JP6766687B2 - Refining method of molten steel - Google Patents

Description

The present invention relates to a refining method for producing molten steel with high cleanliness, particularly a refining method at the end of vacuum degassing refining.

In recent years, steel having excellent mechanical properties has been required in order to improve the performance of mechanical devices and miniaturize mechanical parts. In general, steel is desiliconized, dephosphorized, and decarburized in a converter, and then the composition of the molten steel is prepared in a secondary refining step to remove inclusions in the molten steel. It is produced by continuous casting after reduction, but in order to improve mechanical properties, it is necessary to reduce inclusions in molten steel as much as possible.

For example, in bearing steel, the amount and size of inclusions in the steel determine the rolling fatigue life, so in the secondary refining process, the molten steel is subjected to ladle slag refining treatment (hereinafter referred to as "LF treatment"). In some cases) and vacuum degassing treatment (hereinafter sometimes referred to as "RH treatment") are performed to reduce inclusions in molten steel.

In the RH treatment, two dipping pipes are immersed in the molten steel in the ladle, the vacuum tank connected to the dipping pipe is depressurized, and the molten steel is sucked up into the vacuum tank by the pressure difference from the atmospheric pressure to release the molten steel recirculation gas. This is a process in which the molten steel is supplied into the molten steel from one of the immersion pipes, and the molten steel is circulated between the inside of the vacuum chamber and the ladle to degas and reduce inclusions.

In the RH treatment, the molten steel is strongly agitated, so that the removal of inclusions is promoted, but on the other hand, slag is involved in the molten steel, so that the circulation control of the molten steel is important. , Many techniques related to recirculation control have been proposed.

For example, in Patent Document 1, after performing reduction refining with slag having a basicity of 3 or more, vacuum degassing is performed by using a recirculation type degassing device to set 2/3 of the processing time to high recirculation and 1/3 to weak recirculation. A method for producing bearing steel, which is characterized by refining, has been proposed.

Patent Document 2 states that when a molten steel produced in an arc melting furnace or a converter is transferred to a ladle for refining, the refining in the ladle is set to 60 minutes or less, and the total ring flow rate of the molten steel by the recirculation type degassing device is set to 60 minutes or less. A method for producing high-clean steel has been proposed, which comprises performing degassing for 25 minutes or more, which is 8 times or more that of molten steel.

Patent Document 3 states that when molten steel discharged from a converter or an electric furnace is smelted by a ladle smelting device and then smelted by a recirculation type vacuum degassing device to produce high-cleanliness steel, the recirculation type vacuum degassing is performed. The slag basicity in the refining process performed by the device is 6.5 or more and 13.5 or less, and in the first half treatment of 1/3 to 1/2 of the total processing time of the recirculation type vacuum degassing device, the molten steel ring flow rate is 180 ton / min. As described above, a method for producing high-cleanliness steel has been proposed, which comprises a high recirculation state of 210 ton / min or less and a weak recirculation state of a molten steel ring flow rate of 110 ton / min or more and 140 ton / min or less in the latter half treatment. There is.

Patent Document 4 describes in a method of repressurizing a vacuum refining apparatus in which nitrogen gas is introduced into a vacuum chamber at the end of a vacuum refining process for molten steel to restore the pressure from vacuum to normal pressure, and argon gas or the like is not present on the surface of the molten steel bath. A vacuum recovery method has been proposed in which an active gas is introduced to prevent nitrogen absorption of molten steel.

Patent Document 5 proposes a method for producing high-cleanliness ultra-low carbon steel, which comprises supplying a stirring gas according to the degree of vacuum in the vacuum chamber without bringing slag into the vacuum chamber. ing.

Japanese Unexamined Patent Publication No. 62-03650 Japanese Unexamined Patent Publication No. 2001-342516 Japanese Unexamined Patent Publication No. 2008-133505 Japanese Patent Application Laid-Open No. 05-331526 Japanese Patent Application Laid-Open No. 08-199225

As described above, the mechanical properties of steel are greatly affected by the amount and size of inclusions present in the steel, mainly oxide-based inclusions. Among the oxide-based inclusions in steel, particularly coarse inclusions of about several tens of μm are CaO-containing low melting point inclusions (hereinafter, may be referred to as “CaO-containing inclusions”).

The coarse CaO-containing inclusions are slag-based inclusions generated by the ladle slag used in refining being caught in the molten steel, and CaO in the slag is reduced and mixed into the molten steel, and Al ₂ O ₃ in the molten steel. And the inclusions formed by reacting with MgO-Al ₂ O _3, and further, these inclusions are coarsened inclusions by incorporating the inclusions in the molten steel.

Coarse CaO-containing inclusions worsen the extreme statistical values of quality control indicators and the fatigue life of product characteristics, so it is necessary to reduce the amount and size, but for that purpose, the starting point of agglomeration and coalescence It is effective to reduce the amount and size of the low melting point inclusions, and to reduce the amount and size of the inclusions in the molten steel incorporated into the low melting point inclusions.

In order to reduce the amount and size of these inclusions, in each manufacturing process (ladle refining-RH treatment-continuous casting), prevention of inclusions in the molten steel or inclusions in the molten steel It is necessary to take measures to reduce inclusions such as removing objects.

Entrainment of ladle slag in molten steel increases in frequency when the molten steel flow velocity is high or when the slag / molten steel interface is severely disturbed in the RH treatment, and the amount and amount of slag-based inclusions mixed in are large. Both increase.

After the RH treatment is completed, in order to return the molten steel in the vacuum chamber to the ladle, "reinforcement" is performed to return the decompression state in the vacuum chamber to atmospheric pressure, but at the time of restoration, it is sucked up in the vacuum chamber. Since the molten steel and the molten steel stored in the immersion pipe rapidly descend and return to the ladle, the falling flow velocity of the molten steel induced by the rapidly descending molten steel is very large, and the slag / molten steel interface is violent. It sways and the slag is caught in the molten steel, and the slag that is caught is drawn deep into the molten steel in the ladle.

Further, if the molten steel recirculation gas flow rate at the time of recompression is the same strong recirculation condition as the molten steel recirculation gas flow rate during the RH treatment, the molten steel recirculates at a flow rate exceeding the critical molten steel flow velocity involving slag even during the recompression. Therefore, the slag in the vacuum chamber is always in a state where it is easily caught in the molten steel. Further, at the slag / molten steel interface, there is a molten steel flow velocity that exceeds the critical flow velocity of slag entrainment, and if the contact time between the slag and the molten steel is long, the slag in the vacuum chamber continues to be easily entrained in the molten steel. The amount of slag-based inclusions inside will increase.

In the method of Patent Document 1, the first half 2/3 of the recirculation type degassing treatment is a high recirculation and the second half is a weak recirculation, but Patent Document 1 does not describe the recirculation conditions at the time of recompression. Further, when the latter half of the recirculation type degassing treatment is made weak recirculation, the total amount of oxygen in the molten steel T.I. It may not be possible to reduce O sufficiently.

In the method of Patent Document 2, refining in the ladle is 60 minutes or less, the ring flow rate of the molten steel by the recirculation type degassing device is 8 times or more that of the total molten steel, and degassing is performed for 25 minutes or more. The recirculation conditions at the time of recompression are not described, and the recirculation conditions that affect the slag entrainment in the molten steel are also unclear.

In the method of Patent Document 3, the first half of the total processing time of the recirculation type vacuum degassing device is set to the high recirculation state and the latter half is set to the weak recirculation state, but the same as the method of Patent Document 1. , The recirculation conditions at the time of recompression are not described, and since the latter half is weakly agitated, the total oxygen content of the molten steel is T.I. There is a risk that O cannot be lowered sufficiently.

In the method of Patent Document 4, the gas type and the like at the time of decompression are specified, but Patent Document 4 does not describe the decompression condition for suppressing slag entrainment of molten steel in the vacuum chamber and the ladle. .. In the method of Patent Document 5, an appropriate stirring gas flow velocity according to the pressure in the vacuum chamber is defined, but in Patent Document 5, slag entrainment in molten steel is suppressed in the vacuum chamber and the ladle. The conditions for repressurization are not described.

Therefore, in view of the current state of the prior art, the present invention suppresses slag entrainment of molten steel that occurs during decompression and unsteady state of vacuum degassing of molten steel, and the amount and size of CaO-containing inclusions in the steel. An object of the present invention is to produce steel having high cleanliness, and an object of the present invention is to provide a refining method for molten steel that solves the problem.

The present inventors have diligently studied a method for solving the above problems. As a result, if the pressure in the vacuum chamber is controlled to optimize the descending speed of the molten steel in the vacuum chamber and the immersion pipe during the depressurization of the vacuum degassing treatment, the slag entrainment of the molten steel that occurs during unsteady state and It was found that the infiltration of molten steel in the ladle of slag-based inclusions into the deep part can be suppressed, and the amount and size of CaO-containing inclusions remaining in the steel due to slag can be significantly reduced. This point will be described later.

The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) After vacuum degassing the molten steel in the ladle containing C, Si, Mn, P, and S with a degassing device equipped with a dipping tube in the vacuum tank, the pressure in the vacuum tank is applied. In the refining method, the pressure P1 [Pa] in the depressurized state is restored to the atmospheric pressure P0 [Pa], and the refining of the molten steel is completed.
(I) From the pressure P1 to the pressure P2 defined by the following formula (1), the lowering speed of the molten steel in the vacuum chamber is maintained at 0.20 m / sec or more to repressurize, and then the pressure is restored.
(Ii) A method for refining molten steel, characterized in that the descending speed of the molten steel in the immersion pipe is maintained at 0.15 m / sec or less and the pressure is restored from pressure P2 to atmospheric pressure P0.

P2 = P0-ρ ・ g ・ (hd) ・ ・ ・ (1)
ρ: Molten steel density [kg / m ³ ]
g: Gravity acceleration [m / sec ² ]
h: Height from the molten steel bath surface in the ladle to the molten steel bath surface in the vacuum tank [m]
d: Soaking in molten steel bath in a vacuum chamber [m]

(2) The method for refining molten steel according to (1) above, wherein the vacuum degassing treatment is performed by an RH type vacuum degassing device.

(3) When repressurizing with the RH type vacuum degassing device, the decompression gas is supplied from one or both of a portion where the molten steel recirculation gas is blown and a portion directly supplied into the vacuum chamber. The method for refining molten steel according to (2) above.

(4) The method for refining molten steel according to (3) above, wherein the decompression gas is an inert gas.

(5) The molten steel is C: 1.20% or less, Si: 3.00% or less, Mn: 1.60% or less, P: 0.05% or less, S: 0.05% or less in mass%. The method for refining molten iron according to any one of (1) to (4) above, which comprises the above, and the balance is composed of Fe and unavoidable impurities.

(6) The molten steel further contains, in mass%, Al: 0.20% or less, Cr: 3.50% or less, Mo: 0.85% or less, Ni: 4.50% or less, Nb: 0.20. % Or less, V: 0.45% or less, W: 0.30% or less, B: 0.006% or less, N: 0.060% or less, Ti: 0.25% or less, Cu: 0.50% or less , Pb: 0.45% or less, Bi: 0.20% or less, Te: 0.010% or less, Sb: 0.20% or less, Mg: 0.010% or less, Ca: 0.010% or less, REM The method for refining molten steel according to (5) above, wherein one or more of 0.010% or less and O: 0.003% or less are contained.

According to the present invention, it is possible to suppress slag entrainment of molten steel and penetration of slag-based inclusions into the deep part of the ladle in the pan, which occurs at unsteady state, and CaO-containing inclusions remaining in the steel due to slag. Since the amount and size of the steel can be significantly reduced, it is possible to provide steel having high cleanliness and excellent mechanical properties.

It is a figure which shows the mode in which the immersion ascending pipe and the immersion descending pipe of a vacuum chamber are immersed in molten steel and slag, the inside of the vacuum chamber is depressurized, and the molten steel and slag are sucked up in the vacuum chamber. It is a figure which shows the mode of molten steel and slag during a vacuum degassing treatment. It is a figure which shows the mode of molten steel and slag in a dipping riser pipe and a dipping down pipe at the time of normal pressure recovery. It is a figure which shows the mode of the molten steel and slag in the immersion riser pipe and the immersion drop pipe at the time of recompression of this invention.

The method for refining molten steel of the present invention (hereinafter, may be referred to as "the refining method of the present invention") is
After vacuum degassing the molten steel in the ladle containing C, Si, Mn, P, and S with a degassing device equipped with a dipping tube in the vacuum chamber, the pressure in the vacuum chamber is reduced. In the refining method in which the pressure P1 [Pa] is restored to the atmospheric pressure P0 [Pa] and the refining of the molten steel is completed.
(I) From the pressure P1 to the pressure P2 defined by the following formula (1), the lowering speed of the molten steel in the vacuum chamber is maintained at 0.20 m / sec or more to repressurize, and then the pressure is restored.
(Ii) From the pressure P2 to the atmospheric pressure P0, the falling speed of the molten steel in the immersion pipe is maintained at 0.15 m / sec or less and the pressure is restored.

P2 = P0-ρ ・ g ・ (hd) ・ ・ ・ (1)
ρ: Molten steel density [kg / m ³ ]
g: Gravity acceleration [m / sec ² ]
h: Height from the molten steel bath surface in the ladle to the molten steel bath surface in the vacuum tank [m]
d: Soaking in molten steel bath in a vacuum chamber [m]

Hereinafter, the refining method of the present invention will be described by taking as an example a case where molten steel is subjected to RH type vacuum degassing treatment.

1 to 3 show an aspect of the RH type vacuum degassing treatment. FIG. 1 shows an embodiment in which the immersion rising pipe and the immersion descending pipe of the vacuum chamber are immersed in molten steel and slag, the inside of the vacuum chamber is depressurized, and the molten steel and slag are sucked up in the vacuum chamber. FIG. 2 shows vacuum degassing. The mode of the molten steel and slag during the treatment is shown, and FIG. 3 shows the mode of the molten steel and slag in the immersion rising pipe and the immersion descending pipe at the time of normal repressurization.

As shown in FIG. 1, when the molten steel is subjected to vacuum degassing treatment, a vacuum tank 1 provided with a dipping rising pipe 2a and a dipping descent pipe 2b and connected to an exhaust device (not shown) via an exhaust pipe 4 is provided. Or the pan 2 is raised, the immersion ascending pipe 2a and the immersion descending pipe 2b are immersed in the molten steel 5a through the slag 6a, and the air in the vacuum chamber 1 is exhausted from the exhaust pipe 4 to exhaust the vacuum chamber. The pressure inside the vacuum chamber 1 is reduced, and the molten steel 5a and the slag 6a are sucked up into the vacuum chamber 1.

Since the molten steel 5a in the ladle 2 is covered with the slag 6a, the molten steel 5a and the slag 6a are sucked up in the vacuum chamber 1, and the sucked molten steel 5b is similarly covered with the slag 6b. In a state where the molten steel 5b is covered with the slag 6b, the molten steel recirculation gas 3a (inert gas such as argon and nitrogen) is applied to the molten steel in the immersion riser pipe 2a from the molten steel recirculation gas inlet 3 provided in the immersion riser pipe 2a. Gas) is blown in.

As shown in FIG. 2, the pumping action generated by the blowing of the molten steel recirculation gas 3a raises the molten steel 5b in the immersion riser pipe 2a, and the swelling 5c of the molten steel 5b is formed in the vacuum chamber 1 to form a vacuum chamber. The molten steel 5b in 1 flows toward the immersion descent pipe 2b at a required molten steel flow velocity 5d, and recirculates from the immersion descent pipe 2b to the ladle 2 (see “arrow” in FIG. 2).

The swelling 5c of the molten steel 5b is exposed to the reduced pressure atmosphere of the vacuum tank 1, and the degassing treatment for the molten steel 5b proceeds, and while the molten steel 5b is recirculated, the vacuum degassing treatment for the molten steel 5a proceeds.

The slag 6b covering the molten steel 5b is caught in the molten steel 5b circulating to the ladle 2 when the molten steel flow velocity 5d is high and the disturbance at the slag / molten steel interface is large, so that the slag 6b is in a reduced pressure state in the vacuum chamber 1. The behavior of the molten steel 5b recirculating to the ladle 2 and the behavior of the slag 6b that stays unevenly on the upper part of the immersion descent pipe 2b during the vacuum degassing treatment when the vacuum degassing is completed by returning the pressure to atmospheric pressure. , The cleanliness of the molten steel 5a after the molten steel 5b is recirculated to the ladle 2 is greatly affected.

When the depressurized state in the vacuum chamber 1 is depressurized toward atmospheric pressure, the height of the slag 5c of the molten steel 5b in the vacuum chamber 1 decreases, and accordingly, it exists in the vicinity of the immersion riser pipe 2a. Since the slag 6b flows toward the immersion riser pipe 2a, the molten steel 5b and the slag 6b in the vacuum chamber 1 descend from the immersion riser pipe 2a and the immersion lowering pipe 2b to the ladle 2 as shown in FIG.

Normally, even during recompression, the molten steel recirculation gas 3a is supplied from the molten steel recirculation gas inlet 3 of the immersion riser pipe 2a, so that the molten steel 5b and the slag 6b on the immersion riser pipe 2a side are strongly agitated and the molten steel reflux occurs. continue.

When the molten steel 5b in which the slag 6b is suspended is lowered from the immersion pipe rising pipe 2a to the ladle 2 by strong stirring, a molten steel region in which the slag 6b is suspended is formed in the molten steel 5a, and the molten steel region is in the ladle 2. It penetrates deep into the molten steel 5a and the cleanliness of the molten steel subjected to the vacuum degassing treatment is lowered.

Further, when the molten steel recirculation continues, the molten steel flow velocity 5d from the immersion rising pipe 2a side to the immersion descending pipe 2b side exists on the lower surface of the slag 6b that is biased toward the immersion descending pipe 2b side, and the vacuum tank. The slag 6b in 1 is easily caught in the molten steel flow velocity at the slag / molten steel interface.

When the flow rate of the molten steel recirculation gas 3a at the time of recompression is the same as the flow rate of the molten steel recirculation gas during the RH treatment, the recirculation amount of the molten steel decreases, but the molten steel flow velocity 5d in the vacuum chamber 1 increases, and the molten steel flow velocity 5d. However, when the critical molten steel flow velocity for slag entrainment is exceeded, the slag in the vacuum chamber 1 is always in a state of being easily entrained in the molten steel 5b.

Further, the molten steel flow velocity 5d formed on the lower surface of the slag 6b, which is biased toward the immersion descent pipe 2b side, from the immersion rise pipe 2a side to the immersion descent pipe 2b side exceeds the critical flow velocity of slag entrainment and slag. When the contact time between the 6b and the molten steel 5b becomes long, the slag 6b in the vacuum chamber 1 continues to be easily caught in the molten steel 5b, and the amount of slag-based inclusions in the molten steel 5a increases.

Further, when the pressure is restored, the molten steel 5b and the slag 6b in the vacuum chamber 1 drop sharply, so that the slag / molten steel interface in the ladle 2 violently shakes, and the possibility that the molten steel 5a entrains the slag 6a increases. Furthermore, due to this rapid drop of molten steel 5b and slag 6b, if the molten steel flow velocity exceeds the slag entrainment critical flow velocity near the slag / molten steel interface and the state continues for a long time, the slag system is entrained in the molten steel 5a. The amount of inclusions increases.

As described above, among the non-metallic inclusions in steel, an increase in the amount and size of CaO-containing inclusions inhibits the mechanical properties of steel, particularly ductility, toughness, fatigue properties and the like.

Therefore, based on the above, the present inventors can suppress an increase in the amount and size of CaO-containing inclusions, or reduce the amount and size.
(x) To shorten the contact time between the slag and the molten steel at the slag / molten steel interface where the molten steel recirculation continues (there is a molten steel flow velocity), and
(y) Realized the idea that it is effective to suppress the slag / molten steel interface sway due to the descent of molten steel and slag in the vacuum chamber and immersion pipe and reduce the slag entrainment in the molten steel. We enthusiastically examined the method of doing so.

As a result, the bath immersion of the molten steel in the vacuum chamber (see “d” in FIG. 2) becomes 0 m from the pressure P1 in the depressurized state (see FIG. 2) at the initial stage of recompression, as defined by the above equation (1). When the downward pressure of the molten steel in the vacuum chamber is maintained at 0.20 m / sec or more and the pressure is rapidly restored up to the pressure P2 [Pa].
(a) The rising height of the molten steel in the vacuum chamber rapidly decreases, and the molten steel descends into the immersion riser pipe before the slag existing near the immersion riser pipe flows to the immersion riser pipe side. Inflow into the immersion riser pipe is suppressed and at the same time
(b) The time at which the molten steel flow velocity from the immersion riser pipe side to the immersion down pipe side exists on the lower surface of the slag that is biased toward the immersion descending pipe side, that is, the contact time between the slag and the molten steel in the presence of the molten steel flow velocity. It was found that it was shortened and the slag entrainment of molten steel circulating in the ladle was suppressed.

Further, after rapidly repressurizing the molten steel in the vacuum chamber to a pressure P2 [Pa] where the bath immersion is 0 m, the descending speed of the molten steel in the immersion pipe is maintained at 0.15 m / sec or less, and the depressurization is slowly performed. Then,
(c) The slag / molten steel interface in the ladle is suppressed, reducing slag entrainment in the molten steel and at the same time.
(d) It was found that the infiltration of molten steel in the ladle of slag-based inclusions into the deep part was suppressed.

The refining method of the present invention was carried out based on the above findings, and significantly reduces the amount and size of CaO-containing inclusions remaining in the steel due to slag, and has high cleanliness and excellent mechanical properties. It is possible to manufacture steel.

Hereinafter, the decompression of the refining method of the present invention (recovery from pressure P1 to pressure P2 [Pa] in the decompressed state, and subsequent decompression from pressure P2 [Pa] to atmospheric pressure P0 [Pa] is performed. Pressure) will be described in detail with reference to the drawings.

FIG. 4 shows aspects of molten steel and slag in the immersion ascending pipe and the immersion descending pipe at the time of recompression of the refining method of the present invention.

As shown in FIG. 4, the decompression state at the initial stage of decompression (see FIG. 2) is rapidly recompressed to the pressure P2 at which the bath immersion of molten steel in the vacuum chamber (see “d” in FIG. 2) becomes 0 m. (Hereinafter referred to as "rapid decompression".)

Due to the rapid decompression, the molten steel 5b above the immersion riser pipe 2a rapidly descends into the immersion riser pipe 2a, so that the height of the rise 5c (see FIG. 2) of the molten steel 5b rapidly decreases, and at the same time, On the immersion descending pipe 2b side, the slag 6 and the molten steel 5b rapidly descend into the immersion descending pipe 2b.

With the rapid descent of the slag 6 and the molten steel 5b on the immersion rising pipe 2b side, the slag 6b existing in the vicinity of the immersion rising pipe 2a does not flow to the immersion rising pipe 2a side, so that the molten steel 5b above the immersion rising pipe 2a The swelling 5c rapidly descends into the immersion riser pipe 2a without being covered by the slag 6b. Therefore, slag entrainment of molten steel does not occur in the immersion riser pipe 2a.

Further, due to the rapid decompression, the slag and molten steel are present for a period of time during which the molten steel flow velocity from the immersion rising pipe side to the immersion descending pipe side exists on the lower surface of the slag that is biased toward the immersion descending pipe side, that is, in the presence of the molten steel flow velocity. The contact time is shortened, and slag entrainment of molten steel circulating in the ladle is suppressed.

The pressure P2 [Pa] forming a state in which the bathing of molten steel (see “d” in FIG. 2) in the vacuum chamber is 0 m can be defined by the following equation (1).

P2 = P0-ρ ・ g ・ (hd) ・ ・ ・ (1)
P0: Atmospheric pressure [Pa]
ρ: Molten steel density [kg / m ³ ]
g: Gravity acceleration [m / sec ² ]
h: Height from the molten steel bath surface in the ladle to the molten steel bath surface in the vacuum tank [m]
d: Soaking in molten steel bath in a vacuum chamber [m]

The pressure P2 is the pressure when the bathing of the molten steel in the vacuum chamber becomes 0 m, that is, the pressure when the upper surface of the slag becomes the same height as the bottom surface of the vacuum chamber in the vacuum chamber, and is the atmospheric pressure P0. The pressure is determined by the static pressure (ρ · g · (hd)) of the molten steel in the vacuum chamber (see “h” and “d” in FIG. 2).

When the pressure P1 at the initial stage of recompression is rapidly recompressed to the pressure P2 defined by the above formula (1), the descending speed of the molten steel in the vacuum chamber is set to 0.20 m / sec or more. If the descending speed of the molten steel in the vacuum chamber is less than 0.20 m / sec, the time until the bath immersion of the molten steel in the vacuum chamber becomes 0 m, that is, the time for the slag and the molten steel to come into contact with each other becomes long, and the molten steel Since slag entrainment occurs and coarse slag-based inclusions remain up to the steel material, the descending speed is set to 0.20 m / sec or more. It is preferably 0.40 m / sec or more. The lowering speed of the molten steel depends on the capacity of the vacuum degassing device and the supply speed of the gas supplied into the vacuum chamber for decompression, so the upper limit is not particularly limited.

According to the actual operation, when the bath immersion of the molten steel in the vacuum chamber is about 0.35 m, the pressure can be restored to the pressure P2 in about 1 to 2 seconds. In consideration of this point, the descending speed of the molten steel in the vacuum chamber was set to 0.20 m / sec or more as a feasible descending speed of the molten steel.

Further, after the molten steel in the vacuum chamber is rapidly repressurized to a pressure P2 [Pa] where the bath immersion is 0 m, the descending speed of the molten steel in the immersion tube is maintained at 0.15 m / sec or less, and slowly reaches the atmospheric pressure. (Hereinafter referred to as "slow pressure recovery"). The slow decompression reduces the slag / molten steel interface sway in the ladle, which suppresses slag entrainment in the molten steel and suppresses the infiltration of slag-based inclusions into the deep part of the ladle. To.

When the descending speed of the molten steel in the immersion pipe exceeds 0.15 m / sec in the slow recovery pressure, the slag / molten steel interface sway in the ladle increases, the slag entrainment in the molten steel increases, and the slag entrainment increases. Since the slag-based inclusions penetrate deep into the molten steel in the ladle and the coarse slag-based inclusions remain in the steel material, the descending speed of the molten steel in the immersion pipe is set to 0.15 m / sec or less. It is preferably 0.05 m / sec or less.

For example, in actual operation, when the descent speed of the molten steel in the immersion pipe is set to 0.05 m / sec and the slow depressurization is performed, the time until the slow depressurization is completed is about 30 seconds. The refining method can be carried out reasonably in actual operation.

The lower limit of the falling speed of the molten steel is not particularly limited because it may be appropriately set in consideration of productivity.

In the refining method of the present invention, the molten steel containing C, Si, Mn, P, and S to be subjected to the vacuum degassing treatment may be a molten steel having a normal component composition refined in a normal refining step (primary refining). .. The preferable composition of the molten steel will be described later.

The vacuum degassing treatment (secondary refining) performed following the primary refining is performed using an RH type vacuum degassing device, and when the pressure is restored after the vacuum degassing treatment is completed, the pressure P2 defined by the above formula (1). The lowering speed of the molten steel in the vacuum chamber is maintained at 0.20 m / sec or more and the pressure is restored, and then the lowering speed of the molten steel in the immersion pipe is maintained at 0.15 m / sec or less from the pressure P2 to the atmospheric pressure. After the pressure is restored, the molten steel is cast. The casting may be ordinary casting, but continuous casting is preferable.

For example, if the refining method of the present invention is applied in a refining process in which primary refining is performed in a converter and then secondary refining is performed outside the converter, slag entrainment in molten steel can be suppressed as much as possible. The method is suitable for producing steels that require high cleanliness, for example, steels for bearings.

At the time of recompression, the decompression gas supplied into the vacuum chamber is supplied from one or both of the portion where the molten steel recirculation gas is blown and the portion directly supplied into the vacuum chamber. When the decompression gas is supplied from both portions, it is preferable that the flow rate of the decompression gas supplied from the portion where the molten steel recirculation gas is blown is smaller than the flow rate of the molten steel recirculation gas during the vacuum degassing treatment.

By making the flow rate of the recompression gas supplied from the part where the molten steel recirculation gas is blown at the time of recompression smaller than the flow rate of the molten steel recirculation gas at the time of vacuum degassing treatment, the slag entrainment of the molten steel in the vacuum tank is further improved. It can be suppressed. The decompression gas is preferably an inert gas such as argon or nitrogen that does not easily react with the molten steel from the viewpoint of preventing contamination of the molten steel.

If the molten steel to be subjected to the vacuum degassing treatment is a molten steel having a normal composition, that is, a molten steel containing the basic elements C, Si, Mn, P, and S of the steel, slag may be involved in the molten steel. Since the effect of reducing inclusions in the refining method of the present invention can be obtained while being suppressed, the composition is not limited to the molten steel having a specific composition, but the preferable composition of the molten steel in which the effect of reducing inclusions is remarkably exhibited is described below. explain. Hereinafter,% means mass%.

C: 1.20% or less C is an element effective for ensuring the strength and hardness of steel after quenching. If it exceeds 1.20%, cracks occur during quenching and the cutting tool becomes too hard, which shortens the life of the cutting tool. Therefore, C is preferably 1.20% or less. More preferably, it is 1.00% or less.

For steel types that do not require much strength or hardness, C is not necessarily required, so the lower limit is not particularly limited. However, since C is a basic element of steel and it is difficult to make it 0%, the lower limit is set. Does not include 0%. C is preferably 0.001% or more in order to secure the required strength and hardness.

Si: 3.00% or less Si is an element that enhances hardenability and is effective in ensuring strength and hardness. If it exceeds 3.00%, it becomes too hard and the life of the cutting tool is shortened. Therefore, Si is preferably 3.00% or less. More preferably, it is 2.50% or less.

For steel types that do not require much strength or hardness, Si is not required, so the lower limit is not specified. However, Si is a basic element of steel and it is difficult to set it to 0%, so the lower limit is 0. Does not include%. Si is preferably 0.001% or more in order to secure the required strength and hardness.

Mn: 1.60% or less Mn is an element that enhances hardenability and is effective in ensuring strength and hardness. If it exceeds 1.60%, cracks occur during quenching and the cutting tool becomes too hard, which shortens the life of the cutting tool. Therefore, Mn is preferably 1.60% or less. More preferably, it is 1.20% or less.

A steel type that does not require much strength or hardness does not require Mn, so a lower limit is not particularly set. However, since Mn is a basic element of steel, the lower limit does not include 0%. Mn is preferably 0.01% or more in order to secure the required strength and hardness.

P: 0.05% or less P is an impurity element and is an element that inhibits toughness. If P exceeds 0.05%, the toughness is significantly reduced, so P is preferably 0.05% or less. More preferably, it is 0.03% or less. The lower limit includes 0%, but if P is reduced to 0.0001% or less, the refining cost increases significantly, so 0.0001% is a practical lower limit for practical steel.

S: 0.05% or less S is an impurity element and an element that inhibits toughness, like P. If S exceeds 0.05%, the toughness is significantly reduced, so S is preferably 0.05% or less. More preferably, it is 0.03% or less. The lower limit includes 0%, but if S is reduced to 0.0001% or less, the refining cost increases significantly, so 0.0001% is a practical lower limit for practical steel.

In addition to the above basic elements, the molten steel having a preferable composition is 0.20% or less, Cr: 3.50% or less, Mo: 0.85%, as long as it does not impair the mechanical properties and / or chemical properties of the steel. Below, Ni: 4.50% or less, Nb: 0.20% or less, V: 0.45% or less, W: 0.30% or less, B: 0.006% or less, N: 0.060% or less, Ti: 0.25% or less, Cu: 0.50% or less, Pb: 0.45% or less, Bi: 0.20% or less, Te: 0.01% or less, Sb: 0.20% or less, Mg: It may contain one or more of 0.010% or less, Ca: 0.010% or less, REM: 0.010% or less, O: 0.003% or less.

Al: 0.20% or less Al is a deoxidizing element and is an element that refines crystal grains. If it exceeds 0.20%, coarse oxide-based inclusions are formed and the toughness and ductility are lowered. Therefore, Al is preferably 0.20% or less. More preferably, it is 0.15% or less. From the viewpoint of ensuring the effect of refining the crystal grains, 0.005% or more is preferable, and 0.010% or more is more preferable.

Cr: 3.50% or less Cr is an element that enhances hardenability and is effective in ensuring strength and hardness. If it exceeds 3.50%, the toughness and ductility decrease, so 3.50% or less is preferable. More preferably, it is 2.50% or less. From the viewpoint of ensuring the effect of adding Cr, 0.01% or more is preferable, and 0.05% or more is more preferable.

Mo: 0.85% or less Mo is an element that enhances hardenability and is effective in ensuring strength and hardness. Mo is an element that forms carbides and contributes to the improvement of temper softening resistance. If it exceeds 0.85%, a supercooled structure is formed and the toughness and ductility are lowered. Therefore, Mo is preferably 0.85% or less. More preferably, it is 0.65% or less. From the viewpoint of ensuring the effect of adding Mo, 0.005% or more is preferable, and 0.010% or more is more preferable.

Ni: 4.50% or less Ni is an element that enhances hardenability and is effective in ensuring strength and hardness. If it exceeds 4.50%, the toughness and ductility decrease, so Ni is preferably 4.50% or less. More preferably, it is 3.50% or less. From the viewpoint of ensuring the effect of adding Ni, 0.005% or more is preferable, and 0.010% or more is more preferable.

Nb: 0.20% or less Nb is an element that forms carbides, nitrides, and / or carbonitrides, and contributes to suppressing coarsening of crystal grains and improving tempering and softening resistance. If it exceeds 0.20%, toughness and ductility decrease, so Nb is preferably 0.20% or less. More preferably, it is 0.10% or less. From the viewpoint of ensuring the effect of adding Nb, 0.005% or more is preferable, and 0.010% or more is more preferable.

V: 0.45% or less V is an element that forms carbides, nitrides, and / or carbonitrides, and contributes to suppressing coarsening of crystal grains and improving tempering and softening resistance. If it exceeds 0.45%, toughness and ductility decrease, so V is preferably 0.45% or less. More preferably, it is 0.35% or less. From the viewpoint of ensuring the effect of adding V, 0.005% or more is preferable, and 0.010% or more is more preferable.

W: 0.30% or less W is an element that enhances hardenability and is effective in ensuring strength and hardness. Further, W is an element that forms carbides and contributes to the improvement of temper softening resistance. If it exceeds 0.30%, a supercooled structure is formed and the toughness and ductility are lowered. Therefore, W is preferably 0.30% or less. More preferably, it is 0.25% or less. From the viewpoint of ensuring the effect of adding W, 0.005% or more is preferable, and 0.010% or more is more preferable.

B: 0.006% or less B is an element that enhances hardenability and contributes to the improvement of strength. Further, B is an element that segregates at the austenite grain boundaries, suppresses the grain boundary segregation of P, and contributes to the improvement of fatigue strength. If it exceeds 0.006%, the toughness decreases, so B is set to 0.006% or less. It is preferably 0.004% or less. From the viewpoint of ensuring the effect of adding B, 0.0005% or more is preferable, and 0.0010% or more is more preferable.

N: 0.060% or less N is an element that forms fine nitrides to refine crystal grains and contribute to improvement of strength and toughness. If it exceeds 0.060%, nitride is excessively generated and the toughness deteriorates. Therefore, N is preferably 0.060% or less. More preferably, it is 0.040% or less. From the viewpoint of ensuring the effect of adding N, 0.001% or more is preferable, and 0.005% or more is more preferable.

Ti: 0.25% or less Ti is an element that forms fine Ti nitrides to refine crystal grains and contribute to the improvement of strength and toughness. If it exceeds 0.25%, Ti nitride is excessively formed and the toughness is lowered. Therefore, Ti is preferably 0.25% or less. More preferably, it is 0.15% or less. From the viewpoint of ensuring the effect of adding Ti, 0.005% or more is preferable, and 0.010% or more is more preferable.

Cu: 0.50% or less Cu is an element that contributes to the improvement of corrosion resistance. If it exceeds 0.50%, the hot ductility is lowered and cracks and flaws are generated. Therefore, Cu is preferably 0.50% or less. More preferably, it is 0.30% or less. From the viewpoint of ensuring the effect of adding Cu, 0.01% or more is preferable, and 0.05% or more is more preferable.

Pb: 0.45% or less Pb is an element that contributes to the improvement of free-cutting property. If it exceeds 0.45%, the toughness decreases, so the Pb is preferably 0.45% or less. More preferably, it is 0.30% or less. From the viewpoint of ensuring the effect of adding Pb, 0.005% or more is preferable, and 0.010% or more is more preferable.

Bi: 0.20% or less Bi is an element that contributes to the improvement of free-cutting property. If it exceeds 0.20%, the toughness decreases, so the Bi is preferably 0.20% or less. More preferably, it is 0.16% or less. From the viewpoint of ensuring the effect of adding Bi, 0.005% or more is preferable, and 0.010% or more is more preferable.

Te: 0.010% or less Te is an element that contributes to the improvement of free-cutting property. If it exceeds 0.010%, the toughness decreases, so the Te is preferably 0.010% or less. More preferably, it is 0.006% or less. From the viewpoint of ensuring the effect of adding Te, 0.001% or more is preferable, and 0.002% or more is more preferable.

Sb: 0.20% or less Sb is an element that contributes to the improvement of corrosion resistance, mainly sulfuric acid resistance and hydrochloric acid resistance, and the improvement of free-cutting property. If it exceeds 0.20%, the toughness decreases, so the Sb is preferably 0.20% or less. More preferably, it is 0.15% or less. From the viewpoint of ensuring the effect of adding Sb, 0.01% or more is preferable, and 0.03% or more is more preferable.

Mg: 0.010% or less Mg is an element that contributes to the improvement of free-cutting property. If it exceeds 0.010%, the toughness decreases, so Mg is preferably 0.010% or less. More preferably, it is 0.006% or less. From the viewpoint of ensuring the effect of adding Mg, 0.0005% or more is preferable, and 0.0010% or more is more preferable.

Ca: 0.010% or less Ca is a deoxidizing element and is an element that forms CaO-Al ₂ O ₃ based inclusions having a low melting point and easily aggregate in a deoxidizing reaction. When it exceeds 0.010%, the Al ₂ O ₃ system inclusions are compounded with the low melting point CaO-Al ₂ O ₃ system inclusions and coarsened, and the coarsened CaO-Al ₂ O ₃ system inclusions become coarse. Ca is preferably 0.010% or less because it does not become liquid phase at the rolling temperature and remains coarse in the steel. More preferably, it is 0.006% or less.

Since the smaller the amount of Ca, the more preferable it is, the lower limit is not limited, but since about 0.0001% inevitably remains, 0.0001% is a substantial lower limit in practical steel.

REM: 0.010% or less REM (one or more of rare earth elements, La, Ce, Pr, and Nd) is CaO in molten steel in molten steel sufficiently deoxidized with Al or Al-Si. , It is an element that reduces CaO in inclusions and modifies CaO-Al ₂ O ₃ based inclusions. If it exceeds 0.010%, a low melting point compound phase having a high REM concentration appears in the inclusions, and the aggregation of inclusions is promoted to form coarse inclusions, so that the REM is 0.010. % Or less is preferable. More preferably, it is 0.007% or less.

In the molten steel sufficiently deoxidized with Al or Al—Si, 0.0005% or more is preferable, and 0.0010% or less is more preferable, from the viewpoint of ensuring the effect of adding REM.

O: 0.003% or less O is an element that forms an oxide. If it exceeds 0.003%, coarse oxides are generated and the rolling fatigue life is shortened. Therefore, O is preferably 0.003% or less. More preferably, it is 0.002% or less. The lower limit includes 0%, but if O is reduced to 0.0001% or less, the refining cost increases significantly, so 0.0001% is a practical lower limit for practical steel.

In the composition of molten steel, the balance is Fe and unavoidable impurities. The unavoidable impurities are elements that are inevitably mixed from the steel raw material and / or in the steelmaking process, and are acceptable elements as long as they do not impair the characteristics of the molten steel and the characteristics of the cast steel.

Next, examples of the present invention will be described. However, the conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention. Therefore, the present invention is not limited to this one-condition example. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

(Example)
The molten steel having the composition shown in Table 1 is subjected to primary refining by a converter, LF treatment and / or secondary refining by RH treatment, then the molten steel is continuously cast, and the continuous slab is block-rolled to steel. Manufactured a piece.

Specifically, a molten steel having the above-mentioned composition is melted in a 270-ton converter, deoxidized with any one or more of Si, Mn, and Al at the time of steel ejection, and then a predetermined slag is produced. Ladle refining is performed according to the composition, component adjustment and cleaning treatment are performed using an RH type vacuum degassing device, and after recompression, continuous casting is performed to make slag, and the slag is heated and held in a heating furnace. After that, it was subjected to bulk rolling to obtain steel pieces.

In the above steel pieces, the maximum predicted diameter [μm] of the extreme value statistics of non-metal inclusions in the predicted area of 30,000 mm ^{2 was} estimated by the extreme value statistical method. The maximum predicted diameter of inclusions (√area (max)) estimated by extreme value statistics is, for example, "Metal fatigue micro-defects and effects of inclusions" (Yukitaka Murakami, Yokendo, 1993, p.223). -239). The method used is a two-dimensional method of estimating the maximum inclusion diameter observed within a certain area by a two-dimensional inspection.

Using the above extreme value statistical method, the position of 1/4 of the L cross section of the steel piece (the center line of the loose surface, the center line of the facing surface, and the cross section including the center line of the slab) on the loose surface side. From the image of non-metal inclusions taken with an optical microscope, the maximum predicted diameter of inclusions with inspection reference plane: 100 mm ² (10 × 10 mm), inspection field: 16, and prediction area of 30,000 mm ² √ area (max) was calculated.

Specifically, 16 data (data of 16 fields) of the maximum diameter of the inclusions obtained by the observation are plotted on the extremum probability sheet according to the method described in the above document, and the maximum inclusion distribution straight line (data of the maximum inclusions) ( The maximum predicted diameter of inclusions √area (max) was estimated at an area of 30,000 mm ² by obtaining the maximum inclusions and the linear function of the extremum statistical standardization variable) and extrapolating the maximum inclusion distribution straight line.

Table 1 also shows the composition of the molten steel, the pressure recovery conditions (the falling speed of the molten steel from P1, P2, and P2, the falling speed of the molten steel from P2 to atmospheric pressure), and the maximum diameter of the extreme value statistical prediction [μm]. ..

In Examples 13 to 32, during the vacuum degassing treatment, the pressure P1 to the pressure P2 at the initial stage of depressurization is restored at a molten steel descending speed of 0.20 m / sec or more, and the pressure P2 to the atmospheric pressure is 0.15 m / sec. Since the pressure was restored at the following molten steel descending speed, the maximum diameter of the extremum statistical prediction was 23 μm or less.

In Comparative Examples 1 to 12, one or both of the molten steel descending speed from the pressure P1 to the pressure P2 and the molten steel descending speed from the pressure P2 to the atmospheric pressure at the initial stage of decompression are outside the scope of the present invention during the vacuum degassing treatment. , The maximum diameter of the extreme value statistical prediction exceeds 51 μm.

As described above, it can be seen that in the invention example, the size of coarse inclusions caused by slag is reduced as compared with the comparative example, and a steel having high cleanliness and excellent mechanical properties can be obtained.

As described above, according to the present invention, it is possible to suppress slag entrainment of molten steel that occurs during unsteady state and penetration of slag-based inclusions into the deep part of the molten steel in the ladle, and it remains in the steel due to slag. Since the amount and size of CaO-containing inclusions can be significantly reduced, it is possible to provide steel having high cleanliness and excellent mechanical properties. Therefore, the present invention has high utility in the steel industry.

1 Vacuum increase 2 Ladle 2a Immersion riser pipe 2b Immersion lowering pipe 3 Molten steel recirculation gas inlet 3a Molten steel recirculation gas 4 Exhaust pipe 5a, 5b Molten steel 5c Rise 5d Molten steel flow velocity 6 6a, 6b Slag

Claims (6)

After vacuum degassing the molten steel in the ladle containing C, Si, Mn, P, and S with a degassing device equipped with a dipping tube in the vacuum tank, the pressure in the vacuum tank is reduced. In the refining method in which the pressure P1 [Pa] is restored to the atmospheric pressure P0 [Pa] and the refining of the molten steel is completed.
(I) From the pressure P1 to the pressure P2 defined by the following formula (1), the lowering speed of the molten steel in the vacuum chamber is maintained at 0.20 m / sec or more to repressurize, and then the pressure is restored.
(Ii) A method for refining molten steel, characterized in that the descending speed of the molten steel in the immersion pipe is maintained at 0.15 m / sec or less and the pressure is restored from pressure P2 to atmospheric pressure P0.
P2 = P0-ρ ・ g ・ (hd) ・ ・ ・ (1)
ρ: Molten steel density [kg / m ³ ]
g: Gravity acceleration [m / sec ² ]
h: Height from the molten steel bath surface in the ladle to the molten steel bath surface in the vacuum tank [m]
d: Soaking in molten steel bath in a vacuum chamber [m] The method for refining molten steel according to claim 1, wherein the vacuum degassing treatment is performed by an RH type vacuum degassing device. The claim is characterized in that when the decompression is performed by the RH type vacuum degassing device, the decompression gas is supplied from one or both of a portion where the molten steel recirculation gas is blown and a portion which is directly supplied into the vacuum chamber. The method for refining molten steel according to 2. The method for refining molten steel according to claim 3, wherein the decompression gas is an inert gas. The molten steel contains C: 1.20% or less, Si: 3.00% or less, Mn: 1.60% or less, P: 0.05% or less, S: 0.05% or less in mass%. The method for refining molten steel according to any one of claims 1 to 4, wherein the balance is composed of Fe and unavoidable impurities. In terms of mass%, the molten steel further contains Al: 0.20% or less, Cr: 3.50% or less, Mo: 0.85% or less, Ni: 4.50% or less, Nb: 0.20% or less, V: 0.45% or less, W: 0.30% or less, B: 0.006% or less, N: 0.060% or less, Ti: 0.25% or less, Cu: 0.50% or less, Pb: 0.45% or less, Bi: 0.20% or less, Te: 0.010% or less, Sb: 0.20% or less, Mg: 0.010% or less, Ca: 0.010% or less, REM: 0. The method for refining molten steel according to claim 5, wherein one or more of 010% and O: 0.003% or less are contained.

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