Research On Microstructure And Mechanical Properties Of HG550 Beam Steel Under Ultra-Fast Cooling Conditions

With the rapid development and drastic competition of the Motor Industry,automotive steel has facing challenges include security,the environment,resources,energy and cost aspects.The high strength steel is the preferred material to fulfill the mentioned requirements,but also the major target of automobile steel in the future.The research background is"HuaiSteel high strength automotive steel HG550 TMCP technology research".In this thesis,the contral rolling and ultra-fast cooling parameters to the HG550 steel's microstructure and properties were studied through the thermal simulation testing and trial rolling experiments in the RAL laboratory.The best hot rolling parameters were obtained.The main conclusions of the study are as follows:(1)Through researching on tested steels continuous cooling transformation behavior and single-pass compression deformation experiments,it's found that with the cooling speed increasing,microstructure of tested steels is refined,and the hardness is gradually increasing,at the same time,microstructure transform gradually from the ferrite,pearlite to the mainly consisting of lath bainite.In the wider cooling speed range(1 ?/s?20?/s),LB microstructure is obtained;The application of ultra-fast cooling after deformation can greatly shorten the ferrite phase transformation start time(incubation period),lower ferrite transformation start temperature,and move the CCT curve to the left upper side.The static recrystallization behavior of austenite in the holding time after deformation was researched and the plot the static softening curve was drawn by using double-pass press experiments.(2)The influence of different TMCP parameters to the beam steel was studied.While deformation amount increased,it promotes the ferrite transformation,refines ferrite grain,increases tested steels strength and elongation rate.With the finishing temperature increased,since the grain refinement strengthening and precipitation strengthening reasonable cooperation,tested steels strength and ductility increased.(3)Controlling the finishing rolling temperature and cooling time is constant,when final cooling temperature is 648 ?,it has large cooling speed,beneficial to the grain refinement and precipitation strengthening,tested steels' strength and ductility increased,promote precipitation of carbonitride,to improve the strength and ductility of the tested steels;when final cooling temperature is reduced to below 500 ?,the strength increased because the content of the LB is raised,since the large dislocation density between the laths is not conducive deformation,the elongation rate of HG550 is decreased.Using ultra-fast cooling technology could refine grains and improve banded structure,and can suppress MnS inclusions phase roughening during precipitated,reduce its damage to steel continuity.In short,it could improve the cold-bending property of automobile beam steel.(4)Through researching on different crafts of control rolling and control cooling,the best hot rolling parameters of automotive beam steel HG550 were obtained.The experimental steel which in the conditions of finishing temperature 890 ?,final cooling temperature 650 ?,has the best comprehensive performance.Tensile strength 888MPa,yield strength 655MPa,elongation rate 24.6%,and fully matches the requirement of the high strength beam steel for automobiles.