JPH10110247A - Spring steel excellent in hydrogen embrittlement resistance and fatigue characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an infiltrated-hydrogen-capturing effect in a corrosive environment and to improve corrosion fatigue characteristic by adding carbide-, nitride-, and sulfide-forming elements to a spring steel and dispersing their precipitates finely into the material. SOLUTION: At least one kind selected from the group consisting of, by mass, <=0.5% Ti, <=0.5% Nb, <=0.5%, Zr, <=0.5% Ta, <=0.5% Hf, and <=3.0% Mo is added to a spring steel containing 0.3-<0.7%C, 0.1-4.0% Si, and 0.005-2.0% Mn. At this time, in the case where one or more elements among Ti, Nb, Zr, Ta, and Hf are added, the sum of their contents is regulated to >=0.001%, and, in the case of single addition of Mo alone, Mo is incorporated by 0.05%, and further, 1-200ppm N and 300ppm S are incorporated. Simultaneously, precipitates of <5μm average grain size, consisting of the carbide, nitride, and sulfide of the added alloying elements or a multiple compound thereof, are finely dispersed in the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to spring steel useful as a material for valve springs, suspension springs, stabilizers, torsion bars and the like of internal combustion engines such as automobiles. The present invention relates to spring steel that provides a spring having fatigue characteristics.

[0002]

2. Description of the Related Art The chemical composition of spring steel is JIS G356.
5-G 3567, G 4801, etc.
Hot-rolled wire (hereinafter referred to as “rolled material”) produced from them is drawn, then heat-formed into a spring shape and then quenched and tempered (hot-spring forming), or drawn to a predetermined wire diameter. Various types of springs are manufactured by a method of processing and oil-tempering and then performing spring processing (cold spring forming). In recent years, the characteristics required for springs have become even more severe, and under such circumstances, those obtained by subjecting various alloy steels to heat treatment are often used.

On the other hand, in springs used for automobiles and the like, the stress of the springs has been increased as part of measures to reduce the weight of the springs to reduce exhaust gas and fuel consumption. , 800 MPa or more, high strength spring steel is demanded. However,
In general, as the strength of the spring increases, the susceptibility to defects tends to increase. Particularly, in a spring used in a corrosive environment, the corrosion fatigue life is deteriorated, and there is a concern that the spring may be damaged early. One of the causes of shortening the corrosion fatigue life is hydrogen embrittlement due to hydrogen generated as the corrosion reaction progresses, and as a remedy, increasing the stress by adding a large amount of various alloying elements. This method has been adopted, but this method has an economic problem that the cost of the steel material increases.

[0004] Further, as a method of suppressing hydrogen embrittlement, a method of making crystal grains fine and a method of forming fine precipitates are considered to be effective.
A method of adding a nitride-forming element has been adopted. In spring steels, it has been considered that the above-mentioned method of adding a carbon / nitride forming element is effective in expecting an effect of improving the toughness by refining the crystal grains. Giant carbon / nitride-based inclusions are generated by the addition of the material-forming element, and there is a possibility that fatigue properties, which are important spring properties, are deteriorated.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and its object is to enhance hydrogen embrittlement resistance while increasing strength and stress, It is another object of the present invention to provide a linear, rod-shaped or plate-shaped spring steel which provides a spring (including a valve spring, a suspension spring, a plate spring, etc.) having improved fatigue characteristics.

[0006]

The spring steel according to the present invention which can solve the above-mentioned problems has a Ti: 0.5% or less,
Nb: 0.5% or less, Zr: 0.5% or less, Ta: 0.
At least one element selected from the group consisting of 5% or less, Hf: 0.5% or less, and Mo: 3.0% or less,
When one or more of i, Nb, Zr, Ta and Hf are contained, the total content thereof is 0.001% or more. When only Mo is contained alone, the content is 0.05% or more. 200 ppm S: contains 5 to 300 ppm, and Ti, Nb, Zr, Ta, H
f, Mo, an average particle size of at least one element selected from the group consisting of carbides, nitrides, sulfides, and composite compounds thereof (hereinafter, referred to as carbon, nitrogen, and sulfide).
It is characterized in that precipitates of less than μm are finely dispersed. Surface to be tested: A cross section having a depth of 20 mm ² and a depth of 0.3 mm or more from the surface of the steel and set in an arbitrary direction so as not to include the central portion.

[0007] Further, in the test surface, the Ti,
Precipitates of at least one element selected from the group consisting of Nb, Zr, Ta, Hf, and Mo, which are composed of carbon, nitrogen, and sulfide, and have an average particle diameter of 5 μm or more adversely affect fatigue characteristics. Satisfactory restriction is preferred, which results in a spring steel with even better hydrogen embrittlement resistance and fatigue properties. Size and number of precipitates: 500 or less having an average particle diameter of 5 to 10 μm, 50 or less having an average particle diameter of more than 10 μm and 20 μm or less, and 10 or less having an average particle diameter of more than 20 μm.

If the spring steel of the present invention further contains 1.0% or less of V as another element, V also acts as a carbon / nitride / sulfide forming element, so that Ti, Nb, Zr,
Fine precipitates composed of carbon, nitrogen and sulfide of at least one element selected from the group consisting of Ta, Hf, Mo and V;
Alternatively, if the above requirements are satisfied for coarse precipitates, the characteristics as spring steel can be further enhanced.

Further, in the present invention, in order to further enhance the spring characteristics such as toughness, durability and set resistance, the austenite grain size after quenching and tempering is 20 μm or less, the hardness is HRC 50 or more, and the fracture toughness value ( K _IC ) is 40MPa√
m or more is preferable.

[0010] The spring steel of the present invention has a basic feature in that the type, size and number of carbon, nitrogen and sulfide are specified as described above, and other elements are not particularly limited. Preferred elements to be contained or elements to be excluded are as follows. The reason for determining the preferable contents of the following elements will be described later in detail.

(1) Ni: 3.0% or less (preferably 0.05 to 3.0%), Cr: 5.0% or less (preferably 0.05 to 5.0%), and Cu: 1.0% or less. At least one element selected from the group consisting of 0% or less (preferably 0.01 to 1.0%);

(2) Al: 1.0% or less (preferably 0.005 to 1.0%), B: 50ppm or less (preferably 1 to 50ppm), Co: 5.0% or less (preferably 0.1% or less). 01 to 5.0%) and W: at least one element selected from the group consisting of 1.0% or less (preferably 0.01 to 1.0%).

(3) Ca: 200 ppm or less (preferably 0.1 to 200 ppm), La: 0.5% or less (preferably 0.001 to 0.5%), Ce: 0.5% or less (preferably 0.001-0.5%) and Rem:
At least one element selected from the group consisting of 0.5% or less (preferably 0.001 to 0.5%);

(4) The preferred basic component of steel is C: 0.
3 to less than 0.7%, Si: 0.1 to 4.0% and M
n: 0.005 to 2.0%, the balance being Fe and unavoidable impurities.

(5) The inevitable impurities in the steel are P: 0.0
2% or less, and Zn contained as another impurity is 60%.
ppm or less, Sn is 60 ppm or less, As is 60 ppm
Hereinafter, Sb is preferably 60 ppm or less, and a steel satisfying the requirements of the following formula (I) exhibits more excellent performance as a spring steel. 2.5 ≦ (FP) ≦ 4.5 (I) where FP = (0.23 [C] +0.1) × (0.7 [Si] +1) × (3.5 [Mn]
+1) × (2.2 [Cr] +1) × (0.4 [Ni] +1) × (3 [Mo] +1) (However, [element] represents mass% of each element)

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In spring steel, a method of mainly refining crystal grains has conventionally been employed in order to suppress a decrease in toughness accompanying an increase in strength. From such a viewpoint, various methods have been proposed for adding a carbon / nitride forming element to steel to refine crystal grains and increase toughness.

However, in the field of spring steel, there is no idea to regulate the size of carbon / nitride from the viewpoint of improving hydrogen embrittlement. However, as described above or as described in detail below, an appropriate amount of Ti, Nb, Zr, T
If at least one element selected from the group consisting of a, Hf, and Mo is contained, and precipitates composed of these carbon, nitrogen, and sulfides are finely dispersed, the hydrogen embrittlement resistance of spring steel is dramatically improved. I knew it could be enhanced.

The reason is considered as follows. That is, the hydrogen embrittlement of spring steel is considered to be due to the fact that hydrogen penetrating into the steel diffuses and transfers to the former austenite grain boundary, thereby weakening the bond energy of the grain boundary and causing brittle fracture at that part, and includes the above elements. It is thought that precipitates composed of carbon, nitrogen, and sulfide trap hydrogen that has penetrated into the steel material, thereby suppressing hydrogen embrittlement. There is a concern that the product may be coarsened and the coarse precipitate may cause early deficiency.

As a technique for modifying spring steel focusing on oxide-based coarse precipitates, an average particle diameter of 30 μm existing near the surface is used.
Based on the knowledge that cracks occur starting from precipitates of about m or more, a method was proposed to improve toughness by controlling the composition of oxide-based precipitates in valve spring steel and increasing the ductility of oxide-based precipitates. Have been. However, as the detoxification technology using oxide-based precipitates has progressed, the problem of early breakage, particularly due to Ti-based nitride-based precipitates, has been pointed out. Is underway. However, in order to further increase the stress and strength of a spring steel used in a corrosive environment such as a suspension spring, the above measures for detoxifying oxide-based precipitates alone are not enough, and Improvement of hydrogen embrittlement and corrosion resistance is an essential requirement.

The most effective method for improving the corrosion resistance is to add a large amount of alloying elements. However, in addition to the disadvantage that the material cost is increased, it is necessary to modify the manufacturing process such as annealing and the like, and to improve the economical efficiency. There are disadvantages. However, by adding a small amount of at least one selected from the group consisting of Ti, Nb, Zr, Ta, Hf, and Mo to the spring steel as described above, the average of those elements consisting of carbon, nitrogen and sulfide is obtained. If fine precipitates having a particle diameter of less than 5 μm are generated in a dispersed state, the effect of trapping diffusible hydrogen is exhibited, and the hydrogen embrittlement resistance is enhanced.

On the other hand, if the amount of coarse precipitates increases due to the addition of these elements, there is a possibility that the coarse precipitates may lead to fatigue fracture and deterioration of toughness starting from fatigue.
Therefore, while exerting the effect of improving the hydrogen embrittlement resistance by the addition of the above elements, further research was conducted to prevent the deterioration of the fatigue properties caused by the coarse precipitates becoming the starting point of fatigue. If the cooling rate is well controlled during the solidification process of carbon and the size and number of the above-mentioned elements, carbon, nitrogen, and sulfide, they become the starting point of fatigue and do not cause deterioration of the fatigue characteristics or toughness. I knew that it could be dramatically increased.

Hereinafter, the reasons for limitation related to precipitates defined in the present invention will be described in detail. In the present invention, for improving hydrogen embrittlement resistance, fine precipitates composed of carbon, nitrogen, and sulfide of at least one element selected from the group consisting of Ti, Nb, Zr, Ta, Hf, and Mo are converted into diffusible hydrogen. Finely dispersed for trapping, such diffusible hydrogen trapping effect is effectively exerted by fine precipitates having an average particle size of less than 5 μm, and even if the above-mentioned carbon, nitrogen and sulfide, they exceed 5 μm When the precipitate is a coarse precipitate having an average particle diameter, the effect of improving hydrogen embrittlement resistance as intended in the present invention is not exhibited. That is, the precipitate having an ultrafine size having an average particle diameter of 10 nm to 5 μm effectively acts on the improvement of the hydrogen embrittlement resistance without adversely affecting the fatigue properties, and significantly enhances the overall properties of the spring steel. .

This is because diffusible hydrogen is trapped in a finely dispersed state in spring steel by the fine precipitates, and delayed fracture caused by diffusible hydrogen is prevented. It is thought that diffusible hydrogen is intensively trapped in the coarse precipitates, and rather promotes delayed fracture. In any case, in order to effectively exhibit the hydrogen embrittlement resistance improvement effect of the fine precipitates,
The carbon / nitride / sulfide of the metal element must be extremely fine with an average particle diameter of less than 5 μm. With an average particle diameter of more than 5 μm, the effect of improving hydrogen embrittlement cannot be effectively exhibited. In addition, as described later, the fatigue characteristics starting from the precipitates are deteriorated, and the performance as spring steel is deteriorated.

Here, the finer precipitates having an average particle diameter of less than 5 μm, which are composed of carbon, nitrogen and sulfide, which contribute to the improvement of hydrogen embrittlement resistance, the effect becomes more effective as the size and the number of the fine precipitates are smaller. At present, it has been confirmed that the number on the test surface as described later is 1,0.
When it is present in a dispersed state of at least 00, preferably at least 3,000, more preferably at least 5,000, and still more preferably at least 10,000, the effect of improving hydrogen embrittlement resistance by the diffusible hydrogen trapping effect can be obtained. It has been found to be effective. Moreover, such fine precipitates do not act as starting points for fatigue and the like, and do not adversely affect the fatigue characteristics. The average particle diameter of the precipitate is (major axis + minor axis) /
2 means the ratio of major axis / minor axis of the precipitate is 3.0 or less.

However, the carbon / nitride / sulfide present in the test surface having a depth of at least 0.3 mm from the surface in the cross section of the spring steel and having a width of 20 mm ² set from a region not including the center portion. However, when the size is increased, not only does the effect of improving hydrogen embrittlement resistance be adversely affected, but also the starting point of fatigue has a noticeable adverse effect on the fatigue characteristics of the spring steel. Therefore, in order to clarify the quantitative standard, the size and the number of the coarse precipitates were examined. As a result, the coarse precipitates having an average particle diameter of 5 μm or more composed of the carbon, nitrogen, and sulfide in the test surface were determined. However, it is confirmed that if the cooling conditions during casting and the like are properly controlled so as to satisfy the following requirements, the adverse effects on hydrogen embrittlement resistance and fatigue properties due to the coarse precipitates can be suppressed to a practically negligible level. Was. Size and number of coarse precipitates: average particle diameter 5 to 10 μm
m is 500 or less, average particle diameter is more than 10 μm and 20 μm
m or less, and 10 or less having an average particle diameter of more than 20 μm.

Therefore, according to the present invention, it is necessary to control the size and the number of the carbon, nitrogen and sulfide exceeding 5 μm so as to satisfy the above requirements. The charcoal, nitrogen and sulfide are 1,400
It precipitates at a high temperature of about 1,500 ° C., and tends to grow gradually and become coarse in the subsequent cooling process. Therefore, in order to suppress the amount of coarse precipitates as described above, the cooling rate during casting must be reduced. The temperature may be increased to 0.1 ° C./sec or more, more preferably about 0.5 ° C./sec or more, and the generation of coarse precipitates may be suppressed as much as possible.

Thus, according to the present invention, fine precipitates having an average particle diameter of less than 5 μm, consisting of the above-mentioned carbon, nitrogen and sulfide, are innumerably contained in steel, specifically 1,000 or more, preferably 3,000 or more, more preferably 5,000 or more, and still more preferably 10,000 or more are precipitated in a finely dispersed state, thereby effectively exhibiting a diffusible hydrogen trapping effect and significantly increasing hydrogen embrittlement resistance. It becomes possible. Further, the coarse precipitates having an average particle diameter of 5 μm or more composed of the above-mentioned carbon, nitrogen, and sulfide not only do not exhibit the effect of improving hydrogen embrittlement resistance by the diffusible hydrogen trap, but also exhibit fatigue fracture starting from the coarse precipitates. As a result, precipitates having an average particle diameter of 5 to 10 μm are removed from the precipitates as described above.
00 or less (more preferably 300 or less), 50 or less (more preferably 30 or less) precipitates having the same average particle diameter of more than 10 μm and 20 μm or less, and the same average particle diameter of 20 or less.
By suppressing the number of precipitates exceeding μm to 10 or less (more preferably 5 or less, and still more preferably substantially 0), a spring steel having both excellent hydrogen embrittlement resistance and fatigue characteristics can be obtained.

Next, the reasons for determining the chemical components of the steel used in the present invention will be explained. In the steel used in the present invention, Ti: 0.5% or less (preferably 0.001 to 0. 0%) is used as a metal element for generating the fine carbon, nitrogen and sulfide described above.
5%), Nb: 0.5% or less (preferably 0.001 to
0.5%), Zr: 0.5% or less (preferably 0.00%)
1 to 0.5%), Ta: 0.5% or less (preferably 0.1%).
001 to 0.5%), Hf: 0.5% or less (preferably 0.001 to 0.5%), Mo: 3.0% or less (preferably 0.05 to 3.0%) And the N content is from 1 to 200.
It is essential to control the contents of ppm and S in the range of 10 to 300 ppm.

The elements selected from the group consisting of Ti, Nb, Zr, Ta, Hf and Mo are all elements forming carbon, nitrogen and sulfide, and fine carbon and carbon are formed in the crystal grains and in the grain boundaries in the spring steel. Nitrogen / sulfide precipitates and is an indispensable component for trapping diffusible hydrogen that causes hydrogen embrittlement and enhancing hydrogen embrittlement resistance. It also has the effect of increasing the fineness of the grains, increasing the toughness and increasing the sag resistance of the spring. In order to exhibit these effects effectively, at least one of the above six elements must be contained in an appropriate amount.

Of these elements, Ti, Nb, Zr, T
a and Hf are each 0.001% or more and 0.001% or more in total, and more preferably 0.005%.
By containing the above, the above effects can be significantly exerted. However, if their contents become too large, the carbon, nitrogen, and sulfide coarsen and the number thereof increase, and the adverse effect on the fatigue properties appears remarkably. Preferably 0.
Should be kept below 2%.

Mo forms carbon, nitrogen, and sulfide to increase hydrogen embrittlement resistance and fatigue properties, and also contributes to improvement of hydrogen embrittlement resistance and fatigue properties by increasing grain boundary strength.
Further, the presence of Mo also exerts an effect of increasing the corrosion resistance by absorbing molybdate ions generated during corrosion dissolution. Mo is desirably added in a slightly larger amount than the above-mentioned elements because it has a high solid solubility in steel and it is difficult for carbides to coarsen.
The content is preferably about 5% or more, more preferably 0.1% or more. Therefore, when Mo is contained alone, 0.0
It is effective to contain 5% or more. However, when the Mo content is too large, not only the above-mentioned effects and effects are saturated, but also the coarsening and increase of the number of carbon, nitrogen and sulfide are caused, so that the content is 3.0% or less, preferably 2.0% or less. Should be kept to a minimum.

N and S are the above six elements and carbon, nitrogen,
In order to form a sulfide and effectively exhibit a diffusible hydrogen trap formation and crystal grain refinement effect, at least N is 1 p
pm or more, preferably 5 ppm or more, more preferably 1 ppm or more.
0 ppm or more, S is 5 ppm or more, preferably 10 pp
m or more. However, if the amount is too large, the size and number of the carbon / nitride / sulfide-based precipitates increase, adversely affecting the fatigue properties.
00 ppm or less, preferably 100 ppm or less, more preferably 70 ppm or less, and S is 300 ppm or less, preferably 200 ppm or less, more preferably 15 ppm or less.
It should be kept below 0 ppm.

There are no particular restrictions on other elements contained in the steel used in the present invention. However, other preferable elements are also described from the viewpoint of securing general required characteristics as spring steel or enhancing its performance. Keep it.

First, in the present invention, Ti, Nb, Zr, T
charcoal other than the element selected from the group consisting of a, Hf, Mo
It is effective to contain V as a nitrogen / sulfide forming element in an amount of about 0.005% or more, more preferably 0.01 or more. That is, an appropriate amount of V not only functions to form fine precipitates composed of carbon, nitrogen, and sulfide to further enhance the hydrogen embrittlement resistance and fatigue characteristics, but also exerts the effect of refining the crystal grains and toughness and proof stress. And also contributes to the improvement of 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.
It should be kept below, more preferably below 0.5%.

In the case of a steel material containing V, Ti, N
It is necessary that the fine and coarse precipitates of b, Zr, Ta, Hf, Mo, and V as carbon, nitride and sulfide as a whole satisfy the size and number described above.

The basic components of the spring steel according to the present invention are the following three elements, C, Si, and Mn, and the balance is substantially composed of Fe. The preferable contents thereof are as follows.

C: 0.3% or more and less than 0.7% C is an element that is essentially contained in steel and contributes to improvement in strength (hardness) after quenching and tempering. When the C content is 0.3% or less, the strength (hardness) after quenching and tempering becomes insufficient. On the other hand, when the C content is 0.7% or more, not only the toughness and ductility after quenching and tempering deteriorates, but also the corrosion resistance becomes poor. Adverse effects will appear. Considering the strength and toughness of the spring steel, a more preferable C content is 0.3 to 0.55%. In order to more reliably improve hydrogen embrittlement resistance and corrosion fatigue properties, the C content is preferably 0.3 to 0.55%.
The range of 0 to 0.50% is preferable.

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 viewpoints of strength and hardness as a spring material and suppression of decarburization, the more preferable range of Si is 1.0 to 3.0%.
Range.

Mn: 0.005 to 2.0% Mn has a different effect depending on whether the amount of Mn is 0.005% or more and less than 0.05% or between 0.05% and 2.0%. There is expected. First, the lower limit of the amount of Mn is determined in consideration of the scouring efficiency when performing on a practical scale,
To reduce the amount of Mn to less than 0.005% requires a long time of refining, which significantly increases the cost. Therefore, the lower limit is specified as above for practical reasons.

Next, the amount of Mn is set to 0.005% to 0.05%.
The case where it is specified to be less than the range is a case where other hardenability improving elements (for example, Cr, Ni, Mo, etc.) are sufficiently contained in the steel (about 0.5% or more), and more than that. If the hardenability increases, difficulties such as formation of a supercooled structure appear. In such a case, the Mn content is reduced to 0.1.
When the content is suppressed to less than 05%, a hard structure is not easily formed, and cold working such as wire drawing is easily performed, which is preferable. In addition, formation of coarse MnS which is likely to be a starting point of fracture is also suppressed. On the other hand, the Mn content is 0.05% or more and 2.0%
The case where the content is specified in the following range is a case where the hardenability improving element amount in the steel is small (approximately 0.5% or less), and Mn of 0.05% or more must be contained in order to positively enhance the hardenability. Is preferred. However, if the amount of Mn is too large, the quenching property is excessively improved and a supercooled structure is easily generated, so the upper limit is 2.0%. In this case, there is a possibility that MnS serving as a starting point of destruction may be formed. Therefore, by reducing the amount of S or combining with other sulfide-forming elements (Ti, Zr, etc.), MnS should not be generated as much as possible. Is desirable. It is also effective to include at least one of Cr, Ni, Mo, V, and Cu in this spring steel for the purpose of improving corrosion resistance and the like for the following reasons.

Cr: 5.0% or less (preferably 0.05 to 5.0%) Cr makes amorphous and dense rust generated on the surface layer under corrosive conditions and contributes to improvement of corrosion resistance. In addition to Mn, it effectively acts to improve the hardenability similarly to Mn. Such an effect is effectively exhibited by adding 0.05% or more, but 5.0% or more.
If it is added excessively in excess of the above, carbides are less likely to dissolve during quenching, adversely affecting strength and hardness. More preferable content of Cr is 0.1 to 2.0%.
Range.

Ni: 3.0% or less (preferably 0.05 to 3.0%) Ni enhances the toughness of the material after quenching and tempering, and
It has the effect of increasing corrosion resistance by making the generated rust amorphous and dense, and also has the effect of improving the sag characteristics, which are important as spring characteristics. Such an effect is effectively exhibited by adding 0.05% or more, but is preferably 0.1% or more. However, when the content exceeds 3.0%, the hardenability is excessively increased, and a supercooled structure is easily generated after rolling. The more preferable range of Ni is 0.1 to 1.0%.
Range.

Cu: 1.0% or less (preferably 0.01 to 1.0%) Cu is an element that is electrochemically nobler than iron and has an effect of improving corrosion resistance. Such an effect is effectively exhibited by adding 0.01% or more. However, even if it exceeds 1.0%, no further effect of improving corrosion resistance can be expected, and the material may be brittle due to hot rolling. Come up. A more preferred range for Cu is in the range of 0.1-0.5%. The following elements can be mentioned as further preferable contained elements, and the action of each additive element can be effectively exhibited.

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. 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 contributes to the improvement of corrosion resistance by forming tungstate ions during corrosion dissolution. The effect of these elements is that Al: 0.
005% or more, B: about 1 ppm or more, Co: 0.
Effectively exhibited by addition of about 01% or more and W: about 0.01% or more. However, when Al exceeds 1.0%, the amount of oxide-based precipitates increases and the size thereof becomes coarse. The addition of B and Co saturates at about 50 ppm and 5.0%, adversely affecting the fatigue properties, so that further additions are economically useless and the W content is 1.0%.
If it exceeds, the material toughness will be adversely affected. From these viewpoints, a more preferable content of the above element is Al:
0.01-0.5%, B: 5-30 ppm, Co: 0.
5 to 3.0%, W: 0.1 to 0.5%.

One or more of Ca, La, Ce, and Rem These are all elements that contribute to the improvement of corrosion resistance.
In addition, Ca exerts an action as a strong deoxidizing element, thereby making oxide-based precipitates in steel finer and contributing to improvement in toughness. The reason that these elements increase the corrosion resistance is that
It 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 gather 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 also becomes basic, and as a result, the liquid inside the corrosion pit is neutralized, and the starting point of corrosion fatigue and It is considered that the growth of corrosion pits is significantly suppressed. Such an effect is 0.1 p with Ca
pm or more, La, Ce, and Rem can be effectively exhibited by containing 0.001% or more, and more certainly 0.005% or more, respectively. Severely damaged,
Further, the effects of La, Ce and Rem are saturated at about 0.1% each, so that further addition is economically useless.

Further, P, which is an impurity unavoidably mixed into steel, segregates at the grain boundaries and lowers the grain boundary strength to cause grain boundary destruction. Should. In addition, Zn, Sn, As, and Sb, which are other impurities that may be mixed in the steel material, also tend to cause grain boundary segregation to increase grain boundary strength and promote hydrogen embrittlement. It is desirable to keep it below this level.

Further, in designing the components of the spring steel used in the present invention, it is desirable to adjust the components so as to satisfy the requirement of the following formula (I) in addition to the contents of the individual elements. That is, the hydrogen embrittlement of the spring steel is caused by the intrusion of diffusible hydrogen into the crystal grain boundaries as described above, and the intrusion of the diffusible hydrogen adversely affects the corrosion resistance of the steel. It has been confirmed that the corrosion resistance itself is improved by adding an appropriate amount of Cr, Ni, Mo, Cu, etc. to the steel. However, the addition of a large amount of these alloy elements increases the material cost and further increases the hardenability. It is not possible to neglect the increase in processing costs due to the addition of annealing of rolled material. However,
Regarding C, Si, Mn, Cr, Ni, and Mo in steel,
By adjusting their contents so as to satisfy the relationship of the following formula (I), it is possible to obtain a spring steel which is extremely excellent in corrosion resistance with a small amount of alloying element added and which does not require annealing treatment of a rolled material. Becomes possible. 2.5 ≦ (FP) ≦ 4.5 (I) where FP = (0.23 [C] +0.1) × (0.7 [Si] +1) × (3.5 [Mn]
+1) × (2.2 [Cr] +1) × (0.4 [Ni] +1) × (3 [Mo] +1) (where [element] represents mass% of each element).

However, if the value of FP is less than 2.5, it is difficult to obtain uniform sintering, and it is difficult to stably obtain high strength. A supercooled structure appears and the strength after rolling is 1,300.
MPa or more, so that an annealing process is indispensable when performing a subsequent drawing process, leading to an increase in the number of steps. However,
If the content of each element is adjusted so as to satisfy the relationship of the above formula (I), uniform quenching is performed during quenching and tempering, so that high strength can be stably achieved, and a supercooled structure appears in the rolled structure. Since there is no need to perform an excessive increase in the strength, the drawing process can be performed smoothly without any trouble without requiring any annealing treatment.

When a spring steel having the above component composition is processed into a suspension spring or the like, a slab is hot-rolled and processed into a linear shape and then quenched and tempered, or subjected to an oil tempering process to obtain a predetermined material. After tempering to a linear hardness (tensile strength), it is processed into a spring shape.
μm or less (more preferably 15 μm or less) and hardness is HR
C50 or more (more preferably 52 or more), fracture toughness value K
_IC is 40 MPa√m or more (more preferably, 50 MPa√m).
m or more).

Thus, the prior austenite crystal 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, etc., 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.

[0052]

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.

EXPERIMENTAL EXAMPLE 1 Steel materials having chemical components Nos. 1 to 90 shown in Tables 1 to 6 were melted, cast by an ingot casting method or a continuous casting method, and then 155 mm square billets were produced by slab rolling. Then, it was processed into a wire 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 was 350
The hardness was adjusted so that the hardness became HRC 53 to 55 in the range of ~ 450 ° C x 1 hour.

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.

Ti, Nb, Zr, Ta, Hf, M
EP for measuring the size and number of o, V charcoal, nitrogen and sulfide
MA was used. That is, the area to be inspected (long side / short side = 5, depth 0.3 mm from the surface layer) inside the depth 0.3 mm from the surface of the longitudinal section (passing the center line) of the rotating bending test piece
(The long side is in contact with the part) Automatic operation is performed to cover 20 mm ² to pick up all the precipitates, and the average particle diameter is 3 μm
The size and composition of the precipitates were analyzed. For precipitates having an average particle size of less than 3 μm, the specimens after the delayed cracking test were used, and a total of 20 visual fields of each steel type were observed using EPMA and Auger to identify the composition of the precipitates, Photographing (1,000-20,000 times)
The size and the number were measured according to the formula, and the number was calculated by converting the test area to 20 mm ² . Tables 1, 3,
Tables 5 and 6 show the steel composition of the present invention, Tables 2 and 4 show the steel composition of the comparative examples, and Tables 7 to 12 show the performance test results.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

[Table 4]

[0060]

[Table 5]

[0061]

[Table 6]

[0062]

[Table 7]

[0063]

[Table 8]

[0064]

[Table 9]

[0065]

[Table 10]

[0066]

[Table 11]

[0067]

[Table 12]

From Tables 1 to 12, the following can be considered. No. satisfying all the requirements of the present invention. 1-2
5, 43 to 70 and 76 to 90 are examples satisfying the requirements of the present invention, and good results are obtained in any of hydrogen embrittlement resistance, corrosion fatigue life, and fatigue characteristics. Particularly, regarding hydrogen embrittlement resistance, Ti, Nb, Zr, Ta, H
No. f that does not include Mo. 26, 27, and 28, it can be seen that each of the above embodiments is much better.

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. 4 to 24, 45 to 70) 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, it may be an impurity element such as P or S, or Zn or S
Comparative examples in which the content of n, As, Sb, etc. is large, and the size and number of coarse precipitates are out of the preferred requirements (No. 32,
33, 74, 75), the effect of improving the hydrogen embrittlement crack life is hardly exhibited.

In consideration of the corrosion durability, No. 3 to 8,46
In those containing an appropriate amount of Ni and Cr such as ~ 50,
Nos. It can be seen that the corrosion fatigue life is remarkably increased as compared with Examples 1, 2, 43 to 45 (however, a small amount of Cr is contained). Further, in steel types (Nos. 9 to 12, 51 to 54) in which appropriate amounts of Al, B, Co, and W are positively added to increase strength and toughness, hydrogen embrittlement resistance and
No. 1 in any of the corrosion fatigue life. It shows characteristics comparable to steel types such as 4,46. Steel types (Nos. 13 to 16, 55 to 58) to which appropriate amounts of Ca, La, Ce, and Rem are added for the purpose of improving corrosion resistance have a longer corrosion fatigue life than steel types (Nos. The improvement is clearly shown.

Looking at the effects of the size and number of the precipitates, it can be seen that those satisfying the preferred requirements of the present invention have no precipitate breakage due to the fatigue test and do not adversely affect the fatigue properties. On the other hand, No. 29-31, 71-73
Is a comparative example in which a large amount of coarse precipitates was generated by slowing the cooling rate during solidification, and the probability of breakage due to the coarse precipitates increased, and the fatigue life was extremely reduced.

Regarding C, Si, and Mn, which are the main elements in the steel, if the C content is too large (No. 34), it can be seen that there is a tendency that the fracture toughness value is lowered and the hydrogen embrittlement crack life is also deteriorated. Will be No. in which the amount of Si is slightly insufficient. 35
No. is slightly insufficient in hardness, and conversely is too much. In No. 36, the toughness was slightly lowered, and the life of hydrogen embrittlement cracking in each case was slightly insufficient. Also, if you secure a certain amount of Cr,
Highly cold workable steel can be obtained by keeping the Mn content low (Nos. 82 to 88). Mn, Ni, C
When the amount of r is too large (Nos. 38 to 40), the tendency of insufficient hardness due to the presence of a large amount of retained austenite appears. Further, in Comparative Examples (Nos. 41 and 42) in which the N and S contents deviate from the specified requirements, the number of coarse precipitates composed of carbon, nitrogen, and sulfide increases, and it can be seen that deterioration of fatigue characteristics and the like is remarkable. .

In the embodiments of the present invention, the FP values within the preferred range (Nos. 1, 3 to 5, 9, 10, 13)
~ 24, 43, 46, 47, 51, 52, 55-70)
Thus, direct drawing can be performed without the need for annealing after rolling, so that the manufacturing process can be simplified and the cost can be reduced. Ti, Nb, Zr, Ta,
In the examples (Nos. 1 to 5, 48 to 50, and the like) in which the contents of Hf, Mo, N, and S are within the more preferable ranges, more stable performance in hydrogen embrittlement resistance, corrosion durability, and fatigue characteristics is obtained. In contrast to those obtained, the examples (Nos. 17, 20, and 5) were slightly insufficient in a more preferable range.
9, 62, 65), the examples (Nos. 18, 19, 2) exhibit somewhat lower properties of hydrogen embrittlement and conversely, exhibit higher properties.
1, 22, 60, 61, 63, 64, 66, 67), the fatigue properties show somewhat lower values. However, they still have remarkably excellent hydrogen embrittlement resistance and fatigue characteristics as compared with the comparative examples.

[0074]

The present invention is constituted as described above. By adding an appropriate amount of at least one of Ti, Nb, Zr, Ta, Hf and Mo to a spring steel, the carbon, nitrogen and sulfide thereof can be obtained. To finely disperse the precipitates made of carbonaceous materials, to enhance hydrogen embrittlement by exerting a diffusible hydrogen trapping effect on the precipitates, and to regulate the size and number of the coarse precipitates made of carbon, nitrogen and sulfide. Accordingly, it is possible to provide a spring steel having high strength, high stress, and excellent hydrogen embrittlement resistance and fatigue properties.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Nobuhiko Ibaraki 2 Nadahama-Higashi-cho, Nada-ku, Kobe Inside Kobe Steel, Ltd.Kobe Works (72) Inventor Takenori Nakayama 1-5-5 Takatsukadai, Nishi-ku, Kobe Stock Company Kobe Steel, Ltd.Kobe Research Institute (72) Inventor Takashi Iwata 1-5-5 Takatsukadai, Nishi-ku, Kobe City, Ltd.Kobe Steel Research Institute Kobe Research Institute (72) Inventor Yoshinori Yamamoto Nadahama, Nada-ku, Kobe No. 2, Higashicho Kobe Steel Works, Ltd.Kobe Steel Works (72) Inventor Norio Okochi No. 2, Nadahama-Higashicho, Nada Ward, Kobe City Kobe Steel Works, Ltd. 5 Kobe Steel, Ltd.Kobe Research Institute (72) Inventor Jun Inada 2nd Nadahamahigashi-cho, Nada-ku, Kobe Company Kobe Steel Works Kobe steelworks (72) inventor Takeshi Kuroda Kobe Nada Ward Nadahamahigashi-cho, address 2 Corporation Kobe Steel Works Kobe steelworks

Claims (12)

[Claims] 1. Ti: 0.5% or less (meaning mass%,
The same shall apply hereinafter), Nb: 0.5% or less, Zr: 0.5% or less, Ta: 0.5% or less, Hf: 0.5% or less, Mo: 3.0% or less When at least one of Ti, Nb, Zr, Ta and Hf is contained, the total content thereof is 0.001% or more, and when only Mo is contained alone, the content is 0.05% or more. , N: 1 to 200 ppm, S: 5 to 300 ppm, and Ti, Nb, Zr, Ta, H
Hydrogen resistance, characterized in that precipitates of at least one element selected from the group consisting of f and Mo, which are composed of carbides, nitrides, sulfides or composite compounds thereof, having an average particle diameter of less than 5 μm are finely dispersed. Spring steel with excellent brittleness and fatigue properties. Test surface: set in an arbitrary direction so as to have a depth of 0.3 mm or more from the steel surface and not to include the central part 20
mm ² cross section. 2. The method according to claim 1, wherein the Ti, N in the surface to be inspected is Ti or N.
b, Zr, Ta, Hf, Mo The number of the precipitates of at least one element selected from the group consisting of carbides, nitrides, sulfides, or composite compounds thereof having an average particle diameter of 5 μm or more satisfy the following requirements. The spring steel according to claim 1, which is excellent in hydrogen embrittlement resistance and fatigue properties. Size and number of precipitates: 500 or less having an average particle diameter of 5 to 10 μm, 50 or less having an average particle diameter of more than 10 μm and 20 μm or less, and 10 or less having an average particle diameter of more than 20 μm. 3. A carbide, nitride or sulfide of at least one element selected from the group consisting of Ti, Nb, Zr, Ta, Hf, Mo, and V containing V: 1.0% or less as another element. The spring steel according to claim 1, wherein precipitates having an average particle diameter of 5 µm or less, which are composed of a substance or a composite compound thereof, are finely dispersed. 4. A carbide, nitride or sulfide of at least one element selected from the group consisting of Ti, Nb, Zr, Ta, Hf, Mo and V containing V: 1.0% or less as another element. 3. The spring steel according to claim 2, wherein precipitates having an average particle diameter of 5 μm or more, which are composed of a substance or a composite compound thereof, satisfy the requirements of the above-mentioned (2). 5. The austenite grain size after quenching and tempering is 20 μm or less, the hardness is 50 or more HRC, and the fracture toughness (K _IC ) is 40 MPa @ m or more.
The spring steel according to any one of the above. 6. The composition according to claim 1, further comprising at least one element selected from the group consisting of Ni: 3.0% or less, Cr: 5.0% or less, and Cu: 1.0% or less. Item 6. The spring steel according to any one of Items 1 to 5. 7. Other at least one element selected from the group consisting of Al: 1.0% or less, B: 50 ppm or less, Co: 5.0% or less, and W: 1.0% or less. The spring steel according to any one of claims 1 to 6, comprising an element. 8. As another element, Ca: 200 pp
m, at least one element selected from the group consisting of La: 0.5% or less, Ce: 0.5% or less, and Rem: 0.5% or less. The described spring steel. 9. The steel having a C, Si, Mn content of C:
The spring steel according to any one of claims 1 to 8, wherein the content is 0.3% or more and less than 0.7%, Si: 0.1 to 4.0%, and Mn: 0.005 to 2.0%. 10. The unavoidable impurity in steel is P: 0.02
% Or less. 11. Zn contained as an impurity in steel is 6%.
0 ppm or less, Sn is 60 ppm or less, As is 60 pp
m or less, Sb is 60 ppm or less.
Spring steel according to 0. 12. The spring steel according to claim 9, wherein the steel satisfies the following formula (I). 2.5 ≦ (FP) ≦ 4.5 (I) where FP = (0.23 [C] +0.1) × (0.7 [Si] +1) × (3.5 [Mn]
+1) × (2.2 [Cr] +1) × (0.4 [Ni] +1) × (3 [Mo] +1) (However, [element] represents mass% of each element)