Bake hardening behavior of advanced high strength steel grades

Abstract

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 treatments, following straining induced from part forming, which may lead to increased in-service component performance due to a strengthening mechanism known as bake hardening (BH). This study quantified the BH 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 times and temperatures from 15 to 60 min and 120 to 200 °C, respectively. Samples were prestrained 0, 2, and 5 pct. The effects of time, temperature, and prestrain on the DP and TBF steels were evaluated using uniaxial tensile testing. Because no additional part forming occurs after austenite is quenched to martensite during hot stamping, the PHS grades were not experimentally prestrained and thus BH assessed low temperature tempering (LTT) response. Uniaxial tensile, bend, and microhardness testing were used to evaluate LTT for the same times and temperatures. Yield strength generally increased for the DP and TBF steels with increasing time and temperature. Without prestrain, DP 1000 and TBF 1200 showed similar aging for most conditions. BH exceeded 100 MPa for DP 1000 at the greatest time/temperature. Tensile ductility also remained high for all test conditions without prestrain, typically decreasing by 1 pct or less relative to AR. With the addition of 2 and 5 pct prestrain, different aging behaviors per grade were observed with TBF 1200 showing the greatest BH response followed by TBF 1000. After prestraining, DP 1000 had the smallest BH values. Prestraining TBF 1200 5 pct followed by aging at 200 °C for 60 min resulted in a BH value of 180 MPa. Likewise, BH for TBF 1200 continuously increased with increasing prestrain. Ductility diminished for the DP and TBF steels with increasing prestrain, time, and temperature. Tensile tests for PHS grades showed that yielding transitioned from continuous to discontinuous and yield strengths reached maxima with increasing tempering intensity. The higher carbon PHS 2000 had the greatest BH response and a maximum value of 180 MPa after aging at 200 °C for 15 min. Bend test results for both grades had large scatter and showed no observable trends in bend angle with increasing aging.