中高碳铝镇静钢形成MnS塑性夹杂的工艺开发与实践

 为提高中高碳钢产品的抗疲劳性能,利用中高碳钢的成分特点,研究开发了中高碳铝镇静钢中MnS以Al₂O₃为形核质点的非均质形核工艺,将钢中Al₂O₃脆性夹杂用塑性MnS包裹,解决了疲劳应力钢因脆性非金属夹杂引起的疲劳断裂问题。通过对微细、弥散Al₂O₃夹杂生成条件、MnS非均质形核析出热力学条件的研究,开展了钢中关键元素的成分设计、精炼及连铸集成工艺的设计与开发。工业实践表明,低活度氧条件下进行铝终脱氧可以形成3～5 μm微细弥散的Al₂O₃夹杂,并作为非均质形核的核心在二次枝晶晶间的凝固末端析出弥散、细小的粒状MnS;通过梯度脱氧、真空碳脱氧以及保护浇铸等操作可以有效稳定控制钢中全氧含量,提高钢水洁净度,成品TO]质量分数平均为0.000 618%,较原工艺的0.000 739%降低了16%;成品的夹杂物中MnS及MnS包裹Al₂O₃夹杂所占比例大于96%,与世界领先产品的夹杂物控制水平相当,考虑到产品使用过程中Al₂O₃夹杂外部的MnS包裹层必须足够厚,塑性夹杂才能起作用,建立了MnS “有效包裹率” 的概念,当硬相夹杂物被MnS包裹且硬相夹杂物的最大半径不大于MnS包裹部位半径的1/2时,认为MnS对硬相夹杂物实现了“有效包裹”;MnS塑性夹杂工艺可明显提高材料的疲劳性能,成品的平均断裂韧性为83.47 MPa·m1/2,较原工艺的67.31 MPa·m1/2提高了24%。

Development and practice on forming MnS plastic inclusion in medium high carbon Al killed steel

In order to improve the fatigue resistance of medium and high carbon steel products, based on the composition characteristics of medium and high carbon steel, the process which the heterogeneous nucleation of MnS in steel takes Al₂O₃ as nucleation particle is developed, and the brittle inclusions of Al₂O₃ in steel is wrapped by plastic MnS, the problem of fatigue fracture caused by brittle nonmetallic inclusion is solved. The composition design of key elements in steel, the design and development of refining and continuous casting integrated process were carried out by studying the formation conditions of fine and dispersed Al₂O₃ inclusions and the thermodynamic conditions of MnS heterogeneous nucleation and precipitation. Industrial practice shows that,the final deoxidation by aluminum with low activity oxygen can form 3-5 μm fine dispersed Al₂O₃inclusions, which act as the core of heterogeneous nucleation and precipitate dispersed and fine granular MnS at the solidification end of secondary dendrites; through gradient deoxidation, vacuum carbon deoxidation and protective casting, the total oxygen content in steel can be effectively controlled and the purity of molten steel can be improved,the average mass percent of TO] of the finished product is 0.000 618%, which is 16% lower than 0.000 739% of the original process;the proportion of MnS and MnS wrapped Al₂O₃ inclusions is more than 96% of the total inclusions in the finished product,which is equivalent to the inclusion control level of the world′s leading products, considering that the Al₂O₃ inclusion wrapped by MnS must be thick enough for the plastic inclusion to work,the concept of "effective package rate" of MnS is established,while the maximum radius of hard inclusion was not more than 1/2 of radius of MnS wrapping part, it is considered that MnS realizes "effective wrapping" of hard phase inclusions;the average fracture toughness of the finished product is 83.47 MPa·m1/2, which is 24% higher than 67.31 MPa·m1/2 of the original process.