The armour steel sheet PROTAC 500M, made in the production plants of the largest Slovenian steel producer, is supplied to the world market in quenched and tempered condition. Low temperature tempering is processed at a temperature of 200°C, that is due to low temperature very demanding process of heat treatment. The thermal properties of steel PROTAC 500M are the basis for the design of low temperature heat treatment and they were determined within this thesis. Thermal conductivity, specific heat and temperature conductivity of the steel at room temperature and at a temperature interval between room temperature, and the temperature of 480°C were determined using the Hot Disk TPS 2200 thermal properties analysis instrument in accordance with standard ISO 22007-2. Validation of the obtained parameters in the direct industrial environment was carried out on the basis of the measured thermal properties and the performed termotechnical calculations.