Bake Hardening Behavior of DP, TBF, and PHS Steels with Ultimate Tensile Strengths Exceeding 1 GPa

Third generation advanced high strength steels (AHSS) have been developed combining high strength and formability, allowing for lightweighting of vehicle structural components. These AHSS components are exposed to paint baking operations ranging in time and temperature to cure the applied paint. The paint baking treatment, combined with straining induced from part forming, may lead to increased in-service component performance due to a strengthening mechanism known as bake hardening. This study aims to quantify the bake hardening behavior of select AHSS grades. Materials investigated were press hardenable steels (PHS) 1500 and 2000; transformation induced plasticity (TRIP) aided bainitic ferrite (TBF) 1000 and 1200; and dual phase (DP) 1000. The number designations of these grades refer to minimum as-received ultimate tensile strengths in MPa. Paint baking was simulated using industrially relevant times and temperatures from 15 to 60 min and 120 to 200 °C, respectively. Samples were prestrained 0, 2, or 5 pct to replicate part forming. Bake hardening values ranging from 90 to 140 MPa were observed for DP and TBF grades that were prestrained 2 pct and baked at 170 °C for 20 min. However, ductility diminished for these steels when subjected to 5 pct prestrain with uniform elongations after baking decreasing to 1 pct in some instances. PHS steels, on the other hand, showed substantial increases in yield strength without prestrain. Increases of 122 and 175 MPa were recorded for PHS 1500 and PHS 2000, respectively, following baking at 160 °C for 60 min. However, ultimate tensile strengths decreased due to reduced strain hardening, while total elongations decreased slightly.