【作者】 郭薇；

【导师】 朱苗勇； Lifeng Zhang；

【作者基本信息】 东北大学 ， 钢铁冶金， 2010， 博士

【摘要】 连铸坯质量一直是冶金工作者关注和研究的重点问题之一。深入理解凝固过程枝晶形貌的发展变化及枝晶间的微观偏析对于揭示凝固机理及控制铸坯质量是十分必要的。目前数值模拟已逐渐成为研究微观机理现象必不可少的技术而日趋成熟,对铸坯内部微观机理的模拟研究与深入探讨对于连铸技术的发展具有重要的指导作用。枝晶是连铸坯凝固过程最常见的组织形态,研究枝晶形貌演变过程有助于控制凝固组织形貌。因此本课题针对连铸坯凝固过程,结合宏观传热模型,对钢液凝固过程的枝晶生长形态变化及成分偏析进行研究,并发现了连铸过程和铸造过程在凝固组织形貌分析上的重要区别。本文主要研究内容和获得的结果如下:（1）分析并总结了国内外连铸及铸造过程微观组织及微观偏析模型,针对连铸钢液的凝固过程提出了研究枝晶生长的微观结构参数及其变化规律的数学模型,并结合实际浇注条件对碳钢和不锈钢两种钢种进行数值模拟计算,得出一些重要的微观组织参数,如枝晶生长速率、枝晶尖端半径、二次枝晶臂间距、固液界面处的溶质浓度、温度梯度、冷却速率、过冷度和枝晶尖端温度,随凝固进程及坯壳生长的变化关系。（2）连铸坯凝固微观组织模型的验证与检验。以浇注碳钢连铸坯为研究对象,根据对现场实际浇注冷却后的钢坯试样的二次枝晶臂间距的测量值与模型计算值进行比较分析。确定了模型在连铸范畴内运用是合理的,且预测的微观组织参数能够反映凝固过程的组织变化情况;同时确定出连铸过程的稳定性常数值在0.05-0.085的适用范围,推导并验证了吉布斯-汤姆森系数1.2×10^-8运用在连铸坯凝固过程的合理性。（3）分析了连铸坯凝固末期固液界面前沿碳钢的主要元素C、Si、Mn、P、S和不锈钢的主要元素C、Cr、Ni、P、S的微观偏析程度随凝固进程的变化关系。当固相分率超过0.9时,P和S的偏析显著增加,其中S的偏析最大,而C,Si和Mn（Cr和Ni）的偏析变化不如P和S显著。随着凝固坯壳的不断生长,液相越来越少,残余在液相中的溶质元素也会随着液相的减少而减少;各元素的偏析程度随枝晶生长速率的增加而略趋严重。（4）研究了稳定性常数σ^*等微观参数和拉速等浇注参数对连铸凝固过程各微观结构参数的影响规律。稳定性常数越大,枝晶尖端半径越小,二次枝晶臂间距也越小。稳定性常数从0.025增大到0.05时,碳钢铸坯和不锈钢铸坯的枝晶尖端半径均减小约5～6μm,碳钢铸坯的二次枝晶臂间距减小幅度在100μm范围内,不锈钢铸坯的二次枝晶臂间距减小幅度在10μm范围内。在过热度为20℃下,拉速V_c从0.6m/min变化至1.0m/min时,在坯壳生长方向上,枝晶生长速率、枝晶前沿溶质浓度、温度梯度、冷却速率和过冷度均随着拉速的增加而减小,而枝晶尖端半径、二次枝晶臂间距、枝晶尖端温度则随之增大。（5）研究比较了碳钢和不锈钢两种钢种的微观组织特性。碳钢连铸坯的枝晶生长速率在0.1-0.7mm/s内变化,枝晶尖端半径在12μm内变化,不锈钢连铸坯的枝晶尖端半径要比碳钢小3-4μm;对于二次枝晶臂间距来说,不锈钢则比碳钢要小得多。由于不锈钢其特有的组成和凝固模式,使得不锈钢的各微观参数值均比碳钢小,其凝固组织更为细密。本研究对连铸凝固过程的微观机理进行了探索性的分析,取得了一些初步性的进展,但仍需进行后续的深入研究,为实现预测连铸过程中的微观组织形貌、提高材料组织性能提供理论指导。更多还原

【Abstract】 The quality of the final steel products is the most concerned issue in the industrial practice of steel continuous casting process. It is very important to investigate the dendrite evolution and microsegregation among the dendrites for steel solidification during continuous casting process. The mathematical simulation has become a necessary technology in the micro field. Nowadays researches on the microstructure evolution and the micro-segregation in steel continuous casting process have received more and more interests. Dendrite is one of the most common configurations during solidification. For continuous casting process, the analysis of the dendrite growth and the microstructure shape is quite different from that of ingot casting process. In the present study, the microstructure and microsegregation of steel solidification combined with the macro- heat transfer have been studied for continuous casting process. The main contents and the developments are carried out below:（1） The microstructure and microsegregation models which are summarized both for the continuous casting process and the ingot casting process are presented for the application of steel solidification during continuous casting process. A numerical method is developed to analyze and calculate the microstructure parameters, such as the dendrite tip radius, the dendrite growth velocity, the liquid concentration, the temperature gradient, the cooling rate, the secondary dendrite arm spacing, and the dendrite tip temperature in front of S/L interface, with the variations of solidification progress and solid shell growth for both carbon steel and stainless steel during continuous casting.（2） The current model is well validated by the published models and the measurement data. The results show that the present model is reasonable. Taking carbon steel for actual continuous casting, the analysis of secondary dendrite arm spacing of the samples is compared with the calculation results. The reasonable stability coefficient in the range of 0.05-0.085 and Gibbs-Thomson coefficient 1.2×10^-8 are determined for continuous casting process.（3） The microsegregation of C、Si、Mn、P、S for carbon steel and C、Cr、Ni、P、S for stainless steel with the variations of both solidification progress and solid shell growth are discussed. When the solid fraction is over 0.9, the microsegregation of P and S increases observably, especially the S segregation, while the variation of C, Si and Mn （Cr and Ni） microsegregation is not so obvious. With the solidification progress, the microsegregation degree of each element increases with the increasing of the dendrite growth velocity.（4） The effects of the stability coefficientσ^* and casting speed on the microstructure parameters are studied. For different systems, a* takes different values. The determination of the stability coefficient value has great importance on the dendrite growth shapes forming. The larger the stability coefficient is, the smaller the dendrite tip radius and secondary dendrite arm spacing are. When the stability coefficient changes from 0.025 to 0.05, the dendrite tip radius decreases by 5-6μm for both carbon steel and stainless steel. But the reduced rate for the secondary dendrite arm spacing for carbon steel is within 100μm, which for the stainless steel is only within 10μm. Effects of different casting speed V_c 0.6m/min, 0.8m/min and 1.0m/min on these parameters are also investigated. When casting speed increases from 0.6 m/min to 1.0 m/min, the dendrite growth velocity, the solute concentration, the temperature gradient, the cooling rate and the undercooling decrease, while the dendrite tip radius, the secondary dendrite arm spacing and the dendrite tip temperature increase.（5） Comparisons between the carbon steel and the stainless steel for the above results are carried out, and the same trend for both two steel grades is obtained. When the dendrite growth rate changes from 0.1 mm/s to 0.7 mm/s, the dendrite tip radius for carbon steel changes within 12μm, while dendrite tip radius for stainless steel is 3-4μm smaller than that of carbon steel. For the secondary dendrite arm spacing, the values of stainless steel are much smaller than those of carbon steels. The comparisons show that the micro- structural parameter values of stainless steel are smaller than those of the carbon steel due to its special compositions and solidification mode characteristics.Above all, the current study is just an exploration and has obtained some elementary developments. More research should be done to realize the prediction of the microstructure shape and improve the capabilities of microstructure for the solidification process during continuous casting.更多还原