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

Abstract

Provided are a non-quenched and tempered wire rod and a method for manufacturing the same, which can simultaneously ensure machinability and impact toughness without additional heat treatment by adding a high amount of S and a high amount of N. 【Method for Solving the Problem】The non-quenched and tempered wire rod according to the present invention comprises, 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: 0.01 to 0.05%, Cr: 0.1 to 0.3%, Ti: 0.01 to 0.02%, Ca: 0.0005 to 0.002%, N: 0.007 to 0.02%, and the balance of Fe and inevitable impurities, contains ferrite and pearlite as a microstructure, satisfies the following relational expression 1, and satisfies that the area fraction of MnS ranges from 0.10 to 0.60%. [Relational Expression 1] 20 ≤ [Mn] / [S] ≤ 70

Description

Technical Field

[0001] The present invention relates to a non-quenched and tempered wire rod with improved machinability and impact toughness, and a manufacturing method thereof. More specifically, the present invention relates to a non-quenched and tempered wire rod suitable for use as a material for automobiles or mechanical parts, and a manufacturing method thereof.

Background Art

[0002] Unlike quenched and tempered steel that 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 has advantages not only 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 an environmentally friendly steel material that can be expected to reduce CO2 generated by the operation of the furnace during heat treatment. In the initial stage of development, non-quenched and tempered steel has been applied only to parts that do not require high toughness because it is relatively inferior in toughness compared to quenched and tempered steel.

[0003] However, recently, the requirements for environmental issues and cost reduction have increased, and the demand for improving the toughness of non-quenched and tempered steel has also been increasing. Furthermore, cutting is often performed to ensure the final shape of the part, and machinability is also required at the same time. In order to improve machinability, generally, a large amount of MnS is generated by adding S, which causes a problem that the toughness of the product decreases.

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide a non-quenched and tempered wire rod and a manufacturing method thereof that can overcome the inferior toughness compared to conventional quenched and tempered steel and ensure both machinability and impact toughness without additional heat treatment by adding high S and high N.

Means for Solving the Problems

[0005] The non-quenched and tempered wire rod with improved machinability and impact toughness of the present invention consists of, 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: 0.01 to 0.05%, Cr: 0.1 to 0.3%, Ti: 0.01 to 0.02%, Ca: 0.0005 to 0.002%, N: 0.007 to 0.02%, the balance being Fe and inevitable impurities, contains ferrite and pearlite as the microstructure, satisfies the following relational expression 1, and is characterized in that the area fraction of MnS is in the range of 0.10 to 0.60%. [Relational Expression 1] 20 ≤ [Mn] / [S] ≤ 70

[0006] According to the present invention, in the non-quenched and tempered wire rod with improved machinability and impact toughness, the number density of MnS is 70 pieces / mm 2 or more, and the major axis to minor axis ratio of MnS is preferably 40 or less.

[0007] The non-quenched and tempered wire rod with improved machinability and impact toughness of the present invention preferably has a tensile strength of 700 MPa or more, a yield strength in the range of 350 to 500 MPa, and a yield ratio in the range of 0.45 to 0.65. The non-quenched and tempered wire rod of the present invention has an impact toughness of 60 J / cm 2 or more, and the product of the tensile strength and the impact toughness can be 45000 MPa·J / cm 2 or more.

[0008] The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to the present invention comprises, 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: 0.01 to 0.05%, Cr: 0.1 to 0.3%, Ti: 0.01 to 0.02%, Ca: 0.0005 to 0.002%, N: 0.007 to 0.02%, and the balance being Fe and unavoidable impurities, reheating a steel slab containing ferrite and pearlite as a microstructure in a temperature range of 950 to 1120°C, finish-rolling the reheated steel slab at 750 to 850°C to produce a wire rod, and cooling the wire rod after coiling. The cooling step after coiling includes cooling at an average cooling rate of 0.1 to 5.0°C / s until 400°C. The wire rod contains ferrite and pearlite as a microstructure, satisfies relational expression 1, and preferably has an area fraction of MnS of 0.10 to 0.60%.

Effects of the Invention

[0009] In the non-quenched and tempered wire rod with improved machinability and impact toughness according to an embodiment of the present invention, Ti and Al combine with N to form nitrides such as TiN and AlN. Such nitrides prevent grain boundary growth, refine the grain size, and improve toughness. Further, since the Ca-based oxide due to the addition of Ca acts as a nucleation site for the formation of MnS, the elongation of MnS during rolling is suppressed, improving machinability and toughness. Therefore, it can be used for automotive materials and mechanical part materials that require both machinability and toughness even without heat treatment.

Modes for Carrying Out the Invention

[0010] The non-quenched and tempered wire rod with improved machinability and impact toughness according to the present invention comprises, 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: 0.01 to 0.05%, Cr: 0.1 to 0.3%, Ti: 0.01 to 0.02%, Ca: 0.0005 to 0.002%, N: 0.007 to 0.02%, and the balance being Fe and unavoidable impurities, and contains ferrite and pearlite as a microstructure. It satisfies the following relational expression 1, and the area fraction of MnS is in the range of 0.10 to 0.60%. [Relational expression 1] 20 ≤ [Mn] / [S] ≤ 70

[0011] This specification does not describe all elements of the embodiments, and general content or overlapping content among embodiments in the technical field to which the present invention pertains is omitted. Also, when a certain part states that a certain component "includes", unless otherwise stated to the contrary, it does not exclude other components, but rather means that it may further include other components. Singular expressions include plural expressions unless there are obvious exceptions in the context. Hereinafter, the present invention will be described in detail.

[0012] The inventors have studied from various angles in order to provide a wire rod that can ensure machinability and impact toughness. As a result, it has been found that by appropriately controlling the alloy composition and microstructure of the wire rod, machinability and impact toughness can be ensured without separate heat treatment, and the present invention has been completed.

[0013] The non-quenched and tempered wire rod with improved machinability and impact toughness of the present invention consists of, 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: 0.01 to 0.05%, Cr: 0.1 to 0.3%, Ti: 0.01 to 0.02%, Ca: 0.0005 to 0.002%, N: 0.007 to 0.02%, the balance being Fe and inevitable impurities, includes ferrite and pearlite as the microstructure, satisfies the following relational expression 1, and the area fraction of MnS is in the range of 0.10 to 0.60%. [Relational expression 1] 20 ≤ [Mn] / [S] ≤ 70 Hereinafter, the reasons for the numerical limitations of the alloy component element contents in the embodiments of the present invention will be described. Hereinafter, unless otherwise specified, the unit is weight%.

[0014] The content of C (carbon) is 0.3 to 0.5%. C is an element that plays a role in improving the strength of the wire rod. To exert this effect, it is preferable to contain 0.3% or more of C. However, if the content thereof becomes excessive, there is a risk that the toughness and machinability will deteriorate. Therefore, the upper limit of the content of C is limited to 0.5%.

[0015] The content of Si (silicon) is 0.4 - 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 effects cannot be exerted. On the other hand, when it exceeds 0.9%, the deformation resistance of the steel rapidly increases due to solid solution strengthening, and there is a risk that the cold workability deteriorates. Therefore, in the present invention, the upper limit of the content of Si is limited to 0.9%.

[0016] The content of Mn (manganese) is 0.5 - 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-mentioned effects cannot be exerted. On the other hand, when the content of Mn exceeds 1.2%, the strength of the steel itself is too high, and the deformation resistance of the steel rapidly increases, so there is a risk that the cold workability deteriorates. Therefore, the upper limit of the content of Mn is limited to 1.2%.

[0017] The content of Cr (chromium) 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, without increasing the strength of the steel itself more than necessary, it precipitates carbides in the steel to reduce the amount of solid solution carbon and contributes to the reduction of dynamic strain aging due to solid solution carbon. When the content of Cr is less than 0.1%, the above-mentioned effects cannot be exerted. On the other hand, when it exceeds 0.3%, the strength of the steel itself is too high, and the deformation resistance of the steel rapidly increases, which may deteriorate the cold workability. Therefore, the upper limit of the Cr content is limited to 0.3%.

[0018] The content of P (phosphorus) is 0.02% or less. P is an impurity that is inevitably contained, segregates at grain boundaries, and is the main cause of reducing the toughness of steel and decreasing the resistance to delayed fracture. Therefore, in the present invention, it is desirable to control its content as low as possible. Theoretically, it is advantageous to control the content of P to 0%, but due to inevitable inclusion 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 to be 0.02%.

[0019] The content of S (sulfur) is 0.01 to 0.05%. S is an element that segregates at grain boundaries, greatly reduces the ductility of steel, and is the main cause of forming sulfides in steel and deteriorating the resistance to delayed fracture and stress relaxation characteristics. It is an impurity that is inevitably contained during the manufacturing process. However, it is preferable to actively utilize S to improve machinability as in the present invention. S combines with Mn to form MnS to improve machinability. In the present invention, considering the content of S effective for improving machinability within the range that does not significantly reduce the toughness of steel, it is managed within the range of 0.01% to 0.05%.

[0020] The content of sol.Al is 0.01 to 0.05%. sol.Al is an element that usefully acts as a deoxidizer. To exhibit this effect, it is preferable that sol.Al is contained at 0.01% or more. However, if the content of Al exceeds 0.05%, there is a risk of manufacturing difficulties due to Al oxides generated in the casting process. Therefore, in the present invention, the upper limit of the sol.Al content is limited to 0.05%.

[0021] The content of Ti (titanium) is 0.01 to 0.02%. Ti is an element that plays a significant role in improving the toughness of steel by forming TiN precipitates during the solidification process of steel, suppressing the growth of austenite grain size during the slab heating and hot rolling processes, and refining the grain size of the final microstructure. When the content of Ti is less than 0.01%, it is difficult to ensure a sufficient amount of TiN precipitates to limit the movement of austenite grain boundaries. On the other hand, when it exceeds 0.02%, there is a problem that coarse titanium nitride is generated and the toughness is rather deteriorated. Therefore, in the present invention, the upper limit of the content of Ti is limited to 0.02%.

[0022] The content of Ca (calcium) is 0.0005 - 0.002%. Ca is an essential element for reducing the aspect ratio of MnS and improving machinability and impact toughness. When Ca is added, oxides are formed, which act as nuclei for MnS generation, suppress the elongation of MnS during wire rod rolling, and can maintain a low aspect ratio. The low aspect ratio of MnS not only improves machinability but also alleviates the anisotropy of the fine structure and prevents the deterioration of toughness. However, in order to show this effect, Ca must be contained at least 0.0005%, but when the content of Ca exceeds 0.002%, there may be a risk of making the manufacturing difficult. Therefore, in the present invention, the upper limit of the Ca content is limited to 0.002%.

[0023] The content of N (nitrogen) is 0.007 - 0.02%. N is an essential element for realizing the effect of improving impact toughness by forming nitrides together with Ti and Al and refining the grain size. When the content of N is less than 0.007%, it is difficult to ensure sufficient nitrides, and the amount of precipitates such as Al and Ti decreases, making it impossible to ensure the toughness targeted in the present invention. On the other hand, when the content of N exceeds 0.02%, the amount of solid solution nitrogen that does not exist in nitrides increases, and there is a risk of reducing the toughness and ductility of the wire rod. Therefore, in the present invention, the upper limit of the content of N is limited to 0.02%.

[0024] The balance other than the alloy composition is Fe. The non-quenched and tempered wire rod of the present invention may usually contain other impurities that may be included in the industrial production process of steel. Since these impurities are contents that can be known to anyone 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.

[0025] The non-quenched and tempered wire rod according to an embodiment of the present invention can satisfy the following relational expression 1. In relational expression 1, each of [S] and [Mn] means the content (weight %) of the element. [Relational expression 1] 20 ≦ [Mn] / [S] ≦ 70 Relational expression 1 is an expression regarding machinability. In the present invention, MnS is generated by adding high S and Mn. MnS has a form and directionality that are elongated in the rolling direction as an elongation inclusion, and greatly improves the machinability of the non-quenched and tempered wire rod according to the present invention. However, MnS acts as a starting point and propagation path of cracks during impact and deteriorates the impact toughness. When the ratio of [Mn] / [S] is less than 20, the machinability is satisfactory but the impact toughness may decrease. On the other hand, when it exceeds 70, the machinability may not be sufficient. Therefore, in the present invention, the ratio of [Mn] / [S] is limited to 20 to 70. More preferably, it is 30 to 60.

[0026] In the non-quenched and tempered steel material of the present invention, the area fraction of MnS is 0.10 to 0.60%, preferably 0.15 to 0.50%, and more preferably 0.15 to 0.45%. Further, the non-quenched and tempered steel material according to an embodiment of the present invention can include a number density of MnS of 70 pieces / mm 2 or more, preferably 80 pieces / mm 2 or more, and more preferably 90 pieces / mm 2 or more. When the density of MnS increases, it acts as a stress concentration source during cutting to reduce the cutting resistance and improve the machinability. For this reason, the density of MnS must be at least 70 pieces / mm 2 or more.

[0027] The non-quenched and tempered steel material of the present invention has an aspect ratio of MnS of 40 or less, preferably 30 or less, and more preferably 20 or less. At this time, when the aspect ratio of MnS exceeds 40, the impact toughness will be rapidly reduced. In addition, the non-quenched and tempered steel material of the present invention has a tensile strength of 700 MPa or more. Furthermore, the non-quenched and tempered steel material of the present invention has a yield strength of 350 to 500 MPa. Moreover, the non-quenched and tempered steel material of the present invention has a yield ratio of 0.45 to 0.65.

[0028] In addition, the non-quenched and tempered steel material of the present invention has an impact toughness of 60 J / cm 2 or more, and the product of the tensile strength and the impact toughness is 45000 MPa·J / cm 2 or more. Next, a method for manufacturing a non-quenched and tempered wire rod according to an embodiment of the present invention will be described.

[0029] The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness of the present invention comprises, 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: 0.01 to 0.05%, Cr: 0.1 to 0.3%, Ti: 0.01 to 0.02%, Ca: 0.0005 to 0.002%, N: 0.007 to 0.02%, the balance being Fe and unavoidable impurities, reheating a steel slab containing ferrite and pearlite as a microstructure in a temperature range of 950 to 1120 °C, finish-rolling the reheated steel slab at 750 to 850 °C to produce a wire rod, and cooling the wire rod after coiling, wherein the cooling step after coiling includes cooling at an average cooling rate of 0.1 to 5.0 °C / s to 400 °C, the wire rod contains ferrite and pearlite as a microstructure, satisfies relational expression 1, and the area fraction of MnS is 0.10 to 0.60%. 〔Relational expression 1] 20 ≦ [Mn] / [S] ≦ 70

[0030] Hereinafter, each manufacturing process will be described in more detail. First, heat the bloom that satisfies the above-described component system, and then roll the steel slab to obtain a billet.

[0031] Reheating stage The reheating stage is a process stage for reheating the rolled billet, and is a stage for reducing the rolling load during wire rod rolling. At this time, the reheating can be performed at a temperature of 950 to 1120 °C. If the reheating temperature of the steel slab is less than 950 °C, the rolling load may increase, resulting in manufacturing difficulties. On the other hand, if it exceeds 1120 °C, all the finely generated AlN in the steel slab will be redissolved during reheating, and the effect of grain size refinement will be significantly reduced.

[0032] Wire rod rolling stage In the wire rod rolling stage, the reheated steel slab is hot-rolled into a wire rod. At this time, the finishing rolling temperature of the hot rolling is 750 to 850 °C. If the finishing rolling temperature is less than 750 °C, the rolling load may increase. On the other hand, if it exceeds 850 °C, the crystal grains will coarsen, and it may be difficult to ensure the high toughness aimed at in the present invention.

[0033] Coiling stage Perform a process of coiling the wire rod manufactured as described above into a coil. This coiling temperature is 750 to 850 °C. Since the temperature of the wire rod obtained by the finishing rolling may rise due to transformation heat generation, the temperature of the wire rod immediately before coiling may be higher than the temperature at which the finishing rolling was performed. At this time, depending on the temperature increased by the heat generation, after cooling to the coiling temperature, coiling can be performed, or coiling can be performed without separate cooling. If the temperature during coiling is less than 750 °C, the martensite in the surface layer generated during cooling cannot be recovered by reheating, tempered martensite is generated, and there is a high risk of causing surface defects during cold working. On the other hand, if the temperature exceeds 850 °C, a thick scale is generated on the wire rod surface, not only is it easy to generate surface defects during descaling, but also the cooling time becomes excessive during subsequent cooling, and there is a risk of productivity decline.

[0034] Cooling stage The coiled wire rod can undergo a cooling process, and the cooling rate at this time is characterized in that the average cooling rate is in the range of 0.1 to 5.0 °C / s and it is cooled to 400 °C by 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, the excessive generation of primary ferrite cannot satisfy the target strength, while when it exceeds 5 °C / s, low-temperature structures such as martensite may occur, which may reduce toughness and machinability. Examples

[0035] 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 for 90 minutes under the temperature conditions shown in Table 2 below, then finishing rolling was performed at 800 °C, coiling was performed at 780 °C, and then it was cooled under the temperature conditions shown in Table 2 to manufacture 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 6 were manufactured (Table 1), and the machinability, tensile strength, and impact toughness of the collected wire rod specimens were measured.

[0036] Here, the room temperature tensile strength was measured by sampling from the center of the non-quenched and tempered steel specimen at 25 °C, and the room temperature impact toughness was measured 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. The obtained value was evaluated as the Charpy impact energy value.

[0037] Also, for the machinability evaluation, a wire rod with a diameter of 26 mm was manufactured as a 24-mm diameter cold drawn bar (CD bar) by applying a reduction ratio of 14.8%. The machinability was evaluated using a CNC lathe. After turning the 24-mm diameter CD bar until it reached a diameter of 15 mm, the degree of wear of the turning tool was evaluated. At this time, the cutting conditions were 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 was carried out using cutting oil. As the cutting tool, a cermet tool with a chip breaker was used. The wear depth of the tool was measured by measuring the depth of the flank wear surface after continuously machining 300 parts having the above-described shape. If it exceeded 0.2 mm, it was judged as defective; if it was 0.2 mm or less, it was judged as good. Also, the area fraction of MnS, the number density of MnS, and the aspect ratio of MnS were each photographed 20 times at a magnification of 200 times on the cross-section of the wire rod L using an optical microscope and analyzed using image analysis software.

[0038]

Table 1

[0039]

Table 2

[0040]

Table 3

[0041] As can be confirmed from Tables 1 to 3 above, in the case of Examples 1 to 7, since all of the chemical components, relational expressions, area fraction of MnS, number density, aspect ratio, and manufacturing conditions presented in the present invention are satisfied, a tensile strength of 700 MPa or more, a yield strength of 350 to 500 MPa, a yield ratio of 0.45 to 0.65, a product of tensile strength and impact toughness of 45000 MPa· / cm 2 or more, an impact toughness of 60 J / cm 2 or more, and good machinability can be ensured.

[0042] On the other hand, in the case of Comparative Examples 1 to 9 that do not satisfy one or more of the conditions proposed in the present invention, it can be confirmed that one or more of the properties of tensile strength, impact toughness, tensile strength × impact toughness, and machinability are inferior. Specifically, in Comparative Example 1, since it is outside the range of carbon content and has high strength, tool wear is poor. In Comparative Examples 2 and 3, the impact toughness was insufficient due to excessive Si and Mn contents. Also, in Comparative Example 4, the content of Ti was insufficient and the effect of grain refinement was not sufficiently achieved, resulting in inferior impact toughness. In Comparative Example 5, the impact toughness decreased due to the large aspect ratio of MnS. In Comparative Example 6, the value of the relational expression (1) could not be satisfied, and the fraction and density of MnS were not sufficient, so the cutting tool wear was poor. In Comparative Examples 7 to 9, although all the chemical components were satisfied, they were outside the ranges of heating temperature and cooling rate, and the toughness was inferior or the target strength was not satisfied.

[0043] As described above, although the exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and those having ordinary knowledge in the relevant technical field can understand that various changes and modifications can be made without departing from the concept and scope of the claims described below.

Industrial Applicability

[0044] According to the present invention, since it is possible to provide a non-quenched and tempered wire rod and a method for manufacturing the same that can simultaneously ensure machinability and impact toughness without additional heat treatment, 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: 0.01 to 0.05%, Cr: 0.1 to 0.3%, Ti: 0.01 to 0.02%, Ca: 0.0005 to 0.002%, N: 0.007 to 0.02%, consisting of the balance Fe and inevitable impurities, including ferrite and pearlite as the microstructure, satisfying the following relational expression 1, and having an area fraction of MnS in the range of 0.10 to 0.60%, characterized by a free-cutting wire rod with improved machinability and impact toughness. [Relational Expression 1]20 ≤ [Mn] / [S] ≤ 70

2. The number density of MnS is 70 pieces / mm ２ The non-quenched and tempered wire rod according to claim 1, characterized in that the above is satisfied, and the machinability and impact toughness are improved.

3. The free-cutting wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the aspect ratio of MnS is 40 or less.

4. The free-cutting wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the tensile strength is 700 MPa or more.

5. The free-cutting wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the yield strength is 350 to 500 MPa.

6. The free-cutting wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the yield ratio is 0.45 to 0.

65.

7. The impact toughness is 60 J / cm ２ The non-quenched and tempered wire rod according to claim 1, characterized in that the machinability and impact toughness are improved and the impact toughness is 60 J / cm or more.

8. The product of the tensile strength and the impact toughness is 45,000 MPa·J / cm ２ The non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 1, characterized in that the above is satisfied.

9. 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: 0.01 to 0.05%, Cr: 0.1 to 0.3%, Ti: 0.01 to 0.02%, Ca: 0.0005 to 0.002%, N: 0.007 to 0.02%, consisting of the balance Fe and inevitable impurities, Reheating a steel slab containing ferrite and pearlite as the microstructure in a temperature range of 950 to 1120 °C, Performing finish rolling on the reheated steel slab at 750 to 850 °C to produce a wire rod, and Cooling the wire rod after coiling, The cooling step after coiling includes a step of cooling at an average cooling rate of 0.1 to 5.0 °C / s until 400 °C, The wire rod includes ferrite and pearlite as the microstructure, Satisfying the following relational expression 1, A method for manufacturing a free-cutting wire rod with improved machinability and impact toughness, characterized in that the area fraction of MnS is 0.10 to 0.60%. [Relational Expression 1]20 ≤ [Mn] / [S] ≤ 70

10. The number density of MnS is 70 pieces / mm ２ The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 9, characterized in that the above is satisfied.

11. The manufacturing method of the non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 9, characterized in that the aspect ratio of MnS is 40 or less.

12. The manufacturing method of the non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 9, characterized in that the coiling temperature is 750 to 850 °C.

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