Study On Microstructure And Properties Of High Strength H-beam For Ocean Engineering

With the exploitation of marine resources,the offshore oil and gas resources have become increasingly depleted,the marine industry has begun to develop towards far-reaching waters,and the marine engineering steel demand has started to increase with the application of marine engineering steel due to the limitation of service life.In order to meet the needs of the harsh environment of far-reaching sea,the required marine engineering steel must be high strength and toughness,large size and easy to weld.H-beam for marine engineering has become an important part of marine engineering equipment due to its excellent cross-section characteristics.However,there are still difficulties in the production of H-beam for high-strength and large-size marine engineering and need to be imported from abroad.In this paper,the microstructure and properties of SM490YB H-beam for ocean engineering were analyzed by phase-change experiment and laboratory simulation of controlled rolling and cooling experiments.The microstructures were studied under different finishing temperature,final cooling temperature and cooling rate.The law of change.The main work and research results are as follows:(1)By comparing the steel composition and the corresponding organizational performance control mechanism of H-beam for marine engineering at home and abroad,the development and design of niobium-titanium composite SM490YB test steel,through the control of rolling and cooling process to make Nb,Ti microalloying elements have a useful influence,which is fine grain strengthening and precipitation strengthening to improve the strength and toughness of H-beam.(2)The austenite continuous cooling transition curve(CCT curve)of SM490YB test steel was obtained by using the Origin software to measure the change of the expansion of test steel with time and temperature at different cooling rates by phase change experiment.It provide a reference basis for the test parameters of controlled rolling and controlled cooling.It is found that the Ac3 of the test steel is 802?,Ac1 is 729? and Ms is 436?.With the increase of the cooling rate,the phase transition point decreases gradually,the grain size decreases and the hardness of the microstructure increases.(3)On the basis of the experiment of phase change,the different finishing temperatures were selected for testing.The microstructures of the test steels were measured by optical microscopy and scanning electron microscopy(TEM).The effects of finish rolling temperature on the microstructure and properties of the test steels were obtained.The decrease of the final rolling temperature and ultra fast cooling with each other so that the original air cooling condition of the ferrite+pearlite transformation to bainite+iron ultrafast cold conditions of ferrite,yield strength,tensile strength and other properties have some degrees of increase.When the finish rolling temperature is 850?,the mechanical properties of the test steel are the best,that is,the yield strength is 530MPa,the tensile strength is 648MPa,the transverse impact toughness is 179J,the longitudinal impact toughness is 191J and the elongation is 20%.The main mechanism of strengthening and toughening is precipitation strengthening and fine grain strengthening.The precipitation of the second phase particles after controlled rolling and ultra rapid cooling was observed by transmission electron microscope(TEM),and the formation principle and mechanism of the particles were analyzed.When the finishing temperature is 850?,the size of(Nb,Ti)(C,N)precipitates are close to the critical size of the Orowan mechanism and the cutting mechanism,and the precipitation strengthening effect is the best.(4)Experiments were carried out on the final finish temperature and cooling rate with the best finishing performance.Through the microstructure observation and mechanical property test,the optimum cooling parameters were obtained,and the different cooling The influence of the process on the microstructure mechanical properties of the test steel thickness,and provide the theoretical reference and basis for the industrial application.The final cooling temperature is the key factor affecting the final organization ratio.With the decrease of the final cooling temperature,the bainite content in the same direction of the test steel is gradually increased and the ferrite content is decreased.The cooling rate has a significant effect on the uniformity of the organization in the thickness direction of the test steel.The faster the cooling rate,the similar the grain in thickness direction of the test steel.The results show that the finishing temperature is 850?,the final cooling temperature is 480?,the cooling rate is 45?/s,the mechanical properties of the test steel are the best,the yield strength is 557MPa,the tensile strength is 651MPa,-40? horizontal impact energy is 211 J,-40? vertical impact energy is 225J,the elongation is 19%.