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Key factors of stretch-flangeability of sheet materials

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Jae Ik Yoon at Samsung Electronics Company
Jae Ik Yoon
  • Samsung Electronics Company
Jaimyun Jung
Jaimyun Jung
Jaimyun Jung
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Jung Gi Kim at Gyeongsang National University
Jung Gi Kim
Seok Su Sohn at Korea University
Seok Su Sohn
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Abstract and Figures

Stretch-flangeability evaluated using hole-expansion testing represents the ability of sheet materials to resist edge fracture during complex shape forming. Despite a property imperative for automotive part applications of advanced high-strength steels, factors governing stretch-flangeability are not yet well understood. In this study, the mechanical properties of a selected group of materials with different microstructures were investigated using tensile, fracture toughness, and hole-expansion tests to find the factor governing the stretch-flangeability that is universally applicable to a variety of metallic materials. It was found that the fracture toughness of materials, measured using the fracture initiation energy, is a universal factor governing stretch-flangeability. We verified that fracture toughness is the key factor governing stretch-flangeability, showing that the hole-expansion ratio could be well predicted using finite element analysis associated with a simple ductile damage model, without explicitly taking into account the microstructural complexity of each specimen. This validates the use of the fracture toughness as a key factor of stretch-flangeability.
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METALS
Key factors of stretch-flangeability of sheet materials
Jae Ik Yoon
1
, Jaimyun Jung
1
, Jung Gi Kim
1
, Seok Su Sohn
2
, Sunghak Lee
1,2
, and Hyoung Seop Kim
1,2,3,
*
1
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673,
Republic of Korea
2
Center for Advanced Aerospace Materials, Pohang University of Science and Technology (POSTECH), Pohang 37673,
Republic of Korea
3
Center for High Entropy Alloys, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
Received: 19 December 2016
Accepted: 15 March 2017
Published online:
20 March 2017
ÓSpringer Science+Business
Media New York 2017
ABSTRACT
Stretch-flangeability evaluated using hole-expansion testing represents the
ability of sheet materials to resist edge fracture during complex shape forming.
Despite a property imperative for automotive part applications of advanced
high-strength steels, factors governing stretch-flangeability are not yet well
understood. In this study, the mechanical properties of a selected group of
materials with different microstructures were investigated using tensile, fracture
toughness, and hole-expansion tests to find the factor governing the stretch-
flangeability that is universally applicable to a variety of metallic materials. It
was found that the fracture toughness of materials, measured using the fracture
initiation energy, is a universal factor governing stretch-flangeability. We veri-
fied that fracture toughness is the key factor governing stretch-flangeability,
showing that the hole-expansion ratio could be well predicted using finite ele-
ment analysis associated with a simple ductile damage model, without explicitly
taking into account the microstructural complexity of each specimen. This val-
idates the use of the fracture toughness as a key factor of stretch-flangeability.
Introduction
Currently, automotive industries are subjected to
strong pressure to reduce exhaust emission of their
automobiles, which is an important cause of air pol-
lution. To address this imposition, automotive
industries have focused on the development of a
lightweight body-in-white using high-strength sheet
steel. The development of lightweight automobile
bodies has a variety of other goals as well, such as
increasing fuel efficiency, increasing strength and
safety, and improving durability [1–4]. Advanced
high-strength steels (AHSS) with excellent tensile
properties such as dual phase (DP), transformation
induced plasticity (TRIP), twinning induced plastic-
ity (TWIP), quenching and partitioning (Q & P), and
lightweight steels have been developed to meet these
goals [5–11].
However, formability, which is the ability to be
shaped into a desirable structure without fracture,
becomes a problem in high-strength grade steels.
Generally, the formability of these steels is
Address correspondence to E-mail: hskim@postech.ac.kr
DOI 10.1007/s10853-017-1012-y
J Mater Sci (2017) 52:7808–7823
Metals
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Stretch-flangeability is a crucial parameter in the fabrication of complex-shaped automotive parts, such as door panels, fenders, and body-in-white components, requiring extensive edge stretching during forming operations 12 . It is evaluated by hole expansion ratio (HER), a significant parameter in assessing the local ductility of DP or CP steels, especially during the complex shape forming process in the automotive industry 12,13 . HER values are determined by conducting hole expansion tests (HET) following ISO 16630 14 . ...
... For instance, in order to precisely predict HER values without performing HETs, linear or nonlinear correlations were established based on simple uniaxial tensile properties, including normal anisotropy ( R) 15,16 , post uniform elongation (PUE) 15,17 , strain hardening ratio (n) 18 , and UTS 15,17 . Also, some other mechanical property, such as initial fracture energy, was found to be closely related to stretch-flangeability 12 . However, these relationships are only valid for several AHSS grades, such as interstitial free (IF), bake hardened (BH), mild, and high strength low alloy (HSLA) steels, but not for DP or CP steels. ...
... In this study, 212 datasets from earlier studies 15,17,[31][32][33][34][35][36][37][38][39][40][41][42]12,[43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61] containing chemical compositions, microstructural characteristics and mechanical properties of AHSS, especially DP and CP steels, were analyzed and processed, completing missing values via multivariate imputation by chained equations (MICE). Then three ML algorithms, such as support vector machine (SVM), symbolic regression (SR) and extreme gradient boosting (XGBoost) were employed to determine correlations between HER, UTS or TE with compositions, including carbon (C), carbon equivalent (CE), manganese (Mn), silicon (Si), and chromium (Cr), and volume fractions of different phases, such as ferrite (VF), bainite (VB), martensite (VM) and tempered martensite (VTM). ...
Prediction and optimization of stretch flangeability of advanced high strength steels utilizing machine learning approaches
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Advanced high strength steels (AHSS) exhibit diverse mechanical properties due to their complex chemical compositions and microstructures. Existing machine learning (ML) studies often focus on specific steel grades, limiting generalizability in predicting and optimizing AHSS properties. Here, an ML framework was presented to predict and optimize the stretch-flangeability of AHSS based on composition-microstructure-property correlations, using datasets from 212 steel conditions. Support vector machine, symbolic regression, and extreme gradient boosting models accurately predicted hole expansion ratio (HER), ultimate tensile strength (UTS), and total elongation (TE). Shapley additive explanations revealed the importance of bainite volume fraction (VB), carbon content (C), and chromium content (Cr) for HER, UTS, and TE, respectively. Multi-objective optimization generated 252 optimized conditions with improved comprehensive mechanical properties. The best optimized chemical compositions (0.12wt.% C-1.10Mn-0.15Si-0.47Cr) along with the carbon equivalent (CE) of 0.44 wt.%, and microstructural features (7.2% ferrite, 44.5% bainite, 40.5% martensite, and 7.8% tempered martensite) yielded HER of 119.8%, UTS of 1013.5 MPa, and TE of 22.7%. This systematic framework enables efficient prediction and optimization of material properties (especially HER), with potential applications across various fields of materials science.
View
... DP has large techno-commercial implications [22,42]. However, its usage is often restricted by the so-called hole expansion ratio (HER) or non-uniform elongation [43][44][45][46]. The HER might degrade further during paint-baking, where hydrogen embrittlement is a usual suspect [47,48]. ...
... HER is a measure of a hole's diameter expansion during a 'customized' test [66] for stretch-flangeability. More generically, the HER depends on the post-necking ductility or non-uniform elongation [43][44][45][46]. We have examined, in greater detail [45,66], the role of various microstructural attributes on the HER. ...
View
... On the other hand, strain localization leads to the formation of micro-defects at the interfaces between microconstituents (see Fig. 7), which can act as preferential sites for crack initiation [6,18,50,51]. Although this effect becomes more pronounced by reducing the ductility of steel [24], 500MC, with the highest fraction of defects, and 700MCPlus, with the lowest ductility, both show the smallest reductions in post-punching fatigue limit. ...
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This study aims to investigate the influence and mechanism of strain rate variation on the hole expansion ratio of complex phase advanced high-strength steels. Two different speeds of hole expansion tests were conducted within the quasi-static range, accompanied by a uniaxial tension test with a deformation mode similar to hole expansion for supplementary analysis. Finite element simulation was utilized to analyze the detailed deformation behavior within the material, particularly at the hole edge where cracks occur during hole expansion. The mechanical and fracture properties obtained from the uniaxial tension test were incorporated into the simulation, taking into account the anisotropy of the material to predict the precise location of crack initiation within the hole edge. To account for the strain rate effect on the experimentally determined hole expansion ratio, a ductile fracture model was introduced and its necessity was validated by considering the occurrence of material fracture before and after crack initiation. By utilizing 3D solid elements, considering material anisotropy, and applying the ductile fracture model, the simulation provided reasonable predictions for the hole expansion ratio, which exhibited variation with strain rate.
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Hot-rolled steel grades can serve as cost-effective and high-performance solutions for automotive and heavy transportation chassis and seat parts. These applications generate significant demand for cut-edge quality, stretch flangeability, and fracture toughness. This study investigated two thermomechanically hot-rolled steel grades, 850F and 850CP, both of which exhibited a tensile strength of 850 MPa. To assess the performance in chassis and seat applications, materials were examined using ISO 16630 hole-expansion, mechanical punching, shearing, and fine blanking tests. Thin sheet fracture toughness was determined by essential work of fracture (EWF) tests, with fatigue pre-cracks. The test results indicate that the 850CP material demonstrates only a marginally better hole-expansion ratio (HER), but significantly enhanced cut edge quality in punching, shearing, and fine blanking processes. Another substantial difference is the fracture performance of the materials, with 850CP exhibiting a superior essential work of fracture (We) compared to 850F: 545 kJ/m² vs. 203 kJ/m², respectively. The results indicate that 850CP is a highly suitable material for applications requiring superior cut-edge quality across diverse cutting processes, in conjunction with exceptionally high fracture toughness. The results also demonstrate that the fracture toughness can be a good indicator of the sheared-edge crack propagation behavior.
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Advanced high strength steels (AHSS) exhibit diverse mechanical properties due to their complex microstructures. Existing machine learning (ML) studies often focus on specific steel grades, limiting generalizability in predicting and optimizing AHSS properties. Here, an ML framework was presented to predict and optimize the stretch-flangeability of AHSS based on microstructure-property correlations, using datasets from 212 steel conditions. Support vector machine, symbolic regression, and extreme gradient boosting models accurately predicted hole expansion ratio (HER), ultimate tensile strength (UTS), and total elongation (TE). Shapley additive explanations revealed the importance of bainite, martensite, and ferrite volume fractions for HER, UTS, and TE, respectively. Multi-objective optimization generated 170 optimized conditions with improved comprehensive mechanical properties. The best optimized microstructural features (7.2% ferrite, 44.5% bainite, 40.5% martensite, 7.8% tempered martensite) yielded HER of 113.6%, UTS of 999.6 MPa, and TE of 25.0%. This systematic framework enables efficient prediction and optimization of material properties, with potential applications across various fields of materials science.
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Advanced High Strength Sheet Steels Physical Metallurgy, Design, Processing, and Properties ▶
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Stretch-flangeability representing the capability of a sheet material to form into a complex shaped part is not a well-known sheet metal forming property. We correlate mechanical properties with stretch-flangeability of various advanced high strength steels (AHSSs) to capture the stretch-flanging phenomenon and improve the stretch-flangeability of steel sheet materials. The stretch-flangeability of materials is usually evaluated using a hole expansion test. During the hole expansion test, the stress state in the hole edge part of the specimen is almost the same as that of the uniaxial tensile test. However, a single parameter in tensile properties of the AHSSs exhibits no clear correlation with flangeability estimated as the hole expansion ratio (HER). Because micro-cracks in the hole edge region of the hole expansion testing samples play a significant role in HER values, we propose and demonstrate that fracture toughness is the key factor governing the HER of AHSSs.
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