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

Abstract

Provided are a non-quenched and tempered wire rod capable of simultaneously ensuring machinability and impact toughness without additional heat treatment, and a method for manufacturing the same. 【Solution means】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 0.02%, with the balance being Fe and inevitable impurities. The microstructure is such that the average thickness of the pearlite layer in the L cross-section, which is a cross-section parallel to the rolling direction, is 30 μm or less, and the average grain size of ferrite is 20 μm or less, and the following relational expressions 1 to 5 are satisfied. [Relational expression 1] 20 ≦ Mn / S ≦ 70 [Relational expression 2] 1.4 ≦ Al / N ≦ 7 [Relational expression 3] 0.7 ≦ Mn + Cr ≦ 1.4 [Relational expression 4] 0.2 ≦ C / Mn ≦ 0.7 [Relational expression 5] 0 ≦ Mn c / Mn f ≦ 3

Description

Technical Field

[0001] The present invention relates to a non-quenched and tempered wire rod excellent in 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 excellent in machinability and impact toughness that can ensure both machinability and impact toughness without additional heat treatment and is suitable for use as a material for automotive parts or mechanical parts, 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 heat treatment of QT (Quenching and Tempering), 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 simplification of processes, and improvement of productivity, but also is an environmentally friendly steel material that can be expected to have an effect of reducing CO2 generated by the operation of the furnace during heat treatment. In the early 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. However, recently, the demand side's 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 parts, 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

[0003] An object of the present invention is to provide a non-quenched and tempered wire rod and a method for manufacturing the same 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

[0004] The quality - free 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 0.02%, the balance being Fe and inevitable impurities. The microstructure contains ferrite and pearlite, and the average thickness of the pearlite layer in the L cross - section, which is a cross - section parallel to the rolling direction, is 30 μm or less.

[0005] In the quality - free wire rod of the present invention, the average grain size of the ferrite in the C cross - section, which is a cross - section perpendicular to the rolling direction, is 20 μm or less.

[0006] The quality - free wire rod of the present invention satisfies the following relational expression 1. [Relational expression 1] 20 ≤ Mn / S ≤ 70

[0007] The quality - free wire rod of the present invention satisfies the following relational expression 2. [Relational expression 2] 1.4 ≤ Al / N ≤ 7

[0008] The quality - free wire rod of the present invention satisfies the following relational expression 3. [Relational expression 3] 0.7 ≤ Mn + Cr ≤ 1.4

[0009] The quality - free wire rod of the present invention satisfies the following relational expression 4. [Relational expression 4] 0.2 ≤ C / Mn ≤ 0.7

[0010] The quality - free wire rod of the present invention satisfies the following relational expression 5. [Relational expression 5] 0 ≤ Mn c / Mn f ≤ 3 (In the above formula, Mn c means the average Mn content (at%) contained in the cementite in the pearlite, and Mn f means the average Mn content (at%) contained in the ferrite in the pearlite.)

[0011] The non-quenched and tempered wire rod of the present invention has a tensile strength of 700 MPa or more, and a yield strength that can be 350 to 500 MPa. Further, the yield ratio (yield strength / tensile strength) value is in the range of 0.45 to 0.65, and the impact toughness value at room temperature is 60 J / cm 2 or more, and the product of the tensile strength and the impact toughness value is 45000 MPa·J / cm 2 or more.

[0012] The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness of the present invention includes reheating a steel slab containing 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%, and the balance being Fe and inevitable impurities in a temperature range of 950 to 1100°C, finishing rolling the reheated steel slab at 750 to 850°C to produce a wire rod, and cooling the wire rod after coiling,

[0013] The cooling step after coiling includes a first cooling step of cooling at an average cooling rate of 5 to 100°C / s from the finish rolling temperature to the coiling temperature, a second cooling step of cooling at an average cooling rate of 2 to 5°C / s from the coiling temperature to 700°C after the first cooling, and a third cooling step of cooling at an average cooling rate of 0.1 to 2°C / s from 700°C to 450°C after the second cooling. The microstructure of the wire rod includes ferrite and pearlite, and the average thickness of the pearlite layer in the L cross-section, which is a cross-section parallel to the rolling direction, is 30 μm or less.

Advantages of the Invention

[0014] The non-quenched and tempered wire rod with improved machinability and impact toughness of the present invention has Al combined with N to form AlN nitride. Such nitrides suppress grain boundary growth during heating, refine the thickness of the pearlite layer and the grain size of ferrite, and improve impact toughness. In addition, by controlling the Mn / S ratio to refine the size of MnS in order to improve machinability, it is possible to ensure machinability while minimizing the decrease in impact toughness. Therefore, it is applicable to automotive materials and mechanical part materials that require both machinability and impact toughness even without heat treatment.

Embodiments for Carrying Out the Invention

[0015] 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%, with the balance being Fe and inevitable impurities. The microstructure contains ferrite and pearlite, and the average thickness of the pearlite layer in the L cross-section, which is a cross-section parallel to the rolling direction, is 30 μm or less.

[0016] This specification does not describe all elements of the embodiments, and omits general content or overlapping content among embodiments in the technical field to which the present invention belongs. Also, when a certain part states that a certain component "includes", it means that other components are not excluded and may further include 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.

[0017] The inventors of the present invention have studied from various angles in order to provide a wire rod that can ensure machinability and impact toughness. As a result, they have found that by appropriately controlling the alloy composition and microstructure of the wire rod, it is possible to ensure machinability and impact toughness without separate heat treatment, and have thus completed the present invention.

[0018] 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%, with the balance being Fe and unavoidable impurities. The microstructure contains ferrite and pearlite, and the average thickness of the pearlite layer in the L cross-section, which is a cross-section parallel to the wire rod rolling direction, satisfies 30 μm or less.

[0019] Hereinafter, the reasons for the numerical limitations of the element contents of the alloy components in the embodiments of the present invention will be described. Unless otherwise specified, the unit is wt%.

[0020] The content of C (carbon) may be 0.3 to 0.5%. C is an element that plays a role in improving the strength of the wire rod. In order to exhibit the above-described effects, it is preferable to contain C at 0.3% or more. However, if the content thereof becomes excessive, toughness and machinability may deteriorate. Therefore, it is desirable to limit the upper limit of the C content to 0.5%.

[0021] The content of Si (silicon) may be 0.4 to 0.9%. Si is an element that is useful as a deoxidizer and also plays a role in improving strength. 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 desirable to limit the upper limit of the Si content to 0.9%.

[0022] The content of Mn (manganese) may be 0.5 to 1.2%. Mn is an element that is useful 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, and the deformation resistance of the steel rapidly increases, so the cold workability may deteriorate. Therefore, it is desirable to limit the upper limit of the Mn content to 1.2%.

[0023] The content of P (phosphorus) may be 0.02% or less. P is an impurity inevitably contained, and is the main element that segregates at the grain boundaries, reduces the toughness of the steel, and causes a decrease in the stress corrosion cracking resistance. 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 it is inevitably contained in the manufacturing process, so it is important to manage the upper limit. In the present invention, the upper limit of the P content is managed to 0.02%.

[0024] The content of S (sulfur) may be 0.01 to 0.05%. S is an element that segregates at the grain boundaries, greatly reduces the ductility of the steel, and is the main cause of forming sulfides in the steel, deteriorating the stress corrosion cracking resistance and stress relaxation characteristics, and is an impurity inevitably contained during the manufacturing process. However, in order to improve machinability as in the present invention, S may also be actively utilized. 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 greatly reduce the toughness of the steel, it is managed within the range of 0.01% to 0.05%.

[0025] The content of sol.Al may be 0.015 to 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, if the content of Al exceeds 0.05%, difficulties in manufacturing may occur due to Al oxides generated during the casting process. Therefore, in the present invention, it is preferable to limit the upper limit of the Al content to 0.05%.

[0026] The content of Cr (chromium) is 0.1 to 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 dissolved carbon and contributes to reducing dynamic strain aging caused by dissolved carbon. When the Cr content is less than 0.1%, the above-mentioned effects cannot be exerted, and when it exceeds 0.3%, the strength of the steel itself is too high and the deformation resistance of the steel rises sharply, which may deteriorate the cold workability. Therefore, it is desirable to limit the upper limit of the Cr content to 0.3%.

[0027] The content of N (nitrogen) may be 0.007 - 0.02%. N is an essential element for realizing the effect of improving impact toughness by forming nitrides together with Al and refining the grain size. When the N content is less than 0.007%, it is difficult to ensure sufficient nitrides and the amount of AlN precipitates decreases, so the toughness targeted in the present invention cannot be ensured. When the N content exceeds 0.02%, the amount of dissolved nitrogen that does not exist as nitrides increases, and the toughness and ductility of the wire rod may decrease. Therefore, in the present invention, it is preferable to limit the upper limit of the N content to 0.02%.

[0028] The balance other than 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 these impurities are known to anyone with ordinary knowledge in the technical field to which the present invention belongs, the present invention does not particularly limit their types and contents.

[0029] The non-quenched and tempered wire rod of the present invention contains ferrite and pearlite as the microstructure, and the average thickness of the pearlite layer in the L cross-section, which is a cross-section parallel to the wire rod rolling direction, may be 30 μm or less. When the thickness of the pearlite exceeds 30 μm and a coarse pearlite band is formed, the total area between ferrite / pearlite decreases, the impact energy cannot be dispersed, the propagation of cracks becomes easy, and the impact toughness decreases.

[0030] The average grain size of the ferrite in the C cross-section, which is the cross-section perpendicular to the rolling direction, of the non-quenched and tempered wire rod of the present invention may be 20 μm or less. By finely controlling the grain size of the ferrite, impact toughness can be ensured.

[0031] The non-quenched and tempered wire rod of the present invention can satisfy the relational expressions 1 to 5. In the relational expressions 1 to 4, [Al], "N", [C], [S], [Mn], and [Si] respectively represent the contents (weight %) of the corresponding elements.

[0032] [Relational expression 1] 20 ≤ Mn / S ≤ 70 (machinability) Relational expression 1 is an expression regarding machinability. In the present invention, MnS is formed by adding high S and Mn. MnS has a form and directionality that extends long in the rolling direction as an elongation inclusion, and can greatly improve the machinability of the non-quenched and tempered wire rod according to the present invention. However, since MnS acts as a crack initiation point and propagation path during impact, it has the effect of deteriorating impact toughness. When the ratio of Mn / S is less than 20, the machinability may be satisfactory, but the impact toughness may decrease. When it exceeds 70, the machinability may not be sufficient. Therefore, in the present invention, the ratio of Mn / S can be limited to 20 to 70.

[0033] [Relational expression 2] 1.4 ≤ [Al] / [N] ≤ 7 (impact toughness) Relational expression 2 is an expression regarding toughness. In the present invention, AlN is formed by adding high N and Al. The precipitation of fine AlN in the steel refines the crystal grains and improves the impact toughness of the non-quenched and tempered wire rod according to the present invention. In order to exhibit the above effects, it is advantageous to generate as many AlN precipitates of 50 Nm or less as possible. For this purpose, it is preferable to control the ratio of Al / N to 1.4 to 7. When the ratio of Al / N is less than 1.4, sufficient AlN precipitates cannot be generated. When it exceeds 7, coarse AlN precipitates are formed, and rather, the impact toughness may be inferior. Therefore, in the present invention, the ratio of Al / N is limited to 1.4 to 7, preferably 1.9 to 5.0, and more preferably 3.5 to 5.0.

[0034] [Relational Expression 3] 0.7 ≤ Mn + Cr ≤ 1.4 (Impact Toughness) Relational Expression 3 is an expression regarding impact toughness. Mn and Cr have the effect of refining the interlamellar spacing of pearlite and improving toughness. In order for such an effect to be fully exerted, the total of Mn and Cr must be 0.7 or more. However, if it exceeds 1.4, the pearlite fraction increases and strength etc. increases excessively, deteriorating the impact toughness. Therefore, in the present invention, the total of Mn and Cr is limited to 0.7 to 1.4, preferably 0.8 to 1.3, and more preferably 1.0 to 1.3.

[0035] [Relational Expression 4] 0.2 ≤ C / Mn ≤ 0.7 (Impact Toughness) Relational Expression 4 is an expression regarding impact toughness. When the C / Mn ratio is less than 0.2, hard tissues with low toughness such as martensite and bainite are likely to be generated, which may deteriorate the impact toughness. Conversely, if it exceeds 0.7, the amount of pearlite with a wide interlamellar spacing increases, deteriorating the impact toughness. Therefore, the value of C / Mn is limited to the range of 0.2 to 0.7, preferably 0.3 to 0.6, and more preferably 0.4 to 0.5.

[0036] [Relational Expression 5] 0 ≤ Mn c / Mn f ≤ 3 (Cold Workability) Mn c means the average Mn content (at%) contained in cementite in pearlite, and Mn f means the average Mn content (at%) contained in ferrite in pearlite.

[0037] Relational expression 5 is an expression related to cold workability and indicates the Mn distribution ratio in pearlite. The Mn distribution ratio in pearlite is the value obtained by dividing the average Mn content (at%) contained in the cementite in pearlite by the average Mn content (at%) contained in the ferrite in pearlite. In the present invention, the Mn distribution ratio in pearlite is limited to 0 to 3. The inventors of the present invention have confirmed through a number of experiments that when the Mn distribution ratio in pearlite satisfies 3 or less, the cold workability is improved, and thus the present invention has been proposed. Since Mn is an element that tends to segregate to cementite in pearlite, ordinary pearlite has an Mn distribution ratio of 5 or more. In order to control such an Mn distribution ratio to 3 or less, it is necessary to suppress the diffusion of Mn into the cementite in pearlite, which can be achieved by the post-winding cooling process in which the cooling rate is applied differently according to the temperature range according to the present invention described later for the Mn distribution ratio in pearlite.

[0038] Moreover, the non-quenched and tempered steel material of the present invention may have a tensile strength of 700 MPa or more.

[0039] The yield strength may be 350 to 500 MPa.

[0040] The yield ratio may be 0.45 to 0.65.

[0041] The impact toughness may be 60 J / cm 2 or more.

[0042] The product of the tensile strength and the impact toughness may be 45000 MPa·J / cm 2 or more.

[0043] Next, the manufacturing method of the non-quenched and tempered wire rod of the present invention will be described. The manufacturing method of 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: 0.015 to 0.05%, Cr: 0.1% to 0.3%, N: 0.007% to 0.02%, with the balance being Fe and inevitable impurities. The method includes reheating a steel slab in a temperature range of 950 to 1100°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 post-coiling cooling stage includes a first cooling stage of cooling at an average cooling rate of 5 to 100°C / s from the finish rolling temperature to the coiling temperature, a second cooling stage of cooling at an average cooling rate of 2 to 5°C / s from the coiling temperature to 700°C after the first cooling, and

[0044] after the second cooling, a third cooling stage of cooling at an average cooling rate of 0.1 to 2°C / s from 700°C to 450°C.

[0045] The microstructure of the wire rod includes ferrite and pearlite, and the average thickness of the pearlite layer in the L cross-section, which is a cross-section parallel to the rolling direction, may be 30 μm or less.

[0046] Also, according to the present invention, the average grain size of the ferrite in the C cross-section, which is a cross-section perpendicular to the rolling direction, may be 20 μm or less.

[0047] Further, the present invention can satisfy the following relational expression 1. [Relational expression 1] 20 ≤ Mn / S ≤ 70

[0048] Also, the present invention can satisfy the following relational expression 2. [Relational expression 2] 1.4 ≤ Al / N ≤ 7

[0049] Further, the present invention can satisfy the following relational expression 3. [Relational expression 3] 0.7 ≤ Mn + Cr ≤ 1.4

[0050] Also, the present invention can satisfy the following relational expression 4. [Relational Expression 4] 0.2 ≤ C / Mn ≤ 0.7

[0051] Also, the present invention can satisfy the following Relational Expression 5. [Relational Expression 5] 0 ≤ Mn c / Mn f ≤ 3

[0052] Hereinafter, each manufacturing stage will be described in more detail. First, after heating a bloom that satisfies the above-described component system, the steel slab is rolled to obtain a billet.

[0053] Reheating stage The reheating stage is a stage of 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 1100°C. If the reheating temperature of the steel slab is less than 950°C, the rolling load may increase, causing difficulties in manufacturing. On the other hand, if it exceeds 1100°C, the AlN generated in the steel slab will redissolve during heating, and the grain size refinement effect by AlN will be significantly reduced.

[0054] 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 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, and if it exceeds 850°C, the crystal grains will become coarse, making it difficult to ensure the high toughness targeted in the present invention.

[0055] Coiling stage A process of winding the wire rod produced in the wire rod rolling stage into a coil can be performed. At this time, the winding temperature can be 750 to 850 °C. Since the temperature of the wire rod obtained by the finish rolling may rise due to transformation heat generation, the temperature of the wire rod immediately before winding may be higher than the temperature at which the finish rolling was performed. At this time, according to the temperature increased by the heat generation, after cooling to the winding temperature, winding can be performed, or winding can be performed without separate cooling. If the temperature during winding 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 possibility of causing surface defects during wire drawing. On the other hand, if the temperature exceeds 850 °C, a thick scale is formed 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.

[0056] Cooling stage In the cooling stage, after winding the finish-rolled wire rod, it is a stage of cooling to obtain the non-quenched and tempered wire rod according to the present invention, and it is a process for controlling the Mn distribution ratio contained in the cementite in pearlite and ferrite described above. In order to control the Mn distribution ratio of the cementite in pearlite to 3 or less, it is necessary to suppress the diffusion of Mn to the maximum extent during the cooling process. In order to suppress the diffusion of Mn into cementite to the maximum extent, it is effective to apply different cooling rates according to temperature ranges.

[0057] The first cooling stage (CR1): Finish rolling temperature ~ Winding temperature The first cooling stage can be performed at an average cooling rate of 5 to 100 °C / s from the finish rolling temperature to the winding temperature. The temperature range of the first cooling stage is a region where the diffusion of Mn occurs very quickly. At a cooling rate of less than 5 °C / s, the Mn distribution ratio is likely to exceed 3 due to the diffusion of Mn, and a cooling rate exceeding 100 °C / s has a limit that is difficult to apply commercially. Therefore, it is desirable to perform the first cooling stage at a cooling rate of 5 to 100 °C / S.

[0058] Second Cooling Stage (CR2): Coiling Temperature ~ 700 °C The second cooling stage can be carried out at an average cooling rate of 2 - 5 °C / S from the coiling temperature to 700 °C after the first cooling process. If the cooling rate is less than 2 °C / S, the Mn distribution ratio may exceed 3 due to the diffusion of Mn. If the cooling rate exceeds 5 °C / S, material non-uniformities such as mixed grains may occur due to uneven cooling. Therefore, it is preferable to carry out the second cooling stage at a cooling rate of 2 - 5 °C / S.

[0059] Third Cooling Zone (CR3): Temperature of 700 - 450 °C The third cooling stage can be carried out at an average cooling rate of 0.1 - 2 °C / s from 700 °C to 450 °C after the second cooling process. If the cooling rate is less than 0.1 °C / s, the pearlite lamellar spacing becomes coarsened, making it difficult to ensure the strength targeted in the present invention. If the cooling rate exceeds 2 °C / s, a bainite low-temperature structure may be generated during cooling. Therefore, it is preferable to carry out the third cooling stage at a cooling rate of 0.1 - 2 °C / s.

[0060] Hereinafter, the present invention will be described in more detail through embodiments. However, the description of these embodiments is for illustrative purposes of implementing the present invention, and the present invention is not limited by the description of these embodiments. This is because the scope of the rights of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0061] Example A bloom having an alloy composition as shown in Table 1 below was heated at 1,200 °C for 4 hours, and then hot-rolled into a billet at a finish rolling temperature of 1,100 °C. Thereafter, the billet was heated at 1,100 °C for 90 minutes, and then hot-rolled into a wire rod using a φ25 mm roll at a finish rolling temperature of 800 °C. Next, wire rod specimens of Examples 1 to 7 and Comparative Examples 1 to 13 were produced by applying a three-stage cooling process according to the temperature ranges of CR1 - CR2 - CR3. Thereafter, the microstructure of the cooled wire rod specimens and the Mn distribution ratio of ferrite / cementite are shown in Table 2 below, and the tensile and impact toughness properties were measured and shown in Table 3 below.

[0062] Here, the room temperature tensile strength was measured by sampling from the center of an as-rolled 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, 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 rate of 14.8%. Machinability was evaluated using a CNC lathe. After turning the CD bar with a diameter of 24 mm until it reached a diameter of 15 mm, the segmentation of the turning chip was evaluated. The cutting conditions at this time 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 it was performed using cutting oil. The segmentation of the turning chip was judged based on the number of turns of the turning chip generated during turning. When the turning chip was generated with 5 turns or less, it was judged as good; when it occurred with more than 5 turns and 10 turns or less, it was judged as ordinary; when it occurred with more than 10 turns, it was judged as poor, and it is shown in Table 3.

[0063] Also, the average thickness of the pearlite layer was obtained by taking arithmetic means after photographing 30 locations at a magnification of 200 times at a point 1 / 4 of the wire rod diameter, and the average grain size of ferrite means a value corresponding to the equivalent circular diameter.

[0064]

Table 1

[0065]

Table 2

[0066]

Table 3

[0067] Specifically, Examples 1 to 7 satisfy all the chemical components, relational expressions, and manufacturing conditions shown in the present invention, and satisfy all of a tensile strength of 700 MPa or more, a normal temperature impact toughness of 60 J or more, and a value of tensile strength × impact toughness of 45,000 MPa·J / cm or more and machinability. On the other hand, Comparative Examples 1 to 5 that deviate from the chemical components do not satisfy at least one or more values. Comparative Examples 6 to 9 satisfy the range of the chemical components shown in the present invention, but the values of the relational expressions deviate from the range and the target physical property values cannot be satisfied. And in the case of Comparative Examples 10 to 13 that could not satisfy the heating temperature and cooling conditions among the manufacturing conditions, the tensile strength and impact toughness could not satisfy the target values at the same time.

Industrial Applicability

[0068] According to the present invention, it is possible to provide a non-quenched and tempered wire rod that can ensure both machinability and impact toughness without additional heat treatment, so 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, the microstructure includes ferrite and pearlite, and the average thickness of the pearlite layer in the L cross-section which is a cross-section parallel to the rolling direction is 30 μm or less, and it is characterized in that it is a non-quenched and tempered wire rod with improved machinability and impact toughness.

2. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized in that the average grain size of the ferrite in the C cross-section which is a cross-section perpendicular to the rolling direction is 20 μm or less.

3. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized by satisfying the following relational expression 1. 【Relational expression 1】20 ≤ Mn / S ≤ 70

4. 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】1.4 ≤ Al / N ≤ 7

5. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized by satisfying the following relational expression 3. 【Relational expression 3】0.7 ≤ Mn + Cr ≤ 1.4

6. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized by satisfying the following relational expression 4. 【Relational expression 4】0.2 ≤ C / Mn ≤ 0.7

7. The non-quenched and tempered wire rod with improved machinability and impact toughness according to Claim 1, characterized by satisfying the following relational expression 5. [Relational Expression 5] 0 ≤ Mn ｃ / Mn ｆ ≤ 3 (Mn ｃ is the average Mn content (at%) contained in the cementite in pearlite, and Mn ｆ means the average Mn content (at%) contained in the ferrite in pearlite.)

8. 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.

9. 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 500 MPa.

10. 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.

11. The impact toughness at normal temperature is 60 J / cm ２ The non-quenched and tempered wire rod according to claim 1, characterized in that the above is satisfied, which has improved machinability and impact toughness.

12. The product of the tensile strength and the impact toughness at room temperature is 45000 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.

13. In terms of weight percentage, it 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%, and the balance consists of Fe and inevitable impurities. The step of reheating the steel sheet in the temperature range of 950 to 1100°C, The step of finish rolling the reheated steel sheet at 750 to 850°C to produce a wire rod, and After winding up the wire rod, the step of cooling is included, The cooling step after winding up includes a first cooling step of cooling at an average cooling rate of 5 to 100°C / s from the finish rolling temperature to the winding temperature, After the first cooling, a second cooling step of cooling at an average cooling rate of 2 to 5°C / s from the winding temperature to 700°C, and After the second cooling, a third cooling step of cooling at an average cooling rate of 0.1 to 2°C / s from 700°C to 450°C is included, The microstructure of the wire rod includes ferrite and pearlite, and the average thickness of the pearlite layer in the L cross-section which is a cross-section parallel to the rolling direction is 30 μm or less. A method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness.

14. The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 13, wherein the average grain size of the ferrite in the C cross-section which is a cross-section perpendicular to the rolling direction is 20 μm or less.

15. The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 13, characterized in that the following relational expression 1 is satisfied. [Relational expression 1] 20 ≦ Mn / S ≦ 70

16. The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 13, characterized in that the following relational expression 2 is satisfied. [Relational expression 2] 1.4 ≦ Al / N ≦ 7

17. The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 13, characterized in that the following relational expression 3 is satisfied. [Relational expression 3] 0.7 ≦ Mn + Cr ≦ 1.4

18. The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 13, characterized in that the following relational expression 4 is satisfied. [Relational expression 4] 0.2 ≦ C / Mn ≦ 0.7

19. The method for manufacturing a non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 13, characterized in that the following relational expression 5 is satisfied. [[Relationship formula 5]] 0 ≤ Mn ｃ / Mn ｆ ≤ 3 (Mn ｃ represents the average Mn content (at%) contained in the cementite in pearlite, and Mn ｆ represents the average Mn content (at%) contained in the ferrite in pearlite.)

20. The manufacturing method of the non-quenched and tempered wire rod with improved machinability and impact toughness according to claim 13, characterized in that the winding temperature is 750 to 850 °C.

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