) It is more preferable that the N content in the steel after Step 3 is approximately 37 ppm or less in the implementation of the first to eighth inventions, after adjusting and purging.
• the process 4 can be performed after the process 3.
• Step 4 after the supply of oxidizing gas is stopped, the molten steel is continuously recirculated in the RH unit, and denitrification is performed along with the inclusion removal treatment.
• the cleanliness of the molten steel can be further enhanced by performing a reflux treatment for a certain period of time after supplying the oxidizing gas. Further, the nitrogen concentration contained in the molten steel can be reduced with the molten steel reflux treatment using Ar gas.
• FIG. 1 is a diagram schematically showing the treatment status and the treatment apparatus of steps 1 to 3 in the method of the present invention.
• FIG. 2 is a graph showing the relationship between the purge gas flow rate in step 2 or 3 and the N content in steel after step 3.
• Figure 3 shows the desulfurization rate and CaOZAl O in slag and in slag (FeO +
• Figure 4 shows the desulfurization rate and CaOZAl O in slag and in slag (FeO +
• the method of the present invention is a method for producing ultra-low sulfur, low nitrogen, high cleanliness steel by treating molten steel by the following Step 1 to Step 3 or Step 1 to Step 4.
• Step 1 to Step 3 or Step 1 to Step 4 preferred embodiments for carrying out the method for melting ultra-low sulfur high cleanliness steel according to the present invention will be described in more detail.
• the basic principle of the present invention is that in Step 1, all of the CaO-based flux used for the molten steel desulfurization treatment is added to the upper part of the molten steel that has been produced after the converter blowing and stored in the ladle. However, this does not exclude adding a part of it in step 2. Depending on the target temperature, target A1 content, and target S content, the amount of aluminum (A1) added and the amount of oxidizing gas supplied will be determined. Add the appropriate amount of CaO flux. A predetermined amount of CaO-based flux may be added at once, or may be added in divided portions.
• the CaO-based flux has a high melting point, the CaO-based flux can be further melted and melted by utilizing the high temperature region formed by the supply of the acidic gas in the subsequent step 2. Is preferable.
• quick lime is preferable because it is inexpensive and economically advantageous, and also has good weatherability.
• it is effective to form a force berth slag that covers the surface and area of the molten steel as quickly as possible. It is desirable to use at least%.
• a portion of the CaO-based flux is added to the ladle prior to the addition of A1 to the molten steel during the steel feeding from the converter to the ladle to form a force slag. After that, a predetermined amount of A1 is added to the molten steel in the ladle, and then the remaining CaO flux and A1 are added to the ladle.
• the amount of flux added to form the force berth slag is 1 to 3 kgZt. Further preferred. If the amount of flux is less than 1 kgZ, the force berth lag may become too thin, and there will be a risk of insufficient shielding of atmospheric power. Also, when the amount of flux is more than 3kg Z, slag with a higher melting point is formed and the fluidity is lowered.Therefore, a region not covered with slag is locally generated on the surface of the molten steel, and the atmosphere This is a force that may cause an unsatisfactory interruption.
• the amount of A1 to be added is sol.
• the oxidation of A1 by oxidizing gas is essential in the above step 2
• the total amount of A1 commensurate with the planned amount of A1 acid is added to the molten steel at this stage of steelmaking. May be.
• This A1 is added to the molten steel in the ladle that is still in the middle of steelmaking from the previous stage. In this case as well, in order to dissolve A1 quickly and uniformly, it is possible to use the stirring power of the molten steel stream without waste by matching the position of the dropping point of A1 into the ladle and the falling point of the molten steel flow. is important
• the steelmaking start force is also reduced by 20% of the total steelmaking time.
• Add a part of A1 and then add A1 to more than 0.050% by the time 50% have passed.
• the remaining amount of A1 can be added by the end of the supply of the acidic gas in the second step.
• the converter slag contains Po and is used in the subsequent desulfurization process.
• step 1 not only step 1, but also steps 2 and 3 described later are performed under atmospheric pressure.
• steps 2 and 3 described later are performed under atmospheric pressure. The reason is that, in the present invention, it is not necessary to perform a strong stirring operation under reduced pressure, and in order to carry out the processes of steps 1 to 3 under reduced pressure, the equipment cost and running cost increase.
• step 2 the molten steel and CaO flux are agitated by blowing the stirring gas into the ladle in the ladle at atmospheric pressure to which the CaO flux has been added in step 1, and at the same time, the oxidizing gas is added to the molten steel.
• the oxidizing gas is added to the molten steel.
• Al O produced by the reaction between the acidic gas and molten steel.
• the additive amount of CaO and A1 which are directly targeted in the present invention, includes those charged in the ladle before the start of the steelmaking of the converter power. This means that until the supply of the acidic gas is completed.
• step 2 the acidic gas is supplied to the molten steel by using the oxidative exothermic reaction caused by the reaction between the molten steel components and the oxidizing gas to suppress the heating of the molten steel or the temperature drop. This is because the composition of slag is controlled by generating Al 2 O. This acid gas
• the above-mentioned kind of gas having the ability to oxidize elements in molten steel can be used.
• a method for supplying the acidic gas a method of blowing an oxidizing gas into the molten steel, a lance disposed above the molten steel, or a method of blowing an acidic gas with a nozzle force can be used. From the viewpoints of slag controllability and improvement of slag melting and weatherability by utilizing a high temperature region, a method of spraying on the surface of molten steel using an upper blowing lance is preferred. As a result, the CaO-based flux can be directly heated using the high-temperature region formed by the reaction between the acidic gas and the molten steel in the ladle to promote the CaO-based flux. it can.
• the height of the lance or nozzle from the upper surface (molten surface) of the molten steel is preferably in the range of about 0.5 to 3 m. If the lance or nozzle height is less than 0.5m, the spitting of the molten steel will become severe and the lance or nozzle life may be reduced, while if it exceeds 3m, the acid gas jet will be This is because it reaches the surface of the molten steel and there is a risk that the oxygen efficiency of the smelting may be significantly reduced.
• the supply amount of Sani ⁇ gas in step 2 pure oxygen equivalent amount in 0. 4 Nm 3 Zt least a child and force transducer preferred, and even more preferably to a 1. 2 Nm 3 Zt more.
• This oxygen supply amount is preferred for oxidizing the A1 to obtain a heat source for maintaining the temperature and increasing the temperature of the molten steel.
• the oxygen supply amount of the CaO source added in step 1 is also preferred. It is also a good supply amount for promoting slagui dredging.
• the supply rate of the oxidizing gas is preferably in the range of 0.075-0.24 NmVmin / t in terms of pure oxygen. If the supply rate of the oxidizing gas is less than 0.075 Nm 3 ZminZt, the processing time may be long and productivity may be reduced. On the other hand, if it exceeds 0.24Nm 3ZminZt, the CaO flux can be heated sufficiently, but the supply time of the oxidizing gas is shortened and the amount of Al 2 O generated per unit time is reduced.
• step 2 Al 2 O is generated by supplying the acidic gas performed as described above.
• the acid oxide produced by the reaction between the acid gas and molten steel is mainly Al 2 O.
• Stirring methods in Step 2 include introducing a stirring gas into the molten steel through a lance immersed in the molten steel, and introducing a porous plug force stirring gas installed at the bottom of the ladle. It is preferable to introduce a stirring gas into the molten steel through the lance. The reason for this is that in the case of introducing a porous plug force stirring gas installed at the bottom of the ladle, it is difficult to introduce a gas with a sufficient flow rate.
• the flow rate of the stirring gas is preferably in the range of 0.0033 to 0.02Nm 3 ZminZt.
• the blowing flow rate is less than 0.0033 Nm 3 ZminZt, the stirring force is insufficient, the stirring of the slag and Al 2 O is insufficient, and the oxygen potential of the slag is increased.
• Step 3 This is because the reduction of the oxygen potential of the slag in Step 3 is insufficient, which may be disadvantageous for desulfurization.
• the blowing flow rate exceeds 0.02 Nm 3 ZminZt, the occurrence of splash will increase excessively and the productivity may be reduced. Reduce the oxygen potential of the slag as much as possible and reduce productivity. In order to avoid this, it is more preferable to set the blowing flow rate to 0.015 Nm 3 ZminZt or less.
• step 3 the supply of the acidic gas using the top blowing lance is stopped, and the molten steel by blowing the stirring gas through the lance immersed in the molten steel in the ladle at atmospheric pressure and the like. Continue stirring the slag to desulfurize and remove inclusions.
• the stirring gas blowing time after stopping the supply of the acidic gas is preferably 4 minutes or more, more preferably 20 minutes or less.
• the amount of stirring gas blown is preferably in the range of 0.005 to 0.02 Nm 3 ZminZt. The reason why it is preferable to continue stirring under the above-mentioned conditions for melting extremely low sulfur high cleanliness steel will be described below.
• step 2 in order to increase the oxygen potential of the slag during the supply of the acidic gas, the supply rate of the oxidizing gas is decreased or the atmospheric pressure is reduced. It is conceivable to supply an acidic gas while blowing a large amount of stirring gas into the molten steel.
• Step 2 stirring of the molten steel and the slag in the ladle is carried out by the oxidizing gas supply period. (Step 2) and the subsequent period (Step 3) when no acidic gas is supplied. That is, even after the supply of the acidic gas by the top blowing lance 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 step, the concentration of lower acid oxides in the slag can be reduced and the desulfurization ability of the slag can be maximized. Under normal gas supply conditions, the stirring gas injection time t in step 3 with respect to the acidic gas supply time t in step 2
• the ratio (tZt) is preferably 0.5 or more.
• the acid generated by the supply of the acidic gas in the step 2 Separation of material inclusions is also performed at the same time.
• the gas stirring time by the stirring gas blowing is preferably 4 minutes or longer. If the gas agitation 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 acidic gas in Step 2 in Step 3, and the desulfurization rate is increased. It is also a force that makes it difficult to secure reaction time to sufficiently reduce [O].
• the longer the gas agitation time the lower the effect of low sulfur and cleanliness, but on the other hand, the productivity decreases and the molten steel temperature also decreases. Is preferred.
• the stirring gas blowing in step 3 is also preferably performed by a method in which the stirring gas is introduced through a lance immersed in molten steel.
• the reason for this is, for example, when introducing a stirring gas from a porous plug installed at the bottom of the ladle, it is difficult to introduce a sufficient amount of gas into the molten steel. This is because the FeO and MnO components cannot be sufficiently reduced, and it may be difficult to produce extremely low sulfur steel.
• the method of the present invention is characterized in that gas stirring is performed under atmospheric pressure. It is difficult to stir slag and metal vigorously with a small amount of gas injection like gas stirring under reduced pressure, and it is difficult to perform gas stirring under stable gas flow conditions. is there.
• the flow rate of the stirring gas is preferably 0.0033 to 0.02 Nm 3 ZminZt. If the blowing flow rate is less than 0.0033 Nm 3 ZminZt, the stirring force is insufficient, and the oxygen potential of the slag in the process 3 is insufficiently reduced, and further desulfurization may not be promoted. Moreover, if the blowing flow rate exceeds 0.02 Nm 3 ZminZt, the occurrence of splash becomes extremely large, which may lead to a decrease in productivity. In order to reduce the oxygen potential of the slag as much as possible and to avoid a decrease in productivity, it is more preferable to set the blowing flow rate to not more than 0.005 Nm 3 ZminZt.
• the composition of the slag component after completion of the treatment in Step 3 is defined in the eighth invention, and the ratio of the mass content of CaO to Al 2 O (hereinafter also referred to as “CaOZAl 2 O”) is 0.9 to 2.
• the total mass content of FeO and MnO in the slag (hereinafter also referred to as “FeO + MnO”) is preferably 8% or less.
• Preliminary Test 2 After adding CaO and A1 to the molten steel in Step 1, oxygen gas was blown with an upper blow lance in Step 2, and then Ar gas was blown for 9 minutes. Stirring was performed. 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. The meteorite was not used.
• the meteorite was added so that the meteorite content would be 10-15%, and only Ar gas was blown in without supplying the acidic gas, and only the stirring operation was performed for 13 minutes. In both preliminary tests 2 and 3, the total amount of slag is 18-22kgZt.
• the desulfurization rate after the treatment was measured and arranged as a relationship with the content of CaO / Al 2 O in slag and (FeO + MnO) in slag.
• Figure 3 shows the desulfurization rate and CaOZAl O in slag and in slag (FeO +
• Fig. 4 is a diagram showing the relationship between the desulfurization rate and the CaOZAl O content in slag and (FeO + MnO) content in slag.
• the value of CaOZAlO is set to 0.
• the content is 3% or less, a desulfurization rate of 90% or more is obtained, which is more preferable.
• the CaOZAl O value should be 1.5 or more, and the (FeO + MnO) content should be 1% or less.
• step 3 Steel composition, inclusion control, etc. after completion of step 3
• an ultra-low sulfur, low nitrogen, high cleanliness steel with an S content of 10 ppm or less in the molten steel, a T. [O] of 30 ppm or less, and an N content of 50 ppm or less is produced.
• the temperature at the end of step 3 is about 1590-1665 ° C.
• Table 3 shows a typical composition range of steel at the end of Step 3.
• T. [O] is a force that is lower in the product than at the end of Step 3. This is due to the effect of Step 4 following Step 3. is there.
• N the N content is suppressed to 50 ppm or less at the end of Step 3. Note that the N content may be lower in the product than at the end of Step 3. This is due to the effect of Step 4 following Step 3.
• the molten steel in the ladle is treated without immersing a dip tube such as a snorkel.
• a dip tube of a degassing device When a dip tube of a degassing device is immersed, slag is divided inside and outside the dip tube, and the slag existing in the area where the oxidative gas is supplied is promoted, but the other areas In this case, the slag that exists in the lagoon is delayed in hatching, and the slag existing outside the dip tube is not sufficiently stirred, which may reduce the amount of slag that effectively acts on desulfurization.
• the amount of slag after the completion of step 3 is preferably about 13 to 32 kgZt. If the amount of slag is less than 13 kgZt, it is difficult to obtain a stable desulfurization rate with a small amount of slag. Considering the slag outflow from the converter to the ladle, the amount of slag in the pan after Step 3 is more preferably 16 kgZt or more. In addition, if the amount of slag exceeds 32 kgZt, the time required to control slag component yarn formation becomes longer, and as a result, the processing time may be extended. Considering the ladle capacity (the sum of the molten steel volume and the slag volume in the ladle) and the degree of stirring between the molten steel and slag, this slag amount is more preferably 25 kgZt or less.
• the inclusions spherical when hydrogen-induced cracking resistance is required or when it is necessary to prevent nozzle clogging in the continuous forging process, for example, CaSi, CaAl, FeCa, FeNiCa It is preferable to make the inclusions spherical by adding a Ca-containing substance such as In this case, the CaSi loading is preferably in the range of about 0.2 to 1.2 kgZt.
• the CaO content in the spherical inclusion is preferably 45 to 75%. This is because when the CaO content is less than 45%, the spheroidizing action becomes unstable, whereas when the CaO content exceeds 75%, the stretchability of inclusions increases, which can be the origin of hydrogen-induced cracking.
• meteorite In recent years, meteorites have been difficult to obtain due to the depletion of resources, and their use tends to be restricted due to consideration of environmental issues. It is also suitable as a method for melting mold steel.
• Step 4 is a step that may be carried out after Step 3 in order to perform temperature compensation while suppressing ultrasulfurization and maintaining an extremely low S content, and to improve cleanliness and denitrification. It is. This requires the use of RH equipment.
• the 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.
• the reaction rate between the slag and molten steel is low, resulfurization can be suppressed even when a RH heat treatment is performed using an RH device.
• Step 1 to Step 3 Through the processes of Step 1 to Step 3 described above, desulfurization, denitrification, and steel cleaning to the extremely low sulfur range by using A1 and oxygen and CaO-based flutters are achieved.
• 3 As a component at the end, molten steel with S content of 10 ppm or less, T. [O] of 30 ppm or less, and N content of 50 ppm or less can be obtained at low cost.
• the corresponding product The components in the product have an S content of 10 ppm or less, T. [O] of 30 ppm or less, and an N content of 50 ppm or less without additional treatment such as addition of Ca.
• step 3 S content is 6 ppm or less, T. [O] is 25 ppm or less, and N content is 40 ppm or less. Can be obtained at low cost. Furthermore, by combining step 4, as an ingredient in the product, without additional treatment such as addition of Ca, the S content is 6 ppm or less, T. [O] is 15 ppm or less, and the N content is Steel material of 40ppm or less can be manufactured at low cost.
• the hot metal that has been subjected to hot metal desulfurization and hot metal dephosphorization treatment in advance is charged into a 250 ton (t) -scale top-bottom blow converter, and the C content in the molten iron is 0.03 to 0.2. Rough decarburization blowing was performed until it reached%.
• the end-point temperature is 1630 ⁇ 1690 ° C, and the crude decarburized molten steel is taken out into the ladle, and various deoxidizers and alloys are added at the time of steel making to add the molten steel components in the ladle to C: 0.03-0.
• step 1 at the time of steel extraction under atmospheric pressure, 8 kgZt of quick lime in terms of CaO was added to the molten steel in the ladle within 50% of the previous period of total steel production time. In addition, after adding this quicklime, 400 kg of metal A1 was added all at once.
• Step 2 a lid is placed on the ladle and the gap in the opening is purged with Ar gas, and the immersion lance is immersed in the molten steel in the ladle.
• Ar gas is supplied at a rate of 0.012 Nm 3 ZminZt. Blowing At the same time, the top blowing lance force with a water cooling structure also sprayed oxygen gas on the surface of the molten steel at a supply rate of 0.14 NmVmin / t. At this time, the direct lead distance between the lower end of the lance and the upper surface of the molten steel was 1.8 m, and the oxygen supply time was 6 minutes. In addition, the dip tube was not crushed in the molten steel, and the generated gas, splash, dust, etc. were introduced into the dust collector from the inside of the ladle lid and processed.
• step 3 after the supply of oxygen gas is stopped, the gap in the opening of the ladle lid is continuously purged with Ar gas, and Ar gas is blown for 10 minutes at the above Ar gas supply rate and stirred. Went.
• the composition of the slag component after step 3 is 0.9 to 2.4 for CaOZAl O, (
• FeO + MnO FeO + MnO
• step 4 oxygen gas was also blown with 1.6 Nm 3 Zt for the top blowing lance force installed in the vacuum chamber immediately after the start of RH treatment.
• the straight lance nozzle was used, the vertical distance between the bottom 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 ZminZt.
• the RH device has a dip tube diameter of 0.66 m, a reflux Ar gas flow rate of 2.0 Nm 3 Zmin, 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 slag amount in the melting test is about 18kgZt.
• Tables 4 and 5 show the test conditions for test numbers 1 to 14 of the present invention example and test numbers 15 to 27 of the comparative example, and the desulfurization rate, the S content in the molten steel, and the number of inclusions in the steel. Test results such as T. [0] as an index and N content in steel are shown.
• Comparative example A is JS 3's, comparison example's, ratio ⁇ J B is process 2's! / S! J represents I * ⁇ .
• Desulfurization rate is calculated by ⁇ (Snudged buckwheat [S] — [S] after Step 3) / Salted [S] ⁇ X100 (%).
• Step 2 the immersion lance is immersed in the molten steel in the ladle, and Ar gas is blown at a supply rate of 0.012 Nm 3 ZminZt, and oxygen gas is supplied from the top blowing lancer having a water cooling structure to 0.14 Nm 3 ZminZt. Sprayed onto the surface of the molten steel at a feed rate of. At this time, the vertical distance between the bottom of the lance and the surface of the molten steel (metal surface) was 1.8 m, and the oxygen supply time was 6 minutes.
• Step 4 the upper blowing lance force oxygen gas installed in the vacuum chamber immediately after the start of the RH treatment was blown 1.
• ONm 3 Zt The operating conditions of RH were the same as in the example of the present invention.
• the RH device has a dip tube diameter of 0.66 m, a reflux Ar gas flow rate of 2.0 Nm 3 Zmin, 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.
• Test Nos. 1 to 14 which are tests for the inventive examples, have a significantly improved desulfurization rate as compared to Test Nos. 15 to 27, which are tests for comparative examples.
• the content is greatly reduced, and T. [o], which is an indicator of the number of inclusions, is also reduced.
• test numbers 1 to 14 are examined in detail as follows.
• test numbers 1 to 4 the higher the value of CaOZAl O, the higher the (FeO + MnO) content.
• test numbers 4 and 6 in which sufficient stirring time after the supply of oxygen gas was stopped in step 3 were compared with test numbers 7 and 8 in which the same stirring time was shortened to less than 4 minutes and 3 minutes, In Test Nos. 7 and 8, the S content after Step 3 where the desulfurization rate is low is slightly high. This indicates that the desulfurization effect of the present invention is further increased by setting the inert gas blowing time in Step 3 to 4 minutes or more.
• the gas flow rate of the gap purge at the opening of the ladle lid and the upper surface area of the molten steel in the ladle satisfy the relationship (1) defined in the ninth invention.
• an ultra-low sulfur low nitrogen high cleanliness steel having a lower N content in the steel than in test number 1 of the present invention example that does not satisfy the relationship of the formula (1) could be produced.
• the melting method of the present invention the addition of CaO-based flux, gas stirring of molten steel and flux, and the supply of acidic gas are optimized, and the ladle is covered with a lid. Purging the opening of the pan lid with an inert gas ensures high desulfurization efficiency and at the same time effectively removes inclusions and suppresses the absorption of nitrogen in the molten steel.
• steel with a low N content and high cleanliness can be stably produced. Therefore, the method of the present invention is based on excellent economic efficiency, for example, an extremely low sulfur and low nitrogen content in which the S content in steel is 10 ppm or less, the T. [O] is 30 ppm or less, and the N content is 50 ppm or less.
• a refinement method capable of melting high cleanliness steel it can be widely applied in the steelmaking technical field.

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