Formation Mechanism and Improvement of Magnetic Particle Inspection Defects in Cr5 Backup Roller Forged Ingot

Industrial tests and thermodynamic calculations were utilized to investigate the source and formation of magnetic particle inspection defects identified on the near-surface of the Cr5 back-up roll forged ingot, which was used in large cold rolling mills. The results showed that the linear aggregating SiO2-MnO-Al2O3 liquid inclusions up to 3 mm led to the flaw detection failure. SiO2-MnO-Al2O3 liquid inclusions were firstly formed in the inductive furnace. Due to its low contact angle, a huge amount of SiO2-MnO-Al2O3 liquid inclusions were inherited into the forged ingot. The formation of SiO2-MnO-Al2O3 liquid inclusions was attributed to the over-oxidation and relatively low aluminum content in the molten steel, as calculated by Factsage 8.1. Controlling the amount of aluminum in molten steel during the smelting process could modify the formation of SiO2-MnO-Al2O3 and CaO-SiO2-Al2O3 liquid oxide into solid Al2O3 type inclusions that were easily removed. Besides, the CaO-SiO2-Al2O3 liquid oxide could be transformed from CaO-Al2O3 type oxide by the significant loss of aluminum content during the VD process or slag entrapment. Certain content of aluminum in the molten steel could improve the flaw detection caused by the aggregating SiO2-MnO-Al2O3 inclusions effectively.