MICROSTRUCTURE COMPONENTS AND MECHANICAL PROPERTIES OF AN ACICULAR FERRITE PIPELINE STEEL

Through proper thermo-mechanical controlled processing (TMCP), the acicular ferrite (AF) microstructures of different phase components were obtained for an ultra-low carbon C-Mn-Mo-Nb-V type of pipeline steel. The relation between AF microstructures and mechanical properties was investigated. The results showed that the effective grain size (EGS) was reduced obviously with the decreasing finish rolling temperature (FRT), but was independent on the cooling rate (CR) and simulated coiling temperature (SCT). Lowering FRT or enhancing CR could increase the percentage of granular bainitic ferrite (GF) and bainitic ferrite (BF) in microstructure. The content of high angle grain boundaries (HAGB) was increased with decrease of FRT when FRT was over 800 degrees C, then decreased with FRT at 750 degrees C. It was found that the higher fraction of HAGB could lead to better low temperature toughness. The fine AF microstructure with more GF and BF and less quasi-polygonal ferrite (QF) could result in a good combination of strength and toughness.