Wire, method for manufacturing the same, and tire cord manufactured using the same

Abstract

A wire having excellent drawability according to an embodiment of the present invention contains 0.70 to 1.10 weight percent of carbon (C), 0.15 to 0.50 weight percent of silicon (Si), 0.20 to 0.90 weight percent of manganese (Mn), 0.015 weight percent or less of phosphorus (P), 0.015 weight percent or less of sulfur (S), 0.02 to 0.35 weight percent of chromium (Cr), 0.010 weight percent or less (0 excepted) of aluminum (Al), 0.005 weight percent or less (0 excepted) of nitrogen (N), the balance of iron (Fe), and other unavoidable impurities. According to an embodiment of the present invention, in a cross section cut along a direction perpendicular to a length direction of the wire, a maximum thickness of a decarburized layer observed in a peripheral direction of the cross section is 0.08 mm or less.

Description

Technical Field

[0001] This invention relates to a wire, a method for manufacturing the same, and tire cords manufactured using the same. More specifically, it relates to a wire that improves drawability and has uniform quality by controlling the thickness of the decarburized layer, a method for manufacturing the same, and tire cords manufactured using the same. Background Technology

[0002] Tire cords are reinforcing materials embedded inside the rubber to improve the stability, drivability, and durability of tires installed in vehicles such as automobiles.

[0003] Tire cords are generally made of steel or high-molecular synthetic fibers (polyester, nylon, etc.).

[0004] Compared with polymer synthetic fiber tire cords, steel tire cords have high strength and excellent heat resistance, and therefore have been widely used recently.

[0005] Steel tire cords are manufactured into extremely fine filaments through a drawing process and an array heat treatment process.

[0006] If drawing is performed using extremely fine wires, high work hardening will occur, which may lead to wire breakage due to non-metallic inclusions, surface defects, decarburized layers, residues not removed after pickling.

[0007] If a break occurs during the manufacturing of tire cords, it will not only lead to a decrease in work efficiency and productivity, but may also cause safety accidents to workers. Therefore, reducing the number of breakages is a problem that must be solved.

[0008] In particular, if the decarburized layer on the surface of the wire is thick, it will not only cause wire breakage during the drawing process, but may also lead to a decrease in the fatigue resistance of the tire cord due to uneven internal quality.

[0009] On the other hand, tire cord wire is usually produced using the blast furnace-converter process. This is because the blast furnace-converter process uses iron ore as raw material to produce molten iron and then uses it to produce products. Therefore, compared with the electric furnace process, which uses scrap iron as the main raw material, it can produce products of superior quality.

[0010] However, given the international trend of most industries accelerating towards carbon neutrality, the steel industry urgently needs to develop technologies that can replace the blast furnace-converter process, which emits large amounts of carbon dioxide, and that ensure the quality of the products produced is comparable to or better than that of products from the blast furnace-converter process. Summary of the Invention

[0011] Technical issues The present invention is intended to solve the problems described above. The object of the present invention is to provide a wire with excellent drawability and uniform internal quality, a method for manufacturing the wire thereof, and tire cords manufactured using the wire thereof.

[0012] The purpose of this invention is not limited to the purposes mentioned above, and other purposes not mentioned can be clearly understood by those skilled in the art based on the following description.

[0013] Technical solution The wire with excellent drawing processability according to one embodiment of the present invention comprises 0.70 to 1.10% by weight of carbon (C), 0.15 to 0.50% by weight of silicon (Si), 0.20 to 0.90% by weight of manganese (Mn), less than 0.015% by weight of phosphorus (P), less than 0.015% by weight of sulfur (S), 0.02 to 0.35% by weight of chromium (Cr), less than 0.010% by weight of aluminum (Al), less than 0.005% by weight of nitrogen (N), the balance being iron (Fe) and other unavoidable impurities.

[0014] According to one embodiment of the present invention, in a cross-section cut along a direction perpendicular to the length direction of the wire, the maximum thickness of the decarburized layer observed along the circumferential direction of the cross-section reaches less than 0.08 mm. According to an embodiment of the present invention, the wire satisfies the following formula 1.

[0015] Formula 1: 0.1≤(2×Cr) / Si (In Equation 1, Cr represents the chromium content and Si represents the silicon content, in weight percentage).

[0016] The wire with excellent drawing processability according to one embodiment of the present invention may further contain less than 0.2% by weight (excluding 0) of copper (Cu) and less than 0.15% by weight (excluding 0) of nickel (Ni).

[0017] The final microstructure of the wire with excellent drawability according to one embodiment of the present invention may contain more than 90 vol.% pearlite and the balance ferrite.

[0018] The wire with excellent drawability according to one embodiment of the present invention can achieve a cross-sectional shrinkage rate of more than 30%.

[0019] According to one embodiment of the present invention, the thickness of the decarburized layer can be less than 0.05 mm.

[0020] According to one embodiment of the present invention, the carbon (C) content can reach 0.7 to 0.95% by weight.

[0021] According to one embodiment of the present invention, the tensile strength can reach more than 1000 MPa.

[0022] A method for manufacturing a wire with excellent drawability according to an embodiment of the present invention includes: (a) a smelting step, in which raw materials are smelted; (b) a continuous casting step, in which a semi-finished product is manufactured; and (c) a wire rolling step, in which the semi-finished product is hot rolled.

[0023] The wire rod produced by the (c) wire rolling step contains 0.70 to 1.10% by weight of carbon (C), 0.15 to 0.50% by weight of silicon (Si), 0.20 to 0.90% by weight of manganese (Mn), less than 0.015% by weight of phosphorus (P), less than 0.015% by weight of sulfur (S), 0.02 to 0.35% by weight of chromium (Cr), less than 0.010% by weight of aluminum (Al), less than 0.005% by weight of nitrogen (N), the balance being iron (Fe) and other unavoidable impurities.

[0024] According to one embodiment of the present invention, in a cross-section cut along a direction perpendicular to the length direction of the wire, the maximum thickness of the decarburized layer observed along the circumferential direction of the cross-section reaches less than 0.08 mm.

[0025] According to an embodiment of the present invention, the wire satisfies the following formula 1.

[0026] Formula 1: 0.1≤(2×Cr) / Si (In Equation 1, Cr represents the chromium content and Si represents the silicon content, in weight percentage).

[0027] According to an embodiment of the present invention, in the smelting step (a), the raw material may include at least one of reduced iron, molten iron and scrap iron.

[0028] According to one embodiment of the invention, (a) the smelting step can be performed in at least one of the equipment in the blast furnace and the electric furnace.

[0029] According to one embodiment of the present invention, the wire after the wire rolling step (c) may further contain less than 0.2% by weight of copper (Cu) and less than 0.15% by weight of nickel (Ni).

[0030] According to one embodiment of the present invention, the (c) wire rolling step may include: (c-1) a reheating step, reheating the semi-finished product at a temperature of 1000 to 1170°C for more than 90 minutes; (c-2) a precision rolling step, hot rolling the reheated semi-finished product at an inlet-side temperature of 850 to 1000°C; and (c-3) a winding step, winding the hot-rolled product at a temperature of 800 to 900°C.

[0031] According to one embodiment of the present invention, the cross-sectional shrinkage rate of the wire after the wire rolling step (c) can reach more than 30%.

[0032] According to one embodiment of the present invention, the final microstructure of the wire after the (c) wire rolling step may contain more than 90 vol.% pearlite and the balance ferrite.

[0033] One embodiment of the tire cord of the present invention comprises a plurality of wires. The maximum thickness of the decarburized layer observed in the cross-section of the wires is less than 0.08 mm.

[0034] The wire satisfies the following formula 1.

[0035] Formula 1: 0.1≤(2×Cr) / Si (In Equation 1, Cr represents the chromium content and Si represents the silicon content, in weight percentage).

[0036] According to one embodiment of the present invention, the wire may contain 0.70 to 1.10% by weight of carbon (C), 0.15 to 0.50% by weight of silicon (Si), 0.20 to 0.90% by weight of manganese (Mn), less than 0.015% by weight of phosphorus (P), less than 0.015% by weight of sulfur (S), 0.02 to 0.35% by weight of chromium (Cr), less than 0.010% by weight of aluminum (Al), less than 0.005% by weight of nitrogen (N), the balance being iron (Fe) and other unavoidable impurities.

[0037] According to one embodiment of the present invention, the wire may further contain less than 0.2% by weight of copper (Cu) and less than 0.15% by weight of nickel (Ni).

[0038] According to one embodiment of the present invention, the maximum thickness of the decarburized layer can be less than 0.05 mm.

[0039] According to one embodiment of the present invention, the tensile strength can reach 3.2 GPa or higher.

[0040] The effects of the invention In one embodiment of the present invention, a wire with excellent drawability can achieve excellent drawability and uniform physical properties by controlling the thickness of the decarburized layer on the surface of the wire.

[0041] Furthermore, the tire cord of one embodiment of the present invention can achieve an excellent tensile strength of 3.2 GPa or more by using a wire with excellent drawability.

[0042] The effects of this invention are not limited to those mentioned above, and other effects not mentioned can be clearly understood by those skilled in the art through the description of the scope of protection of the invention. Attached Figure Description

[0043] Figure 1 This is a flowchart illustrating a method for manufacturing a wire with excellent drawability according to an embodiment of the present invention.

[0044] Figure 2 To show Figure 1 The flowchart shows the detailed steps in the wire rolling process.

[0045] Figure 3 This is a flowchart illustrating a method for manufacturing tire cords according to an embodiment of the present invention.

[0046] Figure 4 This is a magnified photograph showing the decarburized layer of the sample from the experimental example. Detailed Implementation

[0047] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, so that those skilled in the art can easily implement the present invention. However, the present invention can be implemented in many different ways, and the present invention is not limited to the following embodiments.

[0048] Furthermore, if it is mentioned that a certain structural element (or region, layer, part, etc.) is "above" other structural elements, "connected" to, or "combined" with other structural elements, then it can be directly "set / connected / combined" on other structural elements, or a third structural element can be set between them.

[0049] Terms such as “include” or “possess” are used to indicate the presence of features, figures, steps, actions, structural elements, components, or combinations thereof described in the specification, and should be understood as not precluding the presence or increased possibility of one or more other features, figures, steps, actions, structural elements, components, or combinations thereof.

[0050] To clearly illustrate the present invention, detailed descriptions of relevant well-known technologies that are irrelevant to the description or may unnecessarily obscure the main idea of ​​the present invention have been omitted. In assigning reference numerals to various structural elements in the various figures in this specification, the same or similar structural elements in the entire text of the specification have been assigned the same or similar reference numerals.

[0051] Furthermore, the meanings of the terms or words used in this specification and the scope of protection of the invention should not be limited to their ordinary meanings or dictionary meanings. Based on the principle that inventors can appropriately define terms and concepts in order to best explain their invention, they should be interpreted as meanings and concepts that conform to the technical ideas of the invention.

[0052] Unless otherwise specified, the notation "A~B" for numerical values ​​A and B means "above A and below B". In this context, the unit only appears in numerical value B, which also implies that the unit applies to numerical value A.

[0053] The embodiments of the present invention will be described in detail below.

[0054] Wire with excellent drawability The wire with excellent drawing processability according to one embodiment of the present invention comprises 0.70 to 1.10% by weight of carbon (C), 0.15 to 0.50% by weight of silicon (Si), 0.20 to 0.90% by weight of manganese (Mn), less than 0.015% by weight of phosphorus (P), less than 0.015% by weight of sulfur (S) (excluding 0), 0.02 to 0.35% by weight of chromium (Cr), less than 0.010% by weight of aluminum (Al) (excluding 0), less than 0.005% by weight of nitrogen (N), the balance being iron (Fe) and other unavoidable impurities.

[0055] The following is a detailed description of the role and content of each alloying element in the wire with excellent drawability according to an embodiment of the present invention.

[0056] Carbon (C) Carbon (C) is an element that effectively increases strength. Although strength increases with increasing carbon content, there is a problem of decreased toughness.

[0057] If the amount of carbon (C) added is insufficient within the preset range, the strength of the tire cords in the final product may not be guaranteed.

[0058] Conversely, if the amount of carbon (C) added exceeds the preset range, the toughness and the reduction of section shrinkage will decrease, which may lead to a decrease in drawability due to the formation of nitrate cementite.

[0059] Therefore, in the wire with excellent drawing processability in one embodiment of the present invention, the carbon (C) content can be controlled to 0.70 to 1.1% by weight, and preferably to 0.7 to 0.95% by weight.

[0060] Silicon (Si) Silicon (Si) is an element that can effectively increase strength, deoxidize, and increase the activity of carbon.

[0061] If the amount of silicon (Si) added is insufficient within the preset range, it will be difficult to ensure the deoxidation effect and strength.

[0062] Conversely, if the amount of silicon (Si) added exceeds the preset range, the toughness decreases, the deformation resistance increases, and the surface oxide layer is difficult to remove during the pickling process.

[0063] Furthermore, if the amount of silicon (Si) added exceeds the preset range, it may lead to an increase in the thickness of the decarburized layer due to excessive decarburization, thereby reducing the workability of the drawing process.

[0064] Therefore, in the wire with excellent drawing processability in one embodiment of the present invention, the silicon (Si) content can be controlled at 0.15 to 0.5% by weight, and preferably at 0.16 to 0.48% by weight.

[0065] Manganese (Mn) Manganese (Mn) is an element that increases the strength of wires and effectively deoxidizes them. It prevents thermal brittleness by forming MnS intercalation compounds. Furthermore, manganese (Mn) also reduces carbon activity.

[0066] If the amount of manganese (Mn) added is insufficient within the preset range, there will be problems such as reduced deoxidation effect, difficulty in ensuring strength, and induction of heat brittleness.

[0067] Furthermore, if the amount of manganese (Mn) added is insufficient within the preset range, the drawing processability may decrease as the thickness of the decarburized layer increases due to the minimal effect of inhibiting decarburization.

[0068] If the amount of manganese (Mn) added exceeds the preset range, segregation may occur in the center of the wire, potentially leading to the formation of a low-temperature microstructure upon cooling. Specifically, martensite may be formed upon cooling.

[0069] Therefore, in the wire with excellent drawing processability in one embodiment of the present invention, the content of manganese (Mn) can be controlled at 0.2 to 0.9% by weight, and preferably at 0.22 to 0.89% by weight.

[0070] Phosphorus (P) Phosphorus (P) is a residual element that must be removed during the steelmaking process. If the phosphorus content is greater than 0.015% by weight, it may lead to embrittlement due to grain boundary segregation and the formation of Fe3P compounds.

[0071] Therefore, in the wire with excellent drawing processability according to an embodiment of the present invention, the phosphorus (P) content can be controlled to be below 0.015% by weight.

[0072] Sulfur (S) Sulfur (S) is a residual element that must be removed during the steelmaking process, and it will form MnS non-metallic inclusions.

[0073] If the sulfur content is greater than 0.015% by weight, the toughness will be reduced due to the excessive formation of sulfides such as MnS.

[0074] Therefore, in the wire with excellent drawing processability according to an embodiment of the present invention, the sulfur (S) content can be controlled to be below 0.015% by weight.

[0075] Copper (Cu) Copper (Cu) is an element that helps increase the strength and corrosion resistance of wire. However, if the amount of copper (Cu) added exceeds the preset range, it may induce surface cracks during high-temperature wire rolling.

[0076] Therefore, in the wire with excellent drawing processability in one embodiment of the present invention, the copper (Cu) content can be controlled to be below 0.2 weight percentage (excluding 0), and preferably controlled to be between 0.02 and 0.19 weight percentage.

[0077] According to one embodiment of the present invention, copper (Cu) may be derived from scrap iron. However, the present invention is not limited thereto, and may be added separately as needed.

[0078] Nickel (Ni) Nickel (Ni) is an element that helps improve the strength and corrosion resistance of wire. Furthermore, nickel (Ni) effectively reduces surface cracks caused by copper (Cu) thickening during hot rolling and also reduces carbon diffusion.

[0079] However, if the amount of nickel (Ni) added exceeds the preset range, it may reduce the fatigue life of the product due to excessive formation of retained austenite. Furthermore, excessive addition of nickel (Ni) can lead to increased manufacturing costs.

[0080] Therefore, in the wire with excellent drawing processability in one embodiment of the present invention, the nickel (Ni) content can be controlled to be below 0.15% by weight (excluding 0), and preferably controlled to be between 0.01 and 0.15% by weight.

[0081] Chromium (Cr) Chromium (Cr) is an element that helps improve the strength and curing ability of wires. Furthermore, chromium (Cr) also enhances the corrosion resistance of wires. Additionally, chromium (Cr) is an effective element in reducing carbon activity.

[0082] If the amount of chromium (Cr) added is insufficient within the preset range, it will be difficult to ensure the required strength, the decarburization inhibition effect will be minimal, and the workability may decrease as the thickness of the decarburized layer increases.

[0083] If the amount of chromium (Cr) added exceeds the preset range, it may induce wire breakage in subsequent drawing processes due to the formation of low-temperature structures. Furthermore, excessive addition of chromium (Cr) can lead to increased manufacturing costs.

[0084] Therefore, in the wire with excellent drawing processability in one embodiment of the present invention, the chromium (Cr) content can be controlled at 0.02 to 0.35% by weight, and preferably at 0.03 to 0.33% by weight.

[0085] Aluminum (Al) Aluminum (Al) is an element that acts as a powerful deoxidizer, and it also enhances toughness and forms non-metallic inclusions due to the finer grain size of austenite crystals.

[0086] However, if the amount of aluminum (Al) added exceeds the preset range, it may cause wire breakage during the drawing process due to the excessive formation of coarse, non-flexible aluminum intervening materials.

[0087] Therefore, in the wire with excellent drawing processability in one embodiment of the present invention, the content of aluminum (Al) can be controlled to be less than 0.01 weight percentage (excluding 0), and preferably controlled to be 0.001 to 0.01 weight percentage.

[0088] Nitrogen (N) Nitrogen (N) is an element that combines with elements such as aluminum (Al), titanium (Ti), vanadium (V), and niobium (Nb) to form precipitates and effectively achieve fine austenite crystal grain size.

[0089] However, if the amount of nitrogen (N) added exceeds the preset range, it may cause strength deviation due to excessive formation of precipitates, which may adversely affect the cold rolling performance during the drawing process.

[0090] Therefore, in the wire with excellent drawing processability in one embodiment of the present invention, the nitrogen (N) content can be controlled to be less than 0.005 weight percentage (excluding 0), and preferably controlled to be between 0.0032 and 0.0044 weight percentage.

[0091] In addition to the steel composition described above, the remaining components may include iron (Fe) and unavoidable impurities. Unavoidable impurities are those introduced during the manufacturing process, which is well known in the art and will therefore be omitted from specific description.

[0092] In one embodiment of the present invention, in addition to the alloy composition described above, the addition of other elements is not excluded. Multiple elements may be included in various ways without infringing upon the technical concept of the present invention. If other elements are included, they may replace the remaining iron (Fe).

[0093] In a wire with excellent drawability according to an embodiment of the present invention, the thickness of the decarburized layer can be less than 0.08 mm, and preferably less than 0.05 mm.

[0094] A decarburized layer is a layer on the surface of a metal material where carbon has been lost. A decarburized layer is formed by causing carbon to be lost from the surface of a metal material exposed to high temperatures.

[0095] In particular, the decarburized layer can have a fatal impact on the process of manufacturing tire cords using wire.

[0096] Specifically, the greater the thickness of the decarburized layer, the lower the surface hardness of the wire, potentially leading to strength loss. This could make the wire more prone to deformation during both dry and wet drawing processes, potentially increasing the risk of wire breakage.

[0097] Furthermore, the greater the thickness of the decarburized layer, the more directly the wire surface comes into contact with oxygen, leading to corrosion. This could potentially make the wire more susceptible to water corrosion during wet drawing processes.

[0098] Furthermore, the greater the thickness of the decarburized layer, the more likely it is that the amount of impurities on the surface of the wire will increase. This could potentially cause micro-defects in the wire during subsequent processes.

[0099] As mentioned above, minimizing the thickness of the decarburized layer is likely to be a very important technique in order to manufacture wires with excellent drawing processability.

[0100] Therefore, the inventors have proposed a scheme to minimize the thickness of the decarburized layer by precisely controlling the content ratio between multiple elements that affect carbon activity.

[0101] In a wire with excellent drawability according to an embodiment of the present invention, silicon (Si) and chromium (Cr) can satisfy the following formula 1.

[0102] 0.1≤(2×Cr) / Si (In Equation 1, Cr represents the chromium content and Si represents the silicon content, in weight percentage.) According to Equation 1, the thickness of the decarburized layer can be controlled by adjusting the ratio between chromium (Cr), which reduces carbon activity, and silicon (Si), which increases carbon activity.

[0103] Specifically, Equation 1 allows for minimizing the thickness of the decarburized layer and taking into account other physical properties required by the wire (e.g., tensile strength, etc.) by designing the weighted value for the chromium (Cr) content (i.e., 2×Cr in Equation 1) to the silicon (Si) ratio to be greater than 0.1, thereby controlling the thickness of the decarburized layer.

[0104] According to one embodiment of the present invention, the thickness of the decarburized layer of the wire satisfying Formula 1 can reach less than 0.08 mm.

[0105] According to one embodiment of the present invention, in order to control the thickness of the decarburized layer to be thinner, Equation 2 or Equation 3 can be satisfied.

[0106] In a wire with excellent drawing processability according to an embodiment of the present invention, silicon (Si), chromium (Cr) and manganese (Mn) can satisfy the following formula 2.

[0107] Formula 2: 1≤(6×Cr+Mn) / (3×Si) (In Equation 2, Cr represents the chromium content, Si represents the silicon content, and Mn represents the manganese content, all in weight percentage.) According to Equation 2, the thickness of the decarburized layer can be controlled by adjusting the ratio between chromium (Cr) and manganese (Mn) which reduce carbon activity and silicon (Si) which increases carbon activity.

[0108] Specifically, Equation 2 allows for minimizing the thickness of the decarburized layer and taking into account other physical properties required by the wire (e.g., tensile strength, etc.) by designing the ratio between the weighted value assigned to the chromium (Cr) content and the sum of the manganese (Mn) content (i.e., [6×Cr+Mn] in Equation 2) and the weighted value assigned to the silicon (Si) content (i.e., 3×Si in Equation 2) to be greater than 1, thereby controlling the thickness of the decarburized layer.

[0109] According to one embodiment of the present invention, the thickness of the decarburized layer of the wire satisfying Formula 2 can reach less than 0.05 mm.

[0110] In a wire with excellent drawing processability according to an embodiment of the present invention, silicon (Si), chromium (Cr), manganese (Mn) and nickel (Ni) satisfy the following formula 3.

[0111] Formula 3: 1 ≤ (12×Cr + 2×Mn + 3×Ni) / (6×Si) (In Equation 3, Cr represents the chromium content, Si represents the silicon content, Mn represents the manganese content, and Ni represents the nickel content, all in weight percentage.) According to Equation 3, the thickness of the decarburized layer can be controlled by adjusting the ratio between chromium (Cr), manganese (Mn), and nickel (Ni) which reduce carbon activity and silicon (Si) which enhances carbon activity.

[0112] Specifically, Equation 3 allows for minimizing the thickness of the decarburized layer and taking into account other physical properties required by the wire (e.g., tensile strength, etc.). The ratio between the sum of multiple values ​​that assign different weights to chromium (Cr), manganese (Mn), and nickel (Ni) (i.e., [12×Cr+2×Mn+3×Ni] in Equation 2) and the value that assigns a weight to the content of silicon (Si) (i.e., 3×Si in Equation 2) is designed to be greater than 1, thereby controlling the thickness of the decarburized layer.

[0113] According to one embodiment of the present invention, the thickness of the decarburized layer of the wire satisfying Formula 3 can reach less than 0.05 mm.

[0114] Ultimately, the wire with excellent drawability according to an embodiment of the present invention can be manufactured by the alloy composition and the manufacturing process described below.

[0115] In particular, the wire with excellent drawing processability of one embodiment of the present invention can be precisely controlled by the content ratio between elements that affect carbon activity through the relationship described above, thereby minimizing the thickness of the decarburized layer.

[0116] In a wire with excellent drawability according to an embodiment of the present invention, the total amount of non-metallic intervening material can reach less than 0.05 vol.%, and the average maximum size of the non-metallic intervening material can reach less than 40 μm.

[0117] Preferably, the total amount of non-metallic intervention can reach 0.021 to 0.025 vol.%, and the maximum size of non-metallic intervention can reach 17.1 to 17.3 μm.

[0118] According to one embodiment of the present invention, the average thickness of the surface oxide layer of the wire can reach less than 8 μm, and preferably less than 7.2 μm.

[0119] Therefore, the surface oxide layer of the wire can be completely removed through the pickling process. This reduces the frequency of wire breakage during the drawing process.

[0120] Finally, according to an embodiment of the present invention, the frequency of cord breakage during the manufacture of tire cords can be significantly reduced by controlling the thickness of the decarburized layer, the total amount of non-metallic intervening materials, the maximum size of the non-metallic intervening materials, and the thickness of the oxide layer within a preset range.

[0121] The tensile strength (TS) of the wire with excellent drawability according to one embodiment of the present invention can reach more than 1000 MPa, preferably 1050 to 1250 MPa.

[0122] In one embodiment of the present invention, the cross-sectional reduction rate of the wire with excellent drawability can reach 30% or more. Here, the cross-sectional reduction rate is expressed as a percentage, representing the reduction in cross-sectional area when the wire breaks after being stretched along its length. For example, the cross-sectional reduction rate can be defined as follows.

[0123] Section reduction rate [(A0-A1) / A0]×100(%) In the formula, A0 means the cross-sectional area of ​​the wire before stretching, and A1 means the cross-sectional area of ​​the wire after stretching.

[0124] The final microstructure of the wire with excellent drawability according to one embodiment of the present invention may contain more than 90 vol.% pearlite and ferrite.

[0125] However, the final microstructure of the wire is not limited to the above and may include other structures that replace ferrite.

[0126] The following is a detailed description of a method for manufacturing a wire with excellent drawability according to an embodiment of the present invention.

[0127] Manufacturing method of wire with excellent drawing processability The following describes a method for manufacturing a wire with excellent drawability according to an embodiment of the present invention.

[0128] Figure 1 This is a flowchart illustrating a method for manufacturing a wire with excellent drawability according to an embodiment of the present invention.

[0129] A method for manufacturing a wire with excellent drawability according to an embodiment of the present invention includes: (a) a smelting step, in which raw materials are smelted; (b) a continuous casting step, in which a semi-finished product is manufactured; and (c) a wire rolling step, in which the semi-finished product is hot rolled.

[0130] According to one embodiment of the invention, (a) the smelting step is a step of producing molten soup by smelting raw materials. (a) The smelting step can be performed in at least one of the following: a blast furnace and an electric furnace.

[0131] If a blast furnace is used, the raw materials may include iron ore and coke. Specifically, the blast furnace process can simultaneously perform the smelting and reduction processes of the raw materials using hot air after they are charged into the blast furnace.

[0132] The molten iron produced in the blast furnace can be made into molten steel with impurities removed by controlling the alloy composition coefficient within a preset range through subsequent steelmaking processes.

[0133] If an electric furnace is used, the raw materials may include at least one of scrap iron and reduced iron. For example, the raw materials may directly contain 0 to 100% by weight of reduced iron and residual scrap iron. Reduced iron may include DRI (Direct Reduced Iron), HBI (Hot Briquetted Iron), LRI (Low Reduced Iron), etc.

[0134] Specifically, the electric furnace process involves melting raw materials using electrical energy after they are loaded into the furnace. For example, the electric furnace can be an EAF (Electric Arc Furnace). An EAF melts raw materials using the heat of an electric arc generated between electrodes. An EAF can be powered by alternating current (AC), but is not limited to this; direct current (DC) power can also be used.

[0135] However, the types of electric furnaces are not limited to those mentioned above. For example, other types of electric furnaces such as ESF (Electric Smelting Furnace) and EIF (Electric Induction Furnace) can also be used.

[0136] A manufacturing method that uses both blast furnaces and electric furnaces involves transferring a portion of the molten iron produced in the blast furnace into an electric furnace to generate the final molten iron.

[0137] The raw materials may include molten iron from blast furnaces, scrap iron, and reduced iron. For example, the raw materials may contain 15-30% by weight of molten iron from blast furnaces, 15-30% by weight of reduced iron, and the remaining percentage of scrap iron.

[0138] If the amount of molten iron charged into the electric arc furnace is insufficient, it may be difficult to completely melt the reduced iron and scrap iron. Conversely, if the amount of molten iron exceeds the preset range, the increased iron production will lead to excessive carbon dioxide emissions from the blast furnace operation.

[0139] If the amount of reduced iron charged into the electric furnace is less than the preset range, the proportion of scrap iron in the raw material will increase, and other elements contained in the scrap iron may cause the physical properties of the wire to deteriorate. Conversely, if the amount of reduced iron charged into the electric furnace exceeds the preset range, the amount of reduced iron that is difficult to melt will be greater than the amount of scrap iron, which may result in the raw material being difficult to melt completely.

[0140] In the case of using an electric furnace as described above, or in the case of using both a blast furnace and an electric furnace, a process can be performed to process the molten broth produced in the electric furnace.

[0141] For example, the molten metal prepared in an electric furnace can undergo a vacuum degassing process. Specifically, the vacuum degassing process involves treating the molten metal with an inert gas at a preset vacuum level. Argon (Ar) can be used as the inert gas.

[0142] Specifically, the vacuum degassing process can control the argon (Ar) circulation flow rate at 900–1100. The process is carried out under the following conditions: vacuum treatment time is controlled to be more than 20 minutes, and vacuum level is controlled to be less than 2 mbar. This allows for control over the maximum size and nitrogen content of non-metallic intervening materials that affect drawing processability.

[0143] According to one embodiment of the present invention, in the (b) continuous casting step, the molten material prepared in the (a) smelting step can be manufactured into a semi-finished product form.

[0144] In this embodiment, the semi-finished product can be a billet. However, the present invention is not limited to this and can also be manufactured into slab or billet shapes.

[0145] Figure 2 To show Figure 1 The flowchart shows the detailed steps in the wire rolling process.

[0146] Reference Figure 2 (c) The wire rolling process may include (c-1) a reheating process, (c-2) a precision rolling process and (c-3) a winding process.

[0147] In the (c-1) reheating step, the semi-finished product may be reheated. The (c-1) reheating step may be performed at a reheating temperature of 1000 to 1170°C and a reheating time of more than 90 minutes.

[0148] If the reheating temperature exceeds the preset range, an excessive decarburized layer may form on the surface. Conversely, if the reheating temperature is below the preset range, defects such as clogging may occur during the rolling process due to increased equipment load in precision rolling.

[0149] In the (c-2) precision rolling step, wire is manufactured by rolling the semi-finished product. The rolling process can be performed multiple times in the (c-2) precision rolling step. The (c-2) precision rolling step can be performed at an inlet-side temperature of 850–1000°C.

[0150] If the temperature at the entry side of the precision rolling mill exceeds the preset range, an excessive decarburized layer may form. Conversely, if the temperature at the entry side of the precision rolling mill is below the preset range, the load on the rolling equipment will increase, potentially causing surface defects such as rolling folds.

[0151] The diameter of the wire after the (c-2) precision rolling step can reach 4mm to 7mm, preferably 4.5 to 6mm.

[0152] However, the diameter of the wire is not limited to what is described above; the diameter of the wire can vary depending on the rolling conditions.

[0153] In the (c-3) winding step, precision-rolled wire can be wound. The (c-3) winding step can be performed at various temperatures. Wire rods can be manufactured by rolling semi-finished products. The (c-2) precision rolling step can be performed at a winding temperature of 800–900°C.

[0154] If the winding temperature exceeds the preset range, the decarburized layer may become excessively thick, resulting in a very thick surface oxide layer, which could cause wire breakage during drawing and twisting. Conversely, if the winding temperature is insufficient within the preset range, a large difference will occur between it and the rolling temperature, potentially leading to poor winding shape.

[0155] According to an embodiment of the present invention, the thickness of the decarburized layer, the total amount of non-metallic inclusions, the maximum size of non-metallic inclusions, and the thickness of the oxide layer can be controlled within a preset range by the alloy composition coefficient and manufacturing process described above, thereby manufacturing wires with excellent drawability and uniform physical properties.

[0156] The following describes a method for manufacturing tire cord using wire produced through the aforementioned alloy composition coefficients and processes, as well as the tire cord itself.

[0157] Tire cord manufacturing method and tire cord Figure 3 This is a flowchart illustrating a method for manufacturing tire cords according to an embodiment of the present invention.

[0158] Reference Figure 3 A method for manufacturing tire cord according to an embodiment of the present invention may include (d) an acid pickling step, (e) a first drawing step, (f) a heat treatment step, (g) an electroplating step, (h) a second drawing step, and (i) a twisting step.

[0159] In the pickling step (d), the surface of the wire is finished by exposing it to the pickling solution. In the pickling step (d), the surface oxide layer of the wire can be removed and an iron oxide layer can be formed on the wire surface. This improves the surface properties of the wire and enhances its corrosion resistance. The pickling solution may contain acidic substances; for example, the pickling solution may include sulfuric acid, hydrochloric acid, etc.

[0160] (e) The first drawing step can be a dry drawing process. For example, in the first drawing step of (e), a dry extension line device can be used to process the wire after wire rolling into wire with a diameter of 1 to 2 mm.

[0161] The heat treatment step (f) can be a patterned heat treatment step. For example, in the heat treatment step (f), the wire can be heat treated at a temperature range of 800 to 900°C.

[0162] (g) The electroplating step can be a step of forming a coating on the surface of the wire. For example, in the electroplating step (g), a brass coating can be formed on the surface of the wire. Specifically, in the electroplating step (g), copper electroplating and zinc electroplating can be performed on the surface of the wire respectively, thereby forming a brass coating through diffusion treatment.

[0163] (h) The second drawing step can be a wet drawing process. For example, in the second drawing step (h), a wire with a diameter of 1 to 2 mm can be drawn into a wire with a diameter of 0.15 to 0.40 mm by using wet drawing with a wet lubricant.

[0164] In the (i) twisting step, tire cords can be manufactured by twisting multiple wires that have passed through the (h) second drawing step to a preset strength.

[0165] According to an embodiment of the present invention, the tire cord manufacturing method can minimize the frequency of wire breakage during the drawing process by using wire with excellent drawability to manufacture tire cord, thereby significantly improving the production efficiency of tire cord.

[0166] In one embodiment of the present invention, tire cords can be disposed in the belt layer area of ​​a vehicle tire to improve tire durability, thereby enhancing vehicle driving stability.

[0167] The tensile strength of the tire cord in one embodiment of the present invention can reach 3.2 GPa.

[0168] Test case Hereinafter, preferred experimental examples are described to aid in understanding the present invention. These examples are for illustrative purposes only and are not intended to limit the invention to the following embodiments.

[0169] Tables 1 and 2 show the alloy composition coefficients and values ​​of Equations 1 to 3 for Test Examples 1 to 9. Table 3 shows the manufacturing process conditions for Test Examples 1 to 9. Table 4 shows the maximum thickness of the decarburized layer for Test Examples 1 to 9. The diameter of the wire samples for Test Examples 1 to 9 was 5.5 mm.

[0170] Regarding the maximum thickness of the decarburized layer shown in Table 4, after cutting the wires of Test Examples 1 to 9 in a direction perpendicular to the length of the wire, the decarburized layer was observed and the thickness of the thickest decarburized layer was measured using an optical microscope along the periphery of the cut surface.

[0171] Regarding the breakage frequencies shown in Table 4, the breakage frequencies generated when manufacturing tire cords using the wires based on Test Examples 1 to 9 were measured. More specifically, the average breakage frequency generated when manufacturing tire cords using three sets of coils wound with the wires of each Test Example is shown.

[0172] Table 1 Table 2 Table 3 Table 4 Figure 4 This is a magnified photograph showing the decarburized layer of the test specimen. Specifically, Figure 4 Part (a) shows the shape and maximum thickness of the decarburized layer observed in the specimen of Test Example 1. Figure 4 Part (b) shows the shape and maximum thickness of the decarburized layer observed in the specimen of Test Example 3. Figure 4 Section (c) shows the shape and maximum thickness of the decarburized layer observed in the specimen of Test Example 5. Figure 4 Section (d) shows the shape and maximum thickness of the decarburized layer observed in the specimen of Test Example 8.

[0173] Referring to Tables 1 to 4, all of Experimental Examples 1 to 9 met the content ranges of each element and manufacturing process conditions as described above.

[0174] The values ​​of Equation 1 for all of Test Examples 1 to 9 reached 0.1 or higher. Therefore, the maximum thickness of the decarburized layer observed in the specimens of Test Examples 1 to 9 all reached 0.08 mm or less. The wire breakage frequency of the specimens of Test Examples 1 to 9 was 4 times or less.

[0175] Reference Figure 4 Observed Figure 4 The maximum thickness of the decarburized layer in the test pieces of Test Examples 1, 3, 5 and 8 all reached less than 0.08 mm.

[0176] Refer to Tables 1-4 and Figure 4 The values ​​of Equation 2 in Test Examples 1 to 5 were all above 1, while the values ​​of Equation 2 in Test Examples 6 to 9 were all below 1. The maximum thickness of the decarburized layer observed in the specimens of Test Examples 1 to 5 was below 0.05 mm, while the maximum thickness of the decarburized layer observed in the specimens of Test Examples 6 to 9 exceeded 0.05 mm.

[0177] Ultimately, the breakage frequency of the test pieces in Examples 1 to 5, where the value of Equation 2 is 1 or higher, reached less than 1 time, which was significantly lower than that of the test pieces in Examples 6 to 9.

[0178] Tables 1-4 and Figure 4The values ​​of Equation 3 in each of Test Examples 1 to 5 reached 1, while the values ​​of Equation 3 in each of Test Examples 6 to 9 were less than 1. The maximum thickness of the decarburized layer observed in the specimens of Test Examples 1 to 5 was less than 0.05 mm, while the maximum thickness of the decarburized layer observed in the specimens of Test Examples 6 to 9 all exceeded 0.05 mm.

[0179] Ultimately, the breakage frequency of the test pieces in Examples 1 to 5, where the value of Equation 3 is 1 or higher, reached less than 1 time, confirming that the breakage frequency was significantly lower than that of the test pieces in Examples 6 to 9.

[0180] According to one embodiment of the present invention, the thickness of the decarburized layer can be controlled to 0.05 mm by extremely precise control of the content ratio between elements that increase carbon activity (silicon (Si)) and elements that decrease carbon activity (manganese (Mn), chromium (Cr), nickel (Ni)).

[0181] Therefore, by preventing excessive decarburization that could damage the surface hardness and strength of the wire and minimizing impurities, the average wire breakage frequency in subsequent drawing processes can be significantly reduced to below 1.

[0182] As described above, preferred embodiments of the present invention have been presented. It is self-evident to those skilled in the art that the present invention may be embodied in other specific embodiments without departing from its spirit or scope, in addition to the comparative examples and embodiments described above. Therefore, the embodiments described above are merely examples and not limiting; thus, the present invention is not limited to the description above, and modifications may be made within the scope of the appended claims and equivalents.

Claims

1. A wire with excellent drawability, characterized in that, It contains 0.70–1.10% by weight carbon (C), 0.15–0.50% by weight silicon (Si), 0.20–0.90% by weight manganese (Mn), less than 0.015% by weight phosphorus (P), less than 0.015% by weight sulfur (S), 0.02–0.35% by weight chromium (Cr), greater than 0 and less than or equal to 0.010% by weight aluminum (Al), greater than 0 and less than or equal to 0.005% by weight nitrogen (N), the balance being iron (Fe) and other unavoidable impurities. In a cross-section cut perpendicular to the length of the wire, the maximum thickness of the decarburized layer along the circumferential direction of the cross-section reaches less than 0.08 mm. The wire satisfies the following formula 1: Formula 1: 0.1≤(2×Cr) / Si In Formula 1, Cr represents the chromium content and Si represents the silicon content, both in weight percentage.

2. The wire with excellent drawability according to claim 1, characterized in that, It also contains 0 to 0.2% by weight of copper (Cu) and 0 to 0.15% by weight of nickel (Ni).

3. The wire with excellent drawability according to claim 1, characterized in that, The final microstructure contains more than 90 vol.% pearlite and the remainder ferrite.

4. The wire with excellent drawability according to claim 1, characterized in that, The cross-sectional shrinkage rate reaches over 30%.

5. The wire with excellent drawability according to claim 1, characterized in that, The thickness of the decarburized layer is less than 0.05 mm.

6. The wire with excellent drawability according to claim 1, characterized in that, The carbon (C) content is 0.7 to 0.95% by weight.

7. The wire with excellent drawability according to claim 1, characterized in that, The tensile strength reaches over 1000 MPa.

8. A method for manufacturing a wire with excellent drawability, characterized in that, include: (a) The smelting step, which involves smelting the raw materials; (b) Continuous casting steps to produce semi-finished products; and (c) Wire rolling step: hot rolling of the semi-finished product. The wire rod produced by the wire rolling step described in (c) contains 0.70–1.10% by weight carbon (C), 0.15–0.50% by weight silicon (Si), 0.20–0.90% by weight manganese (Mn), less than 0.015% by weight phosphorus (P), less than 0.015% by weight sulfur (S), 0.02–0.35% by weight chromium (Cr), greater than 0 and less than or equal to 0.010% by weight aluminum (Al), greater than 0 and less than or equal to 0.005% by weight nitrogen (N), the balance being iron (Fe) and other unavoidable impurities. In a cross-section cut perpendicular to the length of the wire, the maximum thickness of the decarburized layer along the circumferential direction of the cross-section reaches less than 0.08 mm. The wire satisfies the following formula 1: Formula 1: 0.1≤(2×Cr) / Si In Formula 1, Cr represents the chromium content and Si represents the silicon content, both in weight percentage.

9. The method for manufacturing a wire with excellent drawability according to claim 8, characterized in that, In the (a) smelting step, the raw materials include at least one of reduced iron, molten iron and scrap iron.

10. The method for manufacturing a wire with excellent drawability according to claim 8, characterized in that, The smelting step (a) is performed in at least one of the equipment in the blast furnace and the electric furnace.

11. The method for manufacturing a wire with excellent drawability according to claim 8, characterized in that, The wire that has undergone the wire rolling step (c) further contains less than 0.2% by weight of copper (Cu) and less than 0.15% by weight of nickel (Ni).

12. The method for manufacturing a wire with excellent drawability according to claim 8, characterized in that, The wire rolling step (c) includes: (c-1) Reheating step: The semi-finished product is reheated at a temperature of 1000 to 1170°C for no more than 90 minutes; (c-1) A precision rolling step, wherein the reheated semi-finished product is hot-rolled at an inlet-side temperature of 850–1000°C; and (c-3) Winding step: Wind the hot-rolled product at a temperature of 800-900°C.

13. The method for manufacturing a wire with excellent drawability according to claim 8, characterized in that, The cross-sectional shrinkage rate of the wire after the wire rolling step (c) reaches more than 30%.

14. A tire cord, characterized in that, Includes multiple wires, The maximum thickness of the decarburized layer located at the cross-section of the wire reaches less than 0.08 mm. The wire satisfies the following formula 1: Formula 1: 0.1≤(2×Cr) / Si In Formula 1, Cr represents the chromium content and Si represents the silicon content, both in weight percentage.

15. The tire cord according to claim 14, characterized in that, The wire contains 0.70 to 1.10% by weight of carbon (C), 0.15 to 0.50% by weight of silicon (Si), 0.20 to 0.90% by weight of manganese (Mn), less than 0.015% by weight of phosphorus (P), less than 0.015% by weight of sulfur (S), 0.02 to 0.35% by weight of chromium (Cr), greater than 0 and less than or equal to 0.010% by weight of aluminum (Al), greater than 0 and less than or equal to 0.005% by weight of nitrogen (N), and the balance being iron (Fe) and other unavoidable impurities.

16. The tire cord according to claim 15, characterized in that, The wire also contains 0 to 0.2% by weight of copper (Cu) and 0 to 0.15% by weight of nickel (Ni).

17. The tire cord according to claim 14, characterized in that, The maximum thickness of the decarburized layer is less than 0.05 mm.

18. The tire cord according to claim 14, characterized in that, The tensile strength reaches 3.2 GPa or higher.

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