WO2024204819A1

WO2024204819A1 - Wire rod, steel wire, and machine component

Info

Publication number: WO2024204819A1
Application number: PCT/JP2024/013327
Authority: WO
Inventors: 真小此木
Original assignee: 日本製鉄株式会社
Priority date: 2023-03-31
Filing date: 2024-03-29
Publication date: 2024-10-03

Abstract

This wire rod has a predetermined chemical composition, in which: when the diameter of the wire rod is noted as D, the metal structure in a 1/4 D part, which is a part found in a cross section taken perpendicular to the length direction of the wire rod, at the depth of 1/4 D from the surface of the wire rod, includes bainite which accounts for an area ratio of 70% or more; the ratio of 0.05% yield strength to tensile strength (0.05% yield strength/tensile strength) is 0.72-0.40 × (C%) or more, where (C%) is the content of C in mass%; and a value obtained by dividing the standard deviation of Vickers hardness in a cross section perpendicular to the length direction of the wire rod by the average value of Vickers hardness is 0.150 or less.

Description

Wire rods, steel wires, and machine parts

This disclosure relates to wire rods, steel wires, and machine parts.

For machine parts such as steel bolts and screws, high-strength materials are required to reduce weight and size. Traditionally, these machine parts have been manufactured by softening machine structural steel wire rods containing C, Mn, Cr, Mo, etc. through spheroidizing annealing, then forming them into the desired shape through cold heading and rolling, and then quenching and tempering to impart strength.

Because the heat treatments of spheroidizing annealing and quenching and tempering increase manufacturing costs and _CO2 emissions, a non-tempered bolt is known that omits these heat treatments and instead manufactures wire rod whose strength has been increased by adjusting the composition and rapid cooling, and then performs wire drawing to impart strength to the wire rod.
Various wire rods and steel wires for manufacturing mechanical parts such as non-heat treated bolts have been proposed (for example, see Patent Documents 1 to 6).

Patent Document 1: JP-A-2-166229 Patent Document 2: WO 2016/121820 Patent Document 3: WO 2017/122830 Patent Document 4: WO 2018/008698 Patent Document 5: JP-A-2021-183710 Patent Document 6: JP-A-3-6325

Non-tempered bolts are made by forming steel wire whose strength has been increased by wire drawing. As the strength of the steel wire increases, damage and wear to the mold during forming, or the product is more likely to crack during processing, making it difficult to apply this to high-strength bolts.
In addition, conventional non-tempered bolts have a high dislocation density, which means that the bolt's 0.2% yield strength is low, resulting in a problem of large permanent elongation of the bolt. To improve this, it is necessary to add a bluing process in which the bolt is held at 300 to 400°C after being formed into a mechanical part such as a bolt.

The present disclosure aims to provide wire rod and steel wire for obtaining mechanical parts that have excellent strength and formability, and that have high 0.2% yield strength and reduced permanent elongation even if the bluing process after molding into the mechanical parts is omitted, as well as the mechanical parts.

The above problems are solved by the following means.
<1> In mass%,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: 0.003% or less,
Ti: 0 to 0.050%,
B: 0 to 0.0050%,
Cr: 0-1.50%,
Mo: 0 to 0.50%,
Nb: 0 to 0.050%,
V: 0 to 0.20%,
Cu: 0 to 0.50%,
Ni: 0 to 0.70%,
Sn: 0-0.30%,
Sb: 0 to 0.005% and Ca: 0 to 0.0050%,
and the balance being Fe and impurity elements,
When a diameter of the wire is D, a metal structure at a 1/4D portion having a depth of 1/4D from a surface of the wire in a cross section perpendicular to a longitudinal direction of the wire contains bainite in an area ratio of 70% or more,
When the C content in mass% is (C%), the ratio of 0.05% proof stress to tensile strength (0.05% proof stress/tensile strength) is 0.72-0.40×(C%) or more;
A wire having a standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the wire divided by an average value of the Vickers hardness of the wire, the standard deviation being 0.150 or less.
<2> In mass%,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: 0.003% or less,
Ti: 0 to 0.050%,
B: 0 to 0.0050%,
Cr: 0-1.50%,
Mo: 0 to 0.50%,
Nb: 0 to 0.050%,
V: 0 to 0.20%,
Cu: 0 to 0.50%,
Ni: 0 to 0.70%,
Sn: 0-0.30%,
Sb: 0 to 0.005% and Ca: 0 to 0.0050%,
and the balance being Fe and impurity elements,
When the diameter of the steel wire is d, a metal structure at a 1/4d part having a depth of 1/4d from the surface of the steel wire in a cross section perpendicular to the longitudinal direction of the steel wire contains bainite in an area ratio of 70% or more,
When the C content in mass% is (C%), the tensile strength is 610+900×(C%) MPa or more, the reduction in area is 62−32×(C%)% or more, the tensile strength (MPa)×reduction in area (%) is 52,000 or more, and the ratio of 0.2% proof stress to tensile strength (0.2% proof stress/tensile strength) is 0.985−0.083×(C%) or more,
A steel wire, wherein a value obtained by dividing the standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the steel wire by the average value of the Vickers hardness is 0.150 or less.
<3> A mechanical part including a shaft portion,
In mass percent,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: 0.003% or less,
Ti: 0 to 0.050%,
B: 0 to 0.0050%,
Cr: 0-1.50%,
Mo: 0 to 0.50%,
Nb: 0 to 0.050%,
V: 0 to 0.20%,
Cu: 0 to 0.50%,
Ni: 0 to 0.70%,
Sn: 0 to 0.30%,
Sb: 0 to 0.005% and Ca: 0 to 0.0050%,
and the balance being Fe and impurity elements,
When the diameter of the shaft portion is dm, a metal structure at a 1/4 dm portion having a depth of 1/4 dm from a surface of the shaft portion in a cross section perpendicular to a longitudinal direction of the shaft portion contains bainite in an area ratio of 70% or more,
When the content of C in mass% is (C%), the tensile strength of the shaft portion is 580 + 900 × (C%) MPa or more;
The ratio of 0.2% proof stress to the tensile strength (0.2% proof stress/tensile strength) is 0.90 or more,
A mechanical component, wherein a value obtained by dividing the standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the shaft portion by the average value of the Vickers hardness is 0.150 or less.

According to the present disclosure, there is provided a wire rod and steel wire for obtaining mechanical parts that have excellent strength and formability, and that have high 0.2% yield strength and reduced permanent elongation even when the bluing process after molding into the mechanical parts is omitted, as well as mechanical parts.

FIG. 2 is a diagram showing an example of an SEM photograph mainly of a bainite structure. FIG. 13 is a diagram showing another example of an SEM photograph mainly of a bainite structure.

The wire according to the present disclosure will now be described.
In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits. However, when the numerical values before and after "to" are followed by "more than" or "less than," the numerical range does not include these numerical values as the lower or upper limit.
The content of an element in a chemical composition may be expressed by adding "amount" to the element symbol (for example, C amount, Si amount, etc.).
With regard to the contents of elements in chemical compositions, "%" means "mass %".
When the content of an element in a chemical composition is described as "0-", this means that the element does not necessarily need to be contained.
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.
The "surface" of a wire means the "outer peripheral surface."
The "central axis" of a wire means an imaginary line that passes through the center point of a cross section perpendicular to the longitudinal direction of the wire and extends in the longitudinal direction (axial direction).
"1/4D" is synonymous with "D/4".

The inventors of the present disclosure have studied the effects of the structure and mechanical properties of the wire on the properties of steel wire and machine parts after wire drawing, and have found that by setting the composition of the wire within a specific range, making the structure of the wire mainly bainite with the area ratios of ferrite, pearlite, and martensite suppressed, setting the ratio of the tensile strength and 0.05% yield strength of the wire, i.e., 0.05% yield strength/tensile strength (hereinafter referred to as the "0.05% yield strength ratio of the wire") within a specific range, and reducing the standard deviation of the hardness of the wire, it is possible to obtain a steel wire with excellent strength and formability after wire drawing, and a high 0.2% yield strength ratio (0.2% yield strength/tensile strength) and suppression of permanent elongation of the bolt after processing into a non-tempered bolt.

When the wire structure is mainly ferrite or pearlite, the 0.2% yield strength ratio of the bolt is small. Non-tempered bolts have a high dislocation density because their strength is imparted by wire drawing. If there are many mobile dislocations, they will undergo plastic deformation with small strains, lowering the elastic limit and, as a result, reducing the yield strength. In order to increase the yield strength of steel with high dislocation density, it is effective to fix the solute carbon and nitrogen contained in the steel to the dislocations and suppress their movement.

Conventional non-tempered bolts use a heat treatment called bluing, in which the bolt is held at 300 to 400°C to fix carbon to the dislocations, suppressing their movement and increasing the bolt's 0.2% yield strength.
The inventors of the present disclosure have found that by making the structure of the wire rod mainly bainite and setting the 0.05% proof stress ratio of the wire rod within a specific range, the 0.2% proof stress ratio of the non-tempered bolt is increased even without bluing. Wire rods with normal bainite structure have a high mobile dislocation density and therefore a low elastic limit. When solute carbon is fixed to mobile dislocations, the movement of dislocations is suppressed and the elastic limit is increased. The commonly used 0.2% proof stress is affected by plastic deformation, so in this disclosure, 0.05% proof stress, which shows a value closer to the elastic limit, is used. Wire rods with bainite structure having a high 0.05% proof stress have a large amount of solute carbon fixed to dislocations. Since solute carbon is removed from the fixation of dislocations by wire drawing, it is considered that a steel wire drawn from a wire rod with bainite structure having a high 0.05% proof stress has a high solute carbon concentration. It is presumed that a high amount of dissolved carbon fixes carbon to mobile dislocations and suppresses dislocation movement without the need to heat to the bluing treatment temperature of 300 to 400°C, thereby increasing the 0.2% yield strength ratio of the bolt.

If the martensite fraction of the wire is high, it may break during drawing, or cracks may occur during forming, and the 0.2% yield strength ratio of the bolt may decrease. Even if the wire has a bainite-based structure, if the 0.05% yield strength ratio of the wire is low, the 0.2% yield strength ratio of the bolt may decrease. The reason for this is that in order to increase the 0.2% yield strength ratio of the bolt, a sufficient amount of solute carbon is required to fix mobile dislocations after drawing, but when the 0.05% yield strength ratio of the wire is low, the 0.2% yield strength ratio of the bolt decreases due to a lack of solute carbon. In addition, a bainite-based structure reduces work hardening, which is suppressed in high-strain processing such as bolt head processing, reducing the load on the die. Furthermore, the cementite in the bainite structure is fragmented and has high ductility, so work cracks can be suppressed.

If the hardness of the wire varies widely, the 0.2% yield strength ratio of the bolt after forming decreases. In addition, processing cracks are more likely to occur during bolt forming. The reason for this is presumably because the deformation caused by wire drawing becomes uneven, which in turn causes the distribution of dislocations to become uneven.

<Wire rod>
The wire rod according to the present disclosure has been discovered based on the above findings. The chemical composition, metal structure, mechanical properties, and manufacturing method of the wire rod according to the present disclosure will be described below.

[Chemical composition]
The chemical composition of the wire according to the present disclosure is, as essential elements, in mass %,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005 to 0.080%, and N: 0.0010 to 0.0150%,
O: Contains 0.003% or less, with the balance being Fe and impurity elements.
The wire according to the present disclosure may contain other optional elements as necessary.

C: 0.08-0.80%
C is contained to ensure the strength required for mechanical parts. If the C content is less than 0.08%, it is difficult to ensure the necessary strength for mechanical parts. If the C content exceeds 0.08%, the ductility, toughness, and cold forgeability are deteriorated. Therefore, the C content is set to 0.08 to 0.80%. The lower limit of the C content is 0.08%, preferably 0.15%, The upper limit of the C content is 0.80%, preferably 0.75%, and more preferably 0.70%. The C content is preferably 0.15 to 0.75%, and more preferably 0.20 to 0.70%.

Si: 0.03~1.50%
Silicon functions as a deoxidizing element and is an effective element for imparting the necessary strength to mechanical parts. If the amount of silicon is less than 0.03%, these effects are insufficient. If it exceeds 50%, the ductility and toughness of the machine parts will deteriorate, and the deformation resistance of the steel wire will increase, deteriorating the cold forgeability. Therefore, the Si content is set to 0.03 to 1.50%. The Si content is preferably 0.05 to 1.00%, and more preferably 0.10 to 0.60%.

Mn: 0.50-2.00%
Mn is an element necessary for imparting the necessary strength to mechanical parts. If the Mn content is less than 0.50%, the effect is insufficient. If the Mn content exceeds 2.00%, the mechanical parts The toughness is deteriorated, and the deformation resistance of the steel wire is increased, deteriorating the cold forgeability. Therefore, the Mn content is set to 0.50 to 2.00%. The preferable Mn content is 0.70 to 1. 50%, and more preferably 0.90 to 1.20%.

P: 0.050% or less P is contained in the wire rod as an impurity. P segregates at the grain boundaries of mechanical parts and deteriorates toughness, so it is desirable to reduce the P content. For this reason, the upper limit of the P content is set to 0.050%. A preferable upper limit of the P content is 0.020%, and a more preferable upper limit is 0.015% or less. The lower limit of the P content is preferably 0% (i.e., no P is contained), but from the viewpoint of reducing the dephosphorization cost, it may be more than 0% (or 0.0001% or more).

S: 0.050% or less S is contained in the wire rod as sulfides such as MnS. If the S content exceeds 0.050%, the cold heading property of the steel wire is deteriorated and the toughness of the mechanical parts is deteriorated. Therefore, the upper limit of the S content is set to 0.050%. A preferable upper limit of the S content is 0.030%. A more preferable upper limit is 0.015%. Note that the lower limit of the S content may be more than 0% (or 0.001% or more) from the viewpoint of reducing the desulfurization cost.

Al: 0.005-0.080%
Al functions as a deoxidizing element, and also has the effect of forming AlN to refine crystal grains and improve the toughness of machine parts. It also fixes solute N to suppress dynamic strain aging, It has the effect of reducing deformation resistance. If the Al content is less than 0.005%, these effects are insufficient. If the Al content exceeds 0.080%, the effects are saturated and manufacturability may decrease. Therefore, the Al content is set to 0.005 to 0.080%, preferably 0.010 to 0.065%, and more preferably 0.020 to 0.055%.

N: 0.0010-0.0150%
N forms nitrides with Al, Ti, Nb, V, etc., refines crystal grains, and has the effect of improving the toughness of machine parts. If the N content is less than 0.0010%, the amount of nitrides precipitated is If the N content exceeds 0.0150%, the deformation resistance of the steel wire increases due to dynamic strain aging caused by the solute N, and the workability deteriorates. The N content is preferably 0.0020 to 0.0100%, and more preferably 0.0025 to 0.0060%.

O: 0.003% or less O is an impurity and is inevitably contained in steel. If the O content exceeds 0.003%, coarse oxides are formed, which may reduce fatigue strength, so the O content is limited to 0.003% or less. The preferred upper limit of the O content is 0.002%.

Balance: Fe and impurity elements In the chemical composition of the wire according to the present disclosure, the balance is Fe and impurity elements (which may be referred to as "impurities" as appropriate in the present disclosure).
Here, the term "impurities" refers to components contained in raw materials or components mixed in during the manufacturing process and not intentionally added. Furthermore, impurities also include components that are intentionally added but are contained in an amount that does not affect the performance of the steel wire obtained by drawing the wire rod according to the present disclosure.

In addition, the wire rod according to the present disclosure may contain, in mass %, one or more elements selected from the group consisting of the following groups A to D, in place of a portion of the Fe. Note that the elements in the following groups A to D are optional elements, and these elements may not be included, i.e., may be 0%.

[Group A]
One or two selected from the group consisting of Ti: 0.050% or less and B: 0.0050% or less

Ti: 0.050% or less Ti functions as a deoxidizing element and is effective in facilitating the formation of bainite. If the Ti content exceeds 0.050%, these effects become saturated and coarse oxides or nitrides are formed, which may deteriorate the fatigue strength of the mechanical parts. Therefore, if Ti is contained, the Ti content should be 0.002 to 0.050%. The preferred Ti content is 0.003 to 0.050%, and more preferably 0.007 to 0.040%.

B: 0.0050% or less B has the effect of facilitating the formation of bainite. If the B content exceeds 0.0050%, carbides are formed at grain boundaries, which may deteriorate wiredrawability. Therefore, when B is contained, the B content should be 0.0002 to 0.0050%. The preferred B content is 0.0003 to 0.0050%, and more preferably 0.0008 to 0.0030%.

[Group B]
Cr: 1.50% or less,
Mo: 0.50% or less,
One or more selected from the group consisting of Nb: 0.050% or less and V: 0.20% or less

Cr: 1.50% or less Cr is an element necessary for imparting the necessary strength to mechanical parts. If the Cr content exceeds 1.50%, the martensite fraction of the wire increases, deteriorating wiredrawability, and the deformation resistance of the steel wire increases, deteriorating cold forgeability. Therefore, the Cr content is set to 1.50% or less. The preferred Cr content is 0.02 to 0.80%, and more preferably 0.10 to 0.50%.

Mo: 0.50% or less Mo has the effect of imparting the necessary strength to mechanical parts. If the Mo content exceeds 0.50%, the alloy cost increases, and the deformation resistance of the steel wire increases, deteriorating the cold forgeability. Therefore, when Mo is contained, the Mo content is preferably 0.02 to 0.50%. The preferred Mo content is 0.03 to 0.35%, and more preferably 0.05 to 0.25%. The Mo content may be 0.15% or less.

Nb: 0.050% or less Nb has the effect of increasing the strength of machine parts by precipitating carbides and nitrides, the effect of improving toughness by refining crystal grains, the effect of reducing solute N and reducing deformation resistance, etc. If the Nb content exceeds 0.050%, the effect is saturated and the cold forgeability may deteriorate. Therefore, when Nb is contained, the Nb content is preferably 0.002 to 0.050%. The preferred Nb content is 0.001 to 0.040%. The more preferred Nb content is 0.005 to 0.030%.

V: 0.20% or less V has the effect of precipitating carbides and nitrides to increase the strength of mechanical parts. If the V content exceeds 0.20%, the alloy cost increases. Therefore, if V is contained, the V content should be 0.02 to 0.20%. The preferred V content range is 0.01 to 0.15%.

[Group C]
Cu: 0.50% or less,
Ni: 0.70% or less,
One or more selected from the group consisting of Sn: 0.30% or less and Sb: 0.005% or less

Cu: 0.50% or less Cu precipitates finely to impart the necessary strength to mechanical parts and improve corrosion resistance. If the Cu content exceeds 0.50%, hot ductility deteriorates and surface defects are likely to occur. Therefore, if Cu is contained, the Cu content should be 0.02 to 0.50%. The preferred Cu content is 0.02 to 0.30%.

Ni: 0.70% or less Ni has the effect of improving corrosion resistance. If the Ni content exceeds 0.70%, the alloy cost increases. Therefore, if Ni is contained, the Ni content is preferably 0.02 to 0.70%. The Ni content is preferably 0.02 to 0.50%, and more preferably 0.05 to 0.30%.

Sn: 0.30% or less Sn has the effect of improving corrosion resistance. When Sn is contained, the Sn content is preferably 0.002% or more. However, if the Sn content exceeds 0.30%, the ductility decreases and the cold workability deteriorates, so the Sn content is limited to 0.30% or less. The preferred upper limit of the Sn content is 0.20%.

Sb: 0.005% or less Sb has the effect of improving corrosion resistance. When Sb is contained, the Sb content is preferably 0.001% or more. However, if the Sb content exceeds 0.005%, the ductility decreases and the cold workability deteriorates, so the Sb content is limited to 0.005% or less. The preferred upper limit of the Sb content is 0.004%, and the more preferred upper limit is 0.003%.

[Group D]
Ca: 0.0050% or less Ca is contained as a deoxidizing element. Ca has the effect of making oxides fine and improving fatigue strength. If the Ca content exceeds 0.0050%, ductility decreases and cold workability deteriorates, so it is limited to 0.0050% or less. The preferred upper limit of the Ca content is 0.0040%, and the more preferred upper limit is 0.0030%.

[Metal structure of wire]
Next, the metal structure of the wire according to the present disclosure will be described.
In the metal structure of the wire rod according to the present disclosure, the area ratio of bainite at a depth of 1/4D from the surface (outer peripheral surface) of the wire rod in a cross section (C cross section) perpendicular to the longitudinal direction of the wire rod is 70% or more. If the bainite area ratio is less than 70%, the 0.2% proof stress ratio of the mechanical component decreases and the workability deteriorates. The bainite area ratio is preferably 80% or more, and more preferably 90% or more.

(Method of measuring metal structure of wire rod)
The microstructure observation was performed by taking two samples from positions 50 mm apart in the axial direction of the wire, mirror-polishing a cross section (C cross section) perpendicular to the central axis of each wire sample, etching it with picral, and observing the ¼ D portion.
First, the C-section of the wire to be measured is mirror-polished and then etched with picral (a solution of 5% picric acid and 95% ethanol) to reveal the structure.
Next, in the 1/4D portion, a region of 20 μm×20 μm or more in one of four locations spaced 90° apart in the circumferential direction is photographed at a magnification of 5000 times using a field emission scanning electron microscope (FE-SEM). Examples of the bainite structure are shown in FIG. 1A and FIG. 1B.

The metal structure may contain bainite, pearlite, martensite, and ferrite. Of these, martensite and ferrite do not contain carbides within the grains. The pearlite structure is a structure in which cementite and ferrite are arranged alternately and almost parallel to one another. Therefore, structures other than bainite (structures that do not contain carbides within the grains, pearlite structures) in the photographed structure are visually marked, and the area of the regions of structures other than bainite is determined by image analysis (software name: Nireco's small general-purpose image processing analysis system LUZEX_AP). Note that this operation is performed by measuring and calculating two samples, averaging these values, and subtracting the area percentage of structures other than bainite from the total area (100%) to determine the area percentage of bainite in this disclosure.

[Mechanical properties of wire]
0.05% proof stress/tensile strength: 0.72-0.40×(C%) or more In the wire rod according to the present disclosure, when the C content in mass % is (C%), the ratio of 0.05% proof stress to tensile strength (0.05% proof stress/tensile strength) is 0.72-0.40×(C%) or more. When the 0.05% proof stress/tensile strength is 0.72-0.40×(C%) or more, a high 0.2% proof stress ratio can be obtained after molding into a mechanical part without performing a bluing treatment. The 0.05% proof stress/tensile strength is preferably 0.78-0.40×(C%) or more.

The tensile test of the wire rod is carried out using JIS Z2241:2011 No. 14A test piece and following the test method of JIS Z2241:2011 to measure the tensile strength (TS), 0.05% yield strength, and reduction in area. The 0.05% yield strength is the stress at which the plastic elongation becomes 0.05% of the extensometer gauge length according to the offset method defined in JIS Z2241:2011. The tensile test piece is machined and taken from the wire rod immediately after straightening. The tensile test is carried out on three samples and the average value is used.

(Vickers hardness of wire)
The wire rod according to the present disclosure has a value obtained by dividing the standard deviation of the Vickers hardness in a cross section (C cross section) perpendicular to the length direction of the wire rod by the average value of the Vickers hardness of 0.150 or less. When this value is 0.150 or less, the 0.2% proof stress ratio of a mechanical part manufactured from the steel wire through wire drawing is high. In addition, the workability when forming into a mechanical part is improved. The value is preferably 0.100 or less, more preferably 0.050 or less.

The hardness test was carried out by mirror-polishing the C-section of the wire and using a Vickers hardness tester to measure a total of nine points: four points at a depth of 1.0 mm from the surface of the wire, rotated at 90° intervals around the wire, four points at a depth of D/4 (where D is the diameter of the wire), rotated at 90° intervals around the wire, and one point at the center, and then calculating the average value and standard deviation. When the measured values are _x1 , _x2 , ..., _xn , the average value _Xave and standard deviation s were calculated from the following formula.
X _ave = (1/n)×Σx _n
s=((1/n)×Σ(x _n −X _ave ) ² ) ^(1/2)
The test force for measuring Vickers hardness is 9.8 N, and the measurement is performed according to the method of JIS Z2244-1:2020.

(Wire diameter)
The wire diameter D of the wire according to the present disclosure is not particularly limited, but is preferably 3.0 to 25.0 mm, and more preferably 5.0 to 18.0 mm.

[Wire rod manufacturing method]
An example of a method for producing a wire according to the present disclosure will be described.
A steel slab made of predetermined components that satisfy the above-mentioned chemical composition is heated to 1100 to 1250°C and held in the furnace for 90 minutes or more. Thereafter, the slab is hot-rolled at an entry temperature of 750 to 800°C for finish rolling, and then coiled into a ring shape at 780 to 820°C.
After coiling, the wire is primarily cooled to 750° C. at an average cooling rate of 2 to 15° C./s.
Thereafter, secondary cooling is performed from 750° C. to 500° C. at an average cooling rate of 25° C./s or more.
Next, the tertiary cooling is performed from 500° C. to 450° C. at an average cooling rate of 20° C./s or less.
The wire is then held at 420-440°C for 30-100 seconds (primary holding), heated from 490°C to 550°C at an average heating rate of 10°C/s or more, held at 580-620°C for 60-200 seconds (secondary holding), and then water-cooled to obtain the wire.

If the heating temperature is less than 1100°C or the heating time is less than 90 minutes, the carbide is insufficiently dissolved, and the 0.05% yield strength ratio of the wire rod decreases. If the heating temperature exceeds 1250°C, the surface hardness of the wire rod decreases due to decarburization.
If the entry temperature of the finish rolling exceeds 800° C., the gamma grains (austenite grains) become coarse, the ductility of the wire rod decreases, and the workability deteriorates. If the entry temperature is less than 750° C., the gamma grain size becomes non-uniform, and the standard deviation of the hardness of the wire rod increases.
If the coiling temperature exceeds 820° C., the gamma grains become coarse, the ductility of the wire rod decreases, and the workability deteriorates. If the coiling temperature is less than 780° C., the area ratio of bainite decreases, and the 0.05% proof stress ratio of the wire rod decreases.
If the primary average cooling rate to 750°C after coiling is less than 2°C/s, the gamma grains become coarse, the ductility of the wire rod decreases, and the workability deteriorates. If it exceeds 15°C/s, the recovery and recrystallization of the gamma grains are delayed, resulting in mixed grains. As a result, the gamma grain size becomes non-uniform, the standard deviation of the hardness of the wire rod increases, and the 0.2% proof stress ratio and permanent elongation of the mechanical parts deteriorate.
If the secondary average cooling rate from 750° C. to 500° C. is less than 25° C./s, the area ratio of ferrite and pearlite increases, and the 0.05% yield strength ratio of the wire rod decreases.
If the tertiary average cooling rate from 500°C to 450°C exceeds 20°C/s, the standard deviation of the hardness of the wire becomes large, and the formability of the bolt after drawing deteriorates, as well as the 0.2% yield strength ratio and permanent elongation of the bolt deteriorate.
If the first holding time at 420 to 440°C is less than 30 seconds, the hardness of the wire will vary widely, and the 0.2% yield strength ratio and permanent elongation of the mechanical parts will deteriorate. If the first holding time exceeds 100 seconds, the 0.05% yield strength ratio of the wire will decrease.
If the average heating rate from 490°C to 550°C is less than 10°C/s, the 0.05% yield strength ratio of the wire rod will be low, and the 0.2% yield strength ratio and permanent elongation of the mechanical parts will be deteriorated. If the secondary holding temperature is less than 580°C, the 0.2% yield strength ratio and permanent elongation of the mechanical parts will be deteriorated, and if it exceeds 620°C, the strength will be reduced. If the secondary holding time is less than 60 seconds, the 0.05% yield strength ratio of the wire rod will be reduced. If the secondary holding time exceeds 200 seconds, the carbides will coarsen, and the reduction in area of the steel wire after drawing will be reduced.

The wire rod according to the present disclosure is drawn with an area reduction rate of 15 to 50% to produce steel wire, which is then cold heading and rolling processed into the shape of a mechanical part such as a bolt. If the area reduction rate of the wire drawing is less than 15%, the yield strength ratio of the mechanical part decreases and the permanent elongation increases. If the area reduction rate of the wire drawing is 50% or more, the workability of the mechanical part deteriorates. After the wire drawing, zinc plating and baking treatment are performed as necessary.

The steel wire manufactured using the wire rod of the present disclosure has high strength and drawing ability, and is excellent in strength and formability of the bolt. A non-tempered bolt manufactured using such a steel wire has a high bolt 0.2% yield strength ratio and excellent permanent elongation even if the bluing treatment after forming is omitted.
The steel wire and the machine component according to the present disclosure will be described below. Note that the chemical composition of the steel wire and the machine component according to the present disclosure is the same as that of the wire rod described above, and therefore will not be described below.

<Steel wire>
The steel wire according to the present disclosure is obtained by drawing the above wire rod, and plastic flow is observed in the surface layer of the longitudinal cross section of the steel wire. The metal structure and mechanical properties of the steel wire according to the present disclosure will be described.

[Metal structure of steel wire]
In the steel wire according to the present disclosure, when the diameter of the steel wire is d, the metal structure at a 1/4d portion, which is 1/4d deep from the surface of the steel wire in a cross section perpendicular to the longitudinal direction of the steel wire, contains bainite with an area ratio of 70% or more. If the bainite area ratio is less than 70%, the 0.2% proof stress ratio of the mechanical component decreases and the workability deteriorates. The bainite area ratio is preferably 80% or more, and more preferably 90% or more.
The metal structure of the steel wire can be measured in the same manner as the metal structure of the wire rod described above.

[Mechanical properties of steel wire]
Tensile strength: 610+900×(C%)MPa or more Reduction of area: 62-32×(C%)% or more Tensile strength (MPa)×reduction of area (%): 52,000 or more 0.2% yield strength ratio (0.2% yield strength/tensile strength): 0.985-0.083×(C%) or more When the C content in mass % is (C%), the steel wire according to the present disclosure has a tensile strength of 610+900×(C%)MPa or more, a reduction of area of 62-32×(C%)% or more, a tensile strength (MPa)×reduction of area (%) of 52,000 or more, and a 0.2% yield strength ratio (0.2% yield strength/tensile strength) of 0.985-0.083×(C%) or more.
When the tensile strength, reduction in area, and 0.2% yield strength ratio of the steel wire satisfy the above-mentioned relationships, when a non-tempered mechanical part is manufactured from this steel wire, the cold workability is improved and sufficient strength for the mechanical part is obtained.

From the viewpoint of achieving both cold workability and tensile strength when made into a mechanical part, it is preferable that the tensile strength is 640 + 900 x (C%) MPa or more, the reduction in area is 66 - 32 x (C%)% or more, the tensile strength (MPa) x reduction in area (%) is 53,000 or more, and the 0.2% yield strength ratio is 0.990 - 0.083 x (C%) or more.

The tensile strength, 0.2% yield strength ratio, and reduction in area of the steel wire disclosed herein can be measured in the same manner as in the tensile test of the wire rod described above, using a JIS Z2241:2011 No. 14A test piece and following the test method of JIS Z2241:2011.

(Vickers hardness of steel wire)
In the steel wire according to the present disclosure, the standard deviation of the Vickers hardness in a cross section (C cross section) perpendicular to the longitudinal direction of the steel wire is divided by the average Vickers hardness of 0.150 or less. If this value is 0.150 or less, the workability when forming into a mechanical part is improved, and the 0.2% proof stress ratio of the mechanical part is increased. The value is preferably 0.100 or less, and more preferably 0.050 or less.
The hardness test of the steel wire can be carried out in the same manner as the hardness test of the wire rod described above, and the average value and standard deviation can be calculated in the same manner.

(Steel wire diameter)
The wire diameter d of the steel wire according to the present disclosure is not particularly limited, but may be, for example, 2.0 to 23.0 mm, or 4.0 to 18.0 mm.

<Machine parts>
The mechanical part according to the present disclosure is obtained by performing cold heading and rolling on the above-mentioned steel wire and processing it into the shape of a mechanical part such as a bolt. The mechanical part according to the present disclosure includes a shaft portion, and the shaft portion has the following metal structure and mechanical properties.

[Metal structure of shaft]
When the diameter of the shaft portion is dm, the metal structure at a 1/4 dm portion, which is 1/4 dm deep from the surface of the shaft portion in a cross section (C cross section) perpendicular to the longitudinal direction of the shaft portion, contains bainite with an area ratio of 70% or more. If the bainite area ratio is less than 70%, the 0.2% proof stress ratio of the mechanical component decreases and workability deteriorates. The bainite area ratio is preferably 80% or more, and more preferably 90% or more.
The metal structure of the shaft portion can be measured in the same manner as the metal structure of the wire rod described above.

[Mechanical properties of shaft]
Tensile strength of shaft portion: 580+900×(C%) MPa or more In the mechanical component according to the present disclosure, the tensile strength of the shaft portion is 580+900×(C%) MPa or more, where the C content in mass % is (C%). By having such a tensile strength, the mechanical component exhibits high strength.

0.2% Yield Strength/Tensile Strength: 0.90 or More The mechanical component according to the present disclosure has a ratio of 0.2% yield strength to the tensile strength of the shaft portion (0.2% yield strength/tensile strength) of 0.90 or more. The mechanical component according to the present disclosure can obtain a high 0.2% yield strength ratio of 0.90 or more even without bluing treatment. The 0.2% yield strength/tensile strength is preferably 0.92 or more.

The tensile test can be performed using a linear test piece with the shaft cut off, in the same manner as the tensile test of wire material described above, using a JIS Z2241:2011 No. 14A test piece, in accordance with the test method of JIS Z2241:2011.

(Vickers hardness of shaft)
In the mechanical component according to the present disclosure, the standard deviation of the Vickers hardness in a cross section (C cross section) perpendicular to the longitudinal direction of the shaft portion is divided by the average value of the Vickers hardness to be 0.150 or less. When this value is 0.150 or less, the 0.2% proof stress ratio of the mechanical component manufactured from the steel wire through wire drawing is high. In addition, the processability when forming into the mechanical component is improved. It is preferably 0.100 or less, more preferably 0.050 or less.
The hardness test of the shaft portion can be carried out in the same manner as the hardness test of the wire rod described above, and the average value and standard deviation can be calculated in the same manner.

(shaft diameter)
The diameter dm of the shaft portion of the mechanical component according to the present disclosure is not particularly limited, but may be, for example, 2.0 to 23.0 mm, or 4.0 to 18.0 mm.

(permanent set)
The mechanical component according to the present disclosure has a high 0.2% yield strength and is suppressed from permanent elongation.
For permanent elongation, the shaft of the machine part is machined or rolled to form a coarse thread shape. After threading, the machine part is fitted with an adapter as described in JIS B1051:2014 so that the length of the play thread is equal to the outer diameter of the thread. When the tensile strength of the mechanical part is 600 MPa or more and less than 700 MPa, the tensile stress is 440 MPa, when the tensile strength is 700 MPa or more and less than 800 MPa, the tensile stress is 515 MPa, when the tensile strength is 800 MPa or more and less than 900 MPa, the tensile stress is 650 MPa, when the tensile strength is 1000 MPa or more and less than 1100 MPa, the tensile stress is 830 MPa, when the tensile strength is 1100 MPa or more and less than 1200 MPa, the tensile stress is 910 MPa, when the tensile strength is 1200 MPa or more and less than 1300 MPa, the tensile stress is 970 MPa, and when the tensile strength is 1300 MPa or more and less than 1400 MPa, the tensile stress is 1050 MPa. The difference (L1-L0) between the total length L1 of the mechanical part after unloading and the total length L0 of the mechanical part before the test is defined as the permanent elongation. The total length L1 of the mechanical part after unloading and the total length L0 of the mechanical part before the test were measured using a micrometer.

Below, the wire material according to this disclosure will be described in more detail with reference to examples. However, these examples do not limit the wire material according to this disclosure.

<Wire rod production and evaluation>
Steels A to U having the chemical compositions shown in Table 1 were used to manufacture wire rods by heating, hot rolling, cooling, and holding under the conditions shown in Table 2. In Table 1, blanks mean that the element is not included, and the balance is Fe and impurities. In each table, underlines mean that the element is outside the scope of the present disclosure.

The obtained wire was used for structural observation, tensile testing, and hardness measurement using the methods described above. Table 3 shows the structure and mechanical properties of the wire for each test number. In Table 3, F in the remaining structure stands for ferrite, P for pearlite, and M for martensite.

<Steel wire manufacturing>
The wire rods in Table 3 were used for drawing at the area reduction rates shown in Table 4 to obtain steel wires. The obtained steel wires were used for structure observation, hardness measurement, and tensile testing in air at room temperature by the above-mentioned methods to measure tensile strength (TS) and reduction of area (RA). Steel wires in which TS×RA was 52000 or more, TS was 610+900×(C%)MPa or more, RA was 62−32×(C%)% or more, 0.2% yield strength ratio was 0.985−0.083×(C%) or more, and the value obtained by dividing the standard deviation of Vickers hardness by the average value of Vickers hardness was 0.150 or less were judged to have good strength, formability, and yield strength.

<Bolt manufacturing>
The steel wires in Table 4 were used and processed into bolt shapes by cold heading and rolling. Test numbers 1 and 13 had threads of M5 and shank lengths of 100 mm, test numbers 10 and 17 had threads of M6 and shank lengths of 100 mm, and test numbers 2 to 9, 11, 12, 14 to 16, 18 to 20, and 22 to 25 had threads of M8 and shank lengths of 100 mm. These were then electrogalvanized, and after plating, the bolts were baked at 200°C for 120 minutes to obtain bolts. Test number 21 cracked during bolt processing and could not be processed into a bolt shape.
From the shaft of the obtained bolt, a JIS Z2241:2011 No. 14A test piece was obtained by machining, and was measured according to the test method of JIS Z2241:2011. The diameter of the parallel part of the tensile test piece was 4 mm for the bolt with thread part M5, 5 mm for the bolt with thread part M6, and 7 mm for the bolt with thread part M8. The tensile test piece was taken, and the structure observation, hardness measurement, and tensile test were performed by the above-mentioned method, and the tensile strength (TS) and 0.2% proof strength were measured. The 0.2% proof strength ratio of the bolt was obtained by dividing the 0.2% proof strength by the tensile strength. The obtained bolt was also used to measure the permanent elongation by the above-mentioned method. Table 5 shows the TS of each bolt, the 0.2% proof strength ratio of the bolt, and the permanent elongation. It was judged as good when TS was 580 + 900 × (C%) MPa or more, 0.2% proof stress ratio (bolt proof stress ratio) was 0.90 or more, the value obtained by dividing the standard deviation of Vickers hardness by the average value of Vickers hardness was 0.150 or less, and permanent elongation was 1 μm or less.

The above results show that the wire rods and steel wires of test numbers 1 to 19 that meet the requirements of this disclosure have excellent formability during bolt processing, and a high 0.2% yield strength ratio was obtained even without performing a bluing process after bolt processing.

(Additional Note)
The present disclosure includes the following aspects.
<1> Chemical composition, in mass%,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: 0.003% or less, with the balance being Fe and impurity elements;
When a diameter of the wire is D, a metal structure at a 1/4D portion having a depth of 1/4D from a surface of the wire in a cross section perpendicular to a longitudinal direction of the wire contains bainite in an area ratio of 70% or more,
When the C content in mass% is (C%), the ratio of 0.05% proof stress to tensile strength (0.05% proof stress/tensile strength) is 0.72-0.40×(C%) or more;
A wire having a standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the wire divided by an average value of the Vickers hardness of the wire, the standard deviation being 0.150 or less.
<2> Chemical composition, in mass%,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: Contains 0.003% or less, and further contains one or more selected from the group consisting of the following A group to D group, with the balance being Fe and impurity elements;
[Group A]
One or two selected from the group consisting of Ti: 0.050% or less and B: 0.0050% or less [Group B]
Cr: 1.50% or less,
Mo: 0.50% or less,
One or more selected from the group consisting of Nb: 0.050% or less and V: 0.20% or less [Group C]
Cu: 0.50% or less,
Ni: 0.70% or less,
One or more selected from the group consisting of Sn: 0.30% or less and Sb: 0.005% or less [Group D]
Ca: 0.0050% or less. When the diameter of the wire is D, a metal structure at a 1/4D portion having a depth of 1/4D from the surface of the wire in a cross section perpendicular to the longitudinal direction of the wire contains bainite at an area ratio of 70% or more,
When the C content in mass% is (C%), the ratio of 0.05% proof stress to tensile strength (0.05% proof stress/tensile strength) is 0.72-0.40×(C%) or more;
A wire having a standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the wire divided by an average value of the Vickers hardness of the wire, the standard deviation being 0.150 or less.
<3> The wire according to <2>, having a chemical composition including, by mass%, the A group.
<4> The wire according to <2> or <3>, having a chemical composition including, by mass%, the group B. <5> The wire according to any one of <2> to <4>, having a chemical composition including, by mass%, the group C.
<6> The wire according to any one of <2> to <5>, having a chemical composition including, by mass%, the D group.

<7> The chemical composition is, in mass%,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: 0.003% or less, with the balance being Fe and impurity elements;
When the diameter of the steel wire is d, a metal structure at a 1/4d part having a depth of 1/4d from the surface of the steel wire in a cross section perpendicular to the longitudinal direction of the steel wire contains bainite in an area ratio of 70% or more,
When the C content in mass% is (C%), the tensile strength is 610+900×(C%) MPa or more, the reduction in area is 62−32×(C%)% or more, the tensile strength (MPa)×reduction in area (%) is 52,000 or more, and the ratio of 0.2% proof stress to tensile strength (0.2% proof stress/tensile strength) is 0.985−0.083×(C%) or more,
A steel wire, wherein a value obtained by dividing the standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the steel wire by the average value of the Vickers hardness is 0.150 or less.
<8> Chemical composition, in mass%,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: Contains 0.003% or less, and further contains one or more selected from the group consisting of the following A group to D group, with the balance being Fe and impurity elements;
[Group A]
One or two selected from the group consisting of Ti: 0.050% or less and B: 0.0050% or less [Group B]
Cr: 1.50% or less,
Mo: 0.50% or less,
One or more selected from the group consisting of Nb: 0.050% or less and V: 0.20% or less [Group C]
Cu: 0.50% or less,
Ni: 0.70% or less,
One or more selected from the group consisting of Sn: 0.30% or less and Sb: 0.005% or less [Group D]
Ca: 0.0050% or less. When the diameter of a steel wire is d, a metal structure at a 1/4d part having a depth of 1/4d from the surface of the steel wire in a cross section perpendicular to the longitudinal direction of the steel wire contains bainite in an area ratio of 70% or more,
When the C content in mass% is (C%), the tensile strength is 610+900×(C%) MPa or more, the reduction in area is 62−32×(C%)% or more, the tensile strength (MPa)×reduction in area (%) is 52,000 or more, and the ratio of 0.2% proof stress to tensile strength (0.2% proof stress/tensile strength) is 0.985−0.083×(C%) or more,
A steel wire, wherein a value obtained by dividing the standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the steel wire by the average value of the Vickers hardness is 0.150 or less.
<9> The steel wire according to <8>, having a chemical composition including, by mass%, the Group A.
<10> The steel wire according to <8> or <9>, having a chemical composition including, by mass%, the B group.
<11> The steel wire according to any one of <8> to <10>, having a chemical composition including, by mass%, the C group.
<12> The steel wire according to any one of <8> to <11>, having a chemical composition including, by mass%, the D group.

<13> A mechanical part including a shaft portion,
The chemical composition, in mass%, is
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: 0.003% or less, with the balance being Fe and impurity elements;
When the diameter of the shaft portion is dm, a metal structure at a 1/4 dm portion having a depth of 1/4 dm from a surface of the shaft portion in a cross section perpendicular to a longitudinal direction of the shaft portion contains bainite in an area ratio of 70% or more,
When the content of C in mass% is (C%), the tensile strength of the shaft portion is 580 + 900 × (C%) MPa or more;
The ratio of 0.2% proof stress to the tensile strength (0.2% proof stress/tensile strength) is 0.90 or more,
A mechanical component, wherein a value obtained by dividing the standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the shaft portion by the average value of the Vickers hardness is 0.150 or less.
<14> A mechanical part including a shaft portion,
The chemical composition, in mass%, is
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: Contains 0.003% or less, and further contains one or more selected from the group consisting of the following A group to D group, with the balance being Fe and impurity elements;
[Group A]
One or two selected from the group consisting of Ti: 0.050% or less and B: 0.0050% or less [Group B]
Cr: 1.50% or less,
Mo: 0.50% or less,
One or more selected from the group consisting of Nb: 0.050% or less and V: 0.20% or less [Group C]
Cu: 0.50% or less,
Ni: 0.70% or less,
One or more selected from the group consisting of Sn: 0.30% or less and Sb: 0.005% or less [Group D]
Ca: 0.0050% or less. When the diameter of the shaft portion is dm, a metal structure at a 1/4 dm portion that is 1/4 dm deep from the surface of the shaft portion in a cross section perpendicular to the longitudinal direction of the shaft portion contains bainite at an area ratio of 70% or more,
When the content of C in mass% is (C%), the tensile strength of the shaft portion is 580 + 900 × (C%) MPa or more;
The ratio of 0.2% proof stress to the tensile strength (0.2% proof stress/tensile strength) is 0.90 or more,
A mechanical component, wherein a value obtained by dividing the standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the shaft portion by the average value of the Vickers hardness is 0.150 or less.
<15> The mechanical component according to <14>, having a chemical composition including the A group in mass %.
<16> The mechanical part according to <14> or <15>, having a chemical composition including the B group in mass%.
<17> The mechanical component according to any one of <14> to <16>, having a chemical composition including, by mass%, the C group.
<18> The mechanical component according to any one of <14> to <17>, having a chemical composition including, by mass%, the D group.

The disclosure of Japanese Patent Application No. 2023-059430, filed on March 31, 2023, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated.

Claims

In mass percent,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: 0.003% or less,
Ti: 0 to 0.050%,
B: 0 to 0.0050%,
Cr: 0-1.50%,
Mo: 0 to 0.50%,
Nb: 0 to 0.050%,
V: 0 to 0.20%,
Cu: 0 to 0.50%,
Ni: 0 to 0.70%,
Sn: 0 to 0.30%,
Sb: 0 to 0.005% and Ca: 0 to 0.0050%,
and the balance being Fe and impurity elements,
When a diameter of the wire is D, a metal structure at a 1/4D portion having a depth of 1/4D from a surface of the wire in a cross section perpendicular to a longitudinal direction of the wire contains bainite in an area ratio of 70% or more,
When the C content in mass% is (C%), the ratio of 0.05% proof stress to tensile strength (0.05% proof stress/tensile strength) is 0.72-0.40×(C%) or more;
A wire having a standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the wire divided by an average value of the Vickers hardness of the wire, the standard deviation being 0.150 or less.
In mass percent,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: 0.003% or less,
Ti: 0 to 0.050%,
B: 0 to 0.0050%,
Cr: 0-1.50%,
Mo: 0 to 0.50%,
Nb: 0 to 0.050%,
V: 0 to 0.20%,
Cu: 0 to 0.50%,
Ni: 0 to 0.70%,
Sn: 0 to 0.30%,
Sb: 0 to 0.005% and Ca: 0 to 0.0050%,
and the balance being Fe and impurity elements,
When the diameter of the steel wire is d, a metal structure at a 1/4d part having a depth of 1/4d from the surface of the steel wire in a cross section perpendicular to the longitudinal direction of the steel wire contains bainite in an area ratio of 70% or more,
When the C content in mass% is (C%), the tensile strength is 610+900×(C%) MPa or more, the reduction in area is 62−32×(C%)% or more, the tensile strength (MPa)×reduction in area (%) is 52,000 or more, and the ratio of 0.2% proof stress to tensile strength (0.2% proof stress/tensile strength) is 0.985−0.083×(C%) or more,
A steel wire, wherein a value obtained by dividing the standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the steel wire by the average value of the Vickers hardness is 0.150 or less.
A mechanical part including a shaft portion,
In mass percent,
C: 0.08-0.80%,
Si: 0.03 to 1.50%,
Mn: 0.50-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
N: 0.0010-0.0150%,
O: 0.003% or less,
Ti: 0 to 0.050%,
B: 0 to 0.0050%,
Cr: 0-1.50%,
Mo: 0 to 0.50%,
Nb: 0 to 0.050%,
V: 0 to 0.20%,
Cu: 0 to 0.50%,
Ni: 0 to 0.70%,
Sn: 0 to 0.30%,
Sb: 0 to 0.005% and Ca: 0 to 0.0050%,
and the balance being Fe and impurity elements,
When the diameter of the shaft portion is dm, a metal structure at a 1/4 dm portion having a depth of 1/4 dm from a surface of the shaft portion in a cross section perpendicular to a longitudinal direction of the shaft portion contains bainite in an area ratio of 70% or more,
When the content of C in mass% is (C%), the tensile strength of the shaft portion is 580 + 900 × (C%) MPa or more;
The ratio of 0.2% proof stress to the tensile strength (0.2% proof stress/tensile strength) is 0.90 or more,
A mechanical component, wherein a value obtained by dividing the standard deviation of Vickers hardness in a cross section perpendicular to the longitudinal direction of the shaft portion by the average value of the Vickers hardness is 0.150 or less.