EP3867415A1

EP3867415A1 - Method for thermal treatment of steel wire with associated apparatus

Info

Publication number: EP3867415A1
Application number: EP19783063.1A
Authority: EP
Inventors: Christophe Mesplont; Dennis VANLEENE
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 2018-10-16
Filing date: 2019-10-10
Publication date: 2021-08-25
Also published as: BR112021005116A2; CN112840043A; CN112840043B; EA202191049A1; KR20210076909A; WO2020078829A1

Abstract

A method for thermal treatment of a steel wire comprises the known steps of unwinding a steel wire at a wire velocity, guiding the steel wire through a heating section comprising one or more inducting coils, cooling and winding the steel wire on a carrier. Specific about the method is that a protective gas is injected in the heating section at a flow rate that depends on the wire velocity. The inventors found that the flow rate of the protective gas should decrease with increasing velocity. In the limit no protective gas is injected in the heating section when the line is running at the operative speed. This results in a reduced use of of protective gas. When take-up or pay-off spools need to be exchanged and the line needs to run at a lower velocity, the flow rate of the protective gas is increased. The invention allows to control the type and level of oxides formed on the wire. The invention extends also to an apparatus implementing the method wherein the gas flow of the protective gas is made dependent on the wire velocity through a control system.

Description

Method for thermal treatment of steel wire with associated apparatus

Description

Technical Field

[0001] The invention relates to a method for thermal treatment of steel wire and an associated apparatus to execute such method. The thermal treatment of steel wire is used to change the properties of the steel. Particular steel wires where the use of a thermal treatment is required are for example the stress relieving of bead wire (for coiling beads as used in a tire), the annealing of far drawn low carbon wire before further drawing or the tempering of martensitic wire (high carbon) e.g. for use as spring wire.

Background Art

[0002] In the production of steel wires for various uses such as bead wire or low carbon bare or galvanised wire it is many times necessary to thermally treat drawn steel wire. Due to the drawing of the wire the grains of the steel are elongated and at the same time dislocations and defects are introduced. Both lead to an increased strength of the wire but also to a reduced ductility. This is - amongst others - due to the locking of gliding planes by the dislocations and defects when the steel becomes under stress.

[0003] In some cases it is necessary to trade back some of the strength gain to improve ductility. In the case of a steel wire this may be e.g. to increase elongation-at-break. For example a bead wire - that is a bronze or brass coated steel wire - must show a minimum degree of elongation-at-break (2% or more) to be safely mountable in the bead of a tire. Alternatively a spring wire must be thermally treated in order to control the yield point of the wire that has an impact on the spring properties.

[0004] Generally this trade in of tensile strength for ductility is performed by

means of a thermal treatment of the steel wire. Therewith it is meant that the wire is brought to a temperature that in any case remains below the temperature of the A1 line in the iron carbon phase diagram followed by controlled cooling. Due to this treatment the dislocations and defects diffuse and combine resulting in a still strong but more ductile wire. This is also called‘stress relieving’ or‘recovery’. Increasing the temperature and/or treatment time further will lead to recrystallization where strain free grains combine. This may already lead to too soft structures. If the temperature is further raised for a long enough period further grain growth will occur. However, it is not the purpose of the treatment to come to recrystallization - or even more disliked - to grain growth as then the gain in tensile strength due to cold drawing is lost partly or even completely.

[0005] Another example of a thermal treatment is tempering. During tempering of martensitic quenched wire, carbon will diffuse out of heavily stressed martensite and precipitate in the form of carbides, resulting in a more ductile yet strong microstructure that can be coiled for example as a spring.

[0006] Historically, thermal treatment of steel wires has been done by heating the steel wire in a molten lead bath. The temperature of the molten lead can be easily controlled while the heat transfer from lead to wire is optimal resulting in immediate and stable temperatures i.e. isothermal heating. In addition - and this is a not recognised advantage - the molten lead also insulates the steel wire surface from oxidation. The only oxidation that can occur is at the exit of the lead bath but this is countered by covering the molten lead with anthracite releasing coal gas that is burned thereby consuming the oxygen in the vicinity of the steel wire surface.

[0007] However, lead and coal gas have been identified as having serious impact on human health and environment and are therefore more and more banned in a production environment. One therefore must seek alternatives to heat the wire to the required temperature. Alternatives are in the case of steel wire the use of salt baths, or the use of fluidized sand beds or the use of electrical resistance heating. Salt baths bring operating safety risks with them. The heat transfer in fluidized beds is much less than in a lead bath. When using electrical resistance heating, running electrical contacts with a steel wire may result in sparking leading to martensitic steel spots that are not acceptable.

[0008] In order to overcome those drawbacks wire heating by means of induction has been introduced. A landmark publication of that technique is

US4788394. In this heating method the steel wire is lead through an alternating magnetic field inducing eddy currents that heat the wire. This results in fast, contactless heating of the wire up to the desired

temperature where after the wire is lead through a soaking zone i.e. a long, flat box wherein the wire is insulated from its environment. Although the treatment is not as isothermal as when using a lead bath the temperature can be kept sufficiently stable. In order to prevent oxidation of the wire it must be surrounded by a protective atmosphere which can be done by injecting a protective gas into the heating coil.

[0009] Also WO 2014/142355 describes a wire heating system and a wire heating method comprising one or more induction coils followed by a soaking zone. According the system and method the heating of the wire rod is controlled by adjusting the feed current to the induction coils based on the entry wire speed and diameter. In this way the overheating of the wire is prevented even when the speed of the line is reduced for example for a changeover.

[0010] CN 107227400A describes an induction heating apparatus wherein the individual steel wires pass through individual coils of which the power can be individually controlled. This allows to process different wire diameters concurrently on the same line.

[0011] When using a wire heating method by means of induction, the use of a protective atmosphere becomes mandatory as at the open ends air enters the heating tubes leading to the formation of an iron oxide scale at the surface of the wire. The formation of an iron oxide scale makes

subsequent coating operations such as galvanizing or application of a bronze coating more difficult. A thorough descaling by means of acids or mechanical descaling then becomes necessary which adds to the cost of the product and has an environmental impact. However, maintaining a protective atmosphere also adds to the cost of the product which is to be avoided.

Disclosure of Invention

[0012] A primary objective of the inventors was therefore to reduce the cost and the environmental impact of the thermal treatment of steel wire. More specifically the inventors succeeded in a greatly reducing the use of protective gas without increasing the need for additional pickling. The inventors have found a method to control the formation of an oxide scale.

In addition the inventors have succeeded in completely eliminating the use of a protective gas at least during the period wherein the line is running at its operative speed.

[0013] According a first aspect of the invention, a method for thermal treatment of a steel wire according the steps of claim 1 is presented. The method includes the steps of:

(a) Unwinding a steel wire at a wire velocity. Usually this unwinding is done out of a reel, bobbin or spider. The wire velocity is the linear velocity of the wire by which it is pulled through the installation by the wire take-up;

(b) Guiding the steel wire through a heating section for heating the steel wire to a temperature of between 350°C and 750°C. The heating section comprises one or more induction coils that are placed in series. With‘in series’ is meant that when following a single wire it runs through the coils one after the other. Different wires may run side by side through one or more coils in series. Alternatively, for each wire there may be a single series of one or more coils only containing one wire with the coils being arranged side by side.

Preferably the induction coils are powered at a constant power, making the driving electronics simple and reliable and resulting in a stable and equal heat treatment of all wires.

(c) Thereafter the wire is cooled to ambient temperature for example by letting it cool in ambient air, or in a coolant such as water or oil or a combination thereof.

(d) Finally the wire is wound on a carrier at the take up side.

What is now particular about the method is that during the guiding of the steel wire through the heating section a protective gas is injected in the heating section at a flow rate that depends on the wire velocity, more specifically in that the flow rate is decreased with increasing velocity of the steel wire or in that the flow rate is increased with decreasing velocity.

[0014] This is remarkable as in standard practice the amount of protective gas flow is kept constant irrespective of the wire velocity. Such a decrease in gas flow with increasing wire speed or vice versa an increase in gas flow with decreasing wire speed is counterintuitive in that one would expect a gas flow use proportional to the surface area of the wire passing by.

According the inventors it has certain advantages to modulate the protective gas flow with the wire velocity in the opposite way:

• One advantage is that the modulation of the gas flow in the described way results in a controlled, constant growth of the oxides that can be removed in a controlled, constant way in subsequent steps;

• This is possible even when delivering a constant power to the one or more induction coils. This obviates the necessity to have a complex control electronics of the mid frequent power fed to the coils as a function of the wire velocity;

• Obviously less protective gas is needed in particular when running at high velocities.

[0015] Regarding the step (b):

• the heating temperature for stress relieving of bead wire is best

performed between 350°C and 550°C, more preferably between 380°C and 450°C.

• The recrystallization of low carbon steel wire usually takes place at higher temperatures of 720 to 750°C.

• The invention is equally well useable for the tempering of martensitic high carbon steel wire. Martensitic high carbon steel wire is obtained by rapidly cooling a wire after it has been brought to the austenizing temperature of 930 to 1000°C. Tempering takes place at 360 to 550°C and helps the diffusion of some carbon out of the martensitic lattice thereby forming iron carbide precipitates. By tempering some ductility is restored to the otherwise brittle martensitic steel wire.

[0016] In a first particularly preferred embodiment, the flow of protective gas is decreased with increasing velocity of the steel wire or the flow of protective gas is increased with decreasing velocity of the steel wire in a continuous or stepwise manner. [0017] The decrease of flow rate can be continuous with increasing wire velocity. Mutatis mutandis, the increase of flow rate can be continuous with decreasing wire velocity.

[0018] Examples of continuous dependencies are a gas flow that is inversely proportional to the wire velocity or a gas flow that linearly decreases with increasing wire velocity i.e. the relation between gas flow and wire velocity has a constant negative slope in the transition region.

[0019] Alternatively, the decrease in flow rate can be stepwise such as for

example within a first lower range of velocities the gas flow is kept at a high level and when the velocity enters a higher, second range then as long as the wire has a velocity in that range the protective gas flow is reduced to a lower level compared to the high level. Mutatis mutandis the increase in gas flow can be stepwise when the velocity enters the lower, first range of wire velocity.

[0020] Different combinations are possibly such as:

• a continuous decrease in gas flow with increasing wire velocity and stepwise increase in gas flow with decreasing wire velocity.

• Alternatively a stepwise decrease in gas flow with increasing wire velocity and continuous increase in gas flow with decreasing wire velocity .

[0021] In a further preferred embodiment of the method the power, delivered to the one or more induction coils is kept constant with changing wire velocity.

[0022] In a further preferred embodiment after the heating step (b) the wire is immediately guided through a‘soaking zone’ referred to as step (b’). A soaking zone comprises a thermally insulating enclosure wherein the wire is allowed to cool down in a controlled way, more specifically at a slow rate. In a soaking zone, the diffusion phenomena in the steel continue without the need of adding additional heat. For clarity: when a soaking zone is used, it becomes part of the heating section. It follows that the protective gas must also be injected in the soaking zone.

[0023] In a likewise preferred embodiment after the cooling of the steel wire the wire is coated with a metallic coating which comprises a metal or metal alloy selected from the group comprising copper, zinc, tin, bronze, brass or any combination thereof a step that is called (c’) hereinafter.

[0024] The application of the metallic coating can be done in several ways such as for example:

• By hot dipping, for example by guiding, dipping the steel wire through a molten zinc bath thereby obtaining galvanised steel wire;

• By a chemical exchange reaction by guiding, dipping the steel wire through an electrolyte bath such as for example a copper tin sulphate bath for making bead wire;

• By electrolytic deposition of copper and zinc possibly followed by diffusion to obtain brass;

Combinations of two or more of the above coatings techniques is also possible.

[0025] During the thermal treatment of steel wire operative periods in which the wire velocity is kept constant at an operative velocity will alternate with ‘changeover periods’. In‘changeover periods’ pay-off spools that are near to running empty are exchanged with full spools and/or take-up spools that are close to full are doffed and replaced by empty spools. During the changeover period the velocity of the steel wire is reduced to a reduced velocity that is lower than the operative velocity of the steel wire during the operative period. The wire velocity is kept constant at this reduced velocity during the changeover period. This is necessary in order to give operators sufficient time to exchange spools. During changeover all mechanical and chemical properties are kept on target.

[0026] In line with the invention during the changeover period a protective gas flow is injected into the heating section. This gas flow is decreased i.e. there is a reduced injection of protective gas flow when the velocity is increased to the operative velocity. As the operative period is much longer than the changeover period there is a reduced gas flow for an extended period of time. This results in a large saving of protective gas usage during a prolonged time.

[0027] The inventors suggest a way to estimate the minimum flow amount of protective gas changeover periods:

The cross sectional area of the heating section is the volume of the heating section divided by the length of the heating section. The volume of the heating section is the volume surrounding the wire in the heating section from the entry into the heating section to the exit of the heating section. For clarity: if a soaking section is present, the volume of the soaking section must be taken into account.

[0028] As long as the flow of protective gas is larger than 1 to 10 times the

product of the cross sectional area and the difference between the operative velocity and the reduced velocity (hereinafter the‘Product’) there is no oxidation to be expected. The inventors were able to keep the gas flow below ten times the Product and conjecture that is possible to reduce this further to below eight times or even below five times the Product. If the cross sectional area of the heating section is further reduced by for example introducing ceramic tubing into the coils, the minimum flow amount to prevent oxidation will be further reduced.

[0029] An alternative method to calculate the protective gas flow when the wire is running at the reduced wire velocity is to specify the refreshment rate in the volume taken by the heating section. When the gas flow into the heating section is larger than 1 to 6 times the volume of the heating section per minute, there will be a complete refresh in protective gas every minute or every ten seconds in the complete volume.

[0030] In a particularly preferred embodiment the inventors reduced the gas flow to zero when the wire was running at the operative velocity. This presents the ultimate savings in protective gas as then there is no spillage of gas while still delivering a steel wire with the correct oxides.

[0031] In a further preferred embodiment the volume of the heating section is purged with protective gas at the commencement of a changeover period. With‘purging’ is meant a short blow of gas of at least one to ten times the volume of the heating section to quickly and completely remove all possible oxygen remaining in the heating section.

[0032] In another preferred embodiment of the method the reduced velocity

during changeover is less than 50%, or even less than 60% or less than 75% of the operative velocity. In any case the reduced velocity during changeover is larger than zero or more than 5% or even 10% of the operative velocity. [0033] As a protective gas an inert gas such as argon or nitrogen can be used. Alternatively protective gases are reducing gasses such as hydrogen or carbon monoxide although the latter is generally not considered due to its toxicity. Alternatively mixtures of gasses such as a mixture of nitrogen and hydrogen (e.g. resulting from the cracking of ammoniac) can be

considered as a protective gas. However, by far the safest is to use nitrogen, provided there is enough venting.

[0034] According a preferred embodiment the inventors have found that providing a mixture of reducing gas and inert gas as a protective gas makes it possible to tune to the desired oxide scale formation on the steel wire. Indeed for certain wire applications a controlled presence of certain oxides is desirable. In this respect even an oxidizing gas such as air or pure oxygen can be injected together with an inert gas under controlled circumstances.

[0035] According a second aspect of the invention an apparatus, installation for the thermal treatment of steel wire is presented. The apparatus comprises a heating section with one or more induction coils, a take-up section with an adjustable velocity for pulling said steel wire through said heating section. The apparatus is further provided with a controllable supply of gas for injecting protective gas into the heating section. Special about the apparatus is that the flow rate of said supply of protective gas is dependent on said adjustable velocity in that the flow rate of the supply of the protective gas decreases when the velocity of the steel wire increases and/or wherein the supply of protective gas increases when the velocity of the steel wire decreases.

[0036] In a preferred embodiment of the apparatus the heating section also

comprises a soaking section attached to the one or more induction coils. The wire uninterruptedly goes from the induction coils to the soaking section.

[0037] In a further preferred embodiment the dependency of the supply of

protective gas with the velocity of the steel wire is such that the supply of protective gas decreases stepwise when the velocity of the steel wire increases. Mutatis mutandis the flow of gas increases stepwise when the velocity of the steel wire decreases.. Alternatively the relation between wire velocity and flow of protective gas can be continuous for example inversely proportional to the wire speed or linearly proportional with a negative slope. Combinations of stepwise rise in protective gas flow when entering a lower velocity range with a continuous increase in protective gas flow when the wire velocity is reduced are of course also possible, likewise the combination of a continuous increase in flow rate when lowering the speed and a stepwise reduction of gas flow when the wire speed is increased are equally well preferred.

[0038] In a further preferred embodiment the power delivered to the induction coils can or is kept constant at a fixed level independent of the wire velocity. In other words: the apparatus is free of any feedback loop between wire velocity and power delivered to the induction coils.

[0039] In an exemplary embodiment the apparatus will work at an operative

speed. The operative speed is related to the diameter of the steel wire to be annealed. At the operative speed the flow rate of protective gas is low or zero. When the velocity of the wire is reduced the flow rate of the supply of protective gas is reduced by between 1 and 10 times the product of the cross sectional area of said heating section times the difference between the operative velocity and the actual wire velocity.

[0040] In a further embodiment the apparatus is provided with a gas premix unit.

In the gas premix unit a reducing gas is mixed with an inert gas in pre-set ratios prior to being injected as a protective gas in the heating section. Alternatively the gas premix unit can be used to mix an oxidising gas with an inert gas. An oxidising gas is for example oxygen or air. The premix unit allows to tune the composition and amount of the oxides that form on the steel wire.

Brief Description of Figures in the Drawings

[0041] FIGURE 1 shows a schematic representation of a wire processing line comprising the thermal treatment apparatus according the invention;

[0042] FIGURE 2 shows different operating schemes for the method for thermal treatment of steel wires. [0043] FIGURE 3 shows a block diagram illustrating the method and some alternatives comprised in the method.

Mode(s) for Carrying Out the Invention

[0044] FIGURE 1 shows a schematic representation of a bead wire line wherein the apparatus according the invention is included and operated. Note that some steps and baths such as drying steps and water rinsing are omitted from the drawing as these are known to the skilled person and would only complicate the schematic.

[0045] Pay-off spool 102 delivers steel wire 140 for example a cold drawn, high carbon steel wire with a diameter between 0.70 and 3.00 mm e.g. 0.89 mm, 0.96 mm, 1.30 mm, 1.60 mm, or 1.83 mm. In what follows the terms ‘after’ and‘before’ are relative to the pay-off direction of the wire. Due to the cold drawing the tensile strength of the wire is about 1700 to 2700 N/mm² depending on diameter and required tensile strength level. For example a 0.89 Normal Tensile (NT) bead wire has a minimum tensile strength of 1900 N/mm², while a 0.89 High Tensile (FIT) bead wire as a minimum tensile strength of 2150 N/mm² (ISO 16650). Such a hard drawn wire cannot be used safely in a bead of a tire as it does not show sufficient elongation at break. Therefore the steel wire must be thermally treated.

[0046] In a known step the wire 140 is first cleaned in cleaning section 106 to remove any surface residuals. In a next known step the wire is guided through a heating section 111 where the temperature is raised to 480°C. The heating section 111 consists of two induction coils 108, 108’, sequentially organised. The induction coils are fed with mid-frequent power source 112. Immediately after the induction coils a soaking zone 110 is provided that keeps the wire hot until it leaves the heating section 111. The soaking section is a thermally insulated chamber. The exit temperature is about 400°C. The volume of the heating section is the free space inside the heating section wherein the wire travels. It is equal to the average cross section times the length of the heating section. The volume of the heating section is filled with a protective gas - in this case nitrogen - from a central tank 114 through a manifold of feed lines 130 in order to prevent oxidation of the wire surface.

[0047] After exiting the heating section the wire 140’ is cooled to ambient

temperature by means of a water quench or simply in air (step not shown).

[0048] During the heating of the wire the following oxides of iron may form:

• Ferrous oxide, iron(ll)oxide, FeO, wOstite;

• Ferric oxide, iron(lll) oxide, Fe₂03, hematite;

• lron(ll, III) oxide, Fe30₄, magnetite

In particular the Fe(lll) oxides are difficult to remove by an acid bath 116. When using a protective gas, the formation of oxides and in particular Fe(lll) oxides is prevented. When no protective gas is used oxides will grow making the removal of the oxides much more difficult.

When the protective gas is a mixture of a reducing gas (e.g. hydrogen) with an inert gas (e.g. nitrogen) the ratio of reducing vs inert gas can be used to control the formation of the oxides.

[0049] After removal of the oxides the wire is lead through a plating section 118, comprising e.g. copper sulphate with tin dissolved. By chemical exchange a bronze coating is deposited on the steel wire 140’ resulting in bead wire 140”. After drying in a dry oven an optional green adhesion enhancer may be applied on the steel wire in applicator 120 prior to take up on the take- up spool 104.

[0050] If oxides are present on the wire 140’ the exchange reaction in the plating bath 118 is inhibited resulting in not properly coated bead wire with concomitant lack of adhesion or leading to differences in appearance. As mentioned the continuous supply of a protective gas prevents the formation of hard to remove oxides on the wire. It was therefore a surprise to the inventors that the use of a protective gas can be reduced or even stopped when the line is running at its operative speed. The operative speed is that speed at which the wire upon exiting of the heating section has the desired mechanical properties. It varies with the diameter of the wire and is between 100 and 600 meter per minute. Only when the speed of the line is reduced below the operative speed, an increased use of a protective gas becomes necessary. This goes counter the general opinion that a protective gas is always needed. [0051] A reduction in wire speed is needed for example at the run-out of a pay-off spool or at the changeover of a take up spool. The reduced wire speed is one tenth of the operative speed in order to allow operators to exchange spools in a safe way while guaranteeing the product quality throughout the complete spool.

[0052] In order to control the flow of protective gas as a function of the wire

velocity a control system is added to the installation with a velocity sensor 126 that controls a throttle valve 124 through controller 128. When the wire velocity is reduced e.g. in case of spool runout or change in take-up spool the gas flow is increased. When the wire velocity nears the operative velocity the gas flow supply is decreased or even set to nil.

[0053] Different strategies - as illustrated in FIGURE 2 - may exist for coupling the gas flow of the protective gas to the wire velocity (also called‘line speed). In a first strategy indicated with dash-dot line 206, the gas flow‘F’ (expressed in normal litres of gas per minute) is inversely proportional to the wire velocity‘V’ (in meter per minute): wherein <P_red is the gas flow at the reduced wire velocity V_red . The gas flow at reduced wire velocity is set to:

Wherein‘C is a constant between 0.1 and 1.0, for example 0.2 and‘A’ is the average cross sectional area of the heating section in centimetre squared (cm²).

[0054] In a second strategy, indicated by curve 202 in FIGURE 2, the gas flow decreases linearly with the wire velocity in the manner:

Wherein‘V_op’ is the operational velocity and‘F_or’ is a small‘maintenance’ flow of protective gas maintained during the operative period.‘C is again a value of between 0.1 and 0.5 when the velocity is expressed in meter per minute, the cross sectional area in centimetre square and the gas flow in litres per minute. Hence, when going from operative regime to the changeover period, the gas flow is reduced by an amount that is proportional to the cross sectional area of the heating section times the difference in velocity between operational and changeover wire velocity.

[0055] In a third strategy, indicated with 204 in FIGURE 2, the protective gas flow is maintained at a high level‘O_red’ in a speed range from‘V_red’ to some higher speed e.g. 10% above V_red or‘V_red + D’. Once the wire velocity is higher than the latter speed the gas flow is completely switched off.

[0056] FIGURE 3 illustrates the different alternative paths in the method that can be followed to anneal a steel wire. After unwinding of the steel wire at a wire velocity‘V 300, the wire is heated 305 by guided through a heating section. The heating section either consists out of one or more induction coils 302 (path A) or consists out of one or more induction coils 302 followed by a soaking zone 304 (path B). After the heating section, the wire is cooled 306 to ambient temperature. Thereafter the annealed wire can be directly spooled on a carrier (path D) in a winding step 314 or can be coated with bronze coating consisting of copper and tin (path C) in an electrolytic bath 308 or can be hot dip galvanised (path E) by immersion in a molten zinc bath 310 prior to be being wound on a carrier 314. Specific about the method is that the flow of protective gas depends on the wire velocity more specifically that the flow of protective gas is decreased with increasing velocity. The advantage of the method is that the use of protective gas is greatly reduced resulting in a reduced production cost and an improved environmentally friendly operation.

[0057] Remarkable is also that the modulation of the gas flow allows to work at a constant power level fed to the induction coils. This dispenses the need to have an expensive feedback control loop between the wire velocity and the mid frequent wave generators feeding the induction coils.

Claims

1. Method for thermal treatment of a steel wire comprising the steps of

(a) Unwinding a steel wire at a wire velocity;

(b) Guiding said steel wire through a heating section for heating said steel wire to a temperature of between 350°C and 750°C, wherein said heating section comprises one or more induction coils;

(c) Cooling said steel wire to ambient temperature;

(d) Winding said steel wire on a carrier;

characterized in that

during the guiding of said steel wire through said heating section a protective gas is injected in said heating section with a flow rate that is decreased with increasing velocity of said steel wire or with a flow rate that is increased with decreasing velocity of said steel wire.

2. The method according to claim 1 wherein the flow rate of protective gas is decreased continuously with increasing velocity of said steel wire.

3. The method according to claim 1 or 2 wherein the flow rate of protective gas is increased continuously with decreasing velocity of said steel wire.

4. The method according to claim 1 wherein the flow rate of protective gas is decreased stepwise with increasing velocity of said steel wire.

5. The method according to claim 1 or 4 wherein the flow rate of protective gas is increased stepwise with decreasing velocity of said steel wire.

6. The method according a combination of claims 2 and 5 or a combination of claims 3 and 4.

7. The method according to any one of claims 1 to 6 wherein after the step (b) the step

(b’) immediately guiding said steel wire through a soaking zone, said

soaking zone being part of the heating section;

is introduced.

8. The method according to any one of claims 1 to 7 wherein after step (c) the step

(c’) coating said steel wire with a metallic coating comprising a metal or metal alloy selected from the group consisting of copper, zinc, tin, bronze, brass or any combination thereof;

is introduced.

9. The method according to any one of claims 1 to 8 wherein operative periods during which said steel wire runs at an operative velocity are alternated with changeover periods wherein the velocity of said steel wire is decreased to a reduced velocity wherein

- during said changeover period a protective gas flow is injected in the heating section,

- during said operative period there is a reduced injection of protective gas flow compared to the gas flow during changeover.

10. The method according to claim 9 wherein said heating section has a cross sectional area and wherein the flow of protective gas during said exchange period is larger than said cross sectional area times the difference between said operative velocity and said reduced velocity.

11.The method according to claim 9 or 10 wherein the flow of protective gas

during said changeover period is larger than one time the volume of said heating section per minute.

12. The method according to any one of claims 9 to 11 wherein the flow of

protective gas is zero when said steel wire is running at the operative velocity.

13. The method according to any one of claims 1 to 12 wherein the volume of said heating section is purged with protective gas at the commencement of a changeover period.

14. The method according to any one of claims 9 to 13 wherein said reduced

velocity is less than 75% of said operative velocity.

15. The method according to any one of claims 1 to 14 wherein said protective gas is one out of the group consisting of argon, nitrogen, hydrogen, carbon monoxide or a mixture thereof.

16. An apparatus for the thermal treatment of steel wire comprising a heating

section with one or more induction coils, a take-up section for pulling said steel wire through said heating section, said take-up section having an adjustable velocity, a controllable supply of gas for injecting protective gas into said heating section,

characterized in that

the flow rate of said supply of protective gas decreases when the velocity of the steel wire increases and/or wherein said supply of protective gas increases when the velocity of the steel wire decreases. .

17. The apparatus for heating steel wire according claim 16 wherein said heating section further comprises a soaking section attached to said one or more induction coils.

18. The apparatus for heating steel wire according to any one of claims 16 to 17 wherein said apparatus further comprises a premix unit for mixing an inert gas with a reducing gas or an oxidation gas prior to injection the mixture into said heating section.