Wire rod, steel wire, and method for manufacturing same

Abstract

An ultra-low carbon steel wire rod according to the present invention comprises, in weight%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05 to 0.15%, Ti: 0.020 to 0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0%), and the balance of Fe and inevitable impurities, wherein, when the radius of the wire rod is R, Ti4C2S2 precipitates having an average size of 100 nm or more among precipitates observed in a region from the center of a cross-section perpendicular to the longitudinal direction to 0.9 R account for 85% or more by an area fraction, the weight ratio of Mn to S (Mn / S) is 7 to 20, and the tensile strength is 280 MPa or less.

Description

Wire rod, steel wire and method of manufacturing the same

[0001] The present invention relates to an ultra-low carbon steel wire rod, a steel wire, and a method for manufacturing the same.

[0002] When using mechanical structural materials such as reinforcing bars and scaffolding steel plates on site, they require fastening through binding, and heat-treated wire with a diameter of 0.8 to 1.0 mm is mainly used. Although on-site workers sometimes use semi-automatic equipment to bind the binding wire, most are bound by hand, so the binding wire must have low strength.

[0003] A typical binding wire manufacturing process involves producing a wire rod with a diameter of 5.5 to 6.5 mm using a steel mill slab or billet, removing scale through mechanical peeling at a binding wire manufacturer, reducing the diameter (downsizing) through drawing, and finally finishing by annealing in the ferrite single-phase region to reduce the increased strength.

[0004] As mentioned earlier, it is important to reduce the strength of the binding wire, and to achieve this, a soft phase of ferrite must be formed. Additionally, the carbon and nitrogen content, which provide the highest solid solution strengthening effect, must be kept as low as possible. However, since decarburization and denitrification treatments during the steelmaking process significantly increase manufacturing costs and there are limits to reducing carbon and nitrogen content, it is necessary to form precipitates or crystals such as TiN, TiC, and Ti(C,N) by adding alloying elements such as Ti.

[0005] In addition, the precipitates must be coarse because if they are fine, the strength increases due to precipitation strengthening. Since the TiN crystallized in the liquid phase during continuous casting is very high at around 1400℃ and the temperature of the wire rod heating furnace is low at 1200℃ or lower, growth is possible.

[0006] However, precipitates such as TiC, excluding these, have low precipitation temperatures. In the case of TiC, the precipitation temperature is 900°C or lower, and the temperature at which precipitate formation occurs rapidly is around 650°C. However, in the heating furnace during the wire rod manufacturing process, even if these precipitates are precipitated, they are re-dissolved, and since the precipitates formed during Stelmore cooling do not have time to grow due to the rapid Stelmore cooling rate, there was a limit to lowering the carbon content.

[0007] One aspect of the present invention for solving the aforementioned problem is to provide an ultra-low carbon steel wire rod, a steel wire, and a method for manufacturing the same, which can be applied to reinforcing bars, scaffolding steel plates, machine structural products, etc., by lowering the strength of the wire rod through the formation of coarse TiN precipitates and coarse Ti4C2S2 precipitates by adding Ti.

[0008] The technical problems to be solved in this document are not limited to those mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art to which this invention belongs from the description below.

[0009] To achieve the above objective, an ultra-low carbon steel wire rod according to one embodiment of the present invention comprises, in weight%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities, and when the radius of the wire rod is R, among the precipitates observed in the region from the center of the cross-section perpendicular to the longitudinal direction to 0.9R, Ti4C2S2 precipitates with an average size of 100 nm or more have an area fraction of 85% or more, the weight ratio of Mn to S (Mn / S) is 7 to 20, and the tensile strength is 280 MPa or less.

[0010] According to one embodiment of the present invention, the microstructure of the wire may contain ferrite with an area fraction of 99% or more.

[0011] According to one embodiment of the present invention, the average grain size of the ferrite may be 40 μm or more.

[0012] According to one embodiment of the present invention, the wire may have an average thickness of surface scale of 14㎛ or more.

[0013] According to one embodiment of the present invention, the wire may have a tensile strength variation between the coil overlap portion and the center of 67 MPa or less.

[0014] A method for manufacturing an ultra-low carbon steel wire rod according to one embodiment of the present invention may include the steps of: heating a bloom containing, in weight%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities, at 1130~1200℃ for 240~500 minutes and then rolling the bloom to produce a billet; hot rolling the obtained billet; coiling the hot-rolled wire rod at 900~980℃; and cooling the coiled wire rod at a cooling rate of 0.1~1.0℃ / s.

[0015] In one embodiment of the present invention, after the cooling step, the wire rod may have an area fraction of 85% or more of Ti4C2S2 precipitates with an average size of 100 nm or more among the precipitates observed in the region from the center of the cross-section perpendicular to the length direction to 0.9 R, where the radius of the wire rod is R.

[0016] According to one embodiment of the present invention, the hot rolling can be performed at 1130 to 1180°C for 80 minutes or more.

[0017] According to one embodiment of the present invention, after the winding step, the microstructure of the wire may contain ferrite with an area fraction of 99% or more.

[0018] According to one embodiment of the present invention, the average grain size of the ferrite may be 40 μm or more.

[0019] According to one embodiment of the present invention, after the winding step, the wire may have an average thickness of surface scale of 14㎛ or more.

[0020] According to one embodiment of the present invention, after the cooling step, the wire may have a tensile strength of 280 MPa or less.

[0021] According to one embodiment of the present invention, after the cooling step, the wire may have a tensile strength variation between the coil overlap and the center of 67 MPa or less.

[0022] A super low carbon steel wire according to one embodiment of the present invention comprises, in weight%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities, and may have a tensile strength of 330 MPa or less, a yield strength of 200 MPa or less, and a yield ratio of 0.55 or less.

[0023] According to one embodiment of the present invention, the steel wire may have a yield point elongation of 1.0% or less.

[0024] A method for manufacturing an ultra-low carbon steel wire according to one embodiment of the present invention comprises the steps of: maintaining a bloom containing, in weight%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), and the remainder being Fe and unavoidable impurities, at a temperature of 1130~1200℃ for 240~500 minutes, and then rolling the bloom to produce a billet; hot rolling the obtained billet at a temperature of 1130~1180℃ for 80 minutes or more; and coiling the hot-rolled wire rod at a temperature of 900~980℃. The method may include the step of cooling the wound wire at a rate of 0.1 to 1.0℃ / s; the step of drawing the cooled wire; and the step of heating the drawn wire at A3-260℃ or higher and A3-160℃ or lower for at least 2 hours and then cooling it.

[0025] According to one embodiment of the present invention, after the step of cooling after heating, the steel wire may have a tensile strength of 330 MPa or less, a yield strength of 200 MPa or less, and a yield ratio of 0.55 or less at a true strain (e) of 3.62 to 3.98.

[0026] According to the present invention, by lowering the strength of the wire rod through the formation of coarse TiN precipitates and coarse Ti4C2S2 precipitates via the addition of Ti, it is possible to provide an ultra-low carbon steel wire rod, a steel wire, and a method for manufacturing the same that can be applied as reinforcing bars, scaffolding steel plates, machine structural products, etc.

[0027] The effects obtainable from the present invention are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art to which the present invention belongs from the description below.

[0028] FIG. 1 is a drawing showing a cross-section of a wire according to one embodiment of the present invention.

[0029] FIG. 2 is a diagram showing a method for measuring the average size of Ti4C2S2 precipitates in a cross-section of a wire according to one embodiment of the present invention.

[0030] FIG. 3 is a drawing showing the center and overlapping portion of a wire coil according to one embodiment of the present invention.

[0031] FIG. 4 is a drawing showing an analysis area in a cross-section of a wire rod according to one embodiment of the present invention.

[0032] FIG. 5 is a diagram showing a method for measuring tensile strength using a wire rod or steel wire according to one embodiment of the present invention.

[0033] FIG. 6 is a diagram showing the relationship between the yield strength and yield point elongation of a steel wire according to one embodiment of the present invention.

[0034] 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.

[0035] 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.

[0036] 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. For instance, singular expressions in this specification include plural expressions unless the context clearly indicates an exception.

[0037] 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.

[0038] Since wire rods and steel wires used for binding, such as reinforcing bars and scaffolding steel plates, are tied by hand by workers, low product strength is essential; however, there have been limitations in removing carbon and nitrogen to ensure low strength. Accordingly, the present invention aims to lower the strength of the wire rod by making the microstructure of the wire rod a single ferrite phase, adding a large amount of Ti element to remove carbon and nitrogen—which are solid solution interstitial elements that increase strength when dissolved in ferrite—thereby inducing the formation of TiN, and inducing the formation of coarse Ti4C2S2 precipitates to suppress solid solution strengthening and precipitation strengthening effects.

[0039] The ultra-low carbon steel wire rod according to one embodiment of the present invention will be described in detail below.

[0040] In the present invention, the "center" (OR) of the wire refers to the intersection point of the major axis and the minor axis of the cross-section perpendicular to the longitudinal direction of the wire, as shown in FIG. 1 (provided that if all axes of the cross-section are the same, the intersection point of the two axes). If the cross-section of the wire is a circle, the distance from the center of the circle to the outermost surface refers to the radius of the wire, and if it is an ellipse, the distance from the center of the ellipse to the outermost surface of the major axis refers to the radius of the major axis.

[0041] In addition, in the present invention, "cross-section of the wire" refers to a surface cut perpendicular to the longitudinal direction of the wire, "center" refers to an area from the center to 0.5R in the cross-section of the wire, and "surface" or "surface area" refers to an area from the center to 0.9R to 1R in the cross-section of the wire.

[0042] A super low carbon steel wire rod according to one embodiment of the present invention comprises, in weight%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities.

[0043] The reasons for limiting the compositional range of each alloying element are described below. Unless otherwise noted, units are weight percent.

[0044] The content of C and N may be 0.002% or less, respectively.

[0045] C and N are solid solution strengthening elements that most significantly increase material strength, and adding 0.1% improves strength to the level of 100 MPa. If the content of C and N exceeds 0.002%, it may be difficult to achieve the target strength of the present invention. Therefore, it is desirable to maintain the content of C and N at 0.002% or less, and more preferably at 0.0001% to 0.001% or less.

[0046] The Si content may be 0.02% or less.

[0047] Si is a ferrite solid solution strengthening element, and adding 0.1% increases strength to the level of 15 MPa. If the Si content is too high, it may be difficult to achieve the target strength of the present invention. In addition, if the Si content is too high, FeSiO4 is formed, which is not advantageous for scale removal during mechanical stripping, and the remaining scale causes wear on the die during wire drawing, so it is important to include as little Si as possible. Therefore, it is desirable to maintain the Si content at 0.02% or less, and more preferably at 0.01% or less.

[0048] The Mn content is 0.05~0.15%.

[0049] The weight ratio of Mn to S (Mn / S) can be 7 to 20.

[0050] Although Mn has a smaller solid solution strengthening effect compared to Si, a 0.1% increase increases strength to the level of 10 MPa. In other words, to reduce strength, the Mn content must be kept low. However, if the Mn content is low, brittleness may occur due to the formation of S grain boundaries. Therefore, the Mn content is maintained at 0.05~0.15%, preferably 0.05~0.11%, while the weight ratio of Mn to S (Mn / S) is maintained at 7 or higher, preferably 7~20.

[0051] The Ti content can be 0.020~0.100%.

[0052] The addition of Ti is necessary to form TiN and Ti4C2S2 for the removal of N and C present in the matrix structure. If the Ti content is less than 0.020%, it may be difficult to remove dissolved C and N, and if it exceeds 0.100%, clogging of the tundish nozzle may occur, leading to increased manufacturing costs. Therefore, it is desirable to maintain the Ti content at 0.020~0.100%, and more preferably at 0.030~0.100%.

[0053] The content of P and S may be 0.015% or less, respectively.

[0054] The solid solution strengthening effect by P and S is approximately 80 MPa per 0.1%, indicating a significant increase in strength due to solid solution strengthening. If the content of P and S exceeds 0.015%, it may be difficult to achieve the target strength of the present invention, and intergranular embrittlement may cause wire breakage during fresh processing. Therefore, it is desirable to maintain the content of P and S at 0.015% or less.

[0055] 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.

[0056] The microstructure of an ultra-low carbon steel wire rod according to one embodiment of the present invention, comprising the alloy composition as described above, may contain ferrite in an area fraction of 99% or more. It is preferable that the microstructure of the wire rod be a ferrite (α) single-phase structure. A ferrite single-phase structure means containing ferrite in an area fraction of 99% or more, or 100% or more.

[0057] The average grain size of the ferrite in the microstructure of the above wire may be 40 μm or larger. If the grain size of the wire is excessively small, the influence of grain boundaries hindering the movement of domain walls increases, which may lead to an increase in coercivity. Therefore, it is desirable to increase the grain size of the wire to reduce the density of grain boundaries.

[0058] According to one embodiment of the present invention, the ultra-low carbon steel wire rod may have an average thickness of surface scale of 14 μm or more. When the thickness of the surface scale of the wire rod is thick, it is desirable in that the residual scale attached to the matrix can be significantly reduced when scale is peeled off from the wire drawing process, thereby preventing die breakage caused by oxides during the wire drawing process. Therefore, it is desirable that the average thickness of the surface scale be 14 μm or more, and more preferably 15 μm or more.

[0059] In an ultra-low carbon steel wire rod according to one embodiment of the present invention, when the radius of the wire rod is R, among the precipitates observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9R, Ti4C2S2 precipitates with an average size of 100 nm or more may have an area fraction of 85% or more.

[0060] The average size of the Ti4C2S2 precipitate is derived by measuring the area of ​​the TEM (transmission electron microscope) at 100,000x magnification in the 0.9R (R is the radius of the wire) region from the center of the cross-section perpendicular to the length direction as shown in Fig. 2 according to the TEM precipitate extraction method (replica), and then converting the extracted area into a circle shape.

[0061] If the size of the above Ti4C2S2 precipitate is less than 100 nm or the area fraction is less than 85%, an increase in strength due to precipitation strengthening may occur.

[0062] Accordingly, the size of the Ti4C2S2 precipitate is 100 nm or more, and the area fraction of the precipitate is 85% or more, preferably 87% or more, and more preferably 90% or more.

[0063] According to one embodiment of the present invention, the ultra-low carbon steel wire rod may have a tensile strength of 280 MPa or less, and a tensile strength variation between the coil overlap and the center may be 67 MPa or less.

[0064] If the tensile strength is too high, it may be difficult to use it as a binding wire or annealed wire for binding reinforcing bars, scaffolding steel plates, etc., which are the intended purpose of the present invention.

[0065] Furthermore, when wire is cut, the hardness of the cross-section naturally increases due to cutting deformation. At this time, the strength of the surface increases more significantly than that of the center of the material; if the strength difference between the two parts exceeds a certain level, the likelihood of burr formation increases rapidly.

[0066] Generally, wires inevitably have edge portions (overlapping portions) with a relatively higher stacking density than the center portion (center portion) of the coil. At this time, as shown in FIG. 3, the center of the wire coil refers to the center portion of the coil with a relatively low stacking density corresponding to 1 and 2 in FIG. 3 when the wire is divided into eight parts, and the overlapping portion refers to the edge portion of the coil with a high stacking density corresponding to 3 and 4 in FIG. 3.

[0067] Accordingly, when manufacturing wire rods, the transformation of the structure changes due to the difference in cooling caused by the stacking density between the overlapping portion and the center, and as a result, a variation in tensile strength occurs between the overlapping portion and the center. In the present invention, the wire rod can increase product reliability by reducing the variation in tensile strength between the coil overlapping portion and the center to 67 MPa or less.

[0068] Hereinafter, a method for manufacturing an ultra-low carbon steel wire rod according to one embodiment of the present invention will be described.

[0069] A method for manufacturing an ultra-low carbon steel wire rod according to one embodiment of the present invention may include the steps of: heating a bloom containing, in weight%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities, at 1130~1200℃ for 240~500 minutes and then rolling the bloom to produce a billet; hot rolling the obtained billet; coiling the hot-rolled wire rod at 900~980℃; and cooling the coiled wire rod at a cooling rate of 0.1~1.0℃ / s.

[0070] The reason for limiting the component range of each alloy composition above may be the same as described above, and each manufacturing step will be explained in more detail below.

[0071] First, a bloom having the compositional components described above is prepared.

[0072] Afterwards, the bloom can be heated in a steel billet heating furnace at 1130–1200°C for 240–500 minutes and then rolled into a billet.

[0073] Ti4C2S2 is a high-temperature precipitate formed at 1200°C, and it is important to form as much of the Ti4C2S2 precipitate as possible before growing it. If the temperature of the steel billet heating furnace is below 1130°C, operational defects may occur due to roll load during steel billet rolling, and if it exceeds 1200°C, precipitate formation may be insufficient. Therefore, it is desirable to maintain the heating temperature between 1130°C and 1200°C, and more preferably between 1150°C and 1200°C. In addition, if the heating time is less than 240 minutes, precipitate formation may be insufficient, and if it exceeds 500 minutes, operational defects may occur. Therefore, it is desirable that the heating time of the steel billet heating furnace be between 240 and 500 minutes, more preferably between 240 and 400 minutes, and most preferably between 240 and 300 minutes.

[0074] Next, the obtained billet can be subjected to a hot rolling step.

[0075] For example, the above-mentioned billet can be manufactured into a wire rod of a desired size by performing hot rolling sequentially consisting of rough rolling, intermediate rough rolling / finish rolling, and finish rolling.

[0076] The above hot rolling can be performed at 1130~1180℃ for 80 minutes or more, and during finish rolling, a phase transformation from austenite to ferrite begins, the austenite completely disappears, and the microstructure can consist only of ferrite.

[0077] The above hot-rolled wire rod may undergo a winding step at 900~980℃ to reduce the strength of the wire rod through an increase in grain size and to reduce the strength variation within the coil.

[0078] Since it is necessary to reduce strength through ferrite grain growth and the wire manufacturer removes scale by mechanical peeling, it is desirable to maintain a high winding temperature.

[0079] If the winding temperature is less than 900℃, the scale thickness is low at 8㎛ or less, and the average grain size of the ferrite is also small at 25㎛. In addition, if the temperature exceeds 980℃, the average grain size of the ferrite can be increased to 40㎛ or more, but winding defects such as ring indentation due to the high temperature may occur, making it difficult to manufacture the wire rod. Therefore, the above winding step is preferably performed at a temperature of 900~980℃, and more preferably at a temperature of 940~960℃.

[0080] After the above winding, a cooling step is performed.

[0081] The above cooling step should be performed as slowly as possible to reduce strength variation within the coil.

[0082] The above cooling can be performed with the cover closed without applying a blower (air) in the Stelmore cooling zone. At this time, the cooling rate can be maintained at 0.1 to 1.0°C / s and can be performed at 250 to 350°C, preferably 300°C, until entry into the reforming tube.

[0083] If the above cooling rate is less than 0.1℃ / s, equipment investment becomes necessary due to the limitations of the conveyor speed, and if it exceeds 1.0℃ / s, the variation in tensile strength within the coil may increase to 90MPa or more. Therefore, it is desirable to perform the cooling at a speed of 0.1 to 1.0℃ / s, more preferably at 0.2 to 1.0℃ / s, and most preferably at 0.5 to 0.9℃ / s. In addition, if the cooling temperature is less than 250℃ or exceeds 350℃, jamming or sagging may occur when entering the reforming tube, causing the coil shape to become non-uniform.

[0084] In an embodiment of the present invention, the ultra-low carbon steel wire rod manufactured by the above method may have an area fraction of 85% or more of Ti4C2S2 precipitates with an average size of 100 nm or more among the precipitates observed in the region from the center of the cross-section perpendicular to the longitudinal direction to 0.9 R, where the radius of the wire rod is R.

[0085] The microstructure of an ultra-low carbon steel wire rod according to one embodiment of the present invention may contain ferrite with an area fraction of 99% or more, and the average grain size of the ferrite may be 40㎛ or more. In addition, the average thickness of the surface scale may be 14㎛ or more.

[0086] In addition, the ultra-low carbon steel wire according to one embodiment of the present invention may have a tensile strength of 280 MPa or less, and a tensile strength variation between the coil overlap and the center may be 67 MPa or less.

[0087] Hereinafter, an ultra-low carbon steel wire according to one embodiment of the present invention will be described.

[0088] A super low carbon steel wire according to one embodiment of the present invention may comprise, in weight%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities.

[0089] The above ultra-low carbon steel wire may have a tensile strength of 330 MPa or less, preferably 300 MPa or less, and most preferably 280 MPa or less.

[0090] The above ultra-low carbon steel wire may have a yield strength of 200 MPa or less, preferably 180 MPa or less, and a yield ratio of 0.55 or less.

[0091] If the tensile strength and yield strength of the above ultra-low carbon steel wire are too high, or if the yield ratio exceeds 0.55, the strength is too high, making it difficult to apply as a binding wire or annealed wire for binding reinforcing bars, scaffolding steel plates, etc., which is the purpose of the present invention.

[0092] In addition, the ultra-low carbon steel wire of the present invention can reduce yield point elongation to 1.0% or less despite having low yield strength and yield ratio.

[0093] Yield point elongation is a phenomenon that generally occurs during strip winding. While the amount of deformation increases during the initial strip winding, the radius of curvature becomes minimum and the amount of deformation becomes maximum at the moment the strip is completely wound onto the mandrel of the winder. As a result, the yield point decreases, and this decrease is applied to the steel plate as supersaturated deformation energy. This supersaturated deformation energy causes the occurrence of tack defects. However, the ultra-low carbon steel wire of the present invention can reduce the yield point elongation to 1.0% or less despite having low yield strength and yield ratio, thereby reducing the occurrence of tack defects. Preferably, the yield point elongation may be 0%.

[0094] Hereinafter, a method for manufacturing an ultra-low carbon steel wire according to one embodiment of the present invention will be described.

[0095] A method for manufacturing an ultra-low carbon steel wire according to one embodiment of the present invention comprises the steps of: maintaining a bloom containing, in weight percent, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), and the remainder being Fe and unavoidable impurities, at a temperature of 1130~1200℃ for 240~500 minutes, and then rolling the bloom to produce a billet; hot rolling the obtained billet at a temperature of 1130~1180℃ for 80 minutes or more; and coiling the hot-rolled wire rod at a temperature of 900~980℃. The method may include the step of cooling the wound wire at a rate of 0.1 to 1.0℃ / s; the step of drawing the cooled wire; and the step of heating the drawn wire at A3-260℃ or higher and A3-160℃ or lower for at least 2 hours and then cooling it.

[0096] The manufacturing of the above wire rod may be the same as the method for manufacturing an ultra-low carbon steel wire rod according to one embodiment of the present invention described above.

[0097] The drawing of the above wire can be performed by drawing with a total true deformation amount (e) of 3.62 (wire diameter 0.9 mm) to 3.98 (wire diameter 0.75 mm), preferably 3.73, after mechanical peeling.

[0098] After the above freshening step, a step of heating and then cooling can be performed.

[0099] Specifically, the fresh wire can be heated at A3-260°C or higher and A3-160°C or lower for at least 2 hours and then cooled.

[0100] If the above heating temperature is below A3-260℃, the tensile strength is high, which may cause increased hand fatigue when used as a binding wire; if it exceeds A3-160℃, it may be difficult to use as a product due to accelerated oxidation and localized corrosion.

[0101] The ultra-low carbon steel wire according to one embodiment of the present invention manufactured as described above may have a tensile strength of 330 MPa or less, a yield strength of 200 MPa or less, and a yield ratio of 0.55 or less.

[0102] The present invention will be explained in more detail below through the following examples. However, the following examples are merely illustrative of the present invention, and the scope of the present invention is not limited thereto.

[0103] Examples

[0104] A billet having the alloy composition shown in Table 1 below was manufactured. At this time, to coarsen the Ti4C2S2 precipitates, a bloom of 400 mm x 500 mm was prepared, held in a steel billet heating furnace for 300 minutes, and then rolled to produce a billet of 160 mm x 160 mm. Next, the billet was held in a wire rod heating furnace at 1170°C for 85 minutes, then hot-rolled, and finally coiled at the coiling temperature. Subsequently, the coiled wire rod was extremely slow-cooled in a Stelmore cooling zone at a conveyor cooling speed without airflow and with the cover closed. The temperature upon entry into the reforming tube was 300°C.

[0105] The steel billet heating furnace temperature, wire rod winding temperature, and conveyor cooling speed during wire rod manufacturing are shown in Table 2 below.

[0106] Classification CNT iSi Mn SM n / SP Example 1 0.00 20.00 20.03 0.0 20.10.00 5 200.015 Example 2 0.00 20.00 20.05 0.0 20.10.00 5 200.015 Example 3 0.00 20.00 20.08 0.0 20.10.00 5 200.015 Example 4 0.00 20.00 20.10 00.0 20.10.00 5 200.015 Comparative Example 10.00 20.0 020.0150.020.10.005200.015Comparative Example 20.0020.0020.1500.020.10.005200.015Comparative Example 30.0020.0020.0500.020.10.005200.015Comparative Example 40.0020.0020.0500.020.10.005200.015Comparative Example 50.0020.0020.0500.020.10.005200.015

[0107] Classification Steel Bill Heating Furnace Temperature (°C) Wire Rod Winding Temperature (°C) Conveyor Cooling Rate (°C / s) Example 1 1809500.9 Example 2 11809500.9 Example 3 11809500.9 Example 4 11809500.9 Comparative Example 1 1809500.9 Comparative Example 2 11809500.9 Comparative Example 3 12309500.9 Comparative Example 4 12308800.9 Comparative Example 5 123095018

[0108] The wire rod prepared above was drawn with a true strain (e) of 3.73, heated at A3-260°C or higher and A3-160°C or lower for 2 hours, and then cooled to produce a steel wire. The area fraction of Ti4C2S2 precipitates larger than 100 nm, ferrite grain size, scale thickness, tensile strength, and tensile strength deviation within the coil were measured among the observed precipitates of the wire rod prepared above, and the results are shown in Table 3 below. The area fraction of Ti4C2S2 precipitates and the ferrite grain size of the wire rod were identified from the analysis area shown in Fig. 4 within the region from the center of the cross-section perpendicular to the longitudinal direction of the wire rod up to 0.9R, and the scale thickness was identified from the surface (0.9R to 1R in Fig. 4). In addition, the tensile strength, yield strength, yield ratio, and whether yield point elongation occurred of the steel wire prepared above were measured, and the results are shown in Table 4 below. The area fraction of Ti4C2S2 precipitates was measured by preparing a specimen using the TEM precipitate extraction method (replica) as shown in Fig. 2, and obtaining a large number of images at 50,000 times the analysis area shown in Fig. 4.

[0109] Ferrite grain size was measured using EBSD. The specimens were polished and finished with silica gel, and cross-sectional measurements were taken at 100x magnification (Step size: 0.2 mm, tolerance angle: 15°).

[0110] Scale thickness was measured using a cross-section of the wire rod, milled with the cross-section facing the polishing floor, and polished after covering the surface with rubber to prevent scale breakage. The cross-section of the wire rod was photographed at 100x magnification using an optical microscope to measure the scale thickness from the surface of the wire rod, and the average thickness for 5 sheets was presented.

[0111] The tensile strength of the wire rod and steel wire was measured using an Instron 8862, and the tensile speed was set to 20 mm / min. As shown in Figure 5, a wire rod with a diameter of 5.5 mm was cut into 400 mm lengths and subjected to a tensile test. To verify the yield point, a prestrain of 2% was applied to straighten the wire, and the tensile test was performed until fracture occurred.

[0112] The tensile strength variation between the overlapping section and the center of the wire coil was measured by taking one ring from the top part of the wire, cutting it into 400 mm L sizes, and measuring the average tensile strength of the center without overlapping (Fig. 3, 1 and 2) and the overlapping section where the wire overlaps (Fig. 3, 3 and 4).

[0113] The yield strength and yield point elongation of the steel wire were measured as shown in Fig. 6 (in Fig. 6, YS represents yield strength and TS represents tensile strength). Yield strength refers to the maximum strength before strength decreases after elastic deformation, and yield point elongation refers to the section where elongation continues even though stress decreases slightly after passing the yield point.

[0114] The yield ratio of a steel wire refers to the ratio of its yield strength to its tensile strength.

[0115] Classification Observation of Precipitates 100 nm or larger Ti4C2S2 Area Fraction (%) Ferrite Grain Size (㎛) Scale Thickness (㎛) Tensile Strength (MPa) Tensile Strength Variation within Coil (MPa) Example 1 85 4215 280 65 Example 2 89 43 17 27 558 Example 3 90 40 15 27 267 Example 49 44 614 27 66 Comparative Example 1 20 44 16 29 462 Comparative Example 2 35 41 16 29 860 Comparative Example 3 12 42 17 30 558 Comparative Example 48 22 86 29 259 Comparative Example 5 12 42 16 30 592

[0116] Classification Tensile Strength (MPa) Yield Strength (MPa) Yield Point Yield Point Yield Point Elongation Occurrence Example 1 328 177.10.54 No occurrence Example 2 320 169.60.53 No occurrence Example 3 314 160.10.51 No occurrence Example 4 320 172.80.54 No occurrence Comparative Example 1 39 6308.90.78 Occur Comparative Example 2 38 627 7.90.72 Occur Comparative Example 3 410 299.30.73 Occur Comparative Example 4 35 621 7.20.61 No occurrence Comparative Example 5 410 299.30.73 Occur

[0117] Examples 1 to 4 and Comparative Examples 1 to 2 illustrate the effect of Ti content on wire rods and steel wires. As shown in Tables 3 and 4 above, in Comparative Example 1, which contains an insufficient Ti content of 0.015%, and Comparative Example 2, which contains an excessive Ti content of 0.150%, the tensile strength of the wire rod was found to be high at 290 MPa or higher, and it was confirmed that the tensile strength was also very high at 386 MPa or higher when manufactured into steel wire. In contrast, in the case of Examples 1 to 3 according to the present invention, the tensile strength of the wire rod was 280 MPa or lower and the tensile strength of the steel wire was 330 MPa or lower, which was found to be significantly lower compared to Comparative Examples 1 and 2. Furthermore, in the case of Examples 1 to 4, it was confirmed that the yield strength was 200 MPa or lower and the yield ratio was 0.55 or lower, and no yield point elongation was observed. In the case of Comparative Examples 1 and 2, the yield ratio and yield point elongation were found to be significantly higher compared to the Inventive Example. Furthermore, the area fraction of Ti4C2S2 among coarse precipitates of 100 nm or larger observed in the region from the center of the cross-section perpendicular to the longitudinal direction of the wire rod up to 0.9R was 85% or higher, whereas in the Comparative Examples, this fraction was found to be low. From these results, it could be inferred that the occurrence of the yield ratio and yield point elongation is influenced by the behavior of Ti4C2S2 precipitates observed in the wire rod. Example 2 and Comparative Example 3 show the effect of the steel billet heating furnace temperature on the wire rod and steel wire. In Example 2, the steel billet heating furnace temperature was maintained at a low 1180°C to form Ti4C2S2, while in Comparative Example 3, it was maintained at a high 1230°C. As shown in Tables 3 and 4 above, it can be confirmed that the tensile strength of the wire rod and steel wire in Comparative Example 3 is high, and this can be presumed to be the result caused by the almost non-formation of coarse precipitates.

[0118] Example 2 and Comparative Example 4 show the differences in ferrite grain size and scale thickness according to the winding temperature. As shown in Tables 3 and 4 above, in the case of Example 2, where the winding temperature was 950°C, the ferrite grain size was large at 40 µm, whereas in the case of Comparative Example 4, where the winding temperature was low at 880°C, the ferrite grain size was relatively small at 28 µm. In addition, a difference was observed in scale thickness; in the case of Example 2, a thick scale of 14 µm or more was formed, whereas in the case of Comparative Example 4, it was confirmed that the scale formation was less at 6 µm.

[0119] Example 2 and Comparative Example 5 illustrate the effect of conveyor cooling speed on wire rods and steel wires in a Stelmore cooling zone. Example 2 was cooled at a low cooling speed of 1°C / s, while Comparative Example 5 was cooled rapidly at a cooling speed of 18°C / s under conditions where air was applied without a cover. As shown in Tables 3 and 4 above, it was confirmed that there was a significant difference in the variation of the tensile strength of the wire rod within the coil depending on the conveyor cooling speed; in the case of Example 2, it was small at 58 MPa, whereas in the case of Comparative Example 5, it was large at 92 MPa.

[0120] Although embodiments of the invention disclosed above have been illustrated and described, the disclosed invention is not limited to the specific embodiments described above, and various modifications may be made by those skilled in the art to which the disclosed invention belongs without departing from the essence claimed in the claims.

Claims

1. In wt%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities, When the radius of the wire is denoted as R, among the precipitates observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9R, Ti4C2S2 precipitates with an average size of 100 nm or more account for an area fraction of 85% or more, and The weight ratio of Mn to S (Mn / S) is 7 to 20, and Wire rod with a tensile strength of 280 MPa or less.

2. In Paragraph 1, A wire rod whose microstructure contains ferrite in an area fraction of 99% or more.

3. In Paragraph 2, The above-mentioned ferrite wire has an average grain size of 40㎛ or more.

4. In Paragraph 1, The above wire is a wire having an average surface scale thickness of 14㎛ or more.

5. In Paragraph 1, The above wire is a wire having a tensile strength deviation of 67 MPa or less between the coil overlap and the center.

6. A step of manufacturing a billet by heating a bloom containing, in weight%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), and the remainder being Fe and unavoidable impurities, at 1130~1200℃ for 240~500 minutes, and then rolling the bloom into a steel billet; A step of hot rolling the above-mentioned billet; A step of winding the above hot-rolled wire rod at 900~980℃; and A method for manufacturing a wire rod comprising the step of cooling the wound wire rod at a cooling rate of 0.1 to 1.0℃ / s.

7. In Paragraph 6, A method for manufacturing a wire rod in which, after the cooling step, the wire rod has an area fraction of 85% or more of Ti4C2S2 precipitates with an average size of 100 nm or more observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9 R, where R is the radius of the wire rod.

8. In Paragraph 6, A method for manufacturing a wire rod in which the above hot rolling is performed at 1130~1180℃ for 80 minutes or more.

9. In Paragraph 6, A method for manufacturing a wire rod in which, after the above-mentioned winding step, the microstructure of the wire rod contains ferrite with an area fraction of 99% or more.

10. In Paragraph 6, A method for manufacturing a wire rod having an average ferrite grain size of 40㎛ or more.

11. In Paragraph 6, A method for manufacturing a wire rod in which, after the above-mentioned winding step, the average thickness of the surface scale is 14㎛ or more.

12. In wt%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities, The tensile strength is 330 MPa or less, and The yield strength is 200 MPa or less, and Steel wire with a yield ratio of 0.55 or less.

13. In Paragraph 12, The above steel wire is a steel wire having a yield point elongation of 1.0% or less.

14. A step of manufacturing a billet by maintaining a bloom containing, in wt%, C: 0.002% or less (excluding 0), N: 0.002% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.020~0.100%, P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), and the remainder being Fe and unavoidable impurities, at a temperature of 1130~1200℃ for 240~500 minutes, and then rolling the bloom into a steel billet; A step of hot rolling the obtained billet at a temperature of 1130~1180℃ for 80 minutes or more; A step of winding the above hot-rolled wire rod at a temperature of 900 to 980°C; A step of cooling the above-mentioned wound wire at a rate of 0.1 to 1.0℃ / s; The step of drawing the above-mentioned cooled wire; and A method for manufacturing a steel wire comprising the step of heating the above-mentioned fresh wire at A3-260℃ or higher and A3-160℃ or lower for at least 2 hours and then cooling it.

15. In Paragraph 14, A method for manufacturing a steel wire in which, after the step of heating and cooling, the steel wire has a tensile strength of 330 MPa or less, a yield strength of 200 MPa or less, and a yield ratio of 0.55 or less at a true strain (e) of 3.62 to 3.98.

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