Sensors & Transducers


Strip temperature control: IR thermometry for continuous annealing

January 2008 Sensors & Transducers

Steel consumers and competitive market conditions are driving steel mills to produce higher quality products more cost effectively. In order to meet these demands, steel mills must implement better controls to manage the temperatures and surface characteristics of the steel strip during the annealing process.

High strength steels for the automotive industry require tight temperature control in order to maintain material properties and keep scrap rates and coil reheats to a minimum.

Traditionally, single-wavelength and dual-wavelength infrared thermometers (ie ratio or two-colour) have been used for strip temperature control within an annealing line. As the graph in Figure 1 illustrates, the fundamental challenge with using these sensors is that their accuracy can vary greatly with the emissivity of the steel strip. Unfortunately in an annealing process, the emissivity of the steel strip can vary significantly and unpredictably due to:

* Changes in alloy, surface texture, surface oxidation, grain growth, and elemental migration.

* Abnormal operating conditions, such as a furnace leak, a bad roller, or a reheated coil.

Figure 1. Performance of single, dual, and multiwavelength sensors on steel annealing lines. Assumes strip temperature of 1400°F (760°C)
Figure 1. Performance of single, dual, and multiwavelength sensors on steel annealing lines. Assumes strip temperature of 1400°F (760°C)

The best illustration that these temperature errors exist is when product properties do not correlate with the measured strip temperatures, or when a phantom temperature change occurs at the welded seam of the strip. With single- and dual-wavelength sensors, the approach to eliminating errors has been to make adjustments to the sensors for each product, but as Table 1 illustrates, this strategy has limits with being able to satisfy all of the annealing line requirements.

Table 1
Table 1

Williamson's multiwavelength sensor is specifically designed to meet the challenging requirements of annealing lines. Without the need to make any adjustments, this sensor provides more accurate and reliable temperature measurements across a wide range of steel alloys under a variety of operating conditions. With these high performance sensors, it is possible to obtain tighter control of physical properties, as well as reduce scrap rates, coil reheats, and costly coil downgrades.

Williamson's PRO Series multiwavelength sensor was specifically developed to compensate for the challenging requirements of a steel mill annealing line. The sensor uses an ESP algorithm that considers infrared energy, emissivity, and the measured wavelengths to accurately calculate the strip temperature and the surface emissivity. The algorithm is a computer based empirical model that was developed from extensive data that was collected from off-line simulations and on-line tests. As the sample data in Table 2 illustrates, one ESP algorithm can be used for accurate measurements on most steel alloys including, carbon steels, tool steels, dual-phase steels, and high strength steels. Additional ESP algorithms exist for stainless steels, electrical steels, coated steels, and high temperature steels.

Table 2
Table 2

With the ability to provide realtime emissivity measurements, the PRO 110 can be used to verify measurement conditions, as well as optimise the radiant heat transfer and line speeds throughout the furnace. All combined, these capabilities help to produce better quality steel more consistently and more efficiently.

As the unit is able to make more accurate temperature measurements under a wide range of operating conditions, it offers the flexibility of installing sensors in more locations along the annealing line for greater control of process temperatures:

* For straight-pass furnaces, one or more sensors can be placed at the exit of the flame-heating section, the exit of the radiant-heating section and at the exit of the cooling zone.

* For serpentine-pass furnaces, multiple sensors can be placed within several heating and cooling zones.

* For hot dip lines, a sensor is placed at the snout.

* For galvaneal lines, sensors are placed at the exit of the heating zone (often an induction heating system) and just before the 'turn roll' where the coated strip must be below 650°F (350°C) before it contacts the roll and causes surface damage.

Features of multi-wavelength sensors

* Accurate temperature measurements without any adjustments to the sensor on carbon steels, tool steels, high strength steels, electrical steels, stainless steels, and high temperature steels.

* Automatic compensation for strip emissivity and e-slope variation.

* Realtime measurement of strip temperature and surface emissivity for process verification and optimisation.

* Dramatic reduction in sensitivity to warm furnace wall reflections.

Advantages of emissivity measurements

* High emissivity strip absorbs heat more efficiently, resulting in faster line speeds at lower zone temperatures.

* Low emissivity strip has a smoother surface texture resulting in a smoother finish.

* Stable and reasonable emissivity values validate operating conditions, surface texture, and crystal structure.

* Increasing emissivity values indicate oxide growth or surface defects that could be caused by changes in the hydrogen levels of the furnace.

Recommended locations for sensor

1. Direct flame and radiant tube heating zones: installation requires a cooled viewing tube to eliminate interference from reflections off the hot furnace walls.

2. Cooling zones: nNo special mounting provisions required.

3. Inter-connecting ductwork: for low-emissivity strips or advanced alloys, this location requires a cooled viewing tube. For high emissivity or carbon steels, no special mounting provisions are required because the multiwavelength sensor is less sensitive to hot background reflections.

4. Nip measurement technique: single-, dual-, or multiwavelength sensors may be used. Dual- and multiwavelength sensors tolerate misalignment to the roller nip.

For more information contact Andy Brown, Temperature Controls, +27 (0)11 791 6000, [email protected], www.tempcon.co.za



Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Vibration monitoring in three axes
ifm - South Africa Sensors & Transducers
Machine vibrations are important indicators when it comes to assessing the current condition of a machine. ifm’s new and smart IO-Link vibration sensor helps to implement a simple and scalable condition monitoring approach using but a single device.

Read more...
Optimise sludge treatment and reduce operational costs
Sensors & Transducers
The Endress+Hauser inline measuring devices, the Proline Teqwave MW 300 and the Proline Teqwave MW 500, determine the total solids content of wastewater directly through microwave transmission. This is considerably less time consuming than the process of determining total solids content in a laboratory.

Read more...
Family of analysers for smart and efficient chlorine measurement
ABB South Africa Sensors & Transducers
ABB has launched ChloroStar, a family of sensors, transmitters and accessories for accurate and reliable chlorine measurement and analysis that enable users in the water, wastewater and other industries to control chlorine more efficiently, enhancing treatment and increasing process uptime.

Read more...
Leading the way with Level 9 PDS implementation
Sensors & Transducers
Applying a Level 9 Proximity Detection System (PDS) to a mining operation is as much about changes in behaviour as it is about technical solutions, according to Pieter Wolfaardt from Apex, Booyco Electronics’ training partner of choice.

Read more...
Enhanced automated weld inspection system
Sensors & Transducers
SmartRay is offering the release of new hardware for its weld inspection system, JOSY, delivering elevated performance and flexibility in the process.

Read more...
Leaders in sensor technology
Gail Norton Instrumentation Sensors & Transducers
Photoelectric eyes have always needed perfect working conditions, but in the workplace there is no such thing. Steam, water, light and dirt interfere with most photoelectric eyes, but never with a Telco sensor.

Read more...
Easy parameter setting of vision sensors
ifm - South Africa Sensors & Transducers
Contour verification and object inspection are two of the most important functionalities in industrial imaging. With the new O2D5xx series, ifm now offers a solution that perfectly masters both functionalities

Read more...
Keeping a close eye on product quality and purity
Endress+Hauser South Africa Sensors & Transducers
Colour measurements are necessary in many processes to avoid product losses and ensure safe production and batching. The Memosens Wave CKI50 process spectrometer from Endress+Hauser is a compact, robust and process-friendly device allowing quality, batching and phase checks to be performed by a single instrument for the entire visible colour spectrum.

Read more...
The sensory reaction of building management systems
Schneider Electric South Africa Sensors & Transducers
Today’s building management systems (BMS) cannot function properly without sensors. This is so critical that if sensors start failing, they can become ‘blind’.

Read more...
Advanced sensor technology to reduce vehicle fatalities
Senseca Sensors & Transducers
Measurement technology plays a critical role across a range of industries. This is especially true when supplying highly accurate and reliable data for road management entities that assist drivers of vehicles experiencing hazardous weather conditions.

Read more...









While every effort has been made to ensure the accuracy of the information contained herein, the publisher and its agents cannot be held responsible for any errors contained, or any loss incurred as a result. Articles published do not necessarily reflect the views of the publishers. The editor reserves the right to alter or cut copy. Articles submitted are deemed to have been cleared for publication. Advertisements and company contact details are published as provided by the advertiser. Technews Publishing (Pty) Ltd cannot be held responsible for the accuracy or veracity of supplied material.




© Technews Publishing (Pty) Ltd | All Rights Reserved