Study On Microstructure And Mechanical Properties Of TMCP Plate And CGHAZ Of Mo-containing Weathering Bridge Steels

Molybdenum is not only an element to improve corrosion resistance,but also an element to increase strength,which is very significant for the development of weathering bridge steel.However,the amount of molybdenum added to the same grade weathering bridge steel(420MPa)which has been mass produced is not more than 0.20 wt.%.In this paper,molybdenum elements with different contents are added into the alloy system of 0.05C-0.3Si-1.25Mn-0.3Ni-0.45Cr-0.34 Cu weathering bridge steels.On the one hand,the influence of molybdenum content on the structure and mechanical properties of weathering bridge steel plates under Thermo-Mechanical Control Process technology(TMCP)is explored,and the connection of molybdenum content,structure and mechanical properties is established;on the other hand,the influence of molybdenum content on coarse-grained heat-affected zone(CGHAZ)of the bridge steel under thermal simulation is explored,and the molybdenum content is analyzed.And the influence of heat input on the structure and low temperature impact toughness,the results will be conducive to the optimization of the composition of weathering bridge steel and the formulation of welding process,and deepen the application of molybdenum in weathering bridge steel.Molybdenum element of 0.05%,0.20%,0.35% and 0.50% was added into 0.05C-0.3Si-1.25Mn-0.3Ni-0.45Cr-0.34 Cu weathering bridge steels,and rolled into steel plates by the same TMCP technology.The microstructure was analyzed by OM,SEM,EBSD,XRD and TEM,and the mechanical test of tensile,hardness and low temperature impact was completed.The change of microstructure and mechanical properties with the change of molybdenum content in weathering bridge steel was analyzed.The CGHAZ structure was prepared by simulating 20 kJ/cm and 35 kJ/cm heat input with a thermal simulation test machine.The microstructure was observed by OM,SEM,EBSD,XRD and TEM,and the low-temperature impact toughness experiment was focused on.The relationship among molybdenum content,microstructure and low-temperature impact toughness was established.The results show that under the same TMCP process,the Mo content of the weathering bridge steel plate increases from 0.20% to 0.50%,the microstructure transfers to the low temperature phase,the polygonal ferrite decreases,the granular bainite increases,the lath bainite appears;the yield strength increases with the Mo content increasing,and the yield strength increases ~ 70 MPa with the Mo content increasing by 0.15%;but the impact toughness of the steel plate decreases from 210 J to 170 J.At 20 kJ/cm,when the molybdenum content is from 0.05% to 0.50%,the CGHAZ microstructure of weathering bridge steel is Granular bainite and lath bainite.However,with the increase of Mo content,small block M/A in the simulated CGHAZ becomes slender M/A,which is easy to cause the initiation and propagation of microcracks,leading to the decrease of impact toughness.Comparing with the heat input of 20kJ/cm,at 35 kJ / cm the impact toughness decreased to ~20 J,mainly due to the coarsening of the microstructure.