JPH06128689A - Spring steel with excellent set resistance - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a spring steel excellent in fatigue resistance and hydrogen embrittlement fracture resistance. [Composition] C: 0.3-0.6% by weight, Cr: 0.8-2.
94%, Mo: 0.3-2.0%, Ni: 0.1-1.0%,
V: 0.1 to 0.8%, Si: 1.5% or less, Mn: 1.5% or less, and optionally Nb: 0.02 to 0.30%, the balance Fe and impurities. Consists of. Increases sagging resistance without depending on Si.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for springs which is excellent in fatigue resistance and hydrogen embrittlement fracture resistance.

[0002]

2. Description of the Related Art In recent years, in order to reduce the weight of automobiles,
The weight of the suspension spring is reduced. In order to quantify the suspension spring, it is effective to list the design stress of the spring and use the spring under high stress. However, the use of suspension springs under high stress results in increased fatigue. The settling causes the height of the spring to decrease, which in turn causes the performance of the spring, which is a safety component, to deteriorate. Therefore, spring steel with excellent fatigue resistance is required.

As a spring steel having excellent fatigue resistance, S is
There is SUP6 with improved fatigue resistance due to i. Also, S
SUP7, which has a larger amount of Si than UP6, is also widely used. Further, a spring steel having a fatigue resistance exceeding SUP7 is also disclosed in Japanese Patent Laid-Open No. 58-67847.

[0004]

These spring steels having excellent fatigue resistance use Si as a basic element for increasing the fatigue resistance. Si has a function of improving solid resistance by increasing the matrix strength by forming a solid solution in ferrite, and 1.5% by weight or more is required to obtain the function.

However, such Si-dependent steel for springs may induce delayed fracture due to hydrogen under high stress and under use conditions that are disadvantageous in terms of hydrogen fracture embrittlement resistance.
This is a non-negligible problem for automobile suspension springs that are forced to be used under these conditions.

The object of the present invention is to have not only the fatigue resistance and toughness required for spring steel, but also to have the same settling resistance as Si-dependent spring steel and moreover, Another object of the present invention is to provide a spring steel excellent in hydrogen embrittlement fracture resistance.

[0007]

Spring steels used for automobile suspension springs are used under high stress and in an environment unfavorable for hydrogen embrittlement fracture. Under such conditions, Si, which is an element for improving the fatigue resistance, adversely affects delayed fracture due to hydrogen.

[0008] Therefore, the inventors of the present invention planned to increase the sag resistance without depending on Si, and repeated various experiments, and as a result, the composite addition of Ni, Cr, Mo and V was used as a substitute for Si. It was discovered that it could be. It was also confirmed that this composite addition ensures the basic properties such as fatigue resistance and toughness required for spring steel.

The spring steel of the present invention was developed on the basis of the above-mentioned findings. C: 0.3 to 0.6% by weight% and Cr:
0.8-2.94%, Mo: 0.3-2.0%, Ni: 0.1-1.
0%, V: 0.1 to 0.8%, Si: 1.5% or less, Mn: 1.
5% or less and, if necessary, Nb: 0.02 to 0.30%
And the balance is Fe and impurities.

[0010]

The chemical composition of the steel of the present invention will be described in detail below.

Cr, Mo, V or Cr, Mo, V,
By adding Nb in combination, alloy carbide that is not dissolved in austenite during heating during quenching makes austenite crystal grains finer and prevents their coarsening.
The crystal grain boundaries thus refined reduce the amount of dislocation movement and improve the settling resistance. Further, due to this composite addition, even when quenching is performed from the quenching temperature of the spring steel of about 900 ° C., reprecipitation occurs and secondary hardening occurs in the subsequent tempering process. This means that, when aiming at the same tempering hardness, the tempering temperature range is widened, and the desired hardness can be stably obtained. The reasons for limiting each component are as follows.

C forms carbides with Cr, V and Nb, but when quenched, it also forms a solid solution in the matrix and strengthens the matrix. 0.
If it is less than 30%, sufficient hardness cannot be obtained as spring steel,
If it exceeds 0.6%, the toughness is significantly deteriorated in this application where quenching is performed at a high temperature, so the content is made 0.3 to 0.6%.

Cr forms a carbide, and part of it forms a solid solution with the substrate to improve the hardenability, facilitating the quenching and simultaneously increasing the temper softening resistance by tempering. Further, the increase of the amount has the effect of decreasing the activity of C, but the upper limit is set so that the economical efficiency is not deteriorated and the cold workability is not deteriorated.
It was set to 2.94%. Further, from the viewpoint of hardenability, if it is less than 0.6%, it is insufficient, so the lower limit was made 0.6%.

V is an effective element that forms a carbide and reduces the activity of C. This carbide is difficult to form a solid solution in austenite, prevents the growth of crystal grains, and increases the softening resistance. % Or more is required. However, if 0.8% or more, huge primary carbides are generated during solidification and adversely affect the fatigue life, so the content is made 0.8% or less.

Ni forms a solid solution in the matrix to enhance the hardenability, improve the toughness, and stabilize the fatigue life. For this reason, 0.1% or more is necessary. However, if it exceeds 1.0%, it becomes difficult to reduce the strength due to annealing. Therefore Ni is 0.
It is set to 1 to 1.0%.

Mo forms carbides in steel. In particular, Mo carbides precipitated during high temperature tempering prevent movement of dislocations, which improves the settling resistance. If it is less than 0.1%, the effect is not sufficient, and if it exceeds 2.0%, the effect is saturated, so the content is made 0.1 to 2.0%.

Si is used as a deoxidizer, but is 1.5%
When it exceeds, it has a bad influence on hydrogen embrittlement resistance under high stress. Further, it becomes difficult to reduce the hardness due to annealing. Therefore, the content is made 1.5% or less, preferably 0.5% or less, and more preferably 0.3% or less.

Mn is also used as a deoxidizer, but 1.5%
If it exceeds 1.0, it will be difficult to reduce the hardness due to annealing.
% Or less.

Nb forms high melting point fine special carbides,
It effectively prevents the crystal grains from becoming coarse with the rise of the quenching heating temperature. This tendency is recognized when the content is 0.02% or more, and as the addition amount increases, the crystal grains become finer and the toughness improves.
Since the effect is saturated even if the content exceeds 3%, the content is made 0.02 to 0.3% when added.

[0020]

EXAMPLES Examples of the present invention will be described below.

Eight types of steel shown in Table 1 (A1 to A6, B
1, B2) were melted, and a test material having a plate thickness of 3 mm and a width of 200 mm was manufactured through a process of hot rolling-pickling-annealing. A1 to A6 are steels of the present invention, among which A1 to A6
4 contains Nb, and A5 and A6 do not contain Nb. Also, B1 is steel equivalent to SUP7, and B2 is Japanese Patent Laid-Open No.
It is a typical steel for springs proposed in Japanese Unexamined Patent Publication No. 8-67847.

The final hardness of each manufactured test material is HRC4.
After quenching and tempering to 5, 50 and 55,
Apply a load that gives a bending stress of 100 kgf / mm ² and 2
The amount of settling was measured after holding at 00 ° C. for 48 hours. Further, a tensile test piece with a notch shown in FIG. 1 was taken from each test material, and a load corresponding to a stress of 95 kgf / mm ² was applied in 0.2 N hydrochloric acid to measure the time until delayed fracture. . The measurement result of the amount of sag is shown in Table 2, and the result of the delayed fracture test is shown in Table 3.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

B1 is SUP7 and B2 is JP-A-58-67.
No. 849 presented steel, all of which are Si-dependent spring steels having excellent fatigue resistance. The steels A1 to A6 of the present invention contain these S even though the Si content is suppressed to 0.3% or less.
Has the same settling resistance as the i-dependent conventional steels B1 and B2. Regarding the hydrogen embrittlement fracture resistance, the steels A1 to A6 of the present invention in which the Si content is suppressed are significantly superior to the Si-dependent conventional steels B1 and B2. It has been confirmed that the steels A1 to A6 of the present invention are equivalent to the conventional steels B1 and B2 in terms of basic performance such as fatigue resistance and toughness required for steel for springs.

[0025]

As is apparent from the above description, the spring steel of the present invention has the same settling resistance as or higher than that of the Si-dependent conventional steel, and is superior in resistance to the conventional steel. Has hydrogen brittle fracture characteristics. In addition, basic performance such as fatigue resistance and toughness required for spring steel is high. Therefore, it can be used as a suspension spring for an automobile, which is required to be lightweight and used under severe conditions, and exhibits stable performance for a long period of time, which is very effective in terms of safety and security.

[Brief description of drawings]

FIG. 1 is a dimensional diagram of a test piece used for a hydrogen embrittlement fracture test.

Claims (2)

[Claims] 1. C: 0.3 to 0.6% by weight, Cr: 0.8
~ 2.94%, Mo: 0.3-2.0%, Ni: 0.1-1.0
%, V: 0.1 to 0.8%, Si: 1.5% or less, Mn: 1.5
% Or less, and the balance Fe and impurities, which is a steel for springs having excellent fatigue resistance. 2. C: 0.3 to 0.6% by weight, Cr: 0.8
~ 2.94%, Mo: 0.3-2.0%, Ni: 0.1-1.0
%, V: 0.1 to 0.8%, Si: 1.5% or less, Mn: 1.5
% Or less, Nb: 0.02 to 0.30%, and the balance Fe and impurities, characterized in that it is a steel for springs having excellent fatigue resistance.