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dition to the concurrent increase in strength and ductility, grain refinement of cast steels can also minimize cast defects, i.e., porosity and segregation during the continuous and ingot steel casting.[1,2] Because d-ferrite is the first solid phase directly formed from the liquid during solidification of low carbon ferritic steels, the size of the d-ferrite phase directly determines the grain size of the steel ingots or castings at room temperature. Thus, the grain refinement of d-ferrite is critically important in controlling the grain size of low carbon ferritic steels during both steelmaking process and foundry operations. Over the decades, it is widely accepted that grain refinement is strongly influenced by the solutes in steels. However, most of the grain refinement studies and/or practices in steels focused on controlling the grains

MING LI is with the School of Chemical Engineering, The University of Queensland, St. Lucia, QLD 4072, Australia and also with the School of Mechanical and Mining Engineering, The University of Queensland, St Lucia, QLD 4072, Australia. JIAN-MIN LI and MING-XING ZHANG are with the School of Mechanical and Mining Engineering, The University of Queensland. Contact e-mail: [email protected] QING ZHENG is with the Baosteel Research Institute, Baoshan Iron and Steel Co. Ltd., Pu Dian Road 370, Shanghai 200122, China. GEOFF WANG is with the School of Chemical Engineering, The University of Queensland. Manuscript submitted August 18, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

during the heating process e.g., heat treatment or hot forming. This included the addition of strong carbide-forming alloying elements, such as Ti,[3,4] Nb,[5] and Zr,[6] which form small carbide particles, pinning down the growth of austenite grains. The segregating solutes, e.g., B,[7–9] segregates along the austenite grain boundaries, dragging the boundary movement. However, it is still unclear the roles and mechanisms of solutes in controlling the microstructure and grain size of as-case steels, including ingots and castings. Unlike in steels, the effects of solutes on grain refinement of cast light alloys have been comprehensively studied and well understood.[10–13] Generally, it is considered that the solute re-partitioning takes place in the liquid at front of the liquid/solid interface of a growing grain.[2] Such re-partitioning/segregation constructs a solute-enriched region at the front of the solid/liquid interface of the existing nucleated crystal. As a result, a constitutional supercooling zone is generated and a solute suppressed nucleation (SSN) zone is formed. Previous studies[14,15] indicates that at the front of liquid/solid interface of the existing equiaxed grain, the further nucleation can be largely suppressed in the SSN zone. Thus minimizing the width of SSN zone is critical to improve the nucleation efficacy and grain refinement.[12] The Interdependent theory proposed by Easton and StJohn[10] addressed that controlling the alloy chemistry, i.e., adding effective solute can reduce the widt