FINE MICROSTRUCTURE AND TOUGHNESS OF LOW CARBON COPPER CONTAINING ULTRA HIGH STRENGTH NV-F690 HEAVY STEEL PLATE
LIU Dongsheng, CHENG Binggui, CHEN Yuanyuan
Institute of Research of Iron and Steel, Shasteel, Zhangjiagang 215625
Cite this article:
LIU Dongsheng CHENG Binggui CHEN Yuanyuan. FINE MICROSTRUCTURE AND TOUGHNESS OF LOW CARBON COPPER CONTAINING ULTRA HIGH STRENGTH NV-F690 HEAVY STEEL PLATE. Acta Metall Sin, 2012, 48(3): 334-342.
Abstract Advanced NV-F690 heavy steel plates for offshore structure and shipbuilding have been produced via continuous casting of the slab, thermomechanical control rolling of the plate followed by solutionizing (austenitizing), quenching and tempering (QT) steps. The present work is to reveal the microstructure evolution and evaluate the mechanical properties of 80 mm thick plates subjected to the QT process. The microstructures were characterized with SEM, EBSD and TEM. At quarter thickness ($t$/4) where the cooling rate was rapid, the as quenched microstructures consist of mainly lath--like bainite (LB), between the laths exist fine martensite/austenite (M/A) constituents. At center thickness (t/2), the as quenched microstructures consist of granular bainite (GB)+dislocated LB. The fraction of high angle grain boundary (HAGB) at t/4 of the quenched plate is 67.5% while that at t/2 is 63.0%. After tempering at 650 ℃ for 150 min, the average width of the laths is 0.2 μm at t/4 and\linebreak 0.4 μm at t/2. Cell structures with high misorientations exist in the tempered LB at t/4 while these are absent at t/2. The fraction of HAGB is increased to 71.7% at t/4 while that at t/2 is not significantly changed. Profuse Cu precipitation occurs and the M/A constituents decompose into Cr-Mo containing carbides during tempering. Ductile fracture behaviour is observed even at -80 ℃ at both t/2 and t/4 of the 80 mm thick NV-F690 plate treated by the QT process. The impact toughness at t/4 is higher than that at t/2 due to the predominance of finer LB and higher fraction of HAGB. With the increase of the displacement of the ductile crack propagation, the area with dimples and the size of the dimples increase leading to increased Charpy impact absorbed energy of the NV-F690 plate\linebreak at -80 ℃.
