硅锰脱氧55SiCr弹簧钢中镁铝尖晶石的形成及演变

 某钢厂生产的55SiCr弹簧钢采用硅锰脱氧工艺,但在其冶炼过程中存在大量尖晶石类夹杂物,对最终产品的性能十分不利。尖晶石等硬、脆性夹杂物是弹簧在服役过程中疲劳断裂的主要因素之一,因此为明确弹簧钢中该类夹杂物的来源,进而控制并去除钢中非金属夹杂物,通过夹杂物自动分析、扫描电镜和能谱分析等手段,结合FactSage热力学计算分析了55SiCr弹簧钢冶炼过程夹杂物的演变及主要夹杂物的形成机理。分析结果表明,LF精炼后钢中夹杂物数量大幅上升,且其平均成分偏向SiO₂-Al₂O₃-CaO三元相图中高熔点区域;夹杂物主要以SiO₂·Al₂O₃·CaO·MgO为主,多表现为钙铝酸盐包裹或半包裹尖晶石的复合夹杂物类形态,此外还有少量单独的尖晶石夹杂物存在于钢中。对于上述夹杂物的形成及演变进行热力学计算,结果表明,钢液中Mg、Al含量上升将导致钢中析出大量尖晶石夹杂物,并与液态夹杂结合形成含镁复相夹杂物;同时,钢液成分的变化也会导致精炼过程生成的SiO₂·Al₂O₃·CaO·MgO类夹杂物中MgO、Al₂O₃含量大幅增加,在复合夹杂物内部析出尖晶石相。因此,为减少硅锰脱氧弹簧钢中尖晶石类硬脆性夹杂物的生成,需要严格控制钢中Mg、Al含量,尽可能降低夹杂物中MgO、Al₂O₃含量,以实现对弹簧钢中非金属夹杂物的塑性化控制。

Formation and evolution of magnesia-alumina spinel in Si-Mn deoxidized 55SiCr spring steel

The 55SiCr spring steel produced by a steel mill adopts silicon-manganese deoxidization process, but there are a lot of spinel inclusions in its smelting process, which is very unfavorable to the final product performance. Hard and brittle inclusions are one of the main factors during fatigue fracture of spring in service, so in order to clarify the source of spinel inclusions in spring steel and then control and remove non-metallic inclusions in steel, the evolution of inclusions and formation mechanism of main inclusions in 55SiCr spring steel during smelting were analyzed by means of automatic inclusion analysis, scanning electron microscope and energy spectrum analysis, combined with FactSage thermodynamic calculation. The analysis results show that the number of inclusions in the steel after LF refining increases greatly, and its average composition tends to the high melting point region in the ternary phase diagram of SiO₂-Al₂O₃-CaO. The inclusions are mainly SiO₂·Al₂O₃·CaO·MgO, mostly in the form of calcium aluminate-wrapped or semi-wrapped spinel composite inclusions, and a small amount of single spinel inclusions exist in steel. Thermodynamic calculation of formation and evolution for above inclusions shows that increase of Mg and Al content in molten steel will lead to the precipitation of a large number of spinel inclusions in steel and combine with liquid inclusions to form Mg-containing multiphase inclusions. At the same time, the change of molten steel composition will also lead to the substantial increase of MgO and Al₂O₃ contents in SiO₂·Al₂O₃·CaO·MgO inclusions generated during refining, and the spinel phase will be precipitated in the composite inclusions. Therefore, in order to reduce the formation of spinel-like hard and brittle inclusions in Si-Mn deoxidized spring steel, it is necessary to control the contents of Mg and Al in the steel strictly and reduce the contents of MgO and Al₂O₃ in the inclusions as much as possible, so as to realize the plastic control of non-metallic inclusions in spring steel.