JP3064672B2 - High strength spring steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high-strength spring steel.
More specifically, the present invention relates to a high-strength spring steel that can be suitably used for a suspension spring of an automobile or the like.

[0002]

2. Description of the Related Art The chemical composition of spring steel is JIS G356.
5-3567 and G4801. The rolled material made of such a steel material is subjected to wire drawing to a predetermined wire diameter, and then subjected to OT processing and then to spring working (cold winding spring), It is manufactured by subjecting a rolled material to wire drawing and then heating to perform spring forming-quenching and tempering (hot wound spring). However, all of the conventional spring steels have a material strength after quenching and tempering of 160 to 190 kgf / mm ² (HR
C: less than 53), which cannot be said to satisfy the requirements for high strength spring steel.

That is, since the material strength of steel materials generally required as high-strength spring steel is 200 kgf / mm ² (HRC: 53) or more, the required strength is adjusted by adjusting the chemical composition of conventional spring steel. Attempts have been made to get
In conventional steels, the fatigue life and set properties tend to deteriorate as the strength is increased. If the strength is set to 200 kgf / mm ² or more, it becomes impossible to satisfy the extremely important fatigue life and set properties as a spring property.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and its object is to solve the problem.
An object of the present invention is to provide a high-strength spring steel having a strength of not less than kgf / mm ² and excellent performance in toughness, delayed fracture characteristics, fatigue characteristics, and the like.

[0005]

The structure of the high-strength spring steel according to the present invention, which can solve the above-mentioned problems, is expressed by weight%.
C: 0.50 to 0.70%, Si: 1.5 to 2.0
%, Mn: 0.2 to 0.60%, Ni: less than 0.5%,
Cr: 0.5 to 1.2%, Cu: 0.1 to 0.5%,
V: containing 0.05 to 0.5%, and 1.5 × Mn +
It satisfies the condition of Cr + Ni ≦ 1.8, is composed of the balance of iron and inevitable impurities, and has a gist where the hardness after quenching and tempering is HRC 53 or more.

[0006]

According to the present invention, the hardness after quenching and tempering is determined by specifying the chemical composition of the steel material without impairing the toughness, delayed fracture resistance, sag resistance, decarburization resistance, fatigue characteristics, etc.
And attained 53 or more (200 kgf / mm ² or more in tensile strength). The reason for determining the content of each component is as follows.

C: 0.5 to 0.7% C is an indispensable element for ensuring an improvement in strength after quenching and tempering, and if less than 0.5%, the hardness of martensite generated after quenching. And the strength after quenching and tempering cannot be sufficiently increased. But 0.7%
If it exceeds 300, the toughness after quenching and tempering deteriorates, and the fatigue properties for springs cannot be satisfied.

Si: 1.5 to 2.0% Si functions as a solid solution strengthening element. If it is less than 1.5%, the matrix strength becomes insufficient and the purpose of increasing the strength cannot be fulfilled. If it exceeds 0.0%, the decarburization resistance rapidly deteriorates during the heating process during the production process, and the fatigue resistance characteristics deteriorate due to an increase in the decarburization depth.

Mn: 0.2-0.6% Mn is an indispensable component as a hardenability improving element and must be contained at least 0.2%. However, if it is too much, the hydrogen permeability of the material after quenching and tempering is increased, and as a result, hydrogen embrittlement is likely to occur in a corrosive environment. Therefore, the upper limit of the Mn content is set to 0.6%.

Ni: less than 0.5% Ni not only has the effect of improving the toughness of the material after quenching and tempering, but also has the effect of suppressing hot embrittlement of the Cu-containing steel material. The upper limit was set to 0.5% because surface defects easily occur during rolling.

Cr: 0.5 to 1.2% Cr improves hardenability similarly to Mn. In addition, Cr not only has an effect of increasing heat resistance, but also, as a result of various studies, has been found to significantly improve sag resistance, which is an important spring characteristic. In order to effectively exhibit these effects, the content must be 0.5% or more. However, if the content is too large, the toughness after quenching and tempering deteriorates.
0% was made the upper limit.

V: 0.05 to 0.5% V has the effect of reducing the crystal grain size, increasing the proof stress ratio and improving the sag resistance characteristics, and 0.05% or more is required to effectively exhibit such effects. Must be included. However, when the content exceeds 0.5%, the amount of alloy carbide not solid-dissolved in austenite at the time of quenching heating increases, and large carbides remain to reduce the fatigue life.

Cu: 0.1-0.5% Cu is an element that is electrochemically nobler than iron and has the effect of enhancing pitting corrosion resistance in a corrosive environment and enhancing corrosion fatigue characteristics. If it is less than 1%, the effect cannot be exhibited effectively.
However, if it is too large, it causes embrittlement of the material at the time of hot rolling, so it must be suppressed to 0.5% or less.

The essential elements of the high-strength spring steel according to the present invention are as described above, and the balance consists of iron and unavoidable impurities. If necessary, Nb and / or Mo may be added in addition to the above essential elements. By containing an appropriate amount,
Spring characteristics can be further enhanced.

Mo: 0.05-0.5% and / or Nb: 0.05-0.5% Mo is a carbide-forming element and precipitates a fine alloy carbide during tempering to promote the secondary hardening effect. It contributes to improvement of sag resistance and fatigue resistance. If it is less than 0.05%, the effect is insufficient, and the effect is saturated at 0.5%.

Nb has the effect of reducing the crystal grain size, increasing the proof stress ratio and improving the sag resistance characteristics. In order to effectively obtain such effects, 0.05% or more of addition is necessary. However, even if added in excess of 0.5%, no further effect can be obtained. It promotes the formation of coarse carbonitrides and deteriorates fatigue life.

Further, in the present invention, while satisfying the requirement of the above-mentioned essential element content, among these elements, the content of Mn, Cr and Ni satisfies the requirement of 1.5 × Mn + Cr + Ni ≦ 1.8. It has been clarified that the content must be adjusted, and by defining such requirements, a steel with excellent balance of strength and toughness as a spring steel can be obtained. If this value exceeds 1.8, the amount of retained austenite after quenching increases and not only the toughness deteriorates, but also a supercooled structure is easily generated in the cooling process after rolling, so that a high strength spring is formed. The suitability as steel for use is impaired.

Although unavoidable impurities include O and N, especially in the fatigue of a high-strength suspension spring, the inclusion sensitivity is greater than that of the conventional material, so the content of O and N should be minimized. Is desirable.

In conventional component steels such as JIS SUP7,
By lowering the tempering temperature to a lower temperature side, it is possible to increase the tensile strength after quenching and tempering.However, if the tensile strength is increased to 200 kgf / mm ² or more by this method, the toughness of the material is significantly deteriorated, It is no longer suitable as a spring material. For this reason, in the past, a strength level of 190 kgf / mm ² or less had to be allowed. However, in order to reduce the weight of automobiles, it is essential to increase the strength of suspension springs, etc.
A tensile strength of 200 kgf / mm ² or more is being demanded.

In addition, when a high-strength steel having a tensile strength of 200 kgf / mm ² or more is used, in particular, grain boundary fracture due to hydrogen embrittlement tends to occur. The strength as seen in such other high strength bolts steel embrittlement and destruction delay to become 120 kgf / mm ² or more is likely to occur have been reported previously,
As the strength of the spring becomes higher, grain boundary fracture due to hydrogen embrittlement occurs in an environment such as moisture as the strength becomes higher, thereby greatly reducing the fatigue life.

According to the high-strength spring steel of the present invention, the hardness after quenching and tempering is HRC 53 or more by specifying the types of the contained elements and their contents as described above.
To exert 0kgf / mm ² level or higher tensile strength. Moreover, the steel of the present invention exhibits excellent toughness despite having such high strength, and in particular, by specifying the contents of Mn, Cr, and Ni, delayed fracture due to hydrogen embrittlement or the like is suppressed. In addition, the fatigue life can be remarkably improved.

Conventionally, in the quenching process during the manufacturing process of a cross-linked spring or the like, oil quenching is performed to avoid the occurrence of quenching cracks and the like. When such a quenching treatment is performed on the added material, a part of retained austenite remains, which hinders an increase in strength. Therefore, when focusing on high strength, the addition of a large amount of alloying elements should be avoided as much as possible.However, since the spring steel of the present invention suppresses the alloying elements in a small amount as described above, The amount of retained austenite is small, and high strength can be achieved.

As described above, according to the present invention, C, Si, Mn, N
By specifying the content of i, Cr, Cu, V, etc., and particularly by specifying the amount of “1.5 × Mn + Cr + Ni”, it exhibits high strength of 200 kgf / mm ² or more and is important as spring steel. A high-strength spring steel excellent in delayed fracture resistance, sag resistance, fatigue resistance and the like can be obtained.

[0024]

EXAMPLES Next, the present invention will be described specifically with reference to examples, but the present invention is not limited by the following examples. No. shown in Table 1. 1 to No. After melting 16 steel materials, a billet of 115 mm square was manufactured by forging, rolled to a wire diameter of 12.5 mmφ by wire rod rolling, and then drawn to 12 mmφ. This drawn wire has a length of 3
After cutting to 00 mm, quenching and tempering were performed. The quenching is oil quenching at a heating temperature of 950 ° C., and the tempering temperature is 420
° C. Table 2 shows the results of the tensile test after quenching and tempering.
In the case of a high-strength suspension spring, mechanical properties in a corrosive environment become a problem. Therefore, a delayed fracture test was performed in 5% sodium chloride to evaluate hydrogen embrittlement. In this test, a test piece with an annular notch was used, and the test was performed by a load tension method. Furthermore, after rolling the test material to 10 mm and drawing it to 9 mmφ, a spring was manufactured by a normal method, and a fatigue test was performed on a spring sample that had been subjected to shot peening and setting, and the results are shown in Table 2. Also described.

[0025]

[Table 1]

[0026]

[Table 2]

In Tables 1 and 2, No. 1 to 6 are examples satisfying the requirements of the present invention, and include fatigue characteristics, tensile strength,
Good results have been obtained in any of the delayed crack fracture characteristics. On the other hand, No. Nos. 7 to 16 are comparative examples lacking any of the prescribed requirements, and as shown below, are poor in any of the tensile strength, fatigue properties and delayed fracture properties.

No. 7: X value (= 1.5 × Mn% + Ni
% + Cr%) is out of the specified range, the tensile strength is good, but the fatigue properties are poor. No. 8: The tensile strength is 200 kgf / mm due to insufficient C content
^It has not reached ^2, and its fatigue properties are poor. No. 9: Since the amount of C is too large, the aperture value is extremely low, and the fatigue characteristics and delayed fracture strength are poor. No. 10: Sufficient tensile strength cannot be obtained due to insufficient Si content. No. 11, 12: In addition to the X value exceeding the specified range, Mn
Since the amount is out of the specified range, the aperture value is low and satisfactory fatigue characteristics cannot be obtained. Moreover, the delayed fracture characteristics are insufficient. No. 13: The tensile strength is insufficient because the X value is too small and the Cr amount is also insufficient. No. 14: Since the X value and the amount of Cr are too large, the aperture value is particularly low and lacks fatigue characteristics. No. 15: Since the amount of V is too large, the aperture value is low and the fatigue characteristics are poor. Also, the delayed fracture strength is poor. No. 16: Since the Mo amount is too large, the aperture value and the fatigue characteristics are insufficient.

[0029]

The present invention is constituted as described above. By specifying the component composition of the steel material, the hardness after quenching and tempering can be achieved without impairing the toughness, delayed fracture, sag resistance and fatigue characteristics. Thus, it is possible to provide a high-performance spring steel having a tensile strength of 200 kgf / mm ² level or more.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-216951 (JP, A) JP-A-62-274058 (JP, A) JP-B 64-4578 (JP, B2) (58) Field (Int. Cl. ⁷ , DB name) C22C 38/00 301 C22C 38/46

Claims (2)

(57) [Claims] 1. C: 0.50 to 0.70% by weight, S
i: 1.5 to 2.0%, Mn: 0.2 to 0.60%, N
i: less than 0.5%, Cr: 0.5 to 1.2%, Cu:
0.1-0.5%, V: 0.05-0.5%,
A high-strength spring steel which satisfies the condition of 1.5 × Mn + Cr + Ni ≦ 1.8, is composed of the balance of iron and inevitable impurities, and has a hardness after quenching and tempering of HRC53 or more. 2. Nb: 0.05 or more as another component
The high-strength spring steel according to claim 1, which contains 0.5% and / or Mo: 0.05 to 0.5%.