Modification Mechanism of Cerium on the Inclusions in Drill Steel
Yu HUANG1, Guoguang CHENG1(), You XIE2
1 State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China 2 Zenith Steel Group Co., Ltd., Changzhou 213011, China
Cite this article:
Yu HUANG, Guoguang CHENG, You XIE. Modification Mechanism of Cerium on the Inclusions in Drill Steel. Acta Metall Sin, 2018, 54(9): 1253-1261.
Fatigue fracture is the main failure forms of drill steel, and the hard oxide with large size is one of the main reasons for the fatigue fracture of drill steel. Therefore, the miniaturization and softening of inclusion can effectively improve the anti-fatigue performance of drill steel and prolong its service life. Rare earth elements have very good affinity with oxygen and sulfur in molten steel, and the hardness of resulting rare earth compounds is very low. In this work, the rare earth element cerium was added into drill steel to investigate the effect of Ce on the MgAl2O4 and sulfides. The composition, morphology, number, and size of inclusions in drill steel were analyzed by using SEM and EDS. The evolution process and modification mechanism of Ce on MgAl2O4 and sulfides were clarified by experimental results and calculated by thermodynamic software. The type of inclusions in drill steel without Ce addition is MgAl2O4 and (Ca, Mn)S. As the Ce content in drill steel reaches to 0.0078% (mass fraction), the type of inclusions changes to Ce-O and Ce-S. In addition, a few complex inclusions, mixture of Ce-O and MgO, were also found. The size of inclusions in drill steel decreases significantly as the oxides and sulfides were modified into Ce-O and Ce-S. The calculated results show that MgAl2O4 and (Ca, Mn)S in drill steel can be effectively modified into Ce-O and Ce-S as the Ce added into molten steel, and the modification sequence of Ce on the MgAl2O4 is as follows: MgAl2O4→CeAlO3+MgO→Ce2O3+MgO→Ce2O3. The content of Ce in drill steel has great influence on the type of inclusions. The modification mechanism of Ce on MgAl2O4 calculated by Factsage 6.3 agrees well with the experimental observations.
Table 1 Compositions of steels investigated (mass fraction / %)
Fig.1 Typical morphologies of inclusions in steel A (a) Mg-Al-O (b) MnS (c, d) Mg-Al-O+(Mn, Ca)S
Fig.2 Composition of Mg-Al-O inclusions in steel A
Fig.3 The line-scanning and face-scanning mapping of compound inclusions (inset) in steel A (a) Al2O3+(Ca, Mn)S (b) MgAl2O4+(Ca, Mn)S
Fig.4 Typical morphologies of inclusions in steel B (a, b) Ce-O-S (c, d) Ce-O-S+MgO
Fig.5 Composition of Ce-O-S inclusions in steel B
Fig.6 Typical line-scanning mapping of Ce-O-S+MgO inclusions (inset) in steel B
Fig.7 Typical face-scanning mapping of Ce-O-S+MgO inclusions in steel B
Fig.8 Effect of Ce on the composition and morphologies of inclusions
Steel
Proportion of inclusions with different diameter / %
Number density mm-2
<3 μm
3~6 μm
>6 μm
A
51.5
37.3
11.2
14.8
B
62.5
33.7
3.8
15.8
Table 2 Diameter and number density of inclusions in steels
Fig.9 The equilibrium solidification of steel A
Fig.10 Scheil solidification calculations of steel A (a) solidification process (b) variation of solid fraction with S content and Mn content
Fig.11 Stability diagram of Mg-Al-O system of steel A at 1873 K (w—mass fraction of element)
Fig.12 Modification process of cerium on Mg-Al-O inclusions
Fig.13 Stability diagram of Ce-Al-O system of steel B at 1873 K
Equation
/ (J·mol-1)
[Ce]+[S]=CeS(s)
-422100+120.38 T
[Ce]+3/2[S]=1/2Ce2S3(s)
-536420+163.86 T
[Ce]+4/3[S]=1/3Ce3S4(s)
-497670+146.3 T
Table 3 Standard Gibbs free energies of various rare earth inclusions[26,27,28]
Fig.14 Stability diagram of Ce-S system of steel B at 1873 K
Fig.15 A sketch of the formation process of rare earth Ce inclusions
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