Abstract: Taking 25Mn steel flange for example, the effect of the parameters on microstructure and mechanical properties of 25Mn steel flange were studied. Tensile and fracture mechanism of the flange after different tempering temperatures was revealed by means of scanning electron microscope. The experimental results indicated that residual stress and uneven microstructure exists in rolled flange. Quasi cleavage and brittle fracture are its main fracture forms. The homogeneity and refined grains can be obtained in the range of 220-660 ℃ tempering temperature. After low temperature tempering, the fracture form is shear fracture and cleavage fracture since there is the morphology of river pattern and tear edges. The strength of 25Mn flange is decreased with the increase of tempering temperature. Fine granular carbides are precipitated and plasticity reaches a maximum value after 620 ℃ tempering. The elongation and reduction of area is about 29% and 65.32%, respectively. There are a big density and depth of toughening nest and Charpy absorbed energy reaches a peak value of 103 J. The plasticity-toughness is the best. When the tempering temperature is higher than 620 ℃, granular carbides are coarsened evidently and the plasticity-toughness is reduced notably. In order to obtain excellent mechanical properties, for 25Mn steel flange, it is recommended that the optimum heat treatment process is: quenching at 880 ℃ and holding 2 h, 620 ℃ tempering for 10h after cooling in 10%NaCl aqueous solution.

Key words: casting-rolling compound forming;25Mn steel flange;heat treatment;microstructure;fracture mechanism