| Research Abstract |
The purpose of the present research is to analyze the phase equilibria between the carbonitrides in microalloyed steels and to develop a method to calculate the complicated phase relationship. 1.Miscibility gaps in some transition-metal carbide or carbonitride systems have been experimentally studied. The systems investigated were (V,Nb)C, (Ti,V,Nb)C, (V,Nb)C, (V,Nb)(C,N), (Ti,Nb)(C,N)and(Ti,V)(C,N),and the equilibrium compositions were determined by the X-ray measurements and EDX analysis. A thermodynamic calculation of the miscibility gaps was performed on the basis of the experimental results. The Gibbs energy of the carbonitrides has been described using a sublattice model and the interaction energies in the pseudobinary systems were estimated from the difference in lattice constants in components. The miscibility gaps in the 20 (X,Y,Z)C types of carbides,10 (X,Y,Z)N types of nitrides and 10 (X,Y)(C,N) types of carbonitrides were calculated. 2.A thermodynamic model was presented to calculate the composition of austenite as well as that of complex carbonitrides in the microalloyed steels containing Ti,V and Nb. The thermodynamic model was applied to the phase equilibrium calculation between the austenite and (Ti,V,Nb)X, (M1,M2)(C,N),or(Ti,V,Nb)(C,N) type of compounds. Some variations of precipitation mode were observed through the calculation according to the alloy contents and kinds of additional elements. The effect of individual element was briefly summarized as follow;(a) Ti addition to the microalloyed steels enhances the formation of Ti-rich nitrides mainly due to its extreme low solubility in austenite. (b)Complex addition of the elements brings the phase separation of the carbonitrides,and in almost cases the separation between NbC and VC or NbC and TiN is presented. This phenomena can be explained from the view points of the relative stability of binary compounds.
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