The influence of manganese on the bake hardening of hot dip galvanized low carbon steels

This work is a study of the manganese influence on the aging behavior of hot rolled and hot dip galvanized low carbon steels. For this, two steels were evaluated, with Mn content ranged between 0.16% and 0.30%. The work was carried out on industrial scale, in order to verify the tendency of results obtained previously on studies performed on laboratory scale. The evaluation of the aging phenomenon was performed in two ways: accelerated aging tests on hot rolled steels and bake hardening tests on cold rolled and galvanized steels. In both tests, it was verified that the increase of the Mn content might be an effective option in the control of aging in aluminum killed steels, within the limits of the process variables used. In agreement with the results of the accelerated aging and bake hardening tests, a higher carbides’ volumetric fraction was observed in the steel with higher Mn content. The steel with higher Mn content presented an approximated mean bake hardening value of 43 MPa and an approximated mean carbides’ volumetric fraction of 0,02. The values related to the steel with lower Mn content was 64 MPa and 1% respectively. This work corroborates with previous studies, which indicates the manganese precipitates as important carbides nucleation sites, reducing the amount of solid solution carbon in the ferritic matrix, resulting in a higher aging resistance.