JP7457234B2 - Bolts and bolt manufacturing methods - Google Patents

Description

TECHNICAL FIELD The present invention relates to bolts, steel for bolts, and methods of manufacturing bolts.

BACKGROUND OF THE INVENTION As automobiles and industrial machinery become more efficient and lighter, and civil engineering and architectural structures become larger, there is a need for bolts with higher strength.
The bolts are made of mechanical structural alloy steel such as SCM435 and SCM440 specified in JIS G 4053:2016. Bolts are made of mechanical structural alloy steel that is formed into a predetermined shape and then hardened and tempered to adjust its strength.
In order to increase the strength of a bolt, it is sufficient to increase the carbon content of the steel material or to lower the tempering temperature.

However, bolts with a tensile strength exceeding 1200 MPa are susceptible to delayed fracture, a type of hydrogen embrittlement, 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 snow-melting salt is present, the amount of hydrogen that penetrates increases due to salt adhesion, increasing the possibility of delayed fracture.

Therefore, bolts with excellent delayed fracture resistance have been studied.
For example, Patent Document 1 discloses bolts and steel materials that utilize V carbonitride, which acts as a hydrogen trap site, and have a tensile strength of 1200 to 1600 MPa and excellent delayed fracture resistance.

Japanese Patent Application Publication No. 2002-276637

Recently, there has been a demand for a bolt that has even higher strength and superior delayed fracture resistance than the bolt of Patent Document 1.
The object of the present invention is to provide a bolt that exhibits excellent delayed fracture resistance at a high strength level of 1,600 MPa or higher, where delayed fracture is generally likely to occur, a method for manufacturing the bolt, and a bolt that is the material for the bolt. Our goal is to provide steel.

The inventors have determined that by using a steel material that has a predetermined chemical composition and a content of Mo and V that satisfies the following formula (1) as a bolt material, MC and M, which serve as hydrogen trap sites, can be reduced. It has been found that _2C carbide is dispersed in the bolt.
1.26<Mo/1.4+V≦3.70...(1)
As a result, the inventors discovered that a bolt with high strength and excellent delayed fracture resistance could be obtained.
The above problem is solved by the following means.

<1> Chemical composition, in mass%,
C: 0.36 to 0.50%,
Si: 0.02 to 0.10%,
Mn: 0.20 to 0.84%,
Cr: 0.62 to 1.98%,
V: 0.50 to 1.20%,
Mo: 0.99 to 3.50%,
Al: 0.010 to 0.100%,
N: 0.0010 to 0.0300%,
P: 0.015%,
S: 0.015%,
O: 0.0030%, and the balance: Fe and impurities;
And satisfy the following formula (1):
A bolt having a tensile strength of 1600 MPa or more.
1.26<Mo/1.4+V≦3.70 (1)
In formula (1), Mo and V are substituted with the mass percentages of Mo and V contained in the bolt, respectively.
<2> The chemical composition contains, in mass%, Ti: 0.100% or less in place of a portion of the Fe,
Nb: 0.100% or less,
B: 0.0050% or less,
The bolt according to <1>, further comprising one or more selected from the group consisting of W: 2.50% or less and REM: 0.020% or less.
<3> The bolt according to <1> or <2>, wherein the chemical composition further includes, in place of a portion of the Fe, one or more selected from the group consisting of, 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.
<4> The amount of trapped hydrogen after cathodic hydrogen charging for 72 hours at a current density of 0.2 mA/ ^cm2 in a room temperature solution containing 3 mass% sodium chloride and 3 g of ammonium thiocyanate per liter is 7.0 ppm or more;
The bolt according to <1> or <2>, which 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 mass % sodium chloride to which 0.3 g of ammonium thiocyanate has been added per 1 L, is then plated with a hydrogen permeation prevention coating, and is left for 96 hours. After that, when a constant load of 0.9 times the tensile strength is applied, the time until the bolt breaks is 100 hours or more.

<5> Molding steel for bolts having the chemical composition according to any one of <1> to <3> into a bolt shape,
After heating to a temperature of A _c3 point or higher, quenching is performed,
A bolt manufacturing method that involves tempering in a temperature range of 530 to 690°C.
<6> A bolt steel that is a material for the bolt according to any one of <1> to <4>,
Having the chemical composition and tensile strength according to any one of <1> to <3>,
After heating to a temperature of A _c3 point or higher, quenching is performed from the heating temperature to 200°C at a cooling rate of 5°C/second or more, and the temperature range is 530 to 690°C and the holding time is 30 minutes to 4 hours. Steel for bolts that has a tensile strength of 1,600 MPa or more when tempered.

According to the present invention, it is possible to provide a bolt that has a high strength of 1600 MPa or more and exhibits excellent delayed fracture resistance, a manufacturing method thereof, and a steel for bolts that is a material thereof.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment which is an example of this invention is described in detail.
In this specification, the content of each element in the chemical composition expressed as "%" means "mass %."
The content of each element in a chemical composition is sometimes referred to as "element content." For example, the content of C may be expressed as C amount.
A numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as lower and upper limits.
A numerical range in which "more than" or "less than" is added to the numerical value written before and after "~" means a range that does not include these numerical values as the lower limit or upper limit.
The term "process" is included not only in an independent process but also in the case where the intended purpose of the process is achieved even if the process cannot be clearly distinguished from other processes.

<Bolt>
The bolt according to this embodiment has a chemical composition in mass%,
C: 0.36-0.50%,
Si: 0.02 to 0.10%,
Mn: 0.20-0.84%,
Cr: 0.62-1.98%,
V: 0.50-1.20%,
Mo: 0.99-3.50%,
Al: 0.010-0.100%,
N: 0.0010-0.0300%,
P: 0.015% or less,
S: 0.015% or less,
O: 0.0030% or less, and the balance: consisting of Fe and impurities,
and satisfies the following formula (1),
The tensile strength is 1600 MPa or more.
1.26<Mo/1.4+V≦3.70...(1)
However, in formula (1), the mass percentages of Mo and V contained in the bolt are substituted for Mo and V, respectively.

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

(Essential element)
C: 0.36-0.50%
C is an element that improves the strength of steel and increases the strength of bolts. If the C content is less than 0.36%, the strength required for the bolt cannot be obtained. On the other hand, when the amount of C is more than 0.50%, cold workability decreases.
Therefore, the amount of C is set to 0.36 to 0.50%. Note that the preferable amount of C is 0.40 to 0.46%.

Si: 0.02-0.10%
By reducing the content of Si, delayed fracture strength can be improved. In order to increase delayed fracture resistance, the amount of Si is set to 0.10% or less. On the other hand, if it is less than 0.02%, the improvement in delayed fracture strength will be saturated and the cost in the steel manufacturing process will increase.
Therefore, the amount of Si is set to 0.02 to 0.10%. Note that the preferable amount of Si is 0.02 to 0.08%.

Mn: 0.20-0.84%
Mn combines with S to form MnS and prevents grain boundary segregation of S. It also has the effect of improving hardenability. When the amount of Mn is less than 0.20%, grain boundary segregation of S increases and delayed fracture resistance decreases. On the other hand, when the Mn content exceeds 0.84%, cold workability during processing into a part shape decreases.
Therefore, the Mn content is set to 0.20 to 0.84%. Note that the preferable amount of Mn is 0.30 to 0.75%.

Cr: 0.62-1.98%
Cr is an effective element for ensuring the hardenability of steel. If the Cr content is less than 0.62%, the effect of improving hardenability will be insufficient. On the other hand, when the Cr content exceeds 1.98%, the cold workability of the steel decreases. Furthermore, if the Cr content exceeds 1.98%, the desired hydrogen trapping effect cannot be obtained because the precipitation of MC and M ₂ C carbides is inhibited.
Therefore, the Cr content is set to 0.62 to 1.98%. Note that the preferable amount of Cr is 0.70 to 1.60%.

V: 0.50~1.20%
Mo: 0.99-3.50%
V and Mo are important elements in this embodiment. V and Mo are elements that form carbides. By containing an appropriate amount of V in combination with Mo in steel, MC type carbide (Mo, V) C, which is composed of V and Mo, and M ₂ C type carbide (Mo, V) are produced. _2C precipitates. A large amount of fine MC type carbides and fine M ₂ C type carbides can be precipitated by quenching the steel from the austenite region and then tempering it at a high temperature of 550 to 650°C. By precipitation of these fine carbides, the strength of steel can be increased by precipitation strengthening. Further, the fine MC type carbide and the fine M ₂ C type carbide function as hydrogen trap sites and can improve delayed fracture resistance. Trapped hydrogen is hydrogen that is fixed by the MC type carbide and M ₂ C type carbide and cannot move freely in the steel.

In order to obtain the effect as a trap site, it is necessary to contain 0.50% or more of V and 0.99% or more of Mo. On the other hand, if the V content exceeds 1.20% or the Mo content exceeds 3.50%, a large amount of undissolved coarse carbonitrides will remain during quenching heating. In order to dissolve these coarse carbonitrides into austenite, it is 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 V content is set to 0.50 to 1.20%, and the Mo content is set to 0.99 to 3.50%. Note that the preferable amount of V is 0.55 to 1.05%, and the preferable amount of Mo is 1.20 to 3.10%.

Mo/1.4+V
The V and Mo contents need to satisfy formula (1).
1.26<Mo/1.4+V≦3.70...(1)
In formula (1), the mass percentages of Mo and V contained in the bolt are substituted for Mo and V, respectively.

In bolts with high tensile strength of 1600 MPa or more, fine MC type carbides and fine M ₂ C type carbides, which are hydrogen trap sites, are dispersed in large quantities in the steel in order to improve delayed fracture resistance. It is necessary. When the value of formula (1) is 1.26 or less, there are insufficient hydrogen trap sites and the delayed fracture strength decreases. When the value of formula (1) is larger than 3.70, solid solution of Mo and V carbonitrides into the matrix becomes insufficient during heating during quenching, and coarse carbonitrides remain. As a result, the amount of fine Mo and V carbonitrides precipitated during tempering decreases, making it impossible to obtain the desired strength and hydrogen trapping effect.

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. 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:0.0010~0.0300%
N is an element that forms nitrides and carbonitrides and suppresses coarsening of austenite crystal grains during quenching and heating. In order to suppress coarsening of crystal grains, the amount of N needs to be 0.0010% or more. On the other hand, when the amount of N exceeds 0.0300%, coarse nitrides and carbonitrides are generated and become fracture starting points.
Therefore, the amount of N is set to 0.0010 to 0.0300%. Note that the preferable amount of N is 0.0020 to 0.0200%.

P: 0.015% or less P is an impurity. It is preferable that the P content is as low as possible. P segregates at austenite grain boundaries. When the amount of P exceeds 0.015%, the prior austenite grain boundaries after quenching and tempering become brittle, causing intergranular cracking. Therefore, it is necessary to limit the amount of P to a range of 0.015% or less. The preferable upper limit of the P content is 0.012%. P is an impurity element that is inevitably contained in the bolt, but as long as it is within the above range, P may be contained in the bolt in an amount exceeding 0%.

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 steel material. 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%.

O: 0.0030% or less O is an impurity that is inevitably contained. O forms oxides and becomes the starting point of delayed fracture. Therefore, the O content is 0.0030% or less. A preferred upper limit of the O content is 0.0020%, and more preferably 0.0015%. The O content is preferably as low as possible. However, excessive reduction of the O content increases manufacturing costs. Therefore, in consideration of normal industrial production, a preferred lower limit of the O content is 0.0001%, and more preferably 0.0005%, and even more preferably 0.0007%. Within the above range, O may be contained in the bolt at more than 0%.

(Optional element)
The bolt according to the present embodiment may contain one or more of Ti, Nb, B, W, and REM as optional elements in place of a portion of Fe. Since these elements are optional elements, the content may be 0% or may exceed 0%.

Ti: 0.100% or less Ti is an element that combines with N and C to form carbonitrides in steel materials. This carbonitride pins austenite grain boundaries and prevents coarsening of the structure. In order to obtain this effect of preventing coarsening of the structure, 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 during processing into a part shape will decrease due to an increase in material hardness.

Therefore, the Ti content is preferably 0 to 0.100%, more preferably more than 0% to 0.100%, and even more preferably 0.005 to 0.05%.

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

Therefore, the Nb content is preferably 0 to 0.100%, more preferably more than 0% to 0.100%, and even more preferably 0.005 to 0.05%.

B: 0.0050% or less B improves the hardenability of steel even if it is dissolved slightly in austenite. B may be included in the steel material in order to efficiently obtain martensite during carburizing and quenching. On the other hand, when B is added in an amount exceeding 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 to 0.0050%, more preferably more than 0 to 0.0050%, and even more preferably 0.0007 to 0.0030%.

W: 2.50% or less W, like Mo, is an element that causes significant secondary hardening when tempered at high temperatures. By precipitating as M ₂ C type carbide, W can increase the strength of steel through precipitation strengthening. Furthermore, the M ₂ C type carbide functions as a hydrogen trap site and can improve delayed fracture resistance. Therefore, W may be contained in an amount of 2.50% or less. The W content is preferably from 0 to 2.50%, more preferably from more than 0 to 2.50%, even more preferably from 0.20 to 1.80%.

REM: 0.020% or less REM (rare earth elements) is a general term for a total of 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. It is. When REM is contained in the steel material, the extension of MnS particles is suppressed during rolling and hot forging, and the effect of suppressing cracking during cold forging is obtained. However, if the REM content exceeds 0.020%, a large amount of sulfide containing REM is generated, which deteriorates the machinability of the steel. Therefore, REM may be contained in an amount of 0.020% or less. The REM content is preferably 0 to 0.020% in total of the 17 elements, more preferably more than 0% to 2.00%, and even more preferably 0.005% to 0.015%.

(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 optional element. Specifically, these optional elements may be contained in an amount of 0%, or may be contained within the upper limit range described below. Even if these arbitrary elements are included in the bolt within the range described below, the properties of the bolt will not be 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 chemical composition of the bolt in this embodiment consists of Fe and impurities. Here, impurities refer to elements mixed in from ores used as raw materials for steel, scrap, or the environment during the manufacturing process. As impurities other than P, S, and O, for example, Pb, Cd, Co, Zn, Ca, etc. may be present. These impurity elements are allowed to be contained within a range that does not reduce the delayed fracture strength of the bolt. Further, Ni, Cu, and Sn, which are expected to have an anticorrosive effect, and Bi, which are expected to have an effect of improving machinability, can be contained in an amount of 0.20% or less. Cu, Sn, and Bi impair castability when added in large amounts, so the upper limit of each is set to 0.20%. If Ni is added in a large amount, an effect commensurate with cost cannot be obtained, so the upper limit is set at 0.20%.

[Characteristics of bolt]
The bolt according to this embodiment has the above chemical composition and has a tensile strength of 1600 MPa or more.
In addition, the bolt according to the present embodiment has excellent delayed fracture resistance, and specifically, the bolt according to the present embodiment has a current density of The amount of trapped hydrogen after cathodic hydrogen charging at 0.2 mA/ ^cm2 for 72 hours is 7.0 ppm or more, in a solution at room temperature in which 0.3 g of ammonium thiocyanate per 1 L is added to 3 mass% sodium chloride. After cathodic hydrogen charging at a current density of 0.03 mA/ ^cm2 for 24 hours, plating was applied, and after being left for 96 hours, a constant load of 0.9 times the tensile strength was applied, and the time until breakage was It is preferable that the material has the property of not breaking for 100 hours.

The tensile strength, amount of trapped hydrogen, and delayed fracture resistance of the bolt according to this embodiment will be described in detail below.

(Tensile strength)
In the bolt according to this embodiment, the tensile strength measured by taking a tensile test piece from the bolt is 1600 MPa or more. When the tensile strength is 1600 MPa or more, the bolt can be significantly reduced in size and weight. On the other hand, if the tensile strength exceeds 2200 MPa, there is a high possibility that delayed fracture will occur even if the amount of hydrogen that enters is small. Therefore, the tensile strength is preferably 2200 MPa or less.

(Trap hydrogen amount)
In the bolt according to this embodiment, the trap after cathodic hydrogen charging for 72 hours at a current density of 0.2 mA/cm ² in a room temperature solution of 3 mass % sodium chloride added with 3 g of ammonium thiocyanate per liter. It is preferable that the amount of hydrogen is 7.0 ppm or more. When the amount of trapped hydrogen is 7.0 ppm or more, hydrogen that has entered the bolt will diffuse and accumulate at prior austenite grain boundaries, reducing the possibility that delayed fracture will occur. Therefore, it is preferable that the amount of trapped hydrogen is 7.0 ppm or more.

The amount of trapped hydrogen was measured by temperature-programmed hydrogen analysis using gas chromatography. The amount of hydrogen released from the sample from room temperature to 400°C at a heating rate of 100°C/hour was defined as the amount of hydrogen trapped. Note that the room temperature in this embodiment is 25°C.
The amount of trapped hydrogen is measured on a round bar test piece (round bar test piece for investigating the amount of trapped hydrogen) with a diameter of 7 mm and a length of 30 mm taken from a bolt.
However, if it is not possible to collect a round bar of the above size, collect a round bar test piece of the size that can be collected from the target bolt and analyze multiple pieces together. The sample weight required for hydrogen analysis is preferably 2 g or more.

(Delayed fracture strength)
Since the bolt according to this embodiment is used in a real environment, it needs to have sufficient delayed fracture strength. The bolt according to this embodiment was subjected to cathodic hydrogen charging for 24 hours at a current density of 0.03 mA/cm ² in a room temperature solution containing 3 mass% sodium chloride and 0.3 g of ammonium thiocyanate per liter. After applying hydrogen permeation prevention plating and leaving it for 96 hours, it is preferable that when a constant load of 0.9 times the tensile strength is applied, the time to breakage must be 100 hours or more. If it did not break in 100 hours, it was judged that it had particularly excellent delayed fracture strength. Here, the plating is performed to confine hydrogen in the steel material, and Zn plating or the like can be used.
Measurement of delayed fracture strength is carried out on bolts with the bolt in its original shape.

As shown above, the bolt according to the present embodiment has optimized tensile strength, trapped hydrogen amount, and delayed fracture limit hydrogen amount by applying optimal quenching and tempering, which will be described later, to a steel material with an optimal chemical composition. It was planned.

<Method of manufacturing bolts>
Although the method for manufacturing the bolt according to the present embodiment is not particularly limited, an example of the method for manufacturing the bolt according to the present embodiment will be described in detail below using the steel for bolts according to the present embodiment.

(Process of forming into bolt shape)
First, as a bolt material, a bolt steel having the same chemical composition as the above-mentioned bolt is prepared.
The casting is carried out to produce an ingot or slab having the aforementioned chemical composition. The cast ingot or slab is then processed by hot working, such as hot rolling, hot extrusion, or hot forging, to produce steel for bolts having the required rough shape, such as a round bar.
The bolt steel is then subjected to wire drawing, annealing, cold working, thread rolling, etc., to form it into a desired bolt shape. Annealing or spheroidizing annealing treatment may be performed multiple times between multiple cold working processes. Hot working may also be included in the forming process.

(Quenching and tempering process)
After forming into a desired bolt shape, in order to impart strength, the steel is heated to a temperature equal to or higher than the A _c3 point of the steel, and then quenched by water or oil cooling. If the heating temperature for quenching (hereinafter referred to as the "quenching heating temperature") is too low, the solid solution of Mo and V carbonitrides in the matrix becomes insufficient, and coarse carbonitrides remain. As a result, the amount of fine Mo and V carbonitrides precipitated during tempering is reduced, making it impossible to obtain the desired strength and hydrogen trapping effect.

On the other hand, from the viewpoint of operation, if the quenching heating temperature is excessively high, the damage to the furnace body and the auxiliary parts of the heat treatment furnace becomes significant, and the manufacturing cost increases, so it is not preferable. Although it depends on the chemical composition of the steel (A _c3 point), the quenching heating temperature is preferably 870 to 1150°C.

In order to improve the delayed fracture resistance, tempering must be performed after the above quenching treatment. In this embodiment, the tempering temperature must be limited to 530 to 690°C. If the tempering temperature is less than 530°C, precipitation strengthening by Mo carbide and V carbide that precipitate during tempering will be insufficient, and the desired strength cannot be obtained. In addition, the desired hydrogen trapping ability and delayed fracture limit hydrogen amount cannot be achieved.

On the other hand, if the tempering temperature exceeds 690°C, MC type carbides undergo Ostwald growth and do not contribute to precipitation strengthening, making it difficult to obtain a tensile strength of 1600 MPa or higher. Therefore, the tempering temperature is limited to 530 to 690°C.
Note that the preferred range of tempering temperature is 550 to 650°C.

The tempering time is preferably 30 minutes to 4 hours.
If the tempering time is less than 30 minutes, the precipitation strengthening by Mo carbide and V carbide precipitated during tempering becomes insufficient, and the desired strength cannot be obtained, and the desired hydrogen trapping ability and delayed fracture limit hydrogen amount cannot be achieved.
On the other hand, when the tempering time exceeds 4 hours, the precipitated Mo carbides and V carbides become coarse, so that the precipitation strengthening by the Mo carbides and V carbides becomes insufficient, and the desired strength cannot be obtained, and the desired hydrogen trapping ability and delayed fracture limit hydrogen amount cannot be achieved.
Through the above steps, the bolt according to this embodiment is manufactured.

Next, examples of the present invention will be described. However, the conditions shown below are only examples adopted to confirm the feasibility and effects of the present invention, and the conditions of the present invention are limited to these examples. It's not something you can do. In implementing the present invention, various conditions may be adopted as long as they do not depart from the gist and achieve the purpose.

<Molding of various test pieces>
(Preparation of steel bars)
Steels (Steel Nos. A to V) 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. Also, blank spaces in Tables 1-1 and 1-2 indicate that the respective elements were not added.
The balance is Fe and impurities, including P, S, and O.
Incidentally, A _c3 (° C.) in Table 1-2 is a temperature calculated by the following formula.
A _c3 (°C) = 910 - 203 x √C + 44.7 x Si + 104 x V + 31.5 x Mo
In the above formula for A _c3 (° C.), the mass % contents of Mo and V contained in the steel for bolts (product equivalent to a bolt) are substituted for C, Si, V and Mo, respectively.

Next, in order to reproduce bolt manufacturing, quenching and tempering was performed under the following conditions, and then the tensile strength and trapped hydrogen amount of the quenched and tempered bolt equivalent were measured, and the delayed fracture resistance was evaluated using the following method. did.

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

(Implementation of tempering)
After oil quenching, tempering was performed at the temperatures listed in Table 2. The holding time at the tempering temperature was 60 minutes, and the cooling after tempering was air cooling.

(Tensile test piece)
According to the test piece collection method described in JIS G 0416:2019, a smooth tensile test was performed on a round bar with a diameter of 20 mm and a length of 300 mm after the above quenching and tempering treatment, total length: 70 mm, parallel part: diameter 6 mm x length 32 mm. A piece (No. A2 tensile test piece) was taken.

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

(Preparation of delayed fracture test specimens)
From the center of 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 with 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 delayed fracture test piece.
The same results are obtained whether the bolt is evaluated or the test piece is evaluated.

As described above, the production No. shown in Table 2 below was obtained. Tensile test pieces from 1 to 46, production no. 1 to 46 for investigating the amount of trapped hydrogen, and production No. 1 to 46 delayed fracture specimens were obtained, respectively.

<Performance evaluation using each test piece>
(Tensile strength)
Using the tensile test piece produced in the above procedure, a tensile test was conducted in the atmosphere at room temperature in accordance with JIS Z 2241:2011 to determine the tensile strength.

(Trapped hydrogen amount)
The round bar with a diameter of 7 mm and a length of 70 mm prepared by the above procedure was subjected to cathodic hydrogen charging for 72 hours at a current density of 0.2 mA/ ^cm2 in a room temperature solution of 3 mass% sodium chloride to which 3 g of ammonium thiocyanate was added per liter. After that, the sample was heated from room temperature to 400°C at a heating rate of 100°C/hour using a gas chromatograph, and the amount of hydrogen released from the sample was measured.

(Delayed fracture strength)
The delayed fracture test pieces with a notch of φ7 mm × 70 mm (notch part φ4.2 mm, angle 60°) prepared by the above procedure were cathodically charged with hydrogen at a current density of 0.03 mA/ ^cm2 for 24 hours in a room temperature solution of 3 mass% sodium chloride to which 0.3 g of ammonium thiocyanate was added per liter, then plated with Zn to prevent hydrogen permeation, and left for 96 hours. A constant load of 0.9 times the tensile strength was applied, and the time until fracture was measured. If fracture did not occur after 100 hours, the test was discontinued.

The results for 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.

As is clear from Tables 1-1 and 2, the production No. 1 with optimized chemical composition and quenching and tempering conditions. As for Nos. 1 to 16 and 33 to 46, both had high tensile strength and a high amount of trapped hydrogen, and no delayed fracture occurred, so they had excellent strength and delayed fracture resistance. I understand.

On the other hand, production No. 1, in which at least one of the chemical composition and quenching and tempering conditions was not optimized. It can be seen that none of Nos. 17 to 32 had excellent strength or delayed fracture resistance.

According to the present invention, it is possible to provide a bolt that is high in strength and exhibits excellent delayed fracture strength, a method for manufacturing the bolt, and a steel for the bolt that is a material for the bolt.

Claims (6)

The chemical composition is in mass%,
C: 0.36-0.50%,
Si: 0.02 to 0.10%,
Mn: 0.20-0.84%,
Cr: 0.62-1.98%,
V: 0.50-1.20%,
Mo: 0.99-3.50%,
Al: 0.010-0.100%,
N: 0.0010-0.0300%,
P: 0.015% or less,
S: 0.015% or less,
O: 0.0030% or less, and the balance: consisting of Fe and impurities,
and satisfies the following formula (1),
A bolt having a tensile strength of 1600 MPa or more.
1.26<Mo/1.4+V≦3.70...(1)
However, in formula (1), the mass percentages of Mo and V contained in the bolt are substituted for Mo and V, respectively. The chemical composition, in place of a part of the Fe, Ti: 0.100% or less in mass %,
Nb: 0.100% or less,
B: 0.0050% or less,
The bolt according to claim 1, further comprising one or more selected from the group consisting of W: 2.50% or less and REM: 0.020% or less. The chemical composition, in place of a part of the Fe, Pb: 0.05% or less in mass %,
Cd: 0.05% or less,
Co: 0.05% or less,
Zn: 0.05% or less,
The bolt according to claim 1 or 2, further comprising one or more selected from the group consisting of Ca: 0.02% or less and Zr: 0.02% or less. When the amount of trapped hydrogen after cathodic hydrogen charging for 72 hours at a current density of 0.2 mA/cm ² is 7.0 ppm or more in a solution at room temperature in which 3 g of ammonium thiocyanate per 1 L is added to 3 mass % sodium chloride. can be,
After cathodic hydrogen charging for 24 hours at a current density of 0.03 mA/cm ² in a room temperature solution of 3% by mass of sodium chloride and 0.3 g of ammonium thiocyanate per 1 L, zinc plating was performed for 96 hours. The bolt according to any one of claims 1 to 3, wherein the bolt takes 100 hours or more to break when a constant load of 0.9 times the tensile strength is applied after the bolt is left standing. A bolt steel having the chemical composition according to any one of claims 1 to 3 is formed into a bolt shape,
After heating to a temperature of A _c3 or higher, quenching is performed,
A method for manufacturing a bolt , comprising the steps of: performing a tempering treatment in a temperature range of 530 to 690°C to manufacture a bolt having a tensile strength of 1600 MPa or more . The bolt manufacturing method according to claim 5, wherein the tempering time of the tempering treatment is 30 minutes to 4 hours.