JP2024532753A - Spring steel and spring steel wire, and their manufacturing method - Google Patents

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 manufacturing the same.
The present invention relates to a method for producing a steel wire, the method including the steps of: 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 to 850-1000°C and maintaining the temperature for 1 second or more to effect austenitization; and The austenitizing step is followed by a step of quenching at 25 to 80°C and then tempering at 350 to 500°C. The steel further comprises, by weight%, 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%, and the area of the hysteresis loop obtained by a Bauschinger torsion test is 206 ^mm2 or more.
[Selected figure] Figure 4

Description

The present invention relates to spring steel and spring steel wire, and their manufacturing methods, and more specifically to spring steel and spring steel wire with improved resistance to permanent deformation by increasing the dislocation density in the material or decreasing the average crystal grain size, and their manufacturing methods.

In recent years, there has been a strong demand for lighter automotive materials in order to improve fuel efficiency. In particular, in the case of suspension springs, in order to meet the demand for lighter weight, spring designs are now being applied that use high-strength materials with strengths of 1800 MPa or more after quenching and tempering.

However, when currently available spring steels are used under high stress conditions, problems such as deterioration of durability and increased permanent deformation are likely to occur. The permanent deformation (resistance) of a spring is the resistance to plastic deformation caused by dynamic and static loads applied during the use of the spring, and generally refers to the change in height of the spring after a certain period of use relative to its initial height. Therefore, an increase in permanent deformation reduces the spring height, lowering the vehicle height, and as a result, the bumper height is reduced, causing serious problems from the perspective of safety. Therefore, there is a demand for spring steels with high permanent deformation resistance to enable high stress design of springs.

It has become clear that the silicon contained in spring steel is effective in improving permanent deformation resistance, and steel that meets SAE9254 is widely used as spring steel with excellent permanent deformation resistance. However, as the demand for high-stress springs continues to increase, so too does the demand for methods that can further increase permanent deformation resistance.

Patent Document 1 discloses that the permanent deformation resistance is excellent when the total number of (V, Cr) carbides, carbonitrides, and V and Cr composite carbides and composite carbonitrides with a diameter of 50 nm or less is ¹⁰ /μm2 or more in ferrite in the pearlite structure. However, since (V, Cr) carbides, carbonitrides, and V and Cr composite carbides and composite carbonitrides are all mainly composed of V, they rapidly dissolve at temperatures of 850°C or higher. Therefore, in the current spring processing process in which the heating temperature is 900°C or higher, it is difficult to expect the improvement in permanent deformation resistance due to the precipitates disclosed in Patent Document 1. In addition, since the price of V ferroalloy has recently risen exponentially, the contents disclosed in Patent Document 1 may also act as a disadvantage in terms of manufacturing costs.

JP 2002-180199 A

The objective of the present invention to solve the above problems is to provide a spring steel and spring steel wire that have excellent resistance to permanent deformation by increasing the dislocation density in the material or decreasing the average crystal grain size, and a manufacturing method thereof.

The spring steel wire of the present invention having excellent resistance to permanent deformation contains, by weight, 0.4 to 0.7% C, 1.2 to 2.3% Si, 0.2 to 0.8% Mn, 0.2 to 0.8% Cr, with the balance being Fe (iron) and other unavoidable impurities, has a dislocation density of 1.16 x ¹⁰¹⁵ / ^m2 or more, and an average crystal grain size of 8.4 μm or less.

The spring steel wire of the present invention, which has excellent resistance to permanent deformation, further contains, by weight, 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%.

In addition, the spring steel wire of the present invention having excellent resistance to permanent deformation has a hysteresis loop area of 206 ^mm2 or more obtained in a Bauschinger torsion test.

The method for producing a spring steel wire with excellent resistance to permanent deformation according to the present invention 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, with the remainder being Fe (iron) and other unavoidable impurities, heating the drawn steel wire to 850-1000°C and maintaining this temperature for at least 1 second, and quenching the wire at 25-80°C after the austenitizing step, followed by tempering at 350-500°C.

In addition, in the method for producing a spring steel wire having excellent resistance to permanent deformation according to the present invention, the steel further contains, by weight percent, 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%.

The spring steel of the present invention having excellent resistance to permanent deformation contains, by weight, 0.4 to 0.7% C, 1.2 to 2.3% Si, 0.2 to 0.8% Mn, 0.2 to 0.8% Cr, with the balance being Fe (iron) and other unavoidable impurities, has a dislocation density of 0.11 x ¹⁰¹⁵ / ^m2 or more, and an average crystal grain size of 9.6 μm or less.

In addition, the spring steel with excellent permanent deformation resistance according to one embodiment of the present invention further contains, by weight percent, 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%.

The method for producing spring steel with excellent permanent deformation resistance of the present invention includes the steps of producing a billet 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 remainder being Fe (iron) and other unavoidable impurities, heating the billet at 960-1100°C, and finish rolling at 855-920°C.

In addition, in the manufacturing method of spring steel with excellent permanent deformation resistance of the present invention, the billet further contains, by weight percent, 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%.

The present invention provides spring steel and steel wire with improved resistance to permanent deformation by increasing the dislocation density in the material or decreasing the average crystal grain size, and a manufacturing method thereof.

1 is a graph showing the relationship between the average crystal grain size and the area of the hysteresis loop of a steel wire for spring steels according to the present invention and comparative examples. 1 is a graph showing the relationship between the average crystal grain size and the area of the hysteresis loop of a spring steel wire according to the present invention and a comparative example. 1 is a graph showing the relationship between dislocation density and the area of the hysteresis loop of a steel wire in spring steels according to the present invention and a comparative example. 1 is a graph showing the relationship between dislocation density and the area of the hysteresis loop of a spring steel wire according to the present invention and a comparative example.

The spring steel wire of the present invention having excellent resistance to permanent deformation may contain, by weight, 0.4 to 0.7% C, 1.2 to 2.3% Si, 0.2 to 0.8% Mn, 0.2 to 0.8% Cr, with the balance being Fe (iron) and other unavoidable impurities, have a dislocation density of 1.16 x ¹⁰¹⁵ / ^m2 or more, and an average crystal grain size of 8.4 μm or less.

The following describes preferred embodiments of the present invention. However, the embodiments of the present invention may be modified in various different forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the art.

The terms used in this application are merely used to describe specific examples. Thus, for example, singular expressions include plural expressions unless the context clearly requires the singular. Furthermore, it should be noted that the terms "include" or "comprise" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

On the other hand, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Therefore, unless expressly defined herein, certain terms should not be interpreted in an overly ideal or formal sense. For example, in this specification, singular expressions include plural expressions unless there is a clear exception in the context.

In addition, in this specification, the terms "about," "substantially," and the like are used to mean a numerical value or a value close to that value when the tolerances of manufacturing and materials inherent in the referred meaning are presented, and are used to prevent unscrupulous infringers from unfairly using the disclosure content in which precise and absolute numerical values are mentioned to aid in the understanding of the present invention.

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

The reasons for limiting the alloy composition are explained in detail below.

The C (carbon) content may be 0.4-0.7%.

C is an essential element that is added to ensure the strength of the spring. Taking this into consideration, C may be added in an amount of 0.4% or more. However, if the C content is excessive, twin martensite structures are formed during the heat treatments of quenching and tempering, causing cracks in the material and significantly reducing the fatigue life. In addition, if the C content is excessive, defect sensitivity increases, and if corrosion pits occur on the surface, the fatigue life and fracture stress are significantly reduced. Taking this into consideration, the upper limit of the C content may be limited to 0.7%.

The Si (silicon) content may be 1.2-2.3%.

Si is an element that has an excellent effect of dissolving in ferrite to strengthen the strength and improve deformation resistance. Taking this into consideration, 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 becomes saturated and surface decarburization may occur during heat treatment. Taking this into consideration, the upper limit of the Si content may be limited to 2.3%.

The Mn (manganese) content may be 0.2-0.8%.

Mn is an element that plays a role in improving the hardenability of steel and ensuring its strength. Taking this into consideration, Mn may be added in an amount of 0.2% or more. However, if the Mn content is excessive, the hardenability increases excessively, hard tissue is likely to occur during cooling after hot rolling, and the formation of MnS inclusions increases, which may result in a decrease in corrosion fatigue resistance. Taking this into consideration, the upper limit of the Mn content may be limited to 0.8%.

The Cr (chromium) content may be 0.2-0.8%.

Cr is a useful element for ensuring oxidation resistance, temper softening, prevention of surface decarburization, and hardenability. Taking this into consideration, Cr may be added in an amount of 0.2% or more. However, if the Cr content is excessive, the strength may be deteriorated due to a decrease in deformation resistance. Taking this into consideration, the upper limit of the Cr content may be limited to 0.8%.

The spring steel with excellent permanent deformation resistance of the present invention may further contain, by weight percent, 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%.

The V (vanadium) content may be 0.01-0.3%.

V is an element that contributes to improving strength and refining crystal grains. In addition, V can combine with C and N to form carbonitrides, which act as hydrogen trapping sites to inhibit hydrogen penetration into the steel material and reduce the occurrence of corrosion. Taking this into consideration, V may be added in an amount of 0.01% or more. However, if the V content is excessive, manufacturing costs may increase. Taking this into consideration, the upper limit of the V content may be limited to 0.3%.

The Nb (niobium) content may be 0.005-0.05%.

Nb is an element that combines with C and N to form carbonitrides, contributing to refinement of the structure and acting as a trapping site for hydrogen. Taking this into consideration, 0.005% or more of Nb may be added. However, if the Nb content is excessive, coarse carbonitrides may be formed, reducing the ductility of the steel. Taking this into consideration, the upper limit of the Nb content may be limited to 0.05%.

The Ti (titanium) content may be 0.001-0.15%.

Ti is an element that improves strength and toughness through precipitation strengthening and contributes to grain refinement. In addition, Ti can combine with C and N to form carbonitrides, which can act as hydrogen trapping sites and cause precipitation hardening, thereby improving spring properties. In consideration of this, Ti may be added in an amount of 0.001% or more. However, if the Ti content is excessive, the manufacturing cost increases and the spring property improvement effect due to the precipitation saturates. In addition, if the Ti content is excessive, the amount of coarse alloy carbides increases in the base material during austenite heat treatment, acting similarly to nonmetallic inclusions, which may reduce fatigue properties and precipitation strengthening effects. In consideration of this, the upper limit of the Ti content may be limited to 0.15%.

The Mo (molybdenum) content may be 0.01-0.4%.

Mo is an element that combines with C and N to form carbonitrides, contributing to refinement of the structure and acting as a trapping site for hydrogen. Taking this into consideration, Mo may be added at 0.01% or more. However, if the Mo content is excessive, hard structures are likely to form during cooling after hot rolling, and coarse carbonitrides may form, reducing the ductility of the steel. Taking this into consideration, the upper limit of the Mo content may be limited to 0.4%.

The remaining component of the present invention is iron (Fe). However, in the normal steel manufacturing process, unintended impurities may be unavoidably mixed in from the raw materials or the surrounding environment, and it is not possible to eliminate these. These impurities are known to any engineer of normal manufacturing processes, so the contents of all of them will not be specifically mentioned in this specification.

Another aspect of the present invention provides a steel wire having the same composition as the spring steel having excellent resistance to permanent deformation. The reasons for limiting the numerical values of each of the components are as described above.

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

Meanwhile, the inventors of the present invention have investigated various influencing factors that affect the permanent deformation resistance of spring steel and have discovered the following facts:

The permanent deformation of a spring occurs through cyclic plastic deformation or microcreep, which occurs over many loading cycles at stress levels lower than the yield strength of the material. When a material is deformed, new dislocations are generated within the material, or existing dislocations move and merge or disappear, ultimately changing the dislocation density.

Generally, rolling, forming, and processing cause deformation that exceeds the yield point at one time, which increases the dislocation density and results in work hardening. However, when a spring is subjected to cyclic plastic deformation or microcreep at a low stress level that does not exceed the yield point, the dislocation density decreases over a long period of time, and the spring ultimately becomes permanently deformed. However, because springs must operate at a stress level lower than the yield point for stability reasons due to the product's characteristics, it is inevitable that the dislocation density will decrease after a certain period of use.

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

To increase the dislocation density in the material when manufacturing springs, the dislocation density must be increased from the time of manufacturing the wire rod by hot rolling. To achieve this, it is effective to roll at a lower temperature or to cool it. Also, to ensure that dislocations pile up frequently at the grain boundaries, the grains must be refined to shorten the distance that the dislocations travel to the grain boundaries, so that they encounter the grain boundaries more frequently.

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

The spring steel of the present invention, which has excellent resistance to permanent deformation, may also have an average grain size of 9.6 μm or less.

Moreover, the spring steel wire excellent in permanent deformation resistance of the present invention may have a dislocation density of 1.16×10 ¹⁵ /m ² or more.

The spring steel wire of the present invention, which has excellent resistance to permanent deformation, may also have an average crystal grain size of 8.4 μm or less.

Meanwhile, the permanent deformation of a spring means the change in height of the spring after it has been used for a certain period of time compared to its initial height, so it is generally measured in the spring state, but the Bauschinger torsion test allows it to be measured in the steel wire state as well. In the Bauschinger torsion test, a steel wire is twisted at a speed of 15°/min with a load equal to or greater than the yield strength, and then after the load is removed, it is twisted at a speed of 15°/min with a load equal to or greater than the yield strength. The part that overlaps with the torque-twist angle curve at this time is called the hysteresis loop. The larger the area of the hysteresis loop, the greater the spring's resistance to permanent deformation.

Therefore, when the Bauschinger torsion test is performed on the spring steel wire having excellent permanent deformation resistance according to the present invention, the area of the hysteresis loop may be 206 ^mm2 or more.

Next, we will explain the manufacturing method of the spring steel and steel wire of the present invention that have excellent resistance to permanent deformation.

The method for producing spring steel having excellent permanent deformation resistance according to the present invention may include the steps of producing a billet containing, by weight, 0.4-0.7% C, 1.2-2.3% Si, 0.2-0.8% Mn, 0.2-0.8% Cr, the balance being Fe (iron) and other unavoidable impurities, having a mixed structure of ferrite and pearlite as a microstructure, and having a dislocation density of 0.11 x ¹⁰¹⁵ / ^m2 or more, heating the billet at 960-1100°C, and finish rolling and coiling at 855-920°C.

The reasons for limiting the composition ratio of each alloy element are as described above, and each manufacturing step will be explained in more detail below.

As mentioned above, in order to improve the permanent deformation resistance of springs, the dislocation density of spring steel and steel wire must be increased or the crystal grains must be refined. In addition, in order to increase the dislocation density or refine the crystal grains, the billet heating temperature and the finish rolling temperature must be appropriately controlled.

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 be large. Also, if the heating temperature of the billet is too low, the coarse carbides that may be generated during casting will not all dissolve, and the alloy 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 become large, and even if hot rolling is performed under the same rolling conditions, the grain size of the final wire will become large. Taking this into consideration, the upper limit of the heating temperature of the billet may be limited to 1100°C.

The finish rolling temperature in 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 be large. 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 before the start of cooling will be large, and the grain size after final cooling will be large. Taking this into consideration, the upper limit of the finish rolling temperature may be limited to 920°C.

The method for producing a spring steel wire having excellent resistance to permanent deformation according to the present invention may include a step of producing a steel wire by drawing the steel, an austenitizing step of heating the drawn steel wire at 850 to 1000°C and maintaining the temperature for at least 1 second, and a step of quenching the wire at 25 to 80°C after the austenitizing step and tempering the wire at 350 to 500°C.

First, the spring steel of the present invention, which has excellent resistance to permanent deformation, is drawn to produce a steel wire.

Then, the steel wire undergoes an austenitizing step, in which the steel wire is heat treated at a temperature range of 850 to 1000°C.

Meanwhile, induction heat treatment equipment is now often 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 heated sufficiently and may not transform into austenite. Therefore, the heat treatment holding time in the austenitizing step may be 1 second or more.

Next, the steel wire that has undergone the austenitizing step is quenched at a temperature in the range of 25 to 80°C, and then tempered at a temperature in the range of 350 to 500°C. The tempering step is a step in which the present invention secures the desired mechanical properties, and is necessary to ensure toughness and strength.

If the tempering temperature is too low, toughness will not be ensured and there is a risk of breakage during forming and in the finished product state. Taking this into consideration, the tempering temperature may be 350°C or higher. On the other hand, if the tempering temperature is too high, the strength will decrease rapidly and it may become difficult to ensure high strength. Taking this into consideration, the upper limit of the tempering temperature may be limited to 500°C.

Hereinafter, the present invention will be described in more detail through examples. However, the description of such examples is intended to illustrate the implementation of the present invention, and the present invention is not limited by the description of such examples. This is because the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from them.

{Example}
A billet having the alloy composition shown in Table 1 below was produced, and then the billet was heated and finish-rolled under the conditions shown in Table 1 below, and then coiled to produce spring steel.

The spring steel was then drawn in accordance with the ASTM E8 standard, and then heated at 975°C for 15 minutes to undergo an austenitization step. It was then immersed in oil at 70°C for rapid cooling (quenching), and then tempered at 390°C for 30 minutes to produce spring steel wire.

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

The grain size was measured by analyzing the orientation of five randomly selected locations using a JSM 7200F electron backscatter diffraction pattern analyzer (EBSD). The average grain size refers to the average of the grain sizes measured at five random locations.

The dislocation density was measured by taking a photograph using a transmission electron microscope (TEM) (model name: FEI Technai Osiris) and then observing the number of dislocations per unit area.

In the Bauschinger torsion test, a steel wire is twisted at a speed of 15°/min with a load equal to or greater than the yield strength, and then after the load is removed, it is twisted at a speed of 15°/min with a load equal to or greater than the yield strength. The part that overlaps with the torque-twist angle curve at this time is called the hysteresis loop.

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

In Examples 1 to 3, the alloy composition and manufacturing conditions satisfied the requirements proposed by the present invention, i.e., the average crystal grain size of the steel was 9.6 μm or less, the dislocation density of the steel was 0.11×10 ¹⁵ /m ² or more, the average crystal grain size of the steel wire was 8.4 μm or less, the dislocation density of the steel wire was 1.16×10 ¹⁵ /m ² or more, and the area of the hysteresis loop in the Bauschinger torsion test was 206 mm ² or more.

In Comparative Example 1, the alloy composition satisfied the requirements of the present invention, but the finish rolling temperature did not satisfy the range of 855 to 920°C. Therefore, in Comparative Example 1, the average grain size of the steel was 15.6 μm, and the average grain size of the steel wire was 12.3 μm, resulting in coarse grains. As a result, the area of the hysteresis loop obtained in the Bauschinger torsion test was very low at 163 ^mm2 , and the permanent deformation resistance was poor.

In Comparative Example 2, the alloy composition satisfied the range proposed by the present invention, but the billet heating temperature did not satisfy the range of 960 to 1100°C. Therefore, in Comparative Example 2, the average crystal grain size of the steel was 13.8 μm, and the average crystal grain size of the steel wire was 11.4 μm, resulting in coarse crystal grains. As a result, the area of the hysteresis loop obtained in the Bauschinger torsion test was very low at 184 ^mm2 , and the permanent deformation resistance was poor.

In Comparative Example 3, the alloy composition satisfied the range proposed by the present invention, but the finish rolling temperature did not satisfy the range of 855 to 920°C. Therefore, in Comparative Example 3, the average crystal grain size of the steel was 11.4 μm, and the average crystal grain size of the steel wire was 10.7 μm, resulting in coarse crystal grains. As a result, the area of the hysteresis loop obtained in the Bauschinger torsion test was low at 205 ^mm2 , and the permanent deformation resistance was poor.

Figures 1 and 2 are graphs showing the area of the hysteresis loop of steel wire as a function of the average crystal grain size of the steel and steel wire. Referring to Figures 1 and 2, it can be seen that the smaller the average crystal grain size, the larger the area of the hysteresis loop. In other words, it can be confirmed that the smaller the average crystal grain size, the better the permanent deformation resistance.

Figures 3 and 4 are graphs showing the area of the hysteresis loop of steel wire as a function of the dislocation density of steel and steel wire. Referring to Figures 3 and 4, it can be seen that the area of the hysteresis loop increases as the dislocation density increases. In other words, it can be confirmed that the higher the dislocation density, the better the permanent deformation resistance.

The present invention provides spring steel and steel wire with improved resistance to permanent deformation by increasing the dislocation density in the material or decreasing the average crystal grain size, and a manufacturing method thereof.

Claims (9)

In weight percent, it contains C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, and the balance is Fe and other unavoidable impurities.
The dislocation density is 1.16×10 ¹⁵ /m ² or more,
A spring steel wire having excellent resistance to permanent deformation and characterized in that the average crystal grain size is 8.4 μm or less. The spring steel wire with excellent resistance to permanent deformation according to claim 1, further comprising, by weight, 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%. 2. The spring steel wire having excellent resistance to permanent deformation according to claim 1, characterized in that the area of a hysteresis loop obtained in a Bauschinger torsion test is 206 ^mm2 or more. A step of producing a steel wire by wiredrawing a steel containing, by weight%, 0.4 to 0.7% C, 1.2 to 2.3% Si, 0.2 to 0.8% Mn, 0.2 to 0.8% Cr, and the balance being Fe (iron) and other unavoidable impurities;
austenitizing the drawn steel wire at 850 to 1000°C and maintaining the temperature for at least 1 second; and after the austenitizing, quenching the drawn steel wire at 25 to 80°C and tempering the wire at 350 to 500°C. The method for manufacturing spring steel wire with excellent resistance to permanent deformation according to claim 4, characterized in that the steel further contains, by weight percent, 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%. In weight percent, it contains C: 0.4-0.7%, Si: 1.2-2.3%, Mn: 0.2-0.8%, Cr: 0.2-0.8%, and the balance is Fe and other unavoidable impurities.
The dislocation density is 0.11×10 ¹⁵ /m ² or more,
A spring steel having excellent resistance to permanent deformation, characterized in that the average crystal grain size is 9.6 μm or less. The spring steel with excellent resistance to permanent deformation according to claim 6, further comprising, by weight, 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%. A step of producing a billet containing, by weight percent, 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;
The method for producing spring steel having excellent resistance to permanent deformation comprises the steps of heating the billet at 960 to 1100°C, and finish rolling the billet at 855 to 920°C. 9. The method for producing a spring steel having excellent permanent deformation resistance according to claim 8, wherein the billet further contains, by weight%, 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%.

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