EP4640904A1

EP4640904A1 - Non-quenched and tempered steel wire with excellent cold forging property and manufacturing method therefor

Info

Publication number: EP4640904A1
Application number: EP23907284.6A
Authority: EP
Inventors: Jae Han Lim; Hyeon Bae NOH
Original assignee: Hyundai Steel Co
Current assignee: Hyundai Steel Co
Priority date: 2022-12-23
Filing date: 2023-09-01
Publication date: 2025-10-29
Also published as: WO2024135996A1; KR102871471B1; US20250305082A1; JP2026500921A; KR20240101158A; EP4640904A4; CN120344703A

Abstract

Provided are a non-heat-treated steel wire rod with excellent cold forgeability and a method of manufacturing the same, the non-heat-treated steel wire rod including carbon: 0.20 to 0.40 wt%, silicon: 0.10 to 0.30 wt%, manganese: 1.30 to 1.60 wt%, phosphorus: more than 0 wt% and up to 0.05 wt%, sulfur: more than 0 wt% and up to 0.05 wt%, chromium: 0.02 to 0.30 wt%, nickel: 0.02 to 0.30 wt%, molybdenum: 0.02 to 0.30 wt%, vanadium: 0.01 to 0.15 wt%, niobium: 0.01 to 0.05 wt%, aluminum: 0.005 to 0.060 wt%, titanium: 0.005 to 0.020 wt%, copper: 0.01 to 0.30 wt%, boron: 0.0001 to 0.0020 wt%, nitrogen: 0.005 to 0.015 wt%, and a balance of iron and other unavoidable impurities, wherein a sum of Nb and V is 0.02 to 0.2 wt%, and wherein the non-heat-treated steel wire rod satisfies a tensile strength of 900 MPa or more.

Description

TECHNICAL FIELD

The present invention relates to a non-heat-treated steel wire rod with excellent cold forgeability and a method of manufacturing the same, and more particularly, to a non-heat-treated steel wire rod capable of achieving excellent strength and toughness without heat treatment, and a method of manufacturing the same.

BACKGROUND ART

Heat-treated steel wire rods, which are quenched and tempered in the austenite region after cold forging to enhance material strength and toughness, are commonly used for the mechanical structure of vehicle parts.
As shown in FIG. 5, a typical heat-treated steel wire rod is manufactured through a raw material manufacturing step S10, and a parts processing step S20 for processing the manufactured wire rod into parts. Specifically, the raw material manufacturing 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 the above-mentioned steps is post-processed according to product specifications to ultimately manufacture non-heat-treated wire rod products. The post-processing refers to the parts processing step S20, and the parts processing step S20 includes a cold drawing step S21, a spheroidization step S22, a cold forging step S23, a quenching/tempering step S24, a processing step S25, and a product manufacturing step S26.
Meanwhile, a non-heat-treated steel wire rod is a steel produced by omitting heat treatment during the above-mentioned post-processing. Specifically, the spheroidization step S22 to be performed after the cold drawing step S21, and the quenching/tempering step S24 to be performed after the cold forging step S23 may be omitted.
Unlike the heat-treated steel, tensile strength and impact toughness similar to those of the heat-treated steel may be achieved without the above-mentioned heat treatment processes. Thus, process simplification and cost savings may be enabled to achieve economic feasibility, and heat treatment defects and warping may be prevented to produce products that require straightness.
However, because the above-mentioned heat treatment processes are omitted for the non-heat-treated steel wire rod, work hardening continuously occurs during the cold drawing step S21 and the cold forging step S23. This leads to an increase in product strength but also a continuous decrease in ductility and toughness as well as a reduction in mold life during cold forging. As a result, the benefits of omitting heat treatment may diminish.
In addition, because the non-heat-treated steel wire rod does not undergo heat treatment during the parts manufacturing process and thus the mechanical properties of the raw material affect the final parts properties, variations in mechanical properties may pose a significant risk.
The related art documents include Japanese Patent Registration No. 7044197 and Korean Patent Registration No. 10-1262462 .

DETAILED DESCRIPTION OF THE INVENTION

TECHNICAL PROBLEM

The present invention provides a non-heat-treated steel wire rod with excellent cold forgeability, which achieves excellent strength and toughness without heat treatment and exhibits no variation in tensile strength, and a method of manufacturing the same. However, the above description is an example, and the scope of the present invention is not limited thereto.

TECHNICAL SOLUTION

According to an aspect of the present invention, there is provided a non-heat-treated steel wire rod with excellent cold forgeability, the non-heat-treated steel wire rod including carbon (C): approximately 0.20 wt% to 0.40 wt%, silicon (Si): approximately 0.10 wt% to 0.30 wt%, manganese (Mn): approximately 1.30 wt% to 1.60 wt%, phosphorus (P): more than 0 wt% and up to approximately 0.05 wt%, sulfur (S): more than 0 wt% and up to approximately 0.05 wt%, chromium (Cr): approximately 0.02 wt% to 0.30 wt%, nickel (Ni): approximately 0.02 wt% to 0.30 wt%, molybdenum (Mo): approximately 0.02 wt% to 0.30 wt%, vanadium (V): approximately 0.01 wt% to 0.15 wt%, niobium (Nb): approximately 0.01 wt% to 0.05 wt%, aluminum (Al): approximately 0.005 wt% to 0.060 wt%, titanium (Ti): approximately 0.005 wt% to 0.020 wt%, copper (Cu): approximately 0.01 wt% to 0.30 wt%, boron (B): approximately 0.0001 wt% to 0.0020 wt%, nitrogen (N): approximately 0.005 wt% to 0.015 wt%, and a balance of iron (Fe) and other unavoidable impurities, wherein a sum of Nb and V is approximately 0.02 wt% to 0.2 wt%, and wherein the non-heat-treated steel wire rod satisfies a tensile strength of approximately 900 MPa or more.
The non-heat-treated steel wire rod may include a composite precipitate with a diameter of approximately 50 nm or less, and the composite precipitate may be a composite precipitate 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, AIN, and BN.
The composite precipitate may have an austenite grain size of number 10 or above.
According to an aspect of the present invention, there is provided a method of manufacturing a non-heat-treated steel wire rod with excellent cold forgeability, the method including forming a billet by reheating a steel material including carbon (C): approximately 0.20 wt% to 0.40 wt%, silicon (Si): approximately 0.10 wt% to 0.30 wt%, manganese (Mn): approximately 1.30 wt% to 1.60 wt%, phosphorus (P): more than 0 wt% and up to approximately 0.05 wt%, sulfur (S): more than 0 wt% and up to approximately 0.05 wt%, chromium (Cr): approximately 0.02 wt% to 0.30 wt%, nickel (Ni): approximately 0.02 wt% to 0.30 wt%, molybdenum (Mo): approximately 0.02 wt% to 0.30 wt%, vanadium (V): approximately 0.01 wt% to 0.15 wt%, niobium (Nb): approximately 0.01 wt% to 0.05 wt%, aluminum (Al): approximately 0.005 wt% to 0.060 wt%, titanium (Ti): approximately 0.005 wt% to 0.020 wt%, copper (Cu): approximately 0.01 wt% to 0.30 wt%, boron (B): approximately 0.0001 wt% to 0.0020 wt%, nitrogen (N): approximately 0.005 wt% to 0.015 wt%, and a balance of iron (Fe) and other unavoidable impurities, at approximately 1100 °C to 1350 °C, wherein a sum of Nb and V is approximately 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 the cold drawing, wherein, in the forming of the wire rod, rolling is performed while heating in a temperature region above an A3 transformation point.
The temperature region above the A3 transformation point may be approximately 750 °C to 900 °C.
In the forming of the wire rod, cooling may be performed at a cooling rate of approximately 2 °C/s or less after preforming the rolling.
In the cold drawing of the wire rod, a drawing reduction rate may be approximately 30% to 50%.
In the forming of the wire rod, a speed of a conveyor for transporting the wire rod after being rolled may be controlled to approximately 0.2 m/s to 0.7 m/s to control an overlap density of the wire rod transported by the conveyor.
The wire rod manufactured after the cold forging may satisfy a tensile strength of approximately 900 MPa or more.

ADVANTAGEOUS EFFECTS

According to the present invention, a non-heat-treated steel wire rod with excellent cold forgeability, which achieves excellent strength and toughness without heat treatment and exhibits no variation in tensile strength, may be manufactured. The above effects of the present invention are examples, and the scope of the present invention is not limited thereto.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a method of manufacturing a non-heat-treated steel wire rod, according to an embodiment of the present invention.
FIG. 2 is a schematic view specifically showing a wire rod rolling step shown in FIG. 1.
FIGS. 3 and 4 include schematic views of wire rods cooled on a cooling conveyor shown in FIG. 2.
FIG. 5 is a flowchart of a method of manufacturing a heat-treated steel wire rod, according to a comparative example of the present invention.
FIG. 6 includes scanning electron microscopy (SEM) images (a low-resolution (bright-field (BF) mode) image (a), a high-resolution (BF mode) image (b), and a high-resolution (scanning transmission electron microscopy (STEM) mode) image (c)) of an MX precipitate of a sample of Embodiment 1 of the present invention.
FIG. 7 includes SEM images (a low-resolution (BF mode) image (a), a high-resolution (BF mode) image (b), and a high-resolution (STEM mode) image (c)) of an MX precipitate of a sample of Comparative Example 2.

MODE OF THE INVENTION

Hereinafter, the present invention will be described in detail by explaining embodiments of the invention with reference to the attached drawings. The invention may, however, be embodied in many different forms and should not be construed as being 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 invention to one of ordinary skill in the art. Like reference numerals refer to like elements throughout. Further, various elements and regions in the drawings are schematically illustrated. Therefore, the scope of the present invention is not limited by the relative sizes or distances shown in the attached drawings.
The present invention relates to a method of manufacturing a non-heat-treated steel wire rod with excellent cold forgeability by controlling precision rolling conditions and a cooling rate during wire rod manufacturing, and the non-heat-treated steel wire rod with excellent cold forgeability will be described first before describing the manufacturing method.

<Non-Heat-Treated Steel Wire Rod>

The non-heat-treated steel wire rod with excellent cold forgeability according to an embodiment of the present invention includes carbon (C): approximately 0.20 wt% to 0.40 wt%, silicon (Si): approximately 0.10 wt% to 0.30 wt%, manganese (Mn): approximately 1.30 wt% to 1.60 wt%, phosphorus (P): more than 0 wt% and up to approximately 0.05 wt%, sulfur (S): more than 0 wt% and up to approximately 0.05 wt%, chromium (Cr): approximately 0.02 wt% to 0.30 wt%, nickel (Ni): approximately 0.02 wt% to 0.30 wt%, molybdenum (Mo): approximately 0.02 wt% to 0.30 wt%, vanadium (V): approximately 0.01 wt% to 0.15 wt%, niobium (Nb): approximately 0.01 wt% to 0.05 wt%, aluminum (Al): approximately 0.005 wt% to 0.060 wt%, titanium (Ti): approximately 0.005 wt% to 0.020 wt%, copper (Cu): approximately 0.01 wt% to 0.30 wt%, boron (B): approximately 0.0001 wt% to 0.0020 wt%, nitrogen (N): approximately 0.005 wt% to 0.015 wt%, and a balance of iron (Fe) and other unavoidable impurities.
The functions and contents of the components included in the non-heat-treated steel wire rod according to the present invention are as follows. In this case, the unit for the contents of the constituent elements is wt%.

Carbon (C): approximately 0.20 wt% to 0.40 wt%

C is an element that forms an Nb- or V-based precipitate and dissolves in the matrix to increase the strength of steel. Although approximately 0.20 wt% or more of C is required to ensure sufficient strength of steel, when the content of C increases to more than approximately 0.40 wt%, the increase in strength leads to significant decreases in toughness and ductility. Therefore, C is added at approximately 0.20 wt% to 0.40 wt% to ensure excellent strength and toughness of non-heat-treated steel.

Silicon (Si): approximately 0.10 wt% to 0.30 wt%

Si is an element that contributes to high strength and is useful for enhancing fatigue deformation resistance by increasing softening resistance. However, when excessively added as an element that improves deformation resistance, Si may significantly deteriorate the cold forgeability of non-heat-treated steel for which spheroidization is omitted. Considering this, Si is added at approximately 0.10 wt% to 0.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention.

Manganese (Mn): approximately 1.30 wt% to 1.60 wt%

Mn is an element that is useful for deoxidizing steel, and dissolves in the matrix to effectively increase and ensure the strength of non-heat-treated steel. Mn lowers the transformation point to contribute to intermediate pearlite refinement and enhance toughness. When the content of Mn is less than approximately 1.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention, sufficient strength may not be ensured, and when the content of Mn is greater than approximately 1.60 wt%, the increase in strength may result in a decrease in toughness.

Phosphorus (P): more than 0 wt% and up to approximately 0.05 wt%

P is an element that easily segregates to impair the toughness of steel, and has an excellent solid solution strengthening effect to enhance the strength of steel with only a small amount added. P may be added at a content ratio of more than 0 wt% and not more than approximately 0.05 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of P is greater than approximately 0.05 wt%, the toughness of steel may decrease.

Sulfur (S): more than 0 wt% and up to approximately 0.05 wt%

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

Chromium (Cr): approximately 0.02 wt% to 0.30 wt%

Cr not only increases the strength of steel but also enhances hardenability and strength as a ferrite stabilizing element. Cr may be added at a ratio of approximately 0.02 wt% to 0.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of Cr is less than approximately 0.02 wt%, the strength enhancement effect may be insufficient, and when the content of Cr is greater than approximately 0.30 wt%, the improved hardenability may lead to the formation of low-temperature structures during cooling, and cold drawability and forgeability may decrease.

Nickel (Ni): approximately 0.02 wt% to 0.30 wt%

Ni contributes to increasing hardenability and enhancing toughness. Ni may be added at approximately 0.02 wt% to 0.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of Ni is less than approximately 0.02 wt%, the effect of Ni addition may be insignificant, and when the content of Ni is greater than approximately 0.30 wt%, the costs may increase.

Molybdenum (Mo): approximately 0.02 wt% to 0.30 wt%

Mo contributes to enhancing strength and toughness. Mo may be added at approximately 0.02 wt% to 0.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of Mo is less than approximately 0.02 wt%, the effect of Mo addition may be insignificant, and when the content of Mo is greater than approximately 0.30 wt%, hardness may increase to reduce workability, and the manufacturing costs of non-heat-treated steel may increase significantly.

Vanadium (V): approximately 0.01 wt% to 0.15 wt%

V is an element that reacts with C and N to form an NbV (carbide/nitride) composite precipitate and contributes to strength enhancement through precipitation hardening. V acts as a ferrite nucleation site during wire rod rolling to increase the fraction of ferrite and enhance strength and toughness. Considering this, V may be added at approximately 0.01 wt% to 0.15 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of V is less than approximately 0.01 wt%, the effect of V addition is insufficient, and when the content of V is greater than approximately 0.15 wt%, the precipitation hardening effect is insignificant and ineffective.

Niobium (Nb): approximately 0.01 wt% to 0.05 wt%

Like V, Nb is an element that reacts with C and N to form Nb (carbide/nitride). The Nb-based precipitate is an element that enables precipitation hardening and prevents grain boundary coarsening of steel. Like V, Nb acts as a ferrite nucleation site during wire rod rolling to increase the fraction of ferrite and enhance strength and toughness. However, the increase in Nb raises the solutioning temperature, and Nb which does not dissolve during raw material rolling forms a coarse precipitate. The formation of the coarse precipitate does not effectively impede the movement of dislocations, and thus the effect on fatigue life improvement is insignificant. As such, to increase the formation of a fine precipitate by dissolving Nb as much as possible during raw material rolling, Nb may be added at approximately 0.01 wt% to 0.05 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention.
Meanwhile, the sum of Nb and V may be approximately 0.02 wt% to 0.2 wt%. Nb and V are elements that form carbides or nitrides. Although the individual content ranges of these elements are also important, because the two are interchangeable and excessive addition may lead to the formation of a coarse precipitate, the total amount of both elements needs to be appropriately controlled in the present invention.

Aluminum (Al): approximately 0.005 wt% to 0.060 wt%

Al primarily acts as a deoxidizer, and combines with oxygen in steel to form oxides. Al remaining after reacting with oxygen combines with nitrogen to form AlN. Herein, AIN prevents grain boundary coarsening to enhance the toughness of the product. Al may be added at approximately 0.005 wt% to 0.060 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of Al is less than approximately 0.005 wt%, Al is insufficient to form AIN, and when the content of Al is greater than approximately 0.060 wt%, a deterioration in mechanical properties may occur.

Titanium (Ti): approximately 0.005 wt% to 0.020 wt%

Like V and Nb, Ti forms carbonitrides to cause precipitation hardening and enhance strength and toughness. Ti may be added at approximately 0.005 wt% to 0.020 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of Ti is less than approximately 0.005 wt%, the effect of Ti addition is insufficient, and when the content of Ti is greater than approximately 0.02 wt%, the manufacturing costs increase rapidly, and coarse alloy carbides increase and act similarly to non-metal inclusions, thereby deteriorating the fatigue properties and the precipitation hardening effect.

Copper (Cu): approximately 0.01 wt% to 0.30 wt%

Cu is an element effective in increasing the strength and enhancing the toughness of steel. Cu may be added at approximately 0.01 wt% to 0.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of Cu is less than approximately 0.01 wt%, the effect of Cu addition is insufficient, and when the content of Cu is greater than approximately 0.30 wt%, surface enrichment may be caused and cold drawability during a drawing process may decrease.

Boron (B): approximately 0.0001 wt% to 0.0020 wt%

B is an element that segregates at the grain boundaries and enhances the ductility and toughness of steel. B may be added at approximately 0.0001 wt% to 0.0020 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of B is less than approximately 0.0001 wt%, the effect of B addition is insufficient, and when the content of B is greater than approximately 0.0020 wt%, the hardenability of steel may increase and low-temperature structures may be caused during rapid cooling.

Nitrogen (N): approximately 0.005 wt% to 0.015 wt%

N combines with Al, V, and Nb to form nitrides such as AIN, VN, and NbN. These nitrides not only enable grain refinement through the grain boundary pinning effect, but also act as a ferrite nucleation site to increase the fraction of ferrite. N may be added at approximately 0.005 wt% to 0.015 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of N is less than approximately 0.005 wt%, the number of nitride particles formed may be insufficient, and when the content of N is greater than approximately 0.015 wt%, N may dissolve in the matrix to increase strength and decrease cold drawability.
The remaining component of the present invention is iron (Fe). However, because unintended impurities are inevitably introduced from the 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 any one of ordinary skill in the art and thus are not specifically described in this specification.
The non-heat-treated steel wire rod with excellent cold forgeability and the above-mentioned composition may satisfy a tensile strength of approximately 900 MPa or more.
The above-described non-heat-treated steel wire rod with excellent cold forgeability may include a composite precipitate with a diameter of 50 nm or less, and the composite precipitate may be a composite precipitate 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, AIN, and BN. By including the composite precipitate, grain refinement and ferrite fraction increase may be achieved and thus strength and toughness may be enhanced. For optimal effect, the composite precipitate may have a diameter of approximately 50 nm or less.
The grain refinement of the composite precipitate may be determined by measuring the austenite grain size (A, G, S). When the composite precipitate particles are finely dispersed, the austenite grain size may be number 10 or above. The grain refinement is a critical means of enhancing the strength and toughness of steel materials, and the principle thereof is based on the pinning effect by an MX precipitate, which inhibits grain growth. Therefore, the fine dispersion of the composite precipitate particles is crucial.
The fine dispersion of the composite precipitate particles in the non-heat-treated steel wire rod of the present invention may be determined based on the number of composite precipitate particles per approximately 100 µm². To achieve desired strength and toughness of steel materials, 1000 or more composite precipitate particles may be included per approximately 100 µm².
The above-described non-heat-treated steel wire rod according to an embodiment of the present invention may be manufactured as described below.
FIG. 1 is a flowchart of a method of manufacturing a non-heat-treated steel wire rod, according to an embodiment of the present invention, FIG. 2 is a schematic view specifically showing a wire rod rolling step shown in FIG. 1, and FIGS. 3 and 4 include schematic views of wire rods cooled on a cooling conveyor shown in FIG. 2.
Referring to FIG. 1, the non-heat-treated steel wire rod manufacturing method according to an embodiment of the present invention may be divided into two steps. The method includes a raw material manufacturing step S10 for manufacturing a wire rod by a steelmaker, and a parts processing step S20 for manufacturing parts by processing the wire rod manufactured through the raw material manufacturing step. In general, a heat-treated steel wire rod is manufactured by performing a raw material manufacturing step S10 and then a parts processing step S20 as shown in FIG. 5. The parts processing step S20 includes a cold drawing step S21, a spheroidization step S22, a cold forging step S23, a quenching/tempering step S24, a processing step S25, and a product manufacturing step S26.
Meanwhile, the non-heat-treated steel wire rod may be a steel produced by omitting heat treatment during the above-mentioned parts processing step S20. For example, the spheroidization step S22 to be performed after the cold drawing step S21, and the quenching/tempering step S24 to be performed after the cold forging step S23 may be omitted.
Because the non-heat-treated steel wire rod does not undergo the spheroidization step S22 and the quenching/tempering step S24, work hardening continuously occurs during the manufacture 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-mentioned heat treatment processes.
The inventors of the present invention have optimized the process conditions for manufacturing the non-heat-treated steel wire rod, focusing on reducing the variation in mechanical properties during a wire rod rolling step S14.
The non-heat-treated steel wire rod manufacturing method according to an embodiment of the present invention includes forming a billet by reheating a steel material with the above-mentioned composition, at approximately 1100 °C to 1350 °C; forming a wire rod by rolling the reheated billet; cold drawing the wire rod; and performing cold forging after the cold drawing.
Each step will now be described in detail.
To maximize the grain refinement effect of a composite precipitate, fine dispersion of the composite precipitate particles is crucial. In this regard, the fine dispersion may be achieved through re-dissolution and re-precipitation of the composite precipitate within the matrix, which may be controlled through reheating before rolling a raw material bloom. The reheating is performed at a temperature below the liquidus temperature, approximately 1100 °C to 1350 °C, to dissolve the constituent elements of the composite precipitate as much as possible.
The forming of the wire rod may include performing rolling while heating in a temperature region above the A3 transformation point, and the temperature region above the A3 transformation point may be approximately 750 °C to 900 °C. Cooling may be performed at a cooling rate of approximately 2 °C/s or less after preforming the rolling.
Because the spheroidization step S22 and the quenching/tempering step S24 are omitted for the non-heat-treated steel wire rod, the raw material structure is the same as the structure of the final product. Therefore, when the cooling rate after wire rod rolling is high, the transformation to low-temperature structures (e.g., bainite and martensite) may be promoted to cause problems in the parts manufacturing process.
By controlling the cooling rate of the wire rod after hot rolling to approximately 2 °C/s or less for the reasons mentioned above, the microstructure in the wire rod may be controlled to include ferrite and pearlite. The area fraction of the ferrite may be approximately 30% to 60% and the area fraction of the pearlite may be the remaining fraction. Herein, the fraction refers to a ratio of area derived from a microstructural image of the steel material by using an image analyzer.
Firstly, when the microstructure of the wire rod transforms into the low-temperature structures, hardness increases and cold forgeability decreases. This may shorten the mold life and negate the benefits of omitting heat treatment. Secondly, when the low-temperature structures are formed in the center of the microstructure of the wire rod where component segregation is concentrated, chevron cracks may occur during cold drawing due to interfacial differences between phases at the surface and center of the product.
Meanwhile, in the wire rod cold drawing step S21, the drawing reduction rate may be approximately 30% to 50%. Because spheroidization is omitted for non-heat-treated steel during the drawing process of the parts manufacturing process, work hardening continuously occurs. This may shorten the mold life during cold forging and negate the benefits of omitting heat treatment. In the present invention, cold drawing is performed at a reduction rate of approximately 30% to 50% by using the Bauschinger effect 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 susceptible to permanent deformation depending on the direction in which a load is applied, and refers to an effect in which, when a metal subjected to a load beyond its yield point is then exposed to a load in the opposite direction, its yield point is lowered and deformation occurs at a yield strength lower than the original yield strength.
Referring to FIGS. 2 to 4, the non-heat-treated steel wire rod W of the present invention may be primarily processed in the form of a product through the wire rod rolling step S14. In the wire rod rolling step S14, a raw material sequentially passes through a heating furnace 10, a roughing mill 20, a first water box 32, a precision mill 40, a second water box 34, a laying head 50, and a cooling conveyor 60 shown in FIG. 2 so as to be manufactured into the wire rod W, and then the manufactured wire rod W is finally put on a stand 70.
Furthermore, in the present invention, in addition to ductility and toughness enhancement achieved because the spheroidization step S22 and the quenching/tempering step S24 are omitted, for uniform control of tensile strength across the entire coil, the speed of the cooling conveyor 60 for cooling the wire rod W after being rolled is controlled.
Specifically, after being rolled using the precision mill 40, the wire rod W is dropped into a ring shape and transported by the cooling conveyor 60. In this case, the transported ring-shaped wire rod W has overlapping regions P, where several rings overlap each other on the cooling conveyor 60, depending on an overlap density of the wire rod W, resulting in a higher density at the overlapping regions P compared to a central region C. This causes a difference in cooling rate between the center and sides of the wire rod W during cooling. That is, the cooling rate at the overlapping regions P of the wire rod W is lower than that at the central region C of the wire rod W, and thus a uniform tensile strength across the entire wire rod W may not be obtained.
In the present invention, to reduce the variation in tensile strength, the speed of the cooling conveyor 60 may be controlled to approximately 0.2 m/s to 0.7 m/s to control the overlap density of the wire rod W transported by the cooling conveyor 60.
Particularly, referring to (a) of FIG. 4, when the speed of the cooling conveyor 60 is less than approximately 0.2 m/s, the increase in density at the overlapping regions P of the wire rod W results in a variation in tensile strength and a decrease in productivity.
On the other hand, referring to (c) of FIG. 4, when the speed of the cooling conveyor 60 is greater than approximately 0.7 m/s, although the decrease in overlap density of the wire rod W allows for uniform control of the cooling rate, a risk due to malfunction of the cooling conveyor 60 occurs.
Therefore, as shown in (b) of FIG. 4, by controlling the speed of the cooling conveyor 60 to approximately 0.2 m/s to 0.7 m/s, the overlap density of the wire rod W may be controlled uniformly and thus the material properties based on the cooling rate may also be controlled uniformly.
The non-heat-treated steel wire rod of the present invention may satisfy a tensile strength of approximately 900 MPa or more after cold forging without performing heat treatment.
Test examples will now be described for better understanding of the present invention. However, the following test examples are merely to promote understanding of the present invention, and the present invention is not limited to thereto. The content not described herein may be sufficiently inferred by one of ordinary skill in the art, and therefore, a detailed description will not be provided.

<Test Example 1>

A raw material with the composition shown in Table 1 was heat-treated at approximately 1100 °C to 1350 °C, and rolled and then cooled under the conditions shown in Table 2, and the microstructure and material properties thereof are shown in Table 2. For comparison, samples of Comparative Examples 1 and 2 of the present invention were manufactured using heat-treated steel and conventional non-heat-treated steel alloy compositions. Samples of Embodiment 2 and Comparative Examples 3 and 4 were manufactured using the same composition as Embodiment 1.

[Table 1]

Sample	Chemical Composition (wt%)
Sample	C	Si	Mn	P	S	Cr	Ni	Mo	V	Nb	Ti
Embodiment 1	0.30	0.20	1.37	0.006	0.004	0.08	0.02	0.01	0.050	0.031	0.010
Comparative Example 1 (Heat-Treated Steel)	0.45	0.20	0.80	0.010	0.002	0.02	0.02	0.01	-	-	-
Comparative Example 2 (Non-Heat-Treated Steel)	0.45	0.25	1.40	0.011	0.007	0.03	0.01	-	0.05	-	-

[Table 2]

Sample	Rolling Method	Cooling Rate (°C/s)	Low-Temp. Structure Formation	AGS (No.)	Hardness (HV)	Tensile Strength (MPa)	Impact Strength (J/cm²)
Embodiment 1	Controlled Rolling	2.00	-	10.6	232	750	160
Embodiment 2	Controlled Rolling	0.50	-	10.6	232	688	203
Comparative Example 1 (Heat-Treated Steel)	Conventional Rolling	0.50	-	9.1	88	685	55
Comparative Example 2 (Non-Heat-Treated Steel)	Controlled Rolling	0.50	-	10.2	102.5	826	70
Comparative Example 3	Controlled Rolling	3.00	Formed	10.6	238	782	122
Comparative Example 4	Controlled Rolling	5.00	Formed	10.6	245	805	80

FIG. 6 includes scanning electron microscopy (SEM) images (a low-resolution (bright-field (BF) mode) image (a), a high-resolution (BF mode) image (b), and a high-resolution (scanning transmission electron microscopy (STEM) mode) image (c)) of an MX precipitate of the sample of Embodiment 1 of the present invention, and FIG. 7 includes SEM images (a low-resolution (BF mode) image (a), a high-resolution (BF mode) image (b), and a high-resolution (STEM mode) image (c)) of an MX precipitate of the sample of Comparative Example 2.
Referring to Table 2, the samples of the comparative examples with a cooling rate higher than approximately 2 °C/s show the formation of low-temperature structures, and thus may not easily achieve an impact strength value of approximately 125 J/cm². The samples of the comparative examples with a cooling rate lower than approximately 2 °C/s do not show the formation of low-temperature structures, but achieve an impact strength value lower than the target value.
On the other hand, compared to the samples of the comparative examples, the samples of the embodiments of the present invention do not show the formation of low-temperature structures, exhibit similar hardness and tensile strength values, and achieve a high impact strength value of approximately 150 J/cm² or more.
Particularly, referring to FIGS. 6 and 7, the sample of Embodiment 1 shows that the number of composite precipitate particles with an average size of approximately 20 nm in the wire rod W is 3000 or more per approximately 100 µm². The sample of Comparative Example 2 shows that the number of composite precipitate particles with an average size of approximately 34 nm in the wire rod W is 100 or more per approximately 100 µm². The greater the number of composite precipitate particles per an area of approximately 100 µm², the higher the dispersion of composite precipitate particles. Thus, the sample of Embodiment 1 exhibits a higher dispersion of composite precipitate particles compared to the sample of Comparative Example 2. In this regard, it may be understood that the sample of Embodiment 1 exhibits higher values not only in hardness and tensile strength but also in impact strength compared to the sample of Comparative Example 2.

<Test Example 2>

The sample of Embodiment 1 was subjected to cold forging strains of approximately 60% to 80%, and compressive strength values based on the increase in drawing reduction rate (cold drawing reduction of approximately 10% to 50%) are comparatively shown in Table 3.

[Table 3]

Sample		Raw Material	10% Drawing	20% Drawing	30% Drawing	40% Drawing	50% Drawing
Embodiment 1 Tensile Strength (MPa)		750	849	931	948	977	1065
Flow Stress	60% Strain (0.9 Strain)	1083	879	829	770	778	818
	70% Strain (1.2 Strain)	1022	879	804	769	778	818
	80% Strain (1.6 Strain)	898	805	740	699	719	766
Total Work	60% Strain	943	726	722	717	724	764
Energy (Compressed)	(0.9 Strain)
	70% Strain (1.2 Strain)	1248	977	950	937	946	996
	90% Strain (1.6 Strain)	1642	1319	1276	1235	1252	1318

Referring to Table 3, the sample of Embodiment 1 of the present invention shows a tensile strength value of approximately mid-900 MPa or more at a cold drawing reduction rate of approximately 30% to 40%, and achieves a tensile strength of approximately 1000 MPa or more at a cold drawing reduction rate of approximately 50% without heat treatment.
The compressive strength decreases as the drawing reduction rate increases, leading to an enhancement in cold forgeability. This result is attributed to the dislocations that pile up at the grain boundaries during drawing and slide under low stress. The compressive strength increases again at a cold drawing reduction rate of approximately 50%, and thus ferrite and pearlite grain refinement and an increase in total dislocation density based on deformation are expected.

<Test Example 3>

Wire rod samples produced using the same composition and process conditions as the sample of Embodiment 1 were cooled, and then an overlapping region tensile strength and a central region tensile strength of each sample were measured by controlling the conveyor speed under the conditions of Table 4, and variations in tensile strength were calculated and shown in Table 4.

[Table 4]

Sample	Conveyor Speed (Cv)	Overlapping Region Tensile Strength (MPa)	Central Region Tensile Strength (MPa)	Variation (MPa)	Remark
Embodiment 3	0.20 m/s	681	730	49	Strength variation decreases (within 50 MPa)
Embodiment 4	0.50 m/s	735	767	32
Embodiment 5	0.70 m/s	743	772	29
Comparative Example 5	0.10 m/s	656	727	71	Strength variation increases, and productivity decreases
Comparative Example 6	0.80 m/s	745	774	29	Malfunction risk occurs

Referring to Table 4, although the same hot rolling process is performed, when the conveyor speed Cv is approximately 0.1 m/s as in the sample of Comparative Example 5, a significant variation in tensile strength occurs and a decrease in productivity is caused by the low conveyor speed. When the conveyor speed Cv is approximately 0.8 m/s as in the sample of Comparative Example 6, the variation in tensile strength decreases, but an equipment malfunction risk occurs.
On the other hand, the samples of Embodiments 3, 4, and 5 exhibit a tensile strength variation within approximately 50 MPa and do not have any significant issues in terms of productivity.
While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the following claims.

Claims

A non-heat-treated steel wire rod with excellent cold forgeability, the non-heat-treated steel wire rod comprising carbon (C): approximately 0.20 wt% to 0.40 wt%, silicon (Si): approximately 0.10 wt% to 0.30 wt%, manganese (Mn): approximately 1.30 wt% to 1.60 wt%, phosphorus (P): more than 0 wt% and up to approximately 0.05 wt%, sulfur (S): more than 0 wt% and up to approximately 0.05 wt%, chromium (Cr): approximately 0.02 wt% to 0.30 wt%, nickel (Ni): approximately 0.02 wt% to 0.30 wt%, molybdenum (Mo): approximately 0.02 wt% to 0.30 wt%, vanadium (V): approximately 0.01 wt% to 0.15 wt%, niobium (Nb): approximately 0.01 wt% to 0.05 wt%, aluminum (Al): approximately 0.005 wt% to 0.060 wt%, titanium (Ti): approximately 0.005 wt% to 0.020 wt%, copper (Cu): approximately 0.01 wt% to 0.30 wt%, boron (B): approximately 0.0001 wt% to 0.0020 wt%, nitrogen (N): approximately 0.005 wt% to 0.015 wt%, and a balance of iron (Fe) and other unavoidable impurities,
wherein a sum of Nb and V is approximately 0.02 wt% to 0.2 wt%, and

wherein the non-heat-treated steel wire rod satisfies a tensile strength of approximately 900 MPa or more.
The non-heat-treated steel wire rod of claim 1, wherein the non-heat-treated steel wire rod comprises a composite precipitate with a diameter of approximately 50 nm or less, and
wherein the composite precipitate is a composite precipitate comprising one or more selected from Nb, V, Ti, and Al, or one or more selected from composite precipitates comprising TiN, VC, VN, NbC, NbN, AIN, and BN.
The non-heat-treated steel wire rod of claim 2, wherein the composite precipitate has an austenite grain size of number 10 or above.
A method of manufacturing a non-heat-treated steel wire rod with excellent cold forgeability, the method comprising:
forming a billet by reheating a steel material comprising carbon (C): approximately 0.20 wt% to 0.40 wt%, silicon (Si): approximately 0.10 wt% to 0.30 wt%, manganese (Mn): approximately 1.30 wt% to 1.60 wt%, phosphorus (P): more than 0 wt% and up to approximately 0.05 wt%, sulfur (S): more than 0 wt% and up to approximately 0.05 wt%, chromium (Cr): approximately 0.02 wt% to 0.30 wt%, nickel (Ni): approximately 0.02 wt% to 0.30 wt%, molybdenum (Mo): approximately 0.02 wt% to 0.30 wt%, vanadium (V): approximately 0.01 wt% to 0.15 wt%, niobium (Nb): approximately 0.01 wt% to 0.05 wt%, aluminum (Al): approximately 0.005 wt% to 0.060 wt%, titanium (Ti): approximately 0.005 wt% to 0.020 wt%, copper (Cu): approximately 0.01 wt% to 0.30 wt%, boron (B): approximately 0.0001 wt% to 0.0020 wt%, nitrogen (N): approximately 0.005 wt% to 0.015 wt%, and a balance of iron (Fe) and other unavoidable impurities, at approximately 1100 °C to 1350 °C, wherein a sum of Nb and V is approximately 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 the cold drawing,

wherein, in the forming of the wire rod, rolling is performed while heating in a temperature region above an A3 transformation point.
The method of claim 4, wherein the temperature region above the A3 transformation point is approximately 750 °C to 900 °C.
The method of claim 4, wherein, in the forming of the wire rod, cooling is performed at a cooling rate of approximately 2 °C/s or less after preforming the rolling.
The method of claim 4, wherein, in the cold drawing of the wire rod, a drawing reduction rate is approximately 30% to 50%.
The method of claim 4, wherein, in the forming of the wire rod, a speed of a conveyor for transporting the wire rod after being rolled is controlled to approximately 0.2 m/s to 0.7 m/s to control an overlap density of the wire rod transported by the conveyor.
The method of claim 4, wherein the wire rod manufactured after the cold forging satisfies a tensile strength of approximately 900 MPa or more.