Abstract: During the ultrasonic testing of a 45 steel with aluminum and sulfur elements, it was found that there were many subcutaneous defects in the rolled material. The causes of these defects were analyzed by metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that these defects were macroscopic inclusions in steel. Combined with the pouring process of molten steel, it could be seen that the purity of molten steel was poor, and secondary oxidation occurred during the pouring process.The non-metallic inclusions in molten steel were effectively removed by optimizing the smelting process and optimizing the performance of the protective slag. The improved steel had no subcutaneous defects, and the level of non-metallic inclusions was also improved.