JP5545148B2 - Molten steel refining method - Google Patents

Description

  The present invention relates to a molten steel refining method for performing desulfurization refining while circulating the molten steel in a ladle to a vacuum tank in which pressure is reduced.

  Due to the high need for materials that have both high strength and high workability, such as high-strength steel for automobiles, the cleanliness of steel has been increasing. In order to ensure hole expansibility, which is a typical index of workability, it is important to suppress precipitation of MnS, which is an extensible inclusion, and for that purpose, ultra-low sulfurization of steel is performed.

  Low-sulfur steel with an S concentration in the steel of 0.003 to 0.010 mass% can be melted by performing a desulfurization process in the hot metal stage before converter refining. On the other hand, for ultra-low-sulfur steel having an S concentration of 0.0015% by mass or less, further 0.0010% by mass or less in steel, the hot metal desulfurization treatment is not sufficient, and after the refining of the converter is completed. Desulfurization is performed in the molten steel stage. For molten steel desulfurization treatment, desulfurization treatment is performed while performing secondary refining under reduced pressure using a flux injection method, RH vacuum degassing device, etc., in which a powder desulfurization agent is blown into the ladle in the ladle under atmospheric pressure with a carrier gas. A method of performing is used.

  For example, as a desulfurization method for an RH vacuum degassing apparatus that is most widely used as a vacuum degassing facility, Patent Document 1 discloses that a depressurization is performed using an upper blowing lance during refining in an RH vacuum degassing apparatus. Discloses a method of desulfurization treatment by spraying a desulfurizing agent on the surface of the molten steel.

In molten steel refining using an RH vacuum degassing apparatus, a method is sometimes used in which oxygen gas is blown onto the surface of the molten steel in a vacuum tank and Al contained in the molten steel is burned to increase the temperature of the molten steel. In RH vacuum degassing, in the case where such a temperature rise by Al combustion and molten steel desulfurization treatment are used together, if the temperature rise treatment by Al combustion is performed after the desulfurization treatment, S once shifted from molten steel to slag increases. It is said that a phenomenon that returns to molten steel, that is, a so-called sulfite reaction, occurs with an increase in oxygen potential during temperature treatment. In Patent Document 2, it is said that such a desulfurization phenomenon can be prevented by performing a desulfurization process after performing a temperature raising process by burning Al. After completion of the temperature raising treatment, the molten steel is circulated for 3 minutes or more, and the desulfurization treatment is started after the Al ₂ O ₃ generated by the temperature raising treatment is floated and separated.

When the desulfurizing agent is added to the molten steel as powder, a flux mainly composed of CaO and CaF ₂ is used as the desulfurizing agent. In the desulfurization process in the RH vacuum process, a desulfurization reaction after the desulfurization agent added to the molten steel reaches the ladle slag cannot be expected. In Patent Document 3, when the desulfurization treatment is performed using a flux mainly composed of CaO and CaF ₂ by an RH vacuum degassing apparatus, the volume mixing ratio occupied by the particle size of 75 μm or less of the raw material particles constituting the flux is set to 70. It is said that a high desulfurization rate can be obtained by setting the ratio to at least%.

However, it was found that if a flux mainly composed of fine powder of 75 μm or less is used, inclusions resulting from this fine powder flux remain in the molten steel, and conversely deteriorate the cleanliness of the molten steel. When the desulfurization flux is added by the injection method in the RH degassing apparatus using the fine powder flux, when the hot rolled steel sheet is evaluated, there are many A-based inclusions even though the desulfurization treatment is performed. I understood it. When the composition analysis of those inclusions was performed, it was found that the inclusion was a CaO—CaF ₂ —Al ₂ O ₃ —CaS system, and the desulfurization flux remained. Therefore, in Patent Document 4, when producing ultra-low sulfur steel by blowing desulfurization flux into molten steel in secondary refining, desulfurization with a particle size of less than 100 μm is less than 5% by mass and an average particle size is less than 300 μm. An invention using a flux is disclosed.

JP-A-5-214424 JP 2008-169407 A JP-A-8-269533 JP 2002-322510 A

  As described above, when molten steel desulfurization is performed using an RH vacuum degassing apparatus, if the particle size of the desulfurization flux used is reduced, the desulfurization rate is improved, but the A-based inclusions in the steel sheet increase, and the particle size of the desulfurization flux is increased. As a result, the A-based inclusions decrease, but the desulfurization rate decreases.

  In the molten steel refining that recirculates the molten steel in the ladle to a vacuum tank that has been depressurized and refining, when performing desulfurization refining by adding a powder desulfurizing agent to the molten steel, while maintaining an excellent desulfurization rate, It aims at providing the molten steel refining method which can reduce the A-type inclusion in a steel plate.

That is, the gist of the present invention is as follows.
(1) In molten steel refining, in which molten steel in a ladle is circulated in a vacuum chamber with reduced pressure, refining is performed, and the probability of existence of particles having a total particle size of 60% or more of CaO and CaF ₂ and a particle size of 100 μm or less is volume. A powder desulfurizing agent having a ratio of 80% or more is added to the molten steel for desulfurization and refining, and the powder desulfurizing agent is added to the molten steel by blowing it into the molten steel together with a carrier gas from an immersion lance or in a vacuum chamber. It is performed by spraying on the surface of the molten steel, and after the desulfurization refining, Al gas in the molten steel is 0.020 to 0.055% by mass in terms of Al content in the steel by blowing oxygen gas to the molten steel in the vacuum tank. burned so, the molten steel refining method characterized by thereafter further refluxing molten steel between the ladle and the vacuum chamber.
(2) The molten steel refining method according to claim 1, wherein 15 to 45 parts by mass of CaF ₂ is contained when the total of CaO and CaF ₂ in the powder desulfurizing agent is 100 parts by mass.

  The present invention relates to a refining process in which the molten steel in the ladle is recirculated to a vacuum tank in which the pressure is reduced, and after performing desulfurization refining by adding a powder desulfurizing agent to the molten steel, oxygen gas is supplied to the molten steel in the vacuum tank. Even if a powder desulfurization agent with a small particle size is used as a desulfurization agent, the A-based inclusions are not generated in the hot-rolled steel sheet, and the resulfurization is small. It is possible to produce an ultra-low sulfur steel having good workability.

It is a figure which shows the relationship between the amount of Al combustion after a desulfurization process, and the number of A type inclusions of a steel plate. It is a conceptual diagram which shows the condition of the molten steel refining of this invention at the time of using an RH vacuum degassing apparatus, (a) is a figure which shows the condition which blows in a desulfurization agent from an immersion lance, (b) is desulfurization from the lance in a vacuum chamber The figure which shows the condition which sprays an agent on the molten steel surface, (c) is a figure which shows the condition which blows oxygen gas on the molten steel surface from the lance in a vacuum chamber, and burns Al.

The present invention relates to a molten steel refining in which a molten steel in a ladle is recirculated to a vacuum tank in which the pressure is reduced and refined, and a powder desulfurizing agent mainly composed of CaO and CaF ₂ is added to the molten steel to perform the desulfurizing refining Target method. An RH vacuum degassing apparatus is most commonly used as molten steel refining in which the molten steel in the ladle is recirculated to a vacuum tank with reduced pressure. Hereinafter, the case where the RH vacuum degassing apparatus is used will be described with reference to FIG. In the molten steel refining using the RH vacuum degassing apparatus, the ladle 1 is filled with the molten steel 11, the riser pipe 3 (immersion pipe) and the downfall pipe 4 (immersion pipe) of the vacuum chamber 2 are immersed in the molten steel, By performing reflux argon gas blowing 13 from the middle, the molten steel 11 in the ladle rises from the riser pipe into the vacuum tank, and descends from the downcomer pipe 4 via the vacuum tank to return to the ladle. To do. The inside of the vacuum chamber 2 is depressurized by the vacuum exhaust 15, and the alloy 14 can be added.

As the desulfurizing agent, a powder desulfurizing agent containing 60% by mass or more of CaO and CaF ₂ is used. By making the total content of CaO and CaF ₂ 60 mass% or more, the desulfurization ability of CaO can be sufficiently extracted. The CaO and the ratio of CaF _2, when the total of CaO and CaF ₂ is 100 parts by mass, preferably contains CaF ₂ about 15 to 45 parts by weight. When such a ratio is adopted, the effect of lowering the melting point of the desulfurizing agent by CaF ₂ can be exhibited. On the other hand, the amount of expensive CaF ₂ used can be suppressed to suppress an increase in desulfurization cost. As a desulfurizing agent component other than CaO and CaF ₂ , Al ₂ O ₃ , MgO, SiO ₂ , Al, Ca, Mg, Na, K, and the like can be used.

  As described above, in the desulfurization process in the RH vacuum process, a desulfurization reaction after the desulfurization agent added to the molten steel reaches the ladle slag cannot be expected. In order to advance the desulfurization reaction while the desulfurization agent is suspended in the molten steel before it floats and separates, it is effective to increase the reaction interface area between the molten steel and the desulfurization agent. Is preferably as small as possible. When the existence probability of particles having a particle size of 100 μm or less as the desulfurizing agent is 80% or more by volume, a suitable desulfurization ability can be realized. The particle size measurement of the desulfurizing agent can be performed as a laser diffraction / scattering method or the like.

  When the desulfurizing agent having a minute particle size as described above is used, a part of the desulfurizing agent cannot be lifted and separated and is taken into the slab while being suspended in the molten steel. In addition, as described above, since a low-melting-point desulfurizing agent is used, the desulfurizing agent taken into the slab becomes an inclusion that is easily stretched in hot rolling. As a result, the inclusions resulting from these low melting point and minute desulfurization agents become A-type inclusions after hot rolling, which is a factor that deteriorates the workability of the steel sheet.

  In the present invention, in molten steel refining, in which the molten steel in the ladle is circulated to a vacuumed vacuum tank for refining, after desulfurization treatment, oxygen gas is blown to the molten steel in the vacuum tank to burn Al in the molten steel. It is characterized by making it. By burning Al in the molten steel after the desulfurization treatment, the frequency of occurrence of A-based inclusions in the hot-rolled steel sheet can be reduced.

FIG. 1 shows the relationship between the amount of Al combustion and the number of steel plate A-based inclusions when Al in steel is burned by blowing oxygen gas to the molten steel in the vacuum tank after desulfurization treatment in RH vacuum degassing treatment. It shows the relationship. As the desulfurizing agent, a CaO—CaF ₂ —MgO-based component having an average particle size of 75 μm was used, and added to the molten steel as a desulfurizing agent basic unit of 2 to 8 kg / t during the RH vacuum treatment. Thereafter, oxygen gas was sprayed onto the surface of the molten steel in the RH vacuum chamber to burn Al in the steel. In FIG. 1, the amount of Al combustion is plotted on the horizontal axis, and the number of A-based inclusions on the steel sheet is plotted on the vertical axis. As is clear from FIG. 1, when the Al combustion is not performed, the number of A-based inclusions in the steel sheet is 1 / mm ² or more, whereas by performing Al combustion, the number of A-based inclusions is It becomes less than 1 piece / mm ^2, and the number of A-based inclusions decreases as the amount of Al combustion increases.

The following two points can be considered as the reason why the steel sheet A-based inclusions are reduced by burning Al in the molten steel after the desulfurization treatment. First, a large amount of Al ₂ O ₃ is generated in the molten steel due to Al combustion, and the residual desulfurizing agent particles in the molten steel are aggregated and coalesced into Al ₂ O ₃ to promote flotation. Secondly, even if a small amount of desulfurizing agent particles remain in the molten steel, the composition changes to a high melting point composition of Al ₂ O ₃ system, so that it does not stretch during rolling and thus does not become an A-based inclusion.

Conventionally, when oxygen is added to molten steel after desulfurization treatment to cause a reaction to burn Al in the molten steel, the S component taken into the desulfurizing agent returns to the molten steel and the S concentration increases, so-called It was thought that the sulfation phenomenon would occur. Also in patent document 2, it is supposed that the temperature rising process by Al combustion should be performed before a desulfurization process. In the present invention, the purpose is not to raise the temperature, but to suspend and harden the inclusions resulting from the microdesulfurization agent by suspending Al ₂ O ₃ in the molten steel.

As a method for burning Al in molten steel, as shown in FIG. 2 (c), a lance 5 is inserted into the vacuum chamber 2, and oxygen gas is blown from the tip of the lance toward the molten steel surface in the vacuum chamber. It can be carried out. The sprayed oxygen gas 18 reacts with Al in the molten steel to produce Al ₂ O ₃ . The burning Al is added to the molten steel before the oxygen gas is blown. The timing of adding Al may be before desulfurization treatment or after desulfurization treatment. In order to ensure the desulfurization efficiency in the desulfurization treatment, it is necessary to strongly deoxidize the molten steel. Therefore, it is necessary to add at least Al enough to strongly deoxidize before the desulfurization treatment.

After burning Al in the molten steel, the molten steel is circulated between the ladle and the vacuum chamber. Thereby, Al ₂ O ₃ in the molten steel generated by Al combustion sufficiently reacts with the residual fine inclusions caused by the desulfurizing agent in the molten steel, and floats in the ladle 1 and can adhere to and separate from the slag 12. The reflux time of the molten steel after Al combustion is preferably 2 minutes or longer.

  In the present invention, the number of A-based inclusions in the steel sheet can be reduced by burning Al after the desulfurization treatment. In the present invention, as is clear from FIG. 1, the amount of A-based inclusions can be greatly reduced by making the amount of Al combustion by oxygen 0.020% by mass or more in terms of Al content. It is. On the other hand, when the amount of Al combustion increases, a sulfation phenomenon from the ladle slag is observed. If the amount of Al combustion is 0.070% by mass or less, it is possible to produce an ultra-low sulfur steel by suppressing the amount of sulfurization to a predetermined range. About the Al combustion amount performance by oxygen gas blowing, it can calculate from the reduction allowance of Al content in molten steel before and behind oxygen gas blowing. When adding Al during oxygen gas blowing, the amount of Al added during blowing is added to the reduction amount of the Al content in the molten steel before and after oxygen gas blowing.

  The desulfurizing agent can be added to the molten steel by blowing the desulfurizing agent together with the carrier gas into the molten steel from the tip of the immersion lance immersed in the ladle, or by desulfurizing the molten steel surface from the tip of the lance inserted in the vacuum chamber. Either method of spraying the agent can be selected. When blowing into the molten steel using an immersion lance, it is preferable to blow a desulfurizing agent into the molten steel immediately below the riser pipe 3 of the RH vacuum tank 2 as shown in FIG. The blown desulfurizing agent is sucked together with the molten steel into the riser 3 and discharged from the downcomer 4 into the ladle via the vacuum chamber. In order to follow such a path, it is possible to sufficiently ensure the residence time in the molten steel until the desulfurizing agent blown into the molten steel rises and is absorbed by the slag on the surface of the ladle. As shown in FIG. 2 (b), the same applies when the desulfurizing agent is sprayed into the molten steel from the lance 5 inserted into the vacuum vessel. The molten steel to which the desulfurizing agent has been added is stirred in the vacuum vessel. Then, it descends in the molten steel in the ladle via the downcomer, stays sufficiently in the ladle, floats after desulfurization, and is taken into the ladle slag.

  Even in the extremely low-sulfur steel, a slight amount of S remains in the steel sheet, so that a slight amount of sulfide is formed in the steel sheet. When the remaining sulfide is MnS, it is stretched by rolling. In order to control the form so as not to be stretched by rolling, Ca can be added to the molten steel. In the present invention, when Ca is added to molten steel, it is preferable to add Ca after the completion of Al combustion.

  The present invention was applied when molten steel adjusted to a predetermined component in the hot metal pretreatment and converter process was accommodated in a ladle and molten steel desulfurization was performed using an RH vacuum degassing apparatus. The ladle molten steel amount was 400 tons, and the S content in the molten steel was reduced to about 0.003% by the hot metal pretreatment and the converter process.

As the desulfurization agent, a CaO—CaF ₂ —MgO-based desulfurization agent containing CaO: 45 mass%, CaF ₂ : 30 mass%, and MgO: 20 mass% was used. The particle size of the desulfurizing agent is 80% by volume when 100 μm or less, and the average particle size is 75 μm. The particle size of the desulfurizing agent was measured using the method as described above.

  In the treatment by the RH vacuum degassing apparatus, first, Al is added to perform deoxidation. The amount of Al added is targeted at an Al content of 0.040% by mass for the level at which oxygen gas is not blown, and the level at which Al is burned by blowing oxygen gas is further increased by the amount corresponding to the amount of Al burn. Al was added so as to be high.

  After Al deoxidation, desulfurization treatment is performed. Two methods were applied as the desulfurization agent addition method. First, as shown in FIG. 2A, the immersion lance 6 is immersed in the molten steel immediately below the riser pipe 3 of the RH vacuum chamber 2, and the desulfurizing agent is added to the molten steel using argon gas as a carrier gas from the tip of the immersion lance 6. Infused into. Secondly, as shown in FIG. 2B, a desulfurizing agent was sprayed onto the surface of the molten steel from a lance 5 provided in the RH vacuum chamber. At the time of spraying, argon gas was used as a carrier gas. The time required for adding the desulfurizing agent was 15 to 25 minutes.

After the desulfurization treatment was completed, oxygen gas was blown onto the surface of the molten steel from the lance 5 provided in the RH vacuum chamber as shown in FIG. The oxygen blowing time was 3 to 10 minutes, the oxygen flow rate was 2000 to 3000 Nm ³ / hr, and the amount of Al burn was adjusted by adjusting the oxygen blowing amount. The lance used for spraying the desulfurizing agent and the lance used for spraying oxygen gas shared the same lance.

  After completing the Al combustion, the molten steel was circulated for 3 minutes using an RH vacuum degasser. When Ca was added to the molten steel, Ca was added by means of injection, wire charging, alloy charging, etc. after 3 minutes had elapsed from the completion of Al combustion. The Ca content target in the molten steel was set to 0.002% by mass.

The molten steel was formed into a continuous cast slab using a slab continuous casting apparatus, and further hot-rolled to obtain a hot-rolled steel plate having a thickness of 3.2 mm. For inclusion evaluation, samples were taken from both the width center and ¼ width of the hot-rolled steel sheet, and the cross section parallel to the rolling direction and the plate thickness direction was used as the spectroscopic surface, and approximately 3 × 10 mm ^{2 in} the entire thickness direction. Ten fields of view were observed with an optical microscope. Inclusions with a length of 10 μm or more were used as A-based inclusions, and the number of A-based inclusions was measured and evaluated in units of pieces / mm ² .

The results are shown in Table 1. Invention Example No. Nos. 1 to 9 have an Al combustion amount within the preferred range of the present invention, and the number of A-based inclusions is 0.1 / mm ² or less, and the S concentration in the molten steel after treatment is also 0.0010 mass. % Or less. Invention Example No. In Nos. 10 and 12, since the Al combustion amount was lower than the preferred range of the present invention, the number of A-based inclusions was slightly higher. Invention Example No. In Nos. 11 and 13, since the Al combustion amount was higher than the preferred range of the present invention, the sulfurization was slightly larger, but the S concentration in the steel after the treatment could be secured to 0.0015% by mass or less.

Comparative Example No. In Nos. 1 and 2, since Al combustion was not performed, the number of A-based inclusions exceeded 1 / mm ² .

DESCRIPTION OF SYMBOLS 1 Ladle 2 Vacuum tank 3 Rising pipe 4 Falling pipe 5 Lance 6 Immersion lance 11 Molten steel 12 Slag 13 Circulating argon gas blowing 14 Alloy 15 Vacuum exhaust 16 Desulfurizing agent blowing 17 Desulfurizing agent blowing 18 Oxygen gas blowing

Claims (2)

In molten steel refining, in which molten steel in a ladle is circulated in a vacuum chamber with reduced pressure, refining is performed, and the existence probability of particles with a total particle size of CaO and CaF ₂ of 60% by mass or more and a particle size of 100 μm or less is 80 by volume. % Or more of powder desulfurization agent is added to the molten steel for desulfurization and refining. The addition of the powder desulfurization agent to the molten steel is blown into the molten steel with a carrier gas from an immersion lance, or on the surface of the molten steel in a vacuum tank. It is performed by spraying, and after the desulfurization refining, oxygen gas is sprayed on the molten steel in a vacuum tank so that the amount of Al combustion in the molten steel is 0.020 to 0.055% by mass in terms of Al content in the steel. A molten steel refining method, characterized by burning and then circulating the molten steel between a ladle and a vacuum chamber. CaO and CaF in the powder desulfurization agent ₂ When the total amount is 100 parts by mass, CaF ₂ The molten steel refining method according to claim 1, wherein 15 to 45 parts by mass are contained.

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