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The influence of the friction stir process (FSP) on the hole expansion formability of dual-phase 600 steel was examined by experimental and finite element analyses. Due to the microstructure of refining ferrite, martensite, and bainite, the yield strength and tensile strength increased after FSP by 95% and 63%, respectively. FSP led to the formation of (101) || ND and (111) || RD texture components in the processed zone. The observation of (111) || RD texture components in the FSPed steel resulted in an increment in the plastic strain ratio from 0.86 to 0.98. FSP provided a 20% improvement in the hole expansion ratio. The microstructural refinement and crystallographic texture played an important role in this increase. The FSP contributed to the increase in the hole expansion formability capability by delaying the crack formation during hole expansion and by causing the hole to fail at a higher load. It has been observed by experimental and finite element (FE) simulation results that the highest circumferential stress occurs at the hole edge where the main crack is formed. The FE simulation predictions agree with less than 2% error with the experimental observations. [ABSTRACT FROM AUTHOR]

The main purpose of this study is to exhibit failure prediction capability of polynomial-based yield functions with a basic damage model. For this purpose, a constitutive model considering anisotropic plasticity and ductile fracture was developed. In this model, anisotropic plastic behavior of dual phase steels, namely DP600 and DP800, was described by quadratic Hill48 and non-quadratic anisotropic homogeneous the fourth-order polynomial (HomPol4) stress potentials and the generalized plastic work criterion from ductile damage models was used for the prediction of fracture initiation. The model has been implemented into an implicit finite element (FE) code. The parameters of the constitutive model were calibrated with uniaxial tensile tests performed in different directions with respect to the rolling direction of the materials and anisotropic stress potentials were evaluated by comparison of the predicted in-plane variations of the plastic properties (yield stress ratios and Lankford coefficients), and yield locus contours with experimental data. The calibrated model was firstly applied to uniaxial tensile test and then to a hole expansion test to predict fracture. The stroke values at fracture, hole expansion ratios (HER) and fracture locations were investigated. Any significant difference between the anisotropic stress potentials was not observed in terms of HER predictions, however plastic work criterion in conjunction with HomPol4 function predicted the crack initiation locations accurately on the fractured samples. Afterward, the Lode parameter and stress triaxiality effects were investigated in fracture stroke prediction. Since the HomPol4 predictions of fracture initiation locations are accurate, the predicted HomPol4 results from the generalized plastic work criterion were compared with the modified Mohr-Coulomb ductile fracture model results. A significant improvement was observed in the fracture displacement predictions. However, it is seen that the failure location predictions of both models were the same. From these results, it can be concluded that the HomPol4 yield criterion has an effective potential to predict the failure locations even though with a basic damage model. In the current study, the out-of-plane anisotropy effect was assessed as well. To this end, Hill48's parameter correlated with the out-of-plane shear components were adjusted. It was found that the out-of-plane anisotropy has a negligible effect on the predictions of HER and fracture initiation location. [ABSTRACT FROM AUTHOR]

JSW Steel has been producing a number of high strength dual phase steels for automotive applications. These steels are processed through the state-of-the-art continuous annealing lines. The cold rolled steel sheets are passed through different zones including preheating, heating, soaking, slow cooling, rapid cooling, overaging and final cooling in a very short time. The strengthening mechanisms in these processes such as phase transformation, precipitation etc. are a strong function of line speed and strip temperatures. Minor change in any of these parameters causes significant variations in mechanical properties. Optimization of these process parameters becomes challenging due to the associated costs involved in plant scale experimentation. The same can be effectively carried out on a lab scale using thermo-mechanical simulator "Gleeble". Processing routes for the different annealing cycles can be converted into time-temperature plots and programmed in Gleeble for lab scale continuous annealing line (CAL) simulations. In the present work, an offline simulation methodology was established for CAL process through Gleeble. The mechanical properties in Gleeble simulated samples were found to be in close agreement with the actual plant trials. Furthermore, the established methodology was successfully utilized to investigate the effects of different CAL parameters on the mechanical properties of a high strength dual phase steel grade. Consequently, on the basis of physical simulations, the optimum processing conditions were identified to achieve desired mechanical properties during plant processing. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the effect of a post-weld intercritical annealing (PWIA) on the microstructure and tensile behavior of a friction stir (FS)-welded DP600 steel. Results showed that in the FS-welded sample, tempering of the pre-existing martensite in the sub-critical heat-affected zone (HAZ) and formation of ferrite and pearlite in the fine-grained HAZ led to the formation of a softened zone and a significant decrease in the ultimate tensile strength (UTS) with a joint efficiency of 87%. After PWIA, a dual phase microstructure was developed throughout the weldment, which resulted in the elimination of the softened zone and full recovery of the UTS. Measurements by digital image correlation revealed that localization of strain in the softened zones after yielding resulted in the early fracture. However, the strain localization was significantly reduced after PWIA. All samples exhibited features of ductile fracture mode during tensile testing. Ferrite-martensite interface decohesion was the main void nucleation mechanism in the DP600 steel and PWIA sample, while in the FSW sample a fraction of voids were also formed as a result of martensite necking. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the micro-texture and plastic anisotropy of a dual phase steel. A DP800 steel was produced by cold rolling and intercritical annealing of a low carbon steel. Micro-texture analysis on the DP800 steel by means of electron backscatter diffraction revealed that a high orientation density exists along the α and γ fibers. However, the overall texture intensity was not significant. Tensile testing of the DP800 steel at different angles to the rolling direction demonstrated a very low dependency of tensile properties and work hardening behavior to the loading direction. Nevertheless, the r-value showed some directionality with a mean value of 0.851, suggesting a substantial anisotropy for the studied DP800 steel. [ABSTRACT FROM AUTHOR]

In this work, the deformation behavior of fine grained dual phase steel with intragranular martensite distribution as thin boundaries is studied via in-situ EBSD investigation and full field fast Fourier transform based crystal plasticity simulation. The effect of grain size and intragranular martensite fraction on the crystal plasticity of grains is examined by analyzing the geometrically necessary dislocation (GND) density evolution and reorientation magnitude. The size of the grains/subgrains is found to profoundly govern the rotation magnitude and development of GNDs by controlling the effect of intragranular and grain boundary martensite on the stress/strain state of grains during deformation. It is observed that for comparable grain sizes and similar initial orientation, higher intragranular martensite fraction leads to higher initial GND density, lesser lattice rotation, and higher development of GND density under deformation. Alternatively, among grains of similar initial orientation and martensite content, coarse grain evolves with higher GND density and rotation magnitude under deformation than the finer ones. However, the initial GND density is higher in fine grains. The stress-strain development obtained from crystal plasticity simulation corroborates the above observations and demonstrates a strong dependence on the intragranular martensite. On the whole, the combined investigation of experimental and simulation results suggests that intragranular martensite fraction and ferrite grain size are the two major, interdependent parameters that govern the crystal plasticity behavior of dual phase steel studied with martensite morphology as thin boundaries. [ABSTRACT FROM AUTHOR]

Servo press forming machines are advanced forming systems that are capable of imparting interrupted punch motion, resulting in enhanced room temperature formability. The exact mechanism of the formability improvement is not yet established. The contribution of interrupted motion in the ductility improvement has been studied through stress relaxation phenomena in uniaxial tensile (UT) tests. However, the reason for improved formability observed when employing servo press is complicated due to the additional contribution from frictional effects. In the present work, an attempt is made to decouple the friction effect on formability improvement numerically. The improved formability is studied using a hole expansion test (HET). The limit of forming during hole expansion is modeled using the Hosford-Coulomb (HC) damage criteria, which is implemented as a user subroutine in a commercial explicit finite element (FE) software. Only the contribution of stress relaxation is accounted for in the evolution of the damage variable during interrupted loading. Therefore, the difference between simulation and experimental hole expansion ratio (HER) can be used to decouple the friction effect from the overall formability improvement during hole expansion. The improvement in HER due to stress relaxation and friction effect is different. The study showed that the model effectively captures the hole expansion deformation process in both monotonic and interrupted loading conditions. Compared to stress relaxation, friction effect played a major role during interrupted HET. [ABSTRACT FROM AUTHOR]

The mechanical response of dual phase (DP) steel exhibits a complex dependence on the microstructure. The chemical composition and microstructure characteristics of the phases have significant effects on the contrast between the response of the phases, which affects, not only the strength and ductility, but also the anisotropic response of DP steel under strain path changes. In this work, extended dislocation-based models of the ferrite and martensite phases of DP steel are proposed and used in a finite element based representative volume element approach to account for the contrast between the local response of the phases. The flow stress of each phase is computed as a function of the amount of substitutional and interstitial solute elements and the microstructural characteristics of the phase. Particular attention is paid to the phase model of the martensite phase. The model parameter controlling the storage of dislocations is related to the carbon content, which appears to be the most important parameter affecting the strength of martensite and its contrast with the local response of the ferrite phase. The model predicts a significant effect of the contrast between the local responses of the phases and the microstructure characteristics of each phase on the yield locus after prestraining and on the stress–strain behaviour after strain path change, i.e., forward-reverse shear loading and cyclic uniaxial tension–compression loading. [ABSTRACT FROM AUTHOR]

The present investigation is aimed at revealing the effect of martensite content and impingement angle on the erosive wear of dual phase (DP) steels developed via intercritical annealing (ICA) heat treatment by conducting erosion tests using air jet erosion tester at different angles of impact at a constant velocity of 90 m/s, using alumina (Al2O3) as the erodent. DP steels having four different martensite volume fractions (MVF) 0.39, 0.50, 0.65 and 0.79 were developed by holding normalized steel for different intercritical holding times, and their erosion behavior was examined at 15, 45, 70, and 90° angles of impingement. Normalized (N) steel was used as reference material for comparative purpose. The results indicate that steady state erosion rate decreases with increasing angle of impact for N steel as well as DP steels. However, DP steels showed a significantly reduced erosion rate in comparison with N steel reflecting the role of microstructure and consequent increase in hardness. Steady state erosion rate also decreased with increasing martensite content from 39 to 79%, and DP4 steel containing 79% martensite showed the best performance in terms of erosion resistance. The mechanism of material removal changed from micro-ploughing to crater formation with the increase in the angle of impingement. Micro-ploughing, lip formation, and embedded chips were identified as the operative erosion mechanisms for N steel, whereas ploughing, craters and cracks formation were the operative erosion mechanisms for DP steels. [ABSTRACT FROM AUTHOR]

The present study aims at to study the effect of chemical composition, intercritical annealing parameters (temperature and holding time) on microstructure and mechanical properties of dual phase steels. Three normalized steels (A, B and C) with different compositions were subjected to inter-critical annealing at 760°C and 810°C for different holding times (3, 6 and 9 min). The microstructural characterization was carried out using a scanning electron microscope (SEM) and mechanical properties were evaluated using micro-hardness measurements and tensile tests. The strength has been found to increase from 500 MPa (steel A) to >1200 MPa (steel C) at the cost of a decrease in elongation from 25% (steel A) to 7.5% (steel C). Results have been analysed and discussed. [ABSTRACT FROM AUTHOR]

Dual phase steel is usually produced through inter-critical annealing followed by rapid quenching, but this study examines the repetitive corrugation and straightening (RCS) technique as a potential alternative. Dual phase steel was repeatedly corrugated and straightened using a semi-circular die to observe the effect of severe plastic deformation on martensitic transformation and mechanical properties. Using scanning electron microscopy and X-ray analysis, the martensitic phases of the samples were quantified after each RCS cycle. In addition, the crystallite sizes, lattice strains and dislocation densities in the steel were measured using XRD techniques, while the changes in hardness and yield strength were determined using the Vickers hardness test and the tensile test, respectively. An effective strain of 10.5 is introduced by twelve numbers of RCS cycles, and this strain has led to martensitic phase transformation which is revealed through EBSD analysis. The steel's yield strength, hardness, and wear characteristics have been greatly altered as a result. By the end of twelve cycles, the density of dislocations had grown to an order of 1014, and dislocation character found changing from screw into edge with plastic strain. Steel's yield strength has increased from 380 MPa to 720 MPa, while its hardness has increased from 150 HV30 to 280 HV30. [ABSTRACT FROM AUTHOR]

It was observed that the laser welding at low speed or medium speed did not deteriorate the fatigue property significantly, with a drop of about 9–12% in fatigue limit, compared to the parent material. Further, at a high stress amplitude, all samples survived almost similar numbers of fatigue cycles because of similar type of cyclic hardening–softening effect. However, at low stress amplitude, the cyclic softening of martensite was dependent on the magnitude of residual stress. For the joints fabricated at higher welding speed of 0.1 m/s, a decline of 28% in fatigue limit was observed in comparison with that of the parent material. Additionally, faster crack propagation was visible for high speed laser welded sample at both high and low stress amplitudes. It was proposed that the presence of detrimental {001} cleavage plane and high residual stress might have restricted the cyclic softening phenomena for any stress amplitude. [ABSTRACT FROM AUTHOR]

The numerical study on the complex failure mechanism in dual phase steels is significant because of their intricate microstructure. Their mechanical behavior depends on the volume fraction of martensite/ferrite and microstructural morphology, such as size, aspect ratio, interconnectivity, and mechanical behavior of individual phases. Like all multiphase materials, the connectivity between different phases and possible changes in mechanical properties near the interface (interphases) play significant roles in microstructural damage initiation and propagation and, therefore, affect the overall mechanical response. This study focused on the stress–strain behavior of DP-800 steel using finite element analysis of three-dimensional representative volume elements. Based on experimental results, local hardening was considered in the ferrite matrix near the martensite islands. Also, various volume fractions and material properties for different hardening zones were implemented to study the stress–strain behavior of DP steels. Interface elements based on cohesive zone modeling were considered to simulate the damage on the ferrite and martensite interface numerically. Furthermore, hydrostatic stress distribution which is known as an indicator to show the void initiation was used to predict damage zones and the stress distribution is compared with the results obtained from the cohesive zone method. The mechanical behaviors of the finite element models by the proposed methodology, considering cohesive elements and hardening zones in three-dimensional representative volume elements, indicated a better adaptation to the experimental results. [ABSTRACT FROM AUTHOR]

The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated in this paper. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The results showed that the ferrite and bainite can be refined and the volume fraction of bainite can be slightly increased by 5% by decreasing the ferrite transformation temperature from 690 to 635 °C and increasing the bainite transformation temperature from 400 to 450 °C. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7% with the ferrite transformation temperature decreasing. The cracks or voids were smaller in size and also the number in steel with the lower ferrite transformation temperature after expanding. The microstructure separation along the boundary between bainite and ferrite and the slip band in the ferrite appeared at the non-uniform deformation stage. However, fewer appeared in the microstructure of steel with lower ferrite transformation temperature. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio. The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7%. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio. [ABSTRACT FROM AUTHOR]

The application of dual phase steel in the automotive sector is increasing due to its high strength-to-weight ratio and good shock absorption capacity. The formability of dual phase steel is complex due to its multiphase microstructure which results in inaccurate formability prediction using numerical methods. The present study is aimed at analyzing the forming behavior and microstructure evolution of a dual phase steel, DP780, at different strain paths. The forming behavior of this steel was analyzed in terms of forming limit diagram (FLD) using Nakazima tests and finite element method. Various necking criteria were used to construct the FLD. Samples deformed at different dome height for three principal strain paths and strain levels were subjected to detailed microstructure and texture investigations. The microstructure and texture study was mainly focused on analyzing the development of deformation texture, misorientation development and orientation hardness. These studies showed that the formability of DP780 steel is largely affected by the development of γ fiber texture. Also, the evolution of other microstructure and texture quantifiers such as texture intensity, misorientation and relative orientation hardness at different strain paths was correlated with the formability of the material. [ABSTRACT FROM AUTHOR]

The goal of this study was to acquire mechanical properties of a dual-phase grade advanced high strength steel (AHSS) by means of different microindentation loading conditions. Conventional, cyclic, and multi-step indentations were performed on DP 800 sample; and Young’s modulus, hardness values were obtained by the depth-sensing indentation technique. Effects of different load levels (50–300 mN) and type of indentation on the results were also analyzed through ANOVA. The load levels experimented yielded overall material response rather than its constituents (e.g., martensite and ferrite). Both hardness and Young’s modulus tend to decrease with increasing maximum indentation load level, especially from 50 to 100 mN, which is regarded as indentation size effect. [ABSTRACT FROM AUTHOR]

Deformation band localization modes, uniform tensile strength, and uniform elongation of Ferrite-Martensite Dual-Phase (DP) steels are analyzed by finite element (FE) study. Treating the microstructure inhomogeneity as the sole cause of imperfection, failure initiation is predicted as the natural fallout of plastic instability caused by load drop because of localized plastic strain in the Representative Volume Element (RVE) during straining. Strain partitioning between two phases (ferrite matrix and martensite island) are investigated on RVEs, and it reveals that the increase of martensite yield stress decreases the plastic deformation and increases the stress state in martensite. Whereas, a decrease in martensite island volume fraction (Vm) results in the reduction of plastic deformation and stress state in the island. Studies are then carried out to investigate the effects of the ferrite-martensite flow properties and martensite volume fraction on the macroscopic tensile deformation behavior and band localization of DP steels. Micromechanical based FE simulation results emphasize that an increase in initial yield strength and volume fraction of martensite increases the ultimate tensile stress (UTS) with the decrease in uniform elongation. Similarly, as the hardening rate of ferrite increases, it increases the ultimate tensile stress (UTS) and uniform elongation. Additionally, deformation band localization modes alter from inclined to perpendicular to the loading axis with an increase in martensite volume fraction and initial yield strength of martensite. The knowledge of this work can be used to design DP steels with desired mechanical properties. [ABSTRACT FROM AUTHOR]

Abstract In the presented investigation, cold-rolled sheets of a selected dual-phase (DP) steel were heat-treated according to varying thermal profiles, thus reproducing continuous annealing process. Initially the samples were soaked at 780 and 810 °C for 0–60 s followed by water cooling. Next, samples were preliminary treated by applying the same conditions, however after water cooling these compositions were subject to tempering at 230, 380 and 460 °C for both 60 and 240 s. The characterization of the effect of heat-treatment parameters on the mechanical properties and structure is the main objective of this investigation. Mechanical properties of the samples after applied thermal profiles were in line with those requirements imposed on the commercial sheets of DP steels. The obtained results of the investigation showed that tempering deteriorates the Yield Ratio, defined as R p0.2 /R m. This was caused by the martensite decomposition combined with carbide precipitation processes. Transmission electron microscopy observations revealed precipitated carbides, Fe 3 C in martensitic and M 7 C 3 in ferritic areas. The quantitative results of the structural investigation were then applied to predict the R p0.2 and R m using the Perlade model. The results indicate that discrepancies between the measured tensile tests and calculated R p0.2 and R m do not exceed 10%. [ABSTRACT FROM AUTHOR]

Monotonic deformation behavior of ferrite-martensite dual phase steels with martensite volume of 13-43% have been analyzed in the current investigation using micromechanics based finite element simulation on representative volume elements. The effects of martensite volume fraction on the strain partitioning behavior between soft ferrite matrix and hard martensite islands in dual phase steels during tensile deformation have been investigated. As a consequence of strain incompatibility between hard martensite and soft ferrite phases, inhomogeneous deformation and finally deformation localization occur during tensile deformation. Restricted local deformation in ferrite phase caused by the adjacent martensite islands triggers the local stress triaxiality development. As the martensite volume fraction increases, the local deformation restrictions in ferrite phase also increases and which results in higher stress triaxiality development. Similarly the strain partitioning behavior between ferrite matrix and martensite island is also influenced by the volume fraction of martensite. The strain partitioning coefficient increases with increasing martensite volume fraction. [ABSTRACT FROM AUTHOR]

The effect of dew point on the selective oxidation behavior, microstructure and mechanical properties of a high-Al low-Si dual phase steel is elucidated. The results showed that as dew point is increased from-30°C to +5°C, internal oxidation of alloying elements occurs, and consequently the mass of external oxides decreases, which improves the galvanisability of steel sheets. It was also observed that the dew point had an impact on the microstructure and mechanical properties of the experimental steel. With the increase of dew point, the decarburization layer on the surface was increased, leading to inferior mechanical properties. Compared with the industrial steel, the high-Al-low-Si DP steel exhibited excellent galvanisability under identical annealing conditions. [ABSTRACT FROM AUTHOR]

In the present work, tensile behavior of tailor rolled blanks (TRBs) made of DP590 was studied. The TRBs were produced by variable gauge rolling. The tensile specimens of TRB had thickness ratios (Tr) of 0.47, 0.52, 0.68, 0.78 and 0.89, whereas the lengths of thickness transition zone (TTZ) were 58, 75 and 110 mm. The results of tensile test along with SEM and EBSD observations revealed that the total elongation of TRBs is strongly dependent on the TTZ geometry. Accordingly, it was found out that the total elongation increases with increasing Tr. However, unprecedented results were obtained for variations of total percent elongation versus the length of TTZ. In case of tensile specimens with identical gauge length, the total elongation decreases with increasing the length of TTZ. To justify the results, a novel geometrical parameter called effective length ratio (ELR) was introduced. This new parameter showed a meaningful relation with the total elongation. Accordingly, the effects of TTZ position and geometry on the total elongation were studied utilizing ELR parameter. It was concluded that the total elongation increases with increasing the ELR parameter. Furthermore, a linear correlation between ELR and total elongation was established. This can be used to interpolate the tensile behavior of TRBs for any intermediate conditions which can be more applicable for prediction of tensile behavior without performing any tests. Finally, the influence of thickness ratio on the probability of fracture from a certain region was discussed. [ABSTRACT FROM AUTHOR]

In the present study, tensile properties, strain hardening and fracture behavior of dual-phase (DP) steels were correlated with martensite volume fraction ( V ). A series of DP steels with different amounts of V (28-50 %) were produced by cold rolling and subsequent intercritical annealing of a ferrite-pearlite starting structure. Hardness and tensile tests results of DP steels showed that variation of hardness, uniform elongation and total elongation with V was linear and obeyed the rule of mixtures, whereas yield strength and ultimate tensile strength exhibited a nonlinear variation with V . Analysis of strain hardening behavior of DP steels by the Hollomon analysis showed two stages of strain hardening corresponding to ferrite deformation and co-deformation of ferrite and martensite, respectively. The strain hardening exponent of first stage ( n ) increased with increasing V , while the strain hardening exponent of second stage ( n ) as well as transition strain between the deformation stages decreased. [ABSTRACT FROM AUTHOR]

Dual phase steels are advanced high strength alloys typically used for structural parts and reinforcements in car bodies. Their good combination of strength and ductility and their lean composition render them an economically competitive option for realizing multiple lightweight design options in automotive engineering. The mechanical response of dual phase steels is the result of the strain and stress partitioning among the ferritic and martensitic phases and the individual crystallographic grains and subgrains of these phases. Therefore, understanding how these microstructural features influence the global and local mechanical properties is of utmost importance for the design of improved dual phase steel grades. While multiple corresponding simulation studies have been dedicated to the investigation of dual phase steel micromechanics, numerical tools and experiment techniques for characterizing and simulating real 3D microstructures of such complex materials have been emerged only recently. Here we present a crystal plasticity simulation study based on a 3D dual phase microstructure which is obtained by EBdD tomography, also referred to as 3D EBdD (EBdD-electron backscatter diffraction). In the present case we utilized a 3D EBdD serial sectioning approach based on mechanical polishing. Moreover, sections of the 3D microstructure are used as 2D models to study the effect of this simplification on the stress and strain distribution. The simulations are conducted using a phenomenological crystal plasticity model and a spectral method approach implemented in the Düsseldorf Advanced Material Simulation Kit (DAMAdK). [ABSTRACT FROM AUTHOR]

Friction stir lap welding of dual phase 590 steel has been successfully performed within the A to A temperature range upon adjusting the process parameters. The microstructures and the texture have been characterized using scanning electron microscopy and electron back scattered diffraction analysis. The martensite volume fraction increases with an increasing travel speed from 100 to 300 mm/min for a constant tool rotational speed of 300 rpm. Under severe deformation and high strain rate during friction stir welding, the grain orientation 〈101〉 shifts toward the 〈111〉 and 〈001〉 directions compared to the base metal. The base metal shows γ fiber components, whereas the friction stir welded specimen shows strong brass with weak cube fibers. [ABSTRACT FROM AUTHOR]

In this paper, a cellular automata method based model is proposed for simulating phase transformation kinetics of inter-critical heating of dual phase (DP) steel. This developed model deals with the kinetics of pearlite dissolution, ferrite transformation and austenite grain growth based on carbon diffusion process. Diffusion equation is discretized and solved by finite difference method whereas austenite grain growth is controlled by transition rules applied in cellular automata algorithm. The model is operated in the temperature range of 730-890 °C for four different specimens of DP steel. This model predicts appropriately the microstructure and volume fraction of formed austenite during inter-critical heating of DP steel. In addition, this study shows that the presence of carbon and alloying elements enhances carbon equivalent of DP steel, helps in austenite formation. [ABSTRACT FROM AUTHOR]

The mechanical properties of dual Phase steels (DP steels) are strictly related to the spatial distribution and the geometry of the two phases composing the steel, ferrite and martensite. Due to the high costs to obtain images of sections of steel samples, one important industrial problem is the reduction of the number of 2D sections needed to build and simulate a geometric model which may reproduce in a realistic way the 3D geometry of the material. In this context, the availability of suitable techniques of parameter estimation or identification is fundamental to solve the problem. In this work we present a germ-grain model which approximates the main geometric characteristics of the martensite, taking into account the inhomogeneities of the material. The parameters of the model are estimated on the basis of the morphological characteristics of the images of about 150 tomographic sections of a real sample, quantified by the Minkowski functionals. Here we replace the Mahalanobis distance, introduced in previous literature, with the $\mathcal{N}$ -distance, which provides computational advantages. In order to test if the estimated model is reproducing the distribution of the Minkowski functionals of the real material, both confidence bands from the simulated model are computed and compared with the real data and techniques for the detection of functional outliers are applied to quantify the accuracy of fit of the estimated model. [ABSTRACT FROM AUTHOR]

Single Point Incremental Forming (SPIF) is a promising manufacturing technology concerning the production of customized products, low batches or prototyping of ready-to-use parts, given its easy implementation and absence of dedicated tooling. The range of application is wide, covering many materials and virtually unlimited geometries. Indeed, nowadays' process downsides are more related to high forming times and dimensional inaccuracy. There are many processing parameters that can be optimized to circumvent such problems. In this work, focus is given on the effect of increasing tool feed rate. To this end, a dedicated SPIF machine is employed. After forming steel and aluminum sheets, parameters like forming force, maximum wall angle and formability are assessed for a range of velocities from 1500 to 12,000 mm/min. Parameters like step down or tool diameters are kept constant for a clear comparison. It will be shown how the process can be fastened up without seriously compromising its feasibility. [ABSTRACT FROM AUTHOR]

Multi-pulse tempering (MPT) on the resistance spot welding (RSW) is a considerable method to improve the quality of spot weld joints. For its uncertain tempering temperature and large temperature fluctuation, some stable sequential coupling finite element (FE) models for RSW and MPT of DP590 dual phase steel are developed to control the tempering temperature and further determine the experimental parameters. The nugget shape and size of simulation are verified by experimental observation, and the effects of MPT on microstructure, microhardness, and mechanical property during tensile-shear loading condition are experimentally investigated. The results show that MPT and its tempering temperature have no effect on the tensile-shear failure mode and have slight impact on nugget size and tensile-shear peak load. Besides, MPT leads to lower nugget hardness and better mechanical properties of spot weld joints than that of ordinary RSW due to the formation of a tempered martensite, and the higher tempering temperature expands these effects of MPT. However, the extent of the impact by MPT temperature on microstructure and mechanical property of spot weld joints is inferior to that of the impact by the MPT process itself. [ABSTRACT FROM AUTHOR]

A plate of dual phase steel was produced from low carbon steel with intercritical annealing treatment. Its optically determined surface microstructure was utilized to construct three different microstructural models. To describe the ductile damage in the ferritic matrix, the Gurson-Tvergaard-Needleman model was used with the failure in the martensite phase being ignored. The numerical results obtained for the mechanism of void initiation and coalescence were compared with the experimental observations. The numerical results obtained from the randomly extruded 3D model showed a significantly better agreement with the experimental ones than those obtained from the 2D model or the uniformly extruded 3D model. [ABSTRACT FROM AUTHOR]

The blanking/trimming/cropping process introduces a substantial plastic deformation to the sheet metal and causes premature edge fracture during the subsequent forming process. In an attempt to understand how the blanking process affects edge fracture, an experimental and numerical study was undertaken on the plane-strain blanking process. Blanking tests on a DP780 steel sheet were carried out on a special fixture utilizing a in-situ microscope for the digital mage correlation (DIC) deformation measurement. The DIC method provides a detailed deformation field of the specimen that has not been reported in any other publications before. Interrupted tests were carried out to study the crack formation and propagation during the blanking procedure, while scanning electron microscope was applied to examine the blanked surface quality as well as the edge profile after test. Following the experimental study, a detailed finite element model with mesh size of 0.01 mm in the critical region was established for the numerical investigation. The model features (a) a non-associated Hill 48 flow rule, (b) von-Mises yield condition and (c) modified Mohr-Coulomb fracture model. With material parameters calibrated from the in-plane tests as well as accurate boundary conditions measured in actual tests, the finite element model accurately predicted the blanking process quantitatively. The current study also gave quantitative values of the parameters of interest during the blanking test, such as the global load displacement response and the local strain gradient history. The geometrical features of the blanked edge, i.e. the amount of roll over, the extent of the burnished zone and fracture zone were all accurately predicted by the present simulation. [ABSTRACT FROM AUTHOR]

The mechanical properties of DP780 dual phase steel were measured by quasi-static and high-speed tensile tests at strain rates between 0.001 and 1000 s at room temperature. The deformation and fracture mechanisms were analyzed by observation of the tensile fracture and microstructure near the fracture. Dynamic factor and feret ratio quantitative methods were applied to study the effect of strain rate on the microstructure and properties of DP780 steel. The constitutive relation was described by a modified Johnson-Cook and Zerilli-Armstrong model. The results showed that the strain rate sensitivity of yield strength is bigger than that of ultimate tensile strength; as strain rate increased, the formation of microcracks and voids at the ferrite/martensite interface can be alleviated; the strain rate effect is unevenly distributed in the plastic deformation region. Moreover, both models can effectively describe the experimental results, while the modified Zerilli-Armstrong model is more accurate because the strain-hardening rate of this model is independent of strain rate. [ABSTRACT FROM AUTHOR]

Two different kinds of experimental techniques were used to in-situ study the austenite formation during intercritical annealing in C-Mn dual phase steel. The microstructure evolution was observed by confocal laser scanning microscope, and the austenite isothermal and non-isothermal transformation kinetics were studied by dilatometry. The results indicate that banded structure is produced for the reason of composition segregation and the competition between recrystallization and phase transformation. Austenite prefers to nucleate not only at ferrite/ferrite grain boundaries, but also inside the grains of ferrite. Furthermore, the austenitizing process is accomplished mainly via migration of the existing austenite/ferrite interface rather than nucleation of new grains. The incubation process can be divided into two stages which are controlled by carbon and manganese diffusion, respectively. During the incubation process, the nucleation rate of austenite decreases, and austenite growth changes from two-dimensional to one-dimensional. The partitioning coefficient, defined as the ratio of manganese content in the austenite to that in the adjacent ferrite, increases with increasing soaking time. [ABSTRACT FROM AUTHOR]

This work presents relevant metallurgical considerations to produce galvanized dual phase steels from low cost aluminum-silicon steels which are produced by continuous strip processing. Two steels with different contents of Si and Al were austenized in the two-phase field ferrite + austenite (α + γ) in a fast manner to obtain dual phase steels, suitable for hot-dip galvanizing process, under typical parameters of continuous annealing processing line. Tensile dual phase properties were obtained from specimens cooled from temperature below Ar, held during 3 min, intermediate cooling at temperature above Ar and quenching in Zn bath at 465 °C. The results have shown typical microstructure and tensile properties of galvanized dual phase steels. Finally, the synergistic effect of aluminum, silicon, and residual chromium on martensite start temperature ( M), critical cooling rate ( C), volume fraction of martensite, and tensile properties has been studied. [ABSTRACT FROM AUTHOR]

An experimental approach for determining the effect of machining the gage section of specimens for quasi-static, biaxial tension/shear testing of sheet steels is described. This method is demonstrated by comparing the results found by an existing testing method with a reduced thickness (Mohr and Oswald Exp Mech 48:65-77, ) to that of full-thickness specimens. Finally, the same results are compared to a specimen in which half of the thickness is removed. These tests are performed on DP780, a first generation advanced high-strength steel. Analysis of the full-thickness and one of the reduced thickness specimens shows that some second-order effects in measuring stress occur from these procedures, but these effects are small. Direct comparison of the engineering stress versus strain curves for the tested conditions shows that for the steel under consideration, there is little dependence on full-thickness, partial thickness, or through-thickness sampling location for continuum-level anisotropic plasticity properties in multi-axial loading for the material investigated and the methods used in manufacturing the specimens. [ABSTRACT FROM AUTHOR]

In this study, the effects of ferrite morphology and of ferrite volume fraction on the mechanical properties and the fracture toughness values ( KIc) of dual phase steels were investigated. The fracture toughness values were determined using an approach that was different from the standard one. A homogenization heat treatment was applied to the steels. Two different starting microstructures were then obtained by either the full annealing of or the quenching of the steels from austenite before applying intercritical annealing to the steels in the austenite region. After this process, dual phase steels with two types of morphology, coarse and fine, were obtained with 40, 20, and 10% ferrite volume fractions, depending on the starting microstructure. Tempered specimens with 10% ferrite exhibited improved fracture toughness values compared with those of the other steels. [ABSTRACT FROM AUTHOR]

Under certain conditions, strain rate sensitive materials such as dual phase steels, show formability improvement under high strain rate forming which is known as hyperplasticity. In this research, two commercial dual phase steel sheets, DP500 and DP780, were formed under quasi-static conditions using the Nakazima test and under high strain rate conditions by electrohydraulic forming (EHF) into a conical die. Macro-strains, measured from electro-etched circle grids with an FMTI analyzer, showed remarkable formability improvement in EHF specimens. Micro-strains, i.e., the strains in the ferrite and the martensite, were calculated by quantitative metallography of more than 7000 ferrite grains and 10,500 martensite islands. The goal was to investigate the deformation improvement of the constituents under EHF. Around 20 and 100% deformation improvements were observed in ferrite and martensite, respectively. Furthermore, as a micro-mechanical modeling technique, correlation of the micro-strains with the macro-strains was investigated by applying the mixture rule. Results showed a reasonable correlation between the macro and micro-scale strains; however in banded microstructures, the strain in the martensite should be determined precisely for more accuracy. [ABSTRACT FROM AUTHOR]

The modified Mohr-Coulomb and the extended Cockcroft-Latham fracture criteria are used in explicit finite-element (FE) simulations of ductile crack propagation in a dual-phase steel sheet. The sheet is discretized using tri-linear solid elements and the element erosion technique is used to model the crack propagation. The numerical results are compared to quasi-static experiments conducted with five types of specimens (uniaxial tension, plane-strain tension, in-plane shear, 45° and 90° modified Arcan) made from a 2 mm thick sheet of the dual-phase steel Docol 600DL. The rate-dependent J flow theory with isotropic hardening was used in the simulations. The predicted crack paths and the force-displacement curves were quite similar in the simulations with the different fracture criteria. Except for the 45° modified Arcan test, the predicted crack paths were in good agreement with the experimental findings. The effect of using a high-exponent yield function in the prediction of the crack path was also investigated, and it was found that this improved the crack path prediction for the 45° modified Arcan test. In simulations carried out on FE models with a denser spatial discretization, the prediction of localized necking and crack propagation was in better accordance with the experimental observations. In four out of five specimen geometries, a through-thickness shear fracture was observed in the experiments. By introducing strain softening in the material model and applying a dense spatial discretization, the slant fracture mode was captured in the numerical models. This did not give a significant change in the global behaviour as represented by the force-displacement curves. [ABSTRACT FROM AUTHOR]

Damage in a dual phase steel was measured using in situ high-resolution X-ray absorption tomography. A comparison with the behavior of its two constituents ferrite and martensite, taken separately, was also achieved in the present work. The method was particularly useful for analyzing the respective contribution of nucleation and growth of voids in the studied materials. Quantitative analysis of the damage events was carried out on a same 3D region inside the reconstructed volumes at different deformation steps for different samples cut from the three kinds of materials. Void number prediction and growth model, based on local stress triaxiality, show a good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

n in-plane shear specimen made of dual phase steel designed for ductile fracture studies is presented and then analyzed experimentally and numerically. In the experiment, digital image correlation (DIC) technique is utilized to measure the deformation of the specimen. Based on the implicit nonlinear FE solver Abaqus/Standard, numerical analysis of the specimen is performed by using plane stress and solid elements respectively. The elongation of the specimen's gauge length and the shear strain distribution within the shear zone are compared between the experimental and numerical results and a general good agreement is obtained. Thereafter, based on calculated results, the stress state of the shear zone is investigated in detail. It is shown that the shear stress is dominant within the shear zone despite of the emergence of normal stresses. The deformation is concentrated in the shear zone, where the incipient fracture is most likely to occur. The stress triaxiality and the Lode parameter at the fracture initiation are found to be maintained at a relatively low level, which implies that the stress state achieved by the specimen is close to pure shear. The present study demonstrates that the proposed in-plane shear specimen is suitable for investigation of the fracture behavior of high strength materials under shear stress states. [ABSTRACT FROM AUTHOR]

Draw restraining force is used in sheet metal forming processes to regulate material flow and control springback in the parts. In this paper, the influence of draw restraining force on springback behaviour of dual phase steels is investigated, through numerical simulations of the draw bend test. Three grades of dual phase steels are submitted to different normalized back forces in the range from 0.5 to 1.1 of the initial yield stress. All simulations were performed with the DD3IMP in-house code. The study highlights the importance of an accurate modelling of the isotropic work hardening of dual phase steels for an accurate springback prediction. [ABSTRACT FROM AUTHOR]

A major part of the industrial development in sheet metal forming is supported by numerical simulation. While individual simulation solutions for each application are available to study the producibility or serviceability of parts, a more unified approach that considers the results from the previous step along the production chain has not been presented so far. Within a joint research project that is founded by the German Federal Ministry of Education, a simulation strategy to model the process chain of dual phase steels from the hot rolled strip up to the behavior of components under crash conditions is developed. One important aim is to close the virtual process chain that ranges from steel design to crashworthiness simulation. The proposed concept will take into account micro-structural aspects of the material as well as macroscopic approaches, depending on the process step. This contribution presents the current status of the project considering the process steps cold rolling, annealing and deep drawing. [ABSTRACT FROM AUTHOR]

Ductile plug failure of resistance spot welded shear-lab specimens is studied by full 3D finite element analysis, using an elastic-viscoplastic constitutive relation that accounts for nucleation and growth of microvoids to coalescence (The Gurson model). Tensile properties and damage parameters are based on uni-axial tensile testing of the basis material, while the modelled tensile response of the shear-lab specimens is compared to experimental results for the case of a ductile failure near the heat affected zone (HAZ). A parametric study for a range of weld diameters is carried out, which makes it possible to numerically relate the weld diameter to the tensile shear force (TSF) and the associated displacement, u TSF, respectively. Main focus in the paper is on modelling the localization of plastic flow and the corresponding damage development in the vicinity of the spot weld, near the HAZ. For decreasing weld diameter, localization of plastic flow may be observed to occur in the weld nugget, introducing significant shearing. Due to these competing mechanisms a critical transition radius of the weld may be found. However, due to the limitation of the Gurson model in describing ductile failure at very low stress triaxiality, further analysis of the shear failure is omitted. [ABSTRACT FROM AUTHOR]

A steel containing 0.088 wt% C, 1.2 wt% Mn, and 0.78 wt% Cr was rolled at intercritical temperature (790 °C) and quenched to produce dual-phase microstructure. Rolling caused anisotropic increase in tensile strength and little change in ductility. The results suggest that rolling increased strength by a combination of strengthening of the ferrite and an increase in the stress transferred to the martensite. Up to 20% rolling reduction strengthened the ferrite by work hardening, larger reductions then reduced the strength of ferrite, anisotropically, due to increased recovery. Subgrains in ferrite were observed after rolling in the intercritical region which can contribute to the ultimate strength of the rolled material. [ABSTRACT FROM AUTHOR]