Multiscale modelling of stress and strain partitioning in high strength dual phase steels

Multiscale modelling of stress and strain partitioning in DP steel was carried out using both realistic microstructure-based RVE models as well as stochastic microstructures generated by Monte Carlo method. The stochastic microstructure models were shown to resemble that of realistic microstructures, enabling research on the specific aspects of the microstructure that could be difficult to control and study during experimental work. One such feature of the realistic microstructures studied in this work was the grain size and microstructure morphology. The microstructures were generated with varying average grain sizes while all other parameters, such as boundary conditions, material properties and volume fractions of martensite and ferrite were kept constant. It is found that the effect of grain size is much more pronounced during the initial localisation of the plastic deformation at and around the interface of the phases. In addition, the decrease in ductility and increase in strength of the DP steels are directly related to the refinement of grain sizes of each phase and the stress-strain partitioning in between them.

Effect of tensile pre-strain on bending and unloading of automotive steels

In recent years, advanced high strength steels (AHSS) have been used in a wide range of automotive applications; they may have property variations through the thickness and the properties may also be dependent of prior processing including pre-straining. In order to model forming processes precisely using, for example, finite element analysis, it is important that material input data should adequately reflect these effects. It is known that shape defects in roll forming are related to small strains in material that has undergone prior deformation in a different strain path. Much research has already been performed on the change in the Young’s Modulus once a steel sheet has been plastically deformed,however many of these tests have only been conducted using tensile testing, and therefore may not take into account differences in compressive and tensile unloading. This research investigates the effect of tensile pre-straining on bending behaviour for various types of material;in bending, one half of the sheet will load and unload in compression and hence experience deformation under a reversed stress. Four different materials were pre-strained in tension with 1%, 3%, 7%, 11% and 25% elongation. Using a free bending test, moment curvature diagrams were obtained for bending and unloading. The results showed that the characteristics of the moment curvature diagram depended on the degree of pre-straining; more highly strained samples showed an earlier elastic-plastic transformation and a decreased Young's Modulus during unloading. This was compared to previous literature results using only tensile tests. Our results could influence the modeling of springback in low tension sheet operations, such as roll forming.

Development of boundary misorientations during grain growth in silicon steels

Changes in grain size, texture and misorientation distributions have been monitored during extensive normal grain growth in 3%Si steels. The boundary misorientation distributions deviate significantly from the Mackenzie relationship. Comparisons of correlated and uncorrelated distributions show large excesses of low angle boundaries. However, these are not a result of low energy boundaries being favoured during grain growth since the deviation diminishes as growth proceeds. The effect originates in the nucleation of grains in colonies of similar orientation during primary recrystallisation. A slight tendency for promotion of 60º boundaries may indicate some preference for the retention of lower energy twin boundaries during grain growth in silicon steel.