| The development and utilization of marine resources has received increasing attention from all over the world,which has led to the development of offshore steel with superior performance.The TMCP + modulation processing(QT)production mode of traditional offshore steel meets the performance index in strength,but the toughness index is poor.Especially for offshore steel,it is very demanding for low temperature toughness due to the low temperature of service environment and the large temperature difference between the day and night.Therefore,it is urgent to prepare a high strength marine steel with excellent low temperature toughness.In this study,a new type of offshore steel with excellent low temperature toughness was prepared by adding an appropriate amount of austenite stabilizing element Mn and a small amount of microalloying elements while ensuring the high strength(700 MPa)of the offshore steel plate.The performance changes of this offshore steel under different heat treatment parameters were studied separately,and the heat treatment process parameters were gradually optimized.Finally,the quenching + intercritical annealing + tempering(QIAT)three-step heat treatment process was used to control the microstructure of this offshore steel to improve its low temperature toughness.The SEM,XRD,EBSD,and TEM technologies were used to characterize the microstructure and retained austenite of this offshore steel after QT,QIAT,and after different tempering temperatures under QIAT.The mechanical properties of this offshore steel and its variation were tested and studied by the normal temperature tensile,low temperature impact and oscillating impact tests.The intrinsic relationship between the evolution of microstructure and the change of low-temperature toughness under different heat treatment processes was analyzed and discussed,and the mechanism of low temperature toughness change was revealed.The results show that the microstructure of this new offshore steel after QIAT heat treatment was a multiphase structure composed of tempered bainite/martensite,intercritical ferrite and film-like retained austenite,and the film-like retained austenite thickness was only 0.1 ?m.The yield strength was 716 MPa,the tensile strength was812 MPa,and the low temperature toughness at-40°C reached 150 J.Plasticity and low temperature toughness were significantly improved compared to conventional QT processes.Enrichment of Mn in the retained austenite formation region caused a surge in dissipated free energy induced by austenite interface migration,making it a one-dimensional film-like flat growth.The excellent stability of the film-like retained austenite itself,the large number of small-angle grain boundaries and the high-densitylocal orientation difference in the formation area were the reasons for the excellent low temperature toughness of the offshore steel.Under the QIAT process,bainite/martensite and intercritical ferrite gradually decomposed with the increase of tempering temperature,and distributed in granular or short rod shape.At the same time,the content of retained austenite increased,the yield strength and tensile strength of this offshore steel decreased,and the plasticity and low temperature toughness increased.The reason was that the more the austenite content was reversed,the greater the absorption energy of the crack during the stable expansion stage of the crack.This was due to the improvement of the dislocation accumulation in the offshore steel by tempering,which maked the Smith factor distribution uniform,the small-angle grain boundary increased,and some grains underwent secondary crystallization.These contributed to the improvement of low temperature toughness. |
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