Influence Of Purifying Agent On The Non-metallic Inclusions And Total Oxygen Content In Bearing Steel

Bearing steel is one of the most typical special steel grades. It is widely used in the mechanical manufacturing, rail transport, automatic manufacturing and national defense industry. Its quality plays a remarkable role in the precision, property, lifetime and reliability of mechanical product.Due to the high demand on the total oxygen content, the Al deoxidation is largely used in bearing steel, nevertheless Al would result in nozzle clogging and the in continuous casting process formation of inclusions Al2O3which is hardness and easy to aggregate to form cluster inclusions. Once the cluster inclusions can’t float from the molten steel, it will break chains in the rolling process and seriously spoil the property of bearing steel.Based on the principle of slag filtrating and bubble metallurgy, the author added the pre-melted12CaO·7Al2O3and CaCO3into the molten steel and observed the results. The following conclusions were obtained in the experiment condition:(1) The total oxygen content of of G20CrNi2Mo bearing steel which is producted by ESR is20×10-6. The average diameter of the inclusion in the ingot is1.99μm. The SEM results show that the most inclusions are spinel with Al2O3and little MgO. Moreover, the MnS and MnS-spinel are found in the inclusions.(2) The addition of premelted12CaO·7Al2O3purifying agent show a great performance in total oxygen content, the amount, area and the size of inclusions. The smaller purifying agent particles can gain a better effect. However, if the particle is too small it will be residual in the molten steel. The result shows that increasing the dosage of purifying agent can improve the purification effect.(3) After relevant calculation, the study indicates that initial decomposition temperature of CaCO3in the molten steel is much lower than the refining temperature, which determines that the method to generate CO2bubble by the decomposition of CaCO3is a practical proposal. Also, here exists a linear relationship between the size of CaCO3powder and CO2 bubble, which can control the ideal size of CO2bubble by setting the size of CaCO3powder.(4) The addition of CaCO3foaming agent can remove the inclusion in the molten steel. the average diameter of inclusions decrease from1.6μm to1.3μm, the area fraction of inclusions decrease from0.03%to0.025%after adding CaCO3. But if the particle is too small, the decomposition product CaO may be residual in the molten steel and become the big inclusion.(5) The results of SEM indicate that the addition of pre-melted12CaO·7Al2O3purifying agent and CaCO3foaming agent play a meaningless effect on the composition and type of the inclusions. The main way of purifying agent and CO2bubble to remove inclusions relies on physical adsorption.(6) The MgO crucible can influence the composition of inclusions, because large amount of spinel were found in the steel. Refining slag will enhance this influence as well. The majority of inclusions are spherical type MgO in all the experiments with refining slag operation.