Detection of rolled-in surface scale in steel sheets by optical and thermal inspection techniques

Surface scale residuals on steel sheets which are being hot-rolled may produce defects, such as rolled-in scale patches, which may result in substantial downgrading of the finished product. A number of different optical techniques which have been investigated for the detection of rolled-in scale defects are described. A polarization-enhanced optical backscattering approach is shown to provide good defect visibility, at the expense of a substantial drop in the signal level and of a sensitivity to environmental perturbations. The spectral-analysis approach is investigated, but requires in this case the use of long-wavelength, cryogenically cooled detectors which are not convenient for field inspection. Finally, a number of thermographic techniques are analyzed, taking advantage of the naturally high temperature of the steel sheets during hot rolling.Surface scale residuals on steel sheets which are being hot-rolled may produce defects, such as rolled-in scale patches, which may result in substantial downgrading of the finished product. A number of different optical techniques which have been investigated for the detection of rolled-in scale defects are described. A polarization-enhanced optical backscattering approach is shown to provide good defect visibility, at the expense of a substantial drop in the signal level and of a sensitivity to environmental perturbations. The spectral-analysis approach is investigated, but requires in this case the use of long-wavelength, cryogenically cooled detectors which are not convenient for field inspection. Finally, a number of thermographic techniques are analyzed, taking advantage of the naturally high temperature of the steel sheets during hot rolling.