ES2517523T3 - Large strength steel forged part - Google Patents

Abstract

A large strength steel forged piece comprising: compositions comprising basic compositions comprising: C: 0.31% by mass or more and 0.5% by mass or less, Si: a 0 , 02% by mass or more and 0.2% by mass or less, Mn: 0.1% by mass or more and 0.6% by mass or less, Ni: 2.6% by mass or plus and 3.4% by mass or less, Cr: 0.8% by mass or more and 1.9% by mass or less, Mo: 0.25% by mass or more and 0.8 Mass% or less, V: 0.05% mass or more and 0.2% mass or less, and Al: 0.005% mass or more and 0.1% mass or less, and a remainder that includes faith and inevitable impurities; where an S content such as the inevitable impurity is 0.008% by mass or less; and where the large strength steel forged piece consists of a martensitic structure or a mixed structure of martensite and bainite; a grain size of the anterior austenite is 19 μm or more and 70 μm or less; and a maximum block size of martensite is 15 μm or less and a minimum block size of martensite is 0.5 μm or more.

Description

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DESCRIPTION

Large strength steel forged part

5 Field of the invention

The present invention relates to a large strength steel forged piece.

Background of the invention

High strength is required for large crankshafts and intermediate shafts that are used for boats or power generators. In general, these trees are produced by forged pieces of steel. Nowadays, in addition to a further improvement in strength, an excellent toughness is also required for these large forged steel parts, which usually has a reciprocal compensation relationship with

15 the resistance.

In order to improve the strength and toughness of steel forged parts, (1) high strength steel has been developed for a large steel forged piece in which component compositions are limited (see publication of Japan's unexamined patent application No. 2005-344149); (2) forged steel that is manufactured by limiting component compositions and specifying a structure that mainly includes bainite and martensite (see Japan patent No. 3896365); (3) a crankshaft that is manufactured by limiting the component compositions and limiting a grain size of the previous austenite (see Japan's unexamined patent application publication No. 2010-248540); (4) nickel-based thermal refining steel that is manufactured

25 by specifying an amount of aluminum over the grain limits (see Japan's unexamined patent application publication No. 2000-212705); (5) Forged part steel that is manufactured by specifying concentrations of magnesium and aluminum (see Japan's unexamined patent application publication No. 2008-25021 and Japan's unexamined patent application publication No. 2009-173961); and (6) a forged part that is manufactured by specifying a sulfur content rate, hot forging conditions, and the like (see Japan's unexamined patent application publication No. 2003-147436).

However, even the steels (1) to (3) described above cannot show sufficient strength because a block size and grain size of the formed structure may not be

35 and, therefore, the toughness and fatigue resistance cannot be improved in a balanced way. In addition, the aluminum content in the steel (4) described above is high and, therefore, nonmetallic inclusions and intermetallic compounds can be generated to deteriorate toughness and fatigue resistance. It is said that steel (5) improves strength. However, it is not intended that the steel (5) improve the toughness. The forged steel piece (6) is said to have high strength and high toughness. However, the existence of the previously determined amount of sulfur generates nonmetallic inclusions such as MnS, and, as a result, fatigue resistance deteriorates. As described above, none of the conventional steel forged parts have improved strength, toughness and fatigue resistance in a balanced manner.

Summary of the invention

The present invention is achieved in view of the problems that have been described in the foregoing and the object of the present invention is the provision of a high-strength forged piece of steel having a highly balanced strength and toughness. and that has a high resistance to fatigue.

The invention to solve the problem described above is a large strength steel forged piece that includes:

compositions that include 55 basic compositions that include:

C: 0.31% by mass or more and 0.5% by mass or less, If: 0.02% by mass or more and 0.2% by mass or less, Mn: 0.1 % by mass or more and 0.6% by mass or less, Ni: 2.6% by mass or more and 3.4% by mass or less, Cr: 0.8% by mass or more and a 1.9% by mass or less, Mo: 0.25% by mass or more and 0.8% by mass or less,

V: 0.05% by mass or more and 0.2% by mass or less, and Al: 0.005% by mass or more and 0.1% by mass or less, and

65 a remainder that includes Faith and inevitable impurities; where an S content such as the inevitable impurity is 0.008% by mass or less; Y

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where the large strength steel forged piece consists of a martensitic structure or a mixed structure of martensite and bainite; a grain size of the anterior austenite is 19 μm or more and 70 μm or less; and a maximum block size of martensite is 15 μm and a minimum block size of martensite is

5 0.5 μm or less.

The large strength steel forged piece has excellent strength and toughness in a balanced manner and also provides high fatigue resistance by limiting the compositions and structure described above. as described above and the adjustment of the grain size of the anterior austenite and the block size within the range described above.

In the present case, "large size" in the forged steel part of high strength high strength means a forged steel part having a spherical part or a cylindrical part whose diameter 15 is 150 mm or more, a forged part of steel that has a plate-like part whose thickness is 150 mm or more, and a forged piece of steel that has a similar size or a larger size.

As described above, the large strength steel forged part of the present invention has excellent strength and toughness in a balanced manner and also has high fatigue resistance. For this reason, the large strength steel forged piece can preferably be used for large crankshafts, intermediate shafts, and the like used for ships, power generators, or the like.

Brief description of the drawings

25 Figure 1 is a graph illustrating a relationship between a ratio x of a depth and a distance from a surface to a central part of a large strength steel forged piece and a fraction of a martensitic structure fm (x ) (%); Figure 2 is a graph illustrating a relationship between a maximum size of a martensitic block and the absorbed energy of Charpy that is measured by using test specimens of the examples and comparative examples; Figure 3 is a graph illustrating a relationship between the tensile strength and the absorbed energy of Charpy that is measured by using the test specimens of the examples and the comparative examples; Figure 4 is a graph illustrating a relationship between tensile strength and fatigue resistance that is measured by using the test specimens of the examples and comparative examples;

Figure 5 is a reverse pole figure map of the test tube of example 4; Figure 6 is a reverse pole figure map of the test tube of Example 11; Figure 7 is a reverse pole figure map of the specimen of comparative example 3; Figure 8 is a reverse pole figure map of the test specimen of comparative example 7; Figure 9 is a reverse pole figure map of the test specimen of comparative example 9; Figure 10 is a TTT diagram that is used as analysis conditions in an analysis example; Figures 11 are graphs illustrating the temperature dependence of each physical property that are used as the analysis conditions in the analysis example; Figures 12 are graphs illustrating the plastic behavior in each phase that are used as the analysis conditions in the analysis example;

Figure 13A is a graph illustrating the analysis conditions A and B in the analysis example and Figure 13B is a graph illustrating the analysis results; and Fig. 14A is a graph illustrating a relationship between a depth of a large strength steel forge and Brinell hardness in the reference example and Fig. 14B is a graph illustrating a relationship between depth and a fraction of martensitic structure in the reference example.

Detailed description of the preferred embodiments

Hereinafter, embodiments of the high strength large strength steel forging of the present invention will be described in detail.

<Composition>

The large strength steel forged part consists of basic components that include: C: 0.31% by mass or more and 0.5% by mass or less, Si: 0.02% by mass or more and 0.2% by mass or less, Mn: 0.1% by mass or more and 0.6% by mass or less, Ni: 2.6% by mass or more and 3 , 4% by mass or less, Cr: 0.8% by mass or more and 1.9% by mass or less, Mo: 0.25% by mass or more and 0.8% by mass or less, V: 0.05% by mass or more and 0.2% by mass or less, and Al: 0.005% by mass or more and 0.1% by mass or less, and a remainder that includes Faith and inevitable impurities. The reason for the

The limitation of each component will be described hereinafter.

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(C: 0.31% by mass or more and 0.5% by mass or less)

The lower limit of the carbon content (C) is 0.31% by mass and the lower content is preferably 0.33% by mass. On the other hand, the upper limit of the carbon content (C) is 0.5% by mass and the content

5 is preferably 0.4% by mass. Carbon (C) helps improve hardenability and strength. A carbon content that is less than the lower limit described above results in difficulties in obtaining sufficient hardening capacity and strength. Conversely, a carbon content that is greater than the upper limit described above results in extreme deterioration in toughness and promotes the generation of a "A" segregation in a large ingot.

(Yes: 0.02% by mass or more and 0.2% by mass or less)

The lower limit of the silicon content (Si) is 0.02% by mass and the lower content is preferably a

15 0.06% by mass. On the other hand, the upper limit of the silicon content (Si) is 0.2% by mass and the upper content is preferably 0.16% by mass. Silicon (Si) contributes to deoxidation and resistance improvement. A silicon content that is less than the lower limit described above cannot provide a sufficient demonstration of this effect. Conversely, a silicon content that is greater than the upper limit described above results in significant "A" segregation and, therefore, it is difficult to obtain a pure ingot.

(Mn: 0.1% by mass or more and 0.6% by mass or less)

The lower limit of manganese content (Mn) is 0.1% by mass and the lower content is preferably a

25 0.3% by mass. On the other hand, the upper limit of manganese content (Mn) is 0.6% by mass and the upper content is preferably 0.45% by mass. Manganese (Mn) improves hardening capacity and strength. A manganese content that is less than the lower limit described above leads to difficulties in demonstrating the effect described above. Conversely, a manganese content that is greater than the upper limit described above promotes a tempering by tempering.

(Ni: 2.6% by mass or more and 3.4% by mass or less)

The lower limit of the nickel (Ni) content is 2.6% by mass and the lower content is preferably 2.8%

35 in mass. On the other hand, the upper limit of nickel content (Ni) is 3.4% by mass. Nickel (Ni) improves hardening capacity, strength and toughness. A nickel content that is less than the lower limit described above cannot provide a sufficient demonstration of this effect. A nickel content that is greater than the upper limit results in difficulties in obtaining an adequate size of the anterior austenite grains. In addition, a quantity of Ni used, which is of high price, can be reduced, and the production cost can be reduced by adjusting the nickel content less than the upper limit described above.

(Cr: 0.8% by mass or more and 1.9% by mass or less)

The lower limit of the chromium content (Cr) is 0.8% by mass and the lower content is preferably 1.4% by mass. On the other hand, the upper limit of the chromium content (Cr) is 1.9% by mass and the upper content is preferably 1.65% by mass. Chromium (Cr) improves hardening capacity and toughness. A chromium content that is less than the lower limit described above cannot provide a sufficient demonstration of the effect described above. Conversely, a chromium content that is greater than the upper limit described above promotes a "A" segregation.

(Mo: 0.25% by mass or more and 0.8% by mass or less)

The lower limit of the molybdenum (Mo) content is 0.25% by mass and the lower content is preferably 0.4% by mass. On the other hand, the upper limit of the molybdenum (Mo) content is 0.8% by mass and the upper content is preferably 0.6% by mass. Molybdenum (Mo) improves hardening capacity, strength and toughness. A molybdenum content that is less than the lower limit described above cannot provide a sufficient demonstration of the effect described above and promotes "A" segregation. Conversely, a molybdenum content that is greater than the upper limit described above promotes microsegregation and tends to generate a segregation by gravity.

(V: 0.05% by mass or more and 0.2% by mass or less)

65 The lower limit of the vanadium (V) content is 0.05% by mass and the lower content is preferably 0.07% by mass. On the other hand, the upper limit of the vanadium (V) content is 0.2% by mass and the content

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higher is preferably 0.13% by mass. The addition of a small amount of vanadium (V) greatly improves the hardenability and strength. However, vanadium has a small equilibrium distribution coefficient and, therefore, tends to generate microsegregation. A vanadium content that is less than the lower limit described above cannot ensure resistance

5 enough. Conversely, a vanadium content that is greater than the upper limit described above promotes the generation of a microsegregation.

(Al: 0.005% by mass or more and 0.1% by mass or less)

The lower limit of the aluminum content (Al) is 0.005% by mass and the lower content is preferably 0.008% by mass. On the other hand, the upper limit of the aluminum content (Al) is 0.1% by mass and the upper content is preferably 0.03% by mass. Aluminum (Al) is used as a deoxidation agent. In addition, aluminum generates fine compounds such as AlN, and this AlN ends the growth of the grains and can form fine grains. An aluminum content that is less than the lower limit described in what

15 above cannot provide a sufficient demonstration of this effect. Conversely, an aluminum content that is greater than the upper limit described above may generate oxides and intermetallic compounds and may deteriorate toughness and fatigue resistance because aluminum forms bonds with other elements. such as oxygen.

The basic components of the large strength steel forged part have been described above. The remaining components are substantially iron (Fe). However, trace amounts of unavoidable impurities may be included (eg, S, O, P, Cu, Sn, N, and the like). In addition, other elements may be positively included in a range as long as the elements do not provide an adverse effect to the operation and the effect of the large strength steel forged.

25 Examples of these other elements may include Ti, Ca and Mg. From the point of view of a reduction in the generation of coarse inclusions, preferably a total amount of the inevitable impurities is controlled up to 0.5% by mass or less.

(S: 0.008% by mass or less)

The sulfur content (S) is 0.008% by mass or less and preferably 0.003% by mass or less. Sulfur (S), which forms MnS in the steel forged piece, deteriorates fatigue resistance when the content exceeds the upper limit described above. However, in the industrial field, the content never becomes 0% by mass.

35 The contents of the other unavoidable impurities are preferably as follows.

(Or: 0.0025% by mass or less)

The oxygen (O) content is preferably 0.0025% by mass or less and more preferably 0.002% by mass or less. Oxygen (O) binds with various elements to form nonmetallic inclusions and, as a result, deteriorates fatigue resistance. Accordingly, the oxygen content is preferably the upper limit that has been described above or less. However, in the industrial field, the content never becomes 0% by mass.

Four. Five

(P: 0.02% by mass or less)

The upper limit of the phosphorus content (P) is preferably 0.02% by mass or less and more preferably 0.01% by mass or less. A phosphorus content (P) that is greater than the upper limit described above results in a deterioration in hot ductility and tends to generate cracks and other defects during forging.

(Cu: 0.1% by mass or less)

The upper limit of the copper content (Cu) is preferably 0.1% by mass or less and more preferably 0.05% by mass or less. A copper (Cu) content that is greater than the upper limit described above tends to generate cracks and other defects during the forging.

(Sn: 0.03% by mass or less)

The upper limit of the tin content (Sn) is preferably 0.03% by mass or less and more preferably 0.01% by mass or less. A tin content (Sn) that is greater than the upper limit described above may result in a deterioration in toughness.

65 (N: 0.02% by mass or less)

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The upper limit of a nitrogen (N) content is preferably 0.02% by mass or less and more preferably 0.01% by mass or less. A nitrogen (N) content that is greater than the upper limit described above results in a deterioration in hot ductility and tends to generate cracks and other defects during forging.

5 <Structure>

Subsequently, the structure of the large strength steel forged piece will be described.

The large strength steel forged piece consists of a martensitic structure or a mixed structure of martensite and bainite. Both strength and toughness can be satisfied simultaneously in a balanced way by forming the high-strength large-forged steel piece consisting of only those two types of structures. The existence of other structures such as ferrite and perlite in the large strength steel forged piece cannot satisfy

15 both resistance and toughness simultaneously.

A grain size of the anterior austenite in the large strength steel forged piece (an average size) is 19 μm or more and 70 μm or less. The grain size of the anterior austenite affects the block size. A coarse grain size of anterior austenite expands the block size and does not provide sufficient toughness. As a result, it is determined that the upper limit of the grain size of the anterior austenite is 70 μm. Conversely, a too fine grain size that is less than 19 μm results in a deterioration in hardening capacity and results in contamination with proeutectoid ferrite and, therefore, the balance between resistance and toughness it deteriorates. The grain size of the anterior austenite can be measured by a method described in the examples.

25 In a martensitic block size, which is a substructure of the martensitic structure that forms the high-strength steel forge, a maximum block size of it is 15 μm or less and a minimum block size of It is 0.5 μm or more. The size of the martensitic block that is being determined to be the interval described above allows the steel forged to improve strength, toughness and fatigue resistance in a balanced manner. In particular, a reduction in this maximum block size to 15 μm or less allows the steel forge to show a stable toughness. On the other hand, too much reduction in block size increases the grain limit density and gives an increase in the crack growth rate. Consequently, it is determined that the minimum block size is 0.5 μm or more.

35 In the forged steel piece of high strength, it is preferable that a fraction of a martensitic structure fm (x) (%), in which a ratio of a depth to a distance from a surface to a part central is defined as x (0 ≤ x ≤ 1), be

fm (x) = 100, where 0≤x≤0.1; 104-40x≤fm (x) ≤100, in which 0.1 <x≤0.15, 122 -160x ≤ fm (x) ≤ 100, in which 0.15 <x ≤ 0.2; 230-700x ≤fm (x) ≤100, in which 0.2 <x≤0.3; 110-300x ≤fm (x) ≤112-40x, in which 0.3 <x≤ 0.35;

45 (22-20x) / 3≤fm (x) ≤105-20x, in which 0.35 <x≤50.5; (32-40x) / 3≤fm (x) ≤95, in which 0.5 <x≤0.8; y 0 ≤ fm (x) ≤ 95, in which 0.8 <x ≤ 1 (the interval (a) illustrated in Figure 1).

In the present case, a remaining structure is a bainitic structure. This fraction of the martensitic structure can be measured by the use of a method described in the examples, that is, a rule of the mixtures obtained from hardness measurement results.

The "central part" of the large strength steel forged piece means the deepest position with respect to each surface position. For example, when the steel forged part of

55 large dimensions of high strength have a spherical part, the central part means the central point of the spherical part; when the forged part has a cylindrical part, the central part means the central axis of the cylindrical part; and when the forged part has a plate-like part, the central part means the central plane that is at an equal distance from both surfaces. A "distance from the surface to the central part" means a vertical distance from each part of the surface to the central part. For example, when the forged part has a spherical part or a cylindrical part, the distance from the surface to the central part means a radius of the spherical part or the cylindrical part, and when the forged part has a plate-like part, the distance from the surface to the central part means half the thickness of the plate.

A control of the fraction of martensitic structure fm (x) in the large forged steel part of

65 high strength as described above makes it possible to reduce the generation of all internal effort from the surface to the center. As a result, the balance between resistance, toughness and

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Fatigue resistance can be further improved.

When the large strength steel forged piece includes a region of depth in which a fraction of the martensitic structure is smaller than the range described above, the tensile stress may remain in a region near the surface, in particular about x = 0.2. This residual tensile stress results in a reduction in fatigue resistance. The large strength steel forged part is preferably used for a crankshaft and the like. A bending stress is applied repeatedly to a chamfer part of a crankshaft and, therefore, high fatigue resistance is required, in particular, for the surface of the chamfer part. The crankshaft is finished by machine processing after heat treatment, so that the surface layer is ground. A reduction in the residual tensile stress in the range of a certain depth with respect to the surface (approximately 0 ≤ x ≤ 0.3) can provide a higher fatigue resistance when the large forged steel piece of high dimensions Resistance is used for the crankshaft and the like. On the other hand, a fraction of the martensitic structure that includes a region of depth that exceeds the interval

15 which has been described above can lead to an increase in the internal transformation effort caused by tempering and, therefore, temper cracks may tend to be generated.

In the large strength steel forged part, it is preferable that the fraction of the martensitic structure fm (x) (%) be

fm (x) = 100, in which 0≤x≤0.1; 104 -40x5 ≤ fm (x) ≤ 100, in which 0.1 <x ≤ 0.15; 122-160x ≤ fm (x) ≤ 100, in which 0.15 <x ≤ 0.2; 150-300x ≤fm (x) ≤100, in which 0.2 <x≤0.3;

25 105-150x≤fm (x) ≤112-40x, in which 0.3 <x≤0.35; 105-150x ≤fm (x) ≤105-20x, in which 0.35 <x≤ 0.5; 80 -100x≤fm (x) ≤170-150x, in which 0.5 <x≤ 0.8; y 0 ≤fm (x) ≤130-100x, in which 0.8 <x≤ 1,

in order to further reduce the generation of the entire internal effort from the surface to the center (the interval (b) illustrated in Figure 1).

Adjusting the fraction of the martensitic structure to the lower limit value described above or more can further reduce the generation of a tensile stress near the surface. On the other hand, the

Adjusting the fraction of martensitic structure to the upper limit value described above or less can reduce internal tensile stress and, therefore, the risk of tempering cracks can be further reduced.

In order to further improve the operation and the effect described above (a reduction in the generation of all internal effort from the surface to the center), it is preferable that d (fm (x)) / dx≤0, in which 0≤ x≤ 1.

<Performance, application, and the like>

The large strength steel forged piece, which has the compositions and structure described above, has both excellent strength and toughness and has high fatigue resistance. This resistance (tensile strength) is preferably 1,050 MPa or more, and more preferably 1,080 MPa or more. Tensile strength means a value that is measured in accordance with JIS-Z2241 (1998).

The large strength steel forged piece has excellent strength, toughness and fatigue resistance as described above and, therefore, can preferably be used for large crankshafts and intermediate shafts. that are used for boats or power generators. In particular, in order to achieve an improvement of the output power and the formation of

55 smaller engines of diesel engines for ships or diesel engines for power generation on land, for example, additional fatigue resistance and tensile strength are required (for example, a tensile strength of 950 MPa or more) for large crankshafts. The large strength steel forged part can sufficiently meet these requirements.

<Manufacturing method>

A method for manufacturing the large strength steel forging is not particularly limited. The large strength steel forged piece can be obtained by forging and heat treatment of the adjusted steel in the compositions described above. Hereinafter, an example of the method for manufacturing the large strength steel forged piece which is a solid type crankshaft having a diameter will be described

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150 mm or more.

First, a steel is melted in which some previously determined component compositions that have been described above are adjusted by using an electric oven, a high induction oven

5 frequency, a steel converter, and the like. Following the above, impurities (sulfur, oxygen, and the like) are removed (reduced) by vacuum refining and the like. After the impurities have been removed, an ingot is formed from this steel by casting. As a method of casting, ingot casting is mainly used. Continuous casting can also be used when a relatively small piece of steel is formed.

Subsequently, a round bar material is forged before forming a crankshaft. In order to forge the steel within the excellent deformation property range of the steel, the temperature in this operation is preferably 1,150 ° C or more, and more preferably 1,200 ° C or more. A lower heating temperature results in an increase in strain stress and a reduction in manufacturing efficiency. A

The heating period is preferably 3 hours or more. This warm-up period is essential to match the temperatures between the surface and the inside of the ingot. In general, this heating period is proportional to the square of a diameter of a processed object. For example, it is determined that the heating period is 3 hours or more at the time of manufacturing the large crankshaft that has been described above.

After the round bar material has been manufactured by forging, the round bar material is forged in the form of a solid crankshaft. This forging is preferably performed by a continuous grain orientation (CGF) method of forging. The CGF forging method is a method in which an ingot is forged and processed in such a way that an axis center of the ingot becomes a center of the axis part

25 of the solid-type crankshaft and the ingot is forged in an integrated manner and processed in such a way that a part that tends to deteriorate the properties by a central line segregation is included in all of a part of the crankshaft shaft center solid type. Examples of the CGF forging method described above may include a RR forging method and a TR forging method. These methods are preferable because the high purity parts occupy the surface side of the solid type crankshaft and, therefore, the solid type crankshaft tends to be obtained which has excellent strength and fatigue property.

Hereinafter, the forging method will be described using the RR forging method as an example.

35 In the RR forging, the forged material obtained is heated at 1,150 ° C or more for 3 hours or more to heat each crankshaft bend. As a specific procedure, first, the round bar material that is obtained by the procedure described above is processed by a machine to form a material for a RR slab. Following the above, pin shafts, a pair of block parts and stump shafts, which are corresponding to the formation of a cylinder, are partially heated and a vertical force of a press becomes a force in the lateral direction by means of a wedge mechanism and, thus, a cylinder is forged by applying, simultaneously, a compression force in the lateral direction and an eccentric force to the RR material. By repeating this operation in the number of cylinders required, a single crankshaft is formed. In order to forge the steel within the excellent deformation property range of the steel, the temperature in this operation is preferably of

1,150 ° C or more, and more preferably 1,200 ° C or more. A lower heating temperature results in an increase in strain stress and a reduction in manufacturing efficiency. This warm-up period is essential to match the temperatures between the surface and the inside of the ingot. The heating period is determined to be 3 hours or more at the time of manufacturing the large crankshaft described above.

After RR forging, the retained austenite (γ) that is contained in the forge can decompose before the thermal refining treatment (tempering and tempering treatment) is performed. In order to form a finer structure, a phase transformation is used during the thermal refining treatment. At this time, the retained γ continues to exist during heating in the thermal refining treatment until

55 the temperature exceeds the transformation point of Ac1, when the retained γ that exists after forging is stable. This retained γ is the γ that remains during the forging and heat treatment and originally has the same orientation in the anterior austenite after the forging. Consequently, an interface between the retained γ does not form a grain limit, when the transformation of γ advances and the retained γ come into contact with each other. As a result, a grain size of γ after completion of the transformation of γ is a coarse grain size that is similar to the original grain size of γ. Consequently, a decomposition treatment of the retained γ is performed.

An example of the method for decomposing retained austenite includes a method for a maturation treatment in which the forged piece is kept under heating at the transformation point of Ac1 or less (600 ° C to 680 ° C). In this treatment, the heating time maintained is 5 hours or more and preferably 20 hours or more. Through such maturation treatment, retained austenite decomposes and

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It can be reduced to 1% by volume or less. For another example of the method for decomposing retained austenite, a sub-zero treatment can be used.

Subsequently, the thermal refining treatment (tempering and tempering treatment) is performed. First,

5 the forged part is slowly heated (a heating rate of 30 ° C / hour at 70 ° C / hour) to the transformation point of Ac3 or more (840 ° C to 940 ° C) and is maintained for a certain period (3 hours at 9 hours) before tempering. From the point of view of a reduction in the formation of a coarse anterior austenite grain, the quenching is preferably performed at a relatively low temperature (840 ° C to 940 ° C) which is the transformation point of Ac3 or more as it has been described above. A large product, which generates a temperature difference between an outer part and an inner part of the material during heating, is slowly heated to a heating temperature before tempering and is maintained for a certain period in order to equalize the temperature between the surface and the inner part of the steel. The maintenance time required depends on a diameter of a steel product. The larger the dimensions of the product, the longer the maintenance time will become. Consequently, the following tempering is

15 performs after sufficient maintenance time has been taken and the temperature is equal to the inside of the steel product.

Tempering is performed using a cooling agent such as an oil and polymer solution and the martensitic structure or the structure made of martensite and bainite is obtained. In order to obtain such a structure, tempering is performed at an average cooling rate of 3 ° C / minute or more. This cooling rate is more preferably 5 ° C / minute or more and 100 ° C / minute or less, and more preferably 10 ° C / minute

or more and of 60 ° C / minute or less.

In a forged piece of large steel, water quenching has a risk of generating cracks and, for

Therefore, oil quenching and polymer quenching are usually used for tempering a large crankshaft. The cooling rate at the time of tempering depends on the size of a piece of steel forged. An average cooling rate between 800 ° C and 500 ° C of a crankshaft having a diameter of approximately 500 mm is approximately 20 ° C / minute in the tempering with oil and approximately 50 ° C / minute in the tempering with polymer. A crankshaft that has a larger diameter (for example, 1,000 mm) requires a much slower cooling rate.

In order to satisfy both the strength and the toughness of the large strength steel forging, a control of the martensitic structure or the mixed structure of martensite and bainite is required. As a result of a study of the conditions to achieve such a structure even when a rate of

35 quench cooling to apply to a large crankshaft having a diameter of 150 mm or more is approximately 20 ° C / minute (in the case of oil quenching), the inventors of the present invention have managed to determine the compositions of chemical components as described above.

In tempering, the steel forge is cooled to 200 ° C or less. By cooling to 200 ° C or less as described above, the transformation can be completely completed. Insufficient cooling results in the retention of retained austenite not transformed, and this results in a fluctuation in the properties. After quenching, preferably the steel forged piece is subjected to tempering.

45 In annealing, the steel forged piece is slowly heated (a heating rate 30 ºC / hour at 70 ºC / hour) to the previously determined temperature (550 ºC to 620 ºC) and is maintained for a certain period (5 hours at 20 hours). In order to adjust the balance between resistance and toughness and to remove the internal stress (residual stress) during tempering, this annealing is performed at 550 ° C or more. However, too high annealing temperature results in softening caused by the formation of coarse carbides and the recovery of dislocation structures and, therefore, sufficient strength cannot be ensured. Consequently, the annealing temperature is determined to be 620 ° C or less.

The large strength steel forged part can be obtained from the forged part treated with a thermal refining as described above when further processing in

55 finishing machine that includes the grinding of at least a part of the surface, if necessary. The method for manufacturing the large strength steel forging of the present invention is not limited to the manufacturing method described above. The large strength steel forged part can also be manufactured by, for example, free forging. The forged piece of high-strength large steel, other than large crankshafts, can be obtained by a similar manufacturing method.

[Examples]

Hereinafter, the present invention will be described in further detail with reference to the examples. However, the present invention is not limited to these examples.

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[Measurement methods]

Each measurement in the examples is performed using the following methods.

5 1. Grain size (μm) of anterior austenite (γ)

In accordance with ASTM (E112-96), the grain size number was determined by the comparative method with the following procedure and, following the above, a grain size (a nominal grain size) of a grain of austenite anterior.

(one)

A corresponding grain size number N is determined by comparing a 100-magnification photograph in observation under an optical microscope and the conventional diagram.

(2)

The grain size number N is determined according to the number of grains n in a square of 25

mm side (625 mm2: microscopic field) in observation under a 100 magnification optical microscope, and the following formula (1) can be established.

image 1

(3) A grain size d (μm) can be calculated using the following formula (2) because, in 62,500 μm2, there are possibly n grains

image2

In the present case, the grain size of the anterior austenite was measured in 10 places (10 microscopic fields), 25 and each average grain size was calculated.

2. Martensitic block size (μm)

Reference is made to a group of slats, which is a substructure of martensite, which has almost the same orientation as a block (a martensitic block). A malorientation between blocks is 15º or more (high angle grain limit). Consequently, by the following method, a martensitic block size (μm) was calculated from an inverse pole figure map that is obtained by the FESEM-EBSP method.

(1) A reverse pole figure map is obtained by measuring EBSP at intervals of 0.3 μm in a microscopic field of 120 μm  120 μm.

(2)

The regions that are surrounded in a malorientation between adjacent crystals of 15 ° or more are specified from the inverse pole figure map, and each area thereof is determined.

(3)

A block size is determined by calculating the square roots of each area (√ (area)) described above.

The martensite block size was measured in 10 places (10 microscopic fields), and a maximum size and a minimum size in each microscopic field were determined, and then each average size (an average maximum size and an average size was determined) minimum).

45 3. Tensile property (0.2% elasticity limit: YS (MPa), Tensile strength: TS (MPa), Elongation: (%), and Reduction in area: RA (%))

These properties were measured according to JIS-Z2241 (1998). The shape of the specimen was the shape of specimen No. 14 illustrated in JIS-Z2201 (1998), and its resignation was adjusted to Φ6  G. L. 30 mm.

4. Charpy absorbed energy: vE (J)

Charpy's absorbed energy was measured in accordance with JIS-Z2242 (2005). The shape of the specimen was the shape of a 2 mm V-shaped, which is illustrated in JIS-Z2242 (2005). Three test pieces were tested

55 in each sample and the average value was determined as the energy absorbed.

5. Fatigue resistance FS of rotation bending (MPa) and durability limit

A rotation flexural fatigue resistance test was performed according to the following test method, and fatigue resistance was evaluated.

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Test tube: 30 mm smooth test pieces of Φ10 mm  GL (5 pieces) Test method: Flexion by rotation (Stress ratio = -1, Rotation speed = 3,000 at 3,600 rpm) Evaluation method: Stepped method (Step increment 20 MPa)

5 Fatigue resistance [FS] = Braking effort (MPa) -Standard increment (MPa) Durability limit = Fatigue resistance [FS] / Tensile strength [TS]

[Examples 1 to 13 and comparative examples 1 to 14]

10 The types of steel a to q which have compositions shown in Table 1 were produced through fusion. In the present case, "-" in Table 1 represents below the measurable limits. A seventy ton bullion of the type of steel was cast by melting in an electric oven and by refining in a spoon. Ingots of forty kilograms of each type of steel b to q were cast by melting in a high frequency induction furnace.

fifteen

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[Table 1]

Steel type

Composition (% by mass)

C

Yes Mn Neither Cr Mo V To the S OR P Cu Sn N

to

0.35 0.09 0.35 3.04 1.61 0.45 0.10 0.017 0.001 0.0012 0.008 0.03 0.001 0.0049

b

0.35 0.08 0.34 2.74 1.58 0.45 0.10 0.011 0.006 - 0.007 0.01 0.006 0.0054

C

0.35 0.08 0.34 3.23 1.57 0.46 0.10 0.016 0.007 - 0.007 0.01 0.006 0.0050

d

0.32 0.14 0.47 2.78 1.62 0.45 0.10 0.012 0.007 - 0.009 0.02 0.007 0.0048

and

0.32 0.16 0.46 3.22 1.59 0.48 0.10 0.008 0.007 - 0.010 0.01 0.007 0.0055

F

0.23 0.07 0.28 3.04 1.56 0.34 0.11 0.003 0.004 0.0013 0.003 0.01 0.001 0.0040

g

0.32 0.24 0.43 3.29 1.62 0.45 0.10 0.012 0.007 - 0.010 0.02 0.006 0.0051

h

0.34 0.21 1.00 3.01 1.61 0.50 0.10 0.037 0.007 - 0.010 0.01 0.007 0.0053

i

0.345 0.05 0.32 1.61 1.51 0.50 0.15 0.044 0.005 - 0.004 0.01 0.008 0.005

j

0.32 0.07 0.32 3.72 1.66 0.38 0.10 - 0.008 - 0.007 - - -

k

0.35 0.25 0.95 3.04 0.21 0.50 0.11 0.025 0.006 - 0.007 0.01 0.007 0.0063

l

0.38 0.21 1.04 3.05 3.01 0.52 0.14 0.008 0.006 - 0.005 0.01 0.006 0.0072

m

0.35 0.26 1.01 2.98 1.62 0.50 <0.01 0.020 0.007 - 0.010 0.03 0.008 0.0064

n

0.35 0.19 0.28 1.61 1.61 0.51 0.15 0.027 0.005 - 0.004 0.01 0.008 0.0050

or

0.33 0.27 0.64 1.63 1.63 0.25 0.15 0.004 0.002 - 0.007 0.03 0.001 0.0058

p

0.40 0.24 0.97 0.41 1.97 0.27 0.08 0.003 0.003 0.0009 0.006 0.03 0.007 0.0054

that

0.33 0.30 0.43 0.17 3.01 0.47 0.078 0.028 0.0005 - 0.007 0.03 0.001 0.0064

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The ingot (70 tons) of the type of steel was hot forged to form round-shaped forged pieces with a diameter of 500 mm. The types of steel b to q were forged to a size of 90 mm  90 mm  600 mm and, following the above, cooled in the atmosphere. Each forged part of the types of steel a a cooled to room temperature and, following the above, a small piece with a size of 20 mm 

5 20 mm  180 mm was cut from each forged piece. Each small piece was heat treated under the conditions shown in Table 2 that simulated a crankshaft forging process, and each of the treated small pieces was cooled in the oven to prepare specimens.

Following the above, the thermal refining treatment (tempering and tempering treatment) was performed to

10 each test piece in order to ensure the resistance of the crankshaft. The tempering treatment was carried out under conditions of tempering that simulated a heating and cooling rate of a crankshaft with a diameter of 500 mm. Specifically, the tempering treatment was carried out in a way that, using a small simulation furnace, the temperature rose to 870 ° C to 940 ° C at a heating rate of 40 ° C / hour and this temperature was maintained for 3 hours to 8 hours and, following the above, the specimens were cooled in

15 such that an average cooling rate in a temperature range of 870 ° C to 500 ° C was 20 ° C / minute at 50 ° C / minute. In the tempering treatment, the specimens were kept at a temperature of 560 ° C to 610 ° C for 13 hours and cooled in the oven (the treatment conditions for each specimen are shown in Table 2). The specimens (steel forges) of examples 1 to 13 and comparative examples 1 to 14 were obtained by the methods as described above.

twenty

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[Table 2]

Steel type

Forged heating conditions Heat treatment conditions

Tempering conditions

Annealing Conditions

Temperature (ºC)

Maintenance time (h)  Temperature (ºC) Maintenance time (h) Cooling rate (ºC / min)  Temperature (ºC) Maintenance time (h)

Example 1

to 1280 9 870 3 twenty 560 13

example 2

to 1280 9 870 3 twenty 580 13

example 3

to 1280 3 870 8 fifty 590 13

example 4

to 1310 3 870 8 fifty 580 13

example 5

to 1280 9 870 3 fifty 580 13

example 6

to 1280 9 870 3 fifty 600 13

example 7

to 1280 9 870 3 fifty 610 13

example 8

to 1280 9 920 3 fifty 590 13

example 9

to 1280 9 940 3 fifty 580 13

example 10

b 1280 3 870 3 twenty 580 13

example 11

C 1280 3 870 3 twenty 580 13

example 12

d 1280 3 870 3 twenty 580 13

example 13

and 1280 3 870 3 twenty 580 13

comparative example 1

F 1230 3 870 3 twenty 590 13

comparative example 2

g 1230 3 870 3 twenty 580 13

comparative example 3

h 1280 3 870 3 twenty 580 13

comparative example 4

i 1280 3 870 3 twenty 580 13

comparative example 5

J 1280 3 870 3 twenty 580 13

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Steel type

Forged heating conditions Heat treatment conditions

Tempering conditions

Annealing Conditions

Temperature (ºC)

Maintenance time (h)  Temperature (ºC) Maintenance time (h) Cooling rate (ºC / min)  Temperature (ºC) Maintenance time (h)

comparative example 6

k 1280 3 870 3 twenty 580 13

comparative example 7

l 1280 3 870 3 twenty 580 13

comparative example 8

m 1280 3 870 3 twenty 580 13

comparative example 9

n 1280 3 870 3 fifty 600 13

comparative example 10

or 1200 one 870 3 twenty 580 13

comparative example 11

or 1200 one 870 3 twenty 610 13

comparative example 12

p 1200 one 870 3 twenty 580 13

comparative example 13

p 1200 one 870 3 twenty 610 13

comparative example 14

that 1200 one 870 3 twenty 590 13

comparative example 15

to 1280 9 960 3 twenty 610 13

comparative example 16

to 1280 9 1050 3 twenty 612 13

comparative example 17

b 1230 3 960 3 twenty 580 13

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Each state of the microstructures (martensitic structures (M) or bainitic structures (B)) of the specimens was observed in example 1 to 13 and comparative example 1 to 14. In addition, the grain size of the anterior austenite, the size of martensitic block, tensile properties, Charpy absorbed energy, and fatigue properties (FS fatigue resistance and durability limit) were evaluated using the measurement method described above. The measurement result is shown in table 3.

In the present case, "-" in Table 3 represents that the property was not measured.

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[Table 3]

Structure

Property Comprehensive evaluation

Microstructure e

Previous γ grain size Minimum block size (μm) Maximum block size (μm) Related figure Structure Evaluation traction Charpy Fatigue

Minimum (μm)

Maximum (μm) YS (MPa) TS (MPa) HE(%) RA (%) Evaluation vE (J) EV evaluation FS (MPa) Limit of durability

Example 1

M 26 53 - - - 〇 1084 1202 16.0 61.3 〇 71 〇 - - 〇

example 2

M 26 53 - - - 〇 1014 1130 163 61.6 〇 123 〇 524 0.464 〇

example 3

M 19 37 - - - 〇 1052 1153 156 604 〇 108 〇 - - 〇

example 4

M 26 53 05 105 figure 5 〇 1103 1195 130 51.1 〇 85 〇 562 0470 〇

example 5

M 26 44 - - - 〇 1097 1206 172 6369 〇 102 〇 - - 〇

example 6

M 26 44 - - 〇 1047 1153 17.4 65.6 〇 111 〇 - - 〇

example 7

M 26 44 - - - 〇 946 1058 157 67.3 〇 125 〇 563 0.532 〇

example 8

M 37 63 - - - 〇 1000 1102 159 584 〇 126 〇 - - 〇

example 9

M 44 63 - - - 〇 1019 1101 171 65.5 〇 132 〇 - - 〇

example 10

B + M 44 63 - - - 〇 1005 1143 163 621 〇 59 〇 - - 〇

example 11

M 53 63 0.5 13.5 figure 6 〇 1073 1191 170 60.8 〇 64 〇 - 〇

example 12

B + M 37 62 - - - 〇 931 1064 185 63.1 〇 75 〇 - - 〇

example 13

M 53 63 - - - 〇 938 1066 17.3 615 〇 76 〇 - - 〇

comparative example1

B 26 53 - - - 〇 891 1015 165 612  113 〇 - - insufficient resistance 

comparative example2

M 44 125 - - -  976 1118 165 61.0 〇 29  - - insufficient tenacity 

comparative example3

M 105 250 05 24.5 figure 7  943 1072 14.1 48.4 〇 twenty  - - insufficient tenacity 

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Structure

Property Comprehensive evaluation

Microstructure

Previous γ grain size Minimum block size (μm) Maximum block size (μm) Related figure Structure evaluation traction Charpy Fatigue

Minimum (μm)

Maximum (μm) YS (MPa) TS (MPa) HE(%) RA (%) Evaluation vE (J) EV evaluation FS (MPa) Limit of durability

comparative example4

M 22 44 - - - 〇 1047 1172 136 554 〇 29  - - insufficient tenacity 

comparative example5

M 44 88 - - -  insufficient grain size 

comparative example6

B 26 63 - - - 〇 939 1067 173 58.7 〇 2. 3  - - insufficient tenacity 

comparative example7

M 149 250 05 20.5 figure 8  1000 1149 13.3 48.7 〇 42  - - insufficient tenacity 

comparative example8

M 31 74 - - -  887 1029 184 636  77 〇 - - insufficient resistance 

comparative example9

B + M 19 149 05 175 figure 9  1034 1136 16.1 59.4 〇 42  - - insufficient tenacity 

comparative example10

B 26 63 - - - 〇 966 1103 17.0 56.9 〇 3. 4  - - insufficient tenacity 

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Structure

Property Comprehensive evaluation

Microstructure

Previous γ grain size Minimum block size (μm) Maximum block size (μm) Related figure Structure Evaluation traction Charpy Fatigue

Minimum (μm)

Maximum (μm) YS (MPa) TS (MPa) HE(%) RA (%) Evaluation vE (J) EV evaluation FS (MPa) Limit of durability

comparative example11

B 26 63 - - - 〇 833 963 182 65.1  56 〇 458 0476 insufficient resistance 

comparative example12

B 26 37 - - - 〇 919 1078 16.3 603 〇 19  - - insufficient tenacity 

comparative example13

B 26 37 - - - 〇 805 964 182 652  64 〇 465 0.482 insufficient resistance 

comparative example14

B 26 44 - - - 〇 857 1012 16.1 63.5  Four. Five  - - insufficient strength and durability 

comparative example15

M 63 177 - - -  980 1095 12.1 43.8 〇 25  - - insufficient tenacity 

comparative example16

M 88 250 - - -  987 1143 12.6 33.4 〇 17  - - insufficient tenacity 

comparative example17

B + M 63 105 - - -  1093 1201 152 48 〇 22  - - insufficient tenacity 

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Figure 2 illustrates a relationship between a maximum size of the martensitic block and the energy absorbed from Charpy that is measured by using the specimens of the examples and the comparative examples. Figure 3 illustrates a relationship between the tensile strength and the absorbed energy of Charpy that is measured by using the specimens of the examples and the comparative examples. Figure 4 illustrates a relationship between resistance to

5 tensile and fatigue resistance that is measured by using the specimens of the examples and comparative examples.

Inverse pole figure maps of the specimens of example 4, example 11, comparative example 3, comparative example 7 and comparative example 9, respectively, are illustrated in Figure 5, Figure 6, Figure 7, figure

10 8 and figure 9.

[Analysis]

Examples 1 to 13, in which both the compositions and the manufacturing conditions of the steels

15 meeting the requirements of the present invention, they provided forged steel parts with desired properties. Comparative examples 1 to 14, in which the compositions of the steels did not meet the requirements of the present invention, did not provide the forged steel parts with desired properties. Comparative examples 15 to 17, in which, although the compositions of the steels met the requirements of the present invention, the grain size of the previous austenite did not satisfy

20 the requirements of the present invention due to an inadequate tempering temperature did not provide desired forged steel parts.

As illustrated in Figure 2, it is found that a maximum martensite block size of 15 μm or less provides excellent toughness (energy absorbed from Charpy). As illustrated in Figure 3, it is found that the high strength steel for the forged piece of large steel has excellent toughness (a property against impact) even when the high strength steel for the forged part of Large steel has a higher resistance (a resistance of 1,050 MPa or more) than the resistance of conventional steel. In general, a material that has a higher strength has a lower toughness. However, a large strength steel forged piece can be provided having

30 an excellent strength and toughness in a balanced way (for example, a resistance of 1,050 MPa or more) by optimizing chemical components and metal structures.

Figure 4 illustrates a relationship between tensile strength and fatigue resistance. The fatigue resistance of the steel forging of the present invention increases by approximately 10% or more compared

35 with that of conventional steel. The durability limit (= fatigue resistance / tensile strength) is equal to that of conventional steel, and the proportional relationship between tensile strength and fatigue resistance is maintained. In other words, an increase in notch sensitivity associated with a higher resistance is not observed.

40 [Sample analysis]

A heat transfer and thermal stress analysis for the transformation stress in a hardened steel was performed using the FORGE 2009 general purpose software. The specific conditions are as follows. Assuming a round bar shape, modeling was performed using a two-dimensional axisimetric model. In one axis direction, modeling was performed in only one unit of length, and it was determined that the upper surface and the lower surface were an adiabatic state. The heat transfer and thermal stress analysis was performed under conditions where the initial temperature was set uniformly at 870 ° C and the temperature was cooled to approximately room temperature. Each material property used in the analysis is illustrated in Figures 10 to 12. The following analysis conditions A and B are

50 used in the analysis.

(Analysis conditions A)

As illustrated by a dashed line in Figure 13A, a structure fraction in the bar steel

55 round in which 100% of the martensitic structure exists in a relationship of a depth with respect to a distance from the surface to the central part of 0 to 0.35 and 95% of the martensitic structure and 5% of Bainitic structure exist in the central part.

(Analysis conditions B)

60 As illustrated by a continuous line in Figure 13A, a fraction of structure in the round bar steel in which 100% of the martensitic structure exists in a relationship of a depth with respect to a distance from the surface to the central part of 0 to 0.1 and 100% of the bainitic structure exists in the central part.

65

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The analysis results in the analysis conditions A and B are illustrated in Figure 13B. As illustrated in this figure 13B, the analysis conditions A result in a larger internal transformation effort. The analysis conditions B give rise to a residual stress in a position in a depth ratio of approximately 0.2. A fraction of the structure between the analysis conditions A and B enables

5 control all internal effort from the surface to the center with low effort.

[Reference Example 1]

A seventy ton ingot of the type of steel to which it has some component compositions shown

10 in table 1 described above was cast by melting in an electric oven and by refining in a spoon. The ingot (70 tons) of the type of steel was hot forged by means of a free forging press to form round-shaped forged pieces with a diameter of 500 mm (a radius of 250 mm) and then Earlier, the round bar cooled in the atmosphere. The round bar forges were treated with a heat of 1,280 ° C for 3 hours, which simulated the heating before the forging of

15 RR. Before tempering and tempering treatment, a maturation treatment was performed for the round bar forged parts (being maintained at a temperature of 650 ° C for 20 hours), and the round bar shaped forged parts were cooled to room temperature . The tempering conditions were as follows. The round bar forges were heated at a heating rate of 40 ° C / hour and maintained at 870 ° C for 8 hours and, following the above, a polymer quench was performed. Following what

20 above, the round bar forged parts were maintained at 580 ° C for 15 hours as the tempering treatment and cooled in the oven to 350 ° C and, following the above, the round bar forged parts were air cooled to the ambient temperature to obtain a large strength steel forged piece of reference example 1.

25 The forged steel piece of high strength obtained was rectified so that a part of each depth in a direction from the surface to the center (25 mm, 40 mm, 70 mm, 100 mm, 130 mm, 160 mm, 190 mm, 220 mm and 250 mm) forms a surface. Brinell HB hardness, tensile strength (MPa), structure fraction (%) and grain size of anterior austenite (γ) (μm) were measured at each depth. The tensile strength of the large strength steel forged part in

30 reference example 1 is a converted value that is calculated from the Brinell HB hardness measured according to the hardness conversion table (SAE J 417). The methods for measuring and calculating Brinell hardness and a fraction of a martensitic structure are as follows. The measurement result is shown in table 4 together with heating treatment conditions at the time of manufacture.

35 Brinell hardness

Brinell hardness was measured according to JIS-Z2243 (2008).

Fraction of martensitic structure (%)

The fraction of martensitic structure was calculated from the hardness measurement result (Brinell hardness: HB) using the following formula that is obtained from the mixture rule. Structures other than the martensitic structure were determined as the bainitic structure, and a fraction of bainitic structure (%) was also calculated.

Four. Five

HB = HBM  fm (x) / 100 + HBB  (1 -fm (x) / 100)

fm (x): Fraction of martensitic structure (%)

HBM: Brinell hardness of martensite

50 (Actual measured value of the complete martensite part: 368) HBB: Brinell hardness of the bainite (Actual measured value of the complete bainite part: 352)

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[Table 4]

image3

A relationship between each depth and the Brinell hardness of the large strength steel forged part in reference example 1 is illustrated in Figure 14A and a relationship between each depth and a fraction of a martensitic structure is illustrated in Figure 14B

A large strength steel forged part of the present invention has both excellent strength and toughness, and has high fatigue resistance. Consequently, the large strength steel forged piece can preferably be used for large crankshafts

10 dimensions and intermediate trees used for boats or power generators.

Claims (2)

E13002083 10-16-2014 1. A large strength steel forged piece comprising: 5 compositions comprising basic compositions comprising: C: 0.31% by mass or more and 0.5% by mass or less, If: 0.02% by mass or more and 0.2% by mass or less, 10 Mn: 0.1% by mass or more and 0.6% by mass or less, Ni: 2.6% by mass or more and 3.4% by mass or less, Cr: 0.8% by mass or more and 1.9% by mass or less, Mo: 0.25% by mass or more and 0.8% by mass or less, V: 0.05% by mass or more and 0.2% by mass or less, and 15 Al: 0.005% by mass or more and 0.1% by mass or less, and a remainder comprising Fe and unavoidable impurities; where an S content such as the inevitable impurity is 0.008% by mass or less; and where the large strength steel forged piece consists of a martensitic structure or a mixed structure of martensite and bainite; 20 a grain size of the anterior austenite is 19 μm or more and 70 μm or less; and a maximum block size of martensite is 15 μm or less and a minimum block size of martensite is 0.5 μm or more. 2. The large strength steel forged part according to claim 1, wherein 25 a fraction of martensitic structure fm (x) (%) in which a relation of a depth with respect to a distance from a surface to a center is defined as x (0 ≤ x ≤ 1) is fm (x) = 100, in which 0≤x≤0.1; 104-40x≤fm (x) ≤100, in which 0.1 <x≤0.15; 30 122-160x≤fm (x) ≤100, in which 0.15 <x≤0.2; 230-700x ≤fm (x) ≤100, in which 0.2 <x≤0.3; 110-300x ≤fm (x) ≤112-40x, in which 0.3 <x≤ 0.35; (22-20x) / 3≤fm (x) ≤ 105-20x, in which 0.35 <x≤0.5; (32-40x) / 3≤fm (x) ≤95, in which 0.5 <x≤0.8; y 0 ≤fm (x) ≤95, in which 0.8 <x≤ 1. 35 2. 3