CN112840043B

CN112840043B - Method for heat treating steel wire and related apparatus

Info

Publication number: CN112840043B
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: 2023-05-16
Anticipated expiration: 2039-10-10
Also published as: EA202191049A1; WO2020078829A1; BR112021005116A2; EP3867415A1; KR20210076909A; CN112840043A

Abstract

A method for heat treating a steel wire, comprising the known steps of: unwinding the wire at wire speed, guiding the wire through a heating section comprising one or more induction coils, cooling the wire and winding it on a carrier. The method is characterized in that a shielding gas is injected into the heating section at a flow rate that depends on the wire speed. The inventors have found that the flow rate of the shielding gas should decrease as the velocity increases. In the limit, no shielding gas is injected into the heating section when the process line is running at operating speed. This results in a reduced use of shielding gas. When the spool or 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 present invention allows control of the type and level of oxide formed on the filaments. 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 wire speed 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 a steel wire and to a related apparatus for carrying out the method. The heat treatment of the steel wire is used to modify the properties of the steel. Special steel wires requiring heat treatment are for example stress relief of bead wires (for use in winding beads used in tires), annealing of a far drawn mild steel wire before further drawing, or tempering of martensitic (high carbon) wires for example used as spring wires.

Background

In the production of steel wires for various uses, such as bead wires or low carbon bare wires or galvanized wires, it is often necessary to heat treat the drawn wires. As a result of 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 an increase in the strength of the wire, but a decrease in ductility. This is due in particular to the locking of the sliding surfaces caused by dislocations and defects when the steel is under stress.

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

In general, this compromise between tensile strength and ductility is achieved by heat treatment of the steel wire. This means, therefore, that the temperature of the filaments is kept in any case below the temperature of the A1 line in the iron-carbon phase diagram, followed by controlled cooling. As a result of this treatment, dislocations and defects diffuse and combine, resulting in filaments that are still strong but more ductile. This is also known as "stress relief" or "recovery". An increase in temperature and/or processing time will further result in recrystallization, wherein no strained grains are bonded. This may already result in a structure that is too soft. If the temperature is further raised for a sufficient period of time, grain growth will continue to occur. However, the purpose of this treatment is not to recrystallize (or even reverse it) the grains to grow, since at this point the tensile strength of the grains has been partially or even completely lost due to cold drawing.

Another example of a heat treatment is tempering. During tempering of the martensitic quenched filaments, the carbon will diffuse out of the highly stressed martensite and precipitate as carbides, resulting in a more ductile but still strong microstructure that can be coiled into a spring, for example.

Historically, heat treatment of steel wire has been performed by heating the wire in a molten lead bath. The temperature of the molten lead can be easily controlled while optimizing the heat transfer from the lead to the wire, resulting in an immediate 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 oxidation that can occur is at the outlet of the lead bath, but this is counteracted by covering the molten lead with anthracite coal that releases gas, which burns to consume oxygen near the surface of the wire.

However, lead and gas have been identified as having serious impact on human health and the environment, and are therefore increasingly prohibited from use in production environments. Therefore, alternatives to heating the wire to the desired temperature must be sought. In the case of steel wires, alternatives are the use of salt baths, or the use of fluidized sand beds or the use of resistance heating. Salt baths present operational safety risks. The heat transfer in the fluidised bed is much less than in the lead bath. When using resistance heating, electrical contact with the wire while in progress may produce sparks, resulting in unacceptable martensitic steel spots.

To overcome these drawbacks, a method of passing an induction heating wire has been introduced. A marked publication of this technology is US4788394. In this heating method, the wire is guided through an alternating magnetic field which induces eddy currents which heat the wire. This allows the wire to be heated to a desired temperature quickly and contactlessly after it is guided through the soak zone (i.e., the flat long box), wherein the wire is insulated from its environment. Although this treatment is not isothermal as in the case of lead baths, the temperature can remain sufficiently stable. In order to prevent the filaments from being oxidized, the filaments must be surrounded by 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 wire heating method comprising one or more induction coils followed by a soak 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 can be prevented even when the speed of the processing line is reduced (for example, for changeover).

CN107227400a describes an induction heating apparatus in which a single wire passes through a single coil whose power is individually controllable. This allows to handle different wire diameters simultaneously on the same processing line.

When using a method by means of induction heating the wire, the use of a protective atmosphere becomes mandatory, as air enters the heating tube at the open end, so that scale forms on the surface of the wire. The formation of scale makes subsequent coating operations such as galvanization or 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

Accordingly, 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 successfully completely eliminated 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 invention, a method for heat treating a steel wire according to the steps of claim 1 is presented. The method comprises the following steps:

(a) Unwinding the wire at wire speed. Such unwinding is typically accomplished with a reel, spool, or cross-shaft. The wire speed is the wire linear speed at which the wire is pulled through the device by the take-up device;

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

(c) Thereafter, the filaments are cooled to ambient temperature, for example, by cooling the filaments in ambient air or in a coolant such as water or oil or a combination thereof.

(d) Finally, the filaments are wound on a carrier on the take-up side.

Now, a particular feature with respect to the method is that during the guiding of the wire through the heating section, a shielding gas is injected into the heating section at a flow rate that depends on the wire speed, more particularly a particular feature with respect to the method is that the flow rate decreases with increasing speed of the wire or that the flow rate increases with decreasing speed.

This is notable because in standard practice the amount of shielding gas flow remains constant regardless of wire speed. This decrease in gas flow with increasing wire speed, or vice versa, is counterintuitive as one would expect the use of gas flow to be proportional to the surface area of wire passing. According to the present inventors, adjusting the shielding gas flow in the opposite manner using the wire speed has certain advantages:

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

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

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

Regarding step (b):

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

The recrystallization of the low carbon steel wire is generally carried out at a higher temperature of 720 ℃ to 750 ℃.

The invention can equally well be used for tempering martensitic high carbon steel wires. The 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 performed at 360 ℃ to 550 ℃ and helps some of the carbon diffuse out of the martensitic lattice, forming iron carbide precipitates. By tempering, the original brittle martensitic steel wire reverts some of the extensibility.

In a first particularly preferred embodiment, the shielding gas flow decreases in a continuous or stepwise manner as the wire speed increases, or the shielding gas flow increases in a continuous or stepwise manner as the wire speed decreases.

The decrease in flow rate may continue as the wire speed increases. The increase in flow rate may continue with the decrease in wire speed, mutatis mutandis.

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

Alternatively, the decrease in flow rate may be stepwise, e.g. in a first lower speed range, the gas flow is kept at a high level, and when the speed goes into a second higher range, the shielding gas flow is decreased to a lower level than the high level as long as the wire has a speed in this range. The increase in gas flow may be stepwise as the speed enters the first range of lower wire speeds, mutatis mutandis.

Different combinations are possible, such as:

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

Alternatively, the gas flow decreases stepwise as the wire speed increases, and the gas flow increases continuously as the wire speed decreases.

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

In another preferred embodiment, the wire is guided through a "soak zone" immediately 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 particularly at a slow rate. In the soaking zone, the diffusion phenomenon in the steel continues to occur without adding additional heat. For clarity reasons: when a soak zone is used, the soak zone becomes part of the heating section. Therefore, shielding gas must also be injected into the soak 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 may be accomplished in several ways, for example:

galvanized steel wire is obtained by hot dipping, for example by guiding the steel wire through a molten zinc bath, dipping the steel wire in the molten zinc bath;

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

by electrolytic deposition of copper and zinc, it is possible to subsequently obtain brass by diffusion;

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

During the heat treatment of the wire, the operating cycle (in which the wire speed is kept constant at the operating speed) will alternate with the "switching cycle". In the switching cycle, the pay-off spool which is in near-empty operation is exchanged with the full spool, and/or the take-up spool which is in near-full operation is doffed and replaced by the empty spool. During the transition period, the wire speed is reduced to a reduced speed that is lower than the operating speed of the wire during the operating period. During the transition period, the wire speed remains unchanged at this reduced speed. This is necessary to give the operator enough time to replace the spool. During the conversion all mechanical and chemical properties remain within the target range.

According to the invention, during the switching cycle, a flow of shielding gas is injected into the heating section. As the velocity increases to the operating speed, the gas flow decreases, i.e. the injection of the shielding gas flow decreases. Since the operating period is much longer than the switching period, the gas flow decreases over an extended period of time. This results in a substantial saving in the use of shielding 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 of the heating section (from the inlet into the heating section to the outlet from the heating section) surrounding the wire. For clarity reasons: if a soaking zone is present, the volume of the soaking zone must be considered.

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

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

In a particularly preferred embodiment, the inventors reduced the gas flow to 0 when the wire was traveling at operating speed. This ultimately saves shielding gas, as in this way no gas escapes while still transporting the 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. By "purging" is meant blowing in a short time at least 1 to 10 times the volume of the heating zone to rapidly and completely remove all possible oxygen remaining in the heating zone.

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

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

According to a preferred embodiment, the inventors have found that providing a mixture of a reducing gas and an inert gas as a shielding gas allows fine tuning of the desired oxide scale formation on the steel wire. Indeed, for certain filament 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 for heat treating steel wire is presented. The apparatus includes a heating section having one or more induction coils; the wire winding section with adjustable speed is used for pulling the steel wire to pass through the heating section. The apparatus is also provided with a controllable gas supply for injecting a shielding gas into the heating section. The special feature about the apparatus is that the flow rate of the shielding gas supply depends on the above mentioned adjustable speed, because 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, the soaking section being 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 as the wire speed increases. When the wire speed is reduced, the gas flow rate is increased stepwise, mutatis mutandis. Alternatively, the relationship between wire speed and shielding gas flow may be continuous, such as inversely proportional to wire speed or linear with a negative slope. Of course, a combination of stepwise increase of the shielding gas flow rate at the time of entering the lower speed range and continuous increase of the shielding gas flow rate at the time of lowering the wire speed is also possible, and similarly, a combination of continuous increase of the flow rate at the time of lowering the speed and stepwise decrease of the gas flow rate at the time of raising the wire speed is also preferable.

In another preferred embodiment, the power delivered to the induction coil may be kept constant at a fixed level or maintained constant at a fixed level, independent 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 wire to be annealed. At operating speeds, 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 a factor of 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 premix unit. In the gas premix unit, the reducing gas and the inert gas are mixed in a preset ratio before being injected into the heating section as the shielding gas. Alternatively, a gas premix unit may be used to mix the oxidizing gas with the inert gas. The oxidizing gas is, for example, oxygen or air. The premix unit allows fine tuning of the composition and amount of oxides formed on the steel wire.

Drawings

FIG. 1 shows a schematic representation of a wire processing line including a heat treatment apparatus according to the present invention;

fig. 2 shows different operating schemes for 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 an apparatus according to the invention is included and operated. Note that some steps and baths, such as drying steps and water rinsing, are omitted from the figures, as these are known to the person skilled in the art and only complicate the schematic.

The payout spool 102 carries steel wire 140, such as a cold drawn high carbon steel wire, having a diameter between 0.70mm and 3.00mm, such as 0.89mm, 0.96mm, 1.30mm, 1.60mm, or 1.83mm. Hereinafter, the terms "rear" and "front" are relative to the paying-off direction of the wire. Due to cold drawing, the filaments have a tensile strength of about 1700N/mm, depending on the diameter and the desired level of tensile strength ² To 2700N/mm ² . For example, the minimum tensile strength of a 0.89 standard tensile (NT) bead wire is 1900N/mm ² And the minimum tensile strength of the 0.89 High Tensile (HT) bead wire is 2150N/mm ² (ISO 16650). Such hard wires cannot be safely used in the bead of a tire because they do not exhibit sufficient elongation at break. Therefore, the steel wire must be heat treated.

In a known step, the filaments 140 are first cleaned in a cleaning zone 106 to remove any surface residues. In the 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 soak zone 110 is provided which keeps the filaments in a hot state until they leave the heating section 111. The soaking section is an insulated 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. The 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 shielding gas (here nitrogen) to prevent oxidation of the wire surface, which is conveyed from the central tank 114 through the feed manifold line 130.

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

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

iron oxide, iron (II) oxide, feO, wustite;

iron oxide, iron (III) oxide, fe ₂ O ₃ Hematite;

iron (II, III) oxide, fe ₃ O ₄ Magnetite.

In particular, fe (III) oxide is difficult to remove by the acid bath 116. When a shielding gas is used, the formation of oxides, particularly Fe (III) oxides, can be prevented. When the shielding gas is not 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 the reducing gas to the inert gas may be used to control the formation of the oxides.

After removal of the oxide, the wire is directed through an electroplating section 118, which includes, for example, copper sulfate with dissolved tin. By chemical displacement, a bronze coating is deposited on the wire 140', forming a bead wire 140". After drying in the kiln, an optional green adhesion enhancer may be applied to the wire in the applicator 120 prior to the take-up spool 104.

If oxide is present on the wire 140', the displacement reaction in the plating bath 118 is inhibited, producing a bead wire that is not properly coated, and accompanied by a lack of adhesion or resulting in a difference in appearance. As described above, the continuous supply of the shielding gas prevents the formation of oxides on the steel wire that are difficult to remove. Thus, surprisingly, the use of shielding gas may be reduced or even stopped when the processing line is run 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 filaments and is between 100 and 600 meters per minute. The use of the shielding gas needs to be increased only when the speed of the processing line is reduced below the operating speed. Contrary to the general view of always requiring a shielding gas.

For example, when the paying-off spool finishes paying-off or winding-up spool conversion, the wire speed needs to be reduced. The reduced wire speed is one tenth of the operating speed in order to allow the operator to exchange the spool in a safe manner while ensuring product quality over the entire spool.

To control the flow of shielding gas according to the wire speed, a control system is added to the device with a speed sensor 126, which controls the throttle valve 124 via a controller 128. When the wire speed decreases, for example in the case of spool unwinding or spool changing, the gas flow increases. When the wire 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 with the wire speed (also referred to as "linear speed"). In a first strategy, represented by the dash-dot line 206, the gas flow "Φ" (expressed in standard liters of gas per minute) is inversely proportional to the wire speed "V" (in meters per minute):

wherein phi is _red Is the gas flow at reduced filament velocity. The gas flow at reduced filament speed is set to:

Φ _red ＝C·A·V _red

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

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

Φ(V)＝Φ _op +C·A·(V _op -V)

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

In a third strategy, indicated at 204 in FIG. 2, the following "V _red To a higher speed (e.g. above V _red 10% or V _red +Δ) velocity range, the shielding gas flow is maintained at a high level of "Φ _red ". Once the wire speed is higher than the latter speed, the gas flow is completely shut off.

Fig. 3 shows different alternative paths that can be used for wire annealing in this method. After unwinding 300 the wire at wire speed V, the wire is heated 305 by guiding the wire through a heating section. The heating section is made up 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 filaments are cooled 306 to ambient temperature. Thereafter, the annealed wire may be wound directly onto the carrier in winding step 314 (path D), or may be coated with a bronze coating composed of copper and tin in electrolytic bath 308 (path C), or may be hot dip galvanized by immersion in molten zinc bath 310 prior to winding 314 onto the carrier (path E). The method is specific in that the shielding gas flow rate depends on the wire speed, more specifically, the shielding gas flow rate decreases with increasing speed. The method has the advantages of greatly reducing the use of shielding gas, thereby reducing the production cost and improving the environment-friendly operation.

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

Claims

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

(a) Unwinding the wire at wire speed;

(b) Directing the wire through a heating section for heating the 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;

characterized in that the operating cycle is alternated with a switching cycle, in which the wire is run at an operating speed, in which the speed of the wire is reduced to a reduced speed,

during the guiding of the wire through the heating section, a shielding gas is injected into the heating section at a flow rate that decreases with increasing speed of the wire or at a flow rate that increases with decreasing speed of the wire, and wherein during the switching period a shielding gas flow is injected into the heating section, and wherein during the operating period the injected shielding gas flow is reduced compared to the gas flow during switching.

2. The method of claim 1, wherein the flow rate of shielding gas continuously decreases as the speed of the steel wire increases.

3. The method of claim 1, wherein the flow rate of shielding gas continuously increases as the speed of the steel wire decreases.

4. The method of claim 1, wherein the flow rate of shielding gas decreases stepwise as the speed of the steel wire increases.

5. The method of claim 1, wherein the flow rate of shielding gas increases stepwise as the speed of the steel 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 of claim 1, wherein after said step (b), the steps of:

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

8. The method of claim 1, wherein after said step (c), the steps of:

(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. The method of claim 1, wherein the heating section has a cross-sectional area, and wherein a flow rate of shielding gas during the transition period is greater than the cross-sectional area multiplied by a difference between the operating speed and the reduced speed.

10. The method of claim 1, wherein during the transition period, a flow rate of shielding gas per minute is greater than a volume of the heating section.

11. The method of claim 1, wherein the flow rate of the shielding gas is 0 when the wire is traveling at the operating speed.

12. The method of claim 1, wherein the volume of the heating section is purged with a shielding gas at the beginning of a switching cycle.

13. The method of claim 1, wherein the reduced speed is less than 75% of the operating speed.

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

15. An apparatus for heat treating a steel wire comprising a heating section having one or more induction coils; the wire winding section is used for pulling the steel wire to pass through the heating section and has adjustable speed; a controllable gas supply for injecting a shielding gas into the heating section, and a control system with a speed sensor, the control system controlling the throttle valve by a controller to control the flow of shielding gas in dependence of the wire speed,

it is characterized in that the method comprises the steps of,

the controller decreases the flow rate of the supply of the shielding gas when the speed of the wire increases, and/or wherein the controller increases the supply of the shielding gas when the speed of the wire decreases.

16. The apparatus for heat treating steel wire of claim 15, wherein the heating section further comprises a soaking section attached to the one or more induction coils.

17. The apparatus for heat treating steel wire according to claim 15, wherein the apparatus further comprises a pre-mixing unit for mixing an inert gas with a reducing gas or an oxidizing gas before injecting the mixture into the heating section.