Steel wire for spring having excellent cold formability and method for manufacturing same

Abstract

A steel wire for a spring having excellent cold formability according to one embodiment of the present invention contains, in wt%, 0.60% to 0.66% of C, 1.40% to 1.65% of Si, 0.50% to 0.70% of Mn, 0.50% to 0.80% of Cr, 0.10% to 0.20% of V, 0.020% or less of P, 0.020% or less of S, 0.007% or less of N, and a remainder of Fe and other unavoidable impurities, wherein the microstructure has, in area%, 90% to 95% of tempered martensite and 5% to 10% of retained austenite, and the average aspect ratio of Fe-based carbides precipitated during tempering is 1: 3 to 1: 7.

Description

Technical Field

[0001] The present invention relates to a steel wire for a spring having excellent cold formability, a method for manufacturing the same, and the steel wire for a spring can be used in an application of a trunk opening / closing spring. Background Art

[0002] As the demand for vehicle convenience features increases, the use of springs for automatically opening and closing trunks is increasing. A trunk spring or a rear gate spring is a compression spring used in an actuator for automatically opening and closing a vehicle door. Since these trunk springs do not require high fatigue strength and life compared to conventional vehicle engine valve opening and closing springs, cold-drawn high-carbon cold-drawn steel wires are generally used.

[0003] However, as the shift towards electric vehicles increases the demand for reducing energy consumption, the demand for high-strength spring materials is growing. Therefore, an oil quenching and tempering wire (OT-wire) that can achieve a higher strength than a cold-drawn spring is being pursued.

[0004] To improve the formability of OT heat-treated steel wires, Korean Patent Application No. 10-2014-7032284 (May 20, 2013) titled "High-strength steel wire for a spring having excellent coiling performance and hydrogen embrittlement resistance and manufacturing method therefor" discloses the use of a certain fraction of retained austenite and grain refinement. On the other hand, the corresponding invention is applicable to products having a relatively low strength level of about 1.9 GPa. In addition, for an ultra-high-strength steel level of about 2.2 GPa, carbides of V, W, and Mo have been conventionally used and commercialized in springs for opening and closing engine and transmission valves. However, the use of a large amount of such relatively expensive carbide elements may reduce price competitiveness. Therefore, there is a need to develop technologies for optimizing the amount of carbide elements and fatigue life, and to provide a spring steel wire for a spring and a corresponding manufacturing method therefor. Summary of the Invention

[0005] Technical Problem

[0006] In order to solve the above problems, the present invention aims to provide a steel wire for a spring having excellent cold formability, and a method for manufacturing the same by controlling alloy composition, microstructure, and manufacturing process.

[0007] The technical objectives of the present invention are not limited to the above, and those skilled in the art will clearly understand other objectives not described above from the above detailed description.

[0008] Technical solution

[0009] The steel wire for springs with excellent cold formability according to an embodiment may contain, by weight percentage (wt%): 0.60% to 0.66% of C, 1.40% to 1.65% of Si, 0.50% to 0.70% of Mn, 0.50% to 0.80% of Cr, 0.10% to 0.20% of V, 0.020% or less of P, 0.020% or less of S, 0.007% or less of N, and the balance of Fe and inevitable impurities. And its microstructure may contain, by area fraction, 90% or more and 95% or less of tempered martensite and 5% or more and 10% or less of retained austenite, and the average aspect ratio of the precipitated Fe-based carbides may be 1:3 or greater and 1:7 or less.

[0010] The steel wire for springs with excellent cold formability according to an embodiment may have Fe-based carbides precipitated at the prior austenite grain boundaries with an aspect ratio of 1:5 or greater and 1:15 or less.

[0011] The tensile strength of the steel wire for springs with excellent cold formability according to an embodiment may be 2.2 GPa or greater.

[0012] The reduction of area of the steel wire for springs with excellent cold formability according to an embodiment may be 40% or greater.

[0013] The method for manufacturing a steel wire for springs with excellent cold formability according to an embodiment may include: preparing a wire rod that contains, by weight percentage (wt%): 0.60% to 0.66% of C, 1.40% to 1.65% of Si, 0.50% to 0.70% of Mn, 0.50% to 0.80% of Cr, 0.10% to 0.20% of V, 0.020% or less of P, 0.020% or less of S, 0.007% or less of N, and the balance of Fe and inevitable impurities; drawing the wire rod at a reduction of area of 82% to 87% at room temperature to produce a steel wire; reheating the steel wire to a temperature of 900 °C to 970 °C for 30 seconds to 120 seconds, and then quenching the reheated steel wire at a temperature of 30 °C to 90 °C; and tempering the quenched steel wire at a temperature of 420 °C for 30 seconds to 500 seconds.

[0014] A method for manufacturing a spring wire having excellent cold formability according to an embodiment may include controlling Fe-based carbides precipitated during tempering to have an average aspect ratio of 1:3 or greater and 1:7 or less.

[0015] A method for manufacturing a spring wire having excellent cold formability according to an embodiment may include controlling Fe-based carbides precipitated at prior austenite grain boundaries during tempering to have an aspect ratio of 1:5 or greater and 1:15 or less.

[0016] Advantageous Effects

[0017] According to an embodiment of the present invention, a spring wire having excellent cold formability capable of ensuring a high strength of 2.2 GPa or greater and fatigue life, and a method for manufacturing the same may be provided. Detailed Description of Embodiments

[0018] Hereinafter, examples of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided to fully convey the spirit of the present invention to those of ordinary skill in the art to which the present invention pertains. The present invention is not limited to the examples shown herein, but may be implemented in other forms. To make the description of the present invention clear, irrelevant parts are not shown, and the dimensions of components are enlarged for clarity.

[0019] Throughout the specification, when a part is referred to as "comprising", "including" and / or "having" a certain element, it should be understood that unless otherwise indicated, the description does not exclude the presence or addition of one or more elements.

[0020] Unless otherwise clearly indicated in the relevant context, the singular form of the noun corresponding to an item may include one or a plurality of items.

[0021] Hereinafter, the reasons for limiting the alloy element content in the embodiments of the present invention numerically will be described. Unless otherwise specified, the unit is expressed in weight percentage (wt%).

[0022] A spring wire having excellent cold formability according to an embodiment may contain, in weight percentage (wt%): 0.60% to 0.66% of C, 1.40% to 1.65% of Si, 0.50% to 0.70% of Mn, 0.50% to 0.80% of Cr, 0.10% to 0.20% of V, 0.020% or less of P, 0.020% or less of S, 0.007% or less of N, and the balance of Fe and inevitable impurities.

[0023] The content of C (carbon) may be 0.60% to 0.66%.

[0024] C is an element effective for increasing strength through solid solution strengthening. Considering this, C can be added in an amount of 0.60% or more. However, when the C content exceeds 0.66%, the toughness may decrease, resulting in a decline in spring formability.

[0025] The content of Si (silicon) can be 1.40% to 1.65%.

[0026] Si can be used to deoxidize the steel and inhibit the growth of carbides during tempering, enabling an increase in the tempering temperature. When the tempering temperature is increased, the aspect ratio of Fe-based carbides may decrease at the target strength, and the precipitation of Fe-based carbides at the prior austenite grain boundaries may be inhibited, thereby suppressing brittle fracture (tempered martensite embrittlement) caused by carbides precipitated at the prior austenite grain boundaries. When the Si content is less than 1.40%, it may be difficult to increase the tempering temperature, which may lead to a decrease in spring formability due to carbides with a large aspect ratio precipitated at the prior austenite grain boundaries, and when the Si content exceeds 1.65%, it may cause surface decarburization during the wire manufacturing process, thereby reducing the fatigue strength.

[0027] The content of Mn (manganese) can be 0.50% to 0.70%.

[0028] Mn is an element for improving hardenability. When the Mn content is 0.50% or more, an appropriate fraction of retained austenite is ensured, providing improved resistance to hydrogen embrittlement, and when the Mn content exceeds 0.70%, the spring formability may deteriorate due to the non-dissolution of carbides in the segregation band.

[0029] The content of Cr (chromium) can be 0.50% to 0.80%.

[0030] Cr is an effective element for ensuring strength by improving hardenability. In addition, similar to Si, Cr plays a role in inhibiting the growth of carbides during tempering. Considering this, Cr can be added in an amount of 0.50% or more. However, when the Cr content exceeds 0.80%, the spring formability may deteriorate due to the non-dissolution of carbides in the segregation band.

[0031] The content of vanadium (V) can be 0.10% to 0.20%.

[0032] V is a V-based precipitation hardening carbide element. When the V content is less than 0.10%, the precipitation hardening effect becomes insufficient, making it difficult to achieve the target strength, and when the V content exceeds 0.20%, coarse V-based carbonitrides may form, which may lead to a decline in spring formability.

[0033] The content of P (phosphorus) can be 0.020% or less (excluding 0%).

[0034] P is an element that may segregate at grain boundaries and reduce formability. Considering this, the content of P can be limited to 0.020% or less.

[0035] The content of sulfur (S) can be 0.020% or less (excluding 0%).

[0036] Similar to P, S is an element that not only segregates at grain boundaries and reduces formability, but also forms low-melting sulfides and reduces hot rollability. Therefore, the content of S can be limited to 0.020%.

[0037] The content of nitrogen (N) can be 0.007% or less.

[0038] N combines with V added to the steel to form V-based carbonitrides. When the nitrogen content is controlled at 0.007% or less, the manufacturing cost can be reduced, and when the nitrogen content exceeds 0.007%, coarse carbonitrides may form, which may result in poor impact toughness.

[0039] The remaining component of the disclosed invention is iron (Fe). However, in a typical manufacturing process, it may be inevitable to introduce unexpected impurities from raw materials or the surrounding environment, so they cannot be excluded. Since such impurities are well known to those skilled in the field of conventional manufacturing processes, their details are not described in this specification.

[0040] The steel wire for springs having excellent cold formability according to one embodiment may have a microstructure containing 90% or more and 95% or less tempered martensite and 5% or more and 10% or less retained austenite by area fraction.

[0041] When the area fraction of retained austenite is 5% or greater, the sensitivity to hydrogen is reduced, resulting in improved hydrogen embrittlement resistance. When the area fraction of retained austenite exceeds 10%, the durability of the spring may be reduced due to strain-induced martensite formed during spring forming.

[0042] In the steel wire for springs having excellent cold formability according to one embodiment, the average aspect ratio of Fe-based carbides precipitated during tempering can be 1:3 or greater and 1:7 or less.

[0043] When the average aspect ratio of Fe-based carbides precipitated during tempering is 1:3 or greater, a tensile strength of 2.2 GPa or greater, which is the target in the present invention, can be achieved, and when the average aspect ratio exceeds 1:7, sufficient tempering may not occur, which may lead to a decrease in spring formability.

[0044] In addition, in a steel wire for a spring having excellent cold formability according to an embodiment, the aspect ratio of Fe-based carbides precipitated at prior austenite grain boundaries may be 1:5 or greater and 1:15 or less.

[0045] In the case of prior austenite grain boundaries, film-like carbides are precipitated along the grain boundaries. When the aspect ratio of the carbides precipitated at the grain boundaries is 1:5 or greater, high strength can be ensured due to the precipitation strengthening effect of the carbides. However, when the aspect ratio exceeds 1:15, the spring formability may deteriorate.

[0046] In addition, the tensile strength of a steel wire for a spring having excellent cold formability according to an embodiment is 2.2 GPa or greater.

[0047] In addition, the reduction of area of a steel wire for a spring having excellent cold formability according to an embodiment may be 40% or greater.

[0048] A method for manufacturing a steel wire for a spring having excellent cold formability according to an embodiment may include: preparing a wire rod containing, by weight percentage (wt%), 0.60% to 0.66% of C, 1.40% to 1.65% of Si, 0.50% to 0.70% of Mn, 0.50% to 0.80% of Cr, 0.10% to 0.20% of V, 0.020% or less of P, 0.020% or less of S, 0.007% or less of N, and the balance of Fe and inevitable impurities; drawing the wire rod at a reduction of area of 82% to 87% at room temperature to produce a steel wire; reheating the steel wire to a temperature of 900°C to 970°C for 30 seconds to 120 seconds, and then quenching the reheated steel wire at a temperature of 30°C to 90°C; and tempering the quenched steel wire at a temperature of 420°C to 450°C for 30 seconds to 500 seconds.

[0049] Hereinafter, each manufacturing operation will be described in more detail.

[0050] After preparing a wire rod satisfying the above alloy composition, a process for producing a steel wire by drawing, reheating, quenching, and tempering may be performed.

[0051] First, a wire rod satisfying the above alloy composition may be prepared. The reasons for numerically limiting the component ranges of the respective alloy compositions are as described above.

[0052] In the production of the steel wire, the wire rod may be drawn at a reduction of area of 82% to 87% at room temperature to produce a steel wire.

[0053] When the cross-sectional shrinkage rate of the wire during drawing is less than 82%, the drawing amount is insufficient, resulting in a small prior austenite grain size, and thus the spring steel wire according to the present invention may not meet the cross-sectional shrinkage rate of 40% or more for the desired spring formability. In addition, when the cross-sectional shrinkage rate of the wire during drawing exceeds 87%, the limit of drawability is exceeded, resulting in central cracking during the drawing process, and as a result, the final spring steel wire according to the present invention may not meet the cross-sectional shrinkage rate of 40% or more, which is the goal in the present invention.

[0054] Reheating and then quenching includes reheating the steel wire at a temperature of 900 °C to 970 °C for 30 seconds to 120 seconds, and then quenching at a temperature of 30 °C to 90 °C.

[0055] When the reheating temperature is lower than 900 °C or the reheating time is shorter than 30 seconds, the reverse transformation to austenite in the microstructure of the steel wire is incomplete, and there may be undissolved pearlite, which results in the spring steel wire according to the present invention not meeting the target cross-sectional shrinkage rate of 40% or more. When the reheating temperature exceeds 970 °C or the reheating time exceeds 120 seconds, the prior austenite grain size increases, resulting in poor ductility, and the spring steel wire according to the present invention may not meet the target cross-sectional shrinkage rate of 40% or more.

[0056] In addition, when the quenching temperature is 30 °C or higher, quenching cracks can be prevented due to the rapid volume change during the martensite transformation, and when the quenching temperature exceeds 90 °C, the cooling rate is not fast, resulting in a retained austenite fraction of more than 10%.

[0057] The quenched wire can be tempered at a temperature of 420 °C to 450 °C (which is different from the conventional tempering temperature). In this case, the tempering time can be preferably set to 30 seconds to 500 seconds. When tempering for 30 seconds or longer, the tensile residual stress on the surface is sufficiently released, which can improve the fatigue strength, and when the tempering time exceeds 500 seconds, there is no change in the tempering effect, resulting in a cost problem.

[0058] In addition, when the tempering temperature is lower than 420 °C, the average aspect ratio of Fe-based carbides in the steel wire exceeds 1:7, and the aspect ratio of Fe-based carbides precipitated along the prior austenite grain boundaries exceeds 1:15, resulting in poor spring formability. When the tempering temperature is too high, the average aspect ratio of Fe-based carbides becomes less than 1:3, approaching a spherical shape, and the aspect ratio of Fe-based carbides precipitated along the prior austenite grain boundaries becomes less than 1:5, making it difficult to achieve a tensile strength of 2.2 GPa or more (which is the target for the spring steel wire according to the present invention). The upper limit of the tempering temperature can vary depending on the added Si, Cr, and V, but it can preferably be 450 °C or lower.

[0059] Hereinafter, the present invention will be described in more detail by way of embodiments. However, the description of the embodiments is only for illustrative purposes of the implementation of the present invention, and the present invention is not limited by the description of the embodiments. This is because the scope of the rights of the present invention is determined by the matters described in the scope of the claims and matters reasonably inferred therefrom.

[0060] {Examples}

[0061] For the various alloy composition ranges shown in Table 1 below, ingots were produced in a vacuum induction melting furnace. The prepared ingots were subjected to solution heat treatment at 1200 °C for 4 hours in a heating furnace to remove the casting structure. Thereafter, in order to produce the ingots into wire rods, welding was performed at the rear end of a 10-meter-long billet. Then the welded ingots were precision rolled into wire rods with a diameter of 9 mm. Next, the wire rods were drawn at the drawing reduction rates shown in Table 2 to produce cold-drawn steel wires, and then reheated at the reheating temperatures shown in Table 3 below for 68 to 70 seconds, and then quenched in oil at a temperature of 55 °C to 70 °C. The quenched steel wires were heat-treated at the tempering temperatures shown in Table 3 below for 105 to 110 seconds.

[0062] Thereafter, the microstructure of the heat-treated steel wires was then analyzed.

[0063] The fraction of retained austenite was obtained by phase analysis using electron backscatter diffraction (EBSD), and images were taken five times at a magnification of 1,500 times, and the average value was recorded. A confidence index of 0.1 or less was regarded as an error to increase the reliability.

[0064] The aspect ratio of Fe-based carbides was observed by preparing 3-mm disc-shaped TEM samples. Photographs were taken in ten random regions of each specimen at a magnification of 160,000 times, and the average aspect ratio of all Fe-based carbides in the grain interior and grain boundaries was measured using an image analyzer. The aspect ratio of the precipitates at the prior austenite grain boundaries was measured by locating the prior austenite grain boundaries and performing three measurements at a magnification of 160,000 times, and the average value was recorded.

[0065] The tensile test was carried out by cutting the heat-treated material into lengths of 300 mm and performing a tensile test in the form of the original cylinder. The material was gripped 100 mm from both ends and the tensile test was carried out at a speed of 10 mm / minute.

[0066] Table 3 shows the heat treatment temperature, the fraction of retained austenite in the microstructure of the steel wire, and the results of the tensile test.

[0067] [Table 1]

[0068]

[0069] [Table 2]

[0070] Reduction ratio of cross-sectional area (%) in wire drawing Inventive Example 1 83.1 Inventive Example 2 83.1 Inventive Example 3 83.1 Inventive Example 4 83.1 Comparative Example 1 83.1 Comparative Example 2 83.1 Comparative Example 3 83.1 Comparative Example 4 83.1 Comparative Example 5 83.1 Comparative Example 6 83.1 Comparative Example 7 83.1 Comparative Example 8 83.1 Comparative Example 9 83.1 Comparative Example 10 83.1 Comparative Example 11 83.1 Comparative Example 12 83.1 Comparative Example 13 83.1 Comparative Example 14 87.4 Comparative Example 15 81.2

[0071] [Table 3]

[0072]

[0073]

[0074] Inventive Examples 1 to 4 can produce steel wires for springs manufactured according to the alloy composition of Table 1 and the heat treatment conditions of Table 2, wherein the steel wire contains 90% or more and 95% or less tempered martensite and 5% or more and 10% or less retained austenite by area fraction, the Fe-based carbide satisfies an average aspect ratio of 1:3 to 1:7, and the Fe-based carbide precipitated at the prior austenite grain boundary satisfies an aspect ratio of 1:5 to 1:15, thereby satisfying a tensile strength of 2.2 GPa or more and a reduction of area of 40% or more.

[0075] Comparative Example 1 has a C content of more than 0.66% and a retained austenite fraction of more than 10%, and thus has a reduction of area of 38% (which does not satisfy 40% or more). In other words, Comparative Example 1 has a poor reduction of area.

[0076] Comparative Example 2 has a C content of less than 0.60% and a retained austenite fraction of 4.9% (which does not satisfy the range of 5% to 10%), and shows a poor tensile strength of 2.13 GPa.

[0077] Comparative Example 3 has an Si content of less than 1.40% and an average aspect ratio of Fe-based carbide of 1:2 (which does not satisfy 1:3 to 1:7), and thus shows a poor tensile strength of 1.91 GPa.

[0078] Comparative Example 4 has an Si content of more than 1.65% and an average aspect ratio of Fe-based carbides of 1:8 (which does not satisfy 1:3 to 1:7), and thus shows poor spring formability due to a low cross-sectional shrinkage rate of 36%.

[0079] Comparative Examples 5 and 6 do not satisfy the Mn content according to the present invention, and thus do not satisfy the retained austenite fraction, and thus show a poor cross-sectional shrinkage rate.

[0080] Comparative Example 7 has a Cr content of less than 0.50% and a retained austenite fraction of 4.9%, resulting in a poor tensile strength of 2.19 GPa.

[0081] Comparative Example 8 has a Cr content of more than 0.80% and a retained austenite fraction of more than 10%, resulting in a cross-sectional shrinkage rate of 38%, indicating poor spring formability.

[0082] Comparative Example 9 has a V content of more than 0.20%, resulting in the formation of coarse vanadium carbonitrides, which reduces ductility and thus results in a poor cross-sectional shrinkage rate.

[0083] Comparative Example 10 has a reheat temperature lower than 900 °C, producing undissolved pearlite, which results in a decrease in the cross-sectional shrinkage rate, thus giving a poor cross-sectional shrinkage rate of 35%.

[0084] Comparative Example 11 has 973 °C, exceeding 970 °C, resulting in coarsening of the prior austenite grain size and a poor cross-sectional shrinkage rate of 39%.

[0085] Comparative Examples 12 and 13 have a tempering temperature lower than 420 °C, and in the case of Comparative Example 12, the aspect ratio of Fe-based carbides is 1:8 (which does not satisfy 1:3 or more to 1:7), and in the case of Comparative Example 13, the aspect ratio of Fe-based carbides precipitated at the prior austenite grain boundaries exceeds 1:15, and in both cases, the cross-sectional shrinkage rate is poor, being 39%.

[0086] Comparative Example 14 has a drawing amount of more than 87%, which results in internal cracks during drawing, thus resulting in a poor cross-sectional shrinkage rate of the wire.

[0087] Comparative Example 15 is processed with a drawing amount of less than 82%, and the strength of the heat-treated wire cannot be ensured.

Claims

1. A steel wire for springs having excellent cold formability, comprising, by weight percentage (% by weight): 0.60% to 0.66% of C, 1.40% to 1.65% of Si, 0.50% to 0.70% of Mn, 0.50% to 0.80% of Cr, 0.10% to 0.20% of V, 0.020% or less of P, 0.020% or less of S, 0.007% or less of N, and the balance of Fe and inevitable impurities, wherein the microstructure of the steel wire for springs comprises, by area fraction, 90% or more and 95% or less of tempered martensite and 5% or more and 10% or less of retained austenite, and the average aspect ratio of the precipitated Fe-based carbide is 1:3 or greater and 1:7 or less.

2. The steel wire for springs having excellent cold formability according to claim 1, wherein the aspect ratio of the Fe-based carbide precipitated at the prior austenite grain boundary is 1:5 or greater and 1:15 or less.

3. The steel wire for springs having excellent cold formability according to claim 1, having a tensile strength of 2.2 GPa or greater.

4. The steel wire for springs having excellent cold formability according to claim 1, having an area reduction of 40% or greater.

5. A method for manufacturing a steel wire for springs having excellent cold formability, the method comprising: preparing a wire rod comprising, by weight percentage (% by weight): 0.60% to 0.66% of C, 1.40% to 1.65% of Si, 0.50% to 0.70% of Mn, 0.50% to 0.80% of Cr, 0.10% to 0.20% of V, 0.020% or less of P, 0.020% or less of S, 0.007% or less of N, and the balance of Fe and inevitable impurities; drawing the wire rod at room temperature with an area reduction of 82% to 87% to produce a steel wire; reheating the steel wire to a temperature of 900°C to 970°C for 30 seconds to 120 seconds, and then quenching the reheated steel wire at a temperature of 30°C to 90°C; and tempering the quenched steel wire at a temperature of 420°C to 450°C for 30 seconds to 500 seconds.

6. The method according to claim 5, wherein the Fe-based carbide precipitated during the tempering is controlled to have an average aspect ratio of 1:3 or greater and 1:7 or less.

7. The method according to claim 5, wherein the Fe-based carbide precipitated at the prior austenite grain boundary during the tempering is controlled to have an aspect ratio of 1:5 or greater and 1:15 or less.

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