Advanced High Strength Steel Sheets Research Articles

Influence of strain rate on tensile properties and fracture behaviour of DP600 and DP780 dual-phase steels

The influences of the strain rate on the tensile properties and fracture behaviour of DP600 and DP780 advanced high-strength sheet steels have been studied. The variation of their mechanical properties depending on the strain rate have been researched by applying uniaxial tensile tests at three different strain rates (0.001, 0.01, 0.06 s−1). The influences of strain rate on fracture behaviour have been investigated by displaying the fracture surfaces of the material. Strain rate increase has been determined to increase the yield strength, tensile strength, total elongation and hardening rate. The strain hardening coefficient has been found not to be significantly affected by the strain rate. It has been determined that, the fracture has occurred faster during necking while load-carrying capacity has increased with strain rate increase.

A hybrid method for prediction of damage initiation and fracture and its application to forming limit analysis of advanced high strength steel sheet

In this work, damage locus for predicting both ductile damage initiation and fracture of the advanced high strength steel grade JAC780Y was developed. Tensile tests of sheet samples with various geometries were performed and occurred damage was identified by the direct current potential drop method. Then, FE simulations of the corresponding tests were carried out, in which local stress triaxiality and plastic strain values of the critical areas were evaluated. As a result, damage loci for the states of damage onset and fracture could be obtained. Effects of different yield criteria on the resulted limiting curves were also investigated. Furthermore, the damage loci were transformed to strain based forming limit curves. To verify applicability of the determined damage loci and forming limit curves, stretch–forming tests and forming of an industrial part were conducted. The damage loci and transformed forming limit curves could fairly describe both occurred damage initiations and fracture of the steel.

Experimental validation of numerical model for asymmetric deep drawing of DP780 steel sheet using digital image correlation

A validated numerical model for in-plane stress/strain prediction is essential in understanding the deformation behaviour of sheet metal forming process, in particular, asymmetric deep drawing of advanced high strength steel sheet. In this work, the Yld2000-2d anisotropic yield criterion integrated with the initial homogeneous anisotropic hardening model was employed to describe the complex material behaviours of DP780 steel as well as the adoption of Yoshida chord model for elastic modulus degradation. Digital image correlation technique was utilized to measure the in-plane strain and shape deviation of the developed P- channel. The validity of the numerical model was assessed by comparing the predicted strain distribution and twist springback with the measured results. It indicates that the developed numerical model based on the selected constitutive models is acceptable for the deformation analysis, although the predicted discrepancies still exist.

Investigations on fracture curves in strain and stress space for advanced high strength steel forming

Conventional forming limit curves (FLCs) are inappropriate for describing formability for advanced high strength (AHS) steel sheets, since such steel grades experience fracture without localized necking occurrence. The aim of this work was to develop a fracture curve (FC) for the AHS steel grade DP980. The FC was determined by means of the Nakajima stretch forming test and tensile tests of various sample geometries, by which shear fracture governed. An optical strain measurement system was used to capture strain histories of deformed samples up to failure. From these results, fracture strains were gathered and plotted in a strain space. Subsequently, the strain based curve was transformed to space between stress triaxiality and plastic strain. Hereby, effects of anisotropic yield function, namely, the Hill’48 model on obtained stress fracture loci were investigated. In order to verify applicability of the determined limit curves, a Mini-tunnel part was pressed and simulated. It was found that the stress based FC do predict failure of the DP980 steel sheet more accurately than the strain based F C.

Parametric study on numerical simulation of the electromagnetic forming of DP780 steel workpiece with aluminium driver sheet

The purpose of this study is to investigate the influences of numerical parameters on the electromagnetic forming (EMF) simulation. The 3-dimensional coupled electromagnetic- mechanical simulations were conducted to predict the deformation behavior of the advanced high strength steel (AHSS) sheet receiving support in EMF with aluminum driver sheet. Dual phase (DP) 780 steel workpiece was formed into a hemi elliptical protrusion shape with aluminum alloy AA1050 driver sheet using a flat spiral coil actuator and open cavity die. The deformed shape of the DP780 workpiece and the computation time with respect to element size, N cycle number and time step of electromagnetic (EM) solver were analysed.

Ductile fracture prediction for metal sheets using all-strain-based anisotropic eMMC model

In order to predict ductile fracture of metal sheets, the stress invariants based Modified-Mohr-Coulomb (MMC) fracture model was transferred into an all-strain based MMC (eMMC) model under plane stress condition, predicting the fracture strain dependent on strain ratio or Φ angle, instead of stress triaxiality and Lode angle parameter. The strain ratio or Φ angle could be directly measured by digital image correlation (DIC), while the latter required finite element analysis to be determined. This method makes it possible to study material fracture behavior while bypassing plasticity. The eMMC fracture locus can be fully calibrated by fracture strains directly measured from DIC. The fracture strain was also extended by a linear transformation operating to the plastic strain tensor to incorporate the fracture anisotropy. Extensive experimental results of TRIP780 advanced high strength steel and AZ31B-H24 magnesium alloy sheets were used to verify the proposed model. The model was implemented into Abaqus/Explicit as a user material subroutine (VUMAT). Good prediction capability has been demonstrated for these two materials by FE simulation using shell elements.

Application of Gotoh's Orthotropic Yield Function for Modeling Advanced High-Strength Steel Sheets

An accurate description of the directional dependence of uniaxial tensile yielding and plastic flow in advanced high-strength steel sheets may require either a nonassociated plasticity model with separate quadratic yield function and flow potential or an associated plasticity model with nonquadratic yield function. In this paper, Gotoh's fourth-order homogeneous polynomial yield function is applied to model two advanced high-strength steel sheets in an associated plasticity model. Both the parameter selection for calibrating Gotoh's yield function and its positivity and convexity verification are given in some detail. Similarities and differences between the associated plasticity model presented here and the nonassociated one appeared in the literature are discussed in terms of the directional dependence of yield stresses and plastic strain ratios under uniaxial tension and yield stresses under biaxial tension loading.

Application of direct resistance treatment to localized areas on advanced high strength steel sheet

The possibility of creating local-designed variations in microstructure has the potential to radically change how advanced and ultra-high strength steels are used in vehicle components. Such local changes in the microstructure offer the potential for tailored combinations of strength and ductility, both reducing vehicle weight and improving structural performance. This study addresses development of a new technology for creating such local changes in microstructure. The process, termed direct resistance heat treatment (DHRT) is analogous to resistance welding with in situ tempering. Current impulses are used to first locally transform the microstructure (austenitize and quench to martensite) then temper the martensitic microstructure to achieve desired levels of performance. Work here was done on a 1-mm thick 10B21 steel. Practices were first developed to achieve fully martensitic microstructures, and then combinations of temper currents and times were used to achieve differing final hardnesses. Thermal analyses suggested that for the material studied, stable temperatures (during heating) could be achieved in about 50 ms, and the workpiece could effectively quenched in about 100 ms. Mechanical evaluations showed that the locally hardened areas were effective for diverting plastic deformation, even when using notched specimens.

Effect of an aluminum driver sheet on the electromagnetic forming of DP780 steel sheet

Some experimental studies have proved that materials with low electrical conductivity and even nonconductive materials can be formed using driver sheets with high electrical conductivity. This study investigated the effect of an aluminum driver sheet on the forming peak height of advanced high-strength steel sheet (AHSS) in the electromagnetic forming (EMF) process. DP780 workpieces were formed into a hemi-elliptical protrusion shape, analogous to a reinforcement rib, with various thicknesses, sizes, and material types of the aluminum driver sheets. These experiments were performed with a flat spiral coil actuator connected to the EMF system with a high energy capacity. In addition, three-dimensional coupled electromagnetic-mechanical finite element (FE) simulations were conducted to analyze the effect of the aluminum driver sheet on the forming peak height of the high-strength steel sheets and to predict the proper configuration of the aluminum driver sheet.

Influence of surface modification on galling resistance of DC53 tool steel against galvanized advanced high strength steel sheet

The presence of galling is always detrimental when sheet metal forming advanced high strength steel (AHSS). Galling tends to reduce tool life, degrade the quality of components, and increase scrap rate of production. To prevent or postpone the initiation of galling and thereby extend the service life of DC53 tool steel forming tools, surface modification techniques like nitriding treatment and thermal diffusion (TD) carbide treatment can be utilized. A custom-designed sheet strip tribo-tester that includes bending deformation was used to perform tribological tests on tool steels having surface treatments. Comparisons were made of the friction coefficient, surface roughness and critical total sliding distance of individually treated and conventionally quenched/tempered DC53 tool steel dies against galvanized DP600 sheet. The initiation and evolution of galling and other wear mechanisms were subsequently analyzed and compared. Results indicated significant improvement in galling resistance for dies with surface modification. The improvement in galling resistance for the nitrided and TD coated DC53 tool steels, respectively, was attributed mainly to (i) the constitution of complex Fe3N, Fe4N phases in compound layer, and (ii) the good anti-adhesive capability of VC coating with high hardness in TD layer.

Twist springback characteristics of dual-phase steel sheet after non-axisymmetric deep drawing

This paper aims to investigate the twist springback characteristics of advanced high strength steel sheet subjected to deep drawing. A C-rail benchmark, which leads to a particularly pronounced twist springback characteristics, was developed. For an accurate numerical modeling of the process, a non-quadratic anisotropic yield criterion integrated with combined isotropic and kinematic hardening model was used to describe the strain-stress behavior including anisotropy and Bauschinger effects. The corresponding mechanical experiments, namely uniaxial tension and forward-reverse simple shear tests were performed to determine the material parameters. The digital image correlation technique was applied for component tests as well as the deformation and stress-strain analysis. The experimental validation of the elastic-plastic finite element model was assessed by comparing maximum in-plane strain, thickness reduction distribution and twist springback of the drawn rail. To explore the source of twist springback, the deformation associated with in-plane stress and bending moment was analyzed. The results indicate that the bending moment before springback caused by non-symmetric stress states play an important role in twist springback and control. Certain regions of the die radius were varied in a numerical analysis to control the bending moment for the minimization of twist springback as well as the preliminary results of the relationship between the ratio of variable die radius and twist springback.

A study on variable friction model in sheet metal forming with advanced high strength steels

Conventional Coulomb model assumes constant friction coefficient in sheet metal forming simulation. It tends to predict too high friction shear stress in forming advanced high-strength steels (AHSS). Study is conducted by firstly designing an experiment method to develop a pressure-dependent variable friction model for DP780 AHSS sheet sliding against DC53 cold-work tool. The model is then used to simulate the forming and springback of U-shape bending under tension. Comparison of the predicted results with (i) pressure-dependent variable friction model, (ii) constant friction model, and the result of experimental verification indicates significant improvement of springback prediction with the proposed model.

Mechanical Joining of Various Materials by Clinching Method

Clinching is a simple, cheap and efficient method of joining that enables to join two or more sheets without any additional elements such as rivets, bolts or nuts. In addition, clinching does not require a surface preparation e.g. drilling (riveting), cleaning and roughening of the surface (adhesive boding) and other types of surface preparations (arc welding). Clinching is utilized in a wide range of applications and can be applied to different materials such as low carbon steel sheets, high-strength steel sheets, aluminium alloys, magnesium alloys. The paper presents the results of evaluation of clinched joint properties. The advanced high-strength steel sheet DP600 in combination with the drawing grade steel sheets DC06, DX51D+Z and high-strength low alloy steel sheet H220PD were used for experiments. The influence of position of the sheets relative to the punch and die of the tool on the carrying capacities of the clinched joints was observed as well. The tension test and microhardness test were used for the evaluation of clinched joint properties.

Microstructural Developments Leading to New Advanced High Strength Sheet Steels: A Historical Assessment of Critical Metallographic Observations

Journal Article Microstructural Developments Leading to New Advanced High Strength Sheet Steels: A Historical Assessment of Critical Metallographic Observations Get access David K Matlock, David K Matlock Advanced Steel Processing and Products Research Center, The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar Larrin S Thomas, Larrin S Thomas Advanced Steel Processing and Products Research Center, The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar Mark D Taylor, Mark D Taylor Advanced Steel Processing and Products Research Center, The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar Emmanuel De Moor, Emmanuel De Moor Advanced Steel Processing and Products Research Center, The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar John G Speer John G Speer Advanced Steel Processing and Products Research Center, The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 21, Issue S3, 1 August 2015, Pages 5–6, https://doi.org/10.1017/S1431927615000823 Published: 23 September 2015

Failure Performance Analysis of Resistance Spot Welded Advanced High Strength Steel Sheets

A numerical method to predict failure performance was developed for resistance spot welded AHSS (advanced high strength steel) sheets in this work and applied for a TRIP (transformation induced steel sheets) 980 sheet. For the numerical analysis, utilizing the numerical inverse calibration method, hardening data and fracture criteria were characterized for the base and the weld nugget based on the standard simple tension test and a newly designed miniature tension test, respectively. The characterized properties of the base and the weld nugget were then applied to analyze failure performance in the coupon tests of the lap-shear and U-shape tension tests carried out for welded sheets. The analysis was performed under the quasi-static condition in this early effort and showed reasonably good agreement with experiments both in failure modes and strength.

Determination of 3D Ductile Failure Criteria for Advanced High Strength Steel Sheet

In this work, 3D ductile fracture locus was determined for the advanced high strength (AHS) steel sheet grade DP780 using a hybrid approach between experiment and FE simulation. Tensile tests of different sample geometries were performed for the investigated dual phase steel, by which varying stress triaxiality (η) and lode angle (θ) values developed in the material during loading were introduced. During the tests, the direct current potential drop (DCPD) method and digital image correlation (DIC) technique were applied for identifying crack initiation on the micro-scale and fracture of the specimens due to local plastic deformation. Obtained force and displacement curves were correlated with the electric potential curves. Then, the moments of crack onset were determined for various states of stress. In parallel, the most critical areas of deformed samples before fracture were observed by the DIC method. Subsequently, FE simulations of the tensile tests were carried out and calculated local stresses and strains were gathered. The stress triaxialities, equivalent plastic strains and lode angles were evaluated for the corresponding detected areas. These threshold variables obtained from different specimens were plotted as the 3D failure locus for defining crack initiation and fracture occurrence in the DP steel.

다양한 하중경로에서의 DP980 강판의 파단변형률 예측에 관한 연구

Abstract The current study is concerned with the prediction of fracture strains for DP980 steel sheets over a wide range of loading paths. The use of DP980 steel is increasing significantly in automotive industries for enhanced safety and higher fuel efficiency. The material behavior of advanced high-strength steels (AHSSs) sheets sometimes show unpredictable and sudden fracture during sheet metal forming. A modified Lou–Huh ductile fracture criterion is utilized to predict the formability of AHSSs because the conventional forming limit diagram (FLD) constructed based on necking is unable to evaluate the formability of AHSSs sheets. Fracture loci were extracted from three dimensional fracture envelopes by assuming the plane-stress condition to evaluate equivalent plastic strains at the onset of fracture for a wide range of loading paths. Three different types of specimens -- pure shear, dog-bone and plane strain grooved -- were utilized for tensile testing to calibrate the fracture model of DP980 steel sheets. Fracture strains of each loading path were evaluated such that there shows little deviation between fracture strains predicted from the fracture model and the experimental measurements. From the comparison, it is clearly shown that the three dimensional fracture envelopes can accurately predict the onset of the fracture of DP980 steel sheets for complicated loading conditions from compressive loading to shear loading and to equi-biaxial tensile loading.

Constitutive and friction modeling for accurate springback analysis of advanced high strength steel sheets

The present work aims to analyze the influences of the material and friction models and to suggest the optimum selection of the models for springback simulations. Firstly, a transformation-induced plasticity (TRIP) steel was characterized using the conventional and advanced models for the springback prediction in U-draw/bending. A special attention was paid to the friction model, which was derived based on the macroscopically observed frictional behavior and interpreted with the microscopic contact mechanism. The combination of advanced constitutive and friction models could predict springback and punch load quite well. It was found that the hardening law and elastic unloading modulus had the major influences, while the friction model also had a major influence in the case of high blank holding force. A specific value of constant friction coefficient could provide similar predictions as the variable friction coefficient, but the former could not describe a non-uniform distribution of friction coefficient. Next, the analysis was extended to the sensitivity studies that considered wider ranges of material and frictional behaviors. These studies suggested that (1) the plastic anisotropy has to be modeled using an anisotropic yield function if the yield stresses or r-values affecting the relevant stress state are far from the isotropic assumption; (2) the reverse loading behavior has to be characterized using a hardening model that can precisely describe the transient hardening and permanent softening; (3) the elastic modulus should be defined using an advanced model if the material shows a visible reduction of elastic modulus; and (4) the friction coefficient should be carefully defined using an advanced friction model if the blank holding force is high.

Measurement and modeling of simple shear deformation under load reversal: Application to advanced high strength steels

In this paper, the stress–strain behavior under load reversal of advanced high strength steel (AHSS) sheet samples was measured using a modified simple shear (SS) apparatus. The forward–reverse loading behavior was characterized for three different grades of AHSS; namely, DP, TRIP and TWIP steels. For comparison purpose, compression–tension (CT) tests were also carried out for the same materials. For all the cases, a typical complex anisotropic hardening behavior, including the Bauschinger effect, transient strain hardening with high rate and permanent softening, was observed during load reversal. No premature localization and sheet buckling occurred in these experiments, which have been major technical hurdles in CT tests. For example, an engineering shear strain of over 40%, which corresponds to an effective strain of roughly 0.2, at reversal was achieved for DP980 although the uniform elongation of this material in uniaxial tension is only 5%. A recently developed distortional hardening model (HAH) was employed to reproduce the SS stress–strain curves. Using the coefficients determined with these SS data, the CT behavior was predicted with the HAH model and compared with experimental results. This complementary study indicated that the constitutive model determined from the SS flow curves satisfactorily reproduced the CT hardening behavior. As an application, finite element simulations of springback were carried out for 2D draw-bending of a strip sheet.

A study on galling initiation in friction coupling stretch bending with advanced high strength hot-dip galvanized sheet

As a form of surface damage arising between sliding solids, galling has become a critical issue in the use of advanced high strength steel (AHSS) sheet and it will reduce tool life and lower part quality. To clarify the dynamic evolution of galling initiation and to identify its initiation quantitatively, this study was conducted to simulate the sliding of hot-dip galvanized AHSS sheet against cold-work DC53 die with the use of a self-developed sheet/strip tribo-tester. Such tester is capable to yield plastic deformation and stretch bending simultaneously. The test allowed the identification of galling initiation via analyzing the step response of recorded friction coefficient which was subsequently corroborated with the observation of surface morphology on die surface. The difference of sheet surface roughness in vertical direction (ΔRy) was proposed as a quantitative index for the analysis and identification of galling initiation. Dynamic evolution of galling initiation was investigated with the help of SEM analyses on the surface of both slid sheet and die. Wear mechanisms affecting the friction behavior and galling initiation were subsequently suggested and discussed.