Abstract
	The material C-Mn-Nb-V steel was reheated in order to find out structural changes and precipitation behaviour of austenite grains size (AGS). Reheating temperatures were from 950 to 1250 ?C and reheating times were from 10 to 60min. After the reheating some AGS changes were observed. Temperature 1100?C showed that AGS began to rise from 78 to 162?m. It was caused by exceeded precipitation dissolution temperature. This temperature is threshold temperature for abnormal AGS and it is caused by drag effect of Nb carbides or carbonitrides to grain boundary which are soluted during the reheating. After the reheating of material the average AGS by calculating method of grains at the circular line was determinated. Measured data were compared with calculation data. Calculation data were created by two equations on the basic of the regression analysis and one of them Eq.2 is according to Dutta-Sellar model. Eq.1 by MATLAB program showed a good coincidence with measured data, but it is applicable for the fast calculation. Coefficients dependence on input AGS, chemical composition of steel, processing conditions and activation energy of grain growth were used for derived equation (2a). Both equations are suitable for comparison with measured data. Influence of holding time on growing of AGS application by equation (2a) showed the good description of change of AGS, where the input AGS was changing from 40 - 100?m. Determination of  and  deviations of measured and calculated data from equations and  were designed, respectively.