Spring steel and spring wire, and methods for manufacturing the same.

Abstract

The present invention discloses a spring steel and steel wire having excellent resistance to permanent deformation by increasing the dislocation density in the material or decreasing the average crystal grain size, and a method for producing the same. The method includes the steps of producing a steel wire by drawing a steel containing, by weight, 0.4-0.7% C, 1.2-2.3% Si, 0.2-0.8% Mn, 0.2-0.8% Cr, and the balance being Fe (iron) and other unavoidable impurities, heating the drawn steel wire at 850-1000°C and maintaining the temperature for at least 1 second, and after the austenitizing step, quenching at 25-80°C and tempering at 350-500°C. The steel further contains, by weight, at least one selected from the group consisting of 0.01-0.3% V, 0.005-0.05% Nb, 0.001-0.15% Ti, and 0.01-0.4% Mo. The hysteresis loop obtained by a Bauschinger torsion test is The area of ​​the loop is 206 mm 2 The present invention is characterized in that:

Description

[Technical Field]

[0001] The present invention relates to spring steel and spring wire, and methods for manufacturing them, and more particularly to spring steel and spring wire in which permanent deformation resistance is improved by increasing the dislocation density in the material or decreasing the average grain size, and methods for manufacturing them. [Background technology]

[0002] In recent years, there has been a strong demand for lightweight materials in automobiles to improve fuel efficiency. In particular, in the case of suspension springs, to meet the demand for weight reduction, spring designs using high-strength materials with a strength of 1800 MPa or more after heat treatment and tempering are currently being applied.

[0003] However, currently available spring steels are prone to problems such as reduced durability and increased permanent deformation when used under high-stress conditions. Permanent deformation (resistance) of a spring refers to its resistance to plastic deformation caused by dynamic and static loads applied during spring use, and generally means the change in height after a certain period of use relative to the spring's initial height. Therefore, increased permanent deformation reduces the spring height, lowering the vehicle's height and consequently the bumper height, causing serious safety issues. Consequently, spring steels with high permanent deformation resistance are required to enable high-stress spring designs.

[0004] It has become clear that the silicon contained in spring steel is effective in improving permanent deformation resistance, and steel corresponding to SAE9254, which has excellent permanent deformation resistance, has become widespread as a spring steel. However, the demand for high-stress springs continues to increase, and so does the demand for methods that can further increase permanent deformation resistance.

[0005] Patent Document 1 describes a pearlite structure in which the total number of (V,Cr) carbides, carbonitrides, and composite carbides and carbonitrides of V and Cr with a diameter of 50 nm or less is 10 / μm within the ferrite. 2It is disclosed that when the above amounts are present, the material exhibits excellent resistance to permanent deformation. However, (V,Cr) carbides, carbonitrides, and composite carbides and carbonitrides of V and Cr all have V as the main component and therefore melt rapidly at temperatures above 850°C. Consequently, in current spring manufacturing processes where the heating temperature is 900°C or higher, it is difficult to expect the improvement in permanent deformation resistance due to precipitates disclosed in Patent Document 1. Furthermore, since the price of V ferroalloys has risen geometrically recently, the content disclosed in Patent Document 1 may also act as a disadvantage in terms of manufacturing costs. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2002-180199 [Overview of the project] [Problems that the invention aims to solve]

[0007] The objective of the present invention to solve the above-mentioned problems is to provide spring steel and spring wire with excellent permanent deformation resistance by increasing the dislocation density in the material or decreasing the average grain size, and a method for manufacturing the same. [Means for solving the problem]

[0008] The spring steel wire of the present invention, which has excellent resistance to permanent deformation, contains, by weight percent, C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, with the remainder being Fe (iron) and other unavoidable impurities, and has a dislocation density of 1.16 × 10⁻¹⁶. 15 / m 2 The above conditions are met, and the average crystal grain size is 8.4 μm or less.

[0009] Furthermore, the spring steel wire of the present invention, which has excellent resistance to permanent deformation, further comprises one or more selected from the group consisting of V: ​​0.01-0.3%, Nb: 0.005-0.05%, Ti: 0.001-0.15%, and Mo: 0.01-0.4% by weight.

[0010] Furthermore, the spring steel wire of the present invention, which has excellent resistance to permanent deformation, has a hysteresis loop area of ​​206 mm² obtained in the Bauschinger torsion test. 2 That's all.

[0011] Furthermore, the present invention provides a method for manufacturing spring steel wire with excellent permanent deformation resistance, which includes the steps of: producing steel wire by drawing steel containing, by weight %, C: 0.4~0.7%, Si: 1.2~2.3%, Mn: 0.2~0.8%, Cr: 0.2~0.8%, with the remainder being Fe (iron) and other unavoidable impurities; an austenitizing step in which the drawn steel wire is heated to 850~1000°C and maintained for 1 second or more; and a step in which, after the austenitizing step, the steel wire is quenched at 25~80°C and then tempered at 350~500°C.

[0012] Furthermore, in the method for manufacturing spring steel wire with excellent permanent deformation resistance according to the present invention, the steel further comprises one or more selected from the group consisting of V: ​​0.01-0.3%, Nb: 0.005-0.05%, Ti: 0.001-0.15%, and Mo: 0.01-0.4% by weight.

[0013] Furthermore, the spring steel of the present invention, which has excellent resistance to permanent deformation, contains, by weight percent, C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, with the remainder being Fe (iron) and other unavoidable impurities, and has a dislocation density of 0.11 × 10⁻¹⁰. 15 / m 2 The above conditions are met, and the average crystal grain size is 9.6 μm or less.

[0014] Furthermore, the spring steel with excellent resistance to permanent deformation according to one embodiment of the present invention further comprises one or more elements selected from the group consisting of V: ​​0.01-0.3%, Nb: 0.005-0.05%, Ti: 0.001-0.15%, and Mo: 0.01-0.4% by weight.

[0015] Moreover, the method for manufacturing a spring steel excellent in permanent deformation resistance according to the present invention includes a step of manufacturing a billet containing, by weight%, C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, with the balance being Fe (iron) and other inevitable impurities, a step of heating the billet at 960 to 1100°C, and a step of finish rolling at 855 to 920°C.

[0016] Moreover, in the method for manufacturing a spring steel excellent in permanent deformation resistance according to the present invention, the billet further contains at least one selected from the group consisting of V: 0.01 to 0.3%, Nb: 0.005 to 0.05%, Ti: 0.001 to 0.15%, and Mo: 0.01 to 0.4% by weight.

Effects of the Invention

[0017] According to the present invention, it is possible to provide a spring steel and a steel wire having improved permanent deformation resistance by increasing the dislocation density in the material or decreasing the average crystal grain size, and a method for manufacturing the same.

Brief Description of the Drawings

[0018] [Figure 1] It is a graph showing the relationship between the average crystal grain size of the spring steel according to the present invention and the comparative example and the area of the hysteresis loop of the steel wire. [Figure 2] It is a graph showing the relationship between the average crystal grain size of the spring steel wire according to the present invention and the comparative example and the area of the hysteresis loop of the steel wire. [Figure 3] It is a graph showing the relationship between the dislocation density of the spring steel according to the present invention and the comparative example and the area of the hysteresis loop of the steel wire. [Figure 4] It is a graph showing the relationship between the dislocation density of the spring steel wire according to the present invention and the comparative example and the area of the hysteresis loop of the steel wire.

Modes for Carrying Out the Invention

[0019] The spring steel wire of the present invention, which has excellent resistance to permanent deformation, contains, by weight percent, C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, with the remainder being Fe (iron) and other unavoidable impurities, and has a dislocation density of 1.16 × 10⁻¹⁶. 15 / m 2 The above conditions apply, and the average crystal grain size may be 8.4 μm or less.

[0020] Preferred embodiments of the present invention will be described below. However, embodiments of the present invention may be modified into various different forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, embodiments of the present invention are provided to give a more complete explanation of the present invention to a person with average skill in the art.

[0021] The terminology used in this application is solely for illustrative purposes. Therefore, for example, a singular expression includes plural expressions unless the context clearly requires it to be singular. Furthermore, it should be noted that terms such as “includes” or “equipped with” used in this application are used to explicitly indicate the existence of features, stages, functions, components, or combinations thereof described in the specification, and are not used to provisionally exclude the existence of other features, stages, functions, components, or combinations thereof.

[0022] On the other hand, unless otherwise specifically defined, all terms used herein should be considered to have the same meaning as that generally understood by a person of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, no particular term should be interpreted in an overly idealistic or formal sense. For example, in this specification, a singular expression includes a plural expression unless the context clearly indicates otherwise.

[0023] Furthermore, in this specification, "about," "substantially," etc., are used to mean the numerical value or a similar value when a tolerance for manufacturing and material inherent to the meaning referred to is presented, and are used to prevent unscrupulous infringers from unfairly using disclosures that refer to precise and absolute numerical values ​​to aid in understanding the invention.

[0024] The spring steel of the present invention, which has excellent resistance to permanent deformation, contains, by weight percent, C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, with the remainder being Fe (iron) and other unavoidable impurities.

[0025] The reasons for limiting the alloy composition will be explained in detail below.

[0026] The carbon (C) content may be 0.4 to 0.7%.

[0027] Carbon (C) is an essential element added to ensure the strength of springs. Considering this, C may be added in amounts of 0.4% or more. However, if the C content is excessive, twin-type martensitic structures will form during quenching and tempering heat treatments, leading to cracking of the material and a significant reduction in fatigue life. Furthermore, excessive C content increases defect susceptibility, and if corrosion pits occur on the surface, fatigue life and fracture stress will be significantly reduced. Considering this, the upper limit of C content may be restricted to 0.7%.

[0028] The silicon (Si) content may be 1.2 to 2.3%.

[0029] Si is an element that has an excellent effect in strengthening the ferrite by solid-solving within it and improving its deformation resistance. Considering this, Si may be added in an amount of 1.2% or more, and more preferably 1.4% or more. However, if the Si content is excessive, the effect of improving deformation resistance will saturate, and surface decarburization may occur during heat treatment. Considering this, the upper limit of the Si content may be limited to 2.3%.

[0030] The manganese (Mn) content may be 0.2 to 0.8%.

[0031] Mn is an element that plays a role in improving the hardenability of steel and ensuring its strength. Considering this, Mn may be added in amounts of 0.2% or more. However, if the Mn content is excessive, the hardenability may increase excessively, leading to the formation of a hard structure during cooling after hot rolling, increased MnS inclusion formation, and a decrease in corrosion fatigue resistance. Considering this, the upper limit of the Mn content may be limited to 0.8%.

[0032] The chromium (Cr) content may be 0.2 to 0.8%.

[0033] Cr is a useful element for ensuring oxidation resistance, temper softening resistance, prevention of surface decarburization, and hardenability. Considering this, Cr may be added in amounts of 0.2% or more. However, if the Cr content is excessive, the strength may actually be reduced due to a decrease in deformation resistance. Considering this, the upper limit of the Cr content may be limited to 0.8%.

[0034] Furthermore, the spring steel of the present invention, which has excellent resistance to permanent deformation, may further contain one or more elements selected from the group consisting of V: ​​0.01-0.3%, Nb: 0.005-0.05%, Ti: 0.001-0.15%, and Mo: 0.01-0.4% by weight.

[0035] The content of V (vanadium) may be 0.01 to 0.3%.

[0036] V is an element that contributes to improved strength and refinement of crystal grains. Furthermore, V can combine with C and N to form carbon / nitride compounds. These formed carbon / nitride compounds act as hydrogen trapping sites, suppressing hydrogen penetration within the steel and reducing corrosion. Considering this, V may be added in concentrations of 0.01% or more. However, excessive V content can increase manufacturing costs. Therefore, the upper limit of V content may be restricted to 0.3%.

[0037] The niobium (Nb) content may be 0.005 to 0.05%.

[0038] Nb is an element that combines with C and N to form carbon / nitride deposits, contributing to microstructure refinement, and acts as a hydrogen trapping site. Considering this, Nb may be added in amounts of 0.005% or more. However, if the Nb content is excessive, coarse carbon / nitride deposits may form, reducing the ductility of the steel. Considering this, the upper limit of the Nb content may be limited to 0.05%.

[0039] The titanium (Ti) content may be 0.001 to 0.15%.

[0040] Ti is an element that improves strength and toughness through precipitation hardening and contributes to particle refinement. Furthermore, Ti can combine with C and N to form carbon / nitride deposits, which can act as hydrogen trapping sites, leading to precipitation hardening and improved spring properties. Considering this, Ti may be added in amounts of 0.001% or more. However, excessive Ti content increases manufacturing costs and saturates the spring property improvement effect due to precipitates. Additionally, excessive Ti content can increase the amount of coarse alloy carbides in the base material during austenite heat treatment, acting similarly to non-metallic inclusions, potentially reducing fatigue properties and precipitation hardening effects. Considering this, the upper limit of Ti content may be limited to 0.15%.

[0041] The molybdenum (Mo) content may be 0.01 to 0.4%.

[0042] Mo is an element that combines with C and N to form carbon / nitride deposits, contributing to microstructure refinement, and acts as a hydrogen trapping site. Considering this, Mo may be added in amounts of 0.01% or more. However, if the Mo content is excessive, a hard structure is likely to develop during cooling after hot rolling, and coarse carbon / nitride deposits may form, reducing the ductility of the steel. Considering this, the upper limit of the Mo content may be limited to 0.4%.

[0043] The remaining component of this invention is iron (Fe). However, in the normal steel manufacturing process, unintended impurities from the raw materials and surrounding environment inevitably become mixed in, and these cannot be eliminated. Since these impurities are known to any technician in the normal manufacturing process, not all of them are specifically mentioned herein.

[0044] Another aspect of the present invention provides a steel wire having the same composition as the spring steel with excellent permanent deformation resistance. The reasons for the numerical limitations of each component are as described above.

[0045] The spring steel of the present invention, which has excellent resistance to permanent deformation, may contain a mixed structure of ferrite and pearlite as its microstructure by controlling the alloy composition ratio, and may not contain bainite or martensite.

[0046] On the other hand, the inventors of the present invention investigated various factors influencing the permanent deformation resistance of spring steel and found the following facts.

[0047] The permanent deformation of a spring occurs through periodic plastic deformation or microcreep, which takes place over numerous loading cycles at stress levels lower than the material's yield strength. As the material deforms, new dislocations are generated within it, or existing dislocations move, combine with each other, or disappear, ultimately changing the dislocation density.

[0048] Generally, in rolling, forming, processing, etc., in order to apply a deformation amount exceeding the yield point at one time, the dislocation density increases and the work hardening phenomenon appears. However, when subjected to periodic plastic deformation or micro-creep phenomenon at a low stress level that does not exceed the yield point, such as in a spring, rather, the dislocation density decreases over time, and ultimately the spring undergoes permanent deformation. However, due to the characteristics of the product, the spring must operate at a stress level lower than the yield point considering stability. Therefore, it is an inevitable phenomenon that the dislocation density decreases after a certain period of use.

[0049] Therefore, in order to improve the permanent deformation resistance of the spring, it is most preferable to increase the dislocation density in the material during spring manufacturing or to reduce the rate of disappearance by frequently piling up dislocations at the grain boundaries during spring use.

[0050] In order to increase the dislocation density in the material during spring manufacturing, the dislocation density must be increased from the manufacturing of the wire by hot rolling. For this purpose, rolling at a lower temperature or cooling methods are effective. Also, in order to make dislocations frequently pile up at the grain boundaries, the grain size must be refined to shorten the distance that dislocations move to the grain boundaries and make them meet the grain boundaries more frequently.

[0051] Therefore, the spring steel for springs with excellent permanent deformation resistance according to the present invention may have a dislocation density of 0.11×10 15 / m 2 or more.

[0052] Also, the spring steel for springs with excellent permanent deformation resistance according to the present invention may have an average crystal grain size of 9.6 μm or less.

[0053] Also, the spring steel wire for springs with excellent permanent deformation resistance according to the present invention may have a dislocation density of 1.16×10 15 / m 2 or more.

[0054] Also, the spring steel wire for springs with excellent permanent deformation resistance according to the present invention may have an average crystal grain size of 8.4 μm or less.

[0055] On the other hand, permanent deformation of a spring refers to the change in height after a certain period of use relative to the initial height of the spring, and is therefore generally measured in the spring state. However, the Bauschinger torsion test is a method that makes it possible to measure this deformation in the steel wire state as well. In the Bauschinger torsion test, a steel wire is twisted at a rate of 15° / min under a load greater than the yield strength, and then, after the load is removed, it is twisted again at a rate of 15° / min under a load greater than the yield strength. At this time, the portion that overlaps with the torque-twist angle curve is called the hysteresis loop. The larger the area of ​​the hysteresis loop, the greater the resistance of the spring to permanent deformation.

[0056] Therefore, when the Bauschinger torsion test is performed with the spring steel wire of the present invention, which has excellent permanent deformation resistance, the area of ​​the hysteresis loop is 206 mm². 2 That's fine too.

[0057] Next, the methods for manufacturing spring steel and steel wire with excellent permanent deformation resistance according to the present invention will be described.

[0058] The present invention relates to a method for manufacturing spring steel with excellent permanent deformation resistance, comprising, by weight %, C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, with the remainder being Fe (iron) and other unavoidable impurities, a microstructure containing a mixed structure of ferrite and pearlite, and a dislocation density of 0.11 × 10⁻⁶. 15 / m 2 The process may include the steps of manufacturing the billet, heating the billet to 960-1100°C, and finishing rolling and winding it at 855-920°C.

[0059] The reason for limiting the component ratios of each alloying element is as described above, and each manufacturing stage will be explained in more detail below.

[0060] As mentioned above, in order to improve the permanent deformation resistance of springs, the dislocation density of the spring steel and wire must be increased, or the grain size must be refined. Furthermore, in order to increase the dislocation density or refine the grain size, the billet heating temperature and the finish rolling temperature must be appropriately controlled.

[0061] The heating temperature of the billet of the present invention is preferably in the range of 960 to 1100°C. If the heating temperature of the billet is too low, the load on the rolling rolls will increase. Also, if the heating temperature of the billet is too low, all of the coarse carbides that may be generated during casting will not dissolve, so the alloying elements may not be uniformly distributed in the austenite. Taking this into consideration, the heating temperature of the billet may be 960°C or higher. On the other hand, if the heating temperature is too high, the grain size of the billet will increase, and even if hot rolling is performed under the same rolling conditions, the grain size of the final wire rod will be larger. Taking this into consideration, the upper limit of the heating temperature of the billet may be limited to 1100°C.

[0062] The finish rolling temperature of the present invention is preferably in the range of 855 to 920°C. If the finish rolling temperature is too low, the load on the rolling rolls will increase. Taking this into consideration, the finish rolling temperature may be 855°C or higher. On the other hand, if the finish rolling temperature is too high, the austenite grain size will be large before the start of cooling, and the grain size will be large after final cooling. Taking this into consideration, the upper limit of the finish rolling temperature may be limited to 920°C.

[0063] The present invention provides a method for manufacturing spring steel wire with excellent permanent deformation resistance, which may include the steps of: drawing steel to produce steel wire; an austenitizing step in which the drawn steel wire is heated to 850 to 1000°C and maintained at that temperature for 1 second or more; and a step in which, after the austenitizing step, the steel wire is quenched at 25 to 80°C and then tempered at 350 to 500°C.

[0064] First, the spring steel with excellent permanent deformation resistance according to the present invention is drawn to produce steel wire.

[0065] Subsequently, the steel wire undergoes an austenitization stage. In the austenitization stage, the steel wire is heat-treated at a temperature range of 850 to 1000°C.

[0066] On the other hand, induction heat treatment equipment is increasingly being used to manufacture spring steel wire. When using induction heat treatment equipment, if the heat treatment holding time is less than 1 second, the ferrite and pearlite structures may not be sufficiently heated and may not transform into austenite. Therefore, the heat treatment holding time in the austenitization stage may be 1 second or longer.

[0067] Next, the steel wire that has undergone the austenitization step is hardened at a temperature of 25 to 80°C and then tempered at a temperature of 350 to 500°C. This tempering is a step necessary for the present invention to ensure the desired mechanical properties, and is required to ensure toughness and strength.

[0068] If the tempering temperature is too low, toughness may not be ensured, and the product may break during molding and in its finished state. Considering this, the tempering temperature may be 350°C or higher. On the other hand, if the tempering temperature is too high, the strength may decrease rapidly, making it difficult to ensure high strength. Considering this, the upper limit of the tempering temperature may be limited to 500°C.

[0069] The present invention will be described in more detail below through examples. However, these examples are for illustrative purposes only and do not limit the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0070] {Example} After manufacturing a billet having the alloy composition shown in Table 1 below, the billet was heated and finish-rolled under the conditions shown in Table 1 below, and then wound up to produce spring steel.

[0071] Subsequently, the spring steel was drawn according to the ASTM E8 standard, and then subjected to an austenitization step of heating at 975°C for 15 minutes. Next, it was rapidly cooled (quenched) by immersion in 70°C oil, and then tempered by holding at 390°C for 30 minutes to produce spring wire.

[0072] [Table 1]

[0073] The grain size and dislocation density of steel, the grain size and dislocation density of steel wire, the area of ​​the hysteresis loop in the Bauschinger torsion test, and the tensile strength after quenching and tempering heat treatment are shown in Table 2 below.

[0074] The crystal grain size was measured by analyzing the orientation at five arbitrary locations using an electron backscatter diffraction (EBSD) pattern analyzer with the model name JSM 7200F. The average crystal grain size represents the average of the crystal grain sizes measured at these five arbitrary locations.

[0075] Dislocation density was measured by taking photographs with a transmission electron microscope (TEM) model named FEI Technai Osiris and then observing the number of dislocations per unit area.

[0076] The Bauschinger torsion test involves twisting a steel wire at a rate of 15° / min under a load exceeding its yield strength, then removing the load, and finally twisting it again at a rate of 15° / min under a load exceeding its yield strength. The portion of the torque-twist angle curve that overlaps with this twist is called the hysteresis loop.

[0077] The tensile strength after quenching and tempering heat treatment was measured using a universal test machine (UTM).

[0078] [Table 2]

[0079] Examples 1-3 satisfied the alloy composition and manufacturing conditions proposed in this invention. Therefore, the average grain size of the steel was 9.6 μm or less, and the dislocation density of the steel was 0.11 × 10⁻⁶. 15 / m 2 In summary, the average grain size of the steel wire is 8.4 μm or less, and the dislocation density of the steel wire is 1.16 × 10⁻⁶. 15 / m 2 The above, plus the area of ​​the hysteresis loop in the Bauschinger torsion test, is 206 mm². 2 The above conditions were met.

[0080] Comparative Example 1 satisfied the alloy composition proposed in the present invention, but the finish rolling temperature did not satisfy the range of 855-920°C. Consequently, Comparative Example 1 exhibited coarse grains, with an average grain size of 15.6 μm for the steel and 12.3 μm for the steel wire. As a result, the hysteresis loop area obtained in the Bauschinger torsion test was 163 mm². 2 The resistance was very low, resulting in poor resistance to permanent deformation.

[0081] Comparative Example 2 satisfied the alloy composition proposed in the present invention, but the billet heating temperature did not satisfy the range of 960-1100°C. Consequently, Comparative Example 2 exhibited coarse grains, with an average grain size of 13.8 μm for the steel and 11.4 μm for the steel wire. As a result, the hysteresis loop area obtained in the Bauschinger torsion test was 184 mm². 2 The resistance was very low, resulting in poor resistance to permanent deformation.

[0082] Comparative Example 3 satisfied the alloy composition proposed in the present invention, but the finish rolling temperature did not satisfy the range of 855-920°C. Consequently, Comparative Example 3 exhibited coarse grains, with an average grain size of 11.4 μm for the steel and 10.7 μm for the steel wire. As a result, the area of ​​the hysteresis loop obtained in the Bauschinger torsion test was 205 mm². 2 It was low and had poor resistance to permanent deformation.

[0083] Figures 1 and 2 are graphs showing the area of ​​the hysteresis loop in steel wire against the average grain size of steel and steel wire. Referring to Figures 1 and 2, it can be seen that the area of ​​the hysteresis loop increases as the average grain size decreases. In other words, it can be confirmed that the lower the average grain size, the better the resistance to permanent deformation.

[0084] Figures 3 and 4 are graphs showing the area of ​​the hysteresis loop in steel wire with respect to dislocation density. Referring to Figures 3 and 4, it can be seen that the area of ​​the hysteresis loop increases with increasing dislocation density. In other words, it can be confirmed that higher dislocation density results in superior resistance to permanent deformation. [Industrial applicability]

[0085] According to the present invention, it is possible to provide spring steel and steel wire in which permanent deformation resistance is improved by increasing the dislocation density in the material or decreasing the average grain size, and a method for manufacturing the same.

Claims

1. In mass%, it contains C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, with the remainder being Fe and other unavoidable impurities. Dislocation density is 1.16 × 10⁻⁶ １５ / m ２ That's all. A spring steel wire with excellent resistance to permanent deformation, characterized by an average crystal grain size of 8.4 μm or less.

2. The spring steel wire with excellent permanent deformation resistance according to Claim 1, further comprising one or more selected from the group consisting of V: ​​0.01 to 0.3%, Nb: 0.005 to 0.05%, Ti: 0.001 to 0.15%, and Mo: 0.01 to 0.4% by mass.

3. The area of ​​the hysteresis loop obtained in the Bauschinger torsion test is 206 mm². ２ The spring steel wire with excellent resistance to permanent deformation as described in claim 1, characterized in that it is as described above.

4. A step in manufacturing steel wire by drawing steel containing, by mass%, C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, with the remainder being Fe and other unavoidable impurities. The drawn steel wire is heated to 850-1000°C and maintained at that temperature for 1 second or more in an austenitizing step, and The method for manufacturing spring steel wire with excellent permanent deformation resistance according to claim 1, characterized in that, after the austenitizing step, the process includes a step of quenching at 25 to 80°C followed by tempering at 350 to 500°C.

5. The method for manufacturing a spring steel wire with excellent permanent deformation resistance according to claim 4, characterized in that the steel further comprises one or more selected from the group consisting of V: ​​0.01 to 0.3%, Nb: 0.005 to 0.05%, Ti: 0.001 to 0.15%, and Mo: 0.01 to 0.4% by mass.

6. The composition comprises, by mass%, C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, with the remainder being Fe and other unavoidable impurities. Dislocation density is 0.11 × 10 １５ / m ２ That's all. A spring steel with excellent resistance to permanent deformation, characterized by an average grain size of 9.6 μm or less.

7. The spring steel with excellent permanent deformation resistance according to Claim 6, further comprising one or more selected from the group consisting of V: ​​0.01 to 0.3%, Nb: 0.005 to 0.05%, Ti: 0.001 to 0.15%, and Mo: 0.01 to 0.4% by mass.

8. A step in manufacturing a billet containing, by mass%, C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, with the remainder being Fe and other unavoidable impurities. The steps include heating the billet to 960-1100°C, and A method for producing spring steel with excellent resistance to permanent deformation, as described in claim 6, characterized by including a step of finish rolling at 855 to 920°C.

9. The method for producing spring steel with excellent permanent deformation resistance according to claim 8, characterized in that the billet further comprises one or more selected from the group consisting of V: ​​0.01 to 0.3%, Nb: 0.005 to 0.05%, Ti: 0.001 to 0.15%, and Mo: 0.01 to 0.4% by mass.

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