轴承钢中大尺寸夹杂物的特征、来源及改进工艺

以韶钢BOF-ARS (氩站)-LF-RH-CC工艺路线生产GCr15轴承钢为研究背景,采用水浸超声探伤缺陷定位解剖、夹杂物金相显微镜与扫描电镜检验、全冶炼-连铸过程跟踪取样相结合的方法,研究了大尺寸夹杂物的特征和来源,并提出改进工艺。研究结果表明,大尺寸夹杂物主要有两类,一类是含6%~7%SiO₂(质量分数)的低熔点CaO-MgO-Al₂O₃-SiO₂类大颗粒微观夹杂,尺寸分布在50~500 μm范围,另一类是不含SiO₂的CaO-MgO-Al₂O₃类宏观夹杂,尺寸不小于500 μm。前者的主要来源为出钢的过程采用高黏度的低碱度渣与高熔点的石灰混加所引起的化渣不均匀而导致的卷渣;后者主要因为LF精炼工序添加的大量的铝钙精炼渣难以及时熔化而被卷入到钢液内部所导致。因此,精炼渣的设计和造渣工艺优化是改进上述大尺寸夹杂物的关键。改进后造渣工艺为,出钢过程中用钙铝精炼渣取代低碱度渣,并减少LF精炼工序外加的渣料,控制炉渣二元碱度(w(CaO)/w(SiO₂))在5~9范围,Al₂O₃质量分数为23%~28%。改进后炉渣流动性好,水口结瘤现象得到改善,轧材中主要为细小的MgO-Al₂O₃尖晶石及复合硫化物类夹杂,成品探伤合格率得到有效提升。

Characteristics,sources analysis of large size inclusions and technical improvement during bearing steel production

Based on the BOF-ARS(argon stirring)-LF-RH-CC process of bearing steel production in Shaoguan Steel Plant, the characteristics and sources of large size inclusions were explored through the method of water immersion ultrasonic test combined with metalloscope, scanning electron microscope and the systematic sampling during the metallurgical-continuous casting process, and the improved process was proposed. Results showed that there were mainly two kinds of large inclusions, one was low-melting CaO-MgO-Al₂O₃-SiO₂ inclusion from 6% to 7% SiO₂(mass percent), whose size was from 50 to 500 μm, and the other was CaO-MgO-Al₂O₃ without SiO₂, whose size was larger than 500 μm. The source of the former was the slag entrapment caused by the uneven slagging, which was the result of the combined charge of low basicity slag with high viscosity and high melting point lime during steel tapping. The latter was induced by the charge of large bulk of calcium-aluminate slag during the refining process, which was difficult to be melted rapidly and was entrapped into steel. Therefore, the design of refining slag and the optimization of the slagging process were the key points to decrease the large size inclusions. The improved slagging technology was applied by feeding the large bulk of calcium-aluminate slag during tapping in advance, instead of the low-basicity slag, and the addition amount of other slag was reduced during LF refining process. The basicity of refining slag (w(CaO)/w(SiO₂)) was controlled in the range from 5 to 9, and the mass percent of Al₂O₃ was from 23% to 28%. After the improvement, the fluidity of slag was good, and nozzle clogging was reduced. The main inclusions in the products were micro MgO-Al₂O₃ spinel and composite sulfides. The qualified rate of bearing steel products evaluated by ultrasonic test was significantly improved.