JP2018127670A - Method for refining molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method where, upon pressure recovery in vacuum degassing treatment, the slag entrainment of a molten steel caused upon a nonstationary time is suppressed to reduce the amount and size of coarse CaO-containing inclusions in the steel.SOLUTION: Provided is a refining method where a molten steel is subjected to vacuum degassing treatment by a degassing device provided with a vacuum tank and an immersion tank, and the molten steel is refined, in which, after the completion of vacuum degassing treatment and before the start of pressure recovery, the flow rate of a rotary flow gas V[Nm/t/min] per molten steel 1t fed from an immersion tube into the molten steel is controlled so as to satisfy equation (1), and pressure recovery is performed, in which: A[m] denotes the cross-sectional area of tubes feeding the gas into the immersion tube; ρ[kg/m] denotes the density of the molten steel; ρ[kg/m] denotes the density of a rotary flow gas; g[m/s] denotes gravity acceleration; h[m] denotes a molten steel head; P[Pa] denotes the pressure in the vacuum tank; N[number] denotes the number of the gas tubes feeding the gas into the immersion tube; M[t] denotes the weight of the molten steel in a ladle; and V[Nm/t/min] denotes the flow rate of the rotary flow gas during the degassing treatment.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for refining molten steel, and particularly to a method for refining vacuum refining.

  In recent years, steel having excellent mechanical properties has been demanded in order to improve the performance of mechanical devices and downsize peripheral components. In general, steel is subjected to desiliconization treatment, dephosphorization treatment, and decarburization treatment of molten steel in a converter, and then, in the secondary refining process, the composition of the molten steel is adjusted, and inclusions in the molten steel are removed. In order to improve the mechanical properties, it is necessary to reduce inclusions in the 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, ladle slag refining treatment (hereinafter referred to as “LF treatment”). And vacuum degassing treatment (hereinafter sometimes referred to as “RH treatment”) to reduce inclusions in the molten steel.

  In the RH treatment, two dip pipes are immersed in the molten steel in the ladle, the vacuum tank connected to the dip pipe is depressurized, and the molten steel is sucked into the vacuum tank by a differential pressure from the atmospheric pressure, and the molten steel reflux gas is discharged. This is a process for degassing and reducing inclusions by supplying the molten steel from the dip tube into the molten steel and circulating the molten steel between the vacuum chamber and the ladle.

  In the RH treatment, the molten steel is strongly stirred, so that the removal of inclusions is promoted. On the other hand, slag is entrained in the molten steel. Many technologies related to reflux control have been proposed.

  For example, in Patent Document 1, after performing refining and refining with slag having a basicity of 3 or more, vacuum degassing is performed using a reflux degassing apparatus with 2/3 of the treatment time being high reflux and 1/3 being weak reflux. A method for producing a bearing steel 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 flow rate of the molten steel by the recirculation degassing apparatus is completely reduced. There has been proposed a method for producing highly clean steel, characterized in that degassing is carried out for 25 minutes or more at least 8 times the molten steel.

  In Patent Document 3, when a high-cleanness steel is produced by refining molten steel from a converter or an electric furnace with a ladle refining device after refining it with a ladle refining device, a recirculating vacuum degassing is performed. In the first half treatment of 1/3 to 1/2 of the total treatment time of the recirculating vacuum degassing apparatus, the slag basicity in the refining treatment performed in the apparatus is 6.5 to 13.5, and the molten steel ring flow rate is 180 ton / min. As described above, a method for producing a high cleanliness steel has been proposed in which a high reflux state of 210 ton / min or less is set, and in the latter half treatment, a molten steel flow rate of 110 ton / min or more and a weak reflux state of 140 ton / min or less is set. Yes.

  In Patent Document 4, in a re-pressure method of a vacuum refining apparatus in which nitrogen gas is introduced into a vacuum tank at the end of vacuum refining treatment of molten steel and the pressure is restored from vacuum to normal pressure, the surface of the molten steel is free of argon gas or the like. It has been proposed to introduce an active gas to prevent sorption of molten steel.

  Patent Document 5 proposes a method for producing a high cleanliness ultra-low carbon steel, characterized in that a stirring gas is supplied in accordance with the degree of vacuum in the vacuum chamber without bringing slag into the vacuum chamber. ing.

JP 62-063650 A JP 2001-342516 A JP 2008-133505 A JP 05-331526 A JP-A-08-199225

  As described above, the amount and size of inclusions existing in the steel, mainly oxide inclusions, greatly affect the mechanical properties of the steel. Among 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 sometimes referred to as “CaO-containing inclusions”).

Coarse CaO-containing inclusions are slag inclusions generated when ladle slag used in refining is caught in molten steel, CaO in slag is reduced and mixed into molten steel, and Al ₂ O _{3 in} molten steel Inclusions produced by reacting with MgO-Al ₂ O _3, and these inclusions are inclusions that are coarsened by incorporating inclusions in the molten steel.

  Coarse CaO-containing inclusions deteriorate the quality control index extreme value statistics 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 that are the starting point of the steel, and to reduce the amount and size of the inclusions in the molten steel taken into the low melting point inclusions.

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

  The ladle slag is entrained in the molten steel when the flow rate of the molten steel is high or when the slag / metal interface is severely disturbed. Increase.

  After completion of the RH treatment, in order to return the molten steel in the vacuum chamber to the ladle, “return pressure” is performed to return the reduced pressure state in the vacuum chamber to atmospheric pressure. Since the molten steel and the molten steel stored in the dip tube descend rapidly and return to the ladle, the descending flow velocity of the molten steel induced by the rapidly descending molten steel is very large and the slag / metal interface is intense. Peristally, slag is caught in molten steel.

  Moreover, if the molten steel reflux gas flow rate at the time of returning pressure is the same strong circulating flow condition as the molten steel reflux gas flow rate during the RH treatment, the molten steel will return at a flow rate exceeding the critical molten steel flow velocity involving slag even during the returning pressure. As a result, the slag in the vacuum chamber is always placed in a state where it is easily caught in the molten steel.

  In the method of Patent Document 1, the first half of the reflux degassing process is set to high reflux, and the latter half of the third is set to weak reflux. Further, when the second half of the recirculation type degassing treatment is weakly recirculated, the total amount of oxygen T.I. O may not be sufficiently reduced.

  In the method of Patent Document 2, the refining in the ladle is set to 60 minutes or less, and the degassing is performed for 25 minutes or more by setting the ring flow rate of the molten steel by the reflux degassing apparatus to 8 times or more of the total molten steel. The recirculation conditions at the time of return pressure are not described, and the recirculation speed 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 entire processing time of the reflux-type vacuum degassing apparatus is set to a high reflux state and the latter half is set to a weak reflux state. The reflux conditions at the time of return pressure are not described, and since the second half is weakly stirred, the total oxygen content T.I. There is a possibility that O cannot be lowered sufficiently.

  In the method of Patent Document 4, the gas type and the like at the time of return pressure are defined. However, Patent Document 4 describes the return pressure condition for suppressing the slag entrainment of molten steel in the vacuum chamber and the ladle. Absent. In the method of Patent Document 5, an appropriate stirring gas flow rate corresponding to the pressure in the vacuum chamber is defined. However, Patent Document 5 suppresses slag entrainment in molten steel in the vacuum chamber and the ladle. The pressure recovery conditions to be performed are not described.

  In view of the current state of the art, the present invention suppresses slag entrainment in molten steel that occurs during non-steady times during vacuum degassing of molten steel, and the amount of coarse CaO-containing inclusions in the steel It is an object to provide a method for refining molten steel that solves the problem.

  The present inventors diligently studied a method for solving the above problems based on the current state of the prior art. As a result, the slag in the vacuum tank can be reduced if the molten steel reflux gas flow rate (ie, molten steel ring flow rate) is reduced and the molten steel flow rate in the vacuum chamber is reduced to maintain a weak reflux state when the vacuum degassing process is restored. It was found that the perturbation of the / molten steel interface can also be reduced and an environment in which the slag is not easily caught in the molten steel can be formed in the vacuum chamber.

  Furthermore, 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. It was found that the mechanical properties can be enhanced.

  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 with a degassing apparatus including a vacuum tank and a dip tube.
After the vacuum degassing process is completed, when the decompressed state in the vacuum chamber is restored to atmospheric pressure, the circulating gas flow rate V [Nm ³ / t / t of molten steel supplied into the molten steel from the dip tube before the decompression starts. The molten steel refining method is characterized in that the pressure is controlled so as to satisfy the following formula (1).

A [m ² ]: Cross-sectional area of the pipe supplying gas into the dip 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]: Vacuum tank pressure N [lines]: Number of gas tubes supplying gas into the dip tube M [t]: Molten steel weight in the ladle V ₀ [Nm ³ / t / min]: Molten steel reflux gas flow rate during degassing

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

  (3) When returning pressure by the RH type refining apparatus, the returning pressure gas is supplied from one or both of a part for blowing molten steel reflux gas and a part for supplying directly into the vacuum chamber. The method for refining molten steel according to (2).

  (4) The molten steel refining method according to (3), wherein the return pressure gas is an inert gas.

  (5) The molten steel is in mass%, 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 The molten steel refining method according to any one of (1) to (4) above, wherein the balance is made of Fe and inevitable impurities.

  (6) The molten steel is further mass%, Al: 0.20% or less, Cr: 3.50% or less, Mo: 0.85% or less, Ni: 4.50% or less, Nb: 0.20 %: 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 as described in (5) above, containing 0.010% or less and O: 0.003% or less.

  According to the present invention, when the vacuum degassing process is restored, the flow rate of the molten steel can be optimized to suppress the inclusion of CaO-containing inclusions in the molten steel, thereby reducing the amount and size of inclusions in the steel. It is possible to provide steel having excellent mechanical properties.

Maximum particle size and average particle size of CaO-containing inclusions in the case of conventional flow rates that do not reduce molten steel reflux gas flow rate, and maximum particle size and average particle size of CaO-containing inclusions in the case of reducing molten steel reflux gas flow rate It is a figure which shows an example of each ratio of a diameter (invention example). Extreme pressure statistical maximum predicted particle size (comparative example) of CaO-containing inclusions in the case of the conventional flow rate that does not reduce the molten steel reflux gas flow rate at the time of return pressure, and the extreme value of the CaO-containing inclusions in the case of reducing the molten steel reflux gas flow rate It is a figure which shows an example of contrast of a statistical maximum prediction particle diameter (invention example).

The method for refining molten steel of the present invention (hereinafter sometimes referred to as “the refining method 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 apparatus including a vacuum tank and a dip tube,
After the vacuum degassing process is completed, when the decompressed state in the vacuum chamber is restored to atmospheric pressure, the circulating gas flow rate V [Nm ³ / t / t of molten steel supplied into the molten steel from the dip tube before the decompression starts. Min] is controlled so as to satisfy the following formula (1), and the pressure is restored.

A [m ² ]: Cross-sectional area of the pipe supplying gas into the dip 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]: Vacuum tank pressure N [lines]: Number of gas tubes supplying gas into the dip tube M [t]: Molten steel weight in the ladle V ₀ [Nm ³ / t / min]: Molten steel reflux gas flow rate during degassing

  As described above, among the inclusions in steel, in particular, the increase in the number of coarse CaO-containing inclusions and the increase in particle size are factors that hinder mechanical properties, particularly ductility, toughness, impact properties, fatigue properties, and the like. is there. The present inventors diligently studied a technique 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, the recirculation gas flow rate V [Nm ³ / t / min] per 1 ton of molten steel supplied from the dip tube into the molten steel at the time of recompression in the vacuum degassing treatment of the molten steel is expressed by the above formula (1 If the pressure is controlled so as to satisfy (), the generation of harmful CaO-containing inclusions can be suppressed, the inclusions in steel can be reduced in size and diameter, and the mechanical properties can be improved. I found.

  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 vacuum degassing treatment may be a molten steel having a normal component composition refined by a normal refining process (primary refining). In addition, the preferable component composition of molten steel is mentioned later.

  The vacuum degassing treatment (secondary refining) performed after the primary refining may be performed using a refining apparatus that can perform repressure according to the above formula (1) at the time of recuperation. The RH refining apparatus is preferable in that the amount of molten steel reflux gas can be easily controlled according to the above formula (1).

When the RH refining apparatus is used, the return pressure gas is supplied from one or both of the part where the molten steel reflux gas is blown and the part where the molten steel is supplied directly into the vacuum chamber, and the reflux gas flow rate V [Nm ³ / t / min. Can be controlled so as to satisfy the above formula (1). The return pressure gas is preferably an inert gas. After returning pressure according to the above formula (1), the molten steel is cast. Casting may be ordinary casting, but continuous casting is preferred.

  Next, the reason for performing the return pressure by controlling the molten steel reflux gas flow rate V at the time of return pressure so as to satisfy the above formula (1) will be described.

Left side of the above formula (1): (2A ² (ρ _Fe gh + P ₀ ) / ρ _g ) ^1/2 · (N / M)
The left side of the above formula (1) is a condition set to continue the RH process. That is, when “the molten steel static pressure (ρ _Fe · g · h) + the pressure in the vacuum chamber (P ₀ )” is larger than the dynamic pressure “(1/2) · ρ _g · V ² ” of the gas blown into the dip tube. In order to continue the RH treatment under the proper conditions, the molten steel penetrates into the pipe into which the gas is blown, the pipe is blocked, and the next RH treatment may not be performed under the proper conditions. The left side of the above formula (1) was set.

Right side of the above formula (1): (2/3) · V ₀
In the RH treatment of molten steel, the limit 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 Technology Course Material (1984), p.67, p90). According to calculation by the above, it is about 0.7 m / second.

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

The molten steel ring flow rate is proportional to the molten steel flow rate, and the molten steel circulating gas flow rate is proportional to the cube of the molten steel ring flow rate. For example, to make the molten steel flow rate 0.8 m / second less than the critical molten steel flow velocity 0.7 m / second, It is necessary to increase the molten steel ring flow rate by (0.7 / 0.8) times and the molten steel reflux 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 ].

Normally, the flow rate of the molten steel differs depending on the flow rate of the molten steel gas, but the molten steel does not involve the slag at the slag / molten steel interface. If the molten steel reflux gas flow rate V ₀ [Nm ³ / t / min], which can be adjusted as appropriate, is specified, vacuum degassing treatment that does not involve slag at the slag / molten steel interface should be performed within a wide range of molten steel reflux gas flow rates. Can do.

  And when the molten steel reflux gas flow rate is not less than the value (upper limit) of the right side of the above formula (1), the entrainment of slag into the molten steel in the vacuum tank and ladle does not decrease, and the steel It is confirmed that coarse CaO-containing inclusions remain, and when the reflux 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 dip tube. It was confirmed that it was difficult to penetrate the pipe and continue proper operation.

In addition, the inventors of the present invention treated the molten steel containing C, Si, Mn, P, and S with RH, and at the time of return pressure, before starting the return pressure, the molten steel reflux gas flow rate V [Nm ³ / t / min. ] Is controlled so as to satisfy the above formula (1), it was confirmed that the particle size of CaO-containing inclusions in the steel decreased and the extreme statistical maximum particle size dmax also decreased.

  Fig. 1 shows the maximum particle size and average particle size of a CaO-containing inclusion in the case of a conventional flow rate that does not reduce the molten steel reflux gas flow rate, and the maximum particle size of the CaO-containing inclusion in the case of reducing the molten steel reflux gas flow rate An example of each ratio of a diameter and an average particle diameter (invention example) is shown.

  Fig. 2 shows the extreme statistical maximum predicted particle size (comparative example) of inclusions in CaO-containing inclusions in the case of a conventional flow rate that does not reduce the molten steel reflux gas flow rate at the time of return pressure, and CaO-containing inclusions in the case of reducing the molten steel reflux gas flow rate An example of the comparison of the extreme value statistical maximum prediction particle diameter (invention example) of a thing is shown.

  As shown in FIG. 1, when the molten steel reflux gas flow rate is reduced according to the above formula (1) at the time of return pressure, 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 extreme value statistical maximum predicted particle size is also greatly reduced.

  That is, when the vacuum degassing pressure is restored, the slag entrainment in the molten steel can be greatly suppressed by reducing the molten steel circulating gas flow rate (ie, molten steel flow rate) and reducing the molten steel surface flow velocity. It is possible to reduce the amount and size of coarse CaO-containing inclusions generated due to the above. This is the knowledge found by the present inventors and is the knowledge that forms the basis of the refining method of the present invention.

  The molten steel subjected to the vacuum degassing treatment at the reflux gas flow rate satisfying the above formula (1) is a molten steel having a normal component composition, that is, a molten steel containing C, Si, Mn, P, and S as basic elements of the steel. If present, the inclusion reduction effect by controlling the reflux gas flow rate according to the above formula (1) is manifested, and therefore, the present invention is not limited to molten steel having a specific component composition. The composition will be described. Hereinafter,% means mass%.

C: 1.20% or less C is an element effective for securing the strength and hardness of steel after quenching. If it exceeds 1.20%, cracking occurs at the time of quenching, and it becomes too hard and the life of the cutting tool is reduced, so C is preferably 1.20% or less. More preferably, it is 1.00% or less.

  In steel types that do not require much strength or hardness, C is not necessarily required, so the lower limit is not particularly limited, but C is a basic element of steel and is difficult to reduce to 0%. Does not include 0%. C is preferably 0.001% or more from the viewpoint of securing required strength and hardness.

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

  In steel types that do not require much strength or hardness, Si is not required, so the lower limit is not particularly defined. However, since Si is a basic element of steel and it is difficult to make it 0%, the lower limit is 0. % Not included. In terms of securing required strength and hardness, Si is preferably 0.001% or more.

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

  A steel type that does not require so much strength or hardness does not require Mn, so the lower limit is not particularly defined. However, since Mn is a basic element of steel, the lower limit does not include 0%. Mn is preferably 0.01% or more from the viewpoint of securing required strength and hardness.

P: 0.05% or less P is an impurity element and an element that inhibits toughness. When P exceeds 0.05%, the toughness is remarkably lowered. Therefore, 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 on practical steel.

S: 0.05% or less S, like P, is an impurity element and an element that inhibits toughness. When S exceeds 0.05%, the toughness is remarkably lowered. Therefore, 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 on practical steel.

  In addition to the basic elements, Al: 0.20% or less, Cr: 3.50% or less, Mo: 0.85%, within a range that does not impair the mechanical properties and / or chemical properties of steel. 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: One or more of 0.010% or less, Ca: 0.010% or less, REM: 0.010% or less, and O: 0.003% or less may be contained.

Al: 0.20% or less Al is a deoxidizing element and an element that refines crystal grains. If it exceeds 0.20%, coarse oxide inclusions are generated, and the toughness and ductility are lowered. Therefore, Al is preferably 0.20% or less. In terms of ensuring the effect of crystal grain refinement, 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 securing strength and hardness. When it exceeds 3.50%, toughness and ductility are lowered, so Cr is preferably 3.50% or less. More preferably, it is 2.50% or less. In terms of securing 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 securing 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 generated, and the toughness and ductility are lowered. Therefore, Mo is preferably 0.85% or less. More preferably, it is 0.65% or less. In terms of securing 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 securing strength and hardness. If it exceeds 4.50%, the toughness and ductility deteriorate, so the Ni content is preferably 4.50% or less. More preferably, it is 3.50% or less. In terms 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 carbide, nitride, and / or carbonitride, and contributes to suppression of coarsening of crystal grains and improvement of temper softening resistance. If it exceeds 0.20%, toughness and ductility are lowered, so Nb is preferably 0.20% or less. More preferably, it is 0.10% or less. In terms of securing 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 resistance to temper softening. If it exceeds 0.45%, the toughness and ductility deteriorate, so V is preferably 0.45% or less. More preferably, it is 0.35% or less. In terms of securing 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 securing strength and hardness. W is an element that forms carbides and contributes to improvement in 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. In terms of securing 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 improves hardenability and contributes to improvement in strength. B is an element that segregates at the austenite grain boundaries, suppresses P grain boundary segregation, 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. In order to ensure the effect of addition of 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 contributes to improvement of strength and toughness by forming fine nitrides to refine crystal grains. If it exceeds 0.060%, nitrides are excessively generated and the toughness deteriorates, so N is preferably 0.060% or less. More preferably, it is 0.040% or less. In terms of securing 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 contributes to the improvement of strength and toughness by forming fine Ti nitride to refine crystal grains. If it exceeds 0.25%, Ti nitride is excessively generated and the toughness decreases, so Ti is preferably 0.25% or less. More preferably, it is 0.15% or less. In terms of securing 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 contributing to improvement of corrosion resistance. If it exceeds 0.50%, the hot ductility is reduced and cracks and flaws are generated, so Cu is preferably 0.50% or less. More preferably, it is 0.30% or less. In terms 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 improvement of free-cutting properties. If it exceeds 0.45%, the toughness decreases, so Pb is preferably 0.45% or less. More preferably, it is 0.30% or less. In terms of securing 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 improvement of free-cutting properties. If it exceeds 0.20%, the toughness decreases, so Bi is preferably 0.20% or less. More preferably, it is 0.16% or less. In order to ensure 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 contributing to the improvement of free-cutting properties. If it exceeds 0.010%, the toughness decreases, so Te is preferably 0.010% or less. More preferably, it is 0.006% or less. In order to ensure 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 improvement of corrosion resistance, mainly sulfuric acid resistance and hydrochloric acid resistance, and improvement of free-cutting properties. If it exceeds 0.20%, the toughness decreases, so Sb is preferably 0.20% or less. More preferably, it is 0.15% or less. In terms of securing 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 contributing to the improvement of free-cutting properties. If it exceeds 0.010%, the toughness decreases, so Mg is preferably 0.010% or less. More preferably, it is 0.006% or less. In terms of securing 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 inclusions having a low melting point and easily aggregated by a deoxidation reaction. If it exceeds 0.010%, the Al ₂ O ₃ inclusions are complexed and coarsened with low melting point CaO—Al ₂ O ₃ inclusions, and the coarsened CaO—Al ₂ O ₃ inclusions are: Since it does not become liquid phase at the rolling temperature and remains in the steel as it is coarse, Ca is preferably 0.010% or less. More preferably, it is 0.006% or less.

  The lower the Ca, the better, so the lower limit is not limited, but unavoidably about 0.0001% remains, so 0.0001% is the practical lower limit on practical steel.

REM: 0.010% or less REM (rare earth elements, La, Ce, Pr, and Nd) is a molten steel that is sufficiently deoxidized with Al or Al-Si. An element that acts to reduce CaO in inclusions and to modify CaO-Al ₂ O ₃ inclusions. If it exceeds 0.010%, a low melting point compound phase with a high REM concentration appears in the inclusions, which promotes the aggregation of inclusions and produces coarse inclusions. % 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 in terms of securing the effect of adding REM.

O: 0.003% or less O is an element that forms an oxide. If it exceeds 0.003%, a coarse oxide is formed and the rolling fatigue life is lowered. 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 on practical steel.

  In the component composition of the molten steel, the balance is Fe and inevitable impurities. Inevitable impurities are elements that are inevitably mixed in from the steel raw material and / or in the steel making process, and are elements that are allowed in a range that does not impair the characteristics of the molten steel and further the characteristics of the steel from which the molten steel is cast.

  Next, examples of the present invention will be described. However, the conditions in this example are one example of conditions used to confirm the feasibility and effects of the present invention. Therefore, the present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(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 continuous casting to produce steel.

  Specifically, when the molten steel subjected to primary refining in a 270 t converter is discharged, the molten steel is deoxidized with one or more of Si, Mn, and Al, and the deoxidized molten steel is subjected to a predetermined process. Secondary refining was performed by LF treatment using a slag composition, then component composition was adjusted by RH treatment, and after cleaning treatment, continuous casting was performed to obtain a slab. The slab was heated and held in a heating furnace, and then subjected to ingot rolling to obtain a steel slab.

In the steel slab, the extreme statistical maximum predicted diameter [μm] of the nonmetallic inclusions in the predicted area of 30000 mm ^{2 was} estimated by the extreme statistical method. The estimation of the maximum predicted diameter (√area (max) of inclusions by extreme value statistics can be found in, for example, “Effects of Metal Fatigue Micro Defects and Inclusions” (Murakami Takayoshi, Yokendo, 1993, p. 223). -239) The method used is a two-dimensional method of estimating the maximum inclusion diameter observed within a certain area by a two-dimensional inspection.

Using the above-mentioned extreme value statistical method, the position of ¼ on the loose surface side of the L cross section of the steel slab (the cross section including the center line of the loose surface, the center line of this opposing surface, and the center line of the slab) From the image of the non-metallic inclusions taken by the optical microscope, the inspection reference plane: 100 mm ² (10 × 10 mm), the inspection visual field: 16, and the maximum predicted diameter of the inclusion with an area of 30000 mm ² to be predicted √area (max) was calculated.

Specifically, 16 data of the maximum diameter of the inclusions obtained by observation (data of 16 fields of view) are plotted on an extreme value probability sheet according to the method described in the above document, and the maximum inclusion distribution straight line ( The maximum inclusion and the maximum predicted diameter √area (max) of the inclusion in the area of 30000 mm ² were estimated by extrapolating the maximum inclusion distribution straight line.

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

  Invention Example No. Since the molten steel reflux gas flow rate at the time of return pressure is within an appropriate range, 1 to 20 show a good value with a maximum predicted particle size of 16 to 29 μm according to extreme value statistics. Comparative Example No. In Nos. 23, 24, 27, 30, and 33, since the molten steel reflux gas flow rate at the time of return pressure deviates from the upper limit value, no improvement is observed in the extreme value statistical maximum predicted particle size.

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

  As described above, in the invention examples, it is clear that the steel having excellent mechanical properties can be obtained because the inclusions are prevented from becoming coarser than the comparative example of the conventional operation.

  As described above, according to the present invention, it is possible to optimize the flow velocity of the molten steel recirculation during vacuum degassing and to suppress the inclusion of CaO-containing inclusions in the molten steel, so the amount of inclusions in the steel Further, it is possible to reduce the size and provide a steel having excellent mechanical properties. Therefore, the present invention has high applicability 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 apparatus including a vacuum tank and a dip tube,
After the vacuum degassing process is completed, when the decompressed state in the vacuum chamber is restored to atmospheric pressure, the circulating gas flow rate V [Nm ³ / t / t of molten steel supplied into the molten steel from the dip tube before the decompression starts. The molten steel refining method is characterized in that the pressure is controlled so as to satisfy the following formula (1).

A [m ² ]: Cross-sectional area of the pipe supplying gas into the dip 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]: Vacuum tank pressure N [lines]: Number of gas tubes supplying gas into the dip tube M [t]: Molten steel weight in the ladle V ₀ [Nm ³ / t / min]: Molten steel reflux gas flow rate during degassing   The method for refining molten steel according to claim 1, wherein the vacuum degassing treatment is performed with an RH refining apparatus.   The return pressure gas is supplied from one or both of a part for blowing molten steel recirculation gas and a part directly supplied into the vacuum chamber when the return pressure is performed by the RH refining apparatus. The molten steel refining method as described.   The molten steel refining method according to claim 3, wherein the return pressure gas is an inert gas.   The molten steel contains, in mass%, 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. 5. The method for refining molten steel according to any one of claims 1 to 4, wherein the balance is made of Fe and inevitable impurities.   The molten steel is further, 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: 0.00. The method for refining molten steel according to claim 5, comprising one or more of 010% or less and O: 0.003% or less.

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