JP5326310B2 - Method of melting high Mn ultra-low carbon steel - Google Patents

Description

  The present invention relates to a method for melting high Mn ultra-low carbon steel having a carbon content of 0.005 mass% or less and a Mn content of 0.4 mass% or more and 2.0 mass% or less. The present invention relates to a method for melting high-Mn ultra-low carbon steel by improving the Al deoxidation method performed after charcoal refining.

  In recent years, thin steel sheets used for many applications such as steel sheets for automobile exteriors, steel sheets for cans, and steel sheets for home appliances have a carbon content of 0.01 to 0.1 because of their ease of improving workability. The mass of low-carbon steel is being converted to ultra-low carbon steel with a carbon content of 0.005 mass% or less, which can only be achieved by vacuum decarburization refining. It is expanding. In addition, vacuum decarburization refining is decarburization refining performed in the state where atmospheric pressure is lower than atmospheric pressure.

  This ultra-low carbon steel is obtained by decarburizing and refining at atmospheric pressure to the limit that can be economically acceptable using a converter etc. to obtain molten steel from hot metal, and then the obtained molten steel is installed in a vacuum degassing facility. Using vacuum decarburization refining to the target concentration by reaction with oxygen in steel or added oxygen source under reduced pressure, and after this vacuum decarburization refining, adding deoxidizer such as metal Al and deoxidizing treatment It is manufactured by applying component adjustment / inclusion reduction treatment (see, for example, Patent Document 1).

In this melting process, in the converter, the carbon concentration is generally decarburized to about 0.03 to 0.06% by mass, but when decarburized to this level, iron is oxidized in addition to carbon, and the converter end point is reached. The FeO concentration in the slag at the time increases. Moreover, Mn in the hot metal is also oxidized, and the MnO concentration in the slag is increased. A part of this slag is mixed into the molten steel when steel is discharged and flows out into the ladle together with the molten steel. In this state, when deoxidation treatment with Al is performed after vacuum decarburization refining with a vacuum degassing facility, FeO and MnO in the slag react with Al in the molten steel even after the deoxidation treatment, and the Al concentration in the molten steel becomes In the end, there is a problem that the Al content is outside the component specification range, or that multiple adjustments of the Al component are required, resulting in a decrease in productivity. In particular, when the Mn content is high, MnO is generated even during vacuum decarburization and refining, and the amount of MnO in the slag increases, and the above problem becomes apparent. Although cleanliness of the steel by Al ₂ O ₃ which generated also occur lowered, this problem does not interest the present invention.

  By the way, several methods for melting ultra-low carbon steel in which adverse effects due to MnO are suppressed have been proposed. For example, Patent Document 2 discloses that steel is removed from a converter to a ladle without adding alloy iron such as Fe-Mn to the ladle, and in a vacuum degassing facility, oxygen gas is blown into the ladle. When the carbon concentration in molten steel reaches 0.005% by mass or less after carbon refining, Al is added to deoxidize, and then electrolytic metal Mn (JIS: “MMnE”) is added to adjust the Mn concentration. A method for melting ultra-low carbon steel has been proposed. Further, in Patent Document 3, the molten steel after the converter steel is vacuum decarburized and refined with a vacuum degassing facility, and when melting an ultra-low carbon steel containing 0.3 to 3% by mass of Mn, Vacuum decarburization refining is carried out with the Mn concentration in the molten steel before vacuum decarburization refining kept to 0.3% by mass or less, followed by deoxidation treatment, and after the deoxidation treatment, electrolytic metal Mn is added to the vacuum degassing tank Thus, a method for producing a Mn-containing ultra-low carbon steel has been proposed, characterized in that the Mn concentration in the molten steel is adjusted to a desired value.

  If patent document 2 and patent document 3 maintain Mn density | concentration in the molten steel at the time of vacuum decarburization refining at low level, there will be little MnO in slag and the deoxidation process by Al will become easy. However, in Patent Document 2 and Patent Document 3, the electrolytic metal Mn used for Mn adjustment after vacuum decarburization refining is extremely expensive, which causes an increase in manufacturing cost and is not a preferable operation mode.

As an inexpensive Mn source, Mn ore is known, and when melting extremely low carbon steel containing Mn, it is desirable to use Mn ore as a Mn source from the viewpoint of resource saving and energy saving. ing. In addition, Mn ore is charged into the converter when decarburizing and refining the hot metal in the converter, is reduced by the carbon in the hot metal, and the reduced Mn moves into the molten steel, thereby functioning as a Mn source. To do. A part of Mn transferred into the molten steel is oxidized during the decarburization refining in the converter and transferred to slag, but even if this yield is taken into consideration, it is much cheaper than the electrolytic metal Mn.
JP 2002-69527 A JP-A-8-291319 JP 2003-253324 A

  The present invention has been made in view of the above circumstances, and its object is to use Mn ore as a part of the Mn source in order to reduce the amount of expensive electrolytic metal Mn used. Combining decarburization refining under atmospheric pressure and vacuum decarburization refining under reduced pressure in vacuum degassing equipment, the carbon content is 0.005 mass% or less, the Mn content is 0.4 mass% or more, 2 When melting high-Mn ultra-low carbon steel of less than 0.0% by mass, the Al content in the molten steel can be adjusted within the standard by only one Al deoxidation treatment, and multiple adjustments of Al components are required. The object is to provide a high Mn ultra-low carbon steel melting method that can adjust the Al component in the molten steel without reducing the productivity of the vacuum degassing equipment.

In order to solve the above-mentioned problems, the method for melting high-Mn ultra-low carbon steel according to the present invention obtains molten steel by decarburizing and refining hot metal in a converter at atmospheric pressure. 1. The molten steel after steel is discharged from the furnace is subjected to vacuum decarburization refining and Al deoxidation treatment in a vacuum degassing equipment, so that the carbon content is 0.005 mass% or less and the Mn content is 0.4 mass% or more. When melting high-Mn ultra-low carbon steel of 0% by mass or less, the converter uses Mn ore as a Mn source for decarburization and refining. In the Al deoxidation treatment after the vacuum decarburization and refining, The amount of Al-based deoxidizer calculated by the formula (1) is added, and an Al-based deoxidizer is added.
W _AL = (A + α × [O] + β + γ × ΔMn) / B (1)
However, in the formula (1), W _AL : Al-based deoxidizer input amount per ton of molten steel (kg / t), A: Al content target value per ton of molten steel (kg / t), [O]: Al Oxygen concentration in molten steel (ppm) before deoxidation, ΔMn: Difference between Mn concentration in molten steel immediately after steelmaking and Mn concentration in molten steel at the end of vacuum decarburization refining (mass%), B: Al-based deoxidizer Al pure component (−), α: constant (0.001 ≦ α ≦ 0.002), β: constant (0.3 ≦ β ≦ 0.5), γ: constant (1.0 ≦ γ ≦ 1.5) ).

  According to the present invention, Al after vacuum decarburization refining according to the oxidized content of Mn in the molten steel in the period from after the steel from the converter to the ladle to vacuum decarburization refining in the vacuum degassing equipment. Since the amount of Al-based deoxidizer charged during deoxidation treatment is set, Al-based deoxidizer is charged without excess and deficiency, and the Al content in the molten steel is within the standard with only one Al deoxidation treatment. It is possible to adjust, and it is possible to adjust the Al component in the molten steel without requiring multiple adjustments of the Al component and without reducing the productivity of the vacuum degassing equipment.

  Hereinafter, the present invention will be described in detail. First, the background to the present invention will be described.

  The inventors have made Mn in molten steel in the melting process of high Mn ultra-low carbon steel having a carbon content of 0.005 mass% or less and a Mn content of 0.4 mass% or more and 2.0 mass% or less. As a result of various studies for the purpose of reducing the amount of electrolytic metal Mn (MMnE) used for adjustment, the most effective thing is to enable Mn ore which is an inexpensive Mn source in converter decarburization refining As much as possible, this Mn ore was reduced with carbon in the molten steel at the time of converter decarburization and refining, and it was found that the Mn concentration in the molten steel was kept high at the stage of converter decarburization and refining. .

  This is due to the following reason. That is, Mn ore is reduced and Mn produced is transferred into molten steel, and Mn in the molten steel is oxidized and transferred to slag in vacuum decarburization and refining in converter decarburization and vacuum degassing equipment. Not all Mn in molten steel undergoes oxidation loss, and the amount of oxidation loss is only a part. Even if a part of Mn in molten steel caused by Mn ore is oxidized and lost, Mn ore is very cheap as a source of Mn, and even if it is oxidized and lost, the amount of residue increases accordingly. Therefore, the amount of electrolytic metal Mn used is reduced accordingly. This is because the manufacturing cost is reduced as a whole because the cost merit by reducing the amount of electrolytic metal Mn used is great.

  However, in the above smelting process, the undeoxidized molten steel contained in the ladle is transported from the converter to the vacuum degassing facility, and the vacuum degassing facility performs vacuum decarburization refining, which is oxidation refining. Therefore, during this period, Mn in the molten steel is oxidized and MnO in the slag increases. Therefore, in the Al deoxidation treatment performed after the vacuum decarburization refining in the vacuum degassing equipment, the Al content in the molten steel varies due to the reaction between MnO in the slag and Al in the molten steel. In order to adjust the Al component, it was often necessary to add Al again after the Al deoxidation treatment. In other words, due to the re-adjustment of Al, there is a problem that the processing time in the vacuum degassing facility becomes longer and the productivity is lowered.

  Therefore, by changing the Mn concentration in the molten steel, among the Al content added during Al deoxidation, the effect of the Mn concentration in the molten steel on the Al loss content that is oxidized and lost instead of Al in the molten steel. investigated. The Al loss component is oxidized by the Al component spent to deoxidize oxygen in the molten steel, the Al component spent to reduce FeO and MnO in the slag, and the oxygen gas in the atmosphere. Al content.

  The survey results are shown in FIG. In addition, the Mn concentration shown in FIG. 1 is the Mn concentration of the molten steel in the ladle immediately after the steel is drawn, and the Al loss is indicated by the mass per ton of molten steel (unit: kg). As shown in FIG. 1, it can be seen that the Al loss increases in proportion to the oxygen concentration in the molten steel before Al deoxidation, but the Mn concentration range (0.05 to 0.12% by mass) marked with It was found that the level of Al loss was different from the Mn concentration range (0.26 to 0.35 mass%) indicated by ○. In other words, it was confirmed that the Al loss increased as the Mn concentration in the molten steel increased.

  Based on this result, the amount of Al loss was investigated in more detail. As a result, the Al loss was proportional to the difference between the Mn concentration in the molten steel in the ladle immediately after steelmaking and the Mn concentration in the molten steel at the end of vacuum decarburization refining. The minute was found to increase. That is, it was confirmed that the Al loss increased in proportion to the MnO generated in the ladle.

  The present invention was made on the basis of the results of this investigation, and the method for melting high Mn ultra-low carbon steel according to the present invention is performed by decarburizing and refining the molten iron at atmospheric pressure in the converter. Next, the molten steel after the steel output from the converter was vacuum decarburized and Al deoxidized in a vacuum degassing facility, so that the carbon content was 0.005% by mass or less, and the Mn content was 0.00. When melting 4% by mass or more and 2.0% by mass or less of high Mn ultra-low carbon steel, in the converter, Mn ore is introduced as a Mn source and decarburized and refined, and the Al after the vacuum decarburized and refined The deoxidation treatment is characterized by adding an Al-based deoxidizing agent in an amount of Al-based deoxidizing agent calculated by the following equation (1).

W _AL = (A + α × [O] + β + γ × ΔMn) / B (1)
However, in the formula (1), W _AL : Al-based deoxidizer input amount per ton of molten steel (kg / t), A: Al content target value per ton of molten steel (kg / t), [O]: Al Oxygen concentration in molten steel (ppm) before deoxidation, ΔMn: Difference between Mn concentration in molten steel immediately after steelmaking and Mn concentration in molten steel at the end of vacuum decarburization refining (mass%), B: Al-based deoxidizer Al pure component (−), α: constant (0.001 ≦ α ≦ 0.002), β: constant (0.3 ≦ β ≦ 0.5), γ: constant (1.0 ≦ γ ≦ 1.5) ). Here, the constant β is an Al content mainly consumed for reducing FeO in the slag, and in FIG. 1, it corresponds to an Al loss when the oxygen concentration in the molten steel before Al deoxidation is zero. In addition, the constant α and the constant γ are based on the investigation results shown in FIG. The pure Al content (B) is, for example, B = 0.6 when the Al content is 60% by mass.

  Next, a method for melting high-Mn ultra-low carbon steel according to the present invention having the above-described configuration will be described.

The hot metal discharged from the blast furnace is received in a hot metal holding / conveying vessel such as a hot metal ladle or torpedo car. Before the hot metal is decarburized and refined in a converter, the hot metal is subjected to desiliconization or dephosphorization. In the present invention, Mn ore is charged into the furnace at the time of decarburization and refining in the converter, and Mn ore is reduced with carbon in the hot metal in this converter decarburization and refining. In order to efficiently reduce this Mn ore by converter decarburization refining, it is necessary to reduce the amount of slag in the converter, and silicon (Si) in the hot metal by performing desiliconization treatment or dephosphorization treatment This is because the amount of SiO ₂ generated in converter decarburization refining is reduced and the amount of slag in the converter is reduced. The desiliconization treatment is refining for removing in advance silicon that inhibits the dephosphorization reaction in order to promote the dephosphorization reaction.

  What is necessary is just to implement the desulfurization process in a hot metal stage according to the sulfur specification value of the steel type to melt. That is, in the case of low-sulfur steel having a sulfur standard value of 0.010% by mass or less, a preliminary desulfurization treatment is performed using a CaO-based desulfurizing agent or the like.

Hot metal that has been subjected to desiliconization treatment or dephosphorization treatment and, if necessary, desulfurized treatment is charged into a converter, and oxygen gas is blown up or blown down to decarburize and refine the hot metal. In this decarburization refining, while the basicity (mass% CaO / mass% SiO ₂ ) of the slag to be produced is in the range of 2-5, quick lime or dolomite is charged into the converter as a solvent, Mn ore is charged into the converter continuously or intermittently and decarburization refining is performed.

  The end point timing of converter decarburization refining is preferably the time when the carbon concentration of the molten steel produced by decarburization refining is 0.04 to 0.06% by mass. If the carbon concentration of the molten steel is lowered too much, the oxygen potential of the molten steel and the slag in the furnace increases, and Mn reduced to the molten steel is oxidized and transferred to the slag, but the carbon concentration of the molten steel is 0.04% by mass or more. If it is, the transfer of Mn to the slag can be prevented. On the other hand, if the carbon concentration of the molten steel is too high, the processing time for vacuum decarburization refining in the vacuum degassing facility in the next process becomes long, and the productivity decreases. If the carbon concentration of the molten steel is 0.06 mass% or less, the processing time for vacuum decarburization refining can be processed within a predetermined time that does not hinder the continuous casting process.

  This molten steel is discharged from the converter into a ladle, and then the ladle containing the molten steel is conveyed to a vacuum degassing facility such as an RH vacuum degassing apparatus. Immediately after the steel is extracted, a sample for analysis is taken from the molten steel in the ladle to grasp the Mn concentration in the molten steel.

  In the vacuum degassing equipment, vacuum decarburization refining is carried out, and therefore, deoxidation treatment with strong deoxidation elements such as Al, Ti, Si and the like is not performed at the time of steel extraction, and the steel is transported without being deoxidized. Further, at the time of steel output, a part of the slag in the furnace is caught in the molten steel, flows out from the converter into the ladle, and stays on the molten steel in the ladle. Since the slag staying in the ladle may react with a deoxidizer such as Al in the molten steel after deoxidation treatment in the vacuum degassing facility in the next process, the cleanliness of the molten steel may be impaired. A slag modifier such as a slag may be added from above the pan to deoxidize the slag.

  In the vacuum degassing facility, first, vacuum decarburization refining under reduced pressure is performed. This vacuum decarburization refining uses conventional methods such as a method of utilizing dissolved oxygen in molten steel and a method of supplying oxygen gas and iron oxide to molten steel in a vacuum tank of a vacuum degassing facility. carry out. At the end of this vacuum decarburization refining, when the carbon concentration in the molten steel is decarburized to below the target value, an analytical sample is taken from the molten steel, and the Mn concentration and oxygen concentration in the molten steel at the end of the vacuum decarburization refining To figure out.

Then, the Al-based deoxidizing agent input amount (W _AL ) is obtained using the equation (1), and the same amount of the obtained Al-based deoxidizing agent input amount (W _AL ), such as metal Al or Fe—Al alloy, etc. The Al-based deoxidizer is added to deoxidize the molten steel. The vacuum decarburization refining is naturally terminated by deoxidizing the molten steel. However, when an oxygen source such as oxygen gas is supplied, the supply of the oxygen source is stopped before the Al deoxidation process. After the Al deoxidation treatment, vacuum degassing refining is further performed in a vacuum degassing facility, and components such as Mn, Si, Ti, Nb, and V are adjusted as necessary. Electrolytic metal Mn is mainly used to adjust the Mn component. When the component adjustment is completed, the ladle containing the molten steel is conveyed to a continuous casting process, and a slab is produced from the molten steel by continuous casting.

  As described above, according to the present invention, after the steel is discharged from the converter to the ladle, depending on the oxidized content of Mn in the molten steel in the period from vacuum decarburization refining in the vacuum degassing equipment, vacuum is applied. Since the amount of Al-based deoxidizer to be used in Al deoxidation after decarburization and refining is set, the Al-based deoxidizer is charged without excess and deficiency, and Al in molten steel can be obtained only by one Al deoxidation. The content can be adjusted within the standard, and it is possible to adjust the Al component in the molten steel without requiring multiple adjustments of the Al component and without reducing the productivity of the vacuum degassing equipment. Moreover, since Mn ore is used as a Mn source, expensive electrolytic metal Mn can be reduced.

  Explained an example in which a high Mn ultra-low carbon steel having a Mn concentration standard of 0.55 to 0.65 mass% and an Al concentration standard of 0.02 to 0.04 mass% was applied by applying the method of the present invention. To do.

  The hot metal discharged from the blast furnace was dephosphorized, and the hot metal was charged into a converter and supplied with oxygen gas from an upper blow lance to perform decarburization refining. In this decarburization refining, 12 kg of Mn ore per ton of molten steel was added to the converter, and the basicity of the slag in the converter was adjusted to 3.0. Converter decarburization refining was completed when the carbon concentration in the molten steel reached 0.045 mass%.

  The obtained molten steel is discharged from the converter to the ladle, then transported to the RH vacuum degasser, and vacuum decarburized and refined until the carbon concentration reaches 0.001% by mass with the RH vacuum degasser. did. After this vacuum decarburization refining, the same amount of metal Al as the value calculated by the above-mentioned formula (1) is added for deoxidation treatment, and after deoxidation treatment, the electrolytic metal Mn is used to make the Mn concentration 0. Mn adjustment was performed so that it might become 6 mass%, and high Mn very low carbon steel was melted. When calculating using the formula (1), the pure Al content (B) is 1.0, and the Al content target value (A) of the molten steel is 0.3 kg / t (Al target value of the molten steel = 0.03). Mass%), α was 0.0015, β was 0.4, and γ was 1.3.

  After the degassing refining treatment, the molten steel was transferred from the RH vacuum degasser to a continuous casting facility and cast to produce a slab slab. An analytical sample was collected from the slab slab, the Al concentration was analyzed, and the variation in the Al component value between charges was investigated. As a result, the σ value of the standard deviation of the Al component value was 0.0042% by mass, and the conventional σ value was 0.0057% by mass, so the variation in the Al component value of the slab is greatly reduced. I was able to confirm. Further, by applying the present invention, the processing time in the RH vacuum degassing apparatus was 28.0 minutes, which was shortened by about 1.5 minutes compared to the conventional processing time of 29.5 minutes.

It is the result of investigating the Al loss by changing the Mn concentration in the molten steel.

Claims (1)

Decarburization and refining of the molten iron in the converter at atmospheric pressure is performed to obtain molten steel, and then the molten steel that has been discharged from the converter is vacuum decarburized and Al deoxidized in a vacuum degassing facility. When the high Mn ultra-low carbon steel having a carbon content of 0.005 mass% or less and a Mn content of 0.4 mass% or more and 2.0 mass% or less is melted, Mn ore is added and decarburized and refined, and in the Al deoxidation treatment after the vacuum decarburized refinement, an Al-based deoxidizer is added in the amount of Al-based deoxidizer calculated by the following formula (1). A method for melting high-Mn ultra-low carbon steel, characterized by:
W _AL = (A + α × [O] + β + γ × ΔMn) / B (1)
However, in the formula (1), each symbol represents the following.
W _AL : Input amount of Al-based deoxidizer per ton of molten steel (kg / t)
A: Target value of Al content per ton of molten steel (kg / t)
[O]: Oxygen concentration in molten steel (ppm) before Al deoxidation
ΔMn: Difference between the Mn concentration in the molten steel immediately after steelmaking and the Mn concentration in the molten steel at the end of vacuum decarburization (% by mass)
B: Al pure content of an Al-based deoxidizer (-)
α: Constant (0.001 ≦ α ≦ 0.002)
β: Constant (0.3 ≦ β ≦ 0.5)
γ: Constant (1.0 ≦ γ ≦ 1.5)

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