Steel wire rod, steel wire, and methods of manufacturing same

Abstract

Disclosed are a steel wire rod, a steel wire, and methods of manufacturing same. The steel wire rod according to the present invention contains: 0.60-1.00% of C, 0.1-0.4% of Si, 0.2-0.6% of Mn, 0.3% or less (including 0%) of Cr, and the balance being Fe and other inevitable impurities, with a microstructure including pearlite, and has an average lamellar spacing L (unit: ㎛) controlled to fall within a specific range with respect to the carbon equivalent (Ceq).

Description

Wire rods, steel wires and methods for manufacturing them

[0001] The present invention relates to a steel wire used in tire steel cords, saw wires, etc., and a method for manufacturing the same.

[0002] The present invention also relates to a wire rod for manufacturing steel wire and a method for manufacturing the same. More specifically, the present invention relates to a wire rod and a method for manufacturing the same, which can omit isothermal transformation heat treatment during steel wire manufacturing by controlling the degree of supercooling according to carbon equivalent.

[0003] Tire steel cord uses high-carbon steel with a carbon content of approximately 0.6 to 1.0 wt%. For conventional steel cord, controlling the steel to a single pearlite phase as much as possible is advantageous for securing drawability. The pearlite structure rotates in the drawing direction during drawing, and the ferrite and cementite, which are composed of layers of pearlite, are aligned in the drawing direction. As drawing progresses, the interlamellar spacing decreases, and the tensile strength continuously increases due to work hardening, which causes a large number of dislocations to cluster at the ferrite / cementite interface.

[0004] However, when continuous drawing is performed, there is a limit to the amount of processing that the steel can withstand. Therefore, if additional drawing is required to reduce the wire diameter beyond the drawing limit of the steel, an isothermal transformation heat treatment must be performed to release the accumulated processing amount. This isothermal transformation heat treatment is also called LP (Lead Patenting).

[0005] Traditionally, for steel cord products, steel mills produced 5.5mm diameter wire rods. Wire drawing companies would then draw the wire to a diameter of approximately 3mm and perform a primary isothermal transformation heat treatment. This was then further drawn to a diameter of approximately 1.3mm and a secondary isothermal transformation heat treatment was performed.

[0006] In order to omit one isothermal transformation heat treatment in the conventional steel cord manufacturing process, the lamellar layer spacing must be widened as much as possible to lower the tensile strength, increase the amount of work hardening that can be accommodated, and reduce the work hardening rate.

[0007] The lamellar spacing of pearlite is determined by the transformation temperature. However, in steel mills, the microstructure is controlled through continuous cooling after wire rolling. In this case, pearlite is formed at various temperatures through continuous cooling, so the lamellar spacing of pearlite is not constant, and thus the mechanical properties also vary.

[0008] The problem to be solved by the present invention is to provide a wire rod having excellent fresh processability and thus enabling the omission of intermediate constant temperature heat treatment, and a method for manufacturing the same.

[0009] In addition, the problem to be solved by the present invention is to provide a steel wire using the above-mentioned wire and a method for manufacturing the same.

[0010] According to an embodiment of the present invention for solving the above problem, a wire includes C: 0.60 to 1.00% by weight, Si: 0.1 to 0.4%, Mn: 0.2 to 0.6%, and Cr: 0.3% or less (including 0%), the remainder being Fe and other unavoidable impurities, and has a microstructure including pearlite, and the average lamellar spacing L (unit: ㎛) of the pearlite satisfies the following equation 2-1 in relation to the carbon equivalent (Ceq) according to the following equation 1.

[0011] [Formula 1]

[0012] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0013] (In the above formula 1, [ ] is the weight% of the corresponding component.

[0014] [Formula 2-1]

[0015] 0.54-0.35×Ceq ≤ L

[0016] The average lamellar spacing L (unit: ㎛) of the above pearlite can additionally satisfy the following equation 2-2 in relation to the carbon equivalent (Ceq) according to the above equation 1.

[0017] [Formula 2-2]

[0018] L ≤ 0.64-0.35×Ceq

[0019] The above-mentioned wire may have a microstructure containing pearlite at an area ratio of 98% or more.

[0020] The above Ceq may be 0.77 to 1.06.

[0021] According to an embodiment of the present invention for solving the above problem, a method for manufacturing a wire rod includes the steps of heating a steel material including, in wt%, C: 0.60 to 1.00%, Si: 0.1 to 0.4%, Mn: 0.2 to 0.6%, and Cr: 0.3% or less (including 0%), the remainder being Fe and other unavoidable impurities; rolling the steel material into a wire rod and then coiling it; and cooling the wire rod to generate a microstructure including pearlite, wherein the cooling step controls a transformation undercooling ΔTs (℃), which is defined as the difference between an equilibrium transformation temperature at which the free energy of austenite and the free energy of pearlite are equal in austenite to pearlite transformation, and an actual pearlite transformation initiation temperature upon cooling, in relation to a carbon equivalent (Ceq) according to the following Equation 1, so as to satisfy the following Equation 3-1.

[0022] [Formula 1]

[0023] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0024] (In the above formula 1, [ ] is the weight% of the corresponding component.

[0025] [Formula 3-1]

[0026] ΔTs ≤ 64.1×Ceq+6

[0027] The above cooling step controls the transformation supercooling so as to additionally satisfy the following equation 3-2 in relation to the carbon equivalent (Ceq) according to the above equation 1.

[0028] [Formula 3-2]

[0029] 64.1×Ceq-14 ≤ ΔTs

[0030] The above heating is performed at 1000 to 1250°C for 60 to 120 minutes, and the above coiling can be performed at 800 to 900°C.

[0031] According to an embodiment of the present invention for solving the above problem, a steel wire has a wire diameter of 2 mm or less, contains C: 0.60 to 1.00% by weight, Si: 0.1 to 0.4%, Mn: 0.2 to 0.6%, and Cr: 0.3% or less (including 0%), the remainder being Fe and other unavoidable impurities, and has a microstructure including pearlite, and the average lamellar spacing L (unit: ㎛) of the pearlite satisfies the following equation 2-1 in relation to the carbon equivalent (Ceq) according to the following equation 1.

[0032] [Formula 1]

[0033] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0034] (In the above formula 1, [ ] is the weight% of the corresponding component.

[0035] [Formula 2-1]

[0036] 0.54-0.35×Ceq ≤ L

[0037] The average lamellar spacing L (unit: ㎛) of the above pearlite can additionally satisfy the following equation 2-2 in relation to the carbon equivalent (Ceq) according to the above equation 1.

[0038] [Formula 2-2]

[0039] L ≤ 0.64-0.35×Ceq

[0040] The above steel wire may contain pearlite at an area ratio of 98% or more.

[0041] The above Ceq may be 0.77 or more, 0.80 or more, 0.85 or more, or 0.90 or more, and may be 1.06 or less, 1.00 or less, or 0.95 or less.

[0042] A method for manufacturing a steel wire according to an embodiment of the present invention for solving the above problem includes a step of drawing a wire rod having a microstructure including C: 0.60 to 1.00% by weight, Si: 0.1 to 0.4%, Mn: 0.2 to 0.6%, and Cr: 0.3% or less (including 0%), the remainder being Fe and other unavoidable impurities, and wherein the average lamellar spacing L (unit: ㎛) of the pearlite satisfies the following equation 2-1 in relation to the carbon equivalent (Ceq) according to the following equation 1.

[0043] [Formula 1]

[0044] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0045] (In the above formula 1, [ ] is the weight% of the corresponding component.

[0046] [Formula 2-1]

[0047] 0.54-0.35×Ceq ≤ L

[0048] The average lamellar spacing L (unit: ㎛) of the pearlite of the above-mentioned wire can additionally satisfy the following equation 2-2 in relation to the carbon equivalent (Ceq) according to the above equation 1.

[0049] [Formula 2-2]

[0050] L ≤ 0.64-0.35×Ceq

[0051] The above-mentioned wire may have a microstructure containing pearlite at an area ratio of 98% or more.

[0052] The above Ceq may be 0.77 or more, 0.80 or more, 0.85 or more, or 0.90 or more, and may be 1.06 or less, 1.00 or less, or 0.95 or less.

[0053] The above fresh processing step can be performed under the condition that the accumulated strain (ε) according to the following equation 4 is 2.8 or more.

[0054] [Formula 4]

[0055] ε= 2×ln(di / df)

[0056] (In the above equation 4, di is the diameter before fresh processing, and df is the diameter after fresh processing)

[0057] According to the present invention, when a wire rod having a diameter of approximately 4 to 6 mm is hot rolled, coiled, and cooled, cooling is controlled so that the degree of transformation from austenite to pearlite according to the carbon equivalent satisfies Equation 3-1, thereby producing a wire rod having excellent wire drawability. As a result, direct drawing becomes possible up to a diameter of approximately 1.3 mm without intermediate isothermal transformation heat treatment.

[0058] As a result, an average lamellar spacing satisfying Equation 2-1 and a pearlite structure of 98% or more can be secured depending on the carbon equivalent, and by utilizing this, direct drawing is possible without heat treatment from a wire diameter of approximately 4 to 6 mm to approximately 1.1 to 1.3 mm. Through this, steel wires for tire cords and saw wires can be manufactured by omitting one of the isothermal transformation heat treatments that are usually applied twice.

[0059] In addition to the effects described above, specific effects of the present invention are described below while explaining specific details for carrying out the invention.

[0060] Figure 1 schematically illustrates a method for manufacturing a wire according to the present invention.

[0061] Figure 2 schematically illustrates the concept of transformation supercooling (ΔTs).

[0062] Figure 3 shows the degree of transformation supercooling according to the carbon equivalent of the example specimens and the comparative example specimens.

[0063] Figure 4 shows the average lamellar spacing of pearlite according to the carbon equivalent of the example specimens and the comparative example specimens.

[0064] Figure 5 shows the tensile strength of the wire according to the carbon equivalent of the example specimens and the comparative example specimens.

[0065] The advantages and features of the present invention, and the methods for achieving them, will become clearer with reference to the embodiments described in detail below. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms. These embodiments are provided solely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention. The present invention is defined solely by the scope of the claims.

[0066] Hereinafter, the wire rod, steel wire and manufacturing method thereof according to a preferred embodiment of the present invention will be described in detail.

[0067] In the present invention, a method for manufacturing a wire rod is proposed, using a high-carbon steel material containing C: 0.60 to 1.00% by weight, Si: 0.1 to 0.4%, Mn: 0.2 to 0.6%, and Cr: 0.3% or less (including 0%), with the remainder being Fe and other unavoidable impurities, by heating, rolling, coiling, and blowing cooling in a stelmor cooling process. In order to omit the isothermal transformation heat treatment in the subsequent drawing process to manufacture a steel wire, a larger amount of drawing process, i.e., a cumulative strain (ε), should be applied (for example, ε≥2.8) than in the case of performing the heat treatment. Therefore, it is necessary to secure a microstructure composed of a high proportion of pearlite while minimizing the formation of a proeutectoid phase. In order to suppress the occurrence of a sintered phase while maximizing the average lamella spacing in continuous cooling, the inventors of the present invention found that the average transformation undercooling (ΔTs) must be controlled to fall within a certain range in order to manage the change in continuous temperature at which actual transformation occurs from the equilibrium transformation temperature determined by the carbon equivalent of the steel.

[0068] Transformation supercooling is the difference between the equilibrium transformation temperature and the temperature at which the transformation actually occurs, and it is the activation energy that drives the transformation mechanism. As the temperature increases, the free energy of each phase of austenite and pearlite increases. In the low temperature range, the free energy of pearlite is low, and in the high temperature range, the free energy of austenite is low. Therefore, when the temperature is lowered, austenite, which exists at high temperatures, has the same free energy as pearlite at a certain temperature. This temperature is the equilibrium transformation temperature, and it is determined by the alloying components.

[0069] Even if austenite reaches the equilibrium transformation temperature, activation energy is required for transformation to begin, so the temperature can be lowered further to make austenite more unstable than pearlite, thereby initiating transformation.

[0070] At this time, the degree of supercooling ultimately depends on how fast the cooling rate is. The faster the cooling rate, the greater the degree of supercooling and the lower the temperature range where the actual transformation occurs, resulting in a fine interlayer spacing. If the cooling rate is slow, the degree of supercooling decreases and the interlayer spacing becomes coarser.

[0071] The inventors of the present invention have confirmed that when the transformation undercooling ΔTs (℃) in the temperature range where pearlite transformation actually occurs from the equilibrium transformation temperature determined by the alloy composition satisfies Equation 3-1 according to the carbon equivalent according to Equation 1, it is possible to secure a pearlite structure with a high ratio (e.g., 98% or more in terms of area ratio) while maximizing the average lamellar spacing.

[0072] [Formula 1]

[0073] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0074] (In the above formula 1, [ ] is the weight% of the corresponding component.

[0075] [Formula 3-1]

[0076] ΔTs ≤ 64.1×Ceq+6

[0077] When the transformation supercooling degree exceeds the range specified in Equation 1, the lamella spacing becomes finer, the tensile strength increases, and the limit drawing amount decreases.

[0078] In addition, since there is a risk of formation of a nitrate phase if the transformation supercooling is too low, it was confirmed that it is desirable to additionally control the carbon equivalent (Ceq) according to the above equation 1 to satisfy the following equation 3-2.

[0079] [Formula 3-2]

[0080] 64.1×Ceq-14 ≤ ΔTs

[0081] Equation 3, which synthesizes Equation 3-2 into Equation 3-1, is as follows.

[0082] [Formula 3]

[0083] 64.1×Ceq-14 ≤ ΔTs ≤ 64.1×Ceq+6

[0084] The above equation 3 was obtained through linear regression analysis based on the data in Figure 3 below.

[0085] The average lamellar spacing L (unit: ㎛) of pearlite of the wire manufactured through cooling in this supercooling control range can satisfy the following equation 2-1 in relation to the carbon equivalent (Ceq) according to equation 1.

[0086] [Formula 2-1]

[0087] 0.54-0.35×Ceq ≤ L

[0088] In the case of a conventional wire rod manufacturing method, the airflow is increased in the Stelmor cooling facility to rapidly cool the wire rod. However, in this case, since the strength of the wire rod is very high, the limit for drawing is reduced, and in order to increase this, constant temperature heat treatment is required during drawing. However, in the case of the wire rod according to the present invention, by controlling the transformation undercooling as in the above equation 3 during cooling, the lamellar spacing, which is a key microstructural factor of pearlite, can be controlled to satisfy equation 2-1, that is, 0.54-0.35×Ceq ≤ L, and ultimately the tensile strength of the wire rod can be controlled. In this case, since the lamellar interlayer spacing is wide, the tensile strength can also be secured at a low level suitable for drawing, and as a result, the processing limit can be increased in the subsequent drawing, and the work hardening rate can also be reduced.

[0089] In addition, in order to suppress the tensile strength of the wire rod from being too low due to the sintering process, it is preferable that the average lamellar spacing L (unit: ㎛) of pearlite additionally satisfies the following equation 2-2. In addition, since if the lamellar spacing becomes too wide, the drawability of the wire rod may be significantly reduced, in order to suppress this, it is more preferable that the average lamellar spacing L of pearlite additionally satisfies the following equation 2-2.

[0090] [Formula 2-2]

[0091] L ≤ 0.64-0.35×Ceq

[0092] Equation 2, which synthesizes Equation 2-2 into Equation 2-1, is as follows.

[0093] [Formula 2]

[0094] 0.54-0.35×Ceq ≤ L ≤ 0.64-0.35×Ceq

[0095] The above equation 2 was obtained through linear regression analysis based on the data in Figure 4 below.

[0096] The wire rod according to the present invention contains, in wt%, C: 0.60 to 1.00%, Si: 0.1 to 0.4%, Mn: 0.2 to 0.6%, and Cr: 0.3% or less (including 0%), the remainder being Fe and other unavoidable impurities. The unavoidable impurities are made up of various impurities that are inevitably included in the steelmaking process, etc. For example, phosphorus (P) and sulfur (S) are impurities that cannot be filtered out during the steelmaking process, and although the purity and processability are improved when they are managed in small amounts, they can be managed to be less than 0.03% each in consideration of economic feasibility.

[0097] In the wire rod according to the present invention, Ceq is determined according to the contents of C, Mn, Si, and Cr according to the above formula 1. More specifically, the Ceq of the wire rod according to the present invention may be 0.77 to 1.06. For example, the wire rod according to the present invention may have the Ceq of 0.77 or more, 0.80 or more, 0.85 or more, or 0.90 or more, and may be 1.06 or less, 1.00 or less, or 0.95 or less.

[0098] The wire according to the present invention has a microstructure including pearlite. It is preferable that the area ratio of pearlite is 98% or more.

[0099] Hereinafter, the content and reason for addition of each component included in the hot-rolled steel sheet according to the present invention will be described in detail.

[0100] [C: 0.60∼1.00 wt%]

[0101] C is a key alloying element that constitutes cementite during pearlite transformation, and the lamellar spacing and pearlite equilibrium transformation temperature change depending on the C content. When the carbon content is less than 0.6%, the proeutectoid ferrite fraction increases, making it difficult to secure wire workability. Conversely, when the carbon content exceeds 1.00%, the formation and segregation of the proeutectoid cementite phase damage the structural integrity of the core of the wire, which also deteriorates the wire workability.

[0102] Accordingly, the content of C is preferably 0.60 to 1.00 wt%, and in terms of avoiding the above-mentioned disadvantages, it is more preferably 0.70 to 0.90 wt%.

[0103] [Si: 0.1∼0.4 wt%]

[0104] Si is mostly dissolved in ferrite during pearlite transformation, and is hardly distributed in cementite, and its diffusion rate is slower than that of C, so if there is a large amount of Si dissolved, the pearlite transformation becomes slow overall. It is an element that causes a solid solution strengthening effect when dissolved in ferrite. If it is less than 0.1%, the solid solution strengthening effect is insufficient, making it difficult to achieve high strength in the final product, and if it exceeds 0.4%, the ferrite hardens excessively, reducing the drawability.

[0105] Accordingly, the content of Si is preferably 0.1 to 0.4 wt%, and in terms of avoiding the above-mentioned disadvantages, it is more preferably 0.2 to 0.3 wt%.

[0106] [Mn: 0.2∼0.6 wt%]

[0107] Although manganese does not affect the tensile strength of pearlite during isothermal transformation, it can affect the tensile strength during continuous cooling by changing the hardenability of the steel under limited cooling capacity, thereby changing the transformation initiation temperature. If it is less than 0.2%, it is difficult to expect an appropriate level of hardenability improvement effect, and if it exceeds 0.6%, as it is a high-carbon steel, it segregates in the center together with carbon, increasing the risk of martensite defects occurring in the center.

[0108] Accordingly, the content of Mn is preferably 0.2 to 0.6 wt%, and in terms of avoiding the above-mentioned disadvantages, it is more preferably 0.3 to 0.5 wt%.

[0109] [Cr: 0.3 wt% or less]

[0110] Although Cr is not an essential component in the present invention, it is an element that enables isothermal transformation at a lower temperature by separating the temperature ranges where pearlite and bainite occur in the TTT curve during isothermal transformation, and as a result, it is possible to see the effect of high strength in the final product. As the C content increases, in order to obtain appropriate hardenability, it is added in small amounts to replace Mn, which has a risk of center segregation, or Si, which is a ferrite hardening element. When the Cr content exceeds 0.3%, the effect of increasing the strength of the final product, such as in steel wires, is insufficient, so there is no need to add it in excess.

[0111] Accordingly, the Cr content is preferably 0.3 wt% or less, and more preferably 0.1 to 0.2 wt%.

[0112] Method for manufacturing wire rod

[0113] The method for manufacturing a wire rod according to the present invention comprises the steps of preparing a steel material containing, by weight %, C: 0.60 to 1.00%, Si: 0.1 to 0.4%, Mn: 0.2 to 0.6%, and Cr: 0.3% or less (including 0%), the remainder being Fe and other unavoidable impurities, heating the steel material, rolling and coiling the steel material, and cooling the steel material to create a microstructure including pearlite. The steel material for manufacturing the wire rod can be obtained by a method such as casting a steel material manufactured in a converter into a bloom and then rolling it into a billet, or directly casting it into a continuous casting billet to produce a billet having a cross-sectional area of, for example, 160×160 ㎟.

[0114] Figure 1 schematically illustrates a method for manufacturing a wire according to the present invention.

[0115] Referring to FIG. 1, the wire manufacturing method according to the present invention includes a heating step (S110), a rolling and coiling step (S120), and a cooling step (S130).

[0116] The above heating can be performed at 1000-1250°C for 60-120 minutes. The heating step prepares the steel for wire rolling. If the heating temperature is too low, wire rolling may be difficult, and if the heating temperature is too high, the austenite structure may grow excessively.

[0117] Wire rod rolling is performed using a conventional hot wire rod rolling method, and wire rods with a diameter of approximately 4 to 6 mm can be produced.

[0118] Steel cord wire makers remove scale through mechanical exfoliation to produce eco-friendly products. The appropriate scale thickness varies depending on the mechanical exfoliation method, but is typically around 10㎛. If the scale thickness is less than this, mechanical exfoliation is insufficient and it remains on the wire surface, causing dies to break during drawing and creating defects such as surface blemishes. If the scale thickness is too thick, scale loss increases, lowering productivity. Scale thickness is strongly influenced by the coiling temperature. Below 800℃, scale thickness is low, while above 900℃, scale thickness is excessive. Therefore, it is recommended to control the coiling temperature between 800 and 900℃.

[0119] The mechanical properties of pearlite are governed by the lamellar spacing, which in turn is influenced by the degree of supercooling, which is the difference between the equilibrium transformation temperature and the actual transformation temperature. Excessive cooling increases the degree of supercooling, while slow cooling reduces it. If the cooling rate is too slow, proeutectoid ferrite or proeutectoid cementite is formed, depending on the carbon content, reducing the drawability.

[0120] In the method for manufacturing a wire according to the present invention, the cooling step can control the microstructure regardless of the alloy composition by controlling the degree of transformation supercooling to satisfy Equation 3-1, etc., according to the carbon equivalent according to Equation 1 as described above.

[0121] The manufactured wire may have a microstructure containing more than 98% pearlite, and may have a tensile strength of between 800 and 1250 MPa.

[0122] By processing the wire rod manufactured in the above manner, a steel wire having a diameter of approximately 2 mm or less can be manufactured.

[0123] In terms of omitting the isothermal transformation heat treatment during the fresh processing, it is preferable that the fresh processing be performed under the condition that the cumulative strain (ε) according to Equation 4 below is 2.8 or more, more preferably under the condition that the cumulative strain is 2.8 or more and 3.0 or more.

[0124] [Formula 4]

[0125] ε= 2×ln(di / df)

[0126] (In the above equation 4, di is the diameter before fresh processing, and df is the diameter after fresh processing)

[0127] In the case of the present invention, it was confirmed that delamination does not occur even during fresh processing of such high accumulated strain, and thus, constant temperature transformation heat treatment during fresh processing can be omitted.

[0128] Example

[0129] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the examples described below are intended only to illustrate and further concretize the present invention and are not intended to limit the scope of the present invention.

[0130] Steel having the same composition as Table 1 was manufactured in a furnace, then cast into a bloom and then rolled into billets with a cross-sectional area of ​​160×160 ㎟.

[0131] Afterwards, the temperature was maintained at approximately 1100℃ for 90 minutes in a heating furnace, and the wire was rolled to a diameter of 5.5 mm through wire rolling under normal conditions. The coiling temperature was controlled to various degrees from 780℃ to 930℃ as shown in Table 1, and the airflow in the Stelmor cooling was then adjusted to control the transformation supercooling ΔTs (℃) to satisfy Equation 3.

[0132] [Formula 3]

[0133] 64.1×Ceq-14 ≤ ΔT ≤ 64.1×Ceq+6

[0134] Table 1 shows the alloy composition and carbon equivalent of the steel used in the example specimens and comparative example specimens, and Table 2 shows the degree of supercooling, average lamella spacing, and tensile strength of the steel used in the wire production along with the comparative example specimens.

[0135] Carbon equivalent was calculated according to the following equation 1.

[0136] [Formula 1]

[0137] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5

[0138] (In the above formula 1, [ ] is the weight% of the corresponding component.

[0139] The transformation undercooling was described as the difference between the equilibrium transformation temperature and the actual pearlite transformation onset temperature during continuous cooling, as illustrated in Fig. 2. The equilibrium transformation temperature was derived from the alloy composition of the steel through thermodynamic calculations, and the pearlite transformation onset temperature was set as the lowest value in the part where the measured temperature decreased and then increased for the first time, based on the results of measuring the material temperature during the actual STELMOR cooling process. Typically, the actual pearlite transformation starts at a temperature slightly higher than the lowest point of the measured temperature, but since this is affected by the cooling conditions / speed, it is impossible to measure it in an actual wire coil being produced. Instead, the transformation onset temperature was set at the lowest point of the curve where the temperature decreased and then increased, which is easy to measure.

[0140] Additionally, the average lamella spacing was measured by photographing the microstructure of the manufactured wire specimens, and the spacing between the lamellae of pearlite was expressed as an average value.

[0141] Tensile strength was measured using a tensile tester (Zwick Z250).

[0142] [Table 1]

[0143]

[0144] [Table 2]

[0145]

[0146] ※In Table 2 above, Equation 2-1 is 0.54-0.35xCeq, and Equation 2-2 is 0.54-0.35xCeq.

[0147] Referring to Table 1, the exemplary specimens 1-24 and the comparative specimens 1-10 do not show significant differences in alloy composition or carbon equivalent. The difference between the exemplary specimens and the comparative specimens lies in the control of transformation supercooling by adjusting the airflow during stelmore cooling after wire rolling and coiling. In particular, in the case of the exemplary specimens 1-24, the transformation supercooling was controlled to satisfy the aforementioned equation 3-1 according to the carbon equivalent of the wire.

[0148] Figure 3 shows the degree of transformation supercooling according to the carbon equivalent of the example specimens (inventive specimen) and the comparative example specimens (comparative specimen).

[0149] Referring to Table 2 and Fig. 3, it can be seen that the transformation supercooling degree according to the carbon equivalent of the exemplary and comparative example specimens tends to increase as the carbon equivalent increases. However, in the case of the exemplary specimens, it can be seen that the transformation supercooling degree was controlled within a range satisfying Equation 3-1, i.e., ΔTs ≤ 64.1×Ceq+6. In the case of the comparative example specimens, conventional tire steel cord wire cooling with a relatively high airflow rate in the Stelmor cooling was applied.

[0150] In addition, for the example specimens, the transformation supercooling was controlled to a range satisfying Equation 3-2, i.e., 64.1×Ceq-14 ≤ ΔTs. If the transformation supercooling according to the carbon equivalent is too low, it is difficult to suppress the formation of proeutectoid cementite due to the high carbon content, and as a result, the tensile strength is lowered, but it is difficult to secure the drawability due to the stress concentration during draw processing caused by proeutectoid cementite. Therefore, the transformation supercooling of the example specimens was controlled to a range satisfying Equation 3-2.

[0151] By controlling this transformation supercooling, the lamellar spacing, which is a key microstructural factor of pearlite, could be controlled, and ultimately, the tensile strength of the wire could be controlled.

[0152] In the case of comparative specimens, the airflow rate is relatively high in the Stelmor cooling, which results in a faster cooling rate and an initiation of transformation at a lower temperature beyond the thermodynamic equilibrium temperature, which increases the degree of transformation undercooling, makes the lamella spacing finer, and increases the tensile strength.

[0153] Figure 4 shows the average lamellar spacing of pearlite according to the carbon equivalent of the exemplary specimens (inventive material) and the comparative specimens (comparative material). Figure 5 shows the tensile strength of the wire according to the carbon equivalent of the exemplary specimens (inventive material) and the comparative specimens (comparative material).

[0154] Referring to Table 2, Figs. 4 and 5, it can be seen that there is a clear difference in the average lamellar spacing according to the carbon equivalent between the exemplary specimens and the comparative specimens. In the case of the exemplary specimens, when the lamellar spacing L of pearlite is measured in ㎛ units, it can be seen that the average lamellar spacing of pearlite is controlled to satisfy Equation 2-1, that is, 0.54-0.35×Ceq ≤ L, for the carbon equivalent. As can be confirmed in Fig. 5, since the lamellar interlayer spacing is wider than that of the comparative specimens, the tensile strength is also secured at a low level suitable for drawing. In the case of the exemplary specimens, since the tensile strength can be controlled low, the processing limit can be increased during subsequent drawing processing, and the work hardening rate can also be lowered.

[0155] Furthermore, for the example specimens, the average lamellar spacing L of pearlite was controlled to additionally 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 sliding distance within the ferrite increases during drawing, causing more dislocations to concentrate at the interface and generate cracks, which may cause a problem of a decrease in the limit drawing amount. In consideration of this, it is more preferable to satisfy the lamellar spacing according to Equation 2-2.

[0156]

[0157] The above example specimens and comparative example specimens were tested using a draw bench to control the strain per pass to approximately 0.2 and measure the limiting freshness reduction, i.e., strain. The limiting strain was determined by the occurrence of delamination through a torsion test. If no delamination occurred in three tests, it was considered pass, and if delamination occurred even once, it was considered fail.

[0158] The accumulated strain was calculated based on Equation 4.

[0159] [Formula 4]

[0160] ε= 2×ln(di / df)

[0161] (In the above equation 4, di is the diameter before fresh processing, and df is the diameter after fresh processing)

[0162] The change in wire diameter according to fresh processing and the accumulated strain according to Equation 4 are shown in Table 3 below, and whether delamination occurred during a torsion test after fresh processing was evaluated.

[0163] [Table 3]

[0164]

[0165] Referring to Table 3, it can be seen that when a fresh processing of a total strain of 2.2 was applied, no delamination occurred in the torsional test for both the example specimens and the comparative example specimens.

[0166] However, when a drawing process with a total strain of 2.4 was applied, some of the comparative specimens showed delamination during the torsional test, and all of the comparative specimens showed delamination during the torsional test when a drawing process with a total strain of 2.8 or less was applied. These results show that, for the comparative specimens, drawing is possible without delamination up to a final wire diameter of 1.8 mm based on an initial wire diameter of 5.5 mm, but it is impossible to omit the isothermal transformation heat treatment when the final wire diameter is smaller than this.

[0167] In contrast, it can be seen that no delamination occurred in the torsional test for the example specimens even when fresh processing up to a total strain of 2.8 was applied. It can be seen that no delamination occurred in the torsional test for some example specimens even when fresh processing up to a total strain of 3.0 was applied.

[0168] Through the results in Table 3, it can be seen that the exemplary specimens manufactured through the intended supercooling control of the present invention can be drawn with a greater total strain applied amount than the comparative exemplary specimens. Furthermore, it can be seen that, for the exemplary specimens, drawing is possible without isothermal transformation heat treatment up to a final diameter of approximately 1.30 mm based on an initial diameter of 5.5 mm.

[0169] Therefore, it can be concluded that if a wire rod manufactured through a method including transformation supercooling control according to the present invention is drawn under conditions of a cumulative strain of 2.8 or more, a separate isothermal transformation heat treatment can be omitted.

[0170] While the embodiments of the present invention have been described above, it is clear that the present invention is not limited to the embodiments disclosed herein, and that various modifications may be made by those skilled in the art within the scope of the technical concept of the present invention. Furthermore, even if the operational effects according to the configuration of the present invention were not explicitly described and explained while describing the embodiments of the present invention, it is natural that the effects predictable by the corresponding configuration should also be acknowledged.

Claims

1. Contains C: 0.60∼1.00% by weight, Si: 0.1∼0.4%, Mn: 0.2∼0.6%, and Cr: 0.3% or less (including 0%), the remainder being Fe and other unavoidable impurities. It has a microstructure containing pearlite, A wire in which the average lamellar spacing L (unit: ㎛) of the pearlite satisfies the following equation 2-1 in relation to the carbon equivalent (Ceq) according to the following equation 1. [Formula 1] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5 (In the above formula 1, [ ] is the weight% of the corresponding component. [Formula 2-1] 0.54-0.35×Ceq ≤ L 2. In paragraph 1, A wire rod, wherein the average lamellar spacing L (unit: ㎛) of the pearlite further satisfies the following equation 2-2 in relation to the carbon equivalent (Ceq) according to the equation 1. [Formula 2-2] L ≤ 0.64-0.35×Ceq 3. In paragraph 1 or 2, The above wire is a wire having a microstructure containing pearlite at an area ratio of 98% or more.

4. In paragraph 1 or 2, Pre-treated with Ceq of 0.77∼1.

06.

5. A step of heating a steel material containing C: 0.60∼1.00% by weight, Si: 0.1∼0.4%, Mn: 0.2∼0.6%, and Cr: 0.3% or less (including 0%), the remainder being Fe and other unavoidable impurities; A step of rolling the above steel into a wire rod and then coiling it; and A cooling step for cooling the above-mentioned preform to generate a microstructure including pearlite is included. The above cooling step is a wire rod manufacturing method in which the transformation undercooling ΔTs (℃), which is defined as the difference between the equilibrium transformation temperature at which the free energy of austenite and the free energy of pearlite are equal in the transformation from austenite to pearlite and the actual pearlite transformation initiation temperature during cooling, is controlled to satisfy the following Equation 3-1 in relation to the carbon equivalent (Ceq) according to Equation 1 below. [Formula 1] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5 (In the above formula 1, [ ] is the weight% of the corresponding component. [Formula 3-1] ΔTs ≤ 64.1×Ceq+6 6. In paragraph 5, A wire manufacturing method, wherein the cooling step controls the transformation supercooling so as to additionally satisfy the following equation 3-2 in relation to the carbon equivalent (Ceq) according to the equation 1. [Formula 3-2] 64.1×Ceq-14 ≤ ΔTs 7. In paragraph 5 or 6, The above heating is performed at 1000∼1250℃ for 60∼120 minutes, A wire manufacturing method in which the above winding is performed at 800 to 900°C. Has a wire diameter of 8.2 mm or less, Contains C: 0.60∼1.00% by weight, Si: 0.1∼0.4%, Mn: 0.2∼0.6%, and Cr: 0.3% or less (including 0%), the remainder being Fe and other inevitable impurities. It has a microstructure containing pearlite, A steel wire in which the average lamellar spacing L (unit: ㎛) of the pearlite satisfies the following equation 2-1 in relation to the carbon equivalent (Ceq) according to the following equation 1. [Formula 1] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5 (In the above formula 1, [ ] is the weight% of the corresponding component. [Formula 2-1] 0.54-0.35×Ceq ≤ L 9. In paragraph 8, A steel wire wherein the average lamellar spacing L (unit: ㎛) of the pearlite further satisfies the following equation 2-2 in relation to the carbon equivalent (Ceq) according to the equation 1. [Formula 2-2] L ≤ 0.64-0.35×Ceq 10. In clause 8 or 9, The above steel wire is a steel wire containing pearlite at an area ratio of 98% or more.

11. In clause 8 or 9, Steel wire with the above Ceq of 0.77 to 1.

06. A method for manufacturing a steel wire, comprising the step of drawing a wire rod having a microstructure including pearlite, wherein the wire rod contains C: 0.60 to 1.00% by weight, Si: 0.1 to 0.4%, Mn: 0.2 to 0.6%, and Cr: 0.3% or less (including 0%), the remainder being Fe and other inevitable impurities, and wherein the average lamellar spacing L (unit: ㎛) of the pearlite satisfies the following equation 2-1 in relation to a carbon equivalent (Ceq) according to the following equation 1. [Formula 1] Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Cr] / 5 (In the above formula 1, [ ] is the weight% of the corresponding component. [Formula 2-1] 0.54-0.35×Ceq ≤ L 13. In paragraph 12, A method for manufacturing a steel wire, wherein the average lamellar spacing L (unit: ㎛) of pearlite of the above-mentioned wire additionally satisfies the following equation 2-2 in relation to the carbon equivalent (Ceq) according to the above equation 1. [Formula 2-2] L ≤ 0.64-0.35×Ceq 14. In paragraph 12 or 13, A method for manufacturing a steel wire, wherein the above-mentioned wire has a microstructure containing pearlite at an area ratio of 98% or more.

15. In paragraph 12 or 13, A method for manufacturing steel wire, wherein the Ceq is 0.77 to 1.

06.

16. In paragraph 12 or 13, A method for manufacturing a steel wire, wherein the above fresh processing step is performed under the condition that the accumulated strain (ε) according to the following equation 4 is 2.8 or more. [Formula 4] ε= 2×ln(di / df) (In the above equation 4, di is the diameter before fresh processing, and df is the diameter after fresh processing)

Images