本文通过板坯连铸、钢板控轧控冷(TMCP)、固溶淬火回火(QT)工业生产流程, 开发低C含Cu高强韧NV-F690特厚(厚度t为80 mm)船体和海洋平台用钢板. 使用SEM, EBSD和TEM分别研究了淬火(Q)态和QT态钢板的精细组织, 测试了距离钢板表面$t$/4处 (高冷却速率)和芯部$t$/2处(低冷却速率)的室温硬度和拉伸性能, 在-60和-80 ℃下进行了Charpy冲击(Charpy V notch, CVN)示波实验. 结果表明, 淬火速率较大有利于板条组织形成和提高大角度晶界比例, $t$/4处的组织为板条状贝氏体(LB), 板条间存在细小片状马氏体/奥氏体(M/A)组元, 晶粒间大角度晶界(>15o)体积分数为67.5%; t/2处的组织为粒状贝氏体(GB)+LB, 大角度晶界体积分数为63.0%; Q态下的LB具有高位错密度, 但晶粒内不存在Cu析出相. 经过650 ℃回火150 min, 钢板的强韧性匹配优良, 低温下呈韧性断裂, 大量含Cu弥散沉淀相在基体组织内析出. t/2处的M/A组元分解为Cr-Mo碳化物, 贝氏体板条宽度为0.4 μm, 大角度晶界分数为62.5%; t/4处的LB板条回复, 板条内存在与基体取向差较大的亚晶, 大角度晶界分数提高到71.7%, 板条平均宽度为0.2 μm. 在-80 ℃下, NV-F690钢板t/4处的韧性高于t/2处的韧性. 随着纤维断裂位移的增大, 韧窝断裂区比例和韧窝尺寸逐渐增大, NV-F690钢低温Charpy冲击能量逐渐提高.
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 ℃.
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