Wire rod, steel wire and manufacturing method thereof

Abstract

A wire rod according to an embodiment of the present invention comprises, in wt%, 0.69-0.75% of C, 0.10-0.50% of Si, 0.10-0.50% of Mn, 0.010-0.035% of Ti, 0.0030-0.0040% of N, 0.030% or less of P, and 0.030% or less of S, with the remainder comprising Fe and inevitable impurities, wherein [Ti] / [N] satisfies 1 to 3, and the microstructure of the wire contains, in area fraction, 1.5% or less of proeutectoid ferrite in the 1 / 2D to 3 / 2D region, with the remainder being pearlite. ([Ti] / [N] is the ratio between the number of atoms of Ti and the number of atoms of N, [Ti] is (wt% of Ti / 47.887), and [N] is (wt% of N / 14.007).)

Description

Wire rod, steel wire and method of manufacturing the same

[0001] The present invention relates to wire rods, steel wires and methods for manufacturing the same.

[0002] Typically, fine bead wire is made by first drawing a 5.5 mm wire after pickling, then performing an intermediate heat treatment to make it ductile (form fine pearlite), and then performing a second drawing to produce a bead wire of 1.0 mm or less, and then performing a low-temperature heat treatment called bluing to secure elongation. The bead wire according to the present invention is a high-strength grade, and is mainly made of 0.82%C-0.2%Si-0.5Mn general-purpose carbon steel.

[0003] To reduce carbon emissions in recent automobiles, steelmakers are making significant efforts to eliminate heat treatment, a major source of carbon emissions. Intermediate heat treatment is a key process for securing ductility, which is reduced by drawing, through the formation of fine pearlite. However, because product sizes are small (less than 1.0 mm), the total processing volume without heat treatment is high, making it difficult to meet mechanical properties. Furthermore, the increased processing volume due to the omission of the intermediate heat treatment process significantly increases the tensile strength of the product. Therefore, to address the issues that arise during beading (forming) due to high rigidity, a tensile strength similar to that of currently heat-treated materials is required.

[0004] The purpose of the present invention to solve the above-described problems is to provide a wire rod, steel wire, and a manufacturing method thereof, which fix the C content to 0.69 to 0.75%, secure product elongation through suppression of solid solution nitrogen through [Ti] / [N] control, and lower the strength of the wire rod to the level of existing heat-treated materials that have undergone intermediate heat treatment by changing the wire rod manufacturing standard while omitting intermediate heat treatment. Here, [Ti] / [N] refers to the ratio of the atomic numbers of Ti and N.

[0005] The problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0006] According to one embodiment of the present invention, a wire includes C: 0.69 to 0.75% by weight, Si: 0.10 to 0.50%, Mn: 0.10 to 0.50%, Ti: 0.010 to 0.035%, N: 0.0030 to 0.0040%, P: 0.030% or less, and S: 0.030% or less, the remainder being Fe and other inevitable impurities, wherein [Ti] / [N] satisfies 1 to 3, and the microstructure includes proeutectoid ferrite in an area fraction of 1.5% or less in a 1 / 2D to 3 / 2D region and the remainder being pearlite. (Here, [Ti] / [N] is a ratio of the atomic numbers of Ti and N, [Ti] means (weight% of Ti / 47.887) and [N] means (weight% of N / 14.007)).

[0007] Additionally, the wire according to one embodiment of the present invention may have a tensile strength of 850 to 920 MPa.

[0008] Additionally, the wire according to one embodiment of the present invention may have a cross-sectional reduction ratio (RA) of 40 to 60%.

[0009] According to another embodiment of the present invention, a method for manufacturing a wire rod comprises the steps of: heating a billet containing C: 0.69 to 0.75% by weight, Si: 0.10 to 0.50%, Mn: 0.10 to 0.50%, Ti: 0.010 to 0.035%, N: 0.0030 to 0.0040%, P: 0.030% or less, and S: 0.030% or less, the remainder being Fe and other inevitable impurities, and [Ti] / [N] satisfying 1 to 3 at 950 to 1050°C for 90 to 120 minutes; subjecting the heated billet to sintering to prepare a wire rod; and subjecting the wire rod to A e3 +20 to A e3 Finishing rolling step at +80℃; A e3 +50 to A e3 Step of winding at +120℃; A e1A first cooling step of 10 to 25°C / s to -100°C; and a second cooling step of 2 to 5°C / s to 300°C. (Here, [Ti] / [N] is the ratio of the atomic numbers of Ti and N, and [Ti] means (weight % of Ti / 47.887) and [N] means (weight % of N / 14.007))

[0010] In addition, in the method for manufacturing a wire according to one embodiment of the present invention, the speed of the finishing rolling may be 50 to 90 m / s.

[0011] According to another embodiment of the present invention, a steel wire comprises C: 0.69 to 0.75% by weight, Si: 0.10 to 0.50%, Mn: 0.10 to 0.50%, Ti: 0.010 to 0.035%, N: 0.0030 to 0.0040%, P: 0.030% or less, and S: 0.030% or less, the remainder being Fe and other inevitable impurities, wherein [Ti] / [N] satisfies 1 to 3, and the microstructure comprises proeutectoid ferrite in an area fraction of 1.5% or less in a 1 / 2D to 3 / 2D region and the remainder being pearlite. (Here, [Ti] / [N] is a ratio of the atomic numbers of Ti and N, [Ti] means (weight% of Ti / 47.887) and [N] means (weight% of N / 14.007)).

[0012] Additionally, the steel wire according to one embodiment of the present invention may have a diameter of 1.00 mm or less.

[0013] Additionally, the steel wire according to one embodiment of the present invention may have a tensile strength of 2150 to 2250 MPa.

[0014] Additionally, the steel wire according to one embodiment of the present invention may have a twisting number of 30 or more without delamination.

[0015] Additionally, the steel wire according to one embodiment of the present invention may have an elongation of 7.0% or more.

[0016] A method for manufacturing a steel wire according to one embodiment of the present invention may include a step of drawing the wire material; and a step of heat treating the wire material at a temperature of 420 to 480°C for 1 to 2 seconds.

[0017] According to one embodiment of the present invention, the wire rod can satisfy a tensile strength of 850 to 920 MPa, which can secure productivity by increasing the processing speed during drawing.

[0018] In addition, the steel wire according to one embodiment of the present invention can satisfy an optimal tensile strength of 2150 to 2250 MPa, an elongation of 7.0% or more, and a number of twists of 30 or more without delamination of the steel wire without intermediate heat treatment.

[0019] In addition, the reduction of tensile strength required to improve workability during forming of steel wire can be achieved by reducing the amount of air blown during low-temperature rolling and low-temperature coiling of the wire rod and the pearlite formation section.

[0020] Hereinafter, embodiments of the disclosed invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to sufficiently convey the spirit of the disclosed invention to those skilled in the art. The disclosed invention is not limited to the embodiments presented herein and may be embodied in other forms. To clarify the disclosed invention, the drawings may omit portions irrelevant to the description, and the sizes of components may be slightly exaggerated to facilitate understanding.

[0021] Throughout the specification, whenever a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0022] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0023] Hereinafter, the reasons for numerical limitations on the alloy component content in the embodiments of the present invention will be described. Hereinafter, unless otherwise specified, the unit is weight percent.

[0024] According to one embodiment of the present invention, a wire includes, in wt%, C: 0.69 to 0.75 %, Si: 0.10 to 0.50 %, Mn: 0.10 to 0.50 %, Ti: 0.010 to 0.035 %, N: 0.0030 to 0.0040 %, P: 0.030 % or less, and S: 0.030 % or less, the remainder being Fe and other inevitable impurities, wherein [Ti] / [N] satisfies 1 to 3, and the microstructure includes proeutectoid ferrite in an area fraction of 1.5% or less in a 1 / 2D to 3 / 2D region and pearlite for the remainder. (Here, [Ti] / [N] means the ratio of the atomic numbers of Ti and N.)

[0025] The reasons for limiting the composition range of each alloy element are described below.

[0026] The C (carbon) content can be 0.69 to 0.75%.

[0027] C is the element that can most effectively increase the strength of a material, and a 0.10% increase in its content has a strength increase effect of 100 MPa. If the C content is less than 0.69%, the strength of the final product may be inferior, and if it is added in excess of 0.75%, it exceeds the product regulation upper limit. Therefore, it is recommended to add C in the range of 0.69 to 0.75%.

[0028] The content of Si (silicon) can be 0.10 to 0.50%.

[0029] Since Si can form Fe2SiO4 at the interface between the base material and the scale during cooling, thereby lowering the scale exfoliation property, it is desirable to maintain it at 0.50% or less as it can contribute to an increase in relative investment rate when the Si content increases.

[0030] The content of Mn (manganese) can be 0.10 to 0.50%.

[0031] When Mn increases by 0.10%, the strength increases by 20 MPa, and when heat treated, it provides sufficient hardenability to suppress the formation of two-phase structures such as proeutectoid ferrite. In the present invention, it is difficult to secure tensile strength when controlled below 0.10%, and when it exceeds 0.50%, the target strength is exceeded and the lower limit total elongation decreases, so it is preferable to maintain it below that.

[0032] The content of Ti (titanium) can be 0.010 to 0.035%.

[0033] Ti plays a role in increasing the strength of steel wires through TiC precipitates. When added below 0.010%, sufficient TiC precipitates are not formed, making it difficult to achieve high strength. When added above 0.035%, tundish nozzle clogging occurs, resulting in poor productivity. Therefore, controlling the Ti content below that level is desirable.

[0034] The content of N (nitrogen) can be 0.0030 to 0.0040%.

[0035] Nitrogen is an element that excels at increasing strength through solidification. However, excessive content can lead to increased aging strength due to dislocation and N bonding, which reduces the ductility of the material and increases manufacturing costs. Considering this, the upper limit for Ni content can be set at 0.0040%.

[0036] [Ti] / [N]: satisfies 1 to 3. (Here, [Ti] / [N] means the ratio of the atomic numbers of Ti and N.)

[0037] The solid solution nitrogen formed in the ferrite lowers the elongation of the product because it is fixed to the dislocation. Since TiN is formed when Ti is added, it is possible to prevent the decrease in elongation due to the solid solution nitrogen. When [Ti] / [N] is formed below 1, the effect is small, and when it exceeds 3, wire breakage is induced during processing due to the formation of coarse TiN, so it is preferable to control it below that. In the above [Ti] / [N], the atomic weight of Ti is 47.887 and the atomic weight of N is 14.007, and in [Ti] / [N], [Ti] means (weight% of Ti / 47.887) and [N] means (weight% of N / 14.007).

[0038] Although the content of other impurities, phosphorus (P) and sulfur (S), is not specifically stipulated, it is desirable to control them to 0.030% or less each from the perspective of securing ductility, as in conventional steel wires.

[0039] In addition to the above composition, the remaining component is iron (Fe). However, during the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full details are not specifically mentioned in this specification.

[0040] The microstructure of the wire according to one embodiment of the present invention comprises an area fraction of proeutectoid ferrite of 1.5% or less in the 1 / 2D to 3 / 2D region, and pearlite in the remainder. If the area fraction of proeutectoid ferrite exceeds 1.5%, the elongation of the final steel wire (material) after drawing and bluing heat treatment is reduced. In addition, the occurrence rate of kinks, which occur when uniaxial stress is applied after being twisted into an alpha shape, may increase by 30% or more.

[0041] In addition, the wire rod according to one embodiment of the present invention may have a tensile strength of 850 to 920 MPa. If the tensile strength exceeds 920 MPa, the life of the die during drawing may be reduced, and the processing speed may not be increased, resulting in poor productivity.

[0042] Additionally, the wire according to one embodiment of the present invention may have a cross-sectional area reduction ratio (RA) of 40 to 60%. Preferably, it may be 40 to 50%. More preferably, it may be 45 to 50%.

[0043] Hereinafter, a method for manufacturing a wire according to one embodiment of the present invention having the above-described alloy composition will be described.

[0044] The wire of the present invention can be manufactured by manufacturing a billet having the above-described alloy composition, and then reheating it - wire rolling - coiling - cooling it.

[0045] A method for manufacturing a wire rod according to one embodiment of the present invention comprises the steps of: heating a billet containing C: 0.69 to 0.75% by weight, Si: 0.10 to 0.50%, Mn: 0.10 to 0.50%, Ti: 0.010 to 0.035%, N: 0.0030 to 0.0040%, P: 0.030% or less, and S: 0.030% or less, the remainder being Fe and other inevitable impurities, and [Ti] / [N] satisfying 1 to 3 at 950 to 1050°C for 90 to 120 minutes; subjecting the heated billet to sintering to prepare a wire rod; and subjecting the wire rod to A e3 +20 to A e3 Finishing rolling step at +80℃; A e3 +50 to A e3 Step of winding at +120℃; A e1A first cooling step of 10 to 25°C / s to -100°C; and a second cooling step of 2 to 5°C / s to 300°C. (Here, [Ti] / [N] is the ratio of the atomic numbers of Ti and N, and [Ti] means (weight % of Ti / 47.887) and [N] means (weight % of N / 14.007))

[0046] After manufacturing a billet having the above-described alloy composition, the billet is maintained at a heating furnace temperature of 950 to 1050°C for 90 to 120 minutes for normalizing and austenite formation, and then rolled. If the temperature is maintained below 950°C, there is a problem of a long charging time, and if the temperature is maintained above 1050°C, there is a heating furnace load. Therefore, it is preferable to control the temperature to 950 to 1050°C. More preferably, it can be controlled in the temperature range of 950 to 990°C. Even more preferably, it can be controlled in the temperature range of 1030 to 1050°C. In addition, if it is maintained for less than 90 minutes, it may be difficult to form central austenite, and if it is maintained for more than 120 minutes, coarse grain growth may occur. Therefore, it is preferable to maintain the temperature for 90 to 120 minutes.

[0047] The heated billet is rolled under normal rolling conditions. That is, the heated billet is sequentially subjected to rough rolling, finishing rolling, and finishing rolling to produce wire rod.

[0048] The temperature of the finishing rolling after the rolling of the present invention is A to prevent the formation of proeutectoid ferrite. e3 +20 to A e3 +80℃ is preferred. More preferably, it can be controlled in the range of 900 to 1000℃. More preferably, it can be controlled in the range of 950 to 1050℃.

[0049] The winding temperature of the wire of the present invention is A e3 +50 to A e3It can be +120℃. Since the blowing capacity of the stelmore cooling bed is determined, replacing the equipment to increase the blowing amount inevitably causes an increase in manufacturing costs. In addition, the reason why a lot of blowing is required after coiling or the cooling speed must be fast is because the proeutectoid ferrite formation zone must be passed as quickly as possible and the pearlite transformation point must be reached quickly. The low-temperature coiling within the above range of the present invention can make this possible without replacing the blower. However, A e3 The equipment cannot be controlled below +50℃, and A e3 There may be a limit to the reduction in tensile strength when the temperature exceeds +120℃. The temperature range may be preferably 750 to 900℃. More preferably, the temperature range may be 800 to 900℃.

[0050] After winding A e1 It may include a first cooling step proceeding at a cooling rate of 10 to 25 ℃ / s to -100 ℃ and a second cooling step proceeding at a cooling rate of 2 to 5 ℃ / s or less to 300 ℃. A to suppress the formation of proeutectoid ferrite and promote the formation of pearlite. e1 The cooling rate is 10 to 25°C / s to -100°C, and if the cooling rate exceeds 25°C / s in the first cooling stage, additional equipment investment is required due to equipment limitations. Preferably, it can be controlled at 15 to 25°C / s.

[0051] In the second cooling step, it is preferable to cool at a cooling rate of 2 to 5°C / s to control the target tensile strength of the wire rod to 850 to 920 MPa up to 300°C. That is, in order to satisfy the tensile strength of the wire rod according to the present invention, the first cooling step and the second cooling step are essentially included, with the proeutectoid ferrite having an area fraction of 1.5% or less and the remainder being pearlite.

[0052] In addition, in the method for manufacturing a wire according to one embodiment of the present invention, the speed of the finishing rolling may be 50 to 90 m / s.

[0053] Hereinafter, a steel wire having the above-described alloy composition and a method for manufacturing the same according to one embodiment of the present invention will be described.

[0054] According to another embodiment of the present invention, a steel wire comprises C: 0.69 to 0.75% by weight, Si: 0.10 to 0.50%, Mn: 0.10 to 0.50%, Ti: 0.010 to 0.035%, N: 0.0030 to 0.0040%, P: 0.030% or less, and S: 0.030% or less, the remainder being Fe and other inevitable impurities, wherein [Ti] / [N] satisfies 1 to 3, and the microstructure comprises proeutectoid ferrite in an area fraction of 1.5% or less in a 1 / 2D to 3 / 2D region and the remainder being pearlite. (Here, [Ti] / [N] is a ratio of the atomic numbers of Ti and N, [Ti] means (weight% of Ti / 47.887) and [N] means (weight% of N / 14.007)).

[0055] Additionally, the steel wire according to one embodiment of the present invention may have a diameter of 1.00 mm or less.

[0056] In addition, the steel wire according to one embodiment of the present invention can reduce overload of the device during forming by satisfying a tensile strength of 2150 to 2250 MPa. Preferably, the tensile strength may be 2150 to 2230 MPa.

[0057] Additionally, the steel wire according to one embodiment of the present invention may have a twisting number of 30 or more without delamination.

[0058] In addition, the steel wire according to one embodiment of the present invention can secure ductility along with optimal strength with an elongation of 7.0% or more.

[0059] A method for manufacturing a steel wire according to another embodiment of the present invention may include the steps of drawing the wire rod; and performing a bluing heat treatment at a temperature of 420 to 480°C for 1 to 2 seconds. If the bluing heat treatment temperature is lower than 420°C, the elongation recovery is not achieved and there is a significant deviation, and if it exceeds 480°C, a spheroidization phenomenon of plate-like pearlite is observed, significantly reducing the strength and elongation and also lowering the kink.

[0060] The present invention will be described in more detail through the following examples.

[0061] (Example)

[0062] In the present invention, JS-SWRH82A (0.82c-0.25Si-0.5Mn) applied to 0.96 mm fine bead wire was used as Comparative Example 1, and Comparative Examples and Experimental Examples were manufactured with differences in components and wire manufacturing conditions as shown in Tables 1 and 2 below. Bloom and billet rolling were performed, and the heating furnace temperature was maintained at 990 ℃ for 80 minutes for low-temperature rolling and coiling, and the rolling speed was set at 80%. The temperature at the entrance of the finishing rolling was 810 ℃, the diameter after passing through the final rolling mill was 5.0 mm, and the coiling temperature was 780 ℃ (A e3 : 745℃), proeutectoid ferrite is formed (A e1 : 720℃) and the maximum cooling rate of 25℃ / s was applied up to 620℃, which is the pearlite nose entry temperature, and the next pearlite growth and coil shape control section was finished by cooling without blowing. In addition, the temperature at the entry of the reforming tube was set to 320℃.

[0063] As a method for measuring the microstructure of the wire of the present invention, a specimen was etched using 3% nital and the microstructure was observed using a scanning electron microscope (SEM). The microstructure was observed in a cross-section of 2 / D to 3 / 2D, and the central (2 / 2D) ​​location was excluded to minimize the effect of center segregation. The microstructure was measured at a location that could represent the structure, and the proeutectoid ferrite was measured at x1000 magnification, and the pearlite interlayer spacing was measured at x3000 magnification, and only pearlite that was formed straight without being tilted was selected for measurement.

[0064] In addition, in the case of measuring the tensile strength of the wire, a 40 cm long piece was cut from the 2 rings (circumference: 3.2 m) at the rear end of the coil and a tensile test was performed. The tensile speed (cross head speed) was 70 m / m and the length of the balanced section was 30 cm. The diameter before the tensile test was measured using a stereo projector, and the diameter of the necked part after the tensile test was used to measure the cross-sectional reduction ratio (= 1-(after tensile test / before certification test)^2)*100).

[0065] CSiMn[Ti] / [N]TiNPS Comparative Example 10.750.250.50.7500.0100.00390.0150.022 Comparative Example 20.720.250.50.9750.0120.00360.0200.012 Comparative Example 30.630.250.50.0090.00010.00330.0050.007 Comparative Example 40.510.250.50.9510.0130.00400.0030.005 Comparative Example 50.710.250.50.8470.0110.00380.0120.013 Experimental Example 10.720.250.51.0030.0120.00350.0040.001Experimental example 20.710.250.51.8930.0220.00340.0170.009Experimental example 30.720.250.52.6740.0320.00350.0020.010Comparative example 60.710.250.53.0190.0320.00310.0160.025Comparative example 70.720.250.51.0030.0120.00350.0230.019Comparative example 80.720.250.53.5850.0380.00310.0040.001Comparative example 90.720.250.53.1080.0340.00320.0040.001

[0066] Experimental Examples 1 to 3 according to the present invention satisfied all of the alloy composition range, [Ti] / [N], and process control conditions, and as a result, the tensile strength of the wire rod decreased, but the cross-sectional reduction rate increased up to a maximum of 50%. Specifically, the area fraction of proeutectoid ferrite was 1.5% or less, and the remainder was pearlite, and the tensile strength satisfied 850 to 920 MPa. The results are shown in Table 2 below.

[0067] Protease ferrite area fraction (%)Pearlite area fraction (%)Pearlite average size (nm)Tensile strength of wire (MPa)Reduction rate of wire cross-section (%)Heating furnace extraction temperature (℃)Finished rolling entrance temperature (℃)Rolling speed (m / s)Wire diameter (mm)Coiling temperature (℃)First cooling rate after coiling (℃ / s)Second cooling rate after coiling (℃ / s)Comparative example 10.899.21701120411060101010058802520Comparative example 21.798.31781025451050100010058802520Comparative example 33.496.6192910491050100010058802520Comparative example 45.894.2205809541050102010058802520Comparative Example 51.898.217998044105010001005770254Experimental Example 11.498.618191049980920805780254Experimental Example 21.598.517990550985910805785254Experimental Example 31.198.917892046990930805790254Comparative Example 61.898.218093032980920805780254Comparative Example 73.996.11828505298092080578074Comparative Example 81.398.7183125035980920805780254Comparative Example 91.598.5182115038980920805780254

[0068] In addition, the manufactured wire rod was also subjected to drawing processing to 0.96 mm at a drawing speed of 400 m / min using a dry wire drawing machine without intermediate heat treatment after pickling, and bluing treatment was performed by immersing at 450℃ for 2 seconds, and then water cooling was performed to manufacture steel wire. In the case of measuring the tensile strength of the steel wire according to the present invention, the length of the tensile test piece was 40 cm, the tensile speed (cross head speed) was 100 m / m, and the length of the equilibrium part was 30 cm, which is the same as the wire rod. The elongation was determined as the % of the elongated length compared to the initial length of 30 cm, and in the case of torsion, the back load was set to load (kg) x 0.008, the length was set to 100D (D: diameter), and the number of fractures was confirmed while rotating the non-fixed chuck in the same direction while fixing one chuck, and the shape of the broken material after the final destruction was defined as a normal material if it had a right angle. Accordingly, the measured properties of the experimental and comparative examples of the steel wire are shown in Table 3 below.

[0069] The above experimental examples 1 to 3 show the results of low-temperature coiling at 780°C by lowering the rolling speed and not blowing air in the section below 620°C. It was confirmed that the strength of the steel wires of experimental examples 1 to 3 decreased to a maximum of 2210 MPa during drawing and bluing, and the elongation increased to 7.3% or more.

[0070] Fresh material diameter (mm) Fresh processing amount (%) Bluing temperature (℃) Tensile strength (Mpa) Total elongation (%) Twist (times) Comparative example 10.9696.34502, 4206.632 Comparative example 20.9696.34502, 3506.734 Comparative example 30.9696.34502, 2403.50 Comparative example 40.9696.34502, 1303.20 Comparative example 50.9696.34502, 3006.935 Experimental example 10.9696.34502, 2107.336 Experimental example 20.9696.34502, 2157.435 Experimental example 30.9696.34502,2307.331Comparative example 60.9696.3450Single line 00Comparative example 70.9696.34502,1505.934Comparative example 80.9696.3420Single line 00Comparative example 90.9696.3420Single line 00

[0071] Comparative Example 1 is a typical rolling, cooling, and processing condition for a steel grade that undergoes intermediate heat treatment during drawing, and its mechanical properties and microstructure are shown in Table 2 above. It is a steel grade close to eutectoid steel, and its ferrite fraction is less than 1% and it is composed of pearlite more than 99%. When the steel grade is directly drawn as wire rod without intermediate heat treatment, the tensile strength was confirmed to be 2400 MPa or more, the elongation was 6.6%, and the number of twists was 32. As described above, when the strength exceeds 2250 MPa, problems such as equipment load during the tire beading process occur, so a decrease in the strength is necessary. Comparative Examples 3 and 4 show changes in mechanical properties according to carbon content. This is a steel grade with a carbon content that gradually decreases from 0.72% to 0.52%, and the pearlite fraction decreases and the proeutectoid ferrite fraction increases to about 5.8%. It can be confirmed that the strength decreases as the ferrite fraction increases, but the RA increases. When drawing and bluing heat treatment are performed, the tensile strength decreases to the level of 2200 MPa, but there is a problem that the elongation does not recover to about 3%. These results confirm that proeutectoid ferrite is a factor that greatly affects the elongation, and this is because the dislocation density is greatly accumulated in the ferrite during drawing, so the dislocations are not completely annihilated when the same bluing temperature and time as the existing heat-treated material are applied.

[0072] Furthermore, in the case of Comparative Examples 1, 2 and 5, although the range of the alloy composition of the present invention is satisfied, the value of [Ti] / [N] is less than 1, so the effect of preventing the decrease in elongation due to dissolved nitrogen is small, and thus it can be confirmed that the elongation is inferior to that of Experimental Examples 1 to 3 at less than 7.0%.

[0073] Comparative examples 6, 8 and 9 satisfy the content range of the alloy composition according to the present invention, but when [Ti] / [N] exceeds 3, it can be confirmed that the short circuit occurs during the final drawing, which is the result of coarse TiN precipitation or crystallization.

[0074] Comparative Example 7 does not satisfy the range of the first cooling rate according to the present invention, so that the area fraction of the proeutectoid ferrite of the wire exceeds 1.5%, and accordingly, it can be confirmed that the elongation of Comparative Example 7 is inferior to that of Experimental Examples 1 to 3.

Claims

1. Containing C: 0.69 to 0.75% by weight, Si: 0.10 to 0.50%, Mn: 0.10 to 0.50%, Ti: 0.010 to 0.035%, N: 0.0030 to 0.0040%, P: 0.030% or less, and S: 0.030% or less, the remainder being Fe and other unavoidable impurities, [Ti] / [N] satisfies 1 to 3, The microstructure is a wire rod containing 1.5% or less of area fraction of proeutectoid ferrite in the 1 / 2D to 3 / 2D region and the remainder being pearlite. (Here, [Ti] / [N] is the ratio of the atomic numbers of Ti and N, [Ti] means (weight% of Ti / 47.887) and [N] means (weight% of N / 14.007).

2. In claim 1, Wire rod having a tensile strength of 850 to 920 MPa.

3. In claim 1, Wire rod with a reduction in cross-sectional area (RA) of 40 to 60%.

4. A step of heating a billet containing C: 0.69 to 0.75% by weight, Si: 0.10 to 0.50%, Mn: 0.10 to 0.50%, Ti: 0.010 to 0.035%, N: 0.0030 to 0.0040%, P: 0.030% or less, and S: 0.030% or less, the remainder being Fe and other unavoidable impurities, and wherein [Ti] / [N] satisfies 1 to 3 at 950 to 1050°C for 90 to 120 minutes; A step of preparing a wire rod by rolling the above heated billet; The above pre-existing condition is A e3 +20 to A e3 Finish rolling step at +80℃; A e3 +50 to A e3 Coiling step at +120℃; A e1-First cooling step at 10 to 25°C / s to -100°C; and A method for manufacturing a wire rod, comprising a second cooling step at 2 to 5°C / s to 300°C. (Here, [Ti] / [N] is the ratio of the atomic numbers of Ti and N, [Ti] means (weight% of Ti / 47.887) and [N] means (weight% of N / 14.007).

5. In claim 4, A method for manufacturing a wire rod, wherein the speed of the above finishing rolling is 50 to 90 m / s.

6. Containing C: 0.69 to 0.75% by weight, Si: 0.10 to 0.50%, Mn: 0.10 to 0.50%, Ti: 0.010 to 0.035%, N: 0.0030 to 0.0040%, P: 0.030% or less, and S: 0.030% or less, the remainder being Fe and other unavoidable impurities, [Ti] / [N] satisfies 1 to 3, The microstructure is a steel wire containing 1.5% or less of proeutectoid ferrite in the 1 / 2D to 3 / 2D region and the remainder being pearlite. (Here, [Ti] / [N] is the ratio of the atomic numbers of Ti and N, [Ti] means (weight% of Ti / 47.887) and [N] means (weight% of N / 14.007).

7. In claim 6, Steel wire with a diameter of 1.00 mm or less.

8. In claim 6, Steel wire having a tensile strength of 2150 to 2250 MPa.

9. In claim 6, A steel wire having a twisting number of 30 or more times without delamination of the steel wire.

10. In claim 6, A steel wire having an elongation of 7.0% or more.

11. A step of drawing the wire of claim 1 without intermediate heat treatment; and A method for manufacturing a steel wire, comprising the step of performing a bluing heat treatment at a temperature of 420 to 480°C for 1 to 2 seconds.