【作者】 李杰；

【导师】 翟启杰； 龚永勇；

【作者基本信息】 上海大学 ， 钢铁冶金， 2009， 博士

【摘要】 细化金属凝固组织是改善材料综合性能的有效手段,一直是材料界关注的热点课题。本文在前人研究的基础上,研究了脉冲电流细化金属凝固组织的条件和机制,探讨了脉冲电流凝固细晶技术的应用前景。实验研究发现,相同凝固条件以及脉冲电流参数下,以平行电极方式引入脉冲电流的细化效果优于其它电极引入模式,电极的插入深度和过热度对脉冲电流细化纯铝凝固组织的效果没有影响,纯铝凝固组织中等轴晶平均晶粒尺寸在0.3mm左右,等轴晶面积比率在70%左右。采用平行电极引入脉冲电流时,在熔点以上施加脉冲电流没有细化作用;在形核阶段施加脉冲电流细化效果最为显著,且与全程处理时的细化效果相当;在晶体生长阶段施加脉冲电流可促进柱状晶向等轴晶转变。在上述实验的基础上,采用平行电极引入脉冲电流时晶核主要来源于熔体的自由液面。利用游离晶粒隔离实验证实了这一推断,并在此基础上,提出了脉冲电流液面扰动凝固细晶方法。本文结合电磁场理论求解出脉冲电流及其引起的电磁场在导电熔体内的分布。结果显示,脉冲电流的作用区域主要集中在自由液面,且引起的各场强从表面向熔体内部呈指数衰减。本文建立了脉冲电流能量损耗与各参数之间的理论关系式,表明能量损耗不但与脉冲电流峰值、振荡频率和处理时间有关,还与脉冲电流的作用频率、电极有效宽度以及电极之间的距离有关。与实验相结合,发现在处理时间不变的条件下,提高脉冲电流在熔体内的功率损耗,有利于提高细化效率。分析认为,脉冲电流在熔体表面引起的功率损耗,延缓了自由表面的凝固时间,使自由液面不断产生晶核,晶核在自然对流和电磁力的共同作用下,沿着固液界面前沿向下漂移,若固液界面前沿存在热过冷,则漂移的晶核会逐渐长大,并在底部堆积,最终形成细化的凝固组织。本文研究了在实验室条件下脉冲电流对GCr15工业用轴承钢凝固组织的影响规律,发现脉冲电流能有效改善轴承钢的枝晶组织结构,且等轴晶随着放电频率的提高不断增多。在有热顶冒口的工艺条件下,应用较低功率损耗的脉冲电流即可达到较好的细化效果。本文在实验室研究的基础上,设计开发了工业试验用大功率脉冲电源以及相关辅助机构,并成功地在宝钢股份公司特殊钢分公司完成脉冲电流细化3.7吨模铸GCr15轴承钢的工业试验。分析初轧锭及成品轧材的内部质量后发现,经脉冲电流处理后铸锭的各质量指标,诸如疏松、组织致密度、碳化物带状、以及宏观碳偏析指数,均有显著的改善。上述试验为脉冲电流凝固细晶技术的大规模工业应用奠定了坚实的基础。更多还原

【Abstract】 Refining solidification structure of metals is effective method to improving mechanical properties of metal materials. In the past few years, investigators utilized electric current pulse (ECP) for refining grain size during solidification, which become grandully current issue. This work systematically investigated process conditions of refining solidification structure by ECP, and theoretically analyzed refinement mechanism. At last, industrial experiments of refinement technique by ECP were carried out.Pure Al was firstly selected as the experimental material, and ECP employing modes were optimized. Experimental results showed that under the same conditions, refining effect by the parallel electrodes on the melt top surface was better than other modes used in the experiment. The refined equiaxed grain size and area rate of equaixed grain zone was about 0.3mm and 70%. The inserting depth of electrodes and the change of superheat degree had not influence on grain refinement effect. ECP can not influence the macrostructure of pure aluminium when treating at temperature above liquidus, treating of ECP at initial solidification or nucleating stage is most effective, its refinement efficiency was euqivalent to that of treating at the whole solidification processing, but treating of ECP at crystal growth stage also made structure change from columnar to equaixed (Columnar to Equiaxed Transition--CET). Based on all these results, the refinement method of ECP’s agitation on the melt top surface was presented.The distribution of current and magnetic fields caused by ECP in melt was analysed by using electrodynamics theory. The results showed that action zone of ECP focused on the top surface because of skin effect, and presented exponential attenuation from surface toward melt inside. And, power loss had relation not only with peak value current and vibrating frequency, but also with pulse discharging frequency, effective width of electrode, distance between electrodes. Increasing of ECP’s power loss in the melt could increase refining efficiency. It is considered that power loss prolong solidifying time near the top surface of melt, Joule heating casued by ECP in melt continually bring out floating crystal nucleus, which drift downward along the front of solid liquid interface by the natural convection and electromagnetic force. If the front of solid liquid interface exists undercooling, the floating nucleus will grandully grow up and sedimentate at the bottom of ingot, hence the fine equaixed grain is obtained.On the other hand, the influence of ECP on solidification structure of high carbon and chromium bearing steel was investigated. Results showed that under the laboratory conditions, the better refining effect could be achieved by ECP with low voltage and intermediate discharging frequency. With an increase in discharging frequency, the solidified structure and the central shrinkage porosity were gradually improved. With the hot top, lower power loss could achieve the better refining effects. Based on the investigation mentioned above, a large-power ECP generator for industry application were designed and made, subsequently plant trials about applying electric current pulse (ECP) to 3.7 ton weight high carbon and chromium bearing steel ingot were carried out in Baoshan Iron & Steel Co., Ltd. The inspection results by ultrasonic detection and macroscopic examination and so on suggested that compared with samples without ECP treatment, the inner quality of the forged billets treated by ECP was obviously improved, and the center porosity and shrinkage cavity were also reduced. For finished round bars, carbon distribution was more homogeneous. Moreover, maximum width and average width of carbide banding, especially average spacing between carbide bandings were decreased significantly. Therefore, ECP can refine the solidified structure of ingot and enhance its inner quality.更多还原