The present disclosure relates to a wire rod and a steel wire having excellent hydrogen embrittlement resistance, and manufacturing methods therefor.

As reinforcement materials for oil well drilling or transportation, carbon steel of a 0.35% level is used as products after being drawn and rolled. Generally, reinforcement materials are produced by subjecting a wire rod produced in a steel mill to lead patenting (LP) heat treatment in a wire drawing company to form fine pearlite, increasing reduction in wire diameter and strength through dry drawing, securing a shape for a final product through cold rolling, and removing surface residual stress through high-frequency heat treatment.

In the carbon steel of the 0.35% level, silicon (Si) and manganese (Mn) are added in amounts of 0.25% and 0.8%, respectively. Meanwhile, according to demand for high strength, a carbon (C) content is increased up to 0.72% to improve strength. This is because carbon (C) is an element capable of increasing strength most effectively relative to price, but the carbon (C) content exceeding that of eutectoid steel results in formation of grain boundary proeutectoid cementite, leading to a high possibility of hydrogen embrittlement. Meanwhile, strength-increasing elements such as chromium (Cr) and molybdenum (Mo) are rarely added due to deterioration of workability or formation of a low-temperature structure causing wire breakage during processing.

As energy is depleted, an environment for oil well extraction has recently moved from offshore to deep sea. Since oil wells in the deep sea contain large amounts of hydrogen and sulfur compared to the offshore, a steel wire having high hydrogen resistance is required. The steel wire requiring a certain amount of carbon to be used as a reinforcement material results in a microstructure having a mixed structure of ferrite and pearlite, and thus necessitates the use of precipitates capable of trapping hydrogen.

To solve the above-described problems, the present disclosure provides a wire rod and a steel wire, which have excellent crack resistance in an acidic (99.5% H2S + CH3COOH) atmosphere by precipitating fine TiS in a ferrite matrix and thus are preferably applied for deep-sea oil well drilling or transportation, and manufacturing methods therefor.

In addition, the present disclosure provides a wire rod and a steel wire, which have excellent product reliability by mist-spraying a refrigerant onto an overlapping portion of a wire rod coil during initial cooling to reduce a tensile strength deviation between the overlapping portion and a central portion to 50 MPa or less, and further enable drawing and rolling without LP heat treatment in a wire drawing company, and manufacturing methods therefor.

To achieve the above object, a wire rod with excellent hydrogen embrittlement resistance according to an embodiment of the present disclosure comprises, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.20 to 0.50% of silicon (Si), 0.50 to 1.00% of manganese (Mn), 0.050 to 0.100% of titanium (Ti), 0.030 to 0.080% of sulfur (S), and the remainder of iron (Fe) and other inevitable impurities, wherein an atomic ratio of Ti to S (Ti/S) satisfies Formula (1) below, and when a radius of the wire rod is D, a microstructure of the wire rod measured at a position of 1/2D to 3/2D in a radial direction of the wire rod may comprise 20 to 30% of ferrite and residual pearlite in area fraction. Formula (1): 0.5 ≤ [(Ti content (wt%)) / (atomic weight of Ti)] / [(S content (wt%)) / (atomic weight of S)] ≤ 1.8

In addition, in the wire rod according to an embodiment of the present disclosure, TiS precipitates per 300 to 8000 nm² of an equivalent cross-sectional area in ferrite may be 58% or more in area fraction.

In addition, in the wire rod according to an embodiment of the present disclosure, a size of the TiS precipitates may be 10 nm to 50 nm.

In addition, in the wire rod according to an embodiment of the present disclosure, a maximum thickness of coarse cementite in an overlapping portion of a coil may be 100 nm or less.

In addition, in the wire rod according to an embodiment of the present disclosure, a tensile strength deviation between the overlapping portion and a central portion of the coil may be 50 MPa or less.

In addition, the wire rod according to an embodiment of the present disclosure may have an average tensile strength of 650 MPa or more.

In addition, the wire rod according to an embodiment of the present disclosure may have an average reduction of area (RA) of 60% or more.

In addition, a method for manufacturing a wire rod according to an embodiment of the present disclosure may comprise: manufacturing a wire rod by heating a billet comprising, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.20 to 0.50% of silicon (Si), 0.50 to 1.00% of manganese (Mn), 0.050 to 0.100% of titanium (Ti), 0.030 to 0.080% of sulfur (S), and the remainder of iron (Fe) and other inevitable impurities at 1000 to 1100°C and then hot rolling the billet at 900 to 1000°C; coiling the wire rod into a coil shape at a temperature of 780 to 850°C; and cooling the coiled wire rod, wherein the cooling comprises mist-spraying a refrigerant onto an overlapping portion of the wire rod coil having high stacking density during initial cooling to cool the overlapping portion at a cooling rate of 8 to 13°C/s.

In addition, the cooling according to an embodiment of the present disclosure is performed in a Stelmor cooling zone, and may be performed by spraying the refrigerant onto the overlapping portion of the wire rod coil by a mist-spraying device installed at a front end portion of the cooling zone during initial cooling.

In addition, in the wire rod according to an embodiment of the present disclosure, a tensile strength deviation between an overlapping portion and a central portion of the coil may be 50 MPa or less.

In addition, a steel wire with excellent hydrogen embrittlement resistance according to an embodiment of the present disclosure comprises, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.20 to 0.50% of silicon (Si), 0.50 to 1.00% of manganese (Mn), 0.050 to 0.100% of titanium (Ti), 0.030 to 0.080% of sulfur (S), and the remainder of iron (Fe) and other inevitable impurities, and may have a crack resistance of 6.1 hours or more in an acidic (99.5% H₂S + CH₃COOH) atmosphere.

In addition, the steel wire according to an embodiment of the present disclosure may have a tensile strength of 850 MPa or more.

In addition, the steel wire according to an embodiment of the present disclosure may have a yield ratio of 0.90 or more.

In addition, a method for manufacturing a steel wire according to an embodiment of the present disclosure may comprise: manufacturing a wire rod by heating a billet comprising, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.20 to 0.50% of silicon (Si), 0.50 to 1.00% of manganese (Mn), 0.050 to 0.100% of titanium (Ti), 0.030 to 0.080% of sulfur (S), and the remainder of iron (Fe) and other inevitable impurities at 1000 to 1100°C and then hot rolling the billet at 900 to 1000°C; coiling the wire rod into a coil shape at a temperature of 780 to 850°C; cooling the coiled wire rod, wherein the cooling comprises mist-spraying a refrigerant onto an overlapping portion of the wire rod coil having high stacking density during initial cooling to cool the overlapping portion at a cooling rate of 8 to 13°C/s; dry drawing the cooled wire rod with a total reduction ratio of 60% or more without lead patenting (LP) heat treatment; cold rolling the dry-drawn wire rod to a reduction ratio of 60%; and performing high-frequency heat treatment after the cold rolling.

In addition, the high-frequency heat treatment according to an embodiment of the present disclosure may be performed at A1 transformation temperature -30°C to A1 transformation temperature.

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

The wire rod and the steel wire of the present disclosure 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 drilling or transportation.

Furthermore, by mist-spraying a refrigerant onto an overlapping portion of a wire rod coil during initial cooling to cool the overlapping portion, a tensile strength deviation between the overlapping portion and a central portion is reduced to 50 MPa or less, thereby providing excellent product reliability, and drawing and rolling can be performed without LP heat treatment in a wire drawing company.

• FIG. 1 is a view showing a central portion and an overlapping portion of a wire rod coil according to an embodiment of the present disclosure.
• FIG. 2 is a view showing a temperature gap between an overlapping portion and a central portion of a coil in a case of cooling by mist-spraying a refrigerant in an initial cooling step of a wire rod coil according to an embodiment of the present disclosure and in a case of cooling by a conventional method without mist cooling.
• FIG. 3 is a result of measuring crack occurrence time after maintaining a steel wire manufactured according to an embodiment of the present disclosure and a conventional steel wire in an acidic (99.5% H2S + CH3COOH) atmosphere.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are presented to sufficiently convey the spirit of the present disclosure to those skilled in the art to which the present disclosure pertains. The present disclosure is not limited to the embodiments presented herein and may be embodied in other forms. In the drawings, illustration of parts unrelated to the description may be omitted to clarify the present disclosure, and sizes of components may be somewhat exaggerated for understanding.

In addition, when a part "includes" a certain component, this means that it may further include other components, rather than excluding other components, unless specifically stated to the contrary.

Expressions in the singular include expressions in the plural unless the context clearly indicates an exception.

Generally, TiS is known as a precipitate capable of trapping diffusible or non-diffusible hydrogen, but it has not been used in products subject to drawing processing because it may cause wire breakage during processing.

Accordingly, in the present disclosure, it is intended to improve hydrogen embrittlement resistance of a steel wire by precipitating fine TiS precipitates and controlling conditions of drawing and rolling to facilitate hydrogen trapping without causing wire breakage during processing.

A wire rod according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.20 to 0.50% of silicon (Si), 0.50 to 1.00% of manganese (Mn), 0.050 to 0.100% of titanium (Ti), 0.030 to 0.080% of sulfur (S), and the remainder of iron (Fe) and other inevitable impurities.

Hereinafter, reasons for limiting the component composition of the wire rod will be described in detail. All component compositions below refer to wt% unless otherwise specified.

A content of carbon (C) may be 0.20% to 0.40%.

C is an element capable of increasing material strength most effectively, and increasing the C content by 0.1% has an effect of increasing strength by 100 MPa. The C content being less than 0.20%, it may be difficult to achieve strength of a target product, and if the C content exceeds 0.40%, hydrogen embrittlement may deteriorate due to an increase in pearlite fraction. Considering this, the C content is preferably included in an amount of 0.20% to 0.40%, and more preferably 0.25% to 0.35%.

A content of silicon (Si) may be 0.20% to 0.50%.

Si is a solid solution strengthening element capable of increasing strength of a 15 MPa level when added in an amount of 0.20%. However, adding a large amount may affect scale peelability due to formation of FeSiO₄ or the like, so it is preferable to limit an upper limit thereof to 0.50%. If the Si content is less than 0.20%, it may be difficult to secure strength, and if the Si content exceeds 0.50%, there may be difficulty in scale removal at a wire drawing company.

A content of manganese (Mn) may be 0.50% to 1.00%.

Mn increases strength of a 20 MPa level when increased by 0.10%, but is added for the purpose of imparting sufficient hardenability during heat treatment. If the Mn content is less than 0.50%, it may be difficult to achieve target strength and secure hardenability during Stelmor cooling, and if the Mn content exceeds 1.00%, wire breakage occurs during drawing and rolling due to segregation, so it is preferable to limit an upper limit thereof to 1.00%.

A content of titanium (Ti) may be 0.050% to 0.100%.

For trapping of diffusible or non-diffusible hydrogen in steel, fine TiS precipitation in a ferrite matrix is required. If the Ti content is less than 0.050%, sufficient TiS precipitates are not formed, making it impossible to secure crack resistance in an acidic (99.5% H2S + CH3COOH) atmosphere, and if the Ti content exceeds 0.100%, tundish nozzle clogging may occur, so it is preferable to limit an upper limit thereof to 0.100%.

A content of sulfur (S) may be 0.030% to 0.080%.

S is a component necessary for formation of fine TiS precipitates. If the S content is less than 0.030%, formation of TiS precipitates may be difficult, and if the S content exceeds 0.080%, occurrence of center segregation deteriorates, causing wire breakage during processing, so it is preferable to limit an upper limit thereof to 0.080%.

The remaining component of the present disclosure is iron (Fe). However, unintended impurities may inevitably be mixed from raw materials or surrounding environments in a typical manufacturing process, so this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in the present specification.

A microstructure of the wire rod according to an embodiment of the present disclosure may include ferrite and pearlite to secure strength and drawability, and when a radius of the wire rod is D, the microstructure of the wire rod measured at a position of 1/2D to 3/2D in a radial direction of the wire rod may be included as 20 to 30% of ferrite and residual pearlite in area fraction. If the ferrite area fraction exceeds 30%, wire breakage may occur during drawing processing.

In the wire rod according to an embodiment of the present disclosure, considering amounts of Ti and S for forming TiS precipitates affecting hydrogen embrittlement resistance, Formula (1) below may be derived. Formula (1): 0.5 ≤ [(Ti content (wt%)) / (atomic weight of Ti)] / [(S content (wt%)) / (atomic weight of S)] ≤ 1.8

If an atomic ratio of Ti to S (Ti/S) of Formula (1) satisfies Formula (1), fine TiS precipitates having a size of 10 to 50 nm can be generated, and thus hydrogen embrittlement resistance of the wire rod is remarkably improved.

In addition, the wire rod according to an embodiment of the present disclosure may include TiS precipitates having an area fraction of 58% or more per 300 to 8000 nm² of an equivalent cross-sectional area in ferrite.

If TiS precipitates are finely and evenly dispersed in the ferrite matrix, they may provide trap sites for hydrogen permeated into the inside to inhibit movement of hydrogen, thereby preventing hydrogen from aggregating in one place.

As such, in order to be suitable for trapping hydrogen permeated into the inside of the wire rod and inhibiting aggregation of hydrogen, TiS precipitates are preferably included in an area fraction of 58% or more per 300 to 8000 nm² of a converted unit area in ferrite. If TiS precipitates are included in an area fraction of less than 58%, distribution of the precipitates becomes small, reducing positions capable of trapping hydrogen, so an effect of inhibiting hydrogen aggregation is lowered, and thus sufficient hydrogen embrittlement resistance cannot be secured.

In addition, a size of the TiS precipitates is preferably 10 to 50 nm. If the size is less than 10 nm, the size of the precipitate itself is too small to properly perform a role of inhibiting movement of hydrogen permeated into the inside, and if the size exceeds 50 nm, the precipitates are coarse and hydrogen trapping ability is lowered, so the effect of inhibiting hydrogen aggregation may be lowered, and wire breakage may be caused during drawing processing or cold rolling.

In the wire rod according to an embodiment of the present disclosure, a maximum thickness of coarse cementite in an overlapping portion of a coil may be 100 nm or less.

Coarse cementite refers to cementite having a particle size of more than 100 nm and 300 nm or less, and if the maximum thickness of coarse cementite exceeds 100 nm in the overlapping portion of the wire rod coil of the present disclosure, cracks are likely to occur during drawing processing, deteriorating wire drawability.

In addition, in the wire rod according to an embodiment of the present disclosure, a tensile strength deviation between an overlapping portion and a central portion of a coil may be 50 MPa or less.

Generally, in a wire rod, an edge portion (overlapping portion) having relatively higher stacking (overlapping) density than a center portion (central portion) of a coil is inevitably generated. At this time, as shown in FIG. 1, the central portion of the wire rod coil refers to a center portion of the coil having relatively low stacking density corresponding to (1 to 2, 5 to 6) when the wire rod is divided into 8 equal parts, and the overlapping portion refers to an edge portion of the coil having high stacking density corresponding to (3 to 4, 7 to 8).

Accordingly, during manufacture of the wire rod, transformation of structure varies due to a cooling difference according to stacking density between the overlapping portion and the central portion, and as a result, a tensile strength deviation occurs between the overlapping portion and the central portion.

However, in the present disclosure, the wire rod can increase reliability of a product by reducing the tensile strength deviation between the overlapping portion and the central portion of the coil to 50 MPa or less.

In addition, the wire rod according to an embodiment of the present disclosure satisfies the component system and microstructure, and thus may have an average tensile strength of 650 MPa or more and an average reduction of area (RA) of 60% or more.

Next, a method for manufacturing a wire rod with excellent hydrogen embrittlement resistance according to an embodiment of the present disclosure will be described.

A method for manufacturing a wire rod with excellent hydrogen embrittlement resistance according to an embodiment of the present disclosure includes: manufacturing a wire rod by heating a billet comprising, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.20 to 0.50% of silicon (Si), 0.50 to 1.00% of manganese (Mn), 0.050 to 0.100% of titanium (Ti), 0.030 to 0.080% of sulfur (S), and the remainder of iron (Fe) and other inevitable impurities at 1000 to 1100°C and then hot rolling the billet at 900 to 1000°C; coiling the wire rod into a coil shape at a temperature of 780 to 850°C; and cooling the coiled wire rod, wherein the cooling comprises mist-spraying a refrigerant onto an overlapping portion of the wire rod coil having high stacking density during initial cooling to cool the overlapping portion at a cooling rate of 8 to 13°C/s.

The reasons for limiting the component range of each alloying element are as described above, and each manufacturing step will be described in more detail below.

First, a billet having the above-described composition components is manufactured.

Thereafter, the billet may be heated at 1000 to 1100°C under typical conditions and then hot rolled at 900 to 1000°C. For example, the heated billet may be subjected to hot rolling sequentially composed of rough rolling, intermediate rough rolling/finish rolling, and final rolling to be manufactured into a wire rod of a desired size.

Subsequently, a step of coiling into a coil shape is performed.

A coiling temperature is associated with scale peelability at a wire drawing company. Since steel wires for oil wells remove scale through pickling at a wire drawing company, there is a problem in peelability if scale is dense or thick.

Therefore, the coiling step according to an embodiment of the present disclosure is preferably performed in a temperature range of 780 to 850°C. If the coiling temperature is less than 780°C, poor coiling may be induced, and if the coiling temperature exceeds 850°C, scale (FeO) may be excessively formed, and thus there may be a problem that loss occurs and a cooling rate must be increased.

The coiled wire rod falls onto a Stelmor cooling conveyor in a circular shape and is stacked constantly and moved. The moved wire rod coils have a density difference as portions where the overlapping portion and the central portion of each wire rod coil overlap are small and large in a proceeding direction and a vertical direction thereof, and due to such a density difference between the overlapping portion and the central portion, cooling rates for respective parts become different in a process of slow cooling at room temperature during conveyor movement, which eventually causes cooling deviation.

That is, the overlapping portion, which is a portion having relatively high stacking density on the conveyor, has a lower cooling rate than the central portion and is extracted without structure transformation being completely completed until passing through a slow cooling region, and such an untransformed portion is cooled at a high speed while coming into contact with air.

Therefore, the central portion of the wire rod having relatively low density completes structure transformation within the conveyor, but the structure of the overlapping portion having relatively high density comes into contact with air in a state where it is not completely transformed and is rapidly cooled, exhibiting tensile strength much higher than a normal value, thereby causing significant tensile strength deviation between the overlapping portion and the central portion of the wire rod coil, resulting in non-uniform structure.

Therefore, in the present disclosure, the coiled wire rod is cooled, but a refrigerant is mist-sprayed onto the overlapping portion of the wire rod coil having high stacking density during initial cooling to cool the overlapping portion, thereby intending to reduce temperature deviation between the overlapping portion and the central portion.

At this time, by mist-spraying the refrigerant onto the overlapping portion of the wire rod coil having high stacking density during initial cooling to cool the overlapping portion at a cooling rate of 8 to 13°C/s, the temperature deviation between the overlapping portion and the central portion can be reduced.

The cooling may be performed using a Stelmor cooling zone. That is, if the coiled wire rod is transferred to the Stelmor cooling zone and moved while spraying the refrigerant onto the overlapping portion of the coil through a mist-spraying device installed at a front end portion of an entrance of the Stelmor cooling zone, cooling deviation occurs between the central portion and the overlapping portion of the coil at the initial stage of cooling, but when the wire rod coil exits the cooling zone, temperature deviation between the central portion and the overlapping portion of the coil disappears.

That is, in the present disclosure, the overlapping portion of the wire rod coil is intensively cooled in the initial stage of cooling to rapidly lower the temperature of the overlapping portion and passed through the cooling zone, so that a temperature difference between the central portion and the overlapping portion of the coil is reduced after passing through the cooling zone, and transformation is completed regardless of a stacking density difference, thereby preventing occurrence of tensile strength deviation of the wire rod.

At this time, the mist-spraying device is installed at the front end portion of the cooling zone so that initial cooling of the overlapping portion of the coil can be performed up to a distance including a section where heat generation occurs while the wire rod coil passes through the cooling zone.

In addition, the overlapping portion of the wire rod coil is cooled at a cooling rate of 8 to 13°C/s while spraying the refrigerant by the mist-spraying device installed at the front end portion. If the cooling rate is less than 8°C/s, the tensile strength deviation between the overlapping portion and the central portion exceeds 50 MPa, and if the cooling rate exceeds 13°C/s, additional equipment investment is required, so it is preferable to control the cooling rate to be equal to or less than that.

As the refrigerant, typical liquid nitrogen capable of cooling the wire rod coil may be used, and it is sprayed in a mist form during initial cooling to cool the front end portion of the wire rod coil.

Hereinafter, in the manufacturing method of the wire rod of the present disclosure, it will be described with reference to the drawings that a temperature gap between the overlapping portion and the central portion of the coil is reduced when cooling is performed by mist-spraying the refrigerant onto the overlapping portion of the wire rod coil in the initial cooling step.

FIG. 2 is a view showing a temperature gap between an overlapping portion and a central portion of a coil in a case of cooling by mist-spraying a refrigerant in an initial cooling step of a wire rod coil according to an embodiment of the present disclosure and in a case of cooling by a typical method without mist cooling.

As shown in FIG. 2(a), in the wire rod manufacturing method of the present disclosure, the overlapping portion of the coil is cooled by mist-spraying the refrigerant by the mist-spraying device installed at the front end portion of the cooling zone during initial cooling. At this time, the mist-spraying device is installed up to a distance including a section where heat generation occurs, and by passing through this section at a speed of 8 to 13°C/s while cooling by the mist-sprayed refrigerant, the temperature gap between the overlapping portion and the central portion of the wire rod coil can be reduced as a result.

On the other hand, as shown in FIG. 2(b), it can be confirmed that in the case of cooling according to a typical method without mist cooling, the temperature gap between the overlapping portion and the central portion of the coil appears large.

Next, a steel wire with excellent hydrogen embrittlement resistance according to an embodiment of the present disclosure will be described.

A steel wire with excellent hydrogen embrittlement resistance according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.20 to 0.50% of silicon (Si), 0.50 to 1.00% of manganese (Mn), 0.050 to 0.100% of titanium (Ti), 0.030 to 0.080% of sulfur (S), and the remainder of iron (Fe) and other inevitable impurities.

Reasons for limiting the component range of each alloying element are as described above.

The steel wire includes ferrite and pearlite in microstructure, and includes 58% or more of fine TiS precipitates in area fraction per 300 to 8000 nm² of an equivalent cross-sectional area in ferrite. A degree of crack occurrence varies under an acidic (99.5% H₂S + CH₃COOH) atmosphere according to the area fraction of TiS precipitates in such ferrite, and the steel wire of the present disclosure has crack resistance of 6.1 hours or more in an acidic atmosphere, and has excellent crack resistance as cracks are not formed inside even when maintained for 2.5 times or more in an acidic atmosphere compared to when TiS is not added.

In addition, the steel wire according to an embodiment of the present disclosure may have a tensile strength of 850 MPa or more and a yield ratio of 0.90 or more.

Next, a method for manufacturing steel with excellent hydrogen embrittlement resistance according to an embodiment of the present disclosure will be described.

method for manufacturing a steel wire according to an embodiment of the present disclosure comprises:manufacturing a wire rod by heating a billet comprising, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.20 to 0.50% of silicon (Si), 0.50 to 1.00% of manganese (Mn), 0.050 to 0.100% of titanium (Ti), 0.030 to 0.080% of sulfur (S), and the remainder of iron (Fe) and other inevitable impurities at 1000 to 1100°C and then hot rolling the billet at 900 to 1000°C; coiling the wire rod into a coil shape at a temperature of 780 to 850°C; cooling the coiled wire rod, wherein the cooling comprises mist-spraying a refrigerant onto an overlapping portion of the wire rod coil having high stacking density during initial cooling to cool the overlapping portion at a cooling rate of 8 to 13°C/s; dry drawing the cooled wire rod with a total reduction ratio of 60% or more without lead patenting (LP) heat treatment; cold rolling the dry-drawn wire rod to a reduction ratio of 60%; and performing high-frequency heat treatment after the cold rolling.

The method for manufacturing a steel wire of the present disclosure omits an LP heat treatment process performed in a wire drawing company, and draws and rolls the wire rod directly to form a final product. That is, in the present disclosure, a separate LP heat treatment process can be omitted by mist-spraying a refrigerant onto the overlapping portion of the wire rod coil in an initial cooling step during manufacture of the wire rod to cool the overlapping portion at a cooling rate of 8 to 13°C/s, thereby reducing temperature deviation between the overlapping portion and the central portion of the wire rod.

The wire rod manufactured as described above is dry drawn with a total reduction ratio of 60% or more without LP heat treatment. Specifically, the wire rod undergoes drawing processing while passing through 10 to 12 dies, and at this time, a steel wire can be obtained by controlling a processing linear velocity to 2 to 4 m/s and a reduction amount per pass to 10 to 20%.

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

After the cold rolling, high-frequency heat treatment is performed at A1 transformation temperature -30°C to A1 transformation temperature. If a high-frequency heat treatment temperature is less than A1 transformation temperature - 30°C, toughness is not secured, creating a risk of breakage during forming and in a product state, and if the temperature exceeds A1 transformation temperature, strength may decrease.

The steel wire manufactured according to the present disclosure has a tensile strength of 850 MPa or more and a yield ratio of 0.90 or more, and the steel wire has crack resistance of 6.1 hours or more in an acidic (99.5% H2S + CH3COOH) atmosphere, which may be 2.5 times or more compared to when TiS is not added.

That is, according to the present disclosure, by cooling the overlapping portion of the wire rod coil by mist-spraying the refrigerant during initial cooling, the tensile strength deviation between the overlapping portion and the central portion of the coil can be reduced to 50 MPa or less, providing excellent product reliability, and further, drawing and rolling are possible without separate LP heat treatment in a wire drawing company. In addition, the steel wire of the present disclosure precipitates fine TiS in a ferrite matrix and thus has excellent crack resistance in an acidic (99.5% H2S + CH3COOH) atmosphere, so it can be preferably applied for deep-sea oil well drilling or transportation.

Hereinafter, the present disclosure will be described in more detail by the following examples. However, the following examples are only for illustrating the present disclosure, and the scope of the present disclosure is not limited thereto.

Steels satisfying various alloy compositions shown in Table 1 below were steel-made in an electric arc furnace, and then cast under typical conditions to manufacture continuous casting billets (160×160 mm²). Next, a rolling process was performed on the manufactured billets under typical conditions to a wire rod diameter of 14 mm to manufacture wire rods having proeutectoid ferrite of 32% in area fraction.

Units in Table 1 below are parts by weight, and an atomic ratio of Ti to S (Ti/S) was calculated according to Formula (1) below. Formula (1): 0.5 ≤ [(Ti content (wt%)) / (atomic weight of Ti)] / [(S content (wt%)) / (atomic weight of S)] ≤ 1.8
(wherein a mass of Ti is 47.887, and a mass of S is 32.06) Category C Si Mn Ti S Atomic Ratio of Ti to S Example 1 0.34 0.25 0.82 0.079 0.03 1.76 Example 2 0.35 0.24 0.78 0.079 0.050 1.06 Example 3 0.35 0.24 0.79 0.080 0.080 0.67 Comparative Example 1 0.34 0.24 0.79 0.000 0.001 0.07 Comparative Example 2 0.35 0.25 0.78 0.039 0.03 0.87 Comparative Example 3 0.36 0.25 0.79 0.040 0.050 0.54 Comparative Example 4 0.34 0.26 0.81 0.041 0.080 0.34 Comparative Example 5 0.35 0.23 0.81 0.040 0.1 0.27 Comparative Example 6 0.36 0.24 0.80 0.121 0.03 2.70 Comparative Example 7 0.35 0.24 0.80 0.119 0.050 1.59 Comparative Example 8 0.34 0.26 0.79 0.118 0.080 0.99 Comparative Example 9 0.33 0.26 0.78 0.119 0.1 0.80 Comparative Example 10 0.34 0.25 0.82 0.079 0.03 1.76 Comparative Example 11 0.35 0.25 0.79 0.080 0.03 1.79

Subsequently, the rolled wire rods were coiled into a coil shape at a temperature of 780 to 850°C considering scale peelability. Next, the coiled wire rods were transferred to a Stelmor cooling zone, and a refrigerant was sprayed onto an overlapping portion of the coil through a mist-spraying device installed at a front end portion of Stelmor to perform initial cooling at a rate of 8 to 13°C/s, thereby manufacturing wire rods. The coiling temperature, whether mist cooling was applied, and the cooling rate are as shown in Table 2 below. Category Coiling Temperature (°C) Mist Cooling Application Mist Cooling Rate (°C /s) Example 1 829 Applied 12.1 Example 2 831 Applied 12.1 Example 3 831 Applied 12.4 Comparative Example 1 828 Not Applied 0.0 Comparative Example 2 829 Applied 12.2 Comparative Example 3 830 Applied 12.4 Comparative Example 4 830 Applied 12.4 Comparative Example 5 829 Applied 12.0 Comparative Example 6 829 Applied 12.0 Comparative Example 7 828 Applied 12.0 Comparative Example 8 829 Applied 12.3 Comparative Example 9 829 Applied 12.0 Comparative Example 10 888 Applied 11.9 Comparative Example 11 829 Not Applied 4.8

Table 3 below shows average tensile strength of the manufactured wire rods, tensile strength deviation between the overlapping portion and the central portion of the coil, wire rod average reduction of area (RA), and maximum thickness of coarse cementite in the overlapping portion. In addition, Table 4 below shows area fraction of TiS precipitates per 300 to 8000 nm² of equivalent cross-sectional area, TiS precipitate size, wire rod scale thickness, and scale peelability. The tensile strength of the wire rod was measured by performing a tensile test at room temperature at an initial strain rate of 10 mm/min. The maximum reduction of area of the wire rod was measured by enlarging it using a projector. Scale peelability was classified as Good when a residual scale amount was 0.05% or less after applying 5% strain in a tensile tester, and Poor when exceeding 0.05%. At this time, a length of a test piece was 30 cm, and evaluation was performed on 20 cm of a central portion excluding 10 cm of grip engaging portions on both sides. Subsequently, the manufactured wire rods were subjected to drawing processing with a total reduction ratio of 60% or more after removing a part of scale existing on the surface using a mechanical peeling method without LP heat treatment. Thereafter, cold rolling was performed up to a reduction ratio of 60%, and high-frequency heat treatment was performed at A1 transformation temperature -30°C to A1 transformation temperature to manufacture steel wires.

Table 5 below shows values after measuring LP heat treatment application, total reduction ratio during drawing, reduction ratio, workability, tensile strength, yield strength, yield ratio, and crack occurrence time under an acidic atmosphere (99.5% H2S + CH3COOH) of the manufactured steel wires. In addition, results of Example 1 and Comparative Example 1, in which crack occurrence was measured by maintaining under an acidic atmosphere, are shown in FIG. 3. At this time, workability was classified as Good when wire breakage rate per ton was less than 1.5 times, and Poor when it was 1.5 times or more. Category Average Tensile Strength (MPa) Tensile Strength Deviation between Overlapping Portion and Central Portion (MPa) Average Reduction of Area (%) Maximum Thickness of Coarse Cementite in Overlapping Portion (nm) Example 1 678 46 63 95 Example 2 680 45 61 80 Example 3 682 48 60 82 Comparative Example 1 620 89 58 182 Comparative Example 2 671 48 63 78 Comparative Example 3 670 47 62 85 Comparative Example 4 678 49 64 92 Comparative Example 5 672 50 64 84 Comparative Example 6 672 46 62 79 Comparative Example 7 671 48 63 90 Comparative Example 8 670 48 63 85 Comparative Example 9 675 45 62 94 Comparative Example 10 675 49 64 81 Comparative Example 11 612 90 59 179 TiS Precipitate Area Fraction per 300 to 8000 nm² of Equivalent Cross-sectional Area (%) TiS Precipitate Size (nm) Wire Rod Scale Thickness (µm) Scale Peelability Example 1 58 32 4.5 Good Example 2 61 37 4.2 Good Example 3 73 44 4.7 Good Comparative Example 1 0 2 4.2 Good Comparative Example 2 11 21 4.1 Good Comparative Example 3 9 28 3.8 Good Comparative Example 4 8 24 3.9 Good Comparative Example 5 7 28 4.0 Good Comparative Example 6 32 22 3.8 Good Comparative Example 7 38 21 3.2 Good Comparative Example 8 24 19 3.4 Good Comparative Example 9 31 18 3.8 Good Comparative Example 10 58 21 8.2 Poor Comparative Example 11 58 15 5.3 Good Category LP Heat Treatment Application Total Reduction Ratio during Drawing (%) Reduction Ratio (%) Workability Tensile Strength (Mpa) Yield Strength (Mpa) Yield Ratio Crack Occurrence Time in Acidic Atmosphere (hr) Example 1 Not Applied 72 60 Good 892 821 0.92 6.3 Example 2 Not Applied 72 60 Good 890 801 0.9 6.7 Example 3 Not Applied 72 60 Good 888 826 0.93 6.1 Comparative Example 1 Applied 58 60 Good 871 801 0.92 2.2 Comparative Example 2 Not Applied 72 60 Good 888 808 0.91 2.1 Comparative Example 3 Not Applied 72 60 Good 890 801 0.9 2.3 Comparative Example 4 Not Applied 72 60 Good 882 803 0.91 2.1 Comparative Example 5 Not Applied 72 60 Good 879 809 0.92 2.0 Comparative Example 6 Not Applied 72 60 Good 883 812 0.92 3.5 Comparative Example 7 Not Applied 72 60 Good 880 792 0.9 3.1 Comparative Example 8 Not Applied 72 60 Good 886 797 0.9 3.0 Comparative Example 9 Not Applied 72 60 Good 890 801 0.9 3.7 Comparative Example 10 Not Applied 72 60 Poor 850 774 0.91 5.1 Comparative Example 11 Not Applied 72 60 Poor 790 711 0.9 5.2

Comparative Example 1 is a medium carbon steel (Ti and S not applied) currently in commercial use, which is a steel grade manufactured by a conventional method such as not applying mist cooling, and was produced as a product after LP heat treatment for structure refinement in a wire drawing company, drawing and rolling, and high-frequency treatment. In the case of the steel wire of Comparative Example 1, tensile strength was 871 MPa and yield ratio was 0.92, and when maintained under an acidic (99.5% H₂S + CH₃COOH) atmosphere, it was confirmed that internal cracks occurred when maintained for 2.2 hours as shown in Table 4 and FIG. 3 below.

Examples 1 to 3 and Comparative Examples 2 to 9 are examples in which Ti and S contents were varied, and in order to reduce tensile strength deviation between the overlapping portion and the central portion of the wire rod coil, the overlapping portion of the coil was cooled by spraying a refrigerant using a mist-spraying device installed at a front end portion of Stelmor, and then drawn and rolled without LP heat treatment in a wire drawing company, and high-frequency heat treated to manufacture steel wires.

In the case of Examples 1 to 3 and Comparative Examples 2 to 9, tensile strength was at a level of 670 MPa and average reduction of area was at a level of 62% regardless of Ti and S contents. However, when the content values of Ti and S proposed in the present disclosure were not satisfied, it could be confirmed that a difference appeared in the TiS precipitate area fraction (%) per 300 to 8000 nm² of equivalent cross-sectional area. Examples 1 to 3 exhibited an area fraction of 58 to 73%, but Comparative Examples 2 to 5 exhibited an area fraction of a 10% level, and Comparative Examples 6 to 9 exhibited an area fraction of a 24 to 38% level. These results affect internal crack formation time under an acidic atmosphere. In the case of the steel wires of {Examples 1 to 3}, they could withstand at least 6 hours or more 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 could be confirmed that internal cracks occurred when maintained for 3.0 to 5.2 hours, which is slightly improved compared to Comparative Example 1.

Comparative Example 10 is a comparative group for scale peelability evaluation by increasing only the coiling temperature to 888°C compared to Example 1. In the case of Comparative Example 10, tensile strength and wire rod properties appeared similar to Example 1, but it was confirmed that scale peelability at a wire drawing company was poor and affected hydrogen evaluation in the product.

In addition, Comparative Example 11 did not apply mist cooling, and compared to Example 1, tensile strength deviation between the overlapping portion and the central portion of the wire rod coil increased to 90 MPa, and the maximum thickness of coarse cementite in the overlapping portion also became thick, indicating that workability would be poor when LP heat treatment is not applied at a wire drawing company.

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

Through these results, by reducing the tensile strength deviation between the overlapping portion and the central portion of the wire rod coil to 50 MPa or less according to the present disclosure, it is possible to manufacture a steel wire with uniform structure without tensile strength deviation by drawing and rolling without performing LP heat treatment in a wire drawing company, and since internal cracks do not occur even when maintained for 6.1 hours or more in an acidic atmosphere by precipitating fine TiS in a ferrite matrix, which is 2.5 times or more compared to when TiS is not added, it was found that crack resistance is excellent and thus it will be preferably applied for deep-sea oil well drilling or transportation.

Although exemplary embodiments of the present disclosure have been described above, the present disclosure is not limited thereto, and those skilled in the art will appreciate that various changes and modifications are possible without departing from the concept and scope of the appended claims described below.