Mg Treatment And Hot Ductility Of Ti-bearing Micro-Alloyed Steels

The corner crack of microalloyed steel is the most common defect during continuous casting process.During the peritectic transformation from δ phase to γ phase,due to the difference of lattice structure between them,a large volume shrinkage occurred.The thickness of initial solidification shell in mold is not uniform,and surface cracks are likely to occur when local stress is concentrated.The coarseness of the original austenite grains also has an adverse effect on the strand quality,especially the surface cracks for peritectic steel.In this paper,a systematic study on the heterogeneous nucleation mechanism and the transformation behavior of peritectic steel under non-equilibrium conditions were carried out.The nucleation mechanism of MgAl2O4-TiN-δ-Fe was clarified through the experimental of Mg-Ti treatment in the laboratory.Based on the nucleation mechanism,the effect of Mg treatment on the peritectic steel was verified.The conclusions provide technical ideas for control of crack defects in microalloyed steel.A large number of dispersed and Mg-rich TiN particles were formed in the Mg-treated melt.From the wettability of TiN and liquid phase and the lattice misfit between TiN and δphase,it is confirmed that TiN can be used as effective nucleation particles for initical solification of ferrite phase,significantly reducing nucleation undercooling temperature,which plays a role in inducing δ-Fe heterogeneous nucleation in the early stage of solidification.The equiaxed ratio of ferritic stainless steel with Mg treatment increased from 37%to 50%,and the grain size reduced by 26%.A large number of TiN particles with cores of MgO/MgAl2O4 were formed in the molten steelThe refining mechanism ferrite phase with Mg treatment is analyzed based on lattice misfit theory.The lattice misfit between MgO/MgAl2O4 and TiN is 0.068%and 5.02%,respectively,and corresponding value between TiN and δ-Fe is 4.41%obtaind by HRTEM analysis.The crystal relationships among these phases is(200)TiN//(110)δ-Fe and(400)MgAl2O4//(200)TiN.The wetting feathure between TiN and melt promotes the nucleation behavior of liquid on TiN surface.The oxides with high melting point such as Al2O3,MgO and MgAl2O4 provides effective heterogeneous nucleation location for preicipation of TiN.The hot ductility of peritectic steel can be improved by Mg treatment significantly,meanwhile,the porior austenite grain was refined by 18.7%after Mg addition.The size of TiN particles riched with Mg element reduced by 41%.The reduction of area under 780℃ was higher than 60%.Industrial tests confirmed that Mg treatment can effectively suppress surface cracks during continuous cating of peritectic steel.After Mg treatment,the incidence of corner cracks in the continuous casting slab dropped from 3.79%to 0.01%,and the quality of the slab was significantly improved.