Wire rod for welding rods and manufacturing method therefor

Abstract

The present invention relates to a wire rod for welding rods and a manufacturing method therefor. One aspect of the present invention is to provide a wire rod for welding rods and a manufacturing method therefor. A preferred aspect of the present invention is to provide: a wire rod for welding rods, having superior strength and wire drawability as well as excellent cost-effectiveness; and a manufacturing method therefor.

Description

Welding wire rod and method of manufacturing the same

[0001] The present invention relates to a wire rod for welding rods and a method for manufacturing the same.

[0002] In the case of welding wire, a wire with a diameter of about 5.5 mm is first drawn to be processed to a diameter of about 2.2 mm, and then undergoes drawing heat treatment and plating processes, and is then manufactured into a welding rod with a final diameter of 0.8 to 1.6 mm through second drawing.

[0003] At this time, in the case of conventional ultra-high strength wire rods, pre-heat treatment is performed in the first stage of drawing to ensure drawing processability, which leads to increased production costs and reduced productivity.

[0004] Meanwhile, conventional welding wire rods contain about 2-3% of expensive Ni to secure low-temperature impact properties required for multi-layer welding of thick plates. Applying this to the automotive industry leads to increased manufacturing costs, which becomes an obstacle to expanding industrial applications.

[0005] Patent documents 1 and 2 are examples of technologies related to wire rods for welding rods.

[0006] Patent document 1 discloses a special welding rod wire containing, in weight percent, C: 0.03~0.13%, Mn: 1.5~2.5%, Si: 0.5%~1.2%, Mo: 0.2~0.4%, Ti: 0.1~0.3%, N: 70 ppm or less, S: 0.03% or less, and the remainder being Fe and other unavoidable impurities, with an average grain size of 10~30 μm.

[0007] Patent Document 2 comprises, in weight%, C: 0.01~0.15%, Si: 0.001~0.15%, Mn: 0.5~3.0%, P: greater than 0 and less than or equal to 0.03%, S: greater than 0 and less than or equal to 0.03%, Cu: greater than 0 and less than or equal to 0.5%, Ni: 0.05~0.9%, Cr: 0.001~0.1%, Mo: 0.001~0.5%, Al: 0.001~0.05%, B: 0.0005~0.01%, N: 0.001~0.01%, at least one of V, Nb, and Ti, wherein V: 0.05~0.2%, Nb: 0.005~0.1%, Ti: 0.05~0.3%, and the remainder being Fe and unavoidable impurities, wherein the tensile strength is maximum A welding rod wire is disclosed, characterized by having a thickness of 700 MPa and a deviation of 40 MPa or less.

[0008] However, patent documents 1 and 2 have the disadvantage that they are difficult to apply to welding of steel materials for Gigasteel because their tensile strength is low.

[0009] [Prior Art Literature]

[0010] (Patent Document 1) Korean Registered Patent Publication No. 1060785

[0011] (Patent Document 2) Korean Published Patent Application No. 2024-0063539

[0012] One aspect of the present invention aims to provide a wire rod for welding and a method for manufacturing the same.

[0013] A preferred aspect of the present invention is to provide a welding rod wire with excellent strength and drawing processability while having excellent economic efficiency, and a method for manufacturing the same.

[0014] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0015] One embodiment of the present invention provides a welding rod wire comprising, in weight%, C: 0.050~0.160%, Si: 0.0010~0.250%, Mn: 1.0~2.50%, P: 0.030% or less (excluding 0%), S: 0.030% or less (excluding 0%), Cr: 0.40~6.0%, Mo: 0.10~0.650%, Al: 0.20% or less (excluding 0%), Ni: 0.40% or less (excluding 0%), Ti: 0.20% or less (excluding 0%), Nb: 0.10% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities, wherein the microstructure comprises bainitic ferrite, the minimum effective grain size of the bainitic ferrite is 0.50㎛ or more, and the maximum effective grain size of the bainitic ferrite is 20.0㎛ or less.

[0016] The above wire may additionally include one or more of V: ​​0.20% or less, Zr: 0.10% or less, and B: 0.010% or less.

[0017] The above wire may additionally contain Cu: 0.50% or less.

[0018] The above microstructure may include 0.50% or less (including 0%) of retained austenite and the remainder of bainitic ferrite in terms of area %.

[0019] The above bainitic ferrite may have an average effective grain size of 10 μm or less.

[0020] The above wire can have a tensile strength of 840 to 1000 MPa.

[0021] The above wire may have a reduction of area (RA; Reduction of Area) of 60~75%.

[0022] Another embodiment of the present invention comprises the steps of: preparing a billet comprising, in weight percent, C: 0.050~0.160%, Si: 0.0010~0.250%, Mn: 1.0~2.50%, P: 0.030% or less (excluding 0%), S: 0.030% or less (excluding 0%), Cr: 0.40~6.0%, Mo: 0.10~0.650%, Al: 0.20% or less (excluding 0%), Ni: 0.40% or less (excluding 0%), Ti: 0.20% or less (excluding 0%), Nb: 0.10% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; reheating the billet at 1050~1250℃; extracting the reheated billet at 1050~1150℃ and then rolling it into a wire rod to obtain a wire rod; A method for manufacturing a welding rod wire is provided, comprising the steps of: winding the wire at 860 to 940°C; and cooling the wire at a cooling rate of 2.0 to 3.0°C / s in a Stelmor cooling zone.

[0023] The above billet may additionally include one or more of V: ​​0.20% or less, Zr: 0.10% or less, and B: 0.010% or less.

[0024] The above billet may additionally contain Cu: 0.50% or less.

[0025] According to one aspect of the present invention, a wire for welding rods and a method for manufacturing the same can be provided.

[0026] According to a preferred aspect of the present invention, a welding rod wire with excellent strength and drawing processability while having excellent economic efficiency and a method for manufacturing the same can be provided.

[0027] Figure 1 is an IQ (Image Quality) and IPF (Inverse Pole Figure) photograph of Invention Example 2 observed by EBSD.

[0028] Figure 2 shows the IQ (Image Quality) and IPF (Inverse Pole Figure) images of Comparative Example 3 observed by EBSD.

[0029] Figure 3 shows the Image Quality (IQ) and Inverse Pole Figure (IPF) images of Comparative Example 7 observed by EBSD.

[0030] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0031] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0032] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0033] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0034] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0035] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0036] Hereinafter, a wire rod for a welding rod according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise specified, the alloy composition described below refers to weight percent.

[0037] C: 0.050~0.160%

[0038] C is an element that is advantageous for securing the strength of the weldment by increasing hardenability. If the C content is less than 0.050%, there may be a disadvantage in that it becomes difficult to impart sufficient strength to the weld metal when welding ultra-high strength steel with a tensile strength of 1 GPa or more. If the C content exceeds 0.160%, the viscosity of the molten metal decreases during arc welding, resulting in poor bead shape, and there may also be a disadvantage in that the weld metal is excessively hardened, leading to increased brittleness. Therefore, the C content may have a range of 0.050 to 0.160%. The lower limit of the C content is more advantageous at 0.055%, more advantageous at 0.060%, and most advantageous at 0.065%. The upper limit of the C content is more advantageous at 0.155%, more advantageous at 0.150%, and most advantageous at 0.145%.

[0039] Si: 0.0010~0.250%

[0040] Si is an element that promotes the deoxidation of molten metal during arc welding. It is effective in suppressing the occurrence of porous defects such as blowholes, making it an advantageous element for securing the strength of the weldment. If the Si content is less than 0.0010%, there may be a disadvantage in that porous defects such as blowholes are prone to occurring in the weldment due to insufficient deoxidation power. If the Si content exceeds 0.250%, there may be a disadvantage in that it significantly increases the generation of non-conductive slag, thereby worsening the electrodeposition paintability of the weldment. Therefore, the Si content may have a range of 0.0010% to 0.250%. The lower limit of the Si content is more advantageous at 0.0025%, more advantageous at 0.0050%, and most advantageous at 0.010%. The upper limit of the above Si content is more advantageous at 0.18%, more advantageous at 0.16%, and most advantageous at 0.14%.

[0041] Mn: 1.0~2.50%

[0042] Mn is a deoxidizing element and is advantageous for securing the strength of the weldment by promoting the deoxidation of the molten metal during arc welding, thereby suppressing the occurrence of porous defects such as blowholes. If the Mn content is less than 1.0%, deoxidation becomes insufficient within the appropriate range of the Si content mentioned above, and there may be a disadvantage in that porous defects such as blowholes are prone to occurring in the weldment. If the Mn content exceeds 2.50%, the viscosity of the molten metal becomes excessively high, and when the welding speed is high, the molten metal cannot flow properly into the weldment, resulting in a humping bead and making it prone to bead shape defects. Therefore, the Mn content may have a range of 1.0 to 2.50%. The lower limit of the Mn content is more advantageous at 1.1%, more advantageous at 1.2%, and most advantageous at 1.3%. The upper limit of the above Mn content is more advantageous at 2.45%, more advantageous at 2.40%, and most advantageous at 2.35%.

[0043] P: 0.030% or less (excluding 0%)

[0044] P is an impurity element included in steel, and if the content of P exceeds 0.030%, there may be a disadvantage in that the weld metal becomes sensitive to high-temperature cracking. Meanwhile, although it is advantageous for P not to be included in steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is desirable for the content of P to be 0.030% or less (excluding 0%). It is more advantageous for the content of P to be 0.025% or less, more advantageous for 0.020% or less, and most advantageous for 0.015% or less.

[0045] S: 0.030% or less (excluding 0%)

[0046] S is an impurity element included in steel, and if the content of S exceeds 0.030%, there may be a disadvantage of impairing the toughness of the weld metal. Meanwhile, although it is advantageous for S not to be included in the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is desirable for the content of S to be 0.030% or less (excluding 0%). It is more advantageous for the content of S to be 0.025% or less, more advantageous for 0.020% or less, and most advantageous for 0.015% or less.

[0047] Cr: 0.40~6.0%

[0048] Cr is a ferrite-stabilizing element and a hardenable element that is advantageous for securing the strength of the weld metal. If the Cr content is less than 0.40%, there may be a disadvantage in that it becomes difficult to impart sufficient strength to the weld metal when welding ultra-high strength steel with a tensile strength of 1 GPa or more. If the Cr content exceeds 6.0%, there may be a disadvantage in that the formation of a δ-ferrite structure or excessive precipitation of chromium carbides within the structure leads to weld metal embrittlement, i.e., a decrease in toughness. Therefore, the Cr content may have a range of 0.40 to 6.0%. The lower limit of the Cr content is more advantageous at 0.70%, more advantageous at 1.0%, and most advantageous at 1.20%. The upper limit of the Cr content is more advantageous at 5.5%, more advantageous at 5.0%, and most advantageous at 4.5%.

[0049] Mo: 0.10~0.650%

[0050] Mo is a ferrite-stabilizing element and a hardenable element that is advantageous for securing the strength of the weld metal. If the Mo content is less than 0.10%, there may be a disadvantage in that it becomes difficult to impart sufficient strength to the weld metal when welding ultra-high strength steel with a tensile strength of 1 GPa or more. If the Mo content exceeds 0.650%, there may be a disadvantage in that the toughness of the weld metal decreases. Therefore, the Mo content may have a range of 0.10 to 0.650%. The lower limit of the Mo content is more advantageous at 0.15%, more advantageous at 0.20%, and most advantageous at 0.25%. The upper limit of the Mo content is more advantageous at 0.63%, more advantageous at 0.61%, and most advantageous at 0.59%.

[0051] Al: 0.20% or less (excluding 0%)

[0052] Al is a deoxidizing element that is advantageous for securing the strength of the weld metal by promoting the deoxidation of the molten metal during arc welding even in trace amounts. To ensure the aforementioned effect, 0% is excluded as the lower limit of the Al content. However, due to the deoxidizing effect of Al, for example when welding galvanized steel sheets, it may interfere with the oxidation reaction of Zn, thereby increasing zinc vapor pressure and inducing arc instability, which may promote the occurrence of porosity defects in the weld. If the above Al content exceeds 0.20%, the formation of Al-based oxides increases, which may result in a decrease in the strength and toughness of the weld metal in some cases, and may have the disadvantage of making the weld sensitive to electrodeposition coating defects caused by non-conductive oxides. Therefore, the above Al content may have a range of 0.20% or less (excluding 0%). The above lower limit of the Al content is more advantageous at 0.001%, more advantageous at 0.002%, and most advantageous at 0.003%. The upper limit of the above Al content is more advantageous at 0.19%, more advantageous at 0.18%, and most advantageous at 0.17%.

[0053] Ni: 0.40% or less (excluding 0%)

[0054] Ni is an element advantageous for ensuring the strength and toughness of the weld metal. To ensure the aforementioned effects, 0% is excluded as the lower limit of the Ni content. If the above Ni content exceeds 0.40%, there may be a disadvantage of becoming sensitive to cracking. Therefore, the above Ni content may have a range of 0.40% or less (excluding 0%). The above Ni content is more advantageous at 0.001%, more advantageous at 0.002%, and most advantageous at 0.003%. The above Ni content is more advantageous at 0.39%, more advantageous at 0.38%, and most advantageous at 0.37%.

[0055] Ti: 0.20% or less (excluding 0%)

[0056] Ti is a carbonate element that is advantageous for improving the strength of the weld metal by promoting carbonation of the molten metal during arc welding, even in trace amounts. In addition, it facilitates the development of acicular ferrite, which can improve the toughness of the weldment. To ensure the aforementioned effects, 0% is excluded as the lower limit of the Ti content. If the Ti content exceeds 0.20%, the formation of Ti-based oxides increases, which may result in a disadvantage where the strength and toughness of the weld metal decrease in some cases. Therefore, the Ti content may have a range of 0.20% or less (excluding 0%). The lower limit of the Ti content is more advantageous at 0.001%, more advantageous at 0.002%, and most advantageous at 0.003%. The upper limit of the Ti content is more advantageous at 0.19%, more advantageous at 0.18%, and most advantageous at 0.17%.

[0057] Nb: 0.10% or less (excluding 0%)

[0058] Nb is an element advantageous for improving the strength and toughness of weld metal by increasing hardenability and densifying the microstructure. Additionally, it has the effect of improving the flow of molten metal and stabilizing the arc during arc welding. To ensure the aforementioned effects, 0% is excluded as the lower limit of the Nb content. If the Nb content exceeds 0.10%, there may be a disadvantage in that low-melting point compounds are formed at the grain boundaries, making high-temperature cracking more likely to occur. Therefore, the Nb content may have a range of 0.10% or less (excluding 0%). The lower limit of the Nb content is more advantageous at 0.001%, more advantageous at 0.002%, and most advantageous at 0.003%. The upper limit of the Nb content is more advantageous at 0.09%, more advantageous at 0.08%, and most advantageous at 0.07%.

[0059] The remaining component 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 skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0060] The wire of the present invention may additionally include one or more of V: ​​0.20% or less, Zr: 0.10% or less, and B: 0.01% or less.

[0061] V: 0.20% or less

[0062] V is a precipitation strengthening element capable of improving the strength of the weld metal by forming carbonitrides, and is an element advantageous for improving the strength and toughness of the weld metal by increasing hardenability and densifying the microstructure. If the content of V exceeds 0.20%, excessive precipitates are formed, which may have the disadvantage that the toughness of the weld metal may decrease in some cases due to excessive strength. Therefore, the content of V may be in the range of 0.20% or less. It is more advantageous for the content of V to be 0.19% or less, more advantageous for it to be 0.18% or less, and most advantageous for it to be 0.17% or less. Meanwhile, the present invention does not specifically limit the lower limit of the V content, but as an example, the lower limit may be 0.0001%.

[0063] Zr: 0.10% or less

[0064] Zr is an element that promotes the deoxidation of molten metal during arc welding and is advantageous for suppressing the occurrence of porous defects such as blowholes. If the content of Zr exceeds 0.10%, there may be a disadvantage in that the electrodeposition paintability of the weldment is reduced. Therefore, the content of Zr may be in the range of 0.10% or less. It is more advantageous for the content of Zr to be 0.09% or less, more advantageous for it to be 0.08% or less, and most advantageous for it to be 0.07% or less. Meanwhile, the present invention does not specifically limit the lower limit of the Zr content, but as an example, the lower limit may be 0.0001%.

[0065] B: 0.010% or less

[0066] B is an element that is advantageous for increasing hardenability and improving the strength of the weld metal. If the content of B exceeds 0.010%, there may be a disadvantage in that the toughness of the weld metal is reduced due to an excess of hardenability. Therefore, the content of B may be in the range of 0.010% or less. It is more advantageous for the content of B to be 0.009% or less, more advantageous for it to be 0.008% or less, and most advantageous for it to be 0.007% or less. Meanwhile, the present invention does not specifically limit the lower limit of the B content, but as an example, the lower limit may be 0.0001%.

[0067] The wire of the present invention may additionally contain Cu: 0.50% or less.

[0068] Cu: 0.50% or less

[0069] Cu is an element that is advantageous for improving corrosion resistance. If the content of Cu exceeds 0.50%, there may be a disadvantage in that the weld metal becomes more susceptible to cracking. Therefore, the content of Cu may be in the range of 0.50% or less. It is more advantageous for the content of Cu to be 0.45% or less, more advantageous for 0.40% or less, and most advantageous for 0.35% or less. Meanwhile, the present invention does not specifically limit the lower limit of the Cu content, but as an example, the lower limit may be 0.0001%.

[0070] The microstructure of the wire rod of the present invention may include bainitic ferrite. The minimum effective grain size of the bainitic ferrite may be 0.50 μm or larger. If the minimum effective grain size of the bainitic ferrite is less than 0.50 μm, there may be a disadvantage in that the strength of the wire rod increases excessively, thereby relatively reducing the elongation. It is more advantageous for the minimum effective grain size of the bainitic ferrite to be 1.0 μm or larger, more advantageous for it to be 1.5 μm or larger, and most advantageous for it to be 2.0 μm or larger. In the present invention, the upper limit of the minimum effective grain size of the bainitic ferrite is not specifically limited, but as an example, it may be 3.0 μm.

[0071] The above bainitic ferrite may have a minimum effective grain size of 20.0 μm or less. If the maximum effective grain size of the above bainitic ferrite exceeds 20.0 μm, there may be a disadvantage in that the strength of the wire rod becomes relatively insufficient. It is more advantageous for the maximum effective grain size of the above bainitic ferrite to be 19.0 μm or less, more advantageous for it to be 18.0 μm or less, and most advantageous for it to be 17.0 μm or less. In the present invention, the lower limit of the maximum effective grain size of the above bainitic ferrite is not specifically limited, but as an example, it may be 15.0 μm.

[0072] Meanwhile, the above effective grain size may be defined as the average grain size calculated from the number of grains per unit area according to ASTM E2627. Although not specifically limited, for example, to obtain the above effective grain size, measurements may be taken at any point from a minimum of 10 to a maximum of 50 times. Among the effective grain sizes obtained at this time, the smallest value may be defined as the minimum effective grain size, and the largest value may be defined as the maximum effective grain size. In addition, although not specifically limited, for example, to obtain the average effective grain size, it may be defined as the average value of the effective grain sizes obtained by taking measurements at different points of the wire from a minimum of 10 to a maximum of 50 times.

[0073] The above bainitic ferrite may have an average effective grain size of 10 µm or less. If the average effective grain size of the above bainitic ferrite exceeds 10 µm, there may be a disadvantage in that it is difficult to simultaneously secure sufficient strength and elongation of the wire rod according to the Hall-Petch relationship. It is more advantageous for the average effective grain size of the above bainitic ferrite to be 9 µm or less, more advantageous to be 8 µm or less, and most advantageous to be 7 µm or less.

[0074] The microstructure of the wire rod of the present invention may include 0.50% or less (including 0%) of retained austenite and the remainder of bainitic ferrite in terms of area percentage. The bainitic ferrite is a structure advantageous for securing the strength and elongation of the wire rod. Meanwhile, the retained austenite is an impurity structure that is inevitably included during the manufacturing process. Since it is desirable not to include the retained austenite as much as possible, it is advantageous for its fraction to be 0%. However, in the present invention, considering that the retained austenite may inevitably be included, the upper limit may be limited to 0.5%. If it exceeds 0.50%, the dimensions change slightly during the process in which the retained austenite transforms into martensite during wire drawing, and in some cases, there may be a disadvantage in that the dimensional accuracy of the drawn wire rod is reduced during the transformation process.

[0075] As described above, the wire rod of the present invention can have a tensile strength of 840 to 1000 MPa. In addition, the wire rod of the present invention can have a reduction of area (RA; Reduction of Area) of 60 to 75%. Furthermore, the wire rod of the present invention can secure a primary drawing speed of 300 m / min or more and a secondary drawing speed of 250 m / min or more even if the pre-heat treatment process prior to the primary drawing stage is omitted when manufacturing welding rods. Moreover, it can be manufactured at a low cost and can be preferably used as a welding rod for steel materials such as gigasteel-grade automobiles.

[0076] Hereinafter, a method for manufacturing a wire rod for welding according to one embodiment of the present invention will be described.

[0077] First, a billet satisfying the aforementioned alloy composition is prepared. The present invention does not specifically limit the billet preparation process, and any method used in the relevant technical field may be utilized.

[0078] Subsequently, the billet is reheated at 1050 to 1250°C. The temperature range at this time refers to the cracking zone among the preheating zone, heating zone, and cracking zone within the reheating section. That is, the temperature is maintained at a relatively low level in the preheating zone, and the temperature is gradually increased from the heating zone to the cracking zone. If the reheating temperature is below 1050°C, the inside and outside of the billet are not heated uniformly, and the subsequent rolling temperature of the billet is lowered, which may cause deformation of the material's shape during the rolling process and become a hindering factor during the rolling process. If the reheating temperature of the billet exceeds 1250°C, bending of the material may occur, which may cause problems when extracting from the furnace. The lower limit of the reheating temperature of the billet is more advantageous at 1060°C, more advantageous at 1070°C, and most advantageous at 1080°C. The upper limit of the reheating temperature of the above billet is more advantageous at 1240℃, more advantageous at 1220℃, and most advantageous at 1200℃.

[0079] Subsequently, the reheated billet is extracted at 1050–1150°C and then rolled to obtain a wire rod. If the billet extraction temperature is below 1050°C, the rolling load of the billet becomes excessive, causing an overload on the rolling equipment and potentially becoming a cause for malfunction during the rolling process. If the billet extraction temperature exceeds 1150°C, the high-temperature thermal damage to the rolling rolls becomes sensitive, and the temperature during rolling becomes excessively high, which may result in a defective billet shape. The lower limit of the billet extraction temperature is more advantageous at 1060°C, more advantageous at 1070°C, and most advantageous at 1080°C. The upper limit of the billet extraction temperature is more advantageous at 1140°C, more advantageous at 1130°C, and most advantageous at 1120°C. Here, the extraction temperature cannot be higher than the reheating temperature.

[0080] Subsequently, the wire is wound at 860 to 940°C. If the winding temperature is below 860°C, the elongation of the wire may decrease and the winding shape may become defective. If the winding temperature exceeds 940°C, it may adversely affect the material and physical property variations of the wire and cause excessive oxidation scale to form on the surface of the wire. The lower limit of the winding temperature is more advantageous at 870°C, more advantageous at 880°C, and most advantageous at 890°C. The upper limit of the winding temperature is more advantageous at 930°C, more advantageous at 920°C, and most advantageous at 910°C.

[0081] Subsequently, the wound wire is cooled in a Stelmor cooling zone at a cooling rate of 2.0 to 3.0°C / s. If the cooling rate is less than 2.0°C / s, the material of the wire becomes soft, and in some cases, the coil may sag, which may result in winding defects. If the cooling rate exceeds 3.0°C / s, the material of the wire becomes hard, which reduces workability; therefore, pre-heat treatment is required during drawing, and there may be disadvantages such as increased manufacturing costs. The lower limit of the cooling rate is more advantageous at 2.1°C / s, more advantageous at 2.2°C / s, and most advantageous at 2.3°C / s. The upper limit of the cooling rate is more advantageous at 2.9°C / s, more advantageous at 2.8°C / s, and most advantageous at 2.7°C / s.

[0082] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0083] (Example)

[0084] Billets having the alloy compositions listed in Tables 1 and 2 below were prepared, and wire rods were manufactured by reheating the billets, rolling the wire rods, coiling, and cooling under the conditions listed in Table 3 below. The microstructure and mechanical properties of the wire rods manufactured in this manner were measured, and the results are shown in Table 4 below. In addition, welding rods were manufactured by first drawing and second drawing the wire rods manufactured above without a prior heat treatment process, and the first drawing speed and second drawing speed were measured, and the results are shown in Table 4 below.

[0085] The microstructure and effective grain size were observed using an optical microscope after taking a specimen from the above-mentioned wire, polishing the cross-sectional structure of the specimen, and etching it with Nital solution. Additionally, the Kikuchi pattern was analyzed using Electron Backscattered Diffraction (EBSD) to obtain Image Quality (IQ) and Inverse Pole Figure (IPF) maps that visualize information on grain boundaries and grain orientations. Subsequently, the grains were identified by referring to the EBSD IQ and IPF maps along with the microstructure images observed with the optical microscope mentioned above. The effective grain size was calculated from the number of grains per unit area, and the average effective grain size was measured by taking a minimum of 10 and a maximum of 50 measurements at different points in this manner.

[0086] Among the mechanical properties, tensile strength was measured using the test method specified in the ATSM A370 standard.

[0087] Among the mechanical properties, the cross-sectional shrinkage rate was measured using the test method specified in the ATSM A370 standard and expressed as a percentage of the difference between the wire's initial diameter and the diameter at fracture, divided by the initial diameter.

[0088] Steel Grade No. Alloy Composition (Weight%) CSI Mn PSCr Mo Al 10.07 0.06 1.70 0.009 0.006 1.44 0.52 0.003 20.07 0.07 1.69 0.017 0.006 1.44 0.51 0.013 0.08 0.09 1.71 0.009 0.005 1.40 0.53 0.022 40.08 0.11 1.82 0.009 0.007 1.380.560.01550.070.371.650.0110.0060.500.300.00660.060.521.480.0020.0030.030.570.00270.050.261.540.0080.0060.010.350.01080.090.451.600.0050.0050.150.300.001

[0089] Steel Grade No. Alloy Composition (Weight%) NiTiNbVZrBCu 10.0180.0030.0010.001--0.01620.0110.0020.04----30.0120.0350.05-0.002-0.01340.0180.0490.10--0.0010.01353 .000.0400.0030.0010.001-0.00763.420.0020.0010.003-0.0010.01572.000.0040.0020.0010.0030.0010.00982.540.0300.0010.0020.0010.0010.010

[0090] Classification Steel Grade No. Billet Reheating Temperature (°C) Extraction Temperature (°C) Coiling Temperature (°C / s) Cooling Rate (°C) Invention Example 1 10 50 10 50 86 0 2.7 Invention Example 2 2 11 00 11 00 89 5 2.5 Invention Example 3 3 12 00 11 40 90 4 2.0 Invention Example 4 4 ​​12 50 11 50 94 0 3.0 Comparative Example 1 5 11 50 11 50 93 0 3.0 Comparative Example 2 6 10 80 10 90 89 0 2.6 Comparative Example 3 7 12 50 10 50 92 0 2.9 Comparative Comparative Example 48 1230 1140 88 32.3 Comparative Example 5 1100099 58 45 1.9 Comparative Example 6 11270 12509 50 3.2 Comparative Example 7 198 598 08 55 1.8 Comparative Example 8 110 10 100094 5 1.8 Comparative Example 9 110 25 10 159 60 1.6 Comparative Example 10 11280 12609 75 3.1 Comparative Example 1 11104 510309 55 1.9 Comparative Example 12 199 59908 50 3.0

[0091] Classification Microstructure (Area %) BF Minimum Effective Grain Size (㎛) BF Maximum Effective Grain Size (㎛) BF Average Effective Grain Size (㎛) Tensile Strength (MPa) Area Shrinkage (%) Primary Drawing Speed ​​(m / min) Secondary Drawing Speed ​​(m / min) BFRA Invention Example 1 99.99 0.01 2.61 7.57.88 427 430 0250 Invention Example 2 99.98 0.02 2.61 9.87.49 326 8350275 Invention Example 399.990.012.618.15.895967325260 Invention Example 4100.000.618.06.399861375280 Comparative Example 199.450.552.621.38.1101058175215 Comparative Example 299.370.630.414.87.9102454150200 Comparative Example 399.480.523.224. 310.4116851160210Comparative Example 499.640.363.023.710.284058155205Comparative Example 599.990.010.313.26.798557165195Comparative Example 6100.003.520.511.583759290240Comparative Example 7100.000.411.86.1101552245230Comparative Example 899.990. 012.820.27.599756295250Comparative Example 999.990.013.321.512.498955290245Comparative Example 10100.003.822.413.297358280235Comparative Example 1199.990.013.522.112.899257285240Comparative Example 12100.0-0.413.07.298755270225BF: Bainetic Ferrite, RA: Retained Austenite

[0092] Figure 1 shows the Image Quality (IQ) and Inverse Pole Figure (IPF) images of Inventive Example 2 observed by EBSD. Figure 2 shows the Image Quality (IQ) and Inverse Pole Figure (IPF) images of Comparative Example 3 observed by EBSD. Figure 3 shows the Image Quality (IQ) and Inverse Pole Figure (IPF) images of Comparative Example 7 observed by EBSD. As can be seen from Tables 1 to 4 and Figures 1 to 3, Inventive Examples 1 to 4, which satisfy the alloy composition and manufacturing conditions of the present invention, satisfy the microstructure proposed by the present invention, the minimum and maximum effective grain sizes of bainitic ferrite, and the average effective grain size of bainitic ferrite, thereby securing the tensile strength and reduction in area targeted by the present invention. Furthermore, it can be seen that high levels of primary and secondary drawing speeds are achieved even if the pre-heat treatment process prior to the primary drawing stage is omitted during the manufacture of welding rods.

[0093] In the case of Comparative Examples 1 to 4, which do not satisfy the alloy composition of the present invention, it can be seen that the microstructure proposed by the present invention is not secured, or the minimum or maximum effective grain size of bainitic ferrite or the average effective grain size of bainitic ferrite is not satisfied, and thus the tensile strength or reduction in area targeted by the present invention is not secured. In addition, it can be seen that the primary drawing speed or secondary drawing speed during the manufacture of welding rods is at a low level.

[0094] In the case of Comparative Examples 5 to 12, which do not satisfy the manufacturing conditions of the present invention, it can be seen that the minimum or maximum effective grain size of bainitic ferrite or the average effective grain size of bainitic ferrite proposed by the present invention is not satisfied, and thus the tensile strength or reduction in area targeted by the present invention is not secured. In addition, it can be seen that the primary drawing speed or secondary drawing speed during the manufacture of welding rods is at a low level.

Claims

1. In wt%, C: 0.050~0.160%, Si: 0.0010~0.250%, Mn: 1.0~2.50%, P: 0.030% or less (excluding 0%), S: 0.030% or less (excluding 0%), Cr: 0.40~6.0%, Mo: 0.10~0.650%, Al: 0.20% or less (excluding 0%), Ni: 0.40% or less (excluding 0%), Ti: 0.20% or less (excluding 0%), Nb: 0.10% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities, The microstructure contains bainitic ferrite, The minimum effective grain size of the above bainitic ferrite is 0.50㎛ or larger, and A welding rod wire having a maximum effective grain size of 20.0 μm or less of the above bainitic ferrite.

2. In Paragraph 1, The above wire is a welding rod wire further comprising one or more of V: ​​0.20% or less, Zr: 0.10% or less, and B: 0.010% or less.

3. In Paragraph 1, The above wire is a welding rod wire additionally containing 0.50% or less of Cu.

4. In Paragraph 1, The above microstructure is a wire rod for welding rods comprising 0.50% or less (including 0%) of retained austenite and the remainder of bainitic ferrite in terms of area %.

5. In Paragraph 1, The above bainitic ferrite is a welding rod wire with an average effective grain size of 10㎛ or less.

6. In Paragraph 1, The above wire is a welding rod wire having a tensile strength of 840 to 1000 MPa.

7. In Paragraph 1, The above wire is a welding rod wire having a reduction of area (RA; Reduction of Area) of 60~75%.

8. A step of preparing a billet comprising, in wt%, C: 0.050~0.160%, Si: 0.0010~0.250%, Mn: 1.0~2.50%, P: 0.030% or less (excluding 0%), S: 0.030% or less (excluding 0%), Cr: 0.40~6.0%, Mo: 0.10~0.650%, Al: 0.20% or less (excluding 0%), Ni: 0.40% or less (excluding 0%), Ti: 0.20% or less (excluding 0%), Nb: 0.10% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; A step of reheating the above billet at 1050~1250℃; A step of obtaining a wire rod by extracting the above-mentioned reheated billet at 1050~1150℃ and then rolling it into a wire rod; A step of winding the above wire at 860~940℃; and A method for manufacturing a welding rod wire, comprising the step of cooling the wire at a cooling rate of 2.0 to 3.0℃ / s in a Stelmor cooling zone.

9. In Paragraph 8, A method for manufacturing a wire rod for welding rods, wherein the above billet further comprises one or more of V: ​​0.20% or less, Zr: 0.10% or less, and B: 0.010% or less.

10. In Paragraph 8, The above billet is a method for manufacturing a wire rod for welding rods that additionally contains Cu: 0.50% or less.

Images