Effect of microstructure on low-temperature toughness of a low carbon Nb–V–Ti microalloyed pipeline steel

Abstract

In order to further understand the relation between microstructure and low-temperature toughness of high strength pipeline steel, two kinds of microstructures with different features in a low carbon Nb–V–Ti microalloyed pipeline steel were obtained by adjusting thermo-mechanical controlled processing parameters. The microstructural characteristics, including detailed structure of microstructural constituents, as well as their effects on low-temperature toughness were investigated. The results show that under higher reduction in austenite non-recrystallization region and faster cooling rate during accelerated cooling, the microstructure is dominated by acicular ferrite (AF) accompanied by a small amount of fine martensite/austenite (M/A) islands. In contrast, lower reduction and slower cooling rate lead to a predominantly quasi-polygonal ferrite (QF) microstructure with coarse M/A islands. Like AF, QF contains high density tangled dislocations, well developing a lot of dislocation walls and cellular substructures. Compared with QF, AF appears to have finer effective grain size (EGS). The fine EGS and high fraction of high angle grain boundaries (HAGBs) make the cleavage crack propagation direction deflect frequently. The coarse M/A islands can lead to cleavage microcracks at the M-A/ferrite matrix interfaces. Acicular ferrite dominated microstructure exhibits excellent low-temperature toughness because of fine EGS, high fraction of HAGBs and fine M/A islands.