Steel wire rod and steel wire having excellent hydrogen embrittlement resistance and manufacturing method therefor

Abstract

The present invention relates to a wire rod and a steel wire, both having excellent hydrogen embrittlement resistance, and a manufacturing method therefor and, more specifically, to a wire rod having excellent hydrogen embrittlement resistance and comprising, by weight%, C: 0.20-0.40%, Si: 0.20-0.50%, Mn: 0.50-1.00%, Ti: 0.050-0.100%, S: 0.030-0.080%, and the balance of Fe and inevitable impurities, wherein an atomic ratio (Ti / S) of Ti and S satisfies the following formula (1), and when a diameter of the wire rod is D, a microstructure in a region of 1 / 2D to 3 / 2D of a cross section of the wire rod includes 20 to 30% by area fraction of ferrite and the balance of pearlite. Formula (1): 0.5 ≤ (Ti weight) / (Ti mass) / (S weight) / (S mass) ≤ 1.8

Description

Wire rods, steel wires with excellent hydrogen embrittlement resistance and their manufacturing method

[0001] The present invention relates to wire rods, steel wires and methods for manufacturing the same having excellent hydrogen embrittlement resistance.

[0002] For oilfield mining or transportation, carbon steel with a carbon content of 0.35% is used as a reinforcing material, drawn and rolled. Typically, reinforcing material is produced by applying LP heat treatment to wire rod produced at steel mills at a drawing center to form fine pearlite. Dry drawing reduces the wire diameter and increases strength. Cold rolling secures the shape used in the final product. Finally, high-frequency heat treatment removes surface residual stress.

[0003] For carbon steels containing 0.35% Si and 0.8% Mn, respectively, are added. Meanwhile, depending on the demand for high strength, the C content is sometimes increased to 0.72% to improve strength. This is because C is the element that can most effectively increase strength in proportion to its cost, and if added more than that of eutectoid steel, intergranular proeutectoid cementite is formed, which increases the possibility of hydrogen embrittlement. On the other hand, strength-increasing elements such as Cr and Mo are not often added because they lower workability or form low-temperature structures, which cause wire breakage during processing.

[0004] Due to energy depletion, oil well extraction environments have recently shifted from nearshore to deep-sea. Deep-sea wells contain significantly higher levels of hydrogen, sulfur, and other chemicals than nearshore wells, necessitating steel wires with high hydrogen resistance. Steel wires used as reinforcing materials require a certain amount of carbon, resulting in a microstructure composed of a mixture of ferrite and pearlite. This necessitates the use of precipitates capable of trapping hydrogen.

[0005] In order to solve the above-described problem, the present invention aims to provide a wire rod, steel wire, and a method for manufacturing the same, which can be preferably applied for deep-sea oil well mining or transportation because they have excellent crack resistance in an acidic (99.5% H2S+CH3COOH) atmosphere by precipitating fine TiS in a ferrite matrix.

[0006] In addition, the present invention aims to provide wire rods, steel wires and a method for manufacturing the same by reducing the tensile strength deviation between the overlapping portion and the center to 50 MPa or less by spraying a coolant mist on the overlapping portion of the wire coil during initial cooling, thereby improving product reliability, and further enabling drawing and rolling without LP heat treatment in a drawing room.

[0007] In order to achieve the above object, a wire rod having excellent hydrogen embrittlement resistance according to one embodiment of the present invention contains, in wt%, C: 0.20 to 0.40%, Si: 0.20 to 0.50%, Mn: 0.50 to 1.00%, Ti: 0.050 to 0.100%, S: 0.030 to 0.080%, the remainder Fe and other unavoidable impurities, and the atomic ratio of Ti and S (Ti / S) satisfies the following equation (1), and when the diameter of the wire rod is D, the microstructure of the wire rod measured at a position of 1 / 2D to 3 / 2D in the diameter direction of the wire rod may include ferrite with an area fraction of 20 to 30% and the remainder pearlite.

[0008] Equation (1): 0.5 ≤ (Ti weight) / (Ti mass) / (S weight) / (S mass) ≤ 1.8

[0009] In addition, the wire according to one embodiment of the present invention has a cross-sectional area of ​​300 to 8000 nm within the ferrite. 2 The area fraction of TiS precipitates may be greater than 58%.

[0010] Additionally, the size of the TiS precipitate according to one embodiment of the present invention may be 10 nm to 50 nm.

[0011] In addition, in the wire according to one embodiment of the present invention, the maximum thickness of coarse cementite in the coil overlapping portion may be 100 nm or less.

[0012] In addition, the wire according to one embodiment of the present invention may have a tensile strength deviation between the coil overlapping portion and the center portion of 50 MPa or less.

[0013] Additionally, the wire according to one embodiment of the present invention may have an average tensile strength of 650 MPa or more.

[0014] Additionally, the wire according to one embodiment of the present invention may have an average cross-sectional reduction rate (Reduction Area, RA) of 60% or more.

[0015] In addition, a method for manufacturing a wire rod according to an embodiment of the present invention may include a step of heating a billet containing, in wt%, C: 0.20 to 0.40%, Si: 0.20 to 0.50%, Mn: 0.50 to 1.00%, Ti: 0.050 to 0.100%, S: 0.030 to 0.080%, the remainder Fe, and other unavoidable impurities, at 1000 to 1100°C and then hot-rolling it at 900 to 1000°C to manufacture a wire rod; a step of coiling the wire rod into a coil shape at a temperature of 780 to 850°C; and a step of cooling the coiled wire rod, wherein the step of mist-injecting a coolant onto an overlapping portion of the wire rod coil having a high stacking density during the initial cooling and cooling it at a cooling rate of 8 to 13°C / s.

[0016] In addition, cooling according to one embodiment of the present invention is performed in a stelmore cooling bed, and can be performed by spraying refrigerant by a mist sprayer installed at the tip of the cooling bed on the overlapping portion of the wire coil during initial cooling.

[0017] In addition, the wire according to one embodiment of the present invention may have a tensile strength deviation between the overlapping portion and the center of the coil of 50 MPa or less.

[0018] In addition, a steel wire having excellent hydrogen embrittlement resistance according to one embodiment of the present invention contains, in wt%, C: 0.20 to 0.40%, Si: 0.20 to 0.50%, Mn: 0.50 to 1.00%, Ti: 0.050 to 0.100%, S: 0.030 to 0.080%, the remainder Fe and other unavoidable impurities, and may have a crack resistance of 6.1 hours or more in an acidic (99.5% H2S+CH3COOH) atmosphere.

[0019] Additionally, the steel wire according to one embodiment of the present invention may have a tensile strength of 850 MPa or more.

[0020] Additionally, the steel wire according to one embodiment of the present invention may have a yield ratio of 0.90 or more.

[0021] In addition, a method for manufacturing a steel wire according to an embodiment of the present invention comprises the steps of: heating a billet containing, in wt%, C: 0.20 to 0.40%, Si: 0.20 to 0.50%, Mn: 0.50 to 1.00%, Ti: 0.050 to 0.100%, S: 0.030 to 0.080%, the remainder Fe, and other inevitable impurities, at 1000 to 1100°C, and then hot-rolling it at 900 to 1000°C to manufacture a wire rod; coiling the wire rod into a coil shape at a temperature of 780 to 850°C; cooling the coiled wire rod coil; mist-spraying a coolant on an overlapping portion of the wire rod coil having a high stacking density during the initial cooling, and cooling it at a cooling rate of 8 to 13°C / s; and cooling the cooled wire rod to a total area reduction rate of 60% without LP (lead patent) heat treatment. It may include a step of dry drawing, a step of cold rolling at a reduction ratio of 60% after the dry drawing, and a step of high-frequency heat treatment after the cold rolling.

[0022] In addition, the high-frequency heat treatment according to one embodiment of the present invention can be performed at an A1 transformation temperature of -30°C to an A1 transformation temperature.

[0023] In addition, the steel wire according to one embodiment of the present invention may have a crack resistance of 6.1 hours or more in an acidic (99.5% H2S+CH3COOH) atmosphere.

[0024] The wire and steel wire of the present invention have excellent crack resistance in an acidic (99.5% H2S+CH3COOH) atmosphere by precipitating fine TiS in a ferrite matrix, and thus can be preferably applied for deep-sea oil well mining or transportation.

[0025] Furthermore, by spraying a coolant mist on the overlapping part of the wire coil during initial cooling, the tensile strength deviation between the overlapping part and the center is reduced to 50 MPa or less, resulting in excellent product reliability, and drawing and rolling are possible without LP heat treatment in the fresh wire.

[0026] FIG. 1 is a diagram showing the center and overlapping portion of a wire coil according to one embodiment of the present invention.

[0027] FIG. 2 is a diagram showing the temperature difference between the coil overlapping portion and the center portion in the case of cooling by mist spraying refrigerant in the initial cooling stage of the wire coil according to one embodiment of the present invention and in the case of cooling by a conventional method without mist cooling.

[0028] Figure 3 shows the results of measuring the crack occurrence time after maintaining a steel wire manufactured according to one embodiment of the present invention and a conventional steel wire in an acidic (99.5% H2S+CH3COOH) atmosphere.

[0029] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. To clarify the present invention, the drawings may omit portions irrelevant to the description, and the sizes of components may be slightly exaggerated to facilitate understanding.

[0030] Additionally, when a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise specifically stated.

[0031] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0032] TiS is generally known as a precipitate capable of trapping diffusible or non-diffusible hydrogen, but it has not been used in fresh-processed products because it can cause short circuits during processing.

[0033] Accordingly, the present invention aims to improve the hydrogen embrittlement resistance of steel wire by precipitating fine TiS precipitates and controlling the conditions of drawing and rolling, thereby facilitating hydrogen trapping without causing wire breakage during processing.

[0034] According to one embodiment of the present invention, the wire contains, in wt%, C: 0.20 to 0.40%, Si: 0.20 to 0.50%, Mn: 0.50 to 1.00%, Ti: 0.050 to 0.100%, S: 0.030 to 0.080%, the remainder being Fe and other unavoidable impurities.

[0035] Below, the reasons for limiting the composition of the above-mentioned pre-existing materials are explained in detail. Unless otherwise specified, the composition below refers to weight %.

[0036] The carbon (C) content may be 0.20% to 0.40%.

[0037] C is the element that can most effectively increase the strength of a material, and a 0.1% increase in the C content has a strength increase effect of 100 MPa. If the C content is less than 0.20%, it may be difficult to achieve the target strength of the product, and if it exceeds 0.40%, hydrogen embrittlement may be inferior due to an increase in the pearlite fraction. Considering this, the C content is preferably included in an amount of 0.20% to 0.40%, and more preferably, it is included in an amount of 0.25% to 0.35%.

[0038] The silicon (Si) content may be 0.20% to 0.50%.

[0039] Silicon (Si) is a solid-solution strengthening element that can increase strength by 15 MPa when added at 0.20%. However, when added in large amounts, it can affect scale exfoliation by forming FeSiO4, etc. Therefore, it is recommended to limit the upper limit to 0.50%. If the Si content is less than 0.20%, it may be difficult to secure strength, and if it exceeds 0.50%, it may be difficult to remove fresh scale.

[0040] The manganese (Mn) content may be 0.50% to 1.00%.

[0041] Mn increases the strength by 20 MPa when increased by 0.10%, but is added for the purpose of providing sufficient hardenability (hardenability) during heat treatment. If the Mn content is less than 0.50%, it may be difficult to achieve the target strength and secure hardenability during stelmore cooling. If it exceeds 1.00%, segregation may cause wire breakage during drawing and rolling, so it is desirable to limit the upper limit to 1.00%.

[0042] The content of titanium (Ti) can be 0.050% to 0.100%.

[0043] To trap diffusible or non-diffusible hydrogen within a steel, fine TiS precipitation within the ferrite matrix is ​​required. If the Ti content is less than 0.050%, sufficient TiS precipitates are not formed, failing to ensure crack resistance in an acidic (99.5% H2S+CH3COOH) atmosphere. If the Ti content exceeds 0.100%, tundish nozzle clogging may occur. Therefore, the upper limit is preferably set at 0.100%.

[0044] The sulfur (S) content may be 0.030% to 0.080%.

[0045] S is a component necessary for the formation of fine TiS precipitates. If the S content is less than 0.030%, the formation of TiS precipitates may be difficult, and if it exceeds 0.080%, the occurrence of central segregation is hindered, causing short circuits during processing. Therefore, it is desirable to limit the upper limit to 0.080%.

[0046] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0047] According to one embodiment of the present invention, the microstructure of the wire rod may include ferrite and pearlite to secure strength and drawability, and when the diameter of the wire rod is D, the microstructure of the wire rod measured at a position 1 / 2D to 3 / 2D in the diameter direction of the wire rod may include ferrite with an area fraction of 20 to 30% and the remainder pearlite. When the area fraction of the ferrite exceeds 30%, wire breakage may occur during wire drawing.

[0048] According to one embodiment of the present invention, the wire can be derived by the following equation (1) by considering the amount of Ti and S for forming TiS precipitates that affect brittleness resistance.

[0049] Equation (1): 0.5 ≤ (Ti weight) / (Ti mass) / (S weight) / (S mass) ≤ 1.8

[0050] When the atomic ratio of Ti and S (Ti / S) in the above formula (1) satisfies formula (1), fine TiS precipitates having a size of 10 to 50 nm can be generated, thereby significantly improving the brittleness resistance of the wire.

[0051] In addition, the wire according to one embodiment of the present invention has a cross-sectional area of ​​300 to 8000 nm within the ferrite. 2 It may contain TiS precipitates having an area fraction of 58% or more.

[0052] When TiS precipitates are finely and evenly dispersed within the ferrite matrix, they can provide trap sites for hydrogen that has penetrated inside, thereby preventing the movement of hydrogen and preventing hydrogen from agglomerating in one place.

[0053] In this way, TiS precipitates are suitable for trapping hydrogen that has penetrated into the interior of the wire and preventing hydrogen from coagulating, and the area of ​​the converted unit in the ferrite is 300-8000 nm. 2 It is preferable that the TiS precipitate be included in an area fraction of 58% or more. If the TiS precipitate is included in an area fraction of less than 58%, the distribution of the precipitate becomes narrow, reducing the number of locations where hydrogen can be trapped, thereby reducing the hydrogen agglomeration inhibition effect, and thus failing to secure sufficient hydrogen embrittlement resistance.

[0054] In addition, the size of the TiS precipitate is preferably 10 to 50 nm. If it is less than 10 nm, the size of the precipitate itself is too small to properly play the role of preventing the movement of hydrogen that has penetrated inside, and if it exceeds 50 nm, the precipitate is coarse and the hydrogen trapping ability is reduced, which may reduce the effect of preventing hydrogen aggregation, and may cause wire breakage during fresh processing or cold rolling.

[0055] According to one embodiment of the present invention, the wire may have a maximum thickness of coarse cementite in the coil overlapping portion of 100 nm or less.

[0056] Coarse cementite refers to cementite having a particle size of more than 100 nm and less than 300 nm. If the maximum thickness of coarse cementite in the overlapping portion of the wire coil of the present invention exceeds 100 nm, cracks are likely to occur during drawing, thereby deteriorating drawing workability.

[0057] Additionally, the wire according to one embodiment of the present invention may have a tensile strength deviation between the coil overlapping portion and the center portion of 50 MPa or less.

[0058] In general, the wire coil inevitably has a relatively higher stacking (overlapping) density at the edge (overlapping area) than at the center (central area) of the coil. At this time, as illustrated in Fig. 1, the center of the wire coil refers to the central area of ​​the coil with a relatively low stacking density corresponding to (1-2, 5-6) when the wire is divided into 8 equal parts, and the overlapping area refers to the edge area of ​​the coil with a high stacking density corresponding to (3-4, 7-8).

[0059] Accordingly, during the manufacturing of the wire, the transformation of the structure is different due to the cooling difference according to the stacking density between the overlapping portion and the center, and as a result, a difference in tensile strength occurs between the overlapping portion and the center.

[0060] However, in the present invention, the tensile strength deviation between the coil overlapping portion and the center can be reduced to 50 MPa or less, thereby increasing the reliability of the product.

[0061] In addition, the wire according to one embodiment of the present invention may satisfy the above component system and microstructure, and may have an average tensile strength of 650 MPa or more and an average reduction area (RA) of 60% or more.

[0062] Next, a method for manufacturing a wire having excellent hydrogen embrittlement resistance according to one embodiment of the present invention will be described.

[0063] A method for manufacturing a wire rod having excellent hydrogen embrittlement resistance according to one embodiment of the present invention comprises the steps of: heating a billet containing, in wt%, C: 0.20 to 0.40%, Si: 0.20 to 0.50%, Mn: 0.50 to 1.00%, Ti: 0.050 to 0.100%, S: 0.030 to 0.080%, the remainder Fe, and other unavoidable impurities at 1000 to 1100°C and then hot-rolling it at 900 to 1000°C to manufacture a wire rod; coiling the wire rod into a coil shape at a temperature of 780 to 850°C; and cooling the coiled wire rod coil, wherein the method comprises the steps of mist-injecting a coolant onto an overlapping portion of a wire rod coil having a high stacking density during the initial cooling to cool the wire rod at a cooling rate of 8 to 13°C / s.

[0064] The reason for limiting the composition range of each alloy element is as described above, and each manufacturing step is explained in more detail below.

[0065] First, a billet having the above-described composition is manufactured.

[0066] The billet can then be heated to 1000-1100°C under normal conditions and then hot-rolled at 900-1000°C. For example, a hot rolling process consisting of rough rolling, intermediate rough rolling / finish rolling, and finish rolling can be sequentially performed on the heated billet to produce a wire rod of the desired size.

[0067] Next, it goes through a step of winding into a coil shape.

[0068] Coiling temperature is related to the scale exfoliation of fresh yarn. Since oil well wires are pickled to remove scale from fresh yarn, dense or thick scale can cause exfoliation problems.

[0069] Therefore, it is preferable that the coiling step according to one embodiment of the present invention be performed at a temperature range of 780 to 850°C. If the coiling temperature is lower than 780°C, coiling failure may occur, and if it exceeds 850°C, scale (FeO) may be excessively formed, resulting in loss and a need for an increased cooling rate.

[0070] The wound wire coils fall on a stelmor cooling conveyor in a circular shape and move in a regular pile. The moving wire coils have a density difference depending on the number of overlapping parts between the overlapping part and the center of each wire coil in the moving direction and vertical direction. Due to this density difference between the overlapping part and the center, the cooling speed of each part differs during the slow cooling process at room temperature while moving on the conveyor, which ultimately results in a cooling deviation.

[0071] That is, the overlapping area, which is a region with a relatively high stacking density on the conveyor, has a lower cooling rate than the central area, so it is extracted without complete tissue transformation until it passes through the slow cooling area, and this untransformed area cools at a high rate when it comes into contact with the atmosphere.

[0072] Accordingly, the central portion of the wire coil with a relatively low density undergoes tissue transformation within the conveyor, but the tissue of the overlapping portion with a relatively high density is not completely transformed and cools rapidly upon contact with the atmosphere, exhibiting a tensile strength much higher than the normal value. This causes a significant tensile strength difference between the overlapping portion and the central portion of the wire coil, resulting in a non-uniform tissue.

[0073] Therefore, in the present invention, a wound wire coil is cooled, and a refrigerant is mist-sprayed onto the overlapping portion of the wire coil having a high stacking density during the initial cooling, thereby reducing the temperature difference between the overlapping portion and the center portion.

[0074] At this time, by spraying a coolant mist on the overlapping part of the high-density wire coil during the initial cooling and cooling it at a cooling rate of 8 to 13°C / s, the temperature difference between the overlapping part and the center part can be reduced.

[0075] The above cooling can be performed using a stelmore cooling zone. That is, when the coiled wire is transferred to the stelmore cooling zone and the mist sprayer installed at the front end of the entrance to the stelmore cooling zone sprays refrigerant onto the overlapping portion of the coil while moving it, a cooling difference occurs between the center of the coil and the overlapping portion at the beginning of cooling, but when the wire coil exits the cooling zone, the temperature difference between the center of the coil and the overlapping portion disappears.

[0076] That is, in the present invention, by intensively cooling the overlapping portion of the wire coil in the initial stage of cooling to rapidly lower the temperature of the overlapping portion and pass it through the cooling zone, the temperature difference between the central portion of the coil and the overlapping portion is reduced after passing through the cooling zone, so that transformation is completed regardless of the difference in stacking density, and accordingly, no deviation in the tensile strength of the wire is generated.

[0077] At this time, a mist spraying device is installed at the tip of the cooling bed so that initial cooling of the coil overlap can be achieved up to a distance including the section where heat is generated as the coil passes through the cooling bed.

[0078] Additionally, the overlapping portion of the wire coil is cooled at a cooling rate of 8 to 13°C / s by spraying refrigerant using a mist sprayer installed at the tip. If the cooling rate is less than 8°C / s, the tensile strength deviation between the overlapping portion and the center portion exceeds 50 MPa, and if it exceeds 13°C / s, additional equipment investment is required, so it is desirable to control it below that rate.

[0079] As the above refrigerant, ordinary liquid nitrogen capable of cooling the wire coil can be used, and the tip of the wire coil is cooled by spraying it in the form of mist during the initial cooling period.

[0080] Hereinafter, in the method for manufacturing a wire of the present invention, when cooling is performed by spraying a coolant mist onto the overlapping portion of the wire coil in the initial cooling stage, the temperature gap between the overlapping portion and the central portion of the coil is reduced, as described with reference to the drawings.

[0081] FIG. 2 is a diagram showing the temperature difference between the coil overlapping portion and the center portion when cooling is performed by spraying refrigerant mist in the initial cooling stage of the wire coil according to one embodiment of the present invention and when cooling is performed using a conventional method without mist cooling.

[0082] As illustrated in Fig. 2(a), in the wire manufacturing method of the present invention, the overlapping portion of the coil is cooled by mist-spraying refrigerant using a mist-spraying device installed at the tip of the cooling bed during initial cooling. At this time, the mist-spraying device is installed to a distance that includes the section where heat generation occurs, and by passing through this section at a speed of 8 to 13°C / s, the mist-sprayed refrigerant cools it, thereby reducing the temperature gap between the overlapping portion and the center portion of the wire coil.

[0083] On the other hand, as shown in Fig. 2(b), it can be confirmed that when cooling is performed using a conventional method without mist cooling, a large temperature gap appears between the overlapping portion and the central portion of the coil.

[0084] Next, a steel wire having excellent water-resistance according to one embodiment of the present invention will be described.

[0085] A steel wire having excellent brittleness resistance according to one embodiment of the present invention contains, in wt%, C: 0.20 to 0.40%, Si: 0.20 to 0.50%, Mn: 0.50 to 1.00%, Ti: 0.050 to 0.100%, S: 0.030 to 0.080%, the remainder being Fe and other unavoidable impurities.

[0086] The reason for limiting the composition range of each alloy element is as described above.

[0087] The above steel wire has a microstructure that includes ferrite and pearlite, and the converted cross-sectional area within the ferrite is 300 to 8000 nm. 2 It contains fine TiS precipitates with an area fraction of 58% or more. Depending on the area fraction of TiS precipitates in the pearlite, the degree of cracking in an acidic (99.5% H2S+CH3COOH) atmosphere varies. The steel wire of the present invention has excellent cracking resistance in an acidic atmosphere, with a cracking resistance of 6.1 hours or more, and no cracks are formed inside even when maintained in an acidic atmosphere for 2.5 times or more compared to when TiS is not added.

[0088] Additionally, the steel wire according to one embodiment of the present invention may have a tensile strength of 850 MPa or more and a yield ratio of 0.90 or more.

[0089] Next, a method for manufacturing a steel having excellent hydrogen embrittlement resistance according to one embodiment of the present invention will be described.

[0090] According to one embodiment of the present invention, a steel wire comprises a step of manufacturing a wire rod by heating a billet containing, in wt%, C: 0.20 to 0.40%, Si: 0.20 to 0.50%, Mn: 0.50 to 1.00%, Ti: 0.050 to 0.100%, S: 0.030 to 0.080%, the remainder Fe, and other unavoidable impurities, at 1000 to 1100°C, and then hot-rolling at 900 to 1000°C, a step of coiling the wire rod into a coil shape at a temperature of 780 to 850°C, a step of cooling the coiled wire rod, a step of mist-spraying a coolant on an overlapping portion of the wire rod coil having a high stacking density during the initial cooling, and cooling the cooled wire rod at a cooling rate of 8 to 13°C / s, without LP (lead patent) heat treatment, a total area reduction ratio of 60%. The method includes a step of dry drawing, a step of cold rolling up to a reduction ratio of 60% after the dry drawing, and a step of high-frequency heat treatment after the cold rolling.

[0091] The method for manufacturing a steel wire of the present invention omits the LP heat treatment process performed in a wire drawing machine, and allows the wire to be directly drawn and rolled to form a final product. That is, in the present invention, during the initial cooling stage of wire manufacturing, a coolant mist is sprayed onto the overlapping portion of the wire coil to cool it at a cooling rate of 8 to 13°C / s, thereby reducing the temperature difference between the overlapping portion and the center portion of the wire, thereby allowing the omission of a separate LP heat treatment process.

[0092] The wire rod manufactured as described above is dry-drawn to a total area reduction rate of 60% or more without LP heat treatment. Specifically, the wire rod undergoes drawing while passing through 10 to 12 dies. During this process, the wire speed is controlled to 2 to 4 m / s, and the area reduction per pass is controlled to 10 to 20%, thereby producing a steel wire.

[0093] In order to secure the formation and tensile strength of the final product, cold rolling is performed up to a reduction ratio of 60% after the drawing.

[0094] After the above cold rolling, high-frequency heat treatment is performed at A1 transformation temperature -30℃ to A1 transformation temperature. If the high-frequency heat treatment temperature is lower than A1 transformation temperature -30℃, toughness is not secured, and there is a risk of breakage during forming and product state. If it exceeds A1 transformation temperature, strength may be reduced.

[0095] The steel wire manufactured according to the present invention has a tensile strength of 850 MPa or more, a yield ratio of 0.90 or more, and the steel wire can have a crack resistance of 6.1 hours or more in an acidic (99.5% H2S+CH3COOH) atmosphere, which is 2.5 times or more compared to a steel wire without TiS addition.

[0096] That is, according to the present invention, by spraying a coolant mist onto the overlapping portion of the wire coil during initial cooling to cool it, the tensile strength deviation between the overlapping portion and the center of the coil can be reduced to 50 MPa or less, thereby improving product reliability, and further enabling drawing and rolling without a separate LP heat treatment in the wire drawing room. In addition, the steel wire of the present invention exhibits excellent crack resistance in an acidic (99.5% H2S+CH3COOH) atmosphere by precipitating fine TiS in the ferrite matrix, and thus can be preferably applied to deep-sea oil well mining or transportation.

[0097] Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the following examples are intended only to illustrate the present invention and the scope of the present invention is not limited to these examples.

[0098] Example

[0099] Steel satisfying the various alloy compositions shown in Table 1 below was manufactured in a steel mill, and then cast under normal conditions to produce a continuous casting billet (160 x 160 mm2). Next, the manufactured billet was rolled under normal conditions to a wire diameter of 14 mm to produce a wire having an area fraction of 32% proeutectoid ferrite.

[0100] The unit in Table 1 below is part by weight, and the atomic ratio of Ti and S (Ti / S) was calculated according to the following equation (1).

[0101] Equation (1): 0.5 ≤ (Ti weight) / (Ti mass) / (S weight) / (S mass) ≤ 1.8

[0102] (Here, the mass of Ti is 47.887 and the mass of S is 32.06)

[0103] Classification CSiMnTiSTi and S atomic ratio Experimental example 10.340.250.820.0790.031.76 Experimental example 20.350.240.780.0790.0501.06 Experimental example 30.350.240.790.0800.0800.67 Comparative example 10.340.240.790.0000.0010.07 Comparative example 20.350.250.780.0390.030.87 Comparative example 30.360.250.790.0400.0500.54 Comparative example 40.340.260.810.0410.0800 .34Comparative Example 50.350.230.810.0400.10.27Comparative Example 60.360.240.800.1210.032.70Comparative Example 70.350.240.800.1190.0501.59Comparative Example 80.340.260.790.1180.0800.99Comparative Example 90.330.260.780.1190.10.80Comparative Example 100.340.250.820.0790.031.76Comparative Example 110.350.250.790.0800.031.79

[0104] Next, the rolled wire was wound into a coil shape at a temperature of 780 to 850°C, taking scale exfoliation into consideration. Next, the wound wire was transferred to a stelmore cooling bed, and a mist sprayer installed at the tip of the stelmore was used to spray refrigerant onto the coil overlap, thereby initially cooling the wire at a rate of 8 to 13°C / s. The coiling temperature, application of mist cooling, and cooling rate are as shown in Table 2 below.

[0105] Classification Winding temperature (℃) Whether mist cooling is applied Mist cooling rate (℃ / s) Experimental example 1829 Applied 12.1 Experimental example 2831 Applied 12.1 Experimental example 3831 Applied 12.4 Comparative example 1828 Not applied 0.0 Comparative example 2829 Applied 12.2 Comparative example 3830 Applied 12.4 Comparative example 4830 Applied 12.4 Comparative example 5829 Applied 12.0 Comparative example 6829 Applied 12.0 Comparative example 7828 Applied 12.0 Comparative example 8829 Applied 12.3 Comparative example 9829 Applied 12.0 Comparative example 10888 Applied 11.9 Comparative example 11829 Not applied 4.8

[0106] Table 3 below shows the average tensile strength of the manufactured wire, the tensile strength deviation between the overlapping portion and the central portion of the coil, the average area reduction ratio (RA) of the wire, and the maximum thickness of coarse cementite in the overlapping portion. Table 4 below also shows the average tensile strength of the manufactured wire with a converted cross-sectional area of ​​300 to 8000 nm. 2 The area fraction of TiS precipitates, the size of TiS precipitates, the thickness of wire scale, and the scale exfoliation were shown. The tensile strength of the wire was measured by performing a tensile test at an initial strain rate of 10 m / m at room temperature. The maximum area reduction of the wire was measured by magnification using a projector. The scale exfoliation was classified as good if the amount of scale remaining after 5% strain in the tensile tester was 0.05% or less, and poor if it exceeded 0.05%. At this time, the length of the specimen was 30 cm, and the evaluation was performed on the central 20 cm part excluding the 10 cm of the grip engagement area on both sides. Subsequently, the manufactured wire was subjected to drawing with a total area reduction rate of 60% or more after removing some of the scale on the surface using a mechanical exfoliation method without LP heat treatment. Afterwards, cold rolling was performed at a reduction ratio of 60% and high-frequency heat treatment was performed at A1 transformation temperature -30℃ to A1 transformation temperature to manufacture steel wire.

[0107] Table 5 below shows the values ​​of the total area reduction during drawing, reduction ratio, workability, tensile strength, yield strength, yield ratio, and crack occurrence time under an acidic atmosphere (99.5% H2S+CH3COOH) after measuring the application of LP heat treatment to the manufactured steel wire. In addition, the results of Experimental Example 1 and Comparative Example 1, which measured the occurrence of cracks while maintaining the wire in an acidic atmosphere, are shown in Fig. 3. At this time, the workability was classified as good when the wire breakage rate was less than 1.5 times per ton, and as poor when it was 1.5 times or more.

[0108] Average tensile strength (MPa) Overlap-center tensile strength deviation (MPa) Average area reduction ratio (%) Maximum thickness of coarse cementite in the overlapping section (nm) Experimental example 1 678 466 395 Experimental example 2 680 456 180 Experimental example 3 682 486 082 Comparative example 1 620 895 182 Comparative example 2 671 486 378 Comparative example 3 670 476 285 Comparative example 4 678 496 492 Comparative example 5 672 50 6484 Comparative example 6 672 466 279 Comparative example 7 671 486 390 Comparative example 8 670 486 385 Comparative example 9 675 456 294 Comparative example 10 675 496 481 Comparative example 1 1 612 90 59 179

[0109] Conversion cross-sectional area 300~8000㎚ 2 TiS precipitate area fraction (%)TiS precipitate size (nm)Wire scale thickness (㎛)Scale peelabilityExperimental example 158324.5GoodExperimental example 261374.2GoodExperimental example 373444.7GoodComparative example 1024.2GoodComparative example 211214.1GoodComparative example 39283.8GoodComparative example 48243.9GoodComparative example 57284.0GoodComparative example 632223.8GoodComparative example 738213.2GoodComparative example 824193.4GoodComparative example 931183.8GoodComparative example 1058218.2InferiorComparative example 1158155.3Good

[0110] LP heat treatment application Total area reduction rate during freshening (%) Compression ratio (%) Workability Tensile strength (Mpa) Yield strength (Mpa) Yield scattering Crack occurrence time (hr) Experimental example 1 Not applied 7260 Good 8928210.926.3 Experimental example 2 Not applied 7260 Good 8908010.96.7 Experimental example 3 Not applied 7260 Good 8888260.936.1 Comparative example 1 Applied 5860 Good 8718010.922.2 Comparative example 2 Not applied 7260 Good 8888080.912.1 Comparative example 3 Not applied 7260 Good 8908010.92.3 Comparative Example 4 Not Applied 7260 Good 882 803 0.91 2.1 Comparative Example 5 Not Applied 7260 Good 879 809 0.92 2.0 Comparative Example 6 Not Applied 7260 Good 883 812 0.92 3.5 Comparative Example 7 Not Applied 7260 Good 880 792 0.93.1 Comparative Example 8 Not Applied 7260 Good 886 797 0.93.0 Comparative Example 9 Not Applied 7260 Good 890 8010.93.7 Comparative Example 10 Not Applied 7260 Inferior 850 774 0.91 5.1 Comparative Example 11 Not Applied 7260 Inferior 790 711 0.95.2

[0111] Comparative Example 1 is a steel type manufactured using conventional methods, such as not applying mist cooling, using currently commercially available medium-carbon steel (Ti, S not applied). LP heat treatment was performed in the wire drawing process to refine the structure, and the product was produced after drawing, rolling, and high-frequency treatment. The steel wire of Comparative Example 1 showed a tensile strength of 871 MPa and a yield ratio of 0.92, and when maintained in an acidic (99.5% H2S+CH3COOH) atmosphere, internal cracks were confirmed to occur after 2.2 hours of maintenance, as shown in Table 4 and Fig. 3 below.

[0112] Experimental examples 1 to 3 and comparative examples 2 to 9 are examples in which the contents of Ti and S are different. In order to reduce the tensile strength deviation between the overlapping portion and the center portion of the wire coil, a mist sprayer installed at the tip of the stelmore was used to spray a coolant to cool the coil overlapping portion, and then the steel wire was manufactured by drawing, rolling, and high-frequency heat treatment without LP heat treatment in a wire drawing room.

[0113] In the case of Experimental Examples 1 to 3 and Comparative Examples 2 to 9, regardless of the content of Ti and S, the tensile strength was at the level of 670 MPa, and the average cross-sectional reduction rate was at the level of 62%. However, if the content values ​​of Ti and S presented in the present invention are not satisfied, the converted cross-sectional area is 300 to 8000 nm. 2 It can be confirmed that there is a difference in the area fraction (%) of TiS precipitates. In the case of Experimental Examples 1 to 3, it was confirmed that the area fraction was 58 to 73%, but in the case of Comparative Examples 2 to 5, it was confirmed that the area fraction was around 10%, and in the case of Comparative Examples 6 to 9, it was confirmed that the area fraction was around 24 to 38%. These results affect the internal crack formation time in an acidic atmosphere. In the case of the steel wires of Experimental Examples 1 to 3, it was able to withstand at least 6 hours in an acidic atmosphere, but in Comparative Examples 2 to 5, which did not satisfy the Ti and S content values, internal cracks occurred when maintained for 2.0 to 2.3 hours, similar to Comparative Example 1, and in Comparative Examples 6 to 9, it was confirmed that internal cracks occurred when maintained for 3.0 to 5.2 hours, which was a slight improvement over Comparative Example 1.

[0114] Comparative Example 10 is a comparative group for evaluating scale exfoliation properties, with only the coiling temperature increased to 888℃ compared to Experimental Example 1. In the case of Comparative Example 10, the tensile strength and wire properties were similar to those of Experimental Example 1, but it was confirmed that the scale exfoliation of fresh yarn was inferior and that it affected the hydrogen evaluation of the product.

[0115] In addition, in Comparative Example 11, in which mist cooling was not applied, the tensile strength deviation between the overlapping portion and the center portion of the wire coil increased to 90 MPa compared to Experimental Example 1, and the maximum thickness of coarse cementite in the overlapping portion also increased, indicating that the workability would be inferior if LP heat treatment of the fresh yarn was not applied.

[0116] In addition, as a result of measuring the crack occurrence time by maintaining the steel wires of Experimental Example 1 and Comparative Example 1 in an acidic atmosphere, as shown in Fig. 3, it was confirmed that internal cracks occurred in Comparative Example 1 when maintained for 2.2 hours, but cracks occurred in the steel wire of Experimental Example 1 of the present invention when maintained for 6.7 hours.

[0117] Through these results, it was found that by reducing the tensile strength deviation between the overlapping portion and the center portion of the wire coil to 50 MPa or less according to the present invention, a steel wire having a uniform structure can be manufactured without LP heat treatment in the drawing process by drawing and rolling, and by precipitating fine TiS in the ferrite matrix, internal cracks do not occur even when maintained for more than 6.1 hours in an acidic atmosphere, which is 2.5 times longer than when TiS is not added, so that it has excellent crack resistance and can be preferably applied for deep-sea oil well mining or transportation.

[0118] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and spirit of the claims set forth below.

Claims

1. Contains, by weight%, C: 0.20~0.40%, Si: 0.20~0.50%, Mn: 0.50~1.00%, Ti: 0.050~0.100%, S: 0.030~0.080%, the remainder being Fe and other unavoidable impurities. The atomic ratio of Ti and S (Ti / S) satisfies the following equation (1), When the diameter of the wire is D, the microstructure of the wire measured at a position of 1 / 2D to 3 / 2D in the diameter direction of the wire is a wire with excellent hydrogen embrittlement resistance containing 20 to 30% of ferrite and the remainder pearlite in area fraction. Equation (1): 0.5 ≤ (Ti weight) / (Ti mass) / (S weight) / (S mass) ≤ 1.8 2. In paragraph 1, The above wire has a cross-sectional area of ​​300 to 8000 nm in ferrite. 2 A wire having an area fraction of 58% or more of TiS precipitates.

3. In paragraph 2, A wire having a size of the above TiS precipitate of 10 nm to 50 nm.

4. In paragraph 1, The above wire is a wire having a maximum thickness of coarse cementite of 100 nm or less in the coil overlapping portion.

5. In paragraph 1, The above wire is a wire having a tensile strength deviation of 50 MPa or less between the coil overlapping portion and the center.

6. In paragraph 1, The above wire is a wire having an average tensile strength of 650 MPa or more.

7. In paragraph 1, The above wire is a wire having an average reduction area (RA) of 60% or more.

8. A step of manufacturing a wire rod by heating a billet containing C: 0.20 to 0.40%, Si: 0.20 to 0.50%, Mn: 0.50 to 1.00%, Ti: 0.050 to 0.100%, S: 0.030 to 0.080%, the remainder being Fe and other unavoidable impurities, at 1000 to 1100°C and then hot-rolling it at 900 to 1000°C; A step of winding the above wire into a coil shape at a temperature of 780 to 850°C; and A step of cooling the above-mentioned coiled wire, wherein a step of misting a refrigerant onto the overlapping portion of the coiled wire having a high stacking density during the initial cooling is performed to cool it at a cooling rate of 8 to 13°C / s; A method for manufacturing a wire comprising:

9. In paragraph 8, The above cooling is performed in a stelmore cooling bed, and the method for manufacturing wire is performed by spraying coolant by a mist sprayer installed at the tip of the cooling bed onto the overlapping portion of the wire coil during the initial cooling.

10. In paragraph 8, The above wire is a method for manufacturing a wire having a tensile strength deviation of 50 MPa or less between the overlapping portion and the center of the coil.

11. Contains C: 0.20~0.40%, Si: 0.20~0.50%, Mn: 0.50~1.00%, Ti: 0.050~0.100%, S: 0.030~0.080%, the remainder being Fe and other unavoidable impurities, A steel wire with excellent hydrogen embrittlement resistance and crack resistance of 6.1 hours or more in an acidic (99.5% H2S+CH3COOH) atmosphere.

12. In paragraph 11, Steel wire with a tensile strength of 850 MPa or more.

13. In paragraph 11, Steel wire with a yield ratio of 0.90 or higher.

14. A step of manufacturing a wire rod by heating a billet containing C: 0.20 to 0.40%, Si: 0.20 to 0.50%, Mn: 0.50 to 1.00%, Ti: 0.050 to 0.100%, S: 0.030 to 0.080%, the remainder being Fe and other unavoidable impurities, at 1000 to 1100°C and then hot-rolling it at 900 to 1000°C; A step of winding the above wire into a coil shape at a temperature of 780 to 850°C; A step of cooling the above-mentioned coiled wire, wherein a step of misting a refrigerant onto the overlapping portion of the coiled wire having a high stacking density during the initial cooling is performed to cool it at a cooling rate of 8 to 13°C / s; A step of dry drawing the cooled wire rod to a total reduction rate of 60% or more without LP (lead patent) heat treatment; A step of cold rolling up to a reduction ratio of 60% after the above dry rolling; and Step of performing high-frequency heat treatment after the above cold rolling A method for manufacturing a steel wire comprising:

15. In paragraph 14, A method for manufacturing a steel wire, wherein the above high-frequency heat treatment is performed at an A1 transformation temperature of -30°C to an A1 transformation temperature.

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