WO2012098938A1

WO2012098938A1 - Delayed-fracture-resistant boron-containing steel for high-strength bolts, and high-strength bolts

Info

Publication number: WO2012098938A1
Application number: PCT/JP2012/050157
Authority: WO
Inventors: 洋介松本; 淳稲田; 千葉　政道
Original assignee: 株式会社神戸製鋼所
Priority date: 2011-01-18
Filing date: 2012-01-06
Publication date: 2012-07-26
Also published as: JP5608145B2; JP2012162798A

Abstract

The present invention provides, without adding an expensive alloying element such as Cr or Mo in a large amount, a boron-containing steel for high-strength bolts which combines excellent delayed fracture resistance and high strength of 1100MPa or more. This boron-containing steel contains 0.20 to less than 0.40% of C, 0.20 to 1.50% of Si, 0.30 to 2.0% of Mn, up to 0.03% (exclusive of 0%) of P, up to 0.03% (exclusive of 0%)of S, 0.05 to 1.0% of Ni, 0.01 to 1.50% of Cr, up to 1.0% (inclusive of 0%) of Cu, 0.01 to 0.10% of Al, 0.01 to 0.1% of Ti, 0.0003 to 0.0050% of B and 0.002 to 0.010% of N, and contains one ore more elements selected from the group consisting of Cu, Ni and Cr in a total amount of 0.10 to 3.0% with the balance consisting of iron and unavoidable impurities and the [Si]/[C] ratio being 1.0 or more. Further, the boron-containing steel has a ferrite/pearlite dual-phase structure.

Description

Boron-added steel for high strength bolts and high strength bolts with excellent delayed fracture resistance

The present invention relates to a steel for bolts used in automobiles, various industrial machines, and the like, and a high-strength bolt obtained by using this steel for bolts, and particularly has excellent delayed fracture resistance even when the tensile strength is 1100 MPa or more. The present invention relates to a boron-added high-strength bolt steel and a high-strength bolt.

Currently, bolts with a tensile strength of up to 1100 MPa are being made cheaper by shifting to boron-added steel. On the other hand, standard steel such as chromium molybdenum steel (JIS G 4053, SCM, etc.) is still frequently used for high strength bolts having a tensile strength of 1100 MPa or more. This standard steel contains a large amount of high-priced alloy elements such as Cr and Mo. Therefore, there is a demand for an alternative steel of the above chromium molybdenum steel with reduced Cr and Mo in response to a request for reducing the steel material cost. It is growing. However, it is difficult to ensure the strength simply by reducing the alloy elements.

Therefore, the use of boron-added steel, which uses the effect of improving hardenability by adding boron, as a material for high-strength bolts has been studied. However, since the delayed fracture resistance greatly deteriorates as the strength increases, it is difficult to apply to a severe part of the usage environment where delayed fracture is likely to occur.

Various technologies for improving delayed fracture resistance have been proposed so far. For example, Patent Document 1 proposes a technique for improving the corrosion resistance by adding a predetermined amount of Cu to the boron-added steel and reducing the hydrogen generated during the corrosion reaction to suppress the penetration of hydrogen into the steel. Has been. However, it is difficult to ensure corrosion resistance only by containing Cu.

In Patent Document 2 and Patent Document 3, the resistance to delayed fracture is improved by refining crystal grains, but it is difficult to apply to severe environments only by the effect of refining crystal grains.

Patent Document 4 relates to a steel material excellent in delayed fracture evaluation method and delayed fracture resistance, and shows a steel material in which the carbon equivalent is defined and the amount of S is reduced. However, it is difficult to completely suppress the occurrence of delayed fracture only by making extremely low sulfur. Moreover, since it is necessary to reduce sulfur extremely, there exists a possibility of causing the increase in manufacturing cost.

On the other hand, Patent Document 5 is a technique related to wear-resistant steel having excellent toughness and delayed fracture resistance. In the technique disclosed in Patent Document 5, since it is necessary to perform quenching as it is after rolling and to stop quenching in a specific temperature range, the process becomes complicated and there is a possibility that the manufacturing cost increases.

Patent Document 6 relates to a wear resistant steel material excellent in toughness and delayed fracture resistance, and a method for producing the same. In the technique disclosed in Patent Document 6, since it is necessary to process after quenching and tempering, there is a problem that when it is diverted to a bolt, it is difficult to mold the bolt. Furthermore, the steel material which becomes object by patent document 6 is plate shape, and the evaluation of delayed fracture resistance is also performed using the flat plate without a notch. However, since the bolt which is the subject of the present invention has a notch, it is necessary to evaluate the delayed fracture resistance based on a stricter standard than the delayed fracture resistance shown in Patent Document 6.

As described above, all of the technologies proposed so far to improve delayed fracture resistance have problems in strength, delayed fracture resistance in harsh environments, and manufacturing.

JP 2006-118033 A Japanese Patent No. 3535754 Japanese Patent No. 3490293 Japanese Patent No. 4370991 JP 2002-80930 A JP 2002-1105024 A

The present invention has been made under such circumstances, and its purpose is to have a high tensile strength of 1100 MPa or more without adding a large amount of expensive alloy elements such as Cr and Mo, and An object of the present invention is to provide a boron-added high-strength bolt steel exhibiting excellent delayed fracture resistance, and a high-strength bolt made of such a boron-added high-strength bolt steel.

The boron-added high-strength bolt steel of the present invention that has achieved the above-mentioned object is: C: 0.20 to less than 0.40% (0.20% to less than 0.40%) (% means mass%) The same shall apply hereinafter), Si: 0.20 to 1.50%, Mn: 0.30 to 2.0%, P: 0.03% or less (excluding 0%), S: 0.03% or less ( Ni: 0.05 to 1.0%, Cr: 0.01 to 1.50%, Cu: 1.0% or less (including 0%), Al: 0.01 to 0 .10%, Ti: 0.01 to 0.1%, B: 0.0003 to 0.0050% and N: 0.002 to 0.010%, respectively, and a group consisting of Cu, Ni and Cr And a total of 0.10 to 3.0% selected from the group consisting of iron and unavoidable impurities, and a Si content [Si] The content [C] ratio ([Si] / [C]) is 1.0 or more and the structure is a composite structure of ferrite and pearlite (ferrite / pearlite structure). is there.

Further, the present invention is a steel for boron added high-strength bolts, C: 0.20 to less than 0.40%, Si: 0.20 to 1.50%, Mn: 0.30 to 2.0% , P: 0.03% or less (excluding 0%), S: 0.03% or less (not including 0%), Ni: 0.10 to 0.50%, Cr: 0.03 to 0.00. 80%, Cu: 0.50% or less (including 0%), Al: 0.01 to 0.10%, Ti: 0.01 to 0.1%, B: 0.0003 to 0.0050% and N: 0.002 to 0.010% each, and one or more selected from the group consisting of Cu, Ni and Cr in total of 0.15 to 1.5%, the balance being iron and inevitable It is made of impurities, and the ratio of Si content [Si] to C content [C] ([Si] / [C]) is 1.0 or more, Also include those having the gist in that a composite structure of ferrite and pearlite (ferrite-pearlite structure).

Further, the present invention relates to a steel for boron added high-strength bolts, C: 0.20 to less than 0.40%, Si: 0.20 to 1.50%, Mn: 0.30 to 1.5% , P: 0.03% or less (excluding 0%), S: 0.03% or less (not including 0%), Ni: 0.10 to 0.50%, Cr: 0.03 to 0.00. 80%, Cu: 0.50% or less (including 0%), Al: 0.02-0.05%, Ti: 0.02-0.06%, B: 0.0010-0.003% and N: 0.003 to 0.006% each contained, and one or more selected from the group consisting of Cu, Ni and Cr in total of 0.15 to 1.5%, the balance being iron and inevitable It is made of impurities, and the ratio of Si content [Si] to C content [C] ([Si] / [C]) is 1.0 or more, Also include those having the gist in that a composite structure of ferrite and pearlite (ferrite-pearlite structure).

The boron-added high-strength bolt steel of the present invention may further contain at least one selected from the group consisting of Nb: 0.01 to 0.1% and V: 0.01 to 0.1%, if necessary. The characteristics of the steel for boron-added high-strength bolts can be further improved by including these.

In a preferred embodiment of the present invention, the ratio ([Si] / [C]) of the Si content [Si] and the C content [C] is (a) the C content is 0.20% or more and 0.00. When it is less than 25%, it is 2.0 or more. (B) When the C content is 0.25% or more and less than 0.29%, it is 1.5 or more. (C) The C content is 0.29. % Or more and less than 0.40% is 1.0 or more.

In addition, the high-strength bolt of the present invention that has achieved the above object is the structure using the steel (boron-added high-strength bolt steel), formed into a bolt shape, and then subjected to quenching and tempering treatment. The point is that the tensile strength when tempered martensite is 1100 MPa or more.

In the present invention, the chemical composition is strictly defined, and the ratio of the content ratio of Si and C ([Si] / [C]) is controlled within an appropriate range, so that severe fracture is likely to occur. It is possible to realize a boron-added high-strength bolt steel that exhibits excellent delayed fracture resistance even in a difficult environment. Moreover, the high strength bolt excellent in delayed fracture resistance is realizable using this boron addition steel for high strength bolts.

It is explanatory drawing which shows the external appearance shape of the test piece used in the Example. It is a graph which shows the relationship between the value of ratio ([Si] / [C]) and delayed fracture strength ratio 2. The corrosion weight loss and delayed fracture strength ratio 2 of the examples of the present invention and comparative examples in Examples are graphed.

The present inventors show high strength with a tensile strength of 1100 MPa or more without adding a large amount of expensive alloy elements such as Mo and Cr, and excellent delay resistance even at such high strength. In order to obtain a boron-added steel that exhibits destructive properties, we conducted extensive research. As a result, it has been found that, in a boron-added steel having a tensile strength of 1100 MPa or more, reducing the C content as much as possible rather than containing an alloy element is very effective in securing delayed fracture resistance. Although the reduction of C leads to insufficient strength, the Si content is equal to or higher than the C content [ie, the ratio of Si and C content ([Si] / [C]) is 1.0 or higher]. It was found that the strength reduction due to the reduction of the C content can be sufficiently secured.

Moreover, although the corrosion resistance is improved by reducing the C content, it is also effective to control the total content of Cu, Ni, Cr, etc. in order to ensure sufficient delayed fracture resistance under harsh environments, Furthermore, by adjusting other chemical components, it has been found that a boron-added steel having excellent delayed fracture resistance can be realized even with a high tensile strength of 1100 MPa or more, and the present invention has been completed. In addition, the steel material of this invention may implement a spheroidizing annealing process before bolt shaping | molding as needed.

C is an element useful for ensuring the strength of the steel, but increasing its content deteriorates the toughness and corrosion resistance of the steel and tends to cause delayed fracture. On the other hand, Si is also an element useful for ensuring the strength of steel, but the relationship with delayed fracture was unclear. Therefore, the present inventors investigated the influence of Si on delayed fracture. As a result, by increasing the Si content over the C content, it was possible to achieve both a tensile strength of 1100 MPa or more and excellent delayed fracture resistance in a harsh environment.

That is, in order to secure 1100 MPa or more only by adding C alone, it is necessary to increase the amount of C. However, as described above, when the C content is increased, the corrosion resistance of the steel is deteriorated, the amount of hydrogen generated on the steel surface is increased, and as a result, the amount of hydrogen entering the steel is increased and delayed fracture is likely to occur. Become. Even when an element for improving the corrosion resistance such as Cu, Ni, Cr, etc. is added in order to improve the corrosion resistance of the steel containing a large amount of C, a large improvement effect does not appear because the corrosion resistance of the matrix is low.

On the other hand, the combined addition of C and Si can increase the strength with Si, so that the C content can be relatively reduced. That is, it is possible to ensure excellent corrosion resistance and delayed fracture resistance and tensile strength of 1100 MPa or more by lowering the C content of the matrix and ensuring the strength with Si that does not significantly affect the corrosion resistance of steel. It became possible. Further, it has been found that the effect of the corrosion resistance improving element such as Cu, Ni, Cr, etc. appears remarkably by increasing the corrosion resistance of the matrix.

The steel for boron-added bolts of the present invention has a ratio ([Si] / [C]) of the Si content [Si] (mass%) and the C content [C] (mass%) from the above purpose. It is necessary to satisfy 1.0 or more. When ([Si] / [C]) satisfies 1.0 or more, excellent delayed fracture resistance can be secured. The value of the ratio ([Si] / [C]) is preferably 2.0 or more, and more preferably 3.0 or more. However, even if the ratio ([Si] / [C]) satisfies 1.0 or more, if the chemical component composition is out of the proper range, the delayed fracture resistance and other characteristics deteriorate. Occurs.

The value of the ratio ([Si] / [C]) is preferably controlled in an appropriate range according to the C content. Specifically, when (a) C: 0.20% or more and less than 0.25%, the ratio ([Si] / [C]) is set to 2.0 or more, and (b) C: 0.25. % Or more and less than 0.29%, the ratio ([Si] / [C]) is set to 1.5 or more, and (c) C: 0.29% or more (that is, 0.29% or more and 0.40). %), The ratio ([Si] / [C]) value is preferably 1.0 or more.

Moreover, in order to satisfy the basic characteristics as steel for bolts, components such as C, Si, Mn, P, S, Al, Ti, B, N, Cu, Ni, and Cr contained in the steel are appropriately adjusted. There is a need. The reasons for limiting the ranges of these components are as follows.

[C: 0.20 to less than 0.40% (0.20% or more and less than 0.40%)]
C is an element indispensable for forming carbides and securing the necessary tensile strength as high-strength steel. In order to exhibit such an effect, it is necessary to contain 0.20% or more. However, when C is contained excessively, the delayed fracture resistance is deteriorated due to a decrease in toughness and a decrease in ductility. In order to avoid such an adverse effect of C, the C content needs to be less than 0.40%. In addition, the minimum with preferable C content is 0.22%, It is good to set it as 0.25% or more more preferably. Moreover, the upper limit with preferable C content is 0.35%, It is good to set it as 0.30% or less more preferably.

[Si: 0.20 to 1.50%]
Si is an element necessary as a solid solution element that acts as a deoxidizer during melting and strengthens the matrix. By containing 0.20% or more of Si, sufficient strength can be secured. However, if the Si content exceeds 1.50% and is excessively contained, the cold workability of the steel material is lowered even when spheroidizing annealing is performed, and the grain boundary oxidation during quenching (heat treatment) is promoted. Deteriorates delayed fracture resistance. In addition, the minimum with preferable Si content is 0.3%, More preferably, it is good to set it as 0.4% or more. Moreover, the upper limit with preferable Si content is 1.0%, It is good to set it as 0.8% or less more preferably.

[Mn: 0.30 to 2.0%]
Mn is an element that improves hardenability, and is an important element for achieving high strength. The effect can be exhibited by containing 0.30% or more of Mn. However, if the Mn content is excessive, segregation to the grain boundary is promoted, the grain boundary strength is lowered, and the delayed fracture resistance is lowered, so 2.0% was made the upper limit. In addition, the minimum with preferable Mn content is 0.4%, It is good to set it as 0.6% or more more preferably. Moreover, the upper limit with preferable Mn content is 1.5%, It is good to set it as 1.0% or less more preferably.

[P: 0.03% or less (excluding 0%)]
P is contained as an impurity, but if it is present in excess, it causes segregation at the grain boundary, lowers the grain boundary strength, and deteriorates the delayed fracture characteristics. Therefore, the upper limit of the P content is 0.03%. In addition, the upper limit with preferable P content is 0.01%, It is good to set it as 0.005% or less more preferably.

[S: 0.03% or less (excluding 0%)]
If S is present in excess, sulfides segregate at the grain boundaries, leading to a decrease in grain boundary strength and delayed fracture resistance. Therefore, the upper limit of the S content is set to 0.03%. In addition, the upper limit with preferable S content is 0.01%, It is good to set it as 0.006% or less more preferably.

[Ni: 0.05 to 1.0%]
Ni is an element that improves corrosion resistance, and exhibits an effect when added in an amount of 0.05% or more. However, if added in a large amount, the steel material cost increases, so the upper limit is made 1.0%. In addition, the minimum with preferable Ni content is 0.10%, More preferably, it is 0.15% or more. Moreover, the upper limit with preferable Ni content is 0.80%, and a more preferable upper limit is 0.50%.

[Cr: 0.01 to 1.50%]
Cr is an element for improving corrosion resistance, and exhibits an effect by adding 0.01% or more. However, if added in a large amount, the steel material cost increases, so the upper limit is made 1.50%. The preferable lower limit of the Cr content is 0.03%, the more preferable lower limit is 0.05%, and the still more preferable lower limit is 0.10%. Moreover, the upper limit with preferable Cr content is 1.0%, and a more preferable upper limit is 0.80%.

[Cu: 1.0% or less (including 0%)]
Cu is an element for improving corrosion resistance, and in order to exert this effect, 0.05% or more is preferably contained. However, if added in a large amount, the steel material cost increases, so the upper limit is made 1.0%. In addition, the upper limit with preferable Cu content is 0.80%, and a more preferable upper limit is 0.50%.

[Al: 0.01 to 0.10%]
Al is an element effective for deoxidation of steel, and by forming AlN, austenite grains can be prevented from becoming coarse. In order to exert such effects, the Al content needs to be 0.01% or more. However, even if the Al content exceeds 0.10% and becomes excessive, the effect is saturated. In addition, the minimum with preferable Al content is 0.02%, More preferably, it is good to set it as 0.03% or more. Moreover, the upper limit with preferable Al content is 0.08%, More preferably, it is good to set it as 0.05% or less.

[Ti: 0.01 to 0.1%]
Ti is an element effective for fixing N in steel and precipitating TiC to improve delayed fracture resistance. Ti nitrides and carbides are useful for refining crystal grains, and the refining of crystal grains contributes to further improvement of delayed fracture resistance. In order to exhibit these effects effectively, it is necessary to contain Ti 0.01% or more. However, if the Ti content is excessive and exceeds 0.1%, workability is reduced. In addition, the minimum with preferable Ti content is 0.02%, More preferably, it is good to set it as 0.03% or more. Moreover, the upper limit with preferable Ti content is 0.08%, More preferably, it is 0.06% or less, More preferably, it is good to set it as 0.05% or less.

[B: 0.0003 to 0.0050%]
B is an element effective in improving the hardenability of steel, and in order to exhibit the effect, it is necessary to contain 0.0003% or more. However, if the B content becomes excessive and exceeds 0.0050%, the toughness is lowered instead. In addition, the minimum with preferable B content is 0.0005%, More preferably, it is good to set it as 0.0010% or more. Moreover, the upper limit with preferable B content is 0.004%, More preferably, it is good to set it as 0.003% or less.

[N: 0.002 to 0.010%]
N combines with Ti to form TiN in the solidification stage after melting, and has the effect of improving the delayed fracture resistance by miniaturizing crystal grains. Such an effect is effectively exhibited when the N content is 0.002% or more. However, when TiN is formed in a large amount, TiN is not dissolved by heating at about 1300 ° C., and formation of Ti carbide useful for improving delayed fracture resistance is inhibited. Excess N is also harmful to delayed fracture characteristics. In particular, if the N content exceeds 0.010% and becomes excessive, the delayed fracture characteristics are significantly reduced. In addition, the minimum with preferable N content is 0.003%, It is good to set it as 0.004% or more more preferably. Moreover, the upper limit with preferable N content is 0.008%, More preferably, it is good to set it as 0.006% or less.

[One or more selected from the group consisting of Cu, Ni and Cr: 0.10 to 3.0% in total]
Cu, Ni and Cr are all elements that improve corrosion resistance, and by making their total content 0.10% or more, hydrogen penetration into the steel can be suppressed and delayed fracture resistance can be improved. However, when these elements become excessive, the steel material cost increases, so the total amount needs to be 3.0% or less. In addition, the minimum with preferable total content of these elements is 0.15%, and a more preferable minimum is 0.20%. Moreover, the upper limit with preferable total content of these elements is 2.0%, and a more preferable upper limit is 1.5%.

The basic components of the steel for boron-added high-strength bolts of the present invention are as described above, with the balance being iron and inevitable impurities.

In addition to the above components, it is also effective to further contain Nb or V in the boron-added high-strength bolt steel of the present invention, if necessary. Appropriate ranges and actions when these elements are contained are as follows.

[One or more selected from the group consisting of Nb: 0.01 to 0.1% and V: 0.01 to 0.1%]
Nb and V are effective elements for refining crystal grains and improving delayed fracture resistance. In order to exert such an effect, the content of these elements is preferably set to 0.01% or more. However, when these elements are contained excessively, the strength of the hot rolled material becomes higher than necessary. Moreover, the steel material cost also increases. Therefore, the upper limit was made 0.1% respectively. In addition, the minimum with preferable content of these elements is 0.02%, respectively, More preferably, it is good to set it as 0.03% or more. Moreover, the upper limit with preferable content is 0.08%, respectively, More preferably, it is good to set it as 0.06% or less.

In the steel for boron-added high-strength bolts having the above chemical composition, the structure after rolling is basically a composite structure of ferrite and pearlite (indicated as “ferrite / pearlite”). Using this steel material, it is formed into a bolt shape with or without spheroidizing treatment if necessary, and then subjected to quenching and tempering treatment to make the structure tempered martensite, thereby obtaining a predetermined tensile strength. Thus, a bolt having excellent delayed fracture resistance can be obtained. Appropriate conditions for quenching and tempering at this time are as follows.

In the heating at the time of quenching, heating at 850 ° C. or higher is necessary for stable austenitization treatment. However, when heated to a high temperature exceeding 960 ° C., the crystal grains become coarse, which causes deterioration in delayed fracture characteristics. Therefore, in order to prevent coarsening of crystal grains, it is useful to heat and quench at 960 ° C. or lower.

The as-quenched bolts have low toughness and ductility and do not become bolt products as they are, so they need to be tempered. For this purpose, it is effective to perform a tempering treatment at a temperature of at least 200 ° C. On the other hand, when the tempering temperature exceeds 600 ° C., the steel material having the above chemical composition cannot secure a tensile strength of 1100 MPa or more. Moreover, the delayed fracture resistance improves as the prior austenite crystal grain size becomes finer. In order to exert such an effect, it is preferable that the grain size number (JIS G 0551) has a structure of 8 or more.

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.

The steel materials (test Nos. 1 to 24) having the chemical composition shown in Table 1 below were melted and then rolled into a wire having a diameter of 12 mmφ. Table 1 shows the structure of each wire after rolling. Thereafter, quenching was performed from 870 ° C., and tempering was performed within a range in which a tensile strength of 1100 MPa or more could be secured, and then a tensile test was performed. Moreover, corrosion resistance and delayed fracture resistance were evaluated using the test piece with a notch shown in FIG. In the tensile test, the one having no notch in the shaft portion was used, but in the delayed fracture test, the test piece with the notch was used as described above so as to simulate the stress concentration of the screw portion. Further, in the delayed fracture test, the test piece of FIG. 1 having no notch was also evaluated.

Corrosion resistance was evaluated by corrosion weight loss before and after immersion when the test piece was immersed in 15% HCl for 30 minutes. Delayed fracture resistance was implemented by immersing the test piece in 15% HCl for 30 minutes, washing with water and drying, then applying a constant load, and comparing the load that did not break for more than 100 hours. At this time, a value obtained by dividing the load that does not break for 100 hours or more after acid immersion by the maximum load when the tensile test is performed without acid immersion is defined as a delayed fracture strength ratio, and this value (delayed fracture strength ratio) is 0.70. The above was judged as acceptable. The results are shown in Table 2 below together with the structures after quenching and tempering. In Table 2, “Delayed Fracture Strength Ratio 1” indicates the result of evaluation of delayed fracture resistance using a test piece without a notch, and “Delayed Fracture Strength Ratio 2” indicates a test piece with a notch. It shows the results of evaluating delayed fracture resistance. In addition, about the example where predetermined | prescribed tensile strength (tensile strength of 1100 Mpa or more) was not acquired, the test of corrosion resistance and delayed fracture resistance was not implemented. Further, based on these results, the relationship between the value of the ratio ([Si] / [C]) and the delayed fracture strength ratio 2 is shown in FIG.

From these results, we can consider as follows. Test No. 1 to 14 are examples (the steel of the present invention) that satisfy the requirements [chemical composition and ratio ([Si] / [C])] defined in the present invention, and exhibit excellent delayed fracture resistance along with high strength. You can see that it is demonstrating.

In contrast, test no. No. 15 has insufficient C content, so that it cannot secure a tensile strength of 1100 MPa or more by ordinary heat treatment.

Test No. No. 16, since the C content is excessive, the ratio of delayed fracture strength is reduced due to a decrease in ductility (when using a test piece without a notch, a result excellent in delayed fracture resistance is obtained. However, when a notched specimen is used, the delayed fracture resistance is remarkably inferior).

Test No. In No. 17, since the Si content is insufficient, the ratio of [Si] / [C] is less than 1.0, and a tensile strength of 1100 MPa or more cannot be secured by ordinary heat treatment.

Test No. No. 18, although the content of each element is satisfactory, since the ratio of [Si] / [C] is less than 1.0, the corrosion resistance of the steel material deteriorates and the delayed fracture strength ratio (particularly the delayed fracture strength ratio) 2) has fallen.

Test No. No. 19 has an insufficient Mn content, and a tensile strength of 1100 MPa or more cannot be secured under normal heat treatment conditions.

Test No. In No. 20, since the Mn content is excessive, the grain boundary strength is reduced due to segregation, and the delayed fracture resistance is deteriorated (delayed fracture strength ratio 2 is 0.32).

Test No. Since No. 21 does not contain Ni, the corrosion resistance is deteriorated and the delayed fracture resistance is low (the delayed fracture strength ratio 1 is 0.64 and the delayed fracture strength ratio 2 is 0.33).

Test No. No. 22 does not contain essential Ni and Cr, and therefore corrosion resistance deteriorates and delayed fracture resistance is low (delayed fracture strength ratio 1 is 0.62, delayed fracture strength ratio 2 is 0.2. 39).

Test No. Since No. 23 does not contain essential Cr, corrosion resistance deteriorates and delayed fracture resistance is low (delayed fracture strength ratio 2 is 0.41).

Test No. In No. 24, since the total content of Cu, Ni and Cr is less than 0.10%, the corrosion resistance is not sufficient, and the delayed fracture resistance is low (the delayed fracture strength ratio 2 is 0.62).

Also, test No. in Table 2 Among 15 to 24, the delayed fracture strength ratio 1 of those evaluated for delayed fracture resistance is 0.70 or more, but the delayed fracture strength ratio 2 is often less than 0.70. From this, even when it is evaluated that it is excellent in delayed fracture resistance when a test piece without a notch is used for evaluation of delayed fracture resistance as in Patent Document 6, the present invention Thus, when a test piece having a notch is used assuming a bolt, it can be said that the delayed fracture resistance may be inferior. That is, it can be said that the delayed fracture resistance of the bolt steel of the present invention is evaluated under more severe conditions.

Fig. 3 shows test no. 1 to 14 (examples of the present invention) and Test No. 15 is a graph showing the values of corrosion weight loss and delayed fracture strength ratio 2 in an example (comparative example) in which corrosion resistance and delayed fracture resistance tests were carried out among 15-24. From FIG. 3, it can be seen that the inventive example has a smaller corrosion weight loss than the comparative example and has a high delayed fracture strength ratio 2 measured using a notched test piece, that is, excellent delayed fracture resistance. Recognize.

Claims

C: 0.20 to less than 0.40% (% means mass%, the same shall apply hereinafter), Si: 0.20 to 1.50%, Mn: 0.30 to 2.0%, P: 0.03 % Or less (excluding 0%), S: 0.03% or less (not including 0%), Ni: 0.05 to 1.0%, Cr: 0.01 to 1.50%, Cu: 1 0.0% or less (including 0%), Al: 0.01 to 0.10%, Ti: 0.01 to 0.1%, B: 0.0003 to 0.0050%, and N: 0.002 to In addition to each containing 0.010%, one or more selected from the group consisting of Cu, Ni and Cr is contained in a total of 0.10 to 3.0%, the balance being iron and inevitable impurities, and Si The ratio of [Si] to C content [C] ([Si] / [C]) is 1.0 or more, and the structure is a composite of ferrite and pearlite. Delayed fracture resistance excellent boron-added high-strength bolts for steel, which is a tissue.
The boron-added high-strength bolt according to claim 1, further comprising at least one selected from the group consisting of Nb: 0.01 to 0.1% and V: 0.01 to 0.1%. Steel.
C: 0.20 to less than 0.40%, Si: 0.20 to 1.50%, Mn: 0.30 to 2.0%, P: 0.03% or less (excluding 0%), S : 0.03% or less (excluding 0%), Ni: 0.10 to 0.50%, Cr: 0.03 to 0.80%, Cu: 0.50% or less (including 0%), Al: 0.01 to 0.10%, Ti: 0.01 to 0.1%, B: 0.0003 to 0.0050%, and N: 0.002 to 0.010%, respectively, Cu , Ni and Cr are contained in a total of 0.15 to 1.5%, the balance is composed of iron and inevitable impurities, and the Si content [Si] and the C content The ratio of [C] ([Si] / [C]) is 1.0 or more, and the structure is a composite structure of ferrite and pearlite. Delayed fracture resistance excellent boron-added high-strength bolts for steel.
The boron-added high-strength bolt according to claim 3, further comprising at least one selected from the group consisting of Nb: 0.01 to 0.1% and V: 0.01 to 0.1%. Steel.
C: 0.20 to less than 0.40%, Si: 0.20 to 1.50%, Mn: 0.30 to 1.5%, P: 0.03% or less (excluding 0%), S : 0.03% or less (excluding 0%), Ni: 0.10 to 0.50%, Cr: 0.03 to 0.80%, Cu: 0.50% or less (including 0%), In addition to containing Al: 0.02-0.05%, Ti: 0.02-0.06%, B: 0.0010-0.003% and N: 0.003-0.006%, respectively, Cu , Ni and Cr are contained in a total of 0.15 to 1.5%, the balance is composed of iron and inevitable impurities, and the Si content [Si] and the C content The ratio of [C] ([Si] / [C]) is 1.0 or more, and the structure is a composite structure of ferrite and pearlite. Delayed fracture resistance excellent boron-added high-strength bolts for steel.
The boron-added high-strength bolt according to claim 5, further comprising at least one selected from the group consisting of Nb: 0.01 to 0.1% and V: 0.01 to 0.1%. Steel.
The ratio ([Si] / [C]) of the Si content [Si] and the C content [C] is:
(A) When the amount of C is 0.20% or more and less than 0.25%, it is 2.0 or more,
(B) When the amount of C is 0.25% or more and less than 0.29%, it is 1.5 or more,
(C) The boron-added high-strength bolt steel according to any one of claims 1 to 6, which is 1.0 or more when the C content is 0.29% or more and less than 0.40%.
Using the boron-added high-strength bolt steel according to any one of claims 1 to 6, after forming into a bolt shape, quenching and tempering treatment are performed, and the tensile strength when the structure is tempered martensite is high. A high-strength bolt excellent in delayed fracture resistance, characterized by being 1100 MPa or more.