High Strength Steel Sheets Research Articles

Influence of Punch Servo Path on Elliptical Hole Flange Forming

This study investigated the process of forming an elliptical hole flange by using a SPFC780Y high-strength steel sheet with a material thickness of 1 mm. Three servo motion paths on a servo press were compared with a traditional crank motion path on a conventional press. Four punch motion paths were analyzed, namely a crank motion path on a conventional press (type T), a servo motion path with a 3-mm downward motion followed by a 0.5-mm upward motion (type S3), a servo motion path with a 2-mm downward motion followed by a 0.5-mm upward motion (type S2), and a servo motion path with a 1-mm downward motion followed by a 0.5-mm upward motion (type S1). The finished products of an elliptical hole flanging process were evaluated in terms of thickness, force-displacement, stress, strain, and springback. The results reveal that the products produced using the evaluated servo motion paths were superior to those produced using the evaluated crank motion path.

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.

Influence of Final Heat Treatment Modes on Structure and Properties of High Strength Sheet Steel

Influence of Final Heat Treatment Modes on Structure and Properties of High Strength Sheet Steel

Clinching of High-Strength Steel Sheets with Local Preheating

Clinching is a manufacturing method of mechanically joining two or more materials without the use of heat or additional components. This process relies on high plastic deformation to create a secure bond. Clinching technology is widely used for joining materials of various grades and thicknesses. Especially in the automotive industry, clinching is an alternative to resistance spot welding. However, the load-bearing capacity of clinched joints is comparatively lower when compared to resistance spot-welded joints. This research aimed to increase the load-carrying capacity of clinched joints. To enhance the load-bearing capacity of the clinched joints, localized modification of the microstructure was carried out, primarily focusing on the neck area of the joint. The alteration of the microstructure within the clinched joint was accomplished through the application of localized heating using the resistance spot welding method. The microstructure distribution in the clinched joint region was analyzed using light and scanning electron microscopy, as well as microhardness measurements. Two material grades, micro-alloyed steel HX420LAD+Z and dual-phase ferritic–martensitic steel HCT600X+Z, were tested. Each grade underwent five groups of ten samples, which were subjected to identical experimental conditions of local heating by resistance spot welding (RSW) and clinching. The utilization of RSW on the clinched joint region resulted in an average enhancement of 17% in the load-carrying capacity for material HCT600X+Z, and an average increase of 25% for material HX420LAD+Z.

Experiment for Measuring Mechanical Properties of High-Strength Steel Sheets under Cyclic Loading by V-Shaped Double-Shear-Zone Specimens.

The simple shear test shows significant advantages when measuring the hardening and shear properties of thin sheet metal at large strains. However, previous shear tests had an end effect caused by local stress concentration and a boundary effect caused by deformation overflow, resulting in non-uniform strain distribution in the shear zone. Therefore, a unique V-shaped double-shear-zone specimen is proposed to measure the Bauschinger effect under cyclic shear loading conditions in this paper. Simple shear experiments and three different types of cycle shear experiments are conducted to analyze the uniformity of deformation in the shear zone and the effect of pre-strain and the number of cyclic loads on the Bauschinger effect of Q890 high-strength steel sheets. The results indicate that the proposed V-shaped double-shear-zone specimen can still maintain uniform shear deformation in forward/reverse cyclic loading experiments, even at large strains. Q890 high-strength steel exhibits a significant Bauschinger effect, which is more pronounced with the increase in shear pre-strain and loading cycles. The results of this paper provide a new approach for studying the hardening characteristics under large strain and the mechanical properties under cyclic shear loading for metal sheets.

Development of resistance spot welding technology applying multi-stage adaptive control for narrow pitch spot welding of high strength steel sheets

In order to expand the application of high strength steel sheets to automotive bodies, stabilizing nugget diameter of resistance spot welds is required in the presence of several industrial disturbances. This study aims to ensure the nugget diameter of high strength steel sheets regardless weld spacing by utilizing adaptive control technology based on real-time feedback of heat quantity. Short weld spacing leads to expulsion from the sheet surface applying conventional adaptive control welding for mild steel sheets, whereas decrease in nugget diameter due to reduction of effective welding current for high strength steel sheets. On the other hand, two-stage adaptive control method enables to ensure nugget diameter regardless of steel strength even with 10 mm weld spacing, in which heat quantity was controlled independently in each stage of welding. The mechanism of these phenomena was revealed by verifying the influence of physical properties of steel and welding current pattern on the shunting phenomena using numerical simulation.

Wear characteristics of the tool in the cold stamping process of ultra high strength steel sheets by establishing a novel wear test method based on the progressive die

Demands for lightweighting and crashworthiness of the vehicle body have increased. For this purpose, advanced high strength steel to the automobile body was regarded as a solution with respect to manufacturing cost and impact energy absorption. However, increasing the strength of the automotive steel sheet lead to a diversity of problems caused by the tool wear due to higher forming load than that of commonly used steel sheets. Hence, systematic wear experiment and evaluation methods are required to quantitatively and qualitatively evaluate the tool wear amount in the sheet metal forming process. In this study, a methodology is proposed to quantitatively evaluate the wear of sheet metal forming tool based on the experimental results. In order to carry out a systematic wear test and save the time and cost, the progressive die set was designed to be suitable for wear test. The designed testing machine can simultaneously test four types of punches made under various tooling conditions such as materials, shapes, and coatings. Through the measurement of the wear depth, roughness, and surface imaging of the punch and product roughness, which represent the wear characteristics, quantitative evaluation methods for tool wear in the sheet metal forming process are established. By referring to the wear test results, it is confirmed that it is appropriate to analyze the reasonable tool wear characteristics by using the proposed methodology for quantifying the tool wear in the sheet metal forming process.

A uniform method for the characterization of shear fracture strain for high-strength steel sheets under quasi-static and crash loading

In the conflict between lightweight design and crash safety of high-strength steel components failure models for crash simulation have to be calibrated precisely. To predict shear failure strains in a wide strain rate range shear experiments must be performed from quasi-static up to crash-relevant strain rates of about 100 s-1. In particular, asymmetric notched shear tensile specimens have proven useful for this purpose in previous investigations. Using this type of shear specimen, a characterization concept for shear loading in a wide strain rate range is presented for high strength steel sheets of different strength, ductility and sheet thickness in this contribution. Within a round robin test of different laboratories the influence of the degrees of freedom in the clamping area and the free clamping length on the kinematic of the specimens and on the deformation and failure behavior in the shear zone are investigated in detail. The main outcome is that the disadvantages of a fix clamping in terms of shear deformation, shear fracture and homogeneity of the shear zone can be compensated by a large free clamping length. An evaluation method based on digital image correlation (DIC) is presented, which enables an operator- and laboratory-independent determination of shear strains up to failure.

Multi-point incremental bending of doubly curved high strength steel sheet under minimum strain energy loading path

Multi-point incremental bending of doubly curved high strength steel sheet under minimum strain energy loading path

Fracture toughness to assess the effect of trimming on the fatigue behaviour of high-strength steels for chassis parts

High-strength steels are widely used in vehicle body-in-white, offering a good balance between crashworthiness and lightweight design. The increased requirements of heavier electric vehicles, in terms of fatigue resistance and crashworthiness, highlight that chassis parts have remarkable lightweighting potential. However, applying these grades in chassis parts is not straightforward, as the forming processes, like trimming, may introduce surface defects that compromise the fatigue resistance of the component. This work presents a material selection strategy for the applicability of high-strength steels in chassis parts of electric vehicles. The proposed approach allows the evaluation of the key parameters of the chassis parts in a simple way. The crash performance is evaluated through fracture toughness using the essential work of fracture (EWF) methodology. The method is applied to thin high-strength steel sheets employing double-edge notched tensile specimens (DENT). On the other hand, fatigue performance is investigated in terms of fatigue resistance for both notched and unnotched specimens. The results for different complex-phase and dual-phase steels show a good agreement between the EWF and the fatigue notch factor. The method could help apply high-strength steel to chassis parts, as designers will have a tool to focus the expensive fatigue tests on the best material candidates.

Effect of pre-strain on edge formability of high strength steels for chassis applications

There is a high level of uncertainty in the prediction of edge cracking during the design and manufacturing of high strength sheet metal products. Edge ductility is a complex parameter to measure since it is not an intrinsic property of the material, but it also depends on the trimming strategy/parameters that have been applied to the edge (edge preparation method, cutting/punching clearance, etc.). Furthermore, most forming processes involve more than one operation where the material can be deformed in a first stage, then trimmed or punched and further deformed in a final forming step to obtain the desired shape. Therefore, in order to design a robust and safe forming process able to predict the risk of edge cracking, it is crucial to understand the influence of pre-strain on the residual formability of trimmed edges. This paper analyses the effect of different pre-strain levels and paths (uniaxial tension, plane strain and equi-biaxial stretching) on the sheared edge ductility of a Complex Phase (CP) steel and a high strength low alloy (HSLA) steel used for automobile chassis applications. To this purpose, rectangular specimens are cut out from pre-strained samples and punched. Hole Tension Tests (HTTs) in combination with Digital Image Correlation (DIC) are carried out to determine the edge fracture strain of undeformed and pre-strained samples. The results are discussed in terms of major strains obtained from DIC. The proposed methodology allows describing the edge cracking sensitivity of high-strength steel sheets as a function of the pre-deformation state and can be useful to predict the risk of edge cracking more accurately in forming simulations.

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.

Comparison of RSW technologies on DP steels with modified instrumented Charpy impact test

In recent decades the toughness of welded structures becomes more and more important. This trend is particularly valid for the advanced high-strength steels (AHSS), which have reduced toughness comparing to the low-strength structural steels. Dynamic characteristics of the welded joints of the newly developed high-strength steel sheets required by the automotive industry have been neglected, with welding procedures being optimized on the static joint properties, mainly on the tensile-shear force. Up to the present time, testing of dynamic properties of the spot-welded joints has been performed by increased testing speed during tensile testing. The authors have developed a new dynamic testing method and designed new testing equipment for impact bending, which can give a numeric result to characterize the resistance of spot-welded joints against dynamic load. In this paper, this method will be used to evaluate resistance spot-welded joints made on DP600 and DP800 steels with three different technological parameters, comparing long-time welding, short-time welding, and two-pulse welding. The different parameters cause different weld nugget sizes, failure modes, impact energies, and force–time diagrams. All test results show that bigger weld nugget diameters cause better impact energies and impact forces, but the differences are not perfectly correlated.

Improved Cross-tension Strength of a Friction-element-welding Joint by Tempering Treatment Using Electric Heating

Resistance spot welding is widely used in automobile assembly, but reduction of the cross-tension strength (CTS) is inevitable when using high-strength steel sheets. Therefore, we focused on joining high-strength steel sheets using friction element welding (FEW), which is mainly used to join steel sheets and aluminum alloy sheets, to improve the CTS. However, even when using FEW, the CTS decreases at the joint, when a high-carbon-content steel sheet is used as the lower sheet. This CTS decrease is presumed to be due to the low local ductility caused by the large hardness difference (ΔHn) between martensite (M) and ferrite (α) in the inter-critically annealed and quenched area during joining. The local ductility of the material having complex structure with hard and soft phases is affected by the structural morphology and ΔHn. Therefore, we attempted to improve the CTS by creating joints whose α of the inter-critically annealed and quenched area was equiaxed or acicular, and performed tempering treatment using electric heating to reduce ΔHn of this part. The results showed that ΔHn decreased at the annealed joint and the CTS improved. Furthermore, the highest CTS was observed at the joint having acicular α when tempering was performed. It was inferred that the reduction of ΔHn using electric heating improved the local ductility and CTS of the area. Furthermore, it was considered that the local ductility and CTS are also influenced by the α morphology.

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>

Understanding the Formation Mechanism of Al-Rich Interfacial Layer during Galvanizing of Cu Pre-coated High-Strength Steel Sheet

Understanding the Formation Mechanism of Al-Rich Interfacial Layer during Galvanizing of Cu Pre-coated High-Strength Steel Sheet

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.