As the demand for high-performance steels increases every year, improving steel cleanliness is a main task of steelmakers. Nonmetallic inclusions in steel have a detrimental effect
on the performance of steels, such as their strength, toughness, fatigability, cleanliness, surface fi nishing, castability, etc. Thus, control of inclusions is very important in the production
of high-cleanliness steels. The control of inclusions usually means to control the amount, morphology, composition, and size distribution of inclusions and their spacial
distribution in the steel product. Nonmetallic inclusion in steel is always a key topic for steelmaking process during the past five decades.

A wide range of operating approaches including deoxidation, calcium treatment, slag refining, and prevention against reoxidation have been applied throughout the steelmaking
processes to achieve the better control of nonmetallic inclusions in steel. Firstly, accurate characterization of inclusions
is required to clearly understand the source and formation mechanism of inclusions and then to find methods to remove
and control them. The combination of the various methods is the best way to better understand the characteristics of
inclusions. Besides, most of indigenous inclusions are produced during the deoxidation process. The Al2O3-rich inclusions and Al2O3·MgO spinel inclusions formed in Al-killed steels may cause defects on the surface of steel products after polishing due to their poor deformability. The nucleation,
growth, fl uid-flow transport, and removal of inclusions in the molten steel have been widely investigated. For the unremoved inclusions, modifi cation treatment is necessary to make them harmless to steel. Calcium treatment was well used to modify inclusions into calcium aluminates with low melting temperature. The composition of inclusions was greatly influenced by the composition of the top slag and lining refractory materials. A lot of thermodynamic and kinetic models have been developed to predict the formation
of inclusions. Moreover, the oxide metallurgy was applied to utilize inclusion particles to improve steel properties.

 Different grades of steels have diff erent requirements of inclusions. More fundamental research is necessary to determine the target of inclusions in various types of steels.
Although inclusions in the equilibrium state can be well predicted by thermodynamics, kinetic prediction is necessary to the variation of inclusions during the entire steelmaking
process, especially the variation of composition and size of inclusions. During the solidification and cooling process of the steel and the heating of the steel, the equilibrium between
the steel and inclusions varied with temperature, during which the evolution of inclusions needs to be investigated.

The special edition for the Journal of Iron and Steel Research International focused on nonmetallic inclusions in steel. Five articles were collected.

Sunday et al. did an extensive review on types of inclusions in steel, inclusion modification and sulfide shape control during refining in the ladle and summarized several key
aspects of the metallurgy of calcium treatment and inclusion evolution during refining in the ladle; Ying and Zhang et al. performed a kinetic modeling on the reoxidation of
Al–Ti-containing steel by CaO–Al 2O3–MgO–SiO2 slag and predicted the amount and composition of inclusions during refining; Joohyun et al. discussed the eff ect of CaF2 content
in refining slag on the formation of spinel inclusions in Ni–Cr–V-alloyed special steels; Cong et al. investigated the effect of Zr addition on the evolution of inclusions in EH36
shipbuilding steel; Qiang et al. simulated the collision and aggregation of inclusions in a turbulent fluid.

Li-feng Zhang The guest editor of this special column is Li-feng Zhang. Li-feng was born in 1972 and is a professor at the School
of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, China. He received his Bachelor degree of Metallurgical Engineering
from Chongqing University in 1993 and Ph.D degree of Metallurgical Engineering from University of Science and Technology Beijing in 1998. He
worked at Tohoku University (Japan), Technical University of Clausthal (Germany), University of Illinois at Urbana-Champaign (USA), Norwegian University of Science and Technology (Norway)
and Missouri University of Science and Technology (USA). He was a professor at Norwegian University of Science and Technology and an
associate professor at Missouri University of Science and Technology.
Prof. Li-feng Zhang has done extensive theoretical and practical studies on nonmetallic inclusions in steel. His research results have been used
in several industries and promoted the improvement of clean steels in China. He was selected as the Royal Academy of Engineering (RAEng)
Distinguished Visiting Fellow at University of Warwick in 2016. He was the winner of 2017 AIST Richard J. Fruehan Award, 2010 Jerry
Silver Best Paper Award of Association for Iron & Steel Technology (AIST), 2009 Continuous Casting Award Best Paper of AIST, 2007
TMS Young Leader in Light Metals of TMS Society, and 2001 Steel-making Conference Best Paper Award of Iron and Steel Society. He is
a member of editorial board for 8 English journals and 8 Chinese journals.
He has been invited to review papers for over 80 international journals.