STEEL NEAR-NET-SHAPE MATERIAL AND METHOD FOR PRODUCING SAME

Abstract

A steel near-net-shape material having high fatigue strength and high tensile strength is provided. The steel near-net-shape material has a chemical composition containing, in mass %, C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, V: more than 0.10 to 0.40%, and N: 0.003 to 0.030%. The steel near-net-shape material is composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, and satisfies Formula (1). A diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more. [V in precipitates]/[V]≥0.30  (1)

Description

TECHNICAL FIELD

The present invention relates to a steel near-net-shape material that is a near-net-shape material composed of steel, and a method for producing the steel near-net-shape material.

BACKGROUND ART

A steel material for structural steel is used as a starting material for components for machine structural use as typified by automobile components, industrial machinery components, and construction machinery components. Examples of a steel material for structural steel include a carbon steel material for machine structural use, and an alloy steel material for machine structural use.

Components for machine structural use are required to have high fatigue strength. Therefore, the following production method is known as a method for producing a component for machine structural use having high fatigue strength using a steel material that serves as a starting material. First, a steel material is subjected to working such as hot forging to produce a steel material having a desired component shape. The steel material having the desired component shape is subjected to an age hardening treatment to produce a steel near-net-shape material. Cutting of the steel near-net-shape material is performed to produce a component for machine structural use that is the end product. In the above production process, by subjecting the steel material after working to an age hardening treatment, the fatigue strength of the component for machine structural use can be increased.

A steel material to serve as a starting material for a component for machine structural use which is produced by performing an age hardening treatment is proposed, for example, in Japanese Patent Application Publication No. 2011-236452 (Patent Literature 1).

The steel material described in Patent Literature 1 contains, in mass %, C: 0.14 to 0.35%, Si: 0.05 to 0.70%, Mn: 1.10 to 2.30%, S: 0.003 to 0.120%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.40%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.30%, and V: 0.05 to 0.45%, with the balance being Fe and unavoidable impurities, and satisfies the following formulae:

13[C]+8[Si]+10[Mn]+3[Cu]+3[Ni]+22[Mo]+11[V]≤30,5[C]+[Si]+2[Mn]+3[Cr]+2[Mo]+4[V]≤7.3,2.4≤0.3[C]+1.1[Mn]+0.2[Cu]+0.2[Ni]+1.2[Cr]+1.1[Mo]+0.2[V]≤3.1,2.5≤[C]+[Si]+4[Mo]+9[V], and[C]≥[Mo]/16+[V]/3.

By adjusting the chemical composition to satisfy the above parametric formulae, the steel material disclosed in Patent Literature 1 has a microstructure consisting of bainite, improves hot forgeability, and increases hardness after hot forging. It is described in Patent Literature 1 that because the steel material disclosed in Patent Literature 1 has a bainitic structure, it is excellent in machinability. According to Patent Literature 1, hot forging is performed on a steel material having the aforementioned structure to produce an intermediate component. Thereafter, the intermediate component is subjected to cutting into a component having a desired shape. Thereafter, an age hardening treatment is performed. It is described in Patent Literature 1 that by this means a high strength is obtained in the produced component.

However, when the component is produced by performing hot forging, strain is liable to arise in the intermediate component during a cooling process after the hot forging. Therefore, the shape of the intermediate component is liable to be slightly deformed relative to the desired shape. In other words, it is difficult to make the shape of the intermediate component after hot forging close to the final shape due to the influence of thermal strain. Therefore, cutting of the intermediate component after hot forging is performed to make the shape of the intermediate component close to the final shape.

When cutting of an intermediate component after hot forging is performed as described above, the yield decreases. Therefore, in order to increase the yield, a recent trend is to switch from hot forging to cold working as typified by cold forging. When cold working is adopted instead of hot forging, it is possible to make the shape of the intermediate product a near net shape (a shape that is almost the same as the final shape). In this case, the cutting amount in the cutting process of the intermediate component can be reduced. Therefore, the yield improves. In addition, in some cases the cutting process itself can be omitted. In this case, the productivity improves.

However, in cold working as typified by cold forging, the machining load tends to increase compared to hot forging. For this reason, it is necessary to improve the workability of the steel material during cold working (hereinafter, referred to as “cold workability”). Specifically, it is required that the steel material can be worked into a desired shape with a small load, and that the occurrence of cracks during cold working is suppressed. Accordingly, in the case of performing an age hardening treatment after cold working, the steel material that is the object of the processing needs to have excellent cold workability and to also have excellent fatigue strength after the age hardening treatment.

A steel material to serve as a starting material for a component that is to be produced by performing an age hardening treatment after cold forging is proposed, for example, in Japanese Patent Application Publication No. 2019-173168 (Patent Literature 2).

The steel material disclosed in Patent Literature 2 consists of, in mass %, C: 0.02 to 0.25%, Si: 0.005 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less. Al: 0.005 to 0.050%/u. Cr: 0.02 to 0.70%, V: 0.02 to 0.30%, N: 0.003 to 0.030%, Nb: 0 to 0.10%, B: 0 to 0.005%, Ca: 0 to 0.005%, Bi: 0 to 0.10%, Pb: 0 to 0.20%, and the balance: Fe and impurities. In the steel material disclosed in Patent Literature 2, the total content of Cu, Ni and Mo among the impurities is 0.05 mass % or less, the content of Ti among the impurities is 0.005 mass % or less, and the steel material has a chemical composition that satisfies Formula (1). Formula (1) is as follows: [V precipitates]/[content of V]≤0.50. The microstructure of the steel material disclosed in Patent Literature 2 is composed of ferrite, and pearlite and/or bainite. The area fraction of ferrite in the microstructure is 10 to 90%. It is described in Patent Literature 2 that a steel material having the above structure has high cold forgeability, and in a case where the steel material is subjected to an age hardening treatment after cold forging, high fatigue strength is obtained.

CITATION LIST

Patent Literature

• Patent Literature 1: Japanese Patent Application Publication No. 2011-236452
• Patent Literature 2: Japanese Patent Application Publication No. 2019-173168

SUMMARY OF INVENTION

Technical Problem

A component produced from the steel material disclosed in Patent Literature 2 has high fatigue strength. However, in some cases a component is required to not only have high fatigue strength, but to also have high tensile strength. In Patent Literature 2, compatibly achieving both high fatigue strength and high tensile strength is not investigated.

An objective of the present invention is to provide a steel near-net-shape material having high fatigue strength and high tensile strength, and a method for producing the steel near-net-shape material.

Solution to Problem

A steel near-net-shape material according to the present disclosure has a chemical composition consisting of, in mass %,

• • C: 0.03 to 0.25%,
  • Si: 0.02 to 0.50%.
  • Mn: more than 0.70 to 2.50/o,
  • P: 0.035% or less,
  • S: 0.050% or less,
  • Al: 0.005 to 0.050%,
  • V: more than 0.10 to 0.40%,
  • N: 0.003 to 0.030%,
  • Cr: 0 to 0.70%,
  • Nb: 0 to 0.100%,
  • B: 0 to 0.0100%,
  • Cu: 0 to 0.30%,
  • Ni: 0 to 0.30%,
  • Ca: 0 to 0.0050%,
  • Bi: 0 to 0.100%,
  • Pb: 0 to 0.090%,
  • Mo: 0 to 0.05%,
  • Ti: 0 to 0.005%,
  • Zr: 0 to 0.010%,
  • Se: 0 to 0.10%,
  • Te: 0 to 0.10%.
  • rare earth metal: 0 to 0.010%,
  • Sb: 0 to 0.10%,
  • Mg: 0 to 0.0050%,
  • W: 0 to 0.050%, and

the balance: Fe and impurities,

wherein:

a microstructure of the steel near-net-shape material is composed of:

polygonal ferrite having an area fraction of 20 to 90%, and

a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%;

when a content of V in the chemical composition is defined as [V](mass %), and a total content of V in V precipitates in the steel near-net-shape material is defined as [V in precipitates] (mass %), the steel near-net-shape material satisfies Formula (1); and

a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more:

[V in precipitates]/[V]≥0.30  (1).

A method for producing the aforementioned steel near-net-shape material according to the present disclosure includes:

a steel material preparation process of preparing a steel material having a chemical composition consisting of, in mass %,

• • C: 0.03 to 0.25%,
  • Si: 0.02 to 0.50%,
  • Mn: more than 0.70 to 2.50%,
  • P: 0.035% or less,
  • S: 0.050% or less,
  • Al: 0.005 to 0.050%,
  • V: more than 0.10 to 0.40%,
  • N: 0.003 to 0.030%,
  • Cr: 0 to 0.70%,
  • Nb: 0 to 0.100%,
  • B: 0 to 0.0100%,
  • Cu: 0 to 0.30%,
  • Ni: 0 to 0.30%,
  • Ca: 0 to 0.0050%,
  • Bi: 0 to 0.100%,
  • Pb: 0 to 0.090%,
  • Mo: 0 to 0.05%,
  • Ti: 0 to 0.005%,
  • Zr: 0 to 0.010%,
  • Se: 0 to 0.10%,
  • Te: 0 to 0.10%,
  • rare earth metal: 0 to 0.010%,
  • Sb: 0 to 0.10%.
  • Mg: 0 to 0.0050%,
  • W: 0 to 0.050%, and

the balance: Fe and impurities,

wherein:

a microstructure of the steel material is composed of:

polygonal ferrite having an area fraction of 20 to 90%, and

a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%, and

when a content of V in the chemical composition is defined as [V] (mass %), and a total content of V in V precipitates in the steel material is defined as [V in precipitates] (mass %), [V in precipitates]/[V] is 0.05 to less than 0.30;

a cold working process of subjecting the steel material to cold working;

an age hardening treatment process of subjecting the steel material after cold working to an age hardening treatment in which a treatment temperature is set in a range of 500° C. to an A_c1 point, and a holding time at the treatment temperature is set in a range of 15 to 150 minutes;

wherein:

the cold working process includes:

a first-direction cold working process of subjecting the steel material to, from a first direction, cold working in which a working strain amount is 0.05 or more, and

a second-direction cold working process of subjecting the steel material to, from a second direction which is different from the first direction, cold working in which a working strain amount is 0.05 or more; and

a total of a working strain amount generated in the steel material in the first-direction cold working process and a working strain amount generated in the steel material in the second-direction cold working process is 0.20 or more.

Advantageous Effects of Invention

The steel near-net-shape material of the present disclosure has high fatigue strength and high tensile strength. The method for producing a steel near-net-shape material of the present disclosure can produce the aforementioned steel near-net-shape material.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a view illustrating a hydrogen evolution curve obtained in a case where hydrogen was charged into a steel near-net-shape material by a cathodic hydrogen charging method.

DESCRIPTION OF EMBODIMENTS

The present inventors conducted various studies for the purpose of obtaining high fatigue strength and high tensile strength in a steel near-net-shape material, and obtained the following findings.

First, the present inventors conducted studies from the viewpoint of the chemical composition with respect to a steel near-net-shape material in which high fatigue strength and high tensile strength can be obtained in a compatible manner. As a result, the present inventors considered that if the chemical composition of a steel near-net-shape material is a chemical composition consisting of, in mass %, C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, V: more than 0.10 to 0.40%, N: 0.003 to 0.030%, Cr: 0 to 0.70%, Nb: 0 to 0.100%, B: 0 to 0.0100%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, Ca: 0 to 0.0050%, Bi: 0 to 0.100%, Pb: 0 to 0.090%, Mo: 0 to 0.05%, Ti: 0 to 0,005%, Zr: 0 to 0.010%, Se: 0 to 0.10%, Te: 0 to 0.10%, rare earth metal: 0 to 0.010%. Sb: 0 to 0.10%, Mg: 0 to 0.0050%. W: 0 to 0.050%, and the balance: Fe and impurities, there is a possibility that high fatigue strength and high tensile strength will be obtained.

Here, the present inventors considered that if the microstructure of the steel near-net-shape material is a structure mainly composed of martensite, the tensile strength will increase. However, in a case where the microstructure of the steel near-net-shape material having the aforementioned chemical composition is a structure mainly composed of martensite, it is necessary to perform thermal refining treatment (quenching and tempering). In the quenching it is necessary to heat the steel material to a high temperature that is equal to or higher than the A_c3 point. In addition, in the thermal refining treatment, because tempering is also performed after quenching, the number of steps in the production process also increases. Therefore, in the case of making the microstructure of the steel near-net-shape material having the aforementioned chemical composition a structure mainly composed of martensite, the production cost will be high. Here, the phrase “a structure mainly composed of martensite” means a structure in which an area fraction of martensite is 90% or more.

In addition, in a case where the microstructure of a steel near-net-shape material having the aforementioned chemical composition is a structure mainly composed of martensite, in some cases the hardness of the steel near-net-shape material becomes excessively high. In this case, even when high tensile strength is obtained, in some cases the fatigue strength of the steel near-net-shape material decreases.

Therefore, the present inventors investigated means for compatibly obtaining both high fatigue strength and high tensile strength even when, in a steel near-net-shape material having the aforementioned chemical composition, the microstructure is not mainly composed of martensite, and instead the microstructure is composed of polygonal ferrite and a phase composed of pearlite and/or bainite (hereinafter, referred to as a “hard phase”). As a result, the present inventors considered that by utilizing precipitation strengthening by V precipitates, even if the microstructure is not mainly composed of martensite and is instead a structure composed of polygonal ferrite and a hard phase, both high fatigue strength and high tensile strength can be compatibly obtained.

In precipitation strengthening by V precipitates, a large number of nano-sized fine V precipitates are formed in the steel material and therefore the fatigue strength increases. In the present description, V carbo-nitrides (V(C,N)), V carbides (VC), and V nitrides (VN) are collectively defined as “V precipitates”. Almost all of the V precipitates in the steel near-net-shape material are V carbo-nitrides. However, cases can also arise in which some of the V precipitates precipitate as V carbides and/or V nitrides. V carbides and V nitrides have the same effect as V carbo-nitrides. Accordingly, in the present description, the term “V precipitates” includes V carbo-nitrides, V carbides and V nitrides.

The present inventors conducted studies to ascertain to what extent V precipitates need to be present in a steel near-net-shape material in which the contents of the respective elements in the chemical composition are within the respective ranges described above in order for the fatigue strength to increase. Here, in a steel near-net-shape material in which the contents of the respective elements in the chemical composition are within the respective ranges described above, a content of V in the chemical composition is defined as [V] (mass %). In addition, when the mass % of the chemical composition of the steel near-net-shape material is taken as 100%, the total content of V in V precipitates in the steel near-net-shape material is defined as [V in precipitates] (mass %). As a result of such studies the present inventors have discovered that in a steel near-net-shape material in which the contents of the respective elements in the chemical composition are within the respective ranges described above, if Formula (1) is satisfied, even if the microstructure of the steel near-net-shape material is a structure composed of polygonal ferrite and a hard phase, the fatigue strength of the steel near-net-shape material increases sufficiently.

[V in precipitates]/[V]≥0.30  (1)

As described above, in a steel near-net-shape material in which the contents of the respective elements in the chemical composition are within the respective ranges described above, if Formula (1) is satisfied, even if the microstructure of the steel near-net-shape material is a structure composed of polygonal ferrite and a hard phase, the fatigue strength of the steel near-net-shape material increases sufficiently. However, it has been revealed that even if the fatigue strength of the steel near-net-shape material increases sufficiently, in some cases sufficient tensile strength of the steel near-net-shape material is not obtained. Therefore, the present inventors conducted further studies regarding means for enabling both high fatigue strength and high tensile strength to be compatibly obtained. As a result, the present inventors have discovered that in a steel near-net-shape material in which the contents of the respective elements in the chemical composition are within the respective ranges described above, the microstructure is a structure composed of polygonal ferrite and a hard phase, and which satisfies Formula (1), if, in addition, a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more, it is possible to compatibly obtain both high fatigue strength and high tensile strength. This point is described hereunder.

It is considered that a diffusible hydrogen content in a case where a steel near-net-shape material is charged with hydrogen by a cathodic hydrogen charging method has a correlation with the morphology of V precipitates in the steel near-net-shape material. Among V precipitates, there are V precipitates which have a spheric shape and V precipitates which have a plate-like shape. In the following description, a V precipitate which has a spheric shape is referred to as a “spherical V precipitate”. A V precipitate which has a plate-like shape is referred to as a “plate-like V precipitate”.

A spherical V precipitate forms an incoherent interface with the parent phase (α). In this case, the spherical V precipitate acts only as a simple barrier. Specifically, the spherical V precipitate inhibits only the movement of dislocations that directly collide with the spherical V precipitate in question. Therefore, the resistance of spherical V precipitates to movement of dislocations is weak.

On the other hand, a plate-like V precipitate has a NaCl-type crystal structure, and forms a coherent interface or a semi-coherent interface having the Baker-Nutting (BN) relationship with the parent phase (α). Specifically, a plate-like V precipitate forms a coherent interface or a semi-coherent interface in which the {100} plane of the plate-like V precipitate and the {100} plane of the parent phase are parallel and the <100> direction of the plate-like V precipitate and the <110> direction of the parent phase are parallel. The coherent interface or semi-coherent interface forms a coherent strain field around the plate-like V precipitate. The coherent strain field inhibits the movement of dislocations. In other words, a plate-like V precipitate inhibits not only the movement of dislocations which directly collide with the relevant plate-like V precipitate, but also inhibits the movement of dislocations that pass through the area around the plate-like V precipitate. Therefore, the resistance of plate-like V precipitates to the movement of dislocations is stronger than the resistance of spherical V precipitates to the movement of dislocations.

Therefore, in a steel near-net-shape material in which the contents of the respective elements in the chemical composition are within the respective ranges described above, the microstructure is a structure composed of polygonal ferrite and a hard phase, and which satisfies Formula (1), if the proportion of plate-like V precipitates among V precipitates is large, resistance to the movement of dislocations can be further increased, and as a result not only high fatigue strength but also high tensile strength can be obtained.

In this connection, the size of V precipitates (spherical V precipitates and plate-like V precipitates) is at the nano level. Therefore, it is extremely difficult to distinguish between plate-like V precipitates and spherical V precipitates to determine the proportion of plate-like V precipitates among V precipitates by observing the microstructure. On the other hand, hydrogen is easily trapped at a coherent interface and a semi-coherent interface, while it is difficult for an incoherent interface to trap hydrogen. In other words, it is easy for plate-like V precipitates to trap hydrogen, and it is difficult for spherical V precipitates to trap hydrogen. Therefore, in a steel near-net-shape material in which the contents of the respective elements in the chemical composition are within the respective ranges described above, the microstructure is a structure composed of polygonal ferrite and a hard phase, and in which V precipitates are precipitated in an amount satisfying Formula (1), if the amount of trapped hydrogen (that is, the diffusible hydrogen content) when charged with hydrogen by a cathodic hydrogen charging method is large, it means that, in V precipitates which increase the fatigue strength, the proportion of plate-like V precipitates which also increase the tensile strength is large.

For the reasons described above, it is considered that in a steel near-net-shape material in which the contents of the respective elements in the chemical composition are within the respective ranges described above, the microstructure is a structure composed of polygonal ferrite and a hard phase, and which satisfies Formula (1), if, in addition, a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more, high fatigue strength and high tensile strength are obtained. The reasons described above are assumptions. However, the fact that in a steel near-net-shape material in which the contents of the respective elements in the chemical composition are within the respective ranges described above, the microstructure is a structure composed of polygonal ferrite and a hard phase, and which satisfies Formula (1), when, in addition, a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more, high fatigue strength and high tensile strength are obtained, is demonstrated by Examples to be described later.

A steel near-net-shape material of the present embodiment and a method for producing the steel near-net-shape material that have been completed based on the above findings are as follows.

[1]

A steel near-net-shape material, having a chemical composition consisting of, in mass %,

• • C: 0.03 to 0.25%,
  • Si: 0.02 to 0.50%,
  • Mn: more than 0.70 to 2.50%,
  • P: 0.035% or less,
  • S: 0.050% or less,
  • Al: 0.005 to 0.050%,
  • V: more than 0.10 to 0.40%,
  • N: 0.003 to 0.030%,
  • Cr: 0 to 0.70%,
  • Nb: 0 to 0.100%,
  • B: 0 to 0.0100%,
  • Cu: 0 to 0.30%,
  • Ni: 0 to 0.30%,
  • Ca: 0 to 0.0050%,
  • Bi: 0 to 0.100%,
  • Pb: 0 to 0.090%,
  • Mo: 0 to 0.05%,
  • Ti: 0 to 0.005%,
  • Zr: 0 to 0.010%,
  • Se: 0 to 0.10%,
  • Te: 0 to 0.10%,
  • rare earth metal: 0 to 0.010%,
  • Sb: 0 to 0.10%,
  • Mg: 0 to 0.0050%,
  • W: 0 to 0.050%, and

the balance: Fe and impurities,

wherein:

a microstructure of the steel near-net-shape material is composed of:

polygonal ferrite having an area fraction of 20 to 90%, and

a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%;

when a content of V in the chemical composition is defined as [V] (mass %), and a total content of V in V precipitates in the steel near-net-shape material is defined as [V in precipitates] (mass %), the steel near-net-shape material satisfies Formula (1); and

a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more:

[V in precipitates]/[V]≥0.30  (1)

[2]

The steel near-net-shape material according to III, wherein

the chemical composition contains, in lieu of a part of Fe, one or more elements selected from the group consisting of:

• • Cr 0.01 to 0.70%,
  • Nb: 0.001 to 0.100%,
  • B: 0.0001 to 0.0100%,
  • Cu: 0.01 to 0.30%.
  • Ni: 0.01 to 0.30%,
  • Ca: 0.0001 to 0.0050%,
  • Bi: 0.001 to 0.100%,
  • Pb: 0.001 to 0.090%,
  • Mo: 0.01 to 0.05%,
  • Ti: 0.001 to 0.005%,
  • Zr: 0.002 to 0.010%,
  • Se: 0.01 to 0.10%,
  • Te: 0.01 to 0.10%,
  • rare earth metal: 0.01 to 0.010%,
  • Sb: 0.01 to 0.10%,
  • Mg: 0.0005 to 0.0050%, and
  • W: 0.001 to 0.050%.

[3]

A method for producing the steel near-net-shape material according to [1] or [2], including:

a steel material preparation process of preparing a steel material having a chemical composition consisting of, in mass %,

• • C: 0.03 to 0.25%,
  • Si: 0.02 to 0.50%,
  • Mn: more than 0.70 to 2.50%,
  • P: 0.035% or less,
  • S: 0.050% or less,
  • Al: 0.005 to 0.050%,
  • V: more than 0.10 to 0.40%,
  • N: 0.003 to 0.030%,
  • Cr 0 to 0.70%,
  • Nb: 0 to 0.100%,
  • B: 0 to 0.0100%,
  • Cu: 0 to 0.30%,
  • Ni: 0 to 0.30%,
  • Ca: 0 to 0.0050%,
  • Bi: 0 to 0.100%,
  • Pb: 0 to 0.090%,
  • Mo: 0 to 0.05%,
  • Ti: 0 to 0.005%,
  • Zr: 0 to 0.010% M,
  • Se: 0 to 0.10%,
  • Te: 0 to 0.10%,
  • rare earth metal: 0 to 0.010%,
  • Sb: 0 to 0.10%.
  • Mg: 0 to 0.0050%,
  • W: 0 to 0.050%, and

the balance: Fe and impurities,

wherein:

a microstructure of the steel material is composed of:

polygonal ferrite having an area fraction of 20 to 90%, and

a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%, and

when a content of V in the chemical composition is defined as [V] (mass %), and a total content of V in V precipitates in the steel material is defined as [V in precipitates](mass %), [V in precipitates]/[V] is 0.05 to less than 0.30;

a cold working process of subjecting the steel material to cold working;

an age hardening treatment process of subjecting the steel material after cold working to an age hardening treatment in which a treatment temperature is set in a range of 500° C. to an A_c1 point, and a holding time at the treatment temperature is set in a range of 15 to 150 minutes;

wherein:

the cold working process includes:

a first-direction cold working process of subjecting the steel material to, from a first direction, cold working in which a working strain amount is 0.05 or more, and

a second-direction cold working process of subjecting the steel material to, from a second direction which is different from the first direction, cold working in which a working strain amount is 0.05 or more; and

a total of a working strain amount generated in the steel material in the first-direction cold working process and a working strain amount generated in the steel material in the second-direction cold working process is 0.20 or more.

Hereunder, the steel near-net-shape material and the method for producing the steel near-net-shape material of the present embodiment are described in detail. The symbol “%” in relation to elements means “mass %” unless otherwise noted.

[Regarding Steel Near-Net-Shape Material]

In the present description, the term “steel near-net-shape material” means a component obtained by subjecting a steel material to processing by an external force and/or to a heat treatment to impart a shape to the steel material. The steel near-net-shape material may be an end product. Further, the steel near-net-shape material may be subjected to a process such as cutting to produce an end product.

[Chemical Composition]

The chemical composition of the steel near-net-shape material of the present embodiment contains the following elements.

C: 0.03 to 0.25%

Carbon (C) combines with V in the steel material to form V precipitates. V precipitates increase the fatigue strength and tensile strength of the steel near-net-shape material by precipitation strengthening. If the content of C is less than 0.03%, even if the contents of other elements are within the respective ranges of the present embodiment, the aforementioned effect will not be obtained sufficiently. On the other hand, if the content of C is more than 0.25%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material that is a starting material for the steel near-net-shape material will decrease. Therefore, the content of C is to be 0.03 to 0.25%. A lower limit of the content of C is preferably 0.04%, more preferably 0.05%, further preferably 0.06%, further preferably 0.07%, and further preferably 0.08%. An upper limit of the content of C is preferably 0.24%, more preferably 0.23%, further preferably 0.22%, further preferably 0.21%, and further preferably 0.20%.

Si: 0.02 to 0.50%

Silicon (Si) increases the fatigue strength of the steel near-net-shape material. Si also deoxidizes the steel. If the content of Si is less than 0.02%, even if the contents of other elements are within the respective ranges of the present embodiment, the aforementioned effects will not be obtained sufficiently. On the other hand, if the content of Si is more than 0.50%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material that is a starting material for the steel near-net-shape material will decrease. Therefore, the content of Si is to be 0.02 to 0.50%. A lower limit of the content of Si is preferably 0.03%, more preferably 004%, further preferably 0.05%, further preferably 0.06%, and further preferably 0.07%. An upper limit of the content of Si is preferably 0.45%, more preferably 0.40%, further preferably 0.35%, further preferably 0.30%, and further preferably 0.25%.

Mn: more than 0.70 to 2.50%

Manganese (Mn) increases the fatigue strength of the steel near-net-shape material. If the content of Mn is 0.70% or less, even if the contents of other elements are within the respective ranges of the present embodiment, the aforementioned effect will not be obtained sufficiently. On the other hand, if the content of Mn is more than 2.50%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material that is a starting material for the steel near-net-shape material will decrease. Therefore, the content of Mn is to be more than 0.70 to 2.500% n. A lower limit of the content of Mn is preferably 0.75%, more preferably 0.80%, further preferably 1.00%, further preferably 1.20%, further preferably 1.40%, and further preferably 1.50%. An upper limit of the content of Mn is preferably 2.40%, more preferably 2.30%, further preferably 2.20%, further preferably 2.10%, further preferably 2.00%, and further preferably 1.90%.

P: 0.035% or less

Phosphorus (P) is an impurity that is unavoidably contained. In other words, the content of P is more than 0%. P segregates at grain boundaries, which decreases the fatigue strength and tensile strength of the steel near-net-shape material. Therefore, the content of P is to be 0.035% or less. An upper limit of the content of P is preferably 0.030%, more preferably 0.025%, and further preferably 0.020%. The content of P is preferably as low as possible. However, excessively reducing the content of P will raise the production cost. Therefore, when taking into consideration normal industrial production, a lower limit of the content of P is preferably 0.001%, more preferably 0.005%, further preferably 0.008%, and further preferably 0.010%.

S: 0.050% or less

Sulfur (S) is an impurity that is unavoidably contained. In other words, the content of S is more than 0%. S combines with Mn to form MnS, which increases the machinability of the steel material. However, if the content of S is more than 0.050%, coarse MnS will form. The coarse MnS is liable to serve as the origin of a crack during cold working. Consequently, the cold workability of the steel material that is a starting material for the steel near-net-shape material will decrease. Therefore, the content of S is to be 0.050% or less. An upper limit of the content of S is preferably 0.045%, more preferably 0.040%, further preferably 0.030%, and further preferably 0.020%. The content of S is preferably as low as possible. However, excessively reducing the content of S will raise the production cost. Therefore, when taking into consideration normal industrial production, a lower limit of the content of S is preferably 0.001%, more preferably 0.005%, and further preferably 0.006%.

Al: 0.005 to 0.050%

Aluminum (Al) deoxidizes the steel. If the content of Al is less than 0.005%, even if the contents of other elements are within the respective ranges of the present embodiment, the aforementioned effect will not be obtained. On the other hand, if the content of Al is more than 0.050%, even if the contents of other elements are within the respective ranges of the present embodiment, coarse Al-based inclusions such as Al oxides will form in the steel material. The coarse Al-based inclusions are liable to serve as the origin of a crack during cold working. Consequently, the cold workability of the steel material that is a starting material for the steel near-net-shape material will decrease. Therefore, the content of Al is to be 0.005 to 0.050%. A lower limit of the content of Al is preferably 0.005%, more preferably 0.006%, further preferably 0.007%, further preferably 0.008%, further preferably 0.009%, further preferably 0.010%, and further preferably 0.015%. An upper limit of the content of Al is preferably 0.045%, more preferably 0.040%, further preferably 0.030%, further preferably 0.025%, and further preferably 0.020%. Note that in the steel near-net-shape material of the present embodiment, the phrase “content of Al” means the total content of Al.

V: more than 0.10 to 0.40%

Vanadium (V) combines with C and/or N in the steel material to form V precipitates. The V precipitates increase the fatigue strength and tensile strength of the steel near-net-shape material by precipitation strengthening. If the content of V is 0.10% or less, even if the contents of other elements are within the respective ranges of the present embodiment, the aforementioned effect will not be obtained sufficiently. On the other hand, if the content of V is more than 0.40%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material that is a starting material for the steel near-net-shape material will decrease. Therefore, the content of V is to be more than 0.10 to 0.40%. A lower limit of the content of V is preferably 0.11%, more preferably 0.12%, further preferably 0.13%, further preferably 0.14%, and further preferably 0.15%. An upper limit of the content of V is preferably 0.38%, more preferably 0.35%, further preferably 0.33%, further preferably 0.30%, further preferably 0.28%, and further preferably 0.25%.

N: 0.003 to 0.030%

Nitrogen (N) combines with V in the steel material to form V precipitates. The V precipitates increase the fatigue strength and tensile strength of the steel near-net-shape material by precipitation strengthening. If the content of N is less than 0.003%, even if the contents of other elements are within the respective ranges of the present embodiment, the aforementioned effect will not be obtained sufficiently. On the other hand, if the content of N is more than 0.030%, even if the contents of other elements are within the respective ranges of the present embodiment, the numerical proportion of spherical V precipitates among the V precipitates will become large. In such case, the fatigue strength and tensile strength of the steel near-net-shape material will decrease. Therefore, the content of N is to be 0.003 to 0.030%. A lower limit of the content of N is preferably more than 0.003%, more preferably is 0.004%, and further preferably is 0.005%. An upper limit of the content of N is preferably 0.028%, more preferably 0.025%, further preferably 0.023%, further preferably 0.020%, further preferably 0.018%, and further preferably 0.015%.

The balance of the chemical composition of the steel near-net-shape material of the present embodiment is Fe and impurities. Here, the term “impurities” refers to elements which, during industrial production of the steel material that is the starting material for the steel near-net-shape material, are mixed in from ore or scrap that is used as a raw material or from the production environment or the like, and which are not elements that are intentionally contained in the steel near-net-shape material. For example, oxygen (O) is assumed as an impurity. Even if O as an impurity is contained in an amount of 0.040% or less, the advantageous effects of the steel near-net-shape material of the present embodiment are obtained. Note that, it is considered that elements other than O can also be included in the impurities.

[Regarding Optional Elements]

The chemical composition of the steel near-net-shape material of the present embodiment may further contain, in lieu of a part of Fe, one or more elements selected from the group consisting of Cr, Nb, B. Cu, Ni, Ca. Bi, Pb, Mo, Ti, Zr, Se, Te, rare earth metal (REM), Sb, Mg and W. These elements are each an optional element. Hereunder, each optional element is described.

[First Group]

The chemical composition of the steel near-net-shape material of the present embodiment may further contain, in lieu of a part of Fe, one or more elements selected from the group consisting of Cr, Nb, B, Cu and Ni within the respective ranges of contents described in the following. Each of these elements increases the fatigue strength and tensile strength of the steel near-net-shape material.

Cr: 0 to 0.70%

Chromium (Cr) is an optional element and need not be contained. In other words, the content of Cr may be 0%. When contained, that is, when the content of Cr is more than 0%, Cr improves the hardenability of the steel material and increases the fatigue strength and tensile strength of the steel near-net-shape material. If even a small amount of Cr is contained, the aforementioned effects are obtained to a certain extent. However, if the content of Cr is more than 0.70%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material that is the starting material for the steel near-net-shape material will decrease. Therefore, the content of Cr is to be 0 to 0.70%. When contained, the content of Cr is to be 0.70% or less. A lower limit of the content of Cr is preferably 0.01%, more preferably 0.03%, further preferably 0.05%, further preferably 0.07%, further preferably 0.09%, and further preferably 0.10%. An upper limit of the content of Cr is preferably 0.65%, more preferably 0.60%, further preferably 0.50%, further preferably 0.45%, further preferably 0.40%, further preferably 0.35%, and further preferably 0.30%.

Nb: 0 to 0.100%

Niobium (Nb) is an optional element and need not be contained. In other words, the content of Nb may be 0%. When contained, that is, when the content of Nb is more than 0%, Nb combines with C and/or N in the steel material to form Nb precipitates. The Nb precipitates increase the fatigue strength and tensile strength of the steel near-net-shape material by precipitation strengthening. If even a small amount of Nb is contained, the aforementioned effect is obtained to a certain extent. However, if the content of Nb is more than 0.100/a, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material that is the starting material for the steel near-net-shape material will decrease. Therefore, the content of Nb is to be 0 to 0.100%. When contained, the content of Nb is to be 0.100% or less. A lower limit of the content of Nb is preferably more than 0%, more preferably is 0.001%, further preferably is 0.010%, and further preferably is 0.020%. An upper limit of the content of Nb is preferably 0.080%, and more preferably is 0.060%.

B: 0 to 0.0100%

Boron (B) is an optional element and need not be contained. In other words, the content of B may be 0%. When contained, that is, when the content of B is more than 0%. B strengthens crystal grain boundaries of the steel near-net-shape material. As a result, the fatigue strength and tensile strength of the steel near-net-shape material increase. If even a small amount of B is contained, the aforementioned effect is obtained to a certain extent. However, if the content of B is more than 0.0100%, the aforementioned effect will be saturated. In addition, if the content of B is more than 0.0100%, the raw material cost will increase and the producibility will also decrease. Therefore, the content of B is to be 0 to 0.0100%. When contained, the content of B is to be 0.0100% or less. A lower limit of the content of B is preferably more than 0%, more preferably is 0.0001%, further preferably is 0.0010%, further preferably is 0.0020%, and further preferably is 0.00300%. An upper limit of the content of B is preferably 0.0080%, more preferably 0.0070%, and further preferably 0.0060%.

Cu: 0 to 0.30%

Copper (Cu) is an optional element and need not be contained. In other words, the content of Cu may be 0%. When contained, that is, when the content of Cu is more than 0%, Cu improves the hardenability of the steel material and increases the fatigue strength and tensile strength of the steel near-net-shape material. If even a small amount of Cu is contained, the aforementioned effects are obtained to a certain extent. However, if the content of Cu is more than 0.30%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material that is the starting material for the steel near-net-shape material will decrease. Therefore, the content of Cu is to be 0 to 0.30%. When contained, the content of Cu is to be 0.30% or less. A preferable lower limit of the content of Cu is more than 0%, more preferably is 0.01%, further preferably is 0.05%, and further preferably is 0.10%. A preferable upper limit of the content of Cu is 0.29%, more preferably is 0.28%, and further preferably is 0.25%.

Ni: 0 to 0.30%

Nickel (Ni) is an optional element and need not be contained. In other words, the content of Ni may be 0%. When contained, that is, when the content of Ni is more than 0%. Ni improves the hardenability of the steel material and increases the fatigue strength and tensile strength of the steel near-net-shape material. If even a small amount of Ni is contained, the aforementioned effects are obtained to a certain extent. However, if the content of Ni is more than 0.30%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold forgeability of the steel material that is the starting material for the steel near-net-shape material will decrease. Therefore, the content of Ni is to be 0 to 0.30%. When contained, the content of Ni is to be 0.30% or less. A preferable lower limit of the content of Ni is more than 0%, more preferably is 0.01%, further preferably is 0.05%, and further preferably is 0.10%. A preferable upper limit of the content of Ni is 0.29%, more preferably is 0.28%, further preferably is 0.27%, and further preferably is 0.25%.

[Second Group]

The chemical composition of the steel near-net-shape material of the present embodiment may further contain, in lieu of a part of Fe, one or more elements selected from the group consisting of Ca, Bi and Pb within the respective ranges of contents described below. Each of these elements increases the machinability of the steel near-net-shape material.

Ca: 0 to 0.0050%.

Calcium (Ca) is an optional element and need not be contained. In other words, the content of Ca may be 0%. When contained, that is, when the content of Ca is more than 0%, Ca increases the machinability of the steel near-net-shape material. If even a small amount of Ca is contained, the aforementioned effect is obtained to a certain extent. However, if the content of Ca is more than 0.0050%, even if the contents of other elements are within the respective ranges of the present embodiment, Ca will form coarse CaO. In this case, the cold workability of the steel material that is the starting material for the steel near-net-shape material will decrease. Therefore, the content of Ca is to be 0 to 0.0050%. When contained, the content of Ca is to be 0.0050% or less. A preferable lower limit of the content of Ca is more than 0%, more preferably is 0.0001%, further preferably is 0.0010%, and further preferably is 0.0120%. A preferable upper limit of the content of Ca is 0.0045%, and more preferably is 0.0040%.

Bi: 0 to 0.100%

Bismuth (Bi) is an optional element and need not be contained. In other words, the content of Bi may be 0%. When contained, that is, when the content of Bi is more than 0%. Bi increases the machinability of the steel near-net-shape material. If even a small amount of Bi is contained, the aforementioned effect is obtained to a certain extent. However, if the content of Bi is more than 0.100%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material that is the starting material for the steel near-net-shape material will decrease. Therefore, the content of Bi is to be 0 to 0.100%. When contained, the content of Bi is to be 0.100% or less. A preferable lower limit of the content of Bi is more than 0%, more preferably is 0.001%, further preferably is 0.010%, further preferably is 0.020%, and further preferably is 0.030%. A preferable upper limit of the content of Bi is 0.090%, more preferably is 0.080%, further preferably is 0.070%, and further preferably is 0.065%.

Pb: 0 to 0.090%

Lead (Pb) is an optional element and need not be contained. In other words, the content of Pb may be 0%. When contained, that is, when the content of Pb is more than 0%, Pb increases the machinability of the steel near-net-shape material. If even a small amount of Pb is contained, the aforementioned effect is obtained to a certain extent. However, if the content of Pb is more than 0.090%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material that is the starting material for the steel near-net-shape material will decrease. Therefore, the content of Pb is to be 0 to 0.090%. When contained, the content of Pb is to be 0.090% or less. A preferable lower limit of the content of Pb is more than 0%, more preferably is 0.001%, further preferably is 0.010%, further preferably is 0.020%, and further preferably is 0.040%. A preferable upper limit of the content of Pb is 0.080%, and more preferably is 0.070%.

[Third Group]

The chemical composition of the steel near-net-shape material of the present embodiment may further contain, in lieu of a part of Fe, one or more elements selected from the group consisting of Mo, Ti, Zr, Se, Te, rare earth metal (REM), Sb, Mg and W. These elements are impurities.

Mo: 0 to 0.05%

Molybdenum (Mo) is an impurity, and need not be contained. In other words, the content of Mo may be 0%. Mo reduces the cold workability of the steel material that is the starting material for the steel near-net-shape material. If the content of Mo is more than 0.05%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material will decrease. Therefore, the content of Mo is to be 0 to 0.05%. When contained, the content of Mo is to be 0.05% or less. A preferable upper limit of the content of Mo is 0.04%, more preferably 0.03%, and further preferably 0.02%. The content of Mo is preferably as low as possible. However, excessively reducing the content of Mo will raise the production cost. Therefore, a preferable lower limit of the content of Mo is more than 0%, and more preferably is 0.01%.

Ti: 0 to 0.005%

Titanium (Ti) is an impurity, and need not be contained. In other words, the content of Ti may be 0%. Ti combines with N in the steel near-net-shape material to form Ti-based inclusions. The Ti-based inclusions serve as the origin of a crack during cold working. Consequently, the Ti-based inclusions decrease the cold workability of the steel material that is the starting material for the steel near-net-shape material. If the content of Ti is more than 0.005%, even if the contents of other elements are within the respective ranges of the present embodiment, the cold workability of the steel material will decrease. Therefore, the content of Ti is to be 0 to 0.005%. When contained, the content of Ti is to be 0.005% or less. A preferable upper limit of the content of Ti is 0.004%, more preferably is 0.003%, and further preferably is 0.002%. The content of Ti is preferably as low as possible. However, excessively reducing the content of Ti will raise the production cost. Therefore, a preferable lower limit of the content of Ti is more than 0%, and more preferably is 0.001%.

Zr: 0 to 0.010%

Zirconium (Zr) is an impurity, and need not be contained. In other words, the content of Zr may be 0%. If the content of Zr is more than 0.010%, even if the contents of other elements are within the respective ranges of the present embodiment, Zr will form coarse inclusions and thereby cause the fatigue characteristics of the steel material to decrease. Therefore, the content of Zr is to be 0 to 0.010%. When contained, the content of Zr is to be 0.010% or less. A preferable upper limit of the content of Zr is 0.008%, more preferably 0.006%, and further preferably 0.005%. The content of Zr is preferably as low as possible. However, excessively reducing the content of Zr will raise the production cost. Therefore, a preferable lower limit of the content of Zr is more than 0%, and more preferably is 0.002%.

Se: 0 to 0.10%

Selenium (Se) is an impurity, and need not be contained. In other words, the content of Se may be 0%. If the content of Se is more than 0.10%, even if the contents of other elements are within the respective ranges of the present embodiment, Se will embrittle the steel material and cause the strength and fatigue characteristics of the steel material to decrease. Therefore, the content of Se is to be 0 to 0.10%. When contained, the content of Se is to be 0.10% or less. A preferable upper limit of the content of Se is 0.08%, more preferably 0.06%, and further preferably 0.05%. The content of Se is preferably as low as possible. However, excessively reducing the content of Se will raise the production cost. Therefore, a preferable lower limit of the content of Se is more than 0%, and more preferably is 0.01%.

Te: 0 to 0.10%,

Tellurium (Te) is an impurity, and need not be contained. In other words, the content of Te may be 0%. If the content of Te is more than 0.10%, even if the contents of other elements are within the respective ranges of the present embodiment, Te will embrittle the steel material and cause the strength and fatigue strength of the steel material to decrease. Therefore, the content of Te is to be 0 to 0.10%. When contained, the content of Te is to be 0.10% or less. A preferable upper limit of the content of Te is 0.08%, more preferably is 0.06%, and further preferably is 0.05%. The content of Te is preferably as low as possible. However, excessively reducing the content of Te will raise the production cost. Therefore, a preferable lower limit of the content of Te is more than 0%, and more preferably is 0.01%.

Rare earth metal (REM): 0 to 0.010%

Rare earth metal (REM) is an impurity, and need not be contained. In other words, the content of REM may be 0%. If the content of REM is more than 0.010%, even if the contents of other elements are within the respective ranges of the present embodiment, REM will form coarse inclusions and will cause the fatigue characteristics of the steel material to decrease. Therefore, the content of REM is to be 0 to 0.010%. When contained, the content of REM is to be 0.010% or less. A preferable upper limit of the content of REM is 0.008%, more preferably 0.006%, and further preferably 0.005%. The content of REM is preferably as low as possible. However, excessively reducing the content of REM will raise the production cost. Therefore, a preferable lower limit of the content of REM is more than 0%, and more preferably is 0.001%.

Note that, in the present description the term “REM” means one or more types of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. In the present description the term “content of REM” refers to the total content of these elements.

Sb: 0 to 0.10%

Antimony (Sb) is an impurity, and need not be contained. In other words, the content of Sb may be 0%. If the content of Sb is more than 0.10%, even if the contents of other elements are within the respective ranges of the present embodiment. Sb will embrittle the steel material and cause the strength and fatigue characteristics of the steel material to decrease. Therefore, the content of Sb is to be 0 to 0.10%. When contained, the content of Sb is to be 0.10% or less. A preferable upper limit of the content of Sb is 0.08%, more preferably is 0.06%, and further preferably is 0.05%. The content of Sb is preferably as low as possible. However, excessively reducing the content of Sb will raise the production cost. Therefore, a preferable lower limit of the content of Sb is more than 0%, and further preferably is 0.01%.

Mg: 0 to 0.0050%

Magnesium (Mg) is an impurity, and need not be contained. In other words, the content of Mg may be 0%. If the content of Mg is more than 0.0050%, even if the contents of other elements are within the respective ranges of the present embodiment, Mg will form coarse inclusions and cause the fatigue characteristics of the steel material to decrease. Therefore, the content of Mg is to be 0 to 0.0050%. When contained, the content of Mg is to be 0.0050% or less. A preferable upper limit of the content of Mg is 0.0040%, more preferably is 0.0030%, and further preferably is 0.0025%. The content of Mg is preferably as low as possible. However, excessively reducing the content of Mg will raise the production cost. Therefore, a preferable lower limit of the content of Mg is more than 0%, and more preferably is 0.05%.

W: 0 to 0.050%

Tungsten (W) is an impurity, and need not be contained. In other words, the content of W may be 0%. If the content of W is more than 0.050%, even if the contents of other elements are within the respective ranges of the present embodiment, W will reduce the cold workability of the steel material that is the starting material. Therefore, the content of W is to be 0 to 0.050%. When contained, the content of W is to be 0.040% or less. A preferable upper limit of the content of W is 0.030%, more preferably 0.025%, and further preferably 0.020%. The content of W is preferably as low as possible. However, excessively reducing the content of W will raise the production cost. Therefore, a preferable lower limit of the content of W is more than 0%, and further preferably is 0.001%.

[Microstructure]

The microstructure of the steel near-net-shape material of the present embodiment contains polygonal ferrite, and pearlite and/or bainite. In the present description, pearlite and/or bainite are referred to as a “hard phase”. Further, in the present description, the term “bainite” includes martensite. In microstructure observation that is described later, it is extremely difficult to distinguish between bainite and martensite after an age hardening treatment. Therefore, in the present description, bainite and martensite are not distinguished, and are referred to collectively as “bainite”.

The polygonal ferrite area fraction in the microstructure is 20 to 90%. The balance of the microstructure is, as described above, a hard phase. In other words, the microstructure of the steel near-net-shape material is composed of polygonal ferrite having an area fraction of 20 to 90%, and the hard phase having a total area fraction of 10 to 80%.

If the polygonal ferrite area fraction is 20 to 90%, on the precondition that the contents of the respective elements in the chemical composition are within the respective ranges of the present embodiment, Formula (1) is satisfied, and a diffusible hydrogen content when the steel near-net-shape material is charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more, high fatigue strength and high tensile strength are obtained in the steel near-net-shape material.

A preferable lower limit of the polygonal ferrite area fraction in the microstructure of the steel near-net-shape material is 25%, more preferably 30%, and further preferably 35%. A preferable upper limit of the polygonal ferrite area fraction is 80%, more preferably 75%, and further preferably 70%.

The hard phase area fraction in the microstructure is, as described above, 10 to 80%. A preferable lower limit of the area fraction of pearlite in the microstructure is 5%, and more preferably is 10%. A preferable upper limit of the area fraction of pearlite is 50%, and more preferably is 40%. A preferable lower limit of the area fraction of bainite in the microstructure is 5%, and more preferably is 10%, A preferable upper limit of the area fraction of bainite in the microstructure is 80%, and more preferably is 70%.

[Method for Measuring Polygonal Ferrite Area Fraction and Total Area Fraction of Pearlite and Bainite in Microstructure]

The polygonal ferrite area fraction and the total area fraction of pearlite and bainite in the microstructure of the steel near-net-shape material are measured by the following method.

A specimen for microstructure observation is taken from an arbitrary position of the steel near-net-shape material. An arbitrary surface among the surfaces of the specimen is designated as an observation surface. The observation surface is mirror-polished. The observation surface after polishing is etched using a 3% nital etching reagent (ethanol+3% nitric acid solution). An arbitrary five observation visual fields on the etched observation surface are observed with an optical microscope at a magnification of ×400, and photographic images are created. At this time, the position of each observation visual field is a position which is deeper than at least 3 mm from the original surface of the steel near-net-shape material. The size of each observation visual field is set to 200 μm×200 μm. Polygonal ferrite is identified in the photographic images of the respective visual fields. Specifically, a phase having a lamellar structure can be identified as pearlite. A region (white region) in which the brightness is higher than the brightness of the pearlite can be identified as polygonal ferrite. A region (dark region) in which the brightness is lower than the brightness of the polygonal ferrite and the pearlite can be identified as bainite. The polygonal ferrite area fraction (%) is determined based on the total area of polygonal ferrite determined in the five visual fields, and the total area of the five visual fields. Similarly, the total area fraction (%) of pearlite and bainite is determined based on the total area of pearlite and bainite determined in the five visual fields, and the total area of the five visual fields. Note that, in the microstructure, if pearlite is 0%, the total area fraction of pearlite and bainite corresponds to the area fraction of bainite. Similarly, in the microstructure, if bainite is 0%, the total area fraction of pearlite and bainite corresponds to the area fraction of pearlite.

[Regarding Formula (1)]

In the steel near-net-shape material of the present embodiment, the content of V in the chemical composition of the steel near-net-shape material is defined as [V] (mass %). In addition, the total content of V in V precipitates in the steel near-net-shape material when the chemical composition of the steel near-net-shape material is taken as 100% is defined as [V in precipitates] (mass %). In this case, the steel near-net-shape material of the present embodiment satisfies Formula (1).

[V in precipitates]/[V]≥0.30  (1)

Let VP be defined as VP=[V in precipitates]/[V]. VP shows the precipitation proportion of V precipitates in the steel near-net-shape material. Even when the contents of the elements in the chemical composition of the steel near-net-shape material are within the respective ranges of the present embodiment and the microstructure is a structure composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, if VP is less than 0.30, formation of V precipitates in the steel near-net-shape material will be insufficient. In this case, the fatigue strength and tensile strength in the steel near-net-shape material will decrease.

On the other hand, if VP is 0.30 or more, on the precondition that the contents of the respective elements in the chemical composition of the steel near-net-shape material are within the respective ranges of the present embodiment, the microstructure is a structure composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, and a diffusible hydrogen content is 0.10 ppm or less, V precipitates will be sufficiently precipitated in the steel near-net-shape material. Therefore, the fatigue strength and tensile strength of the steel near-net-shape material will be increased by precipitation strengthening by the V precipitates.

A preferable lower limit of VP is 0.31, and more preferably is 0.32. An upper limit of VP is not particularly limited. A preferable upper limit of VP is 0.60, more preferably is 0.55, and further preferably is 0.52.

[Method for Measuring Total Content of V in V Precipitates ([V in Precipitates]) in the Steel Near-Net-Shape Material]

The content of V in V precipitates (that is, [V in precipitates]) in the steel near-net-shape material is determined by an extraction residue analysis method.

Specifically, a sample of approximately 1000 mm³ (approximately 7.8 g) is cut out from the steel near-net-shape material. A 10% AA-based solution (a liquid in which tetramethylammonium chloride, acetylacetone, and methanol are mixed at a ratio of 1:10:100) is prepared. The cut-out sample is immersed in the 10% AA-based solution. Constant-current electrolysis is performed on the immersed sample.

First, pre-electrolysis is performed on the sample. By this means, deposits on the surface of the sample are removed. The following conditions are set for the pre-electrolysis: current: 1000 mA, time: 28 minutes, temperature: room temperature (25° C.). After undergoing pre-electrolysis, the sample is taken out from the solution. After being taken out from the solution, the sample is ultrasonically cleaned in alcohol. By this means, deposits on the surface of the sample are removed. The mass of the sample from which deposits have been removed (the mass of the sample before constant-current electrolysis) is measured.

Next, constant-current electrolysis is performed on the sample after the pre-electrolysis. The following conditions are set for the electrolysis: current: 173 mA, time: 142 minutes, temperature: room temperature (25° C.). The electrolyzed sample is taken out from the solution. After being taken out from the solution, the sample is ultrasonically cleaned in alcohol. By this means, deposits (residue) on the surface of the sample are removed. The solution after electrolysis, and the solution used for the ultrasonic cleaning are suction-filtered using a filter. The mesh size of the filter is 0.2 μm. By this means, residue is collected.

The mass of the sample from which deposits (residue) were removed (mass of the sample after constant-current electrolysis) is measured. Then the “mass of the sample electrolyzed with a constant current” is determined based on a differential value between the measured values of the mass of the sample before and after the constant-current electrolysis.

The residue collected on the aforementioned filter is transferred to a petri dish and dried. The mass of the dried residue is measured. Thereafter, in accordance with JIS G 1258 (2014), the residue is analyzed using an ICP emission spectrometer (high-frequency inductively coupled plasma emission spectrophotometer) to determine the “mass of V in the residue”.

A value obtained by dividing the determined “mass of V in the residue” by the “mass of the sample electrolyzed with a constant current” and shown as a percentage is defined as “[V in precipitates] (mass %)”.

[Regarding Diffusible Hydrogen Content]

In the steel near-net-shape material of the present embodiment, in addition, on the precondition that the contents of the respective elements in the chemical composition are within the respective ranges of the present embodiment, the microstructure is composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, and the steel near-net-shape material satisfies Formula (1), a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more. More specifically, a diffusible hydrogen content in a case where the steel near-net-shape material of the present embodiment is charged with hydrogen by a cathodic hydrogen charging method in a 3% NaCl-3 g/L NH₄SCN aqueous solution under conditions of a current density of 0.1 mA/cm² and a conduction time of 72 hours is 0.10 ppm or more.

In the steel near-net-shape material of the present embodiment, if the diffusible hydrogen content is 0.10 ppm or more, on the precondition that the contents of the respective elements in the chemical composition are within the respective ranges of the present embodiment, the microstructure is composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, and the steel near-net-shape material satisfies Formula (1), high fatigue strength and high tensile strength are obtained.

Although the reason for this is not certain, as described above it is considered that it is due to the morphology of V precipitates. Among V precipitates, spherical V precipitates form an incoherent interface with the parent phase. In this case, the spherical V precipitates themselves become a barrier to the movement of dislocations. However, it is difficult for the parent phase which is in contact with the spherical precipitates at the incoherent interface to become a barrier to the movement of dislocations. On the other hand, around plate-like V precipitates, a coherent interface or a semi-coherent interface is formed. In this case, not only the plate-like V precipitates but also a coherent strain field of the parent phase around the plate-like V precipitates which is in contact with the plate-like V precipitates at the coherent interface or the semi-coherent interface become a barrier to the movement of dislocations. Consequently, the resistance of plate-like V precipitates to the movement of dislocations is stronger than the resistance of spherical V precipitates. Therefore, when VP is the same value (that is, even when the precipitated amount of V precipitates is the same), the fatigue strength and tensile strength become higher when the precipitated amount of plate-like V precipitates is greater.

In this connection, hydrogen is easily trapped at a coherent interface and a semi-coherent interface. On the other hand, it is difficult for hydrogen to be trapped at an incoherent interface. In other words, plate-like V precipitates easily trap hydrogen, and it is difficult for spherical V precipitates to trap hydrogen. Therefore, in a steel near-net-shape material which includes V precipitates in an amount satisfying Formula (1), it is considered that when the aforementioned diffusible hydrogen content is large, the proportion of plate-like V precipitates is large. It is considered that as a result, in the steel near-net-shape material, together with high fatigue strength, high tensile strength is also obtained.

The mechanism described above is an assumed mechanism. However, the fact that in a steel near-net-shape material in which contents of the respective elements in the chemical composition are within the respective ranges described above, the microstructure is composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, and which satisfies Formula (1), if, in addition, a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more, high fatigue strength and high tensile strength are obtained has been demonstrated by Examples to be described later.

A preferable lower limit of the diffusible hydrogen content is 0.11 ppm, more preferably is 0.12 ppm, further preferably is 0.13 ppm, and further preferably is 0.14 ppm. Although a preferable upper limit of the diffusible hydrogen content is not particularly limited, for example, the upper limit is 0.50 ppm, more preferably is 0.45 ppm, further preferably is 0.40 ppm, further preferably is 0.35 ppm, and further preferably is 0.30 ppm.

[Method for Measuring Diffusible Hydrogen Content]

A method for measuring the diffusible hydrogen content is as follows. A round bar specimen having a diameter of 7 mm and a length of 40 mm is cut out from an arbitrary position of the steel near-net-shape material. A cathodic hydrogen charging method is used to introduce hydrogen into the cut-out round bar specimen.

Specifically, the round bar specimen is immersed in a 3% NaCl-3 g/L NH₄SCN aqueous solution. Thereafter, hydrogen is introduced into the round bar specimen by a cathodic hydrogen charging method under conditions of a current density of 0.1 mA/cm² and a conduction time of 72 hours. The timing at which the aforementioned conduction of a current is stopped is taken as the timing at which introduction of hydrogen into the round bar specimen is completed.

After completing the introduction of hydrogen into the round bar specimen, the hydrogen content in the round bar specimen is measured using thermal desorption-gas chromatography. The following treatment is performed depending on the time from completing introduction of hydrogen to the round bar specimen until starting measurement of the hydrogen content in the round bar specimen using thermal desorption-gas chromatography (hereinafter, this time is referred to as “gap time”).

If the gap time is 30 minutes or less, the round bar specimen into which introduction of hydrogen has been completed is used as it is to start measurement of the hydrogen content. On the other hand, if the gap time is to be more than 30 minutes, after introduction of hydrogen to the round bar specimen is completed, the round bar specimen is stored in a state in which the round bar specimen is immersed in liquid nitrogen until starting measurement of the hydrogen content. This is to suppress the release of hydrogen introduced into the round bar specimen to the outside of the round bar specimen during the period until measurement of the hydrogen content is started.

The hydrogen content in the round bar specimen which, depending on the gap time, was subjected to the aforementioned treatment is measured by the following method using thermal desorption-gas chromatography. Specifically, the round bar specimen is heated from room temperature to 400° C. at a heating rate of 100° C./hr. The hydrogen content generated by the rise in temperature is measured at intervals of five minutes. Based on the obtained hydrogen contents, a hydrogen evolution curve as illustrated in FIG. 1 is obtained. The obtained hydrogen evolution curve is used to determine the cumulative hydrogen content released from room temperature to 350° C. The obtained cumulative hydrogen content is defined as the “diffusible hydrogen content (ppm)”.

As described above, in the steel near-net-shape material of the present embodiment, the contents of the respective elements in the chemical composition are within the respective ranges of the present embodiment, the microstructure is composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, the steel near-net-shape material satisfies Formula (1), and a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more. Therefore, in the steel near-net-shape material of the present embodiment, not only is high fatigue strength obtained, but high tensile strength is also obtained.

[Production Method]

A method for producing the steel near-net-shape material of the present embodiment is described hereunder. The production method described in the following is one example of a method for producing the steel near-net-shape material, and the production method is not limited to the following method. In other words, as long as the contents of the respective elements in the chemical composition are within the respective ranges of the present embodiment, the microstructure is composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, the steel near-net-shape material satisfies Formula (1), and a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more, a method for producing the steel near-net-shape material is not limited to the production method described in the following. However, the production method described in the following is a favorable method for producing the steel near-net-shape material of the present embodiment.

The method for producing the steel near-net-shape material of the present embodiment includes a process of preparing a steel material to serve as a starting material for a steel near-net-shape material (steel material preparation process), and a process of producing a steel near-net-shape material from the steel material (steel near-net-shape material production process). Each process is described in detail hereunder.

[Steel Material Preparation Process]

In the steel material preparation process, a steel material to serve as a starting material for a steel near-net-shape material is prepared. Although the shape of the steel material is not particularly limited, for example, the steel material is a steel bar or a wire rod. The composition of the steel material to serve as a starting material for the steel near-net-shape material of the present embodiment is as follows.

[Composition of Steel Material to Serve as Starting Material for Steel Near-Net-Shape Material]

The composition of the steel material to serve as a starting material for the steel near-net-shape material of the present embodiment is as follows. The chemical composition of the steel material to serve as the starting material for the steel near-net-shape material is the same as the chemical composition of the steel near-net-shape material. In other words, the chemical composition of the steel material consists of, in mass %, C: 0.03 to 0.25%. Si: 0.02 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, V: more than 0.10 to 0.40%. N: 0.003 to 0.030%, Cr: 0 to 0.70%, Nb: 0 to 0.100%, B: 0 to 0.0100%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, Ca: 0 to 0.0050%, Bi: 0 to 0.100%, Pb: 0 to 0.090%, Mo: 0 to 0.05%, Ti: 0 to 0.005%, Zr: 0 to 0.010%, Se: 0 to 0.10%, Te: 0 to 0.10%, rare earth metal: 0 to 0.010%, Sb: 0 to 0.10%. Mg: 0 to 0.0050%, W: 0 to 0.050%, and the balance: Fe and impurities.

In the steel material that is the starting material for the steel near-net-shape material of the present embodiment, V in the chemical composition of the steel material is defined as [V] (mass %). The total content of V in V precipitates in the steel material when the chemical composition of the steel material is taken as 100% is defined as [V in precipitates] (mass %). In this case, [V in precipitates]/[V] in the steel material is 0.05 to less than 0.30.

Let VP0 be defined as VP0=[V in precipitates]/[V] in the steel material. It is preferable that in the steel material to serve as the starting material for the steel near-net-shape material, V precipitates are not formed as much as possible, and the amount of dissolved V is large. If a large amount of V precipitates is already formed in the steel material, in an age hardening treatment to be described later in the process for producing a steel near-net-shape material that uses the steel material as a starting material, fine V precipitates will not form, and the V precipitates that are already present in the steel material will coarsen. In this case, in the steel near-net-shape material, it will be difficult for plate-like V precipitates to form, and the proportion of spherical V precipitates will be excessively large. In such case, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by a cathodic hydrogen charging method will be low. Consequently, sufficient fatigue strength and tensile strength will not be obtained in the steel near-net-shape material.

If VP0 of the steel material is 0.30 or more, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by a cathodic hydrogen charging method will be low. Consequently, sufficient fatigue strength and tensile strength will not be obtained in the steel near-net-shape material. Therefore, VP0 of the steel material is to be made less than 0.30. Although a lower limit of VP0 of the steel material is not particularly limited, for example, the lower limit is 0.05.

Note that, VP0 of the steel material can be determined by the same measurement method as the method for measuring VP in the steel near-net-shape material.

In addition, the microstructure of the steel material that is the starting material for the steel near-net-shape material of the present embodiment is the same as the microstructure of the steel near-net-shape material that is described above. In other words, the microstructure of the steel material is composed of polygonal ferrite having an area fraction of 20 to 90%, and a hard phase having an area fraction of 10 to 80%.

If the polygonal ferrite area fraction in the microstructure of the steel material is more than 90%. V precipitates will be formed in an excessively large amount in the steel material. Consequently, VP0 will be 0.30 or more. In this case, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by a cathodic hydrogen charging method will be low. Therefore, sufficient fatigue strength and tensile strength will not be obtained in the steel near-net-shape material.

On the other hand, if the polygonal ferrite area fraction in the microstructure of the steel material is less than 10%, the cold workability of the steel material will be too low. In this case, in a cold working process during a process for producing the steel near-net-shape material to be described later, a crack will occur in the steel material. Therefore, the microstructure of the steel material is composed of polygonal ferrite having an area fraction of 20 to 90%, and a hard phase having an area fraction of 10 to 80%.

Note that, the polygonal ferrite area fraction and the hard phase area fraction in the microstructure of the steel material can be measured by the same measurement method as the method for measuring the polygonal ferrite area fraction and the hard phase area fraction in the microstructure of the steel near-net-shape material.

The steel material to serve as a starting material for the steel near-net-shape material of the present embodiment may be supplied from a third party or may be produced. In the case of producing the steel material, the steel material preparation process includes a process of preparing a starting material (starting material preparation process), and a process of subjecting the starting material to hot working to produce a steel material (hot working process). These processes are described hereunder.

[Starting Material Preparation Process]

A molten steel having the aforementioned chemical composition is produced. A starting material is then prepared using the molten steel. For example, a molten steel having the aforementioned chemical composition is produced using a converter or an electric furnace or the like. The molten steel is used to produce a cast piece by a continuous casting process. Alternatively, the molten steel is used to produce an ingot by an ingot-making process.

[Hot Working Process]

The prepared starting material is subjected to hot working to produce a steel material. In the case of performing hot rolling as the hot working, for example, the following method is available. The hot rolling includes a rough rolling process of subjecting the starting material to rough rolling to form the starting material into a billet, and a finish rolling process of subjecting the billet to finish rolling to make the billet into a steel material. In the rough rolling process, for example, the following processes are performed. The starting material (cast piece or ingot) is heated, and thereafter is subjected to blooming using a blooming mill. As necessary, after blooming, the starting material is further subjected to rolling using a continuous mill to produce a billet. In the continuous mill, horizontal roll stands and vertical roll stands are alternately arranged in a row, and the starting material is rolled using grooves formed in the rolling rolls of the respective stands to thereby form the starting material into a billet. Note that, a billet may be produced directly by a continuous casting process.

In the finish rolling process, for example, the following processes are performed. The billet produced in the rough rolling process is charged into a heating furnace and heated. The heated billet is then subjected to finish rolling (hot rolling) with a finish-rolling mill train to make the billet into a rod having a predetermined diameter. The finish-rolling mill train includes a plurality of stands arranged in a row. Each stand includes a plurality of rolls arranged around a pass line. The billet is rolled using grooves formed in the rolling rolls of the respective stands to produce a steel material (rod).

Note that, the hot working process is not limited to hot rolling. In the hot working process, instead of the hot rolling described above, hot forging may be performed, or a hot extrusion process may be performed.

[Regarding Heating Temperature]

In the hot working process, the heating temperature for heating the steel material immediately prior to performing the final hot working is to be, for example, 1000 to 1300° C. For example, in a case where the hot working process includes a rough rolling process and a finish rolling process, the heating temperature in the heating furnace of the finish rolling process is to be 1000 to 1300° C. If the heating temperature in the heating furnace of the finish rolling process is 1000 to 1300° C., on the precondition that the other production conditions are satisfied, V precipitates formed prior to the hot working process will sufficiently dissolve.

[Regarding Finishing Temperature]

In the hot working process, the steel material temperature after the final rolling reduction is defined as the finishing temperature (° C.). In a case where the hot working process includes a rough rolling process and a finish rolling process, the term “finishing temperature” means the steel material temperature (surface temperature of the steel material) on the exit side of the stand at which rolling reduction was last performed in the finish-rolling mill train in the finish rolling process. The finishing temperature is, for example, 800 to 1200° C. If the finishing temperature is 800 to 1200° C., on the precondition that the other production conditions are satisfied, reprecipitation of V that dissolved in the heating furnace can be sufficiently suppressed.

[Regarding Cooling Rate]

In the hot working process, the cooling rate after hot working is, for example, 0.4 to 4.0° C./s. Here, the cooling rate after hot working is defined as follows. The average cooling rate until the steel material temperature reaches 200° C. from the finishing temperature after hot working is completed is defined as the cooling rate after hot working (° C./s). If the cooling rate after hot working is 0.4 to 4.0° C./s, on the precondition that the other production conditions are satisfied, the polygonal ferrite area fraction will be 20 to 90% and the hard phase area fraction will be 10 to 80% in the steel material, and in addition, [V in precipitates]/[V] in the steel material will be 0.05 to less than 0.30.

The steel material to serve as a starting material for the steel near-net-shape material is produced by the production method described above. Note that, the steel material after the hot working process may be subjected to a normalizing treatment process for the purpose of adjusting the microstructure.

[Normalizing Treatment Process]

The normalizing treatment process is an optional process, and need not be performed. When performing the normalizing treatment process, it suffices that a heat treatment temperature in the normalizing treatment is 1000 to 1300° C., and a cooling rate after holding at the heat treatment temperature is 0.4 to 4.0° C./s. In other words, the heat treatment temperature and the cooling rate of the normalizing treatment are set in the same ranges as the heating temperature and the cooling rate in the hot working process, respectively.

[Steel Near-Net-Shape Material Production Process]

One example of a method for producing a steel near-net-shape material using the above steel material will now be described. The steel near-net-shape material production process includes a process of subjecting the steel material to cold working (cold working process), a process of subjecting the steel material after cold working to an age hardening treatment (age hardening treatment process), and a process of performing cutting of the steel material after the age hardening treatment (cutting process). Here, the cutting process is an optional process. In other words, the cutting process need not be performed. Each process is described hereunder.

[Cold Working Process]

The cold working process includes a first-direction cold working process and a second-direction cold working process. In the first-direction cold working process, the steel material is subjected to cold working in which a working strain amount becomes 0.05 or more from a first direction. In the second-direction cold working process, the steel material is subjected to cold working in which a working strain amount becomes 0.05 or more from a second direction. In addition, in the cold working process a total of a working strain amount generated in the steel material in the first-direction cold working process and a working strain amount generated in the steel material in the second-direction cold working process is 0.20 or more.

The first direction and the second direction are not particularly limited as long as they are different directions to each other. For example, the first direction and the second direction may intersect. Further, the first direction and the second direction may be orthogonal, as in the case of cold drawing and upsetting to be described later.

In short, in the cold working process, the steel material receives loads that are applied from two different directions (a first direction and a second direction). By means of the loads applied from the two directions, the directions of movement of dislocations within grains of the steel material become a plurality of directions, and not just one fixed direction. Therefore, in comparison to a case where cold working is performed only from one direction, cross-slips are more likely to occur within the steel material. When a cross-slip occurs, dislocations are more likely to collide with each other. Therefore, dislocations which have collided and no longer move (sessile dislocations) increase, and dislocations that are left within grains increase. As a result, the dislocation density within grains increases. When the dislocation density increases, strain is formed. In an age hardening treatment process to be described later, to eliminate this strain, plate-like V precipitates are likely to precipitate in portions where the strain is formed. In other words, the formed strain serves as the nuclei of the plate-like V precipitates. When plate-like V precipitates are precipitated, the fatigue strength and tensile strength of the produced steel near-net-shape material are increased by precipitation strengthening.

In other words, it is considered that there is a positive correlation between the amount of strain formed in the steel material and the precipitated amount of plate-like V precipitates in the steel material. In the cold working process, strain is generated in the steel material by carrying out cold working from two different directions (first direction and second direction). The total of the working strain generated by the cold working from two different directions is referred to herein as “total working strain”. By accumulating the total working strain in the steel material, in the age hardening treatment process to be described later, plate-like V precipitates can be sufficiently precipitated so that the diffusible hydrogen content becomes 0.10 ppm or more.

Here, the total of the amount of strain generated by a first cold working process (first-direction working strain amount), and the amount of strain generated by a second cold working process (second-direction working strain amount) is defined as the “total working strain amount”. More specifically, in the present embodiment, the first-direction working strain amount is 0.05 or more, and the second-direction working strain amount is 0.05 or more. In addition, the total working strain amount is 0.20 or more.

If the total working strain amount is 0.20 or more, dislocations which move in multiple directions in the steel material will increase, and as a result the dislocation density in the grains will increase. Therefore, in the age hardening treatment process to be described later, plate-like V precipitates can be caused to precipitate sufficiently so as to satisfy Formula (1) and so that the diffusible hydrogen content becomes 0.10 ppm or more. As a result, in the steel near-net-shape material, sufficiently high fatigue strength and sufficiently high tensile strength are obtained. If the total working strain amount is less than 0.20, the aforementioned effect will not be obtained sufficiently. Therefore, the total working strain amount is to be 0.20 or more. A preferable lower limit of the total working strain amount is 0.23, more preferably is 0.25, and further preferably is 0.28.

An upper limit of the total working strain amount is not particularly limited. However, if the total working strain amount is excessively large, the deformation resistance of the steel material during the cold working process will be excessively high, and an excessive load will be applied to the equipment system. Thus, a preferable upper limit of the total working strain amount is 1.50, more preferably is 1.20, and further preferably is 0.80.

In the cold working process of the present embodiment, furthermore, as described above, not only is it required to make the total working strain amount that is the total of the first-direction working strain amount and the second-direction working strain amount 0.20 or more, but also to make the first-direction working strain amount 0.05 or more and to make the second-direction working strain amount 0.05 or more. If at least one of the first-direction working strain amount and the second-direction working strain amount is less than 0.05, even if the total working strain amount is 0.20 or more, although there may be cases where Formula (1) is satisfied, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by a cathodic hydrogen charging method will be excessively low. When at least one of the first-direction working strain amount and the second-direction working strain amount is less than 0.05, the movement directions of dislocations within grains will be biased. In this case, it will be difficult for cross-slips to occur. Consequently, the dislocation density within the grains will be insufficient. It is considered that, consequently, formation of plate-like V precipitates in the age hardening treatment process will be insufficient.

An upper limit of the first direction working strain and an upper limit of the second direction working strain are not particularly limited. A preferable upper limit of the first-direction working strain amount is, for example, 0.40, and more preferably is 0.30. A preferable lower limit of the first-direction working strain amount is 0.06, and more preferably is 0.08. A preferable upper limit of the second-direction working strain amount is, for example, 0.80, and more preferably is 0.50. A preferable lower limit of the second-direction working strain amount is 0.06, and more preferably is 0.08.

[Preferable First-Direction Cold Working Process and Second-Direction Cold Working Process]

Preferably, cold drawing is performed as the first-direction cold working process, and upsetting is performed as the second-direction cold working process.

In the cold drawing (first-direction cold working process), wire drawing is performed. The wire drawing may be only primary wire drawing, or may be cold drawing that is carried out multiple times such as a primary wire drawing, a secondary wire drawing and the like. After cold drawing (after the first-direction cold working process), depending on the steel near-net-shape material to be produced, the steel material may be cut to an appropriate length.

In the upsetting (second-direction cold working process), working that compresses the steel material in the longitudinal direction is performed. The upsetting may be performed once, or may be performed multiple times.

In a case where cold drawing is adopted as the first cold working process and upsetting is adopted as the second cold working process, by means of the cold drawing and upsetting, the steel material receives loads applied from two directions which are, namely, a direction perpendicular to the longitudinal direction of the steel material, and the longitudinal direction of the steel material. In this case, it is easier for cross-slips to occur, and it is easier for sessile dislocations to increase in the steel material. As a result, the dislocation density in the grains increases and it is easier for strain that becomes the nuclei of plate-like V precipitates to be formed in a large amount in the steel material.

The working strain amount generated in the steel material by cold drawing (first-direction working strain amount) is defined as “cold drawing strain amount”. The working strain amount generated in the steel material by upsetting (second-direction working strain amount) is defined as “upsetting strain amount”. The cold drawing strain amount and the upsetting strain amount are calculated by means of a true strain amount ε(−) by a cylindrical approximation defined by Formula (2).

ε(−)=|In{1+(L−L0)/L0}|  (2)

Specifically, in the case of calculating the cold drawing strain amount, L in Formula (2) represents the length in the wire-drawing direction (longitudinal direction) of the steel material after the cold drawing process. L0 in Formula (2) represents the length in the wire-drawing direction (longitudinal direction) of the steel material before the cold drawing process. Based on the above definitions, the cold drawing strain amount (true strain amount ε) is determined using Formula (2). In a case where cold drawing is carried out multiple times, the cold drawing strain amount (true strain amount ε) in each cold drawing operation is determined, and the total value of those cold drawing strain amounts is adopted as the cold drawing strain amount (true strain amount ε) in the cold drawing process.

On the other hand, in the case of calculating the upsetting strain amount, L in Formula (2) represents the length in the wire-drawing direction (longitudinal direction) of the steel material after the upsetting process. L0 in Formula (2) represents the length in the wire-drawing direction (longitudinal direction) of the steel material before the upsetting process. Based on the above definitions, the upsetting strain amount (true strain amount ε) is determined using Formula (2). In a case where upsetting is carried out multiple times, the upsetting strain amount (true strain amount ε) in each upsetting operation is determined, and the total value of those upsetting strain amounts is adopted as the upsetting strain amount (true strain amount ε) in the upsetting process.

The total value of the determined cold drawing strain amount and upsetting strain amount is adopted as the total working strain amount (−).

As described above, in the cold working process of the production method of the present embodiment, by performing the first-direction cold working process and the second-direction cold working process, working strain amounts from two different directions are imparted to the steel material. At such time, the first-direction working strain amount is 0.05 or more, the second-direction working strain amount is 0.05 or more, and in addition, the total working strain amount is 0.20 or more. By this means, in an age hardening treatment process that is described later, nuclei for forming a large amount of plate-like V precipitates are formed in the steel material. As a result, in the age hardening treatment process, plate-like V precipitates can be caused to sufficiently precipitate so as to satisfy Formula (1) and so that the diffusible hydrogen content becomes 0.10 ppm or more.

[Age Hardening Treatment Process]

The steel material after the cold working process is subjected to an age hardening treatment process. The treatment temperature (° C.) and the holding time (min) at the treatment temperature in the age hardening treatment process are as follows.

Treatment Temperature: 500° C. to A_c1 Point

If the treatment temperature in the age hardening treatment process (hereinafter, also referred to as “age hardening treatment temperature”) is within the range of 500° C. to the A_c1 point, V precipitates can be caused to precipitate in the steel material so as to satisfy Formula (1) and also so that a diffusible hydrogen content in the steel near-net-shape material when charged with hydrogen by a cathodic hydrogen charging method will be 0.10 ppm or more. As a result, in the steel near-net-shape material, high fatigue strength and high tensile strength can be obtained.

If the age hardening treatment temperature is less than 500° C., the precipitated amount of V precipitates will be insufficient. In this case, the steel near-net-shape material will not satisfy Formula (1). On the other hand, it is considered that if the age hardening treatment temperature is more than the A_c1 point, a change from plate-like V precipitates to spherical V precipitates will be promoted. Therefore, the diffusible hydrogen content in the steel near-net-shape material when charged with hydrogen by a cathodic hydrogen charging method will be excessively low. Thus, the age hardening treatment temperature is to be within the range of 500° C. to the A_c1 point. A preferable lower limit of the age hardening treatment temperature is 520° C., more preferably is 540° C., and further preferably is 560° C. A preferable upper limit of the age hardening treatment temperature is 700° C., more preferably is 680° C., and further preferably is 660° C.

Holding Time: 15 to 150 Minutes

The holding time at the age hardening treatment temperature is to be 15 to 150 minutes. If the holding time is 15 to 150 minutes, V precipitates can be caused to precipitate in the steel material so as to satisfy Formula (1) and also so that a diffusible hydrogen content in the steel near-net-shape material when charged with hydrogen by a cathodic hydrogen charging method will be 0.10 ppm or more. As a result, in the steel near-net-shape material, high fatigue strength and high tensile strength can be obtained.

If the holding time is less than 15 minutes, the precipitated amount of V precipitates will be insufficient. In this case, the steel near-net-shape material will not satisfy Formula (1). On the other hand, it is considered that if the holding time is more than 150 minutes, a change from plate-like V precipitates to spherical V precipitates will be promoted. Therefore, the diffusible hydrogen content in the steel near-net-shape material when charged with hydrogen by a cathodic hydrogen charging method will be excessively low. Thus, the holding time is to be 15 to 150 minutes. A preferable lower limit of the holding time is 20 minutes, and more preferably is 30 minutes. A preferable upper limit of the holding time is 120 minutes, and more preferably is 100 minutes.

The steel near-net-shape material of the present embodiment can be produced by the production processes described above. Note that, the production method described above is one example of a method for producing the steel near-net-shape material of the present embodiment. Accordingly, as long as the contents of the respective elements in the chemical composition are within the respective ranges of the present embodiment, the microstructure is composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, the steel near-net-shape material satisfies Formula (1), and a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more, a method for producing the steel near-net-shape material is not limited to the production method described above. However, the production method described above is a favorable method for producing the steel near-net-shape material of the present embodiment.

[Regarding Optional Process]

As described above, the steel material after the age hardening treatment process may be subjected to a cutting process.

[Cutting Process]

The cutting process is an optional process. If performed, in the cutting process, the steel material after the age hardening treatment is subjected to cutting to produce the steel near-net-shape material in a desired shape.

As described above, the steel near-net-shape material of the present embodiment can be produced by the above described production process (cold working process—age hardening treatment process—cutting process, or cold working process—age hardening treatment process) instead of the conventional production process (hot forging process—cutting process). Because a hot forging process can be omitted, the yield can be improved and, furthermore, the productivity can be increased. Hereunder, the steel near-net-shape material of the present embodiment is described specifically by way of Examples.

Examples

Molten steels of each test number having the chemical compositions shown in Table 1-1 and Table 1-2 were produced by vacuum melting. A 150 kg ingot was produced using the respective molten steels. The symbol “−” in the “Chemical Composition” column of Table 1-1 and Table 1-2 means that the content of the corresponding element was less than the detection limit. Note that, in the steel of each test number shown in Table 1-1 and Table 1-2, the content of O was 0.0040% or less.

TABLE 1-1
Test	Chemical Composition (unit is mass % balance in Fe and impurities)
Number	C	Si	Mn	P	S	Al	V	N	Cr	Nb	B	Cu	Ni	Co	Bi	Pb	Mo	Ti	Zr	Se	Te	REM	Sb	Mg	W
1	0.15	0.09	1.42	0.016	0.012	0.025	0.12	0.005	0.09	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
2	0.15	0.08	1.80	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
3	0.24	0.08	1.42	0.014	0.010	0.012	0.18	0.005	0.15	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
4	0.22	0.06	1.65	0.015	0.011	0.013	0.23	0.005	0.14	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
5	0.06	0.09	2.06	0.015	0.009	0.011	0.28	0.004	0.12	—	—	—	—	—	—	—	0.01	0.002	—	—	—	—	—	—	—
6	0.04	0.07	2.11	0.014	0.010	0.012	0.34	0.005	0.21	—	—	—	—	—	—	—	0.61	0.001	—	—	—	—	—	—	—
7	0.14	0.44	1.62	0.016	0.010	0.010	0.23	0.006	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
8	0.19	0.03	1.81	0.011	0.009	0.015	0.25	0.005	0.31	—	—	—	—	—	—	—	—	0.001	—	—	—	—	—	—	—
9	0.20	0.12	1.50	0.009	0.012	0.016	0.19	0.006	0.11	—	—	—	—	—	—	—	0.02	—	—	—	—	—	—	—	—
10	0.10	0.06	2.27	0.015	0.009	0.010	0.21	0.005	0.15	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
11	0.17	0.06	0.71	0.016	0.011	0.011	0.24	0.006	0.16	—	—	—	—	—	—	—	0.01	0.002	—	—	—	—	—	—	—
12	0.15	0.08	1.68	0.033	0.010	0.012	0.22	0.005	0.15	—	—	—	—	—	—	—	0.61	0.001	—	—	—	—	—	—	—
13	0.14	0.06	1.79	0.015	0.048	0.013	0.23	0.007	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
14	0.11	0.07	1.81	0.015	0.011	0.011	0.39	0.005	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
15	0.13	0.09	1.80	0.015	0.010	0.010	0.37	0.006	0.15	—	—	—	—	—	—	—	0.61	0.001	—	—	—	—	—	—	—
16	0.16	0.06	1.79	0.016	0.012	0.011	0.14	0.006	0.16	—	—	—	—	—	—	—	0.01	0.002	—	—	—	—	—	—	—
17	0.18	0.07	1.78	0.014	0.011	0.010	0.11	0.005	0.14	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
18	0.14	0.06	1.77	0.014	0.008	0.043	0.21	0.006	0.13	—	—	—	—	—	—	—	0.61	0.001	—	—	—	—	—	—	—
19	0.15	0.07	1.83	0.016	0.009	0.006	0.19	0.006	0.14	—	—	—	—	—	—	—	0.01	0.002	—	—	—	—	—	—	—
20	0.13	0.08	1.61	0.016	0.012	0.015	0.20	0.005	0.63	—	—	—	—	—	—	—	0.01	0.002	—	—	—	—	—	—	—
21	0.16	0.06	1.81	0.016	0.008	0.014	0.23	0.005	0.04	—	—	—	—	—	—	—	0.61	0.001	—	—	—	—	—	—	—
22	0.15	0.06	1.80	0.014	0.010	0.012	0.24	0.027	0.15	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
23	0.14	0.07	1.78	0.015	0.010	0.013	0.25	0.014	0.14	—	—	—	—	—	—	—	0.01	0.002	—	—	—	—	—	—	—
24	0.16	0.06	1.79	0.015	0.011	0.011	0.23	0.004	0.11	—	—	—	—	—	—	—	0.61	0.001	—	—	—	—	—	—	—
25	0.14	0.06	1.81	0.013	0.011	0.012	0.21	0.005	0.13	—	—	—	—	—	—	—	0.01	0.004	—	—	—	—	—	—	—
26	0.14	0.09	1.30	0.015	0.012	0.011	0.22	0.006	0.10	—	—	0.28	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
27	0.12	0.10	1.79	0.014	0.010	0.010	0.21	0.005	0.10	—	—	—	0.27	—	—	—	0.61	0.001	—	—	—	—	—	—	—
28	0.15	0.08	1.80	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
29	0.15	0.08	1.30	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
30	0.15	0.08	1.80	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.61	0.001	—	—	—	—	—	—	—
31	0.14	0.08	1.52	0.014	0.011	0.011	0.22	0.004	0.11	—	—	—	—	0.0035	—	—	0.01	0.002	—	—	—	—	—	—	—
32	0.14	0.06	1.78	0.011	0.012	0.011	0.23	0.005	0.12	—	—	—	—	—	0.060	—	0.01	0.001	—	—	—	—	—	—	—
33	0.13	0.07	1.61	0.012	0.011	0.014	0.26	0.005	0.11	—	—	—	—	—	—	0.064	0.61	0.001	—	—	—	—	—	—	—
34	0.15	0.07	1.54	0.013	0.012	0.011	0.22	0.004	0.13	0.055	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
35	0.12	0.06	1.66	0.014	0.013	0.012	0.23	0.004	0.14	—	0.0058	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
36	0.12	0.06	1.21	0.016	0.010	0.016	0.30	0.006	0.17	—	—	—	—	—	—	—	0.61	0.001	—	—	—	—	—	—	—
37	0.14	0.07	2.01	0.014	0.009	0.010	0.20	0.005	0.45	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
38	0.12	0.25	1.31	0.015	0.011	0.010	0.23	0.005	0.38	—	—	—	—	—	—	—	0.03	0.001	—	—	—	—	—	—	—
39	0.08	0.33	2.19	0.013	0.010	0.015	0.16	0.006	0.45	—	—	—	—	—	—	—	0.61	0.001	—	—	—	—	—	—	—
40	0.15	0.08	1.80	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
41	0.15	0.08	1.36	0.012	0.011	0.018	0.27	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
42	0.14	0.07	1.90	0.013	0.012	0.011	0.26	0.006	—	—	—	—	—	—	—	—	—	—	0.003	—	—	—	—	—	—
43	0.12	0.07	1.90	0.013	0.016	0.012	0.27	0.006	—	—	—	—	—	—	—	—	—	—	—	0.07	—	—	—	—	—
44	0.13	0.08	1.38	0.012	0.015	0.013	0.27	0.006	—	—	—	—	—	—	—	—	—	—	—	—	0.06	—	—	—	—
45	0.15	0.06	1.90	0.012	0.012	0.011	0.25	0.005	—	—	—	—	—	—	—	—	—	—	—	—	—	0.003	—	—	—
46	0.16	0.06	1.88	0.013	0.010	0.013	0.26	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	0.04	—	—
47	0.15	0.08	1.91	0.015	0.012	0.011	0.26	0.005	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	0.0020	—
48	0.12	0.07	1.88	0.016	0.012	0.012	0.25	0.006	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	0.006

TABLE 1-2
Test	Chemical Composition (unit is mass % balance in Fe and impurities)
Number	C	Si	Mn	P	S	Al	V	N	Cr	Nb	B	Cu	Ni	Co	Bi	Pb	Mo	Ti	Zr	Se	Te	REM	Sb	Mg	W
49	0.26	9.08	1.83	0.016	0.012	0.013	0.21	0.005	0.12	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
50	0.01	0.07	1.78	0.015	0.011	0.011	0.32	0.005	0.14	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
51	0.15	0.01	1.34	0.014	0.010	9.013	0.24	0.004	0.15	—	—	—	—	—	—	—	0.01	0.002	—	—	—	—	—	—	—
52	0.14	0.59	1.91	0.015	0.009	0.012	0.17	0.005	0.17	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
53	0.16	0.08	2.65	0.013	0.012	0.011	0.22	0.007	0.15	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
54	0.18	0.07	2.32	0.014	0.010	9.009	0.19	0.004	0.14	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
55	0.10	0.06	4.69	0.015	0.010	0.013	0.51	0.004	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
56	0.13	9.08	1.82	0.014	0.010	0.012	0.06	0.005	0.15	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
57	0.18	0.07	1.21	0.015	0.009	0.018	0.21	0.006	0.82	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
58	0.15	0.08	1.82	0.016	0.011	0.016	0.16	0.007	1.02	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
59	0.18	0.06	1.82	0.016	0.012	0.015	0.21	0.032	0.18	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
60	0.15	0.07	1.83	0.014	0.010	0.010	0.22	0.004	0.12	—	—	—	—	—	—	—	0.01	0.607	—	—	—	—	—	—	—
61	0.15	0.09	0.81	0.013	0.011	0.099	0.22	0.005	0.11	—	—	—	—	—	—	—	0.08	0.061	—	—	—	—	—	—	—
82	0.15	0.09	1.42	0.016	0.012	0.025	0.12	0.005	0.09	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
63	0.15	0.08	1.80	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	6.601	—	—	—	—	—	—	—
64	0.15	0.09	1.42	0.016	0.012	0.025	0.12	0.005	0.09	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
85	0.15	0.08	1.80	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
66	0.15	0.09	1.42	0.016	0.012	0.025	0.12	0.005	0.09	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
67	0.15	0.08	1.80	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	0.601	—	—	—	—	—	—	—
68	0.15	0.09	1.42	0.016	0.012	0.025	0.12	0.005	0.09	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
69	0.15	0.09	1.42	0.016	0.012	0.025	0.12	0.005	0.02	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
70	0.15	0.08	1.80	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	0.601	—	—	—	—	—	—	—
71	0.15	0.09	1.42	0.016	0.012	0.025	0.12	0.005	0.09	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
72	0.15	0.09	1.42	0.016	0.012	0.025	0.12	0.005	0.02	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
73	0.15	0.08	1.80	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	6.601	—	—	—	—	—	—	—
74	0.15	0.08	1.30	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
75	0.15	0.08	1.80	0.015	0.011	0.015	0.21	0.006	0.16	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
76	0.09	0.07	0.85	0.015	0.011	0.011	0.21	0.005	0.14	—	—	—	—	—	—	—	0.01	0.001	—	—	—	—	—	—	—
77	0.16	0.08	2.09	0.014	0.012	0.012	0.16	0.005	0.46	—	—	—	—	—	—	—	0.02	0.001	—	—	—	—	—	—	—
78	0.21	0.09	2.21	0.012	0.010	0.013	0.21	6.006	0.58	—	—	0.24	0.25	—	—	—	0.01	0.061	—	—	—	—	—	—	—
79	0.15	0.08	1.86	0.012	0.011	0.018	0.27	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
80	0.15	0.08	1.86	0.012	0.011	0.018	0.27	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
81	0.15	0.08	1.86	0.012	0.011	0.018	0.27	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
82	0.15	0.08	1.86	0.012	0.011	0.018	0.27	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
83	0.15	0.08	1.86	0.012	0.011	0.018	0.27	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
84	0.15	0.08	1.86	0.012	0.011	0.018	0.27	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
85	0.15	0.08	1.86	0.012	0.011	0.018	0.27	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
86	0.15	0.08	1.86	0.012	0.011	0.018	0.27	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
87	0.15	0.08	1.86	0.012	0.011	0.018	0.27	0.007	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—

Each produced ingot was used to produce a steel material to serve as a starting material for a steel near-net-shape material. Specifically, each ingot was subjected to hot working (hot forging) to produce a round bar material having a diameter of 42 mm (φ42). The heating temperature in the hot forging was 1200° C., and the finishing temperature was 1000° C. For the test numbers other than Test Nos. 76 to 79, the cooling rate after hot forging was 0.5° C./sec. In Test No. 76, the cooling rate after hot forging was 0.1° C./sec. In Test Nos. 77 and 78, the cooling rate after hot forging was 6.0° C./sec. In Test No. 79, the cooling rate after hot forging was 0.2° C./sec. Round bar materials (steel materials) to respectively serve as a starting material for a steel near-net-shape material were produced by the above production process.

The produced round bar materials were subjected to a cold working process. Specifically, the round bar material of each test number was subjected to cold drawing as a first-direction cold working process, and thereafter was subjected to upsetting as a second-direction cold working process. The cold drawing strain amount and the upsetting strain amount in the cold drawing and upsetting, and the total working strain amount were as shown in Table 2-1 and Table 2-2. Note that, in a case where a crack was confirmed in the round bar material during the cold working process, production was immediately stopped, and it was determined that the cold workability for the relevant test number was low.

TABLE 2-1
	Round Bar Material		Age Hardening Treatment
	(Steel Material)	Cold Working Process	Procss	Steel Near-net-shape Material
		Hard					Age			Hard
	Polygonal	phase		Cold		Total	hardening		Polygonal	phase		Diffusible
	territe area	arear		drawing	Upsetting	working	treatment	Holding	territe	area		hydrogen	Fatigue	Tensile
Test	fraction	function		strain	strain	strain	temperature	time	area	fraction		content	strength	strength
Number	(%)	(%)	v/PD	amount (−)	amount (−)	amount (−)	(° C.)	(min)	fraction (%)	(%)	v/F	(ppm)	(MPa)	(MPa)	Remarks
1	72	28	0.05	0.20	0.10	0.30	600	60	72	23	0.37	0.12	480	852	Inventive Example
2	61	32	0.11	0.20	0.10	0.20	800	80	61	39	0.43	0.16	500	858	Inventive Example
3	39	61	0.18	0.20	0.10	0.30	600	60	39	61	0.52	0.22	530	915	Inventive Example
4	42	58	0.19	0.20	0.10	0.30	600	60	42	58	0.48	0.20	530	928	Inventive Example
5	68	32	0.05	0.20	0.10	0.20	800	80	68	32	0.38	0.14	490	846	Inventive Example
6	61	39	0.05	0.20	0.10	0.30	600	60	61	39	0.36	0.12	500	847	Inventive Example
7	64	36	0.11	0.20	0.10	0.30	600	60	64	36	0.28	0.16	520	896	Inventive Example
8	46	54	0.16	0.20	0.10	0.30	600	60	46	54	0.36	0.15	500	850	Inventive Example
9	51	42	0.15	0.20	0.10	0.30	600	60	51	49	0.42	0.15	510	882	Inventive Example
10	23	77	0.05	0.20	0.10	0.30	600	60	23	77	0.28	0.17	500	848	Inventive Example
11	84	16	0.14	0.20	0.10	0.30	600	60	84	16	0.42	0.21	500	861	Inventive Example
12	63	37	0.11	0.20	0.10	0.20	800	80	63	37	0.36	0.15	510	874	Inventive Example
13	74	26	0.11	0.20	0.10	0.30	600	60	74	26	0.38	0.14	500	858	Inventive Example
14	70	30	0.11	0.20	0.10	0.30	600	60	70	30	0.52	0.26	540	944	Inventive Example
15	66	34	0.13	0.20	0.10	0.20	800	80	66	34	0.46	0.24	550	963	Inventive Example
16	54	46	0.07	0.20	0.10	0.30	600	60	54	46	0.39	0.16	500	848	Inventive Example
17	48	52	0.05	0.20	0.10	0.30	600	60	48	52	0.23	0.12	490	845	Inventive Example
18	66	34	0.10	0.20	0.10	0.30	600	60	66	34	0.42	0.18	500	861	Inventive Example
19	62	38	0.10	0.20	0.10	0.30	600	60	62	38	0.45	0.21	420	848	Inventive Example
20	34	66	0.08	0.20	0.10	0.30	600	60	34	86	0.43	0.20	510	890	Inventive Example
21	72	28	0.13	0.20	0.10	0.30	600	60	72	23	0.41	0.18	500	846	Inventive Example
22	65	35	0.12	0.20	0.10	0.20	600	80	65	35	0.52	0.26	490	847	Inventive Example
23	59	41	0.11	0.20	0.10	0.30	600	60	59	41	0.46	0.24	500	854	Inventive Example
24	62	38	0.13	0.20	0.10	0.30	600	60	62	38	0.39	0.16	510	886	Inventive Example
25	61	39	0.10	0.20	0.10	0.20	600	80	61	39	0.39	0.19	510	880	Inventive Example
26	41	59	0.10	0.20	0.10	0.30	600	60	41	59	0.42	0.19	520	886	Inventive Example
27	34	66	0.08	0.20	0.10	0.30	600	60	34	86	0.44	0.18	510	861	Inventive Example
28	60	40	0.11	0.15	0.10	0.25	600	60	66	40	0.34	0.13	480	849	Inventive Example
29	60	40	0.11	0.20	0.10	0.30	520	60	60	40	0.32	0.18	480	850	Inventive Example
30	57	43	0.11	0.20	0.10	0.30	660	60	57	43	0.46	0.14	490	846	Inventive Example
31	59	41	0.10	0.20	0.10	0.30	600	60	59	41	0.37	0.18	490	851	Inventive Example
32	64	36	0.11	0.20	0.10	0.20	600	80	64	36	0.34	0.15	490	850	Inventive Example
33	57	43	0.11	0.20	0.10	0.30	600	60	57	43	0.39	0.20	490	849	Inventive Example
34	61	39	0.11	0.20	0.10	0.30	600	60	61	39	0.33	0.13	500	848	Inventive Example
35	66	34	0.09	0.20	0.10	0.20	600	80	66	34	0.33	0.15	500	853	Inventive Example
36	87	13	0.11	0.20	0.10	0.30	600	60	87	13	0.44	0.20	500	848	Inventive Example
37	22	78	0.09	0.20	0.10	0.30	600	60	22	78	0.26	0.18	520	886	Inventive Example
38	32	68	0.10	0.20	0.10	0.30	600	60	32	68	0.40	0.19	500	884	Inventive Example
39	35	65	0.05	0.20	0.10	0.30	600	60	35	65	0.32	0.11	500	850	Inventive Example
40	66	34	0.11	0.20	0.10	0.30	600	15	68	34	0.22	0.14	480	848	Inventive Example
41	58	42	0.11	0.20	0.10	0.30	600	60	58	42	0.43	0.21	510	883	Inventive Example
42	61	39	0.12	0.20	0.10	0.20	800	80	61	39	0.39	0.20	500	872	Inventive Example
43	62	38	0.13	0.20	0.10	0.30	600	60	62	38	0.40	0.16	500	864	Inventive Example
44	60	40	0.13	0.20	0.10	0.30	600	60	60	40	0.44	0.17	500	870	Inventive Example
45	63	37	0.11	0.20	0.10	0.20	800	80	63	37	0.38	0.20	520	884	Inventive Example
46	62	38	0.12	0.20	0.10	0.30	600	60	62	38	0.39	0.18	530	890	Inventive Example
47	62	38	0.12	0.20	0.10	0.30	600	60	62	38	0.40	0.19	520	880	Inventive Example
48	58	42	0.11	0.20	0.10	0.30	600	60	58	42	0.36	0.16	490	863	Inventive Example

TABLE 2-2
	Round Bar Material				Age Hardening
	(Steel Material)	Cold Working Process	Treatment Process	Steel Near-net-shape Material
		Hard					Age			Hard
	Polygonal	phase		Cold		Total	hardening		Polygonal	phase		Diffusible
	territe area	area		drawing	Upsetting	working	treatment	Holding	territe	area		hydrogen	Fatigue	Tensile
Test	fraction	function		strain	strain	strain	temperature	time	area	fraction		content	strength	strength
Number	(%)	(%)	v/PD	amount (−)	amount (−)	amount (−)	(° C.)	(min)	fraction (%)	(%)	V/F	(ppm)	(MPa)	(MPa)	Remarks
49	34	68	0.22	—	—	—	—	—	—	—	—	—	—	—	Comparative Example
50	99	1	0.05	0.20	0.10	0.30	600	60	99	1	0.09	0.06	300	522	Comparative Example
51	55	45	0.12	0.20	0.10	0.30	600	60	55	45	0.40	0.17	410	704	Comparative Example
52	70	30	0.07	—	—	—	—	—	—	—	—	—	—	—	Comparative Example
53	38	62	0.12	—	—	—	—	—	—	—	—	—	—	—	Comparative Example
54	1	99	0.12	—	—	—	—	—	—	—	—	—	—	—	Comparative Example
55	74	26	0.11	—	—	—	—	—	—	—	—	—	—	—	Comparative Example
56	42	58	0.05	0.20	0.10	0.30	600	60	42	58	0.10	0.01	380	650	Comparative Example
57	42	58	0.14	—	—	—	—	—	—	—	—	—	—	—	Comparative Example
58	3	97	0.08	—	—	—	—	—	—	—	—	—	—	—	Comparative Example
59	63	37	0.55	0.20	0.10	0.30	600	60	63	37	0.72	0.08	430	803	Comparative Example
60	58	42	0.11	—	—	—	—	—	—	—	—	—	—	—	Comparative Example
61	33	67	0.12	—	—	—	—	—	—	—	—	—	—	—	Comparative Example
62	70	30	0.05	0.05	0.05	0.10	600	60	70	30	0.15	0.02	380	754	Comparative Example
63	58	42	0.11	0.05	0.05	0.10	600	60	58	42	0.20	0.02	420	784	Comparative Example
64	71	29	0.05	0.30	—	0.30	600	60	71	29	0.31	0.06	400	768	Comparative Example
65	69	31	0.11	0.30	—	0.30	600	60	69	31	0.35	0.08	430	794	Comparative Example
66	69	31	0.05	—	0.30	0.30	600	60	69	31	0.28	0.05	410	778	Comparative Example
67	70	30	0.11	—	0.30	0.30	600	60	70	30	0.33	0.07	410	765	Comparative Example
68	72	28	0.05	0.20	0.10	0.30	400	60	72	28	0.14	0.02	410	842	Comparative Example
69	70	30	0.05	0.20	0.10	0.30	750	60	82	18	0.49	0.02	220	557	Comparative Example
70	58	42	0.11	0.20	0.10	0.30	750	60	77	23	0.56	0.02	250	605	Comparative Example
71	70	30	0.05	0.20	0.10	0.30	600	2	70	30	0.19	0.05	420	832	Comparative Example
72	68	32	0.05	0.20	0.10	0.30	640	300	68	32	0.48	0.05	330	701	Comparative Example
73	64	36	0.11	0.20	0.10	0.30	600	1440	64	36	0.50	0.08	380	743	Comparative Example
74	64	36	0.11	0.20	0.10	0.30	640	300	64	36	0.52	0.07	360	725	Comparative Example
75	64	36	0.11	0.20	0.10	0.30	660	150	64	36	0.49	0.08	370	733	Comparative Example
76	93	7	0.30	0.20	0.10	0.30	600	60	93	7	0.33	0.05	330	621	Comparative Example
77	15	85	0.09	—	—	—	—	—	—	—	—	—	—	—	Comparative Example
78	0	100	0.16	—	—	—			—	—	—	—	—	—	Comparative Example
79	86	14	0.34	0.20	0.10	0.30	600	60	86	14	0.42	0.09	430	795	Comparative Example
80	57	43	0.11	0.05	0.05	0.10	600	60	57	43	0.23	0.07	440	815	Comparative Example
81	55	45	0.12	0.30	—	0.30	600	60	55	45	0.43	0.08	390	821	Comparative Example
82	60	40	0.11	—	0.30	0.30	600	60	60	40	0.42	0.07	400	830	Comparative Example
83	58	42	0.11	0.20	0.10	0.30	400	60	58	42	0.16	0.04	360	831	Comparative Example
84	61	39	0.12	0.20	0.10	0.30	750	60	61	39	0.52	0.08	360	652	Comparative Example
85	58	42	0.11	0.20	0.10	0.30	600	2	58	42	0.18	0.05	380	827	Comparative Example
86	63	37	0.11	0.20	0.10	0.30	600	300	63	37	0.56	0.09	450	808	Comparative Example
87	60	40	0.11	0.20	0.10	0.30	—	—	60	40	0.11	0.04	340	823	Comparative Example

The round bar material after the cold working process was subjected to an age hardening treatment process to produce a steel near-net-shape material. In the age hardening treatment, the age hardening treatment temperature (° C.) and the holding time (mins) were as shown in Table 2-1 and Table 2-2.

A steel near-net-shape material of each test number was produced by the above production process.

[Evaluation Tests]

The following evaluation tests were carried out on the round bar material (steel material) to serve as a starting material for the steel near-net-shape material of each test number, and on the steel near-net-shape material of each test number.

[Microstructure Observation Test on Round Bar Material (Steel Material) to Serve as Starting Material for Steel Near-Net-Shape Material]

The microstructure of the round bar material of each test number was observed by the following method. A test specimen was taken from a central part including the central axis of the round bar material of each test number. Among the surfaces of the test specimen, a surface perpendicular to the longitudinal direction of the round bar material was adopted as an observation surface. The observation surface was mirror-polished. The observation surface after polishing was etched using a 3% nital etching reagent (ethanol+3% nitric acid solution). An arbitrary five observation visual fields on the etched observation surface were observed with an optical microscope at a magnification of ×400, and photographic images were created. The size of each observation visual field was set to 200 μm×200 μm. In the photographic images of the respective visual fields, polygonal ferrite and a hard phase (pearlite and/or bainite) were identified by the method described above. The polygonal ferrite area fraction (%) was determined based on the total area of polygonal ferrite determined in the five visual fields, and the total area of the five visual fields. Similarly, the total area fraction (%) of the hard phase (pearlite and bainite) was determined based on the total area of pearlite and bainite determined in the five visual fields, and the total area of the live visual fields. The obtained polygonal ferrite area fractions (%) are shown in the column “Polygonal ferrite area fraction (%)” of the column “Round Bar Material (Steel Material)” in Table 2-1 and Table 2-2. Further, the obtained hard phase area fractions (%) are shown in the column “Hard phase area fraction (%)” of the column “Round Bar Material (Steel Material)” in Table 2-1 and Table 2-2.

[Test to Measure VP0 of Round Bar Material (Steel Material) to Serve as Starting Material for Steel Near-Net-Shape Material]

The VP0 (=[V in precipitates]/[V]) of the round bar material of each test number was determined by the following extraction residue analysis method.

Specifically, a sample of approximately 1000 mm³ (approximately 7.8 g) was cutout from the round bar material. The cut-out sample was used to determine [V in precipitates] (mass %) in the round bar material by the same method (extraction residue analysis method) as the method for measuring VP that is described above. VP0 was determined based on the content of V([V]) in the chemical composition of the round bar material, and [V in precipitates]. The determined values of VP0 are shown in the column “VP0” of the column “Round Bar Material (Steel Material)” in Table 2-1 and Table 2-2.

[Microstructure Observation Test on Steel Near-Net-Shape Material]

The microstructure of the steel near-net-shape material of each test number was observed by the following method. A test specimen was collected from a central part including the central axis of the steel near-net-shape material of each test number. Among the surfaces of the test specimen, a surface perpendicular to the longitudinal direction of the steel near-net-shape material was adopted as an observation surface. The observation surface was mirror-polished. The observation surface after polishing was etched using a 3% nital etching reagent (ethanol+3% nitric acid solution). Using the etched observation surface, the polygonal ferrite area fraction (%) and the area fraction (%) of the hard phase of the steel near-net-shape material were determined by the same method as the method employed in the microstructure observation of the round bar material (steel material). Note that, the positions of the five observation visual fields were each a position at a depth of at least 3 mm from the surface of the steel near-net-shape material. The obtained polygonal ferrite area fractions (%) are shown in the column “Polygonal ferrite area fraction (%)” of the column “Steel Near-net-shape Material” in Table 2-1 and Table 2-2. Further, the obtained hard phase area fractions (%) are shown in the column “Hard phase area fraction (%)” of the column “Steel Near-net-shape Material” in Table 2-1 and Table 2-2.

[Test to Measure VP of Steel Near-Net-Shape Material]

The VP (=[V in precipitates]/[V]) of the steel near-net-shape material of each test number was determined by the following extraction residue analysis method.

Specifically, a sample of approximately 1000 mm³ (approximately 7.8 g) was cut out from the steel near-net-shape material. The cut-out sample was used to determine [V in precipitates] (mass %) in the steel near-net-shape material by the method for measuring VP (extraction residue analysis method) that is described above. VP was determined based on the content of V ([V]) in the chemical composition of the steel near-net-shape material, and [V in precipitates]. The determined values of VP are shown in the column “VP” of the column “Steel Near-net-shape Material” in Table 2-1 and Table 2-2.

[Test to Measure Diffusible Hydrogen Content]

The diffusible hydrogen content of the steel near-net-shape material of each test number was determined by the following method. A round bar specimen having a diameter of 7 mm and a length of 40 mm was cut out from a portion including the central axis of the steel near-net-shape material. Hydrogen was introduced into the cut-out round bar specimen, using a cathodic hydrogen charging method. Specifically, the round bar specimen was immersed in a 3% NaCl-3 g/L NH₄SCN aqueous solution. Thereafter, hydrogen was introduced into the round bar specimen by a cathodic hydrogen charging method under conditions of a current density of 0.1 mA/cm² and a conduction time of 72 hours. The timing at which the aforementioned conduction of a current was stopped was taken as the timing at which introduction of hydrogen into the round bar specimen was completed. After completing the introduction of hydrogen into the round bar specimen, the hydrogen content in the round bar specimen was measured without delay (that is, while the gap time was within 30 minutes) by the following method using thermal desorption-gas chromatography. Specifically, the round bar specimen was heated from room temperature to 400° C. at a heating rate of 100° C./hr. The hydrogen content generated by the rise in temperature was measured at intervals of five minutes. Based on the obtained hydrogen contents, a hydrogen evolution curve as illustrated in FIG. 1 was obtained. The obtained hydrogen evolution curve was used to determine the cumulative hydrogen content released from room temperature to 350° C. The obtained cumulative hydrogen content was defined as the “diffusible hydrogen content (ppm)”. The obtained diffusible hydrogen contents are shown in the column “Diffusible hydrogen content (ppm)” of the column “Steel Near-net-shape Material” in Table 2-1 and Table 2-2.

[Fatigue Test]

The fatigue strength (bending fatigue strength) of the steel near-net-shape material of each test number was measured by the following method. A plurality of Ono type rotating bending fatigue test specimens conforming to JIS Z 2274 (1978) were taken from the steel near-net-shape material. The central axis of each Ono type rotating bending fatigue test specimen was coaxial with the central axis of the steel near-net-shape material. Using the Ono type rotating bending fatigue test specimens, an Ono type rotating bending fatigue test conforming to JIS Z 2274 (1978) was conducted at room temperature in the atmosphere. In the fatigue test, the rotational speed was set to 3000 rpm, and the maximum stress at which no rupture occurred after the number of repetitions of stress loading was 10⁷ cycles was defined as the fatigue strength (MPa). The obtained fatigue strengths are shown in the column “Fatigue strength (MPa)” of the column “Steel Near-net-shape Material” in Table 2-1 and Table 2-2. In the present examples, if the fatigue strength was 480 MPa or more, it was determined that the fatigue strength was high. On the other hand, if the fatigue strength was less than 480 MPa, it was determined that the fatigue strength was low.

[Tensile Test]

The tensile strength of the steel near-net-shape material of each test number was measured by the following method. A No. 14A test coupon specified in JIS Z 2241 (2011) was taken from a position including the central axis of the steel near-net-shape material. The longitudinal direction of the test specimen approximately matched the longitudinal direction of the steel near-net-shape material. The diameter of the parallel portion of the test specimen was 6 mm, and the gage length was 10 mm. Using the test specimen, a tensile test was conducted at room temperature (25° C.) in the atmosphere, and the tensile strength (MPa) was thereby determined. The obtained tensile strengths are shown in the column “Tensile strength (MPa)” of the column “Steel Near-net-shape Material” in Table 2-1 and Table 2-2. In the present examples, if the tensile strength was 845 MPa or more, it was determined that the tensile strength was high. On the other hand, if the tensile strength was less than 845 MPa, it was determined that the tensile strength was low.

[Test Results]

The test results are shown in Table 2-1 and Table 2-2. Referring to Table 1-1, Table 1-2, Table 2-1 and Table 2-2, in the steel near-net-shape materials of Test Nos. 1 to 48, the contents of the respective elements in the chemical composition were within the respective ranges of the present embodiment. In addition, the microstructure was composed of polygonal ferrite having an area fraction of 20 to 90% and a hard phase having an area fraction of 10 to 80%, and VP satisfied Formula (1). Further, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was 0.10 ppm or more. Therefore, the fatigue strength of the steel near-net-shape materials of Test Nos. 1 to 48 was 480 MPa or more, which indicated high fatigue strength. In addition, the tensile strength of the steel near-net-shape materials of Test Nos. 1 to 48 was 845 MPa or more, which indicated high tensile strength.

On the other hand, in Test No. 49 the content of C was too high. Therefore, a crack was confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.

In Test No. 50, the content of C was too low. Therefore, the polygonal ferrite area fraction of the steel near-net-shape material was too high. In addition, VP of the steel near-net-shape material did not satisfy formula (1), and the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. As a result, the fatigue strength and tensile strength were low.

In Test No. 51, the content of Si was too low. Therefore, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test No. 52, the content of Si was too high. Therefore, the cold forgeability of the round bar material was low, and a steel near-net-shape material could not be produced.

In Test Nos. 53 and 54, the content of Mn was too high. Therefore, a crack was confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.

In Test No. 55, the content of V was too high. Therefore, a crack was confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.

In Test No. 56, the content of V was too low. Therefore, VP of the steel near-net-shape material did not satisfy Formula (1), and the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test Nos. 57 and 58, the content of Cr was too high. Therefore, a crack was confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.

In Test No. 59, the content of N was too high. Therefore, in the round bar material serving as a starting material for the steel near-net-shape material, VP0 was 0.30 or more. In other words, coarse V precipitates which serve as an origin of a fatigue fracture within the round bar material were excessively formed. Therefore, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. Consequently, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test No. 60, the content of Ti was too high. Therefore, a crack was confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.

In Test No. 61, the content of Mo was too high. Therefore, a crack was confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.

In Test Nos. 62, 63 and 80, the total working strain amount in the cold working process was too low. Therefore. VP of the steel near-net-shape material did not satisfy Formula (1). In addition, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test Nos. 64, 65 and 81, although the total working strain amount in the cold working process was 0.20 or more, the upsetting strain amount was less than 0.05. Therefore, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. As a result, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test Nos. 66, 67 and 82, although the total working strain amount in the cold working process was 0.20 or more, the cold drawing strain amount was less than 0.05. Therefore, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. As a result, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test Nos. 68 and 83, the age hardening treatment temperature was too low. Therefore, VP of the steel near-net-shape material did not satisfy Formula (1). In addition, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test Nos. 69, 70 and 84, the age hardening treatment temperature was too high. Therefore, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. As a result, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test Nos. 71 and 85, the holding time at the age hardening treatment temperature was too short. Consequently. VP of the steel near-net-shape material did not satisfy Formula (1). In addition, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test Nos. 72 to 75 and 86, the holding time at the age hardening treatment temperature was too long. Therefore, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. Consequently, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test No. 76, although the contents of the respective elements in the chemical composition were within the respective ranges of the present embodiment, the polygonal ferrite area fraction of the round bar material that served as the starting material for the steel near-net-shape material was too high. In addition, VP0 was 0.30 or more. Consequently, the polygonal ferrite area fraction of the steel near-net-shape material was too high. In addition, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test Nos. 77 and 78, although the contents of the respective elements in the chemical composition were within the respective ranges of the present embodiment, the polygonal ferrite area fraction of the round bar material that served as the starting material for the steel near-net-shape material was too low. Therefore, a crack was confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.

In Test No. 79, although the contents of the respective elements in the chemical composition were within the respective ranges of the present embodiment. VP0 of the round bar material that served as the starting material for the steel near-net-shape material was 0.30 or more. Therefore, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel near-net-shape material were low.

In Test No. 87, although the contents of the respective elements in the chemical composition were within the respective ranges of the present embodiment, an age hardening treatment was not performed. Consequently. VP of the steel near-net-shape material did not satisfy Formula (1). In addition, the diffusible hydrogen content of the steel near-net-shape material when charged with hydrogen by the cathodic hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel near-net-shape material were low.

An embodiment of the present disclosure has been described above. However, the foregoing embodiment is merely an example for implementing the present disclosure. Accordingly, the present disclosure is not limited to the above embodiment, and the above embodiment can be appropriately modified within a range that does not deviate from the gist of the present invention.

Claims

1. A steel near-net-shape material, comprising:

a chemical composition consisting of, in mass %,

C: 0.03 to 0.25%,

Si: 0.02 to 0.50%,

Mn: more than 0.70 to 2.50%,

P: 0.035% or less,

S: 0.050% or less,

Al: 0.005 to 0.050%,

V: more than 0.10 to 0.40%,

N: 0.003 to 0.030%,

Cr: 0 to 0.70%,

Nb: 0 to 0.100%,

B: 0 to 0.0100%,

Cu: 0 to 0.30%,

Ni: 0 to 0.30%,

Ca: 0 to 0.0050%,

Bi: 0 to 0.100%,

Pb: 0 to 0.090%,

Mo: 0 to 0.05%,

Ti: 0 to 0.005%,

Zr 0 to 0.010%,

Se: 0 to 0.10%,

Te: 0 to 0.10%,

rare earth metal: 0 to 0.010%,

Sb: 0 to 0.10%,

Mg: 0 to 0.0050%,

W: 0 to 0.050%, and

the balance: Fe and impurities,

wherein:

a microstructure of the steel near-net-shape material is composed of:

polygonal ferrite having an area fraction of 20 to 90%, and

a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%;

when a content of V in the chemical composition is defined as [V](mass %), and a total content of V in V precipitates in the steel near-net-shape material is defined as [V in precipitates] (mass %), the steel near-net-shape material satisfies Formula (1); and

a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more: [V in precipitates]/[V]≥0.30  (1).

2. The steel near-net-shape material according to claim 1, wherein:

the chemical composition contains, in lieu of a part of Fe, one or more elements selected from the group consisting of:

Cr: 0.01 to 0.70%,

Nb: 0.001 to 0.100%,

B: 0.0001 to 0.0100%,

Cu: 0.01 to 0.30%,

Ni: 0.01 to 0.30%,

Ca: 0.0001 to 0.0050%,

Bi: 0.001 to 0.100%,

Pb: 0.001 to 0.090%,

Mo: 0.01 to 0.05%,

Ti: 0.001 to 0.005%,

Zr 0.002 to 0.010%,

Se: 0.01 to 0.10%,

Te: 0.01 to 0.10%,

rare earth metal: 0.001 to 0.010%,

Sb: 0.01 to 0.10%,

Mg: 0.0005 to 0.0050%, and

W: 0.001 to 0.050%.

3. A method for producing the steel near-net-shape material according to claim 1, comprising:

a steel material preparation process of preparing a steel material having a chemical composition consisting of, in mass %,

C: 0.03 to 0.25%,

Si: 0.02 to 0.50%,

Mn: more than 0.70 to 2.50%,

P: 0.035% or less,

S: 0.050% or less,

Al: 0.005 to 0.050%,

V: more than 0.10 to 0.40%,

N: 0.003 to 0.030%,

Cr: 0 to 0.70%,

Nb: 0 to 0.100%,

B: 0 to 0.0100%,

Cu: 0 to 0.30%,

Ni: 0 to 0.30%,

Ca: 0 to 0.0050%,

Bi: 0 to 0.100%,

Pb: 0 to 0.090%,

Mo: 0 to 0.05%,

Ti: 0 to 0.005%,

Zr 0 to 0.010%,

Se: 0 to 0.10%,

Te: 0 to 0.10%,

rare earth metal: 0 to 0.010%,

Sb: 0 to 0.10%,

Mg: 0 to 0.0050%,

W: 0 to 0.050%, and

the balance: Fe and impurities,

wherein:

a microstructure of the steel material is composed of:

polygonal ferrite having an area fraction of 20 to 90%, and

a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%, and

when a content of V in the chemical composition is defined as [V](mass %), and a total content of V in V precipitates in the steel material is defined as [V in precipitates] (mass %), [V in precipitates]/[V] is 0.05 to less than 0.30;

a cold working process of subjecting the steel material to cold working;

an age hardening treatment process of subjecting the steel material after cold working to an age hardening treatment in which a treatment temperature is set in a range of 500° C. to an Ac1 point, and a holding time at the treatment temperature is set in a range of 15 to 150 minutes;

wherein:

the cold working process includes:

a first-direction cold working process of subjecting the steel material to, from a first direction, cold working in which a working strain amount is 0.05 or more, and

a second-direction cold working process of subjecting the steel material to, from a second direction which is different from the first direction, cold working in which a working strain amount is 0.05 or more; and

a total of a working strain amount generated in the steel material in the first-direction cold working process and a working strain amount generated in the steel material in the second-direction cold working process is 0.20 or more.