JP6756264B2 - 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 reduce the size of 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.

In Patent Document 2, when the molten steel produced in an arc melting furnace or a converter is transferred to a ladle and refined, 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 highly 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 Unexamined Patent Publication No. 05-331526 Japanese Unexamined Patent Publication 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 (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 / metal 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 / metal interface is violent. As the slag sways and gets caught in the molten steel, the slag that gets caught is pulled deep into 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, since the molten steel recirculation gas is continuously supplied to the molten steel during the recompression, the slag accumulated in the vacuum chamber at the time of recompression returns to the ladle through the immersion pipe for supplying the molten steel recirculation gas. On the way, the slag is suspended in the molten steel by stirring with the above gas and penetrates into the deep part of the ladle. As a result, the cleanliness of the steel deteriorates.

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 recirculation type degassing steel sheet is weakly recirculated, the total oxygen content in the molten steel is 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.

In view of the current state of the prior art, the present invention suppresses slag entrainment of molten steel that occurs during unsteady state during decompression of vacuum degassing treatment of molten steel, and the amount and size of coarse CaO-containing inclusions in the steel. An object of the present invention is to produce steel having high cleanliness by reducing slag, 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 rising height of the molten steel on the dipping pipe (hereinafter sometimes referred to as "immersion rising pipe") side where the molten steel rises during the repressurization of the vacuum degassing treatment is optimized, it will be accumulated in the vacuum chamber. It is possible to prevent the slag from infiltrating into the immersion riser pipe, and further to prevent the slag-based inclusions from invading deep into the ladle, resulting in the amount of CaO-containing inclusions present in the steel. We found that the size could be significantly reduced.

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

(1) The molten steel containing C, Si, Mn, P, and S is subjected to vacuum degassing treatment with a degassing device equipped with a dipping rising pipe and a dipping descent pipe in the vacuum tank, and then the vacuum tank is depressurized. In the refining method that evacuates from to atmospheric pressure and finishes refining of molten steel
(I) Recirculation gas is blown into the molten steel in the vacuum chamber on the immersion riser pipe side provided with the recirculation gas inlet from the recirculation gas inlet.
(Ii) A method for refining molten steel, which is defined by the following equation (1) and is characterized in that a swelling of molten steel having a height of Δh [m] satisfying the following equation (2) is formed and a decompression treatment is performed.

Δh [m] = 2.0 × 10 ^-3・ H ^-1.3・ ε ^2/3・ ・ ・ (1)
H [m]: Distance from the recirculation gas blowing position of the immersion riser pipe to the molten steel surface in the vacuum tank ε [W]: Molten steel stirring force ε [W] = 0.00835 ・ Uo ²・ as defined by the following formula Q ₀
U _o = (200 / 3π) ・ (Q _o / D ₁ ²・ ρ)
Q _o = 11.4 ・ G ^1/3・ D ₁ ^4/3・ ln (P ₃ / P ₁ ) ^1/3
U _o [m / sec]: Average flow velocity of molten steel in the immersion pipe during recirculation
Q _o [t / min]: Molten steel ring flow rate
G [NL / min]: Flow rate of blown gas
D ₁ [m]: Inner diameter of immersion tube
P ₁ [Pa]: Pressure in the vacuum chamber before the start of repressurization
P ₃ [Pa]: Static pressure at the recirculation gas blowing position
ρ [kg / m ³ ]: Molten steel density Δh [m]> 2d _S2・ ・ ・ (2) d _S2 = (4 / 3π) ・ (V _s / D ₂ ² )
V _s = (D ₁ ²・ d _S1・ π) / 2
d _S2 [m]: Slag thickness in the vacuum chamber V _s [m ³ ]: Slag volume brought into the vacuum chamber D ₂ [m]: Inner diameter of the vacuum chamber d _S1 [m]: Slag thickness in the ladle

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

(3) When the repressurization is performed by the RH type refining apparatus, 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).

(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, wherein the method contains Fe 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, when the pressure is restored in the vacuum degassing treatment, the rising height formed on the molten steel on the immersion riser pipe side is optimized to suppress the flow of slag in the vacuum chamber to the immersion riser pipe side. Since it is possible to suppress the mixing of coarse CaO-containing inclusions into the molten steel, the amount and size of inclusions in the steel can be significantly reduced, and a steel having high cleanliness and excellent mechanical properties is provided. be able to.

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. It is a figure which shows the mode which raised the molten steel and slag in a vacuum chamber. It is a figure which shows the normal mode of a vacuum degassing treatment. It is a figure which shows one aspect of molten steel and slag at the start of recompression. It is a figure which shows one aspect of molten steel and slag which is being recompressed. It is a figure which shows another aspect of molten steel and slag which is undergoing a decompression treatment. It is a figure which shows the mode of molten steel and slag at the final stage of the decompression treatment shown in FIG. It is a figure which shows the concrete relationship between the slag height and the slag thickness during a pressure recovery process.

The method for refining molten steel of the present invention (hereinafter, may be referred to as “refining method for the present invention”)
The molten steel containing C, Si, Mn, P, and S is subjected to vacuum degassing treatment with a degassing device equipped with a vacuum tank and a dipping tube, and then the vacuum tank is depressurized from a reduced pressure state to atmospheric pressure. In the refining method to finish the refining of molten steel
(I) Recirculation gas is blown into the molten steel in the vacuum chamber on the immersion riser pipe side provided with the recirculation gas inlet from the recirculation gas inlet.
(Ii) It is characterized in that a swelling of molten steel having a height of Δh [m] that satisfies the following formula (2) is formed and a decompression treatment is performed, as defined by the following formula (1).

Δh [m] = 2.0 × 10 ^-3・ H ^-1.3・ ε ^2/3・ ・ ・ (1)
H [m]: Distance from the recirculation gas blowing position of the immersion riser pipe to the molten steel surface in the vacuum chamber ε [W]: Molten steel stirring power ε [W] = 0.00835 · U ₀ ² defined by the following formula.・ Q ₀
U ₀ = (200 / 3π) ・ (Q ₀ / D ₁ ²・ ρ)
Q ₀ = 11.4 ・ G ^1/3・ D ₁ ^4/3・ ln (P ₃ / P ₁ ) ^1/3
U ₀ [m / sec]: Average flow velocity of molten steel in the immersion pipe during recirculation
Q ₀ [t / min]: Molten steel ring flow rate
G [NL / min]: Flow rate of blown gas
D ₁ [m]: Inner diameter of immersion tube
P ₁ [Pa]: Pressure in the vacuum chamber before the start of repressurization
P ₃ [Pa]: Static pressure at the recirculation gas blowing position
ρ [kg / m ³ ]: Molten steel density Δh [m]> 2d _S2・ ・ ・ (2) d _S2 = (4 / 3π) ・ (V _s / D ₂ ² )
V _s = (D ₁ ²・ d _S1・ π) / 2
d _S2 [m]: Slag thickness in the vacuum chamber V _s [m ³ ]: Slag volume brought into the vacuum chamber D ₂ [m]: Inner diameter of the vacuum chamber d _S1 [m]: Slag thickness in the ladle

Hereinafter, the refining method of the present invention will be described.

As described above, among the non-metallic inclusions in steel, an increase in the number and size of coarse CaO-containing inclusions is a factor that impairs the mechanical properties of steel, particularly ductility, toughness, fatigue properties and the like. Is. The present inventors have diligently studied a method for suppressing an increase in the number and size of coarse CaO-containing inclusions or reducing the number and size.

As a result, after the vacuum degassing treatment is completed, when the depressurizing gas is supplied into the vacuum chamber to restore the depressurized state in the vacuum chamber to atmospheric pressure, the molten steel on the immersion riser pipe side is piled up in the vacuum chamber. By optimizing the climb height, it is possible to suppress the mixing of harmful CaO-containing inclusions into the molten steel, reduce the number and size of inclusions in the steel, and improve the mechanical properties of the steel. I found it.

First, the height of the molten steel in the vacuum chamber (hereinafter sometimes referred to as the "height of the molten steel") and the thickness of the slag in the vacuum chamber (hereinafter sometimes referred to as the "slag thickness"). The relationship will be explained.

1 to 3 show an aspect of the vacuum degassing treatment. FIG. 1 shows a mode in which the immersion rising pipe and the immersion descending pipe of the vacuum tank are immersed in molten steel and slag, FIG. 2 shows a mode in which the molten steel and slag are raised in the vacuum tank, and FIG. The usual aspects of gas treatment are shown.

When performing a vacuum degassing treatment on molten steel, first, as shown in FIG. 1, a vacuum tank 1 provided with an immersion riser pipe 2a and a immersion lowering pipe 2b and connected to an exhaust device (not shown) via an exhaust pipe 4 Or the pan 2 is raised, and the immersion ascending pipe 2a and the immersion descending pipe 2b are immersed in the molten steel 5a through the slag 6a.

When the vacuum degassing treatment is started, the air in the vacuum chamber 1 is exhausted from the exhaust pipe 4 in the immersed state of the immersion rising pipe 2a and the immersion descending pipe 2b, and the inside of the vacuum chamber 1 is depressurized. As shown, 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 slag 6a is sucked up together with the molten steel 5a in the vacuum chamber 1, and the sucked molten steel 5b is covered with the slag 6b.

In a state where the molten steel 5b is covered with the slag 6b, the recirculation gas 3a (an inert gas such as argon or nitrogen) is introduced into the molten steel in the immersion riser pipe 2a from the recirculation gas inlet 3 provided in the immersion riser pipe 2a. Infuse. Due to the pumping action generated by the blowing of the recirculation gas 3a, the molten steel 5b in the immersion riser pipe 2a rises, a rise 5c of the molten steel 5b is formed in the vacuum tank 1, and the molten steel 5b in the vacuum tank 1 is immersed. It circulates clockwise from the descending pipe 2b to the pan 2 (see "arrow" in FIG. 3).

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 circulates clockwise, the vacuum degassing treatment for the molten steel 5a proceeds.

When the vacuum degassing treatment is completed, the depressurized state in the vacuum tank 1 is returned to the atmospheric pressure. At this time, the behavior of the molten steel 5b returning to the ladle 2 and the upper part of the immersion lowering pipe 2b during the vacuum degassing treatment. The behavior of the slag 6b, which has been unevenly retained, greatly affects the cleanliness of the molten steel 5a after the molten steel 5b returns to the ladle 2.

FIG. 4 shows one aspect of molten steel and slag at the start of recompression. FIG. 5 shows one aspect of molten steel and slag during the pressure recovery treatment.

As shown in FIG. 4, when the depressurized state in the vacuum chamber 1 is returned to the atmospheric pressure, the height of the slag 5c of the molten steel 5b in the vacuum chamber 1 becomes lower, and accordingly, it exists in the vicinity of the immersion riser pipe 2a. The slag flows toward the immersion riser pipe 2a, and the molten steel 5b and the slag 6b descend from the immersion riser pipe 2a to the ladle 2.

Since the recirculation gas 3a is supplied from the recirculation gas inlet 3 of the immersion riser pipe 2a even during the repressurization treatment, the molten steel 5b and the slag 6b on the immersion riser pipe 2a side are strongly agitated. 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 deep portion of the molten steel 5a in the ladle 2 is formed. The cleanliness of the molten steel that has been vacuum degassed is reduced.

Therefore, the present inventors have diligently studied a method of preventing the slag staying in the vacuum chamber 1 from infiltrating into the immersion riser pipe during the pressure recovery treatment. First, if the height of the molten steel rising formed on the immersion rising pipe side in the vacuum chamber is made higher than the slag thickness retained in the vacuum chamber, the present inventors will stay on the immersion descending pipe side in the vacuum chamber. It was thought that the slag would stay as it was and would not flow to the immersion riser pipe side, and the infiltration of slag into the immersion riser pipe would be suppressed.

The above idea will be described based on the drawings.

FIG. 6 shows another aspect of molten steel and slag during the recompression treatment. FIG. 7 shows the mode of molten steel and slag at the final stage of the pressure recovery treatment shown in FIG.

In the mode of the molten steel and the slag during the pressure recovery treatment shown in FIG. 6, a swelling 5c of the molten steel 5b is formed on the immersion rising pipe 2a side, and the molten steel 5b on the immersion rising pipe 2a side is covered with the slag 6b. Not.

That is, after the vacuum degassing treatment shown in FIG. 3 is completed, the depressurization treatment is performed so that the height of the swelling 5c of the molten steel 5b in the vacuum tank 1 stays in the upper part of the immersion descent pipe 2b in the vacuum tank 1. Start with a thickness higher than the thickness of the slag 6b to be slag, and proceed with the decompression treatment while maintaining that state.

Due to the presence of the swelling 5c of the molten steel 5b whose height is higher than the thickness of the slag 6b, the slag 6b that stays in the upper part of the immersion descent pipe 2b in the vacuum chamber 1 during the pressure recovery treatment stays as it is and rises in immersion. Since it does not flow to the pipe 2a side, only the molten steel 5b descends into the ladle 2 from the immersion riser pipe 2a, and the pressure recovery treatment proceeds, and even at the end of the pressure recovery treatment, as shown in FIG. Only the molten steel 5b descends from the immersion riser pipe 2a into the ladle 2.

This point is substantially different from the conventional decompression treatment shown in FIGS. 4 and 5, and is the basic idea of the refining method of the present invention.

While the molten steel 5b descends into the immersion riser pipe 2a, the reflux gas 3a is blown into the molten steel 5b from the recirculation gas injection port 3, but since there is no slag covering the molten steel 5b, the suspended state of the molten steel and the slag is Does not occur.

Based on the above idea, the present inventors have diligently studied the specific relationship between the height of the molten steel rise and the slag thickness. As a result, when the height of the molten steel rising on the immersion rising pipe side is less than twice the slag thickness on the immersion descending pipe side, the slag staying on the immersion descending pipe side flows and stays on the immersion rising pipe side. Then, the pressure recovery treatment shown in FIGS. 4 and 5 was performed, and it was found that slag entrainment occurred in the molten steel during the pressure recovery treatment, and coarse inclusions remained in the steel.

FIG. 8 shows a specific relationship between the rising height of the molten steel and the thickness of the slag during the pressure recovery treatment.

In the refining method of the present invention, the rising height of the molten steel formed on the immersion rising pipe side where the molten steel rises during the repressurization treatment is defined by the following formula (1).

Δh [m] = 2.0 × 10 ^-3・ H ^-1.3・ ε ^2/3・ ・ ・ (1)
Δh [m] can be defined by the distance from the recirculation gas blowing position of the immersion riser pipe to the molten steel surface in the vacuum chamber: H [m] and the molten steel stirring force: ε [W] determined by the following formula.
ε [W] = 0.00835 ・ U _o ²・ Q _o

U _o = (200 / 3π) ・ (Q _o / D ₁ ²・ ρ)
Q _o = 11.4 ・ G ^1/3・ D ₁ ^4/3・ ln (P ₃ / P ₁ ) ^1/3
U _o [m / sec]: Average flow velocity of molten steel in the immersion pipe during recirculation
Q _o [t / min]: Molten steel ring flow rate
G [NL / min]: Flow rate of blown gas
D ₁ [m]: Inner diameter of immersion tube
P ₁ [Pa]: Pressure in the vacuum chamber before the start of repressurization
P ₃ [Pa]: Static pressure at the recirculation gas blowing position
ρ [kg / m ³ ]: molten steel density

Then, the pressure recovery treatment is performed so that the molten steel rise height Δh [m] defined by the above formula (1) satisfies the following formula (2). This point is a feature of the refining method of the present invention.

Δh [m]> 2d _S2 ... (2)
d _S2 is the thickness of the slag in the vacuum chamber, which is determined by the volume V _s [m ³ ] of the slag that enters the vacuum chamber from the immersion riser pipe and the immersion descending pipe when the inside of the vacuum chamber is depressurized. It can be defined by the following formula.

d _S2 = (4 / 3π) · (V _s / D ₂ ² )
V _s = (D ₁ ²・ d _S1・ π) / 2
D ₂ [m]: Inner diameter of vacuum chamber d _S1 [m]: Slag thickness in the ladle

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 refining device, and when the pressure is restored after the vacuum degassing treatment is completed, the molten steel defined by the above formula (1) rises. The pressure recovery treatment is performed so that the height Δh satisfies the above formula (2), and after the pressure recovery, 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 step 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 refining method is suitable for producing steel materials that require high cleanliness, for example, steel materials for bearings.

At the time of recompression, the recompression gas supplied into the vacuum chamber is a molten steel recirculation gas in order to form a rise of height Δh defined by the above formula (1) on the molten steel on the immersion riser pipe side in the vacuum chamber. Is supplied from one or both of the part where the gas is blown and the part which is 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.

The molten steel to be subjected to the vacuum degassing treatment is defined by the above formula (1) if it is a molten steel having a normal composition, that is, a molten steel containing C, Si, Mn, P, and S, which are the basic elements of steel. At a height of Δh, a swelling of molten steel satisfying the above formula (2) can be formed, slag entrainment in the molten steel can be suppressed, and an inclusion reduction effect in the steel material can be obtained. Therefore, a specific component composition can be obtained. The preferable component composition of the molten steel, which is not limited to the molten steel of the above, but which remarkably exhibits the effect of reducing inclusions, will be described. 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. From the viewpoint of ensuring the effect of refining the crystal grains, 0.005% or more is preferable, and 0.010% 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 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 0.001% 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%, nitrides are excessively formed and the toughness is lowered. 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 that easily aggregates in the deoxidizing reaction. If it exceeds 0.010%, the Al ₂ O ₃ based inclusions are compounded with the low melting point CaO-Al ₂ O ₃ based inclusions and coarsened. The coarsened CaO-Al ₂ O ₃ system inclusions do not become liquid phase at the rolling temperature and remain in the steel in a coarse state. Therefore, Ca is preferably 0.010% or less. 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 this example 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 was subjected to primary refining by a converter, secondary refining by LF treatment and RH treatment, and continuously cast to produce steel.

Specifically, when the molten steel that has undergone primary refining in a 270t converter is ejected, the molten steel is deoxidized with one or more of Si, Mn, and Al, and the deoxidized molten steel is subjected to LF treatment. Following the secondary refining of No. 1, the component composition was adjusted by RH treatment under the conditions shown in Table 1, and after cleaning treatment, continuous casting was performed to obtain slabs. This slab was heated and held in a heating furnace and then subjected to block rolling to obtain a steel slab.

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" (Takayoshi Murakami, Yokendo, 1993, p. 223). -239) can be performed. 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. samples were taken from the images of the nonmetallic inclusions captured by the optical microscope examination reference ^{plane: 100mm 2 (10mm × 10mm)} , the inspection field of view: 16, area make predictions: maximum expected inclusions 30,000 mm ² The diameter √ 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.

The results of the above estimation are also shown in Table 1.

Comparative Example No. Examples 1 to 8 are examples in which the rising height (Δh) of the molten steel on the immersion riser pipe side is twice or less the slag thickness (d _S2 ) in the vacuum chamber in the depressurization treatment during the vacuum degassing treatment. Yes, there is no improvement in the maximum predicted particle size of the extreme value statistics.

On the other hand, Invention Example No. 9 to 20 are examples in which the rising height (Δh) of the molten steel on the immersion riser pipe side is made larger than twice the slag thickness (d _S2 ) in the vacuum chamber in the decompression treatment during the vacuum degassing treatment. Compared with the comparative example, the maximum predicted particle size by the extreme value statistics is 21 to 26 μm, which is a good value.

As described above, in the invention example, the coarsening of inclusions is suppressed as compared with the comparative example of the conventional operation, so that it is clear that a steel having excellent mechanical properties can be obtained.

As described above, according to the present invention, the height of the swelling formed on the molten steel on the immersion riser pipe side is optimized when the vacuum degassing treatment is restored, and the slag in the vacuum chamber to the immersion riser pipe side is optimized. Since it is possible to suppress the flow of coarse CaO-containing inclusions in the molten steel, the amount and size of inclusions in the steel can be reduced, the cleanliness is high, and the mechanical properties are excellent. Steel can be provided. Therefore, the present invention has high utility in the steel industry.

1 Vacuum increase 2 Ladle 3 Recirculation gas inlet 3a Recirculation gas 4 Exhaust pipe 5a, 5b Molten steel 5c Rise 6 6a, 6b Slag

Claims (6)

After vacuum degassing the molten steel containing C, Si, Mn, P, and S with a degassing device equipped with a dipping rising pipe and a dipping descent pipe in the vacuum tank, the vacuum tank is depressurized to atmospheric pressure. In the refining method that evacuates to and finishes the refining of molten steel
(I) Recirculation gas is blown into the molten steel in the vacuum chamber on the immersion riser pipe side provided with the recirculation gas inlet from the recirculation gas inlet.
(Ii) A method for refining molten steel, which is defined by the following equation (1) and is characterized in that a swelling of molten steel having a height of Δh [m] satisfying the following equation (2) is formed and a decompression treatment is performed.
Δh [m] = 2.0 × 10 ^-3・ H ^-1.3・ ε ^2/3・ ・ ・ (1)
H [m]: Distance from the recirculation gas blowing position of the immersion riser pipe to the molten steel surface in the vacuum tank ε [W]: Molten steel stirring force defined by the following formula ε [W] = 0.00835 · U _o ²・ Q ₀
U _o = (200 / 3π) ・ (Q _o / D ₁ ²・ ρ)
Q _o = 11.4 ・ G ^1/3・ D1 ^4/3・ ln (P ₃ / P ₁ ) ^1/3
U _o [m / sec]: Average flow velocity of molten steel in the immersion pipe during recirculation
Q _o [t / min]: Molten steel ring flow rate
G [NL / min]: Flow rate of blown gas
D ₁ [m]: Inner diameter of immersion tube
P ₁ [Pa]: Pressure in the vacuum chamber before the start of repressurization
P ₃ [Pa]: Static pressure at the recirculation gas blowing position
ρ [kg / m ³ ]: Molten steel density Δh [m]> 2d _S2・ ・ ・ (2) d _S2 = (4 / 3π) ・ (V _s / D ₂ ² )
V _s = (D ₁ ²・ d _S1・ π) / 2
d _S2 [m]: Slag thickness in the vacuum chamber V _s [m ³ ]: Slag volume brought into the vacuum chamber D ₂ [m]: Inner diameter of the vacuum chamber d _S1 [m]: Slag thickness in the ladle The method for refining molten steel according to claim 1, wherein the vacuum degassing treatment is performed by an RH type refining apparatus. 2. The second aspect of the present invention is that when the repressurization is performed by the RH type refining apparatus, 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 described method for refining molten steel. 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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