Formation mechanism and improvement of detected defects in large forged Cr5 back-up roll

Based on ultrasonic flaw detection on the large forged Cr5 back-up roll during manufacturing process, the flaw detection was analyzed by anatomical sampling, IF→LF→VD→VC smelting process of forged roll was traced and sampled, defects type on the forged roll and source of the whole smelting process was determined by scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS). Combined with FactSgae 8.1 software, the formation mechanism and control of flaws leading to the ultrasonic flaw detection were calculated theoretically. The research results show that the defects leading to the incompatibility of flaw detection are linearly aggregated SiO₂-MnO-Al₂O₃ large-size inclusions, the size of single particle can reach 200 μm, the compositions of inclusion of flaws are consistent with the inclusions before pouring of vacuum casting(VC). A large amount of SiO₂-MnO-Al₂O₃ inclusions are found before the tapping of IF and still a few remained at the end of LF process, while large SiO₂-MnO-Al₂O₃ inclusions are entrapped to the roll melt during VD process under large stirring conditions. SiO₂-MnO-Al₂O₃ inclusions with a low contact angle can be well wetted by the liquid steel, it is difficult to float and remove from the liquid steel during the LF refining process. The inclusions are aggregated and remained during the solidification process to form the area of large inclusions and lead to ultrasonic flaw detection after forging eventually. Thermodynamic calculations show that SiO₂-MnO-Al₂O₃ liquid inclusions will be stable with the steel composition containing mass fraction of Al below 0.003% and O above 0.015%, and SiO₂-MnO-Al₂O₃ liquid inclusions are stable with a higher O content with increasing of Al content in liquid steel.SiO₂-MnO-Al₂O₃ phase stable field will disappear with Al increasing to 0.023% in the liquid steel. Before the tapping of IF and in the subsequent processes, controlling the Al mass fraction in the liquid steel above 0.023% can effectively modify the SiO₂-MnO-Al₂O₃ liquid inclusions into Al₂O₃ or Al₂O₃-rich inclusions with a good removability. Practical results show that the unqualified rate of ultrasonic flaw detection of Cr5 back-up roll forgings is reduced.