The Slag Optimization On Refining Processes Of Ultra-low-oxygen Gear Steel And Research On Magnesia-alumina Inclusions

The fatigue crack is easy to occur in gear steel affecting the transmission life of gears forgears bear the cyclic stress and the impact force under working condition. The transmission lifeof gears is mainly affected by the cleanliness of steel.28MnCr5gear steel is of high standardwhich is introduced from abroad and the the quality between China and international level exists.This work researches on the effect of slag composition on the cleanliness of28MnCr5in refiningprocesses and provides improvement measures.The quality of gear steel is mainly determined by its chemical composition and the chemicalcomposition fluctuation influences the hardenability of steel directly. However, the steelcomposition is influenced by the steel/slag reaction, then the steel/slag equilibrium is calculatedwith an aid of FactSage and the relationship between slag composition and [O]、[Al]、[Ca]、[Mg]、[S] in molten steel is discussed.The main task to improve the cleanliness of ultra-low-oxygen gear steel is to reduce theoxygen content of molten steel for low oxygen content improves the transmission life greatly.The effect of SiO2and CaO/Al2O3ratio of slag on the content of dissolved and total oxygen iscalculated. The present work measured the equilibrium oxygen content in molten steel within6~10seconds with ZrO2solid electrolyte; Simultaneously, the total oxygen content inequilibrium is also measured by the oxygen-nitrogen combined analyzer. The effect of SiO2andCaO/Al2O3ratio of slag on the cleanliness of molten steel is discussed by comparing the contentof dissolved and total oxygen of varying slag composition.The magnesia-alumina spinel inclusions deteriorate the steel quality when the oxygencontent is reduced greatly.The equilibrium reaction between the28MnCr5molten steel and themagnesia-alumina spinel inclusions has been investigated by software Factsage. It is concludedthat the critical [Mg] increases from0.39to0.42×10-6with the [Al] ranging from0.02%to0.04%, which makes the spinel inclusions form in molten steel. It is found that the formation ofspinel inclusions in28MnCr5is unavoidable in the present refining process. The calculationresults show that the spinel inclusions could not be transformed into the low-melting point liquidinclusions when [Mg]>8.5×10-6. When [Mg] is less than that, the spinel inclusions could be transformed according to the route “spinel→CaO-Al2O3-MgO liquid inclusions→CaO” with theincreasing [Ca]. The critical [Ca] is3×10-6which could transform spinel inclusions into liquidinclusions completely. According to the calculation results, the calcium reduced from refiningslag was not enough to make spinel inclusions transformed completely. It is necessary to increase[Ca] by application of calcium wire feeding process.