Microstructural Characteristics with Various Finish Rolling Temperature and Low Temperature Toughness in Hot Rolled Nb–Ti Ferritic Steel

Abstract

In order to investigate the effects of finish rolling temperature on the microstructural characteristics and fracture mechanism of Nb–Ti ferritic steel, the samples with various finish rolling temperature have been observed by optical microscopy, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The results indicated that both ferrite grain size and volume fraction of (Nb,Ti)C precipitates decreased with the decreasing finish rolling temperature, but the size of (Nb,Ti)C precipitates changed little with the finish rolling temperature. All of these factors caused the yield strength, tensile strength and elongation remained similar. However, the low temperature toughness was significantly influenced by the finish rolling temperature. Based on the observation results of fracture surface of samples tested at −40°C, the fracture type of steels with finish rolling temperature of 940°C and 910°C was brittle, while the fracture of steel with finish rolling temperature of 880°C was ductile. At last, the crack initiation and propagation mechanism is elucidated in details. At −40°C, both intergranular cracks and transgranular cracks existed in the steels finish rolled at 940°C and 910°C, and only microvoides were observed in the steel finish rolled at 880°C with high toughness.