Mechanical Behavior Of Dual Phase Steel Research Articles

Effect of the anisotropy of martensitic transformation on ferrite deformation in Dual-Phase steels

The volume increase and shape change during austenite to martensite transformation in dual-phase (DP) steels are largely accommodated in the microstructure by the deformation of the surrounding ferrite matrix. Accurate estimation of transformation-induced deformation of ferrite via experiments and modeling is essential for predicting the subsequent mechanical behavior of DP steels. This study aims to illustrate the disadvantages of simplifying the anisotropic transformation deformation of martensite to isotropic dilatation for modeling the transformation-induced deformation of ferrite. A novel methodology is developed comprising sequential experimental and numerical research on DP steels to quantify transformation-induced strains in ferrite. This methodology combines the results of prior austenite grain reconstruction, phenomenological theory of martensite crystallography and electron backscatter diffraction (EBSD) orientation data to estimate variant-specific transformation deformation. Subsequently, by comparison of full-field micromechanical calculation results on a virtual DP steel microstructure with experimental EBSD kernel average misorientation and geometrically necessary dislocation measurement results it is shown that neglecting the shear deformation associated with the martensitic transformation leads to significant underestimation in the prediction of transformation-induced strains in ferrite.

Effect of strain rate-induced microstructure on mechanical behavior of dual-phase steel

In this work, the microstructure evolution characteristics after 3% pre-strained at different strain rates are elaborated to reveal the effect on the mechanical behavior of dual-phase steel. The results showed the strain field simulated by the representative volume element model shows obvious strain heterogeneity, and the degree of strain localization intensifies with the increase of the strain rate. There is sufficient time to complete the dynamic transition among geometrically necessary dislocations (GNDs)-low-angle grain boundaries (LAGBs)-middle angle grain boundaries (MAGBs)-high angle grain boundaries (HAGBs) at low strain rates (10−4 s−1∼10−3 s−1), thereby releasing strain distortion energy. As the strain rate increases, the transformation process from LAGBs to HAGBs is hindered due to the shortened strain response time. It can promote the accumulation of high-density GNDs and LAGBs inside the grains, which intensifies strain localization. The incomplete evolution of the microstructure caused by the pre-strain history becomes a resistance to dislocation motion during re-deformation, and more importantly, the higher the strain rate, the more severe the hardening rate loss. This leads to an increase in yield strength and a decrease in uniform elongation, which drastically deteriorates the mechanical stability of the steel.

Strain-Hardening Behavior of Dual-Phase Steel under Multistress States

The complexity and nonuniformity of microdeformation in high-strength dual-phase (DP) steels make it difficult to accurately predict the deformation processes in stamping. Here, in situ experiments under three kinds of stress states and a crystal plasticity finite element (CPFE) model were used to predict the strain-hardening behavior of DP steel. The microdeformation in various deformation stages was extracted from metallographs captured during in situ experiments to calculate the strain partitioning functions of each phase. Then, the multistress state mechanical behavior of DP steel was calculated based on the CPFE model combined with the strain partitioning functions of ferrite and martensite. The calculation results showed that (1) the CPFE model could accurately describe the load–stroke curves of the three experiments and that (2) the stress state had a significant effect on the strain partition ratio and the initial critical resolved shear stress of the material. In addition, the stress–strain curves of DP800 steel were predicted under six stress states; these curves did not coincide and had obvious deviations. Finally, the activity of the grain slip systems and the dislocation evolution were analyzed and discussed to predict the microdeformation and stress state effects on strain-hardening behavior.

Micromechanical Modeling and Investigating the Effect of Particle Size and the Interface of Phases on the Mechanical Behavior of Dual-Phase Steels

Dual-phase (DP) ferrite–martensite steels have a fine microstructure that consist of martensite islands, dispersed in a ferrite matrix. In the present work, a micromechanical-based finite element (FE) analysis is carried out in order to investigate the mechanical properties of this type of steels, such as yield stress and tensile strength. The step forward contribution of the present study is to insert the effect of the presence of the interface of the constituting phases in the FE model. In order to do so, a micromechanical FE model is developed in which the effect of the interface of the constituting phases is taken into account by means of cohesive elements. The required parameters of the cohesive elements are determined by a trial and error procedure which compares the numerical results to the experimental ones. Experimental data is used to verify the created FE model. This model is then used to investigate the effect of martensite volume fraction and small-to-large particle size ratio of martensite on the deformation behavior of the DP steel. This study shows that considering the interface of the constituting phases greatly affects the numerical results on stress and strain distributions and produces more realistic findings in agreement with experiments. The results show that applying cohesive elements between two phases have reduced tensile strength, yield stress, and the amount of uniform elongation.

Study of the Mechanical Behavior of Dual-Phase Steel Based on Crystal Plasticity Modeling Considering Strain Partitioning

The similar crystal structures of martensite (BCT) and ferrite (BCC) cause difficulty in distinguishing the grain orientations of individual phases in dual-phase (DP) steels. A dislocation-based multiphase mixed hardening model is presented, considering both ferrite and martensite strain partitioning, to describe the texture-dependent mechanical behavior of DP steels more precisely. This model is based on the ideals that (i) the volume fractions of the constituent phases and the corresponding strain partitioning function are obtained through in situ tensile experimentation, and (ii) the grain orientations of ferrite and martensite are assumed to be in accordance with the overall texture. We applied the model to calculate the macroscopic and microscopic mechanical behavior of DP800 steel using a crystal plasticity finite element (CPFE) code. The results show that the calculated stress-strain response and textural evolution are in good agreement with the experimental results. The dislocation evolution indicates that the rapid hardening of ferrite induces a high hardening rate for DP steels early in plastic deformation. In addition, for the grains corresponding to the texture center orientations of DP800, the activity and dislocation density evolutions of the slip systems are studied.

An RVE procedure for micromechanical prediction of mechanical behavior of dual-phase steel

A “bottom-up” representative volume element (RVE) for a dual phase steel was constructed based on measured microstructural properties (“microproperties”). This differs from the common procedure of inferring hypothetical microproperties by fitting to macroscopic behavior using an assumed micro-to-macrolaw. The bottom-up approach allows the assessment of the law itself by comparing RVE-predicted mechanical behavior with independent macroscopic measurements, thus revealing the nature of the controlling micromechanisms. An RVE for DP980 steel was constructed using actual microproperties. Finite element (FE) simulations of elastic-plastic transitions were compared with independent loading-unloading-loading and compression-tension experiments. Constitutive models of three types were utilized: 1) a standard continuum model, 2) a standard Crystal Plasticity (CP) model, and 3) a SuperDislocation (SD) model similar to CP but including the elastic interactions of discrete dislocations. These comparisons led to following conclusions: 1) While a constitutive model that ignores elastic interaction of defects can be fit to macroscopic or microscopic behavior, it cannot represent both accurately, 2) Elastic interactions among dislocations are the predominant source of nonlinearity in the nominally-elastic region (i.e. at stresses below the standard yield stress), and 3) Continuum stress inhomogeneity arising from the hard martensite / soft ferrite microstructure has a minor role in the observed transitional nonlinearity in the absence of discrete dislocation interactions.

Multi-scale modelling of mechanical behavior of dual-phase steels based on realistic microstructure

Multi-scale modelling of mechanical behavior of dual-phase steels based on realistic microstructure

A Dislocation Density-Based Model for Analyzing Mechanical Behavior of Dual-Phase Steels

The aim of this work is to develop a microstructural analysis of the mechanical behavior of dual-phase steels. Finite element simulations were conducted, and the results were validated by experimental observations. A dislocation density constitutive formulation was used for the ferrite phase, while the Gurson–Tvergaard–Needleman damage model and an elastic–plastic model with isotropic hardening were used for the grain boundaries and martensite, respectively. The deformation and failure behavior of the material predicted by the model developed here showed good agreement with experimental observations. Moreover, the investigation on the effect of grain size and martensite volume fraction revealed that the microstructural inhomogeneity plays an important role in material behavior.

Effect of pre-IC annealing treatments on the final microstructure and work hardening behavior of a dual-phase steel

This paper investigates the relationship between the microstructure and the work hardening behavior of a dual-phase (DP) steel. Various DP microstructures were systematically produced by application of different pre-IC annealing heat treatments as well as changing the IC annealing temperature. It was found that various austenite nucleation sites such as grain boundaries, prior pearlite colonies, martensite particles and cementite particles, have different nucleation and growth effectiveness which significantly influences the microstructure after IC annealing. Following a quantitative analysis of all microstructures, the effect of microstructural parameters including martensite particle size, volume fraction as well as their spatial distribution and morphology, on the mechanical behavior of DP steels is examined by considering true work hardening rate, instantaneous work hardening rate and the dislocation annihilation factor from the Kocks–Mecking analysis. These analyses reveal that at constant ratios of volume fraction to size of martensite particles, there are significant differences in all the three work hardening parameters. It is proposed that these observations are due to the effects of morphology and spatial distribution of martensite particles. Furthermore, it was shown that the contribution of martensite particles to work hardening behavior, via geometrically necessary dislocations, is only significant at the early stages of deformation.

Microstructure Modelling of Dual-Phase Steel Using SEM Micrographs and Voronoi Polycrystal Models

The microstructure of dual-phase (DP) steels is composed of a matrix of ferrite reinforced by islands of martensite and the common interphase boundaries. To study the mechanical behavior of DP steels, steel with 45% ferrite and 55% martensite was fabricated and tested in the laboratory. Two types of finite element models were then created based on SEM images. The first model directly created the grains and boundaries from the SEM images, while the second model used a Voronoi type algorithm to construct geometries which are statistically similar to the SEM images. The models consider the measured morphology of ferrite, martensite, and their phase boundaries. The Gurson damage model was then used for the ferrite and boundary regions. The obtained results correctly predicted the failure mechanisms in a DP material. The results indicate that the deformation is localized due to microstructural inhomogeneities and the nucleation of voids in the boundaries between the ferrite and martensite grains. The good correlation between the numerical and experimental observations from SEM micrographs shows the efficiency of the proposed models in predicting the failure mechanism of DP steels.

Dynamic Tensile Characteristics of DP600 Steel Sheets for Automotive Applications

To reduce fuel consumption and greenhouse gas emission, dual phase (DP) steels have been considered for automotive applications due to their higher tensile strength, better initial work hardening along with larger elongation compared to conventional grade of steels. In such applications, which would create potential safety and reliability issues under dynamic loading, the mechanical behavior of DP steel considering the strain rate must be examined. In the present study, the dynamic tensile behavior of DP600 steel sheets was investigated using a high-speed tensile testing machine at various strain rates. And the quasi-static tensile testing was also conducted on the steel to understand the effect of the strain rate on the tensile property. The fracture mechanisms of the steel were also analyzed. The results show that the mechanical properties of DP600 steel are noticeably influenced by the strain rates. As the strain rate increases, the strength of the steel increases and the obvious yield phenomenon can be observed when the strain rate is above 0.01 s-1. The fracture elongation of DP600 steels decreases with increasing strain rate from 0.001 to 1 s-1, then increases up to the strain rate of 100 s-1 and reaches the lowest value at the strain rate of 1000 s-1. DP600 steel sheet exhibit typical ductile fracture characteristics with dimples morphology of the facture surface when tensile deformed at various strain rates.

Formability of Dual-Phase Steels

The mechanical behavior of dual-phase steels depends on a broad range of metallurgical phenomena. The initial hardening reflects the volume fraction of the martensitic phase via a back stress, but this also depends on the local strength of the martensite. Formability is related not only to strain hardening and other continuum properties such as normal plastic anisotropy, but may also be influenced by fracture phenomena associated with discrete microstructural events such as decohesion of the ferrite-martensite interface. Various micromechanisms of duplex systems are discussed and the influence of the related mechanical behavior in determining formability in different technological forming process is described. The low yield-to-tensile-strength ratio of dual-phase steels is of particular benefit in stamping shallow components such as autobody skin panels; however, denting resistance requires a high yield strength emphasizing the importance of aging after forming.