Structure-Property Relationship in a Micro-laminated Low-Density Steel for Offshore Structure

A low-density steel (7.5gcm(-3)) steel produced by direct-quenching and intercritical annealing (IA) process is designed for offshore structure. The microstructure, mechanical properties, and corrosion resistance of steels are studied. It is found that after IA process, the microstructure is composed of micro-laminated delta-ferrite, tempered martensite, and reverted austenite. Two stage partitioning processes, the reheating process prior to rolling, and IA process lead to enrichment of C, Mn, and Ni elements in the reverted austenite. The enrichment of alloying elements stabilized austenite, and the retained austenite content is up to 18%. The growth of reverted austenite at lath-lath interface is simulated by DICTRA software, and the results show that the growth of reverted austenite are controlled by diffusion of alloying elements. The ductility and toughness has a strong relationship with content of retained austenite. The mechanical properties are excellent with high yield strength (>620MPa) and high total elongation (>40%). The experimental steel has excellent resistance to marine atmospheric corrosion, because of enrichment of Al and Ni in the rust layer, that leads to a compact layer. In addition, the partitioning of Al and Ni into different phase, promotes the corrosion resistance of rust layer.