Warm V-Bending and Hydrogen Embrittlement Properties of Ultrahigh-Strength TRIP-Aided Bainitic Ferrite Steel Sheets

The warm V-bending and hydrogen embrittlement properties of the ultrahigh-strength transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel sheets were investigated to apply for the automotive structural parts manufactured by cold- or warm-press forming. The V-bending tests were carried out at a forming speed of 1 mm/min and a forming temperature (T, °C) of 25 and 100 ℃ using a hydraulic servo type universal testing machine with a 88-degree V-punch and a V-die using V-bend specimens with dimensions of 5 mm width, 50 mm length and 1.2 mm thickness without and with hydrogen. Hydrogen charging was conducted by means of cathodic charging using a 3 wt% NaCl + 3 g/L NH4SCN solution at a current density of 10 A/m2 for 48 h before V-bending. The main results were as follows. (1) The 1100-MPa-grade TBF375 steel with a chemical composition of 0.2C-1.5Si-1.5Mn (mass%) enabled to conduct the V-bending at the forming temperature of 25 ℃ with hydrogen charging. This is considered that the hydrogen embrittlement crack propagation was suppressed owing to the finely and uniformly dispersed retained austenite (γR) although a large amount of retained austenite transformed into martensite at the outside of the V-bending part during V-bending. (2) At T = 100 ℃, the TBF steels were able to perform the 90-degree warm V-bending, considering the springback, by the moderate strain-induced martensitic transformation of γR as the TRIP effect at the plastic deformation region of outside of the specimen owing to the increase in the stability of γR in comparison with that at T = 25 ℃.