JP2017078209A - High strength bolt and steel for high strength bolt - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength bolt having high strength and excellent hydrogen embrittlement resistance.SOLUTION: The high strength bolt has a chemical composition containing, by mass%, C:0.24 to 0.40%, Si:0.01 to 0.50%, Mn:0.5 to 2.0%, P:0.020% or less, S:0.020% or less, Cr:0.2 to 0.7%, Al:0.005 to 0.050%, B:0.001 to 0.005%, Ti:0.007 to 0.100%, N:0.002 to 0.008%, Nd:0.001 to 0.100%, Nb:0 to 0.050%, Mo:0 to 0.30% and V:0 to 0.30% and the balance Fe with impurities and satisfies formula (1). [Psurf]-0.215 Nd≤0.015 (1), where [Psurf] is substituted with average P concentration (mass%) in a range from a surface to 10 μm depth and Nd is substituted with content (mass%) of Nd.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a bolt and a steel for a bolt, and more particularly to a high-strength bolt and a steel for a high-strength bolt excellent in hydrogen embrittlement resistance.

  Bolts are used in industrial machinery used in industry, agriculture, civil engineering, and the like, as well as automobiles, bridges, and various buildings. Bolts are manufactured as follows. For example, medium carbon low alloy steel such as SCM435 and SCr440 standardized in JIS G4053 is formed into a bolt-shaped intermediate product by cold forging. Thereafter, the intermediate product is quenched and tempered to form a bolt. In order to reduce the weight of automobiles and the size of building bolt joints, bolts are required to have higher strength. However, in the bolt made of the medium carbon low alloy steel described above, the hydrogen embrittlement resistance decreases as the tensile strength increases.

  Hydrogen embrittlement is considered to be an embrittlement phenomenon of prior austenite grain boundaries caused by hydrogen that has entered the steel from the environment of use. In order to improve the hydrogen embrittlement resistance, the influence of various factors such as the tempering temperature has been studied.

  For example, Japanese Patent Application Laid-Open No. 2007-146284 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2008-156678 (Patent Document 2) propose bolts excellent in hydrogen embrittlement resistance. In these documents, the resistance to hydrogen embrittlement is enhanced by a combination of adjustment of steel material components and refinement of the steel structure. Specifically, the bolts disclosed in these documents contain Mo and B. Mo forms fine carbides to trap hydrogen and render it harmless. B is concentrated at the grain boundary to increase the grain boundary strength. Further, in these documents, the prior austenite grain size of the bolt is set to 10 μm or less. Thereby, precipitation of film-like carbide is suppressed and hydrogen embrittlement resistance is improved.

  However, Patent Documents 1 and 2 do not sufficiently study P in steel. P segregates at the grain boundaries, embrittles the grain boundaries, and decreases the resistance to hydrogen embrittlement. Furthermore, in patent documents 1 and 2, since Mo which is an expensive alloy element contains 0.3% or more as an essential element, a manufacturing cost becomes high.

JP 2007-146284 A JP 2008-156678 A

  An object of the present invention is to provide a high strength bolt having high strength and excellent hydrogen embrittlement resistance, and a steel for high strength bolts.

The high-strength bolt according to the present invention is, in mass%, C: 0.24 to 0.40%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.020. %: S: 0.020% or less, Cr: 0.2-0.7%, Al: 0.005-0.050%, B: 0.001-0.005%, Ti: 0.007- 0.100%, N: 0.002 to 0.008%, Nd: 0.001 to 0.100%, Nb: 0 to 0.050%, Mo: 0 to 0.30%, and V: 0 It contains ˜0.30% and satisfies the formula (1).
[Psurf] −0.215Nd ≦ 0.015 (1)
Here, an average P concentration (mass%) in a range from the surface to a depth of 10 μm is substituted into [Psurf]. Nd content (mass%) is substituted for Nd.

  The steel for high-strength bolts according to the present invention has the chemical composition described above.

  The high strength bolt according to the present invention has high strength and excellent hydrogen embrittlement resistance. This high-strength bolt is manufactured using the steel for high-strength bolts described above.

FIG. 1 is a diagram showing the relationship between the concentration of P not bonded to Nd in the surface layer of steel and the hydrogen embrittlement strength ratio, which is an index of hydrogen embrittlement resistance. FIG. 2 is a side view of a test piece for a hydrogen embrittlement resistance test.

  The present inventors investigated and examined a method for improving the hydrogen embrittlement resistance while increasing the strength of the bolt, and obtained the following knowledge.

  (A) If P in the steel segregates at the grain boundaries, the hydrogen embrittlement resistance of the steel decreases. Therefore, neodymium (Nd) is included in the chemical composition of the bolt. In this case, Nd combines with P to form NdP. Thereby, even if it does not contain Mo as an essential element, the grain boundary segregation of P is suppressed and the hydrogen embrittlement resistance of steel increases. Since NdP serves as a diffusible hydrogen trap site, the hydrogen embrittlement resistance of the steel is further enhanced.

(B) Here, P not bonded to Nd (hereinafter also referred to as Pfree) exists in the steel. If the Pfree concentration in the range from the surface of the high-strength bolt to the depth of 10 μm (hereinafter referred to as the surface layer) is low, the grain boundary segregation of P in the surface layer is suppressed and the hydrogen embrittlement resistance is improved. Specifically, in a high-strength bolt having a tensile strength of 1000 MPa or more, the Pfree concentration (hereinafter also referred to as [Pfree]) of the steel surface layer satisfies the formula (1).
[Psurf] −0.215Nd ≦ 0.015 (1)
Here, an average P concentration (mass%) in a range from the surface to a depth of 10 μm is substituted into [Psurf]. Nd content (mass%) is substituted for Nd.

  In this case, the high-strength bolt has excellent hydrogen embrittlement resistance while having a high tensile strength of 1000 MPa or more. Hereinafter, this point will be described in detail.

  FIG. 1 is a diagram showing the relationship between the Pfree concentration ([Pfree]) of the steel surface layer and the hydrogen embrittlement strength ratio, which is an index of hydrogen embrittlement resistance. The atomic weight of P is about 31, and the atomic weight of Nd is about 144. Therefore, in mass%, P which is about 0.215 times the Nd concentration binds to Nd as NdP. That is, [Pfree] = [Psurf] −0.215Nd. FIG. 1 was obtained by the following method.

  It has a chemical composition corresponding to SCM435 standardized in JIS G4053, and the P content is changed in the range of 0.009 to 0.040 mass% and the Nd content is changed in the range of 0.002 to 0.040 mass%. Nine kinds of test materials were prepared. Table 1 shows the chemical composition of each test material.

  Each test material hot forged to a diameter of 16 mm was subjected to quenching and tempering treatment, and a test piece with an annular notch shown in FIG. 2 was collected from each test material. The quenching process was performed at 850 ° C., and the tempering process was performed at 400 ° C. × 1 hour. The test piece had a diameter D1 of 8 mm, a V-shaped notch was formed in the center of the test piece, and the test piece diameter D2 at the notch was 6 mm. The radius of curvature of the notch bottom was 0.1 mm, and the notch angle α was 60 °. The average P concentration in the range (surface layer) from the surface of the test piece to the depth of 10 μm corresponds to the P content of each test material.

Hydrogen was introduced into each test piece using a cathodic hydrogen charging method. Specifically, the test piece was immersed in dilute sulfuric acid having a pH of 3.0, and hydrogen was charged to the test piece under cathodic electrolysis conditions with a current density of 1.0 mA / cm ² . Thereafter, a constant load tensile test was performed to determine the maximum load stress MS (MPa) that did not break.

  Further, a round bar tensile test piece having a diameter of 6 mm was collected from each specimen subjected to quenching and tempering treatment. Using a tensile test piece, a tensile test based on JIS Z 2241 was carried out at room temperature (25 ° C.) and in the atmosphere to obtain a tensile strength TS (MPa).

Using the maximum load stress MS and the tensile strength TS, the hydrogen embrittlement strength ratio W was determined by the following equation.
W = MS / TS

  FIG. 1 was created based on the obtained hydrogen embrittlement strength ratio W.

  Referring to FIG. 1, among a plurality of test pieces having a Pfree concentration [Pfree] of 0.002 to 0.037% by mass on the surface layer, the hydrogen embrittlement strength ratio is up to 0.015% by mass or less. W was high. On the other hand, when [Pfree] exceeds 0.015% by mass, the hydrogen embrittlement resistance is significantly reduced as compared with the case where [Pfree] is 0.015% by mass or less. That is, the hydrogen embrittlement strength ratio W with respect to [Pfree] had an inflection point when [Pfree] was 0.015 mass%. Therefore, if [Pfree] is 0.015% or less, excellent hydrogen embrittlement resistance is exhibited.

  Moreover, with reference to FIG. 1, when [Pfree] was 0.015 mass% or less, more excellent hydrogen embrittlement resistance was exhibited as [Pfree] decreased.

  Most of the base points for hydrogen embrittlement of high-strength bolts are the surface of the incomplete threaded part of the bolt or the surface of the threaded bottom part. In the range from the surface of the high-strength bolt to a depth of 10 μm, if the concentration of P (Pfree) not bonded to Nd is 0.015 mass% or less, excellent hydrogen embrittlement resistance is exhibited.

  During the bolt manufacturing process, in the cold forging process, a lubricating film is formed on the surface of the bolt steel before cold forging. Usually, a phosphate film is used as the lubricating film. If quenching and tempering are performed on the steel material on which the phosphate film is formed after cold forging, P in the phosphate film penetrates from the surface of the steel material to the inside during the heat treatment. Therefore, the average P concentration of the surface layer of the high-strength bolt is increased.

  For example, a non-phosphorous lubricating film containing no P is used, or after using a phosphate film, the surface of the steel material is washed or pickled before the quenching process to remove the phosphate film from the surface. . In this case, it can suppress that the average P density | concentration of the surface layer of a high intensity | strength bolt becomes high.

The high-strength bolt according to the present invention completed on the basis of the above findings is mass%, C: 0.24 to 0.40%, Si: 0.01 to 0.50%, Mn: 0.5 to 2. 0%, P: 0.020% or less, S: 0.020% or less, Cr: 0.2 to 0.7%, Al: 0.005 to 0.050%, B: 0.001 to 0.005 %, Ti: 0.007 to 0.100%, N: 0.002 to 0.008%, Nd: 0.001 to 0.100%, Nb: 0 to 0.050%, Mo: 0 to 0.00. It contains 30% and V: 0 to 0.30% and satisfies the formula (1).
[Psurf] −0.215Nd ≦ 0.015 (1)
Here, an average P concentration (mass) in a range from the surface to a depth of 10 μm is substituted for [Psurf]. Nd content (mass%) is substituted for Nd.

  The chemical composition of the high-strength bolt may include one or more selected from the group consisting of Nb: 0.001 to 0.050 mass% and V: 0.01 to 0.30 mass%. . Furthermore, the chemical composition of the high-strength bolt may include Mo: 0.01 to 0.30 mass%.

  The steel for high-strength bolts according to the present invention has the chemical composition described above.

  Hereinafter, the high strength bolt and the steel for high strength bolt according to the present invention will be described in detail. Note that “%” relating to an element means mass% unless otherwise specified.

[Chemical composition]
The chemical composition of the high-strength bolt of this embodiment contains the following elements.

C: 0.24-0.40%
Carbon (C) has a lower alloy cost than other alloy elements, and increases the strength and hardenability of the steel. If the C content is 0.24% or more, a yield strength of 1000 MPa or more is obtained. On the other hand, if the C content is too high, the hydrogen embrittlement resistance of the steel decreases. Therefore, the C content is 0.24 to 0.40%. In order to obtain high strength and hardenability, the preferable lower limit of the C content is 0.27%. In order to obtain high hydrogen embrittlement resistance, the preferable upper limit of the C content is 0.37%.

Si: 0.01 to 0.50%
Silicon (Si) increases the strength and hardenability of the steel. Si further stabilizes the Fe-based carbide ε and increases the temper softening resistance. Since temper softening resistance can be increased by Si, high-temperature tempering can be performed to improve the hydrogen embrittlement resistance of the steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the steel surface decarburizes. Therefore, the Si content is 0.01 to 0.50%. The upper limit with preferable Si content is 0.15%.

Mn: 0.5 to 2.0%
Manganese (Mn) increases the strength and hardenability of the steel. Mn further fixes S in steel as MnS. Thereby, it can suppress that FeS produces | generates in a crystal grain boundary. If the formation of FeS is suppressed, red hot brittleness is suppressed and hot ductility increases. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the hydrogen embrittlement resistance of the steel decreases. Therefore, the Mn content is 0.5 to 2.0%. The upper limit with preferable Mn content is 1.0%.

P: 0.020% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and lowers the toughness and hydrogen embrittlement resistance of the steel. Therefore, the P content is 0.020% or less. The upper limit with preferable P content is 0.015%, More preferably, it is 0.010%. The P content is preferably as low as possible, but considering the production cost, the preferable lower limit of the P content is 0.003%.

S: 0.020% or less Sulfur (S) is an impurity. S combines with Fe to form FeS at the grain boundaries, thereby reducing the hot ductility of the steel. Therefore, the S content is 0.020% or less. The upper limit with preferable S content is 0.015%, More preferably, it is 0.010%. The S content is preferably as low as possible, but considering the production cost, the preferable lower limit of the S content is 0.003%.

Cr: 0.2-0.7%
Chromium (Cr) increases the strength, hardenability, and temper softening resistance of the steel. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, grain boundary segregation of P is promoted. Therefore, the Cr content is 0.2 to 0.7%.

Al: 0.005 to 0.050%
Aluminum (Al) produces fine precipitates (Al ₂ O ₃ and AlN) and suppresses coarsening of the prior austenite grain size. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, this effect is saturated. Therefore, the Al content is 0.005 to 0.050%. In the present embodiment, the Al content is the total Al content.

B: 0.001 to 0.005%
Boron (B) increases the hardenability of the steel. If the B content is too low, this effect cannot be obtained. On the other hand, if the B content is too high, this effect is saturated. Therefore, the B content is 0.001 to 0.005%. The upper limit with preferable B content is 0.003%.

Ti: 0.007 to 0.100%
Titanium (Ti) fixes N in steel as TiN and suppresses the generation of BN. Therefore, when B is contained, the effect of improving the hardenability of B is sufficiently exhibited by containing Ti. Ti further generates fine precipitates (Ti (CN) and TiC) to suppress coarsening of the prior austenite grain size. If the Ti content is too low, these effects cannot be obtained. On the other hand, if the Ti content is too high, these effects are saturated. Therefore, the Ti content is 0.007 to 0.100%. In order to fix N in steel, it is preferable to add a Ti amount that is 3.4% or more of the N amount by mass%.

N: 0.002 to 0.008%
Nitrogen (N) reacts with Al to react with AlN and Ti to form fine precipitates such as Ti (CN). Fine precipitates suppress coarsening of the prior austenite grain size. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, N that could not react with the metal elements forming nitrides and carbonitrides reacts with B to become BN, and the effect of improving the hardenability of B is greatly impaired. Therefore, the N content is N: 0.002 to 0.008%. The upper limit with preferable N content is 0.005%.

Nd: 0.001 to 0.100%
Neodymium (Nd) combines with P to form NdP. Generation of NdP suppresses the grain boundary segregation of P and suppresses the deterioration of the toughness and hydrogen embrittlement resistance of the steel. Since NdP serves as a diffusible hydrogen trap site, the hydrogen embrittlement resistance of the steel is further enhanced. Nd further refines MnS. Since fine MnS serves as a hydrogen trap site, the concentration of diffusible hydrogen in the steel decreases and the hydrogen embrittlement resistance increases. If the Nd content is too low, these effects cannot be obtained. On the other hand, if the Nd content is too high, these effects are saturated. Therefore, the Nd content is 0.001 to 0.100%. A preferable lower limit of the Nd content is 0.005%. The upper limit with preferable Nd content is 0.050%.

  The balance of the chemical composition of the high-strength bolt according to the present embodiment is composed of Fe and impurities. Here, the impurities are mixed from ore as a raw material, scrap, or production environment when industrially manufacturing high-strength bolts, and have an adverse effect on the high-strength bolts of this embodiment. It means what is allowed in the range. For example, when Nd is added with misch metal, other rare earth elements (Ce, La, etc.) contained in the misch metal are contained as impurities in the steel, but these elements are resistant to hydrogen embrittlement and tensile strength. Has no effect.

[Arbitrary elements]
The high-strength bolt described above may further contain one or more selected from the group consisting of Nb and V in place of part of Fe. All of these elements are optional elements, and refine the crystal grains to improve the hydrogen embrittlement resistance of the steel.

Nb: 0 to 0.050%
Niobium (Nb) is an optional element and may not be contained. When contained, Nb produces fine carbides, nitrides, and carbonitrides and refines crystal grains (former austenite grains). The refinement of crystal grains increases the hydrogen embrittlement resistance of steel. Nb particularly produces stable (Nb, Ti) (CN) with Ti and refines the steel. If Nb is contained even a little, this effect can be obtained to some extent. However, if the Nb content is too high, this effect is saturated. Therefore, the Nb content is 0 to 0.050%. The minimum with preferable Nb content is 0.001%. The upper limit with preferable Nb content is 0.030%.

V: 0 to 0.30%
Vanadium (V) is an optional element and may not be contained. When contained, V forms fine carbides and nitrides and refines crystal grains (former austenite grains). This increases the hydrogen embrittlement resistance of the steel. If V is contained even a little, this effect can be obtained to some extent. However, this effect is saturated if the V content is too high. Therefore, the V content is 0 to 0.30%. A preferable lower limit of the V content is 0.01%.

  The high-strength bolt according to the present invention may further contain Mo instead of a part of Fe.

Mo: 0 to 0.30%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo increases the strength, hardenability, and temper softening resistance of the steel. If Mo is contained even a little, this effect can be obtained to some extent. However, if the Mo content is too high, the manufacturing cost increases. Therefore, the Mo content is 0 to 0.30%. A preferable lower limit of the Mo content is 0.01%. The upper limit with preferable Mo content is 0.15%.

  In addition, the steel for high strength bolts by this invention has the above-mentioned chemical composition.

[Pfree concentration of surface layer]
In the high-strength bolt according to the present invention, the Pfree concentration ([Pfree]) of the surface layer is 0.015% or less. As described above, when [Pfree] is 0.015% or less, the hydrogen embrittlement strength ratio W can be kept high, and excellent hydrogen embrittlement resistance can be obtained.

[Pfree] = [Psurf] −0.215 Nd
Here, an average P concentration in a range from the surface to a depth of 10 μm is substituted for [Psurf]. Nd content (mass%) is substituted for Nd.

  The average P concentration is obtained by the following method. A high strength bolt is cut perpendicular to the surface and a sample is taken. The surface to be observed is mirror polished. At the measurement point, the P concentration in the range from the surface to a depth of 10 μm (that is, the surface layer) is obtained with an electron probe microanalyzer (EPMA) at a pitch of 0.5 μm. The average of the obtained P concentrations is defined as the average P concentration (%).

  As described above, when the quenching and tempering treatment is performed with the phosphate soap film, which is a lubricant during cold forging, attached to the bolt surface, P contained in the film penetrates into the steel. The P concentration in the surface layer is increased by P that has entered the steel, and usually exceeds 0.1%. Therefore, in this case, it is difficult to trap all P as NdP with the Nd content (0.001 to 0.100%) defined in the present invention.

  The atomic weight of P is about 31, and the atomic weight of Nd is about 144. Accordingly, if Nd is contained in an amount of about 4.6 times the surface P concentration by mass%, P can be trapped by NdP and detoxified. However, this method greatly increases the manufacturing cost.

  Therefore, in order to improve the hydrogen embrittlement resistance by Nd, it is necessary to prevent P from entering the steel from the steel surface during quenching and tempering. The suppression method is, for example, using a non-phosphorous lubricant film or removing the phosphate film from the bolt surface before quenching and tempering.

[Production method]
An example of the manufacturing method of the high strength bolt by this invention is demonstrated. First, steel for high-strength bolts is manufactured by a known manufacturing method (raw material manufacturing process). Then, a high strength bolt is manufactured using the steel for high strength bolts (bolt manufacturing process). Hereinafter, each step will be described.

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

[Bolt manufacturing process]
In the bolt manufacturing process, high-strength 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. Hereinafter, each process will be described.

[Wire drawing process and cold forging process]
First, a steel wire is manufactured by drawing a wire. The wire drawing may be performed only by primary wire drawing, or may be performed a plurality of times such as secondary wire drawing. At the time of wire drawing, a lubricating film is formed on the surface of the wire. The lubricating film is, for example, a phosphate film or a non-phosphorous lubricating film.

  Preferably, a non-phosphorous lubricating film containing no P is used. Or when a phosphate membrane | film | coat is used, before the below-mentioned hardening process, the steel material (steel wire) surface is wash | cleaned or pickled, and a phosphate membrane | film | coat is removed from the surface. The cleaning is, for example, a well-known alkali cleaning. In this case, the Pfree concentration of the surface layer of the manufactured high-strength bolt is 0.015% or less.

[Cold forging process]
The drawn steel material is cut to a predetermined length, and the forged steel material is cold forged to produce a bolt. Even during cold forging, a lubricating film similar to that in the wire drawing step may be formed again on the surface of the steel material. Further, after forming the lubricating film in the wire drawing step, cold forging may be performed without forming the lubricating film again.

  In addition, you may implement softening heat processing in multiple times before a wire drawing process and a cold forging process for the purpose of softening of the steel material for bolts (wire) which is too strong. Moreover, the said wire drawing process may be abbreviate | omitted.

[Quenching and tempering process]
The bolt manufactured by cold forging is quenched and tempered under known conditions to adjust the tensile strength of the bolt to 1000 MPa or more. As described above, when a phosphate film is formed on the surface of the bolt before quenching and tempering, P penetrates from the bolt surface by quenching and tempering. Therefore, in the present embodiment, a non-phosphorous lubricating film is used as the lubricating film in the wire drawing process and the cold forging process, or the phosphate film is removed from the bolt surface before quenching and tempering. Thus, P does not enter the steel from the steel surface during quenching and tempering, and the Pfree concentration of the surface layer of the high-strength bolt after quenching and tempering is 0.015% or less.

  The high-strength bolt of this embodiment is manufactured by the above manufacturing process.

  Molten steel having the chemical composition shown in Table 2 was produced in a vacuum melting furnace.

  An ingot was produced by the ingot-making method using the molten steel. The ingot was hot forged to produce a round bar having a diameter of 16 mm (corresponding to the steel for high-strength bolts in the present invention). From the manufactured round bar, a hydrogen embrittlement test piece with an annular notch shown in FIG. 2 was collected by machining. The lubricant shown in Table 2 was applied to the surface of the collected hydrogen embrittlement test piece to form a lubricant film. Thereafter, quenching and tempering were performed on the hydrogen embrittlement test specimens and the round bars with the lubricating film attached. The quenching temperature was as shown in Table 2. Tempering was held at 400 ° C. for 1 hour in all test numbers. A round bar tensile test piece having a diameter of 6 mm was collected by machining from a round bar subjected to quenching and tempering. By the above manufacturing process, a hydrogen embrittlement test piece simulating a high-strength bolt and a round bar tensile test piece were manufactured.

[Hydrogen embrittlement resistance test]
Hydrogen was introduced into the hydrogen embrittlement resistant test piece of each test number using the cathode hydrogen charging method. Specifically, the test piece was immersed in dilute sulfuric acid having a pH of 3.0, and hydrogen was charged to the test piece under cathodic electrolysis conditions with a current density of 1.0 mA / cm ² . Thereafter, a constant load tensile test was performed to determine the maximum load stress MS (MPa) that did not break.

[Tensile test]
Furthermore, using a round bar tensile test piece having the same test number, a tensile test based on JIS Z 2241 was carried out at room temperature (25 ° C.) and in the atmosphere to obtain a tensile strength TS (MPa). Using the maximum load stress MS and the tensile strength TS, the hydrogen embrittlement strength ratio W was determined by the following equation.
W = MS / TS

[Surface P concentration measurement test]
The cross section of the notch of the test piece after the hydrogen embrittlement resistance test was polished. The average P concentration in the range from the surface of the notch bottom to a depth of 10 μm was measured using an electron probe microanalyzer (EPMA). Specifically, the P concentration was measured at a pitch of 0.5 μm, and the average value was defined as the average P concentration (mass%).

[Test results]
Table 2 shows the test results of the surface average P concentration, surface Pfree concentration, tensile strength (TS), and hydrogen embrittlement strength ratio (W).

  The chemical compositions of test numbers 1 to 7 were appropriate, and the manufacturing conditions were also appropriate. Therefore, the Pfree concentration in the surface layer of each test number was 0.015% or less. As a result, the tensile strength TS was 1000 MPa or more, the hydrogen embrittlement strength ratio was 0.7 or more, and excellent strength and hydrogen embrittlement resistance were exhibited.

  On the other hand, the C content and Mn content of Test No. 8 were too low. As a result, the tensile strength was less than 1000 MPa and the strength was low.

  The C content of test number 9 was too high. As a result, the hydrogen embrittlement strength ratio W was low.

  The Mn content of test number 10 was too high. As a result, the hydrogen embrittlement strength ratio W was low.

  The P content of test number 11 was too high. As a result, the Pfree concentration in the surface layer was too high, and the hydrogen embrittlement strength ratio was low.

  The N content of test number 12 was too high. As a result, the tensile strength was less than 1000 MPa. This is probably because BN was formed and the hardenability was low.

  In test number 13, Nd was not contained. Therefore, the Pfree concentration in the surface layer was too high, and the hydrogen embrittlement strength ratio was low.

  In Test No. 14, although the chemical composition is appropriate, a phosphate soap film is used as a lubricant during cold forging, and the quenching and tempering treatment is performed with the phosphate soap film adhered to the surface of the test material. Carried out. Therefore, the Pfree concentration in the surface layer was too high, and the hydrogen embrittlement strength ratio was low.

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

Claims (6)

% By mass
C: 0.24 to 0.40%,
Si: 0.01 to 0.50%,
Mn: 0.5 to 2.0%
P: 0.020% or less,
S: 0.020% or less,
Cr: 0.2 to 0.7%
Al: 0.005 to 0.050%,
B: 0.001 to 0.005%,
Ti: 0.007 to 0.100%,
N: 0.002 to 0.008%,
Nd: 0.001 to 0.100%,
Nb: 0 to 0.050%,
Mo: 0 to 0.30%, and
V: A high-strength bolt containing 0 to 0.30%, the balance having a chemical composition of Fe and impurities, and satisfying the formula (1).
[Psurf] −0.215Nd ≦ 0.015 (1)
Here, an average P concentration (mass%) in a range from the surface to a depth of 10 μm is substituted into [Psurf]. Nd content (mass%) is substituted for Nd. The high-strength bolt according to claim 1, wherein the mass% is
Nb: 0.001 to 0.050% and
V: A high-strength bolt containing one or more selected from the group consisting of 0.01 to 0.30%. The high-strength bolt according to claim 1 or 2, in mass%,
Mo: A high-strength bolt containing 0.01 to 0.30%. % By mass
C: 0.24 to 0.40%,
Si: 0.01 to 0.50%,
Mn: 0.5 to 2.0%
P: 0.020% or less,
S: 0.020% or less,
Cr: 0.2 to 0.7%
Al: 0.005 to 0.050%,
B: 0.001 to 0.005%,
Ti: 0.007 to 0.100%,
N: 0.002 to 0.008%,
Nd: 0.001 to 0.100%,
Nb: 0 to 0.050%,
Mo: 0 to 0.30%, and
V: Steel for high-strength bolts having a chemical composition containing 0 to 0.30% and the balance being Fe and impurities. It is steel for high strength bolts according to claim 4, Comprising:
Nb: 0.001 to 0.050% and
V: Steel for high-strength bolts containing at least one selected from the group consisting of 0.01 to 0.30%. The high-strength bolt steel according to claim 4 or 5, wherein the steel is for mass%.
Mo: Steel for high-strength bolts containing 0.01 to 0.30%.

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