Wire rod having excellent drawability, and manufacturing method therefor

Abstract

To provide a machine structure wire rod applicable to automobiles, construction components, and the like, and a manufacturing method therefor, and to provide a wire rod having excellent drawability, and a method for manufacturing the same.SOLUTION: The present invention is a wire rod which, in weight%, consists of C: 0.8-1.2%, Si: 0.01-0.6%, Mn: 0.1-0.6%, Cr: 0.8-2.0%, Al: 0.01-0.06%, N: 0.02% or less (excluding 0), with the remainder being Fe and inevitable impurities, wherein a microstructure includes proeutectoid cementite in a pearlite main structure, includes 20 or more AlN having an average particle size of 30 nm or less per unit area (μm2), and includes a microstructure satisfying the following relational expression 1, wherein [Relational Expression 1] is [(average size of block crystal grains(μm))2 / (length of proeutectoid cementite(μm / 1200 μm2))≤0.5].SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a wire rod having excellent wire drawability and a manufacturing method thereof, and more particularly to a wire rod for machine structures applicable to automobiles, construction parts, etc., and a manufacturing method thereof, and more particularly to a wire rod having excellent wire drawability and a manufacturing method thereof. [Background technology]

[0002] Steel materials for machine structures used in automobiles, construction parts, etc., such as bearings, are usually manufactured by drawing rolled wire rods and then cold working the drawn wire rods into complex shapes. However, since the above-mentioned steel material is a hypereutectoid steel and is difficult to process, it is difficult to directly wiredraw the rolled wire material. For this reason, a softened material is manufactured by performing a spheroidal softening heat treatment, performing material sizing by wiredrawing, and then performing an additional spheroidal softening heat treatment to compensate for the increase in strength due to wiredrawing.

[0003] The spheroidizing heat treatment described above is intended to improve cold workability by spheroidizing cementite in the microstructure and inducing a uniform particle distribution, thereby preventing wire breakage during wire drawing, improving the life of the drawing dies, and reducing the hardness of the material being drawn. However, the above-mentioned spherical softening heat treatment requires a lot of heat treatment cost and production time, which increases manufacturing costs. Furthermore, it does not meet the recent demand for minimizing energy consumption to reduce carbon dioxide emissions. Therefore, in providing wire rods for use in bearings, etc., there has been a demand for the development of wire rods that can omit or shorten the spherical softening heat treatment while ensuring excellent wire drawing properties. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a wire rod to be used for machine structural parts such as bearings, etc., and a manufacturing method thereof. Specifically, the present invention aims to provide a wire rod that can omit or shorten the spherical softening heat treatment and can ensure excellent wiredrawing characteristics and strength, and a manufacturing method thereof. The object of the present invention is not limited to the above-mentioned content. A person having ordinary skill in the art to which the present invention pertains will have no problem in understanding further object of the present invention from the entire description of the present specification. [Means for solving the problem]

[0005] The wire rod of the present invention having excellent wire drawability comprises, by 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 the remainder being Fe and inevitable impurities, The microstructure is mainly pearlite with proeutectoid cementite. AlN with an average particle size of 30 nm or less is used for 2 ) contains 20 or more per It is characterized by having a microstructure that satisfies the following relational expression 1. Here, [Relationship 1] is (average size of block crystal grains (μm)) 2 / (length of proeutectoid cementite (μm / 1200μm 2 ))≦0.5.

[0006] The method for producing a wire rod having excellent wire drawability of the present invention includes the steps of heating a steel slab containing, by weight %, 0.8 to 1.2% C, 0.01 to 0.6% Si, 0.1 to 0.6% Mn, 0.8 to 2.0% Cr, 0.01 to 0.06% Al, 0.02% or less N (excluding 0), and the remainder being Fe and unavoidable impurities, and rolling the steel slab to produce a billet; cooling the produced billet; heating the billet to 950 to 1050°C; rolling the heated billet into a wire rod; and winding the wire rod and cooling it to 550 to 650°C at an average cooling rate of 3°C / sec or more, and cooling it to a temperature of 550 to 650°C at an average cooling rate of 1°C / sec or less, The wire rod rolling is performed so that the austenite grain size (AGS) before finish rolling is 5 to 20 μm, and the finish rolling is performed in a temperature range of 730° C. to Acm with a deformation amount of 0.3 or more. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a wire rod for machine parts such as bearings, which has excellent strength and wiredrawability even when the spherical softening heat treatment is omitted or shortened, and a method for producing the same, which can reduce costs and carbon dioxide emissions in the production process. The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a photograph of the microstructure of Example 1 observed with a scanning electron microscope (SEM) in an embodiment of the present invention. [Figure 2] 1 is a photograph of the microstructure of Comparative Example 5 observed with a scanning electron microscope (SEM) in an example of the present invention. [Figure 3] 1 is a photograph of the microstructure of Example 1 observed by electron backscatter diffraction (EBSD) in an embodiment of the present invention. [Figure 4] 1 is a photograph of the microstructure of Comparative Example 5 observed by electron backscatter diffraction (EBSD) in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] The terminology used herein is for the purpose of describing the invention and is not intended to limit the invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the related definition clearly dictates otherwise. As used herein, the meaning of "comprises" embodies features and does not exclude the presence or addition of other features. Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content.

[0010] The present invention will be described in detail below. The inventors of the present invention recognized that when a spheroidal softening heat treatment is performed on wire rods used for machine structural parts such as bearings, the heat treatment requires a lot of cost and time, and acts as a burden on the environment. Therefore, they conducted extensive research into a method that can ensure excellent wiredrawability during wiredrawing to manufacture parts even if the spheroidal softening heat treatment is shortened or omitted, and as a result, they arrived at the present invention. First, the wire rod, which is one aspect of the present invention, will be described in detail.

[0011] The wire rod of the present invention is composed of, by weight%, C: 0.8-1.2%, Si: 0.01-0.6%, Mn: 0.1-0.6%, Cr: 0.8-2.0%, Al: 0.01-0.06%, N: 0.02% or less (excluding 0), with the remainder being Fe and inevitable impurities. The role and content of each component will be explained below. The % for each component below means % by weight.

[0012] Carbon (C): 0.8~1.2% C is an element added to ensure a certain level of strength. If the C content is less than 0.8%, the strength of the base metal decreases, making it difficult to ensure sufficient strength even after quenching and tempering heat treatments performed after the spheroidizing softening heat treatment and forging process. On the other hand, if the C content exceeds 1.2%, there is a risk that precipitates of new phases such as (FeCr)3C may cause problems such as center segregation during solidification of cast slabs such as blooms. Therefore, the C content is preferably 0.8 to 1.2%, more preferably 0.9 to 1.1%.

[0013] Silicon (Si): 0.01 to 0.6% The Si content is a typical substitutional element that is added to ensure a certain level of strength. If the Si content is less than 0.01%, it is difficult to ensure the strength and sufficient hardenability of the steel. On the other hand, if the Si content exceeds 0.6%, there is a drawback in that the cold forgeability during forging after the spheroidal softening heat treatment is deteriorated. Therefore, the Si content is preferably 0.01 to 0.6%.

[0014] Manganese (Mn): 0.1 to 0.6% Mn is an element that forms a substitutional solid solution in the matrix structure to strengthen the solid solution, and is a typical austenite former that can ensure the desired strength without reducing ductility. If the Mn content is less than 0.1%, the strength achieved by solid solution strengthening is not guaranteed, and the effect of improving toughness cannot be expected. On the other hand, if the Mn content exceeds 0.6%, defects such as chevron cracks may occur due to MnS during forging after the spheroidal softening heat treatment. Therefore, the Mn content is preferably 0.1 to 0.6%.

[0015] Chromium (Cr): 0.8~2.0% Cr, like Mn, is an element that improves the hardenability of steel. If the Cr content is less than 0.8%, it becomes difficult to ensure sufficient hardenability to obtain martensite during the quenching and tempering heat treatments performed after the forging process. On the other hand, if the Cr content exceeds 2.0%, center segregation is promoted, increasing the risk of a large amount of low-temperature structure occurring in the wire rod. Therefore, the Cr content is preferably 0.8 to 2.0%, and more preferably 1.0 to 2.0%.

[0016] Aluminum (Al): 0.01 to 0.06% The Al content not only has a deoxidizing effect, but also precipitates Al-based carbonitrides, which inhibit austenite grain growth and maintain the proeutectoid ferrite fraction close to the equilibrium phase. If the Al content is less than 0.01%, the aluminum content is insufficient, resulting in most of the Al being dissolved in solid solution, and insufficient aluminum nitride (AlN), which inhibits austenite grain growth during heat treatment, is produced. Therefore, the Al content is preferably 0.01% or more. On the other hand, if the Al content exceeds 0.06%, the amount of hard inclusions such as Al2O3 may increase, potentially causing nozzle clogging, particularly during continuous casting. Therefore, the Al content is preferably 0.01 to 0.06%.

[0017] Nitrogen (N): 0.02% or less (excluding 0) The above-mentioned N has a solid solution strengthening effect, but if the content exceeds 0.02%, there is a risk that the toughness and ductility of the material will decrease due to the solid solution nitrogen that is not bonded to nitrides. Therefore, it is preferable to control the N content to 0.02% or less.

[0018] The remainder contains iron (Fe), and in the normal manufacturing process, unintentional impurities may be inevitably mixed in from the raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are obvious to anyone with ordinary skill in the manufacturing process, the entire contents of these impurities will not be specifically mentioned in this specification.

[0019] On the other hand, the microstructure of the wire rod according to one aspect of the present invention includes pro-eutectoid cementite in a pearlite-based structure. Specifically, the pro-eutectoid cementite is formed in a network at the grain boundaries along the prior austenite grains, and complete pearlite is formed within the grains. During cooling, carbon supersaturated in the austenite precipitates as Fe3C, forming pro-eutectoid cementite at the prior austenite grain boundaries. However, due to refinement of the grains, which are the diffusion paths of elements, the pro-eutectoid cementite assumes a network structure.

[0020] AlN precipitates in the above microstructure, and the AlN has an average particle size of 30 nm or less per unit area (μm 2 If the average grain size of the AlN exceeds 30 nm, the effect of suppressing the growth of crystal grains by pinning is significantly reduced, so it is preferable that the size is 30 nm or less, and the number of grains is distributed per unit area (μm 2 If the number of AlN particles per unit area (μm) is less than 20, even if AlN particles are generated, the number of AlN particles that inhibit the growth of crystal grains will be insufficient, and there is a risk that the crystal grains will become coarse. 2 It is more preferable that the number of AlN particles having a size of 30 nm or less per layer is 50 or more.

[0021] Meanwhile, the pearlite and pro-eutectoid cementite preferably contain 10% or less of pro-eutectoid cementite in terms of area fraction, with the remainder being pearlite, and may additionally contain 5% or less of one or more of pro-eutectoid ferrite, bainite, and martensite. If the pro-eutectoid cementite fraction exceeds 10%, toughness may be rapidly reduced, so it is preferable that it does not exceed 10%. Note that, although one or more of pro-eutectoid ferrite, bainite, and martensite may be partially generated during the wire manufacturing process, if it exceeds 5%, fracture is likely to occur during wire drawing, so it is preferable that it does not exceed 5%.

[0022] During spheroidizing heat treatment, the characteristics of the grain boundaries are the main factor that determines the diffusion rate, and therefore the total heat treatment time. During softening heat treatment, the cementite in the pearlite structure changes shape from plate-like to spherical, and the strength of the material decreases depending on the degree of spheroidization. During softening heat treatment, metal atoms move through various diffusion paths via defect spaces within the material, including vacancies, which are atomic-level defects, dislocations (or pipes), which are a type of line defect, and grain boundaries. Compared to atomic defects, dislocations and grain boundaries have relatively larger spaces, making them favorable for fast diffusion.

[0023] In order to eliminate or reduce the time required for softening heat treatment, it is preferable to increase the relative grain boundary area by refining the crystal grains, but this may have adverse effects such as a decrease in equipment life and productivity due to an increase in rolling load. Therefore, the wire rod of the present invention can have excellent wiredrawability even if the spheroidizing softening heat treatment is omitted or reduced by using a fine structure that satisfies the following relational expression 1. [Equation 1] (Average size of block crystal grains (μm)) 2 / (length of proeutectoid cementite (μm / 1200μm 2 ))≦0.5 The block crystal grains refer to a group of crystal grains in which the orientation of ferrite is the same among the cementite and ferrite that constitute pearlite, and the average size refers to the average particle size of the crystal grains.

[0024] The length of the proeutectoid cementite is 2 As described above, the pro-eutectoid cementite is formed along the prior austenite grain boundaries, and therefore the length of the pro-eutectoid cementite means the length measured along the grain boundaries.

[0025] The wire rod of the present invention can be drawn by 15% or more without a spheroidal softening heat treatment before the wire drawing process, has a tensile strength (TS) of 1200 MPa or more, and has a cross-sectional area reduction rate of 20% or more. The wire rod of the present invention can be drawn without the spheroidizing heat treatment. Commonly used materials can develop defects such as chevron cracks even at wiredrawing rates of less than about 10% due to their coarse grain size. However, the wire rod of the present invention does not develop internal defects such as cracks even at wiredrawing rates of over 15%, or even about 30%. This is because the colonies rotate easily during wiredrawing, eliminating external stress and preventing defects such as cracks from developing with only a small amount of wiredrawing. Furthermore, as the wiredrawing rate increases, voids such as dislocations and vacancies are generated, further promoting spheroidizing behavior during the spheroidizing heat treatment after wiredrawing.

[0026] To manufacture mechanical parts, such as bearing steel with complex shapes, wire rod is produced from steel wire, and typically undergoes two spheroidizing heat treatments: a spheroidizing heat treatment and a wiredrawing process for sizing the material. Typical spheroidizing heat treatments are performed at temperatures between Ae1 and Ae1+100°C, and are a heat treatment method that produces carbides with an average aspect ratio of cementite of 3 or less throughout the entire region from the surface to the center. However, the wire rod of the present invention, due to the improved wiredrawability achieved by producing a fine-grained wire rod, can be drawn to a greater extent than conventional materials, promoting the production of spheroidized cementite during spheroidizing heat treatment. Even a single spheroidizing heat treatment after wiredrawing achieves an average aspect ratio of cementite of 3 or less and a low tensile strength of 740 MPa or less, making it easy to perform cold heading or cold forging to produce the final product.

[0027] Next, a method for manufacturing a wire rod according to another aspect of the present invention will be described in detail. In a preferred example of manufacturing the wire rod of the present invention, a steel billet, e.g., a bloom, having the above-described alloy composition is heated and rolled to produce a billet, and the billet is then heated, rolled into a wire rod, coiled, and cooled. Each step will be described in detail below.

[0028] First, a steel slab, e.g., a bloom, having the above-described alloy composition is prepared and heated to 1100-1300°C. If the heating temperature of the slab is less than 1100°C, the temperature is too low to diffuse elements within the slab, making it difficult to eliminate the segregation-enriched layer formed during casting. On the other hand, if the heating temperature exceeds 1300°C, scale forms on the surface of the slab at a rapid rate, causing surface scratches during rolling and reducing productivity due to material loss. On the other hand, the heating time of the slab is preferably 2-10 hours. If the heating time of the slab is less than 2 hours, it is difficult to reach the target temperature inside the slab. If the heating time exceeds 10 hours, the depth of the surface decarburized layer increases, and the decarburized layer may remain even after rolling is completed. Therefore, it is preferable not to exceed 10 hours.

[0029] The heated billet is rolled to produce a billet. The billet produced after the billet rolling is generally cooled to room temperature by air cooling, but in the present invention, a billet at 500°C or higher is cooled at a cooling rate of 5°C / s or more. For this purpose, water cooling is preferably performed. As a specific example, the billet is placed in a water-cooled chamber to minimize the precipitation and coarsening of AlN. If a billet at a temperature below 500°C is water-cooled, AlN precipitates and coarsens, and the AlN does not fully dissolve during billet heating for wire rod production, which is the next step. This makes it difficult to obtain AlN of 30 nm or less.

[0030] The produced billet is heated to a temperature range of 950 to 1050°C. If the billet heating temperature is below 950°C, rollability is reduced, while if the billet heating temperature exceeds 1050°C, rapid cooling is required for rolling, making cooling control difficult and potentially resulting in cracks and other defects, making it difficult to ensure good product quality. The heating time is preferably 80 to 120 minutes. If the heating time is less than 80 minutes, it may be difficult to reach the target temperature deep inside the material, resulting in an atmosphere in which reverse transformation is partially incomplete. If the heating time exceeds 120 minutes, the surface decarburized layer becomes too deep and may remain after rolling is completed, which is undesirable.

[0031] The heated billet is subjected to wire rod rolling to obtain a wire rod. The wire rod rolling is preferably groove rolling, which allows the billet to have the shape of a wire rod. In the present invention, in order to refine the crystal grains during final finish rolling, it is preferable to ensure that the austenite grain size (AGS) before finish rolling is 5 to 20 μm. Thereafter, finish rolling is preferably performed in a temperature range of 730°C to Acm with a deformation amount of 0.3 or more. It is more preferable that the deformation amount is 0.5 or more. Here, Acm means the temperature at which cementite dissolves during heating or precipitates during cooling in hypereutectoid steel.

[0032] If the AGS before the finish rolling is less than 5 μm, the rough rolling is performed at a low temperature, which increases the roll load and shortens the equipment life. On the other hand, if it exceeds 20 μm, an increase in the critical deformation amount is required during the finish rolling, making it difficult to produce wire rod with fine grains. Also, if the finish rolling temperature is lower than 730°C, the load on the rolling rolls increases and shortens the equipment life. Conversely, if it is higher than Acm, phase transformation does not occur, making it difficult to produce fine-grained wire rod. During the above wire rod rolling, it is preferable to satisfy the condition of the following relational expression 2. [Equation 2] 2500*([C]-1) 2 +100000*([Al]-0.035) 2 +(AGS-12.5) 4 / 130+(finishing rolling temperature -760) 2 / 65≦80 (In the above relational expression 2, [C] and [Al] mean the contents (wt%) of C and Al in the alloy composition, the unit of AGS is μm, and the unit of finish rolling temperature is ° C.)

[0033] The carbon content influences the formation of cementite (Fe3C) in the manufactured wire rod and spheroidized heat-treated material, which in turn affects mechanical properties such as tensile strength, so an appropriate carbon content is required. The lower the Al content, the less AlN precipitates, which makes it difficult to suppress grain growth, so optimization is necessary. Furthermore, the higher the AGS before finish rolling, the lower the rolling amount and finish rolling temperature must be to refine the grains. Therefore, from a process cost perspective, it is preferable to manage the AGS and finish rolling temperature appropriately. The above relational expression 2 reflects this technical viewpoint, and if the value of relational expression 2 exceeds 80, it is difficult to expect appropriate cementite formation and grain refinement.

[0034] After the wire rod is rolled, it is coiled and cooled. The cooling is preferably performed by cooling to a temperature range of 550 to 650°C at an average cooling rate of 3°C / sec or more, and then cooling at an average cooling rate of 1°C / sec or less after the temperature reaches 550 to 650°C. If the average cooling rate to the temperature range of 550 to 650°C is less than 3°C / sec, it is difficult to maintain the fine crystal grains obtained during rolling at a temperature below the transformation point. On the other hand, after reaching 550 to 650°C, the cooling rate below that temperature is preferably 1°C / sec or less in order to suppress the formation of low-temperature structures such as bainite and martensite.

[0035] In the present invention, a spheroidized material can be produced by drawing the wire rod produced as described above, heating it to Ae1 to Ae1+100°C, holding it for 5 to 15 hours, and then cooling it to 660°C at a rate of 20°C / hr or less to perform spheroidizing heat treatment. If the heating temperature is less than Ae1, the spheroidizing heat treatment time becomes too long. Conversely, if the heating temperature exceeds Ae1+100°C, the number of spheroidized carbide seeds decreases, and the spheroidizing heat treatment effect may be insufficient. Here, Ae1 refers to the temperature at which austenite is generated during heating or disappears during cooling. If the holding time is less than 5 hours, the spheroidizing heat treatment does not proceed sufficiently, resulting in a problem of an increased aspect ratio of cementite. If the holding time exceeds 15 hours, the cost increases. If the cooling rate exceeds 20°C / hr, there is a risk of pearlite reforming due to the high cooling rate. After the spheroidizing heat treatment, the drawn wire exhibits a weak tensile strength of 740 MPa or less, and the average aspect ratio of cementite is 3 or less, making it easy to carry out cold heading or cold forging to produce the final product. [Example]

[0036] Examples of the present invention will be described below. It should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. (Example) A bloom having the alloy composition (wt %) shown in Table 1 below (the remainder being Fe and unavoidable impurities) was prepared and then rolled to produce a billet. The bloom was cast, homogenized at 1200°C for 4 hours, and then rolled at 1000°C. After rolling, the bloom was air-cooled to 500°C and then placed in a water-cooled chamber at a cooling rate of 5°C / s or more, in the case of water-cooling according to the cooling method shown in Table 2. A wire having a diameter of 9 mm was then produced from the billet under the wire-producing conditions shown in Table 2 below. The microstructure and mechanical properties of the wire thus produced were measured, and the results are shown in Table 3. On the other hand, the wire rods produced as described above were drawn and then subjected to a single soft spheroidizing heat treatment (holding at 780°C for 8 hours, followed by cooling to 640°C at a cooling rate of 15°C / hr), and the average aspect ratio of cementite and tensile strength were measured. The results are shown in Table 4.

[0037] Meanwhile, in Table 2, the austenite grain size (AGS) before finish rolling was measured by cutting the material using a cutting crop performed before finish hot rolling, immediately quenching it, and measuring the AGS using the ASTM E112 method. For the sampled test specimens, measurements were taken at five random points 1 / 4 of the way down from the diameter, and the average values ​​were shown. The average grain size of the blocks was measured using EBSD and the ASTM E112 method. The block size was defined as the size of a region in pearlite where the ferrite crystal orientation is the same and the crystal orientation difference is 15 degrees or more. Among the following examples, Example 1 and Comparative Example 5 were observed and shown in Figures 3 and 4, respectively. The block size was quantified using the ASTM E112 method. The measured material was measured at five random points within one-quarter of the diameter of a test specimen taken after wire rolling and removing the end-end water-cooled portion, and the average value was calculated. Furthermore, the length of pro-eutectoid cementite was measured by photographing five random points within one-quarter of the diameter of a test specimen taken after wire rolling and removing the end-end water-cooled portion using an SEM at 3000x magnification. The total length of the pro-eutectoid cementite was analyzed using Leica's Clemex Vision software, and the average of the five points was calculated.

[0038] The wiredrawability was evaluated by drawing the manufactured 9 mm wire rods at a cross-sectional area reduction rate of 5 to 50%. The center of the L-shaped cross section of the drawn material was photographed at 5000x magnification to check whether defects such as chevron cracks occurred at the pearlite interface, pro-eutectoid cementite interface, etc., and the presence or absence of such defects was indicated by ○ / ×. On the other hand, after the above-mentioned single spheroidizing heat treatment, the average aspect ratio of cementite was measured by photographing three fields of view at 1 / 4 to 1 / 2 points in the diameter direction of the wire rod using an SEM at 3000x magnification, automatically measuring the long axis / shortening of cementite within the field of view using an image measurement program, and then statistically processing the results.

[0039] [Table 1]

[0040] [Table 2]

[0041] In Table 2 above, the column for relational expression 2 is 2500*([C]-1) 2 +100000*([Al]-0.035) 2 +(AGS-12.5) 4 / 130+(finishing rolling temperature -760) 2 where [C] and [Al] are the contents of C and Al (wt%) in the alloy composition, AGS is the average grain size of austenite in μm, and the finish rolling temperature is in °C.

[0042] [Table 3]

[0043] In Table 3 above, pro-eutectoid C means pro-eutectoid cementite, P means pearlite, B means bainite, and M means martensite. Relational formula 1 is expressed as follows: (average size of block crystal grains (μm)) 2 / (length of proeutectoid cementite (μm / 1200μm 2 ))].

[0044] [Table 4]

[0045] As can be seen from Tables 1 to 4 above, the wire rods of Examples 1 to 5, which satisfy the conditions proposed by the present invention, have excellent wiredrawability with a reduction area of ​​20% or more even without spheroidizing heat treatment, and can also maintain a tensile strength of 1200 MPa or more. Furthermore, even with a single spheroidizing heat treatment after wiredrawing, wire rods with an average cementite aspect ratio of 3 or less can be produced. In particular, Figure 1 is a scanning electron microscope (SEM) photograph of the microstructure of the wire rod of Example 1. As can be seen from Figure 1, Example 1 is composed of pro-eutectoid cementite and full pearlite, and the arrows in Figure 1 indicate the pro-eutectoid cementite. As can be seen from Figure 1, the pro-eutectoid cementite is confirmed to be formed along the prior austenite grain boundaries. Figure 3 is an EBSD photograph of Example 1, which confirms that the misorientation of the crystal grains is 2 degrees or more. This confirms that the average block crystal grain size of Example 1 is approximately 4.7 μm, which is significantly smaller than that achieved under typical manufacturing conditions.

[0046] On the other hand, in Comparative Example 1, air cooling was performed after rolling the steel billet, causing coarsening of AlN in the steel material, and in Comparative Example 2, the Al content in the steel composition was low, so AlN could hardly be generated. As a result, in the wire rods of Comparative Examples 1 and 2, 2 The number of AlN particles with a size of 30 nm or less per wire was 20 or less, and the growth of crystal grains could not be suppressed during cooling of the wire, resulting in the size of block crystal grains not being controlled. Comparative Example 3 had a low carbon content, and pro-eutectoid ferrite remained in the wire, and the wiredrawing characteristics were superior to those of the other comparative examples, but the strength was low due to the low carbon content, and the strength of the material was low even after spheroidizing heat treatment, making it difficult to use for various purposes.

[0047] In Comparative Example 4, the AGS size before finish rolling was larger than that of the invention examples due to the high billet heating temperature. Since coarse AGS can be refined by a high critical deformation amount, an insufficient finish rolling deformation amount ultimately resulted in coarse grains appearing in the wire rod, resulting in poor wiredrawability. In Comparative Example 5, the high finish rolling temperature prevented fine grains from being obtained, resulting in coarse grains like in Comparative Example 4, and poor wiredrawability. Figure 2 is a photograph of the microstructure of the wire rod of Comparative Example 5 observed by SEM, and it can be seen that the grain size is larger than that of Figure 1, and the length of the pro-eutectoid cementite formed along the prior austenite grain boundaries is shorter. Figure 4 is an EBSD photograph of Comparative Example 5, and the misorientation of the grains is classified as in Figure 3. When compared with Figure 3, it can be seen that the block grain size of Comparative Example 5 in Figure 4 is coarse.

[0048] In Comparative Example 6, fine crystal grains were not obtained due to the small amount of finish rolling, and coarse crystal grains appeared in the wire rod, resulting in poor wiredrawability. In the wire rod of Comparative Example 7, the fine crystal grains produced by rolling coarsened due to the low initial cooling rate, preventing the formation of fine wire rod crystal grains and resulting in poor wiredrawability. In the case of Comparative Example 8, martensite and bainite appeared due to the fast cooling rate, and it was confirmed that internal cracks occurred even with just 5% wiredrawing.

Claims

1. In weight percent, 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 the remainder being Fe and inevitable impurities, The microstructure is mainly pearlite with proeutectoid cementite. AlN with an average particle size of 30 nm or less is used as a unit area (μm ２ ) contains 20 or more per A wire rod with excellent wire drawability, characterized by having a microstructure that satisfies the following relational expression 1: [Relationship 1] (Average size of block crystal grains (μm)) ２ / (length of proeutectoid cementite (μm / 1200 μm ２ ))≦0.5

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

3. 2. A wire rod with excellent wire drawability according to claim 1, wherein the microstructure is pro-eutectoid cementite with an area fraction of 10% or less, and the remainder is pearlite.

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

5. 2. The wire rod with excellent drawability according to claim 1, wherein the wire rod is not subjected to a spherical softening heat treatment before the wire drawing step, and is drawn by 15% or more during the wire drawing.

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

7. heating a slab containing, by weight percent, 0.8 to 1.2% C, 0.01 to 0.6% Si, 0.1 to 0.6% Mn, 0.8 to 2.0% Cr, 0.01 to 0.06% Al, 0.02% or less N (excluding 0), and the remainder being Fe and unavoidable impurities, and rolling the slab to produce a billet; cooling the produced billet; heating the billet to 950 to 1050°C; rolling the heated billet into a wire rod; and winding the wire rod and cooling it to 550 to 650°C at an average cooling rate of 3°C / sec or more, and cooling it to a temperature of 550 to 650°C at an average cooling rate of 1°C / sec or less, The wire rod is rolled so that the austenite grain size (AGS) before finish rolling is 5 to 20 μm, and the finish rolling is performed at a temperature range of 730° C. to A cm with a deformation amount of 0.3 or more.

8. 8. The method for producing a wire rod having excellent wire drawability according to claim 7, wherein the wire rod is rolled so as to satisfy the condition of the following relational expression 2. [Relationship 2] 2500*([C]-1) ２ +100000*([Al]-0.035) ２ +(AGS-12.5) ４ / 130+ (finish rolling temperature -760) ２ / 65≦80 (In the above Relational Formula 2, [C] and [Al] mean the contents (wt%) of C and Al in the alloy composition, the unit of AGS is μm, and the unit of finish rolling temperature is ° C.)

9. 8. The method for producing a wire rod excellent in wire drawability according to claim 7, wherein the steel billet is heated in a temperature range of 1100 to 1300°C for 2 to 10 hours, and the billet is cooled to 500°C or higher after rolling the steel billet at a cooling rate of 5°C / s or higher.

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

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