In this study, three kinds of bainitic steel plates are manufactured by varying the chemical compositions and their microstructures are analyzed. Tensile and Charpy impact tests are performed at room and low temperature to investigate the correlation between microstructure and mechanical properties. In addition, heat affected zone (HAZ) specimens are fabricated by a simulation of welding processes, and the HAZ microstructure is analyzed. The base steel that has the lowest carbon equivalent has the highest volume fraction of acicular ferrite and the lowest volume fraction of secondary phases, so the strength is the lowest and the elongation is the highest. The Mo steel has a higher volume fraction of granular bainite and more secondary phases than the base steel, so the strength is high and the elongation is low. The CrNi steel has the highest volume fraction of the secondary phases, so the strength is the highest and elongation is the lowest. The tensile properties of the steels, namely, strength and elongation, have a linear correlation with the volume fraction of secondary phases. The Mo steel has the lowest Charpy impact energy at -80 oC because of coarse granular bainite. In the Base-HAZ and Mo-HAZ specimens, the hardness increases as the volume fraction of martensite-austenite constituents increases. In the CrNi-HAZ specimen, however, hardness increases as the volume fraction of martensite and bainitic ferrite increases.

Recently, steel structures have increasingly been required to have sufficient deformability because they are subjected to progressive or abrupt displacement arising from structure loading itself, earthquake, and ground movement in their service environment. In this study, high-strength low-carbon bainitic steel specimens with enhanced deformability were fabricated by varying thermo-mechanical control process conditions consisting of controlled rolling and accelerated cooling, and then tensile and Charpy V-notch impact tests were conducted to investigate the correlation between microstructure and mechanical properties such as strength, deformability, and low-temperature toughness. Low-temperature transformation phases, i.e. granular bainite (GB), degenerate upper bainite(DUB), lower bainite(LB) and lath martensite(LM), together with fine polygonal ferrite(PF) were well developed, and the microstructural evolution was more critically affected by start and finish cooling temperatures than by finish rolling temperature. The steel specimens start-cooled at higher temperature had the best combination of strength and deformability because of the appropriate mixture of fine PF and low-temperature transformation phases such as GB, DUB, and LB/LM. On the other hand, the steel specimens start-cooled at lower temperature and finish-cooled at higher temperature exhibited a good low-temperature toughness because the interphase boundaries between the low-temperature transformation phases and/or PF act as beneficial barriers to cleavage crack propagation.