Non-quenched and tempered wire rod with excellent machinability and impact toughness and method for producing the same

Abstract

Provided are a non-quenched and tempered wire rod capable of controlling the microstructure to ensure impact toughness and wear resistance characteristics of a cutting tool, and a method for manufacturing the same. **Solution**: The present invention relates to a non-quenched and tempered wire rod with improved machinability and impact toughness, and a method for manufacturing the same. The non-quenched and tempered wire rod according to the present invention contains, by weight%, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S: 0.01 to 0.05%, sol.Al: 0.015 to 0.05%, Cr: 0.1% to 0.3%, N: 0.007% to 0.020%, with the balance being Fe and inevitable impurities. The microstructure contains ferrite and pearlite and satisfies the following relational expressions 1 and 2. [Relational expression 1] [N] - [Al] / 1.93 ≤ 0.009 [Relational expression 2] -23[C] + [Si](5 - 2[Si]) - 4[Mn] + 104[S] + 3 ≥ 0

Description

Technical Field

[0001] The present invention relates to a non-quenched and tempered wire rod having excellent machinability and impact toughness and a method for manufacturing the same. More specifically, the present invention relates to a non-quenched and tempered wire rod suitable for use as a material for automotive parts or machine parts that controls the microstructure to ensure impact toughness and wear resistance of cutting tools, and a non-quenched and tempered wire rod having excellent impact toughness and a method for manufacturing the same.

Background Art

[0002] Unlike quenched and tempered steel, which ensures a certain level of strength and toughness through QT (Quenching and Tempering) heat treatment, non-quenched and tempered steel omits the QT heat treatment process. Therefore, non-quenched and tempered steel not only has advantages in terms of economy such as reduction of heat treatment costs, shortening of delivery time due to process simplification, and improvement of productivity, but also has an environmental-friendly steel material with an expected reduction effect of CO 2 generated during the operation of the furnace during heat treatment. In the early stage of development, non-quenched and tempered steel had relatively inferior toughness compared to quenched and tempered steel, so it has been applied only to parts where high toughness is not required. However, recently, as the demands of consumers for environmental issues and cost reduction increase, the demand for improving the toughness of non-quenched and tempered steel is increasing. In addition, since cutting is often performed to ensure the final shape of the parts, machinability is also required at the same time. In order to improve machinability, generally, a large amount of MnS is generated by adding S, and for this reason, a problem occurs in that the toughness of the product decreases.

Summary of the Invention

Problems to be Solved by the Invention

[0003] In the present invention, in order to overcome the impact toughness, which is inferior to that of existing quenched and tempered steel in non-quenched and tempered steel, the present invention provides a non-quenched and tempered wire rod and a method for manufacturing the same that can control the microstructure to ensure impact toughness and wear resistance of cutting tools by ensuring the AlN grain boundary pinning effect, grain refinement through cold rolling at low temperature, and a sufficient fraction of the soft ferrite phase.

Means for Solving the Problems

[0004] The non-quenched and tempered wire rod with improved machinability and impact toughness according to the present invention contains, by weight %, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S: 0.01 to 0.05%, sol.Al: 0.015 to 0.05%, Cr: 0.1% to 0.3%, N: 0.007% to 0.020%, the balance being Fe and inevitable impurities, the microstructure contains ferrite and pearlite, and satisfies the following relational expression 1.

[0005] [Relational Expression 1] [N] - [Al] / 1.93 ≤ 0.009

[0006] The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to the present invention contains, by weight %, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S: 0.01 to 0.05%, sol.Al: 0.015 to 0.05%, Cr: 0.1% to 0.3%, N: 0.007% to 0.020%, the balance being Fe and inevitable impurities. The method includes the steps of reheating a steel slab at 950 to 1100°C, finish rolling the reheated steel slab at 750 to 850°C to manufacture a wire rod, and cooling the wire rod after coiling. The cooling step after coiling includes cooling to 400°C at an average cooling rate exceeding 0.1 and not exceeding 5.0°C / s, and the wire rod satisfies the relational expression 1.

Effect of the Invention

[0007] According to the present invention, in the non-quenched and tempered wire rod with improved machinability and impact toughness, Al combines with N to form AlN nitride. Such nitrides suppress grain boundary growth during heating, refine the grain size, and improve the impact toughness. Also, the area fraction of the ferrite phase is ensured to be 20% to 40% from a point more than 1 / 4 of the wire rod diameter from the surface to the center, further ensuring the impact toughness. And the size of MnS, which can improve machinability but deteriorate impact toughness, is refined to minimize the decrease in impact toughness while ensuring machinability, especially the wear resistance of cutting tools. Therefore, it can be applied to automotive materials or materials for machine parts that require both machinability and impact toughness without heat treatment.

BEST MODE FOR CARRYING OUT THE INVENTION

[0008] The non-quenched and tempered wire rod with improved machinability and impact toughness of the present invention contains, by weight %, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S: 0.01 to 0.05%, sol.Al: 0.015 to 0.05%, Cr: 0.1% to 0.3%, N: 0.007% to 0.020%, the balance being Fe and inevitable impurities, the microstructure contains ferrite and pearlite, and satisfies the following relational expression 1.

[0009] [Relational expression 1] [N] - [Al] / 1.93 ≤ 0.009

[0010] This specification does not describe all elements of the embodiments, and general content in the technical field to which the present invention pertains or overlapping content among the embodiments is omitted. Also, when a certain part "includes" a certain component, this means that other components can be further included without excluding other components unless otherwise stated to the contrary. Singular expressions include plural expressions unless there are obvious exceptions in the context. Hereinafter, the present invention will be described in detail.

[0011] The inventors of the present invention have studied from various angles in order to provide a wire rod capable of ensuring machinability and impact toughness. As a result, it has been discovered that by appropriately controlling the alloy composition and microstructure of the wire rod, machinability and toughness can be ensured without separate heat treatment, and the present invention has been completed.

[0012] The non-quenched and tempered wire rod with improved machinability and impact toughness of the present invention contains C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S: 0.01 to 0.05%, sol.Al: 0.015 to 0.05%, Cr: 0.1% to 0.3%, N: 0.007% to 0.02%, the balance being Fe and inevitable impurities, the microstructure contains ferrite and pearlite, and satisfies the following relational expression 1.

[0013] [Relational expression 1] [N] - [Al] / 1.93 ≤ 0.009

[0014] The reasons for the numerical limitations of the element contents of the alloy components of the present invention will be described below. Unless otherwise specified, the unit is wt% below.

[0015] The content of C is 0.3 to 0.5%. C is an element that plays a role in improving the strength of the wire rod. To exhibit the above-described effects, it is preferable to contain C at 0.3% or more. However, when the content is excessively large, the toughness and machinability may deteriorate. Therefore, it is preferable to limit the upper limit of the content of C to 0.5%.

[0016] The content of Si is 0.4 to 0.9%. Si is an element that plays a role in improving the strength while being a useful element as a deoxidizer. When the content of Si is less than 0.4%, the above-described effects cannot be exhibited. When it exceeds 0.9%, the deformation resistance of the steel rapidly increases due to solid solution strengthening and the cold workability may deteriorate. Therefore, it is preferable to limit the upper limit of the content of Si to 0.9%.

[0017] The content of Mn is 0.5 to 1.2%. Mn is a useful element as a deoxidizer and a desulfurizer. When the content of Mn is less than 0.5%, the above-described effects cannot be exhibited. When the content of Mn exceeds 1.2%, the strength of the steel itself becomes excessively high, the deformation resistance of the steel rapidly increases, and the cold workability may deteriorate. Therefore, it is preferable to limit the upper limit of the content of Mn to 1.2%.

[0018] The content of P is 0.02% or less. P is an impurity that is inevitably contained, segregates at the grain boundaries, and is a main cause for reducing the toughness of the steel and decreasing the resistance to delayed fracture. Therefore, in the present invention, it is preferable to control the content as low as possible. Theoretically, it is advantageous to control the content of P to 0%. However, since it is inevitably contained in the manufacturing process, it is important to manage the upper limit. In the present invention, the upper limit of the P content is managed at 0.02%.

[0019] The content of S is 0.01 to 0.05%. S segregates at grain boundaries, significantly reducing the softness of steel. It is the main cause of forming sulfides in steel, deteriorating the resistance to delayed fracture and stress relaxation characteristics, and is an impurity inevitably contained during the manufacturing process. However, S may be actively utilized to improve machinability as in the present invention. S combines with Mn to form MnS, improving machinability. In the present invention, within the range that does not significantly reduce the toughness of the steel, the content of S effective for improving machinability is considered and controlled within the range of 0.01% to 0.05%.

[0020] The content of Sol.Al is 0.015 - 0.05%. sol.Al is an element that usefully acts as a deoxidizer. In order to exhibit the above-described effects, sol.Al may be contained in an amount of 0.015% or more. However, when the content of Al exceeds 0.05%, manufacturing difficulties may occur due to Al oxides generated during the casting process. For this reason, in the present invention, it is preferable to limit the upper limit of the Al content to 0.05%.

[0021] The content of Cr is 0.1 - 0.3% or less. Cr is an element that plays a role in promoting ferrite and pearlite transformation during hot rolling. Also, it does not make the strength of the steel higher than necessary, and precipitates carbides in the steel to reduce the amount of solid-solution carbon, contributing to the reduction of dynamic strain aging caused by solid-solution carbon. When the content of Cr is less than 0.1%, the above-described effects cannot be exhibited. When it exceeds 0.3%, the strength of the steel itself becomes excessively high, the deformation resistance of the steel rapidly increases, and thus the cold workability may deteriorate. Therefore, it is preferable to limit the upper limit of the Cr content to 0.3%.

[0022] The content of N is 0.007 - 0.02%. N is an essential element for realizing the effect of improving impact toughness by forming nitrides together with Al to refine the grain size. When the content of N is less than 0.007%, it is difficult to ensure sufficient nitrides, so the amount of AlN precipitates decreases and the toughness targeted in the present invention cannot be ensured. When the content of N exceeds 0.02%, the amount of solid solution nitrogen that does not exist as nitrides increases, and the toughness and softness of the wire rod may decrease. Therefore, in the present invention, it is preferable to limit the upper limit of the content of N to 0.02%.

[0023] The balance outside the alloy composition is Fe. The non-quenched and tempered wire rod of the present invention can contain other impurities that can be included in the normal industrial production process of steel. Since such impurities are obvious to those with ordinary knowledge in the technical field to which the present invention pertains, the types and contents thereof are not particularly limited in the present invention.

[0024] The non-quenched and tempered wire rod of the present invention can satisfy relational expressions 1 to 2. In relational expressions 1 to 2, [Al], [N], [C], [S], [Mn], and [Si] respectively represent the contents (weight %) of the corresponding elements.

[0025] [Relational expression 1] (Impact toughness) [N] - [Al] / 1.93 ≤ 0.009

[0026] Relational expression 1 is a mathematical formula related to toughness. In the present invention, AlN is formed by adding high N and Al. The precipitation of fine AlN in steel suppresses the growth of crystal grains, refines the grain size, and improves the impact toughness of the non-quenched and tempered wire rod according to the present invention. In order to exhibit the above-described effect, it is preferable to control the ratio of [N] - [Al] / 1.93 to 0.009 or less. When the value of [N] - [Al] / 1.93 exceeds 0.009, a considerable amount of the nitrogen added in a state where Al is insufficient remains in the steel without binding to Al, which may deteriorate the impact toughness.

[0027] [Relational expression 2] -23[C] + [Si](5 - 2[Si]) - 4[Mn] + 104[S] + 3 ≥ 0 (Machinability)

[0028] Relational expression 2 is an equation related to tool wear in machinability. Generally, when S is added, MnS is formed and acts as a stress concentration source during machining to reduce cutting resistance and improve the tool life through a lubricating effect. However, when the hardness increases due to the action of added alloying elements, tool wear is accelerated, so both must be considered. Relational expression 2 reflects such effects in a complex manner, and when its value is 0 or more, good tool wear can be ensured.

[0029] The non-quenched and tempered wire rod of the present invention contains ferrite and pearlite as its microstructure. At this time, the area fraction of ferrite satisfies 20 to 40% in the region from a point where the wire rod diameter is 1 / 4 or more from the surface to the center part.

[0030] The non-quenched and tempered wire rod of the present invention may have an area fraction of the generated AlN of 0.03% or more.

[0031] The non-quenched and tempered wire rod of the present invention may have a size of the generated AlN of 150 nm or less.

[0032] Also, the non-quenched and tempered wire rod of the present invention may have the number of carbonitrides per unit area with an average circle equivalent diameter of 100 nm or less of 2 pieces / μm 2 or more.

[0033] Also, the non-quenched and tempered steel material of the present invention may have a tensile strength of 700 MPa or more.

[0034] Also, the non-quenched and tempered wire rod of the present invention may have a yield strength of 350 to 450 MPa.

[0035] Also, the non-quenched and tempered wire rod of the present invention may have a yield ratio of 0.45 to 0.65.

[0036] Also, the non-quenched and tempered wire rod of the present invention may have an impact toughness of 60 J / cm 2 or more.

[0037] In addition, the as - rolled wire of the present invention may have a product of tensile strength and impact toughness of 30,000 to 60,000.

[0038] Next, a method for manufacturing the as - rolled wire of the present invention will be described. The as - rolled wire with improved machinability and impact toughness of the present invention can be manufactured by various methods, and the manufacturing method is not particularly limited. For example, it can be manufactured by the following method.

[0039] The as - rolled wire with improved machinability and impact toughness of the present invention contains, by weight%, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S: 0.01 to 0.05%, sol.Al: 0.015 to 0.05%, Cr: 0.1% to 0.3%, N: 0.007% to 0.020%, with the balance being Fe and inevitable impurities. The method includes the steps of reheating a steel slab, hot - rolling the reheated steel slab to produce a wire rod, and cooling the wire rod after coiling. The cooling step after coiling includes cooling at an average cooling rate exceeding 0.1 and not exceeding 5.0 °C / s until 400 °C, and the wire rod satisfies the following relational expression 1.

[0040] [Relational Expression 1] [N] - [Al] / 1.93 ≤ 0.009

[0041] Hereinafter, each manufacturing step will be described in more detail. First, after heating a bloom that satisfies the above - mentioned component system, the steel slab is rolled to obtain a billet.

[0042] Reheating step The reheating step is a step of reheating the rolled billet, and is a step for reducing the rolling load during wire rod rolling. At this time, the reheating can be performed at a temperature of 950 to 1100 °C. If the reheating temperature of the steel slab is less than 950 °C, the rolling load may increase and it may be difficult in production. On the contrary, if it exceeds 1100 °C, the AlN generated in the steel slab will be redissolved during heating, so that the area fraction of the presented AlN becomes less than 0.03%, and the effect of contributing to grain size refinement is significantly reduced.

[0043] Wire rod rolling stage In the wire rod rolling stage, the reheated steel slab is hot-rolled to produce a wire rod. At this time, the finish rolling temperature of the hot rolling can be 750 to 850 °C. If the finish rolling temperature is less than 750 °C, the rolling load may increase. If it exceeds 850 °C, the crystal grains may become coarse and it may be difficult to ensure the high toughness targeted in the present invention.

[0044] Coiling stage The process of coiling the wire rod manufactured as described above into a coil can be performed. At this time, the coiling temperature can be 750 to 850 °C. Since the temperature of the wire rod obtained by the finish rolling may increase due to transformation heat generation, the temperature of the wire rod immediately before coiling may be higher than the temperature at which the finish rolling was performed. At this time, it can be coiled after cooling to the coiling temperature by the temperature increased by the heat generation, or it can be coiled without separate cooling. If the temperature at the time of coiling is less than 750 °C, the surface layer martensite generated during cooling cannot be recovered by reheating, and there is a problem that tempered martensite is generated and surface defects are likely to be induced during wire drawing. On the contrary, if the temperature exceeds 850 °C, not only is a thick scale formed on the surface of the wire rod and surface defects are likely to occur during descaling, but also the cooling time becomes excessive during subsequent cooling and the productivity may decrease.

[0045] Cooling stage The coiled wire rod can perform a cooling process. At this time, the cooling rate is characterized by cooling to 400 °C in the range where the average cooling rate exceeds 0.1 and is 5.0 °C / s or less through air cooling or controlled cooling after hot forging. When the average cooling rate from after coiling to 400 °C is less than 0.1 °C / s, excessive formation of primary ferrite cannot satisfy the targeted strength. When it exceeds 5 °C / s, low-temperature structures such as martensite may occur and the toughness and machinability may be reduced.

[0046] {Example} A bloom having an alloy composition as shown in Table 1 below was heated at 1,200°C for 4 hours, and then the steel slab was rolled at a finishing rolling temperature of 1,100°C to obtain a billet. Thereafter, the billet was heated at 1,090°C for 90 minutes, then finish-rolled at 800°C, coiled at 780°C, and then cooled to produce a wire rod with a diameter of 26 mm. Wire rods having the components of Invention Steels 1 to 7 and Comparative Steels 1 to 7 were manufactured (Table 1), and the tensile strength, impact toughness, wear depth of the cutting tool, and area fraction of ferrite and AlN of the collected wire rod specimens were measured and shown in Table 2 below.

[0047] Here, the room temperature tensile strength was measured by sampling from the center of the non-normalized steel specimen at 25°C, and the room temperature impact toughness was evaluated by the Charpy impact energy value obtained by performing a Charpy impact test on a specimen having a U-notch (U-notch standard sample standard, 10×10×55 mm) at 25°C. Also, for the evaluation of machinability, a wire rod with a diameter of 26 mm was manufactured into a cold-drawn bar (CD-Bar) with a diameter of 24 mm by applying a reduction ratio of 14.8%. The wear degree of the tool was evaluated using a CNC lathe. After turning a CD-Bar with a diameter of 24 mm to a length of 20 mm with a diameter of 15 mm, the wear degree of the tool was evaluated. At this time, the cutting conditions were carried out using cutting oil under the conditions of a cutting speed of 100 mm / min, a feed rate of 0.1 mm / rev, and a cutting depth of 1.0 mm, and the cutting tool used was a cermet tool with a chip breaker. The wear depth of the tool was measured by measuring the flank wear depth after continuously machining 300 parts having the shape mentioned above, and it was judged as defective when it exceeded 0.2 mm and good when it was less than that. In order to measure and quantify the area fraction of very fine AlN of 100 nm or less, a transmission electron microscope specimen was prepared by the replica method and observed at a magnification of 100,000 times, and the arithmetic mean of the AlN area fraction was obtained from 50 images using image analysis software.

[0048]

Table 1

[0049]

Table 2

[0050] The ferrite area fraction was measured in the region from a point 1 / 4 of the wire diameter away from the surface to the center at the surface. As can be confirmed from Table 1 and Table 2, in the case of Examples 1 to 7, all of the chemical components, relational expressions, and manufacturing conditions presented in the present invention are satisfied, and the target impact toughness of 60 J / cm 2 or more, a tensile strength of 700 MPa or more, and good wear characteristics of the cutting tool are shown.

[0051] On the contrary, in the case of Comparative Examples 1 to 10 that do not satisfy all the conditions proposed in the present invention, at least one of the characteristics of tensile strength, impact toughness, and wear depth of the cutting tool was shown to be inferior. Specifically, in Comparative Example 1, since Comparative Steel 1 has a low C content value, it cannot satisfy a tensile strength of 700 MPa or more. In Comparative Examples 2 and 3, the impact toughness was not achieved due to the excess of Si content and Mn content, respectively. Also, in Comparative Examples 4 and 5, since the Al and N contents were lower than the values presented in the present invention, the area fraction of AlN could not satisfy 0.03% or more, and thus a low impact toughness value was shown. In Comparative Examples 6 to 7, although the chemical components presented in the present invention were satisfied, the relational expressions 1 and 2 presented were not satisfied, so the impact toughness was low and the wear of the cutting tool was poor. In Comparative Examples 8 to 10, although the range of chemical components was satisfied, the temperature range of steel slab heating was exceeded or the average cooling rate range up to 400 °C presented during cooling after rolling could not be satisfied, so the impact toughness and tensile strength deviated from the target values or the wear of the cutting tool was not good.

[0052] In the foregoing, exemplary embodiments of the present invention have been described. However, the present invention is not limited thereto, and it will be understood by those having ordinary knowledge in the relevant technical field that various changes and modifications can be made without departing from the concept and scope of the claims described below.

Industrial Applicability

[0053] According to the present invention, it is possible to provide a non-quenched and tempered wire rod with improved impact toughness and machinability while improving environmental problems and cost reduction problems, and thus industrial applicability is recognized.

Claims

1. By weight, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S: 0.01 to 0.05%, sol.Al: 0.015 to 0.05%, Cr: 0.1% to 0.3%, N: 0.007% to 0.02%, the balance being Fe and inevitable impurities, and the microstructure contains ferrite and pearlite, A non-quenched and tempered wire rod with improved machinability and impact toughness, characterized by satisfying the following relational expression 1. [Relational expression 1] [N] - [Al] / 1.93 ≤ 0.009

2. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized by satisfying the following relational expression 2. [Relational expression 2] -23[C] + [Si](5 - 2[Si]) - 4[Mn] + 104[S] + 3 ≥ 0

3. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the area fraction of ferrite measured in the region from a position more than 1 / 4 of the wire diameter away from the surface of the wire rod to the center is 20 to 40%.

4. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the area fraction of the generated AlN is 0.03% or more.

5. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the size of the generated AlN is 150 nm or less.

6. The number of carbonitrides per unit area of carbonitrides having an average equivalent circle diameter of 100 nm or less is 2 / μm ２ The non-quenched and tempered wire rod according to claim 1, characterized in that the above is satisfied, and having improved machinability and impact toughness.

7. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the tensile strength is 700 MPa or more.

8. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the yield strength is 350 to 450 MPa.

9. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the yield ratio is 0.45 to 0.

65.

10. The impact toughness at normal temperature is 60 J / cm ２ The free-cutting wire rod with improved machinability and impact toughness according to claim 1, characterized in that the above is satisfied.

11. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the product of the tensile strength and the impact toughness at room temperature is 30,000 to 60,000.

12. Reheating a steel slab containing, by weight, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S: 0.01 to 0.05%, sol.Al: 0.015 to 0.05%, Cr: 0.1% to 0.3%, N: 0.007% to 0.02%, the balance being Fe and inevitable impurities, in a temperature range of 950 to 1100 °C, Manufacturing a wire rod by finish rolling the reheated steel sheet at 750 to 850 °C, and Cooling the wire rod after coiling, including The cooling step after coiling includes cooling to 400 °C at an average cooling rate exceeding 0.1 and not exceeding 5.0 °C / s. The wire rod is characterized by satisfying the following relational expression 1, and is a method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness. [Relational Expression 1] [N] - [Al] / 1.93 ≤ 0.009

13. A method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 12, characterized by satisfying the following relational expression 2. [Relational Expression 2] -23[C] + [Si](5 - 2[Si]) - 4[Mn] + 104[S] + 3 ≥ 0

14. A method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 12, characterized in that it is the area fraction of ferrite measured in the region from a position more than 1 / 4 of the wire rod diameter away from the surface of the wire rod to the center.

15. A method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 12, characterized in that the area fraction of AlN generated in the reheating step is 0.03% or more.

16. A method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 12, characterized in that the size of AlN generated in the reheating step is 150 nm or less.

17. The number of carbonitrides per unit area of carbonitrides having an average equivalent circle diameter of 100 nm or less of the wire rod is 2 / μm ２ The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 12, characterized in that the above is satisfied.

18. A method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 12, characterized in that the tensile strength is 700 MPa or more.

19. A method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 12, characterized in that the yield strength is 350 to 450 MPa.

20. A method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 12, characterized in that the yield ratio is 0.45 to 0.

65.

21. The impact toughness is 60 J / cm ２ The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 12, characterized in that the impact toughness is 60 J / cm or more.

22. A method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 12, characterized in that the product of the tensile strength and the impact toughness is 30,000 to 60,000.

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