Advanced High Strength Steel Sheets Research Articles

Effect of magnetic field on mechanical properties of an advanced high strength steel sheet

The present study deals with the experimental investigations during plastic deformation of a dual phase steel sheet in uniaxial tension under a magnetic field, applied in the longitudinal direction of the tensile specimen. To ascertain the changes in tensile properties, the experiments are performed with and without magnetic field. The microstructural and texture investigations are done using field emission scanning electron microscopy in the samples taken from the middle of the tensile tested with 20% of gauge length (20% engineering strain) with and without magnetic field. The results show an increase in compressive residual stress, microhardness and anisotropy of the sheet when tested with magnetic field which may affect the formability of tested sheet. It is also observed that the magnetic field affects the texture during plastic deformation and renders a strong texture as compared to that of specimen tested without magnetic field.

Characterizing the mechanical deformation response of AHSS steels: A comparative study of cyclic plasticity models under monotonic and reversal loading

This study evaluates the performance of a user-defined combined hardening modeling method for advanced high-strength steels (AHSS) under monotonic and reversal loading conditions. The plastic behavior of TWIP980 and TRIP980 AHSS sheet metals is investigated using a cyclic plasticity modeling approach. The model incorporates an isotropic von Mises yield criterion and a single-term Chaboche nonlinear kinematic hardening rule. Monotonic and reversal loading stress-strain curves are predicted and compared with experimental results. The model accurately captured the Bauschinger effect for both materials, but it needs help to effectively model the permanent softening behavior observed in TWIP980 steel. Overall, the proposed modeling method agrees well with experimental results for monotonic loading and accurately represents the Bauschinger effect and transient behavior during reversal loading. However, better improvements are needed to capture the permanent softening behavior of TWIP980 steel.

Strength of Bolted Lap Shear Connections with Advanced High-Strength Steel Sheets

Strength of Bolted Lap Shear Connections with Advanced High-Strength Steel Sheets

High cycle fatigue behaviour of advanced high strength steel sheet (HS1000) in automotive application

The demand for high strength steel ranges from 750 to 1200 MPa in automobile industries to reduce cost, and fuel consumption and increase safety. Stress- controlled high cycle fatigue (HCF) tests for HS1000 steel sheets in transverse direction were conducted to determine the fatigue limit. Cycle softening process occurs in HS1000 steel sheet. Based on controlled material data, HS1000 steel sheet is compared with more common steel grades. The fatigue limit of HS1000 steel sheets is much higher than that of lower strength structural steel and has comparable ductility and high fatigue strength.

Numerical Study of Predicting Forming Process Based on Different Hardening Models in Advanced High Strength Steel Sheets

Sheet metal shaping for advanced high strength (AHS) steel sheets is commonly utilized in the manufacturing of automobile components. This helps to decrease the weight of new structures and increase the crashworthiness performance of these structures. The phenomenon known as springback deviation plays a significant role in creating such components via deformation and fracture. This study investigated the production of an S-rail from components made of steel grade 780 with a thickness of 1 millimeter by employing appropriate simulations using finite elements (FE) and generation. The materials were initially tested in order to collect the necessary parameter sets for the models being employed. The Barlat89 yield function, the Y-U kinematic hardening model, and a model combining the Y-U kinematic hardening model with the Barlat89 yield function were all used in the simulations. In addition, the forming limit curves (FLCs were utilized to assess the computed formabilities and served as the failure criteria. The deformation, springback effect, deviations, and twist springback of the components developed after the S-rail forming were subsequently studied, measured, and the findings compared. It was discovered that the proposed yield criteria and kinematic hardening models could accurately predict the formability of the material without causing any damage. The strain distributions acquired from the Y-U kinematic hardening indicated that the sidewall curl area was slightly larger than what was predicted by previous models. The model combining the Y-U kinematic hardening model with the Barlat89 yield function produced the most accurate predictions for overall springback effect, deviations, and twist springback of the AHS steel sheets.

Effects of Diffusible Hydrogen on Fatigue Life of Spot Welds in High-tensile-strength Steel Sheets

Spot-welded advanced high-strength steel (AHSS) sheets are used to decrease car body weight and enhance crashworthiness simultaneously. Such welds are susceptible to hydrogen embrittlement, but research in this area is limited. We have evaluated the hydrogen embrittlement properties of spot welds fabricated with AHSS sheets and clarified the dependence of diffusible hydrogen content and tensile rate on tensile shear strength. However, the performance of spot welds must be evaluated not only by static testing such as tensile shear tests but also by dynamic fatigue testing. Herein, we investigated the effects of diffusible hydrogen on their fatigue life via tensile–shear fatigue tests under cathodic hydrogen charging. The number of cycles to failure and the endurance limit decreased with diffusible hydrogen content. The number of cycles to failure decreased as the constant amplitude-loading frequency declined. The intergranular cracking is more likely to occurs as the amount of diffusible hydrogen increases and as the frequency decreases. These results are explained by hydrogen accumulation and crack growth behavior caused by mechanism for hydrogen-enhanced local plasticity and hydrogen enhancement of the strain-induced generation of vacancies mechanism. Furthermore, the correlation between static testing and dynamic fatigue testing was investigated, and the relationship between fatigue strength and the amount of diffusible hydrogen was in good agreement with the diffusible hydrogen dependence of tensile shear strength obtained from slow rate tensile shear tests under hydrogen charging.

Effect of strength of steel sheets on peel tensile strength and failure mode of dissimilar joint of spot welds

Advanced high strength steel sheet (AHSS) is widely used in the automotive body for weight reduction and the improvement of crash performance. Peel tensile strength of spot welds tends to decrease as the steel sheet strength increases. Therefore, in order to further expand the application of AHSS, technology to prevent fracture of spot welds is required. In order to achieve that, it is necessary to understand the characteristics of spot-welded joints and clarify the controlling factors. However, most of the conventional joint strength evaluations have been conducted on the same kind material joints, and there are few systematic studies on the joint strength and fracture mode of dissimilar material joints that are often used in actual vehicles. Therefore, the purpose of this study is to obtain a guideline for grasping the characteristics of dissimilar joints in case of the plug failure. Using various joint shapes, we compared the peel tensile strength and failure position of the same kind material joints and the dissimilar material joints with the TS1500 MPa class hot-stamped steel sheet. As a result, it was found that the deformation behaviour of the test piece affects the joint strength and fracture position of the dissimilar material joints. Specifically, in dissimilar material joints, cross tension joints and double cup-shape tension joints tended to break on the TS1500 MPa class hot-stamped steel side. On the other hand, L-shape tension joints broke on the lower strength material side. From these results, it is considered important to understand their deformation behaviour in order to analyse and predict the fracture phenomenon of spot welds in actual vehicles and automotive parts.

Effect of strength of steel sheets on tensile shear strength and failure mode of dissimilar joint of spot welds

Advanced high strength steel sheet (AHSS) is widely used in the automotive body for weight reduction and the improvement of crash performance. For example, TS1500 MPa class hot stamped steel (1500HS steel) sheet is applied to many vehicle models. However, there is a concern that performance of spot welded joints will decline as steel strength increases. In order to further expand the application of AHSS, it is very important to have knowledge about the characteristics of spot welded joints. In this report, we investigated the tensile shear strength (TSS) of the 1500HS steel sheet. Then, we compared the joint strength and failure position of the same kind material joints and the dissimilar material joints with the 1500HS steel sheet in case of the plug failure. As a result, it was found that TSS of dissimilar material joints depend on base metal strength on lower strength steel side. However, fracture occurred on the 1500HS steel side except for joints with large difference of base metal strength. The mechanism is thought that the effect of strain concentration at the edge of nugget on the 1500HS steel side due to the increase of rotational deformation around the nugget.

Characterization and Modeling of Anisotropic Fracture of Advanced High-Strength Steel Sheets

<div class="section abstract"><div class="htmlview paragraph">As an engineering approach of balanced complexity and accuracy, the Generalized Incremental Stress-State dependent damage Model (GISSMO) in LS-DYNA<sup>®</sup> has now been widely adopted by the automotive industry to predict metallic materials’ fracture occurrences in both forming and crashworthiness simulations. Calibration of the nominal GISSMO is typically based on material characterization data along a certain representative material orientation. Nevertheless, many rolled or extruded metallic materials, such as advanced high-strength steel (AHSS) sheets, exhibit accentuated anisotropic fracture behavior, even though, notably, some of these materials show comparatively weak anisotropic plasticity in the meantime. Accordingly, in this work, the deformation and fracture behavior of a selected AHSS grade, Q&P980 steel, was first characterized based on a series of mechanical experiments under simple shear, uniaxial tension, plane strain, and equi-biaxial tension conditions. Then, material models were calibrated based on the plasticity and fracture data. Two fracture models, either stress- or strain-based, were applied to fit the fracture loci of the target material, which then could be directly implemented into the material cards in LS-DYNA<sup>®</sup>. Particularly, to simulate the anisotropic fracture behavior of the target material, an extended GISSMO material card (eGISSMO) was introduced and highlighted in this work. Unlike the nominal GISSMO, the eGISSMO integrated the different anisotropic fracture loci and damage accumulation along three material orientations (longitudinal, diagonal, and transverse) into a single material card. In the subsequent validation based on a customized three-point-bending (3PB) testing setup on hat-section samples, only the finite element (FE) model using the calibrated eGISSMO successfully simulated the anisotropic fracture bifurcation observed in the actual experiments.</div></div>

Subzero Material Properties of Advanced High-Strength Cold-Formed Steel Alloys

The overall temperature in high latitude regions has been rapidly increasing in recent years, creating a demand for infrastructure to support increasing human activities. Recent advances in material science have resulted in the development of advanced high-strength steels (AHSS), which are new grades of cold-formed steel (CFS) with unprecedented strength. To design safe infrastructure, the material properties of AHSS under subzero temperatures must be quantified. An experimental investigation following the steady-state test protocol was carried out to quantify the subzero temperature effects on the material properties of AHSS and conventional CFS sheets with yield strengths ranging from 395 MPa to 1200 MPa. Two types of AHSS (dual phase and martensitic) and two types of conventional CFS (mild and high-strength low-alloy) were investigated at temperatures down to −60 ∘C. The stress–strain relationship, elastic modulus, and key stresses and strains were reported from the experiments. The results show that AHSS’s material properties do not degrade but are mildly strengthened at subzero temperatures than at ambient, which indicates that AHSS is a suitable construction material for structural members in high-latitude regions. Furthermore, modeling on stress–strain relationships of AHSS and conventional CFS at subzero temperatures was developed, demonstrating excellent fits with the experiment data.

Development of double-sided friction stir welding of advanced high strength steel sheets

Double-sided friction stir welding (FSW), performed with a pair of rotation tools, was studied in an attempt to increase the joining speed when applied to welding of automotive advanced high-strength steels (AHSS). The experiments were performed on butt joint of 1.6-mm-thick 1180 MPa grade steel sheets, and sound welds were obtained when the travel speed was no more than 4 m/min, fairly comparable with laser beam welding (LBW), one of the conventional automotive welding processes. It was clarified by experimental and numerical analyses that the enhancement in travel speed was due to the unique material flow interactively developed by a pair of rotation tools. Double-sided FSW was performed at 3 m/min of joining speed on butt joint of dissimilar steel sheets in a combination of 1.5 GPa and 590 MPa grades. The mechanical properties of the welded joints were evaluated by tensile, V-bending and Erichsen tests comparing with LBW, which verified that double-sided FSW exhibited sufficient mechanical properties for the press-forming to produce automotive parts. A prototype automotive part was successfully fabricated using the tailor-welded blank welded with double-sided FSW, which demonstrates its applicability to automotive production.

Application of a high-speed infrared thermographic camera to the study of HAZ softening in S960 welded joints

Hot strip rolling of low-carbon advanced high-strength (AHS) sheet steels is difficult because their products require specific chemical compositions to achieve excellent mechanical properties. The formation of a heat-affected zone (HAZ) during welding is therefore a challenge in terms of maintaining high strength values. This fact complicates the formation of the softening area when the hardness in the HAZ rapidly decreases. According to the continuous cooling transformation diagram (CCT), the chemical composition of the base metal is directly related to the fine-grained martensitic microstructure arising from very high cooling rates and microalloying up to 0.2 wt% by Nb, V, and Ti. The gas metal arc welding (GMAW) was used to perform the study, what presents a challenge for thin sheets because of very narrow HAZ formation, suscepible to HAZ softening phenomenon. The width of the HAZ was about 6 mm and determined by applying an infrared thermographic camera to measure the real-time temperature development. The significant reduction in microhardness, known as the softening phenomenon, occurred in the intercritical heat-affected zone or at the boundary between intercritical and subcritical HAZ, according to real-time diagnostics of the temperature development and the microhardness results. The amount of heat produced during welding plays an important role in this context and is directly connected to the heat input of the welding process. The softening area can be reduced and its impact on the mechanical properties can be lowered by appropriately modifying the welding conditions, e.g., optimising the heat input. The purpose of this paper is to point out the change in temperature during the welding process.

Crack Initiation Detection in JAC780Y during Tensile Loading by Using Direct Current Potential Drop and Acoustic Emission Techniques

In this work, the non-destructive and fracture test techniques were studied for micro-crack detection in the advanced high strength steel (AHSS) notched sheets. Crack initiation of JAC780Y during tensile loading was investigated by Direct Current Potential Drop (DCPD) and Acoustic Emission (AE) techniques. The results confirm for the first time that both techniques can be induced to indicate micro-crack from fracture behavior of AHSS sheet in forming and AE technique can detect the crack initiation faster and more effective than DCPD.

Gradient damage model for ductile fracture introducing degradation of damage hardening modulus: implementation and experimental investigations

This study presents a gradient damage model for ductile fracture, in which the damage hardening modulus is degraded by the accumulation of plastic deformation and the volume expansion caused by negative hydrostatic pressure. The proposed model fulfills the thermodynamic requirements, and the governing equations are derived from energy minimization principles. Two parameter studies are carried out to confirm the basic performance of the proposed model, in which some typical ductile fracture responses are demonstrated by changing parameters for degrading the damage hardening modulus. Also, a series of numerical experiments are presented to reveal the ability of the proposed model to successfully simulate the fracture tests of advanced high strength steel sheets with different tensile strengths. It is indeed confirmed by the close agreement with experimental results that the proposed model is capable of realizing the breaking elongation, the transitional behavior from unstable to stable crack propagations, and the corresponding load–displacement curves. Also, the model successfully reproduces and predicts the crack initiation positions in notched specimens with different notch radii.

Effects of Partial Replacement of Si by Al on Cold Formability in Two Groups of Low-Carbon Third-Generation Advanced High-Strength Steel Sheet: A Review

Partial replacement of Si by Al improves the coatability (or galvanizing property) of Si-Mn advanced high-strength steel (AHSS) sheets. In this paper, the effects of the partial replacement on the microstructure, tensile property, and cold formability are reported for the low-carbon third-generation AHSS sheets, which are classified into two groups, “Group I” and “Group II”. The partial replacement by 1.2 mass% Al increases the carbon concentration or mechanical stability of retained austenite and decreases its volume fraction in the AHSSs, compared to Al-free AHSSs. The partial replacement deteriorates the tensile ductility and stretch formability in the AHSSs with a tensile strength above 1.2 GPa. On the other hand, it achieves the same excellent stretch-flangeability as Al-free AHSSs. A complex addition of Al and Nb/Mo further enhances the stretch-flangeability. The cold formabilities are related to the heat treatment condition and microstructural and tensile properties, and the stress state.

Multistage Deep-Drawing with Alternating Blank Draw-in for Springback Reduction in Transfer and Progressive Dies

Cold formed sheet metal parts made of advanced high-strength steels (AHSS) offer a high potential for a lightweight, durable and economic design. However, manufacturing dedicated, high-strength parts with cold forming technologies such as conventional deep-drawing often results in unacceptable shape deviations due to elastic springback after unloading the part from the forming tools. Therefore, various springback compensation methods have been established to ensure dimensional quality of such sheet metal parts. At the Institute for Metal Forming Technology (IFU Stuttgart), deep-drawing with alternating blank draw-in was developed in this context as a new approach to reduce springback and enhance cold forming of AHSS sheet metal parts. Presented work provides numerical sensitivity analysis as well as experimental studies about this new forming method. The asymmetric and alternating blank draw-in, which is changed within a multistage forming process sequence, results in an alternated bending over tool radii and leads to a beneficial stress superposition in the part wall area with reduced springback phenomena. Compared to conventional deep-drawn sheet metal components, springback of a benchmark part geometry could thus be reduced over 75 % by a three-stage forming process with an optimized blank draw-in kinematic.

Constitutive modeling of commercial pure titanium sheet based on non-associated flow rule and differential hardening

The commercial-pure titanium (CP-Ti) sheet has attracted a great interest from biomedical and aerospace industries because of its strong mechanical advantages such as lightweight, high strength, good formability, and corrosion-resistance. However, strong anisotropic features, such as evolutionary yield surface and strength difference in tension and compression, of the CP-Ti require advanced constitutive modeling compared to standard advanced high strength steel sheets. This study took into account the differential hardening behavior and the changing R-value of CP-Ti sheet observed during the uniaxial tensile and bulge tests in developing a material model. The observed behaviors are modeled by Hill48 quadratic function based on non-associated flow rule with equivalent plastic work dependent evolutionary parameters. The developed material model was then implemented into a user material subroutine (VUMAT) for ABAQUS/EXPLICIT and used to simulate a circular deep drawing to verify the developed model. Simulation results are compared with those of a material model coupling Yld2000–2d yield function with associated flow rule. The comparison shows that the developed material model provides not only a good agreement with the experiment for yield and potential surfaces but also accurate predictions in forming simulations.

Influence of strength and notch shape on crack initiation and propagation behavior of advanced high strength steel sheets

Advanced high-strength steels (AHSSs) are widely adopted in car body structures to achieve weight reduction for energy efficiency and crash safety. This study investigates the effects of material strength and notch shapes on crack initiation and propagation behaviors in AHSS sheets used in car body structures. Tensile tests were conducted for three types of Japanese grade AHSS (JSC590, JSC980, and JSC1180) specimens and two notch root radii (R0.5 and R5). The crack initiation and propagation behaviors were found to strongly depend on the material strength and notch root radius. The crack generated from the notch root for the R0.5 specimens, whereas it generated from 1 to 2 mm inside the notch root for the R5 specimens. Unstable crack propagation occurred beyond the maximum load for the R5 and JSC1180 specimens, whereas only stable crack propagation occurred for the R0.5 specimens. From the digital image correlation analyses, we found that the specimen notch radius affects the stress triaxiality and equivalent plastic strain around the notch root. The JSC980 material had the highest crack initiation energy, which correlates with the plastic dissipation energy utilized for thickness reduction. The JSC590 material exhibited the highest crack propagation energy, which correlates with the true fracture strain.

Sheared edge formability characterization of cold-rolled advanced high strength steels for automotive applications

Edge cracking has become a limiting factor in the use of some advanced high strength steels (AHSS) for high-performance automotive applications. This fact has motivated the development of a multitude of experimental tests for edge formability prediction over the last years. In this sense, the Hole Expansion Test (HET) according to ISO16630 has been established in the automotive industry as a standard procedure for edge cracking sensitivity ranking. However, whereas it may be useful for rapid material screening, the results are often not accurate and reliable enough. Consequently, alternative methods based on Digital Image Correlation (DIC) have been proposed aimed at improving the prediction of edge cracking occurrence during forming and obtaining useful strain data that can be implemented in forming simulations. This paper explores the applicability of different DIC-based methods, such as Half-Specimen Dome Tests, Sheared Edge Tensile Tests, and KWI hole expansion tests with a flat nosed punch, for characterizing the edge formability of three cold-rolled AHSS sheets. The results obtained from the different testing methods are compared and validated with a laboratory-scale demonstrator. Finally, the limitations and advantages of the different methods are discussed.

Finite element implementation of hydrostatic pressure-sensitive plasticity and its application to distortional hardening model and sheet metal forming simulations

• The strength-differential (SD) effect of AHSS with ultimate tensile strength exceeding 1 GPa is modeled by a hydrostatic pressure-dependent distortional plasticity theory. • The finite element implementation of the pressure-dependent plasticity theory is proposed in the manuscript. The implemented theory is validated with the comparison study with the associated stand-alone code and experimental measurements. • Both the SD- and strain patch change effects of TIRP1180 are reproduced in finite element simulation by embedding the distortional plasticity model, HAH 20 , in the pressure-dependent yield condition. The effectiveness and applicability of the FE-implemented pressure-dependent distortional plasticity model in sheet metal forming simulations is validated through the springback prediction studies. In this work, the hydrostatic pressure-dependency of the distortional plasticity model HAH 20 was implemented using a finite element (FE) code to account for the strength-differential (SD) effect that has been observed in advanced high-strength steel sheets. To this end, a fully-implicit stress update algorithm formulation was introduced for the pressure-dependent plasticity theory. The implementation was validated by comparing the FE prediction of the material behavior during a number of tests with those of a stand-alone code of the constitutive description as well as with experimental data. In order to investigate the SD effect on the springback simulation results, the U-draw bending test was analyzed within this FE framework. Furthermore, in order to assess the effectiveness and stability of the formulation for a large-scale example, forming simulations of an automotive structural part were conducted. In addition to the SD effect, strain path changes and geometrical aspects were also investigated in this example. It was shown that the distortional plasticity-based pressure-dependent yield criterion well describes the asymmetric SD behavior in sheet metal forming simulations.