Ductility Of Dual-phase Steels Research Articles

Measurement of edge fracture strain of dual-phase steels by in-plane bending test

Edge ductility of dual phase steels is highly affected by the preparation of the edge and especially shear cutting has a detrimental effect on the formability. In this study, a novel in-plane bending test is used to characterize the edge ductility of dual-phase steels. Digital image correlation technique is employed to calculate the fracture strain along the edge in the desired material orientation. The proposed test setup isolates the material-dependent aspects, namely damage and hardening, from those resulting from testing procedure factors such as contact stresses and friction compared to conventional hole expansion capacity (HEC) test. The results show that the edge crack phenomenon depends on the material orientation.

Mechanical and fracture behavior of high strength steels under high strain rate deformation: Experiments and modelling

By the automotive structure design, crashworthiness has become also an important issue so that a better understanding of plastic deformation of material at high velocity is necessary. The effects of strain rate on mechanical properties and fracture mechanism of ferritic-martensitic dual phase (DP) steel grades 780 and 1000 were investigated by both experiments and micromechanics based modeling. For the examined steels, quasi-static (0.001 s−1) and medium strain rate (0.5–1 s−1) tensile tests were carried out on a universal testing machine, while high strain rate (1500-2500 s−1) tests were performed by a Split-Hopkinson tensile bar. Afterwards, FE simulations using 2D representative volume elements (RVEs) were conducted for investigating microstructure effects on local deformation and damage of DP steels under varying strain rates. Flow curves of observed phase constituents at different strain rates were described by using a dislocation based theory and local chemical composition in combination with the Johnson-Cook (JC) hardening model. Furthermore, individual damage criteria based on the rate-dependent JC failure model were applied to describe the local crack mechanisms in DP microstructures. Calculated local stresses, strains and damage developments of deformed phases were studied. It was found that microstructure characteristics especially phase fraction differently affected the strain hardening and ductility of DP steels under low and high strain rate deformation. The damage initiation and propagation in microstructures of both steels at various strain rates predicted by RVE simulations were well correlated with the experimental results.

Micromechanical analysis of orientation dependency on deformation behavior in DP steels by dislocation density-based crystal plasticity simulation

Effect of the initial orientations of ferrite grains on the strain localization and consequently deformation behavior in DP780 steels was studied by employing experimental examination and numerical simulations. Based on dislocation density-based crystal plasticity finite element simulation, it was revealed that the initial orientation of ferrite grains plays a key role in the evolution of shear bands and consequently on the ductility of dual phase steels. In other words, the stored dislocation density within each ferrite grain and consequently the energy absorption capacity of DP steels depends on its initial orientation. Moreover, a qualitative comparison between simulated data and measured texture maps exposes the validity of the analysis.

Evaluation of using distribution functions for mean field modelling of multiphase steels

Abstract Selection and development of distribution functions (descriptors) for the complex phase steels is described in the paper. The objective was to design the multiphase microstructure with smooth gradients of microstructural features, which will prevent deterioration of local ductility of dual phase steels during stretch-forming process. Sensitivity analysis of descriptors with respect to microstructural parameters is described. Identification of descriptors based on experimental data was performed. The descriptors of complex phase microstructure were defined in the form of distribution functions.

Computational modeling of the effect of equiaxed heterogeneous microstructures on strength and ductility of dual phase steels

In this study, a code was developed to create virtual random representative volume elements (RVEs) depicting the actual and highly equiaxed heterogeneous microstructure of ferrite–martensite dual phase (DP) steels. Within this approach it was possible to perform a parametric study of the effects of DP microstructure (e.g., volume fraction, size, and distribution of the martensite; grain size and boundaries of the ferrite; martensite–ferrite interphase) and mechanical properties of the ferrite and martensite phases on the overall stress–strain behavior. A finite strain elastic–viscoplastic constitutive model has been used in conducting these microstructural-based simulations. It is shown that plastic strain localization in the form of localized narrow bands significantly control the ductility and ultimate fracture of DP steels. It was also noticed that by adding viscosity into the material property, the ductility increased significantly without compromising the strength of DP steels. It is shown that by decreasing the size of martensite phase and considering the ferrite–martensite interphase the overall response yields a simultaneous increase in strength and ductility.

Microstructural modeling of dual phase steel using a higher-order gradient plasticity–damage model

This paper focuses on the application of a higher-order gradient-dependent plasticity–damage model for microstructural modeling of dual-phase (DP) steels. Damage evolution is governed by the evolution of a nonlocal plasticity measure which is a function of the local equivalent plastic strain rate and its corresponding first-order gradient. Different two-dimensional representative volume elements of the DP microstructure are virtually generated by varying the martensite phase volume fraction, distribution, and size, and volume fraction and size of dispersed hard inclusions. It is shown that the employed modeling framework is capable of addressing three main issues that are not considered by the current studies on microstructural modeling of advanced high strength steels (AHSS); finite element mesh-dependency, size effects, and additional hardening due to plastic strain gradients. It is concluded that based on the employed mechanical properties of each phase in the DP steel, strength and ductility is governed by damage evolution and not necessarily by plastic strain localization alone. Therefore, it is shown that including nonlocal damage evolution is critical for accurate prediction of the strength and ductility of DP steels. It is also shown that dispersing 5% volume fraction of hard inclusions in the DP steel optimizes both strength and ductility such that a new generation of AHSS might be attained.

The influence of microstructure and composition on the plastic behaviour of dual-phase steels

This paper presents a systematic experimental study of the relative contributions of the martensite volume fraction, morphology and carbon content to the overall strength and ductility of dual-phase steels. To this end, model dual-phase steels combining three volume fractions of martensite, three martensitic carbon contents, and three arrangements of the phases were generated by suitable control of the heat treatments. The microstructures include long and short elongated, as well as equiaxed martensitic islands. The yield and tensile strengths increase with both the amount of martensite and its carbon content. The ductility in general increases with the volume fraction of martensite at constant martensitic carbon content, and conversely increases with the martensitic carbon content for a fixed volume fraction of martensite. Departure from the general trend is observed for combinations of a large volume fraction of martensite with a large martensitic carbon content, and is attributed to damage-induced softening or fracture. Specimens with long elongated martensite demonstrate an improved ductility compared to other microstructures. The plastic flow properties were further investigated by relying on a non-linear homogenization model which explicitly accounts for the microscopic parameters. The micromechanical model captures reasonably well the tensile behaviour for the complete range of martensite carbon contents and volume fractions.

Failure Mode and Ductility of Dual Phase Steel with Edge Crack

Dual phase steels having a microstructure consisting of a ferrite matrix, in which particles of martensite are dispersed, have received a great deal of attention due to their useful combination of high strength, high work hardening rate and ductility, all of which are favorable properties for forming processes. In the present work, various microstructure-level finite element models are generated based on the actual microstructure of DP590 steel, to capture the mechanical behavior and fracture mode. The failure mode of DP steels is predicted using the plastic strain localization theory, mainly resulting from the material microstructure-level inhomogeneity as well as the initial geometrical imperfection. Besides the simulation, tensile test specimens of dog bone type with different edge cracks were prepared on an internally designed blanking tool, and the corresponding deformation processes were recorded via digital image correlation system. It is found that the overall ductility of the DP590 steel strongly depends on the ductility of the ferrite matrix, and pre-existing edge cracks reduce the overall ductility of the steel and change the failure mode.

A Plasticity Induced Anisotropic Damage Model for Sheet Forming Processes

The global fuel crisis and increasing public safety concerns are driving the automotive industry to design high strength and low weight vehicles. The development of Dual Phase (DP) steels has been a big step forward in achieving this goal. DP steels are used in many automotive body-in-white structural components such as A and B pillar reinforcements, longitudinal members and crash structure parts. DP steels are also used in other industrial sectors such as precision tubes, train seats and Liquid Petroleum Gas (LPG) cylinders. Although the ductility of DP steel is higher than classical high strength steels, it is lower than that of classical deep drawing steels it has to replace. The low ductility of DP steels is attributed to damage development. Damage not only weakens the material but also reduces the ductility by formation of meso-cracks due to interacting micro defects. Damage in a material usually refers to presence of micro defects in the material. It is a known fact that plastic deformation induces damage in DP steels. Therefore damage development in these steels have to be included in the simulation of the forming process. In ductile metals, damage leads to crack initiation. A crack is anisotropic which makes damage anisotropic in nature. However, most researchers assume damage to be an isotropic phenomenon. For correct and accurate simulation results, damage shall be considered as anisotropic, especially if the results are used to determine the crack propagation direction. This paper presents an efficient plasticity induced anisotropic damage model to simulate complex failure mechanisms and accurately predict failure in macro-scale sheet forming processes. Anisotropy in damage can be categorized based on the cause which induces the anisotropy, i.e. the loading state and the material microstructure. According to the Load Induced Anisotropic Damage (LIAD) model, if the material is deformed in one direction then damage will be higher in this direction compared to the other two orthogonal directions, irrespective of the microstructure of the material. According to Material Induced Anisotropic Damage (MIAD) model, if there is an anisotropy in shape or distribution of the particles responsible for damage (hard second phase particles, inclusions or impurities) then the material will have different damage characteristics for different orientations in the sheet material. The LIAD part of the damage model is a modification of Lemaitre’s (ML) anisotropic damage model. Modifications are made for damage development under compression state and influence of strain rate on damage, and are presented in this paper. Viscoplastic regularization is used to avoid pathological mesh dependency. The MIAD part of the model is an extension of the LIAD model. Experimental evidence is given of the MIAD phenomenon in DP600 steel. The experimental analysis is carried out using tensile tests, optical strain measurement system (ARAMIS) and scanning electron microscopy. The extension to incorporate MIAD in the ML anisotropic damage model is presented in this paper as well. The paper concludes with a validation of the anisotropic damage model for different applications. The MIAD part of the model is validated by experimental cylindrical cup drawing wheras the LIAD part of the model is validated by the cross die drawing process.

On key factors influencing ductile fractures of dual phase (DP) steels

In this paper, we examine the key factors influencing ductile failure of various grades of dual phase (DP) steels using the microstructure-based modeling approach. Various microstructure-based finite element models are generated based on the actual microstructures of DP steels with different martensite volume fractions. These models are, then, used to investigate the influence of ductility of the constituent ferrite phase and also the influence of voids introduced in the ferrite phase on the overall ductility of DP steels. It is found that with volume fraction of martensite in the microstructure less than 15%, the overall ductility of the DP steels strongly depends on the ductility of the ferrite matrix, hence pre-existing micro-voids in the microstructure significantly reduce the overall ductility of the steel. When the volume fraction of martensite is above 15%, the pre-existing voids in the ferrite matrix does not significantly reduce the overall ductility of the DP steels, and the overall ductility is more influenced by the mechanical property disparity between the two phases. The applicability of the phase inhomogeneity driven ductile failure of DP steels is then discussed based on the obtained computational results for various grades of DP steels, and the experimentally obtained scanning electron microscopy (SEM) pictures of the corresponding grades of DP steels near fracture surface are used as evidence for result validations.

Influence of Martensite Mechanical Properties on Failure Mode and Ductility of Dual-Phase Steels

The effects of the mechanical properties of the martensite phase on the failure mode and ductility of dual-phase (DP) steels are investigated using a micromechanics-based finite element method. Actual microstructures of DP steels obtained from scanning electron microscopy (SEM) are used as representative volume elements (RVEs) in the finite element calculations. Ductile failure of the RVE is predicted as plastic strain localization during the deformation process. Systematic computations are conducted on the RVE to quantitatively evaluate the influence of the martensite mechanical properties and volume fraction on the macroscopic mechanical properties of DP steels. These properties include the ultimate tensile strength (UTS), ultimate ductility, and failure modes. The computational results show that, as the strength and volume fraction of the martensite phase increase, the UTS of DP steels increases, but the UTS strain and failure strain decrease. In addition, shear-dominant failure modes usually develop for DP steels with lower martensite strengths, whereas split failure modes typically develop for DP steels with higher martensite strengths. The methodology and data presented in this article can be used to tailor DP steel design for its intended purposes and desired properties.

Predicting failure modes and ductility of dual phase steels using plastic strain localization

Ductile failure of metals is often treated as the result of void nucleation, growth and coalescence. Various criteria have been proposed to capture this failure mechanism for various materials. In this study, ductile failure of dual phase steels is predicted in the form of plastic strain localization resulting from the incompatible deformation between the harder martensite phase and the softer ferrite matrix. Microstructure-level inhomogeneity serves as the initial imperfection triggering the instability in the form of plastic strain localization during the deformation process. Failure modes and ultimate ductility of two dual phase steels are analyzed using finite element analyses based on the actual steel microstructures. The plastic work hardening properties for the constituent phases are determined by the in-situ synchrotron-based high-energy X-ray diffraction technique. Under different loading conditions, different failure modes and ultimate ductility are predicted in the form of plastic strain localization. It is found that the local failure mode and ultimate ductility of dual phase steels are closely related to the stress state in the material. Under plane stress condition with free lateral boundary, one dominant shear band develops and leads to final failure of the material. However, if the lateral boundary is constrained, splitting failure perpendicular to the loading direction is predicted with much reduced ductility. On the other hand, under plane strain loading condition, commonly observed necking phenomenon is predicted which leads to the final failure of the material. These predictions are in reasonably good agreement with experimental observations.

The effect of microstructure and strain rate on the stage III strain hardening and ductility of dual-phase steels

The effects of microstructure and strain rate on stage III strain hardening (i.e. the strain hardening which occurs just prior to instability) and ductility in dual-phase steels were studied by tensile testing specimens produced by intercritically annealing 0.08C-1.45Mn-0.21Si steel. The steel was cold-rolled 50% and heat treated to obtain either normalized, martensitic or tempered martensitic microstructures prior to intercritical annealing at 717°C for 5 min and ice-water quenching. Some specimens were intercritically annealed directly after cold rolling. Stage III strain hardening was found to be a function of microstructure, with specimens produced from cold-rolled and normalized initial microstructures exhibiting higher strain hardening rates than those annealed with initially martensitic or tempered martensitic microstructures. The difference in strain hardening behavior was attributed to the effects of microstructure on the rate of dislocation substructure generation and consolidation. The strain dependence of strain hardening rates at high strain was shown to be an important factor in controlling the uniform strain. Three strain rates covering two orders of magnitude were used in strain rate jump tests to determine values of strain rate sensitivity. The strain rate sensitivity was 0.002 for all microstructures when tested at the lowest strain rates and 0.005 at the two highest rates. The strain rate dependence of the strain rate sensitivity was attributed to dynamic strain aging effects. Both uniform and post-uniform elongation increased with increasing strain rates. Low uniform elongations at low strain rates arose from lower strain hardening rates observed near instability. Post-uniform elongation increased because strain rate sensitivity increased with increasing strain rate.

A theoretical model for the flow behavior of commercial dual-phase steels containing metastable retained austenite: Part II. calculation of flow curves

The role of metastable retained austenite(γ R), its volume fraction, and mechanical stability on the flow characteristics of a dual phase steel containing 20 vol pct of ‘as quenched’ martensite in a ferrite matrix has been examined in this paper employing the flow curve expressions derived in Part I of this paper. It has been found that for a given volume fraction ofγ R, its mechanical stability plays a crucial role in enhancing the ductility. Whereas highly stableγ R does not contribute either to strength or ductility of the steel, highly unstableγ R which causes an increase in the strength is detrimental to ductility. Aγ R which is moderately stable and undergoesγ R → α′ transformation over a larger strain range is beneficial to enhanced ductility. Increasing amounts of moderately stableγ R significantly increase both the strength and the ductility of dual-phase steels through a sustained work-hardening due toγ R → α′ transformation. Load transfer which is determined by a parameterq has a significant contribution to work-hardening. A value of ∣|q∣|= 4500 MPa has been found to partition realistically the stress and strain in these steels.

Effect of retained austenite on the yielding and deformation behavior of a dual phase steel

Tensile tests were conducted on a vanadium containing dual phase steel at temperatures between −53 and + 187°C to determine the effect of retained austenite stability on tensile properties. The transformation of retained austenite to martensite with stress/strain was shown to be a contributing factor in the yielding and strain hardening behavior of the dual phase steel. Increasing the stability of the austenite, by increasing the test temperature, caused the expected shift in the austenite to martensite strain transformation to higher strains. This led to a lower initial strain hardening exponent which increased with strain, compared to the ambient and sub-zero temperature deformation where the initial strain hardening exponent was higher but decreased with strain. The former behavior, which accentuated the strain hardening ability at higher strains, led to an increase in the uniform and total elongations, suggesting that the ductility of dual phase steels can be further improved by optimizing the stability of the retained austenite.

A consideration of models describing the strength and ductility of dual-phase steels

A consideration of models describing the strength and ductility of dual-phase steels

Influence of martensite composition and content on the properties of dual phase steels

A study has been made of the mechanical properties of dual phase (martensite plus ferrite) structures produced when Fe-Mn-C alloys are quenched from the austenite plus ferrite phase field, so as to give a series of alloys with constant ferrite and martensite compositions but varying percent martensites. It is found that the strength of a dual phase structure is dependent on the ferrite grain size and the volume fraction of martensite, and is independent of the composition and strength of the martensite. In agreement with previous work the ductility of these steels is superior to that for standard HSLA steels at the same tensile strength. As shown in a previous paper the strength and ductility as a function of percent martensite are in agreement with Mileiko’s theory of composites of two ductile phases. This theory and the results indicate that the superior ductility of dual phase steels is largely a consequence of the high strength (fine grained), highly ductile (low interstitial content) ferrite matrix.