Infrared thermography is nowadays used in a wide range of applications. In the steel industry, infrared thermography is mostly used for temperature measurement, which is required for process and quality control. In this article, a high-speed temperature monitoring system for steel strips is proposed. The proposed system is based on infrared line scanners, which are the most suitable devices for temperature monitoring of moving objects. Accurate temperature measurement is critical for this type of application. Therefore, a rigorous methodology to apply quantitative thermography is presented. In addition, this work proposes accurate temperature and spatial calibrations, including a geometrical model, calibration targets, and an optimization procedure. The proposed calibrations open up the possibility of detecting regions of interest in the resulting thermograms, determining the position with accuracy while avoiding distortions. The proposed system is applied to a real industrial application: temperature monitoring in cold rolling. Tests show excellent performance, producing accurate results that provide detailed information about the temperature of very long steel strips.