TADASHI FURUHARA, KENJI TSUZUMI, GORO MIYAMOTO, TAKAFUMI AMINO, and GENICHI SHIGESATO In the present study, bainite transformation kinetics was examined in low C-Mn steels with the addition of small amounts of B and Mo. This addition delays the onset of the bainite transformation. Mo addition causes transformation stasis at temperatures between 873 K and 823 K (600°C and 550°C) just below the bainite-start (B s ) temperature, resulting from an incomplete bainite transformation. Post-stasis transformation after a prolonged hold proceeds by the formation of ferrite with a low dislocation density, and in Mo-containing alloys, often the formation of carbides. The volume fraction at which the transformation stops is higher for lower carbon contents and lower transformation temperatures. By contrast, at 773 K (500°C), the bainite transformation accompanying cementite precipitation occurs regardless of microalloying and is completed after shorter hold times. EDX measurement performed on the Mo-added 0.15 pct C alloy with aberration-corrected STEM revealed that segregation at the bainite/austenite interphase boundary is small for Mn and negligible for Mo in the early stages of stasis, which does not support the incomplete transformation mechanism based on the solute drag theory for the alloys used.
Estimation of changes in mold flux composition and physical properties during casting was investigated for control of slab surface quality. In this study, 0.7 mass% Al steel and normal Al-killed steel were cast with three kinds of mold flux having Al 2 O 3 contents of 1.3 to 6.0 mass% and different basicities. The results can be summarized as follows: (1) The Al 2 O 3 content of these mold fluxes increased to 30 mass% during continuous casting. The composition change of the mold flux can be reproduced by the Equilibrium Effective Reaction Zone Model (EERZM) by fitting parameters, referring to the casting results. (2) The analysis by EERZM revealed that the viscosity of the mold flux and throughput of the molten steel affect the rate of increase of the Al 2 O 3 content in the mold flux. (3) The change of mold flux physical properties was estimated based on flux composition changes. The change of the crystallization temperature and main crystal can be estimated by FactSage. (4) Mold flux viscosity can be estimated by revising the modified Iida's equation, which considers the effect of Al 2 O 3 as an amphoteric oxide.
Estimation of changes in the composition and physical properties of mold flux during continuous casting was investigated for control of slab surface quality. In this study, 0.7 mass% Al steel and normal Al-killed steel were cast with three kinds of mold flux having Al 2 O 3 contents of 1.3 to 6.0 mass% and different basicities. The results can be summarized as follows:(1) The Al 2 O 3 content of these mold fluxes increased to 30 mass% during continuous casting. The composition change of the mold flux can be reproduced by the Equilibrium Effective Reaction Zone Model (EERZM) by fitting parameters, referring to the casting results. (2) The analysis by EERZM revealed that the viscosity of the mold flux and the throughput of the molten steel affect the rate of increase of the Al 2 O 3 content in the mold flux. (3) The change of the physical properties of the mold flux was estimated based on the changes in the flux composition. The change of the crystallization temperature and main crystal can be estimated by FactSage. (4) Mold flux viscosity can be estimated by revising the modified Iida's equation, which considers the effect of Al 2 O 3 as an amphoteric oxide.
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