Microscopic investigation of damage mechanisms and anisotropic evolution of damage in DP600

Abstract

Weight reduction and fuel consumption play an important role on material selection in automotive industry. In this respect, ferritic-martensitic dual phase steels are gaining popularity thanks to their versatile combination of strength and formability. In this study, we investigate evolution of damage and active damage mechanisms in a commercial DP600 steel. Interrupted tensile tests are conducted in both rolling (RD) and transverse directions (TD). Subsequently, damage mechanisms and void evolution is characterized by cross-sectional SEM micrographs. The results reveal that, in both RD and TD, damage occurs by three different damage mechanisms. Namely, void formation due to inclusions, cracking of martensite islands and decohesion between ferrite and martensite. From these damage mechanisms, void formation due to large inclusions occur in the early stages of deformation, whereas the other two are both active throughout the complete stretching. The most commonly observed damage mechanism was martensite cracks and seem to be the primary reason of failure. In addition, void evolution studies clearly show that damaged area as well as number of voids increase more rapidly in RD than TD. Furthermore, in both directions, damage concentrates at the mid plane of the specimens, leading to an inhomogeneous distribution of voids in the thickness direction.

An RVE-Based Study of the Effect of Martensite Banding on Damage Evolution in Dual Phase Steels

“…It is concluded in [8] that for the DP800 they studied no definitive failure mechanism could be found meaning all of them were equally possible. Asik [9], who studied damage mechanisms for the specific DP steel that are used in this study, found that all damage mechanisms are equally likely to occur. A general overview of literature on this subject can be found in [10].…”

“…The second type was inside the ferrite grains with a martensite neighbor and towards the boundary between ferrite and the neighboring martensite island. The choice of these locations was based on the experimental observations, which show that in DP steels there is more than one active damage mechanism [9,28]. By adding the most common damage mechanisms into the model we aim to clarify and give an explanation on how one of the mechanisms becomes dominant over the others and on the effect of martensite banding on the evolution of these damage mechanisms.…”

“…The RVE with banded morphology was generated by considering a commercial DP600 steel (Tata Steel, Ijmuiden, The Netherlands). The same batch of this steel has been investigated experimentally before for damage mechanisms [9] and has the composition given in Table 2. Figure 1 illustrates secondary electron (SE) images obtained from different cross-sections of the steel and the measured volume percentages of martensite distribution along rolling, transverse and normal directions.…”

“…For the simulations, voids are introduced in the structure which are placed based on the most common active damage mechanisms [9,28]. The cylindrical voids were placed to ferrite-martensite boundaries and into ferrite grains but being closer to martensite.…”

“…The first structure resembles the banded martensite distribution of commercial DP600 steels, which has a minimum ultimate tensile strength of 590 MPa. The banded structure in DP steel sheets originates from the elemental segregation during solidification and subsequent rolling process [9,29]. On the other hand, the second distribution (random martensite distribution) is used to compare the evolution of different types of damage in order to clarify the effect of banding on damage evolution in DP structures.…”