JP5919000B2 - Manufacturing method of steel for forging - Google Patents

Description

  The present invention relates to a method for producing forging steel used for producing forgings such as marine parts and power generation parts, and particularly forging steel useful for producing forgings requiring high fatigue characteristics. It relates to a method for manufacturing.

  In order to obtain forging steel that exhibits excellent fatigue properties that are less susceptible to fatigue failure even under harsh usage environments, inclusions in steel should be used to suppress the decrease in fatigue strength due to inclusion defects, which has become a problem in recent years. The formation and formation of coarse inclusions are prevented by controlling the composition and form.

  As a method for controlling the composition and form of inclusions in the molten steel treatment process, the present applicant also adjusts the gas stirring time during ladle refining and the slag thickness in a stationary state, for example, in Patent Document 1, and slag Proposing that highly clean aluminum killed steel can be obtained by adjusting the amount of MgO in the gas and the time of gas stirring to satisfy the prescribed formula, and controlling the reflux time of the molten steel and the amount of molten steel reflux in vacuum degassing refining. Yes.

  Furthermore, in the case of Patent Document 2 and Patent Document 3, the present applicant optimizes the flow rate of each bottom blowing plug and the total flow rate when performing refining by blowing inert gas from the two bottom blowing plugs. Suggests that inclusions can be reduced and high cleanliness steel can be obtained.

JP 2010-189691 A JP 2011-214084 A JP 2011-214083 A

  Patent Document 1 relates to a steel sheet having excellent bendability, and it is considered necessary to control inclusions that particularly affect fatigue characteristics in order to ensure excellent fatigue characteristics.

  In addition, Patent Documents 2 and 3 propose to stably obtain molten steel with reduced inclusions, but machines that require more excellent fatigue properties that are less susceptible to fatigue failure even under severe use environments. When it is intended for use in parts, the forging steel used in the machine part has a suppressed variation in fatigue characteristics depending on the part, specifically, not only in the radial direction but also in the height direction. Is required to be suppressed.

  The present invention has been made paying attention to such circumstances, and the purpose thereof is to suppress variations in fatigue characteristics (specifically, endurance limit ratio) not only in the radial direction but also in the height direction, An object of the present invention is to provide a useful method for producing forging steel that exhibits fatigue characteristics superior to those of conventional forging steel.

The method for producing a forging steel according to the present invention that has solved the above-described problems includes a step of performing ladle refining by gas stirring on a molten steel produced from a converter or an electric furnace, and then performing a degassing process. In the method for producing forging steel, the ladle refining by gas stirring is performed while blowing an inert gas from two bottom blowing plugs provided at the bottom of the ladle, and one bottom blowing plug And the gas flow rate of the other bottom blowing plug so that each gas flow rate of the two bottom blowing plugs satisfies the following formula (1). Of the slag so that the total slag is 0.8 to 1.8 NL / min per ton of molten steel, and the slag composition after the degassing treatment satisfies 3.5 ≦ CaO / SiO ₂ (mass ratio) ≦ 20. Characterize where to adjust ingredients To.
0.1 ≦ R ≦ 0.25 (1)
[However, in the formula (1),
R = Q _small / (Q _large + Q _small )
R: Gas flow ratio Q _Small : Gas flow rate of bottom blowing plug with small gas flow rate (NL / min)
Q _large : Indicates the gas flow rate (NL / min) of the bottom blowing plug with a large gas flow rate. ]

  According to the present invention, by controlling the conditions in the molten steel treatment, it is possible to obtain a forging steel in which the inclusions are prevented from agglomerating and coarsening and the variation in fatigue characteristics is small. In particular, in the present invention, a forging steel in which variation in the radial direction of the forging steel (steel ingot) is suppressed and variation in the height direction can also be reliably obtained.

FIG. 1 is a plan view of a ladle used in the examples.

  The inventors of the present invention have made extensive studies on a method for manufacturing a forging steel in which variation in fatigue characteristics (specifically, a limit of durability) is suppressed not only in the radial direction but also in the height direction. As a result, when manufacturing steel for forging by (melting / primary refining)-> (molten steel treatment)-> (ingot forming), in particular, the bottom stirring method is adopted during the molten steel treatment to control the gas stirring conditions. The inventors have found that if the slag composition after the degassing process is controlled, the inclusion agglomeration and coarsening are suppressed, and as a result, the forging steel with reduced variation is obtained.

  Hereinafter, the conditions at the time of a molten steel process are explained in full detail. In addition, the said melting and primary refining performed before a molten steel process can be performed using a high frequency melting furnace, an electric furnace, a converter, etc. according to a conventional method.

  First, in the present invention, a ladle having two bottom blowing plugs (porous plugs) provided at the bottom is used, and ladle refining is performed while blowing an inert gas (for example, Ar gas) from the two bottom blowing plugs. Therefore, the molten steel is efficiently stirred.

  Further, in the present invention, a difference is provided between the gas flow rate of one bottom blowing plug and the gas flow rate of the other bottom blowing plug so that each gas flow rate of the two bottom blowing plugs satisfies the following formula (1). adjust.

Specifically, of the inert gas flow rates from the two bottom blowing plugs, the larger gas flow rate is Q _large (N (meaning Normal, hereinafter, the same applies to the gas flow rate) L / min), and the smaller gas flow rate. When the flow rate is _small Q (NL / min) and “Q _small / (Q _large + Q _small )” is defined as the “gas flow rate ratio (R)” to be blown, this gas flow rate ratio is 0.1 to 0.3. Be within the range of 25.
0.1 ≦ R ≦ 0.25 (1)
[However, in the formula (1),
R = Q _small / (Q _large + Q _small )
R: Gas flow ratio Q _Small : Gas flow rate of bottom blowing plug with small gas flow rate (NL / min)
Q _large : Indicates the gas flow rate (NL / min) of the bottom blowing plug with a large gas flow rate. ]

When the gas flow rate ratio is smaller than 0.1 (for example, Q _large = 100 NL / min, Q _small = 10 NL / min), for example, when the smaller gas flow rate becomes 10 NL / min or less, the inert gas from the nozzle However, it is difficult to operate because the nozzle cannot be discharged and the nozzle is clogged. Even if the inert gas can be made to flow, the effect is not changed when the gas flow rate is small as compared with the case where the flow rate ratio is 0 (that is, when there is only one bottom blowing plug). Therefore, the gas flow ratio is set to 0.1 or more. Preferably it is 0.15 or more. On the other hand, when the gas flow rate ratio exceeds 0.25, the cleanliness of the molten steel deteriorates, which is not preferable. The gas flow ratio is preferably 0.2 or less.

  In addition, since the total gas flow rate affects the stirring state of the molten steel, in the present invention, the total gas flow rate is defined as 0.8 to 1.8 NL / min per ton of molten steel. When the total gas flow rate is less than 0.8 NL / min per ton of molten steel, the stirring power of the molten steel is too small, so that the aggregation and floating separation of inclusions in the molten steel cannot be promoted by stirring. The degree of cleanliness cannot be improved. The total gas flow rate is preferably 1.0 NL / min or more per 1 ton of molten steel. On the other hand, if the total gas flow rate is larger than 1.8 NL / min per ton of molten steel, the stirring force of the molten steel becomes too large, and a large amount of slag is caught in the molten steel, or the bath surface of the molten steel touches the atmosphere. Since it will be in a state and the cleanliness of molten steel will fall, it is not preferable. The total gas flow rate is preferably 1.6 NL / min or less.

  In the present invention, the ladle having the two bottom blowing plugs provided at the bottom may be used for ladle refining, and the installation position of the plug is not particularly limited. For example, a ladle provided with two bottom-blowing plugs at the bottom as shown in Patent Document 2 and the like described above can be used.

  For other conditions of ladle refining, conventional methods performed by those skilled in the art can be adopted. While stirring the molten steel in the ladle, the temperature and main components are adjusted, and a deoxidizer is added to the molten steel. Then, deoxidation (in order to obtain a desired inclusion composition, it is recommended to deoxidize by adding Al to make Al killed steel), desulfurization, and the like.

Moreover, the degassing process performed after the ladle refining can be performed by, for example, a lid degassing apparatus (VD) by a person skilled in the art. In the present invention, it is necessary to adjust the slag components so that the CaO / SiO ₂ (mass ratio) (ie, slag basicity) satisfies 3.5 or more and 20 or less in the slag composition after the degassing treatment.

When the CaO / SiO ₂ is smaller than 3.5, re-oxidation is likely to occur due to high SiO ₂ and slag is likely to be caught. Therefore, the CaO / SiO ₂ is 3.5 or more. Preferably it is 5.0 or more. On the other hand, when the CaO / SiO ₂ is larger than 20, the slag is not easily caught, and it becomes difficult to control the oxide composition in the molten steel. Therefore, the CaO / SiO ₂ is 20 or less. Preferably it is 10 or less.

  The slag composition can be adjusted by any method as long as it is within the range of the slag basicity, for example, by adjusting the input amount of auxiliary materials (flux, limestone, etc.).

  As described above, the present invention may include a step of performing ladle refining by gas stirring so as to satisfy the above-described conditions for the molten steel produced from the converter or the electric furnace, and then performing a degassing treatment. For example, after the degassing process, a second ladle refining (secondary refining) may be performed to further promote the floating separation of the entrained slag and the deoxidized product.

[Agglomeration]
For ingot making (casting), a generally performed method can be employed. For example, as shown in the examples described later, the ingot is formed by inserting molten steel into a mold from below through an injection tube. It is possible to adopt a bottom pouring method or the like to manufacture.

  By adopting the method described above, inclusions are controlled as follows, and a steel ingot (forging steel) having a small variation in fatigue characteristics regardless of the part can be obtained.

That is, among the oxides contained in the steel, the proportion of coarse CaO and CaS inclusions is small, and the presence ratio of relatively small CaO—Al ₂ O ₃ inclusions is high. The ratio of the number of oxide inclusions (inclusions of 5 μm or more detected by EPMA) whose content is 5% by mass or more and 45% by mass or less is the total inclusions (inclusions of 5 μm or more detected by EPMA) It is 5% or more (preferably 7.5% or more) with respect to the number, and a material exhibiting excellent fatigue characteristics can be obtained. If the CaO-containing inclusions are present in an amount of 5% or more, the effect is exhibited, and a higher abundance ratio is desirable, but in terms of production, it is substantially about 30% or less. The other inclusions present in the forging steel of the present invention are mainly MgO—Al ₂ O ₃ and MnS.

  The present invention is characterized in that, by producing by the above method, a forging steel is obtained in which coarse inclusions are suppressed and fatigue characteristics are high, in particular, variation in fatigue characteristics due to sites is suppressed. The component composition of the steel material used is not particularly limited, but for securing the strength and toughness required for the final forged product, forging steel (forged product) obtained in the melting stage It is preferable to satisfy the following chemical component composition.

[C: 0.2-0.6%]
C is an element that contributes to improving the strength of the forged product. In order to ensure sufficient strength, C is preferably contained in an amount of 0.2% (meaning “mass%”; the same applies to chemical components). The amount of C is more preferably 0.25% or more, and still more preferably 0.30% or more. However, if the amount of C is too large, the toughness of the forged product is deteriorated. The amount of C is more preferably 0.55% or less, still more preferably 0.50% or less.

[Si: 0.05 to 0.50%]
Si acts as an element for improving the strength of the forged product and is preferably contained in an amount of 0.05% or more in order to ensure sufficient strength. The amount of Si is more preferably 0.1% or more, and still more preferably 0.15% or more. However, if the amount of Si is too large, the reverse V segregation becomes remarkable and it becomes difficult to obtain a clean steel ingot. Therefore, the content is preferably 0.50% or less. The amount of Si is more preferably 0.45% or less, still more preferably 0.40% or less.

[Mn: 0.20 to 1.5%]
Mn is an element that enhances hardenability and contributes to improvement in strength. In order to ensure sufficient hardenability and strength, the amount of Mn is preferably 0.20% or more. The amount of Mn is more preferably 0.5% or more, still more preferably 0.8% or more. However, if the amount of Mn is too large, reverse V segregation is promoted. The amount of Mn is more preferably 1.2% or less, still more preferably 1.1% or less.

[Ni: 0.10 to 3.50%]
Ni is an element useful as a toughness improving element, and the amount of Ni is preferably 0.10% or more. More preferably, it is 0.2% or more. However, if the amount of Ni becomes excessive, the cost will increase, so 3.50% or less is preferable. More preferably, it is 3.0% or less.

[Cr: 0.9 to 4%]
Cr is an element effective for improving the hardenability and improving the toughness, and their action is preferably exhibited by containing 0.9% or more. The amount of Cr is more preferably 1.1% or more, still more preferably 1.3% or more. However, if the amount is too large, reverse V segregation is promoted and it becomes difficult to produce highly clean steel. Therefore, the Cr content is preferably 4% or less. More preferably, it is 3.0% or less.

[Mo: 0.10 to 0.70%]
Mo is an element that effectively acts to improve all of hardenability, strength, and toughness, and it is preferable to contain Mo in an amount of 0.10% or more in order to exert these effects. The amount of Mo is more preferably 0.20% or more, and further preferably 0.25% or more. However, since Mo has a small equilibrium partition coefficient and easily causes microsegregation (normal segregation), the amount of Mn is preferably 0.70% or less. More preferably, it is 0.60% or less.

[V: 0.01 to 0.20%]
V has an effect of precipitation strengthening and refinement of structure, and is an element useful for increasing the strength. In order to effectively exhibit such an action, it is recommended to contain V by 0.01% or more. However, the V content is preferably 0.20% or less because the above effect is saturated and is economically wasteful even if contained excessively. More preferably, it is 0.15% or less.

[Al: 0.005 to 0.10%]
Al effectively acts as a deoxidizing element in the steel making process, and also effectively acts on the crack resistance of the steel. Accordingly, Al is preferably contained in an amount of 0.005% or more. More preferably, it is 0.010% or more. However, when the amount of Al increases, Al ₂ O ₃ is produced as inclusions, and these inclusions segregate and aggregate during solidification to produce coarse inclusions, which deteriorates the fatigue characteristics of the forged product. Therefore, the upper limit of the Al content is preferably 0.10%. More preferably, it is 0.08% or less.

[S: 0.008% or less (excluding 0%)]
S is an element that is inevitably included, and is an element that forms coarse sulfides as inclusions due to segregation during solidification and lowers the fatigue strength of the forged product. Accordingly, the S content is preferably 0.008% or less. The amount of S is more preferably 0.006% or less, still more preferably 0.004% or less.

[Ti: 0.005% or less (excluding 0%)]
Ti forms fine inclusions such as TiN, TiC, and Ti ₄ C ₂ S ₂ and is dispersed in the steel, and occludes and captures excess hydrogen in the steel that exceeds the solid solubility limit. It is an element that improves hydrogen cracking. From such a viewpoint, 0.0002% or more of Ti may be contained. However, when the amount of Ti becomes excessive, coarse nitrides are formed as inclusions, and the fatigue strength of the forged product is reduced. Therefore, the Ti content is preferably 0.005% or less. More preferably, it is 0.004% or less, More preferably, it is 0.003% or less.

[Total O: 0.0025% or less (excluding 0%)]
O (oxygen) is an element that forms oxide inclusions such as SiO ₂ , Al ₂ O ₃ , MgO, and CaO and reduces the fatigue strength of the forged product. Therefore, the total O (total oxygen) amount is preferably reduced as much as possible, and is preferably 0.0025% or less. More preferably, it is 0.0015% or less, More preferably, it is 0.0010% or less.

  The recommended composition of steel is as described above, with the balance consisting of iron and inevitable impurities. Examples of the inevitable impurities include Mg. In the case of Mg, it is allowed to be contained in the range of 15 ppm or less.

  Moreover, it is possible to further contain other elements as long as the effects of the present invention are not adversely affected. Examples of other elements that allow positive addition include B (boron), which has an effect of improving hardenability, and W, Nb, Ta, Cu, Ce, Zr, Te, which are solid solution strengthening elements or precipitation strengthening elements. These may be added alone or in combination of two or more. For example, the total amount of these additive elements is preferably about 0.1% or less.

  Using the forging steel obtained by the above method, for example, it is heated and subjected to hot forging to finish a forged product having a cross-sectional diameter of 150 to 700 mm. The forged product thus obtained is widely used effectively in industrial fields such as machinery, ships, and generators, and is suitable for parts that require high fatigue strength.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention. That is, in the following embodiment, a ladle having two bottom blowing plugs as shown in FIG. 1 is used, but the ladle having two bottom blowing plugs is not limited to this.

  In this example, ladle refining was performed on the steel types shown in Table 1. The scrap was melted using an electric furnace and primary refining was performed by a person skilled in the art. Then, 20 to 100 tons of molten steel was transferred to a ladle (capacity: 100 ton class) provided with two bottom blowing plugs at the bottom as shown in the following details and FIG. Then, Ar gas (inert gas) was blown from the two bottom blowing plugs, and the ladle refining was performed by changing the gas flow rate of each plug as shown in Table 2.

The stirring power in this molten steel treatment was controlled by a person skilled in the art using a bottom blowing gas flow rate Qg (ie, equivalent to the sum of the gas flow rates of the two bottom blowing plugs, the same applies hereinafter, unit: NL / min · t). .
The gas stirring time was 200 to 600 minutes in total for the secondary refining.

(Details of ladle)
New pan (after work brick construction)
Ladle inner diameter = 2858mm (If all the permanent bricks are exposed, the ladle inner diameter = 3318mm)
No. 1 porous position is 900mm from the center
d = 900 × 2 = 1800 mm
d / D = 0.54-0.63
No. 2 porous position is 680mm from the center
d = 680 × 2 = 1360 mm
d / D = 0.40-0.48
Electrode PCD; A = 350mm
1.5A = 525 <d
Ladle refractory slag line: MgO-C type wall: MgO-C-based bottom (bottom part): Al ₂ O ₃ -MgO based

In this example, No. Set the gas flow rate of 1 porous to large (Q _large ). The gas flow rate of 2 porous was made small (Q _small ) (in the case where the flow rates of No. 1 porous and No. 2 porous were reversed, that is, the gas flow rate of No. 2 porous was made large (Q _large ), small a No.1 porous gas flow rate even when the (Q _small), it is confirmed that the same effect can be obtained). Moreover, when the number of nozzles of bottom blowing is single (experiment No. 13-16), it is No. Only 1 porous was used, and in Table 2, “Q _small = 0” and “gas flow ratio R = 0” were set.

(Degassing treatment)
Degassing treatment was performed using a lid degasser (VD). The stirring power was controlled by the bottom blowing gas flow rate Qg (NL / min · t) by a person skilled in the art. In addition, as in Table 2, the basicity (slag basicity, C / S) in the slag composition after degassing treatment is as shown in Table 2 (for example, disclosed in JP 2007-231410 A). The component composition of the slag was adjusted by adding flux.

(Agglomeration)
After the above treatment, an ingot was produced by performing a bottom pouring and ingot treatment. Then, it heated to about 1300 degreeC and gave hot forging, and finished it into the forging goods with a cross-sectional diameter of 150-700 mm.

  Using the forged product thus obtained, the following inclusions and fatigue characteristics were evaluated.

(Evaluation of composition and number of inclusions)
The upper equivalent position of the obtained forged product (position within the range of 70 to 100% of the distance from the bottom when the total length is 100%, the steel upper part) and the bottom equivalent position (when the total length is 100%) In the cross section perpendicular to the axis center (cut surface of the end material discarded at each position) of the distance within the range of 0 to 20% from the bottom surface (steel bottom), in the radius (R) direction from the axis center Small pieces were cut out from the R / 3 position toward each other, and after polishing, composition analysis of inclusions was performed by EPMA (observation field size is 100 mm ² , number of fields is 1 field / site) (EPMA apparatus and measurement conditions are as follows) Street).

  Then, in all fields of view, the total number of inclusions having an inclusion major axis of 5 μm or more and the number of oxide inclusions having a CaO content of 5 mass% or more and 45 mass% or less are obtained. The ratio (%) of the number of oxide inclusions in which the CaO content is 5% by mass or more and 45% by mass or less in the number of inclusions (however, the major axis of inclusions is 5 μm or more) was determined. The results are shown in Table 2. In Table 2, the case where the ratio was 5% or more was evaluated as “◯”. Moreover, the case where the said ratio was less than 5% was evaluated as "x".

(EPMA device and measurement conditions)
Manufacturer: JEOL Ltd. Model: JXA-8900RL
Acceleration voltage: 15 kV
Beam current: 1.7 × 10 ^-9 A
Beam diameter: 1μm

  For composite inclusions containing oxides and non-metallic elements other than oxygen (for example, N, S, etc.), those containing 20 mass% or more of oxide inclusions in the above EPMA are used as oxide inclusions. Certified as a thing.

(Measurement of tensile strength)
From the R / 3 part at the top of the steel material of the forged product (round bar) and the R / 3 part at the bottom of the steel material, a tensile test piece (φ6 mm × gauge length 30 mm) One portion of each part was collected at 45 ° from the direction, and a tensile test (JIS Z 2204, 2241) was performed at room temperature.

(Evaluation of fatigue characteristics)
From the R / 3 part position at the top of the steel material and the R / 3 part position at the bottom of the steel material of the obtained forged product (round bar), the following test pieces were sampled and subjected to fatigue tests under the following conditions.

[Fatigue test conditions]
Test piece: 10 mm diameter smooth test piece
(The longitudinal direction of the specimen is inclined 45 degrees from the axial direction of the forged product)
Test method: Rotating bending fatigue test (stress ratio = -1, rotation speed: 3600 rpm)
Fatigue strength evaluation method: Difference method Difference stress: 20 MPa
Number of specimens: 5 each Fatigue strength of each specimen = (breaking stress)-(step stress)
Then, a durability limit ratio (fatigue strength σ _w / tensile strength σ _B ) was determined as an index of fatigue limit.

  This fatigue test is performed on five test pieces, and first, the average value of the endurance limit ratio at the R / 3 part position of the upper part of the steel material is obtained, and further, the average value and the minimum of the endurance limit ratio in the five test pieces are obtained. The difference from the value was obtained, and the variation in the fatigue characteristics in the radial direction was evaluated. The results are shown in Table 2. In Table 2, the case where the difference between the average value and the minimum value is 0.025 or less is evaluated as “◯” (the variation is small. The variation in the radial direction is particularly small), and the case where it exceeds 0.025 is evaluated as “× ".

In addition, the average value “E _TOP ” of the endurance limit ratio at the R / 3 part position of the upper part of the steel obtained using the five test pieces and the R / 3 part of the lower part of the steel obtained using the five test pieces The ratio (E _TOP / E _BOT ) of the average value “E _BOT ” of the endurance limit ratio at the position was determined. The results are shown in Table 2. In Table 2, the case where E _TOP / E _BOT is 0.90 or more and 1.07 or less is evaluated as “◯” (small variation, particularly small variation in the height direction), and less than 0.90 or 1.07 The super case was evaluated as “×”.

  From Table 2, it can be considered as follows. That is, Experiment No. 1-8, as the gas stirring conditions in ladle refining are as specified, and the slag composition after degassing is controlled within the specified range, the desired inclusion composition is obtained, fatigue It can be seen that the variation in characteristics is suppressed regardless of the part. On the other hand, Experiment No. 9-20, because at least one of the gas stirring conditions in ladle refining and the slag composition after degassing treatment is outside the specified range, the desired inclusion composition cannot be obtained, and as a result, the variation in fatigue characteristics is It occurred in at least one of the radial direction and the height direction.

  Specifically, Experiment No. In Nos. 9 and 11, the slag basicity is below the specified range. In Nos. 10 and 12, since the slag basicity exceeded the specified range, the desired inclusions could not be ensured, resulting in variations in fatigue characteristics.

  Experiment No. In Nos. 13 to 16, the inert gas was blown from only one place (the gas flow rate ratio R in ladle refining was zero). In No. 13, since the slag basicity was below the specified range, the desired inclusions could not be secured, resulting in variations in fatigue characteristics.

  Experiment No. No. 17 has a high gas flow rate ratio R and low slag basicity. Nos. 18 and 19 had a high gas flow rate ratio R in ladle refining, and the slag basicity exceeded the specified range, so that none of the desired inclusions could be secured, resulting in variations in fatigue characteristics.

  Experiment No. No. 20 had a high gas flow rate ratio R, so that desired inclusions could not be secured, resulting in variations in fatigue characteristics.

Claims (1)

In the method for producing forging steel, including a step of performing ladle refining by gas stirring, and then degassing the molten steel produced from the converter or electric furnace,
The ladle refining by gas stirring is
It is performed while blowing inert gas from two bottom blowing plugs provided at the bottom of the ladle,
Provide a difference between the gas flow rate of one bottom blowing plug and the gas flow rate of the other bottom blowing plug,
Each gas flow rate of the two bottom blowing plugs satisfies the following formula (1), and the total gas flow rate of the two bottom blowing plugs is 0.8 to 1.8 NL / min per 1 ton of molten steel, And,
The slag composition after the degassing treatment is adjusted so that the slag composition satisfies 3.5 ≦ CaO / SiO ₂ (mass ratio) ≦ 20 . A method for producing forging steel in which variation is suppressed and variation in fatigue characteristics in the height direction is suppressed .
0.15 ≦ R ≦ 0.25 (1)
[However, in the formula (1),
R = Q _small / (Q _large + Q _small )
R: Gas flow ratio Q _Small : Gas flow rate of bottom blowing plug with small gas flow rate (NL / min)
Q _large : Indicates the gas flow rate (NL / min) of the bottom blowing plug with a large gas flow rate. ]