High-strength and high-corrosion-resistant steel wire, and manufacturing method therefor

Abstract

A steel wire according to one embodiment of the present invention can be a high-strength and high-corrosion-resistant steel wire comprising, by wt%, 0.50-0.75% of carbon (C), 0.5-0.9% of manganese (Mn), 0.2-0.5% of silicon (Si), 0.03-0.18% of molybdenum (Mo), and the balance of iron (Fe) and inevitable impurities, wherein an average diameter of a pearlite block of the steel wire is 25 μm or less, and a carbon content of cementite in pearlite is 18 at% or higher.

Description

High-strength, high-corrosion-resistant steel wire and method for manufacturing the same

[0001] The present invention relates to a high-strength, high-corrosion-resistant steel wire and a method for manufacturing the same.

[0002] Armor cables are reinforcing materials that support the loads on flexible pipes transporting crude oil at sea. They require high strength and resistance to H2S environments, and the methods to achieve this are as follows.

[0003] First, the strength of the material itself can be increased by adding a large amount of elements that enhance the strength of the steel. A representative example of such a reinforcing element is carbon. When the carbon content increases, the fraction of the hard phase cementite within the wire increases, and as the lamellae spacing of the pearlite structure becomes denser, the strength of the material can be improved. However, while carbon is an effective element for improving strength, it degrades sour characteristics, so an appropriate content must be selected depending on the usage environment.

[0004] Second, the wire drawing material is produced by drawing and heat-treating rolled wire to form the final wire, and its strength can be significantly enhanced through work hardening during processing. During wire drawing, work hardening can occur due to reasons such as the finening of lamellar spacing, an increase in the work hardening coefficient, and the accumulation of dislocations.

[0005] Third, separately from the above, strength can be improved by increasing the drawing strain. In this case, since the drawing strain of the material is closely related to the ductility of the material, it is advantageous for strength improvement if the material itself is processed easily without breakage during drawing, but it may have a negative effect on corrosion resistance.

[0006] However, since the above methods change the strength of the steel in an interdependent manner rather than acting independently, there is a limit to the increase in strength when controlling them independently to improve strength.

[0007] Furthermore, since sour resistance is generally inversely proportional to strength, it is difficult to improve sour resistance while increasing strength. Therefore, there is a need to develop a method that can secure excellent sour resistance while enhancing strength.

[0008] One aspect of the present invention is to provide a high-strength, high-corrosion-resistant steel wire with excellent sour characteristics, and a method for manufacturing the steel wire, by ensuring excellent sulfide stress cracking (SSC) characteristics of the steel wire.

[0009] One aspect of the present invention is to enable the formation of a fine pearlite structure through lead bath heat treatment, thereby providing a high-strength, high-corrosion-resistant steel wire and a method for manufacturing steel wire.

[0010] The problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by those skilled in the art from the description below.

[0011] As a means to achieve the above-mentioned purpose, a steel wire according to one embodiment of the present invention may comprise, in weight %, 0.50 to 0.75% carbon (C), 0.5 to 0.9% manganese (Mn), 0.2 to 0.5% silicon (Si), 0.03 to 0.18% molybdenum (Mo), and the remainder being iron (Fe) and unavoidable impurities.

[0012] According to one embodiment of the present invention, the average diameter of the pearlite block particles of the steel wire may be 25 μm or less.

[0013] The carbon content of cementite in the pearlite of the steel wire according to one embodiment of the present invention may be 18 atomic percent.

[0014] A steel wire according to one embodiment of the present invention may have a Sulfide Stress Cracking (SSC) fracture time of 720 hours or more measured under an H2S environment.

[0015] A steel wire according to one embodiment of the present invention may have a yield strength of 650 MPa or more.

[0016] A steel wire according to one embodiment of the present invention may have a tensile strength of 1000 MPa or more.

[0017] A method for manufacturing a steel wire according to one embodiment of the present invention comprises the steps of: manufacturing a wire rod by hot rolling a steel material at 950°C to 1100°C, wherein the steel material comprises, in weight percent, 0.50 to 0.75% carbon (C), 0.5 to 0.9% manganese (Mn), 0.2 to 0.5% silicon (Si), 0.03 to 0.18% molybdenum (Mo), and the remainder being iron (Fe) and unavoidable impurities; heating the wire rod to 950 to 1050°C; and heat-treating the heated wire rod in a lead bath at 500 to 600°C. The method may include the step of producing a steel wire by drawing the heat-treated wire rod with a cross-sectional reduction rate of 60 to 80%, wherein the average diameter of the block particles of the steel wire is 25 μm or less and the carbon content of the cementite in the pearlite is 18 atomic percent or more.

[0018] In addition, the above steel wire may have a Sulfide Stress Cracking (SSC) fracture time of 720 hours or more measured under an H2S environment.

[0019] In addition, the above steel wire may have a yield strength of 650 MPa or more.

[0020] In addition, the above steel wire may have a tensile strength of 1000 MPa or more.

[0021] According to the present invention, by adding molybdenum (Mo) to the steel wire, it is possible to provide a high-strength, high-corrosion-resistant steel wire and a method for manufacturing the steel wire that exhibits excellent sulfide stress cracking (SSC) characteristics and excellent sour characteristics.

[0022] In addition, a fine pearlite structure is formed through lead bath heat treatment, thereby providing a method for manufacturing high-strength, high-corrosion-resistant steel wire and steel wire.

[0023] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0024] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0025] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense.

[0026] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0027] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight.

[0028] A steel wire according to one embodiment of the present invention comprises, in weight percent, 0.50 to 0.75% carbon (C), 0.5 to 0.9% manganese (Mn), 0.2 to 0.5% silicon (Si), 0.03 to 0.18% molybdenum (Mo), and the remainder being iron (Fe) and unavoidable impurities, and the average diameter of the pearlite block particles of the steel wire is 25 μm or less, and the carbon content of the cementite in the pearlite is 18 atomic percent or more.

[0029] Hereinafter, the composition range of the steel wire of the present invention will be described in detail.

[0030] Carbon (C) may be 0.50% to 0.75%.

[0031] The carbon mentioned above is an economical element for improving the strength of the steel wire. If the carbon content is less than 0.50 weight%, the strength of the steel wire may decrease. On the other hand, if the carbon content exceeds 0.75 weight%, ductility may decrease. In particular, since sourness decreases as the carbon content increases, it is desirable to limit the upper limit of the carbon content to 0.75 weight%. Accordingly, it is desirable for carbon to be included in an amount of 0.50 to 0.75 weight% with respect to the total weight of the high-strength, high-corrosion-resistant steel wire. More preferably, it may be 0.60 to 0.65%.

[0032] Manganese (Mn) may be 0.5% to 0.9%.

[0033] Manganese is an element with severe central segregation, and if the manganese content exceeds 0.9 weight%, it is highly likely to cause a low-temperature structure. On the other hand, if the manganese content is less than 0.5 weight%, there is a problem in that it is difficult to ensure hardenability. Therefore, it is desirable to include manganese in an amount of 0.5 to 0.9 weight% with respect to the total weight of the high-strength, high-corrosion-resistant steel wire. More preferably, it may be 0.6 to 0.8%.

[0034] The silicon (Si) content may be 0.2% to 0.5%.

[0035] Silicon exhibits the effect of suppressing strength reduction by stabilizing the pearlite layer along with solid solution strengthening. If the silicon content is less than 0.2 weight%, it is difficult to obtain the aforementioned effects, and if the silicon content exceeds 0.5 weight%, there is a problem of reduced wire drawing performance. Therefore, it is preferable that silicon be included in an amount of 0.2 to 0.5 weight% with respect to the total weight of the high-strength, high-corrosion-resistant steel wire. More preferably, it may be 0.2 to 0.3%.

[0036] The molybdenum (Mo) content may be 0.03% to 0.18%.

[0037] Molybdenum is effective in improving corrosion resistance. However, if the molybdenum content is less than 0.03 weight%, it is difficult to obtain the aforementioned effect. If the molybdenum content exceeds 0.18 weight%, hardenability is significantly increased, leading to increased heat treatment time and reduced productivity. Therefore, it is preferable that molybdenum be included in an amount of 0.03 to 0.18 weight% relative to the total weight of the high-strength, high-corrosion-resistant steel wire. More preferably, it may be 0.10 to 0.15%.

[0038] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0039] The average diameter of the pearlite block particles in the high-strength steel wire of the present invention is 25 μm or less. The definition of a pearlite block is a region where the crystal orientation of ferrite is the same, as described in Takahashi et al.’s *Journal of the Japan Institute of Metallurgy* Vol. 42 (1978), p. 702, and was determined by an Electron Backscatter Diffraction (EBSD) device. The diameter of the pearlite block particles was determined from the measurement results obtained by an EBSD device capable of obtaining a ferrite crystal orientation map. Strength and hardness increase as the pearlite block particles become smaller. This is because the fine pearlite structure induces more uniform deformation. Furthermore, by controlling the average diameter of the pearlite block particles to 25 μm or less, a steel wire with improved fatigue resistance can be obtained.

[0040] Meanwhile, the carbon content contained in the cementite within the pearlite of the high-strength steel wire of the present invention will be explained in detail.

[0041] Since cementite is composed of three Fe atoms and one C atom, the carbon content of cementite (Fe3C) in pearlite before drawing is 25 atomic percent. However, as drawing is performed, the carbon content of cementite in pearlite of the steel wire becomes smaller than 25 atomic percent. This means that as drawing is performed, cementite decomposition occurs in addition to plastic deformation of the cementite in pearlite. This cementite decomposition is a phenomenon that occurs because it is energetically stable for carbon to escape from cementite and move into the ferrite matrix.

[0042] The steel wire of the present invention comprises, in weight percent, 0.50 to 0.75% carbon (C), 0.5 to 0.9% manganese (Mn), 0.2 to 0.5% silicon (Si), 0.03 to 0.18% molybdenum (Mo), and the remainder being iron (Fe) and unavoidable impurities. It is preferable that the maximum value of the pearlite diameter of the steel wire is 25 μm or less, and the carbon content of cementite within the pearlite is 18 atomic percent or more. By exhibiting a carbon content of cementite within the above-described range, a steel wire with improved strength and enhanced corrosion resistance can be obtained.

[0043] Meanwhile, the steel wire of the present invention having the alloy composition and microstructure described above has a Sulfide Stress Cracking (SSC) fracture time of 720 hours or more measured under an H2S environment. Here, an H2S environment refers to a sour environment in which hydrogen sulfide is present and hydrogen diffusion occurs. By adding molybdenum (Mo), the diffusion of hydrogen (H) can be suppressed, thereby exhibiting sour resistance.

[0044] For steel wire to be used in a sour environment, the SSC fracture time must be at least 720 hours. For example, it must be approximately 720 to 1,000 hours; if fracture occurs within less than 720 hours, it cannot be used in a sour environment.

[0045] In addition, the steel wire of the present invention may have a yield strength of 650 MPa or more.

[0046] In addition, the steel wire of the present invention may have a tensile strength of 1000 MPa or more. Specifically, after drawing the wire rod, the carbon content contained in the cementite becomes 18 atomic percent or more, causing carbon emitted from the cementite to migrate to the ferrite, thereby causing solid solution strengthening. As a result, it is preferable for the steel wire to exhibit a tensile strength of 1000 MPa.

[0047] Hereinafter, a method for manufacturing a high-strength, high-corrosion-resistant steel wire, which is one aspect of the present invention, will be described in detail.

[0048] One embodiment of the present invention comprises the steps of: manufacturing a wire rod by hot rolling a steel material at 950°C to 1100°C, the steel material comprising, in weight percent, 0.50 to 0.75% carbon (C), 0.5 to 0.9% manganese (Mn), 0.2 to 0.5% silicon (Si), 0.03 to 0.18% molybdenum (Mo), and the remainder being iron (Fe) and unavoidable impurities; heating the wire rod to 950 to 1050°C; and heat-treating the heated wire rod in a lead bath at 500 to 600°C. The present invention provides a method for manufacturing a high-strength, high-corrosion-resistant steel wire, comprising the step of manufacturing a steel wire by drawing the heat-treated wire rod with a cross-sectional reduction rate of 60 to 80%, wherein the average diameter of the pearlite block particles of the steel wire is 25 μm or less and the carbon content of the cementite in the pearlite is 18 atomic percent or more.

[0049] The components included in the above steel material preferably comprise, in weight percent as in the components included in the high-strength, high-corrosion-resistant steel wire described above, 0.50 to 0.75% carbon (C), 0.5 to 0.9% manganese (Mn), 0.2 to 0.5% silicon (Si), 0.03 to 0.18% molybdenum (Mo), and the remainder being iron (Fe) and unavoidable impurities. The reason for limiting the compositional range of each element is as described above.

[0050] In manufacturing wire rods by hot rolling the above steel, it is preferable to perform the hot rolling at a temperature of 950 to 1100°C. At this time, if the steel is hot rolled at a temperature below 950°C, sufficient recrystallization does not occur, resulting in a problem of microstructure non-uniformity. If the steel is hot rolled at a temperature exceeding 1100°C, there is a problem of reduced strength and ductility due to grain coarsening. In addition, excessive decarburization may occur, which can worsen drawing workability. If hot rolling is performed at a temperature below 1000°C, a problem may arise where the roll life decreases as the temperature drops. Therefore, it is more preferable to perform hot rolling at a temperature of 1000 to 1050°C.

[0051] It is preferable to heat the wire rod manufactured by the above hot rolling to 950 to 1050°C. Through this heating, an austenite structure can be formed. If heated to a temperature below 950°C, there is a problem in that austenitizing of the pearlite structure does not occur, and spheroidized cementite may remain. If heated above 1050°C, there is a problem of reduced strength and ductility due to grain coarsening. More preferably, it can be heated to 980 to 1020°C.

[0052] Subsequently, it is desirable to heat-treat the heated wire rod in a lead bath at 500 to 600°C. Through lead bath heat treatment, a pearlite structure can be formed. Additionally, by incorporating molybdenum into the steel, the diffusion rate during lead bath heat treatment is slowed down, thereby forming a fine pearlite structure. This allows for improved strength and elongation. If the temperature during lead bath heat treatment is below 500°C, there is a problem of reduced ductility due to a non-uniform microstructure containing bainite. If the lead bath heat treatment temperature exceeds 600°C, there is a problem of reduced strength due to the formation of coarse pearlite. Therefore, it is desirable to heat-treat the heated wire rod in a lead bath at 500 to 600°C. Furthermore, if the temperature is below 530°C, a low-temperature structure may develop, potentially worsening drawing workability, and if it exceeds 560°C, the interlayer spacing of the pearlite becomes coarse, leading to a problem where the target level of strength and ductility cannot be secured. Therefore, more preferably, lead bath heat treatment can be performed at 530 to 560°C.

[0053] After the above lead bath heat treatment, steel wire can be manufactured by drawing with a cross-sectional reduction rate of 60 to 80%. The cross-sectional reduction rate is a ratio indicating how much the cross-sectional area of ​​the material is reduced through the drawing process. The cross-sectional reduction rate can be calculated using the following formula.

[0054]

[0055] In the above equation, A0 is the initial cross-sectional area, A f represents the cross-sectional area after drawing.

[0056] If the above cross-sectional reduction rate is less than 60%, the amount of fresh processing is insufficient, and sufficient strength cannot be secured. On the other hand, if it exceeds 80%, there is a risk of delamination occurring. Here, delamination refers to a phenomenon in which surface cracks propagate along the length of the steel wire, causing the steel wire to fracture.

[0057] In addition, the average diameter of the pearlite block particles of the above steel wire may be 25 μm or less.

[0058] In addition, the carbon content of cementite in the pearlite of the above steel wire may be 18 atomic percent or more.

[0059] In addition, the above steel wire may have a Sulfide Stress Cracking (SSC) fracture time of 720 hours or more measured under an H2S environment.

[0060] In addition, the above steel wire may have a yield strength of 650 MPa or more.

[0061] In addition, the above steel wire may have a tensile strength of 1000 MPa or more.

[0062] The present invention will be explained in more detail below through embodiments thereof. However, the present invention is not limited to the following embodiments.

[0063] Examples

[0064] Steel billets of Examples 1 to 3 and Comparative Examples 1 to 2 having the composition shown in Table 1 below were hot-rolled at 950 to 1100°C to produce wire rods. Subsequently, the wire rods were heated to 950 to 1050°C, then heat-treated in a lead bath at 550 to 600°C, and then drawn with a cross-sectional reduction rate of 60 to 80% to produce steel wires of Examples 1 to 3 and Comparative Examples 1 to 2.

[0065] Classification C (wg%) Si (wg%) Mn (wg%) Mo (wg%) Example 10.6 20.2 0.7 0.10 Example 20.6 20.2 0.7 0.12 Example 30.6 20.2 0.7 0.15 Comparative Example 10.6 20.2 0.70 Comparative Example 20.6 20.2 0.7 0.25

[0066] Subsequently, the yield strength, tensile strength, cementite carbon content, average diameter of pearlite block particles, and SSC characteristics of the steel wires of the above-mentioned examples and comparative examples were measured, and the results are shown in Table 2 below.

[0067] Tensile strength was measured using a universal test machine (UTM) with the model name KDPI-130 Series.

[0068] The carbon content in cementite was measured by analyzing the orientation using an Electron Backscatter Diffraction (EBSD) analyzer with the model name JSM 7200F.

[0069] The average diameter of the pearlite block grains was determined by averaging three arbitrary locations at positions moved 1 / 4 of the diameter from the surface of the steel wire cross-section in the direction of the center using an Electron Backscatter Diffraction (EBSD) analyzer.

[0070] In addition, the SSC fracture time was measured by determining the time it takes for fracture to occur while force is applied to the specimen under an H2S atmosphere.

[0071] Classification Yield Strength (MPa) Tensile Strength (MPa) Cementite Carbon Content (%) Pearlite Block Grain Average Diameter (μm) SSC Break Time (Hour) Example 1 67 410 38 19 25 720 or higher Example 2 67 910 49 20 21 720 or higher Example 3 68 910 62 21 17 720 or higher Comparative Example 1 62 28 99 15 42 26 Comparative Example 2 58 78 73 16 29 17

[0072] Referring to Table 2 above, it can be confirmed that Examples 1 to 3, which satisfy the alloy composition, cementite carbon content, and average diameter of pearlite block particles of the present invention, show increased yield strength and tensile strength compared to Comparative Examples 1 and 2.

[0073] In addition, in the case of Comparative Example 1, which did not contain molybdenum, failure occurred after 26 hours in the SSC test, and in the case of Comparative Example 2, which contained more than 0.18% molybdenum, failure occurred after 17 hours. On the other hand, Examples 1 to 3, which fall within the scope of the present invention with a molybdenum content of 0.03 to 0.18%, did not experience failure for more than 720 hours in the SSC test. Through this, it can be confirmed that the addition of molybdenum (Mo) suppresses the diffusion of hydrogen (H), thereby exhibiting sour resistance characteristics.

[0074] Accordingly, according to the present invention, by optimizing the alloy composition and controlling the microstructure and manufacturing method, it is possible to improve strength while ensuring excellent resistance to sour.

[0075] 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 concept of the claims set forth below.

Claims

1. A high-strength, high-corrosion-resistant steel wire comprising, in weight percent, 0.50 to 0.75% carbon (C), 0.5 to 0.9% manganese (Mn), 0.2 to 0.5% silicon (Si), 0.03 to 0.18% molybdenum (Mo), and the remainder being iron (Fe) and unavoidable impurities, wherein the average diameter of the pearlite block particles of the steel wire is 25 μm or less, and the carbon content of the cementite in the pearlite is 18 atomic percent or more.

2. In Claim 1, The above steel wire is a high-strength, high-corrosion-resistant steel wire with a Sulfide Stress Cracking (SSC) fracture time of 720 hours or more measured under an H2S environment.

3. In Claim 1, The above steel wire is a high-strength, high-corrosion-resistant steel wire with a yield strength of 650 MPa or higher.

4. In Claim 1, The above steel wire is a high-strength, high-corrosion-resistant steel wire with a tensile strength of 1000 MPa or more.

5. A step of manufacturing a wire rod by hot rolling a steel material containing, in weight percent, 0.50 to 0.75% carbon (C), 0.5 to 0.9% manganese (Mn), 0.2 to 0.5% silicon (Si), 0.03 to 0.18% molybdenum (Mo), and the remainder being iron (Fe) and unavoidable impurities, at 950℃ to 1100℃; A step of heating the above wire to 950 to 1050℃; A step of heat-treating the heated wire rod at 500 to 600°C in a lead bath; and The method includes the step of manufacturing a steel wire by drawing the heat-treated wire rod with a cross-sectional reduction rate of 60 to 80%, and A method for manufacturing a high-strength, high-corrosion-resistant steel wire, wherein the average diameter of the pearlite block particles of the steel wire is 25 μm or less, and the carbon content of the cementite in the pearlite is 18 atomic percent or more.

6. In Claim 5, The above steel wire is a method for manufacturing a high-strength, high-corrosion-resistant steel wire having a Sulfide Stress Cracking (SSC) fracture time of 720 hours or more measured under an H2S environment.

7. In Claim 5, The above steel wire is a method for manufacturing a high-strength, high-corrosion-resistant steel wire having a yield strength of 650 MPa or more.

8. In Claim 5, The above steel wire is a method for manufacturing a high-strength, high-corrosion-resistant steel wire having a tensile strength of 1000 MPa or more.

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