JP7164032B2 - Bolts and steel materials for bolts - Google Patents

Description

The present disclosure relates to bolts and steel materials for bolts.

As the performance of automobiles and industrial machines increases, the weight of automobiles and industrial machines increases, and the size of civil engineering and construction structures increases, bolts with higher strength are required.
The bolts are made of machine structural alloy steel such as SCM435 and SCM440 specified in JIS G 4053:2016. The strength of the bolt is adjusted by quenching and tempering after forming the alloy steel for machine structural use into a predetermined shape.
In order to increase the strength of the bolt, the carbon content of the steel material should be increased or the tempering temperature should be lowered.

However, a bolt with a tensile strength exceeding 1200 MPa poses a problem of delayed fracture, which is a kind of hydrogen embrittlement. Delayed fracture is a phenomenon in which a component placed under static stress suddenly and brittlely fractures after a certain period of time.
Delayed fracture is a phenomenon caused by penetration of hydrogen, and the higher the strength of the steel material, the lower the critical value of hydrogen penetration leading to delayed fracture.
When the bolt is used outdoors, particularly in an environment where seawater, snowmelt salt, and the like splash, the amount of hydrogen entering increases due to salt adhesion, increasing the risk of delayed fracture.

Therefore, conventionally, bolts having excellent delayed fracture resistance have been studied.
For example, Patent Literature 1 discloses a bolt and steel material having a tensile strength of 1617 MPa or more and excellent delayed fracture resistance by utilizing precipitation strengthening by Mo carbide and W carbide that precipitate during tempering.
Further, Patent Document 2 discloses a method for manufacturing a high-strength bolt having a tensile strength of 1600 MPa or more and excellent delayed fracture resistance, which advantageously prevents hydrogen embrittlement represented by delayed fracture.
In addition, Patent Document 3 discloses a high-strength bolt with good delayed fracture resistance and a strength of 1500 MPa or more and a method for improving the delayed fracture resistance using alloy carbides of V, Mo, Ti and Nb. .
Further, Patent Document 4 discloses a high-strength tempered steel excellent in delayed fracture resistance using alloy carbides of V, Mo, Ti and Nb, and a method for producing the same.

Patent Document 1: JP-A-2001-032044 Patent Document 2: JP-A-2007-31736 Patent Document 3: JP-A-2006-131990 Patent Document 4: JP-A-2006-45670

Recently, there has been a demand for bolts that are even more excellent in delayed fracture resistance than the bolts of Patent Documents 1 to 4.
An object of the present disclosure is to provide a bolt exhibiting excellent delayed fracture resistance at a tensile strength level of 1600 MPa or more, where the risk of delayed fracture is generally very high, and a steel material for the bolt as a raw material thereof. to provide.

The inventors have adopted a steel material that has a predetermined chemical composition as the material of the bolt and that the contents of Mo, V, W and Cr satisfy the following formulas (1), (2) and (3). As a result, M ₂ C-type carbides, which act as hydrogen trap sites, are dispersed in the bolt.
2V/(Mo+0.5W)≦0.20 (1)
0.10≦(2V+0.5W)/Mo≦0.40 (2)
0.10≤2Cr/Mo≤0.35 (3)
As a result, the inventors have found that a bolt having high strength and excellent delayed fracture resistance can be obtained.
The above problems are solved by the following means.

<1>
The composition, in mass %,
C: 0.35 to 0.50%,
Si: 0.02 to 0.10%,
Mn: 0.20-0.80%,
Mo: 1.50-5.00%,
W: 0 to 1.00%,
V: 0 to 0.20%,
Cr: 0.20-0.50%,
Al: 0.010 to 0.100%,
N: 0.0010 to 0.0150%,
P: 0.015% or less, and S: 0.015% or less,
contains
The balance consists of Fe and impurities,
and satisfying the following formulas (1), (2), and (3) below,
Tensile strength is 1600 MPa or more,
bolt.
2V/(Mo+0.5W)≦0.20 (1)
0.10≦(2V+0.5W)/Mo≦0.40 (2)
0.10≤2Cr/Mo≤0.35 (3)
However, in formulas (1), (2), and (3), the contents (% by mass) of Cr, Mo, V, and W contained in bolts are substituted for Cr, Mo, V, and W, respectively. , V or W is not included, 0 is substituted for V or W.
<2>
in % by mass,
Ti: 0.100% or less,
Nb: 0.100% or less,
B: 0.0050% or less,
Ni: 0.20% or less,
Cu: 0.20% or less,
REM: 0.020% or less,
2. The bolt according to claim 1, further comprising at least one selected from the group consisting of Sn: 0.20% or less and Bi: 0.20% or less.
<3>
in % by mass,
Pb: 0.05% or less,
Cd: 0.05% or less,
Co: 0.05% or less,
Zn: 0.05% or less,
Ca: 0.02% or less, and Zr: 0.02% or less,
3. The bolt according to claim 1, further comprising at least one selected from the group consisting of:
<4>
M ₂ C _- type carbide having a length of 5 nm or more and containing a total of 70 atomic % or more of Mo, Cr, and at least one of V and W with respect to M (metal element) has a unit area 4. The bolt according to any one of claims 1 to 3, wherein there are 10 or more bolts per 0.01 μm ² .
<5>
In a solution at room temperature to which 3.0 g of ammonium thiocyanate was added per liter of a 3.0% by mass sodium chloride aqueous solution, the material was cathodically charged with hydrogen at a current density of 0.03 mA/cm ² for 24 hours, and then subjected to hydrogen permeation prevention plating. 5. The bolt according to any one of claims 1 to 4, wherein the bolt is left to stand for 96 hours and then breaks for 100 hours or more when a constant load of 0.9 times the tensile strength is applied.
<6>
In a solution at room temperature to which 3.0 g of ammonium thiocyanate was added per 1 L of 3.0% by mass sodium chloride aqueous solution, it was cathodically charged with hydrogen at a current density of 0.2 mA/cm ² for 72 hours and allowed to stand at room temperature for 48 hours. The bolt according to any one of <1> to <5>, which has a post-trapped hydrogen amount of 3.0 ppm or more.
<7>
A steel material for bolts, which is a material for the bolt according to any one of <1> to <6>,
A steel material for a bolt having the composition of the bolt.

According to the present disclosure, it is possible to provide a bolt that exhibits high strength and excellent delayed fracture strength, and a steel material for the bolt that is the raw material for the bolt.

An embodiment that is an example of the present disclosure will be described in detail below.
In addition, in this specification, "%" display of the content of each element in the chemical composition means "% by mass".
The content of each element in the chemical composition is sometimes referred to as "element content". For example, the content of C may be expressed as the amount of C.
A numerical range represented using "to" means a range including the numerical values described before and after "to" as lower and upper limits.
Numerical ranges in which "greater" or "less than" are attached to numerical values written before and after "to" mean ranges that do not include these numerical values as lower or upper limits.
The term "process" includes not only an independent process, but also a process that is indistinguishable from other processes, as long as the intended purpose of the process is achieved.

[Bolt chemical composition]
The chemical composition of the bolt according to this embodiment is as follows.

(essential element)
C: 0.35-0.50%
C is an element that improves the strength of steel and increases the strength of bolts. If the amount of C is less than 0.35%, the strength required for the bolt cannot be obtained. On the other hand, if the C content is more than 0.50%, a large amount of alloy carbide remains undissolved during quenching heating, resulting in a decrease in strength at a predetermined tempering temperature, and the precipitation amount of alloy carbide during tempering is relatively reduced. Therefore, the hydrogen trapping ability is also lowered.
Therefore, the amount of C should be 0.35 to 0.50%. The preferred C content is 0.38-0.45%, and the more preferred C content is 0.40-0.43%.

Si: 0.02-0.10%
By reducing the content of Si, the delayed fracture resistance can be improved. In order to increase the delayed fracture strength, the amount of Si is set to 0.10% or less. On the other hand, even if the amount of Si is less than 0.02%, the improvement in delayed fracture strength is saturated, and the cost in the steelmaking process increases.
Therefore, the Si content should be 0.02 to 0.10%. A preferred Si content is 0.02 to 0.08%, and a more preferred Si content is 0.03 to 0.06%.

Mn: 0.20-0.80%
Mn combines with S to form MnS and prevents grain boundary segregation of S. In addition, it has the effect of improving the hardenability. If the Mn content is less than 0.20%, the grain boundary segregation of S increases and the delayed fracture strength decreases. On the other hand, if the Mn content exceeds 0.80%, the cold workability when working into a component shape is lowered, and quench cracks are likely to occur.
Therefore, the Mn content should be 0.20 to 0.80%. A preferable Mn amount is 0.30 to 0.70%, and a more preferable Mn amount is 0.40 to 0.60%.

Mo: 1.50-5.00%
W: 0-1.00%
V: 0-0.20%
Mo, W and V are important elements in this disclosure. Mo and W are elements that form M ₂ C type carbide. V is an element that forms MC-type carbides, but when an appropriate amount of V is mixed with Mo, M ₂ C-type carbides containing V precipitate. Carbides containing Mo, Cr, and at least one of W and V correspond to these M ₂ C type carbides.
A large amount of fine M ₂ C type carbides can be precipitated by tempering the steel at a high temperature of 570 to 690° C. after quenching the steel from the austenite region. Precipitation of these fine M ₂ C-type carbides can increase the strength of the steel through precipitation strengthening. In addition, fine M ₂ C-type carbides function as hydrogen trap sites and can improve delayed fracture resistance. Trapped hydrogen is hydrogen that is immobilized by the M ₂ C type carbides and cannot move freely in the steel.

In order to obtain the effect as a trap site, it is necessary to contain Mo at 1.50% or more. In addition, by containing an appropriate amount of at least one of W and V, the effect of the M ₂ C type carbide as a trap site is further improved. On the other hand, when the amount of Mo exceeds 5.0%, the amount of W exceeds 1.0%, or the amount of V exceeds 0.20%, undissolved coarse coal is formed during quenching heating. Nitrides remain. In order to dissolve the coarse carbonitrides into the austenite, the heating temperature for quenching must be raised, which causes problems such as distortion during quenching and an increase in oxides on the surface.
Therefore, the Mo content is 1.50 to 5.00%, the W content is 0 to 1.00%, and the V content is 0 to 0.20%.
The preferred Mo content is 2.00 to 4.00%, the preferred W content is 0.02 to 1.00%, and the preferred V content is 0.10 to 0.17%.
A more preferable Mo amount is 2.50 to 3.50%, a more preferable W amount is 2.70 to 3.20%, and a more preferable V amount is 0.12 to 0.15%.

The contents of Mo, W and V must satisfy the following formulas (1) and (2).
2V/(Mo+0.5W)≦0.20 (1)
0.10≦(2V+0.5W)/Mo≦0.40 (2)
In formulas (1) and (2), the contents (% by mass) of Mo, W, and V contained in bolts are substituted for Mo, W, and V, respectively, and when V or W is not included, 0 is assigned to V.

Cr: 0.20-0.50%
Cr is an effective element for ensuring the hardenability of steel, and also has the effect of improving the hydrogen trapping ability by forming a solid solution in M ₂ C type carbide. If the Cr content is less than 0.20%, these effects will be insufficient. On the other hand, if the Cr content exceeds 0.50%, the cementite is stabilized and precipitation of M ₂ C type carbide during tempering is inhibited, so the intended hydrogen trapping effect cannot be obtained.
Therefore, the Cr content should be 0.20 to 0.50%. A preferred Cr content is 0.20 to 0.30%, and a more preferred Cr content is 0.24 to 0.28%.

In order not to inhibit the precipitation of M ₂ C type carbides, the Cr content must satisfy the following formula (3).
0.10≤2Cr/Mo≤0.35 (3)
In the formula (3), the Cr and Mo contents (% by mass) contained in the bolt are substituted for Cr and Mo, respectively.

For high-strength bolts with a tensile strength of 1600 MPa or more, it is necessary to disperse a large amount of fine M ₂ C-type carbides, which are hydrogen trapping sites, in the steel in order to improve the delayed fracture strength.
In formula (1), when the value of "2V/(Mo ₊ 0.5W)" exceeds 0.20, the hydrogen trapping ability of the M2C type carbide is insufficient, resulting in a decrease in delayed fracture strength.
In formula (2), when the value of “(2V+0.5W)/Mo” is less than 0.10, the degree of combination of Mo, W and V in the M ₂ C-type carbide is low, and the M ₂ C-type carbide does not trap hydrogen. Insufficient capacity and reduced delayed fracture strength. On the other hand, when the value of “(2V+0.5W)/Mo” exceeds 0.40, the M ₂ C type carbide becomes unstable and becomes another carbide, so the hydrogen trapping ability is insufficient and the delayed fracture resistance is reduced. do.
In formula (3), when the value of " _2Cr /Mo" is less than 0.10, the hydrogen trapping ability of the M2C type carbide is lowered. When the value of " _2Cr /Mo" exceeds 0.35 _, the precipitation amount of cementite containing a large amount of Cr, _M23C6 or _M7C3 increases.
Therefore, the contents of Cr, Mo, V, and W must satisfy formulas (1), (2), and (3).

Al: 0.010-0.100%
Al is an element that functions as a deoxidizer and forms nitrides to suppress coarsening of austenite grains during heating for quenching. In order to obtain these effects, it is necessary to contain 0.010% or more of Al. On the other hand, if the amount of Al exceeds 0.100%, coarse oxide-based inclusions remain in the steel and act as starting points for bolt fracture.
Therefore, the Al content is set to 0.010 to 0.100%. A preferable Al amount is 0.012 to 0.050%, and a more preferable Al amount is 0.025 to 0.035%.

N: 0.0010 to 0.0150%
N is an element that forms nitrides or carbonitrides and suppresses coarsening of austenite grains during heating for quenching. In order to suppress coarsening of crystal grains, the amount of N must be 0.0010% or more. On the other hand, when the amount of N exceeds 0.0150%, coarse nitrides and carbonitrides are formed, which act as fracture starting points.
Therefore, the amount of N should be 0.0010 to 0.0150%. The preferred N content is 0.0020-0.0100%, and the more preferred N content is 0.0030-0.0050%.

P: 0.015% or less P is an impurity. It is preferable that the amount of P is as low as possible. P segregates at austenite grain boundaries. If the amount of P exceeds 0.015%, the grain boundaries of prior austenite after quenching and tempering become embrittled, causing intergranular cracking. Therefore, it is necessary to limit the P content to a range of 0.015% or less. A preferable upper limit of the amount of P is 0.012%. P is an impurity element, but within the above range, P may be contained in the bolt in excess of 0%.
However, the lower limit of the amount of P may be 0.005% or more from the viewpoint of reducing the P removal cost.

S: 0.015% or less S is an impurity. It is preferable that the amount of S is as low as possible. S exists as Mn sulfide in bolts. Mn sulfide generates hydrogen sulfide in a chemical reaction when the steel surface corrodes. This hydrogen sulfide decomposes to generate hydrogen, which penetrates into the steel and lowers the delayed fracture resistance. In addition, Mn sulfide becomes a starting point of fracture. Therefore, it is necessary to limit the S content to a range of 0.015% or less. A preferable upper limit of the amount of S is 0.012%. S is an impurity element, but within the above range, the bolt may contain more than 0% of S.
However, the lower limit of the S amount may be 0.005% or more from the viewpoint of reducing the S-free cost.

(arbitrary element)
The bolt according to the present embodiment may contain at least one element selected from the group consisting of Ti, Nb, B, Ni, Cu, W, REM, Sn, and Bi as an arbitrary element. Specifically, each of these arbitrary elements may be contained in the range of 0% to the upper limit of each element described later.

Ti: 0.100% or less Ti is an element that combines with N and C in bolts to form carbonitrides. These carbonitrides pin the austenite grain boundaries to prevent coarsening of the structure. In order to obtain the effect of preventing coarsening of the structure, 0.100% or less of Ti may be contained. On the other hand, if the Ti content exceeds 0.100%, the hardness of the material increases, resulting in a decrease in cold workability when working into a component shape.

Therefore, the Ti content is preferably 0.100% or less, more preferably over 0% to 0.100%, and even more preferably 0.005 to 0.050%.

Nb: 0.100% or less Nb is an element that combines with N and C in bolts to form carbonitrides. The carbonitrides pin the austenite grain boundaries and prevent coarsening of the structure. In order to obtain the effect of preventing coarsening of the structure, 0.100% or less of Nb may be contained. On the other hand, if the Nb content exceeds 0.100%, the hardness of the material increases, resulting in a decrease in cold workability when working into a component shape.

Therefore, the Nb content is preferably 0.100% or less, more preferably over 0% to 0.100%, and even more preferably 0.005 to 0.050%.

B: 0.0050% or less B enhances the hardenability of steel even when it is slightly dissolved in austenite. B may be contained in the bolt in order to efficiently obtain martensite during carburizing and quenching. On the other hand, if the B content exceeds 0.0050%, a large amount of BN is formed and N is consumed, resulting in coarsening of the austenite grains.
Therefore, the B content is preferably 0.0050% or less, more preferably greater than 0 to 0.0050%, and even more preferably 0.0007 to 0.0030%.

Ni: 0.20% or less Ni is an element that enhances corrosion resistance and toughness, and may be contained in the bolt. If the amount of Ni is too large, the effect commensurate with the cost cannot be obtained, so the upper limit of the amount of Ni is preferably 0.20%. On the other hand, the lower limit of the Ni content is preferably 0.01%.

Cu: 0.20% or less Cu is an element that enhances corrosion resistance and may be contained in bolts. On the other hand, if the amount of Cu exceeds 0.20%, the hot ductility deteriorates, so the upper limit of the amount of Cu is preferably 0.20%. On the other hand, the lower limit of Cu content is preferably 0.01%.

REM: 0.020% or less REM (rare earth element) is a general term for a total of 17 elements, including 15 elements from lanthanum with atomic number 57 to lutecium with atomic number 71, scandium with atomic number 21, and yttrium with atomic number 39. is. When REM is contained in the bolt, extension of MnS particles is suppressed during rolling and hot forging, and cracking during cold forging is suppressed. However, when the amount of REM exceeds 0.020%, a large amount of sulfide containing REM is generated, and the machinability of the steel material for bolts deteriorates.
Therefore, the total amount of the 17 elements is preferably 0.020% or less, more preferably more than 0% to 0.020%, and even more preferably 0.005% to 0.015%.

Sn: 0.20% or less Sn is an element that enhances corrosion resistance and may be contained in the bolt. If the amount of Sn is too large, the high-temperature ductility decreases and the risk of cracking during casting increases, so the upper limit of the amount of Sn is preferably 0.20%. On the other hand, the lower limit of Sn content is preferably 0.005%, more preferably 0.01%.

Bi: 0.20% or less Bi is an element that improves workability and may be contained in the bolt. If the amount of Bi is too large, the high-temperature ductility decreases and the risk of cracking during casting increases, so the upper limit of the amount of Bi is preferably 0.20%. On the other hand, the lower limit of the amount of Bi is preferably 0.005%, more preferably 0.01%.

(Other arbitrary elements)
The bolt according to the present embodiment may contain at least one element selected from the group consisting of the following elements as an arbitrary element. Specifically, each of these arbitrary elements may be contained in the range of 0% to the upper limit of each element described later. Even if these optional elements are included in the bolt within the range described later, the properties of the bolt are not affected.
Pb: 0.05% or less Cd: 0.05% or less Co: 0.05% or less Zn: 0.05% or less Ca: 0.02% or less Zr: 0.02% or less

The remainder of the bolt chemical composition in this embodiment consists of Fe and impurities. The term "impurity" as used herein means an element that is mixed in from the ore, scrap, or the environment during the manufacturing process that is used as a raw material for steel.

(M ₂ C type carbide)
In the bolt according to the present embodiment, it is preferable that 10 or more M ₂ C type carbides having a length of 5 nm or more exist per unit area of 0.01 μm ² .
Fine M ₂ C-type carbides (carbides containing Mo, Cr, and at least one of W and V) precipitated during tempering have a higher hydrogen trapping ability and delayed fracture resistance than VC, Mo ₂ C, and the like. Contribute to improvement.
Here, fine M ₂ C-type carbides are M ₂ C-type carbides containing 70 atomic % or more of Mo, Cr, and at least one of V and W in total with respect to M (metal element). Specifically, fine M ₂ C type carbides correspond to (Mo, Cr, W, V) ₂ C, (Mo, Cr, W) ₂ C, and (Mo, Cr, V) ₂ C. .
These M ₂ C type carbides have a higher hydrogen trapping ability than VC, Mo ₂ C, etc., and contribute to an improvement in delayed fracture resistance.
Therefore, it is preferable to allow a predetermined amount of M ₂ C type carbide with a length of 5 nm or more to exist.
Therefore, the number density of M ₂ C-type carbides with a length of 5 nm or more (the number of M ₂ C-type carbides with a length of 5 nm or more existing per unit area of 0.01 μm ² ) is preferably 10 or more.
From the viewpoint of improving delayed fracture resistance, the number density of M ₂ C type carbides is more preferably 15 or more per unit area of 0.01 μm ² , and even more preferably 20 or more per unit area of 0.01 μm ² .
However, the upper limit of the number density of M ₂ C type carbides is, for example, 100 or less per unit area of 0.01 μm ² from the viewpoint of suppressing deterioration in elongation and toughness.

The number density of M ₂ C type carbides is measured by using a transmission electron microscope after preparing a thin film test piece by the thin film method.
The components of M ₂ C type carbide are measured by preparing a test piece by the extraction replica method and using a transmission microscope (TEM) equipped with an energy dispersive X-ray spectrometer (EDS).
Specifically, it is as follows.

From any part of the bolt to be measured, a part having a plane (hereinafter also referred to as "measurement plane") located at a depth of 2 mm from the surface of the bolt and parallel to the surface of the bolt is sampled, and a thin film test is performed by the thin film method. Specimens are prepared by strip and extraction replica method.

Here, the preparation of the thin film test piece by the thin film method is as follows. First, the base material is cut to a thickness of 0.5 mm using a precision cutting machine. Next, using emery paper of P320 to 1200, cutting and polishing is performed from both sides to a thickness of 60 μm, and a sample of 3 mmφ is punched out. After that, double-sided jet electropolishing is performed until a hole is formed in the center to obtain a thin film test piece for TEM observation. Electropolishing is performed by Tenupol, using 100 ml of perchloric acid-800 ml of glacial acetic acid solution-100 ml of methanol as the electro-polishing liquid, and electro-polishing conditions of 30 V and 0.1 A.
Moreover, preparation of the test piece by the extraction replica method is as follows. First, the measurement surface of the sample taken from the steel member is electrolytically polished. After electropolishing, the measurement surface of the specimen is subjected to constant potential electrolysis at a potential of −200 mV using a 10% acetylacetone-1% tetramethylammonium chloride (TMAC)-methanol solution. This exposes the M ₂ C type carbide from the measurement surface of the sample. The energization time is 30 to 60 sec.
After attaching an acetylcellulose film to the measurement surface of the sample after electrolysis, the film is peeled off and the M ₂ C type carbide is transferred onto the film. Carbon vapor deposition is performed on the transferred film to prepare a carbon vapor deposition film. The carbon deposition film is immersed in a methyl acetate solution to dissolve the acetyl cellulose film, and is scooped up with a Cu mesh having a diameter of 3 mm to obtain an extraction replica film (test piece by extraction replica method).

Next, the number density of M ₂ C type carbides is measured as follows. With the direction perpendicular to the {001} plane of the iron matrix as the incident direction of the electron beam, an arbitrary field of view of the thin film test piece (the measurement surface thereof) is magnified 400000 times (observation area 0.25 μm × 0.25 μm) for 3 fields. Observe. M ₂ C type carbide is identified by electron beam diffraction pattern analysis. After that, the length and number of all M ₂ C-type carbides present in a 0.1 μm×0.1 μm region at the center of the observation screen were measured, and the number of M ₂ C-type carbides having a length of 5 nm or more was counted. Measurement is performed, and the average value of five fields of view is obtained as the "number density of M ₂ C type carbide".
Here, the length of M ₂ C-type carbide means the maximum length of observed M ₂ C-type carbide.
The TEM observation is performed with an FE-TEM at an acceleration voltage of 200 kV.

Also, the chemical components of the M ₂ C type carbide are measured as follows. An arbitrary field of view (observation area of 0.5 μm×0.5 μm) of the extraction replica film (the measurement surface thereof) as the test piece is observed at a magnification of 200,000. M ₂ C-type carbides are identified by analysis of the electron beam diffraction pattern of TEM and analysis by EDS, and the atomic % of metal elements in the carbides is measured by EDS analysis. . Five samples were measured, and the average value of these metal element concentrations was used.
TEM electron diffraction pattern analysis and EDS analysis are performed with FE-TEM at an accelerating voltage of 200 kV.

(Tensile strength)
The bolt according to the present embodiment has a tensile strength of 1600 MPa or more, which is measured by taking a tensile test piece from the bolt.
The bolt tensile strength is a value measured according to JIS Z 2241:2011.

However, the tensile strength of the bolt is measured by taking a test piece from the bolt as follows.
A No. 14A test piece is cut out from the shaft portion of the bolt so that the diameter of the parallel portion is 50% of the diameter of the bolt, and a tensile test is performed in the air at room temperature (25°C) to determine the tensile strength.

(Trapped hydrogen amount)
The bolt according to the present embodiment was cathodically charged with hydrogen at a current density of 0.2 mA/cm ² for 72 hours in a solution at room temperature to which 3.0 g of ammonium thiocyanate was added per 1 L of a 3.0% by mass sodium chloride aqueous solution. , the amount of trapped hydrogen after standing at room temperature (25° C.) for 48 hours is preferably 3.0 ppm or more.
If the amount of trapped hydrogen is less than 3.0 ppm, hydrogen that has entered the bolt may diffuse and accumulate at the prior austenite grain boundaries, increasing the risk of delayed fracture. Therefore, the amount of trapped hydrogen is preferably 3.0 ppm or more.

The amount of trapped hydrogen is measured by a temperature-programmed hydrogen analysis method using a gas chromatograph. The amount of hydrogen released from the sample from room temperature (25°C) to 400°C at a heating rate of 100°C/hour is defined as the amount of trapped hydrogen.

The measurement of the amount of trapped hydrogen is performed on a round-bar test piece (a round-bar test piece for investigating the amount of trapped hydrogen) with a diameter of 7 mm and a length of 70 mm taken from a bolt.
However, if a round-bar test piece of the above size cannot be collected, a round-bar test piece with a diameter of 5 mm and a length of 20 mm is substituted, charged with hydrogen in the same manner and allowed to stand still. may be measured.

(Delayed fracture strength)
Since the bolt according to the present embodiment is used in an actual environment, it is preferable to have sufficient delayed fracture resistance. Therefore, in the bolt according to this embodiment, at a current density of 0.03 mA/cm ² in a room temperature (25 ° C.) solution containing 3.0 g of ammonium thiocyanate per 1 L of 3.0% by mass sodium chloride aqueous solution, After cathodic hydrogen charging for 24 hours, hydrogen permeation prevention plating is applied, and after standing for 96 hours, when a constant load of 0.9 times the tensile strength is applied, the time until breakage is 100 hours or more. is preferred.
Here, the hydrogen permeation prevention plating is performed to confine hydrogen in the bolt, and hot-dip galvanization is performed.

Measurement of delayed fracture strength is performed on a round bar test piece (delayed fracture test piece) with a notch of 7 mm in diameter and 70 mm in length (notch diameter 4.2 mm, angle 60 °) taken from the bolt. do.
However, if a round-bar test piece of the above size cannot be obtained, a round-bar test piece with a notch of 5 mm diameter (notch diameter: 3.0 mm, angle: 60°) may be substituted. There are no particular restrictions on the length as long as chucking is possible.

<Steel for bolts>
The steel material for bolts according to this embodiment is a steel material that is used as a material for the bolts according to this embodiment. The steel material for bolts according to this embodiment has the same chemical composition as the bolt according to this embodiment.

<Bolt manufacturing method>
An example of a method for manufacturing a bolt according to this embodiment will be described in detail below using the steel material for bolts according to this embodiment.

(Process of forming into a bolt shape)
After obtaining the molten steel having the chemical composition of the bolt according to the present embodiment, the molten steel is cast into an ingot or a slab. The cast ingot or slab is finished into a steel material having a desired rough shape such as a round bar by hot working such as hot rolling, hot extrusion and hot forging. After that, the steel material is subjected to wire drawing, annealing, cold working, thread rolling, etc., and formed into a predetermined bolt shape. Annealing or spheroidizing treatment may be performed multiple times between multiple cold workings. Hot working can also be included in the molding process.

(Process of quenching and tempering)
After forming into a predetermined bolt shape, in order to impart strength, the steel is heated to a temperature above austenitization and then quenched by water cooling or oil cooling. If the heating temperature for quenching (hereinafter referred to as "quenching heating temperature") is too low, solid solution of Mo, Cr, W, and V carbides in the matrix becomes insufficient, and coarse carbides are formed. remain. As a result, the amount of fine M ₂ C-type carbides precipitated during tempering is reduced, making it impossible to obtain the desired strength and hydrogen trapping effect.

On the other hand, if the quenching heating temperature is excessively high, the crystal grains become coarse, the toughness and delayed fracture resistance deteriorate, and the damage to the furnace body and accessories of the heat treatment furnace in operation becomes noticeable, resulting in manufacturing costs. is not favorable because it increases
Therefore, the heating temperature for quenching is preferably 930 to 1050°C. Moreover, the holding time at the quenching heating temperature is preferably 30 to 90 minutes.

In order to improve the delayed fracture strength, it is necessary to perform tempering after performing the above-described quenching treatment. In the present disclosure, the tempering temperature should be limited to 570-690°C.

If the tempering temperature is less than 570° C., precipitation of M ₂ C type carbides during tempering is insufficient, and the desired hydrogen trapping ability and delayed fracture limit hydrogen amount cannot be achieved.

On the other hand, when the tempering temperature is 690° C. or higher, the M ₂ C type carbides undergo Ostwald growth, and the target hydrogen trapping ability and delayed fracture limit hydrogen amount cannot be achieved.
Therefore, the tempering temperature is limited to 570-690°C. A preferable tempering temperature range is 590 to 660°C.
The holding time at the tempering temperature is preferably 30 to 90 minutes, and the tempering cooling rate is preferably 50 to 100°C/s.

Through the steps described above, the bolt according to the present embodiment is manufactured.

As described above, in the bolt according to the present embodiment, steel material having an optimum chemical composition is subjected to optimum quenching and tempering, thereby optimizing tensile strength, trapped hydrogen content, and delayed fracture limit hydrogen content. It is.

Next, examples of the present disclosure will be described, but the conditions shown below are only examples adopted to confirm the feasibility and effect of the present disclosure, and the conditions of the present disclosure are limited to these examples. not something. In carrying out the present disclosure, various conditions can be adopted as long as the purpose is achieved without departing from the gist thereof.

<Molding of various test pieces>
(Preparation of steel bars)
Steels having chemical compositions shown in Tables 1-1 and 1-2 (Steel Nos. A to P and AA to AY) were melted and hot forged to prepare steel bars with a diameter of 20 mm and a length of 1000 mm. . Note that the underlined values in Tables 1-1 and 1-2 are outside the scope of the present disclosure. "-" in Tables 1-1 and 1-2 indicates that each element is not added. In addition, in formulas (1) to (3), "0" is substituted for the contents of the elements indicated by "-". The remainder is Fe and impurities.
However, in the chemical compositions shown in Tables 1-1 and 1-2, oxygen (O) is an element contained as an impurity in steel.

Next, in order to reproduce the bolt manufacturing, quenching and tempering were performed under the conditions shown in Table 2, and then the tensile strength of the quenched and tempered bolt equivalent, the amount of trapped hydrogen, and the delayed fracture strength were measured by the following methods. evaluated with

(Implementation of quenching)
A round bar with a diameter of 20 mm and a length of 1,000 mm was cut into a round bar with a diameter of 20 mm and a length of 300 mm, which was quenched at the temperature shown in Table 2. The holding time at the quenching heating temperature was 60 minutes. After that, quenching was performed in an oil bath maintained at 60°C.

(Implementation of tempering)
After oil quenching, tempering was performed at the temperature shown in Table 2. The holding time at the tempering temperature was 60 minutes, and the cooling after tempering was air cooling (cooling rate 10°C/s).

(Tensile test piece)
A smooth tensile test piece (No. 14A test piece) having a total length of 70 mm, parallel portion diameter of 6 mm, and length of 32 mm was taken from the round bar of 20 mm in diameter and 300 mm in length after the quenching and tempering treatment.

(Preparation of test piece for investigation of trapped hydrogen amount)
A round bar with a diameter of 7 mm and a length of 70 mm was sampled from the round bar with a diameter of 20 mm and a length of 300 mm after the quenching and tempering treatment, and used as a round bar test piece for investigating the amount of trapped hydrogen.

(Preparation of test piece for delayed fracture strength)
A round bar test piece with a notch (notch diameter 4.2 mm, angle 60°) with a diameter of 7 mm and a length of 70 mm was taken from the round bar with a diameter of 20 mm and a length of 300 mm after the quenching and tempering treatment, and the resistance A test piece for delayed fracture strength was used.

As described above, production No. 1 to 45 tensile test pieces, production no. 1 to 45 test pieces for investigating the amount of trapped hydrogen, and manufacturing No. Specimens with a delayed fracture strength of 1 to 45 were obtained, respectively. However, production No. As for No. 34, quenching cracks occurred, so the subsequent tests were discontinued.

<Performance evaluation using each test piece>

(Number density of M ₂ C type carbides with a length of 5 nm or more)
The number density (the number per unit area of 0.01 μm ² ) of M ₂ C type carbides with a length of 5 nm or more was measured as described above. Then, evaluation was made according to the following criteria.
A: The number density of M ₂ C type carbides is 10 pieces/0.01 μm ² or more and 14 pieces/0.01 μm ² or less B: The number density of M ₂ C type carbides is 15 pieces/0.01 μm ² or more and 20 pieces/0 Less than 0.01 μm ² C: The number density of M ₂ C type carbides is 20 pieces/0.01 μm ² or more D: The number density of M ₂ C type carbides is less than 10 pieces/0.01 μm ²

(Tensile strength (TS))
Tensile strength was measured as previously described.
Specifically, using the tensile test piece prepared by the above procedure, a tensile test was performed in the atmosphere at room temperature (25° C.) in accordance with JIS Z 2241:2011 to determine the tensile strength.

(Trapped hydrogen amount)
The amount of trapped hydrogen was measured as described above.
Specifically, 3.0 g of ammonium thiocyanate per 1 L of a 3.0% by mass sodium chloride aqueous solution was added to a round bar test piece having a diameter of 7 mm and a length of 70 mm prepared by the above procedure at room temperature (25 ° C.). Cathodic hydrogen charging was carried out at a current density of 0.2 mA/cm ² for 72 hours in a solution of After that, it was allowed to stand at room temperature (25° C.) for 48 hours. Then, using a gas chromatograph, the temperature was raised from room temperature (25° C.) to 400° C. at a heating rate of 100° C./hour, and the amount of hydrogen released from the round-bar test piece was measured.

(Delayed fracture strength test)
The delayed fracture resistance test was measured as described above.
Specifically, a delayed fracture strength test piece with a φ7 mm × 70 mm notch (notch φ4.2 mm, angle 60 °) prepared by the above procedure was added to 1 L of a 3.0% by mass sodium chloride aqueous solution. After cathodic hydrogen charging at a current density of 0.03 mA/cm ² for 24 hours in a solution at room temperature (25°C) to which 3.0 g of ammonium thiocyanate was added, it was plated with Zn to prevent hydrogen permeation, and left to stand for 96 hours. After that, a constant load of 0.9 times the tensile strength was applied, and the time until breakage was measured. The test was discontinued when no breakage occurred for 100 hours.

Table 2 shows the number density, tensile strength (TS), amount of trapped hydrogen, and presence or absence of delayed fracture of M ₂ C type carbides. Note that the underlined values in Table 2 are outside the scope of the present disclosure. In addition, the sign "-" in Table 2 means that the test was not performed.

As is clear from Tables 1 and 2, production No. 1 was optimized for the chemical composition and conditions for quenching and tempering. 1 to 20 (Disclosure Examples 1 to 20) all had high tensile strength, high trapped hydrogen content, and no delayed fracture, so excellent strength and delayed fracture resistance were obtained. It turns out that there is

Disclosure Examples 16 and 17 satisfy the composition requirements of the present disclosure, but were produced under production conditions in which the quenching conditions were slightly out of the preferred range. Disclosed example 17 is manufactured under manufacturing conditions with a higher quenching temperature than those of other disclosed examples, and its strength is slightly higher than that of other disclosed examples. Therefore, the strength-ductility balance of the other disclosed examples is relatively superior. In addition, Disclosure Example 16 is manufactured under manufacturing conditions with a lower quenching temperature than those of the other Disclosure Examples, and the other Disclosure Examples are relatively superior in terms of strength.

On the other hand, with respect to at least one of the chemical composition and the conditions of quenching and tempering, the production No. 1 has not been optimized. 21 to 45 (Comparative Examples 1 to 25) did not exhibit delayed fracture resistance. In addition, Comparative Examples 5, 13, 19 and 20 have insufficient strength, and Comparative Examples 1 to 4, 6 to 8, 10 to 12, and 16 to 25 do not obtain a sufficient amount of trapped hydrogen. . In Comparative Example 15, although the amount of trapped hydrogen is high, the amount of Al in the steel components is higher than 0.100, so there are many oxide-based inclusions containing Al, many fracture starting points are formed, and the delayed fracture characteristics are reduced. is doing.

The disclosure of Japanese Patent Application No. 2019-091328 is incorporated herein by reference in its entirety.
All publications, patent applications and technical standards mentioned herein are to the same extent as if each individual publication, patent application and technical standard were specifically and individually noted to be incorporated by reference. incorporated herein by reference.

According to the present disclosure, it is possible to provide a bolt that exhibits high strength and excellent delayed fracture strength, and a steel material for the bolt that is the raw material for the bolt.

Claims (7)

The composition, in mass %,
C: 0.35 to 0.50%,
Si: 0.02 to 0.10%,
Mn: 0.20-0.80%,
Mo: 1.50-5.00%,
W: 0 to 1.00%,
V: 0 to 0.20%,
Cr: 0.20-0.50%,
Al: 0.010 to 0.100%,
N: 0.0010 to 0.0150%,
P: 0.015% or less, and S: 0.015% or less,
contains
The balance consists of Fe and impurities,
and satisfying the following formulas (1), (2), and (3) below,
Tensile strength is 1600 MPa or more,
bolt.
2V/(Mo+0.5W)≦0.20 (1)
0.10≦(2V+0.5W)/Mo≦0.40 (2)
0.10≤2Cr/Mo≤0.35 (3)
However, in formulas (1), (2), and (3), the contents (% by mass) of Cr, Mo, V, and W contained in bolts are substituted for Cr, Mo, V, and W, respectively. , V or W is not included, 0 is substituted for V or W. in % by mass,
Ti: 0.100% or less,
Nb: 0.100% or less,
B: 0.0050% or less,
Ni: 0.20% or less,
Cu: 0.20% or less,
REM: 0.020% or less,
2. The bolt according to claim 1, further comprising at least one selected from the group consisting of Sn: 0.20% or less and Bi: 0.20% or less. in % by mass,
Pb: 0.05% or less,
Cd: 0.05% or less,
Co: 0.05% or less,
Zn: 0.05% or less,
Ca: 0.02% or less, and Zr: 0.02% or less,
3. The bolt according to claim 1, further comprising at least one selected from the group consisting of: M ₂ C _- type carbide having a length of 5 nm or more and containing a total of 70 atomic % or more of Mo, Cr, and at least one of V and W with respect to M (metal element) has a unit area 4. The bolt according to any one of claims 1 to 3, wherein there are 10 or more bolts per 0.01 μm ² . After cathodic hydrogen charging at a current density of 0.03 mA/cm ² for 24 hours in a room temperature solution to which 3.0 g of ammonium thiocyanate was added per 1 L of a 3.0% by mass sodium chloride aqueous solution, hydrogen permeation prevention plating was applied. 5. The bolt according to any one of claims 1 to 4, wherein the bolt is left to stand for 96 hours and then breaks for 100 hours or more when a constant load of 0.9 times the tensile strength is applied. In a solution at room temperature to which 3.0 g of ammonium thiocyanate was added per 1 L of 3.0% by mass sodium chloride aqueous solution, it was cathodically charged with hydrogen at a current density of 0.2 mA/cm ² for 72 hours and allowed to stand at room temperature for 48 hours. The bolt according to any one of claims 1 to 5, wherein the post-trapped hydrogen amount is 3.0 ppm or more. A steel material for bolts, which is a material for the bolt according to any one of claims 1 to 6,
A steel material for a bolt having the composition of the bolt.