Abstract：For the gear steel 20CrNiMo rolled material with specification of more than[?80 mm，]it was found that the defect detection rate was low. Moreover， the problem of crack defects on the surface of rolled product was also observed. Based on the analysis of crack defects of the rolled products， the cracks were possibly caused by the crack defects in the billet. In order to verify this point of view and find out the exact reason so as to provide solutions， the shot blasting inspection of billet surface was firstly conducted， and no crack defect was observed. Secondly， the defects of the rolled product prepared from billet with cracks and the surface cracks of rolled product were compared and analyzed by metallographic microscope. It was found that the crack morphology and decarburization degree around were different， which indicated that the cracks on rolled product were not caused by the defects of the billet. Finally， the surface of cast billet was found to be seriously scratched and pitted by blasting the billet after slow cooling and be similar to that of the rolling cracks. The results showed that the cracks and warping defects were caused by scratches and pits in the process of casting but not caused by crack defects of the billet.
2020, Vol. 38 
