STEEL AND STEEL WIRE, WHICH ARE FOR SPRING, AND MANUFACTURING METHODS THEREFOR

Abstract

 Disclosed in the present specification are steel and a steel wire, which are for a spring, and manufacturing methods therefor, the steel and the steel wire having excellent resistance to permanent deformation by having increased in-material dislocation density or reduced average grain diameter. The steel wire for a spring, having excellent resistance to permanent deformation, according to an embodiment of the present invention, comprises, by wt%, 0.4-0.7% of C, 1.2-2.3% of Si, 0.2-0.8% of Mn, 0.2-0.8% of Cr, and the balance of Fe (iron) and other inevitable impurities, wherein the dislocation density thereof can be 1.16 × 10¹⁵/m² or more, and the average grain diameter thereof can be 8.4 µm or less.

Description

[Technical field]

[0001] The present invention relates to steel and steel wire for spring having excellent permanent deformation resistance, and more specifically, to steel and steel wire for spring with improved permanent deformation resistance by increasing in-material dislocation density or reducing an average grain diameter, and methods of manufacturing the same.

[Background Art]

[0002] Recently, there has been a great demand for lighter materials for automobiles in order to improve automobile fuel efficiency. In particular, in the case of suspension springs, a spring design using high-strength materials with a strength of 1800 MPa or more after quenching and tempering is being applied to meet the demand for weight lightening.

[0003] However, when currently available steel for springs are used under high stress conditions, problems such as deterioration of durability and an increase in permanent deformation are likely to occur. Permanent deformation of the spring is resistance to plastic deformation caused by dynamic and static loads applied during use of the spring, and generally refers to the change in height after use for a certain period of time compared to the initial height of the spring. Therefore, an increase in permanent deformation reduces the height of the spring, which lowers the height of the vehicle, and as a result, the height of the bumper is lowered, causing a serious problem from a safety perspective. Therefore, in order to enable high-stress design of springs, steel for spring with high permanent deformation resistance is required.

[0004] As it has been discovered that Si contained in steel for spring materials is effective in improving permanent deformation resistance, steel corresponding to SAE9254 is becoming popular as steel for spring having excellent permanent deformation resistance. However, as the demand for high-stress springs continues to increase, the need for methods to further increase permanent deformation resistance is also increasing.

[0005] Patent Document 0001 discloses that, in the case where ferrite in the pearlite structure contains (V, Cr) carbides, carbonitrides, composite carbides of V and Cr, and composite carbonitrides with a diameter of 50 nm or less in a total of 10/µm² or more, it has excellent permanent deformation resistance. However, since (V, Cr) carbides, carbonitrides, composite carbides of V and Cr, and composite carbonitrides all have V as main components, they rapidly dissolve at temperatures of 850 °C or higher. Therefore, in the current spring processing process where the heating temperature is 900 °C or higher, it is difficult to expect an improvement in permanent deformation resistance by the precipitates disclosed in Patent Document 0001. In addition, as the price of V alloy iron has increased exponentially recently, the content described in Patent Document 0001 may also act as a disadvantage in terms of manufacturing costs.

[0006] (Patent Document 0001) Japanese Patent Publication No. 2002-180199 A (Publication date: 2002.06.26).

[Disclosure]

[Technical Problem]

[0007] To solve the above-described problems, the present invention is directed to providing steel and steel wire for spring having excellent permanent deformation resistance by increasing in-material dislocation density or reducing an average grain diameter, and methods of manufacturing the same.

[Technical Solution]

[0008] A steel wire for spring having excellent permanent deformation resistance according to an embodiment of the present invention may include C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe (iron), and other inevitable impurities, based on weight percent, wherein a dislocation density may be 1.16 × 10¹⁵/m² or more, and an average grain diameter may be 8.4 µm or less.

[0009] In addition, the steel wire for spring having excellent permanent deformation resistance according to an embodiment of the present invention may further include 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%, based on weight percent.

[0010] In addition, the steel wire for spring having excellent permanent deformation resistance according to an embodiment of the present invention may have a hysteresis loop area of 206 mm² or more obtained by a Bauschinger torsion test.

[0011] In addition, a method of manufacturing a steel wire for spring having excellent permanent deformation resistance according to an embodiment of the present invention may include: manufacturing a steel wire by drawing steel including C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe (iron), and other inevitable impurities, based on weight percent; heating the drawn steel wire to 850 to 1000 °C and then maintaining the steel wire for 1 second or more to austenitize; and after the austenitization, quenching the steel wire at 25 to 80 °C and then tempering the steel wire at 350 to 500 °C.

[0012] In addition, in the method of manufacturing a steel wire for spring having excellent permanent deformation resistance according to an embodiment of the present invention, the steel may further include 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%, based on weight percent.

[0013] In addition, the steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention may include C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe (iron), and other inevitable impurities, based on weight percent, wherein a dislocation density may be 0.11 × 10¹⁵/m² or more, and an average grain diameter may be 9.6 µm or less.

[0014] In addition, the steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention may further include 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%, based on weight percent.

[0015] In addition, a method of manufacturing steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention may include: manufacturing a billet including C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe (iron), and other inevitable impurities, based on weight percent; heating the billet to 960 to 1100 °C; and finish rolling the billet at 855 to 920 °C.

[0016] In addition, in the method of manufacturing steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention, the billet may further include 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%, based on weight percent.

[Advantageous Effects]

[0017] Embodiments of the present invention can provide steel and steel wire for spring having improved resistance to permanent deformation by increasing in-material dislocation density or reducing an average grain diameter, and methods of manufacturing the same.

[Description of Drawings]

[0018] 

FIG. 1 is a graph showing the relationship between the average grain diameter of steel for spring and the hysteresis loop area of a steel wire according to an example of the present invention and comparative examples.

FIG. 2 is a graph showing the relationship between the average grain diameter of a steel wire for spring and the hysteresis loop area of a steel wire according to an example of the present invention and comparative examples.

FIG. 3 is a graph showing the relationship between the dislocation density of steel for spring and the hysteresis loop area of a steel wire according to an example of the present invention and comparative examples.

FIG. 4 is a graph showing the relationship between the dislocation density of a steel wire for spring and the hysteresis loop area of a steel wire according to an example of the present invention and comparative examples.

[Modes of the Invention]

[0019] A steel wire for spring having excellent permanent deformation resistance according to an embodiment of the present invention may include C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe (iron), and other inevitable impurities, based on weight percent, wherein a dislocation density may be 1.16 × 10¹⁵/m² or more, and an average grain diameter may be 8.4 µm or less.

[Modes of the Invention]

[0020] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the technical idea of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0021] The terms used in the present application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression, unless the context clearly requires it to be singular. In addition, it should be noted that terms such as "comprise" or "include" used in the present application are used to clearly indicate the presence of features, steps, functions, components, or a combination thereof described in the specification, but is not intended to preliminarily rule out the presence of other features, steps, functions, components, or a combination thereof.

[0022] Meanwhile, unless otherwise defined, all terms used in the present specification should be viewed as having the same meaning as generally understood by those skilled in the art to which the present invention pertains. Therefore, unless clearly defined in this specification, specific terms should not be interpreted in an overly idealistic or formal sense. For example, in this specification, singular expressions include plural expressions unless the context clearly dictates otherwise.

[0023] In addition, in this specification, the terms "about," "substantially," and the like are used in a sense at or close to the value when the manufacturing and material tolerances inherent in the stated meaning are presented, and are used to prevent an unscrupulous infringer from unfairly using the disclosure in which exact or absolute values are mentioned to aid in understanding the present invention.

[0024] Steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention may include C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe (iron), and other inevitable impurities, based on weight percent.

[0025] Hereinafter, the reason for limiting the alloy composition will be described in detail.

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

[0027] C is an essential element added to secure the strength of the spring. Considering this, C may be added in an amount of 0.4% or more. However, when the C content is excessive, a twin-type martensite structure is formed during quenching and tempering heat treatment, resulting in material cracks, and thus fatigue life is significantly reduced. In addition, when the content of C is excessive, defect susceptibility increases, and when corrosion pits occur on the surface, fatigue life or fracture stress is significantly reduced. Considering this, the upper limit of the C content may be limited to 0.7%.

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

[0029] Si is an element that is dissolved in ferrite and has an excellent effect in strengthening strength and improving deformation resistance. Considering this, Si may be added in an amount of 1.2% or more, and more preferably, 1.4% or more. However, when the Si content is excessive, the effect of improving deformation resistance is saturated and surface decarburization may occur during heat treatment. Considering this, the upper limit of the Si content may be limited to 2.3%.

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

[0031] Mn is an element that plays a role in securing strength by improving the hardenability of steel materials. Considering this, Mn may be added in an amount of 0.2% or more. However, when the Mn content is excessive, hardenability increases excessively, making it easy for hard structures to form when cooled after hot rolling, and the generation of MnS inclusions increases, which may deteriorate corrosion resistance and fatigue properties. Considering this, the upper limit of the Mn content may be limited to 0.8%.

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

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

[0034] In addition, steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention may further include 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%, based on weight percent.

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

[0036] V is an element that contributes to strength improvement and grain refinement. In addition, V may combine with C or N to form carbide/nitride, and the formed carbide/nitride acts as a trap site for hydrogen, suppressing hydrogen intrusion into the steel material and reducing corrosion generation. Considering this, V may be added in an amount of 0.01% or more. However, when the V content is excessive, manufacturing costs may increase. Considering this, the upper limit of the V content may be limited to 0.3%.

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

[0038] Nb is an element that combines with C or N to form carbide/nitride, contributing to structure refinement and acting as a trap site for hydrogen. Considering this, Nb may be added in an amount of 0.005% or more. However, when the Nb content is excessive, coarse carbide/nitride may be formed and the ductility of the steel material may be reduced. Considering this, the upper limit of the Nb content may be limited to 0.05%.

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

[0040] Ti is an element that improves strength and toughness through precipitation strengthening and contributes to particle refinement. In addition, Ti may combine with C or N to form carbide/nitride, and the formed carbide/nitride may act as a trap site for hydrogen and improve spring characteristics by causing precipitation hardening. Considering this, Ti may be added in an amount of 0.001% or more. However, when the Ti content is excessive, manufacturing costs increase and the effect of improving spring properties due to precipitates is saturated. In addition, when the Ti content is excessive, since the amount of coarse alloy carbides in the base material increases during austenite heat treatment and it acts like non-metallic inclusions, fatigue properties and precipitation strengthening effects may be reduced. Considering this, the upper limit of the Ti content may be limited to 0.15%.

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

[0042] Mo is an element that combines with C or N to form carbide/nitride, contributing to structure refinement and acting as a trap site for hydrogen. Considering this, Mo may be added in an amount of 0.01% or more. However, when the Mo content is excessive, hard structures are likely to occur when cooled after hot rolling, and coarse carbide/nitride may be formed, thereby 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 the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated in an ordinary steel manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, they are not all specifically mentioned in this specification.

[0044] Another aspect of the present invention provides a steel wire composed of the same composition as the steel for spring having excellent permanent deformation resistance. The reason for limiting the numerical value of each component is as described above.

[0045] The steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention can include a mixed ferrite and pearlite structure as a microstructure by controlling the alloy component composition ratio, and no bainite or martensite may be present.

[0046] Meanwhile, the inventors of the present invention examined various factors influencing the permanent deformation resistance of steel for spring and discovered the following facts.

[0047] Permanent deformation of springs occurs due to cyclic plastic deformation or microcreep that occurs over multiple loading cycles at stress levels below the material's yield strength. When a material is deformed, new dislocations are created within the material, and dislocations that already are present are combined or disappear as they move, ultimately changing the dislocation density.

[0048] In general, since rolling, molding, processing, and the like give an amount of deformation that exceeds the yield point at once, the dislocation density increases and thus a work hardening phenomenon occurs. However, like a spring, when it undergoes cyclic plastic deformation or a microcreep phenomenon at a low stress level that does not exceed the yield point, the dislocation density decreases over a long period of time, and eventually the spring becomes permanently deformed. However, due to the nature of the product, since springs should be operated at a stress level lower than the yield point in consideration of stability, it is inevitable that the dislocation density will decrease after a certain period of use.

[0049] Therefore, in order to improve the permanent deformation resistance of a spring, it is most desirable to increase in-material dislocation density when manufacturing the spring or to reduce the rate at which dislocations frequently pile up and disappear at grain boundaries during spring use.

[0050] In order to increase in-material dislocation density when manufacturing springs, the dislocation density should be increased from the time of manufacturing wire rods through hot rolling, and to achieve this, rolling or cooling at a lower temperature is effective. In addition, in order for dislocations to accumulate frequently at grain boundaries, the grains should be refined to shorten the distance the dislocations move to the grain boundaries so that they reach the grain boundaries more often.

[0051] Therefore, the steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention may have a dislocation density of 0.11 × 10¹⁵/m² or more.

[0052] In addition, the steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention may have an average grain diameter of 9.6 µm or less.

[0053] In addition, the steel wire for spring having excellent permanent deformation resistance according to an embodiment of the present invention may have a dislocation density of 1.16 × 10¹⁵/m² or more.

[0054] In addition, the steel wire for spring having excellent permanent deformation resistance according to an embodiment of the present invention may have an average grain diameter of 8.4 µm or less.

[0055] Meanwhile, since the permanent deformation of a spring refers to the change in height after use for a certain period of time compared to the initial height of the spring, it is generally measured in the spring state, but a method that allows measurement even in the steel wire state is a Bauschinger torsion test. The Bauschinger torsion test was performed by applying a load above the yield strength to the steel wire and twisting it at a rate of 15°/min, and then applying a load above the yield strength after removing the load and twisting it at a speed of 15°/min. At this time, the overlapping part on the torque-twist angle curve is called a hysteresis loop. The larger the hysteresis loop area, the greater the permanent deformation resistance of the spring.

[0056] Therefore, when the Bauschinger torsion test is performed using a steel wire for spring having excellent permanent deformation resistance according to an embodiment of the present invention, a hysteresis loop area may be 206 mm² or more.

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

[0058] A method of manufacturing steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention may include: manufacturing a billet including C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe (iron), and other inevitable impurities, based on weight percent, including a ferrite and pearlite mixed structure as a microstructure, and having a dislocation density of 0.11 × 10¹⁵/m² or more; heating the billet to 960 to 1100 °C; and finish rolling and winding the billet at 855 to 920 °C.

[0059] The reason for limiting the component ratio of each alloy element is as described above, and each manufacturing step will be described in more detail below.

[0060] As described above, in order to improve the permanent deformation resistance of the spring, the dislocation density of the steel and steel wire for spring should be increased or the grains should be refined. In addition, in order to increase the dislocation density or refine the grains, it is necessary to appropriately control the billet heating temperature and finish rolling temperature.

[0061] According to an embodiment of the present invention, the heating temperature of the billet is preferably in a range of 960 to 1100 °C. When the heating temperature of the billet is too low, the load on the rolling roll increases. In addition, when the heating temperature of the billet is too low, all the coarse carbides that may have been generated during casting may not be dissolved, so alloying elements may not be uniformly distributed within the austenite. Considering this, a heating temperature of the billet may be 960 °C or higher. On the other hand, when the heating temperature is too high, the grain diameter of the billet becomes large, so even when hot rolling is performed under the same rolling conditions, the grain diameter in the final wire rod inevitably increases. Considering this, the upper limit of the heating temperature of the billet may be limited to 1100 °C.

[0062] The finish rolling temperature according to an embodiment of the present invention is preferably in a range of 855 to 920 °C. When the finish rolling temperature is too low, the load on the rolling roll increases. Considering this, the finish rolling temperature may be 855 °C or higher. On the other hand, when the finish rolling temperature is too high, the austenite grain diameter before the start of cooling increases, so the grain diameter after final cooling inevitably increases. Considering this, the upper limit of the finish rolling temperature may be limited to 920 °C.

[0063] A method of manufacturing a steel wire for spring having excellent permanent deformation resistance according to an embodiment of the present invention may include: manufacturing a steel wire by drawing the steel; heating the drawn steel wire to 850 to 1000 °C and then maintaining the steel wire for 1 second or more to austenitize; and after the austenitization, quenching the steel wire at 25 to 80 °C and then tempering the steel wire at 350 to 500 °C.

[0064] First, a steel wire is manufactured by drawing the steel for spring having excellent permanent deformation resistance according to an embodiment of the present invention.

[0065] Thereafter, it goes through austenitization. In the austenitization, the steel wire is heat-treated at a temperature range of 850 to 1000 °C.

[0066] Meanwhile, induction heat treatment equipment has recently been increasingly used to manufacture a steel wire for spring. When using the induction heat treatment equipment, when the heat treatment maintenance time is less than 1 second, the ferrite and pearlite structures may not be sufficiently heated and thus may not be transformed into austenite. Therefore, the heat treatment maintenance time in the austenitization may be 1 second or more.

[0067] Next, the steel wire that has gone through the austenitization is quenched in the range of 25 to 80 °C, and tempered in the range of 350 to 500 °C. The tempering is a step to secure the mechanical properties desired by the present invention and is necessary to secure toughness and strength.

[0068] When the tempering temperature is too low, toughness is not secured and there is a risk of damage during molding and product states. Considering this, a tempering temperature may be 350 °C or higher. On the other hand, when the tempering temperature is too high, the strength decreases rapidly, making it difficult to secure high strength. Considering this, the upper limit of the tempering temperature may be limited to 500 °C.

[0069] Hereinafter, the present invention will be described in detail by way of examples. However, the description of these examples is only for illustrating the implementation of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of rights of the present invention is determined by matters stated in the patent claims and matters reasonably inferred therefrom.

[Examples]

[0070] After manufacturing a billet having an alloy composition shown in Table 1 below, the billet was subjected to heating and finish rolling under the conditions shown in Table 1 below, and then wound to prepare steel for spring.

[0071] Thereafter, the steel for spring was drawn in accordance with ASTM E8 standards, and then austenitization was performed by heating at 975 °C for 15 min. Thereafter, the steel was immersed in oil at 70 °C, rapidly cooled (quenched), and then tempered at 390 °C for 30 min to manufacture a steel wire for spring.

[Table 1]

	Alloy composition (wt%)								Steel manufacturing conditions
	C	Si	Mn	Cr	V	Ti	Nb	Mo	Billet heating temperature (°C)	Finish rolling temperature (°C)
Comparative	0.53	1.54	0.69	0.67	-	-	-	-	1025	926
Example 1
Comparative Example 2	0.52	1.51	0.65	0.66	0.11	0.03	-	-	1106	874
Comparative Example 3	0.62	1.63	0.58	0.56	-	0.02	0.03	0.15	924	936
Example 1	0.51	1.48	0.71	0.65	-	-	-	-	1061	861
Example 2	0.59	1.56	0.36	0.31	0.10	-	0.02	0.21	1018	873
Example 3	0.64	1.46	0.47	0.29	-	-	0.03	-	975	855

[0072] The grain diameter and dislocation density of the steel and the grain diameter and dislocation density of the steel wire, the hysteresis loop area of the Bauschinger torsion test, and the tensile strength after quenching and tempering heat treatment are shown in Table 2 below.

[0073] The grain diameter was measured by analyzing the orientations of five random locations using an electron backscatter diffraction (EBSD) pattern analyzer with the model name JSM 7200F. The average grain diameter means the average of the grain diameters measured at five arbitrary locations.

[0074] The dislocation density was measured by taking pictures using a transmission electron microscope (TEM) with the model name FEI Technai Osiris, and then observing the number of dislocations included per unit area.

[0075] The Bauschinger torsion test was performed by applying a load above the yield strength to the steel wire and twisting it at a rate of 15°/min, and then applying a load above the yield strength after removing the load and twisting it at a speed of 15°/min. At this time, the overlapping part on the torque-twist angle curve is called a hysteresis loop.

[0076] The tensile strength after quenching and tempering heat treatment was measured using a universal test machine (UTM).

[Table 2]

	Steel		Steel wire
	Average grain diameter (µm)	Dislocation density (×1015/m2)	Average grain diameter (µm)	Dislocation density (×1015/m2)	Hysteresis loop area for Bauschinger torsion test (mm2)	Tensile strength after quenching and tempering heat treatment (MPa)
Comparative Example 1	15.6	0.08	12.3	0.28	163	1906
Comparative Example 2	13.8	0.09	11.4	0.52	184	1945
Comparative Example 3	11.4	0.09	10.7	0.79	205	1979
Example 1	9.6	0.11	8.4	1.16	359	1915
Example 2	8.4	0.13	7.9	2.74	347	1985
Example 3	6.7	0.18	5.7	3.08	387	2026

[0077] Examples 1 to 3 satisfied the alloy composition and manufacturing conditions proposed in the present invention. Therefore, an average grain diameter of the steel was 9.6 µm or less, a dislocation density of the steel was 0.11 × 10¹⁵/m² or more, an average grain diameter of the steel wire was 8.4 µm or less, a dislocation density of the steel wire was 1.16 × 10¹⁵/m² or more, and a hysteresis loop area of the Bauschinger torsion test was 206 mm² or more.

[0078] In Comparative Example 1, the alloy composition satisfied what was proposed in the present invention, but a finish rolling temperature did not satisfy a range of 855 to 920 °C. Therefore, in Comparative Example 1, coarse grains with an average grain diameter of the steel being 15.6 µm and an average grain diameter of the steel wire being 12.3 µm appeared. Accordingly, because the hysteresis loop area obtained by the Bauschinger torsion test was 163 mm², which is very low, the permanent deformation resistance was poor.

[0079] In Comparative Example 2, the alloy composition satisfied what was proposed in the present invention, but a billet heating temperature did not satisfy the range of 960 to 1100 °C. Therefore, in Comparative Example 2, coarse grains with an average grain diameter of the steel being 13.8 µm and an average grain diameter of the steel wire being 11.4 µm appeared. Accordingly, because the hysteresis loop area obtained by the Bauschinger torsion test was 184 mm², which is very low, the permanent deformation resistance was poor.

[0080] In Comparative Example 3, the alloy composition satisfied what was proposed in the present invention, but a finish rolling temperature did not satisfy a range of 855 to 920 °C. Therefore, in Comparative Example 3, coarse grains with an average grain diameter of the steel being 11.4 µm and an average grain diameter of the steel wire being 10.7 µm appeared. Accordingly, because the hysteresis loop area obtained by the Bauschinger torsion test was 205 mm², which is very low, the permanent deformation resistance was inferior.

[0081] FIGS. 1 and 2 are graphs showing the hysteresis loop area of a steel wire according to the average grain diameter of steel and steel wire. Referring to FIGS. 1 and 2, it can be seen that the smaller the average grain diameter, the larger the hysteresis loop area. In other words, it can be seen that the smaller the average grain diameter, the better the permanent deformation resistance.

[0082] FIGS. 3 and 4 are graphs showing the hysteresis loop area of a steel wire according to the dislocation density of steel and steel wire. Referring to Figures 3 and 4, it can be seen that the larger the dislocation density, the larger the hysteresis loop area. In other words, it can be confirmed that the greater the dislocation density, the better the permanent deformation resistance.

[Industrial Applicability]

[0083] Embodiments of the present invention may provide steel and steel wire for spring having improved resistance to permanent deformation by increasing in-material dislocation density or reducing an average grain diameter, and methods of manufacturing the same.

Claims

1. A steel wire for spring having excellent permanent deformation resistance, comprising:

C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe, and other inevitable impurities, based on weight percent,

wherein a dislocation density is 1.16 × 10¹⁵/m² or more, and

an average grain diameter is 8.4 µm or less.

2. The steel wire for spring of 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%, based on weight percent.

3. The steel wire for spring of claim 1, wherein a hysteresis loop area obtained by a Bauschinger torsion test is 206 mm² or more.

4. A method of manufacturing a steel wire for spring having excellent permanent deformation resistance, the method comprising:

manufacturing a steel wire by drawing steel comprising C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe, and other inevitable impurities, based on weight percent,

heating the drawn steel wire to 850 to 1000 °C and then maintaining the steel wire for 1 second or more to austenitize; and

after austenitization, quenching the steel wire at 25 to 80 °C and then tempering the steel wire at 350 to 500 °C.

5. The method of claim 4, wherein 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%, based on weight percent.

6. Steel for spring having excellent permanent deformation resistance, comprising:

C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe, and other inevitable impurities, based on weight percent,

wherein a dislocation density is 0.11 × 10¹⁵/m² or more, and

an average grain diameter is 9.6 µm or less.

7. The steel for spring of 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%, based on weight percent.

8. A method of manufacturing steel for spring having excellent permanent deformation resistance, the method comprising:

manufacturing a billet comprising C: 0.4 to 0.7%, Si: 1.2 to 2.3%, Mn: 0.2 to 0.8%, Cr: 0.2 to 0.8%, a balance of Fe, and other inevitable impurities, based on weight percent,

heating the billet to 960 to 1100 °C; and

finish rolling the billet at 855 to 920 °C.

9. The method of claim 8, wherein 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%, based on weight percent.

Drawing