DP600 Steel Research Articles

Effect of strain rates on the forming properties of DP Steel at high-temperature

Dual Phase steel grade, with its remarkable combination of features including formability and resistance to high temperatures, has been a revolutionary alloy in modern industry. It is a useful material that may be applied to many different tasks. It presents us with the task of examining its characteristics in response to changes in critical parameters such as temperature and loading rate during stretching operations. Understanding the formability behavior of DP steel sheets, which have a thickness of 1 mm, at various increased temperatures and strain rates was the main goal of the investigation. We investigated and examined the tensile characteristics at 650 and 750 degrees Celsius in the first investigation. On the other hand, because of flow stresses in the material at higher temperatures, elongation has decreased in value as deformation rates have increased. In addition, we plotted the FLDs experimentally and examined the formation behavior while performing the Nakazima Test on specimens with a thickness of 1 mm. Furthermore, a limiting dome height (LDH) study was conducted on a laboratory scale. In terms of temperature, the LDH was shown to be higher at 750 and at 0.001/s

Determination of Modified Mohr-Coulomb Damage Model Parameters for DH780 Steel in Finite Element Analysis

In sheet metal forming processes, tearing problems might be occasionally encountered due to many reasons such as incorrect forming parameters. The trial and error methods that are used to solve such problems, on many occasions, are time-consuming and inefficient in terms of finding the correct forming parameters or die design for the forming process. The finite element analysis method, on the other hand, can be used as a tool that is both time and cost-saving. However, in order to effectively exploit the use of finite element analysis in sheet metal forming operations, the material that is used to be formed needs to be well characterized in terms of its hardening behaviour and failure criteria. In this study, a TRIP-aided DP steel (DH780) has been tensile tested in three different deformation conditions (uniaxial, plane stress and shear) and the parameters of its hardening model (Hollomon) and failure criteria (Modified Mohr-Coulomb) have been determined. According to the simulation results, obtained hardening parameters are able to describe the flow behaviour of the steel and the used failure criterion is able to predict the experimental failure correctly in each deformation condition.

Hydrogen Diffusion and Trapping Behaviour in DP Steels: An Electrochemical Hydrogen Permeation Study

Advanced high-strength steels (AHSSs) are widely used in the automotive industry as they possess high strength-to-weight ratio. This allows designing light vehicles with better passenger safety, lower fuel consumption, and CO2emissions. Dual-phase (DP) steels are very attractive among the AHSSs because of their optimal combination of high tensile strength (up to 1.2 GPa), high ductility (10 % or more), and good formability due to their two-phase microstructure (ferrite and martensite). However, the use of DP steel is associated with an increased risk of mechanical degradation in the presence of hydrogen environment, i.e., hydrogen embrittlement (HE). The HE susceptibility of DP steel is mainly determined by hydrogen entry, diffusion, and trapping events in the microstructure (i.e., hydrogen interaction with metal). There are various methods to study the above mechanism in metals like hydrogen charging-tensile test, thermal desorption spectroscopy, silver decoration, scanning kelvin probe microscopy, electrochemical hydrogen permeation, etc. Among these, electrochemical hydrogen permeation using Devanathan - Stachurski (DS) cell is a simple and most widely used technique.In this work, electrochemical hydrogen permeation was used to understand the role of microstructural features on hydrogen diffusion and trapping behaviour in DP-980 steel. It was identified that the phase fraction and its morphological features influence the hydrogen diffusion behaviour. The challenges in layout, design and integration of conventional DS cell into a tensometer and the effect of pre-strain (ex-situ) and tensile stress (in-situ) loading on hydrogen diffusion and trapping in DP-980 will be discussed. The applied elastic stress (i.e., 50 % of YS) during the permeation test resulted in a slight increase in Dapp from 5.4 ± 0.1 × 10- 7 to 5.9 ± 0.69×10-7 cm2s-1 and a slight reduction in NT from 1.08 ± 0.01 × 1021 to 1.0 ± 0.13× 1021 sites cm-3 compared to the no-stress condition. At 110 % of YS, the highest value of iss is 169 ±10.6 (µAcm-2) and a significant increase in the value of Capp to 3.6 ± 0.3 × 10-4 (molHcm-3 ), whereas no significant change in values of Dapp and NT were observed at 110 % YS compared to no stress and 50 % of YS. This could be due to the elastic deformation of martensite at 110 % of YS. The lattice expansion of martensite under elastic tensile stress can accommodate more interstitial hydrogen, thereby higher steady-state permeation current is observed. At 125 % of YS, the value of Dapp (2.86×10-7 cm2s-1) is approximately 50 % less compared to the value observed for the un-stressed specimen (5.4 ± 0.1×10-7 cm2s- 1). The value of NT (2.11×1021 cm-3) is approximately two times higher than that of the value (1.08×1021 cm-3 ). Plastic deformation of both ferrite and martensite results in increased dislocation density (hydrogen trap sites) at 125 % of YS. The sample failed during the hydrogen permeation test and when it is subjected to 125 % YS. It exhibited distinct fractographic features along the thickness of the sample. Quasi-cleavage and intergranular cracks were observed near the cathodic surface. Whereas, dimples and microvoids which are complete ductile features, were observed near the anodic surface. At 125 % of YS a significant decrease in the value of DL 0.6×10-6 cm2s-1 was observed. Whereas the value of NTrev increased to 4.0 ± 0.2×1019 cm-3 from 3.0 ± 0.15×1019 cm-3.An attempt was made to study the hydrogen transport behaviour at the local spot by appropriate modification of conventional DS cell. The fabrication challenges and the potential application of such an instrument will be elucidated.

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.

The Influence of Springing of High-Strength Steel DP780 on the Limiting Possibilities of its Forming during Electro-Hydraulic Stamping

Using the example of stamping a box-shaped part from high-strength sheet steel by a pulsed electro-hydraulic method, the limiting possibilities of its shaping, taking into account the springing of the material, are studied. The influence of the radius of the curvature and the shape of the surface of the corners of the part on the change in the structure of high-strength steel DP780 and the appearance of defects in it is determined. Relations between the radius of the curvature of the surface of the part and the thickness of the workpiece are obtained, at which there are no defects in the structure of DP780 steel and the limiting possibilities of its deformation are achieved with minimal springing of the material.

Study on the Micro-Texture and Anisotropy of a Cold-Rolled and Intercritically Annealed DP800 Steel

Study on the Micro-Texture and Anisotropy of a Cold-Rolled and Intercritically Annealed DP800 Steel

Influence of Temperature on the Forming Limits of High-Strength Low Alloy, and Dual-Phase Steels

High-strength steels (HSS) appear as a good alternative to common steels to reduce vehicle weight, thus reducing fuel consumption. Despite the excellent mechanical behavior towards its lower weight, its application in industry is still limited, as manufacturing such materials suffers from limitations, especially regarding formability. The literature shows springback to be the most common problem. Among the parameters that can be studied to minimize this problem, the temperature appears, according to the literature, to be one of the most influential parameters in minimizing springback. However, the consequence of the temperature increase on the forming limits of materials is not completely understood. This study proposes to determine the consequences of the use of the temperature rise technique in the forming limits of high-strength steels. Two different steels were studied (HSLA 350/440 and DP 350/600). To evaluate the formability, the Nakazima method was used (practical). Finite element models were made which describe the material as well as Nakazima experimental behavior. To predict the forming limit strains via the numerical method, the thickness gradient criterion was applied. The practical and computational results were compared to validate the finite element model. Four different temperature ranges were analyzed. In general, it was found that 400 °C has a negative impact on the forming limits of both steels. This negative effect was found to be due to the alloying elements, such as silicon and manganese, present in the alloy. These alloying elements take part in the increase and decrease in resistance coefficient at the elevated temperature. For HSLA 350/440 steel, the forming limit strain decreased with an increase in temperature up to 600 °C and then increased at 800 °C; whereas for DP 350/600 steel, the forming limit strain decreased till 400 °C and then increased for 600 °C and 800 °C. Another factor which might have contributed to the behavior of the DP steel is the interaction of hard martensite with soft ferrite phase.

Effect of a Zn interlayer on the formation mechanism and mechanical performance of AA6061/DP590 steel resistance riveting welding

This study focuses on the forming mechanism of resistance riveting welding joints of 6061 Al alloy and DP590 steel plate and the effect of adding Zn interlayer on the mechanical properties of the joints. During the piercing stage, the rivet pierces through the Al plate, the Al plate beneath the rivet melts and reacts with the steel, with intermetallic compounds (IMC) generated. Some of the melted Al and IMCs are squeezed onto the side of the rivet. In the process of adding the Zn interlayer, the Zn is gradually incorporated into the melted Al near the rivet during the piercing process. At the welding stage, as the current is increased, the nugget diameter, peak joint load, and energy absorption values gradually increase. The failure mode of the joint changes from an interfacial fracture to pull-out fracture, eventually changing to base material pull-off, as the welding current increases. Compared to joints without Zn addition, joints with Zn addition exhibit a smaller nugget diameter, and narrower IMC layers at the interface. At a welding current of 13 kA, because the failure mode was a shear fracture of the nuggets, the peak joint load of the joints with zinc added decreased by 215 N compared to the unadded ones. On the contrary, when the current increased to 14 kA, the peak joint load of the joints was elevated by 639 N, because the IMC layer and the aluminum plate became the main load-bearing locations.

Semi-automatic miniature specimen testing method to characterize the plasticity and fracture properties of metals

Miniature-testing is essential to characterize the plastic response of materials for which the extraction of conventional full-size specimens is impossible. Examples are weld regions with high property gradients or electronic circuit components. Here, we present the design of a novel testing system for miniature specimens. After inserting the specimens with wedge-shaped shoulders, the system carries out the clamping, initial geometry scanning, and mechanical testing with digital image correlation in an automated manner. The automation prevents the accidental damaging of the miniature specimens during manual handling and adds to the robustness of the overall procedure. It is shown that the experiments with miniature specimens extracted from DP600 steel and aluminum 7075-T6 are highly reproduceable. More importantly, they lead to the same plasticity and fracture properties as experiments on conventional ten-times larger full-size specimens. A correction procedure accounting for the surface roughness, white layer and heat affect zone induced through wire EDM is provided. In addition to developing and validating the new experimental technique, it is applied to characterize a 22MnB4 rod material and stainless steel 316 L foil.

Effect of compressive load on texture evolution and anisotropic behavior of dual-phase steel under biaxial loading in complete σ11-σ22 space

Advanced high strength steels (AHSS) are typically loaded in a multiaxial stress state during forming process and service. However, the deformation mechanism under multiaxial loading is not clarified, which limits the optimization of sheet metal forming. In particular, due to instability, biaxial compression loading of single thin plate has not been reported, which results in unclear evolution of the yield surfaces in the second, third and fourth quadrants of σ11-σ22 space and corresponding deformation mechanisms. Therefore, the deformation mechanism of AHSS thin plates under biaxial loading was systematically investigated in the complete σ11-σ22 space using a specially designed cruciform specimen and buckling prevention fixture in the current work. The mechanical properties of dual-phase (DP780) steel under different loading paths were studied by uniaxial tension, uniaxial compression, and biaxial loading tests. There is an obvious yield strength difference between the first quadrant and the third quadrant in the σ11-σ22 space. In the second and fourth quadrants of σ11-σ22 space, the compression part makes a greater effect on the yield behavior of the material than the tension part. More specifically, dislocation slip is activated earlier at the boundary under compression loading, resulting in earlier yielding of the material. Based on the analysis of the Taylor factor, the activation of slip systems of DP780 steel before and after deformation is clarified. In addition, a detailed analysis of the microstructure and texture evolution in DP780 steel after deformation is conducted, and a correlation between texture evolution and loading paths is established. It is found that the compression part under biaxial loading results in more grains with low Taylor factor and promotes the transformation of the initial texture to copper or rotated copper texture in DP780 steel.

A comparative study of microstructure and corrosion resistance in DP980 steel joints under different laser welding method

Three different laser welding methods were used to welding 2 mm DP980 steel butt joint, which were laser welding, laser-TIG hybrid welding and laser powder feeding (CoCrNi) welding respectively. An observational analysis was carried out in terms of microstructure, mechanical properties and corrosion resistance. It was discovered that a central microstructure of laser welding and laser-TIG hybrid welding the weld was dominated by slatted martensite. The laser powder feeding welding has resulted in a remarkable reduction of cylindrical crystals and an increased number of isometric crystals in weld center due to the addition of CoCrNi powder. Microstructure was further refined in incomplete phase transition zone by three different laser welding methods. The tempered martensite was also formed in the tempering zone, which softened the joint. The hardness values decreased considerably and tensile samples were also fractured at this location. Comparison with corrosion resistance of their joints through polarization curves and impedance spectra shows that laser powder feeding joint > laser joint > laser-TIG hybrid joint. The corrosion resistance of laser and laser-TIG hybrid joints in the base metal (BM) > heat affected zone (HAZ) > weld zone (WZ) is shown based on scanning vibrating electrode technique (SVET) localized area testing data at different time intervals. The laser powder feeding welding enhances the density of passivation film by reacting Cr to form Cr2O3, thereby improving corrosion resistance of entire joint.

Evaluation of resistance spot weldability and weld performance of zinc-coated martensitic steels

This paper explores the resistance spot weldability of three martensitic (MS) steels of 1500 MPa tensile strength. A weld diameter versus current plot was developed for the three MS steels to investigate their weldable current range and expulsion characteristics. The mechanical performance of the welds was investigated using cross-tension, tensile-shear and hardness tests. It was found that the differences in electrical resistivity during the nugget growth period influenced the expulsion limit and, consequently, the weldable current range. The three MS steels failed in pullout failure mode but exhibited different plug ratios. Finally, the mechanical strength of the three MS steels was observed to be better than the other advanced high-strength steels (AHSS) such as DP600, TRIP1000 and PHS1500 steels.

Micromagnetic and quantitative prediction of yield strength and tensile strength in DP590 steels based on ReliefF + Clustering feature selection method

The correlation between multiple patterns of micromagnetic signatures and the mechanical properties (yield strength (Rp) and tensile strength (Rm) of high-strength steel (referred to as DP590 steel in Chinese standards) was investigated in this study. Feedforward neural network (FF-NN) models with carefully selected magnetic features as input nodes were established for quantitative prediction of yield strength and tensile strength. The accuracy FF-NN models highly relied on the quality of calibration specimens and the way of selecting magnetic features. The variations of the measured target properties were used to evaluate the quality of the calibration specimens. The specimens with similar yield strength (or tensile strength) were merged to share the same target properties in the model training process. The results demonstrated that merging proper target properties (label) could improve the performance of the models in quantitative prediction of yield strength and tensile strength in DP590 steels. In addition, the performances of FF-NN models combined with the algorithms of ReliefF and ReliefF + clustering were evaluated. The comparison results proved that the FF-NN models employing input nodes selection strategy of ReliefF + clustering realized the advantages of smaller dimensions of input nodes, less training time consumption at the cost of slight accuracy reduction.

Analysis of the springback effect of laser welded DP600 high-strength steel thin sheets

Steel sheets are produced with increasingly smaller thicknesses, without compromising the properties of the steel, maintaining ductility, strength, and energy absorption after impact can total depth enhance vehicle energy efficiency, lower manufacturing costs, and address the springback effect. The springback effect of laser-welded DP600 two-phase steel sheets was studied in this research. Parameters such as punching speed, initial bending angle, and time were varied. Tests including tensile tests, Vickers hardness, and optical microscopy analysis were conducted to correlate results with material properties. Autogenous laser welding joined DP600 steel plates at the bending position. Controlling springback ensures better quality in vehicle manufacturing and assembly. The results revealed that increasing the punch descent speed from 4 to 12 mm/min, for an internal angle of 90°, on the first day of observation, led to higher springback values (0.58°–3.3°, respectively), while increasing the initial curvature angle (30°–90°), maintaining a constant speed of 4 mm/min on the first day of observation, and impacting the elastic return (6.74°–0.58°, respectively). Furthermore, the observation time demonstrated a continuous increase in the springback variation during 6 days after the flexural test before stabilization (6.89°– 8.7°), maintaining a constant value of 8 mm/min and the internal angle of doubles of 30°.

Laser surface texturing of dies in strip drawing of DP600 steel sheet

ABSTRACT Friction at the blank-die contact influences material flow, formability, and product surface quality in processes like deep drawing. Surface modification of the dies by laser texturing is a potential technique to alter the tribological characteristics at the interface during sliding of the sheet over the die surface in lubricated condition. In this work, the effect of laser surface texturing of dies has been investigated under high normal loads during strip drawing of dual phase (DP600) steel. The effect of texture parameters and process parameters on the interface friction between the strip and the dies has been studied. It has been found that the friction coefficient reduced, and the surface finish improved significantly with the textured dies when compared to the conventional die. This study emphasises the importance of surface modification of the dies by laser texturing for its potential application in sheet metal forming processes.

Parametric study of local laser heat treatment technology on multi forming of advanced-high strength steel (AHSS) part with complex shape

The effectiveness of local laser heat treatment technology to enhance the in situ formability of steels and aluminum alloys has already been widely acknowledged for the one-step forming of components with simple shape geometries. The present study demonstrates that this technology is also able to significantly improve the formability of a complex shaped multi-forming industrial part. An industrial grade advanced-high strength Dual-Phase DP1000 steel is used to analyze the multi-forming of a complex part to determine the most appropriate local laser heat treatment parameters and optimize in situ softening by correlating yield strength, ultimate tensile strength, elongation at fracture, strain hardening exponent and instantaneous strain hardening with local temperature dynamics during the laser treatment. Additionally, numerical simulation analysis using Autoform software is carried out to validate the selected heat affected zone and the in situ softening, ensuring that they are appropriate for improving the formability of the industrial part. These findings are then expanded to study the experimental forming of five in situ laser heat treated models, followed by comparative analysis with a benchmark. This study provides an insight and fundamental guidelines to perform in situ laser heat treatment on complex industrial parts leading to the production of the industrial multi-formed component with optimized formability.

Probing corrosion using a simple and versatile in situ multimodal corrosion measurement system

In this work, we have developed a unique in situ multimodal corrosion system that is capable of acquiring electrochemical data, sample imaging/visualization and hydrogen collection, simultaneously. Each of these modalities yield valuable information pertaining to the ongoing corrosion process. Combining them can yield holistic information on the role of microstructure, processing history, presence of coatings, etc., on the sequence of steps occurring during the corrosion process, and how they correlate with the acquired electrochemical data. Four materials systems, namely AA6061-T6 aluminum alloy, AZ91 magnesium alloy, galvanized DP590 steel, and pure Zn, were investigated under open circuit potential and under potentiodynamic polarization. The multimodal corrosion system was utilized to observe processes such as surface passivation and dissolution, pit and filiform corrosion initiation and propagation, and was correlated with location and magnitude of hydrogen evolution. This approach is shown to yield a truly multimodal understanding of the ongoing corrosion processes.

Comparison of anisotropic yield functions and calibrations for accurate thickness prediction in hole expansion test

The hole expansion test is an appropriate evaluation method for assessing the formability of automotive parts, where fracture prediction using simulated localized thickness is crucial. However, the accurate thickness prediction in the hole expansion test remains challenging. In this study, the selection of yield functions and calibration strategies were investigated to accurately predict the thickness of advanced high-strength steel subjected to the hole expansion test. An accurate description of the material anisotropy is important for thickness prediction because of the generation of various multiaxial stress states. To describe plastic anisotropy, a series of uniaxial tension, bulge, and in-plane biaxial tension tests were conducted on DP980 steel. Because conventional biaxial tension tests for DP980 steel showed a small strain range owing to its low ductility, a new laser-welding-reinforced biaxial tension test was developed to investigate the anisotropy of the plane strain at large strains. Differences in the ratio of the plastic strain rates were observed between the conventional and new tests. Finally, a hole expansion test and corresponding finite element simulations were conducted with different yield functions and calibrations for the Yld2000-2d yield function. The thickness strain profiles along the hole circumference and 3 mm inward were measured and predicted using simulations. In conclusion, both the yield stress and the ratio of plastic strain rates in the plane strain state were equally important for accurate thickness prediction during the DP980 hole expansion test. Furthermore, the crack location was inconsistent with the thinnest region in the experiment, which might be because of the shear fracture anisotropy encountered in DP980 steel.

Finite Element Analysis of DP600 Steel Strips in Deformation Tests

DP600 steels are high strength steels used in structural elements of vehicles. However, they can be used in other sectors, but this requires a study in different working conditions, with the help of tools that facilitate the realization of predictions about the mechanical properties of these materials.The "Design Expert" software was used during this research. In it, the combinable input variables are introduced, which in this case are only the size of the pin and the folding angle of the sheet, so that employing simple combinatorial operations tells us how many and which experiments we will have to perform to cover the different possibilities.Simulations have been carried out with the Deform-2D software, applying the finite element method, to know in advance what is going to be the behavior of the metal sheets interacting with the rotating pin, in processes like drawing.

A Numerical Prediction for Hole-Splitting Damage of DP Steels Based on Plastic Work Criterion Using a Polynomial Stress Potential

A Numerical Prediction for Hole-Splitting Damage of DP Steels Based on Plastic Work Criterion Using a Polynomial Stress Potential