Temperature Measurement Deviation during Annealing of Multiphase Steels
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INTRODUCTION
PRODUCTION of modern advanced high-strength steels requires tight control of production parameters, such as compositional ranges, rolling reductions, and annealing temperatures. Of these parameters, the strip temperature in a continuous annealing furnace is one of the most difficult to measure accurately. A hurdle in the temperature measurement is the requirement for a noncontact method. Pyrometric techniques have progressed significantly in offering a practical and robust method for measuring the temperature of a moving strip. Nevertheless, discontinuities in the temperature readings concurrent with a change in strip material (and not a change in the actual strip temperature) are not uncommon and have led to the development of productdependent adjustments to annealing patterns. These discontinuities are a sign that pyrometers are still affected adversely by changes in the target’s surface and that pyrometrically determined temperatures should be treated with caution. Three main factors influence the measurement of temperature with pyrometers. The first factor is the emissivity of the target material. Emissivity e describes the intensity of a material’s radiation I as a fraction of the radiation of an ideal black body, IB, given by e ¼ I=IB
½1
Because IB for any given wavelength k is temperature unique as given by Planck’s equation (see Discussion), a constant e would enable the deduction of target’s RICHARD G. THIESSEN, EKATERINA BOCHAROVA, and DOROTHEA MATTISSEN, Project Managers, and ROLAND SEBALD, Team Manager, are with ThyssenKrupp Steel, Center of Materials Excellence, Duisburg, Germany. Contact e-mail: [email protected] Manuscript submitted: September 23, 2009. Article published online May 12, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
temperature simply with a measured I(k). This depends, however, on the assumption that the spectral radiance of a given body has the same relative distribution of intensities as an ideal black body, which is also known as the gray body assumption. In metals, the emissivity can be affected not only by the composition and crystal structure but also by the formation of oxides on the target surface.[1,2] If the emissivity of a surface is not known, then pyrometers that sample the radiation intensity at a single wavelength must be calibrated. Quotient pyrometers sample the intensities at multiple wavelengths, comparing the measured relative intensities with the ratios of the theoretical radiance of a black body and thereby circumventing the need for calibration. It must not be overlooked that this approach is dependent on the gray body assumption. Another technique, laser pyrometry, uses the reflection off the target of a laser with a known intensity to determine the emissivity.[3] This technique, however, depends on the assumption that the entire sampled spectrum has the same emissivity as the emissivity at the single wavelength of the laser. Another technique using two lasers of different wavelengths is being developed,[4] but to the authors’ know
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