JP2018162514A - Steel for steel forging, forging crank throw for assemble crankshaft, and forging journal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for steel forging that exhibits excellent machinability and has high yield stress and high fatigue strength, and a forging crank throw for an assemble crankshaft, and a forging journal.SOLUTION: A steel for steel forging satisfies a predetermined chemical component composition; has the total area ratio of proeutectoid ferrite and perlite of 90% or more relative to the entire metallographic structure; and has V carbides with an equivalent circle diameter of 50 nm or less in a number density of 55 or more and 500 or less per 1 μmof the proeutectoid ferrite; has a yield stress of 420 MPa or more and a fatigue strength of 330 MPa or more.SELECTED DRAWING: None

Description

  The present invention relates to steel for forgings useful as a constituent material of an assembled crankshaft.

In recent years, with the growing awareness of preserving the global environment, diesel engines such as two-stroke engines for large ships have been required to achieve high output and compact size and to reduce CO ₂ emissions. The crankshaft that converts the vertical movement of the piston into the rotational movement via the connecting rod requires a high fatigue strength especially for the fillet part because bending stress repeatedly acts on the fillet part of the crank throw.

  In addition, in an assembly-type crankshaft that is manufactured by joining the journal, which is the central shaft of rotation, and the crank throw of the eccentric part by shrink fitting, the slip at the shrink fitting part is in operation as the engine output increases. May occur. In order to prevent this slip, it is necessary to improve the shrink-fit surface pressure at which the crank throw at the time of shrink-fitting grips the journal. To improve this surface pressure, the yield stress of the material used for the crank throw is improved. Is known to be effective.

  Generally, in order to improve the yield stress and fatigue strength of a steel material, it is effective to perform quenching so that the metal structure has a single-phase structure with high tensile strength such as martensite or bainite. However, it is known that steel exhibiting a martensite or bainite structure has poor machinability compared to ferritic / pearlite steel.

  Since crank throws and journals, which are parts of an assembly-type crankshaft, are formed into these shapes and then finished into a final shape by cutting, excellent machinability is also required. At present, it is difficult to obtain forging steel or forging steel having both high fatigue strength and excellent machinability.

  As steels for forged steel products having excellent strength and machinability, for example, techniques such as Patent Documents 1 and 2 have been proposed. In these techniques, the chemical composition is appropriately adjusted, and the metal structure is mainly composed of a bainite structure, a martensite structure, or a combination structure thereof, and the balance is a pearlite structure, a ferrite structure, or a combination structure thereof. It has excellent strength and machinability.

  In these techniques, since the metal structure is mainly composed of a structure such as bainite or martensite, it is apparent that the machinability is inferior compared with carbon steel having a ferrite / pearlite structure.

  On the other hand, as a steel material that achieves both high strength and machinability, a technique such as Patent Document 3 has also been proposed. In this technique, fine precipitates are present in the form of a dotted line in the ferrite at an average line interval of 15 nm or less, thereby strengthening soft ferrite and achieving both strength and machinability in the ferrite / pearlite structure.

  However, this technology targets relatively small steel materials such as automobile parts, and requires a cooling rate of more than 10 ° C./second. Large forged steel products such as assembled crankshafts have a large mass effect, and it is impossible to perform cooling at such a cooling rate even at the center of the product. Therefore, it is virtually impossible to apply such technology to large forged steel products such as assembled crankshafts.

  For this reason, in large forged steel products such as assembled crankshafts, the technology that achieves both excellent machinability, high yield stress, high fatigue strength, etc. by finely dispersing carbide in ferrite. It has not been completed yet.

Japanese Patent Laying-Open No. 2015-117419 Japanese Patent Laying-Open No. 2015-190040 Japanese Patent No. 5035159

  The present invention has been made in view of the circumstances as described above, and its purpose is to provide excellent machinability, as well as steel for forgings that exhibits high yield stress and high fatigue strength, and an assembly die. It is to provide a forged steel crank throw for a crankshaft and a forged steel journal.

The steel for forgings of the present invention that was able to solve the above problems,
% By mass
C: 0.28% or more and 0.47% or less,
Si: more than 0% and 0.45% or less,
Mn: 0.90% or more and 1.50% or less,
S: more than 0% and 0.006% or less,
Cu: more than 0% and 0.30% or less,
Mo: more than 0% and 0.15% or less and V: 0.06% or more and 0.32% or less, respectively, and the balance is made of iron and inevitable impurities,
The total area ratio of pro-eutectoid ferrite and pearlite is 90% or more in the ratio of the entire metal structure,
In addition, V-based carbides having an equivalent circle diameter of 50 nm or less are present at a number density of 55 to 500 per 1 μm ² in the pro-eutectoid ferrite,
The yield stress is 420 MPa or more and the fatigue strength is 330 MPa or more.

  By forming the forged steel crank throw for the assembled crankshaft and the forged steel journal with the forged steel product according to the present invention as described above, the characteristics of these products become extremely good.

  The present invention is configured as described above, and is excellent by appropriately adjusting the chemical component composition and the metal structure and by appropriately adjusting the number density in pro-eutectoid ferrite in a predetermined size V-based carbide. As a result, a steel for forgings that exhibits high machinability and high yield stress and high fatigue strength has been realized. Such steel for forged products is extremely useful as a material for forged steel crank throws for assembled crankshafts and forged steel journals.

FIG. 1 is a graph showing the influence of the number density of V-based carbides on yield stress. FIG. 2 is a graph showing the effect of the number density of V-based carbides on fatigue strength.

  This inventor examined from various angles aiming at realization of the steel for forgings which exhibits the outstanding machinability, and shows high yield stress and high fatigue strength. In particular, the relationship between the form (size, number density) of precipitates in steel, the yield stress and the strength, and the relationship between the metal structure and the machinability of the steel material were examined.

  As a result, high yield stress and high fatigue strength can be realized by appropriately adjusting the number density in pro-eutectoid ferrite in a predetermined size of V-based carbide, and excellent by adjusting the metal structure appropriately. As a result, the present invention was completed. First, the requirements defined for the steel for forgings of the present invention will be described.

[Total area ratio of proeutectoid ferrite and pearlite: 90% or more]
In order to exhibit excellent machinability, it is necessary that the total area ratio of pro-eutectoid ferrite and pearlite is 90% or more as a percentage of the entire metal structure. The total area ratio of pro-eutectoid ferrite and pearlite is preferably 95% or more, and more preferably 98% or more. The total area ratio of pro-eutectoid ferrite and pearlite may be 100%.

[Number density per 1 μm ^{2 in} pro-eutectoid ferrite of V-based carbide having an equivalent circle diameter of 50 nm or less: 55 or more and 500 or less]
In order to realize the excellent yield stress and fatigue strength of steel for forged steel, the number density of V-based carbides precipitated per 1 μm ² in the pro-eutectoid ferrite needs to be 55 pieces / μm ² or more. The number is preferably 100 / μm ² or more, and more preferably 150 / μm ² or more.

However, if the number density of the V-based carbide is excessive and exceeds 500 / μm ² , the toughness may be lowered. The upper limit of the number density of the V-based carbide is preferably 450 pieces / μm ² or less, and more preferably 400 pieces / μm ² or less.

  The V-type carbides targeted in the present invention are intended to include not only V but also carbides containing carbide-forming elements such as Cr and Mo. Moreover, the size of the target V-based carbide is “the equivalent circle diameter is 50 nm or less” because the V-based carbide having an equivalent circle diameter exceeding 50 nm does not contribute to the improvement of the strength of pro-eutectoid ferrite. It is.

  Note that the “equivalent circle diameter” is a value corresponding to the diameter of a circle with the same area, focusing on the size of the V-based carbide.

  The steel for forgings according to the present invention that satisfies the above requirements exhibits the yield stress and fatigue strength equivalent to the bainite structure and martensite structure. Specifically, the yield stress is 420 MPa or more, and the fatigue strength is 330 MPa or more. The “fatigue strength” is evaluated based on a fatigue limit (hereinafter referred to as “fatigue strength”) measured in accordance with JIS-Z2274: 1978. The lower limit of the fatigue strength is required to be 330 MPa or more, preferably 350 MPa or more. If the fatigue strength of steel for forgings is less than the above lower limit, the durability of these parts will be poor when this steel for forging is applied as a material for forged steel crank throws for forged crankshafts or forged steel journals. May be sufficient.

  It is necessary to adjust the chemical composition of the steel for forgings of the present invention appropriately. That is, C: 0.28% to 0.47%, Si: more than 0% to 0.45% or less, Mn: 0.90% to 1.50%, S: more than 0% to 0.006%, It is a steel for forgings containing Cu: more than 0% and 0.30% or less, Mo: more than 0% and 0.15% or less, and V: 0.06% or more and 0.32% or less. The reason for setting the ranges for these elements is as follows. In addition, the unit of content of each element in the following chemical component composition is “mass%”.

(C: 0.28% to 0.47%)
C is an element effective in securing the strength of the steel material. For that purpose, C is contained by 0.28% or more. However, when the amount of C becomes excessive, the toughness of the steel material is lowered. For these reasons, the C content is set to 0.47% or less. The lower limit of the amount of C is preferably 0.30% or more, and more preferably 0.32% or more. Moreover, it is preferable that the upper limit of C amount is 0.45% or less.

(Si: more than 0% and 0.45% or less)
Si is an element contributing to deoxidation and strength improvement of steel materials. In order to exhibit the effect effectively, Si is preferably contained in an amount of 0.1% or more, more preferably 0.15% or more. However, when the amount of Si is excessive, reverse V homogenization is remarkable and the toughness may be reduced. From such a viewpoint, the Si amount needs to be 0.45% or less. The upper limit of the Si amount is preferably 0.40% or less, and more preferably 0.35% or less.

(Mn: 0.90% to 1.50%)
Mn is an element that contributes to improving the hardenability and strength of the steel material. In order to exert such an effect, it is necessary to contain 0.90% or more of Mn. The lower limit of the amount of Mn is preferably 1.0% or more, more preferably 1.1% or more. However, when the amount of Mn becomes excessive, it transforms into a bainite structure, a martensite structure, or the like, and the machinability may be lowered. From such a viewpoint, the amount of Mn needs to be 1.50% by mass or less. The upper limit of the amount of Mn is preferably 1.4% or less, more preferably 1.35% by mass or less.

(S: more than 0% and 0.006% or less)
S combines with Mn in steel to form MnS. MnS reduces ductility and toughness in the direction perpendicular to the main forging direction. Further, when coarse MnS is present, the fatigue strength is lowered. From such a viewpoint, S is preferably reduced as much as possible, and is made 0.006% or less. Preferably it is 0.004% or less, More preferably, it is 0.002% or less. However, S is an impurity inevitably mixed in the steel, and it is impossible for industrial production to reduce the amount to 0%.

(Cu: more than 0% and 0.30% or less)
When the amount of Cu becomes excessive, hot forgeability is deteriorated. From such a viewpoint, the amount of Cu needs to be 0.30% or less. Preferably it is 0.28% or less. However, since Cu is an element that is difficult to separate from molten steel at the time of smelting, its amount cannot be reduced to 0%.

(Mo: more than 0% and 0.15% or less)
Mo is an element effective for improving the hardenability as well as improving the strength of the steel material. If the amount added is large, the material may be transformed into a bainite structure or a martensite structure, and the machinability may be reduced. From such a viewpoint, the Mo amount needs to be 0.15% or less. Preferably it is 0.10% or less. However, since Mo is an element inevitably mixed in scrap, its amount cannot be reduced to 0%.

(V: more than 0.06% and 0.32% or less)
V is an element effective for improving the fatigue strength of a steel material by precipitating fine V-based carbides in pro-eutectoid ferrite. In order to exert such effects, the V amount needs to be 0.06% or more. Preferably it is 0.08% or more, More preferably, it is 0.10% or more. However, when the amount of V becomes excessive, there is a possibility that the toughness of the steel material is reduced by promoting the occurrence of micro-homogeneity. From such a viewpoint, the V amount is set to 0.32% or less. Preferably it is 0.30% or less.

  The basic components of the steel for forgings of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that impurities that are inevitably brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. are contained in the steel. Such unavoidable impurities include, for example, P, N, O, Ni, Cr and the like in addition to S described above.

  The steel for forgings of the present invention may contain unavoidable impurities as long as the effects of the present invention described above are not adversely affected.

  For example, Ni, which is an unavoidable impurity, increases the content thereof, promotes transformation into a bainite structure or a martensite structure, and there is a possibility that the machinability may decrease. From such a viewpoint, the Ni content is preferably 0.35% or less, more preferably 0.30% or less, and even more preferably 0.25% or less.

  In addition, if the content of Cr, which is an inevitable impurity, is increased, the transformation to a bainite structure or a martensite structure is promoted, and the machinability may be lowered. From such a viewpoint, the Cr content is preferably 0.35% or less, more preferably 0.30% or less, and even more preferably 0.25% or less.

  The steel for forgings of the present invention comprises a melting / casting process for melting / casting a steel material having a chemical composition adjusted as described above, a heating process for heating the steel ingot obtained in the casting process, and a heated steel ingot. A forging process for forging, a heat treatment process for normalizing and tempering a forged product obtained in the forging process (hereinafter sometimes referred to as “workpiece”), and a machining process for machining the workpiece after the heat treatment process, It can manufacture by providing. Details in each of these steps are as follows.

[Melting / Casting Process]
Steel with a predetermined chemical composition is melted using a high-frequency melting furnace, electric furnace, converter, etc., and impurity elements such as sulfur and oxygen are removed by vacuum refining or the like. After the component adjustment, vacuum processing is performed to remove impurity elements and gas components such as O and H. In the case of steel for large forged steel products, ingot (steel ingot) casting is mainly employed for casting.

[Heating process]
Since the forging process is performed in a limited temperature range where the deformability of the material is good, the heating temperature before forging is, for example, 1150 ° C. or higher and 1350 ° C. or lower. At this time, in order to make the temperature of the steel ingot surface and the inside uniform, heating and holding for 0.5 hour or more is necessary. Generally, it is thought that it is proportional to the square of the diameter of a workpiece, and a larger steel material has a longer holding time. The lower limit of the heating temperature at this time is preferably 1200 ° C. or higher, and the upper limit of the heating temperature is preferably about 1280 ° C. or lower.

[Forging process]
A steel ingot heated to the above temperature range is forged. At this time, it is necessary to ensure a forging ratio (JIS G0701: 1957) of 3S or more in order to press-fit casting defects such as a zaku nest and microporosity.

[Heat treatment process]
The steel for forgings of the present invention achieves its purpose by appropriately controlling not only the chemical component composition but also the microstructure, so that heat treatment such as normalization and tempering is performed for the purpose of obtaining the desired microstructure. carry out. In austenitization, at least Ac ₃ points or more (830 ° C. or more) is gradually heated (temperature increase rate: about 30 to 70 ° C./hour) and held for a certain time (0.5 hours or more). At this time, in order to sufficiently dissolve the V-based carbide, it is necessary to perform the treatment at an austenitizing temperature of 850 ° C. or higher.

  However, it is necessary to perform the treatment at an austenitizing temperature of 970 ° C. or lower from the viewpoint of suppressing coarsening of the prior austenite grains. In the case of steel for large forged steel products, a temperature difference occurs between the inside and outside of the material during heating. To do. The holding time is proportional to the diameter of the steel material, and the holding time is longer for larger steel materials. Cooling is not performed until the temperature reaches a uniform level inside the steel material.

  By cooling from the above normalizing temperature to 500 ° C. at an average cooling rate of 5 ° C./min or more, fine V-based carbides can be precipitated in the pro-eutectoid ferrite of the steel. However, when cooling at an average cooling rate exceeding 100 ° C./min, martensitic transformation or bainite transformation may occur, which may reduce the machinability of the steel material. Therefore, the average cooling rate at this time needs to be 100 ° C./min or less. Moreover, in order to complete the pearlite transformation completely, it cools to 400 degrees C or less. Insufficient cooling at this time may cause variations in characteristics.

  Tempering is performed by gradually heating (for example, a heating rate of 30 to 70 ° C./hour) to a predetermined temperature (for example, 550 to 650 ° C.) and holding for a certain time (for example, 5 to 20 hours). This tempering treatment has the effect of adjusting the balance between strength, ductility and toughness of the steel material and removing internal stresses and transformation stresses caused by the phase transformation. Therefore, it is necessary to perform tempering by heating to 550 ° C. or higher. However, when the tempering temperature is high, the material is softened due to the coarsening of the carbide, so that there is a possibility that sufficient strength cannot be secured. Therefore, it is necessary to perform tempering at 650 ° C. or lower.

[Machining process]
If necessary, at least a part of the surface layer of the heat-treated forged product is subjected to finish machining such as grinding, whereby a large forged product such as a forged steel throw for an assembled crankshaft or a forged steel journal can be obtained.

  Hereinafter, based on the examples, the effects of the present invention will be described more specifically, but the following examples are not of a nature that limits the present invention, and the design change in the spirit of the above and the following description is Both are included in the technical scope of the present invention.

  Various steel materials having chemical components shown in Table 1 below are melted in a high-frequency melting furnace and an electric furnace, adjusted to a predetermined chemical component composition, then subjected to vacuum treatment after adjusting the components, and impurities such as impurity elements, O, and H The gas component was removed. After smelting, casting was performed, and ingots (steel ingots) of 50 kg or 90 tons were formed. In addition, using the steel type G shown in Table 1, test No. The example shown in FIG. 8 corresponds to an actual machine.

  For the 90 ton ingot, near net forging was applied to the shape of the actual crank throw. On the other hand, with respect to the 50 kg ingot, the forging conditions of the actual machine were simulated, the steel was forged into a square bar shape and was allowed to cool to room temperature. In all the ingots, a forging ratio (JIS G0701: 1957) of 3S or more was ensured in order to press-fit casting defects such as zaku nests and microporosities.

  The forged ingot was subjected to heat treatment (normalizing treatment, tempering treatment) to ensure mechanical properties. The normalization at this time was raised to an austenitizing temperature (870 to 950 ° C.) as shown in Table 2 below, and maintained until the temperature became uniform to the center of the steel ingot.

  In the case of a large crankshaft, it was gradually heated to a holding temperature, subjected to austenitizing treatment for 22 hours, and then cooled to 200 ° C. or lower. Moreover, regarding tempering, it applied at 620 degreeC for 22 hours. In addition, about the average cooling rate after normalization (average cooling rate from normalization temperature to 500 degreeC), the average cooling rate in the fillet part which is hard to cool down is described.

  For a 50 kg ingot, after cutting a 20 mm × 20 mm × L 200 mm square, it simulates the fillet part of the crankshaft of the actual machine and cools it to 300 ° C. or less within an average cooling rate of 0.5 to 5 ° C./min. After that, tempering was held at 610 ° C. for 10 hours or more and cooled in a furnace or allowed to cool.

[Microstructure observation]
After the heat treatment, a specimen for micro observation was cut out, the surface perpendicular to the forging direction was polished, the polished surface was corroded with nital, and the microstructure was observed with an optical microscope.

[Tensile test]
After the heat treatment, for the small material, the test piece was processed so that the longitudinal direction of the test piece was parallel to the forging direction. Regarding the actual crank throw, after cutting a square material of about 30 mm × 30 mm × L 250 mm from the fillet portion, the test piece was processed. The shape of the tensile test piece was a JIS No. 14A tensile test piece (φ6 mm × GL30 mm or φ14 mm × GL70 mm). The tensile test was performed based on JIS-Z2241: 1998, and tensile properties such as 0.2% proof stress, tensile strength, elongation and drawing were measured.

  The “GL” indicates a distance between gauge points (Gauge Length), that is, an effective distance on which stress is applied to a tensile test piece.

[Rotating bending fatigue test]
After the heat treatment, the rotating bending fatigue test piece was processed so that the longitudinal direction of the test piece was parallel to the forging direction, and the rotating bending fatigue test was performed. Regarding the actual crank throw, a square piece of about 30 mm × 30 mm × L 250 mm was cut out from the fillet portion, and then a test piece was processed. The shape of the rotating bending fatigue test piece is JIS No. 1 rotating bending fatigue test piece (φ8 mm, R30 mm, GL22 mm or φ10 mm, R30 mm, GL30 mm), and the rotating bending fatigue test is based on JIS-Z2274: 1978. Carried out. Regarding fatigue strength, a certain stress level is applied 3 × 10 ⁶ times, and when it is not ruptured, the stress width is sequentially increased, and the fatigue strength is obtained by the difference method in which the largest unbroken stress is the fatigue strength. Three samples of conditions were prepared, and the average value was defined as fatigue strength.

[Measurement of dispersion state and number density of V-based carbide]
After heat treatment, the surface perpendicular to the forging direction is taken as the observation surface, and a thin film or sample extraction replica prepared by electropolishing with an electrolytic solution is prepared, and proeutectoid ferrite is measured with a transmission electron microscope (TEM). The precipitate inside and the dispersion state of the precipitate were observed. At this time, the acceleration voltage was 200 kV, and observation was performed at a magnification of 50,000 to 5,000,000 times.

  In order to identify the precipitate in the pro-eutectoid ferrite, component analysis was performed by energy dispersive X-ray spectroscopy (EDS) using the extracted replica observed by the TEM. Since the EDS analysis was performed and all the precipitates were found to be V-based carbides, all the precipitates in the pro-eutectoid ferrite were handled as V-based carbides.

Using a structural photograph obtained with a thin film or a sample extraction replica, the particle size (equivalent circle diameter) and the number of V-based carbides precipitated in the pro-eutectoid ferrite were measured by image analysis. From the measurement results, the number density of V-based carbides having a particle size of 50 nm or less present per 1 μm ² was calculated from the quotient of the number of V-type carbides having a particle size of 50 nm or less and the measurement area.

[Evaluation criteria]
The observation results of the microstructure, tensile properties (0.2% yield strength, tensile strength, elongation and drawing), fatigue properties, and measurement results of the number density of V-based carbide are shown in Table 3 below. In Table 3 below, “F” indicates pro-eutectoid ferrite, “P” indicates pearlite, and “B” indicates bainite, and the microstructure is pro-eutectoid ferrite + pearlite (F + P), or pro-eutectoid ferrite + pearlite + A bainite (F + P + B) composite structure in which the total area ratio of pro-eutectoid ferrite and pearlite satisfies 90% or more was evaluated as “◯”, and a composite area ratio less than 90% was evaluated as “X”. The yield stress was evaluated based on the upper yield point when the yield point could be observed during the tensile test, and 0.2% proof stress when the yield point could not be observed. And the example which shows the high yield stress of 420 Mpa or more was evaluated as "(circle)", and the example which shows the low yield stress of less than 420 Mpa was evaluated as "*". Regarding the fatigue strength, an example showing a high fatigue strength of 330 MPa or more was evaluated as “◯”, and an example showing a fatigue strength of less than 330 MPa was evaluated as “X”. In the number density of V-based carbides, the column indicated by "-" indicates a case where observation of V-based carbides is not performed.

  From this result, it can be considered as follows. Test No. Examples 1 to 8 are examples that satisfy all of the requirements defined in the present invention, and it can be seen that steel for forgings having high yield stress and high fatigue strength can be realized. Moreover, in these steel materials, since the total area ratio of pro-eutectoid ferrite + pearlite (F + P) satisfies 90% or more, it is expected that excellent machinability is exhibited.

  In contrast, test no. 9 to 16 are examples that do not satisfy any of the requirements defined in the present invention, and it can be seen that the yield stress and the high fatigue strength are reduced in at least one of the points.

  Specifically, Test No. Nos. 9 to 15 are examples in which the average cooling rate from the austenitizing temperature to 500 ° C. is slow, the number density of V-based carbides is reduced, and at least one of yield stress and fatigue strength is reduced. .

  Test No. No. 16 is an example using a steel type H that does not contain V, and even when manufactured under appropriate manufacturing conditions, it is expected that there will be almost no precipitation of V-based carbides, resulting in a decrease in yield stress and fatigue strength. Yes.

Based on these results, the influence of the number density of V-based carbides on the yield stress is shown in FIG. FIG. 2 shows the influence of the number density of V-based carbides on the fatigue strength. As is apparent from FIGS. 1 and 2, in order to ensure a yield stress of 420 MPa or more and a fatigue strength of 330 MPa or more, the number density of V-based carbides in the pro-eutectoid ferrite needs to be 55 pieces / μm ² or more. I understand that there is.

Claims (3)

% By mass
C: 0.28% or more and 0.47% or less,
Si: more than 0% and 0.45% or less,
Mn: 0.90% or more and 1.50% or less,
S: more than 0% and 0.006% or less,
Cu: more than 0% and 0.30% or less,
Mo: more than 0% and 0.15% or less and V: 0.06% or more and 0.32% or less, respectively, and the balance is made of iron and inevitable impurities,
The total area ratio of pro-eutectoid ferrite and pearlite is 90% or more in the ratio of the entire metal structure,
In addition, V-based carbide having an equivalent circle diameter of 50 nm or less exists in a number density of 55 to 500 per 1 μm ² in the pro-eutectoid ferrite,
A steel for forged steel product having a yield stress of 420 MPa or more and a fatigue strength of 330 MPa or more.   A forged steel crank throw for an assembled crankshaft constituted by the steel for forged products according to claim 1. A forged steel journal comprising the steel for forged products according to claim 1.

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