Spring steel wire with improved resistance to permanent deformation and method for manufacturing same

Abstract

The spring steel wire having improved permanent deformation resistance according to one embodiment comprises, by weight, C: 0.39% to 0.51%, Si: 1.30% to 1.70%, Mn: 0.20% to 0.45%, P: 0.015% or less, S: 0.020% or less, Cr: 0.50% to 0.80%, Ti: 0.015% to 0.030%, B: 0.0010% to 0.0040%, N: 0.003% to 0.015%, the remainder being Fe and unavoidable impurities, wherein the spring wire may have a dislocation density in the range of 6.0 * 1013 m <-2 > to 3.0 * 1014 m <-2 >.

Description

Technical Field

[0001] The present invention relates to spring steel wire having improved resistance to permanent deformation and a method for producing the same. Background Art

[0002] Suspension springs are designed to withstand repeated fatigue loading in the elastic range below the yield strength, but plastic deformation occurs in local areas, resulting in permanent deformation. The reason for the local plastic deformation is the dislocation movement caused by repeated fatigue loading at stresses below the yield strength.

[0003] In the microstructure of the current commercial spring, a tempered martensite structure having excellent permanent deformation resistance is utilized. The tempered martensite structure has excellent permanent deformation resistance because the carbides formed during tempering restrict the dislocation movement.

[0004] Traditionally, carbide elements such as Mo, V and W are added as single elements to improve the permanent deformation resistance of tempered martensitic steel. However, adding these relatively expensive carbide elements in large quantities may reduce price competitiveness. Therefore, it is necessary to develop a technology that can optimize the amount of carbide elements and thus provide a manufacturing method. Summary of the invention

[0005] Technical issues

[0006] In order to solve the above problems, the present invention aims to provide a spring steel wire having improved permanent deformation resistance and a method for manufacturing the same by controlling alloy composition and manufacturing process.

[0007] Technical Solution

[0008] The spring steel wire with improved permanent deformation resistance according to one embodiment may include, in terms of weight percentage (wt%): 0.39% to 0.51% C, 1.30% to 1.70% Si, 0.20% to 0.45% Mn, 0.015% or less P, 0.020% or less S, 0.50% to 0.80% Cr, 0.015% to 0.030% Ti, 0.0010% to 0.0040% B, 0.003% to 0.015% N, and the remainder is Fe and unavoidable impurities, and the spring steel wire has a carbon fiber thickness of 6.0×10 13 m -2 Up to 3.0×10 14 m -2 The dislocation density.

[0009] The value of Expression (1) of the spring steel wire having improved permanent deformation resistance according to one embodiment may be greater than 7.

[0010] Expression (1): [Si] + 1.5[Cr] / [Mn],

[0011] In Expression (1), [Si], [Cr], and [Mn] represent the contents (wt%) of the respective components.

[0012] A spring wire having improved permanent deformation resistance according to one embodiment may include tempered martensite as its microstructure.

[0013] The average thickness of the precipitated carbide having a thickness of 100 nm or less in a spring wire having improved permanent deformation resistance according to one embodiment may be 12 nm or less.

[0014] The permanent deformation when compressed at a strength of 1274 MPa in a spring wire having improved permanent deformation resistance according to one embodiment may be 3.0 mm or less.

[0015] A method for manufacturing a spring wire having improved permanent deformation resistance according to one embodiment may include: preparing a billet containing, by weight percentage (wt%): 0.39% to 0.51% of C, 1.30% to 1.70% of Si, 0.20% to 0.45% of Mn, 0.015% or less of P, 0.020% or less of S, 0.50% to 0.80% of Cr, 0.015% to 0.030% of Ti, 0.0010% to 0.0040% of B, 0.003% to 0.015% of N, with the balance being Fe and inevitable impurities; performing finish rolling on the billet at 880°C to 980°C to produce a wire rod; cooling the wire rod; reheating the cooled wire rod to a temperature of 900°C to 980°C, and then quenching the reheated wire rod; and tempering the quenched wire rod at a temperature of 220°C to 300°C.

[0016] Cooling may include: rapidly cooling at a rate of 3°C / sec to 8°C / sec to a temperature of 630°C to 690°C; slowly cooling at a rate of 1°C / sec or less to a temperature of 580°C; and air cooling.

[0017] Advantageous Effects

[0018] According to one embodiment of the invention of the present disclosure, a spring wire having improved permanent deformation resistance and a method for manufacturing the same can be provided by controlling alloy components and manufacturing processes. Detailed Description

[0019] 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 embodied in other forms. For the description of the present invention to be clear, irrelevant components are not shown, and for clarity, the dimensions of the components are exaggerated.

[0020] 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 stated, this description does not exclude the presence or addition of one or more elements.

[0021] Unless otherwise clearly indicated in the relevant context, the singular form of a noun corresponding to an item may include one or a plurality of such items.

[0022] Hereinafter, the reasons for numerically limiting the alloy element content in the embodiments of the present invention will be described. Unless otherwise stated, the unit is expressed in weight percentage (wt%).

[0023] Spring wire having improved resistance to permanent deformation according to one embodiment may contain, by weight percentage (wt%): 0.39% to 0.51% of C, 1.3% to 1.7% of Si, 0.20% to 0.45% of Mn, 0.015% or less of P, 0.020% or less of S, 0.50% to 0.80% of Cr, 0.015% to 0.030% of Ti, 0.0010% to 0.0040% of B, 0.003% to 0.015% of N, with the balance being Fe and inevitable impurities.

[0024] The content of C (carbon) may be 0.39% to 0.51%.

[0025] C is an element that effectively increases strength through solid solution strengthening. Considering this, C can be added in an amount of 0.39% or more. However, when the content of C is excessive, the toughness may decrease, which may lead to sudden brittle fracture. Considering this, the upper limit of the C content can be limited to 0.51%.

[0026] The content of Si (silicon) may be 1.30% to 1.70%.

[0027] Si can be used for deoxidizing steel and is an element that inhibits carbide growth during tempering, thereby enabling an increase in the tempering temperature. Increasing the tempering temperature can form ultra-fine carbides at the target strength, thereby enhancing the precipitation strengthening effect, but may reduce the dislocation strengthening effect due to dislocation annihilation. Therefore, when Si is added, the annihilation of moving dislocations during repeated fatigue can be minimized, thereby improving the resistance to permanent deformation. Considering this, Si can be added in an amount of 1.30% or more. However, when the content of Si is excessive, it may cause surface decarburization during the wire manufacturing process, which may reduce the fatigue strength. Considering this, the content of Si can be limited to 1.70%. Preferably, the content of Si can be 1.32% to 1.68%.

[0028] The content of Mn (manganese) can be 0.20% to 0.45%.

[0029] Mn is an element that improves hardenability. In addition, adding Mn allows S, an impurity in steel, to precipitate as MnS, thereby preventing surface defects due to low-melting-point sulfides. Considering this, Mn can be added in an amount of 0.20% or more. However, when the content of Mn is excessive, the impact toughness may decrease. Considering this, the content of Mn can be limited to 0.45%. Preferably, the content of Mn can be 0.23% to 0.39%.

[0030] The content of P (phosphorus) can be 0.015% or less.

[0031] P is an element that can segregate at grain boundaries and reduce the impact toughness. Considering this, the content of P can be limited to 0.015% or less. Preferably, P can be 0.009% or less.

[0032] The content of S (sulfur) can be 0.020% or less.

[0033] Similar to P, S is an element that not only segregates at grain boundaries and reduces toughness but also forms low-melting-point sulfides, thereby inhibiting hot rolling. Considering this, the content of S can be 0.020% or less. Preferably, S can be 0.004% or less.

[0034] The content of Cr (chromium) can be 0.50% to 0.80%.

[0035] Cr is an element that effectively ensures strength by improving hardenability. In addition, similar to Si, Cr is an element that plays a role in suppressing carbide growth during tempering. Considering this, Cr can be added in an amount of 0.50% or more. However, when the content of Cr is excessive, a chromium oxide layer is formed on the surface, which may increase the C / A ratio of corrosion pits, leading to a notch effect. Therefore, an excessive Cr content is not desirable in terms of corrosion fatigue durability. Considering this, the upper limit of the Cr content can be limited to 0.80%. Preferably, the Cr content can be 0.53% to 0.79%.

[0036] The content of Ti (titanium) can be 0.015% to 0.030%.

[0037] Ti can combine with N introduced into the steel to form titanium nitride, thus preventing B from combining with N. Considering this, Ti can be added in an amount of 0.015% or more. However, when the Ti content is excessive, coarse nitrides may be formed, resulting in poor impact toughness. Considering this, the upper limit of the Ti content can be limited to 0.030%. Preferably, the Ti content can be 0.022% to 0.024%.

[0038] The content of B (boron) can be 0.0010% to 0.0040%.

[0039] B is an element that enhances hardenability. Considering this, B can be added in an amount of 0.0010% or more. However, when the content of B is excessive, Fe 23 (CB)6 carbides may be formed at the grain boundaries, resulting in embrittlement of the austenite grain boundaries and promoting surface defects during continuous casting. Considering this, the upper limit of the B content can be limited to 0.0040%. Preferably, the content of B can be 0.0018% to 0.0022%.

[0040] The content of N (nitrogen) can be 0.003% to 0.015%.

[0041] N can combine with Ti added to the steel to form nitrides. Manufacturing with an N content lower than 0.003% may result in increased manufacturing costs. However, when the N content exceeds 0.015%, coarse nitrides may be formed, resulting in poor impact toughness.

[0042] The remaining component of the invention of the present disclosure 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 such unexpected impurities cannot be excluded. Since such impurities may be well-known to those skilled in the art of conventional manufacturing processes, their details are not described in this specification.

[0043] A spring wire with improved resistance to permanent deformation according to an embodiment may have a dislocation density of 6.0×10 13 m -2 to 3.0×10 14 m -2 .

[0044] When the dislocation density is too high, dislocation annihilation can occur more easily, resulting in poor resistance to permanent deformation.

[0045] The value of Expression (1) for a spring wire with improved resistance to permanent deformation according to an embodiment may be greater than 7.

[0046] Expression (1): [Si] + 1.5[Cr] / [Mn]

[0047] In Expression (1), [Si], [Cr], and [Mn] represent the contents (wt%) of the respective components.

[0048] When the value of Expression (1) is 7 or less, coarse precipitates may be generated, or it may be difficult to satisfy the dislocation density range proposed by the present invention. Therefore, when the value of Expression (1) is 7 or less, the resistance to permanent deformation may decrease.

[0049] A spring wire with improved resistance to permanent deformation according to an example may contain tempered martensite as the microstructure. The tempered martensite structure can improve the resistance to permanent deformation by restricting dislocation movement via carbides formed during the tempering process.

[0050] A spring wire with improved resistance to permanent deformation according to an example of the invention of the present disclosure contains precipitated carbides by the above alloy composition and the following manufacturing method, and the average thickness of the carbides with a thickness of 100 nm or less may be 12 nm or less.

[0051] In the invention of the present disclosure, the average value refers to the average value of the values measured at ten arbitrary points.

[0052] A spring wire with improved resistance to permanent deformation according to an embodiment may have a permanent deformation of 3.0 mm or less when compressed at a strength of 1274 MPa by controlling the microstructure as described above.

[0053] Next, a method for manufacturing a spring wire with improved resistance to permanent deformation according to another aspect of the invention of the present disclosure will be described.

[0054] A method for manufacturing a spring wire with improved resistance to permanent deformation according to an embodiment may include: preparing a billet that contains, by weight percentage (wt%), 0.39% to 0.51% C, 1.30% to 1.70% Si, 0.20% to 0.45% Mn, 0.015% or less P, 0.020% or less S, 0.50% to 0.80% Cr, 0.015% to 0.030% Ti, 0.0010% to 0.0040% B, 0.003% to 0.015% N, with the balance being Fe and unavoidable impurities; performing finish rolling on the billet at 880°C to 980°C to produce wire rods; cooling the wire rods; reheating the cooled wire rods to a temperature of 900°C to 980°C, and then quenching the reheated wire rods; and tempering the quenched wire rods at a temperature of 220°C to 300°C.

[0055] The reasons for numerically limiting the component ranges of each alloy composition are as described above, and the details of the manufacturing operations are provided below.

[0056] After preparing a billet that meets the above alloy composition, a series of processes including finish rolling, cooling, reheating, quenching, and tempering can be carried out.

[0057] First, the billet can be finish rolled at 880°C to 980°C to produce wire rods.

[0058] When the finish rolling temperature is low, surface ferrite decarburization may occur. However, when the finish rolling temperature is high, the grain size becomes coarse, making it difficult to achieve the desired resistance to permanent deformation.

[0059] Cooling can include: rapidly cooling at a rate of 3°C / second to 8°C / second to a temperature of 630°C to 690°C; slowly cooling at a rate of 1°C / second or less to a temperature of 580°C; and air cooling.

[0060] In rapid cooling, rapid cooling is carried out to the pearlite transformation region to avoid the temperature region where surface decarburization is caused by the ferrite phase transformation in the surface layer. Therefore, in rapid cooling, rapid cooling can be carried out at a rate of 3°C / second to 8°C / second to a temperature of 630°C to 690°C.

[0061] When rapid cooling is only carried out to a temperature higher than 690°C, surface decarburization may occur in the two-phase region. In addition, when rapid cooling is carried out to a temperature lower than 630°C, low-temperature structures such as bainite or martensite may be formed.

[0062] In slow cooling, the phase transformation time is ensured to allow phase transformation into pearlite and ferrite. Therefore, in slow cooling, slow cooling can be carried out at a rate of 1°C / second or less to a temperature of 580°C.

[0063] When the cooling rate exceeds 1 °C / second, sufficient phase transformation time cannot be ensured, and low-temperature structures may be generated from the untransformed austenite structure.

[0064] The cooled wire rod can be reheated to a temperature of 900 °C to 980 °C and then quenched.

[0065] When the reheating temperature is high, the grain size may become coarse. However, when the reheating temperature is low, pearlite may not be fully reversed, resulting in ferrite decarburization on the surface due to phase transformation.

[0066] The quenched wire rod can be tempered at a low temperature of 220 °C to 300 °C (which is different from the conventional quenching temperature).

[0067] When the tempering temperature is high, the target tensile strength may not be achieved. However, when the tempering temperature is low, the dislocation density is high, which may lead to permanent deformation due to the annihilation of moving dislocations during fatigue testing.

[0068] Hereinafter, the present invention will be described in more detail by way of embodiments. However, the description of the embodiments is only for explaining 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.

[0069] {Examples}

[0070] For various alloy composition ranges shown in Table 1 below, billets were prepared in a vacuum induction melting furnace. The prepared billets 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 billets into wire rods, welding was performed at the rear end of the 10-meter-long billets. The welded billets were subjected to finish rolling to become wire rods with a diameter of 15 mm. Next, the wire rods were rapidly cooled to a temperature of 650 °C at a cooling rate of 6 °C / second, slowly cooled to a temperature of 580 °C at a rate of 1 °C / second, and then cooled by air cooling. Thereafter, the wire rods were drawn to a diameter of 13.8 mm, reheated and quenched using an induction heating heat treatment method. The quenched wire rods were subjected to tempering to produce specimens.

[0071] The finish rolling temperature, reheating temperature, and tempering temperature are shown in Table 2.

[0072] [Table 1]

[0073]

[0074] [Table 2]

[0075]

[0076] Table 3 below shows the value of Expression (1), the average thickness of the carbide, the dislocation density, and the amount of permanent deformation. The value of Expression (1) is calculated by the following Expression (1).

[0077] Expression (1): [Si] + 1.5[Cr] / [Mn]

[0078] In Expression (1), [Si], [Cr], and [Mn] represent the contents (wt%) of the respective components.

[0079] The average thickness of the carbide is measured using a transmission electron microscope (TEM). Using samples prepared by the replication method, 10 random positions are measured at a magnification of 160K, and then the average thickness of the region identified as the carbide by compositional analysis using energy dispersive X-ray spectroscopy (EDS) is observed. At the same time, the average thickness of the carbide is obtained by measuring the average thickness of the carbide with a thickness of 100 nm or less in the precipitated carbide.

[0080] The dislocation density is measured using X-ray diffraction (XRD). After measuring using Cu-Kα radiation (40 kV 40 mA), 40° < 2θ < 100°, 0.02° / second, the results are analyzed using convolutional multiple whole profile (CMWP) software. The detailed experimental method is based on the content of K. Murasawa in Materials Transactions, Vol. 59, p. 1135, 2018.

[0081] The amount of permanent deformation is measured by compressing the spring product at 1274 MPa for 48 hours and then measuring the height difference before and after compression.

[0082] [Table 3]

[0083]

[0084] Referring to Table 3, Examples 1 to 8 satisfy the alloy composition, the value of Expression (1), and the manufacturing method proposed in the invention of the present disclosure. Therefore, Examples 1 to 8 satisfy a dislocation density of 6.0×10 13 m -2 to 3.0×10 14 m -2 , an average thickness of 12 nm or less for the precipitated carbide with a thickness of 100 nm or less, and an amount of permanent deformation of 3.0 mm or less. That is, Examples 1 to 8 have excellent permanent deformation resistance. However, the C content of Comparative Example 1 is low, resulting in a low dislocation density during quenching, and thus the amount of permanent deformation of 3.0 mm or less is not satisfied. That is, the permanent deformation resistance of Comparative Example 1 is poor.

[0085] Comparative Example 2 has a high C content, so the tempering temperature was increased to meet formability. Therefore, the average carbide thickness in Comparative Example 2 is thick, and the permanent deformation amount of 3.0 mm or less is not satisfied. That is, the permanent deformation resistance of Comparative Example 2 is poor.

[0086] Comparative Example 3 has a high Si content, so the tempering temperature was increased to meet formability. Therefore, Comparative Example 3 does not satisfy a dislocation density of 6.0×10 13 m -2 to 3.0×10 14 m -2 , and the permanent deformation amount of 3.0 mm or less is not satisfied. That is, the permanent deformation resistance of Comparative Example 3 is poor.

[0087] Comparative Example 4 has a low Si content, so the tempering temperature was decreased to obtain the target strength. Therefore, Comparative Example 4 does not satisfy a dislocation density of 6.0×10 13 m -2 to 3.0×10 14 m -2 , and the permanent deformation amount of 3.0 mm or less is not satisfied. That is, the permanent deformation resistance of Comparative Example 4 is poor.

[0088] Comparative Example 5 has a high Mn content and breaks during spring forming. Therefore, it is difficult to commercially utilize Comparative Example 5.

[0089] Comparative Example 6 does not satisfy the permanent deformation amount of 3.0 mm or less due to a low Cr content and a thick average carbide thickness. In other words, the permanent deformation resistance of Comparative Example 6 is poor.

[0090] The finish rolling temperature of Comparative Example 7 is low, resulting in decarburization of ferrite in the wire rod. In other words, it is difficult to commercially utilize Comparative Example 7.

[0091] The reheating temperature of Comparative Example 8 is low, which hinders the sufficient dissolution of pearlite during quenching. Therefore, Comparative Example 8 has a large amount of pearlite remaining in the microstructure, resulting in tissue defects. In other words, it is difficult to commercially utilize Comparative Example 8.

[0092] The tempering temperature of Comparative Example 9 is low, resulting in breakage during spring forming. In other words, it is difficult to commercially utilize Comparative Example 9.

[0093] The tempering temperature of Comparative Example 10 is high, so the target strength of 1950 MPa or higher is not satisfied.

[0094] According to one embodiment of the invention of the present disclosure, a spring wire having improved permanent deformation resistance and a method for manufacturing the same as described above can be provided.

Claims

1. A spring wire with improved resistance to permanent deformation, comprising, by weight percentage (% by weight): 0.39% to 0.51% of C, 1.30% to 1.70% of Si, 0.20% to 0.45% of Mn, 0.015% or less of P, 0.020% or less of S, 0.50% to 0.80% of Cr, 0.015% to 0.030% of Ti, 0.0010% to 0.0040% of B, 0.003% to 0.015% of N, the balance being Fe and inevitable impurities, and The spring wire has a dislocation density of 6.0×10 13 m -2 to 3.0×10 14 m -2 .

2. The spring wire with improved resistance to permanent deformation according to claim 1, wherein the value of expression (1) is greater than 7, Expression (1): [Si]+1.5[Cr] / [Mn], wherein in expression (1), [Si], [Cr] and [Mn] represent the contents (weight %) of the respective components.

3. The spring wire with improved resistance to permanent deformation according to claim 1, comprising tempered martensite as its microstructure.

4. The spring wire with improved resistance to permanent deformation according to claim 1, wherein the average thickness of the carbides with a thickness of 100 nm or less in the precipitated carbides is 12 nm or less.

5. The spring wire with improved resistance to permanent deformation according to claim 1, having a permanent deformation of 3.0 mm or less when compressed at a strength of 1274 MPa.

6. A method for manufacturing a spring wire with improved resistance to permanent deformation, the method comprising: Preparing a billet comprising, by weight percentage (% by weight): 0.39% to 0.51% of C, 1.30% to 1.70% of Si, 0.20% to 0.45% of Mn, 0.015% or less of P, 0.020% or less of S, 0.50% to 0.80% of Cr, 0.015% to 0.030% of Ti, 0.0010% to 0.0040% of B, 0.003% to 0.015% of N, the balance being Fe and inevitable impurities; Performing finish rolling on the billet at 880 °C to 980 °C to produce wire rods; Cooling the wire rods; Reheating the cooled wire rods to a temperature of 900 °C to 980 °C, and then quenching the reheated wire rods; and Performing tempering on the quenched wire rods at a temperature of 220 °C to 300 °C.

7. The method for manufacturing a spring wire with improved resistance to permanent deformation according to claim 6, wherein the cooling comprises: Rapidly cooling at a rate of 3 °C / second to 8 °C / second to a temperature of 630 °C to 690 °C; Slowly cooling at a rate of 1 °C / second or less to a temperature of 580 °C; and Air cooling.

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