KR20140122784A

KR20140122784A - Steel wire having high corrosion resistance, spring for the same and method for manufacturing thereof

Info

Publication number: KR20140122784A
Application number: KR20130039579A
Authority: KR
Inventors: 김관호
Original assignee: 주식회사 포스코
Priority date: 2013-04-11
Filing date: 2013-04-11
Publication date: 2014-10-21

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spring used for automobile parts and the like, and more particularly to a spring steel wire having excellent corrosion resistance, a spring using the same, and a manufacturing method thereof.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel wire for a spring having excellent corrosion resistance, a spring using the steel wire, and a manufacturing method thereof. [0002]

In order to improve the fuel efficiency of automobiles, when the weight of the steel parts to be inserted into the automobile is simply reduced, a load that can be supported per unit weight is determined. Therefore, weight reduction should follow after the high strength of parts first.

However, when the component is made to have a high strength, there is a problem that toughness due to grain boundary embrittlement occurs, premature rupture due to machining or use, and premature rupture due to corrosion fatigue. Therefore, there is a demand for high toughness and corrosion fatigue resistance as well as high strength of automobile parts including automobile parts and springs.

On the other hand, as a conventional technique for improving the corrosion fatigue resistance of the spring, there is a method of increasing the kind and content of the alloy element.

In Patent Document 1, the Ni content is increased to 0.55 wt% to increase the corrosion resistance. In Patent Document 2, the Si content is increased to refine the carbide precipitated at the time of tempering, thereby improving the corrosion fatigue strength. In Patent Document 3, the corrosion resistance of the delayed fracture caused by hydrogen is improved by appropriately matching the Ti precipitate, which is a strong trap site for hydrogen, and the precipitate (V, Nb, Zr, Hf), which is a weak site, to improve spring corrosion fatigue life .

However, as Ni is a very expensive element, when added in large amounts, the cost of material increases. Si is a typical element for promoting decarburization, which may cause a considerable risk of increase in the amount of added Ti. V, Nb The precipitate-forming elements have the problem of deteriorating the corrosion fatigue life by purging coarse carbon and nitride from the liquid phase during solidification of the material.

As a conventional technique for increasing the strength of the 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 by using C, Si, Mn, Cr or the like and a method of increasing the hardness by using a high-priced alloying element Mo, Ni, V, Ti, Nb, And a method of increasing the strength of the steel material by tempering heat treatment. However, these methods have a problem in that the cost cost is increased due to the use of expensive elements.

Further, there is a method of increasing the strength of the steel by changing only 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, if 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 molding and use of the spring.

In addition to the above-mentioned methods, there is a method of utilizing Cr, which is well known as an element for improving corrosion resistance, but a salt spray cycle test results in a problem that the corrosion resistance is lowered when Cr is added.

In order to solve this problem, there is a technique for appropriately controlling the relationship between the Cr content and the Cu + Ni content while limiting the Cr content to 0.25 wt% or less. The present invention relates to a technique for improving corrosion resistance due to the formation of a Cu or Ni-enriched layer on the surface layer as the environment is corroded, but when a certain amount of time is exposed to the environment, a certain amount of corrosion occurs, pit) is generated and the fatigue characteristics are deteriorated. In addition, limiting the Cr content to 0.25% by weight or less at the time when high strength of the spring steel is required causes a decrease in the strength and an increase in the content of Cu + Ni, thereby raising the material cost.

Therefore, in order to improve the corrosion resistance of the spring steel, it is necessary not only to reduce the Cr content but also to manage the other components to secure not only the strength but also the corrosion fatigue life.

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

An aspect of the present invention is to provide a spring steel wire having improved corrosion resistance as well as high strength of a spring by suppressing corrosion pits and growth from control of component composition, a spring using the same, and a method of manufacturing the same.

An aspect of the present invention provides a method of manufacturing a semiconductor device, comprising, by weight, 0.4 to 0.7% of C, 1.0 to 3.0% of Si, 0.3 to 2.5% of Mn, 0.2 to 0.6% of Cr, 0.01 to 2.00% of Ni, And the balance of Fe and other unavoidable impurities, and the content of C, Cr and / or Cr in the above components is not more than 0.05%, not more than 0.05%, not more than 0.02%, P: not more than 0.02%, S: not more than 0.02% Ni is a spring steel wire having excellent corrosion resistance satisfying the relationship of 1.10%? [C] + [Cr] + [Ni]? 1.40%, and a spring using the steel wire.

According to another aspect of the present invention, there is provided a method of manufacturing a steel wire, comprising: preparing a steel wire by drawing a steel wire material satisfying the composition;

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 steel wire at 25 to 80 占 폚 and at 300 to 500 占 폚.

The present invention also provides a method of manufacturing a spring having excellent corrosion resistance by cold forming a steel wire for a spring manufactured by the above method at normal temperature.

According to the present invention, it is possible to provide a spring steel wire and spring which can suppress the corrosion pits and growth of the wire material and improve the corrosion resistance of the spring.

Fig. 1 shows the result of measurement of the corrosion loss after the salt spray test on the test material (invention material and comparative material).
Fig. 2 shows the results of measurement of the corrosion pit aspect ratio after the salt spray test on the test material (invention material and comparative material).
Fig. 3 shows the results of measurement of the corrosion fatigue life after the salt spray test on the test materials (inventive materials and comparative materials).
FIG. 4 shows the results of measurement of corrosion loss according to the sum of [C] + [Cr] + [Ni] components.
FIG. 5 shows the result of measurement of the corrosion-port aspect ratio according to the sum of [C] + [Cr] + [Ni] components.
FIG. 6 shows the measurement results of the corrosion fatigue life according to the sum of [C] + [Cr] + [Ni] components.

Hereinafter, the present invention will be described more specifically.

The present inventors have conducted intensive researches to provide a high strength spring steel having excellent corrosion resistance. As a result, when the content of Cr and Ni is controlled to an appropriate level while reducing the content of Cr in alloy elements, It is possible to manufacture a spring having not only strength but also excellent corrosion resistance.

Hereinafter, the composition of the steel wire for a spring and the spring for manufacturing the spring according to the present invention will be described in detail. In the following, the content unit of each component is in weight% unless otherwise specified.

C: 0.4 to 0.7%

Carbon (C) is an essential element added to secure the strength of the spring. In order to obtain such an effect, it is preferable to contain C in an amount of 0.4% or more. However, when the content of C exceeds 0.7%, twin-type martensite structure is formed during quenching and tempering heat treatment, cracks are generated in the material, so that the fatigue life is remarkably lowered and defect susceptibility is increased, , The fatigue life and breakdown stress are remarkably reduced. Therefore, the upper limit is preferably limited to 0.7%.

Si: 1.0 to 3.0%

Silicon (Si) is dissolved in ferrite to enhance the strength of the base material and improve the deformation resistance. If the content of Si is less than 1.0%, Si is dissolved in the ferrite to enhance the strength of the base material and the effect of improving the deformation resistance is insufficient. Therefore, the content of Si is preferably 1.0% or more, more preferably 1.5 % Or more. However, if the content of Si exceeds 3.0%, the effect of improving the deformation resistance is saturated and the effect of further addition is not obtained. In addition, since the surface decarburization is promoted during the heat treatment, the upper limit is preferably limited to 3.0% .

Mn: 0.3 to 2.5%

Manganese (Mn) is an element which is advantageous for enhancing the strength of steel by improving the ingotability of the steel when it is present in the steel. If the content of Mn is less than 0.3%, it is difficult to obtain sufficient strength and entrapment property required as a material for high strength springs. On the other hand, when the content exceeds 2.5%, the toughness is lowered and the bond sensitivity is increased. , The content of Mn is preferably limited to 0.3 to 2.5% in the present invention.

Cr: 0.2 to 0.6%

Chromium (Cr) is a useful element for ensuring oxidation resistance, temper softening property, surface decarburization prevention and incombustibility. If the content of Cr is less than 0.2%, it is difficult to ensure sufficient oxidation resistance, softness of tempering, surface decarburization and incombustibility. On the other hand, when the content exceeds 0.6%, the deformation resistance is lowered and the strength is lowered. In addition, the pH of the corrosion pit base is lowered to promote corrosion. Therefore, the content of Cr in the present invention is preferably limited to 0.2 to 0.6%.

Ni: 0.01 to 2.00%

Nickel (Ni) is an element added to improve the incombustibility and toughness. If the content of Ni is less than 0.01%, the effect of improving the incombustibility and toughness is not sufficient. On the other hand, The fatigue life is shortened due to an increase in the amount of Ni, and an increase in the manufacturing cost is rapidly caused due to the expensive Ni characteristic, which is not preferable. Therefore, in the present invention, Ni is preferably limited to 0.01 to 2.00%.

Cu: 0.15 to 0.35%

Copper (Cu) is an element added to improve corrosion resistance. When the content is less than 0.15%, the above effect can not be sufficiently expected. On the other hand, when the content is more than 0.35%, Cu causes problems such as cracking in hot rolling I can not. Therefore, in the present invention, Cu is preferably limited to 0.15 to 0.35%.

O: 0.0015% or less (including 0%)

In the present invention, the content of oxygen (O) is limited to 0.0015% or less. When the content of O exceeds 0.0015%, oxide-based nonmetallic inclusions are formed to a great extent and the fatigue life is rapidly lowered.

Al: 0.05% or less (including 0%)

Aluminum (Al) is an element improving the toughness of the crystal grain size and improving the toughness. If the content exceeds 0.05%, the amount of the oxide-based precipitate increases and the size thereof is coarsened, I can not.

P and S: 0.02% or less (including 0%), respectively

In the present invention, the contents of phosphorus (P) and sulfur (S) are limited to 0.02% or less, respectively. P is segregated in grain boundaries and toughness is lowered. Therefore, the upper limit is limited to 0.02% Segregation due to grain boundary deteriorates toughness and emulsions are formed to detrimentally affect the spring characteristics. Therefore, the upper limit is preferably limited to 0.02%.

N: 0.02% or less (including 0%)

Since nitrogen (N) reacts with boron (B) to easily form BN and reduces the effect of quenching, it is desirable that the content of N is as low as possible, but it is limited to 0.02% desirable.

Particularly, it is preferable that the total content of C, Cr and Ni in the above components satisfies 1.10%? [C] + [Cr] + [Ni]? 1.40%.

That is, when the total content of [C] + [Cr] + [Ni] is less than 1.10% (when the content of C and Ni is too low) when Cr is contained at 0.2 to 0.6% It is difficult to secure the desired strength due to the low Ni content, and it is difficult to secure additional corrosion resistance due to the low Ni content. As a result, it is necessary to achieve corrosion resistance only by Cr, and it is difficult to improve the corrosion resistance as desired. On the other hand, when the total content of [C] + [Cr] + [Ni] exceeds 1.40% (the content of C and Ni is too high), the C content is high and sufficient strength is secured. However, There is a problem in that the production cost of the material greatly increases due to the high content of Ni, although there is no great problem in securing corrosion resistance because the Ni content is high.

Therefore, when the relationship between C, Cr and Ni satisfies 1.10%? [C] + [Cr] + [Ni]? 1.40% when Cr is contained in an amount of 0.2 to 0.6% It is possible to secure corrosion resistance at an intended level or higher.

The remainder of the present invention is iron (Fe). However, in the ordinary steel 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 can be known by anyone skilled in the ordinary steel manufacturing process.

The component system of the present invention can provide a sufficient effect by adding the above-described alloying elements, but additionally, B and Ti can be additionally added to provide a spring steel wire with improved strength and toughness of the steel.

B: 0.0005 to 0.030%

The addition of boron (B) has the effect of densifying the rust formed on the surface, enhancing the corrosion resistance and improving the hardenability and increasing the grain boundary strength. However, when the content is less than 0.0005%, the ingot is not secured and the strength required for the steel for spring can not be secured. On the other hand, when the content exceeds 0.030%, the carbon- It is present in the austenite grain boundaries and adversely affects the fatigue characteristics, which is not preferable. Therefore, the content of B in the present invention is preferably limited to 0.0005 to 0.030%.

Ti: 0.005 to 0.50%

Titanium (Ti) is an element that improves the spring characteristics by causing a precipitation hardening action by forming carbon and nitride, and improves the strength and toughness through particle refinement and precipitation strengthening. In addition, Ti functions as a trap site of hydrogen which has entered the steel, thereby suppressing the intrusion of hydrogen in the steel and reducing the occurrence of corrosion.

If the content of Ti is less than 0.005%, the precipitation strengthening and the frequency of precipitates serving as hydrogen trap sites are small, which is not effective. On the other hand, if the content exceeds 0.50%, the production cost increases sharply, The effect is saturated and the amount of coarse alloy carbide which is not dissolved in the base material during the austenite heat treatment is increased, so that the effect is similar to that of the nonmetallic inclusion, so that the fatigue characteristic and precipitation strengthening effect are lowered. Therefore, the content of Ti in the present invention is preferably limited to 0.005 to 0.50%.

Further, the steel wire for a spring according to the present invention may further include V but is not necessarily limited thereto.

V: 0.005-0.50%

Vanadium (V) is an element that improves the spring characteristics by causing a precipitation hardening action by forming carbon and nitride, and improves the strength and toughness through particle refinement and precipitation strengthening. If the content of V is less than 0.005%, precipitation strengthening and the frequency of precipitates acting as hydrogen trap sites are small, which is not effective. On the other hand, when the content exceeds 0.50%, the production cost increases sharply, In the austenite heat treatment, the amount of coarse alloy carbide not dissolved in the base material is increased, and the same action as that of the nonmetallic inclusions is performed, so that the fatigue characteristic and precipitation strengthening effect are lowered.

In the present invention, it is possible to manufacture a steel wire for spring including the above-mentioned component composition. Hereinafter, a method for manufacturing a steel wire for a spring and a spring according to the present invention will be described in detail.

A step of cold-working the steel wire to manufacture a spring, and then, after manufacturing the steel wire by drawing the steel wire material, subjecting the steel wire to a processing heat treatment. The processing heat treatment includes a step of heating the steel wire to austenitize, cooling and tempering the steel wire.

The steel wire for a spring according to the present invention comprises an austenitizing step in which a steel wire having a composition satisfying the above composition is drawn to produce a steel wire, heating the steel wire to QT heat treatment at 850 to 1000 ° C, And annealing the austenitized steel wire at 25 to 80 캜 and tempering at 300 to 500 캜.

At this time, if the holding time after heating is less than 1 minute, the carbide and the ferrite + pearlite or pearlite structure may not be heated sufficiently and may not be transformed into austenite, so that the holding time is preferably 1 minute or more. The oil-cooling temperature is a normal condition and is not particularly limited. If the tempering temperature is lower than 300 ° C, the toughness is not ensured and there is a risk of breakage in the molding and the product condition. If the tempering temperature is higher than 500 ° C, the strength is lowered.

The steel wire for spring manufactured under the above conditions can ensure the desired mechanical properties of the present invention.

In the method of manufacturing the spring of the present invention, the steel wire subjected to the above processing heat treatment is subjected to cold forming at room temperature to manufacture a spring.

The spring thus produced can ensure the desired mechanical properties of the present invention.

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 cast steel having the composition shown in Table 1 was subjected to ordinary wire rolling and cooling to produce steel wire rods. At this time, the content unit of each component is% by weight.

division C Si Mn P S Al Cr Cu Nb Ni Mo Ti V B N C + Cr + Ni Comparison 1 0.49 2.11 0.67 0.002 0.007 0.027 0.69 0.25 - 0.31 - 0.02 0.10 - 0.0039 1.49 Comparative material 2 0.56 1.49 0.61 0.002 0.007 0.025 0.60 0.30 0.02 0.29 0.05 0.05 0.10 0.0015 0.0038 1.45 Comparative material 3 0.45 1.76 0.70 0.003 0.0.06 0.024 0.30 0.30 - 0.29 - 0.05 - - 0.0036 1.04 Comparison 4 0.49 2.00 0.30 0.002 0.008 0.024 0.20 0.20 - 0.30 0.10 0.02 - - 0.0035 0.99 Comparative material 5 0.48 2.12 0.68 0.003 0.007 0.024 0.21 0.26 - 0.31 - 0.02 0.10 - 0.0042 1.00 Inventory 1 0.50 1.83 0.32 0.002 0.007 0.023 0.20 0.20 - 0.52 - 0.02 - - 0.0036 1.22 Inventory 2 0.45 2.01 0.32 0.001 0.009 0.023 0.21 0.20 - 0.52 - 0.02 - 0.0021 0.0032 1.18 Inventory 3 0.47 2.12 0.63 0.002 0.007 0.024 0.50 0.25 - 0.31 - 0.02 0.10 - 0.0042 1.28

As shown in Table 1, inventive materials 1 to 3 have a composition ratio of [C] + [Cr] + [Ni] of 1.18 to 1.28% while satisfying Cr content of 0.2 to 0.6% 1 to 5 were at a lower or higher level than the invention.

The reason why the sum of the contents of [C] + [Cr] + [Ni] is important is to reduce the Cr content and control the content of C and Ni to an appropriate level in order to improve the corrosion resistance of the spring steel. This is because it is necessary to sufficiently secure the corrosion fatigue life.

Each of the above steel wires was subjected to quenching and tempering (QT) heat treatment in a tensile strength of 180 to 210 kgf / mm ² . After QT heat treatment, the reduction rate of the cross section was 30% or more. Thereafter, the resultant was heated at 900 to 1000 ° C for 1 minute or more, then cooled at 25 to 80 ° C, and then tempered at 370 ° C or 390 ° C.

When the above process was completed, a salt spray test was conducted for 14 days to generate corrosion pits and then subjected to fatigue test.

First, the corrosion loss of each test material (inventive material and comparative material) measured after the salt spray test is shown in Fig. 1, and the aspect ratio of the corrosion pits of each test material is shown in Fig. At this time, the aspect ratio is defined as the depth / width of corrosion pits, and the aspect ratio of each test material is the average of the measured values for 10 corrosion pits.

As shown in Fig. 1, it was observed that the corrosion loss at each tempering temperature (370 占 폚 (red line), 390 占 폚 (black line)) was less than that of the comparative article. Further, as shown in Fig. 2, the corrosion pit aspect ratio of the inventive material was smaller than that of the comparative material, and this result means that the corrosion resistance of the inventive material is superior to the comparative material.

The fatigue life of each test material after the above salt spray test was measured by a dual-spindle rotary bending fatigue tester, and the results are shown in FIG. At this time, the rotation speed was 3000 rpm, and the load was applied at 60% of the tensile strength.

As shown in Fig. 3, it can be confirmed that the fatigue life of the inventive material is improved by 50% or more from the comparative material.

In addition, corrosion loss, aspect ratio of corrosion pits and corrosion fatigue life according to the sum of [C] + [Cr] + [Ni] component are shown in Figs.

As shown in Figs. 4 to 6, when the sum of the [C] + [Cr] + [Ni] component is in the range of 1.10 to 1.40%, the aspect ratio of the corrosion loss and the corrosion pit is smaller than that of the test material deviating from the above range, It can be confirmed that the lifetime is also greatly improved.

According to the above results, when the sum of the components [C] + [Cr] + [Ni] in the composition of the steel wire for the spring is controlled to an appropriate amount, corrosion fatigue life of the spring can be increased due to improvement in corrosion resistance of the steel wire itself.

Claims

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises 0.4 to 0.7% of C, 1.0 to 3.0% of Si, 0.3 to 2.5% of Mn, 0.2 to 0.6% of Cr, 0.01 to 2.00% of Ni, 0.15 to 0.35% At most 0.05% of Al, at most 0.02% of P, at most 0.02% of S, at most 0.02% of N, the balance Fe and other unavoidable impurities,
Among these components, C, Cr and Ni satisfy the relationship of 1.10%? [C] + [Cr] + [Ni]? 1.40%.

The method according to claim 1,
Wherein the steel wire further comprises 0.0005 to 0.030% of B and 0.005 to 0.50% of Ti in weight%.

The method according to claim 1,
Wherein the steel wire further comprises 0.005 to 0.50% of V by weight%.

A spring having excellent corrosion resistance made of a steel wire for a spring according to any one of claims 1 to 3.

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises 0.4 to 0.7% of C, 1.0 to 3.0% of Si, 0.3 to 2.5% of Mn, 0.2 to 0.6% of Cr, 0.01 to 2.00% of Ni, 0.15 to 0.35% , C, Cr and Ni in the above components are composed of not more than 1.10%? [C], not more than 0.05% Al, not more than 0.02% P, not more than 0.02% S, not more than 0.02% N, ] + [Cr] + [Ni] <1.40%;
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 steel wire at 25 to 80 占 폚 and at 300 to 500 占 폚.

6. The method of claim 5,
Wherein the steel wire further comprises 0.0005 to 0.030% of B and 0.005 to 0.50% of Ti in terms of weight%.

6. The method of claim 5,
Wherein the steel wire further comprises 0.005 to 0.50% of V by weight%.

A method for producing a spring having excellent corrosion resistance, which comprises forming a spring steel wire manufactured by the manufacturing method of any one of claims 5 to 7 at a room temperature by cold forming.