PDF《博士学位论文公示材料》

―1― 博士学位论文公示材料 学生姓名 迟云广 学号

1310204 二级学科 钢铁冶金 导师姓名 朱苗勇 论文题目 轴承钢冶炼过程夹杂物来源与转变机理研究 论文关键词 轴承钢;

夹杂物;

BOF;

耐火材料;

钢包釉;

夹杂物来源;

夹杂物转变 论文摘要(中文) 目前,国内钢铁企业的冶炼设备和冶炼技术已经达到国际先进水平,然而在控制轴承钢全氧和夹 杂物方面与国外先进钢铁企业仍存在差距.

为了提高轴承钢质量,本文在实验室实验和工业实验的基 础上,运用气体分析、成分分析、扫描电镜分析和热力学分析等多种手段,探讨了转炉冶炼过程成为 轴承钢夹杂物来源的可能性, 分析了耐火材料-钢包釉-钢液之间相互作用行为及其与轴承钢洁净度的关 系,并关注了转炉炉渣对控氮的影响与铝酸钙类型夹杂物的来源.本文试图通过这些研究,阐明生产 过程中更多细节对轴承钢夹杂物控制的重要性,如耐火材料和钢包釉,以期为提高轴承钢质量提供重 要支撑.本文主要研究内容和结论如下: (1)通过工业实验和实验室实验,分析了轴承钢转炉冶炼后的钢液中夹杂物的种类及其来源,研 究了铝脱氧过程夹杂物的生成与转变行为.研究发现:经转炉冶炼后的钢液中主要存在三种类型夹杂 物,分别是硅酸钙夹杂物、含有固体颗粒的硅酸钙夹杂物和(Fe, Mn)O 类型夹杂物.硅酸钙类型夹杂物 主要来源于转炉炉渣, (Fe, Mn)O 类型夹杂物则是在转炉吹炼过程中铁水氧化生成的. 在铝脱氧过程中, 氧化铝夹杂物不仅发生均质形核, 生成大量群簇状氧化铝夹杂物, 而且以(Fe, Mn)O 类型夹杂物为核心, 发生非均质形核.与此同时,(Fe, Mn)O 类型夹杂物逐渐被铝还原,最终生成颗粒状氧化铝夹杂物.此外,硅酸钙类型夹杂物在铝的作用下转变成为铝酸钙类型夹杂物. (2)通过在实验室开展耐火材料和钢包釉分别与铝镇静钢(只对轴承钢转炉冶炼终点钢液进行铝 脱氧得到铝镇静钢)反应实验,研究了耐火材料和钢包釉对钢液中氧化铝夹杂物的作用机理,系统的 对比了耐火材料和钢包釉对夹杂物的影响规律.结果表明:氧化铝耐火材料对钢液中的夹杂物几乎没 有影响;

氧化镁耐火材料可以促使钢液中氧化铝夹杂物转变成为尖晶石夹杂物;

在钢包釉的作用下, 钢液中的氧化铝夹杂物首先转变成为尖晶石夹杂物,再转变生成铝酸钙类型夹杂物.氧化镁耐火材料 与钢包釉相比,其与钢液之间的相互作用向钢液提供了更多的 Mg.由于 Mg 较Ca 在钢液中先生成, 因此氧化铝夹杂物首先转变成尖晶石夹杂物,再生成铝酸钙夹杂物.当形成钢包釉的钢包再次使用时, 尖晶石夹杂物快速生成的原因是耐火材料-钢包釉-钢液的相互作用. (3)通过在氧化镁耐火材料棒表面形成不同的钢包釉来模拟钢包的使用制度,并且在实验室进行 不同钢包釉与轴承钢反应实验,研究了钢包使用制度对轴承钢中的夹杂物和全氧质量分数的影响规律. 研究结果表明:由于钢包釉的脱落且与钢液之间的化学反应,钢包釉会恶化轴承钢的洁净度.随着钢 包釉碱度的降低,钢包釉与钢液的反应性逐渐增强,钢液全氧质量分数由 5.7 * 10-4 %分别增加到 6.4 * ―2― 10-4 %、9.5 * 10-4 %和11.7 * 10-4 %,即轴承钢洁净度的恶化程度逐渐增加.钢包釉不仅恶化当前炉次 的钢液洁净度,而且恶化连续炉次的钢液洁净度.轴承钢生产过程中,钢包专用制度更有利于控制轴 承钢质量.此外,随着浸入时间的增加和钢包釉碱度的减小,钢包釉对耐火材料的侵蚀程度逐渐增加. (4)通过工业实验,在轴承钢生产过程中进行全流程取样,研究了冶炼过程中夹杂物的来源及其 转变行为,分析了转炉炉渣对控氮的影响.研究结果表明:出钢脱氧合金化过程中,转炉钢液中的硅 酸钙类型夹杂物和钢包釉脱落形成的夹杂物在铝的作用下均生成铝酸钙类型夹杂物,(Fe, Mn)O 类型夹 杂物被铝还原成为氧化铝夹杂物.精炼过程中,氧化铝夹杂物首先转变成为尖晶石夹杂物,最终转变 成为铝酸钙类型夹杂物.VD 精炼过程会产生尺寸较大的铝酸钙夹杂物.浇铸过程中,钢液发生二次氧 化会重新生成氧化铝夹杂物.此外,引流砂也是夹杂物的一个重要来源.转炉吹炼初期脱氮很微弱, 转炉脱氮主要集中在吹炼15 %到80 %的过程中,转炉吹炼末期钢液有所增氮.炉渣泡沫化程度好,二 次脱氮作用明显,终点氮的平均质量分数为13.7 * 10-4 %,平均脱氮率为74.2 %,可以明显减少氮化物 夹杂物. ―3― 论文摘要(英文) Though the equipments and technologies of domestic steelmaking plants have reached international advanced level,there is still a big gap in controlling the quality of bearing steel, which is mainly related to total oxygen content and inclusion controlling. In order to improve the quality of bearing steel, the effects of BOF process, refractory and ladle glaze on the inclusions and total oxygen content were studied by thermodynamic analysis, industrial and laboratory experiments. The study is expected to illuminate the effects of BOF process, refractory and ladle glaze on the cleanliness of bearing steel, and provide more theoretical support for improving the quality of bearing steel. The main related conclusions are listed as follows. (1) The types of inclusions in BOF crude steel were studied by industrial experiments, and their evolution during Al deoxidation was also investigated by laboratory experiment in this study. The results show that the BOF crude steel is an important source of inclusions. Three types of inclusions ((Fe, Mn)O inclusions, calcium silicates as well as calcium silicates with some solid particles) were found in BOF crude steel, among which (Fe, Mn)O inclusions are the main types of inclusions. Calcium silicates come from BOF slag while (Fe, Mn)O inclusions form due to the reduction of molten steel. During Al deoxidation, alumina cluster could be formed because of homogeneous nucleation, while some singular alumina particles can also be generated on (Fe, Mn)O inclusions due to heterogeneous nucleation. Besides, (Fe, Mn)O inclusions would be reduced to form alumina by dissolved Al completely. Calcium silicates would transform into calcium aluminates with the help of dissolved Al in liquid steel. (2) The effects of different refractories (Al2O3 refractory, MgO refractory and glazed Al2O3 refractory) on the formation of spinel inclusions and calcium aluminate inclusions were studied by laboratory experiments in this study. It is found that the inclusions in bearing steel were hardly affected by Al2O3 refractory, while alumina inclusions would transform into spinel inclusions with the help of MgO refractory or glazed Al2O3 refractory. Meanwhile, glazed Al2O3 refractory could also promote the transformation of spinel inclusions into calcium aluminate inclusions. In contrast to ladle glaze, MgO refractory would paly a more important role in the formation of dissolved Mg. The formation of dissolved Mg and Ca decides the evolution sequence of alumina, and alumina would transform into spinel inclusions preferentially due to the faster formation of dissolved Mg, then calcium aluminate inclusions form with the formation of dissolved Ca. Ladle glaze is also an important source of calcium aluminate inclusions. It is the reaction between refractory and liquid steel that contributes to the rapid formation of spinel when the new MgO-bearing ladle lining is used, while the reason for the rapid generation of spinel in the subsequent heats is the interaction of refractory-ladle glaze-liquid steel. ―4― (3) Different ladle usage was simulated by MgO refractory rods with different glazes to investigate the effects of ladle usage on the cleanliness of bearing steel. The results show that the effect of different ladle usage on the cleanliness of steel varies greatly. The total oxygen content of steel would increase with the decrease of glaze basicity which increases from 5.7 ppm to 6.4 ppm, 9.5 ppm and 11.7 ppm, respectively. The ladle glaze with a lower basicity would deteriorate the cleanliness of the following production more severely. Meanwhile, inclusions can be generated by the flush-off of ladle glaze, and the initial glaze plays an important role in the evolution of inclusions in the following heats. In order to reduce the negative effect of different ladle usage and improve the steel cleanliness as much as possible, specialized ladles should be used when the bearing steel are to produce. MgO in refractory would be turned into ladle glaze by the mechanical and chemical erosion of ladle glaze. The erosion of refractory would be more seriously by ladle glaze with lower basicity and longer immersion time. Crack formed during serving period could decrease service life of refractory and be very harmful to steel cleanliness. (4) The source of inclusions and their evolution were investigated during industrial procedure. Besides, industrial experiments were carried out to study the effect of BOF slag on the removal of nitrogen. It is found that during deoxidation procedure, a large number of alumina inclusions were generated by the reaction of [Al] and [O]. At the same time, (Fe, Mn)O inclusions were also transformed into alumina inclusions by dissolved Al. Calcium silicates in crude steel and inclusions formed by ladle glaze were evolved into calcium aluminates with the help of dissolved Al. During refining process, some alumina inclusions were removed by refining slag and the rest would transform into spinel inclusions, then calcium aluminates. Calcium aluminates with larger size would appear during VD refining. Alumina inclusion........