Spring wire rod, steel wire, spring with improved fatigue resistance and nitriding properties, and manufacturing method thereof

Abstract

An object of the present invention is to provide a wire rod for springs, a steel wire, a spring, and a method for manufacturing the same, which have improved fatigue resistance and nitriding treatment characteristics. [Solution] The spring wire rod of the present invention has improved fatigue resistance and nitriding properties and contains, by weight, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.5%, Cr: 0.9-1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1-0.25%, V: 0.1-0.25%, with the remainder consisting of Fe and unavoidable impurities, with Cr+Mn being 1.8% or less and Mo / V being 1.5 or less, and the microstructure containing 60% or more pearlite structure in the C cross section.

Description

[Technical Field]

[0001] The present invention relates to a wire rod for springs, a steel wire for springs, and a manufacturing method thereof, which have improved fatigue resistance and nitriding properties, and more particularly to a 2,200 MPa-class ultra-high strength spring steel for transmissions, which is easy to nitride even at high temperatures and has improved fatigue resistance and nitriding properties, a steel wire, a spring, and a manufacturing method thereof. [Background technology]

[0002] As vehicle weight reductions continue to drive demand for lighter automotive parts, springs used in automobile transmissions also need to be made stronger. However, as the wire diameter of spring materials decreases with increasing strength, the spring material becomes more sensitive to inclusions, thereby reducing its fatigue limit. To address this issue, spring manufacturers are using nitriding to increase the surface hardness while maintaining strength, thereby increasing the fatigue limit of spring materials.

[0003] Nitriding is usually performed at temperatures above 500°C, but in the case of spring steel, nitriding is performed at 420-450°C to prevent a decrease in strength, and the heat treatment is performed for a long period of time of 10 hours or more to ensure sufficient nitrogen penetration depth. Generally, the tempering heat treatment temperature for normal spring steel is 450°C or less, so most spring steels lose a lot of hardness if they are nitrided for a long period of time at 420-450°C. Therefore, to prevent a decrease in strength, carbide-forming elements such as Cr, Mo, and V are appropriately added during nitriding.

[0004] However, while adding large amounts of elements such as Cr, Mo, and V, which form carbides, can prevent strength loss during nitriding, it can also cause wire breakage due to the formation of low-temperature structures during wire production, and the pearlite isothermal transformation (LP heat treatment, also known as Lead Patenting, LP) process required to obtain pearlite structures takes so long that production becomes nearly impossible. Meanwhile, spring manufacturers are trying to shorten the nitriding process by performing the process at as high a temperature as possible, while at the same time requiring high-strength wire that does not affect on-site productivity.

[0005] Therefore, there is a demand for the development of high strength wire rods and steel wires that have high fatigue limit and nitriding properties and are also highly productive. Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a wire rod for high-strength springs, a steel wire, a spring, and a method for manufacturing the same, which have improved fatigue resistance and nitriding treatment characteristics. [Means for solving the problem]

[0007] The spring wire rod of the present invention, which has improved fatigue resistance and nitriding properties, contains, by weight, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.5%, Cr: 0.9 to 1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1 to 0.25%, V: 0.1 to 0.25%, and the remainder consisting of Fe and unavoidable impurities, with Cr+Mn being 1.8% or less and Mo / V being 1.5 or less, and the microstructure containing 60% or more pearlite structure in the C cross section.

[0008] Furthermore, the wire rod of the present invention may have an average prior austenite grain size of 25 μm or less.

[0009] In addition, the wire rod of the present invention may have a length of 100 mm when analyzed at an L cross section, and the number of VN precipitates having an average grain size of 10 μm or more at a surface depth of 1 mm per 10 mm may be less than 0.2.

[0010] In addition, the wire rod of the present invention has a Mo+V content of 10 at.% or more in the carbide, an average grain size of 50 nm or less, and a density of 10 or more grains / 10×10 μm 2 It may contain (V,Mo)C carbides distributed in

[0011] The wire rod of the present invention may have a tensile strength of 1,400 MPa or less and a cross-sectional area reduction rate of 40% or more.

[0012] The method for producing a spring wire rod having improved fatigue resistance and nitriding treatment properties according to the present invention comprises, by weight %, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.5%, Cr: 0.9 to 1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1 to 0.25%, V: 0.1 to 0.25%, the remainder being Fe and unavoidable impurities, and Cr+Mn is 1.8 % or less, and Mo / V is 1.5 or less. The method includes the steps of heating the bloom to a temperature of 1,200°C or more and rolling it into a billet, maintaining the billet at a temperature of 1,050°C or more for 180 minutes, rolling the billet at a temperature of 1,000°C or less to obtain a wire rod, coiling the rolled wire rod at a temperature of 900°C or less, and cooling the coiled wire rod at a cooling rate of 2°C / sec or less.

[0013] The method for manufacturing a spring steel wire having improved fatigue resistance and nitriding properties of the present invention includes the steps of heating a spring wire rod to a temperature of 900 to 1,050°C, quenching it to 650 to 750°C for isothermal transformation, and drawing the wire rod to manufacture a steel wire.

[0014] Also, the method may include a step of heating the wire to 650 to 750° C. before the isothermal transformation step, and a step of pickling the heated wire.

[0015] The isothermal transformation time of the present invention may be 150 seconds or less.

[0016] The method may further include subjecting the steel wire to QT heat treatment.

[0017] The spring steel wire of the present invention, which has improved fatigue resistance and nitriding properties, contains, by weight, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.5%, Cr: 0.9 to 1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1 to 0.25%, V: 0.1 to 0.25%, and the remainder consisting of Fe and unavoidable impurities, with Cr+Mn being 1.8% or less and Mo / V being 1.5 or less, and containing 90% or more tempered martensite as a microstructure.

[0018] Furthermore, the steel wire of the present invention may have an average prior austenite grain size of 25 μm or less.

[0019] In addition, the steel wire of the present invention may have a length of 100 mm when analyzed at an L cross section, and the number of VN precipitates having an average grain size of 10 μm or more at a surface depth of 1 mm per 10 mm may be less than 0.2.

[0020] The steel wire of the present invention may have a tensile hardness of 2,100 MPa or more and a cross-sectional area reduction rate of 40% or more.

[0021] In addition, the steel wire of the present invention has a Mo+V content of 10 at.% or more in the carbide, an average grain size of 50 nm or less, and a grain size of 10 or more / 10×10 μm 2 It may contain (V,Mo)C carbides distributed in

[0022] The method for manufacturing a spring having improved fatigue resistance and nitriding properties according to the present invention includes the steps of cold-forming a spring wire into the shape of a spring and annealing the formed spring.

[0023] The spring of the present invention, which has improved fatigue resistance and nitriding properties, contains, by weight, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.5%, Cr: 0.9 to 1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1 to 0.25%, V: 0.1 to 0.25%, and the remainder consisting of Fe and unavoidable impurities, with Cr+Mn being 1.8% or less and Mo / V being 1.5 or less, the surface hardness being 800 Hv or more, and the hardness of the C-section region from 1 / 4 to 3 / 4 of the wire diameter being 600 Hv or more. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a wire rod, steel wire, spring, and manufacturing method thereof, which have excellent productivity by minimizing the generation of low-temperature structures, and which have improved fatigue resistance and nitriding properties by suppressing the size of crystal grains and nitrides and uniformly distributing (Mo, V)C carbides. DETAILED DESCRIPTION OF THE INVENTION

[0025] A spring wire rod having improved fatigue resistance and nitriding properties according to one embodiment of the present invention contains, by weight, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.5%, Cr: 0.9 to 1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1 to 0.25%, V: 0.1 to 0.25%, and the remainder consisting of Fe and unavoidable impurities, with Cr+Mn being 1.8% or less and Mo / V being 1.5 or less, and the microstructure containing 60% or more pearlite structure in C cross section.

[0026] This specification does not describe all elements of the embodiments, and content that is common in the technical field to which the present invention pertains or content that is overlapping between embodiments will be omitted.

[0027] Furthermore, when any part is said to "comprise" a certain component, this does not mean that it excludes other components, but that it may further include other components, unless otherwise specified to the contrary.

[0028] The singular expression includes the plural expression unless the context clearly dictates otherwise.

[0029] The present invention will be described in detail below.

[0030] The following examples are presented to fully convey the concept of the present invention to those skilled in the art, but the present invention is not limited to the examples set forth herein and may be embodied in other forms.

[0031] The inventors of the present invention have discovered that productivity can be improved in the steel wire manufacturing process and nitriding properties can be secured in the spring manufacturing process by controlling the grain size during the wire manufacturing process, suppressing the generation of nitrides, distributing fine carbides as uniformly as possible, and minimizing low-temperature structures during the wire cooling process, and have proposed the present invention.

[0032] A spring wire rod having improved fatigue resistance and nitriding properties according to one embodiment of the present invention contains, by weight, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.5%, Cr: 0.9-1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1-0.25%, V: 0.1-0.25%, and the remainder being Fe and unavoidable impurities.

[0033] The reasons for limiting the numerical values ​​of the alloy element contents in the examples of the present invention will be explained below. Unless otherwise specified, the unit is % by weight.

[0034] The C content is 0.6 to 0.7%. C is an element added to ensure product strength. If the C content is less than 0.6%, the sufficient strength targeted in the present invention cannot be achieved. If the C content exceeds 0.7%, impact properties will be significantly reduced after QT heat treatment, the possibility of low-temperature structures occurring during wire production will be significantly increased, resulting in poor wire quality. Furthermore, the LP heat treatment time during the steel wire manufacturing process will be significantly increased, resulting in poor productivity. Therefore, in the present invention, the C content is limited to 0.6-0.7%.

[0035] The Si content is 2.0 to 2.5%. Silicon is not only used for deoxidizing steel, but is also beneficial for ensuring strength through solid-solution strengthening. It is particularly essential for nitriding materials because it prevents wire rods from losing strength during nitriding. Furthermore, silicon significantly improves deformation resistance, an important spring characteristic. Therefore, in the present invention, silicon is added in an amount of 2.0% or more. However, silicon content exceeding 2.5% can induce surface decarburization, potentially degrading the workability of the material. Therefore, in the present invention, the silicon content is limited to 2.0-2.5% depending on the target strength and the degree of material processing.

[0036] The Mn content is 0.2 to 0.5%. Mn is an element that improves hardenability and is one of the elements that is essential for creating a high-strength tempered martensite structure. It is also an element necessary for fixing the impurity S as MnS and rendering it harmless. However, if the Mn content is less than 0.2%, the above-mentioned effects cannot be fully demonstrated, and if the Mn content exceeds 0.5%, segregation may cause deterioration in quality. Furthermore, since the steel type of the present invention ensures sufficient hardenability, it is not necessary to add Mn in an amount of 0.5% or more. Therefore, in the present invention, the Mn content is limited to 0.2 to 0.5%.

[0037] The Cr content is 0.9 to 1.6%. Cr, together with Mn, is effective in improving hardenability and significantly improves the softening resistance of steel during nitriding, making it an essential element for steels intended for nitriding. If the Cr content is less than 0.9%, the above-mentioned effects cannot be fully exhibited, and if the Cr content exceeds 1.6%, the toughness of the steel wire is significantly reduced. Therefore, in the present invention, the Cr content is limited to 0.9 to 1.6%.

[0038] Cr+Mn is 1.8% or less. If the Cr+Mn content exceeds 1.8%, low-temperature structures such as bainite or martensite may be formed during the cooling process of the wire rod, which may lengthen the time required to complete pearlite transformation during LP heat treatment. In the present invention, by controlling the Cr+Mn content to 1.8% or less, low-temperature structures such as bainite or martensite are reduced to 40% or less in the C-section phase (cross section perpendicular to the rolling direction) during cooling of the wire rod, ensuring a pearlite transformation completion time of 150 seconds or less during LP heat treatment.

[0039] The P content is 0.015% or less. P is an element that segregates at grain boundaries and reduces toughness, thereby reducing hydrogen-delayed fracture resistance. It is therefore preferable to eliminate it from steel materials as much as possible, so the upper limit is set at 0.015%.

[0040] The S content is 0.01% or less. Like P, S not only segregates at grain boundaries and reduces toughness, but also forms MnS, which can reduce hydrogen-delayed fracture resistance, so its addition amount is limited to 0.01% or less.

[0041] The Al content is 0.01% or less. Al is a strong deoxidizing element that removes oxygen from steel to increase its cleanliness. However, since Al can form Al2O3 inclusions that can reduce fatigue resistance, the Al content is limited to 0.01% or less in the present invention.

[0042] The N content is 0.007% or less. N forms coarse AlN or VN precipitates that are not dissolved during heat treatment by bonding with impurities, Al, or V. Therefore, in the present invention, the N content is limited to 0.007% or less.

[0043] The Mo content is 0.1 to 0.25%. Mo is an essential element added to improve softening resistance in materials for nitriding. Mo forms carbides with V to increase the strength of steel during tempering and maintain strength even after long-term heat treatment, so it is added at 0.1% or more. However, adding Mo in excess of 0.25% can inhibit pearlite formation, resulting in the formation of low-temperature structures after wire rolling, which can degrade wire quality. It also inhibits pearlite transformation during LP heat treatment before wire drawing, significantly reducing productivity. Therefore, in the present invention, the Mo content is limited to 0.25% or less.

[0044] The V content is 0.1 to 0.25%. V, along with Mo, is an essential element added to materials for nitriding to improve softening resistance. V also forms carbides, increasing the strength of steel during tempering and maintaining strength even during long-term nitriding. V also accelerates pearlite transformation, suppressing low-temperature microstructures during wire production and shortening the isothermal transformation time during LP heat treatment, thereby improving productivity during the steel wire manufacturing process. However, V can form coarse nitrides during wire production, and increasing the amount added requires a higher furnace temperature during wire rolling. Therefore, the upper limit of V content is limited to 0.25%.

[0045] The Mo / V ratio is 1.5 or less. As mentioned above, Mo is an essential element for improving the softening resistance of materials for nitriding. However, Mo inhibits the formation of the pearlite structure targeted in the present invention, so it is added together with V, which shortens the pearlite transformation time, and the Mo / V ratio is limited to 1.5 or less. By limiting the Mo / V ratio to 1.5 or less, the formation of low-temperature structures during cooling of the wire rod is suppressed, and the time to complete pearlite transformation during LP heat treatment can be shortened.

[0046] The balance outside the alloy composition is Fe. The spring wire rod having improved fatigue resistance and nitriding properties according to the present invention may contain other impurities that are typically contained in the industrial production process of steel. Since these impurities are known to anyone skilled in the art, the present invention does not specifically limit the type and content of the impurities.

[0047] Furthermore, in the spring wire rod according to the present invention having improved fatigue resistance and nitriding properties, the prior austenite grain size of the steel material as a fine structure may be an average of 25 μm or less.

[0048] Furthermore, the spring wire rod according to the present invention, which has improved fatigue resistance and nitriding properties, can contain 60% or more of a pearlite structure and the remaining bainite or martensite structure when viewed from a C-section (a cross section perpendicular to the rolling direction), and preferably contains 80% or more of a pearlite structure.

[0049] In addition, the spring wire rod according to the present invention, which has improved fatigue resistance and nitriding properties, has an average grain size of 10 μm or more at a surface depth of 1 mm per 10 mm in length when analyzed on a 100 mm L cross section (a cross section parallel to the rolling direction), and the number of VN precipitates containing 10 at% or more of V is less than 0.2.

[0050] Furthermore, the spring wire rod according to the present invention, which has improved fatigue resistance and nitriding properties, is 10×10 μm 2There are 10 or more (V, Mo)C precipitates, each of which accounts for 10 at. % or more by area and has an average grain size of 50 nm or less.

[0051] Furthermore, the spring wire rod according to the present invention, which has improved fatigue resistance and nitriding properties, may have a tensile strength of 1,400 MPa or less.

[0052] Next, a method for manufacturing a spring wire rod having improved fatigue resistance and nitriding properties according to an embodiment of the present invention will be described.

[0053] The spring wire rod having improved fatigue resistance and nitriding properties according to the present invention may be manufactured by various methods, and the manufacturing method is not particularly limited. However, as an example, the wire rod may be manufactured by the following method.

[0054] The spring wire rod according to the present invention, which has improved fatigue resistance and nitriding properties, is made of, by weight %, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.5%, Cr: 0.9 to 1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1 to 0.25%, V: 0.1 to 0.25%, and the remainder is bloom ( The method includes the steps of heating the molten metal (bloom) to a temperature of 1,200°C or higher and rolling it into a billet, maintaining the billet at a temperature of 1,050°C or higher for 180 minutes, extracting the billet at 1,030 to 1,060°C and rolling it at a temperature of 1,000°C or lower to obtain a wire rod, coiling the rolled wire rod at a temperature of 900°C or lower, and cooling the coiled wire rod at a cooling rate of 2°C / sec or lower.

[0055] Each manufacturing step will now be described in more detail.

[0056] First, a bloom satisfying the above-mentioned composition is heated to a temperature of 1,200°C or higher, and then the steel slab is rolled to obtain a billet.

[0057] The heating process is a process for removing all nitrides inside the steel, and if the temperature is less than 1,200°C, the nitrides are not sufficiently removed, so the lower limit of the heating temperature is limited to 1,200°C.

[0058] The rolled billet is then maintained at a temperature of 1,050°C or higher for 180 minutes. If the heating temperature is below 1,050°C or the heating time is less than 180 minutes, the V will not be sufficiently dissolved in the material, making it impossible to obtain the desired carbide formation. If sufficient carbide formation is not achieved, the softening resistance of the final product may be poor. Therefore, in the present invention, the billet heating temperature is limited to 1,050°C or higher, and the maintenance time is limited to 180 minutes.

[0059] The heated billet is extracted at a temperature of 1,030-1,060°C and then rolled into wire rod at a temperature of 1,000°C or less. If the wire rod rolling temperature is above 1,000°C, the prior austenite grain size may become large and surface decarburization by silicon may occur during the cooling process of the wire rod. In addition, since the coiling temperature in the subsequent coiling process is limited to 900°C or less, the wire rod rolling temperature is limited to 1,000°C or less as much as possible.

[0060] The rolled wire rod is coiled at a temperature of 900°C or less and slowly cooled at a cooling rate of 0.5 to 2°C / sec to obtain the wire rod according to the present invention. If the cooling rate is less than 0.5°C / sec, decarburization may occur, and if the cooling rate exceeds 2°C / sec, the formation of low-temperature structures may cause fractures in the material. Therefore, in the present invention, the cooling rate after coiling is limited to 0.5 to 2°C / sec.

[0061] The spring wire rod produced according to the present invention can contain 60% or more pearlite structures and the remaining bainite or martensite structures when viewed from a C-section (a cross section perpendicular to the rolling direction), and preferably contains 80% or more pearlite structures.

[0062] In addition, in the spring wire rod according to the present invention, the average prior austenite grain size of the steel material is 25 μm or less. In addition, in a 100 mm long cross section (cross section parallel to the rolling direction) analysis, the average grain size is 10 μm or more at a surface depth of 1 mm per 10 mm, and the number of VN precipitates containing 10 at% or more of V is less than 0.2, and the average grain size is 10 × 10 μm. 2 The Mo+V content in the carbide is 10 at.% or more by area, and there are 10 or more (V, Mo)C precipitates with an average grain size of 50 nm or less.

[0063] That is, the spring wire rod according to the present invention has a controlled prior austenite grain size, suppressed VN nitrides, and uniformly distributed (V,Mo)C carbides, thereby ensuring excellent nitriding characteristics during the spring manufacturing process.

[0064] The spring wire rod according to the present invention may have a tensile strength of 1,400 MPa or less and a cross-sectional area reduction rate of 40% or more.

[0065] Next, a method for manufacturing a spring steel wire using the spring wire rod according to the present invention will be described. The spring steel wire according to the present invention is manufactured by drawing the manufactured wire rod.

[0066] A method for manufacturing a high-strength spring steel wire according to an embodiment of the present invention includes the steps of subjecting a wire rod according to the present invention to LA heat treatment, LP heat treatment, and drawing the wire rod to manufacture a steel wire.

[0067] First, the wire rod manufactured according to the present invention is subjected to low temperature annealing (LA) at 650 to 750°C. Through the LA heat treatment, the strength of the wire rod is reduced to 1,200 MPa or less. In the present invention, the LA heat treatment can be omitted if necessary.

[0068] The LA heat-treated wire is then pickled and heated to a temperature of 900-1050°C for 10 minutes or less, with a holding time of 5 minutes or less. The reheating in the 900-1050°C furnace is intended to obtain an austenite structure, and the heat treatment time is limited to 15 minutes or less. If the heat treatment time exceeds 15 minutes, the austenite structure may become coarse, so the holding time is limited to 15 minutes or less. The heated wire is then passed through a lead patenting (LP) at 650-750°C for 3 minutes or less, where it is quenched and isothermally transformed to obtain a pearlite structure. If the isothermal holding temperature during lead patenting is less than 650°C, a low-temperature structure may be formed, and if it exceeds 750°C, the density of the pearlite structure may decrease. Therefore, the lead patenting temperature is limited to 650-750°C. Furthermore, in the present invention, by controlling the Mo / V ratio to 1.5 or less and the Mn+Cr content to 1.8% or less, the pearlite transformation time can be shortened to 150 seconds or less, and productivity can be ensured in the LP heat treatment process.

[0069] The LP heat-treated wire is then drawn to produce a steel wire. The diameter of the produced steel wire may be 5 mm, and if the LP heat treatment is performed again, the diameter may be 2 mm or less.

[0070] The steel wire produced according to the present invention may have a pearlite structure.

[0071] The pearlitic steel wire according to the present invention can be transformed into a tempered martensite structure through a QT process in order to ensure the ultra-high strength and toughness of 2,100 MPa or more that are the target of the present invention.

[0072] To obtain a tempered martensite structure, first, the pearlitic steel wire manufactured according to the present invention is heated to a temperature of 900 to 1,000°C for 5 minutes or less and maintained at that temperature for 10 minutes or less. The heated steel wire is then quenched in oil at 70°C or less and maintained at that temperature for 2 minutes or less to obtain a martensite structure steel wire.

[0073] Subsequently, the quenched martensite steel wire is reheated at a temperature of 450 to 500°C for 3 minutes or less and maintained at that temperature for 5 minutes or less, and the reheated steel wire is quenched again in oil at 70°C or less to obtain a final tempered martensite steel wire for high strength springs.

[0074] In this case, the quenching temperature is 450 to 550°C. If the quenching temperature is less than 450°C, the strength becomes too high and sufficient toughness cannot be ensured, and if the quenching temperature exceeds 550°C, the strength targeted in the present invention cannot be ensured, so the quenching temperature is limited to 450 to 500°C.

[0075] The high-strength spring steel wire produced according to the present invention can contain 90% or more of tempered martensite as its microstructure.

[0076] In the high-strength spring steel wire according to the present invention, the average prior austenite grain size of the steel material is 25 μm or less, and when analyzed in a 100 mm L-section (a section parallel to the rolling direction), no VN precipitates with an average grain size of 10 μm or more are present within a surface depth of 1 mm per 10 mm. 2 The Mo+V content in the carbide is 10 at.% or more by area, and there are 10 or more (V, Mo)C precipitates with an average grain size of 50 nm or less.

[0077] That is, the spring steel wire according to the present invention has a controlled prior austenite grain size, suppressed VN nitrides, and uniformly distributed (V,Mo)C carbides, thereby ensuring excellent nitriding characteristics during the spring manufacturing process.

[0078] The high-strength spring steel wire according to the present invention may have a tensile strength of 2,100 MPa or more, and a reduction in area (RA) of the steel wire may be 40% or more.

[0079] Next, a method for manufacturing a transmission spring using the high-strength spring steel wire according to the present invention will be described. The transmission spring according to the present invention can be manufactured by a conventional method. However, as an example, it can be manufactured by the following method.

[0080] For example, the high strength spring steel wire according to the present invention can be cold formed into a spring, annealed, nitrided, and shot peened to produce a spring for a final transmission.

[0081] In this case, the nitriding treatment can be carried out at a temperature of 420-450°C for 10 hours or more. If the nitriding temperature is below 420°C, nitrogen cannot penetrate the surface properly, and if the nitriding temperature is above 450°C, the hardness of the core of the material decreases, making it impossible to ensure the spring strength targeted in the present invention. Therefore, in the present invention, the nitriding temperature and maintenance time are limited to a temperature of 420-450°C and 10 hours or more.

[0082] The transmission spring according to the present invention can ensure excellent nitriding characteristics during the spring manufacturing process by controlling the size of prior austenite grains in the spring steel wire, suppressing VN nitrides, and uniformly distributing (V,Mo)C carbides.

[0083] The fatigue limit of a nitriding-treated spring can be improved by 10% or more compared to that before nitriding. Here, fatigue limit means the limit at which the spring can withstand repeated loads of 107 or more times during fatigue testing after design.

[0084] Furthermore, the strength of the spring after nitriding treatment can be suppressed to 10% or less compared to before nitriding treatment.

[0085] Furthermore, the spring after nitriding treatment may have a surface hardness of 800 Hv or more, and a hardness of 600 Hv or more from 1 / 4 to 3 / 4 of the wire diameter in a C cross section (cross section perpendicular to the rolling direction).

[0086] Furthermore, the nitriding treatment according to the present invention can be carried out at a high temperature of 420 to 450° C., so that the nitriding treatment time can be shortened and the nitriding treatment characteristics are excellent.

[0087] The present invention will be described in more detail below through examples. However, the description of these examples is merely for the purpose of illustrating the implementation of the present invention, and the present invention is not limited by the description of these examples. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. Example

[0088] Materials having the alloy compositions shown in Table 1 below were cast into ingots, which were then homogenized at 1,200°C. The temperature was then lowered from 1,050°C to 830°C, hot-rolled to a final wire diameter of 6.5 mm, and cooled at a rate of 1°C / sec to obtain wire specimens for the invention and comparative examples. The wire specimens for the invention and comparative examples were then measured for tensile strength, area reduction (RA), pearlite fraction, average prior austenite size, number of precipitates of 50 nm or less in transmission electron microscope (TEM) analysis, and number of VN nitrides of 10 μm or more in size within 1 mm of the surface depth per 10 mm in a 100 mm long L-plane analysis using a scanning electron microscope (SEM). The results are shown in Table 2 below.

[0089] [Table 1]

[0090] [Table 2]

[0091] Next, 6.5 mm test pieces of the wire rods of the comparative example and the inventive example were heat-treated at 720°C for 2 hours (LA heat treatment), pickled, heated at 980°C for 3 minutes, and then isothermal heat-treated at 680°C (LP heat treatment). The time to transform into pearlite through the isothermal heat treatment (LP heat treatment) was measured and is shown in Table 3 below, along with the Mo / V ratio and Cr+Mn value.

[0092] [Table 3]

[0093] Next, the pearlite wire specimens were drawn to produce steel wires with a wire diameter of 3 mm. The produced steel wires were heat-treated at 950°C for 5 minutes, quenched in oil at 60°C, and tempered at temperatures of 450-500°C for 3 minutes or less to obtain QT steel wire specimens. The tensile strength and RA of the QT steel wire specimens were measured, and the results are shown in Table 4 below. The QT steel wire specimens were then manufactured into springs using the manufacturing method of the present invention, and fatigue tests were conducted under conditions of R (tensile stress / compressive stress) = -1 to measure fatigue strength. The QT steel wires were then nitrided at 450°C, and the hardness was measured at 10 or more points from 1 / 4 to 3 / 4 of the depth of the C cross section, and the average was calculated. A fatigue test was conducted under the same conditions as before the nitriding treatment to measure fatigue strength, and the results are shown in Table 4 below.

[0094] [Table 4]

[0095] As shown in Tables 2 to 4, in Examples 1 and 2, which satisfy all of the alloy compositions and manufacturing conditions of the present invention, the tensile strength of the wire rods is 1,400 MPa or less, the pearlite fraction is 60% or more, and the number of (Mo, V)C precipitates is 10 × 10 μm 2 It was confirmed that the area of ​​the steel wire was more than 10, and the strength of the steel wire after QT heat treatment was more than 2,100 MPa, the RA was more than 40%, and the fatigue strength after nitriding treatment all increased by more than 10%. Meanwhile, in Comparative Example 1, the Mo content was excessive compared to V, and the pearlite fraction was 42%, so pearlite was not sufficiently formed upon cooling, the pearlite transformation time exceeded 150 seconds, and the tensile strength was more than 1,400 MPa. In Comparative Example 2, the V content was excessive at 0.3%, which caused coarse VN to crystallize on the surface, resulting in a low RA of 38% and a fatigue strength of 712 MPa, which was significantly inferior to the other test specimens.

[0096] In Comparative Example 3, excessive Mo was added, resulting in the formation of a low-temperature structure throughout the entire C cross section, resulting in a wire rod with a tensile strength of more than 1,400 MPa and a poor RA of 35%. In addition, the pearlite transformation time exceeded 150 seconds, making it impossible to ensure productivity.

[0097] In Comparative Example 4, the Mo / V ratio was satisfactory, but the Mn+Cr content exceeded 1.8%, resulting in the formation of a low-temperature structure like that of Comparative Example 3, resulting in a wire rod with a tensile strength of more than 1,400 MPa and a pearlite transformation time of more than 150 seconds, making it impossible to ensure productivity.

[0098] In Comparative Example 5, the fatigue strength of the OT steel wire was poor at 793 MPa due to the low V content, and the fatigue strength also decreased after the nitriding treatment, and the fatigue properties were not improved.

[0099] The present invention is not limited thereto, and it should be understood that a person having ordinary skill in the art can make various changes and modifications within the scope of the concept and scope of the claims set forth below. [Industrial Applicability]

[0100] According to one embodiment of the present invention, it is possible to provide a high-strength spring wire rod, steel wire, spring, and a method for manufacturing the same, which have improved fatigue resistance and nitriding treatment characteristics.

Claims

1. A steel sheet containing, by mass%, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.5%, Cr: 0.9-1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1-0.25%, V: 0.1-0.25%, with the remainder being Fe and unavoidable impurities, Cr+Mn is not more than 1.8%; Mo / V is 1.5 or less, The microstructure contains 60% or more pearlite structure in the C section and the remaining bainite or martensite structure, The average prior austenite grain size is 25 μm or less, In a 100 mm L cross section analysis length, the number of VN precipitates having an average grain size of 10 μm or more at a surface depth of 1 mm per 10 mm is less than 0.2, The Mo+V content in the carbide is 10 at. % or more, The average particle size is 50 nm or less, 10 or more pieces / 10×10μm ２ 1. A wire rod for springs having improved fatigue resistance and nitriding treatment properties, characterized by containing (V, Mo)C carbides distributed in the range of 1000 to 10000.

2. 2. The spring wire rod according to claim 1, wherein the wire rod has a tensile strength of 1,400 MPa or less and a cross-sectional area reduction rate of 40% or more.

3. 3. A method for manufacturing a spring wire rod according to claim 1 or 2, heating a bloom containing, by mass%, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.5%, Cr: 0.9 to 1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1 to 0.25%, V: 0.1 to 0.25%, and the remainder consisting of Fe and unavoidable impurities, wherein Cr+Mn is 1.8% or less and Mo / V is 1.5 or less, to a temperature of 1,200°C or more and then rolling the bloom into a billet; maintaining the billet at a temperature of 1,050°C or greater for 180 minutes; rolling the billet at a temperature of 1,000°C or less to obtain a wire rod; coiling the rolled wire at a temperature of 900°C or less; and A method for manufacturing a spring wire having improved fatigue resistance and nitriding properties, comprising the step of cooling the wound wire at a cooling rate of 2°C / sec or less.

4. A step of heating the spring wire rod according to claim 1 or 2 to a temperature of 900 to 1050°C and quenching it to 650 to 750°C to isothermally transform it; The method for manufacturing a spring steel wire having improved fatigue resistance and nitriding properties, comprising the step of drawing the wire rod to manufacture a steel wire.

5. heating the wire to 650 to 750°C before the isothermal transformation; and 5. The method for manufacturing a spring steel wire having improved fatigue resistance and nitriding properties according to claim 4, further comprising the step of pickling the wire heated to 650 to 750°C.

6. 5. The method for producing a spring steel wire having improved fatigue resistance and nitriding properties according to claim 4, wherein the isothermal transformation time is 150 seconds or less.

7. 5. The method of claim 4, further comprising the step of subjecting the steel wire to a QT heat treatment.

8. A steel sheet containing, in mass%, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.5%, Cr: 0.9 to 1.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.007% or less, Mo: 0.1 to 0.25%, V: 0.1 to 0.25%, with the remainder consisting of Fe and unavoidable impurities, wherein Cr+Mn is 1.8% or less and Mo / V is 1.5 or less, The microstructure contains tempered martensite at an area fraction of 90% or more, The average prior austenite grain size is 25 μm or less, The tensile strength is 2,100 MPa or more, and the reduction in area is 40% or more. In a 100 mm L cross section analysis length, the number of VN precipitates having an average grain size of 10 μm or more at a surface depth of 1 mm per 10 mm is less than 0.2, The Mo+V content in the carbide is 10 at. % or more, The average particle size is 50 nm or less, 10 or more pieces / 10×10μm ２ A spring steel wire having improved fatigue resistance and nitriding treatment properties, characterized by containing (V, Mo)C carbides distributed in the range of 0.1 to 1.0.

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