JP2007031746A - Steel for high strength bolt having excellent delayed fracture resistance, and high strength bolt - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for a high strength bolt exhibiting sufficient temper softening resistance even without incorporating large quantities of Mo and V into its composition. <P>SOLUTION: The steel for a high strength bolt having excellent delayed fracture resistance has a composition comprising 0.30 to 0.45% C, ≤0.2% Si, 0.30 to 0.80% Mn, ≤0.015% P, ≤0.015% S, 0.40 to 1.2% Cr, 0.15 to 0.45% Mo, 0.05 to 0.35% V, 0.02 to 0.15% Ti, 0.005 to 0.10% Zr, 0.02 to 0.10% Al and ≤0.010% N, and the balance Fe with inevitable impurities, and in which Zr, Al and N satisfy the inequality (1): [Zr]+0.3[Al]>6.5[N]; wherein, [Zr], [Al] and [N] denote each content (mass%) of Zr, Al and N. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to steel for high-strength bolts and high-strength bolts having excellent delayed fracture resistance.

As general bolt steel, JIS standard steels such as SCM435 and SCM440 are widely used. However, when these steels have a tensile strength exceeding about 1200 N / mm ^2, they are so-called delayed that suddenly brittle fracture after a certain period. Breaking easily occurs. Accordingly, steels for high-strength bolts with improved temper softening resistance have been proposed for the purpose of improving delayed fracture resistance. For example, in Patent Document 1, by increasing the amount of Mo, and in Patent Documents 2 and 3, by adding Mo and V in combination, the temper softening resistance of the steel material is improved and the steel material is tempered at a high temperature, thereby delaying resistance. It has been proposed to produce high-strength bolts with excellent destructibility.
JP-A-5-148576 Japanese Patent Laid-Open No. 5-148580 Japanese Patent No. 2739713

  As described above, if the temper softening resistance of the steel for high-strength bolts is improved by containing Mo or V, the delayed fracture resistance of the high-strength bolts can be improved, but Mo and V are very It is an expensive element, and it is inevitable to increase the cost of the product by adding it. This increase in cost is one of the barriers when putting high-strength bolt steel into practical use.

Therefore, an object of the present invention is to provide a steel for high-strength bolts that exhibits sufficient temper softening resistance without containing a large amount of Mo or V. Furthermore, the present invention is a high strength bolt excellent in delayed fracture resistance obtained by quenching and tempering the steel, particularly excellent in delayed fracture resistance despite having a tensile strength of 1300 N / mm ² or more. Another object is to provide a high strength bolt.

The present invention that has solved the above problems
C: 0.30 to 0.45% (meaning mass%, the same shall apply hereinafter)
Si: 0.2% or less (excluding 0%),
Mn: 0.30 to 0.80%,
P: 0.015% or less (excluding 0%),
S: 0.015% or less (excluding 0%),
Cr: 0.40 to 1.2%,
Mo: 0.15-0.45%,
V: 0.05-0.35%,
Ti: 0.02 to 0.15%,
Zr: 0.005 to 0.10%,
Al: 0.02 to 0.10%,
N: 0.010% or less (excluding 0%)
Containing
The balance is Fe and inevitable impurities,
Zr, Al and N are steels for high-strength bolts excellent in delayed fracture resistance, characterized by satisfying the following formula (1).
[Zr] +0.3 [Al]> 6.5 [N] (1)
[Wherein, [Zr], [Al] and [N] represent the contents (mass%) of Zr, Al and N, respectively. ]

  The steel for high-strength bolts of the present invention further contains Ni and / or Cu in total of 1% or less (excluding 0%), or further containing B in an amount of 0.0050% or less (excluding 0%). May be.

The present invention also provides a high-strength bolt excellent in delayed fracture resistance manufactured from the steel for high-strength bolts. The preferred high strength bolts of the present invention are those produced by quenching bolt-shaped steel at a temperature of 880-940 ° C and then tempering at a temperature of 550-620 ° C. In particular, by such quenching and tempering, an annular V notch having a tensile strength of 1300 N / mm ² or more and formed by cutting (V notch angle 60 °, depth 2 mm, notch bottom radius 0.03 mm) A high-strength bolt excellent in delayed fracture resistance is produced in which a round bar specimen (diameter 10 mm) having no fracture does not break even when stress higher than 1800 N / mm ² is applied for 100 hours in distilled water. Is desirable.

  As a result of conducting various tests in order to solve the problem of adding a large amount of Mo or V that leads to cost increase, the present inventors have mainly focused on contributing to refinement of crystal grains so far. Focusing on the effective use from the viewpoint different from the prior art, the present invention has been made.

  Ti is originally an element that improves the temper softening resistance, and its effect is equal to or greater than that of Mo and V. However, Ti has a very strong affinity with N, and depending on the amount of Ti added, most of it is TiN when steel is melted. Although this TiN is effective for refining crystal grains, its melting point (solution temperature in steel) is very high, and it is difficult to dissolve in steel even at the quenching temperature after bolt forming. As a result, in the conventional product, Ti contributed only to the effect of crystal grain refinement, and did not fully exhibit the effect of improving the temper softening resistance.

  In this regard, if the crystal generated during melting is TiN, not TiN, Ti can be easily dissolved in the steel even at the quenching temperature of the bolt, without adding a large amount of Mo, etc. We thought that good tempering softening resistance could be secured for steel. Thus, it has been found that the effect of improving the temper softening resistance by Ti can be sufficiently ensured by suppressing the generation of TiN by preferentially supplementing N with Zr and, as a result, promoting the generation of TiC.

  In the first place, the presence of N is the cause of TiN generation, so it is better to reduce N as much as possible in order to suppress the generation of TiN. However, N is an impurity inevitably mixed at the time of melting, and in reality, the amount of N cannot be made zero with high-strength bolt steel. In addition, Al added as a deoxidizer during the production of killed steel before the addition of other alloying elements (for example, Mn, Cr, Mo, Ti, etc.) also has a denitrifying action, that is, AlN is formed to form steel. It is known to reduce the nitrogen concentration in it. However, the effect is not as great as the above Zr. Further, the addition of a large amount of Al has a problem of generating a large amount of inclusions and deteriorating delayed fracture resistance.

  Therefore, the present inventors paid attention to Zr having an affinity for N that is larger than that of Ti. That is, by adding Zr, N was fixed as ZrN, and the formation of TiN during the melting of steel was suppressed. From this point of view, it should be possible to suppress the formation of TiN during melting as the amount of Zr increases. However, if the amount of Zr exceeds 0.10%, a large amount of Zr-based inclusions precipitate, The toughness and delayed fracture resistance are also deteriorated.

Therefore, in the steel for high-strength bolts and high-strength bolts of the present invention, Zr and Al in steel are used in combination, and the amount of Zr and Al that does not cause inclusion problems due to excessive addition is expressed by the formula ( 1):
[Zr] +0.3 [Al]> 6.5 [N] (1)
[Wherein, [Zr], [Al] and [N] represent the contents (mass%) of Zr, Al and N, respectively. ]
By controlling to an appropriate range so as to satisfy the relationship, TiN generation is effectively suppressed while suppressing the harmful effects of Zr and Al, and as a result, TiC generation is promoted, thereby tempering Ti. It is characterized by ensuring the improvement effect of softening resistance.

  Furthermore, since TiC has a function as a hydrogen trap site, an increase in the amount of TiC in high-strength bolts leads to an increase in the number of hydrogen trap sites, that is, an increase in hydrogen trapping force, which also improves delayed fracture resistance. It is thought to contribute.

As described above, in the present invention, the effect of improving the temper softening resistance of Ti and the hydrogen trapping action are fully utilized, so that high strength bolt steel excellent in delayed fracture resistance without adding a large amount of Mo and the like and High strength bolts can be manufactured. In particular, according to the present invention, while the tensile strength is 1300 N / mm ² or more, the delayed fracture strength described in the following examples is excellent in strength and delayed fracture resistance to reach 1800 N / mm ² or more. High strength bolts can be manufactured. In this regard, it has been found from the experience of the present inventors that a high-strength bolt having a delayed fracture strength of 1800 N / mm ² or more has an extremely low probability of causing delayed fracture in actual use. It is evaluated as excellent.

BEST MODE FOR CARRYING OUT THE INVENTION

First, the reason for determining the ratio of the chemical component in the steel for high-strength bolts of the present invention and the high-strength bolts will be described.
[C: 0.30 to 0.45% (meaning mass%, the same applies hereinafter)]
C is an element necessary for ensuring the hardenability and strength of the steel material. Therefore, the lower limit of C is set to 0.30%. Preferably, 0.33% or more is contained in the steel. On the other hand, when C is excessive, the toughness of steel and bolts deteriorates, and the delayed fracture resistance also decreases. In addition, problems such as burning cracks may occur. Therefore, the upper limit of the C amount is set to 0.45%. Preferably it is 0.40% or less, More preferably, it is 0.38% or less.

[Si: 0.2% or less (excluding 0%)]
Si is an element added as a deoxidizing agent. However, if the remaining amount of Si is excessive, grain boundary oxidation is promoted during heat treatment such as quenching, and the delayed fracture resistance tends to decrease. Therefore, the amount of Si is suppressed to 0.2% or less. Preferably it is 0.10% or less, More preferably, it is 0.07% or less.

[Mn: 0.30 to 0.80%]
Mn is a hardenability improving element and is useful for achieving high strength. In order to exhibit such an effect, it is necessary to contain 0.30% or more of Mn. Preferably it is 0.40% or more, More preferably, it is 0.45% or more. On the other hand, when Mn is excessive, segregation to the grain boundary is promoted, and the grain boundary strength is lowered, so that the delayed fracture resistance is deteriorated. Therefore, the amount of Mn is suppressed to 0.80% or less. Preferably it is 0.60% or less, More preferably, it is 0.55% or less.

[P: 0.015% or less (excluding 0%)]
P is an element that promotes grain boundary fracture due to grain boundary segregation and degrades delayed fracture resistance. Therefore, it is desirable that the P content is low, and the upper limit is made 0.015%. Preferably it is 0.010% or less, More preferably, it is 0.009% or less, More preferably, it is 0.005% or less.

[S: 0.015% or less (excluding 0%)]
S forms inclusions such as MnS in the steel material, and these become stress concentration points when stress is applied. Therefore, in order to avoid stress concentration and improve fracture resistance, it is necessary to reduce the S content, and the upper limit is set to 0.015%. Preferably it is 0.010% or less, More preferably, it is 0.009% or less, More preferably, it is 0.005% or less.

[Cr: 0.40 to 1.2%]
Since Cr is an element necessary for ensuring workability and hardenability, the lower limit is made 0.40%. Preferably it is 0.80% or more, More preferably, it is 0.95% or more. However, excessive addition of Cr promotes grain boundary oxidation and promotes problems such as an increase in quenching temperature, uneven burning due to excessive hardenability, and quench cracking, so the upper limit is set to 1.2%. Preferably it is 1.10% or less, More preferably, it is 1.05% or less.

[Mo: 0.15 to 0.45%]
Mo has an effect of improving hardenability and an effect of suppressing grain boundary oxidation. Furthermore, since Mo is a precipitation hardening element, it is useful for increasing the resistance to temper softening and securing the strength. Therefore, in the present invention, the lower limit of Mo is set to 0.15%. Preferably it is 0.20% or more, more preferably 0.25% or more.
However, since Mo is a very expensive element, the addition of a large amount causes an increase in cost. Conventionally, in order to improve the temper softening resistance of a steel material, to enable high temperature tempering and to produce a high-strength bolt excellent in delayed fracture resistance, a large amount of Mo has been required. Since a proper amount of Al is added and the effect of improving the temper softening resistance by Ti is utilized, it is not necessary to add a large amount of Mo. Therefore, the upper limit of Mo is set to 0.45%. Preferably it is 0.35% or less, More preferably, it is 0.30% or less.

[V: 0.05 to 0.35%]
V refines the crystal grains and improves the temper softening resistance. Further, since V carbide acts also as a hydrogen trap, it is necessary to add V to some extent in order to realize excellent delayed fracture resistance. Therefore, in the present invention, the lower limit of V is set to 0.05%. Preferably it is 0.08% or more, More preferably, it is 0.10% or more.
However, since V is a very expensive element, adding a large amount causes an increase in cost. In addition, the addition of a large amount may produce giant carbides that do not sufficiently dissolve at normal quenching temperatures, thereby degrading the toughness of the steel material. In this regard, in the present invention, by adding appropriate amounts of Zr and Al, as in the case of Mo, it is not necessary to add a large amount of V. Therefore, the upper limit of V is set to 0.35%. Preferably it is 0.20% or less, More preferably, it is 0.15% or less.

[Ti: 0.02-0.15%]
Ti is an element useful for refining crystal grains. Further, by controlling the relationship among the amounts of Zr, Al and N in a well-balanced manner, the effect of improving the temper softening resistance of Ti, which has not been sufficiently exhibited in the past, can be fully utilized in the present invention. Moreover, Ti carbide becomes a strong hydrogen trap site and contributes to the improvement of delayed fracture resistance. In order to fully utilize these effects, the lower limit of Ti is set to 0.02%. Preferably it is 0.04% or more, more preferably 0.05% or more. However, when the amount of Ti is excessive, a huge precipitate that is not sufficiently dissolved by quenching is generated, and the toughness of the steel material may be deteriorated. Therefore, the upper limit of Ti is set to 0.15%. Preferably it is 0.10% or less, More preferably, it is 0.07% or less.

[Zr: 0.005 to 0.10%]
Zr is an element necessary for reducing the amount of N in the steel and suppressing the formation of TiN as described above. It is also effective for making crystal grains finer. In order to fully exhibit these effects, the lower limit of Zr is made 0.005%. Preferably it is 0.008% or more, More preferably, it is 0.015% or more, More preferably, it is 0.025% or more. However, excessive addition of Zr can generate a large amount of inclusions, and can deteriorate toughness and resistance to delayed fracture. Therefore, in the present invention, the upper limit of Zr is set to 0.10%. Preferably it is 0.080% or less, More preferably, it is 0.050% or less, More preferably, it is 0.040% or less.

[Al: 0.02-0.10%]
Al is an element added as a deoxidizer, and is an element necessary for reducing the amount of N in steel and suppressing TiN generation, like Zr. In order to exert the effects of deoxidation and TiN suppression, the lower limit of Al is made 0.02%. Preferably it is 0.035% or more, More preferably, it is 0.04% or more. On the other hand, as the remaining amount of Al increases, the oxide inclusions increase, and the delayed fracture resistance can decrease. Therefore, the upper limit of Al is set to 0.10%. Preferably it is 0.08% or less, More preferably, it is 0.06% or less.

[N: 0.010% or less (excluding 0%)]
Since N is a cause of TiN generation, it should not be added and is preferably reduced as much as possible. Therefore, the upper limit of N is set to 0.010%. Preferably it is 0.008% or less, More preferably, it is 0.006% or less, Especially 0.005% or less. However, N is inevitably contained in the steel as an impurity, and N is not practically 0%.

  The contained elements defined in the steel for high-strength bolts of the present invention are as described above, and the balance is substantially Fe. However, it is naturally allowed that unavoidable impurities brought in depending on the situation of raw materials, materials, manufacturing equipment, etc., for example, 0.003% or less of O, etc. are contained in the steel. Furthermore, it is also possible to positively contain the following elements in the steel as long as the effects of the present invention are not adversely affected.

[Ni and / or Cu: 1% or less in total (excluding 0%)]
Cu is an element effective for enhancing corrosion resistance and suppressing invasion of hydrogen that adversely affects delayed fracture. Ni is an element effective for increasing the toughness and hardenability of the steel material and improving corrosion resistance and suppressing hydrogen intrusion. Therefore, in order to exert such an effect, when these elements are used positively, Cu and / or Ni are contained in the steel in total, preferably 0.2% or more, more preferably 0.3% or more. Let However, even if Cu is added excessively, the above effect is saturated, while the toughness of the steel material is deteriorated. When Ni is added excessively, the effect is saturated and the cost is increased. Therefore, when these are positively added, it is preferable that Cu and / or Ni be 1% or less in total. The total of these is more preferably 0.7% or less, and still more preferably 0.5% or less.

[B: 0.0050% or less (excluding 0%)]
B is an element useful for improving the hardenability of the steel and cleaning the grain boundaries. In order to exhibit these effects, it is preferably 0.0003% or more, more preferably 0.0010% or more. To contain. However, since the toughness is lowered when the amount of B is excessive, the upper limit of B is preferably set to 0.0050%. More preferably, it is 0.0030% or less, More preferably, it is 0.0020% or less.

  A high-strength bolt excellent in delayed fracture resistance can be produced from the steel for high-strength bolts of the present invention having the above chemical components, for example, by heat treatment under conventional quenching / tempering conditions. However, in order to produce a bolt having better delayed fracture resistance and strength, it is desirable to adopt the quenching and tempering conditions described later.

  The bolt steel of the present invention is processed into a bolt shape, and the bolt-shaped steel is quenched and tempered. There is no particular limitation on the means for processing the steel for high-strength bolts of the present invention into a bolt shape, and any conventionally used processing method can be used. For example, after hot-rolling the steel for high-strength bolts of the present invention, spheroidizing annealing is performed as necessary, then wire drawing is performed, and then cold working such as cold forging and cold forging is performed to perform bolting It can be a shape.

  Next, in the present invention, it is recommended that the bolt-shaped steel is quenched at a heating temperature of preferably 880 ° C. or higher. This is because, by quenching at a heating temperature of 880 ° C. or higher, precipitation hardening type alloy elements such as Ti contained in the steel for high-strength bolts of the present invention can be satisfactorily dissolved in the steel material. In particular, when spheroidized carbides are present in the steel before quenching, it is desirable to quench at 880 ° C. or higher in order to sufficiently dissolve these to ensure a desired tensile strength. A more preferable lower limit of the quenching temperature is 900 ° C. However, when the quenching temperature exceeds 940 ° C., there are restrictions on equipment, an increase in cost, and a problem of uneven firing, so a preferable upper limit of the quenching temperature is 940 ° C. A more preferred upper limit is 920 ° C.

It is desirable to produce a high-strength bolt excellent in delayed fracture resistance by tempering the quenched bolt-shaped steel at a heating temperature of 550 ° C. or higher. This is because fine carbides can be sufficiently precipitated by tempering Ti or the like dissolved in the steel material by quenching at a heating temperature of 550 ° C. or higher. The secondary hardening by the precipitation of fine carbides exhibits the effect of temper softening resistance, and the bolt of the present invention can be given high strength, preferably a tensile strength of 1300 N / mm ² or more. In addition, delayed fracture resistance can be improved satisfactorily by high-temperature tempering at 550 ° C. or higher. A more preferable lower limit of the tempering temperature is 580 ° C. On the other hand, if the tempering temperature is too high, the effect of the softening resistance may be diminished and the desired strength may not be obtained. Therefore, the tempering heating temperature is recommended to be 620 ° C. or lower. A more preferred upper limit is 600 ° C.

Moreover, conditions other than the above in quenching and tempering can be set in consideration of the heating temperature and the like. For example, the following conditions can be adopted.
[Hardening conditions]
・ Holding time: 10 minutes or more (preferably 20 minutes or more)
1 hour or less (preferably 40 minutes or less)
・ Cooling conditions: Oil cooling or water cooling [tempering conditions]
・ Holding time: 30 minutes or more (preferably 70 minutes or more)
3 hours or less (preferably 2 hours or less)
・ Cooling conditions: oil cooling, water cooling or air cooling

  The high-strength bolt of the present invention is optimal as a bolt having high strength and excellent delayed fracture resistance used in the automotive field, the architectural field, the industrial machine field, and the like. Examples of the use of the high-strength bolts of the present invention include high tension bolts, torcia-type bolts, hot-dip galvanized high-strength bolts, rust-proof high-strength bolts, and refractory steel high-strength bolts.

  Sample steels having the composition described in Tables 1 and 2 (Comparative Examples) were hot-rolled to a diameter of 12 mm, and the steel was quenched and tempered under the conditions shown in Table 3. Quenching was oil-cooled after being held at each temperature for 30 minutes, and tempering was performed by heating at each temperature for 90 minutes and then water-cooling. The strength and delayed fracture resistance of each sample thus obtained were evaluated as follows.

[Evaluation of strength]
By cutting each sample obtained as described above, a tensile test piece shown in FIG. 1 was produced, and the tensile strength was measured. These results are shown in Table 3.
A test piece showing a tensile strength of 1300 N / mm ² or more was evaluated as good.

[Evaluation of delayed fracture resistance]
By cutting each sample obtained as described above, a delayed fracture test piece (round) having an annular V notch (V notch angle 60 °, depth 2 mm, notch bottom radius 0.03 mm) shown in FIG. A rod-like shape having a diameter of 10 mm) was prepared, and the delayed fracture strength was measured. The “delayed fracture strength” in the present invention refers to the maximum load stress at which the test piece does not break even when the delayed fracture test piece is loaded with various stresses in distilled water for 100 hours. . These results are shown in Table 3.
A test piece showing delayed fracture strength of 1800 N / mm ² was evaluated as good. In this regard, it has been found from the experience of the present inventors that the probability of delayed fracture occurring in actual use is extremely small with a high strength bolt having a delayed fracture strength of 1800 N / mm ² or more.

From Tables 1-3, No. 1 satisfying the requirements of the present invention. 1-19 shows a 1300 N / mm ² or more tensile strength and 1800 N / mm ² or more delayed fracture strength bolts made from high strength bolts steel defined in the present invention, Re strength and delayed fracture It is shown to be excellent in properties. On the other hand, No. which does not satisfy the requirements of the present invention. Each of 20 to 35 has the following problems.

No. No. 20 had a low C content, and therefore the hardenability of the steel material was low, and since the effect of improving the strength by C was small, the desired tensile strength (1300 N / mm ² or more) could not be ensured.
No. In No. 21, since the amount of C was large, the toughness was lowered and the desired delayed fracture strength (1800 N / mm ² or more) could not be secured.
No. In No. 22, since the amount of Mn was small, the hardenability of the steel material was low, and the desired tensile strength could not be secured.
No. In No. 23, since the amount of Mn was large, segregation at the grain boundary was promoted, and the desired delayed fracture strength could not be ensured.

No. In No. 24, since P and S are large, the grain boundary became brittle, and the desired delayed fracture strength could not be secured.
No. In No. 25, the Cr content was lowered, so the hardenability of the steel material was low, and the desired tensile strength could not be secured.
No. 26 or No. In No. 27, since the amount of Mo or V was lowered, the temper softening resistance was insufficient, and the desired tensile strength could not be secured.

No. In No. 28, since the Ti amount was lowered, the temper softening resistance was insufficient, and the desired tensile strength could not be secured.
No. No. 29 does not contain Zr, and thus TiN generation cannot be sufficiently suppressed. As a result, it is considered that the effect of improving the temper softening resistance by Ti was not sufficiently exhibited. As a result, the desired tensile strength could not be ensured.
No. No. 30 does not satisfy the requirement of formula (1) because both Zr and Al content are insufficient. Therefore, it is considered that TiN generation could not be sufficiently suppressed, the amount of TiC after tempering decreased, and the hydrogen trapping force decreased. As a result, the desired delayed fracture strength could not be ensured.
No. In 31, since the amount of Zr was large, inclusions increased. As a result, the toughness deteriorated and the desired delayed fracture strength could not be ensured.

No. No. 32 had a large amount of N, so a large amount of TiN was produced, and the desired tensile strength could not be secured.
No. In No. 33, since the amount of Al was large, inclusions increased. As a result, the toughness deteriorated and the desired delayed fracture strength could not be secured.
No. In 34, since the amount of Zr and Al was large, inclusions increased and the desired delayed fracture strength could not be secured.
No. 35 is SCM435 of conventional steel. Since Ti, Mo, V and the like are not contained in appropriate amounts, the temper softening resistance is not ensured, and the desired tensile strength cannot be ensured.

It is a schematic side view of the tensile test piece used in the Example. It is a schematic side view of the delayed fracture test piece used in the examples.

Claims (6)

C: 0.30 to 0.45% (meaning mass%, the same shall apply hereinafter)
Si: 0.2% or less (excluding 0%),
Mn: 0.30 to 0.80%,
P: 0.015% or less (excluding 0%),
S: 0.015% or less (excluding 0%),
Cr: 0.40 to 1.2%,
Mo: 0.15-0.45%,
V: 0.05-0.35%,
Ti: 0.02 to 0.15%,
Zr: 0.005 to 0.10%,
Al: 0.02 to 0.10%,
N: 0.010% or less (excluding 0%)
Containing
The balance is Fe and inevitable impurities,
A steel for high-strength bolts excellent in delayed fracture resistance, characterized in that Zr, Al, and N satisfy the following formula (1).
[Zr] +0.3 [Al]> 6.5 [N] (1)
[Wherein, [Zr], [Al] and [N] represent the contents (mass%) of Zr, Al and N, respectively. ]   Furthermore, the steel for high-strength bolts of Claim 1 which contains Ni and / or Cu 1% or less in total (excluding 0%).   Furthermore, the steel for high-strength bolts of Claim 1 or 2 which contains B 0.0050% or less (0% is not included).   A high-strength bolt excellent in delayed fracture resistance, which is manufactured from the steel for high-strength bolts according to any one of claims 1 to 3.   The high-strength bolt according to claim 4, which is manufactured by quenching at a temperature of 880 to 940 ° C and then tempering at a temperature of 550 to 620 ° C. Round bar test piece (diameter 10 mm) having a tensile strength of 1300 N / mm ² or more and having an annular V-notch (V notch angle 60 °, depth 2 mm, notch bottom radius 0.03 mm) formed by cutting The high-strength bolt according to claim 5, wherein the bolt does not break even when stress higher than 1800 N / mm ² is applied for 100 hours in distilled water.

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