EP0586711B1

EP0586711B1 - Steel wire with excellent formability into spring and production thereof

Info

Publication number: EP0586711B1
Application number: EP93906811A
Authority: EP
Inventors: Teruyuki Itami Works Of Sumitomo Murai; Yoshiyuki Itami Works Of Sumitomo Miyamoto; Toshiya Itami Works Of Sumitomo Nakamura
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 1992-03-25
Filing date: 1993-03-22
Publication date: 1996-07-10
Anticipated expiration: 2013-03-22
Also published as: DE69303557T2; WO1993019225A1; DE69303557D1; EP0586711A4; EP0586711A1

Abstract

A steel wire for springs which is excellent in the formability into springs and the prevention of environmental pollution. The wire has a surface coated with an amino acid compound after drawing, quenching or tempering, and the coating serves to improve the lubricity and prevent the seizing and chattering in the step of forming into springs. Although the amino acid compound is thermally decomposed in the stress-relieving annealing step after the spring forming step, it does not emit any harmful gas thereby.

Description

(Technical Field)

The present invention relates to a steel wire which is excellent in the formability into springs and which is environmentally favorable and a method of manufacturing the same.

(Background Art)

If a drawn or hardened and tempered steel for spring was formed into e.g. coil springs as it is, it might seize to coiling pins of a coiling machine or stick slipping might occur between the wire and the pins. This is because of its high surface friction coefficient. The higher the strength of the steel wire or the higher the coiling speed, the more frequently such phenomena occur.
If this happens, the coiling speed will vary, so that it is difficult to form coil springs having a uniform shape. Further, abnormal noise (or chattering) will be produced during coiling, which worsens the working environment.
For this reason, it has been an ordinary practice to apply oil to a spring steel wire of this type before forming it into springs not for rust prevention but for lubrication. But due to recent tendency to use steel wires having higher strength and the attendant increase in the spring forming pressure, it is becoming more and more difficult to lubricate steel wires sufficiently with conventional oils. Another problem with such oils is that they tend to splash all over, polluting the environment.
In order to solve this problem, the present applicant proposed in Unexamined Japanese Patent Publication 3-213735 to form fluororesin coatings on steel wires after hardening and tempering them.
Such a fluororesin coating will not splash and can improve the spring formability. But this coating has one problem in that the resin decomposes and produces a gas containing fluorine when annealing the springs formed, which is an indispensable step to remove strains of the springs. The gas thus produced may react with hydrogen contained in the atmospheric moisture, thus producing hydrogen fluoride gas (HF), which corrodes the springs and is also harmful to human beings. Further, the gas will leak out of the annealing facility and pollute the environment.
In the field of the manufacture of high strength steel wires it is known from Database WPI, Week 9112, Derwent Publications Ltd., London, GB; AN 91-082854 (extract of JP-A-3 027 188) to produce a high strength steel wire by performing the steps of dry-drawing a wire, washing with warm water, passing it through a bath of an aqueous solution of an organic solvent, patenting it and then wet-drawing the wire to receive a wire diameter of less than 0.4 mm, the organic solvent being oxalic acid, tartaric acid, resorcinol, glycine etc. However, this document is not concerned with the manufacture of springs and, especially, does not disclose how the adhesion of the coating to the steel wire can be increased in order to improve its spring formability.
An object of this invention is to provide a steel wire which is free of the above problems and free from seizure and chattering during the forming, which is high in spring formability and thus can be formed into products having uniform shape and dimensions and which never produces any harmful gas during the annealing after the spring forming and to provide a method of manufacturing such a steel wire.

(Disclosure of the Invention)

In order to solve the above problems, according to this invention, a coating of an amino acid compound is formed on the surface of a spring steel wire after drawing or hardening and tempering it, said amino acid compound containing a binder to increase the adhesion of the coating.
The amino acid compounds include amide-bonded compounds of amino acids and fatty acids and its metallic salts. Amino acids are the molecules expressed by R-CH(NH₂)COOH (wherein R is a hydrocarbon residue and include neutral amino acids such as glycine and alanine, acidic amino acids such as asparagic acid and glutamic acid, and basic amino acids such as lysine and hydroxylysine. Fatty acids are the molecules expressed generally by R-COOH and include saturated fatty acids such as palmitic acid and stearic acid and unsaturated fatty acids such as linoleic acid.
The above-listed amino acids and fatty acids are mere examples. Amide-bonded compounds of amino acids and fatty acids are represented by the formula R1 CONH(R2)COOH (wherein R1 and R2 are hydrocarbon residues) and their metallic salts are expressed by the formula R1 CONH(R2)COOM (wherein M is a metal).
Such an amino acid compound should be coated to a steel wire preferably in the amount of between 3 g/m² and 15 g/m². Further, in order to increase the adhesion to a steel wire, the coating contains binders such as an acrylic resin.
The steel wire according to the present invention can be manufactured by applying a solution formed by dispersing a powder of an amino acid compound in an organic solvent or water to a hardened and tempered spring steel wire and drying it naturally or forcibly.
Preferably, the amino acid compound dispersed in the solution should have a particle size of between 0.5 and 30 µm. Such a solution should contain binders for higher adhesion, irrespective of whether the liquid is an organic solvent or water. It may further contain antifoamers, rust preventives or antiseptics.
If water is used, surface active agents should preferably be added to uniformly disperse the amino acid compound.
Further, if water is used, it is preferable to continuously apply the solution to a steel wire while keeping the surface temperature of the steel wire or the temperature of the aqueous solution itself to 60 - 100 °C.
If the solution contains a surface active agent, it is desirable to heat the steel wire until its surface temperature rises to between 100 °C and 200 °C after the solution applied to the wire has become dry.
The amino acid compound used will reveal a lubricity substantially as high as fluororesins. The steel wire having a coating of an amino acid compound will never seize or chatter even if it is strongly rubbed against the coiling pins when it is formed into springs.
Further, since the coating is dried, it will not peel or splash.
The amino acid compound will decompose and produce gases during low-temperature annealing after forming the steel wire into springs. But such gases, composed of C, O, H and N, are harmless both to the end products and human beings.
As described above, the amino acid compound should preferably be applied to the steel wire in the amount of 3 g/m² to 15 g/m². If less than 3 g/m², the adhesion would be so low that the spring formability would be insufficient especially if a high-strength steel wire is used. If over 15 g/m², not only will the material cost be too high but the steel wire will be more likely to slip when wound on a feed roll. This may make it difficult to feed the wire smoothly.
Such a coating should contain a binder because the binder increases the adhesion of the amino acid compound and thus makes the dried coating less likely to peel off even if a high forming pressure is applied thereto. The spring formability thus improves still further.
It is preferable that the amino acid compound powder used in the manufacturing method according to the present invention have a particle diameter of 0.5 - 30 µm. Within this range, the compound can be dispersed uniformly in the solution and thus distributed uniformly in the coating.
If its particle diameter is less than 0.5 µm, the compound power particles tend to be aggregated into large masses instead of being dispersed uniformly. If larger than 30 µm, the powder is more likely to precipitate in the solution. In either case, it is difficult to apply the solution uniformly and the coating formed will not reveal expected effects. An amino acid compound is especially difficult to be dispersed in water because it is usually hydrophobic. Thus, if it is dispersed in water, it is preferable to limit its particle diameter within the above range and add a surface active agent to increase its hydrophilicity.
By uniformly dispersing the amino acid compound in an organic solvent or water, it can be applied uniformly to the steel wire and thus the coating will reveal its expected effects sufficiently.
Especially when applying the aqueous solution, it tends to take a long time to dry it. But this problem can be eliminated by heating beforehand the surface of the steel wire or the solution to a temperature of 60 °C to 100 °C. If less than 60 °C, it will take too long to vaporize water (to dry the solution). If over 100 °C, the solution will boil, so that its water content will splash when applied to the wire surface. This makes uniform coating impossible. When heating the solution, it cannot be heated to a temperature higher than 100 °C because it boils at 100 °C.
Also, as described above, by adding a surface active agent to the solution, the dispersibility of the amino acid compound in water improves. But, after drying, the surface acting agent may remain in the coating. Since it tends to absorb water in the atmosphere, the product is likely to develop rust. An effective way to prevent this problem is to heat the steel wire after drying it, because by heating it to 100 - 200 °C, any residual surface active agent will decompose and be removed. If less than 100 °C, the surface active agent will scarcely decompose. If higher than 200 °C, the amino acid compound will thermally decompose, thus damaging the expected effects of the invention. Such a heating process including the application of the solution may be carried out in the line or off the line after winding the steel wire.

(Brief Description of the Drawings)

Fig. 1 is a graph showing how the amount of decomposition changes with the elapsed time after application, for the coating of the amino acid compound and that of a fluororesin coating; and
Fig. 2 is a graph showing the relation between the amount of decomposition and the temperature for the amino acid compound.

(Best Mode for carrying Out the Invention)

Example 1

A dried coating of an amino acid compound was formed in accordance with the manufacturing method of the present invention, on a silicon-chrome steel oil-tempered wire (SWOSC-V) for valve springs having a diameter of 4.0 mm.
The coating was formed by continuously applying a solution to the surface of the hardened and oil-tempered wire and drying it naturally before taking it up. The solution used was prepared by uniformly dispersing in a low-boiling organic solvent (trichloroethane was used in this Example, containing a binder) 10 vol% of a powder of N-lauroyl-L-lysine, which is a compound of an amino acid and a fatty acid, the powder having a particle diameter of 0.5 - 30 µm. The solution was applied to the wire so that the amino acid compound is present in the solution at the rate of 10 g/m². After allowing the solvent to evaporate, the coated steel wire (Example 1 of the present invention) was wound into a coil.
For comparison purposes, we also prepared two different kinds of spring steel wires using the same oil tempered wire as used in Example 1.
Comparative Material 1 has a fluororesin coating provided on the surface of a hardened and tempered wire. The coating was formed by applying a solution in the form of a fluorine solvent in which is uniformly dispersed 10 vol% of polytetrafluoroethylene (PTFE) powder having a particle diameter of 5 - 10 µm and drying it naturally.
Comparative Material 2 is a steel wire having an oil film formed on its surface by immersing it in a gear oil after hardening and tempering it and then coiling it.
These specimens were formed into 300 springs using a coiling machine. Variations in the free lengths of these spring samples were measured.
The coil springs formed had the following dimensions:

Wire diameter: 4.0 mm
Average coil diameter: 25.0 mm
Free length: 55.0 mm
Number of turns: 6.5

Table 1

Components	C	Si	Mn	Cr
Content (%)	0.55	1.4	0.7	0.7

Table 2

Item	Mean value (mm) of free length	Standard deviation
Specimen
1	55.03	0.080
Comparative material 1	55.03	0.072
Comparative material 2	55.04	0.182

As shown in Table 2, the example according to the present invention showed a very small variation in free lengths comparable to that of the comparative example 1.

Example 2

The coil springs made of the material wire according to the present invention and the coil springs made of the comparative material 1 were subjected to low-temperature annealing (heating temperature: 420 °C, heating time: 30 minutes) to remove strains and the amount of decomposition was measured for the amino acid compound coating (wire according to the present invention) and the fluororesin coating (comparative material 1). The results are shown in Fig. 1.
As is apparent from Fig. 1, the amino acid compound coating according to the present invention decomposed by more than 80% in five minutes and decomposed 100% when ten minutes have passed. The gas produced was composed of C, O, H and N, which are all harmless to human bodies.
In contrast, the fluororesin coating of the comparative material 1 was slow to decompose. Only 50% decomposed in 15 minutes. The gas produced was composed of C, H and F. Fluorine (F) reacts with hydrogen in the atmosphere and produces hydrogen fluoride (HF), which is not only harmful to human bodies but can corrode the products.

Example 3

The same N-lauroyl-L-lysine coating as used for the specimen of the present invention in Example 1 was formed on the surface of a piano wire (SWP-A) having a diameter of 1.2 mm in the same manner as in Example 1 (Specimen 2 of the Invention). Also, we prepared Comparative Material 3 formed by applying the same polytetrafluoroethylene coating as used in Example 1 on the same piano wire as above and Comparative Material 4 formed by forming a gear oil film on the same piano wire as above.
These specimens were formed into 500 springs using a coiling machine. Variations in the free lengths of these samples were measured.
The coil springs formed had the following dimensions:

Wire diameter: 1.2 mm
Average coil diameter: 15.0 mm
Free length: 30.0 mm
Number of turns: 12

Table 3

Item	Mean value (mm) of free length	Standard deviation
Specimen
2	30.02	0.041
Comparative material 3	30.03	0.038
Comparative material 4	30.02	0.124

Example 4

The coil springs formed from the material 2 of the present invention and the ones formed from Comparative Material 3 were subjected to low-temperature annealing under the same conditions as in Example 2 to remove strains. These coatings thermally decomposed in the same way as the coatings of Example 2.

Example 5

A solution containing the same amino acid compound as used for the material of the present invention in Example 1 was applied to the same SWOSC-V wires as mentioned above in the amounts as shown in Table 4 and dried.
Each of the material wires thus obtained was formed into 300 springs having the same dimensions as those of Example 1 using a coiling machine. Variations in free lengths of these springs were measured. The results are shown in Table 4. Table 4

Specimen Amount applied (g/m²) Mean value (mm) of free length Standard deviation

1 2 55.05 0.124

2 4 55.03 0.080

3 8 55.02 0.077

4 1 3 55.04 0.078

5 1 7 55.03 0.081
As will be apparent from this table, Specimen 1, on which the solution is applied in a small amount, showed a rather large variation in free lengths when compared with the other specimens. But its variation is smaller than Comparative Material 2 of Example 1.
Specimen 5 showed a small variation in free lengths. But white powder splashed when coiling it and thus worsened the working environment. Further, slip on feed rolls was observed while coiling.

Experiment 6

An amino acid compound coating was formed on the same SWOSC-V wire 4.0 mm in diameter as used in Example 1 by the second manufacturing method according to the present invention. The solution used was water having uniformly dispersed therein 10 vol% of N-lauroyl-L-lysine powder having a particle diameter of 5 - 30 µm together with an acrylic resin binder and a surface active agent.
In this Example, the hardened and tempered steel wire was immersed in boiled water while moving it continuously to heat its surface to 80 ± 5 °C before winding it into a coil. Then, the above-mentioned solution was applied to its surface in the line and dried naturally to remove its water content, Further, in order to remove the surface active agent in the dried coating, the steel wire was heated to keep its surface temperature at 150 °C for one minute. The wire thus formed was wound into a coil shape.
The thus formed Specimen 4 according to the present invention was formed into 300 coil springs having the same dimensions as the springs formed in Example 1. Variation in free lengths of these springs was measured. The results of measurement are shown in Table 5. For comparison purposes, the data on Specimen 1 of the present invention and Comparative Materials 1 and 2 of Example 1 are also shown. Table 5

Item Mean value (mm) of free length Standard deviation

Specimen 4 55.02 0.081

Specimen 1 55.03 0.080

Comparative material 1 55.03 0.072

Comparative material 2 55.04 0.182
After the measurement, this specimen was subjected to low-temperature annealing at 420 °C for 30 minutes. No harmful gas was produced as with Specimen 1 of the present invention in Example 1. Specimens 1 and 3 of Examples 1 and 3 have disadvantages in that special care is needed in handling the amino acid compound power and that devices for collecting organic solvents have to be provided near the application and drying lines to collect organic solvents because it is not environmentally desirable to leave them uncollected. As for Specimen 4 of the present invention, in which aqueous solution is used, there is no need to exercise special care in handling the solution or to protect the environment.

Example 7

An aqueous solution was applied to steel wires after heating their surfaces to different temperatures and they were checked for how dried. Steel wires used were the same wires as used in Examples 1 and 6, i.e. SWOSC-V wires having a diameter of 4 mm. The aqueous solution used was the same one as used in Example 6, i.e. water having uniformly dispersed therein 10 vol% of N-lauroyl-L-lysine powder having a particle diameter of 5 - 30 µm together with an acrylic resin binder and a surface active agent. The surface temperatures of the steel wires were kept at 50 ± 5, 80 ± 5, 110 ± 5 °C, respectively.
The wire heated to 50 ± 5 °C scarcely showed the effect of preheating and it had to be dried forcibly to shorten the processing time. As for the wire heated to 80 ± 5 °C, the solution dried naturally in several seconds after application and thus for in-line application the preheating was very effective. On the other hand, as for the wire heated to 110 ± 5 °C, the solution applied boiled and scattered on the surface of the steel wire, making uneven the amino acid compound on the wire surface in a speckled pattern. The coating thus formed was unattractive to the eye. Also, due to this uneven coating, spring formability was not stable.

Example 8

Instead of heating wires, the aqueous solution was heated and applied to the wires. It was observed how the coating dried. In this example, the same steel wires and aqueous solution as used in Example 7 were used. The solution applied to the wires was heated to two different temperatures, i.e. 50 °C and 80 °C.
The solution heated to 50 °C scarcely showed the effect of preheating and it had to be dried forcibly when applied in the line. The solution heated to 80 °C dried naturally in seconds and it was not necessary to dry it forcibly when applied in the line. The solution could not be heated above 100 °C because it boiled at 100 °C.

Example 9

Hardened and tempered steel wires SWOSC-V having a diameter of 4.0 mm were immersed in boiled water while moving them continuously to heat their surfaces to 80 ± 5 °C. Then, the same solution as used in Example 6 was applied to the wire surfaces and dried to form a coating of lysine laurate ester containing an acrylic resin binder and a surface active agent. One of the wires thus formed was wound into a coil shape as it is (wire ① ). Another one was wound after heating it at 150 °C for one minute (wire ②). The other one was wound after heating it at 250 °C for one minute (wire ③).
The wires ①, ② and ③ were subjected to an exposure test wherein they were exposed to an atmospheric environment kept at 20 °C with the humidity at 80% to check whether rust develops on the wires.
In the test, the wire ① was the first to develop rust, followed by wired ③. The wire ② was the last to develop rust. In other words, the wire heated to 150 °C had the highest corrosion resistance. As for the wire ③, which was heated to 250 °C, no improvement in spring formability, which is the object of this invention, was observed, because the amount of the amino acid compound decreased due to thermal decomposition.
Fig. 2 shows how the amount of the amino acid compound decreases due to thermal decomposition. As will be apparent from this graph, the amino acid compound is stable at temperature of 200 °C or lower. At a temperature higher than 200 °C, its weight begins to decrease and at around 230 °C, decomposition rate increases sharply.

(Industrial Application)

As described above, according to this invention, the spring steel wire has a coating of an amino acid compound. This coating shows high surface lubricating properties comparable to a fluororesin coating and thus can effectively avoid seizure and chattering when forming springs. Further, an amino acid compound never produces any harmful gas that can corrode the products when they are subjected to low-temperature annealing to remove strain. According to the present invention, therefore, high-quality springs which are small in variations in shape and dimensions can be manufactured without polluting the environment.
Also, according to the manufacturing method of this invention, the amino acid compound coating can be formed uniformly in the line. Thus, the spring formability and productivity improve. The second method in which water is used as a solvent eliminates the necessity of providing any facility for collecting organic solvents and permits carefree handling of the solution. This method is especially favorable from the viewpoint of environmental protection.

Claims

A steel wire excellent in spring formability, having a coating comprising an amino acid compound formed on the surface thereof, wherein said coating is formed after drawing or hardening and tempering the steel wire and said amino acid compound forming said coating contains a binder to increase the adhesion of said coating to the steel wire.
A steel wire as claimed in claim 1, wherein said coating is provided in the amount of between 3 to 15 g/m².
A method of manufacturing a steel wire excellent in spring formability, wherein said method comprises the steps of hardening and tempering a spring steel wire, applying to the surface of the steel wire a solution comprising an organic or aqueous solvent and a powder of amino acid compound having a particle diameter of 0.5 µm to 30 µm and dispersed in said solvent, and drying it naturally or forcibly.
A method of manufacturing a steel wire as claimed in claim 3, wherein said solution contains a binder to increase the adhesion of the amino acid compound to the steel wire.
A method of manufacturing a steel wire as claimed in claim 3 or 4, wherein said solution is an aqueous solution containing asurface active agent for improving the uniformity of dispersion of the amino acid compound in water.
A method as claimed in any of claims 3 - 5, wherein, after heating said steel wire until its surface temperature reaches between 60°C and 100°C, said aqueous solution is applied continuously to the surface of the heated steel wire.
A method as claimed in any of claims 3 - 5, wherein said aqueous solution is applied continuously to the surface of said steel wire after heating said aqueous solution to a temperature between 60°C and 100°C.
A method as claimed in any of claims 3 - 7, wherein, after drying said steel wire naturally or forcibly, said steel wire is heated until its surface temperature reaches between 100°C and 200°C.