Wire rod for spring, method for manufacturing same, and steel wire for spring manufactured using same

Abstract

A wire rod for a spring, according to one embodiment of the present invention, comprises: 0.45-0.65 wt% of carbon (C); 1.20-2.00 wt% of silicon (Si); 0.30-1.00 wt% of manganese (Mn); 0.015 wt% or less (excluding 0) of phosphorus (P); 0.015 wt% or less (excluding 0) of sulfur (S); 0.01 wt% or less (excluding 0) of nitrogen (N); at least one element of 0.20 wt% or less of vanadium (V), 0.050 wt% or less of niobium (Nb), and 0.050 wt% or less of titanium (Ti); and the balance consisting of iron (Fe) and other unavoidable impurities, wherein the following equation 1 is satisfied. [Equation 1] 5 ≤ (V+Nb+Ti) / N (In equation 1, V, Nb, Ti, and N respectively represent the atomic weight ratios of vanadium, niobium, titanium, and nitrogen).

Description

Spring wire, manufacturing method thereof, and spring steel wire manufactured using the same

[0001] The present invention relates to a wire for a spring, a method for manufacturing the same, and a steel wire for a spring manufactured using the same.

[0002] Reducing the weight of vehicle components is the most effective way to improve the driving range of electric vehicles and the fuel efficiency of internal combustion engine vehicles.

[0003] In response to this demand for lightweighting, suspension springs used in recent vehicle suspension systems are being manufactured using high-strength materials with excellent strength after Quenching Tempering (QT) treatment.

[0004] In particular, suspension springs require excellent fatigue resistance and corrosion resistance as they are prone to defects caused by corrosion pitting when exposed to the external environment.

[0005] Specifically, suspension springs can be damaged by cracks that propagate from corrosion pits that originate at points where paint has peeled off, and hydrogen embrittlement can occur as the inflowing hydrogen becomes concentrated in the cracks.

[0006] Accordingly, the automotive parts industry urgently needs to develop technology for spring materials with improved corrosion resistance and corrosion fatigue properties.

[0007] The present invention has been devised to solve the above problems, and its purpose is to provide a spring wire having excellent corrosion resistance and corrosion fatigue resistance properties, a method for manufacturing the same, and a spring steel wire manufactured using the same.

[0008] The tasks of the present invention are not limited to the tasks mentioned above, and other tasks not mentioned will be clearly understood by those skilled in the art from the description below.

[0009] According to one embodiment of the present invention, a spring wire comprises carbon (C) 0.45 to 0.65 wt%, silicon (Si) 1.20 to 2.00 wt%, manganese (Mn) 0.30 to 1.00 wt%, phosphorus (P) 0.015 wt% or less (excluding 0), sulfur (S) 0.015 wt% or less (excluding 0), nitrogen (N) 0.01 wt% or less (excluding 0), and at least one element selected from vanadium (V) 0.20 wt% or less, niobium (Nb) 0.050 wt% or less, and titanium (Ti) 0.050 wt% or less, and the remainder comprises iron (Fe) and other inevitable impurities, and satisfies the following formula 1.

[0010] [Formula 1]

[0011] 5 ≤ (V+Nb+Ti) / N

[0012] (In the above formula 1, V, Nb, Ti and N represent the atomic weight ratio of vanadium, the atomic weight ratio of niobium, the atomic weight ratio of titanium and the atomic weight ratio of nitrogen, respectively).

[0013] According to one embodiment of the present invention, the spring wire may further include 0.15 to 0.60 wt% of copper (Cu), 0.15 to 0.80 wt% of nickel (Ni), 0.20 to 0.50 wt% of chromium (Cr), and 0.05 to 0.20 wt% of molybdenum (Mo).

[0014] The final microstructure of the spring wire according to one embodiment of the present invention may include pearlite of 60% or more.

[0015] According to one embodiment of the present invention, the spring wire has a number of precipitates having a diameter of 100 nm or less in the final microstructure of 2.3×10 7 dog / mm 2 It could be strange.

[0016] In the final microstructure of the spring wire according to one embodiment of the present invention, the average size of the austenite crystal grains may be 20 ㎛ or less.

[0017] The spring wire according to one embodiment of the present invention may have a tensile strength of 1,100 MPa or less.

[0018] A method for manufacturing a spring wire according to one embodiment of the present invention includes (S1) a melting step for melting a raw material, (S2) a continuous casting step for manufacturing a first semi-finished product, and (S3) a rolling step for hot rolling the first semi-finished product.

[0019] The above step (S3) includes (S3-1) a first reheating step of first heating the first semi-finished product, (S3-2) a first rolling step of rolling the first heated first semi-finished product to manufacture a second semi-finished product, (S3-3) a first reheating step of second heating the second semi-finished product, and (S3-4) a second rolling step of rolling the second heated second semi-finished product to manufacture a wire rod.

[0020] According to one embodiment of the present invention, the step (S3-1) may be performed at 1,150 to 1,300°C, and the step (S3-3) may be performed at 950 to 1,050°C.

[0021] According to one embodiment of the present invention, the finishing rolling temperature in the step (S3-4) may be 800 to 1,000°C.

[0022] According to one embodiment of the present invention, the step (S3) may further include a winding step (S3-5) of winding the wire at 850 to 950°C.

[0023] According to one embodiment of the present invention, the step (S3) may further include a cooling step (S3-6) of cooling the wire at a cooling rate of 3°C / sec or less.

[0024] According to one embodiment of the present invention, the wire that has gone through the (S3) step contains 0.45 to 0.65 wt% of carbon (C), 1.20 to 2.00 wt% of silicon (Si), 0.30 to 1.00 wt% of manganese (Mn), 0.015 wt% or less of phosphorus (P) (excluding 0), 0.015 wt% or less of sulfur (S) (excluding 0), 0.001 to 0.010 wt% of nitrogen (N), and at least one element of 0.20 wt% or less of vanadium (V), 0.050 wt% or less of niobium (Nb), and 0.050 wt% or less of titanium (Ti), with the remainder containing iron (Fe) and other inevitable impurities.

[0025] A wire manufactured by a method for manufacturing a wire according to one embodiment of the present invention satisfies the following equation 1.

[0026] [Formula 1]

[0027] 5 ≤ (V+Nb+Ti) / N

[0028] In the above formula 1, V, Nb, Ti and N represent the atomic weight ratio of vanadium, the atomic weight ratio of niobium, the atomic weight ratio of titanium and the atomic weight ratio of nitrogen, respectively.

[0029] The wire manufactured by the method for manufacturing a wire according to one embodiment of the present invention may further include 0.15 to 0.60 wt% of copper (Cu), 0.15 to 0.80 wt% of nickel (Ni), 0.20 to 0.50 wt% of chromium (Cr), and 0.05 to 0.20 wt% of molybdenum (Mo).

[0030] According to one embodiment of the present invention, in the step (S1), the raw material may include at least one of iron ore, reduced iron, and iron scrap.

[0031] According to one embodiment of the present invention, the step (S1) may be performed in at least one of a blast furnace and an electric furnace.

[0032] According to one embodiment of the present invention, a spring steel wire contains carbon (C) 0.45 to 0.65 wt%, silicon (Si) 1.20 to 2.00 wt%, manganese (Mn) 0.30 to 1.00 wt%, phosphorus (P) 0.015 wt% or less (excluding 0), sulfur (S) 0.015 wt% or less (excluding 0), nitrogen (N) 0.001 to 0.010 wt%, and at least one element among vanadium (V) 0.20 wt% or less, niobium (Nb) 0.050 wt% or less, and titanium (Ti) 0.050 wt% or less, and the remainder includes iron (Fe) and other inevitable impurities, and the number of precipitates having a circle-equivalent diameter of 100 nm or less in the final microstructure is 2.1×10 8 dog / mm 2 It could be strange.

[0033] The tensile strength of the steel wire for a spring according to one embodiment of the present invention may be 2000 MPa or more.

[0034] The hydrogen embrittlement fracture strength of the spring steel wire according to one embodiment of the present invention may be 1300 MPa or more.

[0035] A steel wire for a spring according to one embodiment of the present invention can satisfy the following equation 1.

[0036] [Formula 1]

[0037] 5 ≤ (V+Nb+Ti) / N

[0038] (In the above formula 1, V, Nb, Ti and N represent the atomic weight ratio of vanadium, the atomic weight ratio of niobium, the atomic weight ratio of titanium and the atomic weight ratio of nitrogen, respectively).

[0039] According to one embodiment of the present invention, a spring wire having improved corrosion resistance and corrosion fatigue resistance by controlling the formation of fine precipitates and the average size of crystal grains, a method for manufacturing the same, and a spring steel wire manufactured using the same can be provided.

[0040] The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of the claims.

[0041] Figure 1 is a flowchart showing a method for manufacturing a wire for a spring according to one embodiment of the present invention.

[0042] Figure 2 is a flowchart showing detailed steps of the (S3) rolling step shown in Figure 1.

[0043] Figure 3 is a flowchart showing a method for manufacturing a steel wire for a spring according to one embodiment of the present invention.

[0044] Hereinafter, with reference to the attached drawings, preferred embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be implemented in various different forms and is not limited or restricted by the following examples.

[0045] Additionally, when a component (or region, layer, portion, etc.) is referred to as being "on," "connected to," or "coupled to" another component, it means that it can be directly placed / connected / coupled to the other component, or that a third component may be placed between them.

[0046] Terms such as "include" or "have" should be understood to specify the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, but not to exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0047] In order to clearly explain the present invention, a detailed description of a part that is irrelevant to the description or a related known technology that may unnecessarily obscure the gist of the present invention has been omitted, and when adding reference signs to components of each drawing in this specification, the same or similar reference signs are attached to the same or similar components throughout the specification.

[0048] In addition, terms and words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0049] Unless otherwise specified, the notation 'A ~ B' for numerical values ​​A and B means 'A or more and B or less'. In such notation, if a unit is attached only to numerical value B, the unit shall be applied to numerical value A as well.

[0050] Hereinafter, embodiments of the present invention will be described in detail.

[0051]

[0052] Wire for springs

[0053] According to one embodiment of the present invention, a spring wire contains carbon (C) 0.45 to 0.65 wt%, silicon (Si) 1.20 to 2.00 wt%, manganese (Mn) 0.30 to 1.00 wt%, phosphorus (P) 0.015 wt% or less (excluding 0), sulfur (S) 0.015 wt% or less (excluding 0), copper (Cu) 0.15 to 0.60 wt%, nickel (Ni) 0.15 to 0.80 wt%, chromium (Cr) 0.20 to 0.50 wt%, molybdenum (Mo) 0.05 to 0.20 wt%, nitrogen (N) 0.001 to 0.010 wt%, the remainder iron (Fe) and other unavoidable impurities.

[0054] Hereinafter, the role and content of each alloy element included in the spring wire according to one embodiment of the present invention will be described in detail.

[0055]

[0056] Carbon (C)

[0057] Carbon (C) is an effective element for increasing strength. Carbon (C) is incorporated into austenite during heat treatment and forms martensite during quenching. While increasing carbon (C) content improves strength, it also reduces toughness.

[0058] If carbon (C) is added below the preset range, it is difficult to secure the strength of the final product, the spring.

[0059] Conversely, if carbon (C) is added in excess of the preset range, it may form a twin-structured plate-like martensite structure during subsequent quenching tempering (QT) heat treatment, thereby reducing fatigue life and toughness.

[0060] Additionally, when carbon (C) is added in excess of the preset range, surface defect sensitivity increases, which can rapidly reduce fatigue life when corrosion pits occur.

[0061] In addition, when carbon (C) is added in excess of the preset range, cross-sectional shrinkage may decrease along with a decrease in toughness, and fresh workability may decrease due to the formation of proeutectoid cementite.

[0062] Accordingly, in the spring wire according to one embodiment of the present invention, the content of carbon (C) can be controlled to 0.45 to 0.65 wt%, preferably 0.48 to 0.62 wt%.

[0063]

[0064] Silicon (Si)

[0065] Silicon (Si) is an element that is effective in increasing strength and deoxidation, and increases carbon activity. Furthermore, silicon (Si) can contribute to improving the deformation resistance of the final product, the spring.

[0066] If silicon (Si) is added below the preset range, it may be difficult to secure the deoxidation effect and strength.

[0067] Conversely, if silicon (Si) is added in excess of the preset range, it may reduce toughness and make it difficult to remove surface scale during the acid pickling process.

[0068] In addition, if silicon (Si) is added in excess of the preset range, it may excessively promote decarburization, which may reduce fatigue resistance and fresh workability as the thickness of the decarburization layer increases.

[0069] Accordingly, in the spring wire according to one embodiment of the present invention, the content of silicon (Si) can be controlled to 1.2 to 2.0 wt%, preferably 1.24 to 1.95 wt%.

[0070]

[0071] manganese (Mn)

[0072] Manganese (Mn) is an element that increases the strength of wire rods while being effective in deoxidation. It also forms MnS inclusions, preventing red-hot embrittlement. Manganese (Mn) also reduces carbon activity. Furthermore, Mn can enhance hardenability during QT heat treatment.

[0073] When manganese (Mn) is added below the preset range, the deoxidation effect is reduced, it is difficult to secure strength, and there is a problem of causing red-hot embrittlement.

[0074] On the other hand, if manganese (Mn) is added below the preset range, the effect of inhibiting decarburization is insufficient, which may reduce the fresh workability as the thickness of the decarburization layer increases.

[0075] Additionally, if manganese (Mn) is added in excess of the preset range, segregation may occur in the center of the wire rod, which may reduce the wire drawing processability.

[0076] Additionally, if manganese (Mn) is added in excess of the preset range, the corrosion resistance and fatigue properties may deteriorate due to MnS inclusions.

[0077] Accordingly, in the spring wire according to one embodiment of the present invention, the content of manganese (Mn) can be controlled to 0.3 to 1.0 wt%, preferably 0.32 to 0.92 wt%.

[0078]

[0079] Person (P)

[0080] Phosphorus (P) is a residual element that must be removed during the steelmaking process. If the phosphorus content exceeds 0.015 wt%, it can cause grain boundary segregation and the formation of Fe3P compounds, which can lead to embrittlement.

[0081] Accordingly, in the spring wire according to one embodiment of the present invention, the content of phosphorus (P) can be controlled to 0.015 wt% or less.

[0082]

[0083] Yellow (S)

[0084] Sulfur (S) is a residual element that must be removed during the steelmaking process and forms non-metallic inclusions such as MnS.

[0085] When the sulfur (S) content exceeds 0.015 wt%, the corrosion resistance fatigue properties may be deteriorated due to excessive generation of sulfides such as MnS.

[0086] Accordingly, in the spring wire according to one embodiment of the present invention, sulfur (S) can be controlled to 0.015 wt% or less.

[0087]

[0088] copper (Cu)

[0089] Copper (Cu) can increase strength through its solid-solution strengthening effect. Furthermore, copper (Cu) can enhance corrosion resistance by forming a dense oxide layer on the surface of the wire.

[0090] If copper (Cu) is added below the preset range, the effect of improving corrosion resistance may be insufficient, resulting in poor corrosion resistance fatigue properties.

[0091] Conversely, if copper (Cu) is added in excess of the preset range, it may cause cracks on the surface during high-temperature wire rolling.

[0092] Accordingly, in the spring wire according to one embodiment of the present invention, the content of copper (Cu) can be controlled to 0.15 to 0.60 wt%, preferably 0.17 to 0.57 wt%.

[0093]

[0094] Nickel (Ni)

[0095] Nickel (Ni) is an element that increases strength and improves corrosion resistance through its solid-solution strengthening effect. Furthermore, nickel (Ni) can effectively reduce surface cracks caused by copper (Cu)-concentrated layers during hot rolling.

[0096] When nickel (Ni) is added below the preset range, the effect of improving toughness and corrosion resistance is insufficient, making it difficult to secure corrosion-resistant fatigue characteristics.

[0097] However, if nickel (Ni) is added beyond the preset range, the fatigue life of the product may be reduced due to excessive formation of retained austenite. Furthermore, excessive nickel (Ni) addition can lead to increased manufacturing costs.

[0098] Accordingly, in the spring wire according to one embodiment of the present invention, the content of nickel (Ni) can be controlled to 0.15 to 0.80 wt% or less, preferably 0.19 to 0.75 wt%.

[0099]

[0100] chromium (Cr)

[0101] Chromium (Cr) is an element that increases the strength of wire rods and contributes to improved hardenability during QT heat treatment. Chromium also enhances the oxidation resistance of wire rods. Furthermore, chromium effectively reduces carbon activity, thereby minimizing decarburization.

[0102] If chromium (Cr) is added below the preset range, it is difficult to secure the required strength, and the effect of suppressing decarburization is insufficient, which may reduce the fresh workability as the thickness of the decarburization layer increases.

[0103] When chromium (Cr) is added in excess of the preset range, it can reduce fatigue strength by lowering deformation resistance.

[0104] Additionally, if chromium (Cr) is added in excess of the preset range, it may increase the depth of corrosion pitting points, thereby deteriorating corrosion resistance and fatigue properties.

[0105] Furthermore, if chromium (Cr) is added in excess of the preset range, it can cause short circuits during subsequent drawing processes due to low-temperature tissue formation. Furthermore, chromium (Cr) is a high-grade element, and excessive addition can lead to increased manufacturing costs.

[0106] Accordingly, in the spring wire according to one embodiment of the present invention, the content of chromium (Cr) can be controlled to 0.2 to 0.5 wt%, preferably 0.22 to 0.36 wt%.

[0107]

[0108] molybdenum (Mo)

[0109] Molybdenum (Mo) improves the toughness and corrosion resistance of wire rods, and is particularly effective in enhancing hardenability and strength during QT heat treatment. Furthermore, molybdenum (Mo) forms complex precipitates with vanadium (V), niobium (Nb), and titanium (Ti), contributing to precipitate stabilization. Furthermore, it suppresses precipitate coarsening, contributing to grain refinement and enhanced delayed fracture resistance.

[0110] If molybdenum (Mo) is added below the preset range, sufficient corrosion resistance and fine precipitate formation may be insufficient.

[0111] If molybdenum (Mo) is added in excess of the preset range, it may cause the formation of low-temperature structures during wire rolling, thereby reducing the wire drawing processability.

[0112] Additionally, if molybdenum (Mo) is added in excess of the preset range, the precipitates may become coarser and the precipitation strengthening effect may be reduced.

[0113] Additionally, molybdenum (Mo) is a high-grade element, and excessive addition may lead to an increase in manufacturing cost.

[0114] Accordingly, in the spring wire according to one embodiment of the present invention, the content of molybdenum (Mo) can be controlled to 0.05 to 0.2 wt%, preferably 0.06 to 0.17 wt%.

[0115]

[0116] Nitrogen (N)

[0117] Nitrogen (N) combines with elements such as aluminum (Al), titanium (Ti), vanadium (V), and niobium (Nb) to form precipitates and is an effective element for refining the grain size of austenite.

[0118] However, if nitrogen (N) is added in excess of the preset range, a deviation in strength may occur due to excessive formation of precipitates, and this may adversely affect cold workability during fresh processing.

[0119] Accordingly, in the spring wire according to one embodiment of the present invention, the content of nitrogen (N) can be controlled to 0.01 wt% or less (excluding 0), preferably 0.008 wt% or less (excluding 0), and more preferably 0.007 wt% or less (excluding 0).

[0120]

[0121] According to one embodiment of the present invention, a spring wire comprises at least one element among 0.05 to 0.20 wt% of vanadium (V), 0.001 to 0.050 wt% of niobium (Nb), and 0.001 to 0.050 wt% of titanium (Ti) instead of the remaining iron (Fe).

[0122]

[0123] Vanadium (V)

[0124] Vanadium (V) is an element that improves the strength by forming fine precipitates at high temperatures and contributes to grain refinement, thereby improving the delayed fracture resistance of wire rods.

[0125] If vanadium (V) is added below the preset range, it may be difficult to secure sufficient strength and delayed fracture resistance due to insufficient formation of fine precipitates.

[0126] If vanadium (V) is added in excess of the preset range, the precipitate may become coarser and the precipitation strengthening effect may be insufficient.

[0127] In addition, vanadium (V) is a high-grade element, and excessive addition may lead to an increase in manufacturing cost.

[0128] Accordingly, in the spring wire according to one embodiment of the present invention, the content of vanadium (V) can be controlled to 0.20 wt% or less, preferably 0.073 to 0.146 wt%.

[0129]

[0130] niobium (Nb)

[0131] Niobium (Nb) is an element that is effective in improving strength and toughness by forming fine precipitates at high temperatures and in improving delayed fracture resistance by refining crystal grains.

[0132] If niobium (Nb) is added beyond the preset range, the amount of precipitates that do not dissolve in austenite during the solidification process after the continuous casting process increases, making it difficult to expect the effect of fine precipitate formation. Consequently, the properties of the final product (spring) deteriorate, and it may be difficult to secure sufficient delayed fracture resistance.

[0133] Additionally, niobium (Nb) is a high-grade element, and excessive addition may lead to an increase in manufacturing cost.

[0134] Accordingly, in the spring wire according to one embodiment of the present invention, the content of niobium (Nb) can be controlled to 0.05 wt% or less, preferably 0.033 wt% or less.

[0135]

[0136] titanium (Ti)

[0137] Titanium (Ti) is an element that forms fine precipitates at high temperatures to improve strength and toughness, and contributes to improving delayed fracture resistance through grain refinement.

[0138] If titanium (Ti) is added in excess of the preset range, the amount of precipitates that are not dissolved in austenite among the precipitates generated during the solidification process after the continuous casting process increases, making it difficult to expect the effect of forming fine precipitates.

[0139] Additionally, titanium (Ti) is a high-grade element, and excessive addition may lead to an increase in manufacturing cost.

[0140] Accordingly, in the spring wire according to one embodiment of the present invention, the content of titanium (Ti) can be controlled to 0.05 wt% or less, preferably 0.038 wt% or less.

[0141]

[0142] In addition to the aforementioned components, the remainder may contain iron and unavoidable impurities. Unavoidable impurities are impurities introduced during the manufacturing process, and as these are widely known in the field, a detailed description will be omitted.

[0143] In one embodiment of the present invention, the addition of elements other than the aforementioned alloy components is not excluded, and various elements may be included within a range that does not impair the technical spirit of the present invention. When additional elements are included, they may be included to replace the remaining Fe.

[0144]

[0145] The final microstructure of the spring wire according to one embodiment of the present invention may include fine precipitates. As used herein, the term "fine precipitate" refers to a precipitate having a circle-equivalent diameter of 100 nm or less. The fine precipitate may include at least one of vanadium (V), niobium (Nb), and titanium (Ti).

[0146] The final microstructure of the spring wire according to one embodiment of the present invention is a unit area (mm 2 ) 2.3 x 10 7 It may contain more than one fine precipitate.

[0147] In a spring wire according to one embodiment of the present invention, the average size of austenite grains may be 20 μm or less. In the present specification, the austenite grains may be austenite grains observed when a cooled wire is reheated to a temperature of 900°C or higher.

[0148] The inventors of the present invention have found that when the number of fine precipitates and the average size of austenite grains in the microstructure are controlled within the aforementioned ranges, the corrosion resistance fatigue properties can be drastically improved.

[0149] Typically, increasing the strength of a material reduces its toughness, which in turn increases its sensitivity to surface defects. Consequently, in springs, fatigue cracks can propagate from corrosion pits formed at points where paint has peeled off during operation, leading to cracks. Furthermore, externally introduced hydrogen can concentrate in the cracks, potentially leading to component damage due to hydrogen embrittlement. To overcome this problem, austenite grain refinement can be used to enhance toughness, thereby reducing sensitivity to surface defects. Furthermore, hydrogen embrittlement can be improved by dispersing hydrogen diffused into grain boundaries.

[0150] Furthermore, fine precipitates can enhance hydrogen embrittlement resistance by trapping hydrogen that has entered the material through corrosion pits. Conversely, coarse precipitates reduce the total number of trap sites, thereby diminishing the trapping effect of the intruded hydrogen, thereby degrading hydrogen embrittlement resistance. Based on these considerations, fatigue resistance can be improved under corrosive environments by controlling the number of fine precipitates and the average size of the austenite grains.

[0151] According to one embodiment of the present invention, in order to extremely precisely control the number of the aforementioned fine precipitates and the average size of the austenite crystal grains, it has been found through numerous studies and experiments that not only the alloy composition system described above but also the atomic weight ratio and content ratio between the elements contributing thereto must satisfy the following equations 1 and 2.

[0152] Specifically, vanadium (V), niobium (Nb), titanium (Ti), and nitrogen (N) satisfy the following equation 1.

[0153] [Formula 1]

[0154] 5 ≤ (V+Nb+Ti) / N

[0155] In the above formula 1, V, Nb, Ti and N represent the atomic weight ratio of vanadium, the atomic weight ratio of niobium, the atomic weight ratio of titanium and the atomic weight ratio of nitrogen, respectively.

[0156] The atomic mass ratio is the content (weight %) of an element divided by its average atomic mass. The average atomic mass of vanadium (V) is 50.9415, the average atomic mass of niobium (Nb) is 92.906, the average atomic mass of titanium is 47.87, and the average atomic mass of nitrogen is 14.007. The unit of average atomic mass is amu (atomic mass unit).

[0157] According to one embodiment of the present invention, fine precipitates can be formed by controlling the atomic ratio of vanadium, niobium, titanium, and nitrogen, which contribute to the formation of precipitates, by Equation 1.

[0158] According to one embodiment of the present invention, copper (Cu), nickel (Ni), molybdenum (Mo), vanadium (V), niobium (Nb), titanium (Ti), carbon (C), chromium (Cr), phosphorus (P), and sulfur (S) can satisfy the following equation 2.

[0159] [Formula 2]

[0160] 0.8 ≤ Cu+0.5Ni+6Mo(10V+18Nb+9Ti)-0.2C-0.6Cr-10(P+S)

[0161] In Equation 2, Cu, Ni, Mo, V, Nb, Ti, C, Cr, P, and S represent the content ratio of copper, nickel, molybdenum, vanadium, niobium, titanium, chromium, phosphorus, and sulfur, respectively, and the unit is weight%.

[0162] The final microstructure of the spring wire according to one embodiment of the present invention may include ferrite and pearlite. Specifically, pearlite may be at least 60%, with the remainder being ferrite. The final microstructure of the spring wire according to one embodiment of the present invention does not include a martensite structure.

[0163] The tensile strength of the spring wire according to one embodiment of the present invention may be 1100 MPa or less, and preferably 950 to 1100 MPa.

[0164] Hereinafter, a method for manufacturing a spring wire according to one embodiment of the present invention will be described in detail.

[0165]

[0166] Method for manufacturing wire for spring

[0167] Hereinafter, a method for manufacturing a wire for a spring according to one embodiment of the present invention will be described.

[0168] Figure 1 is a flowchart showing a method for manufacturing a wire rod having excellent fresh processing properties according to one embodiment of the present invention.

[0169] A method for manufacturing a wire rod having excellent fresh processing properties according to one embodiment of the present invention comprises (S1) a melting step for melting a raw material, (S2) a continuous casting step for manufacturing a first semi-finished product, and (S3) a rolling step for hot rolling the first semi-finished product.

[0170] According to one embodiment of the present invention, the (S1) melting step is a step of melting raw materials to produce molten metal. The (S1) melting step may be performed by at least one of a blast furnace and an electric furnace.

[0171] When using a blast furnace, raw materials may include iron ore and coke. Specifically, the blast furnace process involves loading the raw materials into the furnace and then simultaneously melting and reducing them using hot air.

[0172] The molten iron produced in the blast furnace can be turned into molten steel with the alloy composition controlled within a preset range and impurities removed through a subsequent steelmaking process.

[0173] When using an electric furnace, the raw material may include at least one of iron scrap and reduced iron. For example, the raw material may include 0 to 100 wt% direct reduced iron and the remainder iron scrap. The reduced iron may include direct reduced iron (DRI), hot briquettized iron (HBI), low reduced iron (LRI), etc.

[0174] Specifically, the electric furnace process can melt raw materials by loading them into an electric furnace and then using electrical energy. For example, the electric furnace can be an Electric Arc Furnace (EAF). An EAF melts raw materials through the heat generated by the arc between electrodes. While an EAF can be an alternating current (AC) system, it is not limited to this; a direct current (DC) system can also be used.

[0175] However, the types of electric furnaces are not limited to those described above. For example, other types of electric furnaces such as ESF (Electric Smelting Furnace) and EIF (Electric Induction Furnace) may also be used.

[0176] The method of using a blast furnace and an electric furnace together is a manufacturing method in which a portion of the molten iron produced in the blast furnace is charged into the electric furnace to ultimately produce molten metal within the electric furnace.

[0177] Here, the raw material may include blast furnace molten iron, iron scrap, and reduced iron. For example, the raw material may include 15 to 30 wt% blast furnace molten iron, 15 to 30 wt% reduced iron, and the remainder iron scrap.

[0178] If the amount of molten iron charged into the electric furnace falls below the preset range, complete melting of reduced iron and scrap iron may be difficult. Conversely, if the amount of molten iron exceeds the preset range, molten iron production increases, potentially resulting in excessive carbon dioxide emissions from blast furnace operations.

[0179] If the amount of reduced iron charged into the electric furnace falls below the preset range, the proportion of iron scrap in the raw material increases, which may degrade the physical properties of the wire rod due to tramp elements contained in the iron scrap. Conversely, if the amount of reduced iron charged into the electric furnace exceeds the preset range, the increased amount of reduced iron, which is more difficult to melt than the iron scrap, may make complete melting of the raw material difficult.

[0180] In the case of using the electric furnace described above or using a blast furnace and an electric furnace together, a process of processing the molten metal produced in the electric furnace can be carried out.

[0181] For example, molten metal produced in an electric furnace can undergo a vacuum degassing process. Specifically, the vacuum degassing process can treat the molten metal using an inert gas under a preset vacuum level. The inert gas can be argon (Ar).

[0182] Specifically, the vacuum degassing process can be performed by controlling the argon (Ar) reflux flow rate to 900 to 1100 ℓ / min, the vacuum treatment time to 20 minutes or more, and the vacuum level to 2 mbar or less. This allows for the control of the maximum size of non-metallic inclusions and the nitrogen content that affect the fresh processability.

[0183] According to one embodiment of the present invention, in the continuous casting step (S2), the molten metal produced in the melting step (S1) can be produced in the form of a first semi-finished product. In this embodiment, the first semi-finished product may be a bloom.

[0184] Figure 2 is a flowchart showing detailed steps of the (S3) rolling step shown in Figure 1.

[0185] Referring to FIG. 2, the (S3) rolling step may include (S3-1) a first reheating step, (S3-2) a first rolling step, (S3-3) a second reheating step, (S3-4) a second rolling step, (S3-5) a coiling step, and (S3-6) a cooling step.

[0186] (S3-1) The first reheating step can reheat the first semi-finished product. (S3-1) The first reheating step can be performed at 1,150°C or higher, preferably 1,150 to 1,300°C, and more preferably 1,200 to 1,300°C.

[0187] (S3-1) If the temperature in the first reheating step is below the preset range, vanadium (V)-based precipitates may be re-dissolved into the matrix, but niobium (Nb)-based precipitates and titanium (Ti)-based precipitates may be difficult to re-dissolved.

[0188] Conversely, (S3-1) if the temperature in the first reheating step exceeds the preset range, it may be advantageous for re-employment of coarse precipitates, but it may lead to limitations in equipment specifications and excessive increase in manufacturing costs.

[0189] (S3-2) The first rolling step can roll the first semi-finished product into a second semi-finished product. In the present embodiment, the second semi-finished product can be manufactured as a billet.

[0190] (S3-3) The second reheating step can reheat the second semi-finished product. The second reheating step can be performed under the conditions of 950 to 1050 ℃ and a reheating time of 70 minutes or more.

[0191] If the temperature during the second reheating step exceeds the preset range, excessive decarburization may occur on the surface. Conversely, if the temperature during the second reheating step falls below the preset range, equipment load during precision rolling increases, potentially resulting in defects such as cobbles during rolling.

[0192] (S3-4) In the second rolling step, the second semi-finished product can be rolled to manufacture a wire rod. (S3-4) The second rolling step can be performed in multiple stages. (S3-4) The second rolling step can be performed under the condition of a finishing rolling temperature of 800 to 1000°C.

[0193] If the temperature of the second rolling stage is below the preset range, the load on the rolling equipment increases, which may cause surface defects such as rolling folds.

[0194] Conversely, if the temperature of the second rolling stage exceeds the preset range, a decarburized layer may be excessively formed, which may reduce the fresh workability.

[0195] (S3-4) The diameter of the wire rod that has gone through the second rolling step may be 4.5 to 26 mm, preferably 5.5 to 23 mm.

[0196] However, the diameter of the wire rod is not limited to the above-mentioned, and the diameter of the wire rod may vary depending on the rolling conditions.

[0197] (S3-5) In the winding step, precision rolled wire can be wound. (S3-5) The winding step can be performed at 850 to 950°C.

[0198] If the coiling temperature is below the preset range, a large difference from the rolling temperature may occur, which may result in a defective coil shape.

[0199] Conversely, if the coiling temperature exceeds the preset range, the thickness of the decarburization layer may be excessively formed, and the thickness of the surface scale may be formed thickly, which may cause quality deterioration during freshening and spring manufacturing.

[0200] (S3-6) In the cooling step, the precision rolled wire can be cooled. (S3-6) The cooling step can be performed up to 600 ℃ at a cooling rate of 3 ℃ / sec or less.

[0201] (S3-6) If the cooling rate in the cooling step exceeds a preset value, hard structures such as bainite or martensite may be formed without pearlite transformation being completed, which may reduce the fresh workability.

[0202] According to one embodiment of the present invention, a wire rod having excellent corrosion resistance and corrosion fatigue resistance can be manufactured by controlling the average size of fine precipitates and austenite crystal grains within a preset range through the above-described alloy composition and manufacturing process.

[0203]

[0204] steel wire for springs

[0205] Figure 3 is a flowchart showing a method for manufacturing a steel wire for a spring according to one embodiment of the present invention.

[0206] A spring steel wire according to one embodiment of the present invention can be obtained by directly drawing the aforementioned wire without going through a softening heat treatment step and then going through a QT (Quenching Tempering) step.

[0207] Referring to FIG. 3, the QT step (QT) may include a (QT1) quenching step, a (QT2) cooling step, and a (QT3) tempering step.

[0208] (QT1) In the quenching step, freshly processed steel wire can be maintained at 850 to 1000°C for more than 3 seconds. If the temperature of the (QT1) quenching step is below 850°C, undissolved pearlite may remain. Conversely, if the temperature of the (QT1) quenching step exceeds 1000°C, the decarburized layer grows and the grains become coarser, making it difficult to secure mechanical properties.

[0209] (QT2) In the cooling step, the steel wire that has undergone the quenching step (QT1) can be cooled to room temperature ~ 80℃. (QT2) The cooling step can be performed using air cooling or water cooling, but is not limited thereto.

[0210] (QT3) In the tempering step, the cooled steel wire is heated to 350 to 450°C. If the temperature in the (QT3) tempering step is less than 350°C, it may be difficult to secure the toughness of the steel wire. Conversely, if the temperature in the (QT3) tempering step exceeds 450°C, it may be difficult to secure the strength of the steel wire.

[0211] According to one embodiment of the present invention, a spring steel wire may include a precipitate containing at least one of vanadium (V), niobium (Nb), and titanium (Ti). Specifically, the precipitate having a size of 100 nm or less in circle-equivalent diameter may have a particle size of 2.1 x 10 8 dog / mm 2 This may include:

[0212] The tensile strength of the steel wire for a spring according to one embodiment of the present invention may be 2,000 MPa or more.

[0213] The cross-sectional shrinkage ratio of the spring steel wire according to one embodiment of the present invention may be 40% or more.

[0214] The hydrogen embrittlement fracture strength of the spring steel wire according to one embodiment of the present invention may be 1300 MPa or more, preferably 1400 MPa or more, and more preferably 1410 MPa or more.

[0215]

[0216] Exam example

[0217] Below, we will examine preferred test examples to aid understanding of the present invention. The following test examples are provided solely to aid understanding of the present invention, and the present invention is not limited to the following examples.

[0218] Tables 1 and 2 show the alloy compositions and formulae 1 and 2 values ​​of examples and comparative examples. In Tables 1 and 2, the unit of content of each element is weight %. Table 3 shows the process conditions of examples and comparative examples.

[0219] Table 4 shows the characteristics of the wire according to the examples and comparative examples shown in Tables 1 to 3.

[0220] In Table 4, the pearlite fraction and martensite formation were determined by dividing one ring of coiled wire into 16 parts and collecting test pieces of the wire cross-section perpendicular to the rolling direction, and measuring all 16 wire cross-sections using an optical microscope with 200x magnification.

[0221] In Table 4, the size of the austenite crystal grains was measured by heating and rapidly cooling the wire from room temperature to 950 ℃, then polishing and etching the test specimens, and measuring the average crystal grain size at eight random locations using an optical microscope with a magnification of 200 times.

[0222] In Table 4, the tensile strength was measured by manufacturing specimens that conform to the ASTEM E-8M standard.

[0223] In Table 4, the number of precipitates per unit area was analyzed using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDXS) by extracting samples using the replica method, and the size and number were measured by taking images at eight random locations in scanning electron microscope (STEM) mode.

[0224] Table 5 shows the characteristics of spring steel wire manufactured using the wire shown in Table 4.

[0225] In Table 5, the number of precipitates per unit area was analyzed using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDXS) by extracting samples using the replica method, and the size and number were measured by taking images at eight random locations in scanning electron microscope (STEM) mode.

[0226] In Table 5, the tensile strength was measured by manufacturing specimens that conform to the ASTEM E-8M standard.

[0227] In Table 5, the cross-sectional shrinkage was measured by extending the wire in the longitudinal direction and measuring the decrease in cross-sectional area when the wire broke.

[0228] In Table 5, the hydrogen embrittlement fracture strength was measured by immersing in a 5% NaCl solution and performing an ultra-low-speed tensile test under a cross-head speed of 0.01 mm / min.

[0229] Classification CSiMnPSCuNiCrComparative Example 10.541.440.630.0100.0100.040.020.65Comparative Example 20.491.780.970.0080.0050.520.290.30Comparative Example 30.521.330.510.0170.0030.160.060.44Comparative Example 40.611.920.480.0050.0030 .030.450.23Comparative Example 50.581.590.330.0060.0050.190.710.35Comparative Example 60.531.250.650.0120.0030.300.330.33Example 10.481.520.410.0080.0020.170.210.27Example 20.501.240.390.0070.0040.210 .190.30Example 30.551.450.830.0050.0110.420.560.33Example 40.621.950.320.0120.0030.510.630.36Example 50.491.580.920.0090.0140.430.520.30Example 60.471.790.650.0140.0100.570.750 .22Comparative Example 70.49ㅍ1.780.970.0080.0050.520.290.30Comparative Example 80.581.590.330.0060.0050.190.710.35Comparative Example 90.501.240.390.0070.0040.210.190.30Comparative Example 100.471.790.650.0140.0100.570.750.22

[0230] ClassificationMoVNbTiNFormula 1Formula 2Comparative Example 10.01---0.00450-0.47Comparative Example 20.01--0.0440.00393.300.40Comparative Example 30.160.0670.005-0.00513.760.53Comparative Example 40.060.1050.023-0.00359.240.51Comparative Example 50.090.075-0.02 10.00485.580.72Comparative Example 60.130.0200.0180.0190.00314.440.69Example 10.090.1420.014-0.003312.470.91Example 20.110.102-0.0210.00546.330.81Example 30.100.1460.033- 0.00637.161.61Example 40.140.0730.0070.0380.00417.871.48Example 50.060.1110.0210.0170.00596.550.98Example 60.170.1250.0270.0290.00667.112.73Comparative Example 7 0.01--0.0440.00393.300.40Comparative Example 80.090.075-0.0210.00485.580.72Comparative Example 90.110.102-0.0210.00546.330.81Comparative Example 100.170.1250.0270.0290.00667.112.73

[0231] Rolling stage 1st reheating temperature (℃) 2nd reheating temperature (℃) Finishing rolling temperature (℃) Coiling temperature (℃) Cooling speed (℃ / sec) Comparative example 11, 18896 1952 8882.5 Comparative example 21, 2451 04599 194 32.2 Comparative example 31, 222 9889 0286 10.8 Comparative example 41, 2631 02194 191 51.5 Comparative example 51, 2579999 599 270.7 Comparative example 61, 261976 86 085 31.1 Example 11, 18398896 490 91.4 Example 21, 25797592 18592 .0Example 31,2289629178750.9Example 41,2591,0449589331.2Example 51,2319969689290.7Example 61,2621,0158989100.8Comparative Example 71,1469901,0451,0611.3Comparative Example 81,2301,0229339315.8Comparative Example 91,1369771,0329281.0Comparative Example 101,1391,0059721,0184.5

[0232] Pearlite fraction (%) for wire for springs Martensite formation Average austenite grain size (㎛) Less than 100 nm Precipitates (10 7 dog / mm 2 )Tensile strength (MPa)Comparative example 193X15.60984Comparative example 275X12.30.61,021Comparative example 389X11.51.21,073Comparative example 496X8.63.51,088Comparative example 595X10.82.31,030Comparative example 687X10.11.7999Example 169X8.48.9982Example 275X10.93. 71,005 Example 392X7.79.21,042 Example 496X9.22.31,067 Example 574X8.83.1993 Example 664X6.54.4972 Comparative Example 771X22.50.4971 Comparative Example 852O18.91.21,224 Comparative Example 969X23.91.4988 Comparative Example 1049O14.11.71,198

[0233] Number of precipitates less than 100 nm for steel wire for separation spring (x10 8 dog / mm 2 )Tensile strength (MPa)Cross-sectional shrinkage (%)Hydrogen brittle fracture strength (MPa)Comparative example 102,00533.5843Comparative example 20.22,01737.6952Comparative example 31.02,00440.8989Comparative example 43.12,04038.21,144Comparative example 51.52,01143.41,023Comparative example 61.22,02138.81,001Example 19.22,05746.31,551Example 23.32,03344.61,43 Example 9: 39.62, 040, 51.21, 658 Example 4: 22.12, 019, 50.81, 565 Example 5: 33.52, 022, 48.51, 410 Example 6: 44.92, 088, 52.81, 635 Comparative Example 7: 0.12, 004, 35.59, 21 Comparative Example 8: 12.010, 39.79, 98 Comparative Example 9: 1.32, 034, 40.71, 231 Comparative Example 10: 1.52, 065, 45.11, 275

[0234] Referring to Tables 1 to 3, in the case of Comparative Example 1, the content of chromium (Cr) was 0.65 wt%, exceeding the maximum value of 0.5 wt%, the contents of copper and nickel were 0.04 and 0.02 wt%, respectively, which were less than the minimum value of 0.15, and the content of molybdenum was 0.01 wt%, which was less than the minimum value of 0.05. In addition, the value of Equation 1 was 0, which was less than the minimum value of 5, and the value of Equation 2 was -0.47, which was less than the minimum value of 0.8.

[0235] In the case of Comparative Example 2, the content of molybdenum was 0.01 wt%, which was less than the minimum value of 0.05, the value of Equation 1 was 3.3, which was less than the minimum value of 5, and the value of Equation 2 was 0.40, which was less than the minimum value of 0.8.

[0236] In the case of Comparative Example 3, the phosphorus content was 0.017 wt%, exceeding the maximum value of 0.015 wt%, the nickel content was 0.06 wt%, less than the minimum value of 0.15, the value of Equation 1 was 3.76, less than the minimum value of 5, and the value of Equation 2 was 0.53, less than the minimum value of 0.8.

[0237] In the case of Comparative Example 4, the copper content was 0.03 wt%, which was less than the minimum value of 0.15, and the value of Equation 2 was 0.51, which was less than the minimum value of 0.8.

[0238] In the case of Comparative Example 5, the alloy composition and the value of Equation 1 were satisfied, but the value of Equation 2 was 0.72, which was less than the minimum value of 0.8.

[0239] In the case of comparative example 6, the value of equation 1 was 4.44, which was less than the minimum value of 5, and the value of equation 2 was 0.69, which was less than the minimum value of 0.8.

[0240] As a result, Comparative Examples 1 to 6 did not satisfy at least one of the conditions of the content ratio of each element, the value of Equation 1, and the value of Equation 2.

[0241] Referring to Tables 4 and 5, no fine precipitates were formed in the spring wire and steel wire according to Comparative Example 1. Accordingly, the hydrogen embrittlement fracture strength of the spring wire was 843 MPa, which was less than the minimum value of 1,300 MPa.

[0242] In the case of comparative example 2, the fine precipitates in the spring wire were 0.6 x 10 7 dog / mm 2 And, in the spring steel wire, fine precipitates 0.2 x 10 8 dog / mm 2 Accordingly, the hydrogen embrittlement fracture strength of the spring steel wire was 952 MPa, which was less than the minimum value of 1,300 MPa.

[0243] In the case of Comparative Example 3, fine precipitates were found in the spring wire. 1.2 x 10 7 dog / mm 2 And, in the spring steel wire, fine precipitates 1.0 x 10 8 dog / mm 2 This was it. Accordingly, the hydrogen embrittlement fracture strength was 989 MPa, which was less than the minimum value of 1,300 MPa.

[0244] In the case of Comparative Example 4, the hydrogen embrittlement fracture strength of the spring steel wire was 1,144 MPa, which was less than the minimum value of 1,300 MPa.

[0245] In the case of Comparative Example 5, fine precipitates were found in the spring wire. 2.3 x 10 7 dog / mm 2 And, in the spring steel wire, fine precipitates 1.5 x 10 8 dog / mm 2 This was it. Accordingly, the hydrogen embrittlement fracture strength of the spring steel wire was 1,023 MPa, which was less than the minimum value of 1,300 MPa.

[0246] In the case of comparative example 6, fine precipitates were found in the spring wire. 1.7 x 10 7 dog / mm 2 And, in the spring steel wire, fine precipitates 1.2 x 10 8 dog / mm 2 This was it. Accordingly, the hydrogen embrittlement fracture strength of the spring steel wire was 1,001 MPa, which is less than the minimum value of 1,300 MPa.

[0247] As a result, referring to Comparative Examples 1 to 6, if any one of the alloy composition, the value of Formula 1, and the value of Formula 2 is unsatisfactory, it can be confirmed that the hydrogen embrittlement fracture strength of the final product, the spring steel wire, is insufficient, and thus the corrosion resistance and corrosion fatigue resistance characteristics are not secured.

[0248] Referring again to Tables 1 to 3, in the case of Comparative Example 7, the content of molybdenum was 0.01 wt%, which was less than the minimum value of 0.05, the value of Equation 1 was 3.3, which was less than the minimum value of 5, and the value of Equation 2 was 0.40, which was less than the minimum value of 0.8.

[0249] In addition, in the case of Comparative Example 7, the temperature in the first reheating was 1,146°C, which was less than the minimum value of 1,150°C, the finishing rolling temperature was 1,045°C, which exceeded the maximum value of 1,000°C, and the coiling temperature was 1,061°C, which exceeded the maximum value of 950°C.

[0250] In the case of Comparative Example 8, the alloy composition and the values ​​of Equation 1 were satisfied, but the value of Equation 2 was 0.72, which was less than the minimum value of 0.8. In addition, in the case of Comparative Example 8, the cooling rate was 5.8 ℃ / sec, which exceeded the maximum value of 3 ℃ / sec.

[0251] In the case of Comparative Example 9, the alloy composition, Equations 1 and 2 were satisfied, but the first reheating temperature was 1,136 ℃, which was less than the minimum value of 1,150 ℃, and the finishing rolling temperature was 1,045 ℃, which exceeded the maximum value of 1,000 ℃.

[0252] In the case of Comparative Example 10, the alloy composition, Equations 1 and 2 were satisfied, but the first reheating temperature was 1,139 ℃, which was less than the minimum value of 1,150 ℃, the coiling temperature was 1,018 ℃, which exceeded the maximum value of 950 ℃, and the cooling rate was 5.8 ℃ / sec, which exceeded the maximum value of 3 ℃ / sec.

[0253] Referring to Tables 4 and 5, in the case of Comparative Example 7, fine precipitates were present in the spring wire. 0.4 x 10 7 dog / mm 2 And, in the spring steel wire, fine precipitates 0.1 x 10 8 dog / mm 2 This was it. Accordingly, the hydrogen embrittlement fracture strength was 921 MPa, which was less than the minimum value of 1,300 MPa.

[0254] Additionally, in Comparative Example 7, the average size of the austenite grains was 22.5 ㎛, exceeding the maximum value of 20 ㎛, and the cross-sectional shrinkage of the spring steel wire was 35.5%, less than the minimum value of 40%.

[0255] In the case of Comparative Example 8, fine precipitates were found in the spring wire. 1.2 x 10 7 dog / mm 2 And, in the spring steel wire, fine precipitates 1.1 x 10 8 dog / mm 2 This was it. Accordingly, the hydrogen embrittlement fracture strength of the spring steel wire was 998 MPa, which was less than the minimum value of 1,300 MPa.

[0256] Additionally, in Comparative Example 8, the tensile strength of the spring wire exceeded the maximum value of 1100 MPa at 1224 MPa. This is thought to be because a martensite structure was formed in the final microstructure of the spring wire, and the pearlite fraction was 52%, which is less than the minimum value of 60%.

[0257] Additionally, in Comparative Example 8, the cross-sectional shrinkage rate of the spring steel wire was 39.7%, which was less than the minimum value of 40%.

[0258] In the case of comparative example 9, fine precipitates were found in the spring wire. 1.4 x 10 7 dog / mm 2 And, in the spring steel wire, fine precipitates 1.3 x 10 8 dog / mm 2 This was it. Accordingly, the hydrogen embrittlement fracture strength of the spring steel wire was 1231 MPa, which was less than the minimum value of 1300 MPa.

[0259] Additionally, in the case of Comparative Example 9, the average size of the austenite grains in the spring wire was 23.9 ㎛, exceeding the maximum value of 20 ㎛.

[0260] In the case of Comparative Example 10, fine precipitates were present in the spring wire. 1.7 x 10 7 dog / mm 2 And, in the spring steel wire, fine precipitates 1.5 x 10 8 dog / mm 2 This was it. Accordingly, the hydrogen embrittlement fracture strength of the spring steel wire was 1275 MPa, which was less than the minimum value of 1300 MPa.

[0261] In addition, in the case of Comparative Example 10, a martensite structure was formed in the final microstructure of the spring wire, and the pearlite fraction was 49%, which was less than the minimum value of 60%, and the tensile strength was 1,198 MPa, which exceeded the maximum value of 1,100 MPa.

[0262] Compared to the comparative examples described above, Examples 1 to 6 all satisfied the alloy composition system, Equations 1 and 2, and process conditions. Accordingly, they satisfied both the properties required for the spring wire described above and the properties required for the spring steel wire.

[0263] As described above, preferred embodiments of the present invention have been described. It will be apparent to those skilled in the art that the present invention can be embodied in other specific forms, in addition to the comparative examples and examples described above, without departing from the spirit or scope thereof. Therefore, the above-described embodiments should be considered illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description, but may be modified within the scope of the appended claims and their equivalents.

Claims

1. Contains 0.45 to 0.65 wt% of carbon (C), 1.20 to 2.00 wt% of silicon (Si), 0.30 to 1.00 wt% of manganese (Mn), 0.015 wt% or less of phosphorus (P) (excluding 0), 0.015 wt% or less of sulfur (S) (excluding 0), and 0.01 wt% or less of nitrogen (N) (excluding 0). Contains at least one element of vanadium (V) 0.20 wt% or less, niobium (Nb) 0.050 wt% or less, and titanium (Ti) 0.050 wt% or less, Contains the remaining iron (Fe) and other unavoidable impurities, Spring wire satisfying the following equation 1: [Formula 1] 5 ≤ (V+Nb+Ti) / N (In the above formula 1, V, Nb, Ti and N represent the atomic weight ratio of vanadium, the atomic weight ratio of niobium, the atomic weight ratio of titanium and the atomic weight ratio of nitrogen, respectively).

2. In paragraph 1, Further comprising 0.15 to 0.60 wt% of copper (Cu), 0.15 to 0.80 wt% of nickel (Ni), 0.2 to 0.5 wt% of chromium (Cr), and 0.05 to 0.20 wt% of molybdenum (Mo). Wire for springs.

3. In paragraph 1, The final microstructure contains more than 60% pearlite. Wire for springs.

4. In paragraph 1, The number of precipitates with a diameter of 100 nm or less in the final microstructure was 2.3ⅹ10 7 dog / mm 2 Lee Sang-in, Wire for springs.

5. In paragraph 1, The average size of austenite grains is less than 20 ㎛. Wire for springs.

6. In paragraph 1, Tensile strength of 1100 MPa or less, Wire for springs. 7.(S1) Dissolution step for dissolving raw materials; (S2) Continuous casting step for manufacturing the first semi-finished product; (S3) A rolling step of hot rolling the first semi-finished product; Including, The above step (S3) is, (S3-1) A first reheating step for first heating the first semi-finished product; (S3-2) A first rolling step of rolling the first heated first semi-finished product to manufacture a second semi-finished product; (S3-3) A second reheating step for reheating the second semi-finished product; and (S3-4) A second rolling step for manufacturing a wire rod by rolling the second semi-finished product subjected to secondary heating; including; Method for manufacturing wire for spring.

8. In paragraph 7, The above step (S3-1) is performed at 1,150 to 1,300 ℃. The above step (S3-3) is performed at 950 to 1,050 ℃. Method for manufacturing wire for spring.

9. In paragraph 8, In the above step (S3-4), the finishing rolling temperature is 800 to 1,000 ℃. Method for manufacturing wire for spring.

10. In paragraph 9, The above step (S3) is, Further comprising a winding step (S3-5) of winding the above wire at 850 to 950 ℃. Method for manufacturing wire for spring.

11. In paragraph 10, The above step (S3) is, Further comprising a cooling step (S3-6) of cooling the above-mentioned wire at a cooling rate of 3 ℃ / sec or less. Method for manufacturing wire for spring.

12. In paragraph 7, The pre-processed material that has gone through the above step (S3) is Contains 0.45 to 0.65 wt% of carbon (C), 1.20 to 2.00 wt% of silicon (Si), 0.30 to 1.00 wt% of manganese (Mn), 0.015 wt% or less of phosphorus (P) (excluding 0), 0.015 wt% or less of sulfur (S) (excluding 0), and 0.01 wt% or less of nitrogen (N) (excluding 0). Contains at least one element of vanadium (V) 0.20 wt% or less, niobium (Nb) 0.050 wt% or less, and titanium (Ti) 0.050 wt% or less, Containing the remaining iron (Fe) and other unavoidable impurities, Method for manufacturing wire for spring.

13. In paragraph 12, The above wire is a method for manufacturing a wire for a spring that satisfies the following formula 1: [Formula 1] 5 ≤ (V+Nb+Ti) / N (In the above formula 1, V, Nb, Ti and N represent the atomic weight ratio of vanadium, the atomic weight ratio of niobium, the atomic weight ratio of titanium and the atomic weight ratio of nitrogen, respectively).

14. In paragraph 12, Further comprising 0.15 to 0.60 wt% of copper (Cu), 0.15 to 0.80 wt% of nickel (Ni), 0.2 to 0.5 wt% of chromium (Cr), and 0.05 to 0.20 wt% of molybdenum (Mo). Method for manufacturing wire for spring.

15. In paragraph 7, In the above step (S1), The above raw material comprises at least one of iron ore, reduced iron and iron scrap. Method for manufacturing wire for spring.

16. In paragraph 7, The above step (S1) is, Performed in at least one of the facilities of a blast furnace and an electric furnace, Method for manufacturing wire for spring.

17. Contains 0.45 to 0.65 wt% of carbon (C), 1.20 to 2.00 wt% of silicon (Si), 0.30 to 1.00 wt% of manganese (Mn), 0.015 wt% or less of phosphorus (P) (excluding 0), 0.015 wt% or less of sulfur (S) (excluding 0), and 0.01 wt% or less of nitrogen (N) (excluding 0). Contains at least one element of vanadium (V) 0.20 wt% or less, niobium (Nb) 0.050 wt% or less, and titanium (Ti) 0.050 wt% or less, Contains the remaining iron (Fe) and other unavoidable impurities, The number of precipitates with a diameter of 100 nm or less in the final microstructure is 2.1ⅹ10 8 dog / mm 2 Lee Sang-in, Steel wire for springs.

18. In paragraph 17, Tensile strength is 2000 MPa or more, Steel wire for springs.

19. In paragraph 17, Hydrogen brittle fracture strength is 1300 MPa or more, Steel wire for springs.

20. In paragraph 17, Spring steel wire satisfying the following equation 1: [Formula 1] 5 ≤ (V+Nb+Ti) / N (In the above formula 1, V, Nb, Ti and N represent the atomic weight ratio of vanadium, the atomic weight ratio of niobium, the atomic weight ratio of titanium and the atomic weight ratio of nitrogen, respectively).

Images