JP5449925B2 - High strength bolt with improved delayed fracture resistance and method for manufacturing the same - Google Patents

Description

  The present invention relates to a high-strength bolt with improved delayed fracture resistance, particularly hydrogen embrittlement resistance under corrosive environment, and a method for producing the same.

  In so-called delayed fracture, which occurs after a certain period of time has passed since the stress is applied to the steel material, various causes are intricately intertwined, and it is difficult to identify the cause. However, it is generally considered that delayed fracture is mainly caused by hydrogen embrittlement. Although factors such as tempering temperature, structure, material hardness, grain size, and various alloying elements are recognized as factors affecting this hydrogen embrittlement phenomenon, measures to prevent hydrogen embrittlement have not been established. In fact, various methods are merely trial and error.

For example, Patent Documents 1 to 3 disclose that by adjusting various alloy elements, high strength bolt steel having excellent delayed fracture resistance can be obtained even if the tensile strength is 140 kgf / mm ² or more. Patent Document 4 discloses a technique capable of improving delayed fracture resistance by dispersing fine compounds in steel. In Patent Document 4, delayed fracture resistance is improved by precipitating many fine compounds by quenching and tempering steel and trapping hydrogen (diffusible hydrogen) moving around in the precipitate.

  In recent years, however, delayed fracture resistance in a more severe corrosive environment has been required, and there has been room for improvement in the conventionally proposed technology.

JP-A-60-114551 JP-A-2-267243 JP-A-3-243745 Japanese Patent No. 4031068

  The present invention has been made in view of the problems as described above, and an object thereof is to provide a high-strength bolt capable of exhibiting excellent delayed fracture resistance even in a severe corrosive environment and a method for manufacturing the same. It is in.

  The quenching and tempering high-strength bolts of the present invention that have achieved the above-mentioned problems are: C: 0.25 to 0.6% (meaning of mass%, hereinafter the same), Si: 0.02 to 0.45%, Mn : 0.2 to 0.8%, P: 0.02% or less (not including 0%), S: 0.025% or less (not including 0%), Al: 0.1% or less (0%) N: 0.001 to 0.02%, Cr: 0.05 to 2.0%, Mo: 0.5 to 2.0%, Ti: 0.02 to 0 0.2% and V: containing at least two or more selected from the group consisting of 0.01 to 0.50%, and the total amount of Ti and V is 0.08% or more, with the balance being iron And an inevitable impurity, the austenite grain size number of the bolt shaft portion is 9.0 or more, and the surface of the bolt shaft portion contains Fe and Cr. It has oxide scale which, with a minimum thickness of oxide scale is 0.5μm or more and an average thickness characterized in that it is a 1.0～4.5Myuemu.

  The high-strength bolt according to the present invention preferably has a shaft hardness of Hv400 or more, and further Ni: 1.0% or less (excluding 0%) and / or Cu: 1.0% or less (0 % Is not included).

In the present invention, a steel material having the above chemical composition is formed into a bolt, heated to 870 to 960 ° C. and quenched, and then at 500 to 610 ° C. in an N ₂ gas atmosphere having an O ₂ concentration of 10 ppm or less. A method for producing a high-strength bolt for tempering is also included.

According to the present invention, since an oxide scale containing Fe and Cr is appropriately formed in the shaft portion of the bolt, even in a high-strength bolt having a Hv of 400 or more (that is, a tensile strength of 1300 MPa or more), it is in a severe corrosion environment. Can exhibit excellent delayed fracture resistance. Therefore, it is possible to reduce the size and weight of the fastening parts, and can greatly contribute to improvement of fuel consumption and CO ₂ reduction through weight reduction of various parts including automobile engine parts.

It is a SEM photograph which shows the state of the scale after hardening and tempering in the example of this invention and a comparative example. It is the EPMA image (X-ray microanalyzer) which shows the state of the scale or rust before the acid immersion in this invention example and a comparative example, and after long-time acid immersion. It is the SEM photograph (Drawing 3 (a)) which showed the scale in the example of the present invention (Experiment No. 8), and the graph (Drawing 3 (b)) which shows the intensity of characteristic X-rays.

  The present inventor examined the influence of the surface condition of the steel material in particular on the hydrogen embrittlement phenomenon that causes the delayed fracture phenomenon of high-strength bolts. As a result, by appropriately adjusting the type and thickness of the oxide scale generated in the quenching and tempering process when manufacturing bolts, hydrogen penetration can be suppressed even in severe corrosive environments, and hydrogen embrittlement resistance can be improved. There was found.

  Furthermore, in the present invention, hydrogen that has permeated through the oxide scale can be trapped by appropriately adjusting the alloy elements to form an alloy compound that has the action of trapping hydrogen. In other words, the delayed fracture resistance can be further improved by the two-stage mechanism called hydrogen entry suppression and hydrogen trap.

First, the oxide scale in the present invention will be described with reference to FIGS. FIG. 1 is an SEM photograph showing the state of the scale after quenching and tempering in the examples of the present invention and comparative examples, and FIG. 2 is the state of the scale or rust before acid immersion in the examples of the present invention and comparative examples and after prolonged acid immersion. Is an EPMA (X-ray microanalyzer) image showing 2 are apparatus: JXA-8800RL (manufactured by JEOL Ltd.), acceleration voltage: 15 kV, and irradiation current: 0.2 μA. In the comparative example in which quenching and tempering was performed in an air atmosphere, the scale formed was not uniform, whereas in the present invention example in which quenching and tempering was performed in an N ₂ atmosphere with a low O ₂ concentration as described later, it was uniform. A scale is formed (FIG. 1). Furthermore, it can be seen from the state before acid immersion in FIG. 2 that Cr is concentrated on the surface layer (scale) in the example of the present invention. The oxide scale in the present invention contains Fe and Cr, and mainly contains (Fe, Cr) ₃ O ₄ (85% or more on a mass basis), and also contains FeCr ₂ O _{4 and the} like. The Fe—Cr scale in the present invention means a scale containing 1.5% by mass or more of Cr and 50% by mass or more of Fe analyzed by EPMA. When this oxide scale containing Fe and Cr (hereinafter referred to as “Fe—Cr scale”) is formed on the bolt surface, it is considered that hydrogen penetration can be suppressed mainly by the following three actions.

The first function is to suppress the intrusion of hydrogen at the early stage of corrosion. Fe-Cr scale has lower solubility in corrosive environments such as acid immersion than other oxide scales such as FeO, Fe ₂ O ₃ , Fe ₃ O _4, etc., and hydrogen penetrates into the iron in the early stage of corrosion. Can be suppressed.

  The second action is to suppress the intrusion of hydrogen even after a long time has passed in a corrosive environment. This effect is mainly due to the fact that (i) the Cr concentration on the surface layer of the steel is reduced and (ii) the rust formed after a long time is a dense rust such as α-phase or amorphous. It is thought that there is.

Regarding (i), since the Fe—Cr scale dissolves when immersed in a corrosive environment for a long time, the effect of suppressing the hydrogen penetration by the Fe—Cr scale is reduced. On the other hand, the surface layer of the bolt has a low Cr concentration because Cr has diffused into the oxide scale during the quenching and tempering process. Normally, the Cr in the steel matrix from dissolving in a Cr ^3+, Fe chlorine ions as compared with the case where melted becomes Fe ²⁺ (Cl ^-) becomes attracting strong, resulting in H ⁺ is Gravitate. However, in the present invention, as described above, since there is less Cr on the surface layer of the iron core, Fe is more likely to dissolve than Cr, and H ⁺ is less likely to be attracted. .

Regarding (ii) above, when an oxide scale containing Fe is immersed in a corrosive environment for a long time, first, Fe (OH) ₂ is produced, and further, rust containing FeOOH is produced via an intermediate product. . The FeOOH can exist as an amorphous or crystalline material, and in the case of a crystalline material, an α phase, a β phase, and a γ phase exist. Normally, when an oxide scale such as FeO, Fe ₂ O ₃ , Fe ₃ O ₄ or the like is immersed in a corrosive environment for a long time, β-phase FeOOH is generated. Since the β-phase FeOOH is sparser than the α-phase and amorphous FeOOH, hydrogen easily enters, and the β-phase has a strong force of attracting chlorine ions (Cl ⁻ ), and as a result, H ⁺ is attracted. However, in the present invention, it has been found that the FeOOH becomes α phase or amorphous when the oxide scale contains Cr. Amorphous or α-phase FeOOH is dense and can suppress the intrusion of hydrogen. Therefore, even if the Fe—Cr scale melts and changes to rust, the intrusion of hydrogen can be suppressed.

  The above hydrogen intrusion after a long time can be confirmed from FIG. That is, in the example of the present invention, almost no Cl can be confirmed in the base iron, whereas in the comparative example, Cl intrudes into the base iron, and as a result, hydrogen is considered to have infiltrated.

The third action is that the local battery action is unlikely to occur and therefore corrosion can be suppressed. Fe-Cr scale has lower electrical conductivity than other oxide scales such as FeO, Fe ₂ O ₃ , Fe ₃ O _4, so it is difficult to form a local battery path, and corrosion due to local battery action can be suppressed. As a result, it is considered that hydrogen is hardly attracted and intrusion of hydrogen can be suppressed.

  In order to effectively exhibit the above action, the thickness of the Fe—Cr scale was determined to be 0.5 μm or more in minimum thickness and 1.0 μm or more in average thickness. The Fe—Cr scale preferably has a minimum thickness of 1.0 μm or more (more preferably 1.5 μm or more), and an average thickness of 1.5 μm or more (more preferably 2.0 μm or more). preferable. On the other hand, when the Fe—Cr scale becomes too thick, it becomes easy to peel off, so the average thickness is set to 4.5 μm or less. The average thickness of the Fe—Cr scale is preferably 4.0 μm or less, more preferably 3.5 μm or less.

  In the present invention, in addition to the hydrogen intrusion suppression action by the Fe-Cr scale, when hydrogen has intruded, Mo-based carbide, Ti-based carbide, V-based carbide, and Cr-based carbide can trap hydrogen, Since the coarsening of crystal grains is suppressed, delayed fracture resistance can be enhanced. The above-mentioned Mo-based carbide, Ti-based carbide, V-based carbide, and Cr-based carbide are fine carbides having an equivalent circle diameter of about 5 to 30 nm, in addition to single carbides containing Mo, Ti, V, and Cr, For example, composite carbide containing two or more of these elements such as V-Mo carbide is also included.

  The chemical component composition and structure of the bolt according to the present invention will be described below.

C: 0.25 to 0.6%
C is an essential element for forming carbides with Mo, Ti, V, Cr and the like and securing strength. Therefore, the C amount is set to 0.25% or more. In particular, in order to realize a hardness of Hv 400 or higher, the C content is preferably 0.30% or higher, more preferably 0.35% or higher. On the other hand, if the amount of C is excessive, the carbide is coarsened and the toughness is lowered, so that the delayed fracture resistance is deteriorated. Therefore, the C amount is set to 0.6% or less. The amount of C is preferably 0.5% or less, and more preferably 0.45% or less.

Si: 0.02 to 0.45%
Si acts as a deoxidizer during the melting of steel, and is an important element in obtaining the Fe—Cr scale in the present invention. In the Si-containing steel, Fe ₂ SiO _{4 is} slightly generated at the interface between the base iron and the scale, and the adhesion of the scale is improved by the anchor effect. Further, like C, it is an element that is also useful in securing strength. Therefore, Si was determined to be 0.02% or more. The amount of Si is preferably 0.03% or more, and more preferably 0.04% or more. On the other hand, when the amount of Si becomes excessive, cold workability deteriorates and grain boundary oxidation in the heat treatment during quenching is promoted to deteriorate delayed fracture characteristics. Therefore, the Si amount is determined to be 0.45% or less. The amount of Si is preferably 0.3% or less, and more preferably 0.2% or less.

Mn: 0.2 to 0.8%
Mn is an element that improves hardenability and is an important element for securing strength. Therefore, the amount of Mn is set to 0.2% or more. The amount of Mn is preferably 0.3% or more, and more preferably 0.4% or more. On the other hand, if the amount of Mn is excessive, segregation at the grain boundary and weakening the grain boundary strength results in deterioration of the delayed fracture resistance. Therefore, the amount of Mn is set to 0.8% or less. The amount of Mn is preferably 0.7% or less, more preferably 0.6% or less.

P: 0.02% or less (excluding 0%)
P causes segregation at the grain boundary and lowers the grain boundary strength, so that the delayed fracture resistance is lowered. Therefore, the P content is determined to be 0.02% or less. The amount of P is preferably 0.015% or less, and more preferably 0.010% or less. The smaller the amount of P, the better. However, since it causes an increase in the manufacturing cost of the steel material, it is difficult to make it 0%, and a residual of about 0.001% is allowed.

S: 0.025% or less (excluding 0%)
S forms sulfides such as MnS and is finely dispersed in the steel. However, if the amount of S is excessive, coarse MnS and the like are formed, resulting in stress-concentrated sites, resulting in a decrease in delayed fracture resistance. Therefore, the S amount is determined to be 0.025% or less. The amount of S is preferably 0.010% or less, more preferably 0.005% or less. The amount of S is preferably as small as P, but it is difficult to reduce it to 0% because it increases the manufacturing cost of the steel material, and a residual amount of about 0.001% is allowed.

Al: 0.1% or less (excluding 0%)
Al is an element that has an effect of suppressing growth of crystal grains by combining with N in steel to generate AlN. Improvement of delayed fracture resistance can be expected by refining crystal grains. In order to effectively exhibit such an effect, the Al content is preferably 0.010% or more, more preferably 0.020% or more. On the other hand, when the amount of Al is excessive, oxide inclusions are generated, and the delayed fracture resistance is deteriorated. Therefore, the Al content is determined to be 0.1% or less. The amount of Al is preferably 0.070% or less, more preferably 0.050% or less.

N: 0.001 to 0.02%
N is an element that contributes to the improvement of delayed fracture resistance by forming nitrides and refining crystal grains. Therefore, the N amount is set to 0.001% or more. The N amount is preferably 0.002% or more, and more preferably 0.004% or more. On the other hand, when the amount of N becomes excessive, the amount of N dissolved in the steel increases and the delayed fracture resistance is deteriorated. Therefore, the N amount is set to 0.02% or less. The N amount is preferably 0.007% or less, and more preferably 0.006% or less.

Cr: 0.05-2.0%
Cr is an element that has an effect of enhancing corrosion resistance and is effective in suppressing hydrogen intrusion by forming the above-described Fe—Cr scale. It is also effective in improving toughness and hardenability. In order to effectively exhibit such an effect, the Cr content is determined to be 0.05% or more. The amount of Cr is preferably 0.07% or more, and more preferably 0.1% or more. On the other hand, when the amount of Cr is excessive, the effect is saturated and the manufacturing cost is increased. Therefore, the Cr content is set to 2.0% or less. The amount of Cr is preferably 1.5% or less, more preferably 1.3% or less.

At least two or more selected from the group consisting of Mo: 0.5-2.0%, Ti: 0.02-0.2%, and V: 0.01-0.50%, and Ti and V Total amount is 0.08% or more Mo, Ti, and V form fine compounds (Mo-based compounds, Ti-based compounds, and V-based compounds) that serve as trap sites for hydrogen that has penetrated into the steel. Moreover, these elements have the effect | action which improves hardenability and are effective in the intensity | strength and toughness improvement of steel materials. In order to effectively exhibit such effects, the Mo amount was set to 0.5% or more, the Ti amount was set to 0.02% or more, and the V amount was set to 0.01% or more. The amount of Mo is preferably 0.6% or more, more preferably 0.8% or more. The amount of Ti is preferably 0.03% or more, more preferably 0.05% or more. V amount is preferably 0.03% or more, more preferably 0.05% or more. On the other hand, when the amount of Mo becomes excessive, the effect is saturated, and the life of the forging tool is reduced due to an increase in deformation resistance. Further, when the Ti amount and V amount are excessive, coarse nitrides and carbides are generated, and the toughness is lowered, so that the delayed fracture resistance is lowered. Therefore, the Mo amount is set to 2.0% or less, the Ti amount is set to 0.2% or less, and the V amount is set to 0.50% or less. The amount of Mo is preferably 1.5% or less, more preferably 1.0% or less. The amount of Ti is preferably 0.15% or less, more preferably 0.10% or less. The amount of V is preferably 0.4% or less, more preferably 0.3% or less.

  Of the above elements, Ti and V have the effect of preventing the coarsening of crystal grains by forming carbides, nitrides, and carbonitrides. Therefore, in order to effectively exhibit such an action, the total amount of Ti and V is determined to be 0.08% or more. The total amount of Ti and V is preferably 0.1% or more, more preferably 0.15% or more. The total amount of Ti and V means the amount of V alone when Ti is not contained, and means the amount of Ti alone when V is not contained.

  The basic components of the steel used in the present invention are as described above, and the balance is substantially iron. However, as a matter of course, it is permissible for steel to contain inevitable impurities brought in depending on the situation of raw materials, materials, manufacturing equipment, and the like. Furthermore, the steel used for this invention may contain Ni and / or Cu as needed.

Ni: 1.0% or less (not including 0%) and / or Cu: 1.0% or less (not including 0%)
Ni and Cu are elements having an action of improving corrosion resistance and suppressing hydrogen intrusion similarly to Cr. Ni is an element effective for improving toughness and hardenability. In order to exhibit such an effect, the Ni content is preferably 0.05% or more, more preferably 0.1% or more, and further preferably 0.2% or more. Further, the amount of Cu is preferably 0.05% or more, and more preferably 0.1% or more. On the other hand, when the amount of Ni is excessive, the above effects are saturated and the manufacturing cost is increased. Further, when the amount of Cu is excessive, the above effects are saturated, and the toughness is lowered to cause the delayed fracture resistance to be lowered. Therefore, the Ni content is preferably 1.0% or less, more preferably 0.7% or less, and still more preferably 0.5% or less. The amount of Cu is preferably 1.0% or less, more preferably 0.7% or less, and still more preferably 0.3% or less.

Austenite grain size number of bolt shaft portion: 9.0 or more Since the toughness is improved by refining the austenite crystal grains, the delayed fracture resistance can be improved. Therefore, the austenite grain size number of the bolt shaft portion is set to 9.0 or more, preferably 10.0 or more, more preferably 12.0 or more. The larger the austenite grain size number, the better, but usually it is 15.0 or less.

  The high-strength bolt in the present invention is made by melting steel having the above chemical components in accordance with a normal melting method, casting, hot rolling, wire drawing, and performing softening heat treatment such as spheroidizing annealing, descaling and finishing. After wire drawing, it can be manufactured by bolting by cold forging or cold forging, and further by quenching and tempering. As described above, the high-strength bolt of the present invention has an Fe—Cr scale on the bolt surface, precipitates fine carbides such as Mo, Ti, V, and Cr, and further refines crystal grains. In order to manufacture such a high-strength bolt, it is important to appropriately control the heating temperature, the tempering atmosphere, and the tempering temperature during quenching in the series of manufacturing processes described above. Each manufacturing condition will be described below.

Heating temperature during quenching: 870-960 ° C
If the heating temperature at the time of quenching is too low, the precipitation hardening type elements (Mo, Ti, V, Cr) are not sufficiently dissolved, so that a sufficient amount of carbide cannot be secured at the time of tempering and the strength is lowered, and in the steel The effect of trapping invading hydrogen is reduced. Furthermore, since the carbide before quenching has a spheroidized structure, if the heating temperature at the time of quenching is too low, the spheroidized carbide remains, leading to a reduction in strength and corrosion resistance. On the other hand, if the heating temperature at the time of quenching is too high, the delayed fracture resistance deteriorates due to the coarsening of the crystal grains. Therefore, the heating temperature during quenching was 870 to 960 ° C. The preferable heating temperature at the time of quenching is 900 to 950 ° C, more preferably 910 to 930 ° C.

Tempering temperature: 500-610 ° C
If the tempering temperature is too low, plate-like cementite precipitates at the crystal grain boundaries to lower the grain boundary strength, and the fine carbides of Mo, Ti, V, and Cr described above do not sufficiently precipitate, so delayed fracture resistance Decreases. On the other hand, if the tempering temperature is too high, the strength decreases. Therefore, the tempering temperature is set to 500 to 610 ° C. A preferable tempering temperature is 510-600 degreeC, More preferably, it is 530-595 degreeC.

Tempering atmosphere: N ₂ atmosphere with O ₂ concentration of 10 ppm or less In the production method of the present invention, quenching is performed in the air, and an oxide scale such as FeO is generated on the bolt surface layer during quenching. Next, when tempering, when tempering in the air, it becomes an oxide scale such as Fe ₂ O ₃ or Fe ₃ O ₄ , whereas when tempering in an N ₂ atmosphere with low O ₂ concentration, it is generated during quenching. The oxidized scale is changed to a (Fe, Cr) ₃ O ₄ main component (85% or more on a mass basis) containing Cr.

In the meaning of being an inert gas, Ar and the like can be cited in addition to N ₂ , but in the present invention, a desired Fe—Cr scale cannot be formed by tempering in an Ar atmosphere having a low O ₂ concentration. Fe-Cr scale in the present invention is formed with low N ₂ tempering atmosphere of O ₂ concentration is not necessarily clear why not be formed in the tempering atmosphere of low Ar or the like having the O ₂ concentration, as follows I can guess. That is, in the N ₂ atmosphere, nitrogen slightly forms nitrogen oxides on the outermost layer side (O ₂ -rich side) of the scale, so that the oxygen concentration of the scale decreases, and Fe ₂ O ₃ and Fe ₃ O ₄ are formed. The Fe-Cr scale is formed without being formed, whereas Ar does not form an oxide like nitrogen, so the oxygen concentration of the scale does not decrease, and the Fe-Cr scale cannot be formed. It is done.

N ₂ gas is considered to have a more specific effect. That is, when tempering in an N ₂ atmosphere, N is concentrated on the surface layer of the scale, and this concentrated N reacts with H ^{+ in a} corrosive environment to become ammonium ions (NH ⁴⁺ ), thus reducing H ^+. As a result, it is presumed that hydrogen penetration is suppressed.

In order to form an oxide scale mainly composed of (Fe, Cr) ₃ O ₄ , the O ₂ concentration in N ₂ is set to 10 ppm or less. The O ₂ concentration is preferably 5 ppm or less, more preferably 1 ppm or less.

  Other conditions for quenching and tempering can be appropriately set in consideration of the heating temperature, the amount of fine carbides deposited, and the like, and can be selected from the following ranges, for example.

Quenching conditions and holding time during heating: 10 minutes to 1 hour (preferably 20 minutes to 40 minutes)
-Furnace atmosphere: Atmosphere-Cooling conditions: Oil cooling or water cooling
Tempering conditions / tempering time: 30 minutes to 3 hours (preferably 70 minutes to 2 hours)
・ Cooling conditions: oil cooling or water cooling

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steels having chemical components shown in Table 1 were melted and cast according to a normal melting method, and then hot rolled to obtain a rolled material having a diameter of 12 mm. Subsequently, wire drawing and spheroidizing annealing (750 ° C., 6 hours, cooling rate: 10 ° C./hour) were performed, and a wire drawing material having a diameter of φ9.7 mm was obtained for bolt forming. The wire drawing material was cold forged to form M8 bolt (JIS B0205 and JIS B0209) test pieces, which were quenched and tempered under the conditions shown in Table 2. Other quenching and tempering conditions were as follows: quenching heating time: 45 minutes, cooling condition: oil cooling, tempering time: 90 minutes, cooling condition: water cooling.

Also those tempered atmosphere was N ₂ in Table 2, to control the atmosphere by the following procedure. First, the inside of a cylindrical furnace of φ400 mm × 400 mmL (atmosphere of 1.013 × 10 ⁵ Pa) was depressurized to 0.4 Pa by a rotary pump. Subsequently, it was replaced with 1.013 × 10 ⁵ Pa of N ₂ gas. If the O ₂ concentration in the atmosphere is about 21%, the oxygen concentration in the N ₂ atmosphere after the substitution is 0.4 / (1.013 × 10 ⁵ ) × 0.21 = 0.8 × 10 ⁻⁶ , The oxygen concentration can be calculated as 0.8 ppm.

  About each said test piece, the crystal grain size and hardness of the bolt shaft part, the surface scale state, and the delayed fracture resistance were evaluated in the following manner.

(1) Measurement of grain size and hardness of bolt shaft part After cutting a test piece with a cross section (cross section) perpendicular to the axial direction of the shaft part, D / 4 position (D is the diameter of the shaft part) It was observed in the area of 0.039 mm ² with an optical microscope (magnification: 400 times) to measure the grain size number according to JIS G0551. The measurement was performed for 4 fields of view, and the average of these values was defined as the austenite grain size. In addition, the hardness was measured by measuring the same region as the crystal grain size measurement with a Vickers hardness meter (load: 10 kg). Measurement was performed at four locations, and the average of these values was taken as the hardness of the bolt shaft.

(2) Analysis of surface scale state The analysis of the scale state of the bolt surface was performed by cutting a test piece with a cross section (cross section perpendicular to the axis) and embedding it in a resin, and a field emission scanning electron microscope (manufactured by Hitachi, S- 4500) was used to observe the entire circumference of the bolt surface at a magnification of 500 times, and after confirming that there were no peculiar points, photographed four locations every 90 ° (1000 times magnification), The area ratio of the surface scale layer was calculated to determine the area of the scale layer. Thereafter, the area of the scale layer was divided by the length of the base iron immediately below the scale layer to obtain an average scale thickness, and the average value of the four taken locations was taken as the average thickness of the scale layer. In addition, the minimum thickness of the scale layer was set to the minimum of the scale thicknesses at the four taken locations.

  Furthermore, for analysis of the component composition of the scale layer, the experiment No. 1 shown in FIG. An example of measurement will be described. First, observation was performed at a magnification of 1000 times (FIG. 3A), mapping of each element was performed at a magnification of 2000 times (not shown), a location where Cr was concentrated was determined, and further enlarged to 5000 times (FIG. 3). 3 (a)), point analysis was performed, and the characteristic X-ray intensity was measured as shown in FIG. 3 (b) to determine the composition of each component. The measurement was performed at four locations every 90 ° with respect to the cross section, and the composition obtained using the average of the X-ray intensity ratios of the detection elements (Fe, Si, Cr, O, etc.) The component composition of the scale. As a result, Experiment No. For 8, 10, 11, 13, 17, and 18, the scale components were Cr: 1.5 mass% or more and Fe: 50 mass% or more. In FIG. 3B, C is also detected, but this is due to C when carbon is deposited or C in the embedded resin.

(3) Evaluation of delayed fracture resistance Each test piece was immersed in a 35% HCl solution for 30 minutes, washed with water and dried, and then subjected to a low strain rate tensile test of 0.01 mm / min. In this test, since it is immersed in a high-concentration HCl solution as high as a 35% HCl solution as described above, it is possible to simulate a state of being immersed for a long time in a normal corrosive environment. The tensile test is performed until the test piece breaks, and the elongation at break is compared with the test piece not immersed in acid, (breaking elongation ratio) = (breaking elongation after acid immersion) / (test without acid immersion) The delayed fracture resistance was evaluated by the breaking elongation of the piece.

  The results are shown in Table 2.

  Experiment No. 2 in Table 2 Since 8, 10, 11, 13, 17, and 18 are appropriately controlled in both chemical composition and manufacturing method, they exhibit high strength, crystal grain size, and scale thickness within an appropriate range, and delayed fracture resistance. It was good.

  In contrast, Experiment No. 1-7, 9, 12, 14-16, 19-28 were not suitable for the chemical composition or production method, and therefore any of the strength, grain size, and scale thickness was out of the scope of the present invention. Destructive properties were insufficient.

  Experiment No. Since Nos. 1 to 6 were tempered in the air, Experiment No. 1 was further performed. Regarding No. 1, since the tempering temperature was low, the thickness of the scale was not sufficient, and the delayed fracture resistance was insufficient.

  Experiment No. Since No. 7 had a low tempering temperature, precipitation of carbides of Mo, Ti, V, and Cr was insufficient, resulting in insufficient delayed fracture resistance.

  Experiment No. No. 9 is an example in which the surface scale layer is removed by cutting after quenching and tempering, and since there is no scale that suppresses hydrogen penetration, the delayed fracture resistance is insufficient.

  Experiment No. No. 12 is an example in which the heating temperature at the time of quenching was high, and the delayed fracture resistance properties became insufficient because the crystal grains became coarse and the toughness decreased.

  Experiment No. No. 14 was an example in which the tempering temperature was high, and the strength was insufficient.

Experiment No. Nos. 15 and 16 are examples in which the tempering atmosphere was Ar and H ₂ , respectively, and the delayed fracture resistance was insufficient because the thickness of the scale was insufficient.

  Experiment No. Nos. 19 to 21 are examples in which any one of Mo, Ti, and V was small. Since precipitation of carbides of these elements was insufficient, the hydrogen trap action was insufficient and the delayed fracture resistance was insufficient. It was. In addition, since the total amount of Ti and V was small, the crystal grain size number was reduced and the strength was lowered.

  Experiment No. No. 22 is an example in which the amount of Mn was large, and the delayed fracture resistance was insufficient due to the decrease in grain boundary strength.

Experiment No. 23 is an example in which the amount of Si was large. Because of the large amount of Si, in addition to FeO, brittle Fe ₂ SiO ₄ is generated, cracks are generated in the scale and the corrosion resistance is reduced, and grain boundary oxidation is observed on the surface layer of the iron core. Delayed fracture characteristics became insufficient.

  Experiment No. Nos. 24 and 25 are examples in which the P amount and the S amount were large, respectively, and the delayed fracture resistance became insufficient due to the decrease in the grain boundary strength.

  Experiment No. No. 26 is an example in which the total content of Ti and V was small. Since the crystal grains were coarsened and the toughness was lowered during quenching, the delayed fracture resistance was insufficient.

Experiment No. Nos. 27 and 28 are examples in which Ti and V were large, respectively, and the carbides of these elements were coarse, so that the function as a hydrogen trap site was not sufficiently exhibited, and the delayed fracture resistance was insufficient. No. 28 is No. 28. Like 23, the amount of Si was large, so cracks occurred in the scale.

Claims (5)

C: 0.25 to 0.6% (meaning mass%, hereinafter the same),
Si: 0.02 to 0.45%,
Mn: 0.2 to 0.8%
P: 0.02% or less (excluding 0%),
S: 0.025% or less (excluding 0%),
Al: 0.1% or less (excluding 0%),
N: 0.001 to 0.02%,
Cr: 0.49 to 2.0% is contained,
Further, it contains Mo: 0.5-2.0% and V: 0.01-0.50%, the balance being iron and inevitable impurities,
The austenite grain size number of the bolt shaft portion is 9.0 or more,
It has an oxide scale containing Fe and Cr on the surface of the bolt shaft portion, the minimum thickness of the oxide scale is 0.5 μm or more, and the average thickness is 1.0 to 4.5 μm. Hardened and tempered high strength bolts. The high-strength bolt according to claim 1, further comprising Ti: 0.02 to 0.2% , wherein the total amount of Ti and V is 0.08% or more.   The high-strength bolt according to claim 1 or 2, wherein the shaft hardness is Hv400 or more.   The high-strength bolt according to any one of claims 1 to 3, further comprising Ni: 1.0% or less (not including 0%). A steel material having the chemical composition according to any one of claims 1, 2, and 4 is molded into a bolt,
After heating to 870-960 ° C and quenching,
The method for producing a high-strength bolt according to any one of claims 1 to 4, wherein tempering is performed at 500 to 610 ° C in an N ₂ gas atmosphere having an O ₂ concentration of 10 ppm or less.