WO2015146331A1 - Steel for high strength bolts having excellent delayed fracture resistance and high strength bolt - Google Patents

Steel for high strength bolts having excellent delayed fracture resistance and high strength bolt Download PDF

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WO2015146331A1
WO2015146331A1 PCT/JP2015/053763 JP2015053763W WO2015146331A1 WO 2015146331 A1 WO2015146331 A1 WO 2015146331A1 JP 2015053763 W JP2015053763 W JP 2015053763W WO 2015146331 A1 WO2015146331 A1 WO 2015146331A1
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steel
delayed fracture
fracture resistance
strength
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PCT/JP2015/053763
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French (fr)
Japanese (ja)
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昌之 坂田
千葉　政道
洋介 松本
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株式会社神戸製鋼所
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Publication of WO2015146331A1 publication Critical patent/WO2015146331A1/en

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    • 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
    • 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
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16B—DEVICES FOR FASTENING OR SECURING CONSTRUCTIONAL ELEMENTS OR MACHINE PARTS TOGETHER, e.g. NAILS, BOLTS, CIRCLIPS, CLAMPS, CLIPS OR WEDGES; JOINTS OR JOINTING
    • F16B31/00—Screwed connections specially modified in view of tensile load; Break-bolts
    • F16B31/06—Screwed connections specially modified in view of tensile load; Break-bolts having regard to possibility of fatigue rupture
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a bolt steel used for automobiles, various industrial machines, and the like, and a bolt obtained using the bolt steel, and exhibits excellent delayed fracture resistance even when the tensile strength is 1100 MPa or more.
  • the present invention relates to steel for high strength bolts and high strength bolts.
  • Standard steels such as SCM435 are frequently used for high-strength bolts with a tensile strength of 1100 MPa or more. Since standard steels such as SCM435 are added with a large amount of alloy elements such as Mo, the cost of steel materials increases. With the request of steel material cost reduction, the request for SCM alternative steel which omitted Mo is increasing. However, simply reducing the alloy elements reduces the hardenability and makes it difficult to ensure strength.
  • Patent Document 1 discloses a technique for improving corrosion resistance of steel by adding a predetermined amount of Cu to boron-containing steel, suppressing intrusion of diffusible hydrogen into the steel, and improving delayed fracture resistance. Proposed. However, it is insufficient to ensure delayed fracture resistance only by adding Cu.
  • Patent Document 2 proposes a technique of adding V in order to improve delayed fracture resistance of boron-containing steel.
  • V is an expensive rare metal like Mo
  • the boron-containing steel to which V is added has a small cost reduction effect as an SCM alternative steel.
  • Patent Document 3 boron that exhibits excellent delayed fracture resistance even in harsh environments by controlling the ratio of the content of Si and C in the chemical composition within an appropriate range.
  • a steel for added high strength bolts was proposed.
  • this boron-added high-strength bolt steel contains V as an essential element, as in Patent Document 2, the cost reduction effect as an SCM alternative steel is small.
  • the present invention has been made in view of such circumstances, and the object thereof is to add resistance to high tensile strength of 1100 MPa or more without adding expensive alloy elements such as Mo and V.
  • An object of the present invention is to provide a steel for high-strength bolts excellent in delayed fracture resistance and a high-strength bolt excellent in delayed fracture resistance made of this steel.
  • the present inventors have intensively studied to provide a steel for high-strength bolts excellent in delayed fracture resistance and a high-strength bolt excellent in delayed fracture resistance made of this steel.
  • the C and Mn content satisfies the relationship of the formula (1) described later, it exhibits excellent delayed fracture resistance even in a harsh environment.
  • the present invention has been completed by finding that the strength can be increased if the contents of Si, Mn, and Cr satisfy the relationship of the formula (2) described later.
  • the steel for high-strength bolts with excellent delayed fracture resistance according to the present invention that has been able to solve the above-mentioned problems is, by mass%, C: 0.20 to 0.35%, Si: 0.3 to 1.0%, Mn: more than 0% to 0.6% or less, P: more than 0% to 0.02% or less, S: more than 0% to 0.02% or less, Cr: 0.3 to 1.5%, Al : 0.01 to 0.1%, Ti: 0.05 to 0.1%, B: 0.0003 to 0.005%, and N: more than 0% and 0.01% or less, (1) and formula (2) are satisfied, and the balance is made of iron and inevitable impurities, and has a gist in that it is a mixed structure of ferrite and pearlite.
  • [] means the content (% by mass) of each element. 1 / ([C] ⁇ [Mn]) ⁇ 5.5 (1) [C] + [Si] / 2 + [Mn] / 2 + [Cr] /3 ⁇ 0.82 (2)
  • the present invention also includes a bolt obtained using the steel for high-strength bolts, wherein the metal structure is a tempered martensite structure and the tensile strength is 1100 MPa or more and the high-strength bolt excellent in delayed fracture resistance. Is done.
  • the tensile strength is controlled by appropriately controlling the chemical component composition and strictly controlling the relationship between the contents of C and Mn and the contents of C, Si, Mn, and Cr. Even if it is 1100 MPa or more, a steel for high-strength bolts excellent in delayed fracture resistance can be realized. If this steel is used, a high-strength bolt excellent in delayed fracture resistance can be provided.
  • FIG. 1A is a schematic diagram showing the shape of a notched test piece
  • FIG. 1B is a schematic diagram showing the shape of a notch
  • FIG. 2 is a graph showing the relationship between the value of 1 / ([C] ⁇ [Mn]) and the delayed fracture strength ratio.
  • the present inventors have repeatedly studied to provide a bolt exhibiting excellent delayed fracture resistance even at a high strength of 1100 MPa or more without adding an expensive alloy element such as Mo or V. .
  • an expensive alloy element such as Mo or V.
  • C is an element useful for ensuring the strength of the steel.
  • increasing its content deteriorates the toughness and corrosion resistance of the steel and tends to cause delayed fracture.
  • the X value when the value on the left side of the above formula (1) is the X value, the X value is set to 5.5 or more.
  • the X value is preferably 6.5 or more, more preferably 7.0 or more.
  • the upper limit of the X value is naturally set from the amount of C and the amount of Mn described later, but even if the X value is excessively increased, the effect on delayed fracture resistance is saturated, so the upper limit is, for example, about 30 Preferably there is.
  • the X value is more preferably 20 or less, and still more preferably 15 or less.
  • the coefficient of each element in the above formula (2) indicates the degree of contribution to strength improvement.
  • this Y value is set to 0.82 or more.
  • the Y value is preferably 0.90 or more, more preferably 1.00 or more.
  • the upper limit of the Y value is not particularly limited, but if the Y value becomes excessively large, the strength of the base material becomes too high and the cold forgeability when forming into a bolt shape deteriorates, so the upper limit is about 1.30. It is preferable that The Y value is more preferably 1.20 or less, and still more preferably 1.15 or less.
  • the steel for high-strength bolts of the present invention satisfies the above-mentioned formulas (1) and (2).
  • C 0.20 to 0.35%
  • Si 0.3 To 1.0%
  • Mn more than 0% to 0.6% or less
  • P more than 0% to 0.02% or less
  • S more than 0% to 0.02% or less
  • Cr 0.3 to 1.5%
  • Al 0.01 to 0.1%
  • Ti 0.05 to 0.1%
  • B 0.0003 to 0.005%
  • N more than 0% and 0.01% or less It is.
  • C is an element for securing the strength and ductility of steel in a well-balanced manner, and is an indispensable element for securing the tensile strength necessary as a high-strength bolt. In order to exert such an effect, C needs to be contained by 0.20% or more.
  • the amount of C is preferably 0.23% or more, more preferably 0.25% or more.
  • the C amount is set to 0.35% or less.
  • the amount of C is preferably 0.32% or less, more preferably 0.30% or less.
  • the Si is an element used as a deoxidizer at the time of melting, has an effect of increasing temper softening resistance, and is an element necessary for increasing strength. Therefore, in the present invention, the Si amount is set to 0.3% or more.
  • the amount of Si is preferably 0.4% or more, more preferably 0.47% or more.
  • the Si amount is 1.0% or less.
  • the amount of Si is preferably 0.60% or less, more preferably 0.55% or less.
  • the amount of Mn needs to be 0.6% or less.
  • the amount of Mn is preferably 0.55% or less, more preferably 0.5% or less.
  • Mn is an element that acts effectively as a deoxidizer during melting and has the effect of increasing the hardenability of the steel and increasing the strength. In order to exhibit such an effect effectively, it is preferable to make it contain 0.1% or more.
  • the amount of Mn is more preferably 0.2% or more, and still more preferably 0.30% or more.
  • the P amount is 0.02% or less.
  • the amount of P is preferably 0.015% or less, more preferably 0.01% or less.
  • the amount of P is preferably reduced as much as possible. However, in order to reduce the amount of P to less than 0.001%, the cost is high, so the lower limit may be 0.001%.
  • the S amount is 0.02% or less.
  • the amount of S is preferably 0.015% or less, more preferably 0.01% or less.
  • the amount of S is preferably reduced as much as possible. However, since the cost is increased to make the amount of S less than 0.001%, the lower limit may be 0.001%.
  • Cr is an element that acts to increase hardenability and strength. It also has the effect of increasing temper softening resistance and improving strength. Further, Cr is an element that contributes to enhancing the corrosion resistance of steel and improving delayed fracture resistance. Therefore, in the present invention, Cr is 0.3% or more.
  • the amount of Cr is preferably 0.5% or more, more preferably 0.75% or more. However, even if an excessive amount of Cr is added, the effect is saturated and the manufacturing cost is increased. Therefore, in the present invention, the Cr content is 1.5% or less.
  • the amount of Cr is preferably 1.4% or less, more preferably 1.3% or less.
  • Al is an element that can be added as a deoxidizer and can prevent austenite grains from coarsening by forming AlN, thereby improving delayed fracture resistance.
  • the Al content is 0.01% or more.
  • the amount of Al is preferably 0.04% or more, more preferably 0.05% or more.
  • the Al content is 0.1% or less.
  • the amount of Al is preferably 0.08% or less, more preferably 0.07% or less.
  • Ti is an element that combines with N and C in steel to precipitate TiN and TiC.
  • TiN and TiC act as hydrogen trap sites and contribute to improving delayed fracture resistance.
  • TiN and TiC effectively act on the refinement of crystal grains and contribute to the further improvement of delayed fracture resistance.
  • it is necessary to contain Ti 0.05% or more.
  • the amount of Ti is preferably 0.051% or more, more preferably 0.052% or more. However, when the amount of Ti is excessive, cold forgeability is reduced. Therefore, in the present invention, the Ti amount is 0.1% or less.
  • the amount of Ti is preferably 0.09% or less, more preferably 0.08% or less.
  • the B is an element that acts to increase the hardenability and strength of steel. Therefore, in the present invention, the B amount is 0.0003% or more.
  • the amount of B is preferably 0.0005% or more, more preferably 0.0010% or more. However, even if B is contained excessively, the effect is saturated and the toughness is decreased. Therefore, in the present invention, the B amount is 0.005% or less.
  • the B amount is preferably 0.004% or less, more preferably 0.003% or less.
  • N is an element that contributes to refinement of crystal grains by forming TiN by combining with Ti in the solidification stage after melting. Delayed fracture resistance is improved by making the crystal grains finer.
  • N is preferably contained in an amount of 0.001% or more, more preferably 0.002% or more, and further preferably 0.003% or more.
  • the N content is 0.01% or less.
  • the N amount is preferably 0.008% or less, more preferably 0.006% or less.
  • the component composition of the steel for high-strength bolts according to the present invention is as described above, and the balance is iron and inevitable impurities.
  • the inevitable impurities mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. is allowed.
  • the metal structure after rolling is a mixed structure of ferrite and pearlite.
  • This mixed structure may partially include a bainite structure.
  • the bainite structure is preferably 5 area% or less with respect to the entire metal structure.
  • the steel for high-strength bolts according to the present invention is obtained by melting and casting a steel that satisfies the above component composition, and then heating it to, for example, 950 ° C. or higher to form a wire or bar shape in a temperature range of 800 to 1000 ° C. Then, after hot rolling or hot forging, it can be produced by gradually cooling to a temperature of 600 ° C. or less at an average cooling rate of more than 0 ° C./second and 3 ° C./second or less.
  • Ti carbides, nitrides, and carbonitrides effective for refining crystal grains can be dissolved in austenite. If the temperature is less than 950 ° C., the amount of carbide, nitride, and carbonitride is reduced, and fine Ti, V carbide, nitride, and carbonitride are less likely to be formed by subsequent hot rolling. Therefore, the effect of crystal grain refinement during quenching is reduced.
  • This temperature is more preferably 1000 ° C. or higher.
  • the upper limit of heating temperature is not specifically limited, For example, what is necessary is just to be about 1350 degreeC.
  • the finish rolling temperature or hot forging temperature is preferably set to 1000 ° C. or lower.
  • the finish rolling temperature or the hot forging temperature is higher than 1000 ° C., Ti, V carbide, nitride, and carbonitride are difficult to precipitate, so that the effect of grain refinement during quenching is reduced.
  • the finish rolling temperature or the hot forging temperature becomes too low, there is an increase in rolling load and an increase in the occurrence of surface flaws, which becomes unrealistic, so the lower limit is preferably set to 800 ° C. or higher.
  • the finish rolling temperature is the average surface temperature that can be measured with a radiation thermometer before the final rolling pass or before the rolling roll group.
  • the average cooling rate after the forging is preferably 3 ° C./second or less.
  • the average cooling rate is more preferably 2 ° C./second or less.
  • the steel for high-strength bolts obtained by cooling is formed into a bolt shape according to a conventional method, and then subjected to quenching and tempering to make the metal structure tempered martensite.
  • a high-strength bolt excellent in destructibility can be obtained.
  • the conditions for quenching and tempering are not particularly limited, and may be performed according to a conventional method.
  • the quenching treatment is preferably performed by heating to 850 to 960 ° C., for example.
  • austenite can be stably formed.
  • the heating temperature is more preferably 880 ° C. or higher, and still more preferably 900 ° C. or higher.
  • the heating temperature is preferably 960 ° C. or lower.
  • the heating temperature is more preferably 950 ° C. or lower, and further preferably 940 ° C. or lower.
  • As-quenched bolts have low toughness and ductility and do not become bolt products as they are. Therefore, after quenching, tempering is performed.
  • the tempering treatment is preferably performed by heating to 300 to 500 ° C., for example.
  • the temperature during the tempering treatment is preferably 300 ° C. or higher, more preferably 330 ° C. or higher, still more preferably 350 ° C. or higher.
  • tempering at a temperature of 500 ° C. or lower is preferable.
  • the tempering temperature is more preferably 480 ° C. or less, still more preferably 450 ° C. or less.
  • the heating and holding time in the tempering process is not particularly limited, and is, for example, about 20 to 60 minutes.
  • the steel for high-strength bolts may be spheroidized prior to forming into a bolt shape.
  • the spheroidizing treatment conditions are not particularly limited, and known conditions can be adopted.
  • the heating temperature for the spheroidizing treatment may be 700 to 800 ° C., for example.
  • carbides in the pearlite structure can be dissolved in the steel.
  • the heating temperature is preferably 710 ° C or higher, more preferably 720 ° C or higher.
  • the heating temperature is preferably 800 ° C. or less, more preferably 790 ° C. or less, and further preferably 780 ° C. or less.
  • the average cooling rate is more preferably 18 ° C./hour or less, and further preferably 16 ° C./hour or less.
  • the shaft portion of the high-strength bolt obtained by quenching and tempering the steel for high-strength bolts specified in the present invention has a metal structure of tempered martensite from the surface layer to the central portion, and can secure a strength of 1100 MPa or more. . What is necessary is just to measure the intensity
  • the high-strength bolt preferably has a grain size number of prior austenite at the shaft portion of 8 or more. This is because the delayed fracture resistance improves as the crystal grains of the prior austenite become finer.
  • the grain size number is more preferably 8.5 or more, and even more preferably 9.0 or more.
  • the upper limit of the grain size number is not particularly limited, but may be, for example, 12 or less.
  • the crystal grain size number may be measured based on JIS G0551 (2013).
  • Table 1 below shows the value of 1 / ([C] ⁇ [Mn]) obtained from the left side of the above formula (1) based on the amount of C and Mn contained in the steel, the amount of C contained in the steel, Si
  • the value of [C] + [Si] / 2 + [Mn] / 2 + [Cr] / 3 obtained from the left side of the above formula (2) based on the amount, the amount of Mn, and the amount of Cr is shown.
  • the value of 1 / ([C] ⁇ [Mn]) is expressed as an X value
  • the value of [C] + [Si] / 2 + [Mn] / 2 + [Cr] / 3 is a Y value. Indicated.
  • the metal structure of the obtained wire was observed.
  • the metallographic structure was obtained by observing an arbitrary region at the D / 4 position with an optical microscope after the wire was cut in a cross section and etched with a nital etchant. D shows the diameter of a wire. The observation magnification was 400 times. As a result, the structure of the wire was a mixed structure of ferrite and pearlite.
  • the metal structure of the obtained specimen was observed in the same procedure as the above wire. As a result, the metal structure was a tempered martensite structure.
  • the grain size number can be obtained in an arbitrary region at the D / 4 position in the longitudinal section of the test material by causing the grain boundary to appear in accordance with the quenching and tempering method defined in JIS G0551 (2013). It was measured. D indicates the diameter of the test material.
  • Delayed fracture resistance was evaluated by cutting out a notched test piece shown in FIG. 1A from the above specimen and conducting a tensile test and a delayed fracture resistance test.
  • FIG. 1B shows the shape of the notch.
  • the reason why the notched specimen is used is to simulate the stress concentration in the threaded portion. That is, a tensile test was performed according to JIS Z2241 (2011) using the above-mentioned notched test piece, and the maximum stress was measured.
  • Kt shown to FIG. 1B has shown the stress concentration factor.
  • the notched test piece was immersed in a 15% HCl aqueous solution for 30 minutes, washed with water and dried, then loaded with a constant load, and a maximum load that did not break for more than 100 hours (hereinafter, 100 hours holding stress)).
  • the value obtained by dividing the 100-hour holding stress by the maximum stress that is, the value of 100-hour holding stress / maximum stress is defined as the delayed fracture strength ratio, and this value is shown in Table 2 below.
  • the case where the delayed fracture strength ratio was 0.70 or more was regarded as acceptable, and it was evaluated that the delayed fracture resistance was excellent.
  • FIG. 2 shows the relationship between the above 1 / ([C] ⁇ [Mn]) value and the delayed fracture strength ratio.
  • No. 1 as a comparative example.
  • No. An X value of 20 is outside the requirement defined in the present invention. Since 20 is a reference example containing Mo, it was not plotted in FIG.
  • No. Examples 10 to 19 are examples that do not satisfy any of the requirements defined in the present invention.
  • No. 10 is an example in which the Y value was less than 0.82 because the amount of C was too small, and a strength of 1100 MPa or more could not be secured.
  • No. No. 11 is an example in which the amount of C is excessive, and it is considered that delayed fracture resistance cannot be improved because toughness and ductility are reduced.
  • No. Nos. 12 to 14 are examples in which the amount of Mn was excessive and the X value was less than 5.5. It is considered that the grain boundary strength was lowered by segregation and delayed fracture resistance could not be improved.
  • No. No. 17 is an example that does not contain Ti, and it is considered that delayed fracture resistance could not be improved because TiC serving as a hydrogen trap site did not precipitate.
  • No. 18 is an example in which the X value was less than 5.5, and the delayed fracture resistance could not be improved.
  • No. No. 19 was an example in which the amount of Ti was too small and the X value was less than 5.5, and the delayed fracture resistance could not be improved.
  • Reference numeral 20 is a reference example simulating the JIS standard SCM435. No. No. 20 has a strength of 1100 MPa or more and can improve delayed fracture resistance. However, since it contains Mo, the cost is high.

Abstract

Provided are: steel for high strength bolts, which exhibits excellent delayed fracture resistance without addition of an expensive alloy element such as Mo or V even if the steel has a high strength, namely a tensile strength of 1,100 MPa or more; and a high strength bolt which is formed of this steel and has excellent delayed fracture resistance. Steel for high strength bolts, which has excellent delayed fracture resistance and contains, in mass%, 0.20-0.35% of C, 0.3-1.0% of Si, more than 0% but 0.6% or less of Mn, more than 0% but 0.02% or less of P, more than 0% but 0.02% or less of S, 0.3-1.5% of Cr, 0.01-0.1% of Al, 0.05-0.1% of Ti, 0.0003-0.005% of B and more than 0% but 0.01% or less of N with the balance made up of iron and unavoidable impurities, while satisfying formula (1) and formula (2). This steel for high strength bolts has a mixed structure of ferrite and pearlite. In formula (1) and formula (2), [ ] denotes the content (mass%) of each element. 1/([C] × [Mn]) ≥ 5.5 (1) [C] + [Si]/2 + [Mn]/2 + [Cr]/3 ≥ 0.82 (2)

Description

耐遅れ破壊性に優れた高強度ボルト用鋼および高強度ボルトHigh strength bolt steel and high strength bolts with excellent delayed fracture resistance
　本発明は、自動車や各種産業機械等に用いられるボルト用鋼、およびこのボルト用鋼を用いて得られるボルトに関し、特に引張強さが１１００ＭＰａ以上であっても優れた耐遅れ破壊性を発揮する高強度ボルト用鋼および高強度ボルトに関する。 The present invention relates to a bolt steel used for automobiles, various industrial machines, and the like, and a bolt obtained using the bolt steel, and exhibits excellent delayed fracture resistance even when the tensile strength is 1100 MPa or more. The present invention relates to steel for high strength bolts and high strength bolts.
　引張強さが１１００ＭＰａ以上の高強度ボルトには、ＳＣＭ４３５等の規格鋼が多用されている。ＳＣＭ４３５等の規格鋼には、Ｍｏ等の合金元素が多量に添加されているため、鋼材コストが高くなる。鋼材コスト低減の要請に伴い、Ｍｏを省略したＳＣＭ代替鋼への要望が高まっている。しかし、合金元素を単純に低減するだけでは、焼入れ性が低下し、強度の確保が困難となる。 Standard steels such as SCM435 are frequently used for high-strength bolts with a tensile strength of 1100 MPa or more. Since standard steels such as SCM435 are added with a large amount of alloy elements such as Mo, the cost of steel materials increases. With the request of steel material cost reduction, the request for SCM alternative steel which omitted Mo is increasing. However, simply reducing the alloy elements reduces the hardenability and makes it difficult to ensure strength.
　そこで焼入れ性を向上し、強度を高めるためにボロンを添加したボロン含有鋼を高強度ボルトの素材として用いることが検討されている。ボロン含有鋼は、引張強さが１１００ＭＰａ未満のボルトでは、既に使用が拡大している。しかし強度上昇に伴い耐遅れ破壊性が大幅に低下するため、使用環境の厳しい部位での適用は困難である。 Therefore, it has been studied to use boron-containing steel to which boron is added in order to improve hardenability and increase strength as a material for high-strength bolts. Boron-containing steel has already been used for bolts having a tensile strength of less than 1100 MPa. However, the delayed fracture resistance greatly decreases as the strength increases, so that it is difficult to apply in a severe environment.
　ボロン含有鋼の耐遅れ破壊性を向上させる技術は、既にいくつか提案されている。例えば特許文献１には、ボロン含有鋼に所定量のＣｕを添加することによって、鋼の耐食性を向上させ、鋼中への拡散性水素の侵入を抑制し、耐遅れ破壊性を向上させる技術が提案されている。しかしＣｕを添加させるだけでは耐遅れ破壊性の確保は不充分である。 Several technologies have already been proposed for improving the delayed fracture resistance of boron-containing steel. For example, Patent Document 1 discloses a technique for improving corrosion resistance of steel by adding a predetermined amount of Cu to boron-containing steel, suppressing intrusion of diffusible hydrogen into the steel, and improving delayed fracture resistance. Proposed. However, it is insufficient to ensure delayed fracture resistance only by adding Cu.
　また特許文献２には、ボロン含有鋼の耐遅れ破壊性を改善するために、Ｖを添加する技術が提案されている。しかしＶは、Ｍｏと同様、高価な希少金属であるため、Ｖを添加したボロン含有鋼は、ＳＣＭ代替鋼としてのコスト低減効果は小さい。 Patent Document 2 proposes a technique of adding V in order to improve delayed fracture resistance of boron-containing steel. However, since V is an expensive rare metal like Mo, the boron-containing steel to which V is added has a small cost reduction effect as an SCM alternative steel.
　本出願人も特許文献３に、化学成分組成のうち、特に、ＳｉとＣの含有量の比を適正な範囲に制御することによって、過酷な環境下でも優れた耐遅れ破壊性を発揮するボロン添加高強度ボルト用鋼を提案した。しかしこのボロン添加高強度ボルト用鋼は、上記特許文献２と同様、Ｖを必須元素として添加しているため、ＳＣＭ代替鋼としてのコスト低減効果は小さい。 The present applicant also discloses in Patent Document 3 boron that exhibits excellent delayed fracture resistance even in harsh environments by controlling the ratio of the content of Si and C in the chemical composition within an appropriate range. A steel for added high strength bolts was proposed. However, since this boron-added high-strength bolt steel contains V as an essential element, as in Patent Document 2, the cost reduction effect as an SCM alternative steel is small.
　このように耐遅れ破壊性を改善するためにこれまで提案されている技術は、いずれも高強度、過酷環境下での耐遅れ破壊性や製造面で問題を有している。 Thus, all of the technologies proposed so far for improving delayed fracture resistance have problems in terms of delayed fracture resistance and manufacturing in high strength and harsh environments.
特開２００６－１１８００３号公報JP 2006-118033 A 特開２００７－２１７７１８号公報JP 2007-217718 A 特開２０１３－２２７６４７号公報JP 2013-227647 A
　本発明はこのような事情に鑑みてなされたものであって、その目的は、ＭｏやＶ等の高価な合金元素を添加することなく、引張強さが１１００ＭＰａ以上の高強度であっても耐遅れ破壊性に優れた高強度ボルト用鋼、およびこの鋼からなる耐遅れ破壊性に優れた高強度ボルトを提供することにある。 The present invention has been made in view of such circumstances, and the object thereof is to add resistance to high tensile strength of 1100 MPa or more without adding expensive alloy elements such as Mo and V. An object of the present invention is to provide a steel for high-strength bolts excellent in delayed fracture resistance and a high-strength bolt excellent in delayed fracture resistance made of this steel.
　本発明者らは、耐遅れ破壊性に優れた高強度ボルト用鋼、およびこの鋼からなる耐遅れ破壊性に優れた高強度ボルトを提供するために、鋭意検討を重ねてきた。その結果、化学成分組成を適切に制御したうえで、ＣとＭｎの含有量が後述する式（１）の関係を満足すれば過酷な環境下でも優れた耐遅れ破壊性を発揮すること、Ｃ、Ｓｉ、Ｍｎ、およびＣｒの含有量が後述する式（２）の関係を満足すれば、強度を高められることを見出し、本発明を完成した。 The present inventors have intensively studied to provide a steel for high-strength bolts excellent in delayed fracture resistance and a high-strength bolt excellent in delayed fracture resistance made of this steel. As a result, after appropriately controlling the chemical composition, if the C and Mn content satisfies the relationship of the formula (1) described later, it exhibits excellent delayed fracture resistance even in a harsh environment. The present invention has been completed by finding that the strength can be increased if the contents of Si, Mn, and Cr satisfy the relationship of the formula (2) described later.
　即ち、上記課題を解決することのできた本発明に係る耐遅れ破壊性に優れた高強度ボルト用鋼とは、質量％で、Ｃ：０．２０～０．３５％、Ｓｉ：０．３～１．０％、Ｍｎ：０％超０．６％以下、Ｐ：０％超０．０２％以下、Ｓ：０％超０．０２％以下、Ｃｒ：０．３～１．５％、Ａｌ：０．０１～０．１％、Ｔｉ：０．０５～０．１％、Ｂ：０．０００３～０．００５％、およびＮ：０％超０．０１％以下を含有すると共に、下記式（１）および式（２）を満足し、残部が鉄および不可避不純物からなり、フェライトとパーライトの混合組織である点に要旨を有するものである。下記式（１）、式（２）において、［　］は、各元素の含有量（質量％）を意味する。
１／（［Ｃ］×［Ｍｎ］）≧５．５　・・・（１）
［Ｃ］＋［Ｓｉ］／２＋［Ｍｎ］／２＋［Ｃｒ］／３≧０．８２　・・・（２） That is, the steel for high-strength bolts with excellent delayed fracture resistance according to the present invention that has been able to solve the above-mentioned problems is, by mass%, C: 0.20 to 0.35%, Si: 0.3 to 1.0%, Mn: more than 0% to 0.6% or less, P: more than 0% to 0.02% or less, S: more than 0% to 0.02% or less, Cr: 0.3 to 1.5%, Al : 0.01 to 0.1%, Ti: 0.05 to 0.1%, B: 0.0003 to 0.005%, and N: more than 0% and 0.01% or less, (1) and formula (2) are satisfied, and the balance is made of iron and inevitable impurities, and has a gist in that it is a mixed structure of ferrite and pearlite. In the following formulas (1) and (2), [] means the content (% by mass) of each element.
1 / ([C] × [Mn]) ≧ 5.5 (1)
[C] + [Si] / 2 + [Mn] / 2 + [Cr] /3≧0.82 (2)
　本発明には、上記高強度ボルト用鋼を用いて得られるボルトであって、金属組織は焼戻しマルテンサイト組織で、引張強さが１１００ＭＰａ以上である耐遅れ破壊性に優れた高強度ボルトも包含される。 The present invention also includes a bolt obtained using the steel for high-strength bolts, wherein the metal structure is a tempered martensite structure and the tensile strength is 1100 MPa or more and the high-strength bolt excellent in delayed fracture resistance. Is done.
　本発明によれば、化学成分組成を適切に制御すると共に、ＣとＭｎの含有量の関係、およびＣ、Ｓｉ、Ｍｎ、およびＣｒの含有量の関係を厳密に制御することによって、引張強さが１１００ＭＰａ以上でも耐遅れ破壊性に優れた高強度ボルト用鋼が実現でき、この鋼を用いれば、耐遅れ破壊性に優れた高強度ボルトが提供できる。 According to the present invention, the tensile strength is controlled by appropriately controlling the chemical component composition and strictly controlling the relationship between the contents of C and Mn and the contents of C, Si, Mn, and Cr. Even if it is 1100 MPa or more, a steel for high-strength bolts excellent in delayed fracture resistance can be realized. If this steel is used, a high-strength bolt excellent in delayed fracture resistance can be provided.
図１Ａは、切り欠き付き試験片の形状を示す模式図であり、図１Ｂは切り欠き部の形状を示す模式図である。FIG. 1A is a schematic diagram showing the shape of a notched test piece, and FIG. 1B is a schematic diagram showing the shape of a notch. 図２は、１／（［Ｃ］×［Ｍｎ］）の値と、遅れ破壊強度比との関係を示すグラフである。FIG. 2 is a graph showing the relationship between the value of 1 / ([C] × [Mn]) and the delayed fracture strength ratio.
　本発明者らは、ＭｏやＶ等の高価な合金元素を添加することなく、引張強さが１１００ＭＰａ以上の高強度でも優れた耐遅れ破壊性を示すボルトを提供するために、検討を重ねた。その結果、引張強さが１１００ＭＰａ以上の高強度ボルトについては、合金元素を含有させるよりも、Ｍｎ含有量を極力低減することが耐遅れ破壊性の確保に有効であることが明らかとなった。また、Ｃは鋼の強度を確保する上で有用な元素であるが、その含有量を増加させると鋼の靭性や耐食性が悪化し、遅れ破壊を引き起こしやすくなった。 The present inventors have repeatedly studied to provide a bolt exhibiting excellent delayed fracture resistance even at a high strength of 1100 MPa or more without adding an expensive alloy element such as Mo or V. . As a result, for high-strength bolts with a tensile strength of 1100 MPa or more, it was revealed that reducing the Mn content as much as possible was more effective in securing delayed fracture resistance than containing alloy elements. C is an element useful for ensuring the strength of the steel. However, increasing its content deteriorates the toughness and corrosion resistance of the steel and tends to cause delayed fracture.
　そこで本発明の高強度ボルト用鋼は、引張強さが１１００ＭＰａ以上の高強度ボルトにおける耐遅れ破壊性を改善するために、ＣとＭｎの含有量が下記式（１）を満足することが重要である。下記式（１）において、［　］は、各元素の含有量（質量％）を意味している。
１／（［Ｃ］×［Ｍｎ］）≧５．５　・・・（１） Therefore, in the steel for high-strength bolts of the present invention, it is important that the contents of C and Mn satisfy the following formula (1) in order to improve delayed fracture resistance in high-strength bolts having a tensile strength of 1100 MPa or more. It is. In the following formula (1), [] means the content (% by mass) of each element.
1 / ([C] × [Mn]) ≧ 5.5 (1)
　本発明では、上記式（１）の左辺の値をＸ値としたとき、このＸ値を５．５以上とする。Ｘ値は、好ましくは６．５以上、より好ましくは７．０以上である。Ｘ値の上限は、後述するＣ量およびＭｎ量から自ずと設定されるが、Ｘ値を過剰に大きくしても、耐遅れ破壊性に及ぼす効果は飽和するため、上限は、例えば、３０程度であることが好ましい。Ｘ値は、より好ましくは２０以下、更に好ましくは１５以下である。 In the present invention, when the value on the left side of the above formula (1) is the X value, the X value is set to 5.5 or more. The X value is preferably 6.5 or more, more preferably 7.0 or more. The upper limit of the X value is naturally set from the amount of C and the amount of Mn described later, but even if the X value is excessively increased, the effect on delayed fracture resistance is saturated, so the upper limit is, for example, about 30 Preferably there is. The X value is more preferably 20 or less, and still more preferably 15 or less.
　このように耐遅れ破壊性を改善するために、Ｃを低減すると、強度が低下する。また、Ｍｎを低減すると、焼入れ性が悪くなり、強度が低下する。 ¡In order to improve delayed fracture resistance in this way, the strength decreases when C is reduced. Moreover, when Mn is reduced, the hardenability deteriorates and the strength decreases.
　そこで本発明では、強度を確保するために、ＣおよびＭｎの含有量を考慮したうえで、ＳｉとＣｒを積極的に添加することが重要である。具体的には、引張強さが１１００ＭＰａ以上の強度を確保するために、Ｃ、Ｓｉ、Ｍｎ、およびＣｒの含有量が下記式（２）を満足することが重要である。式（２）において、［　］は、各元素の含有量（質量％）を意味している。
［Ｃ］＋［Ｓｉ］／２＋［Ｍｎ］／２＋［Ｃｒ］／３≧０．８２　・・・（２） Therefore, in the present invention, it is important to positively add Si and Cr in consideration of the contents of C and Mn in order to ensure strength. Specifically, it is important that the contents of C, Si, Mn, and Cr satisfy the following formula (2) in order to ensure that the tensile strength is 1100 MPa or more. In Formula (2), [] means content (mass%) of each element.
[C] + [Si] / 2 + [Mn] / 2 + [Cr] /3≧0.82 (2)
　上記式（２）における各元素の係数は、強度向上への寄与度を示している。本発明では、上記式（２）の左辺の値をＹ値としたとき、このＹ値を０．８２以上とする。Ｙ値は、好ましくは０．９０以上、より好ましくは１．００以上である。Ｙ値の上限は特に限定されないが、Ｙ値が過剰に大きくなると、母材の強度が高くなり過ぎてボルト形状に成形する時の冷間鍛造性が劣化するため、上限は、１．３０程度であることが好ましい。Ｙ値は、より好ましくは１．２０以下、更に好ましくは１．１５以下である。 The coefficient of each element in the above formula (2) indicates the degree of contribution to strength improvement. In the present invention, when the value on the left side of the formula (2) is a Y value, this Y value is set to 0.82 or more. The Y value is preferably 0.90 or more, more preferably 1.00 or more. The upper limit of the Y value is not particularly limited, but if the Y value becomes excessively large, the strength of the base material becomes too high and the cold forgeability when forming into a bolt shape deteriorates, so the upper limit is about 1.30. It is preferable that The Y value is more preferably 1.20 or less, and still more preferably 1.15 or less.
　次に、本発明の高強度ボルト用鋼の成分組成について説明する。本発明の高強度ボルト用鋼は、上述した上記式（１）および式（２）を満足するものであるが、その前提として、Ｃ：０．２０～０．３５％、Ｓｉ：０．３～１．０％、Ｍｎ：０％超０．６％以下、Ｐ：０％超０．０２％以下、Ｓ：０％超０．０２％以下、Ｃｒ：０．３～１．５％、Ａｌ：０．０１～０．１％、Ｔｉ：０．０５～０．１％、Ｂ：０．０００３～０．００５％、およびＮ：０％超０．０１％以下を含有することが重要である。 Next, the component composition of the steel for high-strength bolts of the present invention will be described. The steel for high-strength bolts of the present invention satisfies the above-mentioned formulas (1) and (2). As the premise thereof, C: 0.20 to 0.35%, Si: 0.3 To 1.0%, Mn: more than 0% to 0.6% or less, P: more than 0% to 0.02% or less, S: more than 0% to 0.02% or less, Cr: 0.3 to 1.5%, It is important to contain Al: 0.01 to 0.1%, Ti: 0.05 to 0.1%, B: 0.0003 to 0.005%, and N: more than 0% and 0.01% or less It is.
　Ｃは、鋼の強度と延性をバランス良く確保するための元素であり、高強度ボルトとして必要な引張強さを確保する上で欠くことができない元素である。このような効果を発揮させるためには、Ｃは０．２０％以上含有させる必要がある。Ｃ量は、好ましくは０．２３％以上、より好ましくは０．２５％以上である。しかし、過剰に含有すると、靭性および延性の低下を招いて耐遅れ破壊性が劣化する。従って本発明では、Ｃ量は０．３５％以下とする。Ｃ量は、好ましくは０．３２％以下、より好ましくは０．３０％以下である。 C is an element for securing the strength and ductility of steel in a well-balanced manner, and is an indispensable element for securing the tensile strength necessary as a high-strength bolt. In order to exert such an effect, C needs to be contained by 0.20% or more. The amount of C is preferably 0.23% or more, more preferably 0.25% or more. However, when it contains excessively, the toughness and ductility will fall and delayed fracture resistance will deteriorate. Therefore, in the present invention, the C amount is set to 0.35% or less. The amount of C is preferably 0.32% or less, more preferably 0.30% or less.
　Ｓｉは、溶製時に脱酸剤として用いられる元素であり、また焼戻し軟化抵抗を高める作用を有し、強度を高めるために必要な元素である。従って本発明では、Ｓｉ量は０．３％以上とする。Ｓｉ量は、好ましくは０．４％以上、より好ましくは０．４７％以上である。しかし、Ｓｉ量が過剰になると、ボルト形状に成形するときの冷間鍛造性が劣化する。従って本発明では、Ｓｉ量は１．０％以下とする。Ｓｉ量は、好ましくは０．６０％以下、より好ましくは０．５５％以下である。 Si is an element used as a deoxidizer at the time of melting, has an effect of increasing temper softening resistance, and is an element necessary for increasing strength. Therefore, in the present invention, the Si amount is set to 0.3% or more. The amount of Si is preferably 0.4% or more, more preferably 0.47% or more. However, when the amount of Si becomes excessive, the cold forgeability when forming into a bolt shape deteriorates. Therefore, in the present invention, the Si amount is 1.0% or less. The amount of Si is preferably 0.60% or less, more preferably 0.55% or less.
　Ｍｎを過剰に含有すると、Ｍｎが結晶粒界に偏析し、粒界強度が低下し、耐遅れ破壊性が著しく低下する。また、Ｍｎ量が過剰になると、耐食性が劣化し、耐遅れ破壊性が低下する。従って本発明では、Ｍｎ量は０．６％以下とする必要がある。Ｍｎ量は、好ましくは０．５５％以下、より好ましくは０．５％以下である。しかし、Ｍｎは、溶製時に脱酸剤として有効に作用すると共に、鋼の焼入れ性を高めて強度を高める作用を有している元素である。こうした効果を有効に発揮させるには、０．１％以上含有させることが好ましい。Ｍｎ量は、より好ましくは０．２％以上、更に好ましくは０．３０％以上である。 When Mn is contained excessively, Mn is segregated at the grain boundary, the grain boundary strength is lowered, and the delayed fracture resistance is markedly lowered. Moreover, when the amount of Mn becomes excessive, corrosion resistance will deteriorate and delayed fracture resistance will fall. Therefore, in the present invention, the amount of Mn needs to be 0.6% or less. The amount of Mn is preferably 0.55% or less, more preferably 0.5% or less. However, Mn is an element that acts effectively as a deoxidizer during melting and has the effect of increasing the hardenability of the steel and increasing the strength. In order to exhibit such an effect effectively, it is preferable to make it contain 0.1% or more. The amount of Mn is more preferably 0.2% or more, and still more preferably 0.30% or more.
　Ｐは、不可避不純物として含有する元素であり、過剰に含有すると、粒界偏析を起こして粒界強度を低下させ、耐遅れ破壊性を悪化させる。従って本発明では、Ｐ量は０．０２％以下とする。Ｐ量は、好ましくは０．０１５％以下、より好ましくは０．０１％以下である。Ｐ量は、できるだけ低減することが好ましいが、Ｐ量を０．００１％未満にするにはコスト高となるため、下限は、０．００１％であればよい。 P is an element contained as an unavoidable impurity, and if contained excessively, it causes segregation at the grain boundary, lowers the grain boundary strength, and deteriorates delayed fracture resistance. Therefore, in the present invention, the P amount is 0.02% or less. The amount of P is preferably 0.015% or less, more preferably 0.01% or less. The amount of P is preferably reduced as much as possible. However, in order to reduce the amount of P to less than 0.001%, the cost is high, so the lower limit may be 0.001%.
　Ｓは、不可避不純物として含有する元素であり、過剰に含有すると、熱間脆性を引き起こすばかりでなく、硫化物が結晶粒界に偏析し、粒界強度の低下を招いて耐遅れ破壊性を低下させる。従って本発明では、Ｓ量は０．０２％以下とする。Ｓ量は、好ましくは０．０１５％以下、より好ましくは０．０１％以下である。Ｓ量は、できるだけ低減することが好ましいが、Ｓ量を０．００１％未満にするにはコスト高となるため、下限は、０．００１％であればよい。 S is an element contained as an unavoidable impurity. If it is excessively contained, it not only causes hot brittleness, but also sulfides segregate at the crystal grain boundaries, leading to a decrease in grain boundary strength and a decrease in delayed fracture resistance. Let Therefore, in the present invention, the S amount is 0.02% or less. The amount of S is preferably 0.015% or less, more preferably 0.01% or less. The amount of S is preferably reduced as much as possible. However, since the cost is increased to make the amount of S less than 0.001%, the lower limit may be 0.001%.
　Ｃｒは、焼入れ性を高め、強度を高めるのに作用する元素である。また、焼戻し軟化抵抗を高め、強度を向上させる作用も有している。更に、Ｃｒは、鋼の耐食性を高め、耐遅れ破壊性を向上させるのにも寄与する元素である。従って本発明では、Ｃｒは０．３％以上とする。Ｃｒ量は、好ましくは０．５％以上、より好ましくは０．７５％以上である。しかし、Ｃｒ量を過剰に添加してもその効果は飽和すると共に、製造コストの増加を招く。従って本発明では、Ｃｒ量は１．５％以下とする。Ｃｒ量は、好ましくは１．４％以下、より好ましくは１．３％以下である。 Cr is an element that acts to increase hardenability and strength. It also has the effect of increasing temper softening resistance and improving strength. Further, Cr is an element that contributes to enhancing the corrosion resistance of steel and improving delayed fracture resistance. Therefore, in the present invention, Cr is 0.3% or more. The amount of Cr is preferably 0.5% or more, more preferably 0.75% or more. However, even if an excessive amount of Cr is added, the effect is saturated and the manufacturing cost is increased. Therefore, in the present invention, the Cr content is 1.5% or less. The amount of Cr is preferably 1.4% or less, more preferably 1.3% or less.
　Ａｌは、脱酸剤として添加されるほか、ＡｌＮを形成することによってオーステナイト粒の粗大化を防止でき、その結果、耐遅れ破壊性を向上させるのに作用する元素である。こうした作用を発揮させるために、Ａｌ量は０．０１％以上とする。Ａｌ量は、好ましくは０．０４％以上、より好ましくは０．０５％以上である。しかし、Ａｌを過剰に含有させてもその効果は飽和すると共に、製造コストの増加を招く。また、Ａｌを過剰に含有すると、冷間鍛造性を悪化させる。従って本発明では、Ａｌ量は０．１％以下とする。Ａｌ量は、好ましくは０．０８％以下、より好ましくは０．０７％以下である。 Al is an element that can be added as a deoxidizer and can prevent austenite grains from coarsening by forming AlN, thereby improving delayed fracture resistance. In order to exert such an effect, the Al content is 0.01% or more. The amount of Al is preferably 0.04% or more, more preferably 0.05% or more. However, even if Al is contained excessively, the effect is saturated and the manufacturing cost is increased. Moreover, when it contains Al excessively, cold forgeability will be deteriorated. Therefore, in the present invention, the Al content is 0.1% or less. The amount of Al is preferably 0.08% or less, more preferably 0.07% or less.
　Ｔｉは、鋼中のＮおよびＣと結合してＴｉＮおよびＴｉＣを析出させる元素である。ＴｉＮおよびＴｉＣは、水素トラップサイトとして作用し、耐遅れ破壊性を向上させるのに寄与する。また、ＴｉＮおよびＴｉＣは、結晶粒の微細化に有効に作用し、耐遅れ破壊性の更なる向上に寄与する。こうした作用を発揮させために、Ｔｉは０．０５％以上含有させる必要がある。Ｔｉ量は、好ましくは０．０５１％以上、より好ましくは０．０５２％以上である。しかし、Ｔｉ量が過剰になると冷間鍛造性の低下を招く。従って本発明では、Ｔｉ量は０．１％以下とする。Ｔｉ量は、好ましくは０．０９％以下、より好ましくは０．０８％以下である。 Ti is an element that combines with N and C in steel to precipitate TiN and TiC. TiN and TiC act as hydrogen trap sites and contribute to improving delayed fracture resistance. In addition, TiN and TiC effectively act on the refinement of crystal grains and contribute to the further improvement of delayed fracture resistance. In order to exhibit such an effect, it is necessary to contain Ti 0.05% or more. The amount of Ti is preferably 0.051% or more, more preferably 0.052% or more. However, when the amount of Ti is excessive, cold forgeability is reduced. Therefore, in the present invention, the Ti amount is 0.1% or less. The amount of Ti is preferably 0.09% or less, more preferably 0.08% or less.
　Ｂは、鋼の焼入れ性を高め、強度を高めるのに作用する元素である。従って本発明では、Ｂ量は０．０００３％以上とする。Ｂ量は、好ましくは０．０００５％以上、より好ましくは０．００１０％以上である。しかし、Ｂを過剰に含有しても、その効果は飽和する他、却って靭性を低下させる。従って本発明では、Ｂ量は０．００５％以下とする。Ｂ量は、好ましくは０．００４％以下、より好ましくは０．００３％以下とする。 B is an element that acts to increase the hardenability and strength of steel. Therefore, in the present invention, the B amount is 0.0003% or more. The amount of B is preferably 0.0005% or more, more preferably 0.0010% or more. However, even if B is contained excessively, the effect is saturated and the toughness is decreased. Therefore, in the present invention, the B amount is 0.005% or less. The B amount is preferably 0.004% or less, more preferably 0.003% or less.
　Ｎは、溶製後の凝固段階でＴｉと結合してＴｉＮを形成し、結晶粒の微細化に寄与する元素である。結晶粒が微細化することにより、耐遅れ破壊性が向上する。こうした作用を有効に発揮させるには、Ｎは０．００１％以上含有することが好ましく、より好ましくは０．００２％以上、更に好ましくは０．００３％以上である。しかし、Ｎを過剰に含有し、ＴｉＮが過剰に形成されると、熱間圧延時に１３００℃程度に加熱してもＴｉＮは溶解せず、Ｔｉ炭化物の形成が阻害される。また、Ｎが固溶の状態で鋼中に存在すると、冷間鍛造性が著しく低下する。従って本発明では、Ｎ量は０．０１％以下とする。Ｎ量は、好ましくは０．００８％以下、より好ましくは０．００６％以下である。 N is an element that contributes to refinement of crystal grains by forming TiN by combining with Ti in the solidification stage after melting. Delayed fracture resistance is improved by making the crystal grains finer. In order to effectively exhibit such an action, N is preferably contained in an amount of 0.001% or more, more preferably 0.002% or more, and further preferably 0.003% or more. However, if N is contained excessively and TiN is excessively formed, TiN is not dissolved even when heated to about 1300 ° C. during hot rolling, and the formation of Ti carbide is inhibited. Further, when N is present in the steel in a solid solution state, the cold forgeability is significantly lowered. Therefore, in the present invention, the N content is 0.01% or less. The N amount is preferably 0.008% or less, more preferably 0.006% or less.
　本発明に係る高強度ボルト用鋼の成分組成は、上記の通りであり、残部は、鉄および不可避不純物である。該不可避不純物としては、原料、資材、製造設備等の状況によって持ち込まれる元素の混入が許容される。 The component composition of the steel for high-strength bolts according to the present invention is as described above, and the balance is iron and inevitable impurities. As the inevitable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. is allowed.
　本発明に係る高強度ボルト用鋼は、圧延後の金属組織がフェライトとパーライトの混合組織である。この混合組織には、ベイナイト組織が一部含まれていてもよい。ベイナイト組織は、金属組織全体に対して５面積％以下であることが好ましい。 In the steel for high-strength bolts according to the present invention, the metal structure after rolling is a mixed structure of ferrite and pearlite. This mixed structure may partially include a bainite structure. The bainite structure is preferably 5 area% or less with respect to the entire metal structure.
　次に、本発明に係る高強度ボルト用鋼を製造する方法について説明する。本発明に係る高強度ボルト用鋼は、上記成分組成を満足する鋼を溶製し、鋳造した後、例えば、９５０℃以上に加熱し、８００～１０００℃の温度域で、線材または棒鋼形状に、熱間圧延または熱間鍛造した後、６００℃以下の温度まで、平均冷却速度を０℃／秒超３℃／秒以下として徐冷することにより製造できる。 Next, a method for producing the steel for high-strength bolts according to the present invention will be described. The steel for high-strength bolts according to the present invention is obtained by melting and casting a steel that satisfies the above component composition, and then heating it to, for example, 950 ° C. or higher to form a wire or bar shape in a temperature range of 800 to 1000 ° C. Then, after hot rolling or hot forging, it can be produced by gradually cooling to a temperature of 600 ° C. or less at an average cooling rate of more than 0 ° C./second and 3 ° C./second or less.
　９５０℃以上に加熱することによって、結晶粒の微細化に有効なＴｉの炭化物、窒化物、炭窒化物を、オーステナイトに固溶させることができる。この温度が９５０℃未満では、炭化物、窒化物、炭窒化物の固溶量が少なくなり、後の熱間圧延で微細なＴｉやＶの炭化物、窒化物、炭窒化物が生成しにくくなる。そのため、焼入れ時の結晶粒微細化の効果が減少する。この温度は、より好ましくは１０００℃以上である。加熱温度の上限は特に限定されないが、例えば、１３５０℃程度とすればよい。 By heating to 950 ° C. or higher, Ti carbides, nitrides, and carbonitrides effective for refining crystal grains can be dissolved in austenite. If the temperature is less than 950 ° C., the amount of carbide, nitride, and carbonitride is reduced, and fine Ti, V carbide, nitride, and carbonitride are less likely to be formed by subsequent hot rolling. Therefore, the effect of crystal grain refinement during quenching is reduced. This temperature is more preferably 1000 ° C. or higher. Although the upper limit of heating temperature is not specifically limited, For example, what is necessary is just to be about 1350 degreeC.
　熱間圧延または熱間鍛造では、上記加熱時に固溶させたＴｉやＶを微細な炭化物、窒化物、炭窒化物として鋼中に析出させればよい。そのためには仕上げ圧延温度または熱間鍛造温度を１０００℃以下にすることが好ましい。仕上げ圧延温度または熱間鍛造温度が１０００℃よりも高くなるとＴｉやＶの炭化物、窒化物、炭窒化物が析出しにくくなるため、焼入れ時の結晶粒微細化の効果が減少する。一方、仕上げ圧延温度または熱間鍛造温度が低くなりすぎると、圧延荷重の増加や表面疵の発生増大があり、非現実的となるためその下限は８００℃以上とすることが好ましい。ここで、仕上げ圧延温度は、最終圧延パス前または圧延ロール群前の放射温度計で測定可能な表面の平均温度とした。 In hot rolling or hot forging, Ti or V dissolved at the time of heating may be precipitated in steel as fine carbide, nitride, or carbonitride. For this purpose, the finish rolling temperature or hot forging temperature is preferably set to 1000 ° C. or lower. When the finish rolling temperature or the hot forging temperature is higher than 1000 ° C., Ti, V carbide, nitride, and carbonitride are difficult to precipitate, so that the effect of grain refinement during quenching is reduced. On the other hand, if the finish rolling temperature or the hot forging temperature becomes too low, there is an increase in rolling load and an increase in the occurrence of surface flaws, which becomes unrealistic, so the lower limit is preferably set to 800 ° C. or higher. Here, the finish rolling temperature is the average surface temperature that can be measured with a radiation thermometer before the final rolling pass or before the rolling roll group.
　熱間圧延または熱間鍛造後の冷却では、後のボルト形状への成形性を向上させるため、金属組織をフェライトとパーライトの混合組織にすることが重要であり、そのためには熱間圧延または熱間鍛造後の平均冷却速度を３℃／秒以下にすることが好ましい。平均冷却速度が３℃／秒より大きくなると、ベイナイトやマルテンサイトが生成するため、ボルト形状への成形性が大幅に悪化する。平均冷却速度は、より好ましくは２℃／秒以下である。 In cooling after hot rolling or hot forging, it is important to change the metal structure to a mixed structure of ferrite and pearlite in order to improve the formability to the bolt shape later. The average cooling rate after the forging is preferably 3 ° C./second or less. When the average cooling rate is higher than 3 ° C./second, bainite and martensite are generated, and thus the formability into a bolt shape is significantly deteriorated. The average cooling rate is more preferably 2 ° C./second or less.
　冷却して得られた高強度ボルト用鋼は、常法に従って、ボルト形状に成形した後、焼入れ処理および焼戻し処理を行い、金属組織を焼戻しマルテンサイトとすることによって、高強度で、しかも耐遅れ破壊性に優れた高強度ボルトが得られる。 The steel for high-strength bolts obtained by cooling is formed into a bolt shape according to a conventional method, and then subjected to quenching and tempering to make the metal structure tempered martensite. A high-strength bolt excellent in destructibility can be obtained.
　焼入れ処理および焼戻し処理の条件は特に限定されず、常法に従って行えばよい。 The conditions for quenching and tempering are not particularly limited, and may be performed according to a conventional method.
　焼入れ処理は、例えば、８５０～９６０℃に加熱して行うことが好ましい。８５０℃以上に加熱することにより、安定的にオーステナイト化できる。加熱温度は、より好ましくは８８０℃以上、更に好ましくは９００℃以上である。しかし、加熱温度が９６０℃を超えると、結晶粒が粗大化し、耐遅れ破壊性が劣化することがある。従って上記加熱温度は、９６０℃以下とすることが好ましい。加熱温度は、より好ましくは９５０℃以下、更に好ましくは９４０℃以下である。 The quenching treatment is preferably performed by heating to 850 to 960 ° C., for example. By heating to 850 ° C. or higher, austenite can be stably formed. The heating temperature is more preferably 880 ° C. or higher, and still more preferably 900 ° C. or higher. However, when the heating temperature exceeds 960 ° C., the crystal grains become coarse and delayed fracture resistance may deteriorate. Therefore, the heating temperature is preferably 960 ° C. or lower. The heating temperature is more preferably 950 ° C. or lower, and further preferably 940 ° C. or lower.
　焼入れしたままのボルトは、靭性および延性が低く、そのままの状態ではボルト製品にならない。そこで、焼入れ処理した後は、焼戻し処理を施す。 As-quenched bolts have low toughness and ductility and do not become bolt products as they are. Therefore, after quenching, tempering is performed.
　焼戻し処理は、例えば、３００～５００℃に加熱して行うことが好ましい。焼戻し処理時の温度は、３００℃以上が好ましく、より好ましくは３３０℃以上、更に好ましくは３５０℃以上である。低温焼戻し脆性を回避するには、３８０℃以上の温度で焼戻しするのが良い。しかし焼戻し温度が５００℃を超えると、１１００ＭＰａ以上の強度を確保することが難しくなる。従って５００℃以下の温度で焼戻しすることが好ましい。焼戻し温度は、より好ましくは４８０℃以下、更に好ましくは４５０℃以下である。 The tempering treatment is preferably performed by heating to 300 to 500 ° C., for example. The temperature during the tempering treatment is preferably 300 ° C. or higher, more preferably 330 ° C. or higher, still more preferably 350 ° C. or higher. In order to avoid low temperature temper embrittlement, it is preferable to temper at a temperature of 380 ° C. or higher. However, when the tempering temperature exceeds 500 ° C., it becomes difficult to secure a strength of 1100 MPa or more. Therefore, tempering at a temperature of 500 ° C. or lower is preferable. The tempering temperature is more preferably 480 ° C. or less, still more preferably 450 ° C. or less.
　焼戻し処理における加熱保持時間は特に限定されず、例えば、２０～６０分程度である。 The heating and holding time in the tempering process is not particularly limited, and is, for example, about 20 to 60 minutes.
　上記高強度ボルト用鋼は、ボルト形状に成形するに先立って、球状化処理を行なってもよい。球状化処理条件は特に限定されず、公知の条件を採用できる。 The steel for high-strength bolts may be spheroidized prior to forming into a bolt shape. The spheroidizing treatment conditions are not particularly limited, and known conditions can be adopted.
　球状化処理するときの加熱温度は、例えば、７００～８００℃とすればよい。７００℃以上に加熱することによって、パーライト組織中の炭化物を鋼中に溶かすことができる。加熱温度は、好ましくは７１０℃以上、より好ましくは７２０℃以上である。しかし加熱温度が８００℃を超えると、冷却中に再生パーライトが生成し、冷間鍛造性が劣化することがある。従って加熱温度は、８００℃以下とすることが好ましく、より好ましくは７９０℃以下、更に好ましくは７８０℃以下である。 The heating temperature for the spheroidizing treatment may be 700 to 800 ° C., for example. By heating to 700 ° C. or higher, carbides in the pearlite structure can be dissolved in the steel. The heating temperature is preferably 710 ° C or higher, more preferably 720 ° C or higher. However, when the heating temperature exceeds 800 ° C., regenerated pearlite is generated during cooling, and cold forgeability may deteriorate. Accordingly, the heating temperature is preferably 800 ° C. or less, more preferably 790 ° C. or less, and further preferably 780 ° C. or less.
　上記加熱温度で加熱した後は、室温まで、平均冷却速度を２０℃／時間以下として冷却することが好ましい。平均冷却速度が２０℃／時間を超えると、パーライト組織が形成されて冷間鍛造性が悪くなることがある。平均冷却速度は、より好ましくは１８℃／時間以下、更に好ましくは１６℃／時間以下である。 After heating at the above heating temperature, it is preferable to cool to room temperature at an average cooling rate of 20 ° C./hour or less. When the average cooling rate exceeds 20 ° C./hour, a pearlite structure may be formed and the cold forgeability may deteriorate. The average cooling rate is more preferably 18 ° C./hour or less, and further preferably 16 ° C./hour or less.
　本発明で規定する高強度ボルト用鋼を、焼入れ処理および焼戻し処理して得られた高強度ボルトの軸部は、表層から中心部まで金属組織が焼戻しマルテンサイトとなり、１１００ＭＰａ以上の強度を確保できる。高強度ボルトの軸部における強度は、ＪＩＳ　Ｂ１０５１（２０００年）に基づいて測定すればよい。 The shaft portion of the high-strength bolt obtained by quenching and tempering the steel for high-strength bolts specified in the present invention has a metal structure of tempered martensite from the surface layer to the central portion, and can secure a strength of 1100 MPa or more. . What is necessary is just to measure the intensity | strength in the axial part of a high intensity | strength bolt based on JISB1051 (2000).
　上記高強度ボルトは、軸部における旧オーステナイトの結晶粒度番号が、８番以上であることが好ましい。旧オーステナイトの結晶粒が、微細化するほど耐遅れ破壊性が向上するからである。結晶粒度番号は、より好ましくは８．５番以上、更に好ましくは９．０番以上である。結晶粒度番号の上限は特に限定されないが、例えば、１２番以下であればよい。 The high-strength bolt preferably has a grain size number of prior austenite at the shaft portion of 8 or more. This is because the delayed fracture resistance improves as the crystal grains of the prior austenite become finer. The grain size number is more preferably 8.5 or more, and even more preferably 9.0 or more. The upper limit of the grain size number is not particularly limited, but may be, for example, 12 or less.
　上記結晶粒度番号は、ＪＩＳ　Ｇ０５５１（２０１３年）に基づいて測定すればよい。 The crystal grain size number may be measured based on JIS G0551 (2013).
　本願は、２０１４年３月２５日に出願された日本国特許出願第２０１４－０６２６５６号に基づく優先権の利益を主張するものである。日本国特許出願第２０１４－０６２６５６号の明細書の全内容が、本願に参考のため援用される。 This application claims the benefit of priority based on Japanese Patent Application No. 2014-062656 filed on March 25, 2014. The entire contents of the specification of Japanese Patent Application No. 2014-062656 are incorporated herein by reference.
　以下、実施例を挙げて本発明をより具体的に説明するが、本発明は下記実施例によって制限を受けるものではなく、前記および後記の趣旨に適合し得る範囲で変更を加えて実施することも勿論可能であり、それらはいずれも本発明の技術的範囲に包含される。 EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and may be implemented with modifications within a range that can meet the above and the gist described below. Of course, these are all possible and are included in the technical scope of the present invention.
　下記表１に示す成分組成を有し、残部は、鉄および不可避不純物の鋼を溶製し、得られた鋳造片を１２００℃に加熱し、圧延もしくは熱間鍛造を行い、６００℃以下の温度まで、平均冷却速度を１℃／秒として徐冷し、直径１２ｍｍの線材を製造した。 It has the component composition shown in Table 1 below, and the balance is made of iron and inevitable impurity steel, and the resulting cast piece is heated to 1200 ° C. and rolled or hot forged to a temperature of 600 ° C. or lower. The wire was slowly cooled at an average cooling rate of 1 ° C./second to produce a wire with a diameter of 12 mm.
　下記表１には、鋼に含まれるＣ量およびＭｎ量に基づいて上記式（１）の左辺から求められる１／（［Ｃ］×［Ｍｎ］）の値、鋼に含まれるＣ量、Ｓｉ量、Ｍｎ量、およびＣｒ量に基づいて上記式（２）の左辺から求められる［Ｃ］＋［Ｓｉ］／２＋［Ｍｎ］／２＋［Ｃｒ］／３の値を示す。下記表１では、１／（［Ｃ］×［Ｍｎ］）の値はＸ値と表記し、［Ｃ］＋［Ｓｉ］／２＋［Ｍｎ］／２＋［Ｃｒ］／３の値はＹ値と表記した。 Table 1 below shows the value of 1 / ([C] × [Mn]) obtained from the left side of the above formula (1) based on the amount of C and Mn contained in the steel, the amount of C contained in the steel, Si The value of [C] + [Si] / 2 + [Mn] / 2 + [Cr] / 3 obtained from the left side of the above formula (2) based on the amount, the amount of Mn, and the amount of Cr is shown. In Table 1 below, the value of 1 / ([C] × [Mn]) is expressed as an X value, and the value of [C] + [Si] / 2 + [Mn] / 2 + [Cr] / 3 is a Y value. Indicated.
　次に、得られた線材の金属組織を観察した。金属組織は、線材を横断面で切断し、ナイタール腐食液でエッチングした後、Ｄ／４位置の任意の領域を光学顕微鏡で観察した。Ｄは線材の直径を示す。観察倍率は、４００倍とした。その結果、線材の組織は、フェライトとパーライトの混合組織であった。 Next, the metal structure of the obtained wire was observed. The metallographic structure was obtained by observing an arbitrary region at the D / 4 position with an optical microscope after the wire was cut in a cross section and etched with a nital etchant. D shows the diameter of a wire. The observation magnification was 400 times. As a result, the structure of the wire was a mixed structure of ferrite and pearlite.
　次に、得られた線材を８７０℃に加熱した後、８７０℃から焼入れ処理を行った。焼入れ処理後、下記表２に示す焼戻し温度（℃）に再加熱し、１時間保持して焼戻し処理を行い、供試材を製造した。 Next, after heating the obtained wire to 870 ° C., quenching was performed from 870 ° C. After the quenching treatment, the sample was reheated to the tempering temperature (° C.) shown in Table 2 below, and held for 1 hour to perform the tempering treatment to produce a test material.
　得られた供試材の金属組織を、上記線材と同じ手順で観察した。その結果、金属組織は、焼戻しマルテンサイト組織であった。 The metal structure of the obtained specimen was observed in the same procedure as the above wire. As a result, the metal structure was a tempered martensite structure.
　次に、得られた供試材について、旧オーステナイト粒の結晶粒度番号を測定した。結晶粒度番号は、得られた供試材をＪＩＳ　Ｇ０５５１（２０１３年）で規定される焼入れ焼戻し法に従って結晶粒界を現出させ、供試材の縦断面におけるＤ／４位置の任意の領域で測定した。Ｄは供試材の直径を示す。 Next, the crystal size number of the prior austenite grains was measured for the obtained specimens. The grain size number can be obtained in an arbitrary region at the D / 4 position in the longitudinal section of the test material by causing the grain boundary to appear in accordance with the quenching and tempering method defined in JIS G0551 (2013). It was measured. D indicates the diameter of the test material.
　次に、得られた供試材から、ＪＩＳ　Ｚ２２４１（２０１１年）で規定される１４Ａ号試験片を切り出し、ＪＩＳ　Ｚ２２４１（２０１１年）に従って引張試験を行い、引張強さを測定した。測定結果を下記表２に示す。本発明では、引張強さが１１００ＭＰａ以上の場合を合格とする。 Next, from the obtained specimen, a No. 14A test piece defined in JIS Z2241 (2011) was cut out, a tensile test was performed according to JIS Z2241 (2011), and the tensile strength was measured. The measurement results are shown in Table 2 below. In this invention, let the case where tensile strength is 1100 Mpa or more be a pass.
　次に、引張強さが１１００ＭＰａ以上であった供試材について、耐遅れ破壊性を評価した。耐遅れ破壊性は、上記供試材から、図１Ａに示す切り欠き付き試験片を切り出し、引張試験および耐遅れ破壊試験を行って評価した。図１Ｂに切り欠き部の形状を示す。切り欠き付き試験片を用いたのは、ねじ部の応力集中を模擬するためである。即ち、上記切り欠き付き試験片を用い、ＪＩＳ　Ｚ２２４１（２０１１年）に従って引張試験を行い、最大応力を測定した。なお、図１Ｂに示したＫｔは応力集中係数を示している。 Next, the delayed fracture resistance of the test material having a tensile strength of 1100 MPa or more was evaluated. Delayed fracture resistance was evaluated by cutting out a notched test piece shown in FIG. 1A from the above specimen and conducting a tensile test and a delayed fracture resistance test. FIG. 1B shows the shape of the notch. The reason why the notched specimen is used is to simulate the stress concentration in the threaded portion. That is, a tensile test was performed according to JIS Z2241 (2011) using the above-mentioned notched test piece, and the maximum stress was measured. In addition, Kt shown to FIG. 1B has shown the stress concentration factor.
　また、耐遅れ破壊試験として、上記切り欠き付き試験片を、１５％ＨＣｌ水溶液に３０分間浸漬し、水洗および乾燥した後、一定荷重を負荷し、１００時間以上破断しない荷重の最大値（以下、１００時間保持応力ということがある）を測定した。 In addition, as a delayed fracture resistance test, the notched test piece was immersed in a 15% HCl aqueous solution for 30 minutes, washed with water and dried, then loaded with a constant load, and a maximum load that did not break for more than 100 hours (hereinafter, 100 hours holding stress)).
　上記１００時間保持応力を、上記最大応力で除した値、即ち、１００時間保持応力／最大応力の値を遅れ破壊強度比と定義し、この値を下記表２に示す。本発明では、遅れ破壊強度比が０．７０以上の場合を合格とし、耐遅れ破壊性に優れると評価した。 The value obtained by dividing the 100-hour holding stress by the maximum stress, that is, the value of 100-hour holding stress / maximum stress is defined as the delayed fracture strength ratio, and this value is shown in Table 2 below. In the present invention, the case where the delayed fracture strength ratio was 0.70 or more was regarded as acceptable, and it was evaluated that the delayed fracture resistance was excellent.
　また、上記１／（［Ｃ］×［Ｍｎ］）の値と、遅れ破壊強度比との関係を図２に示す。図２においては、本発明例であるＮｏ．１～９をプロットすると共に、比較例であるＮｏ．１０～２０のうち、Ｘ値が本発明で規定する要件を外れるＮｏ．１２～１４、１８、１９をプロットした。なお、Ｎｏ．２０のＸ値は本発明で規定する要件を外れているが、Ｎｏ．２０は、Ｍｏを含有した参考例であるため、図２にプロットしなかった。 Also, FIG. 2 shows the relationship between the above 1 / ([C] × [Mn]) value and the delayed fracture strength ratio. In FIG. While plotting 1 to 9, No. 1 as a comparative example. No. 10 to No. 20 where the X value deviates from the requirement defined in the present invention. 12-14, 18, 19 were plotted. In addition, No. An X value of 20 is outside the requirement defined in the present invention. Since 20 is a reference example containing Mo, it was not plotted in FIG.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　表１、表２、および図２から、次のように考察できる。Ｎｏ．１～９は、本発明で規定する要件を満足する例である。鋼の成分組成が適切に制御されているため、１１００ＭＰａ以上の高強度と、優れた耐遅れ破壊性を達成できている。 From Table 1, Table 2, and Figure 2, it can be considered as follows. No. Examples 1 to 9 are examples that satisfy the requirements defined in the present invention. Since the steel component composition is appropriately controlled, high strength of 1100 MPa or more and excellent delayed fracture resistance can be achieved.
　これに対し、Ｎｏ．１０～１９は、本発明で規定するいずれかの要件を満足しない例である。これらのうち、Ｎｏ．１０は、Ｃ量が少な過ぎるため、Ｙ値が０．８２を下回った例であり、１１００ＭＰａ以上の強度を確保できなかった。Ｎｏ．１１は、Ｃ量が過剰な例であり、靭性および延性が低下したため、耐遅れ破壊性を改善できていないと考えられる。Ｎｏ．１２～１４は、Ｍｎ量を過剰に含有し、Ｘ値が５．５を下回った例であり、偏析によって粒界強度が低下し、耐遅れ破壊性を改善できなかったと考えられる。Ｎｏ．１５は、Ｓｉ量が少な過ぎるため、Ｙ値が０．８２を下回った例であり、１１００ＭＰａ以上の強度を確保できなかった。Ｎｏ．１６は、Ｃｒ量が少な過ぎる例であり、Ｙ値が０．８２を下回ったため、１１００ＭＰａ以上の強度を確保できなかった。 On the other hand, No. Examples 10 to 19 are examples that do not satisfy any of the requirements defined in the present invention. Of these, No. 10 is an example in which the Y value was less than 0.82 because the amount of C was too small, and a strength of 1100 MPa or more could not be secured. No. No. 11 is an example in which the amount of C is excessive, and it is considered that delayed fracture resistance cannot be improved because toughness and ductility are reduced. No. Nos. 12 to 14 are examples in which the amount of Mn was excessive and the X value was less than 5.5. It is considered that the grain boundary strength was lowered by segregation and delayed fracture resistance could not be improved. No. 15 is an example in which the Y value was less than 0.82 because the amount of Si was too small, and a strength of 1100 MPa or more could not be secured. No. No. 16 is an example in which the amount of Cr is too small. Since the Y value was less than 0.82, a strength of 1100 MPa or more could not be secured.
　Ｎｏ．１７は、Ｔｉ量を含有しない例であり、水素トラップサイトとなるＴｉＣが析出しなかったため、耐遅れ破壊性を改善できなかったと考えられる。Ｎｏ．１８は、Ｘ値が５．５を下回った例であり、耐遅れ破壊性を改善できなかった。Ｎｏ．１９は、Ｔｉ量が少な過ぎ、またＸ値が５．５を下回った例であり、耐遅れ破壊性を改善できなかった。Ｎｏ．２０は、ＪＩＳ規格のＳＣＭ４３５を模擬した参考例である。Ｎｏ．２０は、１１００ＭＰａ以上の強度を有し、しかも耐遅れ破壊性を改善できているが、Ｍｏを含有しているため、コスト高となっている。 No. No. 17 is an example that does not contain Ti, and it is considered that delayed fracture resistance could not be improved because TiC serving as a hydrogen trap site did not precipitate. No. 18 is an example in which the X value was less than 5.5, and the delayed fracture resistance could not be improved. No. No. 19 was an example in which the amount of Ti was too small and the X value was less than 5.5, and the delayed fracture resistance could not be improved. No. Reference numeral 20 is a reference example simulating the JIS standard SCM435. No. No. 20 has a strength of 1100 MPa or more and can improve delayed fracture resistance. However, since it contains Mo, the cost is high.

Claims (2)

  1. 　質量％で、
    　Ｃ　：０．２０～０．３５％、
    　Ｓｉ：０．３～１．０％、
    　Ｍｎ：０％超０．６％以下、
    　Ｐ　：０％超０．０２％以下、
    　Ｓ　：０％超０．０２％以下、
    　Ｃｒ：０．３～１．５％、
    　Ａｌ：０．０１～０．１％、
    　Ｔｉ：０．０５～０．１％、
    　Ｂ　：０．０００３～０．００５％、および
    　Ｎ　：０％超０．０１％以下を含有すると共に、
    　下記式（１）および式（２）を満足し、
    　残部が鉄および不可避不純物からなり、
    　フェライトとパーライトの混合組織であることを特徴とする耐遅れ破壊性に優れた高強度ボルト用鋼。
    １／（［Ｃ］×［Ｍｎ］）≧５．５　・・・（１）
    ［Ｃ］＋［Ｓｉ］／２＋［Ｍｎ］／２＋［Ｃｒ］／３≧０．８２　・・・（２）
    ［式（１）、式（２）において、［　］は、各元素の含有量（質量％）を意味する。］     % By mass
    C: 0.20 to 0.35%,
    Si: 0.3 to 1.0%,
    Mn: more than 0% and 0.6% or less,
    P: more than 0% and 0.02% or less,
    S: more than 0% and 0.02% or less,
    Cr: 0.3 to 1.5%,
    Al: 0.01 to 0.1%,
    Ti: 0.05 to 0.1%,
    B: 0.0003 to 0.005%, and N: more than 0% and 0.01% or less,
    The following formula (1) and formula (2) are satisfied,
    The balance consists of iron and inevitable impurities,
    High strength steel for bolts with excellent delayed fracture resistance, characterized by a mixed structure of ferrite and pearlite.
    1 / ([C] × [Mn]) ≧ 5.5 (1)
    [C] + [Si] / 2 + [Mn] / 2 + [Cr] /3≧0.82 (2)
    [In Formula (1) and Formula (2), [] means content (mass%) of each element. ]
  2. 　請求項１に記載の鋼を用いて得られるボルトであり、
    　金属組織は焼戻しマルテンサイト組織で、
    　引張強さが１１００ＭＰａ以上であることを特徴とする耐遅れ破壊性に優れた高強度ボルト。     A bolt obtained by using the steel according to claim 1,
    The metal structure is a tempered martensite structure.
    A high-strength bolt excellent in delayed fracture resistance characterized by a tensile strength of 1100 MPa or more.
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