【作者】 董艳伍；

【导师】 李正邦； 姜周华；

【作者基本信息】 东北大学 ， 钢铁冶金， 2008， 博士

【摘要】 由于电渣重熔钢锭具有组织致密、成分均匀、表面光洁和成材率高等优点,因此从电渣重熔技术产生之后,就成为生产某些特殊材料的重要手段。长期以来,电渣重熔工艺制度往往凭经验制定,缺乏理论指导,虽然也有许多研究者通过建立数学模型的方法描述电渣重熔过程,并探索最优的工艺,但仍然有许多需要研究的内容。电渣重熔技术产生以后,开发出了许多渣系用于电渣重熔生产,但往往电耗较高,目前国内某厂电渣重熔冷轧辊用钢时吨钢电耗在1500～1600kWh。同时,电渣重熔后钢中增氢严重,电渣钢锭需要进行长期的扩氢处理,以降低钢中的氢含量。在应用电渣重熔生产某些易偏析钢种时,钢中析出的有害相含量较高,有待于通过研究进行控制。本文从上述电渣重熔过程所存在的问题出发,对其进行了相关的研究,旨在提高钢锭质量、降低电耗,探索降低钢中气体含量方法和控制钢中有害相析出含量的途径。针对电渣重熔电耗较高等问题,在现场原用三七(L0)渣系基础上,添加了降低渣系电导率的氧化物组元,开发了L1、L2和L3渣系,并与传统的电渣重熔用L0渣系进行了对比研究。对新设计渣系进行的熔点、粘度、碱度、密度和电导率等的测算结果表明,新设计渣系完全满足电渣重熔用渣要求,并且新设计的渣系电导率更低,更有利于降低电耗。对渣系渗透性的研究结果表明,L0渣系的渗透率最低,这与众多文献资料报道的使用L0渣系重熔后钢中增氢量最小的结果相一致。本文所测得的L0渣系渗透率数值为0.48×10-6 mol·cm-1·min-1。L2渣系对氢的渗透率比L0渣稍大,渗透率为0.98×10-6 mol·cm-1·min-1,同样具有较低的氢渗透率,适合于生产冷轧辊用钢等氢敏感钢种,而其他渣系的氢渗透率相对较高。影响渣系氢渗透率的因素比较多,首先,光学碱度对渣系氢渗透率有重要的影响,总体趋势是光学碱度越大氢渗透率越大。其次,虽然CaO具有较大的水容量,但CaO含量过高更有利于向钢液中渗氢,而Al2O3作为酸性氧化物而存在,有利于阻止氢的渗透。研究表明,本文中所提出的渣系氢渗透率衡量指数EH越大,氢的渗透率越小使用各渣系在实验室所进行的电渣重熔实验研究表明,重熔初期钢中氢含量最高,此时钢中氢含量主要受渣系中含有水分、渣系氢渗透率、自耗电极中氢含量和大气湿度所控制,而重熔中后期钢中氢含量基本恒定,此时钢中氢含量受渣系氢渗透率、自耗电极中氢含量和大气湿度所影响。使用渗透率最低的L0渣系在大气下重熔后,钢中增氢量很小,使用渗透率较低的L2渣系大气下重熔后,钢中增氢量也不大,而其他渗透率较高的渣系大气下重熔后,钢中氢含量较高。研究发现,使用保护气氛和预熔渣重熔,效果最佳,不仅重熔初期钢中氢含量较低,而且重熔中后期钢中增氢量也最低,因为此时钢中氢含量已经基本排除了重熔初期渣系中所含有的水分的影响,也排除了大气湿度对钢中氢含量的影响。对应用各渣系重熔后的钢锭分析后发现,L0和L3渣系重熔后钢中夹杂物总量较低,而L2稍高,L1渣系重熔后钢中夹杂物最高,并且颗粒较大,在L1和L3渣系重熔后的钢中发现大量的氮化物夹杂,这与Ll和L3渣系渗透率较高密切相关。工业实验的研究结果表明L1渣系与L0渣系电耗情况相当,而L2渣系可大幅度的降低电耗,吨钢电耗比L0渣系降低了近200kWh,钢中氢含量小于2ppm,同时新渣系的使用,大大提高了产品的合格率,为企业带来了更大的经济效益。在前人研究的基础上,从电磁场方程、流体流动方程和热量传输方程出发,建立了电渣重熔数学模型,并以重熔体系的温度场分布作为考查的重点,将铸锭凝固过程的局部凝固时间与枝晶间距联系起来,进而通过控制二次枝晶间距来控制铸锭的凝固质量。研究表明,电渣重熔渣池中心的最高温度达到1800℃以上,直径950mm的重熔钢锭铸锭中心的局部凝固时间最长,达到2300s以上,对应的枝晶间距最大,所检测出的二次枝晶间距的大小在500μm左右,而铸锭边缘部位的局部凝固时间较短,在600s左右,对应的二次枝晶间距相对较小,只有200μm左右,从铸锭边缘到中心,枝晶间距逐渐增大,而铸锭的凝固质量也从边缘到中心逐渐变差。模型经过验证后,使用数学模型对Φ130mm结晶器的电渣重熔工艺制度进行了优化设计,并以IN718合金为研究钢种进行了重熔实验,实验发现,电渣重熔后,IN718合金中的夹杂物含量大幅度的降低,通过合理的控制重熔工艺制度,可以有效的控制IN718合金的凝固质量,使其铸锭表面光洁、成分均匀、无宏观缺陷，降低铸锭中的脆性Laves相的析出含量,并降低合金元素的偏析程度。通过实验分析,还提出了Φ130mm结晶器的最佳控制熔速为66kg/h。对应的铸锭中心局部凝固时间为375s,凝固速度(铸锭上涨速度)为10.1mm/min。在实验室所进行的电渣重熔Cr5冷轧辊用钢的实验研究进一步表明,模型计算可以为工艺的制定提供理论依据,通过采用合理的工艺制度可以有效控制电渣重熔钢锭的由于偏析所析出的有害相含量,从而提高钢锭质量。在现场的电渣重熔实验表明,模型计算的金属熔池形状和深度与硫印实测情况吻合较好,说明模型是比较准确的。对现场实验的Cr5冷轧辊电渣钢锭进行解剖分析后发现,电渣钢锭的大部分元素宏观偏析率基本能够控制在0.95～1.05范围内,气体含量也较低,满足钢种的要求。由钢锭的边缘到中心,钢锭的二次枝晶间距不断增大,所检验出最大二次枝晶间距为557μm。Cr5冷轧辊用钢电渣钢锭中所出现的碳化物相含量由铸锭的边缘到中心不断增多,并且颗粒度也逐渐加大,这与枝晶间距的增大息息相关。因此,以枝晶间距大小作为衡量铸锭凝固质量的一个标准是可行的。经过SEM分析发现,Cr5电渣钢锭中所析出的碳化物主要为M7C3和MC型碳化物。现场电渣钢锭分析发现,钢锭中还存在一定的缺陷,这与熔速过高、工艺制度还不够合理有关,因此应用数学模型进行了进一步的工艺制度的计算,新工艺的使用效果还有待于进一步的实验研究。更多还原

【Abstract】 Electroslag remelting (ESR) has been used for some special materials since its birth due to the ingot quality of sound structure, uniform compositions, surface smoothness and high yield. For a long time, the process parameters were determined by experience, which has less theoretical guidance. A lot of mathematical models had been established for describing the phenomenon of ESR process and explored the optimum process, but still many problems should be studied and solved.The power consumption of ESR process is very high when many existed slags are used. At present, the power consumption is 1500～1600kWh/t steel for ESR cold roll steel in a domestic plant. At the same time, hydrogen pick-up during ESR process is very high for hydrogen sensitive steel; the further long time heat treatment is needed for decreasing the hydrogen content. The content of some segregation phase occurs when some prone segregation alloy is produced by ESR, which needs to be controlled. The existing problems for ESR process have been studied aiming at improving ingot quality, decreasing power consumption and the content of gas and harmful phases in our present work.Slag L1, L2 and L3, in which the oxide components were added to decrease the electric conductivity have been designed, in order to decrease the high power consumption when slag LO containing 70%CaF2 and 30%Al2O3 was used in this plant. It indicates from the results of melting point, viscosity, basicity, density and electric conductivity that the designed slags can meet the requirement of ESR process. At the same time the designed slags have low electric conductivity, which is advantaged for decreasing the power consumption.The experimental results indicate that slag LO has the lowest hydrogen permeability, which is coincident with the low hydrogen content in steel after remelting with slag LO in many literatures. The measured value of hydrogen permeability for slag L0 is 0.48×10-6 mol·cm-1·min-1 in this work. The hydrogen permeability is 0.98×10-6mol·cm-1·min-1 for slag L2, which is lower than slag L1 and L3, so it is a promising slag for remelting some special steels. There are many influence factors on hydrogen permeability. Firstly, optical basicity has great effect on hydrogen permeability and the higher optical basicity is the higher hydrogen permeability for general trend. Secondly, though CaO has bigger water capacity, the higher CaO content in slag attributes to increasing hydrogen content in steel. Whereas, Al2O3 is advantaged for preventing hydrogen permeation for its low water capacity and exists as acid oxide. Research shows that the higher the parameter EH is the higher hydrogen permeability, which is put forward for an evaluation parameter of hydrogen permeability of slag in this paper.The ESR experiment results with all kinds of slag studied in this paper indicate that hydrogen content is highest at the beginning of the remelting process, which is controlled by water in slag, hydrogen permeability of slag, hydrogen content in electrode and atmospheric moisture. Later the hydrogen content in steel is kept at a lower content, which is influenced by hydrogen permeability of slag, hydrogen content in electrode and atmospheric moisture here. The increase of hydrogen is low after remelting with the lowest hydrogen permeability of slag L0. The increase of hydrogen is also low after remelting with slag L2. However, the hydrogen content is higher after remelting with slag L1 and L3. It has been found that not only the hydrogen content is lowest at the beginning of the ESR process but also the increase of hydrogen is lowest when protective atmosphere and premelted slag are adopted. Water capacity and atmospheric moisture have little influence on the process in this case. And then the inclusions in remelted ingots with diferent slags were analyzed. Results show that the amounts, of inclusions in steel in the case of slags LO and L3 are less than that with slags L1 and L2. But a lot of nitride inclusions were found in steel remelted with slags LI and L3, which resulted from their high hydrogen permeability.The power consumption test results at industrial ESR show that the slag L1 and L0 has almost the same power consumption, and slag L2 can decrease the power consumption with 200kWh/t steel. Furthermore the products passing rate is higher than before using the new slag, which will have huge economic benefits.A mathematical model, based on electromagnetic field equation, fluid flow equation, and heat transfer equation, was established for the simulation of the electroslag remelting process. The distribution of temperature field was obtained by solving this model. The relationship between the local solidification time and the interdendritic spacing during the ingot solidification process was established, which has been regarded as a criterion for the evaluation of the quality of crystallization. The results indicate that the temperature of slag center is above 1800℃. For a crucible of 950 mm in diameter, the local solidification time (LST) is more than 2300s at the center of the ingot with the longest secondary interdendritic spacing, which is about 500μm. Whereas LST is only 600s at the edge of the ingot according to the calculated results, which is about 200μm for corresponding secondary interdendritic spacing. Secondary interdendritic spacing increases from the edge to center of ESR ingot, and therefore the secondary interdendritic spacing can be used to estimate the ingot quality.The power supply parameters was calculated using the proofed model for the crucible ofΦ130mm in diameter and the experiment was carried out using alloy IN718 in laboratory. It was found after experiment that the non-metallic inclusions content decrease after ESR process and the alloy ingot was sound structure, uniform compositions, surface smoothness and no macroscopic defect with the rational power supply parameters. The content of Laves phase and the extent of element segregation decrease. At the same time the optimal melting rate is 66kg/h forΦ130mm crucible by the experiment analysis. The corresponding LST in ingot center is 375s, and the solidification rate of the ingot is 10.1mm/min.The further experiment was carried out in laboratory using Cr5 cold roll steel. The power supply was calculated by model for different slags. The analysis results indicate that the carbide content in steel is low using the parameters ascertained by model. The results have proved that model can provide theoretical basis for establishing the ESR power supply parameters.The ESR experiment in the industrial scale shows that the predicted shape of molten pool is in agreement with the measured results after the model has been revised, so the model is accurate. It has been found that macrosegregation of most elements in ESR ingot can be controlled at a range of 0.95-1.05 after the Cr5 cold roll steel had been analyzed. The gas content in steel is low, which meet the requirements of steel. Secondary interdendritic spacing gradually increases from edge to the center of the ingot and the largest one measured is 557μm. The content of carbide phase increases form the edge to the center of the ingot and the graininess gradually augments, which has great relationship with the increase of interdendritic spacing. The carbide existing in Cr5 ESR ingot is mainly M7C3 and MC type analyzed by SEM.Some defects had been found in the ingot, which have great relationship with the high melting rate and indicate that the power supply is not enough reasonable. Further power supply have been calculated by model and the capacitity need to be confirmed.更多还原