Behavior Of Dual-phase Steels Research Articles

Multiscale Material Modeling and Simulation of the Mechanical Behavior of Dual Phase Steels Under Different Strain Rates: Parametric Study and Optimization

Recent studies on developing dual phase (DP) steels showed that the combination of strength/ductility could be significantly improved when changing the volume fraction and grain size of phases in the microstructure depending on microstructure properties. Consequently, DP steel manufacturers are interested in predicting microstructure properties as well as optimizing microstructure design at different strain rate conditions. In this work, a microstructure-based approach using a multiscale material and structure model was developed. The approach examined the mechanical behavior of DP steels using virtual tensile tests with a full micro-macro multiscale material model to identify specific mechanical properties. Microstructures with varied ferrite grain sizes, martensite volume fractions, and carbon content in DP steels were also studied. The influence of these microscopic parameters at different strain rates on the mechanical properties of DP steels was examined numerically using a full micro-macro multiscale finite element method. An elasto-viscoplastic constitutive model and a response surface methodology (RSM) were used to determine the optimum microstructure parameters for a required combination of strength/ductility at different strain rates. The results from the numerical simulations were compared with experimental results found in the literature. The developed methodology proved to be a powerful tool for studying the effect and interaction of key strain rate sensitivity and microstructure parameters on mechanical behavior and thus can be used to identify optimum microstructural conditions at different strain rates.

Micromechanical Finite Element Analysis of the Effects of Martensite Particle Size and Ferrite Grain Boundaries on the Overall Mechanical Behavior of Dual Phase Steel

This paper focuses on micromechanical finite element (FE) modeling of the effects of size and morphology (particularly elongation or aspect ratio (AR) along the loading direction) of martensite particles and the ferrite grains on the overall mechanical behavior of dual-phase (DP) steels. To capture the size-effect of the martensite particles and ferrite grains, the core and mantle approach is adapted in which a thin interphase of geometrically necessary dislocations (GNDs) is embedded at the martensite–ferrite boundaries. It is shown that as the martensite particles size decreases or their aspect ratio increases, both the strength and ductility of DP steel increase simultaneously. On the other hand, as the ferrite grain size decreases or its aspect ratio increases, the overall strength increases on the expense of the ductility. The conclusions from this study can be used in guiding the microstructural design of DP steels.

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

High cycle fatigue micromechanical behavior of dual phase steel: Damage initiation, propagation and final failure

Micromechanical analyses of various materials under high cycle fatigue (HCF) loading have been investigated previously. Experimental findings have shown that failure due to HCF occurred with different pattern comparing to the other loading conditions. The current work tries to carry out both experiments and simulations at micro scale. In this research, scanning electron microscopy (SEM) and metallography images of the specimens are taken to investigate the failure and deformation patterns in dual phase (DP) steel under high cycle fatigue loading. Failure mechanisms of DP steels are also predicted using a real representative volume element (RVE) and microstructural finite element (FE) modeling. The linear damage accumulation rule (Miner's damage rule) is used to determine the material degradation due to fatigue loading. The cyclic loading is implemented in the FE model of the microstructure by developing ANSYS parametric design language (APDL) code to obtain the damage, failure pattern and fatigue life of the RVE. It is also shown that the finite element modeling of the real RVE considering both plasticity and fatigue damage leads to acceptable compatibility between the obtained cracking patterns from SEM images (experiments) and finite element predictions.

Micromechanical finite element analysis of the effects of martensite morphology on the overall mechanical behavior of dual phase steel

Strong and ductile dual-phase (DP) steels are an important class of advanced high strength steels that are commonly used by the automotive industry. Micromechanical computational modeling is imperative for guiding the material design of DP steels with simultaneous enhancements in strength and ductility. In this study, the effect of the morphology of the martensite hard phase on the overall stress-strain response of DP steels is studied. Through generating realistic virtual representative volume elements (RVEs) and adapting a microstructural-based approach using a finite deformation elastic-viscoplastic constitutive model, it was possible to conduct various parametric studies of the effects of martensite phase on the strength and ductility of DP steel. Morphological parameters of the martensite phase that are studied include (1) effect of aspect ratio (equiaxed versus elongated), (2) effects of the combination of various aspect ratio within an RVE, and (3) effect of interconnectivity between martensite particles and its thickness. It is shown that elongated martensite particles lead to a better overall performance than that of equiaxed martensite particles. It is found that by having the martensite phase interconnected, both strength and ductility increase. Furthermore, it is shown that the increase in the volume fraction of equiaxed martensite particles within microstructures of elongated particles reduces the strength and ductility simultaneously. Overall, it is concluded that the morphology of the martensite phase in DP steels impacts more the ductility than strength.

Influence of Heat Treatment Schedule on the Tensile Properties and Wear Behavior of Dual Phase Steels

The aim of present study is to investigate the influence of heat treatment schedule on the microstructure, tensile properties, and wear behavior of dual phase (DP) steels. In this regard, a cold-rolled low carbon steel (0.18% C, 1.25% Mn) is subjected to intercritical annealing (IA), step quenching (SQ), and intermediate quenching (IQ) heat treatment cycles to produce DP steels with different microstructures. The results show that the tensile strength, strength-elongation balance, and wear resistance of DP steels change in the order of SQ < IA < IQ. Variation of work hardening rate (θ = dσ/dϵ) with stress for the IA treated DP steel shows a one stage behavior corresponding to stage III, where θ linearly decreases with increasing flow stress. In contrast, for IQ and SQ treated specimens, a two stage behavior corresponding to an initial rapid decrease of θ with stress (transition stage) followed by stage III hardening are observed. The dominant wear mechanism for DP steels produced by IA and IQ treatments is a combination of abrasion and oxidation, while for the SQ treated specimen, a combination of oxidation and delamination is observed.

Investigation of Galvanic Corrosion Behavior of Dual Phase Steel

Dual phase steels have been widely used in the automotive industry to produce lighter weight vehicles to meet stringent safety and fuel economy regulations. While galvanic corrosion resulting from dissimilar ferrite and martensite phases has been reported, little is known about the corrosion behavior of dual phase steel coupled to a dissimilar metal under corrosive environments. A fundamental study was undertaken to understand the corrosion behaviors of dual phase steel under coupling conditions in NaCl solution. The results show that dual phase steel has a corrosion potential between carbon steel and martensitic steel. Galvanic action between the ferrite and martensite phases leads to selective corrosion of ferrite phase in NaCl solution. When coupled to more noble Ni, dissolution takes place in both ferrite and martensite phases, and ferrite phase dissolves faster. When coupled to less noble Zn, a minor amount of hydrogen resulting from corrosion of a steel-Zn coupling is found to diffuse into the steel.

Investigation of Galvanic Corrosion Behavior of Dual Phase Steel

Dual phase steel has been widely used in the automotive industry to produce lighter weight vehicles to meet stringent safety and fuel economy regulations. One potential concern with dual phase steel is galvanic corrosion when it is exposed to a corrosive environment. In dual phase steel, the dissimilarity in chemical composition and lattice structure could induce different corrosion potentials between the ferrite and martensite phases. As a result, microscopic galvanic couplings could be formed between ferrite grains and their neighboring martensite grains. In this study, the corrosion behaviors of three commercial steel grades: carbon steel, martensitic steel and dual phase steel, are investigated with a variety of electrochemical and surface analytical techniques to understand the mechanism of galvanic corrosion of dual phase steel. The corrosion potential of each steel grade is determined with potentiodynamic technique when the steel is exposed to sodium chloride (NaCl) solution. The study shows that the corrosion potential of dual phase steel is an intermediate between martensitic and carbon steels. In a galvanic coupling of carbon steel/ martensitic steel, carbon steel is found to corrode to protect martensitic steel when exposed to NaCl solution. Examination of dual phase steel after immersion in NaCL solution for an extended period of time indicates the occurrence of galvanic corrosion in the dual phase steel. The corrosion is found to occur primarily in the ferrite phase of dual phase steel.

Mechanical, microstructural behaviour and modelling of dual phase steels under complex deformation paths

This paper aims to identify the mechanisms associated to the transient hardening behaviour of dual phase steels under strain path changes, and to capture the observed material behaviours with appropriate constitutive models. First, three DP steel sheets with different amounts of martensite were tested under monotonic and various strain path changes. Second, microstructural analysis of the materials before and after strain path change were performed by means of SEM, TEM, and EBSD. The contribution of texture evolution on the mechanical behaviour was also assessed using the visco-plastic self-consistent (VPSC) polycrystal plasticity model. Transient hardening behaviour and permanent softening were observed in the tension–tension tests for all the studied DP steels. These behaviours were explained by the development of strain gradients during the first load resulting from strain accommodation incompatibilities between the ferrite and martensite phases. For the purpose of describing the macroscopic material behaviours, the enhanced homogeneous anisotropic hardening (HAH) model (Barlat et al., 2014) integrated with the Yld2000-2d anisotropic yield function were adopted for constitutive modelling. The simulation results were discussed in view of the microstructure evolution.

Investigation on Elastic Behaviour of DP800 Dual Phase Steel

The elastic relaxation behavior of dual phase steel DP800 is studied in this investigation, based on experimental and numerical methods the true stress-true strain curve obtained from a standard uniaxial tensile test differs according to angular rolling direction The relationship between true stress and true strain are presented in the form of power law equation. This form of material constitutive model shows that the strength coefficient and strain hardening exponent vary significantly in describing the nonlinear true stress-true strain relationship of the material. Finite Element (FE) calculations with Belytschko-Lin-Tsay shell element formulation are performed using the non-linear FE code Ls-Dyna to predict the plastic deformation of the material. Power Law Isotropic Plasticity criterion is adopted for these numerical analyses. The local strains in plastic deformations zone and true stress-strains characteristics obtained by experiment are compared. Using the same parameter the simulation was applied in different modes which are known as Isotropic Elastic-Plastic Model and Piecewise Linear Isotropic Plasticity Model providd in Ls-Dyna simulation for comparison. In general, good agreement in results is obtained between Power Law Isotropic Plasticity Model is obtained compared to Isotropic Elastic-Plastic Model and Piecewise Linear Isotropic Plasticity Model. It is demonstratedthat the behavior of the strain and the Power law criterion can be determined from uniaxial tensile test with the aid of non-linear FE analyses.

Effect of martensite morphology on low cycle fatigue behaviour of dual phase steels: Experimental and microstructural investigation

The low cycle fatigue (LCF) behaviour of a dual phase (DP) steel with different martensite morphologies has been investigated in the present work. DP steels with coarse martensite morphologies show inferior LCF life in comparison with fine martensite morphologies for all martensite volume fractions examined. It is suggested that this is be due to the development of larger local plastic strain concentrations in the ferrite with a coarser microstructure, compared to the finer microstructural morphology. Fatigue cracks were observed to initiate inside ferrite grains, and to preferentially propagate through the softer ferrite phase. The average sub-cell size was finer in samples with higher martensite volume fractions, but the sub-cell size was almost unaffected by the martensite morphology.

Micromechanical modelling of bending under tension forming behaviour of dual phase steel 600

A micromechanical modelling based approach by means of a Representative Volume Element (RVE) was employed to predict the flow behaviour and plastic strain of DP600 steel, produced by WISCO. Macroscopic modelling of a classical Bending-Under-Tension (BUT) experiment was employed to acquire strain deformation, and thus the following microscopic modelling was implemented by considering the realistic microstructure morphology. Comparisons between macroscopic behaviour and microscopic behaviour, including strain distribution and stress distribution, were extracted for different boundary conditions of the BUT set-up. The micro–macro modelling approach increases the understanding of the steel microstructure, which will enable this microstructure to be tailored for different applications in automobile industry in the future.

Effect of martensite volume fraction on low cycle fatigue behaviour of dual phase steels: Experimental and microstructural investigation

The low cycle fatigue (LCF) behaviour of a dual phase (DP) steel with different martensite volume fractions has been investigated, with particular focus on fatigue life, cyclic hardening/softening behaviour and microstructural evolution. DP steels with martensite volume fractions between 13% and 88% were produced and their monotonic and cyclic deformation behaviours evaluated. The LCF life has been examined in depth and compared with published literature. It has been concluded that, once normalised for plastic strain amplitude, the fatigue life was found to be significantly reduced by an increase in the martensite volume fraction. All alloys were observed to show some initial cyclic hardening followed by cyclic softening. Clear sub-cell formation occurred in ferrite grains irrespective of the martensite volume fraction, and it is suggested that this cell formation and martensite softening are responsible for the cyclic softening behaviour.

Friction and wear behaviour of dual phase steels in discontinuous sliding contact conditions as a function of sliding speed and contact frequency

Friction and wear rate behaviour of dual phase steel discs (DP600) as a function of sliding speed (0.05–3.75m/s), and contact frequency (0.6–16.5Hz) have been studied under unidirectional ball-on-disc dry sliding conditions. The coefficient of friction (COF) and wear rate exhibit not only a highly dependence on the sliding speed, but also on the contact frequency which appears to be the key factor determining the wear behaviour of the DP600 even at constant sliding speed. The wear mechanism is mainly oxidative. The validity of Garcia–Ramil–Celis model is demonstrated for discontinuous sliding contacts at contact frequencies below 7Hz. However, above 7Hz, the disc behaves as if it were subjected to a continuous sliding contact.

Temperature and strain rate effects on the mechanical behavior of dual phase steel

The effect of temperature and strain rate on the mechanical behavior of a commercial dual phase steel (DP 800) has been investigated experimentally by uniaxial tensile tests in this study, covering temperatures (−60°C to 100°C) and strain rates (1×10–4 to 1×102s−1) encompassing conditions experienced in automotive crash situations. Yield and ultimate tensile strength, ductility, temperature effects and strain rate sensitivity have been determined and discussed. It was found that the Voce equation [σ=σs−(σs−σ0)exp(−εθ0/σs))] can be satisfactorily applied to describe the tensile flow curves by means of a modified Kocks–Mecking model. In this model the parameter θ0 is fixed, whereas both σ0 and σs consist of athermal and thermal stress components. The athermal component is only weakly dependent on temperature through the elastic shear modulus μ. The thermal stress component is governed by temperature and strain rate. Statistical analysis based on the experimental data has allowed all parameters in the Voce equation to be quantified.

Influences of Microstructure Characteristics on Forming Limit Behavior of Dual Phase Steels

In the automotive sector, dual phase (DP) steel grades have been increasingly used for various car body parts due to their good combination of strength and formability. To control mechanical and fracture characteristics of the DP steels, effects of martensitic phase fraction, morphology, and phase distribution must be understood. In this work, DP steel sheets with different martensitic phase fractions and ferritic grain sizes were produced through the intercritical annealing process. Likewise, FE simulations of 2D representative volume elements (RVEs) based on real micrograph were performed for all generated DP microstructures. Flow behaviors of single individual phases in the DP steels were described by a dislocation theory and local chemical compositions. Calculated stress–strain responses were verified with experimental results from tensile tests. Subsequently, stretch-forming tests under different states of stress using the Nakajima samples were carried out for the examined DP steels. Micromechanics RVE models were then applied to predict failure occurrences in the DP microstructures by considering plastic strain instability. Influences of morphologies and properties of constituent phases on localization due to incompatible deformation between martensite and ferrite were discussed with regard to governing stress states.

The influence of temperatures and strain rates on the mechanical behavior of dual phase steel in different conditions

This study deals with the mechanical behavior of DP steel. A commercial dual phase steel (DP 800) was strained to 3.5% followed by annealing at 180°C for 30min to simulate the pressing of the plates and the paint-bake cycle involved in the manufacturing process of automobile body structures. The effect of temperature and strain rate on the mechanical behavior of this material was investigated by uniaxial tensile tests, covering applicable temperatures (−60°C to 100°C) and strain rates (1×10−4 to 1×102s−1) experienced in automotive crash situations. Yield and ultimate tensile strength, ductility, temperature effects and strain rate sensitivity as well as strain hardening rate have been determined and discussed.

Metallurgical Investigations on Hyperplasticity in Dual Phase Steel Sheets

Several researchers have reported that dual phase steel sheets exhibit hyperplasticity, that is, a significant formability improvement under certain high strain rate forming conditions. Hyperplastic behavior of dual phase steels formed using an electrohydraulic forming (EHF) process was previously investigated by the authors at both macro- (Golovashchenko et al., 2013, “Formability of Dual Phase Steels in Electrohydraulic Forming,” J. Mater. Process. Technol., 213, pp. 1191–1212) and microscales (Samei et al., 2013, “Quantitative Microstructural Analysis of Formability Enhancement in Dual Phase Steels Subject to Electrohydraulic Forming,” J. Mater. Eng. Perform., 22(7), pp. 2080–2088). A relative deformation improvement of approximately 20% in ferrite grains and 100% in martensite islands was reported in the EHF specimens compared to specimens formed under quasi-static conditions. In this paper, the remarkable deformation improvements of the constituents are discussed in terms of metallurgical mechanisms of deformation. The nucleation and multiplication of dislocations in ferrite and deformation twinning in martensite were found to be the principal mechanisms responsible for the significant improvements of deformation in EHF. In addition, these mechanisms enhance the plastic compatibility between the two phases which reduces the risk of decohesion and delays the onset of fracture in EHF specimens.

A study of the unloading behaviour of dual phase steel

It is important to understand the strain recovery of a steel sheet in order to predict its springback behaviour. During strain recovery, the stress–strain relation is non-linear and the resulting unloading modulus is decreased. Moreover, the unloading modulus will degrade with increasing plastic pre-straining. This study aims at adding new knowledge on these phenomena and the mechanisms causing them. The unloading behaviour of the dual-phase steel DP600 is characterised experimentally and finite element (FE) simulations of a representative volume element (RVE) of the microstructure are performed. The initial stress and strain state of the micromechanical FE model is found by a simplified simulation of the annealing processes. It is observed from the experimental characterisation that the decrease of the initial stiffness of the unloading is the main reason for the degrading unloading modulus. Furthermore, the developed micromechanical FE model exhibits non-linear strain recovery due to local plasticity caused by interaction between the two phases.

Microstructural deformation pattern and mechanical behavior analyses of DP600 dual phase steel

Deformation pattern is the most important issue which can be effective on the prediction of mechanical behavior of dual phase steel in microscale. In this paper, simulation with large and small deformation theories using the real microstructure is employed to investigate the deformation pattern in microstructure of DP600 dual phase steel. Deformation pattern is mostly effective on the transition of rotation and displacement between ferrite and martensite phases and stress–strain behavior. Different deformation theories affect the predicted stress–strain behavior of dual phase steel using the finite element method. The obtained results from both numerical and experimental methods are used to investigate the effect of deformation in the microstructure. The performed experiments were interrupted in three stages to obtain the deformation pattern of components microstructure: necking initiation, localization and failure. Metallographic and SEM images were prepared to study the ferrite matrix and martensite grains deformation, and failure pattern in the microscale. Localization which causes severe deformation, rotation and displacement of the microstructure is defined as the failure criteria and the main source of void formation and rupture phenomenon. The predicted results such as stress–strain behavior and deformation pattern using small and large deformation theories are compared with the experimental findings and observable voids in the SEM images.