JPH10196697A - High strength spring with excellent environmental brittleness resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure high strength and high environmental brittleness resistance by covering at least a part of the surface of a spring steel base material with a corrosion protective film functioning as a sacrifice anode and adding carbonitride forming elements in the spring steel base material. SOLUTION: Spring steel provided with a tensile strength of 1800MPa or more is used as a base material, and at least a part of the surface of the spring steel base material is covered with a corrosion protective film functioning as a sacrifice anode. In this way, a high stress spring can be electrochemically protected from corrosion effectively. However, hydrogen is generated when the spring steel base material is electrochemically protected from corrosion by means of the sacrifice anode, and a detrimental action such as generation of delayed fracture and promotion of growth of a crack based on corrosion fatigue is exerted. Then, carbonitride forming elements such as Ti, Nb, Ta are added to the spring steel base material so that a great amount of fine carbonitride is deposited in the base material. As a result, impurities such as hydrogen and phosphorus penetrating into the base material can be trapped by means of the carbonitride, so that environmental brittleness can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spring in which at least a part of the surface of a spring steel base material is coated with an anticorrosion film functioning as a sacrificial anode, and is particularly resistant to environmental brittleness such as corrosion fatigue and delayed fracture, and has high strength. The present invention relates to a spring having:

[0002]

2. Description of the Related Art In recent years, from the standpoint of protecting the global environment, there has been an increasing demand for reducing the exhaust gas and fuel consumption of transport vehicles represented by automobiles, and it has been desired to reduce the weight of transport devices to satisfy such requirements. It is rare. As one of the measures to reduce the weight, the use of springs used in transport aircraft has been increased in stress, and high-strength spring steel having a strength after quenching and tempering of 1800 MPa or more is used as a spring base material. Proposed.

However, it is generally known that when the strength of steel is increased, environmental strength such as fatigue and delayed brittleness in a corrosive environment is significantly deteriorated, and the corrosion durability life is shortened. Therefore, in order to increase the stress of the spring, it is necessary to prevent such a decrease in environmental embrittlement. For example, the technology described in Japanese Patent Application Laid-Open No. 62-49035, that is, at least the surface of the spring steel base material is required. It has been proposed to electroplate a metal, for example, zinc, which is lower than the base material of the spring steel, so that the galvanized film functions as a sacrificial anode to electrochemically protect the spring.

[0004]

By the way, when the electroplating is performed as described above, hydrogen generated during the electroplating may enter the spring steel base material and cause so-called plating embrittlement. . Also, when the sacrificial anode mechanism electrochemically protects the spring steel base material,
On the basis of the principle of the local battery, there is a problem that hydrogen is generated on the surface of the steel base material, which is the cathode, and penetrates into the spring steel base material to cause delayed fracture.

Further, when a metal film functioning as a sacrificial anode is formed by a surface treatment other than electroplating, for example, even when zinc is brush-plated as described in JP-A-7-3491, When the function is exerted, hydrogen is generated by the formation of a local battery, and delayed fracture occurs due to intrusion of hydrogen into the spring steel base material, so that sufficient environmental embrittlement cannot be obtained. As described above, conventionally, there has been no spring that satisfies both the need for high stress and the need for strong environmental embrittlement.

The present invention has been made in view of the above problems, and has high strength and high environmental brittleness (characteristics that are unlikely to cause environmental embrittlement such as corrosion fatigue and delayed fracture). It is an object of the present invention to provide a high-strength spring having a good spring strength.

[0007]

According to the present invention, there is provided a spring in which at least a part of the surface of a spring steel base material is coated with an anticorrosion film functioning as a sacrificial anode. Carbonitride is finely dispersed in the spring steel base material by adding a carbonitride forming element to the material.

According to the present invention, at least a portion of the surface of a spring steel base material is coated with an anticorrosion film functioning as a sacrificial anode to perform cathodic protection, and the disadvantage of the anticorrosion film is reduced in environmental brittleness resistance. Carbon nitrides dispersed in the steel matrix are preventing. That is, the carbonitride traps hydrogen generated by forming the anticorrosion film, thereby suppressing corrosion fatigue and delayed fracture.

Specifically, at least one of Ti: 0.001 to 0.5% Nb: 0.001 to 0.5% Ta: 0.001 to 0.5% as the carbonitride forming element And N: 0.001 to 0.02%.

[0010] The anticorrosion film may be a metal film made of a metal or an alloy thereof that is more electrochemically lower than the spring steel base material, or a non-metal film that is more electrochemically lower than the spring steel base material. A composite film in which many base metals or alloys thereof are dispersed is used. Further, as the spring steel base material, C:
0.7% or less, Si: 0.1 to 4.0% and Mn:
Spring steel containing 0.01 to 2.0% can be employed.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A spring steel having a tensile strength of 0 MPa or more is used as a base material, and at least a part of the surface of the spring steel base material is coated with an anticorrosion film that functions as a sacrificial anode. As the anticorrosion film, for example, a film made of a metal that is electrochemically lower than a spring steel base material represented by zinc, aluminum, magnesium, or the like, or an alloy containing at least one of those metal elements may be used. it can. Further, as the anticorrosion film, in addition to the above-mentioned metal film and alloy film, a composite film obtained by dispersing a large number of the above-mentioned metals or their alloys in a nonmetal film can be used. Such a composite film includes, for example, a zinc-rich paint which is a paint containing a large amount of metallic zinc powder (zinc dust) in a coating film (non-metal film), a composite plating, a composite ceramic, and the like. The coating method of the anticorrosive film on the base material of the spring steel includes a vapor-phase coating method such as an electroplating method, a hot-dip plating method, a vacuum deposition method, a sputtering method, an ion plating method, a CVD method and an ion implantation method. And any coating method may be used.

In this way, at least a part of the surface of the spring steel base material is coated with the anticorrosion film, and a part of the anticorrosion film is dissolved by the local battery composed of the anticorrosion film and the spring steel base material.
By constituting the sacrificial anode so as to apply a cathodic current to the spring steel base material, the spring steel base material can be cathodic protected, and the high stress spring can be effectively electrochemically protected.

However, when the spring steel base material is electrochemically protected by the sacrificial anode, as described above, hydrogen is generated, which causes delayed fracture or accelerates crack propagation based on corrosion fatigue. And other adverse effects (see Comparative Examples described later). In particular, the more the anticorrosion film is electrochemically lower than the base material of the spring steel, the higher the anticorrosion effect, but on the other hand, the greater the amount of generated hydrogen, which has a more serious adverse effect. Further, depending on the method of forming the anticorrosion film (coating method), hydrogen is generated at the time of coating the anticorrosion film, causing the same problem as described above. Especially,
When a zinc anticorrosive film is coated on the surface of a spring steel base material by electroplating, hydrogen is generated at the time of the plating, and the danger of plating embrittlement due to hydrogen absorption is extremely high.
Also, even when the anticorrosion film is coated on the surface of the spring steel base material by a vapor phase coating method such as vacuum evaporation or sputtering, as is well known, the oxide film on the surface of the spring steel base material is preliminarily reduced. Is performed, and hydrogen enters the spring steel base material by the reduction process. Furthermore, no matter which coating method is adopted, not a little hydrogen absorption of the spring steel base material occurs during pickling treatment of the oxide scale,
This is one of the factors promoting the embrittlement. As described above, when the anticorrosion film is entirely or locally coated on the surface of the spring steel base material, regardless of the coating method used, the adverse effects of hydrogen (environmental embrittlement such as corrosion fatigue and delayed fracture) may occur. ) Is important.

Therefore, in the spring according to the embodiment of the present invention, a carbonitride forming element such as Ti, Nb or Ta is added to a spring steel base material, and fine carbonitrides are contained in the spring steel base material. Since a large number of particles are dispersed and precipitated, the carbonitride traps impurities such as hydrogen and phosphorus which are generated by the pre-coating process and the coating process of the anticorrosion film and penetrate into the spring steel base material, thereby causing environmental embrittlement. Restrained. In addition, the strength and toughness of the spring steel base material itself is enhanced by fine carbonitrides,
The stress of the whole spring is increased.

In order to enhance environmental brittleness resistance by adding a carbonitride forming element to a spring steel base material, at least one of Ti, Nb and Ta as a carbonitride forming element is added in an amount of 0.001 to 0.001. It is preferable that N be contained in the range of 0.5% and N be contained in the range of 0.001 to 0.02%. This is because at least one element of Ti, Nb and Ta is added as a carbonitride forming element, and at least 0.001 element is added to form a carbonitride.
% Or more, and N in the base material of the spring steel is 0.00%.
On the other hand, if the content of the carbonitride forming element exceeds 0.5% and the content of N exceeds 0.02%, the spring steel is solidified during the manufacturing process. This is because coarse carbonitrides may be generated, and the number of coarse carbonitrides may increase to deteriorate the fatigue characteristics of the spring. In addition, Ti as a carbonitride forming element,
Even when Nb and Ta are added in trace amounts, as long as the total amount thereof is 0.001% or more, at least one element of Ti, Nb and Ta is added as a carbonitride forming element. The effect of is obtained.

Further, in order to avoid the danger that the coarse precipitates serve as starting points to shorten the spring fatigue life, Ti, Nb or T added as a carbonitride forming element is used.
a is adjusted to 0.1% or less and N is 0.01%
Below, it is more desirable to adjust to 0.007% or less.

In the above description, the tensile strength is 1
At least a part of the surface of the spring steel base material of 800 MPa or more is coated with an anticorrosion film functioning as a sacrificial anode, and the spring steel base material is configured to contain a carbonitride forming element, thereby achieving high strength. In addition, while providing a spring having high environmental brittleness resistance, the properties (strength, strength, etc.) of the spring steel base material itself are adjusted by adjusting the components of the spring steel base material as follows.
It is possible to provide an excellent spring with improved toughness, sag and fatigue. Hereinafter, the reason will be described.

C: 0.7% or less C is an element essentially contained in steel, and if present even in a trace amount, contributes to improvement in strength (hardness) after quenching and tempering depending on the amount of C. I do. If the C content is less than 0.3%, the strength (hardness) after quenching and tempering is slightly insufficient, while if it exceeds 0.7%, not only does the toughness and ductility after quenching and tempering deteriorate, but also the corrosion resistance decreases. Will also have adverse effects. In consideration of the strength and toughness of the spring steel, the more preferable C content is 0.3 to 0.55%, and in order to more reliably improve the hydrogen embrittlement resistance and the corrosion fatigue characteristics, the C content is 0.30 to 0.50%.
A range of 0.50% is preferred.

Si: 0.1 to 4.0% Si is an element that contributes to strength improvement as a solid solution strengthening element. If it is less than 0.1%, the matrix strength tends to be insufficient. However, if it is added in excess of 4.0%, the carbide will not sufficiently dissolve during quenching and heating,
Uniform austenitization requires high-temperature heating, decarburization of the surface proceeds, and the fatigue characteristics of the spring deteriorate. From the viewpoint of strength and hardness as a spring material and suppression of decarburization, a more preferable range of Si is 1.0 to 2.0%.
Range.

Mn: 0.01 to 2.0% Mn is preferably contained at 0.01% or more as a hardenability improving element. However, if the content exceeds 2.0%, a large amount of retained austenite appears during quenching, and the strength and hardness may be deteriorated. The more preferred content of Mn is in the range of 0.1 to 0.5%.

P: 0.01% or less P is an impurity unavoidably mixed into steel, causing segregation at the grain boundaries, lowering the strength of the grain boundaries and causing fracture at the grain boundaries. % Or less.

S: 0.01% or less S combines with other elements to form sulfide-based inclusions.
Since the size and number of the sulfide-based inclusions increase with an increase in the content of S and adversely affect the fatigue properties, the S content should be suppressed to 0.01% or less.

Ni: not more than 5.0% Ni, if present even in a trace amount, increases the toughness of the material after quenching and tempering according to the amount thereof, and enhances the corrosion resistance by making the generated rust amorphous and dense. It has an effect and also has an effect of improving the sag property, which is important as a spring property. However, if it is contained more than 5.0%, the hardenability is excessively increased, and a supercooled structure is easily generated after rolling. Become. Ni
Is more preferably in the range of 0.1 to 2.0%.

Cu: 1.0% or less Cu is an element that is electrochemically nobler than iron and has an effect of improving corrosion resistance. Such an effect is exhibited in accordance with the amount of Cu, even if present in a very small amount, but is particularly effectively exhibited by adding 0.05% or more. However, 1.
If it exceeds 0%, no further improvement in corrosion resistance can be expected, but rather the material may be embrittled by hot rolling. The more preferable range of Cu is 0.1 to 0.1.
The range is 5%.

Cr: 5.0% or less Cr makes the rust formed on the surface layer amorphous and dense, contributes to the improvement of corrosion resistance, and effectively acts to improve the hardenability similarly to Mn. . Such an effect is exhibited in accordance with the amount of Cr, even if present in a very small amount.
It is particularly effective when added at not less than 05%. However, if it is added in excess of 5.0%, the carbide hardly dissolves during quenching, which adversely affects the strength and hardness. The more preferable content of Cr is 0.1.
It is in the range of 2 to 1.5%.

V: 0.5% or less If V exists even in a trace amount, it forms fine precipitates of carbon, nitrogen, and sulfide in accordance with the amount of V, and exhibits an effect of further increasing hydrogen embrittlement resistance and fatigue characteristics. In addition, it exerts a crystal grain refinement effect to increase toughness and proof stress, and further contributes to improvement in corrosion resistance and sag resistance. However, if it is too large, the amount of carbides not solid-dissolved in austenite at the time of quenching heating increases, and it becomes difficult to obtain satisfactory strength and hardness, so that it is 0.5% or less, more preferably 0.05% or more. Should be kept below 3%.

Mo: 1.0% or less Mo, if present even in a trace amount, has the effect of improving the hardenability in accordance with the amount present, and also has the effect of increasing the corrosion resistance by adsorbing molybdate ions generated during corrosion dissolution. Further, it also has the effect of increasing the grain boundary strength and improving hydrogen embrittlement resistance. These effects are effectively exhibited by containing 0.05% or more, preferably 0.1% or more. However, their effect saturates at about 1.0%, so further additions are economically entirely useless.

At least one element selected from the group consisting of Al, B, Co, and W is an element that enhances toughness and contributes to improvement in sag resistance, and Al reduces the yield strength by reducing the crystal grain size. B has the effect of increasing the grain boundary strength by improving the hardenability, while Co and W increase the strength and hardness after quenching and tempering, and B makes the rust generated on the surface denser and has corrosion resistance. To increase
W forms tungstate ions at the time of corrosion dissolution and contributes to improvement of corrosion resistance. The action of these elements is Al:
About 0.005% or more, B: about 1 ppm or more, Co:
It is effective when added at about 0.01% or more and W: about 0.01% or more. However, when Al exceeds 1.0%, the amount of oxide-based inclusions increases and the size thereof is coarse. And adversely affect fatigue properties, and the above effect of adding B and Co saturates at about 50 ppm and 5.0%.
Further addition is economically wasteful, and W content is 1.
If it exceeds 0%, the material toughness is adversely affected. From these viewpoints, a more preferable content of the above element is A
l: 0.01 to 0.5%, B: 5 to 30 ppm, Co:
0.5-3.0%, W: 0.1-0.5%.

One or more of Ca, La, Ce, and Rem These elements are all elements that contribute to the improvement of corrosion resistance, and Ca further acts as a strongly deoxidizing element to form oxides in steel. It refines system inclusions and contributes to improvement of toughness. The reason why the corrosion resistance is enhanced by these elements is considered as follows. In other words, as the corrosion of steel progresses, a reaction of Fe → Fe ²⁺ + 2e ⁻ Fe ²⁺ + 2H ₂ O → Fe (OH) ₂ + 2H ⁺ occurs in the corrosion pit which is the starting point of corrosion fatigue, As the inside of the pit acidifies,
Cl ^- ions collect from the outside to maintain electrical neutrality, and the liquidity inside the corrosion pit becomes severe, thereby promoting the growth of the corrosion pit. However, an appropriate amount of Ca, La,
When Ce and Rem are present, they dissolve together with iron, but since these elements are basic elements, the liquidity is also basified. As a result, the liquid inside the corrosion pit is neutralized and the pit after the pit is neutralized. It is considered that the growth was significantly suppressed. These effects are attained by 0.1 ppm or more of Ca and L
In a, Ce, and Rem, 0.001% or more, and more certainly 0.005% or more, respectively, can be effectively used, but when the Ca content is 200 ppm or more, damage to the furnace wall refractories during steelmaking is caused. And La, C
Since the effects of e and Rem saturate at about 0.1% each, further addition is economically useless.

When the spring steel having the above-mentioned composition is processed into a spring such as a suspension spring, the cast slab is hot-rolled and processed into a linear shape, followed by quenching and tempering or oil tempering. After being refined to a strand hardness (tensile strength) of the steel, it is processed into a spring shape.
0 μm or less (more preferably 15 μm or less) and hardness H
RC50 or more (more preferably 52 or more), fracture toughness value K _IC is 40 MPa√m or more (more preferably 50 MPa
以上 m or more).

Thus, the prior austenite grain size was 2
When the thickness is less than 0 μm, the carbon / nitride / sulfide generated at the fine crystal grain boundaries is also extremely fine, and functions as a diffusible hydrogen trap with almost no adverse effect on toughness and fatigue characteristics. It is because it becomes possible to exhibit the effect effectively. In order to obtain such a crystal grain size, the austenitizing heat treatment conditions may be appropriately adjusted.

In order to ensure satisfactory durability and sag resistance as a high-strength suspension spring, the strand hardness after quenching and tempering is also important, so that the suspension spring has satisfactory durability and durability. In order to secure the rust resistance, the wire hardness after quenching and tempering is HRC 50 or more, and the fracture toughness value is 40 MPa√m.
It is preferable to secure the above, and if the HRC is less than 50, the durability and the sag resistance tend to be insufficient, and the fracture toughness value is 40 M
If it is less than Pa√m, satisfactory hydrogen embrittlement resistance is hardly exhibited due to insufficient toughness. The more preferable hardness is HRC5 in consideration of durability, sag resistance, hydrogen embrittlement resistance and the like.
2 or more, and the fracture toughness value is 50 MPa√m or more.

[0033]

EXAMPLES Next, examples of the present invention will be described. However, the present invention is not limited by the following examples, and the present invention can be practiced with appropriate modifications within a range that can conform to the spirit of the preceding and following examples. Of course, it is possible, and all of them are included in the technical scope of the present invention.

Example 1 A steel material having a chemical composition of Nos. 1 to 19 shown in Table 1 was melted, cast by an ingot casting method or a continuous casting method, and then a 155 mm square billet was produced by slab rolling. Further, it was processed into a wire rod having a diameter of 14 mm by hot rolling. Each wire is drawn out to a diameter of 12.5 mm, then quenched and tempered, and a plate-like test piece is produced by machining, followed by a chloride bath (2.2 mol ZnCl ₂ : 5.5 mol).
An anticorrosion film functioning as a sacrificial anode was formed by electrogalvanizing (plating thickness: 10 μm) using NH ₄ Cl (based on NH ₄ Cl), and the test piece was subjected to a test. The tensile strength was adjusted by the tempering temperature.

The “plating brittleness” in the same table means that immediately after the electrogalvanizing (thickness: 10 μm), the tensile strength was 60%.
% While applying a stress of%, and those with cracks are marked with "●" in the same column, while those without cracks are marked with "○" in the same column. ing.

In addition, while applying a constant load of 80% of the tensile strength only to the test pieces which did not crack, a salt spray treatment (35 ° C., 5% NaCl) for 8 hours and constant temperature and humidity for 16 hours A composite cycle test that repeats the treatment (35 ° C, 60% relative humidity) is performed, and the test is stopped when red rust is generated on the surface of the galvanized (anticorrosion film), and cracks occur during the test Are given in the column of “Delayed fracture”, while those that did not crack are marked with a “○” in the same column. Table 1 shows Comparative Examples (Nos. 1 to 6) of the present invention and Examples of the present invention (Nos. 7 to 1).
9) shows the steel material composition and the performance test results for environmental embrittlement (plating embrittlement and delayed fracture).

[0037]

[Table 1]

The following can be considered from Table 1.
As apparent from the comparative examples (Nos. 1 to 6), the tensile strength was 1
When a spring steel base material exceeding 800 MPa is coated with an anticorrosion film (electrogalvanized film in this embodiment) functioning as a sacrificial anode, plating embrittlement or delayed fracture occurs, and the environmental embrittlement of the spring is reduced. In the present invention examples (Nos. 7 to 19) in which the spring steel base material contains a carbonitride forming element as specified in the present invention, even when the anticorrosion film is coated, either of the plating embrittlement or the delayed fracture occurs. Also has not occurred. Therefore, it can be seen from the table that a spring having high environmental brittleness resistance can be obtained according to the present invention.

Example 2 From Example 2 below, the characteristics of the spring steel base material itself can be improved by adjusting the steel composition of the spring steel base material as described above, and the characteristics of the spring can be improved. You can see that you can do it.

After smelting steel materials having the chemical components of Nos. 21 to 53 shown in Tables 2 and 3, they are cast by an ingot casting method or a continuous casting method, and thereafter, 155 mm square billets are produced by slab rolling. It was processed into a wire rod having a diameter of 14 mm by hot rolling. Each wire was drawn out to a diameter of 12.5 mm, and then quenched and tempered, and a fracture toughness test piece, a hydrogen embrittlement test piece, a rotary bending corrosion fatigue test piece and a rotary bending fatigue test piece were produced by machining. The tempering condition is that the hardness is HR in the range of 350 to 450 ° C. × 1 hour.
C53-55 was adjusted.

The fracture toughness test piece is a CT test piece having a length of about 3
mm with a pre-fatigue crack of 10 mm
The test was performed at room temperature in the air using an autograph tensile tester. The corrosion fatigue test was conducted by dropping a 5% aqueous solution of NaCl at 35 ° C. onto the test pieces, and all the test pieces were subjected to a shot peening treatment under the same conditions, and a stress of 784 M
The test was performed at Pa and a rotation speed of 100 rpm. Hydrogen embrittlement cracking test is 0.5mol /
1-H ₂ SO ₄ and 0.01mol / 1-KSCN (potassium thiocyanate) was immersed specimen in the mixed solution was performed by applying a voltage of -700 mV vs SCE using potentiostat. The stress is a bending stress of 1400MP
a. In the rotating bending fatigue test, the test pieces were all subjected to shot peening under the same conditions, and the stress was 881MP.
a, the number of test pieces was 10 each, and the test was stopped at 1.0 × 10 ⁷ times.

EPMA was used to measure the size and number of carbon, nitrogen and sulfide of Ti, Nb and V. That is, the area to be inspected (long side / short side =
5. The long side is in contact with the part 0.3 mm deep from the surface layer) 2
All the inclusions were picked up by automatic operation so as to cover 0 mm ^2, and the size and composition of the inclusions having an average particle diameter of 3 μm or more were analyzed. For precipitates having an average particle size of less than 3 μm, the test piece after the hydrogen embrittlement test was used and EP
Using a MA and an Auger electron spectrometer, a total of 20 fields of each steel type were observed to identify the composition of the precipitate,
Photographed and measuring the size and number by (1,000 to 20,000 times), for the number inspection area 20 mm ²
It was obtained by conversion. Table 2 shows the steel composition of the present invention, Table 3 shows the steel composition of the comparative example, and Tables 4 and 5 show the performance test results.

[0043]

[Table 2]

[0044]

[Table 3]

[0045]

[Table 4]

[0046]

[Table 5]

From Tables 2 to 5, it can be considered as follows. No. satisfying all the requirements of the present invention. 21-41
In Examples of the present invention, good results were obtained in hydrogen embrittlement resistance, corrosion fatigue life, and fatigue properties. In particular, regarding hydrogen embrittlement resistance, No. 42, 43
It can be seen that the example is much better than the comparative example.

Among the examples, those containing an appropriate amount of V showed better results in all of hydrogen embrittlement resistance, corrosion fatigue life, and fatigue properties as compared with other examples not containing V. Further, steel types (Nos. 23 to 41) having a C content within the optimum range of 0.30 to 0.50% have a high fracture toughness value and a long hydrogen embrittlement crack life. Even if the main contained element satisfies the specified requirements, the elemental element P or S, or Zn, Sn, As, S
In the comparative example (No. 46) in which the content of b and the like is large and the size and the number of coarse inclusions deviate from the suitable requirements, the effect of improving the hydrogen embrittlement cracking life is hardly exhibited.

In the case where Ni, Cr and Mo are appropriately contained as in Nos. 22 to 26 in consideration of corrosion durability, No. It can be seen that the corrosion fatigue life is significantly longer than that of Example 21 (however, a small amount of Cr is contained). Furthermore, in order to increase strength and toughness, a proper amount of Al,
Steel type to which B, Co, and W are positively added (Nos. 27 to 30)
For both hydrogen embrittlement resistance and corrosion fatigue life,
o. It shows excellent characteristics on the same level as 22 to 26. Suitable amounts of Ca, La, Ce, for the purpose of improving corrosion resistance
The steel types (Nos. 31 to 34) to which Rem is added clearly show an improvement in the corrosion fatigue life as compared with the comparative steel types (Nos. 42 to 53).

Looking at the influence of the size and the number of the precipitates, it can be seen that those satisfying the preferred requirements of the present invention have no inclusion breakage by the fatigue test and do not adversely affect the fatigue properties. On the other hand, No. Nos. 44 and 45 are comparative examples in which a large amount of coarse inclusions were generated by slowing down the cooling rate during solidification, and the probability of breakage due to the coarse inclusions increased, and the fatigue life was extremely reduced. .

Regarding C, Si and Mn, which are the main elements in the steel, the strength after quenching and tempering is slightly lower when the amount of C is slightly insufficient, and when the amount is too large (No. 47), the fracture toughness is higher. It can be seen that the hydrogen embrittlement crack life tends to decrease as the value decreases. When the amount of Si is slightly insufficient, the hardness is slightly insufficient. In No. 48, the toughness is slightly low. In addition, if the Mn content tends to be insufficient, it tends to be difficult to burn and the hardness tends to be insufficient. Furthermore, when the amounts of Mn, Cr, and Mo are too large (Nos. 49 to 51), the tendency of insufficient hardness due to the presence of a large amount of retained austenite appears.
Comparative examples in which the N and S contents are outside the specified requirements (No.
52, 53), it can be seen that the number of coarse inclusions composed of Ti, Nb charcoal, nitrogen, and sulfide increases, and that the fatigue characteristics and the like are significantly deteriorated.

[0052]

As described above, according to the present invention, at least a part of the surface of the spring steel base material is covered with the anticorrosion film functioning as a sacrificial anode to perform cathodic protection, and the spring steel base material Since the carbonitride is finely dispersed in the spring steel base material by adding the carbonitride forming element, hydrogen generated by forming the anticorrosion film can be trapped, thereby causing corrosion fatigue and delayed fracture. Such as suppression,
Environmental brittleness resistance can be improved.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Nobuhiko Ibaraki 2 Nadahama-Higashi-cho, Nada-ku, Kobe Inside Kobe Steel, Ltd.Kobe Steel Works (72) Inventor Hiroshi Ieguchi 1-5-5 Takatsukadai, Nishi-ku, Kobe Stock Company Kobe Steel, Ltd.Kobe Research Institute (72) Inventor Shigenobu Namba 1-5-5 Takatsukadai, Nishi-ku, Kobe City, Kobe Steel Works Kobe Research Institute

Claims (5)

[Claims] 1. A spring in which at least a part of a surface of a spring steel base material is coated with an anticorrosion film functioning as a sacrificial anode, wherein a carbonitride forming element is added to the spring steel base material, and High strength spring with good environmental embrittlement, characterized in that carbonitrides are finely dispersed in the spring. 2. The spring steel base material comprises: Ti: 0.001 to 0.5% (meaning mass%, the same applies hereinafter) Nb: 0.001 to 0.5% as the carbonitride forming element The high-strength spring with good environmental embrittlement according to claim 1, containing at least one element of Ta: 0.001 to 0.5% and N: 0.001 to 0.02%. 3. The high-strength spring according to claim 1, wherein the anticorrosion film is made of a metal or an alloy thereof that is electrochemically lower than the spring steel base material. 4. The environmental embrittlement according to claim 1, wherein the anticorrosion film is formed by dispersing a large number of metals or alloys which are electrochemically lower than the spring steel base material in a nonmetal film. Good high strength spring. 5. The method according to claim 1, wherein the spring steel base material has C: 0.7% or less,
Si: 0.1 to 4.0% and Mn: 0.01 to 2.0
The high-strength spring having good environmental embrittlement according to any one of claims 1 to 4, wherein the high-strength spring is a spring steel containing 0.1% by weight.