【作者】 于雄；

【导师】 文光华；

【作者基本信息】 重庆大学 ， 冶金工程， 2011， 博士

【摘要】 保护渣是连铸过程中使用的一种合成渣,其性质对连铸工艺顺行和铸坯表面质量至关重要。保护渣加在钢液面,绝热保温;其与钢液接触部分熔化形成熔渣层,防止弯月面钢液被氧化,吸收钢液中上浮的夹杂;熔渣流入结晶器与铸坯之间,液渣膜润滑铸坯,防止粘结漏钢;固渣膜控制传热,以形成均匀的坯壳。高Al含量钢种连铸过程中,由于钢液中的Al易与常规保护渣中的重要组份SiO₂发生反应导致保护渣w（Al₂O₃）/w（SiO₂）及Al₂O₃含量增加,从而引起保护渣性质的改变,影响连铸工艺顺行。因此,设计性能优良的结晶器保护渣是高铝钢连铸工艺顺行的关键。根据高铝钢与保护渣反应的特点,即高Al钢液中的[Al]与常规保护渣中（SiO₂）反应是不可避免的,但该反应在较短时间内可以达到动态平衡,由此,提出了两种高铝钢保护渣的设计思路:（i）低碱度CaO-SiO₂渣系保护渣:该保护渣与高铝钢液中的Al反应达到平衡后,保护渣的性质满足连铸要求;（ii）低SiO₂含量CaO-Al₂O₃渣系保护渣,该渣系能有效的避免保护渣与钢液中的Al发生反应。本文采用半球点法测量了保护渣的熔融特性。试验结果表明对于CaO-SiO₂渣系保护渣,随着w（Al₂O₃）/w（SiO₂）增加,保护渣熔点均呈上升趋势;保护渣中增加Li₂O与B₂O₃含量均能有效的降低其熔点,B₂O₃对熔点的影响尤为明显;对于CaO-Al₂O₃渣系保护渣,增加B₂O₃、Li₂O、Na2O、F-、BaO、SrO等含量均能降低保护渣熔化温度,其中Li₂O对熔点的影响最明显。采用旋转粘度法测量了保护渣的流变特性。低碱度CaO-SiO₂渣系保护渣在高铝钢浇铸过程中变性为CaO-Al₂O₃-SiO₂渣系;随着w（Al₂O₃）/w（SiO₂）增加,熔渣粘度随之改变;Al₂O₃呈现出两性特征;含有Li₂O+B₂O₃组合的保护渣变性过程中粘度变化平稳且处于合理范围内。对于CaO-Al₂O₃渣系保护渣,B₂O₃、Li₂O、Na2O及F-等能降低保护渣的粘度;随着MgO含量增加,CaO-Al₂O₃渣系熔渣粘度先降低后升高;BaO及SrO含量增加,CaO-Al₂O₃渣系熔渣粘度升高。在Riboud模型基础上,建立了适合高Al钢保护渣的粘度计算模型。采用渣膜热流模拟仪研究了保护渣渣膜的传热特性。对于CaO-SiO₂渣系保护渣,随着w（Al₂O₃）/w（SiO₂）增加,渣系的热流密度呈降低趋势,渣膜传热能力减弱;在该渣系中加入一定量的Li₂O及（或）B₂O₃有利于增加渣膜热流密度。对于CaO-Al₂O₃渣系保护渣,B₂O₃降低结晶率,降低渣膜厚度,增加热流密度;Li₂O增加结晶率,降低渣膜厚度,降低热流密度,Na₂O降低结晶率,降低渣膜厚度,增加热流密度;F-增加结晶率,降低渣膜厚度,增加热流密度;MgO及SrO均降低结晶率,降低渣膜厚度,热流密度略有增加,BaO替代CaO后,结晶率降低,降低渣膜厚度,热流密度变化平稳。利用热丝法（Single Hot Thermocouple Technique,简称SHTT）构建保护渣的等温转变（Temperature Time Transformation,简称TTT）曲线,研究不同组成对保护渣结晶孕育时间的影响规律;通过构建保护渣的连续冷却转变（Continuous Cooling Transformation,简称CCT）曲线,探讨保护渣组成对临界冷却速度的影响,而且利用SHTT研究了析晶活化能。对于CaO-SiO₂渣系保护渣,Li₂O对结晶孕育时间影响不大,B₂O₃能有效延长孕育时间。对于CaO-Al₂O₃渣系保护渣,随B₂O₃、Na₂O、MgO、BaO和SrO增加,保护渣的TTT曲线向孕育时间增加的方向移动;Li₂O能有效缩短保护渣的晶体孕育时间,同时,提高保护渣的结晶速度,F-对结晶孕育时间影响不大,但能有效提高结晶速率。B₂O₃和Na₂O降低了结晶速率,结晶活化能增加;Li₂O和F-增加了结晶速率,结晶活化能降低;MgO增加了结晶速率,同时也提高了结晶活化能。将研究所取得的成果应用于太钢无磁钢实际生产,取得了良好的效果。更多还原

【Abstract】 Mold slag is a synthetic slag added to the mold during the continuous casting of steel, and its property significantly determines the stability of the continuous casting process of steel and surface quality of the slab. Slag is fed onto the top of the mold to provide thermal insulation; the molten slag protects the steel meniscus from reoxidation and absorbs inclusions from the steel into the molten slag pool; liquid slag ?ows into between solidified shell and a mold, the liquid slag film helps lubrication between the water cooled mold and the solidified shell surface of the steel to prevent sticking against the mold surface, which occasionally leads to breakout, and solid slag film controls heat transfer to achieve uniform shell.During the continuous casting of high-Al steel, [Al] is likely to react with （SiO₂） in the mold slag, which results in increase of w（Al₂O₃）/w（SiO₂） ratio and the Al₂O₃ content, respectively. The changes of mold slag composition would lead to the property changes, which influence the smooth operation of the continuous casting process. So, how to design a suitable mold slag is a key issue for the continuous casting of high-Al steel.Based on the characteristics of reaction between the [Al] and （SiO₂）, that the reaction between the [Al] and （SiO₂） sourced from normal mold slag is inevitable, and this reaction would reach equilibrium in a comparatively short time, two ideas about how to design a suitable mold slag for high-Al steel were proposed: （i） CaO-SiO₂ mold slag with low basicity, this slag would have a suitable property after the reaction between the [Al] and （SiO₂） reached equilibrium; （ii） CaO-Al₂O₃ mold slag with low SiO₂ content and high Al₂O₃ content, this slag would avoid the reaction between the [Al] and （SiO₂）.Hemisphere point temperature was used to study the melting characteristics of mold slag. The results showed that, for CaO-SiO₂ mold slag, the melting points would increase with the increase of w（Al₂O₃）/w（SiO₂）; B₂O₃ and Li₂O could decrease the melting point, especially B₂O₃ exhibited the potent capability to decrease the melting point; for CaO-Al₂O₃ mold slag, the melting point would decrease with the increase of B₂O₃, Li₂O, Na₂O, F-, BaO, SrO, and among these slags, Li₂O had a more evidence influence than others.Rotation viscosity was used to study the rheological property of mold slag. Low basicity CaO-SiO₂ slag would be transformed into CaO-Al₂O₃-SiO₂ mold slag during casting of high-Al steel; the molten slag viscosity would change with the increase of w（Al₂O₃）/w（SiO₂）, Al₂O₃ presented amphoteric behavior; the viscosity of slag containing both Li₂O and B₂O₃ is suitable and ?uctuates only moderately with the increase in w（Al₂O₃）/w（SiO₂）. For CaO-Al₂O₃ series slag, B₂O₃, Li₂O Na₂O and F- could decrease the viscosity, while the viscosity would decrease and then increase with the increase of MgO. With the increase of BaO and SrO, the viscosity of CaO-Al₂O₃ mold slag would increase. Based on the Riboud model, a suitable viscosity mathematical model for the mold slag with high Al₂O₃ content was established.Heat flux simulator of mold slag film was used to study the heat transfer behavior of slag film. For the CaO-SiO₂ slag, the heat flux through the slag film would decrease with the increase of w（Al₂O₃）/w（SiO₂）; heat flux increases with the increase of Li₂O and/or B₂O₃ content in the mold slag. For the CaO-Al₂O₃ slag, B₂O₃ could decrease the crystallization rate, decrease the thickness of slag film, and increase the heat flux; Li₂O could increase the crystallization rate obviously, decrease the thickness of slag film, and decrease the heat flux; Na₂O could decrease the crystallization rate, decrease the thickness of slag film, and increase the heat flux; F- could increase the crystallization rate, decrease the thickness of slag film, and increase the heat flux; MgO and SrO could decrease the crystallization rate, decrease the thickness of slag film, and increase the heat flux appreciably; using the BaO to replace the CaO, the slag film thickness and crystallization rate would decrease, and the change tendency of heat flux was smoothly.The influence of the slag chemical composition on the incubation time of mold slag was studied by constructing Temperature Time Transformation （TTT） diagram, the influence on the critical cooling rate was obtained by constructing of Continuous Cooling Transformation （CCT） using SHTT, in addition, SHTT was used to studied the kinetics of crystallization and crystallization activation energy of CaO-Al₂O₃ slag. For the CaO-SiO₂ slag, Li₂O have little effect on the incubation time, and B₂O₃ could prolong the incubation time obviously. For the CaO-Al₂O₃ slag, B₂O₃, Na₂O, MgO, BaO and SrO would prolong the incubation time; Li₂O would shorten the incubation time, at the same time, increase the rate of crystallization of the slag; F- had little effect on the incubation time, but could increase the rate of crystallization of the slag. B₂O₃ and Na₂O could decrease the crystal growth rate, increase the crystallization activation energy; F- and Li₂O could increase the crystal growth rate, decrease the crystallization activation energy; MgO could increase the crystal growth rate and the crystallization activation energy. Through the practical casting application of high-Al content non-manganese steel in the Taiyuan Iron&Steel（Group） Co.,Ltd., and expected results was obtained.更多还原