KR102042062B1 - Steel wire rod for cold forging and methods for manufacturing thereof - Google Patents

Abstract

The present invention provides a wire rod and a method of manufacturing the same that can ensure excellent cold workability even if the spherical annealing heat treatment is omitted.
Cold-rolled wire rod according to an embodiment of the present invention in weight%, C: less than 0.01 to 0.15%, Si: 0.3% or less, Mn: 0.2 to 0.75%, P: 0.03% or less, S: 0.03% or less, sol. Al: 0.01 to 0.05%, Cr: 0.5% or less, Ti: 0.005 to 0.05%, N: 0.01% or less, the remainder contains Fe and other impurities, satisfies the following formula (1), and the crystal orientation difference is 15 ° The average particle diameter of the ferrite surrounded by the above ferrite grain boundaries is 15 to 40 µm.
{(A / 4) + [(Bx30) / (2A + 40)]} x (C / 3) ≥ 700 ...... Equation (1)
Here, A is the ferrite grain boundary average circle equivalent diameter (µm) with a crystal orientation difference of 15 ° or more, B is the area fraction (area%) occupied by the ferrite grain grains with a crystal orientation difference of 15 ° or more with respect to the entire ferrite phase, and C is a ferrite Mean hardness (Hv) of the phase.

Description

Wire rod for cold pressing and its manufacturing method {STEEL WIRE ROD FOR COLD FORGING AND METHODS FOR MANUFACTURING THEREOF}

The present invention relates to a cold-rolling wire and a method for manufacturing the same, and more particularly to a cold-rolling wire and suitable for use as a material for automobiles or machine parts.

Common wire products are manufactured as final products by hot rolled wire, cold drawn, spheroidized heat treatment, cold drawn, cold rolled, quenched and considered.

The cold working method has excellent productivity compared to the hot working method and the mechanical cutting method, and is widely used as a material for automobile parts or mechanical parts such as bolts and nuts because of the great effect of reducing the heat treatment cost.

However, in order to manufacture mechanical parts through cold working, deformation resistance during cold working needs to be low and ductility needs to be excellent. If the deformation resistance of the steel is high, the life of the tool used in cold work is reduced, and if the ductility of the steel is low, it is likely to cause cracks during cold work, which may cause defective products.

In the case of cold working wire rods, spheroidizing annealing heat treatment before cold working. In the case of the spheroidizing annealing heat treatment, the steel is softened so that the deformation resistance is reduced, and the ductility is improved, thereby improving the cold workability. However, when the spheroidizing annealing heat treatment is performed, an additional cost is generated and the manufacturing efficiency is lowered, and thus, it is necessary to develop a wire rod which can secure excellent cold workability without additional spheroidizing annealing heat treatment.

The present invention is to provide a wire rod and a method for manufacturing the same that can ensure excellent cold workability even if the spheroidized annealing heat treatment is omitted.

Cold-rolled wire rod according to an embodiment of the present invention in weight%, C: less than 0.01 to 0.15%, Si: 0.3% or less, Mn: 0.2 to 0.75%, P: 0.03% or less, S: 0.03% or less, sol. Al: 0.01 to 0.05%, Cr: 0.5% or less, Ti: 0.005 to 0.05%, N: 0.01% or less, the remainder contains Fe and other impurities, satisfies the following formula (1), and the crystal orientation difference is 15 ° The average particle diameter of the ferrite surrounded by the above ferrite grain boundaries is 15 to 40 µm.

{(A / 4) + [(Bx30) / (2A + 40)]} x (C / 3) ≥ 700 ...... (1)

Here, A is the ferrite grain boundary average circle equivalent diameter (µm) with a crystal orientation difference of 15 ° or more, B is the area fraction (area%) occupied by the ferrite grain grains with a crystal orientation difference of 15 ° or more with respect to the entire ferrite phase, and C is a ferrite Mean hardness (Hv) of the phase.

In addition, according to an embodiment of the present invention, B: 0.006% or less may be further included, and the following Equation (2) may be satisfied.

0≤ [0.31Ti + 1.4B-N] ≤0.004 ...... (2)

In addition, according to an embodiment of the present invention, the following formula (3) can be satisfied.

0.8≤ [(Si + Mn + Cr) / 10C + Ti / C] ≤2.3 ....... Formula (3)

In addition, according to an embodiment of the present invention, the microstructure is composed of pearlite and ferrite, the fraction occupied by ferrite as an area fraction may be 80% or more.

According to another embodiment of the present invention, a method for manufacturing a cold-rolled wire rod according to an embodiment of the present invention is wt%, C: less than 0.01 to 0.15%, Si: 0.3% or less, Mn: 0.2 to 0.75%, P: 0.03% or less, S: 0.03% or less, sol.Al: 0.01 to 0.05%, Cr: 0.5% or less, Ti: 0.005 to 0.05%, B: 0.006% or less, N: 0.01% or less, the remainder includes Fe and other unavoidable impurities, and The content of the component is a step of heating a billet satisfying the following formula (1) and formula (2), hot rolling the heated billet at the finish rolling temperature of 920 to 1020 ℃, and take the hot rolled wire rod, Cooling.

0.8≤ [(Si + Mn + Cr) / 10C + Ti / C] ≤2.3 ... (1)

0≤ [0.31Ti + 1.4B-N] ≤0.004 ...... Equation (2)

In addition, according to one embodiment of the present invention, the cooling step is the first cooling to cool at a cooling rate of less than 5 ℃ / s to less than 20 ℃ / s to the temperature range of less than the winding temperature in the hot rolling below the finish rolling temperature It may include a step.

In addition, according to one embodiment of the present invention, the cooling step is a second cooling step of cooling at a cooling rate of less than 2 ℃ / s to less than 5 ℃ / s from the winding temperature to less than 750 ℃ after the first cooling step It may further include.

In addition, according to an embodiment of the present invention, the cooling step further includes a third cooling step of cooling at a cooling rate of less than 1 ℃ / S to less than 2 ℃ / s from 750 ℃ to less than 650 ℃ after the second cooling step. can do.

In addition, according to an embodiment of the present invention, the cooling step is a fourth cooling step of cooling at a cooling rate of less than 0.8 ℃ / s (excluding 0 ℃ / s) from 650 ℃ to less than 400 ℃ after the third cooling step It may further include.

In addition, according to an embodiment of the present invention, the heating temperature in the step of heating the billet may be 1000 to 1150 ℃.

In addition, according to an embodiment of the present invention, the winding temperature in the winding step may be 800 to 880 ℃.

The wire rod according to an embodiment of the present invention and a method of manufacturing the same can provide a wire rod that can suppress the deformation resistance during cold working even if the spheroidized annealing heat treatment is omitted.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are presented to sufficiently convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein but may be embodied in other forms. The drawings may omit illustrations of parts not related to the description in order to clarify the present invention, and may be exaggerated to some extent in order to facilitate understanding.

Cold-rolled wire rod according to an embodiment of the present invention, in weight%, C: less than 0.01 to 0.15%, Si: 0.3% or less (excluding 0), Mn: 0.2 to 0.75%, P: 0.03% or less, S: 0.03% or less, sol. Al: 0.01-0.05%, Cr: 0.5% or less, Ti: 0.005-0.05%, N: 0.01% or less, the rest contains Fe and other impurities, and satisfies the following formula (1).

{(A / 4) + [(Bx30) / (2A + 40)]} x (C / 3) ≥ 700 ...... Equation (1)

Here, A is the ferrite grain boundary average circle equivalent diameter (µm) with a crystal orientation difference of 15 ° or more, B is the area fraction (area%) occupied by the ferrite grain grains with a crystal orientation difference of 15 ° or more with respect to the entire ferrite phase, and C is a ferrite Mean hardness (Hv) of the phase.

In addition, the cold-rolled wire rod according to an embodiment of the present invention further includes B: 0.006% or less, and may satisfy the following Equation (2).

0≤ [0.31Ti + 1.4B-N] ≤0.004 ...... Equation (2)

In addition, the cold-rolled wire rod according to an embodiment of the present invention may additionally satisfy the following formula (3).

0.8≤ [(Si + Mn + Cr) / 10C + Ti / C] ≤2.3 ....... Formula (3)

Hereinafter, the role and content of each component included in the cold pressing wire rod according to the present invention will be described. % For the following components means% by weight.

The content of C (carbon) is 0.01 to less than 0.15%.

C is an element that improves the strength of the wire rod. To improve the strength of the wire rod, C should be included 0.01% or more, more preferably 0.03% or more. However, if the content is excessive, the deformation resistance of the steel is rapidly increased, thereby deteriorating cold forging. Therefore, the upper limit of the C content is 0.14%. As for the upper limit of C content, it is more preferable that it is 0.12%.

The content of Si (silicon) is 0.3% or less.

Si is an element useful as a deoxidizer. However, if the content is excessive, the deformation resistance of the steel is rapidly increased by solid solution strengthening, which causes the cold forging property to deteriorate. Accordingly, the upper limit of the Si content is 0.3%. More preferably, the upper limit of Si content may be 0.2%.

The content of Mn (manganese) is 0.2 to 0.75%.

Mn is an element useful as a deoxidizer and a desulfurization agent. Mn is preferably contained at least 0.2% for this effect. More preferably 0.3% or more may be included. However, when the content of Mn is excessive, the strength of the steel itself is too high, so the deformation resistance of the steel is rapidly increased, which may cause cold forging. The upper limit of the Mn content is 0.75%. Preferably the upper limit of the Mn content may be 0.7%.

The content of P (phosphorus) is less than 0.03%.

P is an inevitable impurity, and is an element that is the main cause of segregation in the crystal grains to lower the toughness of the steel and reduce the delayed fracture resistance. It is therefore desirable to control the content as low as possible. In theory, the content of P is advantageously controlled to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the content of P is made 0.03%.

The content of S (sulfur) is 0.03% or less.

S is an unavoidable impurity and is an element that segregates at grain boundaries to reduce ductility of steel, forms an emulsion in steel, and deteriorates delayed fracture resistance and stress relaxation characteristics. It is therefore desirable to control the content as low as possible. In theory, it is advantageous to control the content of S to 0%, but it is inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the content of S is made 0.03%.

Sol. The content of Al (soluble aluminum) is 0.01 to 0.05% or less.

Al is an element useful as a deoxidizer. In order to have such an effect in the present invention, Al should be included 0.01% or more. In addition, preferably Al may be included 0.015% or more. More preferably Al may be included 0.02% or more. However, when the content is excessive, the austenitic grain refining effect is increased by AlN formation, and thus cold forging property may be lowered. Therefore, in the present invention, the upper limit of the Al content is 0.05%.

The content of Cr (chromium) is 0.5% or less.

Cr serves to promote ferrite and pearlite transformation during hot rolling. Moreover, carbides in steel are precipitated to reduce the amount of solid solution carbon without increasing the strength of the steel itself more than necessary, thereby contributing to the reduction of the dynamic strain aging by the solid solution carbon. However, when the content is excessive, the strength of the steel itself is excessively high, so the deformation resistance of the steel is rapidly increased, which may cause cold forging. Therefore, the upper limit of Cr content is made into 0.5%. The content of Cr may preferably be 0.4% or less.

The content of Ti (titanium) is 0.005 to 0.05%.

Ti is a carbonitride-forming element and when Ti is included in steel, it advantageously acts to fix C and N, which may advantageously act in cold forging. However, when a large amount of fine Ti (C, N) precipitate is precipitated, the strength of the matrix due to precipitation strengthening is rapidly increased, which may worsen cold forging. Its content, size and distribution should be controlled accordingly. If the Ti content is less than 0.005%, the C and N fixing effects are insufficient. On the contrary, when the Ti content exceeds 0.05%, a large amount of Ti precipitates are formed. Thus, in the present invention, the content of Ti is made 0.005 to 0.05%. Also, preferably, the content of Ti may be 0.005 to 0.03%.

The content of N (nitrogen) is 0.01% or less.

N is an inevitable impurity, and if the content is excessive, the amount of solid solution nitrogen increases, so that the deformation resistance of the steel is rapidly increased, which may deteriorate cold forging. In theory, the content of N is advantageously controlled to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, the upper limit of the content of N in the present invention is 0.01%. Preferably the upper limit of the content of N may be 0.008%. More preferably, the upper limit of the content of N may be 0.007%.

The content of B (boron) is 0.006% or less.

B is a nitride forming element, and when B is included in steel, it is advantageous to fix the solid solution N, which may be advantageous to cold forging. However, when used in combination with Ti, the effect may disappear, so it must be used in combination properly. In addition, when the content of B is excessive, the upper limit should be controlled since the formation of grain boundary BN precipitates may adversely affect the ductility of the steel. The upper limit of the B content is 0.006%. Preferably the upper limit of the B content may be 0.005%. More preferably, the upper limit of the B content may be 0.004%.

According to one embodiment of the invention, the content of Ti, B, N may satisfy the following formula (2).

0≤ [0.31Ti + 1.4B-N] ≤0.004 ...... Equation (2)

If the value of Equation (2) is less than 0, the deformation resistance of the steel may increase rapidly, and the cold forging may deteriorate. If the content of Formula (2) exceeds 0.004, the precipitate may be precipitated excessively, and the ductility of the steel may be weakened. Accordingly, according to one embodiment of the present invention, the value of Equation (2) is 0 to 0.04.

According to one embodiment of the present invention, the content of Si, Mn, Cr, C, Ti satisfies Equation (3) below.

0.8≤ [(Si + Mn + Cr) / 10C + Ti / C] ≤2.3 ...... (3)

If the value of Equation (3) is less than 0.8, the content of C may be excessive and cold forging may deteriorate. On the contrary, when the value of Equation (3) exceeds 2.3, the deformation resistance of the steel may increase rapidly and the cold forging property may deteriorate. Therefore, the value of Formula (3) shall be 0.8-2.3.

The wire rod according to an embodiment of the present invention may include ferrite and pearlite as a microstructure. The fraction of ferrite in the area fraction may be 80% or more (excluding 100%). Perlite may be included in an area fraction of 20% or less (excluding 0%). Such a structure has an advantage of securing excellent cold workability and securing strength after drawing.

In addition, according to an embodiment of the present invention, the average particle diameter of the ferrite may be 15 to 40㎛. Preferably the average particle diameter of the ferrite may be 20 to 35 ㎛. If the average particle diameter of the ferrite is less than 15 μm, the strength may increase due to the refinement of grain boundaries, and thus cold forging may be reduced. On the contrary, even when the average particle diameter of the ferrite exceeds 40 µm, the strength may decrease.

The above-mentioned average particle diameter means the average circular equivalent diameter of the particle | grains detected by observing the longitudinal cross section of the wire rod.

In addition, the microstructure of the wire rod according to an embodiment of the present invention may satisfy the following formula (3). In the present invention, through further experiments, the ferrite grain boundary average circular equivalent diameter (A) with a crystal orientation difference of 15 ° or more, the ratio (B) of ferrite grain boundary grains with a crystal orientation difference of 15 ° or more, and the average hardness (C) of the ferrite phase are represented by the following equation (3): ), It was found that cold forging was improved.

{(A / 4) + [(Bx30) / (2A + 40)]} x (C / 3) ≥ 700 ...... Equation (3)

If the value of Equation (3) is less than 700, cold forging may decrease.

The mean hardness value C means (Hv, 1 / 2d + Hv, 1 / 4d) / 2, each of Hv, 1 / 2d and Hv, 1 / 4d from the surface of the wire rod in a cross section perpendicular to the longitudinal direction of the wire rod. The hardness of the wire rod measured at the 1 / 2d position and the 1 / 4d position in the diameter (d) direction of the wire rod.

Hereinafter, a method of manufacturing a cold-rolled wire rod according to an embodiment of the present invention will be described in detail.

The steel piece which satisfy | fills the component system mentioned above is heated. At this time, the heating temperature may be 1000 to 1150 ℃. The heating temperature may preferably be 1030 to 1130 ° C. If the heating temperature is less than 1000 ° C., the hot deformation resistance may increase, leading to a decrease in productivity. On the contrary, when the heating temperature exceeds 1150 ° C., ferrite grains may coarsen and ductility may decrease.

After that, hot rolling is performed to obtain a wire rod. At this time, the finish rolling temperature may be 920 to 1020 ℃. Preferably the finish rolling temperature may be 930 to 1000 ℃. When the finish rolling temperature is less than 920 ° C, the deformation resistance may increase due to the increase in strength due to the refinement of the ferrite grains. On the contrary, when the finish rolling temperature exceeds 1020 ° C, the ferrite grains may be excessively coarsened and ductility may decrease.

Thereafter, the wire rod is wound and cooled. At this time, the winding temperature of the wire rod may be 800 to 880 ℃. Preferably the winding temperature of the wire rod may be 820 to 860 ℃. When the coiling temperature is less than 800 ° C., martensite generated during cooling may not be recovered by recuperation, and some martensite may be formed to form hard and soft steel, which may lower cold forging. On the contrary, when the coiling temperature exceeds 880 ° C, a thick scale may be formed on the surface, which may cause a problem during descaling, and the cooling time may increase, resulting in a decrease in productivity.

Cooling may be sequentially cooled as follows.

The cooling step may include a first cooling step of cooling at a cooling rate of 5 ° C./s to less than 20 ° C./s from the hot rolling to a temperature range below the winding temperature. The first cooling step may proceed with winding.

After the first cooling step may include a second cooling step of cooling at a cooling rate of less than 2 ℃ / 5 ℃ / s to less than 750 ℃ at the winding temperature.

After the second cooling step may include a third cooling step of cooling at a cooling rate of less than 1 ℃ / s to less than 2 ℃ / s from 750 ℃ to 650 ℃.

After the third cooling step may include a fourth cooling step of cooling at a cooling rate of less than 0.8 ℃ / s (excluding 0 ℃ / s) from 600 ℃ to less than 400 ℃.

Hereinafter, the present invention will be described in detail with reference to examples, but the following examples are provided to illustrate the present invention in more detail, but the scope of the present invention is not limited to these examples.

Example

The steel strip having the composition shown in Table 1 was heated at 1100 ° C. for 3 hours, and then hot rolled to Φ 25 mm to prepare a wire rod. At this time, the finish rolling temperature was set at 950 degreeC, and the rolling ratio was made constant at 80%. Subsequently, the wire is manufactured by cooling and winding to 850 ° C. in the first cooling step CR1, and cooling the second cooling step CR2, the third cooling step CR3, and the fourth cooling step CR4. It was. The cooling progress is shown in the following [Table 2].

Steel grade Alloy composition (% by weight) C Si Mn P S Cr Al Ti B N Formula (1) Formula (2) Inventive Steel 1 0.04 0.08 0.53 0.009 0.007 - 0.025 0.025 - 0.0055 2.15 0.0022 Inventive Steel 2 0.07 0.10 0.65 0.010 0.008 - 0.036 0.014 0.0015 0.0048 1.27 0.0016 Invention Steel 3 0.11 0.11 0.74 0.011 0.005 0.31 0.033 0.022 - 0.0037 1.25 0.0031 Inventive Steel 4 0.14 0.12 0.68 0.012 0.006 0.48 0.042 0.007 0.0023 0.0042 0.96 0.0011 Comparative Steel 1 0.08 0.10 0.37 0.010 0.004 - 0.028 0.018 - 0.0058 0.81 -0.0002 Comparative Steel 2 0.12 0.07 0.49 0.008 0.007 - 0.033 0.005 0.0012 0.0047 0.51 -0.0015 Comparative Steel 3 0.15 0.14 0.56 0.011 0.005 0.22 0.039 0.014 - 0.0062 0.71 -0.0019 Comparative Steel 4 0.20 0.16 0.72 0.010 0.006 0.54 0.046 0.011 0.0020 0.0058 0.77 0.0004 Where formula (1) = ([Si] + [Mn] + [Cr] / 10 [C]) + [Ti] / [C], and formula (2) = 0.31 [Ti] + 1.4 [B]- [N],
[C], [Si], [Mn], [Cr], [Ti], [B], and [N] mean content (wt%) of the corresponding element.

Longitudinal cooling rate (℃ / s) Microstructure
Kinds Ferrite fraction Ferrite Average Particle Size (㎛) A
(Μm) B
(%) C
(Hv) Formula (3) Remarks CR1 CR2 CR3 CR4 7.4 2.6 1.3 0.6 F + P 91 33.2 34.8 68 90.3 775 Inventive Example 1 6.2 3.8 1.6 0.4 F + P 88 30.5 32.9 62.5 95.6 778 Inventive Example 2 5.8 4.3 1.2 0.5 F + P 85 23.7 22.5 57.3 98.7 780 Inventive Example 3 8.5 3.1 1.8 0.7 F + P 81 17.6 18.6 48.7 104.6 746 Inventive Example 4 5.3 2.3 0.7 0.6 F + P 86 25.7 21.7 42.3 99.5 630 Comparative Example 1 4.2 1.6 0.5 0.8 F + P 84 13.5 14.3 31.2 106.7 550 Comparative Example 2 6.7 1.2 1.1 0.9 F + P 79 22.4 26.1 27.6 109.3 532 Comparative Example 3 3.8 2.7 1.3 1.0 F + P 74 14.3 12.1 20.1 124.5 462 Comparative Example 4 Herein, F means ferrite and P means pearlite.
In addition, the formula (3) = {(A / 4) + [(B x 30) / (2A + 40)]} x (C / 3), the mean hardness value C is (Hv, 1 / 2d + Hv, 1 / 4d) / 2.
(Where Hv, 1 / 2d and Hv, 1 / 4d are each measured at 1 / 2D position and 1 / 4D position in the diameter (D) direction of the wire rod from the surface of the wire rod in the cross section perpendicular to the longitudinal direction of the wire rod) Longitude)

Since the steel wire was prepared by applying the amount of wire processing of 10%, 20%, 30% to the cooled wire, respectively, it is shown in the following [Table 3] to evaluate the cold forging. The cold forging evaluation was carried out to evaluate the presence of cracks by testing the notched compression specimens with a true deformation of 0.85. The cracks were evaluated as "GO" when no cracks occurred and "NG" when cracks occurred.

Steel grade Tensile Strength (Mpa) Cold workability Remarks 0% 10% 20% 30% 0% 10% 20% 30% Inventive Steel 1 343 389 432 476 GO GO GO GO Inventive Example 1 Inventive Steel 2 366 402 455 491 GO GO GO GO Inventive Example 2 Invention Steel 3 395 428 472 507 GO GO GO GO Inventive Example 3 Inventive Steel 4 418 443 484 516 GO GO GO GO Inventive Example 4 Comparative Steel 1 374 418 465 505 GO GO GO NG Comparative Example 1 Comparative Steel 2 402 442 481 514 GO GO GO NG Comparative Example 2 Comparative Steel 3 436 480 522 568 GO GO NG NG Comparative Example 3 Comparative Steel 4 468 503 554 597 GO GO NG NG Comparative Example 4

As can be seen in Table 3, in the case of Inventive Examples 1 to 4 satisfying the alloy composition and the manufacturing conditions according to an embodiment of the present invention, the microstructure, the grain orientation average size of the ferrite grain boundary surrounded by a grain boundary of 15 ° or more , The ferrite fraction, the average hardness of the ferrite phase satisfies the scope of the present invention and can be confirmed that excellent cold forging.

On the other hand, in the case of Comparative Example 1 the cooling rate of the third cooling step is 0.7 ℃ / s does not satisfy the scope of the present invention, the value of Equation (3) is 700 or less does not satisfy the scope of the invention fresh processing Cracks are generated at 30%, indicating that the cold workability is inferior.

In Comparative Example 2, the values of Equations (1) and (2) are not satisfied, and the cooling rates of the first cooling step, the second cooling step, and the third cooling step do not satisfy the scope of the present invention. The value of 3) also does not satisfy the scope of the present invention, the size of the ferrite grains also does not satisfy the scope of the present invention, it can be seen that the crack is generated at 30% of the fresh working amount, the cold workability is inferior.

In the case of Comparative Example 3, the content of C does not satisfy the scope of the present invention, does not satisfy the values of formulas (1) and (2), and the cooling rates of the second cooling step and the fourth cooling step are It does not satisfy the range, the ferrite fraction is also not 80%, the value of the formula (3) also does not satisfy the scope of the present invention. Thus, cracks are generated in fresh processing amounts of 20 and 30%, indicating that the cold workability is inferior.

In the case of Comparative Example 4, the content of C does not satisfy the scope of the present invention, does not satisfy the values of formulas (1) and (2), and the cooling rates of the first cooling step and the fourth cooling step are It does not satisfy the range, the ferrite grain size also does not satisfy the scope of the present invention, the ferrite fraction is also not 80%, the value of formula (3) also does not satisfy the scope of the present invention. Thus, cracks are generated in fresh processing amounts of 20 and 30%, indicating that the cold workability is inferior.

As described above, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person of ordinary skill in the art may be within the scope and spirit of the following claims. It will be understood that various changes and modifications are possible.

Claims (11)

By weight percent, C: less than 0.01 to 0.15%, Si: 0.3% or less (excluding 0), Mn: 0.2 to 0.75%, P: 0.03% or less, S: 0.03% or less, sol. Al: 0.01-0.05%, Cr: 0.5% or less, Ti: 0.005-0.05%, N: 0.01% or less, the remainder contains Fe and other impurities,
A cold-rolled wire rod having an average grain diameter of 15 to 40 µm in which the crystal orientation difference is surrounded by a ferrite grain boundary of 15 ° or more, satisfying the following formula (1).
{(A / 4) + [(Bx30) / (2A + 40)]} x (C / 3) ≥ 700 ...... Equation (1)
Here, A is the ferrite grain boundary average circle equivalent diameter (µm) with a crystal orientation difference of 15 ° or more, B is the area fraction (area%) occupied by the ferrite grain grains with a crystal orientation difference of 15 ° or more with respect to the entire ferrite phase, and C is a ferrite Mean hardness (Hv) of the phase. The method of claim 1,
B: more than 0.006%,
A wire rod for cold pressing that satisfies the following formula (2).
0≤ [0.31Ti + 1.4BN] ≤0.004 ...... Equation (2) The method according to claim 1 or 2,
A wire rod for cold pressing that satisfies the following formula (3).
0.8≤ [(Si + Mn + Cr) / 10C + Ti / C] ≤2.3 ...... (3) The method of claim 1,
The microstructure is composed of pearlite and ferrite, and the portion of ferrite occupies 80% or more of the cold-rolled wire rod. By weight, C: 0.01 to less than 0.15%, Si: 0.3% or less (excluding 0), Mn: 0.2 to 0.75%, P: 0.03% or less, S: 0.03% or less, sol.Al: 0.01 to 0.05% , Cr: 0.5% or less, Ti: 0.005 to 0.05%, B: 0.006% or less, N: 0.01% or less, the remainder includes Fe and other unavoidable impurities, and the content of the component is represented by the following formula (1) and formula ( Heating the billet satisfying 2);
Hot rolling the heated billet at a finish rolling temperature of 920 to 1020 ° C .;
Winding the hot rolled wire rod and cooling the wire rod;
Method for producing a cold-rolled wire rod comprising a.
0.8≤ [(Si + Mn + Cr) / 10C + Ti / C] ≤2.3 ... (1)
0≤ [0.31Ti + 1.4BN] ≤0.004 ...... Equation (2) The method of claim 5,
The cooling step is a method for manufacturing a cold-rolled wire rod comprising a first cooling step of cooling at a cooling rate of less than 5 ℃ / s to 20 ℃ / s from the finish rolling temperature to the temperature range of less than the winding temperature in the hot rolling. The method of claim 6,
The cooling step further comprises a second cooling step of cooling at a cooling rate of less than 2 ℃ / 5 ℃ / s from the winding temperature to less than 750 ℃ after the first cooling step. The method of claim 7, wherein
The cooling step further comprises a third cooling step for cooling at a cooling rate of less than 1 ℃ / S to 2 ℃ / s from 750 ℃ to 650 ℃ after the second cooling step. The method of claim 8,
The cooling step further comprises a fourth cooling step of cooling at a cooling rate of less than 0.8 ℃ / s (excluding 0 ℃ / s) from 650 ℃ to less than 400 ℃ after the third cooling step further comprising a cold-rolled wire rod manufacturing method . The method of claim 5,
The heating temperature in the heating step of the billet is 1000 to 1150 ℃ the manufacturing method of the wire for cold pressing. The method of claim 5,
The winding temperature in the winding step is a method for producing a cold-rolled wire rod is 800 to 880 ℃.