Study On Multi-phase Regulation And Mechanical Behavior Of Offshore Structural Steel For Low Temperature Service

With the development of resource exploration from shallow sea to deep sea and arctic regions,the requirements for marterials are becoming higher and higher because of the severer service environment of offshore equipment.Not only high strength,excellent low temperature toughness and good weldability are required in offshore structural steels,low yield-to-tensile ratio and high uniform elongation are also required to ensure the safety and reliability of steel structures.In the present work,the regulation of multi-phase microstructure and mechanical behavior in low-carbon low-alloy steels are studied to develop offshore structural steel used in the arctic region.The effect of chemical composition and processing on the microstructure and mechanical properties was studied by crystallographic characterization method-quantification and visualization of microstructure,which can provide guidance for alloy design and process optimization for offshore structural steel with high performance.The diffusion of carbon atoms from ferrite to untransformed austenite was promoted by intercritical deformation because of the increase of ferrite content and density of dislocations in multi-phase steels.The content of VI/V2 pair was increased to accommodate the transformation strain more effectively with the increase of carbon content in untransformed austenite.The content of V1/V2 pair deceased when the phase transformation products changed from bainite to martensite caused by the further increase of carbon content in austenite.However,the entire martensite was almost occupied by one CP region and all six variants belonging to this CP group had high fraction.The ferrite-acicular ferrite multi-phase steel was characterized by low yield-to-tensile ratio,high uniform elongation and excellent low temperature toughness.The crystallographic studies indicated that the boundaries between adjacent acicular ferrite and bainite are V1/V2 pairs,which are high angle grain boundaries.With the increase of Nb content in ferrite-bainite multi-phase steels,the degree of austenite grains being flattened in the non-recrystallization region and the intercritical region increased,resulting in the increase of dislocation density.As a result,the impact toughness was improved because of microstructure refinement.The precipitation strengthening increased because of the increasing amount of precipitates.With the increase of Nb content in experimental steels,the yield strength increased and the ductile-brittle transition temperature decreased.In large-scale production line,a multi-phase microstructure consisted of ferrite,bainite and retained austenite was obtained by the two-step intercritical heat treatment in low-carbon low-alloy offshore structural steels with a thickness of 90 mm.The microstructure in as rolled plate became uniform after the two-step intercritical heat treatment,and the impact toughness at the different locations in the thickness direction of experimental steel was significantly improved.Moreover,retained austenite occurred strain-induced-martensite-transformation gradually during tensile deformation,resulting in high continuous work hardening rate,which can delay the necking and improve uniform elongation.