Abstract
This paper focuses on the thermodynamic behavior of the precipitation of the second phase and microstructure evolution of the high titanium microalloyed steel containing niobium during tempering in the temperature range of 500–650 °C to reveal the influence of the heat treatment scheme on the toughness and microhardness. The results indicate that FCC_A1#2 phase is isomorphic to FCC_A1#3, which are carbonitrides of Ti or Nb, and precipitates at about 1266 °C. The Ti and Nb elements almost entirely precipitated at 800 °C, and their contents in the FCC_A1#2 phase are much higher than those in FCC_A1#3 in the low-temperature region. The microstructure of the tempered samples is mainly composed of granular bainite (GB), on which martensite/austenite (M/A) islands are distributed. The decomposition extent of M/A islands got higher and transformed into a large number of nano-sized cementite distributed at the grain boundaries of ferrite with the increasing tempering temperature. It is noticeable that those nanoscale spherical carbide particles provide an effective way to enhance the low temperature impact toughness of the steel significantly. The nanoparticles in the experimental steel tempered at 600 °C have an average diameter of 4.4 nm, accounting for nearly 65 % of the total number of particles. The experimental steel manifested the best low-temperature impact toughness of 361 J with a tempering temperature of 600 °C for 60 min after being quenched from 900 °C.

Key words
thermodynamics, precipitation, microstructure, property, heat treatment

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