JP7155644B2 - bolt - Google Patents

Description

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

BACKGROUND ART In recent years, in order to deal with environmental problems and the like, there has been a demand for weight reduction and strength enhancement for members used in automobiles, industrial machinery, buildings, and the like. 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, if the tensile strength of the bolt becomes as high as 1000 MPa or more, the hydrogen embrittlement resistance (delayed fracture resistance) is lowered, and hydrogen embrittlement (delayed fracture) occurs due to penetrating diffusible hydrogen. As materials for such high-strength bolts, SCM steel (JIS G 4053 (2008)) containing a large amount of alloying elements such as Mo and alloy steel containing expensive alloying elements such as V are used. ing. These alloy steels are manufactured into wire rods, which are then drawn and cold forged into bolts.

When the alloy steel described above is used as a bolt, the hydrogen embrittlement resistance is good even in an environment where hydrogen penetrates. However, since these alloy steels contain a large amount of alloying elements, the steel material cost is high. In recent years, the prices of alloying elements have soared, and the supply and demand environment is likely to fluctuate. Therefore, there is a demand for a bolt that can achieve high strength and excellent resistance to hydrogen embrittlement while reducing or omitting these alloying elements to reduce the cost of steel materials.

Expensive alloying elements such as Mo and V should be reduced in order to suppress steel material costs. If the alloying elements are reduced, it is possible to suppress the formation of hard structures such as bainite when the wire rod is produced by hot rolling. Therefore, the softening heat treatment for cold working can be omitted or simplified, and the manufacturing cost can be reduced. However, if the alloying elements are reduced, the hardenability of the steel deteriorates, making it difficult to increase the strength of the bolt by quenching, and the hydrogen embrittlement resistance also deteriorates.

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

Patent Literature 1 discloses that the inclusion of Sn provides excellent corrosion resistance, and suppressing hydrogen entry due to corrosion provides a high-strength bolt steel with excellent delayed fracture resistance. However, the bolt of Patent Document 1 is required to contain Mo or V such that Mo+2V satisfies 0.10 to 1.0% by mass.

Patent Document 2 discloses that the inclusion of Sn or Sb makes it possible to obtain steel for bolts having excellent sulfuric acid corrosion resistance. However, the bolt of Patent Literature 2 has a tensile strength of 700 MPa or less, and no consideration is given to delayed fracture resistance.

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

An object of the present invention is to provide a bolt made of low-alloy steel for machine structural use, having high strength and excellent resistance to hydrogen embrittlement in an environment where hydrogen penetrates.

The present inventors used steel containing C, Si, Mn, Cr, B, Sb, Sn, Bi, etc., which does not contain a large amount of expensive alloying elements such as Mo and V, to improve the tensile strength of the bolt. , investigated and examined the components and structures that affect hydrogen embrittlement resistance.

[Regarding both bolt tensile strength and cold workability]
In order to increase the tensile strength of the bolt to 1000-1300 MPa, sufficient hardenability is required. However, if a large amount of alloying elements are added to improve the hardenability, the cold workability of the wire material, which is the material of the bolt, is lowered. In this case, in order to improve the reduced 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 wire rods. must be performed multiple times. Therefore, in addition to containing a large amount of alloying elements such as Mo and V, the processing cost increases.

Therefore, it is desirable to use a steel material that can be cold-worked without performing multiple softening heat treatments for a long period of time and that has hardenability such that the tensile strength described above can be obtained. C, Si, Mn and Cr are all elements that improve hardenability. Focusing on this point, the present inventors decided to use the following function fn1 with C, Si, Mn, and Cr as parameters to determine the hardenability and cold workability of steel materials for manufacturing bolts. investigated.

fn1 = C + (1/10) x Si + (1/5) x Mn + (5/22) x Cr
The coefficient of each element symbol of fn1 is a numerical value indicating the degree of influence on the quench hardening of steel, and the larger the coefficient, the easier the quench hardening of steel occurs. 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, the hardenability is not sufficient, while if fn1 is too high, the hardenability is too high. Furthermore, as a result of examination of the value obtained from fn1, if fn1 satisfies the formula (1), excellent hardenability can be obtained, and sufficient cold working can be performed without performing multiple softening heat treatments for a long time. I found that I could get sexuality.

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 formula (1).

[About the hydrogen embrittlement resistance of bolts]
Antimony (Sb), tin (Sn), and bismuth (Bi) exhibit the effect of suppressing hydrogen penetration when added in trace amounts. Therefore, the present inventors have investigated the addition of these elements to improve the hydrogen embrittlement resistance of the steel material for manufacturing bolts without impairing the hardenability and cold workability. That is, the following function fn2 with Sb, Sn and Bi as parameters was used to determine the hydrogen embrittlement resistance of the bolt.

fn2=Sb+Sn+Bi
The coefficient of each element symbol in fn2 is a numerical value indicating the degree of influence of each element on the delayed fracture characteristics. Since Sb, Sn and Bi have the same degree of effect of suppressing hydrogen penetration, the coefficient of each element symbol of fn2 is 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 tensile strength of the bolt is as high as 1000 to 1300 MPa, if the formula (2) is satisfied, the corrosion resistance is excellent, so the hydrogen penetration suppression effect is good, and the hydrogen embrittlement resistance is excellent. is 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 formula (2).

The present invention was made based on the above findings, and the gist thereof 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% containing one or more,
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 that satisfies formulas (1) and (2),
A bolt characterized in that the tensile strength of the shaft portion is 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 formulas (1) and (2).
(2) From the group consisting of 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), characterized by containing one or more selected types.
(3) The bolt according to (1) or (2), characterized by containing 0.005 to 0.050% of Nb.
(4) The bolt according to (1) to (3), wherein the area from the bottom surface of the thread of the shank to a depth of 50 μm has a compressive residual stress of 10 to 90% of the tensile strength of the shank.

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

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

The present invention will be described in detail below. First, the reasons for limiting the component composition of the bolt of the present invention will be explained. Hereinafter, "%" for 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 hardening and tempering to 1000 MPa or more. If the C content is less than 0.22%, the above effect 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 deteriorates. Therefore, the steel material before cold working such as wire drawing and cold forging must be subjected to multiple heat treatments for a long period of time for the purpose of softening, which increases the manufacturing cost. When the heat treatment is performed, the hydrogen embrittlement resistance is further deteriorated. Therefore, the C content is 0.22-0.40%. A preferred lower limit for the C content is 0.24%, more preferably 0.26%. A preferable 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 temper softening resistance. Si also deoxidizes the steel. The deoxidation product, MnO—SiO ₂ , is a vitrified soft inclusion, which is refined by stretching and breaking during hot rolling. 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 are reduced. Therefore, the Si content is 0.10-1.50%. The lower limit of the Si content is preferably over 0.35%, more preferably 0.40%, still more preferably 0.45%, still more preferably over 0.50%. A preferable upper limit of the Si content is 1.20%, more preferably 1.00%.

[Mn: less than 0.20 to 0.40%]
Manganese (Mn) increases 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 these inclusions are soft and are stretched and split during hot rolling to be finer, the density of MnO—SiO ₂ is reduced and hydrogen embrittlement resistance is increased. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, it segregates at the grain boundaries and promotes intergranular fracture, resulting in rather low hydrogen embrittlement resistance. Therefore, the Mn content is 0.20-0.40%. A preferred lower limit for the Mn content is 0.22%, more preferably 0.25%. A preferable upper limit of the Mn content is 0.38%, more preferably 0.35%.

[Cr: less than 0.70 to 1.60%]
Chromium (Cr) increases the hardenability and makes the tensile strength of the bolt 1000 MPa or more. Cr also 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, when the Cr content is 1.60% or more, the hardenability becomes too high, and the cold workability of the steel material for bolts deteriorates. Therefore, the Cr content is between 0.40 and less than 1.60%. A preferred lower limit for the Cr content is 0.90%. A preferred upper limit for the Cr content is 1.45%.

[Al: 0.005 to 0.060%]
Aluminum (Al) deoxidizes steel. This effect cannot be obtained if the Al content is less than 0.005%. On the other hand, if the Al content exceeds 0.060%, coarse oxide-based inclusions are formed and the cold workability deteriorates. Therefore, the Al content is 0.005-0.060%. A preferable lower limit of the Al content is 0.010%. A preferred upper limit for 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 nitrides (TiN). The formation of TiN suppresses the formation of BN and increases the amount of dissolved B. As a result, the hardenability of the steel is enhanced. Ti further combines with C to form carbides (TiC) to refine grains. This enhances the hydrogen embrittlement resistance of the bolt. These effects cannot be obtained if the Ti content is less than 0.010%. On the other hand, if the Ti content exceeds 0.050%, a large amount of coarse TiN is produced. In this case, cold workability and hydrogen embrittlement resistance deteriorate. Therefore, the Ti content is 0.010-0.050%. A preferred lower limit for the Ti content is 0.015%. A preferred upper limit for the Ti content is 0.045%.

[B: 0.0003 to 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. In addition, coarse BN is produced to deteriorate cold workability. Therefore, the B content is 0.0003-0.0040%. A preferable lower limit of the B content is 0.0005%. A preferable 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 grains. This effect cannot be obtained if the N content is less than 0.0015%. On the other hand, if the N content exceeds 0.0080%, the effect is saturated. Furthermore, N combines with B to form nitrides, which reduces the solid solution B amount. In this case, the hardenability of steel deteriorates. Therefore, the N content is 0.0015-0.0080%. A preferable lower limit of the N content is 0.0020%. A 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 addition to the above components, the present invention adds one or more of antimony (Sb), tin (Sn), and bismuth (Bi) within the range of 0.001% to 0.10%. It is characterized by These elements exhibit the effect of suppressing hydrogen penetration when added in a very small amount. Although the lower limit is set to 0.001% or more for each, the preferable lower limit for sufficiently exhibiting the effect is set to 0.005% or more for each. Furthermore, each is preferably 0.010% or more. On the other hand, if the upper limit of each content exceeds 0.100%, the hot workability of the steel deteriorates, making continuous casting difficult. Also, preferably, the total concentration of Sb, Sn, and Bi should be 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 from ore, scrap, or the manufacturing environment as raw materials when industrially manufacturing high-strength bolts, and include O, P, S and As, Co , Mg, Zr, Te, Bi, Pb, Zn and the like. Among these, As, Co, Mg, Zr, Te, Bi, Pb, Zn and the like are restricted to the extent that they do not inhibit the effects of the present invention.

[Arbitrary element]
The high-strength bolt described above may further contain one or more selected from the group consisting of Cu, Ni, Mo, and V in place of part of Fe. All of these elements are optional elements and enhance 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 steel. However, if the Cu content exceeds 0.50%, the hardenability becomes too high and the cold workability deteriorates. Therefore, the Cu content is 0.50%. A preferable lower limit of the Cu content for obtaining the above effects more effectively is 0.02%, more preferably 0.05%. A preferable upper limit of the Cu content is 0.30%, 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 enhances 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 deteriorates. Therefore, the Ni content is 0-0.30%. A preferable lower limit of the Ni content for obtaining the above effect more effectively is 0.03%, more preferably 0.05%. A preferable upper limit of the Mo content is 0.20%, more preferably 0.10%.

Mo: 0.05% or less Molybdenum (Mo) is an optional element and may not be contained. When included, Mo enhances the hardenability of steel. However, if the Mo content exceeds 0.05%, the hardenability becomes too high, and the cold workability of the steel material for high-strength bolts deteriorates. Therefore, the Mo content is 0-0.05%. A preferable lower limit of the Mo content for obtaining the above effect more effectively is 0.01%, more preferably 0.015%. A preferred upper limit for the Mo content is 0.03%, 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 refines crystal grains by forming carbides, nitrides or carbonitrides. However, if the V content exceeds 0.050%, carbides and the like are coarsened to deteriorate cold workability. Therefore, the V content is 0-0.050%. A preferable lower limit of the V content for obtaining the above effect more effectively is 0.005%. A preferable upper limit of the V content is 0.030%, 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 refine 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 formed, which deteriorates the cold workability of the steel material. Therefore, the Nb content is 0-0.050%. A preferable lower limit of the Nb content for obtaining the above effect more effectively is 0.005%. A preferable upper limit of the Nb content is 0.040%, 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 oxides are produced, and MnS is coarsened, resulting in a marked decrease in cold workability. Therefore, the O content is 0.0020% or less. A preferred upper limit of the O content is 0.0018%. It is preferable that the O content is as low as possible.

[P: 0.020% or less]
Phosphorus (P) is an impurity. P segregates at grain boundaries to reduce cold workability and hydrogen embrittlement resistance of bolts. Therefore, the P content is 0.020% or less. A preferable upper limit of the P content is 0.015%. The lower the P content is, the better.

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

[0.50≦fn1≦0.85]
As previously mentioned, the function fn1 with parameters C, Si, Mn and Cr can be used to determine the hardenability and cold workability of steels for the manufacture of bolts. Moreover, when the chemical composition of the bolt satisfies the following formula (1), excellent cold workability and hardenability can be 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 formula (1). If the corresponding element is an impurity level, "0" is substituted for the corresponding element symbol in formula (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, bainite is produced when the bolt steel is rolled into a wire rod, increasing strength and hardness. Therefore, sufficient cold workability cannot be obtained unless a long-term softening heat treatment is performed multiple times before the subsequent wire drawing process and cold forging process. If fn1 satisfies the formula (1), it is possible to obtain excellent hardenability and sufficient cold workability without performing multiple softening heat treatments for a long period of time. A preferred lower limit for fn1 is 0.53. A preferred upper limit for fn1 is 0.83.

[0.003 ≤ fn2 = Sb + Sn + Bi ≤ 0.100]
As described above, when the following formula (2) is satisfied, the anti-corrosion property is excellent, so the effect of suppressing hydrogen penetration is good, and 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 formula (2). If the corresponding element is an impurity level, "0" is substituted for the corresponding element symbol in formula (2).

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

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 heating the ingots respectively produced using a to n to 1200 to 1300° C., hot forging was performed assuming hot rolling to produce round bars with 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. A round bar with adjusted tensile strength was machined to produce a test piece with an annular V notch as shown in FIG. In addition, test piece No. obtained from each of steels a to m. In all of a1 to m1, the total area ratio of bainite phase and ferrite phase was 5% or less, and the remainder was tempered martensite.

Numerical values without units in FIG. 2 indicate the dimensions (in mm) of the corresponding portions of the test piece. The "φ value" in the figure indicates the diameter (mm) of the designated portion. "60°" indicates that the V notch angle is 60°. "0.175R" indicates that the V-notch bottom radius is 0.175 mm.

Each steel composition no. Specimen Nos. a to n. Various concentrations of hydrogen were introduced into each of a1 to n1. The electrolytic charging method was carried out as follows. Hydrogen was taken into the test piece by generating an anode potential on the surface of the test piece while the test piece was immersed in the ammonium thiocyanate aqueous solution. After that, a galvanized film was formed on the surface of each test piece to prevent hydrogen from escaping from the test piece. Subsequently, a constant load test was performed in which a constant load was applied so that a tensile stress with a nominal stress of 1080 MPa was applied to the V-notch cross section of the test piece. A temperature programmed analysis method using a gas chromatograph was performed on the test piece that was broken during the test and the test piece that was not broken, and the amount of hydrogen in the test piece was measured. After measurement, the maximum amount of hydrogen in a test piece that did not break was defined as the critical diffusible hydrogen amount Hc for each steel.

Furthermore, based on the hydrogen permeability coefficient Href of steel m having a chemical composition corresponding to SCM435 of JIS G4053 (2008) when immersed for 30 hours, the hydrogen permeability coefficient ratio HR (hereinafter simply referred to as HR) is calculated by the following formula (A ).
HR=Hc/Href (A)

HR is an index of hydrogen permeation suppression characteristics. Based on the obtained HR and fn2 of each steel, FIG. 1 was created. Each steel composition No. Specimen Nos. a to n. fn1, fn2 and HR of a1 to n1 are shown in Table 1-2.

From FIG. 1, it is clear that HR increases significantly as fn increases, that is, as the ratio of Sb, Sn or Bi contents increases, and at fn2=0.003 than when fn2=0. More than 5% higher HR.

When fn2 exceeds 0.002, the effect of suppressing hydrogen penetration appears, HR becomes higher than 1.3, and hydrogen embrittlement resistance is superior to SCM435, which is a typical low alloy steel for gears, crankshafts, etc. properties are obtained. However, if fn2 exceeds 0.1, the hot workability of the steel deteriorates, making continuous casting difficult. Therefore, as shown in equation (2), the upper limit of fn2 is 0.100. A preferred lower limit for fn2 is 0.005.

[Metal structure]
It is preferable that 95% or more of the metal structure of the bolt of the present invention is tempered martensite, and the remaining 5% or less of the metal structure is at least one of bainite and ferrite. If the metal structure is at least one of bainite and ferrite, a high tensile strength of 1000 to 1300 MPa cannot be achieved. In addition, 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 a high tensile strength of 1000 to 1300 MPa can be achieved, a phase other than tempered martensite may be included within the range in which the above tensile strength can be achieved. In the present invention, when at least one of bainite and ferrite is included in addition to tempered martensite, the total area ratio of these phases may be 5% or less. However, bolts having a total area ratio of bainite phase and ferrite phase of more than 5% other than tempered martensite may not achieve a high tensile strength of 1000 to 1300 MPa.

The area ratios of tempered martensite, bainite and ferrite of the bolt are measured as follows. First, cut out a cross section parallel to the central axis so as to pass through the central axis of the bolt, polish the cross section, corrode with 3% nital, and use an optical microscope at a magnification of 400 times to see the central axis from the bottom of the screw. In the direction, observe the position of 1/2 the distance from the thread bottom to the central axis. For 0.5 mm ² in the image taken at an observation magnification of 400 times, the image processing software Winroof2015 is used to derive the area ratio of the bright region when the tempered martensite is binarized as a bright region, and the martensite area ratio and

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

In this case, the tensile stress applied to the bottom of the thread when the bolt is tightened is offset with the compressive residual stress. For this reason, the stress state of the starting point is relaxed, and 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 tensile stress canceling effect due to the compressive residual stress is 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” refers to a range from the surface of the high-strength bolt to a depth of 50 μm toward the central axis. 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). Measurement is performed using characteristic X-ray type: MnKα ray, Cr filter, reference diffraction angle 2θ ₀ : 152.0°, η angle 14.0°, X-ray stress constant K: −336 MPa/deg. The measurement site is centered on the central position of the bottom of the thread, and the direction of measurement is parallel to the longitudinal direction of the thread. Tensile strength is determined according to JIS Z2241 (2011).

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

[Material manufacturing process]
A molten steel having the above chemical composition is produced. A cast slab is produced by a continuous casting method using molten steel. Alternatively, an ingot is produced by an ingot casting method using molten steel. The produced slabs or ingots are bloomed into billets. A steel billet is hot-worked into a steel material (wire rod) for high-strength bolts. Hot working is, for example, hot rolling.

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

[Wire drawing process]
First, a steel wire is manufactured by drawing a wire rod. The wire drawing process may be only the primary wire drawing process, or a plurality of wire drawing processes such as the secondary wire drawing process may be performed. During wire drawing, a lubricating coating 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 P-free lubricating coating is used. Alternatively, when a phosphate coating is used, the surface of the steel material (steel wire) is washed or pickled to remove the phosphate coating from the surface before the quenching process described later. Cleaning is, for example, well-known alkaline 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 a bolt.

[About softening heat treatment]
In the conventional bolt manufacturing method, softening heat treatment is performed multiple times before wire drawing and cold forging for the purpose of softening the bolt steel material (wire rod) having too high strength. However, the bolt according to the invention simplifies such a softening heat treatment by satisfying equation (1). As a result, it is possible to suppress an increase in the manufacturing cost due to the softening heat treatment, and to improve the hydrogen embrittlement resistance of the bolt.

[Threading process]
A high-strength bolt manufactured by cold forging is rolled under well-known conditions to form a screw thread.

[Quenching and tempering process]
The high-strength bolt after threading is quenched and tempered under well-known conditions to adjust the tensile strength of the bolt to 1000 to 1300 MPa. If the tensile strength is 1000 MPa or less, the strength of the bolt is insufficient. On the other hand, if the tensile strength exceeds 1300 MPa, the hydrogen sensitivity increases and the hydrogen embrittlement resistance deteriorates. Therefore, the tensile strength of the bolt is 1000-1300 MPa. When using a P-containing lubricating coating typified by a phosphate coating during the wire drawing process, as described above, the surface of the steel material (steel wire) is preferably washed with an alkali before quenching.

[Compressive residual stress application step]
Preferably, the bolt after quenching and tempering is subjected to a well-known compressive residual stress application process to bring the compressive residual stress in the surface layer of the thread base to 10-90% of the tensile strength of the bolt. A well-known compressive residual stress application process is, for example, shot peening. By appropriately adjusting the shot peening conditions, the compressive residual stress in the surface layer of the thread bottom can be made 10 to 90% of the tensile strength of the bolt.

In the above-described manufacturing method, the threading process (pre-rolling process) is included before quenching and tempering, but instead of the pre-rolling process, the threading process (post-rolling process) may be performed after quenching and tempering. . Even 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, shot peening may not be performed.

The bolt of the present invention is manufactured by the manufacturing process described above.

Steel no. Each molten steel of A to R was produced.

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

Billets having a cross section of 162 mm×162 mm were produced by continuous casting using molten steel Nos. A to R having chemical compositions shown in Table 2. The billets having chemical compositions Nos. A to R thus obtained were once cooled to room temperature, and the presence or absence of surface cracks in the cast slab was visually determined. Table 3 shows the results. Also, billets of chemical composition Nos. A to R were hot-worked (hot rolled) to produce wire rods with a diameter of 11.5 mm.

Steel wires of Test Nos. 1 to 20 in Table 3 were produced by drawing the wire rods obtained from billets of Steel Nos. A to R having chemical compositions in Table 2, respectively. At this time, heat treatment was performed for the purpose of softening. The heat treatment temperature was 750° C., the heat treatment time was 60 minutes, and slow cooling was performed after the heat treatment. Furthermore, after degreasing and pickling, zinc phosphate treatment (75 ° C., immersion time 600 sec) and metal soap treatment (80 ° C., immersion time 180 sec) are performed, and the zinc phosphate film and the metal soap film are removed from the surface. A lubricating treatment film was formed. After that, finish wire drawing was performed to produce a steel wire with 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 bolts shown in FIG. Specifically, cold forging was performed in two steps. In the first step, the shank of the bolt was pressed. In the second step, a mold was designed so as to form the head and flange of the bolt, and the mold was mounted on a hydraulic forging press for cold forging. Each numerical value in the figure indicates the dimension (mm) of the corresponding portion. The "φ value" in the figure indicates the diameter (mm) of the designated portion. "Numerical value °" in the figure indicates the angle (°) of the specified site. "R value" indicates the radius of curvature (mm) of the designated site. "M7×1.0" in the figure indicates that the outer diameter is 7 mm and the pitch is 1.0 mm.

Each bolt formed from the steel wires of test numbers 1 to 20 as described above was visually observed to investigate the presence or absence of cracks. Those in which cracks were observed were determined not to be bolt-formable.

Quenching and tempering treatments were performed at the temperatures shown in Table 3 for bolts with test numbers in which no cracks were observed. Prior to the hardening treatment, the bolt surface was washed with alkali to remove the phosphate coating.

In the quenching treatment, the steel was held at the quenching temperature (°C) shown in Table 3 for 40 minutes and then oil-cooled. In the tempering treatment, the tempering temperature shown in Table 3 was maintained for 70 minutes. A bolt was manufactured by the above steps. In addition, when the tempering temperature for obtaining the desired bolt tensile strength (1000 MPa to 1300 MPa) is less than 435 ° C., it is judged that the strength is insufficient, the hydrogen embrittlement resistance property evaluation is not performed, and the subject of the present invention. decided outside.

The steel wires of test numbers 1 to 12, 15, and 18 to 20 were subjected to thread rolling after quenching and tempering, and residual stress was applied to the surface of the screw bottom along with thread forming. 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 of the screw bottom was measured using an X-ray stress measurement method using X-ray diffraction in accordance with JIS B2711 (2013). Measurement was performed using characteristic X-ray type: MnKα ray, Cr filter, reference diffraction angle 2θ0: 152.0°, η angle 14.0°, X-ray stress constant K: -336 MPa/deg. In addition, the residual stress was measured in a direction parallel to the longitudinal direction of the threaded portion centered on the central position of the threaded bottom portion.

[Metal structure]
The area ratios of tempered martensite, bainite and ferrite were measured for each of the bolts formed from the steel wires of test numbers 1 to 20. For the area ratio of each structure, a cross section parallel to the central axis is cut out so as to pass through the central axis of the bolt. was used to observe the position of 1/2 the distance from the screw bottom to the central axis in the direction of the central axis from the screw bottom at a magnification of 400 times, and the tissue was photographed. The image processing software Winroof2015 is used for a total of 0.5 mm ² of the image taken at an observation magnification of 400 times, and the tempered martensite is binarized as a bright region. and All of the bolts formed from the steel wires of test numbers 1 to 20 had a total area ratio of bainite phase and ferrite phase of 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 treatment or rolling was measured in the air at room temperature (25° C.) for each test number. Table 3 shows the measurement results.

[Hydrogen embrittlement resistance evaluation test]
Annular V-notch test pieces shown in FIG. 2 were prepared for the bolts after quenching and tempering treatment or rolling processing of each test number, and hydrogen of various concentrations was introduced using the electrolytic charging method. The electrolytic charging method was carried out as follows. A bolt was immersed in an ammonium thiocyanate aqueous solution. While the bolt was immersed, an anodic potential was generated on the surface of the bolt to introduce hydrogen into the bolt.

After hydrogen was introduced into the bolt, a galvanized film was formed on the bolt surface to prevent hydrogen from escaping from the bolt. Subsequently, a constant load test was performed with a tensile strength of 95% of the tensile strength of the bolt. A temperature programmed analysis method using a gas chromatograph was performed on the bolts that were broken during the test and the bolts that were not broken, and the amount of hydrogen in the bolts was measured. After measurement, the maximum amount of hydrogen in a test piece that did not break was defined as the critical diffusible hydrogen amount Hc in each test number.

Furthermore, the limit diffusible hydrogen amount of the steel wire of Test No. 20 having a chemical composition corresponding to SCM435 in the JIS standard used for conventional bolts was used as the reference (Href) for the limit diffusible hydrogen amount ratio HR. Using the critical diffusible hydrogen amount Href as a reference, the critical diffusible hydrogen amount ratio HR was obtained using the formula (A). In the evaluation of hydrogen embrittlement resistance, samples with HR higher than 1.3 were accepted (“◯” in Table 3), and samples with HR of 1.3 or less were rejected (“×” in Table 3). .

[Test results]
Table 3 shows the test results. Steel wires that have undergone the pre-rolling process are marked with "○" in the column of "pre-rolling" in Table 3, and steel wires that have not undergone the pre-rolling process are marked with "pre-rolling". "-" is indicated in the column. In addition, steel wires subjected to the post-rolling process are marked with "○" in the column of "post-rolling" in Table 3, and steel wires not subjected to the post-rolling process are marked with "post-rolling". "-" is indicated in the column.

The chemical composition of the high-strength bolts of test numbers 1-12 was suitable. Furthermore, fn1 satisfied equation (1) and fn2 satisfied equation (2). Also, the absolute value of the compressive residual stress on the thread bottom surface of the high-strength bolt satisfies the range of 10 to 90% of the tensile strength of the high-strength bolt. As a result, although the high-strength bolts of these test numbers have a high tensile strength of 1000 to 1300 MPa, the critical diffusible hydrogen amount ratio HR is higher than 1.30, and the hydrogen embrittlement resistance outstanding.

The bolt chemistries of Test No. 13 and Test No. 14 were suitable. Furthermore, fn1 satisfied equation (1) and fn2 satisfied equation (2). As a result, although the bolts of these test numbers had a high tensile strength of 1200 MPa, the critical diffusible hydrogen amount ratio HR was higher than 1.30, indicating excellent hydrogen embrittlement resistance. However, since the absolute value of the compressive residual stress on the bottom surface of the thread of the bolt was less than 10% of the tensile strength of the bolt, the HR was low compared to Test Nos. 1-12.

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

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

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

In test number 18, fn2 exceeded the upper limit of formula (2). As a result, the hot workability deteriorated, and cracks occurred in the slab after continuous casting.

The Mn content of test no. 19 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 embodiments are merely examples 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 of the present invention.

The bolt of the present invention has a high strength of 1000 to 1300 MPa and excellent resistance to hydrogen embrittlement. , hub bolts and other automotive bolts.

Claims (4)

% by 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: 0.005-0.060%, Ti: 0.010-0.050%, B: 0.0003-0.0040%, N: 0.0015-0.0080%, Cu: 0.50% Below, 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% containing one or more,
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 that satisfies formulas (1) and (2),
A bolt characterized in that the tensile strength of the shank is 1000-1200 MPa.
0.50≦C+(1/10)×Si+(1/5)×Mn+(5/22)×Cr≦0.85 (1)
0.004 ≤ Sb + Sn + Bi ≤ 0.100 (2)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formulas (1) and (2). Cu: 0.02 to 0.50%, Ni: 0.01 to 0.30%, Mo: 0.01 to 0.05%, V: selected from the group consisting of 0.005 to 0.050% 2. The bolt according to claim 1, containing one or more. The bolt according to claim 1 or 2, characterized by containing Nb: 0.005 to 0.050%. 4. The shank according to any one of claims 1 to 3, wherein a range from the surface of the thread bottom of the shank to a depth of 50 μm has a compressive residual stress of 10 to 90% of the tensile strength of the shank. of bolts.

Images