JP2019218584A - bolt - Google Patents

Abstract

To provide a bolt consisting of a low alloy steel for mechanical structure, having high strength, and having excellent hydrogen embrittlement property under hydrogen intrusion environment.SOLUTION: There is provided a bolt having a chemical composition containing, by mass%, one or more kind of Si:0.10 to 1.50%, Mn:0.20 to less than 0.40%, Sb:0.001 to 0.100%, Sn:0.001 to 0.100%, Bi:0.001 to 0.100%, and satisfying the formula (1) and the formula (2), and having tensile strength of a shank of 1000 to 1300 MPa. 0.50≤C+(1/10)×Si+(1/5)×Mn+(5/22)×Cr≤0.85 (1) 0.003≤Sb+Sn+Bi≤0.100 (2), wherein content of corresponding elements (mass%) is assigned in each element symbol in the formula (1) and the formula (2).SELECTED DRAWING: Figure 1

Description

  The present invention relates to bolts, and more particularly to high strength bolts.

  2. Description of the Related Art In recent years, in order to cope with environmental problems and the like, members used for automobiles, industrial machines, buildings, and the like have been required to have reduced weight and higher strength. In particular, automotive bolts such as engine cylinder head bolts, connecting rod bolts, and hub bolts are required to have a tensile strength of 1000 MPa or more.

  These automotive bolts are used in a hydrogen intrusion environment. However, when the tensile strength of the bolt is as high as 1000 MPa or more, the hydrogen embrittlement resistance (delayed fracture resistance) characteristic is reduced, and hydrogen embrittlement (delayed fracture) occurs due to diffusing hydrogen that has penetrated. As a material for such a high-strength bolt, an SCM steel (JIS G 4053 (2008)) containing a large amount of an alloy element such as Mo, an alloy steel containing an expensive alloy element such as V, and the like are used. ing. These alloy steels are manufactured into wire rods, and further drawn and cold forged into bolts.

  When the above-mentioned alloy steel is used as a bolt, the hydrogen embrittlement resistance is good even in a hydrogen intrusion environment. However, since these alloy steels contain a large amount of alloying elements, the cost of steel materials is high. In recent years, prices of alloying elements have risen sharply, and the supply and demand environment tends to fluctuate. For this reason, there is a demand for a bolt capable of realizing high strength and excellent hydrogen embrittlement resistance while reducing or omitting these alloy elements to suppress steel material costs.

  In order to reduce the cost of steel, expensive alloy elements such as Mo and V may be reduced. If the alloy element is reduced, the formation of a hard structure such as bainite can be suppressed when the wire is manufactured by hot rolling. Therefore, the softening heat treatment for cold working can be omitted or simplified, and the manufacturing cost is reduced. However, when the alloying element is reduced, the hardenability of the steel material is reduced, so that it is difficult to increase the strength of the bolt by quenching, and the hydrogen embrittlement resistance is also reduced.

  Therefore, high-strength bolts containing boron (B) instead of alloying elements such as Mo and V are being studied. B enhances the hardenability of steel, like alloy elements such as Mo and V. However, when the B-containing steel is used as a high-strength bolt having a tensile strength of 1000 MPa or more, the hydrogen embrittlement resistance may decrease.

  Patent Literature 1 discloses that a steel for a high-strength bolt excellent in corrosion resistance by containing Sn and having excellent delayed fracture resistance by suppressing hydrogen intrusion due to corrosion is obtained. However, the bolt of Patent Document 1 is required to contain Mo or V so that Mo + 2V satisfies 0.10 to 1.0% by mass.

  Patent Document 2 discloses that a steel for bolts having excellent sulfuric acid corrosion resistance can be obtained by containing Sn or Sb. However, the bolt of Patent Document 2 has a tensile strength of 700 MPa or less and does not consider delayed fracture resistance.

JP-A-2014-1442 JP 2000-73139 A

  An object of the present invention is to provide a bolt made of low alloy steel for machine structure, having high strength and having excellent hydrogen embrittlement resistance under a hydrogen intrusion environment.

  The present inventors use a steel that does not contain a large amount of expensive alloying elements such as Mo and V, but contains C, Si, Mn, Cr, B, Sb, Sn, Bi, and the like. Investigations were made on the components and structures that affect hydrogen embrittlement resistance.

[Regarding both tensile strength and cold workability of bolts]
In order to increase the tensile strength of the bolt to 1000 to 1300 MPa, sufficient hardenability is required. However, if a large amount of alloying elements are added in order to enhance the hardenability, the cold workability of the wire material, which is the material of the bolt, is reduced. In this case, in order to improve the lowered cold workability, a long-time softening heat treatment for the purpose of softening the steel material is performed before performing cold working such as wire drawing and cold forging on the steel material such as a wire. Must be performed multiple times. Therefore, in addition to containing a large amount of alloy elements such as Mo and V, the processing cost is increased.

  Therefore, it is desirable to use a steel material that can be cold-worked without performing a long-time softening heat treatment a plurality of times and has a hardenability that can obtain the above-described tensile strength. C, Si, Mn, and Cr are all elements that enhance hardenability. The present inventor pays attention to this point, and determines the hardenability and cold workability of the steel material for manufacturing the bolt using the following function fn1 with C, Si, Mn, and Cr as parameters. investigated.

fn1 = C + (1/10) × Si + (1/5) × Mn + (5/22) × Cr
The coefficient of each element symbol of fn1 is a numerical value indicating the degree of influence on the quench hardening of the steel, and the larger the coefficient, the easier the quench hardening of the steel. The content (% by mass) of the corresponding element is substituted for each element symbol of fn1.

  It has been found that if fn1 is too low, sufficient hardenability cannot be obtained, while if fn1 is too high, hardenability becomes too high. Further, as a result of studying the value obtained from the fn1, if fn1 satisfies the formula (1), sufficient cold working can be performed without performing a long-time softening heat treatment multiple times while obtaining excellent hardenability. I found that I could get sex.

0.50 ≦ C + (1/10) × Si + (1/5) × Mn + (5/22) × Cr ≦ 0.85 (1)
The content (% by mass) of the corresponding element is substituted for each element symbol in the formula (1).

[Regarding hydrogen embrittlement resistance of bolts]
Antimony (Sb), tin (Sn), and bismuth (Bi) exert an effect of suppressing hydrogen penetration by adding a small amount. Therefore, it was studied to improve the hydrogen embrittlement resistance by adding these elements without impairing the hardenability and cold workability of the steel material for manufacturing bolts. That is, it was studied to determine the hydrogen embrittlement resistance of a bolt using the following function fn2 with Sb, Sn and Bi as parameters.

fn2 = Sb + Sn + Bi
The coefficient of each element symbol of fn2 is a numerical value indicating the degree of influence of each element on delayed fracture characteristics. Since Sb, Sn, and Bi have the same effect of suppressing hydrogen intrusion, the coefficient of each element symbol of fn2 was set to 1.
The content (% by mass) of the corresponding element is substituted for each element symbol of fn2.

As a result, even if the bolt has a high tensile strength of 1000 to 1300 MPa, if the formula (2) is satisfied, the corrosion resistance is excellent, the hydrogen intrusion suppression effect is good, and the hydrogen embrittlement resistance is excellent. Was found to be obtained.
0.003 ≦ Sb + Sn + Bi ≦ 0.100 (2)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in the formula (2).

The present invention has been made based on the above findings, and the gist is as follows.
(1) In mass%, C: 0.22 to 0.40%, Si: 0.10 to 1.50%, Mn: 0.20 to less than 0.40%, Cr: 0.70 to 1.60 %, Al: 0.005 to 0.060%, Ti: 0.010 to 0.050%, B: 0.0003 to 0.0040%, N: 0.0015 to 0.0080%, Cu: 0 .50% or less, Ni: 0.30% or less, Mo: 0.05% or less, V: 0.050% or less, Nb: 0.050% or less,
Sb: 0.001 to 0.100%, Sn: 0.001 to 0.100%, Bi: 0.001 to 0.100%
O: 0.0020% or less, P: 0.020% or less, S: 0.020% or less,
The balance consists of Fe and impurities and has a chemical composition satisfying the formulas (1) and (2),
A bolt having a shaft with a tensile strength of 1000 to 1300 MPa.
0.50 ≦ C + (1/10) × Si + (1/5) × Mn + (5/22) × Cr ≦ 0.85 (1)
0.003 ≦ Sb + Sn + Bi ≦ 0.100 (2)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in the formulas (1) and (2).
(2) Cu: 0.02 to 0.50%, Ni: 0.01 to 0.30%, Mo: 0.01 to 0.05%, V: 0.005 to 0.050% The bolt according to (1), wherein the bolt contains one or more selected ones.
(3) The bolt according to (1) or (2), wherein the bolt contains Nb: 0.005 to 0.050%.
(4) The bolt according to (1) to (3), wherein a range from the surface of the screw bottom of the shaft to a depth of 50 μm has a compressive residual stress of 10 to 90% of the tensile strength of the shaft.

  The bolt according to the present invention has high strength of 1000 to 1300 MPa and excellent hydrogen embrittlement resistance.

FIG. 1 is a diagram showing the relationship between the hydrogen permeability coefficient ratio HR and the amounts of Sb, Sn and Bi added to the volt. FIG. 2 is a side view of a test piece with an annular V-notch. FIG. 3 is a side view of the screw manufactured in the example.

  Hereinafter, the present invention will be described in detail. First, the reasons for limiting the component composition of the bolt of the present invention will be described. Hereinafter, “%” for the components means “% by mass”.

[C: 0.22 to 0.40%]
Carbon (C) enhances the hardenability of the bolt, and increases the tensile strength of the bolt after quenching and tempering to 1000 MPa or more. If the C content is less than 0.22%, the above effects cannot be obtained. On the other hand, if the C content exceeds 0.40%, the hardenability becomes too high. In this case, the strength of the steel material for bolts after hot working becomes too high, and the cold workability decreases. For this reason, a long-time heat treatment for softening must be performed a plurality of times on the steel material before cold working such as wire drawing and cold forging is performed, thereby increasing the manufacturing cost. When the heat treatment is performed, the hydrogen embrittlement resistance further decreases. Therefore, the C content is 0.22 to 0.40%. A preferred lower limit of the C content is 0.24%, more preferably 0.26%. A preferred upper limit of the C content is 0.38%, more preferably 0.35%.

[Si: 0.10 to 1.50%]
Silicon (Si) suppresses precipitation of cementite and increases tempering softening resistance. Si further deoxidizes the steel. MnO—SiO ₂ as a deoxidation product is a vitrified soft inclusion, and is stretched and divided during hot rolling to be refined. Therefore, the hydrogen embrittlement resistance is enhanced. If the Si content is less than 0.10%, the above effects cannot be obtained. On the other hand, if the S content exceeds 1.50%, the strength becomes too high. In this case, the ductility and cold forgeability of the steel decrease. Therefore, the Si content is 0.10 to 1.50%. A preferred lower limit of the Si content is more than 0.35%, more preferably 0.40%, more preferably 0.45%, and further preferably more than 0.50%. The preferable upper limit of the Si content is 1.20%, more preferably 1.00%.

[Mn: less than 0.20 to 0.40%]
Manganese (Mn) enhances the hardenability and makes the tensile strength of the bolt 1000 MPa or more. Mn further combines with Si to form inclusions (MnO—SiO ₂ ). Since the inclusions are soft and stretched and cut during hot rolling to be miniaturized, the density of MnO—SiO ₂ decreases, and the hydrogen embrittlement resistance increases. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, segregation at the grain boundaries promotes grain boundary destruction, and the hydrogen embrittlement resistance is rather reduced. Therefore, the Mn content is 0.20 to 0.40%. A preferred lower limit of the Mn content is 0.22%, more preferably 0.25%. The preferred upper limit of the Mn content is 0.38%, more preferably 0.35%.

[Cr: 0.70 to less than 1.60%]
Chromium (Cr) enhances hardenability and makes the tensile strength of the bolt 1000 MPa or more. Cr further enhances the hydrogen embrittlement resistance of the bolt. If the Cr content is less than 0.70%, these effects cannot be obtained. On the other hand, if the Cr content is 1.60% or more, the hardenability becomes too high, and the cold workability of the steel material for bolts decreases. Therefore, the Cr content is less than 0.40 to 1.60%. A preferable lower limit of the Cr content is 0.90%. A preferable upper limit of the Cr content is 1.45%.

[Al: 0.005 to 0.060%]
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, this effect cannot be obtained. On the other hand, if the Al content exceeds 0.060%, coarse oxide-based inclusions are generated, and the cold workability is reduced. Therefore, the Al content is 0.005 to 0.060%. A preferred lower limit of the Al content is 0.010%. A preferable upper limit of the Al content is 0.055%. In the chemical composition of the high-strength bolt according to the present invention, the Al content means the total amount of Al contained in the steel material.

[Ti: 0.010 to 0.050%]
Titanium (Ti) combines with N in steel to form nitride (TiN). By the generation of TiN, the generation of BN is suppressed, and the amount of solute B increases. As a result, the hardenability of the steel material increases. Ti further combines with C to form carbides (TiC) to refine crystal grains. Thereby, the hydrogen embrittlement resistance of the bolt is enhanced. If the Ti content is less than 0.010%, these effects cannot be obtained. On the other hand, if the Ti content exceeds 0.050%, a large amount of coarse TiN is generated. In this case, the cold workability and the hydrogen embrittlement resistance decrease. Therefore, the Ti content is 0.010 to 0.050%. A preferable lower limit of the Ti content is 0.015%. A preferable upper limit of the Ti content is 0.045%.

[B: 0.0003-0.0040%]
Boron (B) enhances the hardenability of steel. B further suppresses the grain boundary segregation of P and enhances the hydrogen embrittlement resistance of the bolt. If the B content is less than 0.0003%, these effects cannot be obtained. On the other hand, if the B content exceeds 0.0040%, the effect of improving hardenability is saturated. Further, coarse BN is generated, and the cold workability is reduced. Therefore, the B content is 0.0003 to 0.0040%. A preferred lower limit of the B content is 0.0005%. A preferred upper limit of the B content is 0.0025%.

[N: 0.0015 to 0.0080%]
Nitrogen (N) combines with Ti in steel to form nitrides and refine crystal grains. If the N content is less than 0.0015%, this effect cannot be obtained. On the other hand, if the N content exceeds 0.0080%, the effect is saturated. Further, N combines with B to form a nitride, thereby reducing the amount of solid solution B. In this case, the hardenability of the steel decreases. Therefore, the N content is 0.0015 to 0.0080%. A preferred lower limit of the N content is 0.0020%. The preferable upper limit of the N content is 0.0070%.

[Sb: 0.001% to 0.100%; Sn: 0.001% to 0.100%; Bi: 0.001% to 0.100%]
In the present invention, in addition to the above components, one or more of antimony (Sb), tin (Sn), and bismuth (Bi) are added in the range of 0.001% to 0.10%, respectively. It is characteristic. These elements exhibit an effect of suppressing hydrogen invasion by adding a small amount. The lower limits are each 0.001% or more, but the preferable lower limits for sufficiently exhibiting the effect are each 0.005% or more. Further, each is preferably at least 0.010%. If the upper limit exceeds 0.100%, the hot workability of steel deteriorates and continuous casting becomes difficult. Preferably, the sum of the concentrations of Sb, Sn and Bi is 0.030 to 0.100%.

  The balance of the chemical composition of the high strength bolt according to the invention consists of Fe and impurities. Here, the impurities are those that are mixed in from the ore, scrap, or production environment as raw materials when industrially producing high-strength bolts, and include O, P, S, As, and Co, which will be described later. , Mg, Zr, Te, Bi, Pb, Zn and the like. Among them, As, Co, Mg, Zr, Te, Bi, Pb, Zn and the like are limited to such an extent that the effects of the present invention are not impaired.

[Arbitrary element]
The above-mentioned high-strength bolt may further contain at least one selected from the group consisting of Cu, Ni, Mo, and V instead of part of Fe. Each of these elements is an optional element and enhances the hardenability of steel.

Cu: 0.50% or less Copper (Cu) is an optional element and may not be contained. When included, Cu enhances the hardenability of the steel. However, if the Cu content exceeds 0.50%, the hardenability becomes too high and the cold workability decreases. Therefore, the Cu content is 0.50%. A preferable lower limit of the Cu content for more effectively obtaining the above effects is 0.02%, and more preferably 0.05%. The preferred upper limit of the Cu content is 0.30%, and more preferably 0.20%.

Ni: 0.30% or less Nickel (Ni) is an optional element and may not be contained. When contained, Ni enhances the hardenability of the steel and further increases the toughness of the steel material after quenching. However, if the Ni content exceeds 0.30%, the hardenability becomes too high and the cold workability decreases. Therefore, the Ni content is 0 to 0.30%. A preferable lower limit of the Ni content for more effectively obtaining the above effects is 0.03%, and more preferably 0.05%. A preferred upper limit of the Mo content is 0.20%, and more preferably 0.10%.

Mo: 0.05% or less Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo enhances the hardenability of the steel. However, if the Mo content exceeds 0.05%, the hardenability becomes too high, and the cold workability of the high-strength bolt steel decreases. Therefore, the Mo content is 0 to 0.05%. A preferable lower limit of the Mo content for more effectively obtaining the above effects is 0.01%, and further preferably 0.015%. A preferred upper limit of the Mo content is 0.03%, and more preferably 0.025%.

V: 0.050% or less Vanadium (V) is an optional element and may not be contained. When included, V enhances the hardenability of the steel. V further forms carbides, nitrides or carbonitrides to refine the crystal grains. However, if the V content exceeds 0.050%, carbides and the like are coarsened and cold workability is reduced. Therefore, the V content is 0 to 0.050%. A preferred lower limit of the V content for more effectively obtaining the above effects is 0.005%. A preferred upper limit of the V content is 0.030%, and more preferably 0.020%.

Nb: 0.050% or less Niobium (Nb) is an optional element and may not be contained. When included, Nb combines with C and N to form carbides, nitrides or carbonitrides, and refines the crystal grains. Nb further enhances the hydrogen embrittlement resistance of the bolt. However, if the Nb content exceeds 0.050%, coarse carbides and the like are generated, and the cold workability of the steel material is reduced. Therefore, the Nb content is 0 to 0.050%. A preferred lower limit of the Nb content for more effectively obtaining the above effects is 0.005%. The preferable upper limit of the Nb content is 0.040%, and more preferably 0.030%.

[O: 0.0020% or less]
Oxygen (O) is an impurity. O forms oxides and lowers cold workability. If the O content exceeds 0.0020%, a large amount of oxide is generated, and MnS is coarsened, so that the cold workability is significantly reduced. Therefore, the O content is 0.0020% or less. The preferable upper limit of the O content is 0.0018%. The O content is preferably as low as possible.

[P: 0.020% or less]
Phosphorus (P) is an impurity. P segregates at the crystal grain boundaries to lower the cold workability, and lowers the hydrogen embrittlement resistance of the bolt. Therefore, the P content is 0.020% or less. A preferable upper limit of the P content is 0.015%. The P content is preferably as low as possible.

[S: 0.020% or less]
Sulfur (S) is an impurity. S forms sulfides to lower the cold workability and lower the hydrogen embrittlement resistance of the bolt. Therefore, the S content is 0.020% or less. The preferable upper limit of the S content is 0.010%, and more preferably 0.008%. The S content is preferably as low as possible.

[0.50 ≦ fn1 ≦ 0.85]
As described above, the hardenability and the cold workability of the steel material for manufacturing a bolt can be determined using the function fn1 with C, Si, Mn, and Cr as parameters. When the chemical composition of the bolt satisfies the following formula (1), excellent cold workability and hardenability are obtained.
0.50 ≦ fn1 = C + (1/10) × Si + (1/5) × Mn + (5/22) × Cr ≦ 0.85 (1)
The content (% by mass) of the corresponding element is substituted for each element symbol in the formula (1). If the corresponding element is at the impurity level, “0” is substituted for the corresponding element symbol in equation (1).

  As described above, if fn1 is too low, sufficient hardenability cannot be obtained, but if fn1 is too high, hardenability becomes too high. If fn1 is too high, when the steel for bolts is rolled into wire, bainite is formed, increasing strength and hardness. Therefore, unless the long-time softening heat treatment is performed a plurality of times before the subsequent wire drawing step and the cold forging step, sufficient cold workability cannot be obtained. When fn1 satisfies the expression (1), sufficient cold workability can be obtained without performing a long-time softening heat treatment a plurality of times while obtaining excellent hardenability. A preferred lower limit of fn1 is 0.53. A preferred upper limit of fn1 is 0.83.

[0.003 ≦ fn2 = Sb + Sn + Bi ≦ 0.100]
As described above, when the following expression (2) is satisfied, the corrosion resistance is excellent, the hydrogen intrusion suppression effect is good, and the excellent hydrogen embrittlement resistance is obtained.
0.003 ≦ fn2 = Sb + Sn + Bi ≦ 0.100 (2)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in the formula (2). If the corresponding element is at the impurity level, “0” is substituted for the corresponding element symbol in equation (2).

  In the following description, the grounds for limiting the range of fn2 will be described. The range of fn2 was revealed by the following experiment.

  The steel composition No. having the chemical composition shown in Table 1-1. a to n were vacuum-melted to produce a 150 kg ingot.

  Steel composition No. After each of the ingots manufactured using the samples a to n was heated to 1200 to 1300 ° C., hot forging was performed assuming hot rolling to produce a round bar having a diameter of 15 mm. The round bar after hot forging was allowed to cool in the atmosphere. Subsequently, the round bar was subjected to quenching and tempering assuming heat treatment after bolt forming, and the tensile strength of the round bar was adjusted to about 1200 MPa. The round bar having the adjusted tensile strength was machined to prepare a test piece with an annular V-notch shown in FIG. In addition, the test piece No. obtained from each of the steels a to m. In all of a1 to m1, the area ratio of the bainite phase and the ferrite phase was 5% or less in total, and the rest was tempered martensite.

  Numerical values in which units are not shown in FIG. 2 indicate dimensions (units: mm) of corresponding portions of the test piece. “Φ numerical value” in the figure indicates the diameter (mm) of the designated part. “60 °” indicates that the V notch angle is 60 °. “0.175R” indicates that the V-notch bottom radius is 0.175 mm.

  Using the electrolytic charging method, each steel composition No. Test pieces No. a to n. Various concentrations of hydrogen were introduced into each of a1 to n1. The electrolytic charging method was performed as follows. With the test piece immersed in an aqueous solution of ammonium thiocyanate, anodic potential was generated on the surface of the test piece, and hydrogen was taken into the test piece. Thereafter, a galvanized film was formed on the surface of each test piece to prevent the dissipation of hydrogen in the test piece. Subsequently, a constant load test was performed in which a constant load was applied so that a tensile stress having a nominal stress of 1080 MPa was applied to the V-notch cross section of the test piece. A test piece that broke during the test and a test piece that did not break were subjected to a temperature rise analysis method using a gas chromatograph to measure the amount of hydrogen in the test piece. After the measurement, in each steel, the maximum hydrogen amount of the test piece that did not break was defined as the critical diffusible hydrogen amount Hc.

Further, based on the hydrogen permeability coefficient Href of a steel m having a chemical composition corresponding to SCM435 of JIS G4053 (2008) at an immersion time of 30 hours, a hydrogen permeability coefficient ratio HR (hereinafter simply referred to as HR) is expressed by the following equation (A). ).
HR = Hc / Href (A)

  HR is an index of hydrogen intrusion suppression characteristics. FIG. 1 was created based on the obtained HR and fn2 of each steel. Each steel composition No. Test pieces No. a to n. Table 1-2 shows fn1, fn2 and HR of a1 to n1.

  From FIG. 1, it is clear that HR is significantly increased as fn2 increases, that is, as the ratio of the content of Sb, Sn or Bi increases, and that fn2 = 0.003 compared to when fn2 = 0. The HR is higher than 5%.

  When fn2 exceeds 0.002, the effect of suppressing hydrogen intrusion is exhibited, HR is higher than 1.3, and hydrogen embrittlement resistance is superior to SCM435 which is a typical low alloy steel such as gears and crankshafts. Characteristics are obtained. However, when fn2 exceeds 0.1, hot workability of steel deteriorates and continuous casting becomes difficult. Therefore, as shown in Expression (2), the upper limit of fn2 is 0.100. A preferred lower limit of fn2 is 0.005.

[Metal structure]
Preferably, 95% or more of the metal structure of the bolt of the present invention is tempered martensite, and the remaining metal structure of 5% or less is at least one of bainite and ferrite. When the metal structure is at least one of bainite and ferrite, a high tensile strength of 1000 to 1300 MPa cannot be achieved. Further, the metal structure of the bolt of the present invention is preferably 100% tempered martensite from the viewpoint of strength, but it is not advantageous from the viewpoint of cost effectiveness to make the metal structure a single phase of tempered martensite. . Therefore, as long as the tensile strength can achieve a high strength of 1000 to 1300 MPa, a phase other than tempered martensite may be included in a range in which the tensile strength can be achieved. In the present invention, when at least one of bainite and ferrite is contained in addition to martensite, these phases may be contained in a total area ratio of 5% or less. However, other than tempered martensite, a bolt having a total area ratio of the bainite phase and the ferrite phase of more than 5% may not be able to achieve a high tensile strength of 1000 to 1300 MPa.

The area ratio of tempered martensite, bainite and ferrite of the bolt is measured as follows. First, a cross section parallel to the central axis is cut out so as to pass through the central axis of the bolt, the cross section is polished, then eroded with 3% nital, and the center axis is cut from the bottom of the screw at 400 times magnification using an optical microscope. In the direction, observe the position of 1/2 of the distance from the screw bottom to the central axis. For 0.5 mm ² in an image photographed at an observation magnification of 400 ×, the area ratio of a bright region when binarizing tempered martensite as a bright region was derived using image processing software Winroof 2015 to obtain a martensite area ratio. And

[About the compressive residual stress at the screw bottom]
Preferably, in the screw bottom of the high-strength bolt according to the present invention, the surface compressive residual stress of the screw bottom is 10 to 90% of the tensile strength of the bolt.

  In this case, the tensile stress applied to the screw bottom when the bolt is fastened is offset by the compressive residual stress. For this reason, the stress state at the starting point is alleviated, and the fracture due to hydrogen embrittlement is less likely to occur. If the compressive residual stress (absolute value) is less than 10% of the tensile strength (absolute value), the effect of canceling the tensile stress by the compressive residual stress becomes insufficient, and excellent hydrogen embrittlement resistance cannot be obtained. On the other hand, if the compressive residual stress (absolute value) exceeds 90% of the tensile strength (absolute value), the above effect is saturated. Therefore, the compressive residual stress is 10-90% of the tensile strength.

Here, the “surface layer portion” indicates a range from the surface of the high-strength bolt to a depth of 50 μm toward the central axis. The compressive residual stress is measured by a known X-ray method. Specifically, an X-ray stress measurement method using X-ray diffraction is used in accordance with JIS B2711 (2013). The measurement is performed using the type of characteristic X-ray: MnKα ray, Cr filter, reference diffraction angle 2θ ₀ : 152.0 °, η angle 14.0 °, and X-ray stress constant K: -336 MPa / deg. The measurement site is centered on the center of the screw bottom, and the measurement direction is parallel to the longitudinal direction of the screw portion. The tensile strength is determined based on JIS Z2241 (2011).

[Production method]
An example of the method for manufacturing a bolt according to the present invention will be described. First, a bolt steel material is manufactured by a well-known manufacturing method (material manufacturing process). Then, a bolt is manufactured using the steel material for the bolt (bolt manufacturing process). Hereinafter, each step will be described.

[Material manufacturing process]
A molten steel having the above chemical composition is manufactured. A slab is manufactured by continuous casting using molten steel. Alternatively, an ingot is manufactured by using a molten steel by an ingot-making method. The produced slab or ingot is slab-rolled into a steel slab. The steel slab is hot-worked to obtain a high-strength bolt steel (wire). The hot working is, for example, hot rolling.

[Bolt manufacturing process]
In the bolt manufacturing process, bolts are manufactured using bolt steel materials. The bolt manufacturing process includes a wire drawing process, a cold forging process, and a quenching and tempering process. Hereinafter, each step will be described.

[Drawing process]
First, a wire is subjected to wire drawing to manufacture a steel wire. The wire drawing may be performed only by primary wire drawing, or a plurality of times of wire drawing such as secondary wire drawing. During drawing, a lubricating film is formed on the surface of the wire. The lubricating coating is, for example, a phosphate coating or a non-phosphorous lubricating coating.

  Preferably, a lubricating coating containing no P is used. Alternatively, when a phosphate film is used, the surface of the steel material (steel wire) is washed or pickled before the quenching step described below to remove the phosphate film from the surface. The cleaning is, for example, a well-known alkali cleaning.

[Cold forging process]
The drawn steel material is cut into a predetermined length, and the cut steel material is subjected to cold forging to manufacture bolts.

[About softening heat treatment]
In the conventional method for manufacturing a bolt, a softening heat treatment is performed a plurality of times before drawing and cold forging in order to soften a steel material (wire) for a bolt having too high strength. However, in the bolt according to the present invention, such a softening heat treatment is simplified by satisfying the expression (1). This can suppress an increase in manufacturing cost due to the softening heat treatment, and can further enhance the hydrogen embrittlement resistance of the bolt.

[Threading process]
Rolling is performed on the high-strength bolt manufactured by cold forging under known conditions to form a thread.

[Quenching and tempering processes]
Quenching and tempering are performed on the high-strength bolt after threading under well-known conditions, and the tensile strength of the bolt is adjusted to 1000 to 1300 MPa. When the tensile strength is 1000 MPa or less, the strength of the bolt is insufficient. On the other hand, when the tensile strength exceeds 1300 MPa, the hydrogen sensitivity increases, and the hydrogen embrittlement resistance decreases. Therefore, the tensile strength of the bolt is 1000-1300 MPa. In the case where a lubricating film containing P typified by a phosphate film is used in the wire drawing step, as described above, preferably, the surface of the steel material (steel wire) is washed with alkali before quenching.

[Compression residual stress applying step]
Preferably, a well-known compressive residual stress applying step is performed on the bolt after quenching and tempering to reduce the compressive residual stress of the surface layer at the bottom of the screw to 10 to 90% of the tensile strength of the bolt. A well-known compressive residual stress applying step is, for example, shot peening. By appropriately adjusting the conditions of the shot peening, the compressive residual stress of the surface layer at the bottom of the screw can be made 10 to 90% of the tensile strength of the bolt.

  In the above-described manufacturing method, a thread forming step (pre-rolling step) is included before quenching and tempering, but instead of the pre-rolling step, a screw processing step (post-rolling step) may be performed after quenching and tempering. . Also in this case, a compressive residual stress of 10 to 90% of the tensile strength of the bolt can be applied to the surface layer of the screw bottom. In the case of the post-rolling process, it is not necessary to perform the shot peening.

  Through the above manufacturing steps, the bolt of the present invention is manufactured.

  Steel No. having the chemical composition shown in Table 2 was used. A to R molten steels were manufactured.

  As shown in Table 2, steel R has a chemical composition corresponding to SCM435 of JIS G4053 (2008).

  A billet having a cross section of 162 mm x 162 mm was manufactured by continuous casting using molten steels of steel Nos. A to R having the chemical compositions shown in Table 2. The billets having the chemical compositions Nos. A to R thus obtained were once cooled to room temperature, and the presence or absence of surface cracks of the slab was visually determined. Table 3 shows the results. Further, billets having chemical compositions Nos. A to R were hot-worked (hot-rolled) to produce wires having a diameter of 11.5 mm.

  The wire rods obtained from the billets of steel Nos. A to R having the chemical compositions shown in Table 2 were drawn to produce steel wires of Test Nos. 1 to 20 in Table 3. At this time, heat treatment for softening was performed. The heat treatment temperature was 750 ° C., the heat treatment time was 60 minutes, and the heat treatment was followed by slow cooling. Further, after degreasing and pickling, a zinc phosphate treatment (75 ° C., immersion time 600 sec) and a metal soap treatment (80 ° C., immersion time 180 sec) are performed, and the surface is treated with a zinc phosphate film and a metal soap film. A lubricated film was formed. Thereafter, finish wire drawing was performed to produce a steel wire having a diameter of 10.5 mm. This steel wire was used as a material for high-strength bolt forging.

  Cold forging was performed on each of the steel wires of test numbers 1 to 20 to produce the bolts shown in FIG. Specifically, cold forging was performed in two steps. In the first step, the bolt shaft was pressed and formed. In the second step, a mold was designed to perform processing for forming the head and flange of the bolt, and mounted on a hydraulic forging press to perform cold forging. Each numerical value in the figure indicates the size (mm) of the corresponding part. “Φ numerical value” in the figure indicates the diameter (mm) of the designated part. "Numeric value °" in the figure indicates the angle (°) of the designated part. “R numerical value” indicates the radius of curvature (mm) of the designated portion. “M7 × 1.0” in the figure indicates that the outer diameter is 7 mm and the pitch is 1.0 mm.

  Each of the bolts formed as described above from the steel wires of Test Nos. 1 to 20 was visually observed to check for the occurrence of cracks. If cracks were observed, bolt forming was not possible.

  Quenching and tempering treatments were performed at the temperatures shown in Table 3 for the bolts of the test numbers for which no cracks were observed. Before performing the quenching treatment, the surface of the bolt was washed with an alkali to remove the phosphate film.

  In the quenching treatment, the mixture was kept at a quenching temperature (° C.) shown in Table 3 for 40 minutes, and then cooled with oil. In the tempering treatment, the tempering temperature shown in Table 3 was maintained for 70 minutes. Through the above steps, a bolt was manufactured. When the tempering temperature for obtaining a desired bolt tensile strength (1000 MPa to 1300 MPa) is lower than 435 ° C., it is determined that the strength is insufficient, and the hydrogen embrittlement resistance evaluation is not performed. Judged outside.

  The steel wires of Test Nos. 1 to 12, 15 and 18 to 20 were subjected to rolling after quenching and tempering, and residual stress was applied to the surface of the screw bottom together with the screw working. The steel wires of Test No. 13 and Test No. 14 were rolled before quenching and tempering. The compressive residual stress of the surface layer at the bottom of the screw was measured using an X-ray stress measurement method using X-ray diffraction in accordance with JIS B2711 (2013). The measurement was performed using the type of characteristic X-ray: MnKα ray, Cr filter, reference diffraction angle 2θ0: 152.0 °, η angle 14.0 °, and X-ray stress constant K: -336 MPa / deg. The measurement site measured the residual stress in the direction parallel to the longitudinal direction of the screw portion, with the center at the center of the screw bottom.

[Metal structure]
The area ratio of tempered martensite, bainite, and ferrite was measured for each of the bolts formed from the steel wires of test numbers 1 to 20. The area ratio of each tissue was determined by cutting out a cross section parallel to the central axis of the bolt so as to pass through the central axis of the bolt, polishing the cross section, and then corroding with 3% nitric alcohol (a nital etching solution) for 5 to 15 seconds. At a magnification of 400 times, a half of the distance from the screw bottom to the center axis was observed in the direction of the center axis from the screw bottom, and the tissue was photographed. With respect to a total of 0.5 mm ^{2 of} an image photographed at an observation magnification of 400 ×, the area ratio of a bright region when binarized using tempered martensite as a bright region was derived using image processing software Winroof 2015, and the martensite area ratio was obtained. And In each of the bolts formed from the steel wires of Test Nos. 1 to 20, the total area ratio of the bainite phase and the ferrite phase was 5% or less, and the remainder was tempered martensite.

[Tensile test]
According to JIS B1051 (2000), the tensile strength (MPa) of the high-strength bolt after quenching and tempering of each test number or rolling was measured at room temperature (25 ° C.) and in the air. Table 3 shows the measurement results.

[Hydrogen embrittlement resistance evaluation test]
With respect to the bolts after the quenching and tempering treatments or the rolling processes of the respective test numbers, annular V-notch test pieces shown in FIG. 2 were prepared, and various concentrations of hydrogen were introduced by using an electrolytic charging method. The electrolytic charging method was performed as follows. The bolt was immersed in an aqueous solution of ammonium thiocyanate. With the bolt immersed, an anodic potential was generated on the surface of the bolt, and hydrogen was taken into the bolt.

  After introducing hydrogen into the bolt, a galvanized coating was formed on the bolt surface to prevent hydrogen from dissipating in the bolt. Subsequently, a constant load test in which a tensile strength of 95% of the tensile strength of the bolt was applied was performed. The bolts that broke during the test and those that did not break were subjected to a temperature rising analysis method using a gas chromatograph to measure the amount of hydrogen in the bolts. After the measurement, in each test number, the maximum hydrogen amount of the test piece that did not break was defined as the critical diffusible hydrogen amount Hc.

  Further, the critical diffusion hydrogen amount of the steel wire of Test No. 20 having a chemical composition corresponding to SCM435 in the JIS standard used for the conventional bolt was used as a reference (Href) of the critical diffusible hydrogen amount ratio HR. Based on the critical diffusible hydrogen amount Href, the critical diffusible hydrogen amount ratio HR was determined using the equation (A). As the evaluation of hydrogen embrittlement resistance, those having an HR higher than 1.3 were accepted ("o" in Table 3), and those having 1.3 or less were rejected ("x" in Table 3). .

[Test results]
Table 3 shows the test results. In addition, "○" is shown in the column of "pre-rolling" in Table 3 for the steel wire which has been subjected to the pre-rolling process, and ""-" Is shown in the column. In addition, "○" is shown in the column of "Post-rolling" in Table 3 for the steel wire subjected to the post-rolling process, and ""-" Is shown in the column.

  The chemical compositions of the high-strength bolts of test numbers 1 to 12 were appropriate. Further, fn1 satisfied Expression (1), and fn2 satisfied Expression (2). Further, the absolute value of the compressive residual stress on the screw bottom surface of the high-strength bolt satisfied the range of 10 to 90% of the tensile strength of the high-strength bolt. As a result, the high-strength bolts of these test numbers have a critical diffusible hydrogen content ratio HR higher than 1.30 and have a high hydrogen embrittlement resistance despite the high tensile strength of 1000 to 1300 MPa. Excellent.

  The bolts of Test Nos. 13 and 14 had appropriate chemical compositions. Further, fn1 satisfied Expression (1), and fn2 satisfied Expression (2). As a result, although the bolts of these test numbers had a high tensile strength of 1200 MPa, the critical diffusible hydrogen content ratio HR was higher than 1.30, and were excellent in hydrogen embrittlement resistance. However, since the absolute value of the compressive residual stress on the screw bottom surface of the bolt was less than 10% of the tensile strength of the bolt, the HR was lower than that of Test Nos. 1 to 12.

  With the bolt of test number 15, fn1 was less than the lower limit of the equation (1). Therefore, the tensile strength was less than 1000 MPa.

  With the bolt of test number 16, fn1 exceeded the upper limit of the equation (1). Therefore, the cold workability of the steel material for the bolt (wire) was low, and cracks were observed in the bolt after cold forging, so that the subsequent treatment and test were not performed.

  With test number 17 volts, fn2 was less than the lower limit of equation (2). Therefore, HR was 1.30 or less, and the hydrogen embrittlement resistance was low.

In Test No. 18, fn2 exceeded the upper limit of Expression (2). Therefore, the hot workability is deteriorated, and the steel slab after continuous casting is cracked.

  In test number 19, the Mn content was too high. Therefore, the HR was as low as 1.30 or less, and the hydrogen embrittlement resistance was low.

  The embodiments of the present invention have been described above. However, the above-described embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

  Since the bolt of the present invention has high strength of 1000 to 1300 MPa and excellent hydrogen embrittlement resistance, it is a member used for automobiles, industrial machines, buildings, etc., particularly, engine cylinder head bolts, connecting rod bolts. Suitable for automobile bolts such as hub bolts.

Claims (4)

In mass%, C: 0.22 to 0.40%, Si: 0.10 to 1.50%, Mn: 0.20 to less than 0.40%, Cr: 0.70 to less than 1.60%, Al: 0.005 to 0.060%, Ti: 0.010 to 0.050%, B: 0.0003 to 0.0040%, N: 0.0015 to 0.0080%, Cu: 0.50% In the following, Ni: 0.30% or less, Mo: 0.05% or less, V: 0.050% or less, Nb: 0.050% or less,
Sb: 0.001 to 0.100%, Sn: 0.001 to 0.100%, Bi: 0.001 to 0.100%
O: 0.0020% or less, P: 0.020% or less, S: 0.020% or less,
The balance consists of Fe and impurities and has a chemical composition satisfying the formulas (1) and (2),
A bolt having a shaft with a tensile strength of 1000 to 1300 MPa.
0.50 ≦ C + (1/10) × Si + (1/5) × Mn + (5/22) × Cr ≦ 0.85 (1)
0.003 ≦ Sb + Sn + Bi ≦ 0.100 (2)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in the formulas (1) and (2).   Cu: 0.02 to 0.50%, Ni: 0.01 to 0.30%, Mo: 0.01 to 0.05%, V: 0.005 to 0.050% The bolt according to claim 1, wherein the bolt contains one or more types.   The bolt according to claim 1, wherein the bolt contains Nb: 0.005 to 0.050%.   The range from the surface of the screw bottom of a shaft part to 50 micrometers depth has a compressive residual stress of 10-90% of the tensile strength of the said shaft part, The Claims any one of Claims 1-3 characterized by the above-mentioned. Bolts.

Images