JPH04247824A - Manufacture of high strength spring - Google Patents

Abstract

PURPOSE:To manufacture a spring having high fatigue strength corresponding to the conversion of the stress and strength of an automobile valve spring, a suspension spring or the like into high one. CONSTITUTION:A quenched and tempered steel wire contg. 0.55 to 0.75% C, 1.00 to 2.50 Si and 0.30 to 1.50% Mn, two or three kinds among 1.00 to 4.00% Ni, 0.50 to 2.50% Cr and 0.10 to 1.00% Mo, one or two kinds of 0.05 to 0.60% V and 0.05 to 0.60% Nb and the balance Fe and having 200kgf/mm tensile strength is heated to 100 to 550 deg.C and 15 formed into a spring. By this invention, a spring having high fatigue strength and a long service life which have not been obtd. in conventional one can be manufactured, and the residual stress of the spring after forming is extremely low, low temp. annealing after the forming ordinary required can be obviated.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing high-strength springs with excellent fatigue strength, which are used for automobile engine valve springs, automobile suspension springs, and the like.

0002

[Prior Art] Some steel wires for valve springs or automobile suspension springs conventionally used in automobile engines, etc.
JISG3561, JISG3565, JISG356
6. So-called oil-tempered wire specified by JIS G3567, JIS G3568, etc. is cold-molded into a spring and used. By the way, in recent years there has been an extremely high demand for higher output automobile engines and lighter vehicle bodies, and in order to meet these demands, there is a strong demand for springs with high fatigue strength, but these cannot be achieved using existing materials specified by JIS. It is becoming difficult to meet this demand. In order to meet this demand for improved fatigue strength, spring steel with an increased amount of alloying elements has been proposed as a material (for example, JP-A-59-17
7351, JP-A-62-107044, JP-A-62-1
77152, Japanese Unexamined Patent Publication No. 2-107746). Also,
In terms of spring manufacturing, it has become common practice to harden the surface by nitriding, shot peening, etc., and to apply appropriate compressive residual stress.

0003

[Problems to be Solved by the Invention] In order to improve the fatigue strength of the springs mentioned above, one effective means is to increase the strength or hardness of the steel materials that make up the springs.
Even if quenched and tempered steel wire (oil tempered wire) with high tensile strength is manufactured using the already proposed high-strength spring steel, the cold workability of the steel wire is low. It could not be processed into actual springs through inter-forming, and there was a limit to the longevity of valve springs, suspension springs, etc. The object of the present invention is to provide a method for manufacturing a high-strength spring from a high-strength hardened and tempered steel wire.

0004

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present inventors have conducted various experiments and found that the workability of high-strength hardened and tempered steel wire is 100°C to 550°C. The inventors completed the invention by discovering that the improvement was significantly improved in the temperature range of . That is, the present invention includes (1) weight % C: 0.55-0.75%, Si: 1.00-2.50%, Mn: 0.30-1.50%, and Ni: 1.00-1.00%. 4.00%, Cr: 0.50-2.50%, Mo: 0.10-1.00% and two or three of them and V: 0.05-0.60%, Nb: 0.05 A hardened and tempered steel wire containing one or two of ~0.60% and the remainder consisting of unavoidable impurities and Fe and having a tensile strength of 200 Kgf/mm2 or more is heated at a temperature of 100°C to 550°C. (2) After forming into a spring by the method described in item (1) above, producing a "spring" without performing low-temperature annealing. The gist of this paper is a method for manufacturing high-strength springs.

[0005] Below, the results of experiments conducted by the present inventors will be shown, and the contents of the present invention will be explained in detail. Oil tempered wires having a diameter of 4.0 mm and a tensile strength of 220 Kgf/mm2 having the chemical components shown in Table 1 were manufactured, and tensile tests were conducted at various temperatures.

[0006]

[Table 1]

FIG. 3 shows the reduction of area and tensile strength of the test piece at that time. FIG. 4 shows the results of measuring the hardness of the test pieces after being stretched at the various temperatures shown in FIG. From Figure 3, it can be seen that as the temperature of tensile processing increases, the ductility improves, and the reduction of area increases, and when the temperature exceeds 100°C, the reduction of area becomes more than 40%, indicating that the workability of the material is extremely improved. . It can also be seen that the tensile strength, which is considered to be a measure of the power required for processing at that time, decreases when the processing temperature is 300°C or higher. Furthermore, according to Figure 4, the strength (hardness) of the material after processing at such elevated temperatures
maintains its initial hardness until the processing temperature is around 550℃,
Furthermore, it can be seen that it becomes rather hard in the intermediate temperature range.

[0008] Figure 3 shows the observation of the fracture state during the tensile test, and the presence or absence of abnormal fracture phenomena originating from minute surface flaws.
○ indicates no abnormal rupture, ● indicates abnormal rupture), and in this experiment, this abnormal rupture did not occur at temperatures above 150°C. Note that the data in FIG. 3 excludes values when this abnormal breakage occurs. This result shows that the effect of fine flaws on breakage can be reduced by processing at elevated temperatures. Additionally, tensile residual stress exists on the inside of springs manufactured using the normal cold forming method, and while low-temperature annealing is usually performed, which has a negative effect on fatigue properties, this stress does not exist in the springs formed using the method of the present invention. The residual stress caused by this process is extremely low, and another feature is that there is no need to perform low-temperature annealing.

[0009] From the above, the tensile strength at room temperature is 200K.
When processing high-strength steel wires exceeding gf/mm2, increasing the processing temperature reduces cracking susceptibility to minute flaws, improves material ductility, and reduces processing power. It became clear that the later strength was greater than the initial strength.
Furthermore, it has also become clear that the normally performed low-temperature annealing after molding can be omitted.

0010

[Operation] The reason for limiting the scope of each component of the present invention and the operation will be explained below. C is an element that affects the strength (hardness) of quenched and tempered steel wire, and if it is less than 0.55%, the required strength cannot be obtained, so it must be avoided.
Since there is no further advantage in terms of strength if the content exceeds 75%, the upper limit was set at 0.75%. Si is an element that is necessary to reduce "settling", which is important in the characteristics of springs, and is
．． If it is less than 00%, the "sag" is too large and it cannot be used as a high-strength valve spring or suspension spring, so it must be avoided. On the other hand, if it is added in an amount exceeding 2.50%, not only no further effect can be obtained, but also production difficulties such as suppression of decarburization increase, so it must be avoided. Mn is an element that deoxidizes and provides hardenability to steel materials, and if it is less than 0.30%, its effect is insufficient, and if it is added in excess of 1.50%, no further effect can be obtained. Therefore, the upper limit is 1
．． It was set at 50%. Ni, Cr, and Mo improve hardenability,
Alternatively, it is an element that improves the strength and toughness of the spring by increasing temper softening resistance or precipitating fine carbides, and it is necessary to add two or three of these elements in combination. Therefore, Ni is 1.00 to 4
．． It is necessary to add 0.00%, but if it is less than 1.00%, no effect will be obtained, and if it is added in excess of 4.00%, no further effect will be obtained. Cr is 0.50
% or more is necessary, and if it exceeds 2.50%, the "settling" property deteriorates, so it must be avoided. Mo has the effect of increasing temper softening resistance and adding strength and toughness to the spring by precipitating fine carbides, so Mo is added in an amount of 0.10 to 1.00%, but less than 0.10%. Therefore, the effect is not recognized and the effect is saturated even if it exceeds 1.00%, so it is excluded. V and Nb are elements added to improve strength and set property by refining crystal grains and precipitation hardening, and are added in amounts of 0.05% or more.
One type or a combination of two types are added within a range of 60% or less, but if each component is less than 0.05%, there is no effect.
Even if it is added in excess of 0.60%, the effect is saturated.

Furthermore, since the present invention is directed to a method of manufacturing a spring having high fatigue strength, the present invention has a tensile strength of 20
Hardened and tempered steel wires with a strength of 0 Kgf/mm2 or more are targeted, and cases with a tensile strength of less than 200 Kgf/mm2 are excluded. The quenched and tempered steel wires in question are not necessarily limited to so-called oil-tempered wires manufactured by quenching in oil at high temperatures (austenite range), but also high-temperature steel wires obtained by air quenching, etc. It also includes high-strength steel wire or quenched and tempered steel wire obtained by high-frequency heating. A feature of the present invention is that when forming this hardened and tempered steel wire into a spring, it is processed in a so-called warm manner. The processing temperature is
If the temperature is less than 100°C, the steel wire has low ductility and is highly susceptible to flaws, so it cannot be formed into a spring. On the other hand, 55
In the case of temperature conditions exceeding 0°C, the strength of the obtained spring will decrease, making it impossible to manufacture high-strength springs, and oxidation will become severe and the surface condition will deteriorate, so countermeasures will need to be taken, so it is excluded. .

0012

[Examples] Examples of the present invention are shown below. (Example 1) A diameter 4 wire with a tensile strength of 220 Kgf/mm2 was made from a steel wire having the chemical composition shown in Table 2.
．． A 0 mm oil-tempered wire was manufactured and molded into an automobile valve spring having the specifications shown in Table 3 at various temperatures.

[0013]

[Table 2]

[0014]

[Table 3]

FIG. 1 shows the relationship between the processing temperature and the breakage rate at that time. As a result, it is clear that since breakage occurs at a high rate at room temperature, the breakage rate decreases by increasing the coiling temperature and becomes zero at 150° C. or higher.

(Example 2) A quenched and tempered steel wire having a tensile strength of 220 Kgf/mm2 and a diameter of 3.2 mm was manufactured from a steel wire having the chemical composition shown in Table 4, and the various properties shown in Table 5 were produced. The molded parts were molded into automotive valve springs at various temperatures.

[0017]

[Table 4]

[0018]

[Table 5]

FIG. 2 shows the relationship between the processing temperature of the spring and the breakage rate. As a result, it is clear that breakage occurred at a high rate at room temperature, but workability improved by increasing the coiling temperature, and the breakage rate became zero at temperatures above 150°C.

(Example 3 and Comparative Example) A tensile strength of 210 Kgf/m was obtained from a steel wire having the chemical composition shown in Table 4 above.
An embodiment of the present invention in which a hardened and tempered steel wire with a diameter of 3.2 mm and a strength of , a comparative example in which molding was performed at room temperature (25° C.). For those valve springs, the maximum stress τmax = 1
A fatigue test was conducted 5 x 107 times at 15Kgf/mm2. The results are shown in Table 6.

[0021]

[Table 6]

As shown in Table 6, it can be seen that the spring according to the present invention has good fatigue properties even if low-temperature annealing after forming the spring is omitted.

(Example 4) A steel wire having a tensile strength of 225Kgf/mm2 and having a diameter of 3mm was made from a steel wire having the composition shown in Table 7.
A 2 mm hardened and tempered steel wire was produced and tested by winding it around a core metal having a diameter of 3.2 mm (self-diameter winding test).

[0024]

[Table 7]

As a result, it was confirmed that when the winding temperature was 150° C. or higher, there was no breakage, and the material had sufficient ductility to be processed into a spring.

(Example 5) A steel wire having a tensile strength of 215 Kgf/mm2 and having a diameter of 3 mm was made from a steel wire having the composition shown in Table 8.
A 4 mm hardened and tempered steel wire was manufactured and wound around a core metal having a diameter of 3.4 mm.

[0027]

[Table 8]

As a result, it was confirmed that when the winding temperature was 150° C. or higher, there was no breakage and the material had sufficient ductility to be processed into a spring. Furthermore, although the above embodiments have all been described with respect to valve springs, it has been confirmed that suspension springs also have similar effects.

[0029]

As described above, according to the present invention, a quenched and tempered steel wire having a tensile strength of 200 Kgf/mm2 or more can be stably formed into a "spring", which has not been possible in the past. It is possible to produce high-strength springs with high fatigue strength and long life, which has great industrial effects.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the relationship between spring forming temperature and breakage rate in Example 1.

FIG. 2 is a chart showing the relationship between spring forming temperature and breakage rate in Example 2.

FIG. 3 is a chart showing the results of a tensile test at various temperatures of a quenched and tempered steel wire with a tensile strength of 220 Kgf/mm2.

4 is a chart showing the hardness of the test piece after the tensile test shown in FIG. 3. FIG.

Claims (2)

[Claims] Claim 1: C: 0.55-0.75%, Si: 1.00-2.50%, Mn: 0.30-1.50%, Ni: 1.00-4. 00%, Cr: 0.50 to 2.50%, Mo: 0.10 to 1.00%, and two or three of them, V: 0.05 to 0.60%, Nb: 0.05 to A quenched and tempered steel wire containing one or two of 0.60% and the remainder consisting of unavoidable impurities and Fe and having a tensile strength of 200 Kgf/mm2 or more is heated to a temperature of 100°C or more and 550°C or less. A method for manufacturing a high-strength spring, characterized by forming the spring into a spring. 2. A method for manufacturing a high-strength spring, which is formed into a spring shape by the method according to claim 1 and then made into a "spring" without performing low-temperature annealing.