DP600 Steel Research Articles

A model to determine the damage parameters for microstructure of DP590 steel using coupled finite element method and Teacher-Learner based optimization

The dual phase (DP) steels are extensively employed in the large strength applications. The component or part sometimes fail under the applied load during the production stage or in the working conditions. The main constituents of dual phase steel viz. ferrite and martensite play an important role in determining the performance of component during service. Analysis of material using these constituents separately makes it convenient to determine the optimum working conditions. Numerical methods can be applied to determine the parameters of the damage model for these constituents. The current work suggests a method to determine the damage parameters using the “Teacher-Learner based optimization” (TLBO) algorithm and finite element method. The artificial microstructure in microstructural analysis of uniaxial tensile test is mapped to Gauss points in a damage included finite element model. The TLBO algorithm is used to get the parameters of the damage model for each of the microstructural constituents employing the homogenization approach and using the data extracted from the macro scale simulation. It is found that the proposed uncoupled elitist-TLBO algorithm and finite element analysis can effectively compute the unknown parameters in the Lemaitre’s damage model for both the microstructural constituents viz. ferrite and martensite in dual phase steel. The so developed modelling work being generic in nature that can also be used for the computation of parameters for different types of models.

Study on the Effect of Pre-strain on Fatigue Properties of Dual-phase Steel

The uniaxial tensile test investigated the fatigue performance of DP780 steel without pre-strain and with 5% and 10% uniaxial pre-strain. Topography of fractured surfaces was observed to study the relationship between pre-strain and fatigue performance. The results showed that pre-strain the specimens would affect the fatigue performance of DP780 steel. It would cause an increase in fatigue life when the stress was comparatively high and the fatigue life was less than 100,000 cycles. However, the fatigue life was reduced when the stress level was comparatively low, and the fatigue life was more than 100,000 cycles. Pre-strain reduced the fatigue strength of samples. Fatigue strength of specimens without pre-strain and 5% and 10% pre-strain were 318 MPa, 310 MPa, and 303 MPa, respectively. The sample fracture was a brittle fracture with a pre-strain of 5%, while it was a ductile fracture with a pre-strain of 10%.

Development of combined hardening model for spring-back simulation of DP600 in multi-stage sheet metal forming

High strength steel DP600 is widely used in automobile industry due to its excellent mechanical property compared to conventional steel, but spring-back issue is considered to be the major challenge of DP600 in forming process, as the spring-back is triggered by the release of complicated internal stress. In multi-stage sheet metal forming, the sheet may be subject to several cycles of loading and reverse loading, Bauschinger effect greatly influence the strain stress behaviour. A well designed material model can be used to accurately predict and compensate spring-back in forming process. A combined hardening model especially designed for multi-stage forming is proposed in this paper, and the iterated compensation method could be applied by using LS-DYNA to get an optimized result. An experimental testing facility was developed for calibration of this model with cyclic tension and compression strain stress curve. The compensation method is applied in the numerical simulation of a multi-stage manufacturing process of an A-pillar, the result demonstrates the feasibility of the combined hardening model.

Non-associated and Non-quadratic Characteristics in Plastic Anisotropy of Automotive Lightweight Sheet Metals

AbstractLightweight sheet metals are highly desirable for automotive applications due to their exceptional strength-to-density ratio. An accurate description of the pronounced plastic anisotropy exhibited by these materials in finite element analysis requires advanced plasticity models. In recent years, significant efforts have been devoted to developing plasticity models and numerical analysis methods based on the non-associated flow rule (non-AFR). In this work, a newly proposed coupled quadratic and non-quadratic model under non-AFR is utilized to comprehensively investigate the non-associated and non-quadratic characteristics during the yielding of three lightweight sheet metals, i.e., dual-phase steel DP980, TRIP-assisted steel QP980, and aluminum alloy AA5754-O. These materials are subjected to various proportional loading paths, including uniaxial tensile tests with a 15° increment, uniaxial compressive tests with a 45° increment, in-plane torsion tests, and biaxial tensile tests using laser-deposited arm-strengthened cruciform specimens. Results show that the non-AFR approach provides an effective means for accurately modeling the yield behavior, including yield stresses and the direction of plastic strain rates, simultaneously, utilizing two separate functions and a simple calibration procedure. The introduction of the non-quadratic plastic potential reduces the average errors in angle when predicting plastic strain directions by the quadratic plastic potential function. Specifically, for DP980, the average error is reduced from 3.1° to 0.9°, for QP980 it is reduced from 6.1° to 3.9°, and for AA5754-O it is reduced from 7.0° to 0.2°. This highlights the importance of considering the non-quadratic characteristic in plasticity modeling, especially for aluminum alloys such as AA5754-O.

Extraordinary strength–ductility–toughness in Fe–0.08C plain low-carbon steel via introducing weblike martensite: Towards the third generation

The effect of large-strain cold rolling followed by intercritical annealing on the microstructure and mechanical properties of plain low-carbon steel was investigated. The grain size of ferrite reduced from 38 μm (for the as-received steel) to about 4 μm (for the dual-phase steel). The martensite phase was connected in 3D space in the dual-phase steel and a weblike structure of martensite was created. The stress-strain curve of dual-phase steel represents the continuous yield behavior without any distinct evidence of the yield point phenomenon. By introducing weblike martensite into the microstructure and refining α grains, the yield strength, ultimate tensile strength, and tensile toughness significantly increased to 740 MPa, 1062 MPa, and 193 J cm−3, respectively. The fracture surface of the DP sample had a dimpled appearance, indicating a ductile fracture mode. The results of the present work indicated that a low-cost plain low-carbon DP steel composed of weblike martensite and fine-grained ferrite exhibits extraordinary strength–ductility–toughness balance. Therefore, the dual-phase steel produced in the present work, by moving towards the third generation, can find wide applications in the automotive industry.

Investigations of Microstructure and Mechanical Properties of Post-Weld Heat-Treated DP780 Steel TIG Welds

This study investigates the microstructure and mechanical properties of DP780 steel that has been tungsten inert gas welded and post weld heat treated. Microscopy studies revealed that the weldment’s microstructure varied from martensite in the fusion zone to a mixture of martensite and ferrite in the heat affected zone (HAZ). This heterogeneity in the microstructure resulted in the formation of hardened and softened zones in the cross section of the weldment. The DP780 as-welded joint exhibited lower strength and ductility [yield strength (YS): 492 ± 5 MPa, ultimate tensile strength (UTS): 668 ± 8 MPa, and percent elongation (%El): 8 ± 1] compared to the base metal (BM) (YS: 538 ± 2 MPa, UTS: 794 ± 5 MPa, and %El: 27 ± 2) due to strain localization in the subcritical HAZ. The weldments subjected to post weld heat treatment (PWHT) at 500°C exhibited lower strength and higher ductility (YS: 471 ± 3 MPa, UTS: 624 ± 5 MPa, and %El: 13 ± 1) than the weldments subjected to PWHT at other conditions: 300°C (YS: 501 ± 7MPa, UTS: 658 ± 6 MPa, and %El: 9 ± 1) and 400°C (YS: 492 ± 3 MPa, UTS: 649 ± 5 MPa, and %El: 11 ± 1). The decrease in strength and ductility after PWHT can be attributed to the tempering of martensite present in the weldment. Erichsen cupping tests indicated a reduction in the formability of the as-welded joint due to the presence of a softened zone. While a significant increase in formability is observed in the weldments subjected to PWHT with an increase in temperature, the formability is still inferior to that of the BM due to the inhomogeneity in the microstructures across the weldment.

Mitigating HAZ softening of friction stir welded ferrite-martensite DP steel followed by post weld heat treatment

This study investigates the development of microstructure and mechanical properties in welded joints of ferrite-martensite DP780 steel, with different cooling pathways applied after post-weld heat treatment (PWHT). The findings suggest that quenching in liquid nitrogen (PWHT-LNQ) effectively mitigates the softening of the heat-affected zone by suppressing the formation of tempered martensite and reducing carbide coarsening. The technique promotes a refined microstructure characterized by the discrete distribution of block martensite within the ferrite islands. This microstructural arrangement significantly enhances the strength and ductility properties of the material. Notably, PWHT-LNQ achieves the highest tensile elongation of 17%, which is approximately 2.2 times that of the as-welded joint.

A non-local approach for Reissner–Mindlin shell elements in dynamic simulations: Application with a Gurson model

Numerical prediction of ductile tearing during a car crash is necessary to ensure the structural integrity of the vehicle. Recently, encouraging results were obtained by Davaze et al., (2021) with a non-local damage approach, compatible with dynamic explicit simulations and preserving parallel computation: an implicit gradient model enabled to reproduce experimental results with DP450 steel sheets with 3D solid elements. However, in the automotive industry, thin sheets of metals are mostly modeled with shell elements for computation time efficiency. Shell elements have a more complex formulation: their kinematic description is enriched accounting for displacements but also rotations, leading to two different strain contributions: membrane strain and bending strain. The transverse strain is computed independently in a post-processing step from local internal variables with a zero normal stress assumption. This formulation is known to lead to strain localization after the onset of necking, while this is not the case with 3D solid elements. The problem is even more challenging when considering shells in association with a softening material accounting for damage as this also leads to strong mesh dependency, as with 3D solid elements. This complex problem is solved using non-local formulations for the material behavior. The proposed solution accounts for both in-plane and in-thickness damage gradients. It consists of a two-step regularization procedure introducing nested wire elements in the shell thickness. Using non-local variables to evaluate the transverse strain enables to obtain a continuous spatial distribution for strains, in agreement with the 3D solid case. An updated Lagrangian formulation is used to account for large strains. The performance of the strategy is demonstrated in three test cases, with a comparison with an experimental result for one of them. The results obtained following the proposed strategy lead are equivalent to the ones obtained with the non-local method proposed by Davaze et al., (2021) and are thus consistent with the experimental data.

Influence of pre‐strain and bake hardening on the static and fatigue strength of a DP600 steel sheet

AbstractThe influence of pre‐strain on the tensile and fatigue properties of a dual phase DP600 steel was studied. The material was pre‐strained by uni‐axial tension in rolling and transverse direction. Thereafter, specimens were cut from the deformed plates in parallel or orthogonal to pre‐strain direction. It was found that pre‐strain increases yield and tensile strength. Results suggested that strain path change primarily affects the elastic‐plastic transition during early stage of reloading. Pre‐strained specimens showed an increase in high cycle regimes as a consequence of yield strength increment, irrespective of imposed pre‐straining direction. A modified stress life equation that accounts for pre‐strain was proposed and showed good agreement with experimental data. Bake hardening enhanced both tensile and high cycle fatigue resistance. Walker equation was successfully fitted to account for tensile mean stress. In low cycle fatigue, negligible influence of pre‐strain was observed due to cyclic softening and residual stress relaxation.

Magnetic oxidized multi-walled carbon nanotubes for activating anti-corrosion phosphate coatings formation on DP steel surface

Magnetic oxidized multi-walled carbon nanotubes (magnetic o-MWCNTs) possess excellent ion adsorption properties. Since magnetic o-MWCNTs can accelerate the DP steels surface to adsorb the metal cations (Me2+) and PO43−, more nucleation sites can be formed on the surface of the steel treated with magnetic o-MWCNTs solution, which provide optimum conditions for phosphate deposition. The scanning electron microscope (SEM) has been applied to observe the coating morphology, indicating that more dense and uniform phosphate coating can be formed on the DP steel surface activated by magnetic o-MWCNTs. Potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) suggested that the adsorption of magnetic o-MWCNTs on the steel surface effectively improved the corrosion resistance of the phosphate coating. The optimal treatment time of magnetic o-MWCNTs was 40 s, which can obtain 44.1% inhibition efficiency relative to the bare steel with phosphate coating. Meanwhile, combined with the friction test, the coefficient of friction decreased from 0.556 to 0.405 for the phosphate coating with treatment by magnetic o-MWCNTs. Finally, the activation mechanism of magnetic o-MWCNTs and the growth mechanism of phosphate coatings were concluded by combining the fourier transform infrared (FT-IR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis.

Study on quarter-wave control of DP980 steel based on a bending force linear combination strategy

Study on quarter-wave control of DP980 steel based on a bending force linear combination strategy

On the Measurement of Nakazima Testing Based Out-of-Plane Forming Limit Curves using 2D Digital Image Correlation

The strain compensation method for measuring in-plane forming limit curves (FLCs) using 2D digital image correlation developed previously [A method of measuring in-plane forming limit curves using 2D Digital Image Correlation, SAE Int. J. Mater. Manf., 2023] was modified and extended to more versatile and popular out-of-plane FLCs. The current study introduces a straightforward strain compensation technique for measuring Nakazima testing based out-of-plane FLCs utilizing an affordable single-camera (2D) DIC system. In this study, forming tests are performed on two automotive-grade sheet metal alloys: DP980 steel and a 6xxx series aluminum alloy using the Nakazima test method. The experiments are conducted on a customized setup that allows for simultaneous optical strain measurements using both a stereo DIC and a 2D DIC system. The FLCs are obtained by applying a temporal FLC computation approach to the two measurement sets. The results show that 2D DIC FLC points match those obtained by stereo DIC for both the materials after applying the proposed strain correction method.

Strain-Hardening Prediction of DP600 Steel Considering the Heterogeneous Deformation Between Ferrite and Martensite

Strain-Hardening Prediction of DP600 Steel Considering the Heterogeneous Deformation Between Ferrite and Martensite

Characteristic of DP600 Steel Produced in Hot Rolling Process

Characteristic of DP600 Steel Produced in Hot Rolling Process

Research on the evolving yielding behaviour and micro-mechanism in biaxial deformation of DP1180 sheet based on crystal plasticity finite element model

Dual-phase steels (DP steels) consist of ferrite and hard martensitic phase. For the high strength and good formability, they are widely used in automotive applications. Compared with the DP steels with lower strength, DP1180 is more special for the volume fraction of martensite is higher than that of ferrite. In this paper, the evolving plastic yielding behaviours and insight into their microscale mechanisms of DP1180 were studied by biaxial tension experiments and crystal plasticity finite element method. The representative volume element (RVE) model of DP1180 taking the dual phase fraction, grain size and texture into consideration is established and the relevant parameters of crystal plasticity are reverse calibrated by the uniaxial tensile test stress strain curve. The simulated biaxial tension tests were conducted by the established RVE model to investigate the yielding micro-mechanisms of DP1180 steel, and the obtained yield loci matches well with the test results. The simulation results show that the texture has a significant effect on the contours of the yield loci. The soft ferrite grains weaken the kinetic constraints to the polycrystalline matrix and further free the rotation and plastic deformation of the neighbouring grains. This study thus provides a comprehensive understanding of the effect of the microstructure (crystal structure, texture and dual phase fraction) on the yielding behaviour of DP1180 steel as well as a method of virtual experiment to get the yield loci of metallic materials.

Influence of process and material parameters on the twist springback prediction of a panel

Both advanced high strength steels and aluminum alloys gained increasing popularity because of the constant effort to meet the social pressure of reducing the vehicle weight while keeping their safety. The ratio between material strength and weight is much higher when compared to other materials but, the high springback after forming is one of the disadvantages of these materials, particularly when exhibiting twisting and side wall curl, which are the two most difficult types of springback to control and compensate in die engineering. This study analysis the influence of several material and numerical parameters on the prediction of the twist springback, using the Twist Die. Two processing conditions were considered: (i) blank-holder force that constrains the metal flow and (ii) a stake bead, which changes the forming conditions from draw to stretch forming, at the end of the process. The modelling of the elastoplastic behavior of the sheet materials uses a wide range of experimental data, for a DP980 steel and a 6xxx aluminum alloy. In addition to the effect of the sheet material on springback, the effect of the following parameters is also analyzed numerically: (i) friction coefficient; (ii) beads; (iii) modulus of elasticity; (iv) work hardening law and (v) yield criterion. All numerical simulations were performed with the finite element in-house code DD3IMP. The process parameter that most affects the twist springback is the presence of the stake bead, reducing significantly the springback. In addition, the reduction of the friction coefficient and of the elastic modulus also leads to an increase in springback. On the other hand, the yield criterion has little influence on springback of this component. The twist springback shows a different trend for the steel and the aluminum alloy that seems to be related with the different yield strength but also with the distinct gap between the punch and the die.

Numerical analysis of the load paths and the resulting damage evolution during deep drawing of dual-phase steel

Within the deep drawing process chain, damage develops and evolves – starting with foreign particles from the slag as well as non-metallic inclusions leading to voids, which develop, accumulate and ultimately lead to failure. This damage accumulation and evolution along the process chain decisively influences the performance of components. Accordingly, the influence on the damage accumulation and evolution offers a lever to increase the performance of components. The final damage state in the component has been shown to be dependent on the prevailing load path by means of the stress and strain states along the process route. The investigation of the stress and strain states requires numerical modelling. From the models, the load path can then be extracted and a correlation can be obtained with the damage that results. The present work investigates the load paths during deep drawing of u-shaped profiles as a function of process set-up and process parameters of dual-phase steel DP800. The resulting load paths are extracted, analyzed and correlated with the numerically predicted damage.

Evaluation of ductile fracture criteria in combination with a homogenous polynomial yield function for edge splitting damage of DP steels

Abstract This study investigates the formability characteristics of dual phase steels (DP600 and DP800) under flange stretching conditions through hole expansion tests. The hole-splitting initiation was numerically predicted using ductile damage functions coupled with an orthotropic plasticity model. Therefore, a polynomial yield criterion coupled with three damage criteria, namely generalized plastic work, void growth model, and a shear ductile fracture model, is implemented into Marc software by the user defined material subroutine. Thus, the fracture stroke, hole expansion ratio, and fracture initiation location were estimated for both steels. The polynomial yield criterion could capture the anisotropic features of the dual phase steels. Furthermore, the stress triaxiality-based criteria were reasonably accurate in stretching limit predictions of both steels’ grades. Nevertheless, plastic work predicted the fracture strokes and hole expansion ratios noticeably lower than the experimental outcomes for both steels. In addition, all the numerical results captured the exact fracture initiation location for DP600, while a slight discrepancy was observed for DP800. All ductile fracture models pointed out the identical crack location, which shows the cruciality of the yield criterion for locating the fracture initiation in hole expansion test. Consequently, both void growth model and shear ductile fracture model showed accurate performances conforming to the stress triaxiality was found to be more dominant than the Lode parameter.

Precipitate characteristic and nanoindentation analysis of resistance element welded DP780 steel and 6061 aluminum alloy joint

Resistance element welding (REW) of DP780 steel to 6061-T6 aluminum alloy using a flat Q235 rivet was carried out. The macrostructure, microstructure and nanoindentation hardness of the joint were investigated. The aluminum alloy nugget zone was characterized by high-angle grain boundaries. The elastic modulus and hardness of seven regions of the joint were measured through nanoindentation. The indentation and hardness results exhibited a greater elastic modulus and harness in the steel nugget zone. The elastic modulus and hardness of the heat-affected zone of aluminum alloy depend on the precipitates and grain size.

On the damage behaviour in dual-phase DP800 steel deformed in single and combined strain paths

Ductile damage in dual-phase steels deteriorates the mechanical properties, resulting in a lowered crashworthiness and a shorter life-time under cyclic loading. However, most works focus on the observation of damage under simple strain paths, even though work pieces are often subjected to combined strain paths during forming. In this work, we set out to characterise the damage behaviour for a DP800 steel subjected to combinations of tensile and bending loads. To this end, we use an automated approach based on machine learning to quantify damage in high-resolution SEM panoramic images, coupled with finite element modelling to determine the stress state, and high-throughput nanoindentation to determine the strain hardening behaviour. We find a strong connection between loading direction and damage formation in both steps of the forming process. By focussing on different possible combinations of positive and negative triaxiality, we show that the combination of two deformation modes with positive triaxiality leads to a promotion of damage formation. A negative triaxiality applied after a positive triaxiality leads to void closure, reducing the number of damage sites and total void area. Importantly, deformation with negative triaxiality carried out before deformation with positive triaxiality inhibits damage nucleation and growth due to strain hardening.