Wire having excellent drawability and method for manufacturing the same

Abstract

The present invention relates to a mechanical structural wire suitable for use in vehicles, building components, and the like, and a method for manufacturing the same, and to a wire having excellent drawability and a method for manufacturing the same.

Description

TECHNICAL FIELD

[0001] The present invention relates to a mechanical structural wire for vehicles, building components, etc., and a manufacturing method thereof, and to a wire having excellent drawability and a manufacturing method thereof. BACKGROUND

[0002] Mechanical structural steel for automobiles, building components, etc., such as bearings, is generally manufactured by drawing a rolled wire and cold working the rolled wire into a complex shape.

[0003] However, since the above-described steel is a hypereutectoid steel, and is difficult to work, it is difficult to directly draw the rolled wire. For this purpose, a spheroidizing softening heat treatment is performed, the material is sized by wire drawing, and then, the strength increase due to wire drawing is corrected by an additional spheroidizing softening heat treatment, thereby preparing a softened material.

[0004] The above-described spheroidizing softening heat treatment aims to improve cold workability, and by such a spheroidizing softening heat treatment, cementite in the microstructure is spheroidized, and uniform particle distribution is induced. Thus, it is possible to prevent breakage during wire drawing, improve the life of a small block being worked, and reduce the hardness of the worked material.

[0005] However, when the above-described spheroidizing softening heat treatment is performed, the heat treatment cost can be high, and a prolonged production time can be required, which can result in an increase in manufacturing cost. In addition, it also fails to meet the current latest requirements for minimizing energy consumption in order to reduce carbon emissions. Therefore, in recent years, in providing a wire for bearings, etc., there is a need to develop a wire that can ensure excellent drawability while omitting or shortening the spheroidizing softening heat treatment. SUMMARY

[0006] TECHNICAL PROBLEM

[0007] One aspect of the present disclosure is to provide a wire for mechanical structural components such as bearings, and in particular, to provide a wire that can omit or shorten a spheroidizing softening heat treatment and ensure drawability and strength, and a manufacturing method thereof.

[0008] The objects of the present invention are not limited to the above. Further subjects of the present invention will not be difficult to understand for those skilled in the art from the general content of the present specification.

[0009] TECHNICAL SOLUTION

[0010] According to one aspect of the present disclosure, there is provided a wire having excellent drawability, the wire comprising, in terms of weight %, C: 0.8 to 1.2 %, Si: 0.01 to 0.6 %, Mn: 0.1 to 0.6 %, Cr: 0.8 to 2.0 %, Al: 0.01 to 0.06 %, N: 0.02 % or less (excluding 0), and Fe and inevitable impurities in the balance,

[0011] wherein the microstructure contains pearlite main structure and pro-eutectoid cementite, and

[0012] contains at least 20 AlN particles having an average particle size of 30 nm or less per unit area (μm 2 ) and

[0013] satisfies the following relational expression 1.

[0014] [Relational expression 1]

[0015] (block grain average size (μm)) 2 / (pro-eutectoid cementite length (μm / 1200 μm 2 )) ≤ 0.5

[0016] According to another aspect of the present disclosure, there is provided a method for manufacturing a wire having excellent drawability, the method comprising: heating a steel billet containing, in terms of weight %, C: 0.8 to 1.2 %, Si: 0.01 to 0.6 %, Mn: 0.1 to 0.6 %, Cr: 0.8 to 2.0 %, Al: 0.01 to 0.06 %, N: 0.02 % or less (excluding 0), and Fe and inevitable impurities in the balance, and rolling the steel billet to prepare a small billet;

[0017] cooling the prepared small billet;

[0018] heating the small billet to a temperature of 950 to 1050 °C;

[0019] wire-rolling the heated small billet to prepare a wire; and

[0020] coiling the wire, cooling the wire to a temperature of 550 to 650 °C at an average cooling rate of 3 °C / sec or more, and then cooling it at an average cooling rate of 1 °C / sec or less for a temperature of not higher than 550 to 650 °C,

[0021] wherein wire-rod rolling is performed before finish rolling so that the austenite grain size (AGS) is 5 to 20 μm, and the finish rolling is performed at a temperature range of 730 °C to Acm and a deformation of 0.3 or more.

[0022] Advantages

[0023] As described above, according to the present disclosure, it is possible to provide a wire rod for a mechanical structural member such as a bearing and a manufacturing method thereof, which has drawing properties and strength although spheroidizing softening heat treatment can be omitted or shortened. Thus, it is possible to achieve cost reduction and carbon reduction effects in the manufacturing process.

[0024] The various advantageous effects and features of the present invention are not limited to the above described content, and can be more easily understood through the description of the specific embodiments of the present disclosure. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a photograph of the microstructure of Inventive Example 1 observed with a scanning electron microscope (SEM) in one embodiment of the present disclosure.

[0026] Figure 2 is a photograph of the microstructure of Comparative Example 5 observed with a scanning electron microscope (SEM) in one embodiment of the present disclosure.

[0027] Figure 3 is a photograph of the microstructure of Inventive Example 1 observed using Electron Backscatter Diffraction (EBSD) in one embodiment of the present disclosure.

[0028] Figure 4 is a photograph of the microstructure of Comparative Example 5 observed using Electron Backscatter Diffraction (EBSD) in one embodiment of the present disclosure. DETAILED DESCRIPTION

[0029] The terms used in the present specification are used to explain specific exemplary embodiments, not to limit the present disclosure. Also, the singular forms are intended to include the plural forms unless the context clearly indicates otherwise.

[0030] The meaning of "include" or "comprise" used in the specification is to embody the configuration, and does not exclude the presence or addition of other configurations.

[0031] Unless otherwise defined, all terms used in the present specification, including technical and scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The definitions of the terms are interpreted as consistent with the related art and the disclosure herein.

[0032] Hereinafter, the present application will be described in detail. The inventors of the present application recognized that, in a wire rod for a mechanical structural member such as a bearing, a large amount of heat treatment cost and time is required when performing spheroidizing annealing, which causes environmental burden. Therefore, even if spheroidizing annealing is shortened or omitted, the inventors of the present application intensively researched a method for securing excellent drawability for manufacturing a member during wire drawing, thereby completing the present application.

[0033] First, in one aspect of the present disclosure, a wire rod will be described in detail.

[0034] The wire rod of the present disclosure contains, in weight %, C: 0.8% to 1.2%, Si: 0.01% to 0.6%, Mn: 0.1% to 0.6%, Cr: 0.8% to 2.0%, Al: 0.01% to 0.06%, N: 0.02% or less (excluding 0), and Fe and inevitable impurities in the balance. Hereinafter, the effects and contents of each component will be described. The % of each component below means weight %.

[0035] Carbon (C): 0.8% to 1.2%

[0036] Carbon (C) is an element added to secure a certain level of strength. When the content of C is less than 0.8%, it can be difficult to secure sufficient strength even after quenching and tempering heat treatment after spheroidizing annealing and forging processing. When the content of C exceeds 1.2%, precipitates of new phases such as (FeCr)3C can cause problems such as center segregation during solidification of a steel billet, for example, a bloom. Therefore, the content of C is preferably 0.8% to 1.2%, and more preferably 0.9% to 1.1%.

[0037] Silicon (Si): 0.01% to 0.6%

[0038] Silicon (Si) is a representative substitution element added to secure a certain level of strength. When the content of Si is less than 0.01%, it can be difficult to secure the strength of steel and sufficient hardenability. When the content of Si exceeds 0.6%, there is a disadvantage that cold forgeability can be deteriorated during forging after spheroidizing annealing. Therefore, the content of Si is preferably 0.01% to 0.6%.

[0039] Manganese (Mn): 0.1% to 0.6%

[0040] Manganese (Mn), which is an element that strengthens solid solution by forming a substitutional solid solution in the matrix structure, is an element that can ensure a desired strength without deteriorating ductility, and is a representative austenite former. When the content of Mn is less than 0.1%, strength by solid solution strengthening is not guaranteed, and improvement of toughness is difficult to expect. In addition, when the content of Mn exceeds 0.6%, during forging after spheroidizing softening heat treatment, defects such as V-shaped cracks can occur due to MnS. Therefore, the content of Mn is preferably 0.1% to 0.6%.

[0041] Chromium (Cr): 0.8% to 2.0%

[0042] Like Mn, chromium (Cr) is an element that improves the hardenability of steel. When the content of Cr is less than 0.8%, during hardening and tempering heat treatment after the forging process, it can be difficult to ensure sufficient hardenability to obtain martensite. When the content of Cr exceeds 2.0%, there can be a high possibility that a large amount of low-temperature structure occurs in the wire due to center segregation. Therefore, the content of Cr is preferably 0.8% to 2.0%, and more preferably 1.0% to 2.0%.

[0043] Aluminum (Al): 0.01% to 0.06%

[0044] Aluminum (Al) is an element that not only has a deoxidizing effect, but also helps to suppress the growth of austenite grains and ensure the proportion of proeutectoid ferrite close to the equilibrium phase by precipitating Al-based carbonitride. When the content of Al is less than 0.01%, dissolved aluminum is insufficient, so most of the Al is dissolved, so that aluminum nitride (AlN) that suppresses the growth of austenite grains during heat treatment cannot be sufficiently generated, and therefore the content of Al is preferably 0.01% or more. At the same time, when the content of Al exceeds 0.06%, hard inclusions such as Al2O3 can increase, and in particular, nozzle clogging due to inclusions can occur during continuous casting. Therefore, the content of Al is preferably 0.01% to 0.06%.

[0045] Nitrogen (N): 0.02% or less (excluding 0%)

[0046] Nitrogen (N) is an element that has a solid solution strengthening effect, but when the content of N exceeds 0.02%, the toughness and ductility of the material can be reduced due to dissolved nitrogen that does not combine with nitrides, so the content of N is preferably 0.02% or less.

[0047] In addition to the above components, the wire according to one aspect of the present disclosure can contain Fe and other unavoidable impurities in a balance. However, in a general manufacturing process, it can be inevitable that unintended impurities are mixed from raw materials or the surrounding environment, and thus, these impurities can not be completely excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification. In addition, effective components other than the above components can be additionally added without being completely excluded.

[0048] Meanwhile, the microstructure of the wire according to one aspect of the present disclosure contains pearlite main structure and pro-eutectoid cementite. Specifically, the pro-eutectoid cementite is formed in a network shape at the grain boundaries along the prior austenite grains, and the generated pearlite is formed within the grain boundaries. During cooling, the supersaturated carbon in the austenite is precipitated as Fe3C, and the pro-eutectoid cementite is formed at the grain boundaries of the prior austenite. Due to the grain refinement as a diffusion path of elements, the pro-eutectoid cementite has a network shape.

[0049] AlN is precipitated within the microstructure, and it is preferable that 20 or more AlN particles having an average particle size of 30 nm or less are distributed per unit area (μm 2 ) If the average particle size of AlN exceeds 30 nm, the effect of suppressing grain growth due to pinning is significantly reduced, and thus it is preferable to have a size of 30 nm or less. If AlN particles are present in an amount of less than 20 per unit area (μm 2 ) even if AlN particles are generated, the number of AlN particles is not sufficient to suppress grain growth, and thus grain coarsening can occur. It is more preferable that the number of AlN particles having a size of 30 nm or less per unit area (μm 2 ) is 50 or more.

[0050] Meanwhile, the pearlite and the pro-eutectoid cementite preferably contain 10% or less of the pro-eutectoid cementite and the remaining portion of the pearlite in terms of area fraction, and can further contain 5% or less of one or more of pro-eutectoid ferrite, bainite, and martensite. When the fraction of the pro-eutectoid cementite exceeds 10%, the toughness can be rapidly reduced, and thus the fraction thereof is preferably not more than 10%. Meanwhile, one or more of the pro-eutectoid ferrite, bainite, and martensite can be generated to some extent during the process of manufacturing the wire, but when the fraction thereof exceeds 5%, a break can easily occur during drawing, and thus the fraction thereof is preferably not more than 5%.

[0051] During spheroidizing softening heat treatment, the characteristics of the grain boundaries are a major factor in determining the diffusion rate, and are used to determine the total heat treatment time. During softening heat treatment, the cementite in the pearlite structure changes its shape from a plate shape to a spherical shape, and the strength of the material decreases according to the degree of spheroidization.

[0052] During the softening heat treatment, metal atoms move through various diffusion paths via defect spaces in the material and diffuse via vacancies as atomic defects and dislocations or pipes as line defect types, grain boundaries, etc. Dislocations and grain boundaries have a relatively large space compared to atomic defects, which facilitates rapid diffusion.

[0053] In order to omit the softening heat treatment or shorten the time, it is preferable to increase the area of the relative grain boundary by grain refinement, but due to an increase in rolling load, adverse effects such as a reduction in equipment life, productivity, etc. can occur. Therefore, the wire of the present disclosure has a microstructure satisfying the following relationship expression 1 so that even if the spheroidizing softening heat treatment is omitted or shortened, a wire having excellent drawability can be obtained.

[0054] [Relationship Expression 1]

[0055] (Average size of block grain (μm)) 2 / (Pro-eutectoid cementite length (μm / 1200 μm 2 )) ≤ 0.5

[0056] The block grain refers to a group of grains having the same ferrite orientation among the cementite and ferrite constituting the pearlite, and the average size refers to the average grain size of the grains.

[0057] The length of the pro-eutectoid cementite refers to the total length of the pro-eutectoid cementite measured within a unit area (1200 μm 2 ). As described above, since the pro-eutectoid cementite is formed along the prior austenite grain boundary, the pro-eutectoid cementite length preferably refers to the length measured along the grain boundary.

[0058] The wire of the present disclosure can be drawn by at least 15% without spheroidizing softening heat treatment before the drawing process, in which the tensile strength (TS) of the wire is 1200 MPa or more, and the cross-sectional area reduction is 20% or more. Even if the spheroidizing softening heat treatment is omitted, the wire of the present disclosure can be drawn. Due to the coarse grain size of the commonly used material, even at a drawing amount of about 10%, defects such as V-shaped cracks can occur. However, even at a drawing amount of more than 15% or about 30%, the wire of the present disclosure does not have defects such as cracks in its interior. This is because the aggregated structure is easily rotated when drawing is applied, thereby reducing external stress and preventing defects such as cracks from occurring in the case of a small amount of drawing. In addition, as the amount of drawing increases, vacancies such as dislocations and vacancies are generated, further promoting spheroidization behavior during the spheroidizing softening heat treatment after drawing.

[0059] To manufacture mechanical parts such as bearing steel having a complex shape, a wire rod is manufactured into a steel wire, which is generally subjected to a two-time spheroidizing softening heat treatment and a drawing process to size the material. A typical spheroidizing softening heat treatment is performed at a temperature of Ae1 to Ae1+100°C, and is a heat treatment method in which an average aspect ratio of carbides of 3 or less of cementite is generated in the entire region from the surface to the center portion after the heat treatment. However, the wire rod of the present disclosure can provide a greater amount of drawing than conventional materials through improved drawability by manufacturing a fine-grained wire rod and promote the generation of spheroidized cementite during spheroidizing heat treatment, thus obtaining cementite having an average aspect ratio of less than 3 and a low tensile strength of less than 740 MPa after drawing by only one spheroidizing heat treatment. Therefore, cold forging or cold heading for manufacturing a final product can be facilitated.

[0060] Next, a method for manufacturing a wire rod according to another aspect of the present disclosure will be described in detail. As a preferred example of manufacturing the wire rod of the present disclosure, a steel billet such as a bloom having the above-described alloy composition can be heated, and the steel billet can be rolled to manufacture a billet bloom, and the billet bloom can be heated, wire-rolled, and cooled to manufacture the wire rod of the present disclosure. Hereinafter, each step is described in detail.

[0061] First, a steel billet such as a bloom having the above-described alloy composition is prepared and heated to a temperature of 1100°C to 1300°C. When the heating temperature of the steel billet is less than 1100°C, the temperature is low and is not sufficient for the elements in the steel billet to diffuse, making it difficult to eliminate the concentrated segregation layer generated during continuous casting. At the same time, when the temperature exceeds 1300°C, an oxide scale can be formed on the surface of the steel billet at a fast rate, making surface defects likely to occur during rolling, or the productivity can be reduced due to material loss. At the same time, the heating time of the steel billet is preferably 2 hours to 10 hours, if the heating time of the steel billet is less than 2 hours, it is difficult to reach the target temperature even inside the steel billet, and if the heating time exceeds 10 hours, the depth of the surface decarburized layer increases, and the decarburized layer can remain even after finish rolling, and thus it is preferable that the heating time does not exceed 10 hours.

[0062] The billet is manufactured by rolling the heated billet. The billet manufactured after the billet is rolled is generally cooled to room temperature by air cooling, but in the present disclosure, the billet having a temperature of 500°C or higher is cooled at a cooling rate of 5°C / sec or higher. To this end, water cooling is preferably performed, and as a specific example, it is preferable to load it into a water cooling chamber to prevent precipitation and coarsening of AlN as much as possible. When the temperature of the billet is 500°C or lower, AlN is precipitated and coarsened, making it difficult to obtain AlN of 30 nm or less, because AlN is not sufficiently dissolved during heating of the billet to manufacture wire rod, which is the next process.

[0063] The manufactured billet is heated to a temperature in the range of 950°C to 1050°C. When the heating temperature of the billet is lower than 950°C, the rolling property can be deteriorated, and when the heating temperature of the billet exceeds 1050°C, the rolling requires rapid cooling, making it not only difficult to control the cooling, but also difficult to secure good product quality due to the occurrence of cracks, etc. The heating time is preferably 80 minutes to 120 minutes. If the heating time is less than 80 minutes, it is difficult to reach the target temperature even in the inside of the material, and an atmosphere in which reverse transformation is not completed can partially occur. If the heating time exceeds 120 minutes, the depth of the surface decarburized layer increases, and the decarburized layer can remain after finish rolling, which is not preferable.

[0064] The heated billet is subjected to wire rod rolling to obtain a wire rod. It is preferable that the wire rod rolling is groove rolling in which the billet has a wire rod form. In the present disclosure, it is preferable that the austenite grain size (AGS) before finish rolling is 5 μm to 20 μm. Thereafter, the finish rolling is preferably performed at a temperature in the range of 730°C to Acm and a deformation of 0.3 or more. More preferably, the deformation is 0.5 or more. Here, Acm refers to a temperature at which cementite in a hypereutectoid steel is dissolved during heating or precipitated during cooling.

[0065] If the AGS before finish rolling is less than 5 μm, there can be a problem in that the rolling roll load is increased, making the life of the equipment shortened, because it is performed by rough rolling at a low temperature. If the AGS before finish rolling exceeds 20 μm, it can be difficult to manufacture a wire rod having fine grains because of the need to increase the critical deformation during finish rolling. In addition, when the finish rolling temperature is lower than 730°C, the rolling roll load can be increased, making the life of the equipment shortened, and when the finish rolling temperature is higher than Acm, no phase transformation occurs, making it difficult to manufacture a fine grain wire rod.

[0066] Meanwhile, when the wire rod is rolled, it is preferable to satisfy the condition of the following relational expression (2).

[0067] [Relational Expression 2]

[0068] 2500*([C]-1) 2 +100000*([Al]-0.035) 2 +(AGS-12.5) 4 / 130+(finish rolling temperature-760) 2 / 65≤80

[0069] In the above relational expression (2), [C] and [Al] refer to the contents (wt%) of alloying elements C and Al, AGS is in units of μm, and the finish rolling temperature is in units of °C.

[0070] The content of carbon affects the formation of cementite (Fe3C) in the wire rod and spheroidized heat-treated material, which affects mechanical properties such as tensile strength, and thus an appropriate amount of carbon is required. As the amount of Al decreases, the amount of precipitated AlN decreases and grain growth cannot be suppressed, and thus an optimal amount thereof is required. Furthermore, the larger the AGS before finish rolling, the lower the rolling amount and finish rolling temperature should be so as to reduce the grain size, and thus from the viewpoint of process cost, it is preferable to control an appropriate AGS and finish rolling temperature. Relational expression 2 reflects this technical viewpoint, and when the value thereof in relational expression 2 exceeds 80, it is difficult to expect appropriate cementite formation and grain refinement effects.

[0071] After wire rod rolling, the wire rod is coiled and cooled. Cooling is preferably performed at an average cooling rate of 3°C / sec or more until a temperature range of 550°C to 650°C, and then after the temperature of 550°C to 650°C, cooling is preferably performed at an average cooling rate of 1°C / sec or less. When the average cooling rate until the temperature range of 550°C to 650°C is lower than 3°C / sec, it is difficult to maintain the fine grains obtained during rolling below the transformation point. Meanwhile, after reaching the temperature of 550°C to 650°C, the cooling rate below this temperature is preferably 1°C / sec or less from the viewpoint of suppressing low-temperature structures such as bainite and martensite.

[0072] In the present disclosure, after drawing the wire prepared as above, the wire can be heated to Ae1 to Ae1+100°C and held for 5 to 15 hours, and then subjected to spheroidizing heat treatment by cooling to 660°C at 20°C / hour or less to prepare a spheroidized material. When the heating temperature is lower than Ae1, there can be a disadvantage that the spheroidizing heat treatment time becomes long, and when the heating temperature exceeds Ae1+100°C, the effect of the spheroidizing heat treatment can be insufficient due to a decrease in spheroidizing carbide seeds. Here, Ae1 refers to a temperature at which austenite is generated during heating, or a temperature at which austenite disappears during cooling. When the holding time is less than 5 hours, there can be a disadvantage that the aspect ratio of cementite increases due to insufficient spheroidizing heat treatment, and when the holding time exceeds 15 hours, there can be a disadvantage that the cost increases. When the heating temperature exceeds 20°C / hour, there can be a disadvantage that the pearlite transformation occurs due to a fast cooling rate. After the spheroidizing heat treatment, the wire has a low tensile strength of 740 MPa or less and a cementite average aspect ratio of 3 or less, which can be advantageous for cold forging or cold forging processing to manufacture a final product.

[0073] Embodiment of Invention

[0074] Hereinafter, the present disclosure will be specifically described through the following examples. However, it should be noted that the following examples are only for describing the present disclosure by way of illustration, and are not intended to limit the scope of rights of the present disclosure. This is because the scope of rights of the present disclosure is determined by matters described in the claims and matters reasonably inferred therefrom.

[0075] (Example)

[0076] A steel billet (bloom) having an alloy composition (wt%, balance of Fe and unavoidable impurities) shown in Table 1 below was prepared, and then subjected to bloom rolling to manufacture a billet. After continuous casting, the billet was subjected to a homogenizing heat treatment at 1200°C for 4 hours, and then rolling was performed at 1000°C. In the case of water cooling after the bloom rolling according to the cooling method disclosed in Table 2, the billet was air-cooled to a temperature of 500°C, and then charged into a water cooling chamber and cooled at a cooling rate of 5°C / sec or more. Thereafter, the manufactured billet was manufactured into a wire having a diameter of 9 mm under the wire manufacturing conditions disclosed in Table 2 below. The microstructure and mechanical properties of the wire thus prepared were measured, and the results are shown in Table 3. Meanwhile, after drawing the prepared wire, the average aspect ratio of cementite and the tensile strength were measured by one softening spheroidizing heat treatment (holding at 780°C for 8 hours, and then cooling to 640°C at a cooling rate of 15°C / hour), and the results are shown in Table 4.

[0077] Meanwhile, in Table 2, the austenite grain size (AGS) before finish rolling was collected by cutting the material by cropping before hot finish rolling and immediately quenching it in water, and the AGS was measured using the ASTM E112 method. For the collected specimen, five arbitrary 1 / 4 points of the distance diameter were measured, and then expressed as an average value.

[0078] The cluster grain average size was measured using EBSD and the ASTM E112 method. The cluster is a region in which the crystal orientation of ferrite in the pearlite is the same, and the size having a crystal orientation difference of at least 15 degrees is defined as the cluster size. In the following examples, Inventive Example 1 and Comparative Example 5 were observed, and are shown in Figure 3 and Figure 4 The size of the cluster was quantified using the ASTM E112 method. For the specimen collected after removing the non-water cooled portion after rolling the wire rod, the measured material was measured at five arbitrary 1 / 4 points of the distance diameter, and then expressed as an average value. In addition, for the specimen collected after removing the non-water cooled portion after rolling the wire rod, the length of proeutectoid cementite was photographed at five arbitrary 1 / 4 points of the distance diameter using SEM at X3000, the total length of the proeutectoid cementite was analyzed using measurement using Clemex vision software of Leica, and the average value of 5 points was obtained.

[0079] Drawability was evaluated by drawing the prepared wire rod having a diameter of 9 mm at a cross-sectional reduction of 5% to 50%, and a central portion of the L section of the drawn material was photographed at a magnification of 5000x to check whether defects such as V-shaped cracks occur at the pearlite interface or the proeutectoid cementite interface, and whether cracks occur was expressed as O / X.

[0080] Meanwhile, the average aspect ratio of cementite after the primary spheroidizing heat treatment was measured by photographing the 1 / 4 to 1 / 2 points in the wire rod diameter direction at 3000x magnification in 3 fields of view using SEM, automatically measuring the long axis / short axis of the cementite within the field of view using an image measurement program, and then statistically processing.

[0081] [Table 1]

[0082] Steel grade C Si Mn Cr Al N Steel grade 1 1.05 0.29 0.30 1.69 0.023 0.006 Steel grade 2 1.01 0.28 0.35 1.33 0.024 0.005 Steel grade 3 0.96 0.25 0.35 1.36 0.023 0.004 Steel grade 4 1.00 0.25 0.33 1.36 0.027 0.006 Steel grade 5 0.95 0.24 0.33 1.39 0.029 0.005 Steel grade 6 1.00 0.30 0.27 1.51 0.025 0.003 Steel grade 7 0.97 0.23 0.33 1.41 0.003 ]]> ​ 0.006 Steel grade 8 0.50 ]]> ​ 0.20 0.27 1.48 0.023 0.006 Steel grade 9 0.97 0.20 0.32 1.34 0.030 0.003 Steel grade 10 1.04 0.22 0.27 1.57 0.030 0.003 Steel grade 11 0.98 0.23 0.27 1.65 0.026 0.003 Steel grade 12 0.98 0.28 0.30 1.50 0.024 0.007 Steel grade 13 1.03 0.25 0.27 1.60 0.021 0.007

[0083] [Table 2]

[0084]

[0085]

[0086] In Table 2, the average aspect ratio of cementite after the primary spheroidizing heat treatment was measured by photographing the 1 / 4 to 1 / 2 points in the wire rod diameter direction at 3000x magnification in 3 fields of view using SEM, automatically measuring the long axis / short axis of the cementite within the field of view using an image measurement program, and then statistically processing. 2+ 100000 * ([Al] - 0.035) 2 + (AGS - 12.5) 4 / 130 + (finish rolling temperature - 760) 2 / 65 The calculation relationship expression 2, where [C] and [Al] are the contents of C and Al in the alloy composition (wt%), AGS is the average size of austenite grains in μm, and the finish rolling temperature is in °C.

[0087] [Table 3]

[0088]

[0089] In Table 3, pro-eutectoid C refers to pro-eutectoid cementite, P refers to pearlite, B refers to bainite, and M refers to martensite. In addition, the relationship expression 1 refers to (average size of cluster grains (μm) 2 / (pro-eutectoid cementite length (μm / 1200 μm 2 ) ).

[0090] [Table 4]

[0091]

[0092] As can be seen from the above Tables 1 to 4, the wire of Inventive Examples 1 to 5 satisfying the conditions proposed by the present disclosure can ensure excellent drawability and a cross-sectional reduction of 20% or more even without performing a spherical softening heat treatment, and at the same time, can ensure a high strength of 1200 MPa or more. In addition, a wire having a pro-eutectoid cementite average aspect ratio of 3 or less can be provided by only one spheroidizing heat treatment after drawing. In particular, Figure 1 is a photograph of the microstructure of the wire of Inventive Example 1 observed using a scanning electron microscope (SEM). Referring to Figure 1 , in Inventive Example 1, the microstructure of the wire is composed of pro-eutectoid cementite and generated pearlite, and in Figure 1 , the arrow indicates the pro-eutectoid cementite. As Figure 1 indicated, it can be determined that the pro-eutectoid cementite is formed along the prior austenite grain boundaries. Figure 3 is an EBSD photograph of Inventive Example 1, and it can be determined that the grain orientation difference is at least 2 degrees, and the average size of cluster grains of Inventive Example 1 is about 4.7 μm, which is very small compared to the normal manufacturing conditions.

[0093] Meanwhile, in Comparative Example 1, air cooling is performed after rolling the steel billet, so the AlN in the steel material becomes coarse, and in Comparative Example 2, the AlN content in the steel composition is low, so AlN is hardly generated. As a result, in the wires of Comparative Examples 1 and 2, the pro-eutectoid cementite length per μm 2The number of AlN having a size of 30 nm or less was 20 or less, and grain growth was not inhibited during cooling of the wire, so that the size of the cluster grains was not controlled. In Comparative Example 3, which has a low carbon content, and proeutectoid ferrite remains in the wire, so that the drawing property is superior to that of the other comparative examples, but the strength is low due to the low carbon content, which makes it difficult to be used for the intended purpose even after spheroidizing heat treatment due to the low strength of the material.

[0094] In Comparative Example 4, the AGS size before finish rolling is larger than that of the inventive example due to a high billet heating temperature. Since coarse AGS can be refined by a high critical strain rate, insufficient finish rolling strain rate eventually leads to the occurrence of coarse grains in the wire, resulting in poor drawability. In Comparative Example 5, fine grains are not obtained due to a high finish rolling temperature, and as in Comparative Example 4, the drawing property is not excellent due to coarse grains. Figure 2 is a photograph of the microstructure of the wire of Comparative Example 5 observed with an SEM, and it can be determined that the size of the grains is larger than that of the inventive example, and the length of proeutectoid cementite generated along prior austenite grain boundaries is shorter. Figure 1 Figure 4 is an EBSD photograph of Comparative Example 5, in which the grain orientation difference is distinguished as Figure 3 Figure 3 It can be seen that the size of the grains is larger than that of the inventive example, and the length of proeutectoid cementite generated along prior austenite grain boundaries is shorter. Figure 4

[0095] In Comparative Example 6, fine grains are not obtained due to a small amount of finish rolling, and coarse grains occur in the wire, resulting in poor drawing property. In the wire of Comparative Example 7, fine grains generated by rolling are coarsened due to a low initial cooling rate, so that fine wire grains are not obtained, resulting in poor drawing property. In the case of Comparative Example 8, martensite and bainite occur due to a fast cooling rate, so it can be determined that internal cracks occur at only 5% of drawing.​​​

Claims

1. A wire rod having excellent drawability, comprising, in weight %: C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr: 0.8 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less but not including 0, and the balance of Fe and unavoidable impurities, wherein the microstructure contains pearlite main structure and pro-eutectoid cementite, and contains μm 2 at least 20 AlN particles having an average particle size of 30 nm or less, and satisfying the following relational expression 1, [Relational expression 1] (cluster grain average size) 2 / (pro-eutectoid cementite length) ≤ 0.5, wherein the colony grain refers to a group of grains having the same ferrite orientation among the cementite and ferrite constituting the pearlite, the colony grain average size refers to the average grain diameter of the grains and has a unit of pm, and the proeutectoid cementite length refers to the total length of the proeutectoid cementite measured in a unit area and has a unit of pm / 1200pm 2 .

2. The wire rod having excellent drawability according to claim 1, wherein the pro-eutectoid cementite is formed at the grain boundaries along the prior austenite grains and is formed in a network shape.

3. The wire rod having excellent drawability according to claim 1, wherein the microstructure comprises 10% or less of the pro-eutectoid cementite and the balance of the pearlite in area fraction.

4. The wire rod having excellent drawability according to claim 1, wherein the wire rod has a tensile strength of 1200 MPa or more and a reduction of area of 20% or more.

5. The wire rod having excellent drawability according to claim 1, wherein the wire rod is not subjected to spheroidizing softening heat treatment before a drawing process and is drawn by at least 15% during wire drawing.

6. The wire rod having excellent drawability according to claim 1, wherein an average aspect ratio of the cementite of the wire rod is 3 or less after wire drawing and spheroidizing heat treatment.

7. A method for manufacturing a wire rod having excellent drawability, comprising: heating a steel billet, the steel billet comprising, in weight %: C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr: 0.8 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less but not including 0, and the balance of Fe and other unavoidable impurities, and subjecting the steel billet to rolling to produce a billet; cooling the produced billet; heating the billet to a temperature of 950°C to 1050°C; subjecting the heated billet to wire rod rolling to produce a wire rod; and coiling the wire rod, cooling the wire rod to 550°C to 650°C at an average cooling rate of 3°C / sec or more, and then cooling it at an average cooling rate of 1°C / sec or less for a temperature of not higher than 550°C to 650°C, wherein the wire rod rolling is performed before finish rolling such that an austenite grain size AGS is 5 μm to 20 μm, and the finish rolling is performed at a temperature range of 730°C to Acm and a deformation of 0.3 or more.

8. The method for manufacturing a wire rod having excellent drawability according to claim 7, wherein the hot rolling is performed under a condition satisfying the following relational expression (2), [Relational expression 2] 2500*([C]-1) 2 +100000*([Al]-0.035) 2 +(AGS-12.5) 4 / 130+(finishing temperature-760) 2 / 65≤80 In the above relational expression (2), [C] and [Al] refer to the contents of the alloying components C and Al in weight %, the unit of AGS is μm, and the unit of finish rolling temperature is °C.

9. The method for manufacturing wire having excellent drawability according to claim 7, wherein a billet is heated to a temperature in the range of 1100°C to 1300°C for 2 hours to 10 hours, and after the billet is rolled, the billet is cooled at a cooling rate of 5°C / sec or more to a temperature of 500°C or more.

10. The method for manufacturing wire having excellent drawability according to claim 7, wherein the billet heating time is 80 minutes to 120 minutes.

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