JP6957890B2 - Refining method of molten steel - Google Patents

Description

The present invention relates to a method for refining molten steel, particularly a method for refining when vacuum refining is completed.

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 peripheral parts. In general, steel is subjected to desiliconization treatment, dephosphorization treatment, and decarburization treatment of molten steel in a converter, and then in a secondary refining step, the composition of the molten steel is adjusted 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 immersion pipes are immersed in the molten steel in the ladle, the vacuum tank connected to the immersion 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 from the immersion pipe into the molten steel 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. , 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 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 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. It has been proposed to introduce an active gas to prevent the adsorption 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 having a size 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 when the ladle slag used in refining is caught in the molten steel, and CaO in the slag is reduced and mixed into the molten steel to form Al ₂ O _{3 in the molten steel.} 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 of the inclusions. It is effective to reduce the amount and size of the low melting point inclusions which are the starting points of the above, and also to reduce the amount and size of the inclusions in the molten steel which are 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), the inclusion of inclusions in the molten steel is suppressed, or the inclusions in the molten steel are suppressed. 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, and the amount and size of slag-based inclusions mixed in both increase. Increase.

After the RH treatment is completed, in order to return the molten steel in the vacuum chamber to the ladle, "re-pressure" is performed to return the depressurized state in the vacuum chamber to atmospheric pressure, but at the time of re-pressure, 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. It sways and the slag gets caught in the molten steel.

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.

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 velocity 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. , The recirculation conditions at the time of recompression are not described, and since the latter half is weakly stirred, the total amount of oxygen in the molten steel 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 depressurization are specified, but Patent Document 4 describes the depressurization condition for suppressing slag entrainment of molten steel in the vacuum chamber and the ladle. No. 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 in molten steel that occurs during unsteady state during depressurization of vacuum degassing treatment of molten steel, and the amount of coarse CaO-containing inclusions in the steel. An object of the present invention is to reduce the size of the molten steel, and an object of the present invention is to provide a refining method for molten steel that solves the problem.

Based on the current state of the prior art, the present inventors have diligently studied a method for solving the above problems. As a result, if the molten steel recirculation gas flow rate (that is, the molten steel recirculation flow rate) is reduced and the molten steel flow velocity in the vacuum chamber is reduced to maintain the weak recirculation state at the time of decompression of the vacuum degassing treatment, the slag in the vacuum chamber is maintained. / It was found that the vibration at the molten steel interface can be reduced, and an environment in which the slag is less likely to be caught in the molten steel can be formed in the vacuum chamber.

Further, under the above environment, slag entrainment in molten steel can be suppressed, and the amount and size of coarse CaO-containing inclusions generated due to slag can be reduced. We have found that it can improve mechanical properties.

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

(1) In a refining method for refining molten steel by subjecting molten steel containing C, Si, Mn, P, and S to vacuum degassing treatment with a degassing device equipped with a vacuum tank and a dipping tube.
After completion of the vacuum degassing, at pressure recovery vacuum state in the vacuum chamber to atmospheric pressure, recirculated gas flow rate V per molten steel 1t of condensate pressure gas supplied from the dip tube in the molten steel [Nm ³ / t / min] A method for refining molten steel, which comprises controlling and repressurizing so as to satisfy the following formula (1).

A [m ² ]: Cross-sectional area of the pipe that supplies gas into the immersion pipe ρ _Fe [kg / m ³ ]: Molten steel density ρ _g [kg / m ³ ]: Circulating gas density g [m / sec ² ]: Gravity acceleration h [m]: Molten steel head P ₀ [Pa]: Pressure in vacuum chamber N [pieces]: Number of gas pipes that supply gas into the immersion pipe M [t]: Weight of molten steel in the pan V ₀ [Nm ³ / t / min]: Flow rate of molten steel recirculation gas during degassing treatment

(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: 0.025% 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 vacuum degassing treatment is depressurized, the flow velocity of the molten steel recirculation can be optimized to suppress the mixing of CaO-containing inclusions in the molten steel, so that the amount and size of inclusions in the steel can be reduced. It is possible to provide steel having excellent mechanical properties.

Maximum particle size and average particle size of CaO-containing inclusions when the flow rate of molten steel recirculation gas is not reduced (comparative example), and maximum particle size and average grain size of CaO-containing inclusions when the flow rate of molten steel recirculation gas is reduced. It is a figure which shows an example of each ratio of the diameter (invention example). Extreme value statistics of CaO-containing inclusions at the conventional flow rate that does not reduce the molten steel recirculation gas flow rate at the time of recompression Maximum predicted particle size (comparative example) and the extreme value of CaO-containing inclusions when the molten steel recirculation gas flow rate is reduced. It is a figure which shows an example of the comparison of the statistical maximum predicted particle diameter (invention example).

The method for refining molten steel of the present invention (hereinafter, may be referred to as "the refining method of the present invention") is defined as the method for refining the molten steel of the present invention.
In a refining method for refining molten steel by subjecting molten steel containing C, Si, Mn, P, and S to vacuum degassing treatment with a degassing device equipped with a vacuum tank and a dipping tube.
After completion of the vacuum degassing, at pressure recovery vacuum state in the vacuum chamber to atmospheric pressure, recirculated gas flow rate V per molten steel 1t of condensate pressure gas supplied from the dip tube in the molten steel [Nm ³ / t / min] Is controlled so as to satisfy the following equation (1), and the pressure is restored.

A [m ² ]: Cross-sectional area of the pipe that supplies gas into the immersion pipe ρ _Fe [kg / m ³ ]: Molten steel density ρ _g [kg / m ³ ]: Circulating gas density g [m / sec ² ]: Gravity acceleration h [m]: Molten steel head P ₀ [Pa]: Pressure in vacuum chamber N [pieces]: Number of gas pipes that supply gas into the immersion pipe M [t]: Weight of molten steel in the pan V ₀ [Nm ³ / t / min]: Flow rate of molten steel recirculation gas during degassing treatment

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

As a result, sometimes pressure recovery in the vacuum degassing treatment of molten steel, the recirculated gas flow rate V per molten steel 1t of condensate pressure gas supplied from the dip tube in the molten steel [Nm ³ / t / min], the above equation (1) It was found that if the pressure is reduced by controlling the filling, the formation of harmful CaO-containing inclusions can be suppressed, the amount of inclusions in the steel can be reduced and the diameter can be reduced, and the mechanical properties can be improved. rice field.

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

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 smelted 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 after the primary refining may be performed by using a refining apparatus capable of performing the depressurization according to the above formula (1) at the time of repressurization. The RH type refining apparatus is preferable in that the amount of molten steel recirculation gas can be easily controlled according to the above formula (1).

When the RH type refining device is used, the decompression gas is supplied from one or both of the part where the molten steel recirculation gas is blown and the part which is directly supplied into the vacuum chamber, and the recirculation gas flow rate V [Nm ³ / t / min. ] Can be controlled so as to satisfy the above equation (1). The decompression gas is preferably an inert gas. After the depressurization is performed according to the above formula (1), the molten steel is cast. Casting may be normal casting, but continuous casting is preferable.

Next, the reason why the recompression is performed by controlling the molten steel recirculation gas flow rate V at the time of recompression so as to satisfy the above equation (1) will be described.

Left side of the equation ^{(1): (2A 2 (} ρ Fe gh + P 0) / ρ g) 1/2 · (N / M)
The left side of the above equation (1) is a condition set for continuously performing the RH process. That is, if the "static pressure of molten steel (ρ _Fe · g · h) + pressure inside the vacuum chamber (P ₀ )" is larger than the dynamic pressure "(1/2) · ρ _g · V ² " of the gas blown into the immersion pipe. In order to continue the RH treatment under the proper conditions, there is a risk that molten steel will invade the pipe into which the gas is blown and the pipe will be blocked, making it impossible to perform the next RH treatment under the proper conditions. The left side of the above equation (1) was set.

Right side of the above equation (1): (2/3) · V ₀
In the RH treatment of molten steel, the critical molten steel flow velocity at which molten steel entrains slag at the slag / molten steel interface is, for example, Asai's formula (Asai: 100th and 101st Nishiyama Memorial Technical Lecture Materials (1984), p.67, p90). According to the calculation by the above, it is about 0.7 m / sec.

The present inventors have conceived that if the molten steel flow velocity is lower than the limit molten steel flow velocity, slag entrainment at the slag / molten steel interface does not occur (coarse inclusions are not increased in the molten steel). The upper limit of the recirculation gas flow rate V [Nm ³ / t / min] was set.

The molten steel ring flow rate is proportional to the molten steel flow velocity, and the molten steel recirculation gas flow rate is proportional to the cube of the molten steel ring flow rate. It is necessary to multiply the molten steel ring flow rate by (0.7 / 0.8) times and the molten steel ^{recirculation gas flow rate by {(0.7 / 0.8) 3} = 0.669 ≈ 2/3} times.

The present inventors have found that the calculation results pursuant to the 2/3 less than the steady process time of the molten steel circulating gas flow rate V ₀ [Nm ³ / t / min], molten steel recirculated gas flow rate V [Nm ³ / t / min ] Was set as the upper limit.

Normally, the molten steel flow velocity differs depending on the molten steel recirculation gas flow rate, but the limit condition that the molten steel does not involve slag at the slag / molten steel interface is not the limit molten steel flow velocity that is substantially constant, but depends on the refining conditions as described above. If specified using the molten steel recirculation gas flow rate V ₀ [Nm ³ / t / min] that can be adjusted as appropriate, vacuum degassing treatment that does not involve slag at the slag / molten steel interface can be performed over a wide range of molten steel recirculation gas flow rate. Can be done.

Then, when the molten steel recirculation gas flow rate is equal to or higher than the value (upper limit) on the right side of the above formula (1), the present inventors do not reduce the entrainment of slag in the molten steel in the vacuum chamber and the ladle, and the steel. It is confirmed that coarse CaO-containing inclusions remain in the mixture, and when the recirculation gas flow rate is less than the value (lower limit) on the left side of the above formula (1), the molten steel supplies gas to the immersion pipe. It was confirmed that it would be difficult to invade the pipe and continue proper operation.

Further, the present inventors, C, Si, Mn, P, and, by RH process the molten steel contains S, pressure recovery at the molten steel reflux gas flow rate V of the condensate pressure gas [Nm ³ / t / min] It was confirmed that the particle size of the CaO-containing inclusions in the steel was reduced and the maximum statistical maximum particle size dmax was also reduced if the control was performed so as to satisfy the above formula (1).

FIG. 1 shows the maximum particle size and average particle size (comparative example) of CaO-containing inclusions when the flow rate of molten steel recirculation gas is not reduced, and the maximum grain size of CaO-containing inclusions when the flow rate of molten steel recirculation gas is reduced. An example of each ratio of the diameter and the average particle diameter (invention example) is shown.

Fig. 2 shows the maximum predicted particle size of CaO-containing inclusions (comparative example) in the case of the conventional flow rate that does not reduce the flow rate of molten steel recirculation gas at the time of recompression, and the inclusion of CaO in the case of reducing the flow rate of molten steel recirculation gas. An example of comparison of the maximum predicted particle size (invention example) of the extreme value statistics of an object is shown.

As shown in FIG. 1, when the flow rate of the molten steel recirculation gas is reduced according to the above formula (1) at the time of recompression, the maximum particle size and the average particle size of the CaO-containing inclusions in the steel are greatly reduced. Further, as shown in FIG. 2, the maximum predicted particle size of the extreme value statistics is also greatly reduced.

That is, if the flow rate of the molten steel recirculation gas (that is, the flow rate of the molten steel ring) is reduced and the surface flow velocity of the molten steel is reduced at the time of decompression of the vacuum degassing treatment, slag entrainment in the molten steel can be greatly suppressed and slag. The amount and size of the coarse CaO-containing inclusions produced due to the above can be reduced. This is the finding found by the present inventors and is the finding that forms the basis of the refining method of the present invention.

The molten steel to be vacuum degassed at a recirculation gas flow rate satisfying the above formula (1) is a molten steel having a normal composition, that is, a molten steel containing the basic elements of steel, C, Si, Mn, P, and S. If there is, the inclusion reduction effect is exhibited by controlling the recirculation gas flow rate according to the above formula (1). Therefore, the component of the molten steel is not limited to the molten steel having a specific component composition, but the inclusion reduction effect is remarkably exhibited. The composition 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 lowered, 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%, toughness and ductility decrease, so Cr is preferably 3.50% or less. 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%, the 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. 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 preferably 0.006% or less. More preferably, it is 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%, 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, 0.005% or more is preferable, and 0.010% 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 the 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.} 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 are released. 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 or 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 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. Inevitable impurities are elements that are inevitably mixed in from the steel raw material and / or in the steelmaking process, and are permissible 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 given a predetermined value. Secondary refining was performed by LF treatment using a slag composition, then the component composition was adjusted by RH treatment, purification treatment was performed, and then continuous casting was performed to obtain slabs. This slab was heated and held in a heating furnace and then subjected to bulk rolling to obtain a steel slab.

In the above steel pieces, the maximum predicted diameter [μm] of the extremum statistics of non-metal inclusions in ^{the predicted area of 30,000 mm 2 was estimated by the extremum statistical method.} The maximum predicted diameter of inclusions (√area (max)) estimated by extreme value statistics is, for example, "Metal fatigue micro-defects and the effects of inclusions" (Y. 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. From the image of the non-metal inclusions taken with an optical microscope, the inspection reference plane: 100 mm ² (10 × 10 mm), the inspection field: 16, and the predicted area of 30,000 mm ² maximum predicted diameter of the inclusions. √ area (max) was calculated.

Specifically, 16 data (data of 16 fields of view) 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) ( By obtaining the maximum inclusions and the linear function of the extremum statistical standardization variable) and extrapolating the maximum inclusion distribution straight line, the maximum predicted diameter of inclusions √area (max) at ^{an area of 30,000 mm 2 was estimated.}

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

Invention Example No. In Nos. 1 to 20, since the flow rate of the molten steel recirculation gas at the time of recompression is within an appropriate range, the maximum predicted particle size based on the extreme value statistics is 16 to 29 μm, which is a good value. Comparative Example No. In 23, 24, 27, 30, and 33, since the molten steel recirculation gas flow rate at the time of recompression is out of the upper limit value, no improvement is seen in the maximum predicted particle size of the extreme value statistics.

Comparative Example No. In 21, 22, 25, 26, 28, 29, 31, and 32, the molten steel recirculation gas flow rate at the time of recompression is out of the lower limit, so the molten steel is inserted into the pipe that blows the recirculation gas into the immersion pipe. After the next RH treatment, an appropriate molten steel recirculation gas flow rate could not be obtained, and it became difficult to continue proper operation.

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, when the vacuum degassing treatment is depressurized, the flow velocity of the molten steel recirculation can be optimized to suppress the mixing of CaO-containing inclusions in the molten steel, so that the amount of inclusions in the steel can be suppressed. And the size can be reduced, and steel having excellent mechanical properties can be provided. Therefore, the present invention has high utility in the steel industry.

Claims (6)

In a refining method for refining molten steel by subjecting molten steel containing C, Si, Mn, P, and S to vacuum degassing treatment with a degassing device equipped with a vacuum tank and a dipping tube.
After completion of the vacuum degassing, the pressure in the vacuum chamber when in the return pressure to return from the vacuum state to the atmospheric pressure, recirculated gas flow rate V [Nm per molten steel 1t of condensate pressure gas supplied from the dip tube into the molten steel ³ / t / min] is controlled so as to satisfy the following formula (1) to repressurize, which is a method for refining molten steel.

V ₀ [Nm ³ / t / min]: Flow rate of molten steel recirculation gas during vacuum degassing treatment
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 decompression 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 described in. 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. The molten steel further contains, in terms of 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: 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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