Non-tempered wire rod for hot forging with improved machinability and impact toughness and its manufacturing method

Abstract

The present invention aims to provide a non-tempered wire rod and a manufacturing method thereof, which overcomes the inferior toughness compared to existing tempered steels, and simultaneously ensures impact toughness and machinability without additional heat treatment by refining grain size through the formation of TiN and AlN and suppressing MnS elongation through the addition of Ca. [Solution] The steel contains, by weight, 0.3-0.5% C, 0.4-0.9% Si, 0.5-1.2% Mn, 0.02% or less P, 0.01-0.05% S, 0.01-0.05% sol. Al, 0.1%-0.3% Cr, 0.01%-0.02% Ti, 0.0005%-0.002% Ca, 0.007%-0.02% N, with the remainder being Fe and unavoidable impurities, contains ferrite and pearlite as a microstructure, and is characterized in that it satisfies the following relational expressions 1 to 3. [Relationship 1] 2≦(Al+Ti) / N≦5 [Relationship formula 2] 10≦S / Ca≦70, [Relationship formula 3] 700≦300+903ХC+95ХSi+154ХMn-2303ХS

Description

[Technical field]

[0001] The present invention relates to a non-tempered wire rod for hot forging having improved machinability and impact toughness and a manufacturing method thereof, and more particularly to a non-tempered wire rod for hot forging having improved machinability and impact toughness suitable for use as a material for automobiles or machine parts, which overcomes the toughness inferior to that of existing tempered steels, and simultaneously ensures impact toughness and machinability without additional heat treatment through grain refinement through the formation of TiN and AlN and suppression of MnS elongation through the addition of Ca, and a manufacturing method thereof. [Background technology]

[0002] Unlike quenched and tempered steel, which secures a certain level of strength and toughness through QT (Quenching and Tempering) heat treatment, non-tempered steel omits the QT heat treatment process. Therefore, non-tempered steel is not only economically advantageous in terms of reducing heat treatment costs, shortening delivery time by simplifying the process, and improving productivity, but is also an environmentally friendly steel material that is expected to reduce CO2 emissions generated by furnace operation during heat treatment. In the early stages of development, non-tempered steel had relatively inferior toughness compared to quenched and tempered steel, so it was only used for parts that did not require high toughness. However, recently, as consumers' demands for environmental issues and cost reduction have increased, there has been an increasing demand for improved toughness in non-tempered steel. In addition, cutting is often performed to secure the final shape of the part, so machinability is also required. In order to improve machinability, S is generally added, which generates a large amount of MnS, which causes a problem of reduced toughness of the product. Summary of the Invention [Problem to be solved by the invention]

[0003] The present invention aims to provide a non-tempered wire rod and a manufacturing method thereof, which overcomes the inferior toughness compared to existing tempered steels, and simultaneously ensures impact toughness and machinability without additional heat treatment by refining grain size through the formation of TiN and AlN and suppressing MnS elongation through the addition of Ca. [Means for solving the problem]

[0004] The untreated wire rod of the present invention with improved machinability and impact toughness contains, by weight, 0.3-0.5% C, 0.4-0.9% Si, 0.5-1.2% Mn, 0.02% or less P, 0.01-0.05% S, 0.01-0.05% sol. Al, 0.1%-0.3% Cr, 0.01%-0.02% Ti, 0.0005%-0.002% Ca, 0.007%-0.02% N, with the remainder being Fe and unavoidable impurities, contains ferrite and pearlite as a microstructure, and satisfies the following relational expression 1. [Relationship 1] 2≦(Al+Ti) / N≦5

[0005] The method for producing a non-heat treated wire rod having improved machinability and impact toughness of the present invention includes the steps of reheating a steel slab containing, by weight, 0.3-0.5% C, 0.4-0.9% Si, 0.5-1.2% Mn, 0.02% or less P, 0.01-0.05% S, 0.01-0.05% sol.Al, 0.1%-0.3% Cr, 0.01%-0.02% Ti, 0.0005%-0.002% Ca, 0.007%-0.02% N, with the remainder being Fe and unavoidable impurities, at 950-1120°C, finish rolling the reheated steel slab at 750-850°C to produce a wire rod, and coiling the wire rod and then cooling it to 400°C at an average cooling rate in the range of 0.1-5.0°C / s. Effect of the Invention

[0006] According to the non-tempered wire rod of the present invention with improved machinability and impact toughness, Ti and Al combine with N to form nitrides of TiN and AlN, which hinder the growth of grain boundaries and refine the grain size, improving toughness. In addition, Ca-based oxides due to the addition of Ca act as nuclei for the formation of MnS, suppressing the elongation of MnS during rolling, improving machinability and toughness. Therefore, even if heat treatment is omitted, it can be applied to materials for automobiles or machine parts, which require both machinability and toughness. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] The untreated wire rod of the present invention with improved machinability and impact toughness contains, by weight, 0.3-0.5% C, 0.4-0.9% Si, 0.5-1.2% Mn, 0.02% or less P, 0.01-0.05% S, 0.01-0.05% sol. Al, 0.1%-0.3% Cr, 0.01%-0.02% Ti, 0.0005%-0.002% Ca, 0.007%-0.02% N, with the remainder being Fe and unavoidable impurities, contains ferrite and pearlite as a microstructure, and satisfies the following relational expression 1. [Relationship 1] 2≦(Al+Ti) / N≦5

[0008] This specification does not describe all elements of the embodiments, and contents common in the technical field to which the present invention belongs or contents overlapping between the embodiments are omitted. In addition, when a part "includes" a certain element, this does not mean excluding other elements, but may further include other elements, unless otherwise specified to the contrary. The expression of the singular number includes the expression of the plural number, unless there is a clear exception in the context. The present invention will be described in detail below. The inventors have investigated from various angles in order to provide a wire rod that can ensure machinability and impact toughness, and as a result, they have discovered that by appropriately controlling the alloy composition and microstructure of the wire rod, it is possible to ensure machinability and toughness without a separate heat treatment, thereby completing the present invention.

[0009] The untreated wire rod of the present invention with improved machinability and impact toughness contains, by weight, 0.3-0.5% C, 0.4-0.9% Si, 0.5-1.2% Mn, 0.02% or less P, 0.01-0.05% S, 0.01-0.05% sol. Al, 0.1%-0.3% Cr, 0.01%-0.02% Ti, 0.0005%-0.002% Ca, 0.007%-0.02% N, with the remainder being Fe and unavoidable impurities, and satisfies the following relational expression 1. [Relationship 1] 2≦(Al+Ti) / N≦5

[0010] The reasons for limiting the numerical values ​​of the element contents of the alloy components of the present invention will be explained below. In the following, the unit is weight % unless otherwise specified. The C content is 0.3-0.5%. C is an element that improves the strength of the wire. To achieve the above-mentioned effects, it is preferable to include 0.3% or more of C. However, if the content is too high, toughness and machinability may deteriorate, so it is preferable to limit the upper limit of the C content to 0.5%.

[0011] The Si content is 0.4~0.9%. Silicon is a useful element as a deoxidizer and also plays a role in improving strength. If the silicon content is less than 0.4%, the above-mentioned effects cannot be obtained, and if it exceeds 0.9%, the deformation resistance of the steel increases rapidly due to solid solution strengthening, which may deteriorate the cold workability. Therefore, in the present invention, the upper limit of the silicon content is set to 0.9%.

[0012] The Mn content is 0.5-1.2%. Mn is an element useful as a deoxidizer and desulfurizer. If the Mn content is less than 0.5%, the above-mentioned effects cannot be exhibited, and if the Mn content exceeds 1.2%, the strength of the steel itself becomes excessively high, the deformation resistance of the steel increases sharply, and the cold workability may deteriorate. For this reason, the upper limit of the Mn content is limited to 1.2%.

[0013] The Cr content is 0.1 to 0.3% or less. Cr is an element that promotes ferrite and pearlite transformation during hot rolling. It also reduces the amount of solute carbon by precipitating carbides in the steel without unnecessarily increasing the strength of the steel itself, and contributes to reducing dynamic strain aging caused by solute carbon. If the Cr content is less than 0.1%, the above effects cannot be achieved, and if it exceeds 0.3%, the strength of the steel itself becomes too high, causing a sharp increase in the deformation resistance of the steel, which can lead to deterioration of cold workability. Therefore, the upper limit of the Cr content is limited to 0.3%.

[0014] The P content is less than 0.02%. P is an inevitable impurity that segregates at grain boundaries and reduces the toughness of steel, which is the main cause of reduced delayed fracture resistance. Therefore, in the present invention, it is preferable to control its content as low as possible. In theory, it is advantageous to control the P content at 0%, but since it is inevitably included in the manufacturing process, it is important to control the upper limit, and in the present invention, the upper limit of the P content is controlled at 0.02%.

[0015] The S content is 0.01-0.05%. S is an element that is inevitably included in the manufacturing process, as it segregates at the grain boundaries, greatly reducing the softness of steel, and forms sulfides in steel, which is the main cause of deteriorating delayed fracture resistance and stress relaxation properties. However, as in the present invention, S is also actively used to improve machinability. S combines with Mn to form MnS, which improves machinability, and in the present invention, the S content is controlled in the range of 0.01% to 0.05%, taking into consideration the S content that is effective in improving machinability and does not significantly reduce the toughness of the steel.

[0016] The content of sol.Al is 0.01~0.05%. Sol. Al is an element that acts effectively as a deoxidizer. To achieve the above-mentioned effect, sol. Al may be contained at 0.01% or more. However, if the Al content exceeds 0.05%, manufacturing difficulties may occur due to Al oxides generated during the casting process. For this reason, in the present invention, the upper limit of the Al content is controlled at 0.05%.

[0017] The Ti content is 0.01~0.02%. Titanium (Ti) is an element that plays a major role in improving the toughness of steel by forming TiN precipitates during the solidification process of steel, suppressing the growth of austenite grains during the heating and hot rolling of slabs, and refining the grain size of the final structure. If the Ti content is less than 0.01%, the TiN precipitates are insufficient, making it difficult to secure a sufficient amount to restrict the movement of austenite grain boundaries. On the other hand, if it exceeds 0.02%, coarse titanium nitrides are formed, which deteriorates toughness, so in the present invention, the upper limit of the Ti content is limited to 0.02%.

[0018] The Ca content is 0.0005~0.002%. Ca is an essential element for reducing the aspect ratio of MnS to improve machinability and impact toughness. When Ca is added, it forms an oxide, which acts as a nucleus for the formation of MnS, suppressing the elongation of MnS during wire rolling and maintaining a low aspect ratio. The low aspect ratio of MnS not only improves machinability, but also relieves the anisotropy of the microstructure and prevents deterioration of toughness. However, to achieve the above effects, Ca must be contained at 0.0005% or more, and if the Ca content exceeds 0.002%, difficulties may occur during manufacturing. For this reason, the upper limit of the Ca content in the present invention is limited to 0.002%.

[0019] The N content is 0.007-0.02%. N is an essential element for improving impact toughness by forming nitrides with Ti and Al to refine the grain size. If the N content is less than 0.007%, it is difficult to secure sufficient nitrides, and the amount of precipitates such as Al and Ti decreases, making it impossible to secure the toughness targeted in the present invention. If the N content exceeds 0.02%, the amount of dissolved nitrogen that does not exist as nitrides increases, which may reduce the toughness and softness of the wire rod. For this reason, the upper limit of the N content is limited to 0.02% in the present invention.

[0020] The balance of the alloy composition is Fe. The non-tempered wire rod of the present invention may contain other impurities that may be contained in the industrial production process of ordinary steel. Since such impurities are known to anyone having ordinary skill in the art to which the present invention pertains, the present invention does not particularly limit the type and content of the impurities.

[0021] The non-heat treated wire of the present invention can satisfy the relations 1 to 3. In formulas 1 to 3, [Al], [Ti], [N], [S], [Ca], [C], [Si] and [Mn] each represent the content (wt%) of the corresponding element. [Relationship 1] 2≦([Al]+[Ti]) / [N]≦5 (toughness) Relational formula 1 is a mathematical formula related to toughness. In the present invention, TiN and AlN are formed by adding high N and Ti and Al. The precipitation of fine TiN and AlN in steel suppresses the growth of crystal grains, thereby refining them and improving the impact toughness of the non-tempered wire rod according to the present invention. In order to achieve the above-mentioned effect, it is advantageous to generate as many TiN and AlN precipitates of about 50 nm as possible, and for this purpose, it is preferable to control the ratio of ([Al]+[Ti]) / [N] to 2-5. If the ratio of ([Al]+[Ti]) / [N] is less than 2, sufficient precipitates cannot be generated, and if it exceeds 5, coarse precipitates are formed, which may result in poor toughness. Therefore, in the present invention, the ratio of ([Al]+[Ti]) / [N] is limited to 2-5.

[0022] [Relationship 2] 10≦[S] / [Ca]≦70 (machinability, impact toughness) Relational formula 2 is a mathematical formula related to machinability and impact toughness. In the present invention, MnS is formed by adding high S. MnS is an elongated inclusion, and has a shape and directionality elongated in the rolling direction, which greatly improves the machinability of the non-tempered wire rod according to the present invention. However, MnS acts as a starting point and propagation path of cracks when impact is applied, which deteriorates the impact toughness. Therefore, it is necessary to suppress the elongation of MnS, so Ca is added, and for this purpose, it is preferable to limit the S / Ca ratio to 10 to 70. If the S / Ca ratio is less than 10, the amount of S relative to Ca is insufficient, making it difficult to ensure the machinability targeted in the present invention. On the other hand, if it exceeds 70, a sufficient amount of Ca oxide that can suppress the elongation of MnS is not generated, so the elongation of MnS cannot be effectively suppressed, making it difficult to ensure toughness. Therefore, in the present invention, the S / Ca ratio is limited to 10 to 70 in order to obtain a combination that improves machinability and impact toughness.

[0023] [Relationship 3] 700≦300+903X[C]+95X[Si]+154X[Mn]-2303X[S] (strength) Relational formula 3 is a mathematical formula related to strength. C, Si, and Mn are elements that have a large effect on solid solution strengthening. On the other hand, S forms MnS and reduces the amount of Mn available for solid solution strengthening, thereby lowering strength. Therefore, in order to ensure a wire strength of 700 MPa or more, which is the target of the present invention, the value of Relational formula 3 is limited to 700 or more.

[0024] The non-tempered wire of the present invention contains ferrite and pearlite as a microstructure, and the average interlayer spacing of the ferrite and pearlite is 10.0 to 15.0 μm, preferably 12.0 to 13.0 μm. In the non-heat treated wire of the present invention, the thickness of the ferrite may be 5.0 to 10.0 μm, and preferably 7.0 to 9.0 μm. Moreover, the aspect ratio of the ferrite is 4 or less, The tensile strength is 700 MPa or more, The yield strength is 350 to 450 MPa. The yield ratio is 0.45 to 0.65. Impact toughness is 60J / cm 2 That's all. The product of tensile strength and impact toughness may be 30,000-60,000.

[0025] Next, a method for producing the non-heat treated wire of the present invention will be described. The non-tempered wire having improved machinability and impact toughness according to the present invention may be manufactured by various methods, and the manufacturing method is not particularly limited. However, as an example, the wire may be manufactured by the following method. The non-tempered wire rod having improved machinability and impact toughness of the present invention contains, by weight, 0.3-0.5% C, 0.4-0.9% Si, 0.5-1.2% Mn, 0.02% or less P, 0.01-0.05% S, 0.01-0.05% sol.Al, 0.1%-0.3% Cr, 0.01%-0.02% Ti, 0.0005%-0.002% Ca, 0.007%-0.02% N, and the remainder being Fe and unavoidable impurities. The method includes the steps of reheating a steel slab having a microstructure of ferrite and pearlite, hot rolling the reheated steel slab to manufacture a wire rod, and coiling and cooling the wire rod.

[0026] Each manufacturing step will now be described in more detail. First, a bloom satisfying the above-mentioned composition is heated, and then the steel piece is rolled to obtain a billet. The reheating step is a step in which the rolled billet is reheated in order to reduce the rolling load during wire rolling. Reheating can be performed at a temperature of 950~1120℃. If the billet reheating temperature is less than 950℃, the rolling load increases and manufacturing can be difficult. On the other hand, if it exceeds 1120℃, the fine AlN formed in the billet is re-dissolved during heating, significantly reducing the effect of grain refinement.

[0027] Wire rod rolling stage In the wire rod rolling stage, the reheated steel billet is hot rolled to produce wire rod. At this time, the finish rolling temperature of the hot rolling may be 750 to 850° C. If the finish rolling temperature is less than 750° C., the rolling load may increase, and if it exceeds 850° C., the crystal grains may become coarse, making it difficult to ensure the high toughness targeted in the present invention.

[0028] Winding stage The wire rod manufactured as above may be wound into a coil shape, and the winding temperature may be 750 to 850°C. The temperature of the wire rod obtained by the finish rolling may increase due to transformation heat, so the temperature of the wire rod immediately before winding may be higher than the temperature at which the finish rolling was performed. In this case, the wire rod may be cooled to the winding temperature according to the temperature increased by the heat, and then wound, or may be wound without separate cooling. If the winding temperature is less than 750°C, the martensite in the surface layer formed during cooling cannot be restored by reheating, and tempered martensite is generated, which may increase the possibility of inducing surface defects during wire drawing. On the other hand, if the temperature exceeds 850°C, a thick scale is formed on the surface of the wire rod, which may easily cause surface defects during descaling, and the cooling time during subsequent cooling may be excessive, which may reduce productivity.

[0029] Cooling Phase The coiled wire rod may be subjected to a cooling process, and the cooling rate is characterized in that the wire rod is cooled to 400°C at an average cooling rate of 0.1 to 5.0°C / s through air cooling or controlled cooling after hot forging. If the average cooling rate to 400°C after coiling is less than 0.1°C / s, the target strength may not be satisfied due to excessive generation of pro-eutectoid ferrite, and if it exceeds 5°C / s, low-temperature structures such as martensite may be generated, decreasing toughness and machinability.

[0030] {Example} Blooms having the alloy composition shown in Table 1 below were heated at 1,200°C for 4 hours, and then rolled at a finish rolling temperature of 1,100°C to obtain billets. The billets were then heated at 1,090°C for 90 minutes, finish rolled at 800°C, coiled at 780°C, and cooled to produce wires with a diameter of 26 mm. Wires having the compositions of inventive steels 1 to 7 and comparative steels 1 to 4 were manufactured (Table 1), and the tensile strength and impact toughness of the wire specimens obtained were measured and are shown in Table 2 below. Here, the room temperature tensile strength was measured at 25°C by taking a sample from the center of the non-tempered steel specimen, and the room temperature impact toughness was evaluated as the Charpy impact energy value obtained by performing a Charpy impact test on a specimen having a U-notch (U-notch standard sample standard, 10 x 10 x 55 mm) at 25°C.

[0031] To evaluate machinability, a 26mm diameter wire rod was cut to a 24mm diameter CD-Bar (Cold Drawn Bar) by applying a reduction rate of 14.8%. Machinability was evaluated using a CNC lathe, and a 24mm diameter CD-Bar was cut to a diameter of 15mm, and then the segmentation of the cut chip was evaluated. The cutting conditions were a cutting speed of 100mm / min, a transfer speed of 0.1mm / rev, and a cutting depth of 1.0mm, and cutting oil was used. The segmentation of the cut chip was evaluated based on the number of turns of the cut chip generated during turning, with a cut chip generated with 5 turns or less being good, a cut chip generated with more than 5 turns but less than 10 turns being normal, and more than 10 turns being poor.

[0032] [Table 1]

[0033] [Table 2]

[0034] As can be seen from Tables 1 and 2, in the cases of Examples 1 to 7, all of the chemical compositions, relational formulas, and manufacturing conditions presented in the present invention were satisfied, and wire rods had a tensile strength of 700 MPa or more and a wire rod strength of 60 J / cm2 The wire rod has a good impact toughness and good machinability. On the other hand, in the case of Comparative Examples 1 to 7 which do not satisfy one or more of the conditions proposed by the present invention, it can be confirmed that one or more of the properties of tensile strength, machinability, and impact toughness are inferior. Specifically, Comparative Example 1 does not satisfy the tensile strength of 700 MPa or more due to the low C content, and Comparative Example 2 has a poor machinability due to the excessive Si content and S / Ca value of only 8. Comparative Example 3 does not satisfy the Mn content condition and does not satisfy the relational expression 3, so the strength is not achieved. Comparative Example 4 does not satisfy the relational expression 1 because the Ti content is excessive. Comparative Examples 5 to 7 satisfy the chemical composition proposed by the present invention, but the toughness and tensile strength values ​​are not the target values ​​because the heating range of the steel billet is exceeded or the average cooling rate range up to 400°C proposed is not satisfied. Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and a person having ordinary skill in the art will understand that various changes and modifications can be made without departing from the concept and scope of the claims set forth below. [Industrial Applicability]

[0035] According to the present invention, it is possible to provide a non-tempered wire rod which has improved machinability and toughness even without heat treatment, and the industrial applicability of the wire rod is recognized.

Claims

1. In weight percent, it contains C: 0.3-0.5%, Si: 0.4-0.9%, Mn: 0.5-1.2%, P: 0.02% or less, S: 0.01-0.05%, sol. Al: 0.01-0.05%, Cr: 0.1%-0.3%, Ti: 0.01%-0.02%, Ca: 0.0005%-0.002%, N: 0.007%-0.02%, and the remainder is Fe and unavoidable impurities. It contains ferrite and pearlite as a microstructure, A non-tempered wire rod having improved machinability and impact toughness, characterized by satisfying the following relational expression 1: [Relationship 1] 2≦(Al+Ti) / N≦5

2. 2. The non-heat treated wire rod having improved machinability and impact toughness according to claim 1, characterized in that the following relational expression 2 is satisfied: [Relationship 2] 10≦S / Ca≦70

3. 2. The non-heat treated wire rod having improved machinability and impact toughness according to claim 1, characterized in that the following relational expression 3 is satisfied: [Relationship 3] 700≦300+903ХC+95ХSi+154ХMn-2303ХS

4. 2. The non-heat treated wire rod according to claim 1, wherein the average interlamellar spacing between the ferrite and pearlite layers is 10.0 to 15.0 μm.

5. 2. The non-heat treated wire having improved machinability and impact toughness according to claim 1, wherein the thickness of the ferrite is 5.0 to 10.0 μm.

6. 2. The non-heat treated wire rod having improved machinability and impact toughness according to claim 1, wherein the aspect ratio of the ferrite is 4 or less.

7. 2. The non-heat treated wire rod having improved machinability and impact toughness according to claim 1, characterized in that the wire rod has a tensile strength of 700 MPa or more.

8. 2. The non-heat treated wire rod having improved machinability and impact toughness according to claim 1, characterized in that the wire rod has a yield strength of 350 to 450 MPa.

9. 2. The non-heat treated wire rod having improved machinability and impact toughness according to claim 1, characterized in that the yield ratio is 0.45 to 0.

65.

10. Impact toughness is 60 J / cm ２ 2. The non-heat treated wire rod having improved machinability and impact toughness according to claim 1,

11. 2. The non-heat treated wire rod having improved machinability and impact toughness according to claim 1, characterized in that the product of tensile strength and impact toughness is 30,000 to 60,000.

12. reheating the steel slab containing, by weight, 0.3-0.5% C, 0.4-0.9% Si, 0.5-1.2% Mn, 0.02% or less P, 0.01-0.05% S, 0.01-0.05% sol. Al, 0.1%-0.3% Cr, 0.01%-0.02% Ti, 0.0005%-0.002% Ca, 0.007%-0.02% N, with the remainder being Fe and unavoidable impurities, and containing ferrite and pearlite as a microstructure, in a temperature range of 950-1120°C; Finish rolling the reheated steel billet at 750 to 850° C. to produce a wire rod; and cooling the wire after winding; The post-coil cooling step includes a step of cooling to 400° C. at an average cooling rate of 0.1 to 5.0° C. / s, and the wire satisfies the following Relational Expression 1. [Relationship 1] 2≦(Al+Ti) / N≦5

13. 13. The method for producing a non-heat treated wire having improved machinability and impact toughness according to claim 12, wherein the wire satisfies the following Relational Expression 2: [Relationship 2] 10≦S / Ca≦70

14. 13. The method for producing a non-heat treated wire having improved machinability and impact toughness according to claim 12, wherein the wire satisfies the following Relational Expression 3: [Relationship 3] 700≦300+903ХC+95ХSi+154ХMn-2303ХS

15. 13. The method for producing a non-heat treated wire having improved machinability and impact toughness according to claim 12, wherein the wire has an average interlamellar spacing of the ferrite and pearlite of 10.0 to 15.0 um.

16. 13. The method for manufacturing a non-heat treated wire having improved machinability and impact toughness according to claim 12, wherein the thickness of the ferrite in the wire is 5.0 to 10.0 μm.

17. 13. The method for producing a non-heat treated wire having improved machinability and impact toughness according to claim 12, wherein the aspect ratio of the ferrite in the wire is 4 or less.

18. 13. The method for producing a non-heat treated wire having improved machinability and impact toughness according to claim 12, wherein the wire has a tensile strength of 700 MPa or more.

19. The method for producing a non-heat treated wire having improved machinability and impact toughness according to claim 12, characterized in that the wire has a yield strength of 350 to 450 MPa.

20. 13. The method for producing a non-heat treated wire having improved machinability and impact toughness according to claim 12, wherein the wire has a yield ratio of 0.45 to 0.

65.

21. The wire has an impact toughness of 60 J / cm ２ The method for producing a non-heat treated wire having improved machinability and impact toughness according to claim 12, characterized in that

22. 13. The method for producing a non-heat treated wire having improved machinability and impact toughness according to claim 12, wherein the wire has a product of tensile strength and impact toughness of 30,000 to 60,000.

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