| With the implementation of China’s energy conservation and CO2 emission reduction,the demand for weight reduction has been put forward in the applications of marine engineering and automotive.The low-density steels have attracted the people’s attention due to its low density and good corrosion resistance.At present,the research on low-density steels mainly focused on the field of automotive sheets,while the research on offshore structure steels with all-δ-ferrite is relatively less.In this paper,the microstructure control and toughening mechanism of all-δ-ferrite low-density steels for offshore structure are systematically investigated and analysed.Also,an innovation control technology for microstructure refinement and toughening of δ-ferrite is proposed.Through composition design,and microstructure control,the strength and low temperature toughness of experimental steels were improved.Considering the interaction of microalloying elements(Nb,Zr),reheating,thermal deformation,tempering,etc.,the relationship between the microstructure evolution and mechanical properties of δ-ferrite steels is clarified.the hot deformation mechanism of Nb-Zr microalloyed δ-ferrite low-density steel is studied.It provides theoretical support for composition design,hot rolling process,microstructure and properties control of low-density steels offshore structure.By comparison and analysis of δ-ferrite for three experimental steels,the effect of Zr on inhibiting the growth of δ-ferrite was clarified.The results shows that Fe2Zr has high thermodynamic stability and has a semi-coherent relationship with the δ-ferrite.It also has a strong pinning effect on the grain boundaries of δ-ferrite and inhibit δ-ferrite coarsening during reheating.Through the comparison and analysis of the flow stress curves and corresponding microstructure of Nb-Zr microalloyed δ-ferrite low-density steel under different thermal deformation mechanisms.At a lower energy dissipation rates(η<0.2),the main thermal deformation mechanisms is dynamic recovery.The activation energy of hot deformation is 318.71 kJ/mol,and the hot deformation constitutive equation of the experimental steel is established.Based on the Prasad instability criterion and the dynamic material model,processing maps of experimental steel was established,and the microstructure of the δ-ferrite under the corresponding hot deformation regime is analysed.It is considered that hot deformed at 950～1000℃ at 3 s-1 is a reasonable hot working regime for the experimental steel.In the hot rolling of the experimental steel in the non-recrystallized zone,the δ-ferrite structure is pancaked,and there are highdensity low-angle grain boundaries and substructures.Under the tempering heat treatment of 750℃ for 2 h,the Nb-Zr microalloyed δferritic low-density steel has good strength and low temperature impact toughness.The fine recrystallized structure between the strip-like δ-ferrite structure with high density of low-angle grain boundaries,which not only increases the crack propagation path,but also improves the crack propagation resistance.The strength,ductility and toughness of the experimental steel are well compatible.The yield strength was 329 MPa,tensile strength was 424 MPa,and the elongation was 35%.the toughness at 0℃ was 29 J.The fracture mode of Nb-Zr microalloyed δ-ferritic low-density steel is a mixed mode,including ductile fracture of banded δ-ferritic and quasi-cleavage fracture at the interface with large misorientation angle.The impact fracture surface presents a layered structure,there are many thin layered belts along the rolling direction.These cracks are perpendicular to the rolling direction and extend along the crack propagation direction.In the process of crack propagation,an obvious δ-ferrite interface separation phenomenon occurs,and a quasi-cleavage fracture occurs at the interface perpendicular to the main fracture surface,which effectively increases the releasing rate with δ-ferrite,thereby improving the toughness of the steels. |
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