CN109136750B

CN109136750B - Corrosion-resistant wire rod and steel wire for spring and manufacturing method thereof

Info

Publication number: CN109136750B
Application number: CN201810895306.6A
Authority: CN
Inventors: 金宽镐
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Co ltd; Posco Holdings Inc
Priority date: 2015-12-10
Filing date: 2016-12-08
Publication date: 2020-12-08
Anticipated expiration: 2036-12-08
Also published as: CN106987765A; CN109136750A; CN106987765B; KR101736614B1

Abstract

One aspect of the present invention relates to a wire rod for a spring excellent in corrosion resistance, comprising, in weight%: 0.5 to 0.7% of C, 1.2 to 2.0% of Si, 0.3 to 1.0% of Mn, 0.01 to 0.50% of Cr, 0.01 to 0.50% of Cu, 0.01 to 0.50% of Ni, 0.0001 to 0.10% of Ti, 0.0001 to 0.0050% of B, and the balance of Fe and other unavoidable impurities, wherein C, Si, Cr, Cu and Ni satisfy the following relation 1, relation 1: ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]) is not less than 1.85, wherein [ Si ], [ Cu ], [ Ni ], [ C ] and [ Cr ] in the relational expression 1 represent values corresponding to the contents of the alloying elements in terms of weight%.

Description

Corrosion-resistant wire rod and steel wire for spring and manufacturing method thereof

The present application claims priority from korean patent application No. 10-2015-0175984 filed at 10/12/2015; this application is a divisional application of chinese patent application No. 201611123930.1 filed on 8/12/2016.

Technical Field

The present invention relates to a wire rod for a spring, a steel wire, and a method for manufacturing the same.

Background

When parts of steel materials for automobiles are simply lightened as a means for improving fuel efficiency of automobiles, a deadly problem of automobile safety may be caused because a supportable load per unit weight is determined. Therefore, the weight of the component should be reduced after the component is strengthened.

However, when the strength of the component is increased, the toughness is lowered by grain boundary embrittlement or the like, early fracture occurs during processing or use, and early fracture occurs due to corrosion fatigue. Therefore, automobile materials and automobile parts including springs need to have high strength, high toughness, and corrosion fatigue resistance.

As a conventional technique for improving the corrosion fatigue resistance of a spring, there is a method of increasing the kinds and addition amounts of alloy elements.

In patent document 1, the effect of improving corrosion resistance is obtained by increasing the Ni content to 0.55 wt%, and in patent document 2, the corrosion fatigue strength is improved by increasing the Si content and refining carbides precipitated at the time of tempering (tempering). Further, in patent document 3, by appropriately adjusting Ti precipitates of strong hydrogen substitution elements (trapping sites) and precipitates of weak hydrogen substitution elements (V, Nb, Zr, Hf), the hydrogen resistance is improved, and the corrosion fatigue life of the spring is improved.

However, addition of a large amount of Ni as an extremely expensive element increases the material cost, Si is a typical element for promoting decarburization, and therefore, there is a considerable risk that the addition amount increases, and precipitate-forming elements such as Ti, V, and Nb crystallize from the liquid phase to form coarse carbonitrides when the material solidifies, thereby adversely decreasing the corrosion fatigue life.

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.

Methods of adding alloying elements to obtain high strength basically include a method of improving through-hardening hardness using C, Si, Mn, Cr, etc. and a method of improving strength of steel through rapid cooling and tempering heat treatment using expensive alloying elements Mo, Ni, V, Ti, Nb, etc. However, this method has a problem of cost increase due to the use of expensive elements.

Further, there is also a method of increasing the strength of a steel material by changing only the heat treatment conditions of the existing components without changing the alloy components. That is, tempering at a low temperature can improve the strength of the raw material. However, when the tempering temperature is lowered, the reduction rate of the fracture surface is lowered, so that there occurs a problem of lowering of toughness, and a problem of early fracture or the like in molding and use of the spring.

In addition to the above methods, there is a method of utilizing Cr, which is known as an element for improving corrosion resistance, but as a result of a brine spray cycle (cycle) experiment, it was found that addition of Cr rather lowers corrosion resistance.

In order to solve the above problem, there is a technique of limiting the content of Cr to 0.25 wt% or less and appropriately controlling the relationship between the Cr content and the Cu + Ni content. That is, as the environment corrodes, a passivation layer of Cu and Ni is formed on the surface layer, thereby improving corrosion resistance. However, a certain amount of corrosion occurs after exposure to the environment for a certain period of time, resulting in the occurrence of pits (pit) on the surface, and thus has a problem of a decrease in fatigue characteristics. Furthermore, in the case of spring steel requiring high strength, the strength is reduced by limiting the Cr content to 0.25 wt% or less, and the material cost is increased by increasing the Cu + Ni content.

Therefore, in order to improve the corrosion resistance of spring steel, not only the content of Cr is reduced, but also the contents of C, Si, Mn and Cr are controlled to appropriate standards to secure strength and corrosion fatigue life.

Documents of the prior art

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

(patent document 2) Japanese laid-open patent No. 2011-074431

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

Disclosure of Invention

Technical problem to be solved

An object of one aspect of the present invention is to provide a wire rod for a spring, a steel wire, and a method for manufacturing the same, which are excellent in corrosion resistance, by appropriately controlling an alloy composition and a manufacturing method.

On the other hand, the technical problem to be solved by the present invention is not limited to the above. The technical problems to be solved by the present invention can be understood through the entire contents of the present specification, and those skilled in the art to which the present invention pertains should not have any difficulty in understanding the additional technical problems to be solved by the present invention.

(II) technical scheme

One aspect of the present invention relates to a wire rod for a spring excellent in corrosion resistance, comprising, in weight%: 0.5 to 0.7% of C, 1.2 to 2.0% of Si, 0.3 to 1.0% of Mn, 0.01 to 0.50% of Cr, 0.01 to 0.50% of Cu, 0.01 to 0.50% of Ni, 0.0001 to 0.10% of Ti, 0.0001 to 0.0050% of B, and the balance of Fe and other unavoidable impurities, wherein C, Si, Cr, Cu and Ni satisfy the following relational expression 1.

Another aspect of the present invention relates to a method for manufacturing a wire rod for a spring excellent in corrosion resistance, including the steps of: casting molten steel to obtain a steel billet, wherein the molten steel comprises, by weight, 0.5-0.7% of C, 1.2-2.0% of Si, 0.3-1.0% of Mn, 0.01-0.50% of Cr, 0.01-0.50% of Cu, 0.01-0.50% of Ni, 0.0001-0.10% of Ti, 0.0001-0.0050% of B, and the balance Fe and other unavoidable impurities; and hot-rolling the steel slab to obtain a wire rod, wherein the molten steel is rapidly cooled to a temperature of less than 1400 ℃ at a speed of 1 ℃/s or more on average in a temperature range of 1400 to 1500 ℃ when cast into a steel slab.

Relation 1: ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]) is not less than 1.85

Wherein [ Si ], [ Cu ], [ Ni ], [ C ] and [ Cr ] in the relational expression 1 are values of contents of the alloy elements expressed by weight%.

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

Moreover, the solution to the technical problem is not exhaustive of the features of the invention. Various features of the present invention and advantages and effects corresponding thereto may be more specifically understood with reference to the following detailed description.

(III) advantageous effects

According to the present invention, a wire rod for a spring, a steel wire, and a method for producing the same, which are excellent in corrosion resistance, can be provided.

Drawings

Fig. 1 shows the results of measuring the amount of corrosion loss after a brine spray experiment comparing the comparative material and the inventive material.

Fig. 2 shows the results of measuring the corrosion pit shape ratio after the brine spray experiment of the comparative material and the inventive material.

Fig. 3 shows the results of measuring corrosion fatigue life after a brine spray experiment comparing the comparative material and the inventive material.

FIG. 4 shows the results of measuring the amount of corrosion loss as a function of ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]).

FIG. 5 shows the results of measurement of the etch pit shape ratio as a function of ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]).

FIG. 6 shows the results of measuring the corrosion fatigue life as a function of ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]).

Fig. 7 shows the results of measuring the number of MnS inclusions in the depth of 1mm from the surface of the comparative material and the inventive material and the number of pits having a shape ratio of 0.25 or more in the corrosion pits formed on the surface after the brine spray test was performed.

Fig. 8 shows the results of measuring the etch pit shape ratio of comparative material 2 by a confocal laser microscope.

Fig. 9 shows the results of measuring the etch pit shape ratio of the inventive material 2 by a confocal laser microscope.

Detailed Description

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

The wire rod for a spring excellent in corrosion resistance according to one aspect of the present invention includes, in weight%: 0.5 to 0.7% of C, 1.2 to 2.0% of Si, 0.3 to 1.0% of Mn, 0.01 to 0.50% of Cr, 0.01 to 0.50% of Cu, 0.01 to 0.50% of Ni, 0.0001 to 0.10% of Ti, 0.0001 to 0.0050% of B, and the balance of Fe and other unavoidable impurities, wherein C, Si, Cr, Cu and Ni satisfy the following relation 1.

First, the alloy composition of the wire rod for a spring excellent in corrosion resistance according to one aspect of the present invention will be described in detail. The following units of the respective alloying elements are weight%.

C:0.5～0.7％

C is an essential element added to ensure the strength of the spring.

In order to secure sufficient strength, the C content is preferably 0.5% or more. However, when the C content exceeds 0.7%, a twin (twin) martensite structure is formed during the quenching and tempering heat treatment, and the material is cracked, so that not only the fatigue life is remarkably reduced, but also the defect sensitivity is high, and when corrosion pits are formed, the fatigue life or the breaking stress is remarkably reduced. Therefore, the C content is preferably 0.5 to 0.7%.

1.2 to 2.0% by weight of Si

Si acts to improve the base material strength and the spring Resistance (Sag Resistance) by being dissolved in ferrite.

However, when the Si content is less than 1.2%, the effect of solid solution of Si in ferrite to increase the strength of the parent material and improve the spring loss resistance is not significant, so the lower limit of the Si content is preferably 1.2%, more preferably 1.5%.

On the contrary, when the Si content exceeds 2.0%, the effect of improving the spring-reducing resistance is saturated, the effect of continuing the addition cannot be obtained, and the surface decarburization is promoted at the time of the heat treatment. Therefore, the upper limit of the Si content is preferably 2.0%.

Mn:0.3～1.0％

Mn is an advantageous element in steel materials to improve hardenability of steel materials to ensure strength.

When the Mn content is less than 0.3%, it is difficult to obtain sufficient strength and hardenability required for a material for a high-strength spring. On the other hand, if the Mn content exceeds 1.0%, the toughness decreases, the defect sensitivity increases, corrosion pits are formed, and the lifetime decreases. Therefore, the Mn content is preferably 0.3 to 1.0%.

Cr:0.01～0.50％

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

When the Cr content is less than 0.01%, it is difficult to secure sufficient oxidation resistance, temper softening, surface decarburization prevention, hardenability effect, and the like. On the contrary, when the Cr content exceeds 0.50%, the spring-back resistance is lowered, which in turn causes a decrease in strength, and the pH of the pit base is lowered to promote corrosion. Therefore, the Cr content is preferably 0.01 to 0.50%.

Ni:0.01～0.50％

Ni is an element added to improve hardenability and toughness.

When the Ni content is less than 0.01%, the effect of improving hardenability and toughness is insufficient, and when the Ni content exceeds 0.50%, the retained austenite amount increases, the fatigue life is reduced, and the manufacturing unit price is sharply increased due to the high-priced characteristics of Ni. Therefore, the Ni content is preferably 0.01 to 0.50%.

Cu:0.01～0.50％

Cu is known as an element for improving corrosion resistance.

When the Cu content is less than 0.01%, it is difficult to sufficiently improve the corrosion resistance, and when the Cu content exceeds 0.50%, problems such as cracks occur during hot rolling. Therefore, the Cu content is preferably 0.01 to 0.50%.

Ti:0.0001～0.10％

Ti is an element that acts as a precipitation hardening action by forming carbonitride to improve spring characteristics, and enhances strength and toughness by grain refinement and precipitation strengthening. Further, Ti functions to suppress the intrusion of hydrogen gas into the steel and to reduce the occurrence of corrosion, as a replacement element for hydrogen gas intruding into steel.

When the Ti content is less than 0.0001%, the effect is not remarkable because the frequency of precipitates serving as precipitation strengthening and hydrogen substituting elements is low. On the contrary, when the Ti content exceeds 0.10%, the manufacturing cost is sharply increased, the spring characteristic improving effect by the precipitates is saturated, the amount of coarse alloy carbides insoluble in the base material is increased at the time of the austenitic heat treatment, and the same effect as that of the nonmetallic inclusions is exerted, so that the fatigue characteristic and the precipitation strengthening effect are lowered.

Therefore, the Ti content is preferably 0.001 to 0.10%.

B:0.0001～0.0050％

B is known as an element that densifies rust generated on the surface, improves corrosion resistance, and improves hardenability to improve grain boundary strength.

When the content of B is less than 0.0001%, hardenability cannot be secured, and thus strength required for the steel material cannot be secured. On the contrary, when the B content exceeds 0.0050%, the fatigue characteristics are adversely affected by coarsening of carbonitride-based precipitates or the presence of boron carbide at austenite grain boundaries. Therefore, the B content is preferably 0.0001 to 0.0050%, more preferably 0.0001 to 0.0045%, and still more preferably 0.0001 to 0.0040%.

The remainder of the composition of the present invention is iron (Fe). In addition, in a general manufacturing process, mixing of unexpected impurities from raw materials or the surrounding environment cannot be avoided, and thus cannot be excluded. Since the impurities are known to those skilled in the general manufacturing process, the present invention does not specifically mention all of them.

On the other hand, the alloy composition may further contain 0.001 to 0.30% by weight of V in terms of% by weight, but is not limited thereto.

V:0.001～0.30％

V is an element that acts as a precipitation hardening action by forming carbonitrides to improve spring characteristics, and enhances strength and toughness by grain refinement and precipitation strengthening.

When the V content is less than 0.001%, the frequency of precipitates used as a precipitation strengthening and hydrogen substituting element is low, and the effect is not remarkable. If the content exceeds 0.30%, the manufacturing cost is rapidly increased, the effect of improving the spring characteristics by the precipitates is saturated, the amount of coarse alloy carbides insoluble in the base material during the austenitic heat treatment is increased, and the same effect as that of the nonmetallic inclusions is exerted, so that the fatigue characteristics and the precipitation strengthening effect are reduced. Therefore, the content of V to be added is preferably 0.001 to 0.30%.

In addition, the excellent corrosion resistance can be ensured by controlling the C, Si, Mn, Cr, Cu, and Ni to satisfy the following relational expression 1 in addition to the above alloy composition.

Relation 1: ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]) is not less than 1.85

Wherein [ Si ], [ Cu ], [ Ni ], C ] and [ Cr ] in the relational expression 1 are values in which the contents of the alloying elements are expressed in weight%.

From the viewpoint of corrosion resistance, it is preferable to increase the contents of Si, Cu, and Ni. Since Si increases corrosion fatigue strength while increasing resistance to elasticity reduction, it is preferable to increase the Si content in the range of 1.2 to 2.0 wt%, since Cu and Ni can amorphize corrosion rust, the rust is easily detached, and the shape ratio of corrosion pits can be reduced, thereby contributing to corrosion resistance, and it is preferable to increase the Si content in the range of 0.01 to 0.50 wt%. In contrast, as the C content increases, the toughness and the corrosion fatigue strength decrease, and Cr increases the pH of the bottom surface portion of the corrosion pit to increase the shape ratio of the corrosion pit, so that it is necessary to define a preferable range. Therefore, the strength and the excellent corrosion resistance can be secured only when the value of ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]) is more than a certain level.

On the other hand, the number of MnS inclusions in the surface of the wire rod and the steel wire for a spring of the present invention to a depth of 1mm is preferably 20/mm²The following. When the wire rod (steel wire) is manufactured into a spring through a spring forming process, the number of MnS inclusions of the billet may be used as a base point for surface Pitting (pitching), and thus, the number of MnS inclusions of the billet is preferably limited to 20/mm²The following.

Further, when the wire rod and the steel wire for a spring of the present invention were sprayed with 5% saline water at a temperature of 35 ℃ for 8 hours, then were kept at a temperature of 35 ℃ and a humidity of 60% for 16 hours, and the above-mentioned experiment was repeated for 14 days, the number of large corrosion pits formed in the entire corrosion pits on the surface, the shape ratio of which was more than 0.25, was 15/mm²The corrosion resistance is excellent as follows.

The shape ratio of the corrosion pits is the depth divided by the width of the corrosion pit, which has a direct effect on the corrosion fatigue life of the spring. In general, as the shape ratio of the corrosion pit becomes smaller, the corrosion fatigue life of the spring increases, and therefore, when the corrosion resistance of the steel for a spring is evaluated, the shape ratio of the corrosion pit gradually becomes the criterion thereof.

Next, the method for producing the wire rod for a spring and the steel wire excellent in corrosion resistance according to the present invention will be described in detail.

The method for manufacturing a wire rod for a spring excellent in corrosion resistance according to another aspect of the present invention includes the steps of: casting molten steel satisfying the alloy composition and the relational expression 1 to obtain a billet; and hot rolling the billet to obtain a wire rod, and rapidly cooling the molten steel to a temperature of less than 1400 ℃ at a speed of 1 ℃/s or more on average in a temperature range of 1400 to 1500 ℃ when casting the molten steel into a billet.

The 1400 to 1500 ℃ temperature is a temperature range in which the production rate of MnS inclusions is most active, and therefore, when the molten steel is cast into a billet, it is preferable that the surface of the billet is rapidly cooled in the temperature range and Pitting (pitching) occurs from the surface of the billet as a base point, and thus, it is preferable that the molten steel is rapidly cooled to a depth of 30 mm. As described above, in the wire rod manufactured by hot rolling such a billet by rapidly cooling the billet at a temperature range of 1400 to 1500 ℃ at an average rate of 1 ℃/s or more, the number of MnS inclusions in a depth of 1mm from the surface of the wire rod is controlled to 20/mm²The steel slab having excellent corrosion resistance can be obtained as follows. The 1400-1500 ℃ temperature range is the temperature range in which the production speed of MnS inclusions is most active, so that rapid cooling in the temperature range can reduce the quantity of MnS inclusions produced by nuclei, and can not provide growth time for the MnS inclusions produced by nuclei, thereby reducing the production quantity of MnS.

The billet produced by the above-described method is hot-rolled to produce a wire rod. The hot rolling can be carried out by a usual method, and is not particularly limited. However, it is preferable that the wire rod is obtained by hot rolling at a temperature in the range of 800 to 1050 ℃.

The method for manufacturing the steel wire for the spring with excellent corrosion resistance comprises the following steps: drawing the wire rod manufactured by the method as described above to manufacture a steel wire; austenitizing, heating the steel wire to 900-1050 ℃ and keeping the temperature for more than 5 seconds; and cooling the austenitized wire rod oil to 25-80 ℃, and tempering at 350-450 ℃.

In the step of drawing the wire rod manufactured by the above-described method to manufacture a steel wire, and heating the steel wire to 900 to 1050 ℃ and maintaining the austenite for 5 seconds or more, when the heating maintaining time is less than 5 seconds, carbide and ferrite + pearlite or pearlite structure are not sufficiently heated and are not transformed into austenite, and therefore, it is preferable to maintain the steel wire for 5 seconds or more and perform the austenitization. The oil cooling temperature is not particularly limited since it is a common condition. Further, when the tempering temperature is less than 350 ℃, toughness cannot be secured, and thus there is a risk of fracture in a molding and product state, and on the contrary, when it exceeds 450 ℃, there is a risk of strength reduction, and thus, the tempering temperature is preferably 350 to 450 ℃.

The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are only for illustrating the present invention and are described in more detail, and do not limit the scope of the present invention. Since the scope of the invention is to be determined by the claims appended hereto and by reasonable analogy with claims from this disclosure.

(example 1)

When casting slabs having the composition shown in table 1 below, slabs were cast at cooling rates shown in table 2 as cooling rates of the slab surface in the temperature range of 1400 to 1500 ℃, and were hot-rolled at 950 ℃ to produce wire rods.

Table 2 shows the results of measuring the number of MnS inclusions produced in the depth of 1mm from the surface of the wire rod and the number of pits formed in the surface after the salt spray test, in which the aspect ratio was 0.25 or more.

The content unit of each component in table 1 below is weight%.

TABLE 1

Steel grade	C	Si	Mn	Cr	Cu	Ni	Ti	B	V	([Si]+[Cu]+[Ni])/([C]+[Cr])
											Comparative Steel 1	0.52	1.25	0.66	0.48	0.13	0.18	0.03	0.0008	0.11	1.56
Comparative Steel 2	0.55	1.36	0.65	0.45	0.17	0.25	0.02	0.0025	0.10	1.78
											Comparative Steel 3	0.67	1.57	0.32	0.29	0.08	0.11	0.05	0.0034	-	1.83
Comparative Steel 4	0.58	1.38	0.44	0.44	0.26	0.22	0.02	0.0019	-	1.82
											Comparative Steel 5	0.63	1.83	0.87	0.49	0.03	0.12	0.02	0.0015	0.10	1.77
Invention steel 1	0.54	1.92	0.61	0.38	0.27	0.48	0.07	0.0024	-	2.90
											Invention steel 2	0.68	1.24	0.32	0.04	0.08	0.15	0.02	0.0030	0.26	2.04
Invention steel 3	0.53	1.45	0.92	0.46	0.22	0.24	0.09	0.0021	0.14	1.93
											Invention steel 4	0.65	1.87	0.48	0.42	0.04	0.07	0.01	0.0014	-	1.85
Invention steel 5	0.57	1.56	0.72	0.19	0.16	0.11	0.03	0.0031	0.08	2.41

TABLE 2

The invention materials 1 to 5 satisfy both the alloy composition of each element defined in the present invention and the relational expression 1, and the comparative materials 1 to 5 satisfy the alloy composition of each element defined in the present invention but do not satisfy the relational expression 1.

In Table 1, the ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]) value of the inventive material was about 1.85 to 2.90, but the ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]) value of the comparative material was 1.56 to 1.83, showing a value smaller than that of the inventive material.

Further, it was found that the comparative materials all produced more than 20 MnS inclusions on the lower surface of the wire rod per mm²However, in the inventive material, the number of MnS inclusions formed in a depth of 1mm from the surface to the center of the wire rod was 20/mm²The following.

The tensile strength of each comparative material and the inventive material in Table 1 is 180 to 210kgf/mm²Is subjected to rapid cooling and tempering heat treatment. The steel sheet is rapidly cooled to 60 ℃ after being heated at 980 ℃ for 5 seconds or more, and then tempered at 370 or 420 ℃. The salt spray test was performed 14 days later, and thus the fatigue test was performed after the generation of the corrosion pits. The salt spray test was carried out by spraying 5% salt water at 35 ℃ for 8 hours using a 100mm phi x100mm bar-shaped sample piece, and then maintaining the sample piece at 35 ℃ and 60% humidity for 16 hours, and the test was repeated for 14 days.

FIG. 1 shows the amount of corrosion loss (weight loss,%) measured for each sample after the brine spray test. It was confirmed that the amount of corrosion loss of the inventive material was smaller than that of the comparative material at each tempering temperature (370 ℃ (quadrilateral point) and 420 ℃ (circular point)).

Fig. 2 shows the corrosion pit shape Ratio (Aspect Ratio) of each sample measured after the brine spray experiment. The aspect ratio is defined as depth (depth)/width (width) of the etch pits, and the aspect ratio of each sample is an average of the measured values of 10 etch pits. The shape ratio of the corrosion pits was measured by a Confocal Laser Microscope (Confocal Laser Microscope) on a sample subjected to the saline spray test under the conditions described above.

It can be confirmed that the shape ratio of the corrosion pits of the inventive material is smaller than that of the comparative material, which indicates that the corrosion resistance of the inventive material is superior to that of the comparative material.

FIG. 3 shows the results of Fatigue life of various sheets measured by a four-joint rotary Bending Fatigue Testing Machine (Dual-Spindle Rotating Bending Testing Machine) after a salt water spray test. The Relative Corrosion Fatigue Life (Relative Corrosion Fatigue Life) is a Corrosion Fatigue Life when the Corrosion Fatigue Life of the comparative material 1 is 1. The rotation speed used in the experiment was 3000rpm, and a load corresponding to about 60% of the tensile strength was applied.

As shown in fig. 3, it was confirmed that the fatigue life of the inventive material was improved by 50% or more as compared with the comparative material.

FIGS. 4 to 6 show the amount of corrosion loss, the etch pit shape ratio, and the relative corrosion life as a function of ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]). When the value of ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]) is 1.85 or more, it is confirmed that the ratio of the amount of corrosion loss to the shape of the corrosion pit is small and the relative corrosion fatigue life is significantly improved.

Fig. 7 shows the number of MnS inclusions (square dots) in a depth of 1mm from the surface of each sample piece and the number of pits (round dots) having a shape ratio of 0.25 or more in the corrosion pits formed on the surface after the salt spray test. The comparative materials each exceeded 20/mm for the number of MnS inclusions in the depth of 1mm from the surface²In contrast, the inventive material is 20 pieces/mm²The following; the comparative material has a number of pits with a shape ratio of 0.25 or more in the corrosion pits formed on the surface after the salt spray test, and more than 15 pits/mm²In contrast, the inventive material is 15 pieces/mm²The following. Therefore, it was confirmed that the corrosion resistance of the inventive material was superior to that of the comparative material.

Fig. 8 and 9 show the results of measurement of the etching pit shape ratio of comparative material 2 and inventive material 2 by a Confocal Laser Microscope (Confocal Laser Microscope), and the difference in etching shape ratio was clearly confirmed.

(example 2)

When the cooling rate of the surface of the slab in the temperature range of 1400 to 1500 ℃ is not satisfied, in order to confirm the inferior corrosion resistance, when slabs having the composition of invention steels 1 and 3 were cast, slabs were cast at the cooling rate in the temperature range of 1400 to 1500 ℃ as shown in the following table 3, and hot-rolled at 950 ℃ to produce wire rods.

Table 3 shows the number of MnS inclusions formed on the surface of the wire rod measured to a depth of 1mm and the number of pits formed on the surface after the salt spray test, in which the aspect ratio was 0.25 or more.

TABLE 3

The comparative materials 6 to 13 satisfied the alloy composition proposed in the present invention, but the cooling rate of the surface of the billet in the temperature range of 1400 to 1500 ℃ was less than 1 ℃/s, so that a large amount of MnS inclusions and corrosion pits were formed, and it was confirmed that the corrosion resistance was poor.

The present invention has been described with reference to the embodiments described above, but it will be understood by those skilled in the art that the present invention may be modified and changed in various forms without departing from the technical idea and technical field of the present invention described in the claims.

Claims

1. A wire rod for springs excellent in corrosion resistance, comprising, in weight%: 0.5 to 0.7% of C, 1.2 to 2.0% of Si, 0.3 to 1.0% of Mn, 0.01 to 0.50% of Cr, 0.01 to 0.50% of Cu, 0.01 to 0.50% of Ni, 0.0001 to 0.10% of Ti, 0.0001 to 0.0050% of B, and the balance of Fe and other unavoidable impurities,

the C, Si, Cr, Cu and Ni satisfy the following relational expression 1,

the number of MnS inclusions in a depth of 1mm from the surface of the wire rod was 20/mm²In the following, the following description is given,

relation 1: ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]) is not less than 1.85

Wherein [ Si ], [ Cu ], [ Ni ], [ C ] and [ Cr ] in the relational expression 1 represent values corresponding to the contents of the alloying elements in% by weight.

2. The wire rod for a spring excellent in corrosion resistance according to claim 1, further comprising 0.001 to 0.30% by weight of V.

3. A steel wire for a spring excellent in corrosion resistance, comprising in wt%: 0.5 to 0.7% of C, 1.2 to 2.0% of Si, 0.3 to 1.0% of Mn, 0.01 to 0.50% of Cr, 0.01 to 0.50% of Cu, 0.01 to 0.50% of Ni, 0.0001 to 0.10% of Ti, 0.0001 to 0.0050% of B, and the balance of Fe and other unavoidable impurities,

the number of MnS inclusions in a depth of 1mm from the surface of the steel wire is 20/mm²In the following, the following description is given,

the C, Si, Cr, Cu and Ni satisfy the following relational expression 1,

after spraying 5% saline water at 35 ℃ for 8 hours to the steel wire, and then maintaining the steel wire at 35 ℃ and 60% humidity for 16 hours, the number of corrosion pits with a shape ratio (depth/width) of more than 0.25 in the entire corrosion pits on the surface of the steel wire was 15/mm, and the number of the corrosion pits was 14 days²In the following, the following description is given,

relation 1: ([ Si ] + [ Cu ] + [ Ni ])/([ C ] + [ Cr ]) is not less than 1.85

4. The steel wire for springs excellent in corrosion resistance according to claim 3, characterized in that the steel wire further comprises 0.001 to 0.30% by weight of V.