Microstructure,precipitated phase and reinforcement mechanism of the Nb-Ti microalloyed S550GD+Z hot-rolled steel plate and hot-dip galvanized plate were analyzed by using TEM,EBSD,physical and micro-chemical phase analysis,and the mechanism of strength improvement of the plate by galvanizing was studied.The tensile test results show that the yield strength of the galvanized plate is increased by about 34.6 MPa compared with the hot rolled plate,of which the strength increment of the zinc layer contributes about 15 MPa,while the calculation shows that the comprehensive increment of strength caused by solution treatment strengthening,grain refinement strengthening,dislocation strengthening and the zinc layer strength increment is increased by about 47.8 MPa.Thus,the strength increment of zinc layer is the main reason for the strength improvement of the Nb-Ti microalloyed S550GD+Z steel plate after galvanizing.
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