Effects of Si and Mn contents on V-bending in high strength TRIP-aided dual-phase steel sheets with polygonal ferrite matrix

Effects of Si and Mn contents on V-bending in high strength TRIP-aided dual-phase (TDP) steel sheets with polygonal ferrite matrix were investigated for automotive applications. V-bending test was performed on a hydraulic testing machine using a rectangular specimen (50 mm in length, 5 mm in width, 1.2 mm in thickness) and 88-degree punch (2.0 mm in punch radius) and 88-degree die (12 mm in die width, 0.8 mm in die radius) at a processing speed of 1 mm/min. The main results are as follows. (1) The 0.2C-(1.0-2.5)Si-(1.0-2.0)Mn, mass% TDP steel sheets were able to perform V-bending by strain-induced martensitic transformation of TRIP effect. On the other hand, ferrite-martensite dual-phase (MDP0) steel sheet of 900 MPa grade was not able to perform 90-degree V-bending because of initiation of crack in tension area. (2) The 0.2C-2.5Si-1.5Mn, mass% TDP-G steel sheet of 980 MPa grade was able to enable the 90-degree V-bending that considered an amount of springback (Δθ=θ2-θ1), in which the θ1 and the θ2 are a bending angle on loading and a bending angle after unloading respectively, of more than 2-degree by controlling a displacement of punch bottom dead center.