WO2023190994A1

WO2023190994A1 - Wire rod

Info

Publication number: WO2023190994A1
Application number: PCT/JP2023/013402
Authority: WO
Inventors: 真小此木
Original assignee: 日本製鉄株式会社
Priority date: 2022-03-31
Filing date: 2023-03-30
Publication date: 2023-10-05

Abstract

A wire rod comprising a chemical composition in a predetermined range, wherein, when the diameter of the wire rod is D, in a cross section perpendicular to the longitudinal direction of the wire rod, the metal structure at the position of a depth 1/4D from the surface includes 70-100% of pearlite in area ratio, the remainder when pearlite is less than 100% comprising one or two selected from the group consisting of ferrite and bainite, wherein a mean value of Vickers hardness in the cross section is greater than or equal to 415×F1-90 and less than or equal to 465×F1-80, and the difference (HVs-HVc) between a Vickers hardness HVs at the position of a depth 0.5 mm from the surface and a Vickers hardness HVc in a central portion is less than or equal to 20. F1=(C%)+0.14×(Si%)+0.20×(Mn%)+0.11×(Cr%)+0.50×(Mo%)

Description

wire

The present disclosure relates to a wire.

Mechanical parts such as steel bolts and screws are made by softening the cold heading wire material by spheroidizing annealing to prevent mold wear during forming and processing cracks in the product. Thereafter, it is manufactured by molding and adding strength through quenching and tempering.

For example, Patent Document 1 describes a steel wire rod with C: 0.2 to 0 that can achieve rapid spheroidization before cold forging and improved deformability to achieve excellent cold forgeability. 6% (the meaning of mass%, the same applies hereinafter), Si: 0.3% or less, Mn: 0.2 to 1.5%, respectively. It has a structure mainly composed of pro-eutectoid ferrite and pearlite, has an average crystal grain size of 6 to 15 μm, and has a pro-eutectoid ferrite volume fraction Vf with respect to the equilibrium pro-eutectoid ferrite volume fraction (Vpf1) expressed by the following formula (1). A steel wire rod having a ratio (Vf/Vpf1) of 0.05 to 0.75 has been proposed.

Further, Patent Document 2 describes steel materials for mechanical structures that have excellent cold workability as hot-rolled, C: 0.10 to 0.40%, Si: 0.30% or less, Mn: 0. 20 to 1.70%, Al: 0.01 to 0.10%, the remainder consists of Fe and unavoidable impurities, the minimum from the surface layer is wire diameter (mm) x 0.1, and the maximum is wire diameter (mm) Steel materials for mechanical structures have been proposed in which the average grain size of ferrite grains and pearlite grains is 10 μm or less over ×0.3.

Furthermore, in Patent Document 3, C: 0.005 to 0.6% (mass%, (same below), pseudo pearlite is 10 area % or more, bainite is 75 area % or less, ferrite is 60 area % or less, satisfying the relationship of (pseudo pearlite area % + bainite area % + ferrite area %) ≧ 90 area % A steel wire rod is disclosed.
Furthermore, in Patent Document 4, as a steel material that can simultaneously achieve the shortening of the spheroidization time before cold forging and the improvement of cold workability, C: 0.005 to 0.60%, Si : 0.01 to 0.50%, Mn: 0.20 to 1.80%, Al: 0.01 to 0.06%, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Cr: 0-1.50%, Mo: 0-0.50%, Ni: 0-1.00%, V: 0-0.50%, B: 0-0.0050% , Ti: 0 to 0.05%, the balance is Fe and impurities, the metal structure includes pearlite, and the Mn content in atomic % contained in cementite in the pearlite is A steel material is disclosed in which the value divided by the Mn content in atomic % contained in ferrite is greater than 0 and less than or equal to 5.0.

Patent Document 1: Japanese Patent Application Publication No. 2000-119809 Patent Document 2: Japanese Patent Application Publication No. 5-339677 Patent Document 3: Japanese Patent Application Publication No. 2006-225701 Patent Document 4: International Publication No. 2015/189978

On the other hand, low-alloy steel cold heading wire rods containing C, Mn, Cr, and Mo have high deformation resistance and low ductility, so sufficient workability cannot be obtained with one round of spheroidizing annealing, and generally 2 It is manufactured by performing spheroidizing annealing twice. However, since an increase in the number of annealing leads to an increase in manufacturing costs and _CO2 emissions, there is a demand for simplifying the spheroidizing annealing.

In order to improve the workability after spheroidizing annealing, the deformation resistance (strength) may be lowered and the ductility may be increased. In order to lower the deformation resistance, it is effective to make cementite spherical, further reduce the number density of spherical cementite, and increase the ferrite crystal grain size. Furthermore, in order to improve ductility, it is effective to make cementite spherical and uniformly disperse it.

However, in low-alloy steels with high Cr and Mo contents, the spheroidization of cementite is delayed and lamellar cementite remains, or the cementite size becomes small, resulting in high deformation resistance and low ductility. . For this reason, conventional low alloy steel requires two spheroidizing annealings in order to obtain sufficient workability.

In view of the above circumstances, an object of the present disclosure is to provide a wire rod that can reduce deformation resistance and improve ductility with one spheroidizing annealing even if the wire rod has a composition with a high Cr content. do.

The above problem is solved by the following means.
<1> The chemical composition is in mass%,
C: 0.30-0.50%,
Si: 0.01-0.45%,
Mn: 0.30-1.00%,
P: 0.030% or less,
S: 0.050% or less,
Al: 0.001-0.080%,
Cr: 0.85-1.50%,
N: 0.0010-0.0200%,
O: 0.004% or less,
Mo: 0 to 0.70%,
Ti: 0 to 0.040%,
B: 0 to 0.0040%,
Nb: 0 to 0.050%,
Cu: 0 to 0.50%,
Ni: 0 to 0.30%,
Sn: 0 to 0.30%,
Sb: 0 to 0.050%,
V: 0 to 0.20%, and Ca: 0 to 0.0050%,
with the remainder consisting of Fe and impurity elements,
When the diameter of the wire is D, in a cross section perpendicular to the longitudinal direction of the wire, the metal structure at a depth of 1/4 D from the surface of the wire is pearlite with an area ratio of 70% or more and 100% or less. , and when the area ratio of the pearlite is less than 100%, the remainder is one or two selected from the group consisting of ferrite and bainite,
The average value of Vickers hardness in the cross section is 415 x F1-90 or more and 465 x F1-80 or less, and the Vickers hardness HVs at a depth of 0.5 mm from the surface of the wire and the Vickers hardness at the center are A wire having a hardness difference (HVs-HVc) of 20 or less.
However, F1 is the content of C, Si, Mn, Cr, and Mo in mass% in the wire, respectively (C%), (Si%), (Mn%), (Cr%), and (Mo %), it is a value calculated by the following formula (1).
F1=(C%)+0.14×(Si%)+0.20×(Mn%)+0.11×(Cr%)+0.50×(Mo%)...(1)
<2> The chemical composition is in mass%,
Ti: 0.002-0.040%,
B: 0.0002 to 0.0040%,
Nb: 0.002 to 0.050%,
Cu: 0.02-0.50%,
Ni: 0.02 to 0.30%,
Sn: 0.002-0.30%,
Sb: 0.001 to 0.050%,
V: 0.02 to 0.20%, and Ca: 0.0002 to 0.0050%,
The wire rod according to <1>, including one or more selected from the group consisting of.
<3> The wire rod according to <1> or <2>, wherein the chemical composition contains Mo: 0.10 to 0.65% in mass %.
<4> The wire according to any one of <1> to <3>, wherein the wire has an aperture of -75×F1+120% or more.

According to the present disclosure, there is provided a wire rod that can reduce deformation resistance and improve ductility through one spheroidizing annealing even if the wire rod has a composition with a high Cr content.

It is a figure mainly showing an example of a SEM photograph of a pearlite structure. It is a figure mainly showing an example of a SEM photograph of a bainite structure.

An embodiment that is an example of the present disclosure will be described.
In the present disclosure, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower limit and upper limit. However, a numerical range in which "more than" or "less than" is attached to the numerical value written before and after "~" means a range that does not include these numerical values as the lower limit or upper limit.
The content of an element in a chemical composition may be expressed by adding "amount" to the element symbol (for example, the amount of C, the amount of Si, etc.).
Regarding the content of elements in the chemical composition, "%" means "% by mass".
When the content of an element in a chemical composition is described as "0~", it means that the element does not need to be included.
The term "process" is included not only in an independent process but also in the case where the intended purpose of the process is achieved even if the process cannot be clearly distinguished from other processes.
The "surface" of the wire means the "outer peripheral surface."
The "central axis" of a wire means an imaginary line passing through the center point of a cross section perpendicular to the longitudinal direction of the wire and extending in the longitudinal direction (axial direction). "Longitudinal axis" is also synonymous.
"1/4D" is synonymous with "D/4".

The wire according to the present disclosure has a predetermined chemical composition described below, has a metal structure that satisfies the following (1), and has a cross section perpendicular to the longitudinal direction (sometimes referred to as "C cross section" in the present disclosure). The Vickers hardness measured by satisfies (2) and (3) below.
(1) When the diameter of the wire is D, the metal structure at a depth of 1/4D from the surface in the C section (sometimes referred to as "1/4D section" in this disclosure) is a pearlite structure. The area ratio of pearlite is 70% or more and 100% or less, and when the area ratio of pearlite is less than 100%, the remainder is one or two selected from the group consisting of ferrite and bainite.
(2) The average value of Vickers hardness of the C cross section is 415×F1-90 or more and 465×F1-80 or less. However, when the contents of C, Si, Mn, Cr, and Mo in mass % are respectively (C%), (Si%), (Mn%), (Cr%), and (Mo%), F1=(C%)+0.14×(Si%)+0.20×(Mn%)+0.11×(Cr%)+0.50×(Mo%).
(3) In cross section C, the difference between the Vickers hardness HVs at a depth of 0.5 mm from the surface of the wire and the Vickers hardness HVc at the center (HVs - HVc) is 20 or less.
Due to the above configuration, the wire rod according to the present disclosure has low strength and can obtain a high drawing area even if the spheroidizing annealing, which is normally performed twice, is reduced to one time, even though the wire rod has a high Cr content. I can do it.

The wire rod according to the present disclosure was discovered based on the following findings.
In order to improve the cold forgeability of the steel wire after spheroidizing annealing, it is effective to lower the deformation resistance (strength) and increase the ductility. In order to lower deformation resistance and increase ductility, the structure of the steel wire may be made coarse in ferrite grain size and fine in size of spheroidal cementite.
However, in steel with a high Cr content, cementite is difficult to become spherical, and it is difficult to make the ferrite grain size coarse. Furthermore, in the conventional technology, when cementite is made fine, the growth of ferrite is suppressed, so that the ferrite grains become fine and it is difficult to reduce the deformation resistance.

However, the inventor of the present disclosure has achieved both coarse ferrite grains and fine cementite grains after annealing even in steel containing 0.85% or more of Cr by improving the manufacturing method and structure of the wire rod. It was found that a reduction in deformation resistance and an improvement in ductility were achieved simultaneously.
When the metal structure of the wire is mainly bainite or martensite, cementite becomes fine after spheroidizing annealing. The strength of steel in which spherical cementite is dispersed in ferrite increases when the distance between cementites is small. Therefore, as the size of the spherical cementite decreases, the spacing between them also decreases, and the strength increases. In particular, steel with a high Cr content suppresses the growth of cementite during annealing, so the size of the cementite is small and the strength is less likely to decrease.
On the other hand, if the metal structure of the wire is mainly pearlite, the size of cementite after spheroidizing annealing will be larger than in the case of bainite or martensite, and as a result, the spacing between cementites will be large, resulting in lower strength.
Moreover, since cementite is not generated in the ferrite portion of the wire after spheroidizing annealing, if the ferrite fraction of the wire is high, the distribution of cementite after spheroidizing annealing becomes uneven. When cementite is unevenly distributed, the microscopic strength becomes non-uniform, deformation becomes non-uniform during processing, ductility (restriction of area) decreases, and processing cracks are more likely to occur.

Steel with a high content of Cr and steel containing Mo, B, etc. have high hardenability, so the structure of the wire rod has a high area ratio of bainite and martensite. In order to suppress these structures, it is necessary to reduce the cooling rate. However, when the cooling rate is decreased, the area ratio of pro-eutectoid ferrite increases and the area ratio of pearlite decreases.

Furthermore, even if the structure of the wire is pearlite, wires with low hardness (strength) have a large thickness of cementite that makes up the pearlite, and spheroidization is delayed. For this reason, it is effective to make the wire structure pearlite with a small lamella interval, that is, pearlite with high hardness (strength), in reducing the strength after spheroidizing annealing. On the other hand, if the hardness (strength) of the wire becomes too high, martensite will be mixed in the structure, and the strength after spheroidizing annealing will increase.

In addition, even if the wire rod has a pearlite structure, wire rods with a large difference in hardness between the surface and the center have large variations in cementite thickness and pearlite block grain size, and the ductility (drawing of area) after spheroidizing annealing increases. becomes lower.
Furthermore, even if the wire rod has a pearlite structure, a wire rod with a low reduction of area will have delayed recrystallization of ferrite grains during spheroidizing annealing, and will have a higher strength after spheroidizing annealing.

The inventor of the present disclosure has proposed that a wire having the above-mentioned characteristics be strained by wire drawing or the like such that the total area reduction rate is 20 to 50%, and then subjected to spheroidizing annealing at a temperature of A _c1 or lower. However, in steel containing 0.85 to 1.50% Cr and 0.30 to 0.50% C, which was difficult to achieve with conventional technology, the ferrite grains are coarse grains, the carbides are fine, and the carbides are fine. It has been found that a structure with a small aspect ratio can be obtained, reducing deformation resistance and improving ductility.

The chemical composition (steel component) of the wire according to the present disclosure is expressed in mass%,
C: 0.30-0.50%,
Si: 0.01-0.45%,
Mn: 0.30-1.00%,
P: 0.030% or less,
S: 0.050% or less,
Al: 0.001-0.080%,
Cr: 0.85-1.50%,
N: 0.0010-0.0200%,
O: Contains 0.004% or less, with the remainder consisting of Fe and impurity elements.
The reason why the amount of each element contained in the wire according to the present disclosure is limited to the above range will be explained below.

C: 0.30-0.50%
C is added to ensure strength as a mechanical component. If the C content is less than 0.30%, it is difficult to ensure the strength required as a mechanical component. On the other hand, when the amount of C exceeds 0.50%, ductility, toughness, and cold forgeability deteriorate. Therefore, the amount of C was set to 0.30 to 0.50%. The preferred range of C content that achieves both high strength, ductility, toughness, and cold workability is 0.33 to 0.45%.

Si: 0.01~0.45%
Si functions as a deoxidizing element, and is an effective element for imparting hardenability, improving temper softening resistance, and imparting the necessary strength to mechanical parts. If the amount of Si is less than 0.01%, these effects are insufficient. If the Si content exceeds 0.45%, the ductility and toughness of the mechanical parts will deteriorate, and the deformation resistance of the steel wire will increase, resulting in poor cold forgeability. Therefore, the amount of Si was set to 0.01 to 0.45%. The preferred range of Si content is 0.03 to 0.40%. A more preferable range of Si amount is 0.05 to 0.30%.

Mn: 0.30~1.00%
Mn is an element necessary to impart hardenability and the necessary strength to mechanical parts. If the amount of Mn is less than 0.30%, the effect is insufficient. When the amount of Mn exceeds 1.00%, the toughness of the mechanical parts deteriorates, and the deformation resistance of the steel wire increases, resulting in deterioration of cold forgeability. Therefore, the Mn amount was set to 0.30 to 1.00%. The preferred range of Mn content is 0.35 to 0.90%. A more preferable range of Mn content is 0.40 to 0.85%.

P: 0.030% or less P is contained in the wire as an impurity. P segregates in the grain boundaries of mechanical parts after quenching and tempering and deteriorates toughness, so it is desirable to reduce it. Therefore, the upper limit of the amount of P was set to 0.030%. The 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. Note that the lower limit of the P amount is preferably 0% (that is, not included), but may be more than 0% (or 0.0001% or more) from the viewpoint of reducing the cost of removing P.

S: 0.050% or less S is contained in the wire as a sulfide such as MnS. These sulfides improve the machinability of wire rods or drawn steel wires. When the amount of S exceeds 0.050%, it deteriorates the cold heading properties of the steel wire and also deteriorates the toughness of mechanical parts after quenching and tempering. Therefore, the upper limit of the amount of S was set to 0.050%. The preferable upper limit of the amount of S is 0.030%. A more preferable upper limit of the amount of S is 0.010%. Note that the lower limit of the S amount 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 to improve hardenability and provide necessary strength to mechanical parts. Furthermore, by containing Cr, the shape of the carbide becomes spherical after annealing, improving cold workability. If the amount of Cr is less than 0.85%, the effect is insufficient. When the Cr content exceeds 1.50%, the time for spheroidization of the carbide becomes long, which increases manufacturing costs, and also increases the deformation resistance of the steel wire and deteriorates cold forgeability. Therefore, the Cr content was set to 0.85 to 1.50%. The preferred range of Cr content is 0.87 to 1.40%. A more preferable range of Cr content 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. It also has the effect of fixing solid solution N, suppressing dynamic strain aging, and reducing deformation resistance. If the amount of Al is less than 0.001%, these effects are insufficient. If the amount of Al exceeds 0.080%, the effect may be saturated and the productivity may be reduced. Therefore, the amount of Al was set to 0.001 to 0.080%. The preferred range of Al content is 0.010 to 0.060%. A more preferable range of Al amount is 0.020 to 0.050%.

N: 0.0010-0.0200%
N forms nitrides with Al, Ti, Nb, V, etc., and has the effect of refining austenite crystal grains and improving the toughness of mechanical parts. If the amount of N is less than 0.0010%, the amount of nitride precipitated will be insufficient and no effect will be obtained. When the amount of N exceeds 0.0200%, the deformation resistance of the steel wire increases due to dynamic strain aging due to solid solution N, resulting in deterioration of workability. Therefore, the amount of N was set to 0.0010 to 0.0200%. The preferred range of N amount is 0.0020 to 0.0080%. A more preferable range of the amount of N is 0.0030 to less than 0.0050%.

O: 0.004% or less O is an impurity and is unavoidably contained in steel. If the O amount exceeds 0.004%, coarse oxides may be formed and the fatigue strength may be reduced, so it is limited to 0.004% or less. A preferable upper limit of the amount of O is 0.003%, and a more preferable upper limit is 0.002%.

Remainder: Fe and Impurity Elements In the chemical composition of the wire according to the present disclosure, the remainder is Fe and impurity elements (which may be appropriately referred to as "impurities" in the present disclosure).
Here, impurities refer to components contained in raw materials or components mixed in during the manufacturing process, but not intentionally included. Furthermore, impurities also include components that are contained in an amount that does not affect the performance of the steel wire obtained by drawing the wire according to the present disclosure, even if the impurities are intentionally contained.

In addition, in the wire according to the present disclosure, instead of a part of Fe, in mass %,
Mo: 0.70% or less,
Ti: 0.040% or less,
B: 0.0040% or less,
Nb: 0.050% or less,
Cu: 0.50% or less,
Ni: 0.30% or less,
Sn: 0.30% or less,
Sb: 0.050% or less,
V: 0.20%, and Ca: 0.0050% or less,
It may contain one or more of the following.
In other words, Mo, Ti, B, Nb, Cu, Ni, Sn, Sb, V, and Ca are optional elements, and these elements do not have to be contained in the wire according to the present disclosure, but if they are contained, shall be within the above range. These arbitrary elements will be explained below.

Mo: 0-0.70%
Mo has the effect of improving hardenability and imparting necessary strength to mechanical parts. When the Mo content exceeds 0.70%, the alloy cost increases and the deformation resistance of the steel wire increases, deteriorating cold forgeability. Therefore, when Mo is included, the amount of Mo is preferably 0.02 to 0.70%. A preferred range of Mo amount is 0.10 to 0.70%, a more preferred range of Mo amount is 0.10 to 0.65%, and an even more preferred range of Mo amount is 0.15 to 0.50%. It is.

Ti: 0-0.040%
Ti functions as a deoxidizing element, forms nitrides and carbides, refines austenite grains, improves the toughness of mechanical parts, promotes the formation of solid solution B, and improves hardenability. , has the effect of fixing solid solution N, suppressing dynamic strain aging, and reducing deformation resistance. When the amount of Ti exceeds 0.040%, these effects are saturated and coarse oxides or nitrides are generated, which may deteriorate the fatigue strength of mechanical parts. Therefore, when Ti is included, the amount of Ti is preferably 0.002 to 0.040%. The preferred range of Ti amount is 0.005 to 0.030%. A more preferable range of Ti amount is 0.010 to 0.025%.

B: 0-0.0040%
B segregates at grain boundaries as solid solution B, improves hardenability, and has the effect of imparting necessary strength to mechanical parts. If the amount of B exceeds 0.0040%, carbides may be generated at grain boundaries, which may deteriorate cold workability. Therefore, when B is included, the amount of B is preferably 0.0002 to 0.0040%. The preferred range of B amount is 0.0003 to 0.0030%. A more preferable range of B amount is 0.0005 to 0.0020%.

Nb: 0-0.050%
Nb has the effect of increasing the strength of mechanical parts by precipitating carbides and nitrides, improving toughness by refining austenite crystal grains, and reducing deformation resistance by reducing solid solution N. be. When the amount of Nb exceeds 0.050%, the effect is saturated and cold forgeability may deteriorate. Therefore, when Nb is included, the amount of Nb is preferably 0.002 to 0.050%. The preferred range of Nb content is 0.001 to 0.030%. A more preferable range of Nb content is 0.005 to 0.020%.

Cu: 0-0.50%
Cu has the effect of improving hardenability, precipitating finely, imparting necessary strength to mechanical parts, and improving corrosion resistance. When the amount of Cu exceeds 0.50%, hot ductility deteriorates and flaws are likely to occur on the surface. Therefore, when Cu is included, the amount of Cu is preferably 0.02 to 0.50%. The preferable range of Cu amount is 0.02 to 0.30%.

Ni: 0-0.30%
Ni has the effect of improving hardenability and imparting the necessary strength to mechanical parts. If the Ni amount exceeds 0.30%, the alloy cost will increase. Therefore, when Ni is included, the Ni amount is preferably 0.02 to 0.30%. The preferable range of Ni content is 0.02 to 0.25%.

Sn: 0-0.30%
Sn has the effect of improving corrosion resistance. When Sn is included, the amount of Sn is preferably 0.002% or more. However, if the Sn amount exceeds 0.30%, ductility will decrease and cold workability will deteriorate, so it is limited to 0.30% or less. A preferable upper limit of the amount of Sn is 0.25%.

Sb: 0 to 0.050%
Sb has the effect of improving corrosion resistance. When Sb is included, the amount of Sb is preferably 0.001% or more. However, if the amount of Sb exceeds 0.050%, ductility decreases and cold workability deteriorates, so it is limited to 0.050% or less. A preferable upper limit of the amount of Sb is 0.040%, and a more preferable upper limit is 0.030%.

V: 0-0.20%
V has the effect of increasing the tensile strength of the wire. However, when the amount of V exceeds 0.20%, the alloy cost increases. Therefore, when V is included, the amount of V is preferably 0.02 to 0.20%. The preferred range of V content is 0.05 to 0.15%.

Ca: 0-0.0050%
Ca is added for the purpose of deoxidation, and has the effect of making oxides finer and improving fatigue strength. However, when the amount of Ca exceeds 0.0050%, ductility decreases and cold working deteriorates. Therefore, when Ca is included, the amount of Ca is preferably 0.0002 to 0.0050%. The preferred range of Ca amount is 0.0005 to 0.0030%.

[Metal structure]
Next, the reason for limiting the metal structure of the wire according to the present disclosure will be described.
In the metal structure of the wire according to the present disclosure, the area ratio of pearlite at a depth of 1/4D from the surface (outer peripheral surface) of the wire in the C cross section is 70% or more and 100% or less. When the pearlite area ratio is less than 70%, the strength after spheroidizing annealing becomes high, or the area of drawing becomes low, resulting in poor workability. A preferable lower limit of the pearlite area ratio is 75%, and a more preferable lower limit is 80%.
When the area ratio of pearlite is less than 100%, the remaining structure other than pearlite includes one or both of ferrite and bainite in a total area ratio of 30% or less. When the area ratio of ferrite is high, the area of area after spheroidizing annealing becomes low, and cold workability deteriorates. Moreover, if the area ratio of bainite is high, the strength after annealing will be high and cold workability will be deteriorated. Therefore, the upper limit of the total area ratio of bainite and ferrite is 30%.
The lower limit of the total area ratio of bainite and ferrite is not particularly limited, and may be 0%, that is, bainite and ferrite are not included, and the pearlite area ratio may be 100%.

(Method for measuring metal structure)
The area ratio of the metal structure at a depth of 1/4D from the surface (outer peripheral surface) of the wire according to the present disclosure is measured by the following method.
Each area ratio (area %) of pearlite, bainite, ferrite, etc. is determined by the following procedure.
First, the C cross section of the wire to be measured is polished to a mirror surface, and then etched with Picral (5% picric acid + 95% ethanol solution) to reveal the structure.
Next, when the diameter of the wire is D, the depth from the surface of the wire is 1/4D, the wire is rotated in the circumferential direction every 90 degrees around the longitudinal axis of the wire, and scanning electron beams are scanned at four locations. A photograph of the tissue is taken at a magnification of 5000 times using a microscope (SEM). The field of view at one location is an area of 25 μm or more in the circumferential direction and 19 μm or more in the depth direction.
In the present disclosure, pearlite is defined as a structure in which cementite phases alternate with ferrite phases substantially parallel to each other and a structure in which a plate-shaped cementite phase exists in a photographed structure photograph. Figure 1 shows an example of pearlite structure. In the microstructure photograph, the dark part shows the ferrite phase, and the whitish part shows the cementite phase. A structure in which a linear cementite phase and a ferrite phase are arranged almost parallel to each other as shown in part A of Figure 1(a), a structure in which a linear cementite phase and a separated cementite phase are arranged in a row as shown in part B, and a structure in part C. The structure in which a plate-like cementite phase was observed was defined as a pearlite structure. In addition, a structure in which divided cementite phases exist in rows as shown in part D in FIG. 1(b) is also considered a pearlite structure.
In addition, the bainite structure has an acicular cementite phase with a ratio of (long axis length)/(short axis length) of 3.0 or more, and a structure in which ferrite phases and cementite phases coexist alternately and almost parallel to each other. Rather, the bainite structure is defined as a structure in which the angles of the long axis direction of the acicular cementite phase differ by 30 degrees or more in various orientations. Figure 2 shows an example of a bainite structure. The bainite structure is not a structure in which ferrite phases and cementite phases exist alternately and parallel to each other as shown in parts E and F in FIG. 2, but a structure in which acicular cementite phases in various orientations exist.
Further, the ferrite phase, which is distinguished from the pearlite structure or the bainite structure, was defined as a ferrite structure (sometimes referred to as "ferrite" in the present disclosure). A structure without a cementite phase, such as part G in FIG. 1(a) and part H in FIG. 1(b), was defined as a ferrite structure.

Furthermore, each structure of pearlite, bainite, and ferrite in the photographed structure photograph is visually marked, and the area of each structure is determined by image analysis (software name: Nireco's compact general-purpose image processing and analysis system LUZEX_AP). Note that this operation is performed by measuring and calculating the photographs of four locations, determining the average value thereof, and using the average value as the area % of each tissue in the present disclosure.
If it is difficult to distinguish between ferrite and martensite, identify the observation position with an indentation, corrode it with Picral, take a photograph of the structure, repolish it, and etch it with nital (3% nitric acid + 97% ethanol solution). to reveal the organization. A tissue photograph of the same location is taken using an SEM at a magnification of 5000 times. Areas that are corroded by nital but weakly corroded by picral are determined to be martensite, areas that are weakly corroded by both nital and picral are determined to be ferrite, and each tissue area is visually marked using the method described above. , area % is determined by image analysis.

[Vickers hardness]
(Average value of Vickers hardness)
The average value of the Vickers hardness of the wire rod according to the present disclosure is determined based on the contents of C, Si, Mn, Cr, and Mo contained in the wire rod, respectively (C%), (Si%), (Mn%), and (Cr %), (Mo%), F1 = (C%) + 0.14 x (Si%) + 0.20 x (Mn%) + 0.11 x (Cr%) + 0.50 x (Mo%) In this case, it is 415×F1-90 or more and 465×F1-80 or less. Note that Mo is an arbitrary element, and when Mo is not included, F1 is calculated by setting (Mo%) to "0".

When the average value of Vickers hardness is less than 415 x F1-90, the thickness of layered cementite constituting pearlite is large, and the spheroidization of cementite is delayed, resulting in high strength after spheroidizing annealing and poor cold workability. to degrade. Moreover, lamellar pearlite remains after spheroidizing annealing, or coarse spherical cementite is generated, resulting in a decrease in ductility. Therefore, the lower limit of the average value of Vickers hardness was set to 415×F1-90. Note that when the average value of Vickers hardness exceeds 465×F1-80, the strength after spheroidizing annealing increases and cold workability deteriorates.

Vickers hardness is determined by mirror-polishing the C cross section of the wire and rotating it at 90° intervals in the circumferential direction around the longitudinal axis of the wire at 0.5 mm depth from the surface of the wire. When the diameter of is D, there are 4 locations rotated at 90° intervals around the longitudinal axis of the wire in the circumferential direction at a depth of 1/4D, and 1 location at the center, a total of 9 locations. was measured. The test force for Vickers hardness measurement was 9.8N, and the measurement was performed according to the method of JIS Z2244-1:2020. The average value of the obtained nine locations was defined as the average value of Vickers hardness.

(HVs-HVc)
In the wire according to the present disclosure, the difference (HVs-HVc) between the Vickers hardness HVs at a depth of 0.5 mm from the surface and the Vickers hardness HVc at the center is 20 or less. If (HVs-HVc) exceeds 20, the ductility after spheroidizing annealing decreases and cold workability deteriorates, so the upper limit of (HVs-HVc) was set to 20. HVs measures the Vickers hardness at four locations rotated at 90° intervals in the circumferential direction around the longitudinal axis of the wire at a depth of 0.5 mm from the surface of the wire on the C cross section of the wire. The values were measured and taken as the average value. HVc was defined as the Vickers hardness at one location in the center of the C cross section of the wire.

[Wire rod drawing]
The aperture of the wire according to the present disclosure is preferably −75×F1+120% or more. If the reduction of area is −75×F1+120% or more, the strength after spheroidizing annealing will be low, and the cold workability will be further improved. Therefore, the lower limit of the aperture of the wire according to the present disclosure is preferably −75×F1+120%.
The aperture is a value measured by performing a tensile test using a 9A test piece of JIS Z2241:2011 according to the test method of JIS Z2241:2011.

(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, more preferably 4.0 to 18.0 mm.

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

(Heating process)
In the heating step, a steel piece 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, when the heating temperature exceeds 1150° C., decarburization of the surface layer becomes noticeable and the surface hardness of the mechanical component decreases.

(Hot rolling process)
In the hot rolling step, the heated steel billet is hot rolled so that the entrance temperature in finish rolling is 700 to 760°C, and then rolled up at a temperature of 710 to 780°C. When the winding temperature is lower than 710°C, the ferrite grains after spheroidizing annealing become finer and the strength becomes higher. If the winding temperature exceeds 780°C, recrystallization of ferrite grains will be delayed during spheroidizing annealing, and the strength will increase. In addition, the entry temperature in finish rolling is the surface temperature of the wire rod on the entry side during finish rolling, and the winding temperature is the surface temperature of the wire rod immediately after being placed on the conveyor after hot rolling. Point.

(First cooling step)
In the first cooling step, after winding, the wire rod, which is at a temperature of 710 to 780°C, is cooled to 560°C or more and 620°C or less at an average cooling rate of 10 to 20°C/s in the temperature range from 700°C to 650°C. . For example, a wire rod wound into a ring shape after hot rolling is cooled to the above average cooling rate. When the average cooling rate is less than 10° C./s, the area ratio of the ferrite structure exceeds 30%. Note that the average cooling rate refers to the surface cooling rate of the wire. Although the upper limit of the cooling rate in the first cooling step is not particularly limited, the upper limit of the cooling rate may be 150° C./s or less due to equipment considerations.

(Second cooling process)
In the second cooling step, the wire after the first cooling step is cooled for 600 seconds or more at an average cooling rate of 0.1 to 0.2° C./s. For example, the wire rod after the first cooling step can be transported and cooled in a furnace with a heat source so as to achieve the above-mentioned average cooling rate. When the average cooling rate exceeds 0.2° C./s, (HVs−HVc) tends to exceed 20. When the cooling time is less than 600 seconds, the area ratio of pearlite in the wire tends to be less than 70%.
Note that the upper limit of the cooling time in the second cooling step is not particularly limited, and may be cooled to room temperature, but depending on the end temperature of the first cooling step, from the viewpoint of productivity improvement, equipment restrictions, etc. Therefore, in the second cooling step, after cooling for 600 seconds or more at an average cooling rate of 0.1 to 0.2°C/s, for example, the second cooling step is finished before the temperature drops to 440°C or less, and the cooling rate is 0.2°C/s. Cooling may be allowed to occur at a cooling rate exceeding .

After the second cooling step, it is left to cool.
The wire rod according to the present disclosure can be manufactured through each of the steps described above. Note that the above manufacturing method is an example, and the method for manufacturing a wire according to the present disclosure is not limited to the above method.

Hereinafter, the wire rod according to the present disclosure will be described in more detail by giving examples. However, these examples do not limit the wire rod according to the present disclosure.

<Manufacture and evaluation of wire rod>
Steels A to D and F to O having the chemical compositions shown in Table 1 were heated under the conditions shown in Table 2, hot rolled, and cooled to produce wire rods with a diameter of 3.5 to 20.0 mm. Note that the entrance temperature of finish rolling in hot rolling was within the range of 710 to 850°C. The second cooling rate "0" in Table 2 indicates isothermal maintenance for the second cooling time at the first cooling end temperature, and after the second cooling, it was allowed to cool. Moreover, "-" means that after the first cooling was completed, the second cooling was not performed and the temperature was left to cool.
In Table 1, a blank column means that the element is not included, and the remainder is Fe and impurities. Moreover, in each table, underlining means that it is outside the scope of the present disclosure.

Using the obtained wire, structure observation, hardness measurement, and aperture value measurement were performed by the method described above.
The structure was observed by polishing a cross section (C cross section) perpendicular to the central axis of the wire, corroding it with Picral, and observing the 1/4D section. The viewing area at one location was 25 μm (circumferential direction)×19 μm (depth direction).
The hardness was measured by polishing a cross section (C cross section) perpendicular to the central axis of the wire, using a Vickers hardness meter, and holding the indenter for 15 seconds at a test force of 9.8 N. At a position 0.5 mm deep from the surface of the wire, rotate at 90° intervals in the circumferential direction around the longitudinal axis of the wire at 4 locations, and at 90° intervals at a position 1/4D deep. Measurements were taken at four locations and one location at the center, and the average value was determined.
Further, for HVs, the Vickers hardness was measured at four locations on the C cross section of the wire at a depth of 0.5 mm from the surface of the wire at 90° intervals and measured, and the average value was taken as the average value. HVc was defined as the Vickers hardness at one location in the center of the C cross section of the wire.
The tensile test was performed using a 9A test piece according to JIS Z2241:2011, and according to the test method of JIS Z2241:2011, specifically, using a linear test piece at a crosshead speed of 10 mm/min at room temperature in the atmosphere. The aperture value (RA) was measured. The tensile test was conducted using one wire of each test number.
The results are shown in Table 3. In addition, in the remaining structure, B means bainite, F means ferrite, and M means martensite.

<Manufacture of steel wire>
Using the wire shown in test number 1 in Table 2, annealing, wire drawing, and annealing were performed under the conditions shown in Table 4 to produce a steel wire.

Using the wire rods shown in test numbers 2 to 11 and 13 to 25 in Table 2, wire drawing and annealing were performed under the conditions shown in Table 5 to produce steel wires.

[evaluation]
A tensile test was conducted on these steel wires to evaluate their mechanical properties. The tensile test was carried out at room temperature in the atmosphere using a linear specimen cut from steel wire, with a gage distance of 100 mm and a crosshead speed of 10 mm/min. The tensile strength (TS) and the aperture value (RA ) was measured. The tensile test was conducted using three steel wires of each test number, and the average value was used. Table 6 shows the tensile test results. When TS was 530 MPa or more or RA was 78.0% or less, workability was determined to be poor. Table 6 also shows the number of times of spheroidizing annealing and the evaluation results of workability.

The example of the present disclosure satisfies the requirements of the present disclosure, and has a TS of less than 530 MPa and an RA of more than 78.0% after one round of spheroidizing annealing.
On the other hand, in the comparative example, the chemical composition and manufacturing conditions are outside the scope of the present disclosure, and the TS is 530 MPa or more or the RA is 78.0% or less after one round of spheroidizing annealing.
From these results, it can be seen that the wire rod according to the present disclosure, which has a Cr content of 0.85% or more, achieves a reduction in deformation resistance and an improvement in ductility by one round of spheroidizing annealing.

The disclosure of Japanese Patent Application No. 2022-060862 filed on March 31, 2022 is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually cited as such. captured by.

Claims

The chemical composition is in mass%,
C: 0.30-0.50%,
Si: 0.01-0.45%,
Mn: 0.30-1.00%,
P: 0.030% or less,
S: 0.050% or less,
Al: 0.001-0.080%,
Cr: 0.85-1.50%,
N: 0.0010-0.0200%,
O: 0.004% or less,
Mo: 0 to 0.70%,
Ti: 0 to 0.040%,
B: 0 to 0.0040%,
Nb: 0 to 0.050%,
Cu: 0 to 0.50%,
Ni: 0 to 0.30%,
Sn: 0 to 0.30%,
Sb: 0 to 0.050%,
V: 0 to 0.20%, and Ca: 0 to 0.0050%,
with the remainder consisting of Fe and impurity elements,
When the diameter of the wire is D, in a cross section perpendicular to the longitudinal direction of the wire, the metal structure at a depth of 1/4 D from the surface of the wire is pearlite with an area ratio of 70% or more and 100% or less. , and when the area ratio of the pearlite is less than 100%, the remainder is one or two selected from the group consisting of ferrite and bainite,
The average value of Vickers hardness in the cross section is 415 x F1-90 or more and 465 x F1-80 or less, and the Vickers hardness HVs at a depth of 0.5 mm from the surface of the wire and the Vickers hardness at the center are A wire having a hardness difference (HVs-HVc) of 20 or less.
However, F1 is the content of C, Si, Mn, Cr, and Mo in mass% in the wire, respectively (C%), (Si%), (Mn%), (Cr%), and (Mo %), it is a value calculated by the following formula (1).
F1=(C%)+0.14×(Si%)+0.20×(Mn%)+0.11×(Cr%)+0.50×(Mo%)...(1)
The chemical composition is in mass%,
Ti: 0.002-0.040%,
B: 0.0002 to 0.0040%,
Nb: 0.002 to 0.050%,
Cu: 0.02-0.50%,
Ni: 0.02 to 0.30%,
Sn: 0.002-0.30%,
Sb: 0.001 to 0.050%,
V: 0.02 to 0.20%, and Ca: 0.0002 to 0.0050%,
The wire according to claim 1, comprising one or more selected from the group consisting of:
The wire rod according to claim 1, wherein the chemical composition includes Mo: 0.10 to 0.65% in mass %.
The wire rod according to claim 2, wherein the chemical composition includes Mo: 0.10 to 0.65% in mass %.
The wire rod according to any one of claims 1 to 4, wherein the aperture of the wire rod is -75×F1+120% or more.