Behavior Of Non-metallic Inclusions In Ultra-low Carbon Al-Si Killed Steel During Refining Process And Their Impact On Microstructure

With the rapid development of China’s economy, the demand for sheet steel is alsoincreasing rapidly. Ultra-low carbon Al-Si killed steel, as a soft magnetic alloy with a lowcarbon content (C≤20ppm), often used as core materials for motors and generatorsworking in the rotating magnetic field, which is a kind of necessary magnetic materials forelectric, telecommunications and military industry. Nowadays Ultra-low carbon Al-Sikilled steel, as the largest amount of magnetic material, has been used as an importantenergy-saving metal functional materials. The magnetic properties of the Ultra-low carbonAl-Si killed steel mainly depend on the grain size of ferrite, the crystallographic texture,and the inclusions in the steel. Non-metallic inclusions’ shape, size and their distributionhas a significant effect on microstructure and properties of the steel.In this paper, the formation of inclusions during the RH refining process and theimpact of ladle slag composition (CaO-SiO2-Al2O3-MgOsat-MnO-FetO slags) on theinclusions were experimentally investigated which depended on the RH refining processand slag-steel equilibrium experiments. The online observation of microstructure of steelduring reheating was carried out by ultra high-temperature laser confocal microscope. Theimpact of inclusions on the grain growth of steel was also studied.The results show that the inclusions are Fe-Mn oxide based complex inclusionsbefore RH refining and the number and size of inclusions are greatly impacted by slagbasicity, Al2O3content and ratio of CaO to Al2O3. The number of inclusions is the lowestwhen the basicity is about1.5and the content about20%. The experimental results byoptimized ladle slag equilibrated with the steel, the final steel at the end of RH refiningprocess contains mainly Al2O3-MgO and Al2O3-MgO-SiO2-MnO complex inclusions.Inclusions are mainly less than2μm which accounts for70%~80%of the total number ofinclusions. The number of inclusions greatly reduced, but the inclusions within the rangeof2~5μm increased and the size of inclusions has a slight tendency to increase. Inaddition, The Mn content of complex inclusions and steel increases with increasing ofMnO content in slag, inclusions less than1μm increases and the average size of inclusionsreduces accordingly. After reheating treatment, the inclusions are mainly Al2O3-MgO-SiO2complexinclusion which accounts for90%of the total number of inclusions or more. the numberof inclusions in steel increases compared those samples without reheating treatment. TheMnO content of complex inclusions is less than1%which rarely contain MnS.Al2O3MgO SiO2MnO based complex inclusions become to Al2O3MgO SiO2basedcomplex inclusions in1350℃with the increasing temperature. the MnO content ofcomplex inclusions is affected by experimental conditions during reheating treatment.Inclusions are mainly less than1μm under our experimental conditions.The inclusions lessthan1μm reduces during reheating treatment when reheating temperature is1350℃,which is different from other heating temperature.Under the experimenta l condition, the phase transition temperature of steelsdecreases with increasing of MnO content in slag but the grain growth needs higherreheating temperature during reheating process, the grains are easy to grow and the stablegrain size is bigger under higher temperature. When the grain size is bigger, the grain sizeof ferrite will become bigger during the cooling process. Depending on the demand oninclusions and grain, reheating under1250℃for30min is the most appropriatecondition and the most appropriate slag is46.11%CaO-21.52%SiO2-Al2O3-9.67%MgO-0.32%FetO-1.4%MnO. In conclusion, for the need of the Ultra-low carbon Al-Si killedsteel in post-processing, we should strictly control MnO content of top slag in RH refiningprocess.