JP2505235B2 - High strength spring steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-strength spring steel, and more particularly to a spring steel excellent in sag resistance.

(Prior Art) Conventionally, Si-Mn-based SUP6 and Cr-Mn-based SPU9 have been mainly used as spring steels for suspension systems for automobiles and the like. In recent years, however, there has been a demand for a reduction in the weight of automobiles, and a reduction in the weight of the suspension itself has also been strongly demanded.
On the other hand, various means have been tried throughout the suspension system, and among them, setting the design stress of the spring high is considered to be effective.

Along with such a high stress design of the spring, when a spring is manufactured by using the above-mentioned conventional steel for spring as a raw material, a problem of increase in fatigue occurs. Especially when used in passenger cars,
The increase in settling led to a decrease in the height of the bumper, which became a major safety issue.

Recently, there is a strong demand for sag resistance, and in response to this demand
SUP7, which has a higher Si content than SUP6 and has the highest Si among spring steels specified in JIS-G4801, has come to be widely used as spring steel for passenger car suspension. However, there is an even stricter demand for a lighter suspension spring, and there is a strong demand for the development of spring steel that is more durable than SUP7.

(Problems to be Solved by the Invention) An object of the present invention is to provide a high-strength spring steel which is particularly excellent in sag resistance of a coil spring.

(Means for Solving the Problem) The present inventors have already reported that Si in steel has an effect on the sag resistance in the past (Japanese Patent Application No. 62-147155). As a result of further research on the influence of various elements on the sag resistance, the present inventor further confirmed that Mn in spring steel was used.
It was found that the sag resistance is improved by decreasing the Cr content and increasing the Cr content. This time Mn is reduced,
By adding Cr, Mo, Ni and at least one of Nb and V, the sag resistance of high strength spring steel is improved and
The present invention has been made with a completely new finding that improvement in toughness can be achieved. That is, the gist of the present invention is C: 0.45 to 0.65%, Si: 1.5 to 2.5%, Mn: 0.2 to 0.5%, Cr: 1 to 3.5%, Mo: 0.05 to 1%, Ni: 0.05 to 2% and Nb: 0.01 to 0.5%, V: 0.01 to 0.5% and at least one of them, the balance being Fe and inevitable impurities.

(Operation) First, the reasons for limiting the components of the present invention will be described.

C is an element for obtaining sufficient strength as a high stress spring steel by heat treatment, and 0.45% for securing sag resistance.
% Is required, but if it exceeds 0.65%, the tendency of graphitization increases and the toughness decreases remarkably.
It was set to 0.65%.

Si forms a solid solution in ferrite to increase the strength of the base material, promotes the refinement of precipitated carbides, and is also effective for the refinement of crystal grains. In particular, when applied to high-strength spring steel in hot forming, the range of toughness is expanded, which is effective for spring setability. For this reason, the Si range is 1.
It was set to 5 or more, but if it exceeds 2.5, the toughness deteriorates significantly, so the range was made 1.5 to 2.5%.

Mn is an element to enhance hardenability and to obtain strength even for spring steel, and it is necessary to have 0.2% or more, but if it exceeds 0.5%, toughness and sag resistance are impaired.
It was set to 0.2 to 0.5%.

Cr must be 1% or more to prevent graphitization and decarburization, and if it exceeds 3.5%, large carbides of Cr are generated,
The range is set to 1 to 3.5% because it adversely affects the spring fatigue property and the spring settling property.

Mo is required to be 0.05% or more in order to secure hardenability and further improve the sag resistance of the spring. However, the effect is saturated at 1% or more, so the range is set to 0.05 to 1%.

Ni is an element effective for imparting spring toughness that is strengthened by C and Cr, and is required to be 0.05% or more, 2%
In the above, residual austenite which adversely affects the sag resistance is generated, so the range is set to 0.05 to 2%.

The above are the basic components of the steel of the present invention, but in the present invention, Nb and V are added for the purpose of further improving the sag resistance. These Nb and V are elements for improving the fatigue resistance of the spring steel by refining the crystal grains and precipitating fine carbides.
If it exceeds 0.5%, its effect is saturated, and alloy carbides that are not dissolved in austenite increase, which reduces the fatigue of spring steel, so the range is Nb: 0.01-0.5.
%, V: 0.01 to 0.5% and at least one of them is included. The higher the hardness and strength, the better the sag resistance. The steel of the present invention has high quenching hardness due to high Cr, Mo and Ni, and also has high softening resistance. Therefore, a high tempering temperature can be used, and high toughness can be obtained. That is, a spring having good sag resistance and high toughness can be obtained.

Although the steel of the present invention has the above components, in this production, a spring steel is obtained through the steps of ordinary steelmaking, ingot casting or continuous casting, disassembly rolling, and further bar or wire rod rolling. Can be done. Thereafter, pre-treatment such as pickling, wire drawing, straightening, cutting, and tapering is performed as needed, and then hot coil spring molding and post-processing are performed to obtain a high-strength coil spring.

Next, the effects of the present invention will be described more specifically with reference to examples.

(Example) Table 1 shows the chemical components of the test steel. First
In the table, A1 to A6 steels are examples of the present invention, and B1 to B9 steels are comparative materials. The test coil spring was hot-rolled at a rolling ratio of 50 or more after casting, and subjected to straightening, constant-length cutting, peeling, hot-spring forming, quenching and tempering. The hardness of the test spring was adjusted to HRC52.5 by changing the tempering temperature.

Table 2 shows the specifications of the test spring. The test spring was subjected to shot peening with an arc height of 0.5 mm and then set so that the shear stress τ of the wire was 126.5 kgf / mm ^2, and then subjected to a spring tightening test.

 Table 3 shows the results of the spring tightening fatigue test.

The residual shear strains of the present invention examples were good results with almost all less than 1.0 × 10 ⁻⁴ , but the residual shear strain was large at comparative springs of 1.0 × 10 ⁻⁴ or more, and the sag resistance was poor. Is clear.

 The comparative test spring was as follows.

B1 has a low C content and B2 has a high Si content.

B3, B4, B7, and B8 have a large amount of austenite with a large amount of Mn, Cr, Mo, and Ni remaining.

B5 and B6 have a large amount of Nb and V and fine crystal grains, but have a large amount of carbide and have a structure in which quenching is insufficient.

B9 has a structure in which Nb and V are not added and crystal grains are large and a large amount of retained austenite is present.

(Effect of the Invention) The present invention is a high-strength spring steel having excellent sag resistance of a coil spring, and has great industrial effects.

Claims (1)

(57) [Claims] 1. By weight%, C: 0.45 to 0.65%, Si: 1.5 to 2.5%, Mn: 0.2 to 0.5%, Cr: 1 to 3.5%, Mo: 0.05 to 1%, Ni: 0.05 to 2%. A high-strength spring steel characterized by containing at least one of Nb: 0.01 to 0.5% and V: 0.01 to 0.5%, and the balance being Fe and inevitable impurities.