Wire rod and steel wire for high strength spring, and manufacturing method therefor

Abstract

Disclosed are: a wire rod and a steel wire for a high stress suspension spring for motorcycles, wherein decarbonization and low-temperature structure occurrence are easily suppressed when the wire rod and the steel wire are cooled down; and a manufacturing method therefor. A steel wire for a high strength spring according to the present disclosure comprises, in percent by weight (wt%), 0.55 to 0.65% of carbon (C), 0.5 to 0.9% of silicon (Si), 0.3 to 0.8% of manganese (Mn), 0.3 to 0.6% of chromium (Cr), 0.015% or less of phosphorus (P), 0.01% or less of sulfur (S), 0.01% or less of aluminum (Al), 0.005% or less of nitrogen (N), and the remainder of iron (Fe) and inevitable impurities, satisfies Formula (1) below, and comprises 90% or more of a tempered martensite structure. In Formula (1), C, Mn, Cr, and Si denote contents (wt%) of the corresponding elements, respectively. (1) 0.77 ≤ C + (1 / 6)*Mn + (1 / 5)*Cr + (1 / 24)*Si ≤ 0.83

Description

Wire rods, steel wires, and manufacturing methods for high-strength springs

[0001] The present invention relates to wire and steel wire for ultra-high strength springs of the 1,800 MPa class, and more specifically, to wire and steel wire for high-stress suspension springs for motorcycles, which are easy to suppress decarburization and low-temperature structure during cooling, and a method for manufacturing the same.

[0002] Just like the automotive materials market, the motorcycle market is also continuously pursuing weight reduction and structural changes, and recently, the demand for high-strength spring steel is increasing as the dual-type suspension used in existing motorcycles is being replaced with a mono-type.

[0003] Conventional spring materials used in motorcycle suspensions are wire-formed steels, which lack the strength and fatigue resistance to be used in mono-type suspensions. Therefore, the use of wire with a tempered martensite structure for automotive applications has been explored. However, automotive suspension springs are difficult to manage, difficult to manufacture, and expensive, making them difficult to apply to motorcycle suspension springs. In particular, automotive suspension springs are relatively thicker in diameter than motorcycle springs, making low-temperature structure management difficult. Therefore, a new, high-strength suspension spring suitable for motorcycle applications is urgently needed.

[0004] The present invention is low C eq And the present invention aims to provide a high-strength motorcycle spring wire, steel wire and a manufacturing method thereof, which are easy to suppress decarburization and low-temperature structure through the use of a minimum of alloying elements and optimization of heat treatment during steel wire manufacturing.

[0005] A wire for a high-strength spring according to one embodiment of the present invention comprises, in wt%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, the remainder being Fe and unavoidable impurities, and satisfies the following formula (1), and the microstructure comprises 80% or more of pearlite and the remainder being ferrite.

[0006] (1) 0.77 ≤ C + (1 / 6)Mn + (1 / 5)Cr + (1 / 24)Si ≤ 0.83

[0007] Here, C, Mn, Cr, and Si represent the content (weight%) of each element.

[0008] Additionally, according to one embodiment of the present invention, the thickness of the ferrite decarburization layer on the surface of the wire may be 1 μm or less.

[0009] Additionally, according to one embodiment of the present invention, a low-temperature structure having a hardness of 430 Hv or more within the C cross-section of the wire may exist at an area fraction of 5% or less.

[0010] Additionally, according to one embodiment of the present invention, the tensile strength of the wire may be 1,200 MPa or less.

[0011] A high-strength spring steel wire according to one embodiment of the present invention comprises, in wt%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, the remainder being Fe and unavoidable impurities, and satisfies the following formula (1), and comprises 90% or more of a tempered martensite structure.

[0012] (1) 0.77 ≤ C + (1 / 6)Mn + (1 / 5)Cr + (1 / 24)Si ≤ 0.83

[0013] Additionally, according to one embodiment of the present invention, the average grain size of the old austenite of the steel wire may be 25 ㎛ or less.

[0014] Additionally, according to one embodiment of the present invention, the steel wire may have a tensile strength of 1,700 MPa or more and a reduction in area (RA) of 35% or more.

[0015] A method for manufacturing a high-strength spring steel wire according to one embodiment of the present invention comprises the steps of: drawing a wire rod that satisfies the following formula (1) and contains, in wt%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, the remainder being Fe and unavoidable impurities; heating the steel wire to a temperature range of 900 to 1,000°C for less than 10 seconds and maintaining the temperature for 5 to 30 seconds; and high-pressure water-cooling the heated steel wire. It includes a step of tempering the water-cooled steel wire by heating it to a temperature range of 400 to 500°C for less than 10 seconds and maintaining it for less than 30 seconds; and a step of water-cooling the tempered steel wire.

[0016] (1) 0.77 ≤ C + (1 / 6)Mn + (1 / 5)Cr + (1 / 24)Si ≤ 0.83

[0017] Additionally, according to one embodiment of the present invention, the microstructure of the wire may include 80% or more of pearlite and the remainder of ferrite.

[0018] Additionally, according to one embodiment of the present invention, the thickness of the ferrite decarburization layer on the surface of the wire may be 1 μm or less.

[0019] Additionally, according to one embodiment of the present invention, the wire may have a low-temperature structure having a hardness of 430 Hv or more in the C cross-section at an area fraction of 5% or less.

[0020] Additionally, according to one embodiment of the present invention, the water-cooled steel wire after tempering may include 90% or more of a tempered martensite structure.

[0021] Additionally, according to one embodiment of the present invention, the average austenite grain size of the heated steel wire may be 25 μm or less.

[0022] According to the present invention, it is optimized (C) in a low-cost component system with a small amount of alloying elements added while excluding expensive alloying elements such as Mo and V as much as possible. eq ) allows for easy control of low-temperature structures, and surface decarburization of the wire can be minimized through low Si content.

[0023] In addition, by optimizing the heat treatment during steel wire manufacturing along with the optimization of alloy elements, it is possible to provide high-strength and high-ductility spring steel wire that is easy to control decarburization and low-temperature structure.

[0024] A wire for a high-strength spring according to one embodiment of the present invention comprises, in wt%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, the remainder being Fe and unavoidable impurities, and satisfies the following formula (1), and the microstructure comprises 80% or more of pearlite and the remainder being ferrite.

[0025] (1) 0.77 ≤ C + (1 / 6)Mn + (1 / 5)Cr + (1 / 24)Si ≤ 0.83

[0026] Here, C, Mn, Cr, and Si represent the content (weight%) of each element.

[0027] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. To clarify the present invention, the drawings may omit portions irrelevant to the description, and the sizes of components may be slightly exaggerated to facilitate understanding.

[0028] Additionally, when a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise specifically stated.

[0029] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0030] In this specification, “low-temperature structure” means bainite and martensite, and from the perspective of a person skilled in the art, it collectively refers to the hard structure of steel formed by rapid cooling of steel.

[0031] In order to achieve high strength, there were cost issues and low-temperature structure control problems in applying tempered martensitic structure steel for automobiles to mono-type suspensions for motorcycles. In the past, when forming tempered martensitic structure, oil quenching was used to secure sufficient hardenability after heating in a heat treatment furnace, so Mn and Cr had to be included in a certain content or more. However, with the development of induction heating heat treatment technology, sufficient cooling capacity can be secured by utilizing water cooling, and since the diameter of spring steel material for motorcycles is relatively smaller than that for automobiles, low C eq The potential for utilizing alloying elements has increased. Therefore, it is now possible to achieve the desired strength while reducing the alloying elements compared to automotive spring steel.

[0032] Low Ceq In this case, since decarburization and low-temperature structure management can be relatively advantageous in small-sized materials, the price of the material can be stabilized, and there is an advantage in that the price can be lowered by reducing alloying elements.

[0033] Accordingly, the high-strength spring wire and steel wire of the present invention contain, in wt%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, the remainder being Fe and unavoidable impurities, and must satisfy the following formula (1).

[0034] Hereinafter, the reasons for numerical limitations on the content of alloying elements in the embodiments of the present invention will be described. Hereinafter, unless otherwise specified, the unit is weight%.

[0035] The C content is 0.55 to 0.65%.

[0036] C is an element added to ensure the strength of the product. If the C content is less than 0.55%, C eq Since the martensite structure is not completely formed during cooling because it is not possible to secure the strength, it is difficult to secure the strength. Even if the complete martensite structure is formed, it may be difficult to secure the target strength. If the C content exceeds 0.65%, the impact properties decrease and cracks (quenching cracks) may occur during water cooling, so the content is limited.

[0037] The Si content is 0.5 to 0.9%.

[0038] Silicon (Si) is not only used for deoxidation of steel, but also contributes to strength enhancement through solid solution strengthening. However, excessive addition can cause surface decarburization and complicate material processing. Therefore, the addition amount is controlled to 0.5 to 0.9%, depending on the target strength and the degree of material processing.

[0039] The Mn content is 0.3 to 0.8%.

[0040] Manganese (Mn) is an essential element for creating high-strength tempered martensite structures, as it enhances hardenability. However, as the Mn content in tempered martensite steels increases, toughness deteriorates. Therefore, Mn addition is controlled to 0.3 to 0.8%.

[0041] The Cr content is 0.3 to 0.6%.

[0042] Cr is an element that is effective in improving hardenability together with Mn and improves the corrosion resistance of steel, so it can be added to a certain level, but it is a relatively expensive element compared to Si and Mn, and C eq Because it increases the amount added, the amount is limited to 0.3 to 0.6%.

[0043] The P content is less than 0.015%.

[0044] Since P is an element that segregates at grain boundaries, lowering toughness and reducing hydrogen fracture resistance, it is desirable to exclude it from steel materials as much as possible, and therefore the upper limit is set at 0.015%.

[0045] The content of S is less than 0.01%.

[0046] S, like P, can segregate at grain boundaries, lowering toughness and forming MnS, which can lower hydrogen catastrophic failure resistance. Therefore, its addition amount is limited to 0.01% or less.

[0047] The Al content is less than 0.01%.

[0048] Aluminum is a powerful deoxidizing element that removes oxygen from steel, improving its purity. However, it can also form Al2O3 inclusions, which can reduce fatigue resistance if added in excess of a certain amount. Therefore, its addition is limited to 0.01% or less.

[0049] The content of N is less than 0.005%.

[0050] N is an impurity or combines with Al or V to form coarse AlN or VN precipitates that do not dissolve during heat treatment. Therefore, it should be kept below 0.005%.

[0051] In addition to the above composition, the remainder is Fe, and includes impurities that are inevitably mixed in during other manufacturing processes. The present invention does not exclude the addition of other alloying elements in addition to the above-mentioned alloy composition.

[0052] In the present invention, in order to suppress low-temperature structures such as martensite or bainite while applying a rapid cooling rate to prevent surface decarburization during pre-cooling, the contents of C, Si, Mn, and Cr among the alloy components must satisfy the following equation (1).

[0053] (1) 0.77 ≤ C + (1 / 6)*Mn + (1 / 5)*Cr + (1 / 24)*Si ≤ 0.83

[0054] If the value of Equation (1) is less than 0.77, it may be difficult to generate martensite structure or secure martensite structure strength at a rapid cooling rate. On the other hand, if it exceeds 0.83, low-temperature structure is generated during cooling, which reduces the workability of the steel and requires additional heat treatment, and it may be difficult to secure an appropriate surface hardness.

[0055] When the low Si content is in the range of 0.5 to 0.9% and the contents of C, Si, Mn, and Cr are controlled to satisfy equation (1), surface decarburization can be suppressed during cooling during the manufacturing process, thereby controlling the thickness of the surface ferrite decarburization layer to 1 ㎛ or less, and a low-temperature structure having a hardness of 430 Hv or more in the C-section can be prevented from being generated. In the present invention, the C-section (Cross-sectional area) means the cross-section of the wire perpendicular to the longitudinal direction.

[0056] The high-strength spring wire according to the present invention can be manufactured through a conventional process for manufacturing spring wire. For example, a billet satisfying the above-described alloy composition and formula (1) can be manufactured through heating, hot rolling, coiling, and cooling.

[0057] The manufactured wire has a microstructure comprising more than 80% pearlite and the remainder ferrite, and a ferrite decarburization layer on the surface can be formed with a thickness of 1 ㎛ or less. In addition, the tensile strength of the wire can be 1,200 MPa or less.

[0058] Next, a method for manufacturing a spring steel wire using the high-strength wire described above is described.

[0059] A method for manufacturing a high-strength spring steel wire according to one embodiment of the present invention comprises the steps of: providing a steel wire having a diameter of 15 mm or less by drawing a wire rod satisfying formula (1) containing, in wt%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, the remainder being Fe and unavoidable impurities; heating the steel wire to a temperature range of 900 to 1,000°C for less than 10 seconds and maintaining the temperature for 5 to 30 seconds; and high-pressure water-cooling the heated steel wire. It includes a step of tempering the water-cooled steel wire by heating it to a temperature range of 400 to 500°C for less than 10 seconds and maintaining it for less than 30 seconds; and a step of water-cooling the tempered steel wire.

[0060] Typically, the manufacturing of spring steel wire involves drawing the wire rod, then processing and heat treating the wire. This heat treatment process involves heating the wire to austenitize it, water cooling it, and then tempering it.

[0061] The high-strength spring steel wire according to the present invention is manufactured by drawing a wire having the above-described alloy composition and satisfying formula (1) to a wire diameter of 15 mm or less used in a motorcycle suspension spring.

[0062] Next, to perform QT ​​heat treatment on the fresh steel wire, it is heated to a temperature range of 900 to 1,000℃ for less than 10 seconds and held for 5 to 30 seconds to austenitize. If the heating time to reach the target temperature range exceeds 10 seconds during austenitization, the grains grow, making it difficult to secure the desired properties. In addition, if the holding time is less than 5 seconds, the pearlite structure may not transform into austenite, and if the holding time exceeds 30 seconds, the grains may coarsen, so it is preferable to control the holding time to 5 to 30 seconds.

[0063] Austenitized steel wires are cooled with water at high pressures sufficient to remove the boiling film. If cooling is performed with oil rather than water, a low C eq Due to this, the desired strength cannot be secured. In addition, since the probability of cracks (quenching cracks) increases if high water pressure sufficient to remove the boiling film is not used during water cooling, water must be sprayed from all sides at the highest possible pressure during cooling.

[0064] Water-cooled steel wire is tempered by heating it to a temperature range of 400 to 500°C for less than 10 seconds and maintaining it for less than 30 seconds. If the tempering temperature is below 400°C, toughness is not secured, making processing difficult and increasing the risk of product damage. If the temperature exceeds 500°C, strength is reduced, so the temperature is limited to the above range. Furthermore, if the tempering temperature is not reached within 10 seconds, coarse carbides are formed, reducing toughness. Therefore, it is essential to ensure that the heating time is within 10 seconds.

[0065] Afterwards, the tempered steel wire is cooled to room temperature in water.

[0066] The spring steel wire manufactured under the manufacturing conditions of the present invention comprises at least 90% of a tempered martensite structure after heat treatment. Furthermore, the steel wire has an average austenite grain size of 25 ㎛ or less, resulting in a tensile strength of at least 1,700 MPa, ensuring the high strength properties required for motorcycle suspension springs. Furthermore, the steel wire exhibits an excellent area reduction ratio (RA) of at least 35%, ensuring high ductility.

[0067] Hereinafter, the present invention will be described in more detail through preferred embodiments.

[0068] Example

[0069] The material having the alloy composition shown in Table 1 below was cast into an ingot, homogenized and heat-treated at 1,200℃, and then hot-rolled to a final thickness of 14㎜ while lowering the temperature from 980℃ to 820℃, and then cooled at a rate of 2.5℃ / s. Table 2 shows the measurement results for the C-section low-temperature structure area fraction, hardness, tensile strength, and decarburization layer thickness of the wire rod manufactured according to the manufacturing process described above. In Table 2, the low-temperature structure area fraction (%) refers to the area fraction of the low-temperature structure on the C-section of the wire rod.

[0070] Alloying element (weight%) CSiMnCrPSAlNComparative example 10.61.50.60.40.0110.004<0.003<0.005Comparative example 20.450.80.80.60.010.005<0.003<0.005Comparative example 30.60.51.00.00.010.004<0.003<0.005Invention example 10.60.80.60.40.0090.005<0.003<0.005Invention example 20.60.60.30.60.0110.005<0.003<0.005

[0071] Equation (1) Low-temperature structure area fraction (%) Wire hardness (Hv) Wire tensile strength (MPa) Ferrite decarburization layer thickness (㎛) Total decarburization layer thickness (㎛) Comparative example 10.80 30 3 1 8 1, 0 3 0 2 6 0.2 Comparative example 20.7 3 7 0 2 3 5 7 6 2-22.8 Comparative example 30.7 8 0 2 6 8 8 1-38.1 Invention example 10.8 1 3 0 2 9 1 9 5 0-34.5 Invention example 20.7 9 5 0 2 8 9 3 0-22.4

[0072] The wires of Comparative Examples 1 to 3 and Invention Examples 1 to 2 were drawn to produce steel wires with a diameter of 12 mm, and heat treatment was performed under the conditions shown in Table 3 below. After austenitization, high-pressure water cooling was performed, and after tempering, cooling was performed using general water cooling.

[0073] Equation (1)Austenitizing temperature (℃)Tempering temperature (℃)Average hardness (Hv)RA (%)Tensile strength (MPa)Comparative example 10.803950430573481,920Comparative example 20.737950430498431,670Comparative example 30.771950430502341,690Invention example 10.813950430550421,820Invention example 20.795950430540481,790

[0074] Referring to Tables 1 to 3, it was confirmed that invention examples 1 and 2, which satisfy all of the alloy composition, formula (1), and manufacturing conditions of the present invention, all have a strength of 1,700 MPa or more, and that no low-temperature structure is formed upon cooling.

[0075] On the other hand, Comparative Example 1 had an excessive Si content, resulting in the formation of a ferrite decarburization layer during cooling. Comparative Example 2 did not achieve the target strength of 1,700 MPa or more because the value of Equation (1) was less than 0.77. Comparative Example 3 had a value of Equation (1) of 0.77 or more and 0.83 or less, but the Cr content was outside the range defined by the present invention, failing to achieve the target strength of 1,700 MPa or more, and the area reduction ratio (RA) was less than 35%.

[0076] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and spirit of the claims set forth below.

[0077] The high-strength spring wire according to the present invention can be applied as a suspension spring for automobiles, motorcycles, various types of transportation, or as a spring used in various industrial fields.

Claims

1. Contains, by weight%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, the remainder being Fe and unavoidable impurities, Satisfies the following equation (1), High-strength spring wire with a microstructure containing more than 80% pearlite and the remainder ferrite. (1) 0.77 ≤ C + (1 / 6)*Mn + (1 / 5)*Cr + (1 / 24)*Si ≤ 0.83 (Here, C, Mn, Cr, and Si represent the content (weight%) of each element.) 2. In paragraph 1, High-strength spring wire with a surface ferrite decarburization layer thickness of 1㎛ or less.

3. In paragraph 1, High-strength spring wire having a low-temperature structure with a hardness of 430 Hv or higher in the C section at an area fraction of 5% or less.

4. In paragraph 1, High-strength spring wire with a tensile strength of 1,200 MPa or less.

5. Contains, by weight%, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, the remainder being Fe and unavoidable impurities, Satisfies the following equation (1), High-strength spring steel wire containing more than 90% tempered martensite structure. (1) 0.77 ≤ C + (1 / 6)*Mn + (1 / 5)*Cr + (1 / 24)*Si ≤ 0.83 (Here, C, Mn, Cr, and Si represent the content (weight%) of each element.) 6. In paragraph 5, High-strength spring steel wire having an average austenite grain size of 25 ㎛ or less.

7. In paragraph 5, High-strength spring steel wire with a tensile strength of 1,700 MPa or more and a reduction in area (RA) of 35% or more.

8. A step of preparing a steel wire having a diameter of 15 mm or less by drawing a wire rod satisfying the following formula (1), which contains C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, the remainder being Fe and unavoidable impurities, by weight%; A step of heating the above steel wire to a temperature range of 900 to 1,000°C for less than 10 seconds and maintaining it for 5 to 30 seconds; A step of high-pressure water cooling the above-mentioned heated steel wire; A step of tempering the water-cooled steel wire by heating it to a temperature range of 400 to 500°C for less than 10 seconds and maintaining it for less than 30 seconds; and A method for manufacturing a high-strength spring steel wire, comprising the step of water-cooling the above-mentioned tempered steel wire. (1) 0.77 ≤ C + (1 / 6)*Mn + (1 / 5)*Cr + (1 / 24)*Si ≤ 0.83 (Here, C, Mn, Cr, and Si represent the content (weight%) of each element.) 9. In paragraph 8, A method for manufacturing a high-strength spring steel wire, wherein the microstructure of the above-mentioned wire comprises 80% or more of pearlite and the remainder of ferrite.

10. In paragraph 8, A method for manufacturing a high-strength spring steel wire having a thickness of a ferrite decarburization layer on the surface of the wire of 1㎛ or less.

11. In paragraph 8, The above wire is a method for manufacturing a high-strength spring steel wire in which a low-temperature structure having a hardness of 430 Hv or higher in the C cross-section exists at an area fraction of 5% or less.

12. In paragraph 8, A method for manufacturing a high-strength spring steel wire comprising 90% or more of a tempered martensite structure after the above tempering and water-cooling.

13. In paragraph 8, A method for manufacturing a high-strength spring steel wire having an average austenite grain size of 25 ㎛ or less in the above-mentioned heated steel wire.