WO2020230872A1 - Bolt and steel material for bolt - Google Patents

Bolt and steel material for bolt Download PDF

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Publication number
WO2020230872A1
WO2020230872A1 PCT/JP2020/019354 JP2020019354W WO2020230872A1 WO 2020230872 A1 WO2020230872 A1 WO 2020230872A1 JP 2020019354 W JP2020019354 W JP 2020019354W WO 2020230872 A1 WO2020230872 A1 WO 2020230872A1
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WO
WIPO (PCT)
Prior art keywords
less
bolt
amount
hydrogen
mass
Prior art date
Application number
PCT/JP2020/019354
Other languages
French (fr)
Japanese (ja)
Inventor
真吾 山▲崎▼
美百合 梅原
Original Assignee
日本製鉄株式会社
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Publication date
Application filed by 日本製鉄株式会社 filed Critical 日本製鉄株式会社
Priority to JP2021519494A priority Critical patent/JP7164032B2/en
Priority to KR1020217036401A priority patent/KR102599767B1/en
Publication of WO2020230872A1 publication Critical patent/WO2020230872A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0093—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for screws; for bolts
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C29/00—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides
    • C22C29/02—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on carbides or carbonitrides
    • C22C29/04—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on carbides or carbonitrides based on carbonitrides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

  • This disclosure relates to bolts and steel materials for bolts.
  • alloy steels for machine structural use such as SCM435 and SCM440 specified in JIS G 4053: 2016 are used.
  • the strength of bolts is adjusted by quenching-tempering after forming alloy steel for machine structure into a predetermined shape.
  • the carbon content of the steel material may be increased or the tempering temperature may be lowered.
  • Delayed fracture is a phenomenon in which a part placed under static stress suddenly breaks brittlely after a certain period of time. Delayed fracture is a phenomenon caused by the intrusion of hydrogen, and the higher the strength of the steel material, the lower the critical value of the amount of hydrogen intrusion leading to delayed fracture.
  • the bolt is used outdoors, especially in an environment where seawater, snowmelt salt, etc. come flying, the amount of hydrogen invading increases due to salt adhesion, and the risk of delayed fracture increases.
  • Patent Document 1 discloses bolts and steel materials having a tensile strength of 1617 MPa or more and excellent delayed fracture resistance, utilizing precipitation strengthening by Mo carbides and W carbides that precipitate during tempering.
  • Patent Document 2 discloses a method for producing a high-strength bolt having a tensile strength of 1600 MPa or more, which advantageously prevents hydrogen embrittlement typified by delayed fracture and has excellent delayed fracture resistance.
  • Patent Document 3 discloses a high-strength bolt having a good strength of delayed fracture resistance of 1500 MPa or more and a method for improving the delayed fracture characteristic, utilizing alloy carbides of V, Mo, Ti and Nb. .
  • Patent Document 4 discloses a high-strength tempered steel having excellent delayed fracture resistance and a method for producing the same, utilizing alloy carbides of V, Mo, Ti and Nb.
  • Patent Document 1 Japanese Patent Application Laid-Open No. 2001-032044
  • Patent Document 2 Japanese Patent Application Laid-Open No. 2007-31736
  • Patent Document 3 Japanese Patent Application Laid-Open No. 2006-131990
  • Patent Document 4 Japanese Patent Application Laid-Open No. 2006-45670
  • the subject of the present disclosure is a bolt that exhibits excellent delayed fracture resistance at a strength level of 1600 MPa or more, which generally has a very high risk of delayed fracture, and a steel material for bolts as a material thereof. To provide.
  • the inventors have adopted a steel material having a predetermined chemical composition as a material for bolts and having Mo, V, W and Cr contents satisfying the following formulas (1), (2) and (3). doing, it found that M 2 C-type carbide as a trap site of hydrogen is dispersed in the bolt. 2V / (Mo + 0.5W) ⁇ 0.20 ... (1) 0.10 ⁇ (2V + 0.5W) / Mo ⁇ 0.40 ... (2) 0.10 ⁇ 2Cr / Mo ⁇ 0.35 ⁇ ⁇ ⁇ (3) As a result, the inventors have found that a bolt having high strength and excellent delayed fracture resistance can be obtained. The above problem is solved by the following means.
  • the composition is by mass% C: 0.35 to 0.50%, Si: 0.02 to 0.10%, Mn: 0.20 to 0.80%, Mo: 1.50 to 5.00%, W: 0 to 1.00%, V: 0 to 0.20%, Cr: 0.20 to 0.50%, Al: 0.010 to 0.100%, N: 0.0010 to 0.0150%, P: 0.015% or less, and S: 0.015% or less, Contains, The rest consists of Fe and impurities Moreover, the following formula (1), the following formula (2), and the following formula (3) are satisfied. Tensile strength is 1600 MPa or more. bolt. 2V / (Mo + 0.5W) ⁇ 0.20 ...
  • An M 2 C type carbide having a length of 5 nm or more and containing at least 70 atomic% or more of Mo, Cr, V and W with respect to M (metal element) is a unit area of the M 2 C type carbide.
  • ⁇ 5> In a solution at room temperature to which 3.0 g of ammonium thiocyanate was added per 1 L of a 3.0 mass% sodium chloride aqueous solution, the current density was 0.03 mA / cm 2 for 24 hours, followed by hydrogen permeation prevention plating.
  • ⁇ 6> In a solution at room temperature to which 3.0 g of ammonium thiocyanate was added per 1 L of a 3.0 mass% sodium chloride aqueous solution, the cathode hydrogen was charged at a current density of 0.2 mA / cm 2 for 72 hours and allowed to stand at room temperature for 48 hours.
  • the bolt according to any one of ⁇ 1> to ⁇ 5>, wherein the amount of hydrogen trapped later is 3.0 ppm or more.
  • a steel material for bolts which is the material of the bolt according to any one of ⁇ 1> to ⁇ 6>.
  • the content of each element in the chemical composition may be referred to as “elemental amount”.
  • the content of C may be expressed as the amount of C.
  • the numerical range represented by using “-” means a range including the numerical values before and after "-” as the lower limit value and the upper limit value.
  • the numerical range when "greater than” or “less than” is added to the numerical values before and after “to” means a range in which these numerical values are not included as the lower limit value or the upper limit value.
  • the term “process” is included in this term not only as an independent process but also as long as the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes.
  • C 0.35 to 0.50% C is an element that improves the strength of steel and increases the strength of bolts. If the amount of C is less than 0.35%, the strength required for bolts cannot be obtained. On the other hand, if the amount of C is more than 0.50%, a large amount of alloy carbide remains undissolved during quenching heating, the strength becomes low at a predetermined tempering temperature, and the amount of alloy carbide precipitated during tempering is relatively reduced. Therefore, the hydrogen trapping ability is also low. Therefore, the amount of C is set to 0.35 to 0.50%. The preferable amount of C is 0.38 to 0.45%, and the more preferable amount of C is 0.40 to 0.43%.
  • the delayed fracture resistance can be improved.
  • the amount of Si is set to 0.10% or less in order to increase the delayed fracture resistance.
  • the amount of Si is set to 0.02 to 0.10%.
  • the preferable amount of Si is 0.02 to 0.08%, and the more preferable amount of Si is 0.03 to 0.06%.
  • Mn 0.20 to 0.80% Mn combines with S to form MnS and prevents grain boundary segregation of S. It also has the effect of improving hardenability.
  • the amount of Mn is less than 0.20%, the grain boundary segregation of S becomes large and the delayed fracture resistance decreases.
  • the amount of Mn exceeds 0.80%, the cold workability at the time of processing into a part shape is lowered, and shrinkage is likely to occur. Therefore, the amount of Mn is set to 0.20 to 0.80%.
  • the preferable amount of Mn is 0.30 to 0.70%, and the more preferable amount of Mn is 0.40 to 0.60%.
  • Mo 1.50 to 5.00% W: 0 to 1.00%
  • V 0 to 0.20%
  • Mo, W and V are important elements in the present disclosure.
  • Mo, and W is an element which forms a M 2 C type carbides.
  • V is an element which forms a MC type carbide, with Mo, By including in combination a V proper amount, M 2 C-type carbide containing V precipitates. It should be noted that these M 2 C type carbides, and Mo, and Cr, and at least one of W and V, the carbide containing appropriate.
  • a large amount of fine M 2 C type carbide can be precipitated by quenching the steel from the austenite region and then tempering it at a high temperature of 570 to 690 ° C.
  • fine M 2 C type carbides are precipitated, it is possible to increase the strength of steel by precipitation strengthening.
  • the fine M 2 C type carbide functions as a hydrogen trap site and can improve the delayed fracture resistance.
  • a trap hydrogen, said fixed by M 2 C-type carbide is hydrogen that can not be freely moved in the steel.
  • the amount of Mo is 1.50 to 5.00%, the amount of W is 0 to 1.00%, and the amount of V is 0 to 0.20%.
  • the preferable amount of Mo is 2.00 to 4.00%, the preferable amount of W is 0.02 to 1.00%, and the preferable amount of V is 0.10 to 0.17%.
  • the more preferable amount of Mo is 2.50 to 3.50%, the more preferable amount of W is 2.70 to 3.20%, and the more preferable amount of V is 0.12 to 0.15%.
  • the amount of Cr is set to 0.20 to 0.50%.
  • the preferable amount of Cr is 0.20 to 0.30%, and the more preferable amount of Cr is 0.24 to 0.28%.
  • the content of Cr is required to satisfy the following formula (3). 0.10 ⁇ 2Cr / Mo ⁇ 0.35 ⁇ ⁇ ⁇ (3)
  • the contents (mass%) of Cr and Mo contained in the bolt are substituted for Cr and Mo, respectively.
  • Al 0.010 to 0.100%
  • Al is an element that functions as an antacid and also forms a nitride to suppress the coarsening of austenite crystal grains during quenching and heating. In order to obtain these effects, it is necessary to contain 0.010% or more of Al. On the other hand, when the Al content exceeds 0.100%, coarse oxide-based inclusions remain in the steel and serve as a fracture starting point of the bolt. Therefore, the amount of Al is set to 0.010 to 0.100%.
  • the preferable Al amount is 0.012 to 0.050%, and the more preferable Al amount is 0.025 to 0.035%.
  • N is an element that forms a nitride or carbonitride and suppresses coarsening of austenite crystal grains during quenching and heating.
  • the amount of N needs to be 0.0010% or more.
  • the amount of N exceeds 0.0150%, coarse nitrides and carbonitrides are generated and serve as the starting point of fracture. Therefore, the amount of N is set to 0.0010 to 0.0150%.
  • the preferable N amount is 0.0020 to 0.0100%, and the more preferable N amount is 0.0030 to 0.0050%.
  • P 0.015% or less
  • P is an impurity.
  • the amount of P is preferably as low as possible. P segregates at the austenite grain boundaries. If the amount of P exceeds 0.015%, the old austenite grain boundaries after quenching and tempering become brittle, causing grain boundary cracking. Therefore, it is necessary to limit the amount of P to the range of 0.015% or less.
  • the upper limit of the preferable amount of P is 0.012%.
  • P is an impurity element, P may be contained in the bolt in an amount of more than 0% as long as it is within the above range. However, from the viewpoint of reducing the cost of removing P, the lower limit of the amount of P may be 0.005% or more.
  • S 0.015% or less
  • S is an impurity.
  • the amount of S is preferably as low as possible.
  • S exists as Mn sulfide in the bolt. Mn sulfide generates hydrogen sulfide by a chemical reaction when the steel surface is corroded. When this hydrogen sulfide is decomposed to generate hydrogen, hydrogen invades into the steel and the delayed fracture resistance is lowered. Further, Mn sulfide serves as a fracture starting point. Therefore, it is necessary to limit the amount of S to the range of 0.015% or less.
  • the upper limit of the preferable amount of S is 0.012%.
  • S is an impurity element, but if it is within the above range, S may be contained in the bolt in an amount of more than 0%. However, from the viewpoint of reducing the cost of removing S, the lower limit of the amount of S may be 0.005% or more.
  • the bolt according to the present embodiment may contain at least one selected from the group consisting of Ti, Nb, B, Ni, Cu, W, REM, Sn, and Bi as an optional element. Specifically, these arbitrary elements may be contained in the range of 0% to the upper limit of each element described later.
  • Ti 0.100% or less
  • Ti is an element that combines with N and C in a bolt to form a carbonitride. This carbonitride pins the austenite grain boundaries to prevent texture coarsening.
  • Ti may be contained in an amount of 0.100% or less.
  • Ti when Ti is contained in an amount of more than 0.100%, the cold workability when processing into a part shape is lowered due to the increase in material hardness.
  • the amount of Ti is preferably 0.100% or less, more preferably more than 0% to 0.100%, still more preferably 0.005 to 0.050%.
  • Nb 0.100% or less
  • Nb is an element that combines with N and C in a bolt to form a carbonitride. This carbonitride pins the austenite grain boundaries and prevents texture coarsening.
  • Nb may be contained in an amount of 0.100% or less.
  • Nb is contained in an amount of more than 0.100%, the cold workability when processing into a part shape is lowered due to the increase in material hardness.
  • the amount of Nb is preferably 0.100% or less, more preferably more than 0% to 0.100%, still more preferably 0.005 to 0.050%.
  • B 0.0050% or less B enhances the hardenability of steel even if it is slightly dissolved in austenite.
  • B may be contained in the bolt in order to efficiently obtain martensite during carburizing and quenching.
  • the amount of B is preferably 0.0050% or less, more preferably more than 0 to 0.0050%, still more preferably 0.0007 to 0.0030%.
  • Ni 0.20% or less
  • Ni is an element that enhances corrosion resistance and toughness, and may be contained in bolts.
  • the upper limit of the amount of Ni is preferably 0.20%.
  • the lower limit of the amount of Ni is preferably 0.01%.
  • Cu 0.20% or less Cu is an element that enhances corrosion resistance and may be contained in bolts. On the other hand, if the Cu amount exceeds 0.20%, the hot ductility decreases, so the upper limit of the Cu amount is preferably 0.20%. On the other hand, the lower limit of the amount of Cu is preferably 0.01%.
  • REM 0.020% or less REM (rare earth element) is a general term for a total of 17 elements, including 15 elements from lanthanum with atomic number 57 to lutetium with atomic number 71, scandium with atomic number 21 and yttrium with atomic number 39. Is.
  • the total amount of the 17 elements is preferably 0.020% or less, more preferably more than 0% to 0.020%, still more preferably 0.005% to 0.015%.
  • Sn 0.20% or less Sn is an element that enhances corrosion resistance and may be contained in bolts.
  • the upper limit of the Sn amount is preferably 0.20%.
  • the lower limit of the Sn amount is preferably 0.005%, more preferably 0.01%.
  • Bi 0.20% or less Bi is an element that enhances workability and may be contained in bolts.
  • the upper limit of the amount of Bi is preferably 0.20%.
  • the lower limit of the Bi amount is preferably 0.005%, more preferably 0.01%.
  • the bolt according to the present embodiment may contain at least one selected from the group consisting of the following elements as an optional element. Specifically, these arbitrary elements may be contained in the range of 0% to the upper limit of each element described later. Even if these arbitrary elements are contained in the bolt in the range described later, the characteristics of the bolt are not affected. Pb: 0.05% or less Cd: 0.05% or less Co: 0.05% or less Zn: 0.05% or less Ca: 0.02% or less Zr: 0.02% or less
  • the rest of the chemical composition of the bolt in this embodiment consists of Fe and impurities.
  • the impurity means an ore used as a raw material for steel, scrap, or an element mixed from the environment of the manufacturing process.
  • M 2 C type carbide Bolt according to the present embodiment, the length of 5nm or more M 2 C type carbides is preferably present more than 10 unit area 0.01 [mu] m 2 per.
  • Fine M 2 C type carbides precipitated in the tempering process (carbide containing Mo and Cr and W and at least one of V) is, VC, compared with a Mo 2 C, etc., high hydrogen trapping ability, the delayed fracture resistance Contribute to improvement.
  • fine M 2 C type carbides are to M (metal element), an M 2 C type carbide containing at least one and a total of 70 atomic% or more of Mo, Cr, and V and W.
  • fine M 2 C type carbides (Mo, Cr, W, V) 2 C, (Mo, Cr, W) 2 C, and corresponds (Mo, Cr, V) 2 C is ..
  • These M 2 C type carbides have higher hydrogen trapping ability than VC, Mo 2 C and the like, and contribute to the improvement of delayed fracture resistance. Therefore, the above length 5nm of M 2 C-type carbide, it is preferable to present a predetermined amount.
  • more length 5nm of M 2 C type carbides number density (number per unit area 0.01 [mu] m 2 or more in length 5nm of M 2 C type carbides present per) is preferably 10 or more.
  • the number density of M 2 C type carbides is 15 or more, more preferably unit area 0.01 [mu] m 2 per more 20 unit area 0.01 [mu] m 2 per more preferably.
  • the upper limit of the number density of M 2 C-type carbide from the viewpoint of suppressing the reduction in elongation and toughness for example, to 100 or less per unit area 0.01 [mu] m 2.
  • Measurements of the number density of M 2 C type carbides a thin film specimen prepared by a thin film method, measured by transmission electron microscopy.
  • Measurement of the components of M 2 C type carbides the test pieces were produced by extraction replica method, performed using the energy dispersive X-ray spectrometer (EDS) with a transmission electron microscope (TEM). Specifically, it is as follows.
  • a part located at a depth of 2 mm from the surface of the bolt and having a surface parallel to the surface of the bolt (hereinafter, also referred to as "measurement surface") is sampled, and a thin film test is performed by the thin film method. Specimens are prepared by the piece and extraction replica method.
  • the production of the thin film test piece by the thin film method is as follows. First, the base material is cut to a thickness of 0.5 mm by a precision cutting machine. Next, using emery paper of P320 to 1200, cutting and polishing is performed from both sides to a thickness of 60 ⁇ m, and a sample of 3 mm ⁇ is punched out. Then, double-sided jet electropolishing is performed, and electropolishing is performed until a hole is formed in the center to obtain a thin film test piece for TEM observation.
  • Electropolishing is performed with Tenupol, 100 ml perchloric acid-800 ml glacial acetic acid solution-100 ml methanol is used as the electrolytic polishing solution, and the electrolytic polishing conditions are 30V and 0.1A.
  • the preparation of the test piece by the extraction replica method is as follows. First, the measurement surface of the sample collected from the steel member is electropolished. The measurement surface of the sample after electropolishing is electrolyzed at a potential of ⁇ 200 mV using a 10% acetylacetone-1% tetramethylammonium chloride (TMAC) -methanol solution. Thus, M 2 C-type carbide is exposed from the measuring surface of the harvest. The energizing time is 30 to 60 sec.
  • An extraction replica film (test piece by the extraction replica method) is obtained by immersing the carbon vapor deposition film in a methyl acetate solution to dissolve the acetyl cellulose film and scooping it up with a Cu mesh having a diameter of 3 mm.
  • any field of view of the thin film test piece (the measurement surface thereof) can be magnified at a magnification of 400,000 times (observation area 0.25 ⁇ m ⁇ 0.25 ⁇ m).
  • M 2 C type carbides are identified by electron diffraction pattern analysis. After that, the length and number of all M 2 C type carbides existing in the region of 0.1 ⁇ m ⁇ 0.1 ⁇ m in the center of the observation screen are measured, and the number of M 2 C type carbides having a length of 5 nm or more is determined.
  • the average value of the five field as "number density of M 2 C type carbides".
  • the length of M 2 C type carbides the maximum length of the M 2 C type carbides observed.
  • the TEM observation is performed by FE-TEM at an accelerating voltage of 200 kV.
  • the chemical components of the M 2 C-type carbide is measured as follows.
  • An arbitrary visual field (visual field having an observation area of 0.5 ⁇ m ⁇ 0.5 ⁇ m) of the extracted replica membrane (the measurement surface thereof) as a test piece is observed at a magnification of 200,000 times.
  • the number of measured pieces is 5, and the average value of these is used as the metal element concentration.
  • the analysis of the electron diffraction pattern of TEM and the analysis by EDS are carried out by FE-TEM at an accelerating voltage of 200 kV.
  • the tensile strength measured by collecting a tensile test piece from the bolt is 1600 MPa or more.
  • the tensile strength of a bolt is a value measured according to JIS Z 2241: 2011.
  • the tensile strength of the bolt is measured by collecting a test piece from the bolt as follows. A No. 14A test piece having a parallel portion diameter of 50% of the bolt diameter is cut out from the bolt shaft portion, and a tensile test is performed in the air at room temperature (25 ° C.) to determine the tensile strength.
  • the cathode hydrogen was charged at a current density of 0.2 mA / cm 2 for 72 hours in a room temperature solution containing 3.0 g of ammonium thiocyanate per 1 L of a 3.0 mass% sodium chloride aqueous solution.
  • the amount of trapped hydrogen after standing at room temperature (25 ° C.) for 48 hours is preferably 3.0 ppm or more. If the amount of trapped hydrogen is less than 3.0 ppm, the hydrogen that has entered the bolt may diffuse and accumulate at the former austenite grain boundaries, increasing the risk of delayed fracture. Therefore, the amount of trap hydrogen is preferably 3.0 ppm or more.
  • the amount of trapped hydrogen is measured by a heated hydrogen analysis method using a gas chromatograph.
  • the amount of hydrogen released from the sample from room temperature (25 ° C.) to 400 ° C. at a heating rate of 100 ° C./hour is defined as the trap hydrogen amount.
  • the trap hydrogen amount is measured on a round bar test piece having a diameter of 7 mm and a length of 70 mm (a round bar test piece for investigating the amount of trap hydrogen) collected from a bolt.
  • a round bar test piece of the above size cannot be collected, a round bar test piece with a diameter of 5 mm and a length of 20 mm is used instead, the same hydrogen charge and standing are performed, and the same temperature rise analysis is performed to obtain a hydrogen trap amount. May be measured.
  • the bolt according to this embodiment Since the bolt according to this embodiment is used in a real environment, it is preferable that the bolt has sufficient delayed fracture resistance. Therefore, in the bolt according to the present embodiment, the current density is 0.03 mA / cm 2 in a room temperature (25 ° C.) solution containing 3.0 g of ammonium thiocyanate per 1 L of a 3.0 mass% sodium chloride aqueous solution. After charging with cathode hydrogen for 24 hours, hydrogen permeation prevention plating is applied, and after leaving for 96 hours, when a constant load of 0.9 times the tensile strength is applied, the time until fracture is 100 hours or more. Is preferable. Here, the hydrogen permeation prevention plating is performed to confine hydrogen in the bolt, and hot dip galvanizing is performed.
  • the delayed fracture strength was measured on a round bar test piece (delayed fracture test piece) with a notch (notch diameter 4.2 mm, angle 60 °) with a diameter of 7 mm and a length of 70 mm collected from a bolt. To do. However, if a round bar test piece having the above size cannot be collected, a round bar test piece with a notch (notch diameter 3.0 mm, angle 60 °) having a diameter of 5 mm may be used instead.
  • the length is not particularly limited as long as it can be chucked.
  • the bolt steel material according to this embodiment is a steel material that is a material for bolts according to this embodiment.
  • the bolt steel material according to the present embodiment has the same chemical composition as the bolt according to the present embodiment.
  • the molten steel is cast into an ingot or a slab.
  • the cast ingot or slab is finished into a steel material having a required rough shape such as a round bar by hot working such as hot rolling, hot extrusion, and hot forging.
  • the steel material is subjected to wire drawing, annealing, cold working, screw rolling, etc. to form a predetermined bolt shape.
  • Annealing or spheroidizing annealing may be performed multiple times in the middle of the plurality of cold workings. It is also possible to include hot working in the molding process.
  • the quenching heating temperature is preferably 930 to 1050 ° C. Further, the holding time at the quenching heating temperature is preferably 30 to 90 minutes.
  • tempering In order to improve the delayed fracture resistance, it is necessary to perform tempering after performing the above quenching treatment. In the present disclosure, it is necessary to limit the tempering temperature to 570 to 690 ° C.
  • the tempering temperature is limited to 570 to 690 ° C.
  • the preferred range of tempering temperature is 590 to 660 ° C.
  • the holding time at the tempering temperature is preferably 30 to 90 minutes, and the tempering cooling rate is preferably 50 to 100 ° C./s.
  • the bolt according to this embodiment is manufactured.
  • the tensile strength, the amount of trap hydrogen, and the amount of delayed fracture limit hydrogen are optimized by subjecting a steel material having an optimum chemical composition to optimum quenching and tempering. It is a thing.
  • tempering After oil quenching, tempering was performed at the temperatures shown in Table 2. The holding time at the tempering temperature was 60 minutes, and the cooling after tempering was air cooling (cooling rate 10 ° C./s).
  • a round bar having a diameter of 7 mm and a length of 70 mm was collected from the round bar having a diameter of 20 mm and a length of 300 mm after the above quenching and tempering treatment, and used as a round bar test piece for investigating the amount of trapped hydrogen.
  • Tensile strength (TS) The tensile strength was measured as described above. Specifically, using the tensile test piece prepared by the above procedure, a tensile test was conducted in the air at room temperature (25 ° C.) in accordance with JIS Z 2241: 2011 to determine the tensile strength.
  • the amount of trap hydrogen was measured as described above. Specifically, at room temperature (25 ° C.), 3.0 g of ammonium thiocyanate was added to 1 L of a 3.0 mass% sodium chloride aqueous solution to a round bar test piece having a diameter of 7 mm and a length of 70 mm prepared by the above procedure. Cathode hydrogen charging was carried out in the solution of the above at a current density of 0.2 mA / cm 2 for 72 hours. Then, it was allowed to stand at room temperature (25 ° C.) for 48 hours. Then, using a gas chromatograph, the temperature was raised from room temperature (25 ° C.) to 400 ° C. at a heating rate of 100 ° C./hour, and the amount of hydrogen released from the round bar test piece was measured.
  • the delayed fracture strength test was measured as described above. Specifically, a test piece having a ⁇ 7 mm ⁇ 70 mm notch (notch ⁇ 4.2 mm, angle 60 °) and delayed fracture resistance prepared by the above procedure is used per 1 L of 3.0 mass% sodium chloride aqueous solution. In a solution at room temperature (25 ° C.) to which 3.0 g of ammonium thiocyanate was added, the cathode was hydrogen-charged at a current density of 0.03 mA / cm 2 for 24 hours, then subjected to hydrogen permeation prevention plating with Zn and left for 96 hours. After that, a constant load 0.9 times the tensile strength was applied, and the time until fracture was measured. If it did not break for 100 hours, the test was terminated.
  • Table 2 shows the results of the number density of M 2 C type carbides, tensile strength (TS), amount of trap hydrogen, and presence / absence of delayed fracture.
  • TS tensile strength
  • the underlined values in Table 2 indicate that the values are outside the scope of the present disclosure.
  • the symbol "-" in Table 2 means that the test was not performed.
  • the disclosure examples 16 and 17 were manufactured under manufacturing conditions that satisfy the composition requirements of the present disclosure, but the quenching conditions are slightly out of the preferable range. Disclosure Example 17 is manufactured under production conditions in which the quenching temperature is higher than that of the other Disclosure Examples, and its strength is slightly higher than that of the other Disclosure Examples. Therefore, the other disclosed examples are relatively superior in terms of strength-ductility balance. Further, the disclosure example 16 is manufactured under production conditions in which the quenching temperature is lower than that of the other disclosure examples, and the other disclosure examples are relatively superior in terms of strength.

Abstract

Provided are a bolt which has a steel composition satisfying expressions (1), (2), and (3) and which has a tensile strength of 1,600 MPa or greater, and a steel material as a raw material for the bolt.　Expression (1): 2 V/(Mo + 0.5 W) ≤ 0.20 Expression (2): 0.10 ≤ (2 V + 0.5 W)/Mo ≤ 0.40 Expression (3): 0.10 ≤ 2 Cr/Mo ≤ 0.35 The content of Cr, Mo, V, and W (mass%) in the steel for the bolt is respectively substituted into Cr, Mo, V, and W in expressions (1), (2), and (3).

Description

ボルト、及びボルト用鋼材Bolts and steel materials for bolts
　本開示は，ボルト、及びボルト用鋼材に関するものである。 This disclosure relates to bolts and steel materials for bolts.
　自動車及び産業機械の高性能化、自動車及び産業機械の軽量化、又は土木建築構造物の大型化に伴い、ボルトの高強度化が要求されている。
　ボルトには、ＪＩＳ Ｇ　４０５３：２０１６で規定されたＳＣＭ４３５、ＳＣＭ４４０などの機械構造用合金鋼が用いられる。ボルトは、機械構造用合金鋼を所定の形状に成形後、焼入れ－焼戻し処理で強度を調整する。
　ボルトを高強度化するためには、鋼材の炭素量を高める、あるいは焼戻し温度を低くすればよい。 Higher strength of bolts is required with higher performance of automobiles and industrial machines, weight reduction of automobiles and industrial machines, and increase in size of civil engineering and building structures.
For the bolts, alloy steels for machine structural use such as SCM435 and SCM440 specified in JIS G 4053: 2016 are used. The strength of bolts is adjusted by quenching-tempering after forming alloy steel for machine structure into a predetermined shape.
In order to increase the strength of the bolt, the carbon content of the steel material may be increased or the tempering temperature may be lowered.
　しかしながら、引張強さが１２００ＭＰａを超えるようなボルトでは、水素脆化の一種である遅れ破壊が問題となる。遅れ破壊は、静的応力下に置かれた部品が、ある時間経過後に突然、脆性的に破壊する現象である。
　遅れ破壊は、水素の侵入に起因する現象であり、鋼材の強度が高くなるほど、遅れ破壊に至る水素侵入量の臨界値が低下する。
　ボルトが屋外、特に、海水、融雪塩などが飛来する環境で使用される場合には、塩分付着によって水素侵入量が多くなり、遅れ破壊の危険性が高まる。 However, for bolts having a tensile strength of more than 1200 MPa, delayed fracture, which is a type of hydrogen embrittlement, becomes a problem. Delayed fracture is a phenomenon in which a part placed under static stress suddenly breaks brittlely after a certain period of time.
Delayed fracture is a phenomenon caused by the intrusion of hydrogen, and the higher the strength of the steel material, the lower the critical value of the amount of hydrogen intrusion leading to delayed fracture.
When the bolt is used outdoors, especially in an environment where seawater, snowmelt salt, etc. come flying, the amount of hydrogen invading increases due to salt adhesion, and the risk of delayed fracture increases.
　そこで、従来から、耐遅れ破壊性に優れたボルトが検討されている。
　例えば、特許文献１には、焼もどし時に析出するＭｏ炭化物、Ｗ炭化物による析出強化を活用した、引張強さが１６１７ＭＰａ以上の、耐遅れ破壊特性に優れたボルトおよび鋼材が開示されている。
　また、特許文献２には、引張強度１６００ＭＰａ以上の、遅れ破壊に代表される水素脆化を有利に防止する、耐遅れ破壊特性に優れた高強度ボルトの製造方法が開示されている。
　また、特許文献３には、Ｖ、Ｍｏ、ＴｉおよびＮｂの合金炭化物を活用した、耐遅れ破壊特性の良好な強度が１５００ＭＰａ以上の高強度ボルトおよびその耐遅れ破壊特性向上方法が開示されている。
　また、特許文献４には、Ｖ、Ｍｏ、ＴｉおよびＮｂの合金炭化物を活用した、耐遅れ破壊特性に優れた高強度調質鋼およびその製造方法が開示されている。 Therefore, conventionally, bolts having excellent delayed fracture resistance have been studied.
For example, Patent Document 1 discloses bolts and steel materials having a tensile strength of 1617 MPa or more and excellent delayed fracture resistance, utilizing precipitation strengthening by Mo carbides and W carbides that precipitate during tempering.
Further, Patent Document 2 discloses a method for producing a high-strength bolt having a tensile strength of 1600 MPa or more, which advantageously prevents hydrogen embrittlement typified by delayed fracture and has excellent delayed fracture resistance.
Further, Patent Document 3 discloses a high-strength bolt having a good strength of delayed fracture resistance of 1500 MPa or more and a method for improving the delayed fracture characteristic, utilizing alloy carbides of V, Mo, Ti and Nb. ..
Further, Patent Document 4 discloses a high-strength tempered steel having excellent delayed fracture resistance and a method for producing the same, utilizing alloy carbides of V, Mo, Ti and Nb.
　　特許文献１：特開２００１－０３２０４４号公報
　　特許文献２：特開２００７－３１７３６号公報
　　特許文献３：特開２００６－１３１９９０号公報
　　特許文献４：特開２００６－４５６７０号公報 Patent Document 1: Japanese Patent Application Laid-Open No. 2001-032044 Patent Document 2: Japanese Patent Application Laid-Open No. 2007-31736 Patent Document 3: Japanese Patent Application Laid-Open No. 2006-131990 Patent Document 4: Japanese Patent Application Laid-Open No. 2006-45670
　最近は、特許文献１～４のボルトよりも、さらに耐遅れ破壊特性に優れたボルトが求められている。
　本開示の課題は、一般的に遅れ破壊が生じる危険性が非常に高い、引張強さが１６００ＭＰａ以上の強度レベルにおいて、優れた耐遅れ破壊特性を示すボルト、およびその素材となるボルト用鋼材を提供することにある。 Recently, there is a demand for bolts having better delayed fracture resistance than the bolts of Patent Documents 1 to 4.
The subject of the present disclosure is a bolt that exhibits excellent delayed fracture resistance at a strength level of 1600 MPa or more, which generally has a very high risk of delayed fracture, and a steel material for bolts as a material thereof. To provide.
　発明者らは、ボルトの素材として所定の化学組成を有し、かつ、Ｍｏ、Ｖ、ＷおよびＣｒの含有量が以下の式（１）、（２）、及び（３）を満たす鋼材を採用することで、水素のトラップサイトとなるＭ２Ｃ型炭化物がボルト中に分散することを見出した。
　２Ｖ／（Ｍｏ＋０．５Ｗ）≦０．２０　　　　　・・・（１）
　０．１０≦（２Ｖ＋０．５Ｗ）／Ｍｏ≦０．４０・・・（２）
　０．１０≦２Ｃｒ／Ｍｏ≦０．３５　　　　　　・・・（３）
　その結果、発明者らは、高強度で、かつ優れた耐遅れ破壊特性を有するボルトが得られることを見出した。
　上記課題は、以下の手段により解決される。 The inventors have adopted a steel material having a predetermined chemical composition as a material for bolts and having Mo, V, W and Cr contents satisfying the following formulas (1), (2) and (3). doing, it found that M 2 C-type carbide as a trap site of hydrogen is dispersed in the bolt.
2V / (Mo + 0.5W) ≤ 0.20 ... (1)
0.10 ≦ (2V + 0.5W) / Mo ≦ 0.40 ... (2)
0.10 ≦ 2Cr / Mo ≦ 0.35 ・ ・ ・ (3)
As a result, the inventors have found that a bolt having high strength and excellent delayed fracture resistance can be obtained.
The above problem is solved by the following means.
＜１＞
組成が、質量％で、
Ｃ　：０．３５～０．５０％、
Ｓｉ：０．０２～０．１０％、
Ｍｎ：０．２０～０．８０％、
Ｍｏ：１．５０～５．００％、
Ｗ　：０～１．００％、
Ｖ　：０～０．２０％、
Ｃｒ：０．２０～０．５０％、
Ａｌ：０．０１０～０．１００％、
Ｎ　：０．００１０～０．０１５０％、
Ｐ　：０．０１５％以下、および
Ｓ　：０．０１５％以下、
を含有し、
残部がＦｅ及び不純物からなり、
かつ、下記式（１）、下記式（２）、および下記式（３）を満たし、
引張強さが、１６００ＭＰａ以上である、
ボルト。
　２Ｖ／（Ｍｏ＋０．５Ｗ）≦０．２０　　　　　・・・（１）
　０．１０≦（２Ｖ＋０．５Ｗ）／Ｍｏ≦０．４０・・・（２）
　０．１０≦２Ｃｒ／Ｍｏ≦０．３５　　　　　　・・・（３）
但し、式（１）、（２）、（３）において、Ｃｒ、Ｍｏ、Ｖ、およびＷには、それぞれボルトが含有するＣｒ、Ｍｏ、Ｖ、およびＷの含有量（質量％）が代入され、Ｖ又はＷが含まれないときはＶ又はＷに０が代入される。
＜２＞
質量％で、
Ｔｉ：０．１００％以下、
Ｎｂ：０．１００％以下、
Ｂ　：０．００５０％以下、
Ｎｉ：０．２０％以下、
Ｃｕ：０．２０％以下、
ＲＥＭ：０．０２０％以下、
Ｓｎ：０．２０％以下、および
Ｂｉ：０．２０％以下
よりなる群から選択される少なくとも１種をさらに含有する、請求項１に記載のボルト。
＜３＞
質量％で、
Ｐｂ：０．０５％以下、
Ｃｄ：０．０５％以下、
Ｃｏ：０．０５％以下、
Ｚｎ：０．０５％以下、
Ｃａ：０．０２％以下、および
Ｚｒ：０．０２％以下、
よりなる群から選択される少なくとも１種をさらに含有する、請求項１又は請求項２に記載のボルト。
＜４＞
　長さ５ｎｍ以上のＭ２Ｃ型炭化物であって、Ｍ（金属元素）に対し、ＭｏとＣｒとＶおよびＷの少なくとも一方とを合計で７０原子％以上含むＭ２Ｃ型炭化物が、単位面積０．０１μｍ２当たり１０個以上存在する、請求項１～請求項３のいずれか１項に記載のボルト。
＜５＞
　３．０質量％の塩化ナトリウム水溶液１Ｌ当たり３．０ｇのチオシアン酸アンモニウムを添加した室温の溶液中で、電流密度０．０３ｍＡ／ｃｍ２で２４時間陰極水素チャージした後、水素透過防止めっきを施し、９６時間放置した後、引張強さの０．９倍の一定荷重を負荷した時の、破断に至るまで１００時間以上である請求項１～請求項４のいずれか１項に記載のボルト。
＜６＞
　３．０質量％の塩化ナトリウム水溶液１Ｌ当たり３．０ｇのチオシアン酸アンモニウムを添加した室温の溶液中で、電流密度０．２ｍＡ／ｃｍ２で７２時間陰極水素チャージし、室温で４８時間静置した後のトラップ水素量が３．０ｐｐｍ以上である＜１＞～＜５＞のいずれか１項に記載のボルト。
＜７＞
　＜１＞～＜６＞のいずれか１項に記載のボルトの素材であるボルト用鋼材であって、
　前記ボルトの組成を有するボルト用鋼材。 <1>
The composition is by mass%
C: 0.35 to 0.50%,
Si: 0.02 to 0.10%,
Mn: 0.20 to 0.80%,
Mo: 1.50 to 5.00%,
W: 0 to 1.00%,
V: 0 to 0.20%,
Cr: 0.20 to 0.50%,
Al: 0.010 to 0.100%,
N: 0.0010 to 0.0150%,
P: 0.015% or less, and S: 0.015% or less,
Contains,
The rest consists of Fe and impurities
Moreover, the following formula (1), the following formula (2), and the following formula (3) are satisfied.
Tensile strength is 1600 MPa or more.
bolt.
2V / (Mo + 0.5W) ≤ 0.20 ... (1)
0.10 ≦ (2V + 0.5W) / Mo ≦ 0.40 ... (2)
0.10 ≦ 2Cr / Mo ≦ 0.35 ・ ・ ・ (3)
However, in the formulas (1), (2), and (3), the contents (mass%) of Cr, Mo, V, and W contained in the bolt are substituted for Cr, Mo, V, and W, respectively. , V or W is not included, 0 is substituted for V or W.
<2>
By mass%
Ti: 0.100% or less,
Nb: 0.100% or less,
B: 0.0050% or less,
Ni: 0.20% or less,
Cu: 0.20% or less,
REM: 0.020% or less,
The bolt according to claim 1, further comprising at least one selected from the group consisting of Sn: 0.20% or less and Bi: 0.20% or less.
<3>
By mass%
Pb: 0.05% or less,
Cd: 0.05% or less,
Co: 0.05% or less,
Zn: 0.05% or less,
Ca: 0.02% or less, and Zr: 0.02% or less,
The bolt according to claim 1 or 2, further comprising at least one selected from the group consisting of.
<4>
An M 2 C type carbide having a length of 5 nm or more and containing at least 70 atomic% or more of Mo, Cr, V and W with respect to M (metal element) is a unit area of the M 2 C type carbide. The bolt according to any one of claims 1 to 3, wherein there are 10 or more bolts per 0.01 μm 2 .
<5>
In a solution at room temperature to which 3.0 g of ammonium thiocyanate was added per 1 L of a 3.0 mass% sodium chloride aqueous solution, the current density was 0.03 mA / cm 2 for 24 hours, followed by hydrogen permeation prevention plating. The bolt according to any one of claims 1 to 4, which takes 100 hours or more to break when a constant load of 0.9 times the tensile strength is applied after being left for 96 hours.
<6>
In a solution at room temperature to which 3.0 g of ammonium thiocyanate was added per 1 L of a 3.0 mass% sodium chloride aqueous solution, the cathode hydrogen was charged at a current density of 0.2 mA / cm 2 for 72 hours and allowed to stand at room temperature for 48 hours. The bolt according to any one of <1> to <5>, wherein the amount of hydrogen trapped later is 3.0 ppm or more.
<7>
A steel material for bolts, which is the material of the bolt according to any one of <1> to <6>.
A steel material for bolts having the composition of the bolt.
　本開示によれば、高強度で、かつ、優れた耐遅れ破壊強度を示すボルト、およびその素材となるボルト用鋼材を提供できる。 According to the present disclosure, it is possible to provide bolts having high strength and excellent delayed fracture resistance, and steel materials for bolts as materials thereof.
　以下、本開示の一例である実施形態について詳細に説明する。
　なお、本明細書中において、化学組成の各元素の含有量の「％」表示は、「質量％」を意味する。
　化学組成の各元素の含有量を「元素量」と表記することがある。例えば、Ｃの含有量は、Ｃ量と表記することがある。
　「～」を用いて表される数値範囲は、「～」の前後に記載される数値を下限値及び上限値として含む範囲を意味する。
　「～」の前後に記載される数値に「超」または「未満」が付されている場合の数値範囲は、これら数値を下限値または上限値として含まない範囲を意味する。
　「工程」とは、独立した工程だけではなく、他の工程と明確に区別できない場合であってもその工程の所期の目的が達成されれば、本用語に含まれる。 Hereinafter, embodiments that are an example of the present disclosure will be described in detail.
In addition, in this specification, "%" notation of the content of each element of a chemical composition means "mass%".
The content of each element in the chemical composition may be referred to as "elemental amount". For example, the content of C may be expressed as the amount of C.
The numerical range represented by using "-" means a range including the numerical values before and after "-" as the lower limit value and the upper limit value.
The numerical range when "greater than" or "less than" is added to the numerical values before and after "to" means a range in which these numerical values are not included as the lower limit value or the upper limit value.
The term "process" is included in this term not only as an independent process but also as long as the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes.
[ボルトの化学組成]
　本実施形態に係るボルトの化学組成は、以下のとおりである。 [Chemical composition of bolts]
The chemical composition of the bolt according to this embodiment is as follows.
（必須元素）
　Ｃ　：０．３５～０．５０％
　Ｃは、鋼の強度を向上させる元素であり、ボルトの強度を高める。Ｃ量が０．３５％未満であると、ボルトとして必要な強度が得られない。一方、Ｃ量が０．５０％よりも多いと、焼入れの加熱時に合金炭化物が多量に溶け残り、所定の焼戻し温度では強度が低くなるうえ、焼戻し時の合金炭化物の析出量が相対的に減少するため、水素トラップ能も低くなる。
　従って、Ｃ量は０．３５～０．５０％とする。なお、好ましいＣ量は０．３８～０．４５％、より好ましいＣ量は０．４０～０．４３％である。 (Required element)
C: 0.35 to 0.50%
C is an element that improves the strength of steel and increases the strength of bolts. If the amount of C is less than 0.35%, the strength required for bolts cannot be obtained. On the other hand, if the amount of C is more than 0.50%, a large amount of alloy carbide remains undissolved during quenching heating, the strength becomes low at a predetermined tempering temperature, and the amount of alloy carbide precipitated during tempering is relatively reduced. Therefore, the hydrogen trapping ability is also low.
Therefore, the amount of C is set to 0.35 to 0.50%. The preferable amount of C is 0.38 to 0.45%, and the more preferable amount of C is 0.40 to 0.43%.
　Ｓｉ：０．０２～０．１０％
　Ｓｉは、含有量を低減することで耐遅れ破壊強度を向上させることができる。耐遅れ破壊強度を高めるため、Ｓｉ量を０．１０％以下とする。一方、Ｓｉ量を０．０２％未満としても耐遅れ破壊強度の向上は飽和し、また製鋼工程におけるコストが増大する。
　従って、Ｓｉ量は０．０２～０．１０％とする。なお、好ましいＳｉ量は０．０２～０．０８％、より好ましいＳｉ量は０．０３～０．０６％である。 Si: 0.02 to 0.10%
By reducing the content of Si, the delayed fracture resistance can be improved. The amount of Si is set to 0.10% or less in order to increase the delayed fracture resistance. On the other hand, even if the amount of Si is less than 0.02%, the improvement in the delayed fracture resistance is saturated and the cost in the steelmaking process increases.
Therefore, the amount of Si is set to 0.02 to 0.10%. The preferable amount of Si is 0.02 to 0.08%, and the more preferable amount of Si is 0.03 to 0.06%.
　Ｍｎ：０．２０～０．８０％
　Ｍｎは、Ｓと結合してＭｎＳを形成し、Ｓの粒界偏析を防止する。また、焼入れ性向上の作用を有する。Ｍｎ量が０．２０％未満であると、Ｓの粒界偏析が大きくなり耐遅れ破壊強度が低下する。一方、Ｍｎ量が０．８０％を超えると、部品形状に加工する際の冷間加工性が低下するうえ、焼割れが生じ易くなる。
　従って、Ｍｎ量は０．２０～０．８０％とする。なお、好ましいＭｎ量は０．３０～０．７０％、より好ましいＭｎ量は０．４０～０．６０％である。 Mn: 0.20 to 0.80%
Mn combines with S to form MnS and prevents grain boundary segregation of S. It also has the effect of improving hardenability. When the amount of Mn is less than 0.20%, the grain boundary segregation of S becomes large and the delayed fracture resistance decreases. On the other hand, if the amount of Mn exceeds 0.80%, the cold workability at the time of processing into a part shape is lowered, and shrinkage is likely to occur.
Therefore, the amount of Mn is set to 0.20 to 0.80%. The preferable amount of Mn is 0.30 to 0.70%, and the more preferable amount of Mn is 0.40 to 0.60%.
　Ｍｏ：１．５０～５．００％
　Ｗ　：０～１．００％
　Ｖ　：０～０．２０％
　Ｍｏ、ＷおよびＶは、本開示において重要な元素である。Ｍｏ、及びＷは、Ｍ２Ｃ型の炭化物を形成する元素である。Ｖは、ＭＣ型炭化物を形成する元素であるが、Ｍｏと共に、適正量のＶを複合して含有させることで、Ｖを含むＭ２Ｃ型炭化物が析出する。なお、これらＭ２Ｃ型炭化物は、Ｍｏと、Ｃｒと、ＷおよびＶの少なくとも一方と、を含む炭化物が該当する。
　微細なＭ２Ｃ型炭化物は、鋼をオーステナイト域から焼入れした後、５７０～６９０℃の高温で焼戻しをすることで、多く析出させることができる。この微細なＭ２Ｃ型炭化物が析出することで、析出強化により鋼の強度を上昇させることができる。また、微細なＭ２Ｃ型炭化物は、水素のトラップサイトとして機能し、耐遅れ破壊特性を向上させることができる。トラップ水素とは、前記Ｍ２Ｃ型炭化物によって固定された、鋼中を自由に移動できない水素である。 Mo: 1.50 to 5.00%
W: 0 to 1.00%
V: 0 to 0.20%
Mo, W and V are important elements in the present disclosure. Mo, and W is an element which forms a M 2 C type carbides. V is an element which forms a MC type carbide, with Mo, By including in combination a V proper amount, M 2 C-type carbide containing V precipitates. It should be noted that these M 2 C type carbides, and Mo, and Cr, and at least one of W and V, the carbide containing appropriate.
A large amount of fine M 2 C type carbide can be precipitated by quenching the steel from the austenite region and then tempering it at a high temperature of 570 to 690 ° C. By this fine M 2 C type carbides are precipitated, it is possible to increase the strength of steel by precipitation strengthening. In addition, the fine M 2 C type carbide functions as a hydrogen trap site and can improve the delayed fracture resistance. A trap hydrogen, said fixed by M 2 C-type carbide is hydrogen that can not be freely moved in the steel.
　トラップサイトとしての効果を得るためには、Ｍｏを１．５０％以上含有させる必要がある。加えて、Ｗ及びＶの少なくとも一方を適量含有することで、Ｍ２Ｃ型炭化物による、トラップサイトとしての効果はさらに向上する。一方、Ｍｏ量が５．０％を超えた場合、Ｗ量が１．０％を超えた場合、またはＶ量が０．２０％を超えた場合は、焼入れ加熱時に未固溶の粗大な炭窒化物が残存する。そして、この粗大な炭窒化物をオーステナイト中に固溶させるために、焼入れ加熱温度を高くする必要が生じ、焼入れ時の歪み発生、表面の酸化物増加の問題が発生する。
　従って、Ｍｏ量は１．５０～５．００％、Ｗ量は０～１．００％、Ｖ量は０～０．２０％とする。
　なお、好ましいＭｏ量は、２．００～４．００％、好ましいＷ量は、０．０２～１．００％、好ましいＶ量は、０．１０～０．１７％である。
　また、より好ましいＭｏ量は、２．５０～３．５０％、より好ましいＷ量は、２．７０～３．２０％、より好ましいＶ量は、０．１２～０．１５％である。 In order to obtain the effect as a trap site, it is necessary to contain Mo in an amount of 1.50% or more. In addition, at least one of W and V by appropriate amounts, by M 2 C-type carbide, the effect of a trap site is further improved. On the other hand, if the amount of Mo exceeds 5.0%, the amount of W exceeds 1.0%, or the amount of V exceeds 0.20%, coarse charcoal that is not solid-solved during quenching and heating. Nitride remains. Then, in order to dissolve this coarse carbonitride in austenite, it is necessary to raise the quenching heating temperature, which causes problems such as strain generation during quenching and an increase in surface oxides.
Therefore, the amount of Mo is 1.50 to 5.00%, the amount of W is 0 to 1.00%, and the amount of V is 0 to 0.20%.
The preferable amount of Mo is 2.00 to 4.00%, the preferable amount of W is 0.02 to 1.00%, and the preferable amount of V is 0.10 to 0.17%.
The more preferable amount of Mo is 2.50 to 3.50%, the more preferable amount of W is 2.70 to 3.20%, and the more preferable amount of V is 0.12 to 0.15%.
　Ｍｏ、ＷおよびＶの含有量は、下記式（１）、及び下記式（２）を満たす必要がある。
　２Ｖ／（Ｍｏ＋０．５Ｗ）≦０．２０　　　　　・・・（１）
　０．１０≦（２Ｖ＋０．５Ｗ）／Ｍｏ≦０．４０・・・（２）
式（１）および式（２）において、Ｍｏ、Ｗ、およびＶには、それぞれボルトが含有するＭｏ、ＷおよびＶの含有量（質量％）が代入され、Ｖ又はＷが含まれないときはＶに０が代入される。 The contents of Mo, W and V need to satisfy the following formula (1) and the following formula (2).
2V / (Mo + 0.5W) ≤ 0.20 ... (1)
0.10 ≦ (2V + 0.5W) / Mo ≦ 0.40 ... (2)
In the formulas (1) and (2), the contents (mass%) of Mo, W and V contained in the bolt are substituted for Mo, W and V, respectively, and when V or W is not included, 0 is substituted for V.
　Ｃｒ：０．２０～０．５０％
　Ｃｒは、鋼の焼入れ性を確保するために有効な元素であるとともに、Ｍ２Ｃ型炭化物に固溶し、水素トラップ能を向上させる効果がある。Ｃｒ量が０．２０％未満であると、これらの効果が不十分となる。一方、Ｃｒ量が０．５０％を超えると、セメンタイトを安定化させ、焼戻し時のＭ２Ｃ型炭化物の析出を阻害するため、目的の水素トラップ効果を得ることができない。
　従って、Ｃｒ量は０．２０～０．５０％とする。なお、好ましいＣｒ量は０．２０～０．３０％、より好ましいＣｒ量は０．２４～０．２８％である。 Cr: 0.20 to 0.50%
Cr, along with an effective element for ensuring the hardenability of steel, a solid solution in M 2 C carbides, an effect of improving the hydrogen trapping ability. If the amount of Cr is less than 0.20%, these effects become insufficient. On the other hand, when the Cr amount exceeds 0.50%, cementite to stabilize, to inhibit the precipitation of M 2 C type carbides during tempering, it can not be obtained hydrogen trapping effect of interest.
Therefore, the amount of Cr is set to 0.20 to 0.50%. The preferable amount of Cr is 0.20 to 0.30%, and the more preferable amount of Cr is 0.24 to 0.28%.
　Ｍ２Ｃ型炭化物の析出を阻害させないためには、Ｃｒの含有量は、下記式（３）を満たす必要がある。
　０．１０≦２Ｃｒ／Ｍｏ≦０．３５　　　　　　・・・（３）
　式（３）において、ＣｒとＭｏには、それぞれボルトが含有するＣｒとＭｏの含有量（質量％）が代入される。 In order not to inhibit the precipitation of M 2 C-type carbide, the content of Cr is required to satisfy the following formula (3).
0.10 ≦ 2Cr / Mo ≦ 0.35 ・ ・ ・ (3)
In the formula (3), the contents (mass%) of Cr and Mo contained in the bolt are substituted for Cr and Mo, respectively.
　引張り強さ１６００ＭＰａ以上の高強度を有するボルトにおいては、耐遅れ破壊強度を向上させるために、水素トラップサイトである微細なＭ２Ｃ型炭化物を大量に鋼中に分散させることが必要である。
　式（１）において、「２Ｖ／（Ｍｏ＋０．５Ｗ）」の値が０．２０超えでは、Ｍ２Ｃ型炭化物の水素トラップ能が不足して耐遅れ破壊強度が低下する。
　式（２）において、「（２Ｖ＋０．５Ｗ）／Ｍｏ」の値が０．１０未満では、Ｍ２Ｃ型炭化物中のＭｏ、Ｗ及びＶの複合度が低く、Ｍ２Ｃ型炭化物の水素トラップ能が不足して耐遅れ破壊強度が低下する。一方、「（２Ｖ＋０．５Ｗ）／Ｍｏ」の値が０．４０超えでは、Ｍ２Ｃ型炭化物が不安定となり、別の炭化物となるため、水素トラップ能が不足して耐遅れ破壊強度が低下する。
　式（３）において、「２Ｃｒ／Ｍｏ」の値が０．１０未満では、Ｍ２Ｃ型炭化物の水素トラップ能が低下する。「２Ｃｒ／Ｍｏ」の値が０．３５超えでは、Ｃｒを多量に含有するセメンタイト、Ｍ２３Ｃ６又はＭ７Ｃ３の析出量が増加する。
　よって、Ｃｒ、Ｍｏ、Ｖ、およびＷの含有量は、式（１）、式（２）及び式（３）を満たす必要がある。 In the bolt having a high strength of at least strength 1600MPa tensile, in order to improve the delayed fracture strength, it is necessary to disperse a large amount in the steel of fine M 2 C-type carbide is hydrogen trap sites.
In the formula (1), the value of "2V / (Mo + 0.5 W)" is the greater than 0.20, delayed fracture strength hydrogen trapping ability is insufficient of M 2 C type carbides is reduced.
In the formula (2), "(2V + 0.5 W) / Mo" The value of less than 0.10, Mo of M 2 C-type carbide, low composite of the W and V, M 2 hydrogen trapping of C type carbide The ability is insufficient and the delayed fracture resistance is reduced. Meanwhile, "(2V + 0.5W) / Mo" value of 0.40 than of, M 2 C-type carbide is unstable, since the different carbides, delayed fracture strength hydrogen trapping ability is insufficient decrease To do.
In the formula (3), the value of "2Cr / Mo" is less than 0.10, the hydrogen trapping ability of M 2 C type carbides is reduced. When the value of "2Cr / Mo" exceeds 0.35, the amount of cementite, M 23 C 6 or M 7 C 3 , which contains a large amount of Cr, increases.
Therefore, the contents of Cr, Mo, V, and W need to satisfy the formulas (1), (2), and (3).
　Ａｌ：０．０１０～０．１００％
　Ａｌは、脱酸剤として機能する元素であるとともに、窒化物を形成して焼入れ加熱時のオーステナイト結晶粒の粗大化を抑制する元素である。これらの効果を得るためには、Ａｌを０．０１０％以上含有させる必要がある。一方、Ａｌ量が０．１００％を超えると、粗大な酸化物系介在物が鋼中に残存して、ボルトの破壊起点となる。
　従って、Ａｌ量は０．０１０～０．１００％とする。なお、好ましいＡｌ量は０．０１２～０．０５０％、より好ましいＡｌ量は０．０２５～０．０３５％である。 Al: 0.010 to 0.100%
Al is an element that functions as an antacid and also forms a nitride to suppress the coarsening of austenite crystal grains during quenching and heating. In order to obtain these effects, it is necessary to contain 0.010% or more of Al. On the other hand, when the Al content exceeds 0.100%, coarse oxide-based inclusions remain in the steel and serve as a fracture starting point of the bolt.
Therefore, the amount of Al is set to 0.010 to 0.100%. The preferable Al amount is 0.012 to 0.050%, and the more preferable Al amount is 0.025 to 0.035%.
　Ｎ：０．００１０～０．０１５０％
　Ｎは、窒化物又は炭窒化物を形成し、焼入れ加熱時のオーステナイト結晶粒の粗大化を抑制する元素である。結晶粒の粗大化を抑制するには、Ｎ量を０．００１０％以上とする必要がある。一方、Ｎ量が０．０１５０％を超えた場合、粗大な窒化物や炭窒化物が生成して、破壊起点となる。
　従って、Ｎ量は０．００１０～０．０１５０％とする。なお、好ましいＮ量は０．００２０～０．０１００％、より好ましいＮ量は０．００３０～０．００５０％である。 N: 0.0010 to 0.0150%
N is an element that forms a nitride or carbonitride and suppresses coarsening of austenite crystal grains during quenching and heating. In order to suppress the coarsening of crystal grains, the amount of N needs to be 0.0010% or more. On the other hand, when the amount of N exceeds 0.0150%, coarse nitrides and carbonitrides are generated and serve as the starting point of fracture.
Therefore, the amount of N is set to 0.0010 to 0.0150%. The preferable N amount is 0.0020 to 0.0100%, and the more preferable N amount is 0.0030 to 0.0050%.
　Ｐ：０．０１５％以下
　Ｐは、不純物である。Ｐ量は極力低いことが好ましい。Ｐは、オーステナイト粒界に偏析する。Ｐ量が０．０１５％を超えると、焼入れ、焼戻し後の旧オーステナイト粒界が脆化して粒界割れの原因となる。このため、Ｐ量を０．０１５％以下の範囲に制限する必要がある。好ましいＰ量の上限は０．０１２％である。Ｐは、不純物元素であるが、上記範囲内であれば、Ｐは、ボルトに０％超えで含有されていてもよい。
　ただし、脱Ｐコスト低減の観点から、Ｐ量の下限は、０．００５％以上でもよい。 P: 0.015% or less P is an impurity. The amount of P is preferably as low as possible. P segregates at the austenite grain boundaries. If the amount of P exceeds 0.015%, the old austenite grain boundaries after quenching and tempering become brittle, causing grain boundary cracking. Therefore, it is necessary to limit the amount of P to the range of 0.015% or less. The upper limit of the preferable amount of P is 0.012%. Although P is an impurity element, P may be contained in the bolt in an amount of more than 0% as long as it is within the above range.
However, from the viewpoint of reducing the cost of removing P, the lower limit of the amount of P may be 0.005% or more.
　Ｓ：０．０１５％以下
　Ｓは、不純物である。Ｓ量は極力低いことが好ましい。Ｓは、ボルト中でＭｎ硫化物として存在する。Ｍｎ硫化物は、鋼表面が腐食する際の化学反応で硫化水素を発生する。この硫化水素が分解して水素を発生することで鋼中へ水素が侵入し、耐遅れ破壊強度を低下させる。また、Ｍｎ硫化物が破壊起点となる。このため、Ｓ量を０．０１５％以下の範囲に制限する必要がある。好ましいＳ量の上限は０．０１２％である。Ｓは、不純物元素であるが、上記範囲内であれば、Ｓは、ボルトに０％超含有されていてもよい。
　ただし、脱Ｓコスト低減の観点から、Ｓ量の下限は、０．００５％以上でもよい。 S: 0.015% or less S is an impurity. The amount of S is preferably as low as possible. S exists as Mn sulfide in the bolt. Mn sulfide generates hydrogen sulfide by a chemical reaction when the steel surface is corroded. When this hydrogen sulfide is decomposed to generate hydrogen, hydrogen invades into the steel and the delayed fracture resistance is lowered. Further, Mn sulfide serves as a fracture starting point. Therefore, it is necessary to limit the amount of S to the range of 0.015% or less. The upper limit of the preferable amount of S is 0.012%. S is an impurity element, but if it is within the above range, S may be contained in the bolt in an amount of more than 0%.
However, from the viewpoint of reducing the cost of removing S, the lower limit of the amount of S may be 0.005% or more.
（任意元素）
　本実施形態に係るボルトは、任意元素として、Ｔｉ、Ｎｂ、Ｂ、Ｎｉ、Ｃｕ、Ｗ、ＲＥＭ、Ｓｎ、及びＢｉよりなる群から選択される少なくとも１種以上を含有してもよい。具体的には、これら任意元素を、各々０％～後述する各元素の上限の範囲で含有してもよい。 (Arbitrary element)
The bolt according to the present embodiment may contain at least one selected from the group consisting of Ti, Nb, B, Ni, Cu, W, REM, Sn, and Bi as an optional element. Specifically, these arbitrary elements may be contained in the range of 0% to the upper limit of each element described later.
　Ｔｉ：０．１００％以下
　Ｔｉは、ボルト中でＮ、Ｃと結合して炭窒化物を形成する元素である。この炭窒化物はオーステナイト結晶粒界をピンニングして組織の粗大化を防止する。この組織の粗大化の防止効果を得るためには、Ｔｉを０．１００％以下含有させてもよい。一方、Ｔｉを、０．１００％を超えて含有させると、素材硬さの上昇に起因して部品形状に加工する際の冷間加工性が低下する。 Ti: 0.100% or less Ti is an element that combines with N and C in a bolt to form a carbonitride. This carbonitride pins the austenite grain boundaries to prevent texture coarsening. In order to obtain the effect of preventing the coarsening of the structure, Ti may be contained in an amount of 0.100% or less. On the other hand, when Ti is contained in an amount of more than 0.100%, the cold workability when processing into a part shape is lowered due to the increase in material hardness.
　従って、Ｔｉ量は０．１００％以下とすることが好ましく、０％超～０．１００％がより好ましく、０．００５～０．０５０％がさらに好ましい。 Therefore, the amount of Ti is preferably 0.100% or less, more preferably more than 0% to 0.100%, still more preferably 0.005 to 0.050%.
　Ｎｂ：０．１００％以下
　Ｎｂは、ボルト中でＮ及びＣと結合して炭窒化物を形成する元素である。この炭窒化物はオーステナイト結晶粒界をピンニングし、組織の粗大化を防止する。この組織の粗大化の防止効果を得るためには、Ｎｂを０．１００％以下含有させてもよい。一方、Ｎｂを、０．１００％を超えて含有させると、素材硬さの上昇に起因して部品形状に加工する際の冷間加工性が低下する。 Nb: 0.100% or less Nb is an element that combines with N and C in a bolt to form a carbonitride. This carbonitride pins the austenite grain boundaries and prevents texture coarsening. In order to obtain the effect of preventing the coarsening of the structure, Nb may be contained in an amount of 0.100% or less. On the other hand, if Nb is contained in an amount of more than 0.100%, the cold workability when processing into a part shape is lowered due to the increase in material hardness.
　従って、Ｎｂ量は０．１００％以下とすることが好ましく、０％超～０．１００％がより好ましく、０．００５～０．０５０％がさらに好ましい。 Therefore, the amount of Nb is preferably 0.100% or less, more preferably more than 0% to 0.100%, still more preferably 0.005 to 0.050%.
　Ｂ：０．００５０％以下
　Ｂは、オーステナイト中に僅かに固溶させただけで鋼の焼入れ性を高める。Ｂは、浸炭焼入れ時にマルテンサイトを効率的に得るためにボルトに含有させてもよい。一方、０．００５０％を超えてＢが含有すると、多量のＢＮを形成してＮを消費するため、オーステナイト粒の粗大化を招来する。
　従って、Ｂ量は０．００５０％以下とすることが好ましく、０超～０．００５０％がより好ましく、０．０００７～０．００３０％がさらに好ましい。 B: 0.0050% or less B enhances the hardenability of steel even if it is slightly dissolved in austenite. B may be contained in the bolt in order to efficiently obtain martensite during carburizing and quenching. On the other hand, if B is contained in excess of 0.0050%, a large amount of BN is formed and N is consumed, which leads to coarsening of austenite grains.
Therefore, the amount of B is preferably 0.0050% or less, more preferably more than 0 to 0.0050%, still more preferably 0.0007 to 0.0030%.
　Ｎｉ：０．２０％以下
　Ｎｉは、耐食性と靭性を高める元素であり、ボルトに含有させてもよい。Ｎｉ量が多量になると、コストに見合った効果が得られないため、Ｎｉ量の上限は０．２０％が好ましい。一方、Ｎｉ量の下限は０．０１％が好ましい。 Ni: 0.20% or less Ni is an element that enhances corrosion resistance and toughness, and may be contained in bolts. When the amount of Ni is large, the effect commensurate with the cost cannot be obtained. Therefore, the upper limit of the amount of Ni is preferably 0.20%. On the other hand, the lower limit of the amount of Ni is preferably 0.01%.
　Ｃｕ：０．２０％以下
　Ｃｕは耐食性を高める元素であり、ボルトに含有させてもよい。一方、Ｃｕ量が０．２０％を超えると熱間延性が低下するため、Ｃｕ量の上限は０．２０％が好ましい。一方、Ｃｕ量の下限は０．０１％が好ましい。 Cu: 0.20% or less Cu is an element that enhances corrosion resistance and may be contained in bolts. On the other hand, if the Cu amount exceeds 0.20%, the hot ductility decreases, so the upper limit of the Cu amount is preferably 0.20%. On the other hand, the lower limit of the amount of Cu is preferably 0.01%.
　ＲＥＭ：０．０２０％以下
　ＲＥＭ（希土類元素）とは、原子番号５７のランタンから原子番号７１ルテシウムまでの１５元素と、原子番号２１のスカンジウム及び原子番号３９のイットリウムと、の合計１７元素の総称である。ボルトにＲＥＭが含有されると、圧延時及び熱間鍛造時にＭｎＳ粒子の伸延が抑制され、冷間鍛造時の割れを抑制する効果が得られる。但し、ＲＥＭ量が０．０２０％を超えると、ＲＥＭを含む硫化物が大量に生成され、ボルト用鋼材の被削性が劣化する。
　従って、ＲＥＭ量は、前記１７元素の合計量で０．０２０％以下とすることが好ましく、０％超～０．０２０％がより好ましく、０．００５％～０．０１５％がさらに好ましい。 REM: 0.020% or less REM (rare earth element) is a general term for a total of 17 elements, including 15 elements from lanthanum with atomic number 57 to lutetium with atomic number 71, scandium with atomic number 21 and yttrium with atomic number 39. Is. When REM is contained in the bolt, the elongation of MnS particles is suppressed during rolling and hot forging, and the effect of suppressing cracking during cold forging can be obtained. However, if the amount of REM exceeds 0.020%, a large amount of sulfide containing REM is generated, and the machinability of the steel material for bolts deteriorates.
Therefore, the total amount of the 17 elements is preferably 0.020% or less, more preferably more than 0% to 0.020%, still more preferably 0.005% to 0.015%.
　Ｓｎ：０．２０％以下
　Ｓｎは耐食性を高める元素であり、ボルトに含有させてもよい。Ｓｎ量が多量になると、高温延性が低下し、鋳造時の割れの危険性が高まるため、Ｓｎ量の上限は０．２０％が好ましい。一方、Ｓｎ量の下限は０．００５％が好ましく、０．０１％がより好ましい。 Sn: 0.20% or less Sn is an element that enhances corrosion resistance and may be contained in bolts. When the Sn amount is large, the high temperature ductility is lowered and the risk of cracking during casting is increased. Therefore, the upper limit of the Sn amount is preferably 0.20%. On the other hand, the lower limit of the Sn amount is preferably 0.005%, more preferably 0.01%.
　Ｂｉ：０．２０％以下
　Ｂｉは加工性を高める元素であり、ボルトに含有させてもよい。Ｂｉ量が多量になると、高温延性が低下し、鋳造時の割れの危険性が高まるため、Ｂｉ量の上限は０．２０％が好ましい。一方、Ｂｉ量の下限は０．００５％が好ましく、０．０１％がより好ましい。 Bi: 0.20% or less Bi is an element that enhances workability and may be contained in bolts. When the amount of Bi is large, the high temperature ductility is lowered and the risk of cracking during casting is increased. Therefore, the upper limit of the amount of Bi is preferably 0.20%. On the other hand, the lower limit of the Bi amount is preferably 0.005%, more preferably 0.01%.
（その他任意元素）
　本実施形態に係るボルトは、任意元素として、次の元素よりなる群から選択される少なくとも１種を含有してもよい。具体的には、これら任意元素を、各々０％～後述する各元素の上限の範囲で含有してもよい。これら任意元素を後述する範囲でボルトに含んでも、ボルトの特性に影響はない。
Ｐｂ：０．０５％以下
Ｃｄ：０．０５％以下
Ｃｏ：０．０５％以下
Ｚｎ：０．０５％以下
Ｃａ：０．０２％以下
Ｚｒ：０．０２％以下 (Other optional elements)
The bolt according to the present embodiment may contain at least one selected from the group consisting of the following elements as an optional element. Specifically, these arbitrary elements may be contained in the range of 0% to the upper limit of each element described later. Even if these arbitrary elements are contained in the bolt in the range described later, the characteristics of the bolt are not affected.
Pb: 0.05% or less Cd: 0.05% or less Co: 0.05% or less Zn: 0.05% or less Ca: 0.02% or less Zr: 0.02% or less
　本実施形態におけるボルトの化学組成の残部は、Ｆｅ及び不純物からなる。ここで、不純物とは、鋼の原料として利用される鉱石、スクラップ、又は製造過程の環境等から混入する元素を意味する。 The rest of the chemical composition of the bolt in this embodiment consists of Fe and impurities. Here, the impurity means an ore used as a raw material for steel, scrap, or an element mixed from the environment of the manufacturing process.
（Ｍ２Ｃ型炭化物）
　本実施形態に係るボルトは、長さ５ｎｍ以上のＭ２Ｃ型炭化物が、単位面積０．０１μｍ２当たり１０個以上存在することが好ましい。
　焼戻し過程で析出する微細なＭ２Ｃ型炭化物（ＭｏとＣｒとＷ及びＶの少なくとも一方とを含む炭化物）は、ＶＣ、Ｍｏ２Ｃ等に比べ、水素トラップ能が高く、耐遅れ破壊特性の向上に寄与する。
　ここで、微細なＭ２Ｃ型炭化物は、Ｍ（金属元素）に対し、ＭｏとＣｒとＶおよびＷの少なくとも一方とを合計で７０原子％以上含むＭ２Ｃ型炭化物である。具体的には、微細なＭ２Ｃ型炭化物は、（Ｍｏ、Ｃｒ、Ｗ、Ｖ）２Ｃ、（Ｍｏ、Ｃｒ、Ｗ）２Ｃ、及び、（Ｍｏ、Ｃｒ、Ｖ）２Ｃが該当する。
　これらＭ２Ｃ型炭化物は、ＶＣ、Ｍｏ２Ｃ等に比べ、水素トラップ能が高く、耐遅れ破壊特性の向上に寄与する。
　そのため、長さ５ｎｍ以上のＭ２Ｃ型炭化物を、所定量存在させることが好ましい。
　よって、長さ５ｎｍ以上のＭ２Ｃ型炭化物の個数密度（単位面積０．０１μｍ２当たりに存在する長さ５ｎｍ以上のＭ２Ｃ型炭化物の個数）は、１０個以上が好ましい。
　耐遅れ破壊特性の向上の観点から、Ｍ２Ｃ型炭化物の個数密度は、単位面積０．０１μｍ２当たり１５個以上がより好ましく、単位面積０．０１μｍ２当たり２０個以上がさらに好ましい。
　ただし、Ｍ２Ｃ型炭化物の個数密度の上限は、伸び及び靱性の低下抑制の観点から、例えば、単位面積０．０１μｍ２当たり１００個以下とする。 (M 2 C type carbide)
Bolt according to the present embodiment, the length of 5nm or more M 2 C type carbides is preferably present more than 10 unit area 0.01 [mu] m 2 per.
Fine M 2 C type carbides precipitated in the tempering process (carbide containing Mo and Cr and W and at least one of V) is, VC, compared with a Mo 2 C, etc., high hydrogen trapping ability, the delayed fracture resistance Contribute to improvement.
Here, fine M 2 C type carbides are to M (metal element), an M 2 C type carbide containing at least one and a total of 70 atomic% or more of Mo, Cr, and V and W. Specifically, fine M 2 C type carbides, (Mo, Cr, W, V) 2 C, (Mo, Cr, W) 2 C, and corresponds (Mo, Cr, V) 2 C is ..
These M 2 C type carbides have higher hydrogen trapping ability than VC, Mo 2 C and the like, and contribute to the improvement of delayed fracture resistance.
Therefore, the above length 5nm of M 2 C-type carbide, it is preferable to present a predetermined amount.
Thus, more length 5nm of M 2 C type carbides number density (number per unit area 0.01 [mu] m 2 or more in length 5nm of M 2 C type carbides present per) is preferably 10 or more.
From the viewpoint of improving the delayed fracture resistance, the number density of M 2 C type carbides is 15 or more, more preferably unit area 0.01 [mu] m 2 per more 20 unit area 0.01 [mu] m 2 per more preferably.
However, the upper limit of the number density of M 2 C-type carbide from the viewpoint of suppressing the reduction in elongation and toughness, for example, to 100 or less per unit area 0.01 [mu] m 2.
　Ｍ２Ｃ型炭化物の個数密度の測定は、薄膜法により薄膜試験片を作製し、透過型電子顕微鏡で測定する。
　Ｍ２Ｃ型炭化物の成分の測定は、抽出レプリカ法により試験片を作製し、エネルギー分散型Ｘ線分析装置（ＥＤＳ）付き透過型顕微鏡（ＴＥＭ）を用いて行う。
　具体的には、次の通りである。 Measurements of the number density of M 2 C type carbides, a thin film specimen prepared by a thin film method, measured by transmission electron microscopy.
Measurement of the components of M 2 C type carbides, the test pieces were produced by extraction replica method, performed using the energy dispersive X-ray spectrometer (EDS) with a transmission electron microscope (TEM).
Specifically, it is as follows.
　測定対象となるボルトの任意の部位から、ボルトの表面から深さ２ｍｍに位置しかつボルトの表面と平行な面（以下「測定面」とも称する）を有する部位を採取し、薄膜法により薄膜試験片および抽出レプリカ法により試験片を作製する。 From any part of the bolt to be measured, a part located at a depth of 2 mm from the surface of the bolt and having a surface parallel to the surface of the bolt (hereinafter, also referred to as "measurement surface") is sampled, and a thin film test is performed by the thin film method. Specimens are prepared by the piece and extraction replica method.
　ここで、薄膜法による薄膜試験片の作製は、次の通りである。まず、精密切断機により元材を厚さ０．５ｍｍに切断する。次に、Ｐ３２０～１２００のエメリー紙を用いて両側から６０μｍ厚まで切削研磨を行い３ｍｍφの試料を打抜く。その後、両面ジェット電解研磨を行い、中心部に穴が開くまで電解研磨を行い、ＴＥＭ観察用の薄膜試験片とする。電界研磨はテヌポールで行い、電解研磨液として１００ｍｌ過塩素酸－８００ｍｌ氷酢酸溶液－１００ｍｌメタノールを用い、電解研磨条件は３０Ｖ、０．１Ａとする。
　また、抽出レプリカ法による試験片の作製は、次の通りである。まず、鋼部材から採取した採取物の測定面を電解研磨する。電解研磨後の採取物の測定面を、１０％アセチルアセトン－１％塩化テトラメチルアンモニウム（ＴＭＡＣ）－メタノール溶液を用いて－２００ｍＶの電位で定電位電解する。これにより、Ｍ２Ｃ型炭化物が採取物の測定面から露出する。通電時間は３０～６０ｓｅｃである。
　電解後の採取物の測定面にアセチルセルロースフィルムを貼り付けた後に、フィルムを剥がし、Ｍ２Ｃ型炭化物をフィルム上に転写する。転写したフィルムにカーボン蒸着を行ない、カーボン蒸着膜を作製する。カーボン蒸着膜を酢酸メチル溶液に浸漬してアセチルセルロースフィルムを溶解し、直径が３ｍｍのＣｕメッシュですくい上げることで抽出レプリカ膜（抽出レプリカ法による試験片）を得る。 Here, the production of the thin film test piece by the thin film method is as follows. First, the base material is cut to a thickness of 0.5 mm by a precision cutting machine. Next, using emery paper of P320 to 1200, cutting and polishing is performed from both sides to a thickness of 60 μm, and a sample of 3 mmφ is punched out. Then, double-sided jet electropolishing is performed, and electropolishing is performed until a hole is formed in the center to obtain a thin film test piece for TEM observation. Electropolishing is performed with Tenupol, 100 ml perchloric acid-800 ml glacial acetic acid solution-100 ml methanol is used as the electrolytic polishing solution, and the electrolytic polishing conditions are 30V and 0.1A.
The preparation of the test piece by the extraction replica method is as follows. First, the measurement surface of the sample collected from the steel member is electropolished. The measurement surface of the sample after electropolishing is electrolyzed at a potential of −200 mV using a 10% acetylacetone-1% tetramethylammonium chloride (TMAC) -methanol solution. Thus, M 2 C-type carbide is exposed from the measuring surface of the harvest. The energizing time is 30 to 60 sec.
After sticking the acetyl cellulose film on the measurement surface of the harvest after electrolysis, the film was peeled to transfer the M 2 C-type carbide on the film. Carbon vapor deposition is performed on the transferred film to prepare a carbon vapor deposition film. An extraction replica film (test piece by the extraction replica method) is obtained by immersing the carbon vapor deposition film in a methyl acetate solution to dissolve the acetyl cellulose film and scooping it up with a Cu mesh having a diameter of 3 mm.
　次に、Ｍ２Ｃ型炭化物の数密度を次の通り測定する。鉄のマトリクスの｛００１｝面に垂直な方向を電子線の入射方向として、薄膜試験片（その測定面）の任意の視野を倍率４０００００倍（観察面積０．２５μｍ×０．２５μｍ）で３視野観察する。Ｍ２Ｃ型炭化物は電子線回折パターン解析にて同定する。その後、観察画面の中心部の０．１μｍ×０．１μｍの領域に存在する全てのＭ２Ｃ型炭化物の長さと数を測定し、５ｎｍ以上の長さを有するＭ２Ｃ型炭化物の数を測定し、５つの視野の平均値を「Ｍ２Ｃ型炭化物の個数密度」として求める。
　ここで、Ｍ２Ｃ型炭化物の長さとは、観察されるＭ２Ｃ型炭化物の最大長さを意味する。
　なお、ＴＥＭ観察は、ＦＥ－ＴＥＭにて加速電圧２００ｋＶにて実施する。 Next, the number density of M 2 C type carbides measured as follows. With the direction perpendicular to the {001} plane of the iron matrix as the incident direction of the electron beam, any field of view of the thin film test piece (the measurement surface thereof) can be magnified at a magnification of 400,000 times (observation area 0.25 μm × 0.25 μm). Observe. M 2 C type carbides are identified by electron diffraction pattern analysis. After that, the length and number of all M 2 C type carbides existing in the region of 0.1 μm × 0.1 μm in the center of the observation screen are measured, and the number of M 2 C type carbides having a length of 5 nm or more is determined. measured, the average value of the five field as "number density of M 2 C type carbides".
Here, the length of M 2 C type carbides, the maximum length of the M 2 C type carbides observed.
The TEM observation is performed by FE-TEM at an accelerating voltage of 200 kV.
　また、Ｍ２Ｃ型炭化物の化学成分は次の通り測定する。試験片としての抽出レプリカ膜（その測定面）の任意の視野（観察面積０．５μｍ×０．５μｍの視野）を倍率２０００００倍で観察する。観察する視野に存在する析出物の成分を、ＴＥＭの電子線回折パターンの解析及びＥＤＳによる分析により、Ｍ２Ｃ型炭化物を同定し、ＥＤＳ分析により、炭化物中の金属元素の原子％を測定する。測定個数は５個とし、金属元素濃度はこれらの平均値を用いる。
　ＴＥＭの電子線回折パターンの解析及びＥＤＳによる分析は、ＦＥ－ＴＥＭにて加速電圧２００ｋＶにて実施する。 The chemical components of the M 2 C-type carbide is measured as follows. An arbitrary visual field (visual field having an observation area of 0.5 μm × 0.5 μm) of the extracted replica membrane (the measurement surface thereof) as a test piece is observed at a magnification of 200,000 times. The components of the precipitates present in the observation field of view, the analysis by the analysis and EDS electron diffraction pattern of the TEM, to identify M 2 C-type carbide, the EDS analysis to determine the atomic percent of metal elements in the carbide .. The number of measured pieces is 5, and the average value of these is used as the metal element concentration.
The analysis of the electron diffraction pattern of TEM and the analysis by EDS are carried out by FE-TEM at an accelerating voltage of 200 kV.
（引張強さ）
　本実施形態に係るボルトにおいて、ボルトから引張り試験片を採取して測定した引張強さは１６００ＭＰａ以上である。
　ボルトの引張強さは、ＪＩＳ　Ｚ　２２４１：２０１１に従って測定される値である。 (Tensile strength)
In the bolt according to the present embodiment, the tensile strength measured by collecting a tensile test piece from the bolt is 1600 MPa or more.
The tensile strength of a bolt is a value measured according to JIS Z 2241: 2011.
　ただし、ボルトの引張強さの測定は、次の通りボルトから試験片を採取して、実施する。
　ボルトの軸部から、平行部の直径がボルトの直径の５０％となる１４Ａ号試験片を切り出し、室温（２５℃）の大気中で引張試験を行い、引張強さを求める。 However, the tensile strength of the bolt is measured by collecting a test piece from the bolt as follows.
A No. 14A test piece having a parallel portion diameter of 50% of the bolt diameter is cut out from the bolt shaft portion, and a tensile test is performed in the air at room temperature (25 ° C.) to determine the tensile strength.
（トラップ水素量）
　本実施形態に係るボルトにおいて、３．０質量％の塩化ナトリウム水溶液１Ｌ当たり３．０ｇのチオシアン酸アンモニウムを添加した室温の溶液中で、電流密度０．２ｍＡ／ｃｍ２で７２時間陰極水素チャージし、室温（２５℃）で４８時間静置した後のトラップ水素量は３．０ｐｐｍ以上が好ましい。
　トラップ水素量が３．０ｐｐｍ未満であると、ボルトに侵入した水素が拡散し、旧オーステナイト結晶粒界に集積して、遅れ破壊が生じる危険性が高まることがある。そのため、トラップ水素量は３．０ｐｐｍ以上であることが好ましい。 (Amount of trap hydrogen)
In the bolt according to the present embodiment, the cathode hydrogen was charged at a current density of 0.2 mA / cm 2 for 72 hours in a room temperature solution containing 3.0 g of ammonium thiocyanate per 1 L of a 3.0 mass% sodium chloride aqueous solution. The amount of trapped hydrogen after standing at room temperature (25 ° C.) for 48 hours is preferably 3.0 ppm or more.
If the amount of trapped hydrogen is less than 3.0 ppm, the hydrogen that has entered the bolt may diffuse and accumulate at the former austenite grain boundaries, increasing the risk of delayed fracture. Therefore, the amount of trap hydrogen is preferably 3.0 ppm or more.
　トラップ水素量は、ガスクロマトグラフによる昇温水素分析法で測定する。昇温速度１００℃／時間で、室温（２５℃）から４００℃ までに試料から放出される水素量をトラップ水素量と定義する。 The amount of trapped hydrogen is measured by a heated hydrogen analysis method using a gas chromatograph. The amount of hydrogen released from the sample from room temperature (25 ° C.) to 400 ° C. at a heating rate of 100 ° C./hour is defined as the trap hydrogen amount.
　トラップ水素量の測定は、ボルトから採取した直径７ｍｍ、長さ７０ｍｍの丸棒試験片（トラップ水素量調査用の丸棒試験片）に対して、実施する。
　ただし、上記大きさの丸棒試験片を採取できない場合、直径５ｍｍ、長さ２０ｍｍの丸棒試験片で代用し、同様の水素チャージと静置を行い、同様の昇温分析により、水素トラップ量を測定してもよい。 The trap hydrogen amount is measured on a round bar test piece having a diameter of 7 mm and a length of 70 mm (a round bar test piece for investigating the amount of trap hydrogen) collected from a bolt.
However, if a round bar test piece of the above size cannot be collected, a round bar test piece with a diameter of 5 mm and a length of 20 mm is used instead, the same hydrogen charge and standing are performed, and the same temperature rise analysis is performed to obtain a hydrogen trap amount. May be measured.
（耐遅れ破壊強度）
　本実施形態に係るボルトは、実環境で使用するため、十分な耐遅れ破壊強度を備えることが好ましい。そのため、本実施形態に係るボルトにおいて、３．０質量％の塩化ナトリウム水溶液１Ｌ当たり３．０ｇのチオシアン酸アンモニウムを添加した室温の（２５℃）溶液中で、電流密度０．０３ｍＡ／ｃｍ２で２４時間陰極水素チャージした後、水素透過防止めっきを施し、９６時間放置した後、引張強さの０．９倍の一定荷重を負荷した時の、破断に至るまでの時間が１００時間以上であることが好ましい。
　ここで、水素透過防止めっきは、ボルト中に水素を閉じ込めるために行うものであり、溶融亜鉛めっきを施す。 (Delayed fracture resistance)
Since the bolt according to this embodiment is used in a real environment, it is preferable that the bolt has sufficient delayed fracture resistance. Therefore, in the bolt according to the present embodiment, the current density is 0.03 mA / cm 2 in a room temperature (25 ° C.) solution containing 3.0 g of ammonium thiocyanate per 1 L of a 3.0 mass% sodium chloride aqueous solution. After charging with cathode hydrogen for 24 hours, hydrogen permeation prevention plating is applied, and after leaving for 96 hours, when a constant load of 0.9 times the tensile strength is applied, the time until fracture is 100 hours or more. Is preferable.
Here, the hydrogen permeation prevention plating is performed to confine hydrogen in the bolt, and hot dip galvanizing is performed.
　耐遅れ破壊強度の測定は、ボルトから採取した直径７ｍｍ、長さ７０ｍｍの切欠き（切欠き部直径４．２ｍｍ、角度６０°）付き丸棒試験片（遅れ破壊試験片）に対して、実施する。
　ただし、上記大きさの丸棒試験片を採取できない場合、直径５ｍｍの切欠き（切欠き部直径３．０ｍｍ、角度６０°）付き丸棒試験片で代用してもよい。長さは、チャッキングできる範囲であれば特に制約はない。 The delayed fracture strength was measured on a round bar test piece (delayed fracture test piece) with a notch (notch diameter 4.2 mm, angle 60 °) with a diameter of 7 mm and a length of 70 mm collected from a bolt. To do.
However, if a round bar test piece having the above size cannot be collected, a round bar test piece with a notch (notch diameter 3.0 mm, angle 60 °) having a diameter of 5 mm may be used instead. The length is not particularly limited as long as it can be chucked.
＜ボルト用鋼材＞
　本実施形態に係るボルト用鋼材は、本実施形態に係るボルトの素材となる鋼材である。そして、本実施形態に係るボルト用鋼材は、本実施形態に係るボルトと同じ化学組成を有する。 <Steel for bolts>
The bolt steel material according to this embodiment is a steel material that is a material for bolts according to this embodiment. The bolt steel material according to the present embodiment has the same chemical composition as the bolt according to the present embodiment.
＜ボルトの製造方法＞
　以下、本実施形態に係るボルト用鋼材を用いて、本実施形態に係るボルトの製造方法の一例について詳述する。 <Bolt manufacturing method>
Hereinafter, an example of a method for manufacturing a bolt according to the present embodiment will be described in detail using the bolt steel material according to the present embodiment.
（ボルト形状に成形する工程）
　本実施形態に係るボルトの化学組成を有する溶鋼を得た後、溶鋼を鋳造によりインゴットまたは鋳片とする。鋳造されたインゴットまたは鋳片は、熱間圧延、熱間押出、熱間鍛造などの熱間加工によって、丸棒など所要の粗形状を有する鋼材に仕上げる。その後、該鋼材に伸線、焼鈍、冷間加工、ねじ転造などを施して、所定のボルト形状に成形する。複数回の冷間加工の中間に、焼鈍または球状化焼鈍処理を複数回施してもよい。また、成形の工程に熱間加工を含めることもできる。 (Process of forming into a bolt shape)
After obtaining a molten steel having the chemical composition of the bolt according to the present embodiment, the molten steel is cast into an ingot or a slab. The cast ingot or slab is finished into a steel material having a required rough shape such as a round bar by hot working such as hot rolling, hot extrusion, and hot forging. After that, the steel material is subjected to wire drawing, annealing, cold working, screw rolling, etc. to form a predetermined bolt shape. Annealing or spheroidizing annealing may be performed multiple times in the middle of the plurality of cold workings. It is also possible to include hot working in the molding process.
（焼入れ・焼戻しを行う工程）
　所定のボルト形状に成形した後、強度を付与するため、鋼をオーステナイト化以上の温度に加熱した後、水冷または油冷によって焼入れ処理を行う。なお、焼入れのための加熱温度（以下、「焼入れ加熱温度」という。）が低すぎると、Ｍｏ、Ｃｒ、Ｗ、及びＶの炭化物のマトリックス中への固溶が不十分となり、粗大な炭化物が残存する。その結果、焼戻し時に析出する微細なＭ２Ｃ型炭化物の量が少なくなるため、目的の強度及び水素トラップ効果を得ることができない。 (Process for quenching and tempering)
After forming into a predetermined bolt shape, in order to impart strength, the steel is heated to a temperature equal to or higher than austenitization, and then quenched by water cooling or oil cooling. If the heating temperature for quenching (hereinafter referred to as "quenching heating temperature") is too low, the solid solution of the carbides of Mo, Cr, W, and V into the matrix becomes insufficient, resulting in coarse carbides. Remains. As a result, the amount of fine M 2 C type carbides precipitated during tempering is small, it is impossible to obtain the strength and hydrogen trapping effect of interest.
　一方、焼入れ加熱温度を過度に高くすると、結晶粒の粗大化を招き、靭性及び耐遅れ破壊特性の劣化を招き、また、操業熱処理炉の炉体および付属部品の損傷が顕著になり、製造コストが上昇するため、好ましくない。
　そのため、焼入れ加熱温度は９３０～１０５０℃とするのが好ましい。また、焼入れ加熱温度での保持時間は３０～９０分とすることが好ましい。 On the other hand, if the quenching heating temperature is excessively high, the grain grains are coarsened, the toughness and the delayed fracture resistance are deteriorated, and the furnace body and accessory parts of the operational heat treatment furnace are significantly damaged, resulting in a manufacturing cost. Is not preferable because it increases.
Therefore, the quenching heating temperature is preferably 930 to 1050 ° C. Further, the holding time at the quenching heating temperature is preferably 30 to 90 minutes.
　耐遅れ破壊強度を向上させるためには、上記の焼入れ処理を行った後に焼戻しを行う必要がある。本開示では、焼戻しの温度を、５７０～６９０℃に限定する必要がある。 In order to improve the delayed fracture resistance, it is necessary to perform tempering after performing the above quenching treatment. In the present disclosure, it is necessary to limit the tempering temperature to 570 to 690 ° C.
　焼戻し温度が５７０℃未満では、焼戻し時に析出するＭ２Ｃ型炭化物の析出が不十分で、目的の水素トラップ能、および遅れ破壊限界水素量を達成することができない。 Is less than the tempering temperature of 570 ° C., M 2 C-type carbide precipitation which precipitated during tempering is insufficient, it is impossible to achieve the hydrogen trapping capacity of the object, and a delayed fracture critical amount of hydrogen.
　一方、焼戻し温度が６９０℃以上の場合は、Ｍ２Ｃ型炭化物がオストワルド成長し、目的の水素トラップ能、および遅れ破壊限界水素量を達成することができない。
　そのため、焼戻し温度は５７０～６９０℃に限定する。なお、焼戻し温度の好ましい範囲は、５９０～６６０℃である。
　また、焼戻し温度での保持時間は３０～９０分とすることが好ましく、焼き戻し冷却速度は５０～１００℃／ｓとすることが好ましい。 On the other hand, if the tempering temperature is above 690 ° C., M 2 C type carbides Ostwald ripening, can not be achieved hydrogen trapping ability of interest, and a delayed fracture critical amount of hydrogen.
Therefore, the tempering temperature is limited to 570 to 690 ° C. The preferred range of tempering temperature is 590 to 660 ° C.
The holding time at the tempering temperature is preferably 30 to 90 minutes, and the tempering cooling rate is preferably 50 to 100 ° C./s.
　以上の工程により、本実施形態に係るボルトが製造される。 By the above steps, the bolt according to this embodiment is manufactured.
　以上に示すとおり、本実施形態に係るボルトは、最適な化学組成を備える鋼材に、最適な焼入れ焼戻しを施すことで、引張強さ、トラップ水素量及び遅れ破壊限界水素量の好適化を図ったものである。 As shown above, in the bolt according to the present embodiment, the tensile strength, the amount of trap hydrogen, and the amount of delayed fracture limit hydrogen are optimized by subjecting a steel material having an optimum chemical composition to optimum quenching and tempering. It is a thing.
　次に、本開示の実施例について説明するが、以下に示す各条件は、本開示の実施可能性及び効果を確認するために採用した一例にすぎず、本開示の条件はこの一例に限定されるものではない。本開示の実施においては、その要旨を逸脱せず、その目的を達成する限りにおいて、種々の条件を採用することができる。 Next, an example of the present disclosure will be described, but the conditions shown below are merely examples adopted for confirming the feasibility and effect of the present disclosure, and the conditions of the present disclosure are limited to this example. It's not something. In implementing the present disclosure, various conditions may be adopted as long as the gist is not deviated and the object is achieved.
＜各種試験片の成形＞
　（棒鋼の準備）
　表１－１及び表１－２に示す化学組成を有する鋼（鋼Ｎｏ．Ａ～Ｐ及びＡＡ～ＡＹ）をそれぞれ溶製し、熱間鍛造により、直径２０ｍｍ、長さ１０００ｍｍの棒鋼を準備した。なお、表１－１及び表１－２において下線を付した数値は当該数値が本開示の範囲外であることを示す。また、表１－１及び表１－２における「－」は各元素が無添加であることを示す。また、式（１）～式（３）において、「－」で表記された元素の含有量は「０」が代入される。そして、残部は、Ｆｅ及び不純物である。
　ただし、表１－１及び表１－２に示す化学組成において、酸素（Ｏ）は鋼中に不純物として含まれる元素である。 <Molding of various test pieces>
(Preparation of steel bar)
Steels having the chemical compositions shown in Table 1-1 and Table 1-2 (Steel Nos. AP and AA to AY) were forged and hot forged to prepare steel bars having a diameter of 20 mm and a length of 1000 mm. .. The underlined values in Table 1-1 and Table 1-2 indicate that the values are outside the scope of the present disclosure. In addition, "-" in Table 1-1 and Table 1-2 indicates that each element is not added. Further, in the formulas (1) to (3), "0" is substituted for the content of the element represented by "-". And the balance is Fe and impurities.
However, in the chemical compositions shown in Table 1-1 and Table 1-2, oxygen (O) is an element contained as an impurity in steel.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　次にボルト製造を再現するため、表２の条件で焼入れ、焼戻しを施し、続いて、焼入れ、焼戻ししたボルト相当品の引張強さ、トラップ水素量の測定、および耐遅れ破壊強度を以下の方法で評価した。 Next, in order to reproduce the bolt production, quenching and tempering were performed under the conditions shown in Table 2, and then the tensile strength, trap hydrogen amount, and delayed fracture resistance of the hardened and tempered bolt equivalent were measured by the following methods. Evaluated in.
　（焼入れの実施）
　上記のようにして得た直径２０ｍｍ、長さ１０００ｍｍの丸棒を切断し、直径２０ｍｍ、長さ３００ｍｍの丸棒を切り出し、表２に記載の温度で焼入れを行った。焼入れ加熱温度での保持時間は６０分とした。その後、６０℃に保持した油槽へ焼入れを行った。 (Implementation of quenching)
A round bar having a diameter of 20 mm and a length of 1000 mm obtained as described above was cut, and a round bar having a diameter of 20 mm and a length of 300 mm was cut out and quenched at the temperatures shown in Table 2. The holding time at the quenching heating temperature was 60 minutes. Then, quenching was carried out in an oil tank kept at 60 ° C.
　（焼戻しの実施）
　油焼入れ後、表２に記載の温度で焼戻しを行った。焼戻し温度での保持時間は６０分とし、焼戻し後の冷却は空冷（冷却速度１０℃／ｓ）とした。 (Implementation of tempering)
After oil quenching, tempering was performed at the temperatures shown in Table 2. The holding time at the tempering temperature was 60 minutes, and the cooling after tempering was air cooling (cooling rate 10 ° C./s).
　（引張試験片）
　上記の焼入れ焼戻し処理後の直径２０ｍｍ、長さ３００ｍｍの丸棒から、全長７０ｍｍ、平行部の直径６ｍｍ、長さ３２ｍｍの平滑引張試験片（１４Ａ号試験片）を採取した。 (Tensile test piece)
From the round bar having a diameter of 20 mm and a length of 300 mm after the above quenching and tempering treatment, a smooth tensile test piece (No. 14A test piece) having a total length of 70 mm, a parallel portion having a diameter of 6 mm and a length of 32 mm was collected.
　（トラップ水素量調査用の試験片作製）
　上記の焼入れ焼戻し処理後の直径２０ｍｍ、長さ３００ｍｍの丸棒から、直径７ｍｍ、長さ７０ｍｍの丸棒を採取し、トラップ水素量調査用の丸棒試験片とした。 (Preparation of test pieces for trap hydrogen amount investigation)
A round bar having a diameter of 7 mm and a length of 70 mm was collected from the round bar having a diameter of 20 mm and a length of 300 mm after the above quenching and tempering treatment, and used as a round bar test piece for investigating the amount of trapped hydrogen.
　（耐遅れ破壊強度の試験片の作製）
　上記の焼入れ焼戻し処理後の直径２０ｍｍ、長さ３００ｍｍの丸棒から、直径７ｍｍ、長さ７０ｍｍの切欠き（切欠き部直径４．２ｍｍ、角度６０°）付き丸棒試験片を採取し、耐遅れ破壊強度の試験片とした。 (Preparation of test pieces with delayed fracture resistance)
From the round bar having a diameter of 20 mm and a length of 300 mm after the above quenching and tempering treatment, a round bar test piece having a notch (notch diameter 4.2 mm, angle 60 °) having a diameter of 7 mm and a length of 70 mm was collected and withstood. A test piece with delayed fracture strength was used.
　以上のようにして、製造Ｎｏ．１～４５の引張試験片、製造Ｎｏ．１～４５のトラップ水素量調査用の試験片、及び製造Ｎｏ．１～４５の耐遅れ破壊強度の試験片を、それぞれ得た。ただし、製造Ｎｏ．３４については焼割れが発生したため、以降の試験を中断した。 As described above, the manufacturing No. Tensile test pieces 1 to 45, manufacturing No. Test pieces for investigating the amount of trap hydrogen of 1 to 45, and production No. Test pieces having a delayed fracture resistance of 1 to 45 were obtained, respectively. However, the production No. Subsequent tests were interrupted for 34 because of burning cracks.
＜各試験片を用いた性能評価＞ <Performance evaluation using each test piece>
（長さ５ｎｍ以上のＭ２Ｃ型炭化物の個数密度）
　長さ５ｎｍ以上のＭ２Ｃ型炭化物の個数密度（単位面積０．０１μｍ２当たりの個数）は、既述の通り測定した。そして、次の基準で評価した。
Ａ：Ｍ２Ｃ型炭化物の個数密度が１０個／０．０１μｍ２以上１４個／０．０１μｍ２以下
Ｂ：Ｍ２Ｃ型炭化物の個数密度が１５個／０．０１μｍ２以上２０個／０．０１μｍ２未満
Ｃ：Ｍ２Ｃ型炭化物の個数密度が２０個／０．０１μｍ２以上
Ｄ：Ｍ２Ｃ型炭化物の個数密度が１０個／０．０１μｍ２未満 (Number density of M 2 C type carbides or length 5 nm)
The number density of M 2 C type carbides having a length of 5 nm or more (number per unit area of 0.01 μm 2 ) was measured as described above. Then, it was evaluated according to the following criteria.
A: Number density of M 2 C type carbide is 10 pieces / 0.01 μm 2 or more and 14 pieces / 0.01 μm 2 or less B: Number density of M 2 C type carbide is 15 pieces / 0.01 μm 2 or more and 20 pieces / 0 .01 μm less than 2 C: M 2 C-type carbide number density is 20 pieces / 0.01 μm 2 or more D: M 2 C-type carbide number density is 10 pieces / 0.01 μm less than 2
（引張強さ（ＴＳ））
　引張強さは、既述の通り測定した。
　具体的には、　上記の手順で作製した引張試験片を用い、ＪＩＳ　Ｚ　２２４１：２０１１に準拠して、室温（２５℃）の大気中で引張試験を行い、引張強さを求めた。 (Tensile strength (TS))
The tensile strength was measured as described above.
Specifically, using the tensile test piece prepared by the above procedure, a tensile test was conducted in the air at room temperature (25 ° C.) in accordance with JIS Z 2241: 2011 to determine the tensile strength.
（トラップ水素量）
　トラップ水素量は、既述の通り測定した。
　具体的には、上記の手順で作製した直径７ｍｍ、長さ７０ｍｍの丸棒試験片に、３．０質量％の塩化ナトリウム水溶液１Ｌ当たり３．０ｇのチオシアン酸アンモニウムを添加した室温（２５℃）の溶液中で、電流密度０．２ｍＡ／ｃｍ２で７２時間陰極水素チャージを行った。その後、室温（２５℃）で４８時間静置した。その後、ガスクロマトグラフを用い、昇温速度１００℃／ 時間で、室温（２５℃）から４００℃まで昇温し、丸棒試験片から放出される水素量を測定した。 (Amount of trap hydrogen)
The amount of trap hydrogen was measured as described above.
Specifically, at room temperature (25 ° C.), 3.0 g of ammonium thiocyanate was added to 1 L of a 3.0 mass% sodium chloride aqueous solution to a round bar test piece having a diameter of 7 mm and a length of 70 mm prepared by the above procedure. Cathode hydrogen charging was carried out in the solution of the above at a current density of 0.2 mA / cm 2 for 72 hours. Then, it was allowed to stand at room temperature (25 ° C.) for 48 hours. Then, using a gas chromatograph, the temperature was raised from room temperature (25 ° C.) to 400 ° C. at a heating rate of 100 ° C./hour, and the amount of hydrogen released from the round bar test piece was measured.
（耐遅れ破壊強度試験）
　耐遅れ破壊強度試験は、既述の通り測定した。
　具体的には、上記の手順で作製したφ７ｍｍ×７０ｍｍの切欠き（切欠き部φ４．２ｍｍ、角度６０°）付き耐遅れ破壊強度の試験片に、３．０質量％の塩化ナトリウム水溶液１Ｌ当たり３．０ｇのチオシアン酸アンモニウムを添加した室温（２５℃）の溶液中で、電流密度０．０３ｍＡ／ｃｍ２で２４時間陰極水素チャージした後、Ｚｎで水素透過防止めっきを施し、９６時間放置した後、引張強さの０．９倍の一定荷重を負荷し、破断に至るまでの時間を測定した。１００時間破断しなかった場合は試験を打ち切りとした。 (Delayed fracture strength test)
The delayed fracture strength test was measured as described above.
Specifically, a test piece having a φ7 mm × 70 mm notch (notch φ4.2 mm, angle 60 °) and delayed fracture resistance prepared by the above procedure is used per 1 L of 3.0 mass% sodium chloride aqueous solution. In a solution at room temperature (25 ° C.) to which 3.0 g of ammonium thiocyanate was added, the cathode was hydrogen-charged at a current density of 0.03 mA / cm 2 for 24 hours, then subjected to hydrogen permeation prevention plating with Zn and left for 96 hours. After that, a constant load 0.9 times the tensile strength was applied, and the time until fracture was measured. If it did not break for 100 hours, the test was terminated.
　Ｍ２Ｃ型炭化物の個数密度、引張強さ（ＴＳ）、トラップ水素量、及び遅れ破壊有無の結果を表２に記載する。なお、表２中の下線を付した数値は当該数値が本開示の範囲外であることを示す。また、表２中の符号 “－”は、試験を行わなかったことを意味する。 Table 2 shows the results of the number density of M 2 C type carbides, tensile strength (TS), amount of trap hydrogen, and presence / absence of delayed fracture. The underlined values in Table 2 indicate that the values are outside the scope of the present disclosure. The symbol "-" in Table 2 means that the test was not performed.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　表１～表２から明らかなように、化学組成、並びに、焼入れ焼戻しの条件について好適化を図った製造Ｎｏ．１～２０（開示例１～２０）は、いずれも、引張強さが高く、また、トラップ水素量が高く、遅れ破壊が生じなかったことから、優れた強度と耐遅れ破壊特性が得られていることが判る。 As is clear from Tables 1 and 2, the production No. which has been optimized for the chemical composition and the conditions for quenching and tempering. In each of 1 to 20 (Disclosure Examples 1 to 20), the tensile strength was high, the trap hydrogen amount was high, and delayed fracture did not occur. Therefore, excellent strength and delayed fracture resistance were obtained. You can see that there is.
　尚、開示例１６、１７は、本開示の組成要件を満たすが、焼入れ条件が好適な範囲から若干外れる製造条件で製造された。開示例１７は、他の開示例に比べて焼入れ温度が高い製造条件で製造されており、その強度は他の開示例に比べて若干大きい。そのため、強度－延性バランスに関しては、他の開示例の方が相対的に優れている。また、開示例１６は、他の開示例に比べて焼入れ温度が低い製造条件で製造されており、強度に関しては、他の開示例の方が相対的に優れている。 The disclosure examples 16 and 17 were manufactured under manufacturing conditions that satisfy the composition requirements of the present disclosure, but the quenching conditions are slightly out of the preferable range. Disclosure Example 17 is manufactured under production conditions in which the quenching temperature is higher than that of the other Disclosure Examples, and its strength is slightly higher than that of the other Disclosure Examples. Therefore, the other disclosed examples are relatively superior in terms of strength-ductility balance. Further, the disclosure example 16 is manufactured under production conditions in which the quenching temperature is lower than that of the other disclosure examples, and the other disclosure examples are relatively superior in terms of strength.
　これに対し、化学組成、並びに、焼入れ焼戻しの条件について、少なくともいずれかについて好適化を図っていない製造Ｎｏ．２１～４５（比較例１～２５）については、いずれも、耐遅れ破壊特性が得られていないことが判る。また、比較例５、１３、１９及び２０は、強度が不十分であり、比較例１～４、６～８、１０～１２、１６～２５はいずれも十分なトラップ水素量が得られていない。比較例１５は、トラップ水素量は高くても鋼成分のＡｌ量が０．１００より高いため、Ａｌを含む酸化物系介在物が多数存在し、破壊の起点が多数形成され遅れ破壊特性が低下している。 On the other hand, with respect to the chemical composition and the conditions of quenching and tempering, at least one of them has not been optimized. It can be seen that none of 21 to 45 (Comparative Examples 1 to 25) has the delayed fracture resistance. Further, Comparative Examples 5, 13, 19 and 20 had insufficient strength, and Comparative Examples 1 to 4, 6 to 8, 10 to 12 and 16 to 25 did not obtain a sufficient amount of trap hydrogen. .. In Comparative Example 15, even if the trap hydrogen amount is high, the Al amount of the steel component is higher than 0.100, so that many oxide-based inclusions containing Al are present, many fracture starting points are formed, and the delayed fracture characteristics are deteriorated. doing.
　なお、日本国特許出願第２０１９－０９１３２８号の開示はその全体が参照により本明細書に取り込まれる。
　本明細書に記載された全ての文献、特許出願、および技術規格は、個々の文献、特許出願、および技術規格が参照により取り込まれることが具体的かつ個々に記された場合と同程度に、本明細書中に参照により取り込まれる。 The entire disclosure of Japanese Patent Application No. 2019-091328 is incorporated herein by reference in its entirety.
All documents, patent applications, and technical standards described herein are to the same extent as if the individual documents, patent applications, and technical standards were specifically and individually stated to be incorporated by reference. Incorporated herein by reference.
　本開示によれば、高強度で、かつ、優れた耐遅れ破壊強度を示すボルト、およびその素材となるボルト用鋼材を提供できる。 According to the present disclosure, it is possible to provide a bolt having high strength and excellent delayed fracture resistance, and a steel material for bolts as a material thereof.

Claims (7)

  1. 組成が、質量％で、
    Ｃ　：０．３５～０．５０％、
    Ｓｉ：０．０２～０．１０％、
    Ｍｎ：０．２０～０．８０％、
    Ｍｏ：１．５０～５．００％、
    Ｗ　：０～１．００％、
    Ｖ　：０～０．２０％、
    Ｃｒ：０．２０～０．５０％、
    Ａｌ：０．０１０～０．１００％、
    Ｎ　：０．００１０～０．０１５０％、
    Ｐ　：０．０１５％以下、および
    Ｓ　：０．０１５％以下、
    を含有し、
    残部がＦｅ及び不純物からなり、
    かつ、下記式（１）、下記式（２）、および下記式（３）を満たし、
    引張強さが、１６００ＭＰａ以上である、
    ボルト。
    　２Ｖ／（Ｍｏ＋０．５Ｗ）≦０．２０　　　　　・・・（１）
    　０．１０≦（２Ｖ＋０．５Ｗ）／Ｍｏ≦０．４０・・・（２）
    　０．１０≦２Ｃｒ／Ｍｏ≦０．３５　　　　　　・・・（３）
    但し、式（１）、（２）、（３）において、Ｃｒ、Ｍｏ、Ｖ、およびＷには、それぞれボルトが含有するＣｒ、Ｍｏ、Ｖ、およびＷの含有量（質量％）が代入され、Ｖ又はＷが含まれないときはＶ又はＷに０が代入される。     The composition is by mass%
    C: 0.35 to 0.50%,
    Si: 0.02 to 0.10%,
    Mn: 0.20 to 0.80%,
    Mo: 1.50 to 5.00%,
    W: 0 to 1.00%,
    V: 0 to 0.20%,
    Cr: 0.20 to 0.50%,
    Al: 0.010 to 0.100%,
    N: 0.0010 to 0.0150%,
    P: 0.015% or less, and S: 0.015% or less,
    Contains,
    The rest consists of Fe and impurities
    Moreover, the following formula (1), the following formula (2), and the following formula (3) are satisfied.
    Tensile strength is 1600 MPa or more.
    bolt.
    2V / (Mo + 0.5W) ≤ 0.20 ... (1)
    0.10 ≦ (2V + 0.5W) / Mo ≦ 0.40 ... (2)
    0.10 ≦ 2Cr / Mo ≦ 0.35 ・ ・ ・ (3)
    However, in the formulas (1), (2), and (3), the contents (mass%) of Cr, Mo, V, and W contained in the bolt are substituted for Cr, Mo, V, and W, respectively. , V or W is not included, 0 is substituted for V or W.
  2. 質量％で、
    Ｔｉ：０．１００％以下、
    Ｎｂ：０．１００％以下、
    Ｂ　：０．００５０％以下、
    Ｎｉ：０．２０％以下、
    Ｃｕ：０．２０％以下、
    ＲＥＭ：０．０２０％以下、
    Ｓｎ：０．２０％以下、および
    Ｂｉ：０．２０％以下
    よりなる群から選択される少なくとも１種をさらに含有する、請求項１に記載のボルト。     By mass%
    Ti: 0.100% or less,
    Nb: 0.100% or less,
    B: 0.0050% or less,
    Ni: 0.20% or less,
    Cu: 0.20% or less,
    REM: 0.020% or less,
    The bolt according to claim 1, further comprising at least one selected from the group consisting of Sn: 0.20% or less and Bi: 0.20% or less.
  3. 質量％で、
    Ｐｂ：０．０５％以下、
    Ｃｄ：０．０５％以下、
    Ｃｏ：０．０５％以下、
    Ｚｎ：０．０５％以下、
    Ｃａ：０．０２％以下、および
    Ｚｒ：０．０２％以下、
    よりなる群から選択される少なくとも１種をさらに含有する、請求項１又は請求項２に記載のボルト。     By mass%
    Pb: 0.05% or less,
    Cd: 0.05% or less,
    Co: 0.05% or less,
    Zn: 0.05% or less,
    Ca: 0.02% or less, and Zr: 0.02% or less,
    The bolt according to claim 1 or 2, further comprising at least one selected from the group consisting of.
  4. 　長さ５ｎｍ以上のＭ２Ｃ型炭化物であって、Ｍ（金属元素）に対し、ＭｏとＣｒとＶおよびＷの少なくとも一方とを合計で７０原子％以上含むＭ２Ｃ型炭化物が、単位面積０．０１μｍ２当たり１０個以上存在する、請求項１～請求項３のいずれか１項に記載のボルト。 An M 2 C type carbide having a length of 5 nm or more and containing at least 70 atomic% or more of Mo, Cr, V and W with respect to M (metal element) is a unit area of the M 2 C type carbide. The bolt according to any one of claims 1 to 3, wherein there are 10 or more bolts per 0.01 μm 2 .
  5. 　３．０質量％の塩化ナトリウム水溶液１Ｌ当たり３．０ｇのチオシアン酸アンモニウムを添加した室温の溶液中で、電流密度０．０３ｍＡ／ｃｍ２で２４時間陰極水素チャージした後、水素透過防止めっきを施し、９６時間放置した後、引張強さの０．９倍の一定荷重を負荷した時の、破断に至るまで１００時間以上である請求項１～請求項４のいずれか１項に記載のボルト。 In a solution at room temperature to which 3.0 g of ammonium thiocyanate was added per 1 L of a 3.0 mass% sodium chloride aqueous solution, the current density was 0.03 mA / cm 2 for 24 hours, followed by hydrogen permeation prevention plating. The bolt according to any one of claims 1 to 4, which takes 100 hours or more to break when a constant load of 0.9 times the tensile strength is applied after being left for 96 hours.
  6. 　３．０質量％の塩化ナトリウム水溶液１Ｌ当たり３．０ｇのチオシアン酸アンモニウムを添加した室温の溶液中で、電流密度０．２ｍＡ／ｃｍ２で７２時間陰極水素チャージし、室温で４８時間静置した後のトラップ水素量が３．０ｐｐｍ以上である請求項１～請求項５のいずれか１項に記載のボルト。 In a solution at room temperature to which 3.0 g of ammonium thiocyanate was added per 1 L of a 3.0 mass% sodium chloride aqueous solution, the cathode hydrogen was charged at a current density of 0.2 mA / cm 2 for 72 hours and allowed to stand at room temperature for 48 hours. The bolt according to any one of claims 1 to 5, wherein the amount of hydrogen trapped later is 3.0 ppm or more.
  7. 　請求項１～請求項６のいずれか１項に記載のボルトの素材であるボルト用鋼材であって、
    　前記ボルトの組成を有するボルト用鋼材。     A steel material for bolts, which is the material of the bolt according to any one of claims 1 to 6.
    A steel material for bolts having the composition of the bolt.
PCT/JP2020/019354 2019-05-14 2020-05-14 Bolt and steel material for bolt WO2020230872A1 (en)

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Citations (3)

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Publication number Priority date Publication date Assignee Title
JP2007031736A (en) * 2005-07-22 2007-02-08 Nippon Steel Corp Method for manufacturing high strength bolt excellent in delayed fracture resistance
JP2007031733A (en) * 2005-07-22 2007-02-08 Nippon Steel Corp STEEL HAVING EXCELLENT DELAYED FRACTURE RESISTANCE AND TENSILE STRENGTH IN CLASS OF >=1,600 MPa, AND METHOD FOR PRODUCING FORMING THEREOF
WO2011111872A1 (en) * 2010-03-11 2011-09-15 新日本製鐵株式会社 High-strength steel and high-strength bolt with excellent resistance to delayed fracture, and manufacturing method therefor

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JP3400886B2 (en) * 1995-03-16 2003-04-28 新日本製鐵株式会社 High tension bolt steel with excellent hydrogen entry prevention effect
JP3857835B2 (en) 1999-07-26 2006-12-13 新日本製鐵株式会社 Steel for high strength bolt and method for producing high strength bolt
JP4427010B2 (en) 2004-07-05 2010-03-03 新日本製鐵株式会社 High strength tempered steel with excellent delayed fracture resistance and method for producing the same
JP4555749B2 (en) 2004-10-08 2010-10-06 新日本製鐵株式会社 Method for improving delayed fracture resistance of high strength bolts

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2007031736A (en) * 2005-07-22 2007-02-08 Nippon Steel Corp Method for manufacturing high strength bolt excellent in delayed fracture resistance
JP2007031733A (en) * 2005-07-22 2007-02-08 Nippon Steel Corp STEEL HAVING EXCELLENT DELAYED FRACTURE RESISTANCE AND TENSILE STRENGTH IN CLASS OF >=1,600 MPa, AND METHOD FOR PRODUCING FORMING THEREOF
WO2011111872A1 (en) * 2010-03-11 2011-09-15 新日本製鐵株式会社 High-strength steel and high-strength bolt with excellent resistance to delayed fracture, and manufacturing method therefor

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