【作者】 李慧改；

【导师】 翟启杰； 郑少波；

【作者基本信息】 上海大学 ， 钢铁冶金， 2008， 博士

【摘要】 非金属夹杂物往往造成钢材表面和内部缺陷,但是目前生产工艺无法将钢中夹杂物完全去除。研究表明:当夹杂物小于3μm、种类适当且分布均匀时能够改善钢的强韧性能。以钛氧化物为主复合了硫化锰和氮化钛的细小夹杂物容易诱发晶内铁素体（IGF）,细化晶粒,提高钢材强韧性。因此控制钢液中钛氧化物的析出以使其尺寸小于3μm,并且在钢中弥散分布成为重点研究课题。由于在钢液凝固过程中析出的夹杂物尺寸相对比较细小,本论文系统研究了钢液凝固过程中超细钛氧化物的析出规律,并通过理论计算和定向凝固实验验证了凝固过程中可以析出超细、弥散分布的钛氧化物夹杂。本文首先从通过热力学计算分析了控制钛的氧化物在凝固过程中析出所需要的热力学条件以及生成的钛氧化物的类型。钛的氧化物易于成为IGF核心,为了使钢中析出钛的氧化物,必须控制钢液中的铝钛比。在凝固温度范围内,Ti的氧化物中Ti₃O₅是最稳定的。当钢液温度为1873K,钢液中Ti含量为0.02%时,铝浓度要小于34ppm。凝固前氧含量不同时,钢液中的[Mn]会影响钛的氧化物夹杂的类型。平衡氧含量为30ppm时,夹杂物的类型为锰钛复合氧化物;平衡氧含量在20ppm时,凝固前期生成钛的氧化物,后期生成锰钛复合氧化物;氧含量在10ppm时只生成钛的氧化物。鉴于钛的氧化物中Ti₃O₅热力学最稳定,本文利用数学模型理论计算了凝固过程中不同氧含量及不同冷速下析出Ti₃O₅的含量及直径。在钢液凝固过程中,由于液相和固相中存在溶解度差异会发生溶质元素的微观偏析,凝固过程中温度的降低,在凝固前沿富集大量溶质,提高了[Ti]和[O]的浓度积,[Ti]、[O]反应生成Ti₃O₅夹杂。计算采用了有限差分法来完成,计算区域为二次枝晶臂间距的一半,溶质的质量传输仅由扩散控制。模型在计算溶质元素的微观偏析的同时考虑了Ti₃O₅的析出对溶质浓度的消耗,使溶质的微观偏析程度和Ti₃O₅析出量的计算更加合理。通过数值模拟表明:凝固前钢液中氧含量越高,生成的Ti₃O₅直径越大:冷速越大,Ti₃O₅直径越小。当冷速为10K/min时,对应于实际生产情况,相当于模铸的冷速,凝固过程中析出的Ti₃O₅尺寸约为2.5μm。在冷速为1000K/min时（薄板坯连铸的冷速）,Ti₃O₅的尺寸为0.7μm,非常细小。从理论上证明凝固过程可以形成超细的钛氧化物。理论分析的基础上,本文采用定向凝固实验研究了氧位、冷却速率对凝固过程中析出的夹杂物的数量、种类和尺寸的影响。定向凝固实验中溶质元素沿凝固方向一维扩散,在凝固前沿的富集造成夹杂物在凝固前沿液相的析出。实验时分别加入锰铁、硅铁、钛铁或者锰铁/硅铁、锰铁/钛铁的混合物研究不同氧化物在凝固过程中的析出规律。实验结果表明,分别添加锰、硅及硅-锰元素时,凝固过程中生成的夹杂物尺寸为1.3μm、1.2μm、1.1μm。添加锰时,夹杂物数量最多,添加硅时数量最少,分别为1.1×10⁵/mm³和5.8×10⁴/mm³。添加锰、钛元素后生成的夹杂物类型受凝固前氧含量的影响,在10ppm平衡氧含量时,凝固过程中析出的均为钛的氧化物;20ppm氧含量时生成的夹杂物为钛的氧化物和锰钛复合氧化物;30ppm氧含量时生成的夹杂物为锰钛复合氧化物。钛氧化物尺寸受冷速的影响较大。冷速越大,其平均尺寸越小。当冷速为9K/min,89K/min以及143K/min时,其平均直径分别为2.0μm,1.2μm以及1.0μm。从实验证明凝固过程能够形成超细夹杂物。针对目前存在的夹杂物检测方法,本文提出了检测超细夹杂物的方法,提取了定向凝固实验得到的试样中的超细钛氧化物。即,酸溶分离;超细孔径的聚碳酸脂膜过滤提取夹杂物;扫描电镜和高分辨透射电镜检测。获得了超细夹杂物的三维形貌、尺寸、化学成分及结构等大量有价值信息。这为进一步分析其晶体长大习性,从而为控制这些夹杂物在钢中的形态打下了基础。通过对钢中夹杂物的结构分析发现,钢中不含锰的钛氧化物的类型为Ti₃O₅:而当钢中含锰时,凝固过程中首先析出Ti₃O₅,然后Ti₃O₅与钢中的[Mn]发生作用生成Ti₂O₃与MnO的超细复合氧化物,为MnS复合和贫锰区形成创造有利条件,而Ti₂O₃周围形成贫锰区有利于IGF的形核。完善了IOF的形成机制。更多还原

【Abstract】 For steels,many efforts have been made to remove the nonmetallic inclusions because of their harmful effects to material quality.However,fine dispersed nonmetallic inclusions（＜3μm） can improve the properties of steel.The fine Ti oxides combined with MnS and TiN may refine the crystal by acting as heterogeneous nucleation sites of intragranular ferrite（IGF）,consequently, enhance the strength and toughness of steel.To effectively utilize the ultraflne inclusions,the key issue is to obtain the fine dispersed Ti oxide inclusions.Inclusions precipitated during solidification are relatively small.In the paper,the precipitation of Ti oxides during solidification was studied systematically.The theoretical and experimental results showed that fine dispersed Ti oxide inclusions could be obtained during solidification of steel. The effect of oxygen content,alloying elements contents on the type of inclusion was studied through thermodynamic calculation.Ti oxide is favorable for IGF nucleation,the A1/Ti ratio should be controlled to ensure that Ti oxides precipitate during solidification of steel.Ti₃O₅ is the stablest among all Ti oxides in the range of solidification temperature.[A1]content should be less than 34ppm when[Ti]content is 0.02%at 1873K.[Mn]in liquid steel can affect the type of Ti oxide for given oxygen content.Inclusions are mainly Ti oxide when oxygen content is 10ppm;they are Ti oxide and Mn-Ti oxide when it is 20ppm;Mn-Ti oxide dominates when it is 30ppm.The content and diameter of Ti₃O₅ precipitated during solidification were calculated at different oxygen content and cooling rate by a coupled mathematical model.The microsegregation of[Ti] and[O]in front of the liquid-solid interface leads to the precipitation of Ti₃O₅ during solidification of steel.Ti₃O₅ inclusions were engulfed by the solid phase immediately resulting in fine dispersed inclusions in steel.The finite difference method was introduced to the calculation.The calculation domain is half of the secondary dendrite arm spacing.The mass transmission is only controlled by diffusion.The consuming of[Ti]and[O]for the precipitation Ti₃O₅ was considered in the model.Results showed that the diameter of Ti₃O₅ is 2.5μm at cooling rate of 10K/min which is in the condition of In-mode casting and it is 0.7μm at cooling rate of 1000K/min which is in the condition of thin slab continuous casting.It proved theoretically that ultrafine inclusions can be formed during solidification.Based on theoretical analysis,the effect of oxygen content and cooling rate on number,type and size of inclusions were researched by directional solidification experiment.The solute dements diffused along one direction and the microsegregation of them resulted in the precipitation of inclusions in unidirectional solidification.Mn-Fe,Si-Fe,Ti-Fe or the mixture of Mn-Fe/Si-Fe Mn-Fe/Ti-Fe were added in Fe to investigate the precipitation of oxide inclusions during solidification.The results showed that the diameters of inclusions are 1.3μm、1.21μm、1.1μm when Mn-Fe,Si-Fe and Mn-Fe/Si-Fe were added to steel.The amount of the inclusions is the most with Mn-Fe addition and the least with Si-Fe addition.When Mn-Fe/Ti-Fe was added to steel,the type of Ti oxide was affected by oxygen content.Inclusions are mainly Ti oxide when oxygen content is 10ppm;Ti oxide and Mn-Ti oxide when it is 20ppm;Mn-Ti oxide dominates when it is 30ppm.The size of Ti oxides is mainly affected by cooling rate and it decreases with the increase of cooling rate.The average diameters are about 2.0μm,1.2μm and 1.0μm at the cooling rates of 9K/min,89K/min and 143K/min,respectively.The result is consistent with the calculation result of the coupled model.It proved experimentally that ultrafine inclusions formed during solidification.To extract effectively and detect exactly the ultrafine oxide inclusions in steel sample made by directional solidification,Nuclepore Track-Etch Membrane was introduced to improve the traditional acid dissolution and electrical extraction methods in filtering process.Threedimensional morphology and the structure of TiOx inclusions were detected by scanning electromicroscopy（SEM） and transmission electromicroscopy（TEM）.Other information such as the size and composition etc.were also obtained.The structural detection results showed that the type of Ti oxide precipitated during solidification is Ti₃O₅ when oxygen content is controlled at a reasonable level.When[Mn]exists in steel,reaction of Ti₃O₅ with[Mn]results in the formation of Ti₂O₃ and MnO ultrafine composite inclusions in the following production process,which is advantageous to the composite of MnS and the formation of Mn depleted zone,which is advantageous for nucleation of IGF.The mechanism for IGF nucleation was developed.更多还原