低氧高碳铬轴承钢LF-VD精炼时洁净度与夹杂物特征变化

Study on the variations of cleanliness and inclusions in low oxygen high-carbon-chromium bearing steel during LF-VD refining process

研究了低氧高碳铬轴承钢在LF-VD精炼时洁净度与夹杂物类型、数量、尺寸等特征的变化规律,并关注了个别大尺寸夹杂物。试验中采用了二元碱度约为7、Al2O3质量分数约30%的高碱度高Al2O3精炼渣。LF精炼前期和中期钢中夹杂物主要为Al2O3和MgO-Al2O3:由于与钢液润湿性较差能够较好地去除,数量密度由LF前期的11.3~15.4个/mm2减小至LF中期的4.9~6.7个/mm2,w(T.O)由0.0012%~0.0014%相应地降低至0.0006%~0.0014%;LF后期钢中夹杂物主要为高熔点的钙镁铝酸盐,w(T.O)=0.0006%~0.0010%。经过VD真空精炼后,夹杂物转变为低熔点的钙镁铝酸盐,数量密度由LF结束时的3.4~4.3个/mm2减少为1.2~2.5个/mm2,w(T.O)降低至0.0004%~0.0005%。软吹过程中,夹杂物平均尺寸略有增加,反映夹杂物之间发生了碰撞、长大,但夹杂物的数量密度维持在1.3~3.0个/mm2。特别地,LF结束、VD结束、软搅拌结束时,自动扫描电镜下检测到的夹杂物的最大尺寸分别为36,16,48μm。

In present paper,variations of total oxygen(T.O) and inclusions(including the type,number density,size,chemistry,etc.) in Al deoxidized high-carbon-chromium bearing steel were investigated by pilot trials,and special attention was paid to large inclusions. During refining,high basicity high alumina content slag was used,which was featured by weight ratio of CaO to SiO2 about 7 and Al2O3 mass fraction about 30 %. Inclusions were mainly solid alumina and MgO-Al2O3 spinel at initial and middle stage of LF refining. Because of poor wettability in molten steel,alumina and spinel indicated efficient removal. As a result,the mass fraction of T.O was obviously reduced from 0.001 2 %-0.001 4 % to 0.000 6 %-0.001 4 %,accompanied by decreased inclusion number density from 11.3-15.4 N/mm2 to 4.9-6.7 N/mm2 from initial to middle stage of LF refining. At the end of LF refining,T.O mass fraction was in the range of 0.000 6 %-0.001 0 % and inclusions were changed into solid calcium magnesia aluminates. After VD vacuum refining,calcium magnesia aluminate inclusions had been largely changed into liquid state and T.O mass fraction was furtherly reduced to 0.000 4 %-0.000 5 %. While inclusion number density witnessed an obvious decrease from 3.4-4.3 N/mm2 at the end of LF to 1.2-2.5 N/mm2 at the end of VD vacuum refining. In the afterwards Ar-bubbling,inclusion number density was roughly kept in the range of 1.3-3.0 N/mm2,while the average size of them indicated a tendency of increase. Particularly,the maximum size of inclusion was about 36,16,48 μm at the end of LF refining,VD refining and Ar gas bubbling,respectively.