Experimental and numerical study on geometrically necessary dislocations and non-homogeneous mechanical properties of the ferrite phase in dual phase steels

Microstructure-based model for the static mechanical behaviour of multiphase steels

Modelling of damage and failure in multiphase high strength DP and TRIP steels

Micromechanical modeling of dual phase steels

A microstructure-based approach by means of representative volume elements (RVEs) is employed to evaluate the flow curve of DP steels using virtual tensile tests. Microstructures with different martensite fractions and morphologies are studied in two- and three-dimensional approaches. Micro sections of DP microstructures with various amounts of martensite have been converted to 2D RVEs, while 3D RVEs were constructed statistically with randomly distributed phases. A dislocation-based model is used to describe the flow curve of each ferrite and martensite phase separately as a function of carbon partitioning and microstructural features. Numerical tensile tests of RVE were carried out using the ABAQUS/Standard code to predict the flow behaviour of DP steels. It is observed that 2D plane strain modelling gives an underpredicted flow curve for DP steels, while the 3D modelling gives a quantitatively reasonable description of flow curve in comparison to the experimental data. In this work, a von Mises stress correlation factor σ3D/σ2D has been identified to compare the predicted flow curves of these two dimensionalities showing a third order polynomial relation with respect to martensite fraction and a second order polynomial relation with respect to equivalent plastic strain, respectively. The quantification of this polynomial correlation factor is performed based on laboratory-annealed DP600 chemistry with varying martensite content and it is validated for industrially produced DP qualities with various chemistry, strength level and martensite fraction.

Correlation between 2D and 3D flow curve modelling of DP steels using a microstructure-based RVE approach

An axisymmetric unit cell model based on a regular array of second-phase particles arranged on a BCC lattice is used to study deformation mechanisms of ferrite-pearlite structural steels. Microstructural characteristics of the steels were parameterized by the pearlite volume fraction, the aspect ratio of the pearlite particles, and the neighboring factor, which represents the ratio of interparticle spacing in the longitudinal direction to that in the transverse direction. FE analyses were carried out to investigate the macroscopic and microscopic response of unit cells with morphological features based on idealizations of the microstructures of the actual steels. Tensile properties of each constituent phase were obtained experimentally and used in the analyses. As compared to traditional axisymmetric models, the BCC cell model appears to be able to capture more realistically the behavior of the materials, and it accurately estimates the tensile behavior of the ferrite-pearlite steels even with a relatively large volume fraction of the pearlite phase. The effects of volume fraction and morphology of the second-phase particles on deformation behavior were also investigated.

Micromechanical modeling of ferrite-pearlite steels using finite element unit cell models

The effect of thermo-mechanical treatment on the mechanical properties of Cu bearing age hardenable steels was examined. Accelerated cooling and direct quenching after controlled rolling enhanced to produce the microstructure dominant of low carbon bainite which caused the improvement of strength and toughness. Addition of Cu over 1% was effective in both e-Cu precipitation strengthening and microstructural control through its effect on hardenability. Studies were focussed on the specific effects of Cu in thermo-mechanical process. It was revealed that the improvement of mechanical properties was partly attributed to the retardation of recrystallization in hot working due to Cu addition over 1% and the suppression of e-Cu precipitation during cooling due to rapid cooling.

Effect of Thermomechanical-Processing on Mechanical Properties of Copper Bearing Age Hardenable Steel Plates

Influence of Microstructure on Yielding Behavior of Heavy Gauge High Strength Steel Plates.

The effect of the specimen geometry and the inclusion content on the critical condition for ductile crack initiation was determined using notched round bar tensile specimens, and the critical void volume fraction and secondary void nucleation effect were investigated using the Gurson-Tvergaard constitutive model. It is shown that the secondary void nucleation from pearlite nodules plays a dominant role in ductile crack initiation, and void nucleation and void growth behavior are strongly affected by the stress triaxiality and the plastic strain. In the large plastic strain and low stress triaxiality region, a large number of secondary voids were nucleated in the early stage of deformation and void volume fraction grew to a critical value in spite of the relatively low void growth rate at low stress triaxiality. On the other hand, in the low plastic strain and high triaxial stress region, void volume can grow rapidly even though the volume of secondary nucleated void was smaller than that of high plastic strain region. Critical void volume fraction decreases largely with increasing in MnS content in the high triaxial stress region; however, the effect of MnS content is small in the low stress triaxiality and large plastic strain region.

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Micromechanism of Ductile Crack Initiation in Structural Steels Based on Void Nucleation and Growth

The effects of the volume fraction and the morphology of a second phase on ductile crack initiation behavior were determined by notched round bar tensile specimens using ferrite-pearlite steels which contain quite small amounts of MnS inclusions. Nominal strain to crack initiation was increased by decreas ng pearlite volume fraction, and by the controlled rolling, which produces an elongated microstructure. The Gurson-Tvergaard (G-T) constitutive model was used to investigate the micromechanism of ductile crack initiation behavior. For evaluating the void nucleation strain, an axisymmetric unit cell model based on a Voronoi tessellation of the BCC lattice (V-BCC modell was applied to determine the microscopic strain inside the pearlite phase which controls secondary void nucleation. The parameter representing the volume fraction of nucleated void, f N , was evaluated by fitting the numerical solution to experimental data for nominal stress/nominal strain curves of the notched round bar specimen. It was found that steels with lover pearlite volume fractions or elongated pearlite nodules have lower f N , and the void growth rate is lower for the steels with lower f N , which requires a large amount of plastic strain for void growth. Ductile cracking was initiated in the region having the highest void volume fraction for all steels, It was shown that the critical void volume fraction for ductile crack initiation is independent of stress triaxiality, and the steels with lower f N show smaller critical void volume fractions.

https://www.jstage.jst.go.jp/article/isijinternational1989/41/1/41_1_76/_pdf

Masayoshi Kurihara

Papers

Micromechanical modeling of ferrite-pearlite steels using finite element unit cell models

Effect of Thermomechanical-Processing on Mechanical Properties of Copper Bearing Age Hardenable Steel Plates

Influence of Microstructure on Yielding Behavior of Heavy Gauge High Strength Steel Plates.

Micromechanism of Ductile Crack Initiation in Structural Steels Based on Void Nucleation and Growth

Micromechanical Modeling of Ductile Crack Initiation Behavior of Two Phase Steels