KR101560882B1 - A high-carbon steel wire having excellent drawability and twist property, and its manufacturing method - Google Patents

Abstract

There is provided a high carbon wire material excellent in freshness and torsion characteristics and a method of manufacturing the same.
The present invention relates to a steel sheet comprising, by weight, 0.5 to 0.7% of C, 1.5 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.003 to 0.05% of Sol.Al, 0.02% or less of P, 0.002 to 0.008% of N, 0.002% or less of O, and the balance of Fe and other unavoidable impurities. The microstructure has a pearlite content of 95% or more in an area fraction and less than 5% A ferrite decarburized layer is formed on the surface of the ferrite decarburized layer, and the ferrite constituting the ferrite decarburized layer is composed of a non-pearlite structure in which the <100> crystal in a direction parallel to the rolling direction of the wire rod occupies 48% And a high carbon wire material excellent in twist characteristics, and a method of manufacturing the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high carbon steel wire having excellent drawability and torsion characteristics,

More particularly, the present invention relates to a high-carbon wire rod having improved freshness and torsional characteristics by controlling the cooling rate in the course of manufacturing a wire rod, and a manufacturing method thereof will be.

The steel wire used for the bead wire is 0.6 wt% or more of high carbon steel, and has a product standard having a tensile strength of 1,600 MPa or more and a diameter of 0.8 mm or more in the final steel wire. Bead wire and the like are reinforcement materials used in tires together with tire cords. The manufacturing method of the steel wire such as the bead wire is generally performed by pickling and coating a wire rod having a diameter of 5.5 mm or more produced in a steel mill and then making it into a steel wire of 0.8 mm or more according to the tire type (PCR or TBR) Plating finishes.

It is a characteristic that a high carbon steel wire such as a bead wire should possess, and has a strength and a torsional characteristic. This is because it is possible to improve the stability of the tire due to the high strength, to reduce the weight of the tire, and because the torsion characteristic is required, not one single wire is used but a plurality of steel wires are twisted to produce a thread product.

The strengthening of the steel wire was achieved through microfabrication and increased throughput. This is because, according to the E-F formula proposed in the 1960s, the strength is greatly increased when the structure is pearlite and when the drawing is applied. That is, since the initial pearlite structure before the drawing is fine, it is a condition that the drawing can be sufficiently applied, and the strength of the final steel wire according to the initial strength increase is also increased. Another way to increase the throughput is to increase the strength effectively.

One of the ways to reduce pearlite interlayer spacing is to use alloying elements. A representative alloying element is C. When 0.1 wt% of C is added, the tensile strength of 80 MPa can be improved. In addition, it is known that Cr improves by about 40 MPa. However, the addition of the alloying element causes problems such as grain boundary formation of crushed stone cementite, formation of Cr-based carbide and increase of completion time, and the optimization for the operating condition should be reset.

In addition, it is the most effective method to increase the strength according to the drawing amount. This is because the randomly formed pearlite is aligned in the longitudinal direction (drawing amount (e): 0.5 level) as the drawing is applied, and the index tends to increase exponentially from this point. However, there is a limitation in increasing the strength through the drawing, because the reduction of the cross-sectional reduction rate occurs due to the freshness. That is, there is a time point at which the ductility greatly decreases, and at this time, delamination occurs (in other words, referred to as the freshness limit) for the heat. Therefore, it can be said that the steel wire or rope which is currently commercialized is produced as a product after the drawing is finished at the stage before the drawing limit. Therefore, the strengthening of the steel wire should not be achieved through the control of the main factors but through the combined action of various factors.

Factors affecting other freshness are in the tissue. Perlite basically consists of 80% or more of ferrite and 10 to 15% of cementite in an area fraction. Therefore, the overall directionality depends on ferrite. A conventional wire rod is subjected to a manufacturing process of "heating-rolling-cooling", in which the pearlite formed has a certain direction in the rolling direction, that is, the RD direction. However, no deformed texture is formed when drawing or drawing. Therefore, most ideal is to form pearlite aggregate structure with RD // <001> of 100%, and when such aggregate structure appears, the product which is not subjected to the subsequent heat treatment can improve the freshness.

Another factor that can affect the freshness is the frictional deformation formed by friction between the die and the workpiece. If friction deformation can be suppressed, the deformation stress value received by the material is lowered, which greatly increases the drawability or draft limit. This means that even if the same stress is applied, friction deformation may be relatively small when a soft tissue exists on the surface.

One of the prior arts which attempted to improve the physical properties of the wire rod by controlling the structure of the wire rod surface is disclosed in Korean Patent Laid-Open Publication No. 2013-0109182 entitled "Metal wire made of an iridium-containing alloy" (Patent Document 1) have. This manufacturing method is characterized in that the iridium alloy ingot is formed into a biaxially oriented wire rod in which the existence ratio of the irregular metal ingot is 50% or more through the biaxial pressing process in the presence of a texture having a crystal orientation with a preferred orientation in the <100> direction Thereby providing an iridium metal wire which is excellent in oxidation resistance at high temperatures and usable for spark plug electrodes.

Korean Patent Laid-Open Publication No. 2013-0109182: Metal wire made of an iridium-containing alloy (Applicant: Tanaka Kikinzoku Kogyo Kogyo Co., Ltd.)

The present invention has been developed as a result of a study to improve the physical properties through the control of the texture of the surface of the wire rod. The steel wire rod is formed by forming an equiaxed crystal ferrite layer on the surface of the wire rod by controlling the cooling rate after the wire rod rolling, The present invention provides a high carbon wire having excellent curvature and torsional properties by controlling the crystal orientation of the carbon nanofibers.

According to an aspect of the present invention,

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.5 to 0.7% of C, 1.5 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.003 to 0.05% of Sol.Al, 0.008%, O: 0.002% or less, the balance Fe and other unavoidable impurities,

The microstructure is composed of not less than 95% of pearlite in an area fraction, less than 5% of non-pearlite structure including protonic ferrite and martensite, and

A ferrite decarburized layer is formed on the surface of the ferrite decarburized layer, and the ferrite constituting the ferrite decarburized layer is a high carbon wire rod having excellent freshness and torsion characteristics in which the <100> crystal in a direction parallel to the rolling direction of the wire rod occupies 48% .

The ferrite decarburized layer is preferably formed at a depth of 70 mu m or more.

The ferrite decarburized layer preferably has an area fraction of 2.7% or more.

In addition, the ferrite constituting the ferrite decarburized layer preferably has an area fraction of 9% or more in the <110> crystal in the direction of twisting the rolling direction of the wire.

It is preferable that the ferrite constituting the ferrite decarburized layer has an area fraction of 3% or more in terms of crystal in the direction of twisting the rolling direction of the wire.

Further, according to the present invention,

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.5 to 0.7% of C, 1.5 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.003 to 0.05% of Sol.Al, 0.008%, O: 0.002% or less, the balance Fe and other unavoidable impurities;

A step of cooling the rolled wire rod in austenite single phase region at a cooling rate of 25 DEG C / s or more, followed by isothermal heat treatment so as to form a ferrite decarburized layer on its surface in a ferrite-austenite abnormal region; And

And finally cooling the isothermal heat-treated wire rod at a cooling rate of 20 ° C / s or more. The present invention also relates to a method of manufacturing a high-carbon wire rod having excellent drawability and torsional characteristics.

The isothermal heat treatment is preferably carried out in a temperature range of 715 to 744 占 폚.

The thickness of the formed ferrite decarburized layer is preferably 70 mu m or more.

The ferrite constituting the ferrite decarburized layer preferably has an area fraction of 48% or more in the <100> crystal direction parallel to the rolling direction of the wire rod.

In addition, the ferrite constituting the ferrite decarburized layer preferably has an area fraction of 9% or more in the <110> crystal in the direction of twisting the rolling direction of the wire.

It is preferable that the ferrite constituting the ferrite decarburized layer has an area fraction of 3% or more in terms of crystal in the direction of twisting the rolling direction of the wire.

According to the method of manufacturing a high-carbon wire rod having excellent freshness and torsional characteristics of the present invention and the high-carbon wire rod manufactured by the method of the present invention, the freshness limit can be increased without adding expensive alloying elements such as Cr and V High-strength, high-carbon wire can be produced without a new alloy design.

Particularly, the ferrite decarburized layer formed on the surface of the wire according to the present invention forms a uniform texture with respect to the rolling direction of the wire, thereby preventing propagation of cracks in the longitudinal direction of the wire at the time of twisting and preventing delamination It greatly improves the freshness limit.

1 is a schematic view showing a cooling condition of a wire rod forming a ferrite decarburized layer according to an embodiment of the present invention.
Fig. 2 is a schematic diagram showing an example of crystal orientation of the ferrite decarburized layer. Fig.

Hereinafter, the technical construction according to the present invention will be described in more detail with reference to various embodiments and the accompanying drawings.

First, the high carbon wire according to the present invention comprises, by weight, 0.5 to 0.7% of C, 1.5 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.003 to 0.05% of Sol.Al, 0.02% , S: not more than 0.02%, N: 0.002 to 0.008%, O: not more than 0.002%, and the balance Fe and other unavoidable impurities. Hereinafter, the critical significance of the characteristics and the composition range of each alloy element will be briefly described.

C (carbon): 0.5 to 0.7 wt%

C is a main element for securing strength, and when C is added, it mostly exists in the form of cementite. Increasing the carbon content has the effect of increasing the fraction of cementite and decreasing the pearlite interlayer spacing, which increases the strength. It is reported that when the carbon content is increased by 0.1 wt%, the strength increase effect of 80 MPa is reported. When the carbon content is less than 0.5% by weight, it is difficult to secure the strength in the present invention. When the carbon content is more than 0.70% by weight, the ferrite decarburized layer can not be forcibly formed in an ideal region (ferrite + austenite) of the present invention. Therefore, the content of C is preferably limited to 0.50 to 0.70% by weight.

Si (silicon): 1.5 to 2.5 wt%

Si is dissolved in a ferrite matrix to increase the strength by solid solution strengthening effect. It is known as an element having a very low solubility in cementite, in ferrite grain, ferrite / cementite grain boundary. In addition, since it enhances the activity of carbon, it plays a role of enabling rapid diffusion of carbon at a high temperature, thereby contributing to formation of a thick ferrite decarburized layer in an abnormal region. If the Si content is less than 1.5% by weight, the target decarburized layer can not be formed. If the Si content exceeds 2.5% by weight, the freshness deterioration and surface scale generation are increased.

Mn (manganese): 0.2 to 0.8 wt%

Mn is an austenite stabilizing element and added for securing entrapmentability. When Mn is added, it is combined with S to form an MnS inclusion, which is a soft inclusion but may cause a break in drawing. When the amount is less than 0.2% by weight, it is difficult to secure the entrapment property, so that the hardened cementite is formed in the grain boundaries. When the amount exceeds 0.8% by weight, the center segregation is significant.

Sol. Al (aluminum): 0.003 to 0.05 wt%

Al generates hard unstrained alumina-based nonmetallic inclusions and deteriorates ductility deterioration and freshness. Conversely, when AlN is formed by bonding with nitrogen, the austenite grain size can be miniaturized, have. If Al is added in excess, it reacts with O in the steel to form a hard inclusion. Therefore, it is preferable that Al does not exceed 0.05% by weight so as to stably form AlN. On the other hand, in order to obtain an AlN effect, it is preferable to include at least 0.003 wt% or more.

P (phosphorus), S (sulfur): 0.02 wt% or less

The lower the content of P and S is, the better the impurity content is. However, if the content is too restrictive, the cost of removing impurities in the steelmaking process increases. In the present invention, P and S are respectively controlled to 0.02% or less from the viewpoint of securing ductility as in the conventional steel wire.

N (nitrogen): 0.002 to 0.008 wt%

N is an element which is adhered to a potential formed on a ferrite base during drawing to cause age hardening. And B and other nitrides are produced, and the effect of making the austenite grains finer is obtained. Also, N in the steel is combined with Al to form AlN, which is pinned to the austenitic grain boundary during heating and rolling, and has the effect of refining austenite grains. There is no great effect when it is added at less than 0.002% in the steel, and it is not included at more than 0.008%, so it is preferable to control the content to 0.002 to 0.008% by weight.

O (oxygen): 0.001 to 0.002 wt%

O can form a composite inclusion with Si or the like, thereby forming a soft inclusion that does not adversely affect the drawing characteristics. If the content is less than 0.001% by weight, it is difficult to control. When the content is more than 0.002% by weight, hard inclusions are formed to deteriorate the freshness characteristics. Therefore, the content thereof is preferably controlled to 0.001 to 0.002% by weight.

The high carbon wire according to the present invention preferably has a non-pearlite structure including a pro-eutectoid ferrite, a cobalt cementite and a bainite in an area fraction of 95% or more of pearlite and 5% or less of a microstructure after rolling. The microstructure of the wire is controlled to have a pearlite structure with an area fraction of 95% or more. Unlike ferrites and other dual phases, the pearlite structure exhibits an exponential increase in strength It is because it is visible.

In the high carbon wire of the present invention, the ferrite decarburized layer is formed on the surface thereof, and the ferrite constituting the ferrite decarburized layer occupies 48% or more of the area fraction of the <100> crystal in the direction parallel to the rolling direction of the wire rod Is required.

The high carbon wire according to the present invention forms an aggregate structure in which crystal directions of the ferrite decarburized layer formed on the surface thereof are arranged in a predetermined direction. More specifically, the ferrite constituting the ferrite decarburized layer occupies 48% or more of the area fraction of the <100> crystal in a direction parallel to the rolling direction of the wire rod. Thus, the ferrite decarburized layer can effectively prevent cracks from being generated and propagated in the longitudinal direction of the wire when the torsional external force is applied, because the crystal orientation of the ferrite decarburized layer occupies a high area fraction of crystals parallel to the rolling direction of the wire rods

Preferably, the <110> crystal in a direction twisted with the rolling direction of the wire material occupies 9% or more in an area fraction, and the crystal in a direction twisted with the rolling direction of the wire material is not less than 3% It is.

At this time, in order to obtain a sufficient crack propagation preventing effect, it is preferable that the ferrite decarburized layer occupies 2.7% or more of an area fraction of the entire wire rods, and the depth is controlled to be 70 탆 or more.

Next, a method for producing the high carbon wire material of the present invention will be described.

The present invention relates to a ferritic stainless steel comprising, by weight, 0.5 to 0.7% of C, 1.5 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.003 to 0.05% of Sol.Al, 0.02% , N: 0.002 to 0.008%, O: 0.002% or less, the balance Fe and other unavoidable impurities, and then subjected to billet-ingot welding, followed by heating and wire rolling. The wire rod rolling conditions are conventional, and for example, under the conditions of wire rod rolling, hot wire rolling can be carried out in a temperature range of 950 to 1050 ° C after heating in a heating furnace at 1000 to 1100 ° C.

Next, in the present invention, the rolled wire is cooled in austenite single phase region at a cooling rate of 25 DEG C / s or more, and isothermalized so as to form a ferrite decarburized layer on its surface in a ferrite-austenite abnormal region.

FIG. 1 is a schematic view showing an example of a cooling condition of a wire rod for forming a ferrite decarburized layer according to the present invention, which is an example of a wire rod containing carbon content of 0.62 wt%.

First, cooling is performed at a cooling rate of 25 ° C / s or higher from the L / H temperature to the austenite single phase region after the final wire rolling is completed within the range of 750 to 850 ° C in the range of the L / H temperature (Laying Head Temp) do. If the L / H temperature is lower than 750 ° C, pro-eutectoid ferrite is formed in the pearlite grain boundaries, and a desired structure can not be obtained. If the L / H temperature is higher than 850 ° C, productivity is lowered and desired strength can not be obtained. Given this L / H temperature range, the cooling section in this austenite single phase region can range from 850 ° C to the Ar transformation temperature.

Further, controlling the cooling rate in the austenite single phase region (delta region) to 25 DEG C / s or more lowers the carbon diffusion rate and facilitates the formation of the ferrite decarburized layer in a subsequent isothermal heat treatment process For example. The maximum value of the cooling rate in this austenite single phase region can be controlled to a temperature range where it is difficult to obtain the pearlite area fraction according to the present invention due to the martensite structure produced by quenching.

The wire passed through the austenite single phase region under the above cooling conditions is isothermalized in the ferrite-austenite anomaly region (? -? Region) for 10 minutes or more, for example. The isothermal heat treatment may be carried out at a temperature range immediately above the A1 transformation temperature at which the pearlite transformation starts just below the Ar transformation temperature at which the ferrite-austenite anomaly starts. During this annealing process, a ferrite decarburized layer is formed in a predetermined crystal orientation on the surface of the wire as shown in Fig. The direction of the determination is as described above.

During the isothermal heat treatment in the ferrite-austenite abnormal region, the ferrite layer is formed because the surface of the wire is first cooled in the ideal region region. However, most of the pro-eutectoid ferrite decarburized layer is formed on the surface of the wire because the center portion of the wire still remains in the austenite single-phase region. Thus, the ferrite decarburized layer formed on the surface of the wire in a certain crystal orientation prevents propagation of cracks formed on the surface when a torsional external force is applied to the wire, thereby improving the drawing limit of the wire rod.

Preferably, the isothermal heat treatment in the ferritic-austenitic region is carried out at a temperature between 715 and 744 ° C. If the isothermal annealing temperature is less than 715 ° C, the desired surface wire structure can not be obtained, whereas if the isothermal annealing temperature exceeds 744 ° C, the obtained structure is randomly formed on the surface and has no directionality.

At this time, in the present invention, it is preferable that the ferrite decarburized layer is formed at a depth of 70 μm or more on the surface portion of the wire rod. If the depth of the ferrite decarburized layer is less than 70 mu m, the above-mentioned surface crack propagation preventing effect is insufficient and the twist characteristics of the wire rod can not be improved. To this end, the isothermal annealing in the ferritic-austenitic region is preferably controlled to be continued for at least 10 minutes.

Finally, in the present invention, the isothermal annealed wire rod is finally cooled at a cooling rate of 20 DEG C / s or more. The pearlite structure is formed through the cooling. When the cooling rate is less than 20 ° C, the spacing between the pearlite layers becomes too large to obtain the required high strength. The faster the cooling rate is, the better it is, but it is believed that there is a normal limit of the manufacturing process.

The high carbon wire material of the present invention produced under the above-mentioned process conditions is composed of non-pearlite structure including a pro-eutectoid ferrite and martensite with a pearlite content of 95% or more in an area fraction thereof and less than 5%.

Further, a ferrite decarburized layer is formed on the surface of the ferrite decarburized layer, and the ferrite decarburized layer has an aggregate structure arranged in a predetermined direction with respect to the rolling direction. More specifically, the ferrite constituting the ferrite decarburized layer occupies 48% or more of the area fraction of the <100> crystal in a direction parallel to the rolling direction of the wire rod. Further, the <110> crystal in a direction twisted with the rolling direction of the wire material occupies more than 9% in an area fraction, and the crystal in a direction twisted with the rolling direction of the wire material preferably occupies not less than 3% .

Thus, the ferrite decarburized layer can effectively prevent cracks from being generated and propagated in the longitudinal direction of the wire when the torsional external force is applied, because the crystal orientation of the ferrite decarburized layer occupies a high area fraction of crystals parallel to the rolling direction of the wire rods .

At this time, in order to obtain a sufficient crack propagation preventing effect, it is preferable that the ferrite decarburized layer occupies 2.7% or more of an area fraction of the entire wire rods, and the depth is controlled to be 70 탆 or more.

Hereinafter, a specific embodiment according to the present invention will be described with reference to Figs. 1 and 2. Fig. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred from them.

 (Example)

In the present invention, the steel sheet has a composition of 0.62% of C, 2.0% of Si, 0.5% of Mn, 0.04% of Sol.Al, 0.0060% of N, 0.0018% of O, 0.015% of P and 0.012% of S Ingot was cast and billet-ingot welded, followed by wire rolling at a heating furnace temperature of 1050 ° C and a hot rolling temperature of 1000 ° C. The cooling conditions for forming the ferrite decarburized layer on the surface are the same as in Fig. First, the L / H temperature was set to 800 ° C or higher, the cooling was performed at a cooling rate of 25 ° C / s in the cooling period of 800 ° C to 725 ° C, and the pearlite was formed in the abnormal region (715 ° C to 744 ° C) Isothermal heat treatment at 715 DEG C for 10 minutes or longer, more specifically at 725 DEG C for 10 minutes or more. A surface ferrite decarburization layer of 70 mu m or more was formed on the surface of the wire by this isothermal heat treatment, and then a wire rod having an interval of pearlite layers of 100 to 200 mu m was produced at a cooling rate of 20 DEG C. At this time, the tensile strength (T / S) is 1015 MPa, the section reduction ratio (RA) is 40% or more, more specifically, 48%, more specifically, minimum RA is 38% and maximum RA is 52%. 95% or more is a pearlite structure, and the remaining 5% or less is a non-pearlite structure (low-temperature structure such as pro-eutectoid ferrite and martensite).

The thickness of the surface portion ferrite decarburized layer observed in the wire rod state is 75 μm, and the area fraction is 2.7%. The ferrite decarburized layer was confirmed to be equiaxed, and its direction was observed from EBSD. EBSD specimen preparation and analysis conditions were as follows: 2000 -> The specimens were polished to 1 μm with a diamond suspension. The specimens were polished by 0.5 μm alumina for 30 minutes or more in order to suppress damage during polishing. The EBSD analysis condition was set to a tolerance angle of 10 ° or more and a direction parallel to the rolling direction RD was confirmed.

 As a result of the EBSD analysis, ferrites formed to have an equiaxed structure on the surface were found to have a <100> crystal orientation of more than 48% in the direction parallel to the rolling direction, 9% of the <110> crystal in the twisted direction, and 3% Respectively. A schematic diagram thereof is shown in Fig.

The wire rods were pickled and dry-drawn using a draw bench. At this time, the drawing conditions were 20% reduction per pass, total reduction amount: 97.2%, drawing speed 0.03 m / s, The Mo-based spray was used as the lubricant. The mechanical properties of the manufactured wire and wire are shown in Table 1 below. Comparative Example 1 is the same as that of Example 1 except that the initial cooling is at 25 ° C / s and the temperature from the end of the transformation to 400 ° C is not more than 20 ° C / s Low cooling rate.

Wire rod Final liner Freshness limit T / S
(MPa) RA
(%) Total reduction amount
(%) The tensile strength
(MPa) torsion
(time) Total reduction amount
(%) The tensile strength
(MPa) torsion
(time) Example 1 1,175 41 97.2 1764 36 97.7 1847 22 Comparative Example 1 1,168 39 97.2 1758 27 97.2 X X

97.2% is one of the typical total amount of reduction applied to the product used for the bead wire. Since the line speed of the seal line is larger than the drawing speed used in this experiment, it is lower than the tensile strength of the product by 100 MPa or more, and this value does not represent the specification of the actual product. The wire rods TS and RA of Example 1 and Comparative Example 1 are at a level of 1170 MPa and the level of RA is 40%. The tensile strength of each final steel wire is similar to that of 1760 MPa, but in the torsion test (100D, where D is the final steel wire diameter), 36 tests were conducted in Example 1 and 27 tests were conducted in Comparative Example 1 Example 1 was confirmed to have a twist characteristic or ductility improvement of 25% level as compared with Comparative Example 1. [

Table 1 also shows the results of experiments up to the limit of freshness in which delamination, which is a phenomenon that cracks propagate in the longitudinal direction at the time of twisting, is destroyed at the initial stage. In the case of Example 1, it was confirmed that delamination occurred in the case of Comparative Example 1 in which the addition amount of 97% was more than 97.2%. This means that no further drawing can be applied. When fresh to 97.7%, the T / S was 1840MPa or more and the number of twist was 22.

It was confirmed that the surface decarburization layer was formed to improve the freshness by having <100> parallel to the RD direction, <110> slightly deviated therefrom, and the like.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (9)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.5 to 0.7% of C, 1.5 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.003 to 0.05% of Sol.Al, 0.008%, O: 0.002% or less, the balance Fe and other unavoidable impurities,
The microstructure is composed of not less than 95% of pearlite in an area fraction, less than 5% of non-pearlite structure including protonic ferrite and martensite, and
A ferrite decarburized layer is formed on the surface of the ferrite decarburized layer, the ferrite decarburized layer is formed at a depth of 70 mu m or more, and the ferrite constituting the ferrite decarburized layer has an area fraction of 48 %, Which is excellent in the freshness and torsion characteristics.
The high carbon wire rod according to claim 1, wherein the ferrite constituting the ferrite decarburized layer has an area fraction of 9% or more of crystals in a direction twisted with the rolling direction of the wire rod.
The high-carbon wire rod according to claim 1, wherein the ferrite constituting the ferrite decarburized layer has an area fraction of 3% or more in the direction of twisting the rolled wire.
delete The high-carbon wire rod according to claim 1, wherein the ferrite decarburized layer has an area fraction of 2.7% or more in the entire cross section of the wire rod.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.5 to 0.7% of C, 1.5 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.003 to 0.05% of Sol.Al, 0.008%, O: 0.002% or less, the balance Fe and other unavoidable impurities;
Subjecting the rolled wire material to a isothermal heat treatment so as to form a ferrite decarburized layer having a thickness of 70 mu m or more on the surface thereof in a ferrite-austenite abnormal region after cooling at a cooling rate of 25 DEG C / s or more in austenite single phase region; And
And finally cooling the isothermal heat-treated wire rod at a cooling rate of 20 ° C / s or more to produce a high-carbon wire rod having excellent drawability and torsional characteristics.
The method of claim 6, wherein the isothermal heat treatment is performed at a temperature ranging from 715 to 744 占 폚.
delete The ferrite structure according to claim 6, wherein the ferrite constituting the ferrite decarburized layer has an area fraction of at least 48% in a direction parallel to the rolling direction of the wire, Characterized in that the crystal has an area fraction of 9% or more and an area fraction of crystals in a direction twisted with the rolling direction of the wire is 3% or more.

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