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Key factors of stretch-flangeability of sheet materials

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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 [14]. 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 [511].
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.
... However, forming a flanged hole in high-strength steels is more difficult than forming one in conventional mild steel. As the strength of the steel sheet increases, edge cracking at the flanged tip is more likely to occur, even under a small deformation, due to the reduced stretch-flangeability [5,6]. The hole expansion rate (HER), a key index indicating the formability of the hole-flanging operation, decreases linearly with an increase in tensile strength for steels exhibiting a tensile strength of 590 MPa or less [7]. ...
... Each punch installed on the facing side of the tool acted as a die during the operation of the corresponding opposite punch. even under a small deformation, due to the reduced stretch-flangeability [5,6]. The hole expansion rate (HER), a key index indicating the formability of the hole-flanging operation, decreases linearly with an increase in tensile strength for steels exhibiting a tensile strength of 590 MPa or less [7]. ...
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The recent stringent regulations on vehicle safety and reducing CO2 emissions have led to a continuous increase in the application of press-hardened steel (PHS) in automobiles. Similar to other high-strength steels, assembling PHS components using the common welding techniques employed in automotive production lines is significantly difficult because of the surface coating layers and the additives within. This difficulty in post-processing, attributed to its high strength, also limits the mechanical fastening of PHS components. Therefore, this study aims to develop a process for forming a structure enabling mechanical fastening by sequentially applying piercing and hole-flanging operations during the hot stamping process. Our experimental apparatus was designed to perform the hole-flanging operation after the piercing operation within a single stroke at a specific temperature during the quenching process of PHS. At high temperatures of 440 °C or higher, the hole-flanging process was conducted in a direction opposite to that of the piercing operation for creating the pilot hole. An extruded collar with a height of 8.0 mm and a diameter of 17.5 mm was achieved, which is hole expansion ratio(HER) of 82.5%.
... The mechanical properties have predominantly been investigated for their correlation with anisotropy [23], strain-rate sensitivity [23,24], and strain-hardening exponent [25], which can be obtained via a uniaxial tensile test. However, Yoon et al. [26,27] reported a comparison of the mechanical properties and HER of various alloys and demonstrated that the HER has a distinct correlation with fracture toughness rather than tensile properties. These works directly compared the fracture toughness and HER of various alloys and corroborated a linear correlation. ...
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The demand for high-strength, lightweight structural materials has been increasing; however, alloys with high mechanical performance often have limitations such as unfavorable bendability and stretch-flangeability. In terms of stretch-flangeability, an unexpected crack occurring on the trim line during sheet metal forming has recently been considered as an issue of concern by the industry and academia, but its cause and solution have not been determined yet. This study investigated the effect of the phase interface generated by deformation-induced martensitic transformation on the stretch-flangeability of metastable ferrous medium-entropy alloys. We prepared three alloys that exhibited different transformation-induced plasticity behaviors by controlling their grain sizes and chemical compositions. It was observed that the transformation activity, which is dependent on the microstructure and phase stability, considerably affects the hole expansion ratio obtained from the hole expansion test. Microstructural analyses revealed that deformation-induced martensite formed a layered structure and the phase interface between the face-centered cubic (FCC) and body-centered cubic (BCC) phases served as a site vulnerable to cracking during stretch-flanging. The decreased transformation activity by grain refinement resulted in an increase in the hole expansion ratio (HER) from 8.6% to 28.4%. On the other hand, when the phase stability is low enough to form athermal martensite during quenching, most of the FCC grains are easily transformed into BCC phase and the phase interface is also decreased during stretch-flanging, resulting in the HER of 22.9%. Therefore, the increment of phase interfaces leads to lower stretch-flangeability. This study sheds lights on expanding the applicability of TRIP-utilized materials to industry by suggesting ways to improve stretch-flangeability.
... In addition, it was observed that the results obtained from the inverse calibration method were consistent with the experimental results. Apart from these works, Yoon et al. (2017) investigated the effect of the fracture toughness on the prediction of hole expansion ratio (HER) of various AHSS. It was seen that the results were in accordance with the experimental results. ...
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Although the uniaxial tensile test gives information about the formability limits of the material, different formability tests are required to obtain the stretch formability limit of the material. Hole expansion test is a process performed to obtain the formability limit considering the stretching conditions of a sheet with a hole. For automotive industry, this forming limit has a crucial role in the reliability of the engineering parts exposed to the several stretching modes. In the present study, the hole expansion tests of twinning induced plasticity (TWIP940) and transformation induced plasticity (TRIP590) steel sheets were simulated using finite element method. Implicit Msc. Marc software in conjunction with the Hypela2 user subroutine file, was used for the numeric solutions. The plastic work-based failure criterion was incorporated into the subroutine and the homogeneous fourth-order polynomial-based yield function (HomPol4), was considered to define the bound of the yield loci. Hole expansion ratio and the failure strain predicted from simulations were compared with the experimental results to assess the capability of the HomPol4 criterion. It was seen that the numerical results were in good agreement with the experimental results for both steel sheets. Moreover, failure stroke values were predicted using the failure criterion. Stroke value was successfully predicted for the TWIP940 steel, which does not exhibit a significant amount of strain localization. On the other hand, a difference between the numerical and experimental results was observed for TRIP590 steel sheet.
... Jha et al. [12] successfully developed a high strength ferritic steel with good ductility for automotive application by mainly using Ti and Mo additions. However, the hole expansion ratio, which is crucial for evaluating the stretch-flangeability, was not measured for the steel [13]. Compared to niobium and titanium carbonitrides, V(C,N) has a much higher solubility in both austenite and ferrite [9]. ...
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The mechanical properties and precipitation behavior of hot-rolled microalloyed steels with varied Nb and V additions were investigated. The steels had a predominantly ferritic microstructure with a very good combination of strength (σUTS = 800-1000 MPa), ductility (total elongation = 16-19%) and hole expansion ratio (32-34%). The strength increased with increasing Nb and V content. The stretch-flangeability of the steels did not deteriorate with increasing strength. The favorable hole expansion behavior of the steels is attributed to the fine ferrite grain size (1.6-1.8 μm) and uniformity of the microstructure, as well as the fine precipitates of Nb and V carbonitrides in the matrix. Nb carbonitrides formed during hot rolling and contributed to the grain refinement through their interaction with recrystallization. The co-precipitation of Nb and V carbonitrides during coiling was confirmed and characterized by 3D atom probe tomography. The formation of these fine precipitates contributed to further increase in the strength of the V-containing steel allowing the steel to achieve strength > 1000 MPa.
... But the current grades of AHSS used in automobiles suffer from certain limitation. The AHSS provide adequate strength but possess poor stretch-flangeability [24], a property that is essential for the fabrication of automotive components, particularly the suspension, long members, wheel rim and disc etc. In general, stretch-flangeability of high strength sheet steel can be improved by increasing the local elongation. ...
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