US20230151472A1 - Steel near-net-shape material and method for producing same - Google Patents

Steel near-net-shape material and method for producing same Download PDF

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US20230151472A1
US20230151472A1 US17/995,359 US202117995359A US2023151472A1 US 20230151472 A1 US20230151472 A1 US 20230151472A1 US 202117995359 A US202117995359 A US 202117995359A US 2023151472 A1 US2023151472 A1 US 2023151472A1
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net
steel
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shape material
precipitates
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Takahisa Suzuki
Kei Miyanishi
Makoto Egashira
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Nippon Steel Corp
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • 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.
  • 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.
  • 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.
  • 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.
  • the fatigue strength of the component for machine structural use can be increased.
  • Patent Literature 1 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).
  • 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:
  • 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.
  • cold workability the workability of the steel material during cold working. 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.
  • Patent Literature 2 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.
  • 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.
  • 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.
  • a steel near-net-shape material according to the present disclosure has a chemical composition consisting of, in mass %,
  • a microstructure of the steel near-net-shape material is composed of:
  • polygonal ferrite having an area fraction of 20 to 90%
  • a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%;
  • 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 aforementioned steel near-net-shape material according to the present disclosure includes:
  • a microstructure of the steel material is composed of:
  • polygonal ferrite having an area fraction of 20 to 90%
  • a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%
  • 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;
  • the cold working process includes:
  • 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 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.
  • 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.
  • 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.
  • 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.
  • 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:
  • 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.
  • 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.
  • the thermal refining treatment because tempering is also performed after quenching, the number of steps in the production process also increases.
  • a structure mainly composed of martensite means a structure in which an area fraction of martensite is 90% or more.
  • the microstructure of a steel near-net-shape material having the aforementioned chemical composition is a structure mainly composed of martensite
  • 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.
  • 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”).
  • 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.
  • V carbo-nitrides V(C,N)
  • V carbides VC
  • V nitrides V precipitates
  • Almost all of the V precipitates in the steel near-net-shape material are V carbo-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.
  • a content of V in the chemical composition is defined as [V] (mass %).
  • 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 %).
  • 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.
  • V precipitates there are V precipitates which have a spheric shape and V precipitates which have a plate-like shape.
  • 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 ( ⁇ ).
  • 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.
  • 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 ( ⁇ ).
  • BN Baker-Nutting
  • 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.
  • 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.
  • 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.
  • V precipitates spherical V precipitates and plate-like V precipitates
  • the size of 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.
  • 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.
  • 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.
  • 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.
  • 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 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.
  • a steel near-net-shape material having a chemical composition consisting of, in mass %,
  • a microstructure of the steel near-net-shape material is composed of:
  • polygonal ferrite having an area fraction of 20 to 90%
  • a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%;
  • a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more:
  • the chemical composition contains, in lieu of a part of Fe, one or more elements selected from the group consisting of:
  • a microstructure of the steel material is composed of:
  • polygonal ferrite having an area fraction of 20 to 90%
  • a hard phase composed of pearlite and/or bainite and having an area fraction of 10 to 80%
  • 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;
  • the cold working process includes:
  • 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 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.
  • the chemical composition of the steel near-net-shape material of the present embodiment contains the following elements.
  • 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 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%.
  • 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%.
  • Phosphorus (P) is an impurity that is unavoidably contained.
  • 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%.
  • Sulfur (S) is an impurity that is unavoidably contained.
  • the content of S is more than 0%.
  • S combines with Mn to form MnS, which increases the machinability of the steel material.
  • 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%.
  • 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 Nitrogen
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • Chromium (Cr) is an optional element and need not be contained.
  • the content of Cr may be 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.
  • 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%.
  • 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%.
  • Niobium (Nb) is an optional element and need not be contained.
  • the content of Nb may be 0%.
  • Nb 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.
  • 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%.
  • Boron (B) is an optional element and need not be contained.
  • the content of B may be 0%.
  • B strengthens crystal grain boundaries of the steel near-net-shape material.
  • the fatigue strength and tensile strength of the steel near-net-shape material increase.
  • the aforementioned effect is obtained to a certain extent.
  • the content of B is more than 0.0100%, the aforementioned effect will be saturated.
  • 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%.
  • 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%.
  • Copper (Cu) is an optional element and need not be contained.
  • the content of Cu may be 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.
  • 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%.
  • 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%.
  • Nickel (Ni) is an optional element and need not be contained.
  • the content of Ni may be 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.
  • 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%.
  • 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%.
  • 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.
  • Calcium (Ca) is an optional element and need not be contained.
  • the content of Ca may be 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.
  • 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%.
  • 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%.
  • Bismuth (Bi) is an optional element and need not be contained.
  • the content of Bi may be 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.
  • 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%.
  • Lead (Pb) is an optional element and need not be contained.
  • the content of Pb may be 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.
  • 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%.
  • 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.
  • Molybdenum (Mo) is an impurity, and need not be contained.
  • 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%.
  • Titanium (Ti) is an impurity, and need not be contained.
  • 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%.
  • Zirconium (Zr) is an impurity, and need not be contained.
  • 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%.
  • Selenium (Se) is an impurity, and need not be contained.
  • 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%.
  • Tellurium is an impurity, and need not be contained.
  • 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 is an impurity, and need not be contained.
  • 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%.
  • 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.
  • Sc scandium
  • Y yttrium
  • Li lutetium
  • content of REM refers to the total content of these elements.
  • Antimony (Sb) is an impurity, and need not be contained.
  • 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%.
  • Magnesium (Mg) is an impurity, and need not be contained.
  • 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%.
  • Tungsten (W) is an impurity, and need not be contained.
  • 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%.
  • the microstructure of the steel near-net-shape material of the present embodiment contains polygonal ferrite, and pearlite and/or bainite.
  • pearlite and/or bainite are referred to as a “hard phase”.
  • 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.
  • 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%.
  • 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%.
  • 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.
  • 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 ⁇ m ⁇ 200 ⁇ m.
  • 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.
  • 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.
  • the content of V in the chemical composition of the steel near-net-shape material is defined as [V] (mass %).
  • 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 %).
  • the steel near-net-shape material of the present embodiment satisfies Formula (1).
  • 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.
  • 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.
  • 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.
  • a sample of approximately 1000 mm 3 (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.
  • pre-electrolysis is performed on the sample.
  • 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.).
  • the sample is 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.
  • 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.
  • 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 %)”.
  • 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.
  • 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 NH 4 SCN aqueous solution under conditions of a current density of 0.1 mA/cm 2 and a conduction time of 72 hours is 0.10 ppm or more.
  • 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.
  • V precipitates form an incoherent interface with the parent phase.
  • the spherical V precipitates themselves become a barrier to the movement of dislocations.
  • plate-like V precipitates a coherent interface or a semi-coherent interface is formed.
  • the mechanism described above is an assumed mechanism.
  • 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.
  • 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.
  • 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.
  • the round bar specimen is immersed in a 3% NaCl-3 g/L NH 4 SCN 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/cm 2 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.
  • 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”).
  • 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.
  • 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)”.
  • 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)
  • 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.
  • 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.
  • 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.
  • 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.
  • a steel material to serve as a starting material for a steel near-net-shape material is prepared.
  • 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.
  • 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.
  • 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.
  • 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 %).
  • [V in precipitates]/[V] in the steel material is 0.05 to less than 0.30.
  • 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.
  • 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.
  • 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.
  • 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%.
  • 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.
  • 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%.
  • 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.
  • 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.
  • a molten steel having the aforementioned chemical composition is produced.
  • a starting material is then prepared using the molten steel.
  • 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.
  • the molten steel is used to produce an ingot by an ingot-making process.
  • the prepared starting material is subjected to hot working to produce a steel material.
  • 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.
  • 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.
  • a billet may be produced directly by a continuous casting process.
  • 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).
  • the hot working process is not limited to hot rolling.
  • hot forging may be performed, or a hot extrusion process may be performed.
  • 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.
  • 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.
  • the steel material temperature after the final rolling reduction is defined as the finishing temperature (° C.).
  • 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.
  • the cooling rate after hot working is, for example, 0.4 to 4.0° C./s.
  • 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.
  • the normalizing treatment process is an optional process, and need not be performed.
  • a heat treatment temperature in the normalizing treatment is 1000 to 1300° C.
  • a cooling rate after holding at the heat treatment temperature is 0.4 to 4.0° C./s.
  • 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.
  • 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).
  • the cutting process is an optional process. In other words, the cutting process need not be performed.
  • the cold working process includes a first-direction cold working process and a second-direction cold working process.
  • the steel material 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.
  • the steel material 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.
  • 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.
  • the first direction and the second direction may intersect.
  • the first direction and the second direction may be orthogonal, as in the case of cold drawing and upsetting to be described later.
  • the steel material receives loads that are applied from two different directions (a first direction and a second direction).
  • 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.
  • 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.
  • plate-like V precipitates are likely to precipitate in portions where the strain is formed.
  • the formed strain serves as the nuclei of the plate-like V precipitates.
  • 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.
  • 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.
  • 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.
  • 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.
  • cold drawing is performed as the first-direction cold working process
  • upsetting is performed as the second-direction cold working process
  • wire drawing 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.
  • the steel material 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.
  • 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.
  • 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.
  • 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.
  • 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 is defined as “cold drawing strain amount”.
  • the working strain amount generated in the steel material by upsetting 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 ⁇ ( ⁇ ) by a cylindrical approximation defined by Formula (2).
  • 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.
  • the cold drawing strain amount (true strain amount ⁇ ) is determined using Formula (2). In a case where cold drawing is carried out multiple times, the cold drawing strain amount (true strain amount ⁇ ) 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 ⁇ ) in the cold drawing process.
  • 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.
  • the upsetting strain amount (true strain amount ⁇ ) is determined using Formula (2). In a case where upsetting is carried out multiple times, the upsetting strain amount (true strain amount ⁇ ) 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 ⁇ ) 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 ( ⁇ ).
  • the first-direction working strain amount is 0.05 or more
  • the second-direction working strain amount is 0.05 or more
  • the total working strain amount is 0.20 or more.
  • 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.
  • age hardening treatment temperature the treatment temperature in the age hardening treatment process
  • 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.
  • 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.
  • 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).
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the steel material after the age hardening treatment process may be subjected to a 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.
  • 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.
  • the steel near-net-shape material of the present embodiment is described specifically by way of 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.
  • 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 ( ⁇ 42). The heating temperature in the hot forging was 1200° C., and the finishing temperature was 1000° C.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 ⁇ m ⁇ 200 ⁇ m.
  • polygonal ferrite and a hard phase 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.
  • 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.
  • a sample of approximately 1000 mm 3 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.
  • 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.
  • 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).
  • 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.
  • a sample of approximately 1000 mm 3 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.
  • 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 NH 4 SCN 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/cm 2 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.
  • 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.
  • 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.
  • an Ono type rotating bending fatigue test conforming to JIS Z 2274 (1978) was conducted at room temperature in the atmosphere.
  • 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 10 7 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.
  • 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.
  • 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.
  • MPa Tensile strength
  • the tensile strength was 845 MPa or more, it was determined that the tensile strength was high.
  • the tensile strength was less than 845 MPa, it was determined that the tensile strength was low.
  • the test results are shown in Table 2-1 and Table 2-2.
  • 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.
  • 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).
  • 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.
  • 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.
  • 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.

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