Control Of Inclusions Of 12MnNiVR Vessel Steel In Rh Refining Process

With the development of application fields of pressure vessel to high pressure,high temperature and large capacity,the quality requirements of pressure vessel steel become higher and higher.Raising nondestructive inspection qualification rate is the main measure to ensure its internal quality.Domestic vessel steel enterprises have carried out extensive studies to improve the inspection qualification rate.Results showed that the general reason for inspection disqualification was silver defects which were caused b y Al2O3 inclusions after rolling.In this study,aiming at reducing the defects in 12 Mn Ni VR vessel steel plate in a domestic steel plant,samples of unqualified vessel steel plate were analyzed by Scanning Electron Microscope?SEM?and Energy Dispersive Spectrometer?EDS?.Total oxyge n content,quantity,size distribution,morphology and composition of inclusions in liquid vessel steel during RH refining and continuous casting were investigated to reveal the causes for inspection disqualification.In view of the RH refining process of vessel steel,liquid region of the top slag and equilibrium between slag and molten steel were calculated by Fact Sage to optimize the RH slag system.Moreover,influence of Ca,O,Al contents on the inclusion formation in steel was also studied and verified by high temperature experiments to optimize Calcium treatment process.Results showed that:?1?Blowing oxygen for rising temperature in the RH refining and reoxidation in the continuous casting were easy to result in excessive Al2O3 inclusions in steel.blowing oxygen flow and circulation time of RH refining should be reasonably controlled.Protective casting should be strengthened,and cast starting and ladle switching operatio n should be normalized.?2?The composition of the optimized slag is: CaO 25?40%,Al2O3 15?30%,MnO 25?35%,Si O2 5?20%.It is beneficial for the optimized slag which is in the liquid zone to absorb the inclusions.At the same time,The equilibrated aluminum content in steel is 300?500ppm,the equilibrated dissolved oxygen is less than 10 ppm.?3?When the calcium content varies in a certain range,it is easy to form liquid inclusions in steel including 3CaO·A1₂O₃,12 CaO·7A1₂O₃ and CaO·A1₂O₃.It was proved that the interval changes with oxygen content,which is 1.585[O]^1.1?[Ca]?0.571 [O]^0.933 obtained by mathematical analysis.