Bake Hardening Behavior of DP, TBF, and PHS Steels with Ultimate Tensile Strengths Exceeding 1 GPa 2020-01-0536
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; dual phase (DP) 1000, and complex phase (CP) 1000. The number designations of these grades refer to as-received ultimate tensile strengths. 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 indices 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 elongation decreasing to 1 pct in some instances. On the other hand, the PHS 1500, PHS 2000, and CP 1000 grades showed limited tensile elongation after prestrain and baking for any time/temperature condition. Without prestrain, increases in yield strength of 85 and 170 MPa were recorded for PHS 1500 and PHS 2000, respectively, and ultimate tensile strengths and total elongations decreased for these grades. CP 1000 showed little change in strength and ductility without prestrain. Microscopy was conducted to relate changes in microstructure with mechanical properties.
Brandon W. Blesi, Charles Smith, David K. Matlock, Emmanuel De Moor