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This study investigates the microstructure, failure characteristics, and mechanical properties of dissimilar resistance spot welds between QP980 quenching and partitioning steel and DP600 dual phase steel. The dissimilar spot welds exhibited a heterogeneous microstructure, with a fusion zone predominantly composed of martensite, significantly overmatching both base materials. A fully martensitic structure with grain size gradient was observed in the upper-critical heat affected zone (UCHAZ) on both sides of the dissimilar joint. In the sub-critical heat affected zone (SCHAZ), no significant phase transformation occurred on both sides. However, at high welding currents, a slight tempering of pre-existing martensite was detected on the QP980 side resulting in a minor hardness drop in this zone. There was a critical welding current at which the failure mode was changed from interfacial mode to pullout mode. The dissimilar QP/DP joint showed a lower tendency to fail via interfacial failure mode compared to similar QP/QP and DP/DP joints, attributed to a higher fusion zone hardness overmatching factor and greater nugget rotation. Pullout failure of dissimilar QP/DP combination was assessed in detail through interrupted tensile shear test. It was found that failure at stronger QP980 side occurred by ductile cracking mechanism initiated from the notch tip, while failure at softer DP600 side occurred by through-thickness localized necking. The former was found to be the prior failure mechanism controlling the load bearing capacity of the dissimilar joint. The mechanical properties of dissimilar QP/DP joints were discussed in terms of peak load and energy absorption, and then were compared to those of similar DP/DP and QP/QP joints in the light of weld microstructure and failure mechanisms.

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 effect of current on resistance spot welding joints on metallurgic and mechanical properties is investigated experimentally using galvanized unequal-thickness DP600 steel sheets. The specimens were chosen to be 1.5 and 1.8 mm thicknesses. The parameters of the welding process were preferred as an electrode force at 5 kN, welding currents at 7, 9, 11, 13 and 15 kA and welding times at 180, 240, 300, 360 and 420 ms. The welded joint specimens were subject to tensile-shear strength testing to determine the ultimate tensile strength. Optimal specimens were inspected using optical microscopes and scanning electron microscopy. The effect of spot welding variables on the ultimate tensile strength was determined and the lobe diagram, which shows optimal current-time values, was obtained. The optimal welding parameters were carried out on new sheet metals successfully. The experimental results show that the welding current plays an important role in the especially crash energy absorption and load-carrying capability. [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]

This research presents the modification of the microstructure on the surface of a dual phase steel (DP) using three different treatments: surface mechanical attrition treatment (SMAT), surface mechanical carburizing treatment (SMCT), and surface mechanical composite coating treatment (SMCCT). We used SMAT to refine the grain size under high strain and strain rate conditions and successfully induced a phase transformation to martensite. The introduction of carbon during SMAT resulted in surface mechano-chemical carburizing, which enhanced the martensite transformation. Furthermore, the addition of silicon during the process led to the formation of SiC and Fe3C particles dispersed within the ferrite and martensite matrix, creating a surface nano-composite. Analysis using SEM and XRD revealed that the carbon and silicon-treated steel exhibited 41.4% martensite, 4.4% bainite, and 54.1% ferrite, indicating increased transformation by these elements. As a result, the alterations in the surface microstructure led to changes in the mechanical properties of the DP steel. Initially, the DP steel had Y.S. of 250 MPa and U.T.S. of 350 MPa. After SMAT treatment, its Y.S. and U.T.S. increased to 280 MPa and 380 MPa, respectively. The addition of carbon and silicon further enhanced the Y.S. and U.T.S. to 350 MPa and 450 MPa. A significant change in microhardness is achieved from 150HV0.05 to 255HV0.05 while after SMCT and SMCCT is 275HV0.05. Despite an increase in surface roughness contributing to heighten friction, there was a significant reduction in wear. This phenomenon could be attributed to grain refinement, dislocation structures, and martensite phase transformations associated with surface hardening. [ABSTRACT FROM AUTHOR]

There are some researches in the literature on the mechanical characteristics of dual-phase (DP) steels used in the automotive industry, but there is no comprehensive research on the corrosion behavior of these steels. In this work, the corrosion behavior of DP steels (DP440, DP590, DP980) exposed to two cycles of accelerated corrosion testing in accordance with Ford CETP 00.00-L-467 was observed. Raman and X-ray diffraction (XRD) techniques were used to classify the corrosion products, and the morphology of the samples was studied using a scanning electron microscope (SEM). Goethite and haematite were the primary chemical compounds determined. In high-mechanical strength DP steels, akaganeite was also identified in corroded specimens. The compounds formed due to corrosion were revealed by SEM images. In this work, according to the results of Raman spectroscopy, which was employed for the first time to reveal corrosion products in high-strength dual-phase steels, it was discovered that corrosion products increased with increasing mechanical strength due to an increasing martensite phase volume percentage. Polarization tests were carried out to support the electrochemical data reported by the Raman analysis. Similarly, an increase in the amount of martensite phase in the microstructure led to a decrease in the material’s corrosion resistance. Polarization experiments were carried out to support the electrochemical data interpreted by Raman analysis. In addition, an increase in the amount of martensite phase in the microstructure led to a decrease in the corrosion resistance of the material. In addition, information regarding the material’s electrochemical performance was obtained through Raman analysis. As shown by Raman, XRD, and polarization tests, the increase in corrosion products formed due to the increase in the amount of martensite led to a decrease in corrosion resistance.

The application of advanced high-strength steels (AHSS) by automakers is now a consolidated reality. In this scenario, Dual Phase (DP) steels deserve special mention due to their versatility in a vast range of applications, since they present an excellent combination between high mechanical strength and high ductility. On the other hand, applying a classical DP steel, with microstructure formed essentially by ferrite and martensite, shear fracture and edge cracking, have regularly been observed in die-bending and flanging operations. In this work the improvement of stretch-flangeability, while still keeping an appropriate ductility, of a classical 980 MPa DP steel was pursued by testing different intercritical annealing conditions in a dilatometer and a Gleeble thermomechanical simulator. According to the results obtained in this study, the stretch-flangeability improvement of a cold-rolled dual phase steel intercritically annealed can be obtained by increasing the soaking and austempering temperatures.

Understanding the effects of martensite volume fractions (V m) in dual phase (DP) steel resulting from heat treatment is crucial for designing structures for mechanical impact resistance and optimizing manufacturing processes. DP steel's material behaviour depends heavily on its microstructure properties. While stress-strain curves for individual phases in DP steels are often determined using empirical models, extensive experimental data is required to establish empirical model constants. This research aims to achieve two main objectives: firstly, to calibrate stress-strain curves for pure ferrite and pure martensite using limited experimental data using micromechanical adaptive iteration algorithm (MAIA). This calibration involves using stress-strain data from DP steels with varying V m during the calibration stage and additional data for verification. Secondly, to conduct a comprehensive sensitivity analysis of MAIA to assess its capabilities and limitations. Microstructure-based finite element (FE) models, simulated with ABAQUS/Standard, are employed to predict stress-strain curves under uniaxial tensile test conditions. The MAIA approach successfully calculated ferrite and martensite stress-strain curves that could predict plastic behaviour of DP steel with different V m, which agreed with experimental work. Key advantages of this approach include avoiding complex 3D microstructure geometries and requiring only two experimentally obtained stress-strain curves with different V m for material constant calibration, along with another curve for validation. However, the experimental data selected for calibration must have a V m difference between 20%–50% and one of the DP steels must have a low martensite volume fraction. The FE micromechanical model could capture the effect of softening of martensite phase and strengthening of ferrite phase as compared to its bulk properties for DP steel. The effect of V m on strain hardening rate was also successfully captured. This technique comes with obvious shortcomings, such as excluding crystal plasticity behaviour, and change in chemical composition within the individual phase with varying martensite volume fraction. [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 manufacturing of three-dimensional sheet metal components, such as car body parts, heavily relies on deep drawing. With the increasing demand for lightweight measures in the automotive industry due to CO2 limitation requirements, various methods are currently employed to reduce the weight of deep drawn components. However, these methods often overlook the potential for influencing the stress-strain dependent damage state within the component, even though it can greatly affect its performance in subsequent applications, and thus offers lightweight potentials. Friction between the deep drawing tools and the sheet metal is a key factor influencing the stress-strain state, and hence, represents a lever that can be utilized to manipulate the damage state in the component. This paper focuses on the numerical analysis of the dependence of damage on friction during deep drawing. Therefore, a rectangular geometry with an asymmetrical material flow is numerically investigated. A friction ratio is introduced with different constant friction coefficients for the corner of the tools and the straight sides. The established load paths at a chosen reference point are then considered in the form of selected stress and strain characteristics and the numerically predicted damage state is compared. The results are used to derive a recommended friction ratio that leads to less damage in the corner of the geometry. Afterwards strip drawing tool surfaces are modified to manipulate their friction properties using machine hammer peening. Subsequently, the influence of the structures of the strip drawing tool surfaces is quantified using strip drawing tests. The identified contact normal stress dependent friction coefficients are then implemented in the numerical simulation and the established numerical predicted damage state is examined to gain a more comprehensive understanding of how the friction ratio and the structure of the tool surfaces influence the damage state.

After closure, voids need to be healed through bonding and interface dissolution to restore the mechanical properties and produce a sound product. For the prediction of void healing and the respective process optimization, a void healing criterion, spanning all three phases is needed. While hot rolling is generally suited for void healing, with large deformations at hydrostatic pressure as well as high temperatures, not all regions in the workpiece offer the same void healing conditions. Due to gradients of these metrics e. g. along the normal direction, a space-resolved approach is needed. Previously, a practical void healing criterion combining void closure and subsequent recrystallization has been proposed and applied to exemplary hot rolling processes. Besides a broader application of the criterion, adding the process parameters roll diameter and temperature, the present paper mainly aims for the experimental validation of this criterion. In bonding trials, a positive influence of temperature and strain on the bond strength has been found. While no clear correlation of the holding time on the bond strength was detectible without concurrent contact pressure the experiments suggested a saturation strain as an indicator for void healing instead. Accordingly, the criterion has been revised and a strain-based criterion proposed, applied, and compared to the previously introduced recrystallization-based void healing criterion.

A proposed alternative method for processing dual phase (DP) steels is the utilization of a cold-rolled duplex ferrite-martensite structure, followed by intercritical annealing. The obtained results demonstrate the feasibility of this approach in producing a desirable microstructure. The microstructure consists of ferrite grains with an average size of less than 6 μm, along with a uniformly distributed fine martensitic phase. Treatment of the steels using this alternative route results in improved strength-ductility properties and a greater work-hardening exponent. These steels exhibit an excellent combination of mechanical properties with tensile strength of 866.69 MPa, total elongation of 24.40%, and the product of tensile strength and total elongation of exceed 20 GPa%.

Dual phase (DP) steels are amongst the most widely used structural steels for automotive applications. It is essential to understand the damage initiation and damage growth in these high strength steels and further shed light on improving mechanical properties. In this work, two DP800 dual phase grades are investigated, which exhibit identical ultimate tensile stress but significantly different elongation in the uniaxial tensile test. To explain the difference in ductility, particularly described by uniform elongation, we investigate the damage initiation and growth mechanisms by analyzing microstructural changes upon deformation, such as voids, dislocation structures and the grain morphology. Furthermore, ferrite micropillars in pre-strained samples are tested in situ to capture the strain hardening capability of ferrite. We found that the DP steel with harder martensite and softer ferrite exhibits more damage initiation sites after deforming to an identical strain. However, void growth is much slower compared to the DP steel grade with fewer initiation sites. We explain the suppressed void growth by significant strain-hardening of ferrite surrounding the voids, which is observed in the micropillar compression experiments. The improved strain hardening of ferrite originates primarily from the difference in chromium content considering the negligible influence of dispersed particles.

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]

Effect of strain rate on the damage behaviour of hydrogen (H)-charged dual phase (DP 780) steel via the in-situ digital image correlation (DIC) technique is investigated in this work. Since stress concentration sites like notches are common in engineering practice, two types of uniaxial tensile tests have been carried out using smooth and notch tensile specimens for detailed analysis. The study reveals the significance of hydrogen embrittlement in DP 780 steel, as no strain rate effect is observed on the mechanical property in the case of an uncharged smooth tensile specimen. However, a significant effect of strain rate is detected after the H-charging. Hydrogen showed a lesser ability to aid the failure process when the applied strain rate is raised, as it could diffuse over a limited distance during the tensile test. The local axial and width strains, along with necking and fracture strains, are quantified for each specimen to understand the strain rate effect better. A centre-line crack is observed in every H-charged specimen's fracture surface owing to the presence of MnS inclusion in DP steel along the central line and its interaction with the atomic hydrogen. Moreover, the degree of hydrogen embrittlement is substantially higher in the notch tensile specimens than in the smooth ones.

The study presents a Machine Learning (ML)-based framework designed to forecast the stress-strain relationship of arc-direct energy deposited mild steel. Based on microstructural characteristics previously extracted using microscopy and X-ray diffraction, approximately 1000 new parameter sets are generated by applying the Latin Hypercube Sampling Method (LHSM). For each parameter set, a Representative Volume Element (RVE) is synthetically created via Voronoi Tessellation. Input raw data for ML-based algorithms comprises these parameter sets or RVE-images, while output raw data includes their corresponding stress-strain relationships calculated after a Finite Element (FE) procedure. Input data undergoes preprocessing involving standardization, feature selection, and image resizing. Similarly, the stress-strain curves, initially unsuitable for training traditional ML algorithms, are preprocessed using cubic splines and occasionally Principal Component Analysis (PCA). The later part of the study focuses on employing multiple ML algorithms, utilizing two main models. The first model predicts stress-strain curves based on microstructural parameters, while the second model does so solely from RVE images. The most accurate prediction yields a Root Mean Squared Error of around 5 MPa, approximately 1% of the yield stress. This outcome suggests that ML models offer precise and efficient methods for characterizing dual-phase steels, establishing a framework for accurate results in material analysis.

The objective of this research was to analyse the stress and strain behaviours of the triaxiality specimens (specimens of different shaping) of dual-phase steels, respectively DP600 and DP800, whose microstructures mainly comprise ferrite and martensite phases. These steels find wide application in the automotive industry, which is constantly searching for better quality materials in the aim of increasing vehicle safety, protecting the environment, and reducing fuel consumption. In this case, for all tested specimens the experimental uniaxial tensile tests were performed at three different strain rates, 0.0083 s−1, 0.042 s−1, and 0.16 s−1, to determine mechanical properties such as stress, strain, yield strength, and ultimate tensile strength of mentioned steels. In addition, uniaxial tensile tests with characteristics similar to those experimental were carried out through the finite element modelling method using the Mat_Picewise_Linear_Plasticity_024 model, to determine the mentioned mechanical properties, but also to determine the failure parameters or plastic strain up to failure, of all triaxiality modelled specimens. The obtained results were validated by comparing the experimental results with numerical simulation results. The comparative scale of accuracy between both steels was made at the fracture strain, and the average accuracy of both steels for the standard (s) specimens was

A dual phase (DP500/800) steel sheet (1.6 mm thickness) was evaluated by means of low cycle fatigue (LCF). The fatigue properties of the material were assessed by means of the cyclic elastic modulus determination at half-life as well as by the plastic strain amplitude measurements. Both methods show a good correlation and high reliability, however, the plastic strain amplitude approach tends to fit better to the experimental results. From a parametric analysis performed, it was observed that the plastic strain amplitude approximation could be more suitable to assess the fatigue properties for LCF of the material due to an improved correlation of the computed cyclic elastic modulus in comparison to the cyclic elastic modulus measurements at half-life. Both methods showed that, for strain amplitudes higher than εa = 0.004, the cyclic elastic modulus tends to be lower than 210 GPa, meanwhile for lower strain amplitudes, the cyclic elastic modulus tends to increase. An evaluation by X-ray Diffraction technique allowed to determine that the increment of residual equivalent strain contributes to the cyclic elastic modulus diminution.

Abstract 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.

Resistance spot welding studies at varying current with an 8 mm electrode on steel grades DP980, DP690, TRIP980 and TRIP780, established that peak strength with sound weld could be achieved at a current range of 9 to 10 kA, where the nugget diameter was between 6.9 and 7.5 mm. The joint efficiency, measured as hardening ratio was highest in TRIP690 at 2.22 and for higher strength grade it was between 1.37 to 1.51. The softening ratio in the HAZ associated with tempering of preexisting martensite, was 0.86 to 0.9. Higher fusion strength was associated with alloying content that increased resistance that increased weld pool fusion that enhanced the nugget diameter and hence the strength. The TRIP steel at high cooling rate showed martensite and retained austenite and the DP steel showed martensite content in higher grade and dispersed ferrite with martensite in the lower DP grade. The Tensile shear strength was lowest (16kN) for DP780, while for other grades it is between 20 to 22 kN. The strength enhancement in TRIP steel was due to higher weld diameter at higher current, where partial pull out failure mode was observed. At lower current, the interfacial pull out failure mode was observed with poor fusion and at high currents partial pull out at HAZ is observed. The Coach peel strength was highest at 3.2 kN for DP780, 2.2 kN for DP980, 2 kN for TRIP 690 and 1.7kN for TRIP 980 and follows the trend in TSS. The observed failure may be attributed to the high fusion strength in TRIP steel due to sound nugget and a microstructure of martensite with retained austenite. The lower strength with DP steels is attributed to low alloyed lath martensite in DP 980 and the lower grade DP780 is softer associated with ferrite dispersion in martensite. At very high currents, the fusion is good but there is pull out at HAZ due to the softening associated with the tempering of preexisting martensite that influenced the failure at HAZ failure mode.

Bu çalışmada özellikle otomotiv endüstrisinde çift fazlı çelik sacların birleşiminde en sık kullanılan nokta direnç kaynak yöntemi tercih edilmiştir. Yapılan işlemler 5 ve 7 kA kaynak akımlarında, 2-6 bar elektrot basınç değerlerinde oluşturulmuştur. Farklı kaynak parametrelerinin kaynak metali sertlik değeri üzerindeki etkileri çoklu doğrusal regresyon analizi ile incelenmiştir. Gerçekleştirilen bu analiz sonucunda elde edilen korelasyon tablosu ve ANOVA analiz değerleri yorumlanmıştır. Kaynak akımı ve elektrot basıncının önem dereceleri belirlenerek, kaynak metali sertlik değeri üzerindeki etkileri karşılaştırılmış ve belirlenen etki değerlerine göre çoklu doğrusal regresyon modeli oluşturulmuştur. İstatistiksel analiz sonuçlarına göre sertlik değerine etki eden en önemli değişkenin kaynak akımı olduğu görülmüştür.

In this study, an electrochemical method was used to permeate hydrogen through annealed DP590 steel under various pre-strain conditions (0-15%). Stress-strain and internal friction-temperature curves of the dual phase (DP) steel were obtained from slow strain-rate tensile tests and internal friction measurements, respectively. The diffusion of interstitial atoms, formation of Cottrell atmospheres, and embrittlement mechanism of DP steel were investigated under different prestress conditions before and after hydrogen permeation. The results show that the tensile strength of DP steel first decreases and then increases and the elongation sharply decreases with increasing pre-strain. The strength and ductility present similar trends with changes in pre-strain before and after hydrogen charging, however, after hydrogen charging, an obvious increase in tensile strength and decrease in elongation are observed. Furthermore, the γ peak amplitude decreases and the Snoek-Ke-Koster (SKK) peak amplitude increases with increasing internal pre-strain according to the friction-temperature curve. The γ peak and SKK peak exhibit the same trends with increasing pre-strain before and after hydrogen charging and both the γ peak and SKK peak decrease with hydrogen charging. The dislocation density in DP steel increases after hydrogen charging.

Research was conducted to investigate the shape memory properties related to the microstructure in an Fe-9.5Ni-6.5Mn (wt.%) dual phase steel developed by intercritical annealing and subsequent ageing. Electron back scattering diffraction (EBSD) analysis and high resolution transmission electron microscope (HRTEM) images revealed that the microstructure of the initial steel in the solution-annealed condition was a typical lath α′-martensite with a high density of dislocations. During the intercritical annealing treatment of the solution-annealed steel at 590 °C in the ferritic-austenitic (α+γ) two-phase region for various annealing times, a part of α′-martensite transformed to the reversed γ-austenite with a difusionless shear nature, resulting in the development of a dual-phase (α′+γ) steel. Also, subsequent ageing treatment of the intercritically annealed steel at 480 °C for 3.6 ks led to formation of the nanoscale θ-NiMn precipitates in the dual-phase microstructure. The experimental results indicated a maximum shape recovery of ∼55% at 1% pre-strain for the intercritically annealed steel for 1.8 ks, followed by subsequent ageing. The presence of austenite phase reinforced by precipitates in the dual-phase steel was the main reason for appearance the shape recovery. In addition, the existence of isolated stacking faults in the reversed austenite region after straining confirmed that the occurrence of the austenite to ε-martensite transformation (γ→ε) under loading and the reverse transformation (ε→γ) upon heating are the main causes of the shape memory effect in the studied steel.

The present paper makes a critical review based on the literature and presents examples of experiments developed by the authors, showing how the hole expansion ratio test (HERT) could be useful to understand and avoid premature cracks caused by flanging operations in sheet metal parts made of advanced high-strength steels. An approach based on damage theory was evaluated along with the mechanical tests necessary to understand the phenomenon, the influence of the trimming process, and the correlation between experimental mechanical testing and simulations. The procedures presented in this work allow for the prediction of edge cracks, often verified after flanging steps during the stamping process, allowing for reductions in tooling costs and setup loops.

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]

High performance sheet metals with a multi-phase microstructure suffer from deformation induced damage formation during forming in the constituent phases but importantly also where these intersect. To capture damage in terms of the physical processes in three dimensions (3D) and its stochastic nature during deformation, two challenges remain to be tackled: First, bridging high resolution analysis towards large scales to consider statistical data and, second, characterising in 3D with a resolution appropriate for sub-micron sized voids at a large scale. Here, we present how this can be achieved using panoramic scanning electron microscopy (SEM), metallographic serial sectioning, and deep-learning assisted automatic image analysis. This brings together the 3D evolution of active damage mechanisms with volumetric and environmental information for thousands of individual damage sites. We also assess potential surface preparation artefacts in 2D analyses. Overall, we find that for the material considered here, a dual phase (DP800) steel, martensite cracking is the dominant but not sole origin of deformation induced damage and that for a quantitative comparison of damage density, metallographic preparation can induce additional surface damage density far exceeding what is commonly induced between uniaxial straining steps.

The aim of this research was to develop a medium Ni–Mn dual phase steel with a combination of relatively high strength and good formability for industrial applications. Accordingly, this research was conducted to examine the effects of intercritical annealing and subsequent ageing treatments on the microstructure and mechanical properties of an Fe-9.5Ni-6.5Mn (wt.%) low carbon martensitic steel. Electron back scattering diffraction (EBSD) analysis revealed that intercritical annealing treatment at 610 °C in the ferrite + austenite (α+γ) dual phase region for various holding times led to formation of the reversed austenite phase in the martensite matrix, producing a medium Ni–Mn dual phase steel. It was resulted that the volume fraction of austenite and its grain size were significantly increased with increasing the holding time. Also, high resolution transmission electron microscope (HRTEM) images illustrated the formation of nano-sized θ-NiMn precipitates within the martensite phase after subsequent ageing at 450 °C for 7.2 ks. Intercritical annealing followed by ageing yielded a good combination of an ultimate tensile strength (∼1160 MPa) and fracture strain (∼11.5%) in the developed dual phase steel compared to the initial martensitic steel. The high strength and good ductility of the developed dual phase steel were respectively attributed to the presence of θ-NiMn precipitates with a hard character and reversely formed austenite with a ductile nature in the microstructure. Furthermore, the fully brittle fracture mode in the aged initial steel changed to a mixture of brittle and ductile behavior in the dual phase steel produced by intercritical annealing and subsequent ageing.

Dissimilarities in metal laser welding lead to brittleness in welded joints due to differences in the thermophysical and chemical properties between dissimilar base materials. To overcome such brittleness, the addition of a preset coating onto the base materials as an interlayer is a method for attaining reliable welded joints. Nd:YAG laser butt welding of DP590 dual-phase steel and 304 stainless, both with a thickness of 1 mm, was performed with a preset nickel coating as an interlayer using an electroplating process. The relationship between the microstructure and the mechanical properties of the welded joints was researched, the microstructure and composition of the weldment were analyzed, and the microhardness, tensile strength and corrosion resistance were tested. The results showed that the preset nickel coating increased the content of Ni element in the welded joints, which is beneficial to the formation of lath martensite. The average hardness of the welded joints increased by 12%, and the tensile strength was higher than 370 MPa. The corrosion rate of the welded joints can be slowed down, and the corrosion resistance can be improved by increasing the nickel coating. [ABSTRACT FROM AUTHOR]

The objective of this research was to analyse the stress and strain behaviours of the triaxiality specimens (specimens of different shaping) of dual-phase steels, respectively DP600 and DP800, whose microstructures mainly comprise ferrite and martensite phases. These steels find wide application in the automotive industry, which is constantly searching for better quality materials in the aim of increasing vehicle safety, protecting the environment, and reducing fuel consumption. In this case, for all tested specimens the experimental uniaxial tensile tests were performed at three different strain rates, 0.0083 s−1, 0.042 s−1, and 0.16 s−1, to determine mechanical properties such as stress, strain, yield strength, and ultimate tensile strength of mentioned steels. In addition, uniaxial tensile tests with characteristics similar to those experimental were carried out through the finite element modelling method using the Mat_Picewise_Linear_Plasticity_024 model, to determine the mentioned mechanical properties, but also to determine the failure parameters or plastic strain up to failure, of all triaxiality modelled specimens. The obtained results were validated by comparing the experimental results with numerical simulation results. The comparative scale of accuracy between both steels was made at the fracture strain, and the average accuracy of both steels for the standard (s) specimens was < 1 %, for the (20a) specimens 1.50 %, for the (4a) specimens 4 %, and for (a) specimens 3.50 %. Finally, it was concluded that the proposed material model and calibrated failure data fitted very well. [ABSTRACT FROM AUTHOR]

The microstructure evolution of a ferrite/bainite dual-phase steel during uniaxial tension was observed in-situ technology. The effects of tensile rates on microstructure, mechanical properties and fracture mechanism were studied. The result shows that stress concentration is easy to form cracks at the interfaces between ferrite and bainite. The sample exhibits high uniform elongation and strength at low-strain rate, which is attributed to the effective interaction between dislocations. The microstructure undergoes orderly and continuous high hardening rate under 'harmonious' conditions. Dislocations are rapidly entangled at high-strain rate and dislocation cells forms. The increased strength of the ferrite brings a reduction in its ability to coordinate more plastic deformation, which eventually leads to local plastic instability and early fracture. [ABSTRACT FROM AUTHOR]

The microscopic hole expansion ratio (μHER) of a ferritic-martensitic (∼10 %) dual phase steel was measured using a novel in-situ scanning electron microscope based miniature hole expansion setup. The effect of extrinsic parameters such as specimen thickness and machining conditions, and intrinsic parameters such as hardness differential (ΔH =H α '- H α) between the soft ferrite matrix and hard martensite islands on μHER values was studied. The miniature HER setup allowed site-specific measurement of microscopic strain localizations in the DP microstructure through high resolution digital image correlation under the triaxial state of stress. The results from these experiments were juxtaposed against another triaxial state of stress ahead of a crack tip, in a fracture toughness (J Ic) test using the single edge notched tensile (SENT) geometry for the same thickness and microstructural conditions of DP steel. It was found that the tempered DP specimen with lower ΔH resulted in a ∼45 % higher μHER as compared to the as-received DP600, although both specimens exhibited similar J Ic values. This apparent discrepancy between the trends in μHER and J Ic values was explained in terms of the differences in failure modes, triaxiality and plastic zone evolution in the two conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Proposed two average plane stress assumptions for 2D RVE modeling of DP steels. • The average plane stress assumptions well approached the intrinsic 3D behavior. • The conclusion is valid for different microstructures and textures. Accurately capturing the microscopic mechanical response of dual-phase (DP) steels is crucial for improving their mechanical properties. Representative Volume Element (RVE) modeling is widely used to probe microstructural behavior, but due to computational restrictions, inherently three-dimensional microstructures are often simplified to two-dimensional models with out-of-plane constraints such as plane stress or plane strain. In this study, the out-of-plane constraint is investigated through 3D and 2D RVE modeling of DP steels. The 3D model was developed with synthesized microstructure and crystal plasticity to simulate microscopic stress state and macroscopic stress strain response in full 3D settings. The 2D models were developed with four different conditions for the out-of-plane effects: plane stress, plane strain, and two newly proposed average plane stress conditions. The results were evaluated by stress-strain curves, out-of-plane stress fields, as well as its statistical analyses under uniaxial tension loading condition. The findings reveal that applying the average plane stress condition to the entire 2D RVE model provides the most accurate approximation, regardless of grain morphology, martensite volume fraction, and texture. [ABSTRACT FROM AUTHOR]

In this study, the nano-to bulk-scale fracture behaviors of a dual phase (DP) steel were investigated by combining a micro-digital-image-correlation (micro-DIC) technique and the finite element method (FEM). The emergence of surface cracks and nano-to bulk-scale strain distributions during plastic deformations were investigated using micro-DIC and macro-DIC techniques. FEM simulations were conducted to calculate the stress state of the damaged regions that could not be directly obtained from experiments. By combining nano- and bulk-scale observations, fracture strain and stress triaxiality of both scales were determined into model parameters for predicting the material damage behavior. The accuracy of the present damage model parameters was confirmed by comparing the load–displacement curves obtained from experimental result and proposed method. This new strategy of combining micro-scale deformation behavior and bulk-scale damage analysis facilitates defining damage model parameters through observing a wide range of regions with only one or two specimens, which is opposed to the conventional method that requires bulk-scale observations in multiple loading condition.

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 paper makes a critical review based on the literature and presents examples of experiments developed by the authors, showing how the hole expansion ratio test (HERT) could be useful to understand and avoid premature cracks caused by flanging operations in sheet metal parts made of advanced high-strength steels. An approach based on damage theory was evaluated along with the mechanical tests necessary to understand the phenomenon, the influence of the trimming process, and the correlation between experimental mechanical testing and simulations. The procedures presented in this work allow for the prediction of edge cracks, often verified after flanging steps during the stamping process, allowing for reductions in tooling costs and setup loops. [ABSTRACT FROM AUTHOR]

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The initial corrosion process of dual phase (DP) steel produced by the continuous annealing process (CAP) in NaCl solution was systematically studied in this paper. The microstructure, nobility of the phases, and micro-galvanic corrosion behavior were characterized by scanning electron microscope (SEM), electron backscattered diffraction (EBSD) technique, scanning Kelvin probe force (SKPFM), and electrochemical impedance spectroscopy (EIS). Results indicate that there are two types of martensite, namely fresh martensite (FM) and tempered martensite (TM), embedded in the ferrite matrix. The two types of martensite are surrounded by dislocations and connect to each other as in a chain-like network at the ferrite grain boundary. Both FM and TM can act as a micro-cathode to promote the anodic dissolution of adjacent ferrite matrix since FM is up to 15 mV noble to the ferrite matrix and TM is to 30 mV noble. The initial corrosion process of DP steel can be divided into two stages. The first stage involves an increasing corrosion rate of the steel due to the successive emergence of cathodic TM and FM, while the corrosion rate decreases in the second stage because of the separation of martensite.

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]

[Display omitted] • Specific spatial microstructure features for studying mechanical properties of DP steel. • Machine learning trained by RVE simulations included plastic and damage behavior for forward prediction. • MCMC inverse analyses solved non-uniqueness of structure–property relationships of steel. • Approach fairly dealt with multi-target optimization and high dimensional problem. This work aims to investigate complex relationship between microstructure characteristics and mechanical properties of dual phase (DP) steel through an inverse analysis based on Markov chain Monte Carlo (MCMC) method combined with meso -scale material modelling. In this framework, a machine learning approach as surrogate model was developed, in which support vector regression (SVR) and artificial neural network (ANN) were trained using results from representative volume element (RVE) simulations coupled with damage model, which were previously calibrated with experimental data of commercial DP steel grades. Moreover, specific microstructure descriptors including Moran's index, martensite band index and martensite orientation were proposed for representing effects of spatial distributions of martensitic phase. As a result, inverse predictions of microstructure characteristics of DP steels for achieving defined yield strength, tensile strength, uniform elongation and toughness were presented. The inverse analysis could solve the non-uniqueness of structure–property relationships of steel, whereby significances of dispersed structures and aligned martensite bands were highlighted in details. The approach fairly dealt with multi-target optimization and high dimensional problem, which can be further applied as a guideline for designing DP microstructures with enhanced mechanical properties. [ABSTRACT FROM AUTHOR]