Ultrahigh-strength wire rod, steel wire and manufacturing methods therefor

Abstract

A wire rod according to one embodiment of the present invention comprises, by wt%, 0.60-1.10% of C, 0.40-1.0% of Si, 0.20-0.60% of Mn, 0.006% or less (excluding 0) of N, and the balance of Fe and other inevitable impurities, wherein the microstructure comprises, by area fraction, 95.0% or more of pearlite and the balance of a non-pearlite structure, and the length of the longest axis of the non-pearlite structure can be 30 μm or less. (The non-pearlite structure means a structure comprising at least one of proeutectoid ferrite, bainite and proeutectoid cementite.)

Description

Ultra-high strength wire rod, steel wire and method of manufacturing the same

[0001] The present invention relates to a high-strength wire rod, a steel wire, and a method for manufacturing the same, which are used in steel cords used as tire reinforcing materials.

[0002] Wire rods can be used to manufacture steel cords used as tire reinforcements, and the steel cords use high-carbon steel containing carbon in the range of 0.6 to 1.0% by weight. Conventional steel contains manganese and silicon in addition to carbon, and in some cases, may contain a small amount of chromium.

[0003] Conventionally, to manufacture steel cord, the wire rod underwent primary drawing, primary isothermal transformation heat treatment, secondary drawing, and secondary isothermal transformation heat treatment. Subsequently, it was brass-plated and wet-drawn. Although the primary isothermal transformation heat treatment may be omitted, the process up to the secondary isothermal transformation is merely a process of reducing the wire diameter to manufacture the final product. Whether the primary isothermal transformation heat treatment is omitted or not, the mechanical properties of the final product are determined by the secondary isothermal transformation heat treatment and the subsequent wet-drawn process. This is because the secondary isothermal transformation heat treatment includes an austenitizing process carried out at around 1000°C. At this temperature, as the austenitizing heat treatment is involved, the effects of the previous drawing and heat treatments applied to the wire rod disappear, a new microstructure is formed, and the mechanical properties of the final product are determined by the subsequent wet-drawn process.

[0004] Therefore, in order to increase the final strength of conventional steel cords, the carbon content was increased to raise the cementite fraction in the microstructure formed during secondary isothermal transformation heat treatment, while the lamellar spacing of pearlite was finely controlled to increase the work hardening rate and tensile strength. Currently commercially available tire cords are products with a tensile strength of 4,000 to 4,300 MPa and contain 0.90 to 0.94% carbon by weight. While it is practically possible to pursue further high strength by increasing the carbon content, increasing the carbon content may lead to a decrease in core ductility due to segregation occurring during steel casting. Furthermore, due to the high carbon content, it may be difficult to secure a sufficient cooling rate to prevent the formation of proeutectoid cementite during cooling after wire rod rolling.

[0005] One aspect of the present disclosure provides a wire rod, a steel wire, and a method for manufacturing the same, having ultra-high strength characteristics by making the lamella spacing very thin.

[0006] One aspect of the present disclosure provides a wire rod, a steel wire, and a method for manufacturing the same, which have ultra-high strength characteristics using a commercially available level of carbon content without increasing the carbon content of the alloy.

[0007] One aspect of the present disclosure provides a wire rod, a steel wire, and a method for manufacturing the same, wherein the lamellar spacing is reduced in an equivalent proportion to the cross-sectional area of ​​the steel wire by drawing without regenerating the microstructure from the wire rod to the final product, and the wire rod has ultra-high strength characteristics.

[0008] One aspect of the present disclosure provides a wire rod, a steel wire, and a method for manufacturing the same, wherein the amount of drawing applied to the final product is 99% or more by continuously increasing the amount of drawing through softening heat treatment while maintaining the pearlite formed in the wire rod state.

[0009] The technical problems to be solved in this document are not limited to those mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art to which this invention belongs from the description below.

[0010] A wire rod according to one embodiment of the present disclosure may comprise, in weight percent, C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), and the remainder being Fe and other unavoidable impurities. The microstructure of the wire rod comprises 95% or more of pearlite and the remainder being non-pearlite in terms of area fraction, and the length of the longest axis of the non-pearlite structure may be 30 μm or less.

[0011] Here, a non-perlite texture refers to a texture containing at least one of proeutectoid ferrite, bainite, or proeutectoid cementite.

[0012] A method for manufacturing a wire rod according to one embodiment of the present disclosure may include a step of casting a billet. The billet may contain, in weight percent, C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), and the remainder being Fe and other unavoidable impurities. The method for manufacturing a wire rod may include a step of heating the billet at 1000 to 1250°C for 60 to 120 minutes. The method for manufacturing a wire rod may include a step of rolling the heated billet. The method for manufacturing a wire rod may include a step of coiling. The method for manufacturing a wire rod may include a step of cooling. The wire rod produced by the above method of manufacturing the wire rod may have a microstructure comprising 95.0% or more of pearlite and the remainder of non-pearlite structure in terms of area fraction, and the longest axis of the non-pearlite structure may be 30㎛ or less.

[0013] Here, a non-perlite texture refers to a texture containing at least one of proeutectoid ferrite, bainite, or proeutectoid cementite.

[0014] A steel wire according to one embodiment of the present disclosure may comprise, in weight percent, C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), and the remainder being Fe and other unavoidable impurities, and may have an average lamellar spacing of 12.5 nm or less.

[0015] A method for manufacturing a steel wire according to one embodiment of the present disclosure may include a step of casting a billet. The billet may contain, in weight percent, C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), and the remainder being Fe and other unavoidable impurities. The method for manufacturing a steel wire may include a step of heating the billet. The method for manufacturing a steel wire may include a step of continuously drawing the manufactured wire rod at a reduction rate of 10 to 20% per pass. The method for manufacturing a steel wire may include a step of performing a softening heat treatment, and the drawing and softening heat treatment may be repeated multiple times.

[0016] According to the present disclosure, it is possible to provide wire rods, steel wires, and methods for manufacturing them that are quality-stable and economical by securing product-level strength without increasing the carbon content.

[0017] According to the present disclosure, a wire rod, a steel wire having ultra-high strength characteristics by making the lamella spacing very thin can be provided, and a method for manufacturing the same.

[0018] The effects according to the present disclosure are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art from the description below.

[0019] Figure 1 shows a conceptual diagram of a conventional steel wire manufacturing process.

[0020] Figure 2 shows a conceptual diagram of a manufacturing process for ultra-high strength steel wire using softening heat treatment according to one embodiment of the present invention.

[0021] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. In this specification, the same reference numerals denote the same elements throughout. Furthermore, various elements and areas in the drawings are depicted schematically. Accordingly, the technical concept of the present invention is not limited by the relative sizes or spacing depicted in the attached drawings. The terms used in this specification are intended to describe the present invention and are not intended to limit the present invention. Additionally, singular forms used in this specification include plural forms unless the relevant definitions clearly indicate otherwise.

[0022] Unless otherwise noted, units are weight percent. Furthermore, when a part is described as "containing" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0023] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms defined in advance are interpreted to have meanings consistent with relevant technical literature and the presently disclosed content.

[0024] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0025] Seonjae

[0026] A wire rod according to one embodiment of the present invention may contain C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), and the remainder may be Fe and other unavoidable impurities. Since cementite may decompose during softening heat treatment when such harsh drawing processing is applied to it, carbon can be bound to the cementite by controlling alloying elements such as Si and Cr.

[0027] The reasons for limiting the above alloy composition are explained in detail below. Unless otherwise specified, all compositions below represent weight percent.

[0028] C (Carbon): 0.60 to 1.10%

[0029] C is a basic alloying element for forming a pearlite structure, and the cementite fraction and lamellar spacing change depending on the C content. If the carbon content is too low, it is difficult to prevent the excessive formation of proeutectoid ferrite, a non-pearlite structure, during wire rod manufacturing. Therefore, the lower limit is preferably 0.60%. More preferably, it is 0.62%. Conversely, if the carbon content is excessive, considering segregation during casting, it is impossible to control the formation of proeutectoid cementite in the segregated areas. Therefore, the upper limit is preferably 1.10%. More preferably, it is 1.08%.

[0030] Si (Silicon): 0.40 to 1.0%

[0031] During pearlite transformation, Si is distributed to ferrite rather than cementite, and exists in high concentrations, particularly near the ferrite / cementite boundary. Consequently, during softening heat treatment following wire drawing, cementite decomposes, making it difficult for carbon to migrate from cementite to ferrite. Therefore, it is an essential element for maintaining a stable cementite shape even during softening heat treatment. If the Si content is too low, it is difficult to sufficiently secure the effect of inhibiting cementite decomposition, so a lower limit of 0.40% is desirable. More preferably, it is 0.41%. On the other hand, if the content is excessive, the ferrite undergoes excessive solid solution strengthening, resulting in insufficient ductility and exhibiting a counterproductive effect of lowering the wire drawing limit. Therefore, an upper limit of 1.0% is desirable. More preferably, it is 0.91%.

[0032] Mn (Manganese): 0.20 to 0.60%

[0033] Although Mn does not affect the change in tensile strength of pearlite during isothermal transformation, in the case of continuous cooling, it can affect the change in tensile strength because it alters the hardenability of the steel under limited cooling capacity, thereby changing the transformation initiation temperature. If the Mn content is too low, it is difficult to expect an appropriate level of improvement in hardenability. Therefore, the lower limit is preferably 0.20%. More preferably, it is 0.23%. On the other hand, if the content is excessive, it becomes centrally segregated along with C, increasing the risk of martensite defects occurring in the center. Therefore, the upper limit is preferably 0.60%. More preferably, it is 0.57%.

[0034] N (Nitrogen): 0.006% or less (excluding 0)

[0035] N, along with C, acts as an interstitial element in steel and exhibits excellent solid solution strengthening effects; however, it can bind to dislocations during drawing processing, causing dynamic strain aging that reduces the ductility of the steel. Therefore, a maximum content of 0.006% is desirable, and a lower content is better. However, since excessively lowering the N content in steel may require significant cost and time in the refining process, it is generally desirable to add at a level of 0.004% or higher.

[0036] A wire rod according to one embodiment of the present invention may further include one or more of Cr: 0.4% or less and B: 0.003% or less in weight%, and when B is included, Ti: 0.01 to 0.02% may be included together to maintain B in a solid solution state.

[0037] Cr (Chromium): 0.4% or less

[0038] Cr is an element that separates the temperature ranges for pearlite and bainite formation in the TTT curve during isothermal transformation, enabling isothermal transformation at lower temperatures and consequently providing a high-strength effect for the final product. As the carbon content increases, Cr can be added in small amounts to replace Mn, which poses a risk of central segregation, or Si, a ferrite-hardening element, in order to obtain appropriate hardenability. Although Cr is not an essential component of the present invention, its addition in small amounts can contribute to high strength. Furthermore, by distributing it within the cementite and strongly binding carbon, it helps suppress the degree of cementite decomposition during softening heat treatment. If the Cr content is excessive, it excessively slows down cementite growth, causing a tendency for the cementite within the pearlite to fragment, which may reduce cold-drawability. Therefore, the upper limit is preferably 0.4%. More preferably, it is 0.39%.

[0039] B (Boron): 0.003% or less

[0040] B is segregated at austenite grain boundaries and slows down ferrite nucleation, thereby reducing the occurrence of proeutectoid ferrite during continuous cooling. This effect is particularly effective in the hypoeutectoid carbon range where the carbon content is lower than the eutectoid composition, and it also enhances cold workability by suppressing the formation of abnormal ferrite in the hypereutectoid region. The effect of B is sufficiently obtained even at 0.003% or less, and since the ferrite generation suppression effect does not increase even if added above this amount, the maximum addition amount is preferably 0.003%.

[0041] Ti (Titanium): 0.01 to 0.02%

[0042] Ti is an element added because, when B is added, B must exist in a solid solution state within the austenite to have a ferrite-inhibiting effect. When B is added to steel, it forms BN and precipitates because it has a strong tendency to bond with nitrogen. If B precipitates without being dissolved, the ferrite-inhibiting effect disappears; therefore, to prevent B from bonding with N, Ti, which has a stronger affinity for N than B, is utilized to form TiN, thereby lowering the dissolved N and allowing B to exist in a solid solution state. Although this may vary depending on the N content in the steel, in the case of N of 0.006% or less according to one embodiment of the present invention, sufficient removal of dissolved N is possible with a Ti content of 0.01 to 0.02%. If the Ti content is too low, the effect of N removal is insufficient, so the lower limit is preferably 0.01%. On the other hand, if the Ti content is excessive, Ti(C, N), etc., precipitates coarsely, reducing cold-drawability; therefore, the maximum content is preferably 0.02%.

[0043] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As such impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0044] The microstructure of a wire rod according to one embodiment of the present invention comprises 95.0% or more of pearlite and the remainder of non-pearlite structure by area fraction, and the length of the longest axis of the non-pearlite structure may be 30 μm or less. Here, the non-pearlite structure refers to a structure comprising at least one of proeutectoid ferrite, bainite, or proeutectoid cementite.

[0045] This is because if coarse ferrite or bainite massivees, or networked proeutectoid cementite are present, stress concentration may occur during wire drawing, leading to cracks. In the case of a finely dispersed non-pearlite structure, problems such as wire breakage do not occur during wire drawing. Therefore, the longest axis length of the non-pearlite structure may be 30 μm or less. Preferably, it may be 23 μm or less, and more preferably 13 μm or less.

[0046] A wire according to one embodiment of the present invention may have an average lamellar spacing of 100 to 250 nm.

[0047] Pearlite microstructure has a lamellar structure in which ferrite and cementite are alternately arranged in thin layers. The lamellar spacing refers to the average distance or thickness between these layered structures. That is, the lamellar spacing can be defined as "(lamellar cementite thickness + lamellar ferrite thickness)" and can be specifically measured by the following method. After polishing the C-section of a steel wire or wire rod, the pearlite is exposed by etching. Subsequently, a micrograph of the sample's microstructure is obtained by taking 10 field-of-view images using a scanning electron microscope (SEM) at a point 1 / 2 of the way from the center of the C-section toward the surface. At this time, the magnification is set to 1,000x. Within the micrograph of the sample, five points are selected where the 5-interval of the lamellar spacing can be measured within the range where the lamellae orientation is aligned. For the five selected points, a straight line is drawn perpendicular to the lamellar, and the length of the 5-interval of the lamellar spacing is calculated. The two points with the smallest 5-interval lamella lengths measured at five selected points are chosen, and the lamella interval is calculated by dividing the measured 5-interval length by 5. In this way, two lamella intervals are obtained from one tissue image, and 20 lamella intervals are obtained from a total of 10 images, after which the average lamella interval can be measured by calculating the arithmetic mean.

[0048] If the lamellar spacing exceeds 250 nm and becomes coarse, the amount of work hardening during wire drawing decreases, resulting in a decrease in the tensile strength that can ultimately be reached. In addition, the number of dislocations piled up at the ferrite / cementite interface becomes excessive, leading to increased pressure and making fracture more likely. Therefore, since the wire drawing limit actually decreases, it is desirable to control it within 250 μm.

[0049] Conversely, if the average lamellar spacing becomes too small, the strength of the pearlite increases and the amount of work hardening increases, resulting in excessive load being applied during wire drawing, which also reduces the wire drawing limit. Therefore, a lamellar spacing of 100 nm or more is desirable.

[0050] According to one embodiment of the present invention, the reduction rate of the lamellar spacing of the wire may be 95.0% or higher. When the wire is drawn at a constant diameter reduction rate by mechanical thinning, the lamellar spacing may be reduced at the same rate. For example, when drawn at a diameter reduction rate of 95.0%, the reduction rate of the lamellar spacing of the wire may be 95.0%.

[0051] In the present invention, mechanical thinning refers to a process of physically reducing the thickness of a material and can be used in the material manufacturing process to obtain a specific thickness or microstructure. In the present invention, mechanical thinning refers to the effect that when the cross-sectional area is reduced during wire drawing, all physical lengths existing on the cross-sectional area are reduced in the same proportion, and thus the lamellar spacing is also reduced in the same proportion.

[0052] According to one embodiment of the present invention, since a high drawing rate of 95.0% or more in diameter reduction rate can be secured through softening heat treatment, the reduction rate of the lamellar spacing of the wire rod may be 95.0% or more. Specifically, it may be 96.0% or more.

[0053] When drawing the above wire rod, the wire diameter reduction rate / lamellar spacing reduction rate may be 0.9 to 1.1. Preferably, the wire diameter reduction rate may be equal to the lamellar spacing reduction rate. That is, the wire diameter reduction rate / lamellar spacing reduction rate may be 1. Here, the wire diameter reduction rate refers to the reduction rate of the wire diameter of the steel wire after drawing relative to the wire diameter of the wire rod before drawing. The lamellar spacing reduction rate refers to the reduction rate of the lamellar spacing of the steel wire after drawing relative to the lamellar spacing of the wire rod before drawing.

[0054] A wire according to one embodiment of the present invention may have a diameter of 4.0 to 6.0 mm.

[0055] A wire according to one embodiment of the present invention may have a cross-sectional reduction rate of 98.85% or more. Preferably, it may be 99.00% or more.

[0056] In manufacturing a wire rod according to one embodiment, a softening heat treatment performed for several seconds during drawing can resolve dislocations within the ferrite generated during the drawing process at regular intervals. This enables additional drawing processing, and by controlling the alloy composition such as Si and Cr described above, the bonding strength within the cementite can be increased and the decomposition of carbon atoms from the cementite into ferrite can be suppressed. That is, the wire rod according to one embodiment can have a cross-sectional reduction rate of 98.85% or more through the control of the alloy composition and the softening heat treatment during the manufacturing process. Preferably, it can have a cross-sectional reduction rate of 99.87% or more.

[0057] A wire according to one embodiment of the present invention may not undergo delamination during a torsion test in a total strain range greater than 0 and less than 3.5.

[0058] According to one embodiment of the present invention, a wire rod can be manufactured to produce a steel wire with a final diameter of 0.15 to 0.35 mm starting from an initial diameter of 4.0 to 6.0 mm. Delamination refers to a phenomenon in which cracks occur in a direction parallel to the longitudinal direction of the steel wire during torsional deformation of a drawn steel wire. Delamination can occur due to mechanical stress, thermal stress, defects, etc., and the occurrence of delamination implies that the drawn steel wire has not secured sufficient ductility, so it cannot be used as a final product. According to one embodiment of the present invention, when drawing from 5.5 mm to 0.2 mm, no fracture occurs during the drawing process at a total strain of approximately 3.31 (True Strain), and delamination may not occur during a torsional test after drawing. Furthermore, even if delamination does not occur, the fracture surface of the steel wire or wire rod that fractures during the torsional test must fracture perpendicular to the longitudinal direction of the steel wire to ensure there are no quality issues.

[0059] steel wire

[0060] A steel wire according to one embodiment of the present invention may comprise, in weight percent, C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), and the remainder being Fe and other unavoidable impurities.

[0061] The reason for limiting the alloy composition above may be the same as the reason for limiting the alloy composition of the wire rod described above. In addition, the steel wire according to one embodiment of the present invention may further include one or more of Cr: 0.4% or less, B: 0.003% or less, and Ti: 0.01 to 0.02% in weight%.

[0062] According to one embodiment of the present invention, the lamellar spacing of the steel wire may be 12.5 nm or less.

[0063] Hereinafter, the lamellar spacing of a steel wire according to an embodiment of the present invention will be explained using FIGS. 1 and FIGS. 2.

[0064] According to FIG. 1, a conventional method for manufacturing steel wire includes primary drawing, primary isothermal transformation heat treatment, secondary drawing, secondary isothermal transformation heat treatment, followed by brass plating and wet drawing. Alternatively, it may include primary drawing, isothermal transformation heat treatment, gold plating, followed by wet drawing. Since the above isothermal transformation heat treatment involves austenitizing at 1000°C or higher, the mechanical thinning effect resulting from the drawing process preceding the heat treatment is eliminated.

[0065] FIG. 2 shows a conceptual diagram of a method for manufacturing steel wire according to an embodiment of the present invention. The method for manufacturing steel wire includes primary drawing, softening heat treatment, secondary drawing, softening heat treatment, tertiary drawing, softening heat treatment, brass plating, and wet drawing. However, this is merely an embodiment, and drawing and softening heat treatment can be repeated multiple times as a cycle, or steel wire can be manufactured through multiple softening heat treatments during drawing. That is, as long as work hardening can be released without creating a new structure, the drawing and softening heat treatments can be performed in any form.

[0066] For example, if the average lamellar spacing is made to be approximately 200 nm in the state of a wire rod with a diameter of 5.5 mm, the lamellar spacing decreases in the same proportion as the wire diameter decreases during the first drawing process. If the wire diameter is reduced from 5.5 mm to 1.3 mm, the lamellar spacing of the 1.3 mm wire becomes approximately 47 nm. At this point, unlike conventional isothermal transformation heat treatment, if heat treatment is performed at an appropriate time between 400 and 600°C to resolve only the dislocations formed in the ferrite without undergoing austenitizing, additional drawing processing becomes possible without any change in the lamellar spacing. If wet drawing is performed down to a final wire diameter of 0.2 mm while the lamellar spacing remains unchanged, the lamellar spacing can be reduced to 7.2 nm. Compared to the lamellar spacing of 23 nm in the conventional final microstructure, the lamellar spacing can be reduced by 319% to 7.2 nm.

[0067] According to one embodiment of the present invention, when a wire with a lamellar spacing of 100 nm is drawn at a diameter reduction rate of 95.0%, a steel wire with a lamellar spacing of 5.0 nm can be manufactured because the lamellar spacing is reduced at the same rate. When a wire with a lamellar spacing of 250 nm is drawn at a diameter reduction rate of 95.0%, a steel wire with a lamellar spacing of 12.5 nm can be manufactured because the lamellar spacing is reduced at the same rate.

[0068] That is, due to the above high fresh processing amount, the lamellar spacing of the steel wire can be 12.5 nm or less. Preferably, it can be 7.2 nm or less.

[0069] A steel wire according to one embodiment of the present invention may have a microstructure comprising 95.0% or more of pearlite and the remainder of non-pearlite in terms of area fraction.

[0070] Here, a non-perlite texture refers to a texture containing at least one of proeutectoid ferrite, bainite, or proeutectoid cementite.

[0071] A steel wire according to one embodiment of the present invention may have a tensile strength of 3,000 to 4,700 MPa. Commercially available tire cords are products with a tensile strength of 4,000 to 4,300 MPa and contain 0.90 to 0.94% carbon by weight. While it is possible to pursue further high strength by substantially increasing the carbon content, the present invention can secure the tensile strength of commercially available tire cords while maintaining the carbon content of commercially available alloys, thereby providing a steel wire through a manufacturing method that is quality-stable and economical. There are two methods to increase the tensile strength of wire drawing: increasing the carbon content to increase the cementite fraction within the pearlite, or increasing the amount of wire drawing to increase the amount of work hardening. As previously mentioned, when increasing the carbon content, the ductility of the core structure may be severely reduced due to segregation during casting, and furthermore, if the cooling rate is not sufficiently secured during wire rod manufacturing, it is difficult to prevent the formation of proeutectoid cementite. Currently, the carbon content of the highest carbon wire rods for tire cords is at the level of 0.90–0.92%, and under conventional technology, it is difficult to stably produce wire rods and steel wires with a higher carbon content than this in terms of quality. A second method to increase tensile strength is to increase the amount of drawing to increase work hardening. When drawing, tensile strength increases due to the proliferation of dislocations caused by plastic deformation and the refinement of lamellar spacing through the aforementioned mechanical thinning. However, since the amount of plastic deformation that can be imparted to the material without cracking is limited, the applied plastic deformation must be relieved after a certain amount of deformation to enable further plastic deformation. Therefore, conventionally, the tensile strength of the final product has been increased by appropriately combining an increase in carbon content and an increase in the amount of drawing. However, according to the present invention, since significantly more drawing can be applied compared to the conventional steel wire manufacturing process, tensile strength can be sufficiently increased without increasing the carbon content, and if the carbon content is increased together, a higher tensile strength than that of conventional products can be secured.

[0072] Tensile strength can be measured by mounting it on a standard tensile testing device.

[0073] A steel wire according to one embodiment of the present invention may have a diameter of 0.1 to 0.5 mm. For example, a steel wire with a diameter of 0.2 mm can be manufactured through the process described above.

[0074] Method for manufacturing wire rods and steel wires

[0075] A method for manufacturing a wire rod according to one embodiment of the present invention may include the steps of: casting a billet comprising, in weight percent, C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), and the remainder being Fe and other unavoidable impurities; heating the billet at 1000 to 1250°C for 60 to 120 minutes; rolling the heated billet; coiling; and cooling.

[0076] The step of casting the billet may include manufacturing the billet through hot rolling or hot forging after casting into a bloom. For example, it may be cast into a bloom or a billet for wire rod rolling, and if cast into a bloom, a billet with a cross-sectional area of ​​160x160 mm² and a length of 6,000 to 12,000 mm may be manufactured through hot rolling or hot forging.

[0077] The step of heating the billet above can be performed at a temperature of 1000°C to 1250°C for 60 to 120 minutes to ensure a uniform temperature distribution over the entire billet and sufficiently reduce deformation resistance during the rolling stage. If the temperature is below 1000°C, it takes longer to ensure a uniform temperature distribution of the billet, which reduces productivity. On the other hand, if the temperature exceeds 1250°C, the time required to ensure temperature uniformity of the billet is reduced, but due to the excessively high temperature, a thick scale forms, resulting in severe loss and a rapid increase in energy consumption, which leads to problems of increased productivity and manufacturing costs.

[0078] In the step of rolling the heated billet, it is preferable that the rolling temperature be 1000 to 1100°C. If the temperature is below 1000°C, defects may occur on the surface of the wire rod due to the rolling load, making it difficult to apply subsequent drawing processing. On the other hand, if the temperature exceeds 1100°C, the size of the austenite grains increases, which may lead to a decrease in strength and ductility. After hot rolling, it is preferable to cool the wire rod to a temperature range suitable for the subsequent coiling process using a conventional water cooling method. According to one embodiment, the step of rolling the wire rod to a wire diameter of 4 to 6 mm may be included.

[0079] The above-mentioned winding step can be performed at 780 to 930°C. Scale removal for the production of eco-friendly products is achieved through mechanical peeling. The appropriate scale thickness varies depending on the mechanical peeling method, but is on average around 10 µm; if the scale thickness is less than this, mechanical peeling is insufficient and the scale remains on the surface of the wire, causing die breakage during drawing processing and forming defects such as surface flaws. If the scale thickness is too thick, productivity decreases because scale loss increases. The scale thickness varies depending on the winding temperature; if the winding temperature is below 780°C, the scale thickness is small, and if it exceeds 930°C, the scale thickness may become excessively thick.

[0080] The above cooling step may include performing air cooling in Stelmor Cooling.

[0081] The wire rod produced by the above manufacturing method has a microstructure comprising at least 95% pearlite by area fraction and the remainder being proeutectoid ferrite, bainite, and proeutectoid cementite.

[0082] The longest axis length of the above remaining proeutectoid ferrite, bainite, and proeutectoid cementite structures may be 30 μm or less.

[0083] A method for manufacturing a steel wire according to one embodiment of the present invention comprises, in weight percent, the step of casting a billet containing C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), and the remainder being Fe and other unavoidable impurities; the step of heating the billet; the step of rolling and cooling the billet into a wire rod; the step of drawing the wire rod thus produced at a reduction rate of 10 to 20%; and the step of performing a softening heat treatment, wherein the drawing and the softening heat treatment can be repeated multiple times.

[0084] The steps of casting the billet, heating, rolling the wire rod, and cooling may be the same as the method for manufacturing the wire rod described above.

[0085] The step of drawing the above wire rod can be carried out with a reduction rate of 10 to 20%. The drawing process can be carried out continuously or in several stages.

[0086] The softening heat treatment step described above is a heat treatment performed to eliminate dislocations generated within the ferrite during drawing without altering the microstructure of the drawn pearlite. Even if dislocations are eliminated through softening heat treatment, the amount of additional drawing that can be applied is not large compared to pearlite that has been completely regenerated through isothermal transformation; therefore, it must be performed within 90% of the drawing amount, and further reduced depending on the initial wire diameter and tensile strength of the wire rod. For example, the softening heat treatment can be performed within a range where the cumulative reduction rate of the area of ​​the C cross-section does not exceed 85%. Unlike conventional methods that created a new microstructure through heat treatment, the softening heat treatment described above can be performed to eliminate work hardening without creating a new microstructure. According to one embodiment of the present invention, the drawing-softening heat treatment process can be repeated multiple times. That is, by eliminating work hardening through softening heat treatment, repeated drawing can be performed without fracture occurring during drawing.

[0087] Referring to FIG. 2, to aid in understanding the new process of the present invention, the softening heat treatment is depicted as being applied once; however, in reality, a large amount of subsequent drawing cannot be applied solely through the softening heat treatment. Therefore, the intermediate softening heat treatment can be performed about 3 to 4 times. Since there is no change in the lamellar spacing due to heat treatment even when the softening heat treatment process is applied multiple times, when manufacturing a final product with a wire diameter of 0.2 mm from a wire rod diameter of 5.5 mm, a reduction rate of 99.7% or more can be applied, and accordingly, a very thin lamellar spacing that is impossible to produce in a conventional process involving isothermal transformation heat treatment can be created. Accordingly, the tensile strength of the final product can be significantly increased by the amount of the reduced lamellar spacing.

[0088] If the softening heat treatment temperature is below 400℃, it takes a long time to eliminate dislocations or the dislocation elimination effect is insufficient, and if it exceeds 600℃, it is difficult to control the decomposition of cementite. In addition, the holding time must be controlled to be long when the heat treatment temperature is low and short when the temperature is high, but this may vary depending on the alloying components that inhibit the decomposition of cementite. If the holding time is less than 10 seconds, dislocation elimination is insufficient, and if it exceeds 30 seconds, it is difficult to control the decomposition of cementite. Therefore, it is preferable that the softening heat treatment be maintained for 10 to 30 seconds at a heating rate of 80℃ / sec or more up to 400 to 600℃.

[0089] The softening heat treatment in the present invention is performed instead of the conventional isothermal transformation heat treatment, and the effect of the softening heat treatment is as described above. That is, through the softening heat treatment, the steel wire produced according to the method for producing steel wire according to one embodiment of the present invention can have a lamellar spacing of 12.5 nm or less.

[0090] After the above softening heat treatment, a water-cooling and heat treatment process can be performed.

[0091] The present invention will be explained in more detail below through examples. However, the description of these examples is merely for illustrating the implementation of the present invention and does not limit the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0092] {Example}

[0093] Steel having the composition shown in Table 1 was steeled in a converter, then cast into a bloom, and subsequently rolled into a steel billet or directly cast into a continuous casting billet to produce a billet with a cross-sectional area of ​​160 x 160 mm². Afterward, it was maintained at a temperature of approximately 1100°C in a heating furnace for 90 minutes and rolled to a wire diameter of 5.5 mm through wire rod rolling under normal conditions. The coiling temperature was varied from 780 to 930°C, and then a wire rod according to one embodiment of the present invention was manufactured through Stelmor cooling.

[0094] Table 1 below shows the alloy composition of the inventive material and the comparative material.

[0095] Classification Alloy Composition (wt.%) CSI Mn NC r BTI Inventive Material 10.6 20.4 10.2 30.00 45 00.00 28 0.015 Inventive Material 20.6 70.4 20.2 50.00 43 00.00 21 0.014 Inventive Material 30.7 10.5 20.2 50.00 51 00.00 27 0.013 Inventive Material 40.7 80.8 90.2 50.00 570 00Inventive Material 50.810.430.310.0041000Inventive Material 60.860.910.320.00440.1600Inventive Material 70.920.410.360.00440.1800Inventive Material 80.950.910.380.00520.2700Inventive Material 91.020.520.530.00390.3200Inventive Material 101.080 .870.570.00390.3900Textbook10.520.450.440.0056000Textbook21.210.470.230.0053000Textbook30.610.250.300.0043000Textbook40.721.120.280.0049000Textbook50.860.410.120.0050000Textbook6 0.810.520.650.0050000Textbook 70.860.410.350.00500.4300Textbook 80.930.450.350.00480.210.00200.006Textbook 90.910.510.450.005200.00180.028Textbook 100.860.550.430.01080.3100

[0096] Table 2 below shows the microstructures of the inventive material and the comparative material in the wire state. The microstructures were observed by examining the C cross-section with an optical microscope at 200x magnification. The length of the longest axis (major axis) of the non-pearlite structure was measured using an Image Analyzer by drawing a straight line through the respective region in each independently existing ferrite, cementite, or bainite region.

[0097] Classification C. Phase fraction (Area %) observed in cross-sectional optical microscope (x200) Non-Pearlite Structure Long axis length (㎛) Pearlite Proeutectoid Ferrite Bainite Proeutectoid Cementite Inventive Material 195.64.40023 Inventive Material 296.23.80022 Inventive Material 397.82.100.113 Inventive Material 498.91.000.18 Inventive Material 598.21.700.18 Inventive Material 699.30.600.17 Inventive Material 799.30.50.10.17 Inventive Material 899.100.80.111 Inventive Material 999.300.60.114 Inventive Material 1098.900.90.24 Comparative Material 189 .110.90041 Study Material 293.404.22.433 Study Material 394.64.31.1035 Study Material 494.52.53.0031 Study Material 594.91.03.01.133 Study Material 694.42.42.01.235 Study Material 794.30.63.12.041 Study Material 894.20.40.35.137 Study Material 993.30.10.56.145 Study Material 1093.00.34.72.041

[0098] Upon observation, structures other than pearlite were observed as proeutectoid ferrite, bainite, and proeutectoid cementite. In the case of non-pearlite structures, the length of the major axis in a single observed field of view did not exceed 30 μm. After descaling and lubrication, wire drawing was performed on the inventive material and comparative materials. During wire drawing, the reduction rate per pass was set between 10 and 20%, and the same pass schedule was applied to all steel grades. Softening heat treatment was performed by drawing from a wire diameter of 5.5 mm to reduction rates where the wire diameters became 2.5 mm, 1.2 mm, and 0.5 mm; the specimens were then held at 500°C for 20 seconds using induction heating and water-cooled. Specimens were taken starting from the wire diameter at which the total cross-sectional area reduction rate was 95%, and torsion tests were performed. Insufficient drawing workability was determined if a wire breakage occurred during drawing, the specimen broke, or the fracture surface of the torsion test was not a right-angle fracture. Table 3 below evaluates the drawing capabilities of the inventive material and the comparative material and indicates the applicable reduction in cross-section.

[0099] Classification Whether wire breakage / fracture occurs during fresh wire (○, X) Twisting fracture shape Whether delamination occurs during twisting (○, X) Cross-sectional reduction rate (%) Right angle of cutting Inventive material 1XX○X99.87 Inventive material 2XX○X99.87 Inventive material 3XX○X99.87 Inventive material 4XX○X99.87 Inventive material 5XX○X99.87 Inventive material 6XX○X99.87 Inventive material 7XX○X99.87 Inventive material 8XX○X99.87 Inventive material 9XX○X99.87 Inventive material 10XX○X99. 87 Comparison Material 1 X ○ X ○ 96.69 Comparison Material 2 ○ Not Performed Not Performed Not Performed 90.45 Comparison Material 3 X ○ X ○ 98.38 Comparison Material 4 X ○ X ○ 97.88 Comparison Material 5 X ○ XX 98.81 Comparison Material 6 X ○ XX 98.81 Comparison Material 7 X ○ XX 98.81 Comparison Material 8 X ○ X ○ 98.38 Comparison Material 9 X ○ X ○ 97.88 Comparison Material 10 X ○ X ○ 97.88

[0100] In the case of the inventive materials, all were capable of being drawn without any problems from a wire diameter of 5.5 mm to 0.2 mm, and normal fracture occurred without delamination during the torsional evaluation after drawing. That is, when steel wire is manufactured using the above-described inventive materials, all can have a lamellar spacing of 12.5 nm or less through mechanical thinning. Furthermore, as mentioned above, by fine-tuning the lamellar spacing, a tensile strength of 3000 to 4700 MPa can be secured without increasing the carbon content. However, in the case of the comparative materials, it was impossible to draw them to an appropriate wire diameter of 0.2 mm, such as fracture occurring during the drawing process or delamination occurring during the torsional test. Through this, it can be seen that the cross-sectional reduction rate of the comparative materials does not reach 99%. 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 set forth below.

Claims

1. In weight%, C: 0.60 to 1.10%, Si: 0.40 to 1.0%, Mn: 0.20 to 0.60%, N: 0.006% or less (excluding 0), the remainder comprising Fe and other unavoidable impurities, and The microstructure comprises more than 95.0% pearlite by area fraction and the remainder being non-pearlite, and A wire having the longest axis length of the above-mentioned non-perlite structure of 30㎛ or less. (Here, non-perlite texture refers to a texture containing at least one of proeutectoid ferrite, bainite, or proeutectoid cementite.) 2. In Claim 1, A wire with an average lamellar spacing of 100 to 250 nm.

3. In Claim 1, A wire in which, when fresh, the diameter reduction rate / lamellar spacing reduction rate of the above wire is 0.9 to 1.

1.

4. In Claim 1, A wire with a diameter of 4.0 to 6.0 mm.

5. In Claim 1, A wire rod further comprising, in weight%, one or more of Cr: 0.4% or less, B: 0.003% or less, and Ti: 0.01 to 0.02%.

6. In Claim 1, Wire rod with a cross-sectional reduction rate of 98.85% or more.

7. In Claim 1, A wire that does not undergo delamination or fractures perpendicular to the longitudinal direction of the wire during a torsional test in an applied total deformation range of greater than 0 and less than 3.

5.

8. A step of casting a billet comprising, by weight, C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), and the remainder being Fe and other unavoidable impurities; A step of heating the above billet at 1000 to 1250℃ for 60 to 120 minutes; A step of rolling the heated billet; A method for manufacturing a wire rod comprising a winding step; and a cooling step, The above wire is, The microstructure comprises more than 95.0% pearlite by area fraction and the remainder being non-pearlite, and A method for manufacturing a wire rod having the longest axis length of the above-mentioned non-perlite structure of 30㎛ or less. (Here, non-perlite texture refers to a texture containing at least one of proeutectoid ferrite, bainite, or proeutectoid cementite.) 9. In weight%, C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), the remainder comprising Fe and other unavoidable impurities, Steel wire with an average lamellar spacing of 12.5 nm or less.

10. In Claim 9, Steel wire with an average lamellar spacing of 7.2 nm or less.

11. In Claim 9, A steel wire whose microstructure comprises more than 95.0% pearlite and the remainder being non-pearlite in terms of area fraction. (Here, non-perlite texture refers to a texture containing at least one of proeutectoid ferrite, bainite, or proeutectoid cementite.) 12. In Claim 9, Steel wire with a tensile strength of 3,000 to 4,700 MPa.

13. In Claim 9, A steel wire having a diameter of 0.1 to 0.5 mm.

14. A step of casting a billet comprising, in weight%, C: 0.60 to 1.10%, Si: 0.4 to 1.0%, Mn: 0.2 to 0.6%, N: 0.006% or less (excluding 0), and the remainder being Fe and other unavoidable impurities; Step of heating the above billet; A step of continuously freshening the above billet at a reduction rate of 10 to 20% per pass; and Includes a step of softening heat treatment; A method for manufacturing steel wire by repeating the above-mentioned fresh and above-mentioned softening heat treatments multiple times.

15. In Claim 14, A method for manufacturing steel wire in which the above softening heat treatment is maintained for 10 to 30 seconds at a heating rate of 80℃ / sec or more up to 400 to 600℃.

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