开浇过程二次氧化对铝脱氧不锈钢中夹杂物的影响

Effect of reoxidation on inclusions in Al-killed stainless steel during the casting start process

为了研究铝脱氧不锈钢开浇过程中二次氧化对钢水洁净度和夹杂物演变的影响,实现钢中夹杂物的有效控制,分别在LF精炼出站、开浇过程中不同时刻取样,采用扫描电镜、ASPEX自动分析仪、热力学计算等不同方法研究了铝脱氧不锈钢中夹杂物的形貌、成分、数量和尺寸分布,确定了铝脱氧不锈钢开浇过程中夹杂物的演变行为和对应机理。研究结果表明,开浇过程钢中氧氮质量分数、夹杂物数密度变化规律类似,20 min时分别增加至7.4×10^(−5)、0.0674%、17.1 mm^(−2),此后随着浇铸过程进行逐渐降低;LF精炼出站时钙处理改性夹杂物效果较好,其类型主要为CaO−Al_(2)O_(3)−SiO_(2)−MgO,开浇过程中二次氧化降低了钙处理操作的作用效果,20 min时夹杂物类型转变为MnO−Al_(2)O_(3)−SiO_(2)−CaO复合夹杂物,浇铸约60 min时,连铸过程中钢水的洁净度基本达到稳定,此时夹杂物类型重新转变为CaO−Al_(2)O_(3)−SiO_(2)−MgO;二次氧化使得钢液中氧质量分数较高,促进了MnO−Al_(2)O_(3)−SiO_(2)−CaO夹杂物的生成,而钢中大尺寸的CaO−Al_(2)O_(3)−SiO_(2)−MnO−(MgO)夹杂物主要通过夹杂物间的碰撞聚合形成;凝固过程中随着温度的降低,促进了MgO·Al_(2)O_(3)尖晶石相和CaO·2MgO·8Al_(2)O_(3)相的析出,提高了夹杂物中Al_(2)O_(3)组分的含量。

Stainless steels are widely used for corrosion resistance and as construction materials.The existence of harmful inclusions probably deteriorates corrosion resistance and easily causes nozzle clogging,surface defects,and the occurrence of cracks.Reoxidation during the casting start process significantly affects the cleanliness of molten steel,which may result in the downgrading or discarding of the steel.The production route of Al-killed stainless steel in this work is“EAF→AOD→LF→Calcium treatment→Continuous casting of round billet.”At LF departure,steel samples were taken at different moments during the casting start process to investigate the effect of reoxidation on the cleanliness of molten steel and the evolution of inclusions in the steel.It aims to achieve effective control of inclusions in the steel.The morphology,composition,amount,and size of inclusions in Al-killed stainless steel were studied using scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS),as well as automated SEM/EDS inclusion analysis(ASPEX).The effects of the oxygen content from reoxidation and the temperature decrease during solidification on the inclusion composition were calculated by the thermodynamic software FactSage 7.2.The evolution behavior and mechanism of inclusions during the casting start process of Al-killed stainless steel were analyzed and discussed.The findings showed that the total oxygen and nitrogen contents,as well as the number density of inclusions in the steel during the casting start process,indicated a similar change trend.They were increased to 7.4×10^(−5),0.0674%,and 17.1 mm^(−2),respectively,at casting 20 min,and then gradually decreased.Inclusions in the steel have been well modified by calcium treatment at LF departure,and its composition was primarily CaO−Al2O3−SiO2−MgO.The effects of calcium treatment were mitigated by reoxidation during the casting start process.Inclusions in the round billet were transformed to MnO−Al2O3−SiO2−CaO at casting 20 min.When the pouring time was 60 min,the cleanliness of the molten steel almost reached a steady state during continuous casting.The contents of total oxygen and nitrogen with the number density of inclusions in the steel were 3.2×10^(−5),0.0628%,and 7.1 mm^(−2),respectively,and inclusions were transformed back to CaO−Al_(2)O_(3)−SiO_(2)−MgO.Furthermore,reoxidation increases the oxygen content in molten steel and promotes the formation of MnO−Al_(2)O_(3)−SiO_(2)−CaO inclusions.Collision and coalescence among inclusions produce large-sized CaO−Al_(2)O_(3)−SiO_(2)−MnO−(MgO)inclusions in the steel.The decrease of the temperature during solidification promotes the precipitation of the MgO·Al_(2)O_(3) spinel phase and CaO·2MgO·8Al_(2)O_(3) phase.As a result,the Al_(2)O_(3) content in inclusions increases.