CN110036131B

CN110036131B - Wire rod and steel wire for spring having excellent anti-corrosion fatigue property, and method for manufacturing same

Info

Publication number: CN110036131B
Application number: CN201780075012.2A
Authority: CN
Inventors: 金宽镐; 金汉辉; 郑会荣; 李炳甲; 全英洙
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2016-12-06
Filing date: 2017-12-06
Publication date: 2021-07-06
Anticipated expiration: 2037-12-06
Also published as: KR101867709B1; EP3553198A4; JP2020509158A; EP3553198A1; US20200063228A1; JP7018444B2; CN110036131A; WO2018106016A1

Abstract

One aspect of the present invention relates to a wire rod for a spring having high strength and excellent corrosion fatigue resistance, in which the combination of the contents of Cr, Cu and Ni is controlled to a suitable level, the maximum depth of corrosion pits is set to a certain level or less, and fine carbides containing Mo are set to a certain level or more.

Description

Wire rod and steel wire for spring having excellent anti-corrosion fatigue property, and method for manufacturing same

Technical Field

The present disclosure relates to a wire rod for a spring, a steel wire having high strength and excellent corrosion fatigue resistance, and a method for manufacturing the same, which may be preferably applied to a suspension spring, a torsion bar, a stabilizer, and the like for a vehicle.

Background

Recently, it has been required to reduce the weight of materials for vehicles to improve fuel efficiency of vehicles, and it has been designed to manufacture suspension springs using high-strength materials having 1800MPa or more after quenching and tempering processes in response to the demand for lightweight materials.

The spring steel is formed into a spring by the following process. After manufacturing the wire rod through the hot rolling process, in the case of hot forming the spring, the wire rod is manufactured into the spring by means of the heating process, the forming process, and the quenching and tempering process, which are sequentially performed, and in the case of cold forming the spring, the wire rod is manufactured into the spring by means of the drawing process and the quenching and tempering process, which are sequentially performed.

Generally, when a material is designed to have high strength, toughness may be deteriorated due to grain boundary brittleness resistance or due to other reasons, and crack sensitivity may also be increased. Therefore, although high strength can be achieved, if the corrosion resistance of the material is reduced, a part such as a suspension spring of a vehicle, which is exposed outside, may have corrosion pits generated in the area where the paint is removed, and the part may be damaged at an early stage due to the diffusion of fatigue cracks from the corrosion pits.

The corrosive environment of the suspension spring may be increased due to the snow melt agent for preventing the road surface from being frozen in winter. Accordingly, there is an increasing demand for steel for springs having high strength and improved resistance to corrosion and fatigue.

Corrosion fatigue of a suspension spring refers to the breaking of the spring. When the paint on the surface of the spring is removed due to gravel or foreign matter on the road surface, the material of the portion without the paint is exposed to the outside, which may cause a pitting reaction, and a corrosion pit may be generated and grown, so that cracks may be generated from the pit and propagated. Hydrogen from an external source may then collect on the cracks and may render the hydrogen brittle, which may lead to the spring breaking.

In order to improve the corrosion fatigue resistance of the spring, methods of increasing the types and contents of alloying elements have been adopted in the prior art. In reference 1, the content of Ni is increased to 0.55 wt% to improve corrosion resistance, thereby increasing corrosion fatigue life, and in reference 2, the content of Si is increased to generate micronized carbides precipitated during tempering, thereby improving corrosion fatigue resistance. In reference 3, Ti precipitates (strong hydrogen trapping sites) are balanced with V, Nb, Zr, and Hf precipitates (weak hydrogen trapping sites) to improve hydrogen delayed fracture resistance, thereby improving corrosion fatigue life of the spring.

However, since Ni is an expensive element, when a large amount of Ni is added, the material cost may increase. For Si, Si is a representative element causing decarburization, and therefore, if the content of Si is increased, a considerable risk may be caused. The elements Ti, V, Nb, etc. that produce precipitates may reduce corrosion fatigue life because the elements may crystallize coarse carbonitrides from the liquid material when the material solidifies.

In order to achieve high strength of the spring, a method of adding an alloying element and a method of lowering a tempering temperature have been used in the prior art. As a method for achieving high strength by adding alloying elements, a method of increasing quenching hardness using C, Si, Mn, and Cr has been adopted, and strength of steel can be increased by rapid cooling and tempering heat treatment using relatively expensive alloying elements such as Mo, Ni, V, Ti, and Nb. However, these techniques may increase material costs.

The strength of the steel is also increased by changing the heat treatment conditions in the ordinary component system without changing the alloy composition. As the tempering temperature is lowered, the strength of the material may increase. However, when the tempering temperature is lowered, a reduction of area (area reduction rate) may be lowered, which may cause deterioration of toughness, and may also cause early breakage of the spring when the spring is formed and used, and other problems.

Therefore, it is necessary to develop a wire rod for a spring, a steel wire, and a method of manufacturing the same, which have high strength and excellent resistance to corrosion and fatigue.

(Prior Art)

(reference 1) Japanese laid-open patent publication No.2008-190042

(reference 2) Japanese laid-open patent publication No.2011-

(reference 3) Japanese laid-open patent publication No.2005-023404

Disclosure of Invention

Technical problem

An aspect of the present disclosure is to provide a wire rod for a spring, a steel wire, and a method of manufacturing the same, which are performed by controlling a combination of contents of Cr, Cu, and Ni to a certain level, controlling a maximum depth of corrosion pits to a certain level or less, and controlling a content of fine carbides containing Mo to a certain level or more.

Meanwhile, the object of the present disclosure is not limited to the above feature. The purpose of the present disclosure may be understood based on the description in the specification, and other purposes of the present disclosure may not be difficult for those skilled in the art to understand in the field containing the present disclosure.

Technical scheme

One aspect of the present disclosure relates to a wire rod for a spring having excellent resistance to corrosion fatigue, the wire rod comprising, in weight%: c: 0.40 to 0.70%, Si: 1.30 to 2.30%, Mn: 0.20 to 0.80%, Cr: 0.20 to 0.80%, Cu: 0.01 to 0.40%, Ni: 0.10 to 0.60%, Mo: 0.01% to 0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities, which satisfies the following formula 1,

the microstructure contains 50 area% or less of ferrite and the balance pearlite,

and the wire rod comprises 8.0 x 10⁴Count/mm²Or higher Mo-based carbides.

Another aspect of the present disclosure relates to a method of manufacturing a wire rod for a spring having excellent resistance to corrosion fatigue, the method comprising: heating a billet to 900 ℃ to 1100 ℃, the billet comprising in weight%: c: 0.40 to 0.70%, Si: 1.30 to 2.30%, Mn: 0.20 to 0.80%, Cr: 0.20 to 0.80%, Cu: 0.01 to 0.40%, Ni: 0.10 to 0.60%, Mo: 0.01% to 0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities, and the billet satisfies the following formula 1;

obtaining a wire rod by finish hot rolling the heated billet at 800 to 1000 ℃; and

the time for winding the wire and cooling the wire so that the wire is maintained at a temperature in the range of 600 ℃ to 700 ℃ is 31 seconds or more.

Formula 1: 0.14-0.70 Cr-0.76 Cu-0.24 Ni-0.47

(wherein each element symbol is a value of the content of each element expressed in wt.)

Another aspect of the present disclosure relates to a steel wire for a spring having excellent corrosion fatigue resistance and a method of manufacturing the same.

The above-described solutions do not necessarily list all features of the present disclosure. Various features of the disclosure, as well as advantages and effects thereof, will be further understood with reference to the exemplary embodiments described below.

Advantageous effects

According to the present disclosure, a wire rod for a spring, a steel wire having high strength and excellent corrosion fatigue resistance, and a method of manufacturing the same can be provided.

Drawings

FIG. 1 is a graph illustrating relative corrosion fatigue life as a function of maximum depth of corrosion pits in accordance with an exemplary embodiment; and

fig. 2 is a graph illustrating a relative corrosion fatigue life according to the number of Mo-based carbides according to an exemplary embodiment.

Detailed Description

Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, the present disclosure may be modified into various other embodiments, and the scope of the present disclosure may not be limited to the exemplary embodiments described below. These embodiments are provided to assist those skilled in the art in understanding the present disclosure.

In order to solve the above problems, in the present disclosure, various effective factors affecting the corrosion resistance of steel for springs have been studied. In addition, corrosion fatigue is a damage to the spring. When the coating on the surface of the spring is removed to generate corrosion pits, cracks are generated and propagated from the corrosion pits, and hydrogen from the outside is concentrated on the cracks, so that the spring may be broken. In view of the above problems, the present disclosure has been made for the following reasons.

Cr, one of alloying elements, is generally known as an element that can improve corrosion resistance, but as a result of a salt spray test, when the Cr content is increased, corrosion fatigue resistance is decreased. In addition, Cu and Ni form corrosion rust on the surface of the amorphous material during the corrosion reaction, so that the corrosion rate is reduced. Therefore, in order to improve the corrosion fatigue resistance of steel for springs, it may be important to control the combination of the contents of Cr, Cu, and Ni at an appropriate level.

In addition, the greater the maximum depth of the corrosion pits generated on the surface of the material in the corrosion reaction, the worse the corrosion fatigue resistance. In particular, the narrower and deeper the width of the corrosion pit, the worse the corrosion fatigue resistance. Therefore, in order to improve the corrosion fatigue resistance of the steel for springs, it may be necessary to control the maximum depth of the corrosion pits to a certain level or less.

In addition, in order to prevent hydrogen from gathering from the outside on the cracks, it may be necessary to capture hydrogen using fine carbides, and carbides containing alloy elements such as V, Ti, Nb, Mo, etc., as main components, which are not cementite, may be used as the fine carbides. Further, nano-sized and fine Mo-based carbides precipitated at 700 ℃ or less can effectively trap hydrogen, and when the carbides contain V, Ti, Nb, etc. as main components in addition to Mo, the carbides can have an excellent hydrogen trapping effect when Mo is contained.

Accordingly, in the present disclosure, a wire rod for a spring, a steel wire, and a method of manufacturing the same, which have high strength and excellent corrosion fatigue resistance, may be provided by: the combination of the contents of Cr, Cu and Ni is controlled, the maximum depth of the corrosion pit is controlled below a certain level, and the content of fine carbide containing Mo is controlled above a certain level.

Wire rod for spring having corrosion fatigue resistance

In the following description, a wire rod for a spring having excellent resistance to corrosion fatigue will be described in more detail.

The wire rod for a spring having excellent corrosion fatigue may include, in wt%: c: 0.40 to 0.70%, Si: 1.30 to 2.30%, Mn: 0.20 to 0.80%, Cr: 0.20 to 0.80%, Cu: 0.01 to 0.40%, Ni: 0.10 to 0.60%, Mo: 0.01% to 0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities, the wire rod may satisfy the following formula 1, the microstructure may include 50 area% or less of ferrite and the balance of pearlite, and the wire rod may include 8.0 x 10⁴Count/mm²Or higher Mo-based carbides.

In the following description, the alloy compositions of the exemplary embodiments will be described in more detail. In exemplary embodiments, the unit of each element content may be weight% unless otherwise specified. Further, the alloy composition may be applied to a method of manufacturing a wire rod, and may also be applied to a steel wire and a method of manufacturing a steel wire.

C: 0.40 to 0.70 percent

C is an essential element added to ensure the strength of the spring. In order to obtain the effect of C, it may be preferable to add 0.40% or more of C. When the content of C exceeds 0.70%, twin-type martensite (t-type martentite) structures may be formed during the heat treatment of the quenching and tempering processes, and cracks may be generated in the material, which may significantly reduce fatigue life, may increase defect sensitivity, and may significantly reduce fatigue life or fracture stress when corrosion pits are generated. Therefore, the preferable content of C may be 0.40% to 0.70%.

A more preferable lower limit content of C may be 0.45%, and a more preferable upper limit content may be 0.65%.

Si: 1.30 to 2.30 percent

Si may be dissolved in ferrite, may enhance the strength of the base material, and may improve deformation resistance.

When the content of Si is less than 1.30%, the effect of Si dissolving in ferrite to increase the strength of the base material and improve the deformation resistance may be insufficient. Therefore, the preferable lower limit content of Si may be 1.30%, and the more preferable lower limit may be 1.45%. When the content of Si exceeds 2.30%, the improvement effect of the deformation resistance may be saturated, so that a significant effect cannot be obtained by the additional addition of Si, and surface decarburization may occur during the heat treatment. Therefore, a preferable upper limit content of Si may be 2.30%, and a more preferable upper limit may be 2.25%.

Mn: 0.20 to 0.80 percent

If Mn is contained in the steel material, Mn can secure the strength of the steel material by improving the hardenability of the steel material.

When the content of Mn is less than 0.20%, it may be difficult to obtain sufficient strength and hardenability required for a spring material having high strength, whereas when the content of Mn exceeds 0.80%, hardenability may excessively increase so that a martensitic hard structure may be easily formed during cooling after a hot rolling process, and MnS inclusions may be increasingly generated, which may reduce corrosion fatigue resistance. Therefore, the preferable content of Mn may be 0.20% to 0.80%.

A more preferred lower limit content of Mn may be 0.30%, and an even more preferred lower limit content may be 0.40%. A more preferred upper limit content of Mn may be 0.75%, and an even more preferred upper limit content may be 0.70%.

Cr: 0.20 to 0.80 percent

Cr can be used for oxidation resistance, temper softening property, and surface decarburization resistance and ensures hardenability.

When the content of Cr is less than 0.20%, it may be difficult to secure sufficient effects of oxidation resistance, temper softening, surface decarburization and hardenability. When the content of Cr exceeds 0.80%, the deformation resistance may be reduced so that the strength may be reduced. Therefore, the preferable content of Cr may be 0.20% to 0.80%.

A more preferred lower limit content of Cr may be 0.22%, and an even more preferred upper limit may be 0.75%.

Cu: 0.01 to 0.40 percent

Cu may be added to improve corrosion resistance. When the content of Cu is less than 0.01%, the improvement effect of corrosion resistance may be insufficient, whereas when the content of Cu exceeds 0.40%, brittleness may be reduced during the hot rolling process, which may cause cracks and other problems. Therefore, the preferable content of Cu may be 0.01% to 0.40%, and the more preferable content of Cu may be 0.05% to 0.30%.

Ni: 0.10 to 0.60 percent

Ni may be added to improve hardenability and toughness. When the content of Ni is less than 0.10%, the effects of hardenability and toughness may be insufficient, whereas when the content of Ni exceeds 0.60%, the amount of retained austenite may increase, which may decrease fatigue life, and may increase manufacturing costs since Ni is expensive. Therefore, the preferable content of Ni may be 0.10% to 0.60%.

Mo: 0.01 to 0.40 percent

Mo can help refine the microstructure by forming carbonitrides with carbon or nitrogen and can act as trapping sites for hydrogen. To obtain this effect, the preferable content of Mo may be 0.01% or more. However, when the content of Mo is excessive, a martensitic hard structure is likely to be generated during cooling after the hot rolling process, and coarse carbo-nitrides may be generated, which may reduce flexibility of the steel. Therefore, a preferable upper limit content of Mo may be 0.40%.

P: 0.02% or less

P is an impurity. P may segregate into grain boundaries and may reduce toughness. Therefore, the upper limit content of P may be preferably controlled to 0.02%.

S: 0.015% or less

S is an impurity. S may segregate into grain boundaries as an element having a low melting point and may reduce toughness, and also a large amount of MnS may be generated, which may reduce corrosion resistance of the spring. Therefore, the upper limit content of S is preferably controlled to 0.015%.

N: 0.01% or less

The nitride (N) may easily generate BN by reacting with boron (B) and may reduce a quenching effect, and thus, the content of N may need to be controlled to be relatively low. In view of the process load, the content of N may be preferably controlled to 0.01% or less.

Iron (Fe) may also be added in exemplary embodiments. In a general manufacturing process, undesirable impurities from raw materials or the surrounding environment are inevitably added, and thus Fe may not be excluded. Those skilled in the art may be aware of these impurities and, therefore, the impurities will not be described in detail in the exemplary embodiments.

Formula 1: 0.14-0.70 Cr-0.76 Cu-0.24 Ni-0.47

(in formula 1, each element symbol is a value of the content of each element expressed in weight%)

Cr, Cu, and Ni may need to satisfy each of the above element contents, and may also satisfy formula 1 above.

Cr is known as an element that can improve corrosion resistance, but as the Cr content in steel for springs increases, the corrosion fatigue resistance may decrease. The reason is that Cr may lower the pH of the pit bottom during the corrosion reaction, so that Cr may generate a strongly acidic atmosphere in the pit and may increase the maximum depth of the pit. Therefore, the higher the Cr content, the worse the corrosion fatigue resistance.

Cu and Ni may form corrosion rust on the surface of the amorphous material in a corrosion reaction, so that Cu and Ni may reduce a corrosion rate. Therefore, the correlation between the contents of Cr, Cu and Ni and the reduction of the corrosion fatigue resistance of the steel for springs was investigated, and the effective rate (effect rate) of Cr was 0.70, the effective rate of Cu was-0.76, and the effective rate of Ni was-0.24. By controlling the correlation so as to satisfy the above formula 1, the corrosion fatigue resistance is improved.

In addition to the above alloy composition, one or more elements selected from 0.01 to 0.20% of V, 0.01 to 0.15% of Ti, and 0.01 to 0.10% of Nb may be added.

V: 0.01 to 0.20 percent

V may improve strength and may contribute to grain refinement. Further, V can serve as a trap site for hydrogen permeating into steel by forming carbonitride together with carbon (C) or nitrogen (N), and makes it possible to prevent hydrogen from permeating into steel and to reduce corrosion of steel.

When the content of V is less than 0.01%, the above effect may be insufficient. When the content of V is excessive, the manufacturing cost may increase. Therefore, a preferable upper limit content of V may be 0.20%.

Ti: 0.01 to 0.15 percent

Ti can improve elasticity by causing a precipitation hardening effect by forming carbonitrides, and can improve strength and toughness by grain refinement and precipitation strengthening. Ti may also act as a trap site for hydrogen permeating into the steel, so that Ti may prevent hydrogen from permeating into the steel and may reduce corrosion of the steel.

When the content of Ti is less than 0.01%, it may not be effective in terms of precipitation strengthening and reduction in precipitation frequency as a hydrogen trapping site. When the content of Ti exceeds 0.15%, the manufacturing cost may be significantly increased, the improvement effect of spring properties due to precipitation may be saturated, and the amount of coarse alloy carbides that are not dissolved into the base material during the austenitic heat treatment may be such that the coarse alloy carbides may function as non-metallic inclusions. Therefore, the fatigue performance and the effect of precipitation strengthening may be reduced.

Nb: 0.01 to 0.10 percent

Nb can contribute to tissue refinement by forming carbonitrides with carbon or nitrogen, and can serve as a trapping site for hydrogen. To obtain this effect, the preferable content of Nb may be 0.01% or more. However, when the content of Nb is excessive, coarse carbo-nitrides may be formed, which may reduce the ductility of the steel. Therefore, a preferable upper limit content of Nb may be 0.10%.

The microstructure of the wire rod in the exemplary embodiment may include 50 area% or less of ferrite and the balance pearlite. The above area fraction is measured without including precipitates.

When the area fraction of ferrite exceeds 50 area%, the strength of the material may be significantly reduced, so that a desired strength level may not be achieved after the final heat treatment.

In addition, the remainder other than ferrite is pearlite. When a hard structure such as martensite exists in addition to ferrite and pearlite, the wire rod may be broken in the process of drawing the wire rod.

The wire in the exemplary embodiment may include 8.0 × 10⁴Count/mm²Or higher Mo-based carbides.

In order to prevent hydrogen from the outside from accumulating on the cracks, it may be necessary to use fine carbides to trap the hydrogen, and carbides containing alloy elements such as V, Ti, Nb, Mo, etc. as main components, other than cementite, may be used as the fine carbides. Carbides containing Mo as a main component may be precipitated in a nano size in a temperature range of 600 to 700 ℃, so that a hydrogen trapping effect may be significantly enhanced. When the carbide contains V, Ti, Nb, or the like as a main component, the hydrogen trapping effect can be significantly enhanced when Mo is contained.

Therefore, it may preferably contain 8.0X 10⁴Count/mm²Or higher, more preferably 8.5X 10, and may contain Mo-based carbides⁴/mm²Or higher Mo-based carbides.

The amount of Mo-based carbide does not significantly change when manufacturing the steel wire, but the amount of Mo-based carbide may be slightly reduced. Therefore, it is more preferable to ensure 9.0 × 10 in the wire state⁴Count/mm²Or higher Mo-based carbides.

The Mo-based carbide may contain 5 wt% or more Mo based on the carbide. This is because, as described above, when the carbide contains V, Ti, Nb, or the like as a main component, the hydrogen trapping effect can be significantly enhanced when Mo is contained.

Wire for manufacturing spring having excellent corrosion fatigue resistanceMethod for producing timber

In the following description, a method of manufacturing a wire rod for a spring having excellent corrosion fatigue resistance will be described in more detail according to exemplary embodiments.

The method of manufacturing a wire rod for springs having excellent resistance to corrosion fatigue may include: heating the billet which meets the alloy composition to 900-1100 ℃; obtaining a wire rod by finish hot rolling the heated billet at 800 to 1000 ℃; and winding the wire rod and cooling the wire rod so that the time for holding the billet at the temperature in the range of 600 ℃ to 700 ℃ may be 31 seconds or more.

Heating small square billet

The billet satisfying the above alloy composition may be heated to 900 to 1100 ℃.

The heating temperature of the billet may be 900 ℃ or more because the alloying elements may be uniformly distributed in austenite by melting all coarse carbides generated during the molding process. When the heating temperature of the billet exceeds 1100 deg.c, the billet may be excessively heated, so that heat consumption may increase and time may be prolonged, which may cause excessive decarburization.

Hot rolling

The wire rod may be obtained by finish hot rolling the heated billet to 800 to 1000 ℃.

The finish rolling temperature may be 800 ℃ or higher to promote the precipitation of fine carbides. When the finish rolling temperature is less than 800 ℃, the load borne in the rolls may increase, and when the finish rolling temperature exceeds 1000 ℃, the size of grains may increase, so that toughness may decrease, and phase transformation may be delayed in the cooling process, and thus, a martensitic hard structure may be generated.

Coiling and cooling

After the wire is wound, the wire may be cooled so that the time for holding the wire at a temperature in the range of 600 to 700 ℃ may be 31 seconds or more.

The reason why the time for maintaining the wire rod at the temperature in the range of 600 ℃ to 700 ℃ is controlled to be 31 seconds or more may be to secure a sufficient time for completing the pearlite transformation without generating the martensitic hard structure during the cooling process and sufficiently precipitate fine carbides containing Mo as a main component.

Steel wire for spring with excellent corrosion and fatigue resistance

The steel wire for a spring having excellent corrosion fatigue resistance in one exemplary embodiment may satisfy the above alloy composition, the microstructure may be a tempered martensite single phase, and the steel wire may include 8.0 × 10⁴Count/mm²Or higher Mo-based carbides. Because the microstructure is a tempered martensite single phase and contains 8.0 x 10⁴Count/mm²Or higher Mo-based carbide, and thus corrosion fatigue resistance can be improved. The tempered martensite single phase may refer to a structure mainly formed of tempered martensite with the balance being an inevitable impurity structure.

In order to prevent hydrogen from the outside from accumulating on the cracks, it may be necessary to use fine carbides to trap the hydrogen, and carbides containing alloy elements such as V, Ti, Nb, Mo, etc. as main components, other than cementite, may be used as the fine carbides. Carbides containing Mo as a main component may be precipitated in a nano size in a temperature range of 600 to 700 ℃, so that a hydrogen trapping effect may be significantly enhanced, and when carbides V, Ti, Nb, and the like are contained as main components, the hydrogen trapping effect may be significantly enhanced when Mo is contained. Therefore, it may preferably contain 8.0X 10⁴Count/mm²Or higher, more preferably 8.5X 10, and may contain Mo-based carbides⁴Count/mm²Or higher Mo-based carbides. In manufacturing the steel wire, Mo-based carbide may be generated in manufacturing the wire rod, and the Mo-based carbide may not be changed but may be slightly reduced during the heating process and the cooling process.

The maximum depth of the corrosion pits of the steel wire in the exemplary embodiment may be 120 μm or less.

The deeper the maximum depth of the corrosion pits generated on the surface of the material in the corrosion reaction, the worse the corrosion fatigue resistance of the steel for springs. In particular, the narrower and deeper the corrosion pits, the greater the stress applied to the pits, which may significantly reduce the corrosion fatigue resistance.

The maximum depth of the etch pits can be measured after 14 repetitions of the following cycle: in this period, the steel wire sample was put into a salt water spray tester, 5% salt water was sprayed on the steel wire sample for 4 hours in an atmosphere of 35 ℃, the steel wire sample was dried for 4 hours in an atmosphere of a temperature of 25 ℃ and a humidity of 50%, and the steel wire sample was wetted for 16 hours until the humidity became 100%. The severest conditions are set in consideration of the use environment of steel for springs, and when the maximum depth of the corrosion pit is 120 μm or less under the above conditions, improved corrosion fatigue resistance can be ensured.

The tensile strength of the steel wire in the exemplary embodiment may be 1800MPa or more.

Method for manufacturing steel wire for spring having excellent corrosion fatigue resistance

A method of manufacturing a steel wire for a spring having excellent corrosion fatigue resistance in one exemplary embodiment may include: obtaining a steel wire by drawing the wire rod manufactured by the method of manufacturing the wire rod described in the above-described exemplary embodiment; austenitizing the steel wire by heating the steel wire to 850 ℃ to 1000 ℃ and holding the heated steel wire for 1 minute or more; and oil cooling the austenitized wire to 25 to 80 ℃ and tempering the wire at 350 to 500 ℃.

When the holding time after heating is less than 1 minute, the structure of ferrite and pearlite may not be sufficiently heated so that the wire rod is not transformed into austenite, and therefore, the heating time may be preferably controlled to 1 minute or more. In addition, the oil cooling temperature is a commonly used condition, and thus, the oil cooling temperature may not be particularly limited.

When the tempering temperature is less than 350 ℃, toughness may not be ensured, and thus, the wire rod may be broken during the forming process and in the product state. When the tempering temperature exceeds 500 ℃, the strength may be reduced. Therefore, the preferred tempering temperature may be 350 ℃ to 500 ℃.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

In the following description, embodiments of the present disclosure will be described in more detail. It should be noted that the exemplary embodiments are provided to describe the present disclosure in more detail, and do not limit the scope of the claims of the present disclosure. The scope of rights of the present disclosure may be determined based on the subject matter recited in the claims and the matters reasonably inferred from the subject matter.

The billet having the composition in table 1 below was heated to 1000 c and finish rolled at 900 c and coiled. During cooling after winding, the temperatures in the range of 600 ℃ to 700 ℃ were maintained for the times listed in table 2 below, and wire rods were manufactured. The microstructure of the wire was observed and listed in table 2 below.

The wire rod was drawn, heated at 975 ℃ for 15 minutes, placed in oil at 70 ℃ and rapidly cooled, and the cooled wire rod was maintained at 390 ℃ for 30 minutes, and a steel wire was manufactured.

The tensile strength, maximum depth of corrosion pits, Mo-based carbide and relative corrosion fatigue life of the steel wires were measured and are shown in table 2 below. All microstructures are martensitic single phase.

After collecting tensile samples of steel wires according to ASTM E8 standard, tensile strength was measured by performing tensile tests.

For the Mo-based carbide, a sample was cut along a cross section, a fine carbide was extracted by a replication method (replica method), and the fine carbide was analyzed using a transmission electron microscope and energy dispersive X-ray spectroscopy, and the contents of carbides containing 5% or more of Mo were listed in the following table 2 according to the results.

The following cycle was repeated 14 times and the maximum depth of corrosion pits and relative corrosion fatigue life measurements were measured: in this period, the steel wire sample was put into a salt water spray tester, 5% salt water was sprayed on the steel wire sample for 4 hours in an atmosphere of 35 ℃, the steel wire sample was dried for 4 hours in an atmosphere of a temperature of 25 ℃ and a humidity of 50%, and the steel wire sample was wetted for 16 hours until the humidity became 100%.

The maximum depth of the etch pits was measured using a confocal laser microscope (confocal laser microscope).

The relative corrosion fatigue life was measured by performing a rotary bending fatigue test at a speed of 3000rpm and the weight applied to the sample was 40% of the tensile strength. The corrosion fatigue life of 10 samples was tested and the fatigue life of 8 samples, except the sample with the highest fatigue life and the sample with the lowest fatigue life, was averaged. The average value was determined as the corrosion fatigue life of the corresponding sample. When the corrosion fatigue life of comparative example 1 was 1, the relative corrosion fatigue lives of the other samples are shown in table 2.

[ Table 1]

In Table 1 above, formula 1 represents the value of 0.70[ Cr ] -0.76[ Cu ] -0.24[ Ni ].

[ Table 2]

In table 2 above, F represents ferrite, P represents pearlite, and M represents martensite.

Inventive examples 1 to 5, which satisfied the alloy compositions and manufacturing conditions described in the present disclosure, had excellent tensile strength and relative corrosion fatigue life. The relative corrosion fatigue life of the comparative example is between 0.97 and 1.28, but the relative corrosion fatigue life of the inventive example is between 3.23 and 8.21, and the relative corrosion fatigue life of the inventive example is significantly increased.

The comparative examples secured a tensile strength of 1800MPa or more, but did not satisfy the alloy composition and manufacturing conditions described in the present disclosure, and therefore, the relative corrosion fatigue life was deteriorated.

The maximum depth of the corrosion pits of the comparative examples was 128 μm or more, and the number of Mo-based carbides turned out to be less than 8X 10⁴Count/mm²。

As in comparative examples 6 and 7, if the alloy compositions described in the exemplary embodiments are not satisfied, the relative corrosion fatigue life is relatively low even if the manufacturing conditions described in the exemplary embodiments are satisfied. In addition, as in comparative examples 8 and 9, even if the alloy compositions described in the example embodiments were satisfied, the relative corrosion fatigue life was relatively low when the retention time at 600 ℃ to 700 ℃ was not satisfied.

Further, when the martensitic hard structure is formed in the wire rod state as in comparative examples 3 to 5, 8, and 9, breakage of the wire rod frequently occurs during the drawing process, and thus it is difficult to manufacture the wire rod as a steel wire.

FIG. 1 is a graph illustrating relative corrosion fatigue life as a function of maximum depth of corrosion pits, according to an exemplary embodiment. The smaller the maximum depth of the corrosion pit is, the larger the relative corrosion fatigue life is, and when the maximum depth of the corrosion pit is more than 120 μm, the relative corrosion fatigue life is significantly reduced.

Fig. 2 is a graph showing relative corrosion fatigue life according to the amount of Mo-based carbide. The greater the amount of Mo-based carbides, the greater the increase in the relative corrosion fatigue life, and when the amount of Mo-based carbides is less than 8.0X 10⁴Count/mm²The relative corrosion fatigue life is significantly reduced.

While exemplary embodiments have been shown and described above, the scope of the present disclosure is not limited thereto, and it will be apparent to those skilled in the art that modifications and variations can be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A wire rod for springs having excellent corrosion fatigue resistance, comprising:

in weight%, C: 0.40 to 0.70%, Si: 1.30 to 2.30%, Mn: 0.20 to 0.80%, Cr: 0.20 to 0.80%, Cu: 0.01 to 0.40%, Ni: 0.10 to 0.60%, Mo: 0.01% to 0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities,

wherein the wire satisfies the following formula 1:

formula 1: 0.14-0.70 Cr-0.76 Cu-0.24 Ni-0.47

Wherein each element symbol is a value of a content of each element expressed in weight%,

wherein the microstructure comprises 50 area% or less of ferrite and the balance of pearlite, and

wherein the wire comprises 8.0 x 10⁴Count/mm²Or higher Mo-based carbides.

2. The wire for springs according to claim 1, further comprising:

one or more elements selected from 0.01 to 0.20% V, 0.01 to 0.15% Ti and 0.01 to 0.10% Nb in weight%.

3. The wire rod for springs according to claim 1, wherein the Mo-based carbide contains 5 wt.% or more Mo based on carbide.

4. A method of manufacturing a wire rod for springs having excellent resistance to corrosion fatigue, the method comprising:

heating a billet to 900 ℃ to 1100 ℃, the billet comprising in weight%: c: 0.40 to 0.70%, Si: 1.30 to 2.30%, Mn: 0.20 to 0.80%, Cr: 0.20 to 0.80%, Cu: 0.01 to 0.40%, Ni: 0.10 to 0.60%, Mo: 0.01% to 0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities, and the billet satisfies the following formula 1,

formula 1: 0.14-0.70 Cr-0.76 Cu-0.24 Ni-0.47

Wherein each element symbol is a value of a content of each element expressed in weight%;

the time for winding the wire and cooling the wire so that the wire is kept at a temperature in the range of 600 ℃ to 700 ℃ is 31 seconds or more.

5. The method of claim 4, wherein the billet further comprises one or more elements selected from 0.01 to 0.20% by weight of V, 0.01 to 0.15% of Ti, and 0.01 to 0.10% of Nb.

6. A steel wire for springs having excellent corrosion fatigue resistance, the steel wire comprising:

in weight%, C: 0.40 to 0.70%, Si: 1.30 to 2.30%, Mn: 0.20 to 0.80%, Cr: 0.20 to 0.80%, Cu: 0.01 to 0.40%, Ni: 0.10 to 0.60%, Mo: 0.01 to 0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities,

wherein the steel wire satisfies the following formula 1:

formula 1: 0.14-0.70 Cr-0.76 Cu-0.24 Ni-0.47

wherein the microstructure is tempered martensite, and

wherein the steel wire comprises 8.0 x 10⁴Count/mm²Or higher Mo-based carbides.

7. The steel wire for springs according to claim 6, further comprising:

8. The steel wire for springs according to claim 6, wherein the Mo-based carbide contains 5 wt.% or more Mo based on carbide.

9. The steel wire for springs according to claim 6, wherein the maximum depth of the corrosion pits of the steel wire is 120 μm or less.

10. The steel wire for springs according to claim 6, wherein the tensile strength of the steel wire is 1800MPa or more.

11. A method of manufacturing a steel wire for a spring having excellent corrosion fatigue resistance, the method comprising:

obtaining the steel wire by drawing a wire rod manufactured by the method of claim 4 or claim 5;

austenitizing the steel wire by heating the steel wire to 850 ℃ to 1000 ℃ and holding the heated steel wire for 1 minute or more; and

the austenitized wire is oil cooled to 25 to 80 ℃ and then tempered at 350 to 500 ℃.