DP600 Steel Research Articles

Novel method to assess anisotropy in formability using DIC

In this study, a novel technique was developed to identify localized neck of a tensile sample based on the curvature of the surface that is expected to work with any metal in which a localized neck forms prior to fracture. Moreover, a MATLAB-based computational tool has been developed to conduct advanced mathematical computations and numerical analysis on data generated from uniaxial tensile test of DP980 steel coupled with Digital Image Correlation (DIC) based on the novel curvature method. The presented curvature technique is a geometry-based approach that uses the specimen's surface profile and groove geometry to detect localized necking, avoiding the limitations of strain-based methods in distinguishing between localized and diffuse necking. A detailed analysis was conducted on the accuracy of the onset and anisotropy of localized neck. The results of the study revealed that specimens fabricated with 30-degree orientation with respect to the Rolling Direction (RD) of the sheet metal, exhibit a higher level of total strain at the onset of the localized necking, indicating the influence of anisotropy on the material's behavior for DP980. Moreover, consistent R-values were observed among specimens with the same orientation with respect to the RD, exhibiting a rising trend of R-value for orientations from zero to 60-degree, followed by a decrease from 60 to 90-degree. Furthermore, the results exhibited a negative linear relationship between R-values and the magnitude of thinning strain.

Study on the fatigue properties of SPR-bonded aluminum-steel sheet joints

This study deals mainly with the influence of adhesive layer on the fatigue behavior and fretting of self-piercing riveted (SPR) joint joining differing thicknesses of AlSi10MnMg aluminum and DP590 steel sheets in lap shear geometry. Fatigue life, failure modes, crack propagation and fretting behaviors of the SPR and SPR-bonded joints were investigated and compared. The results showed that the static strength and fatigue life of SPR-bonded joints were significantly enhanced than that of SPR joints, but the fatigue failure mode of SPR-bonded and SPR joints differed. The SPR-bonded joints all failed in a rivet tail pull-out mode at all tested loads, which accompanied with small cracking in the aluminum sheet in contact with the rivet tail at lower test load. However, the failure mode of the SPR joints shifted from the aluminum sheet cracking to rivet tail pull-out as the fatigue test load increased. And during the aluminum sheet cracking failure, the fatigue crack originated from the faying interface of aluminum and steel sheet at the vicinity of the rivet shank and propagated along the width direction of aluminum sheet. Fretting wear analysis showed that the overall extent of fretting decreased and the location of fretting areas varied due to the adhesive bonding. Fretting of SPR joints mostly existed at the faying interface between aluminum and steel about 1.5mm away from the rivet shank and resulted in fatigue crack initiation, while the fretting region of SPR-bonded joints shifted to the faying interface between rivet tail and aluminum sheet.

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.

Correlation between individual phase constitutive properties and plastic heterogeneities in advanced-high strength dual-phase steels

The present study comprehensively investigates the individual phase constitutive properties and plastic heterogeneities in advanced high-strength steels (AHSS), particularly DP590 and DP780 dual-phase (DP) steels. A machine learning-based model is implemented to identify the ferrite and martensite phases in the microstructures of DP590 and DP780. Then, the constitutive properties of ferrite and martensite phases are successfully obtained through a hybrid approach of in-situ neutron diffraction coupled with the crystal plasticity finite element method (CPFEM). The distinct microstructures between DP590 and DP780 result in different macroscopic and microscopic properties among the two materials. Owing to different martensite volume fractions (Vm) in DP590 (Vm = 8.3 %) and DP780 (Vm = 35.4 %), a noticeable dependency of plastic heterogeneities during deformation on martensite fraction and its spatial distribution is revealed. Compared to DP590, the deformed microstructure of DP780 exhibits a more heterogeneous distribution of stress and strain fields, along with significant formation of plastic strain localization leading to a remarkable increase in strain partitioning index. It shows that a lower fraction of martensite with its discrete distribution decreases martensite ability to hinder the ferrite deformation, thus strain localization is primarily concentrated within the ferrite phase as the predominant failure mode in DP590. In contrast, a higher martensite fraction in DP780 causes more pronounced strain localization which occurs in the ferrite and at the ferrite/martensite interface. In addition, interconnect distribution between martensite islands enhances the inhibition of martensite to ferrite deformation, thereby high strain gradient at their interface leads to prevalence of ferrite/martensite interface decohesion in DP780.

Microscale modeling of damage mechanisms in dual‐phase steel DP800

Microscale modeling of damage mechanisms in dual‐phase steel DP800

Role of Si element in the IMC formation and tensile-shear property of the laser welding-brazing steel-Al dissimilar joints

A comparative investigation was conducted to explore the role of Si in the intermetallic compound (IMC) formation and tensile-shear property of steel-Al joints fabricated via laser welding-brazing (LWB) technique. Three filler wires including ER5356, ER4043 and ER4047 were employed to join the DP780 steel and 5754 Al alloy. The results exhibited that the presence of Si could improve the tensile-shear property of steel-Al joints. With increasing the Si addition, the failure locations of the joints changed from the interface, weld metal to Al alloy base metal. For the joints fabricated with ER5356 filler, the IMCs consisted of Fe2Al5 and Fe4Al13. On the contrary, Fe4(Al, Si)13 and Al8Fe2Si was detected for the joints prepared with ER4043 and ER4047 filler wires. The Si could suppress the generation of brittle Fe2Al5. In addition, Fe4(Al, Si)13 and Al8Fe2Si could resist the crack propagation, guaranteeing the qualified tensile-shear strength of the joints.

The calibration of the MMC damage model under various hardening models and yielding criterions for DP800 steel

The calibration of the MMC damage model under various hardening models and yielding criterions for DP800 steel

Research of microstructure and properties of resistance spot welded joints of galvanized DP steel for automobile

Research of microstructure and properties of resistance spot welded joints of galvanized DP steel for automobile

Optimizing sheet metal edge quality with laser-polishing: surface characterization and performance evaluation

Dual-phase (DP) steels are widely used in the automotive industry due to their exceptional performance. It offers excellent strength, ductility, formability, and weldability. However, there is a high risk of edge cracking, particularly in materials like DP1000 steel, caused by residual damage from blanking, such as microcracks and burrs, which needs further investigation. In this study, the transformative potential of laser-polishing on DP1000 steel was investigated. The goal was to reduce edge crack sensitivity and enhance edge formability. In this work, laser-polished samples produced by various pre-manufacturing techniques such as sawing, punching, and waterjet cutting were examined. Various evaluations were performed on laser-polished samples. Those included white-light-confocal microscopy, scanning electron microscopy, and Electron Backscatter Diffraction (EBSD) analysis. Those evaluations aimed to analyze the microstructural transformation, surface roughness, and micro grain size distribution resulting from laser-polishing. Laser-polishing is a process in which the edge of the sample is remelted locally. Hence, residual damage vanishes, and surface defects disappear, which should be beneficial for edge formability. On the other hand, the cooling rate during re-solidification is high, leading to high strength and reduced ductility compared to the initial DP steel. Therefore, hole expansion tests were conducted to evaluate the edge formability of the steel. The results indicated a significant improvement in the hole expansion ratio of the laser-polished samples compared to samples with conventional manufactured edges. These findings will help to assess the advantages and limitations of laser-polishing in sheet material manufacturing.

Characterization of Microstructure–Mechanical Properties Relationship in Dissimilar Resistance Spot Welding of the Advanced High-Strength Steel DP590 Steel to the Interstitial-Free Steel DC 06: EBSD Study

Characterization of Microstructure–Mechanical Properties Relationship in Dissimilar Resistance Spot Welding of the Advanced High-Strength Steel DP590 Steel to the Interstitial-Free Steel DC 06: EBSD Study

Analysis of the Microstructure and Mechanical Performance of Resistance Spot-Welding of Ti6Al4V to DP600 Steel Using Copper/Gold Cold-Sprayed Interlayers.

In this article, an attempt was made to join DP600 steel and Ti6Al4V titanium alloy sheets by resistance spot-welding (RSW) using an interlayer in the form of Cu and Au layers fabricated through the cold-spraying process. The welded joints obtained by RSW without an interlayer were also considered. The influence of Cu and Au as an interlayer on the resulting microstructure as well as mechanical properties (shear force and microhardness) of the joints were determined. A typical type of failure of Ti6Al4V/DP600 joints produced without the use of an interlayer is brittle fracture. The microstructure of the resulting joint consisted mainly of the intermetallic phases FeTi and Fe2Ti. The microstructure of the Ti6Al4V/Au/DP600 joint contained the intermetallic phases Ti3Au, TiAu, and TiAu4. The intermetallic phases TiCu and FeCu were found in the microstructure of the Ti6Al4V/Cu/DP600 joint. The maximum tensile/shear stress was 109.46 MPa, which is more than three times higher than for a welded joint fabricated without the use of Cu or Au interlayers. It has been observed that some alloying elements, such as Fe, can lower the martensitic transformation temperature, and some, such as Au, can increase the martensitic transformation temperature.

Damage parameters identification of dual-phase 800 steel based on microvoids analysis and response surface methodology

Accurately identifying the unknown parameters of the Gurson-Tvergaard-Needleman (GTN) damage model is essential to better understand the damage behavior of materials and improve the simulation accuracy. In order to solve the problem that traditional parameter identification methods are cumbersome and inefficient, this paper proposed an efficient damage parameter identification strategy with the combination of microscopic analysis, finite element simulation, and response surface analysis. The relationship between the evolution of microvoids and plastic strain in DP800 steel was quantified through the in-depth analysis of the microstructure of tensile specimens. Response surface method and Box-Behnken experimental design were adopted to efficiently identify and optimize the crucial damage parameters (fN, fC, fF) with the difference ratio of fracture strain as the response variable. Through analysis of variance, the factors that have significant influence on the damage parameters were further confirmed. Through the experimental comparison with the simulation results based on the GTN damage model, the effectiveness of the identified damage parameters was verified, and the influence of these parameters on the stress-strain curve was deeply revealed. The results show that the damage parameter identification strategy has achieved remarkable results in accuracy and efficiency, and has important engineering significance for the accurate prediction of the actual dual-phase steel sheet forming process.

Manufacturing of new dual phase steel with a combination of macro-laminated and micro-laminated morphology

ABSTRACT In this study, a new dual phase steel with a combination of macro-laminated and micro-laminated morphology was fabricated for the first time. Friction stir welding, heat treatment, and cold rolling were employed to produce the new laminated DP steel. The microstructure of produced DP steels consisted of elongated ferrite and martensite phases in the St52 layers. The microhardness profiles and stress–strain curves of produced laminated DP steels showed that the best mechanical properties corresponded to the traverse speed of 70 mm/min due to less hardness gradient. The failure morphology indicated good interfacial bonding that prevented delamination during the tensile deformation.

Damage mechanisms analyses in DP steels using SEM images, FEM, and nonlocal peridynamics methods

An investigation is performed to study the damage mechanisms of dual phase steels using experimental analysis, local finite element simulation, and nonlocal peridynamics (PDs) method. The uniaxial tensile tests and preparation of metallography and SEM images have been used to peruse the pattern of deformation and voids in different phases. Understanding of the relationship between real microstructure and damage mechanisms can often not be obtained from microstructural investigation using experiments only. Here, a novel numerical approach is presented to investigate damage mechanisms in multiphase materials at micro scale level using both local and nonlocal numerical analyses and microstructural images from a scanning electron microscope (SEM). The nonlocal PDs method is capable to overcome many challenges associated with convergence problems in other numerical methods and to detail-oriented study of damage initiation and propagation autonomously. In this study, images of real microstructure are used as RVEs and imported to the FEM software and PD-developed code. Moreover, the effects of different constitutive models for the interface of ferrite and martensite phases on the bond breakage and damage patterns have been also investigated.

Design method of immiscible dissimilar welding (Mg/Fe) based on CALPHAD and thermodynamic modelling

Joining dissimilar metals is a major challenge in joining technology; the weldability of immiscible systems is especially challenging. In this study, a design methodology for dissimilar welding is suggested. The Miedema model and Toop model are developed to calculate the thermodynamics of quaternary alloy systems (Mg-Fe-Al-Cu). Finite element modelling (FEM) of temperature fields and the calculation of phase diagrams (CALPHAD) are combined to provide prerequisite information for modelling. As a test subject, laser welded lap configuration joints of AZ31B magnesium alloy and DP590 steel with a copper coating were put into the design scheme. The interfacial elemental diffusion and formation of intermetallics (IMCs) along the interface during the welding process are predicted. This simulation design scheme predicts the interfacial reaction kinetics and identifies whether the intermediate element works or not. The effects of the Cu coating thickness on the weld constitution, interfacial microstructures and mechanical properties were studied. Cu coating promotes the weld formation fostering the metallurgical reaction of the fusion zone (FZ) with the steel brazing interface. The mechanism of interfacial reactions during the welding-brazing process has been clarified. The Vickers hardness distribution across the interface shows that the Cu-IMCs are ductile.

A comparative study of the galvanized DP980 steel and 2 GPa PHS welded joint via laser beam welding and cold metal transfer welding

Galvanized DP980 steel and Al-Si coated 2 GPa press-hardened steel (PHS) were joined via laser beam welding (LBW) and cold metal transfer (CMT) welding, respectively. The weld metal (WM) of LBW joints contains martensite, acicular ferrite, and bainite. The WM of the CMT joint consists entirely of acicular ferrite. The tensile strength of the LBW joint is 895 ± 5 MPa, slightly higher than that of the CMT joint by 50 MPa due to the presence of martensite. The total elongation is 4.1 ± 0.2 % and 5.0 ± 0.3 % for the LBW and CMT joints, respectively. The enhanced plasticity of the CMT joint was due to the soft WM plastic deformation, which increased the total deformation.

Development of a High Ductility DP Steel Using a Segregation Neutralization Approach: Benchmarked Against a Commercial Dual Phase Steel

Commercial dual-phase steels are typically synonymous with a banded distribution of martensite in their microstructures, which can degrade ductility and increase the anisotropy of mechanical properties. The concept of neutralizing the effect of Mn segregation is employed to change the distribution of martensite to a non-banded distribution. To this end, the ratio of austenite and ferrite stabilizer elements has been changed in the composition of dual-phase steel. Microstructural analysis has been carried out on both hot-rolled (ferrite + pearlite) and heat-treated (ferrite + martensite) microstructures by optical microscope and EBSD, respectively. The microstructural examinations have confirmed the non-banded distribution of second phase and more equiaxed ferrite grains in the segregated neutralized grade microstructures compared to a commercially benchmarked dual-phase steel. Tensile properties of two grades have also been assessed in hot-rolled and heat-treated conditions in RD, TD, and 45 deg tensile directions. In the case of heat-treated condition, total elongation in RD direction has been improved from 20.9 pct in benchmark dual-phase steel to 25.4 pct in segregated neutralized dual-phase steel. Tensile anisotropy results showed a significant difference in tensile strength by tensile direction in benchmark dual-phase steel in both hot-rolled (~ 85 MPa) and heat-treated conditions (~ 48 MPa), while the corresponding differences for the segregated neutralized grades were 14 and 15 MPa, respectively.

Effects of Laser Scanning Strategy on Bending Behavior and Microstructure of DP980 Steel

Laser bending is a kind of cumulative forming technology and bending efficiency is one of its most important indexes. This study investigates the bending behavior and the microstructure of DP980 steel plates under different laser scanning strategies, using an IPG laser system. Two sets of experiments varied the accumulated line energy density (AED) by altering the laser scanning velocity and number of scans. The results show that, for the single laser scanning process, the bending angle of the plate increases with AED, due to a larger temperature gradient through the thickness direction; however, this relationship is nonlinear. A higher AED led to a sharper initial increase in bending angle, which then plateaued. Under the same AED conditions, the bending angle of the plate undergoing multiple laser scans increases by at least 26% compared to the single one, due to the microstructure changes. It is revealed that the bending efficiency is affected by both the AED and the resultant microstructure evolution in the DP980 steel. Higher AED values and appropriate peak temperatures facilitate better bending behavior due to the formation of uniform martensite and grain refinement. Conversely, excessive peak temperatures can hinder bending due to grain growth.

Microstructure and cyclic hardening/softening behavior of laser welded high strength steel joints under asymmetrical cyclic loading

The cyclic behavior of materials under load and its influence on the mechanical properties of materials have always been the focus of engineering research. In this study, high strength steel was successfully welded by laser welding. The microstructure of each region of the high strength steel DP590 post-welding was meticulously analyzed. Additionally, 500 cycles of low-cycle fatigue tests of laser welded high strength steel were carried out by using asymmetric stress control method. After fatigue test, tensile test shall be carried out on some samples. Under the specific combination of mean stress and stress amplitude, compare the specimens that have not been fatigue tested with those that have been fatigue tested, the change of properties of laser welded high strength steel joint under asymmetric cyclic load was analyzed. The results showed that, due to cyclic hardening, the tensile strength of the sample increased to 601 MPa and the yield strength to 325 MPa. Compared with the original sample, the tensile strength and yield strength of the sample after asymmetric cycling had increased by 8.65% and 31.3%, respectively.

EBSD Study of the Effect of Post-weld Intercritical Annealing on the Microstructure and Micro-texture of a Friction Stir Welded DP600 Steel

EBSD Study of the Effect of Post-weld Intercritical Annealing on the Microstructure and Micro-texture of a Friction Stir Welded DP600 Steel