Characterisation of Microstructure, Texture and Mechanical Properties in Ultra Low-Carbon Ti-B Microalloyed Steels

Abstract

In the present study, thermo-mechanical controlled processing followed by water quenching has been utilised to produce ultra low-carbon microalloyed steel in a laboratory scale. The variation in microstructure and corresponding mechanical properties at the selected range of finish rolling temperatures (FRT), ($850-750^{\circ}C$) has been evaluated. The microstructures of the steels consisted of polygonal ferrite, acicular ferrite as well as granular bainite with the average ferrite grain sizes less than $5{\mu}m$. Finish rolling at $850^{\circ}C$ produced weak texture. ${\alpha}-fibre$ and ${\gamma}-fibre$ intensified with the decrease in finish rolling temperature to $800^{\circ}C$. Intensities of the beneficial texture components such as, {554}<225> and {332}<113> also reached the highest value at $800^{\circ}C$. Ferrite deformation texture i.e. ${\alpha}-fibre$ dominated at $750^{\circ}C$ FRT. The characteristic ferrite - bainite microstructure with fine ferrite grain size and uniform distribution of fine TiC particles (< 50 nm) resulted in high yield strength (405-507 MPa), moderate tensile strength (515-586 MPa) and high total elongation (19-22%) for the selected range of finish rolling temperatures. Fairly good impact toughness value in the range of 63-74J was obtained at subzero temperature ($-40^{\circ}C$) in the sub-size sample. The above strength - ductility - toughness combination boosts the potentiality of developed steel for the pipeline application.

Keywords

• alloys;
• thermo-mechanical processing;
• microstructure;
• transmission electron microscopy;
• mechanical properties