KR101867709B1 - Wire rod and steel wire for spring having excellent corrosion fatigue resistance and method for manufacturing the same - Google Patents

Abstract

One aspect of the present invention relates to a wire rod for springs with high strength and excellent corrosion fatigue resistance, in which a combination of Cr, Cu, and Ni content is controlled to an appropriate level, the maximum depth of corrosion pits is set to be below a certain level, and fine carbides containing Mo are set to be at least a certain level.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wire rod for a spring having excellent corrosion fatigue resistance,

The present invention relates to a wire rod, a steel wire, and a method of manufacturing the same, which can be preferably applied to a suspension spring for an automobile, a torsion bar, a stabilizer, and the like, which have high strength and excellent corrosion fatigue resistance.

In recent years, a spring design using a high strength material having a strength of 1800 MPa or more after quenching has been applied in order to meet the demand for light weight in suspension spring.

In the case of the hot-formed spring, the hot-rolled spring is subjected to quenching tempering treatment after heating, followed by quenching in the case of the cold-formed spring, followed by quenching tempering treatment after drawing, .

Generally, if the strength of the material is increased, the toughness of the grain will be deteriorated due to grain boundary embrittlement, and the crack susceptibility will increase. Therefore, when the corrosion resistance of the material is lowered, the parts exposed to the outside such as the automobile suspension spring are formed with the corrosion pits where the paint is peeled off. By the propagation of the fatigue cracks starting from the corrosion pits, There is a risk of breakage.

Especially, recently, since there is a lot of spraying of snow remover to prevent freezing of road surface in winter, the corrosion environment of suspension spring becomes more severe, so the demand for spring steel having high strength and excellent internal fatigue characteristic is getting stronger.

When the surface of the spring surface is peeled off by the gravel or other foreign material on the road surface, the material of the part is exposed to the outside, and the corrosion reaction of pitting occurs, and as the generated corrosion pit gradually grows, Cracks are generated and propagated, and at some moment, hydrogen introduced from the outside is concentrated in the crack portion, and the spring is broken due to hydrogen embrittlement.

As a conventional technique for improving corrosion fatigue resistance of a spring, there is a method of increasing the kind and amount of an alloy element. In Patent Document 1, the corrosion fatigue life is increased by increasing the Ni content to 0.55 wt%, and in Patent Document 2, the Si content is increased to refine the carbide precipitated at the time of tempering, Improved strength. In Patent Document 3, the spring corrosion fatigue life can be improved by improving the hydrogen delay fracture resistance by proper combination of the Ti precipitate, which is a strong hydrogen trapping site, and the (V, Nb, Zr, Hf) precipitate, which is a weak hydrogen trapping site.

However, Ni is a very expensive element, and when added in large amounts, it causes a problem of material cost increase. Since Si is a typical element for promoting decarburization, it may cause a considerable risk of increase in addition amount and precipitation of precipitates such as Ti, V, Nb There is a risk that the forming elements may deteriorate the corrosion fatigue life rather than precipitate coarse carbonitride from the liquid phase during solidification of the material.

On the other hand, as a conventional technique for increasing the strength of a spring, there are a method of adding an alloy element and a method of lowering the tempering temperature. A method of increasing the hardness by adding alloying elements is basically a method of increasing the hardness of the hardening by using C, Si, Mn, Cr or the like. The method of quenching and / or quenching using expensive alloying elements Mo, Ni, V, Ti, The strength of the steel is increased by tempering heat treatment. However, this technology has a problem of cost increase.

Further, there is a method of increasing the strength of the steel by changing the heat treatment condition in the existing component system without changing the alloy component. That is, if the tempering temperature is lowered, the strength of the material increases. However, when the tempering temperature is lowered, the reduction rate of the cross section of the material is lowered, so that the toughness is lowered, and problems such as premature rupture occur during spring forming and use.

Therefore, it is required to develop a wire rod, a steel wire and a manufacturing method thereof for a spring having high strength and excellent corrosion fatigue resistance.

Japanese Patent Application Laid-Open No. 2008-190042 Japanese Laid-Open Patent Publication No. 2011-074431 Japanese Patent Application Laid-Open No. 2005-023404

One aspect of the present invention is to control the combination of Cr, Cu and Ni contents to an appropriate level, to set the maximum depth of corrosion pits to a certain level or less, and to set the microcarbon content containing Mo to a certain level or more, And an excellent spring wire material, a steel wire, and a method of manufacturing the same.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

In one aspect of the present invention, there is provided a steel sheet comprising, by weight%, 0.40 to 0.70% of C, 1.30 to 2.30% of Si, 0.20 to 0.80% of Mn, 0.20 to 0.80 of Cr, 0.01 to 0.40% of Cu, 0.10 to 0.60 of Ni 0.01 to 0.40% of Mo, not more than 0.02% of P, not more than 0.015% of S, not more than 0.01% of N, the balance of Fe and other unavoidable impurities,

The microstructure is composed of ferrite of 50% by area or less and the remaining pearlite,

Mo-based carbide of 8.0 x 10 < ⁴ > / mm < 2 > or more.

Another aspect of the present invention is a method for manufacturing a semiconductor device, comprising the steps of: 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% 0.001 to 0.60%, Mo: 0.01 to 0.40%, P: not more than 0.02%, S: not more than 0.015%, N: not more than 0.01%, and balance Fe and other unavoidable impurities, ; ≪ / RTI >

Finishing the heated billet at 800 to 1000 占 폚 to obtain a wire rod; And

And cooling the wire so that the holding time in the temperature range of 600 to 700 ° C is not less than 31 seconds after winding the wire, and more particularly, to a method of manufacturing wire for spring having excellent corrosion fatigue resistance.

Relation 1: -0.14? 0.70 [Cr] - 0.76 [Cu] - 0.24 [Ni]? 0.47

(In the above-mentioned relational expression 1, the symbol of each element represents the content of each element in weight%.)

Another aspect of the present invention relates to a steel wire manufactured using the wire rod and a method of manufacturing the same.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, it is possible to provide a wire rod, a steel wire, and a method of manufacturing them, which have high strength and excellent corrosion fatigue resistance.

1 is a graph showing the relative corrosion fatigue life according to the maximum depth of corrosion pits of embodiments of the present invention.
FIG. 2 is a graph showing the relative corrosion fatigue life according to the number of Mo-based carbides in the examples of the present invention. FIG.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

In order to solve the problems of the conventional techniques described above, the present inventors have studied various influencing factors on the corrosion resistance of the steel for spring, and the corrosion fatigue of the spring has caused the corrosion of the surface of the spring to be peeled off, The following points were obtained in consideration of the phenomenon that the hydrogen introduced from the outside is concentrated on the cracked portion and the spring is broken while the crack is generated and propagated to the starting point.

First, Cr in alloying elements is generally known as an element for improving corrosion resistance. However, it was found that the corrosion fatigue characteristics were lowered as the Cr content increased. In addition, Cu and Ni had an effect of reducing the corrosion rate by amorphizing the corrosion rust formed on the material surface during the corrosion reaction. Therefore, it is very important to adjust the combination of Cr, Cu and Ni contents to an appropriate level in order to improve the internal fatigue characteristics of the spring steel.

Second, it was found that the corrosion fatigue property of spring steel decreases as the maximum depth of corrosion pits generated on the surface of the material during the corrosion reaction increases. Particularly, the corrosion pits have a narrow width and a deeper depth, which significantly degrades the internal fatigue characteristics. Therefore, in order to improve the internal fatigue characteristic of the steel for spring, it is necessary to control the maximum depth of the corrosion pits to a certain level or less.

Third, it is necessary to trap hydrogen with fine carbide in order to prevent the hydrogen introduced from the outside from concentrating in the crack portion. The fine carbide that can be utilized at this time is not cementite but V, Ti, Nb, Mo And the like. Particularly, the Mo-based carbides precipitate very fine at a temperature of 700 ° C or lower and have a considerably large hydrogen trapping effect. When carbides containing V, Ti, Nb, etc. as a main component besides Mo also contain Mo, the hydrogen trap effect outstanding.

From the above knowledge, it is possible to control the combination of Cr, Cu and Ni contents to an appropriate level, to set the maximum depth of corrosion pits to a certain level or less, and to set the fine carbide containing Mo to a certain level or more, The present invention has been accomplished on the basis of these findings.

For spring with excellent corrosion fatigue resistance Wire rod

Hereinafter, a wire for spring having excellent corrosion fatigue resistance according to one aspect of the present invention will be described in detail.

According to one aspect of the present invention, there is provided a spring wire having excellent corrosion fatigue resistance, comprising: 0.40 to 0.70% of C, 1.30 to 2.30% of Si, 0.20 to 0.80 of Mn, 0.20 to 0.80 of Cr, 0.01 to 0.40% of Ni, 0.10 to 0.60% of Ni, 0.01 to 0.40% of Mo, 0.02% or less of P, 0.015% or less of S and 0.01% or less of N and remaining Fe and other unavoidable impurities, And the microstructure is composed of ferrite and the remaining pearlite of 50% by area or less and the Mo-based carbide of 8.0 × 10 ⁴ / mm 2 or more in the microstructure.

First, the alloy composition of the present invention will be described in detail. Hereinafter, the unit of each element content means weight% unless otherwise specified. In addition, the alloy composition of the present invention is equally applied to the wire rod manufacturing method, the steel wire and the steel wire manufacturing method described below.

C: 0.40 to 0.70%

C is an essential element added to secure the strength of the spring. In order to effectively exhibit the effect, it is preferable that the content is 0.40% or more. On the other hand, when the C content exceeds 0.70%, the twin martensite structure is formed during quenching tempering heat treatment to cause cracking of the material. Therefore, not only the fatigue life is significantly decreased but also the defect susceptibility is increased and the fatigue life Or the breaking stress is remarkably lowered, the upper limit thereof is preferably set to 0.70%.

Si : 1.30 ~ 2.30%

Si is dissolved in ferrite to enhance the strength of the base material and improve the deformation resistance.

When the Si content is less than 1.30%, the effect of enhancing the strength of the base material and improving the deformation resistance is solved by incorporating Si in the ferrite, so that the lower limit of Si is preferably 1.30%, and the lower limit is more preferably 1.45%. On the other hand, when the Si content exceeds 2.30%, the effect of improving the deformation resistance is saturated, so that the effect of further addition can not be obtained and the surface decarburization is promoted during the heat treatment.

Mn : 0.20 to 0.80%

Mn, when present in the steel, is an element which is useful for improving the incombustibility of the steel and securing the strength.

When the Mn content is less than 0.20%, it is difficult to obtain sufficient strength and ductility required for the high-strength spring material. Conversely, when the Mn content exceeds 0.80%, the ingot property excessively increases and the martensite hard- In addition, the production of MnS inclusions increases, and the internal fatigue characteristics may be deteriorated. Therefore, the Mn content is preferably 0.20 to 0.80%.

Cr : 0.20 to 0.80%

Cr is a useful element for ensuring oxidation resistance, softening of temper softening, prevention of surface decarburization and incombustibility.

When the Cr content is less than 0.20%, it is difficult to ensure sufficient oxidation resistance, softening of tempering, surface decarburization and ingotability. On the other hand, when the Cr content exceeds 0.80%, the deformation resistance is lowered and the strength may be lowered. Therefore, the Cr content is preferably 0.20 to 0.80%.

Cu: 0.01 to 0.40%

Copper (Cu) is an element added to improve corrosion resistance. When the content is less than 0.01%, the effect of improving corrosion resistance is insufficient. On the other hand, when the content is more than 0.40%, brittleness is lowered during hot rolling, ≪ / RTI > Therefore, the Cu content is preferably 0.01 to 0.40%.

Ni : 0.10 to 0.60%

Nickel (Ni) is an element added to improve the incombustibility and toughness. When the content of Ni is less than 0.10%, the effect of improving the incombustibility and toughness is not sufficient. On the other hand, when the content is more than 0.60%, the amount of retained austenite And fatigue life is shortened, and the expensive Ni characteristic causes an abrupt increase in manufacturing cost. Therefore, the Ni content is preferably 0.10 to 0.60%.

Mo : 0.01 to 0.40%

Mo is an element which forms carbonitride and nitrogen and carbonitride to contribute to texture refinement and acts as a trap site of hydrogen. In order to effectively exhibit such effect, the content of Mo is preferably 0.01% or more. However, if the Mo content is excessive, there is a high possibility that martensitic hard tissue is generated upon cooling after hot rolling, and coarse carbonitride is formed and the ductility of the steel is lowered, so that the upper limit of the Mo content is preferably 0.40%.

P: not more than 0.02%

P is an impurity and segregates in grain boundaries to lower the toughness. Therefore, the upper limit of P is preferably limited to 0.02%.

S: not more than 0.015%

S is an impurity which is segregated in a grain boundary with a low melting point element to not only decrease the toughness but also form a large amount of MnS to detrimentally affect the internal characteristics of the spring, so that the upper limit is preferably limited to 0.015%.

N: not more than 0.01%

Since nitrogen (N) reacts with boron (B) to easily form BN and reduces the effect of quenching, nitrogen should be controlled as low as possible, but it is preferably limited to 0.01% or less considering the process load.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

Relation 1: -0.14? 0.70 [ Cr ] - 0.76 [Cu] - 0.24 [ Ni ]? 0.47

(In the above-mentioned relational expression 1, the symbol of each element represents the content of each element in weight%.)

Cr, Cu and Ni satisfy not only the above-described respective element content but also the above-mentioned relational expression 1.

Cr is generally known as an element for improving corrosion resistance, but the corrosion fatigue property of the spring steel is lowered as the Cr content is increased. This is because when Cr makes a corrosion reaction, it lowers the pH of the pit base (bottom portion) and makes the inside of the pit strong acidic atmosphere to increase the maximum depth of the pit. That is, Cr serves to lower the internal fatigue characteristics as the content increases.

On the other hand, Cu and Ni have the effect of slowing the corrosion rate by amorphizing the corrosion rust formed on the material surface during the corrosion reaction. As a result of studying the correlation between the Cr, Cu, and Ni contents of the spring steels on the degradation of the internal fatigue characteristics, the present inventors have found that the influences thereof are 0.70 for Cr, -0.76 for Ni and -0.24 for Ni And the corrosion fatigue resistance can be improved by controlling the correlation between them to satisfy the above-mentioned relational expression (1).

At this time, in addition to the alloy composition described above, one or more selected from the group consisting of V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% can be further included in weight percent.

V: 0.01 to 0.20%

V is not only an element contributing to strength improvement and grain refinement but also forms carbonitride with carbon (C) or nitrogen (N) to act as a trap site for hydrogen penetrated into steel, thereby suppressing hydrogen intrusion in the steel And reduce the occurrence of corrosion.

When the V content is less than 0.01%, the above-mentioned effect is insufficient. On the other hand, when the V content is excessive, the manufacturing cost increases, so that the upper limit of the V content is preferably 0.20%.

Ti : 0.01 to 0.15%

Ti is an element that improves the spring characteristics by forming a carbonitride and causing a precipitation hardening action and improves the strength and toughness through particle refinement and precipitation strengthening. Further, Ti acts as a trap site for hydrogen that enters the steel, thereby suppressing the intrusion of hydrogen in the steel and reducing the occurrence of corrosion.

When the Ti content is less than 0.01%, the precipitation strengthening and the frequency of the precipitate acting as the hydrogen trap site are small, which is not effective. When the Ti content is more than 0.15%, the production cost increases sharply, The amount of coarse alloy carbide not dissolved in the base material is increased during the heat treatment, and the same action as that of the nonmetallic inclusions is performed, so that the fatigue characteristic and precipitation strengthening effect are lowered.

Nb : 0.01 to 0.10%

Nb is an element which forms a carbonitride with carbon or nitrogen and contributes mainly to texture refinement and acts as a trap site of hydrogen. Therefore, in order to effectively exhibit the effect, it is preferable that the addition amount is 0.01% or more. However, when the Nb content is excessive, coarse carbonitride is formed and the ductility of the steel is lowered, so that the upper limit of the addition amount is preferably 0.10%.

The microstructure of the wire according to the present invention is composed of not more than 50% by area of ferrite and the remaining pearlite. However, here, the area fraction means the measurement excluding precipitates.

If the ferrite content exceeds 50% by area, the strength of the material becomes too low, and the desired level of strength can not be achieved after the final heat treatment.

The remainder excluding the ferrite is pearlite. In the case where hard tissues such as martensite exist in addition to ferrite and pearlite, there is a possibility that the possibility of disconnection in the step of drawing wire is increased.

The wire according to the present invention contains not less than 8.0 × 10 ⁴ / mm ² of Mo-based carbide.

It is necessary to trap hydrogen with a fine carbide in order to prevent the hydrogen introduced from the outside from concentrating in the crack portion. The fine carbide which can be utilized at this time is not cementite but V, Ti, Nb, Mo It is a carbide containing an alloy element as a main component. Particularly, the carbide containing Mo as a main component precipitates very finely at a nanoscale in the temperature range of 600 to 700 ° C, and the hydrogen trap effect is considerably large. When carbides containing V, Ti, Nb and the like as main components also contain Mo Excellent hydrogen trap effect. Therefore, it is preferable that the Mo-based carbide is contained at 8.0 × 10 ⁴ / mm 2 or more. However, since the number of the Mo-based carbides does not largely fluctuate during the production of the steel wire, it may be slightly reduced, and it is more preferable to secure the Mo-based carbide at 9.0 x 10 ⁴ / mm 2 or more in the wire rod state.

At this time, the Mo-based carbide may be a carbide containing 5% by weight or more of Mo based on the carbide. As described above, carbides mainly composed of V, Ti, Nb and the like also have excellent hydrogen trapping effect when containing Mo.

For spring with excellent corrosion fatigue resistance Wire rod  Manufacturing method

Hereinafter, a method for manufacturing a spring wire having excellent corrosion fatigue resistance, which is another aspect of the present invention, will be described in detail.

A method for manufacturing a spring wire having excellent corrosion fatigue resistance, which is another aspect of the present invention, includes the steps of: heating a billet satisfying the alloy composition described above to 900 to 1100 캜; Finishing the heated billet at 800 to 1000 占 폚 to obtain a wire rod; And cooling the wire so that the holding time in the temperature range of 600 to 700 ° C is 31 seconds or more after winding the wire.

Billet  Heating step

The billet satisfying the alloy composition described above is heated to 900 to 1100 占 폚.

The heating temperature of the billet is set to 900 ° C or higher in order to completely dissolve the coarse carbides that can be produced during casting so that the alloying elements are uniformly distributed in the austenite. On the other hand, when the heating temperature of the billet is higher than 1100 DEG C, the billet is heated more than necessary, the amount of heat consumed is increased, and the time is lengthened, thereby deteriorating the decarburization.

Hot rolling step

The heated billet is hot-rolled at 800 to 1000 占 폚 to obtain a wire rod.

The finishing rolling temperature is set to 800 DEG C or more in order to promote precipitation of fine carbides. If the finish rolling temperature is less than 800 ° C, the load of the rolling roll becomes large. If the finish rolling temperature is more than 1000 ° C, the grain size becomes large and the toughness lowers and the transformation during cooling is delayed.

Coiling  And cooling step

After winding the wire rod, the wire rod is cooled so that the holding time in the temperature range of 600 to 700 ° C is 31 seconds or more.

The control is performed such that the holding time in the temperature range of 600 to 700 占 폚 is 31 seconds or more in order to ensure a sufficient time for the pearlite transformation to be completed without generating the martensite hard tissue at the time of cooling, So that the carbide is sufficiently precipitated.

Spring steel wire with excellent corrosion fatigue resistance

The steel wire for spring having an excellent corrosion fatigue resistance, which is another aspect of the present invention, satisfies the alloy composition described above, the microstructure is a tempered martensite single phase, and contains Mo-based carbide at not less than 8.0 x 10 ⁴ / mm 2. Corrosion fatigue resistance can be improved by making the microstructure a tempered martensite single phase and containing Mo-based carbide at 8.0 x 10 ⁴ / mm 2 or more. Tempered martensite single phase means that it consists of tempered martensite except for some unavoidable impurity.

It is necessary to trap hydrogen with a fine carbide in order to prevent the hydrogen introduced from the outside from concentrating in the crack portion. The fine carbide which can be utilized at this time is not cementite but V, Ti, Nb, Mo It is a carbide containing an alloy element as a main component. Particularly, the carbide containing Mo as a main component precipitates very finely at a nanoscale in the temperature range of 600 to 700 ° C, and the hydrogen trap effect is considerably large. When carbides containing V, Ti, Nb and the like as main components also contain Mo Excellent hydrogen trap effect. Therefore, it is preferable that the Mo-based carbide is contained at 8.0 × 10 ⁴ / mm 2 or more, more preferably 8.5 × 10 ⁴ / mm ² or more. On the other hand, the Mo-based carbide is produced at the time of producing the wire rod and thereafter is not largely changed by heating and cooling according to the manufacture of the steel wire, but may be slightly reduced.

At this time, the steel wire of the present invention may have a maximum corrosion depth of 120 탆 or less.

This is because the larger the depth of the corrosion pit generated on the surface of the material during the corrosion reaction, the lower the internal fatigue characteristics of the spring steel. In particular, the corrosion pits are narrower in width and deeper in depth, and the stress applied to the pits is intensified.

At this time, to measure the maximum depth of the corrosion pits, the test piece of the steel wire was put in a salt water spray tester and sprayed with 5% brine for 4 hours in an atmosphere of 35 캜, dried for 4 hours at a temperature of 25 캜 and a humidity of 50% And a cycle of wetting for 16 hours so as to have a humidity of 100% was repeated after 14 cycles. This is the most severe condition considering the use environment of spring steel. Under these conditions, excellent corrosion fatigue resistance can be guaranteed when the maximum depth of corrosion pits is 120 μm or less.

The tensile strength of the steel wire of the present invention may be 1800 MPa or more.

Manufacturing method of spring steel wire excellent in corrosion fatigue resistance

According to another aspect of the present invention, there is provided a method of manufacturing a steel wire for a spring having excellent corrosion fatigue resistance, comprising: obtaining a steel wire by drawing a wire made by the method of manufacturing a wire according to the present invention; An austenitizing step of heating the steel wire to 850 to 1000 占 폚 and holding the steel wire for at least 1 minute; And cooling the austenitized wire rod to 25 to 80 캜, followed by tempering at 350 to 500 캜.

When the holding time after heating is less than 1 minute, the ferrite and pearlite structure are not sufficiently heated and may not be transformed into austenite, so that the heating time is preferably 1 minute or more. The oil cooling temperature is not particularly limited because it is a normal condition.

If the tempering temperature is less than 350 ° C, the toughness is not ensured and there is a risk of breakage in the molding and the product state. If the tempering temperature is higher than 500 ° C, the strength may be lowered. Therefore, the tempering temperature is preferably 350 to 500 ° C.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

The billet having the composition shown in the following Table 1 was heated to 1000 캜, finishing rolled at 900 캜, and then wound. After winding and cooling, the temperature range of 600 to 700 캜 was maintained for the holding time shown in Table 2 below to produce a wire rod . The microstructure of the wire rod was observed and described in Table 2 below.

After the wire rod was fresh, it was heated at 975 캜 for 15 minutes, then immersed in 70 캜 oil and quenched, and then maintained at 390 캜 for 30 minutes to prepare a steel wire.

The tensile strength, corrosion pit maximum depth, Mo-based carbide, and relative corrosion fatigue life of the steel wire were measured and are shown in Table 2 below. All microstructures were martensite single phase.

Tensile strength of the steel wire was measured by tensile test after tensile specimens were taken according to ASTM E 8 standard.

The Mo-based carbides were cut by cross-sectioning the specimens and then analyzed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy Is shown in Table 2 below. ≪ tb > < TABLE >

The test piece was placed in a salt spray tester and sprayed with 5% brine for 4 hours in an atmosphere of 35 ° C, dried for 4 hours at a temperature of 25 ° C and a humidity of 50%, and cured for 16 hours at a humidity of 100% cycle) after 14 cycles. The maximum depth of corrosion pits and the relative corrosion fatigue life were measured.

The maximum depth of corrosion pit was measured with Confocal Laser Microscope.

The relative corrosion fatigue life was evaluated by the rotational bending fatigue test. The fatigue test speed was 3,000 rpm, and the load applied to the specimen was 40% of the tensile strength. The fatigue life was averaged to determine the corrosion fatigue life of the specimen. Table 2 shows the relative corrosion fatigue life of the remaining specimens when the corrosion fatigue life of Comparative Example 1 was taken as 1.

Steel grade Alloy composition (% by weight) Relationship 1 C Si Mn Cr Cu Ni Mo P S N V Ti Nb Comparative River 1 0.53 1.53 0.68 0.73 - - - 0.016 0.008 0.0049 0.51 Comparative River 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 Invention river 2 0.49 1.45 0.48 0.23 0.22 0.52 0.13 0.008 0.006 0.0057 -0.13 Invention 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 Invention 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

Relation 1 in Table 1 means a value of 0.70 [Cr] - 0.76 [Cu] - 0.24 [Ni].

division Steel grade Wire rod
Microstructure
(area%) 600 ~ 700 ℃
Retention time
(sec) Steel wire
The tensile strength
(MPa) Corrosion feet
Maximum depth (㎛) Mo-based carbide (x 10 ⁴ / mm 2) Relative
Corrosion fatigue
life span Comparative Example 1 Comparative River 1 F: 24, P: 76 18 1,852 241 0 1.00 Comparative Example 2 Comparative River 2 F: 36, P: 64 23 1,914 187 0 1.07 Comparative Example 3 Comparative Steel 3 F: 19, P: 49, M: 10 27 2,075 145 2.18 1.16 Comparative Example 4 Comparative Steel 4 F: 17, P: 54, M: 12 29 2,038 238 5.45 1.04 Comparative Example 5 Comparative Steel 5 F: 2, P: 53, M: 19 30 1,986 132 7.96 1.28 Inventory 1 Inventive Steel 1 F: 14, P: 86 32 1,872 117 8.55 3.23 Inventory 2 Invention river 2 F: 34, P: 66 46 1,883 63 12.37 5.74 Inventory 3 Invention steel 3 F: 4, P: 96 92 2,051 78 74.36 6.37 Honorable 4 Inventive Steel 4 F: 25, P: 75 68 2,064 103 30.54 5.86 Inventory 5 Invention steel 5 F: 37, P: 63 115 2,008 112 132.05 8.21 Comparative Example 6 Comparative River 1 F: 32, P: 68 76 1,866 128 0 0.97 Comparative Example 7 Comparative River 2 F: 31, P: 69 51 1,920 141 0 1.02 Comparative Example 8 Inventive Steel 1 F: 6, P: 85, M: 9 30 1,904 176 2.04 1.01 Comparative Example 9 Invention river 2 F: 8, P: 76, M: 16 28 1,923 214 4.75 1.16

In Table 2, F means ferrite, P means pearlite, and M means martensite.

It is confirmed that Examples 1 to 5, in which both the alloy composition and the manufacturing conditions proposed in the present invention are satisfied, are excellent in tensile strength and relative corrosion fatigue life. In the comparative examples, the relative corrosion fatigue life was in the range of 0.97 to 1.28, but in the case of the inventive examples, the relative corrosion fatigue life was greatly increased to 3.23 to 8.21.

In the comparative examples, it was also possible to secure a tensile strength of 1800 MPa or more. However, it is understood that the corrosion fatigue life is prolonged due to the unsatisfactory alloy composition or manufacturing conditions proposed in the present invention.

In the comparative examples, the maximum depth of corrosion pits was more than 128 μm and the number of Mo-based carbides was less than 8 × 10 ⁴ / ㎟.

As in Comparative Examples 6 and 7, when the alloy composition of the present invention is not satisfied, it can be confirmed that the relative corrosion fatigue life is low even if the manufacturing conditions proposed in the present invention are satisfied. In addition, even when the composition of the alloys proposed in the present invention is satisfied as in Comparative Examples 8 and 9, the relative corrosion fatigue life is low when the holding time is not satisfied at 600 to 700 ° C.

Further, when martensitic hard tissues were formed in the wire rod state as in Comparative Examples 3 to 5, 8 and 9, breakage often occurred at the time of drawing, and it was difficult to produce wire rods.

1 is a graph showing the relative corrosion fatigue life according to the maximum depth of corrosion pits of embodiments of the present invention. The relative corrosion fatigue life was found to be larger when the maximum depth of the corrosion pits was smaller. When the maximum depth of the corrosion pits was larger than 120 ㎛, the relative corrosion fatigue life was greatly decreased.

FIG. 2 is a graph showing the relative corrosion fatigue life according to the number of Mo-based carbides in the examples of the present invention. FIG. The relative corrosion fatigue life was greatly increased as the number of Mo - based carbides was increased. When the Mo - based carbide was smaller than 8.0 × 10 ⁴ / mm 2, the relative corrosion fatigue life was greatly reduced.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (11)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.40 to 0.70% of C, 1.30 to 2.30% of Si, 0.20 to 0.80% of Mn, 0.20 to 0.80% of Cr, 0.01 to 0.40% of Cu, 0.10 to 0.60% 0.40%, P: not more than 0.02%, S: not more than 0.015%, N: not more than 0.01%, balance Fe and other unavoidable impurities,
The microstructure is composed of ferrite of 50% by area or less and the remaining pearlite,
Which is excellent in corrosion fatigue resistance including Mo-based carbide of 8.55 × 10 ⁴ / mm 2 or more.
Relation 1: -0.14? 0.70 [Cr] - 0.76 [Cu] - 0.24 [Ni]? 0.47
(In the above-mentioned relational expression 1, the symbol of each element represents the content of each element in weight%.)
The method according to claim 1,
Wherein the wire rod further comprises at least one member selected from the group consisting of V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% by weight.
The method according to claim 1,
Wherein the Mo-based carbide is a carbide containing 5 wt% or more of Mo based on the carbide, and is excellent in corrosion fatigue resistance.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.40 to 0.70% of C, 1.30 to 2.30% of Si, 0.20 to 0.80% of Mn, 0.20 to 0.80% of Cr, 0.01 to 0.40% of Cu, 0.10 to 0.60% Heating the billets satisfying the following relational expression 1 to 900 to 1100 占 폚, comprising: 0.40%; P: not more than 0.02%; S: not more than 0.015%; N: not more than 0.01%; and balance Fe and other unavoidable impurities;
Finishing the heated billet at 800 to 1000 占 폚 to obtain a wire rod; And
And cooling the wire so that the holding time in the temperature range of 600 to 700 ° C is 31 seconds or more after winding the wire.
Relation 1: -0.14? 0.70 [Cr] - 0.76 [Cu] - 0.24 [Ni]? 0.47
(In the above-mentioned relational expression 1, the symbol of each element represents the content of each element in weight%.)
5. The method of claim 4,
Wherein the billet further comprises at least one selected from the group consisting of V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% by weight.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.40 to 0.70% of C, 1.30 to 2.30% of Si, 0.20 to 0.80% of Mn, 0.20 to 0.80% of Cr, 0.01 to 0.40% of Cu, 0.10 to 0.60% 0.40%, P: not more than 0.02%, S: not more than 0.015%, N: not more than 0.01%, balance Fe and other unavoidable impurities,
The microstructure is tempered martensite,
Steel wire with excellent corrosion fatigue resistance including Mo-based carbide of 8.55 × 10 ⁴ / mm 2 or more.
Relation 1: -0.14? 0.70 [Cr] - 0.76 [Cu] - 0.24 [Ni]? 0.47
(In the above-mentioned relational expression 1, the symbol of each element represents the content of each element in weight%.)
The method according to claim 6,
Wherein the steel wire further comprises at least one member selected from the group consisting of V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% in weight%.
The method according to claim 6,
The Mo-based carbide is a carbide containing 5% by weight or more of Mo based on the carbide, and is excellent in corrosion fatigue resistance.
The method according to claim 6,
Wherein the steel wire has excellent corrosion fatigue resistance with a maximum depth of corrosion pits of 120 m or less.
The method according to claim 6,
The steel wire is excellent in corrosion fatigue resistance with a tensile strength of 1800 MPa or more.
A method for producing a steel wire, comprising: obtaining a steel wire by drawing a wire made by the method of claim 4 or 5;
An austenitizing step of heating the steel wire to 850 to 1000 占 폚 and holding the steel wire for at least 1 minute; And
And a step of tempering the austenitized wire material at a temperature of from 25 to 80 ° C and then tempering at a temperature of from 350 to 500 ° C.

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