WO2018106016A1 - Wire rod for springs with excellent corrosion fatigue resistance, steel wire, and manufacturing method thereof - 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 a certain level or greater.

Description

Wire rod, steel wire and method for manufacturing them excellent in corrosion fatigue resistance

The present invention relates to a high-strength, corrosion-resistant wire rod, steel wire and a manufacturing method thereof that can be preferably applied to suspension springs, torsion bars, stabilizers and the like for automobiles.

Recently, in order to improve the fuel efficiency of automobiles, the weight reduction of automotive materials has been greatly demanded. In particular, in the case of suspension springs, spring designs using high-strength materials having a strength of 1800 MPa or more after quenching and tempering have been applied in order to respond to the demand for lightweighting.

The steel for spring is made of hot wire by hot rolling, and in the case of hot forming spring, it is heated and then molded and then quenched and tempered. In the case of cold formed spring, after tempering, quenched and tempered Molding

In general, when the strength of the material is increased, the crack susceptibility is increased along with the decrease in toughness due to grain embrittlement. Therefore, high strength is achieved, but if the corrosion resistance of the material is inferior, parts exposed to the outside, such as automobile suspension springs, are formed with corrosion pits where the paint is peeled off, and the parts are premature due to the propagation of fatigue cracks starting from the corrosion pits. It may be damaged.

In particular, in recent years, there is a lot of spraying snow remover to prevent the freezing of the road surface in winter, so the corrosion environment of the suspension spring is more severe, so the demand for spring steel having high strength and excellent corrosion resistance is increasing day by day.

Suspension corrosion of suspension spring is when the surface of the spring is peeled off by gravel or other foreign material on the road surface, the material of this part is exposed to the outside, causing pitting corrosion reaction, and the generated corrosion pit grows gradually and starts the pit. The cracks are generated and propagated, and at some point, hydrogen introduced from the outside concentrates on the cracks, and the spring is broken due to hydrogen embrittlement.

Conventional techniques for improving the resistance to corrosion fatigue of the spring include a method of increasing the type and amount of alloying elements. In Patent Document 1, the Ni content was increased to 0.55% by weight to increase corrosion fatigue life by improving corrosion resistance, and in Patent Document 2, the Si content was increased to refine corrosion carbide by tempering carbide precipitated during tempering. Improved strength. In addition, Patent Document 3 was able to improve the life of the spring corrosion fatigue by improving the hydrogen delayed fracture resistance by appropriate combination of Ti precipitates, which are strong hydrogen trapping sites, and (V, Nb, Zr, Hf), which are weak hydrogen trapping sites.

However, Ni is a very expensive element, and when a large amount is added, it causes a problem of material cost increase.Si is a representative element that promotes decarburization, and it may cause a considerable risk in increasing the amount of addition, and precipitates such as Ti, V, and Nb Forming elements are at risk of degrading the life of corrosion fatigue by crystallizing coarse carbonitride from the liquid phase during material solidification.

On the other hand, the prior art for increasing the strength of the spring is a method of adding an alloying element and a method of lowering the tempering temperature. As a method of increasing the strength by adding alloying elements, there is basically a method of increasing the quenching hardness using C, Si, Mn, Cr, etc., and quenching using expensive alloying elements Mo, Ni, V, Ti, Nb, etc. The strength of steel materials is raised by tempering heat treatment. However, such a technique has a problem of rising cost.

In addition, there is a method of increasing the strength of the steel by changing the heat treatment conditions in the existing component system without changing the alloy composition. In other words, when the tempering temperature is performed at a low temperature, the strength of the material increases. However, when the tempering temperature is lowered, the cross-sectional reduction rate of the material is lowered, which causes a problem of deterioration of toughness and problems such as premature failure during spring forming and use.

Therefore, the development of high-strength and corrosion resistant spring wire rod, steel wire and their manufacturing method is required.

(Prior art document)

(Patent Document 1) Japanese Unexamined Patent Publication No. 2008-190042

(Patent Document 2) Japanese Unexamined Patent Publication No. 2011-074431

(Patent Document 3) Japanese Unexamined Patent Publication No. 2005-023404

One aspect of the present invention is to control the combination of Cr, Cu, Ni content to an appropriate level, the maximum depth of the corrosion pit to a certain level or less, high-strength and corrosion-resistant fatigue resistance by a certain level of fine carbide containing Mo To provide this excellent spring wire rod, steel wire and their manufacturing method.

In addition, the subject of this invention is not limited to the content mentioned above. The problem of the present invention will be understood from the general contents of the present specification, those skilled in the art will have no difficulty understanding the additional problem of the present invention.

One aspect of the invention is by weight, C: 0.40-0.70%, Si: 1.30-2.30%, Mn: 0.20-0.80%, Cr: 0.20-0.80%, Cu: 0.01-0.40%, Ni: 0.10-0.60% %, Mo: 0.01-0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, remaining Fe and other unavoidable impurities, satisfying the following relational formula 1,

The microstructure is composed of up to 50 area% ferrite and the remaining pearlite,

The present invention relates to a spring wire rod having excellent corrosion fatigue resistance including Mo-based carbides of 8.0 × 10 ⁴ particles / mm 2 or more.

Another aspect of the present invention is by 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: 0.01-0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, remaining Fe and other unavoidable impurities, and the billets satisfying the following relation 1, 900 ~ 1100 ℃ Heating to;

Hot-rolling the heated billet to 800 to 1000 ° C. to obtain a wire rod; And

After winding the wire, the cooling time so that the holding time in the temperature range of 600 ~ 700 ℃ 31 seconds or more; relates to a method for producing a wire rod for excellent corrosion resistance comprising a.

Relationship 1: -0.14 ≤ 0.70 [Cr]-0.76 [Cu]-0.24 [Ni] ≤ 0.47

(In the relation 1, each element symbol is a value representing each element content in weight%.)

In addition, another aspect of the present invention relates to a steel wire manufactured using the wire and the method of manufacturing the same.

In addition, the solution of the said subject does not enumerate all the characteristics of this invention. 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, there is an effect that can provide a high-strength, excellent corrosion resistance to the wire rod, steel wire and their manufacturing method.

1 is a graph showing the relative corrosion fatigue life according to the maximum depth of corrosion pit of embodiments of the present invention.

Figure 2 is a graph showing the relative corrosion fatigue life according to the number of Mo-based carbide of the embodiments of the present invention.

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

In order to solve the problems of the prior arts described above, the present inventors examine various influence factors on the corrosion resistance of the steel for spring, and at the same time, the corrosion fatigue of the spring is generated by the peeling of the surface of the spring, and the corrosion pits are generated. The following findings were obtained from the fact that the cracks are generated and propagated to the starting point, and hydrogen introduced from the outside is concentrated in the cracks and the springs are broken.

First, although Cr is generally known as an element for improving corrosion resistance, the salt spray test shows that corrosion resistance decreases as Cr content increases. In addition, Cu and Ni had an effect of slowing down the corrosion rate by amorphizing the corrosion rust formed on the surface of the material during the corrosion reaction. Therefore, it is very important to adjust the combination of Cr, Cu and Ni content to an appropriate level in order to improve the corrosion resistance of the spring steel.

Second, it was found that the corrosion resistance of the spring steel decreases as the maximum depth of the corrosion pit formed on the surface of the material during the corrosion reaction. In particular, the narrower the shape and the deeper the corrosion pit, the lower the corrosion fatigue characteristics. Therefore, it is necessary to control the maximum depth of the corrosion pit below a certain level in order to improve the corrosion resistance of the spring steel.

Third, it is necessary to trap hydrogen with fine carbides in order to prevent the hydrogen introduced from the outside from concentrating on the crack portion, and the fine carbides that can be utilized are not cementite but V, Ti, Nb, Mo. It is a carbide mainly containing alloying elements, such as these. Particularly, Mo-based carbides precipitate very finely at a temperature of 700 ° C. or below in nano size, and have a great hydrogen trap effect. Carbides including V, Ti, Nb, etc. as well as Mo also have a hydrogen trap effect when they contain Mo. outstanding.

From the above findings, the combination of Cr, Cu, and Ni content is controlled to an appropriate level, the maximum depth of the corrosion pit is below a certain level, and the fine carbide containing Mo is above a certain level, thereby providing high strength and corrosion resistance. It was confirmed that the wire rod, the steel wire, and their manufacturing method can be provided, and the present invention has been completed.

Wire rod for spring excellent in corrosion fatigue

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

Spring wire for excellent corrosion fatigue resistance according to an aspect of the present invention in weight%, C: 0.40 ~ 0.70%, Si: 1.30 ~ 2.30%, Mn: 0.20 ~ 0.80%, Cr: 0.20 ~ 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, remaining Fe and other unavoidable impurities, and the following relation 1 To satisfy the microstructure, the microstructure is composed of ferrite of 50 area% or less and the remaining pearlite, and contains Mo-based carbide of 8.0 × 10 ⁴ / mm 2 or more.

First, the alloy composition of the present invention will be described in detail. The unit of each element content hereafter means weight% unless there is particular notice. In addition, the alloy composition of the present invention is equally applied to the production method of the wire rod, steel wire and steel wire production method to be described below.

C: 0.40 to 0.70%

C is an essential element added to secure the strength of the spring. In order to exhibit the effect effectively, it is preferable to contain 0.40% or more. On the other hand, if the C content is more than 0.70%, the twin type martensite structure is formed during quenching and tempering heat treatment, resulting in material cracking. However, the fracture stress can be significantly reduced. Therefore, it is preferable that C content is 0.40 to 0.70%.

In addition, the lower limit of the C content may be more preferably 0.45%, and the upper limit may be 0.65%.

Si: 1.30-2.30%

Si is dissolved in ferrite and has the effect of strengthening the base material strength and improving the deformation resistance.

When the Si content is less than 1.30%, the lower limit of Si is preferably 1.30%, and the lower limit may be 1.45% because Si is insufficiently effective in solidifying the ferrite to strengthen the base material strength and improving the deformation resistance. On the other hand, when the Si content is more than 2.30%, the effect of improving the deformation resistance is saturated, and the effect of additional addition is not obtained, and the surface decarburization is promoted during the heat treatment. Therefore, it is preferable that the upper limit of Si is 2.30%, and a more preferable upper limit may be 2.25%.

Mn: 0.20 ~ 0.80%

Mn is an element useful for securing strength by improving the hardenability of steel when present in steel.

If the Mn content is less than 0.20%, it is difficult to obtain sufficient strength and hardenability required as a material for high strength springs. On the contrary, if the Mn content is higher than 0.80%, the hardenability is excessively increased and martensite hard tissues are likely to occur during cooling after hot rolling. In addition, the formation of MnS inclusions increases, rather there is a fear that the corrosion resistance fatigue resistance. Therefore, the Mn content is preferably 0.20 to 0.80%.

In addition, the lower limit of the Mn content may be 0.30%, more preferably 0.40%. In addition, the more preferred upper limit of the Mn content may be 0.75%, and the more preferable upper limit may be 0.70%.

Cr: 0.20 ~ 0.80%

Cr is an element useful for securing oxidation resistance, temper softening, surface decarburization prevention and quenching.

If the Cr content is less than 0.20%, it is difficult to secure sufficient oxidation resistance, temper softening, surface decarburization and quenching effects. On the other hand, when the Cr content is more than 0.80%, the deformation resistance may be lowered, which may lead to a decrease in strength. Therefore, the Cr content is preferably 0.20 to 0.80%.

In addition, the lower limit of the Cr content may be 0.22%, and the upper limit may be 0.75%.

Cu: 0.01 ~ 0.40%

Copper (Cu) is an element added to improve the corrosion resistance. If the content is less than 0.01%, the effect of improving the corrosion resistance is insufficient, whereas if it is more than 0.40%, the brittleness during hot rolling causes a problem such as cracking. May cause Therefore, the Cu content is preferably 0.01 to 0.40%. More preferably, the Cu content may be 0.05 to 0.30%.

Ni: 0.10-0.60%

Nickel (Ni) is an element added to improve the hardenability and toughness. If the content is less than 0.10%, the effect of the hardenability and toughness is not sufficient, whereas if the content is higher than 0.60%, the amount of retained austenite This increases the fatigue life, and the expensive Ni properties cause a sharp rise in manufacturing costs. Therefore, the Ni content is preferably 0.10 to 0.60%.

Mo: 0.01 ~ 0.40%

Mo is an element which forms carbon or nitrogen and carbonitrides, contributes to the refinement of the structure, and acts as a trap site for hydrogen, and in order to effectively exhibit such an effect, the content is preferably 0.01% or more. However, if the Mo content is excessive, the martensite hard structure is likely to occur during hot rolling after cooling, and the upper limit of the Mo content is preferably 0.40% because coarse carbonitride is formed and the ductility of the steel is reduced.

P: 0.02% or less

P is an impurity, and it is preferable to limit the upper limit to 0.02% because of a problem of segregation at grain boundaries and deterioration of toughness.

S: 0.015% or less

S is an impurity, and it is preferable to limit the upper limit to 0.015% because it not only lowers the toughness by grain boundary segregation with low melting elements but also forms a large amount of MnS, which adversely affects the corrosion resistance of the spring.

N: 0.01% or less

Nitrogen (N) is easy to form BN by reacting with boron (B), and should be controlled as low as possible because it is an element that reduces the quenching effect, but considering the process load, it is preferable to limit it to 0.01% or less.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

Relationship 1: -0.14 ≤ 0.70 [Cr]-0.76 [Cu]-0.24 [Ni] ≤ 0.47

(In the relation 1, each element symbol is a value representing each element content in weight%.)

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

Cr is generally known as a corrosion resistance improving element, but corrosion resistance decreases with increasing Cr content in spring steel. This is because Cr lowers the pH of the pit base (bottom) during the corrosion reaction, making the inside of the pit a strong acid atmosphere, thereby increasing the maximum depth of the pit. That is, Cr plays a role of lowering corrosion resistance as the content increases.

On the other hand, Cu and Ni have an effect of slowing down the corrosion rate by amorphizing the corrosion rust formed on the material surface during the corrosion reaction. Therefore, the present inventors have studied the correlation between Cr, Cu, and Ni content on the deterioration of the corrosion resistance of the steel for spring, and found that the influence is 0.70 for Cr, -0.76 for Cu, and -0.24 for Ni, respectively. The corrosion fatigue resistance can be improved by controlling their correlation to satisfy the above Equation 1.

In this case, in addition to the alloy composition described above, it may further include at least one selected from V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% by weight.

V: 0.01 ~ 0.20%

V is not only an element that contributes to strength improvement and grain refinement, but also forms a carbon nitride with carbon (C) or nitrogen (N) and acts as a trap site for hydrogen infiltrating into steel, thereby inhibiting hydrogen intrusion inside steel materials. It is an element that serves to reduce the occurrence of corrosion.

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

Ti: 0.01 ~ 0.15%

Ti is an element that improves the spring characteristics by forming a carbonitride to cause precipitation hardening, and improves strength and toughness through particle refinement and precipitation strengthening. In addition, Ti acts as a trap site for hydrogen infiltrating into steel, thereby inhibiting hydrogen ingress and reducing corrosion.

If the Ti content is less than 0.01%, the precipitation reinforcement and the frequency of precipitates acting as hydrogen trapsites are not effective.If the Ti content is more than 0.15%, the manufacturing cost increases rapidly, and the effect of improving the spring properties by the precipitates is saturated and austenite Coarse alloy carbides that are not dissolved in the base material during heat treatment are increased to act as non-metallic inclusions, thereby reducing fatigue characteristics and precipitation strengthening effects.

Nb: 0.01 ~ 0.10%

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

The microstructure of the wire rod according to the present invention consists of ferrite of 50 area% or less and the remaining pearlite. However, the area fraction here means the measurement except for the precipitate.

If the ferrite is more than 50 area%, the strength of the material is so low that the desired level of strength cannot be achieved after the final heat treatment.

In addition, the remainder except for ferrite is pearlite. If there are hard tissues such as martensite in addition to ferrite and pearlite, there is a possibility that the wire will be disconnected at the stage of wire drawing.

Further, the wire rod according to the present invention contains Mo-based carbide at least 8.0 × 10 ⁴ / mm 2.

It is necessary to trap hydrogen with fine carbides in order to prevent the hydrogen introduced from the outside from concentrating on the crack part, and the fine carbides that can be utilized are not cementite but V, Ti, Nb, Mo, etc. Carbide is composed mainly of alloying elements. In particular, carbides containing Mo as the main component are very finely precipitated at a nano size in the temperature range of 600 to 700 ° C., so that the hydrogen trap effect is very large, and carbides containing V, Ti, and Nb as the main component also contain Mo. Hydrogen trap effect is excellent.

Therefore, the Mo-based carbide is preferably included 8.0 × 10 ⁴ / mm 2 or more, more preferably 8.5 × 10 ⁴ / mm 2 or more.

In addition, but not the number of the Mo-based carbide in the manufacture steel wire changes significantly, it may be more desirable than placing it may slightly reduce securing Mo system carbides in a pre-existing condition 9.0 × 10 ⁴ gae / ㎟ above.

In this case, the Mo-based carbide may be a carbide containing 5% by weight or more based on the carbide. As described above, carbides containing V, Ti, Nb, etc. as a main component also have an excellent hydrogen trap effect when they contain Mo.

Manufacturing method of spring wire rod excellent in corrosion fatigue

Hereinafter, another aspect of the present invention will be described in detail a method for producing a spring wire rod excellent in corrosion fatigue resistance.

Another aspect of the present invention provides a method for producing a spring wire rod excellent corrosion resistance step of heating a billet satisfying the above-described alloy composition to 900 ~ 1100 ℃; Hot-rolling the heated billet to 800 to 1000 ° C. to obtain a wire rod; And after winding the wire rod, cooling the holding time in a temperature range of 600 to 700 ° C. to 31 seconds or more.

Billet heating stage

Billets satisfying the above-described alloy composition is heated to 900 ~ 1100 ℃.

The heating temperature of the billet is above 900 ° C to dissolve all coarse carbides that may be produced during casting so that the alloying elements are uniformly distributed in the austenite. On the other hand, if the heating temperature of the billet is more than 1100 ℃, it is heated more than necessary, the heat consumption is high, there is a fear that the decarburization becomes severe as the time is long.

Hot rolling stage

The heated billet is hot rolled to finish at 800 to 1000 ° C. to obtain a wire rod.

The finishing rolling temperature is 800 ° C. or higher to promote precipitation of fine carbides. If the finish rolling temperature is less than 800 ℃, the load of the rolling roll is large, when the finish rolling temperature is greater than 1000 ℃ grain size is large, toughness is lowered, the transformation is delayed during cooling, martensite hard structure may occur.

Winding and cooling stage

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

Controlling the holding time in the temperature range of 600 to 700 ° C to be 31 seconds or more is to ensure sufficient time for the pearlite transformation to be completed without martensite hard structure being formed upon cooling. This is to allow the carbide to precipitate sufficiently.

Steel wire for spring with excellent corrosion fatigue

Another aspect of the present invention, the spring steel wire having excellent corrosion fatigue resistance satisfies the alloy composition described above, and the microstructure is a tempered martensite single phase and includes Mo-based carbide of 8.0 × 10 ⁴ / mm 2 or more. Corrosion fatigue resistance can be improved by making a microstructure a tempered martensite single phase and including Mo-type carbide 8.0 * 10 <^4> / mm <2> or more. Tempered martensite single phase means composed of tempered martensite except for some unavoidable impurities.

It is necessary to trap hydrogen with fine carbides in order to prevent the hydrogen introduced from the outside from concentrating on the crack part, and the fine carbides that can be utilized are not cementite but V, Ti, Nb, Mo, etc. Carbide is composed mainly of alloying elements. In particular, carbides containing Mo as the main component are very finely precipitated at a nano size in the temperature range of 600 to 700 ° C., so that the hydrogen trap effect is very large, and carbides containing V, Ti, and Nb as the main component also contain Mo. Hydrogen trap effect is excellent. Therefore, the Mo-based carbide is preferably included at least 8.0 × 10 ⁴ / mm 2, more preferably 8.5 × 10 ⁴ / mm ² or more. On the other hand, Mo-based carbide is produced during the production of the wire rod, and does not significantly change even after heating and cooling according to steel wire manufacturing, but may be slightly reduced.

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

This is because the corrosion resistance of the spring steel decreases as the maximum depth of the corrosion pit formed on the surface of the material during the corrosion reaction decreases. In particular, the narrower the shape and the deeper the corrosion pit, the deeper the stress applied to the pit, thereby greatly reducing the corrosion resistance.

At this time, the maximum depth measurement of the corrosion pit is put the test piece of the steel wire in the salt spray tester sprayed 5% brine for 4 hours in a 35 ℃ atmosphere, dried for 4 hours in a temperature 25 ℃, 50% humidity, 40 ℃ atmosphere The cycle of moistening for 16 hours to 100% humidity was repeated 14 cycles and then measured. This is to set the harshest conditions in consideration of the use environment of the spring steel, it can ensure excellent corrosion fatigue resistance when the maximum depth of the corrosion pit is 120㎛ or less under these conditions.

In addition, the steel wire of the present invention may have a tensile strength of 1800 MPa or more.

Manufacturing method of spring steel wire with excellent corrosion fatigue resistance

Another aspect of the present invention is a method for producing a spring steel wire having excellent resistance to corrosion fatigue step of obtaining a steel wire by drawing a wire produced by the method for producing a wire according to the present invention; An austenitization step of maintaining the steel wire at 850 to 1000 ° C. for at least 1 minute; And tempering at 350-500 ° C. after cooling the austenitic wire at 25-80 ° C.

If the holding time after heating is less than 1 minute, the ferrite and pearlite structures may not be sufficiently heated and may not be transformed into austenite, so the heating time is preferably 1 minute or more. In addition, since oil-cooling temperature is normal conditions, it does not specifically limit.

If the tempering temperature is less than 350 ℃, the toughness is not secured, there is a risk of damage in the molding and product state, while if the temperature exceeds 500 ℃ tempering temperature is preferably 350 ~ 500 ℃.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

The billet having the composition shown in Table 1 was heated to 1000 ℃ and then rolled to finish rolling at 900 ℃, the winding after cooling to maintain a temperature range of 600 ~ 700 ℃ during the holding time shown in Table 2 to prepare a wire rod. . The microstructure of the wire rod was observed and listed in Table 2 below.

After the wire was fresh, it was heated at 975 ° C. for 15 minutes, quenched in 70 ° C. oil, and then held at 390 ° C. for 30 minutes to prepare a steel wire.

Tensile strength, maximum depth of corrosion pit, Mo-based carbide, and relative corrosion fatigue life of the steel wire were measured and described in Table 2 below. The microstructure was all martensite single phase.

Tensile strength was measured by performing a tensile test after taking the tensile specimen in accordance with the ASTM E 8 standard.

Mo-based carbides were cross sectioned and then fine carbides were extracted by replica method and analyzed using Transmission Electron Microscope and Energy Dispersive X-ray Spectroscopy. Table 2 shows the number of carbides containing 5% or more.

In addition, the specimen was placed in a salt spray tester and sprayed with 5% brine for 4 hours in a 35 ° C atmosphere, dried for 4 hours in a temperature of 25 ° C and a humidity of 50%, and wetted for 16 hours to be 100% humidity in a 40 ° C atmosphere ( cycle) after 14 cycles. Corrosion pit maximum depth and relative corrosion fatigue life were measured.

Corrosion pit maximum depth was measured with a Confocal Laser Microscope.

The relative corrosion fatigue life was tested by rotational bending fatigue test, the fatigue test speed was 3,000rpm, and the load applied to the specimen was 40% of the tensile strength. The fatigue life was averaged to give 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 is 1.

Steel grade	Alloy composition (% by weight)													Relationship 1
	C	Si	Mn	Cr	Cu	Ni	Mo	P	S	N	V	Ti	Nb
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
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
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, relation 1 represents a value of 0.70 [Cr]-0.76 [Cu]-0.24 [Ni].

division	Steel grade	Wire Rod Microstructure (Area%)	600 ~ 700 ℃ holding time (sec)	Tensile Tensile Strength (MPa)	Maximum depth of corrosion feet (㎛)	Mo-based carbide (× 10 4 pieces / mm2)	Relative Corrosion Fatigue Life
Comparative Example 1	Comparative Steel 1	F: 24, P: 76	18	1,852	241	0	1.00
Comparative Example 2	Comparative Steel 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
Inventive Example 1	Inventive Steel 1	F: 14, P: 86	32	1,872	117	8.55	3.23
Inventive Example 2	Inventive Steel 2	F: 34, P: 66	46	1,883	63	12.37	5.74
Inventive Example 3	Invention Steel 3	F: 4, P: 96	92	2,051	78	74.36	6.37
Inventive Example 4	Inventive Steel 4	F: 25, P: 75	68	2,064	103	30.54	5.86
Inventive Example 5	Inventive Steel 5	F: 37, P: 63	115	2,008	112	132.05	8.21
Comparative Example 6	Comparative Steel 1	F: 32, P: 68	76	1,866	128	0	0.97
Comparative Example 7	Comparative Steel 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	Inventive Steel 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.

Inventive Examples 1 to 5, which satisfy all of the alloy composition and manufacturing conditions presented in the present invention, can be confirmed that the tensile strength and the relative corrosion fatigue life are excellent. In the comparative examples, the relative corrosion fatigue life was 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.

Although the tensile strength of 1800 MPa or more can be secured in the comparative examples, it can be seen that the relative corrosion fatigue life is inferior because the alloy composition or the manufacturing conditions presented in the present invention are not satisfied.

In the comparative examples, the maximum depths of the corrosion pits were all 128 µm or more, and the number of Mo-based carbides was all observed to be less than 8 × 10 ⁴ pieces / mm 2.

As in Comparative Examples 6 and 7, if 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 presented in the present invention are satisfied. In addition, it can be seen that the relative corrosion fatigue life is low when the alloy composition presented in the present invention as described in Comparative Examples 8 and 9 does not satisfy the 600 ~ 700 ℃ holding time.

In addition, when the martensitic hard tissue was formed in the wire rod state as in Comparative Examples 3 to 5, 8, and 9, fracture occurred frequently during the wire drawing, and it was difficult to manufacture the steel wire.

1 is a graph showing the relative corrosion fatigue life according to the maximum depth of corrosion pit of embodiments of the present invention. The smaller the maximum depth of the corrosion pit, the greater the relative corrosion fatigue life. When the maximum depth of the corrosion pit was greater than 120 μm, the relative corrosion fatigue life was greatly reduced.

Figure 2 is a graph showing the relative corrosion fatigue life according to the number of Mo-based carbide of the embodiments of the present invention. The more the number of fatigue Mo system carbides is relatively corrosion life was increased significantly, 8.0 × 10 ⁴ number / relative to the ㎟ If the Mo-based carbide smaller than this decreased significantly relative corrosion fatigue life.

Although described with reference to the embodiments above, those skilled in the art will understand that the present invention can be variously modified and changed without departing from the spirit and scope of the invention as set forth in the claims below. Could be.

Claims (11)

1. By 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, remaining Fe and other unavoidable impurities, satisfying the following Equation 1,

   The microstructure is composed of up to 50 area% ferrite and the remaining pearlite,

   Spring wire rod with excellent corrosion fatigue resistance containing Mo x carbide at least 8.0 × 10 ⁴ / mm2.

   Relationship 1: -0.14 ≤ 0.70 [Cr]-0.76 [Cu]-0.24 [Ni] ≤ 0.47

   (In the relation 1, each element symbol is a value representing each element content in weight%.)
2. The method of claim 1,

   The wire is a weight%, V: 0.01 ~ 0.20%, Ti: 0.01 ~ 0.15% and Nb: 0.01 ~ 0.10% of the wire rod for excellent corrosion resistance further comprising the fatigue fatigue resistance.
3. The method of claim 1,

   The Mo-based carbide is a spring wire rod excellent in corrosion fatigue resistance is a carbide containing Mo 5 wt% or more based on the carbide.
4. By 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 Heating a billet comprising 0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, remaining Fe and other unavoidable impurities, and satisfying the following Formula 1 to 900 to 1100 ° C;

   Hot-rolling the heated billet to 800 to 1000 ° C. to obtain a wire rod; And

   After winding the wire, the cooling time so that the holding time in the temperature range of 600 ~ 700 ℃ 31 seconds or more; comprising a good corrosion resistance wire resistance for the spring.

   Relationship 1: -0.14 ≤ 0.70 [Cr]-0.76 [Cu]-0.24 [Ni] ≤ 0.47

   (In the relation 1, each element symbol is a value representing each element content in weight%.)
5. The method of claim 4, wherein

   The billet is a weight%, V: 0.01 ~ 0.20%, Ti: 0.01 ~ 0.15% and Nb: 0.01 ~ 0.10% The method for producing a wire rod for excellent corrosion resistance further comprising corrosion resistance.
6. By 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, remaining Fe and other unavoidable impurities, satisfying the following Equation 1,

   The microstructure is tempered martensite,

   Steel wire for springs with excellent corrosion fatigue, containing Mo x carbides of 8.0 × 10 ⁴ / mm2 or more.

   Relationship 1: -0.14 ≤ 0.70 [Cr]-0.76 [Cu]-0.24 [Ni] ≤ 0.47

   (In the relation 1, each element symbol is a value representing each element content in weight%.)
7. The method of claim 6,

   The steel wire is a spring steel wire excellent in corrosion fatigue resistance further comprising at least one selected from the weight%, V: 0.01 ~ 0.20%, Ti: 0.01 ~ 0.15% and Nb: 0.01 ~ 0.10%.
8. The method of claim 6,

   The Mo-based carbide is a spring steel wire with excellent corrosion fatigue resistance is a carbide containing Mo 5 wt% or more based on the carbide.
9. The method of claim 6,

   The steel wire is a spring steel wire with excellent corrosion fatigue resistance of the maximum depth of the corrosion pit 120㎛ or less.
10. The method of claim 6,

    The steel wire is a spring steel wire with excellent resistance to corrosion fatigue tensile strength of 1800MPa or more.
11. Drawing a wire rod prepared according to claim 4 or 5 to obtain a steel wire;

    An austenitization step of maintaining the steel wire at 850 to 1000 ° C. for at least 1 minute; And

    After cooling the austenitic wire to 25 ~ 80 ℃, tempering at 350 ~ 500 ℃; The method for producing a steel wire for excellent corrosion resistance comprising a.

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