Effects of Start and Finish Cooling Temperatures on the Yield Strength and Uniform Elongation of Strain-Based API X100 Pipeline Steels

Three types of strain-based API X100 pipeline steels composed of polygonal ferrite (PF), acicular ferrite (AF), granular bainite (GB), bainitic ferrite (BF), and martensite–austenite (MA) were fabricated by varying the start cooling temperature (SCT) and finish cooling temperature (FCT). The effect of the microstructure on the yield strength and uniform elongation before and after strain aging was investigated. In the H-H steel fabricated at a high SCT and low FCT, the reduction in uniform elongation after prestraining was small, but the uniform elongation after heat treatment increased because of the presence of many mobile dislocations around AF and GB, which are present in a relatively high fraction. Because the H-L steel fabricated at high SCT and low FCT has a considerable amount of BF as a result of the significant degree of supercooling, carbon atoms were readily trapped at dislocations inside the BF during heat treatment, which reduced the resistance to strain aging. On the other hand, the L-L steel had a large amount of soft PF transformed in the two-phase (austenite + ferrite) region and high fractions of AF and MA because it was fabricated at low SCT and low FCT. As a result, the uniform elongation of the L-L steel was slightly increased after heat treatment because mobile dislocations were readily generated around the PF and MA boundaries under deformation.