【作者】 刘洋；

【导师】 张立峰；

【作者基本信息】 北京科技大学 ， 钢铁冶金， 2018， 博士

【摘要】 本文采用真空感应炉进行钙处理的实验室实验。首先,研究了取样方式对于夹杂物成分和数量的影响。研究结果表明,真空感应炉良好的动力学条件,有利于促进夹杂物的上浮。过程样由石英管取样器穿过钢液上表面来取得,会取到较多的夹杂物,而终点样是将钢液浇铸到锭模,研究的是钢液内部夹杂物,数量相对较少。其次,研究了耐火材料对钙处理过程的影响。研究表明,钢中加入过量的钙,会与耐火材料发生反应。采用镁砂坩埚实验,样品中MgO平均含量为10～20%,钢液中总镁从13 ppm升高到23 ppm;采用刚玉坩埚实验,钢液中的[Al]s含量高于镁砂坩埚实验。坩埚气孔率会影响夹杂物的数量,高气孔率的坩埚会产生更多的夹杂物。钙处理改性夹杂物的过程中,钢中钙的含量由于挥发而持续降低。夹杂物的成分受此影响,转变规律为CaO-CaS→CaO-CaS-Al2O3→CaO-(CaS)-Al2O3→(CaO)-Al2O3。根据测量结果,计算钢液中[Ca]的趋势变化,对结果进行拟合分析,从而得到在真空感应炉中[Ca]的挥发传质系数为(2.35～3.53)×104(m/s)。接着,研究了钢中硫含量对钙处理效果的影响。瞬态CaS夹杂物的形成与分解是硫含量影响钙处理效果的关键。加钙初期CaO-CaS夹杂物的形成符合朗格缪尔化学吸附,采用该模型计算的结果与实际检测结果趋势一致。通过热力学计算研究了 CaS的分解与钢中的钙,氧和硫含量的关系。多数CaS随着钙的挥发而分解,只有少部分CaS会与夹杂物中A1203发生改性反应,生成均匀的Al2O3-CaO-CaS。因此,钢中硫含量越高,加钙初期生成的CaS含量越高,随着钢中钙含量降低,最终夹杂物中Al2O3含量越高。本文还研究了二次氧化对钙处理效果的影响。研究表明,轻度二次氧化对夹杂物的转变规律影响不大,只是使夹杂物中Al2O3含量和数密度升高。持续二次氧化发生后,钢中的铝和钙首先被氧化,当铝消耗完全后,钢液中的硅开始氧化。按照钢液的吸氧速率可分为三个阶段:初始氧化阶段、铝的稳定氧化阶段、硅的氧化阶段。三个阶段的吸氧速率依次升高。夹杂物的转变路径为CaO-CaS-Al2O3→CaO-(CaS)-Al2O3→(CaO)-Al2O3→Al2O3→SiO2。持续二次氧化使钢中夹杂物的数量升高至200个/mm2以上,远高于轻度二次氧化情况。此外,建立了动力学模型预测持续二次氧化过程中钢中夹杂物的转变规律,考虑了夹杂物的上浮去除对夹杂物和钢液成分的影响。最后,针对精准钙处理在实际生产中难以实现的现状,开发了一款在线指导软件。根据现场的操作参数和钢种需求,订制生成数据库,实时给出预测结果,并提供加入钙线长度的参考建议。本软件可以提高钙处理的效果,促进钢铁精炼过程智能化和自动化生产。更多还原

【Abstract】 In the current study,laboratory scale experiments for the modification of inclusions by calcium treatment of linepipe steels were performed with a vacuum induction furnace.The influence of different sampling methods on the amount of inclusions was investigated and validated by the measurement and the numerical simulation.In the vacuum induction furnace,the fluid flow of the molten steel under electromagnetic force favored the floating inclusions to the top so that a large number of inclusions accumulated at the top surface.More inclusions at the top surface were brought in silica pipe samplers,while most removed inclusions at the top would adhere to the inner wall of the crucible and could hardly be poured into the mold with the molten steel for the last tapped sample.The lining refractory has a great influence on the composition of inclusions since the reaction between excessive calcium and lining refractory occurred.For the experiment with a magnesia crucible,MgO content in inclusions are 10%-20%and the T.Mg content in steels increased from 13 ppm to 23 ppm.For the experiment with alumina crucibles,MgO content in inclusions was hardly measured,and more inclusions were found in steel samples when an Al2O3 crucible with a higher apparent porosity was used.Inclusions were transferred from CaO-CaS to CaO-CaS-Al2O3 and then to CaO-(CaS)-Al2O3 and finally to(CaO)-Al2o3 due to the decrease of calcium content during calcium treatment.The mass transfer coefficient of the dissolved calcium in the steel was estimated in the range of 2.35×104-3.53×10-4 m/s.Sulfur content in steel had an obvious effect on the composition of inclusions during calcium treatment.The formation and decomposition of CaS was the key point.Langmuir adsorption model was proposed for the formation of CaO and CaS just after the addition of Ca-Si powders,which was validated by the measurement.The formation of CaS highly depended on oxygen,calcium and sulfur contents in steel in a thermodynamic view.Since initially generated CaS inclusions were mostly decomposed due to the evaporation of calcium,while the extent of the modification reaction on alumina was limited by the increased sulfur content in steel.Therefore,the formation of CaS-CaO-Al2O3 inclusions with a high Al2O3 content was favored in steel with a high sulfur content in steel.Behavior of inclusions during reoxidization after caclium treatment of Al-killed steel was studied.Both Al2O3 content in inclusions and number density were increased after slight reoxidation occurred.Aluminum and calcium were oxidized firstly after the oxidation of the molten steel by the air,and then SiO2 formed due to the oxidation of silicon when the left[Al]s content was little in the molten steel.There are three stages involved during the process,i.e.,the beginning of reoxidation,aluminum oxidation and silicon oxidation,which oxidation rate gradually increased in each stage.The evolution path of inclusions was CaO-CaS-Al2O3-CaO-(CaS)-Al2O3→(CaO)-Al2O3→Al2O3-SiO2,and the number density of inclusions was higher than 200#/mm2.In addition,a kinetic model was proposed for the prediction of the evolution of inclusions during the reoxidation with air and validated with measured resutls,with consideration of the removal of inclusions.Finally,an online software was developed for the guidence of accurate control on inclusions during calcium treatment.The database was establised based on operation parameters and the steel composition in the production so that the predicted results of inclusions composition and suggestions for the addition amount of calcium are instantly available.The application of the software can siginifcantly improve the efficiency of calcium treatment,and make progress in the field of the intelligent and automatic production.更多还原