V-bendability of ultrahigh strength low alloy trip-aided steel sheets with bainitic ferrite matrix

Effect of retained austenite characteristics on V-bending in ultrahigh strength TRIP-aided steel sheets with bainitic ferrite matrix (TBF steel) was 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 top angle punch (2.0 mm in punch top radius) and 88-degree groove angle die (12 mm in die groove width, 0.8 mm in die shoulder radius) in a processing speed of 1 mm/min. The main results are as follows. (1) The 0.2C-1.5Si-1.5Mn (mass%) TBF 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 TBF375 steel sheet of 1100 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.