Abstract
The effect of Ti microalloying on the precipitation of NbC particles in Nb-microalloyed and Nb-Ti-microalloyed steels was investigated by scanning transmission electron microscopy. The experimental results illustrate that NbC precipitates tend to be formed via a conventional “nucleation and growth” mechanism in Ti-free steel, while the particles would be precipitated as a complex form of TiN cuboid core and NbC hemispherical cap in Ti-Nb-microalloyed steel with 0.009 wt.% Ti and 0.0046 wt.% N. Ti microalloying contributed to the refinement of the precipitate size, and an enhancement of the volume fraction of NbC particles was also found based on the experimental observations.
Highlights
Structural steels with heavy thickness are commonly used in complex constructions, such as offshore oil platforms, large-scale bridges and so on [1,2,3], which usually operate in low-temperature environments
To meet the safety requirements [4,5,6], high strength as well as excellent toughness are necessary for heavy structural steels
Thermomechanical control processing (TMCP) has been proven to be an effective approach to the achievement of fine grains and has been widely applied in the manufacturing of hot rolled steels, in which sufficient strain accumulation before phase transformation is generally considered to be an essential issue [7,8]. It is always a challenge for heavy structural steels, because their limited reduction during the hot rolling process cannot bring about enough strain accumulation for the stimulation of a “strain-induced phase transformation” to refine the grain size
Summary
Structural steels with heavy thickness are commonly used in complex constructions, such as offshore oil platforms, large-scale bridges and so on [1,2,3], which usually operate in low-temperature environments. Thermomechanical control processing (TMCP) has been proven to be an effective approach to the achievement of fine grains and has been widely applied in the manufacturing of hot rolled steels, in which sufficient strain accumulation before phase transformation is generally considered to be an essential issue [7,8] It is always a challenge for heavy structural steels, because their limited reduction during the hot rolling process cannot bring about enough strain accumulation for the stimulation of a “strain-induced phase transformation” to refine the grain size. Fine TiN particles acting as nucleation cores for NbC particles and resulting in the formation of TiN-NbC composites during subsequent finish rolling have been reported [17,18] This phenomenon offers an alternative approach to strain-induced precipitation of NbC, which can be reasonably utilized for the microstructural control of heavy structural steels with limited strain accumulation. The results are useful for considering the control of microstructure and mechanical properties for heavy structural steels with limited reduction ratios and low Nb contents
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