JP5856485B2 - Forged product and manufacturing method thereof - Google Patents

Description

  The present invention relates to forged products such as marine parts and power generation components, and more particularly to forged products that require high fatigue characteristics.

  Controlling the composition and form of inclusions in steel has been conventionally carried out in obtaining forged products that exhibit excellent fatigue properties that are less likely to cause fatigue failure even under harsh usage environments.

For example, in Patent Document 1, the dissolved Mg concentration and dissolved Al concentration in steel are controlled so that the oxides generated during the molten steel processing and casting are easily dispersed finely, or the oxidation contained in the steel. It is shown that the fatigue characteristics are improved by controlling the content of MgO and Al ₂ O ₃ in the inclusions in the material to suppress the occurrence of inclusion aggregation and preventing the formation of coarse inclusions. ing.

  In Patent Document 2, the size of the maximum inclusion existing in steel is sufficiently controlled by controlling the amount of solid solution Ca, solid solution Mg, total Ca, total Mg, and S in steel. It is shown that the fatigue characteristics can be improved by reducing the size.

  Further, in Patent Document 3, the density of inclusions having a major axis of 5 to 10 μm observed at the steel cross section at the lower part of the steel ingot for forging, and the inclusions having a major axis of 5 to 10 μm observed at the steel cross section at the upper part of the steel ingot. It is shown that hydrogen cracking can be prevented without reducing the fatigue strength of a steel ingot by controlling the density of inclusions and the density of inclusions having a major axis of 40 μm or more observed in each steel cross section.

  However, there are cases where the fatigue characteristics vary and there is a problem that a homogeneous forged product cannot be obtained. In order to suppress such variations in fatigue characteristics, the following techniques have been proposed.

For example, in Patent Document 4, in view of the fact that the main inclusions observed on the fatigue fracture surface are coarse S-based inclusions and coarse Ti-based inclusions, inclusions with a major axis exceeding 5 μm observed in the steel cross section. Among them, the density of inclusions with a S content of 20% or more is 100 pieces / cm ² or less, and the density of inclusions with a Ti content of 50% or more is 30 pieces / cm ² or less, It is shown that the durability limit ratio of the steel material central part and the steel material peripheral part (1 / 3R position) can be increased together.

In Patent Document 5, the density (D ₀ ) of inclusions having a major axis exceeding 5 μm observed at the axial position of the cross section perpendicular to the axial direction of the forged product at both end faces of the forged product is 70 pieces / cm ² or less. And the ratio of the density (D _R ) of inclusions whose major axis is more than 5 μm observed in the radial direction from the axial position to the radial direction when the radius of the forged product is R and the density D ₀ Forgings in which variation in fatigue characteristics is reduced by setting (D _R / D ₀ ) within the range of 0.5 to 1.50 are shown.

JP 2010-082662 A JP 2009-173961 A JP 2009-007598 A JP 2009-091649 A JP 2010-082662 A

  However, in recent years, with higher performance of marine engines and generators, there has been a demand for better fatigue characteristics that are less likely to cause fatigue failure even under harsh usage environments. Further, it is required that the variation in fatigue characteristics (specifically, the durability limit ratio) is suppressed not only in the radial direction but also in the height direction (axial direction).

  The present invention has been made paying attention to such circumstances, and the purpose thereof is not only in the radial direction, but in the variation in fatigue characteristics (specifically, the durability limit ratio) as compared with conventional forged products. An object of the present invention is to provide a forged product that is suppressed even in the height direction and a useful method for producing the forged product.

The forged product of the present invention capable of solving the above problems is C: 0.2 to 0.6% (meaning “mass%”; the same applies to chemical components), Si: 0.05 to 0.50%, Mn: 0.20 to 1.5%, Ni: 0.10 to 3.50%, Cr: 0.9 to 4%, Mo: 0.10 to 0.70%, V: 0.01 to 0. 20%, Al: 0.005 to 0.10%, S: 0.008% or less (not including 0%), Ti: 0.005% or less (not including 0%), and Total O: 0.0. Satisfying 0025% or less (excluding 0%), the balance being iron and inevitable impurities,
The ratio of the number of oxide inclusions having a CaO content of 5% by mass or more and 45% by mass or less in the total number of inclusions observed in the steel cross section is 5% or more, and
It is characterized in that the major axis of the maximum inclusion observed in the steel cross section is 100 μm or less.

  The present invention also includes a method for producing the forged product.

In the above manufacturing method, the molten steel in the mold is covered with the bath surface of the molten steel when the ingot casting method is performed in which the molten steel is introduced into the mold from below through an injection tube to produce an ingot. After adding the covering material, the heat insulating material is added, and if necessary, the Ca additive is added before or simultaneously with the heat insulating material,
Ca additives to be added as required and the thermal insulation material, the total Ca in terms of amount to the surface area of the bath surface is added so as to satisfy the ^{0.35kg / m 2 ~10kg / m 2} ; with the following formula ( It is added so that the X value shown in 1) satisfies 0.08 to 0.25;

X value = [% Ca] / ([ % Al] +3 [% Fe 2 O 3] +2 [% SiO 2] +2 [% MnO 2] + [% S]) ... (1)
(In the above formula (1), [% Ca] represents the ratio (mol%) of the total amount of Ca contained in the Ca additive and the heat insulating material with respect to the total amount of the components in the heat insulating material, [% Al], [ % Fe ₂ O ₃ ], [% SiO ₂ ], [% MnO ₂ ], and [% S] represent the respective contents (mol%) in the heat insulating material.)

  According to the present invention, since the composition of inclusions is appropriately controlled, it is possible to obtain a forged product in which aggregation of inclusions is suppressed and variation in fatigue characteristics is small. In particular, in the present invention, since the variation in the radial direction of the forged product is suppressed and the variation in the height direction is also suppressed, excellent fatigue characteristics can be ensured more reliably.

  Specifically, “small variation in fatigue characteristics” in the present invention specifically means that the following (I) and (II) are satisfied.

(I) Average of endurance limit ratio (E _TOP ) at R / 3 part of steel upper part (in the cross section perpendicular to the axis center, the R / 3 position from the axis center toward the radius (R) direction; the same applies hereinafter) The difference between the value and the minimum value is 0.025 or less.

(II) The ratio (E _TOP / E _BOT ) of the R / 3 part durability limit ratio (E _TOP ) at the top of the steel material and the R / 3 part durability limit ratio (E _BOT ) at the bottom of the steel material is 0.90 ≦ (E _TOP / E _BOT ) ≦ 1.07 is satisfied.

The “E _TOP ” is a value calculated by dividing the fatigue strength (σ _w ) of the R / 3 part at the top of the steel by the tensile strength (σ _B ), and “E _BOT ” is the R at the bottom of the steel. / 3 parts fatigue strength (σ _w ) divided by tensile strength (σ _B ).

FIG. 1 shows the relationship between the ratio of oxide inclusions having a CaO content of 5% by mass or more and 45% by mass or less and the difference between the average value and the minimum value of the endurance limit ratio of R / 3 part in the upper part of the steel material. It is a graph which shows. FIG. 2 is a graph showing the relationship between the ratio of oxide inclusions having a CaO content of 5% by mass or more and 45% by mass or less and E _TOP / E _BOT .

The inventors of the present invention have made extensive studies to provide a forged product based on Al killed steel with little variation in fatigue characteristics. As a result, the variation in fatigue characteristics is caused by the composition of inclusions. When the relationship between the variation in fatigue characteristics and the composition of inclusions was examined, the existence ratio of CaO—Al ₂ O ₃ inclusions as inclusions was determined. More specifically, if an oxide inclusion having a CaO content of 5% by mass or more and 45% by mass or less is present in a certain amount or more, aggregation of inclusions can be suppressed and fine dispersion can be realized. As a result, it was found that a forged product having a small variation in fatigue characteristics can be obtained regardless of the part.

  As oxide inclusions, those containing less than 5% by mass of CaO are not preferable because they become inclusions that are likely to be coarsened and coarsened. On the other hand, the case where the CaO content exceeds 45% by mass is not preferable because the ratio of coarse CaO-based inclusions and CaS-based inclusions increases.

  In the present invention, as described above, the formation of coarse inclusions is ensured by securing a ratio of the number of oxide inclusions having a CaO content of 5 mass% or more and 45 mass% or less to the total number of inclusions. Can be suppressed. The ratio of inclusions specified above is preferably 7.5% or more.

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 forged product of the present invention are mainly MgO—Al ₂ O ₃ and MnS.

  In addition, the above-mentioned inclusions are intended for objects of 5 μm or more detected by EPMA, as shown in Examples described later.

  In addition, the forged product of the present invention can control the following chemical composition and manufacturing conditions to suppress the longest diameter of the maximum inclusions observed in the steel cross section to 100 μm or less, and the variation in the above-described fatigue characteristics is more sufficient. It is also suppressed.

  The forged product of the present invention satisfies the following chemical composition in order to satisfy the above inclusion composition and exhibit excellent fatigue properties, and to have strength, toughness and the like.

(Chemical composition of steel)
[C: 0.2-0.6%]
C is an element that contributes to improving the strength of the forged product. To ensure sufficient strength, C is contained in an amount of 0.2% or more. The amount of C is preferably 0.25% or more, more preferably 0.30% or more. However, if the amount of C is too large, the toughness of the forged product is deteriorated, so the content is made 0.6% or less. The amount of C is preferably 0.55% or less, more preferably 0.50% or less.

[Si: 0.05 to 0.50%]
Si acts as an element that improves the strength of the forged product, and is contained in an amount of 0.05% or more in order to ensure sufficient strength. The amount of Si is preferably 0.1% or more, 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, so the content is made 0.50% or less. The amount of Si is preferably 0.45% or less, more preferably 0.40% or less.

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

[Ni: 0.10 to 3.50%]
Ni is an element useful as a toughness improving element, and in the present invention, the Ni content is 0.10% or more. Preferably it is 0.2% or more. However, if the amount of Ni becomes excessive, the cost increases, so the content is made 3.50% or less. 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 exhibited by containing 0.9% or more of Cr. The amount of Cr is preferably 1.1% or more, 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 4% or less. 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 Mo is contained in an amount of 0.10% or more in order to exert these effects. The amount of Mo is preferably 0.20% or more, and more preferably 0.25% or more. However, since Mo has a small equilibrium distribution coefficient and easily causes microsegregation (normal segregation), the amount of Mo is set to 0.70% or less. 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, V is contained in an amount of 0.01% or more. However, the V amount is set to 0.20% or less because the above effect is saturated and is economically wasteful even if contained excessively. 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. Therefore, Al is contained 0.005% or 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 amount is 0.10%. 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. Therefore, the S amount is 0.008% or less. The amount of S is preferably 0.006% or less, 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 0.005% or less. 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 0.0025% or less. Preferably it is 0.0015% or less, More preferably, it is 0.0010% or less.

  The basic components of steel used in the forged product of the present invention are as described above, with the balance being 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.

(Method for manufacturing forged products)
In producing the forged product of the present invention, any method can be used as long as the inclusions form a steel that satisfies the above-mentioned regulations. According to a conventional method, melting / primary refining → molten steel treatment → ingot making → forging Although it can be obtained, it is recommended that the conditions in the ingot-making process be as follows in order to surely obtain the inclusion form steel. Hereinafter, it demonstrates in order of a process.

[Melting / Primary refining]
Melting / primary refining can be performed using a high-frequency melting furnace, electric furnace, converter, or the like according to a conventional method. In the present invention, the amount of molten steel is not particularly limited.

[Molten steel treatment]
In accordance with conventional methods, the temperature and main components are adjusted while stirring the molten steel in the ladle by means of bottom blowing gas stirring, etc., and a deoxidizer and the like are added to the molten steel to perform deoxidation, desulfurization, etc. Just do it. In the present invention, since Al killed steel is assumed, deoxidation is performed by adding Al. Furthermore, if necessary, vacuum degassing treatment is performed using a molten steel treatment process using an LF or a lid degassing device (VD), a reflux degassing method (RH), and dehydrogenation and desulfurization from the molten steel are promoted, What is necessary is just to adjust a molten steel component and molten steel temperature.

[Agglomeration]
An ingot is produced by a bottom pouring ingot method in which the molten steel subjected to the molten steel treatment is charged into the mold from below through an injection tube. The ingot size and shape are not particularly limited.

In molten steel injection, in order to suppress oxidation of the bath surface of the molten steel rises in the mold, generally ₂ are for example C-SiO used -CaO-Al ₂ O ₃ based coating material (Mold Additives) inside the mold It is sprayed on the molten steel surface (bath surface). The coating material may be sprayed after the molten steel is injected into the mold until the molten steel reaches the feeder. The coating material that has been sprayed and covered the bath surface is consumed while gradually flowing into the interface between the mold and the molten steel as the molten steel is poured from below and the bath surface rises. Therefore, when the bath surface is exposed due to consumption of the covering material, it is preferable to add the covering material promptly so that the bath surface is not exposed.

  In the present invention, a heat insulating material is used for heat retention at the end of casting. The heat insulating material is basically added after the molten metal reaches the feeder part. Furthermore, in order to obtain the inclusions defined in the present invention by modifying the inclusions generated due to the heat insulating material, the above modification is attempted by adding a Ca additive. The relationship between the heat insulating material and the timing of addition of the Ca additive is as follows.

That is, after the molten steel reaches the feeder part and before the end of casting (in addition to the case where the inside of the feeder is added while rising, at the end of casting [at the end of pouring, that is, the molten metal surface is stopped) Timing] is also included)
(A) a Ca additive and a heat insulating material; or
(B) Of the Ca additive and the heat insulating material, only the heat insulating material (however, only when Ca is contained in the heat insulating material);
It adds to molten steel from the upper part of a casting_mold | template via the coating | covering material which covers a bath surface so that all of the following (condition a)-(condition c) may be satisfy | filled.

Thus, during the time of ingot forming without generating coarse CaO-based inclusions by adding the Ca additive / heat insulating material between the time when the molten steel reaches the feeder part and before the end of casting. Al ₂ O ₃ inclusions generated in the above can be modified to CaO—Al ₂ O ₃ inclusions by addition of Ca to suppress inclusion aggregation. Further, by covering the surface of the molten steel with a heat insulating material, a portion where defects or component segregation occurs can be concentrated on the upper part of the steel ingot. That is, since the upper part of the molten steel is kept warm by covering the surface of the molten steel with a heat insulating material, the cooling rate of the molten steel is reduced, and as a result, the heated molten steel becomes the final solidified part, and defects and component segregation are removed from It can be concentrated on the solidified part.

  As above-mentioned, Ca additive and a heat insulating material are added to molten steel from the upper part of a mold through the coating | covering material which covers a bath surface. This is because when Ca touches the molten steel directly, the risk of forming coarse CaO-based inclusions increases.

The addition of the Ca additive and the addition of the heat insulating material may be performed in a plurality of times.
Hereinafter, the addition conditions a to c of the Ca additive / heat insulating material will be described in detail.

  (Condition a) Addition of Ca additive is performed simultaneously with the heat insulating material or before the addition of the heat insulating material.

  In the present invention, in order to improve the inclusions caused by the heat insulating material in particular, the Ca additive is added in accordance with the addition timing of the heat insulating material.

  From such a viewpoint, when the Ca additive is added before the addition of the heat insulating material, the time from the addition of the Ca additive to the addition of the heat insulating material is preferably within 10 minutes. More preferably, it is within 5 minutes.

[About the form of Ca additive]
The form of the Ca additive may be pure metal Ca or Ca alloy. As Ca alloy, it can select suitably according to steel component specifications, such as Ca-Si alloy and Ca-Ni alloy. Examples of the added shape include powder and plate.

[Insulation material]
In the present invention, the heat insulating material may satisfy Formula (1) in the following (Condition c), and when the heat insulating material contains Ca, it may further satisfy (Condition b). Not. That is, as the component, a known component (the _{Al, Ca, Si, Fe t} O (t _{coefficient), MnO 2, SiO 2,} Al 2 O 3, C, S , etc.) may be used insulation material made of. Not all of the above elements / compounds may be included. Therefore, what contains Ca may be used as a heat insulating material, and what does not contain Ca may be used. Moreover, you may use the heat insulating material containing metals and oxides other than the above.

(Conditions b) the thermal insulation material and Ca additives to be added if necessary, the total Ca in terms of amount to the surface area of the bath surface is added so as to satisfy the ^{0.35kg / m 2 ~10kg / m 2} .

Total amount (kg) of Ca added to the surface area of the bath and Ca contained in the heat insulating material when converted to pure Ca: Total Ca equivalent added amount is less than 0.35 kg / m ² In this case, the inclusion modification effect by Ca is not sufficiently obtained. The total Ca in terms of amount is preferably 0.40 kg / m ² or more, more preferably 0.45 kg / m ² or more. On the other hand, when the total Ca equivalent addition amount exceeds 10 kg / m ² , coarse CaO inclusions are generated, which is not preferable. Preferably it is 8 kg / m ² or less, more preferably 6 kg / m ² or less.

(Condition c) The heat-retaining material and the Ca additive added as necessary are added so that the X value represented by the following formula (1) satisfies 0.08 to 0.25.
X value = [% Ca] / ([ % Al] +3 [% Fe 2 O 3] +2 [% SiO 2] +2 [% MnO 2] + [% S]) ... (1)
(In the above formula (1), [% Ca] represents the ratio (mol%) of the total amount of Ca contained in the Ca additive and the heat insulating material with respect to the total amount of the components in the heat insulating material, [% Al], [ % Fe ₂ O ₃ ], [% SiO ₂ ], [% MnO ₂ ], and [% S] represent the respective contents (mol%) in the heat insulating material.)

The X value represented by the above formula (1) indicates that the Ca is consumed by the heat insulating material (specifically, Ca is oxidized by the lower oxide (for example, SiO ₂ ) in the heat insulating material), and the inclusion is reformed. In order to prevent the Ca that contributes to the deficiency, the addition amount of the Ca additive is adjusted in accordance with the components of the heat insulating material.

  If the X value is smaller than 0.08, the sufficient reforming effect by Ca cannot be obtained, so the X value is set to 0.08 or more in the present invention. The X value is preferably 0.09 or more, more preferably 0.10 or more. On the other hand, when the X value is too large, coarse CaO-based inclusions are generated. Therefore, the X value is 0.25 or less. Preferably it is 0.20 or less.

As the adjustment patterns of (Condition b) and (Condition c), there are the following three patterns depending on whether Ca is contained in the heat insulating material. That is,
(A-1) When adding a Ca additive and a heat insulating material (including Ca) (condition b) and (condition c), the amount of Ca contained in both the Ca additive and the heat insulating material is adjusted.

(A-2) When adding a Ca additive and a heat insulating material (not including Ca) (condition b) and (condition c), the amount of Ca in the Ca additive is adjusted.

(B) When adding only the heat insulating material (including Ca) among the Ca additive and the heat insulating material (condition b) and (condition c), the amount of Ca in the heat insulating material is adjusted.

〔coagulation〕
After casting, the mold is allowed to stand until the molten steel in the mold is completely solidified.

[Forged products]
As the forged product, for example, after removing the solidified steel ingot, it is heated and subjected to hot forging to finish a forged material having a cross-sectional diameter of 150 to 700 mm.

  The forged product of the present invention is widely and effectively used in industrial fields such as machinery, ships, and generators, and is particularly suitable for parts that require high fatigue characteristics.

  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.

  According to a conventional method, after melting scrap using an electric furnace, 20 to 100 tons of molten steel was transferred to a ladle. Then, the molten steel in the ladle is subjected to a molten steel processing step (including deoxidation performed by adding Al) by LF and a lid degassing device (VD), and the molten steel components and molten steel temperature are adjusted, and shown in Table 1 below. Steel of chemical composition (the balance is iron and inevitable impurities) was melted.

  Next, a generally used ingot casting method was adopted, and the molten steel was poured into a 20-90 ton class mold to produce a steel ingot.

At this time, a C—SiO ₂ —CaO—Al ₂ O ₃ -based coating material was sprayed on the surface of the molten steel (bath surface) in the mold in order to suppress molten steel oxidation in the process of the molten steel rising in the mold. The coating material was sprayed after the molten steel was poured into the mold until the molten steel reached the feeder. When the bath surface was exposed due to the consumption of the coating material, the coating material was quickly added to prevent the bath surface from being exposed. In addition, in some examples as a comparative example, even if the coating material is consumed and the bath surface is exposed, no additional coating material is added, and the addition of the Ca additive in the next step is performed while the bath surface is partially exposed. Went.

  The Ca additive (Ca—Si alloy powder) and a heat insulating material were added to the bath surface after the molten steel reached the feeder part and before the end of casting. The heat insulating material was added after the addition of the Ca additive or simultaneously with the addition of the Ca additive. Further, the time from spraying the Ca additive to adding the heat insulating material was set to 0 second (simultaneous) to 600 seconds (10 minutes).

As a heat insulating material, components are FeO: 10 to 20% by mass, Fe ₂ O ₃ : 10 to 20% by mass, Al: 20 to 25% by mass, Al ₂ O ₃ : 25 to 40% by mass, SiO ₂ : 5 to 5%. A material containing 10 mass and substantially free of MnO ₂ and Ca was used. Further, although S is contained in the heat insulating material, the content of S is about 200 ppm (amount of inevitable impurities) and may be considered to be substantially zero (that is, it affects the equation (1)). Not the amount that affects).

  The composition of the heat insulating material was determined by ICP (inductively coupled plasma) emission spectroscopy. Further, the Ca amount of the Ca additive (Ca—Si alloy powder) was also confirmed by the ICP emission spectroscopic analysis method.

  From the amount of Ca contained in the Ca additive and the heat insulating material, the total amount in terms of Ca / the surface area of the bath surface shown in Table 2 was calculated.

In Table 2, a case (the total surface area Ca in terms amount / bath surface) is 0.35 kg / m ² or more 10 kg / m ² or less "○", in particular 6 kg / m ² or less at 0.45 kg / m ² or more in the case of shows as "◎", it is shown as "×" in the case of 0.35kg / m ² less than or 10kg / m ² greater.

Moreover, it calculated | required also about X value from the component composition of Ca additive and a heat insulating material (Table 2).
In Table 2, a case where the X value is 0.08 or more and 0.25 or less is indicated by “◯”, particularly a case where the X value is 0.10 or more and 0.23 or less is indicated by “◎”, and is less than 0.08 or more than 0.25. In this case, “×” is shown.

After casting, the mold was allowed to stand until the molten steel in the mold was completely solidified. Then, after the molten steel was completely solidified, the solidified steel ingot was demolded by a conventional method, and then heated to about 1300 ° C. to perform hot forging to finish a forged product having a cross-sectional diameter of 150 to 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).

And in the entire visual field, the inclusion major axis is 5 μm or more,
(I) The total number of inclusions and (ii) the number of oxide inclusions having a CaO content of 5% by mass or more and 45% by mass or less were determined.

  Then, [100 × (ii) / (i)] (%) is calculated, and the CaO content in the total number of inclusions observed in the steel cross section is 5% by mass or more and 45% by mass or less. The ratio of the number of oxide inclusions was determined.

(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.

Furthermore, the major axis of the maximum inclusion at the shaft center (OR) at the bottom of the steel material was also measured.
These results are shown in Table 2. In Table 2, a case where the ratio was 5% or more was evaluated as “◯”, and a case where the ratio was 7.5% or more was evaluated as “「 ”. Moreover, the case where the said ratio was less than 5% was evaluated as "x". Moreover, the case where the long diameter of the said largest inclusion was 100 micrometers or less was set as the pass.

(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 of φ6 mm × gauge length 30 mm (the longitudinal direction of the test piece is the axis of the forged product) 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 of the forged product 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 durability 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 steel part 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 was 0.90 or more and 1.07 or less was evaluated as “◯”, and the case where it was less than 0.90 or more than 1.07 was evaluated as “x”.

  From Table 2, it can be considered as follows. That is, no. Nos. 1 to 11 are manufactured by the method specified in the present invention, and the inclusions in the steel satisfy the specifications. Therefore, the variation in the fatigue characteristics in the radial direction of the forged product is suppressed, and the height direction Variations in fatigue characteristics are also suppressed.

  In contrast, no. 12 to 28 are not manufactured under the conditions defined in the present invention (in particular, one or more of condition b and condition c), so that the CaO content is 5% by mass or more and 45% by mass or less. The inclusions could not be secured sufficiently, resulting in variations in fatigue characteristics.

The ratio of specified inclusions (oxide inclusions with a CaO content of 5 mass% or more and 45 mass% or less), the minimum value and the average value of the endurance limit ratio, arranged using the results of Table 2 The relationship with the difference is shown in FIG. From FIG. 1, by setting the ratio of the CaO-containing oxide inclusions to 5% or more, the difference between the minimum value and the average value of the R / 3 part R / 3 part in the upper part of the steel material is 0.025 or less. It can be seen that it can be made smaller. In addition, the relationship between the ratio of specified inclusions (oxide inclusions having a CaO content of 5% by mass or more and 45% by mass or less) and E _TOP / E _BOT arranged using the results of Table 2 is shown. It is shown in 2. From FIG. 2, by setting the ratio of the CaO-containing oxide inclusions to 5% or more, E _TOP / E _BOT can be reliably within the range of 0.90 or more and 1.07 or less. It can be seen that variations in the fatigue characteristics of the bottom can also be reduced.

Claims (2)

C: 0.2 to 0.6% (meaning “mass%”; the same applies to chemical components),
Si: 0.05 to 0.50%,
Mn: 0.20 to 1.5%,
Ni: 0.10 to 3.50%,
Cr: 0.9 to 4%,
Mo: 0.10 to 0.70%,
V: 0.01-0.20%,
Al: 0.005 to 0.10%,
S: 0.008% or less (excluding 0%),
Satisfying Ti: 0.005% or less (excluding 0%) and Total O: 0.0025% or less (excluding 0%),
The balance consists of iron and inevitable impurities,
The ratio of the number of oxide inclusions having a CaO content of 5% by mass or more and 45% by mass or less in the total number of inclusions observed in the steel cross section is 5% or more, and
A forged product characterized in that the longest inclusions observed in the steel cross section have a major axis of 100 μm or less. A method for producing a forged product according to claim 1, wherein the ingot casting method for producing an ingot by inserting molten steel into the mold from below through an injection tube is performed.
After the coating material for coating the molten steel bath surface is added to the molten steel in the mold, a heat insulating material is added, and if necessary, before adding the heat insulating material or simultaneously with the heat insulating material. To add,
The heat-retaining material and the Ca additive to be added as necessary are:
With total Ca in terms of amount to the surface area of the bath surface is added so as to satisfy the ^{0.35kg / m 2 ~10kg / m 2} ,
A method for producing a forged product , wherein the X value represented by the following formula (1) is added so as to satisfy 0.08 to 0.25.
X value = [% Ca] / ([% Al] +3 [% Fe ₂ O ₃ ] +2 [% SiO ₂ ] +2 [% Mn
O ₂ ] + [% S]) (1)
(In the above formula (1), [% Ca] represents the ratio (mol%) of the total amount of Ca contained in the Ca additive and the heat insulating material with respect to the total amount of the components in the heat insulating material, [% Al], [ % Fe ₂ O ₃ ], [% SiO ₂ ], [% MnO ₂ ], and [% S] represent the respective contents (mol%) in the heat insulating material.)

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