High-elongation steel cord having m+n multilayer structure

Abstract

The present invention relates to a high-elongation steel cord having a two-layer twisted structure with an M+N configuration in which a sheath layer composed of N strands of steel wires is twisted around a core layer composed of M strands of steel wires. The steel cord of the present invention has an elongation at break of 5% or more, can be lighter than conventional steel cords, and has improved rotational resistance and rubber permeability, and thus can improve the durability of a tire.

Description

M+N multilayer high elongation steel cord

[0001] The present invention relates to a high-elongation steel cord with an M+N multilayer structure, and more specifically, to a high-elongation multilayer steel cord for reinforcing materials for 0-degree belts and protective belts that has high elongation and excellent rubber penetration.

[0002] It is commonly known that rubber products are reinforced with steel cords for pneumatic tires.

[0003] The belts in the belt layer of a tire are manufactured by topping rubber onto uniformly arranged steel cords. To optimize belt stiffness for driving performance, they are designed to have an angle of approximately 15 to 60 degrees relative to the circumferential centerline of the tire. The 0-degree belt is manufactured by winding it once or twice in the circumferential direction of the tire to increase belt stiffness. Since the steel cords inserted inside the 0-degree belt are manufactured by winding them continuously in the circumferential direction, they inevitably have an angle of 0° to 2° relative to the tire centerline (C). If the 0-degree belt is reinforced with standard steel cords having a break elongation of around 2%, the steel cords cannot stretch as much as the tire is lifted, causing the belt shape to become distorted after vulcanization. Therefore, it is necessary to apply high-elongation steel cords for reinforcing the 0-degree belt.

[0004] Currently, 3×7×0.20 HE high-elongation steel cords are commonly used in 0-degree belts, exhibiting high elongation characteristics with a breaking elongation of 5–7% and a low-weight elongation of 1.4–1.6%. Typically, steel cords with high elongation characteristics, such as a breaking elongation of 4% or more and a low-weight elongation of 0.8% or more, are designated by adding "HE" to the specification name. To manufacture such high-elongation steel cords, 21 strands of 0.20 mm steel wire are produced, followed by the production of three 1+6×0.20 mm strands. Subsequently, these three strands are twisted simultaneously to produce a 3×(1+6×0.20) mm steel cord. To produce 3×7×0.20 HE steel cords, three stranding machines are required to produce strands, and one is required to produce the final product, resulting in a total of four stranding machines. However, due to the difference between the hourly production rate of the final product and the hourly production rate of the strand, the number of stranding machines required for strand production can be reduced to two, but there is a problem of low economic feasibility as it still requires two stages of stranding and a large number of stranding machines.

[0005] Meanwhile, the 1+6×0.20 mm strand has steel wires positioned tightly, so rubber cannot penetrate through the gaps between the core wire and the side wire, resulting in low rubber penetration and raising concerns about reduced tire durability and regrowth.

[0006] Therefore, to improve economic efficiency, it is necessary to develop steel cords that can be produced directly from a single stranding machine. Steel cords that can be produced directly from a stranding machine are in the form of single-layer or multi-layer steel cords without strands, such as 1×N, M+N, and M+N+P. The twist length of these steel cords varies depending on the diameter of the steel wire, but most are 10 mm or more, with a breaking elongation of around 2~3% and a low-weight elongation of 0.6% or less, which presents a limitation in that they are difficult to use as a replacement for conventional M×N HE.

[0007] [Prior Art Literature]

[0008] [Patent Literature]

[0009] (Patent Document 1) EP 1756356 B1

[0010] (Patent Document 2) JP 4553074 B2

[0011] (Patent Document 3) EP 2414581 B1

[0012] (Patent Document 4) EP 3027805 B1

[0013] The present invention is intended to overcome the problems of the aforementioned prior art. One objective of the present invention is to provide a high-elongation steel cord for 0-degree belts and protective belt reinforcements that exhibits high elongation characteristics in a multilayer steel cord of an M+N structure, which has superior productivity compared to an M×N structure, enables weight reduction compared to existing steel cords, contributes to improving tire rolling resistance, and improves tire durability by enhancing rubber penetration.

[0014] Another objective of the present invention is to provide an air-filled tire comprising a high-elongation steel cord having high elongation and improved rubber permeability.

[0015] One aspect of the present invention for achieving the above-mentioned purpose is,

[0016] A high-elongation steel cord having a two-layer structure of an M+N structure, wherein a sheath layer composed of N strands of steel wire is twisted around a core layer composed of M strands of steel wire, and the wire diameter (R) of the steel wire constituting the core layer. m ) and the wire diameter (R) of the steel wire constituting the sheath layer n ) is within the range of Formula 1 below, and the number of steel wire strands (N) constituting the core layer m ) and the number of steel wire strands (N) constituting the sheath layer n The present invention relates to a high elongation steel cord of an M+N multilayer structure characterized by satisfying the relationship of Formula 2 below.

[0017] [Formula 1]

[0018]

[0019] In the above formula,

[0020] R m = Wire diameter (mm) of the steel wire constituting the core layer,

[0021] R n = Wire diameter (mm) of the steel wire constituting the sheath layer,

[0022] [Equation 2]

[0023]

[0024] In the above formula,

[0025] N m = Number of steel wire strands constituting the core layer,

[0026] N n0 = Number of steel wire strands in the sheath layer when the core layer is completely wrapped by the sheath layer,

[0027] N n = The number of steel wire strands constituting the sheath layer.

[0028]

[0029] In the steel cord of the present invention, the twisting direction of the steel wires of the core layer and the sheath layer may be the same.

[0030] In the tensile test of the steel cord of the present invention, the elongation at break is 4% or more, and the elongation in the 0.5~10kg range is 0.8% or more.

[0031] Twist angle (θ) in the elliptical shape of the steel wire in the core layer m The major axis of the ellipse R by ) m.max It is called, and the distance from the center of each steel wire in the core layer to the contact point between steel wires is r m It is called, and the code shape of the core layer D m When saying that, the twist length of the core layer (P m ) can satisfy the relationship of Equation 6 below.

[0032] [Equation 3]

[0033]

[0034] In the above formula,

[0035] R m = Wire diameter (mm) of the steel wire constituting the core layer,

[0036] P m = Twist length of core layer (mm),

[0037] P n = Sheath layer twist length (mm),

[0038] θ m = Twist angle of the core layer (degree),

[0039] θ n = Sheath layer twist angle (degree)

[0040] [Equation 4]

[0041]

[0042] [Formula 5]

[0043]

[0044] [Equation 6]

[0045]

[0046] The filament of the above steel cord may have a carbon content of 0.60 to 0.95 weight percent.

[0047] The steel cord of the present invention may have a fracture elongation of 5% or more during a tensile test, and a low-load elongation of 0.8% or more in the 0.5 to 10 kg range.

[0048] In the present invention, the twist angle of the core filament constituting the core layer is smaller than the twist angle of the sheath filament constituting the sheath layer.

[0049] According to the steel cord of various embodiments of the present invention, the carbon content of the raw material used in the manufacture of the steel cord and the strength of the steel wire are the same, so when the EPI is set based on the same belt strength standard, the amount of steel cord used does not change, but the weight of the belt is reduced as the cord diameter becomes smaller and the thickness of the topping rubber becomes thinner.

[0050] When a steel cord according to one embodiment of the present invention is applied to a tire belt, the weight per unit area is lower compared to the steel cord, thereby providing an effect of improving the rolling resistance of the tire and enhancing fuel efficiency performance through weight reduction.

[0051] According to the present invention, a two-layer steel cord with an M+N structure can be manufactured using a single stranding machine, resulting in excellent manufacturing processability and enabling the production of eco-friendly products through the lightweighting of belts and tires.

[0052] FIG. 1 is a schematic cross-sectional view of a steel cord of a high elongation multilayer structure (M+N) according to one embodiment of the present invention.

[0053] FIG. 2 is a schematic side view of a steel cord of a high elongation multilayer structure (M+N) according to one embodiment of the present invention.

[0054] Figure 3 is a load-elongation curve of a steel cord manufactured in an embodiment of the present invention.

[0055] Figure 4 is a load-low load elongation curve of a steel cord manufactured in an embodiment of the present invention.

[0056] The present invention will be described in more detail below.

[0057] In this application, terms such as 'comprising' or 'having' are intended to specify the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0058] As used herein, 'cord' refers to a reinforcing material constituting the reinforcing structure of a tire, meaning a product formed by twisting and combining multiple strands of steel wire (filament).

[0059] One aspect of the present invention is a high elongation steel cord having a two-layer structure of an M+N structure, wherein a sheath layer composed of N strands of steel wire is twisted around a core layer composed of M strands of steel wire, and the wire diameter (R) of the steel wire constituting the core layer m ) and the wire diameter (R) of the steel wire constituting the sheath layer n ) is within the range of Formula 1 below, and the number of steel wire strands (N) constituting the core layer m ) and the number of steel wire strands (N) constituting the sheath layer n The present invention relates to a high elongation steel cord of an M+N multilayer structure characterized by satisfying the relationship of Formula 2 below.

[0060] [Formula 1]

[0061]

[0062] In the above formula,

[0063] R m = Wire diameter (mm) of the steel wire constituting the core layer,

[0064] R n = Wire diameter (mm) of the steel wire constituting the sheath layer,

[0065] [Equation 2]

[0066]

[0067] In the above formula,

[0068] N m = Number of steel wire strands constituting the core layer,

[0069] N n0 = Number of steel wire strands in the sheath layer when the core layer is completely wrapped by the sheath layer,

[0070] N n = The number of steel wire strands constituting the sheath layer.

[0071]

[0072] The high-elongation multilayer steel cord of the present invention has a double-layer structure, i.e., an M+N structure, wherein M core filaments (steel wires) form a core layer and N sheath filaments form a sheath layer surrounding the core layer.

[0073] In the present invention, the steel cord is a two-layer steel cord with an M+N structure. A strand steel cord such as an M×N strand is less economical because M strands of 1×N are produced first and then twisted into an M×N structure. In contrast, a two-layer steel cord with an M+N structure like the present invention can be twisted simultaneously with a single stretching machine, resulting in excellent productivity and economic efficiency.

[0074] Typically, the diameter of the steel wire used for reinforcing rubber products such as tires is 0.10 to 0.50 mm. If the diameter is smaller than 0.10 mm, it is difficult to maintain the tire shape due to low rigidity. At the end of the belt layer reinforced at an angle greater than 0 degrees, the cut surface of the steel wire comes into contact with the rubber. Since cracks can occur in the unplated steel cross-section, causing delamination between the rubber and the steel wire, the diameter of the steel wire must be smaller than 0.50 mm.

[0075] [Formula 1]

[0076]

[0077] R m = Wire diameter (mm) of the steel wire constituting the core layer,

[0078] R n = Wire diameter (mm) of the steel wire constituting the sheath layer

[0079]

[0080] The number of strands (M) of the core layer and the number of strands (N) of the sheath layer of the steel cord of the present invention are 2 < M < 6. If the number of strands (M) of the core layer is less than 2, the elongation of the core layer is equal to the elongation of one steel wire, and the elongation at break is approximately 2%. Even with 2 strands, there is insufficient internal space, making it unsuitable for imparting high elongation characteristics, so 3 strands or more are suitable. If the number of strands (M) of the core layer exceeds 5 strands, the individual steel wires cannot be positioned uniformly, and the steel wires move into the empty space inside the core layer, resulting in a non-uniform cross-sectional shape and potentially causing a deterioration in stranded wire quality; therefore, 5 strands or fewer are suitable.

[0081] In order to stably form an M+N multilayer structure in the present invention, the number of strands in the sheath layer must be sufficient to wrap the core layer, and the number of strands can be set to satisfy the condition of Equation 2.

[0082] [Equation 2]

[0083]

[0084] N m = Number of steel wire strands constituting the core layer

[0085] N n0 = Number of steel wire strands constituting the sheath layer when the sheath layer completely wraps the core layer

[0086] N n = Number of steel wire strands in the sheath layer

[0087]

[0088] In the present invention N n This N n0 If it is less than -3, the inner diameter of the sheath layer becomes smaller than the outer diameter of the core layer when the sheath layer is densely twisted, so a compressive force is generated by the sheath layer on the core layer, causing a problem where the elongation of the core layer decreases. In contrast, the number of strands in the sheath layer is N n0If it is greater than +3, the inner diameter of the sheath layer becomes excessively larger than the outer diameter of the core layer, so the steel wires constituting the sheath layer cannot maintain a uniform spacing and may deviate from the appropriate trajectory of the sheath layer, potentially causing quality degradation.

[0089] The twist direction of the steel wires in the core layer and the sheath layer is the same, either S-twist or Z-twist.

[0090] The high elongation steel cord of the M+N structure of the present invention can secure a break elongation of 4% or more and a low load elongation of 0.8% or more in the 0.5 to 10 kg range, and can prevent the belt shape from becoming tangled after vulcanization in a 0-degree belt.

[0091] FIG. 1 is a cross-sectional view of a steel cord according to one embodiment of the present invention. Referring to FIG. 1, when looking at a cross-section perpendicular to the longitudinal direction of the steel cord, if the steel wire is twisted at a twist angle, the steel wire becomes an ellipse rather than a circle. Accordingly, the cord diameter is deformed.

[0092] When the twist directions differ, it causes problems such as reduced productivity and decreased elongation. The twist angle is determined by the twist length, and the twist angle is defined based on the vertical line in the direction of the cord length. The relationship between the twist angle and twist length of the core layer and the sheath layer is defined by Equation 6, and when the twist angle of the core layer or the sheath layer is 70 degrees or less, the elongation at break and low-load elongation increase significantly. Therefore, the twist length of the core layer (P m ) and the twist length of the sheath layer (P n ) should be set so that the twist angle is 70 degrees or less according to Equations 6 and 8, and more preferably, the twist angle should be 65 degrees or less.

[0093] Although the lower the twist angle, the higher the elongation characteristics, the short twist length causes a decrease in productivity and there is a possibility that the tire durability may be reduced due to increased brittleness caused by excessive plastic deformation during the wire drawing process, so the twist angle must be 30 degrees or more, and more preferably 40 degrees or more.

[0094] Twist angle (θ) in the elliptical shape of the steel wire in the core layer m The major axis of the ellipse R by ) m.max It is called, and the distance from the center of each steel wire in the core layer to the contact point between steel wires is r m It is called, and the code shape of the core layer D m When saying that, the twist length of the core layer (P m ) can be set to satisfy the relationship in Equation 6 below.

[0095] [Equation 3]

[0096]

[0097] In the above formula,

[0098] R m = Wire diameter (mm) of the steel wire constituting the core layer,

[0099] P m = Twist length of core layer (mm),

[0100] P n = Sheath layer twist length (mm),

[0101] θ m = Twist angle of the core layer (degree),

[0102] θ n = Sheath layer twist angle (degree)

[0103] [Equation 4]

[0104]

[0105] [Formula 5]

[0106]

[0107] [Equation 6]

[0108]

[0109] In the present invention, the twist length (P) of the sheath layer n ) and twist angle (θ n ) is the twist length (P) of the core layer explained above. m ) and twist angle (θ m It can be obtained through a process similar to the process of calculating ). The code diameter of the sheath layer is identical to the total code diameter, and this is D cord Assuming that, it can be defined by Equation 7, and the twist length of the sheath layer (P n ) and twist angle (θ n ) can be defined by Equation 8.

[0110] [Equation 7]

[0111]

[0112] [Equation 8]

[0113]

[0114] The twist angle of the core wire constituting the core layer is smaller than the twist angle of the sheath wire constituting the sheath layer. In the tensile test of a steel cord, the elongation at break increases as the elongated length increases, and there is a relationship in which the elongation at break increases as the wire length per unit length of the steel cord increases. If the twist lengths of the core layer and the sheath layer are the same, D m is D cord Because it is smaller, the wire length per unit length of the steel cord becomes shorter for the core layer wire than for the sheath layer wire, and the core layer wire breaks first, determining the elongation at break. As the core layer wire diameter increases and the twist angle increases, the core layer wire length per unit length of the steel cord can be manufactured to be as long as the sheath layer wire. Therefore, it is desirable for the core layer wire to have a smaller twist angle compared to the wire constituting the sheath layer.

[0115] According to the present invention, the metal wire is a steel wire. The steel composition may contain a minimum carbon content of 0.60 wt% and small amounts of manganese, silicon, sulfur, phosphorus, etc. For example, it may contain 0.70 wt% carbon, 0.50 wt% manganese, 0.20 wt% silicon, 0.013 wt% sulfur, and 0.085 wt% phosphorus. In addition, the steel composition may contain other metal alloy components, such as 0.20 wt% to 0.40 wt% chromium, 0.20 wt% copper, and 0.30 wt% vanadium.

[0116] According to the present invention, a metal wire may be plated to improve adhesion performance with rubber. Preferably, it may be coated with a zinc coating or a brass coating. The thickness of the coating may be any parameter known to a person skilled in the art, for example, the thickness of the brass coating is 0.2 micrometers. The plating performed on the surface of the steel wire to ensure adhesion performance between steel and rubber containing sulfur is a plating composed of 2 to 5 metal elements (nickel, manganese, iron, molybdenum, bismuth, cobalt, etc.) including copper or zinc.

[0117] In the present invention, the carbon content of the raw material of the steel cord is 0.60 to 0.95 weight%. If the carbon content of the raw material is less than 0.60 weight%, the strength of the steel cord is insufficient, and if the carbon content of the raw material exceeds 0.95 weight%, the hardness of the steel cord increases, which is disadvantageous for securing fatigue resistance.

[0118] Another aspect of the present invention relates to a tire comprising the steel cord described above. The tire of the present invention is not limited to a radial tire for passenger cars, but can be configured as various types of tires, such as, for example, heavy-duty tires or tires for two-wheeled vehicles.

[0119] Since the steel cord of the present invention can secure a breaking elongation of 4% or more and a low-load elongation of 0.8% or more in the 0.5 to 10 kg range, it can be used to reinforce tire carcasses, belts, 0-degree belts, etc.

[0120] The present invention will be described in more detail below with reference to specific embodiments, but these embodiments are merely for illustrative purposes and are not intended to limit the scope of the invention.

[0121] Examples

[0122] Example 1

[0123] Using a wire rod with a carbon content of 0.82% and a diameter of 5.5 mm, primary drawing, heat treatment, brass plating, and wet drawing were performed to prepare 5 strands of 0.25 mm HT steel wire and 13 strands of 0.20 mm HT steel wire. Subsequently, during stranding, 5 strands of 0.25 mm were positioned to be twisted first as the core layer, and 13 strands of 0.20 HT were positioned to surround the core layer to form a sheath layer, and then twisted simultaneously to manufacture a 5×0.25+13×0.20 HE steel cord. Before the N strands were wrapped, the M strands were pre-twisted by a twisting rotating component to form a core layer by about half of the target twist length. After the N strands wrapped the core layer and entered the main body of the stranding machine, the core layer and the sheath layer were each twisted to the target twist length by the twisting rotating component.

[0124]

[0125] Example 2

[0126] Using a wire rod with a carbon content of 0.82% and a diameter of 5.5 mm, primary drawing, heat treatment, brass plating, and wet drawing were performed to prepare 4 strands of 0.28 mm HT steel wire and 13 strands of 0.18 mm HT steel wire. Subsequently, during stranding, 4 strands of 0.28 mm were positioned to be twisted first as the core layer, and 13 strands of 0.18 mm HT were positioned to surround the core layer to form a sheath layer, and then twisted simultaneously to manufacture a 4×0.28+13×0.18 HE steel cord. The stranding process was carried out in the same manner as in Example 1.

[0127]

[0128] Comparative Example 1

[0129] A wire rod with a carbon content of 0.82% and a diameter of 5.5 mm was used for primary drawing. Afterward, the wire rod was heat-treated and brass-plated, and final wet drawing was performed to prepare 11 strands of 0.33 mm HT steel wire. During stranding, 3 strands of 0.33 HT were positioned to be twisted first as a core layer, and 8 strands were positioned to wrap around the core layer to form a sheath layer, and then twisted simultaneously to produce 3+8×0.33 HT.

[0130]

[0131] Comparative Example 2

[0132] A wire rod with a carbon content of 0.82% and a diameter of 5.5 mm was used for primary drawing. Afterward, heat treatment and brass plating were performed, and final wet drawing was carried out to prepare 21 strands of 0.20 mm HT steel wire. A primary stranding process was performed to produce three strands of 1+6 × 0.20 mm strands, and then these three strands were twisted into a single steel cord through a secondary additional stranding process to produce 3 × 7 × 0.20 HE.

[0133]

[0134] Test Example 1

[0135] For each steel cord obtained in Examples 1 and 2 and Comparative Examples 1 and 2, physical properties were measured by the following method, and the results are shown in Table 1 below.

[0136] - Code: Measured with a micrometer.

[0137] - Twist Length (Pitch): The measured length of 10 twists in the steel cord is divided by 10. (*Pitch = Length of 10 twists / 10)

[0138] - Breaking strength: Breaking strength was measured according to ASTM D2969 using an INSTRON tensile testing machine.

[0139] - Unit weight: The weight of 1m of the cord was measured using a scale.

[0140] - Elongation at break: Measured according to ASTM D2969 using an INSTRON tensile testing machine.

[0141] - Elongation under reduced load: Elongation in the range of 0.5 kgf to 10 kgf was measured in accordance with ASTM D2969 using an INSTRON tensile testing machine.

[0142] - Adhesion: Based on ASTM D2229, the standard for testing the adhesion of steel cords, the Pull Out Force of a 1-inch long specimen was measured, and the coverage % was measured by visually determining the amount of rubber remaining in the pulled-out specimen.

[0143] - Rubber Penetration (Gas Permeability): After preparing an adhesion test specimen in accordance with ASTM D2229, the standard for steel cord adhesion testing, the ends of the steel cord protruding from the rubber were cut with nippers, and the cut surface was evenly ground with sandpaper to prepare the specimen. Using a Rubber Penetration Test facility, a gas injection pipe was installed on one side of the exposed steel cord cross-section of the specimen and a gas exhaust pipe on the opposite side, and the prepared gas was injected at 50 PSI to measure the amount of gas permeated per unit time.

[0144] Item Unit Comparison Example 1 Comparison Example 2 Example 1 Example 2 Specifications - 3 + 8 × 0.33 HT 3 × 7 × 0.20 HE 5 × 0.25 + 13 × 0.20 HE 4 × 0.28 + 13 × 0.18 HE Cord Diameter mm 1.34 1.34 1.20 1.13 Twist Length mm Core Layer 10 Sheath Layer 18 Wire 3.7 Cord 6.2 Core Layer 3.0 Sheath Layer 6.0 Core Layer 3.0 Sheath Layer 6.0 Twist Direction - S / SS / SS / SS / S Twist Angle (degree) Core Layer 77° Sheath Layer 77° Wire 30° Cord 56° Core Layer 55° Sheath Layer 62° Core Layer 53° Sheath Layer 59° Cutting Strength N 2700 1655 1640 1450 Elongation at Break % 2.4 5.6 6.2 5.8 Elongation under Low Load (0.5~10kg) % 0.1 1.5 1.5 1.5 Unit Weight g / m 7.5 25.8 35.7 15.04 Adhesion Kgf / 25.4㎜ 20 20 15 20 15 00 1410 Adhesion Coverage % 90 90 100 100 Rubber Penetration (Gas Permeability) cc N2 / sec05000

[0145] As confirmed by the results of Table 1 above, the steel cord included in the present invention has a significantly higher elongation at break compared to the steel cord of Comparative Example 1 and has an elongation at break equivalent to that of Comparative Example 2, thereby making it applicable for 0-degree belts or protective belts. Furthermore, rubber penetration is also improved, which can improve the durability and recyclability of the tire.

[0146] FIG. 3 shows the load-elongation curve of the steel cord of the present invention and the load-elongation curve of the steel cord of a comparative example, and FIG. 4 shows the low-load elongation curve.

[0147]

[0148] Test Example 2

[0149] Tire belts were manufactured using the steel cords of Example 1, Example 2 and Comparative Examples 1 and 2. For the belts of Example 1, Example 2 and Comparative Examples 1 and 2, the belt strength, steel cord usage, topping rubber usage, and belt weight were measured, and the results are shown in Table 2 below.

[0150] If a 0-degree belt is reinforced with ordinary steel cords having a break elongation of approximately 2%, the steel cords cannot stretch as much as the tire is lifted, causing the belt shape to become distorted after vulcanization. Generally, the belt shape is verified by analyzing the steel cords + rubber rolls or the tire using X-rays; if the steel cords fail to maintain a straight line and become wavy, the shape is considered distorted and judged as "defective." In cases where the distortion is significant, external warping can be identified visually alone. A belt shape classified as "good" means that the belt shape is not distorted.

[0151] Item Unit Comparison Example 1 Comparison Example 2 Example 1 Example 2 Specifications -3+8×0.33 HT3×7×0.20 HE5×0.25+13×0.20 HE4×0.28+13×0.20 HE Belt EPI ends / inch 6.7 1 1.0 1 1.1 1 2.6 Belt Strength kN / inch 18.2 (Same) 18.2 (Standard) 18.2 (Same) 18.2 (Same) Steel Cord Usage kg / tire 1.33 (-21%) 1.68 (Standard) 1.67 (-1%) 1.66 (-1%) Topping Rubber Usage kg / tire 1.59 (+3%) 1.54 (Standard) 1.43 (-7%) 1.37 (-11%) Belt Weight kg / tire 2.92 (-9%) 3.22 (Standard) 3.09 (-4%) 3.03 (-6%) Belt Shape - Poor Good Good Good

[0152] As confirmed by the results in Table 2 above, the steel cord of Comparative Example 1 has low elongation at break and low load elongation, resulting in defects in the belt shape when applied to a 0-degree belt or protective belt, whereas Comparative Example 2 and Examples 1 and 2 have high elongation at break and low load elongation, indicating good belt shape. Furthermore, when the steel cord of the present invention is applied to an air-filled tire, the amount of steel cord used does not change, but as the cord diameter decreases and the thickness of the topping rubber becomes thinner, it can be confirmed that the weight of the belt is reduced.

[0153] Although the present invention has been described with reference to preferred embodiments, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention. Accordingly, the true scope of protection of the present invention must be determined by the claims set forth below.

Claims

1. A high-elongation steel cord having a two-layer structure of an M+N structure, wherein a sheath layer composed of N strands of steel wire is twisted around a core layer composed of M strands of steel wire, and the wire diameter (R) of the steel wire constituting the core layer. m ) and the wire diameter (R) of the steel wire constituting the sheath layer n ) is within the range of Formula 1 below, and the number of steel wire strands (N) constituting the core layer m ) and the number of steel wire strands (N) constituting the sheath layer n A high elongation steel cord of an M+N multilayer structure characterized by satisfying the relationship of Formula 2 below. [Formula 1] In the above formula, R m = Wire diameter (mm) of the steel wire constituting the core layer, R n = Wire diameter (mm) of the steel wire constituting the sheath layer, [Equation 2] In the above formula, N m = Number of steel wire strands constituting the core layer, N n0 = Number of steel wire strands in the sheath layer when the core layer is completely wrapped by the sheath layer, N n = The number of steel wire strands constituting the sheath layer.

2. A high elongation steel cord of an M+N multilayer structure, characterized in that, in claim 1, the twisting direction of the steel wires of the core layer and the sheath layer are the same.

3. The high elongation steel cord of an M+N multilayer structure according to claim 1, characterized in that the steel cord has a breaking elongation of 4% or more during a steel cord tensile test and an elongation of 0.8% or more in the 0.5 to 10 kg range.

4. In claim 1, the twist angle (θ) in the elliptical shape of the steel wire of the core layer m The major axis of the ellipse R by ) m.max It is called, and the distance from the center of each steel wire in the core layer to the contact point between steel wires is r m It is called, and the code diameter of the core layer D m When saying that, the twist length of the core layer (P m A high elongation steel cord of an M+N multilayer structure characterized by satisfying the relationship of Equation 6 below. [Equation 3] In the above formula, R m = Wire diameter (mm) of the steel wire constituting the core layer, θ m = Twist angle of the core layer (degree), [Equation 4] [Formula 5] [Equation 6] In the above formula, P m = Twist length of core layer (mm), θ m = Twist angle of the core layer (degree) 5. In paragraph 4, the twist length of the core layer (P m ) is the twist angle (θ) of the core layer m A high elongation steel cord of an M+N multilayer structure, characterized by being defined by the above formula 6 such that ) is 30 to 70 degrees.

6. A high elongation steel cord of an M+N multilayer structure, characterized in that, in claim 1, the wire of the steel cord has a carbon content of 0.60 to 0.95 weight%.

7. A high elongation steel cord of an M+N multilayer structure, characterized in that, in claim 1, the twist angle of the core wire constituting the core layer is smaller than the twist angle of the sheath wire constituting the sheath layer.

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