EP3553198B1

EP3553198B1 - Wire rod for springs with excellent corrosion fatigue resistance, steel wire, and manufacturing method thereof

Info

Publication number: EP3553198B1
Application number: EP17877467.5A
Authority: EP
Inventors: Kwan-Ho Kim; Han-Hwi KIM; Hoe-Young Jung; Byoung-Gab LEE; Young-Soo Chun
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2016-12-06
Filing date: 2017-12-06
Publication date: 2024-08-14
Anticipated expiration: 2037-12-06
Also published as: EP3553198A4; JP2020509158A; CN110036131A; US20200063228A1; KR101867709B1; JP7018444B2; EP3553198A1; CN110036131B; WO2018106016A1

Description

[Technical Field]

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

[Background Art]

Recently, it has been required to reduce weights of materials used for vehicles to improve fuel efficiency of vehicles, and a suspension spring has been designed to be manufactured using a high strength material having strength of 1800 MPa or higher after a quenching and tempering process so as to respond to the demand for lightweight materials.
Steel used for springs is formed as a spring through the following processes. After manufacturing a wire rod through a hot rolling process, in case of a hot formed spring, the wire rod is manufactured as a spring through a heating process, a forming process, and a quenching and tempering process performed in order, and in case of a cold formed spring, the wire rod is manufactured as a spring through a drawing process and a quenching and tempering process performed in order.
Generally, when a material is designed to have high strength, toughness may be degraded due to grain boundary embrittlement resistance, or for other reasons, and also, crack sensitivity may increase. Thus, although high strength may be achieved, if corrosion resistance of a material degrades, an externally exposed component, such as a suspension spring of a vehicle, may have a corrosion pit created in a region in which paint is removed, and the component may be damaged at an early stage due to fatigue cracks spreading from the corrosion pit.
A corrosion environment of a suspension spring may be increased due to snow melting agents used to prevent a road surface from freezing in winter. Accordingly, demand for steel for springs with high strength and improved corrosion fatigue resistance has increased.
Corrosion fatigue of a suspension spring refers to breakage of a spring. When paint on the surface of a spring is removed due to pebbles on a road surface or foreign objects, a material of the portion with no paint is exposed externally, which may cause a pitting corrosion reaction, and a corrosion pit may be created and grown, such that cracks may be generated and spread from the pit. Then, hydrogen from an external source may be concentrated on the cracks and may cause hydrogen embrittlement, which may lead to spring breakage.
To improve corrosion fatigue resistance of a spring, the method of increasing types and amounts of contents of alloy elements have been used in the prior art. In reference 1, a content of Ni is increased to 0.55 weight% to improve corrosion resistance, thereby increasing corrosion fatigue life, and in reference 2, a content of Si is increased to create micronized carbide precipitated during tempering, thereby improving strength against corrosion fatigue. In reference 3, Ti precipitation, a strong hydrogen trapping site, and a V, Nb, Zr, and Hf precipitation, weak hydrogen trapping sites, are balanced to improve hydrogen delayed fracture resistance, thereby improving a corrosion fatigue life of a spring.
However, as Ni is an expensive element, material costs may increase when a large amount of Ni is added. As for Si, Si is a representative element causing decarburization, and thus, if a content of Si is increased, it may cause substantial risk. Ti, V, Nb, and the like, elements creating precipitation, may degrade the corrosion fatigue life because the elements may crystallize coarse carbonitrides from liquid materials when the materials are solidified.
To achieve high strength of a spring, the method of adding alloy elements and the method of decreasing a tempering temperature have been used in the prior art. As the method of achieving high strength by adding alloy elements, the method of increasing quenching hardness using C, Si, Mn, and Cr has been used, and strength of a steel material may increase through a rapid cooling and a tempering heat treatment using Mo, Ni, V, Ti, and Nb, and the like, relatively expensive alloy elements. The techniques, however, may increase material costs.
Strength of a steel material has also been increased by changing heat treatment conditions in a general component system without changing an alloy composition. When a tempering temperature is deceased, strength of a material may increase. However, when a tempering temperature decreases, an area reduction rate may decrease, which may cause degradations in toughness, and may also cause early breakage of a spring while a spring is formed and used, and other problems.
Thus, it has been necessary to develop a wire rod for springs with high strength and excellent corrosion fatigue resistance, a steel wire, and a method of manufacturing the same.

(Prior Art)

(Reference 1) Japanese Laid-Open Patent Publication No. 2008-190042
(Reference 2) Japanese Laid-Open Patent Publication No. 2011-074431
(Reference 3) Japanese Laid-Open Patent Publication No. 2005-023404
EP 0 657 557 describes a spring steel for a high strength spring which is used for a valve spring of an internal combustion engine.
EP 0 943 697 describes a steel material for springs having a high strength and high toughness after heat treatment. Such a steel is achieved by refining austenite grain boundaries which tend to promote fractures to obtain a steel having a sufficient ductility and a sufficient impact toughness even when the steel is made to have a high strength

[Disclosure]

[Technical Problem]

An aspect of the present disclosure is to provide a wire rod for springs with high strength and excellent corrosion fatigue resistance, a steel wire, and a method of manufacturing the same by controlling a combination of contents of Cr, Cu, and Ni to a certain level, controlling a maximum depth of a corrosion pit to a certain level or less, and controlling a content of fine carbide containing Mo to a certain level or higher.
Meanwhile, the purposes of the present disclosure are not limited to the features described above. The purposes of the present disclosure may be understood on the basis of the descriptions in the specification, and it may not be difficult for a person having skill in the art in the field in which the present disclosure is included to understand additional purposes of the present disclosure.

[Technical Solution]

The invention is as set forth in the appended claims.

[Advantageous Effects]

According to the present invention , a wire rod for springs with high strength and excellent corrosion fatigue resistance, a steel wire, and a method of manufacturing the same are provided.

[Description of Drawings]

FIG. 1 is a graph illustrating a relative corrosion fatigue life depending on a maximum depth of a corrosion pit according to an example embodiment; and
FIG. 2 is a graph illustrating a relative corrosion fatigue life depending on the number of Mo-based carbides according to an example embodiment.

[Best Mode for Invention]

Hereinafter, embodiments of the present disclosure will be described with reference to the accompanied drawings. The present disclosure, however, may be modified to various other embodiments, and the scope of the present disclosure may not be limited to exemplary embodiments described below. These embodiments are provided to help those skilled in the art to understand the present disclosure.
To address the issues described above, in the present disclosure, various effective factors affecting corrosion resistance of steel for springs have been examined. Also, corrosion fatigue is the breakage of a spring. When a corrosion pit is created as paint on a surface of a spring is removed, cracks are generated and spread from the corrosion pit, and hydrogen from outside is concentrated on the cracks such that a spring may be broken.
Cr, one of alloy elements, is generally known as an element which may improve corrosion resistance, but as a result of a salt water spraying test, corrosion fatigue resistance was degraded when a content of Cr increased. Also, Cu and Ni made corrosion rust formed on a surface of a material amorphous during a corrosion reaction such that a corrosion speed decreased. Thus, to improve corrosion fatigue resistance of steel for springs, it is important to control combination of contents of Cr, Cu, and Ni to an appropriate level.
Also, the greater the maximum depth of a corrosion pit generated on a surface of a material in the corrosion reaction, the further the corrosion fatigue resistance was degraded. Particularly, the narrower and deeper the width of a corrosion pit, the further the corrosion fatigue resistance may degrade. Thus, to improve corrosion fatigue resistance properties of steel for springs, it may be necessary to control a maximum depth of a corrosion pit to a certain level or less.
In addition, to prevent hydrogen from outside from being concentrated on cracks, it is necessary to trap hydrogen using fine carbides, and carbides including alloy elements of V, Ti, Nb, Mo, and the like, as main ingredients, not cementite, may be used as the fine carbide. Also, nano-sized and fine Mo-based carbides precipitated at 700°C or less may effectively trap hydrogen, and when carbides include V, Ti, Nb, and other like, besides Mo, as main ingredients, the carbides may have an excellent hydrogen trapping effect when Mo is contained.
Thus, in the present disclosure, a wire rod for springs with high strength and excellent corrosion fatigue resistance, a steel wire, and a method of manufacturing the same are provided by controlling combination of contents of Cr, Cu, and Ni, controlling a maximum depth of a corrosion pit to a certain level or less, and controlling a content of fine carbide including Mo to a certain level or higher.

Wire Rod for Springs with Corrosion Fatigue Resistance

In the description below, a wire rod for springs with excellent corrosion fatigue resistance will be described in greater detail.
A wire rod for springs with excellent corrosion fatigue includes 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 a balance of Fe and inevitable impurities by weight%, the wire rod satisfies equation 1, a microstructure includes 50 area% or less of ferrite and a balance of pearlite, and the wire rod includes 8.55×10⁴ count/mm² or higher of Mo-based carbides.
In the description below, an alloy composition of the example embodiment will be described in greater detail. In example embodiments, a unit of each element content is weight% unless otherwise indicated. Also, the alloy composition is applied to the method of manufacturing a wire rod, and also is applied to a steel wire and the method of manufacturing the steel wire.

C: 0.40 to 0.70%

C is an essential element added to secure strength of a spring. To draw the effect of C, it is necessary to add 0.40% or higher of C. When a content of C exceeds 0.70%, a twin-type martensite structure may be formed during a heat treatment in a quenching and tempering process, and cracks may be created in a material, which may significantly decrease fatigue life, may increase defect sensitivity, and may significantly degrade fatigue life or fracture stress when a corrosion pit is created. Thus, a the content of C is 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%

Si may be dissolved in ferrite, may enhance strength of a base material and may improve deformation resistance.
When a content of Si is less than 1.30%, the effect of Si dissolved in ferrite to enhance strength of a base material and to improve deformation resistance may be insufficient. Thus, a the lower limit content of Si is 1.30%, and a more preferable lower limit may be 1.45%. When a content of Si exceeds 2.30%, the effect of improvement in deformation resistance may be saturated such that no significant effect may be obtained from additionally added Si, and surface decarburization may occur during a heat treatment. Thus, a the upper limit content of Si is 2.30%, and a more preferable upper limit may be 2.25%.

Mn: 0.20 to 0.80%

If Mn is included in the steel material, Mn may secure strength of a steel material by improving hardenability of the steel material.
When a content of Mn is less than 0.20%, it may be difficult to obtain sufficient strength and hardenability required for a material for springs with high strength, whereas, when a content of Mn exceeds 0.80%, hardenability may increase excessively such that a martensite hard structure may easily be creased during cooling after a hot rolling process, and MnS inclusions may increasingly be created, which may degrade corrosion fatigue resistance properties. Thus, a the content of Mn is be 0.20 to 0.80%
A more preferable lower limit content of Mn may be 0.30%, and an even more preferable lower limit content may be 0.40%. A more preferable upper limit content of Mn may be 0.75%, and an even more preferable upper limit content may be 0.70%.

Cr: 0.20 to 0.80%

Cr may be used to prevent oxidation resistance, temper softening properties, and surface decarburization and to secure hardenability.
When a content of Cr is less than 0.20%, it may be difficult to secure the sufficient effect of oxidation resistance, temper softening properties, surface decarburization, and hardenability. When a content of Cr exceeds 0.80%, deformation resistance may degrade such that strength may degrade. Thus, a the content of Cr is 0.20 to 0.80%.
A more preferable lower limit content of Cr may be 0.22%, and an even more preferable upper limit may be 0.75%.

Cu: 0.01 to 0.40%

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

Ni: 0.10 to 0.60%

Ni is added to improve hardenability and toughness. When a content of Ni is less than 0.10%, the effect of hardenability and toughness may not be sufficient whereas, when a content of Ni exceeds 0.60%, the amount of residual austenite may increase, which may decrease fatigue life, and may increase manufacturing costs as Ni is expensive. Thus, a the content of Ni is 0.10 to 0.60%.

Mo: 0.01∼0.40%

Mo may contribute to refining microstructure by forming carbonitride together with carbon or nitrogen, and may work as a trap site for hydrogen. To obtain the effect, the content of Mo should be 0.01% or higher. However, when a content of Mo is excessive, it may be highly likely that a martensite hard structure may be created during cooling after a hot rolling process, and coarse carbonitride may be created, which may degrade flexibility of a steel material. Thus, a the upper limit content of Mo is 0.40%.

P: 0.02% or less

P is an impurity. P may be segregated into a grain boundary and may degrade toughness. Thus, the upper limit content of P should be 0.02%.

S: 0.015% or less

S is an impurity. S may be segregated into a grain boundary as an element having a low melting point and may degrade toughness, and may also create large amount of MnS, which may degrade corrosion resistance properties of a spring. Thus, the upper limit content of S should be 0.015%.

N: 0.01% or less

Nitride (N) may easily create BN by reacting with boron (B), and may decrease a quenching effect, and thus, a content of N may need to be controlled to be relatively low. Considering process load, the content of N should be 0.01% or less.
Iron (Fe) also is added in the example embodiment. In a general manufacturing process, unintended impurities from raw materials or a surrounding environment may inevitably be added, and thus, Fe may not be excluded. A person skilled in the art may be aware of such impurities, and thus, the impurities may not be described in detail in the example embodiment. -0.14 ≤ 0.70[Cr] - 0.76[Cu] - 0.24[Ni] ≤ 0.47
(In equation 1, each element symbol is a value of a content of each element measured by weight%)
Cr, Cu, and Ni need to satisfy each element content described above, and also satisfy equation 1 above.
Cr is known as an element which may improve corrosion resistance, but as a content of Cr increases in steel for springs, corrosion fatigue resistance may degrade. The reason is that Cr may decrease pH of a pit bottom during a corrosion reaction such that Cr may create a strongly acid atmosphere in the pit and may increase a maximum depth of the pit. Thus, the higher the content of Cr, the more the corrosion fatigue resistance may degrade.
Cu and Ni may make corrosion rust formed on a surface of a material amorphous in a corrosion reaction such that Cu and Ni may decrease a corrosion speed. Thus, the correlation between contents of Cr, Cu, and Ni and the decrease of corrosion fatigue resistance of steel for springs was examined, and the effect rate of Cr was 0.70, the effect rate of Cu was -0.76, and the effect rate of Ni was -0.24. By controlling the correlation to satisfy equation 1 above, corrosion fatigue resistance was improved.
In addition to the alloy composition described above, one or more elements selected from among V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% may further be added.

V: 0.01 to 0.20%

V may improve strength and may contribute to grain refinement. Further, V may work as a trap site for hydrogen infiltrating steel by forming carbonitride together with carbon (c) or nitrogen (N) and such that V may prevent hydrogen infiltration in steel and may decrease corrosion of steel.
When a content of V is less than 0.01%, the above-described effect may not be sufficient. When a content of V is excessive, manufacturing costs may increase. Thus, a preferable upper limit content of V may be 0.20%.

Ti: 0.01 to 0.15%

Ti may improve spring properties by causing a precipitation hardening effect by forming carbonitride, and may improve strength and toughness by refining grains and reinforcing precipitation. Ti may also work as a trap site for hydrogen infiltrating steel such that Ti may prevent hydrogen infiltration in steel and may decrease corrosion of steel.
When a content of Ti is less than 0.01 %, it may not be effective in that a frequency of precipitations reinforcing precipitation and working as a hydrogen trap site decreases. When a content of Ti exceeds 0.15%, manufacturing costs may significantly increase, the effect of improvement in spring properties due to precipitations may be saturated, and the amount of coarse alloy carbide which has not been dissolved into a base material during a heat treatment of austenite such that the coarse alloy carbide may work as a non-metal inclusion. Accordingly, the effect of fatigue properties and precipitation reinforcement may degrade.

Nb: 0.01 to 0.10%

Nb may contribute to structure refinement by forming carbonitride together with carbon or nitrogen, and may work as a trap site for hydrogen. To obtain the effect, a preferable content of Nb may be 0.01% or higher. However, when a content of Nb is excessive, coarse carbonitride may be formed, which may degrade ductility of steel. Thus, a preferable upper limit content of Nb may be 0.10%.
A microstructure of a wire rod in the example embodiment includes 50 area% or less of ferrite, and a balance of pearlite. The area fraction above was measured exclusive of precipitation.
When an area fraction of ferrite exceeds 50 area%, strength of a material may significantly decrease such that, after a final heat treatment, a desirable level of strength may not be implemented.
Also, the remainder excluding ferrite is pearlite. When hard structure such as martensite is existed in addition to ferrite and pearlite, a wire rod may be broken in a process of drawing the wire rod.
The wire rod of the invention includes 8.5×10⁴ count/mm² or higher of Mo-based carbides.
To prevent hydrogen from outside from being concentrated on cracks, hydrogen may need to be trapped using fine carbide, and carbide including alloy elements of V, Ti, Nb, Mo, or the like, as main ingredients, not cementite, may be used as the fine carbide. The carbide including Mo as a main ingredient may be precipitated in nano-size within a temperature in a range of 600 to 700°C such that a hydrogen trapping effect may significantly increase. When the carbides include V, Ti, Nb, and the like, as main ingredients, a hydrogen trapping effect may significantly increase when Mo is contained.
Thus, the number of Mo-based carbides should be 8.5×10⁴ count/mm² or higher.
The number of Mo-based carbides may not be significantly changed when a steel wire is manufactured, but the number of Mo-based carbides may decrease slightly. Thus, it may be more preferable to secure 9.0×10⁴ count/mm² or higher of Mo-based carbides in a wire rod state.
The Mo-based carbides include 5 weight% or higher of Mo based on carbides. That is because, as described above, when the carbides include V, Ti, Nb, and the like, as main ingredients, a hydrogen trapping effect may significantly increase when Mo is contained.

Method of Manufacturing Wire Rod for Springs with Excellent Corrosion Fatigue Resistance

In the description below, a method of manufacturing a wire rod for springs with excellent corrosion fatigue resistance will be described in greater detail in accordance with an example embodiment.
The method of manufacturing a wire rod for springs with excellent corrosion fatigue resistance according to the invention includes heating a billet satisfying the alloy composition as defined by the claims to 900 to 1100°C, obtaining a wire rod by finishing-hot-rolling the heated billet at 800 to 1000°C, and coiling the wire rod and cooling the wire rod such that the time for maintaining the billet at a temperature in a range of 600 to 700°C is 31 seconds or longer.

Heating Billet

The billet satisfying the alloy composition as defined in the claims is heated to a temperature indicated in the appended claims. It is heated to 900 to 1100°C.
The heating temperature of the billet is 900°C or higher because, by melting all coarse carbides generated during a molding process, the alloy elements may be uniformly distributed in austenite. When a heating temperature of the billet exceeds 1100°C, the billet may be excessively heated such that heat consumption may increase, and the time may be prolonged, which may cause excessive decarburization.

Hot-Rolling

A wire rod is obtained by finishing-hot-rolling the heated billet to 800 to 1000°C.
The temperature of the finishing-rolling is 800°C or higher to facilitate precipitation of fine carbides. When the temperature of the finishing-rolling is less than 800°C, load taken in a roller may increase, and when the temperature of the finishing-rolling exceeds 1000°C, a size of a grain may increase such that toughness may degrade, and transformation may be delayed in a cooling process, and accordingly, martensite hard structure may be created.

Coiling and Cooling

After the wire rod is coiled, the wire rod is cooled such that the time for maintaining the wire rod at a temperature in a range of 600 to 700°C is 31 seconds or longer.
The reason for controlling the time for maintaining the wire rod at a temperature in a range of 600 to 700°C to be 31 seconds or longer is to secure sufficient time for completing pearlite transformation without creating martensite hard structure during a cooling process, and to sufficiently precipitate fine carbides including Mo as a main ingredient.

Steel Wire for Springs with Excellent Corrosion Fatigue Resistance

A steel wire for springs with excellent corrosion fatigue resistance in an example embodiment satisfies the alloy composition according to the claims, a microstructure being a tempered martensite single phase, and the steel wire including 8.5×10⁴ count/mm² or higher of Mo-based carbides. As a microstructure is a tempered martensite single phase, and 8.5×10⁴ count/mm² or higher of Mo-based carbides are included,
corrosion fatigue resistance may improve. The tempered martensite single phase may refer to a structure mostly formed of tempered martensite with a balance of an inevitable impure structure.
To prevent hydrogen from outside from being concentrated on cracks, hydrogen may need to be trapped using fine carbides, and carbides including alloy elements of V, Ti, Nb, Mo, or the like, as main ingredients, not cementite, may be used as the fine carbides. The carbides including Mo as a main ingredient are precipitated in nano-size within a temperature in a range of 600 to 700°C such that a hydrogen trapping effect may significantly increase, and when carbides include V, Ti, Nb, or the like, as main ingredients, a hydrogen trapping effect may significantly increase when Mo is contained. Thus, it is a feature of the invention to include 8.5×10⁴ count/mm² or higher of Mo-based carbides. The Mo-based carbides are created when a wire rod is manufactured, and the Mo-based carbides may not be changed but may slightly decrease during a heating process and a cooling process when a steel wire is manufactured.
A maximum depth of a corrosion pit of the steel wire in the example embodiment is 120 µm or less.
In a corrosion reaction, the deeper the maximum depth of a corrosion pit created on a surface of a material, the more the corrosion fatigue resistance properties of steel for springs may degrade. Particularly, the narrower and deeper the corrosion pit, the greater the stress applied to the pit, which may significantly degrade corrosion fatigue resistance.
The maximum depth of the corrosion pit may be measured after 14 repetitions of a cycle in which a sample of the steel wire was put in a salt water spray tester, 5% salt water was sprayed onto the steel wire sample for 4 hours in an atmosphere at 35°C, the steel wire sample was dried for 4 hours at atmosphere at a temperature of 25°C and humidity of 50%, and the steel wire sample was wet for 16 hours until humidity became 100%. The harshest condition was set in consideration of usage environment of steel for springs, and when a maximum depth of the corrosion pit is 120 um or less under the above-described conditions, improved corrosion fatigue resistance could be secured.
The tensile strength of steel wire in the example embodiment is 1800MPa or higher.

Method of Manufacturing Steel Wire for Springs with Excellent Corrosion Fatigue Resistance

A method of manufacturing a steel wire for springs with excellent corrosion fatigue resistance in an example embodiment includes obtaining a steel wire by drawing the wire rod manufactured by the method of manufacturing a wire rod described in the aforementioned example embodiment, austenitizing the steel wire by heating the steel wire to 850∼1000°C and maintaining the heated steel wire for 1 minute or longer, and oil-cooling the austenitized wire rod to 25 to 80°C and tempering the wire rod at 350 to 500°C.
When the maintaining time after heating is less than 1 minute, structures of ferrite and pearlite may not be sufficiently heated such that the wire rod may not be transferred to be austenite, and thus, it is necessary to control the heating time to be 1 minute or longer. Also, the oil-cooling temperature is generally used condition, and thus, the oil-cooling temperature may not be particularly limited.
When the tempering temperature is less than 350°C, toughness may not be secured, and accordingly, the wire rod may be broken during a forming process and in a product state. When the tempering temperature exceeds 500°C, strength may degrade. Thus, the tempering temperature is 350 to 500°C.

[Mode for Invention]

In the description below, embodiments of the present disclosure will be described in greater detail. It should be noted that the exemplary embodiments are provided to describe the present disclosure in greater detail
A billet having a composition as in Table 1 below was heated to 1000°C, and was coiled after finishing-rolling at 900°C. In a cooling process after coiling, a temperature in a range of 600 to 700°C was maintained for maintaining times listed in Table 2 below, and a wire rod was manufactured. A microstructure of the wire rod was observed and listed in Table 2 below.
The wire rod was drawn, was heated at 975°C for 15 minutes, was put in oil at 70°C and was rapidly cooled, and the cooled wire rod was maintained at 390°C for 30 minutes, and a steel wire was manufactured.
Tensile strength of the steel wire, a maximum depth of a corrosion pit, a Mo-based carbide, and relative corrosion fatigue life were measured and listed in Table 2 below. All microstructures were martensite single phases.
The tensile strength was measured by, after gathering tensile samples of the steel wire in accordance with ASTM E 8 standard, performing a tensile test.
As for the Mo-based carbide, the sample was cut cross-sectionally, fine carbides were extracted by a replica method, and the fine carbides were analyzed using a transmission electron microscope and energy dispersive X-ray spectroscopy, and the number of carbides including 5% or higher of Mo from the result was listed in Table 1 below.
A cycle in which a sample of the steel wire was put in a salt water spray tester, 5% salt water was sprayed onto the steel wire sample for 4 hours at atmosphere of 35°C, the steel wire sample was dried for 4 hours at atmosphere of temperature of 25°C and humidity of 50%, and the steel wire sample was wet for 16 hours until humidity became 100% was repeated 14 times, and a maximum depth of a corrosion pit and a relative corrosion fatigue life were measured.
A maximum depth of a corrosion pit was measured using a confocal laser microscope.

The relative corrosion fatigue life was measured by performing a rotary bending fatigue test, a speed of the fatigue test was 3,000 rpm, and a weight applied to the sample was 40% of tensile strength. 10 samples were tested for corrosion fatigue life, and fatigue lives of 8 samples excluding the sample having the highest fatigue life and the sample having the lowest fatigue life were averaged. The average value was determined as a corrosion fatigue life of the respective sample. Relative corrosion fatigue lives of the other samples of when a corrosion fatigue life of a comparative example 1 was 1 were listed in Table 2.

[Table 1]

Steel Type	Alloy Composition (Weight%)													Equation 1
Steel Type	C	Si	Mn	Cr	Cu	Ni	Mo	P	S	N	V	Ti	Nb	Equation 1
Comparative Steel
1	0.53	1.53	0.68	0.73	-	-	-	0.016	0.008	0.0049				0.51
Comparative Steel 2	0.50	1.49	0.51	0.11	0.22	0.25	-	0.009	0.005	0.0052	0.11			-0.15
Comparative Steel 3	0.63	1.62	0.40	0.26	0.28	0.62	0.16	0.010	0.010	0.0042		0.02		-0.18
Comparative Steel 4	0.55	1.85	0.61	0.86	0.10	0.19	0.14	0.011	0.007	0.0051	0.10		0.03	0.48
Comparative Steel 5	0.48	2.26	0.59	0.28	0.34	0.58	0.22	0.008	0.007	0.0046	0.08	0.03		-0.20
Inventive Steel 1	0.52	1.51	0.68	0.72	0.14	0.21	0.03	0.012	0.008	0.0045				0.35
Inventive Steel 2	0.49	1.45	0.48	0.23	0.22	0.52	0.13	0.008	0.006	0.0057				-0.13
Inventive Steel 3	0.60	1.52	0.43	0.28	0.15	0.56	0.16	0.010	0.004	0.0044	0.18		0.02	-0.05
Inventive Steel 4	0.53	1.68	0.41	0.33	0.21	0.25	0.36	0.013	0.006	0.0054		0.14		0.01
Inventive Steel 5	0.49	2.17	0.64	0.71	0.06	0.10	0.25	0.009	0.007	0.0047	0.12		0.05	0.43

In Table 1 above, equation 1 indicates values of
0.70[Cr] - 0.76[Cu] - 0.24[Ni].
In Table 2 above, F refers to ferrite, P refers to pearlite, and M refers to martensite.
Inventive Examples 1 to 5 which satisfied the alloy composition and manufacturing conditions described in the present disclosure had excellent tensile strength and relative corrosion fatigue life. Relative corrosion fatigue lives of the comparative examples were between 0.97 and 1.28, but relative corrosion fatigue lives of the Inventive Examples were between 3.23 and 8.21, which were significantly increased.
The comparative examples secured 1800MPa or higher of tensile strength, but did not satisfy the alloy composition and manufacturing conditions described in the present disclosure, and accordingly, relative corrosion fatigue lives were deteriorated.
Maximum depths of corrosion pits of the comparative examples were 128 µm or greater, and the numbers of Mo-based carbides turned out to be less than 8 × 10⁴ count/mm².
As in comparative examples 6 and 7, if the alloy composition described in the example embodiment was not satisfied, even when the manufacturing conditions described in the example embodiment were satisfied, relative corrosion fatigue lives were relatively low. Also, as in comparative examples 8 and 9, even when the alloy composition described in the example embodiment was satisfied, but when the maintaining time at 600 to 700°C was not satisfied, relative corrosion fatigue lives were relatively low.
Also, when a martensite hard structure was formed in a wire rod state as in comparative examples 3 to 5, 8, and 9, breakage of the wire rod frequently occurred during a drawing process, and accordingly, it was difficult to manufacture the wire rod as a steel wire.
FIG. 1 is a graph illustrating a relative corrosion fatigue life depending on a maximum depth of a corrosion pit according to an example embodiment. The smaller the maximum depth of a corrosion pit, the greater the relative corrosion fatigue life, and when a maximum depth of a corrosion pit is greater than 120 µm, relative corrosion fatigue life was significantly degraded.
FIG. 2 is a graph illustrating a relative corrosion fatigue life depending on the number of the Mo-based carbides. The higher the number of Mo-based carbides, the greater the relative corrosion fatigue life was increased, and when the number of the Mo-based carbides was smaller than 8.0×10⁴ count/mm², relative corrosion fatigue life significantly was degraded.
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 could be made without departing from the scope of the present invention as defined by the appended claims.

Claims

A wire rod for springs with excellent corrosion fatigue resistance, comprising:
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, optionally one or more elements selected from among V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% and a balance of Fe and inevitable impurities by weight%,

wherein the wire rod satisfies equation 1 below: -0.14 ≤ 0.70[Cr] - 0.76[Cu] - 0.24[Ni] ≤ 0.47
where each element symbol is a value of a content of each element measured by weight%,

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

wherein the wire rod comprises 8.5×10⁴ count/mm² or higher of Mo-based carbides,

wherein the Mo-based carbides are carbides including 5wt% or higher of Mo,

said Mo-based carbides measured using a transmission electron microscope and energy dispersive X-ray spectroscopy after extracting fine carbides from the cross-section of the wire rod by the replica method.
The wire rod for springs of claim 1, further comprising:
one or more elements selected from among V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% by weight%.
A method of manufacturing a wire rod for springs with excellent corrosion fatigue resistance, comprising:
heating a billet to 900 to 1100°C, the billet comprising 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, optionally one or more elements selected from among V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% and a balance of Fe and inevitable impurities by weight% and satisfying equation 1 below: -0.14 ≤ 0.70[Cr] - 0.76[Cu] - 0.24[Ni] ≤ 0.47
where each element symbol is a value of a content of each element measured by weight%;

obtaining a wire rod by finishing-hot-rolling the heated billet at 800 to 1000°C; and

coiling the wire rod and cooling the wire rod such that the time for maintaining the wire rod at a temperature in a range of 600 to 700°C is to be 31 seconds or longer.
The method of claim 3, wherein the billet further comprises one or more elements selected from among V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% by weight%.
A steel wire for springs with excellent corrosion fatigue resistance, comprising:
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, optionally one or more elements selected from among V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% and a balance of Fe and inevitable impurities by weight%,

wherein the steel wire satisfies equation 1 below: -0.14 ≤ 0.70[Cr] - 0.76[Cu] - 0.24[Ni] ≤ 0.47
where each element symbol is a value of a content of each element represented by weight%,

wherein a microstructure is tempered martensite, and

wherein the steel wire comprises 8.5×10⁴ count/mm² or higher of Mo-based carbides,

wherein the Mo-based carbides are carbides including 5 wt% or higher of Mo,

said Mo-based carbides measured using a transmission electron microscope and energy dispersive X-ray spectroscopy after extracting fine carbides from the cross-section of the wire rod by the replica method,

wherein a maximum depth of a corrosion pit of the steel wire is 120 µm or less, wherein the maximum depth of a corrosion pit was measured using a confocal laser microscope, employing a cycle in which a sample of the steel wire was inserted into a salt water spray tester, 5% salt water was sprayed onto the steel wire sample for 4 hours in an atmosphere of 35°C, the steel wire sample was dried for 4 hours in an atmosphere of a temperature of 25°C and a humidity of 50%, and the steel wire sample was wetted for 16 hours until humidity became 100%, the process being repeated 14 times, and a maximum depth of a corrosion pit was measured, and

wherein tensile strength of the steel wire is 1800MPa or higher, wherein the tensile strength was measured by, after gathering tensile samples of the steel wire, performing a tensile test in accordance with ASTM E 8 standard.
The steel wire for springs of claim 5, further comprising:
one or more elements selected from among V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% by weight%.
A method of manufacturing a steel wire for springs with excellent corrosion fatigue resistance, comprising:
obtaining the steel wire by drawing a wire rod manufactured by the method in claim 3 or claim 4;

austenitizing the steel wire by heating the steel wire to 850~1000°C and maintaining the heated steel wire for 1 minute or longer; and

oil-cooling the austenitized wire rod to 25~80°C and tempering the wire rod at 350~500°C.