The bake hardening value is one of the vital strength indexes of dual-phase steel, representing the strengthening ability of materials after pre-strain and baking, playing an important role in vehicle safety and lightweight design. Studying and improving the strain aging mechanism of dual-phase steel helps one to understand the material characteristics and enhances its utilization value. However, the ultra-high strength dual-phase steel is often prone to fracture outside the gauge length of a tensile specimen of the bake hardening value test. No suitable theory explains the fundamental law of dislocation pinning during the saturation stage at present. This paper used FEA, DIC, SEM, TEM, internal friction, and metallographic methods to study the strain aging behavior of dual-phase steels under different pre-strain, bake time, and bake temperature conditions. The results show that the fracture outside the gauge length is related to factors such as the uneven distribution of pre-strain and the ultra-high upper yield strength. The rolling pin shape tensile specimen testing has successfully solved this testing problem. The measured results at the saturation stage of dislocation pinning are in good agreement with the fitting results of the dislocation pinning strengthen mechanism based on the probability event quantization assumption.
Keywords: Bake hardening value; Dislocation pinning; Dual-phase steel; Strain aging; Tensile specimen; Upper yield stress.
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