镁处理对1215易切削钢中夹杂物的影响

 1215易切削钢中硫化物夹杂不仅影响钢的切削性能,对钢性能的各向异性以及产品质量问题也有重要影响,对钢中硫化物夹杂的调控是改善产品品质的重要途径。采用镁处理技术,对钢中硫化物夹杂的形态、大小和分布进行调控,解析镁对夹杂物的改质影响。通过高温熔炼试验,冶炼不同镁含量的钢锭,采用光学显微镜、扫描电镜及小样电解技术对钢中夹杂物的二维及三维形态和分布进行分析,并结合热力学计算解析夹杂物改质机制。研究表明,镁具有较强的脱氧能力,可改变钢中硫化物形态和分布。钢中镁质量分数从0增加至0.000 6%、0.001 7%,夹杂物形态首先从Ⅰ类球形、椭球形转变至Ⅱ类沿晶分布的簇状、串链状、珊瑚状,然后再转变为多面体形或不规则块状的的Ⅲ类硫化物。镁质量分数进一步增加至0.002 7%,镁对夹杂物的形态、尺寸、分布影响不再显著。钢中的MnS在熔融液态中不会析出,主要在凝固过程固液两相区析出,其析出温度为1 502.0 ℃,对应的凝固分率为0.409。凝固过程中部分MnS会以钢中氧化物夹杂为异质形核点析出,形成内部氧化物、外部硫化物的复合夹杂。钢中Al₂O₃经镁改质转变成为更加细小弥散分布的MgO·Al₂O₃,改质后夹杂物不易聚集长大,成为更多的MnS析出异质形核点,从而促进了MnS析出,夹杂物整体数量密度增大,平均等效直径减小。

Effect of magnesium treatment on inclusions in 1215 free-cutting steel

Sulfide inclusions in 1215 free-cutting steel not only affect the machinability of steel, but also have a big influence on the anisotropy and the quality of steel. The control of sulfide inclusions is the key to improvement of steel quality. In the current study, Mg treatment was employed to control the morphology, size and distribution of inclusions to explore the effect of Mg on the inclusion modification. Through the high-temperature smelting experiment, a series of steel ingots with different Mg contents were prepared. Using the optical microscope, scanning electron microscope and non-aqueous solution electrolytic etching method, the distribution of inclusions and the two-dimensional as well as the three-dimensional morphology of inclusions in the steel were analyzed. Besides, thermodynamic calculation was conducted to figure out the modification mechanism of inclusions. The results show that, Mg has a strong deoxidation ability, which has a significant influence on the morphology and distribution of sulfide inclusions. When the Mg mass fraction increases from 0 to 0.000 6% and 0.001 7%, the morphology of inclusion changes from spherical, ellipsoidal (type I sulfide) to cluster, chain-like, coralloid (type II sulfide), and then to polyhedron, irregular block (type III sulfide). When the Mg mass fraction further increases to 0.002 7%, the effect of Mg is no more significant. MnS can not be generated in the molten steel. MnS mainly precipitates in the solid-liquid two-phase region during the solidification process, the corresponding precipitation temperature and solidification fraction of steel is 1 502.0 ℃ and 0.409, respectively. During the solidification process, part of MnS takes oxide inclusions as heterogeneous nucleation points, and forms complex inclusion with internal oxide and external sulfide. Once the Mg is added, Al₂O₃ inclusions are modified to dispersive distributed tiny MgO·Al₂O₃ inclusions, which has little possibility of aggregation and growing, providing more heterogeneous nucleation points for the precipitation of MnS. Therefore, the number density of inclusions is increased, while the average equivalent diameter is decreased.