WO2020230880A1 - Steel wire and hot-rolled wire material - Google Patents

Abstract

A steel wire which has a component composition comprising 0.10 to 0.60% of C, 0.01 to 0.50% of Si, 0.20 to 1.00% of Mn, 0.030% or less of P, 0.050% or less of S, 0.85 to 1.50% of Cr, 0.001 to 0.080% of Al, 0.0010 to 0.0200% of N, and a remainder made up by Fe, impurity elements and arbitrary elements, and includes a center axis of the steel wire, wherein, in a cross section parallel to the center axis, 95% by area or more of the metal structure is made up by ferrite and a spherical carbide, ferrite grains have an average grain diameter of 10.0 to 30.0 μm, spherical carbide particles each having an equivalent circle diameter of 0.1 μm or more have an average aspect ratio of 2.5 or less, and the number of the spherical carbide particles each having an equivalent circle diameter of 0.1 μm or more is 1.5×10⁶×[C] to 7.0×10⁶×[C] particles/mm² wherein [C] represents the content (% by mass) of C in the steel wire.

Description

Steel wire and hot rolled wire

The present disclosure relates to steel wire and hot rolled wire.
The present application claims priority based on Japanese Patent Application No. 2019-092640 filed in Japan on May 16, 2019, the contents of which are incorporated herein by reference.

Since cold forging is excellent in dimensional accuracy and productivity of molded products, it is possible to switch from conventional hot forging to cold forging when molding mechanical parts such as steel bolts, screws, and nuts. It is expanding. Further, 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 alloy element content increases, the deformation resistance of the steel material increases and the ductility decreases, so that the mold load increases during cold forging, causing wear and damage to the mold. In addition, problems such as processing cracks occurring in the molded parts occur.

In recent years, the shape of parts has become complicated for the purpose of reducing parts manufacturing costs and improving the functionality of parts. Therefore, the steel material used for cold forging is required to be soft and have extremely high ductility. Therefore, conventionally, the hot-rolled material has been softened by heat treatment such as spheroidizing annealing to improve workability.

The workability of cold forging steel includes deformation resistance that affects the mold load and ductility that affects the occurrence of machining cracks. Although the required characteristics differ depending on the use of the steel material, it is usual that both deformation resistance and ductility, or one of them, is required.

Against this background, various methods have been conventionally proposed as techniques for improving the cold forging property of steel materials.

In Patent Document 1, a region in which the average particle size of ferrite grains is 2 to 5.5 μm, the major axis is 3 μm or less, and the ratio of cementite having an aspect ratio of 3 or less is 70% or more with respect to all cementite is defined from the surface. It is disclosed that the cold workability is improved by setting the wire diameter to 10% or more.

In Patent Document 2, the standard deviation of the cementite distance divided by the average value of the cementite distance is set to 0.50 or less, that is, by making the distance between cementites substantially uniform, during cold forging. A steel wire having a reduced deformation resistance and a reduced cracking has been disclosed.

In Patent Document 3, an average particle size of the ferrite grains is not less than 15 [mu] m, 0.8 [mu] m or less average grain size of globular carbides, the maximum particle size 4.0μm or less, 1 mm ² per 0.5 × 10 ⁶ × the number of and ^{C% ~ 5.0 × 10 6 ×} C% number particle size is cold forgeability that excellent disclosed by the maximum distance between 0.1μm or more globular carbides and 10μm or less.

Japanese Patent Application Laid-Open No. 2000-73137 Japanese Patent Application Laid-Open No. 2006-316291 International Publication No. 2011/108459

The method disclosed in Patent Document 1 is effective for processing in which the crack generation position is near the surface of the rolled wire, but is workable for processing in which the crack generation position is inside the rolled wire. The improvement effect is small. In actual cold forging, the rolled wire is cut and then cold forged. Therefore, in many cases, the vicinity of the surface of the rolled wire does not become the crack generation position, and the effect is limited.

In the steel wire disclosed in Patent Document 2, the ferrite grain size and the number density of cementite, which affect the deformation resistance, are not specified, the deformation resistance is high, and the die load becomes high during cold forging. There are challenges.

The steel wire disclosed in Patent Document 3 has a problem that the Cr content is 0.20% or less, the hardenability is low, and the strength of the parts after quenching and tempering becomes unstable as the wire diameter becomes large. ..

Further, in the conventional steel wire used for cold forging, when the content of alloying elements such as Cr is high, cementite does not sufficiently spheroidize after annealing, the deformation resistance is high, and processing cracks are likely to occur. There are challenges.

Therefore, it is an object of the present disclosure to provide a steel wire containing an alloying element and having excellent cold forging property, and a hot-rolled wire rod for manufacturing the steel wire.

The means for solving the above problems include the following aspects.
<1>
Ingredient composition is mass%,
C: 0.10 to 0.60%,
Si: 0.01-0.50%,
Mn: 0.20 to 1.00%,
P: 0.030% or less,
S: 0.050% or less,
Cr: 0.85 to 1.50%,
Al: 0.001 to 0.080%,
N: 0.0010 to 0.0200%, and the balance: Fe and impurity elements.
In a cross section that includes the central axis of the steel wire and is parallel to the central axis.
More than 95 area% of the metallographic structure is composed of ferrite and spherical carbides.
The ferrite has an average particle size of 10.0 to 30.0 μm.
The spherical carbide has an average aspect ratio of 2.5 or less for the spherical carbide having a diameter equivalent to a circle of 0.1 μm or more, and the content (mass%) of C contained in the steel wire is represented by [C]. case, a circle number of equivalent diameter 0.1μm or more of the globular carbides is ^{1.5 × 10 6 × [C]} ~ 7.0 × 10 6 × [C] number / mm ^2, the steel wire.
<2>
The above. <1>, wherein in the cross section, the average particle size of the spherical carbide having a diameter equivalent to a circle of 0.1 μm or more is 0.50 μm or less, and the maximum particle size of the spherical carbide is 3.00 μm or less. Steel wire.
<3>
When the component composition is mass%,
Ti: 0 to 0.050%,
B: 0 to 0.0050%,
Mo: 0 to 0.50%,
Ni: 0 to 1.00%,
Cu: 0 to 0.50%,
V: 0 to 0.50%,
Nb: 0 to 0.050%,
Ca: 0 to 0.0050%,
Mg: 0 to 0.0050%, and Zr: 0 to 0.0050%,
The steel wire according to <1> or <2>, which satisfies one or more of the above.
<4>
The hot-rolled wire rod for manufacturing the steel wire according to any one of <1> to <3>.

According to the present disclosure, a steel wire containing an alloying element and having excellent cold forging property, and a hot-rolled wire rod for manufacturing the steel wire are provided.

It is the schematic explaining the region which measures the particle diameter of a ferrite grain in the L cross section of the steel wire which concerns on this disclosure.

An embodiment which is an example of the present disclosure will be described.
In the present specification, the numerical range represented by using "-" means a range including the numerical values before and after "-" as the lower limit value and the upper limit value. In addition, the numerical range when "greater than" or "less than" is added to the numerical values before and after "to" means a range in which these numerical values are not included as the lower limit value or the upper limit value.
In the numerical range described stepwise in the present specification, the upper limit value or the lower limit value of the numerical range described stepwise may be replaced with the upper limit value or the lower limit value of the numerical range described stepwise. , Or you may replace it with the value shown in the examples.
In addition, the content of elements in the component composition may be expressed as an elemental amount (for example, C amount, Si amount, etc.).
Further, regarding the content of the element in the component composition, "%" means "mass%".
Further, the term "process" is included in this term not only as an independent process but also as long as the intended purpose of the process is achieved even when it cannot be clearly distinguished from other processes.
Further, the "cross section including the central axis of the steel wire and parallel to the central axis" includes the central axis of the steel wire and is cut along the longitudinal direction (that is, the drawing direction) of the steel wire. A cross section parallel to the axial direction (also referred to as an L cross section) is shown.
Further, the "central axis" indicates a virtual line extending in the axial direction through the center point of the cross section orthogonal to the axial direction (longitudinal direction) of the steel wire.
Further, the "surface layer portion of the steel wire" indicates a region having a depth of up to 500 μm from the surface (outer peripheral surface) of the steel wire toward the central axis (in the radial direction).
Further, the notation "numerical value XD" is a position at a depth X times the diameter D from the surface of the steel wire toward the central axis (in the radial direction), where D is the diameter of the steel wire. Is shown. For example, "0.25D" indicates a position at a depth of 0.25 times the diameter D.

The steel wire according to the present disclosure is a steel wire having a predetermined component composition and having a metal structure satisfying the following (1) and (2).
(1) In the L cross section, 95 area% or more of the metal structure is composed of ferrite and spherical carbide.
(2) The ferrite has an average particle size of 10.0 μm or more, and the spherical carbide has an average aspect ratio of a spherical carbide having a circular equivalent diameter of 0.1 μm or more (hereinafter, simply referred to as “average aspect ratio of spherical carbide”). The number (which may be referred to) is 2.5 or less, and the number of pieces per 1 mm ² in the L cross section is 1.5 × 10 ⁶ × [C] to 7.0 × 10 ⁶ × [C] ([[]. C] represents the content (mass%) of carbon (C) contained in the steel wire).

The steel wire according to the present disclosure is a steel wire having excellent cold forging properties due to the above configuration. The steel wire according to the present disclosure was found based on the following findings.

In order to improve the cold forging property of steel wire, it is effective to lower the deformation resistance and increase the ductility. Therefore, the influence of the metallographic structure on the deformation resistance and ductility was examined using a steel containing 0.85% or more of Cr. As a result, it was found that the deformation resistance is affected by the ferrite grain size, the aspect ratio of carbides, and the number density of carbides. In order to reduce the deformation resistance and obtain good workability, it is effective to increase the ferrite grain size and reduce the number density of carbides. On the other hand, it was found that the particle size and aspect ratio of carbides affect ductility. In order to increase the ductility and suppress cracking during molding, it is effective to reduce the particle size and aspect ratio of the carbide.

In the prior art, in order to coarsen the ferrite grains of medium carbon steel containing 0.10 to 0.60% carbon, it is necessary to increase the particle size of the spherical carbide and reduce the number density. It was. Generally, when the particle size of the carbide becomes smaller, the ferrite particle size also becomes finer, so that it is difficult to achieve both coarse graining of the ferrite grain and finer particle size of the carbide. In particular, in steels having a high Cr content, it has been difficult to coarse-grain ferrite because the growth of carbides is suppressed by the solid solution of Cr in the carbides. Therefore, in the prior art, when the carbide is made finer and the ductility is increased, the deformation resistance is increased and the mold life is shortened.

By improving the method for producing wire rods and steel wires, the present inventors can achieve both coarse graining of ferrite grains and miniaturization of carbides even in steel containing 0.85% or more of Cr, and reduce deformation resistance. And succeeded in achieving improvement in ductility at the same time.

Specifically, in a steel containing 0.85% or more of Cr, in order to achieve both coarse graining of ferrite grains and miniaturization of carbides,
(A) The structure of the hot-rolled material shall be a structure mainly composed of bainite having a small proeutectoid ferrite fraction.
(B) After hot rolling, strain is applied to the steel wire by wire drawing with a total surface reduction rate of 20% or more.
(C) Performing spheroidizing annealing at a temperature of Ac ₁ or less,
Found to be important.

As a result, ferrite grains of steel containing 0.85 to 1.50% of Cr and 0.10 to 0.60% of C, which were difficult in the prior art, are made coarse grains, carbides are made fine, and carbon dioxide is made fine. It has become possible to reduce the aspect ratio of carbides. By obtaining such a structure, the deformation resistance was reduced and the ductility was improved.

The reason why the cold forging property of the steel wire having a structure composed of coarse ferrite grains and fine spherical carbides is excellent is that the coarse carbides that are likely to generate molding cracks and the spherical carbide grains having a large aspect ratio. It is considered that the occurrence of cracks can be suppressed by making the diameter finer, and the strength is lowered and the deformation resistance is reduced by making the ferrite grain size coarse.

Further, even in a composition having a high content of alloying elements such as Cr, by improving the deformability of the steel wire, the steel wire according to the present disclosure can be formed into a complicated shape part by cold forging, and the product yield can be obtained. And productivity are improved. Further, with the steel wire according to the present disclosure, it is possible to integrally mold a complex-shaped part having high strength, which has been difficult in the past. That is, the steel wire according to the present disclosure can be suitably used for machine structural steel used as a material for machine parts such as bolts, screws, and nuts.

Since the steel wire according to the present disclosure can suppress molding cracks, it contributes to high functionality by complicating the shape of parts and improvement of productivity of mechanical parts, and is extremely useful in industry.
Hereinafter, the composition and metallographic structure of the steel wire according to the present disclosure will be specifically described.

<Ingredient composition>
The composition of the steel wire according to the present disclosure is, in mass%, C: 0.10 to 0.60%, 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 to 1.50%, Al: 0.001 to 0.080%, N: 0.0010 to 0.0200%, and the balance: It consists of Fe and impurity elements.
However, the steel wire according to the present disclosure may contain an element other than the above instead of a part of Fe, and the component composition is mass%, for example, Ti: 0 to 0.050%, B: 0 to 0. 0050%, Mo: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 0.50%, V: 0 to 0.50%, Nb: 0 to 0.050%, Ca: One or more of 0 to 0.0050%, Mg: 0 to 0.0050%, and Zr: 0 to 0.0050% may be satisfied. Ti, B, Mo, Ni, Cu, V, Nb, Ca, Mg, and Zr are arbitrary elements. That is, these elements do not have to be contained in the steel wire.
Hereinafter, the reason for limiting the range of the amount of each element contained in the steel wire will be described.

(C: 0.10 to 0.60%)
C is contained in order to secure the strength as a mechanical part. If the amount of C is less than 0.10%, it is difficult to secure the required strength as a mechanical part. On the other hand, when the amount of C exceeds 0.60%, ductility, toughness, and cold forging property deteriorate. Therefore, the amount of C was set to 0.10 to 0.60%. The amount of C may be 0.15% or more, 0.20% or more, or 0.25% or more. The amount of C may be 0.55% or less, 0.50% or more, or 0.40% or less.

(Si: 0.01 to 0.50%)
Si is an element that functions as a deoxidizing element, imparts hardenability, improves temper softening resistance, and imparts the strength required for mechanical parts. If the amount of Si is less than 0.01%, these effects are insufficient. When the amount of Si exceeds 0.50%, the ductility and toughness of the mechanical parts are deteriorated, and the deformation resistance of the steel wire is increased to deteriorate the cold forging property. Therefore, the amount of Si was set to 0.01 to 0.50%. The amount of Si may be 0.03% or more, 0.05% or more, or 0.10% or more. The amount of Si may be 0.35% or less, 0.30% or less, or 0.25% or less.

(Mn: 0.20 to 1.00%)
Mn is an element necessary for imparting hardenability and imparting strength required for mechanical parts. If the amount of Mn is less than 0.20%, the effect is insufficient. If the amount of Mn exceeds 1.00%, the toughness of the mechanical parts deteriorates, the deformation resistance of the steel wire increases, and the cold forging property deteriorates. Therefore, the amount of Mn was set to 0.20 to 1.00%. The amount of Mn may be 0.25% or more, 0.30% or more, or 0.35% or more. The Mn content may be 0.90% or less, 0.85% or less, or 0.80% or less.

(P: 0.030% or less)
P is contained in the steel wire as an impurity. It is desirable to reduce P because it segregates at the grain boundaries of mechanical parts after quenching and tempering and deteriorates toughness. Therefore, the upper limit of the amount of P is 0.030%. The upper limit of the preferable amount of P is 0.020%. The upper limit of the more preferable amount of P is 0.015% or less, or 0.012% or less. The lower limit of the amount of P is preferably 0% (that is, it is preferable not to include it), but it exceeds 0% (or 0.0001% or more or 0.005% or more) from the viewpoint of reducing the de-P cost. There should be.

(S: 0.050% or less)
S is contained in the steel wire as a sulfide such as MnS. These sulfides improve the machinability of steel wire. When the amount of S exceeds 0.050%, the cold forging property of the steel wire is deteriorated and the toughness of the mechanical parts after quenching and tempering is deteriorated. Therefore, the upper limit of the amount of S is set to 0.050%. The upper limit of the preferable amount of S is 0.030%. A more preferable upper limit of the amount of S is 0.015% or 0.010%. The lower limit of the amount of S is preferably 0% (that is, it is preferable not to include it), but it exceeds 0% (or 0.0001% or more or 0.005% or more) from the viewpoint of reducing the cost of removing S. There should be.

(Cr: 0.85 to 1.50%)
Cr is an element required to improve hardenability and impart the required strength to mechanical parts. Further, by containing Cr, the shape of the carbide after annealing becomes spherical, and the cold workability is improved. If the amount of Cr is less than 0.85%, the effect is insufficient. When the amount of Cr exceeds 1.50%, the spheroidizing time becomes long, the manufacturing cost increases, the deformation resistance of the steel wire increases, and the cold forging property deteriorates. Therefore, the amount of Cr was set to 0.85 to 1.50%. The amount of Cr may be 0.87% or more, 0.90% or more, or 0.95% or more. The amount of Cr may be 1.40% or less, 1.30% or less, or 1.20% or less.

(Al: 0.001 to 0.080%)
Al has the effect of functioning as a deoxidizing element and forming AlN to refine austenite crystal grains and improve the toughness of mechanical parts. Further, Al has an effect of fixing the 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 manufacturability may be lowered. Therefore, the amount of Al was set to 0.001 to 0.080%. The amount of Al may be 0.010% or more, 0.020% or more, or 0.025% or more. The amount of Al may be 0.060% or less, 0.050% or less, or 0.040% or less.

(N: 0.0010 to 0.0200%)
N has the effect of forming a nitride with Al, Ti, Nb, V and the like, making austenite crystal grains finer, and improving the toughness of mechanical parts. If the amount of N is less than 0.0010%, the amount of nitride precipitated is insufficient and no effect can be obtained. When the amount of N exceeds 0.0200%, the deformation resistance of the steel wire becomes high due to the dynamic strain aging due to the solid solution N, and the workability deteriorates. Therefore, the amount of N was set to 0.0010 to 0.0200%. The range of the amount of N may be 0.0020% or more, 0.0025% or more, or 0.0030% or more. The amount of N may be 0.0080% or less, less than 0.0050%, or 0.0040% or less.

The steel wire according to the present disclosure has Ti: 0 to 0.050%, B: 0 to 0.0050%, Mo: 0 to 0.50%, Ni: 0 to 0 to improve the characteristics described below. 1.00%, Cu: 0 to 0.50%, V: 0 to 0.50%, Nb: 0 to 0.050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, And Zr: 0 to 0.0050% may contain one or more. However, the steel wire according to the present disclosure can solve the problem without containing these elements. Therefore, the lower limit of the content of these arbitrary elements is 0%.

(Ti: 0 to 0.050%)
Ti functions as a deoxidizing element. Furthermore, Ti has the effect of forming nitrides and carbides to refine austenite crystal grains to improve the toughness of mechanical parts, the effect of promoting the formation of solid solution B, and the effect of enhancing hardenability, and solid solution N. It has the effect of fixing and suppressing dynamic strain aging and reducing deformation resistance. If the amount of Ti exceeds 0.050%, these effects may be saturated and coarse oxides or nitrides may be generated to deteriorate the fatigue strength of mechanical parts. Therefore, the amount of Ti may be contained in the range of more than 0 to 0.050%. The amount of Ti may be 0.005% or more, or 0.010% or more. The amount of Ti may be 0.030% or less, or 0.025% or less.

(B: 0 to 0.0050%)
B has the effect of segregating into grain boundaries as a solid solution B to improve hardenability and impart the required strength to mechanical parts. On the other hand, if the amount of B exceeds 0.0050%, carbides may be generated at the grain boundaries to deteriorate the wire drawing workability. Therefore, the amount of B may be contained in the range of more than 0 to 0.0050%. The amount of B may be 0.0003% or more, or 0.0005% or more. The amount of B may be 0.0030% or less, or 0.0020% or less.

(Mo: 0 to 0.50%)
Mo has the effect of improving hardenability and imparting the required strength to mechanical parts. On the other hand, when the amount of Mo exceeds 0.50%, the alloy cost increases and the deformation resistance of the steel wire increases, which deteriorates the cold forging property. Therefore, the amount of Mo may be contained in the range of more than 0 to 0.50%. The amount of Mo may be 0.10% or more, or 0.15% or more. The amount of Mo may be 0.40% or less, or 0.30% or less.

(Ni: 0 to 1.00%)
Ni has the effect of improving hardenability and imparting the required strength to mechanical parts. On the other hand, if the amount of Ni exceeds 1.00%, the alloy cost increases. Therefore, the amount of Ni may be contained in the range of more than 0 to 1.00%. The amount of Ni may be 0.02% or more, or 0.10% or more. The amount of Ni may be 0.50% or less, or 0.30% or less.

(Cu: 0 to 0.50%)
Cu has the effect of improving hardenability, imparting the necessary strength to mechanical parts, and improving corrosion resistance. On the other hand, if the amount of Cu exceeds 0.50%, the alloy cost increases. Therefore, the amount of Cu may be contained in the range of more than 0 to 0.50%. The amount of Cu may be 0.02% or more, or 0.10% or more. The amount of Cu may be 0.40% or less, or 0.35% or less.

(V: 0 to 0.50%)
V has the effect of precipitating carbide VC and increasing the strength of mechanical parts. On the other hand, if the amount of V exceeds 0.50%, the alloy cost increases. Therefore, the amount of V may be contained in the range of more than 0 to 0.50%. The amount of V may be 0.01% or more, or 0.05% or more. The amount of V may be 0.20% or less, or 0.15% or less.

(Nb: 0 to 0.050%)
Nb has the effect of precipitating carbides and nitrides to increase the strength of mechanical parts, the effect of finely granulating austenite grains to improve toughness, the effect of reducing solid solution N, and the effect of reducing deformation resistance. is there. On the other hand, if the amount of Nb exceeds 0.050%, these effects may be saturated and the cold forging property may be deteriorated. Therefore, the amount of Nb may be contained in the range of more than 0 to 0.050%. The amount of Nb may be 0.001% or more, or 0.005% or more. The amount of Nb may be 0.030% or less, or 0.020% or less.

(Ca: 0 to 0.0050%)
(Mg: 0 to 0.0050%)
(Zr: 0 to 0.0050%)
Ca, Mg and Zr may be used for the purpose of deoxidation. These elements have the effect of making oxides finer and improving fatigue strength. On the other hand, if the content of each of these elements exceeds 0.050%, the effect is saturated and coarse oxides are generated, which may deteriorate the fatigue characteristics. Therefore, the Ca amount, the Mg amount, and the Zr amount may each be contained in the range of more than 0 to 0.050%. The amount of Ca, the amount of Mg, and the amount of Zr may be 0.0001% or more and 0.0005% or more, respectively. The amount of Ca, the amount of Mg, and the amount of Zr may be 0.030% or less, or 0.020% or less, respectively.

(Remaining: Fe and impurity elements)
In the composition of the steel wire according to the present disclosure, the balance is Fe and impurity elements.
Here, the impurity element refers to, for example, a component contained in a raw material or a component that is unintentionally mixed in a manufacturing process and is not intentionally contained. Further, the impurity element includes a component contained in an amount within a range that does not affect the performance of the steel wire even if the component is intentionally contained.

Examples of the impurity element include O and the like. O exists in the steel wire as an oxide such as Al and Ti. If the amount of O is high, coarse oxides are formed, which causes a decrease in fatigue strength of mechanical parts. Therefore, the amount of O is preferably suppressed to 0.01% or less.

<Metal structure>
Next, the reasons for limiting the metal structure of the steel wire according to the present disclosure will be described.
In the steel wire according to the present disclosure, 95 area% or more of the metal structure is composed of ferrite and spherical carbide (spherical cementite) in a cross section (L cross section) parallel to the central axis and including the central axis of the steel wire. Will be done. If the metal structure contains a martensite structure, a bainite structure, a pearlite structure, or the like, the deformation resistance increases, the ductility decreases, and the cold forging property deteriorates. Therefore, it is preferable that these structures are not included. Therefore, in the steel wire according to the present disclosure, in the cross section (L cross section) including the central axis of the steel wire and parallel to the central axis, 97 area% or more, 98 area% or more, or 99 area% or more of the metal structure. However, it may be composed of ferrite and spherical carbide (spherical cementite). The fact that 95 area% or more of the metal structure is composed of ferrite and spherical carbide in the L cross section means that 95 area% or more of the metal structure is formed of ferrite and spherical carbide when the L cross section is observed. means.

The area% of ferrite and spherical carbide is calculated by the following procedure. After mirror-polishing the cross section (L cross section) including the central axis of the steel wire and parallel to the central axis, the sample is immersed in nital (5% nitric acid + 95% ethanol solution) at room temperature for 20 seconds to reveal the metal structure. Using this sample, a scanning electron microscope (SEM) was used to measure a depth of 250 μm (the central part of the surface layer in the depth direction) and a depth of 0.25D (of the steel wire) from the surface (outer peripheral surface of the steel wire). 90 μm in the depth direction, centered on the 0.5D depth (central part of the steel wire) at a depth of 0.25 times the diameter D of the steel wire in the direction from the surface to the center. A 120 μm area is photographed with 2 visual fields each, and a total of 6 visual fields are photographed at a magnification of 1000 times. From the obtained tissue photograph, martensite, bainite, and pearlite are visually marked, and the photographed photograph is image-analyzed (software name: Nireco's small general-purpose image processing analysis system LUZEX_AP) to total martensite, bainite, and pearlite. Get area%. The area% of ferrite and spherical carbide can be obtained by subtracting the total area of martensite, bainite and pearlite from the entire imaging field as the area of ferrite and spherical carbide, and dividing this value by the area of the imaging field. it can. The pearlite structure was a structure in which carbides having an aspect ratio (major axis / minor axis) of more than 5.0 were present in layers.

(Ferrite grain)
-Average grain size of ferrite grains-
Coarse-grained ferrite grains reduce deformation resistance and improve die life during cold forging. When the average particle size of the ferrite grains is less than 10.0 μm, the effect of reducing the deformation resistance is small. Therefore, the lower limit of the average particle size of the ferrite grains is preferably 10.0 μm. The preferable lower limit of the average particle size of the ferrite grains is 11.5 μm. A more preferable lower limit of the average particle size of the ferrite grains is 13.0 μm. On the other hand, when trying to obtain a ferrite grain size exceeding 30.0 μm, the annealing time becomes long and the manufacturing cost increases. Therefore, the upper limit of the average particle size of the ferrite grains was set to 30.0 μm. The upper limit of the average particle size of the preferred ferrite grains is 20.0 μm.

-Measurement method of ferrite grains-
The average particle size of ferrite grains can be measured by an electron backscattering diffraction (EBSD) method. Specifically, as shown in FIG. 1, a depth portion 250 μm from the surface of a cross section (L cross section) including the central axis C of the steel wire 10 and parallel to the central axis C (central portion in the depth direction of the surface layer portion). , 0.25D depth part (the part with a depth of 0.25 times the diameter D of the steel wire in the direction from the surface of the steel wire toward the center of the steel wire), 0.5D depth part (the center part of the steel wire) ), The measurement step is 1.0 μm in a region of 500 μm in the depth direction (diametrical direction) and 500 μm in the central axis direction, that is, in a region of 500 μm square shown by A1, A2, and A3 in FIG. The crystal orientation of bcc-Fe at each measurement point in the region is measured. Here, a boundary having an orientation difference of 15 degrees or more is defined as a ferrite grain boundary. Then, a region of 5 pixels or more surrounded by the ferrite grain boundaries is defined as a ferrite grain. Johnson-Saltykov's measurement method ("Measurement Morphology", Uchida Otsuru Farm Shinsha, S47.7.30), which is a method for determining the average particle size of a grain population on the premise of mixing ferrite grains. , Original work: RT DeHoff. F.N.Rbiness. P189). This is performed for two samples, and the average value of the average particle diameters measured in a total of six measurement regions is taken as the average particle diameter of the ferrite grains.

(Spherical carbide)
-Average aspect ratio of spherical carbide-
The spherical carbide means cementite having an aspect ratio of 5.0 or less represented by the major axis / minor axis of the carbide. When the aspect ratio (major axis / minor axis) of the spherical carbide becomes large, cracks are generated from around the strained carbide and it becomes easy to crack. In particular, when the average aspect ratio of the spherical carbide having a circle equivalent diameter of 0.1 μm or more exceeds 2.5, the ductility is lowered and processing cracks are likely to occur. Therefore, the upper limit of the average aspect ratio of spherical carbide having a circle equivalent diameter of 0.1 μm or more is set to 2.5. The preferable upper limit of the average aspect ratio of the spherical carbide having a circle equivalent diameter of 0.1 μm or more is 2.0. A more preferable upper limit of the average aspect ratio of the spherical carbide having a circle equivalent diameter of 0.1 μm or more is 1.8.

-Maximum particle size of spherical carbide-
As long as the above requirements are met, the particle size of the spherical carbide is not particularly specified. However, the maximum particle size of the spherical carbide affects the occurrence of molding cracks. When the maximum particle size is reduced, it is possible to prevent cracks from being generated around the strained carbide, and it is possible to more effectively prevent cracks in the steel wire. For example, when the maximum particle size of the spherical carbide is 3.00 μm or less, the ductility is further improved and it becomes easier to prevent cold forging cracks. Therefore, the upper limit of the maximum particle size of the spherical carbide may be 3.00 μm. The preferable upper limit of the maximum particle size of the spherical carbide is 2.00 μm. A more preferable upper limit of the maximum particle size of the spherical carbide is 1.50 μm.

-Average particle size of spherical carbide-
Further, when the average particle size of the spherical carbide is 0.50 μm or less, the ductility is further improved and it becomes easier to prevent cold forging cracks. Therefore, the upper limit of the average particle size of the spherical carbide may be 0.50 μm. The preferable upper limit of the average particle size of the spherical carbide is 0.40 μm. A more preferable upper limit of the average particle size of the spherical carbide is 0.32 μm.

If the area ratio of cementite other than spherical carbide to the total cementite is less than 5%, the effect on cold forging property is small, and therefore cementite other than spherical carbide of less than 5% may be contained. The average particle size of the spherical carbide means the average number of circle-equivalent diameters of the spherical carbide. The number average is calculated after excluding spherical carbides with a circle equivalent diameter of less than 0.1 μm.

-Number density of spherical carbides-
Spherical carbide having a circle equivalent diameter of 0.1 μm or more per 1 mm ^{2 of} a cross section (L cross section) including the central axis of the steel wire and parallel to the central axis (referred to as “spherical carbide per 1 mm ² ” in the present specification). If there is a.) If the number is less than 1.5 × 10 ⁶ × [C] number of the time of cold forging, there is a crack is generated around carbides, machining cracks occur. For this reason, the lower limit of the circle equivalent diameter 0.1μm or more globular carbides per 1 mm ² and 1.5 × 10 ⁶ × [C] number. Here, [C] indicates the C content in the steel wire represented by mass%. Circle preferable lower limit of the number of 1 mm ² per equivalent diameter 0.1μm or more globular ^{carbide, 3.0 × 10 6 × [C} ] number, or 3.5 × 10 is a ⁶ × [C] number.

On the other hand, if the number of 1 mm ² per equivalent circle diameter 0.1μm or more globular carbides exceeds 7.0 × 10 ⁶ × [C] number, deformation resistance is increased, increasing the mold load. Therefore, the upper limit of the circle equivalent diameter 0.1μm or more globular carbides per ^{1mm 2 7.0 × 10 6 × [} C] number and. Circle equivalent diameter preferred upper limit is 6.5 × 10 ⁶ × the number of 1 mm ² per 0.1μm or more globular carbides [C] number, or 6.0 × 10 ⁶ × [C] number. Note that "[C]" means the C content (mass%) contained in the steel wire as described above. For example, when the C content is 0.35% by mass, [C] = 0.35. Is. The same applies to [Mn], [Si], and [Cr] in the formula for calculating the Ac ₁ temperature, which will be described later, and each means the content (mass%) of each element in the steel material.

-Measuring method of spherical carbide-
The maximum particle size of the spherical carbide, the average particle size of the spherical carbide, the aspect ratio of the spherical carbide, and the number density of the spherical carbide can be obtained by image analysis of a scanning electron microscope (SEM) photograph.

Specifically, after the cross section (L cross section) including the central axis of the steel wire and parallel to the central axis is mirror-polished, the sample is placed in picral (5% picric acid + 95% ethanol solution) at room temperature for 50 seconds. Immerse to reveal the metallographic structure. Next, by SEM, the 250 μm depth part (the central part in the depth direction of the surface layer part), the 0.25D depth part, and the 0.5D depth part from the surface of the steel wire are set to be the center of the measurement field of view. Then, a metal structure of a total of 15 fields of view is photographed at a magnification of 5000 times in a region of 20 μm in the depth direction and 25 μm in the central axis direction. Each of the above parameters of spherical carbide can be obtained by performing image analysis (software name: Nireco's small general-purpose image processing analysis system LUZEX_AP) on the photographed photograph.

The average number of circle-equivalent diameters of spherical carbides of 0.1 μm or more is defined as the average particle size of spherical carbides, and the maximum particle size in the measurement field is defined as the maximum particle size of spherical carbides. The circle-equivalent diameter of the spherical carbide is the diameter of a circle having an area equal to the area of the spherical carbide. The aspect ratio of the spherical carbide of 0.1 μm or more is obtained by the length of the major axis / the length of the minor axis. The number density of spherical carbides is obtained by dividing the number of spherical carbides having a circle-equivalent diameter of 0.1 μm or more by the area of the measurement field of view.

<Steel wire manufacturing method>
An example of a method for manufacturing a steel wire according to the present disclosure will be described. However, the method for manufacturing a steel wire described below does not limit the steel wire according to the present disclosure. That is, regardless of the manufacturing method, the steel wire satisfying the above requirements is the steel wire according to the present disclosure.

An example of the method for manufacturing a steel wire according to the present disclosure is
The process of drawing a wire mainly composed of bainite with a total surface reduction rate of 20 to 50%,
The process of annealing the wire drawn wire by holding it at 650 ° C or higher and Ac ₁ temperature (° C) or lower for 3 hours or longer and cooling it.
To be equipped. In the method of manufacturing this steel wire,
The step of heating the steel piece having the composition of the steel wire according to the present disclosure to 950 to 1150 ° C.
A process of hot-rolling a heated steel piece at a finish rolling temperature of 850 to 1000 ° C. to obtain a wire rod.
After hot rolling, the wire rod at 850 to 1000 ° C. is cooled to a temperature range of less than 400 to 500 ° C. with an average cooling rate of 30 to 250 ° C./s from 850 ° C. to 550 ° C.
A step of holding the cooled wire rod in a temperature range of 400 ° C. to less than 500 ° C. for 20 seconds or more (first holding step) and
Further, a wire rod mainly composed of bainite may be produced by a step of holding the wire rod that has undergone the first holding step within a temperature range of 500 ° C. to 600 ° C. for 30 seconds or longer (second holding step). Hereinafter, each step will be described in detail.

(Heating process)
In the heating step, the steel piece having the component composition of the steel 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. When the heating temperature exceeds 1150 ° C., decarburization of the surface becomes remarkable, and the surface hardness of the final product decreases.

(Hot rolling process)
In the hot rolling step, the heated steel pieces are 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 become finer and a structure having an average grain size of 10.0 to 30.0 μm cannot be obtained after the annealing step. When the finish rolling temperature exceeds 1000 ° C., the transformation completion time in the first holding step becomes long, and the manufacturing cost increases. The finish rolling temperature refers to the surface temperature of the wire rod 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 less than 400 to 500 ° C. For example, the wire rod after hot rolling may be wound into a ring shape and immersed in a molten salt tank so as to have the above average cooling rate. If the average cooling rate is less than 30 ° C./s, the area ratio of ferrite and spherical carbides tends to decrease after the annealing step, and the number density of spherical carbides tends to decrease. On the other hand, if the average cooling rate is 250 ° C./s or more, the manufacturing cost increases. The cooling rate refers to the surface cooling rate of the wire rod. The average cooling rate from 850 ° C to 550 ° C is the value obtained by dividing 300 ° C (= 850 ° C-550 ° C) by the time required to reduce the surface temperature of the wire from 850 ° C to 550 ° C. is there.

(First holding step)
In the first holding step, the cooled wire rod is held at 400 ° C. to less than 500 ° C. for 20 seconds or more. If the holding temperature is less than 400 ° C., the strength after the annealing step becomes high and the cold forging property deteriorates. When the holding temperature is 500 ° C. or higher, the transformation completion time in the first holding step becomes remarkably long, and the untransformed portion remains after the first holding step and the second holding step. The untransformed portion causes disconnection in the wire drawing process and deteriorates the cold forging property after the annealing process.

If the holding time in the first holding step is less than 20 seconds, untransformed portions remain after the first holding step and after the second holding step, which causes disconnection in the wire drawing process and cold after the annealing step. Deteriorates forgeability. From the viewpoint of manufacturing cost, the upper limit of the holding time is preferably 120 seconds. The first holding step is carried out, for example, by immersing the wire rod in the molten salt tank.

(Second holding step)
In the second holding step, the wire rod that has undergone the first holding step 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 disconnection 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 holding time is preferably 150 seconds. The second holding step is carried out, for example, by immersing in a molten salt tank.

After the second holding step, the wire rod cooled to room temperature suppresses proeutectoid ferrite and pearlite, and has a structure mainly composed of bainite. Specifically, in the structure of the wire rod, the area ratio of bainite measured in the C cross section is 50% or more, and the area ratio of martensite is 0% or more. In the C cross section of the wire rod, the area ratio of martensite may be 0%, preferably more than 0%. The present inventors have found that by controlling the structure of the wire rod in this way, the ductility after spheroidizing annealing is increased. The reason for this can be estimated as follows.

The structure of the wire rod produced by hot rolling and cooling by a normal method using subeutectoid steel with a carbon content of 0.50% or less is a mixed structure of ferrite and pearlite. In such a mixed structure, carbon in the steel is unevenly distributed in the pearlite portion. Therefore, after spheroidizing annealing, the carbides are unevenly distributed in the portion that was pearlite before annealing, and the ductility is lowered. When the structure of the wire is a bainite structure or martensite structure in which ferrite is suppressed, carbon in the steel is uniformly distributed, so that carbides are uniformly dispersed after spheroidizing annealing, and ductility is improved. Martensite is effective in improving ductility because it makes carbides after spheroidizing annealing finer, but on the other hand, it increases deformation resistance because it makes the ferrite grain size after annealing finer. Therefore, in order to improve the ductility of the steel wire after spheroidizing annealing and reduce the deformation resistance, it is effective to make the structure of the wire rod mainly composed of bainite.

Incidentally, bainite in the present disclosure includes as with pearlitic ferrite phase (alpha) and cementite phase (Fe 3 _C) are. However, pearlite is a structure in which ferrite phases and cementite phases are alternately and continuously laminated in layers. On the other hand, bainite is a structure in which lath (needle-shaped lower structure) is contained in grains and granular or needle-shaped carbides are dispersed. In this respect, pearlite and bainite are distinguished.

The area ratio (area%) of bainite, ferrite, and martensite of the wire is determined by the following procedure.

First, the C cross section of the wire rod to be measured (hereinafter sometimes referred to as "object") is mirror-polished, and then the object is immersed in picral (5% picric acid + 95% ethanol solution) at room temperature for 50 seconds. To reveal the organization.

Next, at nine points on the C cross section of the object, a microstructure photograph at a magnification of 1000 times is taken using a scanning electron microscope (SEM). The specific positions of the nine measurement points are as described below. Hereinafter, the diameter of the object is referred to as D.
(1) Four locations on the surface layer At a depth position of 250 μm from the surface of the object (the central portion in the depth direction of the surface layer), four locations at 90 ° intervals in the circumferential direction of the object. It is the measurement point on the surface layer. The visual field shape is a rectangle having a length of 80 μm in the depth direction and a length of 120 μm in the circumferential direction, and the center of the visual field is aligned with the above-mentioned measurement position.
(2) Four locations at a depth of 0.25D At a depth of 0.25D from the surface of the object, four locations at 90 ° intervals in the circumferential direction of the object are 0.25D. The measurement point at the depth position. The visual field shape is a rectangle having a length of 80 μm in the depth direction and a length of 120 μm in the circumferential direction, and the center of the visual field is aligned with the above-mentioned measurement position.
(3) One point on the central axis One point on the central axis that overlaps with the central axis (depth position with a depth of 0.5D from the surface) is defined as the measurement point on the central axis. The visual field shape is a square with a length and width of 80 μm centered on the central axis.

In the present disclosure, in the photographed microstructure, cementite and ferrite having (length of major axis) / (length of minor axis) of 5.0 or more are alternately and continuously laminated in layers, and these layers are laminated. A tissue containing no granular or acicular cementite between the two was designated as pearlite. In addition, pearlite includes pseudo pearlite. The pseudo-pearlite was a structure in which the divided cementites were arranged in a row, did not contain granular or needle-like carbides between the rows, and did not contain a lath (needle-shaped substructure) in the grains. Bainite had a structure in which laths were contained in the grains and granular or needle-like carbides were dispersed between the laths and in the laths.

Furthermore, each structure of bainite, ferrite, martensite, pearlite, austenite, and proeutectoid cementite in the photographed microstructure is visually marked. Then, the area of the region of each tissue is obtained by image analysis (software name: Nireco's small general-purpose image processing analysis system LUZEX_AP). In addition, the above-mentioned series of operations is performed on at least two samples, the area ratio of the structure in these samples is measured and calculated, the average value thereof is obtained, and the average value is the area% of each structure of the wire rod in the present disclosure. And.

If it is difficult to distinguish between ferrite and martensite, use the method described below to distinguish between them. An indentation is made on the C cross section of the object so that the observation position can be determined, and the object is immersed in piclar at room temperature for 50 seconds to reveal the tissue and take a tissue photograph. Then, the object is re-polished and immersed in nital (5% nitric acid + 95% ethanol solution) at room temperature for 20 seconds to reveal the tissue. Then, a tissue photograph of the same portion as the picral etching photograph is taken at a magnification of 1000 times using SEM.

Comparing the picral etching photograph and the nital etching photograph, the region that is corroded by nital but weakly corroded by piclar is determined to be martensite, and the region where both nital and picral are weakly corroded is determined to be ferrite. Then, the region of each tissue is visually marked by the above method, and the area% of each tissue is obtained by image analysis.

If it is difficult to distinguish between bainite and pseudo-perlite, use the method described below to distinguish between them. The C cross section of the wire rod or steel wire to be measured is mirror-polished. The tissue is then exposed by immersion in Nital at room temperature for 20 seconds. Then, a histological photograph of the entire area in the nine regions in the C cross section is taken using SEM at a magnification of 5000 times. A structure in which laths (needle-shaped substructures) were present in the grains and granules or needle-like carbides were present was determined to be bainite. The "nine regions in the C cross section" are (1) four locations on the surface layer portion, (2) four locations at a depth position of 0.25D, and (3) one location on the central axis. Is.

(Wire drawing process)
In the wire drawing process, after the second holding step, the wire rod cooled to room temperature is drawn with a total surface reduction rate of 20 to 50%. By performing wire drawing, spheroidization of carbides is promoted during the annealing step, and growth of ferrite grains is promoted. If the total surface reduction rate during wire drawing is less than 20%, these effects are insufficient and the cold forging property deteriorates. Even if the total surface reduction rate exceeds 50%, the effect may be saturated and the steel wire diameter may become smaller, which may limit the application.

The steel wire diameter (diameter) according to the present disclosure is not particularly limited and may be determined according to the intended use. However, when used as a material for mechanical parts such as bolts, screws and nuts, for example, the diameter is Wire drawing is performed so that the steel wire has a diameter of 3.5 to 16.0 mm.

(Annealing process)
In the annealing step, the steel wire obtained by wire drawing is held at 650 ° C. or higher and Ac ₁ temperature (° C.) or lower for 3 hours or longer to cool. Here, Ac ₁ = 723-10.7 × [Mn] + 29.1 × [Si] + 16.9 × [Cr]. If the annealing temperature is less than 650 ° C., the average particle size of the ferrite grains is less than 10 μm, and the cold forging property deteriorates. When the annealing temperature exceeds Ac ₁ , the average particle size of the ferrite grains becomes less than 10 μm, the number of carbides decreases, and the cold forging property may deteriorate. If the holding time is less than 3 hours, the average particle size of the ferrite grains is less than 10 μm, and the cold forging property deteriorates.

Through the above steps, the steel wire according to the present disclosure can be suitably manufactured. However, as described above, the method for producing the steel wire according to the present disclosure is not particularly limited. The method for producing a steel wire composed of the above steps is only a preferable example for obtaining the steel wire according to the present disclosure.

Next, the wire rod according to the present disclosure will be described. The wire rod according to the present disclosure is a hot-rolled wire rod for manufacturing the steel wire according to the present disclosure. When steel wire is manufactured from wire rod, its chemical composition does not change. Therefore, the wire rod according to the present disclosure inevitably has substantially the same chemical composition as the steel wire according to the present disclosure. On the other hand, as long as the steel wire according to the present disclosure can be obtained through arbitrary processing (for example, wire drawing processing, heat treatment, etc.), the metal structure of the wire rod according to the present disclosure and various aspects of spherical carbide are not particularly limited. A preferable example of the metal structure of the wire rod is that the area ratio of bainite is 50% or more and the area ratio of martensite is 0% or more in the C cross section. The present disclosure relates to a wire rod having such a metal structure by applying strain to the steel wire by wire drawing with a total surface reduction rate of 20% or more, and further performing spheroidizing annealing at a temperature of Ac ₁ or less. Steel wire can be obtained.

Hereinafter, the steel wire of the present disclosure will be described in more detail with reference to examples. However, each of these examples does not limit the steel wire of the present disclosure.

[Manufacturing of steel wire]
Using steel pieces of steel grades A to P having the component compositions shown in Table 1, steel wires were produced as follows under the conditions shown in Tables 2-1 to 4-2 described later. In addition, the part indicated by "-" in Table 1 means that the element in the column was not intentionally added.
Further, in Tables 2-1 to 4-2, the underlined portions mean that they are out of the range required in the present disclosure, or are optional but out of the preferable range.

Specifically, the steel wires of test numbers 1 to 16, 32 to 36, and 41 shown in Tables 2-1 to 2-4 were manufactured as follows.

First, the steel pieces were heated and then hot-rolled, and the obtained wire rod was wound into a ring shape and immersed in a molten salt tank installed behind the hot-rolling line and cooled to 470 to 520 ° C. ..

Next, the wire rod immersed in the molten salt tank was first held and second held in the molten salt bath of two tanks. Then, the wire rod cooled to room temperature (25 ° C.) was wire-drawn at the total surface reduction rate shown in Table 2-1 and Table 2-2, and after the wire was drawn, it was heated and annealed. The annealing treatment of the steel wires of test numbers 1 to 12, 15, 32 and 35 was held at 710 ° C. for 5 hours and then air-cooled, and the annealing treatment of the steel wire of test number 16 was held at 760 ° C. for 5 hours. After air cooling, the annealing treatment of the steel wire of test number 33 was held at 740 ° C. for 5 hours and then air cooled, and the annealing treatment of the steel wire of test number 34 was held at 695 ° C. for 5 hours and then air cooled. The annealing treatment of the steel wire of 36 was held at 730 ° C. for 5 hours and then air-cooled, and the annealing treatment of the steel wire of Test No. 41 was carried out after holding at 735 ° C. for 5 hours and then air-cooled.

Through these steps, the steel wires shown in test numbers 1 to 16 and 32 to 36 were manufactured. Since the steel wires of test numbers 13 and 14 were broken during the wire drawing, no annealing treatment was performed. In Table 2-1 and Table 2-2, "-" in the column for manufacturing conditions means that the measurement was not performed, and "-" for the tissue means that the measurement was not performed.

The steel wire of test number 31 was manufactured as follows.
First, the steel pieces were heated and then hot-rolled, and the obtained wire rod was wound into a ring shape and cooled to 470 ° C. by impulse cooling. Then, the obtained wire rod was immersed in two molten salt tanks for first holding and second holding. Then, the wire rod cooled to room temperature (25 ° C.) was drawn at the total surface reduction rate shown in Table 2-2, held at 710 ° C. for 5 hours after the wire drawing, and then air-cooled.

The steel wires of test numbers 17 to 28 and 37 to 40 shown in Tables 2-1 to 2-4 were manufactured as follows.
First, the steel pieces were heated and then hot-rolled, and the obtained wire rod was wound into a ring shape and cooled by an impulse. Then, the wire rod cooled to room temperature (25 ° C.) was wire-drawn at the total surface reduction rate shown in Table 2-1 and Table 2-2, and after the wire was drawn, it was heated and annealed. The steel wires of test numbers 17 to 28 and 37 to 40 were annealed at 760 ° C. for 5 hours, cooled to 660 ° C. at a cooling rate of 15 ° C./h, and then air-cooled.
Through these steps, the steel wires shown in test numbers 17 to 28 and 37 to 40 were manufactured.

The steel wire of test number 29 shown in Table 3-1 and Table 3-2 was manufactured as follows.
After heating the steel pieces, they were hot-rolled, and the obtained wire rod was wound into a ring shape and cooled by an impulse. Then, the wire rod cooled to room temperature (25 ° C.) was heated to 850 ° C. and quenched, and then heated to 650 ° C. and tempered. Then, the wire was drawn at the total surface reduction rate shown in Table 3-1 and then heated and annealed after the wire was drawn.

The steel wire of test number 30 shown in Table 4-1 and Table 4-2 was manufactured as follows.
After heating the steel pieces, they were hot-rolled, and the obtained wire rod was wound into a ring shape and cooled by an impulse. Then, the wire rod cooled to room temperature (25 ° C.) was heated and subjected to the first annealing treatment. Then, the wire rod cooled to room temperature (25 ° C.) was wire-drawn at the total surface reduction rate shown in Table 4-1 and heated after the wire drawing to be subjected to the second annealing treatment. The first annealing treatment and the second annealing treatment were carried out at 760 ° C. for 5 hours, then cooled to 660 at a cooling rate of 15 ° C./h, and then air-cooled.

[Evaluation]
For these steel wires, the metallographic structure of the steel wires other than the test numbers 13 and 14 that were broken by the wire drawing process was observed, and a compression test was performed.

Total area ratio of ferrite and spherical carbide of steel wire, average ferrite grain size, average aspect ratio of spherical carbide, number density of spherical carbide with equivalent circle diameter of 0.1 μm or more, average of spherical carbide with equivalent circle diameter of 0.1 μm or more The particle size (described as "average particle size" in the table) and the maximum particle size of spherical carbide having a circle-equivalent diameter of 0.1 μm or more (described as “maximum particle size” in the table) are the methods described above. Measured according to. The results are shown in the table. In the table, "number / C%" is the number of spherical carbides with a circular equivalent diameter of 0.1 μm or more observed per 1 mm ² L cross section of each steel wire, and the C content (C content) contained in the steel wire. %) Divided by.

The deformation resistance and limit compressibility of the steel wire were measured by a compression test.
The annealed steel wire was drawn with a surface reduction rate of 8% to prepare a columnar test piece having a diameter D and a height of 1.5 D from the drawn steel wire.
The compression test method is a compression test in which the end face is constrained by a die with concentric grooves based on the standards of the Cold Forging Subcommittee of the Japan Society for Plasticity Processing (Plasticity and Processing, vol.22, No.211, 1981, p139). Was done.
The deformation resistance was 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), p135).
The critical compressibility is a curvature of 0.15 mm, a depth of 0.8 mm, and an angle in the circumferential axial direction of a cylindrical test piece having a diameter of 5.0 mm and a height of 7.5 mm manufactured from the drawn steel wire by machining. A compression test was performed using a test piece having a 30 ° notch. When cracks with a length of 0.5 mm or more were observed, it was determined that cracks had occurred, and the maximum compressibility at which cracks did not occur was defined as the limit compressibility.

The table shows the measurement results of deformation resistance and critical compressibility, and also shows the comparison results with ordinary steel wires (test numbers 17 to 28 and 37 to 40). Deformation resistance and / or the steel wire with the test number described as "equivalent" to the limit compressibility has a deformation resistance within ± 20 MPa and less than ± 20 MPa as compared with the normal steel wire (test numbers 17 to 28 and 37 to 40). / Or the critical compressibility is within ± 2%. The steel wire with the test number described as "good" had better characteristics than the normal steel wire, and the steel wire with the test number described as "poor" was inferior to the normal steel wire.

From the above results, the steel wires of test numbers 1 to 12, 33 to 36, and 41 that satisfy all the requirements specified in the present disclosure are compared with the normal steel wires (test numbers 17 to 28 and 37 to 40) in terms of deformation resistance. Was equivalent or superior. Furthermore, it can be seen that the steel wires of test numbers 1 to 12, 33 to 36, and 41 that satisfy all the requirements specified in the present disclosure are superior in the critical compressibility to the normal steel wire. When the structure before wire drawing (that is, the structure of the wire rod) was evaluated for a part of these steel wires, it had a structure mainly composed of bainite (see Table 5 described later).
The ordinary steel wires 17 to 28 and 37 to 40 are manufactured under manufacturing conditions in which it is presumed that the structure before wire drawing does not become a structure mainly composed of bainite. When the structure before wire drawing (that is, the structure of the wire rod) was evaluated for a part of these ordinary steel wires, it was not mainly bainite (see Table 5 described later).
In test number 13, the steel wire could not be manufactured because the wire was broken during the wire drawing. It is presumed that this is because the holding temperature in the first holding step was too high and the hardness of the wire before wire drawing became excessive.
In the steel wire 14, the steel wire could not be manufactured because the wire was broken during the drawing. It is presumed that this is because the holding time in the first holding step was too short and the hardness of the wire before wire drawing became excessive.
In the steel wire of test number 15, the total area ratio of ferrite and spherical carbide was insufficient, and the average particle size of ferrite was too small, so that the deformation resistance and the critical compressibility were inferior to those of the normal steel wire. It is presumed that this is because the total surface reduction rate in wire drawing was insufficient.
In the steel wire of test number 16, the number of spherical carbides having a diameter equivalent to a circle of 0.1 μm or more was insufficient, and the average grain size of ferrite was too small, so that the deformation resistance and the critical compression ratio were superior to those of the normal steel wire. It didn't become a thing. It is presumed that this is because the annealing temperature in the annealing after the wire drawing process exceeded the Ac1 point.
In the steel wire of test number 31, the total area ratio of ferrite and spherical carbide was insufficient, and the number of spherical carbides with a circular equivalent diameter of 0.1 μm or more was insufficient. Therefore, the deformation resistance and the limit compressibility were higher than those of the normal steel wire. It didn't turn out to be excellent. It is presumed that this is because the average cooling rate after finish rolling was insufficient.
In the steel wire of test number 32, the total area ratio of ferrite and spherical carbide was insufficient, and the average particle size of ferrite was too small. Therefore, the deformation resistance was inferior to that of the normal steel wire, and the critical compression ratio was higher than that of the normal steel wire. It didn't turn out to be excellent. It is presumed that this is because the second holding step was not performed.

Furthermore, with respect to the example steel wire within the scope of all inventions and the comparative example steel wire outside the scope of some inventions, the metallographic structure of the wire rod as the raw material was also evaluated. The evaluation method was as described above in the present specification. The evaluation results are shown in Table 5.

The wire rods, which are the materials of the steel wires of test numbers 1 to 12, 33 to 36, and 41 that satisfy all the requirements specified in the present disclosure, have a bainite area ratio of 50% or more in the C cross section before wire drawing. Therefore, the area ratio of martensite was 0% or more.
On the other hand, in the wire rods of test numbers 13 and 14 in which the wire was broken during the wire drawing process, the amount of bainite was insufficient and the amount of martensite was large.
The wire rod of Test No. 19 for steel wire in which the average aspect ratio of the spherical carbide exceeds the upper limit of the present disclosure and the number density of the spherical carbide is less than the lower limit of the present disclosure includes both bainite and martensite. There wasn't.
The wire rods of test numbers 22 and 24 on the steel wire having an average ferrite grain size below the lower limit of the present disclosure contained both bainite and martensite, but the amount was insufficient.

10 Steel wire C Central axis D Diameter of steel wire

Claims (4)

1. Ingredient composition is mass%,
   C: 0.10 to 0.60%,
   Si: 0.01-0.50%,
   Mn: 0.20 to 1.00%,
   P: 0.030% or less,
   S: 0.050% or less,
   Cr: 0.85 to 1.50%,
   Al: 0.001 to 0.080%,
   N: 0.0010 to 0.0200%, and the balance: Fe and impurity elements.
   In a cross section that includes the central axis of the steel wire and is parallel to the central axis.
   More than 95 area% of the metallographic structure is composed of ferrite and spherical carbides.
   The ferrite has an average particle size of 10.0 to 30.0 μm.
   The spherical carbide has an average aspect ratio of 2.5 or less for the spherical carbide having a diameter equivalent to a circle of 0.1 μm or more, and the content (mass%) of C contained in the steel wire is represented by [C]. case, a circle number of equivalent diameter 0.1μm or more of the globular carbides is ^{1.5 × 10 6 × [C]} ~ 7.0 × 10 6 × [C] number / mm ^2, the steel wire.
2. The first aspect of claim 1, wherein in the cross section, the average particle size of the spherical carbide having a diameter equivalent to a circle of 0.1 μm or more is 0.50 μm or less, and the maximum particle size of the spherical carbide is 3.00 μm or less. Steel wire.
3. When the component composition is mass%,
   Ti: 0 to 0.050%,
   B: 0 to 0.0050%,
   Mo: 0 to 0.50%,
   Ni: 0 to 1.00%,
   Cu: 0 to 0.50%,
   V: 0 to 0.50%,
   Nb: 0 to 0.050%,
   Ca: 0 to 0.0050%,
   Mg: 0 to 0.0050%, and Zr: 0 to 0.0050%,
   The steel wire according to claim 1 or 2, which satisfies one or more of the above.
4. The hot-rolled wire rod for manufacturing the steel wire according to any one of claims 1 to 3.

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