Strain localization and damage in dual phase steels investigated by coupled in-situ deformation experiments and crystal plasticity simulations

Effect of martensite morphology and volume fraction on strain hardening and fracture behavior of martensite–ferrite dual phase steel

Mesoscopic origin of damage nucleation in dual-phase steels

Influence of microstructure and pre-straining on the bake hardening response for ferrite-martensite dual-phase steels of different grades

Microtexture development during equibiaxial tensile deformation in monolithic and dual phase steels

Cold-rolled and annealed ferrite-martensite dual phase (DP) steel sheets are a useful material for automotive applications because of their excellent balance of strength and ductility. Although few research papers have examined the crystallography and microstructure of lath martensite in DP steel, the characteristics of lath martensite have been investigated in single-phase martensitic steel. In the present study, the crystallography and microstructure of lath martensite in a ferrite-martensite dual phase were studied using electron microscopy and electron diffraction analysis, and the crystallographic orientation relationship between the ferrite and martensite was analyzed. The main results are as follows: (1) The lath martensite in DP steel consists of some number of packets, and these packets consist of blocks. This structure is the same as the microstructure of single-phase lath martensite. (2) The habit planes of the martensite laths in a packet tend to be parallel to the close-packed plane of the adjacent ferrite grain, whose fraction is about 30%.

Ferritic–martensitic dual-phase (DP) steels are prominent and advanced high-strength steels (AHSS) broadly employed in automotive industries. Hence, extensive study is conducted regarding the relationship between the microstructure and mechanical properties of DP steels due to the high importance of DP steels in these industries. In this respect, this paper was aimed at reviewing the microstructural characteristics and strengthening mechanisms of DP steels. This review article represents that the main microstructural characteristics of DP steels include the ferrite grain size (FGS), martensite volume fraction (MVF), and martensite morphology (MM), which play a key role in the strengthening mechanisms and mechanical properties. In other words, these can act as strengthening factors, which were separately considered in this paper. Thus, the properties of DP steels are intensely governed by focusing on these characteristics (i.e., FGS, MVF, and MM). This review article addressed the improvement techniques of strengthening mechanisms and the effects of hardening factors on mechanical properties. The relevant techniques were also made up of several processing routes, e.g., thermal cycling, cold rolling, hot rolling, etc., that could make a great strength–ductility balance. Lastly, this review paper could provide substantial assistance to researchers and automotive engineers for DP steel manufacturing with excellent properties. Hence, researchers and automotive engineers are also able to design automobiles using DP steels that possess the lowest fuel consumption and prevent accidents that result from premature mechanical failures.

Reliability/unreliability of mixture rule in a low alloy ferrite–martensite dual phase steel

The synergistic effects of ultrafine grains and nano-size Cu-rich precipitates on the mechanical properties of DP steels

Weight minimization triggered the automotive industry to introduce new advanced high strength steels that show ductile fracture by microvoid evolution under deformation, resulting in unexpected failure without significant necking. Therefore, an extensive study was initiated to gain insight on the formation and development of microstructural damage in dual phase (DP) steel from a mechanical point of view.

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.

In this research, the effect of microstructural features on damage behavior is studied with a special emphasis on evolution of plastic deformation heterogeneity in microscale. This is carried out by performing experimental and numerical work on a ferritic-martensitic DP600 steel sheet from the AHSS family. The experimental investigations are focused on two aspects: (i) evolution of plastic deformation heterogeneity and lattice orientations in the grain scale and (ii) active damage mechanisms and distribution and evolution of damage during deformation. The former is carried out by electron backscatter diffraction (EBSD) measurements. The investigations revealed that before the deformation small orientation variations are present and hence there are geometrically necessary dislocations (GND) at the ferrite grain boundaries. With increasing deformation, lattice orientation spread of the grains increases due to heterogeneity in plastic deformation and therefore the GND content also increases. The density of GNDs tend to be higher around grain boundaries, but they also form highly non-uniform distributions inside the ferrite grains. Moreover, investigations around the voids showed high GND content mostly on one side of the voids. For the latter aspect metallographic cross sections are investigated and image analysis are performed. Two main mechanisms are identified for void formation: (a) by cracking of martensite and (b) decohesion at the ferrite grain boundaries. Voids formed by cracking of martensite grow perpendicular to the loading direction after complete failure of the martensite island. Therefore, these voids have a thickness of the martensite island from which they formed within. The voids that are formed at the boundaries stay thin and mostly grow along the loading direction, causing a higher length to thickness ratio. Image analysis of the spatial distribution of the voids revealed a non-uniform distribution along the thickness of the sheet which is correlated with the distribution of martensite bands. Accordingly, in order to capture the effect of lattice orientations and plastic strain heterogeneity, a strain gradient enhanced rate-independent crystal plasticity framework is implemented as a user subroutine for the commercial finite element package Abaqus/Standard. In this generic framework, a lower order approach is employed for the gradient enhancement. It was shown that the model is capable of predicting both structural and microstructural size effects. The model also showed that the local stress and strain states deviated from the macroscopically prescribed conditions on the RVE. Numerical investigations of DP microstructure are focused on the effect of martensite distribution on (i) the stress and strain partitioning, (ii) the response of damage indicators and (iii) the evolution of pre-existing damage incidents. Firstly, it was shown that the average stress and strain of ferrite and martensite are affected significantly by the distribution and content of the phases. In particular, the martensite percentage affects the stress and strain partitioning due to the hardness contrast as well as the induced strain gradients within ferrite. On the other hand, the effect of martensite distribution is more pronounced on the initial hardening rate of the RVE. Secondly, after comparison of locations of experimentally observed damage incidents and numerical ones, it was seen that a plastic work based damage indicator is suitable for predicting damage locations and damage susceptibility of the microstructure. It was found that an increase in martensite content enhances the possibility of damage in both ferrite and martensite and a banded morphology triggers more damage in the martensite phase. Finally, it was found that evolution characteristics of voids are highly dependent on their locations and surroundings in other words the local stress and strain state. Moreover, the evolution does not follow a monotonic trend. Interestingly, in the simulations during tensile stretch some pre-existing voids shrink, which is an indicator of locally compression or shear type of loading. This also shows the variation of local state from the average prescribed condition. In general, a higher void growth rate is observed for the voids located in between individual martensite islands for the other voids.

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.

Micromechanical analysis of martensite distribution on strain localization in dual phase steels by scanning electron microscopy and crystal plasticity simulation

A dislocation model, accurately describing the uniaxial plastic stress-strain behavior of dual phase (DP) steels, is proposed and the impact of martensite content and ferrite grain size in four commercially produced DP steels is analyzed. It is assumed that the plastic deformation process is localized to the ferrite. This is taken into account by introducing a nonhomogeneity parameter, f(ε), that specifies the volume fraction of ferrite taking active part in the plastic deformation process. It is found that the larger the martensite content the smaller the initial volume fraction of active ferrite which yields a higher initial deformation hardening rate. This explains the high energy absorbing capacity of DP steels with high volume fractions of martensite. Further, the effect of ferrite grain size strengthening in DP steels is important. The flow stress grain size sensitivity for DP steels is observed to be 7 times larger than that for single phase ferrite.

The main aim of the research work is to examine the fracture toughness of the dual phase steels prepared using the intermediate quenching method. The ferrite-martensite dual phase (DP) steel is produced using low carbon micro alloyed steel by the heat treatment and intercritical quenching technique using various intercritical temperatures such as 740,760,780,800 and 820oC. The samples of the produced dual phase steel are analyzed for their microstructure using optical microscope. The fracture toughness investigations for the dual phase steel have been carried out using ASTM standard testing procedure. From the results it is observed that the fine distribution and equal volume fraction of ferrite and martensite phases at 780oC. The effect of which, the A780 steel has demonstrated excellent fracture toughness which is the result of intermediate quenching technique. The fractography analysis it is clear that the ductile initiated brittle fracture is occurred which is due to the increased hard martensite phase and the increment in the stress accumulation at the ferrite which also leads to the higher elongation.