目的 探究特厚超高强海工钢在全截面（尤其-80 ℃极低温）的韧性差异及原因,为深海工程装备关键承力构件实现全厚度强韧性协同提升提供理论依据与工艺方向。方法 采用320 mm厚度连铸坯,通过控轧-控冷结合淬火-回火制备100 mm厚F690海工钢,采用OM、SEM和TEM系统研究表层、1/4处及心部的组织演变规律,结合断裂力学理论分析组织与极低温韧性的关联机制。结果 表层因轧制压下量最大和淬火冷却速度最快（10 ℃/s）,形成了均匀细小的奥氏体晶粒（18.5 μm）,但过高的冷速导致马氏体相变过冷度增大,形核率降低,反而形成粗大块状马氏体,降低裂纹扩展阻力,位错塞积形成的位错墙未能有效阻碍裂纹,导致-80 ℃冲击韧性降至57 J;心部因冷却速度缓慢（1~2 ℃/s）及连铸中心偏析,形成混晶奥氏体（晶粒尺寸26.4 μm）及大量粒状贝氏体（体积分数17.8%）,粒状贝氏体中M/A岛作为脆性核心引发了应力集中,其尺寸效应加剧了低温下解理断裂倾向,导致-80 ℃冲击功仅62 J;1/4处冷速适中（5 ℃/s）,形成板条马氏体（宽度376 nm）与少量粒状贝氏体的混合组织（粒状贝氏体体积分数9.8%）,综合力学性能最优,-80 ℃冲击功达176 J。结论 特厚海工钢极低温韧性非均匀性主要受截面冷却速率差异的显微组织特征支配,通过平衡冷却速率、抑制心部偏析及细化表层马氏体亚结构,可实现全截面强韧性协同提升,满足深远海装备对材料极端环境服役可靠性的要求。
Abstract
The work aims to explore the uniformity of toughness of ultra-thick ultra-high strength offshore steel across the entire cross section, especially at -80 ℃ extremely low temperature, and to provide a theoretical basis and technological direction for achieving the coordinated improvement of strength and toughness of key load-bearing components of deep-sea engineering equipment throughout the entire thickness. A 100 mm-thick F690 offshore steel was prepared from a 320 mm thick continuous casting billet through controlled rolling and controlled cooling followed by quenching and tempering. The microstructure evolution patterns at the surface, 1/4 point and the center were studied by OM, SEM and TEM. The correlation mechanism between the microstructure and extremely low-temperature toughness was analyzed based on the fracture mechanics theory. The largest rolling reduction and fastest quenching cooling rate (10 ℃/s) at the surface layer, uniform and fine austenite grains (18.5 μm) were formed, but the excessively high cooling rate led to an increase in the undercooling degree of martensite phase transformation and a decrease in the nucleation rate, resulting in coarse blocky martensite instead. This reduced the crack propagation resistance, and the dislocation walls formed by dislocation pile-up failed to effectively impede the crack, causing the impact toughness at -80 ℃ to drop to 57 J. The slow cooling rate (1-2 ℃/s) and the center segregation of continuous casting at the center, mixed crystal austenite (grain size 26.4 μm) and a large amount of granular bainite (volume fraction 17.8%) were formed. The M/A islands in granular bainite acted as brittle centers, causing stress concentration, and the size effect intensified the cleavage fracture tendency at low temperatures, resulting in an impact energy of only 62 J at -80 ℃. At the 1/4 position, the cooling rate was moderate (5 ℃/s), forming a mixed structure of lath martensite (width 376 nm) and a small amount of granular bainite (volume fraction 9.8%), with the best comprehensive mechanical properties and an impact energy of 176 J at -80 ℃. The non-uniformity of extremely low-temperature toughness of ultra-thick offshore steel is mainly controlled by the microstructure characteristics regulated by the cooling rate across the cross section. By balancing the cooling rate, suppressing the segregation at the center and refining the substructure of martensite at the surface layer, the coordinated improvement of strength and toughness throughout the entire cross-section can be achieved, meeting the requirements of reliable service of materials in extreme environments for deep-sea equipment.
关键词
特厚海工钢 /
厚度截面效应 /
极低温韧性 /
相变机理 /
裂纹扩展
Key words
extra-thick offshore steel /
thickness section effect /
ultra-low temperature toughness /
phase transformation mechanism /
crack propagation
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基金
河南省重大科技专项（251100230200）
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