COATING LINE CONTROL

The present invention relates to controlling a coating line for continuously coating a substrate, such as a metal, typically steel, strip.

The present invention relates particularly, although by no means exclusively, to a method of controlling the thickness of a coating formed on the substrate in the continuous coating line .

The present invention also relates particularly, although by no means exclusively, to a method of controlling an oven in the continuous coating line .

Continuous coating lines for steel strip (and other substrates) comprise a coater for forming a coating on the strip and an oven for drying, curing, or coalescing the coating. Typically, the oven operates at temperatures as high as 240 ⁰ C but this can vary depending on the curing requirements of the specific coating used. Typically, continuous coating lines also comprise a quenching station for cooling coated strip downstream of the oven.

The coating lines may be dedicated continuous paint coating lines or they may be continuous lines for other purposes such as metal coating, eg galvanizing, with in-line painting facilities included to minimise the number of process steps.

The coatings formed on steel strip in continuous coating lines may be corrosion treatment compositions, which are coated as aqueous solutions and then dried. The coatings may also be resins that provide anti-finger proofing properties and corrosion resistance . The coatings may also be paint compositions that provide a

decorative finish and corrosion resistance.

Coating thickness control is an important issue in terms of product quality and cost (minimising excessive paint usage) .

In addition, oven control, particularly at start up of a line, is an important issue in terms of product quality and minimising energy consumption for the oven.

In general, drying, curing or coalescing of coatings may be achieved by heating coated strip using induction heating, convection heating or by radiation heating (including infra-red) , or some combination of these methods .

Typically, the strip temperature is used as a key oven control variable whereby the process is controlled to achieve a specified strip temperature for strip in the oven. This is often referred to as "peak metal temperature" or PMT.

Control of ovens may be by (a) closed loop control using strip temperature feedback from a non- contact radiation thermometer (pyrometer) , (b) open loop control models, or (c) simple zone temperature control whereby the air temperature in a convection oven is held constant.

In the case of a closed loop control, a pyrometer is usually aimed at the strip just downstream of an oven. There are two common problems with this method. First, the specific emissivity of the strip is often unknown or variable or very low, thereby resulting in poor accuracy of temperature measurement. Second, radiation from inside the oven or from the surrounds that is reflected from the strip can be of unknown or variable intensity,

particularly in the case of convection or radiation heating ovens .

Due to the difficulties measuring strip temperature downstream of an oven, oven suppliers and operators in many cases resort to open loop control models to obtain target strip temperatures . These models may take into account an assumed incoming strip temperature and material properties (specific heats, densities etc), strip speed through an oven and the heat transfer properties of the oven .

A problem with control based on open loop control models is that strip properties and heat transfer properties of an oven are not always sufficiently well known, thereby resulting in systematic control errors.

Further, the incoming strip temperature is usually variable (depending on prior processes or ambient temperature conditions) and hence incoming strip temperature variation is also a source of control error .

For the same reasons outlined above, measuring incoming strip temperature to improve oven control is usually considered to be too difficult.

The following discussion is not to be taken as an admission of the common general knowledge in Australia or elsewhere .

The present invention accurately measures strip temperature and this provides opportunities to control coating thickness and drying, curing or coalescing of coatings on metal strip and other substrates .

The present invention is based in part on a realisation that pyrometry measurements on hot coated

metal strip immediately downstream of an oven can provide information that can be used to control coating thickness .

The present invention is also based in part on a realisation that wedge pyrometry, which is based on taking measurements of radiation emitted in a bite between a roll and a metal strip passing over the roll makes it possible to obtain accurate temperature measurements of the strip.

According to the present invention there is provided a method of controlling the thickness of a coating on a metal substrate, such as strip in a continuous coating line that includes a coater for forming the coating and an oven for drying, curing or coalescing the coating, which method comprises:

(a) monitoring the hot coated substrate downstream of the oven and collecting data that provides information on the thickness of the coating while the coated substrate is hot; and

(b) using the collected data to control the thickness of the coating subsequently formed on the substrate in the coater.

Monitoring the coated substrate while it is hot, i.e. immediately downstream of an oven, makes it possible to achieve rapid feed back to the coater and to make any coating thickness adjustments that may be required quickly. This is an advantage because rapid feed-back minimises the amount of material undercoated or overcoated during start-up or other process transitions . Most methods of measuring film thickness require that the coated substrate be cool (less than 100 ⁰ C) while the present invention is suited to measuring hot coated substrates, i.e. substrates at temperatures greater than 100 ⁰ C.

Typically, step (a) comprises monitoring the hot coated substrate by measuring and/or obtaining an indication of the temperature of the coated substrate.

Typically, step (a) comprises monitoring the hot coated substrate by measuring the radiation emitted from a nip between a roll and the coated substrate using a wedge pyrometer, as described herein, and another pyrometer that is directed at a straight section of the coated substrate downstream of the wedge pyrometer and thereby obtaining an indication of the radiation emitted from the hot coated substrate .

More particularly, preferably step (a) comprises estimating the emissivitys of the hot strip by comparing the radiation measured by the pyrometers . Emissivity is correlated to the coating thickness, with zero coating thickness product having the emissivity of the uncoated substrate and higher coating thickness product having an emissivity characteristic of the coating itself. The optimal wavelength at which to operate the other (i.e. non-wedge) pyrometer depends on the thickness range of the coating and how the characteristic absorption length for the coating varies with the wavelength. At some wavelengths, the strip emissivity reaches a maximum faster than at other wavelengths .

Typically, step (a) comprises monitoring the hot substrate downstream of the oven and upstream of a quenching station for the substrate.

Typically, the target temperature for a coated substrate at the oven outlet is 70-120 ⁰ C for a coalesced resin coating and 190-240 ⁰ C for a cured paint coating. These temperature ranges are indicative of "hot" coated substrate temperatures monitored in step (a) .

According to the present invention there is also provided a method of controlling an oven in a continuous coating line that comprises a coater for forming a coating on a metal substrate, such as a strip, and an oven for drying, curing or coalescing the coating on the substrate , which method includes :

(a) using a wedge pyrometer, as described herein, to measure the radiation emitted by a nip between a roll and the substrate upstream of the coater and/or using a wedge pyrometer to measure the radiation emitted by a nip between a roll and the coated substrate downstream of the oven and thereby obtain an indication of the temperature of the uncoated and/or coated substrate; and

(b) controlling the operation of the oven based on the information obtained in step (a) .

One advantage of the present invention as described in the preceding paragraph is more accurate control resulting in fewer defects or a higher throughput. Other advantages are the simplicity, ease of maintenance and cost effectiveness of the system.

The principle of a wedge pyrometer is that by viewing a nip between a roll and a substrate that contacts the roll, multiple reflections between the substrate and the roll cause the substrate to have an emissivity close to unity. In this context, the term "nip" is understood to mean the contact line between the roll and the substrate.

The wedge pyrometer measures a combination of roll and strip temperatures and hence it is an accurate indication of substrate temperature only when the roll

— ■ *7 — ■ temperature is close to the substrate temperature. In practice, this situation occurs most of the time. One exception is during rapid line speed or substrate section changes. During such transitions, a model can be used to calculate a faster responding temperature signal based on the measured signal and some process data.

With the emissivity being close to unity due to multiple reflections, the use of a wedge pyrometer is very robust against substrate emissivity variation and uncertainty in reflected radiation intensity. This is a key problem in oven applications .

The wedge pyrometer may be located perpendicular to the axis of the roll and thereby view the nip in the direction of travel of the substrate .

Alternatively, the wedge pyrometer may be located laterally spaced away from one end of the roll and thereby view the roll nip from that side of the travelling substrate. This arrangement is usually more convenient and works well .

It is an advantage that the metal substrate be subject to fairly high tension so that it does not shift the roll nip location on the roll circumference or interfere with the view of the roll nip by the wedge pyrometer .

Selection of the wavelength range of the wedge pyrometer will depend on the environment and the temperature range .

Typically, the operating wavelength of the wedge pyrometer is as short as the temperature range will allow. However, factors such as spot size and ambient thermal radiation should also be considered.

A good working wavelength for the wedge pyrometer is less than 4 micrometers .

A good working wavelength for a direct view pyrometer in this application is in a range of 4 to 5.5 micrometers for conventional painted product, but pyrometers covering other wavelengths may be suitable or even preferable, depending on the circumstances.

Oven control step (b) may be based on open loop control .

Oven control step (b) may be based alternatively on a closed loop control model .

The coating may be any required coating for an end-use application. For example, the coatings may be corrosion treatment compositions , which are coated as aqueous solutions and then dried in the oven . The coatings may also be resins that provide anti-finger proofing properties and corrosion resistance. Typically, these coatings require curing or coalescing in the oven . The coatings may also be paint compositions that provide a decorative finish and corrosion resistance.

The selection of oven control step (b) , i.e. open loop control etc may be based at least in part on the type of coating .

According to the present invention there is also provided a method of operating a continuous coating line for producing coated metal substrate, such as strip, that comprises the above-described method of controlling the coating thickness.

According to the present invention there is also

provided a method of operating a continuous coating line for producing coated metal substrate, such as strip, that includes the above-described method of controlling the oven of the line .

Typically, the coating thickness is in a range of 5-20 micrometers.

Typically, the line speed is in a range of 100- 200 m/min. The faster the line speed or the more expensive the product, the more beneficial is the control methodology described here in terms of minimising out of specification (wasted) product.

According to the present invention there is also provided a continuous coating line for producing coated metal substrate, such as strip, that includes a coater for forming a coating on a substrate, an oven for drying, curing or coalescing the coating on the substrate, a device for monitoring the hot coated substrate downstream of the oven and collecting data that provides information on the thickness of the coating while the coated substrate is hot, and a controller for controlling the thickness of the coating that is responsive to the data obtained from the monitoring device.

Typically, the monitoring device comprises a wedge pyrometer, as described herein, and another pyrometer that is directed at a straight section of the coated substrate downstream of the wedge pyrometer for obtaining a measure of the radiation emitted and reflected from the hot coated substrate.

According to the present invention there is also provided a continuous coating line for producing coated metal substrate, such as strip, that includes a coater for forming a coating on a substrate, an oven for drying,

curing or coalescing the coating on the substrate, a wedge pyrometer for measuring the radiation emitted by a nip between a roll and the substrate located upstream of the coater and/or a wedge pyrometer for measuring the radiation emitted by a nip between a roll and the coated substrate located downstream of the oven and thereby obtaining an indication of the temperature of the substrate, and a controller for controlling the operation of the oven that is responsive to the information obtained from one or both pyrometer.

The wedge pyrometer may be located perpendicular to the axis of the roll and thereby view the nip in the direction of travel of the substrate.

Alternatively, the wedge pyrometer may be located laterally spaced away from one end of the roll and thereby view the roll nip from that side of the travelling substrate.

The present invention is described further by way of example with reference to the accompanying Figure which is a diagram of a section of one embodiment of a continuous coating line for producing a coated metal substrate in the form of a steel strip that is arranged to operate in accordance with one embodiment of the method of the present invention .

With reference to the Figure, incoming metal steel strip 1 moves initially in a horizontal path and then passes around a lower transfer roll 3 and moves vertically upwardly through a coater 5 and an oven 7. The hot, coated metal strip that emerges from the oven 7 travels upwardly and then passes around an upper transfer roll 9 and moves horizontally to a quench station 23, at which the strip is cooled to ambient temperature.

The continuous coating line shown in the Figure also includes a wedge pyrometer 11 located so as to view a nip between the metal strip and the lower roll 3 to measure the radiation emitted by and thereby obtain an indication of the strip temperature at this point.

The continuous coating line shown in the Figure also includes a wedge pyrometer 15 located so as to view a nip between the coated metal strip and the upper roll 9 to measure the radiation emitted by and thereby obtain an indication of the strip temperature of the hot strip emerging from the oven 7 at this point.

The continuous coating line shown in the Figure also includes a pyrometer 21 located so as to view a straight section of the coated metal strip downstream of the wedge pyrometer 15 to measure the radiation emitted and reflected from the strip at this point.

The continuous coating line shown in the Figure also includes a control system (not shown) that is responsive to the measured temperatures of the wedge pyrometers 13, 15, and the other pyrometer 21 and operates to control the oven, as required.

In the case of coating thickness control, the control system compares the strip temperatures indicated by the wedge pyrometer 15 and the other pyrometer 21 and processes this data, along with an estimate of the reflected radiation temperature, to obtain a measure of the emissivity of the strip. The emissivity is related to the coating thickness through an established correlation and the correlation between these parameters is used to indicate and/or control coating thickness .

The wedge pyrometers 13, 15 and the other pyrometer 21 may be any suitable pyrometers.

The coater 5 may be any suitable coater, such as a roller coater or a spray coater, for forming a coating to one or both surfaces of the strip. The coatings may be aqueous corrosion treatment compositions .

The coatings may also be resins that provide anti-finger proofing properties and corrosion resistance . The coatings may also be paint compositions that provide a decorative finish and corrosion resistance.

The oven 7 may be any suitable oven, such as induction , radiation or convection oven .

Typically, the metal strip is travelling a speed of 40-200 m/min through the coater 5 and the oven 7.

Typically, the target incoming strip temperature is 15-40 ⁰ C.

Typically, the target outlet coated strip temperature is 70-120 ⁰ C for a resin coating and 190-240 ⁰ C for a paint coating.

The applicant has found in plant trials that the above-described use of wedge pyrometers 13, 15 to measure strip temperature and as input data for an oven control system makes it possible to effectively and efficiently control the oven 7.

Many modifications may be made to the embodiment of the present invention described above without departing from the spirit and scope of the invention.

By way of example, whilst the embodiment is described in the context of coating a steel strip, the present invention is not so limited and extends to any

other substrates that require a coating. By way of example, the substrate could be an aluminium substrate.