Patent Application
20250163560
The invention relates to a method and device for controlling the metallic coating thickness on a steel wire.
Steel wires may be used in many applications that are subject to atmospheric corrosion and therefore need to be protected. Using a metallic coating that will be sacrificed to increase the lifetime of the wire is a well-known method. Metallic coatings can be applied by different methods such as e.g. electroplating, physical or chemical vapor deposition or hot dip. Usual metallic coatings can be Sn, Zn, Cu, Al, Cr or their alloys.
While for some applications a very thin coating is sufficient to protect the steel wire against corrosion for its lifetime, in several cases a thick coating is necessary. This is for instance the case when steel wires are cabled or stranded. Their metallic coating can be damaged due to the friction between the wires, which may cause unprotected zones to be quickly corroded, leading to premature fracture.
In that case a thick and homogeneous layer is desired on the whole length of the steel wire.
The precise control of the coating thickness of a metallic layer can be obtained by different methods, but it appears that none of the existing methods is suitable for thick metallic coatings.
For example, CN105886990A discloses a control method and device for the zinc coating thickness of a steel wire. The device comprises a steel wire zinc smearing air knife and an air conveying pipe, and the steel wire zinc smearing air knife is connected with the air conveying pipe; the device is characterized by further comprising a first steel wire diameter measuring instrument, a second steel wire diameter measuring instrument, a PLC and an electromagnetic flow valve; the first steel wire diameter measuring instrument is used for detecting the actual diameter DO of the steel wire before zinc coating, the second steel wire diameter measuring instrument is used for detecting actual diameter D1 of the steel wire after zinc coating. The PLC controls the electromagnetic flow valve according to the data difference between a calculated diameter D2 of the steel wire and the actual diameter D1 of the steel wire, and through control over the air amount input to the steel wire zinc smearing air knife by the air conveying pipe, the zinc coating thickness of the steel wire is controlled.
The control method described in CN105886990A has been tested with the maximum zinc target coating weight of 250 g/m2 on a 2 mm diameter steel wire. A decreasing linear relation between the zinc layer thickness and the air flow is assumed.
It has been observed that, to obtain thick zinc or zinc-aluminum alloy layers with a standard gas-knife nozzle, e.g. with a coating weight above 250 g/m2, for instance above 300 g/m2 or above 350 g/m2, the gas flow needed to be reduced below 5 m3/h, for instance below 3 m3/h or even below 1.5 m3/h.
At those low flow rates, the nozzle, used to remove the excess molten metal at the surface a steel wire was not functioning correctly, causing large variations in the coating thickness. Rather than removing the excess molten metal at the surface of the steel wire, due to the low gas flow, uncontrolled solidification was causing different, unexpected behaviors, deviating from the usual linear decrease of coating weight with increasing gas flow.
As a consequence, a constant coating thickness could not be guaranteed.
The relation between the metallic coating thickness and the coating weight is given by the formula:
Where CW is the coating weight, d is the coating thickness in mm, D is the bright steel wire diameter and s.g. is the density of the metallic alloy, e.g. 7.12 kg/dm3 for pure Zinc.
The purpose of the invention is to solve the issue raised before by providing a control method and device to precisely control the thickness of a metallic coating on a steel wire.
It is a first object of the invention to provide a method to control the thickness of a metallic coating on a steel wire. In particular, the method allows to control the thickness of a metallic coating on a steel wire in conditions deviating from the usual usual linear decrease of coating weight or thickness with increasing gas flow.
The method comprises the following steps:
The method may also include the steps of nozzle detection and calibration. This means that by sending an instruction (e.g. by pressing a button) the steps of producing and storing the first data set, producing and storing the second data set and the obtention of the calibration curve are fully automatised.
The method is particularly suitable for metallic coating thicknesses in the range between 35 μm and 115 μm. This corresponds e.g. with pure Zinc to coating weights between 250 g/m2 and 1200 g/m2. The relation between the metallic coating thickness and the coating weight is given by the formula:
Where CW is the coating weight, d is the coating thickness in mm, D is the bright steel wire diameter and s.g. is the density of the metallic alloy, e.g. 7.12 kg/dm3 for pure Zinc.
The method is valid for any coating weight from 30 g/m2 to 1200 g/m2. The method is more suitable for coating weight ranging from 250 g/m2 to 1200 g/m2, e.g. from 300 g/m2 to 1100 g/m2 or from 350 g/m2 to 1000 g/m2.
With the method disclosed previously the variation of coating thickness along a steel wire is less than 10% of the target coating thickness.
The metallic coating can be consisting of Sn, Zn, Cu, Al, Cr or their alloys.
The metallic coating is preferably consisting of Zinc or Zn—Al or Zn—Al—X alloys, X being Sr, La, Cu, Ti, Ce, Mg, Ni, Si, or Cr.
A zinc aluminum coating may have an aluminum content ranging from 2 percent by weight to 12 percent by weight, e.g. ranging from 3% to 11%.
Other elements such as silicon (Si) and magnesium (Mg) may be added to the zinc aluminum coating. With a view to optimizing the corrosion resistance, a particular good alloy comprises 2% to 10% aluminum and 0.1% to 3.0% magnesium, the remainder being zinc. An example is 5% Al, 0.5% Mg and the rest being Zn.
The method is particularly suitable to control metallic coating thicknesses on steel wire having a diameter between 1 mm and 18 mm. The wire diameter does not have an influence on the method.
The term steel wire comprises but is not limited to:
A typical high carbon steel has a carbon content (% C) ranging from 0.60% to 1.20%, e.g. 0.75% to 1.1%, a manganese content (% Mn) ranging from 0.10% to 1.0%, e.g. from 0.20% to 0.80%, a silicon content (% Si) ranging from 0.10% to 1.50%, e.g. from 0.15% to 0.70%, a sulphur content (% S) below 0.03%, e.g. below 0.01%, a phosphorus content (% P) below 0.03%, e.g. below 0.01%, the remainder being iron, all percentages being percentages by weight. Other elements as copper or chromium may be present in amounts not greater than 0.40%.
A typical low carbon steel has a carbon content (% C) ranging between 0.02% and 0.20%, e.g. 0.05% to 0.1%, a manganese content (% Mn) ranging from 0.10% to 1.0%, e.g. from 0.20% to 0.80%, a silicon content (% Si) ranging from 0.10% to 1.50%, e.g. from 0.15% to 0.70%, a sulphur content (% S) below 0.03%, e.g. below 0.01%, a phosphorus content (% P) below 0.03%, e.g. below 0.01%, the remainder being iron, all percentages being percentages by weight. Other elements as copper or chromium may be present in amounts not greater than 0.40%. Boron may be present in amounts not greater than 0.1%.
Stainless steel wire includes but is not limited to ferritic and austenitic grades such as those selected in the 200 and 300 series.
The 200 series is used for austenitic grades that contain manganese. These chromium manganese steels have a low nickel content (below 5 percent).
The 300 series is used to name austenitic stainless steels with carbon, nickel, and molybdenum as alloying elements. The addition of molybdenum improves corrosion resistance in acidic environments while nickel improves ductility. AISI 304 and 316 are the most common grades in this series.
It is a second object of the invention to disclose a device for controlling a metallic coating thickness on a steel wire comprising:
A display may be foreseen for easier interaction with the process and for visualization of e.g. calibration curves.
In a preferred embodiment the gas nozzle comprises at least two exit supplies. Depending on the design of the at least two exit supplies, different gas pressure can be obtained at different positions on the wire, allowing a better control of the metallic coating thickness.
The angle between the gas direction and the horizontal (perpendicular to the wire) at each exit supply is in the range −90° to +90° and may be different at each exit supply. With this modification, an increasing flow of gas at one of the exit supplies, results in a higher coating weight.
The relation between the gas flow and the coating thickness may thus have a positive coefficient.
The method and device disclosed above is suitable for any type of gas, e.g. nitrogen, argon, helium, hydrogen, or air or CO2.
In
The steel wire 123 with a metallic coating 125 at its surface are also represented.
The present device was tested with nitrogen (N2) as thickness controlling gas. The flow of N2 is indicated in
The flow rate reaching the gas nozzle 119 was between 0 and 11.4 m3/h in normal conditions, i.e. assuming 1 bar pressure behind the proportional valve 113.
The computer program stored in the computer 107 allows different controls of the device. The target coating weight and type of nozzle are first entered in the computer.
For low coating weights, i.e. below 250 g/m2 the standard type of gas-knife nozzles is used with high flow rate, i.e. above 5 m3/h, and a negative linear relation between the coating weight CW and the gas flow rate V is observed of the type:
V=−a CW+b.
A negative coefficient means that the coating thickness decreases when the gas flow increases.
To reach thicker coatings or higher coating weight, e.g. above 250 g/m2 or above 300 g/m2, the gas nozzle is modified such that the gas flows through at least 2 exit supplies, one at the bottom and one at the middle or at the top of the nozzle. As an illustration, the nozzle represented in
The angle between the gas direction and the horizontal (perpendicular to the wire) at each exit supply is in the range −90° to +90° and may be different at each exit supply. With this modification, an increasing flow of nitrogen at one of the exit supplies, results in a higher coating weight.
The relation between the gas flow and the coating thickness may thus have a positive coefficient.
The correlation can also be parabolic:
An example of such a parabolic relation between the coating weight and the gas flow is shown in
The derivative
is calculated at the process value of the coating weight. This is a local linearization of the slope of the step which needs to be taken to go to the next point. The next step in flow is calculated as ΔV=(2 a CWPV+b)ΔCW. The ΔV is than added up to the actual flow rate at that moment.
The target coating weight is entered in the computer 107. The thickness sensor 105 gives a (feed forward) reference towards the proportional nitrogen valve 113. This feed forward is corrected (in positive or in negative direction) to achieve the real flow measured by the digital flow meter 111. Using a feed forward gives a more stable control. A ‘calibration’ of valve/nozzle combination is needed. The calibration starts automatically pushing a button.
The system needs a series of data to make the correlation between flow and coating weight when a modified nozzle for thick coatings is used. Using the coating weight, the system controls and adjusts, step by step, towards the correlated flow rate. A PID controller may be used to compensate the error between the desired flow and the real flow.
In standard (low thickness coating) mode, the scanning makes a relation data set between analogue current (mA) reference for the proportional valve 113 and the flow (m3/h) measured by the digital flow meter 111. This relationship used as a feed forward for valve position.
In thick coating mode, the scanning detects 2 relations. These relations are simultaneously made, thus having the same X-axis. The first data set is a relation between the analogue current (mA) reference for the proportional valve 113 and the coating weight (g/m2) as measured by the thickness sensor 105. The second data set is a relation between the analogue current (mA) reference for the proportional valve 113 and the flow rate (m3/h) as measured by the digital flow meter 111. By combining the first and the second data sets, the relation between the coating weight (g/m2) and the flow rate (m3/h) is retrieved.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22164729.0 | Mar 2022 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2023/055097 | 3/1/2023 | WO |
