Effect of austenitizing temperature on the microstructure and mechanical properties of Nb–Ti microalloyed steel

Abstract

The effect of austenitizing temperature on the microstructure and mechanical properties of Nb–Ti microalloyed steel was investigated. Steels were subjected to different austenitizing treatments (temperatures ranging from 850 °C to 1250 °C for 5-120 min) and rolled after being austenitized at different temperatures (i.e. 1020 °C, 1070 °C and 1150 °C). The results showed that austenite grain coarsening temperature was around 1000 °C. The mean grain size of the rolled steels initially increased and then decreased, but the ferrite content decreased with increasing austenitizing temperature. The precipitates in the prior austenite and rolled steel were both complex Nb–Ti carbonitrides. As the austenitizing temperature increased from 1020 °C to 1150 °C, most precipitates were dissolved and re-precipitated as dispersive particles with mean size decreasing from 30 nm to 10 nm. Meanwhile, the 80 nm to 100 nm rectangular Ti-rich carbonitrides were not dissolved and varied during the subsequent cooling process.The Yield strength and ultimate tensile strength increased but the elongation and reduction in area (in percent) decreased with the elevated austenitizing temperature.

Keywords

• Austenitizing Temperature
• Precipitation
• Nb–Ti Microalloyed Steel
• Coarsening Temperature
• Mechanical Properties ______________