Thermodynamic Study on Structure Designing of Microalloyed Steel.

Thermodynamic study on phase equilibria in microalloyed steels has been carried out and thefollowing results were obtained 1. Phase Equilibria in the Fe-C-Ti，Fe-C-Nb and Fe-C-V Systems The solubility of TiCNbC和VC in austenite was measured by diffusion couple method. It was确认thesolubility in high carbon region is larger than that in low carbon region due to the lowering of theactivity of Ti, Nb and V by carbon in austenite. in consequence of this fact，the phase boundaries of austenite in these systems exhibite a characteristic shape in the verticalsections.2. Miscibility Gap between NbC and VC in the Fe-C-Nb-V System the Miscibility Gap betweenNbC和VC was determined by lattice parameter measurements on carbides extraction from theFe-C-Nb-V specimens. It examined the miscibility gap was almost symmetric with the criticaltemperature of about 1500 C.3. Effect of Alloying elements on the Solubility of VC in Austenite Itwas ascertained that Cr, Mo, W and Mn increase the solubility of VC in austenite, while Si, Al，Ni和Co decrease it.这些事实都很好地解释了相关的互动alloying elements and carbon in austenite.4. Phase Equilibria in the Fe-C-B System Phase Equilibriain the Fe-rich portion of the Fe-C-B系统were analyzed on the basis of thermodynamic calculationsof the Fe-C and Fe-B systems. the effects of B on the austenite field，the liquidus and solidus tcmperatures and the stability of Fe@>D23@>D2C and Fe@>D223@> d2c_6 carbideswere calculated。

Thermodynamic study on phase equilibria in microalloyed steels has been carried out and the following results were obtained:1. Phase Equilibria in the Fe-C-Ti, Fe-C-Nb and Fe-C-V Systems The solubility of TiC, NbC and VC in austenite was measured by diffusion couple method. It was confirmed that the solubility in high carbon region is larger than that in low carbon region due to the lowering of the activity of Ti, Nb and V by carbon in austenite. In consequence of this fact, the phase boundaries of austenite in these systems exhibite a characteristic shape in the vertical sections.2. Miscibility Gap between NbC and VC in the Fe-C-Nb-V System The miscibility gap between NbC and VC was determined by lattice parameter measurements on carbides extraction from the Fe-C-Nb-V specimens. It was examined that the miscibility gap was almost symmetric with the critical temperature of about 1500゜C.3. Effect of Alloying elements on the Solubility of VC in Austenite It was ascertained that Cr, Mo, W and Mn increase the solubility of VC in austenite, while Si, Al, Ni and Co decrease it. These facts were well explained by considering the interactions between alloying elements and carbon in austenite.4. Phase Equilibria in the Fe-C-B System Phase equilibria in the Fe-rich portion of the Fe-C-B system were analyzed on the basis of thermodynamic calculations of the Fe-C and Fe-B systems. The effects of B on the austenite field, the liquidus and solidus tcmperatures and the stability of Fe@>D23@>D2C and Fe@>D223@>D2C_6 carbides were calculated.