Bake hardening effect of the low strength interstitial free steel

This paper investigates the influence of pre-strain and temperature on the bake hardening (BH) effect of the low strength interstitial free (IF) steel with the yield strength of 137 MPa. The tensile specimens were pre-strained to 2-4-6 % at room temperature followed by baking at temperatures of 150-200-250 oC for 20 minutes. The BH strength was determined by a standard procedure based on the difference between the lower yield strength of the baked specimen and the flow stress of the initial one. The microstructure of the IF steels was characterized by optical microscopy and scanning electron microscopy for the purpose of explaining the BH effect. All the initial and baked steels show a microstructure that includes the ferrite phase, of an average grains size of 45 µm. This observation was consistent with the mechanical properties of the initial steel. The BH strengths have been achieved from 12 to 35 MPa, in which the maximum value was found for the specimen that pre-strained to 6 % and baked at 200 oC. The BH strengths increased with increasing the pre-strain, but slightly decreased when the baking temperature was 250 oC. This mechanism is attributed to pinning of dislocation by carbon solute atoms during the baking process, and the BH strength was correlated with grain boundary segregation.