WO2021210577A1 - 鋼素形材、及び、その製造方法 - Google Patents

鋼素形材、及び、その製造方法 Download PDF

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WO2021210577A1
WO2021210577A1 PCT/JP2021/015310 JP2021015310W WO2021210577A1 WO 2021210577 A1 WO2021210577 A1 WO 2021210577A1 JP 2021015310 W JP2021015310 W JP 2021015310W WO 2021210577 A1 WO2021210577 A1 WO 2021210577A1
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steel
content
amount
precipitate
hydrogen
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French (fr)
Japanese (ja)
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鈴木　崇久
慶 宮西
江頭　誠
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Nippon Steel Corp
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Nippon Steel Corp
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Priority to CN202180042287.2A priority Critical patent/CN115917027B/zh
Priority to US17/995,359 priority patent/US20230151472A1/en
Priority to JP2022515395A priority patent/JP7469696B2/ja
Publication of WO2021210577A1 publication Critical patent/WO2021210577A1/ja
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a steel base material, which is a base material made of steel, and a method for manufacturing the same.
  • Structural steel is used as a material for machine structural parts such as automobile parts, industrial machine parts, and construction machine parts.
  • the structural steel material is, for example, a carbon steel material for machine structure, an alloy steel material for machine structure, or the like.
  • Machine structural parts are required to have high fatigue strength. Therefore, the following manufacturing method is known as a method for manufacturing a mechanical structural part having high fatigue strength using a steel material as a material.
  • a process such as hot forging is performed on the steel material to produce a steel material having a desired part shape.
  • a steel material having a desired part shape is subjected to age hardening treatment to produce a steel material.
  • Cutting is performed on the steel profile to manufacture the final product, mechanical structural parts.
  • the fatigue strength of mechanical structural parts can be increased by performing age hardening treatment on the processed steel material.
  • Patent Document 1 A steel material used as a material for mechanical structural parts manufactured by performing an aging hardening treatment is proposed, for example, in Japanese Patent Application Laid-Open No. 2011-236452 (Patent Document 1).
  • the steel material described in Patent Document 1 is C: 0.14 to 0.35%, Si: 0.05 to 0.70%, Mn: 1.10 to 2.30%, S: 0 in mass%. .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%, the balance consisting of Fe and unavoidable impurities, 13 [C] + 8 [Si] + 10 [Mn] + 3 [Cu] + 3 [Ni] + 22 [ Mo] +11 [V] ⁇ 30, 5 [C] + [Si] +2 [Mn] +3 [Cr] +2 [Mo] +4 [V] ⁇ 7.3, 2.4 ⁇ 0.3 [C] + 1.
  • Patent Document 1 has a microstructure of bainite, enhances hot forging property, and enhances hardness after hot forging by adjusting the chemical composition so as to satisfy the above parameter formula. It is described in Patent Document 1 that the steel material disclosed in Patent Document 1 has a bainite structure and is therefore excellent in machinability.
  • an intermediate part is manufactured by performing hot forging on a steel material having the above structure. After that, the intermediate part is cut to obtain a part having a desired shape. After that, aging hardening treatment is performed. It is described in Patent Document 1 that high strength can be obtained in the manufactured parts.
  • the intermediate parts after hot forging are liable to be distorted in the cooling process. Therefore, the shape of the intermediate component is likely to be slightly deformed with respect to the desired shape. That is, it is difficult to bring the intermediate part after hot forging closer to the final shape due to the influence of thermal strain. Therefore, the intermediate part after hot forging is cut to bring it closer to the final shape.
  • the yield will decrease. Therefore, recently, for the purpose of improving the yield, switching from hot forging to cold forging typified by cold forging has been aimed at.
  • the intermediate product can be made into a near net shape (almost the same shape as the final shape). In this case, the cutting amount in the cutting process of the intermediate part can be reduced. Therefore, the yield is improved. Further, the cutting process itself may be omitted in some cases. In this case, productivity is improved.
  • cold working represented by cold forging tends to increase the machining load compared to hot forging. Therefore, it is necessary to improve the workability of the steel material during cold working (hereinafter referred to as cold workability). Specifically, it is required that a desired shape can be processed with a small load and that the occurrence of cracks during cold processing is suppressed. Therefore, when the aging hardening treatment is carried out after the cold working, the target steel material is required to have excellent cold workability and excellent fatigue strength after the aging hardening treatment.
  • Patent Document 2 A steel material used as a material for parts manufactured by performing aging hardening treatment after cold forging is proposed in Japanese Patent Application Laid-Open No. 2019-173168 (Patent Document 2).
  • the steel material disclosed in Patent Document 2 is, in mass%, C: 0.02 to 0.25%, Si: 0.005 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, Cr: 0.02 to 0.70%, V: 0.02 to 0.30%, N: 0 .003 to 0.030%, Nb: 0 to 0.10%, B: 0 to 0.005%, Ca: 0 to 0.005%, Bi: 0 to 0.10%, Pb: 0 to 0. It consists of 20% and the balance: Fe and impurities.
  • the total content of Cu, Ni and Mo in the impurities is 0.05% by mass or less, and the Ti content in the impurities is 0.005% by mass or less.
  • the microstructure of the steel material of Patent Document 2 contains ferrite, pearlite and / or bainite.
  • the area ratio of ferrite in the microstructure is 10 to 90%.
  • Patent Document 2 describes that a steel material having the above structure has high cold forging property and can obtain high fatigue strength when age-hardened after cold forging.
  • Patent Document 2 Parts made of steel disclosed in Patent Document 2 have high fatigue strength. However, parts may be required to have not only high fatigue strength but also high tensile strength. Patent Document 2 does not study both high fatigue strength and high tensile strength.
  • An object of the present invention is to provide a steel profile having high fatigue strength and high tensile strength, and a method for producing the same.
  • the steel profile material according to the present disclosure is The chemical composition is mass%, C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: Over 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, V: Over 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: 0-0.05%, Ti: 0 to 0.005%, Zr: 0 to 0.010%, Se: 0 to 0.10%, Te: 0 to 0.10%, Rare earth elements: 0-0.010%, Sb: 0 to 0.10%, Mg: 0 to 0.0050%, W: 0 to 0.050% and Remaining
  • the method for producing the above-mentioned steel profile according to the present disclosure is as follows.
  • the chemical composition is mass%, C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: Over 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, V: Over 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: 0-0.05%, Ti: 0 to 0.005%, Zr: 0 to 0.010%, Se: 0 to 0.10%, Te: 0 to 0.10%, Rare earth elements: 0-0.010%, Sb: 0 to 0.10%, Mg: 0 to 0.0050
  • the steel material includes a second-direction cold working step in which cold working is performed on the steel material from a second direction different from the first direction so that the working strain amount is 0.05 or more.
  • the total of the amount of machining strain generated in the steel material in the first-direction cold working step and the amount of machining strain generated in the steel material in the second-direction cold working step is 0.20 or more.
  • the steel profile material of the present disclosure has high fatigue strength and high tensile strength.
  • the method for producing a steel profile of the present disclosure can produce the above-mentioned steel profile.
  • FIG. 1 is a diagram showing a hydrogen release curve obtained when hydrogen is charged to a steel profile by the cathode hydrogen charging method.
  • the present inventors conducted various studies in order to obtain high fatigue strength and high tensile strength in the steel profile, and obtained the following findings.
  • the present inventors examined a steel profile material capable of achieving both high fatigue strength and high tensile strength from the viewpoint of chemical composition.
  • the chemical composition of the steel profile is C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: more than 0.70 to 2.50% in mass%.
  • the present inventors considered that if the microstructure of the steel profile is made mainly of martensite, the tensile strength will be increased.
  • the microstructure of the steel profile material having the above-mentioned chemical composition has a martensite-based structure, it is necessary to carry out a tempering treatment (quenching treatment and tempering treatment).
  • quenching treatment it is necessary to heat the steel material to a high temperature of Acc 3 points or more.
  • a tempering process is also performed after the quenching process, so that the number of man-hours during the manufacturing process increases. Therefore, when the microstructure of the steel profile material having the above-mentioned chemical composition is made mainly of martensite, the manufacturing cost becomes high.
  • the martensite-based organization means an organization having a martensite area ratio of 90% or more.
  • the microstructure of the steel profile with the above chemical composition is mainly martensite, the hardness of the steel profile may become excessively high. In this case, even if a high tensile strength is obtained, the fatigue strength of the steel profile may decrease.
  • the present inventors have described that in the steel body profile having the above-mentioned chemical composition, a phase in which the microstructure is composed of polygonal ferrite and pearlite and / or bainite instead of the microstructure mainly composed of martensite (hereinafter, hard phase).
  • hard phase a phase in which the microstructure is composed of polygonal ferrite and pearlite and / or bainite instead of the microstructure mainly composed of martensite.
  • V-precipitate includes V-carbide, V-carbide and V-nitride.
  • the present inventors have investigated how much V precipitates should be present in a steel profile material in which the content of each element in the chemical composition is within the above range to increase the fatigue strength.
  • the V content in the above-mentioned chemical composition is defined as [V] (mass%).
  • the chemical composition of the steel profile is 100% by mass
  • the total content of V in the V precipitate in the steel profile is defined as [V in the precipitate] (% by mass). ..
  • the microstructure of the steel profile is composed of polygonal ferrite and a hard phase. Even with this structure, the fatigue strength of the steel profile is sufficiently increased. However, it has been found that even if the fatigue strength of the steel profile is sufficiently increased, the tensile strength of the steel profile may not be sufficiently obtained. Therefore, the present inventors have further investigated means capable of achieving both high fatigue strength and high tensile strength.
  • the present inventors have found that the content of each element in the chemical composition is within the above range, the microstructure is a structure composed of polygonal ferrite and a hard phase, and the steel element satisfies the formula (1). Furthermore, it has been found that when the amount of diffusible hydrogen when hydrogen is charged by the cathode hydrogen charging method is 0.10 ppm or more in the profile, it is possible to achieve both high fatigue strength and high tensile strength. This point will be described below.
  • the amount of diffusible hydrogen when hydrogen is charged by the cathode hydrogen charging method is considered to have a correlation with the shape of V precipitates in the steel profile.
  • the V precipitate includes a spherical V precipitate and a plate-shaped V precipitate.
  • the spherical V precipitate will be referred to as a spherical V precipitate.
  • the plate-shaped V precipitate is referred to as a plate-shaped V precipitate.
  • the spherical V precipitate forms an unmatched interface with the parent phase ( ⁇ ).
  • the spherical V precipitate acts only as a simple obstacle.
  • the spherical V precipitate inhibits only the movement of dislocations that directly collide with the spherical V precipitate. Therefore, the resistance of the spherical V precipitate to the dislocation motion is weak.
  • the plate-shaped V precipitate has a NaCl-type crystal structure and forms a matching interface or a semi-matching interface having a Baker-Nutting (BN) relationship with the parent phase ( ⁇ ).
  • BN Baker-Nutting
  • the ⁇ 100 ⁇ of the plate-shaped V precipitate and the ⁇ 100 ⁇ of the matrix are parallel to each other, and the ⁇ 100> direction of the plate-shaped V precipitate and the matrix ⁇ 110>
  • a matching interface or a semi-matching interface that is parallel to the direction is formed.
  • This matching interface or semi-matching interface forms a matching strain field around the plate-shaped V precipitate. This matching strain field inhibits the motion of dislocations.
  • the plate-shaped V precipitate inhibits not only the movement of dislocations that directly collide with the plate-shaped V precipitate, but also the movement of dislocations that pass around the plate-shaped V precipitate. Therefore, the resistance of the plate-shaped V precipitate to the dislocation motion is stronger than that of the spherical V precipitate.
  • the microstructure is a structure composed of polygonal ferrite and a hard phase, and V precipitates satisfy the formula (1). If the proportion of plate-like V precipitates in the deposit is large, the resistance to dislocation motion 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-shaped V precipitates
  • the size of V precipitates is at the nano level. Therefore, it is extremely difficult to distinguish between the plate-shaped V precipitate and the spherical V precipitate by observing the microstructure, and to determine the ratio of the plate-shaped V precipitate in the V precipitate.
  • hydrogen is likely to be trapped at the matched interface and the semi-matched interface, and hydrogen is less likely to be trapped at the unmatched interface. That is, the plate-shaped V precipitate is likely to trap hydrogen, and the spherical V precipitate is difficult to trap hydrogen.
  • the content of each element in the chemical composition is within the above range
  • the microstructure is a structure composed of polygonal ferrite and a hard phase
  • an amount of V precipitate satisfying the formula (1) is precipitated. If the amount of hydrogen trapped (that is, the amount of diffusible hydrogen) is large when hydrogen is charged by the cathode hydrogen charging method in a steel profile, plate-shaped V-deposition that also increases tensile strength in V-precipitates that increase fatigue strength. It means that the proportion of things is high.
  • the content of each element in the chemical composition is within the above range, the microstructure is a structure composed of polygonal ferrite and a hard phase, and further in the steel profile material satisfying the formula (1). If the amount of diffusible hydrogen when hydrogen is charged by the cathode hydrogen charging method is 0.10 ppm or more, it is considered that high fatigue strength and high tensile strength can be obtained. The above reason is presumed. However, in the steel body material satisfying the formula (1), the content of each element in the chemical composition is within the above range, the microstructure is a structure composed of polygonal ferrite and a hard phase, and further, cathode hydrogen. Examples described later prove that high fatigue strength and high tensile strength can be obtained when the amount of diffusible hydrogen when hydrogen is charged by the charging method is 0.10 ppm or more.
  • the steel material of the present embodiment completed based on the above knowledge and the manufacturing method thereof have the following configurations.
  • the chemical composition is mass%, C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: Over 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, V: Over 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: 0-0.05%, Ti: 0 to 0.005%, Zr: 0 to 0.010%, Se: 0 to 0.10%, Te: 0 to 0.10%, Rare earth elements: 0-0.010%, Sb: 0 to 0.10%, Mg: 0 to 0.0050%, W: 0 to 0.050% and Remain
  • the steel profile material according to [1].
  • the chemical composition replaces a part of Fe. Cr: 0.01 to 0.70%, Nb: 0.001 to 0.100%, B: 0.0001 to 0.0100%, Cu: 0.01-0.30%, Ni: 0.01-0.30%, Ca: 0.0001 to 0.0050%, Bi: 0.001 to 0.100%, Pb: 0.001 to 0.090%, Mo: 0.01-0.05%, Ti: 0.001 to 0.005%, Zr: 0.002-0.010%, Se: 0.01-0.10%, Te: 0.01-0.10%, Rare earth elements: 0.01-0.010%, Sb: 0.01 to 0.10%, Mg: 0.0005 to 0.0050%, W: Contains one or more elements selected from the group consisting of 0.001 to 0.050%. Steel profile material.
  • [3] The method for producing a steel profile according to [1] or [2].
  • the chemical composition is mass%, C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: Over 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, V: Over 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: 0-0.05%, Ti: 0 to 0.005%, Zr: 0 to 0.010%, Se: 0 to 0.10%, Te: 0 to 0.10%, Rare earth elements: 0-0.010%, Sb: 0 to 0.10%, Mg: 0 to 0.0050%,
  • the steel material includes a second-direction cold working step in which cold working is performed on the steel material from a second direction different from the first direction so that the working strain amount is 0.05 or more.
  • the total of the amount of machining strain generated in the steel material in the first-direction cold working step and the amount of machining strain generated in the steel material in the second-direction cold working step is 0.20 or more. Manufacturing method of steel profile.
  • the steel material shape means a part in which a steel material is processed or heat-treated by an external force to give it a shape.
  • the steel profile may be the final product.
  • the steel body may be further subjected to processing such as cutting to produce a final product.
  • Carbon (C) combines with V of the steel material to form a V precipitate.
  • the V-precipitate enhances the fatigue strength and tensile strength of the steel profile by strengthening the precipitation. If the C content is less than 0.03%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the C content exceeds 0.25%, the cold workability of the steel material, which is the material of the steel element shape material, is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the C content is 0.03 to 0.25%.
  • the lower limit of the C content is preferably 0.04%, more preferably 0.05%, still more preferably 0.06%, still more preferably 0.07%, still more preferably 0.08. %.
  • the preferable upper limit of the C content is 0.24%, more preferably 0.23%, further preferably 0.22%, still more preferably 0.21%, still more preferably 0.20. %.
  • Si 0.02 to 0.50%
  • Silicon (Si) increases the fatigue strength of the steel profile. Si further deoxidizes the steel. If the Si content is less than 0.02%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 0.50%, the cold workability of the steel material, which is the material of the steel element shape material, is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 0.02 to 0.50%.
  • the lower limit of the Si content is preferably 0.03%, more preferably 0.04%, still more preferably 0.05%, still more preferably 0.06%, still more preferably 0.07. %.
  • the preferred upper limit of the Si content is 0.45%, more preferably 0.40%, still more preferably 0.35%, still more preferably 0.30%, still more preferably 0.25. %.
  • Mn Over 0.70 to 2.50% Manganese (Mn) increases the fatigue strength of steel profiles.
  • Mn content is 0.70% or less, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
  • the Mn content exceeds 2.50%, the cold workability of the steel material, which is the material of the steel body shape material, is lowered even if the other element content is within the range of the present embodiment. Therefore, the Mn content is more than 0.70 to 2.50%.
  • the preferable lower limit of the Mn content is 0.75%, more preferably 0.80%, still more preferably 1.00%, still more preferably 1.20%, still more preferably 1.40%. %, More preferably 1.50%.
  • the preferred upper limit of the Mn content is 2.40%, more preferably 2.30%, still more preferably 2.20%, still more preferably 2.10%, still more preferably 2.00%. %, More preferably 1.90%.
  • Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is more than 0%. P segregates at the grain boundaries to reduce the fatigue strength and tensile strength of the steel profile. Therefore, the P content is 0.035% or less.
  • the preferred upper limit of the P content is 0.030%, more preferably 0.025%, still more preferably 0.020%.
  • the P content is preferably as low as possible. However, excessive reduction of P content raises manufacturing costs. Therefore, considering normal industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.005%, still more preferably 0.008%, still more preferably 0. It is 010%.
  • S 0.050% or less Sulfur (S) is an impurity that is inevitably contained. That is, the S content is more than 0%. S combines with Mn to form MnS and enhances machinability of the steel material. However, if the S content exceeds 0.050%, coarse MnS is produced. Coarse MnS tends to be a starting point of cracking during cold working. Therefore, the cold workability of the steel material, which is the material of the steel body shape material, is lowered. Therefore, the S content is 0.050% or less.
  • the upper limit of the S content is preferably 0.045%, more preferably 0.040%, still more preferably 0.030%, still more preferably 0.020%.
  • the S content is preferably as low as possible. However, excessive reduction of S content raises manufacturing costs. Therefore, considering normal industrial production, the preferable lower limit of the S content is 0.001%, more preferably 0.005%, still more preferably 0.006%.
  • Al 0.005 to 0.050%
  • Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, the above effect cannot be sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.050%, coarse Al-based inclusions such as Al oxide are formed in the steel material even if the other element content is within the range of the present embodiment. Coarse Al-based inclusions tend to be the base point for cracking during cold working. Therefore, the cold workability of the steel material, which is the material of the steel body shape material, is lowered. Therefore, the Al content is 0.005 to 0.050%.
  • the preferable lower limit of the Al content is 0.005%, more preferably 0.006%, further preferably 0.007%, still more preferably 0.008%, still more preferably 0.009. %, More preferably 0.010%, still more preferably 0.015%.
  • the preferred upper limit of the Al content is 0.045%, more preferably 0.040%, still more preferably 0.030%, still more preferably 0.025%, still more preferably 0.020. %.
  • the Al content means the total Al content.
  • V Over 0.10 to 0.40% Vanadium (V) combines with C and / or N in the steel to form a V precipitate.
  • the V-precipitate enhances the fatigue strength and tensile strength of the steel profile by strengthening the precipitation. If the V content is 0.10% or less, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the V content exceeds 0.40%, the cold workability of the steel material, which is the material of the steel element shape material, is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the V content is more than 0.10 to 0.40%.
  • the preferable lower limit of the V content is 0.11%, more preferably 0.12%, still more preferably 0.13%, still more preferably 0.14%, still more preferably 0.15. %.
  • the preferred upper limit of the V content is 0.38%, more preferably 0.35%, still more preferably 0.33%, still more preferably 0.30%, still more preferably 0.28. %, More preferably 0.25%.
  • N 0.003 to 0.030%
  • Nitrogen (N) combines with V in the steel material to form a V precipitate.
  • the V-precipitate enhances the fatigue strength and tensile strength of the steel profile by strengthening the precipitation. If the N content is less than 0.003%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the N content exceeds 0.030%, the number ratio of spherical V precipitates in the V precipitates increases even if the other element contents are within the range of the present embodiment. In this case, the fatigue strength and tensile strength of the steel profile are reduced. Therefore, the N content is 0.003 to 0.030%.
  • the preferable lower limit of the N content is more than 0.003%, more preferably 0.004%, still more preferably 0.005%.
  • the preferred upper limit of the N content is 0.028%, more preferably 0.025%, even more preferably 0.023%, even more preferably 0.020%, still more preferably 0.018%. %, More preferably 0.015%.
  • the rest of the chemical composition of the steel profile material of this embodiment consists of Fe and impurities.
  • impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the steel material that is the raw material of the steel element shape material is industrially manufactured, and are intentionally made of the steel element. It means an element that is not contained in the shape material.
  • the impurity is assumed to be oxygen (O), for example. Even if O as an impurity is contained in an amount of 0.040% or less, the effect of the steel profile material of the present embodiment can be obtained.
  • the elements that can be contained in the impurities may be other than O.
  • the chemical composition of the steel profile of the present embodiment further replaces a part of Fe with Cr, Nb, B, Cu, Ni, Ca, Bi, Pb, Mo, Ti, Zr, Se, Te, and rare earth elements. It may contain one or more elements selected from the group consisting of (REM), Sb, Mg and W. All of these elements are optional elements. Hereinafter, each arbitrary element will be described.
  • the chemical composition of the steel profile material of the present embodiment further comprises one or more elements selected from the group consisting of Cr, Nb, B, Cu and Ni instead of a part of Fe within the range of the following contents. It may be contained. All of these elements increase the fatigue strength and tensile strength of the steel profile.
  • Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When it is contained, that is, when the Cr content is more than 0%, Cr improves the hardenability of the steel material and enhances the fatigue strength and the tensile strength of the steel body shape material. If even a small amount of Cr is contained, the above effect can be obtained to some extent. However, if the Cr content exceeds 0.70%, the cold workability of the steel material, which is the material of the steel profile material, is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Cr content is 0 to 0.70%. When contained, the Cr content is 0.70% or less.
  • the lower limit of the Cr content is preferably 0.01%, more preferably 0.03%, still more preferably 0.05%, still more preferably 0.07%, still more preferably 0.09. %, More preferably 0.10%.
  • the preferred upper limit of the Cr content is 0.65%, more preferably 0.60%, still more preferably 0.50%, still more preferably 0.45%, still more preferably 0.40. %, More preferably 0.35%, still more preferably 0.30%.
  • Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When it is contained, that is, when the Nb content is more than 0%, Nb combines with C and / or N in the steel material to form an Nb precipitate. The Nb precipitate enhances the fatigue strength and tensile strength of the steel profile by strengthening the precipitation. If even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.100%, the cold workability of the steel material, which is the material of the steel body shape material, is lowered even if the content of other elements is within the range of the present embodiment.
  • the Nb content is 0 to 0.100%. When contained, the Nb content is 0.100% or less.
  • the preferable lower limit of the Nb content is more than 0%, more preferably 0.001%, still more preferably 0.010%, still more preferably 0.020%.
  • the preferred upper limit of the Nb content is 0.080%, more preferably 0.060%.
  • B 0 to 0.0100% Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When it is contained, that is, when the B content is more than 0%, B strengthens the grain boundaries of the steel profile. As a result, the fatigue strength and tensile strength of the steel profile increase. If B is contained even in a small amount, the above effect can be obtained to some extent. However, if the B content exceeds 0.0100%, the above effect is saturated. If the B content exceeds 0.0100%, the raw material cost will be further increased and the manufacturability will be lowered. Therefore, the B content is 0 to 0.0100%. When contained, the B content is 0.0100% or less.
  • the preferable lower limit of the B content is more than 0%, more preferably 0.0001%, further preferably 0.0010%, still more preferably 0.0020%, still more preferably 0.0030%. Is.
  • the preferred upper limit of the B content is 0.0080%, more preferably 0.0070%, still more preferably 0.0060%.
  • Cu 0 to 0.30%
  • Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When it is contained, that is, when the Cu content is more than 0%, Cu improves the hardenability of the steel material and enhances the fatigue strength and the tensile strength of the steel body shape material. If even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.30%, the cold workability of the steel material, which is the material of the steel body shape material, is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Cu content is 0 to 0.30%. When contained, the Cu content is 0.30% or less.
  • the lower limit of the Cu content is more than 0%, more preferably 0.01%, still more preferably 0.05%, still more preferably 0.10%.
  • the preferred upper limit of the Cu content is 0.29%, more preferably 0.28%, still more preferably 0.25%.
  • Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When it is contained, that is, when the Ni content is more than 0%, Ni improves the hardenability of the steel material and enhances the fatigue strength and the tensile strength of the steel body shape material. If even a small amount of Ni is contained, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.30%, the cold forging property of the steel material, which is the material of the steel element shape material, is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ni content is 0 to 0.30%. When contained, the Ni content is 0.30% or less.
  • the lower limit of the Ni content is preferably more than 0%, more preferably 0.01%, still more preferably 0.05%, still more preferably 0.10%.
  • the preferred upper limit of the Ni content is 0.29%, more preferably 0.28%, still more preferably 0.27%, still more preferably 0.25%.
  • the chemical composition of the steel profile material of the present embodiment may further contain one or more elements selected from the group consisting of Ca, Bi and Pb in the range of the following contents instead of a part of Fe. good. All of these elements enhance the machinability of steel profiles.
  • Ca 0 to 0.0050%
  • Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When it is contained, that is, when the Ca content is more than 0%, Ca enhances the machinability of the steel profile. If even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, coarse CaO is produced even if the other element content is within the range of this embodiment. In this case, the cold workability of the steel material, which is the material of the steel body shape material, is lowered. Therefore, the Ca content is 0 to 0.0050%. When contained, the Ca content is 0.0050% or less.
  • the lower limit of the Ca content is more than 0%, more preferably 0.0001%, still more preferably 0.0010%, still more preferably 0.0020%.
  • the preferred upper limit of the Ca content is 0.0045%, more preferably 0.0040%.
  • Bi 0 to 0.100%
  • Bismuth (Bi) is an optional element and does not have to be contained. That is, the Bi content may be 0%. When it is contained, that is, when the Bi content is more than 0%, Bi enhances the machinability of the steel profile. If even a small amount of Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.100%, the cold workability of the steel material, which is the material of the steel body shape material, is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Bi content is 0 to 0.100%. When contained, the Bi content is 0.100% or less.
  • the preferred lower limit of the Bi content is more than 0%, more preferably 0.001%, even more preferably 0.010%, even more preferably 0.020%, still more preferably 0.030%. Is.
  • the preferred upper limit of the Bi content is 0.090%, more preferably 0.080%, still more preferably 0.070%, still more preferably 0.065%.
  • Pb 0 to 0.090%
  • Lead (Pb) is an optional element and may not be contained. That is, the Pb content may be 0%. When it is contained, that is, when the Pb content is more than 0%, Pb enhances the machinability of the steel profile. If Pb is contained even in a small amount, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.090%, the cold workability of the steel material, which is the material of the steel body shape material, is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Pb content is 0 to 0.090%. When contained, the Pb content is 0.090% or less.
  • the preferable lower limit of the Pb content is more than 0%, more preferably 0.001%, still more preferably 0.010%, still more preferably 0.020%, still more preferably 0.040%. Is.
  • the preferred upper limit of the Pb content is 0.080%, more preferably 0.070%.
  • the chemical composition of the steel profile of the present embodiment is further selected from the group consisting of Mo, Ti, Zr, Se, Te, rare earth element (REM), Sb, Mg and W instead of a part of Fe. It may contain one or more elements. These elements are impurities.
  • Mo 0-0.05% Molybdenum (Mo) is an impurity and may not be contained. That is, the Mo content may be 0%. Mo lowers the cold workability of the steel material, which is the material of the steel shape material. If the Mo content exceeds 0.05%, the cold workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Mo content is 0 to 0.05%. When contained, the Mo content is 0.05% or less.
  • the preferred upper limit of the Mo content is 0.04%, more preferably 0.03%, still more preferably 0.02%.
  • the Mo content is preferably as low as possible. However, excessive reduction of Mo content raises manufacturing costs. Therefore, the preferred lower limit of the Mo content is more than 0%, more preferably 0.01%.
  • Titanium (Ti) is an impurity and may not be contained. That is, the Ti content may be 0%. Ti combines with N in the steel profile to form Ti-based inclusions. Ti-based inclusions serve as a starting point for cracking during cold working. Therefore, the Ti-based inclusions reduce the cold workability of the steel material, which is the material of the steel body shape material. If the Ti content exceeds 0.005%, the cold workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ti content is 0 to 0.005%. When contained, the Ti content is 0.005% or less. The preferred upper limit of the Ti content is 0.004%, more preferably 0.003%, still more preferably 0.002%. The Ti content is preferably as low as possible. However, excessive reduction of Ti content raises manufacturing costs. Therefore, the preferred lower limit of the Ti content is more than 0%, more preferably 0.001%.
  • Zr Zirconium
  • Zr Zirconium
  • the Zr content may be 0%. If the Zr content exceeds 0.010%, Zr forms coarse inclusions and reduces the fatigue properties of the steel material, even if the other element content is within the range of this embodiment. Therefore, the Zr content is 0-0.010%. When contained, the Zr content is 0.010% or less.
  • the preferred upper limit of the Zr content is 0.008%, more preferably 0.006%, still more preferably 0.005%.
  • the Zr content is preferably as low as possible. However, excessive reduction of Zr content raises manufacturing costs. Therefore, the preferred lower limit of the Zr content is more than 0%, more preferably 0.002%.
  • Se 0 to 0.10%
  • Selenium (Se) is an impurity and may not be contained. That is, the Se content may be 0%. If the Se content exceeds 0.10%, Se brittles the steel material and lowers the strength and fatigue characteristics of the steel material even if the content of other elements is within the range of the present embodiment. Therefore, the Se content is 0 to 0.10%. When contained, the Se content is 0.10% or less.
  • the preferred upper limit of the Se content is 0.08%, more preferably 0.06%, still more preferably 0.05%.
  • the Se content is preferably as low as possible. However, excessive reduction of Se content raises manufacturing costs. Therefore, the preferred lower limit of the Se content is more than 0%, more preferably 0.01%.
  • Te 0 to 0.10%
  • Tellurium (Te) is an impurity and may not be contained. That is, the Te content may be 0%. If the Te content exceeds 0.10%, Te brittles the steel material and reduces the strength and fatigue strength of the steel material even if the content of other elements is within the range of the present embodiment. Therefore, the Te content is 0 to 0.10%. When contained, the Te content is 0.10% or less.
  • the preferred upper limit of the Te content is 0.08%, more preferably 0.06%, still more preferably 0.05%.
  • the Te content is preferably as low as possible. However, excessive reduction of Te content raises manufacturing costs. Therefore, the preferred lower limit of the Te content is more than 0%, more preferably 0.01%.
  • Rare earth element 0-0.010%
  • Rare earth elements are impurities and may not be contained. That is, the REM content may be 0%. If the REM content exceeds 0.010%, the REM forms coarse inclusions and reduces the fatigue properties of the steel material, even if the other element content is within the range of this embodiment. Therefore, the REM content is 0-0.010%. When contained, the REM content is 0.010% or less.
  • the preferred upper limit of the REM content is 0.008%, more preferably 0.006%, still more preferably 0.005%.
  • the REM content is preferably as low as possible. However, excessive reduction of REM content raises manufacturing costs. Therefore, the preferred lower limit of the REM content is more than 0%, more preferably 0.001%.
  • the REM in the present specification refers to lutetium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) to having an atomic number of 71, which are lanthanoids. It means one or more elements selected from the group consisting of lutetium (Lu).
  • the REM content in the present specification is the total content of these elements.
  • Sb Antimony
  • Sb is an impurity and may not be contained. That is, the Sb content may be 0%. If the Sb content exceeds 0.10%, Sb embrittles the steel material and lowers the strength and fatigue characteristics of the steel material even if the other element content is within the range of the present embodiment. Therefore, the Sb content is 0 to 0.10%. When contained, the Sb content is 0.10% or less.
  • the preferred upper limit of the Sb content is 0.08%, more preferably 0.06%, still more preferably 0.05%.
  • the Sb content is preferably as low as possible. However, excessive reduction of Sb content raises manufacturing costs. Therefore, the preferred lower limit of the Sb content is more than 0%, more preferably 0.01%.
  • Mg 0 to 0.0050%
  • Magnesium (Mg) is an impurity and may not be contained. That is, the Mg content may be 0%. If the Mg content exceeds 0.0050%, Mg forms coarse inclusions and reduces the fatigue characteristics of the steel material, even if the other element content is within the range of this embodiment. Therefore, the Mg content is 0 to 0.0050%. When contained, the Mg content is 0.0050% or less.
  • the preferred upper limit of the Mg content is 0.0040%, more preferably 0.0030%, still more preferably 0.0025%.
  • the Mg content is preferably as low as possible. However, excessive reduction of Mg content raises manufacturing costs. Therefore, the preferred lower limit of the Mg content is more than 0%, more preferably 0.0005%.
  • W 0 to 0.050%
  • Tungsten (W) is an impurity and may not be contained. That is, the W content may be 0%. If the W content exceeds 0.050%, W lowers the cold workability of the steel material as the raw material even if the content of other elements is within the range of the present embodiment. Therefore, the W content is 0 to 0.050%. When contained, the W content is 0.040% or less.
  • the preferred upper limit of the W content is 0.030%, more preferably 0.025%, still more preferably 0.020%.
  • the W content is preferably as low as possible. However, excessive reduction of W content raises manufacturing costs. Therefore, the preferred lower limit of the W content is more than 0%, more preferably 0.001%.
  • the microstructure of the steel profile of the present embodiment contains polygonal ferrite and pearlite and / or bainite.
  • pearlite and / or bainite are referred to as "hard phase”.
  • bainite includes martensite.
  • the area ratio of polygonal ferrite in the microstructure is 20 to 90%.
  • the rest of the microstructure is the hard phase, as described above. That is, the microstructure of the steel profile is composed of a polygonal ferrite having an area ratio of 20 to 90% and a hard phase having a total area ratio of 10 to 80%.
  • the area ratio of the polygonal ferrite is 20 to 90%, the content of each element in the chemical composition is within the range of the present embodiment, the formula (1) is satisfied, and hydrogen is charged by the cathode hydrogen charging method. Assuming that the amount of diffusible hydrogen in the case is 0.10 ppm or more, the steel profile can obtain high fatigue strength and high tensile strength.
  • the preferable lower limit of the polygonal ferrite area ratio in the microstructure of the steel profile is 25%, more preferably 30%, still more preferably 35%.
  • the preferred upper limit of the polygonal ferrite area ratio is 80%, more preferably 75%, and even more preferably 70%.
  • the area ratio of the hard phase in the microstructure is 10 to 80% as described above.
  • the preferred lower limit of the area ratio of pearlite in the microstructure is 5%, more preferably 10%.
  • the preferred upper limit of the area ratio of pearlite is 50%, more preferably 40%.
  • the preferred lower limit of the area ratio of bainite in the microstructure is 5%, more preferably 10%.
  • the preferred upper limit of the area ratio of bainite in the microstructure is 80%, more preferably 70%.
  • test piece for microstructure observation from any position on the steel profile. Any surface of the surface of the test piece is specified as an observation surface. Mirror polish the observation surface. The observed surface after polishing is etched with a 3% nital corrosive solution (ethanol + 3% nitric acid solution). Arbitrary five observation fields of the etched observation surface are observed with a 400x optical microscope to generate a photographic image. At this time, the position of each observation field of view is set to a position deeper than at least 3 mm from the surface of the original steel profile. The size of each observation field is 200 ⁇ m ⁇ 200 ⁇ m. Polygonal ferrite is identified in the photographic image of each field of view.
  • a phase having a lamellar structure can be identified as pearlite.
  • a region having a higher brightness than pearlite (white region) can be identified as polygonal ferrite.
  • a region having a lower brightness (dark region) than that of polygonal ferrite and pearlite can be identified as bainite.
  • the polygonal ferrite area ratio (%) is obtained based on the total area of the polygonal ferrite obtained in the five fields of view and the total area of the five fields of view.
  • the total area ratio (%) of pearlite and bainite is determined based on the total area of pearlite and bainite obtained in the five visual fields and the total area of the five visual fields.
  • the V content in the chemical composition of the steel profile material is defined as [V] (mass%). Further, when the chemical composition of the steel profile is 100%, the total content of V in the V precipitate in the steel profile is defined as [V in the precipitate] (mass%). In this case, the steel profile material of the present embodiment satisfies the formula (1). [V in precipitate] / [V] ⁇ 0.30 (1)
  • VP [V in precipitate] / [V] is defined.
  • VP indicates the precipitation ratio of V precipitates in the steel profile.
  • the content of each element in the chemical composition of the steel profile is within the range of this embodiment, and the microstructure is a polycarbonate ferrite with an area ratio of 20 to 90% and an area ratio of 10 to 80%. Even if the structure is composed of a hard phase, if the VP is less than 0.30, the formation of V precipitates in the steel profile is insufficient. In this case, the fatigue strength and tensile strength in the steel profile decrease.
  • the VP is 0.30 or more
  • the content of each element in the chemical composition of the steel profile is within the range of the present embodiment, and the microstructure is a polygo having an area ratio of 20 to 90%.
  • the structure is composed of null ferrite and a hard phase having an area ratio of 10 to 80% and the amount of diffusible hydrogen is 0.10 ppm or less
  • V precipitates are sufficient in the steel profile. It is precipitated in. Therefore, the fatigue strength and tensile strength of the steel profile are increased by the precipitation strengthening by the V precipitate.
  • the preferable lower limit of VP is 0.31, and more preferably 0.32.
  • the upper limit of VP is not particularly limited.
  • the preferred upper limit of VP is 0.60, more preferably 0.55, and even more preferably 0.52.
  • V content of the V precipitate in the steel profile is determined by the extraction residue analysis method.
  • a sample of about 1000 mm 3 (about 7.8 g) is cut out from the steel profile.
  • a 10% AA solution (a liquid in which tetramethylammonium chloride, acetylacetone, and methanol are mixed at a ratio of 1:10: 100) is prepared.
  • the cut sample is immersed in a 10% AA solution. Constant current electrolysis is performed on the immersed sample.
  • the conditions for electrolysis are current: 173 mA, time: 142 minutes, and room temperature (25 ° C.). Take out the electrolyzed sample.
  • the removed sample is ultrasonically cleaned in alcohol. As a result, deposits (residues) on the surface of the sample are removed.
  • the solution after electrolysis and the solution used for ultrasonic cleaning are suction-filtered with a filter.
  • the mesh size of the filter is 0.2 ⁇ m. As a result, the residue is collected.
  • the residue collected on the above filter is transferred to a petri dish and dried. Measure the mass of the dried residue. Then, in accordance with JIS G 1258 (2014), the residue is analyzed by an ICP emission spectrometer (high frequency inductively coupled plasma emission spectroscopic analyzer) to obtain the "mass of V in the residue".
  • ICP emission spectrometer high frequency inductively coupled plasma emission spectroscopic analyzer
  • the content of each element in the chemical composition is within the range of the present embodiment, and the microstructure is 20 to 90% polygonal ferrite and 10 to 80% hard phase.
  • the amount of diffusible hydrogen when hydrogen is charged by the cathode hydrogen charging method is 0.10 ppm or more on the premise that the formula (1) is satisfied.
  • the steel profile of the present embodiment is a cathode hydrogen having a current density of 0.1 mA / cm 2 and an energization time of 72 hours in a 3% NaCl-3 g / LNH 4 SCN aqueous solution.
  • the amount of diffusible hydrogen when hydrogen is charged by the charging method is 0.10 ppm or more.
  • the amount of diffusible hydrogen is 0.10 ppm or more
  • the content of each element in the chemical composition is within the range of the present embodiment, and the microstructure is 20 to 90%. It is composed of a polygonal ferrite and a hard phase of 10 to 80%, and high fatigue strength and high tensile strength can be obtained on the premise that the formula (1) is satisfied.
  • the shape of the V precipitate is the cause.
  • the spherical V precipitates form an unmatched interface with the matrix.
  • the matrix in contact with the spherical precipitate at the unmatched interface is less likely to interfere with the dislocation movement.
  • a matching interface or a semi-matching interface is formed around the plate-shaped V precipitate.
  • the resistance of the plate-shaped V precipitate to the dislocation motion is stronger than that of the spherical V precipitate. Therefore, when the VPs are the same value (that is, even if the precipitation amount of the V precipitates is the same), the larger the precipitation amount of the plate-shaped V precipitates, the higher the fatigue strength and the tensile strength.
  • the above mechanism is an estimate.
  • the preferable lower limit of the amount of diffusible hydrogen is 0.11 ppm, more preferably 0.12 ppm, still more preferably 0.13 ppm, still more preferably 0.14 ppm.
  • the preferable upper limit of the amount of diffusible hydrogen is not particularly limited, but is, for example, 0.50 ppm, more preferably 0.45 ppm, still more preferably 0.40 ppm, still more preferably 0.35 ppm, still more preferably. It is 0.30 ppm.
  • the method for measuring the amount of diffusible hydrogen is as follows. A round bar test piece having a diameter of 7 mm and a length of 40 mm is cut out from an arbitrary position of the steel profile. Hydrogen is introduced into the cut out round bar test piece by using the cathode hydrogen charging method.
  • the round bar test piece is immersed in a 3% NaCl-3 g / LNH 4 SCN aqueous solution. Then, hydrogen is introduced into the round bar test piece by the cathode hydrogen charging method under the conditions of current density: 0.1 mA / cm 2 and energization time: 72 hours. The timing at which the energization is stopped is defined as the timing at which the introduction of hydrogen into the round bar test piece is completed.
  • the gap time After completing the introduction of hydrogen into the round bar test piece, measure the amount of hydrogen in the round bar test piece using temperature rising withdrawal gas chromatography. Depending on the time from the completion of the introduction of hydrogen into the round bar test piece to the start of measurement of the amount of hydrogen in the round bar test piece using the temperature rising withdrawal gas chromatography (hereinafter referred to as the gap time), the following Is processed.
  • the gap time is within 30 minutes, start measuring the amount of hydrogen using the round bar test piece for which hydrogen has been introduced.
  • the round bar test piece is stored in a state of being immersed in liquid nitrogen until the measurement of the amount of hydrogen is started after the introduction of hydrogen into the round bar test piece is completed. This is to prevent the hydrogen introduced into the round bar test piece from being released to the outside of the round bar test piece before the measurement of the amount of hydrogen is started.
  • the amount of hydrogen in the round bar test piece is measured by the following method using temperature rising withdrawal gas chromatography. Specifically, the round bar test piece is heated from room temperature to 400 ° C. at a heating rate of 100 ° C./hour. The amount of hydrogen generated by the temperature rise is measured at 5-minute intervals. Based on the amount of hydrogen obtained, the hydrogen release curve shown in FIG. 1 is obtained. Using the obtained hydrogen release curve, the cumulative amount of hydrogen released from room temperature to 350 ° C. is determined. The obtained cumulative hydrogen amount is defined as the diffusible hydrogen amount (ppm).
  • the content of each element in the chemical composition is within the range of the present embodiment, and the microstructure has a polygonal ferrite having an area ratio of 20 to 90% and an area. It is composed of a hard phase having a ratio of 10 to 80%, satisfies the formula (1), and has a diffusible hydrogen amount of 0.10 ppm or more when hydrogen is charged by the cathode hydrogen charging method. Therefore, the steel profile material of the present embodiment not only obtains high fatigue strength, but also high tensile strength.
  • the manufacturing method described below is an example of a manufacturing method for a steel profile, and is not limited thereto. That is, the content of each element in the chemical composition is within the range of this embodiment, and the microstructure is composed of a polygonal ferrite having an area ratio of 20 to 90% and a hard phase having an area ratio of 10 to 80%. As long as the formula (1) is satisfied and the amount of diffusible hydrogen when hydrogen is charged by the cathode hydrogen charging method is 0.10 ppm or more, the method for producing the steel profile is not limited to the production method described below. However, the manufacturing method described below is a suitable manufacturing method for the steel profile material of the present embodiment.
  • the method for manufacturing the steel shape material of the present embodiment is a step of preparing a steel material to be a material of the steel material shape material (steel material preparation process) and a process of manufacturing a steel material shape material from the steel material (steel material shape material manufacturing process). ) And.
  • each step will be described in detail.
  • the steel material to be the material of the steel body shape material is prepared.
  • the shape of the steel material is not particularly limited, but is, for example, steel bar or wire rod.
  • the composition of the steel material which is the material of the steel element shape material of the present embodiment is as follows.
  • the composition of the steel material which is the material of the steel element shape material of the present embodiment is as follows.
  • the chemical composition of the steel material, which is the material of the steel material is the same as the chemical composition of the steel material. That is, the chemical composition of the steel material is C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.
  • V in the chemical composition of the steel material is defined as [V] (mass%).
  • V in the precipitate is defined as "V in the precipitate" (mass%).
  • [V] / [V] in the steel material is 0.05 to less than 0.30.
  • VP0 defined as [V in precipitate] / [V] in steel materials.
  • V precipitates are not formed as much as possible, and it is preferable that the amount of solid solution V is large.
  • fine V precipitates have not been formed in the aging hardening treatment described later during the manufacturing process of the steel profile material using the steel material, and the V precipitates have already been formed.
  • the V precipitate present in the steel material becomes coarse. In this case, it is difficult for plate-shaped V precipitates to be formed in the steel profile material, and the proportion of spherical V precipitates becomes excessively large.
  • the amount of diffusible hydrogen in the steel body shape material becomes low when hydrogen is charged by the cathode hydrogen charging method. Therefore, sufficient fatigue strength and tensile strength cannot be obtained in the steel profile.
  • the VP0 of the steel material is 0.30 or more, the amount of diffusible hydrogen in the steel element shape material when hydrogen is charged by the cathode hydrogen charging method becomes low. Therefore, sufficient fatigue strength and tensile strength cannot be obtained in the steel profile. Therefore, the VP0 of the steel material is set to less than 0.30.
  • the lower limit of VP0 of the steel material is not particularly limited, but is, for example, 0.05.
  • the VP0 of the steel material can be obtained by the same measurement method as the VP measurement method of the steel body shape material.
  • microstructure of the steel material which is the material of the steel material of the present embodiment is the same as the microstructure of the steel material described above. That is, the microstructure of the steel material is composed of 20 to 90% polygonal ferrite and 10 to 80% hard phase.
  • VP0 is 0.30 or more.
  • the amount of diffusible hydrogen in the steel profile when hydrogen is charged by the cathode hydrogen charging method becomes low. Therefore, sufficient fatigue strength and tensile strength cannot be obtained in the steel profile.
  • the microstructure of the steel material consists of 20-90% polygonal ferrite and 10-80% hard phase.
  • the polygonal ferrite area ratio and the hard phase area ratio in the microstructure of the steel material can be measured by the same measuring method as the polygonal ferrite area ratio and the hard phase area ratio in the microstructure of the steel body shape material.
  • the steel material used as the material for the nitrided steel parts of the present embodiment may be supplied from a third party or may be manufactured.
  • the steel material preparation step includes a step of preparing the material (material preparation step) and a step of hot-working the material to manufacture the steel material (hot working step).
  • material preparation step a step of preparing the material
  • hot working step a step of hot-working the material to manufacture the steel material
  • a molten steel having the above chemical composition is produced.
  • Prepare the material using molten steel For example, molten steel having the above-mentioned chemical composition is produced using a converter, an electric furnace, or the like.
  • a slab is manufactured by a continuous casting method using molten steel.
  • an ingot is manufactured by the ingot method using molten steel.
  • the prepared material is hot-worked to produce a steel material.
  • hot rolling includes a rough rolling step of rough rolling the material into billets and a finish rolling step of finishing rolling the billets into steel.
  • the rough rolling process for example, the following process is carried out. After heating the material (slab, ingot), it is lump-rolled using a lump-rolling machine. If necessary, after slabbing, the billet is further rolled in a continuous rolling mill to produce billets.
  • horizontal roll stands and vertical roll stands are alternately arranged in a row, and the material is rolled into billets using the hole molds formed in the rolling rolls of each stand.
  • the billet may be directly manufactured by the continuous casting method.
  • finish rolling for example, the following process is carried out.
  • the billets produced in the rough rolling process are charged into a heating furnace and heated.
  • finish rolling (hot rolling) is performed in a row of finish rolling mills to obtain rods having a predetermined diameter.
  • the finish rolling mill row includes a plurality of stands arranged in a row. Each stand contains multiple rolls arranged around the pass line.
  • a billet is rolled using a hole mold formed in a rolling roll of each stand to manufacture a steel material (bar wire).
  • the hot working process is not limited to hot rolling.
  • hot working step instead of the hot rolling described above, hot forging may be carried out or hot extrusion may be carried out.
  • the heating temperature of the steel material immediately before the final hot working is performed is, for example, 1000 to 1300 ° C.
  • the heating temperature in the heating furnace of the finish rolling step is 1000 to 1300 ° C.
  • the V precipitate generated before the hot working step is sufficiently solidified on the premise that other manufacturing conditions are satisfied.
  • the temperature of the steel material after the final reduction is defined as the finishing temperature (° C).
  • the finishing temperature is the steel temperature at the outlet side of the stand that was last rolled in the finishing rolling mill line in the finishing rolling process (surface temperature of the steel). Means.
  • the finishing temperature is, for example, 800 to 1200 ° C. When the finishing temperature is 800 to 1200 ° C., reprecipitation of V solid solution in the heating furnace can be sufficiently suppressed on the premise that other production conditions are satisfied.
  • 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 from the finishing temperature to 200 ° C. after the completion of hot working is defined as the cooling rate (° C./s) after hot working.
  • the cooling rate after hot working is 0.4 to 4.0 ° C./s
  • the polygonal ferrite area ratio in the steel material will be 20 to 90%, assuming that other manufacturing conditions are met, and the hard phase.
  • the area ratio is 10 to 80%, and the [V] / [V] in the steel material is 0.05 to less than 0.30.
  • the steel material that is the material of the steel profile material is manufactured.
  • the normalizing treatment step may be carried out on the steel material after the hot working step for the purpose of adjusting the microstructure.
  • the normalizing treatment step is an arbitrary step and does not have to be carried out.
  • the heat treatment temperature of the normalizing treatment may be 1000 to 1300 ° C.
  • the cooling rate after holding at the heat treatment temperature may be 0.4 to 4.0 ° C./s. That is, the heat treatment temperature and the cooling rate of the normalizing treatment are in the same range as the heating temperature and the cooling rate in the hot working process.
  • the steel profile manufacturing process includes a process of cold working the steel material (cold working process), a process of performing aging hardening treatment on the steel material after cold working (aging hardening treatment process), and aging hardening treatment. It is provided with a process (cutting process) of performing a cutting process on the subsequent steel material.
  • the cutting process is an arbitrary process. That is, the cutting process does not have to be performed.
  • each step will be described.
  • the cold working step includes a first-direction cold working step and a second-direction cold working step.
  • the steel material In the first-direction cold working step, the steel material is cold-worked so that the working strain amount is 0.05 or more from the first direction.
  • the steel material In the second-direction cold working step, the steel material is cold-worked so that the working strain amount is 0.05 or more from the second direction.
  • the total of the working strain amount generated in the steel material in the first direction cold working step and the working strain amount generated in the steel material in the second direction cold working step is set to 0.20 or more.
  • the first direction and the second direction are not particularly limited as long as they are different directions.
  • the first direction and the second direction may intersect. Further, they may be orthogonal to each other as in the drawing process and the setting process described later.
  • the steel material receives loads from two different directions (first direction and second direction). Due to the load applied from the two directions, the moving direction of the dislocations in the crystal grains of the steel material is not only a fixed direction but also a plurality of directions. Therefore, cross-slip is more likely to occur in the steel material as compared with the case where cold working is performed only from one direction. When cross slip occurs, dislocations are likely to collide with each other. Therefore, the number of dislocations (immobile dislocations) that cannot move while colliding increases, and the number of dislocations left behind in the crystal grains increases. As a result, the dislocation density in the crystal grains increases. As the dislocation density increases, strain is formed.
  • plate-shaped V precipitates are likely to precipitate in the portion where the strain is formed. That is, the formed strain becomes the core of the plate-shaped V precipitate.
  • the fatigue strength and tensile strength of the produced steel profile material are increased by the precipitation strengthening.
  • the plate-shaped V precipitate can be sufficiently precipitated so that the amount of diffusible hydrogen becomes 0.10 ppm or more in the aging hardening treatment step described later.
  • the total processing strain is the sum of the amount of strain generated by the first cold working process (first direction processing strain amount) and the amount of strain generated by the second cold processing process (second direction processing strain amount). Defined as quantity. More specifically, in the present embodiment, the amount of processing strain in the first direction is 0.05 or more, and the amount of processing strain in the second direction is 0.05 or more. Further, the total processing strain amount is set to 0.20 or more.
  • the total processing strain amount is 0.20 or more, dislocations moving in multiple directions increase in the steel material, and as a result, the dislocation density in the crystal grains increases. Therefore, in the aging hardening treatment step described later, the plate-shaped V precipitate can be sufficiently precipitated so that the formula (1) is satisfied and the amount of diffusible hydrogen is 0.10 ppm or more. As a result, a sufficiently high fatigue strength and a sufficiently high tensile strength can be obtained in the steel profile. If the total processing strain amount is less than 0.20, the above effect cannot be sufficiently obtained. Therefore, the total processing strain amount is 0.20 or more.
  • the preferable lower limit of the total processing strain amount is 0.23, more preferably 0.25, and further preferably 0.28.
  • the upper limit of the total processing strain amount is not particularly limited. However, when the total processing strain amount is excessively increased, the deformation resistance of the steel material during the cold processing process becomes excessively high, which imposes an excessive burden on the manufacturing equipment. Therefore, the preferable upper limit of the total processing strain amount is 1.50, more preferably 1.20, and further preferably 0.80.
  • the total machining strain amount which is the sum of the first direction machining strain amount and the second direction machining strain amount, is set to 0.20 or more, but also the first It is also required that the amount of directional machining strain is 0.05 or more and the amount of second directional machining strain is 0.05 or more.
  • the equation (1) may be satisfied even if the total machining strain amount is 0.20 or more.
  • the moving direction of the dislocations in the crystal grains is biased. In this case, cross slip is less likely to occur. Therefore, the dislocation density in the crystal grains is insufficient. Therefore, it is considered that the formation of plate-like V precipitates is insufficient in the aging hardening treatment step.
  • the upper limit of the first direction machining strain and the second direction machining strain is not particularly limited.
  • the preferable upper limit of the amount of processing strain in the first direction is, for example, 0.40, and more preferably 0.30.
  • the preferable lower limit of the amount of processing strain in the first direction is 0.06, and more preferably 0.08.
  • the preferable upper limit of the amount of processing strain in the second direction is, for example, 0.80, and more preferably 0.50.
  • the preferable lower limit of the amount of processing strain in the second direction is 0.06, and more preferably 0.08.
  • first-direction cold working step and second-direction cold working step Preferable first-direction cold working step and second-direction cold working step
  • the first-direction cold working step is a drawing process
  • the second-direction cold working step is a stationary machining.
  • wire drawing is performed.
  • the wire drawing process may be performed only on the primary wire drawing process, or may be performed by drawing a plurality of times such as a secondary wire drawing process.
  • the steel material may be cut to an appropriate length depending on the steel shape material to be manufactured.
  • processing is performed to compress the steel material in the length direction.
  • the stationary processing may be performed once or a plurality of times.
  • the steel material is in the direction perpendicular to the length direction of the steel material due to the drawing process and the stationary processing. And the load is received from two directions in the length direction of the steel material. In this case, cross-slip is more likely to occur, and immobile dislocations are more likely to increase in the steel material. As a result, the dislocation density in the crystal grains increases, and a large amount of strain, which is the core of the plate-shaped V precipitate, is likely to be formed in the steel material.
  • the amount of machining strain generated in the steel material by the drawing process (the amount of machining strain in the first direction) is defined as the amount of drawing strain.
  • the machining strain amount (second direction machining strain amount) generated in the steel material by the step-in machining is defined as the step-in strain amount.
  • L in the formula (2) is the length in the wire drawing direction (longitudinal direction) of the steel material after the drawing process.
  • L0 in the formula (2) is the length in the wire drawing direction (longitudinal direction) of the steel material before the drawing process.
  • L in the equation (2) is the length in the wire drawing direction (longitudinal direction) of the steel material after the installation processing process.
  • L0 in the formula (2) means the length in the wire drawing direction (longitudinal direction) of the steel material before the installation processing process.
  • the total value of the obtained drawing strain amount and the stationary strain amount is defined as the total processing strain amount (-).
  • the amount of machining strain in two different directions is applied to the steel material.
  • the amount of processing strain in the first direction is set to 0.05 or more
  • the amount of processing strain in the second direction is set to 0.05 or more
  • the total amount of processing strain is set to 0.20 or more.
  • An aging hardening process is carried out on the steel material after the cold working process.
  • the treatment temperature (° C.) in the age hardening treatment step and the holding time (minutes) at the treatment temperature are as follows.
  • Treatment temperature 500 ° C. for ⁇ A c1 point age hardening treatment step the treatment temperature if (hereinafter also referred to as age hardening treatment temperature) is 500 ° C. ⁇ A c1 point, satisfy the formula (1), and a cathode hydrogen charging method
  • the V precipitate can be precipitated in the steel material so that the amount of diffusible hydrogen in the steel body shape material when hydrogen is charged is 0.10 ppm or more. As a result, high fatigue strength and high tensile strength can be obtained in the steel profile.
  • the age hardening treatment temperature is from 500 ° C. to 1 point of Ac.
  • the preferred lower limit of the age hardening treatment temperature is 520 ° C, more preferably 540 ° C, still more preferably 560 ° C.
  • the preferred upper limit of the age hardening treatment temperature is 700 ° C., more preferably 680 ° C., and even more preferably 660 ° C.
  • Retention time 15 to 150 minutes
  • the retention time at the aging hardening treatment temperature is 15 to 150 minutes. If the holding time is 15 to 150 minutes, the formula (1) is satisfied, and the amount of diffusible hydrogen in the steel body when charged with hydrogen by the cathode hydrogen charging method is 0.10 ppm or more.
  • the V precipitate can be deposited in the steel material. As a result, high fatigue strength and high tensile strength can be obtained in the steel profile.
  • the holding time is 15 to 150 minutes.
  • the preferred lower limit of the holding time is 20 minutes, more preferably 30 minutes.
  • the preferred upper limit of the holding time is 120 minutes, more preferably 100 minutes.
  • the steel profile material of the present embodiment can be manufactured.
  • the above-mentioned manufacturing method is an example of the manufacturing method of the steel profile material of the present embodiment. Therefore, the content of each element in the chemical composition is within the range of the present embodiment, and the microstructure is composed of a polygonal ferrite having an area ratio of 20 to 90% and a hard phase having an area ratio of 10 to 80%.
  • the method for producing the steel profile is not limited to the production method described below.
  • the manufacturing method described below is a suitable manufacturing method for the steel profile material of the present embodiment.
  • the cutting process may be performed on the steel material after the aging hardening process.
  • the cutting process is an arbitrary process. When it is carried out, in the cutting process, the steel material after the age hardening treatment is cut to produce a steel body shape material having a desired shape.
  • the steel profile material of the present embodiment replaces the conventional manufacturing process (hot forging process-cutting process) with the above manufacturing process (cold working process-aging hardening process-cutting process). Or, it can be produced by a cold processing process-aging curing process). Since the hot forging step can be omitted, the yield can be improved and the productivity can be improved.
  • the steel profile material of the present embodiment will be specifically described with reference to Examples.
  • the molten steel of each test number having the chemical composition shown in Table 1-1 and Table 1-2 was produced by vacuum melting. A 150 kg ingot was produced using molten steel.
  • "-" In the "Chemical composition” column of Table 1-1 and Table 1-2 means that the corresponding element content was below the detection limit. The O content was 0.0040% or less in the steels having the test numbers shown in Table 1-1 and Table 1-2.
  • the steel material used as the material for the steel body shape material was manufactured. Specifically, the ingot was hot-worked (hot forged) to produce a round bar having a diameter of 42 mm ( ⁇ 42). The heating temperature in 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 number 76, the cooling rate after hot forging was 0.1 ° C./sec. In test numbers 77 and 78, the cooling rate after hot forging was 6.0 ° C./sec. In test number 79, the cooling rate after hot forging was 0.2 ° C./sec.
  • a cold working process was carried out on the manufactured round bar material. Specifically, the round bar material of each test number was subjected to a drawing process as a first-direction cold processing step, and then a stationary process was performed as a second-direction cold processing step.
  • the drawing strain amount and the setting strain amount in the drawing process and the setting process and the total processing strain amount are as shown in Tables 2-1 and 2-2.
  • the round bar material after the cold working process was subjected to an aging hardening process to manufacture a steel profile.
  • the age hardening treatment temperature (° C.) and the holding time (minutes) were as shown in Tables 2-1 and 2-2.
  • the microstructure of the round bar of each test number was observed by the following method. Specimens were collected from the central part including the central axis of the round bar of each test number. Of the surfaces of the test pieces, the surface perpendicular to the longitudinal direction of the round bar was used as the observation surface. The observation surface was mirror-polished. The observed surface after polishing was etched with a 3% nital corrosive solution (ethanol + 3% nitric acid solution). Arbitrary five observation fields of the etched observation surfaces were observed with a 400x optical microscope to generate photographic images. The size of the observation field was 200 ⁇ m ⁇ 200 ⁇ m.
  • Polygonal ferrites and hard phases were identified in the photographic images of each field of view by the methods described above.
  • the polygonal ferrite area ratio (%) was determined based on the total area of the polygonal ferrite obtained in the five visual fields and the total area of the five visual fields.
  • the total area ratio (%) of the hard phase (pearlite and bainite) was determined based on the total area of pearlite and bainite obtained in the five visual fields and the total area of the five visual fields.
  • the obtained polygonal ferrite area ratio (%) is shown in the "Polygonal ferrite area ratio (%)" column of the "Round bar material (steel material)” column in Table 2-1 and Table 2-2.
  • the obtained hard phase area ratio (%) is shown in “Hard phase area ratio (%)" in the "Round bar material (steel material)” column in Table 2-1 and Table 2-2.
  • VP0 was determined based on the V content ([V]) of the chemical composition of the round bar and [V in the precipitate]. The obtained VP0 is shown in the "VP0" column of the "Round steel material (steel material)" column in Table 2-1 and Table 2-2.
  • the microstructure of the steel profile of each test number was observed by the following method. Specimens were collected from the central part including the central axis of the steel profile of each test number. Of the surfaces of the test pieces, the surface perpendicular to the longitudinal direction of the steel profile was used as the observation surface. The observation surface was mirror-polished. The observed surface after polishing was etched with a 3% nital corrosive solution (ethanol + 3% nitric acid solution). Using the etched observation surface, the area ratio (%) of the polygonal ferrite area of the steel profile material and the area ratio (%) of the hard phase were obtained by the same method as for observing the microstructure of the round bar material (steel material). rice field.
  • the positions of all five observation fields were at least 3 mm deeper than the surface of the steel profile.
  • the obtained polygonal ferrite area ratio (%) is shown in the "Polygonal ferrite area ratio (%)" column of the “Steel base material” column in Table 2-1 and Table 2-2.
  • the obtained hard phase area ratio (%) is shown in “Hard phase area ratio (%)” in the “Steel base material” column in Table 2-1 and Table 2-2.
  • the amount of diffusible hydrogen in the steel profile of each test number was determined by the following method.
  • a round bar test piece having a diameter of 7 mm and a length of 40 mm was cut out from a portion including the central shaft of the steel body shape material.
  • Hydrogen was introduced into the cut out round bar test piece by using the cathode hydrogen charging method. Specifically, the round bar test piece was immersed in a 3% NaCl-3 g / LNH 4 SCN aqueous solution. Then, hydrogen was introduced into the round bar test piece by the cathode hydrogen charging method under the conditions of current density: 0.1 mA / cm 2 and energization time: 72 hours.
  • the timing at which the energization was stopped was defined as the timing at which the introduction of hydrogen into the round bar test piece was completed.
  • the amount of hydrogen in the round bar test piece was determined by using warm-up withdrawal gas chromatography. Measured by method. Specifically, the round bar test piece was heated from room temperature to 400 ° C. at a heating rate of 100 ° C./hour. The amount of hydrogen generated by the temperature rise was measured at 5-minute intervals. Based on the obtained amount of hydrogen, a hydrogen release curve as shown in FIG. 1 was obtained. Using the obtained hydrogen release curve, the cumulative amount of hydrogen released from room temperature to 350 ° C. was determined.
  • the obtained cumulative amount of hydrogen was defined as the amount of diffusible hydrogen (ppm).
  • the amount of diffusible hydrogen obtained is shown in the "Diffusible hydrogen amount (ppm)" column of the "Steel base material” column in Table 2-1 and Table 2-2.
  • the fatigue strength (bending fatigue strength) of the steel profile of each test number was measured by the following method.
  • a plurality of Ono-type rotary bending fatigue test pieces conforming to JIS Z 2274 (1978) were collected from the steel profile.
  • the central axis of the Ono-type rotary bending fatigue test piece was coaxial with the central axis of the steel profile.
  • An Ono-type rotary bending fatigue test was carried out in accordance with JIS Z 2274 (1978) at room temperature and in an air atmosphere using the Ono-type rotary bending fatigue test piece.
  • the rotational speed and 3000 rpm, the maximum stress the stress load repetition count has not broken after 107 cycles was fatigue strength (MPa).
  • the obtained fatigue strength is shown in the "fatigue strength (MPa)" column of the "steel profile” column in Table 2-1 and Table 2-2.
  • MPa fatigue strength
  • the tensile strength when the tensile strength is 845 MPa or more, it is judged that the tensile strength is high. On the other hand, when the tensile strength was less than 845 MPa, it was judged that the tensile strength was low.
  • Test results are shown in Table 2-1 and Table 2-2.
  • the content of each element in the chemical composition of the steel profile materials of test numbers 1 to 48 is within the range of this embodiment. It was inside. Further, the microstructure consisted of 20-90% polygonal ferrite and 10-80% hard phase, and the VP satisfied the formula (1). Further, the amount of diffusible hydrogen in the steel body shape material when hydrogen was charged by the cathode hydrogen charging method was 0.10 ppm or more. Therefore, the fatigue strength of the steel profiles of Test Nos. 1 to 48 was 480 MPa or more, showing high fatigue strength. Further, the tensile strength of the steel profiles of Test Nos. 1 to 48 was 845 MPa or more, showing a high tensile strength.
  • test number 49 the C content was too high. Therefore, cracks were confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.
  • test number 50 the C content was too low. Therefore, the polygonal ferrite area ratio of the steel profile was too high. Further, the VP of the steel profile did not satisfy the formula (1), and the amount of diffusible hydrogen in the steel profile when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. As a result, the fatigue strength and the tensile strength were low.
  • test number 51 the Si content was too low. Therefore, the fatigue strength and tensile strength of the steel profile were low.
  • test number 52 the Si content was too high. Therefore, the cold forging property of the round bar forged material is low, and it is not possible to manufacture the steel body shape material.
  • test numbers 53 and 54 the Mn content was too high. Therefore, cracks were confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.
  • test number 55 the V content was too high. Therefore, cracks were confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.
  • test number 56 the V content was too low. Therefore, the VP of the steel profile did not satisfy the formula (1), and the amount of diffusible hydrogen in the steel profile when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel profile were low.
  • test numbers 57 and 58 the Cr content was too high. Therefore, cracks were confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.
  • the N content was too high. Therefore, the VP0 of the round bar material, which is the material of the steel profile material, was 0.30 or more. That is, coarse V precipitates, which are the starting points of fatigue fracture, were excessively generated in the round bar material. Therefore, the amount of diffusible hydrogen in the steel profile when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel profile were low.
  • test number 60 the Ti content was too high. Therefore, cracks were confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.
  • test number 61 the Mo content was too high. Therefore, cracks were confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.
  • test numbers 62, 63 and 80 the total processing strain amount in the cold processing process was too low. Therefore, the VP of the steel profile did not satisfy the formula (1). Further, the amount of diffusible hydrogen in the steel body shape material when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel profile were low.
  • the total machining strain amount was 0.20 or more in the cold working process, but the stationary strain amount was less than 0.05. Therefore, the amount of diffusible hydrogen in the steel profile when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. As a result, the fatigue strength and tensile strength of the steel profile were low.
  • the total machining strain amount was 0.20 or more in the cold working process, but the drawing strain amount was less than 0.05. Therefore, the amount of diffusible hydrogen in the steel profile when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. As a result, the fatigue strength and tensile strength of the steel profile were low.
  • the age hardening treatment temperature was too low. Therefore, the VP of the steel profile did not satisfy the formula (1). Further, the amount of diffusible hydrogen in the steel body shape material when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel profile were low.
  • test numbers 69, 70 and 84 the age hardening treatment temperature was too high. Therefore, the amount of diffusible hydrogen in the steel profile when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. As a result, the fatigue strength and tensile strength of the steel profile were low.
  • the holding time at the age hardening treatment temperature was too short. Therefore, the VP of the steel profile did not satisfy the formula (1). Further, the amount of diffusible hydrogen in the steel body shape material when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel profile were low.
  • test numbers 72 to 75 and 86 the holding time at the age hardening treatment temperature was too long. Therefore, the amount of diffusible hydrogen in the steel profile when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel profile were low.
  • test number 76 although the content of each element in the chemical composition was within the range of this embodiment, the polygonal ferrite area ratio of the round bar material, which is the material of the steel profile material, was too high. Further, VP0 was 0.30 or more. Therefore, the polygonal ferrite area ratio of the steel profile was too high. Further, the amount of diffusible hydrogen in the steel body shape material when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel profile were low.
  • test numbers 77 and 78 although the content of each element in the chemical composition was within the range of this embodiment, the polygonal ferrite area ratio of the round bar material, which is the material of the steel profile material, was too low. Therefore, cracks were confirmed in the round bar material (steel material) during the cold working process, and the cold workability was low.
  • test number 79 although the content of each element in the chemical composition was within the range of this embodiment, the VP0 of the round bar material, which is the material of the steel profile material, was 0.30 or more. Therefore, the amount of diffusible hydrogen in the steel profile when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel profile were low.
  • test number 87 although the content of each element in the chemical composition was within the range of this embodiment, the age hardening treatment was not carried out. Therefore, the VP of the steel profile did not satisfy the formula (1). Further, the amount of diffusible hydrogen in the steel body shape material when hydrogen was charged by the cathode hydrogen charging method was less than 0.10 ppm. Therefore, the fatigue strength and tensile strength of the steel profile were low.

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