**1. Introduction**

Cryogenic vessel steel is an essential material for producing storage tanks and is widely used to store and transport liquefied gases. As increasing clean energy sources are developed and applied, cryogenic vessel storage equipment has been finding more applications [1–5]. Cryogenic vessel storage equipment requires stringent demands for the alloy composition, casting slab quality, and mechanical properties of the cryogenic vessel steel.

Importantly, inclusions affect the cleanliness, control, and quality of steel. During the solidification-rolling process, the morphology of inclusions is changed. Rare earth (RE) elements present a remarkable effect on modifying inclusions and in facilitating clean steel production, given their unique electronic layer structures. Earlier investigations have reported that different RE elements could purify molten steel, modify inclusion morphology, increase cryogenic toughness, and improve corrosion resistance in different steel series [6–10]. When RE is added to extra-low sulfur (S < 0.003%) niobium-titanium micro-alloy steel, the deoxidation and inclusion removal rates can be increased, therefore the molten steel can be more effectively purified [11]. After adding La+Ce compounds to ship plate steel, the inclusions mainly exist in the form of spheres and approximate spheres, while the sharp-angled S-Mn-Ca phase and S-O-Mn-Ca-Al phase inclusions are reduced and the hot-rolled steel strip band structure is lightened [12]. The type of inclusions in drill steel without Ce additions are MgAl2O4 and (Ca, Mn)S. As the Ce content in drill steel reaches 0.0078% (mass fraction), the types of inclusions change to Ce-O and Ce-S and the size of the inclusions in the drill steel decrease significantly [13]. Inclusions affect the

**Citation:** Wu, L.; Zhi, J.; Zhang, J.; Zhao, B.; Liu, Q. Effect of Cerium on the Microstructure and Inclusion Evolution of C-Mn Cryogenic Vessel Steels. *Materials* **2021**, *14*, 5262. https://doi.org/10.3390/ma14185262

Academic Editor: Andrea Di Schino

Received: 19 July 2021 Accepted: 9 September 2021 Published: 13 September 2021

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toughness, corrosion resistance, and fatigue properties of steel, and reducing their size weakens the impact of non-metallic inclusions on the properties of steel. The RE elements La, Ce, and Nd play deoxidation and desulfurization roles in molten steel, making the inclusions fine and diffuse in distribution [14]. Gao et al. found that after Ce treatment, irregular Al2O3 inclusions with a size of 10 −15 μm are wrapped by rare earth and then gradually modified into spheroidal CeAlO3, Ce2O3, and Ce2O2S inclusions with a size of ≈5 μm, distributed into interstitial free slabs [15].

It has been proposed in the literature that RE influences the microstructure of a casting slab. For example, with the addition of La into nonoriented electrical steel, the equiaxed crystal ratio increased and the columnar crystals were suppressed and refined [16]. The addition of La+Ce changed the solidification microstructure distribution of thin slabs produced by compact strip production and refined the solidification structure of 430 ferritic stainless steel, thereby improving the comprehensive properties of the material [17,18]. The addition of La had a significant effect on the distribution of arsenic by the formation of arsenic inclusions, and various RE have been widely used in the steelmaking process to improve steel quality [19].

However, there is still no systematic explanation regarding the mechanisms by which RE improves the quality of casting slabs, changes the structure morphology, and modifies the inclusions. Moreover, casting slab quality is the most important index to evaluate the finished product. The quality of a casting slab should be analyzed from a macro quality perspective, microstructure characteristics, and the evolution law of the inclusions. In this study, the characteristics of inclusions and the microstructure of C-Mn cryogenic vessel steels were comparatively studied without Ce and with the addition of different Ce content levels.
