Study On Microstructure And Properties Of Low-Density Steels For Offshore Structure

With the development of marine resources,it is urgent to develop low-density weathering steel plates applied in the tropical marine atmospheric environment.Aluminum is an effective element for improving the corrosion resistance and reducing the density.Two types of Al-bearing steel plates were developed for different grades of offshore-structural steels,and their corrosion resistance behavior was evaluated.The thermoplasticity and recrystallization behavior of the δ-ferritic steel was investigated.The kinetics of the static recrystallization for the δ-ferritic steel was established by Avrami-type empirical equation,and the recrystallization activation energy was 277.2 kJ/mol,while the Avrami coefficient was 0.93.With the addition of Al in the steel,the stacking fault energy of ferrite would be decreased according to first principle calculations,which suppressed recovery and correspondingly promoted recrystallization.The recrystallization of the δ-ferritic steel only starts at a minimum fractional softening of 0.6.The main nucleation mechanism of the static recrystallization of the δ-ferritic steel was strain induced boundary migration.The dynamic recrystallization behavior of the δ-ferritic steel includes continuous dynamic recrystallization and discontinuous dynamic recrystallization.A process of controlled rolling and on-line annealing process was developed,and the mechanical properties meet the requirements of Grade B,while the density is decreased by 5.6%.A multi-phase low-density offshore-structural steel was produced by controlled rolling,direct-quenching and intercritical annealing process.The microstructure evolution was clarified,and the micro-laminated δ-ferrite growth,alloy element parpition,and reverted austenite nucleation&growth took place.The reheating process before rolling is in the dual phase region of δ-ferrite and austenite,which caused the first partitioning process of C,Mn,Ni,Al and Nb.The microstructure of as-rolled samples after direct-quenching was mainly composed of δ-ferrite,martensite and a-ferrite.After the intercritical annealing,the alloy elements were further partitioned,and carbon,manganese and nickel were enriched in the reverted austenite while aluminum diffused into the tempered martensite.The final structure was composed of micro-laminated δ-ferrite,tempered martensite and retained austenite.The mechanical properties meet the requirements of DH620,while the density is reduced by 4.5%.The state of aluminum atom in rust layer was investigated,and it is found that aluminum atom could replace iron atom in α-FeOOH,β-FeOOH,γ-FeOOH,and Fe3O4.Aluminum atom could fix chlorine atom in the iron oxyhydroxide,which inhibited the formation of P-FeOOH and promoted the formation of y-FeOOH and a-FeOOH.Also,aluminum could refine the corrosion products’ grains.Therefore,aluminum improved the corrosion resistance of steel to chloride ion corrosion.