Steel wire rod, steel wire and manufacturing method thereof

Abstract

A steel wire according to an embodiment of the present invention comprises, by wt%: 0.1-0.5% of carbon (C), 0.01-0.6% of silicon (Si), 0.6-1.6% of manganese (Mn), 0.01-1.4% of chromium (Cr), more than 0% and not more than 0.02% of nitrogen (N); at least one selected from the group consisting of 0.01-0.06% of aluminum (Al), 0.005-0.03% of niobium (Nb), 0.001-0.03% of titanium (Ti), or 0.01-0.3% of vanadium (V); and the remainder of iron (Fe) and inevitable impurities. A microstructure of the steel wire includes ferrite and pearlite, and the ferrite has an average grain size of 3 ㎛ or less, a uniform elongation of 3.5% or more, and a yield ratio of 90% or more.

Description

Wire rod, steel wire and method of manufacturing the same

[0001] The present invention relates to a wire rod, a steel wire, and a method for manufacturing the same, which can simultaneously omit spheroidizing soft heat treatment and QT strengthening heat treatment, and more specifically, to a wire rod, a steel wire, and a method for manufacturing the same, which can omit heat treatment by controlling microstructure by optimizing rolling and subsequent process variables.

[0002] Cold heading wire rods are categorized into heat-treated products, manufactured through nitriding and strengthening heat treatments such as QT, and non-heat-treated products, manufactured without separate heat treatment. While heat-treated products are effective in extending die life during cold forging, the long process of nitriding heat treatment reduces productivity. Furthermore, the final QT heat treatment can cause warpage, requiring additional straightening costs. However, non-heat-treatable steels are more cost-effective because they eliminate the spheroidizing and QT heat treatment processes. Furthermore, they offer significant advantages in productivity and efficiency, eliminating the need for warpage correction.

[0003] Non-quenched steels can achieve tensile properties in the final product through subsequent drawing processing alone, eliminating the need for QT heat treatment. Furthermore, they offer the advantage of being able to produce products with a wide range of strengths through controlled drawing processing.

[0004] However, in the case of the microstructure of the wire rod composed of ferrite and pearlite, there is a concern that chevron-shaped internal cracks may occur if the critical processing amount is exceeded during drawing, so there is a limit to the processing amount, and the yield ratio (YR Ratio), which is the ratio of yield strength (YS) to tensile strength (TS), cannot be secured above 90% compared to the QT heat-treated material with a martensite structure, and it is difficult to secure uniform elongation, which represents the ductility from the yield strength (YS) to the tensile strength (TS), so there are restrictions on the use of fastening bolts in the plastic region, etc., and there were limits to completely replacing the QT heat-treated material.

[0005] Therefore, there is a need to develop a product manufacturing method that can secure equivalent properties without the need for conventional QT heat treatment.

[0006] The present invention aims to provide a wire rod, a steel wire, and a method for manufacturing the same, which can secure tensile properties equivalent to those of QT heat-treated materials by compensating for the shortcomings of existing non-heat-treatable steels without changing the chemical composition.

[0007] 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.

[0008] As a means for achieving the above-described object, a steel wire according to an example of the present invention comprises, in weight %, carbon (C): 0.1% or more and 0.5% or less, silicon (Si): 0.01% or more and 0.6% or less, manganese (Mn): 0.6% or more and 1.6% or less, chromium (Cr): 0.01% or more and 1.4% or less, nitrogen (N): more than 0% and 0.02% or less, and at least one selected from the group consisting of aluminum (Al): 0.01% or more and 0.06% or less, niobium (Nb): 0.005% or more and 0.03% or less, titanium (Ti): 0.001% or more and 0.03% or less, or vanadium (V): 0.01% or more and 0.3% or less, and the remainder comprises iron (Fe) and inevitable impurities, and the microstructure comprises ferrite and pearlite, and the average grain size of the ferrite The size may be 3㎛ or less, the uniform elongation may be 3.5% or more, and the yield ratio may be 90% or more.

[0009] Additionally, the steel wire according to an example of the present invention may have a tensile strength of 800 MPa or more and 1300 MPa or less.

[0010] Additionally, the steel wire according to one example of the present invention may have no cracks at an angle of ±45° based on the fresh direction inside the L cross-section.

[0011] In addition, the wire according to one example of the present invention includes, in weight %, carbon (C): 0.1% or more and 0.5% or less, silicon (Si): 0.01% or more and 0.6% or less, manganese (Mn): 0.6% or more and 1.6% or less, chromium (Cr): 0.01% or more and 1.4% or less, nitrogen (N): more than 0% and 0.02% or less, and includes at least one selected from the group consisting of aluminum (Al): 0.01% or more and 0.06% or less, niobium (Nb): 0.005% or more and 0.03% or less, titanium (Ti): 0.001% or more and 0.03% or less, or vanadium (V): 0.01% or more and 0.3% or less, and the remainder includes iron (Fe) and inevitable impurities, and the microstructure is composed of proeutectoid ferrite, residual pearlite, and bainite with an area fraction of 10% or less. And martensite is included, and the average grain size of the above-mentioned proeutectoid ferrite structure may be 5㎛ or less.

[0012] Additionally, the wire according to an example of the present invention may have a tensile strength of 650 MPa or more and 850 MPa or less.

[0013] In addition, a wire rod according to an example of the present invention may have no cracks at an angle of ±45° based on the drawing direction inside the L-section of the wire rod drawn and extruded with a processing amount of 30% or more and 70% or less of the wire rod.

[0014] (Here, L-section means a parallel section in the rolling direction.)

[0015] In addition, a method for manufacturing a steel wire according to an example of the present invention comprises the steps of preparing a billet including, in wt%, carbon (C): 0.1% or more and 0.5% or less, silicon (Si): 0.01% or more and 0.6% or less, manganese (Mn): 0.6% or more and 1.6% or less, chromium (Cr): 0.01% or more and 1.4% or less, nitrogen (N): more than 0% and 0.02% or less, and at least one selected from the group consisting of aluminum (Al): 0.01% or more and 0.06% or less, niobium (Nb): 0.005% or more and 0.03% or less, titanium (Ti): 0.001% or more and 0.03% or less, or vanadium (V): 0.01% or more and 0.3% or less, and the remainder including iron (Fe) and unavoidable impurities, and heating the billet at a temperature of 950°C or more and 1200°C. The method may include a step of heating to below, a step of finishing hot rolling the billet at 700°C or more and 900°C or less to manufacture a wire rod, a step of cooling the finished hot rolled wire rod at a cooling rate of 3°C / s or more and 15°C / s or less, and a step of drawing and extruding the cooled wire rod at a processing amount of 30% or more and 70% or less.

[0016] In addition, the method for manufacturing a steel wire according to an example of the present invention may further include a step of performing a dehydrogenation treatment on the fresh and extruded wire at a temperature of 200°C or higher and 600°C or lower for 30 minutes or longer and 60 minutes or shorter.

[0017] In addition, in the method for manufacturing a steel wire according to an example of the present invention, the step of manufacturing a wire rod by performing the finishing hot rolling may be performed at a temperature of 700°C or more and 800°C or less on the billet.

[0018] In addition, in the method for manufacturing a steel wire according to an example of the present invention, the cooling step can be performed at a cooling rate of 4°C / s or more and 5°C / s or less for the finished hot-rolled wire.

[0019] In addition, the method for manufacturing a steel wire according to an example of the present invention may further include a step of performing a dehydrogenation treatment on the fresh and extruded wire at a temperature of 300°C or higher and 450°C or lower for 30 minutes or longer and 60 minutes or shorter.

[0020] According to an embodiment of the present invention, a steel wire can be manufactured that can secure tensile properties equivalent to those of QT heat-treated steel by complementing the shortcomings of existing non-heat-treated steel.

[0021] In addition, according to an embodiment of the present invention, the yield ratio (YR Ratio), which is the yield strength (YS) / tensile strength (TS) ratio, can be secured to 90% or more, and the uniform elongation representing the ductility from the yield strength (YS) to the tensile strength (TS) can be secured to 3.5% or more, so that a steel wire used for a plastic region fastening bolt, etc. can be manufactured by replacing a QT heat-treated material.

[0022] Preferred embodiments of the present invention are described below. However, the embodiments of the present invention may be modified in various ways, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more fully explain the present invention to those of ordinary skill in the art.

[0023] The terminology used in this application is solely for the purpose of describing specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Additionally, it should be noted that terms such as "comprise" or "have" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

[0024] Meanwhile, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, specific terms should not be interpreted in an overly idealistic or formal sense. For example, singular expressions herein include plural expressions unless the context clearly indicates otherwise.

[0025] In addition, the terms "about", "substantially", etc. in this specification are used in the sense of or close to the numerical value when manufacturing and material tolerances inherent to the meanings mentioned are presented, and are used to prevent unscrupulous infringers from unfairly using the disclosure contents in which exact or absolute numerical values ​​are mentioned to aid in the understanding of the present invention.

[0026] To ensure workability, wire rods are sometimes subjected to spheroidization heat treatment. Spheroidization heat treatment is an additional process that requires significant heat treatment costs and time, reducing productivity and increasing the manufacturing cost of the material.

[0027] In addition, cold forming of the softened material is performed and QT heat treatment is performed to secure the physical properties of the final product. However, since QT heat treatment is performed at a high temperature of over 800℃, it is pointed out as one of the main causes of environmental pollution due to the large amount of CO2 emissions. To this end, many inventors are working to omit spheroidization and QT heat treatment.

[0028] Accordingly, as a result of in-depth research on a method for omitting spheroidization and QT heat treatment in manufacturing cold forged products such as bolts, the present invention was completed by confirming that by optimizing the alloy composition and manufacturing conditions to control the grain size of the final product, it is possible to secure equivalent physical properties even without spheroidization and QT heat treatment.

[0029] Below is a detailed explanation of the steel wire.

[0030] According to an example of the present invention, the steel wire contains, in weight %, carbon (C): 0.1% or more and 0.5% or less, silicon (Si): 0.01% or more and 0.6% or less, manganese (Mn): 0.6% or more and 1.6% or less, chromium (Cr): 0.01% or more and 1.4% or less, and nitrogen (N): more than 0% and 0.02% or less.

[0031] It contains at least one selected from the group consisting of aluminum (Al): 0.01% or more and 0.06% or less, niobium (Nb): 0.005% or more and 0.03% or less, titanium (Ti): 0.001% or more and 0.03% or less, or vanadium (V): 0.01% or more and 0.3% or less, and may contain the remainder iron (Fe) and unavoidable impurities.

[0032] The roles and contents of each component included in the steel wire according to the present invention are described below. The % for the components below means weight %.

[0033] The C content may be between 0.1% and 0.5%.

[0034] Carbon is an element added to ensure a certain level of strength. The carbon content is limited to 0.1% or more to ensure sufficient initial tensile strength accuracy before drawing and to secure a draw work hardening rate by securing an appropriate pearlite structure fraction. Furthermore, while securing initial strength is facilitated when the carbon content exceeds 0.5%, the coarsening of the pearlite structure facilitates the occurrence of internal cracks during drawing. Therefore, a carbon content of 0.1% or more and 0.5% or less is preferable, and more preferably, the carbon content is limited to a range of 0.2% or more and 0.48% or less.

[0035] The Si content may be 0.01% or more and 0.6% or less.

[0036] Silicon (Si) is a representative substitutional element added to ensure a certain level of strength. When the Si content is less than 0.01%, it is difficult to ensure the strength of the steel, and when it exceeds 0.6%, there is a disadvantage in that the yield strength increases significantly, which worsens the cold forgeability, such as breakage of forming fixtures during cold forging. Therefore, it is preferable to have a content of 0.01% or more and 0.6% or less, and more preferably, it is preferable to limit the carbon content to a range of 0.05% or more and 0.4% or less.

[0037] The Mn content may be 0.6% or more and 1.6% or less.

[0038] Manganese (Mn) is an element that strengthens steel by forming a substitutional solid solution within the matrix, and can secure the target strength without reducing ductility. It is a representative austenite former. If the Mn content is less than 0.6%, the strength through solid solution strengthening is not guaranteed, and it is difficult to expect an improvement in toughness. In addition, if the Mn content exceeds 1.6%, it is difficult to suppress central segregation, which may promote the occurrence of internal cracks during drawing. Therefore, it is desirable to limit the Mn content to 0.6% or more and 1.6% or less.

[0039] The Cr content may be 0.01% or more and 1.4% or less.

[0040] Cr, like Mn, is added to ensure the hardenability of steel. If the Cr content is less than 0.01%, it may be difficult to secure sufficient hardenability. In addition, if the Cr content exceeds 1.4%, the effect of improving the hardenability relative to the amount added is minimal, and the formation of coarse iron carbides may be encouraged, which may lower the impact energy absorption capacity. Therefore, it is preferable to limit the Cr content to 0.01% or more and 1.4% or less.

[0041] The N content may be greater than 0% and less than or equal to 0.02%.

[0042] In the present invention, it is preferable to manage the nitrogen (N) content to 0.02% or less. This is because if the content exceeds 0.02%, the material's ductility / ductility may deteriorate due to the dissolved nitrogen that is not combined as a precipitate.

[0043] The Al content may be 0.01% or more and 0.06% or less.

[0044] It is desirable to limit the above Al content to 0.01 to 0.06%. If the aluminum content is less than 0.02%, it is difficult to secure sufficient deoxidation power, and if it exceeds 0.05%, hard inclusions such as Al2O3 may increase, and in particular, nozzle clogging due to inclusions may occur during casting.

[0045] The Nb content may be 0.005% or more and 0.03% or less.

[0046] Nb precipitates during heating or rolling, which helps refine austenite grains and secure the target proeutectoid ferrite fraction. It also ensures sufficient work hardening during drawing, thereby achieving the target tensile strength while suppressing the deterioration of material ductility and toughness. Below 0.005%, it is difficult to achieve the desired grain refinement effect due to insufficient precipitation. Adding more than 0.03% can actually have a negative impact on grain refinement due to coarsening of precipitates.

[0047] The Ti content may be 0.001% or more and 0.03% or less.

[0048] Ti precipitates during heating or rolling, which helps refine austenite grains and secure the target proeutectoid ferrite fraction. It also ensures sufficient work hardening during drawing, ensuring the target tensile strength while suppressing the decline in material ductility and toughness. Below 0.001%, it is difficult to achieve the desired grain refinement effect due to insufficient precipitation. Adding more than 0.03% can actually reduce ductility and toughness due to coarsening of precipitates, or can even cause internal cracks during drawing.

[0049] The V content may be 0.01% or more and 0.3% or less.

[0050] V precipitates during the cooling process, enhancing the work hardening rate during drawing, thereby stably achieving the target tensile strength. However, at levels below 0.01%, the desired effect is difficult to achieve due to insufficient precipitation. Furthermore, at levels exceeding 0.3%, there is a risk of adverse effects, such as internal cracking or reduced ductility / toughness.

[0051] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment can inevitably be mixed in during the typical manufacturing process, this cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0052] Hereinafter, a steel wire according to one embodiment of the present invention having the above-described alloy composition will be described in detail.

[0053] According to an example of the present invention, a steel wire includes, in weight %, carbon (C): 0.1% or more and 0.5% or less, silicon (Si): 0.01% or more and 0.6% or less, manganese (Mn): 0.6% or more and 1.6% or less, chromium (Cr): 0.01% or more and 1.4% or less, nitrogen (N): more than 0% and 0.02% or less, and at least one selected from the group consisting of aluminum (Al): 0.01% or more and 0.06% or less, niobium (Nb): 0.005% or more and 0.03% or less, titanium (Ti): 0.001% or more and 0.03% or less, or vanadium (V): 0.01% or more and 0.3% or less, and the remainder includes iron (Fe) and inevitable impurities, and the microstructure of the steel wire may include ferrite and pearlite, and the average grain size of the ferrite is 3 ㎛. Below, preferably 1.5㎛ or less, most preferably 1.3㎛ or less, the uniform elongation of the steel wire may be 3.5% or more, preferably 4.0% or more, and the yield ratio of the steel wire may be 90% or more, preferably 94% or more, most preferably 94.9% or more. In the case of the steel wire manufactured according to the composition and process of the present invention, a fine structure of 3㎛ or less can be secured, so that not only an excellent yield ratio of 90% or more can be secured without QT heat treatment, but also a uniform elongation of 3.5% or more can be secured.

[0054] In the present invention, the crystal grain size refers to the diameter of a virtual circle having the same area as the crystal grain area. In addition, the average crystal grain size is (measured area x number of crystal grains). 0.5 can be calculated. The grain size can be measured based on the plane parallel to the cross-section in the vertical direction of the rolling (TD plane). At this time, the grain size was evaluated using an image analyzer with a built-in analysis program that assumes that multiple hexagons are connected (ASTM E112 grain measurement method).

[0055] In addition, the steel wire according to an example of the present invention may have a tensile strength of 800 MPa or more and 1300 MPa or less, preferably 1000 MPa or more and 1300 MPa or less, and most preferably 1053 MPa or more and 1190 MPa or less.

[0056] Additionally, the steel wire according to one example of the present invention may have no cracks at an angle of ±45° based on the fresh direction inside the L cross-section.

[0057] (Here, L-section means a parallel section in the rolling direction.)

[0058] In addition, the wire according to one example of the present invention includes, in weight %, carbon (C): 0.1% or more and 0.5% or less, silicon (Si): 0.01% or more and 0.6% or less, manganese (Mn): 0.6% or more and 1.6% or less, chromium (Cr): 0.01% or more and 1.4% or less, nitrogen (N): more than 0% and 0.02% or less, and includes at least one selected from the group consisting of aluminum (Al): 0.01% or more and 0.06% or less, niobium (Nb): 0.005% or more and 0.03% or less, titanium (Ti): 0.001% or more and 0.03% or less, or vanadium (V): 0.01% or more and 0.3% or less, and the remainder includes iron (Fe) and inevitable impurities, and the microstructure is composed of proeutectoid ferrite, residual pearlite, and bainite with an area fraction of 10% or less. And martensite is included, and the average grain size of the above-mentioned proeutectoid ferrite structure may be 5㎛ or less.

[0059] In addition, the wire according to an example of the present invention may have a tensile strength of 650 MPa or more and 850 MPa or less, preferably 700 MPa or more and 850 MPa or less, and most preferably 756 MPa or more and 830 MPa or less.

[0060] In addition, according to an example of the present invention, when a crack occurrence test is performed on a wire rod after drawing and extruding the wire rod with a processing amount of 30% or more and 70% or less, there may be no cracks at an angle of ±45° based on the drawing direction inside the L-section of the wire rod. In this way, by preventing the occurrence of chevron cracks at an angle of ±45°, it is possible to prevent drawing breakage or premature breakage during use of the final product, thereby reducing process costs and improving the service life of the final product.

[0061] (Here, L-section means a parallel section in the rolling direction.)

[0062] In addition, a method for manufacturing a steel wire according to an example of the present invention comprises the steps of preparing a billet including, in wt%, carbon (C): 0.1% or more and 0.5% or less, silicon (Si): 0.01% or more and 0.6% or less, manganese (Mn): 0.6% or more and 1.6% or less, chromium (Cr): 0.01% or more and 1.4% or less, nitrogen (N): more than 0% and 0.02% or less, and at least one selected from the group consisting of aluminum (Al): 0.01% or more and 0.06% or less, niobium (Nb): 0.005% or more and 0.03% or less, titanium (Ti): 0.001% or more and 0.03% or less, or vanadium (V): 0.01% or more and 0.3% or less, and the remainder including iron (Fe) and unavoidable impurities, and heating the billet at a temperature of 950°C or more and 1200°C. The method may include a step of heating to below, a step of finishing hot rolling the billet at a temperature of 700°C or more and 900°C or less, preferably 700°C or more and 800°C or less to manufacture a wire rod, a step of cooling the finished hot rolled wire rod at a cooling rate of 3°C or more and 15°C or less, preferably 4°C / s or more and 6°C / s or less, and a step of drawing and extruding the cooled wire rod at a processing amount of 30% or more and 70% or less.

[0063] In addition, the method for manufacturing a steel wire according to an example of the present invention may further include a step of performing a dehydrogenation treatment on the fresh and extruded wire at a temperature of 200°C or more and 600°C or less, preferably 300°C or more and 450°C or less, for 30 minutes or more and 60 minutes or less.

[0064] The rolling conditions of the present invention are preferably to perform finish rolling at a temperature range of 700°C to 800°C on a billet heated to 950°C to 1200°C. However, if the temperature is lower than 700°C, roll breakage and coiling may occur due to high rolling load, which may result in material cooling inhomogeneity caused by poor coiling shape. On the other hand, if the temperature exceeds 800°C, it may be difficult to secure precipitates stably, which may lead to a problem of difficulty in controlling the target microstructure due to grain coarsening.

[0065] Subsequently, in the present invention, the final hot-rolled wire rod can be cooled at a cooling rate of 3°C / s to 15°C / s to manufacture the final wire rod. If the cooling rate is less than 3°C / s, the target grain refinement may be difficult due to grain coarsening after the end of rolling, which may cause a problem in securing the final desired required properties. On the other hand, if the cooling rate exceeds 15°C / s, the untransformed austenite phase may be transformed into a low-temperature hard structure, which may cause material breakage or wire breakage during drawing, and the pearlite fraction may increase significantly, which may cause internal cracks.

[0066] The wire of the present invention manufactured by the manufacturing process described above may have a microstructure including an area fraction of 30% or more and 90% or less of proeutectoid ferrite, residual pearlite, and 10% or less of bainite and martensite, may have a proeutectoid ferrite structure with an average grain size of 5 ㎛ or less, and may have a tensile strength of 650 MPa or more and 850 MPa or less.

[0067] Afterwards, the tensile strength of the final product is secured through a drawing process. At this time, the appropriate drawing and extrusion processing amount is limited to 30% or more and 70% or less, so that the target tensile strength of 800 MPa or more and 1300 MPa or less can be secured depending on the processing amount. If the processing amount is less than 30%, the target stable tensile properties cannot be secured, and if it exceeds 70%, there is a concern that chevron cracks at an angle of ±45° from the drawing direction may occur inside the material, resulting in drawing fracture or premature breakage during use of the final product. Therefore, the range of drawing processing amount can be limited to the above range.

[0068] In order to secure the shape of the final product, cold forming is performed, and in order to improve the ductility and toughness reduced by the drawing process, secure the yield ratio, and make the elongation uniform, dehydrogenation treatment and stress relief treatment are performed at a temperature of about 200℃ or more and 600℃ or less for 30 minutes or more and 60 minutes or less, so that a steel wire having an average grain size of ferrite structure of 3㎛ or less, a tensile strength of the final product of 800MPa or more and 1300MPa or less, and a uniform elongation of 3.5% or more can be manufactured.

[0069] The present invention is described in detail through the following examples.

[0070] (Example)

[0071] A billet having the component composition shown in Table 1 below was used to manufacture a wire rod using the manufacturing conditions shown in Table 1, and the area fractions of bainite and martensite formed in the wire rod manufactured under these manufacturing conditions, the average grain size of the proeutectoid ferrite structure, and the tensile strength of the wire rod are shown in Table 1. In Table 1, Experimental Examples 1 to 5 satisfy the component range and manufacturing conditions of the present invention, and Comparative Examples 1 to 5 represent cases that deviate from the component range or manufacturing conditions of the present invention.

[0072] In addition, for each wire manufactured as described above, the occurrence of cracks at an angle of ±45° based on the drawing direction was measured inside the L-section of the wire after drawing and extrusion according to the manufacturing conditions of Table 2, and for the steel wire that was additionally dehydrogenated after drawing and extrusion, the tensile strength, uniform elongation, average grain size of the ferrite structure of the steel wire, and yield ratio were measured, and the results are shown in Table 2 below.

[0073] In Tables 1 and 2 below, the average grain size of the ferrite grain structure of the steel wire and the average grain size of the ferrite grain structure of the steel wire were evaluated using the ASTM E112 method, and were measured using a high-resolution SEM electron microscope during the evaluation.

[0074] In addition, the steel wire photographed by an optical microscope was observed to confirm the occurrence of internal cracks, and in the case of tensile strength and yield strength, the steel wire manufactured according to the manufacturing conditions in Tables 1 and 2 was made into specimens according to the standards of ASTM E8 and measured using a tensile tester to derive the yield ratio, and in the case of uniform elongation, the elongation values ​​from the yield strength to the tensile strength were derived from the tensile curve and are shown in Tables 1 and 2 below.

[0075] Classification CSiMnCrNAlNbTiVFinishingHot rolling temperature (℃)Cooling rate (℃ / s)Proeutectoid ferrite structure fraction (area%)Average size of proeutectoid ferrite structure (㎛)Wire rod tensile strength (MPa)Comparative example 10.180.210.75-0.0030.02---7751004.8499Comparative example 20.350.20.7-0.0030.025---770504.7629Comparative example 30.460.20.7-0.0030.03---820706.2690Comparative example 40.390.260.8-0.010.025--0.06850208.5642Comparative example Experimental Example 10.260.21.40.150.0030.015---9607236859 Experimental Example 20.440.650.950.150.0150.03---775404.1811 Experimental Example 30.440.21.30.150.0040.025--0.05750504.4830

[0076] Classification Fresh / Extrusion processing amount (%) Internal crack occurrence Dehydrogenation treatment temperature (℃) / time (hr) Tensile strength of steel wire (MPa) Uniform elongation of steel wire (%) Average grain size of ferrite structure of steel wire (㎛) Yield ratio (%) Comparative example 160X500 / 350 15.5 5.169 Comparative example 260X620 / 0.56 24 5.14 172 Comparative example 375O-108 42.4 1.276 Comparative example 475O500 / 0.59 39 6.5 4.185 Comparative example 530O-99 11.11 172 Experimental example 155X400 / 1.01 05 35.9 0.7 9 5.1 Experimental example 260X450 / 1.01 19 04.3 1.3 9 4.9 Experimental example 360X300 / 0.511444.01.195.6

[0077] As can be seen in Tables 1 and 2 above, among Comparative Examples 1 to 5 that did not satisfy the component contents or manufacturing conditions of the present invention, it was confirmed that the tensile strength of the wire rod and steel wire products was inferior in the case of Comparative Examples 1, 2, and 4. In addition, in the case of Comparative Examples 3 to 4, in which the average size of the proeutectoid ferrite grain structure and the microstructure grain size of the steel wire were coarse, internal cracks occurred in all of them during drawing, and in particular, in the case of Comparative Example 3, an inferior uniform elongation of 2.4% was confirmed due to this. On the other hand, in the case of Inventive Examples 1 to 3, the average size of the proeutectoid ferrite grains before drawing was secured as fine as 4.4㎛, and due to this, no chevron cracks were observed inside the steel wire products despite the high drawing amount, and it was confirmed that a high uniform elongation of 4.0% or more and an excellent yield ratio of 94.9% or more could be secured despite the high tensile strength.

[0078] Meanwhile, although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims described below.

Claims

1. Contains, in weight%, carbon (C): 0.1% or more but not more than 0.5%, silicon (Si): 0.01% or more but not more than 0.6%, manganese (Mn): 0.6% or more but not more than 1.6%, chromium (Cr): 0.01% or more but not more than 1.4%, nitrogen (N): more than 0% but not more than 0.02%, Contains at least one selected from the group consisting of aluminum (Al): 0.01% or more and 0.06% or less, niobium (Nb): 0.005% or more and 0.03% or less, titanium (Ti): 0.001% or more and 0.03% or less, or vanadium (V): 0.01% or more and 0.3% or less, and the remainder includes iron (Fe) and inevitable impurities. The microstructure contains ferrite and pearlite, The average grain size of the above ferrite is 3㎛ or less, The uniform elongation is 3.5% or more, Steel wire with a yield ratio of 90% or more.

2. In claim 1, Steel wire with a tensile strength of 800 MPa or more and 1300 MPa or less.

3. In claim 1, Steel wire without cracks at an angle of ±45° from the fresh direction inside the L-section. (Here, L section means a parallel section in the rolling direction.) 4. Contains, in weight%, carbon (C): 0.1% or more but not more than 0.5%, silicon (Si): 0.01% or more but not more than 0.6%, manganese (Mn): 0.6% or more but not more than 1.6%, chromium (Cr): 0.01% or more but not more than 1.4%, nitrogen (N): more than 0% but not more than 0.02%, Contains at least one selected from the group consisting of aluminum (Al): 0.01% or more and 0.06% or less, niobium (Nb): 0.005% or more and 0.03% or less, titanium (Ti): 0.001% or more and 0.03% or less, or vanadium (V): 0.01% or more and 0.3% or less, and the remainder includes iron (Fe) and inevitable impurities. The microstructure contains sintered ferrite, residual pearlite, and bainite and martensite with an area fraction of less than 10%. A wire having an average grain size of the above-mentioned cornerstone ferrite structure of 5㎛ or less.

5. In claim 4, Wire with a tensile strength of 650 MPa or more and 850 MPa or less.

6. In claim 4, A wire rod that has no cracks at an angle of ±45° based on the drawing direction inside the L-section of the wire rod that has been drawn and extruded with a processing amount of 30% or more and 70% or less of the above wire rod. (Here, L section means a parallel section in the rolling direction.) 7. Contains, by weight%, carbon (C): 0.1% or more but not more than 0.5%, silicon (Si): 0.01% or more but not more than 0.6%, manganese (Mn): 0.6% or more but not more than 1.6%, chromium (Cr): 0.01% or more but not more than 1.4%, nitrogen (N): more than 0% but not more than 0.02%, A step for preparing a billet including at least one selected from the group consisting of aluminum (Al): 0.01% or more and 0.06% or less, niobium (Nb): 0.005% or more and 0.03% or less, titanium (Ti): 0.001% or more and 0.03% or less, or vanadium (V): 0.01% or more and 0.3% or less, and the remainder including iron (Fe) and unavoidable impurities; A step of heating the above billet to a temperature of 950℃ or higher and 1200℃ or lower; A step of manufacturing a wire rod by performing a final hot rolling on the above billet at a temperature of 700℃ or higher and 900℃ or lower; A step of cooling the above-mentioned hot-rolled wire rod at a cooling rate of 3°C / s or more and 15°C / s or less; and A method for manufacturing a steel wire, comprising the step of drawing and extruding the cooled wire rod at a processing amount of 30% or more and 70% or less.

8. In claim 7, A method for manufacturing a steel wire further comprising the step of performing a dehydrogenation treatment on the fresh and extruded wire at a temperature of 200°C or higher and 600°C or lower for 30 minutes or longer and 60 minutes or shorter.

9. In claim 7, A method for manufacturing a steel wire, wherein the step of manufacturing a wire by performing the above finishing hot rolling is performed on the billet at a temperature of 700°C or higher and 800°C or lower.

10. In claim 7, A method for manufacturing a steel wire, wherein the cooling step is performed at a cooling rate of 4°C / s or more and 5°C / s or less on the finished hot-rolled wire.

11. In claim 7, A method for manufacturing a steel wire further comprising the step of performing a dehydrogenation treatment on the fresh and extruded wire at a temperature of 300°C or higher and 450°C or lower for 30 minutes or longer and 60 minutes or shorter.