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 tests were performed on a servohydraulic testing machine at a processing speed of 1 mm/min using a 88-degree V-punch (2.0 mm in punch radius), 88-degree V-die (12 mm in die gap, 0.8 mm in die shoulder radius), and a rectangular specimen (50 mm in length, 5 mm in width, 1.2 mm in thickness). The results are summarized 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 that produced by heat treatment of annealing at 1 173 K (900°C) for 1 200 s followed by austempering at 648 K (375°C) for 200 s, of 1 100 MPa grade was able to enable the 90-degree V-bending that considered an amount of springback (Δθ =θ1-θ2), in which the θ1 and the θ2 were 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.