Abstract
Rare earth (RE) addition to steels to produce RE steels has been widely applied when aiming to improve steel properties. However, RE steels have exhibited extremely variable mechanical performances, which has become a bottleneck in the past few decades for their production, utilization and related study. Here in this work, we discovered that the property variation of RE steels stems from the presence of oxygen-based inclusions. We proposed a dual low-oxygen technology, and kee** low levels of oxygen content in steel melts and particularly in the raw RE materials, which have long been ignored, to achieve impressively stable and favourable RE effects. The fatigue life is greatly improved by only parts-per-million-level RE addition, with a 40-fold improvement for the tension–compression fatigue life and a 40% enhancement of the rolling contact fatigue life. We find that RE appears to act by lowering the carbon diffusion rate and by retarding ferrite nucleation at the austenite grain boundaries. Our study reveals that only under very low-oxygen conditions can RE perform a vital role in purifying, modifying and micro-alloying steels, to improve the performance of RE steels.
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Acknowledgements
The work was supported by the National Natural Science Foundation (grant nos 52031013, U1708252 and 51725103). We are grateful for the support of and collaboration with the manufacturers of RE steels from many iron and steel companies in China, such as Zhejiang Tianma Bearing Co., Ltd.; Baotou Iron and Steel Group; Benxi Steel Group Corporation; Anshan Iron and Steel Group; Shougang Group; HBIS Group Co., Ltd.; Shandong Iron and Steel Group Co., Ltd.; Hubei **nyegang Steel Co., Ltd.; **ning Special Steel Co., Ltd.; **wang Special Steel Co., Ltd.; and Magang (Group) Holding Co., Ltd. We are grateful to H. Ma and J. T. Wang for the calculations of carbon diffusion barriers and to Y. Zhao for the calculation of carbon diffusion coefficients. We are grateful to the referees for their substantial contributions in improving this work. Their constructive suggestions helped us to explore the in-depth mechanisms regarding the addition of RE to improve fatigue life, leading to the key findings of the differences between the traditional Al2O3 inclusions and the RE-oxysulfides/RE sulfides and their effects on fatigue crack initiation. They inspired us to perform reliable experimental procedures to elucidate the effects of RE in solution on carbon diffusion and to deeply understand the RE effect on the phase transition. The referees suggested rigorous scientific experiments to study fatigue performance under the same conditions and suggested carefully raking the details of the oxygen-controlling routes. All their invaluable comments and suggestions helped us significantly improve the quality and readability of this work. We all express heartfelt thanks to the referees and to the editors for their precious time, their openness to encourage in-depth explorations and their high standard to push for rigorous science, from which we have benefited greatly.
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D.L. conceived the study and proposed the main idea of a dual low-oxygen technology for resolving the long-term bottleneck problems of RE steels. P.W. performed nanoindentation, dilatometric experiments and TEM characterization of the sample after the fatigue measurements. X.-Q.C., Y. Chen and Y. Cao organized and performed the first-principles calculations; P.F., H.L., L. Zheng and X.H. performed the 52100 steel industrial experiments and inclusion analyses as well as fatigue measurements; Y. Luan performed the fatigue experiments of 52100 steels; J.G. performed the low-oxygen experiments in mischmetal; Y.Z., L. Zhang, X.M. and C.D. performed the TEM observation and analyses of RE in solution in different steels; C.Y. performed the fatigue experiments of 52100 steels; Z.J. performed the diffusion couple and S355 experiments; Y. Liu performed the inclusion electrolysis experiments; and M.S. performed the engineering validation. D.L., X.-Q.C. and P.W. wrote the manuscript with input from all other authors. All the authors including Y. Li discussed this manuscript.
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Li, D., Wang, P., Chen, XQ. et al. Low-oxygen rare earth steels. Nat. Mater. 21, 1137–1143 (2022). https://doi.org/10.1038/s41563-022-01352-9
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DOI: https://doi.org/10.1038/s41563-022-01352-9
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