Wire rod, steel wire, and manufacturing method thereof

Abstract

According to the present invention, it is possible to provide a high-strength and high-ductility wire rod, a high-strength and high-ductility steel wire, and a manufacturing method thereof by optimizing alloying elements affecting strength and ductility. A high-strength and high-ductility wire rod according to an embodiment of the present invention contains, in wt%, 0.93-1.03% of C, more than 0% and 0.5% or less of Mn, 0.1-0.4% of Si, 0.5-1.5% of Ni, 0.1-0.5% of Cr, and 0.0005% or more and less than 0.0020% of B, with the remainder comprising Fe and inevitable impurities, and the microstructure thereof may include pearlite and proeutectoid cementite.

Description

Wire rod, steel wire and method of manufacturing the same

[0001] The present invention relates to a high-strength, high-ductility wire rod, a steel wire, and a method for manufacturing the same. More specifically, the invention aims to provide a high-strength, high-ductility wire rod, a steel wire, and a method for manufacturing the same by optimizing alloying elements that affect the strength and ductility of the steel wire.

[0002] Generally, cold-drawn pearlitic steel can secure high strength and appropriate ductility. The pearlitic steel is one of the strongest steels and is widely applied in industries such as tire cords and bridge cables.

[0003] There has been research on the reasons why pearlitic steel can secure excellent strength. Through research on the microstructure, the mechanical properties of pearlitic steel can be improved, and it is reported that the rapid increase in strength after drawing is due to the refinement of the lamellar interlayer spacing.

[0004] Experimental results from Mossbauer spectroscopy, EELS, and 3D-AP show that cementite in pearlite undergoes partial decomposition during cold drawing. Since the decomposition of cementite strongly influences deformation mechanisms, including dislocation generation and transport in the drawn wire, and consequently significantly affects mechanical properties, it remains an important topic being studied by many researchers.

[0005] However, the driving force and kinetic interpretation of the above phenomenon are still under debate, and some researchers have reported that the decomposition of cementite occurs because carbon atoms in lamellar cementite become fixed to dislocations within the ferrite.

[0006] Other researchers argue that the driving force behind the decomposition of cementite is the rapid increase in the solid solubility of carbon within the ferrite caused by the rapid increase in lamellar interfaces resulting from drawing (Gibbs-Thompson effect).

[0007] However, to date, the influence of alloying elements on the decomposition of cementite has not been investigated, and the correlation with mechanical properties resulting therefrom has not been published.

[0008] The objective of the present invention is to provide a high-strength, high-ductility wire rod, a steel wire, and a method for manufacturing the same by optimizing alloying elements that affect strength and ductility.

[0009] The problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by those skilled in the art from the description below.

[0010] As a means to achieve the above-mentioned purpose, a high-strength, high-ductility wire according to one embodiment of the present invention comprises, in weight percent, C: 0.93 to 1.03%, Mn: greater than 0 and less than 0.5%, Si: 0.1 to 0.4%, Ni: 0.5 to 1.5%, Cr: 0.1 to 0.5%, B: greater than 0.0005% and less than 0.0020%, the remainder being Fe and other unavoidable impurities, and the microstructure may include pearlite and proeutectoid cementite.

[0011] The microstructure of a high-strength, high-ductility wire according to one embodiment of the present invention may contain 99% or more of pearlite in terms of area fraction.

[0012] A steel wire drawn from a high-strength, high-ductility wire according to one embodiment of the present invention may include modified pearlite such that the carbon content of cementite in the pearlite differs by 15 atomic percent or more compared to the microstructure before drawing.

[0013] A high-strength, high-ductility steel wire according to one embodiment of the present invention may have a carbon content of cementite in pearlite of 5 to 12 atomic percent.

[0014] A high-strength, high-ductility steel wire according to one embodiment of the present invention may have a tensile strength of 3,000 to 3,200 MPa.

[0015] A method for manufacturing a high-strength, high-ductility steel wire according to one embodiment of the present invention may include the steps of: manufacturing a wire rod by hot rolling a steel material at 1000 to 1200°C, wherein the steel material comprises, in weight percent, C: 0.93 to 1.03%, Mn: greater than 0 and less than 0.5%, Si: 0.1 to 0.4%, Ni: 0.5 to 1.5%, Cr: 0.1 to 0.5%, B: 0.0005% or more and less than 0.0020%, and the remainder being Fe and other unavoidable impurities; cooling the hot-rolled wire rod to 10 to 20°C; and drawing the cooled wire rod with a drawing strain (ε) of 3.0 to 3.8%.

[0016] In a method for manufacturing a high-strength, high-ductility steel wire according to one embodiment of the present invention, the cross-sectional reduction rate in the drawing step may be 45% or more.

[0017] According to one embodiment of the present invention, high-strength, high-ductility wire rods, steel wires, and a method for manufacturing the same can be provided by optimizing alloying elements that affect strength and ductility.

[0018] The problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by those skilled in the art from the description below.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight.

[0024] A steel wire having excellent strength according to one embodiment of the present invention comprises, in weight%, C: 0.93 to 1.03%, Mn: greater than 0 and less than or equal to 0.5%, Si: 0.1 to 0.4%, Ni: 0.5 to 1.5%, Cr: 0.1 to 0.5%, B: greater than or equal to 0.0005% and less than or equal to 0.0020%, and the remainder being Fe and other unavoidable impurities.

[0025] The reason for limiting the above alloy composition is explained in detail below.

[0026] The content of C (carbon) can be 0.93 to 1.03%.

[0027] The above C is a key element for securing strength. In the present invention, if the C content exceeds 1.03%, the cross-sectional reduction rate of the steel decreases, and thus, an increase in strength through drawing cannot be expected. On the other hand, if the C content is less than 0.93%, it is difficult to secure the strength targeted in the present invention. Therefore, it is preferable to limit the C content to 0.93 to 1.03%.

[0028] The content of Mn (manganese) may be greater than 0 and less than or equal to 0.5%.

[0029] Mn is an element effective in increasing hardenability. However, Mn is an element with severe central segregation, and if its content exceeds 0.5 wt%, it is highly likely to cause a low-temperature structure, so it is desirable to limit the upper limit to 0.5 wt%.

[0030] The content of Si (silicon) can be 0.1 to 0.4%.

[0031] Since the above Si preferentially dissolves into ferrite and is effective in improving strength, it is desirable to add at least 0.1%. However, if an excessive amount is added, toughness may decrease, so it is desirable to limit the upper limit to 0.4%.

[0032] The content of Ni (nickel) can be 0.5 to 1.5%.

[0033] Ni plays a role in destabilizing the lamellar interface of pearlite to promote the decomposition of cementite, and additionally, it also plays a role in increasing plastic deformation capacity by increasing the number of operable slip systems of cementite within the pearlite. The above effects are significantly observed when containing 0.5% or more. On the other hand, considering manufacturing costs, it is desirable to limit the upper limit to 1.5%.

[0034] The content of Cr (chromium) may be 0.1 to 0.5%.

[0035] Cr improves strength and ductility by refining the lamellar structure of pearlite. If the Cr content is less than 0.1%, there is no sufficient effect of refining the lamellar structure, and if it exceeds 0.5%, it slows down the isothermal transformation rate, thereby worsening productivity. Therefore, it is desirable to limit the Cr content to 0.1 to 0.5%.

[0036] The content of B (boron) may be 0.0005% or more and less than 0.0020%.

[0037] Since B can improve ductility by enhancing hardenability and suppressing intergranular embrittlement, it is desirable to include at least 0.0005%. On the other hand, if it is added excessively to form boron carbonitrides, ductility can rapidly deteriorate. Therefore, it is desirable to limit the upper limit of the boron content to less than 0.0020%.

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

[0039] It is desirable that the microstructure of the wire rod before drawing be composed of pearlite, with a fraction of 99% and the remainder being proeutectoid cementite. The carbon content contained in the cementite (Fe3C) within the pearlite is 25 atomic percent or more. This is because cementite is composed of three Fe atoms and one C atom.

[0040] However, after wire drawing, the carbon content in the cementite within the pearlite is less than 25 atomic percent. This implies that, in addition to plastic deformation, cementite decomposition occurred within the pearlite during wire drawing. Cementite decomposition is a phenomenon that occurs because it is energetically stable for carbon to escape from the cementite and migrate into the ferrite matrix. Carbon fixed beneath the dislocations in the ferrite hinders the movement of dislocations, thereby increasing strength.

[0041] It is desirable to control the difference in the carbon content of cementite in pearlite to be at least 15 atomic percent when comparing the microstructure of the above steel wire with that before drawing. Specifically, the carbon content contained in cementite (Fe3C) in pearlite before drawing is at least 25 atomic percent. According to one embodiment of the present invention, the carbon content of cementite in pearlite after drawing can be reduced by at least 15 atomic percent compared to before drawing. As described above, the decomposition rate of cementite is controlled by appropriately adding Ni and B alloying elements, thereby improving strength and ductility.

[0042] Through this, modified pearlite exists within the microstructure of the drawn steel wire, and it is desirable to control the carbon content of the cementite within the pearlite to 5 to 12 atomic percent. If the carbon content within the cementite is excessive, the decomposition of the cementite does not occur properly, resulting in a low influx of carbon into the ferrite and thus making it impossible to secure sufficient strength. Considering this, the carbon content within the cementite may be 12 atomic percent or less. More preferably, the carbon content of the cementite within the pearlite after drawing can be controlled to 5 to 10 atomic percent. However, if the carbon content within the cementite is excessively low, wire breakage may occur due to the embrittlement of the ferrite. Considering this, the lower limit of the carbon content within the cementite may be restricted to 5 atomic percent.

[0043] The tensile strength of the steel wire after drawing process can be 3000 to 3200 MPa.

[0044] As described above, Ni and B play a role in promoting the decomposition of cementite, causing carbon present in the cementite to be dissolved into the ferrite. As a result, high-strength steel wire can be obtained after drawing.

[0045] Next, a method for manufacturing a high-strength, high-ductility steel wire according to one embodiment of the present invention will be described.

[0046] A method for manufacturing a high-strength, high-ductility steel wire according to one embodiment of the present invention may include the steps of: manufacturing a wire rod by hot rolling a steel material at 1000 to 1200°C, wherein the steel material comprises, in weight percent, C: 0.93 to 1.03%, Mn: greater than 0 and less than 0.5%, Si: 0.1 to 0.4%, Ni: 0.5 to 1.5%, Cr: 0.1 to 0.5%, B: 0.0005% or more and less than 0.0020%, and the remainder being Fe and other unavoidable impurities; cooling the hot-rolled wire rod to 10 to 20°C; and drawing the cooled wire rod with a drawing strain (ε) of 3.0 to 3.8%.

[0047] The reason for limiting the composition ratio of each alloying element is as described above, and each manufacturing step is explained in more detail below.

[0048] In manufacturing wire rods by hot rolling the above steel, it is preferable to perform the hot rolling at 1000 to 1200°C. At this time, if the steel is hot-rolled at a temperature below 1000°C, sufficient recrystallization does not occur, resulting in a problem of microstructure non-uniformity. If the steel is hot-rolled at a temperature exceeding 1200°C, there is a problem of reduced strength and ductility due to grain coarsening. In addition, significant decarburization occurs, which can worsen the drawing workability.

[0049] The above hot-rolled wire rod can be cooled to 10 to 20°C.

[0050] The above-mentioned cooled wire can be drawn with a drawing strain (ε) of 3.0 to 3.8%.

[0051] The cross-sectional reduction rate of the steel wire during fresh processing can be 45% or more.

[0052] The cross-sectional reduction rate is a ratio indicating how much the cross-sectional area of ​​a material is reduced through the drawing process. The cross-sectional reduction rate can be calculated using the following formula.

[0053]

[0054] In the above equation, A0 is the initial cross-sectional area, A f represents the cross-sectional area after drawing.

[0055] A high-strength, high-ductility steel wire according to one embodiment of the present invention can increase ductility by including Ni and B elements. Since Ni increases the number of operable slip systems of pearlite cementite, it is possible to obtain a high-ductility steel wire by including a Ni content of 0.5 to 1.5%. In addition, since B increases grain boundary stability, it is possible to obtain a high-ductility steel wire by including a B content of 0.0005% or more and less than 0.0020%.

[0056] Hereinafter, the structure and operation of the present invention will be explained in more detail through preferred embodiments of the present invention. However, these are presented as preferred examples of the present invention and should not be interpreted in any way as limiting the present invention.

[0057] {Example}

[0058] Reference Examples 1 to 3, Invention Example 1, and Comparative Example 1, having compositions as shown in Table 1 below, were heated at 1000 to 1200°C, rolled into wire rods, and cooled to 10 to 20°C. Then, the carbon content (atomic%) in cementite, tensile strength, and reduction in area of ​​the wire drawn to 3.1% were measured and are shown in Table 2 below.

[0059] The carbon content in cementite was measured by analyzing the orientation using an Electron Backscatter Diffraction (EBSD) analyzer with the model name JSM 7200F.

[0060] Tensile strength was measured using a universal test machine (UTM) with the model name KDPI-130 Series.

[0061] Classification C (Wt%) Si (Wt%) Mn (Wt%) Cr (Wt%) Ni (Wt%) B (Wt%) Reference Example 1 0.970.20.30.20.7 - Reference Example 20.970.20.30.21.0 - Reference Example 30.970.20.30.21.2 - Invention Example 10.970.20.30.21.00.0010 Invention Example 20.970.20.30.20.60.0015 Invention Example 30.970.20.30.21.30.0008 Comparative Example 10.970.20.30.2 - Comparative Example 20.970.20.30.20.20.0020 Comparative Example 30.970.20.30.22.00.0030

[0062] Carbon content (atomic%) of cementite in pearlite after fresh processing (e=3.1) Tensile strength (MPa) Area reduction rate (%) Reference Example 1 1 2 30 1 340 Reference Example 2 9 30 7 5 42 Reference Example 3 8 31 27 43 Invention Example 1 7 30 9 6 46 Invention Example 2 6 30 1 249 Invention Example 3 5 31 45 45 Comparative Example 1 1 7 28 5 232 Comparative Example 2 1 7 28 5 9 33 Comparative Example 3 1 6 28 8 424

[0063] The present invention aims to control the decomposition rate of cementite by adding Ni and B alloying elements, thereby improving tensile properties.

[0064] Referring to Tables 1 and 2, it can be seen that the carbon content of cementite in Reference Example 1, in which the Ni content was increased by 0.7 wt% in Comparative Example 1, decreased to 12 atomic%. The fact that the carbon content of cementite in Reference Example 1 is lower than that of Comparative Example 1 means that cementite decomposition occurred rapidly at the same amount of deformation. As carbon atoms released by cementite decomposition were dissolved in the ferrite, the tensile strength increased by 161 MPa, from 2852 MPa (Comparative Example 1) to 3013 MPa (Reference Example 1).

[0065] In addition, despite the increase in strength, the cross-sectional reduction rate of Reference Example 1 was 40%, which is an 8% increase compared to Comparative Example 1, which was 32%. This shows that the Ni element is effective not only in increasing strength but also in increasing ductility.

[0066] In addition, the carbon content of cementite in Reference Example 3, in which the Ni content was increased to 1.2 wt%, was 8 atomic%, which is significantly lower than that of Comparative Example 1. Furthermore, the carbon content of cementite in pearlite decreased compared to Reference Example 1. This implies that as the Ni content increases, the decomposition rate of cementite also increases. The tensile strength of Reference Example 3 was 3127 MPa, an increase of 257 MPa compared to Comparative Example 1, and the reduction in area was 43%, an increase of 11% compared to Comparative Example 1. This demonstrates that increasing the Ni element content is effective in increasing strength and ductility.

[0067] Meanwhile, according to the scope of one embodiment of the present invention, in the case of Invention Examples 1 to 3 in which Ni and B were added simultaneously, cementite decomposition occurred rapidly at the same amount of deformation, and the carbon content of cementite in pearlite after drawing was all low at 7 or less. Through this, it can be confirmed that the carbon content of cementite in pearlite differs by 15 atomic percent or more compared to the carbon content of cementite in pearlite before drawing, which was 25 atomic percent or more. Accordingly, as carbon atoms released by the decomposition of cementite were dissolved in ferrite, the tensile strength increased to 3000 to 3200 MPa.

[0068] In addition, in the case of Invention Examples 1 to 3, the cross-sectional reduction rate was increased to 45% or more. It can be seen that the cross-sectional reduction rate can be further increased and the drawing processability can be improved by increasing the number of operable slip systems of pearlite cementite and increasing grain boundary stability through the composite addition of Ni and B, thereby obtaining a high-ductility steel wire.

[0069] Meanwhile, referring to Table 2, it can be seen that the cross-sectional reduction rate decreased in Comparative Examples 2 and 3, which had a combined addition of Ni and B. This is the result of the deterioration of ductility due to the formation of boron carbonitrides as a result of excessively including a B content of 0.0020% or more.

[0070] As shown in the results of Table 2 above, it can be seen that in the case of Invention Examples 1 to 3 satisfying the component range of the present invention, high-strength, high-ductility wire rods and steel wires can be manufactured by adding Ni and B.

[0071] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims set forth below.

Claims

1. A wire rod comprising, in weight%, C: 0.93 to 1.03%, Mn: greater than 0 and less than or equal to 0.5%, Si: 0.1 to 0.4%, Ni: 0.5 to 1.5%, Cr: 0.1 to 0.5%, B: greater than or equal to 0.0005% and less than 0.0020%, and the remainder being Fe and other unavoidable impurities, wherein the microstructure comprises pearlite and proeutectoid cementite.

2. In Claim 1, The microstructure of the above wire contains 99% or more of pearlite in terms of area fraction.

3. A steel wire produced by drawing the wire rod of Claim 1, comprising a modified pearlite having a carbon content of cementite in the pearlite that differs by 15 atomic percent or more compared to the microstructure before drawing.

4. In Claim 3, A steel wire having a carbon content of 5 to 12 atomic percent of cementite in the pearlite of the above steel wire.

5. In Claim 3, A steel wire having a tensile strength of 3000 to 3200 MPa.

6. A step of manufacturing a wire rod by hot rolling a steel material containing, in weight%, C: 0.93 to 1.03%, Mn: greater than 0 and less than or equal to 0.5%, Si: 0.1 to 0.4%, Ni: 0.5 to 1.5%, Cr: 0.1 to 0.5%, B: 0.0005% or more and less than 0.0020%, and the remainder being Fe and other unavoidable impurities, at 1000 to 1200℃; A step of cooling the hot-rolled wire rod to 10 to 20°C; and A method for manufacturing steel wire comprising the step of drawing the above-mentioned cooled wire rod to a drawing strain (ε) of 3.0 to 3.8%.

7. In Claim 6, A method for manufacturing steel wire in which the cross-sectional reduction rate in the above-mentioned fresh stage is 45% or more.

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