Non-quenched and tempered steel wire with excellent cold forging performance and method for manufacturing same

Abstract

Provided are a non-quenched and tempered steel wire having excellent cold forgeability and a method for manufacturing the same. According to the embodiment of the invention, the non-quenched and tempered steel wire having cold forgeability includes about 0.20% to 0.40% by weight of carbon (C), about 0.10% to 0.30% by weight of silicon (Si), about 1.30% to 1.60% by weight of manganese (Mn), about 0% by weight (excluding 0% by weight) to 0.05% by weight of phosphorus (P), about 0% by weight (excluding 0% by weight) to 0.05% by weight of sulfur (S), about 0.02% to 0.30% by weight of chromium (Cr), about 0.02% to 0.30% by weight of nickel (Ni), and the balance of steel. The present invention relates to an aluminum alloy, which comprises, by weight, about 0.02% to 0.30% of molybdenum (Mo), about 0.01% to 0.15% of vanadium (V), about 0.01% to 0.05% of niobium (Nb), about 0.005% to 0.060% of aluminum (Al), about 0.005% to 0.020% of titanium (Ti), about 0.01% to 0.30% of copper (Cu), about 0.0001% to 0.0020% of boron (B), about 0.005% to 0.015% of nitrogen (N), and the balance iron (Fe) and other unavoidable impurities, the sum of niobium (Nb) and vanadium (V) is about 0.02% to 0.2% by weight, and the steel wire can achieve a tensile strength of at least 900 MPa.

Description

Technical Field

[0001] The present invention relates to a non - heat - treated wire rod having excellent cold forging properties and a method for manufacturing the same. More specifically, the present invention relates to a non - heat - treated wire rod capable of achieving excellent strength and toughness without heat treatment and a method for manufacturing the same. Background Art

[0002] Heat - treated wire rods, which are quenched and tempered in the austenite region after cold forging to enhance the strength and toughness of the material, are commonly used in the mechanical structures of vehicle components.

[0003] As Figure 5 shown, a typical heat - treated wire rod is manufactured through a raw material manufacturing step S10 and a component processing step S20 for processing the manufactured wire rod into a component. Specifically, the raw material manufacturing step S10 includes a steelmaking and continuous casting step S11, an ingot 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 the above steps is post - processed according to product specifications, and finally a non - heat - treated wire rod product is manufactured. The post - processing refers to the component processing step S20, and the component processing step S20 includes a cold drawing step S21, a spheroidizing treatment step S22, a cold forging step S23, a quenching / tempering step S24, a processing step S25, and a product manufacturing step S26.

[0004] Meanwhile, non - heat - treated wire rods are steels produced by omitting the heat treatment in the above post - processing. Specifically, the spheroidizing treatment step S22 performed after the cold drawing step S21 and the quenching / tempering step S24 performed after the cold forging step S23 can be omitted.

[0005] Different from heat - treated steels, similar tensile strength and impact toughness to heat - treated steels can be achieved without the above heat treatment process. Therefore, process simplification and cost savings can be achieved to realize economic feasibility, and heat treatment defects and warping can be prevented to produce products that require flatness.

[0006] However, since the non - heat - treated wire rod omits the above heat treatment process, work - hardening continuously occurs during the cold drawing step S21 and the cold forging step S23. This results in an increase in product strength, but a continuous decrease in ductility and toughness, as well as a shortening of the die life during the cold forging process. Therefore, the benefits of omitting heat treatment may be reduced.

[0007] In addition, since the non - heat - treated wire rod does not undergo heat treatment during the component manufacturing process, the mechanical properties of the raw material affect the final component properties, and changes in mechanical properties may pose significant risks.

[0008] The related technical literature includes Japanese Patent Registration No. 7044197 and Korean Patent Registration No. 10-1262462. Summary of the Invention

[0009] Technical Problem

[0010] The present invention provides a non-heat-treated wire rod having excellent cold forging properties and a method for manufacturing the same, which can achieve excellent strength and toughness without heat treatment and have no change in tensile strength. However, the above description is only an example, and the scope of the present invention is not limited thereto.

[0011] Technical Solution

[0012] According to one aspect of the present invention, there is provided a non-heat-treated wire rod having excellent cold forging properties, the non-heat-treated wire rod containing about 0.20 wt% to 0.40 wt% of carbon (C), about 0.10 wt% to 0.30 wt% of silicon (Si), about 1.30 wt% to 1.60 wt% of manganese (Mn), more than 0 wt% and at most about 0.05 wt% of phosphorus (P), more than 0 wt% and at most about 0.05 wt% of sulfur (S), about 0.02 wt% to 0.30 wt% of chromium (Cr), about 0.02 wt% to 0.30 wt% of nickel (Ni), about 0.02 wt% to 0.30 wt% of molybdenum (Mo), about 0.01 wt% to 0.15 wt% of vanadium (V), about 0.01 wt% to 0.05 wt% of niobium (Nb), about 0.005 wt% to 0.060 wt% of aluminum (Al), about 0.005 wt% to 0.020 wt% of titanium (Ti), about 0.01 wt% to 0.30 wt% of copper (Cu), about 0.0001 wt% to 0.0020 wt% of boron (B), about 0.005 wt% to 0.015 wt% of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities, wherein the total of Nb and V is about 0.02 wt% to 0.2 wt%, and wherein the non-heat-treated wire rod satisfies a tensile strength of about 900 MPa or more.

[0013] The non-heat-treated wire rod may include composite precipitates having a diameter of about 50 nm or less, and the composite precipitates may be composite precipitates including one or more selected from Nb, V, Ti, and Al, or one or more selected from composite precipitates including TiN, VC, VN, NbC, NbN, AlN, and BN.

[0014] The composite precipitates may have an austenite grain size of 10 or more grades.

[0015] According to one aspect of the present invention, there is provided a method for manufacturing a non-heat-treated wire rod having excellent cold forging properties, the method comprising forming a billet by reheating a steel material at about 1100 °C to 1350 °C, the steel material comprising about 0.20 wt% to 0.40 wt% of carbon (C), about 0.10 wt% to 0.30 wt% of silicon (Si), about 1.30 wt% to 1.60 wt% of manganese (Mn), more than 0 wt% and at most about 0.05 wt% of phosphorus (P), more than 0 wt% and at most about 0.05 wt% of sulfur (S), about 0.02 wt% to 0.30 wt% of chromium (Cr), about 0.02 wt% to 0.30 wt% of nickel (Ni), about 0.02 wt% to 0.30 wt% of molybdenum (Mo), about 0.01 wt% to 0.15 wt% of vanadium (V), about 0.01 wt% to 0.05 wt% of niobium (Nb), about 0.005 wt% to 0.060 wt% of aluminum (Al), about 0.005 wt% to 0.020 wt% of titanium (Ti), about 0.01 wt% to 0.30 wt% of copper (Cu), about 0.0001 wt% to 0.0020 wt% of boron (B), about 0.005 wt% to 0.015 wt% of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities, wherein the sum of Nb and V is about 0.02 wt% to 0.2 wt%; forming a wire rod by rolling the reheated billet; cold drawing the wire rod; and performing cold forging after cold drawing, wherein, when forming the wire rod, the rolling is carried out while heating in a temperature range above the A3 transformation point.

[0016] The temperature range above the A3 transformation point may be about 750 °C to 900 °C.

[0017] When forming the wire rod, after rolling, the cooling may be carried out at a cooling rate of about 2 °C / s or less.

[0018] In the cold drawing of the wire rod, the drawing reduction rate may be about 30% to 50%.

[0019] When forming the wire rod, the speed of the conveyor for conveying the rolled wire rod may be controlled to be about 0.2 m / s to 0.7 m / s to control the overlapping density of the wire rods conveyed by the conveyor.

[0020] The wire rod manufactured after cold forging may satisfy a tensile strength of about 900 MPa or higher.

[0021] Advantageous Effects

[0022] According to the present invention, a non-heat-treated wire rod having excellent cold forging properties can be manufactured, which can achieve excellent strength and toughness without heat treatment and shows no change in tensile strength. The above effects of the present invention are only examples, and the scope of the present invention is not limited thereto. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a flowchart of a method for manufacturing a non - heat - treated wire rod according to an embodiment of the present invention.

[0024] Figure 2 Specifically shows Figure 1 a schematic view of the wire rod rolling step shown in

[0025] Figure 3 and Figure 4 includes a schematic view of a wire rod cooled on the cooling conveyor shown in Figure 2

[0026] Figure 5 is a flowchart of a method for manufacturing a heat - treated wire rod according to a comparative example of the present invention.

[0027] Figure 6 includes scanning electron microscope (SEM) images of MX precipitates of a sample of Embodiment 1 of the present invention (low - resolution (bright - field (BF) mode) image (a), high - resolution (BF mode) image (b), and high - resolution (scanning transmission electron microscope (STEM) mode) image (c)).

[0028] Figure 7 includes SEM images of MX precipitates of a sample of Comparative Example 2 (low - resolution (BF mode) image (a), high - resolution (BF mode) image (b), and high - resolution (STEM mode) image (c)). DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, the present invention will be described in detail by explaining embodiments of the present invention with reference to the accompanying drawings. However, the present invention can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the present invention to those skilled in the art. Throughout this specification, the same reference numerals refer to the same elements. In addition, various elements and regions are schematically shown in the drawings. Therefore, the scope of the present invention is not limited by the relative sizes or distances shown in the drawings.

[0030] The present invention relates to a method for manufacturing a non - heat - treated wire rod having excellent cold forging properties by controlling precision rolling conditions and cooling rates during wire rod manufacturing, and a non - heat - treated wire rod having excellent cold forging properties will be described first before describing the manufacturing method.

[0031] <Non-heat-treated steel wire rod>

[0032] ​The non-heat-treated wire rod with excellent cold forging properties according to an embodiment of the present invention contains about 0.20 wt% to 0.40 wt% of carbon (C), about 0.10 wt% to 0.30 wt% of silicon (Si), about 1.30 wt% to 1.60 wt% of manganese (Mn), more than 0 wt% and at most about 0.05 wt% of phosphorus (P), more than 0 wt% and at most about 0.05 wt% of sulfur (S), about 0.02 wt% to 0.30 wt% of chromium (Cr), about 0.02 wt% to 0.30 wt% of nickel (Ni), about 0.02 wt% to 0.30 wt% of molybdenum (Mo), about 0.01 wt% to 0.15 wt% of vanadium (V), about 0.01 wt% to 0.05 wt% of niobium (Nb), about 0.005 wt% to 0.060 wt% of aluminum (Al), about 0.005 wt% to 0.020 wt% of titanium (Ti), about 0.01 wt% to 0.30 wt% of copper (Cu), about 0.0001 wt% to 0.0020 wt% of boron (B), about 0.005 wt% to 0.015 wt% of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities.

[0033] The functions and contents of the components contained in the non-heat-treated wire rod according to the present invention are as follows. In this case, the unit of the content of the constituent elements is wt%.

[0034] Carbon (C): approximately 0.20 wt% to 0.40 wt%

[0035] C is an element that forms Nb-based or V-based precipitates and dissolves in the matrix to increase the strength of the steel. Although about 0.20 wt% or more of C is required to ensure sufficient strength of the steel, when the C content increases to more than about 0.40 wt%, the increase in strength results in a significant decrease in toughness and ductility. Therefore, about 0.20 wt% to 0.40 wt% of C is added to ensure excellent strength and toughness of the non-heat-treated steel.

[0036] Silicon (Si): approximately 0.10 wt% to 0.30 wt%

[0037] Si is an element that contributes to high strength and can be used to enhance the fatigue deformation resistance by improving the softening resistance. However, when Si is added in excess as an element to improve the deformation resistance, Si may significantly deteriorate the cold forging properties of the non-heat-treated steel that omits the spheroidizing treatment. Considering this, according to an embodiment of the present invention, about 0.10 wt% to 0.30 wt% of Si is added based on the total weight of the non-heat-treated steel.

[0038] Manganese (Mn): approximately 1.30 wt% to 1.60 wt%

[0039] Mn is an element that can be used to deoxidize steel. It dissolves in the matrix, thereby effectively increasing and ensuring the strength of non-heat-treatable steel. Mn lowers the transformation point, thus contributing to the refinement of intermediate pearlite and the improvement of toughness. When the content of Mn is less than about 1.30 wt% based on the total weight of the non-heat-treatable steel according to an embodiment of the present invention, sufficient strength may not be ensured, while when the content of Mn is greater than about 1.60 wt%, the increase in strength may lead to a decrease in toughness.

[0040] Phosphorus (P): greater than 0 wt% and at most approximately 0.05 wt%

[0041] P is an element that is prone to segregation and impairs the toughness of steel, and has an excellent solid-solution strengthening effect. Adding only a small amount can increase the strength of steel. According to an embodiment of the present invention, P can be added in a content ratio greater than 0 wt% and not greater than about 0.05 wt% based on the total weight of the non-heat-treatable steel. When the content of P is greater than about 0.05 wt%, the toughness of the steel may decrease.

[0042] Sulfur (S): greater than 0 wt% and at most approximately 0.05 wt%

[0043] S is an element that impairs workability and material properties. S segregates at grain boundaries, impairs the ductility of steel, and forms sulfides, which are the main reasons for the deterioration of delayed fracture resistance and stress relaxation properties. Therefore, according to an embodiment of the present invention, S can be added in a content ratio greater than 0 wt% and not greater than about 0.05 wt% based on the total weight of the non-heat-treatable steel. When the content of S is greater than about 0.05 wt%, the martensite grain boundaries may crack, the hot workability may decrease, and surface defects such as cracks may occur due to the formation of coarse inclusions.

[0044] Chromium (Cr): approximately 0.02 wt% to 0.30 wt%

[0045] Cr not only increases the strength of steel but also improves hardenability and strength as a ferrite stabilizing element. According to an embodiment of the present invention, Cr can be added in a ratio of about 0.02 wt% to 0.30 wt% based on the total weight of the non-heat-treatable steel. When the content of Cr is less than about 0.02 wt%, the strength enhancement effect may be insufficient, while when the content of Cr is greater than about 0.30 wt%, the improved hardenability may lead to the formation of a low-temperature structure during cooling, and the cold drawing property and forgeability may decrease.

[0046] Nickel (Ni): approximately 0.02 wt% to 0.30 wt%

[0047] Ni helps to improve hardenability and enhance toughness. According to an embodiment of the present invention, based on the total weight of the non-heat-treated steel, about 0.02 wt% to 0.30 wt% of Ni can be added. When the content of Ni is less than about 0.02 wt%, the effect of Ni addition may not be significant, while when the content of Ni is greater than about 0.30 wt%, the cost may increase.

[0048] Molybdenum (Mo): approximately 0.02 wt% to 0.30 wt%

[0049] Mo helps to enhance strength and toughness. According to an embodiment of the present invention, based on the total weight of the non-heat-treated steel, about 0.02 wt% to 0.30 wt% of Mo can be added. When the content of Mo is less than about 0.02 wt%, the effect of Mo addition may not be significant, while when the content of Mo is greater than about 0.30 wt%, the hardness may increase and the machinability may decrease, and the manufacturing cost of the non-heat-treated steel may increase significantly.

[0050] Vanadium (V): approximately 0.01 wt% to 0.15 wt%

[0051] V is an element that reacts with C and N to form NbV (carbide / nitride) composite precipitates and contributes to the strength improvement through precipitation hardening. V acts as a ferrite nucleation site during the wire rod rolling process to increase the fraction of ferrite and improve strength and toughness. Considering this, according to an embodiment of the present invention, based on the total weight of the non-heat-treated steel, about 0.01 wt% to 0.15 wt% of V can be added. When the content of V is less than about 0.01 wt%, the effect of V addition is insufficient, while when the content of V is greater than about 0.15 wt%, the precipitation hardening effect is not significant and ineffective.

[0052] Niobium (Nb): approximately 0.01 wt% to 0.05 wt%

[0053] Similar to V, Nb is an element that reacts with C and N to form Nb (carbide / nitride). Nb-based precipitates are elements that can cause precipitation hardening and prevent the coarsening of the grain boundaries of the steel. Similar to V, Nb acts as a ferrite nucleation site during the wire rod rolling process to increase the fraction of ferrite and improve strength and toughness. However, the increase in Nb raises the dissolution temperature, and Nb that does not dissolve during the raw material rolling process forms coarse precipitates. The formation of coarse precipitates cannot effectively hinder the movement of dislocations, so it has little effect on improving the fatigue life. Therefore, in order to increase the formation of fine precipitates by dissolving as much Nb as possible during the raw material rolling process, according to an embodiment of the present invention, based on the total weight of the non-heat-treated steel, about 0.01 wt% to 0.05 wt% of Nb can be added.

[0054] Meanwhile, the total of Nb and V can be about 0.02 wt% to 0.2 wt%. Nb and V are elements that form carbides or nitrides. Although the content ranges of these elements individually are also important, since the two are interchangeable and excessive addition may lead to the formation of coarse precipitates, it is necessary to appropriately control the total amount of these two elements in the present invention.

[0055] Aluminum (Al): approximately 0.005 wt% to 0.060 wt%

[0056] Aluminum mainly acts as a deoxidizer and combines with oxygen in the steel to form oxides. The remaining Al after reacting with oxygen combines with nitrogen to form AlN. Here, AlN prevents grain boundary coarsening to enhance the toughness of the product. According to an embodiment of the present invention, based on the total weight of the non-heat-treated steel, about 0.005 wt% to 0.060 wt% of Al can be added. When the content of Al is less than about 0.005 wt%, Al is insufficient to form AlN, while when the content of Al is greater than about 0.060 wt%, the mechanical properties may deteriorate.

[0057] Titanium (Ti): approximately 0.005 wt% to 0.020 wt%

[0058] Like V and Nb, Ti forms carbonitrides, thereby causing precipitation hardening and improving strength and toughness. According to an embodiment of the present invention, based on the total weight of the non-heat-treated steel, about 0.005 wt% to 0.020 wt% of Ti can be added. When the content of Ti is less than about 0.005 wt%, the effect of Ti addition is insufficient, while when the content of Ti is greater than about 0.02 wt%, the manufacturing cost increases rapidly, coarse alloy carbides increase and act similarly to non-metallic inclusions, thereby deteriorating the fatigue performance and the precipitation hardening effect.

[0059] Copper (Cu): approximately 0.01 wt% to 0.30 wt%

[0060] Cu is an element effective for increasing the strength of the steel and enhancing toughness. According to an embodiment of the present invention, based on the total weight of the non-heat-treated steel, about 0.01 wt% to 0.30 wt% of Cu can be added. When the content of Cu is less than about 0.01 wt%, the effect of Cu addition is insufficient, while when the content of Cu is greater than about 0.30 wt%, surface enrichment may occur and the cold drawability during the drawing process may decrease.

[0061] Boron (B): approximately 0.0001 wt% to 0.0020 wt%

[0062] B is an element that segregates at grain boundaries and improves the ductility and toughness of steel. According to an embodiment of the present invention, based on the total weight of the non - heat - treated steel, about 0.0001 wt% to 0.0020 wt% of B can be added. When the content of B is less than about 0.0001 wt%, the effect of adding B is insufficient, while when the content of B is greater than about 0.0020 wt%, the hardenability of the steel may increase and a low - temperature structure may be caused during rapid cooling.

[0063] Nitrogen (N): approximately 0.005 wt% to 0.015 wt%

[0064] N combines with Al, V, and Nb to form nitrides, such as AlN, VN, and NbN. These nitrides can not only achieve grain refinement through the grain - boundary pinning effect but also act as ferrite nucleation sites to increase the fraction of ferrite. According to an embodiment of the present invention, based on the total weight of the non - heat - treated steel, about 0.005 wt% to 0.015 wt% of N can be added. When the content of N is less than about 0.005 wt%, the number of formed nitride particles may be insufficient, while when the content of N is greater than about 0.015 wt%, N may dissolve in the matrix to increase strength and reduce cold drawability.

[0065] The remaining component of the present invention is iron (Fe). However, since it is inevitable to introduce unexpected impurities from raw materials or the surrounding environment during general manufacturing processes, the addition of these impurities may not be completely prevented. Such impurities are known to those skilled in the art and are not specifically described in this specification.

[0066] The non - heat - treated wire rod with excellent cold forging properties and the above composition can meet a tensile strength of about 900 MPa or more.

[0067] The non - heat - treated wire rod with excellent cold forging properties described above may contain composite precipitates with a diameter of 50 nm or less, and the composite precipitates may be composite precipitates containing one or more selected from Nb, V, Ti, and Al, or one or more selected from composite precipitates containing TiN, VC, VN, NbC, NbN, AlN, and BN. By including composite precipitates, grain refinement and an increase in the fraction of ferrite can be achieved, and thus strength and toughness can be improved. For optimal effects, the composite precipitates may have a diameter of about 50 nm or less.

[0068] The grain refinement of the composite precipitates can be determined by measuring the austenite grain size (A, G, S). When the composite precipitate particles are finely dispersed, the austenite grain size can be grade 10 or above. Grain refinement is an important means to improve the strength and toughness of steel, and its principle is based on the pinning effect of MX precipitates to inhibit grain growth. Therefore, the fine dispersion of the composite precipitate particles is crucial.

[0069] The fine dispersion of the composite precipitate particles in the non-heat-treated wire rod of the present invention can be determined based on the number of composite precipitate particles per about 100 μm 2 . To achieve the required strength and toughness of the steel, each about 100 μm 2 can contain 1000 or more composite precipitate particles.

[0070] The above non-heat-treated wire rod according to an embodiment of the present invention can be manufactured as described below.

[0071] Figure 1 is a flowchart of a method for manufacturing a non-heat-treated wire rod according to an embodiment of the present invention, Figure 2 is a schematic diagram specifically showing Figure 1 the wire rod rolling step shown in Figure 3 and Figure 4 includes a schematic diagram of a wire rod cooled on a cooling conveyor shown in Figure 2 .

[0072] Referring to Figure 1 , the method for manufacturing a non-heat-treated wire rod according to an embodiment of the present invention can be divided into two steps. The method includes: a raw material manufacturing step S10 for manufacturing a wire rod by a steel mill; and a component processing step S20 for manufacturing components by processing the wire rod manufactured in the raw material manufacturing step. Generally, as Figure 5 shown, a heat-treated wire rod is manufactured by performing the raw material manufacturing step S10 and the subsequent component processing step S20. The component processing step S20 includes a cold drawing step S21, a spheroidizing treatment step S22, a cold forging step S23, a quenching / tempering step S24, a processing step S25, and a product manufacturing step S26.

[0073] Meanwhile, the non-heat-treated wire rod can be steel produced by omitting the heat treatment during the above component processing step S20. For example, the spheroidizing treatment step S22 performed after the cold drawing step S21, and the quenching / tempering step S24 performed after the cold forging step S23 can be omitted.

[0074] Since the non-heat-treated wire rod does not undergo the spheroidizing treatment step S22 and the quenching / tempering step S24, work hardening continuously occurs during the manufacturing process of the wire rod product. This increases the strength, but reduces the ductility and toughness of the product. Therefore, additional control is required during the manufacturing process to ensure excellent strength and toughness without the above heat treatment process.

[0075] The inventors of the present invention optimized the process conditions for manufacturing the non-heat-treated wire rod, focusing on reducing the change in mechanical properties during the wire rod rolling step S14.

[0076] The method for manufacturing a non-heat-treated wire rod according to an embodiment of the present invention includes: forming a steel billet by reheating a steel material having the above composition at about 1100°C to 1350°C; forming a wire rod by rolling the reheated steel billet; cold-drawing the wire rod; and performing cold forging after cold-drawing.

[0077] Each step will now be described in detail.

[0078] In order to maximize the grain refinement effect of the composite precipitate, the fine dispersion of the composite precipitate particles is crucial. In this regard, the fine dispersion can be achieved by the re-dissolution and re-precipitation of the composite precipitate in the matrix, which can be controlled by reheating before rolling the raw material ingot. The reheating is carried out at a temperature below the liquidus temperature of about 1100°C to 1350°C to dissolve the constituent elements of the composite precipitate as much as possible.

[0079] The formation of the wire rod may include rolling while heating in a temperature range above the A3 transformation point, and the temperature range above the A3 transformation point may be about 750°C to 900°C. After rolling, cooling can be carried out at a cooling rate of about 2°C / s or less.

[0080] Since the spheroidizing treatment step S22 and the quenching / tempering step S24 are omitted for the non-heat-treated wire rod, the structure of the raw material is the same as that of the final product. Therefore, when the cooling rate of the wire rod after rolling is high, the transformation to low-temperature structures (such as bainite and martensite) may be promoted, resulting in problems in the component manufacturing process.

[0081] 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. The area fraction of ferrite can be about 30% to 60%, while the area fraction of pearlite can be the remaining fraction. Here, the fraction refers to the ratio of the area obtained from the microstructure image of the steel material using an image analyzer.

[0082] First, when the microstructure of the wire rod transforms into a low-temperature structure, the hardness increases and the cold forging property decreases. This may shorten the die life and offset the benefits of omitting the heat treatment. Second, when a low-temperature structure forms at the center of the wire rod microstructure where the component segregation is concentrated, chevron cracks may occur during the cold-drawing process due to the difference in the interphase interface between the product surface and the center.

[0083] Meanwhile, in the wire rod cold drawing step S21, the drawing reduction rate can be about 30% to 50%. Since the non-heat-treated steel omits the spheroidizing treatment in the drawing process of the component manufacturing process, work hardening constantly occurs. This may shorten the die life during the cold forging process and offset the benefits of omitting the heat treatment. In the present invention, the Bauschinger effect is utilized to perform cold drawing at a reduction rate of about 30% to 50% to increase the strength of the product and enhance the cold forgeability by minimizing work hardening. The Bauschinger effect is a phenomenon in which a metal becomes more easily permanently deformed according to the direction of the applied load, and it refers to the effect that when a metal is loaded beyond its yield point and then loaded in the opposite direction, its yield point decreases and deformation occurs when the yield strength is lower than the original yield strength.

[0084] Referring to Figures 2 to 4 , the non-heat-treated steel wire rod W of the present invention can be preliminarily processed into the form of a product through the wire rod rolling step S14. In the wire rod rolling step S14, the raw material sequentially passes through Figure 2 the heating furnace 10, rough rolling mill 20, first water tank 32, finishing mill 40, second water tank 34, wire laying head 50, and cooling conveyor 60 shown in

[0085] to manufacture the wire rod W, and then the manufactured wire rod W is finally placed on the rack 70.

[0086] In addition, in the present invention, in addition to the enhancement of ductility and toughness achieved by omitting the spheroidizing treatment step S22 and the quenching / tempering step S24, in order to uniformly control the tensile strength of the entire coil, the speed of the cooling conveyor 60 for cooling the wire rod W after rolling is controlled.

[0087] Specifically, after rolling using the finishing mill 40, the wire rod W falls into a loop and is conveyed by the cooling conveyor 60. In this case, the conveyed loop-shaped wire rod W has an overlapping area P, where, according to the overlapping density of the wire rod W, several loops overlap with each other on the cooling conveyor 60, resulting in a higher density at the overlapping area P compared to the central area C. This causes a difference in the cooling rate between the center and the side of the wire rod W during cooling. That is, the cooling rate at the overlapping area P of the wire rod W is lower than the cooling rate at the central area C of the wire rod W, so it may not be possible to obtain a uniform tensile strength across the entire wire rod W.

[0087] In the present invention, in order to reduce the variation in tensile strength, the speed of the cooling conveyor 60 can be controlled to be about 0.2 m / s to 0.7 m / s to control the overlapping density of the wire rod W conveyed by the cooling conveyor 60.

[0088] In particular, referring to Figure 4 (a) of

[0089] On the other hand, referring to Figure 4 (c) of Figure 4 , when the speed of the cooling conveyor 60 is greater than about 0.7 m / s, although the decrease in the overlapping density of the wire rod W allows for uniform control of the cooling rate, there is a risk due to a failure of the cooling conveyor 60.

[0090] Therefore, as Figure 4 shown in (b) of Figure 4 , by controlling the speed of the cooling conveyor 60 to about 0.2 m / s to 0.7 m / s, the overlapping density of the wire rod W can be uniformly controlled, and thus the material properties based on the cooling rate can also be uniformly controlled.

[0091] The non-heat-treated wire rod of the present invention can satisfy a tensile strength of about 900 MPa or more without heat treatment after cold forging.

[0092] Test examples will now be described to better understand the present invention. However, the following test examples are only for facilitating the understanding of the present invention, and the present invention is not limited thereto. For the content not described herein, those skilled in the art can fully infer it, and thus, no detailed description will be provided.

[0093] <Test Example 1>

[0094] The raw materials having the composition shown in Table 1 were heat-treated at about 1100 °C to 1350 °C, rolled and then cooled under the conditions shown in Table 2, and their microstructures and material properties are shown in Table 2. For comparison, the samples of Comparative Examples 1 and 2 of the present invention were manufactured using a heat-treated steel and a conventional non-heat-treated steel alloy composition. The samples of Embodiment 2 and Comparative Examples 3 and 4 were manufactured using the same composition as in Embodiment 1.

[0095] [Table 1]

[0096]

[0097] [Table 2]

[0098]

[0099] Figure 6 Scanning electron microscope (SEM) images (low-resolution (bright field (BF) mode) image (a), high-resolution (BF mode) image (b), and high-resolution (scanning transmission electron microscope (STEM) mode) image (c)) of MX precipitates including the sample of Embodiment 1 of the present invention, Figure 7 SEM images (low-resolution (BF mode) image (a), high-resolution (BF mode) image (b), and high-resolution (STEM mode) image (c)) of MX precipitates including the sample of Comparative Example 2.

[0100] Referring to Table 2, samples of comparative examples with a cooling rate higher than about 2 °C / s showed the formation of a low-temperature structure and thus may not easily reach an impact strength value of about 125 J / cm 2 . Samples of comparative examples with a cooling rate lower than about 2 °C / s did not show the formation of a low-temperature structure but achieved an impact strength value lower than the target value.

[0101] On the other hand, compared with the samples of the comparative examples, the samples of the embodiments of the present invention did not show the formation of a low-temperature structure but showed similar hardness and tensile strength values and achieved a high impact strength value of about 150 J / cm 2 or higher.

[0102] In particular, referring to Figure 6 and Figure 7 , the samples of Embodiment 1 showed that the number of composite precipitate particles with an average size of about 20 nm in the wire rod W was 3000 or more per about 100 μm 2 . The samples of Comparative Example 2 showed that the number of composite precipitate particles with an average size of about 34 nm in the wire rod W was 100 or more per about 100 μm 2 . The higher the number of composite precipitate particles per area of about 100 μm 2 , the higher the dispersion of the composite precipitate particles. Therefore, compared with the samples of Comparative Example 2, the samples of Embodiment 1 showed a higher dispersion of the composite precipitate particles. In this regard, it can be understood that compared with the samples of Comparative Example 2, the samples of Embodiment 1 showed higher values not only in terms of hardness and tensile strength but also in terms of impact strength.

[0103] <Test Example 2>

[0104] A cold forging strain of about 60% to 80% was applied to the samples of Embodiment 1, and the compressive strength values based on the increase in the drawing reduction rate (the cold drawing reduction rate was about 10% to 50%) are comparatively shown in Table 3.

[0105] [Table 3]

[0106]

[0107] Referring to Table 3, the samples of Embodiment 1 of the present invention showed a tensile strength value of about 950 MPa or greater at a cold drawing reduction rate of about 30% to 40%, and achieved a tensile strength of about 1000 MPa or greater at a cold drawing reduction rate of about 50% without heat treatment.

[0108] As the drawing reduction rate increases, the compressive strength decreases, resulting in improved cold forging properties. This result is attributed to the dislocations piled up at the grain boundaries during drawing and sliding at low stress. When the cold drawing reduction rate is approximately 50%, the compressive strength increases again. Therefore, it is expected that the ferrite and pearlite grains are refined and the total dislocation density based on deformation increases.

[0109] <Test Example 3>

[0110] The wire rod samples produced using the same composition and process conditions as the samples of Embodiment 1 were cooled, and then the tensile strength of the overlapping region and the tensile strength of the central region of each sample were measured by controlling the conveyor speed under the conditions of Table 4, and the change in tensile strength was calculated and shown in Table 4.

[0111] [Table 4]

[0112]

[0113] Referring to Table 4, although the same hot rolling process was carried out, when the conveyor speed Cv was approximately 0.1 m / s as in the sample of Comparative Example 5, the tensile strength changed significantly, and the low conveyor speed led to a decrease in productivity. When the conveyor speed Cv was approximately 0.8 m / s as in the sample of Comparative Example 6, the change in tensile strength decreased, but there was a risk of equipment failure.

[0114] On the other hand, the samples of Embodiments 3, 4, and 5 showed a change in tensile strength within approximately 50 MPa and there were no obvious problems in terms of productivity.

[0115] Although the present invention has been specifically shown and described with reference to the embodiments of the present invention, those skilled in the art will understand that various changes in form and details can be made without departing from the scope of the present invention defined by the following claims.

Claims

1. A non - heat - treated steel wire rod with excellent cold - forging property, the non - heat - treated steel wire rod comprising about 0.20 wt% to 0.40 wt% of carbon (C), about 0.10 wt% to 0.30 wt% of silicon (Si), about 1.30 wt% to 1.60 wt% of manganese (Mn), more than 0 wt% and at most about 0.05 wt% of phosphorus (P), more than 0 wt% and at most about 0.05 wt% of sulfur (S), about 0.02 wt% to 0.30 wt% of chromium (Cr), about 0.02 wt% to 0.30 wt% of nickel (Ni), about 0.02 wt% to 0.30 wt% of molybdenum (Mo), about 0.01 wt% to 0.15 wt% of vanadium (V), about 0.01 wt% to 0.05 wt% of niobium (Nb), about 0.005 wt% to 0.060 wt% of aluminum (Al), about 0.005 wt% to 0.020 wt% of titanium (Ti), about 0.01 wt% to 0.30 wt% of copper (Cu), about 0.0001 wt% to 0.0020 wt% of boron (B), about 0.005 wt% to 0.015 wt% of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities, Among them, the sum of Nb and V is about 0.02 wt% to 0.2 wt%, and wherein, the non - heat - treated steel wire rod satisfies a tensile strength of about 900 MPa or higher.

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

3. The non-heat-treated steel wire rod according to claim 2, wherein, the composite precipitates have an austenite grain size of grade 10 or above.

4. A method for manufacturing a non - heat - treated steel wire rod with excellent cold - forging property, the method comprising: A steel billet is formed by reheating a steel material at about 1100°C to 1350°C, the steel material containing about 0.20 wt% to 0.40 wt% of carbon (C), about 0.10 wt% to 0.30 wt% of silicon (Si), about 1.30 wt% to 1.60 wt% of manganese (Mn), more than 0 wt% and at most about 0.05 wt% of phosphorus (P), more than 0 wt% and at most about 0.05 wt% of sulfur (S), about 0.02 wt% to 0.30 wt% of chromium (Cr), about 0.02 wt% to 0.30 wt% of nickel (Ni), about 0.02 wt% to 0.30 wt% of molybdenum (Mo), about 0.01 wt% to 0.15 wt% of vanadium (V), about 0.01 wt% to 0.05 wt% of niobium (Nb), about 0.005 wt% to 0.060 wt% of aluminum (Al), about 0.005 wt% to 0.020 wt% of titanium (Ti), about 0.01 wt% to 0.30 wt% of copper (Cu), about 0.0001 wt% to 0.0020 wt% of boron (B), about 0.005 wt% to 0.015 wt% of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities, wherein the total of Nb and V is about 0.02 wt% to 0.2 wt%; A wire rod is formed by rolling the reheated steel billet; The wire rod is cold drawn; and Cold forging is performed after cold drawing, wherein, when forming the wire rod, rolling is performed while heating in a temperature range above the A3 transformation point.

5. The method according to claim 4, wherein The temperature range above the A3 transformation point is about 750°C to 900°C.

6. The method according to claim 4, wherein When forming the wire rod, after rolling, cooling is performed at a cooling rate of about 2°C / s or less.

7. The method according to claim 4, wherein During the cold drawing of the wire rod, the drawing reduction rate is about 30% to 50%.

8. The method according to claim 4, wherein, When forming the wire rod, the speed of the conveyor for conveying the rolled wire rod is controlled to be about 0.2 m / s to 0.7 m / s to control the overlapping density of the wire rod conveyed by the conveyor.

9. The method according to claim 4, wherein, The wire rod manufactured after cold forging satisfies a tensile strength of about 900 MPa or higher.

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