Untempered steel wire rod with excellent cold forgeability and its manufacturing method

Abstract

The present invention provides a non-heat treated steel wire rod having excellent cold forgeability and a manufacturing method thereof. According to one embodiment of the present invention, the non-heat treated steel wire rod having excellent cold forgeability contains, by weight, carbon (C): about 0.20% to 0.40%, silicon (Si): about 0.10% to 0.30%, manganese (Mn): about 1.30% to 1.60%, phosphorus (P): about more than 0% to 0.05%, sulfur (S): about more than 0% to 0.05%, chromium (Cr): about 0.02% to 0.30%, nickel (Ni): about 0.02% to 0.30%, molybdenum (Mo): about 0.02% to 0.30%, vanadium (V): about 0.01% to 0.15%, Niobium (Nb): approximately 0.01% to 0.05%, aluminum (Al): approximately 0.005% to 0.060%, titanium (Ti): approximately 0.005% to 0.020%, copper (Cu): approximately 0.01% to 0.30%, boron (B): approximately 0.0001% to 0.0020%, nitrogen (N): approximately 0.005% to 0.015%, and the balance includes iron (Fe) and other unavoidable impurities, and the total of the niobium (Nb) and the vanadium (V) is approximately 0.02% to 0.2%, and a tensile strength of approximately 900 MPa or more can be achieved.

Description

[Technical Field]

[0001] The technical idea of ​​the present invention relates to a non-heat treated steel wire rod having excellent cold forgeability and a manufacturing method thereof, and more particularly to a non-heat treated steel wire rod having excellent cold forgeability and excellent strength and toughness even when a heat treatment step is omitted, and a manufacturing method thereof. [Background technology]

[0002] In the case of conventional wire rods used for automotive parts and machine structures, most of them are tempered steels that are quenched and tempered in the austenite region after cold forging to improve the strength and toughness of the material.

[0003] As shown in FIG. 5, conventional tempered steel wire rods are manufactured through a raw material production step (S10) and a part processing step (S20) in which the produced wire rod is processed into parts. Specifically, the raw material production step (S10) includes a steelmaking and continuous casting step (S11), a bloom reheating step (S12), a billet rolling step (S13), a wire rod rolling step (S14), and a wire rod manufacturing step (S15). The wire rod manufactured through these steps is then processed according to product specifications, ultimately producing a non-tempered steel wire rod product. The post-processing refers to the part processing step (S20), which includes a cold wire drawing step (S21), a spheroidizing heat treatment step (S22), a cold forging step (S23), a quenching / tempering heat treatment step (S24), a processing step (S25), and a product manufacturing step (S26).

[0004] Meanwhile, the non-heat treated steel wire rod is a steel that can omit the heat treatment process in the above-mentioned post-processing, and can omit the spheroidizing heat treatment step (S22) performed after the cold wire drawing step (S21) and the quenching / tempering heat treatment step (S24) performed after the cold forging step (S23).

[0005] Unlike tempered steel, this steel can obtain tensile strength and impact toughness similar to those of tempered steel even without the heat treatment process, which simplifies the process and reduces costs, making it economically advantageous. Furthermore, since it is free from heat treatment defects and warpage, it can be used in products that require straightness.

[0006] However, since the heat treatment process described above is omitted in non-heat treated steel wire rods, continuous work hardening occurs during the cold wire drawing step (S21) and the cold forging step (S23). This increases the strength of the product, but causes a continuous decrease in ductility and toughness, and shortens the life of the die during cold forging, which may reduce the benefits of omitting the heat treatment process.

[0007] Furthermore, in the case of non-heat treated steel wire rods, there is no heat treatment process during the part manufacturing process, so the mechanical properties of the raw material affect the properties of the final part, and therefore variations in mechanical properties can pose a major risk.

[0008] Prior art documents include Japanese Patent No. 7044197 and Korean Patent No. 10-1262462. Summary of the Invention [Problem to be solved by the invention]

[0009] The technical problem to be achieved by the technical concept of the present invention is to provide a non-heat treated steel wire rod having excellent strength and toughness, no deviation in tensile strength, and excellent cold forgeability even when a heat treatment process is omitted, and a manufacturing method thereof, but this problem is merely an example, and the technical concept of the present invention is not limited thereto. [Means for solving the problem]

[0010] According to one aspect of the present invention, there is provided a non-heat treated steel wire rod having excellent cold forgeability, the non-heat treated steel wire rod having, by weight %, carbon (C): about 0.20% to 0.40%, silicon (Si): about 0.10% to 0.30%, manganese (Mn): about 1.30% to 1.60%, phosphorus (P): about more than 0% to 0.05%, sulfur (S): about more than 0% to 0.05%, chromium (Cr): about 0.02% to 0.30%, nickel (Ni): about 0.02% to 0.30%, molybdenum (Mo): about 0.02% to 0.30%, vanadium (V): about 0.01% to 0.15%, and niobium (Nb): about 0.0 1% to 0.05%, aluminum (Al): about 0.005% to 0.060%, titanium (Ti): about 0.005% to 0.020%, copper (Cu): about 0.01% to 0.30%, boron (B): about 0.0001% to 0.0020%, nitrogen (N): about 0.005% to 0.015%, and the balance includes iron (Fe) and other unavoidable impurities, and the total of the niobium (Nb) and vanadium (V) is about 0.02% to 0.2%, and a tensile strength of about 900 MPa or more can be achieved.

[0011] According to one embodiment of the present invention, the non-heat treated steel wire rod contains composite precipitates of about 50 nm or less therein, and the composite precipitates may be composite precipitates containing one or more selected from Nb, V, Ti and Al, or may be composite precipitates containing one or more selected from composite precipitates containing TiN, VC, VN, NbC, NbN, AlN and BN.

[0012] According to one embodiment of the present invention, the complex precipitates may have an austenite grain size number of 10 or more.

[0013] According to one aspect of the present invention, there is provided a method for producing a non-heat treated steel wire rod having excellent cold forgeability. The manufacturing method of the non-heat treated steel wire rod comprises, in weight percent, carbon (C): about 0.20% to 0.40%, silicon (Si): about 0.10% to 0.30%, manganese (Mn): about 1.30% to 1.60%, phosphorus (P): about more than 0% to 0.05%, sulfur (S): about more than 0% to 0.05%, chromium (Cr): about 0.02% to 0.30%, nickel (Ni): about 0.02% to 0.30%, molybdenum (Mo): about 0.02% to 0.30%, vanadium (V): about 0.01% to 0.15%, niobium (Nb): about 0.01% to 0.05%, aluminum (Al): about 0.005% to 0.060%, titanium (Ti): about 0.005% to 0.020%, copper (Cu), the niobium (Nb) and vanadium (V) content being about 0.02% to 0.2% in total, at about 1100°C to 1350°C; rolling the reheated billet to form a wire rod; cold drawing the wire rod; and cold forging after the cold drawing; wherein in the step of forming the wire rod, the rolling can be performed while heating in a temperature range equal to or higher than the A3 transformation point.

[0014] According to one embodiment of the present invention, the temperature range equal to or higher than the A3 transformation point may be about 750°C to 900°C.

[0015] According to one embodiment of the present invention, in the step of forming the wire rod, after the rolling, the wire rod may be cooled at a cooling rate of 2° C. / s or less.

[0016] According to an embodiment of the present invention, in the step of cold drawing the wire rod, the wire drawing area reduction rate may be about 30% to 50%.

[0017] According to one embodiment of the present invention, in the step of forming the wire rod, the conveying speed of the conveyor that conveys the wire rod after the rolling is completed can be controlled to about 0.2 m / s to 0.7 m / s, thereby controlling the overlap density of the wire rod conveyed by the conveyor.

[0018] According to one embodiment of the present invention, after the cold forging step, the manufactured wire rod may have a tensile strength of about 900 MPa or more. [Effects of the Invention]

[0019] According to the technical concept of the present invention, it is possible to produce a non-heat treated steel wire rod having excellent strength and toughness, no deviation in tensile strength, and excellent cold forgeability, even if a heat treatment process is omitted. The effects of the present invention described above are merely examples, and the scope of the present invention is not limited by these effects. [Brief explanation of the drawings]

[0020] [Figure 1] 1A to 1C are diagrams schematically illustrating a method for manufacturing a non-heat treated steel wire rod according to an embodiment of the present invention in accordance with a process sequence.

[0021] [Figure 2] 2 is a diagram specifically illustrating the wire rolling step shown in FIG. 1. FIG.

[0022] [Figure 3] 3 is a diagram schematically illustrating a wire being cooled by the cooling conveyor shown in FIG. 2.

[0023] FIG. [Figure 4] 3 is a diagram schematically illustrating a wire being cooled by the cooling conveyor shown in FIG. 2.

[0023] FIG.

[0023] [Figure 5] 3A to 3C are diagrams schematically illustrating a method for manufacturing a tempered steel wire rod according to a comparative example of the present invention in accordance with a process sequence.

[0024] [Figure 6]1A and 1B are scanning electron microscope (SEM) photographs (low magnification (BF mode) photograph (a), high magnification (BF mode) photograph (b), and high magnification (Stem mode) photograph (c)) of MX precipitates in a sample of Example 1 of the present invention.

[0025] [Figure 7] 1A and 1B are scanning electron microscope (SEM) photographs (low magnification (BF mode) photograph (a), high magnification (BF mode) photograph (b), and high magnification (Stem mode) photograph (c)) of MX precipitates in the sample of Comparative Example 2. BEST MODE FOR CARRYING OUT THE INVENTION

[0026] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The present embodiments are provided to more completely explain the technical concept of the present invention to those skilled in the art. The following embodiments may be modified into various other forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more complete and complete and to fully convey the technical concept of the present invention to those skilled in the art. Throughout this specification, the same reference numerals refer to the same elements. Furthermore, various elements and regions in the drawings are shown schematically. Therefore, the technical concept of the present invention is not limited by the relative sizes and spacings shown in the accompanying drawings.

[0027] The technical idea of ​​the present invention relates to a method for producing a non-heat treated steel wire rod having excellent cold forgeability by controlling the conditions of a precision rolling process and the cooling rate during the production of the wire rod. Prior to explaining the production method, the non-heat treated steel wire rod having excellent cold forgeability will first be explained.

[0028] <Non-thermal steel wire rod>

[0029] The non-heat treated steel wire rod having excellent cold forgeability according to one embodiment of the present invention has, in weight percent, carbon (C): about 0.20% to 0.40%, silicon (Si): about 0.10% to 0.30%, manganese (Mn): about 1.30% to 1.60%, phosphorus (P): about more than 0% to 0.05%, sulfur (S): about more than 0% to 0.05%, chromium (Cr): about 0.02% to 0.30%, nickel (Ni): about 0.02% to 0.30%, and molybdenum (Mo): about 0.02%. ~0.30%, vanadium (V): about 0.01% to 0.15%, niobium (Nb): about 0.01% to 0.05%, aluminum (Al): about 0.005% to 0.060%, titanium (Ti): about 0.005% to 0.020%, copper (Cu): about 0.01% to 0.30%, boron (B): about 0.0001% to 0.0020%, nitrogen (N): about 0.005% to 0.015%, and the remainder includes iron (Fe) and other unavoidable impurities.

[0030] The role and content of each component contained in the non-heat treated steel wire rod according to the present invention will be described below, where the contents of the component elements are all expressed in wt%.

[0031] Carbon (C): approx. 0.20%~0.40%

[0032] Carbon (C) is an element that increases the strength of steel by forming niobium (Nb) and vanadium (V)-based precipitates and dissolving in the matrix. To ensure sufficient strength, carbon (C) is required in an amount of at least about 0.20 wt. %, but if the carbon (C) content exceeds about 0.40 wt. %, the toughness and ductility decrease significantly despite the increased strength. Therefore, to ensure the excellent strength and toughness of untempered steel, carbon (C) is added in an amount of about 0.20 to 0.40 wt. %.

[0033] Silicon (Si): approx. 0.10% to 0.30%

[0034] Silicon (Si) is a useful element that contributes to high strength and improves softening resistance and fatigue deformation resistance. However, if added in excess as an element that improves deformation resistance, it can significantly reduce cold forgeability in non-heat treated steels in which spheroidizing heat treatment is omitted. Taking this into consideration, silicon is added in an amount of approximately 0.1 to 0.30 wt.% based on the total weight of the non-heat treated steel according to one embodiment of the present invention.

[0035] Manganese (Mn): Approximately 1.30% to 1.60%

[0036] Manganese (Mn) is useful for deoxidizing steel and dissolves in the matrix to improve strength. It is an effective element for improving strength and ensuring the strength of untempered steel. It also improves toughness by lowering the transformation temperature and contributing to the refinement of intermediate pearlite. When the manganese (Mn) content of the untempered steel according to an embodiment of the present invention is less than about 1.30 wt.% based on the total weight of the untempered steel, the strength is insufficient. When the manganese (Mn) content exceeds about 1.60 wt.%, the toughness may decrease despite the improved strength.

[0037] Phosphorus (P): Approximately more than 0% ~ 0.05%

[0038] Phosphorus (P) is an element that easily segregates and impairs the toughness of steel, but because it has an excellent solid solution strengthening effect, even a small amount of addition can improve the strength of steel. Phosphorus (P) can be added in an amount of more than about 0 wt% and not more than 0.05 wt% based on the total weight of the non-heat treated steel according to one embodiment of the present invention, but if it exceeds about 0.05 wt%, the toughness of the steel may be reduced.

[0039] Sulfur (S): Approximately more than 0% ~ 0.05%

[0040] Sulfur (S) is an element that impairs workability and physical properties. It segregates at grain boundaries, impairing the ductility of steel, and forms sulfides, which are the main cause of degrading delayed fracture resistance and stress relaxation characteristics. Therefore, sulfur (S) may be added in an amount of more than about 0 wt.% and not more than 0.05 wt.% based on the total weight of the non-heat treated steel according to one embodiment of the present invention. If the sulfur (S) content exceeds about 0.05 wt.%, martensite grain boundaries may be destroyed, reducing hot workability, and the formation of large inclusions may cause surface defects such as cracks.

[0041] Chromium (Cr): Approximately 0.02% to 0.30%

[0042] Chromium (Cr) not only increases the strength of steel but also acts as a ferrite stabilizer, improving hardenability and strength. Chromium (Cr) is preferably added in an amount of about 0.02 to 0.30 wt% based on the total weight of the non-heat treated steel according to an embodiment of the present invention. If the chromium (Cr) content is less than about 0.02 wt%, the strength improvement effect is insufficient. If the chromium (Cr) content exceeds about 0.30 wt%, the hardenability is improved, which may lead to the formation of low-temperature structures during cooling, thereby reducing cold drawing and forgeability.

[0043] Nickel (Ni): approx. 0.02% to 0.30%

[0044] Nickel (Ni) contributes to increasing hardenability and improving toughness. Nickel (Ni) can be added in an amount of about 0.02 wt% to 0.30 wt% based on the total weight of the non-heat treated steel according to one embodiment of the present invention. If the amount of nickel (Ni) added is less than about 0.02 wt%, the effect of the addition is minimal, and if it exceeds about 0.30 wt%, the cost may increase.

[0045] Molybdenum (Mo): approx. 0.02% to 0.30%

[0046] Molybdenum (Mo) contributes to improving strength and toughness. Molybdenum (Mo) can be added in an amount of about 0.02 wt % to 0.30 wt % based on the total weight of the non-heat treated steel according to an embodiment of the present invention. If the amount of Mo is less than about 0.02 wt %, the effect of the addition is insufficient. If the amount of Mo exceeds about 0.30 wt %, the hardness increases, reducing workability and significantly increasing the manufacturing cost of the non-heat treated steel.

[0047] Vanadium (V): approx. 0.01% to 0.15%

[0048] Vanadium (V) reacts with carbon (C) and nitrogen (N) to form NbV (carbide, nitride) composite precipitates, contributing to improved strength through precipitation strengthening. It also acts as a nucleation site for ferrite during wire rolling, increasing the ferrite fraction and improving strength and toughness. Taking this into consideration, vanadium (V) may be added in an amount of about 0.01 wt% to 0.15 wt% based on the total weight of the non-heat-treated steel according to an embodiment of the present invention. If the amount is less than about 0.01 wt%, the effect of the addition is insufficient. If the amount is more than about 0.15 wt%, the effect of increasing precipitation strengthening is slight and ineffective.

[0049] Niobium (Nb): Approximately 0.01% to 0.05%

[0050] Niobium (Nb), like vanadium (V), is an element that reacts with carbon (C) and nitrogen (N) to form Nb (carbides and nitrides). Nb-based precipitates are an element that strengthens steel through precipitation and prevents grain boundary coarsening. Like vanadium (V), they act as nucleation sites for ferrite during wire rod rolling, increasing the ferrite fraction and improving strength and toughness. However, an increase in niobium (Nb) increases the solid solution temperature, and niobium (Nb) that does not dissolve during the rolling heat treatment of the raw material forms coarse precipitates. The formation of coarse precipitates cannot effectively prevent the movement of dislocations and has little effect in contributing to improving fatigue life. Therefore, niobium (Nb) can be added in an amount of about 0.01 wt % to 0.05 wt % based on the total weight of the non-heat treated steel according to one embodiment of the present invention in order to increase the formation of fine precipitates by dissolving as much niobium (Nb) element as possible during the rolling heat treatment of the raw material.

[0051] Meanwhile, the total amount of niobium (Nb) and vanadium (V) may be about 0.02% to 0.2%. The two elements form carbides or nitrides, and the respective content ranges are important. However, since the two elements are mutually substitutable, excessive addition can cause the formation of coarse precipitates. Therefore, in the present invention, the total amount of the two elements must be appropriately controlled.

[0052] Aluminum (Al): approx. 0.005% to 0.060%

[0053] Aluminum (Al) primarily functions as a deoxidizer, combining with oxygen in the steel to form oxides. The remaining aluminum combines with nitrogen to form AlN. AlN prevents grain boundary coarsening and improves product toughness. Aluminum (Al) can be added in an amount of about 0.005 to 0.060 wt.% based on the total weight of the untempered steel according to an embodiment of the present invention. However, if the aluminum (Al) content is less than about 0.005 wt.%, there is insufficient Al to form AlN, and if it exceeds about 0.060 wt.%, there is a risk of a decrease in mechanical properties.

[0054] Titanium (Ti): approx. 0.005% to 0.020%

[0055] Titanium (Ti), like vanadium (V) and niobium (Nb), forms carbonitrides and causes precipitation strengthening, thereby improving strength and toughness. It can be added in an amount of about 0.005 to 0.020 wt% based on the total weight of the untempered steel according to one embodiment of the present invention. If the amount is less than about 0.005 wt%, the effect of the addition is insufficient. If the amount is more than about 0.02 wt%, the manufacturing cost increases sharply, and the amount of coarse alloy carbides increases, acting like nonmetallic inclusions, thereby reducing fatigue properties and the effect of precipitation strengthening.

[0056] Copper (Cu): approx. 0.01%~0.30%

[0057] Copper (Cu) is an element effective in increasing the strength and improving the toughness of steel. Copper (Cu) can be added in an amount of about 0.01 to 0.30 wt% based on the total weight of the non-heat treated steel according to an embodiment of the present invention. If the amount of copper is less than about 0.01 wt%, the effect of the addition is insufficient. If the amount of copper is more than about 0.30 wt%, surface segregation may occur, which may reduce the cold drawability during the wire drawing process.

[0058] Boron (B): Approximately 0.0001% to 0.0020%

[0059] Boron (B) is an element that segregates at grain boundaries to improve the ductility and toughness of steel. Boron (B) can be added in an amount of about 0.0001 wt% to 0.0020 wt% based on the total weight of the non-heat treated steel according to an embodiment of the present invention. If the amount of B is less than about 0.0001 wt%, the effect of the addition is insufficient. If the amount of B is more than about 0.0020 wt%, the hardenability of the steel increases and low-temperature structures may occur during rapid cooling.

[0060] Nitrogen (N): Approximately 0.005% to 0.015%

[0061] Nitrogen (N) combines with aluminum (Al), vanadium (V), and niobium (Nb) to form nitrides such as AlN, VN, and NbN. These nitrides not only refine grains through their grain pinning effect, but also act as ferrite nucleation sites, increasing the ferrite fraction. Nitrogen (N) may be added at about 0.0050 to 0.0150 wt.% based on the total weight of the non-heat-treated steel according to an embodiment of the present invention. If the amount of N is less than about 0.005 wt.%, the number of nitrides that can be formed may be insufficient. If the amount of N exceeds about 0.0150 wt.%, it may dissolve in the matrix, increasing strength but reducing cold drawability.

[0062] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintended impurities may be inevitably mixed in from raw materials or the surrounding environment, and it is not possible to eliminate these. These impurities are known to anyone skilled in normal manufacturing processes, so all of their contents will not be specifically mentioned in this specification.

[0063] The non-heat treated steel wire rod having the above-mentioned composition and excellent cold forgeability can satisfy a tensile strength of about 900 MPa or more.

[0064] Furthermore, the above-mentioned unheat-treated steel wire rod with excellent cold forgeability contains composite precipitates of 50 nm or less therein, and the composite precipitates may be composite precipitates containing one or more elements selected from Nb, V, Ti, and Al, or may be composite precipitates containing one or more elements selected from composite precipitates containing TiN, VC, VN, NbC, NbN, AlN, and BN. The inclusion of the composite precipitates can refine the grains and increase the ferrite fraction, thereby improving strength and toughness. For optimal effect, the composite precipitates preferably have a diameter of approximately 50 nm or less.

[0065] The degree of grain refinement of the complex precipitates can be determined by evaluating the austenite grain size (A, G, S), and when the complex precipitates are finely dispersed, the austenite grain size can be 10 or greater. The grain refinement is an important means for improving the strength and toughness of steel materials, and its principle is that the pinning effect of MX-based precipitates inhibits grain growth, so fine dispersion of the complex precipitates is important.

[0066] The presence or absence of fine dispersion of composite precipitates in the non-heat treated steel wire rod of the present invention is about 100 μm. 2 The number of complex precipitates per unit area can be determined by the number of complex precipitates per unit area. 2 Contains over 1000 pieces per lot.

[0067] The non-heat treated steel wire rod according to the embodiment of the present invention described above can be produced by the following method of the embodiment.

[0068] FIG. 1 is a diagram illustrating a method for manufacturing a non-heat treated steel wire rod according to an embodiment of the present invention in a process sequence, FIG. 2 is a diagram specifically illustrating the wire rod rolling step illustrated in FIG. 1, and FIGS. 3 and 4 are diagrams illustrating a wire rod being cooled by a cooling conveyor illustrated in FIG. 2.

[0069] Referring to FIG. 1, a method for manufacturing a non-heat-treated steel wire rod according to an embodiment of the present invention can be divided into two steps. The step includes a raw material manufacturing step (S10) for manufacturing wire rod to be manufactured at a steel mill, and a parts processing step (S20) for processing the wire rod manufactured through the raw material manufacturing step to manufacture parts. Generally, as shown in FIG. 5, the raw material manufacturing step (S10) for heat-treated steel wire rod is followed by a parts processing step (S20). The parts processing step (S20) includes a cold wire drawing step (S21), a spheroidizing heat treatment step (S22), a cold forging step (S23), a quenching / tempering heat treatment step (S24), a processing step (S25), and a product manufacturing step (S26).

[0070] Meanwhile, the non-heat treated steel wire rod is a steel that can omit the heat treatment step among the above-mentioned parts processing step (S20), and can omit the spheroidizing heat treatment step (S22) performed after the cold wire drawing step (S21) and the quenching / tempering heat treatment step (S24) performed after the cold forging step (S23).

[0071] In the case of non-heat treated steel wire rods, the spheroidizing heat treatment step (S22) and the quenching / tempering heat treatment step (S24) are omitted, so work hardening continues to occur during the processing of the wire rod product. This increases the strength of the product but reduces its ductility and toughness, so separate control must be performed in the manufacturing process to ensure that the wire rod has excellent strength and toughness even when the above-mentioned heat treatment steps are omitted.

[0072] The inventors of the present invention have optimized the conditions of the manufacturing process in the step (S14) of rolling the wire rod in manufacturing the non-heat treated steel wire rod, focusing on reducing the variation in mechanical properties.

[0073] A method for manufacturing a non-heat treated steel wire rod according to one embodiment of the present invention includes the steps of reheating a steel material having the above-described composition at approximately 1100°C to 1350°C to form a billet, rolling the reheated billet to form a wire rod, cold drawing the wire rod, and cold forging the wire rod after the cold drawing.

[0074] Each of the steps will be described in detail below.

[0075] To maximize the grain refinement effect of the complex precipitates, it is important to finely disperse the fine complex precipitates. To achieve this, the complex precipitates in the matrix are redissolved and then reprecipitated, which can be controlled through reheating heat treatment before bloom rolling the raw material. During the reheating heat treatment, the heat treatment is performed at a temperature below the liquid phase (approximately 1100°C to 1350°C), thereby controlling the conditions for as much solid solution as possible of the complex precipitate-forming elements.

[0076] The step of forming the wire rod is a step of performing rolling while heating in a temperature range equal to or higher than the A3 transformation point, and the temperature range equal to or higher than the A3 transformation point may be about 750° C. to 900° C. After the rolling, the wire rod may be cooled at a cooling rate of about 2° C. / s or less.

[0077] For non-heat treated steel wire rods, the spheroidizing heat treatment step (S22) and the quenching / tempering heat treatment step (S24) are omitted, so the structure of the raw material becomes the structure of the final product. Therefore, if the cooling rate after rolling the wire rod is fast, the transformation of low-temperature structures (bainite and martensite) is accelerated, which can cause problems in the part manufacturing process.

[0078] For the above reasons, by controlling the cooling rate of the wire rod after hot rolling to about 2°C / s or less, the microstructure in the wire rod can be controlled to include ferrite and pearlite, and the area fraction of the ferrite can be about 30% to 60%, and the area fraction of the pearlite can be the remaining fraction. Here, the fractions refer to the area ratios obtained by using an image analyzer to take a photograph of the microstructure of the steel material.

[0079] First, when the microstructure of the wire rod transforms into a low-temperature structure, the increased hardness reduces cold forgeability, which shortens the life of the die and negates the benefits of omitting the heat treatment process. Second, if a low-temperature structure occurs in the center of the wire rod where element segregation is concentrated, chevron cracks may occur due to differences in the interface between phases at the surface and deep inside the product during the cold drawing process.

[0080] Meanwhile, in the step (S21) of cold drawing the wire rod, the wire drawing reduction rate may be about 30% to 50%. Since non-tempered steels omit spheroidizing heat treatment during the wire drawing process of the part manufacturing process, work hardening continues, which shortens the life of the die during cold forging and may negate the benefits of omitting heat treatment. In the present invention, the Bauschinger effect is used during the cold wire drawing process to minimize work hardening and improve cold forgeability while increasing the strength of the product. The Bauschinger effect is a phenomenon in which metals are prone to permanent deformation in the direction of the load. This effect occurs when a metal that has been subjected to a load above its yield point is subjected to a load in the opposite direction, lowering its yield point and causing deformation below its original yield strength.

[0081] 2 to 4, the non-heat treated steel wire rod W of the present invention can be primarily processed into a product through a wire rod rolling step (S14). The wire rod rolling step (S14) is performed in such a manner that a raw material is produced into a wire rod W by passing through the heating furnace 10, roughing mill 20, first water box 32, precision rolling mill 40, second water box 34, laying head 50, and cooling conveyor 60 in order as shown in FIG. 2, and then the produced wire rod W is finally placed on a placing table 70.

[0082] Furthermore, in the present invention, the spheroidizing heat treatment step (S22) and the quenching / tempering heat treatment step (S24) are omitted, and therefore, in order to improve the ductility and toughness and to uniformly control the tensile strength throughout the coil, the transport speed of the cooling conveyor 60 that cools the wire rod W after rolling the wire rod W is controlled.

[0083] More specifically, after the rolling process using the precision rolling mill 40 is completed, the wire rod W falls in a ring shape and is transported by the cooling conveyor 60. At this time, the transported ring-shaped wire rod W has a higher stacking density at the overlapping portion P than at the center C because several rings overlap each other depending on the overlapping density of the wire rods W on the cooling conveyor 60. This causes a difference in cooling rate between the center and the edge of the wire rod W during cooling. That is, the cooling rate at the overlapping portion P of the wire rod W is slower than that at the center C of the wire rod W, making it impossible to obtain a uniform tensile strength throughout the wire rod W.

[0084] In the present invention, in order to reduce such deviations in tensile strength, the transport speed of the cooling conveyor 60 is controlled to approximately 0.2 m / s to 0.7 m / s, thereby controlling the overlap density of the wire rods W transported by the cooling conveyor 60.

[0085] In particular, referring to FIG. 4(a), when the transport speed of the cooling conveyor 60 is less than approximately 0.2 m / s, the density of the overlapping portion P of the wire W increases, causing deviations in tensile strength, which in turn leads to a decrease in productivity.

[0086] On the other hand, referring to FIG. 4(c), when the transport speed of the cooling conveyor 60 exceeds about 0.7 m / s, the overlap density of the wire W becomes low and the cooling speed can be controlled uniformly, but there is a risk of malfunction of the cooling conveyor 60.

[0087] Therefore, by controlling the transport speed of the cooling conveyor 60 to about 0.2 m / s to 0.7 m / s as shown in FIG. 4(b), the overlap density of the wire W can be controlled to a constant value, and the physical properties due to the difference in cooling speed can be controlled uniformly.

[0088] The non-heat treated steel wire rod of the present invention can achieve a tensile strength of about 900 MPa or more after cold forging without a separate heat treatment.

[0089] Below, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are merely provided to aid in understanding the present invention, and the present invention is not limited to these experimental examples. Contents not described here can be fully inferred by those skilled in the art, and therefore, explanations thereof will be omitted.

[0090] <Experimental Example 1>

[0091] A raw material having the composition shown in Table 1 below was heat treated at approximately 1100°C to 1350°C, rolled, and then cooled under the conditions shown in Table 2 below, and the resulting microstructure and physical properties are shown in Table 2. For comparison, samples of Comparative Examples 1 and 2 of the present invention were produced using alloy compositions of heat-treated steel and general non-heat-treated steel, respectively. In addition, samples of Example 2, Comparative Example 3, and Comparative Example 4 had the same composition as Example 1.

[0092] [Table 1]

[0093] [Table 2]

[0094] FIG. 6 shows scanning electron microscope (SEM) photographs (a) of a low-magnification (BF mode) photograph, (b) of a high-magnification (BF mode) photograph, and (c) of a high-magnification (Stem mode) photograph) of MX precipitates in the sample of Example 1 of the present invention, and FIG. 7 shows scanning electron microscope (SEM) photographs (a) of a low-magnification (BF mode) photograph, (b) of a high-magnification (BF mode) photograph, and (c) of a high-magnification (Stem mode) photograph) of MX precipitates in the sample of Comparative Example 2.

[0095] Referring to Table 2, in the comparative example samples where the cooling rate of the sample was more than about 2°C / s, low-temperature structures were formed, and the impact strength value was about 125 J / cm 2Furthermore, in the comparative example samples in which the cooling rate of the samples was less than approximately 2°C / s, no low-temperature structure was generated, but it was found that the impact strength value was lower than the target level.

[0096] On the other hand, the samples of the examples of the present invention not only did not form low-temperature structures compared to the samples of the comparative examples, but also had similar hardness and tensile strength values, and an impact strength value of about 150 J / cm 2 All of the above were confirmed to be high.

[0097] In particular, referring to FIGS. 6 and 7, from the sample of Example 1, composite precipitates with an average size of about 20 nm were observed in the wire rod W, and the average size of about 100 μm 2 It was confirmed that the number of composite precipitates per unit area was 3,000 or more per unit area. 2 It was confirmed that there were more than 100 particles per 100 μm. 2 Since the greater the number of composite precipitates per area, the higher the dispersion of the composite precipitates, it was confirmed that the sample of Example 1 had a higher dispersion of the composite precipitates than the sample of Comparative Example 2. As a result, it can be understood that the sample of Example 1 had relatively higher hardness, tensile strength, and impact strength values ​​than the sample of Comparative Example 2.

[0098] <Experimental Example 2>

[0099] The samples of Example 1 were subjected to a cold forging deformation rate of approximately 60% to 80%, and the compressive strength was compared with the increase in the wire drawing reduction rate (cold drawing approximately 10% to 50%), and the respective data values ​​are shown in Table 3 below.

[0100] [Table 3]

[0101] Referring to Table 3, it was confirmed that the sample of Example 1 of the present invention exhibited a tensile strength of about 900 MPa or more when cold drawing was about 30% to 40%, and that when cold drawing was about 50%, a tensile strength of about 1000 MPa or more could be achieved without strengthening heat treatment.

[0102] It was also confirmed that increasing the wiredrawing reduction rate reduces compressive strength and improves cold forgeability. This is believed to be due to dislocations that pile up at grain boundaries during wiredrawing sliding under low stress. It was also confirmed that compressive strength increases again at approximately 50% cold drawing deformation, which is believed to be due to refinement of ferrite and pearlite grains and an increase in total dislocation density as a result of the amount of deformation.

[0103] <Experimental Example 3>

[0104] After cooling under the composition and process conditions of the sample in Example 1, the tensile strength of the overlapping portion and the tensile strength of the center portion of the wire sample were measured while controlling the conveyor speed under the conditions shown in Table 4 below, and the deviation of the tensile strength was calculated and shown in Table 4.

[0105] [Table 4]

[0106] Referring to Table 4, even when hot rolling was performed using the same process, when the conveyor speed (Cv) was about 0.1 m / s as in the sample of Comparative Example 5, the deviation in tensile strength was significant and the slow conveyance speed caused problems of reduced productivity. When the conveyor speed (Cv) was about 0.8 m / s as in the sample of Comparative Example 6, the deviation in tensile strength was reduced, but there was a risk of equipment malfunction.

[0107] On the other hand, the samples of Examples 3, 4 and 5 had a deviation in tensile strength of within about 50 MPa, and it was confirmed that there was no major problem in terms of productivity.

[0108] It will be apparent to those skilled in the art to which the technical idea of ​​the present invention pertains that the technical idea of ​​the present invention described above is not limited to the above-described embodiments and accompanying drawings, and that various substitutions, modifications and changes are possible within the scope of the technical idea of ​​the present invention.

Claims

1. In weight percent, carbon (C): about 0.20% to 0.40%, silicon (Si): about 0.10% to 0.30%, manganese (Mn): about 1.30% to 1.60%, phosphorus (P): about 0% to 0.05%, sulfur (S): about 0% to 0.05%, chromium (Cr): about 0.02% to 0.30%, nickel (Ni): about 0.02% to 0.30%, molybdenum (Mo): about 0.02% to 0.30%, vanadium (V ): about 0.01% to 0.15%, niobium (Nb): about 0.01% to 0.05%, aluminum (Al): about 0.005% to 0.060%, titanium (Ti): about 0.005% to 0.020%, copper (Cu): about 0.01% to 0.30%, boron (B): about 0.0001% to 0.0020%, nitrogen (N): about 0.005% to 0.015%, and the balance including iron (Fe) and other inevitable impurities, the sum of the niobium (Nb) and the vanadium (V) is about 0.02% to 0.2%; Untempered steel wire rod with a tensile strength of approximately 900 MPa or more and excellent cold forgeability.

2. The non-heat treated steel wire rod contains composite precipitates of about 50 nm or less therein, 2. The non-heat treated steel wire rod with excellent cold forgeability according to claim 1, wherein the composite precipitates are composite precipitates containing one or more elements selected from Nb, V, Ti and Al, or composite precipitates containing one or more elements selected from TiN, VC, VN, NbC, NbN, AlN and BN.

3. 3. The non-heat treated steel wire rod having excellent cold forgeability according to claim 2, wherein the composite precipitates have an austenite grain size number of 10 or more.

4. In weight percent, carbon (C): about 0.20% to 0.40%, silicon (Si): about 0.10% to 0.30%, manganese (Mn): about 1.30% to 1.60%, phosphorus (P): about 0% to 0.05%, sulfur (S): about 0% to 0.05%, chromium (Cr): about 0.02% to 0.30%, nickel (Ni): about 0.02% to 0.30%, molybdenum (Mo): about 0.02% to 0.30%, vanadium (V): about 0.01% to 0.15%, niobium (Nb): about 0.01% to 0.05%, aluminum reheating a steel material at about 1100°C to 1350°C to form a billet, the steel material containing about 0.005% to 0.060% aluminum (Al), about 0.005% to 0.020% titanium (Ti), about 0.01% to 0.30% copper (Cu), about 0.0001% to 0.0020% boron (B), about 0.005% to 0.015% nitrogen (N), and the balance being iron (Fe) and other unavoidable impurities, and the niobium (Nb) and vanadium (V) are contained in a total amount of about 0.02% to 0.2%; rolling the reheated billet to form a wire rod; cold drawing the wire; and cold forging the wire after the cold drawing. A method for producing a non-heat treated steel wire rod having excellent cold forgeability, characterized in that in the step of forming the wire rod, rolling is performed while heating in a temperature range equal to or higher than the A3 transformation point.

5. 5. The method for producing a non-heat treated steel wire rod having excellent cold forgeability according to claim 4, wherein the temperature range equal to or higher than the A3 transformation point is about 750°C to 900°C.

6. In the step of forming the wire, 5. The method for producing a non-heat treated steel wire rod having excellent cold forgeability according to claim 4, wherein after the rolling, the steel is cooled at a cooling rate of about 2°C / s or less.

7. 5. The method for producing a non-heat treated steel wire rod having excellent cold forgeability according to claim 4, wherein in the step of cold drawing the wire rod, the wire drawing area reduction rate is about 30% to 50%.

8. In the step of forming the wire, 5. The method for producing a non-heat treated steel wire rod having excellent cold forgeability according to claim 4, wherein a transfer speed of a conveyor that transfers the wire rod after the rolling is completed is controlled to about 0.2 m / s to 0.7 m / s, thereby controlling a overlap density of the wire rod transferred by the conveyor.

9. 5. The method for producing a non-heat treated steel wire rod having excellent cold forgeability according to claim 4, wherein the wire rod produced after the cold forging step has a tensile strength of about 900 MPa or more.

Images