CN112840043A

CN112840043A - Method for heat treating steel wire and related apparatus

Info

Publication number: CN112840043A
Application number: CN201980067576.0A
Authority: CN
Inventors: C·梅斯普隆; D·范利内
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 2018-10-16
Filing date: 2019-10-10
Publication date: 2021-05-25
Anticipated expiration: 2039-10-10
Also published as: EA202191049A1; EP3867415A1; WO2020078829A1; BR112021005116A2; CN112840043B; KR20210076909A

Abstract

A method for heat treating steel wires, comprising the following known steps: the steel wire is unwound at a wire speed, guided through a heating section comprising one or more induction coils, cooled and wound on a carrier. The particularity with respect to this method is that the protective gas is injected into the heating section at a flow rate that depends on the speed of the filaments. The inventors have found that the flow rate of the shielding gas should decrease with increasing velocity. In the limit, no shielding gas is injected into the heating section when the processing line is operating at operating speed. This results in a reduction in the use of shielding gas. When the take-up spool or the pay-off spool needs to be replaced and the processing line needs to run at a lower speed, the flow rate of the shielding gas can be increased. The invention allows control of the type and level of oxides formed on the filament. The invention also extends to an apparatus for carrying out the method wherein the gas flow of the shielding gas is made dependent on the speed of the filament through the control system.

Description

Method for heat treating steel wire and related apparatus

Technical Field

The present invention relates to a method for heat treating steel wires and to the related apparatus for carrying out the method. Heat treatment of steel wire is used to modify the properties of the steel. Special steel wires that require the use of heat treatment are for example stress relief of bead wires (for coiling beads used in tires), annealing of far drawn low carbon steel wires before further drawing, or tempering of martensitic steel wires (high carbon) for example used as spring wires.

Background

In the production of steel wires for various purposes, such as bead wires or low carbon bare wires or galvanized steel wires, it is often necessary to heat-treat the drawn steel wires. Due to the drawing of the wire, the grains of the steel are elongated and dislocations and defects are introduced at the same time. Both result in increased strength of the filament, but reduced ductility. This is due in particular to the sliding surface locking caused by dislocations and defects when the steel is under stress.

In some cases, it is necessary to forgo some strength gains to improve ductility. In the case of steel wire, this may, for example, increase the elongation at break. For example, bead wires (i.e., bronze or brass coated wires) must exhibit the lowest degree of elongation at break (2% and above) that can be safely installed in the bead of a tire. Instead, the spring wire must be heat treated to control the yield point of the wire, which has an effect on the spring performance.

Generally, this compromise between tensile strength and ductility is achieved by heat treatment of the steel wire. This means, therefore, that the temperature of the wire is in any case kept below the temperature of line a1 in the iron-carbon phase diagram, followed by controlled cooling. As a result of this treatment, dislocations and defects diffuse and combine, resulting in a still strong but more ductile filament. This is also referred to as "stress relief" or "recovery". An increase in temperature and/or processing time will further result in recrystallization, wherein unstrained grains are bonded. This already may result in a structure that is too soft. If the temperature is further increased for a sufficient time, grain growth will continue to occur. However, the purpose of this treatment is not to recrystallize (or even to the contrary) the grains to grow, since then the tensile strength of the grains has been partly or even totally lost due to cold drawing.

Another example of heat treatment is tempering. During tempering of the martensitic quenched wire, carbon will diffuse out of the high-stress martensite and precipitate out in the form of carbides, resulting in a more ductile but still strong microstructure, which can be coiled into a spring, for example.

Historically, heat treatment of steel wire has been performed 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 the lead to the steel wire is optimized, resulting in an instant and stable temperature, i.e. isothermal heating. In addition (and not a recognized advantage), the molten lead also insulates the wire surface from oxidation. The only possibility for oxidation is at the outlet of the lead bath, but this is counteracted by covering the molten lead with anthracite coal, which releases gas, which burns and thereby consumes oxygen near the surface of the steel wire.

However, lead and coal gases have been identified as having a serious impact on human health and the environment, and are therefore increasingly banned from use in production environments. Therefore, an alternative must be sought to heat the filaments to the desired temperature. In the case of steel wire, an alternative is to use a salt bath, or to use a fluidized sand bed or to use resistance heating. Salt baths pose operational safety risks. The heat transfer in the fluidized bed is much less than in the lead bath. When using resistance heating, electrical contact with the steel wire in travel may produce sparks, resulting in unacceptable martensitic steel spots.

To overcome these drawbacks, methods by means of induction heating wires have been introduced. A symbolic publication of this technology is US 4788394. In this heating method, the steel wire is guided through an alternating magnetic field, which induces eddy currents that heat the steel wire. This allows the filament to be heated to the desired temperature quickly and contactlessly after it is guided through a soaking zone (i.e., a flat long box), wherein the filament is insulated from its environment. Although this process is not as isothermal as when a lead bath is used, the temperature can remain sufficiently stable. In order to prevent the wire from being oxidized, the wire must be surrounded with a protective atmosphere, which can be achieved by injecting a protective gas into the heating coil.

WO2014/142355 also describes a wire heating system and a wire heating method comprising one or more induction coils followed by a soaking zone. According to the system and method, heating of the wire rod is controlled by adjusting the feed current to the induction coil based on the wire entry speed and diameter. In this way, overheating of the wire is prevented even when the speed of the processing line is reduced (for example for changeover).

CN107227400A describes an induction heating device in which a single steel wire is passed through a single coil whose power can be controlled individually. This allows different wire diameters to be processed simultaneously on the same processing line.

When using the method by induction heating of the filaments, the use of a protective atmosphere becomes mandatory, as air enters the heating tube at the open end, causing the formation of scale on the surface of the filaments. The formation of scale makes subsequent coating operations (such as galvanization or the application of bronze coatings) more difficult. At this time, thorough or mechanical descaling by means of acid becomes necessary, which increases the cost of the product and has an environmental impact. However, maintaining a protective atmosphere also increases the cost of the product to be avoided.

Disclosure of Invention

Therefore, the main object of the present inventors is to reduce the cost and environmental impact of heat treatment of steel wire. More specifically, the inventors succeeded in significantly reducing the use of shielding gas without increasing the need for additional pickling. The present inventors have discovered a method of controlling scale formation. Furthermore, the inventors have succeeded in completely eliminating the use of shielding gas at least during the travel of the processing line at its operating speed.

According to a first aspect of the present invention, a method for heat treating steel wire is proposed according to the steps of claim 1. The method comprises the following steps:

(a) the steel wire is unwound at wire speed. Such unwinding is typically accomplished with a spool, spool or cross. The wire speed is the linear speed of the wire at which the wire is drawn through the apparatus by the take-up;

(b) the steel wire is led through a heating section to heat the steel wire to a temperature between 350 ℃ and 750 ℃. The heating section comprises one or more induction coils placed in series. Where "in series" means that the wire will travel through one coil after another as it follows a single wire. Different filaments may travel side by side through one or more coils in series. Alternatively, for each filament there may be a single string of one or more coils containing only one filament, with the coils arranged side by side. Preferably, the induction coil is powered with constant power, so that the drive electronics are simple and reliable, and so that all filaments are heat treated stably and equally.

(c) Thereafter, the wire is cooled to ambient temperature, for example by cooling the wire 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 on the take-up side.

It is now peculiar to the method that the protective gas is injected into the heating section at a flow rate which depends on the speed of the wire during the guiding of the wire through the heating section, more specifically that the flow rate decreases with increasing speed of the wire or that the flow rate increases with decreasing speed.

This is noteworthy because in standard practice, the amount of shielding gas flow remains constant regardless of the filament speed. This decrease in gas flow with increasing filament velocity, or conversely, an increase in gas flow with decreasing filament velocity, is counterintuitive, as one would expect the use of a gas flow that is directly proportional to the surface area of the filament passing over. According to the present inventors, the use of the wire speed to regulate the flow of shielding gas in the opposite way has certain advantages:

one advantage is that adjusting the gas flow in the above-described manner allows a controlled, constant growth of the oxide, which can be removed in a controlled, constant manner in the subsequent steps;

this is possible even when a constant power is delivered to the induction coil or coils. This eliminates the necessity of using complex control electronics that feed the intermediate frequency power to the coil depending on the wire speed;

obviously, less shielding gas is required, especially at high speeds of travel.

With respect to step (b):

the heating temperature for eliminating the stress of the bead wires is preferably between 350 ℃ and 550 ℃, more preferably between 380 ℃ and 450 ℃.

Recrystallization of low carbon steel wire is generally carried out at higher temperatures ranging from 720 ℃ to 750 ℃.

The invention is equally well applicable to the tempering of martensitic high carbon steel wires. Martensitic high carbon steel wire is obtained by rapidly cooling the steel wire after it has reached an austenitizing temperature of 930 ℃ to 1000 ℃. Tempering is carried out at 360 ℃ to 550 ℃ and helps some of the carbon to diffuse out of the martensite lattice, thereby forming iron carbide precipitates. By tempering, the originally brittle martensitic steel wire recovers some ductility.

In a first particularly preferred embodiment, the flow of protective gas is reduced in a continuous or stepwise manner as the wire speed increases, or the flow of protective gas is increased in a continuous or stepwise manner as the wire speed decreases.

The reduction in flow rate may be continuous with increasing filament velocity. The increase in flow rate may continue as the filament speed decreases, mutatis mutandis.

Examples of continuous correlations are gas flow inversely proportional to the filament speed or gas flow decreasing linearly with increasing filament speed, i.e. the relationship between gas flow and filament speed has a constant negative slope in the transition region.

Alternatively, the reduction in flow rate may be stepwise, for example in a first, lower speed range, with the gas flow being maintained at a high level, and as the speed enters a second, higher range, the shield gas flow is reduced to a lower level than the high level, as long as the filaments have a speed in that range. The increase in gas flow rate may be stepwise, mutatis mutandis, as the velocity enters the first range of lower filament velocities.

Different combinations are possible, such as:

the gas flow rate decreases continuously as the filament speed increases, and the gas flow rate increases stepwise as the filament speed decreases.

Alternatively, the gas flow is reduced stepwise as the filament speed increases and the gas flow is continuously increased as the filament speed decreases.

In another preferred embodiment of the method, the power delivered to the one or more induction coils is held constant as the wire speed is varied.

In another preferred embodiment, the wire is immediately guided through the "soaking zone" after the heating step (b), which step is referred to as step (b'). The soaking zone comprises a thermally insulating enclosure in which the steel wire is allowed to cool in a controlled manner, more specifically, at a slow rate. In the soaking zone, the diffusion phenomenon in the steel continues to occur without adding additional heat. For the sake of clarity: when a soaking zone is used, the soaking zone becomes part of the heating zone. Therefore, the shielding gas must also be injected into the soaking zone.

In a likewise preferred embodiment, after cooling the steel wire, the wire is coated with a metal coating comprising a metal or metal alloy selected from the group comprising: copper, zinc, tin, bronze, brass or any combination thereof, hereinafter referred to as step (c').

The application of the metal coating can be accomplished in several ways, for example:

dip the steel wire in a molten zinc bath by hot dipping, for example by guiding the steel wire through the bath, thereby obtaining galvanized steel wire;

dipping the wire in an electrolyte bath by guiding the wire through the bath (such as a bath of copper tin sulfate used for manufacturing bead wires) by a chemical replacement reaction;

copper and zinc are deposited by electrolysis, possibly followed by diffusion to obtain brass;

combinations of two or more of the above coating techniques are also possible.

During the heat treatment of the steel wire, operating cycles (in which the wire speed is kept constant at the operating speed) will alternate with "switching cycles". In the 'conversion period', the pay-off I-shaped wheel which runs nearly in an idle state is exchanged with the full-line I-shaped wheel, and/or the take-up I-shaped wheel which runs nearly in a full line is doffed and replaced by an empty I-shaped wheel. During the change over period the wire speed is reduced to a reduced speed which is lower than the operating speed of the wire during the operating period. The filament speed is kept constant at this reduction rate during the switching period. This is necessary in order to give the operator sufficient time to replace the spool. During the transition, all mechanical and chemical properties are maintained within the target range.

According to the invention, a flow of shielding gas is injected into the heating section during the switching cycle. When the speed is increased to the operating speed, the gas flow is reduced, i.e. the injection of the shielding gas flow is reduced. Since the operation period is much longer than the switching period, the gas flow rate is reduced for an extended period of time. This results in a substantial saving in the use of protective gas over an extended period of time.

The inventors propose a way to estimate the minimum flow of shielding gas during the switching period:

the cross-sectional area of the heating section is equal to 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 filaments in the heating section (from the inlet into the heating section to the outlet of the heating section). For the sake of clarity: if there is a soaking section, the volume of the soaking section must be considered.

As long as the flow rate of the shielding gas is more than 1 to 10 times the product of the cross-sectional area and the difference between the operation speed and the reduction speed (hereinafter referred to as the "product"), oxidation is not expected to occur. The inventors were able to keep the gas flow below 10 times the above product and speculate that it could be further reduced to 8 or even 5 times below the above product. If the cross-sectional area of the heating section is further reduced by, for example, introducing a ceramic tube into the coil, the minimum flow to prevent oxidation will be further reduced.

An alternative method of calculating the shielding gas flow rate when the wire is traveling at a reduced wire speed is to specify an update rate for the volume occupied by the heating section. When the gas flow rate into the heating section is greater than 1 to 6 times the volume of the heating section per minute, the shielding gas in the entire volume will be completely refreshed every minute or every ten seconds.

In a particularly preferred embodiment, the inventors reduced the gas flow to 0 when the wire was traveling at operating speed. This ultimately saves protective gas, since then no gas escapes while still transporting the steel wire with the correct oxide.

In another preferred embodiment, the volume of the heating section is purged with a shielding gas at the beginning of the switching cycle. Wherein "purging" means blowing a gas at least 1 to 10 times the volume of the heating zone for a short period of time to rapidly and completely remove all possible oxygen remaining in the heating zone.

In another preferred embodiment of the method, the reduction speed during the transition is below 50%, or even below 60% or below 75% of the operating speed. In any case, the reduction speed during the transition is greater than 0 or greater than 5% or even 10% of the operating speed.

As the protective gas, an inert gas such as argon or nitrogen can be used. Alternatively, the protective gas is a reducing gas, such as hydrogen or carbon monoxide, although carbon monoxide is generally not considered for use due to its toxicity. Alternatively, a gas mixture, such as a mixture of nitrogen and hydrogen (e.g. resulting from the cracking of ammonia), may be considered to be the shielding gas. However, to date, it is safest to use nitrogen gas provided that there is sufficient venting.

According to a preferred embodiment, the inventors have found that providing a mixture of a reducing gas and an inert gas as a protective gas makes it possible to fine-tune the desired scale formation on the steel wire. Indeed, for certain silk applications, the controlled presence of certain oxides is required. In this regard, an oxidizing gas (such as air or pure oxygen) may even be injected with an inert gas under a controlled environment.

According to a second aspect of the invention, an apparatus and a device for heat treating steel wires are proposed. The apparatus includes a heating section having one or more induction coils; a take-up section having an adjustable speed for pulling the wire through the heating section. The apparatus is further provided with a controllable gas supply for injecting a protective gas into the heating section. The particularity with respect to the apparatus is that the flow rate of the shielding gas supply depends on the above-mentioned adjustable speed, since the flow rate of the shielding gas supply decreases when the wire speed increases, and/or wherein the shielding gas supply increases when the wire speed decreases.

In a preferred embodiment of the apparatus, the heating section further comprises a soaking section attached to the one or more induction coils. The wire extends uninterrupted from the induction coil to the soaking section.

In another preferred embodiment the dependence of the shielding gas supply on the wire speed is such that the shielding gas supply decreases stepwise when the wire speed increases. If necessary, the gas flow is increased stepwise as the wire speed is reduced. Alternatively, the relationship between the filament velocity and the flow of shielding gas may be continuous, such as inversely proportional to the filament velocity or linearly proportional with a negative slope. Of course, a combination of a stepwise increase in the flow of shielding gas upon entry into the lower speed range and a continuous increase in the flow of shielding gas upon reduction of the speed of the wire is also possible, and similarly a combination of a continuous increase in the flow rate upon reduction of the speed and a stepwise decrease in the flow of gas upon increase of the speed of the wire is likewise preferred.

In another preferred embodiment, the power delivered to the induction coil can be held constant at a fixed level or held constant at a fixed level regardless of the wire speed. In other words: the device does not have any feedback loop between the wire speed and the power delivered to the induction coil.

In an exemplary embodiment, the device will operate at an operating speed. The operating speed is related to the diameter of the steel wire to be annealed. At the operating speed, the flow rate of the shielding gas is low or 0. When the wire speed is reduced, the flow rate of the shielding gas supply is reduced by 1 to 10 times the product of the cross-sectional area of the heating section and the difference between the operating speed and the actual wire speed.

In another embodiment, the apparatus is provided with a gas premixing unit. In the gas premixing unit, the reducing gas is mixed with the inert gas in a preset ratio before being injected into the heating section as the shielding gas. Alternatively, a gas premixing unit may be used to mix the oxidizing gas with the inert gas. The oxidizing gas is, for example, oxygen or air. The pre-mixing unit allows to fine-tune the composition and amount of oxides formed on the steel wire.

Drawings

Figure 1 shows a schematic representation of a wire processing line comprising a heat treatment apparatus according to the invention;

figure 2 shows different operating scenarios of the method for heat treating steel wire.

Fig. 3 shows a block diagram illustrating the method and some alternatives involved in the method.

Detailed Description

Fig. 1 shows a schematic representation of a bead wire processing line in which the apparatus according to the invention is included and operated. Note that some steps and baths, such as drying steps and water rinses, have been omitted from the drawings, as these are known to those skilled in the art and only complicate the schematic.

The payout bobbin 102 carries a steel wire 140, such as a cold drawn high carbon steel wire, having a diameter of between 0.70mm and 3.00mm, such as 0.89mm, 0.96mm, 1.30mm, 1.60mm or 1.83 mm. Hereinafter, the terms "rear" and "front" are relative to the direction of the payoff of the filament. Due to cold drawing, depending on diameter and diameterThe desired tensile strength level, the tensile strength of the filaments being about 1700N/mm²To 2700N/mm². For example, the minimum tensile strength of a 0.89 Standard tensile (NT) bead wire is 1900N/mm²And a minimum tensile strength of 2150N/mm for a 0.89 High Tensile (HT) bead wire²(ISO 16650). Such hard wires cannot be safely used in the bead of a tire because they cannot exhibit sufficient elongation at break. Therefore, the steel wire must be heat treated.

In a known step, the filaments 140 are first cleaned in the cleaning section 106 to remove any surface residue. In a next known step the wire is led through a heating section 111, where the temperature is raised to 480 ℃. The heating section 111 is composed of two induction coils 108, 108' organized sequentially. The induction coil is powered by an intermediate frequency power supply 112. Immediately after the induction coil, a soaking zone 110 is provided which maintains the filament in a hot state until the filament exits the heating zone 111. The soaking section is a heat-insulating chamber. The outlet temperature was about 400 ℃. The volume of the heating section is the free space inside the heating section in which the wire moves. This volume is equal to the average cross-sectional area of the heating section multiplied by the length. The volume of the heating section is filled with a protective gas (here nitrogen) to prevent oxidation of the filament surface, which is delivered from the central tank 114 through the feed manifold line 130.

After leaving the heating section, the filaments 140' are cooled to ambient temperature (step not shown) by water quenching or simply by exposure to air.

During heating of the filaments, the following iron oxides may be formed:

ferrous oxide, iron (II) oxide, FeO, wustite;

iron oxide, iron (III) oxide, Fe₂O₃Hematite;

iron (II, III) oxide, Fe₃O₄And magnetite.

In particular, fe (iii) oxides are difficult to remove by the acid bath 116. When a protective gas is used, the formation of oxides, in particular fe (iii) oxides, can be prevented. When no protective gas is used, the oxide will grow, making removal of the oxide more difficult.

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

After removal of the oxides, the wire is guided through a plating section 118, which for example comprises copper sulfate with tin dissolved therein. The bronze coating is deposited on the steel wire 140' by chemical displacement, thereby forming the bead wire 140 ". After drying in the oven, an optional green adhesion enhancer may be applied to the steel wire in applicator 120 before being taken up on spool 104.

If oxides are present on the wire 140', the substitution reaction in the plating bath 118 is inhibited, resulting in bead wires that are not properly coated, and are accompanied by lack of adhesion or resulting in differences in appearance. As described above, the continuous supply of the protective gas prevents the formation of oxides on the steel wire that are difficult to remove. Thus, the inventors were surprised to reduce or even stop the use of protective gas when the processing line is running at its operating speed. The operating speed is the speed at which the wire leaves the heating section with the desired mechanical properties. The speed varies with the diameter of the wire and is between 100 and 600 meters per minute. The use of shielding gas is only required to be increased when the speed of the processing line is reduced below the operating speed. This is contrary to the general view that a shielding gas is always needed.

For example, when the paying-off I-shaped wheel finishes paying-off or takes-up I-shaped wheel conversion, the wire speed needs to be reduced. The reduced wire speed is one tenth of the operating speed to allow the operator to replace the spool in a safe manner while ensuring product quality throughout the spool.

To control the flow of shielding gas as a function of the wire speed, a control system is added to the device with speed sensor 126, which controls throttle valve 124 via controller 128. When the wire speed is reduced, such as in the case of a spool being paid out or a take-up spool being replaced, the gas flow increases. As the filament speed approaches the operating speed, the gas flow supply is reduced to or even set to 0.

As shown in fig. 2, there may be different strategies for correlating the gas flow of the shielding gas to the filament velocity (also referred to as "linear velocity"). In a first strategy, represented by the dotted line 206, the gas flow "Φ" (in gas standard liters per minute) is inversely proportional to the wire speed "V" (in meters per minute):

wherein phi_redIs the gas flow at reduced filament velocity. The gas flow at reduced wire speed was set to:

Φ_red＝C·A·V_red

where "C" is a constant between 0.1 and 1.0 (e.g., 0.2), and "A" is the average cross-sectional area of the heating segment in square centimeters (cm)²) Is a unit.

In a second strategy, shown as curve 202 in fig. 2, the gas flow rate decreases linearly with the wire speed in the following manner:

Φ(V)＝Φ_op+C·A·(V_op-V)

wherein "V" is_opIs the operating speed, phi_op"is the low shielding gas" maintenance "flow maintained during the operating cycle. When the velocity is expressed in meters per minute, the cross-sectional area is expressed in square centimeters and the gas flow rate is expressed in liters per minute, "C'" is also a value between 0.1 and 0.5. Thus, when entering the transition period from the operating state, the gas flow rate is reduced by an amount proportional to the cross-sectional area of the heating section multiplied by the product of the difference between the operating wire speed and the transition wire speed.

In a third strategy, shown as 204 in FIG. 2, at a slave "V_redTo a certain higher speed (e.g. above V)_red10% or V_red+ Δ) velocity range, the shielding gas flow is maintained at a high level_red". Once the filament velocity is higher than the latter, the gas flow is completely shut off.

Figure 3 shows different alternative paths in the method that can be used for annealing of the steel wire. After unwinding 300 the steel wire at a wire speed V, the steel wire is heated 305 by guiding the steel wire through a heating section. The heating section is comprised of one or more induction coils 302 (path a) or one or more induction coils 302 followed by a soak zone 304 (path B). After the heating section, the wire is cooled 306 to ambient temperature. Thereafter, the annealed wire may be directly wound on the carrier in the winding step 314 (path D), or may be coated with a bronze coating composed of copper and tin in the electrolytic bath 308 (path C), or may be hot dip galvanized by immersion in a molten zinc bath 310 (path E) before winding 314 onto the carrier. The particularity with respect to this method is that the shielding gas flow rate is dependent on the filament speed, more specifically the shielding gas flow rate decreases with increasing speed. The method has the advantage of greatly reducing the use of protective gas, thereby reducing production cost and improving environment-friendly operation.

It is also worth noting that the regulation of the gas flow allows operation at a constant power level fed to the induction coil. This eliminates the need for an expensive feedback control loop between the filament speed and the intermediate frequency wave generator feeding the induction coil.

Claims

1. A method for heat treating a steel wire, comprising the steps of:

(a) unwinding the steel wire at a wire speed;

(b) guiding the steel wire through a heating section for heating the steel wire to a temperature between 350 ℃ and 750 ℃, wherein the heating section comprises one or more induction coils;

(c) cooling the steel wire to ambient temperature;

(d) winding the wire onto a carrier;

it is characterized in that the preparation method is characterized in that,

during the guiding of the wire through the heating section, a protective gas is injected into the heating section at a flow rate which decreases with increasing speed of the wire or at a flow rate which increases with decreasing speed of the wire.

2. The method of claim 1, wherein the flow rate of shielding gas is continuously decreased as the velocity of the wire increases.

3. A method according to claim 1 or 2, wherein the flow rate of shielding gas is continuously increased as the speed of the wire is decreased.

4. The method of claim 1, wherein the flow rate of shielding gas is reduced in steps as the velocity of the wire increases.

5. The method of claim 1 or 4, wherein the flow rate of shielding gas is increased stepwise as the velocity of the wire decreases.

6. A method according to the combination of claims 2 and 5 or the combination of claims 3 and 4.

7. The method according to any one of claims 1 to 6, wherein after step (b) the following steps are introduced:

(b') immediately guiding the steel wire through a soaking zone, which is part of the heating section.

8. The method according to any one of claims 1 to 7, wherein after step (c) the following steps are introduced:

(c') coating the steel wire with a metal coating comprising a metal or metal alloy selected from the group consisting of: copper, zinc, tin, bronze, brass, or any combination thereof.

9. Method according to any one of claims 1 to 8, wherein operating cycles, during which the wire is travelling at an operating speed, alternate with switching cycles, during which the speed of the wire is reduced to a reduced speed, wherein

-injecting a flow of shielding gas into the heating section during the switching period,

-reducing the injected flow of shielding gas during said operating period compared to said flow of gas during the transition.

10. The method of claim 9, wherein the heating section has a cross-sectional area, and wherein a flow rate of shielding gas during the replacement cycle is greater than the cross-sectional area multiplied by a product of the difference between the operating speed and the reduced speed.

11. The method of claim 9 or 10, wherein during the transition period, the flow rate of shielding gas per minute is greater than the volume of the heating segment.

12. The method according to any one of claims 9 to 11, wherein the flow rate of the shielding gas is 0 when the steel wire is travelling at the operating speed.

13. The method of any one of claims 1 to 12, wherein the volume of the heating section is purged with a shielding gas at the beginning of a changeover cycle.

14. The method according to any one of claims 9 to 13, wherein the reduction speed is less than 75% of the operating speed.

15. The method of any one of claims 1 to 14, wherein the shielding gas is selected from one of the group consisting of: argon, nitrogen, hydrogen, carbon monoxide or mixtures thereof.

16. An apparatus for heat treating steel wire comprising a heating section having one or more induction coils; a take-up section for pulling the steel wire through the heating section, the take-up section having an adjustable speed; a controllable gas supply for injecting a shielding gas into the heating section,

it is characterized in that the preparation method is characterized in that,

the flow rate of the supply of the shielding gas is decreased when the velocity of the wire is increased, and/or wherein the supply of the shielding gas is increased when the velocity of the wire is decreased.

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

18. The apparatus for heating a steel wire according to any one of claims 16 to 17, wherein the apparatus further comprises a premixing unit for mixing an inert gas with a reducing gas or an oxidizing gas before injecting the mixture into the heating section.