Wire rod, steel wire and method for manufacturing the same

Abstract

This invention discloses wire, steel wire, and a method for manufacturing the same. The wire according to the invention comprises, by weight %: C: 0.60~1.00%, Si: 0.1~0.4%, Mn: 0.2~0.6%, Cr: less than 0.3% (inclusive), and the balance Fe and other unavoidable impurities, having a fine microstructure comprising pearlite, wherein the average interlamellar spacing L (in μm) of the pearlite is controlled within a specific range with respect to carbon equivalent (Ceq).

Description

Technical Field

[0001] This invention relates to steel wires used in tire steel cords, saw wires, etc., and methods for manufacturing the same.

[0002] Furthermore, the present invention relates to wire rods for manufacturing steel wire and methods thereof. More specifically, the present invention relates to a wire rod and method thereof, which, by controlling the subcooling according to the carbon equivalent, eliminates the need for isothermal transformation heat treatment during the manufacturing of steel wire. Background Technology

[0003] Tire steel cord uses high-carbon steel with a carbon content of approximately 0.6 to 1.0% by weight. For conventional steel cord materials, controlling the carbon content to a single pearlitic phase as much as possible helps ensure workability during wire drawing. During wire drawing, the pearlitic structure rotates in the drawing direction, and its layered ferrite and cementite align along the drawing direction. As wire drawing progresses, the interlamellar spacing decreases, and due to work hardening caused by the dense accumulation of dislocations at the ferrite / cementite interface, the tensile strength continuously increases.

[0004] However, the amount of material that steel can withstand during continuous wire drawing is limited. Therefore, when additional wire drawing is required to further reduce the wire diameter beyond the steel's wire drawing limit, isothermal transformation heat treatment must be performed to eliminate the previously accumulated material. This isothermal transformation heat treatment is also known as LP (lead bath hardening).

[0005] In traditional processes, for steel cord products, after steel mills produce wire with a diameter of 5.5mm, wire drawing companies draw it to a diameter close to 3mm and then perform a first isothermal transformation heat treatment. Subsequently, it undergoes a second wire drawing process to a diameter close to 1.3mm, followed by a second isothermal transformation heat treatment.

[0006] In the traditional steel cord manufacturing process, in order to omit one isothermal transformation heat treatment, the pearlite lamellar spacing must be expanded as much as possible to reduce tensile strength, thereby increasing the amount of work hardening that can be tolerated and reducing the work hardening rate.

[0007] The lamellar spacing of pearlite depends on the transformation temperature. However, in steel mills, the microstructure is controlled by continuous cooling after wire rod rolling. In this case, the lamellar spacing of pearlite is inconsistent due to the formation of pearlite at various temperatures through continuous cooling, and the mechanical properties also change accordingly. Summary of the Invention

[0008] Technical issues

[0009] The technical problem to be solved by the present invention is to provide a wire and a method for manufacturing the same, wherein the wire has excellent drawing properties, thereby eliminating the need for intermediate isothermal heat treatment.

[0010] Furthermore, the technical problem to be solved by the present invention is to provide a steel wire utilizing the aforementioned wire and a method for manufacturing the same.

[0011] Technical solution

[0012] The wire according to an embodiment of the present invention for solving the aforementioned technical problem comprises, by weight %: C: 0.60~1.00%, Si: 0.1~0.4%, Mn: 0.2~0.6%, Cr: less than 0.3% (inclusive), and the balance Fe and other unavoidable impurities, having a fine structure comprising pearlite, wherein the average interlamellar spacing L (in μm) of the pearlite is related to the carbon equivalent (Ceq) according to Equation 1 below, and satisfies Equation 2-1 below.

[0013] [Formula 1]

[0014] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0015] In Equation 1, [ ] represents the weight of the corresponding component.

[0016] [Equation 2-1]

[0017] 0.54-0.35×Ceq ≤ L

[0018] The average interlamellar spacing L (unit: μm) of the pearlite is related to the carbon equivalent (Ceq) according to Equation 1, and can further satisfy Equation 2-2 as follows.

[0019] [Equation 2-2]

[0020] L ≤ 0.64-0.35×Ceq

[0021] The wire may have a fine structure containing more than 98% pearlite in terms of area ratio.

[0022] The Ceq value can be 0.77 to 1.06.

[0023] A wire manufacturing method according to an embodiment of the present invention for solving the aforementioned technical problem includes: a step of heating steel, wherein the steel comprises, by weight %, 0.60-1.00% C, 0.1-0.4% Si, 0.2-0.6% Mn, 0.3% or less (inclusive) Cr, and the balance being Fe and other unavoidable impurities; a step of rolling the steel into wire and then coiling it; and a cooling step of cooling the wire to generate a fine microstructure containing pearlite. The cooling step is controlled such that the transformation undercooling ΔTs (°C) is related to the carbon equivalent (Ceq) according to Equation 1 below, satisfying Equation 3-1 below, wherein the transformation undercooling ΔTs (°C) is defined as the difference between the equilibrium transformation temperature (i.e., the temperature at which the austenite free energy equals the pearlite free energy during the austenite-to-pearlite transformation) and the actual pearlite transformation initiation temperature during cooling.

[0024] [Formula 1]

[0025] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0026] In Equation 1, [ ] represents the weight of the corresponding component.

[0027] [Equation 3-1]

[0028] ΔTs ≤ 64.1×Ceq+6

[0029] The cooling step is controlled such that the degree of supercooling is related to the carbon equivalent (Ceq) according to Equation 1, and further satisfies Equation 3-2 as follows.

[0030] [Equation 3-2]

[0031] 64.1 × Ceq⁻¹⁴ ≤ ΔTs

[0032] The heating can be performed at 1000~1250℃ for 60~120 minutes, and the winding can be performed at 800~900℃.

[0033] According to an embodiment of the present invention, a steel wire for solving the aforementioned technical problem has a wire diameter of less than 2 mm. The steel wire, by weight percent, contains C: 0.60~1.00%, Si: 0.1~0.4%, Mn: 0.2~0.6%, Cr: less than 0.3% (inclusive), and the balance Fe and other unavoidable impurities. It has a fine microstructure comprising pearlite, the average interlamellar spacing L (in μm) of which is related to the carbon equivalent (Ceq) according to Equation 1 below, satisfying Equation 2-1 below.

[0034] [Formula 1]

[0035] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0036] In Equation 1, [ ] represents the weight of the corresponding component.

[0037] [Equation 2-1]

[0038] 0.54-0.35×Ceq ≤ L

[0039] The average interlamellar spacing L (unit: μm) of the pearlite is related to the carbon equivalent (Ceq) according to Equation 1, and can further satisfy Equation 2-2 as follows.

[0040] [Equation 2-2]

[0041] L ≤ 0.64-0.35×Ceq

[0042] The steel wire may contain more than 98% pearlite by area ratio.

[0043] The Ceq value can be 0.77 or higher, 0.80 or higher, 0.85 or higher, or 0.90 or higher, and can be 1.06 or lower, 1.00 or lower, or 0.95 or lower.

[0044] A method for manufacturing steel wire according to an embodiment of the present invention for solving the aforementioned technical problem includes a step of drawing a wire. The wire, by weight percent, contains C: 0.60~1.00%, Si: 0.1~0.4%, Mn: 0.2~0.6%, Cr: less than 0.3% (inclusive), and the balance Fe and other unavoidable impurities. It has a fine structure comprising pearlite, wherein the average interlamellar spacing L (in μm) of the pearlite is related to the carbon equivalent (Ceq) according to Equation 1 below, and satisfies Equation 2-1 below.

[0045] [Formula 1]

[0046] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0047] In Equation 1, [ ] represents the weight of the corresponding component.

[0048] [Equation 2-1]

[0049] 0.54-0.35×Ceq ≤ L

[0050] The average interlamellar spacing L (unit: μm) of the pearlite in the wire is related to the carbon equivalent (Ceq) according to Equation 1, and can further satisfy Equation 2-2 as follows.

[0051] [Equation 2-2]

[0052] L ≤ 0.64-0.35×Ceq

[0053] The wire may have a fine structure containing more than 98% pearlite in terms of area ratio.

[0054] The Ceq value can be 0.77 or higher, 0.80 or higher, 0.85 or higher, or 0.90 or higher, and can be 1.06 or lower, 1.00 or lower, or 0.95 or lower.

[0055] The wire drawing process can be carried out under the condition that the cumulative strain (ε) is 2.8 or more according to Equation 4 below.

[0056] [Formula 4]

[0057] ε = 2 × ln(di / df)

[0058] In Equation 4, di is the diameter before wire drawing, and df is the diameter after wire drawing.

[0059] Beneficial effects

[0060] According to the present invention, when a wire with a diameter of about 4 to 6 mm is hot-rolled and then wound, and during cooling, if the undercooling is controlled such that the undercooling degree of the austenite-to-pearlite transformation according to the carbon equivalent satisfies Equation 3-1, a wire with excellent drawing processability can be manufactured. The results show that it is possible to directly draw the wire to a diameter of about 1.3 mm without intermediate isothermal transformation heat treatment.

[0061] The results show that the average interlamellar spacing and pearlite content of Equation 2-1 with respect to carbon equivalent can be ensured, thereby enabling direct drawing from wire diameters of approximately 4-6 mm to approximately 1.1-1.3 mm. This eliminates the need for one of the two isothermal transformation heat treatments typically required, and allows for the manufacture of steel wires for tire cords or saw wires, etc.

[0062] In addition to the effects described above, the specific effects of the present invention will be explained in conjunction with the following description of the specific embodiments of the present invention. Attached Figure Description

[0063] Figure 1 This is a schematic diagram of the wire manufacturing method according to the present invention.

[0064] Figure 2 This is a schematic diagram illustrating the concept of changing the degree of subcooling (ΔTs).

[0065] Figure 3 The transformation undercooling of the example samples and comparative sample at different carbon equivalents is shown.

[0066] Figure 4The average lamellar spacing of pearlite in the example samples and comparative sample samples at different carbon equivalents is shown.

[0067] Figure 5 The tensile strength of the wires of the example specimens and comparative specimens at different carbon equivalents is shown. Detailed Implementation

[0068] The advantages, features, and methods of achieving these advantages and features of the present invention will become clearer upon referring to the following detailed description of the embodiments. However, the present invention is not limited to the embodiments disclosed below and can be implemented in many different ways. These embodiments are merely intended to complete the disclosure of the present invention and to fully inform those skilled in the art of the scope of the invention, which is ultimately defined by the claims.

[0069] The wire, steel wire and manufacturing method thereof according to preferred embodiments of the present invention will be described in detail below.

[0070] This invention aims to provide a method for manufacturing wire rod using high-carbon steel, comprising, by weight, 0.60-1.00% C, 0.1-0.4% Si, 0.2-0.6% Mn, 0.3% or less (inclusive) Cr, and the balance Fe and other unavoidable impurities. After heating, rolling, and coiling, the steel is subjected to forced-air cooling in a Stellmore cooling process. To omit isothermal transformation heat treatment in the subsequent wire drawing process used to manufacture the wire, it is necessary to withstand a greater amount of drawing than when heat treatment is performed, i.e., cumulative strain (ε) (e.g., ε ≥ 2.8). Therefore, it is necessary to minimize the formation of the proeutectoid phase while ensuring a fine microstructure composed of a high proportion of pearlite. To maximize the average lamellar spacing and suppress the formation of the proeutectoid phase during continuous cooling, the inventors have found that the average transformation undercooling (ΔTs) must be controlled within a certain range to manage the continuous temperature change from the equilibrium transformation temperature, which depends on the carbon equivalent of the steel, to the actual transformation.

[0071] Transformation undercooling refers to the difference between the equilibrium transformation temperature and the actual transformation temperature; it is the activation energy that initiates the transformation mechanism. For austenite and pearlite, their free energy increases with increasing temperature. Pearlite has a lower free energy at low temperatures, while austenite has a lower free energy at high temperatures. Therefore, for austenite existing at high temperatures, if the temperature is lowered, its free energy will be the same as that of pearlite at a specific temperature. This temperature is the equilibrium transformation temperature, which depends on the alloy composition.

[0072] Even if austenite reaches the equilibrium transformation temperature, the activation energy is still required to initiate the transformation. Therefore, the temperature must be lowered further so that the instability of austenite is greater than that of pearlite, thereby inducing the transformation to begin.

[0073] At this point, the degree of supercooling ultimately depends on the cooling rate. The faster the cooling rate, the greater the degree of supercooling, and the lower the actual temperature range where the transformation occurs, thus obtaining a fine interlayer spacing. If the cooling rate is slow, the degree of supercooling decreases, and the interlayer spacing becomes coarser.

[0074] The inventors have confirmed that if the transformation undercooling ΔTs (°C) from the equilibrium transformation temperature, which depends on the alloy composition, to the temperature range where the pearlite transformation actually occurs satisfies Equation 3-1, which is related to the carbon equivalent according to Equation 1, a high proportion (e.g., more than 98% in terms of area ratio) of pearlite structure can be ensured, while maximizing the average lamellar spacing.

[0075] [Formula 1]

[0076] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0077] In Equation 1, [ ] represents the weight of the corresponding component.

[0078] [Equation 3-1]

[0079] ΔTs ≤ 64.1×Ceq+6

[0080] If the undercooling exceeds the range specified in Formula 1, the interlayer spacing will become finer, the tensile strength will increase, and the limit of wire drawing will decrease.

[0081] Furthermore, if the undercooling of the transformation is too low, there is a concern about the formation of a proeutectoid phase. Therefore, it is preferable to control it to further satisfy the following equations 3-2 with respect to the carbon equivalent (Ceq) according to Equation 1.

[0082] [Equation 3-2]

[0083] 64.1 × Ceq⁻¹⁴ ≤ ΔTs

[0084] Equation 3, obtained by combining Equation 3-2 and Equation 3-1, is as follows.

[0085] [Formula 3]

[0086] 64.1×Ceq-14 ≤ ΔTs ≤ 64.1×Ceq+6

[0087] Equation 3 is based on the following Figure 3 The data was obtained through linear regression analysis.

[0088] The average pearlite lamellar spacing L (in μm) of the wire manufactured by cooling within such a supercooling control range is related to the carbon equivalent (Ceq) according to Equation 1 and can satisfy Equation 2-1.

[0089] [Equation 2-1]

[0090] 0.54-0.35×Ceq ≤ L

[0091] In conventional wire manufacturing methods, rapid cooling is achieved by increasing the airflow in a Stellmore cooling system. However, in this case, due to the very high strength of the wire, the maximum drawing capacity is reduced. To increase this capacity, isothermal heat treatment is required during the drawing process. However, for the wire according to the present invention, by controlling the transformation undercooling as shown in Equation 3 above during cooling, the lamellar spacing, which is the pearlite core microstructure factor, can be controlled to satisfy Equation 2-1, i.e., 0.54-0.35×Ceq≤L, ultimately controlling the tensile strength of the wire. In this case, due to the wider lamellar spacing, the tensile strength can also be maintained at a lower level suitable for drawing, resulting in an increased processing limit during subsequent drawing processes and a reduced work hardening rate.

[0092] Furthermore, from the perspective of suppressing the excessive reduction in tensile strength of the wire caused by the proeutectoid phase, the average lamellar spacing L (unit: μm) of the pearlite preferably further satisfies the following equation 2-2. Moreover, if the lamellar spacing becomes too wide, the wire's workability may be significantly reduced; from the perspective of suppressing this, the average lamellar spacing L of the pearlite more preferably further satisfies the following equation 2-2.

[0093] [Equation 2-2]

[0094] L ≤ 0.64-0.35×Ceq

[0095] Equation 2, obtained by combining Equation 2-2 and Equation 2-1, is as follows.

[0096] [Equation 2]

[0097] 0.54-0.35×Ceq ≤ L ≤ 0.64-0.35×Ceq

[0098] Equation 2 is based on the following Figure 4 The data was obtained through linear regression analysis.

[0099] The wire according to the present invention contains, by weight %: C: 0.60~1.00%, Si: 0.1~0.4%, Mn: 0.2~0.6%, Cr: less than 0.3% (inclusive), and the balance Fe and other unavoidable impurities. Unavoidable impurities consist of various impurities unavoidably present in processes such as steelmaking. For example, phosphorus (P) and sulfur (S) are impurities that cannot be completely removed during steelmaking; the lower their content, the higher the cleanliness and processability. However, considering economic factors, they can be controlled to less than 0.03% respectively.

[0100] In the wire according to the invention, according to Formula 1, Ceq depends on the content of C, Mn, Si, and Cr. More specifically, the Ceq of the wire according to the invention can be 0.77 to 1.06. For example, for the wire according to the invention, its Ceq can be 0.77 or more, 0.80 or more, 0.85 or more, or 0.90 or more, and can be 1.06 or less, 1.00 or less, or 0.95 or less.

[0101] The wire according to the invention has a fine structure comprising pearlite. Preferably, the pearlite comprises more than 98% by area.

[0102] The following describes in detail the content of each component in the hot-rolled steel plate according to the present invention and the reasons for its addition.

[0103] [C: 0.60~1.00% by weight]

[0104] Carbon (C) is the core alloying element that forms cementite during the pearlite transformation. The interlamellar spacing and the equilibrium transformation temperature of pearlite vary with the C content. If the carbon content is less than 0.6%, the proeutectoid ferrite fraction becomes high, making it difficult to ensure wire drawing workability. On the other hand, if it exceeds 1.00%, the formation and segregation of the proeutectoid cementite phase impair the microstructure integrity of the wire's center, similarly worsening the wire drawing workability.

[0105] Therefore, the C content is preferably 0.60 to 1.00% by weight, and more preferably 0.70 to 0.90% by weight, from the perspective of avoiding the above-mentioned disadvantages.

[0106] [Si: 0.1~0.4% by weight]

[0107] During the pearlite transformation, silicon (Si) is mostly dissolved in ferrite and hardly distributes into cementite. Furthermore, its diffusion rate is slower than that of carbon (C). If a large amount of Si is dissolved in ferrite, the overall pearlite transformation slows down. As an element that causes solid solution strengthening when dissolved in ferrite, if its concentration is less than 0.1%, the strengthening effect is insufficient, making it difficult to achieve high strength in the final product; if it exceeds 0.4%, the ferrite becomes over-hardened, leading to decreased wire drawing processability.

[0108] Therefore, the Si content is preferably 0.1 to 0.4% by weight, and more preferably 0.2 to 0.3% by weight to avoid the above-mentioned disadvantages.

[0109] [Mn: 0.2~0.6% by weight]

[0110] During isothermal transformation, Mn has no effect on the change in tensile strength of pearlite. However, under continuous cooling conditions, it can alter the hardenability of the steel with limited cooling capacity, thereby changing the transformation initiation temperature and potentially affecting the change in tensile strength. If the Mn content is less than 0.2%, it is difficult to expect an appropriate level of hardenability improvement; if it exceeds 0.6%, due to its high carbon content, it will undergo central segregation along with C, increasing the risk of martensite defects in the central region.

[0111] Therefore, the Mn content is preferably 0.2 to 0.6% by weight, and more preferably 0.3 to 0.5% by weight to avoid the above-mentioned disadvantages.

[0112] [Cr: less than 0.3% by weight]

[0113] While Cr is not an essential component in this invention, by separating the pearlite and bainite formation temperature ranges in the TTT curve during isothermal transformation, isothermal transformation can be achieved at lower temperatures, ultimately resulting in a higher strength final product. As the C content increases, a small amount of C is added to replace Mn, which carries a risk of central segregation, or Si, which acts as a ferrite hardening element, to obtain appropriate hardenability. If the Cr content exceeds 0.3%, the effect on increasing the strength of the final product in steel wires such as steel cord is not significant, therefore excessive addition is unnecessary.

[0114] Therefore, the Cr content is preferably 0.3% by weight or less, more preferably 0.1 to 0.2% by weight.

[0115] Wire manufacturing method

[0116] According to the wire rod manufacturing method of the present invention, the method comprises the following steps: preparing steel, which, by weight percent, contains C: 0.60~1.00%, Si: 0.1~0.4%, Mn: 0.2~0.6%, Cr: less than 0.3% (inclusive), and the balance Fe and other unavoidable impurities; heating, wire rod rolling, coiling, and cooling to generate a fine structure containing pearlite. The steel used for manufacturing wire rod can be obtained by casting steel refined in a converter into large square billets and then rolling the billets, or by directly casting into continuously cast small square billets, for example, with a cross-sectional area of ​​160×160mm. 2 Small square billets.

[0117] Figure 1 This is a schematic diagram of the wire manufacturing method according to the present invention.

[0118] Reference Figure 1 The wire manufacturing method according to the present invention includes a heating step S110, a rolling and winding step S120, and a cooling step S130.

[0119] The heating can be performed at 1000~1250℃ for 60~120 minutes. This heating step brings the wire to a rollable state. If the heating temperature is too low, wire rolling may become difficult; if the heating temperature is too high, excessive austenite growth may occur.

[0120] The wire rod rolling is carried out using conventional hot-rolled wire rod rolling methods, which can produce wire rods with a diameter of approximately 4 to 6 mm.

[0121] In steel cord drawing plants, to produce environmentally friendly products, oxide scale is removed through mechanical stripping. The appropriate oxide scale thickness varies depending on the mechanical stripping method, but the average is 10 μm. If the oxide scale thickness is less than this value, mechanical stripping is insufficient, leaving residue on the wire surface. This can lead to die breakage during drawing, resulting in surface scratches and other defects. If the oxide scale thickness is too thick, oxide scale loss increases, thus reducing productivity. The oxide scale thickness is mainly affected by the winding temperature. Below 800℃, the oxide scale thickness is too thin; above 900℃, the oxide scale thickness is too thick. Therefore, the winding temperature is preferably controlled between 800 and 900℃.

[0122] The mechanical properties of pearlite are governed by the lamellar spacing, which is affected by undercooling. Undercooling is the difference between the equilibrium transformation temperature and the actual transformation temperature. If the cooling rate is too high, the transformation undercooling increases; if the cooling rate is slow, the transformation undercooling decreases. If it is too slow, proeutectoid ferrite or proeutectoid cementite will be produced depending on the carbon content, resulting in reduced wire drawing processability.

[0123] In the wire manufacturing method according to the present invention, as previously described, when the cooling step controls the transformation undercooling according to the carbon equivalent shown in Formula 1 to satisfy Formula 3-1, the microstructure can be controlled regardless of the alloy composition.

[0124] The manufactured wires can have a fine structure containing more than 98% pearlite, and the tensile strength can be between 800 and 1250 MPa.

[0125] By drawing the wire manufactured using the method described above, steel wires with a diameter of about 2 mm or less can be produced.

[0126] From the perspective of omitting the isothermal transformation heat treatment in the wire drawing process, the wire drawing process is preferably carried out under the condition that the cumulative strain (ε) is 2.8 or more according to the following formula 4, and more preferably under the condition that the cumulative strain is 2.8 or more and 3.0 or less.

[0127] [Formula 4]

[0128] ε = 2 × ln(di / df)

[0129] In Equation 4, di is the diameter before wire drawing, and df is the diameter after wire drawing.

[0130] In this invention, it has been confirmed that even when wire drawing is performed under such high cumulative strain, delamination will not occur, and therefore the isothermal transformation heat treatment in the wire drawing process can be omitted.

[0131] Example

[0132] The invention will be described in more detail below through embodiments. However, it should be noted that the following embodiments are only used to illustrate the invention by way of example and are not intended to limit the scope of the claims.

[0133] Steel with the composition shown in Table 1 was smelted in a converter, then cast into large square billets, and then rolled to produce steel with a cross-sectional area of ​​160×160mm. 2 Small square billets.

[0134] Subsequently, the wire was held at approximately 1100°C in a heating furnace for 90 minutes and rolled to a diameter of 5.5 mm under standard conditions. As shown in Table 1, the winding temperature was varied and controlled within the range of 780~930°C. The air volume was then adjusted in the Steyrmo cooling process to control the transition supercooling ΔTs (°C) to satisfy Equation 3.

[0135] [Formula 3]

[0136] 64.1×Ceq-14 ≤ ΔTs ≤ 64.1×Ceq+6

[0137] Table 1 lists the alloy composition and carbon equivalent of the steels used in the example samples and comparative sample samples. Table 2 lists the supercooling, average lamellar spacing and tensile strength of these steels when manufacturing wires, and lists them together with the comparative sample samples.

[0138] The carbon equivalent is calculated according to Equation 1 below.

[0139] [Formula 1]

[0140] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0141] In Equation 1, [ ] represents the weight of the corresponding component.

[0142] like Figure 2As shown, the transformation undercooling is denoted as the difference between the equilibrium transformation temperature and the actual pearlite transformation start temperature during continuous cooling. The equilibrium transformation temperature is calculated thermodynamically based on the alloy composition of the corresponding steel. The pearlite transformation start temperature is set as the lowest value of the first rise after the measured temperature decrease, based on the actual material temperature measured during the Stellmore cooling process. Typically, the actual pearlite transformation begins at a temperature slightly above the lowest measured temperature, but this is affected by cooling conditions / rates and therefore cannot be measured on wire coils in actual production. Therefore, the easily measurable temperature, i.e., the lowest point of the curve after the temperature decreases, is set as the transformation start temperature.

[0143] In addition, the average interlamellar spacing is the average value obtained by imaging the microstructure of the manufactured wire sample and measuring the spacing between the pearlite lamellae.

[0144] Tensile strength was measured using a tensile testing machine (Zwick Z250).

[0145] [Table 1]

[0146]

[0147] [Table 2]

[0148]

[0149] ※In Table 2 above, Equation 2-1 is 0.54-0.35×Ceq, and Equation 2-2 is 0.64-0.35×Ceq.

[0150] Referring to Table 1, there were no significant differences between Example Samples 1-24 and Comparative Samples 1-10 in terms of alloy composition or carbon equivalent. The difference between the Example Samples and the Comparative Samples lies in whether the transformation undercooling was controlled by adjusting the air volume in the Stellmore cooling process after the wire was rolled and coiled. In particular, for Example Samples 1-24, the transformation undercooling was controlled according to the carbon equivalent of the wire to satisfy the aforementioned Equation 3-1.

[0151] Figure 3 The transformation undercooling of the example sample (inventive material) and the comparative sample (comparative material) at different carbon equivalents is shown.

[0152] Refer to Table 2 and Figure 3 Regarding the transformation undercooling of the example and comparative samples at different carbon equivalents, it can be seen that the transformation undercooling increases with increasing carbon equivalent. However, for the example samples, the transformation undercooling is controlled within the range satisfying Equation 3-1, i.e., ΔTs ≤ 64.1 × Ceq + 6. For the comparative samples, a conventional tire steel cord cooling process with relatively high airflow in the Steyrmore cooling system was used.

[0153] Furthermore, for the example samples, the transformation undercooling was controlled within the range satisfying Equation 3-2, i.e., 64.1 × Ceq-14 ≤ ΔTs. When the transformation undercooling based on carbon equivalent is too low, it is difficult to suppress the formation of proeutectoid cementite due to the high carbon content. As a result, although the tensile strength decreases, the proeutectoid cementite causes stress concentration during wire drawing, making it difficult to ensure wire drawing processability. Therefore, the transformation undercooling for the example samples was controlled within the range satisfying Equation 3-2.

[0154] By controlling the undercooling in this way, the lamellar spacing, which is the core microstructure factor of pearlite, can be controlled, and ultimately the tensile strength of the wire can be controlled.

[0155] For the comparative example specimens, the airflow in the Stellmore cooling system was relatively high. Therefore, the cooling rate increased, and as the transition began at a lower temperature beyond the thermodynamic equilibrium temperature, the degree of supercooling increased, resulting in finer interlamellar spacing and increased tensile strength.

[0156] Figure 4 The average lamellar spacing of pearlite is shown for example samples (inventive materials) and comparative sample samples (comparative materials) at different carbon equivalents. Figure 5 The tensile strength of the wires of the example specimens (inventive material) and the comparative specimens (comparative material) at different carbon equivalents is shown.

[0157] Refer to Table 2. Figure 4 and Figure 5 It can be seen that there is a significant difference in the average interlamellar spacing based on carbon equivalent between the example samples and the comparative sample. For the example samples, it can be seen that when the interlamellar spacing L of the pearlite is measured in micrometers, the average interlamellar spacing of the pearlite is controlled to satisfy Equation 2-1 with respect to carbon equivalent, i.e., 0.54-0.35×Ceq≤L. Figure 5 As shown, compared to the comparative example, the wider interlamellar spacing ensures a lower tensile strength, making it suitable for wire drawing. For the example specimens, the tensile strength can be controlled to be lower, thereby increasing the processing limit during subsequent wire drawing and reducing the work hardening rate.

[0158] Furthermore, for the example samples, the average lamellar spacing L of the pearlite was controlled to further satisfy Equation 2-2 with respect to carbon equivalent, i.e., L ≤ 0.64-0.35 × Ceq. If the lamellar spacing becomes too wide, the dislocation slip distance within the ferrite increases during wire drawing, leading to more dislocations concentrating at the interface, potentially causing cracks and thus reducing the maximum wire drawing allowance. Considering this, satisfying the lamellar spacing according to Equation 2-2 is more preferable.

[0159] The strain in each pass of the test specimens of the embodiment and the comparative example was controlled at about 0.2 using a wire drawing machine, and the ultimate wire drawing area reduction, i.e., the strain, was measured. The ultimate strain was judged by whether delamination occurred in the torsion test. If delamination did not occur in 3 tests, it was judged as qualified, and as long as delamination occurred even once, it was judged as unqualified.

[0160] For the cumulative strain, it was calculated according to Equation 4.

[0161] [Equation 4]

[0162] ε = 2×ln(di / df)

[0163] In the said Equation 4, di is the diameter before wire drawing, and df is the diameter after wire drawing.

[0164] In Table 3 below, the wire diameter change according to wire drawing and the cumulative strain according to Equation 4 are listed, and based on this, it was evaluated whether delamination occurred during the torsion test after wire drawing.

[0165] [Table 3]

[0166]

[0167] Referring to Table 3, when wire drawing with a total strain of 2.2 was adopted, it could be seen that delamination did not occur in both the test specimens of the embodiment and the comparative example during the torsion test.

[0168] However, when wire drawing with a total strain of 2.4 was adopted, it could be seen that delamination occurred in a part of the comparative example test specimens during the torsion test, and when wire drawing with a total strain of less than 2.8 was adopted, it could be seen that delamination occurred in all the comparative example test specimens during the torsion test. These results indicate that for the comparative example test specimens, based on the initial wire diameter of 5.5 mm, although wire drawing could be performed to the final wire diameter of 1.8 mm level without delamination, if the final wire diameter was less than this value, isothermal transformation heat treatment could not be omitted.

[0169] In contrast, it could be seen that delamination did not occur in the test specimens of the embodiment even when wire drawing with a total strain reaching 2.8 was adopted. It could be seen that delamination did not occur in some test specimens of the embodiment even when wire drawing with a total strain of 3.0 was adopted.

[0170] From these results in Table 3, it could be seen that compared with the comparative example test specimens, the test specimens of the embodiment manufactured by controlling the degree of undercooling designed by the present invention could be wire drawn with a larger total strain application amount. In addition, for the test specimens of the embodiment, based on the initial wire diameter of 5.5 mm, it could be seen that wire drawing could be performed to the final wire diameter of about 1.30 mm without isothermal transformation heat treatment.

[0171] Therefore, it can be concluded that additional isothermal transformation heat treatment can be omitted when wires manufactured by the method according to the invention, which includes control of transformation undercooling, are drawn under conditions of cumulative strain of 2.8 or higher.

[0172] While embodiments of the present invention have been described above, the present invention is not limited to the embodiments disclosed in this specification. It is evident that those skilled in the art can make various modifications within the scope of the technical concept of the present invention. Furthermore, although the effects of the structure according to the present invention have not been explicitly stated or explained in the foregoing description of the embodiments, the effects that can be expected from this structure should also be acknowledged.

Claims

1. A wire, wherein, The wire, by weight percent, contains C: 0.60~1.00%, Si: 0.1~0.4%, Mn: 0.2~0.6%, Cr: less than 0.3% (inclusive), and the balance Fe and other unavoidable impurities, and has a fine structure comprising pearlite, wherein the average interlamellar spacing L (in μm) of the pearlite is related to the carbon equivalent (Ceq) according to Equation 1 below, and satisfies Equation 2-1 below. [Formula 1] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5 In Equation 1, [ ] represents the weight percentage of the corresponding component. [Equation 2-1] 0.54-0.35×Ceq ≤ L.

2. The wire according to claim 1, wherein, The average interlamellar spacing L (unit: μm) of the pearlite is related to the carbon equivalent (Ceq) according to Equation 1, and further satisfies Equation 2-2 as follows. [Equation 2-2] L ≤ 0.64-0.35×Ceq.

3. The wire according to claim 1 or 2, wherein, The wire has a fine structure containing more than 98% pearlite in terms of area ratio.

4. The wire according to claim 1 or 2, wherein, The Ceq value is 0.77~1.

06.

5. A method for manufacturing wire, comprising: The step of heating the steel, by weight %, the steel contains C: 0.60~1.00%, Si: 0.1~0.4%, Mn: 0.2~0.6%, Cr: less than 0.3% (inclusive) and the balance Fe and other unavoidable impurities; The steps of rolling the steel into wire and then coiling it; and A cooling step to cool the wire to generate a fine structure containing pearlite. The cooling step is controlled such that the degree of supercooling ΔTs (°C) is related to the carbon equivalent (Ceq) according to Equation 1 below, and satisfies Equation 3-1 below, where, The transformation undercooling ΔTs (°C) is defined as the temperature at which the free energy of austenite equals that of pearlite during the transformation from austenite to pearlite, i.e., the equilibrium transformation temperature, and the difference between the actual pearlite transformation initiation temperature during cooling. [Formula 1] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5 In Equation 1, [ ] represents the weight percentage of the corresponding component. [Equation 3-1] ΔTs ≤ 64.1×Ceq+6.

6. The wire manufacturing method according to claim 5, wherein, The cooling step is controlled such that the degree of transformation undercooling is related to the carbon equivalent (Ceq) according to Equation 1, and further satisfies Equation 3-2 as follows. [Equation 3-2] 64.1×Ceq-14 ≤ ΔTs.

7. The wire manufacturing method according to claim 5 or 6, wherein, The heating is performed at 1000~1250℃ for 60~120 minutes. The winding is performed at 800~900°C.

8. A type of steel wire, wherein, The steel wire has a diameter of less than 2 mm. The steel wire, by weight percent, contains C: 0.60~1.00%, Si: 0.1~0.4%, Mn: 0.2~0.6%, Cr: less than 0.3% (inclusive), and the balance Fe and other unavoidable impurities. It has a fine structure containing pearlite. The average interlamellar spacing L (unit: μm) of the pearlite is related to the carbon equivalent (Ceq) according to Equation 1 below, and satisfies Equation 2-1 below. [Formula 1] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5 In Equation 1, [ ] represents the weight percentage of the corresponding component. [Equation 2-1] 0.54-0.35×Ceq ≤ L.

9. The steel wire according to claim 8, wherein, The average interlamellar spacing L (unit: μm) of the pearlite is related to the carbon equivalent (Ceq) according to Equation 1, and further satisfies Equation 2-2 as follows. [Equation 2-2] L ≤ 0.64-0.35×Ceq.

10. The steel wire according to claim 8 or 9, wherein, The steel wire contains more than 98% pearlite by area ratio.

11. The steel wire according to claim 8 or 9, wherein, The Ceq value is 0.77~1.

06.

12. A method for manufacturing steel wire, comprising: The wire drawing process, by weight %, comprises C: 0.60~1.00%, Si: 0.1~0.4%, Mn: 0.2~0.6%, Cr: less than 0.3% (inclusive), and the balance Fe and other unavoidable impurities, having a fine structure containing pearlite, wherein the average interlamellar spacing L (in μm) of the pearlite is related to the carbon equivalent (Ceq) according to Equation 1 below, and satisfies Equation 2-1 below. [Formula 1] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5 In Equation 1, [ ] represents the weight percentage of the corresponding component. [Equation 2-1] 0.54-0.35×Ceq ≤ L.

13. The method for manufacturing steel wire according to claim 12, wherein, The average interlamellar spacing L (unit: μm) of the pearlite in the wire is related to the carbon equivalent (Ceq) according to Equation 1, and further satisfies Equation 2-2 as follows. [Equation 2-2] L ≤ 0.64-0.35×Ceq.

14. The method for manufacturing steel wire according to claim 12 or 13, wherein, The wire has a fine structure containing more than 98% pearlite in terms of area ratio.

15. The method for manufacturing steel wire according to claim 12 or 13, wherein, The Ceq value is 0.77~1.

06.

16. The method for manufacturing steel wire according to claim 12 or 13, wherein, The wire drawing process is performed under the condition that the cumulative strain (ε) is 2.8 or higher according to Equation 4 below. [Formula 4] ε = 2 × ln(di / df) In Equation 4, di is the diameter before wire drawing, and df is the diameter after wire drawing.

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