JP7464832B2 - Bolts and bolt steel - Google Patents

Description

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

2. Description of the Related Art With the increasing performance and weight reduction of automobiles and industrial machinery, and the increasing size of civil engineering and architectural structures, there is a demand for bolts with higher strength.
The bolts are made of alloy steel for machine construction, such as SCM435 and SCM440, as specified in JIS G 4053: 2016. The bolts are made by forming the alloy steel for machine construction into a predetermined shape, and then adjusting the strength by quenching and tempering.
To increase the strength of a bolt, the carbon content of the steel can be increased or the tempering temperature can be lowered.

However, bolts with a tensile strength exceeding 1000 MPa are susceptible to delayed fracture, a type of hydrogen embrittlement, which is a phenomenon in which a component placed under static stress suddenly breaks in a brittle manner after a certain period of time.
Delayed fracture is a phenomenon caused by the penetration of hydrogen, and the higher the strength of the steel material, the lower the critical value of the amount of hydrogen penetration that leads to delayed fracture.
When bolts are used outdoors, particularly in environments where seawater or de-icing salt is present, the amount of hydrogen that penetrates increases due to salt adhesion, increasing the possibility of delayed fracture.

Therefore, bolts with excellent resistance to delayed fracture have been studied.
For example, Patent Document 1 discloses a bolt and steel material that utilizes V carbonitride, which acts as a hydrogen trapping site, and has a tensile strength of 1000 to 1600 MPa and excellent delayed fracture resistance.

JP 2002-276637 A

Recently, there has been a demand for bolts having even better delayed fracture resistance than the bolt of Patent Document 1.
An object of the present invention is to provide a bolt that exhibits excellent resistance to delayed fracture at a strength level having a tensile strength of 1000 MPa or more, where the risk of delayed fracture is generally very high, and a steel material for such a bolt.

The inventors discovered that by using a steel material as the bolt material that has a predetermined chemical composition and whose Mo, V, and W contents satisfy the following formulas (1) and (2), M2C carbides, which act as hydrogen trapping sites, are dispersed in the bolt.
0.10<(2V+0.5W+0.4Cr)/Mo<0.50 ... (1)
Mo/95.95+V/50.94+W/183.8-C/6.0>0...(2)
As a result, the inventors discovered 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 is, in mass%,
C: 0.10 to 0.34%,
Si: 0.02 to 0.10%,
Mn: 0.40 to 2.50%,
Cr: 0 to 0.50%,
Mo: 1.50 to 5.00%,
W: 0 to 4.00%,
V: 0 to 1.00%,
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 is Fe and impurities,
At a portion located 2 mm deep from the surface of the bolt shaft and parallel to the surface of the bolt, there are 10 or more _M2C type carbides with a length of 5 nm or more per unit volume of 0.001 ^μm3 .
A bolt having a tensile strength of 1000 MPa or more.
0.10<(2V+0.5W+0.4Cr)/Mo<0.50 ... (1)
Mo/95.95+V/50.94+W/183.8-C/6.0>0...(2)
Here, M represents a metal element, and in formulas (1) and (2), Mo, V, W, Cr, and C are substituted with the contents (mass%) of Mo, V, W, Cr, and C contained in the bolt, respectively, and when the corresponding element is not contained, 0 is substituted for the corresponding element symbol. Also, a composition in which the value of the formula on the left side of formula (2) is more than 0 after the decimal point is considered to satisfy formula (2).
＜2＞
The composition, in 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,
The bolt according to <1>, further containing at least one selected from the group consisting of Sn: 0.20% or less and Bi: 0.20% or less.
＜3＞
The composition, in 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,
The bolt according to <1> or <2>, further comprising at least one selected from the group consisting of:
＜4＞
The bolt according to any one of <1> to <3>, wherein the bolt is subjected to cathodic hydrogen charging at a current density of 0.03 mA/ ^cm2 for 24 hours in a room temperature solution containing 3.0 g of ammonium thiocyanate added per 1 L of a 3.0 mass % aqueous sodium chloride solution, and then subjected to hydrogen permeation prevention plating. The bolt is then left for 96 hours, and then subjected to a constant load of 0.9 times the tensile strength. The bolt thus obtained takes 100 hours or more to break.
＜5＞
The bolt according to any one of <1> to <4>, in which the amount of trapped hydrogen is 3.0 ppm or more after cathodic hydrogen charging at a current density of 0.2 mA/ ^cm2 for 72 hours in a room temperature solution containing 3.0 g of ammonium thiocyanate added per 1 L of a 3.0 mass % aqueous sodium chloride solution and then allowed to stand at room temperature for 48 hours.
＜6＞
A steel material for a bolt, which is a material for the bolt according to any one of <1> to <5>,
A steel material for bolts having the composition of the bolt.

The present invention provides a bolt that is high in strength and exhibits excellent resistance to delayed fracture, as well as the steel material for the bolt.

Hereinafter, an embodiment of the present invention will be described in detail.
In this specification, the "%" designation for the content of each element in a chemical composition means "mass %."
The content of each element in a chemical composition may be expressed as the “element amount.” For example, the content of C may be expressed as the C amount.
A numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits.
When the numerical range described before and after "to" is followed by "greater than" or "less than," it means that the numerical range does not include the numerical value as the lower limit or upper limit.
The term "process" includes not only an independent process, but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[Chemical composition of bolts]
The chemical composition of the bolt according to this embodiment is as follows:

(Essential elements)
C: 0.10 to 0.34%
C is an element that improves the strength of steel and increases the strength of bolts. If the C content is less than 0.10%, the strength required for a bolt cannot be obtained. On the other hand, if the C content is more than 0.34%, a large amount of cementite precipitates during the specified tempering, so that precipitation strengthening of alloy carbides commensurate with the carbon content cannot be obtained, and the hydrogen trapping ability is also reduced, resulting in a decrease in delayed fracture properties.
Therefore, the C content is set to 0.10 to 0.34%, and preferably to 0.20 to 0.32%.

Si: 0.02 to 0.10%
The Si content can be reduced to improve delayed fracture strength. In order to increase delayed fracture strength, the Si content is set to 0.10% or less. On the other hand, if the Si content is less than 0.02%, the improvement in delayed fracture strength saturates and the cost of the steelmaking process increases.
Therefore, the Si content is set to 0.02 to 0.10%, and preferably to 0.02 to 0.08%.

Mn: 0.40 to 2.50%
Mn combines with S to form MnS, preventing the grain boundary segregation of S. It also has the effect of improving hardenability. If the Mn content is less than 0.40%, the hardenability improvement effect is not sufficient. On the other hand, if the Mn content exceeds 2.50%, the cold workability when processing into a part shape is reduced, and quench cracks are likely to occur.
Therefore, the Mn content is set to 0.40 to 2.50%, and preferably to 0.50 to 0.80%.

Mo: 1.50 to 5.00%
W: 0 to 4.00%
V: 0 to 1.00%
Mo, W, and V are important elements in the present invention. Mo and W are elements that form _M2C type carbides. V is an element that forms MC type carbides, and when V is contained in combination with an appropriate amount of Mo, _M2C type carbides containing V are precipitated together with Mo. These _M2C type carbides include carbides containing Mo and at least one of Cr, V, and W.
Fine M ₂ C type carbides can be precipitated in large quantities by quenching steel from the austenite region and then tempering it at a high temperature of 570 to 690°C. The precipitation of these fine M ₂ C type carbides can increase the strength of the steel through precipitation strengthening. Furthermore, the fine M ₂ C type carbides function as hydrogen trapping sites and can improve delayed fracture resistance. Trapped hydrogen is hydrogen that is fixed by the M ₂ C type carbides and cannot move freely through 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, when at least one of W and V is contained in an appropriate amount, the effect of _M2C type carbides as a trap site is further improved. On the other hand, when the Mo content exceeds 5.00%, when the W content exceeds 4.00%, or when the V content exceeds 1.00%, undissolved coarse carbonitrides remain during quenching heating. Then, in order to dissolve these coarse carbonitrides in austenite, it becomes necessary to increase the quenching heating temperature, which causes problems such as distortion during quenching and an increase in oxides on the surface.
Therefore, the Mo content is set to 1.50 to 5.00%, the W content is set to 0 to 4.00%, and the V content is set to 0 to 1.00%.
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 0.35%.

The contents of Mo, W and V must satisfy the formulas (1) and (2).
0.10<(2V+0.5W+0.4Cr)/Mo<0.50 ... (1)
Mo/95.95+V/50.94+W/183.8-C/6.0>0...(2)
In formulas (1) and (2), the contents (mass%) of Mo, V, W, Cr, and C contained in the bolt are substituted for Mo, V, W, Cr, and C, respectively, and when the corresponding element is not contained, the corresponding element symbol is substituted with 0. Furthermore, a composition in which the value of the formula on the left side of formula (2) exceeds 0 after the decimal point is considered to satisfy formula (2).

Cr: 0 to 0.50%
Cr is an element effective for ensuring the hardenability of steel, and also has the effect of improving hydrogen trapping ability by dissolving in _M2C type carbides. However, if the Cr content exceeds 0.50%, it stabilizes _cementite and _M23C6 , and cementite remains even during tempering, making it impossible to obtain the desired hydrogen trapping effect.
Therefore, the Cr content is set to 0 to 0.50%, and preferably to 0.05 to 0.35%.

Here, in a bolt having a high strength of tensile strength of 1000 MPa or more, it is necessary to disperse a large amount of fine M ₂ C type carbides, which act as hydrogen trapping sites, in the steel in order to improve delayed fracture resistance.
In formula (1), if the value of "(2V+0.5W+0.4Cr)/Mo" is 0.10 or less, the degree of combination of Mo with at least one of Cr, W and V in the M ₂ C carbide is low, the hydrogen trapping ability is insufficient, and the delayed fracture resistance strength is reduced. On the other hand, if the value of "(2V+0.5W+0.4Cr)/Mo" is 0.50 or more in formula (1), the M ₂ C carbide becomes unstable and turns into a different carbide, so the hydrogen trapping ability is insufficient and the delayed fracture resistance strength is reduced. During heating for quenching, the M ₂ C carbide cannot be completely dissolved, and the M ₂ C carbide after tempering becomes coarse, and the delayed fracture resistance strength is reduced.
The value of "(2V+0.5W+0.4Cr)/Mo" is preferably 0.15 to 0.45.

In formula (2), if the value of "Mo/95.95+V/50.94+W/183.8-C/6.0" is 0 or less, cementite remains during tempering, and the hydrogen trapping ability of the M ₂ C type carbide decreases.
The value of "Mo/95.95+V/50.94+W/183.8-C/6.0" is preferably 0.0003 or more. However, from the viewpoint of reducing alloy costs, the upper limit of the value of "Mo/95.95+V/50.94+W/183.8-C/6.0" is preferably 0.0200 or less, and more preferably 0.0160 or less.

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

N: 0.0010 to 0.0150%
N is an element that forms nitrides or carbonitrides and suppresses the coarsening of austenite grains during quenching heating. In order to suppress the coarsening of grains, the N content must be 0.0010% or more. On the other hand, if the N content exceeds 0.0150%, coarse nitrides or carbonitrides are generated and become the starting points of fracture.
Therefore, the N content is set to 0.0010 to 0.0150%, and preferably to 0.0020 to 0.0100%.

P: 0.015% or less P is an impurity. It is preferable that the P content is as low as possible. P segregates at the austenite grain boundaries. If the P content exceeds 0.015%, the prior austenite grain boundaries after quenching and tempering become embrittled, causing grain boundary cracking. For this reason, it is necessary to limit the P content to a range of 0.015% or less. The preferred upper limit of the P content is 0.012%. P is an impurity element that is inevitably contained in bolt steel, but within the above range, P may be contained in the bolt at more than 0%.
However, from the viewpoint of reducing the cost of dephosphorization, the lower limit of the P content may be 0.005% or more.

S: 0.015% or less S is an impurity. It is preferable that the S content is as low as possible. S exists as Mn sulfide in the bolt. Mn sulfide generates hydrogen sulfide through a chemical reaction when the steel surface corrodes. This hydrogen sulfide decomposes to generate hydrogen, which penetrates into the steel and reduces the delayed fracture resistance strength. In addition, Mn sulfide becomes the starting point of fracture. For this reason, it is necessary to limit the S content to a range of 0.015% or less. The preferred upper limit of the S content is 0.012%. S is an impurity element that is inevitably contained, but within the above range, S may be contained in the bolt at more than 0%.
However, from the viewpoint of reducing the desulfurization cost, the lower limit of the S content may be 0.005% or more.

(Optional 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, REM, Sn, and Bi as an optional element. Specifically, each of these optional elements may be contained in a range from 0% to the upper limit of each element described below.

Ti: 0.100% or less Ti is an element that combines with N and C in the bolt to form carbonitrides. These carbonitrides pin the austenite grain boundaries to prevent the structure from coarsening. In order to obtain this effect of preventing the structure from coarsening, Ti may be contained in an amount of 0.100% or less. On the other hand, if Ti is contained in an amount exceeding 0.100%, the cold workability when processing into a part shape decreases due to an increase in the material hardness.

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

Nb: 0.100% or less Nb is an element that combines with N and C in steel to form carbonitrides. These carbonitrides pin the austenite grain boundaries and prevent the structure from coarsening. In order to obtain this effect of preventing the structure from coarsening, Nb may be contained in an amount of 0.100% or less. On the other hand, if Nb is contained in an amount exceeding 0.100%, the cold workability when processing into a part shape decreases due to an increase in the material hardness.

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

B: 0.0050% or less B improves the hardenability of steel even when dissolved in austenite in a small amount. B may be contained in bolts to efficiently obtain martensite during carburizing and quenching. On the other hand, if B is contained in excess of 0.0050%, a large amount of BN is formed and N is consumed, resulting in coarsening of austenite grains.
Therefore, the B content is preferably 0.0050% or less, more preferably more than 0 to 0.0050%, and further 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 Ni content is too high, the effect commensurate with the cost cannot be obtained, so the upper limit of the Ni content 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 the bolt. On the other hand, if the Cu content exceeds 0.20%, the hot ductility decreases, so the upper limit of the Cu content is preferably 0.20%. On the other hand, the lower limit of the Cu content is preferably 0.01%.

REM: 0.020% or less REM (rare earth elements) is a collective term for 17 elements, including 15 elements from lanthanum with atomic number 57 to lutetium with atomic number 71, scandium with atomic number 21, and yttrium with atomic number 39. When REM is contained in a bolt, the extension of MnS particles during rolling and hot forging is suppressed, and cracks during cold forging are suppressed. However, if the REM content exceeds 0.020%, a large amount of sulfides containing REM is generated, deteriorating the machinability of the steel material for the bolt.
Therefore, the REM content is preferably 0.020% or less in terms of the total amount of the 17 elements, more preferably from more than 0% to 0.020%, and even more preferably from 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 large, the high-temperature ductility decreases and the possibility of cracking during casting increases, so the upper limit of the Sn amount is preferably 0.20%. On the other hand, the lower limit of the Sn amount is preferably 0.005%, and more preferably 0.10%.

Bi: 0.20% or less Bi is an element that improves workability and may be contained in bolts. If the Bi content is large, high temperature ductility decreases and the possibility of cracking during casting increases, so the upper limit of the Bi content is preferably 0.20%. On the other hand, the lower limit of the Bi content is preferably 0.005%, and more preferably 0.10%.

(Other optional elements)
The bolt according to this embodiment may contain at least one optional element selected from the group consisting of the following elements. Specifically, these optional elements may each be contained in a range between 0% and the upper limit of each element described below. The properties of the bolt are not affected even if these optional elements are contained in the bolt within the range described below.
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 chemical composition of the bolt in this embodiment is composed of Fe and impurities. Here, impurities refer to elements that are mixed in from the ores and scraps used as raw materials for steel, or from the environment during the manufacturing process.

( _M2C type carbide)
In the bolt according to this embodiment, it is preferable that 10 or more M ₂ C type carbides having a length of 5 nm or more are present per unit volume of 0.001 μm ³ .
Fine M ₂ C type carbides (carbides containing Mo and at least one of Cr, W and V) precipitated during the tempering process have a higher hydrogen trapping ability than VC, Mo ₂ C, etc., and contribute to improving delayed fracture resistance.
Here, the M ₂ C type carbide is an M ₂ C type carbide containing Mo and at least one of Cr, W and V in a total amount of 70 atomic % or more relative to M (metal element).
Specifically, fine M ₂ C type carbides include (Mo, Cr, W, V) ₂ C, (Mo, Cr, W) ₂ C, (Mo, Cr, V) ₂ C, (Mo, Cr) ₂ C, (Mo, W) ₂ C, and (Mo, V) ₂ C. These M ₂ C type carbides have higher hydrogen trapping ability than VC, Mo ₂ C, etc., and contribute to improving delayed fracture resistance.
Therefore, it is preferable that a predetermined amount of M ₂ C type carbides having a length of 5 nm or more are present.
Therefore, the number density of M ₂ C type carbides having a length of 5 nm or more (the number of M ₂ C type carbides having a length of 5 nm or more present per unit volume of 0.001 μm ³ ) is preferably 10 or more.
From the viewpoint of improving the delayed fracture resistance, the number density of M ₂ C type carbides is more preferably 15 or more per 0.001 μm ³ unit volume, and further preferably 20 or more per 0.001 μm ³ unit volume.
However, the upper limit of the number density of M ₂ C type carbides is set to, for example, 100 particles per unit volume of 0.001 μm ³ from the viewpoint of suppressing a decrease in elongation and toughness.

The number density of M ₂ C type carbides is measured by preparing a thin film test piece by the thin film method and measuring it with a transmission electron microscope (TEM).
The components of the M ₂ C type carbide are measured using an energy dispersive X-ray analyzer (EDS) attached to the TEM.
Specifically, the following applies:

A section located 2 mm deep from the shank surface of the bolt to be measured and having a surface parallel to the bolt surface (hereafter also referred to as the "measurement surface") is taken, and a thin film test piece is prepared using the thin film method.

The thin film test specimens are prepared using the thin film method as follows. First, the original material is cut to a thickness of 0.5 mm using a precision cutting machine. Next, P320-1200 emery paper is used to cut and polish both sides to a thickness of 60 μm, and a sample with a diameter of 3 mm is punched out. After that, double-sided jet electrolytic polishing is performed until a hole is made in the center, and a thin film test specimen for TEM observation is prepared. Electrolytic polishing is performed using Tenupol, and the electrolytic polishing solution is 100 ml perchloric acid - 800 ml glacial acetic acid solution - 100 ml methanol, and the electrolytic polishing conditions are 30 V, 0.1 A.

Next, the number density of M ₂ C type carbides is measured as follows. The direction perpendicular to the {001} plane of the iron matrix is the incident direction of the electron beam, and three arbitrary visual fields of the thin film test piece (its measurement surface) are observed at a magnification of 400,000 times (observation area 0.25 μm × 0.25 μm). The M ₂ C type carbides are identified by electron beam diffraction pattern analysis. Then, the length and number of all M ₂ C type carbides present in the 0.1 μm × 0.1 μm area in the center of the observation screen are measured, the number of M ₂ C type carbides having a length of 5 nm or more is measured, and the film thickness is further measured to determine the "number density of M ₂ C type carbides" per unit volume of ^0.001 μm3 for each of the three visual fields.
Here, the length of the M ₂ C type carbide means the maximum length of the observed M ₂ C type carbide.
The TEM observation is carried out using an FE-TEM at an accelerating voltage of 200 kV.

Furthermore, it is confirmed by EDS analysis that the metal element M in the M ₂ C type carbides of 5 nm or more observed in the thin film sample contains Mo and at least one of Cr, W and V. However, Fe is excluded from the analyzed elements.

(Tensile strength)
In the bolt according to this embodiment, the tensile strength measured by taking a tensile test piece from the bolt is 1000 MPa or more.
The tensile strength of the bolt is a value measured in accordance with JIS Z 2241:2011.
Specifically, a No. 14A test piece in which the diameter of the parallel part is 50% of the diameter of the bolt is cut out from the shank of the bolt, and a tensile test is carried out in air at room temperature (25° C.) to determine the tensile strength.

(Trapped hydrogen amount)
In the bolt according to this embodiment, the amount of trapped hydrogen after cathodic hydrogen charging for 72 hours at a current density of 0.2 mA/cm in a room temperature solution containing 3.0 g of ammonium thiocyanate added per 1 L of a 3.0 mass % aqueous sodium chloride solution and then leaving the bolt at rest for 48 hours at room temperature (25° C.) is preferably 3.0 ppm or more.
If the amount of trapped hydrogen is less than 3.0 ppm, the hydrogen that has entered the bolt diffuses and accumulates at the prior austenite grain boundaries, increasing the risk of delayed fracture. Therefore, it is preferable that the amount of trapped hydrogen is 3.0 ppm or more.

The amount of trapped hydrogen was measured by a temperature rise hydrogen analysis method using a gas chromatograph. The amount of trapped hydrogen is defined as the amount of hydrogen released from the sample from room temperature to 400°C at a temperature rise rate of 100°C/hour. In this embodiment, room temperature is 25°C.

The measurement of the amount of trapped hydrogen is carried out on a round bar test piece (round bar test piece for investigating the amount of trapped hydrogen) having a diameter of 7 mm and a length of 70 mm taken from the bolt.
However, if it is not possible to obtain a round bar test piece of the above size, a round bar test piece having a diameter of 5 mm and a length of 20 mm may be used instead, and the hydrogen charging and standing process may be carried out in the same manner, followed by the same temperature rise analysis to measure the amount of trapped hydrogen.

(Delayed fracture resistance)
The bolt according to the present embodiment is preferably provided with sufficient delayed fracture resistance strength for use in an actual environment. In the bolt according to the present embodiment, the bolt is subjected to cathodic hydrogen charging at a current density of 0.03 mA/ ^cm2 for 24 hours in a room temperature (25°C) solution containing 3.0 g of ammonium thiocyanate added per 1 L of 3.0 mass% sodium chloride aqueous solution, and then plated to prevent hydrogen permeation. After leaving the bolt for 96 hours, a constant load of 0.9 times the tensile strength is applied. When the bolt does not break within 100 hours, the delayed fracture resistance strength is judged to be excellent.
Here, the hydrogen permeation prevention plating is performed to trap hydrogen within the bolt, and hot-dip galvanizing is used.

The measurement of delayed fracture resistance strength is carried out on a round bar test piece (delayed fracture test piece) with a notch (notch diameter 4.2 mm, angle 60°) of 7 mm diameter and 70 mm length taken from a bolt.
However, if it is not possible to obtain a round bar test piece of the above size, a round bar test piece with a notch of 5 mm diameter (notch diameter 3.0 mm, accuracy 60°) may be used instead. There is no particular restriction on the length as long as it can be chucked.

<Bolt steel material>
The bolt steel according to the present embodiment is a steel material that is a raw material for the bolt according to the present embodiment. The bolt steel according to the present embodiment has the same chemical composition as the bolt according to the present embodiment.

<Bolt manufacturing method>
Hereinafter, an example of a method for manufacturing a bolt according to this embodiment using the bolt steel according to this embodiment will be described in detail.

(Process of forming into bolt shape)
After obtaining molten steel having the chemical composition of the bolt according to the present embodiment, the molten steel is cast into an ingot or a cast piece. The cast ingot or cast piece 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. The steel material is then subjected to wire drawing, annealing, cold working, thread rolling, and the like to be formed into a desired bolt shape. Annealing or spheroidizing annealing treatment may be performed multiple times between multiple cold workings. The forming process may also include hot working.

(Quenching and tempering process)
After forming into a desired bolt shape, the steel is heated to a temperature equal to or higher than the austenitizing temperature, and then quenched by water or oil cooling to impart strength. If the heating temperature for quenching (hereinafter referred to as the "quenching heating temperature") is too low, the solid solution of Mo, W, and V carbonitrides in the matrix becomes insufficient, and coarse carbides remain. As a result, the amount of fine _M2C type carbides precipitated during tempering is reduced, and the desired strength and hydrogen trapping effect cannot be obtained.

On the other hand, if the quenching heating temperature is excessively high, the crystal grains become coarse, leading to deterioration of toughness and resistance to delayed fracture. In addition, damage to the furnace body and auxiliary parts of the operating heat treatment furnace becomes significant, and the manufacturing cost increases, which is undesirable.
For this reason, the quenching heating temperature is preferably set to 970 to 1050° C. Furthermore, the holding time at the quenching heating temperature is preferably set to 30 to 90 minutes.

To improve delayed fracture resistance, tempering is required after the above quenching treatment. Therefore, the tempering temperature must be limited to 570-690°C.

If the tempering temperature is less than 570° C., the precipitation of M ₂ C type carbides during tempering is insufficient, making it impossible to achieve the desired hydrogen trapping ability and delayed fracture limit hydrogen amount.
On the other hand, if the tempering temperature exceeds 690° C., the M ₂ C type carbide undergoes Ostwald ripening, making it impossible to achieve the desired hydrogen trapping ability and delayed fracture limit hydrogen amount.
Therefore, the tempering temperature is limited to 570 to 690°C.
The preferable range of the tempering temperature is 590 to 660°C.
The holding time at the tempering temperature is preferably 30 to 90 minutes.

The bolt according to this embodiment is manufactured through the above process.

As described above, the bolt according to this embodiment is made by subjecting steel material with an optimal chemical composition to optimal quenching and tempering, thereby optimizing the tensile strength, amount of trapped hydrogen, and delayed fracture limit hydrogen amount.

Next, an embodiment of the present invention will be described. However, the conditions shown below are merely examples adopted to confirm the feasibility and effects of the present invention, and the conditions of the present invention are not limited to these examples. In implementing the present invention, various conditions can be adopted as long as they do not deviate from the gist of the invention and achieve the purpose.

<Molding of various test pieces>
(Preparation of steel bars)
Steels (steels Nos. A to Z2 and AA to AV) having the chemical compositions shown in Tables 1-1 and 1-2 were melted and hot forged to prepare steel bars having a diameter of 20 mm and a length of 1000 mm.
In Tables 1-1 and 1-2, underlined values indicate that the values are outside the range of the present invention. In Tables 1-1 and 1-2, "-" indicates that the respective elements are not added. In Formulas (1) and (2), the contents of elements marked with "-" are substituted with "0". The remainder is Fe and impurities.
The values in the "Formula (2)" column in Tables 1-1 and 1-2 are values obtained by rounding off the value of the left side of formula (2) to the nearest five decimal places, and all of Example 3, Comparative Example 13, and Comparative Example 15 satisfy formula (2).

Next, to reproduce the bolt manufacturing process, the bolts were quenched and tempered under the conditions in Table 2. The tensile strength and amount of trapped hydrogen of the quenched and tempered bolts were then measured, and the delayed fracture resistance was evaluated using the following methods.

(Hardening)
The round bar obtained as described above with a diameter of 20 mm and a length of 1000 mm was cut to obtain a round bar with a diameter of 20 mm and a length of 300 mm, and quenched at the temperature shown in Table 2. The holding time at the quenching heating temperature was 60 minutes. Then, quenching was performed in an oil bath held at 60°C.

(Performance 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 cooling after tempering was performed by air cooling.

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

(Preparation of test pieces for investigating trapped hydrogen amount)
From the round bar having a diameter of 20 mm and a length of 300 mm after the above quenching and tempering treatment, a round bar having a diameter of 7 mm and a length of 70 mm was taken as a round bar test piece for investigating the amount of trapped hydrogen.

(Preparation of test pieces for delayed fracture resistance)
From the round bar having a diameter of 20 mm and a length of 300 mm after the above quenching and tempering treatment, a round bar test piece having a notch having a diameter of 7 mm and a length of 70 mm (notch diameter 4.2 mm, angle 60°) was taken as a test piece for delayed fracture resistance.

In this manner, tensile test pieces for Production No. 1 to 54, test pieces for investigating the amount of trapped hydrogen for Production No. 1 to 54, and test pieces for delayed fracture strength for Production No. 1 to 54 were obtained. However, quench cracks occurred in Production No. 40, so further testing was discontinued.

<Performance evaluation using each test piece>
(Number density of _M2C type carbides having a length of 5 nm or more)
The number density (number per unit volume of 0.001 μm ² ) of M ₂ C type carbides having a length of 5 nm or more was measured as described above, and was evaluated according to the following criteria.
A: The number density of _M2C type carbides is 10 pieces/0.001 μm3 ^or more and 14 pieces/0.001 μm3 or ^less . B: The number density of _M2C type carbides is 15 pieces/0.001 μm3 ^or more and 19 pieces/0.001 μm3 or ^less . C: The number density of _M2C type carbides is 20 pieces/0.001 μm3 ^or more and 100 pieces/0.001 ^μm3 or less. D: The number density of _M2C type carbides is 9 pieces/0.001 μm3 or ^less.

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

(Trapped hydrogen amount)
The amount of trapped hydrogen was measured as described above.
Specifically, the round bar test piece having a diameter of 7 mm and a length of 70 mm prepared by the above procedure was subjected to cathodic hydrogen charging at a current density of 0.2 mA/ ^cm2 for 72 hours in a room temperature (25°C) solution containing 3.0 g of ammonium thiocyanate added per 1 L of 3.0 mass% sodium chloride aqueous solution. The test piece was then left to stand at room temperature (25°C) for 48 hours. The amount of hydrogen released from the round bar test piece was measured using a gas chromatograph by increasing the temperature from room temperature (25°C) to 400°C at a rate of 100°C/hour.

(Delayed fracture resistance test)
The amount of trapped hydrogen was measured as described above.
Specifically, the delayed fracture resistance test piece with a diameter of 7 mm and a length of 70 mm (with a V-shaped circular notch in the center of the length, notch depth 1.4 mm, notch angle 60°, notch tip radius 0.175 mm) prepared by the above procedure was cathodically charged with hydrogen at a current density of 0.03 mA/ ^cm2 for 24 hours in a solution at room temperature (25°C) containing 3.0 g of ammonium thiocyanate per 1 L of 3.0 mass% sodium chloride aqueous solution, then plated with Zn to prevent hydrogen permeation, left for 96 hours, and then a constant load of 0.9 times the tensile strength was applied to measure the time until fracture. If fracture did not occur after 100 hours, the test was discontinued.

The results of the number density of _M2C type carbides, tensile strength, amount of trapped hydrogen, and the presence or absence of delayed fracture are shown in Table 2. Note that underlined values in Table 2 indicate that the values are outside the range of the present invention. Also, the symbol "-" in Table 2 indicates that the test was not performed because the necessary conditions such as tensile strength were not met.

As is clear from Tables 1-1, 1-2, and 2, invention examples 1 to 28 (production numbers 1 to 28), which have optimized chemical compositions and quenching and tempering conditions, all have high tensile strength, a high amount of trapped hydrogen, and no delayed fracture, indicating that excellent strength and resistance to delayed fracture were obtained.

Incidentally, although invention examples 1 and 18 satisfy the composition requirements of the present invention, they were manufactured under manufacturing conditions in which the quenching conditions were slightly outside the preferred range. Invention example 18 was manufactured under manufacturing conditions in which the quenching temperature was higher than the other invention examples, and its strength is slightly greater than the other invention examples. Therefore, in terms of the strength-ductility balance, the other invention examples are relatively superior. Moreover, invention example 1 was manufactured under manufacturing conditions in which the quenching temperature was lower than the other invention examples, and its strength is relatively superior to the other invention examples.

In contrast, in Comparative Examples 1 to 26 (Production Nos. 29 to 54), in which no attempt was made to optimize at least either the chemical composition or the quenching and tempering conditions, it was found that the delayed fracture resistance was insufficient.
Furthermore, Comparative Examples 3, 11 and 21 had insufficient strength, and Comparative Examples 1, 2, 4 to 6, 8 to 10, 13 to 15, 17 to 20 and 22 to 26 did not obtain a sufficient amount of trapped hydrogen.
In addition, in Comparative Example 16, although the amount of trapped hydrogen is high, the amount of Al in the steel components is higher than 0.100, so that a large number of oxide-based inclusions containing Al are present, and a large number of fracture starting points are formed, resulting in a decrease in delayed fracture properties.

According to the present invention, it is possible to provide a bolt having high strength and exhibiting excellent delayed fracture resistance, and a steel material for the bolt that is the raw material thereof.

Claims (6)

The composition is, in mass%,
C: 0.10 to 0.34%,
Si: 0.02 to 0.10%,
Mn: 0.40 to 2.50%,
Cr: 0 to 0.50%,
Mo: 1.50 to 5.00%,
W: 0 to 4.00%,
V: 0 to 1.00%,
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 is Fe and impurities,
And satisfy the following formula (1) and the following formula (2),
At a portion located 2 mm deep from the surface of the bolt shaft and parallel to the surface of the bolt, there are 10 or more _M2C type carbides with a length of 5 nm or more per unit volume of 0.001 ^μm3 .
A bolt having a tensile strength of 1000 MPa or more.
0.10<(2V+0.5W+0.4Cr)/Mo<0.50 ... (1)
Mo/95.95+V/50.94+W/183.8-C/6.0>0...(2)
Here, M represents a metal element, and in formulas (1) and (2), Mo, V, W, Cr, and C are substituted with the contents (mass%) of Mo, V, W, Cr, and C contained in the bolt, respectively, and when the corresponding element is not contained, 0 is substituted for the corresponding element symbol. Also, a composition in which the value of the formula on the left side of formula (2) is more than 0 after the decimal point is considered to satisfy formula (2). The composition, in 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. The composition, in 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,
The bolt according to claim 1 or claim 2, further comprising at least one selected from the group consisting of: The bolt according to any one of claims 1 to 3, which is subjected to cathodic hydrogen charging for 24 hours at a current density of 0.03 mA/ ^cm2 in a room temperature solution containing 3.0 g of ammonium thiocyanate added per 1 L of a 3.0 mass % aqueous sodium chloride solution, and then subjected to hydrogen permeation prevention plating and left for 96 hours, and then when a constant load of 0.9 times the tensile strength is applied, it takes 100 hours or more until fracture. 5. The bolt according to claim 1, which is subjected to cathodic hydrogen charging for 72 hours at a current density of 0.2 mA/ ^cm2 in a room temperature solution containing 3.0 g of ammonium thiocyanate added per 1 L of a 3.0 mass % aqueous sodium chloride solution, and then allowed to stand at room temperature for 48 hours, and has an amount of trapped hydrogen of 3.0 ppm or more. A steel material for a bolt, which is a material for the bolt according to any one of claims 1 to 5,
A steel material for bolts having the composition of the bolt.