Effect of Manganese Content on Burring in 0.2%C-TRIP Sheet Steels with Polygonal Ferrite Matrix

Abstract

Effect of manganese content on burring and tapping in 0.2C-1.5Si-(1.0-2.0)Mn (mass%) high strength TRIP-aided sheet steels with polygonal ferrite matrix (TDP steels) was investigated for automotive applications. Burring test was performed on a machining center with rotational speed of 3500 rpm and cutting feed rate of 10 mm/min, using disc specimen of 50 mm in diameter. Moreover, tapping test was performed on a machining center with rotational speed of 20 rpm and cutting feed rate of 20 mm/min after burring. The hardness of flange after burring was evaluated by hardness increment (ΔHV) expressed by ΔHV=HV_max–HV₀, where HV_max and HV₀ are a maximum hardness after deformation and a base material hardness (an initial Vickers hardness).
The hardness increment (ΔHV) increased with increasing of manganese content. The lower the initial carbon concentration in retained austenite (C_γ0), the higher the hardness increment (ΔHV). Strain induced transformation and work hardening contributed to the toughness of flange after burring with TDP steel having a tensile strength of 980 MPa grade.