Effect of finish rolling temperature on microstructure and mechanical properties of medium manganese martensite NM500 steel after reheating treatment

Medium manganese martensitic wear-resistant steel has broad prospect of application in the fields of coal mining,cement mixing and rail transportation with the characteristics of low cost,high strength,high hardness and high wear resistance. The effect of finish rolling temperature on the microstructure and mechanical properties of medium manganese martensitic NM500 steel after heat treatment was investigated using metallographic microscope,scanning electron microscope and transmission electron microscope. The results showed that compared with the hot-rolled specimens,the prior austenite grain size of the experimental steel was significantly refined after the heat treatment of holding at 850 ℃ for 1 h. The martensite microstructure after phase transformation was finer and the low-temperature impact toughness was greatly improved. The lower finish rolling temperature increased the defect density in the matrix and the amount of V(C,N) with reducing the grain size,which had more benefits for the refinement of austenite grains during the reheating process. When the finish rolling temperature deceased from 900 ℃ to 700 ℃, the prior austenite grain size of experimental steel was refined from 10.50 μm to 5.02 μm and the block width decreased from 1.37 μm to 1.06 μm. The dislocation density increased from 1.05×10¹⁵ m^-2 to 1.51×10¹⁵ m^-2. It was found that the refining martensite and increasing dislocation density significantly enhanced the grain refinement strengthening and dislocation strengthening, resulting in the tensile strength, yield strength and Brinell hardness of the experimental steel increasing to 1 860 MPa,1 084 MPa and 540HB, respectively. Meanwhile,the microstructure refinement led to a significant increase in the density of high angle grain boundaries in the matrix, which effectively inhibited crack propagation. The number and the size of dimples present at the fracture surface increased,and the toughness of experimental steel was improved. The impact absorption energy at -40 ℃ reached 25.5 J.