Steel for high-strength bolts and method for producing the same

Abstract

To provide a steel for a high strength bolt excellent in delayed fracture strength, while having tensile strength over 1,300 MPa; and to provide a production method thereof.SOLUTION: A steel for a high strength bolt contains, in terms of mass%, C:0.25-0.55%, Cr:0.8-1.5%, V:0.05-0.50%, Mo:0.50-2.20%, Al:0.01-0.06%, N:0.005-0.030%, and has a component composition suppressed as Si:0.15% or less, Mn:0.4% or less, P:0.015% or less, S:0.010% or less, and V carbide or the like having a particle size of 10 nm or less is deposited in a parent phase so that hydrogen trap energy becomes 53 kJ / mol or more, and that a local limit hydrogen concentration becomes 1.5 ppm or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a steel for high-strength bolts with a tensile strength exceeding 1300 MPa and a method for manufacturing the same, and particularly relates to a steel for high-strength bolts with excellent delayed fracture strength and a method for manufacturing the same.

Background Art

[0002] High-strength bolts (high-strength bolts) with a tensile strength exceeding 800 MPa are used in the civil engineering and construction fields. In recent years, along with the demand for further higher strength, so-called "delayed fracture", in which fracture rapidly progresses after a certain period of time has elapsed since a load is applied to the bolt, has become a problem. For example, when diffusible hydrogen penetrates due to corrosion in the use environment, in particular, the fracture strength at stress concentration parts such as the thread valleys and the lower part of the bolt head neck decreases, and fracture starts and progresses all at once due to insufficient resistance to the applied load.

[0003] Regarding delayed fracture, it has been proposed to disperse oxides, carbides, nitrides, etc. (hereinafter referred to as carbides, etc.) in steel to trap and fix diffusible hydrogen, and increase the critical amount of hydrogen (critical diffusible hydrogen concentration) that causes delayed fracture. For example, steel containing elements that form carbides, etc. such as V and Mo is used, quenched and tempered, and carbides, etc. of these elements are finely dispersed and precipitated in the matrix.

[0004] Incidentally, although fracture is probabilistically evaluated by statistical processing, probabilistic evaluations can also be performed on the amount of hydrogen ingress and the fracture frequency for delayed fracture, and the critical diffusible hydrogen concentration can be determined. Further, in steel grades where delayed fracture becomes prominent, it has also been proposed to evaluate the resistance to delayed fracture by the local critical hydrogen concentration that focuses on the hydrogen concentration at sites that can be fracture initiation points, instead of the critical diffusible hydrogen concentration that is the average hydrogen amount of the entire steel material. As measurement methods for such local critical hydrogen concentration, a constant load test (CLT), a slow strain rate test (SSRT), a conventional strain rate test (CSRT), a four-point bending method, etc. have been proposed (see Non-Patent Documents 1 and 2).

[0005] For example, in Patent Document 1, for medium carbon steel containing 0.55 to 0.80% C by mass and containing V or Mo that forms carbides or the like, quenched and tempered to adjust the tensile strength to 1400 MPa or more, a high-strength bolt excellent in resistance to delayed fracture with a local critical hydrogen concentration measured by the CSRT method of 1.5 ppm or more is disclosed. Here, after heating to 900 °C or higher, quenching is performed, and tempering is performed at a temperature of 550 °C or higher to disperse and precipitate carbides or the like that become hydrogen trap sites.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] As described above, in order to obtain a trap site composed of carbides or the like that trap diffusible hydrogen in steel, after quenching the steel, fine precipitates composed of carbides of V, Mo, Ti, and / or Nb that are secondarily precipitated by tempering at a relatively high temperature near 600°C are considered effective. On the other hand, the details such as the difference in hydrogen trapping ability for each fine precipitate due to these carbide-forming elements are not clear, and the delayed fracture strength will differ depending on the precipitation form of the fine precipitate. Therefore, there has been a demand for high-strength bolt steel that stably has excellent delayed fracture strength.

[0009] The present invention has been made in view of the above circumstances, and its object is to provide high-strength bolt steel having a tensile strength exceeding 1300 MPa and excellent delayed fracture strength, and a method for manufacturing the same.

Means for Solving the Problems

[0010] The inventor of the present invention focused on the fact that the hydrogen trapping ability in steel can be evaluated by hydrogen trap energy (Ea) and trapped hydrogen amount (Ht), and intensively studied methods for increasing Ea and Ht. As a result, among the precipitates formed by V, Mo, Ti, and Nb, which are carbide-forming elements, it was found that V and Mo can make Ea higher than the other two elements. Furthermore, there are peak tempering conditions that can maximize Ht depending on the tempering temperature and holding time, and it was found that V contributes less to the decrease in Ht when deviating from such peak tempering conditions than Mo. That is, V can stably obtain hydrogen trapping ability with respect to changes in tempering conditions compared to Mo.

[0011] That is, the steel for high-strength bolts of the present invention is a steel for high-strength bolts made of tempered martensite steel with a tensile strength of 1300 MPa or more. By mass%, it contains C: 0.25 to 0.55%, Cr: 0.8 to 1.5%, V: 0.05 to 0.50%, Mo: 0.50 to 2.20%, Al: 0.01 to 0.06%, N: 0.005 to 0.030%, and suppresses Si: 0.15% or less, Mn: 0.4% or less, P: 0.015% or less, S: 0.010% or less. The balance is Fe and inevitable impurities, and has a carbide containing V carbide with a particle size of 10 nm or less precipitated in the matrix phase so that the hydrogen trap energy is 53 kJ / mol or more and the local critical hydrogen concentration measured by the CSRT method is 1.5 ppm or more.

[0012] According to such a feature, while suppressing the C content, the formation of V carbide is promoted and the tensile strength is adjusted to 1300 MPa or more, so that a bolt steel excellent in delayed fracture strength can be obtained.

[0013] Also, the steel for high-strength bolts of the present invention is a steel for high-strength bolts made of tempered martensite steel with a tensile strength of 1300 MPa or more. By mass%, it contains C: 0.25 to 0.55%, Cr: 0.8 to 1.5%, V: 0.05 to 0.50%, Al: 0.01 to 0.06%, N: 0.005 to 0.030%, and suppresses Si: 0.15% or less, Mn: 0.4% or less, P: 0.015% or less, S: 0.010% or less. The balance is Fe and inevitable impurities, and has a carbide containing V carbide with a particle size of 10 nm or less precipitated in the matrix phase so that the hydrogen trap energy is 53 kJ / mol or more and the local critical hydrogen concentration measured by the CSRT method is 1.5 ppm or more.

[0014] According to such a feature, a bolt steel with more stable and excellent delayed fracture strength can be obtained.

[0015] In the above invention, it may be characterized in that it may contain Nb: 0.2% or less and Ti: 0.3% or less. According to such a feature, a bolt steel with more excellent delayed fracture strength can be obtained.

[0016] Moreover, the method for manufacturing high-strength bolt steel according to the present invention has a component composition consisting of, by mass%, C: 0.25 to 0.55%, Cr: 0.8 to 1.5%, V: 0.05 to 0.50%, Mo: 0.50 to 2.20%, Al: 0.01 to 0.06%, N: 0.005 to 0.030%, while suppressing Si: 0.15% or less, Mn: 0.4% or less, P: 0.015% or less, S: 0.010% or less, with the balance being Fe and unavoidable impurities, and is a method for manufacturing high-strength bolt steel made of tempered martensite steel having a tensile strength of 1300 MPa or more. After heating the steel ingot to 900°C or higher and quenching it, tempering is performed at a temperature of 550°C or higher and 700°C or lower for a predetermined time so that the hydrogen trap energy is 53 kJ / mol or more and the local critical hydrogen concentration measured by the CSRT method is 1.5 ppm or more, and carbides containing V carbides with a particle size of 10 nm or less are precipitated in the matrix phase.

[0017] According to such a feature, while suppressing the C content, the formation of V carbides is promoted and the tensile strength is adjusted to 1300 MPa or more, so that bolt steel excellent in delayed fracture strength can be obtained.

[0018] Moreover, the method for manufacturing high-strength bolt steel according to the present invention has a component composition consisting of, by mass%, C: 0.25 to 0.55%, Cr: 0.8 to 1.5%, V: 0.05 to 0.50%, Al: 0.01 to 0.06%, N: 0.005 to 0.030%, while suppressing Si: 0.15% or less, Mn: 0.4% or less, P: 0.015% or less, S: 0.010% or less, with the balance being Fe and unavoidable impurities, and is a method for manufacturing high-strength bolt steel made of tempered martensite steel having a tensile strength of 1300 MPa or more. After heating the steel ingot to 900°C or higher and quenching it, tempering is performed at a temperature of 550°C or higher and 700°C or lower for a predetermined time so that the hydrogen trap energy is 53 kJ / mol or more and the local critical hydrogen concentration measured by the CSRT method is 1.5 ppm or more, and carbides containing V carbides with a particle size of 10 nm or less are precipitated in the matrix phase.

[0019] According to such characteristics, it is possible to more stably obtain a bolt steel excellent in delayed fracture strength.

[0020] In the above-described invention, it may be characterized in that it may contain Nb: 0.2% or less and Ti: 0.3% or less. According to such characteristics, it is possible to obtain a bolt steel more excellent in delayed fracture strength.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0022] A high-strength bolt steel and a manufacturing method thereof, which are one embodiment of the present invention, will be described in detail with reference to FIG. 1.

[0023] In this embodiment, among the steels shown in FIG. 1, steels having a series of component compositions typified by steel types 1 to 5 are used. This component composition is designed such that, after obtaining a martensitic steel having a tensile strength of 1300 MPa or more by quenching and tempering, among the carbides of V, Mo, Ti, and Nb, which are effective as hydrogen trap sites, the carbide of V is preferentially and finely precipitated. Further, among these, for steel types 2 to 5, the carbide of Mo is also finely precipitated in combination, and the component composition is considered to obtain a composite effect with the carbide of V.

[0024] More specifically, this steel contains, by mass%, C: 0.25 to 0.55%, Cr: 0.8 to 1.5%, V: 0.05 to 0.50%, Al: 0.01 to 0.06%, N: 0.005 to 0.030%, and has a component composition in which Si: 0.15% or less, Mn: 0.4% or less, P: 0.015% or less, and S: 0.010% or less are suppressed. Here, as the component composition of such high-strength steel, Mo: 0.50 to 2.20% may be further included. Also, Nb: 0.2% or less and Ti: 0.3% or less may be further included. Note that steels 6 to 8 all have no V added and have a component composition different from the above, and were used as comparative examples in the manufacturing tests described later. Note that Ni is inevitably contained because it is included in the raw materials, but it is preferably less.

[0025] And the manufacturing method of the high-strength bolt steel in this example is as follows. First, the steel having the above-described component composition is melted by vacuum melting to produce an ingot. The obtained ingot is forged and shaped, and after appropriate rough machining, it is heated to 900°C or higher for quenching, tempered at a temperature of 550°C or higher and 700°C or lower for a predetermined time to precipitate carbides containing V carbides with a particle size of 10 nm or less in the matrix phase, and a bolt material having a tempered martensite structure with a tensile strength of 1300 MPa or higher is obtained. Then, it is processed into a bolt shape by machining or the like.

[0026] In particular, regarding the quenching temperature, by setting it to 900°C or higher, the carbides existing before quenching are dissolved to increase the C concentration in the matrix phase, ensuring the tensile strength after tempering. Regarding the tempering temperature, by setting it to 550°C or higher, secondary precipitation of carbides occurs, and by setting it to 700°C or lower, the hardness after tempering is ensured. Then, while setting the above-described component composition and adjusting the quenching temperature and tempering temperature within these ranges, by setting the tempering time to a predetermined time, carbides containing V carbides with a particle size of 10 nm or less are precipitated in the matrix phase. As a result, a tensile strength of 1300 MPa or higher can be achieved, and the hydrogen trap energy can be set to 53 kJ / mol or higher, and the local critical hydrogen concentration measured by the normal rate method (CSRT method) can be set to 1.5 ppm or higher. Consequently, the resistance to delayed fracture can be enhanced.

[0027] In delayed fracture, hydrogen diffuses and migrates to the fracture initiation point due to external stress, causing hydrogen embrittlement fracture. However, by setting the hydrogen trap energy (Ea) to 53 kJ / mol, such delayed fracture can be effectively suppressed.

[0028] As the hydrogen trap sites, mainly fine carbides that precipitate secondary during high-temperature tempering around 600°C are used. Therefore, the tempering temperature is set to a temperature of 550°C or higher and 700°C or lower as described above. In particular, in the case of V and Mo, since the hydrogen trap energy (Ea) can be made higher than that of Ti and Nb, and the contribution of V is smaller than that of Mo regarding the decrease when the peak tempering conditions for the trapped hydrogen amount (Ht) deviate, mainly V carbides are used.

[0029] Note that FIG. 2 illustrates TEM observation images of the secondary-precipitated fine carbides ((a) VC, (b) MoC). It can be confirmed that both are cubic crystals and are elongated in the

[0100] α direction on the (001)α plane.

[0030] Carbides and the like as hydrogen trap sites can be evaluated by hydrogen trap energy (Ea) and trapped hydrogen amount (Ht). Hydrogen trap energy (Ea) is an index indicating how strongly hydrogen is trapped, that is, it quantitatively indicates the trap strength and depends on the microstructure of the trap site such as the coherency strain between the precipitate and the matrix. Also, the trapped hydrogen amount (Ht) is an index indicating how much hydrogen is trapped and depends on the number density of trap sites such as precipitates. That is, the larger the values of hydrogen trap energy (Ea) and trapped hydrogen amount (Ht), the higher the hydrogen trapping ability and the higher the resistance to delayed fracture can be achieved.

[0031] Hydrogen trap energy (Ea) is the sum of the binding energy (Eb) of the trap site and the activation energy (Em) of the movement of dissolved hydrogen. Hydrogen trap energy (Ea) can be calculated by the following (Equation 1) (Choo-Lee's equation, presentation: W.Y. Choo and J.Y. Lee: Metall. Trans. A, 13A (1982), 135.) when assuming a thermal deviation ignoring hydrogen diffusion in the temperature-programmed desorption hydrogen analysis. ∂ln(φ / Tp 2 ) / ∂(1 / Tp)=-Ea / R ···(Equation 1) Here, Tp is the peak temperature (K), φ is the heating rate (K / s), and R is the gas constant (8.31 J / mol·K). The method of temperature-programmed desorption hydrogen analysis will be described in detail in the manufacturing test.

[0032] Also, the amount of trapped hydrogen (Ht) depends on the number density of trap sites as described above. For example, by setting the tempering temperature to 550°C or lower, the hydrogen trap energy (Ea) can be increased. However, the precipitation of carbides and the like that serve as trap sites is reduced, resulting in a lower number density of trap sites and a decrease in the amount of trapped hydrogen (Ht). In addition, cementite with a particle size on the order of μm or undissolved carbides present during quenching also have a high hydrogen trap energy (Ea), but the number density as hydrogen trap sites is small, and a sufficient amount of trapped hydrogen (Ht) cannot be ensured. Therefore, in order to maintain a high amount of trapped hydrogen (Ht), a high number density is ensured as hydrogen trap sites by finely precipitated carbides and the like secondary-precipitated as described above.

[0033] Based on the above, the hydrogen trap energy was measured for various steel grades that produce carbides of V, Mo, Ti, and Nb, and steels manufactured under various quenching and tempering conditions. It was found that the hydrogen trap energy (Ea) can be set to 53 kJ / mol or more in order to obtain excellent delayed fracture resistance. In the process, it was found that the hydrogen trap energy (Ea) of carbides by V and Mo is greater than that of carbides by Ti and Nb. Also, the amount of trapped hydrogen (Ht) is changed according to the tempering heating temperature and holding time conditions. It was found that the decrease in the amount of trapped hydrogen (Ht) when the tempering conditions deviate from the peak tempering conditions that maximize this is less for V than for Mo.

[0034] Based on these, the manufacturing method as described above was found, and high-strength bolt steel with a tensile strength exceeding 1300 MPa and excellent delayed fracture strength was obtained. As described above, carbides by V can more stably ensure the amount of trapped hydrogen (Ht) against changes in tempering conditions than Mo. Therefore, when both V and Mo are contained, it is possible to obtain high-strength bolt steel with excellent delayed fracture strength by adjusting the tempering conditions. On the other hand, when hydrogen trap sites are formed only by carbides and the like by V without containing Mo, the adjustment of tempering conditions can be made easier, and high-strength bolt steel with excellent delayed fracture strength can be obtained more stably.

[0035] [Manufacturing Test] Next, the manufacturing tests conducted using each of the above steel grades will be described.

[0036] First, steels having the component compositions of each of Steel Grades 1 to 8 shown in FIG. 1 were melted in a vacuum melting furnace to obtain 50 kg ingots, which were then formed into bars with a diameter of 32 mm by hot forging. Next, as a normalizing treatment, it was held at 920 °C for 2 hours and then air-cooled, and as a spheroidizing annealing treatment, it was held at 760 °C for 3 hours and then cooled to 650 °C at a cooling rate of -15 °C / hour and then air-cooled to obtain test piece materials.

[0037] Next, after rough machining this test piece material, it was heat-treated under the quenching and tempering conditions shown in FIG. 3 and processed into the shape of a JIS No. 4 smooth tensile test piece with a diameter of 6 mm, and the tensile strength was measured.

[0038] Also, a test piece rough-machined from the test piece material into a shape of 12 mm × 12 mm × 30 mm was heat-treated under the same quenching and tempering conditions, and then a hydrogen analysis test piece of 10 mm × 10 mm × 2 mm was cut out. After hydrogen was introduced into the hydrogen analysis test piece by performing cathodic charging at a current density of 0.05 mA / cm 2 for 24 hours, temperature-programmed desorption hydrogen analysis was performed using a quadrupole mass spectrometer. The heating rate was kept constant, but it was selected from several levels between 100 °C / h and 1800 °C / h respectively.

[0039] As shown in FIG. 4, from the values of the hydrogen desorption curves of each obtained test piece, by subtracting the values of the hydrogen desorption curves at the same heating rate of a steel having the same component composition except not containing V, Mo, Ti, and Nb, a hydrogen desorption curve from hydrogen trap sites due to precipitates such as carbides that become hydrogen trap sites was obtained. From the relationship between the peak temperature (Tp) of the hydrogen desorption curve due to hydrogen trapping by this precipitate and the heating rate (φ), the hydrogen trap energy (Ea) was determined using (Equation 1).

[0040] Furthermore, an annular notch specimen as shown in Fig. 5 was fabricated. That is, the above-described specimen material was rough-machined and heat-treated under the same quenching and tempering conditions, and then machined to obtain an annular notch specimen. The notch radius was set to 0.25 mm. The local critical hydrogen concentration was measured by the CSRT method using this annular notch specimen.

[0041] Specifically, hydrogen was introduced by performing cathode charging at a predetermined current density for 5 days, and immediately thereafter, a tensile test was conducted at a crosshead speed of 1 mm / min to obtain the fracture stress. A portion with a length of 10 mm was cut from the fracture surface of the specimen immediately after the tensile test, and the hydrogen amount was measured using a gas chromatograph. In this measurement of the hydrogen amount, the hydrogen release amount was measured by the temperature-programmed desorption method up to 600°C while raising the temperature at a rate of 100°C / hour, and the hydrogen amount measured by the release up to 300°C was defined as the diffusible hydrogen amount.

[0042] The current density of the cathode charging was changed at multiple levels, and the above operations were similarly performed until the measurement of the diffusible hydrogen amount. After taking the double logarithm of the obtained fracture stress and diffusible hydrogen amount, the relationship between the two was linearly approximated. Furthermore, the diffusible hydrogen amount corresponding to the fracture stress that is 0.6 times the fracture stress when no cathode charging was performed was obtained and defined as the local critical hydrogen concentration.

[0043] As shown in Fig. 3, in Examples 1 to 6, the tensile strength was 1300 MPa or more, the hydrogen trap energy (Ea) was 53 kJ / mol or more, and the local critical hydrogen concentration measured by the CSRT method was 1.5 ppm or more. That is, a high tensile strength of 1300 MPa or more and excellent resistance to delayed fracture were achieved simultaneously.

[0044] Furthermore, in Example 5, the tempering time was set to 5 h, which was longer than in other examples, and in Example 6, the tempering temperature was set to 640 °C, which was higher than in other examples. As a result, the hydrogen trap energy became as high as 60 kJ / mol or more, and at the same time, the local critical hydrogen concentration also became higher than in other cases. It is presumed that this is because, in addition to V, Mo was added in combination, and by setting the tempering conditions to higher temperature and longer time, another hydrogen trap site with a high hydrogen trap energy (Ea) was generated. As tempering conditions under which such an effect can be obtained, for example, the tempering time can be 3 h or more, or the tempering temperature can be 630 °C or more.

[0045] On the other hand, in Comparative Examples 1 to 4, although the tensile strength could be made 1300 MPa or more, the local critical hydrogen concentration was less than 1.5 ppm, resulting in inferior delayed fracture resistance. It is considered that Steel Grades 6 to 8 used in Comparative Examples 1 to 3 did not contain V at all, or the hydrogen trap sites were not generated or were insufficient. In Comparative Example 4, although Steel Grade 1 same as that in Example 1 was used, since the tempering temperature was set as low as 530 °C, it is considered that the hydrogen trap sites were not sufficiently generated.

[0046] Incidentally, the composition range of steel that can provide mechanical strength and delayed fracture resistance almost equivalent to those of the high-strength bolt steel according to Examples 1 to 6 described above is defined as follows.

[0047] First, the essential additive elements will be described.

[0048] C is an element effective for ensuring the mechanical strength of the steel after quenching and tempering. On the other hand, if it is contained in excess, it will reduce the delayed fracture resistance and also reduce the manufacturability such as forging property and rolling property during the molding of the bolt material. Considering these, C is in the range of 0.25 to 0.55% by mass.

[0049] Cr is an element effective for enhancing hardenability to obtain a martensite structure and increasing mechanical strength. On the other hand, if contained in excess, it may reduce the workability as a high-strength bolt and lower the stress corrosion cracking resistance due to an increase in grain boundary oxidation. Considering these factors, Cr is in the range of 0.80 to 1.50% by mass.

[0050] V forms fine carbides etc. and effectively acts as a hydrogen trap site to improve stress corrosion cracking resistance. On the other hand, if contained in excess, it may reduce hot workability and machinability and increase the cost of the steel material. Considering these factors, V is in the range of 0.05 to 0.50% by mass.

[0051] Al forms AlN together with N to refine crystal grains and improve toughness. Also, it is used as a deoxidizer during steel melting. On the other hand, if contained in excess, it may generate hard non-metallic inclusions and reduce the fatigue life. Considering these factors, Al is in the range of 0.010 to 0.060% by mass.

[0052] N forms AlN together with Al to refine crystal grains and improve toughness. On the other hand, if contained in excess, it may form non-metallic inclusions due to oxides and nitrides, serving as the starting point of fatigue fracture and reducing the fatigue life. Considering these factors, N is in the range of 0.005 to 0.030% by mass.

[0053] Next, the optional addition elements and the elements whose content is suppressed as impurities will be described.

[0054] Mo improves the hardenability of steel and suppresses the decrease in hardness due to tempering by dissolving in carbides. Furthermore, by precipitating fine carbides through tempering above 550°C and effectively acting as a hydrogen trap site, it can improve stress corrosion cracking resistance, so it may be optionally added. On the other hand, if contained in excess, it may reduce hot workability and machinability and increase the cost of the steel material. Considering these factors, when adding, Mo is in the range of 0.50 to 2.20% by mass.

[0055] Nb and Ti form fine carbides and the like, effectively act as hydrogen trap sites, and improve the hydrogen embrittlement resistance. Also, when contained in excess, Nb saturates its effect, and Ti forms non-metallic inclusions due to nitrides, which become the starting points of fatigue fracture and reduce the fatigue life. Furthermore, in this example, in order to preferentially generate carbides of V, the contents of Nb and Ti are suppressed. Considering these factors, in terms of mass%, Nb is within the range of 0.20% or less and Ti is within the range of 0.30% or less.

[0056] Si is used as a deoxidizer in steel melting, has the effect of improving the strength of steel by solid solution strengthening, and can be added arbitrarily. However, when contained in excess, it promotes grain boundary oxidation of steel, reduces the hydrogen embrittlement resistance, and further reduces the hot workability. Therefore, the content of Si is suppressed to 0.15% or less in terms of mass%.

[0057] Mn is used as a deoxidizer in steel melting and can be added arbitrarily to improve hardenability. However, when contained in excess, it reduces the manufacturability such as forging and turning. Considering these factors, the content of Mn is suppressed to 0.40% or less in terms of mass%.

[0058] P segregates at the austenite grain boundaries of steel, greatly reduces the grain boundary strength, and causes a decrease in toughness and hydrogen embrittlement resistance. Therefore, the content of P is suppressed to 0.015% or less in terms of mass%.

[0059] Although S has the effect of improving machinability, it reduces the hot workability of steel and forms non-metallic inclusions, reducing toughness and hydrogen embrittlement resistance. Therefore, the content of S is suppressed to 0.010% or less in terms of mass%.

[0060] So far, the representative examples according to the present invention and the modified examples based on these have been described. However, the present invention is not necessarily limited to these. Those skilled in the art will be able to find various alternative examples without departing from the scope of the appended claims.

Claims

1. A high-strength bolt steel made of tempered martensite steel with a tensile strength of 1300 MPa or more, by mass%, C: 0.25 to 0.55%, Cr: 0.80 to 1.50%, V: 0.05 to 0.40%, Mo: 0.50 to 2.20%, Al: 0.010 to 0.060%, including N: 0.005 to 0.030%, and Si: 0.15% or less, Mn: 0.40% or less, P: 0.015% or less, S: suppressed to 0.010% or less, having a component composition consisting of the balance Fe and inevitable impurities, characterized in that carbides containing V carbides with a particle size of 10 nm or less are precipitated in the matrix phase so that the hydrogen trap energy is 53 kJ / mol or more and the local limiting hydrogen concentration measured by the CSR method is 1.5 ppm or more. A high-strength bolt steel.

2. A high-strength bolt steel made of tempered martensite steel with a tensile strength of 1300 MPa or more, by mass%, C: 0.25 to 0.55%, Cr: 0.80 to 1.50%, V: 0.05 to 0.50%, Al: 0.010 to 0.060%, including N: 0.005 to 0.030%, and Si: 0.15% or less, Mn: 0.40% or less, P: 0.015% or less, S: suppressed to 0.010% or less, having a component composition consisting of the balance Fe and inevitable impurities, characterized in that carbides containing V carbides with a particle size of 10 nm or less are precipitated in the matrix phase so that the hydrogen trap energy is 53 kJ / mol or more and the local limiting hydrogen concentration measured by the CSR method is 1.5 ppm or more. A high-strength bolt steel.

3. by mass%, C: 0.25 to 0.55%, Cr: 0.80 to 1.50%, V: 0.05 to 0.40%, Mo: 0.50 to 2.20%, Al: 0.010 to 0.060%, including N: 0.005 to 0.030%, and Si: 0.15% or less, Mn: 0.40% or less, P: 0.015% or less, S: suppressed to 0.010% or less, having a component composition consisting of the balance Fe and inevitable impurities, and a method for manufacturing a high-strength bolt steel made of tempered martensite steel with a tensile strength of 1300 MPa or more, After heating a steel ingot to 900 °C or higher and quenching it, tempering is carried out at a temperature of 550 °C or higher and 700 °C or lower for a predetermined time so that the hydrogen trap energy is 53 kJ / mol or higher and the local critical hydrogen concentration measured by the CSR method is 1.5 ppm or higher, and carbides containing V carbides with a particle size of 10 nm or less are precipitated in the matrix phase. A method for manufacturing high-strength bolt steel characterized by this.

4. By mass percentage, C: 0.25 to 0.55%, Cr: 0.80 to 1.50%, V: 0.05 to 0.50%, Al: 0.010 to 0.060%, N: 0.005 to 0.030% is included, and Si: 0.15% or less, Mn: 0.40% or less, P: 0.015% or less, S: Suppressed to 0.010% or less, It has a component composition consisting of the balance Fe and unavoidable impurities, and is a method for manufacturing high-strength bolt steel made of tempered martensite steel with a tensile strength of 1300 MPa or higher. After heating a steel ingot to 900 °C or higher and quenching it, tempering is carried out at a temperature of 550 °C or higher and 700 °C or lower for a predetermined time so that the hydrogen trap energy is 53 kJ / mol or higher and the local critical hydrogen concentration measured by the CSR method is 1.5 ppm or higher, and carbides containing V carbides with a particle size of 10 nm or less are precipitated in the matrix phase. A method for manufacturing high-strength bolt steel characterized by this.

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