JP2000073137A - Steel wire rod excellent in cold workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel wire rod provided with sufficient deformability by the reduction of deformation resistance in low-medium carbon steel and low-medium carbon low alloy steel as hot-rolled and excellent in cold workability even if spheroidizing annealing treatment is simplified or omitted. SOLUTION: This steel wire material is the one contg. by mass, 0.03 to 0.8% C in which the average grain size of ferrite is 2 to 5.5 μm and also, the region in which the ratio of cementite having <=3 μm major axis and <=3 aspect ratio shown by (the major axis/the minor axis) becomes >=70 area % to the whole cementite is the region of >=10% of the wire diameter from the surface.

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 structural component by cold plastic working such as cold forging, cold forging, cold rolling, and the like. The present invention relates to a steel wire rod capable of exhibiting good cold workability even if the spheroidizing annealing step before cold working is simplified or omitted, and achieving energy savings.

[0002]

2. Description of the Related Art Low-medium carbon steel and low-medium carbon low alloy steel wires are widely used for shafts, bolts, nuts, and other mechanical structural parts requiring relatively high strength. This steel wire is manufactured by cold plastic working such as cold forging, cold forging, cold rolling and the like, and such a steel wire is required to have low deformation resistance. In general, carbides in steel are spheroidized for the purpose of imparting deformability by lowering the deformation resistance of the steel wire.

[0003] The spheroidizing treatment is a treatment in which a hot-rolled steel material is reheated and then gradually cooled. However, the spheroidizing process requires a short processing time of 10 hours and a long processing time of 20 hours or more, which is a manufacturing cost bottleneck.

That is, the spheroidizing treatment is usually performed by spheroidizing annealing.
(1) a method of heating to a temperature not lower than the A ₁ transformation point and then slowly cooling;
Various methods are known, such as (2) a method of holding immediately below the A ₁ transformation point, and (3) a method of repeating heating and cooling above and below the A ₁ transformation point. There is a problem that it is necessary to gradually cool, and the processing cost such as time and energy is very high. For these reasons, various studies have been made on means for shortening the time required for the spheroidizing treatment.

[0005] As one of such techniques, a technique of performing cold working such as wire drawing before performing spheroidizing treatment to break carbides in the steel and promoting the segregation and aggregation of carbides in the subsequent spheroidizing annealing. Conventionally, a method for shortening the processing time has been generally performed. However, although this method has the effect of shortening the processing time, the effect of shortening the processing time throughout the entire manufacturing process is not so innovative because a cold working step is added.

[0006] In addition, by devising the hot rolling step of the wire rod, it is possible to shorten the spheroidizing time,
For example, Japanese Patent Application Laid-Open No. 59-166622 utilizes a well-known fact that it is effective to apply spheroidizing annealing after deforming and breaking a carbide in advance by giving a work in advance to a steel containing 2% by mass or less of C. On the other hand, pearlite is divided by giving a plastic deformation of 10% or more in a (Ferrite + Pearlite) two-phase region of 500 ° C. or more of Ac ₁ or less, and the (ferrite + austenite) temperature range is further reduced by the processing heat. A method of gradually cooling a specific temperature range has been proposed.

According to this method, a spheroidizing ratio exceeding 90% can be obtained depending on a cooling rate in a specific temperature range,
As a result, the spheroidization time can be reduced to 1/5 or less.
However, in this method, since the steel material must be processed at a low temperature of Ac ₁ or less, there is a problem that the load of the rolling mill is large and the rolling is not easy.

Further, JP-A-59-136421 discloses that
Similarly, in the steel containing 2% by mass or less of C, the steel material is once cooled to Ar ₁ point during the finish rolling to form a (ferrite + pearlite) structure, and the pearlite is cut by rolling from that temperature, and the processing is continued. There has been proposed a method in which a temperature range of (ferrite + austenite) is set as a finishing temperature and gradually cooled by heat. However, in this method, it is necessary to process a steel material at a temperature of Ar ₁ or lower, which is lower in temperature than the above technique, so that the load on the rolling mill is further increased and rolling is not easy.

As a method of solving the above-mentioned problem of the rolling temperature, for example, in Japanese Patent Application Laid-Open No. Sho 59-136422, rolling is carried out in a metastable austenite temperature range, and Ae ₁ or less Ar ₁ is utilized by utilizing the work-induced transformation. 10% in the above temperature range
A method of giving the above plastic deformation has been proposed. However, even with this method, the rolling temperature must be lower than Ae ₁ , and the rolling load is still large.

[0010] In fact common to the conventional example have been proposed heretofore, rolling at such A ₁ transformation point temperature near zone, the strain is accumulated in the ferrite to produce at the same time as the carbides, subsequent processing heating However, the spheroidizing rate does not sufficiently recover, and the deformation resistance does not sufficiently decrease in spite of the increase in the spheroidization rate, and there is a problem in the life of the cold working tool.

On the other hand, Japanese Patent Application Laid-Open No. 8-246040 discloses that the elementary processes of the following (1) and (2) are performed once or in combination of two or more times during the heating of a steel material, so that the spheroidizing treatment time is reduced. A rapid continuous spheroidizing annealing method that reduces the time to one hour or less is disclosed, and it is believed that the development of this technique will achieve a significant reduction in the spheroidizing time.
However, even in this technique, spheroidizing annealing has not been completely omitted, and further improvement is desired.

(1) Ae ₁ point or more, Ae ₁ point + 150K
The following temperature range, 1K / sec or more was heated heated at a Atsushi Nobori rate, after holding time of less than 0 seconds 600 seconds in the temperature range, Ae ₁ point or more, Ae ₁ point + 50K~Ae ₁ point +1
Cooling in a temperature range of 50 K at a cooling rate of 5 K / sec or less, or maintaining the temperature in the temperature range, (2) A
e ₁ point + 80K or more, Ae ₁ point + 270K or less, heating at a heating rate of 1K / sec or more, heating for 0 to less than 120 seconds, and Ae ₁ point or more , the temperature range of Ae ₁ point ~Ae ₁ point -150K 5K
Cool at a cooling rate of not more than / sec or maintain the temperature within the temperature range.

[0013]

As described above, various techniques for simplifying the spheroidizing annealing treatment have been proposed for low-medium carbon steel, especially medium carbon steel of C: 0.3% or more. However, from the viewpoint of completely omitting the spheroidizing annealing, none of these methods is sufficient and cannot be a definitive method.

The present invention has been made under such circumstances, and an object of the present invention is to reduce the deformation resistance of hot-rolled low-medium-carbon steel or low-medium-carbon low-alloy steel to obtain sufficient deformability. It is intended to provide a steel wire excellent in cold workability even if the spheroidizing annealing treatment is simplified or omitted.

[0015]

Means for Solving the Problems The present invention, which has achieved the above object, is a steel wire rod containing C: 0.03 to 0.8% by mass and having an average ferrite particle diameter of 2 to 5%. The area where the cementite having a length of 5 μm and a major axis of 3 μm or less and having an aspect ratio of 3 or less (major axis / minor axis) of 70% by area or more with respect to the total cementite is 10% or more of the wire diameter from the surface. This is a steel wire rod having a gist at a point that is an area.

The steel wire of the present invention defines the crystal structure in the wire as described above. The specific chemical composition of the wire is Si: 0.004 to 0.5.
% By mass, Mn: 0.05 to 2% by mass, and Al: 0.0
1 to 0.08% by mass, P: 0.03% by mass or less (including 0%), S: 0.03% by mass or less (0%
%) And N: 0.07% by mass or less (including 0%).

In the steel wire of the present invention, if necessary, (1) Cr: 1.5% by mass or less (excluding 0%), Mo: 1.0% by mass or less (excluding 0%), N
i: 1.0% by mass or less (excluding 0%) and B:
At least one selected from the group consisting of 0.0025% by mass or less (excluding 0%), (2) Ti: 0.5% by mass or less (excluding 0%), V: 0.6% by mass or less (Excluding 0%), Nb: 0.4% by mass or less (excluding 0%)
It is also effective to include at least one selected from the group consisting of Zr: 0.3% by mass or less (not including 0%), and the like, whereby the properties of the steel wire can be further improved.

The average grain size of ferrite is the size of an average grain section of ferrite measured in a structure observed by an optical microscope or a scanning electron microscope (SEM) after polishing and etching a cross section of a wire. .

[0019]

DETAILED DESCRIPTION OF THE INVENTION The inventors of the present invention have conducted detailed investigations on cracks occurring during cold working, and have found that almost all parts are near the surface of a rolled wire rod 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 10% of the diameter, leading to cracking. This region corresponds to a region where the strain is the largest during cold working as a part.

Further, the starting point of crack generation is 5.5 μm
It was found that there were fine scratches on the interface between ferrite and the second phase (here, pearlite) of the above size, inside pearlite, or on the surface. Cracks occurring outside this region are caused by coarse inclusions and the like, and are problems that can be solved by steelmaking techniques such as removal of inclusions.

Therefore, it was considered that the frequency of cracks can be suppressed by reducing the grain size of ferrite to 5.5 μm or less and reducing the interface in contact with the second phase.

In addition to such an idea, deformation resistance, which is another property required for cold workability, can be easily understood from the known fact that deformation resistance and deformability are improved by spheroidizing treatment. As described above, when the pearlite has a divided structure as close to spheroidization as possible, the deformation resistance can be effectively reduced.

However, in the ordinary spheroidizing treatment, although the spheroidization of cementite proceeds, it is difficult to control the ferrite particle size. For example, the literature (Materials Sc)
As described in “Ice and Engineering 62, No. 62 (1983), pp. 163 to 171), the average grain size of ferrite can be controlled only to about 5.8 to 15 μm in the entire wire cross section.

The inventors of the present invention have conducted intensive studies and as a result, by using high-frequency heating for a short time together with cold working,
We succeeded in reducing the size of ferrite in the fragmented cementite structure. The principle of this method is as follows.

First, the steel wire is subjected to a skin pass with a reduction in area of about 5%. Next, this wire is heated to a temperature just below the Ac ₃ point in the two-phase temperature region of ferrite (α) + austenite (γ) by rapid heating at a heating rate of 400 ° C./s or more. The Ac ₃ point here is the temperature at which α completely disappears during continuous heating to become a γ single phase. Then, after heating to a predetermined temperature, the heating is immediately stopped, and cooling is performed at a cooling rate of 10 ° C./s or more and a critical cooling rate or less. The above process is repeated three or four times. By such a treatment, the pearlite structure is further divided, and further, α is refined.

This can be considered to be a result of the following organizational change. First, the pearlite structure is rapidly decomposed by heating immediately below the Ac ₃ point, but by immediately starting cooling immediately below Ac ₃ , pro-eutectoid α that has not been transformed into γ remains in the steel. Will be.

On the other hand, cementite (θ) precipitated in a plate shape in pearlite is also decomposed, but is not decomposed uniformly, but preferentially decomposed in the vicinity of a grain boundary where carbon is rapidly diffused or in the vicinity of lattice defects. However, spherical cementite remains. This θ
Is the same as the θ decomposition process in the conventionally known spheroidizing process. However, while the heating temperature in the ordinary spheroidizing treatment is near the _Al point, this rapid heating treatment is characterized by a higher temperature. For this reason, it is a condition under which the refinement of the primary α can be easily achieved.

Further, under cooling under the above conditions, θ which has not been completely decomposed remains as it is, and pearlite transformation proceeds while precipitating new primary α at the γ-α interface.

Thus, a structure in which fine pro-eutectoid cementite, fragmented cementite and pearlite coexist is obtained by one rapid heating and cooling.

By repeating the above processing,
The volume ratio of pearlite exhibiting a lamellar structure is reduced, and a steel material having a microstructure close to that of a spheroidized material composed of divided θ and fine α can be obtained. However, the manufacturing method shown here is merely an example for obtaining the steel structure of the present invention, and does not limit the other manufacturing methods at all.

The feature of the steel wire according to the present invention resides in that its structure is specified. The reason for limiting each requirement specified in the present invention will be described.

Average grain size of ferrite: 2 to 5.5 μm In the steel wire rod of the present invention, the average grain size of ferrite in a region of 10% or more of the wire diameter from the surface is 2 to 5.5 μm.
Needs to be When the average grain size of the ferrite is less than 2 μm, it is effective in suppressing the occurrence of cracks, but the increase in strength due to the refinement of the crystal grains becomes remarkable, causing the deformation resistance to increase. Further, as described above, when the average particle size of ferrite exceeds 5.5 μm, only the same deformation resistance and deformability as those of the conventional spheroidizing material are reached, so the upper limit is set to 5.5 μm or less. As a method for reducing the average particle diameter of ferrite to 5.5 μm or less, high-frequency heating and cooling for a short time may be mentioned as described above.
There is no problem with other methods.

When the major axis is 3 μm or less, it is indicated by (major axis / minor axis).
In the wire rod of the present invention, the major axis of the divided cementite is 3 μm or less, and the aspect ratio indicated by (major axis / minor axis) is 3 or less. First, cementite having a major axis exceeding 3 μm is likely to be a starting point for early generation of cracks and voids at the interface with ferrite. The reason why the aspect ratio is specified to be 3 or less is that cementite longer than this will hinder plastic flow during cold working and tends to be a starting point of void generation particularly near the steel material surface. The frequency of void generation from smaller cementite is significantly reduced.

70% by area or more of the total cementite
Although it is necessary to be 0 area% or more, if the fraction is less than 70 area%, the effect of improving the cold workability by the divided cementite and fine ferrite intended by the present invention does not clearly appear. The upper limit of this fraction is not particularly limited.
Although it depends on the manufacturing method, 10
Desirably, it is close to 0%.

[0035] In the wire rod 10% or more regions present invention with a wire diameter from the surface, said fraction is 70 area% or more of the split cementite average particle size of the total area above tissues (ferrite 2-5. 5 μm, and the fraction of fragmented cementite is 70% by area or more based on all cementite)
In addition to this, in addition to the deformation resistance of a normal spheroidized material, the deformability is dramatically improved, and processing into a complicated shape is facilitated. However, even if only the region of about 10% from the surface has the above-mentioned structure, it has the effect of shortening the time of the current spheroidizing treatment. Since the effect of shortening the spheroidizing annealing is small only in the region of less than 10%, in the present invention, it is specified as 10% or more of the wire diameter from the surface. 10% of the wire diameter from the surface
Means that when viewed as a whole wire, a region of 20% with respect to the wire diameter is a fragmented cementite having the above fraction of 70% by area or more. When it becomes, it means that the above-mentioned fraction is the cementite of 70 area% or more over the entire wire.

The steel wire of the present invention basically has a C of 0.03
However, the specific chemical composition is as follows: Si: 0.004 to 0.5% by mass, Mn:
0.05 to 2% by mass and Al: 0.01 to 0.08% by mass, and P: 0.03% by mass or less (0%
), S: 0.03% by mass or less (including 0%) and N: 0.07% by mass or less (including 0%), respectively. Is as follows.

C: 0.03 to 0.8% by mass C is an essential element for imparting a desired strength to the wire.
For that purpose, it is necessary to contain at least 0.03% by mass or more. However, if the C content is excessive, an excessive increase in deformation resistance and a decrease in toughness are caused, so the upper limit is 0.8% by mass. The preferred lower limit of the C content is 0.2.
0% by mass, and a preferable upper limit is 0.60%.

Si: 0.004 to 0.5% by Mass Si is added as a deoxidizing agent in the steel making process, but for that purpose, it must be contained in an amount of 0.004% by mass or more.
However, when the Si content is excessive, the deformation resistance is significantly increased, so that the upper limit is 0.5% by mass. The preferred lower limit of the Si content is 0.01% by mass, and the preferred upper limit is 0.3% by mass.

Mn: 0.05 to 2% by mass Mn is an element necessary for fixing S, which is an impurity, to render it harmless and for adjusting the strength of the steel after the tempering treatment. . It is also effective for improving toughness. In order to exert these effects, Mn needs to be contained at least 0.05% by mass or more. However, if Mn is contained excessively, quenching properties are improved, and deformation resistance is reduced due to formation of bainite or the like as hot rolled. 2
It is necessary to be not more than mass%. The preferred lower limit of the Mn content is 0.4% by mass, and the preferred upper limit is 1.2% by mass.

Al: 0.01 to 0.08% by mass Al acts as a deoxidizing agent in the steel making process, and is necessary for fixing N as AlN to reduce solid solution N and reduce deformation resistance. It is. In order to exert such effects, it is necessary to contain 0.01% by mass or more. However, if Al is excessively contained, the cold workability is deteriorated as coarse AlN, so the upper limit is 0.08% by mass.

P: not more than 0.03% by mass (including 0%),
S: 0.03% by mass or less (including 0%) These elements segregate at the grain boundaries or exist as compound inclusions, and inhibit the cold workability.
It must be: It is to be noted that each of these elements is preferably set to 0.016% by mass or less.

N: 0.07% by mass or less (including 0%) Since N significantly increases deformation resistance, it must be suppressed to 0.07% or less in order to obtain good cold workability. . From such a viewpoint, the N content is preferably set to 0.010% by mass or less.

The basic chemical composition of the steel wire of the present invention is as described above, and the balance is composed of Fe and unavoidable impurities. In the steel wire of the present invention, if necessary, (1) Cr: 1.5 mass% or less (excluding 0%), Mo: 1.0 mass% or less (excluding 0%), Ni: 1.0 mass% or less (excluding 0%), and B: 0. At least one element selected from the group consisting of 0025% by mass or less (excluding 0%);
5% by mass or less (excluding 0%), V: 0.6% by mass or less (excluding 0%), Nb: 0.4% by mass or less (0%
) And at least one element selected from the group consisting of Zr: 0.3% by mass or less (excluding 0%), etc., is also effective, thereby further improving the properties of the steel wire. Can be improved. The reasons for limiting the range of these elements are as follows. The steel wire of the present invention has
In addition to these, it may contain a trace component that does not impair the properties of the wire, and such a steel wire is also included in the scope of the present invention.

Cr: 1.5% by mass or less (excluding 0%)
Mo): 1.0% by mass or less (excluding 0%), N
i: 1.0% by mass or less (excluding 0%) and B:
Group consisting of 0.0025% by mass or less (excluding 0%)
At least one element selected from the group consisting of Cr, Mo, Ni and B is a quenching adjusting element, and is added to adjust strength and toughness by quenching and tempering. However, an excessive content is not preferable because deformation resistance is increased. From such a viewpoint, Cr has an upper limit of 1.5% by mass, Mo and Ni have an upper limit of 1.0% by mass, and B has an upper limit of 0.0025% by mass. The effect of the addition of these elements increases as the content increases within the above range,
In order to exert the above-mentioned effects, Cr is 0.03% by mass or more, Mo and Ni are 0.01% by mass or more, and B is 0.03% by mass or more.
It is preferable to contain 003% by mass or more.

Ti: 0.5% by mass or less (excluding 0%)
V): 0.6% by mass or less (excluding 0%), N
b: 0.4% by mass or less (excluding 0%) and Zr:
Selected from the group consisting of 0.3% by mass or less (excluding 0%)
At least one of the elements Ti, V, Nb and Zr to be formed forms fine carbonitrides and is added for the purpose of adjusting the balance between the strength and ductility of the heat-treated material by precipitation hardening. However, an excessive content is not preferable because deformation resistance is increased. From these viewpoints, the upper limit is set to 0.2% by mass.
Note that the above effects due to the addition of these elements increase as the content increases within the above range. However, in order to exhibit the above effects, Ti is used in an amount of 0.015% by mass or more, V, N
It is preferable to contain b and Zr in an amount of 0.010% by mass or more.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention. It is included in the technical range of. In the following examples, the microstructure was determined according to the following procedure.

[Survey of Ferrite Particle Size]
The average grain section of ferrite was measured in the structures etched and observed with an optical microscope or SEM. The measurement was performed by observing an area of 100 μm × 250 μm in five or more visual fields and measuring the average.

[Investigation of the Cementite Cementite Phase] A photograph of the structure taken with a SEM at a magnification of 1000 times in the observation visual field of the cross section of the wire rod was subjected to image analysis. The area ratio of the divided cementite in the following portions was measured. The measurement was performed by observing five visual fields or more as in the ferrite particle size survey, and the average was obtained.

[How to determine the major axis and minor axis] The longest section of cementite was defined as the major axis, and the longest section in the direction perpendicular to the major axis was defined as the minor axis.

[Method of Determining Depth of Structure] At an arbitrary radius, observation is made from the surface toward the center in 5% increments, and the above-mentioned [Investigation of ferrite particle size] and [Investigation of fragmented cementite phase] are carried out. The average was determined. The depth from the surface satisfying any of the conditions of the ferrite grain size and the fragmented cementite was defined as the structure depth. At this time, when there was a tissue depth in the middle of the observed dark field, the median value was obtained by tilt average in each case, and the result was defined as the tissue depth (a region satisfying the tissue defined in the present invention).

[0051]

EXAMPLES [Example 1] Using a steel composition equivalent to JIS S30C, a rapid heating test with high frequency was performed to examine the effect on cold workability. That is, a steel wire rod having a chemical composition shown in Table 1 below and having a wire diameter of 16 mm was subjected to a drawing-high-frequency heating test, and a trial production of a fine ferrite + fractionated cementite structure was performed.

[0052]

[Table 1]

Table 2 below shows the trial production conditions and the obtained structures. The “repetition count” in Table 2 means (high-frequency heating →
(Cooling) is shown. The “texture depth” indicates the depth (% of the wire diameter) from the surface of the region where the divided cementite structure is obtained. 0% is not used at all, and 50% is obtained over the entire wire section. Means the state that is being done. Test No. No. 1 is a conventional example, in which ordinary spheroidizing treatment was performed. The conditions of the spheroidizing treatment were heating temperature: 740 ° C., and the entire spheroidizing time was 20 hours.

[0054]

[Table 2]

FIG. The evaluation results of cold workability (crack occurrence limit upsetting rate, tensile strength TS) are shown for samples 1 to 6 (conventional example and those having a structure depth of 50%). As is clear from the results, those satisfying the requirements specified in the present invention (Test Nos. 2 to 4) have not been subjected to the spheroidizing treatment, but have a T like that of the ordinary spheroidized material.
It has S, and there is an increase in the critical upsetting rate of 5% or more, and it can be seen that the cold workability is remarkably improved.

In Test No. where the tissue depth was less than 50%. For those of Nos. 7 to 9, spheroidizing treatment was performed for 2 hours. The evaluation results of the cold workability are shown in Test N
o. FIG. 2 shows the conventional spheroidizing process along with the conventional example 1 as shown in FIG. 2. As is clear from FIG. It can be seen that the cold workability is equivalent to that of the conventional example even in time, and that the processing time can be reduced.

Example 2 Next, in order to examine the influence of the chemical composition, various steel materials shown in Table 3 were prepared.
The tissue was adjusted by wire drawing and high frequency heating in the same manner as described above. At this time, the wire drawing rate was uniformly 5%, the number of repetitions was all four, and the maximum heating temperature was just below the Ac ₃ point in each sample. Sample (N
o. Regarding 1 to 15), those having a structure depth of 50% were subjected to the simple spheroidizing treatment for 2 hours, and those having a structure depth of less than 50% were examined for cold workability.
In addition, samples (N
o. Regarding the samples 16 to 29), the effect was confirmed by performing a simple spheroidizing treatment on all the samples.

[0058]

[Table 3]

Table 4 below shows the obtained structure and the results of the cold workability. As is clear from this table, the composition steel (sample Nos. 1 to 1) corresponding to the preferred chemical composition of the present invention.
In 5), the one where the organization depth becomes 50% remains as it is,
Those less than 50% are subjected to a simple spheroidizing treatment for 2 hours, so that any of the S30Cs shown as conventional examples is used.
It can be seen that the material has a cold workability equal to or higher than that of the spheroidized material.

On the other hand, in the steel composition (sample Nos. 16 to 29) which deviates from the preferred chemical composition of the present invention, the crack generation limit is still low and the deformation resistance is high even after the simple spheroidizing treatment is performed. You can see that it has become.

[0061]

[Table 4]

[0062]

The present invention is configured as described above, and the steel wire according to the present invention can greatly reduce the spheroidizing annealing treatment, and can completely omit the spheroidizing annealing if necessary. Become.

[Brief description of the drawings]

FIG. 1 is a table showing test Nos. It is a graph which shows the evaluation result of cold workability about 1-6.

FIG. 2 shows test Nos. It is a graph which shows the evaluation result of cold workability about 7-9.

Claims (4)

[Claims] 1. A steel wire rod containing C: 0.03 to 0.8% by mass, wherein the average particle diameter of ferrite is 2 to 5.5 μm and the major axis is 3 μm or less (major axis / minor axis). Characterized by the fact that the area in which the cementite having an aspect ratio of not more than 3 is 70% or more of the total cementite is 10% or more of the wire diameter from the surface, the steel having excellent cold workability. wire. 2. Si: 0.004 to 0.5% by mass, M
n: 0.05 to 2% by mass and Al: 0.01 to 0.0
8% by mass, P: 0.03% by mass or less (including 0%), S: 0.03% by mass or less (including 0%), and N: 0.07% by mass or less (0% The steel wire according to claim 1, wherein 3. Cr: 1.5% by mass or less (excluding 0%), Mo: 1.0% by mass or less (excluding 0%), N:
i: 1.0% by mass or less (excluding 0%) and B:
The steel wire according to claim 1, comprising at least one selected from the group consisting of 0.0025% by mass or less (not including 0%). 4. Ti: 0.5% by mass or less (excluding 0%), V: 0.6% by mass or less (excluding 0%), N
b: 0.4% by mass or less (excluding 0%) and Zr:
The steel wire according to any one of claims 1 to 3, wherein the steel wire comprises at least one selected from the group consisting of 0.3% by mass or less (not including 0%).