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.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Articles (AREA)

Priority Applications (2)

Applications Claiming Priority (2)

Publications (1)

Family

ID=73288842

Family Applications (1)

Country Status (3)

Cited By (1)

Citations (3)

Family Cites Families (4)

• 2020
  • 2020-05-14 KR KR1020217036401A patent/KR102599767B1/ko active Active
  • 2020-05-14 JP JP2021519494A patent/JP7164032B2/ja active Active
  • 2020-05-14 WO PCT/JP2020/019354 patent/WO2020230872A1/ja active Application Filing

Patent Citations (3)

Also Published As

Similar Documents

Legal Events