JPH0559431A - Production of spring with high stress excellent in delayed fracture resistance - Google Patents

Abstract

PURPOSE:To obtain the spring with high stress excellent in durability by reducing the contents of P and S as impurities in a steel with specific composition, regulating Mn content to a proper value, adding specific amounts of Mo and further Nb, V, Al, and N, and performing hardening after recrystallization. CONSTITUTION:A steel having a composition which con: of, by weight, 0.45-0.80% C, <=0.50% Si, 0.20-O.50% Mn, 0.10-2.0% Cr, 0.05-0.80% Mo, and the balance Fe with inevitable impurities and further contains one or more kinds among 0.005-0.200% Nb, 0.05-0.50% V, 0.005-0.100% Al, and 0.003-0.030% N and in which the contents of P and S among the impurities are regulated to <=0.015% and <=0.010%, respectively, is used as a stock. This stock is heated up to 800-1000 deg.C at the time of working into a spring and hot-worked at =10% reduction of area, and, after recrystallization, hardening and tempering are done. By regulating the contents of P, S, Mn, and Mo to the above-mentioned values, respectively, the segregation of P and S in the grain boundaries to be the factors for promoting delayed fracture can be remarkably reduced. Moreover, by using one or more elements among Nb, V, Al, and N, crystalline grains can be refined and durability can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high stress spring used in vehicles such as automobiles and construction machinery, and more particularly to a method for manufacturing a high stress spring which is excellent in delayed fracture resistance and durability.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of energy saving and prevention of environmental pollution, there has been a strong demand for weight reduction of passenger cars, trucks, and the like. Stress design is being considered. For that purpose, it is necessary to increase the strength of the spring. Specifically, from the conventional tensile strength of 130 to 140 kgf / mm ² class to the recent 17
A spring having a strength of 0 kgf / mm ² or more is required.

In order to meet such a demand for higher stress, there is a measure of using conventional SUP 9 or SUP 11A of JIS G 4801 to lower the tempering temperature and increase the tensile strength.
There is a problem that durability is deteriorated because the toughness is low and fatigue strength is not accompanied by that alone. Therefore, recently, using SUP 10 of JIS G 4801, which has excellent hardenability and can increase strength,
High-stress springs were developed, but delayed fracture has been raised as a new problem with the increase in strength.

Delayed fracture is a phenomenon in which steel placed under a static load suddenly becomes brittle after a certain period of time, and is attributed to hydrogen embrittlement due to hydrogen invading the steel from the external environment. The fracture mode is grain boundary fracture along the former austenite grain boundary. To prevent this delayed fracture, bolt steel,
High-strength steels such as PC steel rods have been studied in the past, and measures have been taken to reduce impurity elements such as P and S that segregate or precipitate at grain boundaries and cause a decrease in grain boundary strength. .. However, bolt steel has a tensile strength of at most 150 kgf /
While it is about mm ² , the spring steel has 1
Extremely high strength of 70 kgf / mm ² or more is required. In addition, springs are repeatedly stressed during use, such as bolt steel and P
The usage conditions are different from those of C steel bar, and it is more severe. Therefore, only by reducing the impurities such as P and S as described above, a high-strength spring steel having delayed fracture resistance that meets the demand of customers cannot be obtained.

Japanese Unexamined Patent Publication No. 57-169062 discloses a high stress spring steel excellent in delayed fracture resistance. However, since this steel has a high Si content of 1.0 to 2.5%, it has poor fatigue properties and insufficient durability. Further, Japanese Patent Laid-Open No. 63-274739 proposes a spring steel having excellent hardenability and durability, but this steel does not consider impurities such as P and S,
Further, since Mn is as high as 0.90 to 1.50%, a problem remains in terms of delayed fracture resistance.

[0006]

DISCLOSURE OF THE INVENTION The present invention is 170 kgf / m
It is an object of the present invention to provide a method for manufacturing a spring that has a tensile strength of m ² or more and is also excellent in delayed fracture resistance and durability. An object of the present invention is to provide a spring manufacturing method in which the chemical composition, the processing during spring processing, and the heat treatment conditions are optimally selected.

[0007]

[Means for Solving the Problems] The present invention is characterized by using a steel having the following chemical composition (a) as a raw material and processing and heat treating in the following step (b) during spring processing: 170 kgf / mm ²
The gist is a method of manufacturing a high stress spring having the above tensile strength and excellent delayed fracture resistance.

(A) C: 0.45 to 0.80%, Si: 0.1% by weight.
50% or less, Mn: 0.20 to 0.50%, Cr: 0.10 to 2.00%, Mo:
0.05 to 0.80%, Nb: 0.005 to 0.200%, V: 0.05 to
0.50%, Al: 0.005 to 0.100%, N: 0.003 to 0.030%, at least one of which is composed of Fe and inevitable impurities. When P of impurities is 0.015% or less and S is 0.010% or less. (B) Using a certain steel as a material, 800-1000 during spring processing
It is heated to ℃, hot worked at a reduction of 10% or more, recrystallized, and then quenched and tempered.

According to the present invention, the kinds of alloying components of the material steel and their contents are selected as described above, and the optimum conditions of working and heat treatment when performing spring working using the steel are determined. The above-mentioned excellent effects will be obtained as the overall effects of the present invention. The main findings on which the present invention is based are as follows.

Impurities P and S are kept low, and
By adjusting the Mn content to an appropriate range and adding Mo, the segregation of P and S at the grain boundaries, which is a factor promoting delayed fracture, is significantly reduced.

By adding one or more of Nb, V, Al and N, the crystal grains become finer and the durability is improved.

Recently, in order to increase the strength of springs, an improved ausform treatment is performed in the spring working process, that is, hot working is performed in a stable austenite region, and before the influence of the working disappears (before recrystallization occurs, ) A processing method of quenching and then tempering is adopted. The steel material produced by this method has high strength but insufficient toughness or delayed fracture resistance. However, if hot-working in the stable austenite region is followed by recrystallization followed by quenching and tempering, segregation of P and S at the grain boundaries is reduced, the strength is high, and delayed fracture resistance and A spring with excellent durability can be obtained.

[0013]

First, the function of the alloying components of the steel used as the raw material of the method of the present invention and the reasons for limiting their contents will be described. All% related to the content of alloy components are% by weight.

C: 0.45 to 0.80% C is an essential component for ensuring the strength of steel, and is also an effective component for refining crystal grains. Its content is
If it is less than 0.45%, the strength as a high stress spring cannot be obtained. On the other hand, if C exceeds 0.80%, the amount of C that forms a solid solution in the matrix is saturated and does not contribute to the increase in strength, and coarse carbides are generated, leading to deterioration in toughness.

Si: 0.50% or less Si is added as a deoxidizing agent for steel. However, excessive Si deteriorates the toughness of steel and significantly reduces durability.
It is better to keep Si up to 0.50%.

Mn: 0.20 to 0.50% Mn is an element effective for improving the hardenability of steel and ensuring strength and toughness.

If the content is less than 0.20%, the desired strength,
Toughness cannot be secured. On the other hand, Mn is combined with S and finely precipitates at the grain boundaries to reduce the grain boundary strength. As will be described later, even if S and P are suppressed low, if the Mn content exceeds 0.50%, a spring having a high strength of 170 kgf / mm ² or more cannot obtain excellent delayed fracture resistance. Therefore, the proper range of Mn content is 0.20 to 0.50%.

Cr: 0.10-2.0% Cr improves the hardenability of steel and imparts temper softening resistance.
It is an element effective for obtaining a tensile strength of 170 kgf / mm ² or more. Further, by forming a film having excellent corrosion resistance on the surface of the steel, the delayed fracture resistance is improved by suppressing hydrogen intrusion into the steel from the outside. However, its content is 0.
If it is less than 10%, it is not possible to suppress the above-mentioned hydrogen invasion into the steel from the outside. On the other hand, if the Cr content exceeds 2.0%, the toughness deteriorates due to the increase of carbides, so the content of 0.10- It was set to 2.0%.

Mo: 0.05 to 0.80% Mo improves the hardenability of steel similarly to Cr, and also increases the temper softening resistance. It also improves corrosion resistance. Further, it has the effect of suppressing the grain boundary segregation of P and improves the delayed fracture resistance, but if the content is less than 0.05%, the effect of suppressing the grain boundary segregation of P is poor. On the other hand, if the content exceeds 0.80%, the effect is saturated and the material cost only rises, so the content range was made 0.05 to 0.80%.

One of the characteristics of the material steel of the present invention is that it contains one or more of Nb, V, Al and N in addition to the above basic components.

Nb: 0.005 to 0.20% Nb has the effect of improving the strength and toughness of the steel and the refinement of crystal grains. Particularly, in the case of low P, low S and low Mn clean steel, the delayed fracture resistance is remarkable. Improve. In order to secure the effect, it is necessary to add Nb in an amount of 0.005% or more. On the other hand, even if the content exceeds 0.20%, the effect is saturated and the cost is increased.

V: 0.05 to 0.50% V has the effect of making the steel finer or improving the strength of the steel by precipitation hardening. Therefore, when higher strength is required, it may be added as necessary. Its content is 0.
If it is less than 05%, the effect of strengthening cannot be obtained, while 0.50%
If it is contained in excess of the above, the effect will be saturated.

Al: 0.005 to 0.100% Al has an effect of suppressing coarsening of crystal grains of steel. 0.005
Its content is less than%, its action is insufficient, but 0.
If it exceeds 100%, the effect is saturated.

N: 0.003 to 0.030% N also has the effect of suppressing coarsening of the grain size of steel in the same manner as Al at a content of 0.003% or more, but the content is 0.030%.
If it exceeds, the effect will be saturated.

As described above, P and S as impurities
It is also a great feature of the raw material steel of the present invention to keep the value low.

P: 0.015% or less P segregates at the grain boundaries and reduces the grain boundary strength. Tensile strength
In order to improve delayed fracture resistance in a high strength material of 170 kgf / mm ² or more, it is necessary not only to reduce Mn and S but also to suppress P to 0.015% or less. 0.009% or less is preferable.

S: 0.010% or less S is combined with Mn and finely precipitates at the grain boundaries to reduce the grain boundary strength. In particular, in the case of a high strength material of 170 kgf / mm ² or more, it is necessary to suppress P and simultaneously suppress S to 0.010% or less to suppress fine precipitation at grain boundaries. It is more desirable to keep it below 0.007%.

Next, the processing and heat treatment method in the spring processing step will be described.

In the conventional spring manufacturing method, a step of heating, quenching and tempering a hot-rolled material of a low alloy steel as a raw material is adopted. As mentioned above, there is also a processing method (improved ausform processing) in which hot working is performed in the stable austenite region, quenching is performed before the effect of the working disappears (before recrystallization), and then tempering is performed. However, in these manufacturing methods, segregation of P and S and precipitation of film-like carbides at the crystal grain boundaries cannot be avoided, resulting in poor delayed fracture resistance.

In the present invention, the steel having the chemical composition described above is processed in the steps of heating, hot working, recrystallization, quenching and tempering during spring processing. By adopting this step, segregation of P and S at the grain boundaries and precipitation of film-like carbide are suppressed, resulting in a uniform and fine precipitation state, and the delayed fracture resistance and durability are remarkably improved. The conditions of the treatment in these steps are as described below.

Heating temperature: 800 to 1000 ° C. If the heating temperature before processing exceeds 1000 ° C., the crystal grains become coarse, the delayed fracture resistance decreases, and decarburization becomes remarkable. If the heating temperature is lower than 800 ° C, the temperature at the end of hot working will be low, and the necessary quenching temperature cannot be secured, so sufficient strength cannot be obtained.

Hot-working: Reduction of 10% or more Hot-working is carried out in order to improve the delayed fracture resistance and the durability by processing the product into a predetermined product size and, at the same time, refining the crystal grains. If the rolling reduction is less than 10%, the effect of working is insufficient, the crystal grains are not sufficiently refined, and the effects of delayed fracture resistance and durability improvement cannot be obtained. The upper limit of the rolling reduction may be determined depending on the material steel and the size of the product.

Recrystallization After hot working, it is important to recrystallize sufficiently and then perform the subsequent heat treatment of quenching and tempering. When the chemical composition of the above-mentioned material and the conditions described above are satisfied, recrystallization can be performed by, for example, leaving it to stand (cooling) for about 10 seconds after completion of hot working.

Quenching and tempering After recrystallization as described above, heat treatment of quenching and tempering is performed. Quenching is preferably in the range of 700 to 850 ℃ and oil quenching. Tempering may be performed at 300 to 450 ° C.
As a result, a strength of 170 kgf / mm ² or more can be secured.

[0035]

EXAMPLE Steels having the chemical compositions shown in Tables 1 and 2 were melted in a converter, and a material for leaf springs was manufactured by ordinary continuous casting or ingot casting and hot rolling. Steel Nos. 1 to 14 in Table 1
Is a steel having a composition range defined by the present invention, and No. 2 in Table 1
In Nos. 15 to 21, the contents of the alloy components marked with * are out of the range defined by the present invention.

The above materials were processed and heat-treated under the conditions A to J shown in Table 2. Conditions A to F in Table 2 are the conditions of the present invention, and G to F are the conditions for comparison not within the scope of the present invention.

In each case, as heat treatment, quenching and tempering were carried out by adjusting the tensile strength to be 180 to 190 kgf / mm ² within the above temperature range.

The tensile strength of the spring produced by combining the steel types 1 and 2 of Table 1 and the conditions of Table 2 as shown in Table 3, and
The delayed fracture resistance and durability were investigated. The results are also shown in Table 3. For the delayed fracture resistance and durability, a body spring was prepared and used for the test. The delayed fracture resistance was evaluated by the damage probability after leaving it for 1 month in the state of being incorporated in the axle shaft. Durability was evaluated by fatigue life by performing a fatigue test at an average stress of 75 kgf / mm ² , stress amplitude of 45 kgf / mm ² , and 1 Hz, and those exceeding 1 million cycles were evaluated as having excellent durability. It has a fatigue life of 100 in many previous experiments.
This is because the fatigue fracture problem does not occur when it exceeds 10,000 times.

Test Nos. 1 to 14 in Table 3 are examples in which the material steel defined in the present invention and the conditions of working and heat treatment are satisfied. N
o.15 to 25 are comparative examples in which any of the materials, processing, and heat treatment do not satisfy the conditions of the present invention. The former have a 0% failure probability and a fatigue life of more than 1 million cycles. That is, both delayed fracture resistance and durability are excellent.

On the other hand, in the comparative example, delayed fracture resistance,
Many have poor durability. For example, some of them have excellent durability such as No. 16, but they have poor delayed fracture resistance. On the contrary, Nos. 18 and 20 are excellent in delayed fracture resistance but poor in durability. That is, a spring having high strength but excellent in both delayed fracture resistance and durability has not been obtained.

[0041]

[1 in Table 1]

[0042]

[Table 1-2]

[0043]

[Table 2]

[0044]

[Table 3]

[0045]

According to the method of the present invention, the tensile strength is 170
It is possible to manufacture springs with a kgf / mm ² or higher and excellent delayed fracture resistance and durability. Such a spring is extremely useful as a high-stress spring for automobiles, etc., which is particularly aimed at weight reduction.

Claims (1)

[Claims] 1. By weight%, C: 0.45 to 0.80%, Si: 0.50%
Below, Mn: 0.20 to 0.50%, Cr: 0.10 to 2.0%, Mo: 0.05
~ 0.80%, Nb: 0.005 ~ 0.200%, V: 0.05 ~ 0.50
%, Al: 0.005 to 0.100%, N: 0.003 to 0.030% of 1
A steel containing at least one species and the balance consisting of Fe and unavoidable impurities, with P of impurities of 0.015% or less and S of 0.010% or less is used as a material and heated to 800 to 1000 ° C during spring processing and rolled down. Manufacture of high stress springs with a tensile fracture strength of 170 kgf / mm ² or more and excellent delayed fracture resistance, which is characterized by performing hot working at a rate of 10% or more, recrystallization, and then quenching and tempering. Method.