KR20230072500A - steel - Google Patents

Abstract

Provided are steel materials that have excellent machinability, have excellent bending fatigue strength and surface fatigue strength even after vacuum carburizing or the like, and can suppress heat treatment deformation. The steel material according to the present embodiment has a chemical composition of, in 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 to 0.0200% , Al: 0.001 to 0.100%, O: 0.0050% or less, and P: 0.030% or less, the remainder being Fe and impurities, and satisfying the formulas (1) and (2) in the specification. Further, the microstructures of the cross section and the longitudinal section contain ferrite, and the remainder is composed of pearlite and/or bainite, the arithmetic average value of the area fraction of ferrite is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0 % or less, and the ferrite average grain size ratio is 2.00 or less.

Description

steel

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

In this specification, vacuum carburizing treatment also includes vacuum carburizing nitriding treatment. In addition, in this specification, the vacuum carburizing process includes a vacuum carburizing process (including a vacuum carburizing process) and a quenching process after the vacuum carburizing process.

Machine structural parts are represented, for example, by gears, shafts, and the like of automobiles and construction vehicles. As parts for machine structures, alloy steels for machine structures represented by SCr420, SCM420, and SNCM420 specified in JIS G 4053 (2016) are used.

These steel materials are manufactured into mechanical structural parts by the following manufacturing process, for example. Steel materials are forged (hot forged or cold forged) and/or cut 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. Through the above manufacturing process, the mechanical structural parts are manufactured.

As described above, there are cases where cutting is performed on steel materials during the manufacturing process of machine structural parts. Therefore, high machinability is requested|required of the steel materials used as the raw material of mechanical structural parts.

BACKGROUND ART In recent years, for the purpose of improving the fuel efficiency of automobiles, construction vehicles and the like, weight reduction and miniaturization of mechanical structural parts are progressing. Therefore, excellent bending fatigue strength and surface fatigue strength are required for mechanical structural parts.

As a method of increasing the bending fatigue strength and surface fatigue strength of mechanical structural parts, vacuum carburizing treatment is known. In the vacuum carburizing treatment, a hardened layer (carburized layer or carburized nitriding layer) is formed on the surface layer of the mechanical structural parts. By this hardened layer, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are improved.

However, when vacuum carburizing treatment (vacuum carburizing treatment and vacuum carburizing treatment) is performed, mechanical structural parts are easily deformed. In this specification, deformation of parts for machine structures during vacuum carburizing treatment is referred to as heat treatment deformation. Due to heat treatment deformation, the shape of the machine structural component is deformed. Deformation of the shape of mechanical structural parts causes noise and vibration during operation of automobiles and construction vehicles. Therefore, when vacuum carburizing is performed, steel materials capable of suppressing heat treatment deformation are required.

A technique related to suppression of heat treatment deformation is disclosed in Japanese Unexamined Patent Publication No. 2016-191151 (Patent Document 1), Japanese Unexamined Patent Publication No. 2018-028130 (Patent Document 2), and Japanese Unexamined Patent Publication No. 2007-291486 (Patent Document 1). It is disclosed in Document 3) and Japanese Unexamined Patent Publication No. 2010-150566 (Patent Document 4).

In the carburized parts disclosed in Patent Literature 1, in mass%, C: 0.10 to 0.30%, Si: 0.16 to 1.40%, Mn: 1.40 to 3.00%, P: 0.030% or less, S: 0.060% or less, Cr: 0.01 to 0.01% 0.29%, Al: 0.010 to 0.300%, and N: 0.003 to 0.030%, the balance being Fe and impurities. The surface of this carburized part has a flat part and an edge part. The carbon concentration in the surface layer region of the flat portion from the surface of the flat portion to the position at a depth of 0.05 mm is 0.70 to 0.89%, and the carbon concentration in the surface layer region of the edge portion from the surface of the edge portion to the position at a depth of 0.05 mm is 1.20% or less. Further, the depth of the grain boundary oxide layer 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 they are carburized parts having a shape including an edge portion.

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

In the carburized parts disclosed in Patent Literature 3, 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 0.01% 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 remainder being unavoidable impurities and Fe, [Si%] + [Ni% ]+[Cu%]-[Cr%]> has an alloy composition that satisfies the condition of 0.5. In addition, this carburized part is obtained by performing a carburizing process by vacuum carburizing. As a result, in the carburized parts of Patent Document 3, the toughness of the edge portion does not decrease, and since the surface carbon concentration is 0.6% or more at the lowest part, the part with low strength due to lack of carburization does not occur, according to Patent Document 3 are listed.

The steel materials for vacuum carburizing or vacuum carburizing nitriding disclosed in Patent Literature 4, in terms of mass%, 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: 0.012 to 0.025%, the balance being Fe and impurities, O (oxygen) in impurities: 0.0012% or less, and Ti: 0.003% or less, and has a chemical composition that satisfies formulas (1) to (3). Here, Formula (1) is 910-203×C ^0.5 +44.7×Si≤860, and 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 formula (3) is 0.2 × (S / Mn) + P ≤ 0.030. Further, in a cross section parallel to the longitudinal direction, inclusions such as oxide represented by (πLW/4) ^0.5 under predetermined conditions, where L (μm) is the major axis and W (μm) is the minor axis of the inclusions. The maximum equivalent circle diameter of is 35 μm or less. Patent Document 4 describes that the steel material of Patent Document 4 reduces the variation in heat treatment strain during quenching by adjusting the C and Si content, and increases the surface fatigue strength and bending fatigue strength.

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 Literatures 1 to 4 are documents that disclose techniques for improving fatigue strength, and do not disclose any techniques for suppressing heat treatment deformation.

An object of the present disclosure is to provide a steel material that has excellent machinability, excellent bending fatigue strength and surface fatigue strength after vacuum carburizing, and can suppress heat treatment deformation after vacuum carburizing.

The steel materials of this embodiment are

Chemical composition, in 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 to 0.0200%;

Al: 0.001 to 0.100%;

O: 0.0050% or less, and

P: contains 0.030% or less;

The remainder consists of Fe and impurities, and also satisfies formulas (1) and (2),

In the cross section perpendicular to the longitudinal direction of the steel material and circular in shape with a radius R,

The position of the center of the cross section and the position of R/2 in the radial direction from the center of the cross section and the R/2 positions of eight locations arranged at a pitch of 45° around the center of the cross section as nine cross section observation positions. When defined,

The microstructure at each of the above cross-sectional observation positions contains ferrite, and the remainder is composed of pearlite and/or bainite,

The arithmetic average value of the area fraction of ferrite at the nine cross-sectional observation positions is 50 to 70%, and the standard deviation of the ferrite area fraction is 4.0% or less,

Among the average grain diameters of ferrite at the nine cross-sectional observation positions, the ratio of the maximum average grain diameter to the minimum average grain diameter is 2.00 or less,

In the longitudinal section, which is a cross section parallel to the longitudinal direction of the steel member and includes the central axis of the steel member,

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 the longitudinal section observation position of

The microstructure at each longitudinal section observation position contains ferrite, and the remainder is composed of pearlite and/or bainite,

The arithmetic average value of the area fraction of ferrite at the nine positions of longitudinal section observation is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,

Among the average grain diameters of ferrite at the above nine longitudinal cross-section observation positions, the ratio of the maximum average grain diameter to the minimum average grain diameter 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 Formulas (1) and (2). When 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, and can suppress heat treatment deformation after vacuum carburizing.

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 strain amount ratio (%).
2 is a schematic diagram for explaining the cross-section observation position where microstructure observation is performed in the cross section perpendicular to the longitudinal direction of the steel material of the present embodiment.
Fig. 3 is a schematic diagram for explaining the vertical cross-section observation position at which microstructure observation is performed in the longitudinal cross-section parallel to the longitudinal direction of the steel material of the present embodiment and including the central axis.
4 is a schematic diagram of a band organization.
5 is a diagram showing an example of a heat pattern in a vacuum carburizing process and a quenching process.
Fig. 6 is a plan view of an Ono type rotary bending test piece produced in Example.
7 is a diagram showing an example of a heat pattern in a gas carburizing process and a quenching process.
Fig. 8 is a plan view of a test piece for a roller pitching fatigue test produced in Examples.
Fig. 9 is a schematic diagram for explaining a roller pitching fatigue test.
Fig. 10 is a front view of a large roller test piece produced in Examples.
11A is a perspective view of a simulated gear test piece produced in Example.
Fig. 11B is a perspective view of a through hole in Fig. 11A.

The present inventors have developed a steel material that has excellent machinability, has excellent bending fatigue strength and surface fatigue strength when vacuum carburizing is performed to make parts for machine structures, and can suppress heat treatment deformation after vacuum carburizing. About, research and review were conducted.

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

As a result of the examination, the chemical composition, in terms of mass%, was: 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 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: 0 to 0.30%, Mg: 0 to 0.0035%, Ca: 0 to 0.0030%, and rare earth elements: 0 to 0.0050%, the balance being Fe and impurities. It was considered that the back surface had excellent machinability and was likely to have excellent bending fatigue strength and surface fatigue strength after vacuum carburizing.

The present inventors also considered that there is a possibility of having excellent bending fatigue strength after vacuum carburizing, assuming that the content of each element in the chemical composition is within the above range, and if the following formula (1) is satisfied.

Si/Mn≥1.00 (1)

Here, the content (mass %) of the corresponding element is substituted for each element symbol in Formula (1).

In the steel materials of this embodiment, when the ratio of the Si content to the Mn content is 1.00 or more, that is, when formula (1) is satisfied, the inclusions become soft MnO-SiO ₂ . These inclusions are vitrified during hot working (hot rolling), stretched and divided, and refined. Therefore, coarse inclusions that lower the bending fatigue strength can be reduced, and the bending fatigue strength is increased.

The present inventors also studied a means for suppressing heat treatment deformation in vacuum carburizing treatment. The present inventors paid attention to the microstructure of steel materials. If the microstructure at each site in the steel material is as uniform as possible, specifically, if the fluctuations in the phase structure of the microstructure and the fluctuation of crystal grains at each site in the steel material are suppressed, martensite at the time of vacuum carburizing quenching Variation in the occurrence timing of transformation is suppressed. As a result, heat treatment deformation can be suppressed. Therefore, the inventors of the present invention conducted an examination of the phase structure and crystal grain size at each site of the steel material.

The present inventors first paid attention to the variation of the microstructure in the transverse section, which is the 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-section 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 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, and eight R / 2 positions arranged at 45 ° pitch around the center of the cross section, It is defined as 9 cross-sectional observation positions.

The present inventors investigated and examined microstructures at each cross-sectional observation position. As a result of the examination, it was found that deformation of the heat treatment after the carburizing treatment was suppressed if the microstructure at the cross-sectional observation position satisfies the following requirements.

(1) The microstructure at each cross-sectional observation position contains ferrite, and the remainder is composed of pearlite and/or bainite.

(2) The arithmetic average 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) Among the average ferrite grain sizes at nine cross-sectional observation positions, the ratio of the maximum average grain diameter to the minimum average grain diameter is 2.00 or less.

However, even if the steel material has the above-mentioned chemical composition and satisfies the above-mentioned microstructure, heat treatment deformation is still not sufficiently suppressed, and in particular, noise and vibration during operation of automobiles and construction vehicles cannot be sufficiently suppressed. It turned out that there is Therefore, the present inventors conducted further examination.

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

However, only suppression of fluctuations in the microstructure of the cross section of steel materials is limited to suppression of two-dimensional heat treatment deformation. That is, even if the variation of the microstructure of the cross section of the steel material is suppressed, the microstructure of the longitudinal section parallel to the longitudinal direction of the steel material and including the central axis of the steel material may fluctuate. In this case, variations occur in heat treatment deformation. As a result, noise and vibration during driving cannot be sufficiently suppressed.

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

3 central axis positions arranged at R/2 pitch on the central axis of the steel, and 6 R/2 positions arranged at R/2 positions in the radial direction from each central axis position, 9 longitudinal sections Define the observation position.

The present inventors investigated and examined microstructures at each longitudinal section observation position. As a result of the examination, it was found that heat treatment deformation after gas carburizing treatment is sufficiently suppressed if the microstructure at the cross-sectional observation position satisfies the above requirements and the microstructure at the longitudinal cross-section observation position satisfies the following requirements.

(4) The microstructure at each longitudinal section observation position contains ferrite, and the balance consists of pearlite and/or bainite.

(5) The arithmetic average value of the area fractions of ferrite at nine longitudinal cross-section observation positions is 50 to 70%, and the standard deviation of the ferrite area fractions is 4.0% or less.

(6) Among the average grain diameters of ferrite at nine longitudinal cross-section observation positions, the ratio of the maximum average grain diameter to the minimum average grain diameter is 2.00 or less.

However, even if the steel material has the above-mentioned chemical composition and the microstructure at the cross section observation position and the longitudinal section observation position satisfies the above requirements (1) to (6), there are still cases where heat treatment deformation cannot be sufficiently suppressed. . Therefore, the present inventors conducted further examination.

Here, the present inventors paid attention to the martensite transformation after vacuum carburizing. And the present inventors studied in detail about the generation mechanism of martensitic transformation at the time of vacuum carburizing quenching.

The inventors of the present invention first attempted to suppress heat treatment deformation by using a steel material having the above-described chemical composition and making the martensite transformation time at each site of a machine structural component the same as possible. Specifically, an attempt was made to suppress heat treatment deformation by suppressing fluctuations in the microstructure at each site (cross-section observation position, longitudinal section observation position) of the steel material, and also suppressing fluctuations in the Ms point at each site as much as possible. .

However, as a result of investigation by the present inventors, even if the variation of the microstructure in each part of the steel material having the above-mentioned chemical composition is suppressed, the martensite transformation time in each part of the steel material is slightly shifted anyway, and the same time in each part The martensitic transformation has proven to be very difficult. Specifically, when the time at the time of rapid cooling in the vacuum carburizing treatment is divided into minute periods, even if the variation of the microstructure at the cross-section observation position and the longitudinal section observation position of the steel material is suppressed to the limit, martensitic transformation occurs in the steel material. A micro-period in which a part that has undergone (hereinafter referred to as "martensite transformation part") and a part that has not undergone martensite transformation (hereinafter also referred to as "martensite non-transformed part") coexist occurs anyway. Turned out.

It is considered that the microstructure change of steel materials during vacuum carburizing occurs as follows.

When the quenching time (quick cooling time) is divided into minute time, first, martensitic transformation is started in a part of the inside of the steel material. Then, with the progress of time, martensitic transformation proceeds from the central portion toward the surface layer portion. That is, martensitic transformation occurs not from the surface layer of steel materials but from the inside of steel materials.

Due to the vacuum carburizing treatment, the carbon concentration in the steel material surface layer is higher than the carbon concentration in the steel material interior. Therefore, the Ms point of the steel material surface layer is lower than the Ms point of the inside of the steel material. In addition, even if it is possible to make the Ms point uniform at each part inside the steel material, the cooling rate of each part is not completely the same due to the shape of the steel material. Therefore, when the quenching time is divided into minute time periods, martensitic transformation starts from a part where the cooling rate inside the steel material is high among the parts of the steel material. Therefore, at the time of quenching in the gas carburizing treatment, a minute time zone in which the martensite transformed portion and the martensite non-transformed portion coexist is always generated.

Based on the above knowledge, the present inventors do not suppress heat treatment deformation by making the martensite transformation period the same as possible, but in the case of vacuum carburizing, a micro time period in which the martensitic transformation part and the martensite non-transformation part coexist On the premise that necessarily exists, means for suppressing heat treatment deformation were examined.

At the time of quenching, the martensitic non-transformed portion is softer than the martensite-transformed portion. In addition, the martensitic transformation portion having a body-centered cubic lattice structure has a larger volume than the martensite untransformed portion having a face-centered cubic lattice structure. Therefore, at the time of quenching, when a martensitic transformation part and a martensitic untransformed part coexist because a part of a steel material undergoes martensitic transformation, deformation arises in a martensitic untransformed part. It is thought that this deformation causes heat treatment deformation.

Therefore, the inventors of the present invention, in the case of vacuum carburizing, on the premise that there exists a minute period in which the martensitic transformation part and the martensite non-transformation part coexist, the martensite non-transformation at the time when the martensitic transformation part is generated It was thought that if the strength of the portion can be maintained high, the generation of deformation of the martensitic untransformed portion can be suppressed, and as a result, heat treatment deformation can be suppressed.

Therefore, the inventors of the present invention further studied a means for maintaining high strength of the martensite non-transformed portion when the martensite transformed portion was generated during quenching in the vacuum carburizing treatment. In the steel material having the above chemical composition, in order to increase the strength of the martensitic untransformed portion in the temperature range where the martensitic transformation portion is generated, an element that strengthens the martensitic nontransformed portion in the temperature range where the martensitic transformation portion is generated. It is effective to appropriately contain.

In the above-mentioned chemical composition, the present inventors considered that C, Si, Mn, Cr, and Mo are effective as elements that increase the strength of the martensitic untransformed portion in the temperature range where the martensitic transformation portion is generated. Therefore, further studies were conducted on the relationship between these elements and the heat treatment deformation amount at the time of quenching in gas carburizing treatment. As a result, it was found that heat treatment deformation is remarkably suppressed by 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 Formula (2). When 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). 1 shows the F2 value and the maximum deformation amount ratio ( It is a figure showing the relationship of %). The maximum deformation amount 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 obtained by the method described later.

Referring to FIG. 1, in a steel material in which the content of each element in the chemical composition is within the above-mentioned range and the microstructure at the cross-section observation position and the longitudinal section observation position satisfies the above-mentioned conditions (1) to (6), F2 Accompanying the decrease in , the maximum deformation amount ratio decreases. And, when F2 becomes less than 0.800, the maximum deformation amount ratio is remarkably lowered. That is, the maximum deformation amount ratio to F2 has an inflection point around F2 = 0.800. Therefore, if F2 is less than 0.800, heat treatment deformation of steel materials at the time of carburizing quenching can fully be suppressed.

As described above, the inventors of the present invention, in the steel material having the above-mentioned chemical composition, while suppressing fluctuations in the timing of occurrence of martensitic transformation during quenching by suppressing fluctuations in the microstructure of the cross-section observation position and the longitudinal section observation position to some extent, , On the premise that at the time of quenching, a minute time period in which the martensitic transformation part and the martensite non-transformation part coexist occurs anyway, by setting F2 to less than 0.800, it has excellent machinability and excellent after vacuum carburizing treatment It has been found that it has bending fatigue strength and excellent face fatigue strength, and can sufficiently suppress heat treatment deformation after vacuum carburizing.

The steel materials according to this embodiment completed based on the above knowledge have the following structure.

[One]

is steel,

Chemical composition, in 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 to 0.0200%;

Al: 0.001 to 0.100%;

O: 0.0050% or less, and

P: contains 0.030% or less;

The remainder consists of Fe and impurities, and also satisfies formulas (1) and (2),

In the cross section perpendicular to the longitudinal direction of the steel material and circular in shape with a radius R,

The position of the center of the cross section and the position of R/2 in the radial direction from the center of the cross section and the R/2 positions of eight locations arranged at a pitch of 45° around the center of the cross section as nine cross section observation positions. When defined,

The microstructure at each of the above cross-sectional observation positions contains ferrite, and the remainder is composed of pearlite and/or bainite,

The arithmetic average value of the area fraction of ferrite at the nine cross-sectional observation positions is 50 to 70%, and the standard deviation of the ferrite area fraction is 4.0% or less,

Among the average grain diameters of ferrite at the nine cross-sectional observation positions, the ratio of the maximum average grain diameter to the minimum average grain diameter is 2.00 or less,

In the longitudinal section, which is a cross section parallel to the longitudinal direction of the steel member and includes the central axis of the steel member,

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 the longitudinal section observation position of

The microstructure at each longitudinal section observation position contains ferrite, and the remainder is composed of pearlite and/or bainite,

The arithmetic average value of the area fraction of ferrite at the nine positions of longitudinal section observation is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,

Among the average grain diameters of ferrite at the nine longitudinal cross-sectional observation positions, the ratio of the maximum average grain diameter to the minimum average grain diameter is 2.00 or less,

steel.

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 Formulas (1) and (2). When the corresponding element is not contained, "0" is substituted for the element symbol.

[2]

It is the steel material described in [1],

The chemical composition also, instead of part 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 elements: containing at least one element selected from the group consisting of 0.0050% or less,

steel.

Hereinafter, the steel materials of this embodiment are demonstrated in detail. "%" for an element means mass % unless otherwise specified.

[Chemical composition of steel]

The chemical composition of steel materials contains the following elements.

C: 0.18 to 0.25%

Carbon (C) increases the strength of steel materials. If the C content is less than 0.18%, the above effect is not sufficiently obtained even if the other element content is within the range of the present embodiment.

On the other hand, when the C content exceeds 0.25%, the quenching property becomes excessively high even if the other element content 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 the parts for machine structure is remarkably lowered.

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 upper limit of the C content is preferably 0.24%, more preferably 0.23%, still more preferably 0.22%.

Si: 0.70 to 2.00%

Silicon (Si) enhances the hardenability of steel materials and increases the strength of steel materials. Si also enhances the temper softening resistance of the hardened layer when steel materials are used as parts for mechanical structures. Therefore, the surface fatigue strength of the mechanical structural parts is increased. If the Si content is less than 0.70%, the above effect is not sufficiently obtained even if the other element content is within the range of the present embodiment.

On the other hand, when the Si content exceeds 2.00%, the quenchability becomes too high even if the other element content is within the range of the present embodiment. Therefore, the hardness of steel materials after vacuum carburizing treatment increases. Therefore, the machinability of steel materials deteriorates remarkably.

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 upper limit of the Si content is preferably 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 increases the bending fatigue strength and surface fatigue strength of mechanical structural parts. If the Mn content is less than 0.70%, even if the other element content is within the range of the present embodiment, the above effect is not sufficiently obtained.

On the other hand, when the Mn content exceeds 1.50%, the steel materials become too hard even if the other element content is within the range of the present embodiment. In this case, the machinability of steel materials deteriorates.

Therefore, the Mn content is 0.70 to 1.50%. A preferable lower limit of the Mn content is more than 0.70%, more preferably 0.75%, still more preferably 0.80%. The preferable 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 improves the machinability of steel materials. If the S content is less than 0.005%, the above effect is not sufficiently obtained even if the other element content is within the range of the present embodiment.

On the other hand, when the S content exceeds 0.050%, MnS is excessively formed even when the other element content is within the range of the present embodiment. In this case, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are lowered.

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 upper limit of the S content is preferably less than 0.050%, more preferably 0.035%, still more preferably 0.025%.

N: 0.0050 to 0.0200%

Nitrogen (N) combines with Al and Nb to form AlN and NbN. AlN and NbN suppress coarsening of crystal grains at the time of heating in vacuum carburizing treatment by a pinning effect. If the N content is less than 0.0050%, even if the other element content is within the range of the present embodiment, the above effect is not sufficiently obtained.

On the other hand, when the N content exceeds 0.0200%, even if the other element content is within the range of the present embodiment, in the steelmaking process, the surface of the cast steel or ingot produced is liable to be scratched.

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 upper limit of the N content is preferably 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 also combines with N to form AlN. AlN suppresses coarsening of crystal grains at the time of heating in vacuum carburizing treatment by a pinning effect. If the Al content is less than 0.001%, the above effect is not sufficiently obtained even if the other element content is within the range of the present embodiment.

On the other hand, when the Al content exceeds 0.100%, formation of coarse Al oxide is promoted even when other element contents are within the range of the present embodiment. Coarse Al oxide lowers the bending fatigue strength of mechanical structural parts.

Therefore, the Al content is 0.001 to 0.100%. The preferable 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 preferable 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%, still more preferably 0.035%.

O (oxygen): 0.0050% or less

Oxygen (O) is an impurity. O combines with other elements in steel to form coarse oxide-based inclusions. Coarse oxide-based inclusions reduce the bending fatigue strength of mechanical structural parts. When the O content exceeds 0.0050%, the bending fatigue strength of the mechanical structural parts is remarkably reduced even when the other element contents are within the range of the present embodiment.

Therefore, the O content is 0.0050% or less. The upper limit of the O content is preferably 0.0040%, more preferably 0.0030%, still more preferably 0.0020%, still more preferably 0.0015%.

The one where O content is as low as possible is preferable. However, excessive reduction of O content raises manufacturing cost. Therefore, considering normal industrial production, the lower limit of the O content is preferably 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 is segregated at the grain boundary and lowers the grain boundary strength. When the P content exceeds 0.030%, even if the other element content is within the range of the present embodiment, P is excessively segregated at the grain boundary to lower the grain boundary strength, and as a result, the bending fatigue strength and face fatigue strength of mechanical structural parts are improved. It is lowered.

Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 0.025%, more preferably 0.020%, still more preferably 0.015%.

The one where P content is as low as possible is preferable. However, excessive reduction of P content raises manufacturing cost. Therefore, considering normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, still more preferably 0.005%, still more preferably 0.010%.

The rest of the chemical composition of the steel materials according to this embodiment consists of Fe and impurities. Here, impurities mean that they are mixed from ore as a raw material, scrap, or the manufacturing environment when steel materials are industrially manufactured, and are permitted within a range that does not adversely affect the steel materials of the present embodiment. The impurities referred to herein include B, Pb, W, Sb, Bi, Co, Ta, Sn, In, Zr, Te, Se, and Zn. The total content of impurities other than O and P is 0.01% or less. Also, among the impurities, the B content is 0.0003% or less.

[For optional elements]

The chemical composition of the steel of this embodiment is also, instead of 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 elements: You may contain one or more elements selected from the group consisting of 0.0050% or less. These elements are arbitrary elements, and all increase the bending fatigue strength and surface fatigue strength of mechanical structural parts.

Mo: 0.50% or less

Molybdenum (Mo) is an arbitrary element and does not need to be contained. That is, the Mo content may be 0%. When contained, Mo enhances the quenchability of steel materials and increases the bending fatigue strength and surface fatigue strength of mechanical structural parts. When Mo is contained even in small amounts, the above effect is obtained to some extent.

However, when the Mo content exceeds 0.50%, the steel materials become too hard even if the other element content is within the range of the present embodiment. In this case, the machinability of steel materials deteriorates.

Therefore, the Mo content is 0 to 0.50%, and when contained, it is 0.50% or less (ie, 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 upper limit of the Mo content is preferably 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 arbitrary element and does not need to be contained. That is, the Nb content may be 0%. When contained, Nb combines with C and/or N to form Nb precipitates (NbC, NbN, Nb(CN), etc.). Like AlN, Nb precipitates suppress coarsening of crystal grains in gas carburizing due to a pinning effect. Therefore, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are increased. When even a little bit of Nb is contained, the above effect is obtained to some extent.

However, when the Nb content exceeds 0.050%, the Nb precipitate is coarsened even if the other element content is within the range of the present embodiment. In this case, coarsening of crystal grains in the gas carburizing treatment cannot be sufficiently suppressed. As a result, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are lowered.

Therefore, the Nb content is 0 to 0.050%, and when contained, it is 0.050% or less (ie, more than 0 to 0.050%).

The lower limit of the Nb content is preferably 0.001%, more preferably 0.010%, still more preferably 0.015%, still more preferably 0.020%, still more preferably 0.025%. The upper limit of the Nb content is preferably 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 arbitrary element and does not need to be contained. That is, the Cr content may be 0%. When contained, Cr enhances the quenchability of steel materials and raises the bending fatigue strength and the surface fatigue strength of a mechanical structural part. When even a little Cr is contained, the above effect is obtained to some extent.

However, when the Cr content exceeds 0.60%, even if the other element content is within the range of the present embodiment, excessive carburization easily occurs in the surface layer of the mechanical structural component during vacuum carburizing treatment. In this case, coarse cementite is generated at the grain boundary. As a result, the bending fatigue strength of mechanical structural parts is lowered.

Therefore, the Cr content is 0 to 0.60%, and when it is contained, it is 0.60% or less (ie, 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 preferable 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 0.40%.

Ti: 0.020% or less

Titanium (Ti) is an arbitrary element and does not have to be contained. That is, the Ti content may be 0%. When contained, Ti forms Ti precipitates (TiC, TiN, Ti(CN), etc.) similarly to Nb. Ti precipitate suppresses coarsening of crystal grains in gas carburizing treatment by a pinning effect. Therefore, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are increased. When even a little Ti is contained, the above effect is obtained to some extent.

However, when Ti content exceeds 0.020%, Ti precipitate coarsens even if other element content falls within the range of this embodiment. In this case, coarsening of crystal grains in the gas carburizing treatment cannot be sufficiently suppressed. As a result, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are lowered.

Therefore, the Ti content is 0 to 0.020%, and when contained, it is 0.020% or less (ie, 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 upper limit of the Ti content is preferably 0.019%, more preferably 0.017%, still more preferably 0.015%.

Cu: 0.50% or less

Copper (Cu) is an arbitrary element and does not need to be contained. That is, 0% may be sufficient as Cu content. When contained, Cu increases the quenchability of steel materials and increases the bending fatigue strength and surface fatigue strength of mechanical structural parts. When even a little Cu is contained, the above effect is obtained to some extent.

However, when Cu content exceeds 0.50%, steel materials become too hard even if other element content exists in the range of this embodiment. In this case, the machinability of steel materials deteriorates.

Therefore, Cu content is 0 to 0.50%, and when contained, it is 0.50% or less (namely, 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 upper limit of the Cu content is preferably 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 arbitrary element and does not need to be contained. That is, the Ni content may be 0%. When contained, Ni enhances the quenchability of steel materials and increases the bending fatigue strength and surface fatigue strength of mechanical structural parts. When even a little Ni is contained, the above effect is obtained to some extent.

However, when the Ni content exceeds 0.80%, the steel materials become too hard even if the other element content is within the range of the present embodiment. In this case, the machinability of steel materials deteriorates.

Therefore, the Ni content is 0 to 0.80%, and when it is contained, it is 0.80% or less (ie, 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 upper limit of the Ni content is preferably 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 does not need to be contained. That is, the V content may be 0%. When contained, V forms V precipitates (VC, VN, V(CN), etc.) similarly to Nb. The V precipitate suppresses coarsening of crystal grains in the gas carburizing treatment due to the pinning effect. Therefore, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are increased. When even a little V is contained, the above effect is obtained to some extent.

However, when the V content exceeds 0.30%, the steel materials become too hard even if the other element content is within the range of the present embodiment. In this case, the machinability of steel materials deteriorates.

Therefore, the V content is 0 to 0.30%, and when it is contained, it is 0.30% or less (ie, 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 upper limit of the V content is preferably 0.20%, more preferably 0.15%, still more preferably 0.10%.

Mg: 0.0035% or less

Magnesium (Mg) is an optional element and does not need to be contained. That is, the Mg content may be 0%. When contained, Mg, like Al, deoxidizes the steel. In this case, the formation of coarse oxide is suppressed. Therefore, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are increased. When Mg is contained even in small amounts, the above effect is obtained to some extent.

However, when the Mg content exceeds 0.0035%, formation of coarse Mg oxide in steel materials is promoted even if the other element content is within the range of the present embodiment. In this case, the limiting processing rate during hot working is lowered.

Therefore, the Mg content is 0 to 0.0035%, and when it is contained, it is 0.0035% or less (ie, more than 0 to 0.0035%).

The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0003%, still more preferably 0.0005%. The upper limit of the Mg content is preferably 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 does not need to be contained. That is, the Ca content may be 0%. When contained, Ca refines sulfides in steel materials. Ca also promotes spheroidization of sulfides in steel materials. Therefore, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are increased. When Ca is contained even a little, the above effect is obtained to some extent.

However, when Ca content exceeds 0.0030%, even if other element content exists in the range of this embodiment, coarse Ca oxide will generate|occur|produce in steel materials. In this case, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are lowered.

Therefore, the Ca content is 0 to 0.0030%, and when contained, it is 0.0030% or less (ie, more than 0 to 0.0030%).

The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, still more preferably 0.0005%, still more preferably 0.0007%, still more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0025%, more preferably 0.0022%, still more preferably 0.0020%.

Rare earth element (REM): 0.0050% or less

The rare earth element (REM) is an arbitrary element and does not have to be contained. That is, the REM content may be 0%. When contained, REM dissolves into sulfides in steel materials and suppresses the elongation of MnS. As a result, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are increased. When REM is contained even a little, the above effect is obtained to some extent.

However, when the REM content exceeds 0.0050%, coarse oxide is generated even when the other element content is within the range of the present embodiment. In this case, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are lowered.

Therefore, the REM content is 0 to 0.0050%, and when contained, it is 0.0050% or less (ie, more than 0 to 0.0050%).

The lower limit of the REM content is preferably 0.0001%, more preferably 0.0010%, still more preferably 0.0020%. The preferable upper limit of the REM content is 0.0045%, more preferably 0.0040%, still more preferably 0.0035%, still more preferably 0.0030%.

REM in the present specification is a group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 as a lanthanoid to lutetium (Lu) with atomic number 71 It is an element of one or more elements selected from. REM content in this specification is the total content of these elements.

[Regarding Formula (1)]

The chemical composition of the steel materials of this embodiment further satisfies Formula (1) on the premise that each element content is within the range of this embodiment.

Si/Mn≥1.00 (1)

Here, the content (mass %) of the corresponding element is substituted for each element symbol in Formula (1).

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

These inclusions are stretched and divided 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 using the steel of the present embodiment as a material increases. And when F1 is 1.00 or more, bending fatigue strength becomes higher than SCM420H prescribed by JIS G 4052 (2016). Therefore, when F1 satisfies formula (1), that is, when F1 is 1.00 or more, the content of each element is within the range of the present embodiment, and F2 satisfies formula (2), the present embodiment The bending fatigue strength of parts for machine structural parts manufactured using the steel materials of is sufficiently high.

The lower limit of F1 is preferably 1.05, more preferably 1.07, still 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 upper limit of F1 is preferably 2.10, more preferably 2.00, still more preferably 1.70.

[About the micro structure of steel materials]

The microstructure of the steel materials of this embodiment contains ferrite, the balance is pearlite and/or bainite, and the area fraction of ferrite is 50 to 70%.

When the area fraction of ferrite is less than 50%, the area fraction of pearlite and/or bainite is too high in steel materials. In this case, the hardness of steel materials becomes excessively high. As a result, the machinability of steel materials deteriorates.

On the other hand, when the area fraction of ferrite exceeds 70%, the grain size tends to fluctuate during gas carburizing. Therefore, heat treatment deformation occurs excessively at the time of gas carburizing treatment.

When the area fraction of ferrite is 50 to 70% and the remainder other than ferrite in the microstructure is pearlite and/or bainite, the machinability of steel materials is sufficiently high. In addition, heat treatment deformation at the time of gas carburizing treatment can be suppressed.

In the steel materials of the present embodiment, the microstructure at each cross-sectional observation position and each longitudinal cross-section observation position contains 50 to 70% of ferrite in area fraction, and the balance consists of pearlite and/or bainite.

A preferable lower limit of the area fraction of ferrite at each observation position is 52%, more preferably 55%, still more preferably 57%. A preferable upper limit of the area fraction of ferrite at each observation position is 68%, more preferably 65%, still more preferably 63%.

[About variation of microstructure in cross section of steel]

In the steel material of the present embodiment, fluctuations in the microstructure are sufficiently suppressed in a cross section that is a cross section perpendicular to the longitudinal direction of the steel material. Hereinafter, this point is demonstrated.

2 : is a schematic diagram of the cross section which is a cross section perpendicular|vertical to the longitudinal direction of steel materials of this embodiment. Referring to Figure 2, the cross section CS of the steel material is a circular shape of 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 and eight R/2 positions C2 to 45 are arranged around the center of the cross-section CS at a pitch. C9 is defined as nine "cross-sectional observation positions" C1 to C9.

The microstructures at the cross-sectional observation positions C1 to C9 satisfy the following (A) and (B).

(A) The arithmetic average 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 grain diameters of ferrite at cross-sectional observation positions C1 to C9, the ratio of the maximum average grain diameter to the minimum average grain diameter is 2.00 or less.

Hereinafter, (A) and (B) are demonstrated in detail.

[About (A)]

As in the above (A), in the steel material of the present embodiment, the arithmetic average 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, fluctuations in the phase fraction of the microstructure at each cross-sectional observation position C1 to C9 are sufficiently suppressed. Therefore, at the time of gas carburizing, it is possible to suppress fluctuations in the timing of occurrence of martensitic transformation at each cross section observation position C1 to C9.

When the standard deviation of the area fraction of ferrite at the cross-sectional observation positions C1 to C9 exceeds 4.0%, the variation of the phase fraction at each cross-section observation position C1 to C9 is large. In this case, heat treatment deformation at the time of gas carburizing cannot be sufficiently suppressed.

Therefore, the standard deviation of the area fraction of ferrite at the cross-sectional observation positions C1 to C9 is 4.0% or less.

The upper limit of the standard deviation of the area fraction of ferrite is preferably 3.8%, more preferably 3.5%, still more preferably 3.0%. The lower limit of the standard deviation of the area fraction of ferrite is not particularly limited. The lower limit of the standard deviation of the area fraction of ferrite is preferably 0.1%, more preferably 0.5%, still more preferably 1.0%, still more preferably 1.5%.

[About (B)]

Among the average ferrite grain diameters at the cross-section observation positions C1 to C9, the ratio of the maximum average grain diameter to the minimum average grain diameter is referred to as the "average ferrite grain diameter ratio". The ferrite average grain size ratio in a cross section is defined by the following formula.

Ferrite average grain size ratio = (maximum value of average ferrite grain size in C1 to C9)/(minimum value of average ferrite grain size in C1 to C9)

In the steel material of the present embodiment, the average ferrite grain size ratio at the cross-sectional observation positions C1 to C9 is 2.00 or less. In this case, fluctuations in the average grain size of ferrite at each cross-sectional observation position C1 to C9 are sufficiently suppressed. That is, the ferrite grains at each position are even. Therefore, fluctuations in the occurrence of martensitic transformation during the carburizing treatment can be sufficiently suppressed. Therefore, heat treatment deformation of steel materials at the time of carburizing treatment can be suppressed.

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

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

[About variation of microstructure in longitudinal section in steel]

In the steel material of the present embodiment, fluctuations in the microstructure are sufficiently suppressed not only in the above-described cross section but also in a longitudinal section parallel to the longitudinal direction of the steel material and including the central axis of the steel material. In the steel material of the present embodiment, since fluctuations in the microstructure of not only the cross section but also the longitudinal section are sufficiently suppressed, heat treatment deformation occurring three-dimensionally can be sufficiently suppressed. Hereinafter, suppression of fluctuations in the microstructure in the longitudinal section will be described.

3 : is a schematic diagram of the longitudinal cross section which is a cross section parallel to the longitudinal direction of steel materials of this embodiment, and including the central axis. Referring to FIG. 3, in the longitudinal section LS of the steel material, three central axis positions L1 to L3 disposed at a pitch of R / 2 on the central axis CL1 of the steel material, and R / 2 in the radial direction from each central axis position Six R/2 positions L4 to L9 arranged at the positions are defined as nine "longitudinal plane observation positions" L1 to L9.

The microstructures at the above-described nine longitudinal cross-sectional observation positions L1 to L9 in total satisfy the following (C) and (D).

(C) The arithmetic average value of the area fraction of ferrite at the longitudinal section observation positions L1 to L9 is 50 to 70%, and the standard deviation of the ferrite area fraction is 4.0% or less.

(D) Among the average ferrite grain sizes at the longitudinal cross-section observation positions L1 to L9, the ratio of the maximum average grain diameter to the minimum average grain diameter (ferrite average grain diameter ratio) is 2.00 or less.

Hereinafter, (C) and (D) are explained in detail.

[About (C)]

As in the above (C), in the steel material of the present embodiment, the arithmetic average value of the area fraction of ferrite at the longitudinal section 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, fluctuations in the phase fraction of the microstructure at each longitudinal section observation position L1 to L9 are sufficiently suppressed. Therefore, at the time of gas carburizing treatment, it is possible to suppress fluctuations in the timing of occurrence of martensitic transformation at each of the longitudinal cross-section observation positions L1 to L9.

When the standard deviation of the area fraction of ferrite at the longitudinal cross-section observation positions L1 to L9 exceeds 4.0%, the phase fraction at each longitudinal cross-section observation position L1 to L9 varies greatly. In this case, heat treatment deformation at the time of gas carburizing cannot be sufficiently suppressed.

Therefore, the standard deviation of the area fraction of ferrite at the longitudinal section observation positions L1 to L9 is 4.0% or less.

The upper limit of the standard deviation of the area fraction of ferrite is preferably 3.8%, more preferably 3.5%, still more preferably 3.0%. The lower limit of the standard deviation of the area fraction of ferrite is not particularly limited. The lower limit of the standard deviation of the area fraction of ferrite is preferably 0.1%, more preferably 0.5%, still more preferably 1.0%, still more preferably 1.5%.

[About (D)]

Among the average ferrite grain diameters at the longitudinal cross-section observation positions L1 to L9, the ratio of the maximum average grain diameter to the minimum average grain diameter is referred to as "ferrite average grain diameter ratio". The ferrite average grain size ratio in the longitudinal section is defined by the following formula.

Ferrite average grain size ratio = (maximum value of average ferrite grain size in L1 to L9)/(minimum value of average ferrite grain size in L1 to L9)

In the steel material of the present embodiment, the ferrite average grain size ratio at the longitudinal section observation positions L1 to L9 is 2.00 or less. In this case, fluctuations in the average grain diameter of ferrite at each of the longitudinal cross-section observation positions L1 to L9 are sufficiently suppressed. That is, the ferrite grains at each position are even. Therefore, fluctuations in the occurrence of martensitic transformation during the carburizing treatment can be sufficiently suppressed. Therefore, heat treatment deformation of steel materials at the time of carburizing treatment can be suppressed.

When the ferrite average grain size ratio exceeds 2.00, the ferrite grains at each longitudinal cross-section observation position L1 to L9 fluctuate. In this case, heat treatment deformation of steel materials at the time of gas carburizing cannot be sufficiently suppressed. Therefore, the ferrite average grain size ratio at the nine longitudinal cross-section observation positions L1 to L9 is 2.00 or less.

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

[Method of observing microstructure at each observation position, measuring method of area fraction of ferrite and average ferrite grain size ratio]

The method for observing the microstructure at the cross-section observation positions C1 to C9 and the longitudinal section observation positions L1 to L9 of the steel material according to the present embodiment, and the method for measuring the area fraction of ferrite and the ferrite average grain size ratio are as follows.

[Observation method of microstructure of cross-sectional CS]

The method for observing the microstructure of cross-sectional CS is as follows. From the steel material, a sample containing each cross section observation position C1 to C9 is taken. Among the surfaces of the sample, the surface corresponding to the cross-section CS is taken as the observation surface. In the observation plane, the observation field including the cross-section observation position is 0.5 mm × 1.0 mm.

After polishing the observation surface of the sample, the observation surface is etched using 3% nitric acid alcohol (natal corrosion solution). The observation field (0.5 mm x 1.0 mm) of the etched observation surface is observed with an optical microscope at 100 times magnification.

In the observation field, each phase such as ferrite, pearlite, and bainite has a different contrast for each phase. Specifically, in the observation field, ferrite is observed as white, and bainite and pearlite are observed as black than ferrite. Therefore, ferrite can be easily distinguished from other phases (perlite and bainite). Based on the contrast, ferrite is specified.

[About (A)]

The area (μm ² ) of ferrite in each observation field (each cross-section observation position) is obtained. Using the area of ferrite and the area of the observation field, the area fraction (%) of ferrite in each observation field (each cross-sectional observation position) is obtained.

[How to find the arithmetic average value of ferrite area fraction]

The arithmetic average value of the ferrite area fraction (%) in nine observation fields (cross-sectional observation positions) is defined as the arithmetic average value (%) of the ferrite area fractions in the nine cross-section observation positions C1 to C9.

[How to find the standard deviation of the area fraction of ferrite]

From the ferrite area fractions (%) in nine observation fields (cross-section observation positions), the standard deviation (%) of the ferrite area fractions at nine cross-section observation positions C1 to C9 is calculated. The standard deviation referred to here is the sample standard deviation.

[About (B)]

The area (μm ² ) of each ferrite particle observed in each of the above-described observation fields (each cross-sectional observation position C1 to C9) is measured. Calculate the arithmetic average value of the area of each ferrite grain. The circle equivalent diameter of the arithmetic average value of the obtained area is defined as the average grain diameter (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 particles.

The average grain diameter of the ferrite at nine cross-section observation positions C1 to C9 is obtained. Then, among the average grain sizes of these ferrites, the maximum average grain diameter (µm) and the minimum average grain diameter (µm) of ferrite are specified. The ratio of the maximum average grain size to the specified minimum average grain size (ferrite average grain size ratio) is obtained.

[Observation method of microstructure of longitudinal section LS]

The method of observing the microstructure of the longitudinal section LS is as follows. From the steel materials, samples including each of the longitudinal section observation positions L1 to L9 are taken. Among the surfaces of the sample, the surface corresponding to the longitudinal section LS is used as the observation surface. In the observation plane, the observation field including the longitudinal section observation position is set to 0.5 mm x 1.0 mm. More specifically, let the length of 0.5 mm of an observation visual field be the radial direction of steel materials, and let 1.0 mm be the longitudinal direction of steel materials.

After polishing the observation surface of the sample, the observation surface is etched using 3% nitric acid alcohol (natal corrosion solution). The observation field (0.5 mm x 1.0 mm) of the etched observation surface is observed with an optical microscope at 100 times magnification. Each image in the observation field is specified in the same manner as in the microstructure observation of the cross-sectional CS.

[About (C)]

Among the images specified by the above-described method, the area (μm ² ) of ferrite in each observation field (each longitudinal section observation position) is obtained. Using the area of ferrite and the area of the observation field, the area fraction (%) of ferrite in each observation field (each longitudinal section observation position) is obtained.

[How to find the arithmetic average value of the area fraction of ferrite]

The arithmetic average value of the ferrite area fraction (%) in nine observation fields (longitudinal cross-section observation positions) is defined as the arithmetic average value (%) of the ferrite area fractions at nine longitudinal cross-section observation positions L1 to L9.

[How to find the standard deviation of the area fraction of ferrite]

From the ferrite area fractions (%) in nine observation fields (longitudinal plane observation positions), the standard deviation (%) of the ferrite area fractions at nine longitudinal plane observation positions L1 to L9 is calculated.

[About (D)]

The area (μm ² ) of each ferrite particle observed in each observation field described above (each longitudinal section observation position L1 to L9) is measured. Calculate the arithmetic average value of the area of each ferrite grain. The circle equivalent diameter of the arithmetic average value of the obtained area is defined as the average grain diameter (μm) of ferrite at each longitudinal section observation position L1 to L9.

The average grain diameter of ferrite at the nine longitudinal cross-section observation positions L1 to L9 is obtained. Then, among the average grain sizes of these ferrites, the maximum average grain diameter (µm) and the minimum average grain diameter (µm) of ferrite are specified. The ratio of the maximum average grain size to the specified minimum average grain size (ferrite average grain size ratio) is obtained.

[Regarding Formula (2)]

The chemical composition of the steel materials of this embodiment also 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 Formula (2). When the corresponding element is not contained, "0" is substituted for the element symbol.

In the steel material of this embodiment, not only the cross section CS but also the microstructure of the longitudinal section LS are 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 longitudinal-section observation positions L1 to L9 in the longitudinal section LS satisfy (C) and (D) Even if the microstructure is made uniform by satisfying , as described above, at the time of quenching in the vacuum carburizing treatment, a minute time zone in which the martensite transformed portion and the martensite untransformed portion coexist is always generated. If the amount of heat treatment deformation in the martensite untransformed portion is large in this minute time period, heat treatment deformation occurs. Therefore, in the steel materials of this embodiment, Formula (2) is also satisfied.

It is defined as F2=1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo). F2 is an index related to the heat treatment deformation amount of steel materials in gas carburizing treatment for steel materials. Among the elements in the chemical composition described above, C, Si, Mn, Cr, and Mo contained in F2 are particularly martensitic in a minute period in which a martensitic transformed portion and a martensitic untransformed portion coexist during quenching. Increase the strength of the metamorphic part.

Referring to Fig. 1, with a decrease in F2, the maximum strain amount ratio decreases and the amount of heat treatment deformation decreases. And, when F2 becomes less than 0.800, the maximum deformation amount ratio is remarkably lowered. That is, the maximum deformation amount ratio to F2 has an inflection point around F2 = 0.800.

Therefore, on the premise that the content of each element is within the range of the present embodiment, and also when F2 satisfies the formula (2), that is, when F2 is less than 0.800, the heat treatment deformation of steel materials during gas carburizing treatment is sufficiently can be suppressed

The upper limit of F2 is preferably 0.799, more preferably 0.797, still 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, still more preferably 0.775. The numerical value of F2 is a value obtained by rounding to the fourth decimal place.

In the steel material of the present embodiment having the above configuration, 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), and the cross section observation position C1 to The microstructures at C9 and longitudinal section observation positions L1 to L9 are within the scope of the present embodiment. Therefore, the machinability after performing hot working with respect to the steel materials of this embodiment is excellent. Further, when the vacuum carburizing treatment is performed on the steel materials of the present embodiment, the mechanical structural parts have excellent bending fatigue strength and surface fatigue strength, and can sufficiently suppress heat treatment deformation.

[About the micro structure of steel materials]

The steel material of this embodiment is a so-called rolled material (as-roll material). Therefore, in the steel material of the present embodiment, so-called band structures are observed in the above observation fields of the longitudinal section observation positions L1 to L9. Here, the band structure is a well-known microstructure, and as shown in Fig. 4, ferrite (ferrite band) F extending in the longitudinal direction of the steel material and non-ferrite (non-ferrite band) NF extending in the longitudinal direction of the steel material A, refers to a structure that is alternately layered in the diametric direction. Biferrite is pearlite and/or bainite.

[Use of steel materials]

As described above, the steel materials of the present embodiment are suitable for the raw material of mechanical structural parts. The steel materials of this embodiment are particularly suitable for applications such as gears or shafts for automobile applications, construction machinery, industrial machinery, and the like.

[Method of manufacturing steel]

An example of the manufacturing method of the steel materials of this embodiment is demonstrated. The manufacturing method of steel materials demonstrated later is an example for manufacturing the steel materials of this embodiment. Therefore, the steel materials having the structure described above may be manufactured by other manufacturing methods other than the manufacturing method described later. However, the manufacturing method described later is a preferable example of the manufacturing method of the steel materials of this embodiment.

An example of the manufacturing method of the steel materials of this embodiment includes the following process.

(Process 1) Process of preparing materials (material preparation process)

(Process 2) The process of manufacturing steel by hot processing the material (hot working process)

Hereinafter, each process is demonstrated.

[(Process 1) Material Preparation Process]

In the material preparation process, the material of the steel materials of this embodiment is prepared. Specifically, molten steel in which the content of each element in the chemical composition is within the range of the present embodiment, F1 satisfies formula (1), and F2 satisfies formula (2) is manufactured. The refining method is not particularly limited, and a known method may be used. For example, refining (primary refining) in a converter is performed for molten iron manufactured by a known method. The known secondary refining is performed on the molten steel tapped from the converter. In the secondary refining, the content of alloy elements in molten steel is adjusted, and the content of each element is within the range of the present embodiment, and F1 satisfies formula (1) and F2 satisfies formula (2). Produce molten steel having a composition.

A raw material is manufactured by a well-known casting method using the molten steel manufactured by the refining method described above. For example, you may manufacture an ingot by the ingot method using molten steel. Alternatively, blooms or billets may be produced by a continuous casting method using molten steel. By the above method, a raw material (ingot, bloom or billet) is manufactured. In the case of using the continuous casting method, reduction may be applied to the cast steel during solidification.

[(Process 2) Hot Working Process]

In the hot working step, hot working is performed on the raw material (ingot, bloom or billet) prepared in the raw material preparation step to manufacture the steel materials of the present embodiment. The shape of the steel is not particularly limited, but is, for example, a bar or wire. In the following description, as an example, a case where the steel material is a steel bar will be described. However, even if the steel material has a shape other than steel bar, it can be manufactured in the same hot working process.

The hot working process includes the following process.

(Process 21) Bulking rolling process

Heating temperature: 1250 to 1300°C

Retention time: more than 10 hours

(Process 22) Finish rolling process

Heating temperature: 1150 to 1200°C

Retention time: 1.5 to 3.0 hours

Finishing temperature: 950 to 1000°C

(Process 23) Temperature Maintenance Process

Average cooling rate at 900 to 800 ° C: 0.05 ° C / sec or less

(Process 24) Cooling process

Average cooling rate from 800 to 300 ° C: 0.10 to 1.00 ° C / sec

Hereinafter, each process is demonstrated.

[(Process 21) Bulking Rolling Process]

In the bulk rolling process, a billet is manufactured by hot rolling the raw material. Specifically, in the blow-rolling step, the raw material is hot-rolled (bulk-rolled) with a blow-down mill to manufacture a billet. When a continuous rolling mill is disposed downstream of the lump rolling mill, the billet after the lump rolling may also be subjected to hot rolling using the continuous rolling mill to produce a billet having a smaller size. In the continuous rolling mill, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. As described above, in the lump rolling step, the raw material is manufactured into a billet using a lump rolling mill or using a lump rolling mill and a continuous rolling mill.

The conditions in the lump rolling process are as follows.

Heating temperature: 1250 to 1300°C

Retention time: more than 10 hours

The heating temperature in the heating furnace in the lump rolling step is 1250 to 1300°C. The holding time at the heating temperature (1250 to 1300°C) is 10 hours or more. If the heating temperature in the heating furnace in the batch rolling step is 1250 to 1300 ° C, and the holding time at the heating temperature (1250 to 1300 ° C) is 10 hours or more, material preparation on the premise that other manufacturing conditions are met Solidification segregation in the material generated during the process can be sufficiently alleviated. Therefore, the standard deviation of the area fraction of ferrite at each of the cross-section observation positions C1 to C9 and each of the longitudinal cross-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.

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

Moreover, the reduction of area in a lump rolling process is 30% or more. Here, the reduction rate (%) is defined by the following formula.

Area reduction (%) = (1-area of the cross section (cross section perpendicular to the longitudinal direction) of the steel after break rolling/area of the cross section (cross section perpendicular to the longitudinal direction) of the material before break rolling) × 100

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

[(Process 22) Finish Rolling Process]

In the finish rolling step, first, the billet cooled to room temperature is heated using a heating furnace. With respect to the billet after heating, hot rolling is performed using a continuous rolling mill to manufacture a steel bar, which is a steel material.

Conditions in the finish rolling process are as follows.

Heating temperature: 1150 to 1200°C

Retention 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 step is 1150 to 1200 ° C, and the holding time at the heating temperature (1150 to 1200 ° C) is 1.5 to 3.0 hours, on the premise that other manufacturing conditions are satisfied, steel materials (steel bars) ) can sufficiently suppress the fluctuation of the temperature within. Therefore, the ferrite average grain size ratio at the cross-sectional observation positions C1 to C9 and the longitudinal cross-section observation positions L1 to L9 is 2.00 or less.

[finish temperature]

In the finish rolling process, hot rolling (finish rolling) is performed by a continuous rolling mill equipped with a plurality of rolling stands arranged in a line. Hot rolling using a continuous rolling mill WHEREIN: The steel materials temperature at the exit side of the stand which rolled steel materials last is defined as finishing temperature (degreeC). In addition, steel materials temperature means the surface temperature of steel materials.

The finishing temperature is 950 to 1000°C. If the finishing temperature is 950 to 1000 ° C., variations in the austenite grain size in the steel material (steel bar) are sufficiently suppressed on the premise that other manufacturing conditions are satisfied. Therefore, when transforming from austenite to ferrite in the temperature holding step and cooling step described later, fluctuations in the average ferrite grain size are sufficiently suppressed. Therefore, the ferrite average grain size ratio at the cross-sectional observation positions C1 to C9 and the longitudinal cross-section observation positions L1 to L9 is 2.00 or less.

[(Process 23) Temperature Maintenance Process]

In the temperature holding process, the temperature of the steel materials is maintained after the finish rolling process and before the cooling process. Conditions in the temperature holding process are as follows.

Average cooling rate at a steel material temperature of 900 to 800 ° C: 0.05 ° C / sec or less

After the finish rolling step, the average cooling rate at a steel material temperature of 900 to 800°C is suppressed to 0.05°C/sec or less. For example, after the finish rolling step, the average cooling rate is suppressed to 0.05 ° C./sec or less using a slow cooling cover, a warming cover, or a temperature holding furnace for steel materials having a steel material temperature of 900 to 800 ° C.

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

In the steel material after the finish rolling step, austenite gradually transforms into ferrite as the steel material temperature decreases. When the steel material temperature is in the range of 900 to 800 ° C., if there is a temperature fluctuation in the axial direction of the steel material, ferrite generated at a relatively early stage and ferrite generated at a relatively late stage will be mixed after the finish rolling process. In this case, the ferrite grains produced at an early stage tend to become coarser than the ferrite grains transformed at a later stage. As a result, the variation of the ferrite grains increases particularly in the axial direction (longitudinal section) of the steel material.

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

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

[(Process 24) Cooling process]

In a cooling process, the steel materials temperature after a temperature holding process is cooled. 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

Steel materials with a steel material temperature of 800 to 300 ° C. are cooled at an average cooling rate of 0.10 to 1.00 ° C./sec. 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 area of ferrite at the cross-section observation positions C1 to C9 and the longitudinal section observation positions L1 to L9, assuming that other manufacturing conditions are satisfied The arithmetic average of the fractions is 50 to 70%.

The steel materials of this embodiment which have the above-mentioned structure can be manufactured by the above manufacturing process. In addition, as mentioned above, the steel material of this embodiment is a rolled material.

The steel materials of this embodiment are suitable for raw materials for mechanical structural parts manufactured by performing a vacuum carburizing treatment. However, the steel materials of this embodiment may be manufactured into mechanical structural parts by performing surface hardening heat treatment other than vacuum carburizing treatment. Other surface hardening heat treatments include, for example, quenching and tempering, induction quenching and tempering, nitriding treatment (nitrin quenching and tempering), and the like.

[About mechanical structural parts]

Components for mechanical structures are used, for example, for automobiles and construction vehicles. Parts for machine structure are, for example, gears used in steering mechanisms, shafts, and the like.

The parts for machine structure using the steel materials of this embodiment as a material are manufactured by a well-known manufacturing method. For example, parts for mechanical structures are manufactured by the following method.

An example of the manufacturing method of the component for machine structure includes the following process.

ㆍHot working process

ㆍCutting process

ㆍHeat treatment process

Hereinafter, each process is demonstrated.

[Hot working process]

In the hot working process, hot working is performed with respect to the steel materials of this embodiment. Hot working is known hot forging, for example. The heating temperature in the hot working process is, for example, 1000 to 1300°C. The steel materials after hot working are air-cooled (air-cooled). With respect to the steel materials after standing_to_cool, you may perform an annealing process as needed.

[Cutting process]

With respect to the steel materials after a hot working process, a cutting process is implemented and an intermediate product of a predetermined shape is manufactured. At the time of this cutting process, high machinability of steel materials is requested|required. In the cutting process, known cutting is performed. By cutting, it is possible to manufacture parts for mechanical structures with precise shapes, which are difficult only in the hot working process.

[Heat treatment process]

Heat treatment is applied to the intermediate product after cutting. Here, "heat treatment" includes a well-known vacuum carburizing process and a well-known tempering process. In addition, as described above, the vacuum carburizing treatment also includes the vacuum carburizing treatment.

The vacuum carburizing process includes a vacuum carburizing process and a quenching process. In the vacuum carburizing treatment, it is a known technical matter to those skilled in the art that the surface hardness, the core hardness, and the surface carbon concentration of parts for machine structural parts can be appropriately adjusted by appropriately adjusting well-known conditions.

Hereinafter, a well-known vacuum carburizing treatment will be described as an example of the heat treatment process. In addition, it is well known to those skilled in the art that the well-known vacuum carburizing treatment is also performed in the same process as the vacuum carburizing treatment.

[Vacuum carburizing quenching treatment]

The vacuum carburizing quenching process includes a vacuum carburizing process and a quenching process. Hereinafter, a vacuum carburizing process and a quenching process are demonstrated.

[Vacuum carburizing process]

5 : is a figure which shows an example of the heat pattern in vacuum carburizing process S10 and quenching process S20. The vertical axis of FIG. 5 is the treatment temperature (° C.) at the time of vacuum carburizing, and the horizontal axis is the time (minutes). Referring 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 heating process S0 is 900-1100 degreeC, for example.

In the soaking step S1, the soaking treatment is performed by holding the intermediate product at the carburizing temperature Tc for a predetermined time (holding time t1). The holding time t1 at the carburizing temperature Tc in soaking step S1 is, for example, 5 to 120 minutes. The pressure in the furnace in soaking step S1 may be 10 Pa or less, or nitrogen gas atmosphere may be 1000 Pa or less by simultaneously performing nitrogen gas introduction and vacuum evacuation by a vacuum pump.

In the carburizing step S2, the intermediate product is maintained at the carburizing temperature Tc for a predetermined time (holding time t2). 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 known carburizing gas is used. The carburizing gas is, for example, a hydrocarbon gas such as acetylene, propane or ethylene.

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

In the diffusion step S3, the intermediate product is maintained at the carburizing temperature Tc for a predetermined time (holding time t3). 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 residual gas in the carburizing step. Alternatively, introduction of nitrogen gas and evacuation by a vacuum pump may be performed simultaneously to create a nitrogen gas atmosphere of 1000 Pa or less.

[??Ching process]

Quenching process S20 is implemented with respect to the intermediate product after vacuum carburizing process S10. A well-known cooling method may be used for the cooling method after the vacuum carburizing step S10 to the quenching temperature Ts in the quenching step S20. The cooling method may be, for example, air cooling under vacuum, gas cooling, or other methods. When performing cooling by vacuumization, it stands to cool by the pressure of 100 Pa or less, for example. When performing gas cooling, you may use an inert gas, such as nitrogen gas and/or helium gas, as a cooling gas.

Quenching process S20 includes soaking process S4. In the soaking step S4, the intermediate product after the vacuum carburizing step S10 is maintained at the quenching temperature. In quenching process S20, after soaking process S4, an intermediate product is quenched rapidly 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 maintenance at the quenching temperature Ts is not particularly limited, but is, for example, a nitrogen gas atmosphere. The pressure in the furnace may be equal to or less than atmospheric pressure, for example, 400 hPa or less. The cooling method in the quenching process is oil cooling or water cooling. Specifically, the intermediate product maintained at the quenching temperature is immersed in a cooling bath containing oil or water as a cooling medium, and rapidly cooled. The temperature of oil or water as a cooling medium is, for example, 60 to 200°C. Further, if necessary, sub-zero processing may be performed.

[Tempering process]

A well-known tempering process is implemented with respect to the intermediate product after a 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 grinding or shot peening. When performing a grinding process, a cutting process is performed and a shape is provided to steel materials. By cutting, a more precise shape can be given to steel materials. Further, when the shot peening treatment is performed, compressive residual stress is introduced into the surface layer portion of the intermediate product after the vacuum carburizing treatment. The compressive residual stress inhibits the initiation and propagation of fatigue cracks. Therefore, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are increased. The shot peening process may be performed by a known method. The shot peening treatment is preferably performed using shot particles having a diameter of, for example, 0.7 mm or less, under conditions of an arc height of 0.4 mm or more.

Example 1

Steel materials having chemical compositions shown in Table 1 were prepared. In addition, the steel type number A corresponded to SCM420H specified in JIS G 4052 (2016).

"-" in Table 1 means that the corresponding element content is 0% in significant figures (numerical values up to the minimum number of digits) stipulated in the embodiments. In other words, in the corresponding element content, it means that it is 0% when the fraction in the significant digits (number to the minimum digit) stipulated in the above-described embodiment is rounded off.

For example, the Mo content stipulated in this embodiment is stipulated as a numerical value to two decimal places. Therefore, in the steel type number C in Table 1, it means that it was 0% when the measured Mo content was rounded off to the third decimal place.

In addition, the Nb content stipulated in this embodiment is stipulated as a numerical value to three decimal places. Therefore, in the steel type number A in Table 1, it means that it was 0% when the measured Nb content was rounded to the fourth decimal place.

In addition, rounding means rounding down if the number of digits (fractions) below the prescribed minimum number of digits is less than 5, and rounding up if it is 5 or more.

Steel materials were manufactured by the following method. The molten steel was continuously cast to produce cast steel (bloom) as a raw material. The bloom as a material was subjected to a hot working step under the conditions shown in Table 2.

The temperature described in the "heating temperature (°C)" column of the "bulking rolling process" and the "finish rolling process" column is a heating temperature (°C). The time described in the "holding time (hour)" column of the "bulking rolling step" and "finish rolling step" columns is the holding time (hour) at the heating temperature. The temperature in the “finish temperature (° C.)” column of the “finish rolling step” column is the steel material temperature (surface temperature of the steel material) (° C.) at the exit side of the stand rolled down last in the continuous rolling mill in the finish rolling step. The rate described in the "cooling rate (°C/sec)" column of the "temperature holding step" column is an average cooling rate (°C/sec) at a steel material temperature of 900 to 800°C. The speed described in the "cooling rate (°C/sec)" column in the "cooling step" column is an average cooling rate (°C/sec) at a steel material temperature of 800 to 300°C.

In the blow rolling step of the hot working step, the produced bloom was heated at the heating temperature and holding time shown in Table 2. Then, the bloom was blow-rolled using a blow-down mill to manufacture a billet. The prepared 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 x 162 mm. The heating temperature and holding time in the lump rolling process were as shown in Table 2. In addition, the reduction in area in the bulk rolling step was 30% or more under any manufacturing conditions.

With respect to the billet after the bulk rolling process, the finish rolling process was performed under the conditions shown in Table 2, and a steel material (steel bar) having a diameter of 50 mm was manufactured. 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 performed on the billet after heating to manufacture a steel bar. At this time, the finishing temperature (° C.) was as shown in Table 2.

The temperature holding step was performed on the steel material (steel bar) after the finish rolling step. Manufacturing conditions a to i were adjusted so that the average cooling rate would be 0.05 ° C./sec or less by using a slow cooling cover with respect to steel materials having a steel material temperature of 900 to 800 ° C. On the other hand, in the manufacturing condition j, the steel materials having a steel material temperature of 900 to 800°C were cooled without using a slow cooling cover. Therefore, the average cooling rate in the steel material temperature of 900 to 800 ° C. exceeded 0.05 ° C./sec.

After the temperature holding step, the cooling step was performed. Specifically, in each manufacturing condition, the average cooling rate (° C./sec) at a steel material temperature of 800 to 300° C. was as shown in the cooling step column of Table 2.

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

[Evaluation test]

For the steel materials (steel bars) of each test number manufactured in the above manufacturing process, the following items were obtained.

(A1) Microstructure observation in cross section

(A11) The arithmetic mean value of the area fraction of ferrite in the cross section

(A12) standard deviation of the area fraction of ferrite in the cross section

(A13) Average grain size ratio of ferrite in cross section

(B1) Observation of the microstructure of the longitudinal section

(B11) The arithmetic mean value of the area fraction of ferrite in the longitudinal section

(B12) standard deviation of the area fraction of ferrite in the longitudinal section

(B13) Average grain size ratio of ferrite in longitudinal section

In addition, the following evaluation test was implemented with respect to the steel materials of each test number.

(C1) machinability evaluation test

(C2) Bending fatigue strength evaluation test

(C3) Cotton fatigue strength evaluation test

(C4) Heat treatment deformation evaluation test

Details will be described below.

[(A1) Observation of microstructure in cross section]

From the steel material of each test number, nine samples including each cross section observation position C1 to C9 were taken. Among the surfaces of each sample, the surface corresponding to the cross section CS was used as the observation surface. In the observation plane, the observation field including the cross-section observation position was set to 0.5 mm × 1.0 mm.

After polishing the observation surface of the sample, the observation surface was etched using 3% nitric acid alcohol (natal corrosion solution). The observation field (0.5 mm x 1.0 mm) of the etched observation surface was observed with an optical microscope at 100 times magnification. In the observation visual field, the image was specified by contrast.

Observed images are shown in the "Image" column of the "Cross Section" column in Table 3. In the "Upper" column of the "Cross section" column of Table 3, "○" is indicated when the microstructures at all cross-sectional observation positions contain ferrite and the remainder is pearlite and/or bainite. Also in any test number, the microstructure of the cross section contained ferrite, and the balance was pearlite and/or bainite.

[(A11) arithmetic mean value of the area fraction of ferrite in the cross section]

The area (μm ² ) of ferrite in each observation field (each cross-sectional observation position) was determined. The area fraction (%) of ferrite in each observation field (each cross-sectional observation position) was determined using the area of ferrite and the area of the observation field.

The arithmetic average value of the ferrite area fraction (%) in nine observation fields (cross-sectional observation positions) was defined as the arithmetic average value (%) of the ferrite area fractions in the nine cross-section observation positions C1 to C9. The arithmetic average value of the obtained ferrite area fraction is shown in the "ferrite area fraction (%)" column of the "cross section" column of Table 3.

[(A12) standard deviation of area fraction of ferrite in cross section]

From the ferrite area fractions (%) in nine observation fields (cross-section observation positions), the standard deviation (%) of the ferrite area fractions at nine cross-section observation positions C1 to C9 was calculated. The obtained standard deviation is shown in the "Standard Deviation of Ferrite Area Fraction (%)" column in the "Cross Section" column of Table 3.

[(A13) Average ferrite grain size ratio of cross section]

In addition, the area (μm ² ) of each ferrite particle observed in each of the above-described observation fields (each cross-sectional observation position C1 to C9) was measured. At each cross-sectional observation position C1 to C9, the arithmetic mean value of the area of each ferrite grain was obtained. The circle equivalent diameter of the arithmetic average value of the obtained area was defined as the average ferrite grain diameter (μm) at each cross-sectional observation position C1 to C9.

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

[(B1) Observation of microstructure of longitudinal section]

From the steel material of each test number, nine samples including each longitudinal section observation position L1 to L9 were taken. Among the surfaces of each sample, a surface corresponding to the longitudinal section LS was used as the observation surface. In the observation plane, the observation field including the longitudinal section observation position was set to 0.5 mm × 1.0 mm.

After polishing the observation surface of the sample, the observation surface was etched using 3% nitric acid alcohol (natal corrosion solution). The observation field (0.5 mm x 1.0 mm) of the etched observation surface was observed with an optical microscope at 100 times magnification. In the observation visual field, the image was specified by contrast.

Observed images are shown in the "Phase" column of the "Longitudinal section" column in Table 3. In the "Upper" column of the "Longitudinal section" column of Table 3, "○" is indicated when the microstructures at all longitudinal section observation positions contain ferrite and the remainder is pearlite and/or bainite. In any test number, the microstructure of the longitudinal section contained ferrite, and the balance was pearlite and/or bainite.

[(B11) Arithmetic average value of the area fraction of ferrite in the longitudinal section]

The area (μm ² ) of ferrite in each observation field (each longitudinal section observation position) was obtained. The area fraction (%) of ferrite in each observation field (each longitudinal section observation position) was determined using the area of ferrite and the area of the observation field.

The arithmetic average value of the ferrite area fraction (%) in nine observation fields (longitudinal cross-section observation positions) was defined as the arithmetic average value (%) of the ferrite area fractions at the nine longitudinal cross-section observation positions L1 to L9. The arithmetic average value of the obtained ferrite area fraction is shown in the "Ferrite Area Fraction (%)" column of the "Longitudinal Section" column of Table 3.

[(B12) standard deviation of the area fraction of ferrite in the longitudinal section]

From the ferrite area fractions (%) in the nine observation fields (longitudinal plane observation positions), the standard deviation (%) of the ferrite area fractions at the nine longitudinal plane observation positions L1 to L9 was calculated. The obtained standard deviation is shown in the "Standard Deviation of Ferrite Area Fraction (%)" column in the "Longitudinal Section" column of Table 3.

[(B13) Average Ferrite Particle Diameter Ratio of Longitudinal Section]

In addition, the area (μm ² ) of each ferrite particle observed in each of the above-described observation fields (each longitudinal section observation position L1 to L9) was measured. The arithmetic mean value of the area of each ferrite grain was obtained at each longitudinal section observation position L1 to L9. The circle equivalent diameter of the arithmetic average value of the obtained area was defined as the average grain diameter (μm) of ferrite at each longitudinal section observation position L1 to L9.

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

[Evaluation test]

[(C1) machinability evaluation test]

A machinability evaluation test was conducted by the following method. A heat treatment simulating hot forging and a constant temperature annealing treatment were applied to 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. After that, the steel bar was allowed to cool to room temperature. Further, it was heated at 950°C and held at 950°C for 1 hour. Furthermore, after holding|maintaining at 650 degreeC for 2 hours, it stood to cool to room temperature. The steel bar after cooling was machined (cutting) to produce a test piece for machinability evaluation with a diameter of 45 mm and a length of 400 mm.

About the test piece of each test number, outer peripheral turning was performed and tool life was evaluated. Specifically, the outer periphery turning was performed on the test piece of each test number 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 speed 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.

Perimeter turning was performed for 20 minutes under the above-mentioned cutting conditions. After that, the amount of wear on the flank surface of the cutting tool (mm) was measured.

The obtained flank face wear amount (mm) is shown in the "Abrasion amount (mm)" column of Table 3. When the amount of flank wear (mm) was less than 0.25 mm, it was judged that the machinability of the steel material was high. When the obtained flank surface 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 an Ono type rotary bending test piece for evaluation of bending fatigue strength shown in FIG. 6 was processed. Numerical values in Fig. 6 represent dimensions (unit: mm). "φ" in Fig. 6 means a diameter. "R1" means that the curvature radius 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. Thereafter, 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 with a diameter of 35 mm was machined (cutting) to produce an intermediate product of an Ono type rotary bending test piece. The diameter of the cross section of the intermediate product at the bottom of the notch was 8 mm. The intermediate product was subjected to carburizing treatment (gas carburizing quenching and tempering or vacuum carburizing quenching and tempering) to produce an Ono type rotary bending test piece shown in FIG. 6.

The test piece of Test No. 1 was subjected to gas carburizing and tempering, which is one of the carburizing treatment methods generally used for steels having a chemical composition of SCM420H.

On the other hand, with respect to the test pieces of test numbers 2 to 22, vacuum carburizing quenching and tempering were performed. The conditions of the gas carburizing treatment and vacuum carburizing treatment performed were as follows.

[Gas Carburizing and Tempering: Test No. 1]

7 is a diagram showing an example of a heat pattern in gas carburizing treatment (gas carburizing process and quenching process). Gas carburizing treatment and tempering were performed on the test piece of Test No. 1 under the conditions shown in FIG. 7 . In the gas carburizing treatment, gas carburizing step S30 and quenching step S20 were performed.

Specifically, in gas carburizing step S30, heating step S0, carburizing step S2, and diffusion step S3 were performed on the test piece. In the heating step S0, the round bar of Test No. 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 with a carbon potential Cp2 of 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 with a carbon potential Cp3 of 0.80%.

After the diffusion step S3, the quenching step S20 was performed. In quenching process S20, soaking process S4 was implemented. After furnace cooling to 850°C, in soaking step S4, the quenching temperature Ts was 850°C and the holding time t4 was 30 minutes. Thereafter, quenching was performed with oil at 130°C.

After quenching, tempering was performed on the test piece. The tempering temperature was 180°C, and the holding time at the tempering temperature was 120 minutes. After the elapse of the holding time, it was cooled in air.

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

[Vacuum carburizing quenching and tempering]

About the test pieces of test numbers 2-22, the vacuum carburizing process and tempering shown in FIG. 5 were implemented. Specifically, the pressure in the furnace was maintained at 10 Pa or less. In the heating step S0, the round bar of each test number was heated to a carburizing temperature Tc: 950°C. After heating step S0, soaking step S1 was performed. In the soaking step S1, the steel material (round bar) was maintained at a carburizing temperature Tc: 950°C for a holding time t1: 60 minutes.

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

In the soaking step S4 after the diffusion step S3, after the steel material temperature was furnace-cooled to 850°C, the test piece was soaked at a quenching temperature Ts: 850°C for a holding time t4: 30 minutes. Thereafter, quenching was performed with oil at 130°C.

After quenching, tempering was performed on the test piece. The tempering temperature was 180°C, and the holding time at the tempering temperature was 120 minutes. After the elapse of the holding time, it was cooled in air.

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

An Ono rotary bending fatigue test was conducted using an Ono rotary bending test piece after carburizing treatment (gas carburizing quenching and tempering or vacuum carburizing quenching and tempering). A plurality of test pieces were prepared for each test number. A fatigue test was conducted by changing the applied stress for each test piece, and after repeating 10 million times (10 ⁷ times), the highest stress that did not break was taken as the bending fatigue strength (MPa). In the Ohno-type rotational bending fatigue test, the rotational speed was set to 3000 rpm and the stress ratio was set to constant vibration.

A test piece using the steel of Test No. 1 was used as a reference steel. The ratio of the bending fatigue strength of each test number to the bending fatigue strength of the standard steel was defined as the bending fatigue strength ratio. That is, the bending fatigue strength ratio (%) was obtained by the following formula.

Bending fatigue strength ratio (%) = (bending fatigue strength (MPa) of each test number/bending fatigue strength (MPa) of reference steel) × 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 was obtained. On the other hand, when the bending fatigue strength ratio was less than 120%, it was judged that the bending fatigue strength was low.

[(C3) Cotton Fatigue Strength Evaluation Test]

From the steel material (steel bar with 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. Numerical values in Fig. 8 represent dimensions (unit: mm). "φ" in the drawing means a 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. Thereafter, 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 with a diameter of 35 mm was machined (cutting) to produce 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 and tempering under the conditions described above. The intermediate products of Test Nos. 2 to 22 were subjected to vacuum carburizing and tempering under the conditions described above. Through the above steps, a test piece (small roller test piece) for the roller pitching fatigue test shown in FIG. 8 was produced.

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. Numerical values in Fig. 10 represent 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 steel having a chemical composition corresponding to SCM420H specified in JIS G 4053 (2016), surface polished after gas carburizing under the same conditions as the small roller test piece 200 of test number 1, which is the standard steel. one was used. The diameter of the large roller test piece 100 was 130 mm.

In the roller pitching fatigue test, a large roller test piece 100 was pressed against the small roller test piece 200 with surface pressures of various Hertz stresses. The circumferential speed direction of both roller test pieces at the contact portion is set to the same direction, and the slip rate is -40% (the large roller test piece 100 has a 40% larger circumferential speed of the contact portion than the small roller test piece 200). A test was done. The oil temperature of ATF (lubricating oil for AT) supplied as lubricating oil to the contact portion 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 interruptions in the test was 20 million times (2.0×10 ⁷ times). After repeating 2.0×10 ⁷ times for a plurality of test pieces for each test number, the highest stress at which pitching did not occur was taken as the surface fatigue strength (MPa).

A test piece using the steel of Test No. 1 was used as a reference steel. The ratio of the face fatigue strength of each test number to the face fatigue strength of the reference steel was defined as the face fatigue strength ratio. That is, the surface fatigue strength ratio (%) was obtained by the following formula.

Surface fatigue strength ratio (%) = (plane fatigue strength (MPa) of each test number/plane fatigue strength (MPa) of reference steel) × 100

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

[(C4) Heat Treatment Deformation Evaluation Test]

Gear simulated test pieces shown in FIG. 11A were produced from steel materials (steel bars with a diameter of 50 mm) of each test number. 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. Thereafter, 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 (cutting) to prepare a gear simulated test piece before carburizing (gas carburizing, vacuum carburizing).

In Fig. 11A, numerical values accompanied by "mm" indicate dimensions (unit is mm). "φ" in the drawing means a diameter. The gear simulated test piece had a truncated cone shape. The gear simulated test piece had a circular upper surface with a diameter of 22 mm and a circular lower surface with a diameter of 34 mm. The gear simulated test piece had a cylindrical 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 simulated test piece.

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

Specifically, as shown in FIG. 11B, a total of 16 inner diameters were measured 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. In addition, the inner diameter of the through hole TH was measured from the upper end to the lower end in the longitudinal direction at a position of 0.5 mm from the upper end and at a position of 16.5 mm from the upper end. 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. In addition, at each measurement position, the inner diameter was measured at a total of 18 locations (points P1 to P18 in FIG. 11B) at a 10° pitch around the central axis CL2. Therefore, in the through hole TH, the inner diameter of 18 measurement positions x 18 locations = 324 points was measured.

The gear simulated test piece after the internal diameter measurement was subjected to carburizing treatment (gas carburizing quenching and tempering, or vacuum carburizing quenching and tempering) under the above-described carburizing treatment conditions to prepare a gear simulated test piece after carburizing treatment. Specifically, gas carburizing and tempering under the conditions described above were performed on the simulated gear test piece of Test No. 1. The simulated gear test pieces of Test Nos. 2 to 22 were subjected to vacuum carburizing and tempering under the conditions described above. For the simulated gear test pieces of each test number after the carburizing treatment, the internal diameter of the through hole TH was measured in the same manner as the method for measuring the internal diameter of the through hole TH of the simulated gear test specimen before the carburizing treatment.

[Maximum strain amount ratio of heat treatment]

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

The ratio of the maximum heat treatment strain of each test number to the maximum heat treatment strain of the reference steel (test number 1) was defined as the "maximum strain ratio". That is, the maximum strain amount ratio (%) was obtained by the following formula.

Maximum strain ratio (%) = (maximum heat treatment strain of each test number (mm)/maximum heat treatment strain of reference steel (mm)) × 100

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

[Ratio of deformation amount of heat treatment]

For 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 obtained. The value obtained by subtracting the minimum heat treatment deformation amount from the maximum heat treatment deformation amount obtained 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 calculated|required by the following formula.

Deformation difference ratio (%) = (Difference in deformation amount of each test number (μm)/Difference in deformation amount of standard steel (μm)) × 100

The obtained deformation amount difference ratio (%) is shown in the column of “Deformation amount difference ratio (%)” in Table 3. When the obtained deformation amount difference was 90% or less, it was judged that the deformation amount difference was small. On the other hand, when the deformation amount difference exceeded 90%, it was determined that the deformation amount difference was large.

It was judged that the heat treatment deformation could be sufficiently suppressed three-dimensionally when both the maximum deformation amount ratio and the deformation amount difference ratio were 90% or less. When the maximum deformation amount ratio and/or the deformation amount difference ratio exceeded 90%, it was judged that the heat treatment deformation could not be sufficiently suppressed.

[Test result]

Table 3 shows the test results. Referring to Table 3, in the steels of Test Nos. 4 to 6, the content of each element in the chemical composition was appropriate, and F1 and F2 satisfied the formulas (1) and (2). In addition, the manufacturing conditions were also appropriate for the steel materials of Test Nos. 4-6. For that reason, the microstructure of steel materials was suitable. Specifically, the microstructures of the cross sections of Test Nos. 4 to 6 were structures containing ferrite and the remainder being pearlite and/or bainite. In addition, the arithmetic average value of the ferrite area fraction was 50 to 70%, the standard deviation of the ferrite area fraction was 4.0% or less, and the average ferrite grain size ratio was 2.00 or less. Further, the microstructures of the longitudinal sections of Test Nos. 4 to 6 were structures containing ferrite and the remainder being pearlite and/or bainite. In addition, the arithmetic average 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 grain 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. In addition, the bending fatigue strength ratio was 120% or more, the plane fatigue strength ratio was 125% or more, and both the bending fatigue strength and the plane fatigue strength were excellent. In addition, 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 area fraction of ferrite in the steel materials in the cross section and the longitudinal section exceeded 4.0%. As a result, the difference in deformation amount in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

In Test Nos. 7 and 8, the heating temperature in the batch rolling process was too low. Therefore, the standard deviation of the area fraction of ferrite in the cross section and the longitudinal section exceeded 4.0%. As a result, the difference in deformation amount in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

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

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

In Test Nos. 13 and 14, the finish temperature in the finish rolling process was too high. Therefore, the ferrite average grain size ratio in the cross section and the longitudinal section exceeded 2.00. As a result, the difference in deformation amount in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

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

In Test Nos. 17 and 18, the cooling rate of the cooling process was too slow. Therefore, the arithmetic average value of the area fraction of ferrite in the cross section and the longitudinal section exceeded 70%. Therefore, the difference in deformation amount in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

In Test Nos. 19 and 20, the cooling rate of the cooling process was too fast. Therefore, the arithmetic average value of the area fraction of ferrite in the cross section and the longitudinal section was less than 50%. Therefore, the flank wear amount was 0.25 mm or more. As a result, the machinability of steel materials was low.

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

Example 2

As in Example 1, steel materials having chemical compositions shown in Table 4 were prepared.

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

[Evaluation test]

The same measurement and evaluation tests as in Example 1 were conducted in the same manner as in Example 1 with respect to the steel material (steel bar) manufactured in the above manufacturing process. In the (C2) bending fatigue strength evaluation test, (C3) face fatigue strength evaluation test, and (C4) heat treatment deformation evaluation test, the steel material of test number 1 in Table 1 was used as a reference steel.

[Test result]

Table 5 shows the test results. Referring to Table 5, in the chemical composition of the steels of Test Nos. 1 to 18, the content of each element was appropriate, F1 satisfied Formula (1), and F2 satisfied Formula (2). In addition, the manufacturing conditions were also appropriate for the steel materials of Test Nos. 1-18. For that reason, the microstructure of steel materials was suitable. Specifically, the microstructures of the cross sections of Test Nos. 1 to 18 are made of ferrite, pearlite and/or bainite, the arithmetic average value of the area fraction of ferrite is 50 to 70%, and the standard deviation of the area fraction of ferrite is It was 4.0% or less, and the ferrite average grain size ratio was 2.00 or less. The microstructures of the longitudinal sections of Test Nos. 1 to 18 are made of ferrite, pearlite and/or bainite, the arithmetic average value of the area fraction of ferrite is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less. , the ferrite average grain size ratio was 2.00 or less. Therefore, the flank wear amount was less than 0.25 mm, and the machinability was high. In addition, the bending fatigue strength ratio was 120% or more, the plane fatigue strength ratio was 125% or more, and both the bending fatigue strength and the plane 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 Nos. 19 to 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 nos. 24 and 25, F1 was too low. Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was low.

In Test No. 26, the C content was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

In Test No. 27, the Si content was too low. Therefore, F1 did not satisfy Formula (1). Therefore, the bending fatigue strength ratio was less than 120%, and the face fatigue strength ratio was less than 125%. As a result, the bending fatigue strength and face fatigue strength were low.

In Test No. 28, the Si and Mn contents were too low. Therefore, the bending fatigue strength ratio was less than 120%, and the face fatigue strength ratio was less than 125%. As a result, both bending fatigue strength and face fatigue strength were insufficient.

In Test No. 29, the Si content was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

In Test No. 30, the Mn content was too low. Therefore, the bending fatigue strength ratio was less than 120%, and the face fatigue strength ratio was less than 125%. As a result, the bending fatigue strength and face fatigue strength were low.

In Test No. 31, the Mn content was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low. Also, F1 did not satisfy equation (1). Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was insufficient.

In Test No. 32, the Mn content was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

In Test No. 33, the Cr content was too high. Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was low.

In Test No. 34, the Mo content was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

In Test No. 35, the Nb content was too high. Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was low.

In the above, the embodiment of the present disclosure has been described. However, the above-described embodiment is only an example for carrying out the present disclosure. Therefore, this indication is not limited to the above-mentioned embodiment, Within the range which does not deviate from the meaning, the above-mentioned embodiment can be changed suitably and implemented.

Claims (2)

is steel,
Chemical composition, in 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 to 0.0200%;
Al: 0.001 to 0.100%;
O: 0.0050% or less, and
P: contains 0.030% or less;
The remainder consists of Fe and impurities, and also satisfies formulas (1) and (2),
In the cross section perpendicular to the longitudinal direction of the steel material and circular in shape with a radius R,
The position of the center of the cross section and the position of R/2 in the radial direction from the center of the cross section and the R/2 positions of eight locations arranged at a pitch of 45° around the center of the cross section as nine cross section observation positions. When defined,
The microstructure at each of the above cross-sectional observation positions contains ferrite, and the remainder is composed of pearlite and/or bainite,
The arithmetic average value of the area fraction of ferrite at the nine cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,
Among the average grain diameters of ferrite at the nine cross-sectional observation positions, the ratio of the maximum average grain diameter to the minimum average grain diameter is 2.00 or less,
In the longitudinal section, which is a cross section parallel to the longitudinal direction of the steel member and includes the central axis of the steel member,
Three central axis positions arranged at R/2 pitches 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 the longitudinal section observation position of
The microstructure at each longitudinal section observation position contains ferrite, and the remainder is composed of pearlite and/or bainite,
The arithmetic average value of the area fraction of ferrite at the above nine longitudinal cross-section observation positions is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,
Among the average grain diameters of ferrite at the nine longitudinal section observation positions, the ratio of the maximum average grain diameter to the minimum average grain diameter is 2.00 or less,
steel.
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 Formulas (1) and (2). When the corresponding element is not contained, "0" is substituted for the element symbol. According to claim 1,
The chemical composition also, instead of part 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 elements: containing at least one element selected from the group consisting of 0.0050% or less,
steel.

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