快速加热回火对690MPa级海洋工程用钢组织和性能的影响

EFFECT OF TEMPERING TEMPERATURE ON MICROSTRUCTURE AND PROPERTY OF 690 MPa GRADE OCEAN ENGINEERING STEEL UNDER FAST HEATING RATE

设计一种以低碳贝氏体组织为特征的690 MPa级海洋工程用钢,使用感应加热炉对直接淬火态钢板进行快速加热回火,通过拉伸实验、SEM和TEM手段研究了不同回火温度对其组织和性能的影响.结果表明,钢材的性能,特别是强韧性匹配受未转变奥氏体的转变行为以及析出物状态的综合影响.快速加热至550℃进行回火,组织软化,晶内只有少量析出物出现,致使强度降低,未转变奥氏体在冷却过程中转变为马氏体/奥氏体(M/A)岛;快速加热到600℃,由未转变奥氏体转化成的M/A岛尺寸稍有减小,细小的Cu,Nb/Ti复合析出物大量出现,钢材的强度显著上升;快速升温到660℃,组织内部析出物发生粗化,强度稍有下降,未转变奥氏体在板条间生成残余奥氏体薄膜,使得韧性提高,强韧匹配最佳;快速升温到700℃进行回火,析出物进一步粗化,未转变奥氏体进一步长大在室温下转变为尺寸较大的M/A岛,韧性再次下降,强韧性匹配降低.综合考虑,采用600-660℃的快速加热回火工艺可以使钢材达到最佳的强韧性匹配.

Ocean engineering steel has been rapidly developed with the exploration of ocean re- sourses. A 690 MPa grade low carbon bainite steel was designed for ocean engineering, to upgrade per- formance by mierostructure control and the refinement and dispersion control of precipitates. This steel was tempered o^line with rapid heating rate after control rolling and accelerated cooling process. The results show that the mechanical properties, especially the strength-toughness balance, are strongly influenced by the transformation of untransformed austenite and the condition of precipitates. When fast tempering at 550~, microstructure recovered and few precipitates appeared, bringing strength down seriously, untransformed austenite turned into martensite/austenite （M/A） islands on cooling.When the tempering temperature reached 600℃, the size of the M/A islands transformed from untrans- formed austenite decreased slightly, Cu and Nb/Ti precipitates increased greatly, bringing an apparent improvement in strength. When tempering temperature reached 660℃, precipitates got coarsened and made the strength decreases, the untransformed austenite formed retained austenite film between the laths, improving toughness and making the best strength-toughness balance. When the temper- ing temperature reached the top at 700℃, the precipitates got further coarsened, the untransformed austenite on cooling turned into large M/A islands, bringing toughness and the strength-toughness balance down again. In general consideration, fast heating tempering at 600--660 could make the steel has the best strength-toughness balance.