Microstructure and Mechanical Properties of Microalloyed Low Carbon Martensitic Steel
The microalloyed low carbon martensitic steel doped with titanium,niobium and molybdenum was prepared,and then treated by hot rolling+quenching+tempering at different temperatures(170,190,210,230,250℃).The microstructures,tensile properties and impact properties of the test steel in different heat treatment states were studied,and the strengthening and toughening mechanism was analyzed by taking the steel tempered at 190 ℃ as an example.The results show that hot-rolled test steel was composed of ferrite and bainite,the quenched test steel was composed of ferrite and martensite,and the test steel tempered at different temperatures was composed of tempered martensite and a small amount of ferrite.The ferrite volume area fraction of the tempered steel was 15%-20%,which was lower than that of hot rolled and quenched test steel.The tensile strength and Brinell hardness of the quenched steel were the highest.With the increase of tempering temperature,the tensile strength and Brinell hardness of the tempered test steel decreased,and the percentage elongation after fracture,yield strength and impact absorbing energy increased first and then decreased.The test steel tempered at 190 ℃ had the highest percentage elongation after fracture and impact absorbing energy,and the largest proportion of dimbles in fiber zone at tensile fracture and impact fracture.The tensile strength,Brinell hardness and yield strength of the test steel tempered at 190 ℃ were moderate,and the comprehensive mechanical properties were the best.The precipitation of nano-scale small-sized(Ti,Nb,Mo,Cr)C phase in the test steel tempered at 190 ℃ played a precipitation strengthening role,and the nano-scale large-sized(Ti,Nb,Mo,Cr)C phase played a fine crystal strengthening role.The proportion of the large-angle grain boundary was relatively high,accounting for higher crack propagation work.Therefore the strength and toughness of the test steel were improved.
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