JP2006342388A - High strength steel with excellent delayed fracture resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high strength steel having ≥1,200 N/mm<SP>2</SP>tensile strength and being superior in delayed fracture resistance to the conventional one. <P>SOLUTION: The high strength steel with excellent delayed fracture resistance is a high strength steel with ≥1,200 N/mm<SP>2</SP>tensile strength and has a composition containing, by mass, 0.3 to 0.5% C, 0.5 to 2.5% Cr, 0.2 to 1.0% Mn, 0 to 0.01% N, 0.0005 to 0.005% O, 0.001 to 0.025% S, 0 to 2.0% Si and 0.05 to 1.5%, in total, of V and/or Mo and also containing either or both of B and Li within the range satisfying the following inequalities (1) to (4): (1) 0%≤(B content)≤0.01%; (2) 0%≤(Li content)≤0.05%; (3) 0%<(B content +Li content); (4) (V content)+(Mo content)/5≤[(B content)+(Li content)]×300. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention belongs to a technical field relating to high strength steel having excellent delayed fracture resistance, and more specifically, tensile strength of 1200 N / mm ² or more and high strength excellent in delayed fracture resistance. The present invention belongs to a technical field related to steel, and particularly belongs to a technical field related to high-strength steel used for machine structural applications such as structures / transport equipment. The high-strength steel according to the present invention includes steel processed products such as bolts as well as steel before product processing.

In the field of high-strength steel, improving delayed fracture resistance is an eternal solution. Delayed fracture is a time-dependent fracture phenomenon in which, for example, in the case of steel for bolts exhibiting a tensile strength of 1200 N / mm ² or more, the threaded portion and the bolt neck lower portion are brittlely fractured during use. Delayed fracture resistance is a characteristic relating to resistance to delayed fracture, that is, difficulty in causing delayed fracture.

  Various studies have been made to improve the delayed fracture resistance. Recently, hydrogen (diffusible hydrogen), which can enter the steel from the outside and move around in the steel, is concentrated in the steel over time. Attempts have been made to capture this diffusible hydrogen and prevent steel from becoming brittle. For example, Japanese Patent Application Laid-Open No. 2003-105485 (Patent Document 1) discloses a technique for trapping hydrogen and preventing fatigue failure by precipitating carbonitride in a layered structure of tempered martensite and ferrite. ing.

On the other hand, the present inventors have also studied hydrogen trapping means. In Japanese Patent Laid-Open No. 2003-253376 (Patent Document 2), the trapping action of V and Mo carbonitrides is weak, so that trapping is performed on V and Mo carbonitrides in an environment where a thermal load or dynamic stress load is applied. Since the released hydrogen has a risk of increasing delayed fracture, trapping hydrogen with Ti, Zr, Hf, and Nb carbonitrides as carbonitrides to trap more strongly further suppresses delayed fracture We suggest what we can do.
JP 2003-105485 A JP 2003-253376 A

  The inventors of the present invention have further investigated high-strength steel, and found that delayed fracture is caused by the concentration of defects resulting in the concentration of hydrogen, and as described above, hydrogen is generated from carbonitride. Even if it was released, if we could achieve a method to prevent the concentration of defects, we thought that this method could also suppress delayed fracture.

  On the other hand, conventionally, the delayed fracture resistance has been improved by adding an element for improving corrosion resistance such as Ni or Cr. However, the present inventors have found that there is a strong correlation between corrosion resistance and delayed fracture resistance, but it has been found that improving corrosion resistance does not necessarily improve delayed fracture resistance, that is, improving corrosion resistance, We thought that it was more important to suppress the entry of hydrogen than to suppress the generation of hydrogen.

  The present invention has been made paying attention to such circumstances, and an object thereof is to provide a high-strength steel superior in delayed fracture resistance to conventional high-strength steel.

  In order to achieve the above object, the present inventors have intensively studied, and as a result, completed the present invention. According to the present invention, the above object can be achieved.

  The present invention thus completed and capable of achieving the above object relates to a high-strength steel excellent in delayed fracture resistance, and is excellent in delayed fracture resistance according to claims 1 to 6. High strength steel (high strength steel excellent in delayed fracture resistance according to the first to sixth inventions), the high strength bolt according to claim 7 (high strength bolt according to the seventh invention), which is as follows: It is a configuration.

That is, the high-strength steel excellent in delayed fracture resistance according to claim 1 is a high-strength steel having a tensile strength of 1200 N / mm ² or more, and C: 0.3 to 0.5 mass%, Cr: 0 0.5 to 2.5 mass%, Mn: 0.2 to 1.0 mass%, N: 0.01 mass% or less (including 0 mass%), O: 0.0005 to 0.005 mass%, S : 0.001 to 0.025 mass%, Si: 2.0 mass% or less (including 0 mass%), and V and / or Mo in total 0.05 to 1.5 mass% It is a high strength steel excellent in delayed fracture resistance characterized by containing one or more of B, Li so as to satisfy the following formulas (1), (2), (3) and (4) [First invention].
0% by mass ≦ B amount ≦ 0.01% by mass ---------------------- Formula (1)
0% by mass ≦ Li content ≦ 0.05% by mass
0% by mass <B amount + Li amount ------------------------------ Equation (3)
V amount + Mo amount / 5 ≦ (B amount + Li amount) × 300 --------- Equation (4)

  The high strength steel excellent in delayed fracture resistance according to claim 2, wherein at least one of the B and Li is B, and the B concentration on the steel surface is concentrated to 10 times or more of the B concentration in the steel. The high strength steel excellent in delayed fracture resistance according to claim 1 [second invention].

  In the high strength steel excellent in delayed fracture resistance according to claim 3, at least one of the B and Li is B and Li, and the B concentration on the steel surface is 10 times or more the B concentration in the steel. The high-strength steel excellent in delayed fracture resistance according to claim 1, which is concentrated [third invention].

  The high-strength steel excellent in delayed fracture resistance according to claim 4 contains Al: 0.05 to 0.5% by mass, and has high delayed fracture resistance according to any one of claims 1 to 3. It is a strength steel [fourth invention].

  The high-strength steel excellent in delayed fracture resistance according to claim 5 contains at least 0.03 to 0.5 mass% of at least one of Ti, Zr, Hf, and Nb. The high strength steel having excellent delayed fracture resistance as described in [5th invention].

  The high-strength steel excellent in delayed fracture resistance according to claim 6 contains Ni: 2.0% by mass or less and / or Cu: 1.0% by mass or less. This is a high-strength steel excellent in delayed fracture resistance [Sixth Invention].

  A high-strength bolt according to claim 7 is a high-strength bolt comprising the high-strength steel excellent in delayed fracture resistance according to any one of claims 1 to 6 [seventh invention].

  The high strength steel excellent in delayed fracture resistance according to the present invention is more excellent in delayed fracture resistance than the conventional high strength steel. For this reason, it can be suitably used as a structural member such as a structure or a transportation device, or a structural member of a machine, and there is an effect that the durability can be improved by improving the delayed fracture resistance.

  V and Mo not only ensure the hardenability of steel, but also form fine carbonitrides in the steel and suppress the diffusion of hydrogen, which causes delayed fracture. is there. However, the present inventors do not contain Ti, Zr, Hf, and Nb, and in steel added with 1.2% of V and Mo in total, hydrogen is occluded so that hydrogen does not escape from the surface. Using a test piece that has been subjected to prevention plating and left to stand for a certain period of time to make the hydrogen concentration uniform, a delayed fracture test was performed as it was, and using the test piece, stress was applied in advance to such an extent that fatigue did not occur. Comparing with the case where the delayed fracture test was conducted after the stress was cycled within a certain range of the elastic region, it was found that the latter test progressed more in the latter test. This phenomenon also occurs when a heat load is applied in advance. However, this phenomenon does not occur in test pieces that do not occlude hydrogen. Further, it was found that this phenomenon is moderated in steel containing Ti, Zr, Hf and Nb, and based on this, the invention described in Japanese Patent Application Laid-Open No. 2003-253376 was made. In other words, while the hydrogen is occluded, the stress is cycled and the defect is forcibly moved to release the hydrogen trapped weakly at the strain defect part of V or Mo, so that delayed fracture is likely to occur. If Ti, etc. are present, they are re-trapped and can be suppressed. On the one hand, this is considered to suggest that if the movement of the defect is suppressed, the promotion of delayed fracture can be suppressed.

  As a result of further studies by the present inventors, it was found that the promotion of delayed fracture in the above test can be suppressed by adding B or Li to the steel and making it exist in a solid solution state. That is, by allowing B and Li to exist in a solid solution state, it succeeded in suppressing the movement of defects. Thus, the present invention has been completed.

  Hereinafter, the effect and the reason for limiting the numerical values of the high strength steel according to the present invention will be described in detail.

In the high-strength steel according to the present invention, the tensile strength is 1200 N / mm ² or more. This is because if the tensile strength is not 1200 N / mm ² or more, the required characteristics for recent high-strength steel cannot be satisfied. Furthermore, generally, when the tensile strength is generally 1200 N / mm ² or more, delayed fracture is likely to occur, and it is desired to improve the delayed fracture resistance in the case of this strength. In the present invention, carbides, nitrides, and the like are solid-dissolved by quenching at a temperature of 800 to 1200 ° C., and then tempering is performed at a temperature of 450 to 700 ° C. to sufficiently perform precipitation hardening. It is desirable to ensure strength. The point is that the B compound is completely dissolved during quenching, and impurities such as nitrogen and oxygen are deposited as precipitates that are hardened during tempering to prevent precipitation of B nitride (BN). The temperature condition varies greatly depending on the steel type composition, the steel wire diameter, and the like, so it is necessary to perform optimization, and is not particularly defined in the present invention. Even if this strength requirement is not satisfied, application of the present invention is not hindered.

  The high-strength steel according to the present invention contains C, Cr, Mn, N (may not be included), O, S, Si (may not be included) as composition components, and V and / or Mo. And at least one of B and Li.

  Among the above components, C is an essential element for enhancing the hardenability of steel and ensuring high strength. Moreover, C also has the effect | action which makes V, Mo, Ti, etc. exist stably in steel as a carbide | carbonized_material or a composite compound. C must be contained in an amount of 0.3% by mass (hereinafter, mass% is also referred to as%), and 0.35% or more must be contained in order to ensure the strength and manifest the carbide forming effect. Is desirable. However, if the amount of C is too large, not only does the toughness deteriorate and delayed fracture resistance decreases, but the cold workability tends to deteriorate, so the C amount needs to be suppressed to 0.50% or less. Yes, it is desirable to suppress to 0.45% or less. Therefore, in the high-strength steel according to the present invention, C: 0.3 to 0.5%. Desirably, C: 0.35 to 0.45%.

  Cr can increase the strength at a low cost, and has the effect of increasing the corrosion resistance and slowing the corrosion rate. In order to express these effects, addition of 0.5% or more is required. A more preferable lower limit of the Cr content is 1.0%. On the other hand, when the addition amount of Cr increased, a tendency to easily form corrosion pits was recognized. This is because Cr increases the corrosion resistance of the steel as a whole, but also has the effect of lowering the pH at the tip of the corrosion pit. Once the corrosion pit is formed, the pit is expanded in the pit depth direction. It is thought that it will end up. In order to suppress the formation of the corrosion pits and reduce the adverse effect on delayed fracture resistance, it is necessary to suppress the Cr content to 2.5% or less, and desirably to 2.0% or less. Therefore, in the high-strength steel according to the present invention, Cr: 0.5 to 2.5%. Desirably, Cr is 1.0 to 2.0%.

  Mn has an effect of improving the strength. In order to exhibit this effect, it is necessary to add 0.2% or more, and it is desirable to add 0.5% or more. On the other hand, when the addition amount of Mn increased, the tendency to form a corrosion pit easily was recognized like the case of Cr. In order to suppress the formation of the corrosion pits and reduce the adverse effect on delayed fracture resistance, it is necessary to suppress the amount of Mn to 1.0% or less, and it is desirable to suppress it to 0.8% or less. Therefore, in the high-strength steel according to the present invention, Mn is 0.2 to 1.0%. Desirably, Mn is 0.5 to 0.8%.

  N, O, and S have the effect of precipitating nitrides such as V, Mo, and Ti, oxides, sulfides, and complex compounds thereof in steel. These compounds trap diffusible hydrogen and suppress embrittlement of the steel. However, if the amounts of N, O, and S are too large, the precipitates become coarse, so the amounts of N, O, and S need to be smaller than in the case of ordinary steel. In particular, N is liable to bind to B to form BN, and inhibits the solid solution of B. Therefore, it is necessary to make the amount smaller. Quantitatively, it is as follows.

  N forms nitrides and can stably finely disperse these as precipitates. This precipitate (nitride) has an action of trapping diffusible hydrogen (hereinafter also referred to as a hydrogen trap action). That is, there is an effect that precipitates (nitrides) having a hydrogen trapping action can be finely dispersed to improve the hydrogen trapping action. However, N easily forms BN and inhibits the solid solution of B. Therefore, N is required to be 0.010% or less, and is preferably 0.007% or less. Nitride is not necessarily formed, and therefore N may not be contained (N may be 0%). Therefore, in the high-strength steel according to the present invention, N: 0.01% or less (including 0%). Desirably, N is 0.007% or less.

  O forms oxides and can stably finely disperse these as precipitates. This precipitate (oxide) has a hydrogen trap action. In other words, precipitates (oxides) having a hydrogen trapping action are finely dispersed to be present, thereby improving the hydrogen trapping action. In order to exhibit this effect, O needs to be contained in an amount of 0.0005% or more, and is preferably contained in an amount of 0.001% or more. However, if the amount of O becomes excessive, coarse oxides are likely to precipitate, so it is necessary to make the content 0.005% or less, and it is desirable to make it 0.003% or less. Therefore, in the high-strength steel according to the present invention, O: 0.0005 to 0.005%. Desirably, O: 0.001 to 0.003%.

  S forms sulfides, and these can be finely dispersed stably as precipitates. This precipitate (sulfide) has a hydrogen trap action. In other words, precipitates (sulfides) having a hydrogen trapping action are finely dispersed to be present, thereby improving the hydrogen trapping action. In order to exhibit this effect, S must be contained in an amount of 0.001% or more, and is preferably contained in an amount of 0.003% or more. However, if the amount of S becomes excessive, coarse MnS or the like having a weak hydrogen trap action is precipitated, so it is necessary to make it 0.025% or less, and it is desirable to make it 0.015% or less. Therefore, in the high-strength steel according to the present invention, S: 0.001 to 0.025%. Desirably, S: 0.003 to 0.015%.

  Si may be included, but if the content is too large, coarse Si-based precipitates are formed, and the toughness and delayed fracture resistance deteriorate, so the Si content is 2.0% or less. There is a need. Therefore, in the high-strength steel according to the present invention, Si is 2.0% or less (including 0%).

  V and Mo form carbides, nitrides, oxides, sulfides, and complex compounds thereof in steel finely and have a function of suppressing hydrogen diffusion by trapping hydrogen. For that purpose, it is necessary to contain 0.05% or more of V or Mo, or to contain 0.05% or more of V and Mo in total. About Mo, there is also an effect which promotes solid solution of B by the addition. However, if there is too much content of these elements, there is a possibility that a large amount of hydrogen is released due to thermal fluctuations and stress fluctuations that occur during use as a component, etc., and this may promote delayed fracture. It is necessary to make it below, and it is desirable to make it 1.0% or less. Therefore, in the high strength steel according to the present invention, V and / or Mo: 0.05 to 1.5% in total. Desirably, V and / or Mo: 0.05 to 1.0% in total.

B and Li are elements that can stabilize defects (hereinafter referred to as defect stabilizing elements), and have an effect of suppressing the movement of defects and, consequently, the promotion of delayed fracture. . In order to exhibit this effect, it is necessary to contain one or more of B and Li so as to satisfy the following formulas (1), (2), (3) and (4). Therefore, the high-strength steel according to the present invention contains at least one of B and Li so as to satisfy the following formulas (1), (2), (3) and (4). Details will be described below.
0% by mass ≦ B amount ≦ 0.01% by mass ---------------------- Formula (1)
0% by mass ≦ Li content ≦ 0.05% by mass
0% by mass <B amount + Li amount ------------------------------ Equation (3)
V amount + Mo amount / 5 ≦ (B amount + Li amount) × 300 --------- Equation (4)

  B and Li are defect stabilizing elements as described above. If these elements are contained in the steel, they tend to stay in the defects, and the movement of the defects can be suppressed. In addition, it is considered that the defect is moved by the stress load and prevents further movement of the defect. As a result, it is considered that the concentration of defects in the stress concentration portion can be prevented, and thus weakening of steel, which directly causes delayed fracture, can be suppressed. This effect is especially effective in the case of high-strength steel used in mechanical structural applications where delayed fracture resistance is a problem, especially in applications where repeated thermal and dynamic stress loads such as car engine peripherals are applied. Usefulness increases.

  As described above, these elements (B, Li) are trapped in the defect to suppress the movement of the defect, and the effect of stabilizing the defect increases as the addition amount increases, but the addition amount is too large. And trapped by the strain defects around V and Mo, the hydrogen trap ability of the strain defects around V and Mo is reduced, and the effect of suppressing hydrogen diffusion is hindered. Moreover, when it mix | blends abundantly, in order to form a precipitate in steel, it becomes a factor of stress concentration, and also the hydrogen concentration is caused by the distortion | strain by a precipitate, and delayed fracture resistance will fall. Further, since B has a low solid solubility in steel, the amount remaining in the steel is small even if a large amount is added to the molten steel. From these points, the B amount needs to be 0.01% or less (that is, an amount satisfying the formula (1)), and is preferably 0.005% or less. Further, the Li amount needs to be 0.05% or less (that is, an amount that satisfies the formula (2)), and is preferably 0.03% or less. Therefore, regarding the B amount and the Li amount, first, the B amount is an amount that satisfies the equation (1), and the Li amount is an amount that satisfies the equation (2). A desirable amount of B is 0.005% or less, and a desirable amount of Li is 0.03% or less. As will be described later, the amount of each element (B amount, Li amount) is also limited by the equation (4), but in order to prevent the above inconvenience more reliably, each element determined by the equation (4) is used. It is desirable that it be within + 0.002% of the minimum required amount of element (that is, the amount of B and the amount of Li when the value on the left side = the value on the right side in Equation (4)).

  The required amounts of defect stabilizing elements B and Li are not determined by themselves, but are determined according to the amounts of V and Mo that facilitate the formation of unstable defects. That is, it is necessary to change the B amount and the Li amount according to the amounts of V and Mo. As a result of studying the necessary amounts of B and Li, it was found that the formula (4) should be satisfied. Therefore, the required amounts of B and Li are assumed to satisfy the formula (4).

  That is, in the left side of the above formula (4), if the adverse effect level of V is 1, Mo is about 1/5 of the adverse effect, so the Mo amount is divided by 5 and added to the V amount. On the other hand, on the right side, B and Li are at the same level in terms of defect stabilization effect, so these are added as they are. Then, it is necessary to adjust the amount of each element so that 300 times the value on the right side is larger than the value on the left side. Thereby, the hydrogen diffusion suppression effect and hardenability derived from V and Mo, and the defect stabilization effect derived from B and Li can be exhibited in a balanced manner. When the value on the left side is larger than the value on the right side, the defect stabilization effect is not sufficiently exhibited, so that the delayed fracture resistance is reduced in applications where thermal load and dynamic stress load are repeatedly applied. Therefore, the required amounts of B and Li are assumed to satisfy the formula (4). B and Li may contain both of them, or may contain either of them.

  Accordingly, in the high-strength steel according to the present invention, for B and Li, one or more of B and Li are contained so as to satisfy the above formulas (1), (2), (3) and (4). It is said.

  B and Li may contain both of them, or may contain either of them, and therefore, in the formula (1), the case where the amount of B = 0% (that is, B is not contained) is included. However, in the formula (2), the case where the amount of Li = 0% (that is, Li is not contained) is included.

  When the amount of B is 0% in the formula (1) and the amount of Li is 0% in the formula (2), neither B nor Li is contained. This is the case in the high-strength steel according to the present invention. It becomes inconsistent with containing 1 or more types of B and Li. Therefore, it is necessary to satisfy the expression (3). That is, by satisfying this formula (3), the case where neither B nor Li is contained is excluded, and either or both of B and Li are contained.

  In the above formulas (1) to (4), the B content is the B content (mass%), the Li content is the Li content (mass%), the V content is the V content (mass%), and the Mo content. Indicates the Mo content (% by mass).

  As can be seen from the above, the high-strength steel according to the present invention is, in terms of the elements (V, Mo) that form carbonitrides with a large amount of hydrogen trap, and defect stability that suppresses the movement of defects. This is a balance of chemical elements (B, Li). That is, the effect of suppressing the diffusion of hydrogen derived from V and Mo and the effect of stabilizing the defects derived from B and Li can be exhibited in a balanced manner.

  Delayed fracture in steel is considered to be caused by the simultaneous concentration of defects and diffusion of hydrogen, but the high-strength steel according to the present invention has defects caused by defect stabilizing elements (B, Li). Since the migration can be suppressed and the diffusion and concentration of hydrogen can be suppressed by the elements (V, Mo) forming carbonitride, both the behavior of defects and hydrogen can be suppressed. That is, for example, when heat and stress energy are applied, hydrogen is released from defects around the carbonitride, and when hydrogen is diffused, the defect-concentrated part formed in the stress-strained part in the steel where defects tend to concentrate Hydrogen concentrates and causes delayed destruction. However, in the high-strength steel according to the present invention, defects are less likely to be concentrated, so that hydrogen is also less likely to be concentrated, and delayed fracture is unlikely to occur. Therefore, the high strength steel according to the present invention is more excellent in delayed fracture resistance than the conventional high strength steel.

  For this reason, the high-strength steel according to the present invention can be suitably used as a structural member such as a structure or a transportation device or a component of a machine, and the durability can be improved by improving the delayed fracture resistance. become. In particular, it is useful when it is used in applications where thermal loads and dynamic stress loads are repeatedly applied such as car engine peripheral equipment.

B and Li are defect stabilizing elements as described above, but by making them exist in a solid solution state, the defects can be stabilized, the movement of the defects is suppressed, and the delayed fracture is promoted. Can be suppressed. Of these B and Li, Li is easily dissolved, and is easily present in a solid solution state. However, it is known that B is likely to bind to N to form BN or to form Fe ₂₃ (C, B) _{6. When} such a compound is formed, B is solidified. It is difficult to make it exist in a molten state, and the defect stabilization effect by B is significantly reduced. Therefore, when only B is contained in B and Li, it is important that B exists in a solid solution state so that a sufficient defect stabilizing effect can be exhibited.

  Therefore, even when only B is contained in B and Li, intensive studies have been made in order to ensure that a sufficient defect stabilizing effect can be exhibited. As a result, it has been found that when the B concentration on the steel surface is concentrated 10 times or more of the B concentration in the steel, a sufficient defect stabilizing effect can be surely exhibited. When concentrated in this manner, a sufficient defect stabilizing effect can be expressed because the amount of B in a solid solution state in steel is an amount necessary to exhibit a sufficient defect stabilizing effect. It is thought that. That is, the greater the amount of B in the solid solution state in steel, the greater the degree of concentration of B on the steel surface. The greater the degree of concentration of B on the steel surface, the greater the amount of B in steel. When the amount of B in the solid solution state in the steel is large, and the degree of concentration of B concentration on the steel surface is 10 times or more of the B concentration in the steel, B in the solid solution state in the steel This amount is necessary to develop a sufficient defect stabilizing effect, and it is considered that a sufficient defect stabilizing effect can be achieved.

  Therefore, when only B is contained in B and Li, the B concentration on the steel surface is 10 times the B concentration in the steel in order to ensure that a sufficient defect stabilizing effect can be exhibited. What is necessary is just to have thickened above. That is, if the B concentration on the steel surface is 10 times higher than the B concentration in the steel, a sufficient defect stabilizing effect can be surely exhibited, and this promotes delayed fracture. This can suppress the delayed fracture resistance [second invention].

  In addition, when the B concentration on the steel surface is concentrated to 10 times or more of the B concentration in the steel, a barrier effect against hydrogen intrusion is exhibited and hydrogen intrusion is also suppressed. That is, B has the property of concentrating on the steel surface, but this concentration occurs not only on the surface of the steel itself but also on the oxide film formed by heat treatment and the film formed by corrosion, so-called “rust”. . As a result, it was found that the hydrogen reduction reaction was suppressed, the amount of hydrogen generated was suppressed, rust was stabilized, a barrier effect against hydrogen intrusion was exhibited, and hydrogen invasion was also suppressed. In particular, it has been found that when the degree of concentration of B on the steel surface is 10 times or more of the B concentration in the steel, it exerts a barrier effect against hydrogen penetration and suppresses hydrogen penetration. Therefore, if the B concentration on the steel surface is 10 times higher than the B concentration in the steel, it exerts a barrier effect against hydrogen intrusion and suppresses hydrogen intrusion, which also prevents delayed fracture resistance. [Second invention].

  Therefore, when only B is contained in B and Li, if the B concentration on the steel surface is concentrated to 10 times or more of the B concentration in the steel, a sufficient defect stabilizing effect is ensured. Therefore, it is possible to suppress the promotion of delayed fracture, thereby improving delayed fracture resistance, exhibiting a barrier effect against hydrogen intrusion, also suppressing hydrogen intrusion, Since the delayed fracture resistance can be improved, it is possible to ensure a higher level and superior delayed fracture resistance than the conventional high-strength steel [second invention].

  When both B and Li are contained, a defect stabilizing effect is exhibited even by Li in a solid solution state. Therefore, depending on the amount of Li (or the degree of the defect stabilizing effect by Li), solid solution in steel is obtained. The necessary amount of B in the state changes and the necessary amount of B in the solid solution decreases as the amount of Li in the solid solution increases. The greater the amount of B in solid solution, the greater the degree of concentration of B concentration on the steel surface. Therefore, when both B and Li are contained, in order to ensure that a sufficient defect stabilizing effect can be manifested, the B concentration on the steel surface is more than 10 times the B concentration in the steel. It is not always necessary that the B concentration on the steel surface be n times (n <10) the B concentration in the steel, where n is the amount of Li in the solid solution state. The more it is, the smaller it can be.

  Even when both B and Li are contained, when the B concentration on the steel surface is concentrated to 10 times or more of the B concentration in the steel, the barrier effect against the invasion of hydrogen is exhibited, and the invasion of hydrogen. This also suppresses delayed fracture resistance [third invention].

  Therefore, when both B and Li are contained, if the B concentration on the steel surface is concentrated 10 times or more than the B concentration in the steel, a sufficient defect stabilizing effect is surely exhibited. In addition to preventing delayed fracture, this can not only improve delayed fracture resistance, but also exert a barrier effect against hydrogen penetration by concentrating B on the steel surface to suppress hydrogen penetration. This can also improve the delayed fracture resistance, so that it can be more reliably at a higher level and superior in delayed fracture resistance than the conventional high strength steel [third invention].

  For B, segregation at the austenite grain boundary increases the hardenability and also has the effect of shifting the non-recrystallization temperature region to the high temperature side, making it easier to obtain elongated austenite grains. This also has the effect of improving delayed fracture resistance. From the viewpoint of these effects, B needs to exist in a solid solution state.

  It is difficult to confirm the existence form of B, and there are three methods that can be evaluated in the ppm order: (1) α track etching (ATE) method, (2) ion beam analysis method, and (3) atom probe field ion microscopy. Proposed. However, these methods are extremely limited in apparatus, and the measurement conditions are still in the process of being searched, and there are many problems.

  Therefore, in evaluating the degree of the B concentration in the solid solution state in the present invention, the quantitativeness is inferior to those of these methods. However, as a method for more simply evaluating the degree of the B concentration in the solid solution state, the strength by annealing is as follows. After the tissue adjustment, a method is used in which the solid solution B concentration diffused in the film formed on the surface is evaluated using a secondary ion mass spectrometer. More specifically, in this method, after embedding a steel sample in a resin or the like and mirror polishing, a secondary ion mass spectrometer (: SIMS) (CAMECA IMS5F type) is used to draw a line in a 100 × 100 μm region. Line analysis is performed perpendicular to the steel sample with an analysis width of about 10 μm. At this time, oxygen ions are used as primary ions, and irradiation is performed under conditions of 8 kV-1 nA or less to detect secondary ions. The effectiveness of this method will be described below.

  The SIMS line analysis figure at the time of evaluating B concentration is shown in FIG. As shown in FIG. 2, the B ion concentration is concentrated on the surface portion (L1 portion in FIG. 2), and the B ion concentration in the steel (L2 portion in FIG. 2) is almost constant. The average B ionic strength in steel (L2 part in Fig. 2) is a value that is compared to the B concentration in the steel, and the peak of the B curve (L1 part in Fig. 2) corresponding to the surface of the steel and the film formed on the steel. The maximum B strength was a value that was compared with the B concentration on the steel surface, and the ratio between these two values was calculated. Such SIMS line analysis and the measurement of the ratio of the B concentration in the steel material to the B concentration on the steel material surface were carried out three times, and the average value of these ratios was determined. As a result, when only B is contained in B and Li, the amount of B in a solid solution state in the steel is sufficient when the B ion strength on the steel surface is 10 times or more the B ion strength in the steel. As a result, it was found that the amount required for exhibiting a sufficient defect stabilizing effect was obtained, and a sufficient defect stabilizing effect could be surely exhibited. When the B ion strength on the steel surface is less than 10 times the B ion strength in the steel, even if the B concentration in the steel is high, the amount of B in the solid solution state in the steel exhibits a sufficient defect stabilizing effect. Therefore, the amount necessary to do so is not satisfied, and the defect stabilization effect is low.

  Therefore, if the B concentration on the steel material surface is measured, the ratio of the B concentration and the B concentration in the steel material (obtained by analyzing the amount of B added and the amount of B in the steel), the extent of the B concentration in the solid solution state is determined. Can be evaluated. That is, it can be seen from this ratio whether or not the amount of B in a solid solution state in the steel is an amount necessary to exhibit a sufficient defect stabilizing effect. For example, if this ratio is 10 or more, it can be seen that the amount of B in a solid solution state in the steel is an amount necessary to develop a sufficient defect stabilizing effect. That is, such a method is effective in evaluating the degree of the B concentration in the solid solution state.

  The B concentration on the surface of the steel material can be measured using a secondary ion mass spectrometer, and this measurement is easier than in the case of the ATE method, ion beam analysis method, and atom probe field ion microscope method described above. There are advantages that can be made.

  The high-strength steel according to the present invention preferably contains Al: 0.05 to 0.5% by mass [fourth invention]. Details will be described below. Al particularly has the effect of forming fine precipitates with N, trapping hydrogen in the steel, and improving delayed fracture resistance. In addition, the formation of fine precipitates with N not only prevents the austenite grains from coarsening, but also promotes the solid solution of B, so that positive addition is desirable. Further, like B, the hydrogen reduction reaction is suppressed, and the generation and penetration of hydrogen are suppressed. From this point, it is desirable to contain 0.05% or more. However, if added excessively, coarse inclusions are formed, so 0.5% or less is desirable. Therefore, it is desirable to contain Al: 0.05 to 0.5%. A more preferable upper limit is 0.3% from the viewpoint of suppressing the formation of coarse inclusions.

  In the high-strength steel according to the present invention, it is desirable to contain a total of one or more of Ti, Zr, Hf, and Nb in an amount of 0.03 to 0.5% by mass [fifth invention]. Details will be described below. Ti, Zr, Hf, and Nb all have the effect of forming fine precipitates, strongly trapping hydrogen in the steel, and improving delayed fracture resistance. That is, there is an effect of improving delayed fracture resistance by suppressing hydrogen release due to environmental changes or the like. In addition, the action of Nb has not been elucidated, but there is also an effect of stabilizing the solid solution of B. From this point, it is preferable to contain at least 0.03% of one or more of Ti, Zr, Hf, and Nb. However, if these are excessive, the toughness of the steel decreases, so 0.5% or less is desirable. Therefore, it is desirable to contain at least 0.03 to 0.5% of one or more of Ti, Zr, Hf, and Nb.

  The high-strength steel according to the present invention may contain Ni and / or Cu. Ni and Cu contribute to the improvement of corrosion resistance. However, both Ni: over 2.0% and Cu: over 1.0% saturate the effect of improving corrosion resistance and only lead to an increase in cost, so Ni: 2.0% or less, Cu: 1. It is desirable to keep it below 0%. Therefore, when Ni and / or Cu are contained, it is desirable that Ni: 2.0% or less and / or Cu: 1.0% or less [Sixth Invention]. From the viewpoint of the above effects and cost, the Ni content is preferably 1.5% or less, and more preferably 1.0% or less. From the same point, the amount of Cu is preferably 0.8% or less, and more preferably 0.6% or less.

  P is an element that causes grain boundary segregation and degrades delayed fracture resistance. In order to suppress this, the P content is preferably 0.03% or less, more preferably 0.01% or less, and further preferably 0.005% or less.

  When the high-strength steel according to the present invention contains only the above chemical components, the balance consists of Fe (iron) and inevitable impurities (inevitable impurities brought in depending on the situation of raw materials, materials, manufacturing equipment, etc.) It is. The high-strength steel according to the present invention is not limited to the case of containing only the above-described chemical components, but is an element for imparting further properties (the above-mentioned chemistry within a range that does not adversely affect the achievement of the object of the present invention). Other than the components) can be contained, and the case of containing this element is also included in the high-strength steel according to the present invention.

For example, when the high-strength steel according to the first invention (the high-strength steel according to claim 1) contains only the chemical component according to claim 1, the high-strength steel has “tensile strength of 1200 N / mm ² or more. High strength steel, C: 0.3-0.5 mass%, Cr: 0.5-2.5 mass%, Mn: 0.2-1.0 mass%, N: 0.01 mass% The following (including 0 mass%), O: 0.0005-0.005 mass%, S: 0.001-0.025 mass%, Si: 2.0 mass% or less (including 0 mass%) And V and / or Mo in a total amount of 0.05 to 1.5 mass%, and at least one of B and Li is represented by the above formulas (1), (2), (3) and (4). It is contained so as to satisfy, and the balance is composed of Fe and inevitable impurities.

The high-strength steel according to the first invention (the high-strength steel according to claim 1) contains the chemical component according to claim 1 and an element X (for example, X: Co) other than the chemical component according to claims 1-6. In this case, the high-strength steel is “tensile strength: 1200 N / mm ² or more high-strength steel, C: 0.3 to 0.5 mass%, Cr: 0.5 to 2.5 mass%, Mn : 0.2 to 1.0 mass%, N: 0.01 mass% or less (including 0 mass%), O: 0.0005 to 0.005 mass%, S: 0.001 to 0.025 mass% Si: 2.0% by mass or less (including 0% by mass), V and / or Mo in total of 0.05 to 1.5% by mass, and at least one of B and Li In order to satisfy the formulas (1), (2), (3) and (4), X is further contained, and the balance is Fe and inevitable impurities. Is shall ". This high strength steel is also included in the high strength steel according to the present invention.

  The method for evaluating delayed fracture resistance is not particularly limited, and various methods can be used. For example, a constant strain test, a constant load test, a low strain rate test, and the like can be employed. In these tests, any of an acid dipping method, a cathodic charging method, a method using a CCT tester, etc. may be used to cause hydrogen to enter steel. When performing a low strain rate test, perform the test at a crosshead speed of 2 μm / min or less, and compare the sample that has intruded hydrogen with the sample that has not intruded hydrogen in terms of breaking stress and strain. Is desirable.

  The method for evaluating the defect stabilization effect is not particularly limited, but it is difficult to quantitatively evaluate the defect directly. Therefore, delayed fracture resistance (A) when a delayed fracture test is performed using a test piece into which hydrogen has penetrated, and when a delayed fracture test is performed after applying a stress cycle variation to a test specimen into which hydrogen has penetrated The delayed fracture resistance (B) of the former is examined, and the degree of decrease in the delayed fracture resistance (B) of the latter [for example, (B−A) / A] is determined based on the former delayed fracture resistance (A). Ask. Then, although it is indirect, the defect stabilization effect can be evaluated.

As described above, the high-strength steel according to the present invention is a high-strength bolt having a tensile strength of 1200 N / mm ² or more and excellent delayed fracture resistance. Has excellent delayed fracture resistance and excellent durability [seventh invention].

  Examples of the present invention and comparative examples will be described below. The present invention is not limited to this embodiment, and can be implemented with appropriate modifications within a range that can be adapted to the gist of the present invention, all of which are within the technical scope of the present invention. include. Moreover, since the heat treatment conditions for dissolving B in the solution vary depending on the shape and size of the steel material, the heat treatment conditions can be appropriately changed within a range that can meet the purpose.

A steel containing the chemical components (mass%) and 0.05% Si shown in Table 1 and the balance consisting of Fe and inevitable impurities was melted in a 150 kg vacuum melting furnace, cast into a 150 kg ingot, and cooled. Next, this ingot is forged to 25 mmφ, subjected to a solution treatment at 1200 ° C. for 30 minutes and a normalizing treatment, and then quenched at a temperature of 800 to 1200 ° C. to sufficiently solidify the carbonitride. Tempering treatment was performed at a temperature as high as possible within a temperature range of 450 to 700 ° C. and a tensile strength of 1400 to 1500 N / mm ² . In addition, although the amount of C of the steel shown in Table 1 is only 0.3% or more, since the tensile strength did not become 1200 N / mm ² or more in the above process in the steel with less C than 0.3%, The test is not conducted.

  The steel thus obtained (steel after the above tempering treatment) was used as a test steel, and a delayed fracture specimen having the shape shown in FIG. 1 was produced from this steel. Using this delayed fracture test piece, a delayed fracture resistance test was conducted as follows.

After ultrasonically degreasing the above delayed fracture test piece with acetone, it is set in an SSRT test device and an SSRT test is performed at 30 ° C. in the air at a crosshead speed of 2 × 10 ⁻³ mm / min. The elongation E _{0 of the} test piece was measured.

On the other hand, the delayed fracture specimen was ultrasonically degreased with acetone, and then in a liquid of 0.5 mol / liter (hereinafter referred to as L) H ₂ SO ₄ +0.01 mol / L KSCN for 60 minutes at 1 A / dm ² for the cathode. Charged. Thereafter, galvanization was performed for the purpose of preventing hydrogen escape. This galvanization was obtained by flowing a current of 50 A / dm ² for 3 minutes in a bath in which a complexing agent was added to zinc sulfate heptahydrate, sodium sulfate, and sulfuric acid.

After this galvanizing After equilibrating the concentration of hydrogen in the steel and stored at 200 hours at normal temperature, at a crosshead speed of 2 × 10 ^-3 mm / min, subjected to SSRT test, the elongation E ₁ (cathode of the specimen (Elongation after charging) was measured.

Further, after equilibrating the hydrogen concentration in the steel in the same manner as described above, the stress is within a range in which 50% (700 to 750 MPa) of the tensile strength is the lower limit and 75% (1050 to 1125 MPa) of the tensile strength is the upper limit. Repeated loading and unloading at a crosshead speed of 200 × 10 ⁻³ mm / min, 20 cycles of stress cycle fluctuation, and then SSRT test at a crosshead speed of 2 × 10 ⁻³ mm / min from 50% of the tensile strength. The elongation E _{2 of the} test piece (cathode charge → elongation after fluctuation of stress cycle) was measured.

Based on the above measured values (E ₀ , E ₁ , E ₂ ), delayed fracture susceptibility (stress cycle fluctuation) obtained by DF ₁ (%) = 100 × (1−E ₁ / E ₀ ) as a delayed fracture evaluation index None), delayed fracture susceptibility (with stress cycle variation) obtained by DF ₂ (%) = 100 × (1−E ₂ / E ₀ ) was calculated. In the above stress cycle variation, it was confirmed that there was no difference in tensile strength and strain in the absence of hydrogen (delayed fracture specimen after ultrasonic degreasing and before cathodic charging). It was confirmed that the decline due to the change was negligible.

As an evaluation of this result, those with DF ₁ exceeding 70% are inferior in delayed fracture resistance (no stress cycle fluctuation) (hereinafter also referred to as delayed fracture resistance (1)), and DF ₁ is 70% or less (However, more than 60%) has good delayed fracture resistance (1), and those with DF ₁ of 60% or less can be evaluated to have very good delayed fracture resistance (1). Furthermore, it can be determined that the greater the difference between DF ₂ and DF ₁ is, the greater the delayed fracture promotion under stress fluctuation is. When DF ₂ exceeds 70%, it is used in an environment where there is stress fluctuation. Even if it is used in an environment where there is a stress fluctuation, it is inferior in delayed fracture resistance (hereinafter also referred to as delayed fracture resistance (2)), and DF ₂ is 70% or less (but more than 60%). Delayed fracture resistance is good (delayed fracture resistance (2) is good), and those with DF ₂ of 60% or less can be evaluated as having very good delayed fracture resistance (2).

The test results [DF ₁ (%), DF ₂ (%)] are shown in Table 2. As can be seen from Table 2 and Table 1, no. If 1, V, less amount of Mo, V + (the total content of V and Mo) Mo amount does not meet the 0.05 to 1.5%, is inferior to hydrogen diffusion inhibiting effect, DF ₁ is High, more than 80%, inferior in delayed fracture resistance (1).

No. In the case of No. 2, the amounts of V and Mo are large (the amount of V + Mo: more than 1.5%). Although DF ₁ is lower than in the case of 1, the amount of V + Mo does not satisfy 0.05 to 1.5%, and DF ₂ is remarkably high. This is presumably because the amount of hydrogen released due to stress fluctuations was significantly increased.

No. The case of 3 is an example that does not satisfy the above-mentioned formula (4), and DF ₂ is the largest, 89%, more than 80%, and is inferior in delayed fracture resistance (2). No. 4, no. No. 5 is an example that does not satisfy the above formulas (1) and (2). Although low DF ₁ than in the case of 1-3, Example (No.10~17) DF ₂ is high as compared with the case of the present invention.

No. In the case of 6, the amount of N is large and N: 0.01% or less is not satisfied. DF ₁ is low, but DF ₂ is significantly higher. This contains only B among the defect stabilizing elements B and Li, and the amount of solid solution in the steel of B does not satisfy the amount necessary to exhibit a sufficient defect stabilizing effect. This is probably because of this. That is, since only B is contained in B and Li, DF ₂ is low if the amount of solid solution in the steel of B satisfies the amount necessary to exhibit a sufficient defect stabilizing effect. However, since the amount of N is large and exceeds 0.01%, BN is formed and the solid solution of B is inhibited, the amount of B in the solid solution state in the steel is reduced, and sufficient defect stabilization is achieved. This is thought to be because the amount necessary for producing the effect could not be satisfied.

No. In the case of 7, the amount of C is large, C: 0.3 to 0.5% is not satisfied, DF ₁ is high, and is higher than the case of the examples (No. 10 to 17) of the present invention.

No. In the case of No. 8, the amount of O is large, O: 0.0005 to 0.005% is not satisfied, and DF ₁ and DF ₂ are no. Although it is low compared with the case of 1-7, it cannot be said to be sufficient delayed fracture resistance. No. For 9, a number S content, S: 0.001 to 0.025% of not not meet, DF ₁ and DF ₂ is No. Although low compared to that of 1 to 7, DF ₂ is not a 70% or less, since the resistance to delayed fracture (2) has not reached the level that good can not be said sufficient resistance to delayed fracture.

In contrast, no. In the case of 10 to 17 (Examples of the present invention), both DF ₁ and DF ₂ are low and excellent in delayed fracture resistance (1) and (2). In particular, the amount of Al and the amount of Ti-based elements (Ti, Zr, Hf, Nb) satisfying the amounts of Al and Ti-based elements in the fourth and fifth inventions are DF ₁ and DF ₂ The difference is small (No. 13-17).

No. 15-No. In the case of 17, the Al amount and the Ti-based element are made to satisfy the Al amount and the Ti-based element amount according to the fourth and fifth inventions, and the Ni amount and / or the Cu amount are set to the Ni amount according to the sixth invention. The amount of Cu is satisfied. These are no. Among them, DF ₁ and DF ₂ are particularly low among 10 to 17 and excellent in delayed fracture resistance (1) and (2).

  The high-strength steel excellent in delayed fracture resistance according to the present invention is more excellent in delayed fracture resistance than conventional high-strength steel, so it is suitable as a structural member of equipment such as structures and transportation equipment or machines. In particular, it is suitable as a steel for bolts used in applications in which thermal loads and dynamic stress loads are repeatedly applied, such as car engine peripheral devices, and in addition to automobile parts such as gears, gears It can also be suitably used for steel parts in other automotive parts such as sliding parts, shafts, bearings, construction machines, and industrial machines.

It is a figure which shows the shape of the test piece for a delayed fracture resistance test which concerns on an Example. It is a figure which shows the SIMS line analysis figure at the time of evaluating B concentration.

Claims (7)

Tensile strength: 1200 N / mm ² or more high strength steel, C: 0.3-0.5 mass%, Cr: 0.5-2.5 mass%, Mn: 0.2-1.0 mass %, N: 0.01 mass% or less (including 0 mass%), O: 0.0005-0.005 mass%, S: 0.001-0.025 mass%, Si: 2.0 mass% or less (Including 0% by mass), V and / or Mo in a total amount of 0.05 to 1.5% by mass, and at least one of B and Li is represented by the following formulas (1), (2), A high-strength steel excellent in delayed fracture resistance, characterized by containing (3) and (4) so as to satisfy the requirements.
0% by mass ≦ B amount ≦ 0.01% by mass ---------------------- Formula (1)
0% by mass ≦ Li content ≦ 0.05% by mass
0% by mass <B amount + Li amount ------------------------------ Equation (3)
V amount + Mo amount / 5 ≦ (B amount + Li amount) × 300 --------- Equation (4)   The high strength steel excellent in delayed fracture resistance according to claim 1, wherein at least one of B and Li is B, and the B concentration on the steel surface is concentrated to 10 times or more of the B concentration in the steel. .   The high excellent delayed fracture resistance according to claim 1, wherein at least one of B and Li is B and Li, and the B concentration on the steel surface is concentrated to 10 times or more of the B concentration in the steel. Strength steel.   The high strength steel excellent in delayed fracture resistance according to any one of claims 1 to 3, comprising Al: 0.05 to 0.5 mass%.   The high strength steel excellent in delayed fracture resistance according to any one of claims 1 to 4, containing 0.03 to 0.5 mass% in total of one or more of Ti, Zr, Hf, and Nb.   The high strength steel excellent in delayed fracture resistance according to any one of claims 1 to 5, which contains Ni: 2.0 mass% or less and / or Cu: 1.0 mass% or less. A high-strength bolt comprising the high-strength steel excellent in delayed fracture resistance according to any one of claims 1 to 6.

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