JP5608145B2 - Boron-added steel for high strength bolts and high strength bolts with excellent delayed fracture resistance - Google Patents

Description

  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, but standard steels such as SCM are still frequently used for bolts with higher strength. Since a large amount of alloy elements such as Cr and Mo are added to the SCM standard steel, the demand for SCM alternative steel with reduced Cr and Mo is increasing with the demand for reducing the steel material cost. However, it is difficult to ensure the strength simply by reducing the alloy elements.

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

  Various techniques for improving delayed fracture resistance have been proposed so far. For example, Patent Document 1 proposes a technique for suppressing entry of hydrogen into steel by containing a predetermined amount of Cu in boron-added steel. However, it is difficult to ensure corrosion resistance only by containing Cu.

  Further, Patent Document 2 and Patent Document 3 attempt to improve delayed fracture resistance by crystal grain refinement, but it is difficult to apply to a more severe environment only by the effect of crystal grain refinement.

  Patent Document 4 relates to a method for evaluating delayed fracture and a steel material excellent in delayed fracture resistance. As a steel material, a carbon equivalent and an addition amount of S are specified, but it is delayed only by achieving extremely low sulfur. It is difficult to completely suppress the occurrence of destruction, and it is necessary to extremely reduce sulfur, which may increase the production cost.

  On the other hand, Patent Document 5 is a technique related to a wear-resistant steel material excellent in toughness and delayed fracture resistance, but it is necessary to perform quenching as it is after rolling and to stop quenching in a specific temperature range. It may be complicated and increase the manufacturing cost.

  Patent Document 6 relates to a wear-resistant steel material excellent in toughness and delayed fracture resistance, and a method for producing the same. However, since it is necessary to process after quenching and tempering, it is difficult to form a bolt when diverted to a bolt. . Furthermore, the steel material which is the object in Patent Document 6 is plate-like, and the delayed fracture resistance is evaluated using a flat plate without a notch, but the bolt which is the object of the present invention has a notch. Therefore, it is necessary to evaluate the delayed fracture resistance based on a stricter standard than the delayed fracture resistance shown in Patent Document 6.

  All of the technologies proposed so far for improving delayed fracture resistance have problems in strength, delayed fracture resistance in harsh environments, and manufacturing.

JP 2006-118003 A Japanese Patent No. 3535754 Japanese Patent No. 3490293 Japanese Patent No. 4370991 JP 2002-80930 A Japanese Patent Application Laid-Open No. 2002-115024

  The present invention has been made under such circumstances, and its purpose is to have a high strength of 1100 MPa or more without adding a large amount of expensive alloy elements such as Cr and Mo. Another object is to provide a boron-added high-strength bolt steel excellent in 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 according to the present invention that has achieved the above object is C: 0.20 to less than 0.40% (meaning mass%, the same applies hereinafter), Si: 0.20 to 1. 50%, Mn: 0.30 to 2.0%, P: 0.03% or less (not including 0%), S: 0.03% or less (not including 0%), 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 one or more selected from the group consisting of Cu, Ni and Cr in total 0.10 3.0% content, the balance being iron and inevitable impurities, and the ratio of Si content [Si] to C content [C] ([Si] / [C] Together but not less than 1.0, and has a gist in that a ferrite-pearlite structure.

  In the steel for boron-added high-strength bolts of the present invention, it is effective to further contain Nb: 0.01 to 0.1% and / or V: 0.01 to 0.1%, if necessary. The inclusion of these further improves the properties of the boron-added high-strength bolt steel.

  On the other hand, the high-strength bolt of the present invention that has achieved the above-mentioned object is a steel material as described above (steel for boron-added high-strength bolts), and after forming into a bolt shape, quenching and tempering are performed. It has a gist in that the tensile strength when the structure is tempered martensite is 1100 MPa or more.

  In the present invention, the chemical composition is strictly defined, and the ratio of the Si and C content ([Si] / [C]) is controlled within an appropriate range, so that it is excellent even in harsh environments. Boron-added high-strength bolt steel that exhibits delayed fracture resistance can be realized. By using such a steel material, high-strength bolts with excellent delayed fracture resistance can be realized.

It is explanatory drawing which shows the external appearance shape of the test piece used by this invention. 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 have conducted extensive research on boron-added steel that exhibits excellent delayed fracture resistance even at a high strength of 1100 MPa or higher without adding a large amount of expensive alloy elements such as Mo and Cr. . 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 is very effective in securing delayed fracture resistance than containing alloy elements. Reducing C leads to insufficient strength, but by making the Si content equal to or higher than the C content [ie, the ratio of Si and C content ([Si] / [C]) 1.0 or more], it was found that the strength reduction by reducing the C content can be sufficiently secured.

  Although the corrosion resistance is improved by reducing the C content, it is effective to control the total content of Cu, Ni, Cr, etc. in order to ensure sufficient delayed fracture resistance in harsh environments. Furthermore, by adjusting other chemical components, it was found that a boron-added steel having excellent delayed fracture resistance could be realized even with a tensile strength of 1100 MPa or more, and the present invention was completed. Moreover, 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 amount of Si added rather than the C content, it was possible to achieve both a tensile strength of 1100 MPa or more and excellent delayed fracture resistance under harsh environments.

  That is, if it is attempted to secure 1100 MPa or more only by adding C alone, the corrosion resistance of the steel deteriorates, the amount of hydrogen generated on the steel surface increases, and as a result, the amount of hydrogen entering the steel also increases, resulting in delayed fracture. Is likely to occur. Even if the corrosion resistance is improved by adding an element for improving corrosion resistance such as Cu, Ni, Cr, etc., the corrosion resistance of the matrix is low.

  On the other hand, when C and Si are added in combination, the strength can be increased by Si, so that the C content can be relatively reduced. That is, by reducing the C content of the matrix and securing the strength with Si that does not significantly affect the corrosion resistance of steel, it is excellent in corrosion resistance and delayed fracture resistance, and it is possible to ensure a tensile strength of 1100 MPa or more. It became. 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.

  In the steel for boron-added bolts of the present invention, the ratio of the Si content [Si] (mass%) and the C content [C] (mass%) ([Si] / [C ]) Must be 1.0 or more. As a result, excellent delayed fracture resistance is exhibited. 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, there is a disadvantage that the delayed fracture resistance and other characteristics deteriorate. Arise.

  It is also effective to control the appropriate range of the ratio ([Si] / [C]) according to the C content. Specifically, when (a) C: 0.20 to less than 0.25%, the ratio ([Si] / [C]) is set to 2.0 or more, and (b) C: 0.25 to When the ratio is less than 0.29%, the ratio ([Si] / [C]) is set to 1.5 or more. (C) When C is 0.29% or more (that is, 0.29 to 0.40%). Less) and the ratio ([Si] / [C]) is preferably 1.0 or more.

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

[C: 0.20 to less than 0.40%]
C is an element indispensable for forming carbides and securing tensile strength necessary for 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 acts as a deoxidizer during melting and is an element necessary as a solid solution element for strengthening the matrix. By containing 0.20% or more, sufficient strength can be ensured. However, if Si is contained excessively exceeding 1.50%, the cold workability of the steel material is deteriorated even when spheroidizing annealing is performed, and the grain boundary oxidation in the heat treatment at the time of quenching is promoted. Deteriorating delayed fracture. 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 an excessive amount, it causes segregation at the grain boundary, lowers the grain boundary strength, and deteriorates 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%. In addition, the minimum with preferable Cr content is 0.05%, and a more preferable minimum 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 effective for fixing N in steel and precipitating TiC to improve delayed fracture. In addition, the nitrides and carbides generated above are useful for refining crystal grains, and this can further improve 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%, the 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 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 0.001% 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 is combined with Ti to form TiN in the solidification stage after melting, and refines crystal grains to improve delayed fracture resistance. Such an effect is effectively exhibited when the N content is 0.002% or more. However, when a large amount of TiN is formed, it is not dissolved by heating at about 1300 ° C. and inhibits the formation of Ti carbide. Further, excessive N becomes harmful to the delayed fracture characteristics, and particularly when the content exceeds 0.010%, the delayed fracture characteristics are remarkably lowered. 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 the total addition amount thereof 0.10% or more, hydrogen penetration into the steel can be suppressed and delayed fracture resistance can be improved. However, if these elements are excessive, the steel material cost is increased, 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 in the steel for boron-added high-strength bolts of the present invention are as described above, and the balance is composed of iron and inevitable impurities.

  In addition, the boron-added high-strength bolt steel according to the present invention is effective to contain Nb and V, if necessary, in addition to the above components. Is as follows.

[Nb: 0.01 to 0.1% and / or 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 effects, it is preferable to contain Nb or V in an amount of 0.01% or more. However, if these elements are contained excessively, the strength of the material as it is hot-rolled becomes higher than necessary, and the cost of the steel material increases, so the upper limit was made 0.1%. 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.

  The steel for boron-added high-strength bolts having the above chemical composition is basically a mixed structure of ferrite and pearlite (indicated as “ferrite / pearlite”) after rolling, but this steel can be spheroidized if necessary. With or without processing, after forming into a bolt shape, quenching and tempering are performed to make the structure tempered martensite, thereby ensuring a predetermined tensile strength and excellent delay resistance. It will be destructive. 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 in order to stably perform the austenitizing 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 crystal grain coarsening, it is useful to heat and quench at 960 ° C. or lower.

  The as-quenched bolt has low toughness and ductility, and does not become a bolt product as it is, so it needs to be tempered. For this purpose, it is effective to perform a tempering treatment at a temperature of at least 200 ° C. When this temperature exceeds 600 ° C., the steel material having the above chemical composition cannot secure a tensile strength of 1100 MPa or more. Further, the prior austenite grain size is preferable because the delayed fracture resistance is improved as the grain size is reduced. In order to exert such an effect, the grain size number (JIS G 0551) is preferably 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.

  Steel materials (test Nos. 1 to 24) having the chemical composition shown in Table 1 below were melted and then rolled to obtain a wire rod 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. Furthermore, the delayed fracture resistance was evaluated using the test piece of FIG. In the tensile test, a test piece without a shaft notch was used, and in the delayed fracture test, a test piece with a notch was used so as to simulate the stress concentration of the screw part.

  Corrosion resistance was evaluated by corrosion weight loss before and after immersion of a test piece 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, it can be considered as follows. Test No. Examples 1 to 14 are examples (the steel of the present invention) that satisfy the requirements [chemical component composition and ratio ([Si] / [C])] defined in the present invention, and have high strength and excellent delayed fracture resistance. It can be seen that it is exhibiting sex.

  In contrast, test no. In the case of No. 15, the C content is low, so that a tensile strength of 1100 MPa or more cannot be secured by ordinary heat treatment.

  Test No. No. 16 has a higher C content, so the ratio of delayed fracture strength is reduced due to a decrease in ductility (the test piece without notch is excellent in delayed fracture resistance, but the notched test The piece is inferior in delayed fracture resistance).

  Test No. In No. 17, since the Si content is small, 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. In the case of No. 18, although the content of each element is satisfactory, the ratio of [Si] / [C] is less than 1.0, so the corrosion resistance of the steel material deteriorates and the delayed fracture strength ratio (particularly delayed fracture) The intensity ratio 2) is reduced.

  Test No. No. 19 has a low 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. No. 21 has no added Ni, and 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 the corrosion resistance is deteriorated and the delayed fracture resistance is low (the delayed fracture strength ratio 1 is 0.62 and the delayed fracture strength ratio 2 is 0.39).

  Test No. Since No. 23 does not contain essential Cr, corrosion resistance is deteriorated 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). .

  In addition, test No. Among those evaluated for delayed fracture resistance among 15 to 24, delayed fracture strength ratio 1 is 0.70 or more, but delayed fracture strength ratio 2 is often less than 0.70. From this, when a test piece without a notch is used for evaluation of delayed fracture resistance as in Patent Document 6, even if it is evaluated that the delayed fracture resistance is excellent, the present invention When a specimen 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 as in the present invention is a stricter evaluation.

  FIG. 1-14 (invention example) and test no. The value of corrosion weight loss and delayed fracture strength ratio 2 of the example (comparative example) which carried out the test of corrosion resistance and delayed fracture resistance among 15-24 is graphed. 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 (3)

  C: Less than 0.20 to 0.40% (meaning mass%, the same applies hereinafter), Si: 0.20 to 1.50%, Mn: 0.30 to 2.0%, P: 0.03% or less (Not including 0%), S: 0.03% or less (not including 0%), Ni: 0.05 to 1.0%, Cr: 0.01 to 1.50%, Cu: 1.0 % Or less (including 0%), Al: 0.01-0.10%, Ti: 0.01-0.1%, B: 0.0003-0.0050% and N: 0.002-0. In addition to each containing 010%, one or more selected from the group consisting of Cu, Ni and Cr is contained in total of 0.10 to 3.0%, the balance is made of iron and inevitable impurities, and Si is contained The ratio ([Si] / [C]) of the amount [Si] and the content [C] of C is 1.0 or more and is a ferrite pearlite structure Delayed fracture resistance excellent boron-added high-strength bolts for steel.   Furthermore, the steel for boron addition high strength bolts of Claim 1 which contains Nb: 0.01-0.1% and / or V: 0.01-0.1%.   Using the boron-added high-strength bolt steel according to claim 1 or 2, after forming into a bolt shape, quenching and tempering treatment are performed, and the tensile strength when the structure is tempered martensite is 1100 MPa or more. A high-strength bolt excellent in delayed fracture resistance, characterized by being.