Tensile properties and bake hardening response of dual phase steels with varied martensite volume fraction

Abstract

A study has been made of the tensile properties on martensite plus ferrite dual phase (DP) steels. Tensile samples machined out of industrially produced DP steel sheet are quenched from the austenite plus ferrite phase field, so as to give a series of microstructures with varying martensite volume fraction (MVF) from 0.17 to 1. The strength of the produced DP steels is found to linearly increase with increasing the MVF, whereas their ductility showed a nonlinear decrease.Tensile samples of the produced DP-steels were taken to study the bake hardening (BH) response. The samples were aged at 100, 170 or 220 °C after applying pre-straining of 0, 2% or 5%. The change of BH-response with the time is found to go through two stages, an initial small increase in yield strength, followed by a larger strength increase at longer baking times. Prolonging the aging time results in decreasing the BH-response achieved in the second-stage (over-aging). The BH-response in the first stage is improved by increased pre-straining. This trend implies that Cottrell atmosphere formation is highly influenced by pre-straining. However, during the second stage the effect is reversed, with the non-pre-strained condition recording higher BH-responses up to 180 MPa. It is found also that changing the aging temperature does not change the characteristics of the aging-phenomenon, but it affects its rate. Saturation of increasing the BH-response with increasing MVF in the second strengthening stage is observed at MVF-range of 0.22–0.39.