A wire rod according to the present disclosure comprises, by wt%: C: 1.00% to 1.20%, Si: 1.00% to 1.50%, Mn: 0.10% to 0.50%, Cr: 0.30% to 0.80%, and Al: 0.50% to 1.50%, the balance of Fe and other unavoidable impurities, wherein a microstructure observed at a cross-section of 1/2D to 3/2D (D: diameter) comprises 95% or more of pearlite by area fraction, and 5% or less of massive pro-eutectoid cementite having an average thickness of 1 µm or more in an area of 100 µm2.
The present disclosure relates to a hypereutectoid high-strength wire rod, a steel wire, and a manufacturing method thereof, applicable to bridge cables, such as for suspension bridges and cable-stayed bridges, and steel strands for reinforcing concrete, wherein an addition of aluminum to a carbon steel having C of 1.0% or more suppresses massive pro-eutectoid cementite formed at grain boundaries, thereby securing wire drawability and torsion properties.
In suspension bridges or cable-stayed bridges, cables are used to connect piers and decks, and the cables are used as multiple twisted strands or as multiple bundles connected in parallel. Meanwhile, higher strength is also required for bridge cables. This is because an increase in the strength of a steel wire allows for a reduction in the amount of material used and a shortening of a construction period.
Strengthening of a steel wire is in accordance with an empirical formula proposed by Embury et al. in the 1960s. According thereto, methods for most effectively improving strength include 1) increasing a strength of a material, 2) improving a work hardening rate by refining an initial microstructure to hinder movement of dislocations, and 3) increasing a drawing reduction.
An improvement of a work hardening rate, based on an increase of a material strength and a control of a microstructure, mainly utilizes an addition of alloy elements. C provides a solid-solution strengthening effect and a precipitation strengthening effect. Cr increases a wire drawability through a refinement of a microstructure, the refinement resulting from a decrease in C diffusion in austenite and a consequent increase in nucleation sites. Si serves to increase an initial strength through solid-solution strengthening in ferrite and a refinement of a pearlite structure. Finally, an increase in a wire drawing amount is a method for efficiently increasing a strength, wherein an increase in the wire drawing amount exponentially increases the strength because a pearlite structure entirely rotates in a wire drawing direction to form a fiber-like structure. An increase in the wire drawing amount increases an amount of carbon, present in cementite, that diffuses into ferrite. This carbon becoming supersaturated in the ferrite performs a role similar to martensite, and a formation of this structure greatly increases the strength.
The present disclosure relates to a hypereutectoid high-strength wire rod, a steel wire, and a manufacturing method thereof, applicable to bridge cables for suspension bridges, cable-stayed bridges, and the like, and to steel strands for concrete reinforcement. An object of the present disclosure is to provide a wire rod, a steel wire, and a manufacturing method thereof that secure drawability and torsion properties by adding aluminum to a carbon steel comprising 1.0% C or more to suppress massive pro-eutectoid cementite formed at grain boundaries.
A wire rod according to an embodiment of the present disclosure comprises, by wt%: C: 1.00% to 1.20%, Si: 1.00% to 1.50%, Mn: 0.10% to 0.50%, Cr: 0.30% to 0.80%, and Al: 0.50% to 1.50%, the balance of Fe and other unavoidable impurities, wherein a microstructure observed at a cross-section of 1/2D to 3/2D (D: diameter) comprises 95% or more of pearlite by area fraction, and 5% or less of massive pro-eutectoid cementite having an average thickness of 1 µm or more in an area of 100 µm2.
Further, a wire rod according to an embodiment of the present disclosure has an average tensile strength at room temperature of 1630 MPa or more.
Additionally, a reduction of area of the wire rod according to an embodiment of the present disclosure is 14% or more.
Additionally, in an embodiment of the present disclosure, an average scale thickness of the wire rod is 20 µm or less.
A manufacturing method of a wire rod according to another example of the present disclosure comprises: preparing a billet comprising, by wt%: C: 1.00% to 1.20%, Si: 1.00% to 1.50%, Mn: 0.10% to 0.50%, Cr: 0.30% to 0.80%, Al: 0.50% to 1.50%, and the balance of Fe and other unavoidable impurities; heating and hot-rolling the billet; coiling at Acm+20°C to Acm+100°C; and Stelmor cooling to 350°C to 400°C at a rate of 8°C/s to 12°C/s.
A steel wire according to another embodiment of the present disclosure comprises, by wt%: C: 1.00% to 1.20%, Si: 1.00% to 1.50%, Mn: 0.10% to 0.50%, Cr: 0.30% to 0.80%, Al: 0.50% to 1.50%, the balance of Fe and other unavoidable impurities, wherein, after a Zn plating treatment, a tensile strength of the plated wire is 2250 MPa or more, a torsion is 13 times or more, and an elongation is 6.5% or more.
A manufacturing method of a steel wire according to another example of the present disclosure comprises: preparing the above-described wire rod; performing pickling and isothermal heat treatment; performing wire drawing at a drawing reduction of 77% to 85%; and performing Zn plating treatment.
According to the present disclosure, an ultra-high-strength steel wire is manufacturable by utilizing a hypereutectoid wire rod to which aluminum is added, and an application thereof to a bridge cable is effective in reducing an amount of cable used and a construction period.
Hereinafter, preferred embodiments of the present disclosure are described. However, the embodiments of the present disclosure may be modified into various different forms, and the technical idea of the present disclosure is not limited to the embodiments described hereinafter. Furthermore, the embodiments of the present disclosure are provided to more completely describe the present disclosure to a person having ordinary skill in the art.
The terminology used in the present disclosure is for the purpose of describing particular examples only. Accordingly, a singular expression includes a plural expression unless the context clearly dictates otherwise. In addition, it is to be understood that the terms such as "comprising" or "having" as used in the present disclosure specify the presence of a stated feature, step, function, component, or a combination thereof, but do not preclude the presence or addition of another feature, step, function, component, or a combination thereof.
In an absence of a specific definition herein, all terms used in the present disclosure are to be construed as having the same meaning as commonly understood by one of ordinary skill in the art to which the present disclosure pertains. Accordingly, in an absence of a clear definition in the present disclosure, a specific term is not to be interpreted in an overly idealized or formalistic sense.
Further, as used in the present specification, the terms "about," "substantially," and the like mean at or near a stated numerical value, considering inherent manufacturing and material tolerances, and are used to prevent an unscrupulous infringer from unfairly exploiting a disclosure that recites a precise or absolute numerical value for the purpose of aiding in the understanding of the present disclosure.
Absent a specific statement to the contrary in the present disclosure, percentages indicating the content of each element are based on weight.
First, a description of a wire rod according to the present disclosure is provided.
A wire rod according to an embodiment of the present disclosure comprises, by wt%: C: 1.0% to 1.2%, Si: 1.0% to 1.5%, Mn: 0.1% to 0.5%, Cr: 0.3% to 0.8%, and Al: 0.5% to 1.5%, the balance of Fe and other unavoidable impurities, wherein a microstructure observed at a cross-section of 1/2D to 3/2D (D: diameter) comprises 95% or more of pearlite by area fraction, and 5% or less of massive pro-eutectoid cementite having an average thickness of 1 µm or more in an area of 100 µm2.
Hereinafter, the reasons for the numerical limitations of the content of the alloying elements in the examples of the present disclosure will be described.
C is an element for most effectively increasing a material strength, and an increase in the C content by 0.1% increases the strength by 100 MPa. A C content of less than 1.00% makes achieving a target product strength difficult. A C content greater than 1.20% causes wire breakage during wire drawing due to an increased fraction of massive pro-eutectoid cementite, and therefore the C content is controlled to be 1.20% or less. Preferably, the C content is 1.02% to 1.20%, and more preferably, 1.05% to 1.20%.
Si is a solid-solution strengthening element that increases strength by a level of 15 MPa per an addition of 0.1%, and also serves to suppress a decrease in tensile strength during plating, wherein Si segregates at Si-carbide interfaces, slowing a diffusion of carbon. Therefore, a Si content of less than 1.00% decreases tensile strength, and a Si content of greater than 1.50% causes wire breakage during wire drawing due to deterioration of center segregation and increased decarburization, such that the Si content is controlled to be 1.50% or less. Preferably, the Si content is 1.05% to 1.45%, and more preferably, 1.10% to 1.40%.
Mn, while a solid-solution strengthening element, is added in the present disclosure to secure hardenability. However, a sufficient hardenability is secured due to the inclusion of a large amount of hardenability elements such as C and Cr. A high C content resulting in an expectation of center segregation makes it necessary for the Mn content not to be greater than 0.50%. Preferably, the Mn content may be 0.15% to 0.45%, and more preferably, the Mn content may be 0.15% to 0.40%.
An addition of 0.1% of Cr increases a tensile strength by 40 MPa and reduces a cementite thickness, thereby contributing to an increase in a work hardening rate during wire drawing. Therefore, a Cr content less than 0.30% prevents securing a target strength. A Cr content greater than 0.80% causes formation of coarse chromium carbides, which induces wire breakage during processing and delamination during torsion of a plated wire, and thus the Cr content is controlled to be 0.80% or less. Preferably, the Cr content may be 0.35% to 0.75%, and more preferably, 0.60% to 0.80%.
Aluminum (Al) has a high affinity for oxygen and is generally used as a deoxidizer. However, Al is also known as an element that provides a solid-solution strengthening effect in ferrite and refines pearlite while simultaneously suppressing formation of pro-eutectoid cementite. In the present disclosure, an addition of Al to a carbon steel having 1.0% or more of C refines a microstructure and suppresses formation of massive pro-eutectoid cementite, thereby improving strength and torsion properties. Accordingly, the Al content is 0.50% or more. However, an Al content greater than 1.50% causes a nozzle clogging problem due to formation of complex inclusions such as Al2O3, Al-Si-O, and the like, and thus the Al content is controlled to be 1.50% or less. Preferably, the Al content is 0.65% to 1.45%, and more preferably, 0.60% to 1.40%.
The balance of the present disclosure is iron (Fe). However, impurities unavoidably incorporated from raw materials or a surrounding environment during a conventional manufacturing process are not excluded. As these impurities are known to a person skilled in the art, all details thereof are not particularly described in the present specification.
In a wire rod according to an embodiment of the present disclosure, a microstructure observed at a cross-section of 1/2D to 3/2D (D: diameter) comprises 95% or more of pearlite by area fraction, and 5% or less of massive pro-eutectoid cementite having a thickness of 1 µm or more in an area of 100 µm2.
As described above, an increase in the carbon content is an effective method for increasing strength. However, an increase of the carbon content beyond a eutectoid composition results in a formation of pro-eutectoid cementite at grain boundaries. Whereas a film-like pro-eutectoid cementite is re-dissolved during an isothermal heat treatment and thus poses no problem, a massive pro-eutectoid cementite is not re-dissolved, which creates a risk of wire breakage during wire drawing or delamination during a torsion test of a final product. Therefore, a massive pro-eutectoid cementite having an average thickness of 1 µm or more is controlled to be 5% or less in an area of 100 µm2, preferably 4.5% or less, and more preferably 4.0% or less.
Thereby, a wire rod according to an embodiment of the present disclosure secures high strength even before wire drawing, wherein an average tensile strength at room temperature is 1630 MPa or more, a reduction of area is 14% or more, and an average scale thickness is 20 µm or less, thereby providing excellent scale peelability.
The following describes a manufacturing method of a wire rod according to the present disclosure.
A manufacturing method of a wire rod according to an embodiment of the present disclosure comprises: preparing a billet comprising, by wt%: C: 1.00% to 1.20%, Si: 1.00% to 1.50%, Mn: 0.10% to 0.50%, Cr: 0.30% to 0.80%, Al: 0.50% to 1.50%, and the balance of Fe and other unavoidable impurities; heating and hot-rolling the billet; coiling at Acm+20°C to Acm+100°C; and Stelmor cooling to 350°C to 400°C at a rate of 8°C/s to 12°C/s.
Reasons for the numerical limitations of the component ranges of each of the above-described alloy compositions are as described above, and a more detailed description of each manufacturing step is provided below.
A step of preparing a billet may be a conventional billet manufacturing process.
Further, a step of heating and rolling the billet may be performed by a conventional process. For example, after preparing the billet having the above-described alloy composition, the billet is maintained at a heating furnace temperature of 950°C to 1050°C for 90 minutes to 120 minutes for normalizing and austenite formation, and is then rolled. Maintaining the temperature at less than 950°C causes a problem of a lengthened charging time, and maintaining the temperature at greater than 1050°C causes a load on a heating furnace. Therefore, the temperature is preferably controlled to 950°C to 1050°C. Further, maintaining for less than 90 minutes may make austenite formation in a central part difficult, and maintaining for greater than 120 minutes may cause coarse grain growth.
A coiling temperature affects a formation of pro-eutectoid cementite and a scale thickness. Despite a suppression of a formation of massive pro-eutectoid cementite through an Al addition, a formation of film-type pro-eutectoid cementite is unavoidable in a carbon steel having a C content of 1.0% or more at a temperature of an Acm point or less. A coiling temperature less than Acm + 20°C causes an active formation of the pro-eutectoid cementite. A coiling temperature greater than Acm + 100°C causes a formation of a thick scale due to a high temperature, which makes descaling difficult and prolongs a pickling bath retention time. Accordingly, a control of the coiling temperature to be Acm + 100°C or less is preferable.
Stelmor cooling to 350°C to 400°C at a rate of 8°C/s to 12°C/s.
A stable completion of a pearlite transformation occurs upon cooling to 400°C. A temperature exceeding 400°C causes pearlite growth, resulting in a coarsened microstructure and deteriorated processability, and a high temperature during coil handling deteriorates a worker environment. Accordingly, cooling is controlled in a range of 350°C to 400°C at a rate of 8°C/s to 12°C/s.
The following describes a steel wire according to the present disclosure and a manufacturing method thereof.
A steel wire according to an example of the present disclosure comprises, by wt%, C: 1.00% to 1.20%, Si: 1.00% to 1.50%, Mn: 0.10% to 0.50%, Cr: 0.30% to 0.80%, and Al: 0.50% to 1.50%, the balance of Fe and other unavoidable impurities, wherein after a Zn plating treatment, a tensile strength of the plated wire is 2250 MPa or more, a torsion is 13 times or more, and an elongation is 6.5% or more. A tensile strength of the plated wire being less than 2250 MPa lowers a cable cutting force, causing a problem in cable stability and an insignificant reduction in an amount of cable used. A torsion of the plated wire being less than 13 times causes many internal defects, and an elongation being less than 6.5% deteriorates workability, which may limit an application as a steel wire for cables.
A manufacturing method of a steel wire according to an embodiment of the present disclosure comprises: preparing the above-described wire rod; performing pickling and isothermal heat treatment; performing wire drawing at a drawing reduction of 77% to 85%; and performing Zn plating treatment.
The pickling may be performed with 3% to 5% hydrochloric acid at 70°C to 90°C, and the Zn plating treatment step may be performed by a hot-dip galvanizing treatment or an electro-galvanizing treatment, but is not limited thereto.
Hereinafter, the present disclosure will be described in more detail through examples.
A rear end of a billet, which was cast by vacuum melting 75 kg of a steel having a component system as in Table 1, was welded, held for 90 minutes to 120 minutes in a heating furnace at a temperature of 950°C to 1050°C, and then rolled to 11 mm. The manufactured comparative and inventive examples were coiled at the coiling temperatures of Table 2 and finished by Stelmor cooling at 10°C/s.
Additionally, a steel wire was manufactured by pickling the manufactured wire rod at 80°C in 4% hydrochloric acid, performing an isothermal heat treatment at 580°C, and performing dry wire drawing to a total reduction of area of 77.5% (or a total strain of 1.49).
The mechanical properties and microstructure of the wire rod, and mechanical properties after drawing and plating, were measured and are presented in Table 1 and Table 2 below.
Additionally, for a measurement of a tensile strength of the wire rod, a tensile test was performed on a sample cut to a length of 40 cm from two rings (circumference: 3.2 m) at a rear end of a coil. A crosshead speed was 70 mm/m, and a gauge length was 30 cm. A diameter before the tensile test and a diameter of a necked portion after the tensile test were measured using a stereo projector. A reduction of area was measured based on a formula of (1 - (a diameter after the tensile test / a diameter before the tensile test)2) * 100.
For a measurement of a tensile strength of a steel wire according to the present disclosure, a length of a tensile test specimen was 40 cm, a tensile speed (crosshead speed) was 100 mm/m, and a gauge length was 30 cm, which was the same as that of the wire rod. An elongation was defined as a percentage of an elongated length with respect to an initial length of 30 cm. For a torsion test, a back load was set to load (kg) × 0.008, and a length was set to 100D (D: diameter). A number of twists to fracture was confirmed by rotating an unfixed chuck in one direction while one chuck was fixed.
Further, the area fractions of the massive cementite and the pearlite were determined by calculating an average of 10 measurements (n=10) obtained using a scanning electron microscope at a magnification of 3,000x in a region where the massive cementite or the pearlite was disposed parallel to an electron beam without tilting.
A scale thickness of the wire rod was measured using an optical microscope at a magnification of 200x in a region where the scale was present in a relatively intact form, and an average of five measurements (n=5) was calculated.
An evaluation of scale peelability was performed by testing a specimen having a length of 30 cm using a tensile tester until fracture. Upon a visual inspection, an instance of a small amount of residual scale adhering to a surface was evaluated as 'Good,' and an instance otherwise was evaluated as 'Poor.' Alternatively, after measuring a weight before the tensile test and a weight after the fracture, an existence of less than 0.05% of residual scale was evaluated as 'Good,' and an existence of the residual scale exceeding 0.05% was evaluated as 'Poor.'
For Inventive Examples 1 to 3, a tensile strength of a wire rod was 1630 MPa or more and a reduction of area (RA) was 14% or more, preventing an occurrence of wire breakage during wire drawing. A tensile strength of a final plated wire was 2250 MPa or more, a torsion was 14 times or more without an occurrence of delamination during a torsion test, and an elongation was 6.5% or more.
However, for Comparative Example 1 having the lowest C content of 0.95 wt%, properties and a microstructure of the wire rod were within a range of the present disclosure, but a tensile strength of a final plated wire was merely less than 2250 MPa. On the other hand, for Comparative Example 2 having a C content greater than 1.20 wt%, the tensile strength of the wire rod was high, but wire breakage occurred during processing.
A case of Comparative Example 3 having a Si content of 0.80% exhibits favorable wire drawing workability, but also exhibits a large reduction in tensile strength during plating, thereby causing a difficulty in securing target physical properties. In cases of Inventive Examples 4 and 5, having a Si content satisfying a range of the present disclosure, an occurrence of wire breakage during working is prevented, and a decrease in tensile strength after plating is also confirmed to be reduced. However, in a case of Comparative Example 4 having a Si content of 1.60%, wire breakage occurred during wire drawing.
Comparative Example 5, having an Mn content greater than 0.50%, exhibited wire breakage during wire drawing, confirming that a high Mn addition is unsuitable for a carbon steel designed with Al addition and a C content of 1.0% or more.
Due to Cr increasing strength and refining pearlite, and thus being advantageous for wire drawability, for Inventive Examples 1 and 6 satisfying 0.30% to 0.80% of Cr, no wire breakage occurred during processing, and a tensile strength of the plated wire satisfying 2250 MPa or more made it possible to secure high strength, torsion properties, and elongation. However, for Comparative Example 6 having a Cr content of only 0.20%, the tensile strengths of the wire rod and the plated wire were low, and for Comparative Example 7 having a Cr content of 1.00%, wire breakage was induced during processing due to coarse Cr carbides.
Inventive Examples 1, 7, and 8, satisfying an Al content of 0.50% to 1.50%, exhibit an area fraction of 5% or less of a massive cementite having a thickness of 1 µm or more, indicating a suppression effect of the massive cementite by Al. However, for Comparative Example 8 having an Al content of only 0.20%, an area fraction (%) of the massive cementite having a thickness of 1 µm or more was confirmed to be as high as 16%. In addition, for Comparative Example 9 having an Al content of 1.80%, a non-occurrence of wire breakage during wire drawing was observed, but an occurrence of a delamination during a torsion test of a plated wire and a low elongation were confirmed.
In Comparative Example 10, a coiling temperature greater than Acm+100°C results in a scale thickness of 20 µm or more, which is substantially greater than that of the inventive examples, causing poor scale peelability. The poor scale peelability necessitates prolonged immersion in a pickling tank, thereby rendering Comparative Example 10 unsuitable for processing.
While the exemplary embodiments of the present disclosure have been described, the present disclosure is not limited thereto, and it will be understood by those skilled in the art that various changes and modifications may be made without departing from the concept and scope of the patent claims set forth hereinafter.