WO2022071419A1

WO2022071419A1 - Steel material

Info

Publication number: WO2022071419A1
Application number: PCT/JP2021/035934
Authority: WO
Inventors: 聡志賀; 豊根石; 誠江頭; 雄介河原木
Original assignee: 日本製鉄株式会社
Priority date: 2020-09-30
Filing date: 2021-09-29
Publication date: 2022-04-07
Also published as: DE112021005117T5; CN116209782A; JPWO2022071419A1; KR20230072500A; JP7417171B2

Abstract

Provided is a steel material that has excellent machinability, that has excellent bending fatigue strength and surface fatigue strength even after having a vacuum carburization process or the like performed thereon, and that can suppress heat treatment deformation. The steel material according to an embodiment of the present invention has a chemical composition that contains, in mass%, 0.18-0.25% of C, 0.70-2.00% of Si, 0.70-1.50% of Mn, 0.005-0.050% of S, 0.0050-0.0200% of N, 0.001-0.100% of Al, not more than 0.0050% of O, and not more than 0.030% of P, the remaining portion being Fe and impurities, and that satisfies formula (1) and formula (2) described in the description. In the steel material, microstructures in a lateral cross-section and a longitudinal cross-section contain ferrite and the remaining portions are perlite and/or bainite. The arithmetic average value of the area percentage of the ferrite is 50-70%, the standard deviation of the area percentage of the ferrite is not more than 4.0%, and the average particle diameter ratio of the ferrite is not more than 2.00.

Description

Steel material

This disclosure relates to steel materials, and more particularly to steel materials suitable for materials for machine structural parts manufactured by performing vacuum carburizing treatment.

In the present specification, the vacuum carburizing treatment also includes the vacuum carburizing nitriding treatment. Further, in the present specification, the vacuum carburizing treatment includes a vacuum carburizing step (including a vacuum carburizing nitriding step) and a quenching step after the vacuum carburizing step.

Mechanical structural parts are represented by, for example, gears and shafts of automobiles and construction vehicles. As machine structural parts, alloy steel materials for machine structure represented by SCr420, SCM420, and SNCM420 specified in JIS G4053 (2016) are used.

These steel materials are manufactured into machine structural parts by, for example, the following manufacturing process. Forging (hot forging or cold forging) and / or cutting is performed on the steel material to produce an intermediate product having a desired shape. The intermediate product is subjected to heat treatment (quenching and tempering, carburizing treatment, carburizing nitriding treatment, etc.) to adjust the hardness and microstructure of the intermediate product. Machine structural parts are manufactured by the above manufacturing process.

As mentioned above, cutting may be performed on steel materials during the manufacturing process of machine structural parts. Therefore, the steel material used as the material for machine structural parts is required to have high machinability.

In recent years, the weight and size of mechanical structural parts have been reduced for the purpose of improving fuel efficiency of automobiles and construction vehicles. Therefore, excellent bending fatigue strength and surface fatigue strength are required for mechanical structural parts.

Vacuum carburizing treatment is known as a method for increasing the bending fatigue strength and surface fatigue strength of mechanical structural parts. In the vacuum carburizing treatment, a hardened layer (carburized layer or carburized nitrided layer) is formed on the surface layer of the mechanical structural component. This hardened layer improves bending fatigue strength and surface fatigue strength of mechanical structural parts.

By the way, when vacuum carburizing treatment (vacuum carburizing treatment and vacuum carburizing nitriding treatment) is performed, mechanical structural parts are easily deformed. In the present specification, the deformation of the mechanical structural parts during the vacuum carburizing process is referred to as heat treatment deformation. Due to heat treatment deformation, the shape of mechanical structural parts is distorted. Distortion of the shape of mechanical structural parts causes noise and vibration when driving automobiles, construction vehicles, and the like. Therefore, there is a need for a steel material that can suppress heat treatment deformation when vacuum carburizing treatment is performed.

Techniques for suppressing heat treatment deformation include JP-A-2016-191151 (Patent Document 1), JP-A-2018-028130 (Patent Document 2), JP-A-2007-291486 (Patent Document 3), and JP-A-2010. -150566 is disclosed in Japanese Patent Application Laid-Open No. 4 (Patent Document 4).

The carburized parts disclosed in Patent Document 1 are C: 0.10 to 0.30%, Si: 0.16 to 1.40%, Mn: 1.40 to 3.00%, P: in mass%. Contains 0.030% or less, S: 0.060% or less, Cr: 0.01 to 0.29%, Al: 0.010 to 0.300%, and N: 0.003 to 0.030%. The balance is composed of Fe and impurities. This carburized part has a flat surface portion and an edge portion on the surface. The carbon concentration of the flat portion surface layer region from the surface of the flat portion to the position of 0.05 mm is 0.70 to 0.89%, and the edge portion surface layer from the surface of the edge portion to the position of 0.05 mm in depth. The carbon concentration in the region is 1.20% or less. Further, the grain boundary oxide layer depth is 1 μm or less, and the Vickers hardness of the core portion is 260 or more. As a result, Patent Document 1 describes that the carburized parts of Patent Document 1 are excellent in bending fatigue strength even if the carburized parts have a shape including an edge portion.

The carburized parts disclosed in Patent Document 2 are C: 0.10 to 0.30%, Si: 0.16 to 1.40%, Mn: 1.40 to 3.00%, P: in mass%. Contains 0.030% or less, S: 0.060% or less, Cr: 0.01 to 0.29%, Al: 0.010 to 0.100%, and N: 0.003 to 0.030%. The balance is composed of Fe and impurities. This carburized part has a flat surface portion and an edge portion on the surface. The carbon concentration of the flat portion surface layer region from the surface of the flat portion to the position of 0.05 mm is 0.70 to 0.89%, and the edge portion surface layer from the surface of the edge portion to the position of 0.05 mm in depth. The carbon concentration in the region is 1.20% or less. Further, the Vickers hardness at a depth of 0.3 mm from the surface of the flat portion is 650 or more, the grain boundary oxide layer depth is 1 μm or less, and the Vickers hardness of the core portion is 260 or more. As a result, Patent Document 2 describes that the carburized parts of Patent Document 2 are excellent in bending fatigue strength even if the carburized parts have a shape including an edge portion.

The carburized parts disclosed in Patent Document 3 are, in mass%, C: 0.1 to 0.3%, Si: 0.5 to 3.0%, Mn: 0.3 to 3.0%, P:. 0.03% or less, S: 0.03% or less, Cu: 0.01 to 1.00%, Ni: 0.01 to 3.00%, Cr: 0.3 to 1.0%, Al: 0 .20% or less and N: 0.05% or less, the balance is composed of unavoidable impurities and Fe, [Si%] + [Ni%] + [Cu%]-[Cr%]> 0.5 It has an alloy composition that meets the conditions. Further, this carburized part is obtained by carrying out a carburizing treatment by vacuum carburizing. As a result, in the carburized parts of Patent Document 3, the toughness of the edge portion is not lowered, and since the portion having the lowest surface carbon concentration is 0.6% or more, the portion having low strength due to insufficient carburizing does not occur. It is described in Document 3.

The steel material for vacuum carburizing or vacuum carburizing nitriding disclosed in Patent Document 4 has a mass% of C: 0.10 to 0.25%, Si: 0.35 to 1.5%, Mn: 0.4 to 1.5%, P: 0.025% or less, S: 0.015 to 0.05%, Cr: 0.50 to 2.0%, Al: 0.010 to 0.050% and N: 0. It contains 012 to 0.025%, the balance is Fe and impurities, O (oxygen) in the impurities: 0.0012% or less and Ti: 0.003% or less, and the formulas (1) to (3). Has a chemical composition that satisfies. Here, the formula (1) is 910-203 × C ^0.5 + 44.7 × Si ≦ 860, and the formula (2) is 2.0 ≦ (0.31 × C ^0.5 ) × (0). .7 × Si + 1.00) × (3.33 × Mn + 1.00) × (2.16 × Cr + 1.00) ≦ 3.5, and the formula (3) is 0.2 × (S / Mn) + P. ≦ 0.030. Further, in a cross section parallel to the longitudinal direction, the major axis of the inclusion is L (μm) and the minor axis is W (μm), and under predetermined conditions, an oxide represented by (πLW / 4) ^0.5 , etc. The maximum equivalent circle diameter of the inclusions is 35 μm or less. Patent Document 4 describes that the steel material of Patent Document 4 reduces variations in heat treatment strain during quenching and enhances surface fatigue strength and bending fatigue strength by adjusting the C and Si contents. ..

Japanese Unexamined Patent Publication No. 2016-191151 Japanese Unexamined Patent Publication No. 2018-028130 Japanese Unexamined Patent Publication No. 2007-291486 Japanese Unexamined Patent Publication No. 2010-150566

Patent Documents 1 to 4 disclose techniques for improving fatigue strength, and do not disclose any technique for suppressing heat treatment deformation.

An object of the present disclosure is a steel material having excellent machinability, having excellent bending fatigue strength and surface fatigue strength after vacuum carburizing treatment, and capable of suppressing heat treatment deformation after vacuum carburizing treatment. Is to provide.

The steel material of this embodiment is
The chemical composition is by mass%,
C: 0.18 to 0.25%,
Si: 0.70 to 2.00%,
Mn: 0.70 to 1.50%,
S: 0.005 to 0.050%,
N: 0.0050-0.0200%,
Al: 0.001 to 0.100%,
O: 0.0050% or less, and
P: Contains 0.030% or less,
The balance consists of Fe and impurities, and the formulas (1) and (2) are satisfied.
In a cross section perpendicular to the longitudinal direction of the steel material and having a circular shape with a radius R,
The center position of the cross section and the eight R / 2 positions arranged at a pitch of 45 ° around the center of the cross section at R / 2 positions in the radial direction from the center of the cross section. When defined as 9 cross-sectional observation positions,
The microstructure at each cross-sectional observation position contains ferrite and the rest consists of pearlite and / or bainite.
The arithmetic mean value of the area fraction of the ferrite at the nine cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of the ferrite is 4.0% or less.
Of the average grain sizes of ferrites at the nine cross-sectional observation positions, the ratio of the maximum average grain size to the minimum average grain size is 2.00 or less.
In a vertical cross section parallel to the longitudinal direction of the steel material and including the central axis of the steel material.
Three central axis positions arranged at R / 2 pitch on the central axis, and six R / 2 positions arranged at R / 2 positions in the radial direction from each central axis position. , When defined as 9 vertical cross-sectional observation positions,
The microstructure at each longitudinal section observation position contains ferrite and the rest consists of pearlite and / or bainite.
The arithmetic mean value of the area fraction of ferrite at the nine vertical cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of the ferrite is 4.0% or less.
Among the average particle diameters of ferrites at the nine vertical cross-sectional observation positions, the ratio of the maximum average particle size to the minimum average particle size is 2.00 or less.
Si / Mn ≧ 1.00 (1)
1- (0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo) <0.800 (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1) and the formula (2). If the corresponding element is not contained, "0" is substituted for the element symbol.

The steel material according to the present disclosure has excellent machinability, has excellent bending fatigue strength and surface fatigue strength after vacuum carburizing treatment, and can suppress heat treatment deformation after vacuum carburizing treatment.

FIG. 1 is a diagram showing the relationship between the F2 (= 1- (0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo)) value and the maximum deformation amount ratio (%). FIG. 2 is a schematic view for explaining a cross-sectional observation position where microstructure observation is carried out in a cross section perpendicular to the longitudinal direction of the steel material of the present embodiment. FIG. 3 is a schematic view for explaining a vertical cross-sectional observation position in which microstructure observation is carried out in a vertical cross section including the central axis, which is parallel to the longitudinal direction of the steel material of the present embodiment. FIG. 4 is a schematic diagram of the band structure. FIG. 5 is a diagram showing an example of heat patterns in the vacuum carburizing step and the quenching step. FIG. 6 is a plan view of the Ono-type rotary bending test piece produced in the example. FIG. 7 is a diagram showing an example of heat patterns in the gas carburizing step and the quenching step. FIG. 8 is a plan view of the test piece for the roller pitching fatigue test produced in the example. FIG. 9 is a schematic diagram for explaining a roller pitching fatigue test. FIG. 10 is a front view of the large roller test piece produced in the example. FIG. 11A is a perspective view of the gear simulated test piece produced in the example. 11B is a perspective view of the through hole in FIG. 11A.

The present inventors have excellent machinability, have excellent bending fatigue strength and surface fatigue strength when subjected to vacuum carburizing treatment to obtain mechanical structural parts, and after vacuum carburizing treatment. We investigated and examined steel materials that can suppress heat treatment deformation.

The present inventors examined a steel material having excellent machinability and further having excellent bending fatigue strength and surface fatigue strength after vacuum carburizing treatment from the viewpoint of chemical composition.

As a result of the examination, the chemical composition was C: 0.18 to 0.25%, Si: 0.70 to 2.00%, Mn: 0.70 to 1.50%, S: 0.005 in mass%. ~ 0.050%, N: 0.0050 to 0.0200%, Al: 0.001 to 0.100%, O: 0.0050% or less, P: 0.030% or less, Mo: 0 to 0. 50%, Nb: 0 to 0.050%, Cr: 0 to 0.60%, Ti: 0 to 0.020%, Cu: 0 to 0.50%, Ni: 0 to 0.80%, V: A steel material containing 0 to 0.30%, Mg: 0 to 0.0035%, Ca: 0 to 0.0030%, and rare earth elements: 0 to 0.0050%, with the balance being Fe and impurities. For example, it is considered that it has excellent machinability and may have excellent bending fatigue strength and surface fatigue strength after vacuum carburizing treatment.

The present inventors further assume that the content of each element in the chemical composition is within the above range, and further satisfy the following formula (1) to have excellent bending fatigue strength after vacuum carburizing treatment. I thought there was a possibility.
Si / Mn ≧ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

In the steel material of the present embodiment, if the ratio of the Si content to the Mn content is 1.00 or more, that is, if the formula (1) is satisfied, the inclusions are soft MnO-SiO ₂ . The inclusions are vitrified during hot working (hot rolling), stretched and fragmented, and refined. Therefore, coarse inclusions that reduce the bending fatigue strength can be reduced, and the bending fatigue strength is increased.

The present inventors further investigated means for suppressing heat treatment deformation in vacuum carburizing treatment. The present inventors focused on the microstructure of steel materials. If the microstructure at each part of the steel material is as uniform as possible, specifically, if the variation in the phase composition of the microstructure at each part in the steel material and the variation in the crystal grains are suppressed, the vacuum Variations in the timing of martensitic transformation during carburizing and quenching can be suppressed. As a result, heat treatment deformation can be suppressed. Therefore, the present inventors investigated the phase composition and the crystal grain size at each part of the steel material.

The present inventors first focused on the variation in microstructure in the cross section, which is the cross section perpendicular to the longitudinal direction of the steel material. In order to quantify the variation of the microstructure in the cross section, the cross-sectional observation position, which is the observation position of the microstructure in the cross section, was defined as follows.

When the radius of the cross section of the steel material is R, the center position of the cross section and the position of R / 2 in the radial direction from the center of the cross section are arranged at a pitch of 45 ° around the center of the cross section 8 The R / 2 position of the location is defined as the cross-sectional observation position of 9 locations.

The present inventors investigated and examined the microstructure at each cross-sectional observation position. As a result of the examination, it was found that the heat treatment deformation after the carburizing treatment is suppressed if the microstructure at the cross-sectional observation position meets the following requirements.
(1) The microstructure at each cross-sectional observation position contains ferrite, and the balance consists of pearlite and / or bainite.
(2) The arithmetic mean value of the area fraction of ferrite at nine cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.
(3) Of the average particle diameters of ferrites at nine cross-sectional observation positions, the ratio of the maximum average particle size to the minimum average particle size is 2.00 or less.

However, even a steel material having the above-mentioned chemical composition and satisfying the above-mentioned microstructure still does not sufficiently suppress heat treatment deformation, and particularly noise and vibration during operation of automobiles and construction vehicles. It turned out that there are cases where it cannot be sufficiently suppressed. Therefore, the present inventors further investigated.

As a result, the following items were found. In order to suppress noise and vibration during operation, it is effective to three-dimensionally suppress heat treatment deformation of steel materials. As described above, if the phase composition of the microstructure of the cross section of the steel material and the variation of the crystal grains are suppressed, the heat treatment deformation in the direction perpendicular to the longitudinal direction of the steel material can be suppressed.

However, simply suppressing the variation in the microstructure of the cross section of the steel material only suppresses the two-dimensional heat treatment deformation. That is, even if the variation in the microstructure of the cross section of the steel material is suppressed, the microstructure of the vertical cross section which is parallel to the longitudinal direction of the steel material and includes the central axis of the steel material may vary. In this case, the heat treatment deformation varies. As a result, noise and vibration during operation cannot be sufficiently suppressed.

Therefore, the present inventors paid attention not only to the variation in the microstructure of the cross section of the steel material but also to the variation of the microstructure of the vertical cross section of the steel material. Then, in order to quantify the variation of the microstructure in the vertical section, the vertical section observation position, which is the observation position of the microstructure in the vertical section, is defined as follows.

9 are the 3 central axis positions arranged at R / 2 pitch on the central axis of the steel material and the 6 R / 2 positions arranged at R / 2 positions in the radial direction from each central axis position. It is defined as the vertical cross-sectional observation position of the location.

The present inventors investigated and examined the microstructure at each vertical cross-sectional observation position. As a result of the examination, if the microstructure at the cross-sectional observation position meets the above requirements and the microstructure at the vertical cross-section observation position meets the following requirements, the heat treatment deformation after the gas carburizing treatment is sufficiently suppressed. There was found.
(4) The microstructure at each vertical cross-sectional observation position contains ferrite, and the balance consists of pearlite and / or bainite.
(5) The arithmetic mean value of the area fraction of ferrite at nine vertical cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.
(6) Of the average particle diameters of ferrites at nine vertical cross-sectional observation positions, the ratio of the maximum average particle size to the minimum average particle size is 2.00 or less.

However, even if the steel material has the above-mentioned chemical composition and the microstructure at the cross-sectional observation position and the vertical cross-section observation position satisfies the above requirements (1) to (6), the heat treatment deformation is still sufficient. In some cases, it could not be suppressed. Therefore, the present inventors further investigated.

Here, the present inventors focused on martensitic transformation after vacuum carburizing. Then, the present inventors investigated in detail the mechanism of occurrence of martensitic transformation during vacuum carburizing and quenching.

The present inventors first attempted to suppress heat treatment deformation by using a steel material having the above-mentioned chemical composition and making the martensitic transformation time at each part of the mechanical structural parts as similar as possible. Specifically, it is possible to suppress heat treatment deformation by suppressing variations in the microstructure at each part of the steel material (cross-section observation position, vertical cross-section observation position) and by suppressing variations in the Ms point of each part as much as possible. I tried.

However, as a result of the investigation by the present inventors, even if the variation in the microstructure in each part of the steel material having the above-mentioned chemical composition is suppressed, the martensitic transformation time is inevitably slightly deviated in each part of the steel material, and each part. It turned out to be extremely difficult to transform martensite at the same time. Specifically, when the time for quenching in the vacuum carburizing treatment is divided into minute times, even if the variation in the microstructure at the cross-sectional observation position and the vertical cross-sectional observation position of the steel material is suppressed to the utmost limit, the inside of the steel material In a minute time in which a portion having undergone martensitic transformation (hereinafter, also referred to as “martensite transformation portion”) and a portion not undergoing martensitic transformation (hereinafter, also referred to as “martensite untransformed portion”) coexist. It turned out that the band was inevitably generated.

It is considered that the microstructural changes of the steel material during the vacuum carburizing treatment occur as follows.
When the quenching time (quenching time) is divided into minute times, martensitic transformation starts in a part of the inside of the steel material first. After that, as time progresses, martensitic transformation progresses from the central part toward the surface layer part. That is, the martensitic transformation occurs not from the surface layer of the steel material but from the inside of the steel material.

Due to the vacuum carburizing treatment, the carbon concentration on the surface layer of the steel material is higher than the carbon concentration inside the steel material. Therefore, the Ms point on the surface layer of the steel material is lower than the Ms point inside the steel material. Further, even if it is possible to make the Ms points uniform in each part inside the steel material, the cooling rates of each part are not completely the same due to the shape of the steel material. Therefore, when the quenching time is divided into minute times, the martensitic transformation starts from each part of the steel material where the cooling rate inside the steel material is high. Therefore, at the time of quenching of the gas carburizing treatment, a minute time zone in which the martensitic transformed portion and the martensitic untransformed portion coexist always occurs.

Based on the above findings, the present inventors do not suppress the heat treatment deformation by setting the martensitic transformation time as much as possible, but during the vacuum carburizing treatment, the martensitic transformed portion and the martensitic untransformed portion are mixed. On the premise that there is always a minute time zone, the means for suppressing heat treatment deformation was investigated.

At the time of quenching, the untransformed part of martensite is softer than the transformed part of martensite. Further, the martensitic transformational portion having a body-centered cubic lattice structure has a larger volume than the martensitic untransformed portion having a face-centered cubic lattice structure. Therefore, when a part of the steel material undergoes martensitic transformation during quenching, and the martensitic transformed portion and the martensitic untransformed portion coexist, the martensitic untransformed portion is distorted. It is considered that this strain causes heat treatment deformation.

Therefore, the present inventors assume that there is a minute time zone in which the martensitic transformed portion and the martensitic untransformed portion coexist during the vacuum carburizing treatment, and when the martensitic transformed portion is generated, It was considered that if the strength of the untransformed martensite portion could be kept high, the occurrence of distortion in the untransformed martensite portion could be suppressed, and as a result, the heat treatment deformation could be suppressed.

Therefore, the present inventors further investigated a means for maintaining high strength of the martensitic untransformed portion when the martensitic transformed portion is generated during quenching in the vacuum carburizing treatment. In the steel material having the above-mentioned chemical composition, in order to increase the strength of the martensitic untransformed portion in the temperature range where the martensitic transformed portion is generated, the martensitic untransformed portion is strengthened in the temperature range where the martensitic transformed portion is generated. It is effective to appropriately contain the elements to be used.

In the above chemical composition, the present inventors considered that C, Si, Mn, Cr and Mo are effective as elements for increasing the strength of the martensitic untransformed portion in the temperature range where the martensitic transformed portion is generated. .. Therefore, the relationship between these elements and the amount of heat treatment deformation during quenching of the gas carburizing treatment was further investigated. As a result, it was found that the heat treatment deformation was remarkably suppressed by further satisfying the following formula (2) in the steel material having the above-mentioned chemical composition.
1- (0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo) <0.800 (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (2). If the corresponding element is not contained, "0" is substituted for the element symbol.

It is defined as F2 = 1- (0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo). FIG. 1 shows the F2 value and the maximum in a steel material in which the content of each element in the chemical composition is within the above range and the variation in microstructure at the cross-sectional observation position and the vertical cross-sectional observation position satisfies the above-mentioned requirements. It is a figure which shows the relationship with the deformation amount ratio (%). The maximum deformation ratio is an index of heat treatment deformation. The larger the maximum deformation amount ratio, the larger the heat treatment deformation of the steel material. The maximum deformation amount ratio was determined by the method described later.

With reference to FIG. 1, the content of each element in the chemical composition is within the above range, and the microstructure at the cross-sectional observation position and the vertical cross-sectional observation position satisfies the above-mentioned conditions (1) to (6). In the satisfied steel material, the maximum deformation amount ratio decreases as F2 decreases. When F2 is less than 0.800, the maximum deformation amount ratio is remarkably lowered. That is, the maximum deformation amount ratio with respect to F2 has an inflection point near F2 = 0.800. Therefore, if F2 is less than 0.800, heat treatment deformation of the steel material during carburizing and quenching can be sufficiently suppressed.

As described above, the present inventors suppress the variation in the microstructure at the cross-sectional observation position and the vertical section observation position in the steel material having the above-mentioned chemical composition, and suppress the variation in the timing of martensitic transformation during quenching to some extent. On the other hand, it has excellent machinability by setting F2 to less than 0.800 on the premise that a minute time zone in which martensite transformed portion and martensite untransformed portion coexist is inevitably generated at the time of quenching. It has been found that it has excellent bending fatigue strength and excellent surface fatigue strength after vacuum carburizing treatment, and can sufficiently suppress heat treatment deformation after vacuum carburizing treatment.

The steel material according to the present embodiment completed based on the above knowledge has the following constitution.

[1]
It's a steel material
The chemical composition is by mass%,
C: 0.18 to 0.25%,
Si: 0.70 to 2.00%,
Mn: 0.70 to 1.50%,
S: 0.005 to 0.050%,
N: 0.0050-0.0200%,
Al: 0.001 to 0.100%,
O: 0.0050% or less, and
P: Contains 0.030% or less,
The balance consists of Fe and impurities, and the formulas (1) and (2) are satisfied.
In a cross section perpendicular to the longitudinal direction of the steel material and having a circular shape with a radius R,
The center position of the cross section and the eight R / 2 positions arranged at a pitch of 45 ° around the center of the cross section at R / 2 positions in the radial direction from the center of the cross section. When defined as 9 cross-sectional observation positions,
The microstructure at each cross-sectional observation position contains ferrite and the rest consists of pearlite and / or bainite.
The arithmetic mean value of the area fraction of the ferrite at the nine cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of the ferrite is 4.0% or less.
Of the average grain sizes of ferrites at the nine cross-sectional observation positions, the ratio of the maximum average grain size to the minimum average grain size is 2.00 or less.
In a vertical cross section parallel to the longitudinal direction of the steel material and including the central axis of the steel material.
Three central axis positions arranged at R / 2 pitch on the central axis, and six R / 2 positions arranged at R / 2 positions in the radial direction from each central axis position. , When defined as 9 vertical cross-sectional observation positions,
The microstructure at each longitudinal section observation position contains ferrite and the rest consists of pearlite and / or bainite.
The arithmetic mean value of the area fraction of ferrite at the nine vertical cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of the ferrite is 4.0% or less.
Among the average particle diameters of ferrites at the nine vertical cross-sectional observation positions, the ratio of the maximum average particle size to the minimum average particle size is 2.00 or less.
Steel material.
Si / Mn ≧ 1.00 (1)
1- (0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo) <0.800 (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1) and the formula (2). If the corresponding element is not contained, "0" is substituted for the element symbol.

[2]
The steel material according to [1].
The chemical composition further replaces a portion of the Fe.
Mo: 0.50% or less,
Nb: 0.050% or less,
Cr: 0.60% or less Ti: 0.020% or less,
Cu: 0.50% or less,
Ni: 0.80% or less,
V: 0.30% or less,
Mg: 0.0035% or less,
Ca: 0.0030% or less, and
Rare earth element: Contains one or more elements selected from the group consisting of 0.0050% or less.
Steel material.

Hereinafter, the steel material of this embodiment will be described in detail. Unless otherwise specified, "%" for an element means mass%.

[Chemical composition of steel]
The chemical composition of steel contains the following elements.

C: 0.18 to 0.25%
Carbon (C) increases the strength of the steel material. If the C content is less than 0.18%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the C content exceeds 0.25%, the hardenability becomes excessively high even if the content of other elements is within the range of the present embodiment. In this case, the hardness of the mechanical structural parts after the vacuum carburizing treatment becomes excessively high. As a result, the machinability of mechanical structural parts is significantly reduced.
Therefore, the C content is 0.18 to 0.25%. The lower limit of the C content is preferably 0.19%, more preferably 0.20%, still more preferably 0.21%. The preferred upper limit of the C content is 0.24%, more preferably 0.23%, still more preferably 0.22%.

Si: 0.70 to 2.00%
Silicon (Si) enhances the hardenability of the steel material and enhances the strength of the steel material. Si further enhances the temper softening resistance of the hardened layer when steel is used as a mechanical structural component. Therefore, the surface fatigue strength of the mechanical structural parts is increased. If the Si content is less than 0.70%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Si content exceeds 2.00%, the hardenability is too high even if the content of other elements is within the range of the present embodiment. Therefore, the hardness of the steel material after the vacuum carburizing treatment becomes high. Therefore, the machinability of the steel material is significantly reduced.
Therefore, the Si content is 0.70 to 2.00%. The lower limit of the Si content is preferably 0.71%, more preferably 0.72%, still more preferably 0.75%. The preferred upper limit of the Si content is 1.90%, more preferably 1.70%, still more preferably 1.50%, still more preferably 1.47%, still more preferably 1.45%. %.

Mn: 0.70 to 1.50%
Manganese (Mn) enhances the hardenability of steel materials and enhances the bending fatigue strength and surface fatigue strength of mechanical structural parts. If the Mn content is less than 0.70%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Mn content exceeds 1.50%, the steel material becomes too hard even if the content of other elements is within the range of the present embodiment. In this case, the machinability of the steel material is reduced.
Therefore, the Mn content is 0.70 to 1.50%. The preferred lower limit of the Mn content is more than 0.70%, more preferably 0.75%, still more preferably 0.80%. The preferred upper limit of the Mn content is less than 1.50%, more preferably 1.45%, still more preferably 1.40%, still more preferably 1.35%.

S: 0.005 to 0.050%
Sulfur (S) combines with Mn to form MnS. MnS enhances the machinability of steel materials. If the S content is less than 0.005%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the S content exceeds 0.050%, MnS is excessively formed even if the content of other elements is within the range of the present embodiment. In this case, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are reduced.
Therefore, the S content is 0.005 to 0.050%. The lower limit of the S content is preferably 0.010%, more preferably 0.013%, still more preferably 0.015%. The preferred upper limit of the S content is less than 0.050%, more preferably 0.035%, still more preferably 0.025%.

N: 0.0050-0.0200%
Nitrogen (N) combines with Al and Nb to form AlN and NbN. AlN and NbN suppress the coarsening of crystal grains during heating in the vacuum carburizing treatment due to the pinning effect. If the N content is less than 0.0050%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the N content exceeds 0.0200%, scratches are likely to occur on the surface of the manufactured slab or ingot in the steelmaking process even if the content of other elements is within the range of the present embodiment. ..
Therefore, the N content is 0.0050 to 0.0200%. The lower limit of the N content is preferably 0.0100%, more preferably 0.0120%, still more preferably 0.0130%. The preferred upper limit of the N content is less than 0.0200%, more preferably 0.0190%, still more preferably 0.0180%, still more preferably 0.0150%.

Al: 0.001 to 0.100%
Aluminum (Al) deoxidizes steel. Al further combines with N to form AlN. AlN suppresses the coarsening of crystal grains during heating in the vacuum carburizing treatment due to the pinning effect. If the Al content is less than 0.001%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Al content exceeds 0.100%, the formation of a coarse Al oxide is promoted even if the content of other elements is within the range of the present embodiment. The coarse Al oxide reduces the bending fatigue strength of mechanical structural parts.
Therefore, the Al content is 0.001 to 0.100%. The preferred lower limit of the Al content is 0.010%, more preferably 0.020%, still more preferably 0.025%, still more preferably 0.027%, still more preferably 0.030. %. The preferred upper limit of the Al content is 0.090%, more preferably 0.070%, still more preferably 0.050%, still more preferably 0.045%, still more preferably 0.040. %, More preferably 0.035%.

O (oxygen): 0.0050% or less Oxygen (O) is an impurity. O combines with other elements in the steel to form coarse oxide inclusions. Coarse oxide-based inclusions reduce the bending fatigue strength of mechanical structural parts. If the O content exceeds 0.0050%, the bending fatigue strength of the mechanical structural parts is significantly reduced even if the content of other elements is within the range of the present embodiment.
Therefore, the O content is 0.0050% or less. The preferred upper limit of the O content is 0.0040%, more preferably 0.0030%, still more preferably 0.0020%, still more preferably 0.0015%.
It is preferable that the O content is as low as possible. However, excessive reduction of O content raises manufacturing costs. Therefore, when considering normal industrial production, the preferred lower limit of the O content is more than 0%, more preferably 0.0001%, still more preferably 0.0005%, still more preferably 0.0010. %.

P: 0.030% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and lowers the grain boundary strength. If the P content exceeds 0.030%, even if the content of other elements is within the range of the present embodiment, P is excessively segregated at the grain boundaries to reduce the grain boundary strength, and as a result, the machine. Bending fatigue strength and surface fatigue strength of structural parts are reduced.
Therefore, the P content is 0.030% or less. The preferred upper limit of the P content is 0.025%, more preferably 0.020%, still more preferably 0.015%.
It is preferable that the P content is as low as possible. However, excessive reduction of P content raises manufacturing costs. Therefore, when considering normal industrial production, the preferred lower limit of the P content is more than 0%, more preferably 0.001%, still more preferably 0.005%, still more preferably 0.010. %.

The balance of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, the impurities are mixed from ore, scrap, or the manufacturing environment as a raw material when the steel material is industrially manufactured, and are within a range that does not adversely affect the steel material of the present embodiment. Means what is acceptable. The impurities referred to here are B, Pb, W, Sb, Bi, Co, Ta, Sn, In, Zr, Te, Se, Zn and the like. The total content of impurities other than O and P is 0.01% or less. Of the above impurities, the B content is 0.0003% or less.

[About optional elements]
Further, the chemical composition of the steel material of the present embodiment is replaced with a part of Fe.
Mo: 0.50% or less,
Nb: 0.050% or less,
Cr: 0.60% or less,
Ti: 0.020% or less,
Cu: 0.50% or less,
Ni: 0.80% or less,
V: 0.30% or less,
Mg: 0.0035% or less,
Ca: 0.0030% or less, and
Rare earth element: It may contain one or more elements selected from the group consisting of 0.0050% or less. These elements are arbitrary elements, and all of them enhance the bending fatigue strength and the surface fatigue strength of mechanical structural parts.

Mo: 0.50% or less Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, Mo enhances the hardenability of steel materials and enhances bending fatigue strength and surface fatigue strength of mechanical structural parts. If Mo is contained even in a small amount, the above effect can be obtained to some extent.
However, if the Mo content exceeds 0.50%, the steel material becomes too hard even if the content of other elements is within the range of the present embodiment. In this case, the machinability of the steel material is reduced.
Therefore, the Mo content is 0 to 0.50%, and when it is contained, it is 0.50% or less (that is, more than 0 to 0.50%).
The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, still more preferably 0.05%, still more preferably 0.10%. The preferred upper limit of the Mo content is less than 0.50%, more preferably 0.45%, still more preferably 0.40%, still more preferably 0.35%.

Nb: 0.050% or less Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb binds to C and / or N to form Nb precipitates (NbC, NbN, Nb (CN), etc.). Similar to AlN, the Nb precipitate suppresses the coarsening of crystal grains in the gas carburizing treatment due to the pinning effect. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are increased. If even a small amount of Nb is contained, the above effect can be obtained to some extent.
However, if the Nb content exceeds 0.050%, the Nb precipitate will be coarsened even if the content of other elements is within the range of this embodiment. In this case, the coarsening of crystal grains in the gas carburizing treatment cannot be sufficiently suppressed. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are lowered.
Therefore, the Nb content is 0 to 0.050%, and when it is contained, it is 0.050% or less (that is, more than 0 to 0.050%).
The preferred lower limit of the Nb content is 0.001%, more preferably 0.010%, still more preferably 0.015%, still more preferably 0.020%, still more preferably 0.025. %. The preferred upper limit of the Nb content is less than 0.050%, more preferably 0.045%, still more preferably 0.040%, still more preferably 0.035%.

Cr: 0.60% or less Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, Cr enhances the hardenability of steel materials and enhances bending fatigue strength and surface fatigue strength of mechanical structural parts. If even a small amount of Cr is contained, the above effect can be obtained to some extent.
However, if the Cr content exceeds 0.60%, excessive carburizing is likely to occur on the surface layer of the mechanical structural parts during the vacuum carburizing treatment even if the other element content is within the range of the present embodiment. In this case, coarse cementite is formed at the grain boundaries. Therefore, the bending fatigue strength of the mechanical structural parts is reduced.
Therefore, the Cr content is 0 to 0.60%, and when it is contained, it is 0.60% or less (that is, more than 0 to 0.60%). The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%, still more preferably 0.10%. The preferred upper limit of the Cr content is less than 0.60%, more preferably 0.55%, still more preferably 0.50%, still more preferably 0.45%, still more preferably 0. It is 40%.

Ti: 0.020% or less Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, Ti forms Ti precipitates (TiC, TiN, Ti (CN), etc.) as well as Nb. The Ti precipitate suppresses the coarsening of crystal grains in the gas carburizing treatment due to the pinning effect. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are increased. If even a small amount of Ti is contained, the above effect can be obtained to some extent.
However, if the Ti content exceeds 0.020%, the Ti precipitate will be coarsened even if the content of other elements is within the range of this embodiment. In this case, the coarsening of crystal grains in the gas carburizing treatment cannot be sufficiently suppressed. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are lowered.
Therefore, the Ti content is 0 to 0.020%, and when it is contained, it is 0.020% or less (that is, more than 0 to 0.020%).
The lower limit of the Ti content is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%. The preferred upper limit of the Ti content is 0.019%, more preferably 0.017%, still more preferably 0.015%.

Cu: 0.50% or less Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu enhances the hardenability of steel materials and enhances bending fatigue strength and surface fatigue strength of mechanical structural parts. If even a small amount of Cu is contained, the above effect can be obtained to some extent.
However, if the Cu content exceeds 0.50%, the steel material becomes too hard even if the content of other elements is within the range of this embodiment. In this case, the machinability of the steel material is reduced.
Therefore, the Cu content is 0 to 0.50%, and when it is contained, it is 0.50% or less (that is, more than 0 to 0.50%).
The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, still more preferably 0.10%. The preferred upper limit of the Cu content is 0.45%, more preferably 0.40%, still more preferably 0.30%, still more preferably 0.25%.

Ni: 0.80% or less Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, Ni enhances the hardenability of steel materials and enhances the bending fatigue strength and surface fatigue strength of mechanical structural parts. If even a small amount of Ni is contained, the above effect can be obtained to some extent.
However, if the Ni content exceeds 0.80%, the steel material becomes too hard even if the content of other elements is within the range of the present embodiment. In this case, the machinability of the steel material is reduced.
Therefore, the Ni content is 0 to 0.80%, and when it is contained, it is 0.80% or less (that is, more than 0 to 0.80%).
The lower limit of the Ni content is preferably 0.01%, more preferably 0.05%, still more preferably 0.10%. The preferred upper limit of the Ni content is 0.70%, more preferably 0.60%, still more preferably 0.40%, still more preferably 0.20%.

V: 0.30% or less Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V forms V precipitates (VC, VN, V (CN), etc.), similar to Nb. The V precipitate suppresses the coarsening of crystal grains in the gas carburizing treatment due to the pinning effect. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are increased. If V is contained even in a small amount, the above effect can be obtained to some extent.
However, if the V content exceeds 0.30%, the steel material becomes too hard even if the content of other elements is within the range of the present embodiment. In this case, the machinability of the steel material is reduced.
Therefore, the V content is 0 to 0.30%, and when it is contained, it is 0.30% or less (that is, more than 0 to 0.30%).
The lower limit of the V content is preferably 0.01%, more preferably 0.03%, still more preferably 0.04%. The preferred upper limit of the V content is 0.20%, more preferably 0.15%, still more preferably 0.10%.

Mg: 0.0035% or less Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg deoxidizes steel, similar to Al. In this case, the formation of coarse oxides is suppressed. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are increased. If even a small amount of Mg is contained, the above effect can be obtained to some extent.
However, if the Mg content exceeds 0.0035%, the formation of coarse Mg oxides in the steel material is promoted even if the content of other elements is within the range of the present embodiment. In this case, the limit machining rate during hot machining decreases.
Therefore, the Mg content is 0 to 0.0035%, and when it is contained, it is 0.0035% or less (that is, more than 0 to 0.0035%).
The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0003%, still more preferably 0.0005%. The preferred upper limit of the Mg content is 0.0030%, more preferably 0.0028%, still more preferably 0.0025%, still more preferably 0.0020%.

Ca: 0.0030% or less Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca refines sulfides in steel. Ca further promotes spheroidization of sulfides in steel. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are increased. If even a small amount of Ca is contained, the above effect can be obtained to some extent.
However, if the Ca content exceeds 0.0030%, coarse Ca oxides are formed in the steel material even if the content of other elements is within the range of this embodiment. In this case, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are reduced.
Therefore, the Ca content is 0 to 0.0030%, and when it is contained, it is 0.0030% or less (that is, more than 0 to 0.0030%).
The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0002%, still more preferably 0.0005%, still more preferably 0.0007%, still more preferably 0.0010. %. The preferred upper limit of the Ca content is 0.0025%, more preferably 0.0022%, still more preferably 0.0020%.

Rare earth element (REM): 0.0050% or less Rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When contained, the REM dissolves in the sulfide in the steel material and suppresses the stretching of MnS. As a result, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are increased. If even a small amount of REM is contained, the above effect can be obtained to some extent.
However, if the REM content exceeds 0.0050%, coarse oxides will be produced even if the content of other elements is within the range of this embodiment. In this case, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are reduced.
Therefore, the REM content is 0 to 0.0050%, and when contained, it is 0.0050% or less (that is, more than 0 to 0.0050%).
The preferred lower limit of the REM content is 0.0001%, more preferably 0.0010%, still more preferably 0.0020%. The preferred upper limit of the REM content is 0.0045%, more preferably 0.0040%, still more preferably 0.0035%, still more preferably 0.0030%.

In the present specification, the REMs are scandium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to lutetium having an atomic number of 71 (Lutetium). It is one or more elements selected from the group consisting of Lu). The REM content in the present specification is the total content of these elements.

[About equation (1)]
The chemical composition of the steel material of the present embodiment further satisfies the formula (1) on the premise that the content of each element is within the range of the present embodiment.
Si / Mn ≧ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (1).

It is defined as F1 = Si / Mn. Si and Mn form MnO-SiO ₂ in the process of deoxidation. MnO-SiO ₂ has a melting point of about 1250 ° C. Therefore, it is liquid in the molten metal before solidification, but becomes solid in the steel pieces after solidification, and becomes vitrified soft inclusions.

The inclusions are stretched and fragmented during hot working (hot rolling) to be refined. Therefore, the bending fatigue strength of the mechanical structural parts is improved. In order to obtain fine MnO-SiO ₂ , it is necessary to appropriately control the ratio of Si to Mn. This index is F1.

As F1 increases, the bending fatigue strength of the mechanical structural parts manufactured from the steel material of the present embodiment increases. When F1 is 1.00 or more, the bending fatigue strength is higher than that of SCM420H specified in JIS G4052 (2016). Therefore, when F1 satisfies the formula (1), that is, when F1 is 1.00 or more, it is assumed that the content of each element is within the range of the present embodiment and F2 satisfies the formula (2). The bending fatigue strength of the mechanical structural parts manufactured using the steel material of the present embodiment is sufficiently increased.

The preferred lower limit of F1 is 1.05, more preferably 1.07, and even more preferably 1.10. The upper limit of F1 is not particularly limited. However, considering the content of each element in the chemical composition of the present embodiment, the preferred upper limit of F1 is 2.10, more preferably 2.00, and even more preferably 1.70.

[About the microstructure of steel materials]
The microstructure of the steel material of the present embodiment contains ferrite, the balance is pearlite and / or bainite, and the surface integral of ferrite is 50 to 70%.

If the surface integral of ferrite is less than 50%, the surface integral of pearlite and / or bainite is too high in the steel material. In this case, the hardness of the steel material is excessively increased. As a result, the machinability of the steel material is reduced.
On the other hand, if the surface integral of the ferrite exceeds 70%, the crystal grain size tends to vary during the gas carburizing treatment. Therefore, heat treatment deformation occurs excessively during the gas carburizing treatment.

When the surface integral of ferrite is 50 to 70% and the balance other than ferrite in the microstructure is pearlite and / or bainite, the machinability of the steel material is sufficiently enhanced. Further, heat treatment deformation during gas carburizing treatment can be suppressed.

In the steel material of the present embodiment, the microstructure at each cross-sectional observation position and each vertical cross-sectional observation position contains 50 to 70% ferrite in area fraction, and the balance is composed of pearlite and / or bainite.

The preferable lower limit of the surface integral of ferrite at each observation position is 52%, more preferably 55%, still more preferably 57%. The preferred upper limit of the surface integral of ferrite at each observation position is 68%, more preferably 65%, still more preferably 63%.

[Variation of microstructure in cross section of steel material]
Further, in the steel material of the present embodiment, the variation of the microstructure is sufficiently suppressed in the cross section which is the cross section perpendicular to the longitudinal direction of the steel material. This point will be described below.

FIG. 2 is a schematic cross-sectional view of the steel material of the present embodiment, which is a cross section perpendicular to the longitudinal direction. With reference to FIG. 2, the cross-sectional CS of the steel material has a circular shape with a radius R. In this cross-section CS, the center position C1 of the cross-section CS and the position of R / 2 in the radial direction from the center position C1 of the cross-section CS are arranged around the center of the cross-section CS at a pitch of 45 °. The R / 2 positions C2 to C9 at the locations are defined as the "cross-sectional observation positions" C1 to C9 at the nine locations.

The microstructure at the cross-sectional observation positions C1 to C9 satisfies the following (A) and (B).
(A) The arithmetic mean value of the area fraction of ferrite at the cross-sectional observation positions C1 to C9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.
(B) Among the average particle diameters of ferrites at the cross-sectional observation positions C1 to C9, the ratio of the maximum average particle diameter to the minimum average particle diameter is 2.00 or less.
Hereinafter, (A) and (B) will be described in detail.

[About (A)]
As described in (A) above, in the steel material of the present embodiment, the arithmetic mean value of the area fraction of ferrite at the cross-sectional observation positions C1 to C9 is 50 to 70%, and the standard deviation of the area fraction of ferrite. Is 4.0% or less.

Since the standard deviation of the ferrite area fraction is 4.0% or less, the variation in the phase fraction of the microstructure at each cross-sectional observation position C1 to C9 is sufficiently suppressed. Therefore, it is possible to suppress variations in the occurrence timing of martensitic transformation at each cross-sectional observation positions C1 to C9 during the gas carburizing treatment.

If the standard deviation of the area fraction of ferrite at the cross-sectional observation positions C1 to C9 exceeds 4.0%, the phase fraction varies greatly at each of the cross-sectional observation positions C1 to C9. In this case, the heat treatment deformation during the gas carburizing treatment cannot be sufficiently suppressed.
Therefore, the standard deviation of the surface integral of ferrite at the cross-sectional observation positions C1 to C9 is 4.0% or less.
The preferred upper limit of the standard deviation of the surface integral of ferrite is 3.8%, more preferably 3.5%, still more preferably 3.0%. The lower limit of the standard deviation of the surface integral of ferrite is not particularly limited. The preferred lower limit of the standard deviation of the surface integral of ferrite is 0.1%, more preferably 0.5%, still more preferably 1.0%, still more preferably 1.5%.

[About (B)]
Of the average grain sizes of ferrites at the cross-sectional observation positions C1 to C9, the ratio of the maximum average grain size to the smallest average grain size is referred to as "ferrite average grain size ratio". The ferrite average particle size ratio in the cross section is defined by the following equation.
Ferrite average particle size ratio = (maximum value of ferrite average particle size in C1 to C9) / (minimum value of ferrite average particle size in C1 to C9)

In the steel material of the present embodiment, the ferrite average particle size ratio at the cross-sectional observation positions C1 to C9 is 2.00 or less. In this case, the variation in the average particle size of the ferrite at each of the cross-sectional observation positions C1 to C9 is sufficiently suppressed. That is, the ferrite grains at each position are aligned. Therefore, it is possible to sufficiently suppress the variation in the occurrence of martensitic transformation during the carburizing treatment. Therefore, it is possible to suppress heat treatment deformation of the steel material during the carburizing treatment.

When the ferrite average particle size ratio exceeds 2.00, the ferrite grains at each cross-sectional observation position C1 to C9 are scattered. In this case, the heat treatment deformation of the steel material during the gas carburizing treatment cannot be sufficiently suppressed. Therefore, the ferrite average particle size ratio is 2.00 or less.

The preferred upper limit of the ferrite average particle size ratio is 1.95, more preferably 1.90, and even more preferably 1.80. The lower limit of the ferrite average particle size ratio is not particularly limited. The preferred lower limit of the ferrite average particle size ratio is 1.10, more preferably 1.20, still more preferably 1.30, and even more preferably 1.40.

[Variation of microstructure in vertical section in steel material]
Further, in the steel material of the present embodiment, the variation in microstructure is sufficiently suppressed not only in the above-mentioned cross section but also in the vertical cross section which is parallel to the longitudinal direction of the steel material and includes the central axis of the steel material. .. In the steel material of the present embodiment, the variation in the microstructure not only in the cross section but also in the vertical section is sufficiently suppressed, so that the heat treatment deformation generated three-dimensionally can be sufficiently suppressed. Hereinafter, the suppression of the variation in the microstructure in the vertical cross section will be described.

FIG. 3 is a schematic view of a vertical cross section that is parallel to the longitudinal direction of the steel material of the present embodiment and is a cross section including the central axis. With reference to FIG. 3, in the vertical cross-sectional LS of the steel material, the three central axis positions L1 to L3 arranged at the R / 2 pitch on the central axis CL1 of the steel material, and R in the radial direction from each central axis position. The six R / 2 positions L4 to L9 arranged at the / 2 position are defined as the nine “longitudinal section observation positions” L1 to L9.

The microstructures at the above-mentioned nine vertical cross-sectional observation positions L1 to L9 satisfy the following (C) and (D).
(C) The arithmetic mean value of the area fraction of ferrite at the vertical cross-sectional observation positions L1 to L9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.
(D) Among the average particle diameters of ferrites at the vertical cross-sectional observation positions L1 to L9, the ratio of the maximum average particle size to the minimum average particle size (ferrite average particle size ratio) is 2.00 or less.
Hereinafter, (C) and (D) will be described in detail.

[About (C)]
As described in (C) above, in the steel material of the present embodiment, the arithmetic mean value of the area fraction of ferrite at the vertical cross-sectional observation positions L1 to L9 is 50 to 70%, and the standard deviation of the area fraction of ferrite. Is 4.0% or less.

Since the standard deviation of the area fraction of ferrite is 4.0% or less, the variation in the phase fraction of the microstructure at each vertical section observation position L1 to L9 is sufficiently suppressed. Therefore, it is possible to suppress variations in the occurrence timing of martensitic transformation at each vertical cross-sectional observation positions L1 to L9 during the gas carburizing treatment.

If the standard deviation of the area fraction of ferrite at the vertical section observation positions L1 to L9 exceeds 4.0%, the phase fraction varies greatly at each vertical section observation positions L1 to L9. In this case, the heat treatment deformation during the gas carburizing treatment cannot be sufficiently suppressed.
Therefore, the standard deviation of the surface integral of ferrite at the vertical cross-sectional observation positions L1 to L9 is 4.0% or less.
The preferred upper limit of the standard deviation of the surface integral of ferrite is 3.8%, more preferably 3.5%, still more preferably 3.0%. The lower limit of the standard deviation of the surface integral of ferrite is not particularly limited. The preferred lower limit of the standard deviation of the surface integral of ferrite is 0.1%, more preferably 0.5%, still more preferably 1.0%, still more preferably 1.5%.

[About (D)]
Of the average grain sizes of ferrites at the vertical cross-sectional observation positions L1 to L9, the ratio of the maximum average grain size to the smallest average grain size is referred to as "ferrite average grain size ratio". The ferrite average particle size ratio in the vertical section is defined by the following equation.
Ferrite average particle size ratio = (maximum value of ferrite average particle size in L1 to L9) / (minimum value of ferrite average particle size in L1 to L9)

In the steel material of the present embodiment, the ferrite average particle size ratio at the vertical cross-sectional observation positions L1 to L9 is 2.00 or less. In this case, the variation in the average particle size of the ferrite at each of the vertical cross-sectional observation positions L1 to L9 is sufficiently suppressed. That is, the ferrite grains at each position are aligned. Therefore, it is possible to sufficiently suppress the variation in the occurrence of martensitic transformation during the carburizing treatment. Therefore, it is possible to suppress heat treatment deformation of the steel material during the carburizing treatment.

When the ferrite average particle size ratio exceeds 2.00, the ferrite grains at each vertical cross-sectional observation position L1 to L9 are scattered. In this case, the heat treatment deformation of the steel material during the gas carburizing treatment cannot be sufficiently suppressed. Therefore, the ferrite average particle size ratio at the nine vertical cross-sectional observation positions L1 to L9 is 2.00 or less.

[Method of observing microstructure at each observation position, method of measuring surface integral of ferrite and average grain size ratio of ferrite]
The method for observing the microstructure at the cross-sectional observation positions C1 to C9 and the vertical cross-sectional observation positions L1 to L9 of the steel material of the present embodiment, and the method for measuring the area fraction of ferrite and the average grain size ratio of ferrite are as follows. be.

[Method of observing microstructure of cross-sectional CS]
The method of observing the microstructure of the cross-sectional CS is as follows. A sample including each cross-sectional observation position C1 to C9 is collected from the steel material. Of the surfaces of the sample, the surface corresponding to the cross section CS is used as the observation surface. On the observation surface, the observation field of view including the cross-sectional observation position is 0.5 mm × 1.0 mm.

After polishing the observation surface of the sample, etch the observation surface with 3% alcohol nitrate (Nital corrosive liquid). The observation field of view (0.5 mm × 1.0 mm) of the etched observation surface is observed with a 100x optical microscope.

In the observation field of view, the contrast of each phase such as ferrite, pearlite, and bainite is different for each phase. Specifically, in the observation field of view, ferrite is observed to be white, and bainite and pearlite are observed to be blacker than ferrite. Therefore, ferrite can be easily distinguished from other phases (pearlite and bainite). Identify ferrite based on contrast.

[About (A)]
The area of ferrite (μm ² ) in each observation field (each cross-section observation position) is obtained. Using the area of the ferrite and the area of the observation field of view, the surface integral (%) of the ferrite in each observation field of view (each cross-sectional observation position) is obtained.

[How to calculate the arithmetic mean value of ferrite surface integral]
The arithmetic mean value of the ferrite area fraction (%) at the nine observation fields (cross-section observation positions) is the arithmetic mean value (%) of the ferrite area fraction at the nine cross-section observation positions C1 to C9. Define.

[How to find the standard deviation of the surface integral of ferrite]
The standard deviation (%) of the ferrite area fractions at the nine cross-sectional observation positions C1 to C9 is calculated from the ferrite area fractions (%) at the nine observation fields (cross-section observation positions). The standard deviation here is the sample standard deviation.

[About (B)]
The area (μm ² ) of each ferrite grain observed in each of the above-mentioned observation fields (each cross-sectional observation position C1 to C9) is measured. Obtain the arithmetic mean value of the area of each ferrite grain. The circle-equivalent diameter of the arithmetic mean value of the obtained area is defined as the average particle size (μm) of ferrite at each cross-sectional observation position C1 to C9. Here, the equivalent circle diameter means the diameter (μm) of a circle having the same area as the arithmetic mean value of the area of the ferrite grains.

Obtain the average particle size of the ferrites at the cross-sectional observation positions C1 to C9 at 9 locations. Then, among the average particle diameters of these ferrites, the maximum average particle diameter (μm) and the minimum average particle diameter (μm) of the ferrites are specified. The ratio of the maximum average particle size to the specified minimum average particle size (ferrite average particle size ratio) is obtained.

[Method of observing microstructure of longitudinal section LS]
The method of observing the microstructure of the vertical cross section LS is as follows. A sample including each vertical cross-sectional observation position L1 to L9 is taken from the steel material. Of the surface of the sample, the surface corresponding to the vertical cross section LS is used as the observation surface. On the observation surface, the observation field of view including the vertical cross-sectional observation position is 0.5 mm × 1.0 mm. More specifically, the length of 0.5 mm of the observation field of view is defined as the radial direction of the steel material, and 1.0 mm is defined as the longitudinal direction of the steel material.

After polishing the observation surface of the sample, etch the observation surface with 3% alcohol nitrate (Nital corrosive liquid). The observation field of view (0.5 mm × 1.0 mm) of the etched observation surface is observed with a 100x optical microscope. Each phase in the observation field is identified by the same method as the microstructure observation of the cross-sectional CS.

[About (C)]
Among the phases specified by the above method, the area of ferrite (μm ² ) in each observation field (each vertical cross-section observation position) is obtained. Using the area of the ferrite and the area of the observation field of view, the surface integral (%) of the ferrite in each observation field of view (each vertical cross-sectional observation position) is obtained.

[How to find the arithmetic mean value of the surface integral of ferrite]
The arithmetic mean value of the ferrite area fraction (%) in the nine observation fields (vertical cross-section observation positions) is the arithmetic mean value (%) of the ferrite area fraction in the nine vertical cross-section observation positions L1 to L9. Define.

[How to find the standard deviation of the surface integral of ferrite]
The standard deviation (%) of the ferrite area fractions at the nine vertical cross-sectional observation positions L1 to L9 is calculated from the ferrite area fractions (%) at the nine observation fields (vertical cross-sectional observation positions).

[About (D)]
The area (μm ² ) of each ferrite grain observed in each of the above-mentioned observation fields (each vertical cross-sectional observation position L1 to L9) is measured. Obtain the arithmetic mean value of the area of each ferrite grain. The circle-equivalent diameter of the arithmetic mean value of the obtained area is defined as the average particle size (μm) of ferrite at each vertical cross-sectional observation position L1 to L9.

Obtain the average particle size of ferrites at 9 vertical cross-sectional observation positions L1 to L9. Then, among the average particle diameters of these ferrites, the maximum average particle diameter (μm) and the minimum average particle diameter (μm) of the ferrites are specified. The ratio of the maximum average particle size to the specified minimum average particle size (ferrite average particle size ratio) is obtained.

[About equation (2)]
The chemical composition of the steel material of the present embodiment further satisfies the following formula (2).
1- (0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo) <0.800 (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (2). If the corresponding element is not contained, "0" is substituted for the element symbol.

In the steel material of the present embodiment, not only the cross-sectional CS but also the microstructure of the vertical cross-sectional LS is made uniform. However, the microstructures of the cross-sectional observation positions C1 to C9 in the cross-section CS satisfy (A) and (B), and the microstructures of the vertical cross-section observation positions L1 to L9 in the vertical cross-section LS satisfy (C) and. By satisfying (D), even if the microstructure is made uniform, as described above, a minute time zone in which the martensitic transformed portion and the martensitic untransformed portion coexist always occurs at the time of quenching of the vacuum carburizing treatment. If the amount of heat treatment strain in the martensite untransformed portion is large in this minute time zone, heat treatment deformation will occur. Therefore, the steel material of the present embodiment further satisfies the formula (2).

It is defined as F2 = 1- (0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo). F2 is an index relating to the amount of heat treatment deformation of the steel material in the gas carburizing treatment with respect to the steel material. Among the elements in the above-mentioned chemical composition, C, Si, Mn, Cr and Mo contained in F2 are particularly martensite in a minute time zone in which a martensitic transformed portion and a martensitic untransformed portion are mixed at the time of quenching. Increases the strength of untransformed parts.

With reference to FIG. 1, as F2 decreases, the maximum deformation amount ratio decreases, and the heat treatment deformation amount decreases. When F2 is less than 0.800, the maximum deformation amount ratio is remarkably lowered. That is, the maximum deformation amount ratio with respect to F2 has an inflection point near F2 = 0.800.

Therefore, on the premise that the content of each element is within the range of the present embodiment, further, when F2 satisfies the formula (2), that is, when F2 is less than 0.800, the steel material at the time of gas carburizing treatment. The heat treatment deformation of the above can be sufficiently suppressed.

The preferred upper limit of F2 is 0.799, more preferably 0.797, and even more preferably 0.795. The lower limit of F2 is not particularly limited. However, considering the upper limit of the content of each element in the chemical composition of the present embodiment, the preferable lower limit of F2 is 0.765, more preferably 0.770, and even more preferably 0.775. The numerical value of F2 is a value obtained by rounding off to the fourth decimal place.

The steel material of the present embodiment having the above structure has a cross section in which the content of each element in the chemical composition is within the range of the present embodiment, and F1 and F2 satisfy the formulas (1) and (2). The microstructures at the observation positions C1 to C9 and the vertical cross-sectional observation positions L1 to L9 are within the scope of the present embodiment. Therefore, the machinability after hot working on the steel material of the present embodiment is excellent. Further, when the steel material of the present embodiment is subjected to the vacuum carburizing treatment, the mechanical structural parts have excellent bending fatigue strength and surface fatigue strength, and can sufficiently suppress heat treatment deformation.

[About the microstructure of steel materials]
The steel material of the present embodiment is a so-called as-rolled material (azurol material). Therefore, in the steel material of the present embodiment, a so-called band structure is observed in the above observation fields of the vertical cross-sectional observation positions L1 to L9. Here, the band structure is a well-known microstructure, and as shown in FIG. 4, a ferrite (ferrite band) F extending in the longitudinal direction of the steel material and a non-ferrite (non-ferrite band) NF extending in the longitudinal direction of the steel material are used. However, it means a structure that is alternately laminated in the radial direction. Non-ferrites are pearlite and / or bainite.

[Use of steel materials]
As described above, the steel material of the present embodiment is suitable as a material for mechanical structural parts. The steel material of the present embodiment is particularly suitable for applications such as gears or shafts of automobile applications, construction machinery, industrial machinery and the like.

[Manufacturing method of steel materials]
An example of the method for manufacturing a steel material of the present embodiment will be described. The method for manufacturing a steel material described below is an example for manufacturing the steel material of the present embodiment. Therefore, the steel material having the above-mentioned structure may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferable example of the manufacturing method of the steel material of the present embodiment.

An example of the method for producing a steel material of the present embodiment includes the following steps.
(Process 1) Process of preparing materials (Material preparation process)
(Process 2) A process of hot-working a material to manufacture a steel material (hot-working process)
Hereinafter, each step will be described.

[(Process 1) Material preparation process]
In the material preparation process, the steel material of the present embodiment is prepared. Specifically, a molten steel in which the content of each element in the chemical composition is within the range of the present embodiment, F1 satisfies the formula (1), and F2 satisfies the formula (2) is produced. The refining method is not particularly limited, and a well-known method may be used. For example, refining in a converter (primary refining) is performed on hot metal produced by a well-known method. Well-known secondary refining will be carried out on the molten steel discharged from the converter. In the secondary refining, the content of the alloying elements in the molten steel is adjusted so that the content of each element is within the range of the present embodiment, F1 satisfies the formula (1), and F2 satisfies the formula (2). Manufacture molten steel with a chemical composition that meets the requirements.

Using the molten steel produced by the above-mentioned refining method, the material is manufactured by a well-known casting method. For example, an ingot may be manufactured by an ingot method using molten steel. Further, bloom or billet may be produced by a continuous casting method using molten steel. The material (ingot, bloom or billet) is manufactured by the above method. When the continuous casting method is used, reduction may be applied to the slab in the middle of solidification.

[(Step 2) Hot working process]
In the hot working step, the material (ingot, bloom or billet) prepared in the material preparing step is hot-worked to produce the steel material of the present embodiment. The shape of the steel material is not particularly limited, but is, for example, steel bar or wire rod. In the following description, a case where the steel material is steel bar will be described as an example. However, even if the steel material has a shape other than that of steel bar, it can be manufactured by the same hot working process.

The hot working process includes the following steps.
(Step 21) Ingot rolling step Heating temperature: 1250 to 1300 ° C
Holding time: 10 hours or more (process 22) Finish rolling process Heating temperature: 1150 to 1200 ° C
Holding time: 1.5 to 3.0 hours Finishing temperature: 950 to 1000 ° C
(Step 23) Temperature holding step Average cooling rate at 900 to 800 ° C .: 0.05 ° C / sec or less (Step 24) Cooling step Average cooling rate at 800 to 300 ° C: 0.10 to 1.00 ° C / sec Hereinafter, each step will be described.

[(Step 21) Ingot rolling step]
In the lump rolling process, the material is hot-rolled to produce billets. Specifically, in the lump-rolling step, the material is hot-rolled (bulk-rolled) by a lump-rolling machine to manufacture billets. When a continuous rolling mill is located downstream of the lump rolling mill, hot rolling is further performed on the billet after lump rolling using the continuous rolling mill to produce a smaller billet. You may. In a continuous rolling mill, horizontal stands having a pair of horizontal rolls and vertical stands having a pair of vertical rolls are alternately arranged in a row. As described above, in the slab rolling process, the material is produced in billets using a slab rolling mill or using a shunt rolling mill and a continuous rolling mill.

The conditions in the lump rolling process are as follows.
Heating temperature: 1250 to 1300 ° C
Holding time: 10 hours or more The heating temperature in the heating furnace in the lump rolling process is 1250 to 1300 ° C. The holding time at the heating temperature (1250-1300 ° C.) is 10 hours or more. If the heating temperature in the heating furnace in the lump rolling process is 1250-1300 ° C. and the holding time at the heating temperature (1250-1300 ° C.) is 10 hours or more, other production conditions must be satisfied. As a premise, the solidification segregation in the material generated during the material preparation process can be sufficiently alleviated. Therefore, the standard deviation of the surface integral of the ferrite at each of the cross-sectional observation positions C1 to C9 and each of the vertical section observation positions L1 to L9 is 4.0% or less. The upper limit of the holding time at the heating temperature is not particularly limited. However, considering the manufacturing cost, the preferable upper limit of the holding time at the heating temperature is 30 hours.

The billet manufactured by the lump-rolling process is allowed to cool (air-cooled) to room temperature before the finish rolling process.

The surface reduction rate in the ingot rolling process is 30% or more. Here, the reduction rate (%) is defined by the following equation.
Surface reduction rate (%) = (1-Area of cross section (cross section perpendicular to longitudinal direction) of steel material after lump rolling / Area of cross section (cross section perpendicular to longitudinal direction) of material before lump rolling) × 100

If the surface reduction rate in the ingot rolling process is 30% or more, the area of ferrite at each cross-section observation position C1 to C9 and each longitudinal cross-section observation position L1 to L9, assuming that other manufacturing conditions are satisfied. The standard deviation of the fraction is 4.0% or less.

[(Process 22) Finish rolling process]
In the finish rolling process, first, the billet cooled to room temperature is heated using a heating furnace. The billet after heating is hot-rolled using a continuous rolling mill to produce steel bars, which are steel materials.

The conditions in the finish rolling process are as follows.
Heating temperature: 1150-1200 ° C
Holding time: 1.5 to 3.0 hours Finishing temperature: 950 to 1000 ° C

[Heating temperature and holding time]
The heating temperature in the heating furnace in the finish rolling step is 1150 to 1200 ° C. The holding time at the heating temperature (1150 to 1200 ° C.) is 1.5 to 3.0 hours. If the heating temperature of the heating furnace in the finish rolling process is 1150 to 1200 ° C. and the holding time at the heating temperature (1150-1200 ° C.) is 1.5 to 3.0 hours, other manufacturing conditions are satisfied. Assuming that, the temperature variation in the steel material (steel bar) can be sufficiently suppressed. Therefore, the ferrite average particle size ratio at the cross-sectional observation positions C1 to C9 and the vertical cross-sectional observation positions L1 to L9 is 2.00 or less.

[Finishing temperature]
In the finish rolling process, hot rolling (finish rolling) is carried out by a continuous rolling machine equipped with a plurality of rolling stands arranged in a row. In hot rolling using a continuous rolling mill, the temperature of the steel material on the outlet side of the stand where the steel material is finally rolled is defined as the finishing temperature (° C.). The steel material temperature means the surface temperature of the steel material.

The finishing temperature is 950 to 1000 ° C. When the finishing temperature is 950 to 1000 ° C., the variation in the austenite particle size in the steel material (steel bar) is sufficiently suppressed on the premise that other production conditions are satisfied. Therefore, when the austenite is transformed into ferrite in the temperature holding step and the cooling step described later, the variation in the average particle size of ferrite is sufficiently suppressed. Therefore, the ferrite average particle size ratio at the cross-sectional observation positions C1 to C9 and the vertical cross-sectional observation positions L1 to L9 is 2.00 or less.

[(Step 23) Temperature holding step]
In the temperature holding step, the temperature of the steel material is held after the finish rolling step and before the cooling step. The conditions in the temperature holding process are as follows.
Average cooling rate when the steel material temperature is 900 to 800 ° C: 0.05 ° C / sec or less After the finish rolling process, the average cooling rate when the steel material temperature is 900 to 800 ° C is suppressed to 0.05 ° C / sec or less. For example, after the finish rolling process, the average cooling rate of the steel material having a steel material temperature of 900 to 800 ° C. is 0.05 ° C./sec using a slow cooling cover, a heat retaining cover, or a temperature holding furnace. Keep it below.

If the average cooling rate at a steel material temperature of 900 to 800 ° C. is 0.05 ° C./sec or less, temperature variation in the axial direction (longitudinal direction) of the steel material can be suppressed on the premise that other manufacturing conditions are satisfied. Therefore, it is possible to suppress variations in the ferrite transformation timing in the axial direction of the steel material. Therefore, in particular, it is possible to suppress the growth variation of the ferrite grains in the axial direction (longitudinal section) of the steel material. Specifically, the following mechanism works.

In the steel material after the finish rolling process, austenite gradually transforms into ferrite as the steel material temperature decreases. If the temperature of the steel material varies in the axial direction in the range of 900 to 800 ° C., ferrite produced at a relatively early stage after the finish rolling process and ferrite produced at a relatively late stage coexist. It ends up. In this case, the ferrite grains produced at an early stage tend to be coarser than the ferrite grains transformed at a later stage. As a result, in particular, the variation of ferrite grains becomes large in the axial direction (longitudinal section) of the steel material.

If the average cooling rate at a steel material temperature of 900 to 800 ° C. is high, the temperature variation in the axial direction (longitudinal section) of the steel material becomes large. Therefore, the ferrite average particle size ratio at the vertical cross-sectional observation positions L1 to L9 becomes large.

Therefore, in the present embodiment, the average cooling rate when the steel material temperature is 900 to 800 ° C. is suppressed to 0.05 ° C./sec or less. In this case, temperature variation in the axial direction (longitudinal cross section) of the steel material can be suppressed. Therefore, the deviation of the ferrite formation (transformation) timing in the vertical cross section of the steel material is suppressed. As a result, the ferrite average particle size ratio at the vertical cross-sectional observation positions L1 to L9 can be set to 2.00 or less, provided that other manufacturing conditions are satisfied.

[(Step 24) Cooling step]
In the cooling step, the temperature of the steel material after the temperature holding step is cooled. The conditions in the cooling process are as follows.
Average cooling rate at a steel temperature of 800 to 300 ° C: 0.10 to 1.00 ° C / sec An average cooling rate of 0.10 to 1.00 ° C / sec for a steel with a steel temperature of 800 to 300 ° C. Cool with. Assuming that the average cooling rate at a steel material temperature of 800 to 300 ° C. is 0.10 to 1.00 ° C./sec, the cross-sectional observation positions C1 to C9 and the vertical cross-sectional observation positions are assumed to satisfy other manufacturing conditions. The arithmetic mean value of the area fraction of the ferrite in L1 to L9 is 50 to 70%.

By the above manufacturing process, the steel material of the present embodiment having the above-mentioned configuration can be manufactured. As described above, the steel material of the present embodiment is a rolled material.

The steel material of this embodiment is suitable as a material for mechanical structural parts manufactured by performing vacuum carburizing treatment. However, the steel material of the present embodiment may be manufactured as a mechanical structural part by subjecting a surface hardening heat treatment other than the vacuum carburizing treatment. Other surface hardening heat treatments include, for example, quenching and tempering, induction hardening and tempering, and soaking treatment (quenching quenching and tempering).

[About machine structural parts]
Mechanical structural parts are used, for example, in automobiles, construction vehicles, and the like. Mechanical structural parts are, for example, gears and shafts used in steering mechanisms.

The mechanical structural parts made of the steel material of this embodiment are manufactured by a well-known manufacturing method. For example, mechanical structural parts are manufactured by the following method.

An example of a method for manufacturing mechanical structural parts includes the following steps.
-Hot working process-Cutting process-Heat treatment process Each process will be described below.

[Hot working process]
In the hot working step, hot working is performed on the steel material of the present embodiment. Hot working is, for example, well-known hot forging. The heating temperature in the hot working step is, for example, 1000 to 1300 ° C. The steel material after hot working is allowed to cool (air cooling). If necessary, the steel material after allowing to cool may be annealed.

[Cutting process]
A cutting process is performed on the steel material after the hot working process to manufacture an intermediate product having a predetermined shape. During this cutting process, high machinability of steel materials is required. In the cutting process, well-known cutting is performed. By cutting, it is possible to manufacture precision-shaped machine structural parts, which is difficult only with the hot working process.

[Heat treatment process]
Heat treatment is performed on the intermediate product after cutting. Here, the "heat treatment" includes a well-known vacuum carburizing treatment and a well-known tempering step. As described above, the vacuum carburizing treatment also includes the vacuum carburizing nitriding treatment.

The vacuum carburizing process includes a vacuum carburizing step and a quenching step. It is a well-known technical matter to those skilled in the art that in the vacuum carburizing treatment, well-known conditions can be appropriately adjusted to appropriately adjust the surface hardness, core hardness, and surface carbon concentration of mechanical structural parts.
Hereinafter, a well-known vacuum carburizing process will be described as an example of the heat treatment step. It is well known to those skilled in the art that the well-known vacuum carburizing nitriding treatment is also carried out in the same process as the vacuum carburizing treatment.

[Vacuum carburizing and quenching]
The vacuum carburizing and quenching process includes a vacuum carburizing step and a quenching step. Hereinafter, the vacuum carburizing process and the quenching process will be described.

[Vacuum carburizing process]
FIG. 5 is a diagram showing an example of a heat pattern in the vacuum carburizing step S10 and the quenching step S20. The vertical axis of FIG. 5 is the treatment temperature (° C.) during the vacuum carburizing treatment, and the horizontal axis is the time (minutes). With reference to FIG. 5, the vacuum carburizing step S10 includes a heating step S0, a soaking step S1, a carburizing step S2, and a diffusion step S3.

In the heating step S0, the intermediate product charged in the furnace is heated to the carburizing temperature Tc. At this time, the pressure in the furnace is set to 10 Pa or less. The carburizing temperature Tc in the heating step S0 is, for example, 900 to 1100 ° C.

In the soaking step S1, the intermediate product is held for a predetermined time (holding time t1) at the carburizing temperature Tc, and the soaking heat treatment is carried out. The holding time t1 at the carburizing temperature Tc in the soaking step S1 is, for example, 5 to 120 minutes. The pressure in the furnace in the soaking step S1 may be 10 Pa or less, or the nitrogen gas may be introduced and evacuated by a vacuum pump at the same time to create a nitrogen gas atmosphere of 1000 Pa or less.

In the carburizing step S2, the intermediate product is held for a predetermined time (holding time t2) at the carburizing temperature Tc. The holding time t2 at the carburizing temperature Tc in the carburizing step S2 may be appropriately adjusted. The holding time t2 at the carburizing temperature Tc is, for example, 20 to 60 minutes.

As the carburizing gas in the carburizing step S2, a well-known carburizing gas is used. The carburized gas is, for example, a hydrocarbon gas such as acetylene, propane or ethylene.

The carburized gas pressure in the carburizing step S2 is set to a predetermined gas pressure according to the type of carburized gas. When acetylene is used as the carburizing gas, the carburizing gas pressure is, for example, 10 to 1000 Pa. When the carburized gas is propane, the carburized gas pressure is, for example, 200 to 3000 Pa.

In the diffusion step S3, the intermediate product is held for a predetermined time (holding time t3) at the carburizing temperature Tc. Here, the holding time t3 at the carburizing temperature Tc in the diffusion step S3 is appropriately adjusted. The holding time t3 at the carburizing temperature Tc is, for example, 40 to 90 minutes.

The pressure in the furnace in the diffusion step S3 may be 100 Pa or less in order to remove the residual gas in the carburizing step. Alternatively, the introduction of nitrogen gas and the vacuum exhaust by a vacuum pump may be performed at the same time to create a nitrogen gas atmosphere of 1000 Pa or less.

[Quenching process]
The quenching step S20 is carried out on the intermediate product after the vacuum carburizing step S10. As a cooling method up to the quenching temperature Ts in the quenching step S20 after the vacuum carburizing step S10, a well-known cooling method may be used. The cooling method may be, for example, air cooling under vacuum, gas cooling, or any other method. When cooling by vacuuming is performed, for example, cooling is performed at a pressure of 100 Pa or less. When gas cooling is carried out, an inactivated gas such as nitrogen gas and / or helium gas may be used as the cooling gas.

The quenching step S20 includes a heat soaking step S4. In the soaking step S4, the intermediate product after the vacuum carburizing step S10 is held at the quenching temperature. In the quenching step S20, after the soaking step S4, the intermediate product is rapidly cooled and quenched. The quenching temperature Ts is not particularly limited, but is, for example, 800 to 880 ° C. The holding time t4 at the quenching temperature Ts is not particularly limited, but is, for example, 10 to 80 minutes. The atmosphere during holding at the quenching temperature Ts is not particularly limited, but is, for example, a nitrogen gas atmosphere. The pressure in the furnace may be atmospheric pressure or less, for example, 400 hPa or less. The cooling method in the quenching treatment is oil cooling or water cooling. Specifically, an intermediate product maintained at a quenching temperature is immersed in a cooling bath containing oil or water as a cooling medium and rapidly cooled. The temperature of the oil or water that is the cooling medium is, for example, 60 to 200 ° C. Further, if necessary, sub-zero processing may be carried out.

[Tempering process]
A well-known tempering process is carried out for intermediate products after the quenching process. The tempering temperature is, for example, 100 to 200 ° C. The holding time at the tempering temperature is, for example, 90 to 150 minutes.

[Other processes]
If necessary, the intermediate product after the tempering step may be further subjected to a grinding process or a shot peening process. When grinding is performed, cutting is performed to give the steel material a shape. By cutting, a more precise shape can be given to the steel material. Further, when the shot peening treatment is carried out, the compressive residual stress is introduced into the surface layer portion of the intermediate product after the vacuum carburizing treatment. Compressive residual stress suppresses the generation and growth of fatigue cracks. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural parts are increased. The shot peening process may be carried out by a well-known method. For example, it is desirable that the shot peening treatment is performed using shot grains having a diameter of 0.7 mm or less and having an arc height of 0.4 mm or more.

A steel material having the chemical composition shown in Table 1 was prepared. The steel type number A corresponds to SCM420H specified in JIS G4052 (2016).

"-" In Table 1 means that the corresponding element content is 0% in the significant figures (values up to the smallest digit) specified in the embodiment. In other words, it means that the corresponding element content is 0% when the fractions of the specified significant figures (values up to the smallest digit) in the above-described embodiment are rounded off.
For example, the Mo content specified in this embodiment is specified by a numerical value up to the second decimal place. Therefore, the steel type number C in Table 1 means that the measured Mo content was 0% when rounded to the third decimal place.
In addition, the Nb content specified in this embodiment is specified by a numerical value up to the third decimal place. Therefore, the steel grade number A in Table 1 means that the measured Nb content was 0% when rounded to the fourth decimal place.
In addition, rounding means that if the digit (fraction) below the specified minimum digit is less than 5, it is rounded down, and if it is 5 or more, it is rounded up.

Steel materials were manufactured by the following method. The molten steel was continuously cast to produce a slab (bloom) as a raw material. The hot working process was carried out on the bloom material, which is the material, under the conditions shown in Table 2.

The temperature described in the "heating temperature (° C.)" column of the "bulk rolling process" and the "finish rolling process" column is the heating temperature (° C.). The time described in the "holding time (time)" column of the "lump rolling process" and the "finish rolling process" column is the holding time (hour) at the heating temperature. The temperature in the "Finishing temperature (° C)" column in the "Finish rolling process" column is the steel material temperature (surface temperature of the steel material) (° C) on the outlet side of the last rolled stand in the continuous rolling mill in the finish rolling process. Is. The speed described in the "cooling rate (° C./sec)" column of the "temperature holding step" column is the average cooling rate (° C./sec) when the steel material temperature is 900 to 800 ° C. The speed described in the "cooling rate (° C./sec)" column of the "cooling step" column is the average cooling rate (° C./sec) when the steel material temperature is 800 to 300 ° C.

In the slab rolling process of the hot working process, the produced bloom was heated at the heating temperature and holding time shown in Table 2. Then, bloom was lump-rolled using a lump-rolling machine to produce billets. The manufactured billet was water-cooled to room temperature (25 ° C.). The cross section perpendicular to the longitudinal direction of the billet was a rectangle of 162 mm × 162 mm. The heating temperature and holding time in the lump rolling process are as shown in Table 2. The surface reduction rate in the bulk rolling process was 30% or more under all manufacturing conditions.

The billet after the ingot rolling process was subjected to a finishing rolling process under the conditions shown in Table 2 to produce a steel material (steel bar) having a diameter of 50 mm. Specifically, the billet was heated at the heating temperature (° C.) and holding time (hours) shown in the finish rolling process column of Table 2. Finish rolling was carried out on the billets after heating to produce steel bars. At this time, the finishing temperature (° C.) was as shown in Table 2.

A temperature holding process was carried out on the steel material (steel bar) after the finish rolling process. The production conditions a to i were adjusted so that the average cooling rate was 0.05 ° C./sec or less by using a slow cooling cover for the steel material having a steel material temperature of 900 to 800 ° C. On the other hand, under the production condition j, the steel material having a steel material temperature of 900 to 800 ° C. was allowed to cool without using a slow cooling cover. Therefore, the average cooling rate at a steel material temperature of 900 to 800 ° C. was over 0.05 ° C./sec.

After the temperature holding process, the cooling process was carried out. Specifically, under each manufacturing condition, the average cooling rate (° C./sec) when the steel material temperature was 800 to 300 ° C. was as shown in the cooling process column of Table 2.

For steel materials with a steel material temperature of 300 ° C or less, it was allowed to cool (air-cooled) to room temperature. Through the above manufacturing process, the steel materials (steel bars) of test numbers 1 to 22 shown in Table 3 were manufactured. Note that Test No. 1 is an example using SCM420H, which is a reference steel, and uses manufacturing condition a, which is one of the manufacturing methods generally used for steel materials having a chemical composition of SCM420H.

[Evaluation test]
The following items were sought for the steel materials (steel bars) of each test number manufactured in the above manufacturing process.
(A1) Observation of microstructure in cross section (A11) Arithmetic mean value of area fraction of ferrite in cross section (A12) Standard deviation of area fraction of ferrite in cross section (A13) Ferrite average particle size ratio in cross section (A13) B1) Observation of microstructure in vertical section (B11) Arithmetic mean value of area fraction of ferrite in vertical section (B12) Standard deviation of area fraction of ferrite in vertical section (B13) Ferrite average particle size ratio in vertical section The following evaluation tests were carried out on the steel materials of each test number.
(C1) Machinability evaluation test (C2) Bending fatigue strength evaluation test (C3) Surface fatigue strength evaluation test (C4) Heat treatment deformation amount evaluation test The details will be described below.

[(A1) Observation of microstructure in cross section]
Nine samples including the respective cross-sectional observation positions C1 to C9 were taken from the steel material of each test number. Of the surfaces of each sample, the surface corresponding to the cross-sectional CS was used as the observation surface. On the observation surface, the observation field of view including the cross-sectional observation position was set to 0.5 mm × 1.0 mm.

After polishing the observation surface of the sample, the observation surface was etched with 3% alcohol nitrate (Nital corrosive liquid). The observation field of view (0.5 mm × 1.0 mm) of the etched observation surface was observed with a 100x optical microscope. In the observation field, the phase was identified by contrast.

The observed phases are shown in the "Phase" column of the "Cross section" column of Table 3. In the "Phase" column of the "Cross section" column of Table 3, when the microstructure at all the cross section observation positions contains ferrite and the balance consists of pearlite and / or bainite, "◯" is indicated. In all test numbers, the microstructure of the cross section contained ferrite and the balance was pearlite and / or bainite.

[(A11) Arithmetic mean value of surface integral of ferrite in cross section]
The area of ferrite (μm ² ) in each observation field (each cross-section observation position) was determined. Using the area of the ferrite and the area of the observation field of view, the surface integral (%) of the ferrite in each observation field of view (each cross-sectional observation position) was obtained.

The arithmetic mean value of the ferrite area fraction (%) at the nine observation fields (cross-section observation positions) is the arithmetic mean value (%) of the ferrite area fraction at the nine cross-section observation positions C1 to C9. Defined. The arithmetic mean value of the obtained ferrite surface integral is shown in the "ferrite surface integral (%)" column of the "cross section" column of Table 3.

[(A12) Standard deviation of surface integral of ferrite in cross section]
The standard deviation (%) of the ferrite area fractions at the nine cross-sectional observation positions C1 to C9 was calculated from the ferrite area fractions (%) at the nine observation fields (cross-sectional observation positions). The obtained standard deviation is shown in the "Standard deviation (%) of ferrite surface integral" column in the "Cross section" column of Table 3.

[(A13) Ferrite average particle size ratio in cross section]
Further, the area (μm ² ) of each ferrite grain observed in each of the above-mentioned observation fields (each cross-sectional observation position C1 to C9) was measured. Arithmetic mean values of the areas of each ferrite grain were obtained at each cross-sectional observation positions C1 to C9. The circle-equivalent diameter of the arithmetic mean value of the obtained area was defined as the average particle size (μm) of ferrite at each cross-sectional observation position C1 to C9.

The average particle size of the ferrites at the cross-sectional observation positions C1 to C9 at 9 locations was determined. Then, among the average particle diameters of these ferrites, the maximum average particle diameter (μm) and the minimum average particle diameter (μm) of ferrite were specified. The ratio of the maximum average particle size to the specified minimum average particle size (ferrite average particle size ratio) was determined. The obtained ferrite average particle size ratio is shown in the "ferrite particle size ratio" column of the "cross section" column of Table 3.

[(B1) Observation of microstructure in vertical section]
Nine samples including the vertical cross-sectional observation positions L1 to L9 were taken from the steel materials of each test number. Of the surfaces of each sample, the surface corresponding to the vertical cross section LS was used as the observation surface. On the observation surface, the observation field of view including the vertical cross-sectional observation position was set to 0.5 mm × 1.0 mm.

The observed phase is shown in the "Phase" column of the "Vertical section" column of Table 3. In the "Phase" column of the "Vertical section" column of Table 3, when the microstructure at all the vertical section observation positions contains ferrite and the balance consists of pearlite and / or bainite, "◯" is indicated. In all test numbers, the microstructure of the longitudinal section contained ferrite and the balance was pearlite and / or bainite.

[(B11) Arithmetic mean value of surface integral of ferrite in vertical section]
The area of ferrite (μm ² ) in each observation field (each vertical cross-section observation position) was determined. Using the area of the ferrite and the area of the observation field of view, the surface integral (%) of the ferrite in each observation field of view (each vertical cross-sectional observation position) was obtained.

The arithmetic mean value of the ferrite area fraction (%) in the nine observation fields (vertical cross-section observation positions) is the arithmetic mean value (%) of the ferrite area fraction in the nine vertical cross-section observation positions L1 to L9. Defined. The arithmetic mean value of the obtained ferrite surface integral is shown in the "ferrite surface integral (%)" column of the "longitudinal section" column of Table 3.

[(B12) Standard deviation of surface integral of ferrite in vertical section]
From the ferrite area fractions (%) at the nine observation fields (vertical cross-sectional observation positions), the standard deviation (%) of the ferrite area fractions at the nine vertical cross-sectional observation positions L1 to L9 was calculated. The obtained standard deviation is shown in the "Standard deviation (%) of ferrite surface integral" column in the "Vertical section" column of Table 3.

[(B13) Ferrite average particle size ratio in vertical section]
Further, the area (μm ² ) of each ferrite grain observed in each of the above-mentioned observation fields (each vertical cross-sectional observation position L1 to L9) was measured. Arithmetic mean values of the areas of each ferrite grain were obtained at each vertical cross-sectional observation positions L1 to L9. The circle-equivalent diameter of the arithmetic mean value of the obtained area was defined as the average particle size (μm) of ferrite at each vertical cross-sectional observation position L1 to L9.

The average particle size of ferrites at 9 vertical cross-sectional observation positions L1 to L9 was determined. Then, among the average particle diameters of these ferrites, the maximum average particle diameter (μm) and the minimum average particle diameter (μm) of ferrite were specified. The ratio of the maximum average particle size to the specified minimum average particle size (ferrite average particle size ratio) was determined. The obtained ferrite average particle size ratio is shown in the "ferrite particle size ratio" column of the "longitudinal section" column of Table 3.

[Evaluation test]
[(C1) Machinability evaluation test]
The machinability evaluation test was carried out by the following method. A heat treatment simulating hot forging and a constant temperature annealing treatment were carried out on a steel bar having a diameter of 50 mm. Specifically, the steel bar was heated at 1200 ° C. and held at 1200 ° C. for 30 minutes. Then, the steel bar was allowed to cool to room temperature. Further, it was heated at 950 ° C. and kept at 950 ° C. for 1 hour. Further, after holding at 650 ° C. for 2 hours, the mixture was allowed to cool to room temperature. Machining (cutting) was carried out on the steel bar after cooling to prepare a test piece for machinability evaluation having a diameter of 45 mm and a length of 400 mm.

The outer circumference of the test piece of each test number was turned and the tool life was evaluated. Specifically, the test piece of each test number was subjected to outer peripheral lathe processing under the following conditions. The cutting tool used was an uncoated cemented carbide equivalent to P20 specified in JIS B 4053 (2013). The cutting speed was 250 m / min, the feed rate was 0.35 mm / rev, and the depth of cut was 1.0 mm. At the time of turning, water-soluble cutting oil was used.

The outer circumference was turned for 20 minutes under the above-mentioned cutting conditions. Then, the flank wear amount (mm) of the cutting tool was measured.

The obtained flank wear amount (mm) is shown in the "wear amount (mm)" column of Table 3. When the flank wear amount (mm) was less than 0.25 mm, it was judged that the machinability of the steel material was high. When the obtained flank wear amount (mm) was 0.25 mm or more, it was judged that the machinability of the steel material was low.

[(C2) Bending fatigue strength evaluation test]
From the steel material (steel bar with a diameter of 50 mm) of each test number, an intermediate product of the Ono type rotary bending test piece for evaluation of bending fatigue strength shown in FIG. 6 was processed. The numerical values in FIG. 6 indicate dimensions (unit: mm). “Φ” in FIG. 6 means a diameter. "R1" means that the radius of curvature of the notch bottom is 1 mm.

Specifically, the steel material (steel bar with a diameter of 50 mm) of each test number was heated under the conditions of a heating temperature of 1200 ° C. and a holding time of 30 minutes. Then, hot working (hot forging) was performed at a finishing temperature of 950 ° C. or higher to produce a steel bar having a diameter of 35 mm. A steel bar having a diameter of 35 mm was machined (cut) to process an intermediate product of the Ono type rotary bending test piece. The diameter of the cross section of the intermediate product at the notched bottom was 8 mm. Carburizing treatment (gas carburizing quenching and tempering or vacuum carburizing quenching and tempering) was carried out on the intermediate product to prepare the Ono type rotary bending test piece shown in FIG.

The test piece of Test No. 1 was subjected to gas carburizing treatment and tempering, which is one of the carburizing treatment methods generally used for steel having a chemical composition of SCM420H.
On the other hand, the test pieces of test numbers 2 to 22 were vacuum carburized and quenched. The conditions of the gas carburizing treatment and the vacuum carburizing treatment carried out were as follows.

[Gas carburizing and tempering: test number 1]
FIG. 7 is a diagram showing an example of a heat pattern of gas carburizing treatment (gas carburizing step and quenching step). The test piece of test number 1 was subjected to gas carburizing treatment and tempering under the conditions shown in FIG. In the gas carburizing treatment, the gas carburizing step S30 and the quenching step S20 were carried out.

Specifically, in the gas carburizing step S30, the heating step S0, the carburizing step S2, and the diffusion step S3 were carried out on the test piece. In the heating step S0, the round bar of test number 1 was heated to a carburizing temperature Tc: 950 ° C. In the carburizing step S2, the carburizing temperature Tc: 950 ° C. and the holding time t2: 240 minutes were set in an atmosphere where the carbon potential Cp2 was 0.80%.

In the diffusion step S3, the carburizing temperature Tc: 950 ° C. and the holding time t3: 60 minutes were set in an atmosphere where the carbon potential Cp3 was 0.80%.

After the diffusion step S3, the quenching step S20 was carried out. In the quenching step S20, the soaking step S4 was carried out. After cooling to 850 ° C., in the soaking step S4, the quenching temperature was Ts: 850 ° C. and the holding time was t4: 30 minutes. Then, quenching was carried out with oil at 130 ° C.

After quenching, the test piece was tempered. The tempering temperature was 180 ° C., and the holding time at the tempering temperature was 120 minutes. After the holding time had elapsed, it was air-cooled.

By the above gas carburizing method, the C concentration on the surface of the steel material (round bar) was adjusted to 0.80% by mass.

[Vacuum carburizing quenching and tempering]
The test pieces of test numbers 2 to 22 were subjected to vacuum carburizing treatment and tempering as shown in FIG. Specifically, the pressure in the furnace was kept below 10 Pa. In the heating step S0, the round bar of each test number was heated to a carburizing temperature Tc: 950 ° C. After the heating step S0, the soaking step S1 was carried out. In the soaking step S1, the steel material (round bar) was held at a carburizing temperature Tc: 950 ° C. for a holding time t1: 60 minutes.

After the soaking step S1, the carburizing step S2 was carried out. In the carburizing step S2, acetylene was supplied as a carburizing gas into the vacuum carburizing furnace. The carburized gas pressure in the carburizing step S2 was kept below 1 kPa. In the carburizing step S2, the holding time t2 at the carburizing temperature Tc: 950 ° C. was 40 minutes. The carburized gas pressure in the diffusion step S3 was kept below 5 hPa. In the diffusion step S3, the holding time t3 at the carburizing temperature Tc: 950 ° C. was 70 minutes.

In the heat equalizing step S4 after the diffusion step S3, the temperature of the steel material was cooled to 850 ° C., and then the test piece was equalized at a quenching temperature Ts: 850 ° C. and a holding time t4: 30 minutes. Then, quenching was carried out with oil at 130 ° C.

By the above vacuum carburizing method, the C concentration on the surface of the steel material (round bar) was adjusted to 0.80% by mass.

An Ono-type rotary bending fatigue test was performed using an Ono-type rotary bending test piece after carburizing (gas carburizing and tempering or vacuum carburizing and tempering and tempering). Multiple test pieces were prepared for each test number. A fatigue test was carried out by changing the stress applied to each test piece, and after repeating 10 million times ( ¹⁰⁷ times), the highest stress that did not break was taken as the bending fatigue strength (MPa). In the Ono-type rotary bending fatigue test, the rotational speed was 3000 rpm and the stress ratio was double swing.

The test piece using the steel material of test number 1 was used as the reference steel. The ratio of the bending fatigue strength of each test number to the bending fatigue strength of the reference steel was defined as the bending fatigue strength ratio. That is, the bending fatigue strength ratio (%) was obtained by the following equation.
Bending fatigue strength ratio (%) = (Bending fatigue strength (MPa) of each test number / Bending fatigue strength of standard steel (MPa)) x 100

The obtained bending fatigue strength ratio (%) is shown in the "Bending fatigue strength ratio (%)" column of Table 3. When the obtained bending fatigue strength ratio was 120% or more, it was judged that sufficient bending fatigue strength could be obtained. On the other hand, if the bending fatigue strength ratio is less than 120%, it is determined that the bending fatigue strength is low.

[(C3) Surface fatigue strength evaluation test]
From the steel material (steel bar having a diameter of 50 mm) of each test number, an intermediate product of a test piece for a roller pitching fatigue test for a surface fatigue strength evaluation test shown in FIG. 8 was processed. The numerical values in FIG. 8 indicate the dimensions (unit: mm). “Φ” in the figure means the diameter.

Specifically, the steel material (steel bar with a diameter of 50 mm) of each test number was heated under the conditions of a heating temperature of 1200 ° C. and a holding time of 30 minutes. Then, hot working (hot forging) was performed at a finishing temperature of 950 ° C. or higher to produce a steel bar having a diameter of 35 mm. A steel bar having a diameter of 35 mm was machined (cut) to process an intermediate product of a test piece for a roller pitching fatigue test. The intermediate product of Test No. 1 was subjected to gas carburizing treatment and tempering under the above conditions. The intermediate products of test numbers 2 to 22 were vacuum carburized and tempered under the above conditions. Through the above steps, the roller pitching fatigue test test piece (small roller test piece) shown in FIG. 8 was produced.

FIG. 9 is a schematic diagram of a roller pitching fatigue test. As shown in FIG. 9, the small roller test piece 200 was rotated while pressing the large roller test piece 100 against the small roller test piece 200 with a surface pressure described later. The small roller test piece 200 was a test piece for a roller pitching fatigue test produced by the method of the above test piece. The large roller test piece had the shape shown in FIG. The numerical values in FIG. 10 indicate the dimensions (unit: mm). “R700” in the figure indicates that the radius of curvature of the outer peripheral surface was 700 mm.

The large roller test piece 100 uses a steel having a chemical composition equivalent to SCM420H specified in JIS G4053 (2016), and has a surface after gas carburizing treatment under the same conditions as the small roller test piece 200 of test number 1 which is a standard steel. The polished one was used. The diameter of the large roller test piece 100 was 130 mm.

In the roller pitching fatigue test, the large roller test piece 100 was pressed against the small roller test piece 200 at a surface pressure of various Hertz stresses. The peripheral speed direction of both roller test pieces at the contact portion is the same, and the slip ratio is -40% (the peripheral speed of the contact portion is 40% higher for the large roller test piece 100 than for the small roller test piece 200). The test was performed by rotating. The oil temperature of ATF (lubricating oil for AT) supplied to the contact portion as lubricating oil was 90 ° C., and the maximum surface pressure of the contact stress between the large roller test piece 100 and the small roller test piece 200 was 4000 MPa. The number of test terminations was set to 20 million (2.0 × ¹⁰⁷ ). For each test number, the highest stress at which pitching did not occur after repeating 2.0 × 10 ⁷ times for a plurality of test pieces was defined as the surface fatigue strength (MPa).

The test piece using the steel material of test number 1 was used as the reference steel. The ratio of the surface fatigue strength of each test number to the surface fatigue strength of the reference steel was defined as the surface fatigue strength ratio. That is, the surface fatigue strength ratio (%) was obtained by the following equation.
Surface fatigue strength ratio (%) = (surface fatigue strength (MPa) of each test number / surface fatigue strength of standard steel (MPa)) × 100

The obtained surface fatigue intensity ratio (%) is shown in the "surface fatigue intensity ratio (%)" column of Table 3. When the obtained surface fatigue strength ratio was 125% or more, it was judged that sufficient surface fatigue strength could be obtained. On the other hand, if the surface fatigue strength ratio is less than 125%, it is determined that the surface fatigue strength is low.

[(C4) Heat treatment deformation amount evaluation test]
The gear simulation test piece shown in FIG. 11A was produced from the steel material (steel bar having a diameter of 50 mm) of each test number. Specifically, the steel material (steel bar having a diameter of 50 mm) of each test number was heated under the conditions of a heating temperature of 1200 ° C. and a holding time of 30 minutes. Then, hot working (hot forging) was performed at a finishing temperature of 950 ° C. or higher to produce a steel bar having a diameter of 35 mm. A steel bar having a diameter of 35 mm was machined (cut) to prepare a gear simulation test piece before carburizing (gas carburizing, vacuum carburizing).

The numerical value accompanied by "mm" in FIG. 11A indicates the dimension (unit is mm). “Φ” in the figure means the diameter. The gear simulation test piece had a truncated cone shape. The gear simulation test piece includes a circular upper surface having a diameter of 22 mm and a circular lower surface having a diameter of 34 mm. The gear simulation test piece had a columnar through hole TH including the central axis CL2. The diameter (inner diameter) of the through hole TH was 15 mm, and the central axis of the through hole TH coincided with the central axis of the gear simulation test piece.

The inner diameter (diameter) of the prepared gear simulation test piece before carburizing treatment at each position in the longitudinal direction of the through hole TH was measured with a three-dimensional measuring machine. As a three-dimensional measuring machine, a CNC three-dimensional measuring machine (trade name: Crysta-Apex) manufactured by Mitutoyo Co., Ltd. was used.

Specifically, as shown in FIG. 11B, a total of 16 inner diameters are formed at 1.0 mm pitch positions in the range of 1.0 to 16.0 mm from the upper end toward the lower end in the longitudinal direction of the through hole TH. It was measured. Further, the inner diameter was measured at a position 0.5 mm from the upper end and a position 16.5 mm from the upper end toward the lower end from the upper end in the longitudinal direction of the through hole TH. That is, the inner diameter of the through hole TH was measured at 18 measurement positions in the longitudinal direction of the through hole TH. Further, at each measurement position, the inner diameters of a total of 18 points (points P1 to P18 in FIG. 11B) were measured at a pitch of 10 ° around the central axis CL2. Therefore, in the through hole TH, the inner diameter of 18 measurement positions × 18 points = 324 points was measured.

The gear simulated test piece after the inner diameter measurement is subjected to carburizing treatment (gas carburizing quenching and tempering, or vacuum carburizing quenching and tempering) under the above carburizing treatment conditions, and the gear simulated test piece after the carburizing treatment is obtained. Made. Specifically, the gear simulated test piece of test number 1 was subjected to gas carburizing treatment and tempering under the above-mentioned conditions. The gear simulated test pieces of test numbers 2 to 22 were subjected to vacuum carburizing treatment and tempering under the above conditions. The inner diameter of the through hole TH was measured for the gear simulated test piece of each test number after the carburizing treatment by the same method as the method for measuring the inner diameter of the through hole TH of the gear simulated test piece before the carburizing treatment.

[Maximum deformation ratio of heat treatment]
At each point P1 to P18 of each measurement position of the through hole TH, the value obtained by subtracting the inner diameter (μm) after the carburizing treatment from the inner diameter (μm) before the carburizing treatment is the heat treatment at each point P1 to P18 at each measurement position. The amount of deformation was used. For each test number, the maximum amount of heat treatment deformation in the measurement results of a total of 324 points was determined.

The ratio of the maximum heat treatment deformation amount of each test number to the maximum heat treatment deformation amount of the reference steel (test number 1) was defined as "maximum deformation amount ratio". That is, the maximum deformation amount ratio (%) was obtained by the following equation.
Maximum deformation amount ratio (%) = (maximum heat treatment deformation amount (mm) of each test number / maximum heat treatment deformation amount (mm) of reference steel) × 100

The obtained maximum deformation amount ratio (%) is shown in the "maximum deformation amount ratio (%)" column of Table 3. If the obtained maximum deformation amount ratio is 90% or less, it is determined that the maximum deformation amount ratio is small. On the other hand, when the maximum deformation amount ratio exceeds 90%, it is determined that the maximum deformation amount ratio is large.

[Ratio of deformation amount of heat treatment]
In each test number, the maximum heat treatment deformation amount and the minimum heat treatment deformation amount in the measurement results of a total of 324 points were determined. The value obtained by subtracting the minimum heat treatment deformation amount from the obtained maximum heat treatment deformation amount was defined as the deformation amount difference (μm).

The ratio of the deformation amount difference of each test number to the deformation amount difference of the reference steel (test number 1) was defined as the deformation amount difference ratio. That is, the deformation amount difference ratio was obtained by the following equation.
Deformation amount difference ratio (%) = (deformation amount difference of each test number (μm) / deformation amount difference of reference steel (μm)) × 100

The obtained deformation amount difference ratio (%) is shown in the "deformation amount difference ratio (%)" column of Table 3. If the obtained deformation amount difference ratio is 90% or less, it is determined that the deformation amount difference ratio is small. On the other hand, when the deformation amount difference ratio exceeds 90%, it is determined that the deformation amount difference ratio is large.

If both the maximum deformation amount ratio and the deformation amount difference ratio were 90% or less, it was judged that the heat treatment deformation could be sufficiently suppressed three-dimensionally. When the maximum deformation amount ratio and / or the deformation amount difference ratio exceeds 90%, it is judged that the heat treatment deformation cannot be sufficiently suppressed.

[Test results]
Table 3 shows the test results. With reference to Table 3, in the steel materials of test numbers 4 to 6, the content of each element in the chemical composition was appropriate, and F1 and F2 satisfied the formulas (1) and (2). Furthermore, the production conditions of the steel materials of test numbers 4 to 6 were also appropriate. Therefore, the microstructure of the steel material was appropriate. Specifically, the microstructure of the cross section of Test Nos. 4 to 6 contained ferrite and the balance was a structure composed of pearlite and / or bainite. Further, the arithmetic mean value of the area fraction of ferrite was 50 to 70%, the standard deviation of the area fraction of ferrite was 4.0% or less, and the ferrite average particle size ratio was 2.00 or less. Further, the microstructure of the vertical cross section of Test Nos. 4 to 6 contained ferrite, and the balance was a structure composed of pearlite and / or bainite. Further, the arithmetic mean value of the area fraction of ferrite was 50 to 70%, the standard deviation of the area fraction of ferrite was 4.0% or less, and the ferrite average particle size ratio was 2.00 or less.

Therefore, in the machinability evaluation test, the flank wear amount was less than 0.25 mm, and the machinability was high. Further, the bending fatigue strength ratio was 120% or more, the surface fatigue strength ratio was 125% or more, and both the bending fatigue strength and the surface fatigue strength were excellent. Further, the maximum deformation amount ratio and the deformation amount difference ratio in the heat treatment were 90% or less, and the heat treatment deformation was sufficiently suppressed three-dimensionally.

On the other hand, in

test numbers

2 and 3, the holding time of the lump rolling process was less than 10 hours. Therefore, the standard deviation of the surface integral of the ferrite of the steel material in the cross section and the vertical section exceeded 4.0%. As a result, the deformation amount difference ratio in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

In test numbers 7 and 8, the heating temperature in the lump rolling process was too low. Therefore, the standard deviation of the surface integral of the ferrite in the cross section and the vertical section exceeded 4.0%. As a result, the deformation amount difference ratio in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

In test numbers 9 and 10, the heating temperature in the finish rolling process was too low. Therefore, the ferrite average particle size ratio in the cross section and the vertical section exceeded 2.00. As a result, the deformation amount difference ratio in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

In test numbers 11 and 12, the holding time of the finish rolling process was too short. Therefore, the ferrite average particle size ratio in the cross section and the vertical section exceeded 2.00. As a result, the deformation amount difference ratio in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

In test numbers 13 and 14, the finishing temperature of the finishing rolling process was too high. Therefore, the ferrite average particle size ratio in the cross section and the vertical section exceeded 2.00. As a result, the deformation amount difference ratio in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

In

test numbers

15 and 16, the finish temperature of the finish rolling process was too low. Therefore, the ferrite average particle size ratio in the cross section and the vertical section exceeded 2.00. As a result, the deformation amount difference ratio in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

In test numbers 17 and 18, the cooling rate in the cooling process was too slow. Therefore, the arithmetic mean value of the surface integral of the ferrite in the cross section and the vertical section exceeded 70%. Therefore, the deformation amount difference ratio in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

In test numbers 19 and 20, the cooling rate in the cooling process was too fast. Therefore, the arithmetic mean value of the surface integral of the ferrite in the cross section and the vertical section was less than 50%. Therefore, the amount of flank wear was 0.25 mm or more. As a result, the machinability of the steel material was low.

In test numbers 21 and 22, the cooling rate in the temperature holding process was too fast. Therefore, the ferrite average particle size ratio exceeded 2.00 in the microstructure of the vertical cross section. Therefore, the deformation amount difference ratio in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

Similar to Example 1, a steel material having the chemical composition shown in Table 4 was prepared.

Steel materials were manufactured by the following method. With respect to the molten steel, the steel materials (steel bars) of the test numbers 1 to 35 shown in Table 5 were produced in the same manner as in Example 1 using the production condition b in Table 2.

[Evaluation test]
The same measurement and evaluation test as in Example 1 was carried out on the steel material (steel bar) manufactured in the above manufacturing process by the same method as in Example 1. In the (C2) bending fatigue strength evaluation test, (C3) surface fatigue strength evaluation test, and (C4) heat treatment deformation amount evaluation test, the steel material of test number 1 in Table 1 was used as the reference steel.

[Test results]
Table 5 shows the test results. With reference to Table 5, in the chemical composition of the steel materials of Test Nos. 1 to 18, the content of each element was appropriate, F1 satisfied the formula (1), and F2 satisfied the formula (2). Further, the steel materials of test numbers 1 to 18 had appropriate manufacturing conditions. Therefore, the microstructure of the steel material was appropriate. Specifically, the microstructure of the cross section of test numbers 1 to 18 is composed of ferrite, pearlite and / or bainite, and the arithmetic mean value of the area fraction of ferrite is 50 to 70%, and the area fraction of ferrite. The standard deviation of was 4.0% or less, and the ferrite average particle size ratio was 2.00 or less. The microstructure of the vertical cross section of test numbers 1 to 18 consists of ferrite, pearlite and / or bainite, the arithmetic mean value of the area fraction of ferrite is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4. It was 0.0% or less, and the ferrite average particle size ratio was 2.00 or less. Therefore, the flank wear amount was less than 0.25 mm, and the machinability was high. Further, the bending fatigue strength ratio was 120% or more, the surface fatigue strength ratio was 125% or more, and both the bending fatigue strength and the surface fatigue strength were excellent. Further, the maximum deformation amount ratio and the deformation amount difference ratio in the heat treatment were 90% or less, and the heat treatment deformation was sufficiently suppressed.

On the other hand, in test numbers 19-23, F2 was too high. Therefore, the maximum deformation amount ratio in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

In test numbers 24 and 25, F1 was too low. Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was low.

Test number 26 had too high a C content. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

Test number 27 had a Si content that was too low. Therefore, F1 did not satisfy the formula (1). Therefore, the bending fatigue strength ratio was less than 120%, and the surface fatigue strength ratio was less than 125%. As a result, the bending fatigue strength and the surface fatigue strength were low.

Test number 28 had too low Si and Mn contents. Therefore, the bending fatigue strength ratio was less than 120%, and the surface fatigue strength ratio was less than 125%. As a result, both bending fatigue strength and surface fatigue strength were insufficient.

Test number 29 had too high a Si content. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

In test number 30, the Mn content was too low. Therefore, the bending fatigue strength ratio was less than 120%, and the surface fatigue strength ratio was less than 125%. As a result, the bending fatigue strength and the surface fatigue strength were low.

In test number 31, the Mn content was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low. Furthermore, F1 did not satisfy equation (1). Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was insufficient.

Test number 32 had an excessively high Mn content. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

Test number 33 had a Cr content that was too high. Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was low.

Test number 34 had too high Mo content. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

Test number 35 had too high Nb content. Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was low.

The embodiment of the present disclosure has been described above. However, the embodiments described above are merely examples for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and the above-mentioned embodiment can be appropriately modified and implemented without departing from the spirit of the present disclosure.

Claims

It's a steel material
The chemical composition is by mass%,
C: 0.18 to 0.25%,
Si: 0.70 to 2.00%,
Mn: 0.70 to 1.50%,
S: 0.005 to 0.050%,
N: 0.0050-0.0200%,
Al: 0.001 to 0.100%,
O: 0.0050% or less, and
P: Contains 0.030% or less,
The balance consists of Fe and impurities, and the formulas (1) and (2) are satisfied.
In a cross section perpendicular to the longitudinal direction of the steel material and having a circular shape with a radius R,
The center position of the cross section and the eight R / 2 positions arranged at a pitch of 45 ° around the center of the cross section at R / 2 positions in the radial direction from the center of the cross section. When defined as 9 cross-sectional observation positions,
The microstructure at each cross-sectional observation position contains ferrite and the rest consists of pearlite and / or bainite.
The arithmetic mean value of the area fraction of the ferrite at the nine cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of the ferrite is 4.0% or less.
Of the average grain sizes of ferrites at the nine cross-sectional observation positions, the ratio of the maximum average grain size to the minimum average grain size is 2.00 or less.
In a vertical cross section parallel to the longitudinal direction of the steel material and including the central axis of the steel material.
Three central axis positions arranged at R / 2 pitch on the central axis, and six R / 2 positions arranged at R / 2 positions in the radial direction from each central axis position. , When defined as 9 vertical cross-sectional observation positions,
The microstructure at each longitudinal section observation position contains ferrite and the rest consists of pearlite and / or bainite.
The arithmetic mean value of the area fraction of ferrite at the nine vertical cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of the ferrite is 4.0% or less.
Among the average particle diameters of ferrites at the nine vertical cross-sectional observation positions, the ratio of the maximum average particle size to the minimum average particle size is 2.00 or less.
Steel material.
Si / Mn ≧ 1.00 (1)
1- (0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo) <0.800 (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1) and the formula (2). If the corresponding element is not contained, "0" is substituted for the element symbol.
The steel material according to claim 1.
The chemical composition further replaces a portion of the Fe.
Mo: 0.50% or less,
Nb: 0.050% or less,
Cr: 0.60% or less,
Ti: 0.020% or less,
Cu: 0.50% or less,
Ni: 0.80% or less,
V: 0.30% or less,
Mg: 0.0035% or less,
Ca: 0.0030% or less, and
Rare earth element: Contains one or more elements selected from the group consisting of 0.0050% or less.
Steel material.