Search

Nonmetallic inclusions formed during the Ti-stabilized stainless steelmaking process can induce the clogging in the submerged entry nozzle during continuous casting. To investigate the formation mechanism of nonmetallic inclusions in Ti-stabilized stainless steel, steel specimens were taken at different stages in industrial trials. The characteristics of inclusions and the chemical compositions of steel were analysed. After 10 min of Al deoxidation, the main type of inclusions in samples was irregular MgO·Al2O3. With titanium addition after calcium treatment, the irregular dual phase CaO–TiOx–Al2O3–MgO inclusions were formed. The steel compositions of most samples were located in or close to the Al2O3–TiOx liquid oxide phase field. According to experiment and thermodynamic results, the solid CaTiO3 and/or CaO·2Al2O3 inclusions were formed during titanium addition when the molten steel contained more than 20 ppm calcium. The effect of the initial calcium content on the formation of Ca–Al–Ti–O inclusions during titanium addition was discussed. [ABSTRACT FROM AUTHOR]

Formation mechanism of Ti2O3–TiN complex nucleus and its nucleation effect on δ-Fe at solidification front of Fe-18Cr ferritic stainless steel was investigated through experimental and theoretical methods. According to the equilibrium calculation, Ti2O3 will crystallize in the molten steel before solidification and TiN will not form until the solid phases appear. However, complex nucleus and equiaxed grains exist in the edge of Ti-bearing ingot, where the solidification velocity and temperature gradient are high. Therefore, complex nucleus is indicated to be formed at solidification front and a non-equilibrium solidification model for multi-component alloy was established to investigate the dendrite tip interface phenomenon. The calculation results exhibit an element enrichment at solidification front and explain well with the formation of complex nucleus before solidification. It is also found that there is approximate lattice matching between TiN, and Ti2O3, δ-Fe on the basis of disregistry theory. These results are consistent with the observed complex nucleus and the increased equiaxed zone ratio (EZR) of Ti-bearing ferritic stainless steel. [ABSTRACT FROM AUTHOR]

The characteristics and generating mechanism of heterogeneous carbonitride with micron size in H13 + Nb bar are studied. Three types of precipitates are observed according to the number of layers, that is, two-layer ones, three-layer ones, and four-layer ones. Two-layer precipitate contains the oxide core and the outer Ti-V-rich carbonitride, while three-layer precipitate has one more Nb-rich carbonitride at the outside. Four-layer precipitate has a V-rich carbonitride at the outmost layer besides the inner three types of phases similar to that in the three-layer precipitate. The size of these large precipitates can be even up to tens of microns. The oxide cores are mainly stoichiometric spinel MgAl2O4 and generated in the liquid steel before solidification. The compositional characteristics and precipitating sequence of carbonitride can be well explained through calculation by Scheil-Gulliver model in Thermo-Calc. Ti-V-rich phase precipitates first during solidification, after which Nb-rich phase and V-rich phase precipitates successively. Some dissolution occurs between the different layers during the following temperature reducing and heating treatment process. [ABSTRACT FROM AUTHOR]

Equiaxed solidification contributes to the ridging defect resistance of ferritic stainless steel. Ti 2 O 3 accelerating TiN formation to nucleate δ-ferrite was recently recognized as an effective method to promote equiaxed solidification. However, the acceleration mechanism of Ti 2 O 3 is still unclear, and there lacks direct nucleation evidence. Therefore, in this study, the initial solidification state was preserved using a newly designed droplet experiment. The orientation relationship calibrated disregistry calculation was carried out to understand the nucleation phenomena. It is novel to find that the formation of Ti 2 O 3 -TiN originates from the Ti-O-N particle, which forms before solidification of steel. The sequenced precipitation in the order of Ti 2 O 3 and TiN from the Ti-O-N particle follows the orientation relationship of {0001} Ti2O3 // {111} TiN and finally constitutes the Ti 2 O 3 -TiN. Afterward, the δ-ferrite nucleation will be promoted on the surface of the as-formed Ti 2 O 3 -TiN following {001} δ-Fe // {001} TiN. The primary δ-ferrite dendrite preserved in the droplet experiment affirmed the proposed mechanism. These findings will expand the knowledge of oxide accelerating nucleation and could contribute to suitable compositional design for equiaxed solidification. [Display omitted] • Equiaxed solidification was greatly promoted when Ti 2 O 3 accelerated the formation of TiN. • Ti-O-N was confirmed in initial equiaxed dendrites of δ-ferrite using a newly designed droplet experiment. • Ti 2 O 3 -TiN formed through the precipitation of TiN from Ti-O-N following {111}TiN//{0001}Ti 2 O 3. • δ-Fe nucleated on Ti 2 O 3 -TiN following {001}TiN//{001}δFe. [ABSTRACT FROM AUTHOR]