Strain-induced precipitation kinetics of Nb(C,N) and precipitates evolution in austenite of Nb–Ti micro-alloyed steels

Abstract

In this article, the precipitation–temperature–time (PTT) curves were obtained by plotting 1/A r-time curves (A r represents the area under the flow curves, and some details were clarified in the section “1/A method”) using two-stage interrupted compression testing. Based on the measured PTT curves, the L_J model has been applied to calculate the complete PTT curves by adjusting the modification factor, f, to be 0.022, from which it can be seen that they are both typical C-shaped with the nose temperature ranging from 900 to 925 °C. By using additivity rule, the continuous-cooling-precipitation (CCP) curves were established. And the critical cooling rates for precipitation start were also determined for different cooling start temperatures, and the effects of cooling rates and cooling start temperatures on CCP curves were systematically clarified. The evolution of precipitates was found to be in good agreement with PTT curves. Numerous fine and round precipitates were formed during holding after first compression deformation, which could have hindered static recrystallization kinetics and accounted for the plateaus on 1/A r-time curves. In addition, The cubic precipitates are mainly undissoloved TiN-rich (Ti,Nb) (C,N) carbonitrides. And based on the CCP and continuous-cooling-transformation curves of tested steel, the effect of cooling paths on precipitation and transformation were also discussed in details.