Microstructure And Strengthening Mechanism Of High Strength Low Carbon Bainite Steel

Low carbon bainite steel with high strength, low cost, high weldability andexcellent low temperature toughness of the advantages, and it has been attached greatimportance to the academic and engineering. It widely used in petroleum pipelines, oilplatforms, construction machinery, marine facilities, ships and bridges and other fields.However the bainitic transformation is a medium temperature phase transition whichbetween the eutectoid pearlite and matensite. High strength low carbon bainitic steelsare design by low carbon content and ultra-fine processing. Therefore, low carbonbainite has its unique substructure morphology and microstructure.The composition of the experimental material is design by low carbon niobiumvanadium titanium micro-alloying process. And use control the precipitation phasetechnology,at different cooling rate and finish cooling temperature, the low-carbonbainitic steel was obtain which the intensity level was 700Mpa to 900Mpa. Themorphology and strengthening mechanism was researched by optical microscope,scanning electron microscopy, transmission electron microscopy.The result shows that:1) The main microstructure of the bainitic steel is thegranular bainite and lath bainite., the bainite ferrite lath width of granular bainite wasabout 2μm, the bainite ferrite lath width of lath bainite was about 0.2μm. Thestructures are non-carbon bainite. There are some islands of granular bainite, granularshort strip of the M/A. The M/A island was between the bainite ferrite lath.2) The main strengthen way was grain refinement strengthen and precipitationhardening. The effect of refinement strengthen was about 20%, the precipitation wasabout 30~40%, the transformation hardening was about 15%, the solid solutionstrengthen about 7%.With increasing of the yield,the rate of the fine grainstrengthening, precipitation strengthening and solid solution strengthening changeslittle. The strengthening proportion of the lath bainite phase transformation wassignificantly increased.3) Increase the content of the lath bainite is a effective way to increase thestrength of the low-carbon steel, for the lath bainite is a kind of high strengthstructure.