JP2000119806A - Steel wire rod excellent in cold workability, and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel wire rod having excellent cold workability even in as-hot-rolled state and to provide a useful method for manufacturing such a steel wire rod. SOLUTION: This steel wire rod contains 0.03-0.8 mass% C and grain size in this steel wire rod is smaller on the surface side and larger on the central- part side. In the steel wire rod, when a surface layer means the region between the surface and a position at a depth of 5-30% of wire-rod diameter from the surface, the average grain size in the surface layer is <=5 μm. Further, when an outermost surface layer means the region, within the surface layer, between the outermost surface and a position at a depth of 0.3-0.4 mm from the outermost surface, the average grain size in the outermost surface layer is >=2 μm. Moreover, the average grain size in the inner part inside the surface layer is <=10 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel wire used for manufacturing a machine structure by cold plastic working such as cold forging, cold rolling, cold rolling and the like, and such a steel wire. The present invention relates to a method for producing a steel sheet, and particularly, exhibits good cold workability even if heat treatment before cold work is omitted, or when heat treatment is performed, the cold workability after heat treatment is improved. The present invention relates to a steel wire which can be further improved, and a useful method for producing such a steel wire.

[0002]

2. Description of the Related Art Low-medium carbon steel wires and low-medium carbon alloy steel wires are widely used in the manufacture of shafts, bolts, nuts, and the like as machine structural steel. These steel wires are cold forged,
It is manufactured by cold plastic working such as cold rolling and cold rolling, and in that case, firstly, it is required that the deformation resistance is low. This is because the lower the deformation resistance, the longer the life of the processing jig can be achieved.

On the other hand, steel wires are required to have high ductility. And when manufacturing parts with a high processing rate,
If the ductility is low, cracks occur from the surface and cracks occur. In order to avoid such inconveniences, conventionally, a method of increasing ductility while lowering deformation resistance by annealing heat treatment or spheroidizing heat treatment has been adopted. However, there is a problem in that performing such a heat treatment takes much time and consumes much energy.

[0004] It is possible to reduce the deformation resistance in the hot rolling process of a steel wire rod.
No. 1-15129 proposes a technique for lowering the deformation resistance as it is rolled by reducing the cooling rate after rolling. However, when the cooling rate is reduced, the crystal grains are coarsened, and the ductility is rather lowered.

[0005] It is known that ductility and deformation resistance have a correlation with the crystal grain size. As a scale for evaluating ductility, ε _{T is} a fracture strain at the time of a tensile test, and σ _y is a yield stress as an evaluation scale of a resistance change. The relationship between the crystal grains and the following equations (1) and (2) It is known that it can be expressed like an expression (FBPickering "Towards imp
roved toughness and ductility ", Climax Molybdenium
Co. Symp., Kyoto, 1971, 9). ε _T = 1.4-2.9% C + 0.20% Mn + 0.61% Si-2.2% S-3.9% P-0.25% Sn + 0.017d ^-1/2 … (1) σ _y [MPa] = 15.4 (3.5 + 2.1 % Mn + 5.4% Si + 23 %% N + 1.13d- ^{1 / 2} (2) where% represents mass% of each component, and d represents ferrite grain size.

As can be seen from the above equation, when the particle size d is reduced, the breaking strain ε _T increases and the ductility improves, but the yield stress σ _y also increases, and the deformation resistance also increases. When the particle diameter is increased by d, the opposite relationship is established. Equations (1) and (2) above are for low-carbon steel, but the tendency of the relationship between the grain size d, the strain at break ε _T , and the yield stress σ _y is the same for medium-carbon steel. As described above, reducing the deformation resistance while rolling and improving the ductility are mutually contradictory issues, and it has been a difficult technique to achieve both properties at the same time.

For this reason, for example, Japanese Patent Application Laid-Open No. 05-3
JP-A-39-676 and JP-A-05-339677 introduce a technique of forming a fine ferrite + pearlite structure of 10 μm or less only in the vicinity of the surface. However, in such a steel wire rod, when the average particle size of the surface fine layer is close to 10 μm, sufficient ductility cannot be obtained, and since the particle size in a region other than the surface fine layer is 10 μm or more, a hole is formed at the center. There has been a problem that cracks occur in cold processing such as processing.

[0008]

SUMMARY OF THE INVENTION The present invention has been made under such a circumstance, and an object of the present invention is to provide a steel wire rod having excellent cold workability even as hot rolled. It is an object of the present invention to provide a useful method for producing a simple steel wire.

[0009]

According to the present invention which has achieved the above object, the present invention includes C: 0.03 to 0.8%, the crystal grain size in the wire is small on the surface side, and In the case of a large steel wire rod, when the surface layer has a depth from the surface of 5 to 30% of the wire diameter, the average particle size of the surface layer is 5 μm.
The following and 0.3 from the outermost surface of the surface layer
Steel having a point that the average particle size of the outermost surface layer is 2 μm or more, and the average particle size inside the surface layer is 10 μm or less, when a depth position of about 0.4 mm is the outermost surface layer. It is a wire. The steel wire rod of the present invention basically has a structure of ferrite and pearlite, and the “crystal grain size” means the crystal grain size of this structure. Further, the steel wire rod of the present invention is based on the premise that it is subsequently cold-worked, and therefore, it is assumed that the wire diameter is 3 mm or more.

The specific chemical composition of the steel wire of the present invention is as follows: Si: 0.01 to 0.5%;
n: 0.05 to 2%, Al: 0.01 to 0.08%, and P: 0.03% or less (including 0%);
S: 0.03% or less (including 0%) and N: 0.07
% Or less (including 0%).

In the steel wire of the present invention, if necessary, (1) Cr: 1% or less (excluding 0%), M:
o: 1% or less (excluding 0%) and B: 0.002
One or more selected from the group consisting of 5% or less (excluding 0%), (2) Ti: 0.2% or less (excluding 0%), V: 0.2 or less (including 0%) No), Nb: 0.
It is also effective to contain at least one selected from the group consisting of 2% or less (excluding 0%) and Zr: 0.2% or less (excluding 0%), and thereby the steel wire Characteristics can be further improved.

On the other hand, in order to produce the steel wire of the present invention, the surface temperature of the wire is increased from 50 ° C. to 850 ° C. in the hot finish rolling while maintaining the wire center temperature at 850 ° C. or more. seconds of cooling rate (Ar ₁ -150 ℃)
To (Ar ₁ -50 ° C.), and the surface temperature is further reduced to (Ac ₁ + 50 ° C.) to (Ac ₁ + 150 ° C.).
After that, rolling may be performed once or more at a surface finishing temperature of 950 ° C. or less with a surface reduction rate of 25% or more.

In the present invention, the average grain size is determined by a method in which the cross section of a wire is polished and etched and the nodule size of ferrite and pearlite is cut in a structure observed by an optical microscope or a scanning electron microscope (SEM). It was obtained by using the above method. The average particle size refers to an average particle size measured by observing an area of (100 μm in the depth direction × 250 μm in a direction perpendicular to the depth direction) in five or more visual fields. The average particle size of the outermost surface is centered at a depth of 0.3 mm from the surface of the wire (100 μm in the depth direction × 250 μm in a direction perpendicular to the depth direction).
Is the value measured by the above method. Here, the reason why the depth of 0.3 mm from the surface of the wire is set as the outermost surface is that in a portion shallower than this, a normal structure may not be obtained due to decarburization. Further, the depth at which the average particle size is 5 μm is determined by the average particle size measured by the above method.
(100 μm × 250 μm)
Shows the center depth of the region. Ar ₁ is a temperature (° C.) at which pearlite starts to precipitate when a structure composed of austenite is cooled, and Ac ₁ is a temperature at which austenite starts to precipitate from pearlite when heating a structure containing pearlite ( ° C).

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reasons for limiting each requirement of the present invention are as follows. In the present invention, when a region having a depth from the surface of 5 to 30% of the wire diameter (a region composed of ferrite and / or pearlite) is used as the surface layer, the average particle size of the surface layer is 5 μm or less. (However, the lower limit of the average particle size is 2 μm, as is clear from the purpose described later.) This is for the following reasons.

The present inventors have conducted a detailed investigation on the locations where cracks occur during cold working, and have found that they are almost in the vicinity of the surface of the component, ie, the rolled wire in most cases. More specifically, it was found that cracks occurred in a region from the surface of the wire to a depth of about 30% of the diameter, leading to cracking. This corresponds to a region where deformation is greatest during cold working as a part.

Further, it was found that the starting point of the occurrence of cracks was a fine flaw at the interface between ferrite and pearlite having a size of 5 μm or more, inside pearlite, or on the surface. Cracks may occur outside these areas,
Such cracks are caused by coarse inclusions and the like and cannot be solved only by controlling the crystal grain size (that is, a problem outside the scope of the present invention). Therefore, when there are no coarse inclusions, cracks can be effectively prevented by miniaturizing the structure in this region near the surface. Cracks from fine surface flaws can be suppressed only in a region where the structure is fine and is shallower than 5% of the wire diameter from the surface, but cracks generated in a region deeper than that cannot be prevented.

On the other hand, if the diameter is reduced to a region deeper than 30% of the wire diameter, cracks can be prevented, but the deformation resistance of the entire wire material increases, resulting in an adverse effect such as shortening of the tool life. . Similarly to the above, when the average particle size at a depth of 0.3 to 0.4 mm from the outermost surface (outermost surface layer) of the surface layer is 2 μm or less, the deformation resistance extremely increases and shrinks. Absent.

In the steel wire rod of the present invention, the average particle size inside the surface layer must be 10 μm or less, for the following reason. In order to suppress the occurrence of cracks, it is necessary to reduce the particle size as much as possible, but on the other hand, there is a demand to increase the particle size in order to reduce the deformation resistance. In the vicinity of the surface, it is necessary to define the average particle size as described above from the viewpoint of suppressing cracking. Therefore, in the portion from the fine grain region in the vicinity of the surface to the center, it is necessary to have an appropriate grain size for achieving both crack generation and deformation resistance reduction. The present inventors have studied the appropriate grain size from such a viewpoint.

In the cold working, cracks may be generated from the vicinity of the center, and this is particularly remarkable in the case of drilling. The starting point of the generation is at the interface between ferrite and pearlite having a large grain size as in the vicinity of the surface or at the inside of pearlite. It has been found that crack generation in a uniform portion is remarkable. In the processing of the central portion, since the processing rate rarely increases near the surface, it was found that the deformation rate should be 10 μm or less in order to reduce the deformation resistance while suppressing the occurrence of cracks.

The steel wire of the present invention basically has a C of 0.03
The specific chemical composition is as follows: Si: 0.01 to 0.5%, Mn: 0.05
２2%, Al: 0.01 to 0.08%, P: 0.03% or less (including 0%), S: 0.03
% Or less (including 0%) and N: 0.07% or less (0%
), And the reasons for limiting the range of these elements are as follows.

C: 0.03 to 0.8% C is an element necessary for giving a predetermined strength to the wire.
For that purpose, it is necessary to contain at least 0.03% or more. However, when the content of C is excessive, the deformation resistance is significantly increased, so the upper limit must be set to 0.8%. The preferred lower limit of the C content is 0.3%, and the preferred upper limit is 0.5%.

Si: 0.01 to 0.5% Si is added as a deoxidizing agent at the steel making stage, but for this purpose, it must be contained at 0.01% or more. However, if the content is excessive, the deformation resistance is significantly increased, so the upper limit must be 0.5%. Note that a preferable upper limit of the Si content is 0.25%.

Mn: 0.05 to 2% Mn is an element necessary for fixing S, which is an impurity, to render it harmless, and is added to improve the strength and toughness of steel.
To achieve these effects, at least 0.05
However, if it is contained excessively, the quenching property is improved and bainite or the like is generated as it is in hot rolling, which leads to an increase in deformation resistance. Therefore, the content needs to be 2.0% or less. . The preferred lower limit of the Mn content is
0.3%, and a preferable upper limit is 1.0%.

Al: 0.01-0.08% Al acts as a deoxidizer in the steel making process, and
N is fixed as N to reduce solid solution N and reduce deformation resistance. To achieve these effects, 0.0
It is necessary to contain 1% or more. However, if Al is excessively contained, the cold workability is deteriorated as coarse AlN, so the upper limit is 0.1%. The preferred lower limit of the Al content is 0.02%, and the preferred upper limit is 0.0%.
4%.

P: 0.03% or less (including 0%), S:
0.03% or less (including 0%) P or S segregates at the grain boundary or exists as a compound and impairs the cold workability. Therefore, it is necessary to suppress P and S to 0.03% or less. . It is preferable that each of these elements is not more than 0.01%.

N: 0.07% or less (including 0%) N remarkably increases the deformation resistance. Therefore, in order to obtain good cold workability, N must be suppressed to 0.07% or less. .

The basic chemical composition of the steel wire of the present invention is as described above, and the balance consists of Fe and unavoidable impurities. In the steel wire of the present invention, if necessary, (1) Cr: 1% or less (excluding 0%), Mo: 1% or less (excluding 0%) and B:
At least one selected from the group consisting of 0.0025% or less (not including 0%); (2) Ti: 0.2% or less (0%
, V: 0.2 or less (excluding 0%), N
b: 0.2% or less (excluding 0%) and Zr: 0.
It is also effective to include one or more selected from the group consisting of 2% or less (not including 0%), and thereby, the properties of the steel wire can be further improved. The reasons for limiting the range of these elements are as follows. In addition, besides these components, the steel wire of the present invention may contain a trace component that does not impair its properties, and such a steel wire is also included in the scope of the present invention.

Cr: 1% or less (excluding 0%), M
o: 1% or less (excluding 0%) and B: 0.002
Selected from the group consisting of 5% or less (excluding 0%)
One or more elements Cr, Mo and B are quenching adjusting elements and are added to adjust strength and toughness by quenching and tempering. However, an excessive content is not preferable because deformation resistance is increased. From these viewpoints, Cr and Mo
Sets the upper limit to 1%, and B sets the upper limit to 0.0025%
And The effect of the addition of these elements increases as the content is increased within the above range. However, in order to exert the above effect, 0.01% or more for Cr and Mo, and 0.0003% or more for B. It is preferable to include them.

Ti: 0.2% or less (excluding 0%),
V: 0.2 or less (excluding 0%) Nb: 0.2% or less
(Excluding 0%) and Zr: 0.2% or less (0%
One or more elements selected from the group consisting of Ti, V, Nb and Zr are added to form fine carbonitrides and refine the structure near the surface. However, an excessive content is not preferable because deformation resistance is increased. From such a viewpoint, the upper limit is set to 0.2% in each case. The effect of the addition of these elements increases as the content increases within the above range, but in order to exhibit the above effects, it is preferable to contain 0.001% or more of each.

Next, the finish rolling conditions in the hot rolling according to the present invention will be described. In the following temperature control, as a typical control method, there is a method of controlling the amount of water when passing a wire through the water cooling, but the temperature control method employed in the present invention is not limited to such a method ,
Any method such as a method of controlling by gas cooling and a method of controlling by cooling a cooling medium other than water can be adopted.

In the present invention, basically, the vicinity of the surface is rapidly cooled to precipitate ferrite and pearlite, and then the temperature is raised to cause the reverse transformation to austenite, thereby reducing the ultrafine structure near the surface. What you get. For example, in a conventional technique such as Japanese Patent Publication No. 7-116503, reverse transformation is caused by utilizing the heat generated during processing during rolling or by supplying energy from the outside. However, the present inventors have studied and found that, while keeping the temperature inside the wire high, only the surface is cooled, and then the surface is double-heated by the amount of heat inside the wire, whereby reverse transformation can be caused. Was. Further, in the steel wire rod of the present invention, the surface and the inside exhibit a particle size distribution in which the particle size changes stepwise greatly, but in the above-mentioned conventional technology, the particle size changes in a gradient manner. The difference is also in the diameter distribution.

In the method of the present invention, first, the surface temperature is 9
From the temperature of 00 ° C. or higher, the surface of the wire is cooled at a cooling rate of 50 ° C./sec while maintaining the center temperature of the wire at 850 ° C. or higher. The reason why the cooling is performed while maintaining the wire center temperature at or above the predetermined temperature is because it is effective to restore the surface temperature once cooled by water cooling to a constant temperature (a reheating process described later). If the initial temperature is lower than 900 ° C., the calorific value of the wire becomes small, and it becomes impossible to restore the temperature of the surface once cooled to a predetermined temperature. However, if the temperature is too high, the austenite grain size becomes too large, and the structure after rolling becomes a coarse structure such that the average grain size exceeds 10 μm. From this viewpoint, the upper limit of the temperature is The temperature is preferably set to 1100 ° C. or lower. For the same reason, in the present invention, it is important to keep the wire center temperature at 850 ° C. or higher in the cooling process before finish rolling. In the step of restoring the temperature of the surface cooled in the subsequent step, when a heating device for applying energy from the outside is used, the temperature can be reduced to less than 850 ° C. Diameter 5
In this case, the center temperature of the wire is preferably set to 800 ° C. or more.

In the cooling step, the temperature of the surface of the wire is set at 50
(Ar ₁ -150) to (Ar ₁
(−50 ° C.) due to the following reasons. First, the cooling rate is set to 50 ° C./sec or more in order to prevent austenite grain growth during cooling and to prevent ferrite and pearlite from being formed at a high temperature to cause a coarsening of the structure. Thereby, a very fine structure can be obtained by generating fine ferrite and pearlite, and causing the reverse transformation to return to austenite once again by raising the temperature.

In the above cooling, at least (Ar ₁ -50
(° C.) or lower, but the lower limit of the temperature must be (Ar ₁ −150 ° C.). If the cooling temperature is lower than (Ar ₁ -150 ° C.), unless the cooling rate is strictly controlled, a martensite structure may be generated below the Ms point. The structure of martensite does not undergo reverse transformation to austenite even in the subsequent reheating process, and may remain in the rolled material to increase the tensile strength (TS). Therefore, from the viewpoint of producing a stable product, the lower limit of the cooling temperature needs to be (Ar ₁ -150 ° C.).

On the other hand, the upper limit of the cooling temperature needs to be (Ar ₁ -50 ° C.) as described above. When the temperature is cooled to a temperature higher than this temperature, ferrite and pearlite precipitate once, , The effect of refining the structure cannot be obtained even if reverse transformation occurs.

After the cooling, the surface temperature was reduced to (Ac ₁ +5).
The reason for (0) to (Ac ₁ +150) (recuperation process) is to reversely transform the precipitated ferrite or pearlite into austenite. In this process, very fine austenite can be obtained, which is an important requirement for obtaining a microstructure after rolling. The temperature at this time is (Ac ₁ +5
Below 0), the following two adverse effects occur. First, no reverse transformation occurs, and the effect of reducing austenite cannot be obtained. Secondly, the state in which the ferrite is processed during rolling remains on the product wire rod, and becomes extremely hard, resulting in poor cold forgeability.

On the other hand, when this temperature exceeds (Ac ₁ +150), austenite grains grow after the reverse transformation, and the effect of miniaturization cannot be obtained. Note that the present invention is based on the use of reheating of the surface temperature due to heat from the center of the wire rod. However, even if the surface temperature is controlled or heated by supplying energy from the outside, the condition is maintained. The same effect can be obtained by appropriately controlling.

According to the present invention, in the state of austenite which has become very fine due to the reverse transformation, rolling is performed so that the finishing temperature is 950 ° C. or less with a reduction of 25% or more, so that 2 Very fine structures with an average particle size of ５5 μm can be obtained. 25% reduction
% Or the finishing temperature exceeds 950 ° C., a fine structure cannot be obtained because the austenite particle size cannot be sufficiently reduced. This finish rolling step may be repeated a plurality of times.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention. Are included in the target range.

[0040]

EXAMPLE Using a test steel having the chemical composition shown in Table 1 below, a rolling experiment was conducted on a rolling line consisting of a heating furnace, rough rolling, intermediate rolling, water cooling zone 1, finishing rolling, water cooling zone 2, and a cooling conveyor. Was. At this time, rolling was carried out under various conditions (1) to (5) below. The surface temperature during rolling was measured with a radiation thermometer. The center temperature at each stage was determined by numerical analysis using the difference method, taking into account the measured value of the surface temperature, cooling conditions (water-cooled zone, in air), and finish rolling conditions. (1) Finishing temperature during intermediate rolling (2) Surface temperature and center temperature immediately after water cooling zone 1 (3) Surface temperature and center temperature on the entrance side of finishing mill after reheating after water cooling zone 1 (4) (5) Finished surface temperature of finish rolling

[0041]

[Table 1]

The rolled material was evaluated by measuring the tensile strength in a tensile test and the upsetting limit rate in an upsetting test. The relationship between both the tensile strength and the upsetting limit ratio was determined by examining various steel types of the conventional rolled material in which the average grain size of the wire was 15 μm or more in the entire cross-sectional area. It turned out to be. [Upset limit ratio (%)] = 24010 / (tensile strength / MPa) +16 (1)

In the test material, the value calculated from the tensile strength by the above equation (1) (the upsetting limit ratio of the conventional material)
And the measured values of the upsetting margin ratio were compared, and those excellent in comparison with the conventional material were evaluated as で あ れ ば, and those equal to or less than the evaluation were evaluated as ×.

First, No. 1 in Table 1 was used. Using the test materials of 2,
By changing the rolling conditions, wires having different grain sizes and distributions in the cross section of the wires were prepared. The rolling conditions at this time are shown in Table 2 below, and the evaluation results are shown in Table 3 below.

[0045]

[Table 2]

[0046]

[Table 3]

Next, the steel materials having the respective compositions shown in Table 1 were rolled under the rolling conditions shown in Table 4 below, and the obtained rolled materials were evaluated in the same manner as described above. The evaluation results are shown in Table 5 below.

[0048]

[Table 4]

[0049]

[Table 5]

Further, in Table 1 above, Sample No. 2 was rolled under various conditions shown in Table 6 below. The evaluation results are shown in Table 7 below.

[0051]

[Table 6]

[0052]

[Table 7]

From these results, it is apparent that those satisfying the requirements defined in the present invention have excellent cold workability even when hot rolling is performed.

[0054]

The present invention is configured as described above, and a steel wire having excellent cold workability can be realized even when hot rolling is performed.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor: Mamoru Nagao 1-5-5 Takatsukadai, Nishi-ku, Kobe City Inside Kobe Research Institute, Kobe Steel Ltd. (72) Inventor: Koichi Makii 1 Takatsukadai, Nishi-ku, Kobe City Kobe Steel Co., Ltd. Kobe Steel, Ltd. Kobe Research Institute, Ltd. (72) Inventor Hiroshi Hyaku ▲ Saki ▼ 2 Nadahama-Higashi-cho, Nada-ku, Kobe Kobe Steel Co., Ltd. Kobe Steel F-term (reference) 4K032 AA01 AA02 AA04 AA05 AA06 AA11 AA16 AA19 AA21 AA22 AA27 AA29 AA31 AA35 AA36 AA39 BA02 CA02 CC04 CD03

Claims (5)

[Claims] 1. A steel wire rod containing C: 0.03 to 0.8% (meaning by mass%, the same applies hereinafter), and having a crystal grain size in the wire rod small on the surface side and large in the center part. When the surface layer has a depth from 5 to 30% of the wire diameter as the surface layer, the average particle size of the surface layer is 5 μm or less,
When a depth of 0.3 to 0.4 mm from the outermost surface of the surface layer is defined as the outermost surface layer, the average particle size of the outermost surface layer is 2 μm.
m or more, and the average particle size inside the surface layer is 1
A steel wire excellent in cold workability, characterized in that it has a thickness of 0 μm or less. 2. Si: 0.01 to 0.5%, Mn: 0.1 to 0.5%.
P: 0.03% or less (including 0%), S: 0.0
3% or less (including 0%) and N: 0.07% or less (0%
% Of the steel wire rod according to claim 1. 3. Cr: 1% or less (excluding 0%), M
o: 1% or less (excluding 0%) and B: 0.002
The steel wire according to claim 1, wherein the steel wire comprises at least one element selected from the group consisting of 5% or less (excluding 0%). 4. Ti: 0.2% or less (excluding 0%), V: 0.2 or less (excluding 0%), Nb: 0.
4. The composition according to claim 1, wherein the composition contains at least one element selected from the group consisting of 2% or less (excluding 0%) and Zr: 0.2% or less (excluding 0%). 5. Steel wire rod. 5. In producing the steel wire according to any one of claims 1 to 3, in hot finish rolling, the surface temperature is set to 850 ° C. from the temperature of 900 ° C. or more.
While maintaining the above conditions, the surface of the wire is once cooled to a temperature range of (Ar ₁ -150 ° C.) to (Ar ₁ -50 ° C.) at a cooling rate of 50 ° C./sec, and the surface temperature is further reduced to (Ac ₁ + 50 ° C.).
After heating to (Ac ₁ + 150 ° C.), the finishing temperature is set to 950 ° C. or less at a surface reduction of 25% or more and 1%.
A method for producing a steel wire excellent in cold workability, characterized by rolling at least twice.