JP2011117035A - Steel for high-strength bolt - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for high-strength bolt which has sufficient delayed destruction resistance, even in the case of having the high tensile strength of ≥1,350 MPa and the steel can suitably be used to an automobile, various industrial machines, building structural material, etc., under severe environment, such as seaside or coast zone near the seaside. <P>SOLUTION: The steel for high-strength bolt has the composition comprising >0.30-0.55% C, ≤0.3% Si, ≤0.6% Mn, ≤0.025% P, ≤0.030% S, 0.005-0.10% Al, 1.0-2.5% Cr, 0.25-2.0% Mo, ≤0.030% N, 0.05-0.50% Sn, ≥1.4% [Cr+Mo] and the balance Fe with impurities. If necessary, further, the steel can contain one or more selected from specific contents of V, Ti, Zr, Ca, Mg and Ni. Further, if necessary, the steel can contain the specific contents of compounded Ni and Cu. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to steel for high-strength bolts, and more specifically, high-strength bolts having a tensile strength of 1350 MPa or more and excellent delayed fracture resistance, and suitable for use in automobiles, various industrial machines, building structures, and the like. It relates to the steel used as the material.

  As steel for bolts, that is, steel used as a material for bolts, for example, chromium molybdenum steel such as SCM440 defined in JIS G 4053 (2008) is used. Generally, the strength of bolts is adjusted to about 1000 MPa by tensile strength. It had been. This is because if the tensile strength exceeds 1200 MPa, the bolt breaks easily.

  The above fracture is called “delayed fracture”, which is a phenomenon in which steel placed under static load breaks brittlely after a certain period of time. This is a type of hydrogen embrittlement caused by hydrogen that has penetrated into steel due to corrosion. It is considered. This “delayed fracture” was the biggest obstacle to increasing the strength of the bolt.

  On the other hand, in recent years, with the reduction in weight of automobiles and various industrial machines and the increase in size of building structures, there has been an increasing demand for high-strength bolts that can withstand high tightening forces. Development of steel for bolts, particularly steel for bolts having a tensile strength of 1350 MPa or more has become an urgent task.

  Accordingly, various studies have been made to improve the delayed fracture resistance of high strength steel having a tensile strength of 1200 MPa or more. For example, Patent Documents 1 to 7 contain Cr, Mo and V and are hardenable. And high strength bolt steel with improved temper softening resistance.

  Patent Documents 8 to 10 disclose steels for high-strength bolts that contain a trace amount of B to clean the grain boundaries and enhance the bond strength of the grain boundaries to improve delayed fracture resistance.

  Further, in Patent Documents 11 to 14, fine Ti-based precipitates are produced by containing Ti in addition to containing a small amount of B, and this is used as a hydrogen trap site to improve delayed fracture resistance. Technology is disclosed.

Japanese Patent No. 2670937 JP 7-126799 A JP-A-7-278735 JP-A-8-120408 JP-A-8-225845 JP 2000-328191 A JP 2001-32044 A JP-A-5-171356 Japanese Patent Application Laid-Open No. 8-29579 Japanese Patent Laid-Open No. 9-111399 JP-A-10-17985, JP 10-36940 A JP 11-293401 A JP 2003-268495 A

Yasuhiko Omura, Takahiro Kushida, Fukukazu Nakazato, Ryo Watanabe, Satoshi Oyamada: “Hydrogen Occlusion Behavior and Delayed Fracture Evaluation in High-Strength Bolt Exposure to Air” (Iron and Steel, 91 (2005), p. 478) T.A. Omura, T .; Kudo and S.K. Fujimoto: “Environmental Factors Affecting Hydrogen Entry into High Strength Steel due to Atmospheric Corrosion” (Materials Transactions, 47. 6200).

  The ideas of the techniques proposed in the above-mentioned Patent Documents 1 to 14 are all improved in the structure such as tempering at high temperature or grain boundary cleaning, or trapping hydrogen in finely dispersed precipitates. After hydrogen has invaded, delayed destruction is prevented. In the case of the steel for high-strength bolts disclosed in each of the above-mentioned publications, compared to the case where the conventional steel such as SCM440 defined in the above-mentioned JIS G 4053 (2008) is simply strengthened, the resistance to delay is increased. It has been shown that destructibility is improved.

  However, the evaluation of delayed fracture performed in each of the above publications is a relative evaluation between steel materials by an acid immersion test or a cathodic charge test, and an evaluation method that accurately reproduces the severity of the actual environment. It's a bad thing.

  For example, in an actual coastal environment, severe corrosion due to flying salt occurs, so automobiles, various industrial machines, and building structures are exposed to a severer environment than the laboratory evaluation in each of the above publications.

  For this reason, even when steel for high-strength bolts proposed in each of the above-mentioned publications is used in harsh environments such as coastal areas, the amount of hydrogen intrusion due to corrosion increases. When the strength was 1350 MPa or more, it was not possible to suppress the occurrence of delayed fracture.

  The present invention has sufficient delayed fracture resistance even when the tensile strength is high strength of 1350 MPa or more, and is used in automobiles, various industrial machines, and architectures in harsh environments such as coasts or coastal areas close thereto. An object of the present invention is to provide a steel for a high-strength bolt that can be suitably used for a structure or the like, that is, a steel as a material for a high-strength bolt having the above-described characteristics.

  Although "delayed fracture" has been thought to be caused by hydrogen entering the steel from the actual environment, the amount of hydrogen intrusion from the atmospheric environment is very small. The effect of was not clarified.

  Under such circumstances, Omura et al., One of the inventors of the present invention, in Non-Patent Document 1, using typical high-strength bolts, delayed fracture behavior and hydrogen intrusion into steel The correlation of the behavior was investigated in detail, and high-strength bolts exposed to the atmospheric environment occluded an amount of hydrogen that could cause delayed fracture, and the maximum value of the occluded hydrogen concentration was measured by the electrochemical hydrogen permeation method. The value was found to be about 0.1 μA / cm.

  Furthermore, one of the inventors, T.W. Omura et al. Devised a dry and wet cycle hydrogen permeation test method that reproduces hydrogen invasion in a real air environment in Non-Patent Document 2 and investigated the action mechanism of environmental factors affecting hydrogen intrusion. We found experimental conditions that reproduced or made the environment more harsh, and clarified that the hydrogen intrusion promoting factor was a cyclic change in the attached salinity and temperature and humidity. In addition, it is estimated that the action mechanism of the said attached salt is to accelerate | stimulate the hydrolysis reaction of an iron ion and to reduce pH of a water film by concentrating as a chloride ion in a water film.

  Therefore, based on the above knowledge, the present inventors use the above-described wet and dry cycle hydrogen permeation test method in order to reduce the amount of hydrogen intrusion from the actual atmospheric environment and prevent delayed destruction stably and reliably. Various studies were conducted on the component elements that can effectively suppress the amount of hydrogen entering the steel. As a result, an extremely important matter that Sn exerts a great effect on the suppression of hydrogen intrusion has been clarified.

Although the above Sn has been said to be an element effective for acid resistance, there has been no knowledge so far about the effect of Sn on atmospheric corrosion, and further on the effects of hydrogen intrusion and delayed fracture in the atmospheric environment. There was no. Although there are still many unclear points regarding the mechanism by which Sn exerts a tremendous effect on the suppression of hydrogen invasion, it has the effect of preventing Sn from dissolving as Sn ^{2+ and} lowering the pH of the water film. it is conceivable that.

  This invention is completed based on said knowledge, The summary exists in the steel for high strength bolts shown to following (1)-(6).

(1) By mass%, C: more than 0.30% and 0.55% or less, Si: 0.3% or less, Mn: 0.6% or less, P: 0.025% or less, S: 0.00. 030% or less, Al: 0.005-0.10%, Cr: 1.0-2.5%, Mo: 0.25-2.0%, N: 0.030% or less, and Sn: 0.05 A steel for high-strength bolts, which contains ˜0.50%, fn represented by the following formula (1) is 1.4 or more, and the balance is Fe and impurities.
fn = Cr + Mo (1)
However, the element symbol in the formula (1) represents the content in mass% of the element.

  (2) The steel for high-strength bolts as described in (1) above, further containing, by mass%, V: 0.50% or less.

  (3) The composition according to (1) or (2) above, wherein the composition further contains one or two of Ti: 0.10% or less and Zr: 0.10% or less in mass%. High strength bolt steel.

  (4) In the above-mentioned (1) to (3), further containing one or two of Ca: 0.01% or less and Mg: 0.01% or less in mass% Steel for high-strength bolts according to any of the above.

  (5) The steel for high-strength bolts according to any one of (1) to (4) above, which further contains Ni: 3.0% or less in terms of mass%.

  (6) The composition according to any one of (1) to (4) above, further comprising Ni: 3.0% or less and Cu: 0.3 to 1.0% by mass% Steel for high-strength bolts.

  “Impurity” refers to what is mixed from ore, scrap, or the environment as a raw material when manufacturing steel materials industrially.

  Hereinafter, the inventions related to the steels for high-strength bolts shown in the above (1) to (6) are referred to as “present invention (1)” to “present invention (6)”, respectively. Also, it may be collectively referred to as “the present invention”.

  Since the steel for high-strength bolts of the present invention has sufficient delayed fracture resistance even when the tensile strength is 1350 MPa or higher, an automobile placed in a harsh environment such as a coast or a coastal area close thereto. It is suitable as a material for high-strength bolts used in various industrial machines and building structures.

It is a figure which illustrates typically the wet-and-dry cycle hydrogen permeation test method used in the example. It is a figure which shows the wet-and-dry cycle hydrogen permeation test result as a time change of the hydrogen permeation coefficient JL about the steel 2 of the comparative example used in the Example. It is a figure which shows the wet-and-dry cycle hydrogen permeation test result as a time change of the hydrogen permeation coefficient JL about the steel I of the example of this invention used in the Example. It is a figure which illustrates typically the cathode charge constant load test method used in the Example. It is a figure which illustrates typically the cathode charge hydrogen permeation test method used in the Example.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

C: more than 0.30% and not more than 0.55% C has an effect of improving hardenability and improving strength. In order to obtain sufficient hardenability and to obtain a tensile strength of 1350 MPa or more stably and reliably, it is necessary to contain an amount of C exceeding 0.30%. However, even if C is contained in an amount exceeding 0.55%, the effect is saturated and the cold workability is lowered, so that it is difficult to form a bolt by the cold forging method. Therefore, the C content is more than 0.30% and 0.55% or less. In addition, in order to sufficiently exhibit the effect of improving the strength of C, it is desirable that the lower limit of the C content is 0.35%. In this case, a tensile strength of 1400 MPa or more should be secured stably and reliably. Can do. In order to secure a higher tensile strength, it is desirable that the lower limit of the C content is 0.38%. On the other hand, in order to suppress the decrease in cold workability and facilitate bolt forming by the cold forging method, the upper limit of the C content is desirably 0.52%.

Si: 0.3% or less Si is contained as an impurity in the steel, and when the amount exceeds 0.3%, the formability to the bolt by the cold forging method is remarkably lowered. Therefore, the Si content is set to 0.3% or less. In order to facilitate bolt forming by the cold forging method, it is desirable that the upper limit of the Si content is 0.25%.

Mn: 0.6% or less Mn is contained as an impurity in the steel and segregates at the grain boundary to cause delayed fracture of the intergranular crack type, and further forms Mn-based sulfides into the steel. Promotes hydrogen penetration. In particular, when the Mn content exceeds 0.6%, the occurrence of intergranular cracking-type delayed fracture and hydrogen intrusion into the steel become significant. Therefore, the Mn content is set to 0.6% or less. The upper limit of the Mn content is desirably 0.5%.

P: 0.025% or less P is contained as an impurity in steel and segregates at grain boundaries to reduce toughness and delayed fracture resistance. In particular, when its content exceeds 0.025%, toughness and Decrease in delayed fracture resistance becomes remarkable. Therefore, the content of P is set to 0.025% or less. The content of P is preferably as low as possible.

S: 0.030% or less S is contained as an impurity in steel, and usually exists as Mn sulfide together with Mn described above, and promotes hydrogen penetration by generating hydrogen sulfide when dissolved with corrosion. Lowering the delayed fracture resistance. In particular, when the S content exceeds 0.030%, the delayed fracture resistance due to hydrogen penetration is significantly reduced. Therefore, the content of S is set to 0.030% or less. In order to secure better delayed fracture resistance, the content is desirably 0.015% or less, and more desirably 0.010% or less.

Al: 0.005-0.10%
Al is an element effective for deoxidation of steel, and in order to sufficiently secure this effect, it is necessary to contain 0.005% or more. On the other hand, even if Al is contained in an amount exceeding 0.10%, the above effect is saturated, and the formation of a ferrite phase is promoted, and the delayed fracture resistance is lowered. Therefore, the content of Al is set to 0.005 to 0.10%. In order to fully exhibit the deoxidation action of Al, it is desirable that the lower limit of the Al content is 0.02%. Moreover, in order to suppress the formation of the ferrite phase and ensure good delayed fracture resistance, it is desirable that the upper limit of the Al content be 0.05%. The Al content of the present invention refers to acid-soluble Al (so-called “sol.Al”).

Cr: 1.0-2.5%
Cr has the effect of increasing the hardenability and improving the strength without reducing the delayed fracture resistance. In order to obtain a tensile strength of 1350 MPa or more, it is necessary to contain 1.0% or more of Cr. However, even if Cr is contained in excess of 2.5%, the effect is saturated and the cost is increased, and “M” is one or more of Fe, Cr and Mo, and the former austenite grain boundary Coarse M ₂₃ C ₆ type carbide precipitates and the delayed fracture resistance decreases. Therefore, the Cr content is set to 1.0 to 2.5%. In order to ensure good delayed fracture resistance, the upper limit of the Cr content is desirably 1.5%.

  In addition, as for content of Cr, fn represented by said Formula (1) needs to be fn> = 1.4 in said range.

Mo: 0.25 to 2.0%
Mo, like Cr, has the effect of improving hardenability and improving strength without reducing delayed fracture resistance. Mo contributes to precipitation strengthening by forming fine Mo-V carbide together with V, and has the effect of improving strength without lowering the tempering temperature. In order to obtain a tensile strength of 1350 MPa or more, it is necessary to contain 0.25% or more of Mo. However, even if Mo is contained in excess of 2.0%, the effect is saturated and the cost is increased, and “M” is one or more of Fe, Mo and Cr, and the prior austenite grain boundary Coarse M ₂₃ C ₆ type carbide precipitates and the delayed fracture resistance decreases. Therefore, the Mo content is set to 0.25 to 2.0%. In order to sufficiently exhibit the effect of improving the strength of Mo, it is desirable to set the lower limit of the Mo content to 0.5%. In this case, a tensile strength of 1400 MPa or more can be reliably ensured. . In order to ensure good delayed fracture resistance, it is desirable that the upper limit of the Mo content be 1.0%.

  In addition, as for content of Mo, fn represented by said Formula (1) needs to be fn> = 1.4 in said range.

N: 0.030% or less N is present as an impurity in steel, and when its content is excessive, nitrogen blowholes are generated during melting and cause flaws during processing. In particular, when the N content exceeds 0.030%, blowholes are remarkably generated and wrinkles are likely to occur during processing. Therefore, the N content is set to 0.030% or less. The upper limit of the N content is preferably 0.020%.

Sn: 0.05-0.50%
Sn is the most important element in the present invention, and has the effect of greatly reducing the amount of hydrogen intrusion from the actual atmospheric environment. However, if the Sn content is less than 0.05%, the above effect is insufficient, so 0.05% or more must be contained. On the other hand, even if Sn is contained in excess of 0.50%, the effect is saturated, and in addition, cold workability is lowered and it becomes difficult to form a bolt by a cold forging method. Therefore, the Sn content is set to 0.05 to 0.50%. A desirable lower limit of the Sn content is 0.07%, and a desirable upper limit is 0.25%.

fn: 1.4 or more The total content of both Cr and Mo affects delayed fracture resistance. That is, adjustment of only the content of each element alone is less effective, each content is in an appropriate range, and fn represented by the above formula (1), that is, [Cr + Mo] is 1.4. That is necessary. When fn is less than 1.4, sufficient hardenability cannot be ensured. fn may be 4.5 when the Cr and Mo contents are 2.5% and 2.0%, which are the upper limit values, respectively, but is preferably 3.0 or less.

  For the above reasons, the steel for high-strength bolts according to the present invention (1) contains elements from C to Sn in the above-described range, and fn represented by the above formula (1) is 1.4 or more. Therefore, it was defined that the balance was made of Fe and impurities.

  The steel for high-strength bolts according to the present invention may further contain one or more elements selected from V, Ti, Zr, Ca, Mg, and Ni as necessary. . Moreover, the steel for high-strength bolts according to the present invention may further contain a composite of Ni and Cu as necessary.

  Hereinafter, the above optional elements will be described.

V: 0.50% or less V contributes to precipitation strengthening by forming fine Mo—V carbide together with Mo during tempering, and has the effect of improving strength without lowering the tempering temperature. For this reason, in order to acquire said effect, you may contain V. However, even if V is contained in excess of 0.50%, the effect is saturated and the cost increases, and the formation of excess V-based carbides increases the concentration of occluded hydrogen, resulting in delayed fracture resistance. Incurs a decline. Therefore, the content of V is set to 0.50% or less. In order to prevent the formation of excessive V-based carbonitrides and suppress the decrease in delayed fracture resistance, the upper limit of the V content is desirably 0.25%.

  On the other hand, in order to sufficiently obtain the above-described effect of improving the strength of V, it is desirable that the lower limit of the V content is 0.05%. In this case, it is possible to reliably ensure a tensile strength of 1400 MPa or more. it can. A desirable lower limit of the V content is 0.25%.

  Both Ti and Zr have the effect of forming fine carbides to refine crystal grains and improving delayed fracture resistance. For this reason, when it is desired to obtain better delayed fracture resistance, it may be contained in the following range.

Ti: 0.10% or less Ti has a function of forming fine carbonitrides to refine crystal grains and improving delayed fracture resistance. Therefore, Ti may be contained to obtain this effect. . However, even if Ti is contained in excess of 0.10%, the above effect is saturated and the cost is increased, and cold workability is reduced due to the formation of excessive and coarse Ti-based carbonitrides. Therefore, it becomes difficult to form a bolt by a cold forging method. Therefore, the Ti content is set to 0.10% or less. Note that the upper limit of the Ti content is preferably 0.04%.

  On the other hand, in order to reliably obtain the delayed fracture resistance improvement effect by the refinement of Ti crystal grains as described above, the lower limit of the Ti content is preferably 0.005%, and more preferably 0.01%. .

Zr: 0.10% or less Zr has the action of forming fine carbonitrides to refine crystal grains and improving delayed fracture resistance. Therefore, Zr may be contained to obtain this effect. . However, even if Zr is contained in excess of 0.10%, the above effects are saturated and the cost is increased, and cold workability is reduced due to the formation of excessive and coarse Zr-based carbonitrides. Therefore, it becomes difficult to form a bolt by a cold forging method. Therefore, the content of Zr is set to 0.10% or less. The upper limit of the Zr content is preferably 0.04%.

  On the other hand, in order to surely obtain the delayed fracture resistance improving effect by refining Zr crystal grains, the lower limit of the Zr content is preferably 0.005%, and more preferably 0.01%. .

  In addition, said Ti and Zr can be contained only in any 1 type of them, or 2 types of composites. The total content of these elements may be 0.20%, but is preferably 0.10% or less.

  Ca and Mg combine with S in the steel to form a sulfide, and have the effect of improving the hot workability of the steel. For this reason, when it is desired to ensure better hot workability, it may be contained in the following range.

Ca: 0.01% or less Ca combines with S in steel to form a sulfide and has an effect of improving the hot workability of steel. Good. However, even if Ca is contained in an amount exceeding 0.01%, the above effect is saturated and the cost is increased, and a soluble Ca-based oxide is excessively generated and becomes a starting point of pitting corrosion. Degradability is reduced. Therefore, the Ca content is set to 0.01% or less. In addition, it is desirable that the upper limit of the Ca content be 0.003%.

  On the other hand, in order to reliably obtain the effect of improving the hot workability of Ca described above, it is desirable that the lower limit of the Ca content is 0.0003%.

Mg: 0.01% or less Mg combines with S in steel to form sulfides and has an effect of improving the hot workability of steel. Therefore, Mg may be contained to obtain this effect. Good. However, even if Mg is contained in an amount exceeding 0.01%, the above effects are saturated and the cost is increased, and excessively soluble Mg-based oxides are generated as a starting point of pitting corrosion, and delay resistance is increased. Degradability is reduced. Therefore, the Mg content is set to 0.01% or less. Note that the upper limit of the Mg content is preferably 0.003%.

  On the other hand, in order to reliably obtain the effect of improving the hot workability of Mg as described above, it is desirable that the lower limit of the Mg content is 0.0003%.

  In addition, said Ca and Mg can be contained only in any 1 type of them, or 2 types of composites. The total content of these elements may be 0.02%, but is preferably 0.01% or less.

Ni: 3.0% or less Ni has an effect of increasing toughness. Ni exhibits an effect of suppressing hydrogen intrusion by improving the protection of corrosion products, and has an effect of suppressing hydrogen intrusion, although not as much as Sn. Therefore, Ni may be contained in order to obtain the effects described above. In addition, when steel contains the quantity of Cu mentioned later, in order to prevent the hot crack resulting from Cu, it is necessary to contain Ni in combination. However, even if Ni is contained in excess of 3.0%, the above effect is saturated and the cost increases. Therefore, the Ni content is set to 3.0% or less. Note that the upper limit of the Ni content is desirably 1.0%.

  On the other hand, in order to reliably obtain the effect of Ni described above, it is desirable that the lower limit of the Ni content is 0.2%.

Cu: 0.3 to 1.0%
Cu exhibits an effect of suppressing hydrogen intrusion by improving the protection of corrosion products, and has an action of suppressing hydrogen intrusion, although not as much as Sn. Therefore, to obtain this effect, Cu is contained in an amount of 0.3% or more. Also good. However, even if Cu is contained in an amount exceeding 1.0%, the above effect is saturated and the cost is increased, and the cold workability is lowered, so that it is difficult to form a bolt by a cold forging method. Become. Therefore, the Cu content is set to 0.3 to 1.0%. In addition, about Cu, it is desirable to make the upper limit of content into 0.5%.

  In addition, when Cu is contained, it is necessary to contain the above amount of Ni in combination in order to prevent hot cracking due to Cu.

  For the reasons described above, the steel for high-strength bolts according to the present invention (2) is specified to further contain V: 0.50% or less in the steel for high-strength bolts according to the present invention (1).

  Further, the steel for high-strength bolts according to the present invention (3) is the steel for high-strength bolts according to the present invention (1) or the present invention (2), and further Ti: 0.10% or less and Zr: 0.10. % Or less is defined as containing one or two of them.

  The steel for high-strength bolts according to the present invention (4) is added to any one of the steels for high-strength bolts according to the present invention (1) to the present invention (3), Ca: 0.01% or less, and Mg: 0 It was specified to contain one or two of 0.01% or less.

  The steel for high-strength bolts according to the present invention (5) contains Ni: 3.0% or less in addition to the steel for high-strength bolts according to any one of the present invention (1) to the present invention (4). Stipulated.

  The steel for high-strength bolts according to the present invention (6) is the steel for high-strength bolts according to any one of the present invention (1) to the present invention (4), Ni: 3.0% or less, and Cu: 0 .3 to 1.0%.

  The high-strength bolt made of the steel for high-strength bolts of the present invention is a normal method, that is, a slab or steel ingot melted in a converter or electric furnace is made into a slab by split rolling, and a wire rod by hot rolling. In this case, spheroidizing annealing may be performed as necessary, followed by wire drawing and cold forging, and it is not particularly necessary to limit the method for manufacturing such a bolt.

  However, in order to obtain a high tensile strength of 1350 MPa or more stably and reliably, and to ensure the uniformity of the structure, it is most desirable to perform quenching-tempering heat treatment after forming into a bolt shape.

  In the case of bolts made of ordinary bolt steel, the quenching heating temperature is often less than 900 ° C., but when using the high-strength bolt steel of the present invention as a material, In order to sufficiently dissolve carbide-forming elements such as Cr, Mo and V in the matrix during quenching, the heating temperature for quenching is preferably 900 ° C. or higher, and more preferably 910 ° C. or higher. On the other hand, when the heating temperature for quenching exceeds 1000 ° C., the structure becomes coarse and delayed fracture resistance decreases. Therefore, the heating temperature when quenching a bolt made of the steel for high-strength bolts of the present invention is desirably 900 to 1000 ° C, and more desirably 910 to 1000 ° C.

  In the tempering, in order to reduce the dislocation density introduced during quenching and to improve the delayed fracture resistance by spheroidizing the carbide, it is desirable to raise the temperature as much as possible, and for high strength bolts having a tensile strength of 1350 MPa or more. In order to improve delayed fracture resistance, tempering should be performed at a temperature of 500 ° C. or higher, and in order to improve delayed fracture resistance of a high-strength bolt having a tensile strength of 1400 MPa or higher, at a temperature of 600 ° C. or higher. It is desirable to perform tempering.

If the tempering temperature exceeds the Ac ₁ transformation point of the steel, bolt characteristics such as tensile strength and delayed fracture resistance vary greatly. Therefore, tempering is performed at a temperature below the Ac ₁ transformation point as usual.

  In addition, in order to prevent the so-called “immersion P phenomenon” from occurring during heat treatment and to ensure good delayed fracture resistance in a stable and reliable manner, the cold working lubricant does not contain P. It is desirable to use it.

  Hereinafter, the effects of the present invention will be described more specifically with reference to examples.

  31 types of steels A to Z and steels 1 to 5 having the chemical compositions shown in Table 1 were melted in the laboratory, and the resulting steel ingot was heated to 1250 ° C. to perform hot forging and hot rolling. A round steel having a diameter of 20 mm and a steel plate having a thickness of 20 mm were obtained.

  Steels A to U in Table 1 are steels of the present invention examples whose chemical compositions are within the range defined by the present invention.

  On the other hand, steels V to Z and steels 1 to 5 are steels of comparative examples whose chemical compositions deviate from the conditions specified in the present invention.

  Next, the round steel having a diameter of 20 mm and the steel plate having a thickness of 20 mm were held at the temperature shown in Table 2 for 45 minutes and then oil-quenched, further held at the temperature shown in Table 2 for 1 hour, and then allowed to cool in the atmosphere. And tempered.

  A round bar tensile test piece having a parallel part diameter of 6 mm and a parallel part length of 40 mm was sampled from the heat treated round steel subjected to the above quenching and tempering treatment, and the tensile strength at room temperature was measured.

  Further, from the central portion of the heat-treated steel sheet, a disk test piece having a diameter of 70 mm and a thickness of 0.5 mm was collected, and a wet and dry cycle hydrogen permeation test schematically showing the test method in FIG. The hydrogen penetration characteristics into steel were investigated. In FIG. 1, the above “disc test piece” is expressed as “thin plate test piece (0.5 mm)”.

  The wet and dry cycle hydrogen permeation test was specifically performed as follows.

  First, both surfaces of the disc test piece were polished with No. 600 emery paper, and then Ni plating was applied to one side.

  Next, the disk specimen was set so that the Ni-plated surface (hereinafter referred to as “Ni-plated surface”) was on the cell side filled with the 1N (1 N) NaOH aqueous solution shown in FIG. The plated surface was held at a constant potential of 0 V (zero volt) with respect to the reference electrode. A “silver-silver chloride electrode” was used as a reference electrode.

In addition, the surface exposed to the external environment, that is, the surface not labeled with Ni on the side indicated as “external environment” in FIG. 1 is completely dried after attaching artificial seawater of an amount of 0.5 mg / cm ^2. Then, the following conditions (A) to (D) were further repeated in order to perform exposure in the “external environment”. At this time, the interval between the conditions (A) and (B), the interval between the conditions (B) and (C), the interval between the conditions (C) and (D), and the interval between the conditions (D) and (A). Were 20 minutes, 5 minutes, 5 minutes and 20 minutes, respectively. Note that the test areas of both the external environment exposed surface and the Ni plated surface were 23.7 cm ² .

(A) Temperature 10 ° C., relative humidity 60%, period 3 hours,
(B) Temperature 60 ° C., relative humidity 40%, period 3 hours,
(C) Temperature 60 ° C., relative humidity 60%, period 3 hours,
(D) Temperature 60 ° C., relative humidity 90%, period 3 hours.

When the surface exposed to the external environment of the disk specimen set as described above corrodes and generates hydrogen, hydrogen atoms pass through the disk specimen and are released into the test cell. A current value that oxidizes hydrogen atoms that permeate into hydrogen ions was measured, and the measured value was divided by the above test area to obtain a hydrogen permeation current J (μA / cm ² ).

  Next, a hydrogen permeation coefficient JL (μA / cm) is calculated by multiplying the hydrogen permeation current J obtained as described above by 0.05 cm as the thickness L of the disk specimen, and enters from this environment. Using the maximum value of the hydrogen permeation coefficient JL for hydrogen (hereinafter referred to as “environmental hydrogen permeation coefficient JLenv”) as an index of the hydrogen penetration rate, the hydrogen penetration characteristics from the environment of each steel were compared.

  2 and 3 show an example of the hydrogen permeation coefficient JL calculated as described above.

  FIG. 2 shows the temporal change of the hydrogen permeability coefficient JL for the steel 2 of the comparative example of test number 28. As can be seen from FIG. 2, the maximum value of the hydrogen permeability coefficient JL, that is, the environmental hydrogen permeability coefficient. JLenv is 0.16 μA / cm. For steel Y, the environmental hydrogen permeability coefficient JLenv is 0.26 μA / cm. Steel Y is one of the inventors of Omura et al., Who is one of the most severe in Japan in “Environmental factors affecting hydrogen intrusion in steel” (Materials and Environment, 54 (2005), p. 61). "Low-carbon boron steel", a conventional steel that has been reported to evaluate the hydrogen penetration characteristics in the actual coastal environment of Nago City, Okinawa Prefecture, which is considered to be a safe environment, and reported that the environmental hydrogen permeation coefficient JLenv is about 0.1 μA / cm It is. Therefore, it is assumed that the above-described dry and wet cycle hydrogen permeation test in this example is evaluated under conditions that are about twice as severe as the actual environment.

  On the other hand, FIG. 3 shows the temporal change of the hydrogen permeation coefficient JL for the steel I of the inventive example of test number 9, and the maximum value of the hydrogen permeation coefficient JL, that is, the environmental hydrogen permeation coefficient JLenv is shown in FIG. As can be seen from FIG. 3, it is about 0.02 μA / cm. Therefore, it is clear that the hydrogen penetration characteristics of Steel I are significantly reduced compared to Steel 2 and Steel Y described above. That is, the steel I of the present invention is expected to have a large effect of suppressing hydrogen intrusion even in a dry and wet cycle test that is more severe than the actual environment, and to be excellent in delayed fracture resistance.

  Table 2 also shows the tensile strength and environmental hydrogen permeability coefficient JLenv determined as described above.

  From Table 2, in the case of test numbers 1 to 21 using the steels A to U of the present invention example, a tensile strength of 1350 MPa or more is obtained. Further, the environmental hydrogen permeation coefficient JLenv is 0.05 μA / cm or less, and it is clear that a sufficient hydrogen invasion suppressing effect is obtained.

  On the other hand, in the case of the test numbers 22 to 24 and 26 using the steels V to X and the steel Z among the steels of the comparative examples, the target tensile strength of 1350 MPa or more is not obtained.

  In the case of test numbers 25 and 27 to 31 using steel Y and steels 1 to 5 among the steels of the comparative examples, the environmental hydrogen permeability coefficient JLenv is a large value of 0.14 μA / cm or more, and hydrogen penetration It is clear that it has no inhibitory effect.

  Among the steels of the comparative examples, steel 4 is a steel containing Cu and Ni, which is said to be effective in weather resistance. Therefore, in the case of test number 30 using this steel, the environmental hydrogen permeability coefficient JLenv is the above steel. Although smaller than the cases of test numbers 27 to 29 and 31 using 1 to 3 and 5, the effect is not sufficient.

  Among the steels of the comparative examples, Steel 2 is intended to suppress the hydrogen intrusion by fixing a sulfide as an insoluble Ti-based sulfide by containing a large amount of Ti. Tests using this steel In the case of No. 28 as well, the environmental hydrogen permeability coefficient JLenv is a large value of 0.16 μA / cm, and does not have a sufficient hydrogen penetration inhibiting effect.

  Among the steels of the comparative examples, steel 1 contains Sn but its content is lower than the value specified in the present invention. Therefore, in the case of test number 27 using this steel, the environmental hydrogen permeability coefficient JLenv is a large value of 0.15 μA / cm, and a sufficient effect of suppressing hydrogen penetration is not obtained.

  In order to evaluate the delayed fracture resistance of steel materials, the critical hydrogen permeability coefficient JLth, which indicates the limit environment in which the steel material does not break due to hydrogen embrittlement, is sufficiently higher than the above-described environmental hydrogen permeability coefficient JLenv. It is necessary to confirm that it is large, that is, that the durability of the steel material against hydrogen embrittlement is sufficiently large compared to the amount of hydrogen entering from the environment.

  Therefore, from the round steel having a diameter of 20 mm subjected to the quenching and tempering treatment, the diameter of the parallel part is 6.56 mm, the length of the parallel part is 25.4 mm, and the depth of the central part of the parallel part is 1.42 mm. A round bar tensile test piece with a notch having a notch of 0.1 mmR was collected, and a cathodic charge constant load test that was severer than the actual environment was performed. The above-described notched round bar tensile test piece is a test piece that simulates the stress concentration at the bottom of the bolt and has a stress concentration factor of 5.

  In addition, as shown in the schematic diagram of FIG. 4, the cathode charge constant load test was held for 200 hours with a constant potential applied to the silver-silver chloride reference electrode while applying stress in 3% saline. It was carried out by checking whether or not the test piece was broken. The load stress at this time was 90% of the tensile strength shown in Table 2 for each steel in consideration of the use conditions of the high-strength bolts. Then, the load potential at this time was changed in the range of -0.8 V (volt) to -1.5 V (volt), and the limit load potential at which the test piece did not break was first obtained.

  On the other hand, it has been found that the potential applied in the above-described cathode charge constant load test uniquely corresponds to the severity of the environment from the viewpoint of hydrogen penetration (that is, the hydrogen permeability coefficient in that environment) regardless of the material.

  Therefore, in order to obtain the hydrogen permeation coefficient JLth corresponding to the load potential when the test piece broke in the cathode charge constant load test, the following cathode charge hydrogen permeation test was performed.

  A disc test piece having a diameter of 70 mm and a thickness of 0.5 mm was taken from the center of the thickness of the 20 mm thick steel plate subjected to the quenching and tempering treatment, and the so-called “double cell type” cathode charge shown in FIG. The test was conducted using a hydrogen permeation test apparatus.

In the cell on the left side of FIG. 5, that is, a cell filled with 3% NaCl aqueous solution, hydrogen is generated on the surface of the specimen when the potential of the disk specimen is kept at a negative constant potential with respect to the silver-silver chloride reference electrode. To do. Part of the generated hydrogen passes through the test piece as hydrogen atoms and is oxidized into hydrogen ions in a cell filled with 1N (1 N) NaOH aqueous solution, which is the right cell, and the oxidation current value is the potentiometer of the right cell. Measured with a stat. In the right cell, the test piece is held at a constant potential of 0 V (zero volts) with respect to the reference electrode (silver-silver chloride electrode), and all of the permeated hydrogen atoms are oxidized to hydrogen ions. Calculated hydrogen permeation current J (μA / cm ²⁾ the measured oxidation current value was divided by the test area (23.7 cm ^2), by multiplying the thickness L (0.05 cm) disc test piece J The hydrogen permeation coefficient JL (μA / cm) was calculated. Then, the load potential at this time is changed in the range of -0.8 V (volt) to -1.5 V (volt), the correlation is obtained between the load potential and the hydrogen permeation coefficient, and the above-mentioned cathode charge constant load test The hydrogen permeability coefficient corresponding to the limit load potential at which the test piece does not break was defined as the critical hydrogen permeability coefficient JLth.

  Table 2 also shows the critical hydrogen permeability coefficient JLth determined as described above.

  From Table 2, in the case of test numbers 1 to 21 using the steels A to U of the examples of the present invention, the critical hydrogen permeability coefficient JLth is 0.10 to 0.17 μA / cm, and the environmental hydrogen permeability coefficient JLenv is 0. It can be seen that it is sufficiently larger than 02 to 0.05 μA / cm. That is, in the case of steels A to U, the hydrogen concentration JLth causing hydrogen embrittlement is sufficiently larger than the hydrogen concentration JLenv entering from the environment, and therefore it is clear that there is no risk of causing delayed fracture in the actual environment. is there.

  On the other hand, in the case of the test number 25 and the test numbers 27 to 31 using the steel Y of the comparative example and the steels 1 to 5, the critical hydrogen permeability coefficient JLth is 0.08 to 0.12 μA / cm, Although it is not necessarily lower than the case of test numbers 1 to 21 using the steels A to U of the present invention example, the environmental hydrogen permeation coefficient JLenv exceeds JLth in each test number, so that the environment is extremely harsh. Under long-term use, there is a risk of delayed destruction.

  In addition, the above-mentioned Example uses the test piece extract | collected from the heat-treated round steel and the heat-treated steel plate which performed the quenching-tempering process.

  On the other hand, as already described, the high-strength bolt made of the steel for high-strength bolts of the present invention may be manufactured by a normal method, and the bolt manufacturing method is not particularly limited.

  However, in order to obtain a high tensile strength of 1350 MPa or more stably and reliably and to ensure the uniformity of the structure, it is most desirable to perform a quenching-tempering heat treatment after forming into a bolt shape.

  When a quenching-tempering heat treatment is performed, the structure and strength characteristics of the final actual bolt are determined by the chemical composition of the steel and the quenching-tempering conditions, and are almost affected by the intermediate process of bolt manufacturing. Absent.

  For this reason, it can be considered that the test results in this example are comparable to the performance of an actual bolt manufactured by quenching and tempering.

  Since the steel for high-strength bolts of the present invention has sufficient delayed fracture resistance even when the tensile strength is 1350 MPa or higher, an automobile placed in a harsh environment such as a coast or a coastal area close thereto. It is suitable as a material for high-strength bolts used in various industrial machines and building structures.

Claims (6)

In mass%, C: more than 0.30% and 0.55% or less, Si: 0.3% or less, Mn: 0.6% or less, P: 0.025% or less, S: 0.030% or less Al: 0.005-0.10%, Cr: 1.0-2.5%, Mo: 0.25-2.0%, N: 0.030% or less, and Sn: 0.05-0. A steel for high-strength bolts containing 50%, fn represented by the following formula (1) being 1.4 or more, and the balance being Fe and impurities.
fn = Cr + Mo (1)
However, the element symbol in the formula (1) represents the content in mass% of the element.   The steel for high-strength bolts according to claim 1, further comprising, by mass%, V: 0.50% or less.   The steel for high-strength bolts according to claim 1 or 2, further comprising one or two of Ti: 0.10% or less and Zr: 0.10% or less in terms of mass%. .   The high content according to any one of claims 1 to 3, further comprising one or two of Ca: 0.01% or less and Mg: 0.01% or less in mass%. Steel for strength bolts.   The steel for high-strength bolts according to any one of claims 1 to 4, further comprising Ni: 3.0% or less in terms of mass%.   The steel for high-strength bolts according to any one of claims 1 to 4, further comprising Ni: 3.0% or less and Cu: 0.3-1.0% in mass%.

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