利用稀土控制高应力弹簧钢中夹杂物和凝固组织的应用基础研究

The metallurgical process and theoretical study of high stress valve spring are weak links in the special steel field in China, and the product mainly depends on imports. The improvement of anti-fatigue performance is the process bottleneck, while the total oxygen, non-metallic inclusions and solidification microstructures in steel are the main influence factors. Treating steel with rare earth (RE) is expected to be an effective means to solve the fatigue performance of steel since it can coordinately control the factors above. Utilizing these functions of rare earth Ce to deeply deoxidize, modify inclusions and refine microstructures, and aiming at the related scientific problems with the low yield of RE and unstable application effect, a fundamental study will be carried out for the 55SiCr valve spring steel. Firstly, the refining slag systems containing RE will be designed and their microstructures, physicochemical properties, and the absorption behaviors for inclusions will be investigated. Based on this, the effects of the original cleanliness of molten steel and the amount of RE addition on inclusion modification and solidification microstructures will be studied by the theoretical calculation and steel-slag high temperature experiments. The control mechanism of RE on inclusions and on the refining mechanism of solidification microstructures will be revealed, together with the relationship of steel cleanliness and RE addition amount will be obtained. This research will provide a novel research idea, the experimental and theoretical fundamentals for the performance improvement of the high-grade and high-stress spring steel.

高应力气门弹簧冶炼工艺与理论研究是我国特钢行业的薄弱环节，产品至今主要依赖进口，其中抗疲劳性能的提升是工艺突破的瓶颈，而钢中总氧、夹杂物、凝固组织是主要影响因素，稀土处理因其可协同控制以上因素而有望成为解决疲劳性能的有效手段。本研究拟以55SiCr钢种为研究对象，利用稀土铈深度脱氧、变质夹杂、细化组织的功能，围绕处理过程可能出现的与稀土收得率低、效果不稳定相关的基础科学问题，首先设计稀土精炼渣系并研究其微观结构、理化性能及对夹杂物的吸收行为和对钢中稀土收得率的影响。在此基础上通过理论计算和钢-渣高温实验重点研究钢水初始洁净度（主要是氧、硫含量、夹杂物水平）和稀土加入量对夹杂物变质及对凝固组织的影响规律，揭示稀土对夹杂物控制和对凝固组织细化的机制，获得钢水洁净度和稀土加入量的关联性及针对不同洁净度钢水稀土的临界加入量，为高等级高应力弹簧钢的性能提升提供新的研究思路、基础实验数据和理论依据。

55SiCr高应力弹簧钢主要用于制造交通工具的气门、悬架和制动弹簧，随着汽车行业的迅猛发展，对弹簧钢的性能有了更高的要求。夹杂物所引起的疲劳破坏与组织粗化所造成的力学性能下降等是降低弹簧服役寿命的重要原因。稀土兼具净化钢液、改性夹杂及细化凝固组织的作用，可以有效改善钢的性能，但其合理加入量和加入方式一直是困扰稀土钢发展的一大瓶颈。为此以55SiCr弹簧钢为研究对象，通过理论研究、实验室试验、工业试验等手段，首先研究了精炼渣系中CeO2含量对稀土收得率、钢水洁净度和夹杂物的影响，发现随着CeO2含量的增加，稀土收得率、钢水洁净度变差，夹杂物尺寸变大。在此基础上重点研究了钢水初始洁净度和稀土加入量对夹杂物变质及对凝固组织的影响规律，揭示了稀土对夹杂物控制和对凝固组织细化的机制，获得了当55SiCr弹簧钢钢水总氧含量在7~11ppm，S含量在50ppm以内，改性夹杂物和细化凝固组织稀土的合理加入量。对比了稀土加入时机对夹杂物和凝固组织的影响规律，发现稀土加入时机对凝固组织的影响不显著，但对夹杂物类型和尺寸的影响很大。稀土加入得越晚，其产品中的稀土收得率越高，夹杂物尺寸越小。过量稀土的加入不仅生成大量簇群状、不规则的大尺寸夹杂物而且会使钢的凝固组织粗化，力学性能下降。将以上研究成果推广应用于高等级石油套管钢的工业生产中，大大提升了套管的抗硫合格率。另外，对于稀土在钢的应用过程中可能出现的堵水口现象也进行了深入研究，并提出了改进措施。