Influence of Thermo-Mechanical Processing Parameters and Chemical Composition on Bake Hardening Ability of Hot Rolled Martensitic Steels

In recent years, the increasing demand for advanced high-strength steels (AHSS) has mainly been driven by the automotive industry and the need to reduce weight and to improve safety. Besides good ductility and high strength, AHSS have a high bake hardening and ageing effect, giving additional contribution to the strength of structural parts, subjected to the paint baking process. This paper investigates their bake hardening behaviour in dependence of hot rolling parameters and chemical composition, however, focussing on martensitic steels. To simulate the finishing steps of the hot rolling process with slight changes in reduction and temperature and their influence on the final mechanical properties of hot rolled martensitic steels, different thermo-mechanical paths were applied. The increase in strength due to bake hardening was determined for different thermo-mechanical schedules. Additionally, samples of different chemical compositions within the characteristic industrial tolerance range were studied under variation of pre-load conditions, simulating the thermo-mechanical hot rolling process. The samples were then subjected to bake hardening to study the varying chemical composition on this effect. Furthermore, the local use of bake hardening and ageing in hot rolled multiphase steels was investigated. It could be shown that characteristic values integrally describing the ageing effect, depend on the deformation path and the degree of pre-strain, as well as on temperature and duration of the subsequent heat treatment. This partial ageing is stable and has a potential to be used for local strengthening of the steels.