Warm V-Bendabilites and Hydrogen Embrittlement Properties of Ultrahigh-Strength Quenching and Partitioning-TRIP Steel Sheets

The warm V-bendabilities and hydrogen embrittlement properties of ultrahigh-strength Quenching and Partitioning (QP)-Transformation-Induced Plasticity (TRIP) steel sheets were investigated to apply the QP-TRIP steel sheets for automotive structural parts manufactured by cold or warm press forming. V-bending tests were carried out at a crosshead speed of 1 mm/min at V-bending temperatures of 25, 100 and 150 degrees C using a hydraulic servo testing machine with a 88-degree V-punch (punch tip radius R = 2 mm, R/t(0) = 1.7) and a V-dice (dice groove size l = 12 mm, dice shoulder diameter 0.8 mm) using V-bend specimens with dimensions of 5 mm width, 50 mm length and 1.2 mm thickness without and with hydrogen charging. 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/m(2) for 48 h before V-bending. The main results were obtained as follows. (1) QP-A steel enabled to conduct V-bending at a V-bending temperature T = 25 degrees C although the bending angle after unloading (theta(2)) was less than 90-degree. (2) When V-bending tests were carried out at T = 100 degrees C, QP-B, C, and E steels without hydrogen and QP-B steel with hydrogen charging enabled to conduct V-bending. In addition, QP-B steel was also possible to carry out the V-bending at T = 150 degrees C. These results implied that the V-bending at warm temperatures can improve the V-bendabilities of the QP-TRIP steels.