JP7226083B2 - wire and steel wire - Google Patents

Description

The present disclosure relates to wires and steel wires.

Since cold forging is excellent in dimensional accuracy and productivity of finished products, there is an increasing shift from conventional hot forging when forming machine parts such as steel bolts, screws, and nuts.
Also, parts such as bolts and nuts are often used for structural purposes, and for this reason, alloying elements such as C, Mn, and Cr are added to impart strength. However, when the content of alloying elements increases, the deformation resistance of the steel material increases and the ductility decreases. Work cracks may occur in molded parts.

Moreover, in recent years, the shapes of parts have become more complicated in order to reduce part manufacturing costs and to improve the functions of parts. For this reason, steel materials used for cold forging are required to be soft and have extremely high ductility. It is The workability of steel for cold forging includes deformation resistance, which affects die load, and ductility, which affects the occurrence of work cracks. .

Spheroidizing annealing is a treatment for making cementite spherical to improve workability, and is widely used as a softening treatment for steel for cold forging. Since spheroidizing annealing requires a heat treatment time of about 20 hours, in recent years, in order to improve the productivity and cost of parts, there has been an increasing demand for shortening the treatment time, lowering the annealing temperature, or omitting annealing. there is
In addition, alloying elements such as Cr and Mo are sometimes added to steels for machine structural use in order to ensure the strength necessary for machine parts. The addition of these alloying elements to the steel can delay the spheroidization of cementite, result in insufficient softening after annealing, increase strength, and reduce ductility. As a result, the cold forgeability deteriorates. Therefore, when these alloying elements are added to steel, methods such as performing spheroidizing annealing twice or more are performed in order to improve the cold forgeability.

Under such circumstances, various methods have been conventionally proposed as techniques for shortening the spheroidizing annealing of steel materials and improving the cold forgeability after spheroidizing annealing. For example, before spheroidizing annealing, rough drawing with a reduction in area of 20 to 30% is performed to promote spheroidization of cementite and soften the steel material, and spheroidizing annealing is performed multiple times to soften the steel material. The method of making it, etc. is conventionally widely performed.

For example, Patent Literature 1 discloses a method of making a structure of a bainite single phase before spheroidizing treatment. This method is intended to accelerate cementite spheroidization and shorten the time by making the structure before spheroidization treatment a single bainite phase.

In Patent Document 2, cold forging after spheroidization with a proeutectoid ferrite fraction of 5 to 30 area%, the remaining structure being mainly bainite, and the average value of lath spacing of cementite in bainite being 0.3 μm or more A steel wire rod/steel bar having excellent toughness is disclosed.

In Patent Document 3, the hot-rolled wire has a ferrite structure fraction of 30 area % or more, and the sum of the bainite structure and the martensite structure is 50 area % or more of the remainder, so that the wire is spheroidized after rough drawing. A method is disclosed that allows annealing to be performed at low temperatures and in a short period of time.

In Patent Document 4, as a method for manufacturing case hardening steel, a steel material having a ferrite/pearlite structure with a bainite volume fraction suppressed to 50% or less is subjected to wire drawing drawing at a reduction rate of 28% or more, A method for producing cold forging case hardening steel with spheroidizing annealing is disclosed.

Patent Document 5 discloses a method of shortening the spheroidization treatment time and reducing the deformation resistance of the steel material after the spheroidization treatment by specifying the area ratios of pseudo-pearlite, bainite, and ferrite in the steel structure. ing.

Patent Document 6 discloses a steel wire in which the ductility after low-temperature annealing can be increased by specifying the volume fractions and tensile strength of pearlite, proeutectoid ferrite, and bainite in the structure of the steel wire. .

In Patent Document 7, when the content of C is [C%], it contains 75 × [C%] + 25 or more bainite in volume%, and the balance is one or more of ferrite and pearlite. A component steel wire is disclosed.

JP-A-60-9832 Japanese Patent Application Laid-Open No. 2001-89830 JP-A-2006-37159 JP 2006-124774 A JP-A-2006-225701 WO2011/062012 WO2016/121820

In conventional steel wires for cold forging, when the Cr content is high, the cementite is not sufficiently spheroidized after annealing, resulting in high deformation resistance and easy cracking during work.

Therefore, an object of the present disclosure is to provide a wire rod and a steel wire that can achieve reduction in deformation resistance and improvement in ductility after spheroidizing annealing even if the composition has a high Cr content.

The above problems are solved by the following means.
<1> Component composition is mass %,
C: 0.10 to 0.50%,
Si: 0.01 to 0.50%,
Mn: 0.20-1.00%,
P: 0.030% or less,
S: 0.050% or less,
Cr: 0.85-1.50%,
Al: 0.001 to 0.080%,
N: 0.0010 to 0.0200%, and the balance: Fe and impurities,
Contents by mass % of C, Si, Mn, Cr, Cu, Mo, and V contained in the wire are expressed as (C%), (Si%), (Mn%), (Cr%), (Cu% ), (Mo%), and (V%), and when F1 and F2 are values calculated by the following formulas,
F1 is 1.50 or more and 3.00 or less,
The metal structure contains at least bainite and martensite, the total area ratio of the bainite, the martensite, and the ferrite measured in the cross section perpendicular to the longitudinal direction of the wire is 98% or more, and the area of the ferrite ratio is less than 5%, and the martensite area ratio is more than 0%, and satisfies 10 × F1-15% or more and 21 × F1-20% or less,
When the average circle equivalent diameter of the bainite blocks measured in the cross section parallel to the longitudinal direction and including the central axis of the wire is 15 μm or less, and the diameter of the wire is D, (the bainite blocks in the surface layer The average equivalent circle diameter/average equivalent circle diameter of the bainite block at a depth of 0.25D from the surface) is less than 1.00,
A wire having a tensile strength of 1000×F2+200 MPa or more and 1300×F2+190 MPa or less.
F1 = (Mn%) + 1.2 (Cr%) + 1.2 (Cu%) + 0.8 (Mo%)
F2 = (C%) + 0.14 x (Si%) + 0.20 x (Mn%) + 0.11 x (Cr%) + 0.5 x (Mo%) + 1.54 x (V%)
<2> The component composition is mass %,
Ti: 0.050% or less,
B: 0.0050% or less,
Mo: 0.30% or less,
Ni: 0.30% or less,
Cu: 0.50% or less,
V: 0.30% or less,
Nb: 0.050% or less,
Ca: 0.0050% or less,
The wire according to <1>, satisfying one or more selected from the group consisting of Mg: 0.0050% or less and Zr: 0.0050% or less.
<3> Component composition is mass %,
C: 0.10 to 0.50%,
Si: 0.01 to 0.50%,
Mn: 0.20-1.00%,
P: 0.030% or less,
S: 0.050% or less,
Cr: 0.85-1.50%,
Al: 0.001 to 0.080%,
N: 0.0010 to 0.0200%, and the balance: Fe and impurities,
The contents of Mn, Cr, Cu, and Mo contained in the steel wire are represented by (Mn%), (Cr%), (Cu%), and (Mo%), respectively, and F1 is expressed by the following formula where F1 is 1.50 or more and 3.00 or less,
The metal structure contains at least bainite and martensite, and the total area ratio of the bainite, the martensite, and ferrite measured in the cross section perpendicular to the longitudinal direction of the steel wire is 98% or more, and the ferrite The area ratio is less than 5%, the area ratio of martensite is more than 0%, and 10 × F1-15% or more and 21 × F1-20% or less are satisfied,
The average equivalent circle diameter of the bainite blocks measured in a cross section parallel to the longitudinal direction including the central axis of the steel wire is 15 μm or less, and the average aspect ratio of the bainite blocks is more than 1.00 and less than 1.30. steel wire.
F1 = (Mn%) + 1.2 (Cr%) + 1.2 (Cu%) + 0.8 (Mo%)
<4> The component composition is mass %,
Ti: 0.050% or less,
B: 0.0050% or less,
Mo: 0.30% or less,
Ni: 0.30% or less,
Cu: 0.50% or less,
V: 0.30% or less,
Nb: 0.050% or less,
Ca: 0.0050% or less,
Mg: 0.0050% or less, and Zr: 0.0050% or less,
The steel wire according to <3> satisfying one or more selected from the group consisting of:

ADVANTAGE OF THE INVENTION According to this disclosure, the wire rod and steel wire which can achieve reduction in deformation resistance and improvement in ductility after spheroidizing annealing are provided even if the composition has a high Cr content.

Indicates the measurement position of the bainite block FIG. 10 is an SEM image showing the structure of the wire of test number 6 etched with picral at a depth of 0.25D from the surface layer. FIG.

An embodiment that is an example of the present disclosure will be described.
The wire rods and steel wires according to the present disclosure particularly relate to wire rods and steel wires that can be made soft and highly ductile after annealing, such as bolts, screws, nuts, etc. formed by cold forging, rolling, etc. and a steel wire obtained by drawing the wire. It should be noted that the wire rod covered by the present disclosure also includes a "burn-in coil" obtained by winding a hot-rolled steel bar into a coil shape.

In the present disclosure, a numerical range represented using "to" means a range including the numerical values described before and after "to" as lower and upper limits.
In addition, a numerical range when "more than" or "less than" is attached to a numerical value written before and after "to" means a range that does not include these numerical values as lower or upper limits.
Also, the content of an element in the component composition may be expressed as the amount of element (for example, the amount of C, the amount of Si, etc.).
Moreover, "%" means "% by mass" with respect to the content of elements in the component composition.
In addition, when the content of an element in the component composition is described as "0 to", it means that the element does not have to be contained.
In addition, 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.
Also, the "surface" of the wire or steel wire means the "peripheral surface".
In addition, the “L cross section” means a cross section including the central axis of the wire rod or steel wire and parallel to the longitudinal direction (that is, rolling direction or wire drawing direction).
In addition, "C section" means a section perpendicular to the longitudinal direction of a wire rod or steel wire.
The term "central axis" refers to an imaginary line extending in the longitudinal direction (axial direction) passing through the central point of the cross section perpendicular to the longitudinal direction of the wire rod or steel wire.
In addition, the notation "numerical value XD" means the position at a depth X times the diameter D from the surface toward the central axis (toward the radial direction), where D is the diameter of the wire or steel wire. show. For example, "0.25D" indicates a position at a depth of 0.25 times the diameter D.

The wire and steel wire according to the present disclosure have a predetermined chemical composition, the wire has a metal structure that satisfies the following (1) and (3), the steel wire has a metal structure that satisfies (1) and (2), and further The wire satisfies the tensile strength of (4) below.
(1) The metal structure of the wire rod and steel wire contains at least bainite and martensite, and 98 area% or more as measured by the C cross section is bainite, and the area% is 10 × F1-15% or more 21 × F1-20%. It is composed of the following martensite and ferrite, which can be contained in an area percentage of less than 5%. Here, F1 = (Mn %) + 1.2 (Cr%) + 1.2 (Cu%) + 0.8 (Mo%). The chemical composition of the wire rod and the steel wire should be in the range where F1 is 1.50 or more and 3.00 or less.
(2) The average equivalent circle diameter of the bainite block grains measured on the L cross section of the steel wire is 15 μm or less, and the average aspect ratio of the bainite blocks is more than 1.00 and less than 1.30.
(3) When the average equivalent circle diameter of the bainite blocks measured in the L cross section of the wire is 15 μm or less, and the diameter of the wire is D, The average circle equivalent diameter of the bainite block at the depth of ) is less than 1.00.
(4) The tensile strength of the wire is 1000×F2+200 MPa or more and 1300×F2+190 MPa or less. Here, F2 is the content of C, Si, Mn, Cr, Cu, Mo, and V in mass% contained in the wire (C%), (Si%), (Mn%), (Cr %), (Mo%), and (V%), F2 = (C%) + 0.14 x (Si%) + 0.20 x (Mn%) + 0.11 x (Cr%) + 0.5 x (Mo %) + 1.54 x (V %).

The wire rod and steel wire according to the present disclosure can shorten the spheroidizing annealing time due to the above configuration, and become a steel wire having excellent cold forgeability after annealing. The wire and steel wire according to the present disclosure were discovered based on the following findings.
In order to improve the cold forgeability of the steel wire after annealing, it is effective to lower the deformation resistance and increase the ductility. In order to reduce the deformation resistance and increase the ductility of the annealed steel wire, the structure of the annealed steel wire should have coarser ferrite grains and finer spheroidal carbides. However, in a steel with a high Cr content, cementite spheroidization is delayed, so cementite is less likely to be spheroidized, making it difficult to coarsen the ferrite grain size. In addition, in the prior art, when cementite is made finer, the growth of ferrite is suppressed, so the ferrite grains become finer, making it difficult to achieve low deformation resistance.
However, by improving the manufacturing method and structure of the wire rod, the present inventors achieved both coarsening of ferrite grains and refinement of cementite after annealing, even in steel containing 0.85% or more of Cr. It was found that both the reduction in deformation resistance and the improvement in ductility were achieved at the same time.

The present inventors used steel containing 0.85% or more of Cr to examine the effect of the metal structure of the wire before spheroidizing annealing on the deformation resistance and ductility after spheroidizing annealing. It has been found that in a wire rod containing 0.85% or more of Cr, ductility after spheroidizing annealing is increased by suppressing proeutectoid ferrite and pearlite and forming a structure mainly containing bainite and containing martensite. The reason for this can be presumed as follows.
The microstructure of a wire manufactured by hot-rolling and cooling a hypo-eutectoid steel having a carbon content of 0.50% or less by a conventional method has a mixed microstructure of ferrite and pearlite. In such a mixed structure, carbon in the steel is unevenly distributed in the pearlite portion, so after spheroidizing annealing, carbides are unevenly distributed in the pearlite portion before annealing, resulting in a decrease in ductility. If the structure of the wire rod is a bainite structure or a martensite structure in which ferrite is suppressed, the carbon in the steel is uniformly distributed, so that the carbides are uniformly dispersed after spheroidizing annealing, and the ductility is improved. Martensite refines carbides after spheroidizing annealing, and is therefore effective for improving ductility. Therefore, in order to improve the ductility of the steel wire after spheroidizing annealing and to reduce the deformation resistance, it is effective to make the structure of the wire a mixed structure of bainite and martensite.
Furthermore, the inventors have found that by setting the tensile strength of the wire to 1000×F2+200 MPa or more and 1300×F2+190 MPa or less, the strength after spheroidizing annealing is lowered and excellent cold forgeability can be obtained.

In addition, strain is applied to the wire rod having the above characteristics by wire drawing with a total area reduction of 20 to 50%, and then spheroidizing annealing is performed at a temperature of A _c1 or less. In the steel containing 0.85 to 1.50% of Cr and 0.10 to 0.50% of C, which was difficult, the ferrite grains are coarse grains, the carbides are fine, and the carbide aspect ratio is It was found that a small structure can be obtained, a reduction in deformation resistance and an improvement in ductility can be achieved.

[Component composition]
The component composition (steel component) of the wire rod and steel wire according to the present disclosure is, in mass%, C: 0.10 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.20 to 1 .00%, P: 0.030% or less, S: 0.050% or less, Cr: 0.85-1.50%, Al: 0.001-0.080%, N: 0.0010-0. 0200% and the balance: Fe and impurities. In addition, in the wire rod and steel wire according to the present disclosure, instead of part of Fe, Ti: 0.050% or less, B: 0.0050% or less, Mo: 0.30% or less, Ni: 0.30% Below, Cu: 0.50% or less, V: 0.30% or less, Nb: 0.050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and Zr: 0.0050% It may contain one or more of the following. That is, Ti, B, Mo, Ni, Cu, V, Nb, Ca, Mg, and Zr are optional elements, and these elements may not be contained in the wire and steel wire according to the present disclosure. , If it is contained, it shall be within the above range.
Hereinafter, the reason why the amount of each element contained in the wire rod and steel wire according to the present disclosure is limited to the above range will be explained.

C: 0.10-0.50%
C is added to ensure strength as a machine part. If the amount of C is less than 0.10%, it is difficult to ensure the strength required for mechanical parts. On the other hand, if the amount of C exceeds 0.50%, ductility, toughness and cold forgeability deteriorate. Therefore, the amount of C is set to 0.10 to 0.50%. A preferred range of C content that achieves both high strength, ductility, toughness and cold workability is 0.15 to 0.48%.

Si: 0.01-0.50%
Si is an element that functions as a deoxidizing element, imparts hardenability, improves temper softening resistance, and is effective in imparting the necessary strength to mechanical parts. These effects are insufficient if the amount of Si is less than 0.01%. If the amount of Si exceeds 0.50%, the ductility and toughness of the machine parts are deteriorated, and the deformation resistance of the steel wire is increased to deteriorate the cold forgeability. Therefore, the amount of Si is set to 0.01 to 0.50%. A preferable Si content range is 0.03 to 0.35%. A more preferable Si content range is 0.05 to 0.30%.

Mn: 0.20-1.00%
Mn is an element necessary for imparting hardenability and imparting the necessary strength to mechanical parts. If the amount of Mn is less than 0.20%, the effect is insufficient. If the Mn content exceeds 1.00%, the toughness of the machine parts deteriorates, and the deformation resistance of the steel wire increases, degrading the cold forgeability. Therefore, the Mn amount was set to 0.20 to 1.00%. A preferable Mn amount range is 0.25 to 0.90%. A more preferred range of Mn is 0.30 to 0.85%.

P: 0.030% or less P is contained in wire rods and steel wires as an impurity. P segregates at grain boundaries of machine parts after quenching and tempering, deteriorating toughness, and therefore, it is desirable to reduce it. Therefore, the upper limit of the amount of P is set to 0.030%. A preferable upper limit of the amount of P is 0.020%. A more preferable upper limit of the amount of P is 0.015% or less. The lower limit of the amount of P is preferably 0% (that is, it should not be included), but from the viewpoint of reducing the cost of removing P, it may be more than 0% (or 0.0001% or more).

S: 0.050% or less S is contained in wire rods and steel wires as sulfides such as MnS. These sulfides improve the machinability of the steel wire. When the amount of S exceeds 0.050%, the cold forgeability of the steel wire is deteriorated, and the toughness of the machine part after quenching and tempering is deteriorated. Therefore, the upper limit of the amount of S is set to 0.050%. A preferable upper limit of the amount of S is 0.030%. A more preferable upper limit of the amount of S is 0.010%. In addition, the lower limit of the amount of S may be more than 0% (or 0.001% or more) from the viewpoint of reducing the S removal cost.

Cr: 0.85-1.50%
Cr is an element necessary for improving hardenability and imparting the necessary strength to mechanical parts. Furthermore, the inclusion of Cr makes the shape of the carbide after annealing spherical, thereby improving the cold workability. If the Cr content is less than 0.85%, the effect is insufficient. If the Cr content exceeds 1.50%, the carbide takes a long time to form spheroids, which increases the manufacturing cost and increases the deformation resistance of the steel wire, degrading the cold forgeability. Therefore, the Cr content is set to 0.85 to 1.50%. A preferable Cr content range is 0.87 to 1.40%. A more preferable Cr content range is 0.90 to 1.30%.

Al: 0.001-0.080%
Al functions as a deoxidizing element, forms AlN, refines austenite crystal grains, and has the effect of improving the toughness of mechanical parts. In addition, it has the effect of fixing solute N to suppress dynamic strain aging and reducing deformation resistance. These effects are insufficient if the amount of Al is less than 0.001%. If the amount of Al exceeds 0.080%, the effect may be saturated and the manufacturability may be lowered. Therefore, the Al content is set to 0.001 to 0.080%. A preferable Al content range is 0.010 to 0.060%. A more preferable Al content range is 0.020 to 0.050%.

N: 0.0010 to 0.0200%
N has the effect of forming nitrides with Al, Ti, Nb, V, etc., refining austenite crystal grains, and improving the toughness of machine parts. If the amount of N is less than 0.0010%, the amount of nitrides precipitated is insufficient and the effect cannot be obtained. If the amount of N exceeds 0.0200%, the deformation resistance of the steel wire increases due to dynamic strain aging due to solute N, deteriorating workability. Therefore, the amount of N is set to 0.0010 to 0.0200%. A preferable N amount range is 0.0020 to 0.0080%. A more preferable range of N content is 0.0030 to less than 0.0050%.

For the purpose of improving the properties described below, the wire rod and steel wire according to the present disclosure have Ti: 0.050% or less, B: 0.0050% or less, Mo: 0.30% or less, Ni: 0.30 % or less, Cu: 0.50% or less, V: 0.30% or less, Nb: 0.050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and Zr: 0.0050% It may contain one or more of the following.

Ti: 0.050% or less Ti functions as a deoxidizing element, forms nitrides and carbides to refine austenite crystal grains, has the effect of improving the toughness of machine parts, and promotes the formation of solid solution B. , the effect of improving the hardenability, and the effect of fixing solute N to suppress dynamic strain aging and reduce deformation resistance. When the amount of Ti exceeds 0.050%, these effects are saturated and coarse oxides or nitrides are formed, which may deteriorate the fatigue strength of mechanical parts. Therefore, when Ti is included, the Ti amount is preferably more than 0 to 0.050%. A preferable Ti content range is 0.005 to 0.030%. A more preferable Ti content range is 0.010 to 0.025%.

B: 0.0050% or less B has the effect of segregating at grain boundaries as solid solution B to improve hardenability and imparting the necessary strength to mechanical parts. If the amount of B exceeds 0.0050%, carbides may be formed at grain boundaries to deteriorate wire drawability. Therefore, when B is included, the amount of B is preferably more than 0 to 0.0050%. A preferable B content range is 0.0003 to 0.0030%. A more preferable B content range is 0.0005 to 0.0020%.

Mo: 0.30% or less Mo has the effect of improving hardenability and imparting the necessary strength to mechanical parts. If the Mo content exceeds 0.30%, the alloy cost increases and the deformation resistance of the steel wire increases, degrading the cold forgeability. Therefore, when Mo is included, the Mo content is preferably more than 0 to 0.30%. A preferred Mo content range is 0.10 to 0.25%.

Ni: 0.30% or less Ni has the effect of improving hardenability and imparting the necessary strength to mechanical parts. If the amount of Ni exceeds 0.30%, the alloy cost increases. Therefore, when Ni is included, the Ni content is preferably more than 0 to 0.30%. A preferable Ni content range is 0.02 to 0.25%.

Cu: 0.50% or less Cu has the effects of improving the hardenability, precipitating finely, imparting the necessary strength to mechanical parts, and improving corrosion resistance. When the amount of Cu exceeds 0.50%, the hot ductility deteriorates and the surface tends to be scratched. Therefore, when Cu is included, the amount of Cu is preferably more than 0 to 0.50%. A preferable Cu amount range is 0.02 to 0.30%.

V: 0.30% or less V has the effect of precipitating carbide VC and increasing the strength of mechanical parts. If the V content exceeds 0.30%, the alloy cost increases. Therefore, when V is included, the amount of V is preferably more than 0 to 0.30%. A preferable V content range is 0.01 to 0.20%.

Nb: 0.050% or less Nb has the effect of precipitating carbides and nitrides to increase the strength of mechanical parts, the effect of refining austenite crystal grains to improve toughness, the effect of reducing solid solution N, and reducing deformation. It has the effect of reducing resistance. If the amount of Nb exceeds 0.050%, the effect may saturate and the cold forgeability may deteriorate. Therefore, when Nb is included, the Nb content is preferably more than 0 to 0.050%. A preferable Nb content range is 0.001 to 0.030%. A more preferable Nb content range is 0.005 to 0.020%.

Ca, Mg, Zr: Each 0.0050% or less Ca, Mg, and Zr are added for the purpose of deoxidizing elements. These elements have the effect of refining oxides and improving fatigue strength. If the addition amount exceeds 0.050%, the effect is saturated and coarse oxides are formed, which may deteriorate the fatigue characteristics. Therefore, when Ca, Mg, or Zr is included, the Ca content, Mg content, or Zr content is preferably more than 0 to 0.050%. A preferable range of Ca content, Mg content, or Zr content is 0.001 to 0.030%, respectively. A more preferable range of Ca content, Mg content, or Zr content is 0.005 to 0.020%, respectively.

Balance: Fe and Impurities In the component composition of the wire rod and steel wire according to the present disclosure, the balance is Fe and impurities.
Here, the term "impurity" refers to a component contained in raw materials or a component mixed in during the manufacturing process and not intentionally included. Furthermore, the impurities also include components that are included intentionally in amounts within a range that does not affect the performance of the steel wire.
Examples of impurities include O and the like. O is inevitably contained in the steel wire and exists as oxides such as Al and Ti. If the amount of O is high, coarse oxides are formed, which causes deterioration of the fatigue strength of mechanical parts. Therefore, it is preferable to suppress the O content to 0.01% or less.

F1
In the wire rod and steel wire according to the present disclosure, when the mass% values of the contents of Mn, Cr, Cu, and Mo are respectively (Mn%), (Cr%), (Cu%), and (Mo%) , F1 represented by the following formula is 1.50 or more and 3.00 or less.
F1 = (Mn%) + 1.2 (Cr%) + 1.2 (Cu%) + 0.8 (Mo%)
If F1 is less than 1.50, the ferrite area ratio increases and the ductility after annealing decreases. On the other hand, if F1 exceeds 3.00, the area ratio of martensite increases and the possibility of wire breakage during wire drawing increases. Therefore, F1 is set to 1.50 or more and 3.00 or less. A preferable range of F1 is 1.60 or more and 2.80 or less. A more preferable range of F1 is 1.70 or more and 2.60 or less.
In the formula representing F1, Cr, Cu, Mo, and V are optional elements, and F1 is calculated as "0" when they are not contained in the wire or steel wire.

[Metal structure]
Next, reasons for limiting the metal structure of the wire rod and steel wire according to the present disclosure will be described.
The wire rod and steel wire according to the present disclosure have a metal structure containing at least bainite and martensite, and a total area ratio of bainite, martensite, and ferrite when the C cross section is observed is 98% or more, The area ratio of ferrite is less than 5%, the area ratio of martensite is more than 0%, and 10×F1-15% or more and 21×F1-20% or less are satisfied.
The total area ratio of bainite, martensite, and ferrite of 98% or more means that the total area of these structures is 98% or more with respect to the total area of the measurement field in which the structure was observed. If pearlite is contained in the metallographic structure, the ductility of the steel wire after annealing is lowered and the cold forgeability is deteriorated, so it is preferable that pearlite is not contained. Less than 2 area % of proeutectoid cementite and/or austenite is allowed as the residual structure.

<Ferrite>
If the area percentage of ferrite is 5% or more, the distribution of cementite after annealing becomes non-uniform, resulting in non-uniform strength. If the strength becomes non-uniform, deformation concentrates locally during cold forging, and cracks are likely to occur. Therefore, the area percentage of ferrite is set to less than 5%. A preferable area percentage of ferrite is 4% or less. A more preferable area percentage of ferrite is 3% or less. Note that the metallographic structure of the wire and steel wire according to the present disclosure may not contain ferrite. That is, the area percentage of ferrite may be 0%.

<Martensite>
If the area % of martensite exceeds 21×F1-20%, the deformation resistance after annealing increases and the cold forgeability deteriorates. On the other hand, when martensite is included, the carbides after annealing become finer and the ductility improves. For this reason, the area % of martensite is set to more than 0% and 10×F1-15% or more and 21×F1-20% or less. In addition, "more than 0% and 10 × F1-15% or more" means that when the value of "10 × F1-15" is 0 or less, the lower limit of martensite area% is more than 0%. means.
If the area percentage of martensite exceeds 30%, disconnection tends to occur during wire drawing before annealing, so the area percentage of martensite is preferably 30% or less.

<Bainite>
Of the total area ratio of bainite, martensite, and ferrite, the balance excluding the area ratio of ferrite and the area ratio of martensite is bainite. The area percentage of bainite is preferably more than 113-21×F1% and 115-10×F1% or less, more preferably 112-17×F1% or more and 115-10×F1% or less.
Note that bainite in the present disclosure includes a ferrite phase (α) and a cementite phase (Fe ₃ C) like pearlite, but pearlite has a structure in which ferrite phases and cementite phases are alternately and continuously laminated in layers. and bainite is a structure in which laths (needle-like substructures) are included in grains and granular or needle-like carbides are dispersed.

<Average circle equivalent diameter of bainite blocks>
Reducing the average equivalent circle diameter of the bainite blocks promotes spheroidization of cementite during annealing, and shortens the annealing time. If the average equivalent circle diameter of the bainite blocks exceeds 15 μm, the annealing time is lengthened and the manufacturing cost increases. In addition, since the cementite is incompletely spheroidized, the cold forgeability may deteriorate. For this reason, the average circle equivalent diameter of the bainite blocks is set to 15 μm or less. The average equivalent circle diameter of the bainite blocks is preferably 13 μm or less.

<Average equivalent circle diameter of bainite blocks in the surface layer/Average equivalent circle diameter of bainite blocks in the 0.25D portion)>
When the diameter of the wire is D, and (average equivalent circle diameter of bainite blocks in the surface layer/average equivalent circle diameter of 0.25D bainite blocks) is less than 1.00, the cold forgeability after annealing is poor. improves. A preferable ratio of (average equivalent circle diameter of bainite blocks in the surface layer/average equivalent circle diameter of bainite blocks in the 0.25D portion) is 0.98 or less. If the ratio is less than 0.60, the manufacturing cost increases, so the lower limit is preferably 0.60.

<Average aspect ratio of steel wire bainite block>
The aspect ratio of the bainite block is the ratio of the major axis to the minor axis of the bainite block measured in the L cross section, that is, major axis/minor axis. When the average aspect ratio of the bainite blocks of the steel wire measured in the L cross section is more than 1.00, cementite spheroidization is promoted during annealing, and the annealing time can be shortened. If it is 1.00 or less, there is no effect of shortening the annealing time, and if it is 1.30 or more, the effect is saturated. For this reason, the average aspect ratio of the bainite blocks of the steel wire measured in the L cross section was set to more than 1.00 and less than 1.30.

(Method for measuring tissue)
Measurement of the metal structure of the wire and steel wire according to the present disclosure is performed by the following method.

－Measurement of Area Ratio of Metal Structure in Wires and Steel Wires－
Each area ratio (area %) of bainite, ferrite, and martensite is determined by the following procedure.
First, after mirror-polishing the C cross-section of the wire or steel wire to be measured (hereinafter sometimes referred to as the “object”), the structure was exposed by etching with picral (5% picric acid + 95% ethanol solution). let out
Next, when the diameter of the object is D, four points at intervals of 90° in the circumferential direction of the object at a depth position of 250 μm from the surface of the object and At a depth position of 0.25D, at 4 locations in the circumferential direction at intervals of 90°, a total of 8 locations, a region of 80 μm in the depth direction and 120 μm in the circumferential direction, and a central axis portion (depth from the surface 80 μm×80 μm area at one point (0.5D depth position). Photographs of the structure at 1000 times magnification are taken using a scanning electron microscope (SEM) at these nine locations.
In the present disclosure, in the photographed structure photograph, cementite having a ratio of (long axis length) / (short axis length) of 5.0 or more is alternately and continuously laminated with ferrite in layers, and granular between layers, Alternatively, an acicular structure that does not contain cementite was used as perlite. Perlite includes pseudo-perlite. The pseudo-pseudo-perlite had a structure in which divided cementite was arranged in rows, no granular or needle-like carbide was included between the rows, and no lath (needle-like substructure) was included in the grains. The bainite has a structure in which laths are included in grains and granular or needle-like carbides are dispersed between and within laths.

Furthermore, each structure of bainite, ferrite, martensite, pearlite, austenite, and proeutectoid cementite in the photographed structure photograph is visually marked, and the area of each structure is image analyzed (software name: Nireco small general purpose It is obtained by the image processing analysis system LUZEX_AP). In addition, this operation is measured and calculated for at least two samples, the average value thereof is obtained, and the average value is defined as the area % of each tissue in the present disclosure.
If it is difficult to distinguish between ferrite and martensite, specify the observation position by the indentation, corrode with picral, take a picture of the structure, re-polish, and etch with nital (5% nitric acid + 95% ethanol solution). to make the organization appear. A tissue photograph of the same site is taken with a SEM at a magnification of 1000 times. A region corroded by nital but weakly corroded by picral was determined to be martensite, and a region weakly corroded by both nital and picral was determined to be ferrite, and the regions of each structure were visually marked by the above method. , area % is determined by image analysis.
If it is difficult to distinguish between bainite and pseudo-perlite, mirror-polish the C cross section of the target wire rod or steel wire, and at a depth position of 250 μm from the surface of the target, 90 in the circumferential direction of the target. A region of 30 μm in the depth direction and 45 μm in the circumferential direction at 4 locations at 4° intervals and 4 locations at 90° intervals in the circumferential direction at a depth position of 0.25 D from the surface of the object. , and a 30 μm×30 μm region at one point on the central axis portion. After revealing the structure by etching with nital, a photograph of the structure of the entire area is taken with SEM at 5000 times. A structure in which laths (needle-like substructures) exist in grains and granular or needle-like carbides are present was determined to be bainite.

-Measurement of the average circle equivalent diameter of bainite blocks in wire rods and steel wires-
The average equivalent circle diameter of the bainite block can be measured by an electron beam back scattering diffraction (EBSD) method. Specifically, as shown in FIG. 1, the surface layer portion of the L cross section of the wire 10 or steel wire (near the surface: the portion up to 500 μm in the radial direction from the surface of the wire or steel wire), the 0.25D portion (A portion at a depth of 0.25 times the diameter D from the surface of the wire rod or steel wire toward the center of the steel wire), the depth direction (radial direction) at the 0.5D portion (the center portion of the wire rod or steel wire) ) and 500 μm in the direction of the central axis C, the crystal orientation of bcc-Fe is measured at each measurement point in each region with a measurement step of 1.0 μm, and the boundary with an orientation difference of 15 degrees or more is blocked. defined as grain boundaries. A region of 5 or more pixels surrounded by the block grain boundary is defined as a block grain. The average circle-equivalent diameter of block grains is measured by Johnson-Saltykov's method of determining the average grain size of a grain group on the premise of mixed grains ("Morphology Measurement" Uchida Roukakuho Shinsha, S47.7.30 Published, Original: RT DeHoff. F. N. Rbiness. See P189). This is done for two samples, and the average value of the average equivalent circle diameters measured in a total of six measurement regions is taken as the average equivalent circle diameter of the bainite block.

It should be noted that the "average equivalent circle diameter of the bainite blocks in the surface layer/average equivalent circle diameter of the bainite blocks in the 0.25D portion" in the wire is obtained by setting the average equivalent circle diameter of the bainite blocks in the surface layer obtained by the above-described EBSD method to 0. It is obtained by dividing by the average circle equivalent diameter of the bainite block in the 25D portion.

－Measurement of Average Aspect Ratio of Bainite Blocks in Steel Wire－
The average aspect ratio of the bainite blocks of the steel wire is determined as follows. Block grains of the surface layer portion, 0.25D portion, and 0.5D portion are determined by the EBSD method described above. In this manner, 10 block grains are selected in order from the largest equivalent circle diameter in the six regions of one L cross section. The aspect ratio (major axis/minor axis) is measured for 10 selected block grains, and the average value is calculated. In this way, the average value of each of the six regions is calculated, and the average is taken as the average aspect ratio.

<Tensile strength of wire>
In the wire according to the present disclosure, the content of each of C, Si, Mn, Cr, Mo, and V contained in the wire by mass% (C%), (Si%), (Mn%), (Cr% ), (Mo%), and (V%), where F2 = (C%) + 0.14 x (Si%) + 0.20 x (Mn%) + 0.11 x (Cr%) + 0.5 x ( Mo%) + 1.54 x (V%), the tensile strength is 1000 x F2 + 200 MPa or more. Note that Mo and V are optional elements, and F2 is calculated as "0" when these elements are not included.
If the tensile strength of the wire is less than 1000×F2+200 MPa, softening after annealing is insufficient and the cold forgeability deteriorates. On the other hand, if the tensile strength of the wire exceeds 1300×F2+190 MPa, the cold forgeability after annealing deteriorates. Therefore, the tensile strength of the wire is set to 1000×F2+200 MPa or more and 1300×F2+190 MPa or less. If the tensile strength of the wire exceeds 1300 MPa, the wire tends to break during wire drawing before annealing. Therefore, the tensile strength of the wire is preferably 1300 MPa or less.
The tensile strength of the wire is a value measured by performing a tensile test according to the test method of JIS Z2241 (2011) using a 9A test piece of JIS Z2241 (2011).

[Manufacturing method of wire]
An example of a method for manufacturing a wire according to the present disclosure will be described. The wire according to the present disclosure can be suitably manufactured by a method including a heating process, a hot rolling process, a cooling process, a first holding process, and a second holding process. Each step will be described in detail below.

(Heating process)
In the heating step, the billet having the chemical composition of the wire according to the present disclosure is heated to 950 to 1150°C. If the heating temperature is less than 950° C., the deformation resistance during hot rolling increases and the rolling cost increases. On the other hand, if the heating temperature exceeds 1150° C., the decarburization of the surface layer portion becomes significant, and the surface layer hardness of the final product decreases.

(Hot rolling process)
In the hot rolling process, the heated billet is hot rolled at a finish rolling temperature of 850 to 1000°C. If the finish rolling temperature is less than 850° C., the ferrite grains after annealing become finer and the deformation resistance deteriorates. If the finish rolling temperature exceeds 1000°C, the transformation completion time in the first holding step becomes longer, increasing the manufacturing cost. The finish rolling temperature refers to the surface temperature of the wire immediately after finish rolling.

(Cooling process)
In the cooling step, after hot rolling, the wire rod at 850 to 1000°C is cooled from 850°C to 550°C at an average cooling rate of 30 to 250°C/s to 400 to less than 500°C. For example, the wire rod after hot rolling can be coiled into a ring shape and cooled by immersion in a molten salt bath so as to achieve the above average cooling rate. If the average cooling rate is less than 30° C./s, a structure with a ferrite structure area percentage of less than 5% cannot be obtained. If the average cooling rate is 250° C./s or more, the manufacturing cost increases. The average cooling rate refers to the surface cooling rate of the wire.

(First holding step)
In the first holding step, the temperature is held at 400° C. to less than 500° C. for 20 seconds or longer. If the holding temperature is less than 400° C., the area percentage of martensite exceeds 21×F1-20%, which causes wire breakage in the wire drawing process, and the deformation resistance after the annealing process increases, resulting in poor cold forgeability. deteriorate. When the holding temperature is 500° C. or higher, the area percentage of martensite exceeds 21×F1−20%, which causes wire breakage in the wire drawing process and deteriorates the cold forgeability after the annealing process.
In addition, if the holding time in the first holding step is less than 20 seconds, the area% of martensite exceeds 21 × F1-20%, which causes wire breakage in the wire drawing process and cold forging after the annealing process. deteriorate sexuality. From the viewpoint of manufacturing cost, the upper limit of the retention time is preferably 120 seconds. The first holding step is performed, for example, by immersion in a molten salt bath.

(Second holding step)
In the second holding step, the temperature is held at 500° C. to 600° C. for 30 seconds or longer. If the holding temperature is less than 500° C., the strength of the wire rod is high, which causes breakage in the wire drawing process. If the holding temperature is 600° C. or higher, the manufacturing cost increases. From the viewpoint of manufacturing cost, the upper limit of the retention time is preferably 150 seconds. The second holding step is performed, for example, by immersion in a molten salt bath.

The wire diameter D of the wire according to the present disclosure manufactured through the above steps is preferably 3.0 to 25.0 mm, more preferably 5.0 to 18.0 mm.

[Manufacturing method of steel wire]
An example of the steel wire manufacturing method according to the present disclosure will be described. The steel wire according to the present disclosure can be suitably manufactured using the wire rod according to the present disclosure by a method including at least a wire drawing step. The wire drawing process will be described in detail below.

(Wire drawing process)
In the wire drawing step, the wire rod cooled to room temperature after the second holding step is drawn at a total area reduction of 20 to 50%. Wire drawing promotes the spheroidization of carbides and the growth of ferrite grains during the annealing process. If the total reduction in area during wire drawing is less than 20%, these effects are insufficient and the cold forgeability deteriorates. Even if the total reduction in area exceeds 50%, the effect is saturated and the diameter of the steel wire becomes small, restricting the application.
The wire diameter of the steel wire according to the present disclosure produced by drawing the wire rod according to the present disclosure is preferably 2.5 to 21.0 mm, more preferably 4.5 to 16.0 mm. .
When the steel wire according to the present disclosure is manufactured by drawing the wire according to the present disclosure, the area ratio of the metal structure of the steel wire before annealing, which will be described later, is the area ratio of the metal structure before wire drawing. maintained. Moreover, the bainite blocks are also elongated in the longitudinal direction by the wire drawing, but the equivalent circle diameter of the bainite blocks hardly changes before and after the wire drawing.

(annealing process)
In the steel wire according to the present disclosure, cementite is spheroidized and cold forgeability is improved by performing the following annealing process.
In the annealing step, the steel is held at a temperature of 650° C. or more and A _c1 temperature (° C.) or less for 3 hours or more and then cooled. where A _c1 =723−10.7×(Mn %)+29.1×(Si %)+16.9×(Cr %). If the annealing temperature is less than 650°C, the ferrite grains become fine grains and the cold forgeability deteriorates. If it exceeds _Ac1 , the number of carbides decreases and the cold forgeability deteriorates. If the holding time is less than 3 hours, the ferrite grains become fine grains and the cold forgeability deteriorates.

Hereinafter, the wire rod and steel wire according to the present disclosure will be described more specifically with reference to examples. However, these examples do not limit the wire and steel wire according to the present disclosure.

[Manufacture of wire rods and steel wires]
Using billets of steel types A to L having chemical compositions shown in Table 1, wire rods and steel wires were manufactured under the conditions shown in Tables 2 and 4 to 6 as follows. In Table 1, "-" means that the component was not intentionally added, and the underlined part means that it is outside the scope of the present disclosure. Steel grades K and L are comparative examples that do not satisfy the composition range of the present disclosure.

Specifically, wires and steel wires of test numbers 1 to 10 and 23 to 26 shown in Tables 2 to 4 were manufactured as follows.
After the billet was heated and hot-rolled, it was coiled into a ring, immersed in a molten salt bath at the rear of the hot-rolling line, and cooled to 470°C.
Next, the wire rod was obtained by first holding and second holding, and then cooling to room temperature (25° C.).
The obtained wire was drawn at the total area reduction shown in Table 4 to obtain a steel wire.
The steel wire was then heated and annealed. The steel wires of test numbers 1 to 10 and 23 to 25 were annealed at 710° C. for 5 hours and then air-cooled. Note that the steel wire of test number 26 broke during wire drawing.

Wire rods and steel wires of test numbers 11 to 22, 27 and 28 shown in Tables 2 to 4 were produced as follows.
A steel slab was heated, hot-rolled, coiled into a ring shape, air-blast cooled, and cooled to room temperature (25° C.) to obtain a wire rod. The obtained wire was drawn at the total area reduction shown in Table 4 to obtain a steel wire.
The steel wire was then heated and annealed. The steel wires of test numbers 11 to 22, 27, and 28 were annealed at 760°C for 5 hours, cooled to 660°C at a cooling rate of 15°C/h, and then air-cooled. Note that the steel wire of test number 21 broke during wire drawing.

Also, the wire and steel wire of Test No. 29 shown in Tables 5 and 6 were manufactured as follows.
After the billet was heated and hot-rolled, the wire was wound into a ring shape and air-blast cooled to room temperature (25°C). After that, it was heated to 900° C., immersed in a lead bath of 480° C. for 250 seconds and held, and then cooled to room temperature (25° C.) to obtain a wire. The obtained wire was drawn at the total area reduction shown in Table 6 to obtain a steel wire.
After holding the steel wire at 710° C. for 5 hours, it was air-cooled and annealed.

[Tissue observation]
For the wire rods of test numbers 1 to 29, the area % of each structure, the average equivalent circle diameter of the bainite blocks, and the ratio of the average equivalent circle diameter of the bainite blocks in the surface layer portion and the 0.25D portion were determined by the method described above. Measured according to

In addition, the average equivalent circle diameter of the bainite blocks and the average aspect ratio of the bainite blocks of the steel wires before annealing were measured according to the methods described above. The structure before and after the wire drawing process of Test No. 1 was observed to confirm the area ratio of each structure, and it was confirmed that the metal structure area ratio of the wire rod was reflected in the metal structure area ratio of the steel wire. .

For reference, FIG. 2 shows an example of a structure etched with picral at a depth of 0.25D from the surface of the wire of test number 6. As shown in FIG. In the structure photograph, the corroded area is bainite, and the flat area (weakly corroded area) is martensite. The area % of these tissues was obtained by image analysis by the method described above.

[evaluation]
<Tensile test of wire>
A tensile test was performed and measured according to the method described above. The results are shown in Tables 3 and 5.

<Compression test on steel wire after annealing>
A compression test was performed using the annealed steel wire to measure deformation resistance and critical compressibility. The critical compressibility was used as the ductility evaluation index.
The annealed steel wire was drawn at an area reduction rate of 8%, and a cylindrical test piece having a diameter of d and a height of 1.5 d was produced from the drawn steel wire. The compression test method is a compression test by constraining the end face with a concentrically grooved mold based on the standards of the Cold Forging Subcommittee of the Japan Society for Technology of Plasticity (plasticity and processing, vol.22, no.211, 1981, p139). did
The deformation resistance was defined as the equivalent stress when processed at an equivalent strain of 1.6 and a compressibility of 73.6% according to Kosakada's method (K.Osakada: Ann.CIRP, 30-1 (1981), p.135).

The critical compressibility is 0.15 mm in curvature and 0.8 mm in depth in the peripheral axial direction of a cylindrical test piece with a diameter of 5.0 mm and a height of 7.5 mm, which is machined from the drawn steel wire. A compression test was performed using a specimen having a notch with an angle of 30°. When a crack having a length of 0.5 mm or more was observed, it was determined that a crack had occurred, and the maximum compression rate at which no crack occurred was defined as the critical compression rate.

Tables 4 and 6 show the measurement results of deformation resistance and critical compressibility, and also show comparison results with ordinary steel wires (test numbers 11 to 20, 27 and 28). "Equivalent" indicates that the deformation resistance is within ±20 MPa and the critical compressibility is within ±2% compared to the normal steel wire, and "Good" indicates that it is superior to the normal steel wire. Poor" indicates inferior to normal steel wire.

From the above results, the wire rods and steel wires of test numbers 1 to 10 that satisfy all the requirements specified in the present disclosure have deformation resistance after annealing equivalent to that of the comparative ordinary steel wires 11 to 20, respectively, and have excellent critical compressibility. I understand.
The wire rod of Test No. 21 exceeds the upper limits of the present disclosure for martensite area % and tensile strength. This wire was broken during wire drawing.
The wire of Test No. 22 has an area percentage of ferrite exceeding the upper limit of the present disclosure. The limit compressibility of the steel wire after annealing using this wire rod is equivalent to that of the conventional steel wire for comparison.
The wire rod of Test No. 23 has an area percent of martensite below the lower limit of the present disclosure, and the tensile strength of the steel wire exceeds the upper limit of the present disclosure. The deformation resistance of the steel wire after annealing using this wire rod is inferior to that of the normal steel wire for comparison.
The steel wire of Test No. 24 has an average aspect ratio of the bainite blocks below the lower limit of the present disclosure. The deformation resistance of the steel wire after annealing using this steel wire is inferior to that of the normal steel wire for comparison.
The wire rod of Test No. 25 has F1 below the lower limit of the present disclosure and the area % of ferrite exceeds the upper limit of the present disclosure. The limit compressibility of the steel wire after annealing using this wire rod is equivalent to that of the conventional steel wire for comparison.
The wire rod of Test No. 26 has F1 exceeding the upper limit of the present disclosure and the area % of martensite exceeds the upper limit of the present disclosure. This wire was broken during wire drawing.
In the wire rod of test number 29, the ratio of the surface layer portion to 0.25D exceeds the upper limit of the present disclosure. After wire drawing using this wire rod, the deformation resistance of the steel wire after annealing is poorer than that of the normal steel wire for comparison.

INDUSTRIAL APPLICABILITY The wire and steel wire according to the present disclosure can be suitably used, for example, as mechanical structural steel used as materials for machine parts such as bolts, screws, and nuts. Since the wire rod and steel wire according to the present disclosure can suppress forming cracks, they are industrially very useful because they contribute to the enhancement of functionality due to the complication of the shape of parts and the improvement of the productivity of machine parts.
It is possible to provide a wire rod and a steel wire that can shorten the spheroidizing annealing time and achieve excellent cold forgeability after spheroidizing treatment in a steel for machine structural use with a high Cr content. Cold forging can be used to form parts with high strength and complex shapes, which has been difficult, and product yield and productivity are improved.

10 wire rod C central axis D diameter of wire rod

Claims (4)

The component composition is mass%,
C: 0.10 to 0.50%,
Si: 0.01 to 0.50%,
Mn: 0.20-1.00%,
P: 0.030% or less,
S: 0.050% or less,
Cr: 0.85-1.50%,
Al: 0.001 to 0.080%,
N: 0.0010 to 0.0200%, and the balance: Fe and impurities,
Contents by mass % of C, Si, Mn, Cr, Cu, Mo, and V contained in the wire are expressed as (C%), (Si%), (Mn%), (Cr%), (Cu% ), (Mo%), and (V%), and when F1 and F2 are values calculated by the following formulas,
F1 is 1.50 or more and 3.00 or less,
The metal structure contains at least bainite and martensite, the total area ratio of the bainite, the martensite, and the ferrite measured in the cross section perpendicular to the longitudinal direction of the wire is 98% or more, and the area of the ferrite ratio is less than 5%, and the martensite area ratio is more than 0%, and satisfies 10 × F1-15% or more and 21 × F1-20% or less,
When the average circle equivalent diameter of the bainite blocks measured in the cross section parallel to the longitudinal direction and including the central axis of the wire is 15 μm or less, and the diameter of the wire is D, (the bainite blocks in the surface layer The average equivalent circle diameter/average equivalent circle diameter of the bainite block at a depth of 0.25D from the surface) is less than 1.00,
A wire having a tensile strength of 1000×F2+200 MPa or more and 1300×F2+190 MPa or less.
F1 = (Mn%) + 1.2 (Cr%) + 1.2 (Cu%) + 0.8 (Mo%)
F2 = (C%) + 0.14 x (Si%) + 0.20 x (Mn%) + 0.11 x (Cr%) + 0.5 x (Mo%) + 1.54 x (V%) The component composition, in mass%,
Ti: 0.050% or less,
B: 0.0050% or less,
Mo: 0.30% or less,
Ni: 0.30% or less,
Cu: 0.50% or less,
V: 0.30% or less,
Nb: 0.050% or less,
Ca: 0.0050% or less,
The wire according to claim 1, satisfying one or more selected from the group consisting of Mg: 0.0050% or less and Zr: 0.0050% or less. The component composition is mass%,
C: 0.10 to 0.50%,
Si: 0.01 to 0.50%,
Mn: 0.20-1.00%,
P: 0.030% or less,
S: 0.050% or less,
Cr: 0.85-1.50%,
Al: 0.001 to 0.080%,
N: 0.0010 to 0.0200%, and the balance: Fe and impurities,
The contents of Mn, Cr, Cu, and Mo contained in the steel wire are represented by (Mn%), (Cr%), (Cu%), and (Mo%), respectively, and F1 is expressed by the following formula where F1 is 1.50 or more and 3.00 or less,
The metal structure contains at least bainite and martensite, and the total area ratio of the bainite, the martensite, and ferrite measured in the cross section perpendicular to the longitudinal direction of the steel wire is 98% or more, and the ferrite The area ratio is less than 5%, the area ratio of martensite is more than 0%, and 10 × F1-15% or more and 21 × F1-20% or less are satisfied,
The bainite block has an average equivalent circle diameter of 15 μm or less measured in a cross section parallel to the longitudinal direction including the central axis of the steel wire, and the average aspect ratio of the bainite block is more than 1.00 and less than 1.30. steel wire.
F1 = (Mn%) + 1.2 (Cr%) + 1.2 (Cu%) + 0.8 (Mo%) The component composition, in mass%,
Ti: 0.050% or less,
B: 0.0050% or less,
Mo: 0.30% or less,
Ni: 0.30% or less,
Cu: 0.50% or less,
V: 0.30% or less,
Nb: 0.050% or less,
Ca: 0.0050% or less,
Mg: 0.0050% or less, and Zr: 0.0050% or less,
The steel wire according to claim 3, satisfying one or more selected from the group consisting of:

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