A Dislocation-Based Theory for the Deformation Hardening Behavior of DP Steels: Impact of Martensite Content and Ferrite Grain Size

Yngve Bergström,Dirk Sterkenburg,Ylva Granbom

doi:10.1155/2010/647198

Abstract

A dislocation model, accurately describing the uniaxial plastic stress-strain behavior of dual phase (DP) steels, is proposed and the impact of martensite content and ferrite grain size in four commercially produced DP steels is analyzed. It is assumed that the plastic deformation process is localized to the ferrite. This is taken into account by introducing a nonhomogeneity parameter, f(ε), that specifies the volume fraction of ferrite taking active part in the plastic deformation process. It is found that the larger the martensite content the smaller the initial volume fraction of active ferrite which yields a higher initial deformation hardening rate. This explains the high energy absorbing capacity of DP steels with high volume fractions of martensite. Further, the effect of ferrite grain size strengthening in DP steels is important. The flow stress grain size sensitivity for DP steels is observed to be 7 times larger than that for single phase ferrite.

Highlights

The usage of advanced high strength steels, AHSS, has made it possible for the automotive industry to reduce weight and improve safety of vehicles
The properties of dual phase (DP) steels are characterized by a low yield strength due to the absence of Luders bands and a high rate of deformation hardening, which results in a high tensile strength and good formability
The aim of this work is to find a physical description of the deformation behavior of DP steels

Summary

Introduction

The usage of advanced high strength steels, AHSS, has made it possible for the automotive industry to reduce weight and improve safety of vehicles. One frequently used concept is the Ashby-model [11], which is based on the assumption that the dislocations can be placed in two categories: statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) with pile ups of dislocations in arrays. These arrays have never been observed in high stacking fault energy crystals like ferrite. This has been pointed out by Mughrabi [12] who further states that, “The increasing number of SGP (strain gradient plasticity) theories confirms that the search for a really satisfactory theory is still going on. Better microstructurally founded physical models should be pursued with priority before further development of rather formal, generalized concepts in which the physics tends to be hidden” [13]

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