High-strength Dual-phase Steel Research Articles

Analysis of fatigue fracture mechanism of laser spiral welding of dissimilar metals of 6082 high strength aluminum alloy and DP980 high strength dual phase steel

This paper investigates the relationship between weld quality, influencing factors, microstructure and mechanical properties of laser spiral dissimilarity welding of 6082 high-strength aluminum alloy and DP980 high-strength dual-phase steel, focusing on the fatigue properties and fracture mechanism of 6082-DP980 dissimilar metal welded joints. The research shows that intermetallic compound layers (IMCs) dominate the fatigue crack initiation stage and part of the crack extension stage, two fatigue crack initiation modes are obtained, one is the main crack initiation mode along the FeAl and FeAl2 interlayer, and the other is the secondary crack initiation mode along the Fe3Al and FeAl interlayer. In the crack propagation stage, due to the diffusion of Fe and Al elements generated by IMCs during the welding process, solid solution strengthening, intergranular strengthening and grain boundary strengthening, in the fatigue crack propagation stage formed a hardened zone, hindering the main crack to continue to expand along the IMCs, the main crack shifted to the expansion of the 6082 base material, thus extending the fatigue life.

A generalized, computationally versatile plasticity model framework - Part II: Theory and verification focusing on shear anisotropy

Shear-dominated deformation (SHDD) is pivotal in sheet metal forming; however, comprehensive modeling of plastic anisotropy in SHDD, specifically shear anisotropy considering both yield stress and plastic flow, has been inadequately addressed in existing literature. In this work, a generalized constitutive framework is introduced on the basis of stress triaxiality-dependent state variable to accurately emulate plastic anisotropy and the physics-based shear constraint in SHDD. The framework is capable to seamlessly integrate with existing yield criteria, preserving computational efficiency and versatility. Notably, the yield function, anisotropic parameters, and their optimization or analytical determination for the non-shear deformation state remain unaltered. When integrated with the Hill48 yield function, featuring either one or two anisotropic parameters within the generalized constitutive framework, precise characterization of yield strength and plastic flow in SHDD is achieved. The applicability of the framework extends to various anisotropic yield functions such as the widely employed Yld2k-2d and the sixth-order polynomial (Poly6) function as a class of associated flow rule-based yield functions, and one non-quadratic yield function for non-associated flow rule scenarios. Experimental validation with two engineering sheet metals, high-strength dual-phase steel DP980 and high-strength aluminum alloy AA7075-T6, was conducted. Comparative analyses with several recently proposed yield criteria, especially Poly6-18p, highlighted the efficiency of the proposed constitutive framework. Furthermore, this study explores intrinsic shear constraints, particularly the absence of through-thickness strains under in-plane shear stress. Additionally, it offers an enhanced description of plastic anisotropy in shear yield stress within the general framework, providing valuable insights into the complexities of SHDD.

Microstructure-based modelling of formability for advanced high strength dual-phase steels

The macroscopic formability of advanced high strength dual-phase steels (AHSDPSs) relies highly on the microstructure. Therefore, building the relationship between the microstructure and the formability is vital to enhancing the formability by microstructure optimisation. In this study, a formability model based on microstructure for AHSDPSs was proposed. The tensile simulation of representative volume element (RVE) was implemented to acquire the stress-strain curve (SSC). The forming limit curve (FLC) was constructed based on the method developed in our previous research. The simulation of hole-expansion with the pre-damage of the central-hole induced by the blanking process was conducted to obtain the hole-expansion ratio (HER), in which the vital failure model is developed based on the simulated FLC. For a typical advanced high strength dual-phase steel of DP1180 steel, the prediction errors of SSC, FLC and HER are 5.83 %, 6.85 % and 10.71 %, respectively. The results also depicted that the damage of the prefabricated hole has a noticeable influence on the HER, which could not be ignored for the hole-expansion simulation.

Characterization of corrosion products formed in high-strength dual-phase steels under an accelerated corrosion test

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.

Direct application of elasto-visco-plastic self-consistent crystal plasticity model to U-draw bending and springback of dual-phase high strength steel

The incremental elasto-visco-plastic self-consistent polycrystal model (ΔEVPSC) was utilized to describe the constitutive behavior of dual-phase 980 (DP980) steel. A simple baseline modeling approach was chosen: the hardening behavior of each constituent phase in the DP980 steel was described by a simple Voce hardening law without explicitly considering the back stress; and it was assumed that using the same single crystal elastic modulus for ferrite and martensite is sufficiently representative. The adequacy of this baseline modeling approach was evaluated by comparing various mechanical experimental data with model predictions in terms of the stress vs. strain curves obtained from uniaxial tension, tension-compression, and loading-unloading-loading (LUL) tests. Additionally, the evolution of experimental lattice strain data reported in literature was used to validate the phase-specific Voce hardening parameters. Despite its minimalistic modeling description, the baseline ΔEVPSC model successfully captured key features: 1) the Bauschinger effect, 2) the decrease in chord modulus, and 3) the non-linearity in the stress vs. strain curves resulting from the LUL test. All three mentioned characteristics are crucial for accurate prediction of springback in sheet metals. The ΔEVPSC model, interfaced with a finite element solver (Abaqus/standard) as the user material subroutine, was employed to simulate the Numisheet93 benchmark problem. The strip of DP980 was first U-drawn followed by springback. The model-predicted springback profile aligned well with the experimental results only when stress relaxation was properly considered, resulting in improved predictive accuracy compared to predictions based on a distortional plasticity model.

Finite Element Simulation and Microstructural Analysis of Roll Forming for DP590 High-Strength Dual-Phase Steel Wheel Rims.

In this study, finite element (FE) simulation by the software Abaqus was relied on to investigate the roll forming process of a wheel rim made of an innovative dual-phase steel, i.e., DP590, after flash butt welding (FBW). In the simulation, an FE model was generated, including the design of the dies for flaring, three-roll forming, and expansion, and detailed key processing parameters based on practical production of the selected DP590. Combined with the microstructures and properties of the weld zone (WZ) and heat-affected zones (HAZs) after FBW, the distribution of stress/strain and the change in thickness of the base metal (BM), WZ and HAZs were analyzed, and compared in the important stages of roll forming. Theoretically, the variation in the microstructure and the corresponding stress-strain behaviors of the BM, WZ, and HAZs after FBW have led to the thickness reduction of DP590 that originated from softening behaviors occurring at the region of subcritical HAZs (SCHAZs), and a small amount of tempered martensite has evidently reduced the hardness and strength of the SCHAZ. Meanwhile, the distribution of stress/strain has been influenced to some extent. Further, the study includes the influence of the friction coefficient on the forming quality of the wheel rim to guarantee the simulation accuracy in practical applications. In sum, the dual-phase steel has to be carefully applied to the wheel rim, which needs to experience the processes of FBW and roll forming, focusing on the performance of SCHAZs.

Multiscale analysis of microstructure-based bending characteristics of advanced high strength dual-phase steel

Multiscale analysis of microstructure-based bending characteristics of advanced high strength dual-phase steel

Flexible roll forming of surface developable profiles from Dual Phase steel

Flexible Roll Forming (FRF) can roll-form variable cross-sectional profiles for Electric Vehicle (EV) production however, a major limitation exists due to flange wrinkling while forming high-strength steels. Flange wrinkling can be eliminated by reducing the required level of membrane deformation in the longitudinal direction. Although reducing the severity of the profile's transitions minimises the strains, the overall complexity of the parts is also lowered. Origami-based developable profiles can be created from curved creased folding without membrane stretching or compression. In FRF, such types of profiles can be formed by combining a variation in width and depth over the length of the part. This study presents, for the first time, the analyses of forming a developable shape in a FRF operation. Firstly, analytical equations are applied to calculate the strains and forming stability of each pass which is followed by experimental FRF trials on two high-strength Dual Phase steels. Finally, Finite Element Analysis is used to investigate the forming behaviour of the two types of developable profiles. The experimental results show that the forming of one type of developable profile improves the shape, while the numerical analyses showed that an additional top-hat forming is required for the second profile type.

Dependence of mechanical properties on the phase composition of intercritically annealed medium-Mn steel as the main competitor of high-strength DP steels

Depending on the alloy composition, intercritical annealing may provide different phases in the microstructure. For low-alloyed dual-phase (DP) steels it is usually ferrite and martensite, while for medium-Mn steels retained austenite is also formed. In a present study, a wide intercritical temperature range was applied to a 5% Mn steel to investigate possible microstructure combinations: ranging from fully ferritic, through ferritic-austenitic, multiphase, to fully martensitic, which were next investigated in terms of mechanical properties to clarify the behavior of this type of material. The obtained results together with technological issues and economic indicators were next compared to mechanical properties of typical DP steels in order to assess the possibility of replacing this material in car production. The mechanical properties were evaluated using static tensile and hardness tests. The phase composition was determined qualitatively and quantitatively using dilatometry, X-ray diffraction measurements, and electron backscatter diffraction analysis. The results suggest that both initial austenite and martensite fractions have a decisive influence on the yielding and elongation of steel; however, the tensile strength depends mainly on the sum of martensite initially present in the microstructure and the strain-induced martensite formed from the plastically deformed austenite regardless of the initial retained austenite—martensite ratio. The results indicate superior total elongation of medium-Mn steels reaching 30% compared to DP steels with a similar strength level in the range between 900 and 1400 MPa. However, medium-Mn steels could be a significant competitor to dual phase steels only if some technological problems like discontinuous yielding and serrations are significantly reduced.

Semi in-situ investigations on deformation-induced micro-damage in high-strength dual-phase steels

This work investigates deformation-induced micro-damage modes in ferrite-martensite dual-phase (DP) steels utilizing a state-of-the-art semi in-situ deformation apparatus integrated with scanning electron microscope. By integrating secondary electron imaging, electron backscatter diffraction, and electron channeling contrast, we provide comprehensive statistical results on micro-damages and strain localization events of their formation in DP780 and DP980 steels. The current observations indicate that interface decohesion plays a critical role in DP780, particularly under plastic instability. Based on maps of Kernel averaged misorientation, interface decohesion arises from the accumulation of geometrically necessary dislocations, which serve to compensate for the deformation incompatibility between ferrite and martensite. In contrast, the dominant damage mode in DP980 under plastic instability involves ferrite cracking within martensite-surrounded ferritic grains. This type of micro-damage is a result of strain localization by shear bands that accommodate the global deformation while the grain is constrained by martensite islands. Martensite cracking does not show as predominate sites under the condition of plastic instability in both steels. In summary, our findings highlight the importance of deformation incompatibility and constraint on occurrence of micro-damages through distinct strain localization events, which are determined by both microstructure of damage site and its environmental microstructure. In this work, we shed light on the role of microstructural engineering as a critical strategy for enhancing the resistance of DP steels or hetero-phase steels against deformation-induced damage.

Axial load behavior of reinforced concrete columns with high-strength steel coiled strips as confinement

This paper experimentally investigates the axial load behavior of square reinforced concrete columns confined using dual-phase high-strength (100 ksi [690 MPa] yield strength) steel coiled strips. Two phases of testing were conducted on reduced-scale specimens (8 × 8 in. and 10 × 10 in. [203 × 203 mm and 254 × 254 mm]). Varied parameters include con¬finement type (strip versus reinforcing bar), confine¬ment reinforcement ratio, confinement layout (hoops and ties, single spiral, two spirals), and strip anchor¬age. Although the reinforcing-bar-confined columns demonstrated better postpeak residual strength and ductility, important findings include the following: strip-confined columns had peak strengths exceeding the nominal axial strength predicted by code, strip-con¬fined columns were able to achieve similar normalized peak strengths and prepeak stiffness as columns with reinforcing bar hoop confinement, two strip spirals are necessary to achieve the desired postpeak residual strength and ductility for an eight-bar layout, and strip spirals and hoops may provide better restraint against buckling of corner bars compared with reinforcing bar hoops.

Research on Forming Limit Stress Diagram of Advanced High Strength Dual-Phase Steel Sheets.

The Forming Limit Stress Diagram (FLSD) can accurately describe the forming process of high-strength steel. However, obtaining FLSD is relatively difficult. In order to predict fracture in advanced high-strength dual-phase (DP) steels, limit maximum and limit minimum principal strains of sheet were obtained through multiple sets of test and simulation. Two material parameters, strength coefficient K and hardening exponent n are introduced into the FLSD function which is established by the strain-stress transformation function. The function shows that the k-value determines the value of the maximum principal stress, while the n-value affects the curvature of the curve. Verification of correctness by testing and simulation to within 10% accuracy. This paper explores a new approach to FLSD research based on material properties, which can expand the application scope of FLSD.

Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

One of the objectives for the development of high-strength dual-phase (DP) steel is improving the stretch-flangeability. Large-strained sheared edges are deformed and frequently cracked during stretch-flange formation. Considering shearing as the first deformation, the stretch-flange deformation may be regarded as a secondary deformation. To improve the stretch-flangeability of the DP steels, many researchers have analyzed the microvoid formation. However, in these analyses, the shearing process was not considered. With this background, ex-situ mini-bending tests combined with scanning electron microscopy (SEM) monitoring of microvoid formation were conducted during the secondary deformation. Prior to the secondary deformation, several microvoids were observed on the sheared surface and fine subgrains formed in the ferrite. During secondary deformation, the preliminary microvoids present at the ferrite-martensite interface propagated into the ferrite phase. In contrast, this behavior was not observed for the reamed surface deformation, which was formed without preliminary deformation. Furthermore, microvoids were initiated on ferrite grains that were not present at the ferrite-martensite interface, and martensite islands were not cracked during secondary deformation. This result is noteworthy because martensite cracking was the main factor involved in microvoid initiation, in the absence of shearing. Electron backscattering diffraction analysis revealed that the work hardening of ferrite, prior to the secondary deformation, caused a deviation in the strain concentration sites from those found in the reamed surface deformation. Therefore, this study elucidated microvoid formation on preliminary deformed surfaces via shearing and provided insights for material development considering deformations on the sheared surfaces of materials.

Ductile fracture of dual-phase steel at elevated temperatures

Structural steels can be subjected to a range of stress and temperature conditions, and accurate characterization of their limit states is a requirement for structural design. Yet data on ductile fracture at elevated temperatures remains relatively scarce especially for high-strength steel grades. This paper describes an experimental and numerical investigation of the ductile fracture of a high-strength dual-phase steel under multiaxial stress states at temperatures up to 700 °C. Experiments were carried out on five geometries of specimens and at four temperatures to identify the temperature-dependent fracture locus across a range of stress triaxialities. Finite element analyses of the experiments provided the equivalent strain at fracture initiation. It was found that the fracture initiation strain varies markedly with stress triaxiality and increases with temperature. The effect of strain rate become significant at 500 °C. A temperature-dependent plasticity-damage model was calibrated to predict ductile fracture initiation in the dual-phase steel. The proposed model can be used in finite element analyses to support performance-based design of steel structures at elevated temperatures resulting from accidental events such as fire.

Peculiar roles of nickel diffusion in intermetallic compound formation at the dissimilar metal interface of magnesium to steel spot welds

There is an increased need to join magnesium alloys to high-strength steels to create multi-material lightweight body structures for fuel-efficient vehicles. Direct joining of these two metals is difficult, primarily because pure Mg and pure Fe are immiscible. Although a variety of interlayer materials have been used to facilitate the dissimilar joining in the literature, it is not well understood how the interlayer material affects the joint properties. In this study, a newly developed process, ultrasonic interlayered resistance spot welding (Ulti-RSW), was used to join an Mg alloy to a dual phase high-strength steel with two different interlayer materials: austenitic stainless steel and pure nickel. The IMCs formed with the two interlayers were drastically different, and computational thermodynamics and diffusion simulations revealed that such different IMCs resulted from the peculiar difference in nickel dissolution governed by local chemical potential gradients and diffusion kinetics. The joint with austenitic stainless steel interlayer had an under one micron thick Al-Fe IMC layer and a joint efficiency superior than that with the nickel interlayer as well as other joining processes in the literature.

Corrosion mechanism of advanced high strength dual-phase steels by electrochemical noise analysis in chloride solutions

The corrosion behavior of ferrite-martensite and ferrite-bainite dual-phase steels in NaCl, CaCl2, and MgCl2 solutions was studied by cyclic potentiodynamic polarization (CPP) and electrochemical noise (EN) techniques. The results showed a trend for mixed corrosion due to no passivation by a protective oxide layer. For the ferrite-martensite dual-phase steels, the ferrite phase was preferentially corroded compared with the martensite phase in test solutions. Bainite-microstructure in ferrite-bainite dual-phase steels was preferentially corroded compared with ferrite-phase in CaCl2 and MgCl2 solutions. The corrosion behavior in test solutions in dual-phase steels could be divided into three steps: dissolution in anodic-phase/microstructure, non-uniform corrosion, and localized corrosion. The results indicated that factors such as galvanic coupling between ferrite/martensite, ferrite/bainite, metallic-matrix/inclusion-phases, metallic-matrix/secondary phases, and self-corrosion of each phase and microstructure contributed to the non-uniform and localized corrosion in dual-phase steels.

EFFECT OF RETAINED AUSTENITE ON PLASTIC CHARACTERISTICS OF DUAL PHASE STEELS

In terms of the current trend of research and development of new materials and optimization of current materials in the automotive industry, the greatest attention is paid to progressive high-strength dual-phase (DP) steels with increased stampability, which are designed for cold stamping for specific internal car body components of the current market. New grades of DP steels provide a combination of high strength and good formability and contribute to the weight savings of vehicle parts by 10 to 20 %, compared to current DP grades. Thanks to their top properties, DP steels with increased formability can absorb more crash energy using less steel. As a result, high-strength DP780GI and DP780GI-HF materials of first generation (hereinfater DP780GI-HF) were analyzed. The stampability improvement of DP steels was demonstrated by the experimentally determined Forming Limit Curves for both steels.

In-situ analysis of the elastic-plastic characteristics of high strength dual-phase steel

Modeling the elastic behavior of dual-phase steels is complex due to the strain dependency of Young's modulus and high elastic nonlinearity. Since it is assumed that reasons for this are to be found in microstructural behavior, microscopic in-situ analysis are necessary, but due to the overlap of the martensite and ferrite peaks, the evaluation of diffraction profiles is highly complex. Within this work, CR590Y980T (DP1000) is investigated in a continuous cyclic tensile and tension-compression test under synchrotron radiation at High Energy Material Science beamline P07 in Petra III, DESY. On basis of additional EBSD measurements, an evaluation approach is shown to analyze the dual-phase diffraction profiles in such a way that martensite and ferrite can be separated for three lattice planes. The origin of the specific elastic-plastic behavior of dual-phase steels in terms of onset of yielding, anelasticity or early re-yielding is analyzed on the basis of lattice strains and interphase stresses. For this, the time-synchronously measured micro data is correlated with the macro stress-strain relationship and thermoelastic effect. The results help to better understand strain-dependent elastic-plastic behavior of DP steels on a micro level and provide great potential to improve characterization and modeling in terms of springback prediction.

Microstructure-Based Modelling of Elastoplastic Properties and Deformation Characteristics of Advanced High Strength Dual-Phase Steel

Microstructure-Based Modelling of Elastoplastic Properties and Deformation Characteristics of Advanced High Strength Dual-Phase Steel

Optimization of mechanical properties of high strength dual phase steel through thermo-mechanical simulation

Optimization of mechanical properties of high strength dual phase steel through thermo-mechanical simulation