Effect of Prestrain and Bake Hardening Heat Treatment on Fracture Toughness and Fatigue Crack Growth Resistance of Two Dual-Phase Steels

摘要

Fracture toughness (J integral, R-curve) and fatigue crack growth (da/dN) resistance have been studied in two ferrite-martensite DP-600MPa steels broadly used in the automotive industry, with 7% to 12% of martensite volume fraction, and with different silicon and chromium additions. The effect of 10% prestrain followed by a bake hardening heat treatment was verified. This thermo-mechanical treatment was used to simulate the stamping and the paint baking of the wheels. All the mechanical properties are affected by the thermo-mechanical history for both steels. An inverse relationship was found between yield stress and J integral, and between yield stress and fatigue threshold. This behavior was related with small changes in the microstructure of the steels.