Field of the invention.

The present invention relates to a spring wire of a hard drawn pearlitic steel.

Background of the invention.

Chromium-silicon springs are conveniently made from oil-hardened steel wires. Such steel wires are heated above the austenitizing temperature to a temperature ranging from 850 to 950 °C, and subsequently quenched in an oil bath so that a martensitic structure is obtained. The oil-hardened martensitic steel wires are finally subjected to a tempering treatment, e.g. in a lead bath at a temperature ranging from 350 to 550 °C. The duration and temperature of this tempering treatment determine the ultimate hardness and the final tensile strength of the steel wires.

Starting from the above oil-hardened steel wires, springs are manufactured as follows : the steel wires are coiled, stress-relieved at about 400 to 450 °C, subjected to a shot-peening treatment in order to create compressive stresses at the surface and the coiled shot-peened springs are again stress-relieved at about 250 °C.

Coating oil-hardened steel wires with zinc has disadvantages.

Applying a zinc coating in an electrolytic way does not change the mechanical properties of the underlying steel but leads to a zinc coating with a limited adhesion.

Applying a zinc coating by means of a conventional hot dip operation occurs at a temperature of about 470 °C (melting temperature of zinc is about 420 °C). This galvanizing temperature of 470 °C, however, is too high for some steel compositions, since it would weaken too much the mechanical properties (tensile strength, hardness) of these steel compositions.

A patenting treatment instead of oil-hardening (i.e. austenitizing in the range of 850 to 950 °C and transforming from austenite to pearlite/ferrite

at about 550 to 620 °C, e.g. in a lead bath or in a fluidized bed installation) followed by galvanizing and hard drawing may lead to pearlitic steel wires with acceptable mechanical properties, but this technique, although well known in other related fields, has not been applied to spring wires because of the relatively poor coilability of galvanized spring wires.

Very often springs have to operate under high temperatures in the neighborhood of 80 to 100 °C. Apart from stainless steel compositions, some other low-alloyed steel compositions show a high resistance against heat, i.e. have a low relaxation and a high form stability. Such steel compositions are e.g. chromium-silicon steels and may be used for heat-resistant spring wires. Such spring wires are conveniently made according to the above-cited oil-hardening technique. Patenting these steel wires in order to hard draw and galvanize them is not applied, not only because of the above-mentioned resulting poor coilability but also because the presence of silicon in the steel composition leads to an uncontrolled growth of a brittle layer of an iron- zinc alloy during galvanizing.

Summary of the invention.

It is an object of the present invention to avoid the drawbacks of the prior art. It is an object of the present invention to provide for a heat-resistant spring wire.

It is a further object of the present invention to provide for a spring wire with an improved coilability.

According to the invention, there is provided a spring wire in a hard drawn pearlitic state covered with a zinc alloy coating. The steel wire has a carbon content between 0.45 and 0.75 %

(preferably between 0.55 and 0.75 %, e.g. 0.55 %, 0.65 %, or 0.70 %), a silicon content between 1.00 and 1.60 %, and a chromium content between 0.50 and 1.50 % (preferably between 1.00 and 1.50 %). The steel wire may optionally have a vanadium content ranging from 0.0 to 0.35 %.

The zinc alloy coating has an aluminium content between 2 and 12 %, preferably, the metallic fraction of the zinc alloy coating has an aluminium content between 4 and 6.5 %. The zinc alloy coating further has a wetting agent in an amount less than 0.1 % of the zinc alloy.

With respect to the metallurgic structure, the terms "a hard drawn pearlitic structure" refer to a steel wire that is cold drawn by means of e.g. drawing dies and which has a structure of pearlite and/or ferrite. The higher the carbon content, the higher the content of pearlite in comparison with ferrite.

With respect to the steel composition, carbon contents lower than 0.45 % do not longer suffice to obtain the required final tensile strength of the wire at its final diameter. Carbon contents higher than 0.75 % - in combination with the chromium and silicon contents - make the final hard drawing operation difficult to perform without a substantial increase in the level of fractures.

The chromium and silicon contents are chosen in order to obtain a spring wire with a sufficient heat-resistance. Below the mentioned ranges of chromium and silicon, heat-resistance becomes insufficient, above the mentioned ranges of chromium and silicon, the final hard drawing operation may become difficult due to a decrease in ductility and due to the fact that the time necessary for the transformation from austenite to pearlite during patenting may become unduly large. The manganese content ranges from 0.30 to 1.20 %, preferably from

0.50 to 0.90 %. The phosphorous and sulfur contents are preferably kept as small as

possible, e.g. each below 0.035 %, e.g. each below 0.025 % or e.g. the sum of phoshorous and sulfur below 0.025 %. Other elements such as copper are also preferably kept to the level of unavoidable impurities. All mentioned percentages are percentages by weight of the overall steel composition.

With respect to the zinc alloy coating, the aluminium content ranges from 2 to 12 %, e.g. in the metallic fraction of the zinc alloy coating, the aluminium content ranges from 4 to 6.5 %, and is preferably about the eutectic value of 5 %.

A wetting agent is present in an amount sufficient to have wetting of the substrate steel by the alloy. Amounts smaller than 0.1 % are usually sufficient. The wetting agent can be cerium in an amount ranging from 0.01 % to 0.05 % and/or lanthanum in an amount ranging from 0.01 % and 0.06 %. All mentioned percentages are here percentages by weight of the zinc alloy coating.

The inventors have discovered that, in great contrast with a convenient galvanizing operation, the operation with zinc aluminium alloy leads to improved coilability. Indeed, in a convenient galvanizing operation, uncontrolled growth of the iron-zinc reaction layer due to the presence of silicon limits coilability, while the reaction layer in the zinc aluminium coating shows improved adhesion and deformability and therefore enhances the coilability of the wire.

An explanation can be found in the phenomenon that the aluminium present in the zinc-aluminium alloy probably forms very quickly a thin reaction with the iron of the substrate steel and with the zinc during the coating operation and prohibits any further initiation or growth of other reaction layers. in other words, the chromium silicon steel coated with the zinc aluminium alloy only has a thin aluminium iron reaction layer of a rather

uniform thickness around the steel. No other substantial layer of brittle zinc iron reaction layer is present. This aluminium iron reaction layer provides a good adherence between the zinc-aluminium coating, on the one hand, and the substrate steel, on the other hand, so that the zinc- aluminium strongly sticks to the steel wire during coiling. Moreover, the aluminium zinc coating has a good deformability and a low friction coefficient so that hard drawn spring wires coated with the aluminium zinc coating exhibit an improved coilability.

Description of a preferred embodiment of the invention.

A spring wire according to the present invention is made as follows.

A wire rod with following steel composition is chosen :

%C = 0.65 %Mn = 0.70

%Si = 1.20

%Cr = 1.10

%S and %P smaller than 0.020, the balance being iron and unavoidable impurities. The wire rod is hard drawn to an intermediate diameter of about

4.60 mm and is patented at this intermediate diameter. Subsequently the wire is coated with a zinc aluminium coating with following composition : %AI = 5 %Ce about 0.03

%La about 0.03 the balance being zinc and unavoidable impurities. The thus coated wire is hard drawn to a final diameter of 1.40 mm. At this final diameter the spring wire still has a zinc aluminium coating in a thickness ranging from 50 to 100 g/m ² .

Following mechanical properties have been determined on this spring wire as drawn :

- tensile strength : R _m = 2299 MPa

- yield strength at 0.2 % permanent elongation : R _p0 . ₂ = 2063 MPa

- permanent elongation at maximum load : A _g = 0.86 %

- percentage elongation after fracture : A = 0.92 %.

Following test shows the improved coilability properties of an invention spring wire.

An invention spring wire, i.e. a chromium-silicon spring wire with an aluminium-zinc coating has been compared with a stainless steel wire and with a galvanized steel wire.

For each type of wire 500 compression springs have been coiled.

These compression springs have following geometry :

- average spring diameter : D _m = 5.9 mm ;

- number of active coils : n _a = 8

- total number of coils : n _t = 10

- free spring wire length : L ₀ = 25.6 mm

- wire length per spring : t- 185 mm

The speed of coiling has been set to 80 springs per minute and the standard deviation σ on the free wire length has been used as the parameter for coilability. The table hereunder summarizes these results.

Table ;

So an invention spring wire shows the same level of coilability as a non- coated stainless steel wire and an improved level of coilability in

comparison with a galvanized steel wire. In addition thereto, no flaking occurred with an invention spring wire.

The manufactured invention spring wire combines the advantages of heat-resistant spring steel wires with the advantages of metal-coated spring steel wires.