Study On Microstructure And Properties Of High Ductility EH40 Grade Ship Plate Steel

China's offshore oil,natural gas and some renewable resources are abundant,and the potential for development is enormous,and the transportation of these resources requires a large number of reliable ships.As we all know,there are many serious shipwreck accidents every year in the world,most of which were caused by colliding or hitting the rocks.These shipwreck accidents had caused great damage to people's lives and property,and the oil and gas leakage of the ship would cause serious environmental pollution.Therefore,improving the safety of navigation by changing the characteristics of ship plate steel is the top priority of the current research.The ship plate steel of general strength can not meet the performance requirements.But the EH40 grade ship plate steel can ensure the strength,toughness,fatigue performance,corrosion resistance performance and welding performance of the experimental steel,and the reasonable experimental process can further ensure the high ductility of the experimental steel.In this context,the main research contents and research results of this paper are as follows:(1)The variation of microstructure in high temperature deformation process was studied by multi-pass compression experiments.The results show that the larger the deformation of recrystallization zone was,the finer austenite grains were before the deformation of non-recrystallization zone;when the experimental steel was cooled in the partial recrystallization zone,the austenite grains were gradually refined with the increase of cooling rate,and finally the content of ferrite in the room temperature structure increased,and the grains of ferrite were refined;when the total deformation of the austenite zone was the same,the amount of deformation in the non-recrystallization zone played a more significant role in increasing the ferrite content;the reduction of the final pass compression temperature could refine the ferrite grain and increase the ferrite content.(2)The phase transformation behavior of austenite during continuous cooling of experimental steel under deformation and undeformation was studied by thermal simulation experiment.The results show that the deformation extended the phase transformation interval of ferrite and pearlite,increased the initial ferrite transition temperature and the critical cooling rate of ferrite,and reduced the phase transformation interval of bainite and martensite;regardless of deformation or not,with the increase of continuous cooling rate,the ferrite and pearlite in the high temperature gradually decreased,the bainite in the medium temperature and the martensite in the low temperature gradually increased,and the grains were gradually refined.(3)Combined with the continuous cooling transformation curves,the effects of sectional cooling process parameters on the microstructure were simulated.The results show that the content of ferrite decreased and the grains were refined with the increase of cooling rate in the front section;the content of ferrite increased and the grains were coarsened with the increase of holding time;the holding temperature(760?)was higher than the ferrite phase transformation zone,and the structure was basically free of ferrite;the holding temperature(660?)was located in the ferrite phase transition zone,and the content of ferrite was more;the holding temperature(580?)was lower than the ferrite phase transition zone,and bainite was the majority of the structure;with the increase of cooling rate in the latter stage,the bainite content gradually increased,and the bainite laths were gradually refined;with the decrease of the final cooling temperature,the content of ferrite in soft phase decreased gradually,the content of bainite in hard phase increased gradually,and the bainite morphology changed from granular to lath,and the laths were gradually refined.(4)Hot rolling experiment was carried out at laboratory to study the effects of continuous cooling and controlled cooling process parameters on the microstructure and properties.The results show that the strength of the experimental steel increased and the elongation decreased with the decrease of the final cooling temperature in the continuous cooling;the strength of the experimental steel decreased and the elongation increased with the decrease of the temperature after the first air cooling in the controlled cooling(two-stage cooling);the strength of the experimental steel increased and the elongation decreased with the decrease of the temperature after the first water cooling in the controlled cooling(three-stage cooling);when the yield strength was the same level(510MPa),the elongation of two-stage cooling was 4.2%higher than that of continuous cooling;when the yield strength was high(580MPa),the elongation of three-stage cooling was 3%higher than that of continuous cooling.Compared with continuous cooling,controlled cooling made the experimental steel obtain better comprehensive mechanical properties and further improved the elongation on the basis of strength.